Tectono‐metamorphic setting and paragenetic sequence of Au‐U mineralisation in the Archaean Witwatersrand Basin, South Africa

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Abstract
The Witwatersrand Basin is a structural remnant of a once much larger basin that was deposited between 3074 Ma and 2714 Ma in the south‐central portion of the Kaapvaal Craton. The post‐depositional history of the basin is complex and long‐lived. Compressional tectonics during subsidence and basin filling was followed by craton‐wide extension and rift‐basin formation which resulted in the covering of the Witwatersrand succession by the Ventersdorp Supergroup. A prolonged period of subsidence ensued between ca 2.5 and 2.2 Ga which resulted in the deposition of the Transvaal Sequence that also covered the Witwatersrand strata. The craton was subjected to a major thermal perturbation at 2050 Ma, which accompanied the emplacement of the Bushveld and related igneous complexes. The prograde P‐T path applicable to the basin at this stage was catastrophically disrupted by an impacting bolide which resulted in geologically instantaneous exhumation of portions of the basin at 2025 Ma. A record of the above events is preserved in the metamorphic evolution of the Witwatersrand Basin as well as in the paragenetic sequence of mineralisation recognised within the conglomeratic orebodies which host the economic concentrations of Au and U. Many conglomerate ‘reefs’ nevertheless preserve detrital accumulations of heavy minerals, such as pyrite, zircon, chromite, uraninite and gold, which were concentrated by placer processes in dominantly fluviodeltaic depositional systems. At least three stages of post‐depositional mineralisation are recognised. These are invariably associated with the primary, allogenic concentrations of heavy minerals. Pyrite and authigenic rutile were formed at ca 2.5 Ga in a fluid‐circulation event that appears to be related to onset of Transvaal Sequence deposition. Circulation of hydrocarbon‐bearing fluids and precipitation of bitumens by radiolytic polymerisation followed at ca 2.3 Ga. This event, possibly associated with deposition of the upper portion of the Transvaal Sequence, appears to have also been associated with mobilisation of metals. Widespread fluid circulation and resetting of thermochronometers subsequently took place at 2050–2025 Ma during the Bushveld and Vredefort events, which resulted in precipitation of a number of sulfide phases and gold. Inference and logic support the contention that the bulk of fluid and metal involved in the formation of the post‐depositional mineral paragenesis was derived from within the basin, although the possibility does still exist, particularly with respect to the later events, that these may have been derived from outside the basin.

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