The Witwatersrand Basin and Its Gold Deposits

Abstract

The Mesoarchaean Witwatersrand Basin represents the largest known gold anomaly and has produced more gold than any other ore province in the world. Deposition of the predominantly siliciclastic basin fill began shortly after 2985 Ma in a shallow marine environment, maybe along a passive margin to an old continent to the north(west), and under generally cool climatic conditions (West Rand Group). Tectonic inversion following the extrusion of 2914 Ma andesitic lava led to a shift towards continental sedimentation. Following a basin-wide hiatus at around 2900 Ma, the largely arenitic Central Rand Group was deposited into a retroarc foreland basin that regressed as it was filled between 2780 Ma in response to crustal accretion along the western and northern margins of the basin. This stage was accompanied by the largest known concentration of gold in Earth’s history, initially by microbial fixation on emerging colonies of probably cyanobacteria in near-coastal environments, subsequently by mechanical reworking of the gold-rich microbial mats to form conglomerate-hosted placer deposits. The source of the huge amount of gold in the Witwatersrand is thought to be the entire greenstone-dominated Archaean cratonic surface, which was subjected to intensive chemical weathering permitting large-scale leaching of gold by contemporaneous surface waters. Syn-depositional tectonism that extended from the hinterland into the foreland basin led to repeated further physical reworking of gold-rich sediments to form more placer deposits higher up in the stratigraphy, even above the Witwatersrand Supergroup. The Witwatersrand Basin fill was subjected to a series of alteration events, ranging from burial and regional low-grade metamorphism to heating in the course of the emplacement of the 2054 Ma Bushveld Igneous Complex and catastrophic shattering during the 2023 Ma Vredefort impact, which enabled renewed fluid flow long after primary rock porosity had been obliterated. This caused some short-range mobilisation of ore components, including gold, but without changing the overall sedimentological and stratigraphic control on ore grade.