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At least 811 records · Page 45Linked to original sources

USGS2021

Arctic Alaska Basin, Hanna Trough and Beaufortian Rifted Margin Composite Tectono-Sedimentary Elements

The Arctic Alaska Composite Tectono-Sedimentary Element (AA CTSE) as defined for this volume comprises Mississippian to Lower Cretaceous strata beneath the Alaska North Slope and the Beaufort and Chukchi Seas of the Arctic Ocean. The AA CTSE rests on Devonian and older sedimentary and metasedimentary rocks, considered economic basement for petroleum, and is overlain by Cretaceous to Cenozoic syntectonic strata deposited in the foreland of the Chukotka and Brooks Range orogens. The Mississippian – Triassic part of the AA CTSE is divided into a fold-and-thrust belt in the south and a relatively undeformed platform in the north. The Jurassic – Lower Cretaceous part of the AA CTSE is divided into synrift basins in the north and rift-shoulder deposits in the south. The AA CTSE includes oil-prone source rocks in the Triassic, Jurassic, and Cretaceous and proven reservoir rocks spanning the Mississippian to Lower Cretaceous. Much of the central part of the AA CTSE lies in the oil window whereas the northern and southern parts are mainly in the gas window. Known hydrocarbon accumulations in the AA CTSE total more than 30 billion barrels of oil equivalent and yet-to-find estimates suggest a similar volume remains to be discovered

Alaska↗

USGS2022

Colville Foreland Basin and Arctic Alaska Prograded Margin Tectono-Sedimentary Elements, northern Alaska and southwestern Canada Basin

Cretaceous (post-Neocomian)–Quaternary Brookian strata of Arctic Alaska include the Colville Foreland Basin (CFB) and Arctic Alaska Prograded Margin (AAPM) Tectono-Sedimentary Elements (TSEs). The CFB TSE lies beneath the Alaska North Slope and Chukchi Sea shelf, and the AAPM TSE lies beneath the Arctic Alaska continental terrace of the Beaufort Sea shelf and slope. The CFB TSE rests on rocks ranging from pre-Mississippian sedimentary and metasedimentary rocks (Franklinian sequence) to Lower Cretaceous strata, and is present at the land surface onshore and the seafloor offshore. The AAPM TSE generally rests on Lower Cretaceous (Neocomian) sedimentary strata and is present at the seafloor. The CFB and AAPM TSEs contain oil-prone source rocks in Cretaceous and Paleogene strata, and have been charged locally by hydrocarbon migration from pre-Brookian strata. Both TSEs contain proven reservoir rocks throughout the succession. The CFB TSE ranges from oil-window to gas-window maturity. The AAPM TSE is mainly in the gas window. Known hydrocarbon accumulations in the two TSEs total more than 3.4 Bbbl of oil equivalent and yet-to-find estimates suggest an even larger volume remains undiscovered. Current exploration is focused on shallow stratigraphic traps, in which giant oil accumulations have been discovered in recent years.

Alaska↗

USGS2023

Canada Basin tectono-sedimentary element, Arctic Ocean

The Canada Basin (CB) formed during a short period of seafloor spreading inferred to be Early Cretaceous in age. Brookian strata of inferred Early Cretaceous–Holocene age comprise the sedimentary fill of the Canada Basin Tectono-Sedimentary Element (CB TSE). Although the CB has remained tectonically quiet since seafloor spreading ceased, both proximal and distal tectonism (Alpha Ridge magmatism, and the Cordilleran, Brooks Range and Eurekan orogenies) have influenced sediment source areas, dispersal paths and thicknesses in the basin. In the Neogene, the dominant source of sediments was the Mackenzie River, which drains northern portions of the Cordilleran orogen. The CB TSE is one of the most remote and challenging places on Earth to explore. Although regional seismic reflection and refraction data exist, there are no boreholes to constrain interpretations. Existing published estimates of hydrocarbon potential range from limited to moderate to significant.

Geological Society of London Monograph, Arctic Sed↗

USGS2021

Integrated geophysical imaging of rare-earth-element-bearing iron oxide-apatite deposits in the eastern Adirondack Highlands, New York

The eastern Adirondack Highlands of northern New York host dozens of iron oxide-apatite (IOA) deposits containing magnetite and rare earth element (REE)-bearing apatite. We use new aeromagnetic, aeroradiometric, ground gravity, and sample petrophysical and geochemical data to image and understand these deposits and their geologic framework. Aeromagnetic total field data reflect highly magnetic leucogranite host rock and major structures that likely served as fluid conduits for the hydrothermal system. Bandpass filtering of the aeromagnetic data reveals individual deposits that were verified in the field or from historical records. A three-dimensional inversion for magnetic susceptibility images these deposits at depth, allowing inference of plunge directions and relative size. Radiometric data highlight variations in the surface geology and several large tailings piles that contain REE-bearing apatite. Within the host rock, eTh (equivalent Th), K and the eTh/K ratio are variable with high eTh/K near several of the IOA deposits. Areas with elevated K or low eTh/K representing potassic alteration appear to be rare; instead elevated eTh/K ratios likely reflect widespread sodic alteration overprinting potassic alteration. Bouguer gravity anomalies show limited correspondence to the surface geology, radiometric data, or magnetic data, but do exhibit ~10-km wide highs in areas where deposits are observed. Two-dimensional forward models of the gravity and magnetic data show that deeper dense material beneath the leucogranite is quantitatively feasible. If these dense rocks represent intrusions that were emplaced or still cooling at the time of mineralization, they may have served as a heat source that helped to drive the hydrothermal system. Combining datasets, we find that deposits occur towards the distal ends of major structures within the host leucogranite and mostly above gravity highs. The geophysical modeling thus suggests that IOA deposits formed in structural, thermal, and chemical traps near the distal ends of the hydrothermal system.

New York↗

USGS2014

Geophysical expression of a buried niobium and rare earth element deposit: the Elk Creek carbonatite, Nebraska, USA

The lower Paleozoic Elk Creek carbonatite is a 6–8-km-diameter intrusive complex buried under 200 m of sedimentary rocks in southeastern Nebraska. It hosts the largest known niobium deposit in the U.S. and a rare earth element (REE) deposit. The carbonatite is composed of several lithologies, the relations of which are poorly understood. Niobium mineralization is most enriched within a magnetite beforsite (MB) unit, and REE oxides are most concentrated in a barite beforsite unit. The carbonatite intrudes Proterozoic country rocks. Efforts to explore the carbonatite have used geophysical data and drilling. A high-resolution airborne gravity gradient and magnetic survey was flown over the carbonatite in 2012. The carbonatite is associated with a roughly annular vertical gravity gradient high and a subdued central low and a central magnetic high surrounded by magnetic field values lower than those over the country rocks. Geophysical, borehole, and physical property data are combined for an interpretation of these signatures. The carbonatite is denser than the country rocks, explaining the gravity gradient high. Most carbonatite lithologies have weaker magnetic susceptibilities than those of the country rocks, explaining why the carbonatite does not produce a magnetic high at its margin. The primary source of the central magnetic high is interpreted to be mafic rocks that are strongly magnetized and are present in large volumes. MB is very dense (mean density 3200 kg/m 3 ) and strongly magnetized (median 0.073 magnetic susceptibility), producing a gravity gradient high and contributing to the aeromagnetic high. Barite beforsite has physical properties similar to most of the carbonatite volume, making it a poor geophysical target. Geophysical anomalies indicate the presence of dense and strongly magnetized rocks at depths below existing boreholes, either a large volume of MB or another unknown lithology.

Nebraska↗

USGS2014

Geophysical interpretation of U, Th, and rare earth element mineralization of the Bokan Mountain peralkaline granite complex, Prince of Wales Island, southeast Alaska

A prospectivity map for rare earth element (REE) mineralization at the Bokan Mountain peralkaline granite complex, Prince of Wales Island, southeastern Alaska, was calculated from high-resolution airborne gamma-ray data. The map displays areas with similar radioelement concentrations as those over the Dotson REE-vein-dike system, which is characterized by moderately high %K, eU, and eTh (%K, percent potassium; eU, equivalent parts per million uranium; and eTh, equivalent parts per million thorium). Gamma-ray concentrations of rocks that share a similar range as those over the Dotson zone are inferred to locate high concentrations of REE-bearing minerals. An approximately 1300-m-long prospective tract corresponds to shallowly exposed locations of the Dotson zone. Prospective areas of REE mineralization also occur in continuous swaths along the outer edge of the pluton, over known but undeveloped REE occurrences, and within discrete regions in the older Paleozoic country rocks. Detailed mineralogical examinations of samples from the Dotson zone provide a means to understand the possible causes of the airborne Th and U anomalies and their relation to REE minerals. Thorium is sited primarily in thorite. Uranium also occurs in thorite and in a complex suite of ± Ti ± Nb ± Y oxide minerals, which include fergusonite, polycrase, and aeschynite. These oxides, along with Y-silicates, are the chief heavy REE (HREE)-bearing minerals. Hence, the eU anomalies, in particular, may indicate other occurrences of similar HREE-enrichment. Uranium and Th chemistry along the Dotson zone showed elevated U and total REEs east of the Camp Creek fault, which suggested the potential for increased HREEs based on their association with U-oxide minerals. A uranium prospectivity map, based on signatures present over the Ross-Adams mine area, was characterized by extremely high radioelement values. Known uranium deposits were identified in the U-prospectivity map, but the largest tract occurs over a radioelement-rich granite phase within the pluton that is likely not related to mineralization. Neither mineralization type displays a well-defined airborne magnetic signature.

Alaska↗

USGS2020

Assessing uranium and select trace elements associated with breccia pipe uranium deposits in the Colorado River and main tributaries in Grand Canyon, USA

Assessing chemical loading from streams in remote, difficult-to-access watersheds is challenging. The Grand Canyon area in northern Arizona, an international tourist destination and sacred place for many Native Americans, is characterized by broad plateaus divided by canyons as much as two-thousand meters deep and hosts some of the highest-grade uranium deposits in the U.S. From 2015–2018 major surface waters in Grand Canyon were monitored for select elements associated with breccia-pipe uranium deposits in the area, including uranium, arsenic, cadmium, and lead. Dissolved constituents in the Colorado River were monitored upstream (Lees Ferry), in the middle (Phantom Ranch), and downstream (Diamond Creek) of uranium mining areas. Concentrations of uranium, arsenic, cadmium, and lead at these main-stem sites varied little during the study period and were all well below human health and aquatic life benchmark criteria (30, 10, 5, and 15 μg/L maximum contaminant levels and 15, 150, 0.8, and 3.1 μg/L aquatic life criteria, respectively). Additionally, dissolved and sediment-bound constituents were monitored during a wide range of streamflow conditions at Little Colorado River, Kanab Creek, and Havasu Creek tributaries, whose watersheds have experienced different levels of uranium mining activities over time. Samples from the tributary sites contained ≤3.8 μg/L of dissolved cadmium and lead, and ≤17 μg/L of dissolved uranium. Dissolved arsenic also was mostly below human and aquatic life criteria at Little Colorado River and Kanab Creek; however, 63% of water samples from Havasu Creek were above the maximum contaminant level for arsenic. Arsenic in suspended sediment was greater than sediment quality guidelines in 9%, 35%, and 35% of samples from Little Colorado River, Kanab Creek, and Havasu Creek, respectively. At the concentrations observed during this study, tributaries contributed on average only about 0.12 μg/L of arsenic and 0.03 μg/L of uranium to the main-stem river. This study demonstrates how chemical loading from mined watersheds may be reliably assessed across a wide range of flow conditions in challenging locations.

Arizona↗

USGS2019

U-Pb and Lu-Hf isotope, age, and trace-element data from zircons at four sites in the western Alaska Range and Talkeetna Mountains, Alaska

This Division of Geological & Geophysical Surveys (DGGS) Raw Data File presents U-Pb geochronology and Lu-Hf isotopic compositions, age-dating results, and additional trace-elemental composition of zircons from four granitoids sampled during investigations by DGGS geologists in the western Alaska Range and the Talkeetna Mountains. The purpose of the Lu-Hf and U-Pb isotopic study was to better understand the Jurassic through early Tertiary magmatic and tectonic evolution of the Alaska Range and the significance of that magmatism for the formation of metallic mineral deposits. The U-Pb isotopic data also serve to further refine the crystallization age of plutons, which were previously ambiguous in 40Ar/39Ar geochronology studies.

Alaska↗

USGS2016

Isotopic compositions of the elements 2013 (IUPAC Technical Report)

The Commission on Isotopic Abundances and Atomic Weights (ciaaw.org) of the International Union of Pure and Applied Chemistry (iupac.org) has revised the Table of Isotopic Compositions of the Elements (TICE). The update involved a critical evaluation of the recent published literature. The new TICE 2013 includes evaluated data from the “best measurement” of the isotopic abundances in a single sample, along with a set of representative isotopic abundances and uncertainties that accommodate known variations in normal terrestrial materials.

Pure and Applied Chemistry↗

USGS2004

Concentrations of metals and trace elements in blood of spectacled and king eiders in northern Alaska, USA

In 1996, we measured concentrations of arsenic, barium, cadmium, lead, mercury, and selenium in blood of adult king (Somateria spectabilis) and spectacled (Somateria fischeri) eiders and duckling spectacled eiders from northern Alaska, USA. Concentrations of selenium exceeded background levels in all adults sampled and 9 of 12 ducklings. Mercury was detected in all adult spectacled eiders and 5 of 12 ducklings. Lead concentrations were above the clinical toxicity threshold in one duckling (0.64 ppm) and two adult female spectacled eiders (0.54 and 4.30 ppm). Concentrations of cadmium and mercury varied between species; barium, cadmium, mercury, and selenium varied between sexes. In female spectacled eiders, mercury concentrations increased during the breeding season and barium and selenium levels decreased through the breeding season. Selenium declined at 2.3 ± 0.9% per day and levels were lower in spectacled eiders arriving to the breeding grounds in northern Alaska than in western Alaska. The variation in selenium levels between breeding areas may be explained by differences in timing and routes of spring migration. Most trace elements for which we tested were not at levels currently considered toxic to marine birds. However, the presence of mercury and elevated lead in ducklings and adult female spectacled eiders suggests these metals are available on the breeding grounds.

Alaska↗

USGS2007

Release of elements to natural water from sediments of Lake Roosevelt, Washington, USA

Reservoir sediments from Lake Roosevelt (WA, USA) that were contaminated with smelter waste discharged into the Columbia River (BC, Canada) were examined using three measures of elemental release reflecting varying degrees of physical mixing and time scales. Aqueous concentrations of Cd, Cu, Pb, and Zn in the interstitial water of reservoir sediments, in the gently stirred overlying waters of incubated sediment cores, and in supernatants of aggressively tumbled slurries of reservoir sediments generally were higher than the concentrations from a reference site. When compared to chronic water-quality criteria, all three measures of release suggest that slag-contaminated sediments near the U.S.-Canadian border are potentially toxic as a result of Cu release and Pb release in two of the three measures. All three measures of Cd release suggest potential toxicity for one site farther down the reservoir, probably contaminated as a result of transport and adsorption of Cd from smelter liquid waste. Releases of Zn and As did not appear to be potentially toxic. Carbonate geochemistry indirectly affects the potential toxicity by increasing water hardness.

Washington↗

USGS2009

Trout density and health in a stream with variable water temperatures and trace element concentrations: does a cold-water source attract trout to increased metal exposure?

A history of hard-rock mining has resulted in elevated concentrations of heavy metals in Prickly Pear Creek (MT. USA). Remediation has improved water quality; however, dissolved zinc and cadmium concentrations still exceed U.S. Environmental Protection Agency water-quality criteria. Physical habitat, salmonid density, fish health, and water quality were assessed, and metal concentrations in fish tissues, biofilm, and macroinvertebrates were determined to evaluate the existing condition in the watershed. Cadmium, zinc, and lead concentrations in fish tissues, biofilm, and invertebrates were significantly greater than those at the upstream reference site and an experimental site farther downstream of the confluence. Fish densities were greatest, and habitat quality for trout was better, downstream of the confluence, where water temperatures were relatively cool (16??C). Measures of fish health (tissue metal residues, histology, metallothionein concentrations, and necropsies), however, indicate that the health of trout at this site was negatively affected. Trout were in colder but more contaminated water and were subjected to increased trace element exposures and associated health effects. Maximum water temperatures in Prickly Pear Creek were significantly lower directly below Spring Creek (16??C) compared to those at an experimental site 10 km downstream (26??C). Trout will avoid dissolved metals at concentrations below those measured in Prickly Pear Creek; however, our results suggest that the preference of trout to use cool water temperatures may supersede behaviors to avoid heavy metals. ?? 2009 SETAC.

Environmental Toxicology and Chemistry↗

USGS2011

Ages and sources of components of Zn-Pb, Cu, precious metal, and platinum group element deposits in the goodsprings district, Clark County, Nevada

The Goodsprings district, Clark County, Nevada, includes zinc-dominant carbonate replacement deposits of probable late Paleozoic age, and lead-dominant carbonate replacement deposits, copper ± precious metal-platinum group element (PGE) deposits, and gold ± silver deposits that are spatially associated with Late Triassic porphyritic intrusions. The district encompasses ~500 km 2 although the distribution of all deposits has been laterally condensed by late Mesozoic crustal contraction. Zinc, Pb, and Cu production from about 90 deposits was ~160,000 metric tons (t) (Zn > Pb >> Cu), 2.1 million ounces (Moz) Ag, 0.09 Moz Au, and small amounts of PGEs—Co, V, Hg, Sb, Ni, Mo, Mn, Ir, and U—were also recovered. Zinc-dominant carbonate replacement deposits (Zn > Pb; Ag ± Cu) resemble Mississippi Valley Type (MVT) Zn-Pb deposits in that they occur in karst and fault breccias in Mississippian limestone where the southern margin of the regional late Paleozoic foreland basin adjoins Proterozoic crystalline rocks of the craton. They consist of calcite, dolomite, sphalerite, and galena with variably positive S isotope compositions ( δ 34 S values range from 2.5–13‰), and highly radiogenic Pb isotope compositions ( 206 Pb/ 204 Pb >19), typical of MVT deposits above crystalline Precambrian basement. These deposits may have formed when southward flow of saline fluids, derived from basinal and older sedimentary rocks, encountered thinner strata and pinch-outs against the craton, forcing fluid mixing and mineral precipitation in karst and fault breccias. Lead-dominant carbonate replacement deposits (Pb > Zn, Ag ± Cu ± Au) occur among other deposit types, often near porphyritic intrusions. They generally contain higher concentrations of precious metals than zinc-dominant deposits and relatively abundant iron oxides after pyrite. They share characteristics with copper ± precious metal-PGE and gold ± silver deposits including fine-grained quartz replacement of carbonate minerals in ore breccias and relatively low S and Pb isotope values ( δ 34 S values vary from 0–~4‰; 206 Pb/ 204 Pb <18.5). Copper ± precious metal-PGE deposits (Cu, Co, Ag, Au, Pd, and Pt) consist of Cu carbonate minerals (after chalcocite and chalcopyrite) and fine-grained quartz that have replaced breccia clasts and margins of fissures in Paleozoic limestones and dolomites near porphyritic intrusions. Gold ± silver deposits occur along contacts and within small-volume stocks and dikes of feldspar porphyry, one textural variety of porphyritic intrusions. Lead isotope compositions of copper ± precious metal-PGE, gold ± silver, and lead-dominant carbonate replacement deposits are similar to those of Mojave crust plutons, indicating derivation of Pb from 1.7 Ga crystalline basement or from Late Proterozoic siliciclastic sedimentary rocks derived from 1.7 Ga crystalline basement. Four texturally and modally distinctive porphyritic intrusions are exposed largely in the central part of the district: feldspar quartz porphyry, plagioclase quartz porphyry, feldspar biotite quartz porphyry, and feldspar porphyry. Intrusions consist of 64 to 70 percent SiO 2 and variable K 2 O/Na 2 O (0.14–5.33) that reflect proportions of K-feldspar and albite phenocrysts and megacrysts as well as partial alteration to K-mica; quartz and biotite phenocrysts are present in several subtypes. Albite may have formed during emplacement of magma in brine-saturated basinal strata, whereas hydrothermal alteration of matrix, phenocrystic, and megacrystic feldspar and biotite to K-mica, pyrite, and other hydrothermal minerals occurred during and after intrusion emplacement. Small volumes of garnet-diopside-quartz and retrograde epidote-mica-amphibole skarn have replaced carbonate rocks adjacent to one intrusion subtype (feldspar-quartz porphyry), but alteration of carbonate rocks at intrusion contacts elsewhere is inconspicuous. Uranium-lead ages of igneous zircons vary inconsistently from ~ 180 to 230 Ma and are too imprecise to distinguish age differences among intrusion subtypes; most ages are 210 to 225 Ma, yielding a mean of 217 ± 1 Ma. K-Ar and 40 Ar/ 39 Ar ages of magmatic (plagioclase, biotite) and hydrothermal (K-mica) minerals span a similar range (183–227 Ma), demonstrating broadly contemporaneous intrusion emplacement and hydrothermal alteration but allowing for multiple Late Triassic magmatic-hydrothermal events. Imprecision and range of isotopic ages may have resulted from burial beneath Mesozoic and Tertiary strata and multiple intrusion of magmas, causing thermal disturbance to Ar systems and Pb loss from zircons in intrusions. Separate late Paleozoic (zinc-dominant carbonate replacement deposits) and Late Triassic (all other deposits) mineralizing events are supported by form, distribution, and host rocks of metal deposits, by hydrothermal mineral assemblages, isotope compositions, metal abundances, and metal diversity, and by small intrusion volumes. These characteristics collectively distinguish the Goodsprings district from larger intrusion related carbonate replacement districts in the western United States. They can be used to evaluate proximity to unexposed porphyritic intrusions associated with PGE and gold ± silver mineralization.

Nevada↗

USGS2015

Book review: Rare earth elements—A new approach to the nexus of supply, demand and use: exemplified along the use of neodymium in permanent magnets

This book is part of the “Springer Theses” published by Springer, a book series designed to highlight and share outstanding Ph.D. research. As explained by Springer (on the second page), this series “brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences.” I admire Springer for this endeavor because a large number of exceptional theses are never published, and thus are difficult to obtain, while most of those that are published are presented only in highly condensed form as journal articles. Review info: Rare Earth Elements—A New Approach to the Nexus of Supply, Demand and Use: Exemplified Along the Use of Neodymium in Permanent Magnets . By Volker Zerf. 2013. ISBN 978-3-642-35457-1. 157 pp.

Economic Geology↗

USGS2016

Regional geologic and petrologic framework for iron oxide ± apatite ± rare earth element and iron oxide copper-gold deposits of the Mesoproterozoic St. Francois Mountains terrane, southeast Missouri, USA

This paper provides an overview on the genesis of Mesoproterozoic igneous rocks and associated iron oxide ± apatite (IOA) ± rare earth element, iron oxide-copper-gold (IOCG), and iron-rich sedimentary deposits in the St. Francois Mountains terrane of southeast Missouri, USA. The St. Francois Mountains terrane lies along the southeastern margin of Laurentia as part of the eastern granite-rhyolite province. The province formed during two major pulses of igneous activity: (1) an older early Mesoproterozoic (ca. 1.50–1.44 Ga) episode of volcanism and granite plutonism, and (2) a younger middle Mesoproterozoic (ca. 1.33–1.30 Ga) episode of bimodal gabbro and granite plutonism. The volcanic rocks are predominantly high-silica rhyolite pyroclastic flows, volcanogenic breccias, and associated volcanogenic sediments with lesser amounts of basaltic to andesitic volcanic and associated subvolcanic intrusive rocks. The iron oxide deposits are all hosted in the early Mesoproterozoic volcanic and volcaniclastic sequences. Previous studies have characterized the St. Francois Mountains terrane as a classic, A-type within-plate granitic terrane. However, our new whole-rock geochemical data indicate that the felsic volcanic rocks are effusive derivatives from multicomponent source types, having compositional similarities to A-type within-plate granites as well as to S- and I-type granites generated in an arc setting. In addition, the volcanic-hosted IOA and IOCG deposits occur within bimodal volcanic sequences, some of which have volcanic arc geochemical affinities, suggesting an extensional tectonic setting during volcanism prior to emplacement of the ore-forming systems.

Missouri↗

USGS2016

Mineral thermometry and fluid inclusion studies of the Pea Ridge iron oxide-apatite–rare earth element deposit, Mesoproterozoic St. Francois Mountains Terrane, southeast Missouri, USA

Mineral thermometry and fluid inclusion studies were conducted on variably altered and mineralized samples from the Mesoproterozoic Pea Ridge iron oxide-apatite (IOA)-rare earth element (REE) deposit in order to constrain P-T conditions, fluid chemistry, and the source of salt and volatiles during early magnetite and later REE mineralization. Scanning electron microscopy (SEM)-cathodoluminescence and SEM-backscatter electron images show that quartz and rutile precipitated before, during, and after magnetite and REE mineral growth. Ti-in-quartz and Zr-in-rutile equilibration temperatures range from ≤350° to 750°C in the amphibole, magnetite, hematite, and silicified zones where T increased during magnetite and quartz growth and dropped precipitously after fracturing and brecciation. Late drusy quartz cements within a REE-rich breccia pipe record the lowest T (≤315°–400°C). Liquid-, vapor-rich, and hypersaline (±hematite, calcite) fluid inclusions are common and liquid CO 2 is present locally. Salinities define three populations: saline (10–27 wt % NaCl equiv), hypersaline (34–>60 wt % NaCl equiv), and dilute (0–10 wt % NaCl equiv ). The wide range of eutectic melting temperatures (−67° to −19°C) suggests that saline inclusions trapped variable proportions of a CaCl-MgCl-FeCl-bearing fluid end member and an NaCl-KCl fluid end member. Homogenization temperatures and pressures of these saline inclusions suggest they were trapped when fluids unmixed into brine and vapor at T <350°C, P <15 MPa, and a depth of ~1.5 km. Hypersaline inclusions were trapped at low T and P (~200°C and ~1 MPa) along the V + L + H curve when the system vented to the paleosurface. Data for dilute inclusions in late drusy quartz from the REE-rich breccia pipe are indicative of a boiling epithermal environment. The Na/Cl, Na/K, and Cl/Br ratios of fluid inclusion extracts provide evidence for mixtures of magmatic hydrothermal fluids and evaporated seawater. Extracts from magnetite, hematite, and pyrite plot in the magmatic-hydrothermal field, indicating that Fe was derived from a magmatic source. Their enrichments in Mg and Ca are consistent with a mafic magmatic source. The positive correlation between Na/Mg and Na/Ca ratios may be due to halite saturation or albitization of igneous rocks. Extracts from barite in the REE-rich breccia pipes are enriched in Na and Br and plot near the seawater evaporation trend. He is highly enriched relative to Ne and Ar in fluid inclusion extracts, which precludes air as a source of He. Although the He is mostly of crustal origin, pyrite with a 3 He/ 4 He (R/R A ) of 0.1 contains up to 12% mantle He. Many extracts have low 20 Ne/ 22 Ne ratios due to nucleogenic production of 22 Ne in high F/O minerals such as fluorapatite or F biotite. The arrays of data for 3 He/ 4 He (R/R A ) and 22 Ne/ 20 Ne suggest that volatiles were derived from two sources, a moderate F mafic magma containing mantle He and a high F silicic magma with crustal He. Together with other evidence cited in this report, these data (1) support a magmatic hydrothermal origin for the Mesoproterozoic magnetite-apatite deposit with ore fluids derived from a concealed mafic to intermediate-composition intrusion, (2) suggest that the REE minerals in breccia pipes were either derived from apatite or precipitated in response to decompression and cooling during breccia pipe formation, (3) provide evidence for the influx of basinal brine, magmatic fluids from granitic intrusions, and meteoric water after breccia pipe formation, and (4) show that Pea Ridge was relatively unaffected by the late Paleozoic Mississippi Valley-type (MVT) Pb-Zn system in overlying Cambrian sedimentary rocks.

Missouri↗

USGS1971

Fractionation of minor elements between galena and sphalerite, Darwin lead-silver-zinc mine, Inyo County, California and its significance in geothermometry

Minor element abundances were determined for galena and sphalerite from the Darwin mine, Inyo County, California. The temperature of deposition was calculated from the distribution coefficients determined experimentally by Bethke and Barton (1971) for cadmium and manganese in coexisting galena and sphalerite. The fractionation of cadmium between coexisting sphalerite and galena indicates a temperature of deposition of 416 degrees C+ or -20 degrees , the fractionation of manganese between the same mineral pair a temperature of 377 degrees C+ or -32 degrees . The fractionation of selenium gave unreasonably high temperatures. Three pairs gave temperatures between 568 degrees and 625 degrees C, the fourth pair a temperature of 889 degrees C. The reason for this descrepancy is not known.

California↗

USGS1975

Geochemical studies in the Park City district; II, Sulfide mineralogy and minor-element chemistry, Mayflower mine

Fissure-filling and replacement Pb-Zn-Cu-Au-Ag ores of the Mayflower mine occur in calcareous sedimentary and intrusive wall rocks over a vertical interval of 3,000 feet. The ores are exceptional in the district for their chalcopyrite and gold content and for the unusual associated gangue minerals anhydrite, chlorite, and hematite. High oxygen fugacities are indicated for many stages of mineralization. Sphalerite compositions are highly variable in the range 0.09 to 5.9 weight percent Fe, an indication of large fluctuations in sulfur fugacity. Silver is carried in tetrahedrite-tennantite (0.2 to 16 wt %), enargite (0.5 to 0.8 wt %), in coupled substitution with Bi + Sb in galena (0.02 to 2.9 wt %), and in the minerals argentite, matildite, chalcocite, and bornite. Content of As, Se, Te, and other minor elements is small. Fractionation of Cd between sphalerite and coexisting galena is relatively consistent whereas the fractionation of Mn between sphalerite and galena is variable; calculated temperatures of formation are high and unreliable.

Economic Geology↗