A University of Nevada Reno The Geology and Mineralization of the Challc Mountain and Westgate Mining Districts Churchill County, Nevada .A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology by Dennis Paul Bryan December 19/2 The thesis of Dennis Paul Bryan is approved: 0- C~. lltL/ Department chairman -/ Dean, Graduate School University of Nevada Reno December 1972 ACKNOWLEDGEMENTS The writer is grateful for the assistance of many individuals during the course of this study. Dr. Anthony L. Payne originally suggested Chalk Mountain for possible investigation. Dr. Arthur Baker III, the major thesis advisor, suggested combining both Chalk Mountain and Westgate as a topic and subsequently offered encouragement and assistance throughout the scope of the project. Drs, E. R. Larson, Anthony L. Payne and D. B. Slemmons provided helpful suggestions. Messrs. E. J. Murphy and M. P. Allen of the Nevada Bureau of Mines and Geology provided assistance with the microscopic and analytical aspects of the study. Special thanks go to James Green and Robert Barnes for permission and assistance in mapping on the Chalk Mountain property. G. E. Schrock assisted with the underground mapping, Wayne Kemp critically read the report and provided helpful comments. ii ABSTRACT The Chalk Mountain and Westgate districts were mined for lead and silver during the early part of this century with the Chalk Mountain Mine the major producer from the area. Sedimentary rocks at Chalk Mountain consist entirely of dolomite of Mesozoic age and those at Westgate consist of dolomite, limestones, shales, and calcareous sandstones, also of Mesozoic age. A Late Paleozoic, to Early Mesozoic volcanic unit occurs as the upper plate of a thrust fault in both districts. Igneous rocks consisting of quartz porphyry, quartz monzonite, and granite porphyry were intruded during the Mesozoic while an extensive biorite dike is Cenozoic in age. S k a m zones developed in the dolomite adjacent to the quartz monzonite and quartz porphyry intrusives at Chalk Mountain. Thrusting and deformation of the Mesozoic sediments probably occurred during the Cretaceous. Predominantly normal faulting of Tertiary to Quaternary age produced the physiographic features of the districts and continues to the present day. The ore deposits of the districts are oxidized to known depths of 500 feet ana consist mostly of secondary lead minerals, limonites, and occasional primary sulfides. The ore deposits xn some cases are related to s k a m formation and appear both spacialiy and genetically associated with the Mesozoic intrusives. iii CONTENTS Page ACKNOWLEDGEMENTS ............................................ i ABSTRACT.................................................... ii ILLUSTRATIONS................................................ v INTRODUCTION ................................................ 1 Location. .................. 1 Scope of the Study................ 1 Previous W o r k .......................................... 3 GENERAL GEOLOGY, ............................................. 4 STRATIGRAPHY............................................ 4 Late Paleozoic to Early Mesozoic Volcanics ........ 4 Mesozoic Sediments .................... 7 Gabbs Formation ..... ......... ..... 8 Sunrise Formation ..... .................. 9 Westgate Formation............ 10 INTRUSIVE IGNEOUS ROCKS . .......................... 12 Granite Porphyry............ 12 Quartz Porphyry Dikes.................... 13 Syenite Porphyry S i l l s .............. 14 Porphyritic Quartz Monzonite . .................. 15 Quartz Porphyry.................................... 18 Diorite Porphyry .............. .......... 20 STRUCTURE.............................................. 24 Pre-Tertiary Structures............................ 24 Chalk Mountain................................ 24 iv Page Westgate.................................... 27 Thrust Faulting. ..... .................. 28 Tertiary and Quartemary Structures.............. 29 METAMORPHISM.......................................... 34 Dynamic and Regional Metarcorphism ................ 34 Contact Metamorphism. ..... .................. 35 Chalk. Mountain.................... 36 S k a m s .................................. 38 Westgate .............. ...... ........ 42 GEOLOGIC HISTORY.......... 44 ORE DEPOSITS........................ ...................... 46 History of the Districts ........ ...... ........ 46 Chalk Mountain ............ 47 Chalk Mountain Mine ........................ 51 Westgate ....... ................................ 56 Oxidation of the Ore Deposits...................... .. 59 Ore Associated with Skarns............................ 64 DISCUSSION OF THE ORE DEPOSITS.............................. 69 APPENDIX..................... '............................. 73 REFERENCES CITED............................................ 76 V ILLUSTRATIONS Plate Page 1 Geologic Map of the Chalk Mountain and Westgate Mining Districts ............................ In pocket 2 Geologic Map of the Chalk Mountain M i n e .............. In pocket Figure 1. Location map cf the Chalk Mountain and Westgate Mining Districts.......................... ............ 2 2. Photograph of Chalk Mountain................ .. 17 3. Photomicrograph of quartz monzonite . ................... 17 4. Photomicrograph of quartz porphyry. ................... 22 5. Photograph of diorite d i k e . ...................... .. 22 6. Photograph of the Westgate district................... 25 7. Photograph of south end of Chalk Mountain........ .. 25 8. Photograph of thrust fault at Westgate. ........ 30 9. Photograph of scarp on Chalk Mountain ................... 30 10. Photograph of silica associated with faulting ........... 32 11. Photograph of 1954 fault scarp.................. 32 12. Photomicrograph of phiogopite and chlorite............... 37 13. Photomicrograph of serpentine ........ ........... 40 14. Photomicrograph of olivine pseudomcrphs . ........... 40 15. Photomicrograph of endoscam. ........................... 43 16. Photomicrograph of chiastolite. ............ 43 17. Photograph of oxide vein at Chalk Mountain........ .. 48 18. Photograph of Chalk Mountain Mine ..................... 48 vi Figure Page 19. Strike-frequency diagram of oxide veins at Chalk Mountain .............................. ..... 49 20. Photomicrograph of dolomite breccia.................... 55 21. Photomicrograph of oxidized galena .................... 61 22. Photomicrograph of boytroidal limonite ................ 61 23. Geologic Map of the Northern Thrust Area at Chalk Mountain . 66 1 INTRODUCTION Location Hie Chalk Mountain and Westgate mining districts are situated near the eastern edge of Fairview Valley some 38 air miles east-southeast of Fallon, Nevada in Churchill County (Figure 1). The Westgate district lies at the southern end of the Clan Alpine Mountains, Chalk Mountain lies in the valley 2 miles to the west of Westgate and is approximately 2 miles long and a mile wide, rising to a height of about 1,000 feet above the surrounding terrain. Both districts are midway between the Fairview and Wonder mining districts and are sometimes considered to be a part of the former which lies S miles to the south. Both areas are easily accessible from U. S. Highway 50 whicn is along their southern edges. Scope of the Study Approximately 9 square miles of bedrock x^ere mapped at a scale of 1:12,000. Nearly a mile of underground workings were mapped at a scale, of 1 inch to 50 feet. Emphasis was placed on relating the geol- ogy of Chalk Mountain to that of Westgate and investigating the miner- alization at Chalk Mountain, the more important mining district. Field work was conducted mostly during the summer of 1971 with total days in the field amounting to approximately 35. Fifty-four thin sections and 9 polished sections were investigated and 51 vein samples were analyzed for up to 5 elements each. 2 Figure ! Location Hap of the Chalk Mountain end Weslgate Mining D is tric ts JP 3 Previous Work The first documentation of the ora deposits at Chalk Mountain and Westgate was by Schrader (1947) in a U. S. Geological Survey open file report on the Carson Sink area. Schrader included a map of Chalk Mountain on a scale of 1 inch to 3,500 feet. Speed and Wilden inves- tigated the geology and mineral deposits of Churchill County, including Chalk Mountain and Westgate, in a preliminary report by the U. S. Geological Survey in 1968 and included a 1:24,000 scale map of Westgate. The geology and biostratigraphy of the Westgate area was described in detail by J. I. Corvalan in 1962 in an unpublished Ph.D. thesis from Stanford University. Corvalan's map was at a scale of 1:12,000. D. C. Thorstsnson (1968) investigated the geology of Chalk Mountain in an unpublished report from Northwestern University, Thorstenson’s map was at a scale of 1 inch to 3,000 feet. Neither of tnese papers dealt with the ore deposits of the districts. 4 GENERAL GSOLOCY The rocks of the Chalk Mountain and Westgate districts range in age from Permian (?) to Recent. Fossils are abundant in the Mesozoic strata at Westgate but are absent at Chalk Mountain where the age of the sediments has been inferred indirectly. The sedimentary units consist of dolomites, limestones, shales, and calcareous sandstones of Mesozoic age, and Quaternary valley fill. A Late Paleozoic to Early Mesozoic unit occurs as the upper plate of a thrust fault. Igneous recks include granite porphyry, quartz monzoaite, quartz porphyry, and minor felsic dikes and sills