THE PARTITIONING OF COPPER BETWEEN SILICATE MELTS AND 2-PHASE AQUEOUS FLUIDS - AN EXPERIMENTAL INVESTIGATION AT 1-KBAR, 800-DEGREES-C AND 0.5-KBAR, 850-DEGREES-C

Experiments were performed in the three phase system high-silica rhyolite melt + low-salinity aqueous vapor + hydrosaline brine, to investigate the partitioning equilibria for copper in magmatic-hydrothermal systems at 800 degrees C and 1 kbar, and 850 degrees C and 0.5 kbar. D-Cu(aqm/mlt) and apparent equilibrium constants, K-Cu,Na(aqm/mlt), between the aqueous mixture (aqm = quenched vapor + brine) and the silicate melt (mit) are calculated. D-Cu(aqm/mlt) increases with increasing aqueous chloride concentration and is a function of pressure. K-Cu,Na(aqm/mlt) = 215(+/- 73) at 1 kbar and 800 degrees C and K-Cu,Na(aqm/mlt) = 11(+/- 6) at 0.5 kbar and 850 degrees C. Decreasing pressure from 1 to 0.5 kbar lowers K-Cu,Na(aqm/mlt) by a factor of approximately 20. Data revealed no difference in K-Cu,Na(aqm/mlt) or D-Cu(aqm/mlt) as a function of the melt aluminium saturation index. Within the 2-phase field the K-Cu,Na(aqm/mlt) show no variation with total aqueous chloride, indicating that copper-sodium exchange between the vapor, brine and silicate melt is independent of the mass proportion of vapor and brine. Model copper-sodium apparent equilibrium constants for the hydrosaline brine and the silicate melt revealed a negative dependence on pressure. Model apparent equilibrium constants for copper-sodium exchange between the brine and vapor were close to unity at 1 kbar and 800 degrees C.