The Stibnite-Yellow Pine gold-antimony-tungsten-mercury (Au-Sb-W-Hg) mining district, an important source of antimony (Sb) and tungsten (W) in World War II with a major (similar to 7 Moz) Au resource, is located a few kilometers west of the Thunder Mountain volcanic field on the northwestern edge of the Eocene Challis magmatic complex in west-central Idaho. Using new field studies,Ar-40/Ar-39 and U/Pb geochronology, and major and trace element and isotope geochemistry, we document the stratigraphy of the western part of the Thunder Mountain volcanic field and the ages of related dikes in the Stibnite-Yellow Pine mining district and Pistol Creek dike swarm. The volcanic succession is divisible into a lower section of dacitic tuffs and lavas (ca. 49.5-48.0 Ma) and an upper section of rhyolitic lavas and tuffs (ca. 47.6-46.5 Ma), locally capped by trachyandesite flows (ca. 42.6 Ma). Dikes in the Stibnite-Yellow Pine mining district similarly range from basaltic andesite to rhyolite. Dike ages range from dacitic dikes as old as ca. 49.3 Ma to rhyolitic dikes as young as ca. 46.9 Ma; the emplacement age of basaltic andesite dikes has not been determined. Geochemical and isotopic compositions of igneous rock units of the western Challis magmatic complex indicate they can be clustered into four petrologic groups, which differ in source characteristics and/or in fractionation history. Although Challis magmatism was synchronous with extensional tectonism and detachment faulting, the broader tectonic setting of Challis magmatism is controversial. Despite exhibiting the geochemical hallmarks of subduction-related magmatism, most authors interpret these igneous rocks to have been derived by partial melting of the previously subduction-modified lithospheric mantle that was heated by upwelling asthenosphere in the Eocene. Epithermal Au-Ag mineralization in the Thunder Mountain mining district, located centrally in the Thunder Mountain volcanic field about 15 km northeast of Stibnite, is dated here at 46.0 +/- 0.4 Ma. This age postdates the episodes of Au and W mineralization in the nearby Stibnite-Yellow Pine mining district but overlaps in age with its youngest mineralization episode of Sb-Hg ores.

First posted October 5, 2023 For additional information, contact: Geology, Minerals, Energy, & Geophysics Science CenterU.S. Geological SurveyBuilding 19, 350 N. Akron Rd.P.O. Box 158Moffett Field, CA 94035 During the World War and Cold War eras (1910s–1990s), domestic consumption of numerous mineral commodities relied increasingly on imported supplies. Consumption reliance has since expanded to include 50 “critical minerals” (elements and mineral commodities) that are mostly to entirely imported and subject to curtailment by suppliers or supply chain disruption. New domestic supplies of critical minerals are being pursued by mining companies and by several federal departments and agencies. Information on domestic deposits and resources of critical minerals is being compiled by the U.S. Geological Survey Mineral Resources Program, which has organized investigations by mineral system, deposit type, and commodity.Production, reserves, resources, and inventories of 21 critical minerals in domestic magmatic-hydrothermal deposits related to subduction-generated magmatism, and in tailings, slag, slimes, and electrolyte from copper concentrators, smelters, and refineries that processed some deposits, are largely restricted to Western States and Alaska. The critical mineral commodities Al, Sb, As, Bi, Co, fluorite, Ga, Ge, In, Mn, Ni, Nb, Pd, Pt, potash, Re, Ta, Te, Sn, W, and V are variably concentrated in porphyry/skarn copper-(molybdenum), skarn-replacement-vein (S-R-V) tungsten, polymetallic sulfide S-R-V intermediate sulfidation (IS), high-sulfidation gold-silver, low-sulfidation gold-silver, and lithocap alunite deposit types. These deposit types occur in porphyry copper-molybdenum-gold, alkalic porphyry, porphyry tin (granite related), and reduced intrusion-related mineral systems.Production, reserves, and resources of Co, Ni, Nb, Pd, Pt, Ta, Sn, and V in subduction-related deposits in Western States are insignificant to small, mostly equivalent to months to a few years of recent annual domestic consumption (2016–2020). Significant inventories, equivalent to 2 or more years of consumption of aluminum, antimony, potash, and tungsten in unmined S-R-V tungsten, polymetallic sulfide S-R-V-IS, and lithocap alunite deposits vary from approximately 2 to 8 years. Several decades of consumption of arsenic, bismuth, fluorite, gallium, germanium, and indium exist in some polymetallic sulfide S-R-V-IS and lithocap alunite deposit types.Based on concentrations of critical minerals in reserves, resources, drill holes, and deposit domains (ore types), and in captive refinery records, the largest domestic inventories of Sb, As, Bi, Re, and Te, and possibly Ga, Ge, In, Sn, and W, are in porphyry copper-molybdenum (Cu-Mo) deposits in Alaska, Idaho, Utah, and Arizona, and in interim products of processing porphyry Cu-Mo deposit ores for recovery of copper and molybdenum. Concentrations of critical minerals in archival specimens and sample collections, although somewhat biased by collection and conservation decisions and categorization, are broadly proportionate to those in reserves, resources, and drill holes. These concentrations imply significant inventories of some critical minerals in deposits for which production, resources, and refinery records are unavailable or incomplete.Because of the large masses of ores mined and processed annually (hundreds of millions of metric tons) and in reserves and resources (hundreds of millions of metric tons to billions of metric tons), calculated inventories of critical minerals in porphyry Cu-Mo deposits are equivalent to decades and centuries of recent consumption. However, these inventories should not be considered consumable supplies without reserve definition and development of economically viable mining plans and recovery techniques. An expeditious strategy for elimination or reduction of import reliance is recovery, and improved recovery efficiency, of Sb, As, Bi, Re, and Te, and possibly Ga, Ge, In, Ni, Sn, Ti, and W; during concentration and refining of copper and molybdenum minerals in ores of operating porphyry Cu-Mo mines; and in unmined porphyry Cu-Mo resources. These chalcophile, siderophile, and lithophile critical minerals, often undetectable in ore, are concentrated (hundreds of parts per million to percents) in slimes and electrolyte during copper electrorefining or could be recovered, in part, during sulfide concentration and smelting. Other than rhenium (recovered during molybdenum refining) and tellurium, all have been routinely discarded.Subsidization (for example, commodity price guarantees, tax credits, recovery technology development), political initiative, and (or) sustained market favorability could support new production of critical mineral commodities from subduction-related magmatic-hydrothermal deposits in Western States. In addition, insufficient domestic refining capacity could relegate the large inventories of critical minerals in porphyry Cu-Mo reserves and resources (for example, Pebble, Alaska; Resolution and Copper World [Rosemont], Arizona) to exportation in concentrates and importation insecurity, fortifying their present status.

Carlin-type gold deposits (CTDs) of Nevada are the largest producers of gold in the United States, a leader in world gold production. Although much has been resolved about the characteristics and origin of CTDs in Nevada, major questions remain, especially about (1) the role of magmatism, whether only a source of heat or also metals, (2) whether CTDs only formed in the Eocene, and (3) whether pre-Eocene metal concentrations contributed to Eocene deposits. These issues are exemplified by the CTDs of the Cortez region, the second largest concentration of these deposits after the Carlin trend. Carlin-type deposits are notoriously difficult to date because they rarely generate dateable minerals. An age can be inferred from crosscutting relationships with dated dikes and other intrusions, which we have done for the giant Cortez Hills CTD. What we term "Cortez rhyolites" consist of two petrographic-geochemical groups of siliceous dikes: (1) quartz-sanidine-plagioclase-biotite-phyric, high-SiO2 rhyolites emplaced at 35.7 Ma based on numerous 40Ar/39Ar dates and (2) plagioclase-biotite-quartz +/- hornblende-phyric, low-SiO2 rhyolites, which probably were emplaced at the same time but possibly as early as similar to 36.2 Ma. The dikes form a NNW-trending belt that is similar to 6 to 10 km wide x 40 km long and centered on the Cortez Hills deposit, and they require an underlying felsic pluton that fed the dikes. Whether these dikes pre-or postdated mineralization has been long debated. We show that dike emplacement spanned the time of mineralization. Many of both high-and low-SiO2 dikes are altered and mineralized, although none constitute ore. In altered-mineralized dikes, plagioclase has been replaced by kaolinite and calcite, and biotite by smectite, calcite, and marcasite. Sanidine is unaltered except in a few samples that are completely altered to quartz and kaolinite. Sulfides present in mineralized dikes are marcasite, pyrite, arsenopyrite, and As-Sb-bearing pyrite. Mineralized dikes are moderately enriched in characteristic Carlin-type elements (Au, Hg, Sb, Tl, As, and S), as well as elements found in some CTDs (Ag, Bi, Cu, Mo), and variably depleted in MgO, CaO, Na2O, K2O, MnO, Rb, Sr, and Ba. In contrast, some high-SiO2 rhyolites are unaltered and cut high-grade ore, which shows that they are post-ore. Both mineralized and post-ore dikes have indistinguishable sanidine 40Ar/39Ar dates. These characteristics, along with published interpretations that other giant CTDs formed in a few tens of thousands of years, indicate the Cortez Hills CTD formed at 35.7 Ma. All Cortez-area CTDs are in or adjacent to the Cortez rhyolite dike swarm, which suggests that the felsic pluton that fed the dikes was the hydrothermal heat source. Minor differences in alteration and geochemistry between dikes and typical Paleozoic sedimentary rock-hosted ore probably reflect low perme-ability and low reactivity of the predominantly quartzofeldspathic dikes. Despite widespread pre-35.7 Ma mineralization in the Cortez region, including deposits near several CTDs, we find no evidence that older deposits or Paleozoic basinal rocks contributed metals to Cortez-area CTDs. Combining our new information about the age of Cortez Hills with published and our dates on other CTDs demonstrates that CTD formation coincided with the southwestern migration of magmatism across Nevada, supporting a genetic relationship to Eocene magmatism. CTDs are best developed where deep-seated (similar to 6-8 km), probably granitic plutons, expressed in deposits only as dikes, established large, convective hydrothermal systems.

Porphyry and epithermal mineral deposits form large economic ore bodies that provide the global economy with copper, molybdenum, gold, silver and other byproducts (Re, Te, Se). They form in the upper crust and are related to sulfur- and water-rich intermediate to silicic magmatic sources of hydrothermal fluids that move upward and produce extensive hydrolytic and alkali wall-rock alteration, quartz veins, and sulfides. Porphyry-type deposits are formed above magma chambers where fluids hydrofracture rock at 700–350 °C and at pressures ranging from supra-lithostatic to supra-hydrostatic. The depth of formation ranges from 2 to 10 km and influences orebody geometries and the types and mineralogy of veins, sulfides and wall-rock alteration. The temporal evolution of hydrothermal events is documented by cross-cutting veins and is commonly characterized by a decline in fluid temperature and concordant evolution from potassic alteration to sericitic alteration, with attendant increase in sulfidation state of copper-iron sulfides. In some localities porphyry copper deposits transition upwards to lower temperature base metal lodes (350–200 °C) and eventually the formation of near surface (<1.5 km depth) intermediate- and high-sulfidation epithermal deposits (~300–120 °C). Extensional environments are often characterized by porphyry molybdenum and low-sulfidation epithermal deposits. In the base metal lode and epithermal environments, mixtures of magmatic and meteoric fluids produce ore fluids at hydrostatic pressures that advect freely both vertically and laterally along permeability provided by faults, joints, and porous lithologies. Wall-rock alteration ranges from hydrolytic to alkali-carbonate, and from high- to low-sulfidation state sulfide assemblages, respectively. In porphyry, base metal lode, and epithermal environments, geology and the zonation of wall-rock alteration, veins, sulfide assemblages, and metals are useful for exploration.

The Atastra Creek siliceous hot spring deposit, or sinter, occurs in the Paramount-Bald Peak alteration zone, due north of the Bodie precious metals mining district in the Miocene Bodie Hills volcanic field, California and Nevada, U.S.A. Distinctive features include its geomorphically intact geyser vent mounds, the presence of growth-fault-stepped sinter terraces that developed westward along a NNW trending fault, and atypical Hg, Sb and As concentrations. High-temperature (>75 degrees C), subaerial, proximal apron sinter occurs in two settings - (1) radiating botryoidal geyserite in vent mounds and (2) bedded geyserite (columnar, nodular, size-sorted geyser eggs) intercalated with mid-to low-temperature (<60 degrees C), middle to distal apron terrace sinter. The active geyser vent mounds migrated from south to north across the apron terrace through time. A newly recognized, visually striking type of sinter-termed silica infiltrate herein-formed directly beneath the subaerial geyserite deposits. It is characterized by mainly parallel, thin, irregular beds of white silica with narrow borders of dark blue-gray silica, as well as swirly to globular white silica, all chaotically linked together by silica 'necks' and 'bridges'. The silica infiltrate permeates the pre-existing sinter, and is interpreted to represent super-heated fluid injections into the immediately overlying geyser vent mounds and pools. The adjacent sinter sheets preserve fluid-flow directions of warm-water channels that traversed the discharge apron, including microbial streamer fabric and wavy laminated sinter with flattened bubbles. More distal sinter textures are recorded by rimmed terracettes with mammillated and palisade microbial fabrics and rare, warm pool-related stromatolites. However, plants are absent, probably due to the high metalloids, in particular As. Post-depositional events include an inferred hydrothermal eruption breccia and an interpreted drop in the water table accompanied by incursion of steam acid condensate, as evidenced by pervasive hydrothermal alteration of the Atastra Creek deposits. Hence, the well-exposed sedimentary facies distributions, well-preserved geomorphology, and stratigraphic and structural relationships together allow an unusually complete, paleoenvironmental reconstruction of the Atastra Creek paleohydrothermal deposit. (C) 2018 Published by Elsevier B.V.

First posted June 3, 2019 For additional information, contact: Director, Geology, Geophysics and Geochemistry Science CenterU.S. Geological SurveyBox 25046, MS-973Denver, CO 80225-0046 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 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.

The rhyolite of Bodie Hills (California) is characterized compositionally and the geochronology of selected phases is studied. Sanidine (BHs) from the rhyolite is well suited as a 40Ar/39Ar reference material with high K/Ca and radiogenic yield. Intercalibration with GA1550 biotite from the Dromedary igneous complex (New South Wales, Australia) yields an age of 9.7946 +/- 0.0031 Ma for BHs relative to an age of 98.79 +/- 0.54 Ma for GA1550 and a calibration factor (R-BHs/GA1550) of 0.096719 +/- 0.000032. BHs is also intercalibrated with sanidines of the Taylor Creek Rhyolite (TCs; R-TCs/BHs of 2.90874 +/- 0.00067), Fish Canyon Tuff (FCs; R-FCs/BHs of 2.88339 +/- 0.00088), and rhyolite of Alder Creek (ACs; R-ACs/BHs of 0.12028 +/- 0.00024). These calibration factors yield ages of 28.344 +/- 0.011 Ma, 28.099 +/- 0.013 Ma, and 1.1809 +/- 0.0024 Ma for TCs, FCs, and ACs, respectively, relative to GA1550. Full propagation of errors increases these uncertainties and that of BHs to +/- 0.9% of their ages. Calibration of BHs using the astronomically tuned age of FCs determined by Kuiper et al. (2008) yields an age of 9.8295 +/- 0.0036 Ma. Stepwise heating of BHs reveals the same small, progressive increase in age across the age spectrum reported for FCs, ACs, and other potassium feldspars (e.g., Foland and Xu, 1990; Phillips et al., 2017). This increase is consistent with mass fractionation of argon during step heating and favors use of single-step fusion ages of all sanidines for monitor and calibration purposes. Zircons from the rhyolite of Bodie Hills are strongly zoned in U and Th, and U/Pb geochronologic analyses suggest multiple generations of zircon growth. The youngest ages indicate a crystallization age of 9.97 +/- 0.08 Ma (2 sigma); arguably similar to the 40Ar/39Ar results when residence time is considered, but previous episodes of zircon growth began at least 400 ka prior to eruption. Precise (U - Th)/He thermochronology of these zircons is difficult because crystal-to-crystal variations in the magnitude and complexity of their U + Th zoning complicates accurate alpha ejection calculations. Replicate conventional (single-crystal) (U - Th)/He ages are more widely dispersed than predicted by analytical uncertainties, but laser ablation (U - Th)/He ages are more reproducible, with an inverse variance-weighted mean of 9.71 +/- 0.58 Ma.

The Great Basin of the western United States, the northern component of the Basin and Range Province, is a region of Cenozoic lithospheric extension with multiple periods and types of igneous activity. The composition and volume of Cenozoic magmas reflect a complex interaction between mantle-derived magmas and highly diverse crust, where both mantle sources and magmatic processes were modulated by tectonic environment. The Fish Creek Mountains in north-central Nevada underwent multiple igneous events ranging from ca. 40 Ma to 1 Ma that span all of the complex Cenozoic tectono-magmatic episodes of the Great Basin. The Fish Creek Mountains, therefore, is an ideal location to evaluate the different sources and processes involved in magma generation. Many plutons were emplaced in the region between about 40 and 38 Ma, several of which host base and precious metal deposits. Between 36 and 33 Ma, lava fields and calderas of the 37-19 Ma Ignimbrite Flare-up were emplaced. Both these and the preceding plutons resulted from southwestward rollback of the Farallon plate beneath North America during by far the most voluminous phase of Cenozoic magmatism. The lavas range from rare basalt and basaltic andesite to andesite, dacite, and rhyolite, have continental arc-like incompatible element patterns, and high initial Sr-87/Sr-86 and low eNd that require a metasomatized lithospheric mantle source combined with minor crustal component. Ignimbrites of the 34.4 Ma Cove Mine (trachydacite to rhyolite) and 34.0 Ma Caetano calderas (rhyolite to high-silica rhyolite) are abundantly porphyritic, include hydrous phases, were largely derived from partial melts of crustal rocks, but likely include 20-30% of a mantle-derived component. Igneous activity ceased in the region as the rollback-arc migrated to the southwest, but at 24.9 Ma a new caldera formed in the southern Fish Creek Mountains that was filled by ignimbrites of the Fish Creek Mountains Tuff. Intracaldera rhyolite ignimbrites range from aphyric, pumice-rich deposits at the base to progressively more quartz-feldspar physic ignimbrites at higher levels; all flow units lack hydrous phases. No contemporaneous mafic or intermediate igneous activity accompanied caldera formation, but initial Sr-87/Sr-86 values in the Fish Creek Mountains tuffs are lower than in the Caetano Tuff, suggesting a greater mantle contribution to the 24.9 Ma ignimbrites. After another hiatus in igneous activity, the region was intruded and overlain by basalt to rhyolite dykes and lavas of the northern Nevada rift between 16.8 and 15.1 Ma. The primarily tholeiitic igneous suite is of the same age, chemistry, and isotopic composition as the Grande Ronde Formation of the Columbia River flood basalts, and evolved members (trachydacite and rhyolite) are crustally contaminated. The youngest northern Nevada rift lava is an alkali olivine basalt with isotopic affinity to basalts of the eastern Snake River Plain. After 10 Ma of quiescence, the region was locally covered by mafic lava flows with high-alumina olivine tholeiite compositions, represented by the 5.4 Ma Pumpernickel Valley flows. Their mid-ocean ridge-like incompatible element compositions indicate a depleted mantle source for the lavas, but radiogenic isotopic compositions indicate that the lavas of this region include a significant contribution from a mafic to ultramafic, high-Sr-87/Sr-86 source. The final igneous event in the Fish Creek Mountains region, the 4.0 to 1.0 Ma Buffalo Valley volcanic field, includes flows and spatter cones of transitional to alkalic basalt that are divided into two geochemical groups with identical isotopic compositions. They represent variable, low percent partial melts of the asthenosphere at different depths, yielding different rare earth element characteristics. Similar to the Lunar Crater volcanic field, the Buffalo Valley rocks represent a rare case where the lithosphere in the central Great Basin is now thin enough to allow melting of the underlying asthenosphere. Cenozoic magmatism in the northern Great Basin exhibits several transitions in magma sources and tectonic setting with time. Magmatism began as pre-extension, subduction-related, primarily lithospherically-derived magmas emplaced on/in tectonically-thickened crust. The onset of extension was partially driven by impinge. ment of the Yellowstone plume that resulted in emplacement of rift-related volcanic and intrusive rocks in the northern Nevada rift, followed by the eruption of extension-related HAOT lavas along the northwest margin of the Great Basin. Finally, lithospheric thinning allowed for partial melting of the asthenosphere and eruption of alkaline basaltic lavas.

First posted May 21, 2019 For additional information, contact: Director, Geology, Geophysics and Geochemistry Science CenterU.S. Geological SurveyBox 25046, MS-973Denver, CO 80225-0046 The purpose of this report is to summarize geochemical, petrographic, and geochronologic data for samples, principally those of unmineralized Tertiary volcanic rocks, from the Tonopah, Divide, and Goldfield mining districts of west-central Nevada (fig. 1). Much of the data presented here for the Tonopah and Divide districts are for samples collected by Bonham and Garside (1979) during geologic mapping in and around those districts, whereas much of that for samples from the Goldfield district were obtained by Ashley (1974; 1979; 1990a). Additional data were derived from samples collected between 2012–2015, as part of the Mineral Resources Program funded project titled: “Magmatic-tectonic history and component sources of major precious metal deposits in the southern Walker Lane”. A small amount of additional geochemical data for samples from each of the districts were compiled from other sources. Individual sample collectors are identified by appropriate entries in the “Collector” field (appendix 1) and published sources of geochemical data are defined by entries in the “Chem_Src” data field (appendix 1).

Partially eroded stratovolcanoes worldwide, notably Mounts Rainier and Adams in the Cascades and several volcanoes in Japan, record episodic periods of eruption and geothermal activity that produce zones of hydrothermal alteration. The partly eroded core of late Pleistocene Brokeoff volcano on the south side of Lassen Peak exposes the upper 1 km of multiple ancient (ca. 410-300 ka) magmatic-hydrothermal alteration zones in a 3.5 by 5 km area that allows characterization of the three-dimensional hydrothermal evolution of the volcano. Both acid- and neutral-pH hydrothermal solutions produced distinctive alteration mineral assemblages in close proximity. Early hydrothermal activity is characterized by alunite-rich alteration that is temporally and spatially related to shallow intrusions in the center of the volcano. Younger acid alteration and a large area of neutral-pH alteration formed along the volcano's flanks. The neutral-pH alteration is vertically zoned over 1000 m from shallow zeolite +/- adularia through intermediate argillic (smectite-pyrite +/- illite) to deep propylitic (chlorite-calcite-albite-illite) alteration. Pleistocene alteration is partly overprinted by surficial, steam-heated alteration related to Lassen's modern hydrothermal activity. A large (similar to 3.5 km(2)), shallow (<= 300 m), ca. 1.5 Ma alunite-rich magmatic-hydrothermal alteration zone is exposed on the northeast flank of the nearby Maidu volcanic center. Hydrothermal activity occurred just prior to, or at the beginning of, major eruptive periods on both volcanoes. Fluid flow and hydrothermal alteration were controlled by primary permeability (lateral fluid flow driven by paleotopographic ground-water gradients in volcaniclastic rocks), steeply dipping fractures in more deeply exposed parts of alunite-rich alteration zones, and inferred basement structures. Stable-isotope data indicate hydrothermal fluids were mixtures of variably exchanged meteoric and magmatic waters. Variations in types of hydrothermal alteration and fluid compositions may reflect a temporal increase in depth of magma emplacement and degassing that resulted in increased interaction with and neutralization by wall rocks during ascent of magmatic gases. Alteration zones on both volcanoes are not strongly mineralized, probably reflecting relatively low amounts of magmatic acid, sulfur, and chlorine consistent with degassing of small volumes of pyroxene andesite magma compared to large dacite and/or granodiorite batholiths that form significant epithermal and porphyry deposits.

Successive caldera-forming eruptions from ca. 30 to 25 Ma generated a large nested caldera complex in western Nevada that was subsequently dissected by Basin and Range extension, providing extraordinary crosssectional views through diverse volcanic and plutonic rocks. A high-resolution oxygen isotopic study was conducted on units that represent all major parts of the Job Canyon, Louderback Mountains, Poco Canyon, and Elevenmile Canyon caldera cycles (29.225.1 Ma), and several Cretaceous plutons that flank the Stillwater caldera complex. We provide new oxygen and strontium isotope data for 12 additional caldera centers in the Great Basin, which are synthesized with >150 published oxygen and strontium isotope analyses for regional Mesozoic basement rocks. Stillwater zircons span a large isotopic range (delta O-18(zircon) of 3.6 parts per thousand-8.2 parts per thousand), and all caldera cycles possess low-delta O-18 zircons. In some cases, they are a small proportion of the total populations, and in others, they dominate, such as in the low-delta O-18 rhyolitic tuffs of Job Canyon and Poco Canyon (delta O-18(zireon) = 4.0 parts per thousand-4.3 parts per thousand; delta O-18(magma )= 5.5 parts per thousand-6 parts per thousand). These are the first low-delta O-18 rhyolites documented in middle Cenozoic calderas of the Great Basin, adding to the global occurrence of these important magma types that fingerprint recycling of shallow crust altered by low-delta O-18 meteoric waters. The appearance of low-delta O-18 rhyolites in the Stillwater caldera complex is overprinted on a Great Basin-wide trend of miogeoclinal sediment contribution to silicic magmas that elevates delta O-18 compositions, making identification of delta O-18 depletions difficult. Though not a nominally low-delta O-18 rhyolite, the tuff of Elevenmile Canyon possesses both low-delta O-18 and high-delta O-18 zircon cores that are overgrown by homogenized zircon rims that approximate the bulk zircon average, pointing to batch assembly of isotopically diverse upper crustal melts to generate one of the most voluminous (2500-5000 km(3)) tuff eruptions in the Great Basin. Despite overlapping in space and time, each caldera-forming cycle of the Stillwater complex has a unique oxygen isotope record as retained in single zircons. Most plutons that were spatially and temporally coincident with calderas have isotopic compositions that diverge from the caldera-forming tuffs and cannot be their cogenetic remnants.

First posted November 7, 2018 For additional information, contact: Contact Information, Geology, Minerals, Energy, & Geophysics Science Center—Menlo ParkU.S. Geological Survey345 Middlefield RoadMenlo Park, CA 94025-3591 Epithermal gold-silver deposits are vein, stockwork, disseminated, and replacement deposits that are mined primarily for their gold and silver contents; some deposits also contain substantial resources of lead, zinc, copper, and (or) mercury. These deposits form in the uppermost parts of the crust, at depths less than about 1,500 meters below the water table, and at temperatures below about 300 °C. Most epithermal gold-silver deposits are genetically related to hydrothermal systems associated with subaerial volcanism and intrusion of calc-alkaline magmas along convergent plate margins. These deposits formed throughout most of geologic time, although most known deposits are Cenozoic, which reflects preferential preservation of these shallowly formed deposits in tectonically unstable regions. Epithermal gold-silver deposits range in size from tens of thousands to greater than 1 billion metric tons of ore and have gold contents of 0.1 to greater than 30 grams per metric ton and silver contents of less than 1 to several thousand grams per metric ton. Historically, these deposits have been an important source of gold and silver and are estimated to contain about 8 percent of global gold. The wide range of tonnage-grade characteristics makes epithermal gold-silver deposits an attractive target for small and large exploration and mining companies.This report constitutes a new descriptive model for epithermal gold-silver deposits. It summarizes characteristics of known deposits, including their geological, geophysical, geochemical, and geoenvironmental aspects. Models concerning the genesis of epithermal gold-silver deposits are discussed. The application of descriptive and genetic aspects of the model to mineral exploration and resource assessment of undiscovered deposits is described. Finally, areas where additional research is needed to better understand the genesis of these deposits are identified. An extensive summary table outlining the characteristics of about 100 epithermal gold-silver deposits is included as an appendix; this summary table includes most of the world's largest epithermal gold-silver deposits, and many smaller, well-studied deposits.

Geologic mapping, new U-Pb zircon ages, and new and published 40Ar/39Ar sanidine ages document the timing and extent of Oligocene magmatism in the southern Stillwater Range and Clan Alpine Mountains of western Nevada, where Miocene extension has exposed at least six nested silicic calderas and underlying granitic plutons to crustal depths locally ≥9km. Both caldera-forming rhyolitic tuffs and underlying plutons were emplaced in two episodes, one from about 30.4–28.2Ma that included the Deep Canyon, Job Canyon, and Campbell Creek calderas and underlying plutons, and one from about 25.3–24.8Ma that included the Louderback Mountains, Poco Canyon, and Elevenmile Canyon calderas and underlying plutons. In these two 1–2m.y. periods, almost the entire Mesozoic upper crust was replaced by Oligocene intrusive and extrusive rocks to depths ≥9km over an estimated total area of ~1500km2 (pre-extension). Zircon trace element geochemistry indicates that some plutonic rock can be solidified residual magma from the tuff eruptions. Most plutons are not solidified residual magma, although they directly underlie calderas and were emplaced along the same structures shortly after to as much as one million years after caldera formation. Magma chambers and plutons grew by floor subsidence accommodated by downward transfer of country rocks. If other Great Basin calderas are similar, the dense concentration of shallowly exposed calderas in central Nevada is underlain by a complexly zoned mid-Cenozoic batholith assembled in discrete pulses that coincided with formation of large silicic calderas up to 2500–5000km3.

Porphyry Cu and porphyry Mo deposits are large to giant deposits ranging up to >20 and 1.6 Gt of ore, respectively, that supply about 60 and 95% of the world’s copper and molybdenum, as well as significant amounts of gold and silver. These deposits form from hydrothermal systems that affect 10s to >100 km3 of the upper crust and result in enormous mass redistribution and potential concentration of many elements. Several critical elements, including Re, Se, and Te, which lack primary ores, are concentrated locally in some porphyry Cu deposits, and despite their low average concentrations in Cu-Mo-Au ores (100s of ppb to a few ppm), about 80% of the Re and nearly all of the Se and Te produced by mining is from porphyry Cu deposits. Rhenium is concentrated in molybdenite, whose Re content varies from about 100 to 3,000 ppm in porphyry Cu deposits, ≤150 ppm in arc-related porphyry Mo deposits, and ≤35 ppm in alkali-feldspar rhyolite-granite (Climax-type) porphyry Mo deposits. Because of the relatively small size of porphyry Mo deposits compared to porphyry Cu deposits and the generally low Re contents of molybdenites in them, rhenium is not recovered from porphyry Mo deposits. The potential causes of the variation in Re content of molybdenites in porphyry deposits are numerous and complex, and this variation is likely the result of a combination of processes that may change between and within deposits. These processes range from variations in source and composition of parental magmas to physiochemical changes in the shallow hydrothermal environment. Because of the immense size of known and potential porphyry Cu resources, especially continental margin arc deposits, these deposits likely will provide most of the global supply of Re, Te, and Se for the foreseeable future. Although Pd and lesser Pt are recovered from some deposits, platinum group metals are not strongly enriched in porphyry Cu deposits and PGM resources contained in known porphyry deposits are small. Because there are much larger known PGM resources in deposits in which PGMs are the primary commodities, it is unlikely that porphyry deposits will become a major source of PGMs. Other critical commodities, such as In and Nb, may eventually be recovered from porphyry Cu and Mo deposits, but available data do not clearly define significant resources of these commodities in porphyry deposits. Although alkali-feldspar rhyolite-granite porphyry Mo deposits and their cogenetic intrusions are locally enriched in many rare metals (such as Li, Nb, Rb, Sn, Ta, and REEs) and minor amounts of REEs and Sn have been recovered from the Climax mine, these elements are generally found in uneconomic concentrations. As global demand increases for critical elements that are essential for the modern world, porphyry deposits will play an increasingly important role as suppliers of some of these metals. The affinity of these metals and the larger size and greater number of porphyry Cu deposits suggest that they will remain more significant than porphyry Mo deposits in supplying many of these critical metals.

The southern segment of the ancestral Cascades magmatic arc includes numerous volcanic fields; among these, the Bodie Hills volcanic field (BHVF), astride the California-Nevada border north of Mono Lake, is one of the largest (>700 km(2)) and most well studied. Episodic magmatism in the BHVF spanned about 9 million years between about 15 and 6 Ma; magmatic output was greatest between ca. 15.0 to 12.6 Ma and ca. 9.9 to 8.0 Ma.About two dozen contiguous and coalescing eruptive centers above middle- to shallow-crustal-level reservoirs generated several trachyandesite stratovolcanoes and numerous silicic trachyandesite to rhyolite flow dome complexes whose compositional variations are consistent with fractionation of observed phenocryst phases. BHVF rocks have high-potassium calc-alkaline compositions consistent with generation of subduction-related continental margin arc magmas beneath thick continental crust. Radiogenic isotope ratios in BHVF rocks vary considerably but suggest somewhat enriched, crustal sources; isotopic ratios for some of the more primitive units are consistent with more depleted, mantle sources. Neither age nor whole-rock compositions of BHVF rocks are well correlated with isotopic variations. Textures and compositions of phenocrysts in BHVF rocks are in accord with the associated magma reservoirs evolving via open-system behavior. Reservoir recharge and subsequent incomplete homogenization are evidenced by the broad compositional diversity characteristic of many BHVF eruptive units. Significant compositional diversity among the products of coeval eruptive centers further suggests that centers responsible for BHVF magmatism were underlain by small, discrete, compositionally distinct, and closely spaced reservoirs.Volcanic rocks of the BHVF host quartz-adularia and quartz-alunite epithermal gold-silver deposits, from which about 3.4 Moz. of gold and 28 Moz. of silver have been produced. The volcanic rocks and contained deposits are broadly coeval, which suggests that the associated magmas are the sources of heat, fluids, and metals involved in deposit genesis. Characteristics of the quartz-adularia deposits are consistent with derivation from near-neutral pH fluids at <= 250 degrees C, whereas those of the quartz-alunite systems require more acidic, oxidized, and sulfur-rich fluids at temperatures <250 degrees C. Economically viable precious metal accumulations are in fault-hosted vein deposits in the Bodie and Aurora districts. Circulation of hydrothermal fluids through permeable pyroclastic deposits but lacking prominent structural conduits resulted in large areas of altered but unmineralized rock.

First posted October 4, 2016 Revised October 28, 2016 For additional information, contact: Contact Information, Mineral Resources Program U.S. Geological Survey 12201 Sunrise Valley Drive 913 National Center Reston, VA 20192 http://minerals.usgs.gov/ The U.S. Department of the Interior has proposed to withdraw approximately 10 million acres of Federal lands from mineral entry (subject to valid existing rights) from 12 million acres of lands defined as Sagebrush Focal Areas (SFAs) in Idaho, Montana, Nevada, Oregon, Utah, and Wyoming (for further discussion on the lands involved see Scientific Investigations Report 2016–5089–A). The purpose of the proposed action is to protect the greater sage-grouse (Centrocercus urophasianus) and its habitat from potential adverse effects of locatable mineral exploration and mining. The U.S. Geological Survey Sagebrush Mineral-Resource Assessment (SaMiRA) project was initiated in November 2015 and supported by the Bureau of Land Management to (1) assess locatable mineral-resource potential and (2) to describe leasable and salable mineral resources for the seven SFAs and Nevada additions.This chapter summarizes the current status of locatable, leasable, and salable mineral commodities and assesses the potential of selected locatable minerals in lands proposed for withdrawal that span the Nevada, Oregon, Idaho, and Utah borders. In this report, the four study areas evaluated were (1) the Sheldon-Hart Mountain National Wildlife Refuge Complex SFA in Washoe County, Nevada, and Harney and Lake Counties, Oregon; (2) the Southeastern Oregon and North-Central Nevada SFA in Humboldt County, Nevada, and Harney and Malheur Counties, Oregon; (3) the Southern Idaho and Northern Nevada SFA in Cassia, Owyhee, and Twin Falls Counties, Idaho, Elko County, Nevada, and Box Elder County, Utah; and (4) the Nevada additions in Humboldt and Elko Counties, Nevada.