COPPER SOLUBILITY IN A BASALTIC MELT AND SULFIDE LIQUID/SILICATE MELT PARTITION COEFFICIENTS OF CU AND FE

Abstract

The solubility of copper in a sulfur-saturated basaltic melt has been determined at 1245°C as a function of fO2 and fS2. Copper solubilities at log fO2 values between -8 and -11 fall into two distinct populations as a function of fS2. At log fS2 values < -1.65, sulfide liquid that coexists with the basaltic glass quenches to sulfur-poor bornite solid solution. At log fS2 values in excess of -1.65, the sulfide liquid quenches to a complex intergrowth of sulfur-rich bornite and intermediate solid solution. Copper solubilities in the low-fS2 population range from 594 to 1550 ppm, whereas those in the high-fS2 population range from 80 to 768 ppm. Sulfide liquid/silicate liquid partition coefficients (D) for Cu and Fe range from 480 to 1303 and 0.7 to 13.6, respectively. Metal-sulfur complexing in the silicate liquid is shown to be insignificant relative to metal-oxide complexing for Fe but permissible for Cu at high fS2 values. On log DFe (sulfide-silicate) and log DCu (sulfide-silicate) vs.12(log fS2 - log fO2) diagrams, both fS2 populations show distinct but parallel trends. The observation of two D values for any fS2/fO2 ratio indicates nonideal mixing of species involved in the exchange reaction. The two distinct trends observed for both Cu and Fe are thought to be due to variations in activity coefficient ratios (e.g., γFeO/γFeS and γCuO0.5/γCuS0.5). Results of the experiments suggest that accurate assessments of fS2/fO2 ratios are required for the successful numerical modeling of processes such as the partial melting of sulfide-bearing mantle and the crystallization of sulfide-bearing magmas, as well as the interpretation of sulfide mineralogical zoning. In addition, the experiments provide evidence for oxide or oxy-sulfide complexing for Cu in silicate magmas and suggest that the introduction of externally derived sulfur to mafic magma may be an important process for the formation of Cu-rich disseminated magmatic sulfide ore deposits.