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At least 1,063 records · Page 59Linked to original sources

USGS1983

Groundwater resources of the Dallas-Monmouth area, Polk, Benton, and Marion Counties, Oregon

The Dallas-Monmouth area is in the west-central Willamette Valley of western Oregon. It comprises a total of about 400 square miles in Polk, Benton, and Marion Counties. Tertiary consolidated rocks underlie the entire area. These rocks include marine sandstone, siltstone, shale, tuff, basalt, and gabbro. In the lowlands, the consolidated rocks are overlain by unconsolidated deposits of clay, silt, sand, and gravel which reach a maximum thickness of about 125 feet locally. The consolidated rocks generally are poor water-bearing formations and yield only small quantities of ground water to wells. From 1956 to 1976, about 6 percent of all wells completed in basalt and 14 percent of those completed in marine sedimentary rocks were reported to be "dry holes." The median yield of wells completed in the consolidated rocks was less than 10 gallons per minute, and the maximum reported yield was 200 gallons per minute. Shallow ground water in the consolidated rocks is generally of good quality, but with increasing depth in these rocks the water contains increasing concentrations of dissolved minerals. It is estimated that about 5 percent of all the wells completed in the consolidated rocks yield poor-quality water. The best water-bearing units are sand-and-gravel beds in the unconsolidated deposits. Near Independence, the sand-and-gravel beds are hydraulically continuous and exceed a saturated thickness of 10 feet over an area covering several tens of square miles chiefly within the Willamette River flood plain. Properly constructed wells in these deposits can generally obtain yields of 100 to 500 gallons per minute, and large quantities of additional ground water can be developed from this source. Outside the above area, sand-and-gravel beds generally are less than 10 feet thick, lenticular in shape, local in extent, and generally yield less than 30 gallons per minute to wells. Sand-and-gravel beds probably are absent in the Baskett Slough drainage basin, in the E. E. Wilson Wildlife Management area, and in the Luckiamute and Little Luckiamute River valleys upstream from the U.S. Highway 99W bridge.

Oregon↗

USGS1963

Geology of the Anlauf and Drain Quadrangles, Douglas and Lane Counties, Oregon

The Anlauf and Drain quadrangles, Oregon, lie about 20 miles south of the city of Eugene, in Douglas and Lane Counties. They constitute an area of about 435 square miles that includes parts of both the Cascade Range and Coast Range physiographic provinces. A sequence of lower Tertiary sedimentary and volcanic rocks with a maximum thickness of about 20,000 feet is exposed in the area. The oldest part of this sequence is the Umpqua formation of early Eocene age consisting of a lower member of vesicular and amygdaloidal olivine basalt flows, a middle member of water-laid vitric and lapilli crystal tuff, and an upper member of interbedded fissile siltstone and basaltic sandstone which contains a 300-foot tongue of massive to thick-bedded basaltic sandstone near its top. These rocks are predominantly of marine origin, although the general absence of pillow structures which are common in basaltic lavas of equivalent age elsewhere in the Coast Ranges suggests that some of the flows were poured out subaerially. The overlying tuff member, however, contains Foraminifera and in places has a lime content slightly in excess of 10 percent. Mollusca and Foraminifera indicate that the Umpqua formation is of early Eocene age and is a correlative of the Capay formation of California. The Tyee formation of middle Eocene age overlies the Umpqua formation and consists of more than 5,000 feet of rhythmically deposited sandstone and siltstone in beds 2 to 30 feet thick. The basal part of each bed consists of medium- to coarse-grained sandstone that grades upward into fine-grained sand- stone and siltstone. The principal constituents of the sandstone are quartz, partly a1tered feldspar, mica, clay, and fragments of basalt, fine-grained argillaceous rocks, and mica schist. Other detrital minerals include epidote, garnet, blue-green hornblende, tourmaline, and zoisite. The depositional environment of the Tyee formation is poorly known, although the rhythmic-graded bedding suggests turbidity currents. About 500 feet of sandstone and siltstone assigned to the Spencer formation of late Eocene age unconformably overlies the Tyee formation. The Spencer formation, better exposed in the east-central part of the Coast Ranges, contains marine fossils but also has thin impure coal beds, indicative of strand-line accumulation. The sandstone in the Spencer formation is very similar to that in the Tyee formation, from which it was probably derived. The Fisher formation contains about 5,500 feet of nonmarine pyroclastic and volcanic rocks that are related to the volcanic rock sequences of the western Cascade Range. The formation is characterized by a wide variety of rock types, including conglomerate, tuffaceous sandstone and siltstone, vitric and crystal tuff, waterlaid and mudflow breccia, and andesitic lava flows. These rocks gen- erally occur in lenticular beds that have little stratigraphic significance. The rocks apparently accumulated on a plain slightly above sea level that was subjected alternately to fiooding by running water and to desiccation. Fossil leaves from the lowermost part of the Fisher formation are of late Eocene age; the upper part of the formation is of early, and possibly niiddle, Oligocene age. A few exposures of olivine basalt were mapped in the extreme northern part of the Anlauf quadrangle. The flows, more extensively exposed to the north, overlie the Fisher formation, and, therefore, are tentatively considered to be post-Oligocene in age. All these stratigraphic units, but principally the Fisher formation, are cut by dikes, sills, and stocklike bodies of 'porphyritic basalt, diabase, and norite. Contemporaneously with the emplacement of most of these rocks, in late Miocene (?) time, hydrothermal solutions locailly altered the sedimentary and extrusive igneous racks and deposited cinnabar and other sulfide minerals, carbonates, and silica. Three parallel nartheastward-trending

Bulletin↗

USGS2005

Evolution of the magmatic-hydrothermal acid-sulfate system at Summitville, Colorado: Integration of geological, stable-isotope, and fluid-inclusion evidence

The Summitville Au-Ag-Cu deposit is a classic volcanic dome-hosted high-sulfidation deposit. It occurs in the Quartz Latite of South Mountain, a composite volcanic dome that was emplaced along the coincident margins of the Platoro and Summitville calderas at 22.5??0.5 Ma, penecontemporaneous with alteration and mineralization. A penecontemporaneous quartz monzonite porphyry intrusion underlies the district and is cut and overlain by pyrite-quartz stockwork veins with traces of chalcopyrite and molybdenite. Alteration and mineralization proceeded through three hypogene stages and a supergene stage, punctuated by at least three periods of hydrothermal brecciation. Intense acid leaching along fractures in the quartz latite produced irregular pipes and lenticular pods of vuggy silica enclosed sequentially by alteration zones of quartz-alunite, quartz-kaolinite, and clay. The acid-sulfate-altered rocks host subsequent covellite+enargite/luzonite+chalcopyrite mineralization accompanied by kaolinite, and later barite-base-metal veins, some containing high Au values and kaolinite. The presence of both liquid- and vapor-rich fluid inclusions indicates the episodic presence of a low-density fluid at all levels of the system. In the mineralized zone, liquid-rich fluid inclusions in healed fractures in quartz phenocrysts and in quartz associated with mineralization homogenize to temperatures between 160 and 390 ??C (90% between 190 and 310 ??C), consistent with the range (200-250 ??C) estimated from the fractionation of sulfur isotopes between coexisting alunite and pyrite. A deep alunite-pyrite pair yielded a sulfur-isotope temperature of 390 ??C, marking a transition from hydrostatic to lithostatic pressure at a depth of about 1.5 km. Two salinity populations dominate the liquid-rich fluid inclusions. One has salinities between 0 and 5 wt.% NaCl equivalent; the other has salinities of up to 43 wt.% NaCl equivalent. The occurrence of high-salinity fluid inclusions in vein quartz associated with mineralization, as well as in the deep stockwork veins, suggests that brines originating deep in the system transported the metals. The ??34S values of sulfides in magnetite (-2.3???) and of sulfate in apatite (5.4???) in unaltered quartz latite indicate that ??34S???S was near 0???. The ??34S values of coexisting alteration alunite and pyrite are 18.2??? to 24.5??? and -8.1??? to -2.2???, respectively. Deep in the system, most of the change in ??34S values occurs in the sulfates, indicating that the fluids were initially H2S-dominant, their redox state buffered at depth by equilibration with igneous rocks. However, in the main alteration zone, most of the change in ??34S values occurs in pyrite, indicating that the fluids moved off the rock buffer and became SO42- -dominant as pyrite precipitated and SO2 disproportionation produced the sulfuric acid requisite for acid leaching. The ??34S values of the late-stage barite and sulfides indicate that the system returned to high H2S/SO42- ratios typical of the original rock-buffered fluid. The ??DH2O of alunite parent fluids was near -45??? and their ??18O ranged from 7??? to -1???, depending on the degree of exchange in the alteration zone at low water-rock ratio, or mixing with unexchanged meteoric water. The low ??D values of some alunite samples are interpreted to result from postdepositional exchange with later ore fluids. Fluid exsolved fr om the magma at depth had ??DH2O and ??18OH2O values near -70??? and 10???, respectively. During and following migration to the top of the magma chamber, the fluid underwent isotopic exchange with the partially crystallized magma and its solid and cooler, but still plastic, carapace just below the transition from a lithostatic to hydrostatic pressure regime. These evolved magmatic fluids had ??DH2O and ??18OH2O values close to -40??? and 5???, respectively, prior to release into the superjacent hydrostatically pressured fracture zone, wherein the fluids separat

Chemical Geology↗

USGS1983

Peralkaline and peraluminous granites and related mineral deposits of the Arabian Shield, Kingdom of Saudi Arabia

In the Precambrian Arabian Shield, granitoid plutonic rocks are widespread and range in age from 800 to 550 Ma old; but the mineral-resource potential associated with these plutonic rocks is restricted mainly to the younger, postorogenic granites. Two granite types of current economic interest are zirconium- niobium-enriched per alkaline granites and tin-tungsten-enriched peraluminous granites. Both types are highly evolved, are enriched in lithium, rubidium, and fluorine, and have distinctive mineralogy, textures, and chemistry. The zirconium-niobium-enriched granites are related to medium- to large-sized plutons and complexes of peralkaline granite, and the tin-tungsten-enriched granites are related to medium-sized plutons of biotite or biotite-muscovite granite. Existing geochemical and geologic data for many parts of the Arabian Shield were compiled as a basis for evaluating the resource potential of the granites of the Shield. Commodities associated with granites that have potential for economic mineral deposits include tin, tungsten, molybdenum, beryllium, niobium, tantalum, zirconium, uranium, thorium, rare-earth elements, and fluorite. Prospecting methods useful in discriminating those granites having significant economic potential include reconnaissance geologic mapping, petrographic and mineralogic studies, geochemical sampling of rock and wadi sediment, and radiometric surveying.

Open-File Report↗

USGS2020

Mineral deposits of the Mesoproterozoic Midcontinent Rift system in the Lake Superior region – A space and time classification

The Mesoproterozoic Midcontinent Rift System (MRS) of North America hosts a diverse suite of magmatic and hydrothermal mineral deposits in the Lake Superior region where rift rocks are exposed at or near the surface. Historically, hydrothermal deposits, such as Michigan’s native copper deposits and the White Pine sediment-hosted stratiform copper deposit, were major MRS metal producers. On-going exploration for and potential development of copper-nickel sulfide deposits hosted by the Duluth Complex of Minnesota and the opening of the Eagle nickel mine in Michigan indicate an expanding interest in MRS magmatic deposits. MRS hydrothermal and magmatic mineral deposits, many of which are significant past, present, and likely future providers of critical minerals, here are placed into a space and time metallogenic framework. To construct this framework, regional MRS mineral deposits extracted from the U.S. Geological Survey Mineral Resources Data System (MRDS) and the Ontario Ministry of Energy, Northern Development and Mines Mineral Deposit Inventory (MDI) were supplemented by other known and recently recognized mineral deposits described in the literature. All mineral deposits were classified by deposit type, host rock age and type, and estimated timing of mineralization. Deposits were then put into a tectonic evolutionary framework (stages) for the MRS, which shows that deposits formed within discrete spatial and temporal stages of rift evolution.

Lake Superior region↗

USGS2017

Rare earth mineral potential in the southeastern U.S. Coastal Plain from integrated geophysical, geochemical, and geological approaches

We combined geophysical, geochemical, mineralogical, and geological data to evaluate the regional presence of rare earth element (REE)−bearing minerals in heavy mineral sand deposits of the southeastern U.S. Coastal Plain. We also analyzed regional differences in these data to determine probable sedimentary provenance. Analyses of heavy mineral separates covering the region show strong correlations between thorium, monazite, and xenotime, suggesting that radiometric equivalent thorium (eTh) can be used as a geophysical proxy for those REE-bearing minerals. Airborne radiometric data collected during the National Uranium Resource Evaluation (NURE) program cover the southeastern United States with line spacing varying from ∼2 to 10 km. These data show eTh highs over Cretaceous and Tertiary Coastal Plain sediments from the Cape Fear arch in North Carolina to eastern Alabama; these highs decrease with distance from the Piedmont. Quaternary sediments along the modern coasts show weaker eTh anomalies, except near coast-parallel ridges from South Carolina to northern Florida. Prominent eTh anomalies are also observed over large riverbeds and their floodplains, even north of the Cape Fear arch where surrounding areas are relatively low. These variations were verified using ground geophysical measurements and sample analyses, indicating that radiometric methods are a useful exploration tool at varying scales. Further analyses of heavy mineral separates showed regional differences, not only in concentrations of monazite, but also of rutile and staurolite, and in magnetic susceptibility. The combined properties suggest the presence of subregions where heavy mineral sediments are primarily sourced from high-grade metamorphic, low-grade metamorphic, or igneous terrains, or where they represent a mixing of these sources. Comparisons between interpreted sources of heavy mineral sands near the Fall Line and igneous and metamorphic Piedmont and Blue Ridge units showed a strong correspondence with rocks closest to the Fall Line and poor correspondence with rocks farther inland. This strongly suggests that the primary source of those heavy minerals, especially monazite, is the rocks that formed the rocky coast that was present during opening of the Atlantic Ocean, which in turn indicates the importance of coastal processes in forming heavy mineral sand concentrations. Furthermore, narrow radiometric eTh and K anomalies are associated with major rivers, indicating limited spatial influence of fluvial processes. Later coastal plain sediment deposition appears to have involved reworking of sediments, providing an “inheritance” of the rocky coast composition that persists for some distance from the Fall Line. However, this inheritance is reduced with distance, and sediments within ∼100 km of the coast in Georgia and Florida exhibit properties indicative of mixing from multiple sources.

GSA Bulletin↗

USGS1960

Metallization and post-mineral hypogene argillization, Lost River tin mine, Alaska

The Lost River tin and tungsten deposit occurs in a buried granite pluton and in associated rhyolite dikes that intrude Paleozoic limestone. The dikes and parts of the granite were greisenized and then argillized irregularly. Metallization accompanied greisenization rather than argilli-zation, although both processes probably were closely related in time. Iron-zinc ratios in sphalerite indicate that the ore minerals were deposited at a temperature between 425°C and 740°C. This temperature is within the range of the temperatures at which topaz, a comman associate of the ore minerals, has been synthesized in the laboratory. The temperature of deposition of the ore minerals is above the temperature interval in which clay minerals are stable. Thus, clay minerals could not have formed while ore was being deposited. As temperatures fell and entered the stability range of the clay minerals, argillic alteration encroached upon greisen ores and wall rocks. Reaction rims between quartz and topaz indicate that kaolinite could have formed according to the following reaction: topaz + quartz + water + limestone -* kaolinite + fluorite + Cajj"C (Al,F)2Si04 + Si02 +3H2O+ CaCOa - Al2Si20B(OH)41 + CaF2 + H2CO3 The clay minerals, which formed from diverse rock types, consist of kaolinite, dickite, mixed-layered chlorite-montmorillonite, and minor montmorillonite, accompanied by variable amounts of muscovite and zinnwaldite. The dickite is most common in and near late veins that cut the earlier-formed greisen. Temperature is believed to be the principal agent governing the relations between ore deposition and argillization . Similar relations are to be expected in other high-temperature deposits where abundant veining and fracturing indicate that wall rocks reached an isothermal condition above a maximum temperature of 480°C during ore deposition.

Bering land bridge↗

USGS1980

Intrusions and intrusive complexes in an ophiolite near San Luis Obispo; a chemical and petrologic study

The San Luis Obispo ophiolite is a sequence of mafic and ultramafic rocks that lie as a thrust-like slice on top of Franciscan melange in the southern California Coast Range. The ophiolite comprises the classic Steinmann Trinity of basal serpentinized ultramafic rocks overlain sequentially by spilitized pillow lavas and cherts. The ophiolite is intruded by numerous diabasic dikes and several large intrusive complexes that contain peridotite, gabbro, diabase and felsic rocks. Field relations indicate that this ophiolite originated as a sequence of submarine volcanic flows and breccia units extruded onto an ultramafic basement. Later the volcanic and ultramafic rocks were intruded along their contact by mafic magma that crystallized as a stratiform complex of cumulate peridotite and gabbro. Next, this stratiform complex was intruded by diorite and albite-granite (trondhjemite) sills, which may have been produced by partial melting of gabbros in the upper levels of the stratiform complex. Finally all earlier units were intruded by diabasic sills and dikes that probably were late feeders for the pre-existing volcanic unit. All the ophiolite rocks underwent subsequent highly localized brecciation, under greenschist-facies conditions, in which felsic rocks were incipiently melted by frictional heating. The host rocks and intrusions were metamorphosed at low temperature and the mafic rocks were altered by Na- and Si-metasomatism. Despite this pervasive alteration, evidences of the formational stages of the ophiolite are preserved in disequilibrium mineral assemblages, primarily within mafic and ultramafic rocks of the intrusive complexes. Whole-rock analyses of the ophiolite suite indicate that alteration has obscured but not obliterated the original chemical character of the mafic rocks. The ophiolite diabases and basalts contain up to 6 weight percent Na 2 0 and 58 percent Si0 2 , in contrast to normal Si0 2 contents of 50 percent or less and Na 2 0 contents of less than 3 percent for unaltered oceanic and continental tholeiite basalts. However, the low K 2 0 contents (less than 0.8 percent) of all units in the ophiolite suite indicate that these rocks are not part of an alkalic kindred, but rather are altered tholeiites. Compositions of relict amphibole and plagioclase in the mafic rocks indicate that the ophiolite formed in a high-temperature environment and the deep seated rocks were metamorphosed under amphibolite-facies conditions. A later change in the environmental conditions allowed subsequent low-temperature (greenschist facies) metamorphism and metasomatism. The structure and chemistry of the San Luis Obispo ophiolite support the hypothesis that this body may represent a fragment of oceanic crust and mantle. The volcanic rocks and the intrusive complexes probably formed at a mid-ocean ridge, where conditions of high heat-flow provided the requisite high-temperature environment and promoted the production of mafic magmas. The change to low-temperature conditions probably was due to inception or acceleration of crustal spreading at that portion of the ridge where this ophiolite formed. The nature of brecciation in the ophiolite indicates that the low-temperature environment also was one of greater tectonic disturbance, relative to conditions at the ridge. This tectonically active environment probably corresponds to an island arc where the ophiolite-bearing plate was subducted in an adjacent trench. The absence of a high-pressure mineral assemblage in the San Luis Obispo ophiolite suggests that this segment of ocean-crust and upper mantle was not buried in a subduction zone, but rather was rafted on top of subduction melange (Franciscan Formation) into its present position.

California↗

USGS2023

Sandstone-hosted uranium deposits of the Colorado Plateau, USA

More than 4,000 sandstone-hosted uranium occurrences host over 1.2 billion pounds of mined and in situ U 3 O 8 throughout the Colorado Plateau. Most of the resources are in two distinct mineral systems with deposits hosted in the Triassic Chinle and Jurassic Morrison Formations. In the Chinle mineral system, base metal sulfides typically accompany mineralization. The Morrison mineral system is characterized by V/U ratios up to 20. The uranium source was likely volcanic ash preserved as bentonitic mudstones in the Brushy Basin Member of the Morrison Formation, and lithic volcanic clasts, ash shards, and bentonitic clay in the lower part of the Chinle Formation. Vanadium originated from two possible sources: iron–titanium oxides that are extensively altered in bleached rock near deposits or from similar minerals in variably bleached red beds interbedded with and beneath the Morrison. In Chinle-hosted deposits, in addition to volcanic ash, a contributing source of both vanadium and uranium is proposed here for the first time to be underlying red beds in the Moenkopi and Cutler Formations that have undergone a cycle of reddening-bleaching-reoxidation. Transport in both systems was likely in groundwater through the more permeable sandstones and conglomerate units. The association of uranium minerals with carbonate and more rarely apatite, suggests that transport of uranium was as a carbonate or phosphate complex. The first comprehensive examination of paleoclimate, paleotopography, and subsurface structure of aquifers coupled with analysis of the geochronology of deposits suggests that that there were distinct pulses of uranium mineralization/redistribution during the period from about 259 Ma to 12 Ma when oxidized mineralizing fluids were intermittently rejuvenated in the Plateau in response to changes in tectonic regime and climate. Multiple lines of evidence indicate that deposits formed at ambient temperatures of about 25 °C to no greater than about 140 °C. In both systems, deposits formed where groundwater flow slowed and was subject to evaporative concentration. Stagnant conditions allowed for prolonged interaction of U- and V-enriched groundwater with ferrous iron-bearing reductants, such as illite and iron–titanium oxides, and more rarely organic material such as plant debris. Paragenetically late in the sequence, reducing fluids introduced additional organic matter to some deposits. Reducing fluids and introduced organic matter (now amorphous and altered by radiolysis) may originate from regional petroleum systems where peak oil and gas generation was from ∼ 82 to ∼ 5 Ma. Our novel analysis indicates that these reducing fluids bleached rock and protected affected deposits from remobilization during exposure and weathering that followed uplift of the Plateau (∼80 to 40 Ma).

Arizona, Colorado, Nevada, New Mexico, Utah↗

USGS1984

Geochronology of Precambrian granites and associated U-Ti-Th mineralization, northern Olary province, South Australia

Proterozoic granitoids and metamorphic rocks in the Olary province of the Willyama block of South Australia host ore-grade amounts of U-Th-Ti and U-Fe-Ti-Th minerals. U-Pb-Th isotope analyses on zircons from all granitoids associated with the Crocker Well brannerite deposit indicate that these granitoids were intruded within a short time span, close to the 1579.2??1.5 m.y. age of the brannerite-bearing host-rock. Though the early Paleozoic Delamerian orogeny was intense in this region, the zircon isotopic systems remained unaffected; rather, the best-defined zircon chords on concordia plots show a welldefined lower intercept of 43.8??6.5 Ma, which can only be associated with early Tertiary block faulting. Pb-U-Th isotope analyses on brannerite from the Crocker Well deposit and davidite from the Mt. Victoria deposit and the Radium Hill deposit yield badly scattered and discordant apparent ages that suggest a primary age at least as old as the age of the Crocker Well granitoids, followed by a severe disturbance in the early Paleozoic. ?? 1984 Springer-Verlag.

Contributions to Mineralogy and Petrology↗

USGS1955

Uranium deposits of the northern part of the Boulder Batholith, Montana

Uranium minerals and radioactivity anomalies occur in many silver-lead veins and chalcedony veins and vein zones in the Boulder batholith of southwestern Montanao Pitchblende has been identified in a few silver-lead veins. These veins occupy shear zones along which there is no evidence of large-scale lateral displacement. The wall rock adjacent to the veins is intensely silicified and sencitized quartz monzonite and granodiortte. The veins have yielded substantial quantities of lead, silver, zinc, and gold. The silver-lead veins consist principal1y of galena, spha1erite, tetrahedrite, cha1copyrite and pyrite in a gangue of light to dark gray quartz, altered rock, gouge, and subordinate chalcedony and carbonate minerals. No anomalous radioactivity nor uranium minerals have been found in similar veins in pre-batholithic rocks of the area. Chalcedony veins and vein zones, some of which are ttraniferous, are distinctly different from the silver-lead veins and, with a single except1on, are known only in the batholith. The chalcedony vein zones consist of one or more discontinuous stringers or veins of cha1cedony and microcrystalline quartz in silicified and sericitized quartz monzonite and granodiorite, and in less strongly altered alaskite. On1y small amounts of silver ore have been produced from these chalcedony veins and vein zones. All of the veins are ear1y Tertiary in age, but the silver-lead veins probably are older than the chalcedony veins. Uranium is closely associated with chalcedory and microcrystalline quartz in both types of veins. This association suggests that all of the uranium in the area is of the same age. If so, some of the silver-lead veins must have been reopened during the period of chalcedony vein formation.

Trace Elements Memorandum↗

USGS2006

Major- and trace-element concentrations in rock samples collected in 2004 from the Taylor Mountains 1:250,000-scale quadrangle, Alaska

The Kuskokwim mineral belt of Bundtzen and Miller (1997) forms an important metallogenic region in southwestern Alaska that has yielded more than 3.22 million ounces of gold and 400,000 ounces of silver. Precious-metal and related deposits in this region associated with Late Cretaceous to early Tertiary igneous complexes extend into the Taylor Mountains 1:250,000-scale quadrangle. The U.S. Geological Survey is conducting geologic mapping and a mineral resource assessment of this area that will provide a better understanding of the geologic framework, regional geochemistry, and may provide targets for mineral exploration and development. During the 2004 field season 137 rock samples were collected for a variety of purposes. The 4 digital files accompanying this report reflect the type of analysis performed and its intended purpose and are available for download as an Excel workbook, comma delimited format (*.csv), dBase 4 files (*.dbf) or as point coverages in ArcInfo interchange format (*.e00). Data values are provided in percent, pct (1gram per 100grams), or parts per million, ppm (1gram per 1,000,000grams) per the column heading in the table. All samples were analyzed for a suite of 42 trace-elements (icp42.*) to provide data for use in geochemical exploration as well as some baseline data. Selected samples were analyzed by additional methods; 104 targeted geochemical exploration samples were analyzed for gold, arsenic, and mercury (auashg.*); 21 of these samples were also analyzed to obtain concentrations of 10 loosely bound metals (icp10.*); 33 rock samples were analyzed for major element oxides to support the regional mapping program (reg.*), of which 28 sedimentary rock samples were also analyzed for total carbon, and carbonate carbon.

Open-File Report↗

USGS2011

Microbial reduction of structural iron in interstratified illite-smectite minerals by a sulfate-reducing bacterium

Clay minerals are ubiquitous in soils, sediments, and sedimentary rocks and could coexist with sulfate‐reducing bacteria (SRB) in anoxic environments, however, the interactions of clay minerals and SRB are not well understood. The objective of this study was to understand the reduction rate and capacity of structural Fe(III) in dioctahedral clay minerals by a mesophilic SRB, Desulfovibrio vulgaris and the potential role in catalyzing smectite illitization. Bioreduction experiments were performed in batch systems, where four different clay minerals (nontronite NAu‐2, mixed‐layer illite‐smectite RAr‐1 and ISCz‐1, and illite IMt‐1) were exposed to D. vulgaris in a non‐growth medium with and without anthraquinone‐2,6‐disulfonate (AQDS) and sulfate. Our results demonstrated that D. vulgaris was able to reduce structural Fe(III) in these clay minerals, and AQDS enhanced the reduction rate and extent. In the presence of AQDS, sulfate had little effect on Fe(III) bioreduction. In the absence of AQDS, sulfate increased the reduction rate and capacity, suggesting that sulfide produced during sulfate reduction reacted with the phyllosilicate Fe(III). The extent of bioreduction of structural Fe(III) in the clay minerals was positively correlated with the percentage of smectite and mineral surface area of these minerals. X‐ray diffraction, and scanning and transmission electron microscopy results confirmed formation of illite after bioreduction. These data collectively showed that D. vulgaris could promote smectite illitization through reduction of structural Fe(III) in clay minerals.

Geobiology↗

USGS1990

Geology and mineralization at the Ishmas Kabir gold prospect, Ishmas gold district, Kingdom of Saudi Arabia

Ishmas Kabir ancient gold mine is located approximately 60 km southeast of the city of Ranyah and 9 km northwest of Jabal Ishmas in the southwestern part of the Ishmas gold district. The mine workings are located near the eastern edge of a flat plain covered by eolian sand and drained by Wadi Bishah. Two elongate dumps, 75 m wide and 530 m long (combined length), comprise the workings. Rising 4 m above the plain, they are prominent features in this area of low relief. Core drilling (1,023 m total depth drilled) was conducted to define the extent of potential gold occurrences at the prospect. The two northeasternmost of the four holes drilled unequivocally intersected the gold-bearing main vein zone. Drill hole 1, the northeasternmost hole drilled, passed through significant lengths of altered wall rocks + quartz veins containing > 100 ppb Au (threshold value). The average gold concentration of all intercepts in drill holes 1 through 4 is 278 ppb; the total length of drill core that assayed above the gold threshold value is 13.46 m. The highest gold concentration (1,620 ppb) reported in geochemical assay results was obtained from a 0.33 m length of core in drill hole 1. Gold-intercept occurrences are not economic in the rocks drilled; no further deep drilling is recommended at this time. Gold concentrations in samples obtained from mine dumps are much higher than in drill-core samples. Samples collected by earlier workers assayed 3-7 g/t, and tonnage estimates of mine spoil have been estimated to be as high as 120,000 tons. A possible sampling bias in earlier studies may favor larger gold-enriched, samples relative to samples from quartz tailings. Further work at Ishmas Kabir should be limited to defining gold resource potential in the dumps and in the oxidized, near-surface quartz-vein system. Primary gold at the Ishmas Kabir gold prospect occurs with quartz veins and associated sericite + pyrite alteration hosted by quartz diorite and mafic dikes. The highest gold concentrations both in dumps (earlier studies) and drill core (this investigation) are found with limonite + hematite in breccia zones, in stringer veins, and in sericitic alteration envelopes surrounding quartz veins. Secondary gold is significant in this system and attests to the importance of oxidizing, acidic solutions. Quartz veins intersected by drill holes are surrounded by mylonite schist. Quartz and carbonate veins less than 5 mm thick are boudined, whereas thick quartz veins (£ 1.2 m) have disrupted and brecciated margins; mylonitized country rock envelops quartz-vein fragments. Sulfide mineralization associated with vein formation predates this rock-deformation event. Contemporaneous brittle and ductile deformation of quartz veins and country rocks occurred during the Nabitah orogeny. Supergene gold enrichment took place much later.

Open-File Report↗

USGS1991

Regional study of mineral resources in Nevada: Insights from three-dimensional analysis of gravity and magnetic anomalies

A three-dimensional interpretation of the basins of Nevada was developed based on statewide data bases of digital-gravity, magnetic, geologic, well, and topographic information. An iterative technique was applied to isostatic residual gravity anomalies in Nevada in order to define the location and shape of pre-Tertiary basement and to produce a gravity map that reflects variations of density within the pre-Tertiary basement. The dominant feature of the basement gravity of Nevada is an enormous area of low gravity that spans the entire state between latitudes 37°N and 40.5°N. This regional low strongly correlates with the distribution of middle and late Tertiary volcanic rocks and may reflect silicic intrusions within the mid-crust and upper crust that are the counterparts of volcanic rocks at the surface. Although 80% of Nevada is covered by Cenozoic deposits, these deposits are thicker than 1 km over only about 20% of the state. The remaining 60% of Nevada may have pre-Tertiary basement rocks within reach of current mineral-exploration techniques. Aeromagnetic profiles from the National Uranium Resource Evaluation (NURE) were analyzed in order to produce a map showing the location of shallow magnetic sources in Nevada. This analysis shows that 46% of the state has magnetic sources, generally Mesozoic and Cenozoic igneous rocks, within 1 km of the surface. A linear magnetic anomaly in north-central Nevada has been interpreted by others as a rift zone active during middle Miocene time. The rift also is evident in NURE magnetic profiles, but our interpretation suggests that the magnetic expression of the rift continues south-southeast with similar strike to at least 38°N and perhaps to the amagmatic zone (lat. 37°N). The survival since the middle Miocene of this narrow crustal feature, essentially linear over a distance of 500 km, is difficult to interpret in light of later Basin and Range deformation. Our analysis of gravity anomalies shows that many deep Cenozoic basins are located near the rift, yet only two basins cut across it, and at least five others change strike near the rift, as if to avoid it. The rift may have remained linear because it is associated with crustal structures that acted to resist subsequent deformation.

Nevada↗

USGS1955

Ore deposits on northwestern Chichagof Island, Alaska

The area mapped includes most of northwestern Chichagof Island. The work, started in 1946, is a continuation of the geologic mapping done in the adjoining Chichagof mining district by Reed and Coats. The gold-bearing zone recognized by these writers continues through the area mapped by the writer to the northern shore of Chichagof Island. Stratified rocks ranging in age from Paleozoic through Early Cretaceous were intruded by gabbro, quartz diorite, norite-gabbro, and by younger quartz diorite. The igneous rocks from oldest to youngest are: gabbro, diorite, quartz diorite, the rocks associated with the nickel deposits, which include gabbro-norite and quartz diorite, and basalt. The diorite is believed to have been formed during Early Cretaceous time, and the gabbro is older by an unknown amount. The gabbro and diorite are believed to be, over large areas, recrystallized older rock. The oldest group of quartz diorite intrusives in places cuts the diorite and is therefore younger. The quartz diorite is minerally somewhat foliated and slightly metamorphosed. The igneous rocks associated with the nickel deposits are the youngest of the major igneous rock groups. They intruded the Cretaceous graywacke after it had been folded to essentially its present position. Gold is the most economically important mineral commodity mined to date. The Apex and El Nido are the two largest mines in the area, but some gold has been recovered from the Goldwin and Cobol properties and small amounts from some other small prospects. In addition to gold the area includes nickel-bearing deposits at Mirror Harbor on Chichagof Island and at Bohemia Basin on Yakobi Island. Copper mineralization occurs in the area north of Goulding Harbor. An attempt has been made to outline areas favorable for prospecting based on the regional geology and, on the number and kind of quartz veins found. In general the best area for prospecting appears to be on a northwest-trending zone which extends from the head Pinta Bay to the northern end of Althorp Peninsula.

Alaska↗

USGS1981

Geochemical survey of Shining Rock Wilderness, Heywood County, North Carolina

The U.S. Geological Survey (USGS) made a reconnaissance geochemical survey of the Shining Rock Wilderness (fig. 1) to test for unidentified or unexposed mineral deposits that might be recognized by their geochemical halos. Similar geochemical survey based on trace-element analyses have been credited with the discovery of many types of mineral deposits (Hawkes and Webb, 1962). Although no metallic mineral deposits are known in the study area, the same sequence of rocks that is exposed there contains small deposits of copper-bearing sulfides 10-18 mi (16-28 km) west and southwest of the wilderness (Gair and Slack, 1979). Keith (1907, p. 8) reports minor amounts of copper sulfides and iron sulfides in schist and in quartz veins cutting schist near Retreat, 4 mi (6.5 km) northwest of the wilderness. An old gold mine, the Boylston mine, on Forge Mountain 10 mi (16 km) east of the wilderness (Nitze and Hanna, 1896, p. 181-191) is in metasedimentary rocks of the Great Smoky Group (Hadley and Nelson, 1971) which are probably younger than the rocks exposed in the wilderness.

North Carolina↗

USGS1980

Summaries of data on and lists of references to metallic and selected nonmetallic mineral deposits in the McCarthy Quadrangle, Alaska

These summaries of data on metallic and selected nonmetallic mineral occurrences and lists of selected references to .them in Geological Survey, U.S. Bureau of Mines, and State of Alaska Division of Geological and Geophysical Surveys (and predecessor State and Territorial agencies) reports and maps and a very.few journal articles are designed to aid in library research on the mineral resources of the McCarthy quadrangle, Alaska. The references listed are selected in the sense that mainly statistical reports such as the annual Minerals Yearbook of the U.S. Bureau of Mines and many annual and biennial reports of the Alaska Division of Geological and Geophysical Surveys and its predecessor agencies are not included. Also not included are data on claims about which little more than their locations is known (for example, localities 126- 130 in MacKevett and Holloway, 1977 (OF 77-169A). These omissions should not be interpreted as a judgement that the claims are not on valid mineral occurrences, but only chat there are insufficient data to describe any mineral deposits that might be present. Geochemical anomalies determined by analyses of rock and stream-sediment samples in which no metallic mineral was indentified are also omitted.

Alaska↗