Transactions of the Geological Society of South Africa, 71, 103-111
The general aspects of the relationship between geology and underground water storage are discussed. Conditions and the probability of locating subsurface reservoirs in the various groups and formations of the Basement Complex of Rhodesia are described. A conclusion is drawn that greenstones and phyllites of the Bulawayan Group and the paragneisses of the Karoi-Miami Area are the most favourable formations for underground water storage whereas the granites and metasediments are the least favourable. Furthermore as the country becomes urbanized and industrialized it is virtually certain that, in spite of conservation, ground-water potential is bound to deteriorate.
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Transactions of the Geological Society of South Africa, 71, 113-123, 1 A3 map.
The re-mapping of the Que Que Gold Belt, first mapped from 1926 to 1931 has been undertaken and a revision of the original stratigraphic column is postulated. The oldest system, the Sebakwian, is now thought to be represented only by small ultramafic and sedimentary inclusions in the Rhodesdale Gneiss; the majority is thought to be a narrow ultramafic intrusion of post-Bulawayan age, emplaced along the edge of the Gold Belt. An abundance of fresh pyroxene in the lavas of part of what is thought to be the Bulawayan Group suggested a post-Bulawayan age, but evidence seems to indicate that the lavas, named the Maliami River Formation, are less metamorphosed Bulawayan, related to the more normal greenstones. Together with the exposures near Gatooma, they are thought to be the only unaltered lavas of the Bulawayan Group so far recognized in Rhodesia. The conglomerate, assumed by Macgregor to be the base of the Bulawayan Group, is thought to be merely a minor unconformity in the Lower Bulawayan lavas.
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Transactions of the Geological Society of South Africa, 71, 125-133
The schist belt rocks of the Rhodesian Basement Complex, in the West Nicholson-Beit Bridge District, and their contiguous metamorphosed granulite equivalents are described. It appears that there is no structural or metamorphic discontinuity between the adjacent facies in the area examined. A pre-Great Dyke age for the granulite metamorphism is postulated.
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Transactions of the Geological Society of South Africa, 71, 135-146, 3 pl.
The Indarama Mine provides an interesting structural control of mineralization in the Basement Schists. The orebodies occupy a complex fracture-system, resulting from two episodes of folding which deform Bulawayan greenstones. The effects of this deformation are apparent locally and regionally. Serpentinized ultrabasic rocks, formerly correlated with the Sebakwian System have here been found intruded into the Bulawayan System. The mineralization is of two main generations; earlier gold-arsenic and later gold-antimony. Associated with the latter are the rare minerals berthierite, here the chief gold carrier, and aurostibnite. A final phase of the mineralization is represented by stibnite, here not normally gold-bearing.
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Transactions of the Geological Society of South Africa, 71, 147-158
The observed relationship between economic concentrations of gold in a hydrothermal quartz vein occupying a fissure in Archaean pillow-lava and physical variations in the host-rock resulting from intrusion, fracturing and folding is described.
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Transactions of the Geological Society of South Africa, 71, 159-174
The Barton Farm Magnesite Mine is one of two in Rhodesia producing magnesite for refractory purposes. The deposit which lies about 6 miles S.S.E. of Gatooma in dolomitic argillites and arkoses of the Barton Farm Formation of the Shamvaian Group is described. Insufficient evidence is available to determine the origin of the deposit. It appears to be either a primary sediment, or a replacement body.
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Transactions of the Geological Society of South Africa, 71, 175-188, 6 pl.
The Mashaba Igneous Complex is a layered predominantly ultramafic intrusion of ancient Precambrian age. It consists of a sheeted portion and a dyke portion presumed to be the feeder of the main part of the intrusion. The ring form of the dyke appears to be unique in ultramafic rocks. The Complex is briefly described together with its subsequent deformation. This is held to be due to the emplacement of the Younger Granite of the area and has resulted in the formation of important deposits of chrysotile asbestos.
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Transactions of the Geological Society of South Africa, 71, 189-194
The lithology and stratigraphy of the various Precambrian rock units in the area around Shabani are described together with their mutual structural relationships. A table is presented showing the lithological units in the Shabani area.
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Transactions of the Geological Society of South Africa, 71, 195-204, 7 pl.
Chrysotile asbestos is a widespread mineral found in the majority of the Rhodesian ultrabasic bodies. Economic deposits are found in those areas where all the controlling factors of suitable host-rock, the presence of solutions and structural control have been satisfied. The tendency has been to assume a universal growth mechanism but field evidence indicates that fibre occurs as stress-controlled dilation-seams, as a recrystallization product of serpentine minerals and as a result of serpentinization of olivine. The Shabani orebodies are located in a partially serpentinized dunite with localized structural control a noticeable feature. The Sheffield Claims deposit is a classic example of the recrystallization of picrolite to chrysotile fibre. The Gaths deposit differs from Shabani in that the structural control is more regional. King Mine is an example of fibre formation in a serpentinite with a typical structural pattern.
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Transactions of the Geological Society of South Africa, 71, 205-213
The science of stratigraphy originated in Europe and North America and was concerned with the chronological classification of strata with reference to their fossil content. Stratigraphy has been forced to broaden its basis partly owing to the demands put upon it by other branches of geology, notably the petroleum geologists, and partly by the need to interpret successions in Precambrian rocks which are devoid of diagnostic fossils. Stratigraphic interpretation in Precambrian rocks is further complicated by the vast amount of diastrophism that most of them have undergone. Thus no relative age relationships can be established by the conventional means of superposition and fossil content. The determination of radioactive ages and palaeomagnetic pole positions are two means by which it is possible to establish age relationships in Precambrian rocks. The proper interpretation of these is, however, dependent upon a thorough knowledge of the rocks concerned and involves an understanding of their origin, composition, metamorphism, deformation history and other factors. Thus if stratigraphy is to be appliable to the Precambrian it must further broaden its scope to include all these factors. The objective of stratigraphy should remain the same throughout the geological column; it is the methods of achieving this objective that must change, adapting themselves to varying circumstances. Three types of stratigraphic terminology are gaining acceptance today; lithostratigraphic involving mappable rock units, chronostratigraphic involving rocks deposited within a specific interval of geological time, and biostratigraphic involving rock units characterized by their fossil assemblages. It has been found that the mapping and understanding of the Deweras and the Lomagundi rocks in the NW of Rhodesia has been facilitated by the use of the lithostratigraphic terminology in place of the rather hybrid, but basically chronostratigraphic, terminology used hitherto. This is because recognisable rock units have been used as a basis of subdivision, without yet worrying whether or not they have been arranged in strictly chronological succession. It is suggested that the lithostratigraphic terminology should be adopted for naming rock units throughout the Precambrian of Rhodesia.
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Transactions of the Geological Society of South Africa, 71, 21-32, 3 pl.
Geochemical maps are presented to illustrate the regional distribution of certain major elements and their ratios in areas of granite, greenstone and serpentine and related rocks. The analytical data was compiled from the records of the Rhodesia Geological Survey, and was used in preference to any other data available due to the high standard of analytical reliability. Eighty-eight samples of granite are represented, fifty-seven samples of greenstone and thirty-seven samples of serpentine and related rocks. Analytical data for the arenaceous and argillaceous rocks of the Basement Complex were not compiled, due to the paucity of the data. In relation to the geological complexity and size of the Basement Complex the analytical data provides a scanty and incomplete cover and for this reason the interpretation of the patterns is limited to those which are believed to be significant on a regional scale. The geochemical patterns for the granite reflect the presence of two belts which are potassium-rich and these border the northern and southern margins of the craton. This distribution is interpreted as evidence of geochemical fractionation in the granite magma resulting in the concentration of potassium in the upper levels of the craton which, owing to erosion, is now exposed mainly at the shoulders. The geochemical patterns for the greenstone and serpentine and related rocks reveal regional, and some local difference in the concentrations of certain of the major elements and their ratios. The geochemical abundances of the chemical elements in granite and basalt of the earth's crust are compared with the averages of the Rhodesian rocks. The Rhodesian rocks are remarkably similar to the crustal average, except that the granite is relatively poor in potassium and to a small extent in iron and magnesium, whereas calcium is slightly higher than the average. In relation to mineral deposits the geochemical pattern for potassium in granite appears to be significant. The important mineralized pegmatites at Miami and Bikiti are associated with potassium-rich granites on the northern and southern margins of the craton respectively, and there is evidence of a similar association at Kamativi. Other pegmatite belts are well known but the granite in their vicinities is not sampled. The geochemical patterns for the greenstone and serpentine and related rocks are of little significance in relation to mineral deposits due mainly to the lack of data. It is believed, however, that certain types of ore deposits which occur in these rocks are related to recognizable features of primary geochemical dispersion and that these features could be revealed by studies in the regional geochemistry of the minor elements. It is concluded that the study reveals geochemical features on a regional scale which are of fundamental significance to the geology of the Rhodesian Basement Complex, and also are of economic interest. The patterns illustrate a large area for which there is no analytical data, and the results of the study generally indicate the wide field of research which exists in the regional geochemistry of the Basement Complex.
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Transactions of the Geological Society of South Africa, 71, 215-224
Considerable discussion is taking place at present regarding stratigraphic nomenclature generally, and numerous reports and papers have been published. A fair measure of agreement on principles has been achieved between most countries. Disagreement centres mainly on the scope of stratigraphy and the number of sets of stratigraphic units necessary. A simplified definition of stratigraphy is suggested, and the case for two sets of units, chronostratigraphic and lithostratigraphic, is reviewed. It is concluded that only the largest-scale chronostratigraphic units can be recognized in the Precambrian, and that therefore we should use only the lithostratigraphic terms with the possible use, in addition, of the "System".
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Transactions of the Geological Society of South Africa, 71, 225-254, 3 pl.
The Barberton Mountain Land forms one of the best developed and best preserved remnants of Archaean strata in South Africa. The volcanics and sediments are similar to those associated with geosynclines and island arcs. The Onverwacht Series of the Swaziland System, consists mainly of basaltic and ultrabasic rocks together with siliceous sediments and constitutes the initial magmatic or ophiolite assemblage of the Swaziland geosyncline. The Fig Tree Series, composed primarily of greywackes, shales, banded cherts and banded ironstones, displays characteristics of a flysch assemblage of rocks while finally, the overlying Moodies Series, made up of conglomerates, quartzites, shales and jaspilites, displays the characteristics of a molasse assemblage of a geosyncline. The entire area is surrounded and swamped by granites and gneisses, thereby making it impossible to identify the miogeosynclinal and the cratonic areas associated with the Barberton Mountain Land remnant. Furthermore, the granites are not typical of normal geosynclinal areas and constitute an anomalous event, probably much later than the orogenic episode. Evidence is presented that the Onverwacht and Fig Tree Series of rocks were deposited in a deep trough (pillow lavas, turbidites). A brief account of the structural history of the area is given, and some notes on the mineralization are included. A model is suggested for the major evolutionary events in the development of the Barberton Mountain Land. The first event was the geosynclinal cycle, while the granites make up the second event. It is suggested that the Barberton Mountain Land represents a fold belt in which the geosynclinal and granite cycles are well-displayed, and, as a result, may assist in a better understanding of the evolution and development of other, less well-developed schist belt remnants in Archaean granite terrains of continental shield areas.
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Transactions of the Geological Society of South Africa, 71, 255-270
The Swaziland System is typical of the ancient greenstone belts of the world. Basal parts of these belts (Onverwacht type) are strikingly like emergent island arcs, and must have evolved upon a thin, unstable crust. In contrast, the succeeding parts of most greenstone belts (Fig Tree and Moodies types) have been derived from, and deposited upon, a progressively more stable, thickening, granitic crust. Moreover, at present, the greenstone-granite terrains of the Barberton type form the thickest, coolest, and most stable shield areas of the world. The enveloping granites exceed the greenstones in volume by an order of magnitude. Some are over 3,000 million years old, and most surficial granite exceeds 2,500 million years in age. Granite must have been added to the green stone-granite terrains during both orogenic and anorogenic periods. Much of it has apparently been passively accreted from below. The basalts and peridotites of the Onverwacht type are depleted in the radiogenic nucleides K, U, and Th. Lavas of this composition have been erupted from the mantle in island-arc environments for over 3,000 million years. They may represent the only primary mafic magma-types. Their constancy in composition throughout decipherable geologic time suggests that their source in the upper mantle also has been of uniform composition. This indicates that the first-order differentiation of the upper mantle and protocore preceded the emplacement of the Onverwacht lavas. The small amounts of K, U, and Th in them indicate that similar, localized source-regions of the mantle cannot have been the source of the associated granites. The origin of the widespread granite appears to be the result of large-scale melting in the mantle prior to, and during, Onverwacht times.
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Transactions of the Geological Society of South Africa, 71, 271-275
In September, 1947, a small booklet was published entitled "An Outline of the Geological History of Rhodesia" by A. M. Macgregor. This was only 20 years ago but it was the basis upon which Rhodesian rocks are classified at the present time. The granites and gneisses, together with the relatively small, irregularly-shaped, infolded belts known as "gold belts" or "schist belts" were together called the "Basement Complex". This description of Macgregor's still fits in with our present conception of the "Rhodesian Basement" about which our discussions have been centred during these last three days. Our basement is old and dimly-lit but it is clear from this symposium that a great deal of very diligent probing into its darker corners has taken place during the past 20 years. In my attempt to sum up our deliberations, I shall try to deal with generalities rather than with individual papers. If I do refer to any specific paper, it is not because it is of any special merit - in my view, every paper presented was of a high standard and each contributed greatly towards new ideas and knowledge of the numerous facets concerning the Basement Complex. I shall now attempt to sum up and comment upon the most important issues discussed. 1. Correlation of rock units and nomenclature. 2. Structural and metamorphic considerations. 3. Regional geochemistry. 4. Economic aspects. 5. Statistical information. 6. The problem of water in the Basement rocks.
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Transactions of the Geological Society of South Africa, 71, 3-7
Rhodesia is a deeply eroded Precambrian shield area or craton situated in the centre of the southern part of the African continent. Post-Cambrian rocks occur only in the west where they extend into the Tertiary to Recent Kalahari basin, and in the down-faulted and down-warped Zambezi and Limpopo valleys. To the north of the Rhodesian craton an area of intensely deformed and metamorphosed rocks extends to the Bangweala craton in northern Zambia. A similar zone to the south separates the craton from the Transvaal or Kaapvaal craton of South Africa, while to the east is the lower-lying plain of the Mozambique Metamorphic Zone which near the coast is covered by Karroo and Cretaceous sedimentary rocks. Rocks similar to the Rhodesian Basement Schists occur in the Transvaal craton around Barberton and in the Murchison-Pietersburg area. The Rhodesian craton measures about 300 miles from north to south and 400 miles from east to west. It is bounded on three sides by mobile zones over 100 miles wide in which there have been repeated episodes of deformation and metamorphism, with a general tendency, through time, to sink relative to the craton. Little is known as yet about the geology of these mobile zones, but in the northern or Zambezi Zone there appear to have been episodes at around 2,650, 2,100, 1,650-1,900, and 450-650 million years. Activity occurred in the eastern or Mozambique Zone where it impinges on Rhodesia at about 850-950 and 450-600 million years, and in the southern or Limpopo Zone at around 2,650 and 1,750-2,000 million years. The following is a simplified stratigraphical column of Rhodesian rocks. Phanerozoic: Kalahari System (Tertiary), Cretaceous System, Karroo System (late Carboniferous to Jurassic), Sijarira Group (late Precambrian or early Palaeozoic). Precambrian: Umkondo Group (older than 1,100 million years), Piriwiri and Lomagundi Group (older than 1,650 million years), Deweras Group, Great Dyke (older than 2,530 million years). Basement Complex: Granitic rocks of various ages. Basement Schists - Shamvaian Group (older than 2,650 million years). Bulawayan Group (probably about 2,900-3,200 million years), Sebakwian Group (possibly older than 3,400 million years). The papers read at this Symposium dealt with various aspects of the Rhodesian Basement Complex and some of its mineral deposits, showing recent advances in knowledge and outlining unresolved problems. One of these problems is the stratigraphy and nomenclature of the Basement Schists.
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Transactions of the Geological Society of South Africa, 71, 35-45
Attempts have been made to distinguish the underlying regional element in the distribution pattern of exploited gold deposits in the rocks of the Rhodesian basement. Up to eight-order polynomial trend surfaces have been employed to study the areal variations in five production variables. Two principal trends appear to influence the patterns of gold mineralization, one between north- north-west and northwest, and the other between east-northeast and northeast. The regional distribution of gold deposits in Rhodesia can, therefore, be studied by reference to the geometry of interference patterns produced by the super-imposition of two surfaces. Two broad areas are characterized by the optimum regional developments of trends of tonnage of mineralized rock, ounces of gold emplaced, and grade of ore. The distribution pattern of gold mineralization in Rhodesia appears to be the mirror-image of that in the Kaapvaal crustal fragment to the south.
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Transactions of the Geological Society of South Africa, 71, 47-51
The so-called Basement Complex rocks of Rhodesia can be shown to comprise a number of discernible units and to have been involved in at least four distinct orogenic episodes. The major structural units are shown on a map. It is proposed that the use of the term "Basement Complex" be discontinued and in future the specific units be referred to by name.
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Transactions of the Geological Society of South Africa, 71, 53-78, 3 pl.
The area described is situated near the centre of the Rhodesian craton and was relatively stable during the younger episode of folding and granite intrusion that affected the Basement Complex. It lies between the Chilimanzi and Insiza domes and it was possible to recognize older, pre-dome structures here. Detailed mapping has shown narrow fold belts, each of a different age and trend. The Selukwe Schist Belt was found to be the inverted limb of a nappe structure, thrust over a micro-cratonic basement of gneiss that was later reactivated. There is evidence that the fold belts may have been the sites of small eugeosynclines, so that a lithological correlation between them is not reliable. Differences of tectonic style between the fold belts show that there was an apparently progressive thickening of the competent superstructure. The large gregarious batholiths were mantled gneissic domes that represent a second phase in the development of the shield.
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Transactions of the Geological Society of South Africa, 71, 79-88, 1 pl.
This paper deals with the end-products of metamorphism of Bulawayan greenstones, a metanorite and certain Shamvaian calc-silicate rocks along the Zambezi Metamorphic Zone north of Sipolilo. An outline of the geology is given so that the setting can be visualized. Their very complex tectonic history is only dealt with in the broadest terms, the imprint of two major events, widely separated in time is clearly discernible. The Great Dyke which intervenes in time, helps in the dating of one of the events.
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Transactions of the Geological Society of South Africa, 71, 89-90
The rocks belonging to the Basement Schists in the vicinity of Gatooma consist of representatives of the Sebakwian, Bulawayan and Shamvaian Groups. The Sebakwian is only poorly represented by ultramafic rocks and some sediments. The Bulawayan Group has been divided into two formations. The lower, or Mafic Formation, consists mainly of mafic lavas (greenstones), but includes some metamorphosed, mafic, contemporaneous, intrusives while the upper, or Felsic Formation, consists of mafic lavas and sundry quartz-mica schists which include rhyolitic rocks, sediments derived from these by contemporaneous erosion, and rocks of an indeterminate origin. The Shamvaian Group consists of an upper Pebbly Quartzite Formation and a lower Barton Farm Formation. The former consists of quartzites and poorly developed conglomerates, and the latter of coarse- to fine-grained clastic sediments, dolomitic rocks and an associated magnesite deposit. The clastic sediments were derived from fresh volcanic material. The name Umniati Group has been provisionally given to the suite of volcanic rocks which lies beneath the Proterozoic to the west of Gatooma and which hitherto has been regarded as part of the Bulawayan Group. The suite consists of acid to intermediate lavas, agglomerates and tuffs, together with some minor coarse-grained rocks that are probably contemporaneous intrusives. They are remarkably fresh, both mineralogically and texturally; unaltered pyroxene occurs in some types. This is in marked distinction to the Bulawayan Group volcanics which are almost completely altered to a fine-grained aggregate of chlorite, epidote, leucoxene, minor plagioclase and quartz. The evidence available suggests that the Group is younger than the Shamvaian but older than the Deweras Group, which, with the Lomagundi Group, forms the Proterozoic rocks to the N.W. of Gatooma. The N.W. portion of the Rhodesdale Gneiss outcrops in the area. The Gneiss as a whole is an elongated body 75 miles from north to sout
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Transactions of the Geological Society of South Africa, 71, 9-19
Data concerning gold production, between 1898 and 1932, from 120 of the more important mines in Rhodesia have been subjected to analyses of the general statistics of population frequency distributions. General predictors of the relative economic potential of any host-rock, associated sulphide, or linear structure have been shown by the analyses to be the number of mines containing the geological feature and the third and fourth moments of the frequency distribution of the average size of mines. Greenstones, and then schists, emerge from the analyses as the most important host-rocks for gold mineralization. Optimum conditions also appear to prevail where chalcopyrite, and then stibnite, are present in the ore, in addition to pyrite. There seems to be a preferential concentration of gold mineralization in orebodies which dip into the quadrant between north and east. The distribution patterns suggest that the gold was introduced into the Rhodesian Basement in the greenstones, and that the mineralization is not a direct hydrothermal product of the generation and emplacement of the granites.
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Transactions of the Geological Society of South Africa, 71, 91-102, 5 pl.
The Mont d'Or sediments are structurally homogeneous arkose and greywacke in which six distinct sets of fissures were recognized. The oldest are group six, marked by saddle veins. Those of sets one to four were contemporaneous and are related to the last episode of E.N.E. folding. Set five is related to the younger Surprise Fault zone. There were three main stages of mineralization in the sets one to four fissures, each separated from the other by minor deformation stages. Early white quartz was followed by grey quartz, carbonate and pyrite, which were followed by two divergent types of mineralization, pyrrhotite/chalcopyrite, and galena/sphalerite in stage three. The mineralization is related to a molybdenite/pyrite/chalcopyrite source in soda-rich aplogranite plutons. The mineralization of set five veins is different, with cobalt and nickel arsenides present.
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Transactions of the Geological Society of South Africa, 71, p33
In a paper published in "Some Ore Deposits in Southern Africa" (1964), the writer gave examples of the relationship between the gold deposits and their local structure in Rhodesia. This indicated that they were found under idealized physicochemical conditions following faulting, folding and shearing. The ore bodies were gold-bearing quartz veins in fissures, gold-bearing quartz and sulphide mineral replacements of wall rock in shear zones and silicification accompanied by gold-bearing sulphide mineral impregnations of banded ironstones. The attitude and position of both the bodies and shoots within the bodies were primarily controlled by the structures produced during regional deformation as a result of compressive forces acting on a northerly-trending axis. Further consideration emphasizes that the influence of structure on the localization of economic gold mineralization is the most important factor controlling individual, Rhodesian-type gold deposits. An analysis of a large number of fracture planes was made by plotting the poles on an equal area stereographic projection. This indicated broad, highly generalized stress directions which resulted from the action of a compressive force acting in a northerly-trending axis. The gold mineralization ranges in age from Pre-Bulawayan to Post-Shamvaian and was distributed in patterns corresponding to antecedent, contemporary and recurrent structures. This view contrasts with the longheld belief which associates the ore bodies with the contraction of the intrusive granitic masses.
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