The geological setting of the platinoid and base metal sulfide mineralization in the Platreef of the Bushveld Complex in Drenthe, North of Potgietersrus

Abstract

On the farm Drenthe, the Platreef consists of a complex assemblage of norites, melanorites, pyroxenites, serpentinites, and xenoliths of dolomite. It is a zone of up to 250 m thick between the basal contact of the complex and the overlying main zone norites. The composition of the orthopyroxene shows a rapid increase from En 60 to En 70 in the lower 30 m of the sequence, and a sharp drop from En 70 to En 60 at the top of the Platreef. This compositional break at the upper contact is due to a change from primary bronzite in the Platreef to inverted pigeonitc in the main zone.High-temperature metamorphic assemblages are preserved in the larger xenoliths, whereas their outer margins and the smaller xenoliths contain retrograde metamorphic mineral assemblages. The igneous rocks display irregularly distributed zones of alteration to bronzite bastitites and serpentinites. The alteration process was probably dominated by degassing of the xenoliths.The distribution and composition of the major sulfide minerals indicate a change from a sulfur-rich to a metal-rich environment during the crystallization of the Platreef. Pyrite, monoclinic pyrrhotite, pentlandite, and chalcopyrite dominate the lower part of the Platreef; and hexagonal pyrrhotite, pentlandite, and chalcopyrite (+ or -cubanite) prevail in the upper portion. Whole-rock analyses show an upward decrease in the Ni/Cu ratio from 1.95 to 1.70. Large variations occur in the distribution and grade of Ni, Cu, and platinum-group elements, although highest values are associated with a zone of serpentinization. The Ni and Cu values are unreliable indicators in predicting Pt and Pd concentrations in zones of major metal enrichment. Ni and Cu correlate strongly while Pt and Pd have more significant correlations with Ni than Cu, although the minerals of the platinum-group elements are primarily associated with chalcopyrite. The Pt/Pd ratio decreases upward in the sequence.The mineralization is considered to have been caused by the breakup of dolomite xenoliths that added CO 2 , H (sub 2>) O, and S to the magma, thus simultaneously lowering the solubility of sulfur in the magma and adding sulfur from an external source, which resulted in the precipitation of an immiscible sulfide liquid.