Abstract: Sedimentologic Synthesis of Fossil Gold, Uranium, and Pyrite Concentrations in Proterozoic Witwatersrand Placers, South Africa
W. E. Lawrence Minter
Placer orebodies were developed during deposition of the Proterozoic upper Witwatersrand Group in braided-river and fluvial-fan environments 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 local marginal fault scarps and yoked basins, into which wet alluvial fans, partly composed of Witwatersrand material, were deposited.
Economically important placers are present as regressive deposits on disconformities or unconformities at the base of sedimentary units, transgressive deposits on angular unconformities, 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, which were derived from the granitic-greenstone basement, probably survived exposure to subaerial and shallow-water environments because of a low oxygen pressure or reducing atmosphere. These economic placer minerals were concentrated hydraulically and preserved in pebble-supported conglomerates and typically trough-cross-bedded pebbly quartz arenites in various environments.
Mapped details of the internal and external geometry of the placers, various scalar parameters, and a paleocurrent analysis define facies and complicated distribution patterns of placer concentrates for exploration and mining activities.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC