THE BLACK BUTTE CU-(CO-AG) DEPOSIT, MONTANA: A THERMODYNAMIC MODEL
Thermodynamic modeling accounting for mineralogy, paragenesis, and replacement textures indicate that mineralizing fluids at Black Butte were reduced, low pH, and saline, similar to mineralizing fluids proposed for other deposits in the main Belt-Purcell Basin (e.g., Sullivan). Initial reduced Ba-, Co-, Fe-rich (Cu-poor) ascending fluid migrated upward along syn-sedimentary faults into shallow sediments. Interaction of these fluids with biologically reduced sulfur and seawater led to the deposition of fine-grained, variably Co-Ni-rich pyrite and barite under intermediate oxidation state conditions. The later Cu-depositing hydrothermal fluids had similar characteristics to that responsible for early diagenetic pyrite deposition, but were necessarily hotter (>200°C) to transport sufficient Cu for main stage mineralization. Modeling shows that main stage ore minerals were precipitated due to fluid/rock interaction at the site of deposition and a concomitant increase in fluid oxidation state (fluid buffered by barite dissolution), increase in pH (fluid buffered by carbonate dissolution), and fluid cooling (suggested by quartz deposition), resulting in pyrite, barite, and carbonate replacement within the host strata.