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At least 649 records · Page 36Linked to original sources
USGS1976

Fluid inclusions as a guide to porphyry copper deposits

Fluid-inclusion studies of 36 porphyry copper deposits, mainly in theUnited States, demonstrate that all but 3 evolved through a hydrothermal stage characterized by very high salinities, generally in excess of about 35 weight percent NaC1 equivalent, and boiling temperatures of these fluids ranged from about 250° to 700° C for various stages and deposits. Highsalinities, shown by halite-bearing inclusions, and boiling, suggested by coexisting gas and liquid-rich inclusions, are considered diagnostic of epizonal intrusions which are the most favorable parents for porphyry copper mineralization. Depth of emplacement of many copper-bearing stocks is deduced from fluid inclusions to have been about 6,000 to 10,000 feet (1,800 to 3,000 metres); fluid pressures during mineralization are interpreted generally to be less than 500 bars. Moderate-salinity (less than about 12 percent) and-moderate-temperature (<350°C) fluids are noted in all porphyry copper deposits and were responsible for the deposition of most copper and molybdenum in deposits such as Bagdad, Esperanza, Mineral Park, Morenci, Ray, Sierrita, in the southwestern United States and several in southern British Columbia. However, with only three exceptions, highly saline fluids apparently were present at an early stage and also deposited metals. The relative amounts and economic importance of copper and molybdenum deposited from high and moderate salinity fluids varies within the porphyry deposit class. There is compelling geologic and geochemical evidence that chloride ions are important for transport of metals in hydrothermal solutions, but the porphyry coppers stand out as a class associated with fluids of especially high salinity during at least one stage of their formation. Halite cubes in fluid inclusions are an effective although rough indicator of those salinities and can be conveniently monitored during petrographic study of thin sections. It is suggested that fluid inclusions be used to characterize favorable intrusions, metal anomalies, and cap rocks as an additional method of detecting possible disseminated porphyry copper-type metallization. The presence of halite-bearing inclusions, especially coexisting with gas-rich inclusions, is considered a favorable geochemical anomaly in the search for porphyry ore.

Open-File Report↗
USGS1977

Preliminary data on some Precambrian deposits of zinc-copper-lead sulfides and zinc spinel (granite) in Colorado

Precambrian sulfide deposits in the Southern Rocky Mountains in Colorado are being studied and re-evaluated according to geologic concepts which were developed in recent years in other parts of the world during successful research regarding economic massive sulfide deposits. These studies, initiated in 1974 in Colorado by the U.S. Geological Survey, have indicated that a new look at areas containing long dormant mines and prospects may well lead to the discovery of minable Precambrian sulfide deposits. The zinc, copper, and lead contents of ores investigated to date, supplemented by silver and gold contents, indicate that many long-forgotten deposits are minable in terms of grade. The deposits occur in Precambrian rocks metamorphosed to the lower amphibolite facies in one major region and to the upper amphibolite facies in other regions. Field studies have provided ample evidence indicating that the search for commercial tonnages can he facilitated by using newer concepts of economic geology regarding ore-host rock associations as prospecting guides and by using structural considerations aimed toward learning how metamorphism and folding have modified the shapes and distribution of the ore bodies. A newly recognized concept, originating from the current studies by the U.S. Geological Survey in Colorado, concerns the potential economic significance of gahnite, a zinc spinel. The field and laboratory data indicate that gahnite can be used by geologists as a prospector's guide to ore and can be considerd by mining engineers as a potential major ore mineral, contributing significant amounts of zinc to the sulfide ores in many of the deposits. Studies to date indicate that the Gunnison area, the Salida area, and the Guffey area are particularly favorable to the search for minable deposits, and the potential is equally present in many other areas.

Colorado↗
USGS1983

Reconnaissance geology of the Thaniyah Quadrangle, sheet 20/42 C, Kingdom of Saudi Arabia

The Thaniyah quadrangle, sheet 20/42 C, is located in the transition zone between the Hijaz Mountains and the Najd Plateau of southwestern Saudi Arabia between lat 20?00' and 20?30' N., long 42?00' to 42?30' E. The quadrangle is underlain by Precambrian metavolcanic, metasedimentary, plutonic, and dike rocks. Metavolcanic rocks consist of metamorphosed basalt and andesite with minor dacite and rhyolite and underlie three discontinuous northwest-trending belts. Metasedimentary rocks are confined to small areas underlain by quartzite, metasandstone, marble, and calc-silicate rock. Plutonic rocks include an extensive unit of tonalite and quartz diorite and a smaller unit of diorite and quartz diorite, which occupy much of the central part of the quadrangle. A small body of diorite and gabbro and a two-part zone of tonalite gneiss are also present. All of these plutonic rocks are assigned to the An Nimas batholith. Younger plutonic rocks include extensive graphic granite and rhyolite in the northeastern part of the quadrangle and several smaller bodies of granitic rocks and of gabbro. The metavolcanic rocks commonly have strong foliation with northwest strike and steep to vertical dip. Diorite and quartz diorite are sheared and brecciated and apparently syntectonic. Tonalite and quartz diorite are both foliate and nonfoliate and were intruded in episodes both preceding and following shearing. The granitic rocks and gabbro are post-tectonic. Trends of faults and dikes are mostly related to the Najd faulting episode. Radiometric ages, mostly from adjacent quadrangles, suggest that the An Nimas batholith is 835 to 800 Ma, gabbro and granite, except the graphic granite and rhyolite unit, are about 640 to 615 Ma, and the graphic granite and rhyolite 575 to 565 Ma old. Metavolcanic rocks similar to those hosting copper and gold mineralization in the Wadi Shuwas mining district adjacent to the southwestern part of the quadrangle are abundant. An ancient copper mine was discovered at the edge of the tonalite gneiss belt east of Wadi Ranyah. Granite and gabbro have economic potential as building stone.

Open-File Report↗
USGS1976

Fluid-Inclusion Petrology - Data from Porphyry Copper Deposits and Applications to Exploration

Fluid-inclusion studies of 37 porphyry copper deposits, mainly in the United States, demonstrate that all but 3 evolved through a hydrothermal stage characterized by very high salinities, generally in excess of about 35 weight percent NaCI equivalent. Temperatures of these fluids ranged from about 250 deg to 700 deg C for various stages and deposits. Most systems boiled. High salinities, shown by halite-bearing inclusions, and boiling, suggested by coexisting gas and liquid-rich inclusions, are considered to be diagnostic of epizonal intrusions which are the most favorable parents for porphyry copper mineralization. Depth of emplacement of many copper-bearing stocks is deduced from fluid inclusions to have been about 6,000 to 10,000 feet (1,800 to 3,000 metres); fluid pressures during mineralization are interpreted generally to be less than 500 bars. Moderate-salinity (less than about 12 percent) and moderate-temperature <350 deg C) fluids are noted in all porphyry copper deposits and were responsible for the deposition of most copper and molybdenum in deposits, such as Bagdad, Esperanza, Mineral Park, Morenci, Ray, Sierrita, in the Southwestern United States and several in southern British Columbia. However, with only three exceptions, highly saline fluids apparently were present at an early stage and also deposited metals. The relative amounts and economic importance of copper and molybdenum deposited from high- and moderate-salinity fluids varies within the porphyry deposit class. There is compelling geologic and geochemical evidence that chloride is important for transport of metals, but the porphyry coppers stand out as a class associated with fluids of especially high salinity during at least one stage of their formation. Halite cubes in fluid inclusions are an effective, although rough, indicator of those salinities and can be conveniently monitored during petrographic study of thin sections. Thus, fluid inclusions can be used to characterize favorable intrusions, metal anomalies, and caprocks as an additional method of detecting possible disseminated porphyry copper metallization. The presence of halite-bearing inclusions, especially coexisting with gas-rich inclusions, is considered to be a favorable fluid anomaly in the search for porphyry ore.

Professional Paper↗
USGS1997

New K-Ar and 40Ar/39Ar ages of plutonism, hydrothermal alteration, and mineralization in the central Wasatch Mountains, Utah

Twenty-one new K-Ar and 10 new 40 Ar/ 39 Ar ages are reported for igneous and hydrothermal minerals from intrusive rocks of the Wasatch igneous belt in the central Wasatch Mountains. Interpretation of our new data combined with previously published K-Ar ages and with new 40 Ar/ 39 Ar and U-Pb ages reported by Vogel et al. (1997) suggests that the Clayton Peak stock was emplaced at about 36 to 35 Ma, the Alta stock at about 35 to 33 Ma, and the Little Cottonwood stock at about 31 to 30 Ma. Biotite K-Ar ages progressively increase from west to east in the Little Cottonwood stock, which is consistent with more rapid cooling of the eastern part of the stock and with other evidence suggesting about 15 degrees of eastward tilting of the central Wasatch Mountains following emplacement of the Wasatch igneous belt. Most porphyry stocks in the Park City mining district were emplaced at about 41 to 40 Ma; the Ontario stock was emplaced at about 36 Ma. Vein deposits in the Park City mining district formed at about 36 to 33 Ma. The Park Premier stock was emplaced in several pulses between about 35 to 32 Ma. Alteration related to porphyry copper mineralization in the Park Premier stock formed at 33.5 Ma and advanced argillic alteration and gold mineralization formed at 31.4 Ma. Molybdenum mineralization in the eastern part of the Little Cottonwood stock formed between 26 to 23.5 Ma.

Utah↗
USGS2013

A framework for quantitative assessment of impacts related to energy and mineral resource development

Natural resource planning at all scales demands methods for assessing the impacts of resource development and use, and in particular it requires standardized methods that yield robust and unbiased results. Building from existing probabilistic methods for assessing the volumes of energy and mineral resources, we provide an algorithm for consistent, reproducible, quantitative assessment of resource development impacts. The approach combines probabilistic input data with Monte Carlo statistical methods to determine probabilistic outputs that convey the uncertainties inherent in the data. For example, one can utilize our algorithm to combine data from a natural gas resource assessment with maps of sage grouse leks and piñon-juniper woodlands in the same area to estimate possible future habitat impacts due to possible future gas development. As another example: one could combine geochemical data and maps of lynx habitat with data from a mineral deposit assessment in the same area to determine possible future mining impacts on water resources and lynx habitat. The approach can be applied to a broad range of positive and negative resource development impacts, such as water quantity or quality, economic benefits, or air quality, limited only by the availability of necessary input data and quantified relationships among geologic resources, development alternatives, and impacts. The framework enables quantitative evaluation of the trade-offs inherent in resource management decision-making, including cumulative impacts, to address societal concerns and policy aspects of resource development.

Natural Resources Research↗
USGS2004

Mineral Deposit Data for Epigenetic Base- and Precious-metal and Uranium-thorium Deposits in South-central and Southwestern Montana and Southern and Central Idaho

Metal deposits spatially associated with the Cretaceous Boulder and Idaho batholiths of southwestern Montana and southern and central Idaho have been exploited since the early 1860s. Au was first discovered in placer deposits; exploitation of vein deposits in bedrock soon followed. In 1865, high-grade Ag vein deposits were discovered and remained economically important until the 1890s. Early high-grade deposits of Au, Ag and Pb were found in the weathered portions of the veins systems. As mining progressed to deeper levels, Ag and Pb grades diminished. Exploration for and development of these vein deposits in this area have continued until the present. A majority of these base- and precious-metal vein deposits are classified as polymetallic veins (PMV) and polymetallic carbonate-replacement (PMR) deposits in this compilation. Porphyry Cu and Mo, epithermal (Au, Ag, Hg and Sb), base- and precious-metal and W skarn, W vein, and U and Th vein deposits are also common in this area. The world-class Butte Cu porphyry and the Butte high-sulfidation Cu vein deposits are in this study area. PMV and PMR deposits are the most numerous in the region and constitute about 85% of the deposit records compiled. Several types of syngenetic/diagenetic sulfide mineral deposits in rocks of the Belt Supergroup or their equivalents are common in the region and they have been the source of a substantial metal production over the last century. These syngenetic deposits and their metamorphosed/structurally remobilized equivalents were not included in this database; therefore, deposits in the Idaho portion of the Coeur d'Alene district and the Idaho Cobalt belt, for example, have not been included because many of them are believed to be of this type.

Open-File Report↗
USGS2002

Tectonics and distribution of gold deposits in China - An overview

Gold exploration in China has expanded rapidly during the last two decades since a modern approach to economic development has become a national priority. China currently produces 180 tonnes (t) of gold annually, which is still significantly less than South Africa, USA, and Australia. However, China is now recognized as possessing significant gold resources in a wide range of mineral deposit types. Present estimates of gold resources in China exceed 4,500 t, which comprise 60% in gold-only deposits, more than 25% in base metal-rich skarn, porphyry, and vein deposits, and more than 10% in placer accumulations. The major gold provinces in China formed during the main episodes of Phanerozoic tectonism. Such tectonism involved interaction of China's three major Precambrian cratons, North China, Tarim, and Yangtze (or South China when combined with Cathaysia block), with the Angara (or Siberian), Kazakhstan–Kyrgyzstan, and Indian cratons. Resulting collisions included deformation of accreted oceanic sequences between the cratonic blocks. The most important ore-forming orogenies were (1) the late Paleozoic Variscan (405–270 Ma), which led to amalgamation of the Angara, North China and Yangtze cratons, (2) the Indosinian (270–208 Ma), which led to the collision of North China and South China cratons, (3) the Yanshanian (208–90 Ma), which was largely influenced by the subduction of the Izanagi–Pacific plates beneath eastern China, and (4) the Himalayan (<90 Ma) indentation of the Indian continent into Eurasia. No important Precambrian gold systems are recognized in China, mainly because of reworking of exposed Precambrian rocks by these younger orogenies, but there are a few Caledonian (600–405 Ma) gold-bearing systems in northern Xinjiang. Most of China's orogenic, epithermal, and Carlin-like gold deposits are in the reworked margins of major cratonic blocks and in metasedimentary rock-dominated fold belts adjacent to these margins. Accordingly, the major gold provinces are present along the northern, southeastern and southern margins of the North China craton, along the southwestern and northwestern margins of the Yangtze craton, in the Tianshan and Altayshan orogenic belts in northern Xinjiang, and throughout the southeastern China fold belt. Gold-placer deposits derived from these primary deposits are concentrated in the northernmost part of northeastern China and along the northwestern margin of the Yangtze craton. The major provinces with significant gold in porphyry-related copper systems and base metal skarns are present in the Yangtze River area along the northeastern and southeastern margin of the Yangtze craton, in the fold belt in southwestern China, and scattered through northern China. Three-quarters of the Chinese gold-only deposits occur within the North China craton margins. Half are located in the uplifted Precambrian metamorphic rocks and most of the remainder are hosted in the Phanerozoic granitoids that intruded the reworked Precambrian terranes. The abundance of granite-hosted gold contrasts the North China craton with other Precambrian cratons, such as those in Western Australia, central Canada, and Zimbabwe, where gold is mainly hosted in the Archean greenstone belts. This difference may be explained by the multiple episodes of Phanerozoic tectonism along the North China craton margins resulting from the collision of the Angara, North China, and South China cratons, and from subduction of the Izanagi–Pacific oceanic plates underneath the eastern China continent.

Mineralium Deposita↗
USGS1973

Gold-bearing arsenian pyrite determined by microprobe analysis, Cortez and Carlin Gold Mines, Nevada

Studies of polished sections and chemical analyses made by electron microprobe show that gold and arsenic in the unoxidized ores from the Cortez and Carlin mines are most abundant in pyrite. Gold, as particles too small to be seen under the microscope, along with arsenic is concentrated in tiny pyrite grains (<0.005 mm) and in thin rims of larger pyrite grains. It is concentrated also in arsenopyrite which is sparsely distributed in the Cortez ore. Traces of gold are contained in sphalerite and chalcopyrite sparsely disseminated in Carlin ore. Mercury and antimony occur also in the pyrite, and antimony is in illite as well. Little if any gold or arsenic is contained in quartz, carbonate, clay, and carbonaceous material. In oxidized ore, the sulfur and carbonaceous material have been removed and gold and arsenic occur in iron oxide pseudomorphs after pyrite. Gold can be seen in the oxidized ore, indicating that it has been mobilized and concentrated. No association was found between gold and the carbonaceous material. During mineralization, calcite was removed from the fractured silty carbonate of the Roberts Mountains Formation, creating micropore space in which pyrite, quartz, and illite were deposited. The average tenor of the pyrite, if all the gold were in the pyrite, would be 0.10 percent at Cortez and 0.14 percent at Carlin. These calculated values are similar to the analytical values. The calculated tenor of the carbonaceous material, however-if such material were considered to be the mineral host of the gold-would be 0.28 percent at Cortez and 1 percent at Carlin. These figures are unreasonable when compared with the analytical data, which show that the carbonaceous material contains no detectable amounts of gold. © 1973 Society of Economic Geologists, Inc.

Nevada↗
USGS1981

Geologic setting, petrology, and geochemistry of zoned tungsten-bearing skarns at the Strawberry Mine, central Sierra Nevada, California

The Strawberry mine, 90 km northeast of Fresno, California, occurs on the margin of a small roof pendant of Early Jurassic metasedimentary rocks and middle Cretaceous metaigneous rocks. Middle Cretaceous granitic intrusions surround and intrude the roof pendant. Adjacent to one granodiorite intrusion, several subvertical marble layers are replaced by scheelite-bearing skarns. The skarns form spears that project as much as 300 m upward and into the marble layers. Between the marble and granodiorite, the skarns comprise a metasomatic column and are subdivided into four zones: (1) wollastonite skarn, adjacent to and replacing marble, and containing wollastonite, grossularite, idocrase, and diopside; (2) garnet skarn, containing andradite-grossularite, diopside-hedenbergite, and scheelite; (3) pyroxene skarn containing hedenbergite-diopside, plagioclase, magnetite, and scheelite; and (4) hornblende skarn, adjacent to granodiorite, and containing hornblende, plagioclase, microcline, magnetite, and scheelite. Important features of the skarn zonation are: sharp contacts between skarn zones; relative constancy of whole-rock, mineral, and modal compositions within each skarn zone; major changes in whole-rock, mineral, and modal compositions across contacts between skarn zones; and systematic replacement between skarn zones, with any given skarn zone replaced by the next lower and adjacent skarn zone toward the granodiorite. Volume remained approximately constant in the replacement of marble by various skarn zones. Marble and skarn exhibit uniformly low porosity. Data on mineral paragenesis and mineral composition indicate that in the replacement of any skarn the most calcic mineral is consumed and the residual calc-silicate minerals in the replacing skarn increase in abundance and are richer in total Fe and MnO. This mineral paragenesis can be related to systematic variations in whole-rock composition. From the marble to the granodiorite, skarns are progressively depleted in CaO and CO 2 , and progressively enriched in SiO 2 , total Fe, MgO, MnO, Al 2 O 3 , Na 2 O, K 2 O, and WO 3 . The skarns probably formed when an aqueous fluid containing a solute fraction relatively depleted in Ca and CO 2 , and relatively enriched in Si, Fe, Mg, Mn, Al, Na, K, and W, was expelled from a granitic magma into country rock. The tendency toward local equilibrium between fluid and marble resulted in formation of skarns with systematic stepwise changes in mineral and rock composition at contacts between skarns. Achievement of local equilibrium occurred mainly by infiltration metasomatism with a possible minor component of diffusion metasomatism.

California↗
USGS1996

Significance of tourmaline-rich rocks in the North Range group of the Cuyuna iron range, east-central Minnesota

Concentrations of tourmaline in Early Proterozoic metasedimentary rocks of the Cuyuna iron range, east-central Minnesota, provide a basis for redefinition of the evolutionary history of the area. Manganiferous iron ore forms beds within the Early Proterozoic Trommald Formation, between thick-bedded granular iron-formation having shallow-water alepositional attributes and thin-bedded, nongranular iron-formation having deeper water attributes. These manganese-rich units were previously assumed to be sedimentary in origin. However, a reevaluation of drill core and mine samples from the Cuyuna North range has identified strata-bound tourmaline and tourmalinite, which has led to a rethinking of genetic models for the geology of the North range. We interpret the tourmaline-rich rocks of the area to be a product of submarine-hydrothermal solutions flowing along and beneath the sediment-seawater interface. This model for the depositional environment of the tourmaline is supported by previously reported mineral assemblages within the Trommald Formation that comprise aegirine; barium feldspar; manganese silicates, carbonates, and oxides; and Sr-rich barite veins.In many places, tourmaline-rich metasedimentary rocks and tourmalinites are associated locally with strata-bound sulfide deposits. At those localities, the tourmaline-rich strata are thought to be lateral equivalents of exhalative sulfide zones or genetically related subsea-floor replacements. On the basis of the occurrence of the tourmaline-rich rocks and tourmalinites, and on the associated minerals, we suggest that there is a previously unrecognized potential for sediment-hosted sulfide deposits in the Cuyuna North range.

Economic Geology↗
USGS1996

Clay alteration and gold deposition in the genesis and blue star deposits, Eureka County, Nevada

The Genesis and Blue Star sedimentary rock-hosted gold deposits occur within the 40-mile-long Carlin trend and are located in Eureka County, Nevada. The deposits are hosted within the Devonian calcareous Popovieh Formation, the siliciclastic Rodeo Creek unit and the siliciclastic Vinini Formation. The host rocks have undergone contact metamorphism, decalcification, silicification, argillization, and supergene oxidation.Detailed characterization of the alteration patterns, mineralogy, modes of occurrence, and associated geochemistry of clay minerals resulted in the following classifications: least altered rocks, found distal to the orebody, consisting of both metamorphosed and unmetamorphosed host rock that has not been completely decalcified; and altered rocks, found proximal to the orebody that have been decalcified. Altered rocks are classified further into the following groups based on clay mineral content: silicic, 1 to 10 percent clay; silicic-argillic, 10 to 35 percent clay; and argillic, 35 to 80 percent clay. Clay species identified are 1M illite, 2M 1 illite, kaolinite, halloysite, and dioctahedral smectite. An early hydrothermal event resulted in the precipitation of euhedral kaolinite and at least one generation of silica. This event occurred contemporaneously with decalcification which increased rock permeability and porosity. A second clay alteration event resulted in the precipitation of hydrothermal 1M illite which replaced hydrothermal kaolinite and is associated with gold deposition. Silver and silica deposition is also associated with this phase of hydrothermal alteration. Hydrothermal alteration was followed by supergene alteration which resulted in the formation of supergene kaolinite, halloysite, and smectite as well as the oxidation of iron-bearing minerals. Supergene clays are concentrated along faults, dike margins, and within rocks containing carbonate. Gold mineralization is not associated with supergene clay minerals within the Genesis and Blue Star deposits. Rocks classified as silicic-argillic in the Popovich Formation represent the most significant gold host. Silicic-argillic rocks commonly exhibit bedding-parallel alteration zones. This pattern of alteration indicates that stratigraphy as well as northwest-trending structures played a significant role in the migration of gold-bearing fluids. Based on K-Ar age determinations of hydrothermal 1M illite associated with gold, the main event of mineralization in the Genesis and Blue Star deposits occurred between 93 and 100 Ma, during mid-Cretaceous time.

Economic Geology↗
USGS1894

Natural mineral water of the United States: Section in Fourteenth Annual Report of the United States Geological Survey to the Secretary of the Interior, 1892-1893: Part 2 - Accompanying papers

Aside from the geological interest attached to the subject of mineral waters the facts that within the limits of the United States there are between 8,000 and 10,000 mineral springs, and that the waters from nearly 300 are annually placed upon the market to the extent of over 21,000,000 gallons, at a valuation of nearly \$5,000,000, show plainly that the subject is also one of considerable economic importance. That this importance is an increasing one is evident when a comparison of these figures is made with the figures for 1883, the first year they were compiled. The production then was 7,529,423 gallons, with a valuation of \$1,119,603, and the total number of springs known to be utilized for commercial purposes was only 189.

Annual Report↗
USGS1982

Mineral resource potential map of the Gila-San Francisco Wilderness Study Area, Graham and Greenlee counties, Arizona

The mineral resource potential of the Gila-San Francisco Wilderness Study Area (AZ-040-022/023/024) is low (fig. 2). Although favorable geologic environments for placer gold deposits and manganese vein deposits are present in the study area, the probability of discovering economically exploitable deposits of these metals is low, and not encouraging. Even more speculative is the study area's porphyry copper potential which is based solely on the possibility of favorable host terranes underlying the study area at depth. The study area does contain substantial deposits of pumice, but their economic significance is probably minor. A part of the study area has been previously designated a Known Geothermal Resource Area (KGRA).

Arizona↗
USGS2022

Tungsten skarn potential of the Yukon-Tanana Upland, eastern Alaska, USA—A mineral resource assessment

Tungsten (W) is used in a variety of industrial and technological applications and has been identified as a critical mineral for the United States, India, the European Union, and other countries. These countries rely on W imports mostly from China, which leaves them vulnerable to supply disruption. Consequently, the U.S. government has a current initiative to understand domestic resource potential. The eastern Alaska portion of the Yukon-Tanana Upland (YTU), is prospective for W skarn deposits, the major source of global W supply. The regional geology consists of juxtaposed Paleozoic lithotectonic packages that were reaccreted to North America in the Mesozoic . Multiple subsequent episodes of arc-related magmatism intruded the lithotectonic packages, accompanied by W skarn formation mostly associated with 100–90 Ma intrusions; major W skarn deposits in Canada are part of the same metallogenic event (e.g., Mactung, Cantung). In this paper, we present an assessment for undiscovered W skarn resources for parts of the lesser-explored western (Alaskan) portion of the YTU. We used GIS proximity analysis to map the intersection of pluton and carbonate-bearing rocks to define three permissive tracts for W skarn deposits. The permissive tracts were qualitatively assessed by mineral potential mapping using region-wide sediment geochemistry and mineral concentrate datasets. This analysis showed that much of the western YTU has high potential for undiscovered W skarn deposits, whereas the eastern and southern YTU had only isolated areas of medium to high potential. Historical production and the quality of the geochemistry data of the western YTU tract (ca. 9200 km 2 ) permitted a quantitative assessment of undiscovered W resources. Probabilistic estimates by a panel of 20 experts predicted a 70% chance of one to three undiscovered W skarn deposits in the western YTU tract. The rationale for favorability employed by the expert panel included favorable lithology, previous production, clustering of previously mined deposits, W placers in the area, lack of recent exploration, pan concentrates containing W minerals, and W geochemical anomalies. Estimates were combined with a global grade and tonnage model for W skarns in a Monte Carlo simulation and provided a median estimate of undiscovered resources of 94 kt WO 3 . If the undiscovered W skarn deposits are located close to infrastructure (e.g., near Fairbanks, or close to roads and/or power grid), application of an economic filter indicates that the median total economically recoverable WO 3 is 63 kt with a net present value (NPV) of $330 million USD (2008 dollars). Whereas if deposits are far from infrastructure, median recoverable WO 3 is only 30 kt and the NPV is $44 million. Our models for contained WO 3 resources and NPV estimates for the western YTU tract are considerably lower than the known resources in skarns in adjacent areas in Canada. Estimates for the western YTU are also lower than preliminary estimates for undiscovered W skarn deposits in areas of the western conterminous United States. We speculate that lower permeability and continuity of favorable carbonate rock horizons in the relatively higher-grade metamorphic country rocks in the Alaska portion of the YTU may explain some of the differences in prospectivity. More detailed geologic mapping, modern geochemistry, and geophysical surveys are needed to refine the resource potential of the whole YTU. Regardless, quantitative mineral resource assessment provides a useful tool for making first-order regional estimates of undiscovered resources, identifying target areas for new data acquisition, and guiding research on the fundamental controls of district-scale metallogenic endowments.

Alaska↗
USGS1956

Geologic map of the Tennessee Pass area, Eagle and Lake counties, Colorado

The triangular area bounded by the most productive mining districts of central Colorado- Leadville, Climax-Kokomo, and Gilman-Red Cliff- is of economic interest not only because of its close spatial relation to these districts but because it is closely related geologically. It contains the same rocks as the adjoining mining districts and is closely related to them structurally. It shows widespread evidence of weak mineralization at the surface and may contain ore deposits at depth, as the zones that have been productive in the adjoining mining districts lie beneath a cover of younger rocks in much of the area.

Colorado↗
USGS1963

Studies of fluid inclusions iii: Extraction and uantitative analysis of inclusions in the milligram range

A method has been developed for the extraction and chemical microanalysis of individual fluid inclusions , or groups of inclusions , in the milligram range . Usable quantitative analyses for Na, K, Ca, Mg, CI, B, and SO* have been obtained of mineral samples containing several milligrams of inclusion fluid , and with increased experimental errors, on fractional milligram samples. The steps involved are: 1) concentration of inclusions by sample selection and cutting; 2) electrolytic cleaning; 3) crushing in soft copper sample tube in vacuo; 4) conversion of emitted water to hydrogen and determination of its volume; 5) mass spectrometric determination of the D/H ratio if desired; 6) leaching of the crushed mineral to dissolve soluble salts remaining; 7) microanalysis of the filtrate by sensitive colorimetric and flame photometric methods. The method has been applied to determine the composition of fluid inclusions in mineral samples from several types of deposits, with special attention to a series of samples from Mississippi Valley-type ore deposits.

Economic Geology↗
USGS1993

Summary of the mineral- and energy-resource endowment, BLM roswell resource area, east-central New Mexico

In this summary of two comprehensive resource reports produced by the U.S. Bureau of Mines and the U.S. Geological Survey for the U.S. Bureau of Land Management, we discuss the mineral- and energyresource endowment of the 14-millon-acre Roswell Resource Area, New Mexico, managed by the Bureau of Land Management. The Bureau and Survey reports result from separate studies that are compilations of published and unpublished data and integrate new findings on the geology, geochemistry, geophysics, mineral, industrial, and energy commodities, and resources for the seven-county area. The reports have been used by the Bureau of Land Management in preparation of the Roswell Resource Area Resource Management Plan, and will have future use in nationwide mineral- and energy-resource inventories and assessments, as reference and training documents, and as public-information tools. In the Roswell Resource Area, many metals, industrial mineral commodities, and energy resources are being, or have been, produced or prospected. These include metals and high-technology materials, such as copper, gold, silver, thorium, uranium and/or vanadium, rare-earth element minerals, iron, manganese, tungsten, lead, zinc, and molybdenum; industrial mineral resources, including barite, limestone/dolomite, caliche, clay, fluorspar, gypsum, scoria, aggregate, and sand and gravel; and fuels and associated resources, such as oil, gas, tar sand and heavy oil, coal, and gases associated with hydrocarbons. Other commodities that have yet to be identified in economic concentrations include potash, halite, polyhalite, anhydrite, sulfur, feldspar, building stone and decorative rock, brines, various gases associated with oil and gas exploration, and carbon dioxide. ?? 1993 Oxford University Press.

Nonrenewable Resources↗