Laboratory partitioning of platinum-group elements (PGE) and gold with application to magmatic sulfide–PGE deposits

Abstract

Apparent diversity in laboratory partitioning of platinum-group elements (PGE)–Au between (Fe,Ni)-sulfide liquid and S-saturated basaltic melt is resolved by recognizing the marked control exerted by variation in metal/S ratio of the sulfide liquid. Partition coefficients ( D) increase markedly for Os, Ir and Pd and somewhat for Pt with increase in S content. In addition, the partitioning is markedly dependent on concentration of PGE–Au in the sulfide fraction, for all precious metals in metal-rich sulfide, and for Ru, Pt and Pd in relatively S-rich sulfide. Reversal of partitioning of PGE–Au is presently demonstrated for experiments with metal-rich sulfide liquids. Erratic partitioning behaviour, with anomalously high D(PGE–Au) values, is minimised by degassing starting materials. Summary D(PGE–Au) values for metal-rich sulfide liquids and near-natural PGE–Au abundances are: Os(2.2), Ir(1.8), Ru(2.4), Pt(1.4), Pd(2.9), Au(0.9) (×10 3) for the CCO oxygen buffer and molar Ni/(Fe+Ni)=0.02, and Os(3.7), Ir(3.2), Ru(4.4), Pt(4.6), Pd(5.0), Au(3.0) (×10 3) for the IQF buffer and Ni/(Fe+Ni)=0.12. Thus, PGE–Au would not be extensively fractionated for equilibrium between sulfide and deep mantle melts. For more S-rich sulfide liquids (WM oxygen buffer) and 100–1000 ppm in the sulfide, D(PGE–Au) values are: Os(30), Ir(26), Ru(6.4), Pt(10), Pd(17) (×10 3) at Ni/(Fe+Ni)=0.36, and Os(10), Ir(51), Ru(3.5), Pt(13), Pd(25), Au (1.2) (×10 3) at Ni/(Fe+Ni)=0.66. The laboratory partitioning is notably differential within the PGE–Au group, with relatively high Ds for Ir and Pd and low Ds for Ru and Au. Although separation of an early-magmatic sulfide liquid would result in significant concentration of some PGE (e.g., Ir and Pd), sulfide liquid immiscibility alone would not account for the high abundance of PGE–Au in the reef deposits of the Bushveld and Stillwater Complexes.