Petrology of volcanic rocks associated with silver-gold (Ag-Au) epithermal deposits in the Tonopah, Divide, and Goldfield Mining Districts, Nevada

du Bray, Edward A.- edubray@usgs.gov, John, David A.- djohn@usgs.gov, Colgan, Joseph P.- jcolgan@usgs.gov, Vikre, Peter G.- pvikre@usgs.gov · 2019 · Report

Miocene calc-alkaline volcanic rocks, part of the southern segment of the ancestral Cascades magmatic arc, are spatially, temporally

This 2019 document, Petrology of volcanic rocks associated with silver-gold (Ag-Au) epithermal deposits in the Tonopah, Divide, and Goldfield Mining Districts, Nevada, is preserved in the Mountain Man Mining Library for research and reference. Original source: pubs.usgs.gov.

U.S. Department of the Interior U.S. Geological Survey Scientific Investigations Report 2019-5024 Petrology of Volcanic Rocks Associated with Silver-Gold (Ag-Au) Epithermal Deposits in the Tonopah, Divide, and Goldfield Mining Districts, Nevada

Cover.  Prominent lava flow and volcanic cinder accumulations of the trachyandesite of Red Mountain, at Red Mountain north of Tonopah, Nevada. (Photograph by Edward A. du Bray, U.S. Geological Survey, 2015.)

Petrology of Volcanic Rocks Associated with Silver-Gold (Ag-Au) Epithermal Deposits in the Tonopah, Divide, and Goldfield Mining Districts, Nevada By Edward A. du Bray, David A. John, Joseph P. Colgan, Peter G. Vikre, Michael A. Cosca, and Leah E. Morgan Scientific Investigations Report 2019-5024 U.S. Department of the Interior U.S. Geological Survey

U.S. Department of the Interior DAVID BERNHARDT, Secretary U.S. Geological Survey James F. Reilly II, Director U.S. Geological Survey, Reston, Virginia 2019 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit ://www.usgs.gov or call 1-888-ASK-USGS. For an overview of USGS information products, including maps, imagery, and publications, visit ://store.usgs.gov. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: du Bray, E.A., John, D.A., Colgan, J.P., Vikre, P.G., Cosca, M.A., and Morgan, L.E., 2019, Petrology of volcanic rocks associated with silver-gold (Ag-Au) epithermal deposits in the Tonopah, Divide, and Goldfield mining districts, Nevada: U.S. Geological Survey Scientific Investigations Report 2019-5024, 22 p., ://doi.gov/10.3133/sir20195024. ISSN 2328-0328 (online)

Acknowledgments Geologic investigations carried out for this study were completed as part of the "Magmatic-tectonic history and component sources of major precious metal deposits at Tonopah and Goldfield, Nevada Project," funded by the U.S. Geological Survey Mineral Resources Program. Constructive reviews by A.K. Gilmer and G.E. Graham (U.S. Geological Survey) are much appreciated and helped clarify data presentation.

Contents Acknowledgments iii Abstract 1 Introduction 1 Geologic Setting of Ag-Au Deposits in the Tonopah, Divide, and Goldfield Mining Districts 3 Tonopah Mining District 3 Divide Mining District 6 Goldfield Mining District 6 Analytical Methods 8 Petrographic Characteristics 8 Major Oxide Geochemistry 9 Trace Element Geochemistry 9 Petrogenesis of Volcanic Rocks in the Tonopah, Divide, and Goldfield Mining Districts 16 Conclusions 19 References Cited 20 Figures

1. Index map showing the location of the Tonopah, Divide, and Goldfield mining districts, west-central Nevada relative to the inferred extent of ancestral and modern High Cascades magmatic arcs in northern California and western Nevada 2

2. Total alkali-silica variation diagram showing compositions of Miocene volcanic rocks associated with mineralization in the Tonopah, Divide, and Goldfield mining districts, Nevada 10

3. Variation diagrams showing abundances of major oxides (weight percent) in Miocene volcanic rocks associated with mineralization in the Tonopah, Divide, and Goldfield mining districts, Nevada 11

4. Trace-element, tectonic-setting-discrimination variation diagram showing the composition of volcanic rock units associated with mineralization in the Tonopah, Divide, and Goldfield mining districts, Nevada 14

5. Chondrite normalized rare-earth element diagrams for volcanic rocks associated with mineralization in the Tonopah, Divide, and Goldfield mining districts, Nevada 15

6. Primitive mantle-normalized trace element diagrams for volcanic rock units associated with mineralization in the Tonopah, Divide, and Goldfield mining districts, Nevada 17 Tables

1. Characteristic features of eruptive centers and volcanic rock units associated with mineralization in the Tonopah, Divide, and Goldfield mining districts, Nevada 4

2. Summary of petrographic characteristics for volcanic rock units associated with mineralization in the Tonopah, Divide, and Goldfield mining districts, Nevada 7

3. Representative compositions of volcanic rock units associated with mineralization in the Tonopah, Divide, and Goldfield mining districts, Nevada 12

Conversion Factors International System of Units to U.S. customary units Multiply By To obtain Length millimeter (mm) inch (in.) meter (m) foot (ft) kilometer (km) mile (mi) Area square kilometer (km2) square mile (mi2) U.S. customary units to International System of Units Multiply By To obtain Mass ounce, avoirdupois (oz) gram (g) Pressure bar kilopascal (kPa) Ages are expressed in Ma for mega-annum (million years ago), and m.y. for million years Abbreviations HFSE High-field-strength element HREE Heavy rare-earth element IUGS International Union of Geological Sciences LREE Light rare-earth element LILE Large-ion lithophile element REE Rare-earth element USGS U.S. Geological Survey Chemical Compounds Al2O3 Aluminum oxide CaO Calcium oxide FeO* Total iron expressed as ferrous oxide K2O Potassium oxide MgO Magnesium oxide MnO Manganese oxide Na2O Sodium oxide P2O5 Phosphorus pentoxide SiO2 Silicon dioxide TiO2 Titanium dioxide

Elements Ag Silver Ar Argon As Arsenic Au Gold Ba Barium Bi Bismuth Ca Calcium Ce Cerium Co Cobalt Cr Chromium Cs Cesium Cu Copper Dy Dysprosium Er Erbium Eu Europium Fe Iron Ga Gallium Gd Gadolinium Hf Hafnium Ho Holmium La Lanthanum Lu Lutetium Mo Molybdenum Nb Niobium Nd Neodymium Ni Nickel O Oxygen P Phosphorous Pb Lead Pr Praseodymium Rb Rubidium S Sulfur Sb Antimony Sc Scandium Se Selenium Sm Samarium Sn Tin Sr Strontium Ta Tantalum Tb Terbium Te Tellurium Th Thorium Tm Thulium U Uranium Vanadium W Tungsten Y Yttrium Yb Ytterbium Zn Zinc Zr Zirconium

Petrology of Volcanic Rocks Associated with Silver-Gold (Ag-Au) Epithermal Deposits in the Tonopah, Divide, and Goldfield Mining Districts, Nevada By Edward A. du Bray, David A. John, Joseph P. Colgan, Peter G. Vikre, Michael A. Cosca, and Leah E. Morgan and fractionation of the observed phenocryst minerals. In addition, these rocks have negatively sloping rare-earth element patterns consistent with partial melting in a high-pressure, garnet stable regime. Elevated strontium concentrations and small negative europium anomalies are consistent with partial melting in a plagioclase-unstable setting. However, larger negative europium anomalies among the more silica-rich volcanic rocks indicates a progressively greater role for plagioclase fractionation among these rocks. The importance of hornblende in the petrogenesis of these rocks is reflected in subtly U-shaped middle rare-earth element pattern segments. Increasing lead/cerium and decreasing phosphorus pentoxide/potassium oxide with increasing silicon dioxide are characteristic of these volcanic rocks. These characteristics and their distinctive pre-Cenozoic xenolith content suggest a significant role for crustal contamination in their petrogenesis. Diagnostic textural features preserved by phenocrysts, especially plagioclase, constitute additional evidence that open-system behavior, including reservoir-scale mixing, recharge, and assimilation, was critical to the petrogenesis of volcanic rocks in the Tonopah, Divide, and Goldfield mining districts. Introduction The Tonopah, Divide, and Goldfield mining districts are currently inactive mining areas in west-central Nevada centered around the town of Tonopah, approximately 380 kilometers (km) southeast of Reno along U.S. Highway 95. The Tonopah, Divide, and Goldfield mining districts are historic mining areas in west-central Nevada near the towns of Tonopah and Goldfield, approximately 276 and 302 km, respectively, southeast of Reno (fig. 1). Significant amounts of silver (Ag) and gold (Au) were produced from epithermal vein deposits in the Tonopah (Nolan, 1935; Bonham and Garside, 1974; 1979; Ashley, 1990a), Divide (Bonham and Garside, 1979; Erdman and Barabas, 1996), and Goldfield (Ransome, 1909; Ashley, 1974; 1979; 1990b; Ashley and Silberman, 1976) mining districts. These deposits are spatially, Abstract Miocene calc-alkaline volcanic rocks, part of the southern segment of the ancestral Cascades magmatic arc, are spatially, temporally, and likely genetically associated with precious metal epithermal deposits in the Tonopah, Divide, and Goldfield Districts of west-central Nevada. In the Tonopah mining district, volcanic rocks include the Mizpah Trachyte, Fraction Tuff, and Oddie Rhyolite; in the Divide mining district, they include the Heller Tuff, Brougher Rhyolite, trachyandesite of Red Mountain, Divide Andesite, and volcanics of Donovan Peak (which includes rhyolite, dacite, and rhyodacite units); in the Goldfield mining district they include the Milltown Andesite, an unnamed porphyritic andesite, and latite. All these rocks are porphyritic and contain phenocryst assemblages that include plagioclase, pyroxene, hornblende, biotite, quartz, alkali feldspar, and olivine. These mostly subalkaline, metaluminous, calc-alkalic, and magnesian rocks range from basaltic trachyandesite to rhyolite and contain 54 to 78 weight percent silicon dioxide. In the Divide mining district, the Divide Andesite and the volcanics of Donovan Peak are compositionally distinct from volcanic rocks in the other two mining districts. These rocks define a somewhat more restricted range of silicon dioxide content; are more alkalic; have greater titanium dioxide, sodium oxide, barium, hafnium, lanthanum, niobium, tantalum, yttrium, ytterbium, and zirconium abundances; and lower magnesium oxide, strontium, and vanadium abundances. Elevated zirconium contents are particularly characteristic of these rocks, which are also distinctly younger than most of the rocks in the other two mining districts. The alkalic character (principally higher sodium oxide abundances) and elevated zirconium contents characteristic of the Divide Andesite and the volcanics of Donovan Peak suggest that distinctive sources and (or) processes contributed to the petrogenesis of these rocks. In the Tonopah, Divide, and Goldfield mining districts the geochemistry of Oligocene and Miocene volcanic rocks constrain the processes that contributed to the petrogenesis of these rocks. Specifically, major oxide compositional variation among these rocks is consistent with crystallization

2    Volcanic Rocks Associated with Silver-Gold Epithermal Deposits in Nevada temporally, and likely genetically related to Miocene volcanic rocks (approximately 22 to 16 Ma) that are manifestations of magmatism associated with the southern segment of the ancestral Cascades continental magmatic arc (du Bray and others, 2014; John and others, 2015). Although the charac­ teristics of deposits in these mining districts are reasonably well known, the geochemistry and petrographic features of the associated igneous rocks have not been well character­ ized. Herein, we (1) synthesize and interpret available and new geochemical, geochronological, and petrographic data for unmineralized volcanic rocks in the Tonopah, Divide, and Goldfield mining districts, (2) characterize compositional similarities and differences among these rocks, and (3) use compositional characteristics to interpret the tectonic and petrologic processes responsible for genesis of these rocks. Only unaltered samples of geologic units that have potential genetic associations with mineralizing processes in the three mining districts are included in this synthesis. Two principal factors identify the igneous rocks most likely to have contributed to mineralizing processes: (1) genetically associated magma reservoirs must have been within 10 km of the deposits of interest and (2) igneous rock ages must be within several million years of deposit ages. In the Tonopah mining district, these units include the Fraction Tuff, Mizpah Trachyte (composed of trachyandesite, andesite, trachydacite, and dacite), and Oddie Rhyolite. In the Divide mining district, these include the Heller Tuff (of Bonham and Garside, 1979), Brougher Rhyolite, trachyandesite of Red Mountain (composed of basaltic trachyandesite and trachyandesite), Divide Andesite (composed of trachydacite and rhyolite), and the volcanics of TONOPAH MINING DISTRICT CALIFORNIA NEVADA JUAN DE FUCA (FARALLON) PLATE PACIFIC PLATE NORTH AMERICAN PLATE Cascadia Subduction zone OREGON IDAHO Southern Segment Ancestral Cascades Arc Terrane Sierra Nevada Batholith SA N A N D R E A S FAULT W A L K E R

L A N E PACIFIC OCEAN Mendocino Triple Junction DIVIDE MINING DISTRICT GOLDFIELD MINING DISTRICT KLONDYKE MINING DISTRICT Tonopah Reno SACRAMENTO 100 KILOMETERS 100 MILES 38º 120º 126º 124º 118º 116º 122º 42º 40º EXPLANATION Ancestral arc rocks Mesozoic plutons Modern arc rocks Major Quaternary eruptive center Fault Thrust Fault—Sawteeth on upper plate Strike-slip fault—Arrows show relative motion Base from U.S. Geological Survey digital data, 1:2,000,000, 2018 Lambert Conformal Conic projection Standard parallels 20º N. and 60º N. Central meridian 121º W., Latitude of origin 40º N. North American Datum of 1927 (NAD 27) Figure 1.  Index map showing the location of the Tonopah, Divide, and Goldfield mining districts, west-central Nevada, relative to the inferred extent of ancestral (green polygons) and modern High Cascades (cross hatched polygons) magmatic arcs in northern California and western Nevada (modified from Colgan and others, 2011; John and others, 2012).

Geologic Setting of Ag-Au Deposits in the Tonopah, Divide, and Goldfield Mining Districts    3 Donovan Peak (Bonham and Garside, 1979). The rhyodacite subunit of the volcanics of Donovan Peak (as subdivided by Bonham and Garside, 1979) is composed of trachydacite and rhyolite, whereas the dacite subunit is composed of trachy­ andesite and trachydacite. In the Goldfield mining district, rocks that have a potential genetic relation with mineralization include the Milltown Andesite (composed of basaltic trachy­ andesite, trachyandesite, andesite, trachydacite, and dacite), an unnamed porphyritic andesite (composed of trachyandesite, andesite, and dacite), and latite. Other igneous rocks in the three mining districts are probably either too old or young and (or) their associated reservoirs so distal that they likely did not contribute heat, fluids, or geochemical components required for deposit formation. Diverse types of volcanic centers are responsible for eruption of volcanic rocks in these three mining districts; characteristics of the associated erupted products are simi­ larly diverse (table 1). Widespread, intermediate composition lava flows and their volcaniclastic equivalents were likely erupted from lava dome complexes, whereas more evolved lavas were emplaced as shallow intrusions, formed extrusive lava domes, or accumulated as lava flows erupted from and arrayed around central vents. Ignimbrite deposits included in the Fraction and Heller Tuffs were erupted from relatively poorly constrained caldera sources, one or more of which may be partly coincident with the Tonopah and (or) Divide mining districts. Geochemical, petrographic, and geochronologic data for volcanic rocks in the Tonopah, Divide, and Goldfield mining districts were systematically compiled by du Bray and others (2019b). Some of the data for volcanic rocks in the Tonopah and Divide mining districts pertain to samples collected during geologic mapping in these areas and analyzed by Bonham and Garside (1979), whereas data pertinent to volcanic rocks in the Goldfield mining district were, in part, derived from samples collected and analyzed by Ashley (1974; 1979; 1990b). In 2013, new petrographic data were derived from systematic re-examination of hundreds of thin sections originally prepared as part of investigations carried out by Bonham and Garside (1979) in the Tonopah and Divide mining districts and by Ashley (1974; 1979; 1990b) in the Goldfield mining district. Additional petrographic, geochemical, and geochronologic data were obtained for samples collected by the authors between 2012-2017, as part of the project titled "Magmatictectonic history and component sources of major precious metal deposits in the southern Walker Lane" funded by the U.S. Geological Survey's (USGS) Mineral Resources Program. A small amount of additional geochemical data for samples from each of the mining districts were compiled from other sources. All of the data used to make the interpretations described in this report are presented in du Bray and others (2019a, b). Geologic Setting of Ag-Au Deposits in the Tonopah, Divide, and Goldfield Mining Districts Basement rocks in the Tonopah, Divide, and Goldfield mining districts are dominated by lower Paleozoic siliciclastic metasedimentary rocks of the "Nolan belt domain" (Crafford, 2007; 2008) that were deposited on lower Paleozoic to Neoproterozoic siliciclastic rocks of the western North American continental margin. Basement rocks are overlain by Miocene and volumetrically minor Oligocene (approximately 25 to 16 Ma) volcanic rocks that are manifestations of magmatism associated with the southern segment of the ancestral Cascades arc (du Bray and others, 2014). Metasedimentary basement rocks 10 km north of Tonopah, Nevada, were intruded by Triassic granitic rocks (John and McKee, 1987). Similar basement rocks, 15 km south of the Tonopah mining district in the Klondyke mining district, include a distinctive Cretaceous muscovite granite (104.0±2.0 Ma, Bonham and Garside, 1979). Basement rocks in the Goldfield mining district, exposed north and northeast of the main mining district, and in deeper workings in the mining district, are intruded by Jurassic quartz monzonite (164.3±1.8 million years ago [Ma], uranium-lead (U-Pb) zircon age via sensitive high-resolution ion microprobe (SHRIMP), and 159.2±0.8 Ma, U-Pb zircon age via laser ablation mass spectrometry; du Bray and others, 2019b). Tonopah Mining District The geologic setting of silver-gold quartz-adulariamuscovite (sericite) veins in the Tonopah mining district is complex and incompletely understood. Arc magmatism in the Tonopah mining district was extinguished by northward migration of the Mendocino triple junction and transition to a transform plate margin in this region at about 12 Ma (Putirka and others, 2012). Prior to this transition, arc magmatism was volumetrically dominated by lava flows and lava dome com­ plexes of the Mizpah Trachyte and ash-flow tuff deposits of the Fraction Tuff. These volcanic rocks contain 8-26 volume percent phenocrysts (table 2), principally plagioclase, pyroxene, and hornblende±biotite; quartz, alkali feldspar, or olivine are present in some samples. New argon (40Ar/39Ar) dates suggest that the Mizpah Trachyte was erupted between about 21.4 and 20.9 Ma (du Bray and others, 2019b). Mineralized veins are hosted primarily by poorly characterized silicic eruptive rocks of the Tonopah Formation (Bonham and Garside, 1979) and by intermediate-composition lava flows and breccias of the Mizpah Trachyte near the south end of a presumed dome complex (John and others, 2015). As mapped by Bonham and Garside (1979) the Mizpah Trachyte in the Tonopah mining district is overlain by multiple cooling units of the rhyolitic Fraction Tuff. They subdivided the Fraction Tuff into a lower unit, the Tonopah Summit Member, and an upper unit, the King Tonopah Member

4    Volcanic Rocks Associated with Silver-Gold Epithermal Deposits in Nevada Table 1.  Characteristic features of eruptive centers and volcanic rock units associated with mineralization in the Tonopah, Divide, and Goldfield mining districts, Nevada.—Continued [m, meter; km, kilometer; km2, square kilometer; wt, weight; %, percent; greater than; approximately; Ma, million years ago; SiO2, silicon dioxide; K, potassium; Ar, argon] Eruptive center/unit Volcanic landform Eruptive products Composition Average SiO2 content (wt. %) Area/dimensions Age range

Notable features Tonopah mining district Mizpah Trachyte Flow dome complexes Dominantly lava flows but also minor debrisflow deposits, volca­ niclastic sedimentary rocks, and scarce, shallow intrusions Trachyandesite, trachydacite, andesite, and dacite 16 km2 exposed area; ~11×8 km, elongated north-south 21.41 to Somewhat compositionally diverse unit. Deposits form flow domes and sequences of lava flows. Debris flow deposits, including lahar deposits, are volumetrically minor. Fraction Tuff Ignimbrite sheet Lithic-rich ash-flow tuff Rhyolite Approximately 50 km2 ex­ posed in an essentially circular area north of route US 6 20.19 to The most widespread, voluminous ash-flow tuff in the in the Tonopah mining district. Tuff commonly con­ tains abundant andesitic flow rock, rhyolitic, and pre-Cenozoic lithic fragments, which inhibited welding. Source caldera may be near Tonopah where a >1,100-m-thick section of Fraction Tuff likely accumulated within its source caldera. Oddie Rhyolite Plugs and domes Hypabyssal intrusions and lava domes Rhyolite 2.0 km2 exposed area; numerous elliptical to irregular masses 0.25 to 0.75 km in diameter 17.29 to Steep sided, endogenous to exogenous domes; columnar jointed in some places. Rarely fresh; altered by late stage magmatic and hydrothermal fluids. Divide mining district Heller Tuff Ignimbrite sheet Lithic-rich ash-flow tuff Trachydacite, dacite, and rhyolite Approximately 150 km2 exposed in an es­ sentially circular area south of route US 6 17.43 to Moderate volume ash-flow tuff. Probably erupted from a caldera centered just north of Donovan Peak. Contains conspicuous quartz and biotite phenocrysts. Brougher Rhyolite Lava flows and domes Flow domes, lava flows Rhyolite 35 km2 exposed area; larg­ est dome cluster km across 17.18 to Forms topographically prominent outcrops throughout the Tonopah and Divide mining districts. Domes are unaltered and postdate mineralization in Tonopah mining district and most mineralization in the Divide mining district. Trachyandesite of Red Mountain Isolated/ coalescing stratovolca­ noes(?) Lava flows Trachyandes­ ite, basaltic trachyandesite, andesite, and basaltic andesite 98 km2 exposed area; extensive outcrop area on east flank of central San Antonio Mountains; smaller centers south and west of Tonopah 17.25 to Large volume of lava flows likely the products of multiple eruptive centers. Contains distinctive, commonly altered olivine phenocrysts. Divide Andesite Flow dome complex Hypabyssal intrusion and lava flows Rhyolite and trachydacite 19 km2 exposed area; 9×3 km ~east-west elongate area 17.6 to Misclassified as andesite by Knopf (1921). Most samples contain distinc­ tive hornblende phenocrysts but are composed of low-silica rhyolite. Probably a resurgent intrusion related to Heller Tuff magmatism. Rhyolite of Donovan Peak Plugs, domes, and dikes Lava domes Rhyolite 1.1 km2 exposed area; several small, 0.5 kmdiameter plugs 16.64 to Volumetrically minor component of vol­ canics of Donovan Peak. Relatively crystal-poor rhyolite. Rhyodacite of Donovan Peak Plugs, dikes, and domes Lava domes and flows Trachydacite, rhyolite, and minor trachy­ andesite 5.3 km2 exposed area; 0.25-0.5 km diameter domes confined to the Divide mining district 16.93 to Prominent, dark-weathering lava flows capping peaks and ridges in the Divide mining district. Dacite of Donovan Peak Plugs Hypabyssal intrusions Trachydacite and trachyandesite 1.6 km2 exposed area; two irregularly-shaped masses in the Divide mining district; larger mass is 0.65×2 km 16.70 Ma Unit is the most crystal-rich component of the volcanics of Donovan Peak and contains relatively abundant plagioclase phenocrysts.

Geologic Setting of Ag-Au Deposits in the Tonopah, Divide, and Goldfield Mining Districts    5 (itself composed of lower and upper cooling units). Our work suggests that essentially all the Miocene ash-flow tuff in the Tonopah mining district, where it is at least 1,000 meters (m) thick (base not exposed), is a single unit of Fraction Tuff. It is thickest and most widespread north of Tonopah, where it may constitute the eruptive product associated with a caldera whose northern margin is about 9 km north of Tonopah (John and others, 2015). The average (given because ash-flow tuff eruption is an essentially geologically instantaneous event, whereas the eruption of lava flow complexes typically spans considerable time intervals) of new 40Ar/39Ar dates of the Fraction Tuff suggest that the ash flows were erupted 20.04 Ma (du Bray and others, 2019b). Geochemical, petrographic, and geochronologic characteristics of all ash-flow tuff exposed north of Tonopah suggest that these rocks represent a single eruptive sequence, referred to hereafter as the Fraction Tuff, and that the Tonopah King and Tonopah Summit Members do not constitute distinct, mappable units. A diverse group of volcanic rocks overlie or intrude the Fraction Tuff in the area around Tonopah. The Miocene Siebert Formation of Bonham and Garside (1979) includes thick, voluminous volcaniclastic deposits that contain abundant blocks of Mizpah Trachyte and Fraction Tuff, and thinner, less voluminous ash-fall deposits. We tentatively interpret the Siebert Formation to have accumulated in the basin formed during collapse of the caldera that erupted the Fraction Tuff following eruption of the tuff. In the Tonopah mining district, new 40Ar/39Ar dates (du Bray and others, 2019b) indicate that the Oddie Rhyolite (erupted between about 17.3 to 16.6 Ma), Brougher Rhyolite (erupted between about 17.2 to 16.6 Ma), trachyandesite of Red Mountain (erupted between about 17.3 to 16.0 Ma), and volcanics of Lime Mountain (erupted between about 16.8 to 12.4 Ma) represent magmatism that is at least 2 million years (m.y.) younger than that associated with the Fraction Tuff and deposition of the Siebert Formation and thus postdate mineralization in the main mining district, and are consequently substantially younger than other volcanic rocks in the Tonopah mining district. Silver-gold deposits in the Tonopah mining district, from which greater than (>)174 million ounces (Moz) silver and 1.86 Moz gold were produced (mostly from 1910 to 1930; Bonham and Garside, 1979), consist of high-to low-angle quartz-sulfideadularia-muscovite veins that replaced breccia and gouge in fault zones, and to a lesser extent, filled open-spaces. The principal hydrothermal sulfide minerals include argentite, polybasite, pearcite, sphalerite, galena, chalcopyrite, pyrite, electrum, and pyrargyrite. In addition to adularia and muscovite, the veins contain hydrothermal calcium-magnesium-manganese (Ca-Mg-Mn) carbonate minerals and minor barite. New 40Ar/39Ar dates for most adularia from quartz-adularia-muscovite veins in altered Mizpah Trachyte range from approximately 20.5 to 19.9 Ma (du Bray and others, 2019b). Dates of the veins suggest nearly simultane­ ous eruption of the Fraction Tuff, caldera formation(?), and vein mineralization (John and others, 2015). Table 1.  Characteristic features of eruptive centers and volcanic rock units associated with mineralization in the Tonopah, Divide, and Goldfield mining districts, Nevada.—Continued [m, meter; km, kilometer; km2, square kilometer; wt, weight; %, percent; greater than; approximately; Ma, million years ago; SiO2, silicon dioxide; K, potassium; Ar, argon] Eruptive center/unit Volcanic landform Eruptive products Composition Average SiO2 content (wt. %) Area/dimensions Age range

Notable features Goldfield mining district Milltown Andesite Stratovol­ cano(?) Dominantly lava flows but also debris-flow deposits, volcaniclas­ tic sedimentary rocks, and scarce, shallow intrusions Trachyandesite, trachydacite, andesite, dacite, and basaltic trachy­ andesite Full extent not delineated; discontinuously covers an area at least 10×14 km throughout the entire Goldfield mining district 22.27 to Possibly a geologically composite unit. Characterized by very broad compo­ sitional range, including rocks with 54 to 69 wt percent silica. Porphyritic andesite Lava flows and domes Lava flows, lava domes, and carapace breccia deposits Andesite, dacite, trachydacite, and trachyan­ desite Full extent not delineated; discontinuously cov­ ers an area at least 8×14 km and largely coextensive with expo­ sures of the Milltown Andesite 22.36 to Contains less, but distinctly coarser grained plagioclase, less clinopyrox­ ene, more hornblende, and occasional quartz phenocrysts, which distinguish this unit from the Milltown Andesite. Includes a distinct marginal breccia facies. Latite Lava flows and domes Lava flows Latite Approximately 10 km2 exposed area in an approximately rectangular-shaped area several kilometers north-northeast of Goldfield Rhyolite of Wildhorse Spring Plugs and lava flows Lava flows and flow breccia deposits Rhyolite Full extent not delineated; discontinuously covers an area at least 5×5 km in the northeast part of the Goldfield mining district. 21.48 to Quartz-rich high silica rhyolite forms a series of lava domes. Relation­ ship to other volcanic rocks in the Goldfield main mining district are not established. 1K-Ar and 40Ar/39Ar ages (du Bray and others, 2019b)

6    Volcanic Rocks Associated with Silver-Gold Epithermal Deposits in Nevada Divide Mining District The geologic setting of the Divide silver-gold mining district, centered about 8 km south of Tonopah, Nev., is similar to the Tonopah mining district with Miocene volcanic rocks overlying Paleozoic basement rocks and Fraction Tuff. Divide mining district volcanic rocks contain 6-21 volume percent phenocrysts (table 2), principally plagioclase, biotite, and hornblende; quartz and alkali feldspar are present in samples of the more silicic units, especially the Heller Tuff and the Oddie and Brougher Rhyolites. Except for the Heller Tuff, named for prominent exposures at Heller Butte immediately south of Tonopah, most Miocene tuff south of Tonopah (including the Divide mining district) was assigned to the "Tonopah Summit member" of the Fraction Tuff by Bonham and Garside (1979). The Heller Tuff, is megascopically similar to the Fraction Tuff but is distinguished by generally lower silica contents (averages, about 70 and 75 weight percent, respectively) and distinctly greater total phenocryst contents (24 versus 6 percent, respectively), especially quartz (4 percent versus a trace amount, respectively ) and biotite (2 percent versus a trace amount, respectively ) (du Bray and others, 2019b). The average (given because ash-flow tuff eruption is an essentially geologically instantaneous event) of new 40Ar/39Ar dates for the Heller Tuff is 17.34 Ma, excluding one imprecise age (du Bray and others, 2019b), which indicates that the Heller Tuff is also about 3 m.y. younger than the Fraction Tuff. Ash-flow tuff exposures south of Tonopah Summit, a low pass about 4 km southeast of the town of Tonopah, correlated by Bonham and Garside (1979) with the Tonopah Summit Member of the Fraction Tuff, are distinctly different from the Fraction Tuff but indistinguishable in age, petrography, and geochemistry from those of the Heller Tuff. Therefore, we have reassigned those exposures to the Heller Tuff. Exposures of the Heller Tuff ash-flow tuff sheet are most widely distributed and thickest south of Tonopah Summit, which suggests that these deposits may have been erupted from an inferred caldera whose southern margin is about 14 km south of Tonopah, where Bonham and Garside (1979) described voluminous megabreccia deposits and a possible caldera margin in the Klondyke mining district. The eastern part of the Divide mining district and the large area farther east are dominated by additional Mio­ cene volcanic rocks, including the Divide Andesite (erupted between about 17.6 to 17.4 Ma) and the compositionally diverse volcanics of Donovan Peak (erupted between about 17.3 to 16.5 Ma). These rocks consist of lava flows, lava dome complexes, and hypabyssal plugs. The Divide Andesite, as well as significantly less voluminous rocks included in the volcanics of Donovan Peak, may represent resurgent magma­ tism associated with the inferred southern caldera. Similarly, the arcuate array of Brougher Rhyolite (erupted between about 17.2 to 16.6 Ma) flow domes east and south of Tonopah Sum­ mit may represent ring-fracture hosted, moat intrusions and flows, associated with the inferred southern caldera. Ages of air-fall tuffs of the Siebert Formation in the Tonopah and Divide mining districts range broadly from about 17.5 to 15.5 Ma (Bonham and Garside, 1979; du Bray and others, 2019b). Ages of the Divide Andesite, erupted between 17.6 to 17.3 Ma, the volcanics of Donovan Peak, erupted between about 17.3 to 16.5 Ma, and the parts of the trachyandesite of Red Mountain erupted between about 17.3 and 16.0 Ma (du Bray and others, 2019b), are commensurate with the age of epithermal precious metal deposits in the Divide mining district. Trachyandesite of Mud Lake lava flows, erupted about 13.7 Ma (du Bray and others, 2019b) southeast of the Divide mining district, are considerably younger than mineralization in the Divide mining district. Mineralized veins in the Divide mining district are in Miocene volcanic rocks, principally the Oddie Rhyolite, Heller Tuff, and Siebert Formation (Nolan, 1935; Bonham and Garside, 1979). New adularia 40Ar/39Ar dates of quartz-adularia veins in the Divide mining district suggest mineralization occurred between about 17.3 and 16.8 Ma (du Bray and others, 2019b). About 3 Moz of silver and 30,000 oz of gold were produced; cerargyrite was the principal ore mineral, although sphalerite, argentiferous galena, chalcopyrite, molybdenite, electrum, acanthite, pyrargyrite, and possible tetrahedrite have been identified in dump samples (Bonham and Garside, 1974; Graney, 1987; Erdman and Barabas, 1996). Goldfield Mining District The Goldfield mining district is also dominated by Miocene volcanic rocks, principally lava flows but also brec­ cias and lava domes. These rocks represent the approximate southernmost extent of magmatism related to the ancestral Cascades arc (du Bray and others, 2014), which ended with northward migration of the Mendocino triple junction past the Goldfield mining district at about 13 Ma (Putirka and oth­ ers, 2012). Volcanic rocks associated with mineralization in the Goldfield mining district contain 17-33 volume percent phenocrysts (table 2), principally plagioclase, pyroxene, horn­ blende, and biotite. These rocks are cut by a dense network of west-northwest and north-to northeast-striking normal faults. Ashley (1990a) reports that silicic ash-flow tuff and flows (the Vindicator Rhyolite and the Morena and Sandstorm Rhyolites of Ransome, 1909) are associated with eruptions from a 6-kmdiameter caldera delineated by a series of poorly defined, presumed ring fractures. However, geophysical data suggest that these fractures are related to pluton emplacement rather than caldera collapse (Blakely and others, 2007). Milltown Andesite lava flows are volumetrically domi­ nant in the Goldfield mining district and are host rocks for deposits in the southern part of the main mining district near Florence Hill. New 40Ar/39Ar ages for the Milltown Andesite indicate eruption about 22.3 to 21.9 Ma (du Bray and oth­ ers, 2019b). Other important units in the Goldfield mining district include a series of unnamed porphyritic andesite lava flows (dacite of Ransome, 1909 and porphyritic rhyodacite

Geologic Setting of Ag-Au Deposits in the Tonopah, Divide, and Goldfield Mining Districts    7 Table 2.  Summary of petrographic characteristics for volcanic rock units associated with mineralization in the Tonopah, Divide, and Goldfield mining districts, Nevada. Mineral abundances are averages of microscope-based estimates, relative to total rock, calculated from data presented by du Bray and others (2019b). [Qtz, quartz; Alk-, alkali feldspar; Plag, plagioclase; Hb, hornblende; Bi, biotite; Pyx, pyroxene; Ol, olivine; Opq, opaque iron-titanium (Fe-Ti) oxides; , crystals. TR, trace. Capitalized entries in the Cpx/Opx column define whether clinopyroxene or orthopyroxene form the dominant pyroxene phenocryst population. Accessory minerals: AP, apatite; TI, titanite; ZR, zircon; AL, allanite; %, percent; mm, millimeter; —, not detected] Map unit Average abundances (%) Estimated average crystal size (mm) Size of largest crystal (mm) Accessory minerals Qtz Alkfspr Plag Hb Bi Pyx Cpx/ Opx Ol Opq Total

Color index Qtz Alkfspr Plag Hb Bi Pyx Ol Opq Qtz Alkfspr Plag Hb Bi Pyx Ol Opq Tonopah mining district Mizpah Trachyte — — TR — — — — 0.10 — — — 0.40 AP, ZR Fraction Tuff TR TR TR — — — TR TR — — — — 0.31 ZR, AL, TI Oddie Rhyolite — — — TR — — — — — — 0.17 AP, ZR Divide mining district Heller Tuff TR — — — — — — — 0.40 ZR, AL, TI, AP Brougher Rhyolite TR — — — — — — — 0.26 TI, AL, AP, ZR Trachyandesite of Red Mountain — — TR — — — 0.4 0.03 — — — — Divide Andesite — — — — — — — — — 0.08 — — — — 0.44 AP, ZR Rhyolite of Donovan Peak — TR TR TR — — — 0.05 — — 0.24 ZR, AP Rhyodacite of Donovan Peak — — — — — — 0.07 — — 0.28 AP, ZR Dacite of Donovan Peak — — TR TR — — — — 0.05 — — — 0.24 AP Goldfield mining district Milltown Andesite — — — — — — 0.08 — — — 0.37 AP Porphyritic andesite — — — — — — 0.31 AP, TI Latite TR — TR — — — — — — — — — 0.40 AP, ZR Rhyolite of Wild­ horse Spring — — — — — — — — — 0.50 TI

8    Volcanic Rocks Associated with Silver-Gold Epithermal Deposits in Nevada of Ashley, 1974), latite (Ransome, 1909), and volumetrically minor rhyolites including the Sandstorm and Morena Rhyolites of Ransome (1909) and the rhyolite of Wildhorse Spring (Ashley, 1974) erupted onto an Oligocene erosion surface of Jurassic quartz monzonite and Paleozoic siliciclastic rocks. Porphyritic andesite, exposed east of the main mining district and in mines in the main mining district, is a second important deposit host and is thought to be temporally coincident with deposit formation (Ransome, 1909). New 40Ar/39Ar dates for the porphyritic andesite indicate eruption between about 22.3 to 21.9 Ma (du Bray and others, 2019b). Latite ~22.7 Ma, du Bray and others, 2019b), which partly covers Morena Rhyolite and the erosion surface, is the third principal deposit host rock, especially of Au-Ag deposits in the main mining district immediately northeast of Goldfield (Ransome, 1909; Searls, 1948). North and east of the main mining district, latite consists of gray-purple weathering lava flows and lesser tuffs in which the dull luster of feldspar and mafic phenocrysts reflects partial alteration to very fine-grained clay-mica minerals, quartz, and calcite. In the main mining district, latite has been pervasively altered to quartz, alunite, pyrophyllite, kaolinite, pyrite, illite, montmorillonite and other minerals, rendering distinctions with similarly altered porphyritic andesite and Milltown Andesite problematic. A new 40Ar/39Ar date of sanidine indicates eruption of the Morena Rhyolite 25.17±0.03 Ma (du Bray and others, 2019b). New 40Ar/39Ar dates of biotite and sanidine in rhyolite of Wildhorse Spring indicate eruption between about 21.5 Ma and 21.2 Ma, respectively, (du Bray and others, 2019b). The rhyolite of Wildhorse Spring is temporally but not spatially associated with mineralized rocks in the Goldfield mining district. A new 40Ar/39Ar date of sanidine indicates eruption of the Sandstorm Rhyolite 21.56±0.05 Ma (du Bray and others, 2019b). A series of younger, middle Miocene mafic lava flows, including the Mira Basalt, basalt of Blackcap Mountain, and Malpais Basalt, overlie mineralized, early Miocene rocks in the Goldfield mining district. Miocene volcanic rocks in parts of the Goldfield mining district are extensively faulted. Breccia fragments and gouge in faults immediately north and northeast of the town of Goldfield (main mining district) have been replaced and encrusted by quartz, alunite, kaolinite and numerous gold-copper-silverantimony-arsenic-bismuth-tin-tellurium-selenium-sulfur (Au-Cu-Ag-Sb-As-Bi-Sn-Te-Se-S) minerals. Miocene volcanic rocks and pre-Tertiary rocks adjacent to mineralized faults have been extensively altered to quartz, alunite, kaolinite, pyrite, illite and other aluminosilicate and aluminosulfate minerals. The mineralized faults are localized in the southwestern part of a 40 square kilometer (km2) area of intensely altered rocks (Ransome, 1909; Ashley and Albers, 1975; Ashley, 1990a; Vikre and Henry, 2011). About 4.19 Moz of gold and 1.45 Moz of silver were produced from main mining district fault zones and adjacent wall rocks, mostly from 1903-1940 (Albers and Stewart, 1972). Most new 40Ar/39Ar dates of alunite in mineralized fault zones, and in adjacent wall rocks, are between 21.75 and 19.2 Ma (du Bray and others, 2019b). Analytical Methods Standard petrographic microscope techniques were employed to identify phenocryst minerals and estimate their abundances in 456 samples of volcanic rocks from the Tonopah, Divide, and Goldfield mining districts. Phenocryst size and crystallinity, rock textures, groundmass characteristics, accessory mineral assemblages, and qualitative relative alteration intensity were also determined for each sample. New whole-rock compositions for 190 samples analyzed between 2012 and 2018 were carried out in analytical laboratories under contract to the U.S. Geological Survey (du Bray and others, 2019b). Pertinent analytical methods are described by Taggart (2002). Major oxide abundances (recalculated to 100 percent, volatile-free) were determined by wavelength dispersive x-ray fluorescence spectrometry. A 55-element method that employs a combination of inductively coupled plasma-atomic emission spectrometry and inductively coupled plasma-mass spectrometry was used to determine trace element abundances. Compositions for an additional 108 igneous rock samples in the Tonopah, Divide, and Goldfield mining districts are presented by du Bray and others (2019b). Only unaltered samples, unaffected by post-magmatic hydrothermal alteration or weathering, were used in our petrologic investigation of the Oligocene and Miocene volcanic rocks in the Tonopah, Divide and Goldfield mining districts. Du Bray and others (2019b) define the geochemical parameters that were used to identify altered samples and remove them from the interpreted data compilation. Volcanic rock compo­ sitions have been classified in accord with the International Union of Geological Sciences (IUGS) nomenclature system (Le Maitre, 2002). Petrographic Characteristics Essentially all volcanic rocks associated with mineralization in the Tonopah, Divide, and Goldfield mining districts are porphyritic. Petrographic characteristics of each volcanic rock unit in these mining districts are relatively distinct. As summarized (table 2), most phenocrysts are fine (<1 millimeter [mm]) to medium grained (1-5 mm). Euhedral, albite-twinned plagioclase laths are ubiquitous in these volcanic rocks. Almost all plagioclase phenocrysts are oscillatory zoned and some, especially in intermediate-composition lava flows, are variably sieve textured. Plagioclase in intermediate composition lava flows consists of multiple populations defined by combinations of (1) grain size, (2) presence of distinctive reaction rims, (3) zones that contain mineral and (or) glass inclusions, and (4) resorption textures. Brown to green pleochroic hornblende forms euhedral to subhedral acicular crystals; in weathered or altered samples hornblende is completely replaced by black amorphous material. Clinopyroxene forms pale tan to pale green subhedral to euhedral phenocrysts in intermediate composition lava flows. Biotite is subhedral, tan

Trace Element Geochemistry to deep reddish-brown, and like hornblende, is completely altered to amorphous material in many samples. Olivine, in some samples of the trachyandesite of Red Mountain, forms distinctive but variably altered subhedral phenocrysts. Fine-grained opaque iron-titanium (Fe-Ti) oxide crystals are ubiquitous in all these rocks. Quartz and alkali feldspar occur only in the most felsic rocks of the Tonopah, Divide, and Goldfield mining districts. Quartz forms variably resorbed and embayed, rounded, anhedral to subhedral phenocrysts and sanidine forms variably and weakly perthitic Carlsbad-twinned, euhedral to subhedral phenocrysts. The groundmass of these volcanic rocks contains microphenocryst (typically 0.05 to 0.2 mm long) assemblages dominated by plagioclase and Fe-Ti oxide minerals but also includes variable combinations of quartz, alkali feldspar, clinopyroxene, hornblende, and biotite. Metastable and variably devitrified volcanic glass is also a major groundmass component. The Fraction and Heller Tuffs also contain abundant, locally derived lithic fragments and much less abundant but distinctive, crustally derived xenoliths, especially fragments of Mesozoic granitic and Paleozoic metasedimentary rocks. Major Oxide Geochemistry Compositions of volcanic rocks in the Tonopah, Divide, and Goldfield mining districts vary essentially continuously from about 54 to 78 weight percent silicon dioxide (SiO2), although rocks with 66 to 71 weight percent SiO2 are somewhat under-represented (figs. 2-3). Samples from the Divide mining district contain about 61 to 77 weight percent SiO2, those from the Tonopah mining district contain about 60 to 78 weight percent SiO2, and those of most samples from Goldfield mining district are restricted to between about 54 and 69 weight percent SiO2. The volcanic rocks in these mining districts are transitionally alkaline; many of their compositions cluster between the line that separates alkaline and subalkaline compositions on a total alkali-silica diagram (fig. 2) and the alkaline-subalkaline dividing line of Irvine and Baragar (1971). Intermediate composition rocks, with <65 weight percent SiO2, have whereas the more felsic rocks have (fig. 3). Relative to standard metrics (in cited sources), compositions (table 3) of most volcanic rocks in the Tonopah, Divide, and Goldfield mining districts are metaluminous to weakly peraluminous (Shand, 1951), calc-alkalic to weakly alkali-calcic (Frost and others, 2001); magnesian (calc-alkaline) to weakly ferroan (tholeiitic) (Frost and others, 2001); and follow a calc-alkaline (Irvine and Baragar, 1971) differentiation trend. However, several aspects of the volcanics of Donovan Peak, and perhaps the Divide Andesite in the Divide mining district, are distinct relative to compositions of rocks in the Tonopah and Goldfield mining districts. At any given silica content, the Divide rocks are more alkalic, contain distinctly greater abundances of TiO2 and sodium oxide (Na2O), lower magnesium oxide (MgO) abundances, and have higher ferrous oxide (FeO*, total iron expressed as ferrous oxide)/MgO than the Tonopah and Goldfield rocks. Among volcanic rocks in the Tonopah, Divide, and Goldfield mining districts, concentrations of TiO2, aluminum oxide (Al2O3), FeO*, MgO, calcium oxide (CaO), and phosphorous pentoxide (P2O5) decrease relatively systematically with increasing SiO2 (fig. 3). Na2O abundances do not vary systematically with changing silica content. Abundances of K2O increase broadly with increasing SiO2 content, forming an array coincident with high-K compositions (Gill, 1981). The Heller Tuff contains distinctly lower SiO2 and characteristically elevated TiO2, Al2O3, FeO*, MgO, and CaO abundances relative to those of the Fraction Tuff. Trace Element Geochemistry Volcanic rocks of the Tonopah, Divide, and Goldfield mining districts have trace element abundances similar to those of other convergent-margin, subduction-related igneous rocks. In particular, these rocks are enriched in large-ion lithophile elements (LILE) and depleted in high-field-strength elements (HFSE) (du Bray and others, 2019b). Abundances of niobium, lead, rubidium, tantalum, thorium, ( Nb, Pb, Rb, Ta, Th) and U increase, whereas those of cobalt, copper, scandium, strontium, vanadium, yttrium, ytterbium, and zinc (Co, Cu, Sc, Sr, V, Y, Yb, and Zn), and europium/europium* (Eu/Eu*) and total rare-earth element (REE) content, decrease with increasing silica content. Barium and zirconium (Ba and Zr) abundances vary inconsistently in samples with 54 to about 69 weight percent SiO2 and then decrease dramatically in more silica-rich samples. Abundances of chromium hafnium, lanthanum, molybdenum, and nickel (Cr, Hf, La, Mo, and Ni), and lanthanum/lutetium exhibit no systematic variation with respect to increasing silica content. Abundances of most trace elements in samples of volcanic rocks from the Tonopah and Goldfield mining districts are essentially indistinguishable. However, at any given silica content, abundances of Sr and V in samples of volcanics of Donovan Peak and the Divide Andesite, in the Divide mining district, are low, whereas abundances of Ba, Hf, La, Nb, Ta, Y, Yb, Zr, and total REEs are high relative to those for Tonopah and Goldfield mining district volcanic rocks. The Heller Tuff contains abundances of Rb and Y that are low and abundances of Ba, Hf, La, Zr, and total REEs that are high relative to those for Tonopah and Goldfield mining district volcanic rocks. Abundances of the metals, including Cu, Mo, Pb, and Zn, in volcanic rocks of the Tonopah, Divide, and Goldfield mining districts are similar to average abundances in basalt and granite (Turekian and Wedepohl, 1961). Most volcanic rocks in the Tonopah, Divide, and Goldfield mining districts have similar to values for most subduction-related igneous rocks (Gill, 1981). Similarly, large Ba/Nb and Ba/Ta values reflect negative Nb-Ta anomalies, a recognized characteristic of subduction-related magmatic arc

10    Volcanic Rocks Associated with Silver-Gold Epithermal Deposits in Nevada Rhyolite Dacite Andesite Basaltic andesite Trachydacite Trachyandesite Basaltic trachyandesite Alkaline Subalkaline Na2O + K2O, in weight percent SiO2, in weight percent EXPLANATION Tonopah district Divide district Mizpah Trachyte Fraction Tuff Oddie Rhyolite Porphyritic andesite Milltown Andesite Rhyolite of Wildhorse Springs Goldfield district Heller Tuff Divide Andesite Brougher Rhyolite Rhyodacite of Donovan Peak Dacite of Donovan Peak Rhyolite of Donovan Peak Trachyandesite of Red Mountain Figure 2.  Total alkali-silica variation diagram showing compositions of Miocene volcanic rocks associated with mineralization in the Tonopah, Divide, and Goldfield mining districts, Nevada. Field boundaries from Le Maitre (2002). Alkaline-subalkaline dividing line (red) from Irvine and Baragar (1971). (Na2O, sodium oxide; K2O, potassium oxide; SiO2, silicon dioxide).

Trace Element Geochemistry    11 Shoshonitic High-K Low-K Medium-K SiO2, in weight percent FeO*, in weight percent MnO, in weight percent MgO, in weight percent TiO2, in weight percent Al2O3, in weight percent K2O, in weight percent P2O5, in weight percent CaO, in weight percent Na2O, in weight percent SiO2, in weight percent A. TiO2 C. AI2O3 E. FeO* G. MnO I. MgO B. CaO D. Na2O F. K2O H. P2O5 EXPLANATION Tonopah district Divide district Goldfield district Mizpah Trachyte Fraction Tuff Oddie Rhyolite Porphyritic andesite Milltown Andesite Rhyolite of Wildhorse Spring Heller Tuff Divide Andesite Brougher Rhyolite Rhyodacite of Donovan Peak Dacite of Donovan Peak Rhyolite of Donovan Peak Trachyandesite of Red Mountain Figure 3.  Variation diagrams showing abundances of major oxides (weight percent) in Miocene volcanic rocks associated with mineralization in the Tonopah, Divide, and Goldfield mining districts, Nevada. Field boundaries on potassium oxide (K2O) versus silicon dioxide (SiO2) diagram from Le Maitre (2002); high K-shoshonitic dividing line from Ewart (1982). (TiO2, titanium dioxide; Al2O3, aluminum oxide; FeO*, total iron expressed as ferrous oxide; MnO, manganese oxide; CaO, calcium oxide; Na2O, sodium oxide, K2O, potassium oxide; P2O5, phosphorous pentoxide).

12    Volcanic Rocks Associated with Silver-Gold Epithermal Deposits in Nevada Table 3.  Representative compositions of volcanic rock units associated with mineralization in the Tonopah, Divide, and Goldfield mining districts, Nevada (du Bray and others, 2019b).—Continued [Major oxides (recalculated to 100 percent, volatile free) are in weight percent; FeO* is total iron expressed as ferrous oxide, FeO. Total_I, is prenormalization total; Trace elements are in parts per million; LOI—loss on ignition.Unit names: Fraction, Fraction Tuff; Heller, Heller Tuff; Brougher, Brougher Rhyolite; Red Mtn; trachyandesite of Red Mountain; Mizpah, Mizpah Trachyte; Oddie, Oddie Rhyolite; Divide, Divide Andesite; Rhd Don; volcanics of Donovan Peak-rhyodacite; Dac Don, volcanics of Donovan Peak-dacite; Rhy Don, volcanics of Donovan Peak-rhyolite; and, porphyritic andesite; Mill­ town, Milltown Andesite; Wildhorse, rhyolite of Wildhorse Spring; Long, longitude; lat, latitude; —, no data; SiO2, silicon dioxide; TiO2, titanium dioxide; Al2O3, aluminum oxide; FeO*; total iron expressed as ferrous oxide; MnO, manganese oxide; MgO, magnesium oxide; CaO, calcium oxide; Na2O, sodium oxide; K2O, potassium oxide; P2O5, phosphorus pentoxide; Ba, barium; Cs, cesium; Rb, rubidium; Sr, strontium; Y, yttrium; Zr, zirconium; Hf, hafnium; Nb, niobium; Th, thorium; U, uranium; Ga, gallium; La, lanthanum; Ce, cerium; Pr, praseodymium; Nd, neodymium; Sm, samarium; Eu, europium; Gd, gado­ linium; Tb, terbium; Dy, dysprosium; Ho, holmium; Er, erbium; Tm, thulium; Yb, ytterbium; Lu, lutetium; Co, cobalt; Cr, chromium; Ni, nickel; Sc, scandium; V, vanadium; Cu, copper; Mo, molybdenum; Pb, lead; Zn, zinc; Sn, tin; W, tungsten; Ta, tantalum] Sample 13-T-1 14-T-02 00-T-2 00-T-6 GF93-12 GF87-61A GF93-17C GF03-1 Unit Fraction Fraction Fraction Heller Heller Heller Brougher Brougher Red Mtn Red Mtn Mizpah Mizpah Mizpah Oddie Divide Divide Rhd Don Rhd Don Dac Don Rhy Don and and Milltown Milltown Milltown Wildhorse Long 117.1726 117.2230 117.2443 117.2352 117.1726 117.2265 117.2540 117.2133 117.2523 117.2430 117.2213 117.2444 117.2118 117.2234 117.2131 117.1400 117.2038 117.1989 117.1839 117.2037 117.2166 117.1448 117.2186 117.2111 117.2319 117.1294 Lat SiO2 TiO2 Al2O3 FeO* MnO — MgO CaO Na2O K2O P2O5 Total_I LOI — — Ba Cs — Rb Sr Y Zr — Hf — Nb Th U Ga La Ce Pr — Nd — Sm —

Trace Element Geochemistry    13 Table 3.  Representative compositions of volcanic rock units associated with mineralization in the Tonopah, Divide, and Goldfield mining districts, Nevada (du Bray and others, 2019b).—Continued [Major oxides (recalculated to 100 percent, volatile free) are in weight percent; FeO* is total iron expressed as ferrous oxide, FeO. Total_I, is prenormalization total; Trace elements are in parts per million; LOI—loss on ignition.Unit names: Fraction, Fraction Tuff; Heller, Heller Tuff; Brougher, Brougher Rhyolite; Red Mtn; trachyandesite of Red Mountain; Mizpah, Mizpah Trachyte; Oddie, Oddie Rhyolite; Divide, Divide Andesite; Rhd Don; volcanics of Donovan Peak-rhyodacite; Dac Don, volcanics of Donovan Peak-dacite; Rhy Don, volcanics of Donovan Peak-rhyolite; and, porphyritic andesite; Mill­ town, Milltown Andesite; Wildhorse, rhyolite of Wildhorse Spring; Long, longitude; lat, latitude; —, no data; SiO2, silicon dioxide; TiO2, titanium dioxide; Al2O3, aluminum oxide; FeO*; total iron expressed as ferrous oxide; MnO, manganese oxide; MgO, magnesium oxide; CaO, calcium oxide; Na2O, sodium oxide; K2O, potassium oxide; P2O5, phosphorus pentoxide; Ba, barium; Cs, cesium; Rb, rubidium; Sr, strontium; Y, yttrium; Zr, zirconium; Hf, hafnium; Nb, niobium; Th, thorium; U, uranium; Ga, gallium; La, lanthanum; Ce, cerium; Pr, praseodymium; Nd, neodymium; Sm, samarium; Eu, europium; Gd, gado­ linium; Tb, terbium; Dy, dysprosium; Ho, holmium; Er, erbium; Tm, thulium; Yb, ytterbium; Lu, lutetium; Co, cobalt; Cr, chromium; Ni, nickel; Sc, scandium; V, vanadium; Cu, copper; Mo, molybdenum; Pb, lead; Zn, zinc; Sn, tin; W, tungsten; Ta, tantalum] Sample 13-T-1 14-T-02 00-T-2 00-T-6 GF93-12 GF87-61A GF93-17C GF03-1 Unit Fraction Fraction Fraction Heller Heller Heller Brougher Brougher Red Mtn Red Mtn Mizpah Mizpah Mizpah Oddie Divide Divide Rhd Don Rhd Don Dac Don Rhy Don and and Milltown Milltown Milltown Wildhorse Eu — Gd — Tb — Dy — Ho — Er — Tm — Yb — Lu — Co — — Cr — — — — — — — — — — — — — — Ni — Sc — — — — — — — — — — — — — — Cu — — — — — — — — — — — — Mo — — — — — — — — — — Pb Zn Sn — — — — — — — — — — W — — — — — — — Ta — — — —

14    Volcanic Rocks Associated with Silver-Gold Epithermal Deposits in Nevada rocks. Furthermore, almost all volcanic rocks from these mining districts have relative abundances of Rb and Y+Nb consistent with arc magma compositions (fig. 4). Chondrite normalized REE patterns for volcanic rocks of the Tonopah, Divide, and Goldfield mining districts are negatively sloping and include variably developed negative Eu anomalies (fig. 5). values, and therefore chondrite normalized pattern slopes, are not correlated with silica content; these values span a limited range from about 15 to 25. Light REE (LREE) pat­ tern segments are distinctly more steeply sloped than heavy REE (HREE) pattern segments (fig. 5). HREE pattern segments for volcanic rocks in all three mining districts, especially those for the felsic units, are distinctly U-shaped. Chondrite-normalized REE patterns for intermediate composition rocks from the Tonopah, Divide, and Goldfield mining districts are completely overlapping and indistinguishable. Chondrite-normalized REE patterns for felsic rocks from the Tonopah and Divide mining districts are also essentially indistinguishable. Samples of the Oddie Rhyolite, and to a lesser extent the Fraction Tuff, from the Tonopah mining district have well-developed negative Eu anomalies (Eu/Eu* as small as 0.31) and samples with the lowest Eu/Eu* values have the highest silica contents. REE abundances for the felsic volcanic rocks from the Divide mining district vary over a broad range. As a group, REE patterns for the Divide Andesite and volcanics of Donovan Peak define a relatively broad REE abundance array. However, patterns, and therefore REE abundances, for each of the four units define relatively limited and distinct compositional ranges. Among volcanic rocks in the Divide mining district, the Brougher Rhyolite and the rhyolite of Donovan Peak have the largest negative Eu anomalies (Eu/Eu* values of 0.55 and 0.53, respectively). Diagnostically, most samples Y + Nb, in parts per million Volcanic arc Within plate Syn collisional Rb, in parts per million EXPLANATION Mizpah Trachyte Fraction Tuff Oddie Rhyolite Tonopah district Divide district Porphyritic andesite Milltown Andesite Rhyolite of Wildhorse Springs Goldfield district Heller Tuff Divide Andesite Brougher Rhyolite Rhyodacite of Donovan Peak Dacite of Donovan Peak Rhyolite of Donovan Peak Trachyandesite of Red Mountain Figure 4.  Trace-element, tectonic-setting-discrimination variation diagram showing the composition of volcanic rock units associated with mineralization in the Tonopah, Divide, and Goldfield mining districts, Nevada. Tectonic setting-composition boundaries from Pearce and others (1984). (Rb, rubidium; Y, yttrium; Nb, niobium).

Trace Element Geochemistry    15 D. Divide, felsic composition (>65 % SiO2) samples C. Divide, intermediate composition (<65 % SiO2) samples A. Tonopah, intermediate composition (<65 % SiO2) samples B. Tonopah, felsic composition (>65% SiO2) samples E. Goldfield, intermediate composition (<65 % SiO2) samples Lanthanum (La) Cerium (Ce) Praseodymium (Pr) Neodymium (Nd) Samarium (Sm) Europium (Eu) Gadolinium (Gd) Terbium (Tb) Dysprosium (Dy) Holmium (Ho) Erbium (Er) Thulium (Tm) Ytterbium (Yb) Lutetium (Lu) Lanthanum (La) Cerium (Ce) Praseodymium (Pr) Neodymium (Nd) Samarium (Sm) Europium (Eu) Gadolinium (Gd) Terbium (Tb) Dysprosium (Dy) Holmium (Ho) Erbium (Er) Thulium (Tm) Ytterbium (Yb) Lutetium (Lu) EXPLANATION Mizpah Trachyte EXPLANATION Fraction Tuff Oddie Rhyolite EXPLANATION Dacite of Donovan Peak Trachyandesite of Red Mountain EXPLANATION Milltown Andesite Porphyritic andesite EXPLANATION Brougher Rhyolite Divide Andesite Rhyodacite of Donovan Peak Rhyolite of Donovan Peak Heller Tuff Rare-earth elements Sample/chondrite Rare-earth elements Figure 5.  Chondrite normalized rare-earth element diagrams for volcanic rocks associated with mineralization in the Tonopah, Divide, and Goldfield mining districts, Nevada. Chondrite abundances from Anders and Ebihara (1982). (SiO2, silicon dioxide; greater than; less than). The unlabeled tick between Neodymium and Samarium corresponds to Promethium which does not occur in nature, and therefore this tick is never labelled.

Volcanic Rocks Associated with Silver-Gold Epithermal Deposits in Nevada of the Heller Tuff have negative Eu anomalies that are much smaller than those that are characteristic of the Fraction Tuff. Limited REE data for volcanic rocks in the Goldfield mining dis­ trict yield relatively consistently negatively sloped patterns with small negative Eu anomalies (Eu/Eu* ranges from 0.73 to 0.88). Volcanic rocks in the Tonopah, Divide, and Goldfield mining districts have primitive mantle-normalized patterns that are also similar to rocks characteristic of other continental margin, subduction-related magmatic arc complexes (fig. 6). These patterns are gently negatively sloping and have well developed negative Nb-Ta anomalies similar to those charac­ teristic of subduction-related arc magmas (Wood and oth­ ers, 1979; Gill, 1981; Pearce and others, 1984). Primitive mantle-normalized patterns for these rocks also have distinctly positive Pb and variably developed negative P and Ti anomalies. Most of the volcanic rocks in the Tonopah, Divide, and Gold­ field mining districts have similar primitive mantle-normalized trace element patterns. Only the rhyolitic rocks in each of these three mining districts have somewhat distinctive patterns. Larger negative Ba, P, and Ti anomalies and previously described middle REE depletions are especially noteworthy features of patterns for these rhyolitic rocks. Petrogenesis of Volcanic Rocks in the Tonopah, Divide, and Goldfield Mining Districts The subalkaline, metaluminous, and magnesian character of intermediate composition volcanic rocks that dominate the Tonopah, Divide, and Goldfield mining districts is consistent with their genesis in a subduction related magmatic setting within the southern segment of the ancestral Cascades arc. Putirka and others (2012) suggest that the Mendocino triple junction passed north of the Goldfield and Tonopah regions about 13 and 12 Ma, respectively. Subsequently, subduction of the Farallon Plate beneath North America in this region ceased and additional subduction-related inputs to ongoing magmatism ended. Volcanologic processes that contributed to development of volcanic centers in the Tonopah, Divide, and Goldfield mining districts are constrained by their petrographic and geochemical characteristics. Plagioclase in many of the constituent volcanic rocks is oscillatory zoned, partly resorbed, sieve textured, and includes variable, within-sample size populations; these features suggest that evolution of the associated magma reservoirs involved open-system behavior, especially periodic recharge. Similarly, the geochemistry of these volcanic rocks, especially those that are part of the stratovolcano repre­ sented by the Milltown Andesite, and the petrologically diverse, dome-forming rocks included in the Mizpah Trachyte, vary broadly, also indicative of recharge and incomplete homogenization of reservoir contents. Major oxide compositional variations among these rocks are in accord with crystallization and fractionation of the observed phenocryst assemblages. Consistent linear decreases, especially of CaO, but also Al2O3, with increasing silica content suggests that plagioclase crystallization and fractionation contributed significantly to the compositional evolution displayed by this suite of rocks. Importantly, Sr abundances that decrease linearly from about 1,600 parts per million (ppm) in rocks with about 54 weight percent SiO2 to 150 ppm in rocks with about 77 weight percent SiO2 and Eu/Eu* that decreases from about 0.85 to about 0.50 in these same rocks indicate significant plagioclase fractionation from their parental magmas. Decreasing TiO2, FeO*, and MgO abundances with increasing silica content (fig. 4) are consistent with clinopyroxene, hornblende, biotite, and lesser Fe-Ti oxide crystallization and fractionation. Concomitant K2O abundance increases with increasing SiO2 content signal its incompatibility and concentration in evolving, residual magma. The importance of accessory minerals (especially apatite and Fe-Ti oxides) in the petrogenesis of volcanic rocks in the Tonopah, Divide, and Goldfield mining districts is demonstrated by increasingly well-developed negative P and Ti anomalies among rocks with higher silica contents (fig. 6). Relatively elevated Na2O and TiO2 abundances characteristic of the volcanics of Donovan Peak and, to a lesser extent, the Divide Andesite, in the Divide mining district, are off trend from compositional arrays defined by Tonopah and Goldfield mining district volcanic rocks. Together with the off trend MgO depletion characteristic of the volcanics of Donovan Peak, these variations suggest a compositionally distinct source for most magmas erupted in the Divide mining district. The preponderance of volcanic rocks in the Tonopah and Goldfield mining districts have Zr abundances that vary from about 140 to 190 ppm, a range commensurate with Zr abundances buffered by zircon saturation in calc-alkaline magmas, as per relations described by Watson (1979). Among these rocks, those with >72 weight percent silica have markedly lower Zr abundances, which probably represents accessory zircon crystallization and fractionation from their parental magmas. Importantly, at any given SiO2 content, volcanic rocks in the Divide mining district have significantly higher Zr abundances (as much as 562 ppm) than Tonopah and Goldfield volcanic rocks, which reflect increased Zr solubility in alkaline silicate melts (Watson, 1979). Like most other Miocene volcanic rocks associated with the southern segment of the ancestral Cascades arc, Sr abundances (1,100 and 750 ppm at 57 and 63 weight percent SiO2, respectively) of volcanic rocks in the Tonopah, Divide, and Goldfield mining districts are significantly elevated relative to those characteristic of Andean arc andesites, which contain 600-900 ppm Sr (Hildreth and Moorbath, 1988). Partial melting at the base of the crust-subcontinental mantle section, at plagioclase-unstable pressures (Green, 1982) greater than about 20 kilobars (kb) (70 km depth) and relatively hydrous conditions (Moore and Carmichael, 1998), yields high-Sr magmas. Otherwise, conditions that favor plagioclase stability promote preferential Sr retention in plagioclase, its principal

Petrogenesis of Volcanic Rocks in the Tonopah, Divide, and Goldfield Mining Districts    17 D, Divide, felsic composition (>65 % SiO2) samples C, Divide, intermediate composition (<65 % SiO2) samples A, Tonopah, intermediate composition (<65 % SiO2) samples B, Tonopah, felsic composition (>65% SiO2) samples E, Goldfield, intermediate composition (<65 % SiO2) samples Rubidium (Rb) Ytterbium (Yb) Minerals Minerals Sample/primitive mantle Sample/primitive mantle Sample/primitive mantle Barium (Ba) Thorium (Th) Niobium (Nb) Tantalum (Ta) Potassium (K) Lanthanum (La) Cerium (Ce) Lead (Pb) Strontium (Sr) Phosphorus (P) Neodymium (Nd) Zirconium (Zr) Hafnium (Hf) Samarium (Sm) Europium (Eu) Titanium (Ti) Dysprosium (Dy) Yttrium (Y) Holmium (Ho) Rubidium (Rb) Ytterbium (Yb) Barium (Ba) Thorium (Th) Niobium (Nb) Tantalum (Ta) Potassium (K) Lanthanum (La) Cerium (Ce) Lead (Pb) Strontium (Sr) Phosphorus (P) Neodymium (Nd) Zirconium (Zr) Hafnium (Hf) Samarium (Sm) Europium (Eu) Titanium (Ti) Dysprosium (Dy) Yttrium (Y) Holmium (Ho) F, Goldfield, felsic composition (>65 % SiO2) samples 1,000 1,000 1,000 EXPLANATION Mizpah Trachyte EXPLANATION Rhyolite of Wildhorse Spring EXPLANATION Fraction Tuff Oddie Rhyolite EXPLANATION Dacite of Donovan Peak Trachyandesite of Red Mountain EXPLANATION Milltown Andesite Porphyritic andesite EXPLANATION Brougher Rhyolite Divide Andesite Rhyodacite of

Donovan Peak Rhyolite of

Donovan Peak Heller Tuff Figure 6.  Primitive mantle-normalized (Sun and McDonough, 1989) trace element diagrams for volcanic rock units associated with mineralization in the Tonopah, Divide, and Goldfield mining districts, Nevada. (SiO2, silicon dioxide; greater than; less than).

18    Volcanic Rocks Associated with Silver-Gold Epithermal Deposits in Nevada mineralogic residence, and consequent Sr-poor magmas. Accordingly, volcanic rocks from all three mining districts probably reflect partial melting in a relatively hydrous, high-pressure regime beneath a thick section composed of continental crust and subcontinental mantle. Volcanic rocks in the Tonopah, Divide, and Goldfield mining districts have relative abundances of Rb and Y+Nb consistent with their genesis in a volcanic arc setting (fig. 4). Negative Nb-Ta anomalies (fig. 6) are similarly in accord with genesis of the associated magmas in a subduction-related magmatic arc setting. The amplitude of these anomalies appears minimally correlated with host rock composition, which suggests that these anomalies are not fractionation-related phenomenon. The values of Ba/Nb further corroborate a volcanic arc setting for genesis of volcanic rocks in the Tonopah, Divide, and Goldfield mining districts. Gill (1981) suggests that most magmatic arc rocks have Volcanic rocks in these three mining districts with 54 to 68 weight percent SiO2 have Ba/Nb that range from 50-150 and cluster around 100. Ba/Nb values for these rocks do not covary systematically with respect to either host rock age or composition, although values generally decrease among rocks with greater than 72 weight percent SiO2, probably because of feldspar and biotite crystallization and fractionation. REE abundance variations further constrain the petrogenesis of volcanic rocks in the Tonopah, Divide and Goldfield mining districts. Moderately steep, negatively sloped chondrite-normalized REE patterns such as those characteristic of these volcanic rocks (fig. 5) are similar to those of Andean magmatic arc rocks and are likely additional manifestations of high-pressure partial melting in a garnet-stable regime (Hildreth and Moorbath, 1988), because garnet is a HREE reservoir. MREE to HREE depletion, as defined by subtly U-shaped MREE to HREE pattern segments, is characteristic of the volcanic rocks, especially the more evolved rhyolitic rocks, in the Tonopah, Divide, and Goldfield mining districts (fig. 5). Amphibole fractionation has been inferred in the genesis of MREE depletion and development of U-shaped REE patterns because mineral/melt partition coefficients for amphibole are greatest among the MREE, especially dysprosium (Dy) (Davidson and others, 2007). Consequently, U-shaped MREE to HREE pattern segments for volcanic rocks in the three min­ ing districts probably reflect varying degrees of amphibole fractionation. Weak negative Eu anomaly development characteristic of intermediate composition rocks reflects relatively high pressure melting conditions in a relatively hydrous regime (Moore and Carmichael, 1998) that favored garnet rather than plagioclase (a principal Eu reservoir) stability. Consequently, small negative Eu anomalies that typify the intermediate composition rocks in the three mining districts (fig. 5) are consistent with source-region plagioclase instability and consequent Eu partitioning into partial melts rather than into residuum. In contrast, significantly larger negative Eu anomalies among the more evolved, mostly rhyolitic rocks in these three mining districts suggests that plagioclase fractionation contributed significantly to the compositional evolution of these rocks. Lead/cerium (Pb/Ce) increases from ~0.1 to ~0.4 with increasing SiO2 content among volcanic rocks of the Tonopah, Divide, and Goldfield mining districts. Increasing Pb/Ce values may result from (1) greater subducted slab contributions to mantle-derived melts or (2) increased assimilation of Pb-rich crustal rocks during reservoir melting, assimilation, and homogenization processes. Similarly, elevated Ba/Nb may reflect contributions of subducted slab components, by slab dehydration, to mantle wedge-derived magmas (Hawkesworth and others, 1995; Pearce and Peate, 1995; Cousens and others, 2008; Schmidt and others, 2008). Consequently, the observed positive and negative correlations between Pb/Ce and Ba/Nb, respectively, with SiO2 content suggest that the most SiO2-rich rocks in these areas represent magmas that assimilated relatively greater amounts of crustal material, an inference that could be evaluated using, presently scant, radiogenic isotope data. Farmer and others (2002), Cousens and others (2008), and du Bray and others (2014) suggest that magmatism represented by the southern segment of the ancestral Cascades arc involved significant crustal contamination; an array of geochemical parameters indicate that magmas represented by Miocene volcanic rocks in the Tonopah, Divide, and Goldfield mining districts were affected by similar, variable crustal contamination. Progressively lower P2O5/K2O among increasingly silicic compositions and higher values with increasing MgO abundances are consistent with primary, mafic magmas having assimilated crustal contaminants, because crustal materials generally have P2O5/K2O 0.1 (Farmer and others, 2002). P2O5/K2O decreases with increasing SiO2 and increases with higher MgO contents among rocks of the three mining districts, which suggests that the associated magmas evolved to intermediate or silicic compositions by variable contamination of primary mafic partial melts by crustally derived inputs. Decreasing CaO/Al2O3 and increasing lanthanum/samarium (La/Sm) and Zr/Sm with increasing SiO2 are additional measures reflective of crustal contamination (Cousens and others, 2008). Correspondingly, CaO/Al2O3 decreases, and La/Sm and Zr/Sm increases with increasing SiO2 among rocks of the Tonopah, Divide, and Goldfield min­ ing districts further corroborate the role of progressive crustal contamination in the petrogenesis of intermediate-to felsiccomposition rocks in these areas.

Conclusions    19 Conclusions Precious metal epithermal deposits in the Tonopah, Divide, and Goldfield mining districts of west-central Nevada are spatially, temporally, and likely genetically associated with Miocene calc-alkaline volcanic rocks that are part of the southern segment of the ancestral Cascades magmatic arc. Geochemical characteristics of these volcanic rocks constrain the tectonic setting in which they evolved and corroborate their relations with ancestral Cascades arc magmatism. Specifically, relative abundances of Rb, Y, and Nb in these rocks are commensurate with compositions typical of arc magmas, as are their elevated Ba/Nb and negative Nb-Ta anomalies. Arc-related magmatism in each of these mining districts was extinguished by northward passage of the Mendocino triple junction between about 13 and 12 Ma. Specific Miocene volcanic rock units are essentially unique to each of the three mining districts. Although the probable source of mineralizing fluids in the Tonopah mining district is not well established, its mineral deposits are prin­ cipally hosted by the Mizpah Trachyte, which forms a small field of coalesced lava domes, and the Fraction Tuff, likely erupted from a caldera whose northern margin is about 9 km north of Tonopah. Mineral deposits in the Divide mining district are spatially and temporally associated with (1) the Heller Tuff, likely erupted from a caldera whose southern margin is preserved about 14 km south of Tonopah, and (2) the Divide Andesite and volcanics of Donovan Peak, both likely manifestations of post caldera resurgent magmatism. At Goldfield, mineral deposits are principally associated with the Milltown Andesite, an unnamed porphyritic andesite, and latite. All these volcanic rocks have petrographic and geochemical characteristics consistent with their genesis in a magmatic arc setting. Almost all are porphyritic, containing 5 to 35 percent phenocrysts, principally composed of plagioclase, pyroxene, and hornblende±biotite; quartz, alkali feldspar, or olivine form phenocrysts in some of these rocks. Geochemical compositions of these rocks range essentially continuously from basaltic trachyandesite and basaltic andesite to rhyolite, with SiO2 contents that range from 54 to 78 weight percent. Most have compositions that are transitional from alkaline to subalkaline, metaluminous, calc-alkalic to alkali-calcic, and magnesian (calc-alkaline) to weakly ferroan (tholeiitic). Compositions of the Divide Andesite and the volcanics of Donovan Peak in the Divide mining district are somewhat distinct relative to volcanic rocks in the Tonopah and Goldfield mining districts. Specifically, they are more alkalic; have greater TiO2, Na2O, Ba, Hf, La, Nb, Ta, Y, Yb, and Zr abundances; and lower MgO, Sr, and V abundances than their Tonopah and Goldfield mining district analogs. None of the unaltered volcanic rocks in the Tonopah, Divide, and Goldfield are particularly metal enriched. Significantly elevated Zr contents characteristic of volcanic rocks from the Divide mining district, including the Heller Tuff, are particularly noteworthy and indicate equilibration with relatively alkaline magma. The Divide mining district rocks have Na2O contents that are distinctly greater and K2O contents at the high end of the range defined by rocks from the Tonopah and Goldfield mining districts. These alkalinity characteristics suggest that the petrogenetic evolution of the Miocene volcanic rocks in the Divide mining district involved a source and (or) processes dissimilar to those that controlled the magmatic evolution of volcanic rocks in the other two mining districts. Numerous geochemical characteristics similarly constrain the petrogenetic processes that contributed to compositional evolution among Miocene volcanic rocks in the Tonopah, Divide, and Goldfield mining districts. For example, negatively sloping, relatively HREE depleted REE patterns are consistent with high-pressure, garnet stable, plagioclase unstable partial melting. Elevated Sr concentrations and generally small negative Eu anomalies characteristic of these volcanic rocks are additional manifestations of partial melting in a plagioclase unstable regime. Progressively larger negative Eu anomalies among the more silica rich volcanic rocks in the three mining districts reflect the increased importance of plagioclase fractionation from magmas represented by those rocks. Subtly U-shaped middle chondrite-normalized REE patterns reflect the importance of amphibole crystallization and fractionation from the associated magmas. Major oxide compositional variation among these rocks is consistent with crystallization and fractionation of the other observed phenocryst minerals, including clinopyroxene, biotite, and Fe-Ti oxides. Well-developed negative P and Ti anomalies on extended trace element diagrams substantiate the importance of accessory minerals, especially apatite and Fe-Ti oxides, to the compositional evolution of magma reservoirs represented by volcanic rocks in the three mining districts. Other geochemical parameters suggest that magmas represented by volcanic rocks in these three mining districts underwent variable amounts of crustal contamination during ascent and storage in shallow crustal reservoirs. Specifically, increasing Pb/Ce and decreasing P2O5/K2O with increasing SiO2 abundances are well known manifestations of crustal contamination. Diagnostic textural features preserved by phenocrysts, especially plagioclase, constitute additional evidence that open-system behavior, including reservoir-scale mixing, recharge, and assimilation, were critical to the petrogenesis of volcanic rocks in the Tonopah, Divide, and Goldfield mining districts.

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References Cited    21 du Bray, E.A., John, D.A., Vikre, P.G., Colgan, J.P., Cosca, M.A., Morgan, L.E., Fleck, R.J., Premo, W.R., and HolmDenoma, C.S., 2019b, Petrographic, geochemical, and geochronologic data for Cenozoic volcanic rocks of the Tonopah, Divide, and Goldfield mining districts, Nevada: U.S. Geological Survey Data Series 1099, ://doi. org/10.3133/ds1099. Erdman, C.P., and Barabas, A.H., 1996, Precious metal mineralization at Gold Mountain, Tonopah Divide district, Esmeralda County, Nevada, in Coyner, A.R, and Fahey, P.L., eds., Geology and ore Deposits of the American Cor­ dillera, Symposium Proceedings, April 10-13, 1996: Reno, Geological Society of Nevada, Symposium Proceedings no. 95-1, p. 329-351. [Also available at ://.org/ publications/?itemid=95-1.] Ewart, A., 1982, The mineralogy and petrology of Tertiary— Recent orogenic volcanic rocks with special reference to the andesitic-basaltic compositional range, in Thorpe, R.S., ed., Andesites: New York, John Wiley and Sons, p. 25-87. Farmer, G.L., Glazner, A.F., and Manley, C.R., 2002, Did lithospheric delamination trigger late Cenozoic potassic volcanism in the southern Sierra Nevada, California?: Geo­ logical Society of America Bulletin, v. 114, p. 754-768. [Also available at ://doi.org/10.1130/

Frost, B.R., Barnes, C.G., Collins, W.J., Arculus, R.J., Ellis, D.J., and Frost, C.D., 2001, A geochemical classifica­ tion for granitic rocks: Journal of Petrology, v. 42, p. 2033-2048. [Also available at ://doi.org/10.1093/ petrology/42.11.2033.] Gill, J., 1981, Orogenic andesites and plate tectonics: New York, Springer-Verlag, 390 p. Graney, J.R., 1987, Hasbrouck Mountain, Nevada, preciousmetal mineralization in a fossil hot springs environment, in Johnson, J.L., and Abbott, E., eds., Bulk mineable pre­ cious metal deposits of the western United States April 6-8, 1987, Guidebook for Field Trips, 1987 Symposium: Reno, Geological Society of Nevada, p. 120-125. [Also available from the Nevada Bureau of Mines and Geol­ ogy, Open-File Report 10-8, at ://ronhess.info/MD/ scans/1490/14900273.pdf.] Green, T.H., 1982, Anatexis of mafic crust and high pressure crystallization of andesite, in Thorpe, R.S., ed., Andes­ ites—Orogenic andesites and related rocks: New York, John Wiley and Sons, p. 465-487. Hawkesworth, C., Turner, S., Gallagher, K., Hunter, A., Bradshaw, T., and Rogers, N., 1995, Calc-alkaline magmatism, lithospheric thinning and extension in the Basin and Range: Journal of Geophysical Research, Solid Earth, v. 100, p. 10,271-10,286. [Also available at ://doi.org/10.1029/94JB02508.] Hildreth, W., and Moorbath, S., 1988, Crustal contributions to arc magmatism in the Andes of central Chile: Contri­ butions to Mineralogy and Petrology, v. 98, p. 455-489. [Also available at ://link.springer.com/content/ pdf/10.1007%2FBF00372365.pdf.] Irvine, T.N., and Baragar, W.R.A., 1971, A guide to the chemi­ cal classification of the common volcanic rocks: Canadian Journal of Earth Sciences, v. 8, p. 523-548. [Also available at ://doi.org/10.1139/e71-055.] John, D.A., du Bray, E.A., Blakely, R.J., Fleck, R.J., Vikre, P.G., Box, S.E., and Moring, B.C., 2012, Miocene mag­ matism in the Bodie Hills volcanic field, California and Nevada: A long-lived eruptive center in the southern segment of the ancestral Cascades arc: Geosphere, v. 8, p. 44-97, accessed 2015, at ://doi.org/10.1130/ GES00674.1. John, D.A., du Bray, E.A., Henry, C.D., and Vikre, P.G., 2015, Cenozoic magmatism and epithermal gold-silver deposits of the southern ancestral Cascade arc, western Nevada and eastern California, in Pennell, W.M., and Garside, L.J., eds., New concepts and discoveries, Sparks, Nev., May 14-23, 2015, Symposium Proceedings: Geological Soci­ ety of Nevada Symposium Proceedings, no. 15-CD, p. 611-645, 1 CD-ROM. [Also available at ://.org/ publications/?itemid=15-CD.] John, D.A., and McKee, E.H.,1987, K-Ar ages of granitic plutonism and hydrothermal alteration in western part of the Tonopah 1°×2° quadrangle, Nevada: Isochron/West, no. 48, p. 16-18. Knopf, A., 1921, Contributions to economic geology, 1920, Part I, Metals and Nonmetals except fuels—The Divide silver district, Nevada,: U.S. Geological Survey Bul­ letin 715-K, p. 147-170. [Also available at ://doi. org/10.3133/b715K.] Le Maitre, R.W., 2002, Igneous rocks—A classification and glossary of terms, 2d ed.: Cambridge University Press, 236 p. [Also available at ://doi.org/10.1017/ CBO9780511535581.] Moore, G., and Carmichael, I.S.E., 1998, The hydrous phase equilibria (to 3 kbar) of an andesite and basaltic andesite from western Mexico—Constraints on water content and conditions of phenocryst growth: Contributions to Mineral­ ogy and Petrology, v. 130, p. 304-319. [Also available at ://doi.org/10.1007/s004100050367.] Nolan, T.B., 1935, Underground geology of the Tonopah mining district, Nevada: Nevada University Bulletin 23, no. 5, 49 p. [Also available at ://pubs..unr.edu/ Underground-geol-Tonopah-mining-p/b023.htm.]

22    Volcanic Rocks Associated with Silver-Gold Epithermal Deposits in Nevada Pearce, J.A., Harris, N.B.W., and Tindle, A.G., 1984, Trace ele­ ment discrimination diagrams for the tectonic interpretation of granitic rocks: Journal of Petrology, v. 25, p. 956-983. [Also available at ://doi.org/10.1093/petrology/25.4.956.] Pearce, J.A., and Peate, D.W., 1995, Tectonic implications of the composition of volcanic arc magmas: Annual Review of Earth and Planetary Sciences, v. 23, p. 251-285. [Also available at ://www.annualreviews.org/doi/pdf/10.1146/ annurev.ea.23.050195.001343.] Putirka, K., Jean, M., Cousens, B., Sharma, R., Torrez, G., and Carlson, C., 2012, Cenozoic volcanism in the Sierra Nevada and Walker Lane, California, and a new model for litho­ sphere degradation: Geosphere, v. 8, p. 265-291, accessed 2015, at ://doi.org/10.1130/GES00728.1. Ransome, F.L., 1909, The geology and ore deposits of Gold­ field, Nevada: U.S. Geological Survey Professional Paper 66, 258 p. [Also available at ://doi.org/10.3133/pp66.] Schmidt, M.E., Grunder, A.L., and Rowe, M.C., 2008, Seg­ mentation of the Cascade arc as indicated by Sr and Nd isotopic variation among diverse primitive basalts: Earth and Planetary Science Letters, v. 266, p. 166-181. [Also available at ://doi.org/10.1016/j.epsl.2007.11.013.] Shand, S.J., 1951, Eruptive rocks: New York, John Wiley, 488 p. Searls, F., 1948, A contribution to the published informa­ tion on the geology and ore deposits of Goldfield, Nevada: Nevada Bureau of Mines and Geology Bulletin 48, 24 p. [Also available at ://pubs..unr.edu/ Geol-ore-deposits-Goldfield-p/b048.htm.] Sun, S.-S., and McDonough, W.F., 1989, Chemical and isoto­ pic systematics of oceanic basalts—Implications for mantle composition and processes, in Saunders, A.D., and Norry, M.J., eds., Magmatism in the Ocean Basins: Geological Society of London Special Publication 42, p. 313-345. [Also available at ://sp.lyellcollection.org/content/ specpubgsl/42/1/313.full.pdf.] Taggart, J.E., Jr., 2002, Analytical methods for chemical analysis of geologic and other materials, U.S. Geological Survey: U.S. Geological Survey Open-File Report 02-0223. [Also available at ://doi.org/10.3133/ofr02223.] Turekian, K.K., and Wedepohl, K.H., 1961, Distribution of the elements in some major units of the Earth's crust: Geological Society of America Bulletin, v. 72, p. 175-192. [Also available at :// pubs.geoscienceworld.org/gsa/gsabulletin/article/72/2/175/ 5262/distribution-of-the-elements-in-some-major-units.] Vikre, P.G., and Henry, C.D., 2011, Quartz-alunite altera­ tion cells in the ancestral southern Cascades magmatic arc, in Steininger, R., and Pennell, B., eds., Great Basin Evolution and Metallogeny, 2010 Symposium, May 14-22, Symposium Proceedings: Reno, Nev., Geological Society of Nevada, Symposium Proceedings, no. 10-1, v. I, p. 701-745. [Also available at ://.org/ publications/?itemid=10-1.] Watson, E.B., 1979, Zircon saturation in felsic liquids; experi­ mental results and applications to trace element geochemis­ try: Contributions to Mineralogy and Petrology, v. 70, p. 407-419. [Also available at ://link.springer.com/content/ pdf/10.1007%2FBF00371047.pdf.] Wood, D.A., Joron, J.L., and Treuil, M., 1979, A re-appraisal of the use of trace elements to classify and discriminate between magma series erupted in different tectonic settings: Earth and Planetary Science Letters, v. 45, p. 326-336. [Also available at

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du Bray and others—Volcanic Rocks Associated with Silver-Gold Epithermal Deposits in Nevada—Scientific Investigations Report 2019­-5024 ISSN 2331-1258 (online) ://doi.org/10.3133/sir20195024

Plates & figures from the original