PLATINUM, PALLADIUM AND NICKEL CONCENTRATIONS IN COEXISTING SULFIDE AND BASALTIC MELTS AS A FUNCTION OF SULFUR FUGACITY

Layered mafic intrusions (LMI), a type of magmatic sulfide ore deposit, contain alternating layers of silicate and sulfide minerals and account for over 80% of the world’s platinum group element (PGE) reserves. The alternating layers are hypothesized to be the product of liquid immiscibility, the process through which sulfide melt is exsolved from the pre-existing silicate melt once sulfide saturation has been reached within a magma chamber. Platinum (Pt) and Palladium (Pd), the most common PGEs found in LMI, are concentrated into the sulfide melt relative to the silicate melt. This study examined the concentrations of Pt, Pd and Nickel (Ni) in the quenched sulfide melt and exsolved PGE-rich metal sulfide phases, while independently varying the sulfur fugacity (fS₂) of the system, in order to better constrain the formation of PGE-rich sulfide ore bodies in LMI. Experiments were conducted in a vertical tube furnace equipped for gas-mixing at 1100°C and 1 atm, with oxygen fugacity buffered to QFM (quartz-fayalite-magnetite) and sulfur fugacity measured by analyzing the composition of pyrrhotite. Fused quartz capsules (sealed at one end) were loaded in such a way to allow for immiscible sulfide and silicate melts synthesized at experimental run conditions to equilibrate with Pt, Pd and Ni metals. Elemental concentrations were determined for sulfide and silicate melts by using a CAMECA SXFiveFE EPMA. Three discernable PGE-metal sulfide phases were found to have exsolved from the sulfide melt during quench. Cu- and Ni-rich Pd-bearing phases (~25 – 52 wt.% Pd) were irregular in shape and found to be adjacent to one another. A separate Pt- and Pd-rich phase (~55 and 25 wt.%, respectively) exhibited a dendritic-like texture and was the only Pt-rich phase found in the sulfide run products. Concentrations for Pt, Pd, and Ni when averaging the quenched sulfide melt and PGE-metal sulfide phases ranged from 0.4 – 12.6 wt.%, 1.8 –34.9 wt.%, and 4.6 – 6.4 wt.%, respectively. Pt and Pd concentrations in the quenched sulfide melt generally increased with increasing fS₂. Cu/Ni ratios in the quenched sulfide melt generally decreased with increasing fS₂. Pt, Pd and Ni concentrations were below EPMA detection limits in all basalt run products, requiring future data collection using LA-ICPMS.