The world’s largest gold province: Implications on Archaean atmospheric evolution

Abstract

The majority of gold within the Witwatersrand Basin, the world's largest gold-producing province, in the Archaean Kaapvaal Craton, South Africa, occurs together with rounded pyrite and uraninite, and locally bitumen, on degradational surfaces of fluvial conglomerates that were laid down between 2.90 and 2.84 Ga. Although most of the gold takes a textural position late in a complex paragenetic sequence, available morphological, mineralogical, geochemical and isotopic data all indicate that this hydrothermal gold, analogous to some pyrite and uraninite, was derived from the local mobilisation of detrital particles. Rounded, cletrital pyrite is common in the coarser grained fractions of the siliciclastic basin fill. Together with rounded uraninite, which is particularly abundant in the older beds, it provides important constraints on the redox potential of the Meso- to Neoarchaean atmosphere and hydrosphere. In combination with eukaryotic steroids previously documented from the Pilbara Craton, Australia, the ambient Neoarchaean oxygen fugacity is calculated as having been approximately 10-3, in equilibrium with a relatively acidic hydrosphere (pH = 6). An acidic meteoric palaeoenvironment is supported by intense chemical weathering below erosional unconformity surfaces in the Witwatersrand basin fill. In contrast,near-shore shallow marine deposits contain magnetite. This supports a postulated reducing hydrosphere/atmosphere and highlights that total sulphur concentrations in the ancient ocean were orders of magnitudes lower than in modern sea water.