Date of Award
Master of Science (MS)
The Black Butte Copper Project is a sedimentary rock hosted, Cu-rich massive sulfide deposit hosted in rocks of the ca. 1.5-1.325 Ga Mesoproterozoic Belt Supergroup. Copper-bearing sulfides, mainly chalcopyrite and tennantite, are hosted in debris flows, black shale, dolostone and sedimentary pyrite layers of the Newland Fm. of the Lower Belt Group in the Helena Embayment, an E-W striking limb of the greater NNW trending Belt Basin. Mineralization is hosted within two separate but contemporaneous deposits, Johnny Lee, which hosts Cu sulfides in an upper and lower sulfide zone within sedimentary pyrite and debris flow conglomerates, and Lowry, where Cu sulfides are hosted in a middle and lower sulfide zone in dolomite dissolution breccias and veins cutting the host Newland Fm. in addition to sedimentary pyrite and debris flows. Mineralization at Black Butte has many characteristics of sedimentary exhalative (sedex) type deposits- massive, laterally extensive pyrite hosted in black shales, accessory silver, formation within a continental rift, evidence of seafloor hydrothermal venting- but, unlike most sedex deposits, Black Butte is a Cu sulfide deposit that lacks Pb and Zn. Hence, the deposit model assigned to the Black Butte Project is a hybrid between sedex, based on the environment of ore formation, and stratabound sedimentary hosted Cu, based on the Cu dominant ore. This research aimed to apply robust and innovative geochemical methods to characterize the conditions of formation of the Lowry deposit. Sulfur stable isotope analysis of chalcopyrite from the Lowry deposit show a range of 34S between +/- 5‰ of zero. When plotted with other S isotopes of pyrite and chalcopyrite from the Black Butte Project, a broad range of S-isotope values in pyrite reflect bacterial reduction of marine sulfate (BSR), while the more restricted range in chalcopyrite can be explained by higher temperature thermochemical sulfate reduction (TSR). Carbonate isotopes from hydrothermal dolomite range from +16 to +22‰ in 18O and 0 to -5‰ in 13C, and are depleted in both 18O and 13C compared to sedimentary carbonate from the Newland Fm. Bitumen occurs in recrystallized dolomite and veins along with quartz, pyrite and chalcopyrite, and has reflectance and Raman spectroscopy values consistent with maximum mean temperatures of 223± 8°C (n=8) with a range of 212.8 – 239.5°C and 247± 40°C (n=7) with a range of 219 – 265°C respectively. Sr isotopes from Lowry and Johnny Lee dolomite reflect the modeled Sr values for Proterozoic seawater. Pb isotopes of Lowry sulfides and dolomite plot within an upper crustal source. These geochemical data, combined with observations of mineralized drill core through hand sample and microscopy, suggest that the Lowry deposit formed after deposition of the lower Newland Fm. at temperatures near or below 250°C. Limestone beds were altered to dolomite during diagenesis creating open space for hydrocarbon migration. High-temperature thermochemical sulfate reduction (TSR) replaced bacterial sulfate reduction (BSR) and altered hydrocarbons to bitumen and porewater sulfate to H2S, creating favorable depositional sites for metalliferous fluids. Fluids were sourced from connate brines circulating in the Helena Embayment of the Mesoproterozoic Belt-Purcell basin that scavenged metals from an underlying crustal source and deposited sulfides and gangue upon encountering favorable facies while migrating along structural pathways.
Allard, John, "Mineralogy, Isotope Geochemistry and Geothermometry of the Lowry Deposit, Black Butte Copper Project, Meagher County, Montana" (2022). Graduate Theses & Non-Theses. 275.