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[Display omitted] • Reinterpretation of previous drill holes intercepting carbonate rocks of the Roberts Mountains autochthon can identify normal faults based on omissions in the expected stratigraphy. • Exposures of carbonate rocks are related to Mesozoic anticlines exhumed in the footwalls of normal faults. • Gold deposits hosted in the siliciclastic rocks of the Roberts Mountains allochthon could be the product of the Carlin-type ore fluid (e.g., Hilltop) or the product of coeval or younger surficial/basinal fluids remobilizing gold from elsewhere. This study integrates results from previous studies with new reconnaissance-scale mapping of rock types, structures, and hydrothermal alteration at Goat Ridge and Tenabo to provide a district-scale cross section and palinspastic restoration of an area in Nevada with high potential for Carlin-type mineralization in the footwall of a dismembered thrust fault. Reinterpretation of legacy drill data in light of recent advances in the understanding of Cenozoic normal faulting in the area with the expected stratigraphy of the Roberts Mountains autochthon allows for deciphering whether previous drill holes piercements of faults are the Roberts Mountains thrust or are tilted normal faults that down-dropped the deep-water siliciclastic rocks of Roberts Mountains allochthon onto the coeval carbonate rocks of the Roberts Mountains autochthon. A cross-sectional reconstruction shows that ∼11.1 km (120 %) of extension was accomplished along one set of faults that initiated at 60-70°and tilted 40°E. Previously underappreciated Mesozoic folds are present, which likely formed with horizontal fold hinges. Because of Cenozoic normal faulting and associated tilting, the hinge lines currently plunge at moderate angles and are offset as they step across each crosscutting fault. The anticlinal fold hinges produced the modern exposures of lower plate rocks in the range. The Eocene paleogeologic map that results from a plan-view structural restoration shows that the hinge lines of the original folds trended northwest-southeast in the southern half of the range but turned northward toward the northern end of the range. Mineralization in the range includes silver-dominated polymetallic veins/lodes, weak porphyry-type, Carlin-type, and siliciclastic-hosted gold deposits of uncertain origin. Porphyry-style and silver-dominated polymetallic vein mineralization is associated with ∼39–40 Ma granodiorites, with the silver-dominated polymetallic veins interpreted as the distal veins around weakly mineralized and/or deeply buried porphyry system(s), probably of the quartz monzonitic-granitic porphyry Mo-Cu subclass of porphyry molybdenum deposits. Crosscutting relationships demonstrate that at least one of "upper plate" gold deposits is Eocene, but a Miocene age cannot be ruled out for the other gold deposits. [ABSTRACT FROM AUTHOR]

Veins and pervasive wall-rock alteration composed of coarse muscovite±quartz±pyrite are documented for the first time in a porphyritic granite at Luhr Hill in the Yerington District, Nevada. Coarse muscovite at Luhr Hill occurs at paleodepths of ~6-7 km in the roots of a porphyry copper system and crops out on the scale of tens to hundreds of meters, surrounded by rock that is unaltered or variably altered to sodic-calcic assemblages. Coarse muscovite veins exhibit a consistent orientation, subvertical and N-S striking, which structurally restores to subhorizontal at the time of formation. Along strike, coarse muscovite veins swell from distal, millimeter-thick muscovite-only veinlets to proximal, centimeter-thick quartz-sulfide-bearing muscovite veins. Crosscutting relationships between coarse muscovite veins, pegmatite dikes, and sodic-calcic veins indicate that muscovite veins are late-stage magmatic-hydrothermal features predating final solidification of the Luhr Hill porphyritic granite. Fluid inclusions in the muscovite-quartz veins are high-density aqueous inclusions of ~3-9 wt% NaCl eq. and <1 mol% CO that homogenize between ~150 and 200 °C, similar to fluid inclusions from greisen veins in Sn-W-Mo vein systems. Our results indicate that muscovite-forming fluids at Luhr Hill were mildly acidic, of low to moderate salinity and sulfur content and low CO content, and that muscovite in deep veins and alteration differs in texture, composition, and process of formation from sericite at shallower levels of the hydrothermal system. Although the definition of greisen is controversial, we suggest that coarse muscovite alteration is more similar to alteration in greisen-type Sn-W-Mo districts worldwide than to sericitic alteration at higher levels of porphyry copper systems. The fluids that form coarse muscovite veins and alteration in the roots of porphyry copper systems are distinct from fluids that formed copper ore or widespread, shallower, acidic alteration. We propose that this style of veins and alteration at Luhr Hill represents degassing of moderate volumes of overpressured hydrothermal fluid during late crystallization of deep levels of the Yerington batholith. [ABSTRACT FROM AUTHOR]

This study combines results from reconnaissance-scale mapping of hydrothermal alteration, rock types, and structures to provide a district-scale cross section and associated palinspastic reconstruction of an area with two previously undescribed Laramide (~70 Ma) porphyry systems at Sheep Mountain and Copper Basin (Crown King). Extension at the district scale is placed in an arc-scale context using an original compilation of strike and dip measurements on Tertiary rocks to reconstruct the Laramide porphyry belt prior to extension. The study area contains five sequential, partially superimposed sets of normal faults that are nearly planar where exposed. Dips of all normal faults initiated at 60° to 70° and rotated during slip to angles as gentle as 20°. A palinspastic reconstruction reveals that two, spatially distinct hydrothermal systems overlie different cupolas of a Late Cretaceous pluton. Hydrothermal alteration is zoned from greisen to potassic to transitional greisen-potassic assemblages from deep to shallow structural levels. The reconstruction is used to identify two covered exploration targets. The prospects may be porphyry molybdenum systems of the quartz monzonitic-granitic porphyry Mo-Cu subclass, joining others in an arc that is best known for porphyry copper deposits. The Laramide porphyry belt prior to extension displays a variably well-defined axis, ~100 km wide, with gaps and clusters of deposits along its 700-km strike length. The majority of deposits lie along the axis, but others lie in fore- or rear-arc positions. The interpreted preextensional geometry of the Laramide porphyry belt resembles other porphyry belts and the distribution of active volcanoes at convergent margins. [ABSTRACT FROM AUTHOR]

This study integrates a cross-sectional restoration of normal faults and porphyry- related hydrothermal alteration zones with six new U-Pb zircon dates to constrain the ages of mineralization and large-magnitude Cenozoic extension at Spruce Mountain, northeastern Nevada (USA). Paleozoic sedimentary rocks and sparse rhyolitic intrusive rocks host a porphyry molybdenum deposit dated as ca. 38 Ma that contains associated skarn, carbonate replacements, fissure veins, and jasperoid. At least six crosscutting sets of normal faults with variable dip directions (east, west, and north) and angles (<15° to >60°) are identified at Spruce Mountain, reflecting overprinting phases of extension. Based on the restored cross section, normal faulting at Spruce Mountain resulted in ~6.9 km (120%) of total extension and ~35° of net eastward tilting. The first four fault sets, late Eocene (ca. 38 Ma) or older, collectively accommodate most of the total extension (~5.4 km). Later faults probably were active in the Miocene and Quaternary. All restored faults had initial dips of 45° or greater, and the restored preextensional structure of Spruce Mountain consists of west-vergent folds and gentle westward dips of Paleozoic rocks. Spruce Mountain is classified as a rhyolitic porphyry Mo or Climax-type deposit, with extensive skarn. Eocene rhyolite porphyry dikes associated with porphyry Mo mineralization locally intrude the early generations of normal faults at Spruce Mountain. However, many normal faults also postdate molybdenum mineralization and have partially dismembered the system. Cross-sectional restoration of the postmineralization faulting suggests that the original deposit had an ~6 km long by ~3 km deep elliptical pattern with carbonate replacement deposits surrounding skarn. [ABSTRACT FROM AUTHOR]

The root zone of a porphyry system is a specific region beneath a porphyry orebody that was a site of focused fluid flow, as evidenced by abundant quartz veins, widespread wall-rock alteration, or porphyry dikes merging downward into a porphyritic granite cupola. These zones constitute an important source region of ore fluids and other components, and in certain geologic terrains the characteristics of root zones may point to previously undiscovered deposits. The root zones of four Laramide porphyry copper systems in Arizona recently have been characterized at a reconnaissance level: the Miami Inspiration system associated with the Schultze Granite, the Sierrita-Esperanza system associated with the Ruby Star Granodiorite, the Ray system associated with the Granite Mountain pluton, and the Kelvin-Riverside system associated with the Tea Cup pluton. The two well-studied root zones related to the Jurassic Yerington batholith in Nevada, and associated with the Yerington mine and the Ann-Mason deposit, provide a basis of comparison. All six systems occur in areas with unusually large exposures in both lateral and vertical paleodirections, locally to paleodepths of > 10 km, because of postore extensional faulting and associated tilting. No two systems are 'alike, but many share the presence of the following hydrothermal characteristics: quartz veins and potassic alteration, sodic-calcic and sodic alteration, calcic alteration, and relatively coarse grained muscovite-quartz (greisen). Quartz veins and potassic 'alteration are focused centrally, directly above related cupolas; sodic-calcic and sodic alteration, calcic alteration, and evidence for leaching of silica are observed on the deep flanks of certain systems; and greisen occurs directly beneath ore within and beneath coeval cupolas in many systems. Certain systems exhibit evidence of multiple cycles of release of magmatic fluid followed by incursion of saline ground waters, which are analogous to the biological cycle of exhale- inhale, respectively. The characteristics of the root zones provide important constraints on the exsolution and transport of the magmatic aqueous phase that leads to ore formation, the variable incursion of external fluids into the hydrothermal system, and the degassing of magmatic volatiles that may not be related directly to porphyry ore formation. The most robust conclusions are drawn from the localities that offer the greatest quality of exposure and degree of continuity (including compelling structural reconstructions) between the roots and the ore deposit, from the studies that identify timelines linking processes in the roots with those in the mineral deposit, and from systems in which the deposit itself is well characterized. [ABSTRACT FROM AUTHOR]

Fluorine-rich hydrothermal mineral assemblages developed at Henderson, Colorado, as 12 rhyolitic stocks of Oligocene age were emplaced, crystallized, and cooled inside a much larger and slightly older, rhyolitic intrusion, forming a Climax- type molybdenum deposit. The stocks are grouped into three intrusive centers, and the mineral assemblages, including ore minerals, are grouped according to their temperatures of formation as either high-, moderately high, moderate-, or low-temperature assemblages. The latter three groups are termed lower temperature assemblages, which are subdivided into two suites on the basis of position: above intrusive centers and on the flanks of the Seriate center. Integration of the mineralogy, sequence, and spatial positions of mineral assemblages defines the geochemical evolution of hydrothermal fluids, places qualitative constraints on mass transfer, and provides insight into the development of the geochemical halos that are used in exploration for porphyry orebodies. A reconnaissance fluid inclusion study targeted the lower temperature assemblages, and a geologic assessment of results indicates that we were unable to identify fluid inclusions from several assemblages, After pooling meaningful results, the ranges of temperatures of formation for each group of lower temperature assemblages are moderately high, 600° to 460°C: moderate, 530° to 310°C; and low, 390° to 200°C, with significant differences in salinity between the two suites of assemblages, The suite formed above intrusive centers was deposited by more saline fluids (salinity commonly 28-65 wt % NaCl ± KCl equiv) than the suite formed on the flanks of the Seriate center (mostly <29 wt % NaC1 + KCl equiv). The lower temperature assemblages described here do not overlap in space with higher level propylitic alteration, but fluid inclusions in one sample of propylitized rock indicate temperatures of 320° to... [ABSTRACT FROM AUTHOR]

At Henderson, Colorado, 12 rhyolitic stocks of Oligocene age crystallized in three intrusive centers whose tops are 1 km below the surface of Red Mountain: Henderson (oldest), Seriate, and Vasquez (deepest and youngest). A variety of fluorine-rich hydrothermal mineral assemblages are grouped according to their temperatures of formation, either high-, moderately high, moderate-, or low-temperature (the latter three groups are termed lower temperature assemblages). The groupings were based on relative age relationships in areas free of cyclical events, and absolute temperature estimates based on fluid inclusion study later were made for many mineral assemblages, as detailed in a companion paper. Numerous crosscutting contacts of individual stocks combined with limits of associated high-temperature assemblages can be interpreted as spatially extensive time lines, thereby enabling temporal correlation of spatially separated events. High-temperature silicic (quartz-fluorite) and intense potassic (quartz-K-feldspar-molybdenite) assemblages developed in numerous cycles, each corresponding to emplacement of a stock and deposition of molybdenite. Molybdenite occurs without pyrite mostly in the high-temperature assemblages. In contrast, lower temperature assemblages broadly envelop intrusive centers, rather than single stocks. Some of the moderately high temperature, less intense potassic assemblages (including mottled K-feldspar-quartz and magnetite-K-feldspar) also formed during multiple cycles. Moderate-temperature sericitic assemblages (including topaz with pyrite or magnetite and sericite with pyrite or magnetite) only locally exhibit evidence for multiple cycles, and low-temperature intermediate argillic assemblages (pyrite-day and several assemblages containing base metal sulfides, F-bearing, Mn-rich garnet, and rhodochrosite) were deposited in only a single event. These lower temperature assemblages terminate 700 m below the present surface... [ABSTRACT FROM AUTHOR]

• Many roots zones of porphyry copper systems across Arizona contain coarse muscovite alteration. • Coarse muscovite alteration assemblages vary with associated magmatic composition. • Coarse muscovite veins show systematic transitions in mineralogy and morphology with depth. • In porphyry Cu systems, coarse muscovite alteration is developed in the roots of systems. • In porphyry Mo-Cu systems, coarse muscovite alteration is typically well mineralized. Coarse muscovite veins and alteration occur in porphyry copper and porphyry molybdenum-copper systems within the Laramide arc in Arizona, as well as at the Yerington district in Nevada. This work describes coarse muscovite in veins and altered wall rock in porphyry systems in this region and documents mineral assemblages, mineral compositions, spatial and temporal relationships, and hydrogen isotopic compositions. Coarse hydrothermal muscovite is documented in the roots of porphyry Cu ± Mo systems, as well as in and above the ore bodies in porphyry Mo-Cu systems, and it is compared to coarse hydrothermal muscovite (greisen) in lode Sn-W-Mo systems. Basin and Range extension has exposed coarse hydrothermal muscovite in several Laramide and Jurassic porphyry Cu (±Mo) systems, at paleodepths of ~3 to 12 km: Miami-Inspiration, Sierrita-Esperanza, Copper Basin (Crown King), Granite Mountain (roots of the Ray porphyry system), Gunnison (Texas Canyon stock), Grayback (Kelvin-Riverside district), Sycamore Canyon, the New Cornelia mine (Ajo district), and two systems in the Yerington district. Muscovite is the dominant mica in these coarse muscovite veins and associated alteration, with common K-feldspar and albite (An 00-06), common accessory hematite, rutile, pyrite, and apatite, and rare accessory chalcopyrite, fluorite, molybdenite, wolframite, and scheelite. Coarse hydrothermal muscovite yields δD compositions that suggest formation from fluids that are dominantly magmatic-hydrothermal in origin. Whole-rock compositions of coarse hydrothermal muscovite show common gains in K and loss of Ca ± Na. Coarse muscovite veins and alteration in porphyry copper systems postdate mineralized potassic veins and form too deeply to overlap with shallower acidic forms of alteration (sericitic, advanced argillic). Variation in mineral assemblage, mineral compositions, and mineralization of coarse hydrothermal muscovite correlate with the composition of Laramide stocks. Porphyry Mo-Cu systems contain coarse muscovite alteration assemblages with the highest mineral diversity and trace-element enrichment. Coarse muscovite veins and alteration in porphyry Mo-Cu systems related to stocks ranging from quartz monzonite to granite in composition form at shallower paleodepths and occur within and above the associated orebodies. In contrast, coarse muscovite veins and alteration associated with subalkaline porphyry copper systems occur at deeper levels, in some cases overlapping with the bottom of potassic alteration and the ore body but extending well into the roots of the system in the underlying granitoid cupola. In these latter systems, zones of coarse muscovite alteration typically are poorly mineralized and mineral assemblages are less varied. These characteristics suggest that coarse muscovite-forming fluids are predominately of magmatic-hydrothermal origin and exsolved from late-stage, fractionated magmas of the larger pluton that sourced porphyry stocks and dikes responsible for porphyry copper mineralization. In some instances, however, the exposed coarse muscovite alteration is associated with a petrologically unrelated, commonly more felsic, later intrusion, rather than being related to late exsolution of fluid from the same crystallizing stock or batholith. [ABSTRACT FROM AUTHOR]