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

Abstract

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°C and 1-4 kbar fluid pressure. The fugacities of oxygen and sulfur, and the chemical species in the fluid phase, were controlled by the presence of graphite and of a sulfide melt of pyrrhotite composition and by external control of the fugacity of hydrogen. It has 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 effects on the partitioning. The following partition coefficients have been obtained: D Pd = (5.5 ±0.7) × 10 4 ; D Rh = (2.7 ± 0.6) × 10 4 ; D Ru = (2.5 ± 0.7) × 10 3 ; D Ir = (3.1 ±0.8) × 10 5 ; D Os = (2.3 ±0.3) × 10 2 ; D Au = ( 1.6 ± 0.4) × 10 4 . These values are significantly higher than those reported in the recent literature ( et al., 1991; et al., 1992); this is attributed to their having been obtained under hydrous conditions. Their magnitudes demonstrate that the segregation of sulfide liquid from silicate magma is an effective mechanism for concentrating PGE and Au, as has been concluded from the study of deposits of believed magmatic origin.