Metamorphosed Triassic and Jurassic volcanic and sedimentary rocks have been mapped, described, and measured in the Singatse, Buckskin, and northern Wassuk Ranges near Yerington, west-central Nevada. Herein, we establish new formation names for these rocks and correlate them regionally with other Triassic-Jurassic rocks, in part by use of fossil and radiometric ages. From oldest to youngest, rocks in the Singatse Range consist of a Middle Triassic or older volcanic sequence (McConnell Canyon volcanics), an Upper Triassic sequence of interbedded fine-grained clastic sedimentary rocks, carbonate rocks, tuffaceous sedimentary rocks, and tuffs (Malachite Mine Formation and tuff of Western Nevada Mine), a thick Upper Triassic limestone (Mason Valley Limestone), an uppermost Triassic and Lower Jurassic siltstone sequence (Gardnerville Formation), an Early and/or Middle Jurassic limestone-gypsum-quartzite sequence (Ludwig Mine Formation), and Middle Jurassic volcanic rocks. The sequence is exposed in septa between two Middle Jurassic batholiths and was folded and metamorphosed during emplacement of the batholiths. The Middle Jurassic volcanic rocks are best exposed in the Buckskin Range to the west, where they consist of a lower andesitic sequence (Artesia Lake volcanics) and an upper sequence of more felsic, porphyritic rocks (Fulstone Spring volcanics). The Triassic and Early Jurassic rocks are also exposed in the Wassuk Range to the east and include a thick section of andesitic and silicic volcanics, which may be in part equivalent to the McConnell Canyon volcanics, the lower part of which is intruded by the possibly cogenetic Middle Triassic Wassuk diorite and associated quartz monzonite and quartz porphyry. The McConnell Canyon volcanics apparently formed as part of an Early to early Late Triassic continental-margin volcanic arc that extended from the Mojave Desert area to northern California and Nevada. Volcanism waned in Late Triassic time, and the volcanic rocks were covered by interbedded volcaniclastic, clastic sedimentary, and carbonate rocks that include the Malachite Mine Formation and tuff of Western Nevada Mine. Late Triassic carbonate sequences, such as the Mason Valley Limestone, succeed the interbedded rocks, but this appears to have taken place earlier to the north, whereas volcanism persisted for a longer time to the south. Fine-grained siliciclastic sedi ments, with minor carbonate and local volcanic-derived strata, were deposited above the more massive carbonates in a wide area during latest Triassic and Early Jurassic deposition of the Gardnerville Formation and correlative rocks. The Ludwig Mine Formation is part of a sequence of quartz-rich sandstone, evaporates, and carbonates that is widespread in western Nevada and lies on top of and ties together diverse older rock sequences of quite different character. In addition to the arc volcanic, carbonate, and clastic sequence of Yerington and surrounding regions, these older rock sequences include thick, lithologically different, basinal turbidite-mudstone sequences of similar Late Triassic to Early Jurassic age to the north, strata of the shelf terrane to the northeast and east, and probably also rocks of the North American continental platform and parts of the Sierra Nevada. The Artesia Lake and Fulstone volcanics comprise a Middle Jurassic volcanic center related to the Yerington batholith and to nearby igneous centers that is part of a volcanic arc that extended from north of the Yerington district southward through the Mojave Desert and Arizona.

Abstract Porphyry deposits arguably represent the most economically important class of nonferrous metallic mineral resources. These magmatic-hydrothermal deposits are characterized by sulfide and oxide ore minerals in vein-lets and disseminations in large volumes of hydrothermally altered rock (up to 4 km 3 ). Porphyry deposits occur within magmatic belts worldwide and are spatially, temporally, and genetically related to hypabyssal dioritic to granitic intrusions that are porphyritic and that commonly have an aplitic groundmass. The preponderance are Phanerozoic and most typically Cenozoic in age, which reflects the dominance of magmatism related to subduction tectonics and preservation in young rocks. Porphyry deposits are here grouped into five classes based on the economically dominant metal in the deposits: Au, Cu, Mo, W, and Sn. For each porphyry class, the major metal concentration is enriched by a factor of 100 to 1,000 relative to unmineralized rocks of a similar composition. The mass of porphyry deposits ranges over four orders of magnitude, with the mean size of a deposit ordered Cu > Mo ~ Au > Sn > W. Hydrothermal alteration is a guide to ore because it produces a series of mineral assemblages both within the ore zones and extending into a larger volume (>10 km 3 ) of adjacent rock. The typically observed temporal evolution in porphyry ores is from early, high-temperature biotite ± K-feldspar assemblages (potassic alteration) to muscovite ± chlorite assemblages (sericitic alteration) to low-temperature, clay-bearing assemblages (advanced argillic and intermediate argillic alteration), which is consistent with progressively greater acidity and higher fluid-to-rock ratios of fluids, prior to their eventual neutralization. Although advanced argillic alteration is relatively late in the deposits where it is superimposed on ore and potassic alteration, in the deposits where advanced argillic alteration (especially as quartz + alunite) is preserved spatially above ore and commonly extending to the paleosurface, it can form early, broadly contemporaneous with potassic alteration. In contrast, assemblages of Na plagioclase-actinolite (sodic-calcic alteration) and albite-epidote-chlorite-carbonate (propy-litic alteration) form from a fluid with low acidity and commonly lack ore minerals. Geologic, fluid inclusion, and isotopic tracer evidence indicate magmatic fluids dominate acidic alteration associated with ore and non-magmatic fluids dominate sodiccalcic and propylitic alteration. Veins contain a large percentage of ore minerals in porphyry deposits and include high-temperature sugary-textured quartz veinlets associated with ore minerals and biotitefeldspar alteration and moderate-temperature pyritic veins with sericitic envelopes. The compositions of igneous rocks related to porphyry deposits cover virtually the entire range observed forpresentday volcanic rocks. Mineralizing porphyries are intermediate to silicic (>56 wt % SiO 2 ) and their aplitic-textured groundmass represents crystallization as a result of abrupt depressurization of water rich magma; however, small volumes of ultramafic to intermediate rocks, including lamprophyres, exhibit a close spatial and temporal relationship to porphyry ore formation in some deposits. The understanding of porphyry systems depends critically on determination of the relative ages of events and correlation of ages of events in different locations, which in part depends on exposure. Systems with the greatest degree and continuity of exposure generally have been tilted and dismembered by postmineralization deformation. Most porphyry intrusions associated with ore are small-volume (<0.5 km 3 ) dikes and plugs that were emplaced at depths of 1 to 6 km, though some were emplaced deeper. Deposits commonly occur in clusters above one or more cupolas on the roof of an underlying intermediate to silicic intrusion. Altered rocks extend upward toward the paleosurface, downward into the granitoid intrusion from which the porphyry magma and aqueous fluids were generated, and laterally for several kilometers on either side of a deposit. The underlying magma chambers operated as open systems via mafic magma recharge, wall-rock assimilation, crystallization, and intrusion, but mineralizing intrusions did not erupt. Present-day distributions of hydrothermally altered rock and sulfide-oxide ore minerals are time-integrated products of fracture-guided fluid flow. We distinguish three spatial configurations characteristic of all five classes of porphyry deposits, the first of which has two variants: (1a) sericitic alteration largely lies above and beside potassic alteration in a bell- or hood-shaped volume that narrows upward, as at Chorolque, Henderson, and San Manuel-Kalamazoo; (1b) sericitic alteration is present with advanced argillic alteration, and the latter in some cases forms a broader zone at higher levels in the system, as at Batu Hijau, Cerro Rico, and El Salvador; (2) intense sericitic and local advanced argillic alteration cuts through enclosing potassic alteration near ore but also extends above potassic alteration in an upwardly expanding zone with an overall geometry of a funnel, as at Butte, Chuquicamata, and Resolution; (3) sodic-calcic, in addition to potassic, alteration is widespread in the center of the system and has an inverted cup-shaped volume under potassic alteration, with fingerlike projections of sodic alteration extending up through the overlying orebody, as at Yerington. Metal grades are directly related to where ore minerals originally precipitate and the degree of subsequent remobilization. Precipitation of metals is a function of multiple variables, typically including temperature, acidity, and iron and sulfide availability. Hence, the shape of an orebody depends on the number and positions of mineralizing versus barren intrusions; the proportions, shapes, and orientations of veins, lodes, or breccias; and pressure-temperature changes and wall-rock reactions that govern ore mineral stability. Geochronology and thermal models suggest that durations of hydrothermal activity of 50,000 to 500,000 yr are common, but several large porphyry Cu deposits include multiple events spanning several million years. Crosscutting relationships, including offset veins, provide definitive evidence for the relative ages of hydrothermal events at a particular spatial location. Intrusive contacts that cut off older veins and are in turn cut by younger veins provide time lines that permit correlation of spatially separated events. Most porphyry deposits exhibit multiple intrusions, each associated with a series of hydrothermal veins formed over a declining temperature interval. The high-temperature starting point of hydrothermal fluid compositions varies systematically between porphyry classes and must reflect magma composition and chemical partitioning between melt, mineral, and aqueous fluid. Although the data are sparse, the magmas and associated high-temperature ore fluids vary such that oxidation state, sulfidation state, and total sulfur content are highest for porphyry Cu and Au classes, slightly lower for Mo, lower yet for Sn, and lowest for W. Nearly all classes and subclasses, however, have examples that diverge to low a K+ / a H+ and high sulfur fugacity at lower temperature to produce advanced argillic alteration and high-sulfidation state ore minerals. Just as with the spectrum of global magmatism, the breadth of porphyry mineralization shares fundamental processes yet maintains distinctive geologic characteristics. In spite of a century of study and economic impact, many questions remain unanswered.

Abstract Geologic mapping provides many types of information essential both in exploration for new mineral deposits and during subsequent mining. Geologic mapping of outcrops is used to describe the primary lithology and morphology of rock bodies as well as age relationships between rock units. This information allows delineation of ore-bearing host rocks and postore rocks that obscure or truncate ores. Mapping gathers structural information, including attitudes of veins and postore faults that can be used to predict the geology in the subsurface or laterally under postore rocks, and improves the utility of geophysical data for refinement of subsurface targets. Mapping of the mineralogy of hydrothermal alteration zones, ore minerals, igneous rocks hosting ores, and oxidized and leached rocks that commonly occur at the surface above sulfide-bearing ores can be used in conjunction with geochemical data to produce zonation patterns to target potential ore or to define prospective corridors of exotic mineralization. Similarly, regional geologic mapping in regions with both Paleozoic-Mesozoic overthrusts and Cenozoic normal faults such as the Paleozoic and Mesozoic thrust belt of the United States Cordillera and Basin and Range Province can define prospective windows into basement where mineralization such as Carlin-type gold deposits may occur. In general, geologic mapping underpins the construction of three-dimensional geologic models or hypotheses that guide exploration and discovery and, when geologic time is considered, produces the fourdimensional space-time models necessary for understanding of primary ore formation processes and postdepositional modification by secondary surficial and tectonic processes. Geologic mapping has been used extensively for exploration for more than 100 years and we predict it will continue to be essential although the tools for recording, compiling, and synthesizing data are evolving rapidly and improve data integration in the office and most recently in the field. Both traditional and future methods rely on field identification skills of the geologist to record salient new geologic data. This review describes the traditional paper- and pencil-based mapping system developed and used extensively by the Anaconda Company from 1900 to 1985 and, because of its versatility, adapted by many other geologists in industry and academia. This and similar systems allow geologically complex and diverse data to be recorded and plotted on a base map, including lithology, rock alteration and mineralization features, relative age relationships, and structural features such as faults and veins. Traditional paper-recorded geologic mapping data are now commonly converted to digital format in the office. We document use of mapping at different stages of the mine-life cycle from general regional-scale geologic mapping to regional- to district-scale exploration targeting, to deposit assessment and ore-reserve definition, through mine planning and production. Examples of mapping described herein include the Ann Mason porphyry copper deposit, Yerington district, Nevada; the Bajo de la Alumbrera mine; Argentina; the El Abra-Fortuna-Chuquicamata districts of Chile; and the Pioneer Mountains of Montana. Beyond the use of traditional paper-based mapping methods, recent technological advances include global positioning systems, pen tablet computers, palm computers, and laser ranging devices that all support direct (paperless) field-based digital geologic mapping. Improvements in computation speed, memory, data storage, battery life, durability, screen visibility, and portability have made digital mapping practical in general field mapping, mine sites, and advanced projects. Portable digital-electronic instrumentation allows the field geologist rapid access to digital data bases that include geologic maps and photographic and remotesensing imagery with automatic registration and scale independence. Another example described here, using digital mapping systems in the heavily forested portions of the Pioneer Mountains of Montana, shows how on-line GPS communicating directly to the pc tablet and digital orthophotographs made mapping sufficiently effective so as to discover a previously unknown granitic pluton with a concentric breccia zone. These new digital mapping tools may thus improve the efficiency of mapping and support a scientist in the field with unprecedented opportunities to map where field work has been difficult before. Visualization of geophysical or geochemical data together with geology and synoptic aerial imagery at any scale while mapping provides an integrated data base that facilitates identification of crucial geologic relationships. Digital techniques improve the potential for making conceptual leaps by exploring the available integrated data sets as a field geologist maps, and may in the future lead to more comprehensive three-dimensional geologic models for mineral deposits by effectively using information technology. The authors conclude that both paper and digital systems are powerful and each has certain advantages. However, the central challenge remains the training and nurturing of highly skilled field geologists motivated to practice their profession, welcoming both the rigors of intensive field work and the excitement of scientific discovery. It is surmised here that digital mapping technology may help attract an increasingly computer-literate cadre of new practitioners of mapping into mineral resource exploration.

Abstract Intrusion-related hydrothermal systems represent a large variety of geologic environments that in some cases form large metallic mineral deposits. The deposits examined in this trip represent the spectrum from systems dominated by magmatic fluid (Birch Creek, California and Yerington, Nevada) to those systems in which intrusions serve as heat engines to drive convectively circulating brines derived from sedimentary rocks (Hum-boldt, Nevada). In these examples, nonmagmatic fluids are largely excluded from more deeply emplaced intrusions in a compressive environment, and the hydrothermal composition and ores (e.g., granite W-F, Cu porphyry and skarn) are dictated by the composition of the magma and its mechanism of crystallization and aqueous fluid generation. Magmatic fluids are less important in the shallow crustal ore environment, but apparently contribute to acidic alteration zones located vertically above source intrusions. Using Humboldt as an example, we propose that the Fe oxide Cu-Au ores in the shallow environment require an abundant source of sedimentary brines (typical of evaporitic environments), high fracture permeability (promoted by an exten-sional setting) to allow aqueous fluid flow and dike emplacement, and shallowly emplaced intrusions to serve as heat sources.

Abstract A large lithophile element-bearing hydrothermal system is associated with the well-exposed Birch Creek bi-otite-muscovite granite and its metamorphic aureole in the White Mountains of eastern California. Elements enriched include F, Be, W, Zn, Pb, Ag, Cu, Au, Bi, and Sn although historic production (of Pb, Ag, Au, W) has been minor and likely resources are small. This system is one of several dozen Late Cretaceous intrusion-centered hydrothermal systems that are associated with two-mica granites along the Cordilleran miogeocline in the Great Basin. As a group, these occurrences resemble other Mesozoic W-Sn-F-Be bearing systems in circum-Pacific; however, these seemingly lack the economic deposits—an observation that begs the question: Why not? These granites are also of considerable interest because of their structural characteristics, notably intensely deformed and attenuated margins. They have figured prominently in literature on emplacement mechanisms for granites. This paper provides an overview of collaborative studies by several groups looking at the magmatic, structural, metamorphic, and hydrothermal development of the Birch Creek system. Field, petrological, and geochemical studies demonstrate that the Birch Creek pluton (82 Ma, U-Pb, Ar-Ar) was episodically emplaced with alternating major pulses from at least two, probably three distinct magma sources (two crustal, one subcrustal). Compositions are peraluminous and range from biotite granodiorite to muscovite-biotite granite with episodic aplite formation. Hydrothermal features developed concurrently with each of these events. Field relations reveal a detailed history of fluid release from the evolving magma chamber. These fluids created high-temperature K feldspar-bearing quartz veins that are early and proximal to the magma chamber at any given time. With time, assemblages become K feldspar-destructive (albitization: albite-muscovite-fluorite ± quartz), and finally, only muscovite-stable (greisenization: muscovite-pyrite-fluorite ± quartz). This pattern is consistent with simple models of fluid evolution from the magma. Early veins, like the aplite dike swarms, have concentric and radial orientations consistent with formation in a localized, magma-chamber focused stress regime. Fractures hosting the later associations are consistently northeast oriented and controlled by far-field stress. Features developed in Upper Proterozoic-Lower Cambrian carbonate and clastic host rocks can be linked to the intrusive history via map patterns and crosscutting relationships. Early grossular-rich garnet- and diop-sidic pyroxene-bearing skarnoids (in mixed siltstone-limestone units) and marbles (in massive limestone and dolomite) form and are then deformed (flattened) during early stages of pluton emplacement. Stable isotope data demonstrate at least some of this local metasomatic exchange was accompanied by magmatic fluid influx, whereas the marbles were largely impermeable and escaped metasomatism. “Anhydrous” calcic skarns consist of more iron-rich garnet plus salitic pyroxenes, idocrase, sodic plagioclase, quartz, and fluorite. In dolomite, equivalent vein skarns consist of humite-group minerals plus calcite and variable quantities of diopside, chlorite, spinel and grossular. Hydrous skarn assemblages formed next: an older group is characterized by combinations of clinozoisite-epidote, albitic plagioclase, fluorite, chlorite, and Mg-rich biotite; a younger group is characterized by muscovite, fluorite, pyrite, and fluorphlogopite. Scheelite, beryl, sphalerite, and other sulfides accompany these hydrous skarns. Structurally controlled, distal quartz-carbonate-sulfide veins and replacement bodies extend over 5 km from the intrusion. The skarns, replacement bodies and veins all formed from magmatic fluids as inferred from isotopic and fluid inclusion data. An integrated time-space view of the hydrothermal, structural, and magmatic development is obtained from the field relationships and other physical and chemical constraints. Deformation in the form of locally intense subsolidus foliations, folded dikes and veins, and many local shears can be linked unequivocally to particular magmatic events during emplacement. These relationships demonstrate that much, perhaps all, of the intense deformation found around the western margin of the intrusion is syn-intrusive and driven by magma emplacement. Furthermore, these patterns help establish clear links between hydrothermal events in the intrusion and in the host. Skarns, both anhydrous and hydrous, formed in response to fluid release from each of the principal intrusive pulses. Overprinting events are found locally, yet their scarcity is consistent with a slowly evolving temperature field around the intrusion and predominant fluid flow upwards rather than outward. Similarities and differences with other magmatic-hydrothermal systems, particularly those related to granitic composition rocks, are interesting to consider for this system. Level of exposure, early and continuous fluid production, and the lack of large-scale internal communication in the evolving magma chamber(s) all may have contributed to the lack of economically significant mineralization.

This Paper provides the log for the first full day of the field trip. Day 1 will be spent in the Birch Creek area, east of Bishop in the southeastern White Mountains. There are several goals to the Birch Creek day: 1. To see one of the best-exposed examples of a Cretaceous two-mica granite pluton in the Great Basin with its distinctive style of emplacement and hydrothermal alteration; 2. To examine the relationships among magmatism, structural development, and emplacement; 3. To examine the links between magmatism and hydro-thermal activity; 4. To ask the questions: Why are these Late Cretaceous systems in the Great Basin not well mineralized? What differs between these systems and better-mineralized Late Mesozoic examples elsewhere in the circum-Pacific? 5. To consider how the characteristics of and processes in these granite-related systems compare with other intrusion-related hydrothermal systems, specifically in the Yerington district and in the Humboldt mafic complex. See the accompanying paper ( Barton, 2000 ) for an introduction to the Birch Creek system as a whole and its broader context. Some local terminology used in this field guide is defined there. It is important to get an early start as this is the most challenging day of the field trip because of logistics and length and because it is second only to the skarn traverse at Yering-ton in the physical exertion required. If the upper Mollie Gibson road is impassable due to snow, ice, or obstacles, hiking in and/or alternative stops can be considered.

ABSTRACT The geometry of the Middle Jurassic Yerington Batholith has been reconstructed by removing the effects of Ceno-zoic normal faulting, which has exposed a cross section of the batholith from less than 1 to more than 6 kilometers paleodepth. The batholith is a composite pluton approximately 15 kilometers in diameter and extends at least 6 and possibly 8 to 9 kilometers in vertical dimension. Total volume of the batholith exceeds 1,000 cubic kilometers. It was emplaced into a Triassic-Jurassic volcanic and sedimentary rock sequence by bulk assimilation and ductile deformation of wallrocks. The roof is at approximately 1 kilometer depth and is formed by cogenetic volcanic sequences. The upper mineralized portion of the batholith and its roof are preserved because the batholith has dropped down more than 2.5 kilometers along steeply dipping faults. Porphyry copper and copper skarn mineralization are spatially and temporally associated with emplacement of granite porphyry dikes that are cogenetic with and grade downward into the Luhr Hill Granite. This youngest phase of the batholith is estimated to be about 65 cubic kilometers in volume and was emplaced into the center of the batholith, largely at depths of 5 to 9 (?) kilometers. The Luhr Hill Granite has low copper content (10 ppm) and copper-zinc ratio (0.25) relative to the early and voluminous McLeod Hill Quartz Monzodiorite phase of the batholith (60 ppm copper and copper-zinc ratio of 1). Zinc decreases with differentiation and increasing silica content in the batholith and thus behaves compatibly, whereas copper content does not vary significantly with differentiation except for its sharp decrease in the Luhr Hill Granite. Whole rock chemical variations are consistent with low contents of copper (less than 150 ppm) and significant contents of zinc (about 350-800 ppm) in biotite, one of the early crystallizing and fractionating phases. Application of the theoretical model of Cline and Bodnar (1991) for crystallization of granite at 2 kilobars pressure indicates that hypersaline magmatic ore fluids would have separated late during crystallization and extracted most copper but less than 25 percent of zinc from the magma; zinc would have been sequestered in earlier-crystallized biotite. The fluids from the Luhr Hill Granite apparently migrated from 5 to 9 kilometers depth upward into granite cupolas at 4 to 5 kilometers depth, where they caused hydrofracturing leading to emplacement of granite porphyry dikes along which fluids continued to move upward and outward from the cupolas. The dominance of copper sulfide and lack of zinc sulfide in the Yerington District is consistent with mineralization caused by magmatic ore fluids rich in copper and sulfur but poor in zinc. Metal zoning from inner porphyry copper with or without molybdenum to intermediate skarn copper to outer replacement/skarn copper-iron and vein copper-gold is generally consistent with declining temperature of magmatic hydrothermal fluids, but magnetite-rich iron-replacement ores poor in sulfide may be derived in part from non-magmatic fluids that stripped iron during sodic-calcic alteration of the batholith. Exploration criteria for porphyry copper deposits following the Yerington model should focus on shallowly’ emplaced batholiths with a late and relatively deep granite phase depleted in copper and having a low copper-zinc ratio.

Abstract The Yerington district, Nevada, hosts at least four porphyry copper deposits and several small Fe oxide-copper-gold lodes within a middle Jurassic batholith and its volcanic cover. The contact aureole of the batholith contains early garnet-pyroxene hornfels and endoskarn, later copper-bearing andradite skarn deposits, and latest-stage large Fe oxide-copper-gold replacement deposits. The Jurassic host rocks have been faulted and tilted 60° to 90° W by Cenozoic normal faulting ( Proffett, 1977 ) so that the modern exposures represent cross sections of a complex paleohydrothermal system from the volcanic environment to about 7 km depth. This paper summarizes field, petrologic, and geochemical data that support the origin of hydrothermal wall-rock alteration and ore deposition due to two different types of fluids. Magmatic brines were derived from the crystallization of the youngest equigranular intrusion of the Yerington batholith, the Luhr Hill granite. Brines separated from the granite and were emplaced upward together with granite porphyry dikes to produce copper-iron sulfdes and associated K silicate alteration in the porphyry copper deposits and copper skarns. In the upper part of the hydrothermal system, magmatic fluids are an important source of acids and sulfur that produced sericitic and advanced argillic alteration. A second type of ore fluid is brine derived from formation waters trapped in the Triassic-Jurassic sedimentary section intruded by the batholith. These fluids were heated by the batholith and circulated through its crystalline parts. Hornfels and endoskarn were produced along the contact of an early intrusion. Following intrusion of the porphyry dikes, sedimentary brines circulated up to 3 km into the batholith and upon heating produced sodic-calcic alteration there. Ascent of these brines, particularly after the waning of magmatic fluid input, may have caused shallow-level chlorite-dominated alteration in igneous host rocks and Fe oxide-Cu-Au lodes and replacement deposits in the batholith and its contact aureole, respectively.

THE PURPOSE of this one-day tour is to examine time-space relationships of hydrothermal alteration features and associated porphyry Cu(Mo) and Cu-Fe-Au mineralization, and the relationship of these hydrothermal features to the magmatic history of the Yerington batholith. The batholith exposures we will examine represent an intermediate between the Birch Creek and Buena Vista end members. At Yerington, mag-matic brines were essential to formation of porphyry Cu(Mo) deposits, but at the same time a huge hydrothermal system driven by the batholithic heat was dominated by sedimentary brines and produced Fe oxide-Cu-Au ores distal to the porphyry centers. It will be advantageous to read the papers in this guidebook by Dilles and Proffett (1995) and Dilles et al. (2000) that provide summaries of the magmatic and hy-drothermal histories, respectively, of the Yerington batholith. The descriptions and maps below are lengthy because of the complexity of the exposures and large size of the intrusion-related hydrothermal system. This tour can be accomplished in a leisurely fashion in two days, but can be done in one. There are two short hikes and nine roadside stops.

Abstract The Buckskin Range lies approximately 4 km west of the Yerington porphyry copper district and hosts the Artesia Lake and Fulstone Spring volcanic sequences that structurally overlie the Yerington batholith. Hy-drothermal alteration minerals characteristic of advanced argillic, sericitic, and marginal porphyry copper-type alteration assemblages have been detected via infrared spectrometry, X-ray diffraction, petrography, micro-probe analysis, and hand-lens based-field mapping in the central Buckskin Range. It is postulated that high-level alteration in the Artesia Lake Volcanics may be contemporaneous with the main event of sericitic alteration and pyrite deposition in the deeper porphyry copper environment. The presence of sericitic alteration underlying or overprinting hypogene advanced argillic assemblages may imply that fluids responsible for porphyry copper mineralization have ascended to epithermal depths. The spatial relationships of hydrothermal alteration in the Buckskin Range suggest an evolution of low-pH, sulfide-bearing fluids to nearly neutral, oxide-rich hydrothermal fluids. Sulfide-rich, feldspar-destructive advanced argillic and sericitic alteration is crosscut and overlain by feldspar-stable, oxide-rich sericite-hematite-chlorite alteration. Sericite-hematite-chlorite alteration is abruptly overlain by potassium-added, feldspar-stable calcite-chlorite-hematite alteration, produced by late sodic-calcic or potassium-enriched fluids possibly derived from sedimentary or evaporitic brines.

THE CONTACT between the Yerington batholith and metased-imentary and metavolcanic rocks east of Ludwig, Nevada, is exposed over 3.5 km of paleodepth (Fig. 1 ) due to 90° of westward rotation during Basin-and-Range faulting ( Proffett, 1977 ; Geissman et al., 1982 ). Here we have the opportunity to study the effects of depth, distance, and time in the generation of metamorphic rocks and skarns. As we walk over the terrain, keep reminding yourself that “original up” is to the west. The main emphasis of the trip will be to examine the structural and lithologic controls on the formation of calc-sil-icate hornfels, skarn, and related ores, the mineralogy of these rocks, and their temporal relation to quartz monzodior-ite and granite porphyry intrusions of the Yerington batholith. This examination will yield a conceptual framework for understanding skarn-forming processes and will generate ideas useful in mineral exploration. Some background material on terminology and phase equilibria is given in the first section of the chapter, preceding the description of individual stops. The actual trip log gives descriptions of outcrops for each stop, followed by background material, interpretation, and application where appropriate. An overall summary and conclusions is beyond the scope of this chapter. Many of the conclusions presented below rely on the parallel studies of igneous and hyrothermal events associated with emplacement of the Yerington batholith ( Proffett, 1977 ; Proffett and Dilles, 1984 ; Dilles, 1987 ; Dilles and Einaudi, 1992 ; Dilles et al., 1992 ; Dilles and Proffett, 1995 ), summaries in this Fieldtrip Guidebook ( Dilles et al., 2000 , etc.), as well as studies by others on skarn deposits around the world (cited individually in text).

Abstract The Humboldt mafic complex, west-central Nevada, is a large composite Middle Jurassic basaltic-composition volcano-plutonic center that has exceptionally extensive (>900 km 3 ), intense (nearly complete leaching of many elements) sodium-rich hydrothermal alteration. Mapping of exposures at multiple structural levels allows assessment of the time-space development of hydrothermal alteration and cogenetic magnetite and hematite ± copper sulfide mineralization. Alteration varies from early, deep and proximal marialitic scapolite-hornblende to shallow and distal albite-actinolite-chlorite and chlorite-carbonate assemblages. These associations reflect large compositional changes in host rocks (mass-transfer), whereas distal and deep propylitc assemblages are less intensely modified. Substantial quantities of iron are present in massive, breccia-form, and stratabound magnetite and hematite bodies at intermediate and shallow depths. Lesser amounts of copper, cobalt, and other metals are sporadically enriched at shallow levels. Field, petrological, and geochemical constraints require that the fluids were dominantly or entirely non-magmatic, external brines that circulated in response to the heat and permeability increases associated with repeated basaltic intrusion. The Humboldt system represents a mafic end-member among iron oxide-rich copper-bearing hydrothermal systems ( Barton and Johnson, 1996 ) and, in the larger context, and an end-member in the spectrum of igneous-related hydrothermal systems.

The purpose of this portion of the field trip is to examine the spatial and temporal distribution of sodium-rich hy-drothermal alteration and iron oxide mineralization generated by a saline hydrothermal system driven by the Middle Jurassic Humboldt mafic complex. The Humboldt system is of particular interest because it represents a basaltic mag-matic end member of intrusion-driven hydrothermal systems and, in this case, one where the fluids are largely, perhaps entirely, externally derived brines. Outcrops of Jurassic rocks within the complex record multiple, mutually crosscutting magmatic and hydrothermal events at different structural levels as exposed by mid-Tertiary extension. Mapping of selected areas across this large igneous complex allows definition of the relationships among the magmatic, structural, and hy-drothermal features. In turn, these enable an interpretation of the overall temporal and spatial evolution of a large intrusion-driven hydrothermal system ( Johnson and Barton, 2000 ). From geology and geochemistry, one can estimate that upwards of 15 billion tonnes of iron and 35 million tonnes each of copper and zinc were moved by the hydrothermal system ( Johnson, 2000 ). What happened to these metals and what are the implications for other systems? These are among the issues to consider.