Fractionation of platinum, palladium, nickel, and copper in sulfide–arsenide systems at magmatic temperature

Experimentally derived phase relations of arsenide in sulfide melt are presented to quantify the fractionation paths of As-bearing sulfide melts. When a natural sulfide melt reaches arsenide saturation, a separate Ni–PGE-rich arsenide melt exsolves. The arsenic saturation concentration in an Fe–Ni–Cu sulfide melt is between 0.5 and 1.5 wt%. The affinities of the chalcophile metals for an immiscible arsenide melt follow the order Pt > Pd > Ni ≫ Fe ≈ Cu. In natural systems, arsenide exsolution will be triggered by the activity of the nickel arsenide components dissolved in sulfide melt, Ni being the most common base metal with strong affinity to the As n− anionic species. Arsenic may have a major effect on the fractionation paths of sulfide melts even if no separate arsenide phase forms. Arsenic, and probably many other chalcogens and metalloids in magmatic melts, may undergo associations with Pt and Pd well before discrete PGE minerals become stable phases.