Abstract

The Uitkomst Complex is a Ni-Cu-PGE-Cr-Co mineralized composite mafic-ultramafic intrusion situated in South Africa. The platinum-group elements (PGEs), Re and Au contents in base metal sulfides (BMS), and Re-Os isotope compositions were determined, as well as the occurrence of associated platinum-group minerals (PGMs) in the various mineralized zones of the complex. BMS within the complex have elevated PGE and Au contents, with the highest values (average of ~10 ppm total PGE) for the massive sulfide body, much lower than the values for BMS from the nearby Bushveld Complex. Pentlandite, as major PGE carrier, hosts most of the Pd, whereas pyrrhotite is characterized by elevated iridium subgroup platinum-group element (IPGE; Os, Ir, Ru) content lacking Pd. Chalcopyrite occasionally serves as a carrier of Au. Minor pyrite concentrates Rh and Re. Mass balance calculations show that BMS play a crucial role in IPGE deportment, whereas they accommodate only up to ~60% of the total Pd. The initial 187Os/188Os values for the whole Uitkomst suite indicate a significant but variable contribution of crustal material to mantle-derived parental melts. Platinum-group elements forming inclusions or presented in solid solution within BMS appear to contribute less to the overall PGE grade than the associated PGMs. The data support the model of a sulfide-oversaturated Uitkomst magma forming a monosulfide solid solution (mss) cumulate as massive sulfide orebody under low-R factor conditions in a shallow conduit after escape of an evolved PGE-rich sulfide liquid from the chamber, leaving the mss relatively poor in PGEs.

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