Complex multistage genesis for the Ni–Cu–PGE mineralisation in the southern region of the Platreef,Bushveld Complex,South Africa

Abstract

In the southern portion of the Platreef, Ni–Cu–Pt-group element (PGE) mineralisation results from a complex interplay of pre-, syn- and post-magmatic processes. Sulphide minerals throughout the successio comprise < 1% to > 25% (mode) and occasionally up to 45% over short intersections of core. Generally, these occur as centimetre to millimetre-sized fractionated blebs and interstitial grains of pentlandite, pyrrhotite and chalcopyrite. Where present, zones of massive sulphides rich in chalcopyrite are found close to the footwall contact. More compositionally complex sulphides are associated with felsic melt phases that pervasively infiltrated the package soon after its partial or complete crystallisation. There is a close correlation between Cu and Ni concentrations but a poor correlation between PGE and base metal contents. Thus high Cu+Ni values do not necessarily indicate high PGE grade, although the highest PGE grades are located within sulphide-rich zones towards the floor. Platinum-group minerals (PGMs) are dominated by Pd-bearing tellurides, antimonides, bismuthides, bismutho-antimonides and complex bismuthotellurides. Pt-bearing phases mostly occur as Pt-arsenides and antimonides. PGMs are found in rims around orthopyroxenes, as discrete grains within secondary silicates and as grains adjacent to, or along the margins of, composite sulphides. Rarely are they found as inclusions within sulphide minerals. The present-day distribution of sulphide minerals and PGEs results from a continuum of processes, beginning with scavenging of PGEs by immiscible sulphide droplets. Subsequently, these were modified by the incorporation of meta-sedimentary-derived S, As, Te, Bi and Sb from the devolatilisation of floor rocks and xenoliths. Felsic melts redistributed some of the sulphides and PGMs as they percolated through the Platreef. Finally, late-stage metasomatic fluids altered the primary silicates and led to further remobilisation of the PGEs to form PGMs disseminated within tremolite, talc and serpentine. This final stage may account for the observed decoupling of Cu and Ni from Pt and Pd.