The mineralogy and petrology of platinum-group element-bearing sulphide mineralisation within the Grasvally Norite–Pyroxenite–Anorthosite (GNPA) member, south of Mokopane, northern Bushveld Complex, South Africa

Abstract

The Grasvally Norite–Pyroxenite–Anorthosite (GNPA) member is a 400 to 800 m thick cumulate package located in the northern limb of the Bushveld Complex, south of the town of Mokopane. On the farm Rooipoort it forms the lowermost unit of the magmatic stratigraphy, overlying Transvaal Supergroup sediments, whereas further south on the farm Grasvally it overlies Lower Zone rocks of the Bushveld Complex. The GNPA member is divided into three units; the Lower Mafic Unit (LMF), the Lower Gabbronorite Unit (LGN) and the Mottled Anorthosite Unit (MANO). Platinum-group element (PGE) mineralisation is closely associated with base metal sulphides (BMS) and is confined to the LMF and MANO where PGE grades range from 1 to 4 ppm (3PGE+Au). A number of distinct BMS assemblages are observed throughout the area and are interpreted to be the result of a combination of primary magmatic processes and low temperature alteration. In areas where the GNPA member is underlain by Lower Zone rocks, a pyrrhotite–chalcopyrite–pentlandite sulphide assemblage dominates, representing initial orthomagmatic sulphide mineralisation. Late-stage low temperature alteration has significantly altered much of the sulphide mineralogy, producing two secondary pyrite–chalcopyrite–pentlandite±pyrrhotite±millerite and pyrite–pentlandite±millerite sulphide assemblages. The primary assemblage was variably altered by crystallisation of pyrite and millerite from pyrrhotite and pentlandite at temperatures below 230°C. Sulphide replacement was associated with the precipitation of quartz and secondary silicates. This replacement of sulphides is more prevalent towards the base of the unit where the GNPA member is underlain by quartzites. These features suggest a strong footwall control over the low temperature alteration and thus the extent of the development of the secondary sulphide assemblages.