Witwatersrand gold particle chemistry matches model of metamorphosed, hydrothermally altered placer deposits

Analyses of gold particles from the Ventersdorp Contact Reef in the Klerksdorp and Carletonville goldfields of the Late Archaean Witwatersrand Basin reveal a wide range in Au, Ag and Hg concentrations of 80.9 to 92.9, 6.0 to 17.6, and 0.6 to 5.8 wt.%, respectively. However, individual gold particles are generally homogeneous. This observation is predicted by modelling of Ag and Hg diffusion through gold at the peak metamorphic temperature of about 300 °C. The only exception is gold in hydrothermal quartz veins, the formation of which can be ascribed to the mobilisation of originally detrital gold particles during a chloritisation event that was triggered by tectonically expelled fluids, as a consequence of the 2023 Ma Vredefort meteorite impact event. Inter-particle homogenisation on a hand-sample scale was generally not achieved. The mean gold composition for the same reef in different mines (kilometre scale) can vary drastically. These results confirm the model of a metamorphosed and hydrothermally altered placer deposit. Furthermore they indicate that, in spite of a significant post-depositional modification of the composition of individual gold particles, the mean gold composition at a given reef locality may be used to distinguish different source areas of the originally detrital particles. Received: 3 December 1996 / Accepted: 19 May 1997