Geological Society of America

T11 Vapors, Brines, Sulfides, and Mines: Understanding Metal Mobility in Magma-Hydrothermal Systems and Their Supergene Successors

Sunday, 31 October 2010
8:00 AM - 12:00 PM
Room 601

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

Paper number: T11-12
Program: Technical Sessions
Time: 10:55 AM – 11:10 AM

Authors

Brian Tattitch Presenting Author
University of Bristol, Bristol, N/A
Philip M. Candela
University of Maryland, College Park, College Park, MD
Philip Piccoli
University of Maryland, College Park, College Park, MD
Robert J. Bodnar
Virginia Polytechnic Institute and State University, Blacksburg, VA
L. Fedele
Virginia Tech, Blacksburg, VA

Abstract

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.