SULFIDE MINERALS IN THE BUSHVELD COMPLEX, SOUTH AFRICA

The origin of platinum-group elements (PGE) deposits in large layered intrusions has been controversial. While ore minerals are well-studied in the mined ore zones, details of metal sulfides and associated trace minerals in rocks away from the ore zones have not been well-examined. This study reports early results on petrographic association, composition and distribution of sulfide minerals in the Bushveld Complex.

From microscope observations, it is found that base metal sulfides are largely presented as interstitial phases and as inclusions in the primary igneous minerals: plagioclase, orthopyroxene, clinopyroxene and olivine. Pyrite (FeS2), Pyrrhotite (FeS), Pentlandite (Fe,Ni)9S8, and Chalcopyrite (CuFeS2) are major disseminated sulfide minerals in Bushveld rocks. Minor amount of Siegenite (Ni,Co)3S4 and Millerite (NiS) are found in the Lower and Upper Critical Zones. Most of sulfide minerals are found individually, and the sizes vary from 0.001-0.08mm. Numerous grains are scattered as interstitial phases as a group or a trace line wrapping particular minerals. The modal abundance of sulfide contents broadly increases from top to bottom of the intrusions, excluding the Lower Zone; however, the distribution is not uniform within each zone. Pyrite is found mainly in the Marginal Zone, and less commonly in the Upper and Lower Critical Zones. Chalcopyrite is distributed throughout the Bushveld rocks, and Cu:F:S mole ratio vary from 1:1:2 to 0.77-0.99: 0.94-1.04: 2. Chalcopyrite is typically intergrown with pyrrhotite, and for the latter Fe:S ranging from 0.89-0.95:1. Pentlandite (Fe+Ni):S ratios range from 8.30-8.65:8.

Based on the ongoing observations, we propose that the variation of ore components could be used to construct quantitative models and explain the crystallization and migration processes of base metal sulfides and PGE.