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At least 775 records · Page 43Linked to original sources
USGS2017

Global nonfuel mineral exploration trends 2001-2015

The mission of the U.S. Geological Survey (USGS) National Minerals Information Center (NMIC) is to collect, analyze and disseminate information on the domestic and international supply of and demand for minerals and mineral materials essential to the U.S. economy and national security. Understanding mineral exploration activities and trends assists government policy makers, minerals industry decision makers and research entities in identifying where future sources of mineral supply are likely to be discovered, the amount and type of these resources and factors that may affect exploration and development.

Mining Engineering↗
USGS2010

A review of silver-rich mineral deposits and their metallogeny

Mineral deposits with large inventories or high grades of silver are found in four genetic groups: (1) volcanogenic massive sulfide (VMS), (2) sedimentary exhalative (SEDEX), (3) lithogene, and, (4) magmatichydrothermal. Principal differences between the four groups relate to source rocks and regions, metal associations, process and timing of mineralization, and tectonic setting. These four groups may be subdivided into specific metal associations on ternary diagrams based on relative metal contents. The VMS deposits rarely contain more than 15,600 t Ag (500 Moz). Grades average 33 g/t Ag. Variable Ag- Pb-Zn-Cu-Au ± Sn concentrations are interpreted as having been derived both from shallow plutons and by leaching of the volcanic rock pile in regions of thin or no continental crust and the mineralization is syngenetic. Higher silver grades are associated with areas of abundant felsic volcanic rocks. The SEDEX deposits rarely contain more than 15,600 t Ag (500 Moz). Grades average 46 g/t Ag. Silver, lead, and zinc in relatively consistent proportions are leached from sedimentary rocks filling rift-related basins, where the continental crust is thin, and deposited as syngenetic to diagenetic massive sulfides. Pre-mineral volcanic rocks and their detritus may occur deep within the basin and gold is typically absent. Lithogene silver-rich deposits are epigenetic products of varying combinations of compaction, dewatering, meteoric water recharge, and metamorphism of rift basin-related clastic sedimentary and interbedded volcanic rocks. Individual deposits may contain more than 15,600 t Ag (500 Moz) at high grades. Ores are characterized by four well-defined metal associations, including Ag, Ag-Pb-Zn, Ag-Cu, and Ag-Co-Ni-U. Leaching, transport, and deposition of metals may occur both in specific sedimentary strata and other rock types adjacent to the rift. Multiple mineralizing events lasting 10 to 15 m.y., separated by as much as 1 b.y., may occur in a single basin. Gold is absent at economic levels. The magmatic-hydrothermal silver-rich deposits are epigenetic and related to cordilleran igneous and volcanic suites. Six magmatic-hydrothermal districts each contain more than 31,000 t Ag (1,000 Moz) with grades of veins >600 g/t Ag. Mineralization occurs as veins, massive sulfides in carbonate rocks, and disseminated deposits including porphyry silver deposits, a proposed exploration model. Most deposits are epithermal with low-sulfidation alteration assemblages. Deposits are often telescoped and well-zoned. All large and high-grade magmatic-hydrothermal deposits appear confined to regions of relatively thick continental crust above Cenozoic consuming plate margins on the eastern side of the Pacific Rim. Silver in these deposits may be partly derived by hydrothermal leaching of rocks under or adjacent to the deposits. Specific metal associations in SEDEX and lithogene deposits may reflect confinement of fluid flow to and derivation of metals from specific source rock types. Variable metal associations in VMS and magmatichydrothermal deposits may reflect derivation of metals from a more diverse suite of rocks by convecting hydrothermal systems and processes related to the generation of magma. The discovery rate for silver-rich deposits has accelerated during the past decade, with new deposit types, metal associations, and exploration models being identified that provide numerous exploration and research opportunities.

Book chapter↗
USGS2023

New high resolution airborne geophysical surveys in Nevada And California for geothermal and mineral resource studies

The U.S. Geological Survey (USGS) and the Department of Energy (DOE) are collaborating to acquire high-resolution airborne magnetic and radiometric data to support geologic and geophysical mapping and modeling that will assist geothermal and critical mineral studies. Coordinated with these efforts are programs supporting geologic mapping and airborne LiDAR (light detection and ranging) surveys that yield detailed surface topographic models of the terrain over the same regions spanned by the geophysical surveys. The collaboration leverages resources from the USGS and DOE to acquire large regional datasets that will provide fundamental data necessary to map surface and subsurface geology and structure to benefit mineral and resource program objectives of both agencies. Such regionally uniform datasets are important for geothermal research to assist in identifying geologically favorable settings and as invaluable inputs in predictive models targeting undiscovered resources that use knowledge-driven (e.g., play fairway analysis) or data-driven approaches (e.g., machine-learning methods) to reduce risk associated with resource exploration. These data will also serve a wide range of other related activities from hazard (earthquake, volcano, landslide, environmental) and resource (water, mineral, energy) studies, to mapping and land management. Surveys were conducted in two areas that were selected because they host substantial geothermal and mineral potential in California and Nevada. The data will aid several ongoing USGS and DOE projects aimed at characterizing geothermal and mineral systems, understanding the factors controlling their occurrence, and improving future national resource assessments. The first of these surveys (referred to as GeoDAWN) was collected over northern and western Nevada and eastern California and spans areas of major resource potential associated with the Walker Lane and western Great Basin. This includes Clayton Valley, which hosts substantial lithium brine and clay resources, and the Humboldt Mafic Complex, which constitutes a potentially important resource of critical minerals (including cobalt, rare earth elements, platinum group elements, iron, chromium, nickel, and copper). The second survey area (referred to as GeoFlight) is focused over the Salton Trough in southern California that contains some of the largest and hottest known hydrothermal systems in the world, as well as a substantial lithium brine resource that could potentially meet the nation’s lithium demand for electric vehicles. Data from both surveys will be made publicly available through USGS publications and online data repositories. Future efforts under this collaboration are presently being evaluated and may involve acquisition of other data sets such as airborne gravity, electromagnetic or hyperspectral data to address research targets.

Conference Paper↗
USGS2025

Seamless Geologic Map Database for the Intermountain West, United States: A foundational dataset for mineral systems analysis

The Intermountain West has a complex geologic history, resulting in the formation of a diverse array of mineral deposits. Effective mineral exploration requires understanding the spatial and temporal relationships among geologic processes and events, a key focus of the mineral systems approach to exploration. This paper presents the Intermountain West Seamless Geologic Map Database, a unified dataset designed to support mineral exploration. Integrating geologic provinces, structural settings, and hydrothermal alteration, the database leverages the Seamless Integrated Geologic Mapping (SIGMa) extension to the USGS Geologic Map Schema (GeMS) to standardize geologic data from varied sources. SIGMa's hierarchical stratigraphic organization and feature-level metadata enhance data interoperability and reusability, enabling seamless query, analysis, and visualization of lithology, structural features, mineral deposits, geochronology, hydrothermal alteration. volcanic activity, and By providing a regionally consistent and dynamically evolving geologic map, this database provides a foundational framework for mineral exploration and geologic research. It also allows for an efficient workflow that expedites the publication of integrated geologic map databases.

Conference Paper↗
USGS2025

40Ar/39Ar geochronology supporting mineral resources research at USGS Denver

The 40 Ar/ 39 Ar geochronology method is used to date potassium-bearing rocks and minerals, based on the decay of 40 K to 40 Ar, which provides important temporal constraints for geological events. The USGS Denver Argon Geochronology Laboratory dates samples from a variety of projects, mainly in the USGS Mineral Resource Program and the National Cooperative Geologic Mapping Program, facilitating in-depth research into the timing of geological events and processes. Recent applications of this method include geochronological studies in the Yellow Pine district, revealing mineralization ages that range from 51 to 70 Ma, and investigations into the Bear Lodge alkaline complex, providing insights into the timing of REE carbonatite formation. Additionally, studies at Alunite Ridge highlight the method's potential for understanding concealed mineral deposits. Ongoing projects continue to leverage 40 Ar/ 39 Ar data to enhance understanding of geologic frameworks and mineral resources, underpinning its importance in modern geochronology and mineral resource assessment.

Conference Paper↗
USGS2024

Reactivity of clay minerals with acids and alkalies

One-g samples of a montmorillonite, a metabentonite, an illite, two kaolinites, and three halloysites were treated with 50 ml of hydrochloric acid (6⋅45 N, 1:1), acetic acid (4⋅5 N, 1:3), sodium hydroxide (2⋅8 N), sodium chloride solution (pH 6⋅10; Na = 35‰; Cl = 21⋅5‰), and natural sea water (pH 7⋅85; Na = 35⋅5‰; Cl = 21⋅ 5‰) for a 10-day period in stoppered plastic vials. The supernatant solutions were removed from the clay minerals and analyzed for SiO 2 , Al 2 O 3 , CaO, MgO, Na 2 O, and K 2 O. All the solutions removed some SiO 2 , Al 2 O 3 , and Fe 2 O 3 from the samples, but the quantities were small. Sodium hydroxide attacked the kaolin group minerals more strongly than it did montmorillonite, metabentonite, or illite. Halloysite was more strongly attacked by hydrochloric acid than was any of the other experimental minerals. Hydrochloric acid removed iron oxide coatings from soil clay minerals, but acetic acid did not remove them completely. The samples most strongly attacked by HCl and NaOH were examined by X-ray diffraction. Acid treatment did not destroy the structure of the clays, but the halloysite structure was partially destroyed. Sodium hydroxide attacked the halloysite structure, as shown by chemical analysis and X-ray diffraction. These experiments show that treatment in dilute acids has no harmful effect in the preparation of clays for X-ray diffraction. Acetic acid is preferred to hydrochloric acid for this purpose. Hydrochloric acid cleans clay minerals by removing free iron oxide from the surface; acetic acid is less effective.

Clays and Clay Minerals↗
USGS2018

Mineral constraints on arctic caribou (Rangifer tarandus): a spatial and phenological perspective

Arctic caribou ( Rangifer tarandus ) have the longest terrestrial migration of any ungulate but little is known about the spatial and seasonal variation of minerals in summer forages and the potential impacts of mineral nutrition on the foraging behavior and nutritional condition of arctic caribou. We investigated the phenology, availability, and mechanistic relationships of calcium, phosphorus, magnesium, sodium, potassium, iron, manganese, copper, and zinc in three species of woody browse, three species of graminoids, and one forb preferred by caribou over two transects bisecting the ranges of the Central Arctic (CAH) and Western Arctic (WAH) caribou herds in Alaska. Transects traversed three ecoregions (Coastal Plain, Arctic Foothills and Brooks Range) along known migration paths in the summer ranges of both herds. Concentrations of mineral in forages were compared to estimated dietary requirements of lactating female caribou. Spatial distribution of the abundance of minerals in caribou forage was associated with interactions of soil pH and mineral content, while temporal variation was related to plant maturity, and thus nitrogen and fiber content of forages. Concentrations of sodium were below caribou requirements in all forage species for most of the summer and adequate only on the Coastal Plain during the second half of summer. Phosphorus declined in plants from emergence to senescence and was below requirements in all forages by mid‐summer, while concentrations of copper declined to marginal concentrations at plant senescence. Interactions of sodium with potassium, calcium with phosphorus, and copper with zinc in forages likely exacerbate the constraints of low concentrations sodium, phosphorus, and copper. Forages on the WAH contained significantly more phosphorus and copper than forages collected on the CAH transect. We suspect that migrations of caribou to the Arctic Coastal Plain may allow parturient females to replenish sodium stores depleted by foraging inland through the long arctic winters, while also extending the availability of adequate phosphorus, if animals are able to selectively track emerging waves of forage.

Alaska↗
USGS2013

Mass-balance modeling of mineral weathering rates and CO2 consumption in the forested, metabasaltic Hauver Branch watershed, Catoctin Mountain, Maryland, USA

Mineral weathering rates and a forest macronutrient uptake stoichiometry were determined for the forested, metabasaltic Hauver Branch watershed in north-central Maryland, USA. Previous studies of Hauver Branch have had an insufficient number of analytes to permit determination of rates of all the minerals involved in chemical weathering, including biomass. More equations in the mass-balance matrix were added using existing mineralogic information. The stoichiometry of a deciduous biomass term was determined using multi-year weekly to biweekly stream-water chemistry for a nearby watershed, which drains relatively unreactive quartzite bedrock. At Hauver Branch, calcite hosts ~38 mol% of the calcium ion (Ca 2+ ) contained in weathering minerals, but its weathering provides ~90% of the stream water Ca 2+ . This occurs in a landscape with a regolith residence time of more than several Ka (kiloannum). Previous studies indicate that such old regolith does not typically contain dissolving calcite that affects stream Ca 2+ /Na + ratios. The relatively high calcite dissolution rate likely reflects dissolution of calcite in fractures of the deep critical zone. Of the carbon dioxide (CO 2 ) consumed by mineral weathering, calcite is responsible for approximately 27%, with the silicate weathering consumption rate far exceeding that of the global average. The chemical weathering of mafic terrains in decaying orogens thus may be capable of influencing global geochemical cycles, and therefore, climate, on geological timescales. Based on carbon-balance calculations, atmospheric-derived sulfuric acid is responsible for approximately 22% of the mineral weathering occurring in the watershed. Our results suggest that rising air temperatures, driven by global warming and resulting in higher precipitation, will cause the rate of chemical weathering in the Hauver Branch watershed to increase until a threshold temperature is reached. Beyond the threshold temperature, increased recharge would produce a shallower groundwater table and reduced chemical weathering rates.

Maryland↗
USGS1982

A topology of mineralization and its meaning for prospecting

Epigenetic mineral deposits are universal members of an orderly spatial and temporal arrangement of igneous rocks, endomorphic rocks, and hydrothermally altered rocks. The association and sequence of these rocks is invariant whereas the metric relations and configurations of the properties of these rocks are unlimited in variety. This characterization satisfies the doctrines of topology. Metric relations are statistical, and their modes are among the better guides to optimal areas for exploration. Metric configurations are graphically irregular and unpredictable mathematical surfaces like mountain topography. Each mineral edifice must be mapped to locate its mineral deposits. All measurements and observations are only positive or neutral for the occurrence of a mineral deposit. Effective prospecting is based on an increasing density of positive data with proximity to the mineral deposit. This means sampling for maximal numbers of positive data, pragmatically the highest ore-element assays at each site, by selecting rock showing maximal development of lode attributes.

Book chapter↗
USGS1986

Oxygen isotope compositions of selected laramide-tertiary granitoid stocks in the Colorado Mineral Belt and their bearing on the origin of climax-type granite-molybdenum systems

Quartz phenocrysts from 31 granitoid stocks in the Colorado Mineral Belt yield ??18O values less than 10.4???, with most values between 9.3 and 10.4???. An average magmatic value of about 8.5??? is suggested. The stocks resemble A-type granites; these data support magma genesis by partial melting of previously depleted, fluorine-enriched, lower crustal granulites, followed by extreme differentiation and volatile evolution in the upper crust. Subsolidus interaction of isotopically light water with stocks has reduced most feldspar and whole rock ??18O values. Unaltered samples from Climax-type molybdenumbearing granites, however, show no greater isotopic disturbance than samples from unmineralized stocks. Although meteoric water certainly played a role in post-mineralization alteration, particularly in feldspars, it is not required during high-temperature mineralization processes. We suggest that slightly low ??18O values in some vein and replacement minerals associated with molybdenum mineralization may have resulted from equilibration with isotopically light magmatic water and/or heavy isotope depletion of the ore fluid by precipitation of earlier phases. Accumulation of sufficient quantities of isotopically light magmatic water to produce measured depletions of 18O requires extreme chemical stratification in a large magma reservoir. Upward migration of a highly fractionated, volatile-rich magma into a small apical Climax-type diapir, including large scale transport of silica, alkalis, molybdenum, and other vapor soluble elements, may occur with depression of the solidus temperature and reduction of magma viscosity by fluorine. Climax-type granites may provide examples of 18O depletion in magmatic systems without meteoric water influx. ?? 1986 Springer-Verlag.

Contributions to Mineralogy and Petrology↗
USGS2021

LA-ICPMS U-Pb dating reveals cassiterite inheritance in the Yazov granite, Eastern Siberia: Implications for tin mineralization

U-Pb dating of cassiterite and zircon from the Yazov granite (Transbaikalia region, Eastern Siberia, Russia) and cassiterite from spatially associated tin mineralization in the Tuyukan ore district in the Tonod uplift was conducted using in situ laser ablation inductively coupled plasma mass spectrometry. These analyses allow comparison of isotopic systematics for both minerals, especially related to transport in granitic magma. These data are also useful for understanding possible genetic links between the granite and the tin mineralization. Most of the U-Pb zircon analyses define a 206 Pb/ 238 U age of 719 ± 15 Ma for the granite; in addition, several zircon cores define an inheritance age of 1839 ± 21 Ma. U-Pb data for 10 nearly concordant analyses of disseminated cassiterite from the same samples yield a 206 Pb/ 238 U age of 1838 ± 34 Ma. This is the first documented evidence of cassiterite inheritance in granitic magma. These data indicate the robust character of U-Pb isotope systematics in cassiterite, comparable to that in zircon. The presence of numerous inclusions of cassiterite in zircon from the Yazov granite (revealed by nanotomography) supports the interpretation of inherited cassiterite included during Neoproterozoic zircon crystallization. The data indicate that high tin concentrations in the Yazov granite are due to the incorporation of older cassiterite crystals from country rock, not coeval cassiterite crystallization. Cassiterite samples from two ore occurrences spatially associated with the Yazov granite yield Pb-Pb isochron ages of 1.86–1.82 Ga, indicating that tin mineralization occurred in the Paleoproterozoic, nearly 1 Ga before emplacement of the Yazov granite. Tin mineralization of the ore region is probably related to ~ 1.85 Ga Chuya-Kodar tin-bearing granitic rocks that host tin deposits. These results have broad implications for understanding how critical elements, such as tin, may become enriched in rare-metal granites and how they are related to regional to global geodynamic processes.

Siberia↗
USGS2024

Polyphase stratabound scheelite-ferberite mineralization at Mallnock, Eastern Alps, Austria

A peculiar type of stratabound tungsten mineralization in metacarbonate rocks was discovered and explored at Mallnock (Austria) during the late 1980s. It is the only tungsten occurrence in the Eastern Alps in which scheelite is associated with wolframite (96 mol% ferberite). The tungsten prospect is located in the Austroalpine Drauzug-Gurktal Nappe System recording polyphase low-grade regional metamorphism. Raman spectroscopy of carbonaceous material yield maximum metamorphic temperatures of 296 ± 27 °C and 258 ± 27 °C, which are assigned to Variscan and Eoalpine metamorphism, respectively. Scheelite and ferberite occur as polyphase stockwork-like mineralization in Fe-rich magnesite in the northern ore zone (Mallnock North), whereas in the western ore zone (Mallnock West), scheelite-quartz veinlets are exclusively hosted in dolomitic marbles. LA-ICP-MS analyses of scheelite and ferberite yield low contents of Mo, Nb, Ta, and rare earth elements, but high contents of Na and Sr. Uranium is particularly high in scheelite (up to 200 µg/g) and makes this mineral a suitable target for U–Pb dating. In situ U–Pb dating of scheelite yielded an early Permian age (294 ± 8 Ma) for Mallnock West and a Middle Triassic age (239 ± 3 Ma) for Mallnock North. A monzodioritic dike close to Mallnock yielded a U–Pb apatite date of 282 ± 9 Ma and supports the polyphase formation of this mineralization. The U–Pb scheelite ages indicate that a model for tungsten metallogeny in the Eastern Alps must also consider remobilization of tungsten by metamorphic fluids. In the Alps, the Permian to Triassic period (ca. 290–225 Ma) is characterized by an overall extensional geodynamic setting related to the breakup of Pangea. Lithospheric thinning caused higher heat flow, low-P metamorphism, and anatexis in the lower crust, which led to enhanced crustal fluid flow in the upper crust. These processes were not only responsible for the formation of metasomatic hydrothermal magnesite and siderite deposits in the Eastern Alps but also for this unique magnesite-ferberite-scheelite mineralization at Mallnock.

Mount Mallock↗
USGS2026

The use of fluorite geochemistry and machine learning to identify critical mineral systems

Fluorite (CaF 2 ) is a potential pathfinder to critical mineral and rare earth element (REE) deposits but its application has been limited to a narrow range of mineralization types. I show that fluorite is a robust recorder of mineralization fertility by applying statistical and machine-learning methods to a new global fluorite geochemical database. Distinct median rare earth and trace element patterns are observed among deposit types and genetic environments. Fluorite associated with carbonatites and REE deposits are relatively enriched in Sr and have minimal Eu anomalies. These characteristics define new bivariate discrimination diagrams that correctly identify 78% of carbonatite-related fluorite and 88% of fluorite from REE deposits. Random forest classifiers were developed for a wide range of mineralization types and genetic settings. Trained solely on rare earth element patterns, these models achieve accuracies of 77–79%. Higher classification accuracies (up to 88–96%) are obtained when including elements such as Sr, highlighting the significance of trace elements for optimal fluorite classification. The recognition of diagnostic fluorite compositional fingerprints, particularly in REE-fertile systems, underscores its potential as a pathfinder and indicator for critical mineral exploration in F-bearing environments.

Mineralium Deposita↗
USGS2003

Mineral precipitation and dissolution at two slag-disposal sites in northwestern Indiana, USA

Slag is a ubiquitous byproduct of the iron- and steel-refining industries. In northwestern Indiana and northeastern Illinois, slag has been deposited over more than 52 km2 of land surface. Despite the widespread use of slag for fill and construction purposes, little is known about its chemical effects on the environment. Two slagdisposal sites were examined in northwestern Indiana where slag was deposited over the native glacial deposits. At a third site, where slag was not present, background conditions were defined. Samples were collected from cores and drill cuttings and described with scanning electron microscopy and electron microprobe analysis. Ground-water samples were collected and used to assess thermodynamic equilibria between authigenic minerals and existing conditions. Differences in the mineralogy at background and slag-affected sites were apparent. Calcite, dolomite, gypsum, iron oxides, and clay minerals were abundant in native sediments immediately beneath the slag. Mineral features indicated that these minerals precipitated rapidly from slag drainage and co-precipitated minor amounts of non-calcium metals and trace elements. Quartz fragments immediately beneath the slag showed extensive pitting that was not apparent in sediments from the background site, indicating chemical weathering by the hyperalkaline slag drainage. The environmental impacts of slag-related mineral precipitation include disruption of natural ground-water flow patterns and bed-sediment armoring in adjacent surface-water systems. Dissolution of native quartz by the hyperalkaline drainage may cause instability in structures situated over slag fill or in roadways comprised of slag aggregates.

Indiana↗
USGS2005

Effects of a clearcut on the net rates of nitrification and N mineralization in a northern hardwood forest, Catskill Mountains, New York, USA

The Catskill Mountains of southeastern New York receive among the highest rates of atmospheric nitrogen (N) deposition in eastern North America, and ecosystems in the region may be sensitive to human disturbances that affect the N cycle. We studied the effects of a clearcut in a northern hardwood forest within a 24-ha Catskill watershed on the net rates of N mineralization and nitrification in soil plots during 6 years (1994-1999) that encompassed 3-year pre- and post-harvesting periods. Despite stream NO3- concentrations that increased by more than 1400 ??mol l-1 within 5 months after the clearcut, and three measures of NO3- availability in soil that increased 6- to 8-fold during the 1st year after harvest, the net rates of N mineralization and nitrification as measured by in situ incubation in the soil remained unchanged. The net N-mineralization rate in O-horizon soil was 1- 2 mg N kg-1 day-1 and the net nitrification rate was about 1 mg N kg-1 day-1, and rates in B-horizon soil were only one-fifth to one-tenth those of the O-horizon. These rates were obtained in single 625 m2 plots in the clearcut watershed and reference area, and were confirmed by rate measurements at 6 plots in 1999 that showed little difference in N-mineralization and nitrification rates between the treatment and reference areas. Soil temperature increased 1 ?? 0.8??C in a clearcut study plot relative to a reference plot during the post-harvest period, and soil moisture in the clearcut plot was indistinguishable from that in the reference plot. These results are contrary to the initial hypothesis that the clearcut would cause net rates of these N-cycling processes to increase sharply. The in situ incubation method used in this study isolated the samples from ambient roots and thereby prevented plant N uptake; therefore, the increases in stream NO3- concentrations and export following harvest largely reflect diminished uptake. Changes in temperature and moisture after the clearcut were insufficient to measurably affect the net rates of N mineralization and nitrification in the absence of plant uptake. Soil acidification resulting from the harvest may have acted in part to inhibit the rates of these processes. ?? Springer 2005.

Biogeochemistry↗
USGS2008

What was the groundwater quality before mining in a mineralized region? Lessons from the Questa Project

The U.S. Geological Survey, in cooperation with the New Mexico Environment Department and supported by Molycorp, Inc (currently Chevron Minerals), has completed a 5-year investigation (2001-2006) to determine the pre-mining ground-water quality at Molycorp's Questa molybdenum mine in northern New Mexico. Current mine-site ground waters are often contaminated with mine-waste leachates and no data exists on premining ground-water quality so that pre-mining conditions must be inferred. Ground-water quality undisturbed by mining is often worse than New Mexico standards and data are needed to help establish closure requirements. The key to determining pre-mining conditions was to study the hydrogeochemistry of a proximal natural analog site, the Straight Creek catchment. Main rock types exposed to weathering include a Tertiary andesite and the Tertiary Amalia tuff (rhyolitic composition), both hydrothermally altered to various degrees. Two types of ground water are common in mineralized areas, acidic ground waters in alluvial debris fans with pH 3-4 and bedrock ground waters with pH 6-8. Siderite, ferrihydrite, rhodochrosite, amorphous to microcrystalline Al(OH)3, calcite, gypsum, barite, and amorphous silica mineral solubilities control concentrations of Fe(II), Fe(III), Mn(II), Al, Ca, Ba, and SiO2, depending on pH and solution composition. Concentrations at low pH are governed by element abundance and mineral weathering rates. Concentrations of Zn and Cd range from detection up to about 10 and 0.05 mg/L, respectively, and are derived primarily from sphalerite dissolution. Concentrations of Ni and Co range from detection up to 1 and 0.4 mg/L, respectively, and are derived primarily from pyrite dissolution. Concentrations of Ca and SO4 are derived from secondary gypsum dissolution and weathering of calcite and pyrite. Metal:sulfate concentration ratios are relatively constant for acidic waters, suggesting consistent weathering rates, independent of catchment. These trends, combined with lithology, mineralogy, and mineral solubility controls, provide useful constraints on pre-mining ground-water quality for the mine site where the lithology is known.

Geosciences Journal↗
USGS1958

Role of clay minerals in the transportation of iron

The clay minerals have iron associated with them in several ways: 1. (1) as an essential constituent 2. (2) as a minor constituent within the crystal lattice where it is in isomorphous substitution and 3. (3) as iron oxide on the surface of the mineral platelets. Nontronite, “hydromica,” some chlorites, vermiculite, glauconite and chamosite contain iron as an essential constituent. Kaolinite and halloysite have no site within the lattice for iron, but in certain environments iron oxide (goethite or hematite) is intimately associated as a coating on the micelles. Analyses of clay minerals show that the content of Fe 2 O 3 varies: 29 per cent (nontronite), 7·3 per cent (griffithite), 4.5 per cent (“hydromica”), 5.5 per cent (chlorite), 4 per cent (vermiculite) and 18 per cent (glauconite). The FeO content is: 40 per cent (chamosite), 7.8 per cent (griffithite), 1–2 per cent (“hydromica”), 3 per cent (glauconite) and 2 per cent (chlorite). The iron associated with the clay minerals remains stable in the environment in which the minerals occur, but if either pH or Eh or both are changed the iron may be affected. Change of environment will cause: 1. (1) removal of iron by reduction of Fe 3+ to Fe 2+ ; 2. (2) ion-exchange reactions; 3. (3) instability of the crystal lattice. Experiments using bacterial activity to produce reducing conditions with kaolinite and halloysite coated with iron oxides and with nontronite in which ferric iron is in the octahedral position within the lattice showed that ferric oxide is removed at Eh +0·215 in fresh water and at Eh +0.098 in sea water. Hematite, goethite, and indefinite iron oxides were removed at different rates. Red ferric oxides were changed to black indefinite noncrystalline ferrous sulphide at Eh −0.020 but reverted to ferric oxide under oxidizing conditions. Nontronite turned bright green under reducing conditions and some of the ferrous iron remained within the lattice on a return to oxidizing conditions. Bacterial activity seems to be necessary for maintaining reducing conditions in the environments studied.

Geochimica et Cosmochimica Acta↗
USGS1962

Problem of the thermodynamic status of the mixed-layer minerals

Minerals that show mixed layering, particularly with the component layers in random sequence, pose problems because they may behave thermodynamically as single phases or as polyphase aggregates. Two operational criteria are proposed for their distinction. The first scheme requires two samples of mixed-layer material which differ only in the proportions of the layers. If each of these two samples are allowed to equilibrate with the same suitably chosen monitoring solution, then the intensive parameters of the solution will be invariant if the mixed-layer sample is a polyphase aggregate, but not otherwise. The second scheme makes use of the fact that portions of many titration curves of clay minerals show constancy of the chemical activities of the components in the equilibrating solutions, suggesting phase separation. If such phase separation occurs for a mixed-layer material, then, knowing the number of independent components in the system, it should be possible to decide on the number of phases the mixed-layer material represents. Knowledge of the phase status of mixed-layer material is essential to the study of the equilibrium relations of mineral assemblages involving such material, because a given mixed-layer mineral will be plotted and treated differently on a phase diagram, depending on whether it is a single phase or a polyphase aggregate. Extension of the titration technique to minerals other than the mixed-layer type is possible. In particular, this method may be used to determine if cryptoperthites and peristerites are polyphase aggregates. In general, for any high-order phase separation, the method may be used to decide just at what point in this continuous process the system must be regarded operationally as a polyphase aggregate.

Geochimica et Cosmochimica Acta↗