INVESTIGATIONS OF ORE-FORMING CONDITIONS AND FLUID SOURCES AT THE BLACK BUTTE COPPER PROJECT, BELT BASIN, MONTANA

The Black Butte Project (aka Sheep Creek) is a sediment-hosted Cu (Co, Ag) deposit at the northern margin of the Helena Embayment of the Mesoproterozoic Belt-Purcell Basin. The deposit is unusual in that it has the geologic setting of a sedimentary-exhalative (SEDEX) deposit but a metal endowment more typical of a sediment-hosted stratiform copper deposit. Black Butte is hosted by the Newland Formation, a black shale/dolostone with interbeds of debris flow conglomerate shed off inferred syn-sedimentary growth faults. Two centers of Cu-(Co-Ag) mineralization 2 km apart have been drilled on the property, termed the Johnny Lee and Lowry deposits. Both deposits contain an upper sulfide zone where chalcopyrite ± tennantite replace semi-massive, fine-grained, sedimentary pyrite along bedding as well as filling debris flow matrices. Textural evidence shows that introduction of copper occurred while the enclosing pyrite-rich sediments were still incompletely lithified. Beneath the upper sulfide zone, the Lowry deposit contains crosscutting veins and dissolution breccias in host dolostone filled with Fe-(Cu) sulfides, gangue minerals (dolomite, quartz) and local bitumen. Despite its classic SEDEX-style depositional environment, the high Cu/(Zn+Pb) ratio of Black Butte has led to speculation that the Cu-rich ore fluids may have been sourced from either 1) Cretaceous or younger magmatic-hydrothermal activity; or 2) the Neihart Formation, a thick, hematite-bearing sandstone that rests at the bottom of the Belt Supergroup. The present study is using ore microscopy, fluid inclusions, and stable isotopes of S and C-O to better understand the formation of the Lowry deposit and its relationship to Johnny Lee, as well as ⁸⁷Sr/⁸⁶Sr and Pb isotopes to constrain the origin of the ore-forming fluids. Preliminary analyses of fluid inclusions in epigenetic quartz, as well as bitumen reflectance measurements, give temperatures of 144-179°C and 212-269°C, respectively, and salinities near 10 wt. % eq. NaCl. A limited number (n = 6) of ⁸⁷Sr/⁸⁶Sr analyses of Sr-rich gangue minerals from the Johnny Lee deposit and host dolomite in the Newland Fm. suggests that the ore fluids were sourced from Mesoproterozoic sea water, not from Mesozoic or Cenozoic magmatic-hydrothermal activity. More ⁸⁷Sr/⁸⁶Sr and Pb isotope analyses are in progress to further discriminate between potential metal and fluid sources, and to test whether the Neihart Fm. could have served as a regional aquifer for migration of brines in the Helena Embayment during opening of the Belt Basin.