The Mesoarchean Witwatersrand Basin in the central Kaapvaal craton, South Africa, has been the largest gold-producing province in history. Although mining has reached a very mature state, this ore province remains the biggest regional gold anomaly in the world. Most recent research on the Witwatersrand gold deposits has focused on postdepositional processes, often on a microscale, thereby constraining conditions of gold transport in the host conglomerates. Here we review past and current observations on the geologic setting of the orebodies and first-order controls on gold mineralization, all of which strengthen the argument for a primarily syngenetic model. The Witwatersrand deposits are regarded as remnants of a gold megaevent at 2.9 Ga when environmental conditions are suggested to have been suitable for intense gold flux off the Archean land surface and early photosynthesizing microbes could act as trap sites for riverine and possibly shallow-marine gold. Sedimentary reworking of gold-rich microbial mats led to rich placer deposits which, in turn, became sources of younger placers higher up in the stratigraphy. The same gold concentration mechanism most likely operated on all Mesoarchean land masses, not only on the Kaapvaal craton. The uniqueness of the Witwatersrand gold province is explained by exceptional preservation of these easily erodible, largely continental sediments beneath a thick cover of flood basalt and a later impact melt sheet in the middle of a buoyant craton, with little tectonic overprint over the past two billion years.

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Major, trace, and rare earth element data from shales of the Witwatersrand Supergroup in South Africa were analysed to evaluate their provenance, weathering, and tectonic setting. The studied rocks were affected by minimal hydrothermal and biogenic modification. Geochemical analyses indicate that the Witwatersrand shales are compositionally highly variable, both laterally and within individual stratigraphic shale units, and are characterised by high Chemical Index of Alteration (CIA) values. High CIA, Chemical Index of Weathering (CIW) and Index of Compositional Variability (ICV) values suggest that the source area was subjected to prolonged intense chemical weathering associated with fluctuating palaeo-climate. The Zr/Sc and Th/Sc ratios revealed that Witwatersrand shales were derived from a mixed, predominantly mafic source area. Noteworthy enrichment in incompatible elements in some Witwatersrand formations indicates the presence of granitic sources. Tectonic discrimination diagrams based on immobile trace elements (Th-La-Sc, Sc-Th-Zr/10) and diagnostic trace element ratios (Th/Sc, Zr/Hf, La/Th, La/Sc) indicate that the West Rand Group source area was a passive margin, whereas the Central Rand Group shales were derived from an active continental margin, thus supporting a foreland basin setting. Geochemical signatures illustrated by Hf and Zr content show various cycles of crustal recycling that influenced source area sediment composition, diagenetic and provenance history.

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