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At least 829 records · Page 46Linked to original sources
USGS2005

Iowa magnetic and gravity maps and data: A web site for distribution of data

Magnetic anomalies are due to variations in the Earth's magnetic field caused by the uneven distribution of magnetic minerals (primarily magnetite) in the rocks that make up the upper part of the Earth's crust. The features and patterns of the magnetic anomalies can be used to delineate details of subsurface geology, including the locations of buried faults and magnetite-bearing rocks and the depth to the base of sedimentary basins. This information is valuable for mineral exploration, geologic mapping, and environmental studies. The Iowa magnetic map is constructed from grids that combine information collected in nine separate magnetic surveys conducted between 1953 and 1972. The data from these surveys are of varying quality. The design and specifications (terrain clearance, sampling rates, line spacing, and reduction procedures) varied from survey to survey depending on the purpose of the project and the technology of that time. Every attempt was made to acquire the data in digital form. All survey grids have been continued to 305 m (1,000 ft) above ground and merged together to form the State compilation.

Iowa↗
USGS2021

Groundwater flow conceptualization of the Pahute Mesa–Oasis Valley Groundwater Basin, Nevada—A synthesis of geologic, hydrologic, hydraulic-property, and tritium data

This report provides a groundwater-flow conceptualization that integrates geologic, hydrologic, hydraulic-property, and radionuclide data in the Pahute Mesa–Oasis Valley (PMOV) groundwater basin, southern Nevada. Groundwater flow in the PMOV basin is of interest because 82 underground nuclear tests were detonated, most near or below the water table. A potentiometric map and nine sets of hydrostratigraphic and hydrologic cross sections supplement the conceptualization. Potentiometric contours indicate that groundwater in the PMOV basin generally flows south-southwest and discharges at Oasis Valley. Groundwater encounters an alternating sequence of low- and high-transmissivity rocks, referred to as dams and pools, respectively, as it moves from east to west across eastern Pahute Mesa. Flow from all Pahute Mesa nuclear tests is to Oasis Valley and is well-constrained by water-level data. Flow converges along a corridor of high transmissivity between Pahute Mesa and Oasis Valley. The location of the lateral PMOV basin boundary is well defined, and this boundary, with a few minor exceptions, represents a no-flow boundary. Some boundary uncertainty exists in the northeastern part of the basin, but potential flow-rate estimates across the northeastern boundary resulting from this uncertainty are small relative to the basin groundwater budget. Recharge in the PMOV basin is derived from episodic pulses of modern water and the diffuse percolation of old water (greater than 1,000 years). Episodic recharge is a minor recharge component observed as a rise in groundwater levels that occurs 3 months to 1 year following a wet winter. Minor amounts of episodic recharge through an unsaturated zone in excess of 1,000 feet (ft) requires preferential flow through faults and fractures. The dominant recharge component is slow, steady, diffuse percolation of old water through the unsaturated zone. A large component of old water recharging the groundwater system is consistent with observations of isotopically light deuterium and oxygen 18 compositions in water from wells on Pahute Mesa and central Oasis Valley. About half the recharge in the PMOV basin is derived from the eastern Pahute Mesa area. The remaining recharge is derived primarily from other highland areas including Timber Mountain, Belted and Kawich Ranges, and Black Mountain. The PMOV groundwater system is nearly steady state, where recharge is balanced by the 5,900 acre-feet per year of natural discharge at Oasis Valley. This assumption is reasonable because the basin is dominated by steady-state conditions, where long-term changes in groundwater storage are minimal. Total groundwater withdrawals from 1963 to 2018 have amounted to less than 10 percent of annual groundwater discharge and less than 0.2 percent of the basin’s groundwater storage. Therefore, present-day (2020) conditions are considered representative of predevelopment (pre-1950) conditions in nearly all areas of the basin. The lower PMOV basin boundary is defined at 4,000 ft below the water table to encompass all underground nuclear tests and tritium plumes. This boundary defines the lower boundary of radionuclide migration. However, nearly all flow and tritium transport occur in the upper 1,600 ft of the saturated zone because a transmissivity-with-depth relation indicates that greater than 90 percent of the transmissivity contributing to groundwater flow occurs within 1,600 ft of the water table. Rocks at deeper depths have low transmissivity because argillic and mineralized alterations plug the fractures. Volcanic rocks form the primary aquifers and confining units in the PMOV basin. Volcanic hydrogeologic units (HGUs) and hydrostratigraphic units (HSUs) have transmissivity distributions that span up to eight orders of magnitude with considerable overlap between distributions. Despite the large overlap between units, mean transmissivities of aquifers are one-to-two orders of magnitude greater than the confining units. However, all volcanic-rock HGUs and HSUs are composite units, meaning that they can function spatially as either an aquifer or confining unit .

Nevada↗
USGS2016

Geochemistry, Nd-Pb Isotopes, and Pb-Pb Ages of the Mesoproterozoic Pea Ridge Iron Oxide-Apatite–Rare Earth Element Deposit, Southeast Missouri

Iron oxide-apatite and iron oxide-copper-gold deposits occur within ~1.48 to 1.47 Ga volcanic rocks of the St. Francois Mountains terrane near a regional boundary separating crustal blocks having contrasting depleted-mantle Sm-Nd model ages (T DM ). Major and trace element analyses and Nd and Pb isotope data were obtained to characterize the Pea Ridge deposit, improve identification of exploration targets, and better understand the regional distribution of mineralization with respect to crustal blocks. The Pea Ridge deposit is spatially associated with felsic volcanic rocks and plutons. Mafic to intermediate-composition rocks are volumetrically minor. Data for major element variations are commonly scattered and strongly suggest element mobility. Ratios of relatively immobile elements indicate that the felsic rocks are evolved subalkaline dacite and rhyolite; the mafic rocks are basalt to basaltic andesite. Granites and rhyolites display geochemical features typical of rocks produced by subduction. Rare earth element (REE) variations for the rhyolites are diagnostic of rocks affected by hydrothermal alteration and associated REE mineralization. The magnetite-rich rocks and REE-rich breccias show similar REE and mantle-normalized trace element patterns. Nd isotope compositions (age corrected) show that: (1) host rhyolites have ɛ Nd from 3.44 to 4.25 and T DM from 1.51 to 1.59 Ga; (2) magnetite ore and specular hematite rocks display ɛ Nd from 3.04 to 4.21 and T DM from 1.6 to 1.51 Ga, and ɛ Nd from 2.23 to 2.81, respectively; (3) REE-rich breccias have ɛ Nd from 3.04 to 4.11 and T DM from 1.6 to 1.51 Ga; and (4) mafic to intermediate-composition rocks range in ɛ Nd from 2.35 to 3.66 and in T DM from 1.66 to 1.56. The ɛ Nd values of the magnetite and specular hematite samples show that the REE mineralization is magmatic; no evidence exists for major overprinting by younger, crustal meteoric fluids, or by externally derived Nd. Host rocks, breccias, and magnetite ore shared a common origin from a similar source. Lead isotope ratios are diverse: (1) host rhyolite has 206 Pb/ 204 Pb from 24.261 to 50.091; (2) Pea Ridge and regional galenas have 206 Pb/ 204 Pb from 16.030 to 33.548; (3) REE-rich breccia, magnetite ore, and specular hematite rock are more radiogenic than galena; (4) REE-rich breccias have high 206 Pb/ 204 Pb (38.122–1277.61) compared to host rhyolites; and (5) REE-rich breccias are more radiogenic than magnetite ore and specular-hematite rock, having 206 Pb/ 204 Pb up to 230.65. Radiogenic 207 Pb/ 206 Pb age estimates suggest the following: (1) rhyolitic host rocks have ages of ~1.50 Ga, (2) magnetite ore is ~1.44 Ga, and (3) REE-rich breccias are ~1.48 Ga. These estimates are broadly consistent and genetically link the host rhyolite, REE-rich breccia, and magnetite ore as being contemporaneous. Alteration style and mineralogical or textural distinctions among the magnetite-rich rocks and REE-rich breccias do not correlate with different isotopic sources. In our model, magmatic fluids leached metals from the coeval felsic rocks (rhyolites), which provided the metal source reflected in the compositions of the REE-rich breccias and mineralized rocks. This model allows for the likelihood of contributions from other genetically related felsic and intermediate to more mafic rocks stored deeper in the crust. The deposit thus records an origin as a magmatic-hydrothermal system that was not affected by Nd and Pb remobilization processes, particularly if these processes also triggered mixing with externally sourced metal-bearing fluids. The Pea Ridge deposit was part of a single, widespread, homogeneous mixing system that produced a uniform isotopic composition, thus representing an excellent example of an igneous-dominated system that generated coeval magmatism and REE mineralization. Geochemical features suggest that components in the Pea Ridge deposit originated from sources in an orogenic margin. Basaltic magmatism produced by mantle decompression melting provided heat for extracting melts from the middle or lower crust. Continual addition of mafic magmas to the base of the subcontinental lithosphere, in a back-arc setting, remelted calc-alkaline rocks enriched in metals that were stored in the crust. The St. Francois Mountains terrane is adjacent to the regional T DM line (defined at a value of 1.55 Ga) that separates ~1600 Ma basement to the west, from younger basements to the east. Data for Pea Ridge straddle the T DM values proposed for the line. The Sm-Nd isotope system has been closed since formation of the deposit and the original igneous signatures have not been affected by cycles of alteration or superimposed mineralizing events. No evidence exists for externally derived Nd or Sm. The source region for metals within the Pea Ridge deposit had a moderate compositional variation and the REE-rich breccias and mineralized rocks are generally isotopically homogeneous. The Pea Ridge deposit thus constitutes a distinctive isotopic target for use as a model in identifying other mineralized systems that may share the same metal source in the St. Francois Mountains terrane and elsewhere in the eastern Granite-Rhyolite province.

Missouri↗
USGS1973

Aspects of the magmatic geochemistry of bismuth

Bismuth has been determined in 74 rocks from a differentiated tholeiitic dolerite, two calc-alkaline batholith suites and in 66 mineral separates from one of the batholiths. Average bismuth contents, weighted for rock type, of the Great Lake (Tasmania) dolerite, the Southern California batholith and the Idaho batholith are, 32, 50 and 70 ppb respectively. All three bodies demonstrate an enrichment of bismuth in residual magmas with magmatic differentiation. Bismuth is greatly enriched (relative to the host rock) in the calcium-rich accessory minerals, apatite and sphene, but other mineral analyses show that a Bi-Ca association is of little significance to the magmatic geochemistry of bismuth. Most of the bismuth, in the Southern California batholith at least, occurs in a trace mineral phase (possibly sulfides) present as inclusions in the rock-forming minerals.

Geochimica et Cosmochimica Acta↗
USGS2003

P-T-t paths and differential Alleghanian loading and uplift of the Bronson Hill terrane south central New England

Late Paleozoic U-Pb ages of sphene and 40 Ar/ 39 Ar cooling ages of amphibole and muscovite from rocks of the Bronson Hill terrane in Connecticut and central Massachusetts reflect a late Paleozoic (Alleghanian) overprint on Acadian metamorphic rocks. Prograde Alleghanian sphenes crystallized during the Late Pennsylvanian, and eliminate the possibility that amphibole ages reflect delayed Permian cooling from Devonian Acadian metamorphism. Fourteen new amphibole ages from Connecticut form a north-to-south trend of decreasing age from 294 to 245 Ma, while in Massachusetts four new amphibole ages together with three others from the literature produce a random Carboniferous pattern. Seven new muscovite ages support existing data indicating uniform cooling throughout the Bronson Hill terrane through ∼350°C in the Early Triassic. The rate of Permian cooling defined by amphibole-muscovite pairs increases from ∼4°C/my in northern Connecticut to ∼50°C/my near Long Island Sound. Hinged loading and hinged but delayed exhumation in the southern part of the Bronson Hill terrane (with the hinge in central Connecticut) explain these ages and cooling rates as well as a southerly increasing metamorphic field gradient. One-dimensional thermal modeling indicates that loading of Bronson Hill rocks must have begun by the Late Mississippian. The time of peak Alleghanian metamorphic temperature decreases southward from Early Permian in northern Connecticut to Late Permian to the south. These results demonstrate that the metamorphic effects of the Alleghanian orogeny are not restricted to the Avalon terrane of southeastern New England. On the contrary, the Alleghanian orogeny reset 40 Ar/ 39 Ar mineral ages, recrystallized minerals, partially melted felsic rocks, and transposed fabrics at least as far west as the Bronson Hill terrane in south-central New England.

American Journal of Science↗
USGS2003

A special issue devoted to gold deposits in northern Nevada: Part 2. Carlin-type Deposits

This is the second of two special issues of Economic Geology devoted to gold deposits in northern Nevada. Readers interested in a general overview of these deposits, their economic significance, their context within the tectonic evolution of the region, and synoptic references on each gold deposit type are directed to the preface of the first special issue (John et al., 2003). Volume 98, issue 2, contains five papers that address regional aspects important to the genesis of gold deposits in northern Nevada and five papers that present detailed studies of epithermal deposits and districts. All of the regional papers are pertinent to Carlin-type gold deposits, because they address the age of mineralization (Arehart et al., 2003), origin and evolutionary history of the northwest-striking mineral belts that localize many deposits (Grauch et al., 2003), nature of the middle and lower crust below these mineral belts (Howard, 2003), district- to deposit-scale stream sediment and lithogeochemical anomalies (Theodore et al., 2003), and stratigraphy and structure of a district located along a northeast-striking lineament (Peters et al., 2003). The nine papers in this second special issue focus on an array of problems pertinent to genetic and exploration models for Carlin-type deposits in northern Nevada (Fig. 1). These investigations sort out and characterize the sequence of deformational, igneous, and hydrothermal events in mining districts, constrain the age of mineralization, map paleothermal gradients, identify structures and lithologies that are preferentially mineralized, ascertain processes of ore formation, determine sources of ore fluid components, and define fluid flow paths. A common theme among these papers is inheritance, whereby older features in the mineral belts influence ore formation in subsequent Carlin-type hydrothermal systems. Three types of inheritance are inferred by one or more of these investigations: (1) structural inheritance, where older faults are reactivated during subsequent contractional and/or extensional tectonic events producing permeable fracture systems that focused flow of ore fluids; (2) alteration inheritance, where one or more preore alteration events produced reactive host rocks that are preferentially mineralized; and (3) geochemical inheritance, in which Au and other elements are recycled from older mineralization into younger Carlin-type deposits. Despite the similar age, tectonic setting, alteration types, mineral parageneses, and geochemical signatures of the deposits studied, these papers do not lead to consensus regarding genetic models for Carlin-type deposits. Rather, the separate investigations by different workers, utilizing both similar and unlike approaches, result in markedly different conclusions. Some of this disparity probably is due to real differences in the origin of different districts; however, the opposing conclusions arrived at by investigations on neighboring deposits in a single district are more problematic and most likely are due to difficulties resulting from the superposition of different types and ages of gold mineralization or to substantial variations in the hydrology and proportions of fluid components derived from deep and shallow sources in each deposit. Further work is needed to validate and understand the significance of these differences.

Nevada↗
USGS2022

Indicator mineral analyses of stream-sediment samples using automated mineralogy and mineral chemistry: Applicability to exploration in covered terranes in eastern Alaska, USA

In the past two decades, significant research efforts have been devoted to porphyry copper indicator mineral (PCIM) identification and mineral chemistry to assist exploration. Such studies are important in the Yukon-Tanana upland region of eastern Alaska because well-established geochemical exploration techniques, such as the geochemistry of stream-sediment samples, are less effective in the search for mineral resources owing to cover materials that subdue geochemical signatures. The unglaciated Yukon-Tanana upland region is well-endowed with mineral resources, including porphyry Cu-Mo-Au deposits at Taurus, Bluff, Dennison, and Oreo. Using automated scanning electron microscope (SEM) techniques, numerous PCIMs have been identified in rocks and stream-sediment samples that would likely not have been observed using visual binocular microscope methods. Sulfide minerals such as chalcopyrite, chalcocite, covellite, bornite, molybdenite, and pyrrhotite were identified in rock samples, and more than half of the stream-sediment samples from drainages containing known porphyry mineral occurrences contain chalcopyrite with or without bornite. Many drainages without known occurrences also contain these minerals, which suggests potential for additional occurrences. Svanbergite, an aluminum phosphate sulfate (APS) mineral [SrAl 3 (PO 4 )(SO 4 )(OH) 6 ], was identified in some of the altered or mineralized rock samples from Taurus, and in stream drainages containing porphyry occurrences and may represent the best indicator mineral for Taurus-like porphyry deposits. Apatite chemistry has great potential as a tool for assessing the presence of porphyry mineralization. Grains from rock and stream-sediment samples that contain high Cl (>∼0.4 wt%), Fe (>0.4 wt%), and Mn (>2000 ppm) clearly distinguish apatite from other sources (metamorphic, unmineralized igneous rocks). This study is one of the first to document new streamlined indicator mineral sample collection and processing methods and to combine automated SEM techniques and chemistry of minerals derived from both bedrock and surficial (stream) sediments as they apply to porphyry exploration.

Alaska↗
USGS2009

Stable isotope and petrologic evidence for open-system degassing during the climactic and pre-climactic eruptions of Mt. Mazama, Crater Lake, Oregon

Evaluation of the extent of volatile element recycling in convergent margin volcanism requires delineating likely source(s) of magmatic volatiles through stable isotopic characterization of sulfur, hydrogen and oxygen in erupted tephra with appropriate assessment of modification by degassing. The climactic eruption of Mt. Mazama ejected approximately 50 km 3 of rhyodacitic magma into the atmosphere and resulted in formation of a 10-km diameter caldera now occupied by Crater Lake, Oregon (lat. 43°N, long. 122°W). Isotopic compositions of whole-rocks, matrix glasses and minerals from Mt. Mazama climactic, pre-climactic and postcaldera tephra were determined to identify the likely source(s) of H 2 O and S. Integration of stable isotopic data with petrologic data from melt inclusions has allowed for estimation of pre-eruptive dissolved volatile concentrations and placed constraints on the extent, conditions and style of degassing. Sulfur isotope analyses of climactic rhyodacitic whole rocks yield δ 34 S values of 2.8–14.8‰ with corresponding matrix glass values of 2.4–13.2‰. δ 34 S tends to increase with stratigraphic height through climactic eruptive units, consistent with open-system degassing. Dissolved sulfur concentrations in melt inclusions (MIs) from pre-climactic and climactic rhyodacitic pumices varies from 80 to 330 ppm, with highest concentrations in inclusions with 4.8–5.2 wt% H 2 O (by FTIR). Up to 50% of the initial S may have been lost through pre-eruptive degassing at depths of 4–5 km. Ion microprobe analyses of pyrrhotite in climactic rhyodacitic tephra and andesitic scoria indicate a range in δ 34 S from −0.4‰ to 5.8‰ and from −0.1‰ to 3.5‰, respectively. Initial δ 34 S values of rhyodacitic and andesitic magmas were likely near the mantle value of 0‰. Hydrogen isotope (δD) and total H 2 O analyses of rhyodacitic obsidian (and vitrophyre) from the climactic fall deposit yielded values οf −103 to −53‰ and 0.23–1.74 wt%, respectively. Values of δD and wt% H 2 O of obsidian decrease towards the top of the fall deposit. Samples with depleted δD, and mantle δ 18 O values, have elevated δ 34 S values consistent with open-system degassing. These results imply that more mantle-derived sulfur is degassed to the Earth’s atmosphere/hydrosphere through convergent margin volcanism than previously attributed. Magmatic degassing can modify initial isotopic compositions of sulfur by >14‰ (to δ 34 S values of 14‰ or more here) and hydrogen isotopic compositions by 90‰ (to δD values of −127‰ in this case).

Oregon↗
USGS2016

Paleozoic magmatism and porphyry Cu-mineralization in an evolving tectonic setting in the North Qilian Orogenic Belt, NW China

The NWW-striking North Qilian Orogenic Belt records the Paleozoic accretion–collision processes in NW China, and hosts Paleozoic Cu–Pb–Zn mineralization that was temporally and spatially related to the closure of the Paleo Qilian-Qinling Ocean. The Wangdian Cu deposit is located in the eastern part of the North Qilian Orogenic Belt, NW China. Copper mineralization is spatially associated with an altered early Paleozoic porphyritic granodiorite, which intruded tonalites and volcaniclastic rocks. Alteration zones surrounding the mineralization progress outward from a potassic to a feldspar-destructive phyllic assemblage. Mineralization consists mainly of quartz-sulfide stockworks and disseminated sulfides, with ore minerals chalcopyrite, pyrite, molybdenite, and minor galena and sphalerite. Gangue minerals include quartz, orthoclase, biotite, sericite, and K-feldspar. Zircon LA-ICPMS U–Pb dating of the ore-bearing porphyritic granodiorite yielded a mean 206 Pb/ 238 U age of 444.6 ± 7.8 Ma, with a group of inherited zircons yielding a mean U–Pb age of 485 ± 12 Ma, consistent with the emplacement age (485.3 ± 6.2 Ma) of the barren precursor tonalite. Rhenium and osmium analyses of molybdenite grains returned model ages of 442.9 ± 6.8 Ma and 443.3 ± 6.2 Ma, indicating mineralization was coeval with the emplacement of the host porphyritic granodiorite. Rhenium concentrations in molybdenite (208.9–213.2 ppm) suggest a mantle Re source. The tonalities are medium-K calc-alkaline. They are characterized by enrichment of light rare-earth elements (LREEs) and large-ion lithophile elements (LILEs), depletion of heavy rare-earth elements (HREEs) and high-field-strength elements (HFSEs), and minor negative Eu anomalies. They have ε Hf ( t ) values in the range of +3.6 to +11.1, with two-stage Hf model ages of 0.67–1.13 Ga, suggesting that the ca. 485 Ma barren tonalites were products of arc magmatism incorporating melts from the mantle wedge and the lithosphere. In contrast, the 40-m.y.-younger ore-bearing porphyritic granodiorite is sub-alkaline and peraluminous. They are enriched in LREEs and LILEs, depleted in HFSEs, and show weak negative Eu anomalies. They display ε Hf ( t ) values of captured or inherited zircons in the range of +8.5 to +10.0, and younger two-stage Hf model ages of 0.78 Ga and 0.86 Ga, similar to those of ca. 485 Ma tonalite. The ca. 445 Ma zircons have ε Hf ( t ) values of −2.1 to +9.9, with two-stage Hf model ages of 0.75–1.27 Ga. Moreover, they have relatively high oxygen fugacity than that of the precursor barren tonalite. The ca. 445 Ma magmas at Wangdian thus formed in a subduction setting, and incorporated melts from the subduction-modified lithosphere that had previously been enriched by additions of chalcophile and siderophile element-rich materials by the earlier magmatism and metasomatism during the Paleo Qilian-Qinling Ocean subduction event.

North Qilian Orogenic Belt↗
USGS1969

Distribution of scandium between coexisting biotite and hornblende in igneous rocks

Scandium analyses of more than 90 pairs of coexisting biotite and hornblende from igneous rocks of various provinces (including Southern California, Boulder, Sierra Nevada, Boulder Creek batholiths and the Jemez Mountains volcanic rocks) indicate that the distribution ratio (K d = Sc hornblende /Sc biotite ) for most samples closely approached that of an equilibrium distribution. Median K d values for the igneous samples range from 4.8 to 8.0, which are higher than similar values derived from published data on metamorphic samples and apparently not related to the mode of crystallization (volcanic, hypabyssal, or plutonic). A correlation between Kd and mafic index, (FeO + Fe 2 O 3 )/(FeO + Fe 2 O 3 + MgO) X 100, of both the minerals and the host rock, and between K d and SiO 2 content of the host rock, can be established only for the Southern California batholith samples. However, whether this correlation reflects temperature dependence, compositional dependence, or both, cannot be specified uniquely with present data. The present data also cast doubt on the validity of the so-called “scandium geothermometer.”

GSA Bulletin↗
USGS2013

Porphyry copper assessment of Southeast Asia and Melanesia: Chapter D in Global mineral resource assessment

The U.S. Geological Survey collaborated with member countries of the Coordinating Committee for Geoscience Programmes in East and Southeast Asia (CCOP) on an assessment of the porphyry copper resources of Southeast Asia and Melanesia as part of a global mineral resource assessment. The region hosts world-class porphyry copper deposits and underexplored areas that are likely to contain undiscovered deposits. Examples of known porphyry copper deposits include Batu Hijau and Grasberg in Indonesia; Panguna, Frieda River, and Ok Tedi in Papua New Guinea; and Namosi in Fiji. This assessment covers the countries of Cambodia, Indonesia, Lao People&rsquo;s Democratic Republic, Malaysia, Myanmar, Papua New Guinea, Singapore, Thailand, and parts of southeastern China, India, the Solomon Islands, Vanuatu, and Fiji. Twenty-two geographic areas were delineated as tracts that are permissive for porphyry copper deposits in Southeast Asia. Permissive tracts are grouped into four broadly defined geographic/geologic areas, as follows: (1) the Indochina Peninsula area, (2) Indonesian and Malaysian Islands, (3) New Guinea Island and Papuan New Guinea islands, and (4) Melanesia. Individual tracts range from less than 1,000 to more than 350,000 square kilometers in area. Permissive tracts are based on mapped and inferred subsurface (<1 kilometer depth) distributions of igneous rocks of specific age ranges that define magmatic arcs and magmatic belts that are likely to contain porphyry copper deposits. Most of these magmatic arcs are subduction-related, although some have porphyry-style deposits occurring in postcollisional and (or) poorly understood tectonic settings. Although maps at a variety of different scales were used in the compilation, the final tract boundaries are intended for use at a scale of 1:1,000,000. Global grade and tonnage models for porphyry copper deposits were evaluated. Most of the known deposits are best described as fitting the copper-gold (Cu-Au) subtype of porphyry copper deposit. For some permissive tracts, a general porphyry copper-gold-molybdenum (Cu-Au-Mo) model was used. Assessment participants estimated numbers of undiscovered deposits at different levels of confidence for most of the permissive tracts. These estimates were combined with grade and tonnage models using a Monte Carlo simulation to estimate undiscovered resources. Additional resources in extensions of deposits with identified resources were not evaluated. Assessment results, presented in tables and graphs, show mean amounts of metal and mineralized rock in undiscovered deposits at different quantile levels, as well as the arithmetic mean for each tract. This assessment estimated a mean of 89 undiscovered porphyry copper deposits for the assessed permissive tracts in Southeast Asia and Melanesia. About 288 million metric tons (Mt) of copper and 18,000 metric tons (t) of gold, as well as byproduct molybdenum and silver, could be associated with undiscovered deposits. This represents about four times the number of deposits with identified resources (23) already discovered in Southeast Asia; reliable reported identified resources for those 23 deposits total 84 Mt of copper and 6,000 t of gold. Eleven permissive tracts have no known porphyry copper deposits with reported resources. Three of those 11 tracts lacked sufficient information for a probabilistic assessment and are discussed in qualitative terms. On a regional basis, both the Indochina Peninsula area and the Indonesian-Malaysian Islands area are estimated to contain about 10 times as much in place copper in undiscovered porphyry copper deposits as has been identified to date. For the New Guinea Island areas, the ratio of undiscovered to identified copper resources is about 2. Some parts of the region have a long history of porphyry exploration cycles and mine development, interrupted at times by political and social unrest, environmental concerns, and natural disasters. Changes in mining laws within the region and the recent high price of gold on the world market have prompted renewed interest in porphyry copper deposits in Southeast Asia and Melanesia. However, predicted undiscovered deposits may not be found, and if found, may not be developed. This assessment includes an overview of the assessment results with summary tables. Detailed descriptions of each tract are included in appendixes, with estimates of numbers of undiscovered deposits, and probabilistic estimates of amounts of copper, molybdenum, gold, and silver that could be contained in undiscovered deposits for each permissive tract. A geographic information system (GIS) that accompanies the report includes tract boundaries and a database of known porphyry copper deposits and significant prospects.

Scientific Investigations Report↗
USGS2016

Rare earth element deposits in China

China is the world’s leading rare earth element (REE) producer and hosts a variety of deposit types. Carbonatite- related REE deposits, the most significant deposit type, include two giant deposits presently being mined in China, Bayan Obo and Maoniuping, the first and third largest deposits of this type in the world, respectively. The carbonatite-related deposits host the majority of China’s REE resource and are the primary supplier of the world’s light REE. The REE-bearing clay deposits, or ion adsorption-type deposits, are second in importance and are the main source in China for heavy REE resources. Other REE resources include those within monazite or xenotime placers, beach placers, alkaline granites, pegmatites, and hydrothermal veins, as well as some additional deposit types in which REE are recovered as by-products. Carbonatite-related REE deposits in China occur along craton margins, both in rifts (e.g., Bayan Obo) and in reactivated transpressional margins (e.g., Maoniuping). They comprise those along the northern, eastern, and southern margins of the North China block, and along the western margin of the Yangtze block. Major structural features along the craton margins provide first-order controls for REE-related Proterozoic to Cenozoic carbonatite alkaline complexes; these are emplaced in continental margin rifts or strike-slip faults. The ion adsorption-type REE deposits, mainly situated in the South China block, are genetically linked to the weathering of granite and, less commonly, volcanic rocks and lamprophyres. Indosinian (early Mesozoic) and Yanshanian (late Mesozoic) granites are the most important parent rocks for these REE deposits, although Caledonian (early Paleozoic) granites are also of local importance. The primary REE enrichment is hosted in various mineral phases in the igneous rocks and, during the weathering process, the REE are released and adsorbed by clay minerals in the weathering profile. Currently, these REE-rich clays are primarily mined from open-pit operations in southern China. The complex geologic evolution of China’s Precambrian blocks, particularly the long-term subduction of ocean crust below the North and South China blocks, enabled recycling of REE-rich pelagic sediments into mantle lithosphere. This resulted in the REE-enriched nature of the mantle below the Precambrian cratons, which were reactivated and thus essentially decratonized during various tectonic episodes throughout the Proterozoic and Phanerozoic. Deep fault zones within and along the edges of the blocks, including continental rifts and strike-slip faults, provided pathways for upwelling of mantle material.

Book chapter↗
USGS2012

Two-mica granites of northeastern Nevada

The field settings are described and analytical data are presented for six two-mica granites from north-eastern Nevada. High δ 18 O and 87 Sr/ 86 Sr values indicate that all are S-type granites, derived from continental crust. The major element chemistry and accessory mineral contents of these rocks also are characteristic of S-type granites. Chemical, X ray, and other data are presented for the micas recovered from these granites. The muscovites are notably high in Fe 2 O 3 , FeO, and MgO. Except for one hydrobiotite, each of the biotites has an MgO content near 6.0 weight percent. Two different types of two-mica granites are recognized in the area of this study. One type is distinguished by the presence of many biotite euhedra within muscovite phenocrysts and by an unusual suite of accessory minerals completely devoid of opaque oxides. This type probably resulted from anatexis of late Precambrian argillites under conditions of relatively low oxygen fugacity, along a line that roughly coincides with the westward disappearance of continental basement. In the other textural type of two-mica granite the micas are equigranular and there is a greater variety of accessory minerals. The magmatic evolution of this type also appears to reflect the influence of late Precambrian argillites; there may be age differences between the two types of two-mica granites.

Journal of Geophysical Research Solid Earth↗
USGS1984

An evaluation of the applicability of the telluric-electric and audio-magnetotelluric methods to mineral assessment on the Arabian Shield, Kingdom of Saudi Arabia

Feasibility studies of two electromagnetic methods were made in selected areas of the Jabal Hibshi (1:250,000) quadrangle, 26F, in the Kingdom of Saudi Arabia in March of 1983. The methods tested were the natural source-field telluricelectric and audio-magnetotelluric methods developed and extensively used in recent years by the U.S. Geological Survey in some of its domestic programs related to geothermal and mineral resource assessment. Results from limited studies in the Meshaheed district, the Jabal as Silsilah ring complex, and across a portion of the Raha fault zone clearly demonstrate the appropriateness of these sub-regional scale, reconnaissance-type studies to mineral resource assessment. The favorable results obtained are largely attributed to distinctive and large contrasts in the electrical resistivity of the major rock types encountered. It appears that the predominant controlling factor governing the rock resistivities is the amount of contained clay minerals. Accordingly, unaltered (specifically, non-argillic) igneous and metamorphic rocks have very high resistivities; metasedimentary rocks of the Murdama group that contain several percent clay minerals have intermediate values of resistivity; and highly altered rocks, containing abundant clay minerals, have very low values of resistivity. Water-filled fracture porosity may be a secondary, but important, factor in some settings. However, influences from variations in interstitial or intercrystalline, water-filled porosity are probably small because these types of porosity are generally low. It is reasonable to expect similar results in other areas within the Arabian Shield.

Open-File Report↗
USGS2018

Magmatic origin for sediment-hosted Au deposits, Guizhou Province, China: In situ chemistry and sulfur isotope composition of pyrites, Shuiyindong and Jinfeng deposits

The southwest Guizhou Province, China, contains numerous sediment-hosted Au deposits with Au reserves greater than 700 tonnes. To date, the source of ore fluids that formed the Guizhou sediment-hosted Au deposits is controversial, hampering the formulation of genetic models. In this study, we selected the Shuiyindong and Jinfeng Au deposits, the largest strata-bound and fault-controlled deposits in Guizhou, respectively, for detailed research on pyrite chemistry and S isotope composition using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS), respectively. Petrography and pyrite chemistry studies distinguished five generations of pyrite. Among these, pre-ore pyrite 2 and ore pyrite are the most abundant types in the deposits. Pre-ore pyrite 2 is anhedral to euhedral and with ~2,639 ppm As and wider ranges of Cu, Sb, and Pb (<~22–4,837 ppm, <~6 to 532 ppm, and <~4 to 1,344 ppm, respectively). Gold in pre-ore pyrite 2 is below the detection limit of LA-ICP-MS (~2 ppm). Pre-ore pyrite 2 is interpreted to have a sedimentary (syngenetic or diagenetic) origin. Ore pyrite commonly rims the four identified pre-ore pyrites or occurs as individual, anhedral to euhedral crystals. Ore pyrite is enriched in Au (~641 ppm), As (~9,147 ppm), Cu (~1,043 ppm), Sb (~188 ppm), Hg (~43 ppm), and Tl (~22 ppm) in both deposits. Ore pyrite formed mainly by sulfidation of Fe in Fe-bearing host rocks, mainly Fe dolomite, and As, Cu, Sb, Hg, and Tl, also in ore fluids, were incorporated into ore pyrite. In situ δ 34 S isotope ratios for pre-ore pyrite 2 and ore pyrite were measured by LA-MC-ICP-MS. Pre-ore pyrite 2 from Shuiyindong and Jinfeng deposits resulted in δ 34 S values ranging from −0.8 to +3.4‰ and from 5.1 to 10.5‰, respectively. Analyses of ore pyrite from the Shuiyindong have δ 34 S values that vary from −3.3 to +2.5‰, with a median of 0.7‰; analyses of ore pyrite from the Jinfeng range from 8.9 to 11.2‰, with a median at 10.3‰. Available bulk and in situ δ 34 S data in the literature for pre-ore pyrites 1 and 2 and ore-related sulfide minerals including ore pyrite, arsenopyrite, and late ore-stage stibnite, realgar, orpiment, and cinnabar from several Guizhou sediment-hosted Au deposits were compiled for comparison. Pre-ore-stage pyrites from Guizhou sediment-hosted Au deposits have a broad range of δ 34 S values, from −33.8 to + 17.9‰ (including in situ and available bulk δ 34 S data). Ore-related sulfide minerals in all Guizhou sediment-hosted Au deposits, except Jinfeng, have very similar δ 34 S values, and most data plot between ~−5 and +5‰. In the Jinfeng deposit, the ore-related sulfide minerals exhibit δ 34 S values ranging from 1.9 to 18.1‰, with most data plotting between 6 and 12‰. The broad range of S isotope compositions for the sedimentary pyrites (pre-ore pyrites 1 and 2) indicate that S in these pre-ore pyrites was most likely generated by bacterial reduction from marine sulfate. The narrow range of δ 34 S values (~−5–+5‰) for ore-related sulfide minerals in all Guizhou sediment-hosted Au deposits, excepting the Jinfeng deposit, suggests that the deposits may have formed in response to a single widespread metallogenic event. As the S isotope fractionation between hydrothermal fluids and sulfide minerals in a sulfide-dominated system is small (<2‰) at ~250°C, the initial ore fluids that formed the Guizhou sediment-hosted Au deposits would have had δ 34 S values similar to the ore-related sulfide minerals, between ~−5 and +5‰. At Jinfeng, initial ore fluids may have mixed with local fluids with heavier δ 34 S, possibly basin brine ( δ 34 S basin brine >18‰), resulting in elevated δ 34 S values of ore-related sulfide minerals and especially late ore-stage sulfide minerals. Although few igneous rocks are exposed in the mining area around these deposits, there is evidence of magmatic activity ~20 km away. Furthermore, gravity and magnetic geophysical investigations indicate the presence of a pluton ~5 km below the surface of the Shuiyindong district. Based on in situ S isotope results and recent data indicating proximal intrusions, we interpret a deep magmatic S source for the ore fluids that formed the Guizhou sediment-hosted Au deposits. However, as the age for Au mineralization of Guizhou sediment-hosted Au deposits is still debated, the mineralization-magma connection remains hypothetical. Identifying an ore fluid source and time frame for Guizhou Au mineralization continues to be a critically important research goal for this district.

Yunnan-Guizhou-Guangxi region↗
USGS2004

Magnetic properties experiments on the Mars Exploration Rover Spirit at Gusev Crater

The magnetic properties experiments are designed to help identify the magnetic minerals in the dust and rocks on Mars—and to determine whether liquid water was involved in the formation and alteration of these magnetic minerals. Almost all of the dust particles suspended in the martian atmosphere must contain ferrimagnetic minerals (such as maghemite or magnetite) in an amount of ∼2% by weight. The most magnetic fraction of the dust appears darker than the average dust. Magnetite was detected in the first two rocks ground by Spirit.

Science↗
USGS2008

Limestone - A Crucial and Versatile Industrial Mineral Commodity

Limestone, as used by the minerals industry, is any rock composed mostly of calcium carbonate (CaCO3). Although limestone is common in many parts of the United States, it is critically absent from some. Limestone is used to produce Portland cement, as aggregate in concrete and asphalt, and in an enormous array of other products, making it a truly versatile commodity. Portland cement is essential to the building industry, but despite our Nation's abundance of limestone, there have been cement shortages in recent years. These have been caused in part by a need to find new areas suitable for quarrying operations. To help manage our Nation's resources of such essential mineral commodities, the U.S. Geological Survey (USGS) provides crucial data and scientific information to industry, policymakers, and the public.

Fact Sheet↗