Geology of the Cortez Quadrangle Nevada By JAMES GILLULY and HAROLD MASURSKY With a section on Gravity and Aeromagnetic Surveys By DON R. MABEY GEOLOGICAL SURVEY BULLETIN 1175 Prepared in cooperation with the Nevada State Bureau of Mines The area includes a thick section of Paleozoic and Tertiary rocks, a window of the Roberts thrust, and a bonanza silver camp UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1965 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page Abstract____________________-_---_-__--___-_---_--_-__--------_- 1 Introduction __________________________'__________-_______---_--_-_- 3 Geologic interest of the Cortez quadrangle.____--___-.__--__--__-- 3 Location and accessibility___________.._______--_-_--__-_--_-____ 4 Physical features____-_-__-_________.._-____--_----___--------_- 5 Climate and vegetation__-________--_-____-_------__--------_- 8 Previous work__-__-____---_-__-_--_------_-_----__---_---__- 8 Fieldwork.._.___...___.___----- __.- ...._._.._ 9 Acknowledgments ___-____----------_----__------------------.-- 9 General geology_________-___---___-__..-_--_-__---------_---_-_--_- 10 Principal features__--___---__-_---_-.--_-.-----_--_-----.--_- 10 Stratigraphy of the lower plate of the Roberts thrust (eastern or car­ bonate facies)__-_----__-----__--------_----------------_--_- 13 Cambrian System____»___.__._.___________._______________ 13 Hamburg Dolomite.___________________________________ 13 Cambrian and Ordovician hiatus____________________________ 16 Ordovician System____-_-____--_-_-_-_--_---__-----___-_-. 17 Eureka Quartzite__ __-___-_-_--_-__-_---_---___-_-_____ 17 Hanson Creek Formation___-_---____--_----_-_---___-_- 21 Silurian System_____-___-___-_-__---__-_-_-___-__----__-__ 25 Roberts Mountains Limestone._____-----_-_-_---_______ 25 Devonian System________________________________________ 29 Wenban Limestone.___---_--_----__---_----_-_----_--- 29 Pilot Shale__...__...__._.__..... ----- ------- 38 Stratigraphy of the upper plate of the Roberts thrust (western or siliceous facies)__----__-----------------_-----_---------__--- 40 \prdovician System_._--_-_______-_____--____-____-_-____- 40 Valmy Formation_-_______-_-_______-_-__--_---_-_____ 40 Vinini Formation___-_-_--_--_-----_----_---___-__-___- 48 Silurian System________-_______-_-_____--____--_---_-_____ 54 Fourmile Canyon Formation__-______----__-__--________ 54 Elder Sandstone-.------------------------------------- 57 Devonian System..____________--____--_--_-__-______-__... 59 Slaven Chert____.-_.-___._-_..___---__--__---_.__... 59 Trace elements in the sedimentary rocks.____-----_-_-----_._... 61 Paleogeography prior to the Antler orogeny_____________________ 62 Pre-Pennsylvanian unconformity (Antler orogeny)_________________ 64 Paleozoic rocks younger than the Antler orogeny--_---__--_-___-__ 65 Pennsylvanian or Permian System.__-_____-_-________._._._- 65 Brock Canyon Formation__-_--___--_--___________.._ 65 Post-Permian hiatus-__-_-_-----_______----__-___-____.________ 66 Rocks of Mesozoic age_.--_----_--l_---_----_---__-_-__._.___-. 67 Quartz monzonite of the Mill Canyon stock___--___-__-_____ 67 Quartz porphyry and augite syenite._________________________ 72 in IV CONTENTS General geology Continued Page Tertiary System.._____-_-_-----__--_-___-__--_-___--_-_____-__ 73 Caetano Tuff. ____-__-_.--__---_--____---__-_--.-_____-___ 73 Quartz porphyry dikes___-_-_-------___-__-____-____--_____ 78 Volcanics of Fye Canyon________--________.__-_-___________ 79 Gravels of Horse Canyon and the Toiyabe Range____________ 81 Basaltic andesite______-__-_-__-_--_-_____-__-_____________ 82 Dolerite dikes.____________________________________________ 84 Rhyelites of Horse and Willow Canyons__-----_---_--________ 85 Pliocene sediments and tuff__----_-_-_----__-_-_____________ 87 Quaternary System____________________________________________ 87 Structural geology.__-_________-__-------_-----__-_-_-_-___________ 89 General features._________._._-__----_-______------_-_-__-_-___ 89 Structures related to the Antler orogeny__________________________ 90 Roberts thrust._------___------------------------------_-. 90 Structure of the lower plate of the Roberts thrust _____________ 93 Structure of the upper plate of the Roberts thrust _____________ 93 Age of the Roberts thrust______--------------_---_---------- 94 Structures of the post-Triassic(?)-pre-Tertiary orogeny..., _ _________ 94 Structures of the ear'ier volcanic rocks___________________________ 94 Basin Range faults______-_____-__------_--__--_-----__-___-___ 95 Mining history.___________________________________________________ 97 The mines.______---___-________.---_---------------_____-_-_-____ 100 Mines of the Cortez Silver Mines Co___________________________ 100 Mill Creek mines.-----------.--------------------------------- 104 Cbpper Canyon area.__________________________________________ 104 Turquoise deposits.___--_-____-_------___--_-_---_-___--_---.__ 105 Barite__ ____________________________________________________ 105 Quicksilver _-_-________-_____--_---_-_______--__------_-______ 105 Gravity and aeromagnetic surveys, by Don R. Mabey_____-_____--__._ 105 Gravity anomalies.____________________________________________ 106 Rock densities______-_----__-____-_-________-__-____-_-_-- 107 Gravity profiles___-_----____----__---_-__________-____-____-__ 107 Magnetic anomalies__________________________________________ 110 References cited.__________________________________________________ 111 Index ___________________._.-___.--____--___________________----_- 115 ILLUSTRATIONS [Plates are in pocket] PLATE 1. Geologic map and sections of the Cortez quadrangle, Nevada 2. Map showing location of fossil collections. 3. Gravity and aeromagnetic contour map and profiles. CONTENTS V Page FIGURE 1. Index map, showing location of the Cortez quadrangle and topographic features of north-central Nevada._______ 4 2. Scarp of the Cortez Mountains from the west.___________ 6 3. Scarp of the Cortez Mountains from the north west. ______ 7 4. Cortez Canyon, Toiyabe Range, and Grass Valley playa from the air _______________________________________ 8 5. Photomicrograph of quartzite from the Eureka Quartzite... 20 6. Photomicrograph of quartzite from the Valmy Formation. _ 42 7. Photomicrograph of limestone from the Valmy Formation. _ 43 8. Diagram showing the graptolite zones of Elles and Wood (1914) found in the formations of the Cortez quadrangle. _ 50 9. Dolomitic siltstone of the Fourmile Canyon Formation.__ 55 10. Outcrop of chert of the Slaven Chert___--..___________ 60 11. Photomicrograph of biotite-quartz monzonite from the Mill Canyon stock_______..__._.__._____.________ 69 12. Photomicrograph of hornblende-biotite granodiorite from the Mill Canyon stock.___.__._____.._--...__,______ 71 13. Map showing principal claims of the Cortez Silver Mines Co. 101 14. Composite map of the workings of the Cortez Silver Mines Co._-.._---------.-__--------___----------.-----. 102 15. Composite section of the mines of the Cortez Silver Mines Co.--._-_------------__-__---_-__---_--------__-. 103 TABLES Page TABLE 1. Paleozoic rocks of the Cortez quadrangle, showing distribu­ tion between eastern, western, and "overlap" facies___ 12 2. Graptolites of the Cortez quadrangle, Nevada__________ 46 3. Trace-element composition of formations of the siliceous and carbonate facies._______________________________ 63 4. Analyses and norms of rocks of the Mill Canyon stock.... 70 5. Sequence of rock types in an ash-flow unit_______________ 75 6-10. Analyses and norms: 6. Vitrophyre and welded tuff of the Caetano tuff____. 77 7. Quartz porphyry of dikes in the Cortez quadrangle... 79 8. Rhyolite from the volcanics of Fye Cany on.--...--.. 81 9. Basaltic andesite from the Cortez Mountains and Sho- shone Range.___________-_______-_____-_-_--._ 84 10. Intrusive rhyolite of Horse Canyon----,-------___--_ 86 11. Production of gold, silver, copper, lead, and zinc, in terms of recovered metal, from the Cortez district, 1902-57~___-_ 99 12. Densities of representative samples of major bedrock varieties in the Cortez quadrangle.___________________ 107 GEOLOGY OF THE CORTEZ QUADRANGLE, NEVADA By JAMES GILLTTLY and HAROLD MASUBSKY ABSTRACT The Cortez quadrangle, in north-central Nevada, is of interest geologically because of the formerly active bonanza silver camp at Cortez and because of the exposure of the Roberts thrust, one of the great structural features of the Cordillera. The area embraces the extreme north end of the Toiyabe Range and the south­ ernmost part of the Cortez Mountains, together with very small parts of the Shoshone and Simpson Park Ranges and the great intermontane valleys, Grass and Crescent Valleys. The rocks exposed involve representatives of Paleozoic, Mesozoic, and Cenozoic time. The Paleozoic rocks include representatives of three depositional fades, of which the first two have been brought into juxtaposition by movement of many miles on the Roberts thrust and the third has been deposited upon the eroded stumps of the mountains formed during the thrusting. The rocks of the lower plate of the Roberts thrust, the eastern or carbonate facies, include representatives of Cambrian, Ordovician, Silurian, and Devonian ages, all closely resembling their correlatives in the Eureka district and else­ where in eastern Nevada. The formations are: Hamburg