DETRITAL ORIGIN OF HYDROTHERMAL WITWATERSRAND GOLD - A REVIEW

The Witwatersrand 'basin' is the largest known gold province in the wo rld. The gold deposits have been worked for moren than 100 years but t here is still controversy about the ore forming process. Detailed petr ographic studies often reveal that the gold is late in the paragenetic sequence, which has led many researchers to propose a hydrothermal or igin for the gold. However, observations, such as the occurrence of ro unded, disc-like gold particles next to irregularly shaped or idiomorp hic secondary gold particles in the same sample, suggest an initial de trital gold source within the Witwatersrand strata. Mineral chemical a nd isotopic data, together with SEM cathodoluminescence imaging and fl uid inclusion studies, provide evidence for small-scale variations in the fluid chemistry - a requirement for the short-range mobilization o f the gold. The existing data and observations on the Witwatersrand ro cks support a model of hydrothermally altered, metamorphosed placer de posits, with at least two subsequent gold mobilization events: hydroth ermal infiltration in early Transvaal time (2.6-2.5 Ga) and during the 2.020 Ga Vredefort impact event.