Fluid inclusion insights into the origins of fluids and metals in porphyry copper deposits

Models of the evolution of the hydrothermal systems that form porphyry Cu (Mo-Au) deposits are compromised because aqueous magma-derived fluids in the ore zones of most deposits have changed from their original magmatic compositions as a result of cooling, depressurising, mineral precipitation, brine-vapour unmixing and fluid-rock\ud reactions. However, in deep quartz-rich, sulfide-poor veins from numerous porphyry type deposits, we have identified parental fluids trapped in inclusions at near magmatic\ud temperatures and pressures above the brine-vapour unmixing solvus. We have analysed these inclusions for bulk salinity, density, solute chemistry, helium isotopic ratios and elemental composition, These parental inclusions contain 35 - 70 volume per cent bubble, are low to moderate salinity, contain up to ten mol per cent CO2, and commonly contain a chalcopyrite daughter crystal. Our results indicate that these Cu-rich fluids transport Cu from a plutonic complex below upward into a hydrothermal system, where decompression, cooling, unmixing and water-rock\ud reaction drive ore-mineral precipitation. Na/CI ratios greater than one indicate that in addition to chlorine, sulfur and/or carbonate must play a key role in Cu transportation. Helium isotope ratios indicate that between - 15 and 100 per cent of helium in these fluids is mantle-derived. We suggest that in addition to He, volatiles from mafic magmas in the mantle are also likely to supply CO2 Cu and S to the fluids that form porphyry copper deposits

Ore fluid chemistry in super-giant porphyry copper deposits

Laser ablation inductively Coupled Plasma Mass Spectrometry (ICPMS) analysis of fluid inclusions trapping mineralising solutions in major porphyry-copper deposits indicates that copper concentrations can reach 10_3 - 10_4 ppm in both dilute, CO_2 -bearing fluids and in high salinity brines. Non-C1 transport of Cu is implicated. Au has been determined at 0.2 - 0.4 ppm in the low salinity fluids and may be co-transported with Cu. In contrast, high concentrations of Fe, Mn, Zn and Pb are only attained in brines and correlate strongly with Cl-indicating chloride transport. Physical and/or temporal separation of these fluid types can therefore produce metal fractionation within porphyry systems. Evidence concerning the behaviour of Mo is less clear but brine transport appears to be favoured; phase separation is one mechanism that may account for the fractionation of Mo from Cu and the formation of quartz-molybdenite veins