The Effect of CO2 on Copper Partitioning in Sulfur- Free and Sulfur-Bearing Felsic Melt-Vapor-Brine Assemblages
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Abstract
Analysis of fluid inclusions from porphyry copper deposits (PCD) reveals that magmatic vapor and/or brine are vital for the removal of copper from arc magmas and its transport to the site of ore formation. Experiments in melt-vapor-brine systems allow for investigating the partitioning of copper between silicate melts and volatile phases under magmatic conditions. The presence of CO2 affects both the pressure of vapor saturation and the composition of exsolving volatile phases. However, PCD are primarily sulfide ore deposits, and the role of sulfur must also be examined as part of magmatic-hydrothermal experiments. Therefore, the partitioning of copper in CO2 ± S-bearing experiments was examined in an attempt to provide insights into copper partitioning and the generation of PCD.
I present the results from experiments performed at 800 °C and 100 MPa in CO<sub>2</sub>-bearing melt-vapor-brine systems with X<sub>CO2</sub> = 0.10 and 0.38. The compositions of vapor and brine inclusions and run-product glasses were used to determine the compositions of the magmatic phases. The partitioning of copper between brine and vapor (D<sub>Cu</sub> <super>b/v</super> ±2σ) increases from 25(±6) to 100 (±30) for sulfur-free experiments and increases from 11(±3) to 95(±23) for sulfur-bearing experiments as X<sub>CO2</sub> is increased from 0.10 to 0.38. The partitioning of copper between vapor and melt (D<sub>Cu</sub> <super>v/m</super> ±2σ) decreases from 9.6(±3.3) (sulfur-free, HCl-bearing), 18(±8) (sulfur-bearing, HCl-free), and 30(±11) (sulfur-bearing, HCl-bearing) at X<sub>CO2</sub> = 0.10, to 2(±0.8)(HCl-free) at X<sub>CO2</sub> = 0.38, sulfur-free or sulfur-bearing. These data demonstrate that copper partitioning in sulfur-free, CO<sub>2</sub>-bearing systems is controlled by the changes in the salinity of the vapor and brine corresponding to changes in X<sub>CO2</sub>. Sulfur-bearing experiments demonstrate that magmatic vapors are enriched in copper in the presence of sulfur at low X<sub>CO2</sub>. However, the enrichment of copper in the magmatic vapor is suppressed for sulfur-bearing systems at high X<sub>CO2</sub>.
The MVPart model presented by Candela and Piccoli (1998) was modified to incorporate CO<sub>2</sub> to predict trends in efficiency of removal of copper into exsolving CO<sub>2</sub>-bearing magmatic volatile phases. The CO<sub>2</sub>-MVPart model predicts two to three times lower efficiency for CO<sub>2</sub>-rich (X<sub>CO2</sub> = 0.38) magmatic volatile phases compared to low-CO<sub>2</sub> (X<sub>CO2</sub> ≤ 0.10) systems.