Abstract

Placer ore bodies were developed during deposition of the Proterozoic Upper Witwatersrand Group in braided river environments on fan deltas around the margin of the Witwatersrand Basin, which was created by gentle synclinal warping of the Kaap-Vaal Craton. Tectonic instability around the margins of the Witwatersrand Basin, during the final stages of filling and during deposition of the overlying Ventersdorp Group, created marginal fault scarps to the basin in places, and yoked basins, into which humid alluvial fans, partly composed of Witwatersrand material, were deposited.

These various environments of deposition are illustrated with examples from four placers that are being exploited at present. Stratigraphically, the economically important placers occur as: regressive deposits on unconformities at the base of sedimentary units; transgressive deposits on angular unconformities at the base of sedimentary units; or as terminal deposits on disconformities at the top of sedimentary units.

Heavy detrital particles of gold, and hydraulically equivalent-sized particles of uraninite and pyrite were hydraulically concentrated and preserved in pebble-supported conglomerate and typically trough-crossbedded pebbly quartz arenite facies. These economic placer minerals, which were derived from the granitic-greenstone basement, probably survived exposure in subaerial and shallow water environments because of a low oxygen pressure in the atmosphere.

Mapped details of the internal and external geometry of the placers, various scalar parameters, and paleocurrent analysis are used to define the facies and complicated distribution patterns of placer concentrates for exploration and mining activities.