High-temperature carbonate replacement mineralization, metamorphism, deformation, and intrusion in the Bryant District, Beaverhead County, Montana




McGuire, James B.

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The Bryant District, in the southwest Montana fold-and-thrust belt, contains many structurally and lithologically controlled high-temperature carbonate replacement Pb-Zn-Cu-Ag-Au deposits. Laramide crustal shortening prepared Cambrian and Devonian carbonate strata for fluid circulation through thrusting and folding. Thrust-controlled mineralization is present in the Lion Mountain mines, whereas fold-controlled chimney-style mineralization is present in the Cleve-Avon mines. Pb isotopic evidence indicates that base and precious metals in the district were not directly sourced from phases of the adjacent Late Cretaceous Pioneer Batholith. Ore Pb was probably scavenged from the Middle Proterozoic Belt Supergroup that underlies the district. A small pluton, satellitic to the Pioneer Batholith, underlies the Hecla Dome and may have set up a hydrothermal system which produced carbonate replacement as well as Mo skarn mineralization. Fluid inclusions in ore-associated quartz from the Bryant District are CO2-rich and have salinities from 3 to 8 wt% NaCl equivalent with homogenization temperatures from 260 to 330°C. Fluorine-deficient porphyry Mo deposits in Montana and Idaho, most notably Cannivan Gulch 5 km northwest of the Bryant District, have remarkably similar fluid inclusion characteristics, suggesting that the Bryant District carbonate replacement deposits may be the distal portion of a porphyry Mo system. The mineralization in the Bryant District appears to be related to a late satellitic intrusion of the Pioneer Batholith. Such an intrusion was hypothesized (Winchell, 1914; Karlstrom, 1948) and recently proven by exploratory drilling on the Hecla Dome. Field evidence suggests that Pb-Zn-Cu-Ag-Au mineralization took place after most Laramide shortening, intrusion of the bulk of the Pioneer Batholith, regional metamorphism, and contact metamorphism related to the emplacement of an intrusion below the Hecla Dome