Noble metals in the marginal rocks of the Bushveld Complex

Abstract

Two major subdivisions of marginal rocks occur in the eastern Bushveld Complex. A pyroxenitic group borders the lower and lower critical zones, and a gabbroic group borders the upper critical and main zones. Three main facies of the gabbroic group can be recognized on the bases of field relations, petrography, and chemical composition. Five sampling sites were chosen that would best represent the varieties of marginal rocks by study of the field relations and bulk rock compositions of 184 previously analyzed samples. These sites were resampled and the rocks analyzed for major and trace elements and the noble metals (Ir, Rh, Pt, Pd, and Au). The field relations, petrography, and mineral and bulk rock composition of the samples are presented. The range in noble metal abundances is: Ir: 0.2 to 0.6, Rh: 3 to 12, Pt: 30 to 50, Pd: <10 to 20, and Au 10 to 40 ppb. The values are considerably higher than corresponding abundances in most basalts and chilled margins of other layered complexes--especially for Au. All marginal rocks have broadly similar chondrite-normalized noble metal distribution patterns, with enrichment in Rh and Au relative to Pt and Pd.Abundances of noble metals in a liquid from which the UG-2 chromitite layer and the Merensky Reef may have crystallized is calculated from marginal rock compositions, assuming a conical magma chamber. The liquid is composed of a approximately 52/48 percent pyroxenitic/gabbroic mixture and has a composition of Ir: 0.41, Rh: 7.7, Pt: 40, Pd: 16, and Au: 21 ppb. Maximum distribution ratio values (D (super sulfide liquid) (super silicate liquid) ) computed for the mixture, assuming no fractional crystallization, are between 1.8 and 26 times higher in the UG-2 (chromite) environment than in the Merensky Reef (silicate) environment. Distribution ratio values of Ir: 6,600, Rh: 1,100, Pt: 3,900, Pt: 4,000, Au: 480, and Cu: 480 were obtained for the Merensky Reef. With 50 percent fractional crystallization and incompatible behavior of the metals, the distribution ratios attain more realistic values of Ir: 3,300, Rh: 550, Pt: 1,950, Pd: 2,000, Au: 240, and Cu: 240.