THE SOLUBILITY OF RUTHENIUM IN SULFIDE LIQUID: IMPLICATIONS FOR PLATINUM GROUP MINERAL STABILITY AND SULFIDE MELT-SILICATE MELT PARTITIONING

Abstract

To more completely assess the primary magmatic origin of Ru–Os–Ir (IPGE) alloys, we conducted experiments to evaluate the effects of T, fO2, fS2 and melt composition on the solubility of Ru in molten Fe–Ni–sulfide. Fe–Ni–S melt+Ru were held in olivine crucibles, and experiments were done in a vertical-tube gas-mixing furnace at 1200–1400 °C for 1–5 days. At constant fO2 and fS2, Ru solubility increases with T, and a similar result is obtained if fO2 is varied parallel to the fayalite–magnetite–quartz buffer (FMQ), with fS2 levels to maintain sulfide liquid saturation. At log fS2 of −1.7, Ru solubility decreases from ∼11 wt.% at log fO2 of −10.8, to ∼0.3 wt.% at log fO2 of −8.1. At a log fO2 of −8.6, a similar reduction in Ru solubility occurred as log fS2 decreased from −0.9 to −3.0. Substitution of Ni for Fe in the sulfide results in an increase in Ru solubility, with values ranging from ∼3 wt.% at Fe/Ni of 36 to ∼10 wt.% at Fe/Ni of 6 (log fO2, fS2 of −9.1, −1.7, respectively). Dilution of Ru with a 1:1 mix of Os+Ir results in a 4- and 10-fold decrease in melt Ru content for alloys with ∼60 and 35 mol% Ru, respectively. For the fO2–fS2 conditions required for sulfide liquid saturation in natural basaltic magmas, pure Ru solubility in molten sulfide is expected to exceed 10 wt.%, and dilution by Os+Ir is still likely to require wt.% levels of Ru for IPGE alloy saturation. Since Ru abundances of ore-grade massive sulfide is