GOLD-BEARING VEINS IN THE GARNET MINING DISTRICT, GRANITE COUNTY, MONTANA: SYN-TECTONIC MINERALIZATION ALONG THE LEWIS AND CLARK LINE SHEAR ZONE

Five known gold-bearing vein systems in the Garnet mining district of the central-western Montana Rockies follow the fringe of the Late Cretaceous Garnet stock, a satellite pluton of the Boulder batholith. The syn-tectonic Garnet stock intruded the Lewis and Clark line transpressional shear zone at a depth of 6 km. The stock provided gold-bearing hydrothermal fluids, and the Lewis and Clark line provided structural control for emplacement of the stock and deposition of hydrothermal veins. The stock has a cross-section rather like a cocktail glass; a narrow feeder dike in Proterozoic rocks flares upward and outward in the Lower Paleozoic section. A composite down-plunge projection indicates that the veins filled dilational steps perpendicular to the slip vector in a set of imbricated, SW-directed thrusts. The veins occur in both the margin of the stock and in strata of the contact aureole on the NE limb of the Deep Creek anticline, and sub-parallel axial-plane cleavage. The intrusion accentuated the anticline and the magma was confined to the adjacent syncline, which was thermally softened and thinned under the load of the stock. Mineral assemblages in the veins and in the altered wallrocks indicate that the host rocks had cooled to about 300 C when the hydrothermal ore fluids were injected and deposited the gold. Evidently, late-stage hydrothermal fluids had accumulated in the interior of the Garnet stock, and, upon reaching a critical fluid pressure, had hydro-fractured the crystallized margin of the stock and the contact aureole, and then filled the fractures with gold-bearing quartz veins. The veins therefore comprise parts of the kinematic assemblage of the Lewis and Clark line shear zone, a conclusion that should enhance exploration and development of gold deposits in the district.