Magmatic sulfides and Au:Cu ratios in porphyry deposits: an experimental study of copper and gold partitioning at 850°C, 100 MPa in a haplogranitic melt–pyrrhotite–intermediate solid solution–gold metal assemblage, at gas saturation

Abstract

The behavior of copper and gold in the phase assemblage: pyrrhotite (Po)–intermediate solid solution (Iss)–haplogranitic melt–gold metal (capsule) at 850°C, 100 MPa and gas saturation ( a H 2O ≈0.9) has been investigated. The results are presented as Nernst partition coefficients. The partition coefficient, D Cu Po/Melt=2.6±(0.3)×10 3 (1 σ), was determined for Po and melt in equilibrium with an Iss of composition Cu (19±1)Fe (30±1)S (51±1) (2 σ). For gold, the partition coefficients are: D Au Iss/Melt=5.7±(2.2)×10 3 (1 σ) between Iss and melt, and D Au Po/Melt=140±40 (1 σ) between Po and melt; these imply D Au Iss/Po=42±9 (1 σ) between Iss and Po. The oxygen fugacity for the experiments, log f O 2 =−13.2±0.3 (1 σ), was determined by using the hydrogen sensor technique. The sulfur fugacity, log f S 2 =−1.0±0.7 (1 σ), was calculated from the composition of Po. In our experiments, melts at Iss saturation have Cu/Fe by weight of ca. 1/100. The results indicate that crystallization of Iss may deplete a melt in gold, and crystallization of Po may deplete a melt in copper. However, a melt saturated with Iss (at a given bulk composition, temperature, pressure, oxygen and sulfur fugacity and FeO concentration) will have a fixed concentration of Cu, preventing strong depletion of the melt in Cu until Iss is fractionated from the melt or changes in environmental conditions destabilize Iss. A thermodynamic model for the partitioning of Cu and Au between Po and melt in terms of oxygen and sulfur fugacity indicates that D Cu Po/Melt and D Au Po/Melt may increase greatly with increasing sulfur fugacity and decreasing oxygen fugacity within the Po stability field. The model also suggests that destabilization of both Iss and Po relative to magnetite due to changes in oxygen and sulfur fugacities can result in the release of Cu and Au from the sulfides to the melt.