Copper solubility in a basaltic melt and sulfide liquid/silicate melt partition coefficients of Cu and Fe

摘要

The solubility of copper in a sulfur-saturated basaltic melt has been determined at 1245degreesC as a function of fO(2) and fS(2). Copper solubilities at log fO(2) values between - 8 and - 11 fall into two distinct populations as a function of fS(2). At log fS(2) values < - 1.65, sulfide liquid that coexists with the basaltic glass quenches to sulfur-poor bornite solid solution. At log fS(2) 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-fS(2) population range from 594 to 1550 ppm, whereas those in the high-fS(2) 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 fS(2) values. On log D-Fe (sulfide-silicate) and log D-Cu (sulfide-silicate) vs. 1/2(log fS(2) - log fO(2)) diagrams, both fS(2) populations show distinct but parallel trends. The observation of two D values for any fS(2)/fO(2) 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., γ(FeO)/γ(FeS) and γ(CuO0.5)/γ(CuS0.5)). Results of the experiments suggest that accurate assessments of fS(2)/fO(2) 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. Copyright (C) 2002 Elsevier Science Ltd. [References: 29]