The Origin of the Gold and Uranium Ores of the Black Reef Formation, Transvaal Supergroup, South Africa

The Black Reef Formation is a laterally extensive sedimentary succession at the base of the Transvaal Supergroup. The quartz-pebble conglomerates host erratic concentrations of gold and uranium; however, in areas that are located spatially above the gold and uranium-bearing reefs of the Witwatersrand Supergroup, the concentrations can reach ore-grade. Although the depositional environment was similar to that of the Witwatersrand Supergroup, the processes that lead to the high gold and uranium concentrations in the Black Reef were quite different. The Black Reef experienced intense, post-depositional hydrothermal alteration by circulating aqueous and hydrocarbon fluids (oils) that deposited large volumes of native gold, uranium minerals and pyrobitumen. The gold, which is filamentous, is concentrated along As-Ni-rich surfaces of pyrite, and with uraninite in pyrobitumen. In both environments, the gold is interpreted to be the product of chemically-triggered precipitation from hydrothermal fluid(s)..