FLUID INCLUSION EVIDENCE FOR EVOLUTION OF HYDROTHERMAL FLUIDS FROM THE PORPHYRY COPPER DEPOSIT IN BUTTE, MT

The physical and chemical evolution of the hydrothermal fluids, that formed the porphyry copper-molybdenum deposit in Butte, Montana has been reconstructed based on geologic relations, petrographic observations, cathodoluminescent textures, and fluid inclusion analyses.

Deep, low salinity (3 wt% NaCl equiv.), CO2-bearing, magmatic fluids exolved from a crystallizing magma and were trapped in the one phase field between 500 and 600 degrees C and at a pressure near 2 kilobars. This copper-rich fluid was the source fluid for mineralization at shallower levels in the deposit, although the veins in which this fluid is trapped contain little copper mineralization. Depressurization at high temperatures, due to pressure fluctuation from lithostatic to hydrostatic, caused boiling which produced a brine containing 29-45 wt% NaCl equiv. and a low salinity vapor. The vapor and brine, produced by boiling, physically separated and both were trapped under a wide range of P-T conditions. Homogenization behavior of halite-bearing fluid inclusions indicates that most were trapped at pressures above the boiling curve for a H2O-NaCl fluid. Vapor-rich and halite-bearing inclusions are most common in chalcopyrite and magnetite veins with potassic or chlorite-sericite alteration.

Later pyritic veins with sericitic alteration formed predominantly from low salinity, CO2-bearing magmatic fluids. These veins formed mostly under hydrostatic pressure near 600 bars, at temperatures around 400 to 450 degrees C. While low salinity fluids were trapped above the solvus, some high salinity magmatic fluids are also trapped in quartz precipitated in pyrite-quartz veins, indicating that boiling occurred. Pyrite-quartz veins also contain many inclusions with intermediate salinities, suggesting that some mixing of magmatic fluid with meteoric fluid is likely.

High grade Main Stage veins, containing covellite, enargite, chalcocite, sphalerite, galena, rhodochrocite, and/or pyrite formed in meter-scale fractures under hydrostatic pressure. Inclusions in these veins contain low salinity fluids with a small amount of CO2 indicating that magmatic and meteoric fluid inputs are both likely. No high salinity inclusions have been identified in Main Stage veins.