ORIGIN AND COMPOSITIONS OF FLUIDS THAT FORM PORPHYRY CU (MO-AU) DEPOSITS

Porphyry Cu-(Mo-Au) deposits form above crystallizing magmas in the upper crust. In the ore zones of most porphyry-type deposits quartz-sulfide veins contain abundant high salinity brine and low salinity vapor fluid inclusions derived from unmixing of parental fluids.

In deeper parts of many deposits, quartz rich veins are dominated by moderate salinity inclusions containing 35-70 volume % bubble and commonly a chalcopyrite daughter crystal. These inclusions trapped single phase, parental magmatic fluids at temperatures and pressures above their solvus. We have identified such inclusions in deep quartz-rich veins from a number of porphyry-type deposits including Climax, CO; Butte, MT; Henderson, CO; Mineral Park, AZ; El Salvador, Chile; El Teniente, Chile, Los Pelambres, Chile, and the Mammoth Breccia, AZ. We have analyzed such inclusions in representative samples from the above deposits for their bulk salinity, density, solute chemistry, and He, H, and C isotopes.

Although liquid CO2 is rarely visible in inclusions, evidence for CO2 comes from the formation of clathrates upon fluid cooling. Clathrate melting temperatures, ice melting temperatures, and inclusion bubble sizes indicate that these fluids generally contain <10 mol % CO2 and< 10 wt% NaCl equiv.

Inclusion fluids analyzed by ion chromatography have Na/Cl ratios between 0.9 and 1.5 and Cl/Br ratios between 1400 and 3000. The Cl/Br ratios are near the center of the range of values obtained from other pluton-related ore deposit types, but the Na/Cl ratios are higher and indicate substantial concentrations of other anions including HCO3- and HSO4-.

Inclusions in deep samples with potassic alteration and later pyrite-dominated veins with sericitic alteration have He isotopic ratios as high as 3 R/Ra indicative of a significant component of mantle-derived magmatic fluids. H and C isotopes also suggest a magmatic fluid source.

At Butte, LA-ICP-MS data indicate that chalcopyrite-bearing fluid inclusions contain ~0.5 wt% Cu. The 37.3 million tonnes of Cu production and reserves at Butte require 7.46 billion tonnes of fluid. If 3% of water was extracted from the magma, and all Cu was precipitated, then only 100 km3 of magma containing °Ý150 ppm Cu is required to form the enormous Butte deposits.