AN EXPERIMENTAL ASSESSMENT OF PLATINUM, PALLADIUM AND NICKEL PARTITIONING BETWEEN COEXISTING SULFIDE AND BASALTIC MELTS

This study examined the equilibrium concentrations of platinum, palladium and nickel in coexisting experimentally synthesized sulfide and basaltic melts to evaluate the formation of platinum group element (PGE)-rich sulfide ore bodies in layered mafic intrusions (LMI). Metallic Pt, Pd, and Ni were equilibrated with a silicate melt, a sulfide melt, and pyrrhotite at 1100 °C in fused quartz tubes to mimic the formation of PGE-rich LMI. Experiments were conducted in a vertical tube furnace equipped for gas-mixing with O₂ fugacity buffered to a value equivalent to QFM (quartz-fayalite-magnetite), and S₂ fugacity (fS₂) measured by analyzing the composition of quenched pyrrhotite. Whereas O₂ was kept constant, fS₂ ranged from 10^-10 to 10^-2 bars, allowing for an analysis of its potential impacts on PGEs in LMIs. Major elemental concentrations in the quenched sulfide and silicate melts were determined by using a CAMECA SXFiveFE EPMA. Trace metal concentrations in the quenched silicates were ascertained by using an Agilent 7500ce ICPMS coupled with a Geolas laser ablation system. Nernst style partition coefficients between sulfide and silicate melts for a given element were determined (D_x^sul/sil). D_x^sul/sil was defined as the concentration of an element in the quenched sulfide melt divided by the concentration in the quenched silicate melt. D_Pt^sul/sil was found to be 1.4 x 10⁵ to 1.5 x 10⁷, decreasing with increasing fS₂. Palladium behaved similarly, as D_Pd^sul/sil ranged from 1.8 x 10⁵ to 5.3 x 10⁵ and decreased with increasing fS₂. D_Ni^sul/sil ranged from 3.1 x 10² to 4.5 x 10³ and showed no trend over the range of fS₂ studied. Cu/Ni and Pt/Pd in the sulfide melts was 0.9 to 3.3 and 0.1 to 0.5, respectively. These ratios generally decreased with increasing fS_2. Metal saturated low fS₂ sulfide-silicate melt systems produced sulfide melts with lower Fe concentrations and higher Pt and Pd concentrations, resulting in higher D_PGE^sul/sil values. The results of this study suggest that the scavenging of Pt and Pd from the silicate melt is favored in orthomagmatic sulfide mineral assemblages with low fS₂. Further, the Fe content of the sulfide melt may be a useful proxy for determining PGE enrichment in PGE-rich LMI.