The porphyry Cu-Mo deposit in Butte, Montana, formed where magmatic hydrothermal fluids, introduced with injections of porphyrytic dikes, fractured and permeated the Butte Quartz Monzonite. These fluids formed a stockwork of quartz and quartz-sulfide veinlets with a variety of styles of potassic and sericitic alteration envelopes. The distribution of vein and alteration types and the distribution of fluid inclusions in these veins record the progressive pressure, temperature, and compositional evolution of the hydrothermal fluids that formed this world-class deposit. Deep drilling and 1,300 m of offset along the Continental fault provide a vertical view of almost 3 km through the Butte deposit. Deep veins within and below the highest Mo grades are quartz dominated with thin K-feldspar or, less commonly, biotitic alteration rims. Fluid inclusions in deep veins trapped a single phase aqueous fluid containing 2 to 5 wt percent NaCl equiv and 2 to 8 mol percent CO2 at temperatures between 575°and 650°C and pressures between 200 and 250 MPa, corresponding to depths between 6 and 9 km. Although Cu grades are low in this region, abundant chalcopyrite daughter minerals in fluid inclusions indicate that the fluids were Cu rich. Fluids that formed these veins transported Cu from the magma below, upward into the region of Cu mineralization with only minor Cu precipitation. Over a kilometer above the bulk of deep quartz and quartz-molybdenite veins, the highest Cu grades are in and around chalcopyrite-bearing quartz-sulfide veins with biotitic alteration (early dark micaceous veins), and their upward, equivalent magnetite-chalcopyrite-pyrite-quartz veins with wide K-feldspar, green sericite, and chlorite alteration (pale-green sericitic veins). These veins contain more evidence for brine-vapor unmixing than any other vein type. The upward progression of early dark micaceous veins to pale-green sericitic veins formed where low salinity, CO2-bearing fluids, similar to those trapped in deep quartz veins, ascended, depressurized, sometimes unmixed, and cooled from ~650°C at 90 MPa to ~475°C at ~50 MPa. As low salinity, CO2-bearing, aqueous fluids, similar in composition to fluids trapped in deep quartz veins, cooled at shallow depths, they formed late pyrite-quartz veins with sericitic alteration. These veins formed from fluid cooling at temperatures between 370°and 450°C at transiently hydrostatic pressures between 40 and 70 MPa, corresponding to depths of 4 to 7 km. Most pyrite-quartz veins formed at pressures and temperatures above the H2O-NaCl-CO2 solvus, but evidence for brine-vapor unmixing is also present. Pyrite-quartz veins formed at progressively greater depths as the hydrothermal system cooled, overprinting much previous mineralization. Late Cu-Pb-Zn-Ag-As-rich Main stage veins formed from dilute fluids containing <3 wt percent NaCl equiv and <2 mol percent CO2. These fluids were trapped between 230°and 400°C under hydrostatic pressures between 20 and 60 MPa and depths of 2 to 6 km. No evidence of boiling is observed ...
