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• University of Nevada Reno Geology and Uranium Content of kiddle Tertiary Ash-Flow Tuffs in the Southern Nightingale Mountains and Northern Truckee Range, Washoe County, Nevada A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology by Julia Anne Benham ih July 1982 1 ■•its LIMRAnr The thesis of Julia Anne Benham is approved: Tkesi ' to /68 >% University of Nevada Reno July 1982 ii ACKNOWLEDGMENTS Mr. Harold F. Bonham, Jr. (Nevada Bureau of Mines and Geology) suggested this area and topic. He and Dr. Donald C. Noble (University of Nevada-Reno) provided guidance and erudite discussions throughout the investigation, and, most importantly, taught me how to work with, map, and interpret ash-flow tuffs. Thank you. I acknowledge Dr. L. T. Larson, Dr. J. Lintz, Dr. J. Nelson, and Dr. E. Kersten for their help and guidance. My graduate education was supported by an H.E.W. Fellowship, and partial thesis funding was provided by the Mackay Research Fund. Dr. J. Firby was responsible in both areas of financial assistance and I sincerely thank him. Many thanks to M. Todd, who served as my Field Assis­ tant during the summer of 1981. Conoco, Inc. granted me the use of various equipment. Many fellow graduate students and friends provided interest and encouragement throughout the project. Thank you: Delores Cates, Nancy Wolverson, Judith Wright-Clark, Jaye Up De Graff, Peggy Hester McGrew, Fred and Kathy Strawson, Larry and Sue Amateis, Dick and Sue Nosker, John Brown, Glenn Gierzycki, Steve Park, Walter Crone, Doug Bruha, Don Hudson, and Becky Weimer-McMillion. Above all, my deepest and sincerest thanks to Michael R. Smith for moral and emotional support, stimulating discussions, and constructive critical review throughout the investigation. iii DEDICATION To: Elzena Posey Benhara, my Grandmother, (Winter, 1892 to Summer, 1981) tor whom this thesis is dedicated in most respectful, thoughtful and loving memory. IV ABSTRACT A composite thickness of 940 meters of nine, latitic to rhyolitic, Middle Tertiary ash-flow tuff cooling units, and 200 meters of the younger, overlying Chloropagus Formation are exposed in the southern Nightingale Mountains and northern Truckee Range, Washoe County, Nevada. The basal six Middle Tertiary ash-flow tuffs were emplaced between 29.6 and 27 million years ago, and the upper three units were emplaced between 23.0 and 20.5 million years ago. All nine ash-flows are hydrothermally altered to a minor degree. Uranium is associated with fault zones or the Middle Tertiary ash-flows, where it occurs in or near the basal porous glassy zone of the Tuff of Rattlesnake Canyon. The Chloropagus Formation consists of andesitic to basaltic lava flows with minor dacite, ash-flows, and thin sedimentary horizons. Structure throughout this area is due to the interaction between Basin and Range extension and Walker Lane strike slip movement. V TABLE OF CONTENTS Page ACKNOWLEDGM E N T S ..................... ii DEDICATION .............................. iii ABSTRACT.............. iv ILLUSTRATIONS ........................ vii INTRODUCTION.......................... 1 GENERAL STATEMENT.............................. 1 LOCATION AND PHYSIOGRAPHIC SETTING ............ 3 METHODS AND PROCEDURES ........................ 5 PREVIOUS WORKERS . ............................. 6 THE ASH-FLOW TUFF COOLING UNIT CONCEPT ........ 7 REGIONAL GEOLOGY .................. 11 NATURE AND DISTRIBUTION OF ROCK TYPES.......... 11 TECTONIC SETTING .............................. 12 DESCRIPTIVE GEOLOGY ............ 15 GENERAL STATEMENT.............................. 15 MESOZOIC R O C K S .......... 16 Nightingale Sequence ........................ 16 Granitic Intrusive Rocks .................... 19 CENOZOIC ROCKS ................................ 22 Middle Tertiary Ash-Flow Tuffs .............. 22 General Statement.......................... 22 Ash-Flow Tuff Cooling Unit A .............. 28 Ash-Flow Tuff Cooling Unit B .............. 33 Age and Correlation of Ash-Flow Tuff Cooling Units A and B ................ 35 Tuff of Rattlesnake C a n y o n ................ 37 Coyote Spring T u f f ........................ 44 Tuff of Jackass S p r i n g .................... 49 Ash-Flow Tuff Cooling Unit E .............. 52 Age and Correlation of Ash-Flow Tuff Cooling Unit E ...................... 55 Ash-Flow Tuff Cooling Unit F .............. 55 Age and Correlation of Ash-Flow Tuff Cooling Unit F ...................... 57 Chimney Springs T u f f ...................... 58 VI Page Tuff of Gary's R i d g e ...................... 60 Andesite Dikes .............................. 63 Chloropagus Formation........................ 66 Andesite, Basalt, and Basaltic andesite. 69 Welded Dacite.............................. 70 Ash-Flow Tuffs ............................ 71 Miscellaneous Sedimentary Rocks............ 77 Emplacement of the Chloropagus Formation . 80 Basalt Flows ................................. 81 Surficial Deposits .......................... 82 STRUCTURAL GEOLOGY .............. 84 GENERAL......................................... 84 LOCAL STRUCTURE................................ 84 Regional Tilting ............................ 85 Unconformities .............................. 86 F a u l t s ....................................... 87 ECONOMIC GEOLOGY .................. 92 GENERAL......................................... 92 URANIUM MINERALIZATION ........................ 92 OTHER PROSPECTS AND MINERALIZATION ............ 96 Crosby Mine.................................. 97 Other Prospects.............................. 98 GEOLOGIC HISTORY .................. 100 RECOMMENDATIONS AND CONCLUSIONS................ 104 REFERENCES CITED .................. 107 % vii ILLUSTRATIONS Figures Page 1 Index M a p .................... 2 2 Generalized distribution of 34 to 17 million year old silicic ash-flow tuffs in western Nevada...................... 24 3 Photograph of the southwest facing escarpment of the southern Nightingale Mountains . 25 4 Stratigraphic column of Middle Tertiary ash- flow tuff cooling units in the southern Nightingale Mountains and northern Truckee Range, Nevada .................... 26 5 Histograms of phenocryst mineralogy of Middle Tertiary ash-flow tuffs in the southern Nightingale Mountains and northern Truckee Range, Nevada .................. 27 6 Correlation chart for Middle Tertiary ash-flow T u f f s ................................ 29 7 Photograph of an andesite dike............ 65 8 Schematic, generalized stratigraphic section of the Chloropagus Formation in the northern Truckee Range and southern Nightingale Mountains, Nevada ........ 68 9 Histograms showing phenocryst mineralogy of ash-flow tuffs within the Chloropagus Formation in the northern Truckee Range, Nevada........................ 73 10 Photographs of the basal vitrophyre and densely welded zone of the upper ash-flow tuff cooling unit (Tcafb) within the Chloropagus Formation in the northern Truckee Range 76 Plates 1 Geology of the southern Nightingale Mountains and northern Truckee Range, Washoe County, Nevada.............................. in pocket 2 Interpretive Cross Sections for the southern Nightingale Mountains and northern Truckee Range, Washoe County, Nevada........ in pocket i 1 INTRODUCTION GENERAL STATEMENT The ultimate purpose of this study is to determine and interpret the stratigraphy and map the extent of uranium bearing ash-flow tuff cooling units in the southern Nightin­ gale Mountains and northern Truckee Range, Nevada (Figure 1). Additional objectives of this study include: determination not only of the Middle Tertiary ash-flow tuff stratigraphy, but that of younger volcanic rocks as well; definition of the uranium—rich ash—flow tuff cooling units and the nature of any uranium mineralization; discussion of possible models relating welding, crystallization, weathering and erosion of these units to the formation of uranium deposits; and, finally, to hypothesize a possible tectonic regime responsi­ ble for this geologic setting. This thesis consists of a geologic map (1:12,000) (Plate 1), interpretive cross sections (Plate 2), and descriptions, based on hand and petrographic analysis, of the various ash-flow tuff cooling units and all other rock units encountered in the thesis area. This paper utilizes and follows the terminology of Smith (1960) and Ross and Smith (1960); those papers which best define, describe and explain ash-flow tuffs and the cooling unit concept. CALIFORNIA 3 LOCATION AND PHYSIOGRAPHIC SETTING Situated along the southeastern margin of Winnemucca Dry Lake, near the junction of Washoe, Pershing, and Churchill Counties, this study area is in the southern Nightingale Mountains and northern Truckee Range, Washoe County, Nevada (Figure 1). The area mapped, approximately 15 square miles (24 sq. km.), lies north of the town of Nixon, within the northeast quarter of the U.S.G.S. fifteen minute quadrangle map of Nixon, Nevada; in parts or all of: sections 33, 34 and 35, T.25N., R.24E. and sections 2, 3, 4, 9, 10, 11, 14, 15, 16, 17, 19, 20, 21, 22, 23, 26, 27, 28, 29, 30, 31, 32, 33, 34 and 35, T.24N., R.24E., Mount Diablo Base and Meridian (Plate 1). The Nightingale Mountains and northern Truckee Range are an essentially continuous north-south trending mountain block which rises abruptly on its western side from the val­ ley occupied by Winnemucca Dry Lake (Figure 1). Elevations range from 5,840 ft (1,780 m) in the southern Nightingale Mountains, to slightly less than 4,000 ft (1,220 m), on the alluvial plain proximal

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