FLUID INCLUSIONS IN THE BUTTE, MONTANA MAIN STAGE VEINS: RECONSTRUCTING FLUID ORIGIN AND FATE

The Butte District consists of a large porphyry copper deposit cut by a system of Main Stage veins that are 1-3 kilometers long and up to several meters wide. The Main Stage veins are zoned with copper sulfides in the central zone, Mn-Zn-Pb minerals in the periphery, and a copper-zinc sulfide transition zone between. Measurements on fluid inclusions in quartz and rhodochrosite from Main Stage veins help constrain the temperature, pressure, and chemical conditions that prevailed during ore formation, and illuminate the relationship between the porphyry copper stage fluids and Main Stage fluids. We measured 77 fluid inclusions from the central copper zone and 73 from the peripheral (Mn-Zn) zone; 90% were in quartz. Fluid inclusions in Main Stage veins are liquid-dominated (5-20 vol. % vapor) and 95% homogenize between 225 and 294^oC. Homogenization temperatures corrected for a maximum estimated pressure of 70 MPa yield trapping temperatures of 265-350^oC. Ice melting temperatures show differences in fluid salinity across the district, and appear to vary independently from homogenization temperature. Ice melts between -0.4 and -4.7^oC, with a mean of -2.1^oC. Rupturing fluid inclusions by crushing under oil reveals substantial non-condensable gas in the vapor phase, at a pressure several times greater than atmospheric. Raman spectroscopic measurements of fluid inclusions from the older porphyry copper system show that the dominant non-condensable gas is CO₂. Dissolved CO₂ must be considered when calculating salinity (total dissolved solids) from melting point data, because it substantially lowers the ice melting temperature.

We calculated the CO₂ melting point depression correction by using estimates of CO₂ concentration and applied them to the experimental ice melting temperatures. The lack of observed CO₂-H₂O clathrates upon inclusion cooling suggest that the maximum CO₂ concentration is 0.85 mol% in the Butte Main Stage fluids. Corrected ice melting temperatures greater than 0^oC indicate that many inclusions contain less than 0.85 mol% CO₂. Fluid salinities corrected for 0.20 mol% CO₂ are between 0.05 and 6.92 wt % NaCl equivalent. The maximum calculated fluid salinity with corrections for 0.85 mol% CO₂ is 5.14 wt. % NaCl equivalent.