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At least 757 records · Page 42Linked to original sources
USGS2026

Shallow differentiation of primitive arc magmas at the Jurassic Emigrant Gap mafic complex, Sierra Nevada, California

The Emigrant Gap composite pluton exposes ultramafic to silicic intrusive rocks that preserve the chemical evolution of primitive mafic arc magmas and their open-system interactions in the upper crust during mid-Jurassic growth of the Sierra Nevada batholith (California). We present field and petrographic observations and mineral and whole-rock chemistry of the ~35-km 2 ultramafic to dioritic Emigrant Gap mafic complex and an adjacent penecontemporaneous ~90-km 2 granodiorite that together make up the composite pluton. In the Emigrant Gap mafic complex, four roughly central masses of dunite, wehrlite, and olivine clinopyroxenite are surrounded by weakly layered gabbronorite and non-layered diorite. The ultramafic rocks are cumulates formed from near-liquidus minerals of primitive arc magmas that accumulated in steep feeder zones with substantial modification by melt–mush reaction as primitive liquids repeatedly transited the mush-filled conduits. The dominant gabbronoritic rocks are the variably accumulative products of more advanced crystallization–differentiation of arc tholeiitic basalts and basaltic andesites. The adjacent granodiorite intrusion originated separately and preserves field and geochemical evidence for assimilation of metasedimentary rocks. Open-system hybridization between the gabbronoritic mushes and the granodioritic magma produced an intervening body of two-pyroxene diorite. We infer that the ultramafic rocks and gabbronorite of the Emigrant Gap mafic complex crystallized from near-primitive arc basaltic to basaltic andesitic magmas at ~0.15–0.3 GPa, with estimated f O 2 of ≥FMQ +1 and dissolved H 2 O concentrations of only ~0.5–2 wt %. Notably, the Emigrant Gap composite pluton is distinct from other Mesozoic plutons in the Sierra Nevada batholith because of (1) its abundance of mafic and ultramafic rocks that crystallized from relatively primitive mafic melts and (2) the low inferred H 2 O concentrations of its parental magmas, indicated by a near absence of igneous amphibole and by the intermediate rather than calcic compositions of plagioclase. A Jurassic regional extension event probably accounts for the formation of relatively dry primitive arc magmas, as well as for their ascending to the upper crust.

California↗
USGS1978

Reconnaissance geology and geochronology of the Precambrian of the Granite Mountains, Wyoming

The Precambrian of the western part of the Granite Mountains, Wyoming, contains a metamorphic complex of gneisses, schists, and amphibolites that were derived through amphibolite-grade metamorphism from a sedimentary-volcanic sequence perhaps similar to that exposed in the southeastern Wind River Mountains. Whole-rock Rb-Sr dating places the time of metamorphism at 2,860?80 million years. A high initial 87Sr/ 86 S r ratio of 0.7048 suggests that either the protoliths or the source terrane of the sedimentary component is several hundred million years older than the time of metamorphism. Following an interval of 300:t100 million years for which the geologic record is lacking or still undeciphered, the metamorphic complex was intruded by a batholith and satellite bodies of medium- to coarse-grained, generally massive biotite granite and related pegmatite and aplite. The main body of granite is dated at 2,550?60 million years by the Rb-Sr method. Limited data suggest that diabase dikes were emplaced and nephrite veins were formed only shortly after intrusion of the granite. Emplacement of the granite at about 2,550 million years ago appears to be related to a major period of regional granitic plutonism in the Precambrian of southern and western Wyoming. Granites, in the strict sense, that are dated between 2,450 and 2,600 million years occur in the Teton Range, the Sierra Madre, the Medicine Bow Mountains and the Laramie Range. This episode of granitic plutonism occurred some 50 to 100 million years later than the major tonalitic to granitic plutonism in the Superior province of northern Minnesota and adjacent Ontario-the nearest exposed Precambrian W terrane that is analogous to the Wyoming province. Initial 87Sr / 86Sr ratios of some of the Wyoming granites are higher than expected if the rocks had been derived from juvenile magmas and it is likely that older crustal rocks were involved to some degree in the generation of these granites. Slightly to highly disturbed Rb-Sr and K-Ar mineral ages are obtained on rocks of the metamorphic complex and on the granite. These ages range from about 2,400 to 1,420 million years and are part of a regional pattern of lowered mineral ages of Precambrian W rocks of southern Wyoming. A major discontinuity in these mineral ages occurs along a line extending from the northern Laramie Range, through the northern part of the Granite Mountains, to the southeastern Wind River Mountains. North of this line, Rb-Sr and K-Ar biotite ages are 2,300 million years or greater, whereas to the south, the biotite ages decrease drastically over a short distance, to a common range of 1,600-1,400 million years. We suggest that these lowered ages represent regional cooling below the 300 0 C isotherm as a consequence of uplift and erosion of the large crustal block occurring south of the age discontinuity. In this interpretation, the westerly-trending age discontinuity would be a zone of major crustal dislocation that resulted from vertical tectonics in late Precambrian X or early Precambrian Y time.

Professional Paper↗
USGS1968

SrRbK and Sr isotopic relationships in ultramafic rocks, southeastern Alaska

Geologic evidence suggests that a series of ultramafic complexes of the ‘Duke Island type’ located along a 560 km-long belt in southeastern Alaska crystallized from magmas of ultramafic composition. Some geologists have proposed that these magmas were derived by fractional fusion of ultramafic material in the upper mantle. The 87 Sr/ 86 Sr ratios, corrected for growth of radiogenic 87 Sr, in total rock samples and in minerals from four ultramafic complexes range from 0.7026 to 0.7068, and the K/Rb ratios in total rock samples range from 300 to 1200. Other workers have reported that the isotopic composition of strontium in alpine peridotites is significantly different from that of oceanic volcanic rocks, but the 87 Sr/ 86 Sr ratios of the Alaskan ultramafites and of oceanic volcanic rocks are similar.

Alaska↗
USGS1977

A summary of the geology and mineral resources of the Paris Plateau-House Rock Valley area, Coconino County, Arizona

The Paria Plateau-House Rock Valley area of north-central Arizona is located on the southwestern edge Of the Colorado Plateau physiographic province in an area underlain by about 5,000 meters of fossiliferous marine and continental sedimentary rock ranging in age from Precambrian through Quaternary. The area, which lies north of the Grand and Marble Canyons, is bounded on the west by the East Kaibab monocline and on the east by the Echo monocline. The Paria Plateau, bounded on the South by the scenic Vermilion Cliffs, is composed of continental red-beds of Triassic and Jurassic age, which dip gently northward at 2? to ? away from the north end of the Marble Platform upon which the Paria Plateau sits.

Open-File Report↗
USGS2007

Bald Mountain gold mining district, Nevada: A Jurassic reduced intrusion-related gold system

The Bald Mountain mining district has produced about 2 million ounces (Moz) of An. Geologic mapping, field relationships, geochemical data, petrographic observations, fluid inclusion characteristics, and Pb, S, O, and H isotope data indicate that An mineralization was associated with a reduced Jurassic intrusion. Gold deposits are localized within and surrounding a Jurassic (159 Ma) quartz monzonite porphyry pluton and dike complex that intrudes Cambrian to Mississippian carbonate and clastic rocks. The pluton, associated dikes, and An mineralization were controlled by a crustal-scale northwest-trending structure named the Bida trend. Gold deposits are localized by fracture networks in the pluton and the contact metamorphic aureole, dike margins, high-angle faults, and certain strata or shale-limestone contacts in sedimentary rocks. Gold mineralization was accompanied by silicification and phyllic alteration, ??argillic alteration at shallow levels. Although An is typically present throughout, the system exhibits a classic concentric geochemical zonation pattern with Mo, W, Bi, and Cu near the center, Ag, Pb, and Zn at intermediate distances, and As and Sb peripheral to the intrusion. Near the center of the system, micron-sized native An occurs with base metal sulfides and sulfosalts. In peripheral deposits and in later stages of mineralization, Au is typically submicron in size and resides in pyrite or arsenopyrite. Electron microprobe and laser ablation ICP-MS analyses show that arsenopyrite, pyrite, and Bi sulfide minerals contain 10s to 1,000s of ppm Au. Ore-forming fluids were aqueous and carbonic at deep levels and episodically hypersaline at shallow levels due to boiling. The isotopic compositions of H and O in quartz and sericite and S and Pb in sulfides are indicative of magmatic ore fluids with sedimentary sulfur. Together, the evidence suggests that Au was introduced by reduced S-bearing magmatic fluids derived from a reduced intrusion. The reduced character of the intrusion was caused by assimilation of carbonaceous sedimentary rocks. Tertiary faults dismember the area and drop down the upper part of the mineralizing system to the west. The abundant and widespread kaolinite in oxide ores is relatively disordered (1A polytype) and has ??D and ??18O values suggestive of a supergene origin. The deep weathering and oxidation of the ores associated with exhumation made them amenable to open-pit mining and processing using cyanide heap leach methods. ?? 2007 Society of Economic Geologists, Inc.

Economic Geology↗
USGS2014

Karst in the United States: A digital map compilation and database

This report describes new digital maps delineating areas of the United States, including Puerto Rico and the U.S. Virgin Islands, having karst or the potential for development of karst and pseudokarst. These maps show areas underlain by soluble rocks and also by volcanic rocks, sedimentary deposits, and permafrost that have potential for karst or pseudokarst development. All 50 States contain rocks with potential for karst development, and about 18 percent of their area is underlain by soluble rocks having karst or the potential for development of karst features. The areas of soluble rocks shown are based primarily on selection from State geologic maps of rock units containing significant amounts of carbonate or evaporite minerals. Areas underlain by soluble rocks are further classified by general climate setting, degree of induration, and degree of exposure. Areas having potential for volcanic pseudokarst are those underlain chiefly by basaltic-flow rocks no older than Miocene in age. Areas with potential for pseudokarst features in sedimentary rocks are in relatively unconsolidated rocks from which pseudokarst features, such as piping caves, have been reported. Areas having potential for development of thermokarst features, mapped exclusively in Alaska, contain permafrost in relatively thick surficial deposits containing ground ice. This report includes a GIS database with links from the map unit polygons to online geologic unit descriptions.

Open-File Report↗
USGS1954

Progress report on the Happy Jack mine, Which Canyon area, San Juan county, Utah

The Happy Jack mine is in the White Canyon area, San Juan county, Utah. Production is from high-grade uranium deposits in the Shinarump conglomerate of the Triassic age. In this area the Shinarump beds range from about 16 to 40 feet in thickness and the lower part of these beds fills an east-trending channel this is note than 750 feet wide and 10 feet deep. The Shinarump conglomerate consists of beds of coarse- to fine-grained quartzose sandstone, conglomerate, siltstone, and claystone. Carbonized wood is abundant in these beds, and in the field it was classified as mineral charcoal and coal. Intra-Shinarump channels, cross-stratification, current lineation, and slumping and compaction structures have been recognized in the mine. Steeply dipping fractures have dominant trends in four directions -- N 65°W, N 60°E, N 85°E, and due north. Uranium occurs as bedded deposits, as replacement bodies in accumulations of "trash", and as replacements of larger fragments of wood. An "ore shoot" is formed where the three types of uranium deposits occur together; these ore shoots appear to be elongate masses with sharp boundaries. Uranium minerals include uraninite, sooty pitchblende(?), and the sulfate--betazippeite, johannite, and uranopilite. Associated with the uraninite are the sulfide minerals covellite, bornite, chalcopyritw, and pyrite. Galena and sphalerite have been found in close association with uranium minerals. The gaunge minerals include: limonite and hematite present in most of the sandstone beds throughout the deposit, jarosite that impregnates much of the sandstone in the outer parts of the mine workings, gypsum that fills many of the fractures, and barite that impregnates the sandstone in at least one part of the mine. Secondary copper minerals, mainly copper sulfates, occur throughout the mine, but most abundant near the adits in the outermost 30 feet of the workings. The minerals comprising the bulk of the country rock include quartz, feldspar, and clay minerals. The amount of uranium minerals deposited in a sandstone bed is believed to have been determined by the position of the bed in the channel, the permeability of the sandstone in the bed, and the amount of carbonized wood and plant remains within the bed. The beds considered most favorable for uranium deposition contain an abundance of claystone and siltstone both as matrix filling and as fragments and pebbles. Suggested exploration guides from uranium ore bodies include the following: (1) interbedded siltstone lenses, (2) claystone and siltstone cement and pabbles, (3) concentrations of "trash", (4) covelllite and bornite, (5) chalcopyrite, and (6) carbonized wood.

Utah↗
USGS1983

Petrographic (thin section) notes on selected samples from hornblende-rich metamorphic terranes in the southernmost Sierra Nevada, California

Medium- to high-grade metamorphic rocks that are commonly hornblende-rich, and probably largely of 'oceanic' affinity, are widespread in the southernmost Sierra Nevada, California. These metamorphic rocks are largely amphibolite, mafic and felsic gneiss, granofels, and hypersthene granulite The mineral assemblages suggest that these rocks are at least in part of granulite grade, represent relatively deep crustal levels, and may be exposed parts of the root zone of the Sierra Nevada batholith. Access to the largest area of these rocks is relatively limited and for this reason petrographic data (textures and mineral content based on thin section study) are summarized here. Directions to readily accessible localities are presented, however, where the major metamorphic rock types can be examined and sampled.

Open-File Report↗
USGS1994

Composition and sources of atmospheric dusts in snow at 3200 meters in the St. Elias Range, southeastern Alaska, USA

Dusts in snow from the accumulation zone in the St. Elias Range appear from their chemical compositions to have come from terranes of rocks of ferromagnesian composition. These dusts, with respect to their composition and to the moderate degree of variation that occurs through a depositional year, are similar those deposited in Greenland. The high portion of the St. Elias Range is isolated from dominance by any local dust source terranes, because of altitude and the extent of the surrounding glacierized and snow-covered region. In Greenland the altitude is typically lower, but local sources are even less likely to dominate the character of the dusts deposited into the ice record there. The similar compositions and moderate compositional variations of dusts from these two places bear on the question of whether the dusts that are transported over long distances by the atmosphere under modern and glacial-period conditions are uniform and representative of a broad regional or even hemispheric background dust. The dusts in the snow were measured by means of a suite of major, minor, and trace rock-forming metals chosen to give information about rock types, their constituent minerals, degree of degradation (weathering), and energies of atmospheric uptake from source. The variations in amounts of rock dust through the year in the St. Elias Range snowpack have no time-stratigraphic correspondence to the also large variations in concentrations of other species that are not constituents of rock-derived dusts, such the anions chloride, sulfate, and nitrate; the highs and lows of the two types of materials are apparently completely independent. The structure revealed by the moderately fine-scale sampling of the present study (≈ 10 increments/y) serves as a background for the interpretation of analysis of ice core samples, in which annual layers may be too compressed to permit analysis of sub-annual samples.

Alaska↗
USGS2010

Episodic intrusion, internal differentiation, and hydrothermal alteration of the Miocene Tatoosh intrusive suite south of Mount Rainier, Washington

The Miocene Tatoosh intrusive suite south of Mount Rainier is composed of three broadly granodioritic plutons that are manifestations of ancestral Cascades arc magmatism. Tatoosh intrusive suite plutons have individually diagnostic characteristics, including texture, mineralogy, and geochemistry, and apparently lack internal contacts. New ion-microprobe U-Pb zircon ages indicate crystallization of the Stevens pluton ca. 19.2 Ma, Reflection-Pyramid pluton ca. 18.5 Ma, and Nisqually pluton ca. 17.5 Ma. The Stevens pluton includes rare, statistically distinct ca. 20.1 Ma zircon antecrysts. Wide-ranging zircon rare earth element (REE), Hf, U, and Th concentrations suggest late crystallization from variably evolved residual liquids. Zircon Eu/Eu*–Hf covariation is distinct for each of the Reflection-Pyramid, Nisqually, and Stevens plutons. Although most Tatoosh intrusive suite rocks have been affected by weak hydrothermal alteration, and sparse mineralized veins cut some of these rocks, significant base or precious metal mineralization is absent. At the time of shallow emplacement, each of these magma bodies was largely homogeneous in bulk composition and petrographic features, but, prior to final solidification, each of the Tatoosh intrusive suite plutons developed internal compositional variation. Geochemical and petrographic trends within each pluton are most consistent with differential loss of residual melt, possibly represented by late aplite dikes or erupted as rhyolite, from crystal-rich magma. Crystal-rich magma that formed each pluton evidently accumulated in reservoirs below the present level of exposure and then intruded to a shallow depth. Assembled by episodic intrusion, the Tatoosh intrusive suite may be representative of midsized composite plutonic complexes beneath arc volcanoes.

Washington↗
USGS1989

Geology and K-Ar geochronology of the Paradise Peak Mine and the relationship of pre-Basin and Range extension to Early Miocene precious metal mineralization in west-central Nevada

The Paradise Peak mine is a major gold-silver-mercury deposit located in the southwestern part of the Paradise Range near the eastern edge of the Walker Lane in the western Great Basin, Nevada. The Tertiary volcanic rocks of the area can be divided into a sequence of 26- to 24-Ma intermediate lavas, a sequence of 24- to 22-Ma silicic ash-flow tuffs, and a sequence of 20- to 15-Ma intermediate lavas. We classify these rocks as the older, middle, and younger sequences. Sedimentary rocks, and latites and basalts 12 Ma old or younger, locally overlie the younger intermediate lavas south of the mine. Silicified rhyolite tuff in the lower part of the middle tuff sequence is the principal host of the precious metal ore. Most ore occurs under an acid-leached zone of alunitic alteration and is contained in hydrothermal breccias that crosscut early quartz-pyrite and alunite alteration.Field relations and K-Ar ages of hypogene alunite indicate that precious metal mineralization and alunitic and silicic alteration formed at about 19 to 18 Ma, several million years after eruption of the main host rock. Mineralization formed during high-angle faulting related to crustal extension. Low-angle normal faults are present 5 km northeast of the Paradise Peak mine, but they formed more than 2 Ma after formation of the Paradise Peak deposit. Silicic and alunitic alteration and precious metal mineralization are generally absent in areas of low-angle faulting.Regional stratigraphic relations and K-Ar ages indicate that volcanism changed from silicic ash-flow tuffs to intermediate lavas at about 20 to 19 Ma. Regionally extensive angular unconformities indicate that a period of "pre-Basin and Range" crustal extension occurred between about 22 to 19 Ma. This extension was penecontemporaneous with the shift in the style of volcanism and with gold-silver mineralization in the Paradise Peak mine and in the Goldfield and Tonopah districts of western Nevada. The close temporal and spatial relationships of precious metal mineralization with pre-Basin and Range extension suggest that extension was a major factor in the genesis of early Miocene precious metal deposits in the western Great Basin.

Economic Geology↗
USGS2012

Geologic and mineralogic controls on acid and metal-rich rock drainage in an alpine watershed, Handcart Gulch, Colorado

The surface and subsurface geology, hydrothermal alteration, and mineralogy of the Handcart Gulch area was studied using map and drill core data as part of a multidisciplinary approach to understand the hydrology and affects of geology on acid-rock drainage in a mineralized alpine watershed. Handcart Gulch was the locus of intense hydrothermal alteration that affected an area of nearly 3 square kilometers. Hydrothermal alteration and accompanied weak mineralization are spatially and genetically associated with small dacite to low-silica rhyolite stocks and plugs emplaced about 37-36 Ma. Felsic lithologies are commonly altered to a quartz-sericite-pyrite mineral assemblage at the surface, but alteration is more variable in the subsurface, ranging from quartz-sericite-pyrite-dominant in upper core sections to a propylitic variant that is more typical in deeper drill core intervals. Late-stage, hydrothermal argillic alteration [kaolinite and(or) smectite] was superimposed over earlier-formed alteration assemblages in the felsic rocks. Smectite in this late stage assemblage is mostly neoformed resulting from dissolution of chlorite, plagioclase, and minor illite in more weakly altered rocks. Hydrothermally altered amphibolites are characterized by biotitic alteration of amphibole, and subsequent alteration of both primary and secondary biotite to chlorite. Whereas pyrite is present both as disseminations and in small veinlets in the felsic lithologies, it is mostly restricted to small veinlets in the amphibolites. Base-metal sulfides including molybdenite, chalcopyrite, sphalerite, and galena are present in minor to trace amounts in the altered rocks. However, geologic data in conjunction with water geochemical studies indicate that copper mineralization may be present in unknown abundance in two distinct areas. The altered rocks contain an average of 8 weight percent fine pyrite that is largely devoid of metals in the crystal structure, which can be a significant source of trace metals in other areas with acid rock drainage. Thus, elevated base-metal concentrations in the trunk stream and discrete springs in the study area, as determined in previous studies, are likely derived from discrete metal-rich sources, rather than the abundant pyrite veins or disseminations. Pyrite is oxidized in nearly all outcrops examined. Drill core data show that zones of pyrite oxidation range in depth from 100 meters below the surface at higher elevations to just a few meters depth at the lowest elevations in the study area. However, discrete pyrite oxidation zones are present in drill core to depths of several hundred meters below the pervasive near-surface oxidation zones. These deeper discrete oxidation zones, which are present where fresh pyrite predominates, are spatially associated with fractures, small faults, and breccias. Quartz-sericite-pyrite-altered rocks containing unoxidized pyrite likely have the highest acid-generating capacity of all alteration assemblages in the study area. Hydrothermal alteration has left these rocks base-cation leached and thus acid-neutralizing potential is negligible. In contrast, propylitic-altered felsic rocks commonly contain trace to minor calcite and abundant chlorite, which provide some amount of acid-neutralization despite the presence of a few percent pyrite.

Colorado↗
USGS1984

Geophysical well logs for eleven drill holes at the Colorado School of Mines Experimental Mine Site, Idaho Springs, Colorado

The following geophysical well log measurements were made in eleven drill holes above the Colorado School of Mines Experimental Mine at Idaho Springs, Colorado: (1) acoustic velocity (2) resistivity, (3) caliper, (4) gamma-gamma density, (5) neutron-thermal neutron, (6) gamma ray, (7) induced polarization (IP), (8) self potential (SP), and magnetic susceptibility. The density and acoustic velocity logs indicate extensive fracturing in each of the drill holes. Variations in the relative amount of felsic or mafic mineral components in the rocks can be inferred from the magnetic susceptibility and gamma ray well log responses. Zones containing metallic sulfide mineralization are interpreted from the IP well log response.

Open-File Report↗
USGS1985

Availability and quality of ground water in the Piedmont Province of Virginia

The Piedmont Province of Virginia has an ample supply of groundwater, (perhaps as much as 1.5 billion gallons are in storage per sq mi) generally suitable for domestic and small supply needs. The source of this groundwater is precipitation, which is stored in the pore spaces of the regolith and in fractures in the underlying bedrock of crystalline rocks. Water within the sedimentary rocks of the sedimentary basins is stored in bedding planes, fractures, and in pore spaces in the rock, and in the regolith. Well yields can be minimized in both terrains by constructing wells along lineaments and in valleys. Groundwater in the crystalline rock is generally slightly mineralized and acidic (pH is greater than 7.0). Dissolved solids concentration in deep wells ( < 500 ft) in sedimentary rock may exceed tolerable limits. Land disposal of solid wastes and sewage for domestic septic systems present the major threat to groundwater quality. A greater understanding of the groundwater system in the Virginia Piedmont could be used to anticipate future shortages so that preventive measures could be implemented to protect the groundwater reservoir. (Author 's abstract)

Water-Resources Investigations Report↗