THE EFFECT OF CO2 ON COPPER PARTITIONING IN A FELSIC MELT-VAPOR-BRINE ASSEMBLAGE

Analysis of fluid inclusions from porphyry copper deposits (PCD) reveals that magmatic vapor and/or brine are vital for the extraction of Cu from the melt and transport to the site of mineral precipitation. Experiments in melt-vapor-brine systems allow us to investigate the partitioning of copper between silicate melts and vapor and/or brine volatile phases under magmatic conditions. In many deposits fluid inclusions contain small amounts of CO₂, but with a much smaller concentration range than arc systems in general. To examine the effect of CO₂ on Cu partitioning and generation of PCDs, we have incorporated CO₂ into a magmatic hydrothermal assemblage similar to CO₂-free experiments described in the literature. Experiments were performed at 800^oC and 100 MPa in a system comprising a synthetic silicate melt (Starting composition: 100 MPa granite minimum melt) and coexisiting vapor and brine (NaCl-KCl-FeCl₂-HCl-CuCl-H₂O-CO₂), with bulk CO₂ contents of 10 mol% and 35 mol%.

The compositions of vapor and brine inclusions and run-product glasses were determined by microthermometry and LA-ICPMS. These results were used to calculate Nernst-type partition coefficients for Cu (D_Cu) and apparent equilibrium constants for Cu/Na exchange (K_Cu,Na) among the three phases melt, vapor, and brine. Calculated partition coefficients and equilibrium constants (±2s) are (10 mol% CO₂) D^b/v_Cu= 21 ± 7, D^b/m_Cu= 190 ± 60, D^v/m_Cu= 9.2 ± 4.2 and K^b/v_Cu,Na= 1.1 ± 0.3, K^b/m_Cu,Na= 27 ± 8; (35 mol% CO₂) D^b/v_Cu= 100 ± 40, D^b/m_Cu= 170 ± 50, D^v/m_Cu=1.7 ± 0.8 and K^b/v_Cu,Na= 1.2 ± 0.4, K^b/m_Cu,Na= 30 ± 10. The observed increase in the Cu concentration of the brine relative to the vapor correlates with the increase in salinity of the brine (70wt% to 78wt% NaCl_eq) and decrease in the salinity of the vapor (3wt% to 1wt% NaCl_eq) as the CO₂ content increases from 10 mol% to 35 mol%. This correlation suggests that the primary impact of increasing CO₂ concentration on volatile Cu partitioning is a shift in Cl-complexed Cu, correlated with salinity, towards more enriched brines, less enriched vapors and a vapor dominated system with low metal extraction potential. These data also provide the baseline for investigating the impact of interaction between CO₂ and other potential complexing ligands, such as H₂S, on Cu partitioning.