LOW SALINITY FLUID INCLUSIONS IN MAIN STAGE VEINS OF THE BUTTE, MONTANA PORPHYRY COPPER DEPOSIT

Main Stage mineralization in the Butte District consists of large veins, which are zoned on a district scale from an assemblage of covellite, enargite, chalcocite, pyrite and quartz in the central zone, to peripheral veins containing sphalerite, galena, rhodochrosite, and quartz.

Fluid inclusions in five samples were studied-- two in quartz from the central zone and three in quartz and rhodochrosite from the periphery. Only one type of fluid inclusion was present, characterized by 70% liquid plus 30% vapor. No vapor-rich or halite bearing inclusions were observed. Primary and secondary inclusions, distinguished by their location along a quartz or rhodochrosite growth zone or in a trail of inclusions, respectively, were microthermometrically analyzed to determine homogenization temperature and ice melting temperature. In the peripheral veins, both the quartz and rhodochrosite contained good inclusions. Ice melting temperature (Tm) in the inclusions ranged from-1.4 to -2.5C, corresponding to 2.4 to 4.1wt% NaCl equivalent in the aqueous solution. Homogenization temperatures (Th) ranged from 200 to 268C. In the periphery samples, secondary inclusions were observed in one sample along trails.

In the Central zone samples, primary and secondary inclusions were analyzed to yield Tm measurements from -0.4 to 2.5C, (0.7 to 4.2 wt% NaCl equiv.), and Th ranged from 252C to 300C. The homogenization temperatures indicate maximum trapping temperatures in the range of 270C to 330C, assuming a maximum hydrostatic pressure of 600 bars, which corresponds to an estimated maximum burial depth of 6 km, based on fluid inclusions in the earlier porphyry copper stage (Rusk, et al, 1999). The salinities and temperatures are distinctly lower than in the porphyry copper mineralization stage, where Th ranges from 340C to 500C and salinities range from 3 to 40 wt% NaCl equivalent.