THE BLACK BUTTE CU-(CO-AG) DEPOSIT, MONTANA: A THERMODYNAMIC MODEL

The Black Butte Cu-(Co-Ag) sediment-hosted deposit occurs in dark, turbidite-rich shale and siltstone with minor dolomite beds exposed in the northern Helena Embayment, Belt Purcell Basin. The deposit consists of two main stratigraphic zones (the lower and upper sulfide zones) that contain very fine-grained, early diagenetic pyrite and barite overprinted by chalcopyrite-rich ore assemblages. Highest Co and Ni concentrations were found in the upper sulfide zone, where reconnaissance QEMSCAN^© analyses of 16 samples suggest cobaltian pyrite, cobaltite, and cattierite are important Co-bearing minerals. These Co-bearing minerals range in size from <30μm (~70%) to around 90μm and occur in discrete layers or as irregular patches. They are almost exclusively associated with pyrite and only subordinately with chalcopyrite and other late stage phases. Nickel occurs in pyrite and in siegenite. Silver occurs as widely disseminated acanthite, which shows no correlation with Co and Ni distribution.

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.