STABLE ISOTOPE (S, C, O) STUDY OF PYRITE, CHALCOPYRITE, BARITE, AND DOLOMITE FROM THE BLACK BUTTE CU-(CO-AG) DEPOSIT, MONTANA

The Black Butte Cu-(Co-Ag) deposit (formerly Sheep Creek) is comprised of chalcopyrite-dominated sulfide assemblages that mineralized pyritic stratigraphy within the turbidite and debris flow-bearing Mesoproterozoic Newland Formation exposed in the northern Helena Embayment, Belt Purcell Basin. In the area studied, an up to 21-m-thick lower fine-grained pyrite (PY1) horizon and overlying strata was overprinted by coarse crystalline “hydrothermal” dolomite and coarse pyrite (PY2). A thicker (≤ 100m) upper horizon contains abundant barite and less hydrothermal dolomite. In both zones, paragenetically late chalcopyrite ± quartz replaces fine-grained pyrite, barite, and carbonate.

The δ³⁴S values of sulfides from progressive paragenetic stages include -12.0 to +10.8‰ relative to V-CDT (median of ~-1‰) for PY1, a broader range of -8 to +30‰ with mostly positive values for PY2, and -7.8 to +6.6‰ (median ~-2) for ore stage chalcopyrite. Stratiform barite intergrown with fine-grained pyrite and discordant barite from the upper sulfide zone has δ³⁴S of 15.7 to 24.7‰ and δ¹⁸O of 11.3 to 16.4‰ relative to V-SMOW. Hydrothermal dolomite associated with PY2 in the lower zone has δ¹³C ranges from -6.2 to -0.8‰ relative to V-PDB with δ¹⁸O mostly 20 ± 2‰.

Textural observations and isotopic data are consistent with PY1 formation by biogenic sulfate reduction of seawater sulfate under relatively open conditions. Large isotopic enrichments and a decrease in δ¹⁸O at higher δ³⁴S in coeval barite characterize isotopic patterns similar to those in modern seafloor cold seep barite deposits and diagenetic settings where sulfate is reduced by anaerobic oxidation of methane. Isotopic values of PY2 record continued biogenic sulfate or thermochemical sulfate reduction (TSR) under relatively closed conditions. Isotopic values of hydrothermal dolomite reflect mixing of sedimentary formation waters and low δ¹³C hydrothermal fluids derived from oxidation of mobilized organic carbon within basement strata at elevated temperatures. Finally, isotopically light chalcopyrite ± quartz replaced isotopically light PY1 and barite, and carbonate as a consequence of TSR at elevated temperatures. Thus, Black Butte represents a pyrite ± barite accumulation that was overprinted by higher-temperature Cu-bearing hydrothermal fluids.