PHASE-EQUILIBRIUM CONSTRAINTS ON THE MAGMATIC ORIGIN OF LAURITE + RU-OS-LR ALLOY

Abstract

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 (XRu > 0.85) at 1250°C, and slightly expands to encompass more Ru-poor compositions (XRu > 0.6) at 1200°C. At this f(S2), laurite remains very close to pure RuS2. An increase in sulfur fugacity to 10.39 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 + lr. 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.