Synsedimentary to Diagenetic Cu±Co Mineralization in Mesoproterozoic Pyritic Shale Driven by Magmatic-Hydrothermal Activity on the Edge of the Great Falls Tectonic Zone–Black Butte, Helena Embayment, Belt-Purcell Basin, USA: Evidence from Sulfide Re-Os Isotope Geochemistry

Abstract The ca. 1,500 to 1,325 Ma Mesoproterozoic Belt-Purcell Basin is an exceptionally preserved archive of Earth and its paleoenvironmental conditions. also host world-class base metal sediment-hosted mineralization produced in a variety settings from the rift stage basin evolution through subsequent influence East Kootenay Grenvillian orogenies. mineral potential this has not been fully realized yet. New rhenium-osmium (Re-Os) data presented here for chalcopyrite, pyrite, black shale contribute refine robust genetic model origin Black Butte copper±cobalt±silver (Cu±Co±Ag) deposit hosted by >1,475 Newland Formation Helena Embayment Montana, USA. Chalcopyrite Re-Os yield isochron age (1,488±34 Ma, unradiogenic initial 187Os/188Os composition Osi-chalcopyrite=0.13±0.11) that overlaps with geological Formation. Further, synsedimentary diagenetic massive pyrite evidence resetting (1,358±42 Ma) coincident timing orogeny. Osi-chalcopyrite at 1,488 (0.13±0.11) argues derivation Os magmatic source isotopic inherited upper mantle (Osmantle 1,475 Ma=0.12±0.02). suggests limited contamination continental crustal source. This our new sulfur signatures chalcopyrite (–4.1 +2.1‰-VCDT) implies dominantly metals. We integrate results previously published geochemical conceptualize which Cu metals were largely contributed moderate-temperature, reduced magmatic-hydrothermal fluids carrying species flowing as highly metalliferous within sequence. A subsidiary during thermally forced diagenesis possible. replaced locally units close sediment-water interface, i.e., ideal locus where could cool solubility would fall. suggest was coeval enhanced thermal gradient triggered until 1,455 tholeiitic dike swarm intruded into Archean basement rocks intersected NE-SW-trending Great Falls Tectonic Zone.