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 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.

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 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 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.