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Hydrothermal alteration data mapped using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) were compiled into hydrothermal alteration polygons for use in an assessment of porphyry copper mineral resource potential in the southwestern United States. Hydrothermal alteration polygons along with geochemistry, gravity and magnetic, lithologic, and deposit and prospects data were compiled in a GIS to produce a quantitative set of physical properties for each polygon that were effectively used in making estimates of undiscovered deposits for each permissive tract. Results show a higher estimate of potential undiscovered deposits (17 vs 14) for permissive tracts when ASTER alteration data were used in the assessment.

A probabilistic mineral resource assessment of metal resources in undiscovered porphyry copper deposits of the Ural Mountains in Russia and Kazakhstan was done using a quantitative form of mineral resource assessment. Permissive tracts were delineated on the basis of mapped and inferred subsurface distributions of igneous rocks assigned to tectonic zones that include magmatic arcs where the occurrence of porphyry copper deposits within 1 km of the Earth's surface are possible. These permissive tracts outline four north-south trending volcano-plutonic belts in major structural zones of the Urals. From west to east, these include permissive lithologies for porphyry copper deposits associated with Paleozoic subduction-related island-arc complexes preserved in the Tagil and Magnitogorsk arcs, Paleozoic island-arc fragments and associated tonalite-granodiorite intrusions in the East Uralian zone, and Carboniferous continental-margin arcs developed on the Kazakh craton in the Transuralian zone. The tracts range from about 50,000 to 130,000 km 2 in area. The Urals host 8 known porphyry copper deposits with total identified resources of about 6.4 million metric tons of copper, at least 20 additional porphyry copper prospect areas, and numerous copper-bearing skarns and copper occurrences. Probabilistic estimates predict a mean of 22 undiscovered porphyry copper deposits within the four permissive tracts delineated in the Urals. Combining estimates with established grade and tonnage models predicts a mean of 82 million metric tons of undiscovered copper. Application of an economic filter suggests that about half of that amount could be economically recoverable based on assumed depth distributions, availability of infrastructure, recovery rates, current metals prices, and investment environment.

The regional distribution of arsenic and 20 other elements in stream-sediment samples in northern Nevada and southeastern Oregon was studied in order to gain new insights about the geologic framework and patterns of hydrothermal mineralization in the area. Data were used from 10,261 samples that were originally collected during the National Uranium Resource Evaluation (NURE) Hydrogeochemical and Stream Sediment Reconnaissance (HSSR) program in the 1970s. The data are available as U.S. Geological Survey Open-File Report 02-0227. The data were analyzed using traditional dot maps and interpolation between data points to construct highresolution raster images, which were correlated with geographic and geologic information using a geographic information system (GIS). Wavelength filters were also used to deconvolute the geochemical images into various textural components, in order to study features with dimensions of a few kilometers to dimensions of hundreds of kilometers. The distribution of arsenic, antimony, gold, and silver is different from distributions of the other elements in that they show a distinctive high background in the southeast part of the area, generally in areas underlain by the pre-Mesozoic craton. Arsenic is an extremely mobile element and can be used to delineate structures that served as conduits for the circulation of metal-bearing fluids. It was used to delineate large crustal structures and is particularly good for delineation of the Battle Mountain-Eureka mineral trend and the Steens lineament, which corresponds to a post-Miocene fault zone. Arsenic distribution patterns also delineated the Black Rock structural boundary, northwest of which the basement apparently consists entirely of Miocene and younger crust. Arsenic is also useful to locate district-sized hydrothermal systems and clusters of systems. Most important types of hydrothermal mineral deposit in the northern Great Basin appear to be strongly associated with arsenic; this is less so for low-sulfidation epithermal deposits. In addition to individual elements, the distribution of factor scores that resulted from principal component studies of the data was used. The strongest factor is characterized by Fe, Ti, V, Cu, Ni, and Zn and is used to map the distribution of distinctive basalts that are high in Cu, Ni, and Zn and that appear to be related to the Steens Basalt. The other important factor is related to hydrothermal precious metal mineralization and is characterized by Sb, Ag, As, Pb, Au, and Zn. The map of the distribution of this factor is similar in appearance to the one for arsenic, and we used wavelength filters to remove regional variations in the background for this factor score. The resulting residual map shows a very strong association with the most significant precious metal deposits and districts in the region. This residual map also shows a number of areas that are not associated with known mineral deposits, illustrating the utility of the method as a regional exploration tool. A number of these prospective areas are distant from known significant mineral deposits. The deconvolution of the spatial wavelength structure of geochemical maps, combined with the use of large regional geochemical data sets and GIS, permits new possibilities for the use of stream-sediment geochemistry in the study of large-scale crustal features as well as the isolation of mineral-district scale anomalies.

Alkaline-related epithermal vein, breccia, disseminated, skarn, and porphyry gold deposits form a belt in the southern Rocky Mountains along the eastern edge of the North American Cordillera. Alkaline igneous rocks and associated hydrothermal deposits formed at two times. The first was during the Laramide orogeny (about 70–40 Ma), with deposits restricted spatially to the Colorado mineral belt (CMB). Other alkaline igneous rocks and associated gold deposits formed later, during the transition from a compressional to an extensional regime (about 35–27 Ma). These younger rocks and associated deposits are more widespread, following the Rocky Mountain front southward, from Cripple Creek in Colorado through New Mexico. All of these deposits are on the eastern margin of the Cordillera, with voluminous calc-alkaline rocks to the west. The largest deposits in the belt include Cripple Creek and those in the CMB. The most important factor in the formation of all of the gold deposits was the near-surface emplacement of relatively oxidized volatile-rich alkaline magmas. Strontium and lead isotope compositions suggest that the source of the magmas was subduction-modified subcontinental lithosphere. However, Cripple Creek alkaline rocks and older Laramide alkaline rocks in the CMB that were emplaced through hydrously altered LREE-enriched rocks of the Colorado (Yavapai) province have 208Pb/204Pb ratios that suggest these magmas assimilated and mixed with significant amounts of lower crust. The anomalously hot, thick, and light crust beneath Colorado may have been a catalyst for large-scale transfer of volatiles and crustal melting. Increased dissolved H2O (and CO2, F, Cl) of these magmas may have resulted in more productive gold deposits due to more efficient magmatic-hydrothermal systems. High volatile contents may also have promoted Te and V enrichment, explaining the presence of fluorite, roscoelite (vanadium-rich mica) and tellurides in the CMB deposits and Cripple Creek as opposed to deposits to the south. Deep-seated structures of regional extent that formed during the Proterozoic allowed the magmas to rise to shallow crustal levels. Proterozoic sites of intrusions at 1.65, 1.4, and 1.1 Ga were also important precursors to alkaline-related gold deposits. Many of the larger gold deposits are located at sites of Proterozoic intrusions, and are localized at the intersection of northeast-trending ductile shear zones formed during Mesoproterozoic deformation, and an important north-trending fault formed during 1.1 Ga rifting.

Approximately 99 percent of past production and remaining identified resources of gold, silver, copper, lead, and zinc in the United States are accounted for by deposits that originally contained at least 2 metric tonnes (t) gold, 85 t silver, 50,000 t copper, 30,000 t lead, or 50,000 t zinc. The U.S. Geological Survey, beginning with the 1996 National Mineral Resource Assessment, is systematically compiling data on these deposits, collectively known as "significant" deposits. As of December 31, 1996, the significant deposits database contained 1,118 entries corresponding to individual deposits or mining districts. Maintaining, updating, and analyzing a database of this size is much easier than managing the more than 100,000 records in the Mineral Resource Data System and Minerals Availability System/Minerals Industry Location System, yet the significant deposits database accounts for almost all past production and remaining identified resources of these metals in the United States. About 33 percent of gold, 22 percent of silver, 42 percent of copper, 39 percent of lead, and 46 percent of zinc are contained in or were produced from deposits discovered after World War II. Even within a database of significant deposits, a disproportionate share of past production and remaining resources is accounted for by a very small number of deposits. The largest 10 producers for each metal account for one third of the gold, 60 percent of the silver, 68 percent of the copper, 85 percent of the lead, and 75 percent of the zinc produced in the United States. The 10 largest deposits in terms of identified remaining resources of each of the five metals contain 43 percent of the gold, 56 percent of the silver, 48 percent of the copper, 94 percent of the lead, and 72 percent of the zinc. Identified resources in significant deposits for each metal are less than the mean estimates of resources in undiscovered deposits from the 1996 U.S. National Mineral Resource Assessment. Identified resources are roughly the same magnitude as cumulative past production. Assuming that roughly the same proportion of resources in undiscovered deposits will occur in significant deposits, a substantial number of significant deposits remain to be discovered.

4 Geologic Features 4 Introduction 4 Regional Environment 7 Physical Description of Deposits 8 Geophysical Characteristics 8 Ore Characteristics 9 Geochemical Characteristics 9 Hydrothermal Alteration 10 Weathering 10 Petrology of Associated Igneous Rocks 10 Theory of Deposit Formation 11 Geoenvironmental Features 11 Environmental Features Before Mining 12 Potential Environmental Considerations Related to Mining or Mineral Processing 12 Potential Health Considerations 13 References Cited 13

This report, identified as “Geologic database for digital geology of California, Nevada, and Utah — an application of the North American Data Model" has been approved for release and publication by the Director of the USGS. Although this database has been subjected to rigorous review and is substantially complete, the USGS reserves the right to revise the data pursuant to further analysis and review. Furthermore, it is released on condition that neither the USGS nor the United States Government may be held liable for any damages resulting from its authorized or unauthorized use.

In southern Rocky Mountains, catchments characterized by acidic, metalliferous waters that are relatively unaffected by human activity usually occur within areas that have active or historical mining activity. The US Geological Survey has utilized these mineralized but unmined catchments to constrain geochemical processes that control the surfaceand ground-water chemistry associated with near surface acid weathering as well as to estimate premining conditions. Study areas include the upper Animas River watershed, Lake City, Mt. Emmons, and Montezuma in Colorado and Questa in New Mexico. Although host-rock lithologies range from Precambrian gneisses to Cretaceous sedimentary units to Tertiary volcanic complexes, mineralization is Tertiary in age and associated with intermediate to felsic composition, porphyritic plutons. Pyrite is ubiquitous. Variability of metal concentrations in water is caused by two main factors: mineralogy and hydrology. Parameters that potentially affect water chemistry include: host-rock lithology, intensity of hydrothermal alteration, sulfide mineralogy and chemistry, gangue mineralogy, length of flow path, precipitation, evaporation, and redox conditions. Springs and headwater streams have pH values as low as 2.5, sulfate up to 3700 mg/L and high dissolved metal concentrations (for example: A1 up to 170 rag/L; Fe up to 250 mg/L; Cu up to 3.5 mg/L and Zn up to 14 mg/L). With the exception of evaporative waters, the lowest pH values and highest Fe and A1 concentrations occur in water draining the most intense hydrothermally altered areas consisting of the mineral assemblage quartz-sericite-pyrite. Stream beds tend to be coated with iron floc, and some reaches are underlain by ferricrete. When iron-rich ground water interacts with oxygenated waters in the stream or hyporheic zone, ferrous iron is oxidized to ferric iron, which is less soluble, leading to the precipitation of iron oxyhydroxides. Ground-water wells have been drilled and sampled in two unmined, alpine catchments to characterize constituent concentrations, to identify hydrogeochemical processes controlling constituent concentrations, to determine rock hydraulic properties, and to delineate flow paths. By using an integrated approach to investigating surface and ground waters in these acidic catchments a more complete understanding of the hydrogeologic framework is gained.

Gold deposits in Costa Rica are found in lavas and pyroclastic rocks in volcanic arcs ranging in age from Miocene to the present. Although the arcs and some individual volcanos vary widely in alkalinity and other compositional parameters, there seems to be no close relationship between composition and associated type of epithermal precious-metal deposits.

Stockwork molybdenite deposits in the western conterminous United States may be divided into two groups based on the chemistry of the source pluton: granodiorite systems and granite systems. These two groups show significant differences in age, tectonic setting at time of formation, and orebody geometry.Granite molybdenite systems develop in, and adjacent to, epizonal plutons that meet Tuttle and Bowen's (1958) definition of granite. Cenozoic source plutons for these systems were emplaced in the eastern Rockies and Great Basin coincident with, or just prior to, regional extensional tectonism. Mineralization and alteration patterns in granite molybdenite systems generally conform to those described at the Climax and Henderson deposits.Major element data displayed on Ab-Or-Q and A-K-F plots 1 can characterize potassic, quartz-sericite, argillic, and silicic alteration assemblages and lead to evaluation of the probable depth to mineralization in prospects. Observed alteration reflects hydrogen and alkali metasomatism in fluorine-rich systems. Minor elements concentrated in granite molybdenite systems include Be, Cs, F, Li, Mo, Nb, Rb, Sn, Ta, Th, U, and W.Geophysical and geological evidence indicates that the cylindrical source plutons associated with known granite molybdenite systems are cupolas on the tops of batholiths, suggesting a genetic link between batholiths and ore. A granite molybdenite system appears to be the result of the following: lithophile element concentration by convection-aided thermogravitational diffusion in the upper part of a batholithic magma chamber; transfer of the resulting high silica, volatile-rich magma from the roof zone of the batholith to a high-level cylindrical cupola; establishment of thermogravitational diffusion in the cupola causing further lithophile element concentration; formation of potassic alteration and an ore shell as a result of volatile saturation, resurgent boiling, hydrofracturing, and boiling; and the evolution of peripheral alteration assemblages as meteoric waters enter the magmatic fluid plume above the cupola. Stacked orebodies represent discrete episodes of resurgent boiling, hydrofracturing, and mineralization.Computer-assisted evaluation of major element analyses of igneous rocks, using chemical criteria developed from known granite molybdenite systems, may locate areas with a high exploration potential. These techniques suggest that several late Precambrian bimodal basaltrhyolite terrains in the eastern United States and Canada have exploration potential for molybdenum.

Anomalously high fluorine values have been found in biotite, sericite, and spessartine garnet associated with the various intrusive and alteration sequences at the Henderson molybdenite deposit in Clear Creek County, Colorado. Twelve-element microprobe analyses of these minerals throughout this Tertiary rhyolite complex indicate that the highest fluorine values are found in vein-related, magnesium-rich biotite (up to 7.5 wt % F) and that the lowest fluorine values are found in sericite from the outer zone of pervasive sericitic alteration ( 4.5 at a distance of 500 m from the 0.1 percent MoS 2 zone. These results provide evidence that fluorine was an essential component of the hydrothermal fluid at Henderson, that significant fluorine gradients are found within the sericitic alteration zone, and that the highest activity of fluorine was spatially associated with molybdenite mineralization.

The 1000-Ma-old composite Pikes Peak batholith consists largely of medium- to coarse-grained biotite granite but is intruded by several smaller granite plutons. Two of these, the Tarryall Mountains batholith and the Redskin Granite (Hawley, 1969; Hawley and Wobus, 1977), are here interpreted as cupolas atop the main mass of the Pikes Peak batholith. Chemical analysis for 38 elements in rocks from the Redskin Granite, the Tarryall Mountains batholith, and the biotite granite of the Pikes Peak batholith support the contention that the compositions of the smaller plutons are unlikely to have evolved from that of the main mass of the batholith by crystal fractionation. It is believed that these compositions arose through liquid-state fractionation processes, possibly the convection-aided thermogravitational model of Shaw et al. (1976). It is important to recognize that the chemical signature of this fractionation process can be discerned in holocrystalline rocks as well as in their glassy equivalents because the process may be instrumental in the formation of some kinds of lithophile ore deposits.

In 1984 and 1985, at the request of the Bureau of Land Management, the U.S. Geological Survey and the U.S. Bureau of Mines conducted surveys to assess the mineral resources (known) and mineral resource potential (undiscovered) of 2,ZOO acres of the 5,838acre Pinnacles Wilderness Contiguous Wilderness Study Area (CA-040-303) in the southern Gabilan Range near Hollister, Calif.. In this report, the area studied is referred to as "the wilderness study area", or simply "the study area." No active mines, prospects, or identified mineral or energy resources were identified in the study area. Two areas were determined to have low potential for mineral resources. One of these areas has low potential for gold, and silver resources. The other has low potential for diatomite and oil and gas resources. Character and Setting The study area consists of five parcels with a total area of 2,200 acres located about 30 mi south of Hollister, Calif, (fig. 1). These parcels are adjacent to and contiguous with the Pinnacles National Monument. The study area is located in the southern Gabilan Range, part of the Coast Ranges physiographic province. The oldest rocks in the study area are Cretaceous (144-66 million years before present, Ma) granitic rocks, which are cut by a graben (downdropped fault block) that preserves Miocene (24-5 Ma) volcanic rocks (fig. 2) (see appendix for geologic time chart). Overlying the volcanics are Miocene marine and continental sedimentary rocks, including units formed by erosion of the older igneous rocks. Although there has been some mining activity in the area in the past, there is no present mining or exploration activity. Identified Resources and Mineral Resource Potential There are no active mines, prospects, or identified mineral or energy resources in the study area. Two areas have mineral or energy resource potential (fig. 2). That part of the study area that overlies the source area of the volcanic rocks, represented by a swarm of rhyolitic dikes, has low potential for gold and silver in hot-spring deposits. Those areas underlain by Miocene sedimentary rocks have low potential for diatomite in shale deposits and oil and gas resources.