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At least 1,027 records · Page 57Linked to original sources
USGS2004

The late cretaceous Donlin Creek gold deposit, Southwestern Alaska: Controls on epizonal ore formation

The Donlin Creek gold deposit, southwestern Alaska, has an indicated and inferred resource of approximately 25 million ounces (Moz) Au at a cutoff grade of 1.5 g/t. The ca. 70 Ma deposit is hosted in the Late Cretaceous Kuskokwim flysch basin, which developed in the back part of the arc region of an active continental margin, on previously accreted oceanic terranes and continental fragments. A hypabyssal, mainly rhyolitic to rhyodacitic, and commonly porphyritic, 8- × 3-km dike complex, part of a regional ca. 77 to 58 Ma magmatic arc, formed a structurally competent host for the mineralization. This deposit is subdivided into about one dozen distinct prospects, most of which consist of dense quartz ± carbonate veinlet networks that fill north-northeast–striking extensional fractures in the northeast-trending igneous rocks. The sulfide mineral assemblage is dominated by arsenopyrite, pyrite, and, typically younger, stibnite; gold is refractory within the arsenopyrite. Sericitization, carbonatization, and sulfidation were the main alteration processes. Fluid inclusion studies of the quartz that hosts the resource indicate dominantly aqueous ore fluids with also about 3 to 7 mol percent CO 2 ± CH 4 and a few tenths to a few mole percent NaCl + KCl. The gold-bearing fluids were mainly homogeneously trapped at approximately 275° to 300°C and at depths of 1 to 2 km. Some of the younger stibnite may have been deposited by late-stage aqueous fluids at lower temperature. Measured δ 18 O values for the gold-bearing quartz range between 11 and 25 per mil; the estimated δ 18 O fluid values range from 7 to12 per mil, suggesting a mainly crustally derived fluid. A broad range of measured δ D values for hydrothermal micas, between –150 and –80 per mil, is suggestive of a contribution from devolatilization of organic matter and/or minor amounts of mixing with meteoric fluids. Gold-associated hydrothermal sulfide minerals are characterized by δ 34 S values mainly between –16 and –10 per mil, with the sulfur derived from diagenetic pyrite and organic matter within the flysch basin. A smaller group of δ 34 S measurements, which shows values as depleted as –27 per mil, suggests a different local sulfur reservoir in the basin for the later hydrothermal episode dominated by stibnite. Initial ϵ Nd of –8.7 to –3.1 and 87 Sr/ 86 Sr measurements of 0.706 to 0.709 for the ore-hosting dikes also indicate a crustal reservoir for some of the Late Cretaceous magmatism. Overlapping lead isotope data for these intrusive rocks and for sulfide minerals suggest a crustal contribution for the lead in both. Copper- and gold-bearing stockwork veinlets in hornfels occur at Dome, a prospect located at the northern end of the Donlin Creek deposit. These stockworks are cut by the younger auriferous gold veins that define the main Donlin Creek gold mineralization. Highly saline, gas-rich, heterogeneously trapped fluids deposited the stockworks at temperatures approximately 100°C hotter than those of the main gold-forming event at Donlin Creek. The genetic relationship of the Dome prospect to the main Donlin Creek gold resource is equivocal. The epizonal Donlin Creek deposit shows affinities to the gold systems interpreted by various workers as orogenic or intrusion related; it shows important differences from typical epithermal and Carlin-like deposits. The ore-forming fluids were derived by either broad-scale metamorphic devolatilization above rising mantle melts or exsolution from a magma that was dominated by a significant flysch melt component.

Economic Geology↗
USGS1963

The peg claims spodumene pegmatites, Maine

The Peg Claims pegmatites are located southwest of Rockland in the towns of Warren and Cushing, Knox County, Maine . These pegmatites are representative of a group of zoned, granitic, lithium-bearing pegmatites in which spodumene is present nearly from wall to wall. The pegmatites are discordant, steeply dipping, tabular bodies in the Penobscot quartz-mica schist near the Waldoboro granite. A narrow quartz-tourmaline-muscovite-apatite aureole is commonly developed around each of the pegmatites and around inclusions of country rock in the pegmatites . The pegmatites are zoned, and each may be subdivided into: (1) a narrow quartz-muscovite border zone; (2) a narrow albite-quartz-muscovite wall zone; (3) and an albite-quartz- spodumene -perthite core that constitutes most of the pegmatite . Cross-cutting, fine-grained, quartz-albite- spodumene -muscovite lenses are found within pegmatite cores. The bulk mineralogy and variations in the mineralogy from zone to zone of the largest body (Dike 1) were determined by megascopic mineral point-counts. There is a decrease of albite, quartz, and muscovite and an increase of spodumene and perthite from the border and wall zones into the core. The alkali contents of Dike 1 and of each of its zones were determined by flame photometer and X-ray fluorescence analyses. The amount of Na20 decreases and the amounts of K 2 O, Li 2 0, Rb 2 0, and CS 2 O increase from the border and wall zones into the core. Estimated bulk compositions of Dike 1 and of each of its zones were computed. It is concluded that the distribution, structural features, bulk chemistry, and the mineralogical, chemical, and textural zoning of the pegmatites are consistent with development by fractional crystallization of a pegma-titic fluid in a restricted system.

Maine↗
USGS1989

The Relief Canyon gold deposit, Nevada: A mineralized solution breccia

The Relief Canyon gold deposit in the Humboldt Range of western Nevada is a low-grade, high-tonnage orebody of Tertiary or younger age. The host rocks include limestones of the Triassic Cane Spring Formation, which are overlain by shales of the Triassic Grass Valley Formation. The rocks were folded and metamorphosed to greenschist grade during Jurassic and Cretaceous regional tectonic activity. Mesozoic thrusting may have occurred along the shale-limestone contact, but evidence has been obscured by later hydrothermal activity. The sedimentary rocks were nominally offset along several Late Tertiary normal faults related to uplift of the range.The upper part of the Cane Spring Formation is composed of a poorly sorted breccia composed of limestone clasts with a clay matrix. Irregular pockets within this zone are filled with clay- to pebble-sized fragments derived from the Grass Valley shale. The enclosing limestone beds were tilted moderately to the southwest during Mesozoic deformation, whereas bedding within these pockets is generally horizontal, indicating post-tilting deposition of the sediments. The sediments show graded bedding and other sedimentary features that indicate deposition from flowing water. Thermally mature carbon derived from the limestone is also concentrated in small pockets in the matrix. The breccia unit is likely the product of low-temperature solution brecciation. Ground water dissolved much of the limestone directly beneath the shales, progressively creating irregular cavities and the breccia. Sediments derived from the overlying Grass Valley shale were fiuvially deposited as a matrix to the developing solution breccia.Episodic pulses of hydrothermal fluids were introduced along faults and possibly mixed with the ground water in the breccia zone. Initially, jasperoids formed along the faults, but later hydrothermal pulses introduced gold, silica, and fluorine into both the early jasperoids and the unconsolidated cave-fill sediments to form the orebody. Continued solution-related brecciation chaotically disrupted the gold deposit.Gold, fluorite, pyrite, silver, calcite, and fine-grained silica are the principal hydrothermal minerals in the deposit. Gold was deposited as micron-sized flakes of native gold and rarely as electrum during a relatively late stage of silicification of the jasperoids, the carbon-rich zones, and the clay-rich matrix of the breccia. Fluorite was deposited with and later than the gold in the jasperoids, and it in part replaced the clay-rich breccia matrix. Antimony, arsenic, mercury, and thallium are directly associated with gold in the orebody.The deposit formed at a relatively shallow depth. On the basis of fluid inclusion data, late-stage hydrothermal fluids related to gold and fluorite deposition were extremely dilute and had temperatures near 200 degrees C. The fluid inclusions in fluorite show no evidence for boiling, but porous crackle breccias in the jasperoids suggest that hydrobrecciation took place.

Economic Geology↗
USGS2013

Using simulated maps to interpret the geochemistry, formation and quality of the Blue Gem Coal Bed, Kentucky, USA

This study presents geostatistical simulations of coal-quality parameters, major oxides and trace metals for an area covering roughly 812 km 2 of the Blue Gem coal bed in southeastern Kentucky, USA. The Blue Gem, characterized by low ash yield and low sulfur content, is an important economic resource. Past studies have characterized the Blue Gem's geochemistry, palynology and petrography and inferred a depositional setting of a planar peat deposit that transitioned to slightly domed later in its development. These studies have focused primarily on vertical geochemical trends within the coal bed. Simulated maps of chemical elements derived from 45 measured sample locations across the study area provide an opportunity to observe changes in the horizontal direction within the coal bed. As the Blue Gem coal bed shows significant vertical chemical trends, care was taken in this study to try to select samples from a single, middle portion of the coal. By revealing spatial distribution patterns of elements across the middle of the bed, associations between different components of the coal can be seen. The maps therefore help to provide a picture of the coal-forming peat bog at an instant in geologic time and allow interpretation of a depositional setting in the horizontal direction. Results from this middle portion of the coal suggest an association of SiO 2 with both K 2 O and TiO 2 in different parts of the study area. Further, a pocket in the southeast of the study area shows elevated concentrations of elements attributable to observed carbonate-phase minerals (MgO, CaO, Ba and Sr) as well as elements commonly associated with sulfide-phase minerals (Cu, Mo and Ni). Areas of relatively high ash yield are observed in the north and south of the mapped area, in contrast to the low ash yields seen towards the east. Additionally, we present joint probability maps where multiple coal-quality parameters are plotted simultaneously on one figure. This application allows researchers to investigate associations of more than two components in a straight-forward manner useful in guiding resource exploration.

Kentucky↗
USGS1968

A lead isotope study of galenas and selected feldspars from mining Districts in Utah

The leads in feldspars from the main Tertiary intrusive bodies in each of three mining regions in Utah are isotopically similar to the lead deposits immediately associated with them. These deposits are the largest and also the least radiogenic in each region, whereas the smaller deposits are more radiogenic. Throughout each region the 'ore leads appear to be mixtures in various proportions of lead derived from the intrusive magma and a radiogenic lead component derived from the upper crustal rocks through which the mineralizing fluids passed. Linear relationships are exhibited between the lead isotope ratios within each mining region, and these enable the ages of the upper crustal basement rocks to be determined. A simple tvo-stage model theory for the development of the lead ores yields the folloving data: Oquirrh Mountains region, t 1,650 4-150 m.y., p-Cottonwood-Park City region, t = 2,415 4-30 m.y., p- = 8.87. Tintlc region, t 2,075 4-30 m.y., Milford region, t 1 765 4-70 m.y., p-T ( Us ) = , = 8.83. An alternative "mixing" model is also proposed which requires only 1,650-m.y. and 2,400-m.y. events in these regions. In either case these ages confirm and supplement our present geochronological knowledge of the basement rocks of Utah .

Utah↗
USGS2004

Chapter G: Tentative Correlation Between CIPW Normin pl (Total Plagioclase) and Los Angeles Wear in Precambrian Midcontinental Granites-Examples from Missouri and Oklahoma, with Applications and Limitations for Use

The normative chemical classification of Cross, Iddings, Pirsson, and Washington (CIPW) is commonly used in igneous petrology to distinguish igneous rocks by comparing their magmatic chemistries for similar and dissimilar components. A potential use for this classification other than in petrologic studies is in the rapid assessment of aggregate sources, possibly leading to an economic advantage for an aggregate producer or user, by providing the opportunity to determine whether further physical testing of an aggregate is warranted before its use in asphalt or concrete pavement. However, the CIPW classification currently should not be substituted for the physical testing required in specifications by State departments of transportation. Demands for physical testing of aggregates have increased nationally as users seek to maximize the quality of the aggregate they purchase for their pavements. Concrete pavements are being laid with increased thicknesses to withstand increasing highway loads. New pavement mixes, most notably Superior Performance Asphalt Pavement ('Superpave'), are designed for additional service life. For both concrete and asphalt, the intent is to generate a durable pavement with a longer service life that should decrease overall life-cycle costs. Numerous aggregate producers possess chemical-composition data available for examination to answer questions from the potential user. State geological surveys also possess chemical-composition data for stone sources. Paired with the results of physical testing, chemical- composition data provide indicative information about stone durability and aggregate strength. The Missouri Department of Transportation has noted a possible relation among coarse-grained Precambrian granites of the midcontinental region, correlating the results of abrasion testing with the contents of normative minerals, also known as normins, calculated from chemical composition data. Thus, normin pl ( total plagioclase) can predict, by way of simple regression, the Los Angeles wear for granite samples collected in Missouri. The results of this abrasion testing were extended to another granite in Oklahoma where normin pl predicted Los Angeles wear to within 0.6 percent. This relation may also exist for granitic rocks outside the Oklahoma-Missouri region, as well as for other igneous-rock types.

Bulletin↗
USGS1967

Aurorite, argentian todorokite, and hydrous silver-bearing lead manganese oxide

During a study of hypogene manganese minerals, three silver-bearing manganese oxides were identified in "black calcite" associated with silver ores at the Aurora mine (Treasure Hill), Hamilton, Nevada. Specifically these are: (1) argentian chalcophanite (aurorite) (Ag 2 Ba,Ca,Pb,-K 2 ,Cu,Mn (super +2) )Mn 3 (super +4) O 7 . 3H 2 O; (2) argentian todorokite (Ag 2 ,K 2 ,Ca,Ba,-Na 2 ,Cu,Pb)Mn 4 (super +4) O 9 . 5H 2 O; and (3) hydrous silver-bearing lead manganese oxide (Pb,Cu,Ba,Sb,Ag 2 ,Ca,K 2 )Mn 5 (super +4) O 11 . 5H 2 O. Based on the large concentration of silver in argentian chalcophanite (7.50% Ag 2 O) and the low zinc content (0.25% ZnO) this mineral is recognized as a new species and named "aurorite."Physical and optical properties, and chemical analyses of the three minerals are presented together with X-ray powder diffraction data for aurorite and argentian todorokite. All chemical analyses were done with the electron microprobe analyzer. Due to extremely small particle size and textural relations, no X-ray powder data were obtained for the hydrous silver-bearing lead manganese oxide. Associated minerals include cryptomelane, pyrolusite, birnessite(?), cerargyrite, native silver, quartz, and manganoan calcite. Although the general uniform dark color of the "black calcite" and apparent uniform distribution of the inclusions suggests contemporaneous formation, examination of polished sections at high magnifications shows a definite paragenetic sequence. Manganoan calcite is replaced by fine-grained intergrowths of cryptomelane, pyrolusite, birnessite(?), and cerargyrite. Native silver formed early and the silver-bearing manganese oxides formed late in the sequence; aurorite fills microfractures that cut through all other minerals.These three silver-bearing manganese oxides were recognized only at the Aurora mine, Hamilton, Nevada, although black calcite veins were studied from numerous areas in the western United States. The geologic features of silver-bearing black calcites are described by Hewett and Radtke in the preceding paper.

Nevada↗
USGS2016

Book review: New concepts and discoveries: the Geological Society of Nevada 2015 Symposium Proceedings

The Nevada Geological Society has a long history of convening meetings and workshops focused on the geology and metallogeny of the western United States relevant to the mineral exploration and mining community across the Great Basin. One outgrowth of the Geological Society of Nevada’s 2015 Symposium is a two-volume set, edited by W.M. Pennell and L.J. Garside, entitled New Concepts and Discoveries . The symposium was held in Sparks, Nevada, May 14–23, 2015, with more than 1,000 attendees, 59 talks in 10 thematic sessions, 7 field trips, and 10 short courses, all focused on serving the geologic, exploration, and mining community. The attractively produced, hardbound, two-volume set includes a CD-ROM containing all the manuscripts as well as numerous abstracts from presentations arranged by the thematic session in which they were presented. The papers range from detailed case study descriptions of individual deposits to important syntheses covering the geologic evolution and resulting metallogeny of the Great Basin and beyond.

Economic Geology↗
USGS1945

The preparation of polished thin sections

The relations in ores between opaque and translucent minerals can not be determined satisfactorily by independent examination of either polished surfaces or thin sections. Previous attempts to prepare polished thin sections by Tolman and Rogers, and thinned polished sections by Donnay, appear not to have been entirely successful. Canada balsam, the cementing medium used, is not sufficiently tenacious to overcome the stresses imparted to the surface of the polished grain by the polishing process, which cause the grains to crackle and curl away from the glass slide. This paper describes two mounting media which largely overcome these stresses. These media are the thermoplastics glycol phthalate and Gelva V 7. Glycol phthalate is best suited to the polishing of thin sections on a cloth lap. Gelva V 7 is preferable if sections are to be polished on a Graton-Vanderwilt polishing machine. Photomicrographs of the results are appended.

Economic Geology↗
USGS2022

Early Neoproterozoic gold deposits of the Alto Guaporé province, southwestern Amazon craton, western Brazil

The Alto Guaporé gold province, southwestern Amazon craton, contains gold deposits that have been mined since the beginning of the 18th century and these deposits, together, have modern-day, pre-mining gold resources of at least 1.8 Moz. The ore is associated with quartz vein systems along the southeastern part of the Aguapei belt, a ~35-km-wide and ~500-km-long, NNW-trending shear zone formed due to reactivation of a terrane-bounding suture. The Aguapei belt evolved by ca. 1150 to 1100 Ma rifting and deposition of siliciclastic sediments in an aulacogen basin, followed by deformation and low-grade metamorphism of the sedimentary sequences during 1100 to 900 Ma terrane collision along the craton margin. The deformation was characterized by a compressional regime until ca. 950 Ma and transition to a transpressional setting during the final 50 m.y. The gold deposits are hosted in a variety of structures that are second-order to the main Aguapei shear zone. The Ernesto and Pau-a-Pique deposits are located ~40 km apart and at jogs along the Aguapei belt. They are marginal to pre-ore igneous rocks, with Ernesto hosted in the basal part of the metasedimentary Fortuna Formation that overlies tonalite and Pau-a-Pique at the contact between metasedimentary rocks and diorite. Three deformational phases comprise the compressional (D 1 to D 2 ) to transpressional (D 3 ) tectonic events. In the Pau-a-Pique deposit and the deeper level of the Ernesto deposit, the ore-bearing veins are bedding parallel and follow D 2 strike-slip and reverse fault zones, respectively. However, the veins formed during D 3 reactivation of the older structures by an array of oblique accommodation faults. In contrast, ores at shallower levels of Ernesto, both in discordant and bedding-parallel veins, are hosted within a ~20-m-thick rigid metaconglomerate with associated dilation due to the structural complexity as sedimentary rocks of the Aguapei Group were folded around the dome-shaped roof of the pre-ore tonalite. The ores in both deposits, as well as in many other deposits of the province, are characterized by disseminated and vein-hosted pyrite. Gold occurs mainly as inclusions in the pyrite, with other hydrothermal phases comprising muscovite, Fe-Ti oxides, and minor apatite, chalcopyrite, and galena. Fluid inclusion data, coupled with stable isotope geochemistry and geothermometry, indicate that gold precipitated from a low-salinity, CO 2 -rich fluid at ~300°C and ~2.5 kbar. The source for the fluid and gold was the interbedded pelites during devolatilization of the Aguapei Group sequence. The aqueous-carbonic fluid inclusions and the narrow range of δ 18 O values of quartz (12 ± 1 ‰ ) from many auriferous veins from the central part of the province represent a regional ore-forming fluid. The broad range of δ D for hydrous minerals (–116 to –55 ‰ ) reflects influx of small amounts of meteoric water into the steeply dipping shear zones during postgold exhumation. The 40 Ar/ 39 Ar geochronology from hydrothermal muscovite indicates a widespread hydrothermal event along the belt between 928 and 920 Ma. Collectively, the geological, geochronological, and geochemical data suggest that metamorphic fluids migrated laterally into and then upward along the Aguapei belt and deposited gold in lower-order structures where strain gradients existed between lithounits. The province has many characteristics of large orogenic gold provinces worldwide and represents a highly prospective and underexplored target region for early Neoproterozoic gold, a time period that generally is not well endowed in gold ores.

Alto Guaporé gold province, Amazon craton↗
USGS1984

U-Pb isotope systematics and apparent ages of uranium ores, Ambrosia Lake and Smith Lake districts, Grants mineral belt, New Mexico

Orebodies occur in continental sandstones of the Upper Jurassic Morrison Formation and comprise two main types of mineralization; primary ore formed early in the history of the host rock and resulted in tabular, peneconcordant orebodies rich in organic material; redistributed ore apparently was formed by oxidative destruction and reconcentration of primary ore at some time after Laramide deformation and resulted in roll-type orebodies lacking organic material. U-Pb isotope ages of four redistributed ores from the Ambrosia Lake District give concordant to nearly concordant ages of 3.3 to 12.5 m.y., supporting the concept that most redistributed ore formed in the late Tertiary or later. U-Pb isotope ages of primary ores from the Ambrosia Lake District show pronounced normal discordance, evidently due to coupled, continuous loss of Pb and radioactive daughters of 238 U. Data for ores from the Smith Lake District consistently give pre-Tertiary apparent ages.

Economic Geology↗
USGS2017

Undiscovered porphyry copper resources in the Urals—A probabilistic mineral resource assessment

A probabilistic mineral resource assessment of metal resources in undiscovered porphyry copper deposits of the Ural Mountains in Russia and Kazakhstan was done using a quantitative form of mineral resource assessment. Permissive tracts were delineated on the basis of mapped and inferred subsurface distributions of igneous rocks assigned to tectonic zones that include magmatic arcs where the occurrence of porphyry copper deposits within 1 km of the Earth's surface are possible. These permissive tracts outline four north-south trending volcano-plutonic belts in major structural zones of the Urals. From west to east, these include permissive lithologies for porphyry copper deposits associated with Paleozoic subduction-related island-arc complexes preserved in the Tagil and Magnitogorsk arcs, Paleozoic island-arc fragments and associated tonalite-granodiorite intrusions in the East Uralian zone, and Carboniferous continental-margin arcs developed on the Kazakh craton in the Transuralian zone. The tracts range from about 50,000 to 130,000 km 2 in area. The Urals host 8 known porphyry copper deposits with total identified resources of about 6.4 million metric tons of copper, at least 20 additional porphyry copper prospect areas, and numerous copper-bearing skarns and copper occurrences. Probabilistic estimates predict a mean of 22 undiscovered porphyry copper deposits within the four permissive tracts delineated in the Urals. Combining estimates with established grade and tonnage models predicts a mean of 82 million metric tons of undiscovered copper. Application of an economic filter suggests that about half of that amount could be economically recoverable based on assumed depth distributions, availability of infrastructure, recovery rates, current metals prices, and investment environment.

Urals↗
USGS1967

Ground-water hydrology of the Punjab region of West Pakistan, with emphasis on problems caused by canal irrigation

Rising water tables and the salinization of land as the result of canal irrigation threaten the agricultural economy of the Punjab. Since 1954 the Water and Soils Investigation Division of the West Pakistan Water and Power Development Authority has inventoried the water and soils resources of the Punjab and investigated the relations between irrigation activities, the natural hydrologic factors, and the incidence of waterlogging and subsurface-drainage problems. This report summarizes the findings of the investigation, which was carried out under a cooperative agreement between the Government of Pakistan and the U.S. Agency for International Development, and its predecessor, the U.S. International Cooperation Administration. Leakage from the canal systems, some of which have been in operation for more than 100 years, is the principal cause of rising water levels and constitutes the major component of ground-water recharge in the Punjab. Geologic studies have shown that virtually the entire Punjab is underlain to depths of 1,000 feet or more by unconsolidated alluvium, which is saturated to within a few feet of land surface. The alluvium varies in texture from medium sand to silty clay, but sandy sediments predominate. Large capacity wells, yielding 4 cfs or more, can be developed almost everywhere. Ground water occurring within a depth of 500 feet below the surface averages less than 1,000 ppm of dissolved solids throughout approximately two-thirds of the Punjab. It is estimated that the volume of usable ground water in storage in this part of the alluvial aquifer is on the order of 2 billion acre-feet. In the other one-third of the Punjab, total dissolved solids range from 1,000 to about 20,000 ppm. In about one-half of this area (one-sixth of the area of the Punjab) some ground water can be utilized by diluting with surface water from canals. The ground-water reservoir underlying the Punjab is an unexploited resource of enormous economic value. It is recognized that the scientific management of this ground-water reservoir is the key to permanent irrigation agriculture in the Punjab. The West Pakistan Water .and Power Development Authority has prepared a long-range program for reclaiming the irrigated lands of the Punjab. The essential feature of this program is a proposed network of tubewells (drilled wells) located with an .average density of about one per square mile. Groundwater withdrawals will serve the dual purpose of helping to supply irrigation requirements and of providing subsurface drainage. Despite the feasibility and inherent advantages of tubewell reclamation methods, it is inevitable that just as the superposition of the canal system on the native environment caused undesirable side effects, large-scale ground-water withdrawals again will disturb the hydrologic regimen. The distribtution of withdrawals and maintenance of a favorable salt balance are two distinct, but related aspects of the ground-water budget that present potential hazards that must be considered in the design and management of the tubewell projects. The availability of ground water for irrigation diminishes from northeast to southwest, or downgradient along the doab (an area lying between two rivers) and is negligible in the centers of the lower parts of the doabs, where the ground water is too highly mineralized for use. Ground-water supplies must be developed in areas where they are available and it might become necessary, under a program of maximum exploitation of ground-water resources, to transfer supplies from outside sources to points of use in the lower parts of the doabs. Several factors inherent in the tubewell system will tend to depreciate the quality of ground water with time. Among these are the addition of salts leached from the soils, increased concentration of salts due .to repeated cycles of recirculation, and the possible lateral and upward encroachment of saline water in response to pumping. It is reasonably ce

Water Supply Paper↗
USGS2018

Major element and oxygen isotope geochemistry of vapour-phase garnet from the Topopah Spring Tuff at Yucca Mountain, Nevada, USA

Twenty vapour-phase garnets were studied in two samples of the Topopah Spring Tuff of the Paintbrush Group from Yucca Mountain, in southern Nevada. The Miocene-age Topopah Spring Tuff is a 350 m thick, devitrified, moderately to densely welded ash-flow tuff that is zoned compositionally from high-silica rhyolite to latite. During cooling of the tuff, escaping vapour produced lithophysae (former gas cavities) lined with an assemblage of tridymite (commonly inverted to cristobalite or quartz), sanidine and locally, hematite and/or garnet. Vapour-phase topaz and economic deposits associated commonly with topaz-bearing rhyolites (characteristically enriched in F) were not found in the Topopah Spring Tuff at Yucca Mountain. Based on their occurrence only in lithophysae, the garnets are not primary igneous phenocrysts, but rather crystals that grew from a F-poor magma-derived vapour trapped during and after emplacement of the tuff. The garnets are euhedral, vitreous, reddish brown, trapezohedral, as large as 2 mm in diameter and fractured. The garnets also contain inclusions of tridymite. Electron microprobe analyses of the garnets reveal that they are almandine-spessartine (48.0 and 47.9 mol.%, respectively), have an average composition of (Fe 1.46 Mn 1.45 Mg 0.03 Ca 0.10 )(Al 1.93 Ti 0.02 )Si 3.01 O 12 and are comparatively homogeneous in Fe and Mn concentrations from core to rim. Composited garnets from each sample site have δ18O values of 7.2 and 7.4‰. The associated quartz (after tridymite) has δ 18 O values of 17.4 and 17.6‰, values indicative of reaction with later, low-temperature water. Unaltered tridymite from higher in the stratigraphic section has a δ 18 O of 11.1‰ which, when coupled with the garnet δ 18 O values in a quartz-garnet fractionation equation, indicates isotopic equilibration (vapour-phase crystallization) at temperatures of ~600°C. This high-temperature mineralization, formed during cooling of the tuffs, is distinct from the later and commonly recognized low-temperature stage (generally 50–70°C) of calcite, quartz and opal secondary mineralization, formed from downward-percolating meteoric water, that locally coats fracture footwalls and lithophysal floors.

Nevada↗
USGS1946

Hydrothermal alteration in the Castle Dome copper deposit, Arizona

Hydrothermal alteration of the quartz monzonite host rock in the Castle Dome copper deposit consists of three phases. Very weak propylitic alteration of biotite and plagioclase occurs in the marginal part of the mineralized area. Where mineralization is stronger most of the plagioclase and some of the orthoclase and biotite are replaced by a montmorillonitetype clay, resembling beidellite, and hydrous mica. The third phase is localized along quartz-pyrite veins where the wall rock, commonly as much as an inch from the vein, is replaced by quartz and seriate. In contrast to the localization of quartz-sericite alteration along the veins, clay alteration is general and can be seen between the veins wherever the rock is not completely sericitized. Thus these two types of alteration cannot be separated areally. The most intense clay alteration together with moderate quartz-sericite alteration occurs in the ore body, whereas the strongest quartz-sericite alteration, where pyrite veins are largest and most numerous, is in a zone along the north side of the ore body.

Arizona↗
USGS1949

Geochemical prospecting for ores; a progress report

Methods of prospecting for mineral deposits by means of chemical studies of residual soil, alluvium, glacial moraine, vegetation, and natural water were first applied on a systematic scale in Scandinavia and Russia about 15 years ago. Since the war, work in this field has been undertaken by several independent groups in the United States and Canada, including the Geochemical Prospecting Unit of the U. S. Geological Survey.Best results have been obtained by systematic analysis of zinc in residuum, ground water, and vegetation; copper and lead in gossan and residuum; tin in residuum and alluvium; molybdenum in residuum; and cobalt, nickel, and gold in vegetation. In addition, studies of plant ecologies and toxicity symptoms in vegetation in relation to metal-rich soil appear to be promising.Rapid colorimetric and spectrographic tests for traces of metals in soil, water, and vegetation have been developed for use under field conditions.

Economic Geology↗
USGS2016

Oxygen, hydrogen, sulfur, and carbon isotopes in the Pea Ridge magnetite-apatite deposit, southeast Missouri, and sulfur isotope comparisons to other iron deposits in the region

Oxygen, hydrogen, sulfur, and carbon isotopes have been analyzed in the Pea Ridge magnetite-apatite deposit, the largest historic producer among the known iron deposits in the southeast Missouri portion of the 1.5 to 1.3 Ga eastern granite-rhyolite province. The data were collected to investigate the sources of ore fluids, conditions of ore formation, and provenance of sulfur, and to improve the general understanding of the copper, gold, and rare earth element potential of iron deposits regionally. The δ 18 O values of Pea Ridge magnetite are 1.9 to 4.0‰, consistent with a model in which some magnetite crystallized from a melt and other magnetite—perhaps the majority—precipitated from an aqueous fluid of magmatic origin. The δ 18 O values of quartz, apatite, actinolite, K-feldspar, sulfates, and calcite are significantly higher, enough so as to indicate growth or equilibration under cooler conditions than magnetite and/or in the presence of a fluid that was not entirely magmatic. A variety of observations, including stable isotope observations, implicate a second fluid that may ultimately have been meteoric in origin and may have been modified by isotopic exchange with rocks or by evaporation during storage in lakes. Sulfur isotope analyses of sulfides from Pea Ridge and seven other mineral deposits in the region reveal two distinct populations that average 3 and 13‰. Two sulfur sources are implied. One was probably igneous melts or rocks belonging to the mafic- to intermediate-composition volcanic suite that is present at or near most of the iron deposits; the other was either melts or volcanic rocks that had degassed very extensively, or else volcanic lakes that had trapped rising magmatic gases. The higher δ 34 S values correspond to deposits or prospects where copper is noteworthy—the Central Dome portion of the Boss deposit, the Bourbon deposit, and the Vilander prospective area. The correspondence suggests that (1) sulfur either limited the deposition of copper or was cotransported with copper, and (2) sulfur isotope analysis may be useful in evaluating southeast Missouri iron deposits for copper and possibly for gold.

Missouri↗
USGS1952

Supergene and hydrothermal dispersion of heavy metals in wall rocks near ore bodies, Tintic district, Utah

Part I, T. S. Lovering. Preliminary work in the Tintic district , Utah , determined the relative distance of migration of ore metals in moist carbonate and silicic wall rocks near ore , and in ground water having a very sluggish circulation through mineralized ground. The analyses of efflorescences in the Tintic Standard mine openings at suitable localities indicated that under the conditions existing during the past 25 years lead has not moved perceptibly; gold has migrated only a few inches; copper has moved a few score feet at most and only in acid solutions; and zinc has traveled hundreds of feet, but not as far as some silver. Silver may travel far in the oxidized zone, but if it comes in contact with sulfides its migration is quickly halted. The chief chemical factors that influence the migration of metals in supergene solutions are the hydrogen ion concentration (pH) of hydrolysis of the metal ion, the solubility of sulfates of the metals , and the position of the metals in Schuermann's series if the solutions are in contact with sulfides for a considerable time.

Utah↗