West Rand Group Research Articles

The Witwatersrand Basin, South Africa: Geological framework and mineralization processes

The Witwatersrand Basin formed over a period of 360 Ma between 3074 and 2714 Ma. Pulses of sedimentation within the sequence and its precursors were episodic, occurring between 3086-3074 Ma (Dominion Group), 2970-2914 Ma (West Rand Group) and 2894-2714 Ma (Central Rand Group). Detritus was derived from a mixed granite-greenstone source of two distinct ages; the first comprises Barberton-type greenstone belts and granitoids > 3100 Ma old, and the second consists of the greenstone belt-like Kraaipan Formation and associated granitoids ≤ 3100 Ma old. Subsequent granitoid plutonism was episodic and coincided with hiatuses in sediment deposition, but continued throughout the evolution of the basin. Many of the provenance granitoids are characterized by hydrothermal alteration, are geochemically anomalous with respect to Au and U, and may represent viable source rocks for palaeoplacer mineralization. Tectonically, the basin evolved in response to processes occurring within a Wilson cycle, associated with the encroachment and ultimate collision of the Zimbabwe and Kaapvaal cratons. Metamorphism of the Witwatersrand Basin occurred at ca. 2500, 2300 and 2000 Ma. The first two events coincided with the progressive loading of the basin by Ventersdorp and Transvaal cover sequences, whereas the last reflects intrusion of the Bushveld Complex and/or the Vredefort catastrophism. Mineralization is concentrated in the conglomerates of the Central Rand Group and is represented by a complex paragenetic sequence initiated by early accumulation of detrital heavy minerals. This was followed by three stages of remobilization caused by metamorphic fluid circulation. An early event of authigenic pyrite formation at 2500 Ma was followed at 2300 Ma by maturation of organic material, fluxing of hydrocarbon bearing fluids through the basin and the radiolytic fixation of bitumen around detrital uraninite. This was followed at around 2000 Ma by peak metamorphism which resulted in the widespread redistribution of gold and the formation of a variety of secondary sulphides. Post-depositional fluid conditions were such that metal solubilities were low and precipitation mechanisms very effective, resulting in the superimposition of both primary and secondary mineralization.

The late Archean to Mesoproterozoic major unconformity-bounded units of the Kaapvaal Province of southern Africa

The late Archean to Mesoproterozoic (⩽3.1−1.4 Ga) sedimentary and volcanic successions in the Kaapvaal Province of southern Africa previously have been subdivided into numerous lithostratigraphic units that generally were believed to have been deposited in local or regional basins. However, these lithostratigraphic units are remnants of thirteen unconformity-bounded sequences or units 10 3 m thick, with composite strike lengths of 250–1100 km, and with durations of 10 7 to 10 8 yr. All of these units rest somewhere on the crystalline basement. The stratotypes (but not the full geographic extents) of these sequences, from the base upward, are the Dominion Group, West Rand Group of the Witwatersrand Supergroup, Central Rand Group of the Witwatersrand Supergroup, Klipriviersberg Group of the Ventersdorp Supergroup, Platberg Group of the Ventersdorp Supergroup, Pniel Group of the Ventersdorp Supergroup, Black Reef Quartzite Formation and Chuniespoort Group of the Transvaal Supergroup, Pretoria Group of the Transvaal Supergroup, and the interval represented by the Dullstroom Formation, Rooiberg Group, and the Loskop Formation. Three or more major unconformity-bounded units occur in younger Proterozoic rocks in the Kaapvaal Province but are not definitive of it; two occur in the Soutpansberg Group, and one is most of the Waterberg Group. Some of these unconformity-bounded units are remarkably similar to Phanerozoic cratonic and miogeoclinal sequences. They are the natural building blocks for reconstructing Archean and Proterozoic depositional basins, plate tectonic events, continents and a provincial time scale.

A precise UPb zircon age for the Archaean Pongola Supergroup volcanics in Swaziland

UPb dating of single abraded zircon grains from a Pongola Supergroup rhyodacite sample collected in southern Swaziland yields a crystallization age of 2985±1 Ma. This date is significantly older than our previously reported UPb age determined on highly discordant and unabraded zircons from the same sample. The new zircon age indicates that the formation of the Pongola volcanics occurred not much later than the emplacement of the youngest granitoids thought to be broadly coeval with the Dominion Group (Robb et al., 1991), and before deposition of the Lower West Rand Group (Barton et al., 1989).

Changing tectono-sedimentary scenarios relevant to the development of the Late Archaean Witwatersrand Basin

Recent studies of the Witwatersrand Basin indicate that contrary to earlier proposals the shape and size of the basin and style of sedimentation were very different early and late in basin history. This reflects a geotectonic evolution which may involve an overall Wilson cycle. Progressive stages prior to, during and after basin development are described in order to map this evolution. Stage 1 saw the structural framework of the Kaapvaal Craton which was to accommodate future basin development. Fault zones representing lineaments and sutures associated with the structural evolution of various greenstone belts of the Kaapvaal Craton were rejuvenated (especially during late stages of Witwatersrand Supergroup deposition) to act as primary synsedimentary fault zones. Stage 2 involved the development of the Dominion Basin which has been interpreted as an extensional basin associated with NNE trending faults, although volcanics show calc-alkaline affinities. Stage 3 saw widespread deposition over the Kaapvaal Craton of a variety of subtidal sediments of the West Rand Group and its correlatives including the Pongola Sequence. This epicontinental style of sedimentation was either associated with a thermal cooling phase related to the Dominion Group extension or with the initiation of a foreland basin. The conformable or disconformable relationship between the West Rand Group and the underlying Dominion Group would point to the former. During stage 4 major changes in sedimentary patterns occurred. Upper Central Rand Group sediments were deposited and draped over compressive basement block-faults, which interacted with the sedimentation causing reverse faults/ monoclines and locally provided sources for coarse gravel. The basin was shrinking considerably in size and fluvial sediments derived from developing pediment areas and uplifted basement interacted with a reduced marine water body. The basin was controlled by NE-SW directed compression which caused a relative tectonic escape of the central part of the Kaapvaal Craton to the SE. The reason for these changes was the initial orogenesis associated with the subduction history preceding and the ultimate A-subduction history during the collision of the Zimbabwe Craton with the Kaapvaal Craton. Stage 5 saw the outpouring of the Klipriviersberg Group flood basalts into the Witwatersrand Basin. Early in this stage the tectonism was similar to that for stage 4, but late in the extrusive history a change in feeder dyke patterns probably heralds the major extensional tectonics developed during stage 6, involving the deposition of the patterns probably heralds the major extensional tectonics developed during stage 6, involving the deposition of the Platberg Group. The latter developed in an extensional rift basin caused by the tectonic inversion of faults from their previous reverse state to a normal movement. This has been interpreted as the result of impactogenal rifting. In the aftermath the collapse of the orogen allowed the deposition of the Pniel sequence and Wolkberg Group in extensional rift basins and the associated thermal cooling led to the development of the Chuniespoort Group chemical sedimentation. At no time did the Vredefort Dome affect this sedimentary history, in fact this was a post-Bushveld event which was followed by the emplacement of the Johannesburg Dome. The Vredefort Dome did, however, have a major structural impact on the Witwatersrand Basin and may have played a major role in its long term preservation up to the present day.

α-Spectrometric investigations of radioactive disequilibrium in early Precambrian quartz pebble conglomerates

Sample of fresh and weathered quartz pebble conglomerates of the West Rand Group of the 2.5–2.8 Ga Witwatersrand gold-uranium deposit (South Africa) were investigated. Uranium and thorium element and isotope analyses were carried out by α-spectrometry. Two different kinds of sample treatment were presented: bulk rock analysis and leaching process, and a detailed description of data evaluation from the α-spectrometric measurements were given. Radioactive disequilibrium found in bulk rocks and leached solutions of all weathered samples indicate U redistribution processes, mainly U loss in the weathered zone of conglomerates. It could be shown that the use of α-spectrometry and in addition the use of two kinds of sample treatment is a good method for studying the behaviour of uranium during weathering processes.