Significance of granite-greenstone terranes in the formation of Witwatersrand-type gold mineralisation – A case study of the Neoarchaean Black Reef Formation, South Africa

Abstract Geological characterisation and U-Pb geochronology of detrital zircon grains has proven valuable in finding potential source terranes of siliciclastic sediments. Sediment transport and reworking from gold-bearing Archaean/Palaeoproterozoic granite-greenstone rocks are significant in the formation of conglomerate-hosted (Witwatersrand-type) gold deposits. We further support this link with an example from the Neoarchaean Black Reef Formation, in the Carletonville Goldfield, South Africa. In this study, the detrital and hydrothermal pyrite grains are petrographically distinguishable. Association of gold with solely detrital heavy minerals such as pyrite and not with hydrothermal minerals is similar to other Witwatersrand-type examples, which were formed during reducing atmospheric conditions from granite-greenstone protosource rocks. Textural evidence, together with gold occurrence data, support gradual homogenisation of primary sedimentological and mineralogical features due to post-depositional alteration. Uranium-Pb dating of the Black Reef Formation detrital zircon grains yields an age between 3149 and 3061 Ma, indicating that that the provenance of Black Reef Formation sediments could have been derived from a mixture of the hinterland proto granite-greenstone rocks, and from the reworking of Witwatersrand auriferous conglomerates. We further infer a favourable palaeo-drainage for primary sedimentary gold deposition from an isopach model and palaeocurrent data. The fluvial origin of the Black Reef Formation basal conglomerate provides an ideal setting for gold mobilisation from the sediment-source rocks. Notwithstanding the contribution of hydrothermal activity, sedimentary processes are interpreted to have been primary in the initial concentration of gold through reworking of the Witwatersrand auriferous conglomerates. This has important implications for the genesis of the younger Witwatersrand-type gold mineralisation. The fact that the Witwatersrand-type gold deposits, a target for exploration, occur spatially and temporally within the Archaean/Palaeoproterozoic geological record demonstrates that the Witwatersrand Basin is not unique in terms of composition and paragenesis, but is unique in terms of physical size and magnitude of its gold and uranium endowment.