Cryptic mineral variation and PGE distribution in the Merensky Reef and its discontinuous mela-gabbronoritic hanging wall (“Brown Sugar Norite” - BSN) at Two Rivers Platinum Mine, Eastern Bushveld Complex, South Africa

Abstract

Two Rivers Platinum Mine, situated on the southern sector of the eastern limb of the Bushveld Complex, extracts Platinum Group Elements (PGE) from the UG-2 chromitite layer. A trial operation of the Merensky Reef (MR) exposed unusual occurrences of mela-gabbronoritic lenses, ‘brown sugar norite’ (BSN in mine terminology), within the pyroxenite of the hanging wall. The BSN is a fine-grained mela-gabbronorite, which appears to only occur where the upper chromitite stringer of the Merensky Reef unit is present, and forms laterally inconsistent, flat lenses. Textural features, mineral chemistry of the various rock types, Sr analyses of plagioclase separates and whole rock geochemistry indicate multiple magma injections, fractionation and disequilibrium crystallization of the magma within the Merensky interval. Whole rock MgO and enstatite content of orthopyroxene are higher in BSN compared with the surrounding Merensky Reef pyroxenite, thus supporting the notion of the BSN having formed from a more primitive magma. Both the pyroxenite and BSN plagioclase have 87Sr/86Sr ratios representative of Critical Zone magma; that of the BSN has a lower initial 87Sr/86Sr isotope ratio relative to that of the pyroxenite, suggesting a more primitive magma source. It is suggested that the unusual, discontinuous BSN lenses within pyroxenite may have formed by magmatic erosion of a new magma injection, which affected the crystal mush of the earlier deposited BSN layer. This resulted in isolated lenses of relict, but more primitive orthopyroxene of the BSN in a residual plagioclase-rich melt from an earlier pyroxenitic MR magma. In both rock types (MR pyroxenite and BSN), the PGE occur as discrete platinum-group minerals (PGM), and in solid solution in sulphides. At Two Rivers Platinum Mine, the presence of PGE in sulphides has been identified by EMPA and LA-ICP-MS. The PGE occur mineralogically either in solid solution or as PGM inclusions of different size. Pentlandite, chalcopyrite, pyrrhotite and rare pyrite occur disseminated as individual grains with associated PGM throughout the Merensky pyroxenite interval, but more enriched near the bottom and top chromitite stringers. PGM encountered are moncheite, platarsite and irarsite, relatively enriched in Pt, but poor in Pd and Rh. Our results show that most of the Pd and Rh are incorporated in pentlandite, whereas pyrrhotite, chalcopyrite, and pyrite are almost devoid of PGE. Textural features of the encountered PGM grains suggest their redistribution through late stage magmatic (probably hydrothermal) processes. The BSN contains relatively little if any base metal sulphides and PGM, indicating its formation from magma depleted in PGE.