PHASE-EQUILIBRIUM CONSTRAINTS ON THE MAGMATIC ORIGIN OF LAURITE + Ru Os Ir ALLOY

To more completely assess the origin of associated laurite (RuS2) and Ru–Os–Ir (IPGE) alloy which are found as inclusions in near-liquidus phenocrysts such as chromian spinel, we conducted experiments to evaluate the effects of T and f (S2) on phase relations in the system Ru–Os–Ir–Cu–S. Cu–S melt (added as a flux) + IPGE metals were held in silica crucibles, and experiments were done in both vertical-tube gas-mixing furnace apparatus [low f (S2)] and evacuated silica tubes [higher f (S2) buffered by Pt–PtS] at 1200–1250°C for 1–3 days. At constant f (S2) of 10−1 atm, the two-phase field of laurite + alloy is restricted to only the most Ru-rich bulk compositions ( X Ru > 0.85) at 1250°C, and slightly expands to encompass more Ru-poor compositions ( X Ru > 0.6) at 1200°C. At this f (S2), laurite remains very close to pure RuS2. An increase in sulfur fugacity to 10−0.39 atm at 1200°C and 10−0.07 atm at 1250°C resulted in a considerable expansion of the two-phase field, with both laurite and alloy dissolving more Os + Ir. For example, at 1250°C and f (S2) of 10−0.07, the Os and Ir content of laurite increases to ~20 and ~12 at.%, respectively. Coexisting alloys in both sets of high- f (S2) experiments contain less than 15 at.% Ru. The compositions of laurite and IPGE alloy defined by high- f (S2) experiments show remarkable similarity to coexisting PGM compositions preserved in natural chromitite from several localities. If such phases are the product of entrapment at the magmatic stage, then high- f (S2) conditions are inferred. Similarly, the bulk compositions of laurite from suites in which IPGE alloy is absent also suggest similarly high f (S2), if high-temperature entrapment is assumed. Limits on the f (O2) of magmas that may precipitate alloy–laurite pairs stem from the requirement that such magmas remain sulfide-liquid-undersaturated, at least until PGM are trapped in their phenocryst host. Calculations suggest that for this to occur at high f (S2), laurite + IPGE alloy precipitation requires the involvement of relatively oxidized, low-FeO magmas.