Thermodynamic properties of copper chloride complexes and copper transport in magmatic-hydrothermal solutions

Abstract

The behaviour of copper in hydrothermal waters and brines is poorly known at the pressure–temperature–salinity conditions typical of magmatic-hydrothermal systems, severely limiting our understanding of how much copper can be transported and deposited in such environments. We need to know the identity of relevant copper complexes and have reliable thermodynamic properties for them in order to understand and predict the solubilities of copper-bearing minerals, the partitioning of copper between liquid and vapour and which physico-chemical factors and processes control copper deposition in magmatic-hydrothermal systems. Under saline conditions, copper chloride complexes are likely to be the most important aqueous species of copper and recent experimental studies have shown good agreement for their derived properties up to approximately 300 °C, vapour-saturated pressure and up to approximately 9 m total chloride. There is still a need, however, to have reliable properties for higher temperature and pressure conditions. In this paper we present new equation-of-state parameters and partial molal properties for aqueous Cu(I) chloride complexes (CuCl (aq), CuCl 2 −, CuCl 3 2− and CuCl 4 3−) regressed from experimentally derived log K values derived between 25 and 350 °C and vapour-saturated pressure. The results are used to calculate formation constants for a wide range of temperature and pressure (0–1000 °C and 1–5000 bar). The extrapolation of the properties is tested by calculating chalcopyrite solubilities and comparing them with measured values from Seyfried and Ding (2003) [Seyfried, W.E., Ding, K., 1993. The effect of redox on the relative solubilities of copper and iron in Cl-bearing aqueous fluids at elevated temperatures and pressures: an experimental study with application to subseafloor hydrothermal systems. Geochimca et Cosmochimca Acta 57, 1905–1917]; 400 °C, 500 bars, 0.4–2 m chloride and Hemley et al. (1992) [Hemley, J.J., Cygan, G.L., Fein, J.B., Robinson, G.R., Jr., D'Angelo, W.M., 1992. Hydrothermal ore-forming processes in the light of studies in rock-buffered systems. I, Iron–copper–zinc–lead sulfide solubility relations. Econ. Geol. 87, 1–22]; 300–500 °C, 500–2000 bars, 1 m chloride. There is good agreement with these two experimental datasets, which indicates that our extrapolated thermodynamic properties are reliable at least over these ranges of pressure, temperature and chloride concentration. The new properties are used to calculate chalcopyrite solubilities under similar conditions of two magmatic-hydrothermal copper deposits for which copper concentrations have been measured in individual fluid inclusions: the Starra iron oxide–Au–Cu deposit, Australia; and the Bajo de la Alumbrera porphyry copper deposit, Argentina. In both cases our calculated copper concentrations are consistent with the measured values in inclusions that trapped pre-mineralising and mineralising fluids. More generally, calculated chalcopyrite solubility at different temperature, pressure, pH, chloride concentrations and oxidation states indicates that hypersaline, neutral-weak acidic, and intermediate-reduced brines can transport thousands of ppm copper at 400 °C and above, raising interesting questions about the interpretation and importance of liquid-vapour partitioning of copper. Although cooling is probably the major factor responsible for copper deposition in magmatic-hydrothermal environments, fluid mixing, boiling and fluid–water interaction may be more important in other geological environments, e.g., iron–oxide–copper–gold and epithermal ore deposits.