DEPARTMENT OF THE INTERIOR HUBERT WORK, Secretary UNITED STATES GEOLOGICAL SURVEY GEORGE OTIS SMITH, Director Bulletin 762 GEOLOGY AND ORE DEPOSITS OF THE ROCHESTER DISTRICT, NEVADA BY ADOLPH KNOPF WASHINGTON GOVERNMENT PRINTING OFFICE 1924 ADDITIONAL COPIES OF THIS PUBLICATION MAY BE PROCURED FROM' THE SUPERINTENDENT OF DOCUMENTS GOVERNMENT PRINTING OFFICE WASHINGTON, D. C AT 15 CENTS PER COPY CONTENTS Preface, by F. L. Ransome____________________________________________ Outline of the report__________________________________________ Part I. General features_____________________________ Geography. ___________ __________________.______. Situation of the district ________________________ Physical features _______________- _________________ 1 Climate and vegetation____ ________________ 3 History of mining______________________________ _ 4 Output of silver and gold_______ ________________ fi Geologic investigations of the district-^_________________ 7 Bibliography____________________________________ !) Geology of the Star Peak Range______________________ 9 Part II. General geology of the district____________________ 13 Triassic rocks _______________________________ 13 Introductory outline________________________- 13 Rochester trachyte____________________________. 14 General features____ ______ 14 Petrography______________________________. 14 Thickness and age________ _ ________ 17 Dumortieritized trachyte _____ _____________ 18 Keratophyres________________ _______________ 20 Occurrence and character___________________. 20 Petrographic features__ __________________ 20 Relation of keratophyres to the Rochester trachyte____ '-2 Age and correlation_________________________ 22 Nenzel rhyolite breccia_____ _ _ __________ 23 Occurrence and character. _ _ _________ 23 Stratigraphy and structure____________________ 23 Rhyolite tuffs___________'.__________________ 26 Weaver rhyolite_______________ _____ 20 Occurrence and character __ 2U Petrography_______ _ 27 Tuffaceous shales ____ _ 28 Limestone____________ __ 29 General features____ __ 20 Age__________________________________ 29 Relation of limestone and volcanic rocks _ 30 Metadiorite _______ - 31 Post-Triassic, probably late Jurassic igneous rocks __ 32 Aplite______________ - 32 General features_____ ; >2 Petrography_____ 32 Genetic relations____ --_ 3$ Granite porphyry __ '>* Tertiary (?) rocks 35 Pliocene (?) alluvial-cone detritus ___- 35 Occurrence and character - 35 Age__ 36 Basalt______________ - 36 Quaternary alluvium - 37 in IV CONTENTS Part II. General geology of the district Continued. Page Geologic structure ___ ____ _______________ 38 Folding and faulting__________________________ 3S Schistosity____________________________'___ 40 Physiographic history__________________________,__ 41 Part III The ore deposits___________________________ 43 General features _______________________________ 43 Relation of ore deposits to water level_________________ 43 Silver veins__________________________________ 44 Distribution and general character_________________ 44 Nature of the ore________________-_ _______ 43 Enrichment by supergene argentite________________ 40 Wall-rock alteration_________________________ 50 Gold veins___________'_____________________ 51 General character.. _____:___________________ 51 Nature of the gold ore-forming solutions_____________ 52 Other mineral deposits__________________________ 53 Cinnabar _______________________________ 53 Miscellaneous deposits_______________________ 54 Genesis of the ores of the district___:________________ 55 Placers ________________ _________________ 58 Part IV. Mines and prospects_________________________ 59 Silver mines and prospects_________ ______________ 59 Eochester mine_____________________________ 59 Bonghton & Hackley mine_ ____________________ 64 Nenzel Grown Point property_________ ________ 65 Buck & Charley mine ______ ____ _________ 66 Octopus mine_______ ___ ________________ . 67 Raven prospect____________ ________________ 68 Abe Lincoln mine__ --__ _ 68 Kaiser Bill prospect_______________ _________ 68 Nevada Packard mine , 69 Gold mines and prospects 74 Lincoln Hill mine__ 74 Oro Fino mine____ ___ ____ _ 75 Hagan prospect________________________ 75 Index ______________ - - 77 ILLUSTRATIONS Page PLATE I. Geologic map of the Rochester district _ __ _ 22 II. A, B, Silver-bearing stockwork at the Nevada Packard mine_ 46 III. A, Incipient replacement of sphalerite by supergene argentite; B, Advanced replacement of sphalerite by supergene argentite 47 IV. Map of Nevada Packard mine workings-____ __ 72 FIGURE 1. Index map showing the location of the Rochester district__ 2 2. Diagrammatic section through the south end of the Star Pe'ak Range____________________________ 12 3. Geologic section through the north end of Nenzel Hill____ 24 4. Geologic section from portal of Boughton & Hackley tunnel to northeast corner of North Crown Fraction claim________ 65 5. Diagrammatic section through the Nevada Packard mine . along the line of tunnel A___ 72 PEEFACE By F. L. RAN SOME The modern development of the Rochester district dates from the later part of the year 1912. Early in 1913 F. C. Schrader was de­ tailed by the United States Geological Survey to make a reconnais­ sance study of the district, and his brief examination was followed by an excellent report, which was published in 1915. The con­ tinued productivity of the Rochester mines and the interesting char­ acter of the ore deposits made it evident that more detailed geologic work was desirable, and after the completion of a topographic map, in 1916, Mr. Knopf was instructed to make a geologic study of the district early in 1917. The preparation of his report, like many other investigations by the Geological Survey then in progress, was interrupted by the urgent war demand for information in regard to particular mineral resources and was further delayed by Mr. Knopf's withdrawal from full-time participation in the work of the Survey and his acceptance of a professorship at Yale University. His new duties left only a small part of his time available for the investigation that he has now satisfactorily completed. As Mr. Knopf shows, the ore deposits of the Rochester district are mmeralogically of unusual character. Dumortierite, a rare alumi­ num borosilicate, is in some parts of this district extraordinarily abundant and is accompanied by tourmaline and andahisite minerals, which also are rather unusual in association with silver deposits. The boron-bearing minerals were evidently formed at high temperature, and Mr. Knopf concludes that there were three distinct stages in the deposition of these minerals. The principal silver-bearing veins were probably formed later and at lower temperature. The report shows that the original vein material was of too low grade to be ore. The district owes its economic development to downward enrichment, effected largely by the replacement of sphalerite by argentite a process that is fully discussed in its appropriate, place. It follows that the silver deposits of the Rochester district are not likely to extend to great depth. The same conclusion, based apparently on the observed decrease in tenor of the ore on the deeper mine levels, finds! expression also in the annual report of the Rochester Silver Corporation for 1921. Mr. Knopf has given the explanation for the observed decrease. VI PREFACE The following report, although it deals with only a small area, throws considerable new light on the geology of the Humboldt Range. The range has long been noted as a collecting ground for Triassic fossils, but our knowledge of the beds in which these ancient marine shells are found left much to be desired. Mr. Knopf has not only supplied some of this deficiency but has shown that the light- colored felsitic rocks of the Humboldt Range, which in early reports were regarded as metamorphosed sediments and later were classed generally as rhyolites, include rocks belonging to the group of trachytes a group that is rather sparsely represented in North America, OUTLINE OF THE REPORT Rochester is a silver-mining district in the Humboldt Range of western Nevada. Silver was discovered there in 1912, and to the end of 1922 the district produced 7,000,000 ounces of silver. It is second in yearly outpuj; among Nevada's many silver districts, but it ranks far below Tonopah, the leading district. The prevailing rocks of the district are a thick series of Triassic volcanic rocks overlain by limestone of the same age. The volcanic rocks consist of felsitic trachyte and keratophyre, aggregating 10,000 feet in thickness, and an overlying series of rhyolite, 1,000 feet thick. Above the igneous rocks lies the limestone, of which only 2,000 feet is exposed in the district, although else­ where in the Star Peak Range it apparently attains a far greater thickness. At or near the end of the Jurassic period this series of volcanic rocks and limestone came within the influence of the great revolution and crustal dis­ turbances that affected the western Cordilleran region at that time. A long train of events started then, which culminated in the formation of the ore deposits of the district. First the rocks were folded into a broad anticline, which consti­ tutes one of the major structural features of the range. This arching of the rocks was followed by the intrusion of granitic magma, which within the district proper is represented by fine-grained white granite, or aplite. The bodies of aplite are comparatively large for rock of this kind, and their size implies, therefore,