Geology and mineral deposits of the Needle Mountains District, Southwestern Colorado

Abstract

The Needle Mountains district is set in highly fractured Precambrian granitic rocks that have intruded and metamorphosed older Precambrian metasedimentary rocks. The Precambrian rocks are overlain by outliers of Paleozoic strata and intruded by an upper Tertiary stock. Past mineral production reportedly was limited to silver and gold ores. Some evidence indicates economic potential for basemetal deposits. Mineral deposits are spatially related to a conspicuously altered composite stock, about 2,600 feet wide and 3,500 feet long. The stock consists mainly of an older porphyry body intruded by a younger porphyry body. The older body is composed of pervasively altered granite porphyry or quartz porphyry and related brecciated rocks. A coarser grained core is gradational with a finer grained, flow-structured outer phase. The younger body is composed of variably altered rhyolite porphyry that contains sanidine, plagioclase, quartz, biotite, and sphene phenocrysts set in an aphanitic to microgranular, flow-structured goundmass. The stock was evidently emplaced at a shallow depth. The older body in the stock has been altered to quartz-sericite-pyrite or quartz-sericite-kaolinite-pyrite assemblages in nearly all exposed rocks. Features suggest Mo-Cu resource potential below the surface in the western part of the stock. In places, fractures in Precambrian rocks have been filledmore » with quartz or quartz-pyrite veins. The area of intersecting structural zones includes the Chicago Basin stock and a concentration of metallized veins. Dump rock from mine workings along veins contains significant quantities of sphalerite, galena, and chalcopyrite, and lesser tetrahedrite in a gangue of quartz pyrite, rhodochrosite, and fluorite. 9 figures, 7 tables.« less