Fluid inclusion evidence for the formation of main stage polymetallic base-metal veins, Butte, Montana, USA

The Butte porphyry Cu-Mo deposit is cut by the Butte Main Stage, a system of veins that constitute one of the world’s largest Cordilleran-style base-metal lode deposits. The vein system is zoned from a central Cu-rich zone containing covellite, chalcocite, digenite, and enargite to an intermediate zone containing both Cu and Zn sulfides, to a peripheral zone dominated by sphalerite, galena, and rhodochrosite.\ud \ud We examined fluid inclusions in ~50 veins from throughout the lateral and vertical extent of the deposit and conducted microthermometry on 13 of these samples. Fluid inclusions in Main Stage veins are similar in appearance throughout the central, intermediate, and peripheral zones such that only one type of fluid inclusion dominates all samples observed. At room temperature the fluid inclusions are liquid-rich, with 20 volume % bubble (B20 inclusions). Most inclusions analyzed contain between 1 and 4 wt. % NaCl equivalent, and between 0.2 and 1 mol % CO2 . Most inclusions homogenize to liquid between 250°C and 300°C. Even though there is considerable overlap, there is a weak trend from higher to lower homogenization temperatures and salinities from the central zone to the peripheral zone.\ud \ud Vapor-rich inclusions are rare and were identified in only one Main Stage vein, thus, we infer that nearly all inclusions were trapped in the liquid field at pressures\ud above the boiling curve. Maximum estimated depth of formation for Main Stage veins is 6 km. At such pressures, an isochoric temperature adjustment of up to about 50°C is\ud required, indicating that most Main Stage veins formed at temperatures between about 250°C and 350°C. \ud \ud We suggest that Main Stage veins formed where single-phase B60 fluids, which formed pre-Main Stage pyrite-quartz veins with sericitic alteration, decompressed and mixed with meteoric water in a hydrostatic pressure regime. Pre-Main Stage brines were not likely involved in Main Stage vein formation, and the role of pre-Main Stage vapor in the formation of Main Stage veins is not known