Lode Mineralization in the Judith Mountains, Montana

ABSTRACT Lode mineralization in the Judith Mountains ranges from epithermal gold-silver deposits associated with the Gold Hill igneous complex to disseminated gold ± silver in limestone and skarn, gold ± silver + quartz ± calcite veins, base-metal deposits in igneous and carbonate rocks, and disseminated copper ± molybdenum ± precious metals hosted by intrusive rocks. Open-space filling of fissure veins, breccias, and minor stockworks, and replacements of igneous and sedimentary rocks are present, with some deposits characterized by all these modes of deposition. Alkalic to alkali-calcic laccoliths, stocks, dikes, and sills, intimately associated with many lode deposits, mostly have intruded the Madison Group (Mississippian), but strata as young as the Colorado Shale (Cretaceous) are also intruded. Gold replacements and disseminations hosted by solution and tectonic breccias in the upper Madison Group are also present. Metal production from the Judith Mountains is dominated by gold, with estimates ranging from 200,000 oz to as much as 700,000 oz. Silver, lead, copper, and zinc were minor. Most of the gold came from the Spotted Horse, Maginnis, and Cumberland mines that are associated with the Gold Hill igneous complex and the Gilt Edge mine hosted by upper Madison breccias. The lode deposits formed by hydrothermal processes associated with Laramide igneous activity; most ore deposition occurred during or shortly after intrusion. Intrusions structurally prepared host rocks for mineralization by doming, faulting, and fracturing, thus creating passageways for mineralizing solutions. The metals probably were derived mainly from asthenospheric and lithospheric mantle during evolution of the igneous rocks.