Aqueous fluid inclusions in the Butte, Montana Main Stage vein system

The Butte District consists of a porphyry copper deposit cut by a system of Main Stage veins that are 1--3 kilometers long and typically 0.5--5 meters wide. Main Stage veins are zoned from copper sulfides in the Central Zone, outward to chalcopyrite sphalerite veins, then the Peripheral Zone of sphalerite, galena and rhodochrosite. Microthermometry on aqueous fluid inclusions in Main Stage quartz, rhodochrosite, sphalerite and fluorite reveals that the primary and secondary Main Stage fluid inclusions contain 5--30 volume percent vapor and homogenize between 194 and 346°C. Homogenization temperatures corrected for an estimated pressure of 70 MPa yield trapping temperatures of 240--425°C, with a mean of 323°C. Fluid inclusion ice melts between 0.0 and -4.7°C, with a mean of -2.1°C. Calculated fluid salinities corrected for a CO2 molality of 0.20 yield an average of 2.89 weight percent NaCl equivalent. No homogenization temperature or salinity gradients are apparent within the District.