The Mesoproterozoic Belt Supergroup, which spans western Montana and central to northern Idaho in the USA and southeastern British Columbia in Canada, where it is known as the Purcell Supergroup, hosts several mineralized systems, including the world-class Coeur d'Alene Ag-Pb-Zn district and the Idaho cobalt belt in the southeastern Lemhi subbasin. In both regions, metamorphism, including the involvement of metamorphic fluids, is suspected to have played an important role in the formation and remobilization of ore deposits. Scapolite, a halogen-bearing mineral common in salty metamorphic rocks and in specific Belt lithologies, preserves a geochemical record of these fluids. We integrated Cl/Br ratios in scapolite with whole-rock geochemistry to reconstruct fluid evolution during metamorphism and evaluate the role of these fluids in mineralization. Our data show that halite components were sequestered in scapolite during metamorphism in the saltiest protoliths of the middle and upper Belt strata in the northern part. During halite formation, residual bittern brines were produced and subsequently infiltrated deeper stratigraphic levels. Such residual bittern brine fluid signatures have previously been identified in mineralized zones of the Coeur d'Alene district. Scapolite from the Lemhi subbasin shows that residual bittern brine fluids dominated ore-bearing lithologies there as well. Cobalt mineralization in the Lemhi region has previously been attributed to interaction between residual bittern brines and magmatic fluids released from a 1349 ± 76 Ma intrusion. A new U Pb titanite age from a scapolite-rich biotitite records metasomatism by non-magmatic, highly Br-rich fluids at 1231 ± 18 Ma, indicating fluid rock interaction that likely (re-) mobilized ore metals. These fluids are interpreted as a “pure” residual bittern brine end-member, compositionally similar to those linked to Cu Co mineralization in the Zambian Copperbelt. Our results show that residual bittern brines played a central role in the metallogenic evolution of the Belt Basin, with minimal evidence for a substantial halogen contribution from halite-dissolution or magmatic fluids. • Chlorine from halite is incorporated into scapolite during metamorphism. • Residual bittern brines formed during evaporation and infiltrated sediments. • No evidence of halite dissolution fluids in metasomatized or mineralized rocks. • Long-lived high-salinity pore-waters enabled base- and precious-metal mobility. • We document the most Br-rich scapolite yet reported in nature.
Rein et al. (Sun,) studied this question.