Experimental partitioning of osmium, iridium and gold between basalt melt and sulphide liquid at 1300°C

Abstract

The partitioning of Os, Ir and Au between FeNi sulphide liquid and basalt melt was determined at 1300°C and low pressure using internally buffered charges in alumina containers sealed in silica tubes. Six runs were at the oxygen fugacity of the iron‐silica phase‐fayalite (IQF) buffer and one at iron‐wüstite (IW), with Os, Ir and Au concentrations in the sulphide ranging from approximately 45 to 400 ppm for each metal. The monosulphide liquid (FeNi) varied from 0.02 to 8.9 at % Ni. The major element composition of silicate glass and sulphide run products was quantitatively determined by electron microprobe analysis. The noble metal content of the sulphide phase was calculated from the initial platinum‐group element (PGE) content added and corrected for dilution by constituents derived from the silicate melt, which was mainly iron. Noble metals in silicate glasses were determined by radiochemical neutron activation. Average distribution coefficients (D) for the IQF buffered runs were D(Os) = 3720 ± 1320, D(Ir) = 3180 ± 640 and D(Au) = 3030 ± 980. D(PGE) decreases with increasing oxygen fugacity with values of 980, 1300 and 1400 for Os, Ir and Au respectively for the IW buffered run. No significant correlation of D with sulphide phase bulk composition or total PGE content was observed. The range of D values for Os, Ir and Au, 3720 to 3030 for IQF buffered runs, attests to the efficiency of immiscible sulphide liquid as a concentrator of noble metals from basic silicate magma. However, the marked fractionation of noble metals observed in sulphide ores is not easily modelled by the small variations in D values of individual metals. In addition, these experimental D values are much smaller than values inferred from some PGE ore deposits.