Partitioning of Cu, Ni, Au, and platinum-group elements between monosulfide solid solution and sulfide melt under controlled oxygen and sulfur fugacities

Abstract

Abstract-We have performed six experiments in which we equilibrated monosulfide solid solution (mss) with sulfide melt in evacuated silica capsules containing solid buffers to fix oxygen and sulfur fugacity, at temperatures of 950°C, 1000°C and 1050°C at bulk concentrations of ϳ50 ppm for each of the PGE and Au, 5% Ni, and 7% Cu. Concentrations of O, S, Fe, Ni and Cu were determined by electron microprobe, whereas precious metal concentrations were determined by laser-ablation inductively-coupled mass spectrometry. Partition coefficients of all elements studied show minimal dependences on oxygen fugacity from the IW to the QFM buffers when sulfur fugacity is fixed at the Pt-PtS buffer. Cu, Pt, Pd and Au are strongly incompatible and Ru remains moderately to strongly compatible under all conditions studied. At all oxygen fugacities, at the Pt-PtS sulfur buffer, Ir and Rh remain highly compatible in mss. In the single run at both low oxygen and low sulfur fugacity Ir and Rh were found to be strongly incompatible in mss. At QFM and Pt-PtS the partition coefficient for Ni shows weak temperature dependence, ranging from 0.66 at 1050°C to 0.94 at 950°C. At lower oxygen and sulfur fugacity Ni showed much more incompatible behavior. Comparison with the compositions of sulfide ores from the Lindsley deposit of Sudbury suggests that the sulfide magma evolved under conditions close to the QFM and Pt-PtS buffers. The compatible behavior observed for Ni, Ir and Rh at Lindsley and most other magmatic sulfide deposits hosted by mafic rocks requires equilibration of mss and sulfide liquid at moderately high sulfur fugacity and low temperatures near to the solidus of the sulfide magma. We argue that this constraint requires that the sulfide magma must have evolved by equilibrium crystallization, rather than fractional segregation of mss as is commonly supposed.

Critical Factors Controlling Pd and Pt Potential in Porphyry Cu–Au Deposits: Evidence from the Balkan Peninsula

“…Arc magmas with high potential to produce hydrothermal systems with ideal chemistry for transporting precious metals (origin of Au and Cu deposits) are characterized by fO 2 more than two log units above FMQ, where fO 2 is oxygen fugacity and FMQ is the fayalite-magnetite-quartz oxygen buffer [68,69]. It is consistent with the abundance of magnetite in all porphyry Cu-Au-Pd deposits of the Balkan Peninsula [14][15][16][17][18][19].…”

Contrasting mechanisms for crustal sulphur contamination of mafic magma: evidence from dyke and sill complexes from the British Palaeogene Igneous Province

“…In PGE mineralized settings (e.g. the Bushveld Complex) this feature is well documented for S-saturation in PGE reefs (Maier 2005, and references therein) as the partition coefficient of Pd into sulphide liquid is several orders of magnitude greater than that for Cu (Mungall et al 2005;Mungall & Brenan 2014). However, in a setting where first-stage S-saturation has already occurred deeper in the system, the concentration of chalcophile elements (particularly Pd-group PGE) would be much lower in the remaining silicate magma that continued its ascent to the surface.…”

Paradoxical co-existing base metal sulphides in the mantle: The multi-event record preserved in Loch Roag peridotite xenoliths, North Atlantic Craton

“…The partition coefficient of Pt (D Pt ) decreases with decreasing ƒS 2 from S-oversaturated (pyrrhotite-rich) to S-undersaturated (Fe metal-rich) conditions (Mungall et al, 2005) and thus leads to PGE alloy nucleation. During incongruent partial melting of mantle BMS (forming an initial Cu-Ni-rich sulphide liquid and a residue of Fe-Ni-rich Mss), very high PGE tenors will be achieved in the residue due to high overall D PGE , leading to saturation for Pt-Ir alloys by just 10% partial melting (Mungall & Brenan, 2014).…”