Syn-sedimentary 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 Mesoproterozoic Earth and its paleo-environmental conditions. The Belt-Purcell Basin is also host to world-class base metal sediment-hosted mineralization produced in a variety of settings from the rift stage of basin evolution through to the subsequent influence of East Kootenay and Grenvillian orogenies. The mineral potential of this basin has not been fully realized yet. New rhenium-osmium (Re-Os) data presented here for chalcopyrite, pyrite and black shale contribute to refine a robust genetic model for the origin of the Black Butte copper ± cobalt ± silver (Cu ± Co ± Ag) deposit hosted by the ca. >1,475 Ma Newland Formation in the Helena Embayment of the Belt-Purcell Basin in Montana, USA. Chalcopyrite Re-Os data yield an isochron age (1,488 ± 34 Ma, unradiogenic initial 187Os/188 Os composition Osi-chalcopyrite = 0.13 ± 0.11) that overlaps with the geological age of the Newland Formation. Further, the Re-Os data of syn-sedimentary to diagenetic massive pyrite yield evidence of resetting with an isochron age (1,358 ± 42 Ma) coincident with the timing of the East Kootenay Orogeny. The unradiogenic Osi-chalcopyrite at ca. 1,488 Ma (0.13 ± 0.11) argues for derivation of Os from a magmatic source with a 187Os/188 Os isotopic composition inherited from the upper mantle in the Mesoproterozoic (Osmantle 1,475 Ma= 0.12 ± 0.02). The unradiogenic Osi-chalcopyrite also suggests limited contamination from a continental crustal source. This source of Os and our new sulfur isotopic signatures of chalcopyrite [–4.1 to +2.1 ‰ - VCDT] implies a dominantly magmatic source for metals. We integrate our new results and previously published geological and geochemical evidence to conceptualize a genetic model in which Cu and metals were largely contributed by moderate-temperature, reduced magmatic-hydrothermal fluids carrying reduced sulfur species with a magmatic origin and flowing as highly metalliferous fluids within the shale sequence. A subsidiary derivation of metals during thermally forced shale diagenesis is possible. Chalcopyrite mineralization replaced locally massive syn-3 sedimentary to diagenetic pyrite units close to the sediment-water interface, i.e., an ideal locus where magmatic-hydrothermal fluids could cool and the solubility of chalcopyrite would fall. We suggest that Cu mineralization was coeval with the timing of an enhanced thermal gradient in the Helena Embayment triggered until ca. 1,455 Ma by tholeiitic dike swarm that intruded into Archean basement rocks and intersected the NE–SW-trending Great Falls Tectonic Zone.