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Geology of the Questa Molybdenite Deposit, Taos County, New Mexico William G

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Geology of the Questa Molybdenite Deposit, Taos County, New Mexico William G New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/17 Geology of the Questa molybdenite deposit, Taos County, New Mexico William G. Gustafson, D. G. Bryant, and T. L. Evans, 1966, pp. 51-55 in: Taos-Raton-Spanish Peaks Country (New Mexico and Colorado), Northrop, S. A.; Read, C. B.; [eds.], New Mexico Geological Society 17th Annual Fall Field Conference Guidebook, 128 p. This is one of many related papers that were included in the 1966 NMGS Fall Field Conference Guidebook. Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. These books have set the national standard for geologic guidebooks and are an essential geologic reference for anyone working in or around New Mexico. Free Downloads NMGS has decided to make peer-reviewed papers from our Fall Field Conference guidebooks available for free download. Non-members will have access to guidebook papers two years after publication. Members have access to all papers. This is in keeping with our mission of promoting interest, research, and cooperation regarding geology in New Mexico. However, guidebook sales represent a significant proportion of our operating budget. Therefore, only research papers are available for download. Road logs, mini-papers, maps, stratigraphic charts, and other selected content are available only in the printed guidebooks. Copyright Information Publications of the New Mexico Geological Society, printed and electronic, are protected by the copyright laws of the United States. No material from the NMGS website, or printed and electronic publications, may be reprinted or redistributed without NMGS permission. Contact us for permission to reprint portions of any of our publications. One printed copy of any materials from the NMGS website or our print and electronic publications may be made for individual use without our permission. Teachers and students may make unlimited copies for educational use. Any other use of these materials requires explicit permission. This page is intentionally left blank to maintain order of facing pages. 51 GEOLOGY OF THE QUESTA MOLYBDENITE DEPOSIT, TAOS COUNTY, NEW MEXICO By WILLIAM G. GUSTAFSON, DONALD G. BRYANT, AND THOMAS L. EVANS Molybdenum Corporation of America INTRODUCTION Carpenter (1960, p. 79-86) published a report describing The Questa molybdenite deposit, on the west slope of the events leading to formation of the deposits. Regional the Taos Range in northern New Mexico, is in Sulfur reconnaissance mapping was done by McKinlay (1956, 31 Gulch, a side canyon of the Red River, at altitudes rang- p.; 1957, 23 p.). ing from 7,900 to 10,000 feet. The Taos Range is part of the Sangre de Cristo Mountains, a north-trending uplift PRECAMBRIAN ROCKS near the southern end of the Rocky Mountains. The range Precambrian granite and granite gneiss crop out on the is underlain by Precambrian metamorphic rocks and a north side of Red River Canyon in the vicinity of the Tertiary complex of volcanics and intrusives. Questa deposit. The rocks are gray, medium to coarse Open-pit mining was started in 1964 and the first ore grained, and contain quartz, orthoclase with minor biotite, was sent to the present mill at the start of 1966. The cur- and have foliated and non-foliated facies. The Precam- rent milling rate is in excess of 10,000 tpd. brian granite is distinguishable from Tertiary granite by being gray rather than pink, and has coarser texture. PREVIOUS INVESTIGATIONS Scattered remnants of conglomerate, sandstone, and silt- Early investigations of the Questa molybdenite deposit stone which occur directly above the granite and gneiss were made by Larsen and Ross (1920, p. 567-573), and arc included with the Precambrian of Figure 1. The age of Vanderwilt (1938, p. 599-643). These, and Schilling's these sediments is uncertain but is presumed to be either (1956, 87 p.) early work, describe vein deposits mined by late Paleozoic or early Tertiary. underground methods. More recently, Schilling (1965, p. 26-34) described the stockwork deposit now being mined. TERTIARY ROCKS ANDESITE (TA) QUESTA MINE Andesite porphyry and andesite breccia are the oldest Questa volcanic rocks in the area. Both andesites are characterized by phenocrysts of andesine, but the composition of the Toos matrix is variable. The breccias are cemented by hematite, and contain epidote, apatite, and alunite in the matrix. The Santa Fe groundmass of porphyritic andesite consists of either fine- .4( grained orthoclase and biotite, or glass fragments. The mafic content of andesites is remarkably low and consists Albuquerque of scattered phenocrysts of biotite. Andcsites low in hema- tite or hydrothermal chlorite are often light gray in con- trast to darker colors usually associated with andesites. RHYOLITE (TR) Socorro Overlying the andesite is a sequence of rhyolite porphyry, tuff, and tuff breccia, which caps most of the ridges be- tween the pit area and the west edge of the mountains. • Roswell The rhyolite breccia, on the ridge west of the pit, consists of rhyolite porphyry fragments in a matrix of glass, and fine-grained quartz and orthoclase which is strongly flow- banded. •Corlsbad Farther west, in the area around Goat Hill (Fig. 1), the rhyolites are tuffs and tuff breccias which are characteristi- cally fragmental, without flow-banding, and have a high clay-sericite content. North and west of Goat Hill the rhyolites are flow- banded, dense welded tuffs containing abundant quartz Index Map of Questa Molybdenite Deposit. phenocrysts. 52 NEW MEXICO GEOLOGICAL SOCIETY—SEVENTEENTH FIELD CONFERENCE EXPLANATION CC CC w Alluvium 7 Surf/c/a, doposits young•r than YO/Conic.. a Tap Quartz Porphyry Granite Gronlf• Porphyry V 713 Rhyolit• cr•0 •••••••.. 7_ sto0?, Ands•ite 4000 9 z -4100 ■•■••, U Precambrian Undiff ttttt kited A 10000 10000- - 9500- Contact,.dashrld whore -9000 position is approximate. 9000- - 8500 8500- Fa ult, do•h•d wh•r• position is approximot•. - 8000 8000 - 7500 7500- Roods 7000 7000 500 0 5001000 2000 N FIGURE 1 Geology of the Questa Molybdenite Dcposit. GRANITE PORPHYRY ( TGP ) westward-plunging extension of the stock. Available infor- mation indicates that dips are steep along the north and A granite porphyry plug at the west edge of the pit un- south contacts. derlies the ridge between Sulfur and Iron Gulches. The The Sulfur Gulch stock is a mafic-deficient granite with plug is nearly oval in plan and is about 600 feet in diameter. aplitic texture, and is usually referred to as "the aplite." The granite porphyry is composed of quartz, orthoclase, It consists of interlocking grains of quartz, orthoclase, and and biotite phenocrysts up to 1/2 inch in diameter, set in albite in nearly equal proportions, with phenocrysts of bio- a groundmass of quartz, orthoclase, and sericite. Hydro- tite and albite. Biotite constitutes less than one percent thermal alteration is pervasive and feldspars are altered to of the rock except in areas of molybdenum mineralization. quartz and sericite. Previous sites of biotite are indicated Decreases in the amount of albite in the matrix are fre- by residual concentrations of quartz, sericite, and rutile. quently compensated by an increase in albite phenocrysts. The entire plug is cut by numerous quartz veinlets, and In the ore body, the intrusive has been subjected to re- shows pervasive silicification. crystallization which has destroyed the original texture. There has also been a reduction of albite, development of GRANITE (TG) myrmekite and quartz veinlets, and enlargement of quartz Several Tertiary granitic stocks are present in the Taos grains. Range, but this paper is concerned with only one, the Sul- QUARTZ PORPHYRY (TQp ) fur Gulch stock (Fig. 1). The stock intrudes Precambrian granite and granite A plug and numerous dikes are designated as quartz gneiss, and Tertiary volcanics. The Questa molybdenite de- porphyry on Figure 1. These rocks consist of phenocrysts posit is localized along the granite-andesite contact of the of quartz and feldspar, Vs inch to 1/4 inch in diameter, set C) 0 C cr) C) tri zr=1 x 1-71 t=1 C") z 7:1 tri X FIGURE 2 The Questa Open-Pit Mine, Spring of 1966 (Photograph by Harold L. James; symbols are those used in Figure 1) 54 NEW MEXICO GEOLOGICAL SOCIETY SEVENTEENTH FIELD CONFERENCE in a finer grained silicic groundmass. The quartz porphyry of mineralized fractures have been recognized. Sheeting plug (Fig. 1) is exposed at the top of Goat Hill. Numerous parallel to the surface of the stock is usually prominent but dikes extend radially outward from the plug. The remain- not preferentially mineralized. The remaining fractures are der occur along the ridge between the pit and Goat Hill. short, discontinuous, without preferential orientation, and Considerable variation in groundmass textures, not appar- exhibit little or no offset. Veined fractures range from hair- ent in hand specimens, exists between these various quartz line cracks to fissures two feet wide. In places, the veins and porphyry intrusions. Several have a relatively coarse- veinlets are close enough and contain sufficient ore-grade grained aplitic groundmass, while the groundmass textures mineralization to class the entire mass, wallrock and veins of others is submicroscopic. Compositional variation is also together, as ore, which justifies classification of the deposit significant. While most of the porphyries are granitic, a as a stockwork. few are, in fact, quartz monzonite porphyry. Continued investigation will, it is hoped, provide a basis for a more ALTERATION AND MINERALIZATION suitable classification of these rocks. Seventeen hydrothermal minerals, containing a mini- INTRUSIVE BRECCIA mum of fourteen introduced elements, have been identified Breccia dikes 5 to 20 feet wide intrude aplite on High- in the molybdenite deposit.
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