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OFFICIAL USE ONLY Geology and Mineralogy This document consists of 58 pages, plus 1 figure. Series A UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY THORIUM AND RARE EARTH MINERALS IN THE POWDERHORN DISTRICT, GUNNISON COUNTY, COLORADO* By Jerry C. Olson and Stewart R. Wallace February 1954 Trace Elements Investigations Report 353 This preliminary report is distributed without editorial and technical review for conformity with official standards and nomenclature. It is not for public inspection or quotation. *91This report concerns work done on behalf of the Division of Raw Materi.ls of theU. S, Atomic Energy Commission. OFFICIAL USE ONLY iAL .USNLY 2 USGS - TEI-353 GEOLOGY AND MINERALOGY Distribution (Series A No. of copies Argonne National Laboratory . .e .s 1 Atomic Energy Commission, Wpshington. .0 .0 1 . .e .e .O Division of Raw Materials, Albuquerquei . 1 " -e -o ." Division of Raw Materials, Butte. .O .e .0 1 . ." .s .e Division of Raw Materials, Denver . O e . 1 . ." .o ." Division of Raw Materials, Douglas. .0 . 1 ." .e Division of Raw Materials, Hot Springs. .s .o . .s . 1 .O .0 Division of Raw Materials, Ishpeming. ." 1 Division of Raw Materials, Phoenix. e. 1 Division of Raw Materials, Richfield. 1 Division of Raw Materials, Salt Lake City . 1 Division of Raw Materials, Washington . 3 Exploration Division, Grand Junction..Operations Office. 1 Grand Junction Operations Office. 1 Technical Information Service, Oak Ridge. .0.0.0 6 Tennessee Valley Authority, Wilson Dam. 1 U. S. Geological Survey: Fuels Branch, Washington . 1 Geochemistry and Petrology Branch, Washington 1 . ." Geophysics Branch, Washington . ." 1 . .e Mineral Deposits Branch, Washington . ." 2 . ." E. H. Bailey, Menlo Park. ." 1 . .e A. L. Brokaw, Grand Junction. .e 1 . .e J. h. Cooper, Denver . .. .". 1 N. Denson, Denver. .. ." 1 . ." C. E. Dutton, Madison . .e 1 . ." W. L. Emerick, Plant City . e . 1 . ." L. S. Gardner, Albuquerque. ." 1 M. R. Klepper, Spokane. ." 1 A. H. Koschmann, Denver . .". 2 . .o R. A. Laurence, Knoxville . ." 1 D. M. Lemmon, Washington. .. .". 1 . .e J. D. Love, Laramie... .e 1 . ." P. C. Patton, Denver. .o ." ." ." ." ." ." 2 . J. F. Powers, Salt Lake City. ." .o ." ." ." ." .s 1 Q. D. Singewald, Beltsville . ." ." ." ." .e ." ." . " 1 J. F. Smith, Jr., Denver. ." .o ." .e .s ." .o . ." 1 A. E. Weissenborn, Spokane. ." ." ." ." ." ." ." . .s 1 TEPCO, Denver . ." ." ." ." ." ." ." 2 TEPCO, RPS, Washington, (Including master). 52 3 CONTENTS Page Abstract . .. .0 .0 .0 .0 .0 .0 .0 .0 .0.0.0.".0.0.0 .0.0.0.0.0.0.0. 0 0 :5 Introduction . 0 .0 . .6 Geographic setting. ... .7 Previous work . .9 Field work and acknowledgments. 9 General geology. .. 10 Pre-Cambrian metamorphic complex. 10 Intrusive rocks . 12 Marble of Iron Hill. 13 Sedimentary and volcanic rocks. ... 15 Structure . ,15 Mineral deposits . 16 Thorium deposits.,. * 19 Structural features. * 20 Mineralogy . 21 Relation to alkalic igneous rocks. 23 Age . ........... * 25 Effects of weathering and alteration 26 Distribution of rare earths. .28 . Little Johnnie claims. ... 31 General geology . 0 . 0 . 32 Pre-Cambrian rocks, . 32 Quart z-biotite schist and quartzite* . 32 Metarhyolite . -----.. " 33 Amphibolite .- . - 34 Granite,......... -... * 35 Quartz veins . ... * 35,. Rocks related to the Iron Hill complex. 36 Gabbro . ..... .. -....-.. * 36 Syenite. - - - - - - - - 38 Carbonate. veins. 39 Breccia veins. 40 Sodic amphibole rock . .41 Sedimentary and volcanic rocks. .41 Structure . ...... o 42 Thoriumdeposit . * 43 Outlook for future exploration . ... 0.0 * 46 Literature cited . 54 4 ILLUSTRATIONS Page Figure 1. Index map of Colorado showing location of Powderhorn. district, Gunnison County . .... 8 2. Generalized geologic map of Powderhorn district, Colorado, showing localities examined . 11 3. Geologic map and sections of the Little Johnnie thorium deposit, Gunnison County, Colorado. In envelope TABLES Table 1. Niobium content of 11 samples of veins and carbonate rocks, Powderhorn district. 18 2. Abundance of certain minor elements in spectrographic analyses of mineralized shear zones and carbonate veins . 24 3. Analyses, in percent, of carbonate rock containing rare-earth-bearing apatite. 30 4. Notes on localities showing abnormal radioactivity, Powderhorn district . .0. .9. .. 0.. r THORIUM AND RARE EARTH MINERALS IN THE POWDERHORN DISTRICT GUNNISON COUNTY, COLORADO By Jerry C. Olson and Stewart R. Wallace ABSTRACT Thorium has been found since 1949 in at least 33 deposits in an area 6 miles wide and 20 miles long in the Powderhorn district, Gunnison County, Colo. The district is composed largely of pre-Jurassic metamorphic and igneous rocks, which are chiefly if not entirely pre-Cambrian in age. The metamorphic and igneous rocks are overlain by sandstone of the Morrison formation of Jurassic age, and by volcanic rocks of the Alboroto group and Hinsdale formation of Miocene and Pliocene (?) age, respectively. The thorium deposits occur in or near alkalic igneous rocks in which such elements as titanium, rare earths, barium, strontium, and niobium occur in greater-than-average amounts. The greatest mass of the alkalic igneous rocks -- the Iron Hill composite stock; -- occupies an area of 12 square miles in the southeastern part of the district. The age of the thorium deposits, like that of the alkalic igneous rocks, is not known other than pre-Jurassic. The thorium veins and mineralized shear zones range from a few inches to 18 feet in thickness and from a few feet to 3,500 feet in length. The veins are composed of calcite, dolomite, siderite, ankerite, quartz, barite, pyrite, sphalerite, galena, goethite, apatite, alkali feldspar, and many other minerals. The thorium occurs at least partly in thorite or hydro- thorite. Sparse xenotime has been tentatively identified in one deposit. b Several minerals containing rare earths of the cerium group as major constituents are found in carbonate veins near Iron Hill. Bastnaesite has been identified by X-ray methods, and cerite and synchisite are probably present also. The fluorapatite in some veins and in parts of the carbonate rock mass that occupies 2 square miles in the central part of the Iron Hill complex contains rare earths of the cerium group, generally in amounts of a fraction of a percent of the rock. The radioactivity of the deposits appears to be due almost entirely to thorium and its daughter products. The Th02 content of selected high- grade samples from the Little Johnnie vein is as much as 4 percent. The Th02 content of the veins is generally less than 1 percent, however, and is only 0.05 to 0.1 percent in many of the veins studied. The Little Johnnie vein, which was mapped in detail, can be traced discontinuously for a distance of more than 3,500 feet. The thorium- bearing material occurs as irregular veinlets and thin films introduced into the fault zone. The mineralized shear zone ranges from less than 6 inches to 5 feet in thickness. Near its west end the vein is broken by many faults in a zone that marks the edge of a roughly circular fault block,l1}miles in diameter, that has dropped 1,000 feet or more since the deposition of Miocene volcanic rocks that now floor the Milkranch basin. INTRODUCTION Thorium deposits were discovered in the Powderhorn district in 1949. This district has long been known for its occurrences of alkalic igneous rocks, of which the best known is the Iron Hill complex (Larsen, 1942). 7 Some of these alkalic rocks contain titanium, barium, strontium, rare earths, thorium, and niobium in greater percentages than those of most igneous rocks. A study of the thorium deposits was undertaken to obtain more information on the geologic relation of minor elements, particularly rare earths and thorium, to alkalic igneous rocks, and to evaluate the economic significance of the thorium deposits. Geographic setting The Powderhorn district is an elongate area about 20 miles long and 6 miles wide that trends northwestward from the vicinity of Iron Hill to the Lake Fork of the Gunnison River, in the southwestern part of Gunnison County, Colo. (fig. 1). Powderhorn, a small settlement in the south-central part of the district, is about 30 miles by road southwest of Gunnison, the nearest town. The district is drained by three northward-flowing tributaries of the Gunnison River: Willow Creek, Cebolla Creek, and the Lake Fork of the Gunnison. These streams and their tributaries are deeply incised and in places have cut canyons as much as 1,000 feet deep in the pre-Cambrian rocks which underlie much of the district. The interstream areas consist of dissected mesas that slope gently northward toward the Gunnison River. Many of these are capped by remnants of nearly flat-lying Mesozoic sedi- mentary rocks and Tertiary volcanic rocks. The altitude ranges from 7,500 feet on the Lake Fork of the Gunnison to more than 10,000 feet in the southeastern part of the district. DNr-rTETMENT OF THE INTERIOR TRACE ELEMENTS INVESTIGATIONS UNITED STATES GEOLOGICAL SURVEY REPORT 353 * 107' 109- 1081 106 10S6 In'4. Ifl3* S* 1- 41* N 't R 410 w m W SO J LDE R 400 0 40 I I F DENVER w I GRAND Q O JUNCTIO 390 5.. GUNNISON +tC 0UN POWDKRH RN PUEBLO roe shown oA Figj 2) INSDALE (A 368 OUNTY SAGUAC4E4 COUNTY SI 0 37 I I _____________________________ N [W M I *~I I ~1~I A AJ~IAiA II . QIfLAi/MA FIGURE .- INDEX MAP OF COLORADO SHOWING LOCATION OF POWDERHORN DISTRICT, 50 GUNNISON CgNTY 100 MIle 1 1i 1 GPO 834685 I 9 Previous work The first comprehensive geologic study of the region embracing the Powderhorn district was made by Hunter (1925), who studied the pre- Cambrian rocks of the Gunnison River region in 1911-1913. In part of this work Hunter was associated with E. S. Larsen, Jr., and Whitman Cross (Cross and Larsen, 1935). Larsen also studied the area subsequently and his Professional Paper (Larsen, 1942) on the Iron Hill complex contains a wealth of data on the petrography, mineralogy, and geochemistry of the alkalic rocks.
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