The nature and origin of PGE mineralization in the Rooipoort area, northern Bushveld Complex, South Africa

The Grasvally Norite-Pyroxenite-Anorthosite (GNPA) member within the northern limb of the Bushveld Complex is a PGE-Ni-Cu mineralized, layered package of mafic cumulates. This magmatic sulfide deposit is developed at the equivalent stratigraphic position to the Platreef, being overlain by Main Zone gabbronorites and in places resting unconformably on metasediments from the Transvaal Supergroup. Parental magmas to the GNPA member were of a ‘hybrid’ composition containing both B1 and B2/B3 type magma components which were strongly crustally contaminated and S saturated at the time of emplacement. At depth, the assimilation of crustal S was crucial for ore genesis. Although parental magma(s) experienced a second localised contamination event, interaction with the local footwall at the time of emplacement, did not have any control on the genesis of sulfide mineralization. A single primary sulfide liquid, enriched in PGE, Ni, Cu and semi-metals was distributed throughout the succession during multiphase emplacement of the GNPA member. The distribution and mineralogy of platinum-group and chalcophile elements results from the complex behaviour of these elements during both sulfide fractionation and hydrothermal processes. The primary assemblage is characterised by IPGE-rich pyrrhotite, IPGE-, Rh, and Pd-rich pentlandite, chalcopyrite, and associated Pt-As and Pd-Bi-Te minerals. Secondary assemblages in addition contain Pd- and Rh-rich pyrite and millerite, and discrete minerals including Pd antimonides and arsenides. Whilst correlations between the GNPA/Platreef and Upper Critical Zone remain relatively speculative, the northern limb deposits are thought to have formed from compositionally similar or related magmas, which were poorer in Mg, richer in Ca and Fe and Pd dominant relative to the magma(s) that formed the Upper Critical Zone. It is proposed that with depth the Platreef may progressively transform into a layered succession that is exposed south of the Ysterberg-Planknek Fault and represented by the GNPA member. The Platreef can therefore possible be viewed as a marginal facies of the GNPA member, and sulfide-rich magma which escaped up the margins of the northern limb chamber.