Platinum-gold-copper mineralization, central Medicine Bow Mountains, Wyoming

Abstract

A diversified assemblage of platinum-palladium, gold, and base-metal deposits occurs within the Precambrian igneous- metamorphic terrane of the central Medicine Bow Mountains, Wyoming. The mineralized localities investigated lie in a 125-square-mile area adjacent to and structurally disrupted by the Mullen Creek-Nash Fork shear zone, a major Precambrian tectonic discontinuity that splits the Medicine Bow Mountains into two distinct geologic provinces. Investigations of the lithologic and structural associations of six-teen mined deposits and five mineralized prospects and studies of the mineralogical and chemical characteristics of the ores demonstrate that three principal types of deposits are present: 1) an association of platinum-group elements with hydrothermal copper ores in sheared metagabbroic rocks; 2) numerous gold- and copper-bearing quartz-carbonate veins and less commonly lead- and silver-bearing quartz-carbonate veins associated principally with hydrothermally altered mafic rocks; and 3) a highly metamorphosed body of massive and disseminated zinc, copper and lead sulfides in amphibolites and calc-silicate rocks. structural, mineralogical, and chemical data of country rocks and hydrothermally altered wallrocks indicate that Pt-Pd-Cu and Au-Cu deposits have formed from fluids of meta- morphic origin. Solutions permeating weakly to strongly cataclastic rocks throughout a six to eight mile wide belt south of the Mullen Creek-Nash Fork shear zone have induced pervasive retrograde metamorphic effects. Metamorphic solutions laden with metals, silica, and other dissolved species leached from local country rocks are believed to have diffused into relatively low pressure dilatant zones in shears and fault intersections and there deposited metalliferous veins and sulfide replacement bodies. Mineralogical and textural characteristics of the Zn-Cu- Pb sulfide body and its lithologic associations point to an origin as a metamorphosed syngenetic exhalative sedimentary massive sulfide deposit.