Osmium isotopes and crustal sources for platinum-group mineralization in the Bushveld Complex, South Africa

Economic mineralization is found in the Merensky Reef and the UG2 and UG1 chromitites and includes osmium-bearing laurite (Ru, Ir, Os)S ₂ . The laurite-bearing samples have ¹⁸⁷ Os/ ¹⁸⁶ Os ratios of 1.28-1.60, which are more radiogenic than predicted for mantle derived, 2.0 Ga Os(~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. 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. -from Authors