FLUID INCLUSION EVIDENCE FOR MAGMATIC FLUID EVOLUTION IN THE PORPHYRY COPPER DEPOSIT; BUTTE, MONTANA

The porphyry copper deposit in Butte, Montana formed where quartz porphyry dikes injected into the Butte Quartz Monzonite. Magmatic fluids injected during and after dike emplacement formed veins of quartz, pyrite, chalcopyrite, molybdenite and magnetite in fractures as fluids cooled, depressurized, and reacted with wall rocks. Hydrothermal biotitic breccias surrounding the quartz porphyry dikes, contain broken fragments of deep quartz veins. The dikes and early veins are cut by later quartz-sulfide veins indicating several pulses of fluid injection. Deep quartz veins contain >95% quartz, 0-5% molybdenite, anhydrite, and minor chalcopyrite, bodered by alteration envelopes of hydrothermal Kfeldspar, biotite, and plagioclase. Sulfur isotopes and fluid inclusions indicate that these veins precipitated between 500 and 600 deg C and 1.7 to 2.2 kbars. Although little chalcopyrite, pyrite, galena, or sphalerite are found in these veins, LAICPMS analysis of fluid inclusions suggest high concentrations of Cu, Fe, Pb, and Zn. Shallower veins of quartz, chalcopyrite, pyrite, and magnetite, with alteration envelopes of sericite, chlorite, Kfeldspar and calcite (pale green sericitic alteration), contain halite-bearing fluid inclusions that homogenize by halite dissolution between 270 and 320 deg C (35-39 wt % NaCl). These veins also contain many inclusions that homogenize to vapor between 400 and 500 deg C. Halite-bearing inclusions and vapor-rich inclusions are not observed along the same healed fracture. Differences in homogenization behavior between the two types of inclusions preclude trapping of two immiscible fluids at or near the conditions of unmixing, unless changes in inclusion size or composition have occurred since trapping. LAICPMS analyses indicate that both the vapor and the brine inclusions are rich in metals including Cu, Pb, Zn, Mn, As, and Fe. Geochemical modeling suggests that precipitation of chalcopyrite, pyrite, magnetite, and quartz veins with pale green sericitic alteration would occur upon cooling to 450 or 500 deg C fluids similar to those trapped in early barren quartz veins. Further cooling, to temperatures of 300 degrees or less, accompanied by wall rock reaction would lead to the precipitation of sphalerite, galena and minor chalcopyrite accompanied by propylitic alteration.