A new look at sulphide mineralisation of the northern limb, Bushveld Complex: a stable isotope study

摘要

The Platreef is the main platinum-group element (PGE)-bearing horizon in the northern limb of the Bushveld Complex, South Africa. It is considered to be richer in sulphides than other similar horizons within the Bushveld Complex, in particular the Merensky Reef. Previous work has indicated that assimilation of dolomite may be a mechanism to add sulphur to the magma from which the Platreef formed. Sulphur isotope data presented in this study indicates an additional sulphur source contributing to the Platreef. In the southern Platreef, the Duitschland Formation of the Transvaal Supergroup forms part of the direct footwall. Within this sequence are pyrite-rich shales, which we suggest contributed to the sulphur budget of the Platreef. Local variations in sulphur isotopes, as well as a decrease in crustal sulphur in the Platreef further away from the footwall, also indicate local rather than regional contamination processes. Platreef samples have delta~(18)O values that are higher than expected in a mantle-derived magma, but there is no apparent systematic variation in delta~(18)O with distance from the footwall. This may possibly indicate contamination of the Bushveld magma pre-intrusion, probably in a staging chamber. However, when compared to data from the central sector of the Platreef itself, analysis from this study have lower delta~(18)O values indicating changes in the degree of contamination with varying footwall lithology. Also, differences between plagiodase and pyroxene delta~(18)O values indicate exchange .with fluids. It is possible that late-stage deuteric fluids may have caused this exchange.