Needle Mountain Mining District, La Plata County, Colorado, USAi

Regional Level Types
Needle Mountain Mining District	Mining District
La Plata County	County
Colorado	State
USA	Country

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 presumably taken from small fissure veins. Although the economic potential of the district has not been adequately explored, some evidence indicates potential for base metal 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. Least altered samples of the coarser porphyry contain sanidine and quartz phenocrysts set in a fine-grained micrographic groundmass of quartz and alkali feldspar. 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 groundmass. Mineralogical, textural, and structural criteria indicate that the stock was 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. A zonal distribution of sericite polytypes indicates a focus of intense alteration on the west side of the body. There, molybdenite is locally concentrated along thin quartz veinlets and is disseminated through some of the intensely altered rocks, and As, Au, Cu, Sb, and Sn occur in anomalous concentrations. These features suggest Mo-Cu resource potential below the surface in the western part of the stock, whereas other evidence indicates potential subsurface metallization in the eastern part of the stock.

In places, fractures in Precambrian rocks have been filled with quartz or quartz-pyrite veins. Veins are controlled mostly by north- or east-trending regional fractures, but also by fractures discordant with these trends. Many veins containing ore shoots of base metals are in a north-trending belt about 2 miles wide and 4 miles long. The northern part of the metallized belt is intersected by an east-trending zone of fractures. 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. Sphalerite appears to decrease relative to galena away from the stock. Fluorite was found around the stock and in the area to the southeast. Anomalous amounts of Ag, As, Au, Ba, Bi, Cd, Mn, Mo, Sb, Sn, and W, in addition to abundant Cu, Pb, and Zn, are associated with the veins. The distribution patterns for several metal anomalies indicate a focus of metallization southeast of the Chicago Basin stock. Evidence suggests that a pluton related to the Chicago Basin stock may underlie the area southeast of the stock.