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Minerals & deposits

Find USGS publications on mineral resources and deposits. Search commodities and geographic names, then check source reports for geology and assessment methods.

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USGS2014

Concealed basalt-matrix diatremes with Cu-Au-Ag-(Mo)-mineralized xenoliths, Santa Cruz Porphyry Cu-(Mo) System, Pinal County, Arizona

The Santa Cruz porphyry Cu-(Mo) system near Casa Grande, Arizona, includes the Sacaton mine deposits and at least five other concealed, mineralized fault blocks with an estimated minimum resource of 1.5 Gt @ 0.6% Cu. The Late Cretaceous-Paleocene system has been dismembered and rotated by Tertiary extension, partially eroded, and covered by Tertiary-Quaternary basin-fill deposits. The mine and mineralized fault blocks, which form an 11 km (~7 miles) by 1.6 km (~1 mile) NE-SW–trending alignment, represent either pieces of one large deposit, several deposits, or pieces of several deposits. The southwestern part of the known system is penetrated by three or more diatremes that consist of heterolithic breccia pipes with basalt and clastic matrices, and subannular tuff ring and maar-fill sedimentary deposits associated with vents. The tephra and maar-fill deposits, which are covered by ~485 to 910 m (~1,600–3,000 ft) of basin fill, lie on a mid-Tertiary erosion surface of Middle Proterozoic granite and Late Cretaceous porphyry, which compose most xenoliths in pipes and are the host rocks of the system. Some igneous xenoliths in the pipes contain bornite-chalcopyrite-covellite assemblages with hypogene grades >1 wt % Cu, 0.01 ounces per ton (oz/t) Au, 0.5 oz/t Ag, and small amounts of Mo (<0.01 wt %). These xenoliths were derived from mineralized rocks that have not been encountered in drill holes, and attest to additional, possibly higher-grade deposits within or subjacent to the known system. The geometry, stratigraphy, and temporal relationships of pipes and tephras, interpreted from drill hole spacing and intercepts, multigenerational breccias and matrices, reequilibrated and partially decomposed sulfide-oxide mineral assemblages, melted xenoliths, and breccia matrix compositions show that the diatremes formed in repeated stages. Initial pulses of basalt magma fractured granite, porphyry, and other crustal rocks during intrusion, transported multi-sized fragments of these rocks upward, and partially melted small fragments. Rapid decompression of magma induced catastrophic devolatilization that ruptured overlying rocks to the surface, and generated fragment-volatile suspensions that abraded conduits into near-vertical cylindrical structures. Fragments entrained in suspensions were milled and sorted, and ejected as basal surge, pyroclastic deposits, and airfall tephra that built tuff rings around vents and filled vent depressions. Comminuted m- to mm-sized fragments of wall rocks in magma and suspensions that remained in conduits solidified as heterolithic breccias. Subsequent pulses of basalt magma ascended through the same conduits, brecciated older heterolithic breccias, devolatilized, and quenched, leaving two or more generations of nested and mingled heterolithic breccias and internal zones of fluidized fragments. Tephra and maar-fill deposits from later eruptions are composed of more hydrous and oxidized minerals than earlier tephras, reflecting a higher proportion of water in transport fluid which, based on fluid inclusion populations in mineralized xenoliths, was saline water and CO 2 . The large vertical extent (~600 m; ~2,000 ft) of basalt matrix in pipes, near-paleosurface matrix vesiculation, and plastically deformed basalt lapilli indicates that diatreme eruptions were predominantly phreatic. Diatreme xenoliths represent crustal stratigraphy and, as in the Santa Cruz system, provide evidence of concealed mineral resources that can guide exploration drilling through cover. Vectors to the source of bornite-dominant xenoliths containing >1% Cu and significant Au and Ag could be determined by refinement of breccia pipe geometries, by reassembly of mineralized fault blocks using modal, chemical, and temporal characteristics of hydrothermal mineral assemblages and fluid inclusions, and by paleodrainage analysis.

Arizona↗

USGS2014

Effect of sulfate and carbonate minerals on particle-size distributions in arid soils

Arid soils pose unique problems during measurement and interpretation of particle-size distributions (PSDs) because they often contain high concentrations of water-soluble salts. This study investigates the effects of sulfate and carbonate minerals on grain-size analysis by comparing analyses in water, in which the minerals dissolve, and isopropanol (IPA), in which they do not. The presence of gypsum, in particular, substantially affects particle-size analysis once the concentration of gypsum in the sample exceeds the mineral’s solubility threshold. For smaller concentrations particle-size results are unaffected. This is because at concentrations above the solubility threshold fine particles cement together or bind to coarser particles or aggregates already present in the sample, or soluble mineral coatings enlarge grains. Formation of discrete crystallites exacerbates the problem. When soluble minerals are dissolved the original, insoluble grains will become partly or entirely liberated. Thus, removing soluble minerals will result in an increase in measured fine particles. Distortion of particle-size analysis is larger for sulfate minerals than for carbonate minerals because of the much higher solubility in water of the former. When possible, arid soils should be analyzed using a liquid in which the mineral grains do not dissolve, such as IPA, because the results will more accurately reflect the PSD under most arid soil field conditions. This is especially important when interpreting soil and environmental processes affected by particle size.

Soil Science Society of America Journal↗

USGS1999

Environmental characteristics of clays and clay mineral deposits

Clays and clay minerals have been mined since the Stone Age; today they are among the most important minerals used by manufacturing and environmental industries. The U.S. Geological Survey (USGS) supports studies of the properties of clays, the mechanisms of clay formation, and the behavior of clays during weathering. These studies can tell us how and where these minerals form and provide industry and land-planning agencies with the information necessary to decide how and where clay and clay mineral deposits (fig. 1) can be developed safely with minimal effects on the environment. The term "clay" is applied both to materials having a particle size of less than 2 micrometers (25,400 micrometers = 1 inch) and to the family of minerals that has similar chemical compositions and common crystal structural characteristics (Velde, 1995) described in the next section. Clay minerals have a wide range of particle sizes from 10's of angstroms to millimeters. (An angstrom ( ) is a unit of measure at the scale of atoms.) Thus, clays may be composed of mixtures of finer grained clay minerals and clay-sized crystals of other minerals such as quartz, carbonate, and metal oxides. Clays and clay minerals are found mainly on or near the surface of the Earth.

Information Handout↗

USGS2019

Rare earth element mineral deposits in the United States

Because of their unique special chemical properties, many of the metals in the group of rare earth elements (REEs) have essential applications in 21st century technologies. Examples of products that use REEs are cell phones, computers, fluorescent and light-emitting-diode lights, flat-screen television and computer monitors, and in high-strength magnets used by clean energy technologies such as the generators of wind turbines and batteries of hybrid and electric vehicles. REEs are used in many defense applications, such as in components of jet engines, missile guidance systems, antimissile defense systems, satellites, and communication systems. The rare earth elements have become vital to manufacturing numerous high-tech products, which has been accompanied by a large increase in their demand. At the same time, there has been concern by the United States and many other Nations about the near-monopoly of mining, processing, and supply of REEs from one Nation, China. Between 2011 and 2017, China produced approximately 84 percent of the world’s REEs, and during this time the United States only produced REEs between 2012 and 2015. The U.S. production came entirely from the Mountain Pass mine in California, providing only about 4 percent of the world REE supply. Because REEs are essential for technological applications and are primarily supplied by one Nation, there has been an increased concern in identifying new sources of REEs, including economic REE deposits. In response to these concerns, since 2009, the U.S. Geological Survey (USGS) has conducted numerous studies focused on the distribution, geology, and potential resources for the REE-bearing mineral deposits in the United States. The basic characteristics of mineral deposit types that host REE enrichments in the United Sates are summarized in this report, with selected examples. Several types of REE-enriched mineral deposits are reviewed, including deposits in carbonatites, alkaline igneous rocks, sedimentary phosphate-rich rocks (phosphorite), regions containing REE-rich veins, iron oxide deposits containing REE-bearing apatite, monazite-xenotime-bearing placer deposits (heavy-mineral sands), and ion-adsorption clay deposits (REE in granite-derived regolith). A better understanding of these mineral deposits will assist in identifying domestic resources to help alleviate the dependence on imported REEs. The economic development of REE mineral deposits is affected by many factors beyond mining, such as commodity prices and mineral processing costs. Most of the REEs are hosted by minerals that have complex chemical formulas; this presents more challenges to process and extract the REEs. Continued advancements and refinements in mineral processing techniques may allow REE deposits with complex mineralogy to be economically developed in the future.

Circular↗

USGS1983

Petroleum and mineral resources of Antarctica

No known petroleum or mineral resources occur in Antarctica. The data on these subjects have been collected, mainly since the IGY (International Geophysical Year), 1957-58, as a part of other research carried out by geologists and geophysicists from a number of countries. Specific resource-related studies have not been made. Wright and Williams (1974) summarized what was known of Antarctic mineral resources a decade ago. The U.S. Geological Survey has been actively pursuing various investigations in Antarctica since 194 7. In the course of this work and that of our colleagues elsewhere in the United States and in other countries, much information relevant to petroleum and mineral resources has been obtained. Since 1976, modern state-of-the-art multichannel seismic reflection and aeromagnetic surveys by several countries over the continental margin of Antarctica have indicated thick sedimentary basins. However, no offshore drilling beneath the continental shelf has taken place since the DSDP (Deep Sea Drilling Project) holes in the Ross Sea in 1973. Geologic field investigations begun at the turn of the twentieth century have been intensified in the past two decades; most rock outcrops have been visited and samples collected. Technology to exploit resources, particularly in the Arctic, has been developing at a rapid rate, and much of it could be applied to Antarctica. As a result of the petroleum price increases of the past decade, the attention of a number of countries has turned to Antarctica, but under the policy of "voluntary restraint" adopted by the Antarctic Treaty nations, no active petroleum or mineral exploration is taking place. The Antarctic treaty countries are in the process of negotiating an Antarctic mineral resources regime that is anticipated to be completed within the next several years. Therefore it seemed timely to us to readdress the question of petroleum and mineral resources. These reports review and summarize the available information. The first report summarizes the information relevant to petroleum resources. Although uneconomic at present, petroleum is generally considered more likely to be exploited (if supergiant fields were ever found) in the next few decades than hard minerals. The second report reviews the reported occurrences of minerals in Antarctica and discusses their significance. The final report discusses the Dufek layered mafic intrusion, second only to the Bushveld Complex in size in the world; the Dufek intrusion might be considered as a potential target for mineral exploration.

Circular↗

USGS2015

The Ebola virus disease outbreak and the mineral sectors of Guinea, Liberia, and Sierra Leone

The mineral sector plays a key role in the economies of Guinea, Liberia, and Sierra Leone. The onset of the Ebola virus disease (EVD) outbreak in early 2014, together with changes in mineral market conditions, raised questions regarding the status of mining operations and of mineral development and exploration projects in all three countries. Mineral projects were the underpinnings of World Bank short-term forecasts of increases in gross domestic product (GDP) for all three countries and were expected to be the basis of future economic growth. The significant delay or cancellation of these projects could result in a major economic setback for all three countries. In response to the uncertainty surrounding the status of mineral projects in Guinea, Liberia, and Sierra Leone, the National Minerals Information Center compiled information on the distribution of mines, mineral facilities, and mineral projects under development in the three countries. This fact sheet provides information on the role that the mineral sector plays in their respective economies, on the operating status of mining projects through yearend 2014, and on the coordinated actions by mining companies to support governments and international relief organizations in their efforts to contain the EVD outbreak.

Fact Sheet↗

USGS2020

International geoscience collaboration to support critical mineral discovery

The importance of critical minerals and the need to expand and diversify critical mineral supply chains has been endorsed by the Federal governments of Australia, Canada, and the United States. The geoscience organizations of Geoscience Australia, the Geological Survey of Canada and the U.S. Geological Survey have created the Critical Minerals Mapping Initiative to build a diversified critical minerals industry in Australia, Canada, and the United States by developing a better understanding of known critical mineral resources, determining geologic controls on critical mineral distribution for deposits currently producing byproducts, identifying new sources of supply through critical mineral potential mapping and quantitative mineral assessments, and promoting critical mineral discovery in all three countries.

Fact Sheet↗

USGS2023

National map of focus areas for potential critical mineral resources in the United States

Introduction The U.S. Geological Survey (USGS) launched the Earth Mapping Resources Initiative (Earth MRI) to modernize the surface and subsurface geologic mapping of the United States, with a focus on identifying areas that may have the potential to contain critical mineral resources. EarthMRI can inform strategies to ensure secure and reliable domestic critical mineral supplies for the United States as mandated by Executive Order 13817 and the Infrastructure and Jobs Act of 2021 (Public Law 117–58, 135 Stat. 529). Earth MRI is a collaborative effort between the USGS and the State geological surveys as represented by the Association of American State Geologists to identify, prioritize, and acquire new geoscience data for geographic areas, or focus areas, across the Nation that have potential to host critical mineral resources. Mapping of focus areas was based on a framework of mineral systems and their associated mineral deposit types that could possibly host critical minerals. Using readily available geologic, geophysical, geochemical, and mineral deposit data, teams of USGS scientists worked with representatives of State geological surveys in a series of workshops to outline focus areas that contain evidence of key features for one or more mineral systems. These areas can be used to guide future efforts to collect new geologic, geophysical, geochemical, and topographic data that focus on critical minerals through Earth MRI.

Fact Sheet↗

USGS2025

Critical minerals in mine waste

Introduction Critical minerals are commodities with vulnerable supply chains that play a vital role in supporting the United States’ economy, national defense and security, emerging technologies, and energy independence. The prosperity of our Nation depends on generating a resilient supply of domestic critical minerals; mine waste may be an untapped source of these commodities. Mine waste from centuries of legacy mining persist on the landscape and may contain critical minerals and other valuable commodities previously deemed uneconomic to recover. At modern mines, the financial viability of recovering byproduct critical minerals, which are not the primary target, may be marginal and can ultimately destine them to mine waste. Further, mine waste can be a liability for the mining company or, at legacy mines, the taxpayer because of its effect on the landscape. The U.S. Geological Survey (USGS) has several initiatives to evaluate critical mineral resources in various types of waste. This factsheet highlights studies of mine waste carried out by USGS scientists at the Geology, Energy & Minerals Science Center in collaboration with other science centers funded through the USGS Mineral Resources Program. Recovery of critical minerals from mine waste can aid in remediation efforts and increase domestic supply of vital mineral resources.

Fact Sheet↗

USGS1978

Maps showing mineralogical data for nonmagnetic heavy-mineral concentrates in the Talkeetna Quadrangle, Alaska

Reconnaissance geochemical and mineralogical sampling was done in the Talkeetna Quadrangle during 1975 and 1976 as part of the Alaska Mineral Resource Assessment Program (AMRAP). These maps show the distribution of gold, scheelite, chalcopyrite, arsenopyrite, galena, fluorite, cinnabar, and malachite in the nonmagnetic fraction of heavy-mineral concentrates. Heavy-mineral concentrate samples were collected at 812 sites from active stream channels. The heavy-mineral concentrates were obtained by panning stream sediment in the field to remove most of the light minerals. The panned samples were then sieved through a 20-mesh (0.8 mm) sieve in the laboratory, and the minus-20-mesh fraction was further separated with bromoform (specific gravity, 2.86) to remove any remaining light-mineral grains. Magnetite and other strongly magnetic heavy minerals were removed from the heavy-mineral fraction by use of a hand magnet. The remaining sample was passed through a Frantz Isodynamic Separator and a nonmagnetic fraction was examined for its mineralogical content with the aid of a binocular microscope and an x-ray diffractometer. The nonmagnetic concentrates primarily contain phyllite fragments, muscovite, sphene, zircon, apatite, tourmaline, rutile, and anatase. Most ore and ore-related minerals also occur in this fraction.

Alaska↗

USGS1978

Mineralogical map showing the distribution and abundance of gold, scheelite, chalcopyrite, arsenopyrite, minium, and sapphire corundum in heavy-mineral concentrates in the Seward and Blying Sound quadrangles, Alaska

Reconnaissance geochemical and mineralogical sampling was done in the Seward and Blying Sound quadrangles during 1975 and 1976 as part of the Alaska Mineral Resources Assessment Program (AMRAP). These maps show the distribution and abundance of gold, scheelite, chalcopyrite, arsenopyrite, minium and sapphire corundum in heavy-mineral concentrates. Heavy-mineral concentrate samples were collected at 525 sites from active channels and, locally, from the interface of streambeds with intermediate- to low-tide beaches. The heavy-mineral concentrates were obtained by panning stream sediments in the field to remove most of the light minerals. The panned samples were sieved though a 20-mesh (0.8 mm) screen in the laboratory, and the minus-20 mesh fraction was further separated with bromoform (specific gravity: 2.86) to remove any remaining light-mineral grains. Magnetite and other strongly magnetic heavy minerals were removed from the heavy-mineral fraction by using a hand magnet. The remaining sample was passed through a Frantz Isodynamic Separator 1 and a nonmagnetic fraction was obtained at a setting of 0.6 amperes. A split of this nonmagnetic fraction was examined for its mineralogic content using a binocular microscope and X-ray diffraction. The nonmagnetic concentrates primarily contain muscovite, sphene, zircon, apatite, rutile, and anatase. Small amounts of other minerals such as gold, scheelite, minium, sapphire corundum, and most sulfides will also be found in this fraction. 1 The use of trade names is for descriptive purposes only and does not constitute endorsement of those products by the U.S. Geological Survey.

Alaska↗

USGS2012

Lineament analysis of mineral areas of interest in Afghanistan

During a preliminary mineral resource assessment of Afghanistan (Peters and others, 2007), 24 mineralized areas of interest (AOIs) were highlighted as the focus for future economic development throughout various parts of the country. In addition to located mineral resources of value, development of a viable mining industry in Afghanistan will require the location of suitable groundwater resources for drinking, processing of mineral ores for use or for export, and for agriculture and food production in areas surrounding and supporting future mining enterprises. This report and accompanying GIS datasets describe the results of both automated and manual mapping of lineaments throughout the 24 mineral occurrence AOIs described in detail by Peters and others (2007; 2011). For this study, we define lineaments as "mappable linear or curvilinear features of a surface whose parts align in a straight or slightly curving relationship that may be the expression of a fault or other linear zones of weakness" as derived from remote sensing sources such as optical imagery, radar imagery or digital elevation models (DEMs) (Sabins, 2007). Water wells in bedrock aquifers are generally more productive where boreholes intersect fractures or fracture zones. Lineament identification and analysis have long been used as a reconnaissance tool to identify such favorable conditions for groundwater resources in carbonate bedrock environments (Lattman and Parizek, 1964; Siddiqui and Parizek, 1971). More recently, lineament analysis has been used to identify areas of greater well yields in other bedrock settings, such as crystalline bedrock (Mabee and other, 1994; Moore and others, 2002). Lineaments provide an indication of bedrock areas that warrant further investigation for optimal water well placement. They may also indicate areas of preferential flow and storage of groundwater, and, thus, areas with a greater density of lineaments may indicate greater secondary porosity. Lineaments may indicate structurally trending mineralized areas (for example, Mars and Rowan, 2007), or locations of near-surface water resources, especially when surface vegetation growth coincides with lineaments. The purpose of this report and accompanying GIS data is to provide lineament maps that give one indication of areas that warrant further investigation for optimal bedrock water-well placement within 24 target areas for mineral resources (Peters and others, 2011). These data may also support the identification of faults related to modern seismic hazards (for example, Wheeler and others, 2005; Ruleman and others, 2007), as well as support studies attempting to understand the relationship between tectonic and structural controls on hydrothermal fluid flow, subsequent mineralization, and water-quality issues near mined and unmined mineral deposits (for example, Eppinger and others, 2007).

Open-File Report↗

USGS2015

Results of mineral, chemical, and sulfate isotopic analyses of water, soil, rocks, and soil extracts from the Pariette Draw Watershed, Uinta Basin, Utah

In 2010, Utah Department of Environmental Quality (DEQ) Division of Water Quality (UDWQ, 2010) determined that water quality in Pariette Draw was in violation of Federal and State water quality criteria for total dissolved solids (TDS), selenium (Se), and boron (B). The measure of total dissolved solids is the sum of all the major ion concentrations in solution and in this case, the dominant ions are sodium (Na) and sulfate (SO4), which can form salts like thenardite (Na2SO4) and mirabilite (Na2SO4⋅H2O). The Utah Department of Environmental Quality (2010) classified the contamination as natural background and from nonpoint sources related to regional lithology and irrigation practices. Although the daily loads of the constituents of concern and water chemistry have been characterized for parts of the watershed, little is known about the controls that bedrock and soil mineralogy have on salt, Se, and B storage and the water-rock interactions that influence the mobility of these components in ground and surface waters. Studies in the Uncompahgre River watershed in Colorado by Tuttle and others (2014a, 2014b) show that salt derived from weathering of shale in a semiarid climate is stored in a variety of minerals that contribute solutes to runoff and surface waters based on a complex set of conditions such as water availability, geomorphic position (for example, topography controls the depth of salt accumulation in soils), water-table fluctuations, redox conditions, mineral dissolution kinetics, ion-exchange reactions, and secondary mineral formation. Elements like Se and B commonly reside in soluble salt phases, so knowledge of the behavior of salt minerals also sheds light on the behavior of associated contaminants. The goal of this study was to establish a process-based understanding of salt, Se, and B behavior to address whether these contaminants can be better managed, or if uncontrollable natural processes will overwhelm any attempts to bring Pariette Draw into compliance with respect to recently established total maximum daily limits (TMDLs). We collected data to refine our knowledge about the role of rock weathering and soil formation in the transport and storage of salt in the watershed and to show how salt is cycled under irrigated and natural conditions. Our approach was to sample rock, soils, and sediment on irrigated and natural terrain for mineralogical analysis to determine the residence of salt and associated Se and B, classify minerals as primary (related to rock formation) or secondary weathering products, and characterize mineral dissolution kinetics. Mineral and chemical analyses and selective extractions of rocks and soils provide useful information in understanding solute movement and mineral dissolution/ formation. The resulting data are critical in determining residence of salt, Se, and B in weathered rock and soil and understanding the mobility during water-rock-soil interactions. This report summarizes our methods for sample and data collection and tabulates the mineral, chemical, and isotopic data collected.

Utah↗

USGS1976

Laboratory manual: mineral X-ray diffraction data retrieval/plot computer program

The Mineral X-Ray Diffraction Data Retrieval/Plot Computer Program--XRDPLT (VanTrump and Hauff, 1976a) is used to retrieve and plot mineral X-ray diffraction data. The program operates on a file of mineral powder diffraction data (VanTrump and Hauff, 1976b) which contains two-theta or 'd' values, and intensities, chemical formula, mineral name, identification number, and mineral group code. XRDPLT is a machine-independent Fortran program which operates in time-sharing mode on a DEC System i0 computer and the Gerber plotter (Evenden, 1974). The program prompts the user to respond from a time-sharing terminal in a conversational format with the required input information. The program offers two major options: retrieval only; retrieval and plot. The first option retrieves mineral names, formulas, and groups from the file by identification number, by the mineral group code (a classification by chemistry or structure), or by searches based on the formula components. For example, it enables the user to search for minerals by major groups (i.e., feldspars, micas, amphiboles, oxides, phosphates, carbonates) by elemental composition (i.e., Fe, Cu, AI, Zn), or by a combination of these (i.e., all copper-bearing arsenates). The second option retrieves as the first, but also plots the retrieved 2-theta and intensity values as diagrammatic X-ray powder patterns on mylar sheets or overlays. These plots can be made using scale combinations compatible with chart recorder diffractograms and 114.59 mm powder camera films. The overlays are then used to separate or sieve out unrelated minerals until unknowns are matched and identified.

Open-File Report↗

USGS1983

Application of total-count aeroradiometric maps to the exploration for heavy-mineral deposits in the coastal plain of Virginia, with a section on field-spectrometer-data reduction

Total-count contoured aeroradiometric maps for the Coastal Plain of Virginia were used in an effort to locate economic heavy-mineral placer deposits. The principle behind this approach is that heavy- mineral suites commonly contain radioactive minerals that, if the concentration of heavy minerals is exposed at or within inches of the surface, enable the deposit to be located by use of airborne instruments because of its radiometric contrast with the host sediment. Detailed and regional geologic maps, soil maps, land-use and land- cover maps, information on fertilizer use, and ground-spectrometer data were used to study aeroradiometric anomalies for efficient exploration. Aeroradiometric anomalies in the Coastal Plain of Virginia have three general causes. First, the most intense anomalies are associated with cultural features, such as roads made of granitic material. Second, most anomalies of high to intermediate intensity are associated with land used for agricultural purposes and evidently are caused by applications of radioactive fertilizer. Third, anomalies of intermediate to low intensity are associated with heavy-mineral deposits. Results of this study show that aeroradiometric anomalies associated with heavy-mineral accumulations in the Coastal Plain of Virginia have ground radiometric spectra in which thorium is the strongest component and uranium and potassium are lesser components. Heavy-mineral accumulations found in this study by use of the aeroradiometric data are not considered to be of economic importance, mostly because of the low percentage of economic minerals in the heavy-mineral suites and also because of other factors such as the very fine grained nature of the host sediments and competing land use.

Virginia↗

USGS2009

Mesozoic magmatism and base-metal mineralization in the Fortymile mining district, eastern Alaska — Initial results of petrographic, geochemical, and isotopic studies in the Mount Veta area

We present here the initial results of a petrographic, geochemical, and isotopic study of Mesozoic intrusive rocks and spatially associated Zn-Pb-Ag-Cu-Au prospects in the Fortymile mining district in the southern Eagle quadrangle, Alaska. Analyzed samples include mineralized and unmineralized drill core from 2006 and 2007 exploration by Full Metal Minerals, USA, Inc., at the Little Whiteman (LWM) and Fish prospects, and other mineralized and plutonic samples collected within the mining district is part of the USGS study. Three new ion microprobe U-Pb zircon ages are: 210 ± 3 Ma for quartz diorite from LWM, 187 ± 3 Ma for quartz monzonite from Fish, and 70.5 ± 1.1 Ma for altered rhyolite porphyry from Fish. We also present 11 published and unpublished Mesozoic thermal ionization mass spectrometric U-Pb zircon and titanite ages and whole-rock geochemical data for the Mesozoic plutonic rocks. Late Triassic and Early Jurassic plutons generally have intermediate compositions and are slightly foliated, consistent with synkinematic intrusion. Several Early Jurassic plutons contain magmatic epidote, indicating emplacement of the host plutons at mesozonal crustal depths of greater than 15 km. Trace-element geochemical data indicate an arc origin for the granitoids, with an increase in the crustal component with time. Preliminary study of drill core from the LWM Zn-Pb-Cu-Ag prospect supports a carbonate-replacement model of mineralization. LWM massive sulfides consist of sphalerite, galena, and minor pyrite and chalcopyrite, in a gangue of calcite and lesser quartz; silver resides in Sb-As-Ag sulfosalts and pyrargyrite, and probably in submicroscopic inclusions within galena. Whole-rock analyses of LWM drill cores also show elevated In, an important metal in high-technology products. Hypogene mineralized rocks at Fish, below the secondary Zn-rich zone, are associated with a carbonate host and also may be of replacement origin, or alternatively, may be a magnetite-bearing Zn skarn. Cu-Zn-Pb-Ag-Au showings at the Oscar pros-pect occur in marble-hosted magnetite and pyrrhotite skarn that is spatially related to the stocks, dikes, and sills of the Early Jurassic syenite of Mount Veta. Mineralized rocks at the Eva Creek Ag-Zn-Pb-Cu prospect are within 1.5 km of the Mount Veta pluton, which is epidotized and locally altered along its contact with metamorphosed country rock east of the prospect. We report five new sulfide Pb-isotopic analyses from the LWM, Oscar, and Eva Creek prospects and compare these sulfide Pb-isotopic ratios with those for sulfides from nearby deposits and prospects in the Yukon-Tanana Upland and with feldspar Pb-isotopic ratios for Mesozoic plutons in the region. Disparities between the Pb-isotopic ratios for sulfides and igneous feldspars are consistent with a carbonate-replacement model for both the LWM and Eva Creek prospects. The presence in the Fortymile district of base-metal sulfides within both calc-silicate-rich skarns and the calc-silicate-free carbonate replacement deposits may reflect multistage mineralization by magmatic-hydrothermal systems during the emplacement of two or more magmatically unrelated igneous intrusions. Alternatively, all of the mineralized occurrences could be products of one regionally zoned system that formed during the intrusion of a single pluton. In addition to the likely origin of some of the base-metal occurrences by intrusion-related hydrothermal fluids, proximity of the LWM prospect to the northeast-striking, high-angle Kechumstuk Fault suggests that fluid flow along the fault also played an important role during carbonate-replacement mineralization.

Alaska↗

USGS1973

United States mineral resources

The work on this volume began in January 1972, but in a broader sense its production began many years ago. The chapters were written by geologists most of whom have had many years of experience studying the geology of mineral deposits, and more particularly the commodities about which they have written here. A total of nearly 2,300 man-years of professional experience in the geology of mineral resources is represented by the authors of the volume, and about 30 man-years went directly into its preparation. Each chapter contains not only a synthesis of the state of knowledge of the geology of the commodity, but also an appraisal of the known resources, and an examination of the geologic possibilities for finding additional deposits. In January 1972, responsibility for the preparation of the volume was assigned to us as co-editors, and we were given a tentative list of commodities and authors. We provided each author with a suggested outline of general topics to be covered, and some guidelines as to scope and philosophy of approach, but beyond that we avoided any attempt to fit each chapter into a stereotype. Moreover, the types of commodities range from the major metals and industrial minerals such as copper, silver, and fluorspar, which have been the subject of geologic research for years, to other commodities that are of such varied geologic nature (such as pigments or gemstones) or of such minor present importance (such as scandium or thallium) that they cannot be treated from the same viewpoint as the major minerals. The chapters range, therefore, from comprehensive summary reports to general essays that reflect the individuality of the authors as well as the variation among commodities. Throughout the book the emphasis is on geology, but each chapter contains some summary information on uses, technology, and economics. These summaries are not meant to be exhaustive, however, and additional details are in the 1970 edition of "Mineral Facts and Problems" (Bulletin 650 of the U.S. Bureau of Mines) ; indeed, we regard that book and the present volume as being complementary. In the examination of the geologic possibilities for finding new deposits-in many respects the principal innovative contributions of this volume-we asked the authors to frankly apply the limits of their ingenuity and not only to summarize current theories but also to express their own intuitive ideas, however speculative and unconventional they may seem, that have come from years of study devoted to the origin of mineral deposits. Readers will see that some authors have speculated more courageously than others. In any case, we believe readers will find all the chapters interesting, and many stimulating; and a few we believe can be frankly characterized as intellectually exciting. Most chapters include a section on prospecting techniques, and a summary of geologic or related problems on which the authors believe research might be most fruitful in the continuing efforts to find new resources. An integral part of the book is the bibliographic material cited at the conclusion of each chapter, in lieu of repetition of detailed descriptions already in print. Index and "spot" maps are not included in most chapters because they are available elsewhere, and in many cases with more detail than could possibly be included here. Maps showing the distribution of known deposits of many commodities in the United States are available in the Mineral Resource (MR) map series of the U.S. Geological Survey and in the National Atlas of the United States. The first three chapters deal not with resources of specific commodities but with general information that is pertinent to the study of mineral resources. In the introductory chapter we discuss the purposes of the book, the distinctions between reserves and various categories of resources, and some general conclusions drawn from our view of the book in its entirety. In the second chapter V. E. McKelvey discusses the problems of mineral-resource estimates and public policy. In the third chapter, R. L. Erickson discusses some new points of view on the relation of reserves and resources to the crustal abundance of elements. We acknowledge with thanks the cooperation of our colleagues in all phases of the preparation of this volume. Whatever success the book may attain is due entirely to a total effort. A paragraph of acknowledgment originally submitted as a part of the chapter on "Nuclear fuels" is given here instead because we feel it applied to all chapters: "The writers have drawn freely from published information, not all of which is cited, and from their colleagues, none of whom are given specific credit. The reader should be aware that the paper could not have been written without these sources." We extend specific thanks to Michael Fleischer for preparation of summaries of geochemical information that are included in many chapters.

Professional Paper↗

USGS2023

Timing of rhyolite intrusion and Carlin-type gold mineralization at the Cortez Hills Carlin-type deposit, Nevada, USA

Carlin-type gold deposits (CTDs) of Nevada are the largest producers of gold in the United States, a leader in world gold production. Although much has been resolved about the characteristics and origin of CTDs in Nevada, major questions remain, especially about (1) the role of magmatism, whether only a source of heat or also metals, (2) whether CTDs only formed in the Eocene, and (3) whether pre-Eocene metal concentrations contributed to Eocene deposits. These issues are exemplified by the CTDs of the Cortez region, the second largest concentration of these deposits after the Carlin trend. Carlin-type deposits are notoriously difficult to date because they rarely generate dateable minerals. An age can be inferred from crosscutting relationships with dated dikes and other intrusions, which we have done for the giant Cortez Hills CTD. What we term “Cortez rhyolites” consist of two petrographic-geochemical groups of siliceous dikes: (1) quartz-sanidine-plagioclase-biotite-phyric, high-SiO 2 rhyolites emplaced at 35.7 Ma based on numerous 40 Ar/ 39 Ar dates and (2) plagioclase-biotite-quartz ± hornblende-phyric, low-SiO 2 rhyolites, which probably were emplaced at the same time but possibly as early as ~36.2 Ma. The dikes form a NNW-trending belt that is ~6 to 10 km wide × 40 km long and centered on the Cortez Hills deposit, and they require an underlying felsic pluton that fed the dikes. Whether these dikes pre- or postdated mineralization has been long debated. We show that dike emplacement spanned the time of mineralization. Many of both high- and low-SiO 2 dikes are altered and mineralized, although none constitute ore. In altered-mineralized dikes, plagioclase has been replaced by kaolinite and calcite, and biotite by smectite, calcite, and marcasite. Sanidine is unaltered except in a few samples that are completely altered to quartz and kaolinite. Sulfides present in mineralized dikes are marcasite, pyrite, arsenopyrite, and As-Sb–bearing pyrite. Mineralized dikes are moderately enriched in characteristic Carlin-type elements (Au, Hg, Sb, Tl, As, and S), as well as elements found in some CTDs (Ag, Bi, Cu, Mo), and variably depleted in MgO, CaO, Na 2 O, K 2 O, MnO, Rb, Sr, and Ba. In contrast, some high-SiO 2 rhyolites are unaltered and cut high-grade ore, which shows that they are post-ore. Both mineralized and post-ore dikes have indistinguishable sanidine 40 Ar/ 39 Ar dates. These characteristics, along with published interpretations that other giant CTDs formed in a few tens of thousands of years, indicate the Cortez Hills CTD formed at 35.7 Ma. All Cortez-area CTDs are in or adjacent to the Cortez rhyolite dike swarm, which suggests that the felsic pluton that fed the dikes was the hydrothermal heat source. Minor differences in alteration and geochemistry between dikes and typical Paleozoic sedimentary rock-hosted ore probably reflect low permeability and low reactivity of the predominantly quartzofeldspathic dikes. Despite widespread pre-35.7 Ma mineralization in the Cortez region, including deposits near several CTDs, we find no evidence that older deposits or Paleozoic basinal rocks contributed metals to Cortez-area CTDs. Combining our new information about the age of Cortez Hills with published and our dates on other CTDs demonstrates that CTD formation coincided with the southwestern migration of magmatism across Nevada, supporting a genetic relationship to Eocene magmatism. CTDs are best developed where deep-seated (~6–8 km), probably granitic plutons, expressed in deposits only as dikes, established large, convective hydrothermal systems.

Nevada↗