The fluid/melt partitioning of Cu, Ag, and Au in felsic upper-crustal magmatic-hydrothermal systems

The understanding of metal extraction from magmas into magmatic-hydrothermal fluids is crucial for the development of models on ore genesis and predictions of metal endowments in porphyry systems. The propensity of a metal to concentrate into either the fluid or melt phase can be expressed using the fluid/melt partition coefficient ( D f / m ). While D Cu f / m has been studied in the past, scattered data exist for D Au f / m and almost no data are available for D Ag f / m . Therefore, experiments were conducted at 800 °C and 175 MPa in René41 alloy cold seal pressure vessels to test the effects of the Cl concentration (0–70 wt% NaCl Eq ) in the fluid phase and aluminum saturation index [ASI = molar Al / Na + K + 2Ca , studied range: 0.85–1.4] of the melt phase on the fluid/melt partitioning of Cu, Ag, and Au. Experimental results were used to create empirical equations describing the partitioning of each metal as functions of these variables. Copper and Ag solubility in the silicate melt increase about sixfold over the range of ASI studied, while Au solubility remains constant at 11.1 ± 1.6 ng/g. All three metals show a strong tendency to form chloride complexes in the fluid phase and are more soluble in fluids associated with metaluminous melts. Values of D Cu f / m , D Ag f / m , and D Au f / m vary within the ranges of ∼50–500, ∼5–400, and 250–3500 respectively, generally increasing with increasing Cl concentration in the fluid, and decreasing (Cu, Ag) or increasing (Au) with increasing melt ASI. A simple magma degassing model suggests that over 60% of the Cu and 80% of the Au in the system are concentrated in the fluid phase by the time rapid fluid mobilization occurs during progressive crystallization of an initially water saturated rhyolite melt body emplaced in the upper crust. Modelling also highlights the importance of having a high initial water concentration in magmas for forming magmatic-hydrothermal ore deposits.