PLATINUM-GROUP MINERALS AND OTHER DETRITAL COMPONENTS IN THE KAROO-AGE SOMABULA GRAVELS, GWERU, ZIMBABWE

Abstract

Heavy-mineral concentrates were obtained from a recent exploration campaign for diamond in the Somabula gravels, a paleoplacer of Upper Karoo (Triassic, ~200 Ma) age near Gweru, Zimbabwe. The polymict spectrum of heavy minerals in the Somabula placers comprises staurolite, zircon, monazite, columbite–tantalite, cassiterite, chromite, native gold, platinum-grou p minerals (PGM), and other components, which suggests derivation from a variety of source lithologies. Age determinations revealed that most of the detrital grains of zircon, monazite and columbite–tantalite originate from Pan-African source rocks. Altogether, 622 detrital PGM grains were identified. Pt and Pt alloys, mainly with Fe, Rh, Pd and Ru, predominate (79%), followed by Os–Ir–Ru–Rh–Pt alloys (10%), rustenburgite [Pt 3Sn] (9%), and rare Pt-rich phases (1%). The compositions of the alloys suggest high-temperature crystallization from a complex Pt–Pd–Rh–Os–Ir–Ru–Fe solid solution in a sulfur-poor magmatic environment, probably mafic–ultramafic rocks. The suite of PGM markedly differs from that of detrital PGM originating from the Great Dyke and from the primary platinum-group element mineralization of the Main Sulfide Zone of the Great Dyke. The mineralogical evidence and Os isotope data obtained for the Os–Ir–Ru alloy rule out the possible derivation of the PGM from the Great Dyke. Instead, they are likely derived from late Archean greenstone belts of the Zimbabwe craton.