UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress Cataloging in Publication Data Schmitt, Leonard J. Geology and mineral deposits of the Needle Mountains district, southwestern Colorado. Bibliography: p. 39 Supt. of Docs. no.: I 19.2:1434 1. Mines and mineral resources-Colorado-Needle Mountains. 2. Geology-Colorado-Needle Mountains. I. Raymond, William H., joint author. II. Title. III. Series: United States Geological Survey Bulletin 1434 QE75.B9 no. 1434 [TN24.C6] 557.3'08s [553'.09788′29] 76-608394 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Stock No. 024-001-03029-2 FRONTISPIECE. Eolus Granite intruded by Tertiary stock in Chicago Basin. PLATE 1. Geologic map of the Needle Mountains district, southwestern ...... In pocket FIGURE 1. Index map of southwestern Colorado showing the location of the 2. Drainage map of the southern Needle Mountains V FIGURE Page 3. Map of the Chicago Basin stock showing locations of altered rock samples and their alteration mineral assemblages .... 4. AKF compatibility diagram for older intrusive rocks in the 5. AKF compatibility diagram for rock samples of dikes related to the older intrusive body in the Chicago Basin stock 6. AKF compatibility diagram for samples of younger intrusive rocks from the Chicago Basin stock..... 7. Sketch map of the mineralized area showing interrelations be- TABLES Page 5. Rapid rock analyses of samples from dikes related to the older intrusive body in the Chicago Basin stock 6. Limits of detection by different analytical methods for elements considered geochemically significant and levels of concentra- tion considered anomalous .... 7. Rapid rock analysis of altered sample of the younger intrusive GEOLOGY AND MINERAL DEPOSITS OF THE NEEDLE MOUNTAINS DISTRICT, SOUTHWESTERN COLORADO By L. J. SCHMITT and W. H. RAYMOND 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 presumably taken from small fissure veins. Although the economic potential of the district has not been adequately explored, some evidence indicates 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. 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 quartzsericite-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, molybenite 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 quartzpyrite 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, 1 |