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At least 451 records · Page 25Linked to original sources

USGS1989

Characterization of alkyl carbon in forest soils by CPMAS 13C NMR spectroscopy and dipolar dephasing

Samples obtained from forest soils at different stages of decomposition were treated sequentially with chloroform/methanol (extraction of lipids), sulfuric acid (hydrolysis), and sodium chlorite (delignification) to enrich them in refractory alkyl carbon. As revealed by NMR spectroscopy, this treatment yielded residues with high contents of alkyl carbon. In the NMR spectra of residues obtained from litter samples, resonances for carbohydrates are also present, indicating that these carbohydrates are tightly bound to the alkyl carbon structures. During decomposition in the soils this resistant carbohydrate fraction is lost almost completely. In the litter samples the alkyl carbon shows a dipolar dephasing behavior indicative of two structural components, a rigid and a more mobile component. As depth and decomposition increase, only the rigid component is observed. This fact could be due to selective degradation of the mobile component or to changes in molecular mobility during decomposition, e.g., because of an increase in cross linking or contact with the mineral matter of the soil.

Science of the Total Environment↗

USGS2009

Weathering of the New Albany Shale, Kentucky: II. Redistribution of minor and trace elements

During weathering, elements enriched in black shale are dispersed in the environment by aqueous and mechanical transport. Here a unique evaluation of the differential release, transport, and fate of Fe and 15 trace elements during progressive weathering of the Devonian New Albany Shale in Kentucky is presented. Results of chemical analyses along a weathering profile (unweathered through progressively weathered shale to soil) describe the chemically distinct pathways of the trace elements and the rate that elements are transferred into the broader, local environment. Trace elements enriched in the unweathered shale are in massive or framboidal pyrite, minor sphalerite, CuS and NiS phases, organic matter and clay minerals. These phases are subject to varying degrees and rates of alteration along the profile. Cadmium, Co, Mn, Ni, and Zn are removed from weathered shale during sulfide-mineral oxidation and transported primarily in aqueous solution. The aqueous fluxes for these trace elements range from 0.1 g/ha/a (Cd) to 44 g/ha/a (Mn). When hydrologic and climatic conditions are favorable, solutions seep to surface exposures, evaporate, and form Fe-sulfate efflorescent salts rich in these elements. Elements that remain dissolved in the low pH (<4) streams and groundwater draining New Albany Shale watersheds become fixed by reactions that increase pH. Neutralization of the weathering solution in local streams results in elements being adsorbed and precipitated onto sediment surfaces, resulting in trace element anomalies. Other elements are strongly adsorbed or structurally bound to solid phases during weathering. Copper and U initially are concentrated in weathering solutions, but become fixed to modern plant litter in soil formed on New Albany Shale. Molybdenum, Pb, Sb, and Se are released from sulfide minerals and organic matter by oxidation and accumulate in Fe-oxyhydroxide clay coatings that concentrate in surface soil during illuviation. Chromium, Ti, and V are strongly correlated with clay abundance and considered to be in the structure of illitic clay. Illite undergoes minimal alteration during weathering and is concentrated during illuvial processes. Arsenic concentration increases across the weathering profile and is associated with the succession of secondary Fe(III) minerals that form with progressive weathering. Detrital fluxes of particle-bound trace elements range from 0.1 g/ha/a (Sb) to 8 g/ha/a (Mo). Although many of the elements are concentrated in the stream sediments, changes in pH and redox conditions along the sediment transport path could facilitate their release for aqueous transport.

Applied Geochemistry↗

USGS2004

Speciation and characterization of arsenic in gold ores and cyanidation tailings using X-ray absorption spectroscopy

The knowledge of mineralogy and molecular structure of As is needed to better understand the stability of As in wastes resulting from processing of gold ores. In this study, optical microscopy, scanning electron microscopy, electron microprobe, X-ray diffraction and X-ray absorption fine structure (XAFS) spectroscopy (including both XANES and EXAFS regimes) were employed to determine the mineralogical composition and local coordination environment of As in gold ores and process tailings from bench-scale tests designed to mimic a common plant practice. Arsenic-bearing minerals identified in the ores and tailings include iron (III) oxyhydroxides, scorodite (FeAsO4??2H2O), ferric arsenates, arseniosiderite (Ca2Fe3 (AsO4)3O2??3H2O), Ca-Fe arsenates, pharmacosiderite (KFe4 (AsO4)3(OH)4??6-7H2O), jarosite (K2Fe6(SO4)4 (OH)12) and arsenopyrite (FeAsS). Iron (III) oxyhydroxides contain variable levels of As from trace to about 22 wt% and Ca up to approximately 9 wt%. Finely ground ore and tailings samples were examined by bulk XAFS and selected mineral grains were analyzed by microfocused XAFS (micro-EXAFS) spectroscopy to reconcile the ambiguities of multiple As sources in the complex bulk EXAFS spectra. XANES spectra indicated that As occurs as As5+in all the samples. Micro-EXAFS spectra of individual iron (III) oxyhydroxide grains with varying As concentrations point to inner-sphere bidentate-binuclear arsenate complexes as the predominant form of As. There are indications for the presence of a second Fe shell corresponding to bidentate-mononuclear arrangement. Iron (III) oxyhydroxides with high As concentrations corresponding to maximum adsorption densities probably occur as nanoparticles. The discovery of Ca atoms around As in iron (III) oxyhydroxides at interatomic distances of 4.14-4.17 A?? and the coordination numbers suggest the formation of arseniosiderite-like nanoclusters by coprecipitation rather than simple adsorption of Ca onto iron (III) oxyhydroxides. Correlation of Ca with As in iron (III) oxyhydroxides as determined by electron microprobe analysis supports the coprecipitate origin for the presence of Ca in iron (III) oxyhydroxides. The samples containing higher abundances of ferric arsenates released higher As concentrations during the cyanidation tests. The presence of highly soluble ferric arsenates and Ca-Fe arsenates, and relatively unstable iron (III) oxyhydroxides with Fe/As molar ratios of less than 4 in the ore and process tailings suggests that not only the tailings in the impoundment will continue to release As, but also there is the potential for mobilization of As from the natural sources such as the unmined ore. ?? 2004 Elsevier Ltd.

Geochimica et Cosmochimica Acta↗

USGS1975

Origin of magnetite and pyrrhotite in carbonaceous chondrites

CARBONACEOUS chondrites, although comprising only about 2% of known meteorites, are extremely interesting for scientific investigation. Their mineral constitution, and the correspondence between their bulk chemical composition and the solar abundance of condensable elements, indicate that minimum chemical fractionation and thermal alteration have occurred. The mineral phases observed in these primitive chondrites are sufficiently unique, with respect to other meteorite classes, to have elicited considerable speculation about the physical environment in which they formed 1–7 .

Nature↗

USGS2023

Stream sediment geochemistry in mineral exploration: A review of fine-fraction, clay-fraction, bulk leach gold, heavy mineral concentrate and indicator mineral chemistry

Stream sediment surveys support early-stage reconnaissance mineral exploration and regional assessment programmes, enhanced by recent improvements in analytical method detection limits, continuously improving mineral chemistry, and new approaches to the interpretation of geochemical data. Sediment surveys may be used to predict catchment basin lithology, mineralization type based on pathfinder geochemistry, and geological features based on indicator mineral chemistry. Sediment surveys that target a finer-fraction sediment sample led to the discovery of the La Colosa gold deposit, Colombia. The Batu Hijau porphyry Cu–Au deposit in Indonesia was discovered based on an anomalous clay-sized fraction sample 12 km downstream. In an arid region with poorly developed drainages and minor topographic relief, the Ag-base-metal Navidad District in Argentina was discovered with clay-fraction sediment geochemistry. Heavy mineral concentrate (HMC) sediment surveys that include mineral chemistry determinations have led to global diamond discoveries. HMC surveys contributed to discovery of the Ring of Fire Ni–Cu–PGE and chromite district, Ontario, Canada. Discoveries and geochemical mapping can assist advancement of the application of stream sediment geochemistry in those global areas for which lithologies and deposits are exposed. Stream sediment surveys continue to be one of the most cost-effective geochemical methods for covering large areas for mineral exploration.

Geochemistry: Exploration, Environment, Analysis↗

USGS1983

Preliminary report on geology, geochemical exploration, and biogeochemical exploration of the Red Mountain stockwork, Yellow Pine District, Valley County, Idaho

New exploration targets for gold, molybdenum, tungsten, and tin are indicated by the systematic distribution of metals in soil and plants of the Red Mountain stockwork and its environs. The stockwork is built of countless quartz veins and veinlets, extensively argillized and containing sparsely disseminated gold, pyrite, arsenoDyrite, Dyrrhotite, fluorite, and other minerals. The stockwork developed along the outer ring-fracture zone of the Eocene Quartz Creek cauldron where subsidence strain, unrelieved by radial faulting, produced internal deformation and intense small-scale fracturing in rocks of the Idaho batholith. The stockwork, cropping out as a fault-bounded polygon 2,700 ft long and 2,000 ft wide, contains (underlies?) a large, virtually barren quartz body 1,350 ft long and 350 ft wide. Deformed, shattered granite flanks and presumably underlies the stockwork, which may plunge southward beneath inclusion-bearing granodiorite and alaskite of the Idaho batholith suite. Narrow dikes of rhyolite and latite radiate from two centers within the quartz body. Many dikes and small bosses of rhyolite and latite intrude stockwork and granite. The radial habit of some dikes and the high frequency of small intrusives concentrated within the sericite and kaolinite alteration zones suggest that a Tertiary porphyry is concealed beneath the mountain. Clay-mineral alteration zones mapped in residual soil extend far outward from the stockwork. Valleys filled with Quaternary deposits bound the stockwork-granite complex on the east, north, and west, effectively concealing elements of a crudely elliptical substructure that differs from the fault-dissected, mappable part of the complex. Metal anomalies in soil of the 3,600-ft x 8,400-ft gridded area are mostly weak and small. In contrast, metal anomalies in plants are strong, large, and consistent with the inference of a concealed elliptical substructure that may be hoodlike and may contain more than one mineralized zone. Gold in ashed sapwood of douglas-fir (Pseudotsuga menziesii) indicates a major gold anomaly, peak value 14.2 ppm Au, on inclusion-bearing granodiorite in an unprospected area south of the exposed stockwork. Locally, the gold anomaly is accompanied by a sizable tin anomaly, peak value 100 ppm Sn, in douglas-fir. Molybdenum in ashed leaves of beargrass (Xerophyllum tenax), peak value >500 ppm Mo, indicates extensive anomalies within a 2-milelong semi-elliptical belt of Mo values exceeding the 20 ppm median. Most of the belt is on valley fill, but the southern segment of it, near the major gold anomaly, is mainly on inclusion-bearing granodiorite. Here, where beargrass is sparse, the peak value of Mo in beargrass drops to 70 ppm. Where beargrass is absent, ashed leaves of sedge (Carex geyeri) have anomalous values of 100 ppm Mo. An additional target for molybdenum is indicated by molybdenite from the inaccessible Dart of the main adit. The molybdenite, seen only in dump specimens, is in weakly silicified granite that may underlie part of the stockwork. The source body of the molybdenite is blind, and no clue to it is given by samples of soil or plants. Just north of the major gold anomaly, 2 ppm values of W in beargrass coincide approximately with a tungsten anomaly in soil (peak value 22 ppm W) in an area of float containing disseminated scheelite in weakly silicified inclusion-bearing granodiorite. Other areas of 2 ppm W values in beargrass are nearly coextensive with the Moin-beargrass anomalies on Quaternary deposits of Quartz Creek valley. Some of the target areas for gold, molybdenum, tungsten, and tin have not been circumscribed by our sampling.

Open-File Report↗

USGS1997

Relation of hydrogeologic characteristics to distribution of radioactivity in ground water, Newark Basin, New Jersey

The distribution of radioactivity in ground water in the Newark Basin is controlled by the lithology of the aquifer and the degree of contact between the water that flows through the fractured strata and the radioactive lithologic units. The primary water-bearing lithologic units of the Newark Basin that contain elevated levels of radioactivity are the arkosic sandstones of the Stockton Formation and the black mudstones of the lower part of the Passaic Formation. Lithologic and hydrogeologic characteristics of these rock aquifers that affect the spatial distribution of radionuclides in the water include the concentration of radioactive minerals in the strata; the continuity of radioactive strata; the orientation, depth, interconnectedness, and continuity of regional and local fracture patterns; and the geochemical environment, which affects radionuclide solubility. The stratigraphic zones of uranium enrichment and the continuity of the radioactive strata differ between the Stockton Formation and the lower part of the Passaic Formation. Uranium enrichment in both formations, however, is identified in permeable sections of strata that have sharp contacts with, or that are enclosed by, less permeable zones. In the Stockton Formation, basal conglomerates overlying fine-grained mudstones are the most radioactive lithology. In interbedded red and black mudstones of the lower part of the Passaic Formation, the uranium-rich zones are the black and white carbonate-rich siltstone and mudstone laminae overlying massive fine-grained mudstones. Most samples of the arkosic sandstones of the Stockton Formation tended to be slightly enriched in uranium relative to thorium, whereas all samples of the black mudstones of the lower part of the Passaic Formation were heavily enriched in uranium with respect to thorium, with a maximum concentration of uranium of 6,200 parts per million (ppm). Spatial orientation of radioactive zones is controlled by the depositional environment of the host lithology. Radioactive strata in the arkosic sandstones of the Stockton Formation, which is deltaic or fluvial/deltaic in origin, arc stratigraphically discontinuous and nonplanar. In contrast, the radioactive lacustrine black mudstones of the lower part of the Passaic Formation are laterally continuous and planar. Geophysical logs of boreholes indicate that the number and depths of probable waterbearing fractures are greater in the arkoses of the Stockton Formation and red mudstones of the lower part of the Passaic Formation than in the interbedded red and black mudstones of the lower part of the Passaic Formation, despite the fact that fracture-density counts in cores indicate that black mudstones are more highly fractured than the other rock types. The total number of fractures is greater and bedding-plane fractures are generally more common in the lower part of the Passaic Formation than in the Stockton Formation. Fractures in the Stockton Formation form a well-connected network consisting of both bedding-plane and several sets of high-angle fractures present on a regional scale. The large-scale continuity of interconnected bedding-plane fractures and high-angle regional fractures may be partly responsible for the higher yields observed for wells in the Stockton Formation than for wells in the Passaic Formation and may facilitate migration of dissolved radionuclides far from their source. The well-connected fracture network may increase the likelihood that circulating water comes in contact with radioactive strata. Water in the black mudstones of the lower part of the Passaic Formation may also contain high levels of radioactivity, primarily as a result of the high frequency of radioactive strata and of fracturing, rather than the presence of a well-connected regional fracture system. Ground-water chemistry is dominated by the calcium cation and the bicarbonate anion, both derived from abundant secondary calcite in the fractures. The concentrations of uranium and radium in water that comes into contact with radioactive rock are controlled by the water chemistry. Uranium is the dominant alpha emitter in the oxidizing waters in the red mudstones of the Passaic Formation, whereas radium-226 (226Ra) is the dominant alpha emitter in the suboxic (reducing) water in the black mudstones of the lower part of the Passaic Formation. Because the lithology of the Stockton Formation and, in places, the lower part of the Passaic Formation varies greatly, wells can intercept multiple fracture zones that can contain oxic or suboxic waters, thereby resulting in the presence of both uranium and radium in detectable quantities in the same water sample.

New Jersey↗

USGS1997

Geochemistry and microbiology of iron-related well-screen encrustation and aquifer biofouling in Suffolk County, Long Island, New York

Iron-related well-screen encrustation and aquifer biofouling has decreased the specific capacity of several production wells in Suffolk County, N.Y., and has forced the Suffolk County Water Authority to adopt a costly well-reconditioning and replacement program. The specific-capacity declines are the result of the precipitation of iron oxyhydroxides and the growth of iron bacteria on the well screens and in the pore spaces of the surrounding formation. Mineralogic and chemical analyses indicate that the inorganic part of the encrusting material consists primarily of amorphous ferric hydroxide (Fe(OH)3 ); minor components of the material include goethite (FeOOH), hematite (Fe2 O 3 ), and quartz (SiO 2 ). The weight percent of ferric hydroxide in the material ranged from 32.3 to 98.6 percent and averaged 64.3 percent. Equilibrium modeling indicated that during pumping the well waters were supersaturated with respect to goethite, hematite, magnetite, and quartz and were under-saturated with respect to ferric hydroxide. Theoretical Eh values computed for the ferrous/ferric-iron redox couple and the oxygen/water redox couple averaged 390 millivolts and 810 millivolts, respectively, indicating that the waters were in a state of redox disequilibrium. The disequilibrium condition arises from the mixing of ground water with a low dissolved-oxygen concentration with oxygenated ground water during operation of the well. The low pH of the ground water contributes to the disequilibrium condition by slowing the rate of iron oxidation after the introduction of oxygen. Chemical and mineralogical data indicate that most of the encrusting material in the wells was deposited while the wells were shut down, probably in response to the use of treated water of higher pH to keep pump turbines wet while the wells were not in operation; the increased pH of water in the static water column increases the rate of ferrous-iron oxidation and causes the well water to become increasingly saturated with respect to ferric hydroxide. The median half-time of oxidation in samples of untreated ground water (pH 4-5) was 4.19 days, whereas the average half-time of oxidation in treated water (pH 7-8) was 11.9 minutes Equilibrium modeling indicated that treated waters generally were supersaturated with respect to ferric hydroxide, whereas untreated well waters were not. Field and laboratory data indicate that iron bacteria play an important role in the encrustation and biofouling process in Suffolk County. Filamentous iron bacteria were common in the affected wells. The most common species was Gallionella ferruginea, an effective biofouling agent that prefers water with low, but detectable, dissolved-oxygen concentrations and high dissolved-iron concentrations; this species was more common in biofilm samples from the Magothy aquifer than in those from the upper glacial aquifer. Iron bacteria also were found in sediment cores from several locations in the aquifer and in drilling water. Lignite could act as a carbon source for heterotrophic iron bacteria, which could accelerate the formation of iron-bacteria biofilms in wells screened in some parts of the Magothy aquifer. Iron-bacteria biofilms alter the chemistry of well water by removing iron, manganese, and sulfate from solution and by increasing the pH. Sulfur-reducing bacteria and iron-sulfide mineral phases were observed in some samples of encrusting material, indicating that these bacteria could contribute to well-screen encrustation in some geochemical environments.

Water-Resources Investigations Report↗

USGS2024

Deep-ocean macrofaunal assemblages on ferromanganese and phosphorite-rich substrates in the Southern California Borderland

Mineral-rich hardgrounds, such as ferromanganese (FeMn) crusts and phosphorites, occur on seamounts and continental margins, gaining attention for their resource potential due to their enrichment in valuable metals in some regions. This study focuses on the Southern California Borderland (SCB), an area characterized by uneven and heterogeneous topography featuring FeMn crusts, phosphorites, basalt, and sedimentary rocks that occur at varying depths and are exposed to a range of oxygen concentrations. Due to its heterogeneity, this region serves as an optimal setting for investigating the relationship between mineral-rich hardgrounds and benthic fauna. This study characterizes the density, diversity, and community composition of macrofauna (>300 μm) on hardgrounds as a function of substrate type and environment (depth and oxygen ranges). Rocks and their macrofauna were sampled quantitatively using remotely operated vehicles (ROVs) during expeditions in 2020 and 2021 at depths above, within, and below the oxygen minimum zone (OMZ). A total of 3,555 macrofauna individuals were counted and 416 different morphospecies (excluding encrusting bryozoans and hydrozoans) were identified from 82 rocks at depths between 231 and 2,688 m. Average density for SCB macrofauna was 11.08 ± 0.87 ind. 200 cm −2 and mean Shannon-Wiener diversity per rock (H′ [loge] ) was 2.22 ± 0.07. A relationship was found between substrate type and macrofaunal communities. Phosphorite rocks had the highest H′ of the four substrates compared on a per-rock basis. However, when samples were pooled by substrate, FeMn crusts had the highest H′ and rarefaction diversity. Of all the environmental variables examined, water depth explained the largest variance in macrofaunal community composition. Macrofaunal density and diversity values were similar at sites within and outside the OMZ. This study is the first to analyze the macrofaunal communities of mineral-rich hardgrounds in the SCB, which support deep-ocean biodiversity by acting as specialized substrates for macrofaunal communities. Understanding the intricate relationships between macrofaunal assemblages and mineral-rich substrates may inform effects from environmental disruptions associated with deep-seabed mining or climate change. The findings contribute baseline information useful for effective conservation and management of the SCB and will support scientists in monitoring changes in these communities due to environmental disturbance or human impact in the future.

California↗

USGS2007

Geology and Mineral Resources of the East Mojave National Scenic Area, San Bernardino County, California

The rocks of the East Mojave National Scenic Area (EMNSA) record a history of dynamic geologic events that span more than 1,800 million years (m.y.). These geologic events contributed significantly to development of the spectacular vistas and panoramas present in the area today. The oldest rocks underlie much of the northern part of the EMNSA. These rocks were subjected to extreme pressures and temperatures deep in the Earth's crust about 1,700 million years ago (Ma). They were subsequently intruded by granitic magmas from about 1,695 to 1,650 Ma, by additional granitic magmas at about 1,400 Ma and, later, at about 1,100 Ma, by iron-rich magmas that crystallized to form dark igneous rocks termed diabase. Unusual potassium- and magnesium-rich rocks, emplaced at about 1,400 Ma, crop out in a few places within and near the EMNSA. Their distinctive composition results from very small degrees of partial melting of mantle peridotite that was highly enriched in incompatible trace elements. At Mountain Pass, just outside the northeast boundary of the EMNSA, the potassium- and magnesium-rich rocks are accompanied by a rare type of carbonatite, an igneous rock composed of carbonate minerals, that contains high-grade rare earth element mineralization. Subsequent to these igneous-dominated events, sedimentary strata began to be deposited at about 1,000 Ma; mostly sandstone and shale were deposited initially in marine and, less commonly, in continental environments along the west edge of the core of the North American continent. Sedimentation eventually culminated in the widespread deposition of thick marine limestones from about 400 to about 245 Ma. These limestones represent a continental-shelf environment where shallow-water limestone formed to the east and deeper water limestone formed to the west. The end of the formation of these sedimentary deposits probably was caused by uplift of the shelf, which marked the beginning of a long period of tectonic upheaval. At about 170 Ma, widespread emplacement of coarse-grained granitic magmas began again in the region; some of these magmas also erupted as volcanic rocks. Additional episodes of magmatism took place at about 100 Ma and at 75 Ma. Most of the metallic-mineral occurrences in the EMNSA are associated with the igneous rocks that range in age from 170 to 75 Ma. During each of these magmatic events, the previously deposited sedimentary strata were buckled and broken as the entire region, part of a continental-scale fold and thrust belt, underwent crustal shortening and compression. A period of tectonic quiescence characterized the region from about 65 Ma to about 20 Ma. The quiet period ended abruptly with widespread volcanism along the southern and eastern parts of the EMNSA. The major gold deposits in the Castle Mountains are associated with this episode of volcanism. During this volcanic outburst, the crust extended laterally in several areas that border the EMNSA: along the lower Colorado River 65 km to the east, in the Kingston Range 20 km to the north, and in the central Mojave Desert 75 km to the southwest. This extensional deformation is characterized by the superposition of upper-crustal rocks over midcrustal rocks along large flat-lying faults, several of which project beneath rocks now exposed at the surface in the EMNSA. The near-surface rocks of the EMNSA, however, apparently escaped much of this intense extensional deformation. High-angle faults, which cut several of the mountain ranges, possibly have undergone several periods of movement, which date back to approximately 70 to 100 Ma. Some faults are of local importance to the physiographic development of the mountain ranges and intervening basins, and, in places, the faults seem to have localized various kinds of ore bodies and mineral occurrences. Volcanism and extensional deformation waned from 14 to 11 Ma. By approximately 10 Ma, widespread erosion had produced broad erosional dome-shaped mountains in the n

Bulletin↗

USGS2006

Controls of bedrock geochemistry on soil and plant nutrients in Southeastern Utah

The cold deserts of the Colorado Plateau contain numerous geologically and geochemically distinct sedimentary bedrock types. In the area near Canyonlands National Park in Southeastern Utah, geochemical variation in geologic substrates is related to the depositional environment with higher concentrations of Fe, Al, P, K, and Mg in sediments deposited in alluvial or marine environments and lower concentrations in bedrock derived from eolian sand dunes. Availability of soil nutrients to vegetation is also controlled by the formation of secondary minerals, particularly for P and Ca availability, which, in some geologic settings, appears closely related to variation of CaCO3 and Ca-phosphates in soils. However, the results of this study also indicate that P content is related to bedrock and soil Fe and Al content suggesting that the deposition history of the bedrock and the presence of P-bearing Fe and Al minerals, is important to contemporary P cycling in this region. The relation between bedrock type and exchangeable Mg and K is less clear-cut, despite large variation in bedrock concentrations of these elements. We examined soil nutrient concentrations and foliar nutrient concentration of grasses, shrubs, conifers, and forbs in four geochemically distinct field sites. All four of the functional plant groups had similar proportional responses to variation in soil nutrient availability despite large absolute differences in foliar nutrient concentrations and stoichiometry across species. Foliar P concentration (normalized to N) in particular showed relatively small variation across different geochemical settings despite large variation in soil P availability in these study sites. The limited foliar variation in bedrock-derived nutrients suggests that the dominant plant species in this dryland setting have a remarkably strong capacity to maintain foliar chemistry ratios despite large underlying differences in soil nutrient availability.

Utah↗

USGS2009

Nitrogen dynamics across silvicultural canopy gaps in young forests of western Oregon

Silvicultural canopy gaps are emerging as an alternative management tool to accelerate development of complex forest structure in young, even-aged forests of the Pacific Northwest. The effect of gap creation on available nitrogen (N) is of concern to managers because N is often a limiting nutrient in Pacific Northwest forests. We investigated patterns of N availability in the forest floor and upper mineral soil (0-10 cm) across 6-8-year-old silvicultural canopy gaps in three 50-70-year-old Douglas-fir forests spanning a wide range of soil N capital in the Coast Range and Cascade Mountains of western Oregon. We used extractable ammonium (NH 4 + ) and nitrate (NO 3 - ) pools, net N mineralization and nitrification rates, and NH 4 + and NO 3 - ion exchange resin (IER) concentrations to quantify N availability along north-south transects run through the centers of 0.4 and 0.1 ha gaps. In addition, we measured several factors known to influence N availability, including litterfall, moisture, temperature, and decomposition rates. In general, gap-forest differences in N availability were more pronounced in the mineral soil than in the forest floor. Mineral soil extractable NH 4 + and NO 3 - pools, net N mineralization and nitrification rates, and NH 4 + and NO 3 - IER concentrations were all significantly elevated in gaps relative to adjacent forest, and in several cases exhibited significantly greater spatial variability in gaps than forest. Nitrogen availability along the edges of gaps more often resembled levels in the adjacent forest than in gap centers. For the majority of response variables, there were no significant differences between northern and southern transect positions, nor between 0.4 and 0.1 ha gaps. Forest floor and mineral soil gravimetric percent moisture and temperature showed few differences along transects, while litterfall carbon (C) inputs and litterfall C:N ratios in gaps were significantly lower than in the adjacent forest. Reciprocal transfer incubations of mineral soil samples between gap and forest positions revealed that soil originating from gaps had greater net nitrification rates than forest samples, regardless of incubation environment. Overall, our results suggest that increased N availability in 6-8-year-old silvicultural gaps in young western Oregon forests may be due more to the quality and quantity of litterfall inputs resulting from early-seral species colonizing gaps than by changes in temperature and moisture conditions caused by gap creation.

Forest Ecology and Management↗

USGS2017

New zircon (U-Th)/He and U/Pb eruption age for the Rockland tephra, western USA

Eruption ages of a number of prominent Quaternary volcanic deposits remain inaccurately and/or imprecisely constrained, despite their importance as regional stratigraphic markers in paleo-environment reconstruction and as evidence of climate-altering eruptions. Accurately dating volcanic deposits presents challenging analytical considerations, including poor radiogenic yield, scarcity of datable minerals, and contamination of crystal populations by magma, eruption, and transport processes. One prominent example is the Rockland tephra, which erupted from the Lassen Volcanic Center in the southern Cascade arc. Despite a range in published eruption ages from 0.40 to 0.63 Ma, the Rockland tephra is extensively used as a marker bed across the western United States. To more accurately and precisely constrain the age of the Rockland tephra-producing eruption, we report U/Pb crystallization dates from the outermost ∼2 μm of zircon crystal faces (surfaces) using secondary ion mass spectrometry (SIMS). Our new weighted mean 238 U/ 206 Pb age for Rockland tephra zircon surfaces is 0.598 ± 0.013 Ma (2σ) and MSWD = 1.11 (mean square weighted deviation). As an independent test of the accuracy of this age, we obtained new (U-Th)/He dates from individual zircon grains from the Rockland tephra, which yielded a weighted mean age of 0.599 ± 0.012 Ma (2σ, MSWD = 5.13). We also obtained a (U-Th)/He age of 0.628 ± 0.014 Ma (MSWD = 1.19) for the Lava Creek Tuff member B, which was analyzed as a secondary standard to test the accuracy of the (U-Th)/He technique for Quaternary tephras, and to evaluate assumptions made in the model-age calculation. Concordance of new U/Pb and (U-Th)/He zircon ages reinforces the accuracy of our preferred Rockland tephra eruption age, and confirms that zircon surface dates sample zircon growth up to the time of eruption. We demonstrate the broad applicability of coupled U/Pb zircon-surface and single-grain zircon (U-Th)/He geochronology to accurate dating of Quaternary tephra, and highlight the challenges and opportunities of this technique.

Quaternary Science Reviews↗

USGS1964

Bottom sediments of Saginaw Bay, Michigan

Saginaw Bay is a southwest extension of Lake Huron on the east shore of the Southern Peninsula of Michigan. It is a shallow-water derivative of the Pleistocene Lake Saginaw. Sixty-one bottom samples were collected on a semigrid pattern and analyzed physically. Findings were treated statistically. Sediments range in size from large pebbles to clay. Medium- to fine-grained clear quartz sand is common to all parts of the bay. Currents and wave action are primarily responsible for both median diameter and sorting distribution patterns. Only a very general correlation can be established between depth and median diameter. Heavy minerals occur in abundance locally and show an affinity to shallow-water areas subject to prevailing currents. Shape also locally determines heavy mineral concentrations. Only general conclusions can be established from roundness and sphericity and acid-soluble content. Increased organic content is correlative with quiet water environments. The shallow-water, heterogeneous nature of Saginaw Bay is not conducive to the recognition of sedimentary criteria suitable for correlations in other than a local environment.

Journal of Sedimentary Petrology↗

USGS2012

The physical hydrogeology of ore deposits

Hydrothermal ore deposits represent a convergence of fluid flow, thermal energy, and solute flux that is hydrogeologically unusual. From the hydrogeologic perspective, hydrothermal ore deposition represents a complex coupled-flow problem—sufficiently complex that physically rigorous description of the coupled thermal (T), hydraulic (H), mechanical (M), and chemical (C) processes (THMC modeling) continues to challenge our computational ability. Though research into these coupled behaviors has found only a limited subset to be quantitatively tractable, it has yielded valuable insights into the workings of hydrothermal systems in a wide range of geologic environments including sedimentary, metamorphic, and magmatic. Examples of these insights include the quantification of likely driving mechanisms, rates and paths of fluid flow, ore-mineral precipitation mechanisms, longevity of hydrothermal systems, mechanisms by which hydrothermal fluids acquire their temperature and composition, and the controlling influence of permeability and other rock properties on hydrothermal fluid behavior. In this communication we review some of the fundamental theory needed to characterize the physical hydrogeology of hydrothermal systems and discuss how this theory has been applied in studies of Mississippi Valley-type, tabular uranium, porphyry, epithermal, and mid-ocean ridge ore-forming systems. A key limitation in the computational state-of-the-art is the inability to describe fluid flow and transport fully in the many ore systems that show evidence of repeated shear or tensional failure with associated dynamic variations in permeability. However, we discuss global-scale compilations that suggest some numerical constraints on both mean and dynamically enhanced crustal permeability. Principles of physical hydrogeology can be powerful tools for investigating hydrothermal ore formation and are becoming increasingly accessible with ongoing advances in modeling software.

Economic Geology↗

USGS2012

Sulfur, carbon, hydrogen, and oxygen isotope geochemistry of the Idaho cobalt belt

Cobalt-copper ± gold deposits of the Idaho cobalt belt, including the deposits of the Blackbird district, have been analyzed for their sulfur, carbon, hydrogen, and oxygen isotope compositions to improve the understanding of ore formation. Previous genetic hypotheses have ranged widely, linking the ores to the sedimentary or diagenetic history of the host Mesoproterozoic sedimentary rocks, to Mesoproterozoic or Cretaceous magmatism, or to metamorphic shearing. The δ 34 S values are nearly uniform throughout the Blackbird dis- trict, with a mean value for cobaltite (CoAsS, the main cobalt mineral) of 8.0 ± 0.4‰ ( n = 19). The data suggest that (1) sulfur was derived at least partly from sedimentary sources, (2) redox reactions involving sulfur were probably unimportant for ore deposition, and (3) the sulfur was probably transported to sites of ore for- mation as H 2 S. Hydrogen and oxygen isotope compositions of the ore-forming fluid, which are calculated from analyses of biotite-rich wall rocks and tourmaline, do not uniquely identify the source of the fluid; plausible sources include formation waters, metamorphic waters, and mixtures of magmatic and isotopically heavy meteoric waters. The calculated compositions are a poor match for the modified seawaters that form vol- canogenic massive sulfide (VMS) deposits. Carbon and oxygen isotope compositions of siderite, a mineral that is widespread, although sparse, at Blackbird, suggest formation from mixtures of sedimentary organic carbon and magmatic-metamorphic carbon. The isotopic compositions of calcite in alkaline dike rocks of uncertain age are consistent with a magmatic origin. Several lines of evidence suggest that siderite postdated the emplacement of cobalt and copper, so its significance for the ore-forming event is uncertain. From the stable isotope perspective, the mineral deposits of the Idaho cobalt belt contrast with typical VMS and sedimentary exhalative deposits. They show characteristics of deposit types that form in deeper environments and could be related to metamorphic processes or magmatic processes, although the isotopic evidence for magmatic components is relatively weak.

Idaho↗

USGS2006

Water and Sediment Chemical Data and Data Summary for Samples Collected in 1999 and 2001 in the Goodpaster River Basin, Big Delta B-2 Quadrangle, Alaska

We report the chemical analysis for water and sediment collected from the Big Delta B-2 quadrangle. These data are part of a study located in the Big Delta B-2 quadrangle that focused on the integration of geology and bedrock geochemistry on with the biogeochemistry of water, sediments, soil, and vegetation. The discovery of the Pogo lode gold deposit in the northwest corner of the quadrangle was the impetus for this study. The study objectives were to create a geologic map, evaluate the bedrock geochemical influence on the geochemical signature of the surficial environment, and define landscape-level predevelopment geochemical baselines. Important to baseline development is an evaluation of what, if any, geochemical difference exists between the mineralized and non-mineralized areas within a watershed or between mineralized and non-mineralized watersheds. The analytic results for the bedrock, soils, and vegetation are reported elsewhere. Presented here, with minimal interpretation, is the analytic data for the water and sediment samples collected in the summers of 1999 and 2001, and a summary statistics of these analyses.

Open-File Report↗

USGS1956

Uranium content and leachability of some igneous rocks and their geochemical significance

The uranium content and its leachability in 442 igneous rocks if wide variety were measured. Four-gram samples, crushed below 38 mesh, were leached in in 0.05 M HNO 3 for half an hour at 80 to 85° C; the leachate and undissolved residue were analyzed fluorimetrically. This study explored the relation of uranium content and leachability to petrography. With few exceptions, the relations between uranium content, uranium leachability, rock leachability, rock type, and mineral composition seem to be random. Uranium content and leachability in homogeneous outcrops vary erratically, in heterogeneous outcrops the variation appears nonsystematic. Correlations with geologic environment indicate that uranium content and leachability are largely controlled by geologic processes during and after crystallization.

Trace Elements Investigations↗