ABSTRACT

The Rochester and Silver Star mining districts in the Highland Mountains of southwestern Montana contain precious metal deposits localized along shear zones in Precambrian basement crystal line rocks. The rocks that host these deposits are of Archean ancestry, originally a part of the Archean Wyoming province, but were subsequently regionally deformed and metamorphosed in Early Proterozoic time, 1.9-1.8 Ga. These rocks define a large, elliptical, northeast-trending, doubly plunging antiform or dome cored with leucocratic, augen-bearing gneiss and migmatite overlain by more than 5 km of feldspathic gneiss and mafic, mylonitic gneiss. The Highland Mountains dome, interpreted to be a gneiss dome similar to those described in orogenic zones elsewhere in the world, formed by the differential vertical rise of a mobile core, accompanied by penetrative metamorphism of the overlying rocks. Anatectic melts, composed mainly of aplite and pegmatite derived from the core, were injected into the overlying rocks; most of these leucocratic granitic rocks are marked by mappable zones of sills and small dikes that were preferentially emplaced several hundred meters above the mobile core. The dome is cut by two sets of faults: (1) northwest-trending high-angle faults that appear to have been active during dome formation, as well as in Laramide time; and (2) west-dipping Sevier-age thrust and reverse faults related to basement-involved thrusting during deformation in the adjacent Cordilleran fold-and-thrust belt. Many of these faults are now filled with non-mineralized quartz veins. Ore-bearing quartz veins in the Rochester and Silver Star districts are located within or adjacent to the zone of granitic sill injection where these rocks have been cut by either the high-angle or thrust faults. Although the granitic sills are not extensively mineralized, hydrothermal fluids migrating upwards along fault and shear zones, appear to have scavenged both precious metals and minor amounts of base metals from this zone and redeposited them in fault-controlled fissures.