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At least 1,405 records · Page 78Linked to original sources
USGS1991

Metasomatic oxidation of upper mantle periodotite

Examination of Fe3+ in metasomatized spinel peridotite xenoliths reveals new information about metasomatic redox processes. Composite xenoliths from Dish Hill, California possess remnants of magmatic dikes which were the sources of the silicate fluids responsible for metasomatism of the peridotite part of the same xenoliths. Mo??ssbauer spectra of mineral separates taken at several distances from the dike remnants provide data on Fe3+ contents of minerals in the metasomatized peridotite. Clinopyroxenes contain 33% of total iron (FeT) as Fe3+ (Fe3+/FeT=0.33); orthopyroxenes contain 0.06-0.09 Fe3+/FeT; spinels contain 0.30-0.40 Fe3+/FeT; olivines contain 0.01-0.06 Fe3+/FeT; and metasomatic amphibole in the peridotite contains 0.85-0.90 Fe3+/FeT. In each mineral, Fe3+ and Fe2+ cations per formula unit (p.f.u.) decrease with distance from the dike, but the Fe3+/FeT ratios of each mineral do not vary. Clinopyroxene, spinel, and olivine Fe3+/FeT ratios are significantly higher than in unmetasomatized spinel peridotites. Metasomatic changes in Fe3+/FeT ratios in each mineral are controlled by the oxygen fugacity of the system, but the mechanism by which each phase accommodates this ratio is affected by crystal chemistry, kinetics, rock mode, fluid composition, fluid/rock ratio, and fluid-mineral partition coefficients. Ratio increases in pyroxene and spinel occur by exchange reactions involving diffusion of Fe3+ into existing mineral grains rather than by oxidation of existing Fe2+ in peridotite mineral grains. The very high Fe3+/FeT ratio in the metasomatic amphibole may be a function of the high Fe3+/FeT of the metasomatic fluid, crystal chemical limitations on the amount of Fe3+ that could be accommodated by the pyroxene, spinel, and olivine of the peridotite, and the ability of the amphibole structure to accommodate large amounts of 3 + valence cations. In the samples studied, metasomatic amphibole accounts for half of the bulk-rock Fe2O3. This suggests that patent metasomatism may produce a greater change in the redox state of mantle peridotite than cryptic metasomatism. Comparison of the metasomatized samples with unmetasomatized peridotites reveals that both Fe2+ and Fe3+ cations p.f.u. were increased during metasomatism and 50% or more of iron added was Fe3+. With increasing distance from the dike, the ratio of added Fe3+ to added Fe2+ increases. The high Fe3+/FeT of amphibole and phlogopite in the dikes and in the peridotite, and the high ratios of added Fe3+/added Fe2+ in pyroxenes and spinel suggest that the Fe3+/FeT ratio of the metasomatic silicate fluid was high. As the fluid perolated through and reacted with the peridotite, Fe3+ and C-O-H volatile species were concentrated in the fluid, increasing the fluid Fe3+/FeT. ?? 1991 Springer-Verlag.

Contributions to Mineralogy and Petrology↗
USGS1992

Host-rock controlled epigenetic, hydrothermal metasomatic origin of the Bayan Obo REEFe-Nb ore deposit, Inner Mongolia, P.R.C.

Bayan Obo, a complex rare earth element (REE) Fe Nb ore deposit, located in Inner Mongolia, P.R.C. is the world's largest known REE deposit. The deposit is chiefly in a marble unit (H8), but extends into an overlying unit of black shale, slate and schist unit (H9), both of which are in the upper part of the Middle Proterozoic Bayan Obo Group. Based on sedimentary structures, the presence of detrital quartz and algal fossil remains, and the 16-km long geographic extent, the H8 marble is a sedimentary deposit, and not a carbonatite of magmatic origin, as proposed by some previous investigators. The unit was weakly regionally metamorphosed (most probably the lower part of the green schist facies) into marble and quartzite prior to mineralization. Tectonically, the deposit is located on the northern flank of the Sino-Korean craton. Many hypotheses have been proposed for the origin of the Bayan Obo deposit; the studies reported here support an epigenetic, hydrothermal, metasomatic origin. Such an origin is supported by field and laboratory textural evidence; 232 Th/ 208 Pb internal isochron mineral ages of selected monazite and bastnaesite samples; 40 Ar/ 39 Ar incremental heating minimum mineral ages of selected alkali amphiboles; chemical compositions of different generations of both REE ore minerals and alkali amphiboles; and evidence of host-rock influence on the various types of Bayan Obo ores. The internal isochron ages of the REE minerals indicate Caledonian ages for various episodes of REE and Fe mineralization. No evidence was found to indicate a genetic relation between the extensive biotite granitic rocks of Hercynian age in the mine region and the Bayan Obo are deposit, as suggested by previous workers.

Applied Geochemistry↗
USGS1952

Preliminary summary review of thorium-bearing mineral occurrences in Alaska

Thorium-bearing minerals are known at 47 localities in Alaska. At these localities the thorium occurs as a major constituent or in minor amounts as an impurity in one or more of the following 12 minerals: allanite, columbite, ellsworthite, eschynite, gummite, monazite, orangite, parisite, thorianite, thorite, xenotime, and zircon. In addition other minerals, such as biotite and sphene, are radioactive and may contain thorium. Several unidentified columbate minerals with uranium or thorium and uranium as major constituents have been recognized at some localities. The distribution, by type of deposit, of the 57 thorium occurrences is as follows: lode - 3, lode and placer - 1, granitic rock - 3, granitic rock and related placer - 14, and placer - 26. Of the four lode occurrences only the radioactive veins at Salmon Bay in southeastern Alaska and the contact metamorphic deposit in the Nixon Fork area of central Alaska warrant further consideration, although insufficient data are available to determine whether these two deposits have commercial possibilities. The remaining occurrences of thorium-bearing minerals in Alaska are limited to placer deposits and disseminations of accessory minerals in granitic rocks. In most of these occurrences the thorium-bearing minerals occur in only trace amounts and consequently warrent little further consideration. More data are needed to determine the possibilities of byproduct recovery of thorium-bearing minerals from several of the gold and tin placers.

Alaska↗
USGS1983

Hashemite, Ba(Cr,S)O4, a new mineral from Jordan.

Hashemite, Ba(Cr,S)O4, the isostructural chromate analogue of baryte, has been found in west-central Jordan. It is associated with chromian ettringite, apatite and calcite in a phosphatic carbonate rock analogous to the Hatrurim formation in Israel. The mineral is orthorhombic, Pnma, with a 9.112(2), b 5.541(1), c 7.343(1) A, Z = 4. Strongest XRD lines are 3.516(100), 3.171(80), 3.669(60), 2.175(60), 2.150(45) A. Hashemite occurs as small, euhedral, dark brown, commonly zoned crystals with an average D 4.59 g/cm3 and H. 31/2. It is biaxial positive; dark varieties have alpha 1.952(2), beta 1.960(2), gamma 1.977(2); light varieties have alpha 1.810(2), beta 1.813(2), gamma 1.824(2), 2Vgamma 35o-57o.-J.A.Z.

American Mineralogist↗
USGS2018

Intermediate sulfidation type base metal mineralization at Aliabad-Khanchy, Tarom-Hashtjin metallogenic belt, NW Iran

The Aliabad-Khanchy epithermal base metal deposit is located in the Tarom-Hashtjin metallogenic belt (THMB) of northwest Iran. The mineralization occurs as Cu-bearing brecciated quartz veins hosted by Eocene volcanic and volcaniclastic rocks of the Karaj Formation. Ore formation can be divided into five stages, with most ore minerals, such as pyrite and chalcopyrite being formed in the early stages. The main wall-rock alteration is silicification, and chlorite, argillic and propylitic alteration. Microthermometric measurements of fluid inclusion assemblages show that the ore-forming fluids have eutectic temperatures between −30 and −52 °C, trapping temperatures of 150–290 °C, and salinities of 6.6–12.4 wt% NaCl equiv. These data demonstrate that the ore-forming fluids were medium- to high-temperature, medium- to low-salinity, and low-density H 2 O–NaCl–CaCl 2 fluids. Calculated δ 18 O values indicate that ore-forming hydrothermal fluids had δ 18 O water ranging from +3.6‰ to +0.8‰, confirming that the ore–fluid system evolved from dominantly magmatic to dominantly meteoric. The calculated 34 S H2S values range from −8.1‰ to −5.0‰, consistent with derivation of the sulfur from either magma or possibly from local volcanic wall-rock. Combined, the fluid inclusion and stable isotope data indicate that the Aliabad-Khanchy deposit formed from magmatic-hydrothermal fluids. After rising to a depth of between 790 and 500 m, the fluid boiled and subsequent hydraulic fracturing may have led to inflow and/or mixing of early magmatic fluids with circulating groundwater causing deposition of base metals due to dilution and/or cooling. The Aliabad-Khanchy deposit is interpreted as an intermediate-sulfidation style of epithermal mineralization. Our data suggest that the mineralization at Aliabad-Khanchy and other epithermal deposits of the THMB formed by hydrothermal activity related to shallow late Eocene magmatism. The altered Eocene volcanic and volcaniclastic rocks, especially at the intersection of subvolcanic stocks with faults were the most favorable sites for epithermal ore bodies in the THMB.

Ore Geology Reviews↗
USGS1956

Summary of reconnaissance for radioactive deposits in Alaska, 1945-1954, and an appraisal of Alaskan uranium possibilities

In the period 1945-1954 over 100 investigations for radioactive source materials were made in Alaska. The nature of these investigations ranged from field examinations of individual prospects or the laboratory analysis of significantly radioactive samples submitted by prospectors to reconnaissance studies of large districts. In this period no deposits of uranium or thorium that would warrant commercial exploitation were discovered. The investigations, however, disclosed that radioactive materials occur in widely scattered areas of Alaska and in widely diverse environments. Many igneous rocks throughout Alaska are weakly radioactive because of uranium- and thorium-bearing accessory minerals, such as allanite, apatite, monazite, sphene, xenotime, and zircon; more rarely the radioactivity of these rocks is due to thorianite or thorite and their uranoan varieties. The felsic rocks, for example, granites and syenites, are generally more radioactive than the mafic igneous rocks. Pegmatites, locally, have also proved to be radioactive, but they have little commercial significance. No primary uranium oxide minerals have been found yet in Alaskan vein deposits, except, perhaps, for a mineral tentatively identified as pitchblende in the Hyder district of southeastern Alaska. However, certain occurrences of secondary uranium minerals, chiefly those of the uranite group, on the Seward Peninsula, in the Russian Mountains, and in the vicinity of Kodiak suggest that pitchblende-type ores may occur at depth beneath zones of alteration. Thorite-bearing veins have been discovered on Prince of Wales Island in southeastern Alaska. Although no deposits or carnotite-type minerals have been found in Alaska, several samples containing such minerals have been submitted by Alaskan prospectors. Efforts to locate the deposits from which these minerals were obtained have been unsuccessful, but review of available geologic data suggests that several Alaskan areas are potentially favorable for carnotite-type deposits. The chief of these areas is the Alaska Peninsula-Cook Inlet area which encompasses most of the reported occurrences of the prospectors' carnotite-type samples. Alaska is also potentially favorable for the occurrence of large bodies of the very low-grade uraniferous sedimentary rocks, such as phosphorites and black shales. This type of deposit, however, has not received much study because of the emphasis on the search for bonanza-type high-grade ores. Uraniferous phosphorites similar to those of Idaho, Montana, and Wyoming occur in northern Alaska on the north flank of the Brooks Range; black shales comparable to the uraniferous shales of the Chattanooga formation of southeastern United States have been noted along the Yukon River near the international boundary. Placer deposits in Alaska have some small potential for the production of the radioactive elements as byproducts of gold- and tin-placer mining. the placer area believed to have the relatively greatest potential in Alaska lies in the Kahiltna River valley where concentrates are known to contain such commercial minerals as ilmenite, cassiterite, platinum, and gold in addition to uranothorianite and monazite. The possibilities of the natural fluids--water and petroleum--have not yet been tested in Alaska to any great extent. Studies of fluids are in progress to determine whether they may be used to discover and define areas potentially favorable for the occurrence of uraniferous lodes.

Alaska↗
USGS1987

Age of uranium mineralization at the Jabiluka and Ranger deposits, Northern Territory, Australia: New U-Pb isotope evidence

The Ranger and Jabiluka uranium deposits are the largest in the Alligator Rivers uranium field, which contains at least 20 percent of the world's low-cost uranium reserves. Ore occurs in early Proterozoic metasediments, below an unconformity with sandstones of the 1.65-b.y.-old Kombolgie Formation. This study has used U-Pb isotope data from a large number of whole-rock drill core samples with a variety of mineral assemblages and textures. Ranger samples indicate a well-defined age of 1,737 + or - 20 m.y., consistent with earlier suggestions of the antiquity of this deposit. This age is distinctly pre-Kombolgie, so the Ranger deposit cannot have been formed by processes requiring its presence. The age is consistent, however, with mineralization related to heating associated with either the emplacement of early postmetamorphic granites, or possibly, with intrusion of the nearby Oenpelli Dolerite. In contrast, data for the least-altered Jabiluka ores yield a concordia intercept age of 1,437 + or - 40 m.y.--significantly younger than the Ranger age and also younger than the Kombolgie Formation. This age may correspond to a regional thermal event, as indicated both by mafic dikes of roughly this age and a zircon lower intercept age from a nearby granite-gneiss. Thus our data indicate that there must be at least three distinct periods of major U mineralization in the Alligator Rivers uranium field (including the previously determined approximately 900-m.y. age of the Nabarlek deposit), despite the fact that all of the deposits share strong similarities in their host rock, mineralogy, alteration, and geologic setting.Both Ranger and Jabiluka reflect a common, profound isotopic disturbance at about 400 to 600 m.y. This disturbance, which was especially pronounced at Jabiluka, may correspond to the development of basins and associated basalt flows to the west and southwest, as suggested by Crick et al. (1980).

Economic Geology↗
USGS1999

Petrology and geochemistry of late-stage intrusions of the A-type, mid-Proterozoic Pikes Peak batholith (Central Colorado, USA): Implications for petrogenetic models

The ~1.08 Ga anorogenic, A-type Pikes Peak batholith (Front Range, central Colorado) is dominated by coarse-grained, biotite ?? amphibole syenogranites and minor monzogranites, collectively referred to as Pikes Peak granite (PPG). The batholith is also host to numerous small, late-stage plutons that have been subdivided into two groups (e.g. Wobus, 1976. Studies in Colorado Field Geology, Colorado School of Mines Professional Contributions, Colorado): (1) a sodic series (SiO2= ~44-78 wt%; K/Na=0.32-1.36) composed of gabbro, diabase, syenite/quartz syenite and fayalite and sodic amphibole granite; and (2) a potassic series (SiO2= ~ 70-77 wt%; K/Na=0.95-2.05), composed of biotite granite and minor quartz monzonite. Differences in major and trace element and Nd isotopic characteristics for the two series indicate different petrogenetic histories. Potassic granites of the late-stage intrusions appear to represent crustal anatectic melts derived from tonalite sources, based on comparison of their major element compositions with experimental melt products. In addition, Nd isotopic characteristics of the potassic granites [??(Nd)(1.08 Ga) = -0.2 to -2.7] overlap with those for tonalites/granodiorites [ca 1.7 Ga Boulder Creek intrusions; ??(Nd)(1.08 Ga) = -2.4 to -3.6] exposed in the region. Some of the partial melts evolved by fractionation dominated by feldspar. The late-stage potassic granites share geochemical characteristics with most of the PPG, which is also interpreted to have an anatectic origin involving tonalitic crust. The origin of monzogranites associated with the PPG remains unclear, but mixing between granitic and mafic or intermediate magmas is a possibility. Syenites and granites of the sodic series cannot be explained as crustal melts, but are interpreted as fractionation products of mantle-derived mafic magmas with minor crustal input. High temperature and low oxygen fugacity estimates (e.g. Frost et al., 1988. American Mineralogist 73, 727-740) support a basalt fractionation origin, as do ??(Nd) values for sodic granitoids [??(Nd)(1.08 Ga) = +2.2 to -0.7], which are higher than ??(Nd) values for Colorado crust at 1.08 Ga (ca -1.0 to -4.0). Enrichments in incompatible elements (e.g. rare earth elements, Rb, Y) and depletions in compatible elements (e.g. Cr, Sr, Ba) in the sodic granitoids compared to coeval mafic rocks are also consistent with fractionation. Accessory mineral fractionation, release of fluorine-rich volatiles and/or removal of pegmatitic fluids could have modified abundances of Ce, Nb, Zr and Y in some sodic granitoid magmas. Gabbros and mafic dikes associated with the sodic granitoids have ??(Nd)(1.08 Ga) of -3.0 to +3.5, which are lower than depleted mantle at 1.08 Ga, and their trace element characteristics suggest derivation from mantle sources that were previously affected by subduction-related processes. However, it is difficult to characterize the mantle component in these magmas, because assimilation of crust during magma ascent could also result in their observed geochemical features. The Pikes Peak batholith is composed of at least two petrogenetically different granite types, both of which exhibit geochemical characteristics typical of A-type granites. Models proposed for the petrogenesis of the granitoids imply the existence of mafic rocks at depth and addition of juvenile material to the crust in central Colorado at ~ 1.1 Ga.

Precambrian Research↗
USGS1973

Origin of Comb Layering and Orbicular Structure, Sierra Nevada Batholith, California

A new descriptive term, comb layering , is proposed to replace the informal term Willow Lake-type layering , first introduced by Poldervaart and Taubeneck (1959) to describe layering in granitoid rocks in which constituent crystals are oriented approximately perpendicular to individual layers. The term schlieren layering is proposed to describe the “normal” layering of granitic rocks defined by alternating layers enriched or depleted in the normal mafic minerals. In such layers, elongate minerals commonly lie in the plane of the layering. Comb layering is widespread in plutonic rocks of California and is commonly associated with orbicular diorites. Evidence from a detailed study of three localities in the Sierra Nevada indicates that comb layering forms chiefly in overturned troughs along overhanging walls of plutons or along walls of dikes or pipes that cut country rocks adjoining plutons. Orbicular rocks associated with the comb layering are formed by a nucleus surrounded by multiple comb layers. The growth direction in comb layers can be determined by the branching and widening of plagioclase and hornblende crystals and is invariably toward the parent pluton. Field data indicate that comb layering cannot have formed from silicate magma, and further suggest that the layers have been deposited by large volumes of aqueous fluids that migrated upward along contacts between magma and wallrock or along the interface between magma and previously solidified melt. Comb layering and orbicules are largely restricted to structural traps into which upwardly migrating, solute-rich water was channeled owing to its low density. The comb layers grew on the solid walls of fluid-filled channels, whereas orbicules formed by precipitation of comb layers on hobbling inclusions suspended within the upward-flowing fluid.

California↗
USGS2002

Rocks and geology in the San Francisco Bay region

The landscape of the San Francisco Bay region is host to a greater variety of rocks than most other regions in the United States. This introductory guide provides illustrated descriptions of 46 common and important varieties of igneous, sedimentary, and metamorphic rock found in the region. Rock types are described in context of their identification qualities, how they form, and where they occur in the region. The guide also provides discussion about of regional geology, plate tectonics, the rock cycle, the significance of the selected rock types in relation to both earth history and the impact of mineral resources on the development in the region. Maps and text also provide information where rocks, fossils, and geologic features can be visited on public lands or in association with public displays in regional museums, park visitor centers, and other public facilities.

California↗
USGS1978

Chabazite in siliceous tuffs of a Pliocene lacustrine deposit near Durkee, Baker County, Oregon

A relatively pure chabazite deposit of at least 75 X 10 6 metric tons occurs near the Durkee type locality of erionite. This erionite is in a welded tuff interbedded with lacustrine rocks- of a closed Pliocene basin. The zeolites, chabazite, erionite, clinoptilolite, and other authigenic silicate minerals are confined to a 27-square-kilometer area of altered rocks enclosed in relatively freshwater lacustrine beds. About 135 meters of the lower part of the 350-m stratigraphic section at the southeastern part of the lake basin contain zeolitic tuff. The chabazite is an early product of the reaction between glass of rhyolitic or dacitic composition and alkaline (pH>9) saline lake waters. Erionite, clinoptilolite, analcime, and potassium feldspar follow chabazite in the paragenetic sequence. At the erionite type locality, chabazite and clinoptilolite form megascopic crystal clusters within fractures and cavities in unaltered welded tuff. Elsewhere in the basin, most zeolite crystals in the altered airfall or reworked tuffs are less than 80 micrometers long. Chemical analyses, unit-cell determinations, and mean indices of refraction show that fine-grained chabazite varies in composition and physical parameters.

Oregon↗
USGS2025

Comparative properties of saponitic fault gouge and serpentinite muds cored from mud volcanoes of the Mariana subduction zone

We obtained 12 core samples for physical and chemical characterization from three serpentinite mud volcanoes (Yinazao, Asùt Tesoru, and Fantangisña) located on the forearc of the Mariana subduction system, that were drilled during International Ocean Discovery Program Expedition 366. Two samples from the Fantangisña mud volcano are interpreted to be clay-rich fault gouges derived from the subduction channel. Their bulk compositions are intermediate between the serpentinites and oceanic basalts. The oceanic crustal materials in the gouges have been thoroughly metasomatized and the serpentinites extensively altered to the trioctahedral, Mg-rich smectite clays saponite and corrensite. The only relict phases in clasts of crustal rock are accessory Ti- and P-bearing minerals. The two fault gouge samples have lower frictional strengths ( μ < 0.2) than the serpentinites ( μ = 0.2–0.4), and their measured permeabilities are also somewhat lower. Their physical and compositional properties correspond to saponitic gouges from other faults that juxtapose serpentinite against crustal rocks, in particular gouges from the two creeping traces of the San Andreas Fault recovered in the core from the San Andreas Fault Observatory at Depth. The décollement beneath Fantangisña mud volcano is thus expected to be very weak and likely characterized by stable slip.

Geophysics, Geochemistry, Geosystems↗
USGS2020

Cretaceous to Oligocene magmatic and tectonic evolution of the western Alaska Range: Insights from U-Pb and 40Ar/39Ar geochronology

New U-Pb and 40 Ar/ 39 Ar ages integrated with geologic mapping and observations across the western Alaska Range constrain the distribution and tectonic setting of Cretaceous to Oligocene magmatism along an evolving accretionary plate margin in south-central Alaska. These rocks were emplaced across basement domains that include Neoproterozoic to Jurassic carbonate and siliciclastic strata of the Farewell terrane, Triassic and Jurassic plutonic and volcanic rocks of the Peninsular terrane, and Jurassic and Cretaceous siliciclastic strata of the Kahiltna assemblage. Plutonic rocks of different ages also host economic mineralization including intrusion-related Au, porphyry Cu-Mo-Au, polymetallic veins and skarns, and peralkaline intrusion-related rare-earth elements. The oldest intrusive suites were emplaced ca. 104–80 Ma into the Peninsular terrane only prior to final accretion. Deformation of the northern Kahiltna succession and underlying Farewell terrane occurred at ca. 97 Ma, and more widespread deformation ca. 80 Ma involved south-vergent folding and thrusting of the Kahiltna assemblage that records collisional accretion of the Peninsular-Wrangellia terrane and juxtaposition of sediment wedges formed on the inboard and outboard terranes. More widespread magmatism ca. 75–55 Ma occurred in two general pulses, each having distinct styles of localized deformation. Circa 75–65 Ma plutons were emplaced in a transpressional setting and stitch the accreted Peninsular and Wrangellia terranes to the Farewell terrane. Circa 65–55 Ma magmatism occurred across the entire range and extends for more than 200 km inboard from the inferred position of the continental margin. The Paleocene plutonic suite generally reflects shallower emplacement depths relative to older suites and is associated with more abundant andesitic to rhyolitic volcanic rocks. Deformation ca. 58–56 Ma was concentrated along two high-strain zones, the most prominent of which is 1 km wide, strikes east-northeast, and accommodated dextral oblique motion. Emplacement of widespread intermediate to mafic dikes ca. 59–51 Ma occurred before a notable magmatic lull from ca. 51–44 Ma reflecting a late Paleocene to early Eocene slab window. Magmatism resumed ca. 44 Ma, recording the transition from slab window to renewed subduction that formed the Aleutian-Meshik arc to the southwest. In the western Alaska Range, Eocene magmatism included emplacement of the elongate north-south Merrill Pass pluton and large volumes of ca. 44–37 Ma andesitic flows, tuffs, and lahar deposits. Finally, a latest Eocene to Oligocene magmatic pulse involved emplacement of a compositionally variable but spatially concentrated suite of magmas ranging from gabbro to peralkaline granite ca. 35–26 Ma, followed by waning magmatism that coincided with initiation of Yakutat shallow-slab subduction. Cretaceous to Oligocene magmatism throughout the western Alaska Range collectively records terrane accretion, translation, and integration together with evolving subduction dynamics that have shaped the southern Alaska margin since the middle Mesozoic.

Alaska↗
USGS2012

Mineralization, alteration, and hydrothermal metamorphism of the ophiolite-hosted Turner-Albright sulfide deposit, southwestern Oregon

The Turner-Albright sulfide deposit, part of the Josephine ophiolite, formed on and below the seafloor during Late Jurassic volcanism at a back arc spreading center. Ore fluids were probably localized by faults which were active on the seafloor at the time of sulfide deposition. The uppermost massive sulfide formed on the seafloor at hydrothermal vents, similar to sulfide-depositing hot springs on modern spreading centers. The bulk of the sulfide mineralization formed below the seafloor within olivine basalt hyaloclastite erupted near the time of mineralization. Infiltration of hydrothermal fluid into the hyalo-clastite altered the rock to quartz + Fe-chlorite and quartz + sericite + Fe-chlorite. Intense alteration resulted in replacement of the protolith by quartz + sulfide. The fluid responsible for the hydrothermal alteration was evolved seawater with low p H and Mg and high Fe. Sulfide δ 534 S values average 4.7‰ and indicate contribution of sulfur from both basalt and seawater sulfate. The average value of sulfide δ 34 S and the difference between sulfide and contemporaneous seawater sulfate δ 34 S values are similar to ophiolite-hosted sulfide deposits in Cyprus. Chlorite-rich hydrothermally altered basalt has δ 18 O values of 3.0–6.8‰ due to high-temperature (250°–350°C) reaction with a seawater-derived ore fluid. Quartz in silicified basalt has higher δ 18 O values of 12.3–15.5‰, probably reflecting lower-temperature recrystallization of amorphous silica formed during initial alteration of basalt. Mudstone and clinopyroxene basalt above the sulfide horizons were not altered by the ore-transporting hydrothermal fluid, but these rocks were hydrothermally metamorphosed by altered seawater heated by deep circulation into hot oceanic crust. This subseafloor metamorphism produced a mineral assemblage typical of prehnite-pumpellyite facies metamorphism. Exchange with altered seawater increased the whole-rock δ 18 O of the basalts to values of 9.4–11.2‰.

Journal of Geophysical Research Solid Earth↗
USGS2009

A comparison of pre- and post-remediation water quality, Mineral Creek, Colorado

Pre- and post-remediation data sets are used herein to assess the effectiveness of remedial measures implemented in the headwaters of the Mineral Creek watershed, where contamination from hard rock mining has led to elevated metal concentrations and acidic pH. Collection of pre- and post-remediation data sets generally followed the synoptic mass balance approach, in which numerous stream and inflow locations are sampled for the constituents of interest and estimates of streamflow are determined by tracer dilution. The comparison of pre- and post-remediation data sets is confounded by hydrologic effects and the effects of temporal variation. Hydrologic effects arise due to the relatively wet conditions that preceded the collection of pre-remediation data, and the relatively dry conditions associated with the post-remediation data set. This difference leads to a dilution effect in the upper part of the study reach, where pre-remediation concentrations were diluted by rainfall, and a source area effect in the lower part of the study reach, where a smaller portion of the watershed may have been contributing constituent mass during the drier post-remediation period. A second confounding factor, temporal variability, violates the steady-state assumption that underlies the synoptic mass balance approach, leading to false identification of constituent sources and sinks. Despite these complications, remedial actions completed in the Mineral Creek headwaters appear to have led to improvements in stream water quality, as post-remediation profiles of instream load are consistently lower than the pre-remediation profiles over the entire study reach for six of the eight constituents considered (aluminium, arsenic, cadmium, copper, iron, and zinc). Concentrations of aluminium, cadmium, copper, lead, and zinc remain above chronic aquatic-life standards, however, and additional remedial actions may be needed. Future implementations of the synoptic mass balance approach should be preceded by an assessment of temporal variability, and modifications to the synoptic sampling protocol should be made if necessary.

Colorado↗
USGS2012

Zinc isotope investigation of surface and pore waters in a mountain watershed impacted by acid rock drainage

The pollution of natural waters with metals derived from the oxidation of sulfide minerals like pyrite is a global environmental problem. However, the metal loading pathways and transport mechanisms associated with acid rock drainage reactions are often difficult to characterize using bulk chemical data alone. In this study, we evaluated the use of zinc (Zn) isotopes to complement traditional geochemical tools in the investigation of contaminated waters at the former Waldorf mining site in the Rocky Mountains, Colorado, U.S.A. Geochemical signatures and statistical analysis helped in identifying two primary metal loading pathways at the Waldorf site. The first was characterized by a circumneutral pH, high alkalinity, and high Zn/Cd ratios. The second was characterized by acidic pHs and low Zn/Cd ratios. Zinc isotope signatures in surface water samples collected across the site were remarkably similar (the δ 66 Zn, relative to JMC 3-0749-L, for most samples ranged from 0.20 to 0.30‰ ± 0.09‰ 2σ). This probably suggests that the ultimate source of Zn is consistent across the Waldorf site, regardless of the metal loading pathway. The δ 66 Zn of pore water samples collected within a nearby metal-impacted wetland area, however, were more variable, ranging from 0.20 to 0.80‰ ± 0.09‰ 2σ. Here the Zn isotopes seemed to reflect differences in groundwater flow pathways. However, a host of secondary processes might also have impacted Zn isotopes, including adsorption of Zn onto soil components, complexation of Zn with dissolved organic matter, uptake of Zn into plants, and the precipitation of Zn during the formation of reduced sulfur species. Zinc isotope analysis proved useful in this study; however, the utility of this isotopic tool would improve considerably with the addition of a comprehensive experimental foundation for interpreting the complex isotopic relationships found in soil pore waters.

Colorado↗
USGS1991

Petrogenesis and geological history of a uranium source rock: a case study in northeastern Washington, U.S.A.

A small (4 km 2 ) drainage basin in northeastern Washington contains highly uraniferous groundwater and highly uraniferous peaty sediments of Holocene age. The U is derived from granitic bedrock that underlies the entire drainage basin and that contains 9–16 ppm U. This local bedrock was studied by petrographic, chemical and isotopic methods to determine conditions of its petrogenesis and post-emplacement history that may have contributed to its present high U content and source-rock capability. The original magma was derived by anatexis of Precambrian continental crust of probable mixed metaigneous and metasedimentary character. Mineral-melt partitioning controlled the enrichment of U in chemically evolved phases of the crystallizing melt. Following emplacement in the upper crust at ∼100Ma, the pluton interacted with meteoric-hydrothermal water at ambient temperatures 300°C. Locally intense fracturing promoted alteration, and fracturing and alteration probably continued during later regional uplift in the Eocene. Regional uplift was followed by low-temperature alteration and weathering in the middle to late Tertiary. The combined result of hydrothermal alteration and low-temperature alteration and weathering was the redistribution of U from primary mineral hosts such as allanite to new sites on fracture surfaces and in secondary minerals such as hematite. Zones of highly fractured and altered rock show the most obvious evidence of this process. A model is proposed in which high-angle fractures beneath the drainage basin were the sites of Tertiary supergene enrichments of U. Recent glacio-isostatic uplift has elevated these older enriched zones to shallow levels where they are now being leached by oxidizing groundwater. The chemistry, mineralogy, texture and geological history of this U source-rock suggest criteria for locating other granitic terrane that may contain uraniferous waters and associated young surficial U deposits. The details of U distribution and mobility at this site also apply to the general topic of U mobility in granitic rocks.

Washington↗
USGS1972

Reconnaissance geology of the Precambrian rocks in the Bi'r Ghamrah quadrangle, Kingdom of Saudi Arabia

Three sequences of volcanic and sedimentary rocks are identified in the Precambrian rocks of the Bi'r Ghamrah quadrangle at the eastern edge of the Precambrian Shield in central Saudi Arabia. The oldest sequence is called the Bi'r Khountina Group. It consists of conglomerate marble, andesite, and graywacke. Unconformably overlying this group is a sequence of graywacke with minor lava called the Murdama Group. In a small area in the southern part of the quadrangle, these rocks are unconformably overlain by rhyolitic tuff and rhyolite tentatively correlated with the Shammar Rhyolite. The older of these sedimentary and volcanic rocks were intruded by diorite and gabbro and by a large pluton of alkalic granite. A contact metamorphic aureole was formed in the Bi'r Khountina and Murdama Groups adjacent to the granite, and feeder dikes of the Sbmmmar Rhyolite(?) intrude the granite. The Bi'r Khountina Group is folded into a south-plunging asymmetrical anticlinorium, the west limb of which is repeated across northwest-trending faults. The Murdama Group appears to have been folded along the same axes, but the contact aureole against the alkalic granite and the imprint of the west-northwest striking Najd fault zone cause the rocks of the Murdama Group to appear to trend westward. Results of spectrographic and chemical analyses of wadi sand, heavy-mineral concentrates, and detrital magnetite show small anomalies. The ultramafic rocks intruded prior to the deposition of the Murdama Group are the source of anomalous chromium and lanthanum and of threshold nickel, scandium, and vanadium. The intrusive rocks younger than the Murdama Group are sources for anomalous lead and threshold silver, boron, barium, beryllium, zirconium, lanthanum, and tin. One small ancient working, probably opened for gold, is present, and at least four places in the Precambrian part of the quadrangle ere potentially favorable for gold, silver, and lead. Chromite is a potential resource in the northeastern part of the quadrangle.

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