The Driekop platinum pipe, eastern Bushveld Complex, South Africa – Focus on geochemical, mineralogical and genetic aspects

Abstract Platiniferous dunite pipes, discovered and mainly mined in the 1920s, are discordant orebodies crosscutting the cumulates of the Critical Zone of the eastern Bushveld Complex. The near-vertical Driekop pipe consists of a magnesian dunite, about 300 m in diameter, that hosts an inner, more fayalitic dunite pipe, in a pipe-in-pipe configuration. The latter comprises the platinum-bearing patchy orebody, which was 18 to 24 m in diameter and was mined to a depth of about 200 m. Rock and ore samples as well as concentrates of platinum-group minerals (PGM) were studied by whole rock geochemistry including platinum-group element (PGE) analyses, ore microscopy, scanning electron microscopy with mineral liberation analysis, and electron probe microanalysis. The new findings are presented and metallogenetic models of the pipes are discussed. The ores of the Driekop pipe display similar C1 chondrite-normalised PGE patterns as those of the Mooihoek and Onverwacht pipes, indicating an analogous first-order metal input. The mineralisation is poor in sulphides, Pt is the dominant PGE of the three pipes (92 to 96%), and Pt-dominated PGM prevail. At Driekop, Pt-Fe alloys predominate the assemblage of more than 30 different PGM, followed by mainly Pt compounds with pnictogens (Sb > As – Bi) and Sn as ligands. The PGE mineralisation is a consanguineous component of the magma. Crystallisation of the melt resulted in the coeval formation of Fe-rich olivine and PGM precursor phases at magmatic temperatures. The mineralisation was introduced in the form of nanoparticles and small droplets of PGM (predominantly Pt alloys), which had acted as collectors of other PGE and ligands like As, Sb, Bi, and Sn during generation and ascent of the melt. Down temperature, the proto-PGM recrystallised and coagulated to larger grains, finally leading to the formation of various, in part composite PGM grains. On further cooling, PGM overgrowing Pt-Fe alloys formed by reaction of semi-metal ligands mobilized by supercritical magmatic to hydrothermal fluids. Redistribution of PGE/PGM apparently only occurred on the scale of milli- to centimeters. A primarily magmatic origin of the platiniferous pipe and its mineralisation is proposed here with later modification of the PGM assemblage. Although various hypotheses exist, source and emplacement history of the different host rock and ore components still need provision of convincing evidence. Accordingly, even 100 years after their discovery, the genesis of the platiniferous pipes of the Bushveld Complex remains enigmatic.