SOLUBILITY OF COPPER IN A SULFUR-FREE MAFIC MELT

The solubility of Cu in S-free mafic melts has been measured at a series of f(O2) values and temperatures of 1245 and 1300 degrees C, At constant temperature Cu solubility increases from 0.04 wt% at log f(O2) = -11.9 to 1.10 wt% at log f(O2) = -7.4. Copper solubilities were in excess of 8 wt% in two runs controlled at very high f(O2) conditions of 10(-1.4) and 10(-2.2). Partitioning of Cu between metal and glass shows a strong f(O2) dependence, with D-Cu(met/gl) ranging from 90 at log f(O2) = -7.4 to 2190 at log f(O2) = -11.9. Slopes of Cu solubility and D-Cu(met/gl) vs, log f(O2) suggest that Cu dissolves predominantly in the +1 valence state. Copper solubility decreases with increasing temperature at constant f(O2), similar to experimental results for Ni, Co, and Mo (Dingwell et al., 1994; Holzheid et al., 1994). The data are consistent with Cu dissolution as an oxide (represented by CuO0.5) and suggest that changes in f(O2) (Fe2+/Fe3+ variations and Cu1+ complexation with Fe3+) may have large effects on the distribution of Cu between silicate and sulfide magmas. Results also suggest that the extraction of oxide-bonded Cu in mafic magmas by externally derived S may be an important mechanism in the generation of Cu-rich sulfide ores.