Re–Os isotope and PGE constraints on the timing and origin of gold mineralisation in the Witwatersrand Basin

Analyses of the Re–Os isotope system and PGE abundances have been used to investigate the timing and origin of mineralisation in the Vaal Reefs section of the Witwatersrand Basin, the world's largest gold deposit. Consistent interelement PGE fractionation in both organic and sulphide phases intimately associated with gold mineralisation suggests that the Re–Os isotope system was fractionated during gold mobilisation. The Re–Os isotope data on organic separates intimately associated with the gold in this part of the basin indicate open system behaviour until ∼ 2.26 ± 0.19 Ga (2σ), of the order of 450 Myr younger than the minimum age of sedimentation. The overlying Ventersdorp Volcanics preserve a Re–Os isochron age of 2.43 ± 0.21 Ga, ∼ 250 Ma younger than their U–Pb zircon emplacement age. These data indicate substantial post-sedimentary noble metal mobility within portions of the Witwatersrand Basin. Re–Os model age calculations on Os-rich fractions suggest that noble metals were derived from a 3.0–3.1 Ga terrain of granite–greenstone affinities, significantly younger than the Barberton Greenstone Belt. These data support a hybrid model involving post-depositional mobilisation of ancient, probably detrital, noble metals for the origin of mineralisation in this part of the Witwatersrand Basin. Such a model is applicable to other areas of gold mineralisation in the basin that display similar textural characteristics and geochemistry.