The origin of the gold and uranium ores of the Black Reef Formation, Transvaal Supergroup, South Africa

Abstract

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). The uraninite occurs either as precipitates in pyrobitumen or is “invisible” in the pyrite, occurring as nanoparticles in this mineral. This indicates that conditions were sufficiently oxidizing for the hydrothermal transport of uranium, and contrasts with the case for the Witwatersrand reefs where much of the uranium is detrital. Luminescent radiation damage associated with vein quartz and pyrobitumen, as well as the diffuse distribution pattern of uranium in the pyrobitumen may indicate that some uranium was transported by liquid hydrocarbons. Pyrite in the Black Reef (sedimentary, detrital and hydrothermal pyrite) is highly enriched in gold, uranium and other redox-sensitive trace elements. Provenance analyses using the trace element compositions of the pyrite show that it was not the result of erosional reworking of the underlying Witwatersrand Reefs. However, the close spatial association of high gold and uranium concentrations in the Black Reef with ore-bearing Witwatersrand reefs suggests strongly that gold and uranium, whether in pyrite or occurring as native gold and uraninite, were hydrothermally recycled (uranium may have been recycled by hydrocarbon fluids) from the Witwatersrand Reefs.