A Brief Overview of the Economic Geology of South AfricaM. G. C. Wilson, Council for Geoscience
Comparative production and reserve figures for selected minerals and metals of which South Africa is a major producer:
The geological history of South Africa is a long and complex one going back some 3 700 million years (Ma). The foundation upon which the geological formations of South Africa have subsequently developed is the Kaapvaal Craton, which occupies the north-eastern part of the country. This ancient crustal block is made up largely of Archaean tonalitic and trondhjemitic gneisses and granitoids, along with lesser volumes of metamorphosed volcano-sedimentary rocks known as greenstone belts. The greenstone occurrences are economically important, hosting many gold, antimony, copper, zinc, iron, asbestos, talc, mercury, magnesite, barite and gemstone deposits.
The Barberton Supergroup is the most significant gold-producing greenstone belt in South Africa, whilst the Murchison Belt, made up of rocks of the Gravelotte Group, continues to be an important source of antimony, gold, zinc and copper. The Archaean granites and gneisses are generally poorly mineralised, but do host some pegmatite minerals (including quartz, feldspar, mica, beryllium, spodumene, lepidolite, tantalum, scheelite and tin), corundum, graphite, copper and gold in places. Both the gneisses and greenstones have been folded, faulted and intruded by younger igneous bodies. In terms of modern thinking the Kaapvaal Craton probably formed by the accretion of many smaller terranes or crustal blocks, and was welded together and strengthened by underplating and igneous intrusions.
A period of extensional tectonics followed, resulting in the formation of large sedimentary basins, such as the Witwatersrand and Pongola basins, and the outpouring of the Dominion and Nsuze Group lavas respectively. The lower sediments of the Witwatersrand Supergroup were laid down before the onset of compressive tectonic activity, and prior to the deposition of the gold- and uranium-rich conglomerates and sandstones of the Central Rand Group. The Central Rand Group strata, which are the world's largest repository of gold and contain an estimated 82 000 t of the metal at economically viable grades, were deposited between 3 074 and 2 714 Ma. Similar sediments of the Pongola Supergroup and the Pietersburg Group were deposited during the same time period and are also known to host gold, though at lower concentrations. Shrinkage of the Witwatersrand basin during a period of compressive tectonism resulted in reworking of the Central Rand Group sediments and concentration of the gold and uranium to economic grades. Further hydrothermal remobilisation and concentration of the mineralisation occurred, probably as a result of thermal metamorphism caused by the intrusion of the Bushveld Complex, to the north. Volcanics and sedimentary rocks of the Ventersdorp Supergroup which overlie the Witwatersrand Supergroup covered a large part of the central Kaapvaal Craton, and economic gold concentrations are known along parts of its basal contact with the Witwatersrand strata.
Collision between the Zimbabwe and Kaapvaal Cratons resulted in crustal thickening, which gave rise to the high-grade metamorphism associated with the Limpopo Belt. The oblique nature of this collision is thought to have initiated or re-activated major transcurrent fault systems, resulting in important structures, such as the Thabazimbi-Murchison lineament, which prepared the Craton for later Proterozoic basin development. The first of these basins developed between 2 600 and 2 100 Ma, beginning in the northeast of the country, along the Thabazimbi-Murchison lineament, and spreading westwards and southwards to become what are preserved today as the Transvaal and Griqualand West basins. At the fullest extent of its development this composite basin is thought to have covered an area in excess of 600 000 km2.
Initial volcanic extrusions in the northeast of the basin were covered by the largely clastic sediments of the Black Reef-Vryburg Formations, which host a few small gold deposits. In the quiescent period that followed vast thicknesses of carbonate sediments accumulated, and these are exploited in several places for limestone (both for cement and agricultural use) and metallurgical dolomite. More recent travertine and surficial calcrete deposits, developed over dolomites, are also exploited at several places including Ulco near Barkly West, Slurry near Mafikeng and Lichtenburg. Zinc and lead mineralisation is hosted near the base of the carbonate-dominated sequence and is mined near Reivilo, in the North West Province. In the Pilgrim's Rest area of Mpumalanga, flat-bedded epigenetic gold reefs are hosted within and close to the upper contact of the Malmani Subgroup dolomites. Extensive karsting of the Campbell Group dolomites in the Griqualand West area resulted in the formation and accumulation of substantial manganese deposits which, along with the vast deposits in the Kalahari Manganese Field to the north, around Hotazel, constitute what may be the largest land-based repositories of manganese on earth, with more than 80 % of the known reserve base. Fluorspar deposits are also developed within the dolomite formations; overlying these are substantial deposits of banded iron formation which are exploited for iron ore in several places, the most important being Sishen and Thabazimbi. The iron formation also hosts amphibole asbestos, which was extensively mined in the past. A silicified form of the asbestos, known as tiger's eye, is prized as a gemstone and is unique to South Africa. The Transvaal basin hosts 37 % of the world's andalusite in the pelitic strata of the Pretoria Group, where these rocks fall within the metamorphic aureole of the Bushveld Complex that intruded the Transvaal Supergroup sediments about 2 050 Ma.
The Bushveld Complex is the world's largest known layered intrusion and has an estimated areal extent of 66 000 km2. It contains an ultrabasic to basic unit, up to 9 km thick, known as the Rustenburg Layered Suite, which crops out as eastern, western and northern lobes, associated with acid (largely granitic) rocks termed the Lebowa Granite and Rashoop Granophyre Suites. Though several intrusive centres have been recognised for Bushveld Complex rocks, there are many broad similarities in the basic-ultrabasic succession which has traditionally been divided into a Lower, Critical, Main and Upper Zone. The economically viable chromite reserves of the Bushveld Complex, most of which are hosted in the Critical Zone, are estimated at 68 % of the world's total, whilst the Complex also contains 56 % of all known platinum-group metals. The Merensky Reef, which has developed near the top of the Critical Zone, can be traced along strike for 240 km and is estimated to contain 60 0001 of platinum-group metals to a depth of 1 200 m below surface. The pyroxenitic Plat Reef horizon, north of Potgietersrus, contains a wide zone of platinum mineralisation, along with higher grades of nickel and copper than occur in the rest of the Complex. Vanadium-bearing minerals occur in concentrations of between 0,3 and 2 % in up to 21 layers of titaniferous magnetite in the Upper Zone of the Complex, which contains almost 45 % of the world's vanadium.
Significant tin, fluorite and copper mineralisation is hosted within the acid phase of the Bushveld Complex. This mineralisation is a product of granite fractionation, with most of the tin having been mined from the now defunct Rooiberg and Potgietersrus tin fields. More than half of South Africa's proved resources of fluorspar are located in the Bushveld Complex and environs within 13 deposits, the most important of which is the Vergenoeg Mine. Nickel-bearing pipes are present near Rustenburg, and magnesite, a weathering product of the ultramafic rocks, was exploited near Burgersfort. Substantial quantities of black norite and red syenite are quarried as dimension stone for the construction industry, most being exported.
A very much smaller, though economically important, intrusive of similar age to the Bushveld Complex is the alkaline (carbonatite) Complex at Phalaborwa, in the east of the Northern Province. It is unique in that it is the only carbonatite in the world currently being mined for its copper content. By the time that the Palabora open-pit copper mine closes in 2002, an estimated 3,4 Mt of copper metal will have been extracted along with more than 200 Mt of titaniferous magnetite and significant amounts of nickel, lead, selenium, tellurium, baddeleyite, gold, silver, rare-earth elements, uranium and platinum-group metals. The Complex contains an estimated 298 Mt of merchant-grade phosphate (in the form of apatite) per 100 m of depth and, in addition, is the world's largest resource of high-grade vermiculite. Both the apatite and vermiculite are being mined.
Basin development continued in the northern Kaapvaal Craton with the formation of the Waterberg basin and the Soutpansberg-Blouberg troughs, and their subsequent infilling with lavas and predominantly clastic, red-bed sediments. Though only minor copper, lead and barite mineralisation has been proved in these Proterozoic basins to date, they remain ideal targets for major volcanic- and sediment-hosted base,-metal and uranium mineralisation. After these basins had been infilled, at about 1 800 Ma, the Craton stabilised. The only other economically significant geological events that affected the Kaapvaal Craton during Proterozoic times were the intrusion of the Pilanesberg Alkaline Complex and the Premier diamond pipe at 1 300 Ma. The Premier pipe yielded the world's largest gem diamond, and large gems continue to be recovered from time to time. The focus of tectonic activity and mineralisation within South Africa then moved to the Namaqua-Natal Metamorphic Belt, to the south and west of the Kaapvaal Craton.
The Namaqua-Natal Metamorphic Province forms an arcuate belt, up to 400 km wide, which is draped onto and around the western and southern margins of the Kaapvaal Craton. The rocks making up this province are believed to have formed adjacent to a foreland made up of the Kaapvaal Craton, Kheis Province and Kaaien Terrane. An orogenic event resulted in the welding together of accreted terranes to form the Bushmanland Craton, by 1 750 Ma. Rifting along northwest-trending axes, between 1 600 and 1 400 Ma, led to the formation of intracratonic basins in the thicker western portion of Namaqualand in which supracrustal sequences, such as the Bushmanland Group, were deposited. This group hosts a cluster of four zinc-lead-copper-silver deposits, some of which are capped with barite. Two of these deposits, which occur in the Aggeneys area, are currently being mined, one for base metals and the other for barite. Other smaller sedimentary exhalitive deposits are also known in the area, but the most significant mineralisation is copper, which is hosted in intrusive noritoids of the Koperberg Suite around Springbok. An estimated 2 Mt of copper is contained in these basic bodies, much of which has already been mined.
In the thinner eastern portion of Namaqualand the rifting resulted in the formation of an oceanic basin, the remains of which are represented by the Areachap Terrane which hosts several zinc-copper deposits, two of which were mined, viz. Prieska and Areachap. Further east in the Natal Metamorphic Province a small fragment of Kibaran-aged oceanic crust has also been preserved as the Tugela terrane ophiolite. Though there are copper-nickel-chromium showings associated with mafic/ultramafic complexes in this terrane, and though several small gold, zinc, lead and silver deposits are known, no large viable deposits have been found. The Tugela-Areachap ocean is believed to have been subducted between 1 400 and 1 200 Ma, during a period of convergence and plate collision related to the Kibaran orogeny. This collision caused intense deformation, metamorphism, widespread melting and the generation of vast quantities of granitoid magmas. Greisen-associated tin-tungsten, rare earths and pegmatite mineralisation are all developed within these Kibaran-aged granitoids.
During the late Precambrian period the Pan-African orogeny was associated with the formation of a 3 000-km-long chain of geosynclines around the southern and western Cape, and up into Namibia. Into these geosynclinal basins the Gariep Supergroup and the Malmesbury, Kango, Kaaimans, Gamtoos, Nama and Vanrhynsdorp Groups were deposited; however, no economically significant mineralisation, except for limestone, has yet been discovered in the thick, largely sedimentary deposits. Emplacement of the Cape Granite Suite took place between 600 and 500 Ma, and these granites are only sparsely mineralised (the Riviera tungsten deposit being one of the more important deposits) though, where weathered, they do yield valuable kaolin clay in places. On the east coast the Pan-African event was marked by tectonothermal reworking of basement rocks. The post-tectonic rise of these Pan-African orogens saw the accumulation of shallow-marine, deltaic and fluviatile deposits within the east-west-trending Cape Supergroup trough, as well as in KwaZulu-Natal (Natal Group). This trough is believed to have formed in response to an aborted rift, which is thought to be the precursor to the breakup of Gondwanaland. The sediments within the Cape and Natal basins appear to be devoid of significant mineralisation.
Permo-Carboniferous glaciation followed when South Africa was migrating over the South Pole, leading to the deposition of glacial sediments of the Dwyka Group. Following this, parts of the vast Karoo basin, which covers about two-thirds of South Africa, were filled with basinal muds, deltaic sands and less extensive, coal-forming peats. The latter occur in the north-eastern part of the basin, and resulted in the formation of the extensive Ecca Group coal measures, which provide the main source of energy for South Africa and support a massive export industry. The essentially lacustrine and fluvial flood-plain and channel accumulations that followed, have been termed the Beaufort Group and these contain small sandstone-hosted uranium and molybdenum deposits. Some heavy-mineral placers are also developed in the Karoo sediments. Extensive basic and acid lavas of the Lebombo and Drakensberg Groups cap the Karoo Supergroup and their extrusion preceded the fragmentation of Gondwanaland. South Africa began breaking away from Australia in the northeast around 200 Ma, and this breakup proceeded southwards and then westwards until the proto-Atlantic was formed about 120 Ma. This was accompanied and followed by widespread anorogenic alkaline magmatism of the kimberlitic, carbonatitic and ring-complex types. The kimberlites in particular are economically significant as some of them host important diamond deposits.
Since the breakup of Gondwanaland, Cretaceous and Cenozoic basins and structural traps have developed around the coastal areas in which deposits of oil and natural gas have accumulated, and some success has been achieved in the delineation and exploitation of these resources. A thick blanket of terrestrial and freshwater sediments of Cenozoic age, loosely known as the Kalahari Group sands, was deposited in the Northern Cape. Substantial quantities of minerals and metals occur in coastal dune sands, though their concentrations are only economically viable in a few places. Major mining operations at Richards Bay, on the KwaZulu-Natal coast, and at the Namakwa Sands project, on the Western Cape coast, extract large tonnages of titanium minerals, zircon and monazite. Diamonds have been concentrated in alluvial deposits in the Northern Cape and North West Provinces, and have been exploited for over a century. South Africa's long and varied geological history is certainly reflected in its extraordinary array of mineral deposits and occurrences.
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