The geology of some ore deposits in southern Africa, 1, 1-23

Two diagrams are presented covering the known extent of the Witwatersrand Basin: No. 1 portray's the surface geology of this area: No. 2 attempts, by the removal of younger, obscuring formations, to reveal the complete distribution of the Witwatersrand System. Linked with these diagrams, an historical review is presented, illuminating the events which led up to the discovery of the gold-bearing reefs of the Witwatersrand, the discovery itself and the progress of exploration and development of the Witwatersrand proper, of remote outcrop areas and, eventually, of the hidden extensions of the System.
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The geology of some ore deposits in southern Africa, 1, 109-111

That portion of the Witwatersrand Basin which lies to the east of East Rand Proprietary Mines Ltd, and which extends as far as Heidelberg comprises the East Rand goldfield and is the area described in the papers which follow. Historically, the Main Reef group was followed along outcrop both eastwards and westwards from the Central Rand. Eastwards, this group was traced to Boksburg, where the beds of the Witwatersrand System plunge below a cover of rocks of the Karoo System to reappear for a comparatively short distance near Benoni. This gap, known as the Boksburg Gap, was not bridged for a considerable time as it was believed that the Main reef had petered out at East Rand Proprietary Mines and made again at Kleinfontein and Modderfontein. This concept has now been disproved as it is known that conglomerates of the Main Reef group are continuous from the Central to the East Rand. Surface exposures of Witwatersrand beds are scarce over most of the East Rand, but crop out again south of Nigel and can be clearly followed to Balfour and beyond. This goldfield has celebrated severty-five years of production, as the Nigel Gold Mine was started in 1888 and Van Ryn Estates in 1892. The last mine to be brought to production was Spaarwater Gold mines in 1947. Altogether, twenty-seven mines have been established on the East Rand. Ten of these have ceased production either because they became unprofitable to work or because they were amalgamated with adjoining properties. Lying some thirty miles east of the East Rand goldfield, a new mining area has been established. Auriferous conglomerates belonging to the Upper Witwatersrand were discovered below a covering of Karoo and Transvaal beds, largely by recognition of magnetometric anomalies caused by concealed magnetic beds of the Lower Witwatersrand. The first mine, Winkelhaak Gold Mine, started in 1958 and two other mines, Leslie and Bracken Gold Mines, have since been brought to production. This goldfield is now known as the Evander Area. Well to the south, in the vicinity of Balfour and Greylingstad, several small gold mines were established early in the century, but ceased production many years ago.
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The geology of some ore deposits in southern Africa, 1, 113-123


The area dealt with in these notes lies some 30 miles east of Johannesburg, along the continuation of the outcrop of the Witwatersrand rocks, and includes those mines shown on the locality plan. The mines are referred to in these notes by their colloquial names
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The geology of some ore deposits in southern Africa, 1, 125-160

The geology of five gold mines on the East Rand is described. The geological succession of the younger systems, Karoo, Transvaal and Ventersdorp, is outlined and mention made of economic deposits therein. The Upper Witwatersrand succession is described in some detail, particularly that of the Kimberley Series and of the Footwall Beds below the Main Reef Leader. The nature and possible origin of payshoots, and the association of payable reefs with unconformities are discussed. The conclusion is reached that the period of mineralization antedates all faulting and dyke intrusion, that it is probably contemporaneous with reef formation and that slow contemporaneous earth movement has affected the distribution of the sediments and of payable reefs. Nine types of hypabyssal intrusives have been recognized. Plans and sections illustrating the geology, structure, age relationships of dykes and faults, distribution of the Footwall Beds and other features are presented.
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The geology of some ore deposits in southern Africa, 1, 161-190

A compilation is given of both published and hitherto unpublished data pertaining to the geology of the southern and south-eastern sectors of the East Rand Basin. The general stratigraphic column is described briefly while the sections including the principal economic reefs of the East Rand, namely the Main Reef and Kimberley Reef groups of sediments, are described in considerable detail. The conditions of deposition of these sediments are discussed and it is concluded that the economic conglomerates were formed at base level in a marine neritic environment. Structural details and age relationships of intrusives are given while the metamorphism of both the sediments and intrusives is discussed with special reference to chloritoid and ilmenite-leucoxene-rutile aragenetic sequence.
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The geology of some ore deposits in southern Africa, 1, 191-218

The thin sediments of the Black Reef Series, forming the basal member of the Transvaal System, occupy a semi-circular synclinal trough broadly reflecting the underlying structure of the Ventersdorp and Witwatersrand Systems. Two isolated basins are preserved along the north-eastern extension of the main synclinal axis, the outer of which overlies the East Rand Goldfields. In contrast to other areas, the Black Reef Series within the Witwatersrand basin seldom exceeds 100 ft in thickness and the sediments are composed mainly of hard vitreous quartzites with interbedded layers of carbonaceous shales, while irregular pyrite-rich basal conglomerates are developed in places. The Black Reef Series is virtually unaffected by most of the major faults of post-Ventersdorp age traversing the underlying formations, but became involved in folding of some magnitude. In the subsidiary East Rand basin, contours suggest a relationship between minor or secondary basin structures of the Black Reef and those of the Main Reef Leader. The Government Areas channel and layered deposits are by far the most important of the various economic occurrences described. Payable quantities of gold are distributed through widths of over 40 ft in these channel fillings and data obtained from mining operations established a clear relationship between the distribution of gold in the channel deposits and that of the sub-outcropping Kimberley conglomerates that were eroded. High osmiridium values were associated with the gold recovered from the Black Reef. The ratio of gold and osmiridium recoveries was found to vary in different sections of the mine, and mining and mill records established a correlation between the occurrence of the two precious metals in the Black Reef and the underlying Kimberley conglomerates. It is concluded that gold, osmiridium and some of the pyrite in the Black Reef represent the re-distribution products of eroded Kimberley conglomerates. The Black reef stoping at Randfontein Estates follows lines that are parallel to the strike of sub-outcropping auriferous Witwatersrand conglomerate groups, and the accumulation of gold appears to have been controlled to some extent by the weathering characteristics of soft underlying shale formations which formed areas of depression where gold particles collected - a feature also observed in one of the payable areas of the Geduld Proprietary Mine. The geological conditions and environmental factors do not appear to offer a satisfactory explanation for the occurrence of payable quantities of gold in some of the minor Black Reef areas worked. The absence of payable Black Reef occurrences in apparently favourable areas referred to is also not understood. It is submitted that the physical processes of flotation and flocculation promoted by the abundance of hydrocarbon materials present at the time may have caused fold particles liberated by erosion to become dispersed and carried away by skin flotation
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The geology of some ore deposits in southern Africa, 1, 219-282

The South Rand Goldfield is one of the two minor fields which, together with the seven major gold-producing areas, are located within the Witwatersrand Basin. It is contained in the area between the Sugarbush Fault and the Vaal River and acts as host to the south-easternmost exposures of the Witwatersrand System within the confines of the basin. The nearest gold producers are the Witwatersrand Nigel Mine, in the East Rand Basin, 26 miles to the northwest of the centre of the South Rand Goldfield, and the Bracken Mine, in the Kinross Goldfield, 29 miles to the northeast. Five gold mines - Edenkop, Kildare, Heidelberg-Roodepoort, Southeast Witwatersrand, and Hex River - have been intermittently active in the 75 years since gold was first discovered at the Heidelberg-Roodepoort Mine in 1887. All members of the Witwatersrand System are present in the area, but in a much-attenuated form, thinning progressively southeastwards from the East Rand Basin, through Heidelberg, so that in the extreme south of the area, at the Hex River Mine, they have the following thicknesses: Hospital Hill Series - 2,000 ft; Kimberley-Elsburg Series - 1,100 ft. The Lower Division is thus 5,300 ft thick in this locality, and the Upper Division 1,900 ft, making a total thickness of the Witwatersrand System of 7,200 ft. In the northernmost portion of the area the Lower and Upper Divisions are 8,900 and 4,700 ft thick respectively, and the System as a whole 13,600 ft thick. The Witwatersrand strata are overlain by amygdaloidal, porphyritic and massive andesite lavas of the Lower Series and tuffs, thin lavas, and sediments of the Middle Series of the Ventersdorp System. Above these are the Black Reef and Dolomite Series of the Transvaal System, and the Middle Stage of the Ecca Series of the Karoo System. Underlying the Witwatersrand System are the Old Granite and the Moodies Series of the Swaziland System, the arenaceous members of which have been mistakenly ascribed in the past to the Orange Grove Quartzites at the base of the Witwatersrand System. Conglomerates in the South Rand area are substantially lesser in numbers and extent than in the goldfields to the north, and there are only two which contain economic quantities of gold. There are no payable reefs in the Main Stage, none in the Bird Stage, only one in the Kimberley Stage, and none in the Elsburg Stage of the Upper Division. The Ventersdorp Contact Reef, if present at the base of the Ventersdorp System, has not been shown to carry economic values of gold. Many persistent auriferous conglomerate horizons recognized over extensive areas in other goldfields, such as the Main Reef Leader (Nigel Reef) near the base of the Main Stage in the East Rand Basin, are not developed at all in the South Rand area. An anomalous feature of this area when compared with the Central Rand, East Rand and Kinross Goldfields, is the development of economic banket in the Coronation Reef of the Government Reef Series. This is the only instance outside the Klerksdorp Goldfield where this horizon has been successfully exploited. Intensive prospecting operations over an area of about 1,900 square miles in the South Rand Goldfield have failed to reveal any deposits of gold other than those contained in three narrow payshoots in the Kimberley Reef in four mines and one payshoot in the Coronation Reef in one mine. The area has suffered considerable deformation in the form of folding and faulting that resulted from two stress fields in which compression was in a more-or-less horizontal plane, and a stress field where vertical forces were operative. Two sets of folds are present with axial plane traces treding N 50 W and N 30 W, and these can be correlated with the longitudinal and transverse folds recognized in the East, Central and West Rand areas, and in the Vredefort Dome locality. The interference of the fold systems has given rise to a series of structural culminations and depressions which, in turn, have produced the two regional synforms and two regional antiforms that control the distribution and preservation patterns of all pre-Karoo rocks in the area. The axes of these structures trend north-north-westwards and the regional plunge is in the same direction, resulting in a large mass of granite and Swaziland rocks terminating the South Rand area to the south and southeast. On the north the boundary of the area is formed by the impressive Sugarbush Fault, a displacement which has a general east-north-easterly strike, is of the normal type, and has a downthrow to the south of up to 16,000 ft. A very considerable area of development of Witwatersrand rocks has been preserved from erosion because of this fault which is one of a series of conspicuous normal, and attendant antithetic, faults that follow parallel, sinuous courses, the latter characteristic being the result of bending of these relatively early fault planes about the axes of the transverse folds. These and two other groups of normal faults are considered to be the products of the vertical stress field, while the remaining thrust and wrench faults, which abound in the area, can be satisfactorily assigned to the two horizontal stress fields.
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The geology of some ore deposits in southern Africa, 1, 25-61

Some generalizations about the stratigraphy have been extracted from an abundance of previous work to form a background for the structural aspect, which in the past has not received the attention it warrants. The closure of the basin rim allows a view of the goldfields as a unit, structurally as well as stratigraphically. The various structural segments of the rim are defined and described. A structural analysis of each peripheral segment gives rise to some generalizations about basin-edge structures. A distinction is made between primary features (pre-basin or concurrent with basining) and secondary or post-depositional. Primary peripheral subsidiary basins control the siting of transgressive overlaps. It is these that constitute the host rocks of gold and uranium concentrations.
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The geology of some ore deposits in southern Africa, 1, 283-321

A detailed description of the Upper Division of the Witwatersrand System and the Ventersdorp System in the West Rand area is presented. Attention is drawn to the fact that in the area north of the Witpoortjie Fault there is a marked thinning of the strata from the centre of the area towards the north and west. The area is unique in that reefs within every group of the Upper Division of the Witwatersrand System, numbering about 20 in all, are or have been exploited for their gold or uranium content. Value distribution in the various reefs is also discussed, and it is demonstrated that during the deposition of the Main Reef Group there were a number of entry points about the periphery of the depositional basin, while during the deposition of the Livingstone, Bird and Kimberley Reefs there was only, one major source area which lay towards the north. The area is highly faulted and dislocation took place both before and after the deposition of the Dolomite Series. Some folding is also present. Recent mapping, together with information from boreholes, has shown that there exists a folded basin of Ventersdorp sediments on Rietvalei. These rocks were previously, ascribed to the Dwyka Series.
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The geology of some ore deposits in southern Africa, 1, 323-386

This paper describes the geology of the sector of the Witwatersrand Basin between Randfontein and the Mooi River, generally known today as the West Wits Line. It refers briefly to previous work by others. It discusses the surface geology of the area of which a map in colour is provided, and in more detail the Transvaal and post-Transvaal geology as disclosed by drilling. The sub-Transvaal geology is depicted on another colour map on which the Black Reef contours are also shown and the geology of the Witwatersrand and Ventersdorp Systems is dealt with in considerable detail, particularly the sections of the Systems which contain economic or potentially economic reef (conglomerate) horizons. The economic geology of the area is discussed in all its major aspects, the reefs, their gold content, distribution, composition etc. The mineralogy of the three principal economic reefs is described. The composition and mineralogy, of the "carbon" in these conglomerates is given and a possible source of its origin suggested. The dolomite water problem is dealt with and the information at our disposal is summarised. A short chapter is devoted to the Statistics of production, taxation, dividends etc., of the mines on the West Wits Line, followed by a brief discussion of the relative importance of this area as a gold producer to South Africa, the Free World and the world as a whole. Finally, brief reference is made to the possible extensions of the economic reefs in the general area covered by the West Wits Line.
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The geology of some ore deposits in southern Africa, 1, 387-391

To the south of Randfontein, rocks of the Witwatersrand and Ventersdorp Systems are concealed below an extensive flat cover of the Dolomite Series. The dolomite outcrops extend southwards to beyond the Johannesburg-Potchefstroom National Road where outcropping quartzites of the Timeball Hill Stage of the Pretoria Series dip to the south to form a prominent ridge. Owing to post-Transvaal faulting, a narrow strip of dolomite outcrops farther south, followed by a second range of Timeball Hill sediments, giving rise to a repetition of the main topographical and surface geological features of the area on which the Western Areas Gold Mine is situated. One of the most prominent geological features observed in the outcropping Witwatersrand rocks to the north of the dolomite-covered area consists of a large upthrown block of outcropping Lower Witwatersrand rocks situated in the "Witpoortje gap" between the Witpoortje and Roodepoort faults. The cut-off against the former fault determines the mining limits of Randfontein Estates and neighbouring mines to the west of the gap, while the continuation of a part of this fault system farther south, known as the West Rand Fault, defines the eastern limit of the reefs mined at the Venterspost Gold Mine. To the east of the Roodepoort Fault, reefs of the Main Reef Group have been mined to a depth of nearly 9,000 ft at the Durban Roodepoort Deep Mine and have been intersected in boreholes drilled by the Johannesburg Consolidated Investment Co Ltd as far south as the farm Doornkop 239 IQ. Along the east side of this fault the Ventersdorp Contact Reef is being mined by South Roodepoort Main Reef Areas Ltd
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The geology of some ore deposits in southern Africa, 1, 393-398

In the Rietkuil Syncline, which is situated ten miles to the west of Klerksdorp, there occurs a remarkable development of auriferous and uraniferous reefs within the Lower Division of the Witwatersrand System. The reefs are unique in that they represent the only economic reefs thus far encountered below the Main Reef Group.
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The geology of some ore deposits in southern Africa, 1, 399-416

Five large producing gold mines are situated within an elliptical basin that lies to the east and south of Klerksdorp. A sixth mine is being developed. A detailed description is given of the Upper Witwatersrand beds which are almost invariably obscured by younger rocks. Attention is drawn to the fact that there is a marked thinning of the strata from the south of Buffelsfontein towards the sub-outcrop. The most important auriferous reef is the Vaal Reef, which lies within the Bird Reef Stage, and is generally payable throughout the area. A small amount of the gold produced in the area is obtained from the Elsburg and Ventersdorp Contact Reefs. The area is highly faulted and two ages of faults have been recognized. A structure contour plan of the producing mines is submitted.
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The geology of some ore deposits in southern Africa, 1, 417-448

The stratigraphical succession is more complete and thicknesses somewhat greater than in the type section around Welkom. Isopach maps reveal the nature of marginal unconformities associated with the Basal Reef, the "B" Reef and the upper portion of the Elsburg Stage. Regular overlaps developed in the direction of the limit of the depositional basin. It is deduced that downwars and arches were produced during Kimberley times and were intensified subsequently. Folding on an axis parallel to the elongation of the sedimentary basin was particularly prominent during Elsburg time. A marginal synclinal trough formed and was filled with coarse sediments as subsidence, due to folding, continued. These coarse sediments contain the gold-bearing conglomerates called Elsburg Reefs, which are limited to a narrow strip along the western limb of the syncline. The folding ceased towards the end of Elsburg time, when a new zone of disturbance, the Border Fault, was developed about two miles farther west. Lava of the Ventersdorp Lower Volcanic Stage poured out over the newly-formed sediments, and its weight and the depletion of the magma chamber eventually caused the sediments to tilt westwards. A northerly plunge of the axis of the marginal syncline probably was produced during a later Stage of Ventersdorp time. Step faults associated with the tilting have dislocated the Witwatersrand beds. Later faults are tentatively considered to have been brought about by the processes responsible for the plunge of the axis of the fold. A less spectacular marginal downwarp occurs west of the De Bron Fault in the east. The Witwatersrand beds are tilted towards the De Bron Fault in the southeasterly portion of the area, resulting in a broad zone of interference structures trending north-east across the Freddies Mine. The intimate relationship between the nature of the subsidence and the composition and distribution of the sediments that entered the basin is clearly revealed and its importance in unravelling the succession, facies changes, areal distribution of conglomerates, and the nature of the environments in which they were emplaced cannot be over-emphasized. Sedimentational features in the various reefs show that there might be a relationship between directions of transport of sedimentary material, especially heavy minerals, and pay-shoots. The conglomerates are considered to have been deposited subaqueously near the shoreline of a body of water and within range of the sorting action of currents and waves.
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The geology of some ore deposits in southern Africa, 1, 451-506

The five mines whose geology is described are Free State Geduld, Western Holdings, Welkom, President Steyn, and President Brand lying to the west of the Central (De Bron) Horst. The paper deals with the stratigraphy, structural geology, historical geology, and lithology of the Upper Witwatersrand, Ventersdorp, and Karoo rocks occurring within the area.
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The geology of some ore deposits in southern Africa, 1, 507-548

The stratigraphy and structure of the Upper Division of the Witwatersrand System in the southernmost mines of the Orange Free State Goldfield are briefly outlined. Several breaks in the succession are analyzed with the aid of stratigraphical maps and are interpreted as disconformities associated with either a transverse swell in the sedimentary basin or with the southern extremity of the basin itself. Towards the east the Virginia Section borders against the limit of the basin farthest removed from the original geanticlinal area. The land on this side is assumed to have had a very low relief and to have contributed very little sedimentary material to the basin. The limits of the basin varied widely during Upper Witwatersrand times, being at their narrowest between Basal Reef time and the end of the Kimberley stage. It is contended that most bankets were formed in a neritic environment, closely associated with marginal unconformities and disconformities, the surface of which were profiles of equilibrium. Close to these planes conditions favoured the concentration of heavy minerals and it is under these conditions that payable concentrations of gold and uraninite accumulated. The geological history is described with the aid of a series of paleogeological block diagrams.
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The geology of some ore deposits in southern Africa, 1, 549-599

The distribution of sediments including gold round the margins of the Rand Basin has indicated four distinct points of entry, of sediments into the basin, confirming the outline of the subsidence, arrived at by combined structural and sedimentological criteria. Gold in payable concentrations is one of the criteria for the recognition of the proximity of the edge of the basin. The structural setting of the deltas relates sedimentation to structure, which results in a view of the basin from which the controversial issues concerning the sedimentological environment can be largely resolved. The structural control of sedimentation seems to fit the available facts so closely that the hydrothermal theory seems unnecessary and untenable. The second part of the paper on the broader issues of basin tectonics and the folding associated with subsidence reviews the Vredefort phenomenon in its basin environment as a structural unit with vertical mobility. The floor of the basin consisted of several such segments, with differential subsidence which produced a characteristic pattern of folding in which crustal shortening played little part. This pattern is shown to be related to the distribution of payable gold, in that the structurally depressed portions of the frame sited the gold-bearing deltas as transgressive overlaps outside the structural lip.
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The geology of some ore deposits in southern Africa, 1, 63-108

This account of the geological setting of the gold mines of the Central Rand represents a compilation and collation of previously published data. It constitutes a summary of the version of the geology which has received general acceptance over the years. A general review covers the nature of all geological formations encountered in the area as a whole, and this is followed by a description of tile particular reefs worked in the various mines, the relative economic importance of each reef, the mineralogy, of the reefs, and the distribution pattern of the gold in relation to pebble size, reef widths, mineral assemblages and sedimentary features. In the final section data are given concerning tonnage, of ore treated, amounts of gold recovered, and average grades of ore worked. The Central Rand is contained within a distance of 29 miles in a west-east direction from the Roodepoort Fault, through Johannesburg, to the Boksburg Gap. At the end of 1962, 10 major deep-level mines and 12 small outcrop mines were working. Two other deep-level mines are located within the geographic confines of the area, but because of significant differences in geology, have not been considered as typical members of the Central Rand group of mines. The total thickness of the Swaziland System in the Central Rand area is 700 ft., of the Old Granite an unknown figure, of the Witwatersrand System 24,300 ft., of the Ventersdorp System 5,000 ft., of the Transvaal System 3,600 ft., and of the Karoo System 200 ft. The five subdivisions of the Witwatersrand System have the following thicknesses: Hospital Hill Series 4,900 ft.; Government Reef Series, 6,300 ft.; Jeppestown Series, 3,700 ft.; Main-Bird Series, 3,300 ft,; Kimberley-Elsburg Series, 6,100 ft. The age of the Old Granite which underlies the Lower Division of the Witwatersrand System has been determined as 3,200±65 million years, whereas the age of the Ventersdorp lavas which overlie the Upper Division is said to be 2,120±10 million years. The Central Rand forms part of the northern limb of a synclinorium, the axial plane trace of which strikes ENE. The southward dips of successively higher strata on the limb become progressively flatter. Younger folds with axial trends in a NNW direction are superimposed upon older east-west folds. Cleavage parallel to the axial planes of the older folds is developed in quartzites and shales, whereas small puckers in shales are products of the later folding. Longitudinal faults, such as the Rietfontein, Witpoortje, Roodepoort and Doornkop Faults, are responsible for the most significant displacements of the strata. The oldest faults many of which are of a thrust nature, strike approximately east-west. The next group is predominantly, normal faults that strike north-eastwards and north-westwards. The youngest faults have a north-south strike. A total of 2,000 ft, of conglomerate represents about 8 pet cent. of the thickness of the Witwatersrand thickness of the Witwatersrand thickness of the Witwatersrand System. With the exception of two thin bands in the Government Reef Series, all the conglomerates occur in the Upper Divisioin. There is a general coarsening in grain-size of all strata upwards from the base of the Witwatersrand System, and there is also a progressive thickening upwards of the various conglomerate groups. Ten separate conglomerate horizons have been mined in the Upper Division, nine of these occurring in the Main-Bird Series and one in the Kimberley Stage. The three horizons of greatest economic importance have proved to be the Main Reef, the Main Reef Leader and the South Reef, all developed in the Main Reef Leader and the South Reef, all developed in the lower portion of the Main-Bird Series. In addition, banded pyritic quartzites, present between the Main Reef and the Main Reef Leader, and the Black Reef conglomerate at the base of the Transvaal System, have been mined on the Central Rand. The reefs are not payable over the whole area, and different conglomerate horizons attain their optimum economic development, in different sections of the Central Rand. If all reefs are considered collectively, there is no obvious direct relationship between reef widths, pebble sizes and gold content, but along a single reef horizon lateral variations in reef width or pebble size are frequently associated with sympathetic lateral variations in the amount of gold present. Generally, reefs with larger and better-sorted pebbles contain more gold. The gold is either evenly distributed over the full width of the reef, or is concentrated near the footwall or hanging wall contact. Frequently, the greater the sulphide content of the conglomerate, particularly pyrite, the higher the amount of gold. Paystreaks in the Main Reef Leader, the most important reef horizon, swing from NW-SE, through east-west, to NE-SW, from west to east across the Central Rand. They form a braided pattern. Material originally flowed from the north and northwest along south-easterly and south-westerly directions into the basin. The land surface from which the sediments were derived occupied a position, with time, progressively nearer to the present outcrop. The strata presently preserved, as followed stratigraphically upwards, thus represent material originally deposited closer and closer to the shoreline. Maximum gold concentration took place in those reefs (Main Reef, Main Reef Leader and South Reef) that were deposited at a certain optimum distance from the shoreline, where conditions of concentration were most favourable. In the 76 years since gold was first won from the Central Rand, no less than 140 mines have been in operation at various times. Less than 50 of these became significant producers. A total of 1,023 million tons of Witwatersrand ore has been treated, from which 261 million ounces of gold have been recovered. The average grade of all reefs worked is thus 5.1 dwts. per ton. In addition, 425,000 tons of Black Reef conglomerate have yielded 195.000 ounces of gold at an average grade of 9.2 dwts. per ton. The gold won from the Central Rand won from the Central Rand represents more than 40 per cent. of all gold produced from all goldfields within the Witwatersrand Basin.
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The geology of some ore deposits in southern Africa, 2, 1-7

A summarized history of gold mining in Southern Rhodesia is given, followed by descriptions of the geological setting in which gold deposits in the territory are found. Associated minerals and factors controlling ore deposition are briefly discussed, and, in addition, the nature of the main gold-bearing occurrences is outlined.
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The geology of some ore deposits in southern Africa, 2, 129-168

Chromite deposits of the eastern part of the Bushveld Complex occur in the Critical Zone, which is divisible along its length into western, central, and southern sectors. The sectors differ in sequence of rock units and in sequence of associated chromite seams. In the western sector, the Critical Zone is poorly exposed, and the number and relative positions of chromite seams are uncertain. In the central and southern sectors the Critical Zone is divisible into a lower pyroxenite series and an upper anorthosite series. In the central sector, the sequence of major rock units in the pyroxenite series is everywhere the same. The chromite seams in the series are mainly in the lower two-thirds. Most are enclosed in pyroxenite, but a few are in olivine-bearing rocks or in anorthosite. Remarkable persistence over long distances characterizes certain seams. Chromite seams in the anorthosite series occur in a variety of rock associations: the persistence of the seams is incompletely known. In the southern sector, the pyroxenite series differs markedly from that of the central sector. Chromite seams occur almost entirely in the top 100 to 200 ft. of the series. The number of seams and the sequence of rock units in the pyroxenite series change rapidly southward in the sector, and seams cannot be correlated from part to part of it. In the anorthosite series, thick chromite seams occur at several horizons, commonly in association with pyroxenite units. The seams at Dwars River bridge, however, are in anorthosite. Chemical analyses of pure chromite minerals from various seams are summarized and discussed. Even for seams of the central sector, analyses are as yet insufficient to indicate the full pattern of chemical variation. A general decrease in Cr2O3 content and in Cr:Fe and Mg:Fe ratios upward in the Critical Zone, however, appears to be indicated. Analyses of chromites from different points indicate that there are significant variations in composition within individual seams. Correlation of rock units and chromite seams of the central sector with those of the southern sector involves inferences as to changes in sequence and thickness of units between Steelpoort and Grootboom. Further information is needed to resolve the problem; several different correlations are consistent with the limited data available. The sequences of rock units and their structural relationships are the framework within which the origin of the chromite seams must be examined. The persistence and concordance of rock units, apart from local features explainable by movement during consolidation, are inconsistent with origin of the Critical Zone of the seams by multiple intrusions. Differentiation in situ, complicated by intermittent motion of the magma, is part of the general mechanism involved. The possible role of successive heaves of magma is not yet established.
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The geology of some ore deposits in southern Africa, 2, 15-27

The Cam & Motor Mine is situated four miles east of Gatooma in the Hartley District. It is the largest gold producer in S. Rhodesia. A series of steeply dipping hydrothermal gold-bearing quartz veins are being mined to a vertical depth of 6,550 ft.
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The geology of some ore deposits in southern Africa, 2, 169-182

Geological surveys for economic purposes have supplied information on the chromite seams in the western and southern sectors of this belt not available to Cameron in his paper on the chromite seams of the eastern part of the Bushveld. On the farm Naboom in the western sector, steeply dipping layered chromite seams, in their normal environment of lower Bushveld silicate rocks, lie in their normal position with respect to the Merensky reef, the Leoloberg gabbro and the layered titanomagnetite bands. The reported xenolithic character of the chromite in this area is therefore disproved. A disturbed zone, some three miles wide, along the Steelpoort River valley separates the central and southern sectors. The contact between the Bushveld rocks and the underlying Pretoria series rises rapidly to the south, progressively eliminating the chromite seams and eventually the Merensky Reef. In the five miles of outcrop south of the farm Thorncliffe, the Main Chromite Subzone shows a very marked decrease in thickness. A small outcrop of chromite, of controversial structure, occurs near the Loskop Dam, near the interior contact of the Mafic zone with the domed rocks of Transvaal and older systems near Groblersdal. Comparison of the section of the layered zone of the Mafic portion of the Bushveld, in the eastern and western belts, indicates that the chromite seams of both belts can be correlated to a remarkable extent.
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The geology of some ore deposits in southern Africa, 2, 183-202

Recent unpublished studies of the chromite-bearing seams of the Western Belt of the Bushveld Complex indicate clearly that all the thicker seams are continuous and persistent both along strike and to depths of over 5,000 ft vertically below surface. Chromite is a convenient "sack" name for the complex spinel mineral. Titanium and vanadium are present in small quantities. The composition of chromite is closely related to its associated silicate host rocks. Evidence indicates a variation in composition of grains of chromite within a single sample. A system of symbols is proposed to designate the seams. Chromite-bearing seams are found in various layers, the highest being the Bastard Merensky pyroxenite. Two prominent seams occur in the Upper Group, four in the Middle Group and seven in the Lower Group. Narrow chromite-bearing seams occur below the Lower Group. The seams dip inwards into the Bushveld basin at angles of 8° to 25°. Post-Bushveld faults displace the seams at a number of points, the most severe being related to the emplacement of the Pilanesberg Alkaline Complex. Two wide breaks in continuity occur in the northern portion of the belt. The explanation for these breaks is controversial. The Bushveld contains the major proportion of the known chrome ore reserves of the world and the reserves can be measured in thousands of millions of tons.
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The geology of some ore deposits in southern Africa, 2, 203-207

Deposits of refractory chrome ore are situated in pyroxenites in the central part of a structural basin at the western extremity of the Bushveld Igneous Complex. They take the form of seams or zones, three in number, of accumulated chromite crystals formed during magmatic segregation. These seams differ from one another in their characteristics, which are described separately. Partial chemical analyses of the seams are included. The high alumina content, together with the low percentage of ferrous iron, renders the ore more suitable for refractory purposes than other Bushveld chrome ores.
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The geology of some ore deposits in southern Africa, 2, 209-224

The Great Dyke measures 332 miles in length and averages 3.5 miles in width. It is composed of four igneous complexes. The complexes of the Dyke exhibit a marked layering and rhythmically repeated zoning of the component rock-types. Gabbroic rocks overlie the uppermost pyroxenite in each complex. In cross-section the dip of the layering is inwards. The chromite seams are considered to have been formed by magmatic segregation and are confined to the olivine-rich types. They usually occur at the base of a zone consisting of dunite and harzburgite followed upwards by pyroxenite. Chemical analyses of 32 samples, representing nearly all the chromite seams exposed in the Dyke, indicate a sympathetic variation in the composition of the chromite with that of the silicate rocks. High Cr/Fe ratios (in the Hartley Complex they increase from 1.93 in Seam No. 1 to 4.2 in Seam No. 7) are also indicative of high chromic oxide values (these increase from 51.02 to 59.04 per cent. Cr2O3 downwards). The ratio RO/R2O3 in the samples of the Dyke are close to unity and vary from 0.98 to 1.05. From the results obtained and compared with those of the Bushveld Igneous Complex it is suggested that the Dyke magmas were probably derived from the same source as the Bushveld and that the more ultramafic phase of the igneous cycle commenced before that of the Bushveld.
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The geology of some ore deposits in southern Africa, 2, 225-237

The Merensky Reef of the Bushveld Igneous Complex is at present the sole source of platinum from this formation. This "reef" is a pegmatitic pyroxenite lying within a regularly layered suite of anorthositic and pyroxenitic rocks. The Mafic zone of the Bushveld Complex shows a regular succession across its outcrops and the Merensky Reef occupies a specific position in this sequence of layers. The reef contains the platinum-group metals, partly in the metallic state alloyed with iron, and partly as sulphides, arsenides and sulpharsenides, associated with nickel, copper and iron sulphides and chromite. The reef has been traced for a total of 132 miles in the Eastern and Western Mafic belts. Geological evidence indicates the Merensky Reef to be the basal portion of a relatively thin unit-heave of magma, which shows a pattern of magmatic differentiation. This heave is one of the many unit-heaves that make up the Mafic zone of the Complex. Although the genesis of the reef appears to be clearly the result of differentiation, many unsolved problems still exist. The layering of the reef and its associated rocks is amazingly regular, but this regularity is disturbed by local depressions and domes whose origin presents an unsolved problem.
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The geology of some ore deposits in southern Africa, 2, 239-302

Past production plus ore reserves amount to 1.5 million tons of copper metal. The ore has been found in twenty-odd chonoliths composed of various combinations of anorthosite, diorite, norite, hypersthenite and other related petrologic types which are grouped herein as "noritoid". There are about a thousand outcrops of these rocks within a thousand square miles which represent only a small part of a very much larger basement shield area. In the O'Okiep copper district a stratigraphic column has been established for the predominantly potassic gneisses and granulites interbedded with minor amounts of schist, metaquartzite, etc. It is thought that the gneisses and granulites were formed by multiple waves of granitizing fluids which to some considerable extent invaded along the bedding instead of flooding across it. Granites of three different ages are recognized. Many of the noritoid bodies which have been explored appear to pinch out entirely at depth. Seventy-three percent of the copper metal in ore, which occurs almost entirely in the noritoid chonoliths, is centred about one stratigraphic horizon in their wall rock, 15% a second stratigraphic horizon and 6% at a third. On the other hand some noritoid bodies do not contain ore at these favourable horizons or at any other. Much of the noritoid is associated with "steep structures" not previously reported in the literature. Some of these simulate complicated anticlinal and synclinal diapirs. They are noteworthy for their lack of persistence upwards, downwards and along strike. One, with an amplitude of 1,100 feet, has maximum dimensions in plan of 150 x 600 feet. To explain these features it is postulated that fluids related to the last granitization travelled up the bedding and gathered in such structural traps as pre-existing small anticlines, monoclines and faults. There they softened the rocks which, under tangential stresses, were folded into embryo "steep structures" which became more effective traps for further softening solutions until the mature "steep structures" were ultimately formed. Later fluids of greater potency leached principally Na and Ca from the wall rocks through which they travelled and deposited them as leucocratic noritoid in the "steep structures". In some places later fluids brought Mg and Fe and, from limited horizons at some localities, Cu and S. Some of the noritoid in some places evidently formed magma but replacement of the previously arrived leucocratic noritoid as well as country rock seems to have been the predominant emplacement process. The dispersed channels travelled by, the hypothetical fluids have not been recognized. It is suggested that cooling and lower pressures might release "mineralizers" from rocks formed by metasomatic replacement in much the same manner as is conventionally attributed to magmas. One hundred million years after the last granitization of the presently exposed horizon in the district, pegmatite fluids were generated at some horizon now eroded and injected downwards as post-noritoid dykes.
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The geology of some ore deposits in southern Africa, 2, 29-40

The Dalny Mine, a 22,000 ton per month gold mine in Southern Rhodesia, is situated on an extensive shear zone in a wide regional belt of shearing. The shearing appears to be closely associated with major regional drag folding. The sheared ground is mainly associated with a "greenstone" horizon in the Bulawayan System of the Basement or Archean Complex. Arsenical ore has been emplaced along the shear mainly, by replacement but aided by some fissure filling. The main gangue mineral in the channel is quartz, which contrasts sharply with the heavily carbonated and partly sericitized wall rocks. Generally the wall rocks are clearly defined and barren of gold. Two main dyke series cross the ore channel; one series is younger than the ore, the other is almost contemporaneous with the ore.
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The geology of some ore deposits in southern Africa, 2, 303-314

Messina is situated six miles south of the Limpopo River, the international boundary between the Republic of South Africa and Southern Rhodesia. It lies 1 degree north of the Tropic of Capricorn, at an altitude of 1,900 ft above sea-level, and the climate is dry and sub-tropical. The line of rich copper outcrops was no doubt originally discovered several hundred years ago and exploited by the "Ancients". Later, these people were overrun by Bantu who took over the workings and produced copper artifacts.
In 1903, Colonel John P Grenfell, the founder of the Messina (Transvaal) Development Co. Ltd was shown the workings and was sufficiently impressed to prospect some of them. All four mines were discovered from information revealed by these ancient workings. It is, however, also true that "blind" orebodies often have been located which never reached the surface. In no instance were the ancient workings an indication of the size of the underlying orebodies, and mining operations have invairably proved that the deposits are of greater extent than indicated on surface.
The mines vary in depth from 1,900 ft at Artonville to 3,800 ft at Harper, as developed at the end of 1962. The temperature gradient as measured in Harper mine, between 1,400 ft and 3,800 ft levels, is 1.67 °F per 100 ft. The temperatures on 3,800 ft and 100 ft levels are 123°F and 74 °F respectively. Ore production today, totalling 75,000 to 80,000 tons per month, comes from the four mines Messina, Harper, Campbell and Artonvilla, situated along a strike length of 11 miles. To date, mining operations have yielded 22,500,000 long tons of ore from the four mines. The ore is being extracted by shrinkage stoping and, exceptionally, by open underhand stoping, and is concentrated by flotation. The sulphide ore is smelted and fire refined on the site. The lack of impurities in the Messina copper allows fire-refined copper conforming to specification to be directly produced from the blister copper. About 1,000 long tons of fire-refined copper, assaying 99,9 per cent copper, are produced per month.
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The geology of some ore deposits in southern Africa, 2, 315-337

Exploration of a large low-grade copper deposit in carbonatite near Phalaborwa in the north-eastern Transvaal was undertaken by surface diamond drilling followed by underground drilling, development and bulk sampling. This work has indicated that ore available for an open-pit mine is of the order of several hundred million tons at a grade of 0.7% copper. The main mineral constituents of carbonatite at Loolekop are calcite and magnetite. The carbonatite has on the whole rather vague boundaries, being interbanded with an older rock, phoscorite, which consists of olivine, magnetite and apatite. The central mainly carbonatitic portion occupies an elliptical area on plan, approximately 2,200 ft by 1,200 ft. Diamond drilling has proved that the carbonatite extends as a vertical pipe to 3,500 ft below outcrop.
Two main periods of carbonatite emplacement are recognized. The older carbonatite has a rough vertical mineral banding which follows a concentric elliptical pattern parallel to the general pipe outline. Phoscorite is disposed conformably to this banding. A period of fracturing preceded the second main stage of carbonatite emplacement. The fractures controlled the distribution of a younger carbonatite which transgresses the banded structure of the older rocks. The principal locus of fracturing was at the centre of Loolekop where an elongated pipelike body of younger carbonatite was emplaced. This occurrence is about 2,000 ft long and has a maximum width of 400 ft.
Elsewhere the older carbonatite and phoscorite are riddled with dykes and veins of younger carbonatite varying in width from many feet to less than an inch. These dykes and veins strike in certain referred directions. Post-carbonatite shearing, principally a rejuvenation of movement along the main directions of fracturing, was an important geological event. Copper mineralization is suspected to have taken place in two main stages. The first of these appears to have predated the younger carbonatite. Structural control of the initial mineralization in which bornite was dominant is vague. Shattering of the older banded carbonatite and phoscorite probably provided minute cracks which guided hydrothermal mineralizising solutions. The second stage of mineralization, in which chalcopyrite is dominant, is intimately associated with the younger carbonatite in the main central body and in the multitude of associated dykes and veins. Sulphides lie mainly along fractures in the younger carbonatite. In addition to chalcopyrite and bornite, minor amounts of chalcocite, valleriite and cubanite are found throughout the deposit. Magnetite forms 25% by weight of the ore-body. The magnetite is titaniferous. The titanium concentration is laterally zoned, increasing outward from the centre of the deposit. Apatite is a minor accessory mineral in the greater part of the deposit. In certain wide zones towards the periphery, this mineral accounts for about 20% by weight of the ore.
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The geology of some ore deposits in southern Africa, 2, 339-351

The geology, structure and mineralization of the Mangula copper deposits are dealt with in a generalized manner. A marked relationship between structure and mineralization and a characteristic association of magnetite with the copper sulphides is described. Ideas regarding the genesis of the orebodies are put forward; they are thought to be hypogene and developed in depth under pressure. Brief descriptions of the Norah and Silverside deposits are given. They are considered to be epigenetic.
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The geology of some ore deposits in southern Africa, 2, 353-366

The paper deals with the Alaska Copper Mine in the Lomagundi District, Southern Rhodesia. Copper mineralization is found here within rocks of the Lomagundi System. The interpreted stratigraphic sequence, which deviates from previous descriptions, is outlined. Structurally crumpled and folded rocks of the Dolomite Stage fill the interior of a basin-shaped relic of a powerful thrust breccia. Mineralization is confined to the rocks within the basin and is enriched by supergene and oxidational processes. Payable ore occurs in anticlinal layers separated by low grade zones and in an extended oxidation funnel. The origin of the primary low grade protore is considered epigenetic and appears to correspond to mineralization and ore deposits in the metallogenetic province of the larger Lomagundi District; it is considered to be synorogenic with the Lomagundi folding.
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The geology of some ore deposits in southern Africa, 2, 367-382

The Tsumeb ore deposit is located in the moderately folded upper dolomite formations of the Damara-Otavi System. For purposes of structural mapping these have been subdivided into eight principal zones including several marker beds. A pinch-and-swell plug of pseudo-aplite, accompanied by numerous veins, invades the folded and brecciated dolomite in the mine; the rock appears to include mobilized sedimentary material. The upper half of the Tsumeb orebody was essentially a lens of complex ore developed in thin-bedded dolomite about the elongate pseudo-aplite mass and plunging south roughly with the bedding to a depth of 1,800 feet. In the lower half a second lens asserts itself on the north to build an elliptical shell of massive ore enclosing a core with irregular mineral veins, pods and disseminations. This flat-cylindrical pipe breaks through the chert-rich dolomite at a steep angle to the north to a known depth of some 4,000 feet. The main hypogene minerals are galena, sphalerite and tennantite, with subordinate chalcocite and bornite. Lead is the dominant metal. The ore was relatively rich in copper and poor in zinc in the uppermost levels and behaves similarly, below a depth of 2,700 feet. Widespread oxidation reached 1,000 feet below the surface; thence to 2,600 feet the ore was essentially sulphidic; the deep level ore is again largely oxidic. The localization of the ore-bearing pipe is conceivably related to the crossing of east-trending folds by deep-seated fissures conforming to a regional north-east lineament.
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The geology of some ore deposits in southern Africa, 2, 383-392

The rocks of the Transvaal System in this area generally strike roughly, east-west, with steep dips to the south, except that the rocks of the Rosseauspoort range, which are separated from the rocks; south of it by the Gatkop thrust fault, strike roughly north-south, with low dips to the east. The area was affected by intense faulting, the major components of the fault system being two low-angle strike faults formed by overthrusting from the south. The thrust faulting duplicated, and in places triplicated, the banded ironstones of the Dolomite Series, and erosion carved out parallel east-west trending ranges of banded ironstone. The hematite deposits ate situated in the lower portions of the banded ironstone formation, near to or at the contact with the underlying dolomite. The iron ore occurs as big tabular bodies with a westerly pitch. The physical properties and grade of the ore deteriorate with increasing depth, arid at depth hematite-calcite and hematite-talc rocks appear or, the ore horizon. It is anticipated that hematite-dolomite rocks will also appear at greater depths. The genesis of the ore is visualised as a sequence of hypogene and supergene processes, the former most probably associated with the thrust faulting, while the latter was controlled by the evolution of the present land surfaces.
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The geology of some ore deposits in southern Africa, 2, 393-403

High-grade hematite deposits are intermittently present on and west of the Gamagara Hills over a total strike length of about 40 miles. The Sishen mining area is situated at the northern extremity of this iron ore field. The Gamagara Series rests unconformably on the Campbell Rand Series and the Lower and Middle Griquatown stages of the Pretoria Series, all the formations having been affected by thrust faulting from the west. Small and relatively unimportant deposits of high-grade hematite ore are situated in, and were formed by the transformation of, the banded ironstone of the Lower Griquatown stage. Enormous reserves of high-grade hematite ore were formed by the transformation of the Gamagara basal shale and the Gama-gara conglomerate.
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The geology of some ore deposits in southern Africa, 2, 405-413

A deposit of haematite is situated at Lat. 26°15' Long. 31° E in Swaziland. It occurs as enrichments of folded banded iron stones in the Fig Tree Series of the Precambrian Swaziland System. The orebody outcrops over a strike length of 6,500 feet. The maximum true thickness of the orebody is 320 feet, and the dip is usually between 40° and 60°. Four main ore types are recognized (1) massive hard blue; (2) laminated blue; (3) hard greasy and (4) crumbly greasy. 47.0 million tons of ore are indicated. This reserve lies in a northern block with 12.1 million tons at 61.1%. Fe and a central block of 34.9 million tons at 62.6% Fe. There are additional reserves at depth in the southern block, in float ore and in a biscuity hematite that requires beneficiation by screening. Prospecting this deposit between April 1958 and June 1960 cost 0.81 pence per ton of indicated ore.
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The geology of some ore deposits in southern Africa, 2, 41-58

The Dawn Mine is engaged principally oil working the old Barberton gold deposit which, for many years, lags, virtually unexplored within a few feet of the Queens Mine, an important producer for nearly 50 years . Although worked sporadically on a small scale at various times in the past, the Barberton was always considered to be too unimportant to merit serious attention, and it is only very recently that its true value has been recognized. Development has shown that the Barberton Reef improves considerably both in appearance and gold content at moderate depths. Possible physical and chemical reasons for this are discussed here for the first time. An attempt is made to fit the Queens Barberton shear system into a sub-regional fracture-pattern to be used as a guide for future prospecting in the area.
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The geology of some ore deposits in southern Africa, 2, 415-440

These ore deposits are of major economic importance as a source of iron as well as of manganese ores well suited to metallurgical uses. The ore-bearing belt lies almost in the centre of the sub-continent in semi-desert country. This belt is aligned along a zone of severe tectonic deformation trending northwards from a point 130 miles west to 200 miles north-west of the famous diamond mining centre of Kimberley. Although ore is known to occur along some 150 miles of strike, the main concentrations of ore are found in two well defined areas, namely, a 40 mile stretch immediately north of the town of Postmasburg, and a 30 mile stretch located from 40 miles west to 60 miles north-west of the town of Kuruman. Neither the manganese nor the associated iron ores are sedimentary deposits in the strict sense. They clearly represent metasomatic enrichments and replacements in a variety of rocks of widely differing geological ages. In the Postmasburg area the manganese ores are replacements either of Gamagara Series shales or siliceous tectonic breccias where these rocks lie on or are sunken into the dolomite of the Transvaal System. In the Kuruman area the manganese orebodies represent enrichments of presumed originally highly manganiferous oolitic limestones interbedded in the banded ironstones of the Upper Griquatown succession. Over 12,000,000 tons of manganese ore have been produced during the last 30 years in the Postmasburg area. It is estimated that there are still more than 20,000,000 tons of ore in reserve containing above 30 % Mn. In the newly discovered Kuruman area manganese ore reserves of ~40 % Mn content are considered to exceed 50,000,000 tons. Lower grade ores down to 30 % Mn content largely with an excess of lime may attain to an ore reserve of the order of 1,000 million tons.
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The geology of some ore deposits in southern Africa, 2, 441-461

The lithium, caesium and beryllium bearing pegmatite described is the largest of a group occurring in the mineralized area of the Bikita Tinfield, Southern Rhodesia. It forms a dyke striking north-north-east and dipping eastward at angles from 14 to 45 degrees. The dyke has an explored length of over 5,000 feet, with maximum true width of 210 feet. Age determinations indicate that it is approximately 2,650 million years old. Wall rock alteration is insignificant. The pegmatite is remarkably zoned, thus permitting selective mining. The pegmatite has been developed at close intervals to a depth of 200 feet, and drilled down to 500 feet, thereby establishing reliable figures for reserves which are being actively exploited. The minerals of economic value which are recovered are, in order of relative abundance; petalite, lepidolite, spodumene, pollucite, beryl, eucrytite and amblygonite. Cassiterite, tantalite and microlite were recovered in the past, but no economic deposits of these minerals remain. All these minerals occur in well-defined zones, and the deposits of petalite, lepidolite, pollucite and beryl are amongst the largest in the world. The zoning is asymmetrical with respect to the walls, and conforms to the American classification of border, wall, intermediate and core zones, of which only the latter two types are productive. Although some of the Bikita zones conform to the sequence of mineral assemblages enumerated for American pegmatites, other zones present at Bikita are not listed in the American classification. This feature, coupled with possible nonrecognition of valuable zones at surface due to weathering, suggests that full development is required for potentially interesting pegmatites. Many features of this unique deposit suggest that it was probably formed by stages of crystallization in a closed system. An unusual feature is that the core zone is fine-grained, and the only coarse zones lie near the hangingwall contact. No firm evidence exists for replacement.
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The geology of some ore deposits in southern Africa, 2, 463-495

A detailed description is given of three of the major lithium and beryllium-bearing pegmatites occurring in the Karibib district, South-West Africa. A fairly systematic pattern of internal zoning is found which reflects the crystallization history of pegmatite magmas. The overall zonal pattern of the pegmatites from their contacts corewards is as follows:
1. Granitic zone of albite-perthite-quartz-muscovite.
2. Cleavelandite-quartz-muscovite with accessory beryl, frondelite and sometimes columbite-tantalite.
3. Lithium-bearing ore zones.
4. Cleavelandite-beryl-columbite-tantalite-cassiterite.
5. Quartzose core, either pure or mixed with adjacent mineral phases.
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The geology of some ore deposits in southern Africa, 2, 497-508

The broad geological features, including the mineralogy and petrology, of the Empress deposit are given, together with a description of the orebody and its economic aspects. In addition, the various methods of exploration employed in proving the deposit are summarized, these include geochemical and geophysical techniques and the sinking of 105,000 feet of diamond drill boreholes.
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The geology of some ore deposits in southern Africa, 2, 509-520

A nickel deposit that occurs in ultrabasic rocks near the village of Bindura has recently been prospected. Finely disseminated nickeliferous and other sulphides occur in dunite, serpentinite and serpentine-talc-carbonate rocks which, in the richer portions, contain as much as 4% nickel. Sulphide veining on a small scale is present, but it is quantitatively of minor importance only. Secondary nickel minerals are present in the zone of weathering, but have no economic significance. The ultrabasics, are considered to be of early Precambrian age and occur in sill-like form in older sediments and lavas. Younger basic intrisoves are present. Recovery of sulphides from the serpentinites by flotation is poor due to the fineness of the sulphide particles that necessitates very fine grinding to effect a satisfactory liberation and to the high talc content of the host rock. The cut-off grade of the ore is thus affected adversely, as is also the reserve. The lack of a customs smelter and refinery in Southern Rhodesia or elsewhere in Southern Africa for that matter, weighs heavily against exploiting the deposit commercially. The genesis of the sulphides is problematical as some factors indicate that they were syngenetic segregations from the ultra-basics, while others favour an epigenetic origin possibly involving leaching of nickel from silicates followed by precipitation of sulphides. Juvenile fluids unrelated to the host rocks are not considered as a possible source, notwithstanding the presence of younger granite and basic intrusives in the near vicinity. Nickeliferous sulphide deposits in ultrabasic rocks are not a rarity, but this is the first known occurrence in Southern Rhodesia of sufficient magnitude and grade to suggest commercial exploitation. It thus opens up new vistas for prospectors who in past years have critically examined the numerous bodies of serpentinite occurring in the so-called Gold Belts of Southern Rhodesia for asbestos, chromite and platinum, but not for nickel.
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The geology of some ore deposits in southern Africa, 2, 521-533

The Potgietersrus tin-fields extend northwards from about 15 miles north-west of the town of Potgietersrus and embrace a narrow strip of country, some 14 miles in length. The cassiterite occurs in pipe-like bodies, lenticular bodies, disseminated impregnations and fissures in granitic rocks of the Bushveld Igneous Complex. The granites display a great variety of types forming a regular succession from floor to roof, the types being of different ages. The tin mineralization occurs mostly in the Bobbejaankop granite and its roof phase, but the deepest portions of the Main granite are also enriched in places. Mining at Zaaiplaats Mine is at present confined to three main ore shoots. Winzes extend from top to bottom of each ore shoot. Stopes off these winzes are from 6 to 50 feet high. The pay limit at present is 0.3% metallic tin. Values seldom exceed 0.7 per cent. The operating company, which employs 30 Europeans and 450 Bantu workers on the average, has a milling capacity of 200 tons per 24 hours. Recovery is 75 per cent. Groenfontein Tin Mine is the only other producer in this area, and employs about 5 Europeans and 100 Bantu workers. At present only pipes and lenticular orebodies are being mined.
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The geology of some ore deposits in southern Africa, 2, 535-541

A description is given of the various types of tin deposits in the Protectorate of Swaziland. All these deposits are related to the intrusion of the G4 granite and, in particular, to its pegmatitic phase. Details of the composition of the G4 granite and mineralogy of the related pegmatites are given. The more important eluvial and alluvial deposits have been worked out.
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The geology of some ore deposits in southern Africa, 2, 543-570

In South Africa crocidolite asbestos is found in seams interbedded with the banded ironstone of the Transvaal System. Unweathered specimens of the asbestos and of the country-rock have only become available during the past ten years. The mineralogical composition of unweathered banded ironstone differs markedly from its weathered counterpart. A brief description is given of the main features of the distribution, geology and petrography of the crocidolite deposits of the Cape Province. Five new chemical analyses are given. The writers have concluded that the crocidolite crystallized in situ during the lithification of material precipitated chemically and biochemically in large shallow basins. The fibrous nature of crocidolite resulted from the orientation of the crystals by an initiating surface of magnetite. There is no evidence to indicate that the fibrous nature of crocidolite is in any way related to dynamic metamorphism. The development of areas of greater concentration of crocidolite is related to gentle pre-Loskop folding, which took place prior to the crystallization of the asbestos. The geological structure has been complicated subsequently by more intense folding during post-Waterberg time, but there is no association between this period of folding and the formation of crocidolite. There is no longer any doubt as to the persistence of crocidolite in depth, both as regards quantity and quality, and the asbestos industry is assured of an adequate supply of raw material for the foreseeable future.
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The geology of some ore deposits in southern Africa, 2, 571-578

This paper is intended as a supplement to the paper on "Crocidolite Asbestos in the Cape Province" by Cilliers and Genis. It suggests a possible mechanism whereby riebeckite crystallites could have been orientated to form crocidolite. The growth of crocidolite fibres is likened to dendritic growth during which a constant number of growth points maintained a constant areal distribution, resulting in perfectly parallel fibres. The magnetite screens, which invariably cap the fibre seams, exerted a two-fold control over fibre growth. They provided the constant number of growth points per unit and also exercised a thermal control over the final fibre lengths. The close association of crocidolite asbestos and amosite asbestos in the Transvaal is briefly discussed and it is suggested that these two types and amphibole asbestos has a common prent material. Amosite is thought to have formed in the same way as crocidolite, but at a slightly higher temperature.
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The geology of some ore deposits in southern Africa, 2, 579-591

After describing the properties and uses of amosite, the paper outlines the relationship of the amosite zones to the general geology of the area and discusses the origin of the asbestos together with its approximate data of formation. It is concluded that there is no reason why economical deposits of the fibres should not persist to great depths below surface.
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The geology of some ore deposits in southern Africa, 2, 59-75

The general geology of the Swaziland System in which the deposits occur is described. The deposits are classified as (i) quartz vein deposits, (ii) "Acid" reef deposits, (iii) impregnation deposits and (iv) the zone deposits. A description is given of the geology of the G.4 individual mines. It is concluded that the intrusion of the G granite (dated at 3070±60 million years) and its accompanying structural deformation was responsible for gold and other mineralization. Production figures of the various mines are given.
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The geology of some ore deposits in southern Africa, 2, 593-624

Ultrabasic rocks were intruded into basement gneiss at the margin of the developing Bulawayan eugeosyncline. The ultrabasic rocks segregated into dunes peridotites, harzburgites, pyroxenites and gabbros. The pyroxenites and gabbros were altered to actinolite and actinolite-feldspar rocks by automagmatic water. A diabase dyke swarm was intruded in a N40°-70°E direction. The dyke intrusion was followed by a period of folding, faulting, acid igneous intrusions and hydrothermal activity. The chrysotile asbestos bodies are localized in the central portion of the partly serpentinized dunite where hydrothermal solutions were active and thrust-faulting was a marked feature. The fibre bodies are separated along strike and down-dip by dykes and talc zones. Hydrothermal solutions altered dunite adjacent to fractures to serpentine with excess magnesium and silica going into solution. Fibre grew from these solutions in original fractures, the walls of which moved apart under the force exerted by the growing fibres in areas where local stress had lessened the load on the fractures. Fibre growth only teached economic proportions in structurally suitable areas, generally confined to the centre of the ultrabasic mass. The hydrothermal solutions changed character in the later stages with an increase in the carbon dioxide content altering the silky, fibre to brittle fibre in a carbonated serpentine and finally to fibre pseudomorphs in talc-carbonate rocks. All the rocks and events in the area discussed are considered to be of Precambrian age.
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The geology of some ore deposits in southern Africa, 2, 625-669

A general survey of the geology of the Eastern Transvaal is given. The geology and nature of the chrysotile deposits both in serpentinized dolomite and massive serpentinite is described. Before dealing with the origin of serpentinite and chrysotile a brief survey of the mineralogy of the serpentine minerals is given. Different theories on the origin of ultrabasic rocks and serpentinite as well as chrysotile are discussed. It is concluded that chrysotile asbestos deposits have developed in serpentinized dolomite and serpentinite along tensional fractures produced mainly by faulting. The cause of brittleness in chrysotile asbestos is also briefly discussed.
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The geology of some ore deposits in southern Africa, 2, 671-728

Diamonds occur along the West Coast of Southern Africa from the Olifants River in the south to Angola and beyond in the north. The usual mode of occurrence is in raised beaches or beach places of Tertiary or Quaternary age. An account is given of the better-known occurrences, particularly those of The Consolidated Diamond Mines of South-West Africa. Limited (C.D.M.) and its associated companies together with an outline of the history, of the deposits. The modes of origin and the sources of the diamonds are discussed and summarized.
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The geology of some ore deposits in southern Africa, 2, 77-90

The current views on the stratigraphy and tectonic history of the Swaziland System are recorded. The regional geology of an area west of Barberton is discussed as a representative example of the whole. The close relationship between the auriferous ores and the regional and local tectonic features is demonstrated; the influence of the host rock on the mineralization is mentioned, and short descriptions are given of some of the typical or more important orebodies.
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The geology of some ore deposits in southern Africa, 2, 9-13

Many of Southern Rhodesia's gold ores appear to be directly related to structural influences, probably produced by regional deformation. Briefly, the deposits were formed under idealized phsysico-chemical conditions following faulting, folding or shearing. Types of orebody, namely gold veins in fissures, gold replacements in shears, and gold impregnations in banded ironstone formations, are mentioned and typical examples are briefly quoted. Finally, regional structural aspects are referred to, and the use of structure as an aid to exploration is suggested.
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The geology of some ore deposits in southern Africa, 2, 91-128

Apart from being an unique geological phenomenon the Bushveld Igneous Complex is becoming increasingly important economically, as is borne out by the value of its mineral production which, during 1961, exceeded R21,000,000. The emplacement of this vast igneous body occupying about 260,000 square miles, including the areas covered up with younger formations, occurred in phases comprising extrusive, hypabyssal and plutonic environments. In this way originated a galaxy of igneous rocks, viz. in chronological order: andesitic lavas; felsitic, leptitic and granophyric rocks, also formed at and near the earth's surface and according designated epicrustal in origin; diabase in sheet-like bodies; layered mafic and ultra-mafic rocks constituting the main plutonic phase; pegmatitic rocks comprising granite pegmatite and mafic and ultramafic pegmatoid; and finally Bushveld granite forming a tremendous pluton.
To the economic geologist appeal most the mafic and ultramafic rocks and the Bushveld granite, the former with their deposits of chromite, platinum, vanadiferous magnetite, nickeliferous sulphide ore and magnesite, and the Bushveld granite as the ore-bringer of tin and fluorspar. Although in many respects the structure of the Complex is basin-shaped, the concept of a lopolith does not readily apply and other interpretations are rife, especially with respect to the mafic portion. The structure is evidently composite, having originated from several centres. The mechanics of cone-sheet fracturing seem to have operated. Several theories have been advanced to explain the diversity of the rock-types and no consensus of opinion prevails as to whether magma surges, differing in composition and emplaced successively, were the main cause of this diversity or whether differentiation in place was primarily effective. In any case the general trend of the differentiation in the mafic magma, as displayed in the consolidated products, was from magnesium-rich undersaturated rocks towards iron-rich and alkaline oversaturated types.
Assimilation of country rocks and sodium-metasomatism no doubt contributed considerably to the formation of the silica-rich rocks generally. The aureole of contact metamorphism, although spectacular and providing an outstanding field for the study of thermal-metamorphic changes especially in argillaceous rocks, has not yielded much of economic importance, chrysotile and andalusite being the only commodities. Certain anomalies prevail with regard to several aspects of the economic mineralization. The phenomenal reserves of chromite are associated with pyroxenite and not with dunitic rocks, as is common in many other localities in the world. The reason could be an original high iron content in the magma. However, recent discoveries of chromitite prove associations of this ore, at least to some extent, also with dunitic rocks in the Complex. The scarcity of nickeliferous sulphide ore is noteworthy and could be due to a general poverty in sulphur of the magma. Although the Merensky Reef pegmatoid evidently represents a product of magmatic differentiation in place, its coarse texture and the marginal distribution of its oxidic minerals (chromite, the marginal distribution of its oxidic minerals (chromite, magnetite and ilmenite) reflect a pegmatitic environment of recrystallization. It is not considered possible to identify the vicinity of the Merensky Reef horizon in the layered succession with exactitude by use of the optical constants of the orthopyroxene and plagioclase of the associated noritic rocks.
Several other gabbroic complexes in South Africa are possibly of the same age as the Bushveld Complex. The age-relationship of most of these is, however, doubtful. One outstanding example, viz. the Tromsburg Complex in the Southern Free State, more than 300 miles away from the margin of the Bushveld, was discovered by gravimetric means and was proved by diamond-drilling through an average thickness of 3,500 ft. of younger Karoo beds. Except for the presence of magnetic iron ore in three of these outlying bodies and chromite in the Koringkoppies occurrence, west-northwest of Thabazimbi, they are not known to contain any mineral deposit of possible economic significance.
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