- The Nature of Fluids in Hydrothermal Copper and Molybdenum Ore Deposits - An experimental and analytical study (2012)
- The evolution of magmatic-hydrothermal fluids in porphyry Cu and porphyry Mo deposits was studied using synthetic and natural fluid inclusions by optical microscopy, microthermometry, Raman spectroscopy, and LA-ICP-MS. The partitioning of Cu between vapor and brine in aqueous NaCl-S ± KCl ± FeCl2-rich fluids was investigated by means of hydrothermal experiments in rapid quench autoclaves at 600-800°C, 70-130 MPa and at both oxidizing and reducing fO2, covering all geologically relevant conditions. Resulting partition coefficients (DCuvap/brine) are between 0.2 and 0.4 for the range of studied S-concentrations, fluid pH, fO2, and P-T conditions. These values indicate that Cu does not partition into the vapor phase at any plausible condition in contrast to data from natural quartz-hosted vapor and brine inclusions which appear to indicate Cu enrichment in the vapor. The formation of such Cu-rich vapor-type fluid inclusions was investigated in hydrothermal re-equilibration experiments. For this purpose, coexisting vapor and brine inclusions of known composition were re-equilibrated in a fluid of slightly different composition and lower pH than the trapped one at 800°C, 70-130 MPa. This procedure led to a dramatic increase in Cu concentrations in the vapor phase from 0.3 ± 0.03 to 5.7 ± 3.3 wt% after re-equilibration and the change of DCuvap/brine from a true value of 0.4 ± 0.05 to an apparent value of 8.3 ± 4.9. This post-entrapment modification can be traced back to the difference in fluid pH between the trapped and the surrounding fluid, inducing diffusion of H+ out of the inclusion and the diffusion of Cu+ (and Na+, Ag+) into the inclusion in order to maintain charge balance. Moreover, the presence of larger amounts of S within vapor inclusions as compared to brine inclusions can bind larger amounts of Cu. The re-equilibration of trails of vapor and brine inclusions in a natural quartz sample in a fluid similar to the trapped one, yet more acidic, showed that this modification process can be reversed, resulting in the loss of major amounts of Cu from natural vapor inclusions. The composition of metal-bearing melts and magmatic fluids of Mo-mineralized granites resembling porphyry Mo deposits was investigated using natural melt-, fluid-, and solid-inclusions in quartz crystals found in miarolitic cavities from minor Mo-occurrences in Colorado and Norway. Melt inclusions from the Treasure Mountain Dome are highly enriched in Mo (4-43 ppm), also melt inclusions from the Drammen and Glitrevann granites (5-32 ppm, and 12 ppm, respectively), resembling highly fractionated melts. Copper concentrations are low (<1-30 ppm) in the melts, but high in the fluids: intermediate density, supercritical fluid inclusions of these three locations host 6-1900 ppm, 8-3500 ppm, and 5-180 ppm Cu, respectively. The comparison of these results with data from economic porphyry Mo and porphyry Cu (Mo, Au) deposits shows no difference in Mo concentrations in the particular melts and fluids. Hence, other factors probably control the evolution of a granitic intrusion into large, economic Mo deposits, minor Mo occurrences, or just into barren plutons. These may be mainly the size, position, and geometry of the parental magma chamber, the multiplicity of intrusions maintaining a constant flux of metalliferous, S-rich, hot melts, and the extent of fluid focusing within small rock volumes forming high-grade ore shells.