Copper Partitioning between Amphibole and Silicate Melts: the Effects of Temperature, Melt Compositions, Oxygen Fugacity and Water Concentrations

Porphyry copper deposits commonly occur in arc-related settings where ore-metals are transported by magmas from the mantle wedge to shallow depths, and subsequently partition into the exsolving volatile phase. The partitioning of Cu between crystallizing silicate, oxide and sulfide minerals, sulfide melts and magmatic volatiles will determine the efficiency of Cu transfer into the magmatic-hydrothermal system. Understanding the Cu partitioning behavior between crystallizing mineral phases and silicate melt during crystallization fractionation is therefore fundamentally important. Among the crystallizing phases, amphibole is stable across a wide pressure (P) - temperature (T) range in hydrous arc magmas. Therefore, if the partition coefficients of Cu between amphibole and silicate melts are well constrained, the measured variation of Cu concentrations in natural amphibole crystals can be used to reconstruct the evolution of the Cu concentration in the silicate melt.

In this study, a series of experiments were conducted by piston cylinder apparatus over a wide range of melt compositions (andesitic to rhyolitic) to determine the amphibole/melt partition coefficient of Cu. The experiments were run at T = 740 - 990 °C, P = 0.7 GPa, and oxygen fugacity (fO₂) between NNO +0.75 and NNO +2. The metal activities were imposed by using Au₉₇Cu₃ and Au₉₂Cu₈ alloy capsules. The apparent Cu solubilities in both the silicate melt and amphibole phases decrease with decreasing temperature. The Cu concentrations in a dacite melt increase approximately by factor of 3 while fO₂ increases from NNO +0.75 to NNO +2. However, the amphibole/melt partition coefficient of Cu remains nearly constant at a value of 0.067 ± 0.013 (1 σ), indicating that the partitioning of Cu is not significantly affected by melt composition, fO₂ and water concentrations. Therefore, determination of Cu concentrations in amphiboles may be a suitable tool to monitor the evolution of the Cu budget of ore-related magma reservoirs during magma evolution in porphyry cooper systems. In addition, our results showed that Cu is always incompatible in amphibole; therefore, occasionally measured high Cu concentrations in amphibole are likely an artifact of the presence of submicroscopic sulfide inclusions.