OSMIUM ISOTOPES AND CRUSTAL SOURCES FOR PLATINUM-GROUP MINERALIZATION IN THE BUSHVELD COMPLEX, SOUTH-AFRICA

The Bushveld Complex in South Africa is the largest layered mafic intrusion in the world and a major producer of platinum-group elements (PGEs). Economic mineralization is found in the Merensky Reef and the UG2 and UG1 chromitites and includes osmium-bearing laurite (Ru, Ir, Os)S2. The laurite-bearing samples have Os-187/Os-186 ratios of 1.28-1.60, which are more radiogenic than predicted for mantle-derived, 2.0 Ga Os (approximately 0.9) and indicate a system-wide assimilation of crustal Os. The UG1 chromitite is stratigraphically the lowest and the least radiogenic (1.28), whereas the Merensky Reef is the highest and most radiogenic (1.41-1.60; Hart and Kinloch, 1989). Radiogenic Os from crustal assimilation in mantle-derived magmas or from hydrothermal fluids can account for these high isotopic ratios. Assimilation models require that Os, and by inference other PGEs, are carried in Al-rich tholeiitic (A) magmas rather than high-Mg ultramafic (U) magmas. Pyritic black shales in contact with the intrusion may be high in radiogenic Os, and less than 1% shale assimilation in an A magma can explain the observed ratios. Late-stage hydrothermal fluids driven by the heat of the intrusion could also have carried Os and other PGEs from adjacent rocks into specific layers of the Bushveld. The Os-isotope data add to mounting evidence that PGEs in layered mafic intrusions may be derived from crustal sources and can no longer be considered as solely a product of mantle-derived magmatic processes.