Copper partitioning between silicate melts and amphibole: Experimental insight into magma evolution leading to porphyry copper ore formation

A series of piston cylinder experiments were conducted to determine the partition coefficient of Cu between amphibole and andesitic to rhyolitic silicate melts. The experiments were run at T = 740-990 degrees C, P = 0.7 GPa, oxygen fugacity (fO(2)) ranging from that of the Ni-NiO buffer (NNO) to 2.3 log units higher (NNO + 2.3 and dissolved water in variable concentrations (under-saturated to saturated conditions). Fixed metal activities were imposed by using Au97Cu3 and Au92Cu8 alloy capsules, which allowed simultaneously determination of Cu solubility. Our data demonstrate that Cu solubilities in both the silicate melt and amphibole decrease with decreasing temperature. The solubility of Cu decreases by a factor of 6 from 990 to 740 degrees C in equilibrium with andesitic to rhyolitic melt compositions. The average amphibole/silicate melt partition coefficient of Cu [D-cu, (amph/melt)] is 0.066 +/- 0.006, and is essentially constant without showing any correlation with silicate melt composition, dissolved water concentration, temperature or fO(2). The low D-cu' (amph/melt) value suggests that amphibole crystallizing at any stage of calc-alkaline magma evolution is unable to scavenge a significant fraction of the initially available Cu from the melt. However, D-cu (amph/ melt) is high enough to yield precisely measurable Cu concentrations in natural amphiboles. As D-cu (amph/melt) results constant along the liquid line of descent of calc-alkaline magmas, amphibole compositions may thus be used as a proxy to monitor the evolution of the Cu concentration in the silicate melt (not the bulk magma if sulfide is present). This may be useful for understanding the metallogenic evolution of intrusive rocks, in which silicate melt inclusions in minerals are generally absent. As most porphyry-type Cu ore deposits are associated with upper crustal intrusions, in-situ microanalysis of inclusion-free amphiboles in such rocks may help understand ore genesis and might also be used in mineral exploration to assess the fertility of prospective magmatic systems. (C) 2016 The Authors. Published by Elsevier B.V.