Base Metal Sulphide Minerals in the Upper Critical Zone (UCZ) of the Bushveld Igneous Complex (BIC)

English Abstract:

We report on the distribution and textures of base metal sulphide minerals in mafic cumulates of the Upper Critical Zone of the western Bushveld Complex. Samples from a 828 m section of a single exploration borehole on Turffontein Farm 34 km east of Rustenburg were examined in the petrographic microscope and with scanning electron microscopy (SEM) with energy dispersive X-ray fluorescence spectroscopy for point analysis and mapping. Primary sulfide melt inclusions hosted by silicate primocrysts were not observed in any samples, but pockets of sulfide minerals associated with late-crystallizing interstitial phases including quartz, albite, biotite, rutile, and apatite are common. The samples include orthocumulate textured pyroxenite and chromitite, and meso- to adcumulate textured norite, gabbronorite, and anorthosite. Two principal generations of sulfide minerals are recognized, based in part on the identities of the associated silicate minerals. In those instances where sulfide minerals occupy positions apparently interstitial to the magmatic silicate minerals with pristine textures, the sulfide mineral assemblage is pyrrhotite + chalcopyrite + pentlandite mainly associated with anhydrous minerals (quartz, pyroxene, albite, plagioclase). However most occurrences show hydrothermal overprints, being associated with pyrite, millerite, galena and commonly showing disseminated halos of chalcopyrite within zones of hydrous alteration (actinolite/tremolite, chlorite) of the host silicate minerals. Rare platinum group minerals (sperrylite, cooperite) were observed in samples from the Merensky Reef and the UG2, UG3, and MG4 chromitite layers. Other occurrences of sulfide minerals were hosted by grain boundaries and cleavage fractures of silicate minerals, indicating local transport and redeposition of sulfur and base metals during alteration. A possible interpretation of the observations is that later hydrothermal fluids may have potentially affected the origin of sulphide mineral inclusions.