Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 299-311

The Steelpoort Park Granite is a lenticular, pad-shaped intrusion, approximately 16 miles long, which cuts obliquely across almost the entire Upper and Main Zones of the eastern section of the Bushveld Complex. The petrography of this granite is discussed and compared with that of Bushveld granite. A remarkable contact zone, in which extensive brecciation and assimilation have taken place is dealt with in detail. That the granite is younger than the gabbro is demonstrated by the intrusive relationships. Magnetitite seams occurring in the country-rock are classified and compared with those in other areas of the Complex. Special reference is made to some irregularities in the normal sequence of the magnetitite seams. Ten new chemical analyses of magnetitite (4), granite (2), hybrid rock (2), fayalite diorite (1), and leptite (1) are presented.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 108-207

The Bushveld Complex, which is the largest mafic and ultramafic complex of its kind in the world does not contain copper and nickeliferous ores, with the exception of the Merensky Reef and the sulphide pegmatoids of the Western Transvaal. Ores belonging to the latter type have almost been worked out. However, minor quantities of disseminated sulphides are found throughout the whole sequence of layered rocks of the Main Plutonic Phase, the Hypabyssal Phase, the Late Plutonic Phase and the Volcanic Phase. The investigation reported here is restricted to the rocks of the Main Plutonic Phase and the Late Hypabyssal Phase.
From field observations and bore-hole investigations it is clear that cross-cutting relationships exist between the rocks of the Basal Zone and the sediments of the Pretoria Series in the Western Transvaal. Huge blocks of metasediment were lifted from the floor to form inclusions in the plutonic rocks. The bore-hole results reveal that in the deepest parts of the basin the floor of the Complex has not yet been explored, particularly in the region west of the Pilanesberg.
It was found that disseminated sulphide seldom exceeds 2% by volume of the rock. Abnormal concentrations of sulphide are known only from certain parts of the Basal Zone, from sulphide-bearing pegmatoids, from the Merensky Reef and from the mineralized anorthosites below the Main and the Uppermost Magnetitite Seams. Generally the sulphides constitute approximately 0.05% by volume of the layered rocks. The sulphides are the three modifications of pyrrhotite (troilite, hexagonal and monoclinic pyrrhotite, distinguished from one another by X-ray diffraction and etching methods), pentlandite, chalcopyrite, cubanite, pyrite, mackinawite, sphalerite, bornite galena(?), safflorite-loellingite linnaeite, millerite, chalcopyrrhotite(?) and the nickel-arsenic-bearing minerals, gersdorffite, maucherite, niccolite, rammelsbergite(?) and ullmannite(?). The secondary minerals are valleriite, melnikowite-pyrite, bravoite and covellite. Not much attention was paid to the minerals containing the platinum group metals, although a few have been identified; sperrylite, braggite, cooperite and michenerite(?).
A differentiation trend was established in the mineralogical composition of the sulphide phase. From the base to the top of the Main Plutonic Phase there is a decrease in pentlandite and an increase in chalcopyrite. The latter reaches its maximum in the mineralized anorthosite below the Main Magnetitite Seam. In the Upper Zone chalcopyrite decreases with an increase of pyrrhotite. The sulphides in any pegmatoid, which is considered to be a differentiation product of the host-rock, are also enriched in copper sulphide and pyrrhotite in comparison with the adjoining country-rock. In addition to this differentiation trend, the sulphide of the mafic rock, which contains little sulphide (0.1%), consists mainly of chalcopyrite. Mackinawite in pentlandite appears in these rocks below the Merensky Reef, in association with metal-rich sulphides such as troilite and cubanite, Galena(?) is present only in higher horizons of the layered sequence.
All the sulphides involved in the Fe-Ni-S system exsolved from a solid solution of the monosulphide (FeNi)1-xS above 863°C. The coarse-grained pentlandite in the sulphur-deficient sulphide phase formed by reaction between (NiFe)3±xS2 and pyrrhotite at 610°C. Exsolution flames and bodies of pentlandite, as well as coarse-grained pentlandite, the latter being found in association with sulphur-rich sulphides, are exsolved from the sold solution of the monosulphide below ~845°C. It would appear that mackinawite exsolved only from pentlandite that originated by reaction between (NiFe)3±xS2 and (NiFe)1-xS. According to Kullerud (1962) pyrrhotite or Fe1-xS exsolved from pentlandite below 580°C. It is suggested that Fe1-xS inverted to mackinawite at ˜93°C, perhaps at higher pressures. Yund and Kullerud (1966) mentioned that 4-5% by weight of Cu could be in solid solution in Fe1-xS at 700°C. A great deal of the copper sulphide in the ores in the Bushveld Complex thus also exsolved from the solid solution of monosulphide. Cubanite is not stable above ~440°C and would exsolve from a solid solution of chalcopyrite. A degree of solid miscibility exists between nickeliferous ores from volcanic rocks (e.g. the Igdlukunguaq extrusive basaltic "dyke"), hypabyssal rocks (e.g. the Petolahti diabase) and plutonic rocks (e.g. the Bushveld, Insizwa and Sudbury), decreasing in the order given. At certain localities, in the Western Bushveld, south of Potgietersrus and in the vicinity of Burgersfort, disseminated sulphides are found in the Basal Zone mainly in those regions where a definite cross-cutting relationship exists between the plutonic and the metasedimentary rocks. Sedimentogenetic sulphides are known from the Pretoria Series. The metasediments in the Western Transvaal and at Potgietersrus also contain some sulphide, mainly pyrrhotite or pyrite, of possible sedimentogenetic origin. Arsenic minerals and graphite are present in the metasediments and in the adjacent plutonic rocks, as well as in the sulphide pegmatoids in the Western Bushveld. To account for the abnormal quantity of disseminated sulphide and sulphide pegmatoids in the regions mentioned, it is postulated that assimilation took place of S, As and C from the metasediments by the plutonic rocks. Where the Merensky Reef is in contact with metasediments (north of Potgietersrus and south-east of Groblersdal) abnormal quantities of compounds containing the platinum group metals, arsenic and bismuth(?) have been noticed. This phenomenon is attributed to the assimilation of As and Bi(?) from the metasediments. The As and Bi apparently reacted with the platinum group metals which were in solid solution in the Cu-Ni-Fe sulphides to produce arsenides and bismuthides. In this area S was also assimilated to a considerable degree. The intercumulus material in certain rock types in the Western Transvaal consists largely of sulphide It is thus expected that a pegmatoidal fluid which originated out of the intercumulus liquid would be sulphur-rich and would give rise to fairly massive sulphide pegmatoid. Owing to the fact that Ni2+ replaces Fe2+ diadochically in the early ferromagnesian silicates, the Ni content decreases from the bottom to the top of the layered sequence. The Upper Zone which is relatively Fe-rich has, however, also a fairly large Ni content. Cu, Co and Zn vary sympathetically but remain more or less constant throughout the layered sequence. These elements also tend to replace Fe in the silicates and oxides. Cu, Ni, Co and Zn vary sympathetically with S. Most of the disseminated sulphide in the Bushveld Complex crystallized from an immiscible sulphide liquid trapped in the intercumulus pores. In the Skaergaard intrusion it was found that Cu sulphides crystallized when the sulphur content of the magma was relatively low, whereas Fe sulphides are accompanied by a relatively high sulphur content. By examining the Bushveld Complex on the basis of these facts, certain concentrations of sulphide in the layered rocks of the Complex are explained. During the crystallization of the mafic rocks in the Critical Zone the S content in the magma was relatively low and consequently mainly chalcopyrite was formed. Not enough Cu was present to remove all the S in the magma and its content thus gradually increased. Soon after a change in the composition of the magma in the zone of crystallization, a copper-nickeliferous immiscible liquid formed, representing the Merensky Reef. The Merensky Reef could also be explained by crystallization of a sulphide-oxide-silicate eutectic at a high oxidation potential. During the crystallization of the Main Zone the conditions were the same as those in the Critical Zone, except that the oxidation potential of the magma during the crystallization of the upper part of the Main Zone was relatively high. The last liquid crystallized at a sulphide-oxide-silicate eutectic. Immediately after this event introduction of a new surge of magma or a change in the composition of the magma took place whereby the Main Magnetitite Seam crystallized. The mineralized anorthosite below the Uppermost Magnetitite Seam is a repetition of the former mineralized anorthosite.
Back to title list




Namaqualand Metamorphic Complex. Special Publication No.10, 173-191

The Upington geotraverse spans a critical region of the Namaqua mobile belt, namely the region between the Namaqua Province and the Kheis Province and adjacent Kaapvaal craton. Eighteen rock units from this eastern region of the Namaqua mobile belt were investigated by some or all of the following isotopic methods: Rb-Sr, Pb-Pb, Th-Pb total-rock and U-Pb zircon age measurements. The polyphase deformation and metamorphism experienced by these rocks has disturbed the isotope systematics to various degrees. Despite the resulting large uncertainties in the radiometric ages, which preclude the establishment of a detailed chronology of the separate events, the Namaqua tectogenesis in this region is bracketed in the interval approximately 1 300 to 1 100 Ma ago. The isotopic data do not indicate the reworking or isotopic resetting of pre-existing crust older than 2 000 Ma, and in this marginal region of the Namaqua Province a major contribution of mantle-derived material to the crust approximately 1 300 to 1 200 Ma ago is implied. The Sr- and Pb-isotopic compositions of meta-volcanic rocks extruded at this time are compatible with an interpretation of a mantle source into which some recylcing of crustal material has taken place. This situation is envisaged to reflect an accreting continental margin. Combined with the apparent absence of Archaean crust under the Namaqua Province and the evidence suggesting extensive reworking and isotopic resetting the data are interpreted as reflecting continent-continent convergence during the Namaqua tectogenesis.
Back to title list




Some sedimentary basins and associated ore deposits of South Africa. Special Publication No.6, 151-160

It is recommended that the term refractory clay be used for all clays with a fusion point higher than Seger cone 27, and that formal usage of the ambiguous term fireclay be avoided. The area investigated on the East Rand is situated between 28°15' and 29°00' E and between 26°00' and 26°30'S. The Karoo sequence covers about 75 percent of this area. Refractory clay deposits occur at or near the base of the Vryheid Formation. An isopach map of the Karoo sequence is presented and can serve as a prospecting guide; areas where the Karoo cover is thicker than about 30 m can be eliminated due to excessive overburden. The Karoo rocks attain a maximum thickness (316 m) near Leslie, from which point a prominent channel in which thicknesses of more than 60 m were recorded extends up to north of Weigedag. The thickness over most of the remaining area is less than 60 m. A generalized pre-Karoo geological map is presented and could be used in conjunction with the isopach map as an aid in town planning, the planning of other prospecting operations and geohydrological studies. However, no definite correlation between the distribution of the clay deposits and the pre-Karoo geology has been established. On the West Rand three main groups of Vryheid formation outliers (south of Bank Station, at Westonaria and at Lawley) and numerous smaller outliers were delineated by using a combination of borehole and gravimetric data. Ten reconnaissance boreholes were drilled, eight of which intersected refractory clay. Thickness of the clay layer varies between 15 and 40 m, with an average of about 25 m. The average overburden/clay ratio indicated by the boreholes is 0,27. The abnormal thickness of these deposits may be attributed to a gradual subsidence of the basins during deposition. All except two of the 32 samples tested for refractoriness were refractory clays with Seger cone values ranging between 29-and 35. Most of the samples were either plastic or semi-plastic types, but about one third can be classified as semi-flint. The alumina content (calculated on a calcined basis) of the refractory clay samples varies between about 29 and 41 percent, with an average of 37 percent. The area to the south of Westonaria appears to be the most promising for further prospecting; in one of the holes drilled here, a 36,5 m thick clay layer was intersected which gave Seger cone values of between 34-and 35. Total indicated resources of clay similar to that found in the reconnaissance boreholes could be in excess of 200 million tons. In view of the relatively high alumina content of the clay and the apparently large quantities available, these deposits should be considered seriously as a potential non-bauxitic source of aluminium.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 208-227

Using a variety of geological exploration techniques, apatite mineralisation from the eastern portion of the mafic rocks of the little known Villa Nora occurrence of the Bushveld Igneous Complex was investigated. On both mineralogical and chemical information the area examined can be correlated with the Upper Zone of the Bushveld Complex. The investigation revealed a number of narrow, elongated, lensoid bodies of apatite-bearing magnetitite in which hydroxy-fluorapatite is intimately associated with titaniferous magnetite, ilmenite, and usually either fayalitic olivine or plagioclase. The most important types of magnetitite include olivine-apatite magnetitite, apatite magnetitite feldspar-apatite magnetitite and feldspathic magnetitite. These magnetitites are generally concordant with the layering of the host-rocks but occasionally are of a discordant nature. In the southernmost portion of the area examined appreciable layered apatite mineralisation, without associated magnetite, and some thin bands of apatite-free magnetite were revealed. Detailed petrographic and mineralogic work has been carried out on specimens from a number of the magnetitites and their well-layered host-rocks, which include individual layers, up to twenty feet thick, of ferrotroctolite, olivine ferrogabbro and impure anorthosite. The main mass of the complex, which can be correlated with the Main Zone of the Bushveld Complex, consists essentially of vast amounts of a coarse-grained, pseudostratified leucogabbro and leuconorite with a general dip towards the red Bushveld-type Granite in the south and south-east. Six new chemical analyses are presented, three of various types of magnetites, one each of the iron-rich troctolite and olivine gabbro and one of a pure apatite concentrate separated from a specimen of apatite magnetitite. The maximum recorded content of phosphorus pentoxide of the rocks examined is approximately ten per cent, but the average grade of the magnetitites is of the order of only five to seven per cent of phosphorus pentoxide and is considered to be subeconomic. The host-rocks, both above and below the magnetitites, also contain an appreciable amount of apatite. Minor lead-silver-copper mineralisation, which also occurs in the area, is probably related to intrusive tongues from the Bushveld Granite and is likewise considered to be uneconomic. Genetically the rocks of the upper zone of the Villa Nora occurrence are thought to be the late-stage products of a normal trend of iron-enrichment of a tholeiitic magma. The magma which formed the apatite-bearing magnetitites is considered to have originated essentially by accumulation of a residual liquid and segregation of an immiscible phase coupled with some gravity differentiation in situ, on injection into the host rocks. Apatite-bearing magnetitites appear to be unknown from other areas of the Bushveld Complex and also from other large layered mafic intrusions.
Back to title list




Namaqualand Metamorphic Complex. Special Publication No.10, 1-29

The Namaqua mobile belt which is traditionally considered as part of the Kibaran system of mobile belts in Africa, includes rock units belonging to both the Eburnian (2 000 Ma) and Kibaran (1 000 Ma) age groups. The Namaqualand geotraverse straddles tectonic domains representing this radiometric age variation. The main effects of the Namaqua tectogenesis (1 100-1 200 Ma) are bounded in the south by the southern Cape conductive belt, and in the north by the Namaqua front, the latter a major zone of movement coinciding with a gravity signature. The investigation in the Namaqualand geotraverse and environs are related mainly to three east-west-trending tectono-stratigraphic units, viz. the centrally situated Orange River igneous belt (1 700-2 000 Ma) flanked on both sides by metasedimentary belts interpreted as Eburnian in age, but spatially associated with 1 200 Ma augen gneisses. The Aggeneys metasedimentary belt consists of a metaquartzite/schist association indicative of a shoreline environment and is known for occurrences of stratabound sulphide bodies. The Grunau metasedimentary belt comprises a monotonous metashale/wacke succession. The two main periods of igneous activity are 1 730-2 000 Ma and 1 000-1 200 Ma. The Vioolsdrif and Little Namaqualand suites are regionally the most extensively developed intrusives representing the two age groups respectively. The 2 000 Ma metavolcanics of the Orange River sequence together with the Vioolsdrif Suite constitute the calc-alkaline Orange River igneous belt. The main stratigraphic units of the metavolcanics are outlined and interpreted as terrigenous with eruptive centres concentrated in the west. The metamorphic history and zonation are complex with major retrograde metamorphism. Because of the intense and penetrative overprinting of the Namqua deformation, little is known of the earlier structural development apart from the knowledge that several pre-Namaqua imprints exist. The most important of these is the post-Vioolsdrift Gladkop deformation associated with high-grade metamorphism suggested to be late-Eburnian. The regional corss-section straddling the three major tectono-stratigraphic units depicts the Orange River igneous belt bounded on both sides by a ramp and an extensive flat. The tectono-stratigraphic units are interpreted as first order thrust sheets, the movement (beetween 1 100-1 200 Ma) of which gave rise to the Namaqua and late-Namaqua structures. Due to their ramp configuration the major Groothoek, Lower Fish River and Skelmfontein thrusts were recognised as such and their geometry of movement reconstructed. Componental movement within the major thrust sheets gave rise to the smaller scale thrust geometry established for the Dabenoris Mountains. It is suggested that the family of structures - thrusts, late folds and late shears - might be process-related as in examples described for upper-crustal regimes elsewhere. A schematic reconstruction of the sequence of thrusting explains the observed Namaqua (augen gneiss-related) and late-Namaqua (Spektakel-related) structural development within a time interval of 100 Ma. The metamorphic development and zonation are explained in terms of the structural model. It is suggested that gravity spreading of large volumes of hot augen gneisses, produced during the Namaqua orogeny, acted as the driving force of the thrusting under infrastructural conditions. The Namaqua mobile belt is the product of two orogenies. All the supracrustals are associated with the geosynclinal phase of the Orange River orogeny (Eburnian). The main effect of the Namaqua orogeny (Kibaran) was the production of silicic granitoids which spread laterally to produce infrastructural thrusting.
Back to title list




Contributions to the geology of the Barberton Mountain Land. National Geodynamics Programme, Barberton Project. Special Publication No.9, 185-187, map

Detailed and reconnaissance mapping on scales varying between 1:6 000 and 1:50 000 were carried out in the southern part of the Barberton greenstone belt over a period from 1978-1981 as part of an on-going mapping project. The accompanying map is a compilation of the field-data gathered to date, initially plotted at 1:25 000 and here reduced to 1:50 000. The area mapped covers, in part, that investigated by Viljoen and Viljoen (1969a) during the Upper Mantle Project, and their maps were initially used as a basis for this work. In places no attmpts have ben made to alter their lithologic boundaries. For example, the lithologic subdivisions with the Komati Formation and the Kromberg fold area have been almost entirely reproduced from Viljoen and Viljoen (1969b ,c) as indicated on the inset location maps in Fig. 1. The area to the west of this map is presently under investigation by Isabella Paris (Department of Geology, University of the Witwatersrand) and Simon Lamb (Department of Earth Sciences, Cambridge University). It is anticipated that the compilation of a provisional geological map and a restored cross-section of the entire southern part of the Barberton greenstone belt will be attempted after the completion of their field work. (A 1:25 000 geological map of the southern part of the Barberton greenstone belt and adjacent granitic terrane is currently being compiled from the regional mapping of M.J. Viljoen, R.P. Viljoen and C.R. Anhaeusser but was not available in time for inclusion in this volume, Ed.). To a great extent, the mapping to date has been directed towards improving present knowledge of the structural history of this part of the belt, in addition to attempting to gain further insight into the almost ubiquitous alteration (metasomatic) processes which affect all rock types within the belt. Deformation and metasomatism have considerably altered the original characteristics of the rocks. Both these processes were heterogeneous on a regional and local scale, thus hindering correlation using a purely lithological approach to mapping. Silicification, ferruginization and carbonitization have, to some degree, affected most of the rocks (as indicated on the map) although only in a very qualitative manner. For the sake of clarity in Fig. 1 this metasomatic overprint has not been shown everywhere. Three periods of deformation have been distinguished on the map. Some details of the most conspicuous, second period of deformation, related to regional horizontal shortening, are described by De Wit et al. (1983 - this volume). Structures associated with the earliest period of deformation have been described elsewhere (see later). Both these periods of deformation have resulted in significant structural duplication and transposition of stratigraphic sections and the work to date indicates that the stratigraphic succession as presently accepted for the Barberton Mountain Land may need considerable modification. A provisional attempt at this is shown on the stratigraphic correlation charts by using a simple numbering system, correlating the lithologies of this map with formations as defined by previous workers.
Back to title list




Namaqualand Metamorphic Complex. Special Publication No.10, 45-66

Fourteen rock units in the western portion of the Namaqua mobile belt were investigated by some or all of the following isotopic methods: Rb-Sr, Pb-Pb, Th-Pb total-rock and Rb-Sr mineral age measurements. Two major tectogenetic events are recognised. The early Orange River event is represented by the emplacement and metamorphism of the volcanic rocks of the Haib Subgroup and the emplacement of granitoids of the Vioolsdrif and Gladkop suites in the regions of the Richtersveld Province and the Steinkopf domain. The later Namaqua event is represented mainly by extensive granitoid emplacement and high-grade granultie facies metamorphism in the Okiep Copper District, with minor granitoid and dyke emplacement taking place in the Steinkopf domain. The presence of 1 800 Ma old rocks in the Steinkopf domain and their inferred presence in the Okiep Copper District implies the southward extension of rocks coeval with those present in the Richtersveld Province. The polymetamorphism associated with these tectonic events had far-reaching effects in the form of long continued isotopic resetting of rock and mineral systems. In the instances where the regression of the total-rock isotopic data define errorchron results, geological disturbance is inferred. The Sr-isotopic results indicate substantial crustal reworking and two periods of mantle differentiation producing continental crust at approximately 2 000 and approximately 1 200 Ma ago. These ages correspond to the two tectogenetic events. The Pb-isotopic data for these rocks indicate f2 values for the source regions that are compatible with crustal reworking for the felsic rocks or a significant amount of crustal recycling into the mantle for the mafic rocks. In terms of modern geodynamic processes this can occur at active continental plate margins. On this basis a magmatic arc environment is envisaged for the rocks generated during the Orange River event. The deformation fabrics associated with the Orange River and Namaqua tectogenetic episodes are dated by the time of emplacement of syn- and post-tectonic granitic rocks. The regional fabric observed in the Steinkopf domain and in the Okiep Copper District was not synchronously developed.
Back to title list




Namaqualand Metamorphic Complex. Special Publication No.10, 31-44

In terms of outcrop area in the Namaqualand geotraverse, the grey and pink gneisses of the Steinkopf area constitute a major lithological unit forming a tectonic domain on its own. Three major gneiss units are distinguished, mainly on compositional grounds, namely the Steinkopf Gneiss (granodioritic to granitic), Brandewynsbank Gneiss (granitic) and the Noenoemaasberg Gneiss (leucogranitic). Together they form a grey gneiss complex similar to the basement complexes of a number of well-known mobile belts. The gneisses exhibit well-developed banding shown to be of secondary origin through processes of mechanical flattening of primary heterogeneities coupled with metamorphic and metatectic differentiation. One of the products of these processes is a remarkably fine (millimetre scale) banding in parts of the Noenoemaasberg Gneiss and is defined by the continuous repetition of fine-grained plagioclase-poor and plagioclase-rich bands. In general, by distinguishing secondary from primary features, it is shown that the precursors to the three gneiss units were homogeneous in terms of composition, texture and structure. The gneisses do not exhibit primary banding. Since sedimentary processes incorporate mechanisms of extreme compositional and textural differentiation, culminating in bands, these gneisses, which are devoid of such primary characteristics, are best considered to be of igneous origin. Intrusive relations are established on grounds of the recognition of xenoliths (on mesoscopic to macroscopic scale) and observation of contamination and assimilation at the contacts. It is shown that the Steinkopf Gneiss is intrusive into metasediments of the Khurisberg and Eenriet Subgroup (i.e. not basement), that the Brandewynsbank Gneiss is intrusive into the Steinkopf Gneiss and that the Noenoemaasberg Gneiss is intrusive into both. Intimate spatial association and a common structural and metamorphic history lead to the grouping of the three orthogneiss units into the Gladkop Suite of meta-intrusives. Both the Gladkop and the Vioolsdrif suites are intrusive into the Eenriet Subgroup and are in turn intruded by the Klein Namaqualand Suite. Isotopic characteristics confirm the suggested time-equivalence while chemical, stratigraphic, structural and metamorphic characteristics point to a spatial separation of the two intrusive suites during early Proterozoic times.
Back to title list




A symposium on mineral deposits and the transportation and deposition of metals. Special Publication No.5, 275-295

Crystallisation of the sheet-like granites of the Bushveld Complex began with hornblende granites at the outer margins and migrated black towards volatile enriched biotite granites located at the more central cores. This pattern of centripetal crystallisation and change in the dark mineral species was accompanied by an anomalous reduction in the total mafic mineral content with respect to the light minerals, quartz and feldspars. Deuteric autopneumetasomatism in the late crystallising cores occurred as a result of continued enrichment of the melt in the non-essential residual and volatile constituents. This deuteric alteration, responsible for the perthitisation of the feldspars, chloritisation of the biotites and oxidation of released iron dust from the feldspar lattice, gives rise to the typical "red granites" almost always associated with the mineralisation. Late deuteric growths of actinolite spherulites in quartz indicates the end product in this sequence. Tensional fracturing of the earlier crystallising peripheral rind, due to volumetric contraction on final crystallisation of the core, resulted in the escape of the volatiles and depressurisation of the system. Pneumatolytic-pegmatites developed in the fracture/joint system, particularly in flat open fracture voids, so forming flat manto ore bodies. These, with the enclosing deuterically altered granites, form a pseudo-dome in outcrop. The resulting mineral assemblages consist either of actinolite-fluorite or of magnetite-siderite-fluorite, but subsequent alteration leads both to a common hematite-quartz-fluorite assemblage. Where evidence of the above is found, fracturing of the roof allowed the escape of tin-bearing complexes and thus "tin" is present in the roof rocks. The pattern holds true for both the Rooiberg and Union Tin occurrences and helps to explain the characteristics of the mineral associations observed. A review of mineralisation patterns elsewhere reveals certain diagnostic criteria which can assist in determining if a crystallisation cell fractured its roof or remained intact. Thus, in contradistinction to the above, where fracturing did not take place, the Zaaiplaats type of situation developed with fluorite-cassiterite mineralisation within the granites. Telescoping may occur if fracturing of the roof is late. Certain related mineral associations also occur indicating various stages of development or zonation between the Zaaiplaats and Rooiberg models which are regarded as two extreme opposites or end members in a range of alternatives.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 23-35

The chromite deposits and associated rocks found on the farms Grasvally 293 KR and Zoetveld 294 KR form part of the Basal Zone of the Complex south of Potgietersrus. The rocks consist predominantly of harzburgite, two chromitite seams and minor layers of pyroxenite The two chromitite seams are contained in the harzburgite which is extensively serpentinized. The process of serpentinization appears to be dictated by the circulation of ground-water. Intrusive into the rocks of the Basal Zone is a layered sequence of noritic and anorthositic rocks of the Main Zone which have partially eliminated and extensively faulted the chromitite seams. The rocks of the Main Zone cut across the rocks of the Basal Zone after the consolidation of the latter. This discordant relationship may be due to either additions of parental magma to a depleted chamber or to movement of the magma in the same chamber as a result of warping of the floor, peripheral faulting or slumping of a pile of cumulates elsewhere in the chamber. Laboratory studies undertaken in conjunction with field observations indicate that the Basal Zone was built up by cumulates of olivine, chromite and pyroxene. The lack of compositional breaks, systematic variation in grain size and penecontemporaneous deformation features imply that the floor of accumulation was even and that magmatic current action had little influence on the evolution of the cumulates. To account for the rapid alternation of rock types it is suggested that pressure changes within the magma chamber influenced the sequence of crystallization of the constituent phases by oscillating the crystallization curves across phase boundaries in the system.
Back to title list




Some sedimentary basins and associated ore deposits of South Africa. Special Publication No.6, 65-72

A 15 m thick stromatolitic Limestone bed, intercalated in the upper part of the Duitschland Formation, Transvaal Supergroup, in the northeastern Transvaal, is described. Evidence is presented demon strating that the Duitschland Formation constitutes the base of the Pretoria Group and that the Tongwane Formation represents the top sequence of the Chuniespoort Group. This stromatolitic limestone bed contains disseminated chalcopyrite, bornite and chalcosite with accessory galena, molybdenite, tennantite, mawsonite and a mercury- silver alloy over most of its 100 km strike length. The mineraliza tion is believed to be of syngenetic origin, with possible precipi tation of copper as carbonate by photosynthesis, and subsequent reduction into sulphides during early diagenesis. It is postulated that the lake or epeitic sea was enriched in copper during a short time in its evolution, osssibly by brine discharge. None of the occurrences studied is of economic importance.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 242-265

The environmental relationships of the Transvaal sediments affirm that they occur as inliers encompassed by the Complex or as rocks constituting the floor thereof. Owing to the confusion of sedimentary bedding with simulated load-pressure "bedding" and deformational affine gliding caused by the superincumbent loading by the Complex, all inliers were previously relegated as xenoliths. The constant strike direction maintained by the axial planes of folding within the postulated floor and some inlier sediments, refutes earlier contentions that all these rocks are xenolithic within the Complex. Rotation of the conjugate folding compared with similar folding in undoubted floor sediments to the south, could have been caused by later regional folding or by the intrusion of the Pilanesberg Complex. In close proximity to the Bushveld floor the concomitant mafic sills in the Transvaal sediments change their normal gabbroic character to noritic, as a result of sedimentary assimilation. Fractionation of the noritic sills has caused them to be confused with Bushveld differentiates. Field and petrographic evidence suggests no consanguinity between the sill norites and the chilled Marginal Bushveld Norite. Mapping has confirmed the overriding regularity and persistence of the Bushveld layering. Lithological zoning of the Basal Zone from the mapping was confirmed by the results of bore-holes which were made available after completion of the mapping. A constant stratigraphical position of the Hendriksplaas norite in the area mapped and in the Eastern Transvaal makes the previous hypothesis of it being an uplifted portion of the Marginal Norite untenable. The jointing and faulting of the earlier post-Bushveld tectonic period can be related to the compressive stress which caused the primary conjugate folding in the area. Later westerly movement of all the rocks in the south along an existing fault can be attributed to the emplacement of the Pilanesberg Alkaline Complex. This intrusion produced radial faulting which, where co-incident with pre-existing major shearing, rejuvenated that faulting and created a weakness direction for the injection of the Pilanesberg swarm.
Back to title list




Upper Mantle Project - South African National Committee Symposium, July 1969. Special Publication No.2, 445-448

Measurements of heat flow are given for 15 localities in South Africa. The values range from 1.0 to 1.8 fcal/cm} sec. Shield values tend to be lower than those outside the shield.
Back to title list




Upper Mantle Project - South African National Committee Symposium, July 1969. Special Publication No.2, 477-484

Recent body-wave researches in southern Africa have shown unusually high apparent velocities over certain distance intervals from the source region on the Witwatersrand. Surface wave studies have yielded phase velocity vs. period relations for several paths in this region, and these confirm unusually high Sn velocities in the Southern Transvaal and Northern Orange Free State determined in the body-wave studies. Crust and upper mantle velocities for the region to the west of a line through Kariba and Pretoria are lower than those to the east of this line. Further critical information from this region requires superior methods of locating the seismic event, recording the arrivals in the field, and analysing the data; an advanced system for recording and digital handling of body and surface waves has been constructed, and is briefly described.
Back to title list




Upper Mantle Project - South African National Committee Symposium, July 1969. Special Publication No.2, 417-427

In the present communication the relationship between the potassium and phlogopite contents of peridotite xenoliths is considered, as well as trends possibly relating the alkali abundance relation ships of the xenoliths and their host kimberlites. The potassium contents of the peridotites are discussed in relation to experimental petrology on basalt formation.
Back to title list




Upper Mantle Project - South African National Committee Symposium, July 1969. Special Publication No.2, 351-357

A further occurrence of a diamond-bearing eclogite from the Roberts Victor Mine is reported. Some of the diamonds are unusual in that they appear to have formed around nuclei of rutile. The significance of the occurrence of diamond in eclogite is discussed.
Back to title list




Upper Mantle Project - South African National Committee Symposium, July 1969. Special Publication No.2, 359-369

The Matsoku pipe, which is noted for the high proportion of xenoliths it contains, was visited early in 1968 by a combined team of geologists and geochemists from the Universities of Edinburgh and Cape Town. From the samples collected, 42 ultramafic xenoliths are here described and the volumetric proportions of their constituents measured. Twenty-four proved to be peridotites, 14 pyroxenites, three eclogites and one, a rock of unusual composition, consists of nearly equal amounts of olivine, clinopyroxene and garnet. One abnormal eclogite is particularly rich in opaque minerals. Particular attention has been paid to a study of the garnets from 36 garnet-bearing xenoliths. Their refractive indices and cell edge lengths were determined and are compared with those of garnets from similar rocks which we studied previously. Garnets having aberrant compositions are few. Multiple garnet compositions were found in two instances. This study indicates that the peridotite and pyroxenite xenoliths form a continuous series and that garnets from pyroxenites do not occupy an independent compositional field.
Back to title list




Upper Mantle Project - South African National Committee Symposium, July 1969. Special Publication No.2, 395-416

Descriptions are given of the non-eclogitic ultramafic xenoliths that occur in kimberlite. The localities in Southern Africa at which such rocks have been found are listed with the rock types that occur. Thirty-one of these 64 localities have been visited, specimens in older collections have been examined, and the mineral assemblages which can be confirmed are recorded. The garnet- lherzolite assemblage, olivine-orthopyroxene-clinopyroxene-garnet, which is frequently assumed to be most similar to the material in the Upper Mantle, occurs at more localities than the other assemblages. Olivine-orthopyroxene, olivine-orthopyroxene-clinopyroxene, and olivine-orthopyroxene-garnet are almost as common, and these lherzolites and harzburgites are often found in the same kimberlite occurrence. Xenoliths consisting of garnet-spinel and of olivine- orthopyroxene-clinopyroxene-amphibole are newly recorded. Foliated xenoliths are figured for the first time.
Back to title list




Upper Mantle Project - South African National Committee Symposium, July 1969. Special Publication No.2, 371-393

Eclogite xenoliths that occur in kimberlite are described. A compilation has been made of all the kimberlites in Southern Africa which contain such xenoliths, and the types of eclogite which have been found are listed. Of the 60 localities to which reference has been made, confirmation of the existence of one or more types of eclogite xenolith can be given for 42 of them. The occurrence of some type of eclogite xenolith is reported for the first time from 15 kimberlites. Thirty-two types of eclogite are listed, of which 13 have been found at only one locality; we have confirmed the existence of 25 of these mineral assemblages. Six localities have yielded more than five of these rock types. An association between corundum eclogites and diamond eclogites has been noted. Of those localities with the former, only one does not contain kyanite eclogite, but the reverse is untrue for four localities.
Back to title list




Upper Mantle Project - South African National Committee Symposium, July 1969. Special Publication No.2, 335-349

A twofold division of the Precambrian rocks of southern Africa is based on the recognition of metamorphic facies series in which ancient cratonic areas characterised by Low Pressure/Temperature Facies Series and younger metamorphic belts metamorphosed within the Intermediate Pressure/Temperature Facies Series are recognised. The possibility that this duality represents part of an evolutionary sequence of metamorphic belts and a thickening of the crust with geological time is discussed. Comments on the distribution of kimberlites and alkaline intrusions are made.
Back to title list




Upper Mantle Project - South African National Committee Symposium, July 1969. Special Publication No.2, 9-28

Geological and geochemical investigations of the mafic and ultramafic rocks occurring in the Barberton Mountain Land of the Eastern Transvaal Lowveld commenced in August 1965. The main results of this study are presented in nine accompanying papers dealing with various aspects of the geology of this region pertinent to studies relating to the nature and evolution of the earth's primitive crust and upper mantle.
According to the recommendations of the South African National Upper Mantle Committee, the present project has been grouped under the broad heading, "Ultrabasics in Fold Belts". Although the term "fold belt" has been used, it should be stressed from the outset that the Barberton belt, although strongly folded, is not a true orogenic fold mountain belt of the Alpine type; nor are the contained ultramafic rocks of the true Alpine type (Hess and Thayer: personal communications). As will be shown, they have a geological and geotectonic setting which is unique to the early Precambrian. It is suggested that the Barberton ultramafic rocks, as well as those occurring in similar geotectonic environments in other early Precambrian greenstone belts, warrant recognition as a new class of igneous rock, significantly different from the Alpine type. Up to the time of the present investigation this distinctive class of early Precambrian ultramafic rock has remained largely undescribed or undefined.
The Barberton area was chosen for detailed investigations because of certain unique features. With an age of close to 3,400 million years the supracrustal layered rocks of the area are among the oldest known on earth. In addition, they are exceptionally well developed, preserved and exposed. With respect to the present study it was hoped that the fortuitous combination of the above factors might have resulted in the preservation of the primary nature of some of the earliest material ever to have tapped the earth's mantle. At the same time, the granitic rocks of the region, which everywhere envelop and intrude the supracrustal rocks of the Barberton Mountain Land, constitute the oldest assemblage of granitic rocks known on earth. The latter are exceptionally preserved and exposed, and it became clear that a consideration of these could well yield fundamental data concerning the processes responsible for the earliest development and evolution of the continental crust. The present paper is a brief introduction to the geology of the Barberton region, in which mention is made of previous ideas concerning the general geology as well as the authors' present views, based on the geological and geochemical data acquired over the last four years. The paper also presents the geological setting of the areas and topics to be discussed in more detail in subsequent papers.
Back to title list




Upper Mantle Project - South African National Committee Symposium, July 1969. Special Publication No.2, 153-188, 13 PL.

This paper is an attempt to synthesize and classify from all available data the complex granitic rocks which surround and intrude the Barberton Mountain Land. The area discussed covers 14,000 square km (9,000 square miles) and includes the granitic terrain in Swaziland to the south-east, the granitic area in the Transvaal up to the Transvaal Drakensberg escarpment in the west, and the Nelspruit granitic terrain to the north as far as White River. Most of the granitic rocks have been examined by the authors on a reconnaissance scale, but as part of the geological investigations of the Onverwacht Group the granitic terrain immediately to the south of the Mountain Land and in contact with the latter was mapped in detail. Although representing a small portion of the area discussed in the paper, this terrain, as will be shown, probably represents one of the most important with respect to the relationships of the granitic rocks to each other, to the rocks of the Swaziland Sequence, and to the granitic rocks of Swaziland. It was the geological and geochemical study of this area, therefore, which prompted this paper as well as the accompanying paper which discusses the geochemistry of the granitic rocks of the region.
Back to title list




Upper Mantle Project - South African National Committee Symposium, July 1969. Special Publication No.2, 245-274

The Barberton model, it is maintained, can be applied in a similar way to the Shield area of India, and although little data is available concerning the Siberian and Brazilian Shields, it is contended that the Barberton granite-green stone model will hold for these areas as well. From a literature review it is clear that the Barberton region, with respect to development, state of preservation and exposure is exceptional and probably better than in any similar region thus far described. It is therefore suggested that in the way outlined in the present paper, the model proposed and the details thereof as outlined in the accompanying papers could well be used as a standard reference and a valuable aid to the geological interpretation of granite-greenstone terrains of any shield area.
Back to title list




Upper Mantle Project - South African National Committee Symposium, July 1969. Special Publication No.2, 87-112

The origin of ultramafic rocks is one of the most enigmatic of geological problems and has for many years attracted the attention of numerous field and laboratory investigators. A large amount of literature exists on the subject but to date no entirely satisfactory explanation as to the origin, mode of intrusion, and manner and cause of the serpentinization of ultramafic rocks has been forthcoming. The main controversy centres around the existence or otherwise of a primary ultramafic magma and the manner of emplacement of the latter if it does exist. In the rocks of the Barberton Mountain Land of South Africa, evidence has recently been found which may contribute considerably to an undertaking of some aspects of this problem. Unlike most geological environments which contain ultramafic rocks, certain parts of this region are remarkably well preserved, particularly in the southern part of the Barberton belt where a rather unique sequence of structurally undisturbed mafic and ultramafic rock types occurs. Although having suffered low-grade regional metamorphism, original field relationships as well as undeformed diagnostic volcanic structures are still clearly to be seen. In these ultramafic rocks, and for the first time as far as the authors are aware, unequivocal pillow structures, commonly regarded as being evidence of extrusion of subaqueous molten magma, have been observed. This, together with other evidence from the area, strongly supports the existence of a mobile largely extrusive magma of peridotitic composition.
Back to title list




Upper Mantle Project - South African National Committee Symposium, July 1969. Special Publication No.2, 189-219

See abstract of paper entitled "A proposed new classification of the granitic rocks of the Barberton region", same authors, item 119 in this Bibliography.
Back to title list




Upper Mantle Project - South African National Committee Symposium, July 1969. Special Publication No.2, 55-85

As noted in an accompanying paper, tholeiitic successions of the calc-alkaline type constitute the main rock types of the lowermost volcanic assemblages of most early Precambrian greenstone belts. In the Barberton belt, however, a substantial succession of highly distinctive and as yet undescribed or undefined rock types has been encountered below a normal tholeiitic succession of the calc-alkaline type. The tholeiitic succession constitutes the upper three formations of the Onverwacht Group and is described in another paper. The lower formations of the Onverwacht Group together constitute a distinctive succession of what has been termed the Lower Ultramafic Unit, and are described in detail in this paper. These formations, known from the base to the top as the Sandspruit, Theespruit and Komati formations, attain their maximum development and are best preserved in the southern part of the Barberton Belt which has consequently been chosen as the type locality. In general the rocks are steeply dipping in this area and occupy the cores of two major anticlinal structures, viz. the Onverwacht and Steynsdorp anticlines. Intrusion of granitic rocks has detached these two anticlines and eliminated the median syncline (now occupied only by rocks of the upper formations) which must have existed previously. The Onverwacht anticline is an asymmetrical ENE-trending structure with a somewhat reduced and flattened eastern limb but with a well-developed and well-exposed east-west trending western limb which has consequently been established as the type column for the upper two formations of the Lower Ultramafic Unit. In this area only a narrow sliver of rocks belonging to the lowermost Sandspruit formation is present so that the type section for the latter has been established some several kilometres to the south-east in a large, well-preserved but detached xenolithic mass within the intrusive tonalitic gneisses.
Back to title list




Upper Mantle Project - South African National Committee Symposium, July 1969. Special Publication No.2, 275-295

In this paper an attempt has been made to synthesize all the work carried out in the Barberton region over the last four years, and to assess the significance of the results with respect to the Upper Mantle Project. All available evidence, field, petrological and geochemical, has been used in an endeavour to ascertain as closely as possible the probable chemical and mineralogical composition of the earth's upper mantle at this stage in earth history. The latest ideas concerning the physical and chemical constitution of the Upper Mantle are reviewed briefly and an attempt made to reconcile these with the findings of the present investigation. It is contended that many of the mafic and ultramafic rocks, especially in the lower three formations of the Onverwacht Group, are unique but have as yet never before been recognised as such. It is believed that extensive, almost complete, melting of the Upper Mantle took place under special geological conditions which apparently only existed in early Precambrian times. Besides the frequent occurrence of rocks which represent almost complete melts of the mantle, a variety of mafic and ultramafic rocks formed as a result of partial melting of the mantle are exposed on the surface and can be studied in detail.
Back to title list




Upper Mantle Project - South African National Committee Symposium, July 1969. Special Publication No.2, 29-53

For most aspects of Upper Mantle studies it is essential to gain an appreciation of the geochemistry of the rock types under investigation. Reliable chemical data should yield fundamental clues that could ultimately lead to deductions concerning the composition of the Upper Mantle itself and the melts derived therefrom. Emphasis has consequently been placed on the geochemistry of the Barberton rocks, and numerous total major silicate and trace element analyses have been carried out by a number of organizations.
One of the main problems that faces the investigator, not only in the Barberton region but in all the greenstone belts of the shield areas, is the fact that most of the rocks and particularly the volcanics have undergone metamorphism which varies in intensity and type from the amphibolite facies to the lower greenschist facies and includes also carbonation, hydration, silicification and serpentinization. Although most of the rocks on which important conclusions have been based have generally suffered only a low-grade regional metamorphism, it is impossible to be entirely confident that the analyses represent precisely the original composition of the material under consideration. In addition to the above problem, the effect of primary genetic chemical changes, both horizontally and vertically within the same horizon or at higher or lower stratigraphic positions in the same rock mass, probably also exist and should be taken into account and not confused with chemical changes induced by metamorphism. Despite these problems, however, the available data allow for the selection of superior analyses, and it is clear that, as in the case of a number of the Canadian and Australian greenstone belts, the metamorphism in many areas has been essentially of an isochemical type. The authors are confident that the screened superior analyses reflect very closely the original chemical systems of the rocks under discussion allowing for meaningful comparisons to be made with well-established classes of igneous rocks. Before discussing chemical changes induced by metamorphism and other processes, however, it is necessary to enquire into the precision and accuracy of the techniques employed in obtaining the chemical data. If comparisons and deductions based on chemical data are to be made, then sound analysis is clearly also of considerable importance.
Back to title list




Upper Mantle Project - South African National Committee Symposium, July 1969. Special Publication No.2, 113-151

The upper three formations of the Onverwacht Group are confined to the southern and central portions of the Barberton Mountain Land and appear to have developed in a smaller and more discrete depository than the three lower formations which were originally far more widespread.
The main rock types encountered are pillowed and massive tholeiitic lavas with minor amounts of the "primitive" mafic and ultramafic rocks similar to the material that characterizes the lower formations of the Group. Acid volcanics although not abundant are nevertheless conspicuous and appear to terminate volcanic cycles. They are frequently overlain by substantial chert horizons which are closely related to the acid magmatism. In addition a variety of mafic and felsic pyroclasts are typical and appear to have been deposited subaqueously.
The pillowed tholeiites at the base of the sequence approach closely the composition of oceanic tholeiites, whereas those at the top have affinities with continental tholeiites. There are thus slight yet significant, chemical variation trends from the base to the top of the sequence. Alkali basalts or their metamorphic equivalents were not encountered in the sequence, although trachyandesites have been reported from the overlying Fig Tree Group.
Fundamental changes, probably related to a thickening of the sialic crust, probably took place in the upper mantle during a time break, manifested by a carbonate and chert sediment (the Middle Marker) which separates the three lower formations from the three upper formations. In general the rocks encountered above this marker are closely comparable to the mafic to felsic units of both the Canadian and Australian greenstone belts. Significant differences lie in the fact that the upper three formations of the Onverwacht Group contain less acid volcanics than their counterparts in Canada and Australia and are also characterized by a relative abundance of more "primitive" mafic and ultramafic rock types. These differences could conceivably be due to the fact that the Barberton rocks are significantly older than their counterparts in the other shield areas.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 266-282

A gravimetric survey has been carried out over all areas underlain by rocks of the Bushveld Complex in the area west of the Pilanesberg. The results of this survey indicate two areas where substantial thicknesses of Bushveld rocks are present. One of these areas is located immediately west of the Pilanesberg while the other is located some forty miles to the west. Interpretations of the gravity results show that several inliers of sediments of the Pretoria Series, previously though to be xenoliths in the Bushveld Complex, are in fact upfolded or upfaulted portions of the floor of the Complex. Six boreholes have been drilled through the basic rocks of the Bushveld Complex and have penetrated the underlying sedimentary rocks of the floor at depths consistent with those interpreted from the gravity results. Detailed density measurements were made on borehole cores and it is shown that the mean density of the Bushveld Complex is 3.11 gms/cc in the area immediately west of the Pilanesberg and 3.21 gms/cc for the area forty miles to the west. A density contrast between the floor and Bushveld rocks of 0.30 and 0.40 gms/cc for these two areas respectively was used in interpretative calculations.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 46-58

Three patterns of compositional variations of coexisting silicates and chromite have been recognized thus far in the Transition and Critical Zones in the eastern part of the Bushveld Complex: broad changes in composition of bronzite, plagioclase, and chromite upward in the rock sequence, variations in the composition of bronzite and chromite within chromitic intervals, and fine-scale variations in the composition of bronzite. Broad upward changes are consistent with evolution of the Critical and Transition Zones at least mainly by fractional crystallization and gravitative accumulation of crystals. Variations within chromitic intervals are ascribed primarily to intermittent changes in oxygen fugacity. Fine-scale variations may be due to post-cumulus reactions between crystal and liquid, to subsolidus equilibration, or both.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 5-22

Mainly on the basis of field-relationships, the basic portion of the western part of the Bushveld Igneous Complex is subdivided into eight units: a basal norite unit, a harzburgite unit, a pyroxenite unit, an anorthosite unit, a norite unit, a porphyritic unit, a gabbro unit and a ferrogabbro unit. Floor- relations indicate that the basic rocks apparently follow concordantly on the Magaliesberg quartzite of the Pretoria Series. Towards Rustenburg, in places, the lower basic rocks are intrusive into this quartzite and portions of the quartzite have been lifted upwards by the basic rocks. To the west and the north the invasion of the quartzite becomes more extensive and immediately south-west of the Pilanesberg the basic rocks cut through the quartzite down into the underlying Magaliesberg shale. The roof of the basic rocks is formed by quartzite and granophyre or granite, and quartzite seems to grade into the granophyre. Large-scale intrusive relations are shown by the uppermost ferrogabbro unit, north of the Pilanesberg, where, in two places, it cuts across the whole sequence; a band of the porphyritic pyroxenite unit which lies immediately below the Merensky Reef north of the Pilanesberg but cuts across the norite unit south of the Pilanesberg; a large body of the porphyritic pyroxenite unit intrusive into the basal norite unit south-east of Rustenburg; and the norite unit which cuts across the pyroxenite unit in places. Small-scale intrusive relations are mostly presented by inclusions of one rock unit in another, tongues of one cutting another, and other contact-phenomena. Compositions of the plagioclase, orthopyroxene and clinopyroxene not only indicate that upwards in the sequence the pyroxenes become more iron-rich and the plagioclase more sodic but also that there are compositional "breaks" between succeeding units. Chemical analyses confirm the existence of these breaks. It is concluded that differentiation was one long-continuous process, which started in depth and came to an end at the present site. The magma was probably formed by partial melting within a mantle of inhomogeneous peridotitic composition. It was emplaced in separate surges in the upper part of the Pretoria Series and each succeeding heave was responsible for the formation of a new unit and went further into the sediments than the previous one. In this process a "cake" of basic rocks was built layer by layer so that each successive layer overlaps the previous one. Sagging of the floor and tilting towards the centre of the complex resulted in a tilted, funnel-shaped structure which is built of units fitting like saucers on one another so that each successive saucer overlaps the previous one.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 312-325

The Rooiberg felsite series in the Swaershoek Mountains consists predominantly of felsite and two minor, but persistent, interbedded sedimentary-pyroclastic horizons. The felsites were divided into four groups, mainly on the basis of texture and lithology. The two most important varieties are dark-coloured, porphyritic felsite (F3) and "pale" felsite (F2) which contains insignificant phenocrysts. These felsites are massive and generally do not display volcanic flow-top structures; all evidence indicates that these rocks were erupted as thick lava flows. "Variable" felsite (Fl) commonly exhibits flow-top phenomena, contains lenses of pyroclastic rocks and this formation clearly consists of a succession of numerous thin lava flows. A characteristic quartz-feldspar porphyry (F4) appears to have been intruded as a sill. There is no evidence that any of the felsites were deposited as either ignimbrites or tuff-lavas. Evidence adduced by Menge (1963) that the agglomerate, which underlies the "Union Tin" shale, is an ignimbrite is not convincing. A cyclic pattern involving the intercalations of sedimentary rocks has been discerned. The cycle starts with the appearance of flow-banding in the upper portions of thick lava flows, ranges upwards through the appearance of numerous thin flows and flow-top phenomena followed by pyroclastic rocks and finally sedimentary rocks. Hydrothermal cassiterite mineralization on Doornkom 376 KR is invariably associated with earlier chloritized zones which are also epigenetic. The controls of chloritization, which was a replacement process, are favourable flow-banded felsite zones and fractures. Three new chemical analyses of felsites are presented.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 689-714

The Losberg Intrusion is a flat-lying layered body of post-Transvaal and pre-Karroo age. This intrusion is one of a group of mafic bodies with tholeiitic affinity that extend along a north-south line from Rhodesia into the Orange Free State including such bodies as the Great Dyke and Bushveld Complex. This 400 foot thick highly fractionated intrusion contains three cumulate rock units, which range in composition from ultramafic to acidic. This geochemical investigation complements the detailed petrological study undertaken by Abbott and Ferguson (1965); eighteen of their samples were analyzed for 10 major and 11 trace elements. The ratios of FeO:MgO, Rb:K and the contents of K, Rb, Ba, Ti, Th, U and Zr in the Losberg suite of rocks are higher than those of the Bushveld, suggesting that the partial differentiation took place before emplacement. The general regularity of the geochemical trends across zones of abrupt phase layering would suggest that we are dealing with the differentiation of a single magma in situ. Symmetrical concentrations versus height curves showing elements associated with high temperatures of crystallization aggregating towards the floor and roof of the intrusion are suggestive of early simultaneous consolidation from these margins. Anomalous, in this respect, is the behaviour of K and Rb which display a non-reversible increase with height. It is therefore suggested that a late-stage acid residue permeated the roof rocks.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 576-593

This paper concerns five layered intrusive complexes that occur in southern Africa, the Great Dyke, and the Bushveld, Losberg, Usushwana and Trompsburg complexes. By means of Sr isotopic measurements an attempt has been made to establish if genetic links exist between the complexes and their petrologically distinct layers, both mafic and acid. Using a value of 1.30 x 10(-11) yr for the decay constant of 87Rb, the ages obtained from total-rock isochrons are: 2532±89 m.y. for the Great Dyke, 1954±30 for the Bushveld, 1881±282 for the Losberg, 2874±30 for the Usushwana and 1372±142 for the Trompsburg. The primary 87Sr/86Sr ratios of the acid and mafic rocks from each complex have been determined where possible. The data for the mafic rocks from the Usushwana, Great Dyke and Trompsburg form a trend that is interpreted as evidence for a two-stage model of mantle development; the 87Sr/86Sr ratios for the Bushveld and Losberg are higher, and possible causes of this anomaly are discussed. The primary 87Sr/86Sr ratios for the acid and mafic members of the individual complexes overlap, though only slightly so in the case of the Bushveld; the significance of these data is also discussed.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 283-298

A geological investigation of the northern half of the Marble Hall Fragment revealed that the major geological feature is a block of folded metasediments of the Transvaal System, enclosed in the Bushveld granite. Irregular bodies of diorite and albite diorite, classified with the upper portion of the Main Plutonic Phase of the Bushveld Igneous Complex, and a sheet of pyroxene granulite of the Maruleng type (?) intruded in the Transvaal System. The Karroo System is represented by remnants of horizontal beds of the Ecca Series. A few dolerite dykes encountered in the mapped area are also grouped with the Karroo System. The interference-folding is the result of two successive stages of deformation. The first phase of deformation is believed to have taken place either just before or in the initial stages of the emplacement of the Main Plutonic Phase. It probably marked the first stages of instability of the floor of the Transvaal System before emplacement of the Complex. The deformation took place by a mechanism of shear-folding, and a series of north-south-trending folds ensued. Of these, the most notable was the Swartkop-Marble Hall Anticline that was overfolded to the west. The intensity of all these folds apparently decreased to the south and the west. The Swartkop-Marble Hall Anticline is considered to be the continuation of the Dennilton Plunging Anticline east of Dennilton. The folding took place at deep levels, presumably under high load pressures, so that the formation of b-lineations ensued. The second phase of deformation took place either during the period of emplacement of the Main Plutonic Phase or, less probably, during the emplacement of the Bushveld Granite. The mechanism was most probably that of oblique flexure-slip and the geometrical arrangement that of a conical fold. This pattern of folding led to tension-faults in the outer limb of the Swartkop-Marble Hall Anticline and to thrust-type faulting in the core areas. The diorite and related rocks intruded after, or towards the end of, the second phase of deformation and caused widespread sodium metasomatism. Simultaneously, a medium grade of thermal metamorphism affected the country-rock. Finally, the Bushveld Granite enveloped the complex of folds and intrusions to give the present structure of the Marble Hall fragment. At first, high-grade thermal metamorphism was superimposed on the earlier metamorphic phenomena, but later, regional retrogressive metamorphism followed.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 74-79

The pipe-like body under consideration occurs in the Critical Zone of the Bushveld Complex and contains an assortment of rounded inclusions set in a matrix of ultramafic pegmatoid. The inclusions show a zonation, being crowded towards the centre of the pipe, and consist of felspathic amphibolite with lesser amounts of amphibolite, chromitite and mottled anorthosite. The higher temperature mineral assemblage of the inclusions of felspathic amphibolite, as compared with the mottled anorthosite of the country-rock, suggests that the xenoliths have been derived from a great depth. It is concluded that the emplacement of the contents of the pipe was produced by the aggregation of volatile fluids rich in ultramafic constituents in the mafic Bushveld magma and that the pressure of the volatiles exceeded the lithostatic pressure. The zoning of inclusions in the pipe could be a result of "fluidization" or flowage-differentiation.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 59-66

Thirty-two plagioclase feldspars were separated from a 526-foot borehole core of rhythmically layered mafic rocks belonging to the Critical Series of the Bushveld Complex. These low-temperature feldspars were analyzed for SiO2, Al2O3, Fe2O3, FeO, MgO, CaO, Na2O, K2O, TiO3, MnO, Sr, Ba and Ga. By interpreting the textures of the rocks as products of crystal-settling, an attempt has been made to correlate the major and trace-elements in the plagioclase with real and apparent differentiation trends. The most notable feature of the cumulus minerals is the increase of An with differentiation coupled with the concomitant decrease of Ab. Also noteworthy is the sympathetic trend displayed by the increase of the Fe³⁺/Fe³⁺ + Fe²⁺ ratio with differentiation. It is concluded that this reversal of the normal differentiation trend was produced by an increase in PH2O. An unusual aspect of the zoning is that all the cumulus plagioclases have margins more calcic than the cores. The generally more calcic bulk composition of the plagioclases from the plagioclase-rich cumulates as compared with those from the other cumulates is attributed to a larger and more calcic development of the outer margin. The generally smooth and progressive change in chemical composition of the cumulus phases coupled with the more irregular behaviour of the intercumulus plagioclases suggests differentiation from a single magma. The erratic chemical behaviour of one of the cumulus plagioclases is attributed to complex compositional zoning.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 289-305

Reconnaissance mapping was carried out in an area in Namaqualand some 12 000 km² in extent between the existing geological maps of Bitterfontein in the south and the Richtersveld in the north. In the east the basal augen gneisses are succeeded by leptites, aluminous rocks, quartzites and mafic gneisses. These paragneisses accumulated over 2 600 m.y. ago and they have been folded and sheared during three events of deformation, each consisting of several episodes with accompanying metamorphism. The earliest deformation recognisable, which resulted in the tectonic banding, was followed prior to ˜1850 m.y. by isoclinal folding about northwards-plunging axes. The associated metamorphism ranges from green schist facies in the west at the coast, to granulite facies in the east. Subsequent east-west folds, commonly monoclinal in style, with concomitant shearing and tectonic sliding are responsible for the present east-west banding and large-scale interference structures. Granites were emplaced during the fourth episode of this first event, and cordierite and wollastonite were formed in the gneisses. A related east-west belt of pegmatites occurring to the north of the present area separates the Namaqualand gneisses from the less- recrystallised rocks to the north. The ˜1000 m.y. age prevalent in Namaqualand rocks is thought to refer to this episode. During the second event shearing took place and the basal Stinkfontein quartzites were deposited about 900 m.y. ago. Large phyllonite zones developed during the third event, when the Upper Stinkfontein Formation accumulated, and numerous mafic dykes (˜870 m.y.) and the Richtersveld granite (q 850 m.y.) were emplaced. Following deposition of the Nama sediments (˜600 m.y.) rejuvenation of movement occurred along north-south lines of structural weakness. East-west faulting and the intrusion of dolerites of probable Karoo age (190-150 m.y.) subsequently took place.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 97-109

In the Copper Queen area two elliptical granite-gneiss domes have produced a metamorphic aureole in sediments belonging to the Piriwiri Group. There is no discordance between the domes and sediments, and this feature coupled with their shape and alignment suggest a similarity to en-echelon fold patterns. The domes are composed largely of gneisses which have bands of amphibolite and occasional skarns considered to be metamorphosed relicts of the basal portion of the Piriwiri Group, in addition to intrusions of younger granitic rocks. Field and petrographic observations suggest that the domes are part of a reactivated pre- Piriwiri basement sequence which was mobilized and deformed with the overlying Piriwiri sediments during the Magondi Orogeny. During this Orogeny fresh granitic material was introduced from depth by anatexis and palingenesis and then intruded into pre-Piriwiri rocks concomitant with the rise of the domal rocks in a plastic state.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 157-161

Three granite bornhardts within an 80 km radius of Salisbury have been studied in detail. The weathered surface features on these hills, particularly the development of gnammas, have been measured and compared. Particular attention is paid to the drainage channels or gullies which have been eroded into the upper slopes of the bornhardts. Geomorphic analysis of a specific small, perched drainage basin is applied as for that of a normal-scale land-scape eroded into granite.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 253-264

The Towla-Jopempi area is situated within the northern marginal zone of the Limpopo Mobile Belt, and, when compared with surrounding areas, is metamorphically, structurally and lithologically anomalous. Mount Towla and the two prominences known as Jopempi West and East, which make up the Jopempi Range, are capped by flat-lying metasedimentary sequences displaying mineral assemblages stable in the almandine-amphibolite facies of regional metamorphism and which are lithologically similar to the sediments of the cratonic schist belts. These sequences are underlain by the fine-grained and finely banded Towla gneiss, the well developed foliation planes of which are flat-lying. The gneiss contrasts markedly, both texturally and as regards metamorphic grade displayed (almandine-amphibolite facies), with the surrounding granulitic gneisses typical of the northern marginal zone of the Limpopo Mobile Belt. At Mount Towla a disconformable relationship, possibly of tectonic origin, exists between the Towla gneiss and the metasedimentary capping. Due to poor exposures, the extent of the Towla gneiss is not readily ascertainable but appears to be in the order of 750 km}. In the north, where the contact with surrounding granulitic gneisses is exposed, the Towla gneiss is frequently mylonitised. Flanking the Towla- Jopempi area in the south, is the characteristically linear cataclastic zone of the mobile belt. This zone appears to be truncated by the Towla-Jopempi area. A sill of coarse-grained serpentinised peridotite is intrusive into the metasedimentary sequence at Jopempi West. Silicate analyses of this peridotite and of a similar intrusive from the Beit Bridge area are presented and compared. Chemical comparisons are also made with other Southern African ultramafic rocks. Brief descriptions of the stratigraphy, lithology, petrography and structure of the Towla-Jopempi area are given and various possibilities are suggested in an attempt to account for the anomalous nature of the area.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 277-287

The purpose of the paper is to examine, in a particular instance, the extent to which photo-interpretation can be used as an aid to mapping in areas where the tectonic history has been long and complex. The area chosen is in Namaqualand, northwestern Cape Province, where the tectonic and metamorphic history has been worked out in some detail. The rocks are high-grade gneisses and other metasediments which on the whole, are not easy to differentiate from each other on air photos except where differences in composition are considerable. The first deformational event consists of four periods of folding, with some accompanying shearing; the first episode (F1) resulted in penetrative isoclinal folding, which is responsible for the layering most prominent now, and only relict fold closures show its tectonic origin: these can occasionally be seen on the photographs. The second episode (F2) also consisted of tight, penetrative folds which are deformed by the more gentle, open structures of F3. Interference patterns produced by these two phases are clearly seen on air photos. The fourth phase (F4) consists of broad open folds which have a different trend from, and deflect, the F3 folds, and can easily be recognized on air photos. The second deformational event consisted mainly of episodes of shearing and fracturing and although these structures are in general clearly visible on the photos, their age relationships are difficult to determine. It is concluded that, given good exposures, photo-interpretation may be of more use in areas of complex structure than has hitherto been realised.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 345-347

The generalized structural sequence in the metamorphic country rocks of the Okiep-Nababeep District is: (1) Brandberg gneiss: (2) Springbok granulite, quartzite and schist; (3) Nababeep gneiss; (4) Concordia gneiss and wolframite schist; and (5) Ratelpoort granulite, quartzite and schist with augen gneiss. This sequence has been interpreted in terms of a major recumbent fold, the Nababeep nappe, based on the correlation of various units. Various intrusive bodies, such as the economically, important noritoid suite, were emplaced during and after this deformation. The metamorphic events and their age determinations are described. The Editor states that this is an extended abstract of the paper read at the Symposium and that supplementary data have been published in "The copper deposits of the O'okiep District, South Africa; new data and concepts" by E.F. Stumpfl, T.N. Clifford, A.J. Burger and D. van Zyl, Mineral. Deposita, 11(1). p. 46-70, 1976.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 223-234

The copper-zinc mine at Copperton, Prieska district, Cape Province, is situated in Precambrian gneisses of the Copperton Formation which is partly covered by Dwyka tillite. A study of borehole core reveals lithological differences between acidic gneisses of the footwall and siliceous to pelitic gneisses of the hanging wall. The formation may represent a metamorphosed volcano-sedimentary, sequence of which the orebody probably forms a syngenetic part. A pholyphase tectonic history is recognized. During the early metamorphic episode conditions close to 700°C and 5 kb in the near-granulite grade were attained, probably accompanied by the severe regional (F2) deformation. A Rb/Sr isochron of 1 312±110 m.y. (I = 0,7024±0,0027) dated an isotopic homogenization ascribed to this event. During a second phase of activity, high-grade assemblages were partially retrograded to lower amphibolite assemblages. This was effected by influx of water and mild (F3) deformation. A dyke post-dating the early metamorphism and with a late-metamorphic mineral assemblage gives a maximum age of 1 020 Ma for the second event. Using conservative estimates for mantle Sr isotope ratios and average Copperton Rb/Sr ratio, a maximum age of 1 500 Ma is derived for the Copperton Formation. A syntectonic origin preceding the early metamorphism is envisaged for the volcanic and sedimentary rocks. As part of the Sonama Crustal Province or Namaqua Mobile Belt the Copperton Formation and Kheis Group have a common (Kibaran) tectonic history. However, the Kheis Group is stratigraphically older and may underlie the Copperton Formation. The Hartebeest Pan Formation north of Copperton may prove to be a correlate of the Copperton Formation.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 87-98

The Bon Accord deposit, now almost completely mined out, formed a lens close to the contact of a tightly folded band of Moodies quartzite and an ultramafite of the Jamestown Igneous Suite. The deposit had a unique mineralogy and, among other rare minerals, contained liebenbergite, trevorite, willemseite, nimite, and bonaccordite. Five distinct rock types ranging from a massive trevorite-rich variety (mass percent NiO = 36) to a schistose chlorite-talc variety (mass percent NiO = 10) were present. The size of the orebody is estimated to have been about 20 tons.
Chemically, the Bon Accord deposit differs from known Archaean nickel deposits in that it is practically devoid of sulphur and much richer in nickel. By the use of a Harker diagram it is shown that the ore from the orebody has some genetic relationship to peridotitic komatiite. A Ni-Co variation diagram illustrates the chemical similarity of the Bon Accord material with nickel-rich meteorites and its dissimilarity with awaruite and josephinite.
These phenomena, together with the fact that the internal structure of the massive high-nickel rocks closely resembles that of iron meteorites, lead the author to propose a meteorite origin for the deposit. Extrapolation on the Harker diagram indicates that the meteorite probably consisted of roughly equal amounts of nickel and iron, i.e. Ni49,5 Fe50,5. It had an approximate mass of 5 tons and a volume of 0,6 cubic metres. This meteorite apparently fell 3 giga-years ago, a time of intense meteorite bombardment of the moon, and was enveloped in peridotitic komatiite. Reaction with the magma and later reaction with the crystalline komatiites during a complex geological history caused oxidation of the NiFe under subliquidus conditions, and partial dispersion of the nickel and iron on the outer margins of the body. These reactions gave rise to the various assemblages, eg. trevorite-liebenbergite, trevorite-willemseite, ferroan trevorite-willemseite-nimite, nickloan talc-nickeloan clinochlore-nickeloan magnetite.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 433-447

Petrographic studies on units of the Matchless amphibolite belt have resulted in the recognition of three different types of amphibolite, namely epidote amphibolite, porphyroblastic amphibolite, and chlorite-amphibole schist. Carbonated talc schists are occasionally associated with the belt. Petrochemical evidence indicates that the amphibolites are igneous in origin and are genetically related to the ultrabasic talc schists. The amphibolite and talc schists form part of an originally differentiated igneous sequence, having affinities with oceanic tholeiites and are believed to have been extruded subaqueously.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 297-322

Wegmann suggested a megaboudin origin for steep structures of the Okiep Copper District. This paper shows that minor shears in more or less horizontally disposed Nababeep gneiss have formed because extensional stress worked for a lengthy period along the foliation. An inverse (synclinal) steep structure from the district is analysed in detail. Its kinematics of formation are very similar to that deduced for minor shears. This study on comparatively small outcrops seems to indicate that the development of the steep structures (and the emplacement of the associated copper-bearing noritoid bodies) took place at great depth during a post-F2 phase of anorogenic tectogenesis.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 355-383

The magnetic and gravity methods of geophysical prospecting have been successfully used in the search for bodies of basic rock, some of which contain copper in economic amounts. The successful application of the methods is due to the distinct contrast in the magnetic susceptibility and density between the basic rocks and the country rocks. Although these methods cannot directly detect copper mineralization they provide a rapid and economical way of outlining the basic host rock. The geophysical surveys perform two distinct functions. The first is to detect and locate 211 basic bodies large enough to warrant further investigation. The second function is to outline the detected basic bodies in detail and provide information about the subsurface configuration and possible composition of the bodies. This information is of great assistance when considered in conjunction with geological evidence, geochemistry and other geophysical methods in planning an effective drilling programme to explore these potentially copper-bearing bodies. The results of both magnetic and gravity surveys at three prospects where clear-cut anomalies have been explored by diamond drilling are used to illustrate the application of the two methods in the Okiep Copper District.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 417-432

The oldest granitic rocks east of the confluence of the Khan and Swakop rivers are granitic gneisses, which are unconformably overlain by metasediments of the Nosib Group. The syn- to late-tectonic rocks of the Red granite-gneiss and Salem granite suites occur on the higher-temperature side of a boundary marking the start of anatexis in gneisses. On a regional scale the Red granite-gneiss is the most highly radioactive of the igneous rocks and is the product of anatexis of Nosib Group metamorphites and pre-Nosib gneisses. Residual anatectic melts crystallized post-tectonically as the Alaskitic Pegmatitic granite. The Salem granite suite comprises three members occurring in synclinal structures above the level of the Chuos Formation. Derivation of the melts, possibly by anatexis of Khomas Subgroup metasediments in depth, and subtle intrusion resulted in the virtual confinement of these granites to the stratigraphic level of the Khomas Subgroup. The youngest member of the suite, the leucogranite, is the mow highly radioactive. With respect to uriniferous alaskitic pegmatitic granites of "Rossing" type, it is concluded that uranium was extracted from Nosib and pre- Nosib rocks during progressive regional metamorphism, incorporated into anatectic melts, and concentrated in residual fractions which crystallized near the base of the Damara Group. In the search for such bodies attention should be directed to the cover rocks around domes of Red granite-gneiss in the central parts of the Damara orogen. Other very late- to post-tectonic granites occur as stocks, sheets and anastomosing veins. Of these, the Bloedkoppie granite is most highly radioactive; it is also the protore of uranium mineralization in the overlying calcretes. Calcrete horizons overlying the Red granite-gneiss, the Alaskitic Pegmatitic granite and the leucogranite member of the Salem granite suite are potential hosts for uranium mineralization.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 215-222

Numerous reverse faults with curved fault lines occur along the course of the Hartbees River in the zone of convergence between two major trends of the gneisses of the Namaqualand Metamorphic Complex. These faults lie obliquely across a north-west zone of incipient wrenching separating an east-west trend of the lithological banding in the west from a north-west trend in the east. The zone can also be taken as the most westerly north-west fracture of a series of major faults in the direction farther to the east. The fault pattern is typically that described as resulting from convergent wrench faulting and the associated structures are similar to those formed in models of strike-slip movement.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 181-188

This paper summarizes the results of recent research on the Namaqua sector of the Namaqua-Natal Mobile Belt and outlines the problems encountered in linking the timing of depositional, structural and metamorphic events to the geotectonic evolution of the Proterozoic crust in southern Africa. An attempt is also made to clear up the presently confusing terminology. It is proposed that the Namaqua Mobile Belt developed on pre-existing Archaean sialic crust which was part of the Kalahari-Malagasy Protoshield - a crustal segment of subcontinental proportions. Transformation of this crust, together with its supracrustal cover of possible Proterozoic age, took place during a tectogenetic cycle which started at least ~1 800 Ma ago. Field evidence and Eburnian minimum ages from mafic gneisses suggest that an early phase of high grade metamorphism yielding granulites and charnockites in the Namaqua belt may possibly be related to the Vioolsdrif igneous episode. The widespread Kibaran ages recorded from the Namaqua belt are ascribed to reheating, significant magmatism and final uplift leading to closing of isotopic systems.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 269-295

Steep structures, megabreccias and basic rocks are spatially associated features that transect the entire Precambrian metamorphic succession in the Okiep Copper District in Namaqualand. Steep structures are typically narrow antiformal linear features along which continuity of the flatlying country rocks is interrupted by piercement folding and shearing. Megabreccias form steep plan-oval bodies that lie along steep structures. Most of the disoriented blocks in megabreccia have been derived from higher in the succession. The basic rocks, forming 0,7% of the outcrop area in the central part of the District, range in composition from intermediate to ultrabasic, predominant being diorite and anorthosite. Many intrusives are composite. The bodies of basic rock are generally small, irregular, and of varied form. Most of the basic rocks lie along steep structures. Isopleths and trend diagrams indicate a preferred easterly alignment of steep structures and basic rocks. In the main, development of steep structures was succeeded by formation of megabreccia, both types of structure having been invaded by basic magma. It would appear necessary to invoke alternating conditions of tensional and compressive stress to account for the main characteristics of steep structures, megabreccias and basic intrusives. The copper deposits are confined to the basic rocks, the copper content of which ranges from a mere trace to several percent. Some 20 deposits have been mined since 1852. Past production and reserves of sulphide ore at the end of 1974 totalled 84 000 000 metric tons averaging 2,1% copper.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 131-148

The Dikoloti and Lentswe nickel-copper occurrences are located within 20 Km of the Pikwe and Selebi deposits in the complexly folded and metamorphosed Archaean gneisses of the Central Zone of the Limpopo Mobile Belt of eastern Botswana.
Situated on the eastern limb of a large synformal structure, Lentswe and Dikoloti occupy the same "stratigraphic" position in the succession established by Wakefield (1974) for the Selebi-Pikwe area. This "stratigraphic" position is different from that at Pikwe and Selebi. The massive sulphide body at Dikoloti is folded about an early D1 isoclinal axis whereas at Lentswe the sulphides have been concentrated in a late D3 shear. Pyrrhotite and small amounts of pentlandite and chalcopyrite are associated with altered, amphibolitized and serpentinized metapyroxenite and metaperidotite. The pentlandite is found as discrete grains and in approximately equal amounts as flames and exsolution lamellae. Chalcopyrite occurs as irregular grains or as a replacement of the other sulphides and gangue minerals. Pyrrhotite/pentlandite and pyrrhotite/chalcopyrite ratios are 24/l and 34/l respectively. Chemical analyses of the host rocks confirm their normative peridotitic and pyroxenitic composition. The host rocks have low Al2O3 contents and MgO/SiO2 ratios greater than 0,5.
The sulphides and ultramafic host rocks were emplaced in the enclosing gneisses at an early stage in the development of the Limpopo Belt and have similar histories. Metamorphic and deformational features displayed by the sulphides include recrystallization, twinning and "durchbewegung" structures. Sustained moderate temperatures during the latter periods of the metamorphic cooling history favoured the exsolution of pentlandite from the pyrrhotite. Magnetite encloses small amounts of pyrrhotite, pentlandite and chalcopyrite and in addition contains exsolved ilmenite and spinel suggesting a high temperature magmatic origin for both oxides and sulphides.
The Dikoloti and Lentswe occurrences are compared with the Pikwe and Selebi deposits and it is concluded that although many similarities exist, the differences in host rocks and the pyrrhotite to pentlandite and chalcopyrite ratios preclude an origin from a common magma.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 405-415

The intracratonic branch of the Damara fold belt consists of geosynclinal sediments and subordinate volcanics. Several lines of evidence make it probable that the geosyncline was formed on sialic crust. A discussion of the diverse mineralizations and ore deposits distinguishes mineralizations (1) of the geosynclinal phase and (2) of the orogenic phase. The role of metamorphism in the formation of ore deposits seems to have been rather small. The paucity of large ore deposits in the metamorphic part of the orogenic belt is attributed to the early loss of permeability caused by intense deformation prior to the onset of medium and high-grade metamorphism and to the negligible amount of late- orogenic faulting. The fold belt does not contain those types of ore deposits for which a connection with the subduction process has been proposed.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 351-353

This is an extended abstract of a report on a project which was undertaken as a complementary study on the geochronological and mineralogical investigations being carried out by other workers. To date some 90 samples from this district have been analysed for major, minor and a number of trace elements. A striking feature of the chemistry of gneisses from this district is a marked potassium enrichment compared to granitic rocks from elsewhere. This is coupled with a low K/Rb ratio (averaging 160), indicative of the so-called "low-pressure granulites". The preservation of the low K/Rb is related to the stability of biotite. Structural models are mentioned. One considers that the sequence reflects tectonic duplication by recumbent folding, with late refolding into open structures. In order to test this model, the chemical compositions of representative granulites and gneisses from different structural levels have been examined using cluster analysis techniques. The data reveal three compositional groups, (a) Granulites, including the Springbok upper and lower granulites, and the Red granulite; (b) Gneisses including the Augen, Brandberg and Nababeep gneiss, as well as the Gray granulite; and (c) Hornblende-hypersthene gneiss - a basic granulite within the Nababeep gneiss. Average compositions of each of these groups and of other important rocks from the Nababeep District are tabled and discussed. It is considered that they support but do not necessarily prove a tectonic reduplication model. They also give some indication as to the origin and genesis of these rocks. One worker recognized three intrusive granites in the area - the Modderfontein, Concordia and Rietberg granites. They are compared and discussed.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 385-401

The Natural Remanent Magnetization (NRM) of basic intrusives which are the host rocks of copper mineralization in the Okiep Copper District has been measured using a spinner magnetometer. The direction of NRM is very stable (D = 130°, I = -55°) whereas the intensity of NRM has a wide range of values. A revision of interpretation procedures for magnetic surveys has been made taking NRM into account. Preliminary studies conducted on some of the gneissic and granitic rocks into which the bodies of basic rock have been emplaced do not disclose any meaningful grouping of remanent magnetization directions. The paleomagnetic pole determined for the Okiep basic rocks is located at 21°W, 9°N (δp = 6°, δm = 9°).
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 77-86

The Monarch mine is the easternmost operating antimony mine of the "Antimony Line", Murchison Range, north-eastern Transvaal. The rocks of the "Antimony Line" comprise a suite of talc and carbonate schists situated within a broad zone (˜200 m) of essentially quartz-chlorite schists. The Monarch orebody is situated within an envelope of carbonate rocks which occur as a knot in the zone of talc and talc-carbonate schists. The carbonates in the vicinity of the mineralization are frequently coloured a distinctive green by chromian muscovite and the orebody consists of a suite of en echelon highly siliceous, massive, fine-grained carbonate reefs. The mineralization occurs as both massive and disseminated stibnite. The results of a geochemical traverse across the strike of the ore zone show that both Sb and As increase in concentration by an order of magnitude over the centre of the ore zone relative to the periphery of the ore zone. The Ni/Cr ratio has a consistent value of 0,5 away from the ore zone but increases to approximately 2,0 in the ore zone. Trace element geochemistry presented shows the geochemical similarity between talc schists and carbonate schists relative to the chlorite schists. The "honeycomb dolomite" is shown to have the highest concentrations of Sb, As, Au, W, Br, for the rock types considered. Zones of green chromian muscovite development are often closely related to the orebodies and mineralized zones. They are enriched in Sb, As, Au, W, Br, and K2O relative to the dull grey zones where the mineral is not developed. The Cr, Co, Ni, Mg, Si, and Fe values remain little affected. It is proposed that the development of the green muscovite is related to the introduction of potassium which in turn is related to the Sb mineralization.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 331-344

Quantitative determinations (by atomic absorption) of the trace element content of the gneiss wall rocks of four copper-bearing mafic bodies in Namaqualand, South Africa, indicate that Cu, as well as Ni and Zn in one case, has migrated from the ore magmas into the adjacent walls. It is suggested that migration of these occurred by diffusion through an intergranular aquatic phase and overgrain boundaries. This resulted in a logarithmic decrease in concentration away from the orebody. A relationship between the grade of mineralization and the width of the dispersion aureole seems to exist. The magnetite and biotite phases of the wall rocks have anomalous Cu contents for distances from the contact similar to the extent of the whole-rock anomaly. Reaction between the diffusing ions and the wall rocks (adsorption and/or incorporation by these phases) resulted in the destruction of an ideal diffusion process which limited the distance these ions could have migrated. A depletion of Pb, which is considered to be connected with homogenization and alteration of the feldspar phase, was detected adjacent to some of the ore bearers. Secondary migration through brecciated parts of the wall rocks influenced the extent and form of these aureoles. This puts an unfortunate limitation on their exploration value.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 235-265

Gamsberg is a steep-sided, basin-shaped inselberg, situated in the Namaqualand district of the Cape Province. The rocks form part of a Precambrian metavolcano- sedimentary succession, the Bushmanland sequence, situated in the Namaqua Mobile Belt. Lithologically the units consist of a basal granite gneiss overlain by pelitic schist, metaquartzite, iron formation, psammitic schist, and amphibolite. The iron formation display's distinct ore mineral zoning. An economically important stratiform sulphide-bearing unit as well as magnetite, hematite, and barite mineralization is recognized. The sulphide minerals include pyrite, pyrrhotite, marcasite, sphalerite, and galena. Fine-grained quartzitic pelitic and calcareous metasediments constitute the host rock. Gangue and ore mineral associations imply a stratiform sulphide deposit of marine or possible marine-volcanic origin. The reconstituted sediments and volcanic beds have undergone medium grade metamorphism during the main metamorphic event. Quartz + muscovite is a stable paragenesis and temperature/pressure conditions deduced from the rock-forming minerals indicate a maximum temperature of 670°C and a maximum pressure of 4,5 kb. Results obtained from application of the sphalerite geobarometer support the above maximum pressure conditions. The metavolcano-sedimentary rocks have been subjected to polyphase deformation expressed in five episodes. This deformation is also reflected in the ore fabric.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 349-350

Of the 700 or more noritoid bodies discovered so far in the Okiep Copper District only, about 27 have yielded economic copper concentrations. Three petrographic types are of particular relevance in the context of presently exploited orebodies: (1) hypersthenite, consisting almost entirely, of hypersthene, with some significant Fe-Ti-oxides, and bornite and chalcopyrite; (2) hypersthene diorite, carrying plagioclase, pyroxene, mica, oxide, and chalcopyrite and/or bornite; and (3) mica diorite that has been affected by intensive hydration resulting in the replacement of pyroxene by mica and chlorite, and in which chalcopyrite is the main copper bearer. More than 150 quantitative electron probe analyses have been carried out in conjunction with microscopic studies of representative suites of samples of producing and non- producing noritoid bodies. Some aspects of the results are summarized here.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 99-106

The granite boulders contained in conglomerate beds of the Moodies Group, having been derived from terranes older than the intruding 3 300 m.y. Kaap Valley pluton, may provide valuable evidence for the reconstruction of a primordial basement on which the Swaziland Supergroup was deposited. Boulders in the conglomerate were derived from several granitic sources. Subsequent to their deposition the conglomerate beds were subjected to dynamic metamorphism which has differentially affected minerals in the different boulders. U and Pb isotope measurements indicate an overprint of the Nelspruit migmatic episode which has adversely contributed to recrystallization of zircon and apatite. The zircon data suggest either (a) complete recrystallization with or without subsequent diffusive loss of lead; or (b) continuous diffusive loss of lead from a pre-metamorphic crystallization. The age of the Nelspruit metamorphism is reflected by a 3 140±45 m.y. old authigenic apatite from a quartzite immediately below the Onverwacht Series. Some conglomerate boulders indicate tentative pre-metamorphic ages considerably older than 3 300 m.y.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 149-177

Cordierite-garnet-hypersthene-biotite granulites occur distributed over an area of 4 000 sq Km in the Limpopo metamorphic complex south of the Soutpansberg in northern Transvaal. The commonly observed stable coexistence of the assemblage cordierite-garnet-hypersthene in this area is ascribed to the reaction garnet + quartz = cordierite + hypersthene. This reaction is largely controlled by a reduction in pressure at constant temperature. Chemical zoning is absent and the grains of garnet and cordierite, in thin sections, were found to be constant in composition. Cordierite rims in contact with garnet are, however, slightly enriched in MgO relative to internal compositions. The MgO/FeO ratio of biotite is dependent on its mineralogical association. Element partitioning data for the pair cordierite-garnet showed some scatter related to the edge compositions of cordierite, but it still proved possible to draw Kd-lines. It is therefore suggested that the studied mineral assemblages closely approach widespread equilibrium related to a period of uplift. This suggestion is substantiated by the regular distribution data for the pairs cordierite-hypersthene and garnet- hypersthene. Considerable scatter of distribution data for mineral pairs involving biotite is related to the instability of biotite whenever it is associated with cordierite. A P-T estimate of 800°C and 7,5 kb is based on Currie's geothermometer and geobarometer and is substantiated by independent field and experimental evidence.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 323-329

Sulphides of copper have been detected in variable amounts in virtually all Okiep basic intrusive bodies. Petrological and mineralogical studies of the basic rocks shown evidence of a close relation between silicate petrology and sulphide mineral contents. A classification of the sulphide-bearing intrusives according to silicate mineral composition and copper ore-bearing potential is proposed.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 55-76

The Murchison Range greenstone belt is situated in the north-eastern part of the Kaapvaal craton and is currently the focus of a geological research programme. From the data gathered to date the regional geology of the belt is reassessed and a revised broad stratigraphic subdivision presented. The lowermost stratigraphic division occurs along the south flank of the Murchison Range and comprises an assemblage of magnesian metabasalts and talcose serpentinitic rocks equated with the Lower Ultramafic Unit of the Barberton greenstone model. An early layered mafic intrusion (the Rooiwater Igneous Complex) forms the northern part of the range and the central sector of the belt is comprised of a rock assemblage equated with the Mafic to Felsic Unit of the Barberton model. One of the main rock assemblages within this unit is a basic lava suite containing sequences having a somewhat elevated magnesium content when compared to normal tholeiites. These lavas are associated with a group of chloritic schists displaying some distinctive and unusual chemical characteristics which include a very low calcium content and elevated aluminium, iron and magnesium contents. They are considered to represent sediments of probably largely volcaniclastic origin. Interlayered in the chlorite schists are more felsic horizons in the form of porphyries, quartz-sericite schists, quartzites and minor conglomerates. Whereas some of these are probably of primary volcanic origin, the quartzites and conglomerates are clearly sedimentary having been largely derived from the products of acid magmatism. A variety of iron-rich formations are often associated with these more felsic rocks.
Carbonate rocks are well developed and widespread in the belt. They generally take the form of replacement masses but there appear to be some primary sedimentary types as well. Talc schists are often closely associated with, and gradational to carbonate zones, particularly in the environs of an important carbonate and talc-carbonate assemblage known as the "Antimony Line". Antimony mineralization occurs within thickened carbonate masses along this "Line" and is more specifically associated with siliceous and often fuchsitic zones within these carbonate masses.
The carbonate rocks along the "Antimony Line" show anomalous Ni, Cr, Co, Sb, and As concentrations and this, together with their distinctive major element chemistry, make it clear that they are not of a well-documented sedimentary type. Arguments for and against syngenetic and epigenetic origins for the antimony mineralization associated with these characteristic carbonate rocks are presented by no final conclusion is reached.
Structurally the Murchison fold belt takes the form of an ENE trending keel. Cross-fold trends are super-imposed on and disrupt this pattern. In part this overprint is due to the emplacement of tonalitic granite magmas which invade the belt in large areas.
A range of other mineral deposits in the Murchison Range are briefly discussed. Pyritic copper/zinc mineralization of probably volcanogenic type is associated with a quartz porphyry assemblage. Emeralds occur in biotite schists along the southern contact of the belt where leucogranitic and pegmatitic phases of a muscovite granite have invaded ultramafic schists. Small gold occurrences are widespread in the area, either associated with quartz and carbonate material in crosscutting shears, or associated with banded, often sulphidic iron formation. Mercury mineralization in the form of cinnabar occurs at one main locality in carbonate rocks north of the "Antimony Line".
Titaniferous magnetic bands as well as a Cu/Ni sulphide band occur in the gabbroic phase of the Rooiwater Igneous Complex.
Back to title list




Upper Mantle Project - South African National Committee Symposium, July 1969. Special Publication No.2, 433-444

A summary is given of the stratigraphic relationships of the Swaziland System, the Pongola System and the surrounding granite-gneiss terrain. Age measurements by the Rb-Sr whole-rock method have yielded a consistent chronology for the felsic plutonic rocks of this area. The same method applied to stratified rocks also yielded satisfactory isochrons, but the calculated ages are lower than their probable times of deposition (obtained from their field relationships to dated plutonic rocks). The stratified rocks appear to be more vulnerable to open-system behaviour at times long after their deposition. An age in excess of 3,000 m.y. for the Swaziland System is confirmed, but it has so far not been possible to determine the precise time of deposition.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 715-720

Anorthositic gneiss is found as elongated bodies conformable with the general stratification of intensively deformed Precambrian rocks in the far northern Transvaal. It is pointed out that the gneiss should be regarded as belonging to the "orogenic plutonic" type of anorthosite, to be distinguished from the gravity-stratified rocks of similar chemical composition in the Bushveld Complex.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 477-491

This paper describes in detail the succession in a layered intrusion of Precambrian age in Canada, which is considered by the author to show similarities to the Bushveld Igneous Complex.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 425-440

Fields of composition and magmatic trends of mafic complexes in various geological environments are shown by Mg:Ca:Fe and Mg:Fe:Al cation percentage plots of 730 rock analyses: 1. Bushveld Complex; Great Dyke. 2. Stillwater Complex; Skaergaard intrusion. 3. Basic igneous complexes of north-eastern Scotland. 4. Bay of Islands Complex. 5. Alps, Lewisian (Scotland), New Caledonia and Sittampundi (Madras) complexes. 6. Various mafic complexes in mountain belts. 7. Hayachine, Miyamori, Horoman (Japan), and Union Bay (Alaska) complexes. Thirty new rock analyses for the Table Mountain and Blow Me Down Mountain masses of the Bay of Islands complex are presented and the plots include substantial new data for the Stillwater and Lewisian complexes. The fields of composition and magmatic trends of a number of stratiform complexes are compared. Comparison is also made with fragmented layered masses and with other mafic masses in mountain belts. The general similarity of the composite field of composition of these alpine-type masses suggests the derivation of some alpine-type masses by tectonic fragmentation and metamorphism of stratiform complexes.
Back to title list




The Limpopo Belt. A contribution to the National Geodynamics Programme. Special Publication No.8, 1-3

To date, 26 articles and theses have been published on research undertaken by members of this Working Group and these are listed below. In addition, three colour maps, one compiled by M.K. Watkeys of the Limpopo Belt as a whole, a second compiled by P.C. Horrocks and R.E.P. Fripp of the area south of Messina and a third one compiled by R.E.P. Fripp of the Sand River Gneisses type area, have been published and are included in the pocket at the end of this volume. In this volume are collected a number of additional contributions to the understanding of the geology of the Limpopo Belt. Most of these contributions, like those previously published, are addressed to specific topics and not to an overall synthesis of the state of knowledge of the tectonic history of the Limpopo Belt. The last contribution, however, is an attempt to draw together the varying views of the nature of the Limpopo Belt and to discuss their weaknesses and strengths. The direction of possible future research is also discussed.
Back to title list




The Limpopo Belt. A contribution to the National Geodynamics Programme. Special Publication No.8, 5-8

Recently, a number of reviews of the Limpopo Belt have been composed (Robertson and Du Toit, 1981; Barton and Key, 1981; Light, 1982; Tankard et al, 1982). However, due to the Belt straddling eastern Botswana, southern Zimbabwe and the northern Transvaal, no single map of the whole Belt was available until, as part of the South African Contribution to the International Geodynamics Project, a 1:1 000 000 colour geological map of the region was compiled from published maps, together with unpublished material from various theses. A combination of the structural and stratigraphic subdivisions reveals the Limpopo Belt to be composed of seven tectono-stratigraphic domains, separated from each other by a variety of contacts. Adjacent to the Rhodesian Craton, Domain I consists of a high-grade, granite-greenstone terrane with east-north-east-trending structures. It has been interpreted as a wide ductile shear zone. Towards the east, it is dislocated from the lower- grade rocks to the north by thrusts which do not necessarily follow the orthopyroxene isograd. In the west, a nongradational, but still structural, contact is apparent through the Tuli-Sabi Straightening Zone. In Zimbabwe, Domain I is overthrust by Domain II along shallow dipping zones. Within Domain II, the lithologies of the Central Zone and the Rhodesian Craton are juxtaposed in a wide dextral straightening zone which has been transected by the sinistral Triangle Shear Zone. In Botswana, Domain I is parted from Domain II by the Letlhakane Fault and other steep, normal or dextral wrench faults. Although the lithologies are similar to Domain 9, in Domain II the structures are more northerly trending. Both Domains II and III seem to merge into Domain IV through the disappearance of the cratonic lithologies. In to former case, the structures revert from east-north-east trends to more northery trends which typify tyhe paragneisses and metasediments of Domain IV. To the south, in Domain, V, these lithologies display east-north-east structures in a ductile shear zone with a possibly sinistral sense of movement, which was reactivated during later faulting. Farther west, the Sunnyside Shear Zone has an apparently sinistral direction of movement whilst the Shapane Hill Thrust is directed northerly. These struc taures separate Domains IV and VI. Within the latter, the Central Zone lithologies have east-north-east to north-west trends and have been remobilised to form the Early Proterozoic Mahalapye Complex comprising migmatites and a variety of granites. The southem boundary of this domain is marked by a wide zone of ultramylonites which form the sinistral Palala Shear Zone. The southernmost portion of the Limpopo Belt, Domain VII, consists of high-grade lithologies that have been correlated with the Kaapvaal Craton and which display complex but dominantly westerly-trending strucwres. The contact with the rest of the Limpopo Belt is masked by Proterozoic and Phanerozoic cover but seems to be defined by the Palala Shear Zone. This domain may coalesce with the lower-grade cratonic rocks by a decrease in metamorphic grade, without any structural discontinuity, as represented represented by the Hout River Transition. However, the offset of the Rhenosterkoppies greenstone fragment is suggestive of the contact being a sinistral shear zone. The eastern margin of Domain VII may be demarcated by the Kudu's River Lineament, pre viously termed the Moletsi Shear Zone, which is a sinistral shear zone. Thus, far from being a simple linen zone, the Limpopo Belt comprises a number of domains, each with its own internal strati graphic, domains, each with its own internal stratigraphic, structural and metamorphic history. Most of the studies within the Belt have been of a preliminary, if not pioneering nature. Until the detailed geology of each domain is unravelled and the interrelationship between domains established, any model for the Limpopo Belt must be regarded with healthy scepticism.
Back to title list




The Limpopo Belt. A contribution to the National Geodynamics Programme. Special Publication No.8, 9-18

Isotopic studies of the Sand River Gneisses provide evidence both for the behaviour of certain parent-daughter isotopic systems during metamorphism and the pattern of early crustal evolution in southern Africa during the Archaean. In these gneisses, which have been metamorphosed several times and once to granulite facies, different parent-daughter isotopic systems have behaved independently of one another. This independent behaviour seems to have been controlled by the bulk chemical compositions of the different rock facies. In general, whole rock Rb-Sr systems, after being reset during an early ~3 790 Ma metamorphism, appear to have been the most resistant to resetting during subsequent metamorphism. U-Th-Pb whole rock and zircon systems have, however, been affected by the more recent metamorphic events. Pb-isotopic analyses suggest that the source of the Sand River Gneisses and of several other rock units in the Central Zone of the Limpopo Mobile Belt contains remnants of early, perhaps >4 000 Ma old, mantle differentiates with anomalously high μ-values.
Back to title list




The Limpopo Belt. A contribution to the National Geodynamics Programme. Special Publication No.8, 19-25

The geology of an area in the Central Zone of the Limpopo Mobile Belt near Zanzibar in eastern Botswana is described and the results of chemical and Rb-Sr isotopic analyses of some rock units exposed there are presented. This terrane contains the grey granodioritic Zanzibar Gneiss which has been intruded by mafic dykes. Subsequently, both the gneiss and the dykes were intruded by granite. Based on comparison with the rocks in the Central Zone near Messina, the results suggest that: (1) Zanzibar Gneiss is at least ~3 230 Ma old; (2) it was intruded by mafic dykes ~2 960 Ma ago; (3) both these units were intruded by granite ~2 550 Ma ago and (4) this region reached its present level of exposure ~1 965 Ma ago.
Back to title list




The Limpopo Belt. A contribution to the National Geodynamics Programme. Special Publication No.8, 27-37

Major element concentrations, K/Rb ratios and Rb-Sr isotopic analyses are presented for several mafic dykes intruding the previously deformed supracrustal rocks and the Messina Layered Intrusion in the Central Zone of the Limpopo Belt near Messina, northern Transvaal. In addition, similar data are presented for amphibolite samples from a layered succession of amphibolite and quartzite within the sequence of supracrustal rocks. On an AFM diagram, the chemical analyses for all of the mafic rocks show tholeiitic affinities. At least three dykes were emplaced during a period of igneous activity 3 000±50 Ma ago for which no coeval regional deformational event has been recognized. The other dykes and the amphibolite yield ages that are clearly too young to reflect emplacement. These ages may reflect local metamorphic events and one of these metamorphic events may have accompanied copper mineralization at Messina ~1 850 Ma ago.
Back to title list




The Limpopo Belt. A contribution to the National Geodynamics Programme. Special Publication No.8, 39-41

Pb-isotopic data are presented for eight samples from the Messina Layered Intrusion. These data indicate that U-Pb isotopic systems and Rb-Sr isotopic systems have behaved independently of one another during polyphase metamorphism. The emplacement age of the Messina Layered Intrusion is at least 3 270 Ma.
Back to title list




The Limpopo Belt. A contribution to the National Geodynamics Programme. Special Publication No.8, 43-44

Pb-isotopic data are presented for granulite facies banded iron-formation from the succession of supracrustal rocks in the Central Zone of the Limpopo Mobile Belt. These data yield an age of about 1 950 Ma and suggest that under some metamorphic conditions. U-Pb systems in banded iron-formation are easily disturbed.
Back to title list




The Limpopo Belt. A contribution to the National Geodynamics Programme. Special Publication No.8, 45-46

The Palala Granite (McCourt, 1983) was emplaced in pre-Transvaal times (more than 2 200 Ma ago) into the supracrustal rocks of the Central Zone of the Limpopo Mobile Belt (Visser, 1953). The position of this body entirely within the Palala shear zone, a portion of the major shear zone separating the Central Zone from the Kaapvaal Craton to the south (Watkeys, 1983, Fig. 1 in pocket at back of Volume; Barton and Key, 1981; McCourt, 1983), suggests that the Palala Granite was emplaced along this crustal discontinuity. Subsequently, metamorphism related to movements along the shear zone has imparted a strong gneissosity to the granite and the present mineralogy is altered under conditions of greenschist grade of metamorphism.
Back to title list




The Limpopo Belt. A contribution to the National Geodynamics Programme. Special Publication No.8, 47-53

Date reported in this paper include results of a K-Rb-Sr study on albites from the West Lode (Campbell Mine), a similar study on apparently fresh country rock from Artonvilla Mine, and lead isotope studies on sulphide minerals from the Artonvilla, Campbell and Messina mines. The work was aimed at characterizing the mineralizing solution with respect to 87Sr/86Sr ratios, K/Rb ratios and lead isotopes, and at determining the age of mineralization. 87Sr/86Sr ratios of the Campbell albites are around 0,72 and indicate extensive equilibration of the ore fluid with crustal rocks, or a crustal origin. The lead in the deposits shows high 207/206 ratios and must, therefore, be crustally derived. K-Rb-Sr characteristics of the apparently fresh country rock from the Artonvilla Mine indicate that a fluid penetrated into apparently fresh country rock. Age indications are weak. An age-range in excess of 1 000 Ma is indicated by the Rb-Sr characteristics of the Campbell Mine albites, and the lead isotopic results favour the hypothesis that the ore was first emplaced more than 1 000 Ma ago and was partly remobilized at a later (possibly Karoo) stage.
Back to title list




The Limpopo Belt. A contribution to the National Geodynamics Programme. Special Publication No.8, 55-64

Isotopic studies of rocks in the Southern Marginal Zone of the Limpopo Mobile Belt indicate that, although these rocks may be highly metamorphosed equivalent of granite-greenstone terranes of the Kaapvaal Craton, they were intensely metamorphosed between about 2 650 Ma ago and about 2 600 Ma ago. This metamorphism largely obliterated the isotopic memory of older ages in the area creating an ~2 600 Ma whole rock age veil over the Southern Marginal Zone and may be related to a thrusting of the rocks of the Kaapvaal Craton over those of the Central Zone of the Limpopo Mobile Belt, exposing lower crustal rocks. A group of granitic intrusions was emplaced along a north-easterly trend oblique to the Belt both on the Craton and in the Southern Marginal Zone during and subsequent to this ~2 600 Ma metamorphism.
Back to title list




The Limpopo Belt. A contribution to the National Geodynamics Programme. Special Publication No.8, 65-80

Over 5 000 Km2 of the Central Zone of the Limpopo Belt has been mapped in Zimbabwe. The Archaean terrane comprises a c. 3 800 Ma orthogneissic basement, with c. 3 500 Ma mafic dykes, overlain by an extensive supracrustal succession dominated by migmatitic garnetiferous paragneisses with intercalated calc-silicate gneisses, magnetite-quartzites, quartzites and mafic granulites or amphibolites. Injected into these lithologies are c. 3 300 Ma old mafic-ultramafic complexes and c. 3 000 Ma old mafic dykes. This succession underwent high-grade metamorphism at about 3 150 Ma when temperatures of about 850°C and pressures of about 11 kb were achieved. Later, rapid uplift was followed by the reintroduction of water into the granulites resulting in widespread retrogression to medium-grade. This was accompanied by the major deformational events of the region and by the intrusion of the c. 2 700 Ma old Bulai Gneiss. Subsequently, tectonic activity has been confined to faulting which influenced the deposition of the Proterozoic Waterberg and Phanerozoic Karoo successions.
Back to title list




The Limpopo Belt. A contribution to the National Geodynamics Programme. Special Publication No.8, 81-88

An area of about 200 Km² between Messina and Tshipise in the Limpopo Mobile Belt has been studied. The region is underlain by poly-deformed high-grade metamorphic rocks made up of a variety of basement and supracrustal gneisses. The basement consists of grey banded granodioritic gneisses, while the supracrustal rocks in the area consist mainly of quartzo-feldspathic gneisses with lesser intercalations of garnet-cordierite-sillimanite gneiss, quartzite, banded magnetite quartzite, pyroxenitic amphibolite, calc-silicate gneiss and marble. Intrusive rocks of the Messina Layered Intrusion include gabbroic and anorthositic gneisses, while minor serpentinites and pyroxenites also occur. Tholeiitic dykes of various ages are common in the region. It is suggested that the supracrustal gneisses formed sometime between 3 570 and 3 270 Ma ago during a period of subsidence. Burial to depths greater than 25 Km caused early high-grade regional metamorphism probably about 3 150 Ma ago where pressures reached about 7 kb and possibly even 10 kb while temperatures reached up to 900°C. Subsequent retrogression from 2 900 Ma to 2 400 Ma was probably induced by uplift and erosion with pressures dropping to 5 or 6 kb and temperatures falling to 600°C. Early isoclinal folding together with subsequent refolding and anatexis affected the rocks. Relative tectonic stability was achieved by about 2 200 Ma ago.
Back to title list




The Limpopo Belt. A contribution to the National Geodynamics Programme. Special Publication No.8, 89-102

A small part of the Central Zone of the Limpopo Mobile Belt has been mapped in detail. Two main lithostratigraphic units occur in the area: (i) a suite of hypersthene-bearing granodioritic and quartz dioritic gneisses called the Sand River Gneisses and (ii) a suite of para- and orthogneisses, consisting mainly of quartzite, metapelitic gneiss and pyroxene amphibolites called the Belt Bridge Complex. The former were intruded by the mafic Causeway Dykes about 3 570 Ma ago as well as the mafic Stockford Dykes about 3 060 Ma ago. This younger dyke suite is also recognized within the Beit Bridge Complex. The Sand River Gneisses are the oldest recognized rocks in Africa, and have been dated at about 3 790 Ma. They contain evidence for an early high pressure granulite metamorphism and early tectonic events (M1-D1/D2) for which a pressure of about 13 kb and a temperature of about 920°C pertained. This event is not recognized in the Beit Bridge gneisses, and the latter have undergone a later high- grade metamorphic event (M2) in which pressures of about 9 to 12 kb and temperatures of about 820°C to 990°C pertained. This metamorphism postdates an episode of recumbent folding (D3 - nappes?) which affected both the Sand River Gneisses and the Beit Bridge Complex, and is also related to the main fabric-forming event (D4) in the latter. Subsequently all of the gneisses have been intruded by Stockford age dykes, and deformed by two events of regional upright folding (D5 and D6), and as possible thrust blocks, which probably culminated about 2 600 Ma ago. The original lithologic nature of the Sand River Gneisses is obscure. Geochemical studies, together with the lithological associations evident from the mapping, suggests that the Beit Bridge Complex represents an intensely deformed and metamorphosed equivalent of some elements of a modern ocean floor, including basaltic intrusive and extrusive rocks, and siliceous, pelitic and calcareous sediments. From this and other evidence it is postulated that the Central Zone of the Limpopo Mobile Belt near Messina may represent, or at least contain certain components of, a convergent plate margin, including a magmatic arc, back-arc basin and continental margin.
Back to title list




The Limpopo Belt. A contribution to the National Geodynamics Programme. Special Publication No.8, 103-112

The high-grade supracrustal rocks of the Beit Bridge Complex east of Messina have been subdivided into three lithostratigraphic groups. Their principal lithologies are metaquartzite, various quartzofeldspathic gneisses, amphibolite, metapelite, calc-silicates and marble. The general features of the groups are briefly outlined and the petrology and geochemistry of various more significant lithologies are described in detail. Scapolite-bearing rocks are considered to be the metamorphic equivalents of shales containing saline minerals.
Back to title list




The Limpopo Belt. A contribution to the National Geodynamics Programme. Special Publication No.8, 113-119

The salient features of the Archaean lithologies in the Koedoesrand area, north-west Transvaal, are described and discussed. Supracrustal rocks are well-developed and have been subdivided into the Mount Dowe and Gumbu groups. Ultramafic lithologies intrusive into these supracrustal rocks are correlated with the Messina Suite. Together, the Mount Dowe Group, the Gumbu Group and the Messina Suite form the Beit Bridge Complex. Grey biotite gneiss outcropping along the Mogalakwena River, appears to intrude supracrustal rocks of the Gumbu Group. Deformation was complex and three phases of folding are postulated. Metamorphism reached granulite facies and diagnostic minerals are well-preserved in both the paragneisses and the mafic granulite. The Palala Shear Zone in the southern part of the area, has a width of 10 km and an outcrop length of 25 km. It is characterized by mylonite, ultramylonite and flaser gneiss. The presence of the Beit Bridge Complex and the existence of dome and basin type interference structures, places the Koedoesrand area in the Central rather than the Southern Marginal Zone of the Limpopo Mobile Belt.
Back to title list




The Limpopo Belt. A contribution to the National Geodynamics Programme. Special Publication No.8, 121-142

The rock types are subdivided into a basement migmatitic tonalite and trondhjemite gneiss, termed the Baviaanskloof Gneiss, which is overlain by the supracrustal Bandelierkop Formation composed of a basal ultramafic member overlain by the mafic member and an upper pelitic member. Macroscopic and mesoscopic fold morphologies and statistical data point to three major deformation events of which D1 is an inferred isoclinal event, possibly related to a very early deformation which predates the high-grade Limpopo metamorphic and deformational events. This is succeeded by D2 which developed co-axially to D1 and which imparted a strong foliation to all the rocks in the Southern Marginal Zone. The third fold event D3 is orientated obliquely to D1 and D2 and is particularly well-developed in the south and south-west of the area but is less penetrative elsewhere. D4 is a post-folding and post-metamorphic shearing event which traverses the rocks in north-eastern (Kudu's River Lineament) and east-north-eastern directions, breaking down the high-grade mineral assemblages to much lower grades. The Southern Marginal Zone is divided into two high-grade metamorphic zones separated by the M3 orthopyroxene isograd. The rocks north of the isograd were subjected to two granulite grade metamorphic events (M1 and M2) while the M3 orthopyroxene isograd developed during the last metamorphic event. By utilizing metamorphic textures in fold morphologies it was possible to establish a sequence of tectono-metamorphic events that took place during Archaean crustal development in the Southern Marginal Zone. The possible greenstone origin for the metamorphosed Bandelierkop Formation is suggested by exposures in the Hout River in which supracrustal rocks, at the lower amphibolite facies, become progressively metamorphosed to granulite grade over a distance of some 10 km. Within the same distance weakly deformed greenstones developed progressively stronger foliation (D2) which ultimately becomes rotated by the D3 deformation. The possible greenstone origin for the ultramafic and mafic rocks of the Southern Marginal Zone is further substantiated by petrochemical data which indicates a similarity between these rocks and komatiites (peridotitic) and tholeiites of the Archaean greenstone terranes. The pelitic member of the Bandelierkop Formation is also chemically akin to greywackes and shales from the Fig Tree Group in the Barberton greenstone belt and quartz-sericite schists from the Sutherland greenstone belt. Repeated granite plutonism is important in the establishment of the chronological order of tectonometamorphic events in the Southern Marginal Zone. The Matok pluton (2 600 Ma) was emplaced after M2 and D2 but before M3 and D3 whereas the Palmietfontein granite plutons (2 456 Ma) invaded the high-grade rocks after all the major tectono-metamorphic events had ceased in the Southern Marginal Zone.
Back to title list




The Limpopo Belt. A contribution to the National Geodynamics Programme. Special Publication No.8, 143-167

The second (M2) regional granulite event in the Southern Marginal Zone of the Limpopo Metamorphic Complex in South Africa is revealed by the reaction garnet + quartz = cordierite + hypersthene. The sliding equilibrium of this reaction was controlled by the divariant coexistence of garnet + cordierite + hypersthene + biotite in metapelites over a surface area of at least 5 000 km². The nature of this continuous reaction is preserved within the rocks in the form of symplectic intergrowths of cordierite and second generation hypersthene coronas surrounding garnet in samples with X MgO = 59-70 (cordierite- garnet granulites of Group 1), whereas the same reaction has run to completion in samples with X MgO >70 (garnet-free cordierite granulites of Group 2). The cordierite-free garnet granulites (Group 3) with X MgO <59 show no sign of reaction. Chemical zonation of garnet in a few samples apparently resulted from its breakdown to produce cordierite + hypersthene, suggesting that the edges, rather than the cores, are more representative of the M2 conditions of metamorphism. Zoning of cordierite in six samples is in the opposite direction (more Mg-rich rims) to that of garnet (more Fe-rich rims) and probably resulted from later retrograde exchange. The systematic Fe-enrichmesnt of the cordierite-garnet-hypersthene subtriangles in a AFM-diagram is a good indication that equilibrium during the breakdown of garnet, was attained under conditions of decreasing pressures which descended to approximately 8,2-7,2 kb at constant temperatures of ˜800°C. The pressure gradient during the M2 event is illustrated by the regular regional distribution within the orthoproxene zone of cordierite-garnet-hypersthene subassemblages with similar F/M ratios in the AFM-diagram. The metamorphic conditions during the first granulite event (M1) were >800°C at total pressures of at least 9,5-10 kb. The evidence for near isothermal decreasing pressure pressure recorded by the development of cordierite and hypersthene during the second granulite event (M2) strongly suggests syn-metamorphic erosion of the tectonic pile. The systematic regional variation in the recorded pressures is therefore likely to be due to differential uplift which resulted in differences in the erosion rate.
Back to title list




The Limpopo Belt. A contribution to the National Geodynamics Programme. Special Publication No.8, 169-174

In the area described three tectonic domains are identified: the Central Zone and Southern Marginal Zone of the Limpopo Mobile Belt and the later, cross-cutting Mahalapye Plutonic Block. The two zones of the Mobile Belt reflect different Archaean events related to differential movement of the Rhodesian and Kaapvaal cratons across and along the Limpopo Mobile Belt. Since Archaean times the Limpopo Mobile Belt together with the adjacent cratons has formed a stable shield area. Thus the c. 2 200 Ma Mahalapye Plutonic Block is part of north-westerly migration of granite emplacement in the Kaapvaal Craton throughout the late Archaean and early Proterozoic. These granites are believed to be related to movement of the crust over a mantle hot spot. The trend of Proterozoic planar tectono-metamorphic fabrics is north-west to south-east as opposed to the east-north-east to west-south-west trend of the Archaean Limpopo Mobile Belt.
Back to title list




The Limpopo Belt. A contribution to the National Geodynamics Programme. Special Publication No.8, 175-180

The deformation of the Limpopo Belt and adjacent cratons may be described in terms of several different structural domains each with its own movement pattern. This paper discusses the origins of the deformation and the probable structure of the domains at depth based on observations mainly from the northern sector of the Limpopo Belt.
Back to title list




The Limpopo Belt. A contribution to the National Geodynamics Programme. Special Publication No.8, 181-190

A model for the tectonic evolution of the Limpopo Mobile Belt since the Transvaal Era is proposed. This model is based on recent work in the Transvaal, Soutpansberg and Karoo age rocks to the south of and overlying the Mobile Belt in the northern Transvaal, South Africa. Evidence is provided that indicates the structural evolution to result predominantly from crustal extension, with the consequent development of extensive normal faults. Periods of rebound occurred causing strike slip faulting in both pre- and post-Soutpansberg times. Isostatic readjustment consequent on crustal thinning has led to uplift of more than 15 km of supracrustal rocks within the Mobile Belt region since the last major thermal event at ˜2 700 Ma. This uplift is still proceeding at present.
Back to title list




The Limpopo Belt. A contribution to the National Geodynamics Programme. Special Publication No.8, 191-203

In this paper, the most significant aspects of the Limpopo Belt are reviewed and the various published models of the tectonic history of the Belt are discussed. No rationalization or synthesis of ideas is attempted but possible avenues for future research are outlined, results from which could, with in a reasonably short time, give a much clearer indication of the nature of the Belt.
Back to title list




Upper Mantle Project - South African National Committee Symposium, July 1969. Special Publication No.2, 305-333

The Rhodesian Basement Complex consists of a typical Archaean granite-greenstone Complex, together with later ultramafic and granite intrusions. An upper limit of 2,000 m.y. has been arbitrarily assigned to the Archaean. The schist belt rocks are sedimentary and volcanic successions which are divided into pre- Sebakwian rocks, the Sebakwian, Bulawayan and Shamvaian Groups. Pre-Sebakwian rocks are found only in the Selukwe area and consist of remnants strewn through a gneiss complex. The existence of the Sebakwian Group is based on the recognition of an unconformity between it and the overlying Bulawayan Group together with an abrupt change in metamorphic grade and structural elements across the unconformity. Much of the Group in the type area at Que Que is now regarded as a later ultramafic intrusion. The volcanics of the Bulawayan Group range from olivine basalt to rhyolite in composition and are calc-alkaline in nature. The Umniati Group and Maliami River formation are calc-alkaline, andesitic and dacitic volcanics, predominantly pyroclastic, whose relationship to the remainder of the Bulawayan Group is debatable. There is no clear-cut distinction between the sediments of the Bulawayan and Shamvaian Groups and the sediments of both groups have been divided into four associations. The quartz- mica schist association occurs in the Bulawayan Group and includes some of the rhyolites of the volcanics. The banded ironstone and phyllite associations occur in both the Shamvaian and Bulawayan Groups. The conglomerate-grit association occurs in the Shamvaian Group and is the association of the type Shamvaian, the Shamva grits. The granites and gneisses have been divided into three types, the Tonalite-Gneiss Complex, the Intermediate Granites and the Late Granites. There is an increase in K2O and a decrease in Na2O content from the Tonalite-Gneiss Complex to the Late Granites. Various ultramafic intrusions, some of which are layered, were emplaced in pre-Sebakwian, post- Sebakwian and post-Bulawayan times. Many of these were once included in the Sebakwian Group. It is possible that the faults and fractures in the Basement Complex can be referred to as a wrench fault system. Evidence from radiometric dating, the structure, lithology and sequence of the schist belts and the granite sequence suggests that there are two structural provinces in the Basement Complex. The older centred on Shangani has a general north-north-west trend, whilst the younger is in two units, one to the north-west and the other to the south-east of the older. The exposed portion of the Rhodesian craton is 400 miles long and from 100 to 200 miles wide. Within this relatively small and almost completely accessible area all the typical constituents of the so-called Archaean granite-greenstone complexes may be found. Because of the small size, accessibility and complete range of constituent rock units the Rhodesian craton forms an ideal area for detailed study. Such a study has, in effect, been in progress for many years with the mapping programmes of the Geological Survey, the investigations of the mining houses and the researches of various universities. As a result a great deal of basic information, more especially concerning the field relations and distributions of the rocks, is available, but unfortunately much of it is unpublished. The information does, however, constitute a suitable base from which to launch what might be called the second phase of the study which would be concerned in more detail with specific topics and problems. This paper presents some of the information gathered during a larger investigation into the Basement Complex, much of which is still to be published.
Back to title list




Upper Mantle Project - South African National Committee Symposium, July 1969. Special Publication No.2, 221-244

Studies in the Barberton area have revealed the close genetic connection between magma derived from the upper mantle and virtually all the ore deposits of the region. There is little doubt that many of the important minor, and some major, elements which are eventually concentrated on the crust of the earth by geological and biological processes, giving rise to various ore deposits, have been derived directly from the upper mantle through the medium of ultramafic, mafic and felsic magmatic rocks. It is, therefore, of importance to consider the chemical coherence between various elements as an indicator as to which of the above main rock classes will have concentrations of particular elements. Making use of the Barberton region as a model of a typical early precambrian granite-greenstone terrain it is possible to predict on theoretical grounds the location of ore deposits in other greenstone belts and conversely to make use of known mineralization in certain greenstone belts to predict the location of possible similar mineralization in apparently unmineralized belts. The Barberton region is particularly well suited to the study of the origin and genesis of ore deposits in view of the particularly clear relationships that often exist between mineralization, stratigraphy and structure. It is considered imperative that to gain an insight into the ultimate origin of any mineral deposit, it is often advantageous to focus attention on areas which, though poorly mineralized, have not been subjected to complicating processes such as intense metamorphism and wall rock alteration which in many well-mineralized area have often obscured fundamental relationships between mineralization and host rocks.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 721-733

Easterly dipping volcanic rocks of Stormberg age (Karroo System) comprise the north-south trending Lebombo belt, 24-32 Km wide and extending a distance of about 720 Km southwards from the Limopopo along the Republic of South Africa-Mozambique border. In addition to the well-known feeder-dykes and their extrusive derivatives, an older dyke swarm and large bodies of plutonic layered and/or differentiated basic rocks penetrating both the Precambrian foundation and the younger lavas are recognised. Basic intrusive rocks recognised thus include:
(1) Older dolerite dykes of pre-Loskop age trending NE-SW and penetrating only Precambrian rocks.
(2) Older basic plutonic masses penetrating Precambrian rocks but overlain by Karroo sediments and lavas.
(3) Younger dolerite dykes trending N-S, and cutting across all formations including late rhyolites of Stormberg age.
The older, layered, intrusive, plutonic rocks are considered to represent an early stage in Stormberg vulcanicity and though cutting and locally structurally controlled by Precambrian rocks they are overlain by sediments mainly of the Cave Sandstone stage. Younger and often less differentiated basic intrusions are always associated with and generally concordant with the host basalts. The location of all basic intrusive bodies and volcanicity displays a dependence upon crustal structural control and is closely linked with the Lebombo monocline situated along the warped eastern margin of the Precambrian Transvaal craton. This zone of weakness probably forms an integral part of the southerly extension of the East African Rift System, an area affected by the Mozambique orogeny. That basic magna of olivine basalt composition was available at more than one centre of the Karroo petrographic province is evidenced by similar occurrences as in the Drakensberg area and therefore is of more than local consequence. Ancient cratonic margins are the domain where intrusions of basic magma can be expected and orogenic and mobile belts of intercratonic areas have repeatedly repeatedly acted as zones of crustal instability evidenced by volcanic activity representing mantle disturbance.
Back to title list




Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 326-335

Andalusite and staurolite hornfelses occurring in the north-eastern metamorphic aureole of the Bushveld Complex are described and the relationship between these two minerals is discussed on the basis of their meso-molecular normative composition. It is adduced that the composition of biotite in a metamorphosed Pelitic sediment plays an important role in determining whether staurolite (and/or almandine in certain cases) will develop or not. Biotite reaches a "saturation point" with respect to iron under conditions of both thermal as well as regional metamorphism. Any excess iron remaining after the formation of biotite would be available for the formation of staurolite and/or garnet in non-cordieritic rocks. It was thus found that staurolite develops in rocks which have a mg-value lower than 0.43.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 380-390

Two characteristics of chromite make it especially informative as a petrogenetic indicator. Because it involves 5 chemical components instead of 3, chromite is more responsive than most associated silicates to variations in the parent magma. Once formed, especially in massive segregations, chromite is much more stable physically than any related silicates. Segregated chromite should reflect variations of parent magma more accurately than accessory chromite grains because it is less likely to be equilibrated with post-cumulus silicates or to be hydrothermally altered. Settled or cumulus textures characterize chromitites in stratiform complexes, and distinctive relict settled textures, more or less deformed by magmatic flowage, are preserved in most podiform chromitite in alpine peridotite. For these and other reasons, chromitite segregations are interpreted as prima facie evidence that their primary host-rocks differentiated by crystal settling from fluid mafic magma. Compositionally, chromitites may be said to predict Mg-Fe differentiation trends of later silicates. In Stratiform chromitites, total Fe content increases substantially or rapidly as Cr2O3 decreases below about 55%, but in podiform chromitite's, Al2O3 increases reciprocally and total Fe stays about the same. Chromites can be diagnostic in identifying the original genetic type of highly metamorphosed ultramafites like those near Red Lodge, Montana, U.S.A. The podiform chromitites are believed to show that crystal settling on a large scale is one of the important processes in the upper 25-30 Km of the mantle. All economically important stratiform chromitites are Precambrian in age and occur in stable or shield areas. Podiform Podiform deposits, with one possible exception, occur exclusively along island arcs and mobile mountain belts of Paleozoic along island arcs and mobile mountain belts of Paleozoic or younger age. The disparity in ages between the stratiform and major podiform chromitites implies differentiation in the upper mantle during Precambrian time or changes in the mechanisms which move mantle rocks up into the crust.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 80-107

Three cross-cuts in the Union Platinum Mine as well as a number of boreholes on Swartklip 988, in the Rustenburg district, have been logged and petrographically investigated. The total thickness of rocks investigated is more or less 920 meters, Ten representative rock samples and one chromite concentrate were chemically analysed. On account of petrographic data and rock relationships, the conclusion has been made that the rhythmic layering could be best explained if one accepts that different heaves of homogeneous magma were emplaced at intervals after which fractional crystallization and gravitative differentiation took place. The rocks investigated could be subdivided into four sub-units. The conclusion has been made that the pegmatitic reef is a coarse crystallization product of the magma from which the sub-unit crystallized and of which it forms the base. The chemical analyses of the chromite concentrate show that no platinum minerals occur within the chromite. The high Fe2O3 content of the chromite is attributed to solid solution between magnetite and chromite. The textural relationships of the chromite with respect to the silicates show that the chromite in the chromitite bands is not an early crystallization product, but crystallized after the orthopyroxene in the porphyritic melanorite and even later than the plagioclase in the pseudoreefs in which the plagioclase is interstitial to the orthopyroxene, in other words, these chromitite bands are obviously not early crystal accumulations. The possibility is suggested that the chromite in these rocks unmixed as a chromiferous liquid at an early stage, after the crystallization of some silicates, and which settled as a result of gravity.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 661-688

Three varieties of layered ultramafic intrusives have been recognized in the lower 'part of the Onverwacht Volcanic Group which constitutes the base of the Barberton greenstone belt. The geology and geochemistry of one of these varieties, viz., the Kaapmuiden type, in the north-eastern part of the belt is discussed in detail. Three discrete bodies have been emplaced as near contemporaneous sills into a sequence of "primitive" mafic volcanics with associated sediments and ultramafics. All have undergone pronounced magmatic differentiation, yielding zones of dunite-peridotite, orthopyroxenite, websterite and anorthositic gabbro and norite. The proportion of these various rock types indicate derivation from an ultramafic magma. Geochemical data, in addition to confirming the differentiation trends adduced from petrological evidence, supports the contention that the parental magma which gave rise to the Kaapmuiden bodies was of peridotitic composition. The composition of this magma corresponds closely to a newly described class of "primitive" peridotitic magma, including extrusive varieties, recently described by the authors, from the southern part of the Barberton area. This magma, termed peridotitic komatiite, is thought to be widespread in the basal portions of early Precambrian greenstone belts, particularly those of Southern Africa.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 67-73

From chemical analyses and optical investigations it has become apparent that orthopyroxene (Mg67Fe33) which has formed from pigeonite by inversion, appears as a new phase approximately 3300 feet above the Merensky Reef. Primary orthopyroxene continued to crystallise, together with pigeonite, over a thickness of rocks of about 1000 feet and disappeared when the Mg:Fe ratio in the Ca-poor pyroxene was 65:35. Pigeonite continues to be present as a primary phase for the next 3500 feet and then disappears abruptly below a thin band of pyroxenite, which consists mainly of orthopyroxene (Mg73Fe27). From here on, orthopyroxene crystallised from the magma and the second phase-change from orthopyroxene to pigeonite takes place about 1500 feet below the Main Magnetite band. The orthopyroxene, which has formed from pigeonite by inversion, often displays a preferred orientation in this respect that its crystallographic c-axis lies close to or in the plane of layering. Furthermore, groups of grains of this orthopyroxene have the same optical orientation over fairly large areas, and therefore constitute optical units. The grains in each unit contain pre-inversion exsolution-lamellae of augite which are orientated at random in the orthopyroxene, and post-inversion exsolution-lamellae which are orientated parallel to the (100) plane of the orthopyroxene throughout a unit. This is explained as being due to directed pressure from the superincumbent crystal mass during the inversion of the pigeonite, and to the sluggishness with which this inversion has taken place. A similar phenomenon has been encountered on the contact of metamorphosed xenoliths of Dullstroom lava, where units of the orthypyroxene cut across the contact.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 336-366

The association of argillaceous material and ultramafic igneous rocks was investigated at their regional contact, and in foreign inclusions in the Critical Zone of the Bushveld Complex. Quite a few exposures are known where mafic or ultramafic rocks come into contact with quartzite, but it is entirely a different story where shale is the argillaceous material. Two reasonably clear contacts were investigated in the Eastern Transvaal on the farms Glen Ora 339 KT and on Aapiesdoorndraai 298 KT. In both cases upper Magaliesberg shales and peridotite came into contact. At the Glen Ora 339 KT contact a breccia developed consisting of schistose fragments in a groundmass of granofels. The fragments are rich in biotite and in addition contain quartz, cordierite, plagioclase and a very little K-feldspar. The matrix between the fragments is coarser-grained, contains less biotite and cordierite and is rich in K-feldspar. The contact on Aapiesdoorndraai 298 KT is marked by the presence of hornfels. It is well-bedded and has a general dip of 10-15°W i.e. towards the peridotite. An aluminous-hornfels, containing cordierite and some times sillimanite and/or andalusite was mobilized, disrupting a pyroxene hornfels resulting in fragments of pyroxene hornfels embedded in a matrix of granofels. Two varieties of granofels were recognized, the one being rich in mica and friable, whereas the other is more massive and contains K-feldspar, cordierite and plagioclase. A vertical dykelet of quartz monzonite outcrops in the peridotite perpendicular to the contact between the sedimentary rocks and peridotite. The quartz monzonite is similar in composition to the granofels. The absence of zircon and its chemical composition more or less proves its origin from argillaceous material. Two occurrences of xenolithic corundum-sillimanite rocks were investigated on the farms Thorncliffe 374 KT and Maandagshoek 254 KT in the Critical Zone of the Bushveld Complex. The xenolith on Thorncliffe is near the middle group of chromitite seams and is confined on both sides by anorthosite. The following symmetrical layers were identified in this xenolith: lowing symmetrical layers were identified in this xenolith: (i) Anorthosite country rock containing small amounts of picotite. (ii) A spinel zone, containing two varieties of spinel-pleonast and a variety tending towards hercynite. Plagioclase, sapphirine and sillimanite are also present. (iii) A plagioclase-sillimanite zone. (iv) A corundum zone, consisting mainly of corundum in a groundmass of sillimanite. Small amounts of plagioclase are found surrounding the corundum. (v). The main sillimanite zone in the centre of the xenolith. This zone also contains scattered tabular crystals of corundum, rutile and sapphirine, tabular crystals of corundum, rutile and sapphirine. Mullite was also identified in the sillimanite. Quartz is seldom encountered, but when present is surrounded by a different host of minerals, i.e. K-feldspar, plagioclase, biotite, tourmaline and rutile. The occurrence on Maandagshoek 254 KT is a xenolith in a leucocratic norite near the upper chromitite seams. It is difficult to tell whether this occurrence has the same symmetrical layering as on Thorncliffe. The following zones were identified: (i) A spinel-plagioclase zone. (ii) A corundiferous, pinite-spinel zone. (iii) A banded spinel-cordierite-sillimanite zone. (iv) A poorly banded spinel-plagioclase zone. (v) A thin seam of clinozoisite. (vi) Adhering anorthosite. The thickness of the zones varies considerably and a certain degree of decomposition has taken place with the development of diaspore, pinite and clinozoisite. Mullite was found associated with the sillimanite and K-feldspar, sanidine, biotite and graphite were also encountered. The finely banded character of the xenolith from Maandagshoek, the abundance of cordierite and the presence of graphite provides conclusive evidence that this rock represents originally argillaceous sedimentary material. An analogous origin can be derived from the corundiferous rocks at Thorncliffe. In the metamorphism of these argillaceous rocks the first stage was the expulsion of felsic constituents, followed by desilication and impregnation of the xenoliths with calcium. The femic constituents of the spinel were derived from the ferro-magnesian minerals in the anorthosite. Thus desilication, de-alkalisation, migration of iron and magnesium and calcification can explain the fate of the aluminous sediments in the gabbroic magma.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 201-216

Despite their remarkable diversity in composition, texture and age, most Rhodesian pegmatites are post-Bulawayan in age, while some were still active as a result of events associated with the Zambezi and Mozambique metamorphic belts. The history of these pegmatites was, therefore, a prolonged and complex one involving successive multiple intrusions, with intense structural movement and a significant degree of late-stage albitization and hydrothermal activity. Most of these pegmatites show significant lithium mineralization the form of petalite, a quartz-spodumene intergrowth, eucryptite and lepidolite. As a result of field and petrographic studies, together with a limited programme of experimental hydrothermal work, a paragenetic sequence is established and it is concluded that most Rhodesian lithium pegmatites are products of open-system magmatic crystallization accompanied by intensive late-stage replacement. Despite its theoretical feasibility an entirely primary source of crystallization is found to be unconvincing. Nevertheless, this is not the case for most of the world's spodumene occurrences in which the latter being coarsely crystalline is undoubtedly of magmatic origin, with crystallization proceeding under essentially closed system conditions. Finally, some broad regional trends are suggested, based on various stages of this established paragenesis of lithium pegmatites and their associated late-stage albitization.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 217

Fourteen samples of the Umkondo dolerite of Rhodesia have been studied by the Rb-Sr total-rock method of isotopic age determination. The results do not define a fully satisfactory isochron and the possibility of crustal contamination is examined by reference to geochemical data. The imperfect isochron and also the difficulties inherent in the age-determination of basic rocks result in consid erable uncertainty in the calculated age. Excluding two points, the mean age is 1376±92 My; an alternative interpretation divides the samples into two groups both of which give ages close to 1100 My, but with different initial Sr87/Sr86 ratios. The latter result is in agreement with the published biotite age for the Premier Mine dolerite sill (correlated with the Umkondo on palaeomagnetic grounds), and with the majority of published K-Ar data on sup posedly related dolerites. The 1100 My interpretation is preferred at this stage, but certain inconsistencies will be checked by additional field and laboratory work. Both of the suggested results are compatible with the inference that the Mashonaland dolerites are older than the Umkondo dolerites. The stratigraphic implications are discussed.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 361-385

The Johannesburg-Pretoria granite inlier, occupying an ovoid area of approximately 700 sq Km constitutes one of a number of domical "windows" of ancient granite basement (~3 200 m.y. old) exposed on the Kaapvaal craton in South Africa. Preliminary investigations involving the re-mapping of the area have led to the establishment of a number of granite varieties, each displaying distinctive field characteristics and possessing significantly variable geochemical, mineralogical and textural properties. The oldest granites are represented by a variety of hornblende or biotite tonalitic gneisses. These grade into a complex array of migmatites and gneisses, the latter, in turn, passing transitionally into a suite of grey, homogeneous and porphyritic granodiorites. The youngest granitic rocks are represented by transgressive felsitic and porphyritic dykes. A geochemical study, involving the collection of over 500 granite samples, was undertaken over the dome. Thirty-three of these samples were chosen for chemical analysis and the results are presented and discussed. After additional selective screening, between 400 and 425 of the samples were partially analysed to determine their Na2O, K2O, Sr, and Rb contents. In addition, gamma ray spectrometry was employed to determine the gamma radiation emitted by each sample. Attempts have been made to establish the areal variability of these geochemical data using up to fifth order polynomial trend surfaces. The distribution patterns presented display a high degree of correspondence, not only between themselves, but also with the geology of the granites and the available gravity intepretation of the area.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 235-244

Two major stratigraphical units can be distinguished in the centre of the Limpopo orogenic belt near Messina: an old granitic basement and a suite of cover-rocks known as the Messina Formation. Both have been folded and metamorphosed. Evidence is presented that the granitic basement of tonalitic composition suffered partial anatexis in anticlinal areas resulting in rocks which tend towards the composition of hornblende syenite. Comparison of the petrography of these rocks with the data of experimental anatexis suggests that melting took place at temperatures above 625°C and at a pressure close to 10 kilobars. These can be taken to be the conditions which operated at the peak of the regional metamorphism and contemporaneous deformation. By analogy, it is pointed out that the anorthositic gneiss, calcsilicate rock and hornblendite in the area can be regarded as "restites", i.e. former calcareous shale and marl which have lost K-Na feldspar and quartz during high-grade metamorphism and deformation.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 245-251

Early Precambrian gneisses, migmatites and granitoid rocks underlying an area of some 40 000 sq Km in eastern Botswana are outlined. The majority of these rocks fall within the Shashe-Limpopo metamorphic (mobile) belt and are here assigned to the Shashe-Limpopo Gneiss Complex. The latter is broadly subdivided into two units on field and photogeological criteria, but the subdivision is not thought to be of stratigraphic significance. Assemblage IIA comprises a heterogeneous sequence of varied, predominantly quartzo-felspathic gneisses, mafic and metasedimentary rocks, which have been subjected to high-grade metamorphism, regional migmatization and intense deformation. For the most part, the sequence is regarded as being of supracrustal origin. It is best developed in the southern sector, and in the extreme southwest is has undergone remobilization on a regional scale. Assemblage IIB is more homogeneous, particularly in the south, where it consists predominantly of migmatitic and granitoid gneisses which are structurally conformable with the Assemlage IIA sequence. In the northern sector similar rocks predominate, but other lighologies are also developed which suggest an affinity with Assemblage IIA. Porphyroblastic granitic gneisses occur widely, particularly in the Assemblage IIB sequence in the northern sector. In the northern sector a number of intrusive phases of granodioritic, adamellitic and granitic composition have been distinguished. Their internal fabrics indicate emplacement under synkinematic to post-kinematic conditions. The relationships of certain of the intrusive phases suggest a petrogenetic link with the evolution of the Shashe-Limpopo Gneiss Complex. Similarities with the geology of the Northern Transvaal and Rhodesia are noted and parallels are drawn with a proposed classification of the Rhodesian granites.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 197-200

An occurrence of granite near Shabani is described and shown to be post-Bulawayan in age. Petrographic and chemical analyses of the granite, together with structural evidence, indicate a magmatic origin, emplaced along a low pressure fracture striking N30-40°E. This direction is sub-parallel to the regional compressive force operating during the evolution of the adjacent Bulawayan geosyncline. The potash-rich granite is thought to be a palingenetic derivative of the down-warped gneiss basement. Hydrothermal solutions associated with the granite are shown to be the probable source of chrysotile mineralization formed under suitable structural controls in the adjoining ultramafic rocks.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 111-120

Field data collected during mineral exploration and subsequent study show that the Precambrian rocks known as the Madziwa batholith are composed of three granite batholiths, named Mgadzi, Madziwa and Chizinga. These, and the Mtoko batholiths to the east, were emplaced as late or post-orogenic granites. Their intrusion was controlled by the pre-existing foliation and fracture pattern. Regional metamorphism raised the schists to the greenschist facies, while a contact aureole of pyroxene-hornfels facies developed where the granites intruded the schists. Metamorphic continuity between the gneisses and schists is absent. The contact between the gneisses and granites is gradational. Basic rocks were subsequently intruded along foliation and fracture trends, while dolerites cut all the rocks. Analyses of 12 samples show the chemical composit ion of the gneisses to be comparable with pelitic sediments. The hybrid and granite rocks show a progressive decrease in calcium and iron and an increase in alkalinity, with the replacement of sodium by potassium in the granite. The schists and gneisses appear to constitute mantled gneiss domes overlying the intrusive granite batholiths.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 167-187

The Selebi-Pikwe nickel-copper deposits of Eastern Botswana are located in complexly folded and metamorphosed Archaean paragneisses of the central zone of the Limpopo Orogenic Belt. Occurring in, and folded with, the thick succession of paragneisses are mafic and ultramafic intrusive rocks. Sulphides of copper and nickel have been detec-ted in a number of the mafic and ultramafic bodies but only Selebi, and the smaller Selebi North deposits carry sufficient amounts to be of economic interest. The sulphides occur in or adjacent to silllike bodies of amphibolite as irregular masses and disseminated discrete grains. The ore bodies, whose presence was initially indicated by geochemical soil sampling, are clearly out lined by both geochemical and geophysical prospecting techniques. Exploration by diamond drilling and underground development has been completed at Selebi and Pikwe. The deposits are all primary sul phide ores overlain by approximately 35 m of ferruginous gossan in which there is no secondary mineral enrichment. The dominant sulphide is pyrrhotite with chalcopyrite and pentlandite as the main sources of copper and nickel. In addition there are small amounts of bravoite, marcasite and pyrite. Pentlandite occurs mainly as irregular grains and areas interstitially between pyrrhotite grains, but also as large discrete grains and as thin exsolution lamellae in pyrrhotite. At Pikwe there is about two and a half times as much sulphide as at Selebi with a higher pyrrhotite/chalcopyrite and pyrrhotite/pentlandite ratio. The genesis of the ore and origin of the host amphibolite is uncertain. The amphibolite is generally considered to be an ancient intru sive sive sill. Some factors indicate the late introduction of the sulphides whilst others suggest an age contemporaneous with the amphibolite, and subsequent mobilization to occupy their present position. There is strong evidence for structural control in the location of the sulphides.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 127-130

Granites, gneisses and metasediments in the southern Urungwe District are characterized by large microcline porphyroblasts. Rocks of a central belt contain numerous large microcline porphyroblasts but with increasing distance from this core, the microcline crystals decrease in size and abundance. At the margins, in place of microcline, microcline perthite and orthoclase are developed. The distribution of these potash feldspar indicators shows that there was a thermal metamorphism with associated potash metasomatism. An earlier regional metamorphism is also recognized.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 131-147

The granitic rocks are subdivided into (1) the Ancient Gneiss Complex which includes the widespread tonalitic gneisses and migmatites, (2) the Tonalite Gneiss Domes, (3) the Granodiorite Suite, (4) the Nelspruit Migmatites and Gneisses, (5) the Homogeneous Hood Granite and (6) the Granite Plutons. The petrological terminology applied to the acid plutonic rocks is based on the system of Harpum. It is concluded that the tonalitic gneisses were derived by in situ granitization of volcanic and sedimentary rocks which had suffered metamorphism of Abukuma type to the cordierite amphibolite facies. Doubt exists as to the age relationship between these metamorphites and the Swaziland Sequence but it is provisionally suggested that the metamorphites pre-date the Swaziland Sequence. Following the deposition, and accompanying the deformation of the Swaziland Sequence the Ancient Gneiss Complex basement was reactivated to give the tonalitic diapir domes which are confined to the western and southern flanks of the Swaziland Sequence outcrop. The Granodiorite Suite consists of predominantly leucotonalitic and trondhjemitic rocks with more basic differentiates. Its relation to the Swaziland Sequence is not known, but it is suggested that the Suite was emplaced at some time during the deformation of the Swaziland Sequence. The Homogeneous Hood Granite, which represent a major period of potassium accretion to the crust at about 3 000 My, intrudes both the Swaziland and Pongola Sequences. The granite grades from the underlying Ancient Gneiss Complex through a zone of migmatites to form sheet-like bodies. The final granitic event was the emplacement of transgressive plutons. An early series of plutons of granodioritic composition is recognized in the Transvaal. In Swaziland two ages can be demonstrated but the plutons of both ages are composed of potash-rich granites. The tectonic setting of the granitic rocks are discussed. A distinction is drawn between the tectonic styles of granitic rocks emplaced in orogenic zones and the apparently non-orogenic Homogeneous Hood Granites and Granite Plutons.
Back to title list




Some sedimentary basins and associated ore deposits of South Africa. Special Publication No.6, 73-82

Fluorite mineralisation occurs in most of the carbonate horizons of the Transvaal Supergroup, but only the economically important deposits within the Malmani Dolomite, south of Zeerust, are well known. A study of the lanthanide geochemistry of all South African fluorite deposits reveals particular characteristic patterns which relate to the genesis of the different types of fluorite. For the fluorites in the Zeerust dolomites, two distinct trends are discernible. These are: (1) a relative light lanthanide enriched trend with a slight negative Eu anomaly and an average enrichment of 2,8 relative to chondrites, and (2) a trend in which light and heavy lanthanides are balanced, the Eu anomalies are developed erratically, together with other erratic values, and there is no enrichment relative to chondrites. The granular blockspar type of ore represented by the first trend is interpreted as primary syn-sedimentary endogenic mineralisation as a special stage in the dolomite evaporite facies, while the algal, banded, breccia and pipe-form ores represented by the second trend are interpreted as deposits in which solution and karst features formed the ore traps for remobilised diagenetic mineralisation.
Back to title list




A symposium on mineral deposits and the transportation and deposition of metals. Special Publication No.5, 117-132

The Selebi-Pikwe nickel-copper deposits consist of sill-like bodies of amphibolite and sulphide enclosed in deformed Archaean gneiss of the Limpopo fold belt. Cu/Cu+Ni ratios vary from 0,67 at Selebi to 0,45 at Pikwe, and range from 0,50 to 0,39 within the Pikwe orebody. The ore mineralogy is essentially similar in both deposits but there are differences in the relative abundances of the sulphide phases. Pyrrhotite + pyrite : pentlandite ratios and the relative proportions of pentlandite exsolution lamellae are discussed because of their metallurical significance and the liberation characteristics of the ore minerals are examined. Data on the appreciable development of colloform marcasite in the zone of supergene alteration at Pikwe has been related to ore treatment problems in the early production period at the mine. The sulphides are consid ered to share an intrusive magmatic origin with the host amphibolite which is chemically of troctolitic composition. The various sulphide occurrences in the area are believed to be genetically related and may have been derived from the differentiation of sulphide which separated from mafic/ultramafic parental material.
Back to title list




Some sedimentary basins and associated ore deposits of South Africa. Special Publication No.6, 91-101

The regional sediment transport directions, major provenance areas and the controlling palaeotectonic and palaeogeographic frameworks of sedimentation have been reconstructed for the Great Karoo Basin during the Permian. Analyses of this magnitude can be useful in regional exploration programmes for coal, uranium and fossil heavy mineral beach sand deposits. The strong palaeogeographic control on coal deposition is demonstrated by the fact that some of the most important deposits accumulated in topographically low lying areas on the pre-Karoo surface. Such areas formed sheltered environments ideal for the growth and accumulation of organic material. Elsewhere relatively slow rates of subsidence of a broad, protected, low lying delta plain controlled the deposition of coal. North of the main Karoo Basin many of the coal deposits are confined to structurally controlled linear basins. Hundreds of sedimentary uranium occurrences of varying grade and size occur within a broad, discontinuous belt in the Lower Beaufort of the southwestern portion of the Karoo Basin. The uranium mineralization occurs in a variety of fluvial deposits usually rich in carbonaceous material. Minute tuffaceous fragments, reflecting contemporaneous vulcanism, form a minor but significant constituent in some of the uraniferous sandstones. The uranium occurrences are confined largely to the Southern and Western Facies of the Lower Beaufort, and occur mainly within the confines of the Karoo Trough. The original source of the uranium is not known, but it is suggested that the predominantly granitic provenance area lying to the south and west of the basin provided an adequate source. The uranium also could have been derived from the leaching of the fine-grained tuffaceous material found in these sediments. Consolidated heavy mineral beach deposits have been found in the predominantly fluvio-deltaic Middle Ecca Group of the Northern Facies at a number of widely separated locations. These deposits were formed by shore line processes, such as the reworking of delta-front sands, during periods of temporary marine regression.
Back to title list




Some sedimentary basins and associated ore deposits of South Africa. Special Publication No.6, 219-228

Since the discovery of diamonds in the Lichtenburg area in 1921, many theories have been advanced as to their origin. While much important attention has been aid to the diamonds, it is felt that not sufficient attention has been paid to the gravels themselves particularly on a regional scale. A palae ocurrent study indicates at least three source areas for the gravels, namely southeastern Botswana, the Swartruggens region and probably southern to central Transvaal. A detailed study of the gravels, which is made more complex by the incorporation of Dwyka Diamictite in some of the gravels, supports the palaeocur rent findings. The conclusions drawn are that the basement structure controlled the locality of gravel deposits and thakt there has been warping since the deposition of the gravels causing a shift in the watershed and the capture of the Molopo river's catchment area by the Crocodile drainage basin.
Back to title list




A symposium on mineral deposits and the transportation and deposition of metals. Special Publication No.5, 133-158

The geology and geochemistry of an important antimony ore body in the Murchison greenstone belt of South Africa is described. The body, termed the United Jack Complex, represents a mineralized "carbonate centre" on the so-called "Antimony Line". The United Jack Complex, together with three other major antimony ore bodies, all situated within a distance of 12 km along the "Antimony Line", constitute the world's largest antimony deposit. Consolidated Murchison Mine is currently responsible for about 40% of the free world antimony production. A range of chloritic and quartzitic schists constitute the country rocks of the "Antimony Line" and display some unusual chemical characteristics such as high Mg, Fe, Ni and Cr contents and a very low Ca content. An assemblage of talc- chlorite schists and a variety of grey and green siliceous carbonates constitute the United Jack Complex. The latter are massive or schistose, and consist of quartz-magnesite-dolomite rocks which contain generally lesser amounts of fuchsite and chlorite. They form the "core" or central assemblage of the "Antimony Line" as developed in the United Jack area. Several magnesium-rich carbonates and siliceous cherty rocks within the above carbonates contain stibnite mineralization and, where rich enough, constitute the exploited reefs. Two narrow bands of fine-grained and bedded, quartz-albite-magnesite chert, containing thin layers of stibnite or arsenopyrite, occur in close proximity to the main antimony-bearing carbonates. It is suggested that they could be the equivalents of the gold-bearing sulphide-carbonate iron-formations previously mined for gold elsewhere along the "Antimony Line". Geochemically the carbonate rocks consists of a mixture of elements characteristic of ultrabasic rocks, viz. Mg, Ni, Cr, and Co, and the volatile elements Sb and As, Ba, W, B and Hg. To a limited extent, K and Rb show significant increases in and close to the reef zone. The results of a microprobe study on some of the main rock types are presented and the distribution of the elements noted above, is discussed. Evidence for and against syngenetic and epigenetic models for the origin of the carbonate rocks and of the antimony mineralization is presented. In general, the question of Archaean carbonates is discussed and a distinctive massive, magnesium-rich carbonate variety, considered to be different to the carbonate facies iron-formations, is recognized. Finally, ideas regarding the genesis of the "Antimony Line" assemblage are presented. The widespread chlorite schist country rocks have recently been shown to represent altered Mg-rich pelites (Pearton, 1979a). An original peridotitic horizon has also undergone alteration and is now represented by the talc schist zone. The latter, due to its incompetent nature, was the site of major preferential carbonate replacement which took place in two phases. An early, high temperature CO, and Ca event was followed by a second, lower temperature episode of carbonation which resulted in the introduction of Sb and As As together with B, W, Hg, and Au. Late phase Sb and Si permeated tectonically brecciated areas of the massive Mg-rich carbonates giving rise to significant mineralization. Local K-metasomatism resulted in the introduction of K, Rb and B and to the development of the green fuchsitic carbonates.
Back to title list




Some sedimentary basins and associated ore deposits of South Africa. Special Publication No.6, 183-196

In reviewing the principles of stratigraphic classification, a distinction is made between observational, analytical and inferential or interpretive categories of classifications. Two observational classes, the lithostratigraphic and biostratigraphic, and one ana lytical class, the chronostratigraphic, have formally codified terminologies. However, there are many informal but useful categories of classification that have become important as a means of cor relation and eventual synthesis. New stratigraphic "tools" require new schemes of classification, as they present data in a different way. Earlier examples are units based on geochemical, petrological and sedimentological studies; later examples are those based on wire-line logs, magnetic reversals of the sea floor and high-resolution reflection seismic profiles. From the last-named example, an aspect of stratigraphy relegated previously to the interpretive function provides the stratigrapher with new observa tional units - a succession of unconformity-bound stratigraphic sequences. These litho-tectonic sequences are referred to as the tectonostratigraphic classification (a new term), which provides the framework for the study of the tectonic control of deposition, and for the interpretation and classification of sedimentary basins. Basins developed during the Mesozoic and Cenozoic Eras are now generally analysed in a plate-tectonic context, hence the importance of the tectonostratigraphic classification. A report to the South African Committee for Stratigraphy by the Jurassic-Cretaceous Working Group revealed a number of requirements that had to be met to complete a change-over from the status quo to modern formal stratigraphic classification, terminology and proce dure. The newly-established and the re-defined formal lithostra tigraphic units are classified and provided with stratotype measured section descriptions where these are needed. Emphasis is placed upon the basis of correlation of lithostratigrapic units with their stratotypes and on the extension of correlation beyond the limits of the defined units. The relationships between lithostratigraphic and chronostratigraphic units, facies, transgressive-regressive cycles and larger unconformity-bound sequences, here called tectonostrati graphic units, are illustrated in a schematic model of the South African coastal deposits. Painstaking and methodical stratigraphic analysis and interpretation is required to unravel the geological history of a basin or to tap its resources; for the Jurassic-Cretaceous interval the main interest lies in the realisation of its hydrocarbon potential.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 565-575

The Newer Gabbros of Aberdeenshire are interpreted as having originated as a single layered intrusion of Bushveld type, which was deformed and disrupted during the later part of the Caledonian orogeny. In particular, evidence from the Belhelvie and Insch masses, although incomplete, indicates that the origi nal intrusion displayed extreme cryptic variation (FO86-5, An81-40, En85-25 and Ca42Mg51Fe7 to Ca4OMg5 Fe55), as well as small-scale sedimentary structures (rhythmic layering, mineral- graded layering, etc.). The parent magma is inferred to have been of high-alumina, tholeiitic character, and the reconstructed frac tionation series shows a marked resemblance to the Bushveld layered sequence.
Back to title list




Geodynamics of the Cape Fold Belt. Special Publication No.12, 57-64

In this paper the currently available geophysical data in and around the Cape Fold Belt are reviewed. Although the data are incom plete and although some geophysical methods have not been applied in the area, the author has been able to develop lithospheric models which not only account for the geophysical anomalies, but can also explain some of the anomalous features of the fold belt. The main component of this model is a weak zone (containing dense magnetic material) that coincides with the Southern Cape Conductive Belt and which most probably contains some serpentinized basalt. This weak zone is wedged between the mechanically more competent Namaqua-Natal Belt to the north and a rigid crustal block intruded by Cape Granite to the south. The way in which these properties deteffnined the geodynamic behaviour of the lithosphere is explained.
Back to title list




Geodynamics of the Cape Fold Belt. Special Publication No.12, 65-74

The Little Karoo between the Langeberg and Klein Swartberg ranges is underlain mainly by formations of the Table Mountain and Bokkev eld groups. Systematic mapping reveals that the floor of the geosyn cline sank more quickly to the south, resulting in thicker accumu lation of particularly Bolaas sediments with concurrent facies change from sandstone to shale. Using the Cedarberg Formation as a marker in the Table Mountain Group it has been established that the Worcester fault does not end at Suurbraak but continues to Riversdale and is accompanied by a nearly parallel branch fault (Tradouw) of regional extent. Details of the Warmwaterberg, Touwsberg and Kango multiple faults have also been traced out. Geometric analysis of folds, bed ding, cleavage and lineation leads to the conclusion that the sedi mentary formations were well lithified before folding; maximum crus tal stress acted horizontally from south to north producing an essen tially homogeneous array of structural elements on various scales; gravity sliding was only locally important on the steepened north limb of the Langeberg mega-anticline; the thickness of the Table Mountain Group broadly determined the spacing of mega-folds. The horizontal shortening and intensity of deformation are less than anticipated.
Back to title list




Geodynamics of the Cape Fold Belt. Special Publication No.12, 33-45, 13 FIGS

The Kango inlier of pre-Cape formations has been remapped to establish its tectono-stratigraphic features in greater detail within the framework of geodynamic history of the Cape Fold Belt. The lowest known unit of the Kango Group, the Matjies River Formation, was deposited in an environment varying between unstable shelf (Nooitgedacht Member) and isolated platform (Kombuis Member). No indications of a stratigraphic base were found. The main source area of the fine detritus lay to the west. The Groenefontein Formation accumulated in a subsiding trough deep enough for transport by turbidity currents (Nelsrivier Member), but elevated temporarily when shallow water sandstone of the Brakkerivier Member was laid down. Renewed sinking to bathyal depth resulted in the deposition of turbidites of the Huis Rivier Formation; transport was in a westerly direction along the trough axis, with the main supply from the steep flanks north and south. Major uplift with associated mild folding exposed the newly formed sediments especially to the south, subaerial erosion planing down the upland ranges and moving coarse detritus to the coast. Fanglomerates of the Vaartwell Formation accumulated as polymictic (Rietkloof) and monomictic (Andriesberg) deposits near sea-level. Westward transgression resulted in upward fining of the Vaartwell to form the infralittoral Uitvlugt Formation and circalittoral Gezwinds Kraal Formation. Fluvio-deltaic sediments of the Schoongezigt Formation were laid down in the east. The basin was then subjected to orogeny and uplift before fluvial sediments of the Schoemanspoort Formation washed into isolated depressions at the foot of the elevated highlands. The regional setting is visualized as an embayment connected in the south-east to the open sea. The source areas were probably non-volcanic. The depositional and deformational history of the Kango Group fits a marginal basin association related to the miogeosynclinal zone.
Back to title list




Contributions to the geology of the Barberton Mountain Land. National Geodynamics Programme, Barberton Project. Special Publication No.9, 169-183

The Boesmanskop Syeno-granite Complex consists of four intrusive bodies in the Archaean granite-greenstone terrane of the south-western part of the Barberton Mountain Land. The complex, phases of which have yielded an U-Pb mineral age of about 3 130 Ma and a Rb-Sr whole rock age of about 2 850 Ma, is emplaced along a tectonic lineament that has been subjected to faulting, shearing, dyke empacement and thermal spring activity from Archaean times to the present. Examination of the Boesmanskop suite yields rocks ranging in composition from syenite to granite (sensu stricto), indicating that the parental magma evolved along an oversaturated trend extending from the critical undersaturation line (the alkali feldspar join) in petrogeny's residua system, away from the thermal maximum towards granite compositions. This type of trend, although not uncommon, does not appear to have been previously documented in Archaean syenites. Trace element modelling indicates that compositional variations within the Boesmanskop Syenogranite Complex am due to both crystal and liquid fractionation processes. The main magma chamber underwent fractional crystallization to form a suite of "cumulus-enriched" syenites, quartz-syenites and granites. The finer-grained Weergevonden Syeno-granite assemblage formed in an isolated magma chamber in which cooling was more rapid, thereby preserving "liquid" composition. The source rocks from which the Boesmanskop magma was derived may have been either mafic or felsic although the present study favours an intermediate parent. It is envisaged that melting had a deep-seated origin, occurring at high pressures where the composition of the granite eutectic is relatively SiO2-depleted.
Back to title list




Geodynamics of the Cape Fold Belt. Special Publication No.12, 165-175

Various aspects of the CFB studied during this IGP project are listed. Restraints are dealt with before several strain models are discussed and the most applicable ones elected. The bulk deformation principle is applied using the available strain data for six structural zones across the fold belt. This is done in steps, with homogeneous strain followed by simple shear. A tectonic history is outlined with the aid of four dated consecutive paroxyms of the orogeny and relying on correlation of these with micro- and meso-structures, metamorphic analysis and geophysical crustal structure. Sequential, coaxial deformation in segments is demonstrated. The sequence is determined by inherent weaknesses in the cover and basement. Finally, some other very recent findings from areas outside the geotraverse are compared with the laner and off-shore information from the Agulhas Bank is used to augment the tectonic model and predict its full extent on the African continent. The regularity of four coaxial and co-zonal sets of structures is explained by the absence of static and localised heat sources. This may be a common association for all fold belts with similar regularities.
Back to title list




Contributions to the geology of the Barberton Mountain Land. National Geodynamics Programme, Barberton Project. Special Publication No.9, 1-20

The regional geological setting of komatiitic rocks within the Komati Formation of the Onverwacht Group is outlined. Evidence initially put forward for the extrusive origin of these rocks is pre-examined in view of a recent suggestion that komatiitic rocks in some areas of the Komati Formation were originally emplaced as vertical dykes in a primitive oceanic spreading centre. The detailed anatomy of komatiite and komatiitic basalt flows from the Komati Formation type section is discussed. Komatiite flows are texturally asymmetrical with a basal olivine cumulate zone and an upper zone of bladed (plate) and random spinifex. Chilled and palagonitic flow contacts are frequently observed and the anatomy and internal textures are compared to textural features of similar flows from Munro Township, Ontario. Geochemical variations across sections of three typical flows are examined in detail. Interpretation of the variations found for elements which we believe to have been immobile during the metamorphic reconstitution of the rocks are consistent with olivine accumulation in the lower sections of the units and, thus, are equally consistent with an effusive origin for these units. A three-fold classification of komatiitic basalts, based on field criteria and textures, is proposed. Pillowed, sphenilitic and fine-grained massive komatiitic basalts from the the first type and are equated with the Barberton type of lava originally proposed by Viljoen and Viljoen (1969a). A massive flow of this type has been studied in detail and in section shows a wide range of chemical compositions (11-19% MgO). These variations am consistent with the igneous differentiation of the flow whereby clinopyroxene and olivine accumulated in the lower portions. Pillowed and massive komatiitic basalts form the second type which are usually poorly exposed and often sheared, resulting in low "spikey" outcrops. The pillows, where formed, are relatively small (usually < 50 cm in length), and well-developed pillow horizons are confined to more massive outcrops. This type of komatiitic basalt is equated with the Geluk type of Viljoen and Viljoen (1969a). Finally, a new class of spinifex-textured and coarse-grained massive lava is recognized as the third type of komatiitic basalt. This type is characterized by the development of randomly orientated amphibole needles and often contains narrow layers of well-aligned sub-vertical (to the contacts) amphibole blades. Textural and geochemical evidence indicates that the amphibole needles are pseudomorphs of clinopyroxene and may have been zones through which significant amounts of fluid passed. The detailed field and geochemical studies outlined in the paper substantiate and reinforce the existing interpretation of the Komati Formation as being a largely extrusive pile of komatiitic lavas.
Back to title list




Contributions to the geology of the Barberton Mountain Land. National Geodynamics Programme, Barberton Project. Special Publication No.9, 221-223, map

Since the publication of the 1:50 000 geological map of the Barberton granite-greenstone terrane by the Geological Survey of South Africa in 1956 the region has been critically pre-examined as part of two CSIR sponsored, internationally linked, scientific programmes. Between August 1965 and its completion in July 1969, the Upper Mantle Project concentrated specifically on aspects of the geology of the volcano-sedimentary sequences found in the Barberton greenstone belt and considerable emphasis was placed on investigations of the ultramafic, mafic and felsic rocks found throughout the region. This work, coupled with the ongoing mapping undertaken in the area by the Economic Geology Research Unit of the University of the Witwatersrand, led to revised accounts of the geology and evolutionary development of the Barberton greenstone belt being presented. Investigations concentrating mainly on the granitic terrane flanking the Barberton greenstone belt were undertaken as part of South Africa's contribution to the International Geodynamics Programme during the period April 1975 and July 1981. This project involved regional as well as detailed field mapping supplemented by geochemical, geochronological and structural studies, all of which were undertaken in an attempt to decipher the evolutionary history of the Archaean crust in the area. Because of the many and varied changes that emerged from the remapping programmes it was considered opportune to compile a map of the Barberton greenstone belt and surrounding granitic terrane which kwould provide a summary account of the principal findings of the research and mapping undertaken in the region since 1965. As a result a 1:25 000 colour map of the Barberton Mountain Land in the Eastern Transvaal and Swaziland was fist made available for distribution at the Geocongress '81 symposium held in Pretoria in July 1981 when the results of the Geodynamics Programme were presented. The three most imoportant objectives of the map are: (1) to show the names and locations locations of the more important geological features in the region, including prominent synclines and anticlines, layered ultramafic complexes, granitic plutons and batholiths as well as the principal ore deposits (gold, chrysotile asbestos, iron, magnesite). (2) to illustrate the nature and distribution of the principal rock types and formations that constitute the Onverwacht, Fig Tree and Moodies groups in the Barberton greenstone belt and surrounding xenoliths, and (3) to provide a summary of the nature and distribution of the granitic rocks encountered in the Eastern Transvaal and Swaziland.
Back to title list




Contributions to the geology of the Barberton Mountain Land. National Geodynamics Programme, Barberton Project. Special Publication No.9, 21-29

Large-scale upright folds and structural dislocations formed within the rocks along the southern margin of the Barberton greenstone belt have created a complex tectono-statigraphy which invalidates the existing proposed stratigraphy within this part of the belt. Horizontal shortening and vertical extension overlapped in time with the emplacement of the surrounding granitoids, and were preceded by thrusting and napppe tectonics within the greenstone belt. Remnants of granitoid gneisses are present within the belt, with a tectono-metamorphic history predating this early deformation. This confirms the presence of pre-greenstone belt sialic crust. These remnants were probably part of the so-called Ancient Gneiss Complex. A close field relationship between these gneisses and structurally overlying diamictite units which contain large clasts of similar gneiss, suggest an unconformable relationship between rocks which comprise part of the Barberton Sequence (previously the Swaziland Supergroup) and those of the Ancient Gneiss Complex. Sedimentological and structural data suggest that much of the lowermost stratigraphy of the greenstone belt may be repeated "higher" in the present stratigraphic column, further into the greenstone belt.
Back to title list




Contributions to the geology of the Barberton Mountain Land. National Geodynamics Programme, Barberton Project. Special Publication No.9, 45-62

Sheet-like potassic granitoids constituting the Lochiel Granite (Mpuluzi and Pigg's Peak batholiths) were intruded at 3,0 Ga along the interface between greenstones of the 3,5 Ga-old Onverwacht Group (Barberton Sequence) and underlying sodic granitoids and gneisses up to 3,5 Ga old. Where a salient of the Barberton greenstone belt, known as the Motjane schist belt, projects into the Lochiel Granite in the Motjane Valley of north-west Swaziland, the best-exposed contact was studied by detailed mapping of 4 ha at scales of 1:50 and 1:500, and by geometric and strain analysis. Four generations of folding, of which only the younger two have been previously reported, have affectd the pillowed amphibolite-facies. Onverwacht metabasites; the second (F0\lb) and third (F0\2) fold generations are the most significant. The F0\lb isoclinal folds were originally recumbent and reclined; they indicate a period of horizontal translation, possibly in a thrust or nappe regime. A subsequent period of intense shortening along north-west-trending axes, during which rocks were subjected to more than 80% shortening by oblate strain, caused the north-west-verging keel-like form of the Motjane schist belt. The schist belt extended laterally parallel to its length and, to an even greater extent, in a vertical direction. Maximum extension in the study area during the D2 event was more than 300%; extension was downwards and towards the sagging mass of the main Barberton greenstone belt. Very close spatial, geometric, and tectono-chronometric similarities exist between the D2 structures in the Onverwacht Group, in the marginal Lochiel Granite and in all five generations of dykes and veins injected from the granite. Consequently there is little doubt that all phases of the Lochiel intrusion in the study area were emplaced during the entire duration of the D2 event and that this emplacement was responsible for D2 deformation. Emplacement is deduced to be entirely magmatic, early-to-late synkinematic with respect to D2, extremely forceful, and nondiapiric. The spreading direction of the sheet-like Lochiel granitic body was most probably south-eastward from an entry point into the upper crust north-west of the Motjane schist belt.
Back to title list




Contributions to the geology of the Barberton Mountain Land. National Geodynamics Programme, Barberton Project. Special Publication No.9, 153-167

A total of 13 late granite plutons have now been recognized in the Achaean granite-greenstone terrane of the Eastern Transvaal and Swaziland. These plutons, although characterized by diverse ages, origins and compositions, collectively represent the final magmatic event in the development of the Archaean granitic crust in the region. Five sub-types are defined principally on the basis of compositional criteria. These sub-types are:
1. the Dalmein-type plutons which are granodioritic in composition, range in age between approximately 3,2-2,9 Ga and probably formed by partial melting of pre-existing tonalitic or trondhjemitic gneisses;
2. the Boesmanskop-type plutons which are represented by a differentiated syeno-granite complex dated at between 3,1-2,8 Ga and are considered to have formed by melting of an intermediate (dioritic) parent under conditions of high confirming pressure;
3. the Cunning Moor-type plutons which are tonalitic in composition, have been dated at approximately 2,8 Ga and have an origin related either to fractionation from a Dalmein-type magma or partial melting of older tonalitic/trondhjemitic gneisses;
4. the Sicunusa-type plutons which are granitic in composition, have been dated at approximately 2,6 Ga and formed by differentiation of a Dalmein-type magma in which K-feldspar was a liquidus phase and fO2 was relatively low; and
5. the Mpageni-type plutons which are similar to the Sicunusa-type plutons in terms of bulk composition but are dated at approximately 2,5 Ga and formed by differentiation of Dalmein-type magma in which K-feldspar was not on the liquidus and fO2 was higher than in the former case. The data presently available provide only a limited insight into the role and significance of these plutons in Archaean crustal evolution and additional studies are required.
Back to title list




Contributions to the geology of the Barberton Mountain Land. National Geodynamics Programme, Barberton Project. Special Publication No.9, 81-101

Detailed mapping on the scale of single outcrops was carried out on migmatite exposures in the tonalite-trondhjemite gneiss terrane south-west of the Barberton greenstone belt and in the marginal potash-rich gneisses of the Nelspruit batholith to the north of the belt. Four processes, all arteritic in nature, are recognized as being mainly responsible for the formation of the migmatites encountered in the study area: (a) intrusion of tonalitic or trondhjemitic magma into pre-existing greenstone remnants of variable composition; (b) anatexis of tonalite or trondhjemite gneisses with the emplacement of the resulting melt into either the gneisses or the greenstone remnants; (c) syntectonic emplacement of mafic dykes into pre-existing gneisses, and (d) granitization and assimilation of pre-existing greenstone remnants. The observations indicate that certain migmatites formed through the introduction of felsic magma into a once extensive oceanic (ensimatic) crust, thereby reflecting processes of early continental development. Other migmatites suggest that mafic magmatism was active subsequent to the development of early tonalitic-trondhjemitic gneisses and that later greenstone volcanics may have been deposited in the presence of sialic crust. Pressures and temperatures involved in the migmatite zones appear, at least locally, to have exceeded the tonalite solidus so that anatexis occurred in preference to sub-solidus metamorphic differentiation.
Back to title list




Contributions to the geology of the Barberton Mountain Land. National Geodynamics Programme, Barberton Project. Special Publication No.9, 103-116

Tonalite and trondhjemite gneiss plutons are best exposed in the region south-west of the Barberton greenstone belt where thirteen discrete bodies have now been identified. Certain of these are clearly defined in terms of a regular elliptical shape and a prominent greenstone selvedge, whereas others may be irregularly shaped and incorporate large greenstone remnants. Most of the tonalite-trondhjemite bodies are characterized by subtle, yet significant, chemical differences. The compositional ranges within individual plutons indicate typical fractionation trends whilst bulk chemical variations between certain of the plutons are attributed to small differences in the degee of partial metting that occurred in their formation. Distinguishing between the various bodies is best achieved through their Sr signature, which may vary by a factor of five from one body to another. Rare earth element data, where available, also serves to characterize the various tonalite-trondhjemite types. When considering the data from the Barberton region together with that from the Ancient Gneiss Complex in Swaziland, two broad tonalite-trondhjemite gneiss sub-types emerge, the first, a high Sr (low Rb/Sr), low E REE sub-type and the second, a lower Sr (higher Rb/Sr), higher E REE sub-type. It is suggested that these differences may be attributable to a melt origin at different crustal levels, where isochemical mineral phase changes in the residual material results in different distributions of trace elements between melt and restite. Hence, the characteristics of the first sub-type are compatible with a melt that equilibrated with a dominantly homblende-bearing residue (i.e. partial melting at amphibolitic grades) whilst the second sub-type appears to have formed from a melt equilibrating with a residue in which a significant clinopyroxene component existed (i.e. partial melting in an environment tending towards granulite grades). This scheme obviates the need to invoke a polygenetic origin for the tonalites and trondhjemites from the Barberton area and the Ancient Gneiss Complex.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 1-6

Four stages are recognised in the evolution of the earth's granitic crust, spanning most of early and mid-Precambrian time. The great bulk of granitic rock was present in some form, although not necessarily its present form, early in the history of the earth's crust. Since 2,5 billion years ago there has been a gradual reduction in the regions of continental crust where heat flow has been high enough to lead to the production of new granites.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 7-11

In this general review of the granite problem three different modes of formation of granitic rocks are discussed: - (1) magmatic formation by intrusion of granitic magma, (2) anatectic formation by partial melting in situ, and (3) metasomatic formation by feldspathization. A study of macro- and microphotographs, chiefly from localities in Central Europe and Scandinavia, has demonstrated that each kind of formation imprints its own characteristic petrographic details. In the second part of the paper the origin of granites in the deep crust of the earth is discussed from a petrogenetic and geochemical point of view. As an example of the Ivrea zone in the Western Alps is presented in detail. Here, by means of recent geophysical investigation, it has been proved that a portion of the upper mantle, together with coherent units of the lower crust, was lifted to the present surface. A geological profile across the Ivrea zone shows a strongly deformed series of two alternating rock types of very different composition: - (a) a rock group of basic, mainly gabbroic composition (pyriclasites or amphibolites according to mineral facies) (b) a group of silica-rich rocks such as leucocratic gneisses, leptites, granulites and "stronalites", etc. The bulk composition of this layered series varies from aplitic to gabbroic depending on the thickness and frequency of the layers. There is no evidence of any differentiation in situ from the upper mantle to the lower crust.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 13-16

After many false starts, experiment and observation have now amalgamated to show that the production of melts of the granite class is a natural culmination of metamorphic processes in the crust. Throughout prograde metamorphism, most rocks lose water under conditions where PH2O = PTOTAL. This condition ceases at the high amphibolite facies grade where melting commences and then the condition PFLUID = PMELT = PTOTAL > PH2O is attained and as progressive melting occurs, PTOTAL departs more and more from PH2O. There is good evidence to show (Winkler, 1967; Fyfe, 1970; Brown, 1970: Brown and Fyfe, 1970) that by the time melting commences, most hydrated minerals in the crust will have dehydrated. The significant exceptions are the common hydrated phases of the amphibolite facies: muscovite, biotite and the amphiboles. To answer the question, "how does the crust melt?, we must answer the question, "how do muscovite-biotite-hornblende schists melt?" Or in some cases we may be concerned with the melting of old granitic or old granulitic basement where little water is present at all.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 17-23

From the increasing volume of geological and geochronological data available in recent years, it has been concluded that Africa represents a segment of primeval crust more than 3 000 m.y. old that has been affected by a number of major orogenies including: at least two major orogenic events pior to 2 500 m.y. ago, for which the best evidence is preserved in a number of nuclei designated nos. 1 to 8 in Figure 1; and a sequence of polyepisodic orogenies at 1 850±250 m.h. ago (Eburnian and Huabian orogenic episodes), 1 100±200 m.y. ago (Kibaran orogeny), 600±100 m.h. ago (Damaran-Katangan or Pan-African orogeny), and during the middle Paleozoic-early Mesozoic (Acadian and Hercynian orogenies) and the Tertiary (Alpine orogeny) (Figure 1; Clifford, 1970).
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 25-33

In the Moldanubian basement of the southwestern Black Forest a low-amplitude, asymmetric antiform is exposed. This structure is predominantly built up of three different types of granite of Hercynian age, emplaced in a shell-like fashion and sequence. The core of this antiform consists of Malsburg granite, which intruded the shell of older Blauen granite. Relicts of an outermost, still older granite layer are represented by the Klemmbach-Schlachtenhaus granite. A series of mineralogical and geochemical investigations revealed statistically significant and systematic changes in the modal and chemical composition of the three granites. The oldest granite is characterised by high concentrations of quartz and Na-rich plagioclase, and very low contents of biotite, apatite and zircon. The amount of quartz decreases systematically with decrease in age of the granites, whereas the K-feldspar content, the An-values of the plagioclase and the sum of the mafic constituents increase significantly. Thse changes in mineral composition are even more clearly expressed by changes in concentration of the major and trace elements Na, K, Ca, Ti, P, Zr, Ba, Sr, and Rb. The evolutionary trend, in time and space, of the three granites, indicates that they originated through partial, or complete, anatexis, under rising P-T conditions. This model is originated through partial, or complete, anatexis, under rising P-T conditions. This model is consistent with results obtained in laboratory studies of the melting behaviour of sedimentary rocks. The first melt produced has a leucogranitic composition. This stage is preserved in the outermost, i.e. oldest granite layer (Klemmbach-Schlachtenhaus granite). With increasing temperature the anatectic melt altered to a more granitic composition as represented by the next younger layer, i.e. that of the Blauen granite. Finally, a melt of granitic to granodioritic composition was produced. This stage is represented by a Malsburg granite, which in contrast with the older granites shows pronounced compositional variations on a regional scale. A subdivision of this pluton into zones of equal composition was derived statistically. The spatial sequence and distribution of these zones follow regular patterns which can be explained by the normal differentiation of a magma intruded from the southeast. This primary distribution of material was subsequently affected, locally, by a late or submagmatic phase of K-feldspar metasomatism. A possible mechanism for this reaction is discussed.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 35-44

Several granitic emplacements in the Pyrenees and Ireland are found to have widely differing characteristics, particularly with respect to their relationships with the country rock. The Pyrenean intrusion was a simple event during Hercynian times which was completed before the granite crystallized. The Irish emplacements of Caledonian age, however, were complicated and protracted with development ceasing only when normal crustal temperatures were achieved.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 45-51

Mesoscopic structures are compared in five widely separated Precambrian terrains; in Western Australia, where there are at least seven fold episodes, Uganda (at least six episodes), eastern peninsular India (at least six episodes), southwestern Greenland (at least six episodes), and the Outer Hebrides of Scotland (at least six episodes). All the areas are predominantly gneissose so that there are strong lithological similarities between them. Also the types of structures developed in them, as well as metamorphic grade and syntectonic mineralization, display certain similarities. However, these similarities are considered to be only superficial for it is possible to demonstrate that the relationship between the structures, their styles and orientation, have together resulted in five unique structural complexes characteristic of the deformational history of each terrain. Furthermore, it has been shown within at least one terrain (the Outer Hebrides) that these structural characteristics are constant over considerable distances so that, contrary to first impressions, while each terrain is consistent within itself, it is in fact structurally distinct from the others.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 53-58

The diverse kinds of ptygmas may be, and are here, categorized non-genetically on the basis of their configurations and their structural interrelationships with their host rocks. Although most previously reported features and the "new" data recorded in this paper are more or less consistent with their formation, involving modified responses to stresses, it appears unlikely that any one mode of origin accounts for all ptygmas. Consequently, close study of each occurrence is needed. Such study is often doubly rewarding because it leads to resolution of certain enigmas relating, not only to the genesis of the ptygmas, but also to the formation and mobilization of spatially associated gneisses, granites, and migmatites.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 59-67

The published work up to 1970 on the granitic rocks of the Precambrian Rhodesian Craton has been summarized. The craton is about 630 Km long east-northeast, 400-500 Km wide and about 30-40 Km thick. It became stabilized shortly before it was fractured along its length and intruded by the Great Dyke at around 2 550 m.y. Since then it has moved vertically and been deeply eroded, largely before the middle Precambrian, a thickness of some 5-15 Km of rock having been removed and the middle levels of the Archaean crust exposed. It is surrounded by younger mobile belts that at intervals have relieved crustal strains. The Basement Schists, originally probably over 10 Km thick, consist of metamorphosed basaltic, andesitic and felsic lavas with their hypabyssal equivalents, and sedimentary rocks, derived in part from granitic sources, which show that from the earliest times granitic rocks were exposed to erosion. The Basement Schists appear to have once covered the cratonic area and extended beyond it, and at intervals to have suffered deformation and granitic intrusion. Now only their root-relicts are seen as the schist belts embedded in the Gneiss Complex which seem to have formed a floor to them. The Gneiss Complex, which outcrops around and intrudes the Basement Schists, has deformed them into synclinal structures, split them, and produced cuspate embayed contacts, probably during gravitational sinking and upward intrusion. Schist inclusions are abundant in the complex. The heterogeneous rocks forming the complex are tonalitic granitic gneisses, banded gneisses and migmatites which have undergone several periods of partial melting and deformation, largely by plastic flow. The marginal granitic gneisses are generally conformable with the schist walls, but the internal banded gneisses and migmatites show polyphase melting and complex deformation. Activity in the complex seems to have ended during, or soon after, the deposition of the Basement Schists. The massive and porphyritic granites are discrete bodies, ranging from stocks to large batholiths with sharp, steeply-dipping contacts that are composed of homogeneous and usually equigranular tonalite or adamellite. They are scattered throughout the craton, and intrude the Basement Schists and Gneiss Complex. They are of various early Precambrian ages and the majority are older than the Great Dyke. Some of the younger ones have been dated at around 2 650 m.y. The tonalites are largely confined to a central zone about 600 Km long north-northeast and 150 Km wide. The adamellites occur mainly as large batholiths in the eastern half of the craton. Pegmatites and quartz veins containing economic minerals are associated with some of the adamellites and tonalites.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 69-78

Forty-two new K-Ar age determinations from the southern part of the Rhodesian Archaean (Basement Complex) Craton are presented and discussed. Thirty-nine of these determinations are on samples of granitic rocks. The results show concentrations at approximately 2 900 and 2 600 m.y. These may be regarded as the minimum ages of intrusion for two widespread but separate pre-Great Dyke granitic events. These events are equated with the two granitic episodes evident from field relationships in the Mashaba-Fort Victoria area.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 79-84

The granites and gneisses of an area in the southern part of the Rhodesian Basement Complex are briefly described and discussed. Two major pre-Great Dyke granitic events can be recognised. These are separated in time by the Mashaba Igneous Complex, a predominantly ultramafic layered intrusion. The younger event is largely adamellitic; the older event is largely tonalitic but includes a granodioritic phase. Evidence of even earlier granites, some of which were microcline-bearing, comes from clastic sediments low in the preserved sequence of the Victoria Greenstone Belt. There is a strong possibility that some of the gneisses of the area represent remnants of this granitic floor.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 85-95

The area described extends across between the Shangani and Chilimanzi "gregarious batholiths". An older tonalite gneiss complex was mapped, overlying large, concordant tonalite batholiths as structural domes. This gneissic complex underlies the Selukwe Schist Belt cover as an older gneissic basement that was partially reactivated. Relicts of narrow folded migmatite belts occur in the gneiss. The petrology and structures of the various rock types in this older tonalite gneiss complex are described and their origins discussed.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 121-125

Preliminary mapping in portions of the Madziwa and Uzumba granitic terrains, which flank the Shamva Greenstone Belt in the northeast of Rhodesia, shows that the Madziwa terrain consists mainly of tonalitic gneiss and granodiorite, while the northern part of the Uzumba terrain is occupied by hornblende tonalite and intrusive adamellite. Greenstone belt relicts which have been metamorphosed to the cordierite-amphibolite facies are scattered through the Madziwa tonalitic gneiss. These granitic rocks and their inclusions are described. The features of two greenstone-tonalite contacts are examined and the role of metamorphism and granitization in the formation of the tonalites is discussed.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 149-155

Malawi lies in the intercratonic Mozambique Belt. Although the influence of the late-Precambrian Mozambiquian Orogeny was widespread it is now known that older cycles such as the Irumide (900-1 300 m.y.) and Ubendian (> c. 1 800 m.y.) have also played a major part in the tectonic evolution of the Malawi basement. The Malawi (formerly Nyasa) Province of the Mozambique Belt is subdivided into a northern sub-province, within which Ubendian structures have been recognized, and a southern sub-province where they have not so far been recognized. The geology of two areas, one from each sub-province, is briefly described. The Nyika area lies in the northern sub-province. Ubendian paragneisses form two major tectono-metamorphic units: the high-grade Nyika Gneisses with their downgraded equivalents and the medium-grade Misuku Gneisses. Pre- and early syntectonic granites are associated with each of these units. The Nyika Granite was intruded into Nyika Gneisses during a late-Ubendian fold episode and is unconformably overlain by the Mafingi Group. The Mafingi Group and the underlying basement were deformed in the Irumide Orogeny which was accompanied by greenschist facies metamorphism. The Rumpi Igneous Complex and other minor intrusives were emplaced late in the structural sequence. The Dedza-South Lilongwe area lies in the southern sub-province. Older paragneisses include banded charnockitic gneisses (intruded pre-tectonically by the Linthipe meta-anorthosite), pelitic gneisses, marbles, calc-silicate gneisses and quartzites. They were intruded and metasomatized by the late-syntectonic Dedza Perthitic Complex and appear to be unconformably overlain by the Mchinji Group. The Dzalanyama Granite intruded the lower formations of the Mchinji Group and suffered brittle deformation and greenschist facies metamorphism. The structural history of the two areas is broadly similar, but there are important lithological differences, the reasons for which are not yet clear.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 163-166

The formation of weathered materials in fissures within granite at the Lesapi dam site is studied by means of chemical and mineralogical analysis. The evolution of the different minerals is followed and some physical properties of the weathered product are described. The effect of this material on dam construction is discussed.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 189-196

The copper ore bodies currently being worked at Inyati Mine are shown to be associated with ancient basic dykes. A petrological study of the relationship between the ore and gangue minerals advances evidence that the ore shoots are primarily of magmatic origin. A younger intrusive granite is accompanied by pronounced metasomatic alteration of the country rock.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 219-225

The correct dating of episodes of granite activity is of vital importance to geology. Recent technical advances in potassium-argon dating enable many of the hidden sources of geological error to be quantified, but good geochronology still depends upon a coherent interpretation of the available isotopic, geological and petrological evidence. The theory, advantages and limitations of the conventional K-Ar, total degassing 40Ar/39Ar and 40Ar/39Ar age spectrum dating methods are outlined briefly. The sampling of granites for potassium-argon dating and the interpretation of the apparent ages and other data obtained from total degassing and age spectrum analysis of granites and granite minerals is discussed in some detail. It is concluded that dating of a particular granite can arise from a consideration of the concordance of the ages obtained from it, within a framework of regional age studies, and from the results of more sophisticated 40Ar/39Ar age spectrum dating programmes (few of which have reached the publication stage).
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 227-234

The orogenic zones and related metamorphic belts of Africa are broadly characterised by particular metamorphic facies series reflecting the probable pressure and temperature conditions prevailing during their evolution. During the early Precambrian metamorphism developed under low pressure conditions at high thermal gradients whereas the metamorphic belts found in later Precambrian time are characterised by intermediate pressure and temperature facies series related to a corresponding decrease in the geothermal gradient.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 235-244

Two major stratigraphical units can be distinguished in the centre of the Limpopo orogenic belt near Messina: an old granitic basement and a suite of cover-rocks known as the Messina Formation. Both have been folded and metamorphosed. Evidence is presented that the granitic basement of tonalitic composition suffered partial anatexis in anticlinal areas resulting in rocks which tend towards the composition of hornblende syenite. Comparison of the petrography of these rocks with the data of experimental anatexis suggests that melting took place at temperatures above 625°C and at a pressure close to 10 kilobars. These can be taken to be the conditions which operated at the peak of the regional metamorphism and contemporaneous deformation. By analogy, it is pointed out that the anorthositic gneiss, calc silicate rock and hornblendite in the area can be regarded as "restites", i.e. former calcareous shale and marl which have lost K-Na feldspar and quartz during high-grade metamorphism and deformation.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 265-276

Potash-rich granulite gneisses from within the northern granulite zone of the Limpopo Mobile Belt, a similar anatectically derived, partly intrusive potash granite, the Chibi batholith situated along the southern edge of the Rhodesian craton and the distinctive potash-rich, intrusive Porphyritic Granite, which is found close to the orthopyroxene isograd, are described. The regional, field, petrographic, structural and chemical aspects of these rocks are discussed together with their modes of emplacement and their geochronology. It is concluded that the Chibi Granite and granitic phases in the Granulite Gneiss are related and were derived by partial melting during the high temperature granulite facies metamorphism of the Limpopo Mobile Belt. The Chibi Granite represents a low- to high-level intrusion of dry Mobile Belt-drived, near minimum melt of pre-existing granitic material. Structural evidence suggests that the syntectonic Porphyritic Granite was a later, although related event. It may have been derived by partial melting deep within the Mobile Belt during the waning stages of metamorphism.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 307-317

Massive anorthosites, rapakivi granites and related monzonites and norites, acid volcanic suites and slightly younger tholeiitic and alkali basaltic rocks were emplaced between 1 000 and 2 000 m.y. ago in a belt extending from the Urals to the western United States. The same belt was the site of earlier metamorphic and tectonic activity in the period 1 800 to 2 000 m.y. ago and was characterized by general low-pressure, high-temperature, mineral assemblages, and rapid changes in metamorphic facies. The emplacement of the "post-orogenic" ingneous suites was commonly controlled by tectonic lineaments and graben-like features which later controlled the deposition of continental sandstones and the emplacement of younger more restricted magmatic suites. Mineral ages from the belt are commonly 500 to 800 m.y. younger than Rb-Sr whole-rock or U-Pb concordia on zircons, suggesting the continued presence of high thermal gradients in the area after the end of regional metamorphism and magmatism. The 1 800 to 2 000 m.y. metamorphism, the widespread "post-orogenic" magmatism, the graben formation, the loss of radioactive daughter elements and, perhaps, the control of subsequent metamorphic events during the formation of the Grenville fold belt, are believed to have been controlled by a single major feature in the crust and the upper mantle. The character of the metamorphism and the type of magmatic rocks found in this belt suggests that this area may have been the site of a major upwelling of mantle material in the period from 1 000 to 2 000 m.y. ago. The concentration of labradoritic anorthosites within a relatively limited part of the earth's history is ascribed to a unique combination of high heat flow and relatively stable (thick?) crust.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 319-332

The Fiskenaesset Complex is proposed as a model to demonstrate that the problematical, world-wide occurrences of anorthosites, interlayered in high-grade metamorphic terrains are not of sedimentary origin. They are all igneous intrusions of great age, formed at least 3 000 m.y. ago and were emplaced into high-level supracrustal rocks, or between them and their gneissic basement, at a time when the earth's crust was thin. They were subsequently infolded into the basement and drawn out under catazonal conditions to impart conformity of layering or pseudo-stratification. Their constant association with high-grade granulites and charnockitic gneisses is therefore due to this process and not to cogenesis. These anorthosites are commonly fine-grained, highly calcic and chromitite layered, and are almost invariably intercalated with gabbros, pyroxenites, peridotites and dunites or their metamorphic equivalents. In many respects they have more in common with lunar anorhtosites than with other terrestrial anorhtosites. These latter which occur in the younger layered complexes and as the massif type of deposit, tend to be coarser-grained, are more sodic and have assciated iron and vanadium oxides rather than chromium.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 333-338

The geology of the Proterozoic to Palaeozoic granites and metamorphic basement rocks of the northern Pampean Ranges of Argentina is reviewed. The structure, petrology and genesis of the granitic rocks in the eastern and western regions are compared. In the eastern region the metamorphic rocks are principally slates, phyllites and schists which exhibit an increasing grade of metamorphism from north to south. Into these are intruded small concordant tonalite to granite bodies which are correlated to an early phase of regional metamorphism and migmatization. Later intrusions of batholithic masses of adamellite to granite cut across pre-existing structures. A later phase of pegmatites and granitic veins is held largely responsible for a second migmatization phase. In the southern part of the region the early tonalites form batholiths and the late adamellites form small bodies and veins. Migmatites of various types are common throughout the area. In the western region metamorphic rocks predominate over granites which appear as small concordant bodies. Basic and ultrabasic intrusions, marble, amphibolite and paragneiss are abundant in the metamorphic suite. Metamorphism reached the amphibolite and granulite facies, the highest grade being in the westernmost part of the region. The process of metamorphism and granite formation appears to have been essentially isochemical. The small granitic bodies are interpreted as high temperature anatectic phenomena related to two phases of deformation. Cordierite and andalusite are known from practically all of the eastern region and this area is correlated with the andalusite-sillimanite low pressure facies series of the Abukuma type of metamorphism. The westernmost area of the western region is assigned to the kyanite-sillimanite facies of the Barrovian type of metamorphism. The northern Pampean Ranges are compared with Hercynotype and Alpine type orogenic belts of Europe and it is concluded that it is intermediate between the two.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 339-343

The Brandberg Intrusion, an outstanding landmark which covers an area of approximately 450 sq Km in northwestern South West Africa, comprises five granitic phases. These rocks were introduced during the Jurassic period along the contact between highly folded Damara schists and the Salem Granite, large-scale fracturing and syntexis of the overlying Karoo sediments and basalts occurring at a later date. The initial emplacement of the main mass of hornblende granite was followed by a small intrusion of aegirine-augite granite close to the centre of this body and then by two aplite phases. Finally dykes of brandbergite, an extremely fine-grained rock, were intruded into the country rocks to the northeast of the main body. The Brandberg Intrusion is regarded as having been one of the feeder channels for the acid Karoo lavas which cap the basalts in the area to the north.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 345-359

The Opemisca Lake pluton in west central Quebec intruded the core of an anticlinally folded greenstone sequence of metamorphosed volcanic, intrusive and sedimentary rocks. On a regional scale the pluton is concordant with the steeply to vertically dipping wall rocks, but in detail the contacts show considerable discordance. The pluton is composed chiefly of a granite. The essential minerals of the granite are highly sodic plagioclase, potash feldspar, quartz and hornblende. Quartz increases in abundance from the margin to the centre of the intrusion, whereas the reverse is true for hornblende. However, the compositional changes in the interior of the pluton are so gradual that they impart to it an appearance of monotonous uniformity. Only in the marginal granite is there a pronounced increase in hornblende content and in the inhomogeneity of the granite - features attributable to contamination of the granite by mafic and ultramafic wall rocks. A few small satellitic stocks pierce the country rocks about 2 Km south of the pluton. The stocks are chemically and mineralogically similar to the normal graphite in the pluton, but their distinctive fabric provides evidence of the changes that took place in the granite during crystallization. Textures in the rocks of the pluton and its satellitic stocks indicate that hornblende and highly sodic plagioclase were the first of the essential minerals to crystallize. They were followed very soon afterwards by potash feldspar, and lastly by quartz. The potash feldspar crystals generally grew to a large size and engulfed many of the earlier minerals. Fractionation during crystallization was never of great magnitude and a high degree of equilibrium was maintained between crystals and melt. The microscopic evidence is in complete agreement with the crystallization trend inferred from liquidus relations in the system SiO2 (Q) - KAlSi3O8 (Or) - NaAlSi3O8 (Ab) - CaAl2Si2O8 (An). Experimental microscopic and field data also suggest that during most of its cooling history the magma was relatively dry.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 387-409

A geochemical study of the Opemisca Lake granite pluton in west central Quebec was undertaken to investigate the variation in the chemical composition of the rocks within the pluton. Fifty-one samples were collected and analysed for the following elements or oxides: Ag, B, Ba, Be, Bi, Ca, Co, Cu, CO2, Fe, H2O, K2O, Mo, Mn, Na2O, Ni, P2O5, Pb, S, Sn, Sr, Ti, V, W and Zn. Complete chemical analyses were carried out on 18 of the samples. Variation diagrams for the major and trace elements indicate that assimilation of a diverse assemblage of wall rocks gave rise to contaminated granites which vary considerably but erratically in composition along the margin of the pluton. In contrast, the smooth and systematic compositional variation on the normal (uncontaminated) granite in the interior of the intrusion is best ascribed to differentiation. Quantitatively, contaminated granites are defined as those which (a) have a differentiation index ((1/3 Si + K) - (Ca + Mg) less than 10 (b) contain at least 1,95% Ca, and not less than a combined total of 3,9% Ca and Fe (c) have a calcium content higher than that calculated by means of the formula Ca = 6Sr + 1,34. The behaviour of most of the trace elements can be explained in terms of empirical laws formulated by Goldschmidt (1937) and Ringwood (1955). Isopleth maps were constructed to outline the areas of contaminated and uncontaminated granite and the distribution patterns of certain elements in the pluton. Most of the elements depicted show a pronounced concentric or rudely concentric zonal arrangement. For certain elements, the pattern is noticeably disrupted towards the margin of the intrusion. The distribution of the elements is readily explained by the combined effects of wall rock assimilation and differentiation during cooling of the magma.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 411-417

The southernmost extension of the Namaqualand granite-gneiss complex northwest of Vanrhynsdorp, Cape Province, consists of mainly mesozonal biotite gneiss, augen and flaser-gneiss, and granulite. These rocks were formerly believed to represent the granitized equivalents of Malmesbury sediments, transformed into the above gneisses by two distinct cycles of metamorphism and metasomatism. Recent geochemical analyses and petrographical considerations have revealed that the Malmesbury sediments cannot be the parent rocks of the gneiss complex and did not undergo large-scale metasomatic granitization. A comparison of analytical data of gneisses and Malmesbury sediments with recent results of experimental petrology supports the above conclusion. It is suggested that regional metamorphism and possibly partial mobilization of predominantly arkosic sediments of the Kheis System gave rise to the basement gneisses in which remnants of the transformed strata are still preserved.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 419-432

The Chinamora batholith consists of a series of granitic rock types which are arranged in roughly concentric zones. The core of the batholith is a coarse-grained, often porphyroblastic, granite which is skirted by rocks which range from granodiorite to tonalite in composition. Tonalite is prevalent in the southern quadrants, except where the central granite is intrusive and transgresses the schist belt. Coarse-grained granodiorite, including areas of tonalite, skirts the northwestern quadrant and a medium-grained granodiorite skirts the northeastern quadrant. In this quadrant there is a medium-grained transition zone up to 3 Km wide, between the central granite and the medium-grained granodiorite. Large xenoliths of tonalite occur amidst the granite in the central portion of the batholith. It is probable that the batholith was originally a mantled gneiss dome mainly composed of tonalite, which was modified subsequently by the addition of potassium to form granodiorite, and in turn to form a granite core. Three samples of granite were dated by the potassium-argon method and their ages were reported to be 1 429 m.y., 1 376 m.y., and 874 m.y. respectively. These dates are much younger than the age of the Chinamora batholith which was probably emplaced at 2 900 m.y. or earlier, and they are taken to indicate continued thermal activity which terminated in the late Precambrian. Aerial photographs on a scale of 1:80 000 indicate numerous strongly foliated zones which form a complex system of synforms and antiforms upon which is superimposed a rectangular grid of deeply incised lineations. These features appear to be transgressed by the granite and are taken as additional evidence of potassium metasomatism in the core of the batholith. These structures are believed to result from the rotation of the direction of pressure in a clockwise direction, starting in the north-northeast and terminating in the south-southwest. The complex structure and the presence of large xenoliths in the central portion are taken to indicate that the present erosion surface is close to the roof of the batholith. No evidence was obtained to indicate a genetic relationship between the granite and the mineral deposits which surround it due, possibly, to a lack of analytical data. The presence of a transgressive phase of the granite on the southern flank of the batholith may account for the lack of gold mineralization in that vicinity. In conclusion it is believed that the results of this work justify the reconnaissance approach which was used.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 433-441

The petrochemistry of some important granitic rocks which occur in India is studied. An attempt is made to differentiate between rocks of magmatic origin and those of replacement origin on the basis of petrochemical data derived from 30 chemical analyses. It is concluded that petrochemical studies are of some help, both in separating rocks of different origins and in defining the origin of a particular granite.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 443-446

Features of albite rims from granitic rocks in central South West Africa are described. These show the following features: limitation to plagioclase/potash feldspar contacts, sharp inner edges, convex outer edges, optical continuity with the plagioclase, twin planes continuous with those in the plagioclase, sharp change in extinction of twin lamellae and a general composition range from An0 to An3. Formation by migration of albite exsolved from potash feldspar is postulated. Homogeneous non-perthitic edges of many perthite crystals are the result of this migration of exsolved albite. Analysis of the feldspars will therefore not give their original bulk compositions at the time of solidification. A wide range of 2Vx values of the potassic phase of the perthite indicates that composition, state of ordering and possibly amount and fineness of submicroscopic twinning differ from crystal to crystal.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 447-453

Structural, mineralogical and chemical evidence from the northern margin of the Matopos granite indicates that a contact facies of more basic granitoid rocks, including the "older gneiss" resulted from the interaction of Matopos granite and Bulawayo greenstone belt rocks. Mafic concentrations in the contact zone are regarded as xenoliths stoped from the greenstone belt. Xenoliths in progressive stages of digestion show the mineralogical changes which take place during granitization. Detailed study of microperthitic M-twinned microcline megacrysts indicates growth in a partial melt and the six specimens analysed have near maximum triclinicity and an average composition of Or75(Ab+An)25.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 455-474

Both red and grey Nchanga granites are overlain to the west and north by Lower Roan arkose and shale of the Katanga System. Aplite dykes cutting the granite terminate at the contact and subscribe boulders to the overlying arkose. The basal arkose and, to a lesser extent, the Lower Banded shale were protected from effects of the Lufilian orogeny by the rigidity of the underlying granite, but higher beds of the Lower Roan and Upper Roan are involved in recumbent overfolds. The western corner of the Nchanga granite, near overfolds in the Chingola pit, is intensively sheared. Uranium at Shinkolobwe, emplaced in overthrust folds of Upper Roan, is well dated at 620 million years giving a minimum age of sedimentation of the lower part of the Katanga System. Rubidium-strontium ages of microcline from veins in the ore shale and arkose at Roan Antelope and Musoshi indicate an age near 840±40 million years, with an initial 87Sr/86Sr ratio of 0,709. Rb-Sr determinations on the Nchanga granite indicate an age of 570±40 m.y., with an initial ratio of 0,795. These results by Snelling et al. (1964) have been confirmed by Cahen et al. (1970). Pb-U ages of zircons from Nchanga granite are 600 m.y., but from the granites of Mufulira West and Roan Antelope give 1 975 and 1 730 million years respectively; here the whole-rock Rb-Sr method yields only minimal ages. As the zircon and whole-rock Rb-Sr ages for the Nchanga granite contradict the geological data, further investigations are required. The Rb-Sr whole-rock age is not that of magmatic crystallization, but of a later metamorphic event.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 475-477

The simplest chronology of events compatible with field observations is given for the early Precambrian crust of Greenland, and the characteristics of the main rock units are described. The earliest rocks are potash-bearing granitic gneisses (3 980 m.y. Rb-Sr whole-rock isochron age) developed by deformation from an early igneous granitic suite. Subsequent intrusion of locally abundant basic dykes was followed by the deposition of supracrustal sequences (especially basic volcanic rocks and aluminous pelites) and the emplacement of extensive layered basic igneous complexes with which prominent layers of calcic anorthosite are associated. The supracrustal rocks and the original basement gneisses were tectonically interleaved to form conformable layers from a few metres to several kilometres thick. A major calc-alkaline suite of tonalites, granodiorites and diorites was implaced as usually sub-concordant sheets and makes up more than half of the total early Precambrian crust of Greenland. This suite together with the older rocks was deformed to give large-scale fold interference patterns and was metamorphosed under amphibolite and granulite facies conditions. The high-grade rocks have yielded Pb-Pb whole-rock ages of about 2 800 m.y. Supracrustal sequences do not seem to have been affected by the highest grades of metamorphism and are found intercalated with the gneisses in the southern part of the old gneiss block. These yield K-Ar ages of about 2 500 m.y. Stabilisation of the craton was immediately preceded by the intrusion of basic to ultrabasic dykes in several major swarms and widespread wrench faulting in the period 2 600 to 2 000 m.y. ago.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 479-493

The Hercynian batholith of Central and Northern Portugal is composed mainly of granodiorites and granites of calc-alkali character. Among these, the porphyritic granites are probably the most frequent rock types. In the Aregos region (Northern Portugal) the porphyritic granitic rocks contain numerous xenoliths or enclaves of various orogins. The granitic rocks and their enclaves, and the biotites from the same rocks, have been analysed for both major and trace elements. The geological, petrographic and chemical evidence shows that these porphyritic granites are intrusive magmatic rocks, and an origin by anatexis if favoured in the light of the present results. The most frequent type of enclave is a massive or slightly oriented quartz-plagioclase-biotite rock, which may contain cores with different mineralogical composition: biotite-sillimanite-muscovite, biotite-sillimanite-cordierite or biotite-cordierite-muscovite and variable amounts of quartz and plagioclase. The hypothesis that these enclaves represent restite, or residual rocks after the process of anatexis leading to the formation of a granitic magma, is supported by the following observations: muscovite and potassium feldspar are generally absent in the enclaves; very low "anomalous" K/Rb ratios are found in these rocks; and the major and trace elements patterns of the biotites from the enclaves are similar to those of the biotites from the enclosing granitic rock, suggesting a genetic relationship.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 495-498

On the basis of observations two different zones can be recognized within the Catalan coastal range. The one, which extends to the north of the Tordera River and constitutes the so-called "Costa Brava Catalana", is characterized outwardly by markedly porphyroblastic feldspars and a reddening of the granite. The other, situated to the south of the Tordera (the Maresma Zone), comprises more basic granites, granodiorites and adamellites. In the northern zone microclinization and other mineralogical and structural transition phenomena are apparent, these resulting from strong directional stresses. Highly tectonized rocks are particularly noticeable in the Llafranc, Palamos and San Feliu de Guixols localities, the intensity of the tectonism apparently increasing northwards.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 499-505

During the Hercynian (Upper Carboniferous), a syntectonic metasomatic monzogranite was generated along an anticlinal hinge in mid-Palaeozoic metamorphic "Greenstone Series" at Pormenaz in the Aiguilles Rouges (External Crystalline Massifs of the French Alps). Potash feldspar porphyroblasts were formed in metamorphic ophiolite (prasinite) by metasomatic addition of K2O and SiO2 supplied by remobilization of the subjacent granodioritic basement. The potash feldspar is different in each step between massive prasinite and the resulting porphyritic low-silica "granite", showing the evolution: augen perthitic microcline (banded and augen prasinite) - triclinic orthoclase (orthogneiss) - prismatic monoclinic orthoclase ("granite"). The granite evolved in five kinematic stages: porphyroblastesis, diapirism, cataclasis, recrystallization and erosion. According to geochronometric and stratigraphic data, the total duration of the process, from porphyroblastesis to the time of erosion, is estimated to be about 60 m.y.
Back to title list




Symposium on Granites, Gneisses and Related Rocks. Salisbury 1973. Lister, L A, Ed. Special Publication No.3, 507-509

This is a short review of some of the more general aspects of mobile belts, including their possible origin. W. Q. Kennedy was the first geologist to draw attention to the disparate nature of what he termed the Pan-African (+/- 500 m.y.) thermo-tectonic episode, and it is worth quoting his comment from the Leeds Institute of African Research annual report for 1964: "In some instances (e.g. the Damaran episode in South West Africa, the Gairezi movements in Southern Rhodesia and Mozambique, the Lufilian or Katangan orogeny in Northern Rhodesia (Copper Belt and Katanga, etc.) the orogeny was of normal type involving geosynclinal sedimentation, and progressive regional metamorphism. Elsewhere, however, as in the Mozambique Belt and in the Precambrian areas surrounding the West African craton, the structural differentiation process was very different in character and involved only basement rocks which record the event mainly by granitization or as a 'thermal effect' (metamorphic) resulting in low 'thermal effect' (metamorphic) resulting in low (+/- 500 m.y.) absolute ages impressed on the crystalline rocks". The most obvious characteristic of the gneiss belts, or as they are now called, mobile belts, is their shape. They occur as elongate, linear to markedly arcuate belts or zones between or bounding cratons, ranging up to several thousand kilometres long by hundreds wide, although usually smaller. In places older belts become incorporated into new cratons and are cut by later marginal ones or may become the site of successive movements.
Back to title list




Petrogenesis of the volcanic rocks of the Karoo province. Special Publication No.13, 281-287

Rb-Sr geochronological data are reported for three widely separated Karoo-related complexes, the ages of which could provide important age constraints on the timing of Karoo vulcanicity. For the Messum Complex in Namibia/South-West Africa data for one whole-rock, one plagioclase and three biotite samples define an isochron yielding an age of 132,0±2,2 m.y. Conflicts with some other age determinations from the same area are noted, but the age reported is considered to be a reliable minimum age both for the Messum Complex and for the Etendeka volcanics which it is believed to intrude. Data for 13 whole-rock samples from the Komatipoort Complex are scattered, but data for the olivine gabbros (including two plagioclase separates) and for the granophyric gabbros define two almost linear trends with apparent ages of 448±75 m.y. and 248±17 m.y., both ages being higher than the inferred age of the Lebombo basalts into which the Complex is intrusive. Evidence that some of the plagioclase crystals in the Komatipoort gabbro are xenocrystic is noted, and it is suggested that the observed pseudoisochron behaviour is a consequence of the incorporation in the magma, without isotopic equilibration, of lower-crust derived xenocrysts. Data for ten whole-rock samples from the Birds River Complex define an errorchron, but criteria based on field evidence and geochemical parameters suggest that the data for four samples are unreliable. Data for the remaining six samples define an isochron age of 196,8±11,2 m.y. While accepting that the rejection of data points may be subjective, the result reported is interpreted as a minimum age for the onset of volcanism in the Central Karoo area.
Back to title list




Petrogenesis of the volcanic rocks of the Karoo province. Special Publication No.13, 273-280

Rb-Sr whole-rock age measurements are reported for 86 samples of felsic volcanics from the Lebombo and Nuanetsi areas on the eastern continental margin, and for 20 samples from the Etendeka area on the western continental margin, southern Africa. The data for the main succession of Jozini rhyolites are subdivided according to area: southern Lebombo and Swaziland (179±4 m.y.); central and northern Lebombo (177±6 m.y.); Nuanetsi (191±9 m.y.). The Kuleni rhyolites of the southern Lebombo have an age of 145±3 m.y., and intrusive as well as extrusive felsic rocks of the Bumbeni Complex indicate an age of 133±4 m.y. Geographic variations in the initial 87Sr/86Sr ratios are noted, and the possible effect of anatectic derivation of the felsic volcanics is discussed in the light of observed initial ratios. The number of Bumbeni and Nuanetsi samples was small, but in other respects the ages reported for the Lebombo and Nuanetsi areas are considered to accurately reflect extrusion times. In contrast, the data for the Etendeka area quartz latites are characterized by scatter in excess of experimental error, and an errorchron age of 154±21 m.y. is indicated. A high initial 87Sr/86Sr ratio of 0,7181±11 suggests crustal derivation for these volcanics. The apparent age exceeds that obtained in other studies, and is considered to be anomalously high either because of hydrothermal activity that introduced Sr-rich zeolites and calcites into the volcanics, or because of isotopic heterogeneity in the crustal source rocks prior to their anatexis.
Back to title list




Petrogenesis of the volcanic rocks of the Karoo province. Special Publication No.13, 77-86

The field relationships, petrography and mineralogy of the Rooi Rand dyke swarm are discussed briefly and the geochemistry of the Rooi Rand dyke rocks is considered in detail. This major basaltic dyke swarm crops out in the southern Lebombo and extends from Hluhluwe (Natal, South Africa) in the south to central Swaziland in the north, a distance of approximately 200 km. The Rooi Rand dykes show comparatively rational compositional variation and consequently are of particular importance amongst the basaltic rocks of the Karoo volcanic province. Fractionation of a gabbroic assemblage dominated by clinopyroxene and plagioclase appears to account for most of the chemical variation noted in these rocks, though much of this may have taken place at a comparatively high pressure. Additionally, some variability in incompatible elements may be attributed to variations in the source material.
Back to title list




Petrogenesis of the volcanic rocks of the Karoo province. Special Publication No.13, 331-339

New Pb-isotopic data for Karoo volcanics and minor intrusive rocks from the Lebombo-Nuanetsi area of southern Africa are reported. In the Lebombo the volumetrically significant acid rocks exhibit very little variation in Pb-isotopic composition and plot within the range of isotopic compositions of the underlying basic rocks. Although the basic rocks may have undergone small amounts of crustal contamination, these data are compatible with the derivation of both the basic and acid rocks from the same source. There is no evidence for any variation in the Pb-isotopic composition of either the acid or basic rocks along the length of the Lebombo. In Nuanetsi each of the three formations (Letaba, Sabie River and Nuanetsi rhyolite) have discrete Pb-isotopic compositions, which do not overlap with each other. Although possibly representing derivation from a heterogeneous source region, these data are compatible with contamination of magmas with a homogeneous Pb-isotopic composition by continental crust of the Limpopo belt. Sr- and Pb- isotopic mixing models are considered and it is suggested that the Letaba Formation may represent the uncontaminated Pb-isotopic composition, while the Sabie River and Jozini rhyolite represent those compositions which have undergone contamination. Despite the overprinting of crustal contamination, the 207Pb/204Pb ratios of the Nuanetsi magmas are greater than those of the Lebombo and are also distinctly different from those of the Etendeka. These differences must reflect Etendeka. These differences must reflect fundamental differences in the source region on the scale of hundreds of kilometres. On the basis of the substantial 207Pb/204Pb differences these heterogeneities must have been in existence for at least 1 000 m.y.
Back to title list




Petrogenesis of the volcanic rocks of the Karoo province. Special Publication No.13, 87-104

Nephelinite lavas, referred to as the Mashikiri Formation, and dykes are well preserved in the northern Lebombo and north- eastern Soutpansberg regions in South Africa and in the Sabi region of Zimbabwe. These undersaturated rocks represent the earliest manifestation of extrusive Karoo volcanism in south-east Africa. The nephelinites overlie volcanoclastic sediments and massive sandstones and are volumetrically insignificant compared to the overlying picrite and basaltic rocks. Petrographic and geochemical characteristics of the nephelinites contrast strongly with the overlying, predominantly tholeiitic rocks. The nephelinites are typically porphyritic with clinopyroxene as the dominant phenocryst phase, but also contain olivine, nepheline and titanomagnetite as phenocryst phases. These rocks are characterized by SiO2 contents which vary between 40,4 and 45,0%, MgO values which range between 2,6% and 12,0% and are enriched in incompatible elements, in particular, Ba, Sr and Nb. They are strongly nepheline normative and this criterion has been used by Bristow and Cox (1984) as a simple means of distinguishing between the nephelinitic and tholeiitic volcanics of the Lebombo and Sabi regions. Considered as a group the nephelinites from the northern Lebombo and Sabi show chemical trends compatible with low-pressure fractional crystallization of clinopyroxene, titanomagnetite and olivine. However, there are indications that the northern Lebombo and Sabi nephelinite suites crystallized from parental magmas which differed slightly in initial composition but which were derived from a common source or primary magma. The differences in initial compositions of the parental melts are attributed to small variations in the degree of high pressure fractionation during genesis of the parental nephelinite magmas. High-pressure clinopyroxenes found in some phenocrysts are considered to represent relics of this process. Geochemical and geochronological data suggest that nephelinite volcanism was associated with carbonatite complexes emplaced in south-east Zimbabwe and north-east South Africa at the onset of the Karoo volcanic cycle. The petrography and chemistry of the Lebombo and Sabi nephelinites and adjoining carbonatite complex rocks are typical of the Group II nephelinites of Le Bas (1978b) which consist of clinopyroxene rich rocks, ijolites and carbonatites and are typically associated with the onset of rifting at plate interiors. The carbonatites and nephelinites of south-east Zimbabwe and South Africa were emplaced during proto- rifting of eastern Gondwanaland. On the basis of geochemical data and experimental studies of undersaturated rocks it is concluded that the Karoo carbonatites and nephelinites found in the northern Lebombo and Sabi areas developed in response to small degrees of partial melting (< 8%) of a mantle source enriched in CO2 and incompatible elements. Mantle metasomatism is invoked to account for the increased concentration of CO2 and enrichment in compatible elements.
Back to title list




Petrogenesis of the volcanic rocks of the Karoo province. Special Publication No.13, 105-123

Picrite basalt lavas with unusual chemical relationships form extensive outcrops in the northern Lebombo, Nuanetsi and Tuli regions of south-east Africa. These lavas, referred to as the Letaba River Formation, overlie nephelinitic lavas or Karoo sediments and attain a maximum thickness of some 4 km in the Pafuri region of the northern Lebombo. They are characterized by an abundance of low- and high-pressure olivine phenocrysts, contain rare high-pressure orthopyroxene phenocrysts mantled by clinopyroxene, and opaque oxides and plagioclase commonly set in a fine-grained or glassy matrix. Coarse-grained dykes with similar mineralogy accompany the picrite basalts and are referred to as picrites. The picrite basalts show a range of MgO from about 9-23% MgO with an average of about 15% MgO. Field and petrographic criteria, e.g. extreme thickness and volume, absence of preceding low-MgO basalts and the presence of euhedral and skeletal olivine and clinopyroxene crystals, suggest that the picrite basalts crystallized from Mg-rich parental magmas. If equilibrium crystallization and compensated crystal settling are disregarded then olivine-liquid equilibria (KD/Fe-Mg, D\Ni 01) suggest that the parental liquids were characterized by MgO abundances of at least 13,5%. Chemically the picrite basalts are strongly hypersthene normative and are classified as tholeiitic on the basis of major element chemistry. However, they show abnormally high abundances of K (up to 4% K2) and related incompatible elements (Ti, P, Rb, Ba, Sr, Zr, Nb and LREE) relative to other picritic lava sequences characterized by more or less the same range in MgO. Whole-rock major and compatible element variations found in the picrite basalts can be explained in terms of relatively high-pressure (corresponding to depths of 18-36 km) olivine + orthopyroxene crystal fractionation followed by low-pressure olivine + orthopyroxene (±clinopyroxene) crystal fractionation and accumulation. However, the incompatible elements show considerable scatter on variation diagrams and do not show the anticipated negative correlation when plotted against MgO. The considerable range in incompatible element abundances at a particular MgO value is indicative of strong source area heterogeneity and indicates that a single parental magma hypothesis is untenable. Instead a model involving small degrees of partial melting of a heterogeneous source has been proposed and is discussed by Cox et al. (1984) in this volume.
Back to title list




Petrogenesis of the volcanic rocks of the Karoo province. Special Publication No.13, 69-75

The primary classification of Lebombo lavas is based on chemical composition. Acid or felsic lavas can be distinguished from all others by both high SiO2 and low MgO contents. The remaining rocks, termed mafic volcanics, have MgO > 2% and SiO2 < 60%. Within this group rocks with > 5% normative nepheline are distinguished as nephelinites, the remainder being termed basic volcanics, comprising picrite basalts, basalts, tholeiitic andesites, shoshonites and absarokites. Within the basic group rocks with MgO < 5% are designated tholeiitic andesites and shoshonites, rocks with MgO > 9% are picritic basalts, and the remainder (MgO = 5-9%) are basalts. Distinctions within these groups, between high-K, intermediate-K and low-K lineages, are made on the basis of a K2O vs. MgO diagram. Most groups of rocks can readily be distinguished petrographically except basalts from tholeiitic andesites and, to some extent, from shoshonites and absarokites.
Back to title list




Petrogenesis of the volcanic rocks of the Karoo province. Special Publication No.13, 295-329

Karoo volcanics of both mafic and felsic composition cover large areas of southern Africa, and are characterized by considerable ranges in their initial 87Sr/86Sr (i. e. R0) ratios. Mafic volcanics from the eastern and western continental margins show the greatest range in R0 ratios, i.e. Lebombo-Nuanetsi area: 0,7035-0,7134 and Etendeka: 0,7030-0,7135 respectively. In the Central Karoo area the mafic rocks show a smaller spread of ratios: 0,7046-0,7094. In both the Lebombo and Etendeka areas R0 ratios of 0,704 are characteristic of late stage dykes. In the Lebombo monocline basalts from the southern region show the greatest range in R0 ratios whereas mafic rocks from the northern region and Nuanetsi exhibit more restricted ranges in R0 ratios (0,7042-0,7092). The viability of crustal contamination as a process causing the high and variable R0 ratios of Karoo mafic rocks is evaluated using isotopic, major and trace element and geological data. Apart from the Effingham dolerite suite in northern Natal, contamination is rejected as the major process capable of consistently explaining the R0 ratios of the mafic rocks. The R0 ratios essentially reflect those of the mantle source areas of the mafic rocks. Support for this conclusion comes from complementary studies of mantle xenoliths in southern African kimberlites. These indicate a complex history of metasomatic enrichment of the subcontinental lithosphere such that high 87Sr/86Sr ratios can evolve in quite short periods of time. Metasomatism also leads to enhanced concentrations of incompatible elements. Most Karoo mafic rocks have source areas located in the old enriched subcontinental lithosphere. Voluminous silicic volcanics are confined to the Etendeka and Lebombo regions. In the Etendeka, quartz latites generally have R0 ratios of >0,718, are unrelated to the mafic rocks and are considered to be partial melts of the Pan-American basement rocks. In the Lebombo, the main rhyolites have R0 ratios = 0,7044±2 (southern Lebombo), 0,7048±1 (northern Lebombo) and 0,7085±6 (Nuanetsi). Only in the northern Lebombo is there close agreement between the R0 ratios of the rhyolites and the restricted range in R0 ratios for the associated mafic rocks. On the basis of R0 ratios along the whole length of the Lebombo, the rhyolites appear to be genetically unrelated to the mafic volcanics. Although the origin of the Lebombo rhyolites is not fully resolved, the available isotopic evidence suggests they are probably of lower crustal derivation. The association of high R0 in mafic volcanics with rifted continental margins may suggest that the pattern of continental rifting is influenced by the distribution of zones of enriched lithospheric mantle, in that the latter is hotter, less viscous and therefore more easily penetrated by material from the asthenosphere. The occurrence of voluminous rhyolites in the rifted margin areas may be a result of the increased heat supply to the crust from these zones of geochemically and thermally anomalous lithospheric mantle.
Back to title list




Petrogenesis of the volcanic rocks of the Karoo province. Special Publication No.13, 125-147

The Sabie River Basalt Formation is a well-defined and stratigraphically continuous group of broadly basaltic rocks which is present throughout the whole eastern zone of the Karoo province. It is traceable from Zululand (Natal) northwards along the Lebombo monocline into the Transvaal, and thence into Zimbabwe in the Tuli, Nuanetsi and Sabi areas. In this northern area it lies conformably on picrite basalts of the Letaba Formation but elsewhere rests directly on Karoo sedimentary rocks. In most areas it is terminated upwards by the incoming of Karoo rhyolitic extrusives. The rocks of the formation are divided into high-K, intermediate-K and low-K lineages. Within the first of these, the more primitive types are termed absarokites and the more evolved, shoshonites. The other two lineages are divided into basalts and tholeiitic andesites. Rocks of the high-K and intermediate-K lineages are almost entirely confined to the northern half of the area. A relatively sharp transition to a sequence dominated by low-K types takes place in the vicinity of the Komati River, near the south end of the Kruger National Park. Petrographically most of the rocks are basaltic in appearance, and often porphyritic with plagioclase as the most prominent phenocryst phase. Shoshonitic and absarokitic rocks are occasionally identifiable by alkali feldspar overgrowths on plagioclase, but most taxonomic distinctions are made geochemically. Parental magmas in the northern part of the area were picritic liquids with a wide range of K-contents. Plagioclase fractionation was suppressed in the more K-rich varieties, and the fractionation of ferromagnesian minerals alone (principally clinopyroxene) led to the formation of the basalts and tholeiitic andesites of the intermediate-K lineage and to shoshonites and absarokites. In low-K magmas fractionation was dominated by the crystallization of gabbroic assemblages leading to the formation of the Fe-rich basalts and tholeiitic andesites of the low-K lineage. In the southern part of the area possible picritic parents are not exposed, and petrogenetic relationships are complicated by the operation of several processes including gabbro fractionation, at least some contamination by a granitic component, and by the mixing of an additional component believed to be derived from domains of trace-element enriched mantle.
Back to title list




Petrogenesis of the volcanic rocks of the Karoo province. Special Publication No.13, 171-194

Rhyolites exposed in the Lebombo-Nuanetsi area, southern Africa, have an estimated thickness of 5 000 m and a volume of 35 000 km forming one of the world's largest rhyolite provinces. The rhyolites overlie the main Karoo basalt succession thus marking the end of the main phase of Karoo volcanism. They have been dated in the South Lebombo at 179±4 m.y. The rhyolites crop out along the Lebombo Mountain Range where they are continuously exposed for more than 600 km. These volcanics occur in a monoclinal flexure so that it is not possible to get a three- dimensional view of the sequence. The major rhyolite unit is the Jozini Rhyolite Formation which crops out along the whole length of the Lebombo. It shows very little chemical variation along its length and is enriched in incompatible elements relative to most rhyolites world-wide (av. SiO2 = 70,7%; Rb = 130 ppm; Ba = 1 400 ppm; Sr = 150 ppm; Zr = 1 100 ppm; Nb = 80 ppm; Y = 120 ppm). The second major unit, the Mbuluzi Rhyolite Formation, crops out in Swaziland and is distinguished from the underlying Jozini Formation by the presence of quartz phenocrysts. It is very similar geochemically to the Jozini Formation, though with a lower Zr content (850 ppm). There are also some minor intercalated rhyolite units; the most important of which, the Mkutshane Beds, occurs towards the base of the basalt succession and has distinctly lower trace element abundances than the other rhyolite units in this province. Rhyolites from the Nuanetsi area are generally similar to the Jozini Formation but are slightly more evolved with higher K2, Rb, Ba and Nb, and lower Y and Z contents. The majority of the rhyolites all plot on isochrons of approximately 180 m.y. with initial 87Sr/86Sr ratios increasing towards the north [i.e. southern Lebombo and Swaziland (87Sr/86Sr)o = 0,704; northern Lebombo = 0,705; Nuanetsi = 0,7081. These initial ratios are generally similar to the dolerite intrusions and to the lowest ratios observed in the underlying basalts. The Mkutshane Beds, however, are enriched in radiogenic Sr (and Pb) with initial ratios in the range 0,715-0,741. These data together with Pb-isotope data suggest that the Mkutshane Beds are either crustal melts or have been highly contaminated by the crust. All the rest of the rhyolites are considered to have been ultimately derived from the same source as the basic magmas and were not contaminated by the ancient continental crust during transit. Geochemical modelling indicates that Karoo basaltic material is a suitable source for the rhyolites and an approximately 10% partial melt of a typical Karoo basalt will satisfy the requirements of the trace elements. Minor differences in trace element content among the major rhyolite units, for example between the Jozini Formation (Zr = 1 100 ppm) and the Mbuluzi Formation (Zr = 850 ppm), are attributed to differences in source composition rather than differences in the process involved. Crustal tension and continental breakup provide an environment where basaltic sills, emplaced in the lower crust at the time of flood basalt volcanism, can be melted to generate the rhyolitic magmatism.
Back to title list




Petrogenesis of the volcanic rocks of the Karoo province. Special Publication No.13, 149-169

Thick sequences of incompatible-element enriched picrite basalts constitute the Letaba Formation and occur at or near the Karoo volcanic succession in the northern Lebombo, Nuanetsi and Tuli areas of South Africa and Zimbabwe. Many of the major element compositional trends shown by the picrite basalts can be explained by polybaric fractionation of olivine + orthopyroxene (at high pressure) and olivine + clinopyroxene (at low pressure). However, there is little apparent correlation between major and trace element variations. Examination of the data for the picrite basalt suite reveals planar arrays of data points in many trivariate diagrams. The planar arrays appear due to the compositional interaction between the trend produced by polybaric fractionation and the trend produced by a second petrogenetic process. This second process is considered to be the partial melting of a two-component mantle source. One component is depleted in basaltic major element constituents but enriched in incompatible minor and trace elements, the other has a relatively normal mantle composition. Both components have well-documented analogues in the suite of mantle xenoliths brought to the surface as nodules in southern African kimberlites. Calculated partial melting models indicate that the primitive members of the picrite suite could have originated by about 5% melting of a source whose average composition was modestly enriched (5-10 times "Primitive" mantle values) in incompatible elements. The basalts of the Sabie River Formation which overlie the Letaba Formation in the northern Lebombo and Nuanetsi areas could be derived by fractional crystallization of some of the underlying picrite basalts (those which are less enriched in incompatible elements). The basalts of the Sabie River Formation in the southern Lebombo have markedly lower concentrations of incompatible elements than their northern equivalents. They could be produced from the same type of mantle source as the northern Lebombo rocks but at approximately fourfold higher degrees of partial melting. Alternatively, and more probably, the differences between the basic rocks of the northern and southern Lebombo reflect compositional differences between their mantle source regions.
Back to title list




Petrogenesis of the volcanic rocks of the Karoo province. Special Publication No.13, 389-395

During the analytical phase of the work which is reported in this volume the major analytical product consisted of total rock analysis using X-Ray Fluorescence (XRF) techniques. The majority of the XRF analyses using conventional techniques, together with direct XRF analysis of rock slabs (see below for description of this technique), were carried out at the Geochemistry Department of the University of Cape Town (UCT). However, a significant number of samples were also analysed by XRF at the Geology Department of Rhodes University (RU) and at the Department of Geology and Mineralogy of the University of Oxford. A great many mineral analyses were carried out by electron microprobe with approximately equal amounts being done at Oxford and UCT, with additional analyses also being obtained at RU. Throughout the duration of the project it has been our philosophy to select samples on the basis of stratigraphy, petrography, XRF total rock analysis and (to a lesser extent) mineral chemistry for additional analytical work. The additional analytical data obtained consisted of trace element analysis by spark-source mass spectrography (SSMS) at the Research School of Earth Sciences at the Australian National University (ANU); Sr-isotope determinations at the Bernard Price Institute for Geophysics of the University of the Witwatersrand (BPI), at the University of Oxford, at the Open University (OU), at the Massachussetts Institute of Technology (MIT) and at the University of Texas at Dallas (UTD)-, Nd-isotope determinations at the OU and MIT; Pb- isotope determinations at Leeds University (LU), UTD and BPI; Ar- isotope determinations at LU and at Cambridge University.
Back to title list




Petrogenesis of the volcanic rocks of the Karoo province. Special Publication No.13, 1-26

The Karoo rocks are amongst the earliest manifestations of igneous activity accompanying the breakup of Gondwanaland during the early Mesozoic. Remnants of the once extensive lava sequence and the associated dolerite sills are found throughout southern Africa south of latitude 15°S, the present outcrop being ca. 140 000 km². The main peak of volcanic activity is dated at about 190 m.y. and is probably coincident with the earliest stages of the opening of the Indian Ocean by the separation of Antarctica from southern Africa. A conspicuous later peak of activity occurred at about 120 m.y. (Lower Cretaceous) at the time of the opening of the South Atlantic, the rocks of this age being mainly developed in SWA/Namibia. Over most of southern Africa Karoo tectonic are extensional in nature and are strongly controlled by existing basement structure. Events leading up to the volcanism began with the establishment of a sedimentary trough in the southern part of the continent during the Ordovician. The trough received the sedimentary accumulations of the Cape and Karoo Supergroups (the Karoo basin) with, during the Karoo period, a thinner sequence of shallow marine and continental sediments stretching far to the north. The tectonism of the Cape Fold Belt was initiated relatively early in the Karoo period. gnow succeeding volcanic rocks can be broadly subdivided into those of the central parts of the continent (e.g. the Karoo basin dolerites, the lava sequences of Lesotho and neighbouring areas of the Cape Province) which are overwhelmingly basaltic in nature with the exception of some more dacitic types at the base locally. These sequences extend into the Transvaal (Springbok Flats), Namibia (Mariental) and Botswana but the major outcrops in Namibia consist of the mixed basaltic-acid (latite and quartz- latite) succession of the Etendeka of Lower Cretaceous age. In the east a monoclinal lava sequence is found stretching from northern Mozambique to Natal, and probably marks the boundary between normal continental crust and thinned continental crust bordering the Indian Ocean. The rocks of this zone are varied, consisting of picrite basalt, nephelinites, basalts, tholeiitic andesites, and great thicknesses of rhyolites in the southern part of the structure termed the Lebombo Monocline. An alkalic series including phonolites overlies the Karoo rocks in the Zambezi valley. Large plutonic complexes are developed principally in Namibia (e.g. Erongo, Brandberg, Okonjeje) and the Nuanetsi-Sabi area of south-east Zimbabwe (e.g. Northern Ring, Mateke, Dembe-Divula, Marangudzi). Syenitic Plutons of Cretaceous age are present in southern Malawi (Chilwa Series). Carbonatites are present both here and in south-east Zimbabwe, where they are of early Karoo age (ca. 200 m.y.). Extensive (ca. 200 m.y.). Extensive geochemical studies demonstrate the strongly bimodal (acid-basic) nature of the Karoo sequences, especially along the margins of southern Africa. The Karoo province provides excellent material for testing a number of rival hypotheses of magma generation. These are (a) the role of crustal contamination versus an enriched lithospheric source for the generation of continental basalts, and (b) the origin of silicic volcanics. Are these fractionation products of associated basalts or are they generated by partial melting in the crust or mantle?
Back to title list




Petrogenesis of the volcanic rocks of the Karoo province. Special Publication No.13, 247-266

The results of over one hundred new conventional K-Ar age determinations and seventeen 40Ar/39Ar age spectrum analyses undertaken on thirty igneous rocks from the SWA/Namibian, Central, Lebombo and Nuanetsi areas of Karoo magmatism are presented and compared with previously published K-Ar dates. Notwithstanding the large amount of work that has been done, many important sections of Karoo igneous rocks remain undated and much of the data that has been produced over the past 15 years can be shown to be complex and, in some instances, ambiguous. A major discrepancy still exists between K-Ar and Rb-Sr dating of the Nuanetsi lava sequence. Nevertheless, a critical reappraisal and geochronological interpretation of the evidence currently available does suggest that Mesozoic magmatism in southern Africa was broadly episodic and that the principal maximum of magmatic and volcanic activity probably occurred at different times in each of the overlapping Karoo petrographical provinces. Karoo magmatism appears to be closely related to the tectonic disruption of Gondwanaland. Magmatic activity maxima, around 204, 193, 178, 165, 150, 137 and 120 m.y., i.e. with a periodicity of around 14±3 m.y., can be recognized widely throughout southern Africa as ring-complex, dyke or sill intrusions or as episodes of complete or partial "over-printing" of the K-Ar ages of older rocks; most are also represented by surface volcanism, but no episode was represented everywhere by lava outpourings. In the Central area, for example, the principal maximum of activity is represented by the extrusion of voluminous flood basalt lavas around 193 m.y., in the early Jurassic (Pliensbachian), whereas, in the northern Lebombo and Nuanetsi area, our evidence suggests that the principal magmatic maximum, involving the extrusion of nephelinites, picrite basalts and tholeiitic basalts followed by flood rhyolitic ignimbrites was around 178 m.y. at the top of the lower Jurassic. In the southern Lebombo (including Swaziland), there is evidence to suggest extensive volcanism during both the 193 m.y. and 178 m.y. maxima. In the Etendeka and Lupata areas it is likely that the principal maximum of magmatic activity - represented by flood basalts in both of these areas - was much later, around 120 m.y., in the Lower Cretaceous. Thus the principal foci of Karoo magmatism appear to have moved northwards with time, away from a central (Lesotho) area in which magmatism was most voluminous initially. In addition, it would appear that as continental disruption progressed, Karoo magmatism in southern continental Africa passed into, and is continued by, the magmatism of Madagascar, the Indian Ocean, the Deccan, the Red Sea and the East African Rift on the one hand and by magmatism in the South Atlantic Ocean on the other. Related magmatism is known in Liberia, South America, Tasmania and Antarctica. The magnetic polarity reversal recognizable 300 m above the base of the Lesotho Formation in the Bushman's Pass section is definitively dated at 193±5 m.y. (identical in age to that known from the Freetown Complex in Liberia). A similar reversed to normal polarity change is known from the Olifants River section. Here, however, the lavas with reversed polarity that make up the first 1 000 m of the Olifants River section may be some 16 m.y. younger than the basalts of the Lesotho Formation and, therefore, cannot belong to the same magnetozone as those at the base of the Bushman's Pass. Indeed, it is probable that between -205 m.y. and -170 m.y. the presence of six magnetozones, each with a duration of about 7 to 8 m.y., can be recognized in Karoo igneous rocks and, thus, there may be a close temporal relationship between the episodic maxima and minima of Karoo magmatism and the sequence of palaeomagnetic reversals.
Back to title list




Petrogenesis of the volcanic rocks of the Karoo province. Special Publication No.13, 27-67

Outcrops of the Jurassic Karoo basalts and dolerites in central South Africa are remnants of an intracratonic continental igneous suite whose volume probably once exceeded 1,5 x 10 km². Basic rocks with 6-8% MgO, 50-55% SiO2 and Mg-number = 53-67 are overwhelmingly dominant in the Central area suite. Application of discriminant analysis to a data set of major and trace element analyses of the basic rocks confirms field and petrographic evidence that a number of compositionally distinct basalt magma types exist in the Central area. One of these types, the Lesotho basalt type, accounts for the main bulk of the intrusive dolerites and the flood lava sequence. In comparison, the other types are relatively trivial in volume and are located at the base of the volcanic succession. Minor quantities of corundum- normative andesite-dacite occur interbedded with the lowermost basalt in some areas and they are shown to be crustal melts. Lesotho basalts have Mg-numbers = 53-67 and this range overlaps completely with values for all the other basalt types. High Cr/Ni and Sc/Ni ratios and similar absolute levels of Co, Cr and Ni in all types indicate that the basalts evolved from primary picritic mantle-derived magmas with between 12 and 18% MgO by similar amounts of polybaric fractionation dominated by olivine. The geological and petrographic character of the basalts, together with their major- and trace-element and isotopic compositions, suggests that crustal contamination of the magmas has been minimal and cannot explain the chemical differences between the basalt types. The observed differences in initial 87Sr/86Sr and 143Nd/144Nd ratios, the variable Zr/Y, Zr/Nb, Ti/Zr and Ce/Sm with relatively constant Tb/Yb ratios, and variable incompatible element abundances with similar Cr, Ni, Co and Mg-numbers exhibited by the different basalts discount iso- or polybaric fractional crystallization models for relationships between the different types. Consideration of possible relationships by different models of partial melting of a homogeneous source also fails to explain all the isotopic and trace element characteristics of the basalt of the basalt types. We conclude that the mantle source rocks for the Central area basalts must be chemically heterogeneous. In general Central area mantle sources are enriched in Rb, Ba, U, Th, K, Nb, La, Ce, Sr and P relative to MORB sources. In addition the Karoo sources also exhibit relative LREE and Rb/Sr enrichment and this is consistent with 87Sr/86Sr and 143Nd/144Nd being respectively higher and lower than bulk earth values. The enriched source for Central area was probably located in the subcontinental lithosphere which underwent partial melting during the tensional episode related to the breakup of Gondwanaland.
Back to title list




A symposium on mineral deposits and the transportation and deposition of metals. Special Publication No.5, 1-23

A review of available field and geochronological data allows the recognition of granite-greenstone terrane of different ages within the Basement Complex of the Rhodesian Archaean craton. A terrane about 3 500 m.y. old occurs in the southern and central parts. Its greenstone belt remnants equate largely with the Sebakwian Group of the Rhodesia Geological Survey. Nappe tetonics were a feature of this terrane and sialic crust was present at an early stage. The main greenstone belts are younger and can be divided into the widespread Upper Greenstones and the problematical Lower Greenstones which together correspond largely to the Bulawayan Group of the Rhodesia Geological Survey. The Lower Greenstones were deformed and eroded prior to the deposition of the Upper Greenstones; their age is not known but they may be part of a terrane incorporating tonalites approximately 3 000 m.y. old. The Upper Greenstone are about 2 700 m.y. old, and in the west and south-east are unconformably overlain by the sedimentary Shamvaian Group. Starting from the Belingwe greenstone belt major stratigraphic units of the main greenstone belts can be correlated over the large central region of the craton. The lower Greenstones are most fully developed at Belingwe as the remains of possibly two major ultramafic/mafic-felsic bimodal sequences. Elements of these units can be recognised forming the lowest parts of some of the main greenstone belts particularly around the Shangani batholith. The volcanism of the Upper Greenstones began with a widespread thick tholeiitic pile which included high-magnesium spinifex textured flows. Peridotitic flows occur near the base in some areas. In the east this thick basaltic pile is overlain by fine-grained shallow water sedimentary rocks succeeded by, and in part interbedded with, a further but thinner development of tholeiitic lavas. In the west it passes upwards into bimodal volcanism on different scales in which tholeiitic and, in places, high-magnesium flows alternate with felsic flows and pyroclasts together with sediments derived from the felsic rocks. This total western succession is the time equivalent of, and can be traced laterally into, the eastern sedimentary unit and its overlying basalts. The western bimodal assemblage is succeeded by an andesite-dominant calc-alkaline suite which was not developed in the east. Two suites of post-Upper Greenstones granite can be recognised. The Sesombi Suite of tonalites occurs in the west parallel to the north-north-east-trending line of the western succession. The later Chilimanzi Suite of adamellites, the last major granite event predating the 2 500 m.y. old Great "Dyke" occurs both in the east and west with the largest masses in the east. The intrusions of this suite display an east-north-easterly trend parallel to the Limpopo mobile belt. The five known occurrences of Rhodesian Archaean stromatolites all occur in the Upper Greenstones. Nickel mineralisation is apparently also confined to the Upper Greenstones. The Upper Greenstones constitute the ramains of a widespread cover sequence deposited on a basement which consisted of the circa 3 500 m.y. granite-greenstone terrane, the Lower Greenstones tonalite intruded at about 3 000 m.y. and possibly gneisses dated at 2 800 m.y. The extent of this basement is largely delineated by the Mashaba Ultramafic Suite of layered intrusions and sills, and by the Mashaba-Chibi dyke swarms which together are regarded as the time equivalents of the lower part of the Upper Greenstones. An origin for the Upper Greenstones in north-north-easterly-trending rifting within this earlier basement is postulated. The rifting was initiated by intracratonic block movements between the mobile belts to the noth and south. The Great "Dyke" of Rhodesia may be a last attempt at a Rhodesian greenstone belt.
Back to title list




A symposium on mineral deposits and the transportation and deposition of metals. Special Publication No.5, 25-38

Gold mineralisation in the Gatooma area is examined with reference to three gold deposits: the Dalny, a complex network of quartz-carbonate-sulphide stringers in a shear zone; the Venice, a sulphide-impregnated zone containing pyrite, arsenopyrite and scheelite; and the Golden Valley, a massive quartz vein containing relatively minor amounts of pyrite and galena, and abundant scheelite. Using these deposits, and published experimental data, a model is presented to explain the evolution of Archaean vein and lode gold deposits. The nature of the mineralisations were defined by temperature, fO2, fS2, pH and the bulk chemistry of the fluid which, in turn, were controlled by the nature of the heat source and the chemistry, mineralogy, competence and thermal conductivity of the country rocks. Magmatic, magmatic-meteoric and metamorphic fluids at temperatures above 400°C readily extracted gold from the country rocks. Migration of the fluids away from the heat source resulted in the progressive deposition of : (a) barren quartz veins; (b) gold-scheelite-quartz veins bearing minor sulphides, e.g. Golden Valley; (c) gold-sulphide-quartz-carbonate veins, e.g. Dalny. The distribution of many of the ore bodies of the Venice deposit, the major element chemistry of the wallrocks, the presence of cherty horizons in the mineralised bodies, the lack of well-defined wallrock alteration, and the dissemination of gold and abundant sulphides at the interfaces between volcanic horizons, are all consistent with low temperature deposition due to interaction between hot basaltic lavas and seawater. Subsequent remobilisation of the mineralisation yielded fracture-controlled auriferous lodes in the vicinity of the volcanogenic mineralisation. The model indicates that, in general, as the depositional temperatures decreased the abundance of sulphide minerals and proportion of silver increased and the gold content and fineness decreased.
Back to title list




A symposium on mineral deposits and the transportation and deposition of metals. Special Publication No.5, 39-48

The region studied lies across the main Bulawayo-Hartley greenstone belt, between the Sherwood Star Mine 15 Km north of Que Que and the Connemara Mine, 25 Km to the south. The geological succession consists of Bulawayan Group lavas, tightly folded on north to north-west axes before deposition of the unconformably overlying Shamvaian Group sediments. The Shamvaian sediments were progressively infolded on north-east axes during and following their depsosition. Serpentinites were intruded along the eastern margin of the schist belt, near its contact with the Rhodesdale Gneiss, and granodiorites were intruded in the north-west and east. The main gold mineralization was late in the tectonic history and related to major shear zones. Total recorded gold production of the area up to 1965 was 205 170 Kg (6 597 114 ozs). On parameters of tectonic setting, structure and ore mineral associations, the mined deposits fall into four main groups. 1. The Sessombi Group, accounting for 2,8% of the total gold produced occurs around the margin of the Sessombi Granodiorite in Bulawayan andesitic volcanics. The quartz veins are heavily mineralized with pyrite, chalcopyrite, pyrrhotite and sphalerite and are grouped around small intrusive quartz-diorite stocks. 2. The Bell-Sherwood Star Group has produced 15,6% of the total gold. This occurs in two broad north-north-west-trending belts on the eastern side of the main shist belt syncline, in a variety of host rocks. The mineralization is controlled by intense shattering on north-east- and north-trending shear zones. Quartz-stringer stockworks and disseminations with heavy pyrite-arsenopyrite mineralization were introduced. 3. The Que Que Group, including the Globe and Phoenix Mine, has accounted for 71,2% of the total gold. This occurs as a narrow zone stretching 15 Km south-south-east from the Indarama Mine to the Gaika Mine. It strikes obliquely across the eastern margin of the schist belt, through a wide range of host rocks. It is characterized by an early pyrite, arsenopyrite, loellingite mineralization, followed later by a complex antimony-gold mineralization. The main minerals are stibnite, berthierite, sphalerite, tetrahedrite, bournonite, jamesonite and galena. The gold appears very late in the paragenesis, accompanied by micro-intergrowths of stibnite, and small amounts of aurostibite. The veins show a marked structural control. 4. The Connemara Group occurs in a Bulawayan Group banded ironstone host rock, striking southwards along the eastern schist belt margin. This has accounted for 8,6% of the total gold. Although heavily mineralized with pyrite and pyrrhotite, the higher gold values are generally in the strongly fractured cherty material, and the oreshoots are structurally controlled. A few other mines are present, such as the quartz veins in the Rhodesdale gneiss, the Golden Ridge in ferruginous slate and the rather unusual complex zincblende, galena, pyrite tetrahedrite mineralization in acid volcanic tuffs at the Cactus Mine. These account for less than 2% of the total gold production. The emplacement and possible source of the mineralization are discussed. The recognition of the metallo-tectonic groups as separate exploration targets is advocated. A strong structural control by major dislocation zones ("master shears"), which might be related to the Limpopo Mobile Belt, was recognized. This was combined with local thermal and pressure gradients around the younger tonalites, which were responsible for the clustering of the mines.
Back to title list




A symposium on mineral deposits and the transportation and deposition of metals. Special Publication No.5, 49-53

The Shamva Greenstone Belt is an assemblage of Archaean volcanic and sedimentary rocks in the north-east of Rhodesia. Up to December 1965 a total of 104 gold mines had been worked within the Belt, yielding 56 550 Kg of gold. A study of the distribution of these gold mines shows that 50% of them are associated with greenstone formations of the Bulawayan Group, 40% occur in sedimentary and tuffaceous rocks of the Shamvaian Group, and ten per cent are developed in the Tafuna Formation, and enigmatic assemblage of basaltic lavas and agglomerates which could be either pre- or post-Shamvaian. However, the Bulawayan Group has yielded only two per cent of the total gold output whereas the Tafuna Formation has provided 12% and the Shamvaian Group 86%. The gold mines in the Bulawayan group are mostly developed on quartz veins but there is no predictable pattern to their distribution. In the Shavaian Group, however, virtually all the gold comes from a unique deposit of pyritiferous, tuffaceous sediments which are exposed over an area of only five square kilometres. These sediments have been formed from felsic tuffs which were deposited in shallow water and broken up by current action into confused pebble beds. Gold and pyrite were introduced by fumarolic activity during the volcanism, and the gold was later concentrated along north-east and east-north-east trends during major post-Shamvaian golding. Smaller amounts of gold also occur in other tuffaceous horizons in the Shamvaian; in quartz veins parallel to the regional fold axis within Shamvaian sediments; and to a very minor extent in high level feldspar porphyry intrusions which are believed to be the magma sources from which the Shamvaian tuffs were derived. The gold in the Tafuna Formation has been derived from quartz veins which occur along curved fissures in greenstone. The reefs follow definite trends in north-north-westerly and north-north-easterly directions, both notably different from quartz vein trends in the Bulawayan and Shamvaian Groups. It is concluded that the most favourable target areas for gold exploration in the Shamva Greenstone Belt are in the Tafuna Formation and in the tuffaceous deposits of the Shamvaian Group. A search for tuffaceous deposits within sedimentary formations elsewhere in the Rhodesian Craton is advocated.
Back to title list




A symposium on mineral deposits and the transportation and deposition of metals. Special Publication No.5, 55-60

The ore body of the Redwing Mine is a thin, undulating felsite sheet intruded into an Archaean diorite stock which itself is intrusive into mafic volcanics of the Umtali Greenstone Belt. The felsite which averages 4 m in thickness has a surface strike of nearly 3 Km and has been worked for gold at several places along the outcrop. Prior to the intrusion of the felsite the diorite stock was intruded by several steeply dipping veins and vertical quartz porphyry dykes. Compression from the north and south caused brittle fracture within the felsite and the formation of numerous easterly-dipping parallel, low angle fracture planes. Mineralization in the form of quartz with gold associated with pyrite, galena, arsenopyrite, pyrrhotite, chalcopyrite and sphalerite along these planes gave rise to a series of parallel veins in the felsite. Mineralization is not confined to the felsite and the veins pass across the contact for a short distance into the surrounding diorite. Continuing deformation folded the felsite and veins into anticlines and synclines of small amplitudes which plunge at a shallow angle to the east. A gentle overall southerly dip was also imparted to the body. No defined payshoots have been recognized although there is a suggestion that concentrations of veins occur in broad belts trending east-west across the southerly dip of the felsite. This is roughly parallel to the direction of plunge of the folds although there is no apparent enrichment in any one structure. The ore body is faulted by a series of sub-parallel west-north-west-striking faults with consistent small downthrows to the north. A prominent east-north-east scissor fault following the path of an old dyke causes a small displacement in the felsite in the central section of the workings. The Rezende Shear, a long steep northerly-dipping mineralized zone which strikes west-north-west displaces the felsite by an unknown amount. The last recognizable event is the emplacement of thin horizontal basic sheets. The early quartz veins which intrude the diorite stock, the felsite and the Rezende Shear have similar sulphide minerals suggesting a common mineralizing source operative throughout several periods of tectonic and igneous activity.
Back to title list




A symposium on mineral deposits and the transportation and deposition of metals. Special Publication No.5, 61-65

The Toronto Gold Mine is located on the southern limb of the synclinal Umtali schist belt. Zones of arsenopyrite-stibnite mineralisation within a tabular and steeply dipping body of felsite contain significant amounts of associated gold, although the previously exploited ore bodies are found on the contacts of serpentinite dykes which are intrusive into the felsite. This is compared with the Redwing and Old West mines on the northern limb of the synclinorium, where gold is associated with galena and pyrite mineralisation in a tabular but shallow dipping felsite body. The felsites are considered to be of pyroclastic origin, with the associated gold-bearing sulpharsenide mineralisation derived from the contemporaneous exhalation of volcanic gases.
Back to title list




A symposium on mineral deposits and the transportation and deposition of metals. Special Publication No.5, 67-98

The detailed stratigraphy of the Shangani and Inyati belts is described and the regional setting of nickel deposits within this stratigraphy is discussed. The lower-most or Leechdale Formation of both belts consists largely of talcose serpentinites and magnesium metabasalts with interlayered magnetite iron-formations. The overlying Esmyangene Formation consists of felsic tuffs and agglomerates and is terminated by a sulphide iron-formation and associated carbonaceous shales. The uppermost formation of the Sebakwian Group, viz. the Makwe Formation, commences with serpentinized peridotites and is terminated by high magnesium metabasalts. The latter sequence contains local accumulations of mafic agglomerates. The Bulawayan Group overlies the above and commences with a sequence of distinctive high magnesium metatholeiites termed the Ensangu Formation. This is followed by the Glencoe Formation occupying the core of the Shangani fold structure which is comprised of acid tuffs with sulphidic iron-formations. The tuffs are intruded by the Glencoe layered mafic complex. A layered differentiated ultramafic body intrudes the tuffs of the Esmyangene Formation in both belts. It commences with a basal dunite-peridotite zone now altered to serpentinite and secondary birbirite. This is followed upward by a metapyroxenite, with the final phase layer consisting of a meta-gabbro. The mineralized Shangani and Damba nickel deposits are described. Shangani consists of an anomalous "mushroom" shaped serpentinitic complex apparently "branching" from the upper contact of the layered body and largely surrounded by acid tuffs. Massive nickel sulphides occur on the footwall contact on the underside of each of the "lobes" of the mushroom and disseminated sulphides occur higher up. The whole complex including the two ore bodies is strongly structurally controlled and plunges to the south-east at intermediate angles. The Damba mineralization is associated with the basal serpentinized peridotite flows of the Makwe Formation. Lower grade disseminated mineralization occurs within ultramafics away from the footwall contact but often within depressions with ultramafics away from the footwall contact but often within regional depressions within the latter. The geochemistry of all the rocks described above is discussed and it is shown that in many respects they compare closely with similar rock from other greenstone belts, notably the Barberton belt. The serpentinized peridotites and magnesian metabasalts from the Makwe Formation are similar to peridotitic komatiites and basaltic komatiites of the Geluk type. Of interest are the high magnesium metatholeiites of the Ensangu Formation which compare with similar rocks from South African greenstone belts other than Barberton. The Shangani layered body compares most closely with the Kaapmuiden bodies of Barberton. Evidence is presented which suggests that the Inyati and Shangani belts, now separated by 20 Km of granite, once formed part of the same unit. Reconstructing the belts, an interesting pattern emerges with respect to the distribution of ultrabasic rocks of the two belts, viz. the layered body and the Makwe peridotites and high Mg metabasalts. These form a regional ultramafic centre which pinches out to the north and south and attains its maximum development in the vicinity of Shangani and south of Damba. The Shangani nickel deposit is situated close to this centre while the Damba deposits are situated on the flanks. The nickel-bearing Shangani ultramafic magma is considered to have been emplaced into an acid agglomerate vent as a subvolcanic dyke/lopolith. Other ultrabasic magmas are considered to have been emplaced into the same vent at slightly different times. Some reached the surface to form the Makwe peridotite flows, whereas some magmas were injected at depth into the Esmyangene tuffs where they underwent magmatic differentiation to form the Shangani layered ultramafic body. The above conclusion is based, in part, on geochemical and aeromagnetic data from the layered body. Anomalously high nickel values, accompanied by very high magnetics, occur over the basal birbiritic serpentinites of the layered body just north of the Shangani deposit, and are considered to indicate an ultramafic feeder locus within the agglomerate vent.
Back to title list




A symposium on mineral deposits and the transportation and deposition of metals. Special Publication No.5, 99-107

The Perseverance deposit occurs within an assemblage of pyroclastics and metavolcanics of the Felsic Formation of the Bulawayan Group in the Chakari-Lower Umfuli greenstone belt. It is one of several deposits recently discovered in Rhodesia by purely geochemical techniques. There were no signs of ancient workings of later prospecting activity and yet the orebody is represented by a well-exposed gossan. The deposit is lenticular both in strike ouline and vertical profile, striking approximately north-north-east and dipping steeply east-south-east. The ore occurs as lenses of massive sulphide or as disseminations within a talc-antigorite and andesite host rock. An additional zone of ore occurs near the base of a serpentinised dunite sill in the hanging wall of the Main Orebody. In the Main Orebody the dominant sulphide is pyrrhotite with pentlandite and chalcopyrite being of economic interest. Pentlandite occurs as irregular interstitial grains and patches between pyrrhotite grains or as larger discrete grains and exsolved lamellae in pyrrhotite; calcopyrite occurs as irregular exsolved blebs or wisps in pyrrhotite. The Hanging Wall Orebody shows exsolved pentlandite and chalcopyrite in a silvery coloured pyrrhotite. Various secondary sulphides and several oxides have been observed. The Hanging Wall Orebody was probably evolved by immiscible sulphide segregation within a sill. It is not clear whether the Main Orebody is of volcanogenic origin, a concordant sill cogenetic with the volcanic sequence, or an off-shoot of the hanging wall serpentinised dunite sill.
Back to title list




A symposium on mineral deposits and the transportation and deposition of metals. Special Publication No.5, 109-116

Mineralized and unmineralized serpentinites at Hunters Road occur in a sequence of intermediate to felsic volcanics of Archaean age adjacent to the Rhodesdale Batholith. There appears to be no significant difference in the major element chemistry of the mineralized and unmineralized serpentinites, which are both considered to be derived from dunite. The main disseminated sulphide minerals present in the mineralized serpentinite are pyrite, pyrrhotite, violarite, pentlandite, chalcopyrite and millerite. This mineralization is considered to have been the result of a combination of both magmatic and metamorphic processes.
Back to title list




A symposium on mineral deposits and the transportation and deposition of metals. Special Publication No.5, 159-166

The Monarch Mine is located in a zone of talc-carbonate and quartz-carbonate schists within the quartz-chlorite schists of the central Murchison Range. The most common rock types in the vicinity of the mine are talc schists, quartz-chlorite schists, grey carbonate schists, green carbonate schists, chert-carbonate rocks and chlorite-dolomite schists. These rock types are described and compared with one another and with typical geological materials. Major element and Ni, Cr and Sb data are presented for each of the six rock types. The significance of certain aspects of the geochemistry is discussed and variation diagrams are used to illustrate genetic relationships of the carbonate bearing rocks. The grey, green and chert-carbonate schists are shown to be likely derivatives of talc-carbonate schists, which in turn probably represent peridotitic komatiites which have been subjected to carbonate alteration. The chlorite-dolomite schists are shown to have developed by calcium-magnesium carbonate alteration of the quartz-chlorite schists.
Back to title list




A symposium on mineral deposits and the transportation and deposition of metals. Special Publication No.5, 167-179

The antimony ores of the Murchison Range in the eastern Transvaal lowveld are contained in a layered vulcano-sedimentary assemblage - a greenstone belt - which belongs to the early Precambrian Swaziland Sequence. Similar to the other greenstone belts of the Kaapvaal Craton, the Murchison Range takes the form of a narrow synclinorial inlier within the younger complex granite basement. The belt is structurally strongly deformed but exhibits only a mild regional metamorphic overprint. The antimony ores of the Murchison Range show a stratabound relationship with their siliceous carbonatic host rocks. Two antimony-ore types can be distinguished: (a) an almost monomineralic stibnite ore, and (b) a Ni-rich berthierite ore. In addition to the above-mentioned ores, a stratabound, banded pyrite-arsonopyrite ore occurs to the north of the antimony ores. The texture of both types is that of a metamorphic sulphide ore which recrystallized and was mobilized during metamorphism. An R-mode factor analysis of geochemical data revealed the presence of three dominant factors: (a) a Cu-Zn-Pb-Co (sulphide mineral) factor; (b) a Ni-Cr (ultrabasic) factor; (c) a Sb-Hg-As (low-temperature hydrothermal) factor. The analysis suggests that primarily the antimony ores were formed by synsedimentary volcanogenic submarine-hydrothermal processes. Based on their findings the authors present a conceptual model for the ore formation. A direct genetic relationship of the ore with the intruding basement granites, or a post-metamorphic epithermal ore formation along shear zones seems improbable.
Back to title list




A symposium on mineral deposits and the transportation and deposition of metals. Special Publication No.5, 181-203

Mineral exploration carried out in the Mphoengs Schist Belt, 140 Km south-west of Bulawayo, has led to the discovery of prominent gossans outcropping discontinuously over a strike lenth of 18 Km. Investigations, which have included diamond drilling, revealed the presence of extensiv massive sulphide deposits occurring as strata bound pyrite-pyrrhotite bodies interbedded with a succession of metavolcanic and metsedimentary rocks, the latter forming part of a northward protruding arm of the Tati greenstone belt on the south-western edge of th Rhodesia craton. The sulphide mineralization occurs in close association with thin, but persistent, carbonate units in a sequence of rocks that includes a variety of amphibole and chlorite schists, quartz-feldspar porphyries, agglomerate and tuffs as well as banded iron-formations and phyllites. The main minerals making up the sulphide bodies are pyrrhotite and pyrite. Abundant magnetite is intergrown with the sulphides and marcasite, chalcopyrite, and cubanite occur in only minor amounts. Core samples were analysed for Cu, Ni, Au, Ag, Co, Pb, Zn and Sb but the ores were found to be essentially barren of precious and base metal mineralization. A large tonnage low grade source of sulphur was, however, established. Details of the geology, geochemistry, and mineralogy fo the Mphoengs sulphide deposits are provided - the study being offered as an example of a barren Archaean pyrrhotite-pyrite iron-formation.
Back to title list




A symposium on mineral deposits and the transportation and deposition of metals. Special Publication No.5, 205-208

Sulphides from 28 samples of Archaean banded iron-formation from 17 localities in Rhodesian greenstone belts have δ34S values between -3,9 and +3,5‰. These data support geological and geochemical evidence for a volcanic exhalative origin for the sulphides, with the sulphur in the hydrothermal mineralizing fluids being of magmatic derivation. There is no evidence that these fluids incorporated isotopically heavy sulphate, or that bacterial and/or abiological sulphate reduction contributed significant amounts of sulphide.
Back to title list




A symposium on mineral deposits and the transportation and deposition of metals. Special Publication No.5, 209-217

Stratiform deposits of disseminated copper and iron sulphides are associated with basic volcanic rocks of the Malaputese Formation, one of four supracrustal units occurring within an inlier of Zambezi Metamorphic Zone rocks in north-west Rhodesia. Granulite to upper amphibolite metamorphism of the supracrustal rocks accompanied an initial intense F1 period of deformation. Leucocratic paragneiss, metapelites, quartzite and mafic rocks of the Malaputese Formation suffered widespread retrogressive metamorphism to the greenschist facies during intrusion of potassic granites which are now exposed over much of the inlier. Two later phases of folding (F2 and F3) affected both granitic rocks and the supracrustal units, producing a variety of large-scale interference patterns in the Malaputese. At the Gwai River Mine and adjacent properties sulphides occur in thinly bedded quartzites, calc-silicate rocks and drab grey strata often containing appreciable amounts of cordierite in unusual mineral parageneses. The available evidence suggests that the sulphides were emplaced in volcanogenic sediments near the top of a mafic sequence of lava prior to the main phase of folding and metamorphism. The attitude and shape of the ore bodies is controlled by the fold structures as a result of deformation, disruption and recrystallization of pre-existing, conformable layers of disseminated sulphide.
Back to title list




A symposium on mineral deposits and the transportation and deposition of metals. Special Publication No.5, 219-227

The stratigraphy and sedimentary environment of EoCambrian sediments of the unmetamorphosed Sijarira Group in north-central Rhodesia are discussed briefly to demonstrate a distinct facies change between perilacustrine southern deltaic sediments and northern orthoquartzite carbonate facies. An earlier classification of thse latter into the Lomagundi Group is refuted and they are shown to be a different manifestation of the same cycle of sedimentation as deposited the southern facies. Cyclicity of sedimentation was eustatically controlled and copper content of filial soils is enhanced over rocks representing the first transgression of water level. The analogy with the Katangan copper deposits is demonstrated. A ready source of copper existed in the Copper Queen Formation to the south-east: the mechanism for deposition existed, but copper was precluded from the western basin by a peninsula of granite which effectively bisected the basin until after the first critical transgression.
Back to title list




A symposium on mineral deposits and the transportation and deposition of metals. Special Publication No.5, 229-245

The Selukwe Schist Belt consists of Sebakwian Group rocks lying upside down in the eroded remnant of a major recumbent F2 fold, known as the Selukwe nappe. The Mont d'Or Formation tonalite pluton, which was intruded into the schist belt after the F2 folding, is dated as 3 420 m.y. old. The oldest formation is the Selukwe Greenstone Formation of high-magnesia metabasalts, with many ultramafic horizons and minor metapelite beds forming cyclic sequences. The metabasalts suffered penecontemporaneous erosion and were emplaced at or close beneath the surface. The Selukwe Ultramafic Formation complexes lie at or close beneath the top of the metabasalts, and contain important chromitite horizons. The chromitite is stratiform, and is formed of Mg, Cr-rich spinel. Primary textures and relict sedimentation structures occur. The very elongated lenticular ore bodies probably formed under the initial influence of convection currents in the magma chamber, and their form has been accentuated by later tectonisms. Although similar in chemistry and crystal form to many Alpine-type ores, they do not show any major signs of remelting. The Selukwe chromitites show chemical variation from bottom to top of the ore bodies by cyclic decrease in Cr and increase in Fe. The Wanderer Erosion Surface transects the Selukwe Greenstone and Ultramafic Formations; detritus from these formations, including chromitite, is found in the meta-arenites of the Wanderer Basal Beds. These also contain granitic boulders. The overlying Iron-formation Beds indicate deepeing of the basin. The succeeding Tibilikwe Greenstone Formation of tholeiitic metabasalts and dolerites, is of unknown, but considerable thickness and ends the stratigraphic sequence. Rocks of Selukwe Schist Belt-type, characterized by high-Mg chromitite are widely distributed through the banded gneisses in the southern portion of teh Rhodesian craton. They are absent only from areas of later granitic intrusion. The Mont d'Or Formation, previously thought to predate the Selukwe Ultramafic Complex, is now known to consist of a central tonalite pluton, with peripheral relicts of Wanderer and Tibilikwe Greenstone Formation rocks. F1 shearing and folding affected the Selukwe Ultramafic Complex before the Wanderer erosion surface formed. F2 structures are associated with the nappe movement across Selukwe. F3, F4 and F5 fold trends developed later. Probably, the Tebekwe Ultramafic Complex was emplaced into the Mont d'Or tonalite between the F2 and F3 folding episodes.
Back to title list




A symposium on mineral deposits and the transportation and deposition of metals. Special Publication No.5, 247-252

Norway, once one of the world's major chrome producers, has small deposits of chromite in alpine-type ultramafic rocks found within the Basal Gneiss Sequence and Cambro-Silurian rocks affected by the Caledonian Orogeny. The ultramafic rocks have been divided into three different types, two of which have been mined previously for chromite. The chromite derived from each is chemically distinctive; the distinctive features are probably related to metamorphism. The alpine-type ultramafic bodies within the Cambro-Silurian sequences (Type One) would be the most suitable exploration target should the need arise for Norway to exploit her own chrome resources, but most deposits are sub-economic at present. In some rocks the exsolution of chrome-spinel from chrome-magnetite has been recognised and in others there appears to be an intimate relationship between chromites with different lattice parameters, at least one of which is non-cubic.
Back to title list




A symposium on mineral deposits and the transportation and deposition of metals. Special Publication No.5, 253-261

Geochemical patterns for the major and trace elements were studied in the Chinamora batholith and Leviathan stock. The Chinamora is characterised by a central zone in which K, Pb and g radiation are concentrated. Barium and Sr are enhaced in the peripheral zone. Nickel, Zn, Cr and Ti form patterns which are elongated east-west and Cu, Li, F and Mn extend diagonally from south-west to north-east across the batholith. The Li, F, Mn and Ba peaks include a group of mineralised pegmatite deposits, but there is no correlation of any element analysed with the gold deposits. The east-west patterns are believed to result from the ingestion of roof-rocks in the batholith, whereas most of the south-west to north-east patterns appear to be directly associated with the pegmatite mineralizations. The central patterns result from K-metasomatism which is younger than the mineralisation and from K-Ar age data, it may have terminated during the Proterozoic. Geochemical patterns in the Leviathan are on a very much smaller scale and the patterns of K, Na, Pb, Cr and Ba are elongated to the east-north-east. By contrast most other elements form a fabric which has a northerly direction. This includes zones of Al depletion and Si enrichment and the best gold occurrences. The east-north-east-trending patterns are likely to be primary and the northerly-trending patterns are probably secondary and result from contamination by the roof-rocks. Exploration for gold should follow the northerly trend and not the easterly trend investigated in the past. The nature and origin of K-metasomatism in the Chinamora are not explained adequately by recent concepts, for the Archaean metasediments are dominantly Na-rich and there is no indication of the ingestion of K-rich metasediments by the granitic rocks. It is suggested that K was concentrated downwards during the freezing of the crust to accumulate at the magma interface. A K-rich melt could be mobilised by heat flow during a subsequent phase of magmatic activity, as for example by the Mashonaland dolerite, and this could result in the metasomatism of tonalite and gronodiorite near the surface.
Back to title list




A symposium on mineral deposits and the transportation and deposition of metals. Special Publication No.5, 263-274

The famous Pb-Zn-Ba ore deposits in North Derbyshire (England), at Laisvall (Sweden) and at Pine Point (Western Canada) are closely associated with bitumens. Similar cases have been noticed in France in the Saint-Privat barite deposit (Lodève basin) and in the Les Malines lead-zinc accumulations. Ore bitumens as well as samples of their potential source-rocks were studied using organic geochemical techniques. In the Saint-Privat veins, located in a faulted structure, bitumens are more or less biodegraded, depending upon the relative abundance of sulphides in their neighbourhood. Autunian shales, wallrock of the veins, were identified as source-rock of bitumens. At Les Malines mine, situated in a limestone dome featuring karst phenomena, sulphides bitumens are drastically biodegraded whereas barite bitumens are less affected. Triassic shales, unconformably overlying the karst structure, were recognised as parent sediments for the bitumens. In both case history studies, bacterial degradation of bitumens was demonstrated. The phenomenon is presumably related to the activity of both hydrocarbon-oxidizing and sulphate-reducing bacteria. These last species generate H2S required for metallic sulphide precipitation. Biodegradation of bitumens is not a recent weathering effect. The low maturation stage of bitumens trapped in the ore bodies provides strong evidence in favour of a low temperature deposition process for the metalliferous accumulations. Hence, the non-sedimentary, hydrothermal origin of the two ore deposits has been discarded. Organic geochemical data support the concept of a genetic link between the oil accumulation and the ore deposit. A metallogenic model, based on mixing a metal (Pb-Zn-Ba), hydrocarbon (crude oil) and a sulphur carrying fluid, is proposed to explain the ore genesis.
Back to title list




Some sedimentary basins and associated ore deposits of South Africa. Special Publication No.6, 1-21

The major stratigraphic aspects of lacustrine rock units are geometry (thickness and lateral extent), facies patterns, and vertical sequence. Sizes and shapes of modern lakes are varied, but many large lakes are subcircular to elongate. In cross-section most thick lacustrine units are broadly lenticular, with deposition greatest in the centre of the basin where tectonic subsidence was the most active. The bottom sediments of modern lakes encompass a wide variety of lithofacies. In clastic sediments, there may be concentric belts of gravel, sand, sandy marly mud, and mud, which are controlled by wave base and nearshore wave agitation. Facies patterns among chemical and organic sediments are not predicted so easily. However, two carbonate models are recognized, one with increasing carbonte content toward the centre of the lake and the other with high carbonate concentrations near the margins. The former results from nearshore mixing by terrigenous sediment and the latter from greater carbonate productivity in shallow water. Similarly, there are two organic facies patterns. Offshore increases in organic matter result from deposition and preferred preservation below wave base. In contrast, nearshore concentrations of organic matter are caused by in-place accumulations of plant remains. Few ancient lakes are either sufficiently well preserved or have been studied in sufficient detail to construct even general facies maps. One obvious exception is the Green River Formation of Paleocene-Eocene age, the most extensively studied lacustrine rock unit in the world. In the Green River Formation, the general facies pattern in northeast Utah and northwest Colorado is one of coarse clastics concentrated around the margins and organic-rich mudstone in the central part of the basin. There also is a general basinward increase in carbonate. Most lakes go through more than one cycle of lake formation and filling. The resulting vertical sequence is a composite of many complete and incomplete cycles. Lacustrine rocks contain a variety of allocyclic sequences: glacial varves, nonglacial varves, transgressive-regressive cycles, and various longer patterns represented by bundles of varves or other cyclic deposits.
Back to title list




Some sedimentary basins and associated ore deposits of South Africa. Special Publication No.6, 23-31

The fossil gold placer on Mount Robert near Potgietersrus, northern Transvaal, occurs in the Uitkyk Formation. This formation consists of arenaceous rocks with interlayered conglomerates and shales, and occurs at the top of the Archaean Pietersburg Sequence which forms the Pietersburg greenstone belt. The host rock of the occurrence consists of conglomerates. Its fragments indicate that the provenance area consisted of acid porphyritic lava, chert, banded iron-formation, quartzite, basic lava, vein quartz, and shale. It is suggested that the Uitkyk sediments were transported over short distances and originated from the erosion of a greenstone terrane. The mineralogy of the ore is relatively simple and resembles that of the much younger Witwatersrand banket. Rounded allogenic and, to a lesser extent, idiomorphic to hypidiomorphic authigenic pyrite form the main constituents. Less abundant but genetically interesting ore minerals that have been found so far are leucoxene-rutile, chromite, molybdenite, zircon, carbonaceous matter, and brannerite. The Mount Robert occurrences can be regarded as a primitive forerunner of the Witwatersrand goldfield. Ineffective sedimentary enrichment processes and an invironment unfavourable for life-forms that could have acted as biogenic gold and uranium concentrators are regarded as possible reasons for the low gold content and scarcity of uranium-bearing minerals in the investigated Uitkyk conglomerates.
Back to title list




Some sedimentary basins and associated ore deposits of South Africa. Special Publication No.6, 33-55

A review has been undertaken of the very large volume of information that has been published in the 89 years since gold was first discovered on the Witwatersrand. The literature has been surveyed for opinions voiced and theories propounded for the nature of the depository in which the sediments were laid down and the environmental conditions under which the gold- and uranium-bearing reefs were formed. The review has not been directed towards examining the arguments for and against the various hypotheses that have been advanced concerning the source of the metals and the mineralizing processes that emplaced them in the conglomerates, quartzites, and carbon seams. It is considered that about 90 contributions to the literature can be ranked as having added significantly to a better understanding of the depositional environment. It is apparent that, in the early period of mining operations, speculation favoured the conglomerates to have been formed on a beach. As limited amounts of quantitative data became available, preference was given to a deltaic and, later, a valley-flat environment. With the advent of detailed sedimentological studies, conclusions became far more objective, and systematic observation replaced subjective speculation. The result is that a fair degree of consensus now prevails on deposition having taken place along the interface between a fluvial system that brought the sediments and heavy minerals from an elevated source-area to the northwest and a lacustrine littoral system that reworked the material and redistributed the finer sediments along the shoreline. The goldfields were formed as fluvial fans that built up at several points along the northwestern periphery of the intermontane, intracratonic lake or shallow-water inland sea. Each fan was the result of sediment accumulation at the mouth of a river, which discharged through a canyon and flowed across a relatively narrow piedmont plain, before entering the basin. The review has also covered ideas on the origin of the carbon that is present in the sediments, since it has now been shown to be an important environmental indicator. Evidence is strongly in favour of the material representing the remains of algal colonies that flourished, at certain times only, about the mouths of the rivers. There is a suggestion that the plant-like forms might even have been more akin to lichens. Other ideas are subscribed to by many geologists, and there can be little doubt that, when the centenary of the Witwatersrand goldfields is celebrated in 1986, disputes will still be raging as to how and where the auriferous sediments formed.
Back to title list




Some sedimentary basins and associated ore deposits of South Africa. Special Publication No.6, 57-64

Selective removal of gold and uranium from Witwatersrand ores, with waste or barren rock being reduced to a minimum, has become economically attractive in the light of ever-increasing mining costs. The feasibility of selective mining by means of a mechanical device known as the "rock-cutter" is investigated in the present pilot geostatistical survey by a quantitative study of the interrelationships between gold, uranium and other minerals present in the Vaal Reef at Hartebeestfontein and Zandpan gold mines and also in the Ventersdorp Contact Reef at Venterspost gold mine. In the reefs examined, radioactivity, due mainly to the presence of uraninite, can be used with a high degree of confidence to localize areas also rich in gold, and therefore to positon the rock-cutter. The phyllosilicates chlorite, muscovite and pyrophyllite as well as the heavy minerals pyrite, chromite and zircon are not sufficiently well correlated to either gold or uranium to provide meaningful information for positioning the rock-cutter.
Back to title list




Some sedimentary basins and associated ore deposits of South Africa. Special Publication No.6, 83-89

The highly carbonaceous pyritic shales of the White Band Formation in southern Africa are considered to be lithostratigraphically and broadly chronostratigraphically equivalent to the oil-shale-bearing component of the Irati Formation in South America. The two formations extend over an enormous area and contain similar fossil assemblages notably lacking in unequivocal marine forms. Deposition apparently took place in a series of extensive relatively deep-water sub-basins within an inland sea. Large reserves of exploitable oil-shale have been demonstrated in the Irati Formation. Kerogen-rich shales are known to be present in the White Band as well, but their abundance, grade and distribution have not been established. The high degree of thermal metamorphism imposed on the White Band by deep burial in the southern axial and central parts of the Great Karoo basin is a severly limiting element. In the northwestern part of this basin and in the Kalahari Karoo basin, the shales have been buried less deeply, and the regional degree of diagenesis is lower; intrusive dolerite sills, however, have caused extensive contact metamorphism. Nonetheless, it is in these latter areas that a potential exists for locating bodies of shale of suitable grade that are also free from the effects of dolerite intrusion. There is at present a high element of risk attached to ventures aimed at oil-shale exploitation due to the competition from other, cheaper sources of energy, to the major technical problems associated with environmental impact and to the unfavourable energy- and cost-effectiveness of oil production from shales. In spite of these negative factors, the promise of the Brazilian industry suggests that a reassessment of the White Band shales is overdue.
Back to title list




Some sedimentary basins and associated ore deposits of South Africa. Special Publication No.6, 103-111

The Ecca Group in the northern part of the Karoo Basin is subdivided into a limited number of regressive cycles of sedimentation. The base of each cycle is defined by a major transgression. Each transgression was succeeded by rapid delta progradation into the basin, followed by fluvial aggradation. This resulted in a typical deltaic upward-coarsening sequence which is overlain, in the case of the most extensive regressions, by a fluvial succession in which one or more upward-fining sequences occur. In the ideal complete cycle, the vertical arrangement of depositional environments from the base upward is: (1) Shelf and pro-delta (2) Delta front (3) Distributary channel or delta plain (4) Fluvial valley fill and (5) Transgressive marine. A maximum of eight cycles of sedimentation was identified in the eastern part of the area and only two in the southwestern part. The apparent decrease in the number of recognisable cycles in the south-westerly direction is due to "shale-out" of the uppermost and lowermost cycles of the Vryheid Formation. From this, it follows that the contact between the Pietermaritzburg Shale Formation and the Vryheid Formation, as well as that between the latter and the Volksrust Shale Formation, is diachronous. The cycles become progressively more regressive from the base of the Vryheid Formation to the cycle containing the major coal seams, and then progressively less regressive from the latter to the top of the Vryheid Formation. The influence of the Karoo floor topographic patterns was propagated upward during sedimentation to at least the time of deposition of the cycle containing the major coal seams. The sediments in the western part of the basin were derived from a source to the north and those in the eastern part from a source to the northeast.
Back to title list




Some sedimentary basins and associated ore deposits of South Africa. Special Publication No.6, 113-129

In the Davel coal field the Karoo Supergroup consists of a basal gravelly sedimentary unit (the Dwyka Formation) followed upwards by a shale (the Pietermaritzburg Formation) and an arenaceous unit (the Vryheid Formation). The Vryheid Formation consists, from its base upwards, of the Lower Transition member (fine-grained arkose); the Lower Sandstone member (coarse-grained arkose); the Coal Member (arkose with subordinate shale and the A, B, C and D coal beds in downward order); the Upper Sandstone member (coarse-grained arkose and shale) and the Upper Transition member (fine-grained arkose). Sixteen lithofacies types are present which in turn constitute three types of lithofacies cycles namely: (i) An upward-fining tilloid cycle which represents the Dwyka Formation and which was deposited during the regression of the Dwyka glaciers. (ii) Upward-coarsening shale - arkose cycles which represent prograding deltaic increments and which are represented in the Lower and Upper Transition members of the Vryheid Formation. (iii) Upward-fining arkose - shale cycles which are of a fluvial origin and build the Lower Sandstone, Coal and Upper Sandstone members. In the Coal member these cycles are capped by coal beds which were deposited in backswamps of meandering river systems. A structure contour map of the base of the C coal bed suggests a dendritic channel system with flow towards the south. This channel system had a marked influence on all subsequent sedimentation. Arenaceous material was deposited in active channels whereas muds and coals were deposited on their flood-plains. Channel closure led to the filling of channels by muds, the establishment of swamps and the deposition of coal beds within them. Thickest coal beds and coal of best quality are thus present on the floodplains of active channels and within abandoned channels.
Back to title list




Some sedimentary basins and associated ore deposits of South Africa. Special Publication No.6, 131-150

Oil accumulations in the Middle Ecca sequence near Dannhauser and Wakkerstroom are the most important ones found to date in the northern part of the Karoo basin. At Dannhauser the oil occurs mainly in tight impermeable sandstones above the coal seams, while at Wakkerstroom it occurs in sandstones with more variable porosity and permeability at about the level of the coal seams. The oil was generated from organic matter of aquatic and land-plant origin in source rock shales closely associated with the reservoir rocks. Oil shows are not unually found close to dolerite intrusions, and there is evidence that some oil was destroyed by these. Organic maturity data and inferred paleotemperatures for sediments not thermally affected by intrusions show that oil generation and migration largely could have preceded intrusion; this confirms the field data, and is supported by a reconstruction of the burial history. A former thickness of overburden of between 2 500 and 3 000 m is estimated. It has been found that good porosity and permeability are of secondary origin due to leaching of carbonates and other soluble material which took place when the sandstones were already well-compacted, and probably during the period of dolerite intrusion. A good development of secondary prosity therefore may have largely post-dated oil migration. One late accumulation of oil in secondary pore spaces in a dolerite contact zone is known, however. Some cases of low measured oil saturation may be due to some dilution by leaching fluids, ineffective secondary migration, or some escape of gas or light fractions of the oil, but the latter are usually still abundant. Chromatographic analysis reveals a lack of any large-scale biodegration or flushing by meteoric water, and the quality and composition of the oils is such that they would be easily producible if other conditions were favourable.
Back to title list




Some sedimentary basins and associated ore deposits of South Africa. Special Publication No.6, 161-181

Exploration drilling has been concentrated on the southern Cape area where a nearly complete sedimentary record is present extending from Portlandian to Recent. The major stages of continental rifing and separation can be recognised, and there is some evidence to suggest that mid-Jurassic pre-rift sediments are preserved locally. The onset of rifting and taphrogenic sedimentation is believed to have commenced in the Portlandian with the rapid deposition of continental conglomerates and red-beds and grey non-marine to marine shales. The onset of drifting (separation) between South America and Africa terminated this stage and is marked by seismic reflection horizon "C" which coincides with a regional unconformity. Palaeontological dating places the associated hiatus at the Hauterivian-Barremian boundary (˜121 m.y. BP). The discrepancy with the earliest dated sea-floor magnetic anomaly (M12, ~tilde;128 m.y.) requires further consideration. The post-drift sediments accumulated under relatively stable shelf conditions at medium to slow rates. Regional unconformities are present in the mid-Aptian, Cenomanian to Turonian and at the Maastrichtian-Palaeocene boundary. The Tertiary sediments accumulated at slow to very slow rates under stable shelf conditions, and it is only at this time that limestones achieved any prominence. A major regional unconformity is present in the Upper Oligocene to Lower Miocene. Information on the Neogene is sparse owing to the limited number of samples available. The biostratigraphic framework is based on foraminifera. Ostracodes are of use in zoning Late Jurassic to Early Cretaceous sediments and, together with charophytes, can be applied in non-marine facies. Radiolaria, although widespread, usually are preserved too poorly to be of assistance.
Back to title list




Some sedimentary basins and associated ore deposits of South Africa. Special Publication No.6, 197-203

The Agulhas Bank is located landward of the right-lateral transcurrent Agulhas marginal fracture zone, which has played a significant part in the break-up of west Gondwanaland (Africa and south America). Four lithotectonic units, bounded by unconformities, have been recognised in the Mesozoic succession on the Agulhas Bank. They are correlated with the following stages in the evolution of the Agulhas Bank area: "rifting" stage (Hauterivian and older), during which the pre- and lower Sundays River sequences accumulated, and the "drifting" stage (Barremian through Late Cretaceous), during which the upper Sundays River and Agulhas sequences were deposited. Rifting on the Agulhas Bank took place prior to the development of a regional unconformity represented by seismic reflector horizon C. This unconformity, which is underlain by sediments not younger than Hauterivian, marks the onset of drifting between the African and South American plates. Syntectonic sedimentation characterised the rifting period, so that the Enon Conglomerate Formation grades vertically and laterally into the Swartkops Sandstone Member, the red beds of the Kirkwood Formation and the grey, argillaceous sediments of the Colchester Member and Infanta Formation. Permanent marine conditions had become established on the Agulhas bank towards the end of the rifting stage, when the lower Sundays River sequence was deposited. The drifting stage was marked by the deposition firstly of the deltaic/shallow marine upper Sundays River sequence (late Early Cretaceous) and secondly of the Agulhas marine sequence (Late Cretaceous). The seaward limit of the upper Sundays River sequence is characterised by offlaps shoreward of and sub-parallel to the Agulhas marginal fracture ridge, which is considered to have been formed soon after drifting commenced. The Agulhas sequence (Late Cretaceous) was deposited as a seaward-thickening wedge of marine sediments on a fairly stable shelf, which subsided most probably as result of progressive cooling of the lithosphere.
Back to title list




Some sedimentary basins and associated ore deposits of South Africa. Special Publication No.6, 205-217

An attempt is made to establish the depth intervals over which hydrocarbon generation and primary migration could have taken place in sediments on the Agulhas Bank. A semi-theoretical approach was employed, in which full use was made of geochemical and related organic maturity data and borehole temperature measurements. The generation of oil and gas from source rocks is largely dependent on temperature and time, as well as on the quality of the available organic material. A study of the literature reveals that there is general agreement that the temperature required for the beginning of oil generation from good-quality Lower Cretaceous source shales is in the range of 55° to 60°C. However, the interval of intense hydrocarbon generation from such shales is shown to occur between ca. 75°C and 135°C. An oil-gas transition stage exists to about 160°C, beyond which the dry-gas generation stage is reached. At a temperature of approximately 200°C sediments no longer have commercial gas potential. Time is an important factor in the maturation process. Younger sediments need to be subjected to higher temperatures than older sediments before intense oil generation begins. A short discussion is given on the theory that water released during the dehydration of certain clay minerals could provide the fluids necessary for primary migration of petroleum. This dehydration process is temperature dependent and occurs within the 80°C to 130°C temperature interval of the main phase of oil generation. The oil source potential of the Mesozoic shales on the Agulhas Bank was assessed on the basis of (i) their organic carbon content, which is proportional to the total organic matter; (ii) the ratio of lipid (oil-generating) to lignitic organic matter; (iii) other geochemical parameters such as extract/organic carbon ratios and proportions of desorbed gases; and (iv) those parameters related to the depositional environment of the sediments. On the basis of geochemical maturation data such as vitrinite reflectance, it has been determined that the average palaeogeothermal gradient for the Agulhas Bank was about 3,5°C/100 m. Applying this temperature gradient, the optimum depth interval for oil generation and migration on the Agulhas Bank is calculated to range between 2 000 and 3 600 m. Maturity maps indicating areas on the Agulhas Bank of more or less favourable oil generating potential are presented. Burial history diagrams were constructed for various stratigraphic units present in the Agulhas Bank borehole sections. These diagrams show the sedimentation rate as a function of time and depth, and incorporate curves defining the temperature-depth limits over which oil could have been generated.
Back to title list




Contributions to the geology of the Barberton Mountain Land. National Geodynamics Programme, Barberton Project. Special Publication No.9, 31-44

The Schapenburg greenstone remnant is one of a number of Archaean xenoliths preserved in the granitic terrane in the southern part of the Barberton Mountain Land. The greenstone belt, which is approximately 12 Km long and 2,5 Km wide where best developed, is made up largely of cyclic repetitive, regularly layered, volcano-sedimentary units. These units are generally comprised of peridotitic komatiite flows at the base, overlain by basalt komatiite flows. Banded silicate-facies iron-formation usually terminates or caps each successive cycle. The mafic and ultramafic volcanics are generally serpentinized or altered to a wide variety of schists but locally the rocks show well-preserved spinifex textures and relic olivine crystals. Unlike other greenstone xenoliths in the Barberton area the Schapenburg belt also contains a well-developed metagreywacke-metapelite sequence together with calc-silicate interlayers. The clastic metasediments are considered to be turbidites and contain mainly grunerite±almandine±cordierite±andalusite±staurolite ±epidote/zoisite as well as quartz and magnetite. The calc-silicate units, which are probably altered pelagic calcareous marls, are dominated by diopside±garnet±microcline±epidote/zoisite + quartz assemblages. Incipient anatectic melting of the metagreywackes has occurred and indications are that the rocks were subjected to low to medium grades of metamorphism with temperatures ranging from 500-700°C and pressures ranging from 2,5-3,5 kb (Abukuma facies series of Winkler, 1967). The granitic rocks intruding the Schapenburg belt include components of the first and second magmatic cycles (as defined by Anhaeusser and Robb, 1981). Trondhjemite and tonalite gneisses and migmatites constitute the oldest intrusives (˜3,4-3,2 Ga), whereas granodiorites, adamellites, and K-rich marginal migmatites, associated with the Heerenveen and Mpuluzi batholiths south of the Barberton greenstone belt, represent the latest granitic events in the area (˜3,2-3,0 Ga). The Schapenburg greenstone belt underwent an early deformation history linked with the emplacement of the Archaean granites and a later deformation involving north-west-east fracturing and dyke intrusion. This last event produced left lateral strike separation of the formations as well as segmentation of the belt. From the field and geochemical evidence presented it is concluded that the Schapenburg greenstone remnant represents a xenolith comprising the lowermost formation of the Onverwacht succession (Tjakastad Subgroup) of the Barberton greenstone belt. The presence of the cyclical volcano-sedimentary units, consisting of peridotitic komatiite, basaltic komatiite and banded iron-formation, provides supportive evidence for the interpretation of Viljoen and Viljoen (1969c) who maintain that the Komati Formation is an accumulation of largely effusive komatiitic rocks with some penecontemporaneous sills.
Back to title list




Contributions to the geology of the Barberton Mountain Land. National Geodynamics Programme, Barberton Project. Special Publication No.9, 63-72

The results of Rb-Sr isotopic analyses of suites of samples from 16 units in the Barberton granite-greenstone terrane are presented. The data suggest that emplacement of these rock units has occurred episodically over a time span from ~3 500 Ma to ~2 750 Ma ago. They also suggest that no regional metamorphism has affected this area more recently than ~3 050 Ma ago.
Back to title list




Contributions to the geology of the Barberton Mountain Land. National Geodynamics Programme, Barberton Project. Special Publication No.9, 73-79

Isotopic data from the Barberton granite-greenstone terrane of southern Africa indicate that tectonic activity in this region was episodic over a time span from ~3 560 to ~2 200 Ma ago and that the leucocratic rocks in the area have a close genetic relationship to amphibolitic rocks compositionally similar to those preserved in the Barberton greenstone belt. The early period of tectonic activity involved formation and then partial melting of amphibolitic rocks to form tonalitic and trondhjemitic magmas that magmatically and diapirically intruded other amphibolitic rocks. Later tectonic activity involved partial melting and remobilization of pre-existing crustal rocks. The rocks exposed in the Barberton granite-greenstone terrane were never deeply buried nor subjected to regional high-grade metamorphism more recently than ~3 430 Ma ago. Crystallization ages and emplacement ages may be quite different for individual units. These aspects constrain any model for crustal evolution for this region.
Back to title list




Contributions to the geology of the Barberton Mountain Land. National Geodynamics Programme, Barberton Project. Special Publication No.9, 117-130

The granitic terrane extending for over 80 Km to the north of the Barberton greenstone belt is largely underalin by a suite of essentially cogenetic rocks (the Nelspruit batholith) comprising a dominant, homogeneous, K-rich, porphyritic phase, an assemblage of potassic gneisses and migmatites occurring together with remnants of older soda-rich gneisses and greenstone remnants, and a homogeneous, medium-grained, granodioritic phase. The porphyritic granite is considered to have been derived by widespread partial melting of a heterogeneous crust domianted by tonalite and trondhjemite gneisses. The magma was emplaced passively and coalesced at a relatively high level in the crust to form a sheet-like body. On cooling, the magma underwent in situ crystal fractionation with K-feldspar phenocrysts nucleating late in the paragenetic sequence. Localized metasomatic development of microcline megacrysts also occurred. The potassic gneisses and migmatites topographically underlie the sheet-like porphyritic and granodioritic phases and are regarded as reflecting a slightly deeper level in the crust. The causative processes responsible for the magmatic pulse which resulted in the development of the Nelspruit batholith approximately 3,15 Ga ago are as yet uncertain, but it is speculated that widespread upper mantle/lower crustal degassing may have initiated the melting event.
Back to title list




Contributions to the geology of the Barberton Mountain Land. National Geodynamics Programme, Barberton Project. Special Publication No.9, 131-151

The Archaean granitic terrane south and south-west of the Barberton greenstone belt consists predominantly of a suite of tonalitic and trondhjemitic gneisses into which have been emplaced two large multi-component granitoid bodies known as the Heerenveen and Mpuluzi batholiths. Although geochronologic and Sr-isotopic studies demonstrate that there is little distinction between the ages and initial ratios of the various phases associated with these batholiths, each body displays contrasting textural and geochemical characteristics. The oldest phase is represented by coarse porphyritic granitic rocks into which is intruded a medium- to fine-grained homogeneous leucogranitic phase. Both phases are components of a "bimodal association" that is, in turn, intruded by a third phase which includes medium-granied pink or grey granodiorite and adamellite dykes feeding a homogeneous sheet-like carapace overlying the coarser porphyritic granites. A fourth phase, consisting predominantly of potassic migmatites and gneisses, rims the batholiths and represents the product of interaction between the batholith magmas and components of the pre-existing crust in the region. Geochemically, the Heerenveen batholith has trondhjemitic affinities whreas the Mpuluzi batholith consists predominantly of potassic granites (granodiorite, adamellite, granite sensu stricto). Together with the Nelspruit batholith north of the Barberton greenstone belt the three granitic bodies show a progression in actual values of K2, Na2, Rb and Sr, with the Nelspruit body having intermediate chemical characteristics. The batholiths, which represent components of the second magmatic cycle (as defined by Anhaeusser and Robb, 1981), are considered to have formed from the partial melting of tonalitic or trondhjemitic source rocks similar to those classified in the first magmatic cycle. Petrogenetic modelling of Rb, Sr, and Ba abundances suggests that the Mpuluzi batholith resulted from a smaller degree of partial melting than the Heerenveen body. The Nelspruit batholith formed under conditions intermediate between the two. On cooling, the magmas appear to have experienced in situ crystal fractionation which, in the Heerenveen batholith, was controlled by plagioclase + quartz, with K-feldspar being either "intercumulus" or late stage in character. By contrast, crystal fractionation in the Mpuluzi batholith was controlled by plagioclase + quartz + K-feldspar with the latter mineral being a liquidus phase during most of the solidification history. Finally it is speculated that all the components of the second magmatic cycle in the Eastern Transvaal and Swaziland are broadly coeval and are a direct response to a period of widespread upper mantle/lower crustal de-gassing that may have initiated the melting event approximately 3 200 Ma ago.
Back to title list




Contributions to the geology of the Barberton Mountain Land. National Geodynamics Programme, Barberton Project. Special Publication No.9, 189-219

The Barberton Project, which formed part of the National Geodynamics Programme, has largely been concerned with a detailed study of the nature and distribution of the granitoid rocks surrounding and intruding the Barberton greenstone belt and its satellite xenoliths. During the course of the investigation it became evident that geochemical information would be necessary not only to characterize the diverse granite types found throughout the study region but also to be able to gain an insight into the petrogenesis of these rocks. For the purposes outlined above a large number of samples were collected during the mapping programme and analysed for major, trace and, in some instances, rare earth elements. These results were employed to assist with the routine taxonomic classification of the granitic rocks and the selected trace and rare earth elements were used to place constraints on the possible parentage of certain granitic varieties. The principal conclusions that have been formulated from the large data base accumulated during the tenure of the Barberton Project are embodied in a number of theses as well as published papers that have appeared since the project commenced in April, 1975. A complete list of these publications was provided by Anhaeusser (1981a). The data has also been used extensively in discussions and geochemical modelling recorded in many of the contributions that appear in this volume (Anhaeusser, 1983). Approximately 700 complete rock analyses were considered during various phases of the Barberton Project. Some of the data was in existence following studies undertaken prior to the commencement of this investigation and included results from the Upper Mantle Project (Viljoen and Viljoen, 1969) as well as independent research carried out in the Barberton district and in Swaziland (Anhaeusser, 1969, 1972; Condie and Hunter, 1976; Glikson, 1976; Hunter, 1968, 1970, 1973; Hunter et al., 1975, 1978,; Van Eeden and Marshall, 1965; Visser et al., 1956; Wuth, 1980). This widely dispersed analytical data has been assembled in this volume together with all the data acquired by the authors during the involvement with the Barberton Project. Listed for most samples are the major element analyses and Rb, Sr and Ba trace elements. In addition, mesonormative values have been computed for nearly all the analyses (see Table III). Most of the samples are granitic in character but a few analyses from some of the greenstone xenoliths in the region have also been listed. These include ultramafic, mafic and felsic metavolcanic rocks, calc-silicate rocks and a few uncommon rocks such as marundite (corundum-margarite rocks) as well as rocks rich in barite. REE analyses of selected samples from a wide range of granite types in the Barberton Mountain Land are included in Tables IV-VI. This data is discussed by Robb (1981b) as well as in contributions appearing in this volume. For practical reasons it has not been possible to provide a single map showing the location of all the samples included in this listing of geochemical data. Anyone requiring details of sample localities can obtain this information from the authors. The samples are also located on 1:50 000 scale topocadastral maps housed at the Economic Geology Research Unit, University of the Witwatersrand, Johannesburg. In order, however, to provide a rough guide to the general location of the samples, the numbering scheme given the samples helps provide a guide to their whereabouts in the field. In Table I, and listed alphabetically, are the key letters or codes defining either the farm or the general region where the sample or series of samples was collected. For example the samples prefixed by the code letter A are located on the farm Boekenhoutrand 722 IT and have been described and located in detail in publications by Robb (1981a, b). In some cases there are numerous samples with the same code, as is the case with the TP series that commences on page 6 of the listed data in Table III. These samples are roughly located by making use of a farm (Aarnhemburg 155 IT) that is approximately centrally positioned with respect to the samples which, in this case, are from the Theespruit pluton. Where the same code has been used more than once in Table I (e.g. GS, H, HH, J, K, N, TS, VG) the page on which the analysis is listed is highlighted and from Table II it can, in most cases, be established roughly where the sample originated. As an example, samples coded H from Batavia 151 IT are listed on pages 22 and 23 of the geochemical data (Table III) and are located on 1: 50 000 topocadastral maps in the Badplaas area (2530 DC BADPLAAS), whereas the remaining samples coded H are from Swaziland and their locations may be obtained by consulting the references of Hunter (1968, 1970, 1973) or Hunter et al. (1975, 1978).
Back to title list




Petrogenesis of the volcanic rocks of the Karoo province. Special Publication No.13, 195-245

The volcanic rocks of the Etendeka Formation from north-west SWA/Namibia have a present-day coverage of 78,000 Km² and comprise a series of interbedded basalts, latites and quartz latites, together with four varieties of intrusive dolerite. Apart from one group of dolerites (regional dolerites) which have mineralogical and geochemical similarities to the Lesotho Formation lavas from the Central area, the Etendeka volcanics differ from all other Karoo volcanics by virtue of their Cretaceous age, stratigraphy, mineralogy, geochemistry, and range in mineralogical, elemental and isotopic compositions for the basaltic rocks. Thus the most voluminous basic rocks (Tafelberg basalt type) consist of basalts and dolerites with SiO2 = 48.9-57.8%, Mg-number = 68-28, and initial 87Sr/86Sr = 0.7078-0.7135. The interbedded basalt-latite-quartz latite sequence exhibits many regular and rational compositional variations for major and trace elements, as well as incompatible inter-element and Sr- and Nd-isotopic ratios. These relationships preclude the derivation of these three rock types by any simple melting of fractional crystallization process involving a homgeneous source or parental magma. Consideration of certain critical inter-element and isotopic ratio inter-relationships also rules out any magma mixing model, whether by simple mixing, or by mixing and subsequent fractional crystallization, that might seek to derive the more evolved basaltic rocks by magma mixing of any of the less evolved basic magmas with either latite or quartz latite. Detailed assimilation-fractional crystallization (AFC) modelling, using a variety of actual and putative rock compositions, either as starting materials or as contaminants, cannot consistently explain inter-element and isotopic ratio variations in either the less evolved or more evolved basaltic rocks. The former have variable initial 87Sr/86Sr ratios that almost encompass the range shown by the latter and it is postulated that the evolved basic magmas have been derived by crystal fractionation from less evolved basic magmas which had a range in isotopic compositions. This is supported by quantitative major and trace element modelling across the basaltic spectrum. The majority of the parental basic magmas are considered to be derived from heterogeneously enriched lithospheric mantle, evidence for which is provided by metasomatized mantle-derived nodules contained in kimberlite. Although these basic magmas have "calc-alkaline" affinities there is no compelling evidence for the operation of subduction-related enrichment processes, which would be of Proterozoic age because of Sr- ann Nd- model age considerations, in the mantle source areas of these magmas. Instead, the majority of the basic rock types are considered to be continental flood basalts with their own distinctive lithospheric mantle sources, the only exception being late-stage MORB-like intrusives (Horingbaai dolerites) which are thought to be asthenospheric melts emplaced during an advanced stage of rifting and crustal thinning.
Back to title list




Petrogenesis of the volcanic rocks of the Karoo province. Special Publication No.13, 267-271

The Dokolwayo kimberlite pipe in northern Swaziland is known, on the basis of field evidence, to be post-Ecca Group (and possibly post-Beaufort Group), but pre-Lebombo Group in age. The kimberlite contains abundant phlogopite, but due to alteration effects and to the presence of other mica of presumed crustal origin the mica is not ideal for dating purposes. A Rb-Sr errorchron age of 203±7 m.y. (Ro = 0.7088±0.0023) was obtained from eleven selected samples of the mica, and an 40Ar-39Ar age spectrum analysis on one of these samples gave an age of 198±2 m.y. By both methods it was possible to recognize an older mica component, but the mean age for the selected samples of 200±5 m.y. is considered to be reliable. This is a maximum age for the Karoo volcanism, at least in the central Lebombo area.
Back to title list




Petrogenesis of the volcanic rocks of the Karoo province. Special Publication No.13, 289-293

A precise Rb-Sr internal isochron age of 182±2 m.y. has been obtained for the Tandjiesberg dolerite sheet in southern Namibia. Observed variations in present-day whole rock 87Sr/86Sr ratios with height in the sill are accouted for by magmatically induced variations in Rb/Sr ratio and by superimposed alteration effects. The absence of significant variation in 143Nd/144Nd ratio is attributable to constancy of Sm/Nd ratio and insensitivity to alteration. Mild oxygen isotope exchange is evident in the altered marginal zones of the sill. Homogeneity of magmatic isotope ratios is indicated. The magmatic Nd and Sr initial ratios of 0.51228 [nNd(182 m.y.) = -2] and 0.70621 [nSr(182 m.y.) = +27] fall off to the right of the Nd-Sr mantle array, in the "enriched" region. The inferred values of these parameters in the subcontinental mantle source of this Karoo tholeiite remain unconstrained.
Back to title list




Petrogenesis of the volcanic rocks of the Karoo province. Special Publication No.13, 341-354

143Nd/144Nd, 87Sr/86Sr, Sm and Nd analyses are reported on suites of Karoo volcanic rocks from the four sub-areas of Nuanetsi-north Lebombo, south Lebombo, the Central area, and north-west SWA/Namibia. Only seven (12%) of the samples analysed have positive nNd values similar to those found in the majority of recent mantle-derived rocks. Most of the rest have negative nNd (-1.0 to -17.1) and positive nSr (+3.0 to +240) and thus must contain at least a contribution from source areas which were both old, and had lower Sm/Nd and higher Rb/Sr ratios than the bulk earth. Such trace element ratios are typical of the continental crust, and it is envisaged that crustal contamination will tend either to generate some broad mixing relation between the original magma and a crustal component, and/or disrupt any pre-existing relationship between isotope and parent/daughter trace element ratios with the result that the two become "decoupled". Such decoupling is observed in some basalts from the south Lebombo which may have been contaminated with Archaean crust, but the majority of the Karoo volcanics studied have not been affected significantly by crustal contamination processes. Rather, different styles of trace element enrichment are recognized in both mantle xenoliths and Karoo basalts, and with time these result in different trends on nNd-nSr diagrams. The low 143Nd/144Nd and high 87Sr/86Sr ratios of many basalts and mantle xenoliths suggest that they at least are derived from within the continental lithosphere; since that is where variations in Sm/Nd and Rb/Sr ratios are likely to persist for long enough to generate the observed range in Nd- and Sr-isotopes. Finally, the evolution of the subcontinental lithosphere beneath southern Africa is provisionally described in terms of a two-stage model. Much of the lithosphere appears to have stabilized 1.4-1.0 b.y. ago, which is also the time of significant crustal growth within the Namaqua-Natal mobile belt. It is argued that the two are related, and that increasing the area of stable continental crust also increased the volume and perhaps even the thickness, of material incorporated into the subcontinental lithosphere. Karoo magmatism at ~190 m.y. was then followed by, and may have been responsible for, a second mantle enrichment event now observed in, for example, the modal metsomatism of K-richterite-bearing peridotite xenoliths.
Back to title list




Petrogenesis of the volcanic rocks of the Karoo province. Special Publication No.13, 355-388

The extrusive and intrusive rocks of the Karoo Igneous Province are dominantly of basaltic or rhyolitic (sensu lato) composition. There are, however, a considerable variety of other rock types within the province including picritic basalts, nephelinites, shoshonites, latites, andesites and dacites. Detailed geochemical comparisons of basic rocks in different geographic areas show that there are three "clans" of basic magma types in the Karoo Igneous Province. These clans have their type areas in the Nuanetsi-Lebombo area, the Central Karoo area and the Etendeka area of SWA/Namibia. There is little or no relationship between the composition of Karoo basic magmas and the age of character of the major crustal tectonic units on which they were erupted. Detailed studies have shown that most differences in incompatible element ratios and in Sr- and Nd-isotope ratios between Karoo basic magmas are not due to crustal contamination, partial melting or fractional crystallization processes. They are therefore considered to indicate mantle heterogeneity on a variety of scales. Many of all of the mantle source regions for Karoo basalts must be enriched in incompatible elements and comparison with mantle nodules included as xenoliths in kimberlite suggests that "mantle metasomatism" may be the enrichment process. Rhyolites and quartz latites in the Karoo Igneous Province are located only near the continental margin of southern Africa. They are thought to have been produced by anatexis at or near the base of the crust during crustal thinning which immediately preceded the disruption of Gondwanaland. The Karoo basalts are not concentrated near the continental margins and the acme of Karoo volcanism pre-dates the disruption of Gondwanaland. The distribution, age and inferred lithospheric source of Karoo magmas is consistent with a model which relates the generation of Jurassic-Cretaceous Gondwana flood basalts to a major convective roll in the mantle induced by subduction beneath the Pacific margin of Gondwanaland.
Back to title list




Namaqualand Metamorphic Complex. Special Publication No.10, 67-82

A protracted history of episodic granitoid plutonism characterizes the Precambrian terrain covered by the Namaqualand geotraverse. The transect from the Orange River in the north to Springbok in the south is subdivisible into at least three tectonic domains, each possessing its own distinctive history of granitoid development. Record of events responsible for the formation and consolidation of the earliest known continental crust in the region is only preserved in the Vioolsdrif domain. Mid-Proterozoic mantle differentiation to produce new continental crust by calc-alkaline magmatism occurred 2 000-1 900 Ma ago and provided the protolith for subsequent Precambrian intracontinental events such as reworking and final cratonization. Granitoids in the Vioolsdrif domain show I-type petrographic and geochemical characteristics, with intermediate rock types predominating. Later silicic plutonism at 1 730 Ma represents the products of the first reworking of the original crustal protolith in this region. The early igneous history of the Steinkopf domain is dominated by silicic plutonism, which probably occurred at about the same time as similar events were taking place in the Vioolsdrif batholith. In contrast to the predominantly I-type Vioolsdrif granites, the Gladkop gneisses show a distinct S-type geochemistry, but which may have resulted from assimilation of sedimentary country rock by the granitoid precursors, as well as by tectonic admixing during the tectonothermal processes which produced the gneisses. Post-Gladkop granitoids are not very well developed in the Steinkopf domain, but where studied have proven to be silicic S-type granitoids characterized by muscovite and garnet, and perhaps by a strongly heterogeneous soruce region. Post-Gladkop granitoid plutonism is well developed in the Okiep domain, which is composed mainly of augen gneisses of the Little Namaqualand Suite, and thick sheet-like plutons of later granitoids of the Spektakel Suite. The granitoid precursors to the augen gneisses were dominantly silicic and depleted in Sr relative to older granitoids in the geotraverse. Generation of parental silicic magmas to the Little Namaqualand Suite by crustal anatexis probably accompanied the onset of high-grade metamorphism about 1 250 Ma ago. The crustal source to the Little Namqualand granites was probably similar in age and composition to the intermediate-silicic Vioolsdrif-Gladkop granitoids. The syntectonic Little Namaqualand Suite therefore represents the products of a second reworking of the 2 000-1 900 Ma old crustal protolith in south-western Africa. In contrast, the late to post-tectonic Spektakel granitoids have geochemical and isotopic features that permit parental magmas which are too mafic for their derivation by crustal anatexis. This particular magmatic event at about 1 170 Ma ago may represent the products of mantle-derived potassic syenitoid melts which were allowed access to the Okiep crustal segment.
Back to title list




Namaqualand Metamorphic Complex. Special Publication No.10, 83-89

890 Samples from the Proterozoic Namaqua and Richtersveld provinces of South Africa were analysed by X-ray fluorescence for major and trace elements. High K2/Na2 and Rb/Sr ratios coupled to relatively high concentrations of Ba, Zr and Y characterise and distinguish the average analysis of teh granitic rocks of this region from those of many other portions of the earth's Precambrian crust. Geochemical differences throughout the Namaqua Province were found to be relatively slight and insignificant. The general siliceous nature of the rocks and their high K2/Na2 and Rb/Sr ratios of approximately 2 are manifested throughout the region. The average analysis of samples from the structurally higher Richtersveld Province was consistently more basic than the underlying members of the Namaqua Province with the K2/Na2 ratio close to unity and the Rb/Sr ratio about one half. Chemical evidence lends general support to the structural and stratigraphic model proposed by the Geological Department of teh O'okiep Copper Company involving an intercalation of sedimentary strata and sub-horizontally implaced granitic sills as opposed to arguments expressed by others favouring a metasomatic conversion and re-mobilisation of a former sedimentary assemblage. Rb/Sr ages published on the various stratigraphic members in the Okiep Copper District are in the range of 1 270 Ma to 1 170 Ma. However, the initial 87Sr/86Sr ratios are problematic giving intercept values between 0,728 and 0,708. This was ascribed by Clifford et al. (1975) to the anatectic reworing of differing source materials with diapiric emplacement and fractionation approximately 1 200 Ma ago. However, by using known average Rb/Sr values for each rock unit and assuming a common low isotopic source for all the igneous rocks their primary age can alternatively be calculated to range between 1 800 Ma and 1 250 Ma with the older age corresponding approximately to the youngest ages determined on the lower grade and structurally higher units of the Richtersveld Province. It is therefore held that the intrusive and metamorphic history of the Okiep area was probably more protracted than present isochrons suggest. The proposed model calls for an extensive episode of Proterozoic crustal thickening via processes of granite underplating from below and sedimentary accumulations from above, coupled to repeated episodes of tectonic reworking, to accomodate the complex interplay of metasediments, granulites, gneisses and granites in evidence today.
Back to title list




Namaqualand Metamorphic Complex. Special Publication No.10, 91-100

The interpretation of geoelectrical and gravitational data for the Namaqua-Natal belt is discussed in the light of the main tectonic features of this region. Geoelectrical results indicate that a large fraction of the exposed part of the geotectonic province has cratonic geoelectrical properties, but do not support a continent-continent collision as origin for the Namaqua-Natal belt. Magnetovariational data suggest that the Namaqua-Natal belt could have formed part of an Andean mountain belt, at least towards the end of its evolution. The gravitational data are aslo in agreement with such a cordilleran model. It is shown by means of modelling and age correlation that gravity anomalies can indicate the northern boundary of the mobile belt. The gravity signature across the Namaqua-Natal front resembles gravity profiles across similar boundaries in the Canadian, Indian and Australian shields. Magnetovariational data and static magnetic anomalies indicate the position of the southern boundary of the Namaqua-Natal belt and suggest that it did not form as an intracratonic mobile belt, but originated along a continental edge.
Back to title list




Namaqualand Metamorphic Complex. Special Publication No.10, 101-110

The metaquartzite-schist association at Pella and Dabenoris is tentatively correlated with the metasediments of the Aggeneys Subgroup which forms an ecconomically important component of the Proterozoic Namaqua mobile belt. Their age remains unresolved at present as they occur as tectonic slices within the 1 730 to 2 000 Ma Vioolsdrif-Haib Igneous Complex. Using a stratigraphic and structural approach, the metasediments were subdivided into the arenaceous Dabenoris and Pella formations and the argillaceous Kabas Formation. Lithofacies analyses and sedimentary structures form the basis for interpretation of the palaeoenvironments, from which depositional models suggestive of transgressive and regressive clastic shoreline association are inferred.
Back to title list




Namaqualand Metamorphic Complex. Special Publication No.10, 111-122

The Proterozoic (1 700 to 1 950 Ma) calc-alkaline volcanics and composite batholithic intrusives of the eastern Richtersveld display strong similarities with present-day Cordilleran-type continental margins with regard to composition, sequence of intrusion and deposition, relative proportions of their members and palaeo-physiographic setting. In the south strongly recrystallized metavolcanics of the Richtersveld overlie equivalents of the Namaqualand Metamorphic Complex. Early Richtersveld deformation is extremely heterogeneous and metamorphism increases gradually from low-grade in the central parts to high-grade in the south. Estimates based on the volume and spatial distribution of the intrusives show that the total amount of mantle-derived material appears to be an order of magnitude less than that of the Cenozoic Andes and that it could have been produced during plate tectonic activity of rather short duration. Fluid dynamic calculations from the average spacing and volume of the granodioritic bodies suggest a viscosity ratio of about 1 between intrusives and wall rock during onset of intrusive activity. Considerations based on physical and mathematical models of the deformational effects of high-viscosity diapirs show that most of the heterogeneous structures of the Richtersveld can be explained by diapiric instrusion of Vioolsdrif granodiorite. Transient geothermal models with 700°C hot Vioolsdrif intrusives as heat source suggest that the volume as well as the temperature of these rocks may have been sufficient to generate the metamorphic regime encountered in the north-eastern Richtersveld. Magmatism, deformation and metamorphism may therefore have been mutually interdependent processes which were originally initiated by convective influx of heat from the mantle.
Back to title list




Namaqualand Metamorphic Complex. Special Publication No.10, 123-129

The marginal zone of the mid-Proterozoic Richtersveld Province along the Orange River near Goodhouse, South Africa contains grey gneisses and anatectic granites that were dated by the U-Pb and/or Rb-Sr methods. Zircons from two widely separated localities of gneisses, considered on geological grounds to be older than the Orange River Group supracrustal sequence and the Vioolsdrif granitoids, define a Concordia intercept age of 1 862±17 Ma. This date, regarded as a minimum age, is indistinguishable from published ages for the Vioolsdrif granodiorite. Anatectic granitoids considered to be derived from migmatisation and palingenesis of the Vioolsdrif granitoids, and collectively termed Goodhouse granite, define a Rb-Sr whole-rock isochron with an age of 1 846±76 Ma and an initial 87Sr/86R ratio of 0,704 5±0,001 3. This age is compatible, within error, with the Concordia intercept age of 1 868±41 Ma determined from zircons of two Goodhouse granite samples. Derivation of the Goodhouse granite from the Vioolsdrif granitoids is compatible with Sr-isotopic data and with geochemistry. Since the Goodhouse granite cuts the penetrative fabric in the Orange River supracrustal suite and in the Vioolsdrif granitoids the above Rb-Sr age of 1 846±76 Ma establishes a minimum age for this fabric while its maximum age is 1 880-1 900 Ma. If correlations of structures across the Province boundaries are correct these data constrain the timing of the main fabric in the neighbouring Namaqua Province. Alternatively the structure in the western Richtersveld Province reflects a local tectono-metamorphic event.
Back to title list




Namaqualand Metamorphic Complex. Special Publication No.10, 131-137

The detailed aeromagnetic data of the Okiep Copper District and surrounding area (8 000 Km2) are ideally suited to extract statistical information pertaining to the magnetic and related structural pattern of the area. Altogether 578 first-order aeromagnetic anomaly trends and 250 aeromagnetic lineaments were recognised. The survey area was visually divided into 13 blocks, each block having a distinct dominant aeromagnetic trend. Aeromagnetic anomaly trends striking N60xE have statistical dominance. This trend is perpendicular to the direction of primitive horizontal stress measured in mines in the Okiep Copper District. A positive relationship exists between teh aeromagneitc anomaly trends and the structural pattern of the area. Aeromagnetic lineaments are straight-line disruptions in the magnetic contour pattern and are taken to reflect the basement fracture pattern. A statistical analysis of aeromagnetic lineaments in the survey area revealed three pairs of orthogonal lineament directions. Primary directions of shear faults and breccia faults correspond with the inferred basement fractures. In addition, basement fractures appear to have played a role in the localisation of basic dykes which constitute the host rocks of the Okiep copper deposits.
Back to title list




Namaqualand Metamorphic Complex. Special Publication No.10, 139-146

The results of a 215 Km gravity traverse conducted with high precision along the N7 national road between Garies and Vioolsdrif showed that the thickness of the earth's crust is very consistent along the direction of the traverse. A gravity model shows (that for the densities assumed), undulations of the crust-mantle interface could attain a maximum amplitude of 1,5 Km. A medium wave-length component of the gravity field suggests that the lateral density variations in the upper crust range between 2 650 Kg/m3 and 2 700 Kg/m3. High and low density zones show poor correlation with major outcropping lithology. A magnetic survey along the traverse disclosed a high susceptibility causative body, 21 Km wide, situated between Okiep and Steinkopf. The calculated depth of burial of this body is 3 Km. The major lithological units exposed along the traverse each display a characteristic level of radioactivity. These levels increase in a step-like fashion from the oldest Vioolsdrif Suite rocks to the younger granites of the Spektakel Suite.
Back to title list




Namaqualand Metamorphic Complex. Special Publication No.10, 147-171

The objective of the National Geodynamics Project study was to investigate the nature of the north-east boundary of the high-grade Namaqualand Metamorphic Complex. In order to achieve this, detailed mapping, structural analyses and petrological investigations were carried out by members of the Precambrian Research Unit at the University of Cape Town. Some of the results have already been published by Vajner (1974a b, 1975, 1978), Van Bever Donker (1977, 1978, 1980), Van Zyl (1979, 1981) and Stowe (1979, 1980) and details will be documented in bulltin form. The aims of this report are to synthesise the stratigraphy, structure and igneous activity and metamorphism of the area and to consider the implications for structural evolution and geodynamic processes in the region.
Back to title list




Namaqualand Metamorphic Complex. Special Publication No.10, 193-198

The area investigated is underlain by metasedimentary rocks intruded by granitic rocks. Situated in the north-western Cape Province, it comprises some 1 800 Km² along the Orange River, west of Upington. Several shear zones divide the area into blocks; from the west to east these are the Kakamas shear zone, the Duivelsnek shear zone, the Neusspruit lineament, the Brakfontein shear zone and the Keboes shear zone. Four deformation phases were recognised, the first of which (F1) resulted in the formation of the main penetrative foliation and isoclinal folds. The main deformation event was the second phase of folding, characterised by strong flattening in the vicinity of the major shear zones. Axial planes of this generation of structures generally dip to the north-east. The dip varies between 30x and 50x and increases to about 60x in the vicinity of the shear zones. The effects of the third deformational phase were prominent in the region west of the Neusspruit lineament, where major interference patterns between the north-east-trending F3 and the F4 folds were formed, whereas only one interference dome has been observed to the east of the lineament. The final deformational phase F4 manifested itself as sets of kink folds in several places. The main metamorphism M1 reached high-medium to high-grade conditions and is thought to coincide with F2, possibly even persisting from F1 onwards. A second metamorphic event, M2, is characterised by contact metamorphic assemblages as a result of the emplacement of hypersthene-bearing granitoids. These intrusions undoubtely postdate F2, but appear to have undergone some shearing. Vertical displacement along the two main shear zones (Neusspruit lineament and Keboes shear zone) was between 5 and 10 Km, where the former is thought to have accommodated considerably more displacement than the latter. Horizontal displacement along the Keboes and Brakfontein shear zones amounted to some 16 Km dextral. Horizontal movement along the Neusspruit lineament amounted to some 9 Km. A dynamic model consisting of a combination of block faulting and wrench-fault tectonics and crustal shortening as a result of crustal arching explains the kinematics of the structures mentioned above.
Back to title list




Geodynamics of the Cape Fold Belt. Special Publication No.12, 1-6

The Cape Fold Belt (CFB) is a composite of two orogenies: the Saldanian tectonism of the Pan African event (Hartnady et al., 1974) and the Cape foldings of the Gondwanide orogeny. The National Geodynamics Project successor to the Upper Mantle Project, aimed to elucidate the nature of deformative processes in the deeper lithosphere, particularly in eroded orogens. Although the geotectonic setting of the CFB did not ideally meet the requirements, it was chosen for investigation because of the magnificent exposures and high relief in inviting fold and analysis from mega-scale to micro-scale. Together with a study of cleavage the new data were expected to clarify the kinematics and dynamics of deformation in the fold belt, including the relative involvement of basement and cover during the Gondwanide orogeny. The CFB consists of a western branch of open, upright megafolds, monoclines and normal fautls nearly parallel to the north-north-west strike of the Cape Supergroup from Stellenbosch to Vanrhynsdorp (~270 Km); a southern branch of northward verging folds sliced by thrusts and normal faults trending roughly east-west from Swellendam to Port Alfred (~600 Km); and an intervening syntaxis area about 100 Km wide with varied upright folds and faults of north-east strike, as between Gordons Bay and Bredasdorp. In all three domains the highest ranges are built mainly of quartz arenites of the Table Mountain Group. In the planning stage of the programme seven profiles across the mobile belt were considered: two in the north-west between Clanwilliam and Tulbagh and five across the southern fold belt from Robertson to Alexandria, omitting the syntaxis region south-west of Worcester. Photogeologic study and field reconnaissance led to the final selection of a geotraverse spanning the Outenique and Swartberg ranges between George and Beaufort West. This would include two windows of pre-Cape formations, zones of maximum fold intensity in basement as well as cover, and the broad tract of diminishing deformation toward the undisturbed Karoo platform. With funds provided by the CSIR the work was carried out by students under leadership of Prof. I.W. Halbich and Dr A.E. Schoch. Fieldwork (outside the geotraverse) started in 1972 by arrangement with the Geological Survey, while the main programme was launched in mid-1974. Mr D.K. Toerien of the Geological Survey concurrently provided information far beyond the boundaries of the study strip.
Back to title list




Geodynamics of the Cape Fold Belt. Special Publication No.12, 7-19

The pre-Cape metasedimentary sequence of the George area has been reclassified in accordance with the new stratigraphic code. The Kaaimans Group consists of seven formations named in tectonic sequence as the order of sedimentary superposition is obscured by metamorphism and tectonism. The rocks are primarily fine-grained metapelites and psammites and their field features are described with particular emphasis on lithostratigraphy and structure. The metasedimentary rocks have been intruded by subconcordant bodies of syntectonic granite showing well-developed foliation and lineation parallel to the structure of the envelope. Two phases of pre-Cape deformation are distinguished. During F1 the penetrative foliation S1 and mineral lineation l1 were formed. L1 is interpreted as a stretching lineation parallel to the a1 kinematic direction. It was regenerated during F2 and originally northward facing overfolds became subparallel to l1 (a1 w a2). The F3 folds of post-Cape age are plane and non-plane noncylindrical, with axial trends oriented approximately east-west. Associated with these is a system of conjugate folds and well-developed S3 crenulation cleavage. As a result of F3 folding the lineation locus of deformed l1 is discontinuous. Thus F1, F2 and F3 are characterized by north-south compression. The fourth period of deformation, F4, is featured by brittle fracturing ascribed to north-south extension after the peak of orogenesis involving all the formations.
Back to title list




Geodynamics of the Cape Fold Belt. Special Publication No.12, 21-32

The granitic plutons of the George area encompass two cycles of magmatic activity. The first and major intrusion occurred during the early Cambrian when a bulging sill of Maalgaten granite and an overhead sheet of Rooiklip granite gneiss were emplaced in south-dipping formations of the Kaaimans Group. Textural evidence on the internal structure of the Maalgaten pluton indicates that the upper part cooled more rapidly than the lower and that synintrusive deformation (F1) accompanied its consolidation. The Rooiklip sheet, injected at a higher level, is noted for the presence of fluorite, suggesting emplacement at somewhat lower pressure. Dykes of fine-grained quartz-muscovite albitite intruded the metasediments to the north of the Maalgaten granite contact as late-magmatic differentiates. The Kleinfontein granite is probably also a syntectonic intrusion emplaced during the second period of deformation (F2). Isotopic ages indicate that the second cycle of granite magmatism occurred during the Ordovician when localized anatexis produced the Modderkloof granodiorite and Rooiklip leucogranite. The isotopic ages thus far determined can be taken only as a first approximation (no isochrons obtained), but are in accord with field evidence that these bodies crystallized when F2 deformation was ending. Petrochemical data indicate that the various granites do not represent a differentiation series evolved from a single source. The first period of metamorphism (M1) associated with F1 produced high-grade contact effects round the Maalgaten granite concurrently with lower-grade regional metamorphism farther away. During the second metamorphic event (M2) thermal conditions corresponded to upper low-grade while intensive deformation (F2) took place in the metasediments and the older granites. Associated with the Permian orogeny of the Cape Fold Belt the pre-Cape formations were subjected to lower low-grade retrogressive metamorphism (M3) under further diminishing pressures.
Back to title list




Geodynamics of the Cape Fold Belt. Special Publication No.12, 47-56

The structural history of the Kango Group begins with a sinking basin in which sediments of the Goegamma Subgroup accumulated until major uplift with associated F1 folding of north-westerly trend ended the cycle. A period of relative stability and erosion led to fluvio-littoral sedimentation followed by a second pulse of subsidence and final uplift of the Kansa Subgroup. Severe diastrophism at this time produced tight F2 overfolds striking east-west and verging north, while earlier structures in the Goegamma floor were in part transposed to line up with the superjacent fold elements. The climax was reached when two sheets of Kango strata broke away from the autochthon and were thrust northward probably 5-10 Km. Isotopic dating of post-thrust diabase indicates that the Kango tectonic cycle and ended about 800 Ma BP. The deformation of the Cape orogeny was constrained by a comparable array of crustal stresses that generated giant F3 folds verging north and exemplified by the broad anticline framing the Kango inlier. The depth to which this structure affected the pre-Cape rocks is controversial. The final episode in the tectonic evolution of the Kango was a response to crustal extension whereby major gravity faults let down the rock formations to the south. A geometrical analysis of meso- and microstructural elements reveals that very mild cross folding apparently occurred about south-plunging axes that approximate to the a-kinematic lineation of rotated pebbles in the conglomerate units. This effect may relate to the grain of already tectonized basement.
Back to title list




Geodynamics of the Cape Fold Belt. Special Publication No.12, 75-100

The meso- to mega-structural style across the fold belt is zonally arranged in Cape and Karoo strata and may change abruptly. Two new profiles are used in the analysis (Folder 1). Zone 1, the northernmost, is characterised by listric thrusts and initial folding with only a few per cent of shortening in Karoo beds. Strain in quartz is at low levels and deformation lamellae occur only in single non-regular sets as elongated subgrains. Equilibrium conditions were maintained at intra-crystalline strain rates of 10(-9)/s while temperatures reached 210°C and pressures were ~1,5 kb. Quartz microfabrics are weak but distinctly symmetric to the mesofabric. These features evolved during a fourth paroxysmic event 230 Ma ago. Southwards, zone 2 displays mild low amplitude upright and rounded folds that post-date listric thrusts. About 8% shortening has occurred. A fanning weakly developed axial plane solution cleavage, also found in zone 3, resembles stylolites along which water was extruded. There is evidence of internal longitudinal strain related to folding, but the microstructure and P, T, e conditions are still very similar to those of zone 1. A distinct quartz fabric strongly resembling that of zone 1 becomes passively rotated. Folding with a cleavage-forming paroxysm occurred 230 Ma ago and listric thrusting may relate to an earlier, third event around 247 Ma. (The abstract continues with discussions of all 6 zones.) The quartz microstructure of the fold belt reveals that very few deformation lamellae develop as slip planes, yet their fabric under all conditions is symmetric to that of mesostructures and readily interpreted according to theoretical slip models. Model experiments that test cover-basement interaction at cover material viscosity ratios similar to those expected during long-term natural stresses in rocks, demonstrate that sequential folding may be controlled by basement discontinuities in irregular fashion, as has occurred during the Cape orogeny. At low temperature and load and high pore water pressure, flexing by elastic rebound, following on sudden rupture of a sedimentary sequence under tectonic stress, should be considered a viable alternative to some slow strain folding.
Back to title list




Geodynamics of the Cape Fold Belt. Special Publication No.12, 101-113

Mega-folds in the Cape and Karoo rocks along 22°30'E longitude are of variable style, but have initially all formed by flexural slip, aided locally by gravity. Asymmetrical limb attenuations of second-order flattened flexural-slip folds in the Table Mountain Group, and the homogeneous structural impress in the cover rocks agree better with a crustal compression model for the Cape Fold Belt than with gravity-gliding.
The principal stress axes, V1 and V3, were probably rotated about V2 ˜ B during deformation to assume different orientations with respect to the horizontal at different times. In the Table Mountain Group bedding-plane slip led to overthrusting along normal limbs of megafolds. Structures similar to those of the Fold Zone in the western Cape developed at the base of the Cedarberg Formation in Meirings Poort. The typical fold style and lack of cleavage in the Witteberg and Ecca groups, coupled with the absence of metamorphic minerals (except micas and quartz) along the entire profile imply low temperature (˜300°C) and low confining pressure (2-3 kb) during tectogenesis. Soft sediment deformation in the Dwyka Tillite suggests that the tectogenesis commenced about the beginning of the Permian. There is no regular change in fold intensity from Meirings Poort to the contact of the Beaufort Group on Zeekoegat, the spacing between mega-folds being rather unequal. From there on northwards the fold intensity decrease abruptly. The major folds of the Groot Swartberg and Droekloof ranges are probably related to the thickness and distribution of lithostratigraphic units along the east-west zones determined by epeirogenesis during sedimentation.
Back to title list




Geodynamics of the Cape Fold Belt. Special Publication No.12, 115-123

Second-order disharmonic folding and northward directed overthrusting have been known in the Cape Fold Belt (CFB) for some time. Here it is shown that the former is a result of decoupling along numerous horizons, expecially in the Nardouw Subgroup. The most profound décollement, however, occurs along the Cedarberg Shale Formation (Oc) which, although only several tens of metres thick, is a continuous stratigraphic horizon within some 3 000 m of Table Mountain Group arenites in the southern and western Cape Province. Detachment along Oc leads to disharmony of major folds where fold intensities are moderate, but develops into overthrusts on steep and overturned limbs where intensities increase. Along the Swartberg Range deformation intensities generally increase eastwards. East of Prince Albert the first mappable forelimb thrusts occur and the thrust-plane terminates in Oc. Folding seems to have been succeeded by thrusting and was then resumed to deform the thrust-plane. This has resulted in the juxtaposition of two styles and attitudes of mega-folds. Sequential folding triggered by primary irregularities in the stratigraphic column, may produce such styles. A penetrative S1 cleavage is, statistically speaking, axial planar to the first generation of folds. A second fracture cleavage S2, with a slip component, develops on normal limbs of mega-folds. One set of stresses produces bedding décollement along normal limbs and S2. At this stage of deformation large topographic differences may have existed near the vertical limbs of mega-folds. Consequently a gravity component was locally added to the regional horizontal stress with the result that the maximum principal stress vector becomes inclined to the north. The same stress-orientation explains the thinning of normal limbs of second-order folds (Chapter 9). Single and conjugate kinks represent the latest structures which again favour near horizontal north-south compression. Southward-dipping thrusts in the Meirings Poort profile are readily interpreted as branches of a sole that follows a stratigraphic horizon in the Bokkeveld Group and ends in a northward-dipping bedding décollement along Oc. On a regional scale thrust phenomena become more frequent east of Meirings Poort towards Snykloof, Toverwater Poort and the Baviaanskloof Range. Fold intensities also increase up to longitude 24° east. The Baviaanskloof thrust history fits the multi-stage deformation model rather well.
Back to title list




Geodynamics of the Cape Fold Belt. Special Publication No.12, 125-130

The "Fold Zone" of the western Cape Province is re-interpreted using the facts gathered by Blignault (1970). A similar fold zone has been discovered beneath the same horizon, the Cedarberg Shale Formation in the southern Cape Province at Meirings Poort and Swartberg Pass, and a new model is developed to explain both phenomena without relying on glacial action. In either case the localised disharmonic folds parallel the prevailing tectonic trends, the vergency is towards the tectono-sedimentary basin axis and an erosion surface truncates the structures. The fact that this erosion was by ice in the west and by water in the south has no bearing on the origin of the folds. The latter are explained as synsedimentary slump-folds sliding down the regional gradient during a period of change in epeirogenic and environmental conditions. This also led to restricted glaciation in the west and to local and intermittent lifting of the sedimentary interface above base level of deposits, followed by erosion and overall deposition of the Cedarberg Shale marker. These events happened some 440 Ma ago during retrograde metamorphism in the basement rocks (Chapter 12). In the southern CFB some of the synsedimentary slumping and erosion is probably masked by later tectonic folding and décollement.
Back to title list




Geodynamics of the Cape Fold Belt. Special Publication No.12, 131-148

Illite crystallinity and cleavage style of pelites are compared along a geotraverse across the Fold Belt in an attempt to trace the metamorphic history of the cover rocks. A survey of recent European literature on illite crystallinity studies leads to criteria for the recognition of low-grade metamorphic zones. Previous work on metamorphism in the Cape Fold Belt and the constraints for illite crystallinity work are outlined. A brief summary is given of experimental procedures for selecting the most effective techniques of preparation and analysis, followed by a discussion on the results. A crystallinity index U S Hbrel is defined using a silicon standard, and it is linked directly to published Weber- and K bler-indices via a correlation curve. The microstructure (optical and electron scan) of cleavage from a representative suite of rocks including the crystallinity samples, is illustrated and classified. The cleavage classes or styles correlate very well with the grade of metamorphism as defined by illite crystallinity. Three crystallisation temperatures determined on fluid inclusions of syntectonic quartz veins fit quite well into this picture. The youngest and most northerly exposed rocks (Beaufort Group-middle Permian) affected by tectogenesis display only a very feebly developed, single, axial plane solution cleavage occasionally resembling stylolites. Scarcely discernible preferred orientation of extremely small illite grains is occasionally seen parallel to this cleavage. A middle anchi-grade of metamorphism is never exceeded. P-T estimates are: 1,5 kb/210°C. Two or more transverse cleavages are found only in Bokkeveld (Devonian) and older beds. These structures first developed as solution planes along which parallel recrystallisation of chlorite and sericite subsequently took place. The same minerals also occur as books parallel to bedding and truncated by the later cleavage. It seems as if load metamorphism and tectonic regional metamorphism of very similar grades (upper anchi-grades) were reached at P-T conditions = 1,5 kb/300°C. Epizonal grades are attained farthest south in the Outeniqua Mountains where P-T is estimated at >2,5 kb/350°C, and biotite has occasionally grown parallel to the cleavage.
Back to title list




Geodynamics of the Cape Fold Belt. Special Publication No.12, 149-164

The timing of the Cape orogeny and its sequential development with the aid of 40Ar/39Ar step heating analysis on selected fine-grained rocks from the southern Cape Fold Belt is discussed. Four age steps, some of which are isochrons, are repeatedly confirmed from various parts of the belt and interpreted as successive deformations producing distinct co-zonal cleavages. The Cape Fold Belt therefore originated as a single phase, multiple event orogen during the Permian and the Triassic. The syntectonic sedimentary history and metamorphic evolution are correlated with the tectogenesis.
Back to title list




Geodynamics of the Cape Fold Belt. Special Publication No.12, 177-184

Prerequisities and prejudices that go along with geodynamic modelling are mentioned and the evidence not available for the Cape Fold Belt and most of the contemporaneous southern hemisphere mobile zones is listed. A subduction model does not seem to apply to these orogens, but several characteristics of ensialic Pan African belts are also recognised in the Gondwanide orogenic and tectono-sedimentary cycles. The rather fragmentary nature of all data suggests that the diastrophic history of the Late Palaeozoic to Early Mesozoic evolves from that of earlier cycles. The CFB orogeny was short-lived, which is compatible with a mantle that can not sustain long-term convection.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 36-45

In the Stoffberg area a portion of the Main Zone, and the whole of the Upper Zone of the Bushveld Igneous Complex have been emplaced. The floor of the Bushveld Complex consists of the Dullstroom andesite which belongs to the Smelterskop Stage of the Pretoria Series, whereas the "roof" is made up of various types of felsitic rocks (Rooiberg felsite). The structure of the region is characterised by a gentle westward dip. A fault, referred to as the Laersdrif Fault, has displaced the sedimentary and the igneous rocks. The maximum separation of the Main Magnetitite Seam is about 20,000 feet, and the vertical displacement about 3,700 feet. North of the fault the dip is from 0 to 15°W, and south of it the dip gradually increases to about 25°W. The gabbroic rocks of the Main Zone are composed of layers of hypersthene gabbro, hyperite and norite. A layer of magnetite gabbro is interlayered in the gabbroic rocks. Farther south this horizon is occupied by a peculiar type of black gabbro which was found to owe its colour to the presence of minute grains of magnetite in the plagioclase feldspar. On the ground of the magnetite contents, the black gabbro is correlated with the layer of magnetite gabbro farther north. The magnetite gabbro of the Upper Zone follows on the gabbroic rocks of the Main Zone. In the southern part of the area the two zones are separated from each other by a large xenolith of pyroxene hornfels and by micro-granite of uncertain derivation. Magnetitite seams are present only in the magnetite gabbro suite which is made up of various rock types including hypersthene gabbro, hyperite, norite and olivine-hypersthene gabbro. In all these rocks magnetite is present in amounts of from 0.5% to as much as 15%. Four subzones of magnetitite viz., A, B, C and D occur in the magnetite gabbro of the Upper Zone. The seams of Subzone B are economically the most important in that they contain about 1.86% V2O6. The so-called Main Seam of this Subzone is some 6 feet thick in the Stoffberg area. Higher up in the sequence the V2O6 content of the seams decreases and is about 0.6% in Subzone C, and about 0.3% in Subzone D. Several magnetitite pipes of a composition similar to that of the nearest seams are present in the area.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 228-241

The area investigated at Magnet Heights covers six miles of strike of the best exposure of the uppermost 6,000 feet (Upper Zone) of the layered sequence of the Bushveld Complex. Monomineralic accumulate rocks are magnetitite, anorthosite and pyroxenite. The remaining layered rocks are predominantly gabbroic. Each of the 30 odd iron ore seams can be identified by its V2O5 content, thickness and field relationships. Anorthosite is more abundant than magnetitite and is often intimately associated with the latter, e.g. the Main Seam is underlain with a sharp contact by five feet of pure anorthosite. Most of the seams have a transitional upper and a sharp lower contact, some are gradational both ways, but Seam No. 13 has a sharp upper contact and it is transitional downwards into gabbro. Pyroxenite is the rarest monomineralic rock and only one true layer was detected. There is no fundamental difference in the mineralogy of the ore in the gabbroic rocks, magnetitite seams and plugs. In the plugs and uppermost seams there is greater evidence of volatile activity. The constituents of the magnetitite which are predominantly magnetite, ulvospinel, ilmenite and pleonaste are to all intents similar to those previously described from the Skaergaard and other basic intrusions. A phase not previously described is a mineral, probably diaspore exsolved on the (111) plane of the magnetite. Previous workers identified it as pleonaste. The rhythmic occurrence of magnetitite seams is probably due to intermittent precipitation of magnetite near the floor of the magma chamber. Homogenisation of the magma may have taken place by periodic convective overturns. The magnetitite plugs are more difficult to explain but seem to be pegmatoid bodies.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 367-379

Published accounts of the occurrence and the distribution of shock-metamorphic features, shatter-cones, and shock-breccias, and geometric analogies with known impact-structures, appear to have established the Sudbury lopolith of Canada and the Vredefort ring-structure of South Africa as produced by hypervelocity impacts. The Vredefort structure consists of a central uplift (the Vredefort dome) surrounded by a ring-syncline, which in turn is surrounded by a ring-anticline, the entire complex probably having been produced by a surficial (cometary impact?) explosion. The Bushveld Igneous Complex of South Africa is of about the same age, 1,900 to 2,000 m.y., as the Vredefort ring, and fills three ring-structures which here are inferred to have formed by impacts simultaneously with the Vredefort event. Geologic and gravity maps indicate the Bushveld Complex to be a composite mass that fills three adjacent ring-synclines, each larger than the adjacent Vredefort syncline. The mafic igneous rocks of the Bushveld are limited to the deep, inner parts of the ring synclines, whereas the capping silicic rocks lap onto the highly deformed central uplifts of each circular structure.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 391-424

The lower, ultramafic parts of the Stillwater Complex and the Muskox intrusion are composed of stacks of cyclic units in which the cumulus phases are repeated in a regular order. The successions of cumulus phases in any one cyclic unit are found to correspond to liquidus mineral sequences in experimentally determined simplified basalt systems. Stacks of successively higher cyclic units tend to begin with progressively lower-temperature cumulus phases, to contain more iron-rich minerals, and to gradually become thinner upward in the section. Chromite cumulates are an early and integral part of the cyclic units in both intrusions, and, in the Stillwater Complex, platinum group metals are concentrated in the chromitites. Study of the published descriptions of the Great Dyke and Bushveld Complexes suggests that similar cyclic units may be present in their ultramafic sections. If this is so, then the similar mechanisms of interrupted fractionation must have operated to form the major style of layering in the four complexes. In addition, the character of the cyclic units indicates some differences in original magma compositions.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 441-476

From successions of cumulus minerals contained within repeated stratigraphic (cyclic) units in the layered series of the Muskox intrusion it is shown that the order of crystallization of the magma changed from olivine; clinopyroxene; plagioclase; orthopyroxene during the early stages of solidification to olivine; orthopyroxene; clinopyroxene; plagioclase during later stages. At the level where the change is apparent, the intermediate order olivine; clinopyroxene; orthopyroxene; plagioclase is locally evident. A crystallization model is presented whereby the different crystallization paths of the magmatic liquid may be compared, and it is shown that they probably developed through a process combining (a) differentiation by fractional crystallization; (b) repeated replenishment of the magma composition through additions of new magma; and (c) contamination of the magma with salic material melted from the intrusion's roof-rocks. The model is then used to examine the crystallization sequences of several other layered intrusions in relation to the compositions of their chilled margins. Finally, it is shown that the four minerals listed above may have as many as 30 different crystallization orders in basaltic magmas, and that these may give rise to several dozen different cumulate sequences depending on the exact path of the magma. Many of the crystallization paths require only slightly different starting compositions for the magma - with water content and the ratios CaO/Al2O3 and Na2O/Al2O3 being especially important parameters - or slightly different load pressures.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 492-510

The post-Permian Dufek intrusion of the northern Pensacola Mountains consists of an extremely thick series of layered mafic cumulate rocks showing progressive, strong Fe enrichment stratigraphically upwards. The body is incompletely exposed in two non-overlapping partial stratigraphic sections each about 2 Km thick: one, in the Dufek Massif, forms a lower - but not basal - part; the other, in the Forrestal Range, forms an upper part. The total thickness is estimated to be at least 6 or 7 Km. The mafic rocks are exposed over an area of about 8,000 sq Km, but geophysical data suggest that they occupy a far greater area beneath the ice sheets. These chemically and mineralogically varied rocks are believed to form a single comagmatic series. Smooth chemical trends through the layered mafic series and into a 300-m-thick granophyre cap point to a close genetic tie between the contrasted rocks. Trends of most elements broadly parallel those of the Skaergaard and other highly differentiated, layered mafic intrusive bodies. Conspicuous differences exist, however, and are believed to be due to differences in composition of parental magmas. The Dufek intrusion, for example, remains SiO2-saturated throughout, so that normative olivine is excluded and minor quartz is present even in rocks with SiO2 as low as 45%. Chilled border rocks, where exposed, are highly enriched in Fe and so do not represent the parent-magma, which is believed more likely to resemble hypersthene tholeiite of Ferrar sheets of the Transantarctic Mountains. Labradorite cumulate comprises, at many levels, relatively thin but conspicuous interlayers with sharp basal contacts and gradational tops which may be basal members of much thicker "cyclic" units. The rocks define a felsic chemical trend, at variance with a mafic trend, which leads smoothly into capping granophyre. Scour features at the bases of labradorite-rich layers indicate operation of lateral currents, probably convective in origin. Silicic products of differentiation include dikes of alaskite, aplite, and pegmatite in addition to capping granophyre. Differences in major- and minor-element chemistry of dike-rocks appear related more or less to stratigraphic height of host-rock layers. Significant differences exist between dike-rocks and granophyre and point to a different origin.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 511-531

Turkey is a country where ultramafic rocks and podiform chromite deposits are widespread. Extrusive submarine "ophiolitic" flows as spilites, sodium-keratophyres, pillow-lavas, their diabases and associated radiolarian cherts also have a regional distribution. Although these two rock units often are associated in space they are not in time. Most of the ultramafic massifs in Turkey are in an allochtonous or semi-autochonous position and nothing is known about their exact magmatic age. Recent investigations show that the concept of Dubertret, viz. that there is no fundamental difference between stratiform ultramafic and the alpine-type of ultramafic complexes, can no longer be upheld. Analyses of ultramafic rocks, as well as gabbroic rocks, diabases and extrusive basic rocks from Turkey are presented, together with graphs on which the Mg:Fe ratio is plotted against the SiO2 content of these rocks, for two different districts. Analyses of chromite ores of a large district are given and the ratios of Cr:Fe against Cr:Al are presented on a graph. The origin of nodular ore is discussed, and microtube photos show that a certain exchange of Al, Fe, Mg and Ni takes place during the process of their formation. The shapes of some typical bodies of Turkish chromite are given. It is suggested that the Turkish ultramafic bodies are sliced-off parts of the upper mantle which were still hot enough to yield to flowage. The result was the disruption of once continuous bodies of chromite which now show foliation, lineation and boudinage. During this process the lithological differences existing in the upper mantle were not destroyed as a whole. In some rare cases it is possible to demonstrate that the ultramafic rocks have a gneissoid structure in which older strained olivines are recemented by broken-up and recrystallized unstrained olivines. The thermal effect of these almost crystalline bodies on the country-rock was slight. The "ophiolitic" extrusive suite with its diabases might have been derived from deeper parts of the mantle by refusion. It ascended along zones of weakness and penetrated deep into the crust. It was revived several times during tectonic disturbances.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 532-546

The sequence of rocks occurring around the North and East Ranges of the Sudbury Nickel Irruptive is distinctly different from the sequence around the South Range. On the North and East Ranges, 1,000 to 1,500 feet of orthocumulus "felsic norite" are overlain by about 700 feet of orthocumulus "oxide-rich gabbro". The gabbro grades over 300 feet into 4,500 feet of granophyre, the "micropegmatite". On the South Range the "upper gabbro" and micropegmatite resemble the North Range oxide-rich gabbro and micropegmatite. The upper gabbro is underlain by the mesocumulus "South Range norite" which differs from the felsic norite in containing a higher proportion of hypersthene to augite and less quartz. The South Range norite is separated from the underlying footwall rocks by a 1,500-2,000 foot thick layer of norite (the "quartz-rich norite") across which the content of quartz and micrographic intergrowth of quartz and feldspar increases progressively from 8 to 24 modal per cent and the average grain size of plagioclase decreases from 1.10 to 0.75 mm towards the lower contact. Cryptic variation is present in the Irruptive rocks of the north and east ranges with plagioclase becoming more sodic and the pyroxene more iron-rich upwards in the sequence. On the South Range, plagioclase also becomes more sodic upwards. Little cryptic variation in pyroxene composition has been observed over the lower two thirds of the exposed part of the South Range norite and no fresh material has been found higher in the sequence than this. The average compositions of both hypersthene and augite become increasingly enriched in iron across the quartz-rich norite from the inner to the outer contact. This reversal in the normal trend of cryptic variation is accompanied by a marked increase in the zoning of individual hypersthene grains. The preferred model for the Irruptive is that of a funnel-shaped intrusion. The initial Irruptive magma is thought to have been quartz-rich and the quartz-rich norite is interpreted as representative of this composition, un-modified by crystal accumulation or adcumulus diffusion. After accumulation of the South Range norite, the remaining magma intruded farther along the structure exploited by the intrusion, giving rise to a peripheral sill of felsic norite, oxide-rich gabbro and micropegmatite continuous inwards and downwards with the main body of the Irruptive. Field evidence indicates that the South Range has been elevated 3 miles with respect to the North Range along steeply dipping faults that parallel the long axis of the Sudbury basin. The South Range rocks consequently represent a deeper section through the funnel than those of the North Range.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 547-564

The Giles Complex is a series of deformed mafic-ultramafic sheets, now found as isolated intrusions, scattered over an area of 2,500 sq km in the Musgrave Block of Central Australia. The sheets which are late Precambrian in age were emplaced into a granulite facies terrain, the metamorphism of which has been variously dated from 1,300 to 1,600 million years. The intrusions display many characteristics of stratiform bodies, including prominent large- and small-scale mineralogical layering of the ratio type. There is also a depth stratification developed between intrusions, those in the western portion of the complex being emplaced at a higher level than those of the central zone which were intruded into the lower crust. The evidence for the high-pressure development of some intrusions of the central zone can be summarized as follows: (1) sub-solidus reaction relationships between olivine + plagioclase -> orthopyroxene + clinopyroxene + spinel, and orthopyroxene + plagioclase -> garnet. (2) spinel and rutile exsolution in pyroxenes (3) high R2O3 contents in the pyroxenes (4) unusually high KD (Mg/Fe) values for coexisting pyroxenes (requiring high liquidus temperature and hence high pressures) (5) dominance of orthopyroxene rather than olivine in the early crystallization sequence (6) when observed chill-zones are very thin (e.g. South Mount Davies is 4,200 metres thick and has a chilled zone of a few centimetres). There is a broad correlation between structural height and degree of fractionation. In general the higher level gabbros are more highly fractionated, although no overall marked iron enrichment has been observed. At least two of the sheets have different bulk-compositions which may be the result of progressive movement of liquids away from the partially crystalline lower sheets into higher zones. Final differentiates of the fractionation series may be represented by the Tollu Volcanics, a group of acid and basic volcanics found in close association with the high level gabbros at the western end of the Musgrave Block. The volcanics and gabbros have the same initial Sr87/Sr86 ratios and therefore can be considered to be comagmatic. The best age estimate for both rock suites is 1,100 million years. High-temperature deformation of the sheets, particularly in the central zone, resulted in the formation of localized gabbro-gneiss zones approximately parallel to the main igneous layering. In many respects this resembles the flow-layering, described by Thayer (1963) as being characteristic of Alpine ultramafic bodies. This feature together, with rotation of the intrusions into a near vertical position by later deformations, illustrates the possible relationship between flat-lying stratiform intrusions and those of the Alpine ultramafic association. In the central zone, numerous suites of minor mafic intrusions were emplaced after the crystallization of the Giles Complex. The major deformation phases of the Giles Complex and the injection of many of the minor bodies occurred before major uplift of the central zone. Economically, the Giles Complex has proved disappointing since major chromite and nickel deposits are absent from the ultramafic zones. Low grade lateritic nickel deposits are known, these being due to the weathering of transgressive picrites. Vanadiferous magnetites occur in a high-level gabbro in the western zone, but this and the nickel deposits are too far removed from industrial centres to have been exploited. However, it should be possible to locate stratigraphically favourable portions of the Complex where there may be areas of potential economic interest.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 594-609

The ultramafic sequences of rocks of the Great Dyke are envisaged as a succession of rhythmically layered units the recognition and understanding of which is critical to any synthesis of events. An hypothesis of formation of these rhythmic units by separate inflows of contrasted ultramafic magmas is considered and rejected. Evidence for intermittent accessions of fresh inflows of basic magma to a large magma chamber is found to be acceptable only for one general level of the Sebakwe sequenee. The rhythmic units are considered to have originated by crystallization from a large chamber of convecting basic magma, in which convection currents were only at intervals able to break up a non-convecting bottom layer of magma. This synthesis owes much to the regional mapping of Worst and to the interpretative work of Jackson, who are further acknowledged in detail in the text.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 610-620

A gravity survey covering the central strip of Rhodesia has resulted in the first gravity map of this country, with gravity stations approximately five miles apart along the main roads in the region Salisbury-Sinoia-Hartley-Gwelo-Bulawayo-Beit Bridge-Fort Victoria-Salisbury. This provides a framework for detailed gravity traverses across the Great Dyke. Twelve of these, distributed along its length, have been completed and show very large anomalies. An iterative computer program has been written to generate a subsurface structure to account for the observed gravity anomaly. The derived Dyke cross-sections are presented and compared with the structure proposed by Worst (1960).
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 621-644

The fracture system, which was filled almost immediately with magma, had a shear and dilation origin. Distinct mineralogical and chemical similarities exist among the variety of dyke-rocks occupying the fractures. These include melanorite, melagabbro, gabbro, leuco-gabbro, quartz gabbro and quartz norite. These similarities, together with new Rb-Sr isotope data permit correlation of the Southern Satellite Dykes with the Great Dyke. A feeder-intrusion relationship is postulated. An initial magma type of alkaline olivine basalt or picrite is indicated by the study of a chilled dyke margin. It is possible that this was the composition of the magma of the Great Dyke as well.
Back to title list




Symposium on the Bushveld Igneous Complex and other Layered Intrusions, Pretoria 1969. Special Publication No.1, 645-60

The Usushwana Complex, originally defined to cover only those ultramafic, mafic and acid rock types occurring between the Usushwana and Usutu Rivers, now includes, as a result of more recent mapping, gabbroic and granophyre rocks cropping out at the base of the Insuzi Series building the Makumulu Plateau. Similar rock types extend across the Swaziland border into the adjacent areas of the Transvaal. The Complex comprises two dyke-like bodies striking north-westwards through Amsterdam (in the Republic of South Africa) and Mhlambanyati respectively, linked by a sheet of gabbro and granophyre at the base of the Insuzi Series. The Usushwana Complex intrudes various tonalitic and other gneisses, and a homogeneous biotite granite which has been dated at 3070±60 m.y. The continuity of the outcrops of the Complex in Swaziland is broken by two younger plutons of coarse-grained porphyritic granite A provisional age for these granites is 2,650 m.y. The Usushwana Complex was emplaced during the early Precambrian. Fragmentary layering is preserved particularly in the Embo area. Elsewhere there is a simple layering of gabbro below and granophyre above. The layers dip gently towards the centre of the Complex. Gravity data indicate that the Mhlambanyati occurrence has the form of a dyke. Lack of detailed information from areas other than Mhlambanyati precludes a detailed reconstruction of the mode of emplacement but many of the features are similar to those found in the Great Dyke of Rhodesia. Low-grade copper and nickel mineralization is associated with two layers at Embo. Titaniferous magnetite bands occur in gabbro south-east of Mhlambanyati and in the Makumulu area. Eighteen chemical analyses are given of rock types occurring in the Mhlambanyati area.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 3-5

There are enough sulphide and other types of ore deposit in gneissic metamorphic terranes to make it clear that mineral deposits can occur within and survive all ranks of metamorphism. The textural and mineralogical effects of metamorphism on sulphide ores are well documented (McDonald, 1967). The problem in exploration is to predict their setting and distribution.
Plate tectonics has greatly helped to supply predictive models of metallogenesis and to systematize the occurrence of mineral deposits in time and space within Phanerozoic orogenic belts: by relating massive sulphide and exhalative deposits to island arc volcanism; porphyry copper, molybdenum, tin, and tungsten deposits and a broad class of hydrothermal vein deposits to plutonic and volcano-plutonic complexes above a subducting Benioff zone; alpine nickel, nickel-copper, chromite, and asbestos deposits to obducted segments of ocean floor; and some lead and zinc deposits to shelf sediments above former continental rises (Sawkins, 1972). Different families of deposits lie on subparallel zones along the linear mountain belts with distinctive ages and/or host-rocks lithologies (Sutherland Brown, 1974).
Attempts to extend this tectonic and metallogenic framework to the Precambrian metamorphic terranes have met with only modest success, and indeed most of the direct correlations have been from Phanerozoic island arc calc-alkaline suites and related massive sulphides to the Archaean greenstone belts with ages in the 3 500 to 2 500 million year range Gangster, 1972; Hutchinson, 1973). This leaves a discomfiting gap of about 2 billion years in which the theories fit only rarely and locally, and in which most of the tectonic story is represented by dominantly sialic gneiss terranes of various radiometric ages. What types of deposits should (and do) occur in these?
To examine this question a five-fold classification is useful:

(1) Magmatic ores associated with alkaline, mafic, or ultramafic igneous rocks, e.g. of nickel, chromium, platinum, titanium, diamonds, and rare earths.
(2) Sedimentary first-generation ores of possible exhalative origin, e.g. of iron formation and lead-zinc deposits.
(3) Sedimentary second-generation ores of placer and residual type derived by concentration through weathering and deposition, e.g. of gold, diamonds, tin, bauxite, nickel-laterite and uranium.
(4) Oceanic volcanogenic hydrothermal ores associated with island-arc or ocean-floor centres of an evolved, differentiated nature, e.g. copper, copper-zinc, copper-lead-zinc massive sulphides.
(5) Continental hydrothermal ores of Cordilleran-type (Sawkins, 1972) occurring as disseminated or vein deposits related to high-level plutonism and plutonic-volcanic complexes, e.g. the Andean porphyry copper deposits (Sillitoe, 1973).
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 7-40

Various examples are presented illustrating the application of conventional geophysical methods in metamorphic terranes, and the problems that may arise. Background physical properties may be highly variable and may transgress formation boundaries. Foliation and structure may be complex and render interpretation difficult. Magnetics are generally useful in deriving structure and tracing formations. Induced polarization and resistivity frequently apply, but may be limited by conducting cover, depth of weathering, and accessory magnetite and pyrite. Electromagnetic methods are hampered by conducting cover, weathering, the not infrequent stringered nature of the sulphide occurrences, and their often limited strike length due to plunge or lensing. Gravity is applied usually as an adjunct to suggest massive sulphides or basic intrusives. Drill hole methods are gaining in importance with the search to greater depths and the increased costs of drilling. New developments aiding in exploration are discussed briefly.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 41-51

In some regional fold belts characterized by greenschist facies metamorphism, a steeply plunging mineral streaking lineation commonly forms as an integral part of the fabric defining the regional schistosity. Such metamorphic areas are also characterized by a sequence of fold phases passing from schistose deformation, through crenulate strains to late kink-fold formation. It is also common in such regions for zones to develop where the stratigraphy or bedding is almost everywhere parallel to the schistosity. Steeply plunging intrafolial folds form within such zones, with their axes parallel to the mineral streaking lineation, and with their axial planes parallel to the regional schistosity. Although the subhorizontal first phase fold axes defined by the intersection of bedding (stratigraphy) and schistosity are often highly variable in plunge, the steeply oriented intrafolial folds, mineral streaking and elongation of strain markers (e.g. conglomerate pebbles), are all parallel and extremely constant in orientation regionally (except where affected by subsequent crenulate strain). In areas characterized by these structural relationships, some metal deposits are observed to be strongly elongate parallel to the mineral streaking lineation (and to the axes of intrafolial folds). Examples of such deposits can be found among the volcanogenic massive copper-zinc-iron pyrite deposits of New Brunswick, the Tasman Geosyncline (Australia) and restricted areas of the Canadian Archaean Shield. Some massive nickel sulphide deposits of the Western Australian Archaean also exhibit these structural relationships. Water is known to substantially reduce the strength of silicate bonds during strain. Syntectonic hydrothermal activity (either associated with the metamorphism or the emplacement of these metal deposits) is considered to locally accelerate strain rates, causing intrafolial folds to form within the associated host rocks.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 107-128

The Richardson's Kop deposit is a dilation quartz breccia situated within an intrusive tonalitic pluton. Dominant ore minerals found in the breccia are pyrite, wolframite, sphalerite, and galena. A zone of most intense alteration adjacent to the breccia is characterized by local concentrations of fluorine-rich topaz and the complete absence of mafic minerals in the tonalite. Structural analysis reveals a link between ore mineralization and reactivated primary jointing inherited from intrusion of the pluton. A statistically significant correlation is observed between silver and tungsten both within the ore body and within the adjacent tonalite. On the basis of field observations and experimental data a magmatic hydrothermal model is proposed to explain the mineralization at Richardson's Kop. Intrusion of a water-rich tonalitic magma was followed by the development of a free tungsten-bearing aqueous fluid which migrated through the outer crystalline shell of the pluton. Deposition of wolframite at Richardson's Kop was facilitated by a decrease in temperature and extensive leaching of Fe2+ and Mn2+ in the breccia zone. Continued lateral migration and cooling of the tungsten-bearing fluid resulted in scheelitization of early wolframite and eventually, deposition of scheelite and gold. The geographical distribution of wolframite, scheelite and gold deposits in the Essexvale pluton is entirely consistent with this model.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 189-203

Regional stream sediment geochemistry programmes have been carried out in the Vioolsdrif and Aggeneys areas of the north-western Cape Province on a sample density of 1 per Km2. Orientation work in these areas indicated that the minus 200-mesh fraction has a normal distribution and more clearly defined threshold values than the minus 80-mesh for the majority of elements. The bedrock geology strongly controls the geochemistry of the minus 200-mesh fraction in the arid north-western Cape environment. Pb and Zn in the regional surveys proved extremely effective indicators of base metal deposits in which the elements are present. Cu and Mn are less effective in this respect.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 205-214

A base metal exploration boom since 1971 has given a sudden upsurge in geochemical activity in the north-west Cape. The climate is semi-arid, with red desert soils resting directly on bedrock or with an intervening concretionary horizon. Recognition of endogenous or exogenous material is critical in interpreting soil sample results. Endogenous calcrete predominates and regosol profiles are common. Mechanical dispersion processes dominate in sediments from upper stream courses while hydromorphic factors are important in pans. Trials with vegetation, ground water and vapour/aerosol prospecting may increase depth penetration and be useful in areas covered by deep sand.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 449-459

The Lomagundi Group in the Sinoia area consists of the orthoquartzite-carbonate facies Mucheka Formation, overlain by the Nyagari Formation of slate with "mountain sandstone" lenses. The two formations are separated by a local disconformity in the south-east, against the Biri granite basement dome. Lateral facies variations within the formations also reflect the influence of the basement dome during sedimentation. Detailed mapping of the structure has shown three deformation episodes. In the first deformation the succession was tightly folded into a synclinorium on NNE trending folds (F1) above a basal decollement plane. A cleavage (S1) was developed in the slates and flattening structures that mark the XY plane of the strain ellipsoid in the other beds. Estimates of minimum finite strain were made from "pock marks" in the quartzites and stromatolites in the dolomite. In a second deformation, large scale open cross folding (F2) occurred on NW trending axes that resulted in the complex outcrop pattern now present. A third ENE trending deformation belt (F3) crosses the north-west corner of the area mapped.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 463-475

Structural, metamorphic, igneous and geochemical features characterize regimes of polyphase deformed rocks in the Baltic Shield and are significant features in accounting for marked variations in economic potential. The products of late Archaean times are interpreted in terms of a 'hot line' tectonic model with exposed rocks representing major volcanogenic zones that were metamorphosed and deformed in varying heat flow environments and variably affected by mantle degassing. Dispersal rather than concentration of base metals resulted. Mineralization in early Proterozoic times was largely associated with exhalative activity and later basic plutonic injection in a NW-SE trending zone that stretches across Finland, possibly following a major crustal lineament active in Archaean times. Elsewhere broadly coeval cover and basement deformation, igneous emplacement and channelled uprise of volatiles resulted in marked contrasts in the Svecokarelian regime. Late Proterozoic times were marked by emplacement of large masses of high temperature anorthosite in the Sveconorwegian province. Mineralization was widespread and variable with some possibly related to concentration of exhalite deposits formed in Archaean times in a stable continental region between major volcanogenic zones. Features of Precambrian rocks in parts of southern Africa that are comparable to those in the Baltic Shield are assessed in relation to the nature, location and genesis of base metal sulphide deposits. The possibility of exhalative activity in stable continental regions between major volcanogenic zones in Archaean times is suggested, as is the concentration of resultant sulphides in a regime of polyphase deformed rocks associated with anorthosite emplacement in late Proterozoic times in the north-western Cape Province.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 477-480

[This is an extended abstract of the paper prepared for presentation at the Symposium on Mineralization in Metamorphic Terranes. A more detailed account has been published by the author under the title "Petrochemical studies of the ore environment at Broken Hill, New South Wales" Parts 1-4, Trans. Inst. Min. Metall. Lond. Sect. B, 85, 33-46, 118-131; 132-141, 221-233 (1976) - Editor].
It has been well known for at least most of this century that the stratiform type of sulphide occurrence in the Broken Hill district of New South Wales is usually accompanied by thin beds of iron-rich sedimentary rock, loosely referred to as banded iron formation (BIF). Such an association is now being recognized as a widespread phenomenon all over the world and throughout a large part of geological time, and the geochemistry of these small, impure "banded iron formations" promises to provide an extremely powerful exploration tool - perhaps a major "break-through" in mineral exploration technique.
BIF's of this general association are most commonly of oxide or silicate type, though carbonate and sulphide units do occur. In the Archaean, oxide and silicate BIF are well-known associates of stratiform ore in the Yilgarn Province of Australia and the Superior Province of Canada; in the Proterozoic, oxide-rich units are prominent in the Cloncurry and Broken Hill districts of Australia and at Gamsberg and elsewhere in southern Africa. Perhaps the most notable Palaeozoic occurrences are those associated with the stratiform ore bodies of the Bathurst, New Brunswick, field, and in Mesozoic and Tertiary terranes they are well known as associates of volcanogenic stratiform mineralization in Cyprus, Japan and the islands of the south-west Pacific. The banded iron formations at Broken Hill appear to have considerable significance not only in exploration geochemistry but also in ore genesis theory and metamorphic petrology. What appears to be a faithful preservation of original constitutional features suggests negligible diffusional movement during regional metamorphism, even though the latter has reached sillimanite grade. Further, the constitutional features concerned link the genesis of the BIF's incontrovertibly with that of the ore body, and indirectly indicate the nature of the environment prevailing during sedimentary ore formation.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 481-489

Silver-lead-zinc mineralization in the Proterozoic Willyama Complex, New South Wales, has undergone multiple metamorphism and deformation, thus providing the opportunity for studying the effects on the ores of both prograde (amphibolite to granulite facies) metamorphism and retrograde metamorphism. Two types of silver-lead-zinc deposits are recognized in the Willyama Complex, viz. stratiform bodies known as the "Broken Hill type" and veins known as the "Thackaringa type". These exhibit differences in both mineralogy and textural relationships of the minerals. Electron microprobe analyses have also demonstrated differences in the composition of sphalerite from the two types of deposits. Isotopic studies of lead and sulphur provide evidence of a genetic relationship between the Broken Hill and Thackaringa types. On the basis of a close spatial relationship between retrograde rocks and Thackaringa type deposits, it is proposed that these veins were derived from the Broken Hill type by partial remobilization during retrograde metamorphism.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 491-508

A nickel sulphide-bearing ultramafic horizon at Spargoville, Western Australia, is an example of a magmatic ore association which occurs in a lightly metamorphosed Archaean greenstone environment. Sulphides were originally concentrated at the base of an ultramafic sill which intruded a basic volcanic pile. During a major orogenic event the sequence was overturned. Later faulting has sliced up the resultant hanging wall contact to produce three mineralized horizons, superimposed in plan. Silicate/silicate, silicate/sulphide, and sulphide/sulphide textures within the ore-bearing horizon are described. Primary olivine is preserved in the core of the host ultramafic unit, and tremolite-chlorite-carbonate alteration reaches a maximum towards the margins. Metamorphic olivine is present. Primary ultramafic textures include a "pseudo spinifex" variety consisting of randomly oriented bladed olivines, and the true spinifex or quench type. The textural stages of hydration metamorphism from olivine to a tremolite-chlorite mat are traced. Primary textures between sulphides and silicates are occasionally preserved, but have mostly been destroyed with silicate recrystallization. Sulphide/sulphide relationships show typical magmatic features with a metamorphic overprint. The ultramafic metamorphic mineralogy at Spargoville is consistent with a formational temperature between 420°C and 680°C at 2 kb but the presence of metamorphic olivine may indicate an extension to 770°C. However, comparison of metamorphic textures in massive sulphides with experimental work indicates a metamorphic temperature range between 300°C and 600°C. The preservation of primary silicate mineralogy and textures supports the contention that metamorphic reactions have been retarded by low PH2O and PCO2.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 509-527

Petrochemical fields and trends are presented for podiform chromitite-bearing ultramafic masses occurring amongst Precambrian gneisses in the Beartooth Mountains and formed by tectonic disruption and metamorphism of an ultrabasic-basic igneous complex. Comparison with petrochemical features of the nearby stratiform chromitite-bearing Stillwater complex indicates considerable mobility of most elements except Mg, Ni and Cr during metasomatic activity, with Sr, Ti, Zn, and Mn being particularly mobile. Plots of Cr against both Ni and MgO permit definition of fields useful in the identification of the original genetic type of metamorphosed and tectonically fragmented ultramafic masses. As well as being of petrogenetic significance, these fields may be an aid in exploration for and evaluation of chromite deposits.
Back to title list




Mineralization in Metamorphic Terranes. Special Publication No.4, 529-544

The Avoca deposit consists of conformable, lenticular, cupriferous and lead-zinc pyritic ore zones in a sequence of Lower Palaeozoic sediments, tuffs and lavas. Copper stockworks and magnetite-rich zones also occur. Ore fabrics provide critical genetic information. In the pyritic zones primary colloform, melnikovitic and zoned crystalline pyrite probably formed in the sediment pore waters. Minor carbonaceous horizons indicate that reducing environments sometimes existed at the sediment/water interface and aqueous ionic sulphur may have been produced through bacterial reduction of seawater sulphate. This could have created framboidal pyrite on slow reaction with iron during diagenesis. Siliceous stockworks stratigraphically below the pyritic zones do not contain appreciable colloform pyrite, are demonstrably epigenetic and probably formed from chlorite-rich, metal-laden hydrothermal fluids. Towards the base of the pyritic zones void volume (porosity) increases and this may have originated from leaching by circulating pore waters following pyrite formation. These pore waters would have had increased acidity in a sulphide-rich environment and mixing with hydrothermal brines would have been highly corrosive towards primary pyrite. Strong corrosion of pyrite caused a rapid increase in reduced sulphur species and these reacted with copper, zinc and lead from the brine to form sulphide. Effects of fluid mixing coupled with decrease in temperature and pressure gave stockwork zones at lower levels. Corroded, unzoned crystalline pyrite indicates ore deposition in the presence of a corrosive medium; possibly chloride-rich. This is consistent with present-day geothermal systems. Recently discovered stratiform, polymetallic, metamorphosed sulphide deposits in the NW Cape may have similar ore fabrics indicating corrosion of primary iron sulphide by later hydrothermal fluids.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 1-4

The topic of "mineralogy and petrology applied to exploration" is actually a topic for a rather extensive book or a long series of lectures. Anyone familiar with the scope and the broad spectrum of these applications knows that in the time of 40 minutes only some basic principles can be mentioned, and perhaps also some suggestions for possible future improvements. The days when mineralogy was the "unappreciated stepchild of the mineral industry" (Von Rahden, p. 509) are probably gone forever. Yet, the temptation by the non-informed administrator to try to "save money" by short cuts, leaving out critical mineralogical investigations, is probably still strong. When I gave an early version of my paper to Prof. Ramdohr for his comments, he wrote in the margins, at this place: "This is hardly necessary to say in South Africa". After having learned that in Transvaal alone there are at least 11 laboratories of mineralogy and petrology applied to the Mineral Industry, not counting the University Departments, I certainly agree with the statement of Prof. Ramdohr. With every day that I stay in this country, I am more convinced that - with my talk - I am "carrying coal to Newcastle". After so many interesting discussions with South African colleagues, I have, in fact, lost any hope of offering this audience new examples or ideas on applications of mineralogy and petrology to exploration. But, perhaps it is not entirely out of place to point to two vital roles mineralogy and petrology can play in a mining company. First, if the mineralogist knows his trade he can be a useful and important liaison officer between exploration and mining, and ore dressing and smelting and - last but not least - management. Next, he can also play a key role in the research for more useful and new genetic models which we all need as guides in exploration. The ideal mineralogist-petrologist should in fact be able - in co-operation with the field geologist - to bring together in his work the five scales of the investigation of an ore deposit: the regional scale, the local or mine scale, the outcrop scale, the hand-specimen scale, and the microscopic or mineral scale. Only in bringing together the information from these five dimensions, can we hope to find the best exploration guides. This leads us to a first suggestion or conclusion: The mineralogist-petrologist should normally not confine his work to the laboratory only. As we will see in a moment, many properties of ore and gangue minerals can be understood and better interpreted, if the field relations also are known and observed by the mineralogist-petrologist himself. Why are mineralogical tests so vital to many phases of exploration? The reason for this is quite simple and straightforward: the major exploration guides for ores, as well as the selection of specific metallurgical processes for an ore, are a direct function of the mineralogical properties of ore and gangue minerals. Unless we know them we walk in the dark in the search for new ores, and even more so in the selection of the proper beneficiation method. In the following we want to concentrate on applications in mineral exploration. I should like to emphasise, however, that an integrated exploration campaign will always also be oriented towards the possible future concentration techniques. (A number of papers in this conference will deal with mineralogical methods applied to ore-dressing problems; and it will be easy to see that many of the ore- dressing properties of minerals can be recognised and reported, at least in a preliminary way, during an exploration campaign.) The application of mineralogy and petrology to exploration starts, in fact, not with the microscope; and it often does not end with it either, but the microscopic information is fed back into the exploration field-work, and must result in discussions between the mineralogist-petrologist and the exploration geologist. Mineral exploration today often starts with LANDSAT or ERTS photographs, on which not only different rock units and structures are recognised but also delicate alteration haloes or trains of glacial boulders on fossil and recent moraines, as well as many other valuable details. Going down in scale, mineral and rock features are, of course, the main focus in field mapping. Likewise, geochemical sampling is normally accompanied by notations on colour and types of minerals, soils or rock\fragments. From the outcrop and the hand-specimen scale, including, of course, drill-core data, the next step is then microscopic examination. The main focus of this exposé will now be on the mineralogical and petrological features which serve as tools for exploration. Before looking at some classical and, perhaps, some new examples, four circumstances of general interest ought to be mentioned. These are: 1. grain size as the basic reason for the importance of microscopy (of rocks and ores); 2. the fallacy of assuming that a Δc, i.e. any difference in composition, always implies a migration, i.e. a Δc/Δt), Δt being a difference in time which means a migration; 3. the necessity to stick to an inductive approach which is always a factual approach; we must try not to let a deductive model and theory override our factual knowledge. This means, we should attempt to differentiate between observations and interpretations; and 4. some remarks on semantics; or: the hidden suggestiveness of certain terms (such as "mineralization" instead of mineral content; "dolomitised" instead of dolomitic, "rounded" instead of round, etc.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 5-20

A review of 1 130 analyses of spinal-group minerals shows that their compositional ranges differ in, and may in part be distinctive of, layered mafic complexes, alpine-type peridotites, metamorphosed mafic igneous rocks, and nodules of deep-seated origin. Spinels of slowly cooled rocks show a more restricted compositional range than those of quenched rocks. Chromian titanomagnetite, titanomagnetites with exceptionally high contents of TiO2 (ca. 26%), and phases intermediate between magnetite and aluminian spinel are common in quenched rocks, but rare or absent in the plutonic environment. Metamorphism may reconstitute solid solutions within the latter miscibility gap, but unmixing occurs again during temperature decline. Progressive degrees of unmixing and coarsening of intergrowths of chromian titanomagnetite and low-Cr, aluminian spinel are documented by microprobe analyses of phases in the Tugela Rand Complex. Metamorphic recrystallization may also enhance prospects for beneficiation of titaniferous and vanadiferous ores, as in the ores of the Rooiwater Complex. A quantitative model is presented for chromite and titanomagnetite crystallization in Karoo magmas, based on whole-rock levels of Ti, V and Cr at successive stages of crystallization, as calculated by applying the Rayleigh fractionation equation to whole-rock levels of the incompatible element Zr. The crystallochemical behaviour of V is seen to be radically different from that of Cr. Where Cr is rapidly depleted from melts by partitioning into chromite and pyroxene, the bulk distribution coefficient for V remains close to unity until magnetite is nucleated at a late stage, when rapid depletion of V occurs. Possible applications of this model to the Bushveld Complex are explored. The partitioning of Al and Cr into coexisting ortho- and clinopyroxenes and spinels of equilibrated rocks is such that (Al/Cr)px/(Al/Cr)sp distribution coefficients decline as the Al content of the spinel phase increases. These empirical relationships persist through a wide range of low-pressure rocks and others in the spinel- and garnet-peridotite facies. The data indicate that pressure, per se, exerts little influence on the Al/Cr ratios of spinels, which are largely a function of partitioning between the stable phases. A preliminary survey suggests that a broad pattern of declining Cr levels prevails within chromites of low-pressure, mafic igneous rocks of progressively younger age in southern Africa. Contrasts between chromite compositions in Barberton komatiites and associated intrusives, and detrital chromites found in the Witwatersrand sediments, encourage speculation that the provenance areas of the latter sediments included younger greenstone terrains that have been removed by erosion, or have not as yet been recognised as such. Significant levels of Zn in chromites of both the Barberton komatiitic suite and the Witwatersrand suite may be a pointer to the existence of sulphide ore bodies in ultramafic rocks within the provenance areas of Witwatersrand sediments.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 21-30

In Western Australia, the presence of Zn-rich chromites (>0,6 atomic% Zn) in ultramafic flow sequences provides a guide to the presence of Archaean volcanic peridotite-associated nickel deposits. The Zn-rich chromites are not specific to the host olivine peridotite units, but occur throughout both fresh and weathered ultramafic rocks in the overlying and laterally extensive sequence, thus providing an effective early-phase exploration tool. Reconnaissance studies in other mineralized terrains suggest that Zn-rich chromites may be specific exploration indicators elsewhere. Distinctive Al- and Mg-poor ferrochromites with variable Ti, V, Mn and Zn contents are a widespread oxide phase in magmatic and metamorphosed magmatic sulphide ores of several types. They are inferred to have crystallized from sulphide melts. They are specific indicators of magmatic sulphides, and their preservation, in at least some gossans, adds an important mineralogical parameter to previously documented geochemical and textural criteria for the recognition of former Fe-Ni-Cu sulphides in oxidised surface exposures. Thus chrome-spinels of contrasting composition from both nickel ores and hosting ultramafic sequences, show considerable potential as mineralogical indicators of the presence of magmatic sulphide ores. Their exploration potential warrants further study.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 31-37

In gold exploration and metallurgical testing, the role of the mineralogist is to assist in solving problems with regard to (a) sampling, (b) gold occurrences, and (c) petrologic classifications of host rocks. The presence of "coarse" gold, as a sampling problem, can be overcome by various methods including "screen fire assays", screened analyses and special gravity techniques which may be used to determine distributions of "coarse" versus fine gold. Determinations of mineralogic associations of gold usually require special separation techniques accompanied by fire-AA-mineralogic assays of the products. After accurate mineralogic analyses are obtained for various products, quantitative distributions of refractory gold associations can be calculated by the use of simultaneous equations. Petrologic classifications are required by the geologist for mapping and interpretation of the various rocks. After the petrology and mineralogic differences of the rock units are determined, X-ray diffraction analyses of analytical pulps can be used for lithologic logging of drill cores. XRD logging is an efficient method for correlating lithologic units and alteration trends throughout a gold prospect.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 39-57

Information on the origin, transport and distribution of economically important minerals in Precambrian placers can be obtained from geochemical and fluid inclusion studies of some of their principal detrital components, i.e. pyrite, quartz and gold. Examination and analysis of fluid inclusions and their daughter minerals in pyrite and trace element analyses of single grains not only allow a classification of pyrite types but also permit statements to be made about their origin. Such studies give information regarding ore forming processes and the distribution of different pyrite types in fossil placers. Quartz pebbles, similarly, have characteristic daughter minerals and fluid compositions which in some cases can be used as pathfinders. Regional trends in the distribution of certain pebble types have been observed. The silver content (fineness) of gold particles has shown occasionally a high degree of correlation with the distribution of sedimentological units. Reworking and sedimentary redistribution of gold from older sediments could be recognised.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 59-66

An understanding of the distribution and concentration of gold within the Witwatersrand conglomerate reefs is of fundamental importance to the gold mining industry. The ability to finger print gold-bearing reefs would have an important bearing on mine planning and the development of new methods of mining. From sedimentological studies it is possible to establish the environment in which deposition of gold and associated minerals took place. However, it is not possible to identify sources supplying detrital material on account of the nature of the preserved conglomerates. One possible way of overcoming this problem is to geochemically characterise those detrital minerals associated with the gold. As quartz and pyrite are prominent components of the reef, they are ideal for such a study. In this paper, it is demonstrated that detrital pyrite and quartz have specific geochemical signatures which may be used to characterise individual reefs. Furthermore, evidence is provided of the presence of more than one population of detrital components within reefs such as the Ventersdorp Contact Reef and the Steyn Reef. The recognition of different detrital components suggests that a number of source areas were responsible for providing detritus to the Witwatersrand basin. Through the distinguishing characteristics of detrital mineral populations, geochemistry can play a role in interpreting their distribution patterns within gold-bearing reefs. Geochemistry, thus, may provide a valuable tool in the understanding of the sedimentology and ultimately the distribution and concentration of the gold.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 67-77

Investigations of pre-Witwatersrand and Witwatersrand conglomerates from South Africa reveal low uranium concentrations and radioactive disequilibria in samples from weathered surface exposures. The disequilibria prove the occurrence of unsystematic recent uranium losses and/or gains in the rock altered in the supergene environment. Gamma-spectrometric surveys, thus, are of very limited use for detecting uranium anomalies in geological environments similar to the investigated terrains. However, uranium concentrations of original rock can be estimated from weathered samples by making use of lead-isotope analyses. The results of the lead-isotope analyses indicate that the fresh-rock uranium concentrations estimated from the weathered samples are higher than the measured uranium concentrations of the same samples, i.e. the investigated samples suffered severe uranium losses. Finally, a method is proposed to evaluate the original uranium content of weathered rocks by interpreting their REE concentrations.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 79-86

Epigenetic quartz-gold veins in Archaean greenstone belts of the Yilgarn Block, Western Australia, have formed from metamorphic H2O-CO2 fluids. A regional temperature control on gold deposition is suggested by their virtual restriction to areas of greenschist to transitional amphibolite facies metamorphism. Despite the discordant form of individual veins, the vein sets are commonly stratabound, often being confined to tholeiitic metabasalt/metadolerite units in mixed tholeiitic-komatiitic mafic/ultramafic sequences. The physical properties of these units have played a major role in controlling fracturing, commonly hydraulic fracturing, and fluid access to the system, whereas their mineralogy and geochemistry have controlled gold deposition within the veins. Fluid/rock interaction involving the generation of biotite (or sericite)-Fe sulphide-carbonate alteration zones is an integral part of gold precipitation from Au(HS)-[2] or Au (HCO3)-[2] complexes in the ore fluid. Deposition may be promoted by several mechanisms including increase or decrease in f(O2) with or without decreases in pH at constant temperature. Better understanding of gold deposition processes allows formulation of criteria for gold exploration. For example, the most prospective stratigraphic targets are competent, less mafic, basaltic units in greenschist to transitional amphibolite facies areas. The mineralogically distinctive alteration zones, indicating significant fluid/rock interaction, are important, more local, mineralogical guides to mineralization.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 87-96

The distribution of ferruginous bauxites along the eastern seaboard of South Africa is shown in a small-scale map and their genesis is briefly described. The aluminium oxide, iron oxide, primary slightly-altered iron and titanium oxides and layer silicate minerals in some typical ferruginous bauxites from the eastern seaboard of South Africa have been investigated by X-ray powder diffraction (XRD), infra-red spectroscopy (IR), thin section, electron optical (SEM and TEM) and chemical extraction analysis. The dominant aluminium oxide mineral is well-crystalline gibbsite. Traces of boehmite were identified in some samples. The dominant iron oxide mineral is finely-divided goethite containing from 20 to 25 mole per cent AlO(OH), this being the mineral which gives the bauxites their characteristic yellowish colour. The reddish bauxites also contain finely-divided Al-substituted hematite which masks the colour of the goethite. The aluminium incorporated in the iron oxide structures is not recoverable by the normal Bayer Process. Relatively small amounts of kaolinite (booklet morphology), hallosite (tubular morphology) and quartz have also been identified. Partly altered remnant primary iron and titanium oxides have also been found (ranging from 5 to 12%): magnetite, titano-magnetite, titano-maghemite, ilmenite, anatase and rutile. Gibbsite in these bauxites is thought to form partly from feldspar directly and partly via halloysite, indicating two stages of desilication. This is determined by the variations in the intensity of leaching of the profile. A knowledge of the amount of "available" aluminium and reactive silica together with the detailed mineralogy (including the amount of aluminium substituted in the iron oxide structures) of these bauxites is critical in determining the economic potential of these materials. This information should also be of future importance in indicating more efficient ore-extraction procedures and the recovery of potential by-products such as iron and titanium.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 97-109

The first part of this paper is concerned with macerals, maceral groups, microlithotypes and minerals associated with coal. The heterogeneous composition of hard coals makes it impossible for their ranks to be determined by proximate or ultimate analysis. The rank of a particular coal can be ascertained only by determination of the reflectance of the vitrinite present. The applicability of coal petrology to the solution of technological problems is then discussed. The type of work and the results that are possible are explained in relation to carbonisation, liquefaction, gasification and combustion.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 111-115

The maceral composition and the reflectance of a coal are two petrographic parameters which can be very useful for correlations with certain chemical and physical parameters of coal. Certain macerals in a coal are selectively grouped together and termed "total reactives" according to their behaviour during the carbonisation process. The vitrinite and exinite groups are common reactive macerals but a certain group which previously had been regarded as inert is in fact reactive and is termed "reactive semifusinite". The identification of reactive semifusinite is difficult and three methods are described to obtain the amount of reactive semifusinite which is grouped together with the vitrinite and exinite to form the total reactives in a coal. Some correlations were developed between the petrographic parameters and chemical and physical properties of coal, such as, volatile matter content, ultimate carbon content, free swelling index and Roga index. Correlations between petrographic parameters and the coking properties as well as the hydro-liquefaction behaviour are discussed.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 117-128

For several years the Chamber of Mines of South Africa has been researching coal hardness with the view of deriving a coal cuttability index for the various seams in South Africa. This work has involved the in situ measurement of coal cuttability with the use of a penetrometer developed by the Chamber of Mines. Correlations between coal petrology and coal strengths have been noted elsewhere. The aim of this paper is to synthesise preliminary analytical and cuttability results in order to determine if any correlations exist between these factors. Results were obtained from various seams of thirteen collieries in the Transvaal and Orange Free State and included petrographic, proximate and certain physical analyses as well as cuttability tests. Note: This paper merely presents the results obtained so far in this line of research, and does not include an in-depth discussion of the results. Microlithotype and further statistical analyses are still in progress and it is considered premature to draw any final conclusions based only on the data currently available.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 129-135

The application of wavelength dispersive XRF spectrometry to the determination of major and trace elements in South African coals, coal ash and fly ash has been investigated. Fourteen coals, the corresponding coal ash and 13 fly ash samples from three power stations have been analysed for sulphur and 27 inorganic trace elements. Also, petrographic and mineralogical analyses were made on the coal samples. Major element data have been reported elsewhere (Willis).
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 137-148

Coals from South Africa are generally regarded as characteristically high in ash content and difficult to wash. The urgent need to wash these coals was highlighted in 1970 when South Africa penetrated the Japanese market for the supply of low-ash coals. As a result considerable research and development was undertaken, culminating in the construction of several plants capable of washing coal from the No. 2 Seam of the Witbank Basin to approximately seven percent ash. The aim of the current study has been to analyse the quality of the coal passing through the beneficiation plant at one colliery in the Witbank area. This has included monitoring quality variations in the run of mine coal as related to the various washed fractions using ultimate and coal petrographic analyses. The possibility of obtaining improved beneficiation steps (i.e. multiple products) also has been investigated. When linked to the uses to which those fractions could be put, the significance of the developments becomes more profound. A brief comparison of other beneficiation methods for fine coal is made. The conclusion is drawn that the present dense-medium cyclone is the most efficient for the coals in question. The beneficiation potential of South African coals varies considerably from area to area and is linked largely to the types and forms of minerals and organic components present. Future generations of coal preparation engineers should be made aware that beneficiation methods may not only produce coal fractions with varying ash and calorific values, but also differing organic compositions. This fact will become increasingly important in the rationalisation of South African coal reserves in the not too distant future.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 149-155

As part of an investigation into ways of improving the quality of Soderberg electrodes made in South Africa, a mineralogical, geochemical and petrographical examination was made of some South African anthracites in comparison with anthracites from North Vietnam and Germany. The results of X-ray diffraction on raw and calcined anthracites and the analyses of the macerals in raw anthracites indicated that the sample from North Vietnam, which is an anthracite that is used in Japan for Soderberg-electrode paste, has a very high vitrinite content, low sulphur and phosphorus contents, and, when it has been calcined, contains more graphite than the other samples examined. The samples from the South African collieries, Balgray and Brockwell, contain more sulphur and phosphorus, and much less vitrinite, than the North Vietnamese samples. The anthracite from Natal Anthracite Colliery Ltd contains sulphur and phosphorus in amounts similar to those of the North Vietnamese anthracite, but it has a low vitrinite content. All three South African anthracites contain similar, low amounts of graphite when they have been calcined. The German anthracite, when calcined, contains intermediate amounts of vitrinite and graphite. Chemical analyses of the ash showed that there are major differences in the composition of the ash and the mineralogy of the various anthracites. The results of this preliminary investigation indicate that the most important requisite for graphitisation is a high vitrinite content.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 157-161

The technological importance of the petrographic analyses on coal is now widely accepted and automation of equipment became imperative. The progressive automation of equipment used for petrographic analyses at the South African Iron and Steel Industrial Corporation (ISCOR), Research and Process Development Department is discussed and evaluated.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 163-168

A quantitative X-ray diffraction method is described for the determination of minerals in coal without any prior concentration of the mineral matter. The results on 20 colliery products indicate that illite is overestimated by a factor of 1,7 when Fithian-type material is used as a standard and expandable clays remain undetected in South African coals. However, under the experimental conditions used, i.e. a total measuring time of about 20 minutes per sample, the general measurement precision appears to be of an acceptable order (standard deviation of duplicates from their means of ˜0,04 mass per cent for quartz in the concentration range 0,05 to 7,23%. Detection limits appear to be adequate at 0,04 to 1% depending on the "crystallinity" of the mineral being determined. The concentration of minerals in coal products, thus, can be determined rapidly and relatively cheaply on a routine basis, using an integrated approach. A reference intensity of 0,248 (relative to fluorite) is derived for "illite" in coal.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 169-174

The number 2 coal seam of the Witbank basin, South Africa, has a well-defined geological structure that is developed over most of the basin. A systematically selected suite of samples provide a unique opportunity to study the distribution of trace elements both laterally and vertically across the No. 2 seam. The results of this study are then compared to the trace-element geochemistry of the Nos. 4 and 5 seams which occur at stratigraphically higher levels. A total of 147 coal samples were analysed for 15 trace elements by instrumental neutron-activation analysis. Detailed trace-element profiles across the No. 2 seam indicate that trace elements, associated with the inorganic minerals in coal, reflect changes in the geological structure of the No. 2 seam. Trace elements associated with the organic phase in coal, notably Br and Sb, have a very narrow range in concentrations both vertically and laterally across the No. 2 seam. Changes in the concentration of the organic associated elements reflect changes in the depositional environment of the coal seam. Trace-element concentrations in the Nos. 4 and 5 seams are very similar to those of the No. 2 seam and only by using multivariate techniques is it possible to separate the three seams on the basis of trace-element concentrations. A 98% correct classification of seams is obtained in this study.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 175-200

A review is given of the role of ore-dressing mineralogy in the various stages of the development and exploitation of ores, from exploration, through laboratory and pilot-scale metallurgical testing to plant optimisation and control. Ore-dressing mineralogical techniques which are described and evaluated include heavy media cyclone feasibility testing, mineral identification, determination of mineral proportions, location of elements and determination of mineral compositions, measurement of liberation and locking characteristics, characterisation of mineral surfaces and data presentation. Comments are made on likely future developments in ore-dressing mineralogy and areas where additional research is needed.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 201-208

The behaviour of minerals during flotation of a fine-grained pyrite-rich base-metal ore has been investigated by studying samples of the feeds, concentrates and tailings from every flotation cell in the mill of Brunswick Mining and Smelting at Bathurst, New Brunswick, Canada. The samples were analysed for Zn, Pb, Cu, Fe, Ag, Sb, Sn, Bi, In, Hg, Mo and As; materials balances were determined; and recoveries of all metals from each cell, from each circuit and from the mill were calculated. The percentages of sphalerite occurring as free grains in each sample were determined with an image analyser, and size analyses were performed for free and unliberated sphalerite. The results show that the behaviour of the mineral is defined much better by the recovery of free mineral grains than by the recovery of the element. The recovery of free sphalerite in the zinc circuit is 95 weight percent, whereas, Zn recovery from the same circuit is 88 weight per cent the difference being due to unliberated sphalerite. This high recovery of free sphalerite shows a good performance for the zinc circuit. The unliberated sphalerite recovered in the Zn concentrate occurs in combined grains that contain an average of 68 weight per cent sphalerite. The sphalerite in these combined grains is about 10 to 60 ,um in diameter. The unliberated sphalerite in the mill tailings occurs in grains that contain an average of 9 weight per cent sphalerite, and the sphalerite in these grains is of all sizes up to 100μm. These results show that the only way to obtain better zinc recoveries by flotation from this ore is by regrinding the middlings.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 209-215

The ore microscope has been applied to a wide variety of ore-dressing problems which are of past and present concern in the Viburnum Trend, south-east Missouri, USA. The ore microscope has been used to investigate lead, zinc, copper, and cobalt-nickel concentrates and tails from most of the mills and laboratory and bench-scale test products. A variety of cobalt and nickel minerals have been recognised in the ores and mill products of the Viburnum Trend. These include siegenite, bravoite, nickelean carrollite, fletcherite, millerite, polydymite and vaesite. Their recognition is important to efforts directed at diminishing deleterious cobalt in zinc concentrates and nickel in the copper concentrates of the Viburnum trend and to attempts to concentrate these metals in the Fredericktown portion of the Old Lead Belt. The ore microscope is especially valuable for direct observation of the extent and character of mineral locking in mill products. Sphalerite locked with galena is the cause for the high zinc content in some of the lead concentrates of the Viburnum Trend. Dolomite locked with sphalerite forms a magnesia problem in most Viburnum Trend zinc concentrates. Siegenite locked with sphalerite causes objectionable cobalt to be present in some zinc concentrates. Siegenite locked with chalcopyrite carries nickel, an undesirable constituent at the copper smelter, into the copper concentrates. Siegenite locked with marcasite, pyrite and chalcopyrite contributes to the difficulty of adequately concentrating cobalt and nickel from south-east Missouri Lead District ores. Ore microscopy is equally valuable in providing a means to determine whether a given mineral species is present as free particles and whether the ore does not need additional grinding to release them. The recognition that the large amounts of lead formerly present in Viburnum copper concentrates occurred as free galena particles led to greater agitation during flotation to improve lead-copper separation. The identification of free sphalerite in certain lead concentrates led to improved zinc recovery at one of the mills. It has been proven that the ore microscope is a useful instrument for solving many Missouri Lead Belt mill problems and that it can be equally effective in the study of beneficiation problems in other mining districts.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 217-223

Results of mineralogical investigations on feeds and plant products from the Rosh Pinah plant are discussed. The causes leading to poor recoveries and grades were found to be mainly flotable gangue and carbonaceous matter, and poorly liberated sulphide minerals. A method of solving the problems encountered with the beneficiation of the ore is described.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 225-227

An empirical procedure, based on ore microscopy, offers a simple basis for a quick but sufficiently accurate assessment of the mass distribution of ore minerals over defined categories of middlings. It has been developed for a complex sulphide ore at the Rammelsberg Mine of Preussag AG Metall, Federal Republic of Germany. The composition of the middlings is known to be the limiting factor influencing the three objectives: enrichment, recovery and selectivity. Complex sulphide ore processing, as at the Rammelsberg concentrator, often involves coarse primary grinding followed by flotation of middlings, which are then reground. This process frequently is influenced by severe selectivity problems in the separation of PbS and ZnS. These problems differ with grain size; at coarser sizes the "intergrowth problem" predominates and at the opposite extreme the "slime problem". It is shown, that the intergrowth problem overwhelmingly influences recovery. The total recovery of lead in the rougher stage amounts to up to 85%; three-quarters of the remaining 15% is lost in coarse, extremely intergrown particles and the remaining quarter of the losses only being attributed to poorly floating fine particles. The total losses caused by the "slime problem" are about 10%, whilst 25% are irrecoverable due to particle composition.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 229-234

Secondary enrichment of gold is well known in Australian gold deposits. Many examples are documented of weathered zones containing almost pure gold that give way beneath the water-table to primary sulphide, sulph-arsenide or carbonate veins containing gold alloyed with much silver and/or copper. Other examples of secondary enrichment yield gold of high fineness, yet the primary vein contains only gold in atomic or exceedingly small particles within pyrite or arsenopyrite. The chemical composition of secondary gold may be very variable. For example, not only do silver and copper vary from location to location, but also in some soil environments iron is an important trace component in the gold alloy. However, in some "ironstone" environments of deeply weathered rocks near Kalgoorlie in Western Australia, secondary gold is free from iron and is very rich in gold, e.g. Au 99,46; Ag 0,64 (wt per cent). Most quartz-bearing gold nuggets clearly have been shed from quartz veins. However, the fineness of their gold is commonly found to be higher than that of the parent vein, and is due to displacement of Ag and other elements by supergene processes. Many large nuggets are devoid of quartz and have an irregular rounded or mammillary form. These are commonly found in regions of greenstone or slate, for example at Coolgardie in Western Australia. For many years argument has raged as to whether these nuggets have resulted from accretion in situ in the weathered zones or soils, or merely represent eroded rich patches in primary vein systems. In the Coolgardie and Kalgoorlie regions, for example, metamorphic or igneous fluids have produced irregular masses of gold in ultramafic schists or carbonate lenses. Some of these have been discovered below the supergene zone in recent years in such mines as the Barbara and the Surprise near Coolgardie and from older workings in the Oroya Shoot at Kalgoorlie. Erosion of these ore bodies could account for some of the nuggets found in soils and laterites near the old lodes. However, several nuggets from the laterites and deep soils of the Coolgardie district (many 5 to 20 ounce nuggets have recently been found with metal detectors) are composed of Au 96,5; Ag 3,5 (wt per cent). Textural and probe studies of these nuggets normally show that the gold encloses pisolitic concretions of Fe-rich laterite or bauxite or other soil detritus. In some nuggets, the ovoid inclusions of hematitic iron oxide become partly hydrated near the edge of nuggets. This results in expansion cracks bursting through the apparently solid gold surface. In several regions where nuggets of this type are found, there is no obvious vein or lode source of the accreted gold. In many of these the source of the migrating gold appears to be finely divided gold (atomic or colloidal) or gold-bearing sulphides or arsenides that are disseminated throughout the country rocks. Some of the nuggets from the Cloncurry region of Queensland are found where little or no basement gold mineralization is known. Large clots of a gold-silver alloy with just over 99% gold have accreted there in a bauxitic matrix. Other examples of supergene gold include: (a) octahedra of very pure gold (0,5 mm diameter) set on felted surfaces of Mn oxide, or in puggy clays; (b) filamentous and arborescent forms in several types of decomposed rock; (c) dendritic or paint gold on cracks in weathered rock, clay and laterite; and (d) gold impregnations of fossil wood and coaly matter. Variations in the non-gold components of solid gold reflect the chemical properties of gold-bearing non-hydrothermal solutions. These are commonly thiosulphate and chloride-complex solutions, but the role of the "humic acids" must not be ignored.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 235-237

Four types of zinc oxide ores of Belgian origin are identified on the basis of their mineralogical and chemical analyses. The ore samples specifically respond to some acid and alkaline reagents used at various concentrations. The results of mineragraphic examinations appear to be diagnostic when explaining the recoveries after leaching.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 239-243

This paper covers the study of two minerals used in ceramics, glass works and building industries, in which some anomalous behaviour has been observed in different stages of their processing. The methods of investigation employed were: optical microscopy, chemical analysis and industrial evaluation. The differences in behaviour are related to the textures, size and distribution of the crystals, cataclastic effects, origin and geological provenance. Information about these minerals is given with respect to their occurrence for different mines in Argentina with a geological review of the mines and technical specifications of the products. The observations carried out will enable the different industries to make better use of the minerals which will be of benefit to manufacturing processes and the finished products. The main objective of this paper is to draw attention to the necessity of carrying out mineralogical studies prior to the use of the minerals for their correct treatment and use.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 245-251

This paper gives the results of an empirical study of the floatability of sphalerite in terms of mineralogy and chemistry in natural untreated ore samples from a complex sulphide ore that had been partly exposed to supergene alteration. It is shown that the sphalerite in the study material had been altered by one or both of two reactions in the supergene zone. Firstly, it was enriched in copper (giving rise to chalcopyrite disease) and, secondly, it was depleted in iron. These reactions ran parallel to the well-known reaction in which pyrrhotite is oxidised to pyrite in the deeper parts of the weathering ore body. The floatability of the sphalerite is investigated on a relative basis, i.e. the floatability of sphalerite from near-surface samples is compared with that of sphalerite from depth samples. The loss of iron from the sphalerite correlates negatively with its performance in the Partridge and Smith flotation cell and, depending on the mineralogy of the primary ore, the study material can be divided into three major groups: ores rich in pyrrhotite or pyrite (II), those rich in magnetite, and those rich in chalcopyrite. Each of these groups show distinct patterns in terms of the relative floatability of galena, chalcopyrite and sphalerite, which indicate that the sphalerite that reports anomalously in the copper concentrate should show a larger range in iron content than that recovered in the zinc circuit.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 253-260

The Moncorvo iron ore deposits, north-eastern Portugal, are situated in an area which consists of Lower Palaeozoic and Upper Precambrian (Cambrian ?) formations which were cratonised during the Hercynian orogeny. The ore deposits consist of a folded Ordovician stratigraphic unit which has a thickness rarely exceeding 150±20 m and grading to quartzites along the strike. The proven, possible and probable ore reserves total about 1 000 million tonnes but the deposits have as yet not been intensely mined due to their high phosphorus content which concentration testworks were unable to reduce substantially. Initially, the samples selected for these tests were collected from trenches and a few exploratory drives and then classified by both chemical analysis and physical appearance in hand specimen. Following an intensive drilling campaign, more than 100 selected specimens of the ore deposits were studied under transmitted and reflected light. Then it was possible to identify the main ore types in the deposit: magnetitic, specularitic, specularitic/martitic, martitic and limonitic ores, associated with different phosphates such as apatite, lazulite, rockbridgeite, millisite, wavelite, phosphorus-bearing limonitic material and other minor phosphorus minerals. The collation of all mineralogical data has proved that the classification of ore types based on chemical analysis and physical appearance is very misleading. It has allowed the definition of "ore blocks" of different mineralogical composition to which specific types of beneficiation methods may be successfully applied.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 261-269

The term "auriferous pyrite" relates to an association of gold with pyrite in a variety of textural forms that affect metallurgical treatment. Extraction of the gold depends not only on the grain size and distribution of the gold in pyrite, but also on the crystal size and surface area of the pyrite. Occlusions of medium to coarse gold may be liberated by moderately fine grinding. The amenability of finely dispersed (submicroscopic) gold depends in part on the activity of the pyrite surface for oxidation. Two types of auriferous pyrite at Carlin, Nevada, have been examined by optical microscopy and SEM methods before and after aqueous oxidation test work to show how differences in morphological features can affect rates of oxidation and amenability to gold extractions by methods of chlorination and low temperature oxidation.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 271-274

The remobilisation of detrital gold grains by volatiles emanating from intruding dykes of various compositions is a common occurrence on most gold mines of the Witwatersrand Supergroup. This effect is normally of limited extent and does not appreciably affect gold-recovery rates. However, at Unisel Gold Mines Limited, a basic intrusive of lamprophyric composition has severely affected the nature of the gold mineralization of the Basal Reef in places. Super-heated volatiles emanating from the intrusive have remobilised gold and pyrite causing a significant percentage of the initial detrital gold to be locked up, in finely divided form, in recrystallized pyrite and in porous concretionary pyrite. This phenomenon has adversely affected the recovery of gold during cyanidation after normal run of mine milling. In areas affected by the intrusive gold recovery has been reduced by about five per cent. This paper outlines the types of pyrite occurring in the Basal Reef at Unisel and describes the modes of occurrence of gold and how predictions have been made regarding the recoverability of gold depending on the influence of the lamprophyric intrusive.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 275-286

Detailed ore microscopy investigations of silver-lead-zinc ore bodies 1-7 at Hilton Mine in north-west Queensland have delineated highly complex and diverse silver mineral distributions. Silver minerals, in approximate order of importance, include pyrargyrite, freibergite, native silver, argentian chalcopyrite, stephanite, argentopyrite and dyscrasite. Hilton Mine silver mineralogy within the block examined is characteristically heterogeneous in terms of: (a) its spatial distribution; (b) its silver mineral assemblages from site to site, and (c) its physical and textural location with respect to other ore-forming constituents. The diverse silver mineralogy with its variable textural associations is shown to strongly influence the flotation distribution of total silver reporting to each of the three products: lead concentrate, zinc concentrate and tailings of a Mount Isa lead-zinc concentrator circuit. The uniform poor recovery of Hilton silver to the lead concentrate is shown to be readily predictable on mineralogical and textural grounds.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 287-293

During weathering, curite can be transformed into a copper, a nickel or a cobalt uranyl phosphate, if these ions are avilable. However, in the Shinkolobwe ore, Zaïre, where all three types of metal ions are released in the weathered rock, only the copper species, metatorbernite, is encountered in the oxidation zone. Synthesis of the three types of uranyl phosphate in the laboratory has revealed that the copper uranyl phosphate precipitates selectively because of its lower solubility.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 295-300

Intensive mineralogical studies carried out on platiniferous ores and numerous plant products over a period of 12 years at the Johannesburg Consolidated Investment Co. Ltd (JCI) Minerals Processing Research Laboratory in the Republic of South Africa, indicate two distinct modes of occurrence of the platinum-group metals. These are: (a) discrete platinum-group mineral particles, and (b) submicroscopic "solid solution" platinum group elements (PGE) in base-metal sulphides. The bulk of Pt, all Ru, Os and Ir, as well as some Pd seem to form discrete platinum-group mineral particles, usually amenable to gravity concentration or flotation. Most of the Pd occurs in solid solution within base-metal sulphides and, thus, is recoverable only by flotation. Virtually all Rh is found in solid solution in base-metal sulphides of the Merensky Reef but forms discrete mineral particles in the UG-2 chromitite. A possible third mode is suspected, namely that minor to considerable amounts of some of the platinum-group metals may occur in the form of solid solution within oxides and silicates, thus rendering this proportion of precious metals theoretically unrecoverable with present metallurgical techniques.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 301-313

The uranium mineralization in the south-western Karoo is restricted to very fine-grained sandstones and siltstones in the Paalhuis Member of the Karoo Sequence, and is palaeogeographically and stratigraphically controlled. It is confined to lenses of limited extent that are distributed at random within the lower third portion of sandstone bodies. The mineralized sediment is relatively enriched in carbonaceous matter and sulphides, and is cemented by calcite. The uranium is considered to have been transported by surface and ground waters in the hexavalent state as uranyl ions which were either removed from the water by adsorption on to collectors (such as organic matter and oxyhydroxides of iron, titanium and manganese) and on clays, or complexed with carbonate, phosphate or fluoride. The uranium-bearing solutions were either entrapped within the detritus during deposition or circulated afterwards through the unconsolidated sediments. Then, under suitable physicochemical conditions, the hexavalent uranium was reconverted to the tetravalent state and precipitated in the form of cryptocrystalline coffinite, uraninite or pitchblende, suggesting that they were precipitated from solution at low temperatures. The ore minerals covered quartz and feldspar with a thin film and replaced or infilled open spaces in the groundmass and in organic debris. In the outcropping ore bodies, the tetravalent uranium became unstable and a great variety of minerals carrying hexavalent uranium accumulated at the surface. These concentrations, sometimes of very high grade, are of no economic interest because of their limited magnitude and erratic distribution.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 315-323

A description of the geology, mineralogy, geochemistry and S-isotope composition of pyritic ores of southern Tuscany, currently representing the major resource in the district, is given. Pyrtite occurs mostly as massive, high-grade, coarse-grained bodies either enclosed within Paleozoic-Triassic (?) phyllitic-quartzitic terrains, in association with anhydrite-dolomite lenses, or at the contact between the latter and the overlying calcareous dolomitic formation (Upper Trias). Smaller bodies are either associated with dislocations or in contact with granitic rocks related to the Mio-Pliocenic Apenninic orogeny. Among the many proposed genetic models, the authors favour a volcanic-sedimentary origin for the massive deposits associated with the Paleozoic-Triassic formations. Partial remobilisation during the Apenninic orogeny would have produced the emplacement of the smaller discordant deposits.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 325-336

Pyrometallurgical processes are complex reactions involving, typically, a large number of crystalline and liquid phases, both metal and oxide, as well as a gas phase. Knowledge of heterogeneous equilibria involving all these phases is needed in order to define the limiting conditions for the reactions and to optimise operational parameters. Equilibrium data for selected systems are reviewed and their relationship to thermodynamic properties of individual phases are discussed. Examples of activity-composition relations for a number of alloy and oxide (silicate) phases are presented. These include the alloy systems Pt-Me (Me = Fe, Ni, Co, Cr, Mn), Mo-Cr, and Fe-Ni, and the oxide solid solutions MgO-MeO (MeO = "FeO", CoO, NiO, MnO), CaO-MnO, Al2O3-Cr2O3, Mg2SiO4-Fe2SiO4, Ca2SiO4-Fe2SiO4, MeAl2O4-MeCr2O4 (Me = Fe, Mg, Co), Fe-Ni-O. Special attention is given to equilibrium relations in chromium oxide-containing silicate systems under strongly reducing conditions at high temperatures. It is shown that the solubility of chromium oxide in relatively acid silicate melt increases drastically, and solidus and liquidus temperatures decrease drastically, with decreasing oxygen pressures below a certain threshold value. This is because increasing amounts of chromium are converted from Cr3+ to Cr2+ in the liquid with decreasing oxygen pressures in this low-pressure region.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 337-349

Determination of the phases present in samples of pre-reduced chromite pellets was carried out by the use of optical, X-ray diffractometric and electron-microprobe techniques. It was found that from a chromite-rich pellet, segregation of iron and chromium occurred in successive stage. The chromite grains undergo a peripheral metamorphosis to iron deficient chromite (termed aluminian chromite), the iron from this diffusion forming the carbide Fe3C. Later chromium diffusion results in double carbides of iron and chromium of the M7C3 and M23C6 types. These carbides, in the initial stages of metallisation, are caught up in inter-phase stress fractures formed along the boundary between aluminian chromite and chromite proper. On further reduction, diffusion of the carbides from the aluminian chromite phase to the periphery of the original grain then occurs. Blebs and spherules of the two carbides, then, form outside the original grain boundaries. Further reduction/diffusion eventually results in the original chromite being completely converted to aluminian chromite. Magnesium diffusion and/or counter diffusion and spinel overgrowths are features of the phase changes in the chromite during reduction roasting.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 351-363

A 48 MVA submerged-arc furnace, which has been used for the production of high-carbon ferrochromium from chromite ore of the Bushveld Complex, was excavated approximately one month after it had been switched off. A mineralogical study was made of samples from various regions of the furnace so that the reduction mechanism involved in the reduction and smelting of South African chromium ore under actual plant conditions would be elucidated. The ultimate purpose of the study was an improvement of the efficiency of the process by the application, wherever possible, of the knowledge gained. The conclusions drawn from the profiles of chromite particles in different stages of reduction, which were based on electron-microprobe analyses, were of particular interest. The profiles showed that the order of reduction of oxides in chrome spinel to metallic phases is different in local ores from that in other ores. The role of silica in the reduction mechanism and the phases in which residual chromium is found are also discussed.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 365-367

As the correlation between mineralogical phase and chemical composition influences the type of application at different high temperatures, we studied the mineralogical phases of nine crystal glass fibres of the temperature ranges 1 150°C (Type 1), 1 400°C (Type 2) and 1 500°C (Type 3) at various high temperatures. The methods used in the study were microscopy, X-ray diffraction, transmission electron microscopy and differential thermal analysis. The investigations showed that mullite forms in glassy fibres of the system Al2O3. SiO2 from 850°C to 990°C as 2/1 mullite; 3/2 mullite appeared above 990°C besides the crystallization of cristobalite. Fibres with 95% Al2O3 include the phases delta-Al2O3 and α-Al2O3 and mullite. Delta-Al2O3 is stable up to 1 100°C. α-Al2O3 and mullite are the only stable phases at 1400°C. These different crystal phases influence the quality of the technical fibre according to the stability field of glass and crystals. This study has determined that it is possible to identify different fibres from different productions by their mineralogical compositions and to relate them to the high temperature application.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 369-373

Zeolite Y is a synthetic mineral which is used extensively in catalysis, especially for cracking gas oil. A process is discussed whereby a calcined South African kaolinite is reacted with aqueous sodium hydroxide to extract only the silica from the material. Together with metakaolinite, which is also dissolved in the alkali solution, NaY zeolite crystallizes inside the leached matrix when the mixture is heated. By pre-selecting the reaction parameters, particularly those concerning the reaction mixture, it is possible to control the composition and concentration of the zeolite formed. When the sodium cations are exchanged for either rare earth-hydroxyl or aluminium-hydroxyl cations, active cracking catalysts are produced.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 375-382

The paper describes two research programmes: small-scale experiments in which cubes (with sides of 20 mm) were heated with coke, coal, or graphite to temperatures of between 1 200 and 1 500°C for 1, 2 or 3 hours in an argon atmosphere; and large-scale experiments in which 4 Kg charges of ore and coal, or of ore and coke, in stoichiometric proportions, were heated to temperatures between 1300 and 1 600°C for up to four hours. The reacted charges were examined by microscope, by X-ray diffraction analysis, and by X-ray microanalysis using an energy-dispersive system on a scanning electron microscope. It was found that the early stages of reduction involve complex mineralogical changes including the breakdown of braunite and gangue minerals, the reduction of the higher manganese oxides to manganous oxide and of hematite to metallic iron, and the formation of slag as a result of reaction between gangue and manganous oxides. Further reduction of the ore involves the carburisation of the metallic phase and the reduction of solid manganous oxide, or of manganous oxide dissolved in the slag, by solid carbon or carbon dissolved in the metal.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 383-391

The results of a series of experimental runs undertaken on silicate melts at 1 300°C and 1 400°C and at an oxygen fugacity of -log 8,50 have been combined with data taken from previous work completed at a temperature of 1 200°C. Sulphur solubility was established as a function of sulphur fugacity and temperature for a constant value of 17 weight per cent FeO in the silicate melt. Sulphide capacities (log Cs) calculated from the experimental data and plotted against 1/T°K gave rise to separate isobaric, sulphur fugacity trends which approached predicted behaviour only at temperatures in excess of 1 400°C. However, it was possible to use the trends to establish the sulphur solubility of the silicate melt at 1 350°C and 1 450°C. The results of the study indicate that the field of sulphide saturation increases with increasing temperature. In addition, it was shown that the influence of temperature on sulphur solubility in the field of undersaturation falls from a factor of about 8,5 per 100°C at 1 000°C to about three times per 100°C at 1400°C. Although the absolute amount of sulphur in the melt decreases with decreasing sulphur fugacity, the above ratios are virtually independent of sulphur fugacity. The expanded field of sulphur saturation associated with an increase in temperature would favour the formation of a sulphide phase in magmas emplaced at high temperature, particularly if this is associated with the addition of sulphur from the surrounding country rocks. The identification of the field of sulphur saturation in this study in the temperature range present in a Cu-Ni smelter, together with the similarity of the silicate melt to the composition of slag produced during the smelting of Merensky Reef ore, provides an indication of the intensive parameters required when matte-slag systems are modelled experimentally.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 393-402

Differential Thermal Analysis (DTA) curves are presented for vitreous blast-furnace slag with a high magnesia content and an explanation for the various peaks on the curves is given. It is shown how DTA data can be used to estimate the slag content of mixtures of unhydrated Portland cement and granulated blast-furnace slag. DTA thermogravimetry (TG), X-ray diffraction (XRD) and transmission electron microscopy (TEM) data are presented for blast-furnace slag hydrated in ambient conditions, in boiling water and in steam at 215°C. The products of hydration are mentioned. Particular attention was paid to the role played by the magnesium in the slag during hydration.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 403-408

This paper is presented to demonstrate the value of high resolution transmission electron microscopy for solving detailed problems in applied structural mineralogy. The results of three investigations are cited with this objective in mind. In the first case, high-resolution methods of transmission electron microscopy show that crocidolite contains defects, previously discovered in anthophyllite, which are thought to promote the reaction to a higher biopyribole. Hence there is evidence from common mechanisms for the conversion of amphiboles to micas. Electron-microscopic methods show, in the second case study. that a natural Rodoya chromite is exsolved by spinodal decomposition into two phases, one of which is intermediate in a magnesio-chromite-chromite series and the other being close to magnetite. Prior to the electron-microscopic study these effects were unknown in chromites. In the third example of a successful high-resolution study, uraninite, within a sample of thucholite, is shown for the first time to be present in the form of finely dispersed 1-5 mm grains. By analogy with previous work on a carbonaceous iron-doped polymer char, it is suggested that uraninite grains arise from coalescing uranium atoms during the transformation of the hydrocarbon matrix to a high-rank structure.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 409-412

Mass measurements confirm that the adsorption of polyacrylamide on kaolin reduces the water adsorption at a given relative humidity. Correlation of these mass measurements with two ratios of infra-red spectral band intensities, shows that the amount of water adsorbed on kaolin can be derived directly from spectra if care is taken. The ratios of spectral intensities give an indication of the concentration of clay in the spectral beam and the water adsorbed on the kaolin. The adsorption of polyacrylamide on kaolin results in a shift of the second highest energy hydroxyl absorption to higher wave number. The adsorption of polyacrylamide by a hydrogen-bonding mechanism to the external surface hydroxyls indicates the presence of a previously unsuspected absorption band due to these hydroxyls in the region 3 700-3 660 cm-1.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 413-417

The standard method of studying uranium distribution in geological material by registration of fission tracks from the thermal neutron-induced fission of 235U has been adapted for utilisation in the SAFARI-1 reactor at Pelindaba. The theory of fission-track registration as well as practical problems are discussed. The method has been applied to study uranium distribution in a variety or rock types and the results are discussed in this paper. The method is very sensitive and uranium present in quantities far below the detection limit of the microprobe have been detected.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 419-424

Zircons occur as secondary minerals in association with gold in the Upper Witwatersrand System, and an understanding of their provenance could contribute to the understanding of the source of the uranium and gold mineralization in this formation. Since zircon is a minor constituent of the rock, a very sensitive analytical method is necessary for its study. Developments to the method of neutron-activation analysis were made to enable quantitative results to be obtained for eleven elements in samples of zircons with masses as low as 10 µg. The limit of detection for scandium, for example, was 1 ppm in a sample of 1 µg. These elements included four of the rare earths, and uranium, thorium and scandium. In comparison with the compositions for zircons reported in the literature, the zircons from the three areas studied in the Witwatersrand show a great degree of similarity. However, there are differences between the samples from the Kimberley and Vaal reefs. By the use of multivariate statistics, it was shown that this difference enables these two populations to be classified with a success rate of 85%. The difference arises from differences in the average thorium content and in the distribution patterns of the rare earths.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 425-434

A correction technique applied to analytical measurements obtained by X-ray fluorescence analysis of compressed powders is extended to include, in addition to corrections for elemental matrix effects, corrections for mineralogical effects. This extension results in a 37% relative improvement in the accuracy of determination of manganese in manganese ores of the Middelplaats Mine, which is located on the Kalahari Manganese Field in the Republic of South Africa. This application of mineralogical correction factors requires accurate measurement of the modal mineralogy of samples analysed. A method used to calculate modal mineralogy from elemental data is presented, and checks on the validity of the method are described. Since knowledge of ore mineralogy allows the calculation of ore density, these mineralogical data have an application in ore reserve calculation, as well as a potential application in ore-beneficiation studies. A mineralogical approach to correction factors in X-ray fluorescence analysis may be applied successfully in other situations where time considerations preclude sample fusion, and regular, accurate determinations are required of samples with fairly constant matrices.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 435-442

The paper describes the equipment and method developed for the on-stream analysis of apatite in the slurries of a beneficiation plant. The equipment uses a divergent beam of X-rays that irradiates the slurry as it flows through a sample presenter. The diffracted beam is focused by an elastically bent quartz monochromator crystal into a scintillation detector. The P2O5 content and percentage solids in the slurry are determined from peak and background intensities by the use of a quadratic equation. Measurements showed that the accuracy of these determinations is comparable with that of chemical analysis, which is the method used at present. In addition, one analysis takes two minutes as against the 20 minutes required for each chemical analysis.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 443-447

A selection of stereological relationships is derived for the measurement and prediction of mineral liberation. The Steiner model for liberation is used to develop approximate relationships and a new method is proposed which has a sounder theoretical base than that of Steiner. The new method produces a direct relationship between the apparent liberation as measured on a section through the particles and the true liberation. This relationship is surprisingly simple and has the form Lm(D) = Lm(αD) where Lm is the apparent liberation, Lm(D) the true liberation at mesh size D and αD is the length of the largest linear intercept through particles of mesh size D. The new model provides a prediction of the liberation as a function of particle mesh size from a knowledge of the linear intercept distribution of mineral phase in the ore.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 449-464

The image analyser, in use at the Council for Mineral Technology, is a modern television-based image-analysing system. It provides full programmable control of the analysing system, data processing by means of a built-in microcomputer, six image memories (or bit planes) for the storage of digital images and modules for interactive operations and individual analyses. Each of the phases in an image is detected sequentially and digitised according to the amount of light reflected from or transmitted by that phase. From these digital images a number of basic measurements (area, counts perimeter and various projections) are obtained with great speed and accuracy. By the use of stereological principles and relationships, many additional parameters such as proportions of phases (modal analysis), size measurements and form analysis can be calculated. The capability of the instrument to do sequential image transformations extends its application and versatility, since the modification of images by transformations and interactive operations usually forms part of the analytical process. The instrument lends itself to a wide variety of applications in mineralogy and extractive metallurgy, since it can analyse microscopic and macroscopic images obtained from polished and thin sections of rocks, ores, mineral aggregates and grain mounts of mineral-dressing products, and also from photographic documents of similar products. Brief descriptions of typical applications are included.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 465-467

Fully automated image analysis nowadays is a very powerful tool in the field of quantitative mineralogy. In this paper, improved methods for classification of coal using the image analyser Leitz TAS are described. Starting only with reflectivity measurements, the results are improved by using image transformations based on mathematical morphology. An outlook on further developments is given.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 469-473

This paper presents a review of the methods presently in use and being developed at the Bureau de Recherches Géologiques et Minières laboratory for assessing mineral liberation. Most techniques use an image analysis of a section for data collection, and thus, measurements can be made on a routine basis for areal analysis. Linear analysis, with stereological reconstruction to account for bias in sectioning, has been presented in various previous papers and is only briefly outlined. New developments include neighbourhood behaviour analysis through geometric covariograms obtained by image analysis, and the assessment of potential preferential fractures at mineral boundaries derived from the study of mechanics of solids made up of two materials. Applications are presented for the case of a complex sulphide ore in Brittany.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 475-478

This paper describes the value of quantitative mineralogical data to the mineralogist. The traditional methods of mineralogical analysis are shown to be too slow for many industrial applications. Image analysis is put forward as a viable alternative which is capable of producing large amounts of accurate data in a short time. Brief descriptions are provided of the various categories of image analysis equipment that are available and criteria are provided to help the mineralogist select the most appropriate equipment for a particular purpose. The sampling and specimen preparation problems associated with image analysis are described and mention is made of the stereological transformations that are needed to convert image analysis data into three-dimensional information. Examples are given of the application of image analysis during prospecting and in mineral processing operations.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 479-483

The technique by which polished sections are coated with iridescent films to increase the contrast between phases is explained. The apparatus used is described, and materials found useful for the coating process are listed. The technique is of most use to the mineralogist in helping him to distinguish various minerals in modal analysis.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 485-489

Two types of magnetohydrostatic separators were developed for use with the standard Frantz Isodynamic separator. The first separator is suitable for use on fairly large amounts of material in continuous operation. It consists of a Perspex chute between inclined pole-pieces, with a widening gap in the direction of inclination. Particles of different densities follow paths parallel to the density gradient, and, as the magnetic field decreases in strength, they drop down in different parabolic paths. The second separator consists of a modified glass T-piece, which the operator holds by hand. It is suitable for the very quick separation of small amounts of mineral concentrates. The lightest fractions are separated first, and then, the progressively heavier ones. This apparatus permits the economic use of some of the more expensive paramagnetic liquids.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 491-498

Magnetic techniques are available today for the separation, characterisation and preparation of samples for mineralogical studies. In particular, the methods derived from the high intensity-high gradient and from the magnetohydrostatic separation techniques provide increased degrees of resolution and reliability. In this context, the basic features of the densimetric and susceptibility fractionation methods are described. The basic theories, useful field geometries and procedures for application are reviewed. The novel magnetic differential susceptibility and the three-dimensional axial separation systems are looked at, and their potential uses for mineralogical studies are discussed.
Back to title list




ICAM 81. Proceedings of the First International Congress on Applied Mineralogy. Special Publication No.7, 499-506

Techniques employing X-ray, laser, and ion beams in destructive and non-destructive analysis are discussed briefly and examples of their application are given. The effects of electron-beam analysis are discussed in more detail, together with examples of its application in the biological and medical field, environmental studies, mineralogy, pyrometallurgy and hydrometallurgy. Attention is drawn to the possible misinterpretation of electron-microprobe results with special reference to detection limits, the smearing of sample surfaces, spatial resolution and migration or loss of elements under the electron beam.
Back to title list




Namaqualand Metamorphic Complex. Special Publication No.10, map

No abstract.
Back to title list




Contributions to the geology of the Barberton Mountain Land. National Geodynamics Programme, Barberton Project. Special Publication No.9, P VII

The Geodynamics Project was the major international programme of research in the solid earth sciences in the 1970s. The aim of this Project was to advance our understanding of the dynamics and dynamic history of the solid earth. Much attention was focused on the deep-seated foundations of geological phenomena, especially evidence for motions at depth. On the international level, the Project was run by the Inter-Union Commission on Geodynamics of the International Union of Geological Sciences (IUGS) and the International Union of Geodesy and Geophysics (IUGG). South African participation was co-ordinated and financed by the S. A. Committee for the International Union of Geological Sciences (SACUGS) acting on behalf of the Council for Scientific and Industrial Research (CSIR). Guided by reports and opinions from many academic earth scientists SACUGS defined a framework for a National Programme in South Africa and an illustrated Interim Report on this Programme was presented to the Inter-Union Commission in Grenoble in August, 1975. The comprehensive theory of plate tectonics was first presented in 1968. This valuable model provided a set of well-defined statements about the spreading of sea-floor, convergence of sea-floor and movement of continents. There were also predictions concerning the development of geosynclines and orogenic belts, which could be tested in a clear and decisive way. Because these themes were to play an important role in the Geodynamics Project, and four eroded orogens cover large areas in southern Africa, it was soon decided that a major thrust of the National Programme would be directed to these eroded orogenic belts. Well-exposed narrow strips across four orogens were selected for intensive and multidisciplinary research, with emphasis on their margins against neighbouring cratonic regions: Permo-Triassic Cape Fold Belt, Damara Orogen, Namaqua-Natal Mobile Belt (3 strips), Limpopo Belt. The second component of the National Programme was centred on areas of the Kaapvaal craton which presented striking unsolved geodynamic problems - the Vredefort structure (south-west of Johannesburg) and the granitic terrane surrounding the Barberton Mountain Land. The third component of the National Programme dealt with the geodynamics of southern Africa, and the growth of the ocean basins around the subcontinent, during the Mesozoic and Cenozoic. One part of this research (e.g. mapping and understanding the origin of major features of our surrounding ocean basins, reconstructing the supercontinent of Gondwanaland, mapping of deep structures having high electrical conductivity) was undertaken within other Programmes of the CSIR. Another part focused on the geochemistry of the Karoo Volcanics - a grand volcanic episode which preceded and accompanied the break-up of Gondwanaland. Geochemical and isotopic parameters of the volcanics were studied intensively to establish the mantle constitution and mantle processes which led to the Karoo eruptions and their possible relation to the disruption of Gondwanaland. Within the second and third components of the National Programme many advances were made by "standing on the shoulders" of the achievements of the international Upper Mantle Project of the 1960s. The scientific programme which I have briefly sketched was actively pursued at six South African universities between 1974 and 1980. The inherent need for multidisciplinary "input" led to unprecedented co-operation between earth scientists at these universities. The university workers and scientists from the Geological Survey, Atomic Energy Board (now Nuclear Energy Corporation) and the CSIR gained new insights concerning existing research facilities and capabilities across South Africa. Some young scientists came from overseas to work on the National Programme; some departments forged new links with overseas university workers who were interested in the same fundamental problems in geodynamics. Throughout this period, the Geological Institute of the University of Gottingen in Germany maintained a very strong research effort on the Damara Orogen and also co-operated generously with South African workers on Damaran problems. The principal results of the National Programme were presented at an international symposium, sponsored by The Geological Society of South Africa and the CSIR, held at the University of Pretoria from 6th to 10th July, 1981. Individual contributors became fully aware of the discoveries within the national enterprise and the importance of interpreting their results within the broad perspectives of the national and international ventures. Participants in this symposium - entitled Geocongress '81 - were also able to gauge the effectiveness of focusing many earth science disciplines in narrow intensive study strips, a strategy which had not been attempted before in South African geological research. Sponsored by the CSIR, The Geological Society of South Africa is now publishing a series of volumes which present some of the results of these intensive studies on South African geodynamics problems. The present volume forms part of the series. Within it, the reader will also find references to other papers on the particular field of study published in journals during the course of the Geodynamics Project.
Back to title list




Contributions to the geology of the Barberton Mountain Land. National Geodynamics Programme, Barberton Project. Special Publication No.9, XI-XIV

The South African contribution to the Upper Mantle Project, which terminated in 1969, concentrated mainly on establishing details of the nature and distribution of the mafic and ultramafic volcanic and plutonic sequences in the Barberton greenstone belt. During this study reconnaissance investigations were made in the surrounding Archaean granitic terrane and it was realized that the granites and their contained greenstone xenolithic remnants presented a unique environment in which to undertake research that might contribute towards an understanding of the early crustal evolution of southern Africa. With this in mind participation was eventually sought in the National Geodynamics Programme and the Barberton Project commenced in April, 1975. The Project initially involved the study of a selected strip of country in the Eastern Transvaal Lowveld extending from south of Badplaas, northwards to Bushbuckridge - a total distance of approximately 180 km. This area is underlain by Archaean granites and greenstone xenoliths and the excellent exposure available in the region provides an unrivaled opportunity to investigate some of the oldest rocks on earth. The aim of the investigations was to ascertain details concerning the geochemical, petrological, isotopic and field geological relationships of the rocks in the area with a view to understanding the complex interaction of the granites and greenstones and to ultimately contribute towards establishing the nature and evolutionary history of the earth's primitive crust, including the development of cratons and shield areas that form the nuclei of continents. The Barberton Project was tackled in three stages. The first stage comprised regional geological mapping of extensive tracts of granitic terrane north and south of the Barberton greenstone belt (Fig. 1). The region extending from Nelspruit and Kaapmuiden northwards to Bushbuckridge, and occupying an area of approximately 2 500 km², was mapped by L.J. Robb and was described in an M.Sc. dissertation (Robb, 1977) as well as in several other publications (McCarthy and Robb, 1978; Robb, 1978, 1979). Regional mapping in the tract of granite country immediately north of the Barberton greenstone belt and south of the area mapped by Robb, was undertaken by D.A. van Nierop. These field investigations, in the Kaapsehoop-Nelspruit-Kaapmuiden area, terminated at the end of 1979 and the results are incorporated in a paper in this volume describing the geology of the Nelspruit batholith (Robb et al., 1983). Aspects of the tectonic history of the region immediately north of the Barberton greenstone belt were also described by Fripp et al., (1980). South of the Barberton greenstone belt regional investigations, over an area of approximately 2 000 km², were carried out between the Swaziland border and the Transvaal Drakensberg Escarpment. Studies were undertaken in the Carolina-Badplaas-Lochiel-Amsterdam area by C.R. Anhaeusser who found it advantageous to the project to extend the mapping beyond the boundaries of the originally defined Geodynamics study strip. Throughout the first stage of the project mapping was carried out using 1:30 000 scale aerial photographs and 1:50 000 topocadastral maps. Data recorded included rock type, texture and structural details, and samples were collected on a systematic basis for petrological and geochemical study. Selected exposures were sampled for reconnaissance geochronological purposes. The second stage comprised further, more detailed, mapping of selected areas, granitic plutons, migmatite platforms and greenstone enclaves that had been encountered during the regional mapping programme. Most of this detailed mapping was undertaken in the southern and western parts of the Barberton Mountain Land (Anhaeusser and Robb, 1980). Granitic bodies singled out for special study included the Kaap Valley tonalite pluton, the Doornhoek and Batavia trondhjemite plutons, and the Dalmein granodiorite/adamellite pluton (mapped by L.J. Robb). In addition, the Theespruit and Steynsdorp trondhjemite gneiss plutons as well as the Boesmanskop Syeno-granitic Complex were mapped by C.R. Anhaeusser. Detailed mapping of selected migmatite exposures in the region was undertaken by L.J. Robb who prepared 1:50 outcrop maps showing the principal components of the migmatites as well as their detailed field relations (structure, fabric, composition). During the second stage of the project numerous greenstone xenoliths found in the granitic terrane south-west of the Barberton greenstone belt were mapped by C.R. Anhaeusser using 1:10 000 scale aerial photographs. Two areas were subsequently singled out for detailed description and include the Weergevonden greenstone remnant south of the Boesmanskop syenite pluton (Anhaeusser, 1980) and the Schapenburg greenstone remnant in the Jessievale region (Anhaeusser, 1983). The third stage of the Barberton Project involved petrological and geochemical analysis of selected samples collected during the early phases of the investigation. Isotopic and geochronological investigations followed the field studies in the Nelspruit-Bushbuckridge area and were carried out by E.S. Barton, A.J. Burger, H.J. Welke and L.J. Robb. Similar investigations were later undertaken by J.M. Barton, Jr. in other parts of the Barberton Mountain Land with particular attention being given to the areas south-west and south-east of Badplaas as well as in Swaziland (Barton, 1981a, b; Barton et al., 1980). A synthesis of all the available geochronologic and isotopic data applicable to the Barberton granite-greenstone terrane appears in this volume (Barton, 1983; Barton et al., 1983). The third stage also involved an appraisal of all the data gathered during the course of the Barberton Project and these findings were employed in an attempt to ascertain the genetic or evolutionary history of the various components making up the Archaean granite-greenstone crust in the Barberton Mountain Land. Constraints were imposed on the various interpretations of the data using geochemical modelling techniques coupled with the isotopic information acquired from the region as a whole. The earlier results of these investigations were made available in a Ph.D. thesis (Robb, 1981a) and in a number of papers published in local and international journals (Anhaeusser, 1978, 1980, 1981; Anhaeusser et al., 1979; Anhaeusser and Robb, 1980, 1981; Robb, 1981b; Robb and Anhaeusser, 1979). Most of the latest results of the Geodynamics research efforts in the Barberton Mountain Land are contained in the papers making up this Special Publication.
Back to title list




The Limpopo Belt. A contribution to the National Geodynamics Programme. Special Publication No.8, P VII

The Geodynamics Project was the major international programme of research in the solid earth sciences in the 1970s. The aim of this Project was to advance our understanding of the dynamics and dynamic history of the solid earth. Much attention was focused on the deep-seated foundations of geological phenomena, especially evidence for motions at depth. On the international level, the Project was run by the Inter-Union Commission on Geodynamics of the International Union of Geological Sciences (IUGS) and the International Union of Geodesy and Geophysics (IUGG). South African participation was co-ordinated and financed by the S.A. Committee for the International Union of Geological Sciences (SACUGS) acting on behalf of the Council for Scientific and Industrial Research (CSIR). Guided by reports and opinions from many academic earth scientists SACUGS defined a framework for a National Programme in South Africa and an illustrated Interim Report on this Programme was presented to the Inter-Union Commission in Grenoble in August, 1975. The comprehensive theory of plate tectonics was first presented in 1968. This valuable model provided a set of well-defined statements about the spreading of sea-floor, convergence of sea-floor and movement of continents. There were also predictions concerning the development of geosynclines and orogenic belts, which could be tested in a clear and decisive way. Because these themes were to play an important role in the Geodynamics Project, and four eroded orogens cover large areas in southern Africa, it was soon decided that a major thrust of the National Programme would be directed to these eroded orogenic belts. Well-exposed narrow strips across four orogens were selected for intensive and multidisciplinary research, with emphasis on their margins against neighbouring cratonic regions: Permo-Triassic Cape Fold Belt, Damara Orogen, Namaqua-Natal Mobile Belt (3 strips), Limpopo Belt. The second component of the National Programme was centred on areas of the Kaapvaal craton which presented striking unsolved geodynamic problems - the Vredefort structure (south-west of Johannesburg) and the granitic terrane surrounding the Barberton Mountain Land. The third component of the National Programme dealt with the geodynamics of southern Africa, and the growth of the ocean basins around the subcontinent, during the Mesozoic and Cenozoic. One part of this research (e.g. mapping and understanding the origin of major features of our surrounding ocean basins, reconstructing the supercontinent of Gondwanaland, mapping of deep structures having high electrical conductivity) was undertaken within other Programmes of the CSIR. Another part focused on the geochemistry of the Karoo Volcanics - a grand volcanic episode which preceded and accompanied the break-up of Gondwanaland. Geochemical and isotopic parameters of the volcanics were studied intensively to establish the mantle constitution and mantle processes which led to the Karoo eruptions and their possible relation to the disruption of Gondwanaland. Within the second and third components of the National Programme many advances were made by "standing on the shoulders" of the achievements of the international Upper Mantle Project of the 1960s. The scientific programme which I have briefly sketched was actively pursued at six South African universities between 1974 and 1980. The inherent need for multidisciplinary "input" led to unprecedented co-operation between earth scientists at these universities. The university workers and scientists from the Geological Survey, Atomic Energy Board (now Nuclear Energy Corporation) and the CSIR gained new insights concerning existing research facilities and capabilities across South Africa. Some young scientists came from overseas to work on the National Programme; some departments forged new links with overseas university workers who were interested in the same fundamental problems in geodynamics. Throughout this period, the Geological Institute of the University of Gottingen in Germany maintained a very strong research effort on the Damara Orogen and also co-operated generously with South African workers on Damaran problems. The principal results of the National Programme were presented at an international symposium, sponsored by The Geological Society of South Africa and the CSIR, held at the University of Pretoria from 6th to 10th July, 1981. Individual contributors became fully aware of the discoveries within the national enterprise and the importance of interpreting their results within the broad perspectives of the national and international ventures. Participants in this symposium - entitled Geocongress '81 - were also able to gauge the effectiveness of focusing many earth science disciplines in narrow intensive study strips, a strategy which had not been attempted before in South African geological research. Sponsored by the CSIR, The Geological Society of South Africa is now publishing a series of volumes which present some of the results of these intensive studies on South African geodynamics problems. The present volume forms part of the series. Within it, the reader will also find references to other papers on the particular field of study published in journals during the course of the Geodynamics Project. Accordingly, each volume will allow the reader to fully assess the state of knowledge on the area concerned and the present understanding of the deep-seated foundations of the phenomena observed. The full impact of these researches will become clear when we can compare the results obtained at the very different "depth levels" dealt with in the individual intensive study areas. I hope that when the present series of volumes has been published it will be possible for the co-ordinators to produce a synthesis of the gains to geody- namics which exploits this great range of structural levels and the four billion years of earth history involved. On behalf of the many researchers who took part in this project, I wish to record our deep appreciation to five officers of the CSIR whose continued efforts on our behalf were vital to the success of this venture: the late Dr C. van der Merwe Brink (President and Chairman of SACUGS), his successor, Dr. C.F. Garbers, Dr P. Malherbe, Dr W.T. de Kock, Dr J.A. Brink and Mr O.A. van der Westhuysen.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, P VII

The Geodynamics Project was the major international programme of research in the solid earth sciences in the 1970s. The aim of this Project was to advance our understanding of the dynamics and dynamic history of the solid earth. Much attention was focused on the deep-seated foundations of geological phenomena, especially evidence for motions at depth. On the international level, the Project was run by the Inter-Union Commission on Geodynamics of the International Union of Geological Sciences (IUGS) and the International Union of Geodesy and Geophysics (IUGG). South African participation was co-ordinated and financed by the S. A. Committee for the International Union of Geological Sciences (SACUGS) acting on behalf of the Council for Scientific and Industrial Research (CSIR). Guided by reports and opinions from many academic earth scientists SACUGS defined a framework for a National Programme in South Africa and an illustrated Interim Report on this Programme was presented to the Inter-Union Commission in Grenoble in August, 1975. The comprehensive theory of plate tectonics was first presented in 1968. This valuable model provided a set of well-defined statements about the spreading of sea-floor, convergence of sea-floor and movement of continents. There were also predictions concerning the development of geosynclines and orogenic belts, which could be tested in a clear and decisive way. Because these themes were to play an important role in the Geodynamics Project, and four eroded orogens cover large areas in southern Africa, it was soon decided that a major thrust of the National Programme would be directed to these eroded orogenic belts. Well-exposed narrow strips across four orogens were selected for intensive and multidisciplinary research, with emphasis on their margins against neighbouring cratonic regions: Permo-Triassic Cape Fold Belt, Damara Orogen, Namaqua-Natal Mobile Belt (3 strips), Limpopo Belt. The second component of the National Programme was centred on areas of the Kaapvaal craton which presented striking unsolved geodynamic problems - the Vredefort structure (south-west of Johannesburg) and the granitic terrane surrounding the Barberton Mountain Land. The third component of the National Programme dealt with the geodynamics of southern Africa, and the growth of the ocean basins around the subcontinent, during the Mesozoic and Cenozoic. One part of this research (e.g. mapping and understanding the origin of major features of our surrounding ocean basins, reconstructing the supercontinent of Gondwanaland, mapping of deep structures having high electrical conductivity) was undertaken within other Programmes of the CSIR. Another part focused on the geochemistry of the Karoo Volcanics - a grand volcanic episode which preceded and accompanied the break-up of Gondwanaland. Geochemical and isotopic parameters of the volcanics were studied intensively to establish the mantle constitution and mantle processes which led to the Karoo eruptions and their possible relation to the disruption of Gondwanaland. Within the second and third components of the National Programme many advances were made by "standing on the shoulders" of the achievements of the international Upper Mantle Project of the 1960s. The scientific programme which I have briefly sketched was actively pursued at six South African universities between 1974 and 1980. The inherent need for multidisciplinary "input" led to unprecedented co-operation between earth scientists at these universities. The university workers and scientists from the Geological Survey, Atomic Energy Board (now Nuclear Energy Corporation) and the CSIR gained new insights concerning existing research facilities and capabilities across South Africa. Some young scientists came from overseas to work on the National Programme; some departments forged new links with overseas university workers who were interested in the same fundamental problems in geodynamics. Throughout this period, the Geological Institute of the University of Gottingen in Germany maintained a very strong research effort on the Damara Orogen and also co-operated generously with South African workers on Damaran problems. The principal results of the National Programme were presented at an international symposium, sponsored by The Geological Society of South Africa and the CSIR, held at the University of Pretoria from 6th to 10th July, 1981. Individual contributors became fully aware of the discoveries within the national enterprise and the importance of interpreting their results within the broad perspectives of the national and international ventures. Participants in this symposium - entitled Geocongress '81 - were also able to gauge the effectiveness of focusing many earth science disciplines in narrow intensive study strips, a strategy which had not been attempted before in South African geological research. Sponsored by the CSIR, The Geological Society of South Africa is now publishing a series of volumes which present some of the results of these intensive studies on South African geodynamics problems. The present volume forms part of the series. Within it, the reader will also find references to other papers on the particular field of study published in journals during the course of the Geodynamics Project. Accordingly, each volume will allow the reader to fully assess the state of knowledge on the area concerned and the present understanding of the deep-seated foundations of the phenomena observed. The full impact of these researches will become clear when we can compare the results obtained at the very different "depth levels" dealt with in the individual intensive study areas. I hope that when the present series of vole umes has been published it will be possible for the co-ordinators to produce a synthesis of the gains to geodynamics which exploits this great range of structural levels and the four billion years of earth history involved. On behalf of the many researchers who took part in this project, I wish to record our deep appreciation to five officers of the CSIR whose continued efforts on our behalf were vital to the success of this venture: the late Dr C. van der Merwe Brink (President and Chairman of SACUGS), his successor, Dr. C.F. Garbers, Dr P. Malherbe, Dr W.T. de Kock, Dr J.A. Brink and Mr O.A. van der Westhuysen.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, XI-XIII

Intensive coordinated research across the Damara Orogen started in 1974 as part of the South African contribution to the International Geodynamics Project. The focus of attention was a selected study strip across the western end of the north-east-trending branch of the orogen. Two main factors determined the location of this study strip, (i) an almost 100 per cent exposure, and (ii) a good coverage of basic, large-scale geological mapping (Fig. 1). The strip, 50 km wide and 535 km long, extended from the Naukluft Mountains in the south (24°20'S) to the southern edge of the Kamanjab Inlier in the north. Two south-westerly branches were added to the strip in order to encourage investigation of the uranium-rich province along 'the lower Khan and Swakop Rivers, the unique, westerly verging structures along the lower Ugab River in the southern coastal branch and the junction between the coastal and north-eastern arms of the orogen. Several basic mapping projects were undertaken in order to complete the mapping coverage (Miller, 1973b; Faupel, 1974; Hill, 1975; Bunting, 1977; Sawyer, 1976, 1981; Klein, 1980; Watson, 1982). Although research was concentrated along the study strip, projects outside it were also included in the overall investigation where it was felt these would provide answers to specific problems. The South African project ran in parallel with a multidisciplinary research effort by the Geology, Mineralogy and Geochemical Institutes of the University of Gottingen under the leadership of Prof. Henno Martin. The University of Leeds and the Open University also undertook their own research as well as joint projects with local geologists. The positive cooperation and free exchange of information on all sides was most gratifying and was of tremendous benefit to the whole Damara project. In this volume, sedimentological and stratigraphic papers appear first in chronological order. Then follow sections covering petrologic aspects, structure, metamorphism, geochronology, geophysics and economic geology. Finally, papers on geodynamic modelling of the orogen are followed by a comprehensive synthesis that collates all published data. The maps in the back pocket of the volume incorporate conclusions reached in the final summary. Additional sets of the maps are available at R10,00 per set from the South West African Branch of the Geological Society of South Africa, P.O. Box 699, Windhoek, South West Africa/Namibia. At the onset of the project, the Damara Orogen appeared to be fairly simple, but as detailed work progressed, an extremely complex history became apparent and gradually revealed striking similarities to the Caledonian and Appalachian Orogens at all stages of their development. Particularly significant has been the recognition of: three early major rift zones; a major evaporitic sequence within the southern rift; indications suggesting that rift evolution in the coastal branch was somewhat ahead of that in the north-eastern branch; major facies changes between shelf and deep-water successions and several turbidite sequences in the deeper parts of basins; the ocean floor affinity of the Matchless Amphibolite Member and the within-plate composition of earlier(?) volcanics; syntectonic flysch deposition within the orogen and extensive molasse deposition of different ages on the marginal platforms; a syn- to post-tectonic plutonic suite consisting almost entirely of granite (97 per cent); a progressive southward migration in the locus of deformation related to a general south-eastward tectonic transport direction during (limited?) north-westward subduction of the African cratons beneath a South American craton and of the Kalahari Craton beneath the Congo Craton; extensive thrusting along the southern margin of the orogen involving cover and penetratively deformed basement and associated tectonic emplacement of Alpine-type serpentinites; a deformational history lasting about 120 m.y. in the high-temperature, granite-rich, central region of the north-east branch but having only a 20 m.y. duration in the intensely thrusting southern margin; a structural and metamorphic polarity in the form of paired high-temperature-low-pressure and low-temperature-high-pressure belts in both branches of the orogen; a 40 m.y. period of uplift-related, high-temperature metamorphism in the central regions after cooling along the margins has started; structural-stratigraphic zones characterised by contrasting lithofacies, structural, metamorphic and magmatic evolution and bounded by major, deep-penetrating lineaments which had a profound influence throughout the depositional and tectonic history of the orogen and are even the locus of post-Karoo faulting; contrasting crustal sections within and marginal to the orogen; a thick, shallow, highly conductive zone extending from the north-eastern branch across Botswana into Zambia; a possible direct and dynamically active connection with the Katanga Belt which may have been the source for easterly derived semipelitic sediments. Results obtained during the Geodynamics Project led to several interpretive models for the Damara Orogen - aulacogen, conventional plate tectonics, crustal delamination - some involving continental separation, others not. There is general agreement that plate tectonics did play a role in the evolution of the orogen but none of the proposed models satisfactorily explain all features of the Damara Orogen. Because of such excellent exposure and a reasonably good data base, the Damara Orogen offers itself as one of the best examples for the study of the highly controversial issue of whether plate tectonics was active in the Proterozoic or not and how it evolved through time. Many problems remain, not only for the improved understanding of the Damara in the local context but also for global Proterozoic crustal evolution.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 1-20

Continental evaporitic playa-lake deposits of a Na-HCO3-(K-B-Cl-S) brine type have been identified within cyclical dolomitic-pelitic sediments of the Nosib Group (1000 800 m.y.) along the southern margin of the Damara Orogen. The sequence is terminated by a SO4(2-)-enriched, marine sabkha facies that probably developed during a marine ingression following the continental rifting stage. The saline crust horizons were partly fluidised due to increased pore pressure of the interstitial brines after the evaporitic sequence had been overriden by nappes derived from the Khomas Trough of the Damara Geosyncline. Forming discordant breccias, the evaporitic dolomite mush with included autolithic and xenolithic rock fragments was squeezed into the thrust planes of the nappes. These dolomite breccias, being impregnated with hypersaline interstitial brines, acted as lubricants and largely influenced the late-orogenic tectonic activity along the margins of the orogen. The most voluminous intrusion occurred into the base of the Naukluft Nappe Complex and subsequently formed a plate of albite-dolomite. This intrusion triggered a further nappe movement of about 50 km. The mineral composition of mobilised evaporites and in situ sediments is extremely variable due to numerous palaeoevaporitic pseudomorphs and hydrothermal-metasomatic Na-K-B-Mg-Si reactions. Hypersaline residual solutions derived from the brines were enriched in NaCl-KCl-CaCl2 and locally formed hydrothermal megaquartz bodies with dolomite-calcite and giant talc crystals. Locally stratiform copper sulphide horizons are developed in the primary sediments of the Duruchaus Formation. The genetic development from diagenesis to hydrothermal crystallisation can be reconstructed from analyses of fluid inclusions. Similar phenomena, though less prominent, have also been observed in the northern Damara Orogen. The described sequence may directly be compared with evaporitic sequences of the Adelaide System in South Australia and lithologically resembles certain sediments of the Katanga Supergroup in the East African Copper Belt. Obviously, rift sediments of a playa environment occur more frequently than hitherto known in the Upper Proterozoic palaeorift systems of Pangea.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 21-36

Three pre-orogenic turbidite sequences are developed in the north-east-trending branch of the Damara Orogen. They are all related to rift structures of the early Damaran "graben stage" assumed by Martin and Porada (1977). The Chausib turbidite sequence (Chausib Member) of which c. 1 500 m are preserved, is situated near the southern margin of the Damara Orogen. The turbidites are quartzitic in composition and are intercalated in biotite and graphite schist. They are derived from and partly interfinger with the Hakos quartzite of the Hakos Formation. The Tinkas turbidite sequence (Tinkas Member), 4 000-5 000 m thick, is exposed in a narrow zone, 300-350 km long, in the centre of the Damara Orogen. The calcareous (calc-silicate) Tinkas turbidites are genetically related to the northerly adjoining dolomites and limestones of the Karibib Formation and are intercalated in Kuiseb schist-type metapelites and siltstones. The Tinkas Member grades upwards into and is partly coeval with the Kuiseb Formation. It is proposed to partly correlate the Karibib Formation, the Tinkas Member and the Kuiseb Formation. The spatial relationships of source area to basin and the southward gradation of the Tinkas Member into the Kuiseb Formation suggest the existence of an extensive basin ("Khomas Trough") towards the south. The Chausib turbidites in the southern Damara Orogen may have been deposited near the other side of this basin. The Okonguarri turbidite sequence (Okonguarri Formation) occurs in the northern Damara Orogen and may be up to 4 900 m thick. Two types of turbidites are developed: calcareous turbidites which are clearly related to stromatolitic reefs and reef detritus of mainly the Okotjize Formation, and siliceous turbidites which are probably derived from Nosib-type deposits The calcareous turbidites have been transported episodically from a northern slope into the Okonguarri basin and are concentrated in nine "dolomite zones". The siliceous turbidites are derived from the east and have been transported along the axis of the basin. It is proposed to correlate the Okonguarri Formation with the Okotjize Formation and with the Khan Formation of the central Damara Orogen. Geodynamically, the development of the rapidly subsiding turbidite depositories marks the change from a mature graben or continental rift environment to the initial stage of a more general subsidence.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 37-41

A reconnaissance facies analysis of the Rooikuiseb area has led to a palaeogeographical interpretation of the Damaran deposits in the Okahandja Lineament area. Lower Damaran Nosib Group sediments were deposited on a stable continental margin in the form of fluviatile arenites. Slow subsidence on this margin resulted in the deposition of lower Swakop Group carbonates and muds in a shallow shelf environment. The onset of north-westward subduction, south-east of the study area, resulted in differential subsidence in the Okahandja Lineament area. This produced a basin in the Southern Zone in which about 10 000 m of flysch deposits of the Kuiseb Formation were deposited. The source for this sediment was the magmatic arc forming in the Central Zone above the subduction zone.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 43-63

New data on stratigraphic and lithofacies relationships of rocks of the Swakop Group of the Southern Margin Thrust Belt are presented. These form the basis for a proposed, comprehensive revision of the lithostratigraphic classification of the Swakop Group in this region. The Kudis Subgroup forms the main lower division of the Swakop Group and is comprised of 4 lithologically distinct formations. These are considered to be largely time-equivalent lithofacies that have been superposed by thrusting and folding. Included in the Kudis Subgroup now is the Auas Formation which is redefined to consist only of the prominent quartzites of the Auas Mountains type area. Original distribution of the 4 lithofacies units of the Kudis Subgroup reflects a gradual transition from shallow-water shelf carbonates of the Corona Formation preserved in the lowest tectonic unit, to progressively deeper water, upper and lower continental slope and rise facies associations represented by diverse clastic rocks of mainly resedimented origin of the Blaukrans, Hakos and Auas Formations in the higher nappes. The Vaalgras Subgroup is a newly defined lithostratigraphic unit which represents the main upper division of the Swakop Group in the Southern Margin Thrust Belt. It is confined to the highest tectonic of the Southern Margin Thrust Belt in which it forms a complete stratigraphic succession conformably overlying the Kudis Subgroup. The Vaalgras Subgroup consists of the basal Chuos Formation and a sequence of areally extensive but thin quartzites interbedded with schists and amphibolites which is stratigraphically equivalent to the former "Auas Formation" of past classification schemes. This sequence is now subdivided into 4 regionally important formations which consist, in ascending order, of the Melrose, Mahonda, Gomab River and Kleine Kuppe Formations. Lithofacies interpretations of the rocks of the Vaalgras Subgroup indicate a sustained passive margin environment of deposition that was terminated only by the onset of deposition of the Kuiseb Formation in the southern Damara margin. Extensive lateral and vertical interfingering between the highly immature, metasedimentary pelitic and psammitic schists of the Kuiseb Formation with the Kleine Kuppe Formation is tentatively interpreted in terms of a model of syntectonic sedimentation in a foredeep environment.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 65-71

The Damara Sequence along the lower Ugab River is entirely turbiditic and consists of 3 schist units separated by 2 limestones, all of which are correlated with the Swakop Group. The basal unit is correlated with horizons further east that directly overlie Nosib rocks. The lowermost turbidites therefore imply that the coastal arm of the Damara Orogen had developed into a deep-water ocean by the end of Nosib times, the Pan-African South Atlantic Ocean. Indications are that all sediments were derived from the east. Ice rafting of glacial debris is suggested by the occurrence of a 1,5 m granite boulder in an area where the turbidites are considered to be most distal.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 73-88

The Kuiseb Formation is a flysch-like quartz-plagioclase-mica schist succession with quartz-rich and mica-rich layers and forms the upper unit of the Swakop Group in the Damara Orogen. It is the only stratigraphic unit in most of the Okahandja Lineament Zone and the structurally asymmetric high- pressure-low-temperature Southern Zone; in these two zones, it occupies, together with intrusive granite, a volume of about 160 000 km3. The average composition in the Okahandja Lineament and Southern Zones lies between averages for shale and greywacke. An excess of K2 over Na2 distinguishes the succession from most greywackes and implies a granitic/rhyolitic source area. Three groups of samples are distinguished on the basis of maturity index; northern and southern groups increase in maturity southwards. The central group is associated with a 350 km-long linear belt of ocean-floor basic volcanic rocks and is distinctly less mature. The Kuiseb Formation sediments appear to have been derived from several sources. The central and southern groups of samples formed during a spreading phase in a relatively narrow Damaran ocean and are associated with passive margin sediments. The northern group of samples appears to overlie the central and southern groups, was derived from a rhyolite-rich magmatic arc to the north that is now represented by the granite-rich Central Zone, and was deposited in a fore-arc basin bordering an active continental margin during closure of the Damaran ocean and north-westward subduction of the Kalahari Craton beneath the Congo Craton. The contact between spreading phase Kuiseb and the overlying fore-arc deposits has been obliterated by two intense transposition cleavages that formed during two phases of isoclinal folding and culminated in extensive thrusting along the leading edge of the subducting continent, the Southern Margin Zone. The relationship between Southern Zone and Southern Margin Zone bears similarities to that between a fore-arc basin and subduction complex. These fore-arc deposits in the Okahandja Lineament and Southern Zones must be younger than lithostratigraphic equivalents in the Northern and Central Zones.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 89-114

A sedimentary facies and depositional environmental analysis of the Nama Group indicates that the uppermost portion, i.e. the Fish River Subgroup, was deposited in a foreland basin. The formation of this basin probably commenced during deposition of the lower Schwarzrand Subgroup and possibly even before, during deposition of the basal Kuibis Subgroup. The Nama Group was deposited at the end of a geosynclinal cycle and was folded during the final deformational events of the Damara Geosyncline. The sediments of the lower Nama Group were derived mainly from the Kalahari Craton to the east and were deposited in a northern Zaris and a southern Witputs basin, which were separated by the Osis ridge. The sediments of the upper Nama Group were supplied from an arc-shaped region of uplift to the north and west, related to the Damara Geosyncline. The change in palaeogeography from the lower to the upper Nama Group took place gradually and is illustrated by a number of facies maps. In terms of Pretorius's model (Pretorius, 1979), the Osis ridge and the Zaris and Witputs basins form part of a much larger series of radial upwarps and downwarps, while the arc-shaped region of uplift to the west and north appear to be part of a large concentric upwarp. The sediments of the Nama Group were mainly deposited in fluvial and shallow marine depositional environments. Some offshore deposits occur but no turbidites were observed. The Fish River Subgroup is a molasse-like deposit although its various facies extend over large areas. Those regressions which were the response to uplifts in the Damara Geosyncline have been dated by body and trace fossils and this may assist in unravelling the thermo-tectonic events which took place during the later stages of development of the Damara Geosyncline. The lower Nama Group, i.e. the sediments below the base of the Nomtsas Formation of the Schwarzrand Subgroup, contains an Ediacaran fauna and is probably Vendian in age. The upper Nama Group is probably Cambrian in age as is indicated by the presence of Phycodes pedum and Diplichnites. Palaeomagnetic studies support this contention. The occurrence of Diplocraterion in the lower Schwarzrand Subgroup does not seem to be indicative of a Cambrian or younger age as is generally accepted. The trace fossils generally occur within the transgressive and regressive intertidal to very shallow subtidal clastic sediments. The Ediacaran fauna occurs in intertidal but mainly shallow subtidal clastic sediments, which were laid down landwards from carbonates, in which Cloudina and stromatolites occur. Neither trace nor body fossils have been found in the few offshore sediments of the Nama Group.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 115-138

Ortho-amphibolites occur in the southern and central parts of the north-east-trending branch of the Damara Orogen. Southern Margin and Central Zone amphibolites are interbedded with shelf carbonates, slope mixtites and deep-water fans. The Matchless Member amphibolites are interbedded with quartzose mica schist, the highest unit of the stratigraphic succession. Mobility of Si, (sigma)Fe, Mn, Mg, Ca, Na, K, P, CO2, H2O, Rb, Ba, Sr and possibly LREE and immobility of Co, V, Sc, Ga, Zr, Nb, Y and HREE are indicated during metamorphism and reaction with country rock. Central Zone amphibolites are alkaline: the remainder are hypersthene normative, subalkaline and tholeiitic. The stratigraphically lower amphibolites have a within-plate chemistry; their distribution and associated rock types indicate a continental origin. The Matchless amphibolites have an ocean-floor chemistry. The Damaran sedimentary and orogenic cycle was initiated by continental rifting in three parallel zones in which alkaline acid volcanics occur locally. Widespread subsidence of the rift zones and the intervening areas followed and led to deposition of carbonate and clastic rocks under shallow marine conditions. During renewed rifting, submarine, alkaline basic lavas were extruded in the Central Zone and continental tholeiites along the southern margin of the orogen. The Southern Margin Zone amphibolites are interbedded with continental slope mixtites and continental rise deep-water fans. The southernmost of the three rift troughs developed into a zone of spreading in which crustal thinning may have been accomplished by rotation of rifted basement slices on listric faults. Spreading led to continental breakup and the formation of oceanic crust. It is suggested that the Matchless amphibolite represents ocean-floor material extruded through and onto thick. semi-pelitic sediments covering a buried, mid-oceanic ridge in a Red Sea type of ocean that may have only been 1 000 km wide. Reversal of spreading led to north-westerly directed subduction, continental collision, formation of paired metamorphic belts and the development of an asymmetric structure across the orogen.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 139-145

Ten widely spaced samples of the Matchless and neighbouring amphibolites have been analysed for major and 24 minor elements (including Rare Earths). The mafic rock is of uniform character and occurs almost continuously over a 350 km distance within a thick sequence of clastic sediments. The mafic rocks were probably deposited on a sialic crust and they have the typical composition of an ocean-ridge tholeiite. The geologic evidence for an origin at diverging or converging plate boundaries is not convincing. An explanation of these rocks as indicators of abnormal heat flow in a chemically normal mantle under a thin crust (less than or equal to 30 km) is in accordance with petrologic experiments.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 147-155

Along the southern margin of the Damara mobile belt are over 60 ultramafic bodies. Field and laboratory studies indicate that these fulfil the criteria for Alpine serpentinites, i.e. (a) the bodies occur in a linear belt parallel to the axis of the Damara mobile belt; (b) the bodies were tectonically emplaced; (c) olivine covers a limited composition range (Fo90-Fo95). A large data base of chemical analyses of serpentinites derived from layered complexes, Alaskan complexes, komatiites and Alpine complexes was subjected to multivariant analyses. It was shown that there is a difference in chemistry in rocks from these four groups, and that the Damara serpentinites most closely resemble Alpine material.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 157-172

The Tumas Dome is exposed over about 120 km2 in the Central Zone of the Damara Orogen about 55 km east of Walvis Bay. It consists essentially of medium- to coarse-grained augen gneiss of the Abbabis Metamorphic Complex. The dome is almost surrounded by supracrustal rocks of the younger Damara Sequence and is divided into northern and southern parts by a narrow, east-trending infold of Damaran rocks. The basement gneisses in the central part of the dome contain structural fabrics that predate deposition of the Damara Sequence. The basement and its cover rocks were deformed together in two main folding events during the 550±100 m.y. old, Pan-African Damaran orogeny, and new fabrics were superimposed on the gneisses. Minor structures such as foliations, ductile shear zones, folds and joints in both the basement and the cover rocks indicate that during the first Damaran event the cover rocks were folded into recumbent folds about west to north-west-trending fold axes. During the second Damaran event, folding occurred about north to north-east-trending axes and upright folds were formed. During both these events, the basement rocks were uparched into broad antiformal structures and a result of the interference of these structures was the formation of the Tumas Dome. Concomitant with formation of the dome, the overlying supracrustal rocks were downfolded into synformal keel-like structures.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 173-174

The Ogden Rocks Formation (Hogden Bay Formation, Miller, 1973) was formerly described as a sequence of porphyroblastic gneisses possibly equivalent to parts of the Brak River Formation of the Lower Ugab area. During 1982-83, the structure and metamorphism of the Damaran rocks in the Lower Ugab Structural Domain (LUD) from the coast all along the Ugab River up to the Omangambo granite pluton was studied (Freyer, in prep.). In the course of this investigation it was discovered that these gneisses are blastomylonites intercalated with sheared marble bands and phyllonites.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 175-184

At least four phases of deformation have been recognised in the south-eastern portion of the Damara Orogen in an area around Omitara. Isoclinal north-east-south-west-trending and south-east vergent folds and numerous thrusts with large displacements were produced during the first phase, resulting in tectonic thickening and repetition of parts of the Damara Sequence. Although the axial traces of the F1 and F3 folds are generally subparallel, the F1 folds are more open and their axial planes much steeper than those of the F1 folds. Relatively steep thrusts and reverse faults were also produced during the third phase of deformation, but only in the extreme south-eastern Omitara area was the displacement of the order of at least several kilometres. Over-thrusting during both D1 and D3 was approximately from the north-west towards the south-east, but no distinct stretching lineations were found to obtain more accurate displacement vectors. A zone of east-west-trending en echelon F2 folds in the central, southern and eastern Omitara area has been interpreted as a north-east-south-west-trending sinistral shear zone with a total relative displacement of at least 15 km. Late to post-D3 left-lateral shearing in the north-western Omitara area also resulted in a complex F3 fold pattern in the north-east-south-west-trending Otjivero structure north-west of Omitara. Open, north-south-trending minor folds and crenulations were produced during the last phase of deformation (D4). At least two apparently uneconomic copper deposits in the Omitara area are associated with major thrust zones and it is suggested that they have been deposited from metamorphic fluids escaping along the thrust zones.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 185-197

Approximately 50 km south of the present southern margin of the late Precambrian to early Palaeozoic Damara Orogen, the Naukluft Nappe Complex is exposed. This nappe complex overlies the autochthonous Nama beds which belong to a platform area adjacent to the Damara mobile belt. The total displacement from the north-west to south-east amounts to 50-80 km. The base of the Naukluft Nappes is formed by a dolomite horizon varying in thickness between zero and about 30 metres, the Sole Dolomite. Recent investigations of the mineral content, fluid inclusions, grain fabric and deformation lead to the conclusion that continental playa-lake evaporites have to be assumed as source rocks of this dolomite. Thirty-four different minerals have been identified up to now in the Sole Dolomite. The main components are sparitic dolomite, albite. quartz, tourmaline, Mg-ricbeckite, talc and sericite. The Sole Dolomite contains numerous rock fragments of granites, granodiorites, gneisses and mica schists with tourmaline and Mg-riebeckite as metasomatic minerals. Very frequent are fragments of quartz-albitolites containing more than 50 per cent of albite and various amounts of tourmaline and dolomite. The Sole Dolomite is extremely rich in fluid inclusions of different, but mostly high, salinity. The trapping temperatures are about 100°C higher than the metamorphic temperatures of the overlying Naukluft Nappes and about 200°C higher than in the underlying autochthonous Nama beds. The mineral assemblage of the Sole Dolomite resulted from intense interaction of highly concentrated, hot, hydrothermal waters with the rock fragments and the dolomite matrix, but not from metamorphic reactions. The Sole Dolomite is typically a compact, sometimes well laminated, sparitic rock rarely displaying south-east-facing internal folds. Its base forms a sharp boundary against the limestones of the underlying autochthonous-parautochthonous Nama Group. The minerals of the Sole Dolomite display no primary recrystallisation and no crystal plastic deformation. No preferred lattice orientation is developed. All deformation found in the Sole Dolomite is of the brittle type. Cataclastic mylonitisation without any preferred lattice orientation has been encountered only in the lowermost few centimetres of the Sole Dolomite. This cataclastic mylonite displays grain sizes of less than 1 micron. In contrast to the Sole Dolomite, the underlying Nama limestones have been transformed over a vertical distance of several metres into fine-grained mylonites which reveal a well developed, preferred lattice orientation. From the present facts, the formation of the Sole Dolomite may be interpreted as a discordant intrusion under high pore-fluid pressure into the base of the nappe complex. The development seems to be characterised by the following steps: 1. During increasing burial, the playa-lake sediments were transformed into a tourmaline-bearing albite-dolomite rock rich in interstitial brines. Dewatering of hydrous carbonates and silicates, crystallisation and compactional disequilibrium may have produced abnormally high pore-fluid pressure. 2. During Damaran deformation, a nappe sequence was formed and thrust south-eastwards. At the same time, the evaporitic sequence progressively subsided together with the Damara foreland due to a southward migration of the Damara front. During this stage the pore-fluid pressure increased further by aquathermal-pressuring and deformation. 3. The now highly mobile material intruded into higher levels along the structural discontinuity at the base of the earlier formed nappe complex. Here, it acted as a lubricant and led to further south-eastward movement of the nappe complex. Fluid inclusion development points to osmotic inflow of water from surrounding rocks during this stage of the Sole Dolomite development. 4. The final displacement of the nappes with the Sole Dolomite at its base must have taken place after lithification of the intrusion. Otherwise, the intense low temperature mylonitisation of the underlying autochthonous Nama limestones cannot be adequately explained. Due to the high stiffness of dolomite under confining pressure, only the lowermost few centimetres of the Sole Dolomite plate were cataclastically mylonitised without any preferred lattice orientation.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 199-207

The relationships between the Naukluft Nappe Complex and the Damara mobile belt are investigated with the help of lithofacies comparisons. Some metamorphic features and the origin of the Sole Dolomite, which forms the sole of the upper nappe complex, are discussed. Before crystallisation, the Sole Dolomite consisted of a mobilised, mylonitic sludge with a high content of hot saline fluid. The saline fluid was probably derived from evaporite beds in the lower Damaran Duruchaus Formation which fills a deep basin some 90 km to the north-east of the nappe complex, but may originally have had a wider extent. It is concluded that the Upper Naukluft Nappe Complex had its root zone in the thrust belt forming the southern margin of the Damara Orogen, and not in a hypothetical collision suture somewhere farther north.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 209-217

Observations made in the Gross Barmen area show that the early structural and metamorphic history of the Okahandja Lineament Zone is essentially the same as that established some 200 km farther south-west in the Namib Desert. However, in the Gross Barmen area, a set of late (F5), small-scale folds and an associated crenulation cleavage is peculiar to the Donkerhuk Granite thermal aureole. The folds show an increase in amplitude and tightness towards, and wavelength away from the granite-country rock contact. Furthermore, the axial planes, hinges and a pre-existing schistosity (S3) are rotated as the contact is neared. The progressive change in the late fold geometry can be explained in terms of the different contributions of distortion, bulk rotation and translation components of the total strain. If the final emplacement of the Donkerhuk Granite involved lateral spreading, then the total strain will vary with distance from the granite contact. Thus, the Donkerhuk Granite has both a contact strain aureole and a contact thermal aureole, the latter being more extensive.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 219-223

The P-T conditions of metamorphism in the central Damara Orogen are determined by application of dolomite-calcite solvus thermometry on the natural occurrence of the five-mineral assemblage forsterite + diopside + tremolite + dolomite + calcite. The stability conditions of this assemblage are known from experimental investigation (Käse and Metz, 1980). Going from Karibib south-westwards to Swakopmund, the temperature of metamorphism along the present-day surface increases from 555°C to 645°C. The determination of temperature of metamorphism leads to an estimation of PTOTAL for the areas where the univariant five-mineral assemblage was found. The pressure of metamorphism during the formation of the five-mineral assemblage was 2,6 kb north-east of the Khan/Swakop confluence and 3,4 kb near Swakopmund.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 225-231

The natural gamma-activity of 312 Damaran rocks was measured in order to determine their U, Th and K contents and to calculate their heat production rates. The majority of Damaran igneous rocks has unusually high Th/U ratios of 7 and more, and elevated heat generation rates of 6-21 HGU (2,5-8,8.10(-6) W/m3). The most radioactive unit is the red granite for which one extreme value is 74 HGU (31.10(-6) W/m3). Part of the basement also has elevated Th contents and heat productivity if a comparison is made with the low-Ca granites of Turekian and Wedepohl (1961). Only the metapelites have almost normal Th/U ratios of 5-6 and heat generation rates around 4-5 HGU (1,7-2,1.10(-6) W/m3). On the whole, the Damara Orogen is a Th-rich province with elevated U concentration. The large contrast in heat generation from granite to granite and between basement and cover seems to have caused considerable differences in regional metamorphic temperatures. Only rough estimates can be made, however, mainly because of the unknown pre-metamorphic distribution of heat sources with depth. In the Central Zone these sources contributed 100-150°C per 5 HGU (2,1.10(-6) W/m3) of heat generation at the present surface.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 233-241

Metamorphism in the Damara Orogen has been investigated with special reference to the question of one or more metamorphic events and to differences observed in the areas north and south of the Okahandja Lineament. The strong contrast in pressure and temperature conditions is attributed to different radioactivities of rocks, as recently established, and to opposite vertical movements in these areas related to the general concept of crustal shortening. Mantle heat flow may be taken as uniform throughout. South of the lineament, shortening by 1/3 of initial width but without further addition of heat is needed, while in the north, wide-spread heat advection due to granite intrusions is essential to explain the thermal peak of regional metamorphism. Evolution of the main phase of deformation and its sharp termination is related to thermal effects of the Central Zone of the orogen.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 243-253

The regional metamorphic map of the Damara Orogen indicates that the Southern Margin Zone and the Southern Zone are characterised by a typical Barrovian progression, while the pressure is distinctly lower in the Central Zone. Quantitative P-T estimates in parts of the southern Damara Orogen suggest that the pressure is even higher than for a Barrovian progression for the early syntectonic M1 metamorphism and that the regional P-T variations are rather complex. In the Southern Margin Zone of the Damara Orogen, both temperature and pressure increase quite rapidly in a north-westerly direction, as exemplified in the Omitara area. The highest P-T conditions of 588±50°C and 1 002±120 MPa are reached near the northern limits of the Southern Margin Zone, where the high-pressure assemblages kyanite + staurolite + garnet + biotite + chlorite + muscovite + quartz and kyanite + zoisite + plagioclase + quartz are found in metapelites and calc-silicates respectively. The pressure decreases north-westwards from 1000 MPa to about 450 MPa across the Southern Zone (Khomas Trough). The occurrence of the univariant assemblage kyanite + staurolite + garnet + biotite + muscovite + quartz across the whole width of this zone indicates that the temperature re- mains constant. Near the Okahandja Lineament the P-T conditions are approximately 600°C and 400 MPa. In the Central Zone the temperature increases to at least 650°C while the pressure remains more or less constant at approximately 400 to 425 MPa.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 255-265

Detailed microtextural analysis in two critical areas of the southern Damara Orogen suggests that all the metamorphic minerals grew during an early syntectonic as well as a post-tectonic event of regional metamorphism. Petrographic and garnet zonation studies indicate that the temperature has decreased considerably between these two metamorphic episodes. This is documented by discontinuous garnet growth in the north-western Omitara area where both events reached lower amphibolite facies conditions. Quantitative reconstruction of the changes in P-T conditions by garnet-biotite geothermometry and garnet-plagioclase geobarometry support the microtextural evidence for polymetamorphism. Furthermore, the tectonothermal evolution of the northern Gamsberg-Kuiseb River area in the west was remarkably similar to that of the north-western Omitara area in the east. In both areas, the two metamorphic events reached peak temperatures of between 570±50°C and 588±50°C. Between M1 and M2, the temperature decreased well below 500±50°C which explains the discontinuous garnet growth and the absence of late-tectonic staurolite. The maximum pressure of 902±120 MPa to 1 002±120 MPa was reached during the peak of the first metamorphic event. Towards the end of M1 the pressure decreased by approximately 200 MPa, but it did not increase again during M2. The second metamorphic episode was, therefore, more of a thermal event resulting from post-tectonic thermal relaxation.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 267-272

The Naauwpoort Formation consists of over 6 600 m of alkali rhyolites which were extruded during intracontinental rifting. It occurs at the top of the Nosib Group and also interfingers with the carbonates at the base of the Swakop Group. Zircons from two groups of samples from quartz porphyry lavas near the base of the Naauwpoort Formation give U-Pb ages of 728±40 and 750±65 m. y. (2σ). Related subvolcanic rocks suggest that rifting and deposition of the Nosib Group covered a time span of about 100 m.y. The Naauwpoort age provides a reliable maximum age limit to the start of (i) Otavi Group sedimentation and (ii) deposition of part of the Swakop Group.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 273-280

Porphyritic Salem Suite granitic rocks in the low-grade Northern Zone and adjoining portion of the high-grade Central Zone give respective ages of 589±40 and 546±30 m.y. (2 σ) which are indistinguishable within the limits of uncertainty. A complex, diachronous tectonic history is evident. Ages of deformation and granite intrusion in the Northern Zone and the coastal region of the Huab and Ugab Rivers appear to be similar. Deposition of the Mulden Group, the single intense phase of deformation and the low-grade metamorphism of the Northern Zone were completed between 550 and 600 m.y. ago and prior to the first granite intrusions. This was between 54 and 140 m.y. before completion of deformation in the adjoining, fundamentally different, high-grade, tectonically complex, granite-rich Central Zone. D1 in the southern Central Zone is older than 595 m.y. D2 may have been coeval in both the Northern and Central Zones, but a northward decrease in age across the Central Zone cannot be ruled out. D3 decreases in age northwards across the Central Zone and is at least 20 m.y. and may be as much as 120 m.y. younger in the north. The peak of metamorphism could have been coeval in the Northern and Central Zones. D3 in the northern Central Zone (479±16 m.y. - Haack et al., 1980) is the youngest major deformation event in the orogen. The Central Zone was finally thrust against and over the Northern Zone subsequent to D3 in the former.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 281-287

The age of the Salem-type granites in the central part of the Damara orogenic belt has an important bearing on the timing of tectonic and metamorphic events. Results of U-Pb (zircon) and Rb-Sr (whole rock) age measurements in a suite of reconnaissance samples from various localities within the central Damara belt indicate ages of ~550 m.y. for most samples, but an apparent Rb-Sr whole-rock errorchron age of ~720 m.y. is obtained for certain of the granite and granodiorite samples. A more detailed study of the granites from the vicinity of Goas, south of Karibib, gives a U-Pb (zircon) age of 580±30 m.y. that is considered to be the most reliable indication of the time of intrusion of this granite. From the same locality, an unexpectedly high Rb-Sr whole-rock isochron age (720±77 m.y.; Ro = 0,711±0,002) is obtained. Samples from a single outcrop of the granite mostly have insufficient spread in their Rb/Sr ratios to give a reliable age, but three biotite-rich samples indicate an age of 535±8 m.y. Derivation of the granite, at least in part, by anatexis of Kuiseb schists is indicated. The Rb-Sr isochron age is considered to be an example of incomplete isotopic equilibration, during anatexis, on an outcrop-to-outcrop scale. The data for the biotite-rich samples and for granodiorite contact zone samples demonstrate the danger of sampling for Rb-Sr geochronology solely on the basis of securing the maximum variation in Rb/Sr ratios.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 289-298

The geology and Rb-Sr geochronology of four mineralised and radiometrically anomalous granites and alaskites have been studied in an attempt to establish their characteristic isotopic features. A four-fold classification of the Damaran intrusives from the central belt of the orogen into syn- to post-tectonic Salem-type granites and red granites, and late to post-tectonic leucogranites and alaskites is proposed. Whole-rock Rb-Sr ages are presented for the mineralised Ida Dome alaskite, a mineralised red granite on the farm Otjua 37, and the radioactive Salem granite and leucogranite on Stinkbank 62. The most important isotopic feature which appears to characterise the mineralised granites and alaskites is a high initial Sr-isotope ratio, and this ratio seems to increase with both time and uranium content.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 299-306

K-Ar age determinations on the fine mineral fractions (< 2μm) of phyllites and schists were carried out on samples from two separate areas of the north-south-trending coastal branch of the Damara Orogen and additionally from the Tsumeb region. From the northern Damara branch (Sesfontein area), two groups of ages were obtained. One group, around 490 m.y., is interpreted as the cooling age of a regional metamorphism having its peak in this region around 530 m.y. The second, younger group of ages around 460 m.y. seems to be influenced by the mineral composition of the samples taken for determination and as yet is not interpreted in the sense of a second regional metamorphic event. In the southern part of the northern branch (Brandberg West area), the main regional metamorphic event appears to have occurred around 490 m.y. Younger ages in this region may also have been influenced by the mineral composition of the samples or may have been rejuvenated by the intrusion of late to post-tectonic granites. The K-Ar age determinations on the rocks of the Tsumeb area can be related to a main regional metamorphic event dated at about 460 m.y. in this region.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 307-319

Structural investigations in the Northern Zone of the Damara Orogen, south of the Kamanjab Inlier, lead to the conclusion that older north-south-trending folds were overprinted by south-south-east- north-north-west shortening. The area subjected to this early Damara deformation-possibly older than 600 m.y. - is called the Ugab Structural Domain. The younger overprint is related to the main phase of the Damara deformation. Radiometric investigations point to long lasting heat generation in the northern Damara Orogen. The syntectonic metamorphic front of the main phase of the Damara deformation reached the folded Nama beds at the southern rim of the Damara Orogen at about 530 m.y. ago. In contrast to the Northern Zone, overthrust tectonics plays an important role along the southern margin of the Damara Orogen. Folding and early stages of nappe formation of the Naukluft Nappe Complex started about 530-550 m.y. ago, and possibly was active until about 517 m.y. Further nappe transport took place between 500 and 490 m.y. ago. The final emplacement of the Naukluft Nappe Complex might have taken place at about 480 m.y. during uplift of the Khomas Trough. The polarity of the structural development points to an altogether north-west directed movement of the Kalahari Craton against the Congo Craton.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, P321

The base metal deposits in the Otavi Mountainland of the southern Owambo basin, Namibia, are considered to be of two mineralogical types. Both types are rich in lead and zinc, but distinctive features are that in the Tsumeb type, copper and silver are economically important, arsenic is an important constituent, and traces of germanium, gallium, vanadium and other rare sulphides are present; in the Berg Aukas type, these features are absent. The two types occur at different levels within the Otavi Group dolomites. The Tsumeb ore body, which is representative of the former ore type, has been described by Söhnge (1964) and Hughes (1979). Various hypotheses have been advanced as to the genesis of the bodies within which the base metal mineralisation occurs and also as to the source of the ore fluids. For example, cryptovolcanic and karst origins have been proposed for the pipe structure, contained quartzofelspathic rock (pseudo-aplite) has been considered to be of sedimentary, igneous of hybrid origin, and it has been suggested that carbonate minerals associated with the ore body could be of carbonatitic or normal hydrothermal origin. Sr-isotopic and rare-earth element studies of the Tsumeb pipe carbonates (Allsopp and Ferguson, 1970) are compatible with their derivation from the enclosing carbonate sediments. U-Pb age measurements on zircons contained in the pseudo-aplite of the Tsumeb pipe (this paper) support a sedimentary origin for the pseudo-aplite. Lead isotopic compositions were measured on some 60 galena samples of both mineralisation types from mines and prospects throughout the Otavi Mountainland (including the Tsumeb, Kombat and Berg Aukas mines). The isotopic compositions clearly distinguish the two types of mineralisation and respectively define essentially linear trends on 207Ph/204Pb v. 206Pb/204Pb and 208Pb/204Pb v 206Pb/204Pb diagrams. Two-stage evolution is unlikely to account for the observed linear trends because the ages indicated by the lower intercepts with the lead growth curve are geologically implausible and because, on the thorogenic lead plot, both the observed degree of linearity and the relationship of the trends to the growth curve are unlikely for such a model. The observed trends are interpreted in terms of a mixing model involving a common lower end member but distinct and more radio- genic upper end members for the Tsumeb and Berg Aukas trends. Percolation of metal-bearing fluids, derived from basin dewatering, through distinct sedimentary horizons and karst features, with their distinct lead isotopic compositions representing the upper end members for the mixing process, could account for the data. A study of the lead isotopic ratios, as well as the uranium and lead concentrations, of the more important country rocks is in progress and these data should provide constraints on the mode and source of mineralisation. The model proposed is analogous to that being advanced for the Mississippi Valley type deposits (e.g. Doe and Delevaux, 1972). According to such a model' the lead isotopic data provide only indirect constraints on the time of mineralisation: the isotopic composition of the common end member in the mixing process is compatible with derivation from a well mixed crustal source (such as the Damaran sediments), and the indicated age of about 600 m.y. is the best estimate at present available for the time of mineralisation. This is consistent with geological evidence that the mineralisation pre-dates the about 550 m.y. tectonothermal event that affected the Owambo basin.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 323-338

Rb-Sr and 40Ar/39Ar ages are reported on a wide variety of rocks and minerals from the Damara orogenic belt, Namibia. Rb-Sr whole-rock ages include 764±60 Ma for the Lofdal Nepheline Syenite; 548±31 Ma for the Salem Granitic Suite at Otjozondjou and Omangambo: 495±15 Ma for the Sorris-Sorris Granite; 563±63 Ma for the Salem granite near Swakopmund; and 495±12 Ma and 458±8 Ma on alaskitic granites from Valencia and Rössing respectively. Secondary, or metamorphic. ages were obtained on whole-rock samples of the Naauwpoort volcanics (521±45 Ma) and the Kuiseb schists (548±56 Ma). Rb-Sr biotite ages range from 562-416 Ma (omitting one age of 1 037 Ma from south of the Damara outcrop) and the youngest ages are from the centre of the belt where they tend to be 30-60 Ma younger than those on the margins. This diachronous cooling pattern is confirmed by 40Ar/39Ar analyses on selected micas and hornblende, which also allows some constraints to be placed on the rate of cooling. Overall. the development of spectacular late dome-like structures. the wide variation in metamorphic conditions, and the diachronous cooling pattern testify to a prolonged period of uplift - presumably as a result of crustal thickening. However, the available trace element analyses of granites indicate that many have an intraplate affinity which is at least consistent with the palaeomagnetic results that show there were no detectable relative movements between the bordering Kalahari and Congo Cratons. On balance, it would appear that significant crustal thickening took place in an intracratonic environment, but further work on the nature of intracontinental tectonic processes is needed before such a model can be usefully tested.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 339-354

A study of the aeromagnetic data covering the western portion of the Damara Orogen was undertaken in order to determine whether any additional information relating to the occurrence of uraniferous granite in the area could he derived from these data. The study included palaeomagnetic surveys and an interpretation of the regional structure, coupled with computer modelling of a geomagnetic section across the belt. A number of features are evident from this study, viz: (i) All currently known uraniferous alaskitic granite occurrences of economic interest are hallmarked, on a semi-regional basis, by prominent negative geomagnetic anomalies. The relationship is a stratigraphic one since the palaeomagnetic studies have shown that these negative anomalies arise from a pervasive remagnetisation of the Damaran rocks during the 500 m.y. event. The remagnetisation is retained chiefly in rocks of the Khan Formation - a stratigraphic level with which the uraniferous granites are approximately associated; (ii) A number of structural lineaments and broader lineament zones are, apart from the Okahandja Lineament. recognised and named for the first time, i.e. - the Uis lineament zone, - the Omaruru Lineament. - the Welwitschia lineament zone, and; - the less prominent Wlotzka and Abbabis lineament zones. Computer modelling studies support the hypothesis that these geomagnetic lineament zones are in fact fault-controlled geanticlinal ridges bounded by relatively rapid monoclinal downfolding of the stratigraphy; (iii) A post-F3 (north-east) structural phase, F4, oriented north-north-east is concluded to be of particular significance to the emplacement of uraniferous granite since, firstly, the major fold axes of t be domes and structures with which these occurrences are associated mostly have this orientation and, secondly, the currently known occurrences are exposed along the north-north-easterly trending Welwitschia lineament zone.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 355-367

Extended explosion-seismic profiles were executed within the Damara Orogen and the adjacent craton to the south by a combined group from South Africa and four Universities of the Federal Republic of Germany. The three seismic profiles, within the south-west-north-east-trending orogen and on the craton, show markedly different crustal structures. A profile north of Okahandja in the centre of the orogen, where there is intense invasion by granitoid masses, suggests a relatively simple crust with the absence of a high-velocity lower crust. The profile near Windhoek, near the southern margin of the orogen, suggests a more complex crust, with a section of Damaran strata considerably thinner than the northern profile, 3 km compared to 15 km, before they are transformed into more dense, high-grade rocks. Further south, on the craton itself, the profile indicated a more complex crustal structure with about 20 km of high-velocity lower crust. The simple crustal section of the northern profile, coupled with the absence of a high-velocity lower crust, contrasts sharply with the complex cratonic crust to the south, where there is a remarkable thick- ness of rock of "intermediate" to "mafic" character. A genetic model is considered in which a crustal section similar to the present-day craton moved into the orogen. The lower crust was reduced and restructured by two mechanisms: firstly, by a crustal thinning or necking; secondly, through metamorphism, the original layering was transformed, and the lower crust underwent a profound chemical reconstitution. The "partial melt" fraction worked its way upward so that transformed strata and granitic eruptives make up the bulk of the upper crust. If the dense "restite" merged with the pre-existing upper mantle, the relatively thinner crust is accounted for. Certain features of the tectonic model of Martin and Porada (1977) are also compatible with the seismic data.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 369-379

The results of the electrical sounding studies confirm the existence of a prominent conductive structure as found by magnetovariational investigations occurring in the Central Zone of the eugeosynclinal Swakop Basin. The sounding results show further that the north-east-striking structure is steep-sided and occurs at a depth of 3-10 km below the surface. It has a resistivity of less than 20 ohm.m and its minimum thickness is probably 20 km. The electrical results also favour a connection laterally, but at a depth of at least 20 km, with a conductive zone which was detected by an ultra-deep sounding near Okahandja and another on the pre-Damara Kamanjab Inlier. This finding is in line with the results of previous crustal sounding investigations. The strong development of a conductive zone in mobile belts confirms that these are weak portions of the crust. The crust in the Damara Orogen above the conductive structure is moderately resistive and is typical of Fractured Terrane of other mobile belts. However, the post-tectonic Donkerhuk Granite is highly resistive which indicates that the fracturing which determines the electrical properties occurred syntectonically and is associated with the dominant Damaran episode of deformation and metamorphism. The deformation also affected the resistivities within the zone of pre-Damara inliers in the Southern portion of the Central Zone and extended beyond the border of the orogen in the south whilst the Kamanjab Inlier remained stable and intact as shown by its highly resistive properties. The observation that faults are better aligned with the trend of the conductive structure within its borders than outside and its association with the high-temperature-medium-pressure metamorphic zone in the Central Zone suggests that the ultramafic material which ultimately gave rise to the conductive structure was emplaced during Damaran times. Its low resistivity is ascribed to a compositional change although saline water in fractures is also a possible cause. The only known rock which has the necessary low resistivity is serpentinite. The geoelectrical model cannot distinguish between a subduction zone origin and the shallow emplacement of asthenospheric material in a weak zone. The source of water to produce sufficient serpentinised ultramafic material remains a problem. Several post-Karoo alkaline igneous complexes occur along the course of the conductivity anomaly which seems to mark a line of weakness. The conductive belt shows a close association with the Central Zone of the Damara Belt in the west but also with the Zambezi Belt in the east, and provides the much-sought-after structural connection between these two Pan-African mobile belts. This weak zone delineates the most likely structural boundary between the Congo and Kalahari Cratons.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 381-384

The mineralisation at Gorob has many of the characteristics of the proximal volcanogenic sulphide deposit of the Besshi-type. Clusters of similar deposits are found along the length of the Matchless Member amphibolite and define four volcanic centres. The sequence containing these deposits was formed in a submarine, tensional tectonic environment and was overturned for a distance of at least 250 kilometres during later deformation.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 385-395

Local evaporitic deposits in an early intracontinental rift-fill contain copper mineralisation. The rifting environment should also be suitable for placer gold. Local intracontinental alkaline extrusives and associated sediments may have potential for sub-aqueous volcano-exhalative mineralisation; there are associated subvolcanic carbonatites and differentiated gabbro. Widespread shelf deposits containing abundant carbonate overlie the rift-fill; amphibolites are interbedded in this succession in regions where ferruginous and pyritic quartzites also occur. Graphitic schist along the southern margin of the orogen suggests local anoxic conditions of deposition and potential for mineralisation in the transition zone between graphitic schists and more proximal rock types. Low-grade iron-formation occurs extensively in the northern and north-western parts of the orogen. Rifting eventually culminated in spreading in the Southern Zone. Banded iron-formation associated with basic volcanics in continental slope and deep-water fan deposits along the southern margin should be considered in terms of the sedimentary iron facies. The Hope, Matchless and Otjihase copper deposits are associated with mid-ocean ridge volcanics interbedded with quartzose schist. Magmatic/hydrothermal deposits of the orogenic phase are related to north-westward subduction, ocean closure and continental collision. Uraniferous alaskites were emplaced at deep stratigraphic levels after prolonged differentiation of crust-derived magmas. There is potential for strata-bound tin and tungsten mineralisation in skarns and possibly quartzite mantos adjacent to the more than 200 plutons of granite. Hydrothermal tin/tungsten veins occur. Tin-bearing pegmatites are confined to distinct structural/metamorphic zones in the northern half of the granite belt, i.e. that half furthest from the supposed suture zone. Thrusts in the low-temperature Northern and Southern Margin Zones were potential channels for transport of metals in hydrothermal solutions generated during prograde metamorphism. There would appear to be considerable potential for Mississippi Valley-type mineralisation in the Tsumeb Outjo area in such local depositional environments as fore- and back-reef areas. compaction or drape structures, stratigraphic pinch-outs and landslide breccias. High-level mineralisation (porphyry copper deposits, sulphide, and precious metal veins) may have been a source for some mineralisation in successor basin deposits of the Mulden and Nama Groups.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 397-407

A large number of Sr, Pb and Nd-isotope results are now available on a variety of rocks and minerals from the Damara orogenic belt, Namibia. This offers an unusual opportunity to contrast results from different decay schemes and to combine them to constrain models of the chemical evolution of a segment of continental crust. The exposed basement rocks are at least 2,0 Ga old and both Pb and Sr-isotope ratios suggest that they have unusually enriched U/Pb and Rb/Sr ratios. Within the Damara sedimentary pile at 500 Ma, there was a striking increase in 87Sr/86Sr and decrease in 143Nd/144Nd with depth. Model Nd ages of the lower (Nosib) sediments range from 1,7 and 2,1 Ga, indicating that they were largely derived from the pre-Damara basement. Detrital zircons from these sediments also have model ages of 1,0-2,0 Ga (Briqueu et al., 1980). In contrast, the upper Damara sediments have model Nd-ages of about 1,0 Ga due to the influx of large volumes of material from a much younger source terrain. Combined Nd and Sr isotopes demonstrate that at 500 Ma, the analysed sediments and basement orthogneiss had relatively high 87Sr/86Sr ratios, reflecting the relative increase in Rb/Sr in the upper continental crust. Similar results were obtained from the alaskites and one leucocratic granite, but other, more voluminous diorites and granite show no such displacement to high 87Sr/86Sr, suggesting that they were derived from the lower crust and uppermost mantle. Pb and Nd isotopes indicate that both the alaskites and the uraniferous ore-bearing fluids were generated from enriched crustal material at least 2,0 Ga old. Assuming that the radiogenic Pb isotopes and the relatively high 87Sr/86Sr are from the same source region, it is inferred that the unusually enriched province remobilised in the Damara event was restricted to the upper levels of that segment of the continental crust.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 409-421

The variation in transport direction has been mapped for different phases across the Damaran belt. The earliest movement phase occurred in the northern arm of the belt to produce the south-east-verging Sesfontein thrust and nappe zone. This was followed by south-west-verging structures, only weakly developed in the northern arm of the belt but forming a wide zone of intense deformation in the main Damaran belt where folds produced during this phase (K2, K = kinetic or movement phase) are in the form of large flattened tubes or sheath folds, commonly over 10 km in wavelength. This deformation is considered to be due to a large, low-angle shear zone. On a regional scale, there was differential sinistral movement on this shear, with the rocks of the northern sector being sheared furthest to the south-west. Probable oceanic material was incorporated in this shear zone although there is no evidence for the existence of a large ocean; the oceanic material and locally thick sedimentary deposits may reflect localised basin development along this low-angle, strike-slip or transform fault. The third movement phase (K3) involved a change in relative plate movement so that the main Damaran plate over-rode the southern foreland with south-east directed movement. This produced locally intense deformation and several fold and thrust phases which affected both the over-riding and over-ridden plates. The central zone of the belt was the site of high-temperature metamorphism and granite intrusion synchronous with the K2 movement but continuing until after K3 movement. The granites may have formed by oblique subduction of oceanic material formed in basins along the shear zone, by local crustal delamination, by shear heating along deeper parts of the zone and/or by radioactive self-heating due to local thickening of the crust on this zone.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 423-429

A four-stage model of continental collision for the Damara Orogen is proposed. Early rifting produced a Damara ocean, which was probably several thousand kilometres wide. This was followed by continental convergence with subduction towards the north-west, during which the Khomas Trough developed and granites were intruded in the central Damara Orogen. Continental collision resulted in tight south-east-vergent folding and thrusting in the southern portions of the orogen and this was associated with the high-pressure M1 metamorphic episode. A period of oroclinal bending followed and was responsible for sinistral shear zones in the eastern portion of the collision zone. The buoyancy of the thickened continental crust resulted in suture progradation and a large boundary thrust developed to the south of the former suture, along which intracontinental subduction continued. The resultant isotherm pattern of this phase, together with uplift in the Southern Zone, explain the decrease in both temperature and pressure during the final stages of deformation. During the last phase, intracontinental subduction ceased and resulted in thermal relaxation and the post-tectonic M2 metamorphic episode in the southern Damara Orogen.
Back to title list




Evolution of the Damara Orogen of South West Africa/Namibia. National Geodynamics Programme. Special Publication No.11, 431-515

The Damara Sequence and rocks associated with the Damara Orogen cover most of South West Africa/Namibia. The Damara Orogen consists of coastal and north-east-trending branches which can be divided into contrasting tectonostratigraphic zones. Metamorphic grade increases from the margins to the granite-intruded belts in the west of the coastal branch and the centre of the north-east branch. The Damara episode was initiated between 900 and 1000 m.y. ago during widespread fluviatile deposition (Nosib Group) from local sources within and marginal to intracontinental rifts trending north-north-west, north-east and south away from a triple junction situated west of Swakopmund. In the northern of 3 north-east-trending rifts, late Nosib, bimodal alkaline igneous activity from both sodic and potassic centres lasted from 840 to 730 m.y.; playa-lake and sabkha conditions developed in the southern and possibly the northern rifts. The coastal branch of the orogen developed into a deep Pan-African South Atlantic ocean by the end of the Nosib stage. General subsidence that followed in the north-east-trending branch of the orogen is ascribed to a gradual evolution from rifting to spreading (Swakop Group) and is reflected initially in turbidites in the northern and southern grabens and carbonate-pelite-quartzite associations in the shallower, intervening and marginal areas (lower Swakop Group). Overlying mixtite deposits with both glaciogenic and mass-flow characteristics and local, interbedded metabasites mark a period of crustal instability that led to the evolution of the area between the southern and central grabens into a narrow, deep-water, Southern Zone ocean within which marginal deep-water fans were deposited. Extensive, coeval platform carbonates (Karibib Formation) occur to the north. Eventual continental rupture and the formation of a buried mid-ocean ridge in this Southern Zone ocean is suggested by the composition and form of a narrow, linear, 350 km-long belt of metabasite (Matchless Member) which is interbedded with schistose metapelites and metagreywackes (lower Kuiseb Formation) that interfinger with and overlie the deep-water fans. This stage was probably reached about 700 m.y. ago. Throughout this evolutionary period, the northern graben continued to deepen but never ruptured (16km of sediments), the Northern Platform remained an extremely stable carbonate depository (Otavi Group marker stromatolites occur over hundreds of kilometres), and the Pan-African South Atlantic received only turbiditic equivalents of the Swakop Group. Many of the Swakop Group clastic sediments may have been derived from the east and have the isotopic characteristics of a 1000 m.y.-old source. The structural grain of the orogen points to a reversal of spreading and to north-westward subduction of the African cratons below a South American craton and of the Kalahari Craton below the Congo Craton. D1 recumbent folding was followed by intrusion of 650 m.y.-old granitic rocks, uplift and erosion of the north-western coastal arm, and deposition of a northern molasse (Mulden Group). D2 deformation in the coastal arm marked the continental collision phase in this region; it folded the molasse succession, effected large-scale south-easterly overthrusting, resulted in the formation of paired metamorphic belts and was followed by the intrusion of 590 m.y.-old granite. The final, large-scale deformational event in this region caused westward-vergent back folding which, in its turn, was followed by intrusion of 570 m.y.-old, post-tectonic granites. Further south, in the Central Zone, widespread intrusion of 550 m.y.-old, syntectonic granites and extrusion of their volcanic equivalents in a 150km-wide, high-temperature-low-pressure zone along the leading edge of the Congo Craton was accompanied by uplift, erosion and the deposition of K-rich greywackes as a fore-arc sequence (upper Kuiseb Formation) above the earlier, spreading-phase deposits in the closing Southern Zone ocean. Sedimentological aspects of the Damara succession along the southern margin of the orogen suggest that the lower Nama Group, which contains a unique Ediacara fauna and was derived from easterly sources, was deposited between about 650 (700?) and 550 m.y. ago during deformation north of the Southern Zone ocean. Only when the Southern Zone ocean had been "filled" were Central Zone-derived sediments able to reach the Southern Foreland and platform as a red, distal molasse that contains the Cambrian trace fossil Phycodes pedum (upper Nama Group). About 542 m.y. ago, during the final major deformation event in the Central Zone (D3 doming), the fore-arc deposits and the underlying passive-margin sediments to the south were deformed for the first time and the locus of intense deformation was transferred from the Central Zone to the southern margin of the orogen where large-scale, south-eastward thrusting of basement and cover under conditions of low-temperature-high-pressure metamorphism occurred. Serpentinites intruded along thrust planes. Syntectonic deposits and a high-level nappe complex formed at the toe of the thrust front. Post-tectonic, prograde regional metamorphism in the Southern Zone peaked about 534 m.y. ago and was followed by intrusion of the Donkerhuk Granite which has an age of 523 m.y. The above events attest to a gradual southward progression of the locus of deformation, to closure of the Pan-African South Atlantic ocean about 50 m.y. prior to closure of the north-eastern branch of the orogen, and to deformation lasting 110 m.y. in the Central Zone but less than 20 m.y. along the intensely thrusted southern margin of the orogen. Isostatic uplift and rise of isotherms in the Central Zone caused melting of both basement and cover rocks in this region for a period of 70 m.y. after the peak of metamorphism in the Southern Zone, i.e. until 460 m.y. ago. Final gravity emplacement of the high-level nappe complex about 480 m.y. ago was probably due to this post-tectonic, isostatic uplift. The pre-Karoo erosive bevel (350 m.y.), which coincides approximately with the present erosion surface in SWA/Namibia, points to almost complete removal of the Damaran mountain chain within 180 m.y. of the cessation of tectonism. The Damaran granitic rocks are Hercynotype; granites make up 96 per cent of the more than 200 plutons. Average compositions have a slightly less calc-alkaline character than typical compressional margin granitic suites. Early granites have I-type chemistries and appear to have been derived from deep crustal sources, whereas most of the young granites have intermediate to S-type compositions and were generated at various crustal levels.
Back to title list




Upper Mantle Project - South African National Committee Symposium, July 1969. Special Publication No.2, 5-8

Summary of the project's history.
Back to title list