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

Apparent diversity in laboratory partitioning of platinum-group elements (P GE)-Au between (Fe,Ni)-sulfide liquid and S-saturated basaltic melt is reso lved by recognizing the marked control exerted by variation in metal/S rati o of the sulfide liquid. Partition coefficients (D) increase markedly for O s, Ir and Pd and somewhat for Pt with increase in S content. In addition, t he partitioning is markedly dependent on concentration of PGE-Au in the sul fide fraction, for all precious metals in metal-rich sulfide, and for Ru, P t and Pd in relatively S-rich sulfide. Reversal of partitioning of PGE-Au i s presently demonstrated for experiments with metal-rich sulfide liquids. E rratic partitioning behaviour, with anomalously high D(PGE-Au) values, is m inimised by degassing starting materials. Summary D(PGE-Au) values for meta l-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)(X 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) (X 10(3)) for the IQF buffer and Ni/(Fe + Ni)= 0.12. Thus, PG E-Au would not be extensively fractionated for equilibrium between sulfide and deep mantle melts. For more S-rich sulfide liquids (WM oxygen buffer) a nd 100-1000 ppm in the sulfide, D(PGE-Au) values are: Os(30), Ir(26), Ru(6. 4), Pt(10), Pd(17) (X 10(3)) at Ni/(Fe + Ni)= 0.36, and Os(10), Ir(51), Ru( 3.5), Pt(13), Pd(25), Au (1.2) (X 10(3)) at Ni/(Fe + Ni) = 0.66. The labora tory partitioning is notably differential within the PGE-Au group, with rel atively 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 concentrat ion 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 B ushveld and Stillwater Complexes. (C) 1999 Elsevier Science B.V. All rights reserved.