DISTRIBUTION OF PD, RH, RU, JR, OS, AND AU BETWEEN SULFIDE AND SILICATE METALS

Experiments have been conducted on the partitioning of noble metals in the presence of an H-O-C-S fluid in a high pressure gas apparatus at 1200-1300-degrees-C and 1-4 kbar fluid pressure. The fugacities of oxy gen and sulfur, and the chemical species in the fluid phase, were cont rolled by the presence of graphite and of a sulfide melt of pyrrhotite composition and by external control of the fugacity of hydrogen. It h as been found that PGE and Au partition strongly into a sulfide liquid in equilibrium with a silicate melt. The compositions of the sulfide and silicate melts as well as the redox conditions have only weak effe cts on the partitioning. The following partition coefficients have bee n obtained: D(Pd) = (5.5 +/- 0.7) X 10(4); D(Rh) = (2.7 +/- 0.6) X 10( 4); D(Ru) = (2.5 +/- 0.7) X 10(3); D(Ir) = (3.1 +/- 0.8) X 10(5); D(Os ) = (2.3 +/- 0.3) X 10(2); D(Au) = (1.6 +/- 0.4) X 10(4). These values are significantly higher than those reported in the recent literature (FLEET et al., 1991; CROCKET et al., 1992); this is attributed to the ir having been obtained under hydrous conditions. Their magnitudes dem onstrate that the segregation of sulfide liquid from silicate magma is an effective mechanism for concentrating PGE and Au, as has been conc luded from the study of deposits of believed magmatic origin.