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USGS2006

Sustained long-period seismicity at Shishaldin Volcano, Alaska

From September 1999 through April 2004, Shishaldin Volcano, Aleutian Islands, Alaska, exhibited a continuous and extremely high level of background seismicity. This activity consisted of many hundreds to thousands of long-period (LP; 1–2 Hz) earthquakes per day, recorded by a 6-station monitoring network around Shishaldin. The LP events originate beneath the summit at shallow depths (0–3 km). Volcano tectonic events and tremor have rarely been observed in the summit region. Such a high rate of LP events with no eruption suggests that a steady state process has been occurring ever since Shishaldin last erupted in April–May 1999. Following the eruption, the only other signs of volcanic unrest have been occasional weak thermal anomalies and an omnipresent puffing volcanic plume. The LP waveforms are nearly identical for time spans of days to months, but vary over longer time scales. The observations imply that the spatially close source processes are repeating, stable and non-destructive. Event sizes vary, but the rate of occurrence remains roughly constant. The events range from magnitude ∼0.1 to 1.8, with most events having magnitudes <1.0. The observations suggest that the conduit system is open and capable of releasing a large amount of energy, approximately equivalent to at least one magnitude 1.8–2.6 earthquake per day. The rate of observed puffs (1 per minute) in the steam plume is similar to the typical seismic rates, suggesting that the LP events are directly related to degassing processes. However, the source mechanism, capable of producing one LP event about every 0.5–5 min, is still poorly understood. Shishaldin's seismicity is unusual in its sustained high rate of LP events without accompanying eruptive activity. Every indication is that the high rate of seismicity will continue without reflecting a hazardous state. Sealing of the conduit and/or change in gas flux, however, would be expected to change Shishaldin's behavior.

Alaska↗

USGS1999

Spatial distribution of chemical constituents in the Kuskokwim River, Alaska

The effects of lithologic changes on the water quality of the Kuskokwim River, Alaska, were evaluated by the U.S. Geological Survey in June 1997. Water, suspended sediments, and bed sediments were sampled from the Kusko-kwim River and from three tributaries, the Holitna River, Red Devil Creek, and Crooked Creek. Dissolved boron, chromium, copper, manganese, zinc, aluminum, lithium, barium, iron, antimony, arsenic, mercury, and strontium were detected. Dissolved manganese and iron concentrations were three and four times higher in the Holitna River than in the Kusko-kwim River. Finely divided ferruginous materials found in the graywacke and shale units of the Kuskokwim Group are the probable source of the iron. The highest concentrations of dissolved strontium and barium were found at McGrath, and the limestone present in the upper basin was the most probable source of strontium. The total mercury concentrations on the Kuskokwim River decreased downstream from McGrath. Dissolved mercury was 24 to 32 percent of the total concentration. The highest concentrations of total mercury, and of dissolved antimony and arsenic were found in Red Devil Creek. The higher concentrations from Red Devil Creek did not affect the main stem mercury transport because the tributary was small relative to the Kuskokwim River. In Red Devil Creek, total mercury exceeded the concentration at which the U.S. Environmental Protection Agency (USEPA) indicates that aquatic life is affected and dissolved arsenic exceeded the USEPA's drinking-water standard. Background mercury and antimony concentrations in bed sediments ranged from 0.09 to 0.15 micrograms per gram for mercury and from 1.6 to 2.1 micrograms per gram for antimony. Background arsenic concentrations were greater than 27 micrograms per gram. Sites near the Red Devil mercury mine had mercury and antimony concentrations greater than background concentrations. These concentrations probably reflect the proximity to the ore body and past mining. Crooked Creek had mercury concentrations greater than the background concentration. The transport of suspended sediment-associated trace elements was lower for all elements in the lower river than in the upper river, indicating storage of sediments and their associated metals within the river system.

Alaska↗

USGS1985

Origin and tectonic evolution of the Maclaren and Wrangellia terranes, eastern Alaska Range, Alaska

Major portions of the eastern Alaska Range, south of the Denali fault, in the McCarthy, Nabesna, Mount Hayes, and eastern Healy quadrangles, consist predominantly of the Maclaren and Wrangellia tectono-stratigraphic terranes. The Maclaren terrane consists of the Maclaren Glacier metamorphic belt and the regionally deformed and metamorphosed East Susitna batholith. The Maclaren Glacier metamorphic belt is composed of argillite, metagraywacke, and sparse andesite flows that are progressively regionally metamorphosed from lower greenschist facies to middle amphibolite facies near the East Susitna batholith. The East Susitna batholith is composed of gabbro, quartz diorite, granodiorite, and sparse quartz monzonite. Isotopic ages are as old as a K-Ar hornblende age of 87.5 m.y., possibly reset, and a U-Pb zircon age of 70 m.y. The batholith is intensely deformed and regionally metamorphosed under conditions of the middle amphibolite facies. The Wrangellia terrane is divided into two subterranes: (1) the Slana River subterrane, composed of late Paleozoic andesite to dacite flows, tuff, limestone, and argillite, unconformably overlying massive basalt flows of the Triassic Nikolai Greenstone, Late Triassic limestone, and younger Mesozoic flysch; and (2) the Tangle subterrane, a deeper-water equivalent of the Slana River subterrane, composed of late Paleozoic and Early Triassic aquagene tuff, chert, minor andesite tuff and flows, limestone, unconformably overlying pillow basalt and massive basalt flows of the Triassic Nikolai Greenstone, and Late Triassic limestone. Both subterranes are intruded by locally extensive gabbro and diabase dikes and by cumulate mafic and ultramafic sills. Less extensive terranes (two) are the Clearwater terrane, a sequence of intensely deformed chlorite schist, muscovite schist, marble, and greenstone of Late Triassic age; and an unnamed terrane of ultramafic and associated rocks of presumable Paleozoic or Mesozoic age. Each terrane or subterrane generally has (1) a distinctive time-stratigraphic sequence reflecting a unique geologic history; (2) a missing provenance for bedded sedimentary or volcanic rocks; and (3) bounding thrust or strike-slip faults, interpreted as accretionary sutures. The Maclaren and Wrangellia terranes are juxtaposed along the Broxson Gulch thrust, which consists of an imbricate series of north-dipping thrust faults. Paralleling the Broxson Gulch thrust, a few kilometres to the south, is the north-dipping Eureka Creek thrust, along which are juxtaposed the Slana River and Tangle subterranes. The Maclaren terrane is correlated with the Kluane Schist and the Ruby Range batholith in the southern Yukon Territory, which represent the northward extension of the Taku and Tracy Arm terranes. If correct, this correlation defines a minimum displacement of the Maclaren terrane along the Denali fault of ∼400 km. The Maclaren terrane is interpreted to have formed in a synorogenic Andean-type arc setting on the west margin of Mesozoic North America in the middle to late Mesozoic and early Cenozoic. The Wrangellia terrane is interpreted to have initially formed in an island-arc setting during the late Paleozoic. Subsequently in the Late Triassic, the Wrangellia terrane underwent rifting near the paleoequator, with formation of the Nikolai Greenstone and associated mafic and ultra-mafic igneous rocks. In the middle and late Mesozoic, Wrangellia migrated toward, and was accreted during, the middle Cretaceous to the Maclaren terrane along the Broxson Gulch thrust. Subsequent dispersion of both the Maclaren and Wrangellia terranes along the Denali fault and the Broxson Gulch thrust commenced during the early Tertiary and continues through the present.

Alaska↗

USGS1984

Geomorphic domains and linear features on Landsat images, Circle quadrangle, Alaska

A remote sensing study using Landsat images was undertaken as part of the Alaska Mineral Resource Assessment Program (AMRAP). Geomorphic domains A and B, identified on enhanced Landsat images, divide Circle quadrangle south of Tintina fault zone into two regional areas having major differences in surface characteristics. Domain A is a roughly rectangular, northeast-trending area of relatively low relief and simple, widely spaced drainages, except where igneous rocks are exposed. In contrast, domain B, which bounds two sides of domain A, is more intricately dissected showing abrupt changes in slope and relatively high relief. The northwestern part of geomorphic domain A includes a previously mapped tectonostratigraphic terrane. The southeastern boundary of domain A occurs entirely within the adjoining tectonostratigraphic terrane. The sharp geomorphic contrast along the southeastern boundary of domain A and the existence of known faults along this boundary suggest that the southeastern part of domain A may be a subdivision of the adjoining terrane. Detailed field studies would be necessary to determine the characteristics of the subdivision. Domain B appears to be divisible into large areas of different geomorphic terrains by east-northeast-trending curvilinear lines drawn on Landsat images. Segments of two of these lines correlate with parts of boundaries of mapped tectonostratigraphic terranes. On Landsat images prominent north-trending lineaments together with the curvilinear lines form a large-scale regional pattern that is transected by mapped north-northeast-trending high-angle faults. The lineaments indicate possible lithlogic variations and/or structural boundaries. A statistical strike-frequency analysis of the linear features data for Circle quadrangle shows that northeast-trending linear features predominate throughout, and that most northwest-trending linear features are found south of Tintina fault zone. A major trend interval of N.64-72E. in the linear feature data, corresponds to the strike of foliations in metamorphic rocks and magnetic anomalies reflecting compositional variations suggesting that most linear features in the southern part of the quadrangle probably are related to lithologic variations brought about by folding and foliation of metamorphic rocks. A second important trend interval, N.14-35E., may be related to thrusting south of the Tintina fault zone, as high concentrations of linear features within this interval are found in areas of mapped thrusts. Low concentrations of linear features are found in areas of most igneous intrusives. High concentrations of linear features do not correspond to areas of mineralization in any consistent or significant way that would allow concentration patterns to be easily used as an aid in locating areas of mineralization. The results of this remote sensing study indicate that there are several possibly important areas where further detailed studies are warranted.

Alaska↗

USGS2007

Determining the pattern of cementum annuli and relationship to reproduction in male sea otters

Since the early 1990s, the southwestern Alaskan sea otter (Enhydra lutris) population has declined dramatically and the cause has yet to be determined. Population trajectories of large mammals are determined by three factors: survival rate, reproduction rate, and age of first reproduction (AFR). Of these three, AFR should respond first to environmental change. Life history theory predicts that AFR will be older with bottom-up causes (ie, food limitation) and younger when the cause of the decline is top-down (ie, predation), as there is usually abundant resources in this scenario. Traditionally, determining AFR required lethal sampling, which may not always be possible. Work on many mammalian species suggests that the width of annual cementum layers in teeth may decline when breeding begins. If so, examining teeth annuli may provide a nonlethal alternative for determining AFR. Ongoing research has shown this relationship in female sea otters, but male sea otters have not been tested. Sea otter testes and premolar teeth slides were collected by subsistence hunters working with the US Fish and Wildlife Service and the Alaska Sea Otter and Steller Sea Lion Commission from Alaska (1994– 2005). We determined the pattern in cementum annuli thickness for male sea otters across age by measuring annuli at three sites on each of the two slide sections available. We found that cementum annuli layers decreased with age, but found no correlation between cementum annuli and sexual maturity in male sea otters. This lack of correlation may be due to sampling error or different energy expenditures during reproduction for each sex. Since females expend large amounts of energy through gestation and lactation, we hypothesize that the width of female cementum annuli decreases at a much sharper rate when they reach AFR. The southwest Alaskan sea otter population has plummeted up to 90% since the early 1990s and the reason is unknown.1 Declines may be due to a bottom-up source caused by lack of food and habitat that occurs when populations are dense, in which case we would expect reproduction rates to decrease and age of first reproduction (AFR) to increase. However, circumstantial evidence suggests that the decline may be due to a top-down source, ie, predation.2 In this case, we would expect reproduction rates to increase and AFR to decrease due to lowered densities that coincide with top-down declines.3 An ongoing study of female sea otters has shown that past reproduction can be determined by analysis of cementum annuli in the teeth, and used to calculate AFR.4 Cementum annuli are annual deposits of minerals that occur on the root of a tooth that correspond to the growth of the animal (Figure 1). During summer and spring, growth is fast which accounts for the white area, but during winter, growth slows down and the cementum is compressed which accounts for a darkened line. In females, the relationship between cementum width declines and reproduction likely reflects the large expenditure of energy that is required during gestation and lactation. However, male and female mammals expend energy for reproduction in very distinct ways,3 and the same pattern of cementum decline may not be present in males. Since shifts in the life history strategy of males are poorly understood, we are interested in testing the relationship between cementum declines and reproduction in male sea otters using cementum analysis.

Ethnicity and Disease↗

USGS2004

Multistage hydrothermal silicification and Fe-Tl-As-Sb-Ge-REE enrichment in the Red Dog Zn-Pb-Ag district, northern Alaska: Geochemistry, origin, and exploration applications

Geochemical analyses of major, trace, and rare earth elements (REE) in more than 200 samples of variably silicified and altered wall rocks, massive and banded sulfide, silica rock, and sulfide-rich and unmineralized barite were obtained from the Main, Aqqaluk, and Anarraaq deposits in the Red Dog Zn-Pb-Ag district of northern Alaska. Detailed lithogeochemical profiles for two drill cores at Aqqaluk display an antithetic relationship between SiO2/Al2O3 and TiO2/Zr which, together with textural information, suggest preferential silicification of carbonate-bearing sediments. Data for both drill cores also show generally high Tl, Sb, As, and Ge and uniformly positive Eu anomalies (Eu/Eu* > 1.0). Similar high Tl, Sb, As, Ge, and Eu/Eu* values are present in the footwall and shallow hanging wall of Zn-Pb-Ag sulfide intervals at Anarraaq but are not as widely dispersed. Net chemical changes for altered wall rocks in the district, on the basis of average Al-normalized data relative to unaltered black shales of the host Kuna Formation, include large enrichments (>50%) of Fe, Ba, Eu, V, S, Co, Zn, Pb, Tl, As, Sb, and Ge at both Red Dog and Anarraaq, Si at Red Dog, and Sr, U, and Se at Anarraaq. Large depletions (>50%) are evident for Ca at both Red Dog and Anarraaq, for Mg, P, and Y at Red Dog, and for Na at Anarraaq. At both Red Dog and Anarraaq, wall-rock alteration removed calcite and minor dolomite during hydrothermal decarbonation reactions and introduced Si, Eu, and Ge during silicification. Sulfidation reactions deposited Fe, S, Co, Zn, Pb, Tl, As, and Sb; barite mineralization introduced Ba, S, and Sr. Light REE and U were mobilized locally. This alteration and mineralization occurred during Mississippi an hydrothermal events that predated the Middle Jurassic-Cretaceous Brookian orogeny. Early hydrothermal silicification at Red Dog took place prior to or during massive sulfide mineralization, on the basis of the dominantly planar nature of Zn-Pb veins, which suggests filling of fractures that developed in previously lithified rock. Uniformly low Ca and Mg and uniformly negative Ce anomalies in highly siliceous Red Dog wall rocks reflect hydrothermal decarbonation reactions and pervasive silicification owing to conductive cooling of oxidized metalliferous fluids. Similar Ca and Mg depletions are evident at Anarraaq but generally lack associated silicification, possibly because temperatures of the hydrothermal fluids were too low (<180??C) or because the thermal contrast between the fluids and wall rocks was smaller owing to the greater depth of alteration and mineralization there, compared with Red Dog. Chalcophile element anomalies (Fe, Zn, Pb, Tl, As, Sb) in wall rocks at both Red Dog and Anarraq are attributed to sulfidation reactions, coeval with subsurface Zn-Pb-Ag mineralization, during the mixing of oxidized metalliferous fluids with H2S-rich fluids derived locally within the Kuna Formation. Sedimentary wall rocks in the Red Dog district are characterized by a distinctive suite of geochemical anomalies, especially for Zn, Pb, Tl, As, Sb, Ge, and Eu/Eu*. At the Aqqaluk deposit, wall rocks without visible sphalerite or galena (<300 ppm Zn + Pb) have anomalous Eu/Eu*, Tl, Sb, and As for up to ???100 m stratigraphically below Zn-rich silica rock. At Anarraaq, the Tl anomaly is most extensively developed, and enrichment relative to unaltered black shale of the Kuna Formation is present up to 62 m above the highest Zn-Pb sulfide zones. The magnitude of the enrichment and systematic behavior of Tl in the district make Tl a promising geochemical exploration guide for Red Dog-type Zn-Pb-Ag deposits elsewhere. ?? 2004 by Economic Geology.

Economic Geology↗

USGS2014

Carbonate rocks of the Seward Peninsula, Alaska: Their correlation and paleogeographic significance

Paleozoic carbonate strata deposited in shallow platform to off-platform settings occur across the Seward Peninsula and range from unmetamorphosed Ordovician–Devonian(?) rocks of the York succession in the west to highly deformed and metamorphosed Cambrian–Devonian units of the Nome Complex in the east. Faunal and lithologic correlations indicate that early Paleozoic strata in the two areas formed as part of a single carbonate platform. The York succession makes up part of the York terrane and consists of Ordovician, lesser Silurian, and limited, possibly Devonian rocks. Shallow-water facies predominate, but subordinate graptolitic shale and calcareous turbidites accumulated in deeper water, intraplatform basin environments, chiefly during the Middle Ordovician. Lower Ordovician strata are mainly lime mudstone and peloid-intraclast grainstone deposited in a deepening upward regime; noncarbonate detritus is abundant in lower parts of the section. Upper Ordovician and Silurian rocks include carbonate mudstone, skeletal wackestone, and coral-stromatoporoid biostromes that are commonly dolomitic and accumulated in warm, shallow to very shallow settings with locally restricted circulation. The rest of the York terrane is mainly Ordovician and older, variously deformed and metamorphosed carbonate and siliciclastic rocks intruded by early Cambrian (and younger?) metagabbros. Older (Neoproterozoic–Cambrian) parts of these units are chiefly turbidites and may have been basement for the carbonate platform facies of the York succession; younger, shallow- and deep-water strata likely represent previously unrecognized parts of the York succession and its offshore equivalents. Intensely deformed and altered Mississippian carbonate strata crop out in a small area at the western edge of the terrane. Metacarbonate rocks form all or part of several units within the blueschist- and greenschist-facies Nome Complex. The Layered sequence includes mafic meta¬igneous rocks and associated calcareous metaturbidites of Ordovician age as well as shallow-water Silurian dolostones. Scattered metacarbonate rocks are chiefly Cambrian, Ordovician, Silurian, and Devonian dolostones that formed in shallow, warm-water settings with locally restricted circulation and marbles of less constrained Paleozoic age. Carbonate metaturbidites occur on the northeast and southeast coasts and yield mainly Silurian and lesser Ordovician and Devonian conodonts; the northern succession also includes debris flows with meter-scale clasts and an argillite interval with Late Ordovician graptolites and lenses of radiolarian chert. Mafic igneous rocks at least partly of Early Devonian age are common in the southern succession. Carbonate rocks on Seward Peninsula experienced a range of deformational and thermal histories equivalent to those documented in the Brooks Range. Conodont color alteration indices (CAIs) from Seward Peninsula, like those from the Brooks Range, define distinct thermal provinces that likely reflect structural burial. Penetratively deformed high-pressure metamorphic rocks of the Nome Complex (CAIs ≥5) correspond to rocks of the Schist belt in the southern Brooks Range; both record subduction during early stages of the Jurassic–Cretaceous Brooks Range orogeny. Weakly metamorphosed to unmetamorphosed strata of the York terrane (CAIs mainly 2–5), like Brooks Range rocks in the Central belt and structural allochthons to the north, experienced moderate to shallow burial during the main phase of the Brooks Range orogeny. The nature of the contact between the York terrane and the Nome Complex is uncertain; it may be a thrust fault, an extensional surface, or a thrust fault later reactivated as an extensional fault. Lithofacies and biofacies data indicate that, in spite of their divergent Mesozoic histories, rocks of the York terrane and protoliths of the Nome Complex formed as part of the same lower Paleozoic carbonate platform. Stratigraphies in both

GSA Special Papers↗

USGS2010

Nesting ecology of boreal forest birds following a massive outbreak of spruce beetles

We studied breeding dark-eyed juncos ( Junco hyemalis ), yellow-rumped warblers ( Dendroica coronata ), and spruce-nesting birds from 1997 to 1998 among forests with different levels of spruce ( Picea spp.) mortality following an outbreak of spruce beetles ( Dendroctonus rufipennis ) in Alaska, USA. We identified species using live and beetle-killed spruce for nest sites and monitored nests to determine how the outbreak influenced avian habitat selection and reproduction. We tested predictions that 1) nesting success of ground-nesting juncos would increase with spruce mortality due to proliferation of understory vegetation available to conceal nests from predators, 2) nesting success of canopy-nesting warblers would decrease with spruce mortality due to fewer live spruce in which to conceal nests, and 3) both species would alter nest-site selection in response to disturbance. Juncos did not benefit from changes in understory vegetation; nesting success in highly disturbed stands (46%) was comparable to that in undisturbed habitats throughout their range. In stands with low spruce mortality, nesting success of juncos was low (5%) and corresponded with high densities of red squirrels ( Tamiasciurus hudsonicus ). Yellow-rumped warblers nested exclusively in spruce, but success did not vary with spruce mortality. As disturbance increased, nesting warblers switched from selecting forest patches with high densities of live white spruce ( Picea glauca ) to patches with beetle-killed spruce. Warblers also placed nests in large-diameter live or beetle-killed spruce, depending on which was more abundant in the stand, with no differences in nesting success. Five of the 12 other species of spruce-nesting birds also used beetle-killed spruce as nest sites. Because beetle-killed spruce can remain standing for >50 years, even highly disturbed stands provide an important breeding resource for boreal forest birds. We recommend that boreal forest managers preserve uncut blocks of infested forest within managed forest landscapes and practice partial harvest of beetle-killed spruce rather than commercial clear-cutting of infested stands in order to sustain breeding bird populations until natural reforestation occurs. Because breeding densities do not always reflect fitness, assessing impacts of a massive natural disturbance should include measuring impacts of changes in vegetation on both reproductive success and predator–prey dynamics.

Journal of Wildlife Management↗

USGS2009

The role of ridge subduction in determining the geochemistry and Nd–Sr–Pb isotopic evolution of the Kodiak batholith in southern Alaska

The Paleocene Kodiak batholith, part of the Sanak–Baranof belt of Tertiary near-trench intrusive rocks, forms an elongate body (~ 150 km long) that transects Kodiak Island from SW to NE. The batholith consists of three zones (Southern, Central, and Northern) of kyanite-, muscovite-, and garnet-bearing biotite tonalite and granodiorite and less abundant granite that intruded an accretionary prism (Kodiak Formation, and Ghost Rocks Formation). Small and likely coeval bodies (Northern, Western, and Eastern satellite groups) of quartz gabbro, diorite, tonalite, granodiorite, and leucogranite flank the batholith. The batholith is calc-alkalic, has an aluminum saturation index of > 1.1, FeO t /(FeO t + MgO) ~ 0.65 (at SiO 2 = 65 wt.%), and increases in SiO 2 (~ 61 wt.%–73 wt.%) and decreases in TiO 2 (~ 0.9 wt.%–0.3 wt.%) from SW to NE. As a group, the granitic rocks have light REE-enriched chondrite-normalized patterns with small or no negative Eu anomalies, primitive mantle-normalized negative anomalies for Nb and Ti, and positive anomalies for Pb. Small to large negative anomalies for Th are also distinctive. The quartz gabbros and diorites are generally characterized by generally flat to light REE chondrite-normalized patterns (no Eu anomalies), and mantle-normalized negative anomalies for Nb, Ti, and P. Pb isotopic compositions ( 206 Pb/ 204 Pb = 18.850–18.960; 207 Pb/ 204 Pb = 15.575–15.694; 208 Pb/ 204 Pb = 38.350–39.039) are intermediate between depleted mantle and average continental crust. The Southern zone and a portion of the Central zone are characterized by negative ε Nd values of − 3.7 to − 0.3 and T DM ages ranging from ~ 838 Ma to 1011 Ma. Other granitic rocks from the Central and Northern zones have higher ε Nd values of − 0.4 to + 4.7 and younger T DM ages of ~ 450 to 797 Ma. Granitic and mafic plutons from the Eastern satellites show a wide range of ε Nd values of − 2.7 to + 6.4, and T DM ages from 204 Ma to 2124 Ma. 87 Sr/ 86 Sr values of the Southern and Central zones overlap and tend to be slightly more radiogenic ( 87 Sr/ 86 Sr > 0.70426) than the Northern zone ( 87 Sr/ 86 Sr < 0.70472). 206 Pb/ 204 Pb values increase slightly from the Southern and Central zones toward the Northern zone. There is no clear correlation of the major or trace elements with ε Nd , Pb or Sr isotopic values. Kodiak Formation and the Ghost Rocks Formation overlap the isotopic compositions (e.g., 206 Pb/ 204 Pb = 18.978 to 19.165, 87 Sr/ 86 Sr of 0.705715 to 0.707118, and ε Nd of − 6.7 to − 1.5 at 59 Ma) and T DM values (959 to 1489 Ma) of the batholith. Production of large volumes of granitic rocks in the Sanak–Baranof belt, and particularly on Kodiak Island, reflects a sequence of processes that includes underplating of mantle-derived mafic (possibly from the mantle wedge) and intermediate rocks under the accretionary flysch, interlayering of mantle-derived and flyschoid rocks, and partial melting of the mixed lithologic assemblages. Limited degrees of fractional crystallization or assimilation and fractional crystallization influenced compositions of the granitic rocks. The contribution of mantle-derived rocks that resided in the accretionary prism for only a short period of time prior to partial melting likely exceeds 40% (up to 80%). The balance (60 to 20%) is from a recently recycled crustal component represented by the Kodiak Formation. This type of progressive intracrustal melting from mixed sources controlled the geochemical character of the batholith and is most consistent with the hypothesis that the granitic rocks are associated with a slab-window produced by collision of a spreading oceanic center and a subduction zone and migration beneath the accretionary prism.

Gulf of Alaska↗

USGS1971

The Shublik Formation and adjacent strata in northeastern Alaska description, minor elements, depositional environments and diagenesis

The Shublik Formation (Middle and Late Triassic) is widespread in the surface and subsurface of northern Alaska. Four stratigraphic sections along about 70 miles of the front of the northeastern Brooks Range east of the Canning giver were examined and sampled in detail in 1968. These sections and six-step spectrographic and carbon analyses of the samples combined with other data to provide a preliminary local description of the highly organic unit and of the paleoenvironments. Thicknesses measured between the overlying Kingak Shale of Jurassic age and the underlying Sadlerochit Formation of Permian and Triassic age range from 400 to more than 800 feet but the 400 feet, obtained from the most completely exposed section, may be closer to the real thickness across the region. The sections consist of organic-rich, phosphatic, and fossiliferous muddy, silty, or carbonate rocks. The general sequence consists, from the bottom up, of a lower unit of phosphatic siltstone, a middle unit of phosphatic carbonate rocks, and an upper unit of shale and carbonate rocks near the Canning River and shale, carbonate rocks, and sandstone to the east. Although previously designated a basal member of the Kingak Shale (Jurassic), the upper unit is here included with the Shublik on the basis of its regional lithologic relation. The minor element compositions of the samples of the Shublik Formation are consistent with their carbonaceous and phosphatic natures in that relatively large amounts of copper, molybdenum, nickel, vanadium and rare earths are present. The predominantly sandy rocks of the underlying Sadlerochit Formation (Permian and Triassic) have low contents of most minor elements. The compositions of samples of Kingak Shale have a wide range not readily explicable by the nature of the rock: an efflorescent sulfate salt contains 1,500 ppm nickel and 1,500 ppm zinc and large amounts of other metals derived from weathering of pyrite and leaching of local shale. The only recorded occurrence of silver and 300 ppm lead in gouge along a shear plane may be the result of metals introduced from an extraneous source. The deposits reflect a marine environment that deepened somewhat following deposition of the Sadlerochit Formation and then shoaled during deposition of the upper limestone-siltstone unit. This apparently resulted from a moderate transgression and regression of the sea with respect to a northwest-trending line between Barrow and the Brooks Range at the International Boundary. Nearer shore facies appear eastward. The phosphate in nodules, fossil molds and oolites, appears to have formed diagenetically within the uncompacted sediment.

Open-File Report↗

USGS1949

Calculated gravity anomaly produced by possible fault under line 27-48

The seismograph data from Line 27-48 of Party 47 indicates the possibility of a high angle fault win a displacement of roughly 1,000 feet under South Barrow Test Well No. 2. This fault vas discussed in E. Wiancko's report for United Geophysical Company entitled "Report of Reflection Seismograph Survey in the Barrow Area During 1948". Mr. Delta. and Mr. English recently expressed a desire to know the magnitude of the calculated gravity anomaly produced by this probable fault. Their purpose was to determine the feasibility of proving and locating this fault, if exists, by several traverses with the gravimeter. As interpreted in Wiancko's report, the fault is nearly east-west in strike and is a reverse fault with the up-thrust block on the north side. The assumptions made for the calculation of the theoretical anomaly were that the density of the basement rock was 2.7, the density of the overlying rocks was 2.2 and the fault itself was taken directly from Plate I, Drawing No. 2044, of Wiancko's report, simplified, and reduced by a factor of 2. The calculated anomaly is shown in Fig. 1. A polar chart similar to the type described on pages 153-154 of Heiland's "Geophysical Exploration" was used in computing the value of the anomaly. It was assumed that the feature is two-dimensional. Broken lines indicate values of limited accuracy. It should be mentioned here that Mr. Roland Henderson and Mr. Isidore Zietz of the Section of Geophysics of the Geological Survey carried out the calculations and plotted Fig. 1.

Alaska↗

USGS1994

Seismic evolution of the 1989-1990 eruption sequence of Redoubt Volcano, Alaska

Redoubt Volcano in south-central Alaska erupted between December 1989 and June 1990 in a sequence of events characterized by large tephra eruptions, pyroclastic flows, lahars and debris flows, and episodes of dome growth. The eruption was monitored by a network of five to nine seismic stations located 1 to 22 km from the summit crater. Notable features of the eruption seismicity include : (1) small long-period events beginning in September 1989 which increased slowly in number during November and early December; (2) an intense swarm of long-period events which preceded the initial eruptions on December 14 by 23 hours; (3) shallow swarms (0 to 3 km) of volcano-tectonic events following each eruption on December 15; (4) a persistent cluster of deep (6 to 10 km) volcano-tectonic earthquakes initiated by the eruptions on December 15, which continued throughout and beyond the eruption; (5) an intense swarm of long-period events which preceded the eruptions on January 2; and (6) nine additional intervals of increased long-period seismicity each of which preceded a tephra eruption. Hypocenters of volcano-tectonic earthquakes suggest the presence of a magma source region at 6-10 km depth. Earthquakes at these depths were initiated by the tephra eruptions on December 15 and likely represent the readjustment of stresses in the country rock associated with the removal of magma from these depths. The locations and time-history of these earthquakes coupled with the eruptive behavior of the volcano suggest this region was the source of most of the erupted material during the 1989-1990 eruption. This source region appears to be connected to the surface by a narrow pipe-like conduit as inferred from the hypocenters of volcano-tectonic earthquakes. Concentrations of shallow volcano-tectonic earthquakes followed each of the tephra eruptions on December 15; these shocks may represent stress readjustment in the wall rock related to the removal of magma and volatiles at these depths. This shallow zone was the source area of the majority of long-period seismicity through the remainder of the eruption. The long-period seismicity likely reflects the pressurization of the shallow portions of the magmatic system. ?? 1994.

Journal of Volcanology and Geothermal Research↗

USGS2009

Occurrence and distribution of iron, manganese, and selected trace elements in ground water in the glacial aquifer system of the northern United States

Dissolved trace elements, including iron and manganese, are often an important factor in use of ground water for drinking-water supplies in the glacial aquifer system of the United States. The glacial aquifer system underlies most of New England, extends through the Midwest, and underlies portions of the Pacific Northwest and Alaska. Concentrations of dissolved trace elements in ground water can vary over several orders of magnitude across local well networks as well as across regions of the United States. Characterization of this variability is a step toward a regional screening-level assessment of potential human-health implications. Ground-water sampling, from 1991 through 2003, of the National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey determined trace element concentrations in water from 847 wells in the glacial aquifer system. Dissolved iron and manganese concentrations were analyzed in those well samples and in water from an additional 743 NAWQA land-use and major-aquifer survey wells. The samples are from monitoring and water-supply wells. Concentrations of antimony, barium, beryllium, cadmium, chromium, cobalt, copper, iron, lead, manganese, molybdenum, nickel, selenium, strontium, thallium, uranium, and zinc vary as much within NAWQA study units (local scale; ranging in size from a few thousand to tens of thousands of square miles) as over the entire glacial aquifer system. Patterns of trace element concentrations in glacial aquifer system ground water were examined by using techniques suitable for a dataset with zero to 80 percent of analytical results reported as below detection. During the period of sampling, the analytical techniques changed, which generally improved the analytical sensitivity. Multiple reporting limits complicated the comparison of detections and concentrations. Regression on Order Statistics was used to model probability distributions and estimate the medians and other quantiles of the trace element concentrations. Strontium and barium were the most frequently detected and usually were present in the highest concentrations. Iron and manganese were the next most commonly detected and next highest in concentrations. Iron concentrations were the most variable with respect to the range of variations (both within local networks and aquifer-wide) and with respect to the disparity between magnitude of concentrations (detections) and the frequency of samples below reporting limits (nondetections). Antimony, beryllium, cadmium, silver, and thallium were detected too infrequently for substantial interpretation of their occurrence or distributions or potential human-health implications. For those elements that were more frequently detected, there are some geographic patterns in their occurrence that primarily reflect climate effects. The highest concentrations of several elements were found in the West-Central glacial framework area (High Plains and northern Plains areas). There are few important patterns for any element in relation to land use, well type, or network type. Shallow land-use (monitor) wells had iron concentrations generally lower than the glacial aquifer system wells overall and much lower than major-aquifer survey wells, which comprise mostly private- and public-supply wells. Unlike those for iron, concentration patterns for manganese were similar among shallow land-use wells and major-aquifer survey wells. An apparent relation between low pH and relatively low concentrations of many elements, except lead, may be more indicative of the relatively low dissolved-solids content in wells in the Northeastern United States that comprise the majority of low pH wells, than of a pH dependent pattern. Iron and manganese have higher concentrations and larger ranges of concentrations especially under more reducing conditions. Dissolved oxygen and well depth were related to iron and manganese concentrations. Redox conditions also affect several trace elements such as arsenic and copper; however, a comparison of redox categories, based in part on iron and manganese concentrations, indicated that the concentrations of many redox-sensitive elements were not significantly different among redox categories. Some of the redox-related patterns were not what would be expected on the basis of solubility constraints. Furthermore, barium is affected by redox conditions in at least one well network even though it is not a redox-sensitive element. Concentrations of barium in portions of the glacial aquifer system are limited by sulfate, which is strongly affected by redox conditions. Few samples had concentrations of any trace element that exceeded drinking-water standards (Maximum Contaminant Levels), for compounds regulated in drinking water or Health-Based Screening Levels for unregulated trace elements. More unregulated trace elements had concentrations greater than benchmarks than regulated trace elements. More samples had manganese concentrations greater its benchmark than any other element in the glacial aquifer system wells. Of the 1,590 wells sampled for manganese, only 556 are for private or public drinking-water supplies, and of those, 9.9 percent (55) exceeded the manganese Lifetime Health Advisory. Concentrations of arsenic, selenium, and uranium less frequently exceeded Maximum Contaminant Levels. There are 29 wells that had 2 element concentrations that exceeded their respective benchmarks. Most concentrations that exceeded a health-based benchmark were from wells in the West-Central area (Iowa, Minnesota, North and South Dakota, Nebraska, and Kansas); however, there is little geographical pattern to the wells with element concentrations of concern.

Scientific Investigations Report↗

USGS1959

History of Imuruk Lake, Seward Peninsula, Alaska

A study of Imuruk Lake, a large, shallow lake in north-central Seward Peninsula, Alaska, illuminates the climatic history of northwestern Alaska and the tectonic history of central Seward Peninsula during Pleistocene and Recent time. Special interest attaches to the older lake sediments, because they contain evidence concerning the climate, fauna, and flora that existed in the vicinity of Bering Strait at a time when the Bering land bridge was open and when animal and plant populations were being exchanged between the eastern and western hemispheres. The lake is 8 miles long and less than 10 feet deep; bottom sediments consisting of reworked wind-blown silt bury a rolling bedrock topography of much greater relief. Analysis of the hydrologic regime indicates that much of the water draining into the lake is lost by evaporation; smaller quantities are lost by discharge through the outlet, the Kugruk River, and by leakage into the lava flows along the lake shore. Changes in the duration and temperature of the summer ice-free season would result in changes in the amount of water lost by evaporation and thus in appreciable changes in lake level. Imuruk Lake occupies an initial low area on basaltic lava flows of Quaternary age, but the initial low area has been modified by faulting and now lies in a poorly defined graben. Topographic evidence confirmed by study of lacustrine terraces indicates that until recently Imuruk Lake drained westward into the Noxapaga River instead of eastward into the Kugruk River. A history of repeated warping of the lake basin, on which is superimposed a history of oscillating lake level which is due to changes in climate, is recorded by three systems of abandoned shore-line features found along the shores: a warped shore cliff of probable Illinoian age, a double set of warped terraces of probable Wisconsin age, and a low, horizontal terrace of Recent age. Bones of bison, horse, and mammoth were found in peaty sediments containing many twigs but no large wood; their presence indicates that these mammals, at least, were capable of surviving in a tundra environment during cold stages of the Pleistocene epoch and at a time when the Bering land bridge was in existence nearby. The sediments filling the deeper parts of the bedrock basin of Imuruk Lake probably contain an uninterrupted pollen record that reflects vegetation changes in central Seward Peninsula beginning in middle Illinoian time and terminating a few thousand years ago. Core drilling and pollen analysis of these sediments would greatly amplify our understanding of late Pleistocene events in the vicinity of the Bering land bridge.

Alaska↗

USGS2024

Special topic—Eruption plumes and clouds

Introduction Explosive eruptions create plumes of volcanic ash and gas that can rise more than 30,000 feet (9.1 kilometers [km]) above sea level within minutes of eruption onset. The resulting clouds disperse under prevailing winds and may cause hazardous conditions hundreds to thousands of kilometers from the volcano, including in international airspace. Rapid detection and characterization of explosive activity is vital to mitigate the wide-ranging effects of volcanic ash. Ashfall thicknesses as small as a millimeter or so on the ground can affect infrastructure, agriculture, and air quality, requiring extensive clean-up procedures (Schuster, 1981; Warrick and others, 1981, U.S. Geological Survey, 2022). Volcanic clouds also pose substantial threats to aircraft. Since 1953, 88 encounters between airplanes and ash clouds have been documented worldwide (International Civil Aviation Organization, 2015, appendix F), resulting in aircraft damage and, in 9 cases, engine failure (Guffanti and others, 2010). In 1982, two large passenger planes suffered complete engine failure owing to eruptions in Indonesia (Global Volcanism Program, 1982) and a similar incident occurred over Alaska in 1989 (Casadevall, 1994). In all three cases, they were able to restart some engine capability and land safely once they emerged from the ash clouds, although with substantial damage (Guffanti and others, 2010). The clear threat to aviation has led to establishment of nine Volcanic Ash Advisory Centers (VAAC) around the world to monitor and rapidly disseminate information about volcanic eruptions to the aviation community. U.S. Geological Survey (USGS) volcano observatories issue the Volcano Observatory Notice for Aviation that informs of preeruptive unrest or eruptive activity. When ash-producing eruptions do occur, volcano observatories work closely with their regional VAAC to ensure consistency and accuracy in eruption onset time, cloud altitude, ash production, and duration as reported in Volcanic Ash Advisories. Explosive volcanism in the United States and Commonwealth of the Northern Mariana Islands prompts 50–100 such advisories in any given year (table J1). This collaborative effort is greatly aided by USGS detection and monitoring of eruption clouds to ensure a timely and coordinated response. To support these efforts to provide guidance on ash transport and fallout, the USGS developed the Ash3d volcanic ash dispersion model ( https://vsc-ash.wr.usgs.gov/ash3d-gui ) (Schwaiger and others, 2012). Automated simulations are run daily by the USGS for volcanoes that are in elevated states of unrest, and in response mode when eruptions occur. During eruptions, the model output is provided to local National Weather Service Weather Forecast Offices to guide them in the issuance of their information products (such as special weather statements, ashfall advisories, or ashfall warnings), as well as to State and local governments and the public. Characterization of the eruption source is needed to estimate the parameters used to initialize the Ash3d model, and by the Anchorage and Washington VAACs to initialize other dispersion models that inform forecasts for the airborne volcanic cloud. The source parameters that can be provided by observation during an eruption include eruption start time, eruption cloud height over time, and eruption duration. Other, nonobservable source parameters, such as mass eruption rate and grain-size distribution, are based on empirical correlations and study of historical deposits. The goal is to provide a time series of cloud heights, mass eruption rates, and particle-size distributions that accurately reflects current conditions. When feasible, the USGS also provides guidance on the nature of ongoing eruptions and forecasts future activity using petrologic monitoring of collected tephra samples. The aims of providing accurate observable parameters are achieved through analysis of (1) near-real-time meteorological satellite data, (2) ground-based cameras (see of Flinders, A.F., Lowenstern, J.B., Coombs, M.L., and Poland, M.P., eds., Recommended capabilities and instrumentation for volcano monitoring in the United States: U.S. Geological Survey Scientific Investigations Report 2024–5062–G, 11 p., https://doi.org/10.3133/sir20245062g.">chapter G , this volume; Orr and others, 2024), (3) weather radar, (4) volcanic lightning detection, and (5) ground-based ash sensors and sampling. Explosive eruptions can be detected by a variety of geophysical monitoring, including infrasound (see of Flinders, A.F., Lowenstern, J.B., Coombs, M.L., and Poland, M.P., eds., Recommended capabilities and instrumentation for volcano monitoring in the United States: U.S. Geological Survey Scientific Investigations Report 2024–5062–C, 11 p., https://doi.org/10.3133/sir20245062c.">chapter C , this volume; Lyons and others, 2024) and seismicity (see of Flinders, A.F., Lowenstern, J.B., Coombs, M.L., and Poland, M.P., eds., Recommended capabilities and instrumentation for volcano monitoring in the United States: U.S. Geological Survey Scientific Investigations Report 2024–5062–B, 9 p., https://doi.org/10.3133/sir20245062b.">chapter B , this volume; Thelen and others, 2024). However, those methods cannot quantify the altitude, ash content, and dispersal dynamics of resulting volcanic clouds. Ideally, all available sources of monitoring data are synthesized to develop a coherent understanding of eruptive activity. The guidance summarized here provides a framework for characterizing volcanic clouds in the atmosphere and tracking the evolution of explosive eruption dynamics.

Scientific Investigations Report↗

USGS2013

Lithofacies, age, depositional setting, and geochemistry of the Otuk Formation in the Red Dog District, northwestern Alaska

Complete penetration of the Otuk Formation in a continuous drill core (diamond-drill hole, DDH 927) from the Red Dog District illuminates the facies, age, depositional environment, source rock potential, and isotope stratigraphy of this unit in northwestern Alaska. The section, in the Wolverine Creek plate of the Endicott Mountains Allochthon (EMA), is ~82 meters (m) thick and appears structurally uncomplicated. Bedding dips are generally low and thicknesses recorded are close to true thicknesses. Preliminary synthesis of sedimentologic, paleontologic, and isotopic data suggests that the Otuk succession in DDH 927 is a largely complete, albeit condensed, marine Triassic section in conformable contact with marine Permian and Jurassic strata. The Otuk Formation in DDH 927 gradationally overlies gray siliceous mudstone of the Siksikpuk Formation (Permian, based on regional correlations) and underlies black organic-rich mudstone of the Kingak(?) Shale (Jurassic?, based on regional correlations). The informal shale, chert, and limestone members of the Otuk are recognized in DDH 927, but the Jurassic Blankenship Member is absent. The lower (shale) member consists of 28 m of black to light gray, silty shale with as much as 6.9 weight percent total organic carbon (TOC). Thin limy layers near the base of this member contain bivalve fragments (Claraia sp.?) consistent with an Early Triassic (Griesbachian-early Smithian) age. Gray radiolarian chert dominates the middle member (25 m thick) and yields radiolarians of Middle Triassic (Anisian and Ladinian) and Late Triassic (Carnian-late middle Norian) ages. Black to light gray silty shale, like that in the lower member, forms interbeds that range from a few millimeters to 7 centimeters in thickness through much of the middle member. A distinctive, 2.4-m-thick interval of black shale and calcareous radiolarite ~17 m above the base of the member has as much as 9.8 weight percent TOC, and a 1.9-m-thick interval of limy to cherty mudstone immediately above this contains radiolarians, foraminifers, conodonts, and halobiid bivalve fragments. The upper (limestone) member (29 m thick) is lime mudstone with monotid bivalves and late Norian radiolarians, overlain by gray chert that contains Rhaetian (latest Triassic) radiolarians; Rhaetian strata have not previously been documented in the Otuk. Rare gray to black shale interbeds in the upper member have as much as 3.4 weight percent TOC. At least 35 m of black mudstone overlies the limestone member; these strata lack interbeds of oil shale and chert that are characteristic of the Blankenship, and instead they resemble the Kingak Shale. Vitrinite reflectance values (2.45 and 2.47 percent Ro) from two samples of black shale in the chert member indicate that these rocks reached a high level of thermal maturity within the dry gas window. Regional correlations indicate that lithofacies in the Otuk Formation vary with both structural and geographic position. For example, the shale member of the Otuk in the Wolverine Creek plate includes more limy layers and less barite (as blades, nodules, and lenses) than equivalent strata in the structurally higher Red Dog plate of the EMA, but it has fewer limy layers than the shale member in the EMA ~450 kilometers (km) to the east at Tiglukpuk Creek. The limestone member of the Otuk is thicker in the Wolverine Creek plate than in the Red Dog plate and differs from this member in EMA sections to the east in containing an upper cherty interval that lacks monotids; a similar interval is seen at the top of the Otuk Formation ~125 km to the west (Lisburne Peninsula). Our observations are consistent with the interpretations of previous researchers that Otuk facies become more distal in higher structural positions and that within a given structural level more distal facies occur to the west. Recent paleogeographic reconstructions indicate that the Otuk accumulated at a relatively high paleolatitude with a bivalve fauna typical of the Boreal realm. A suite of δ 13 C org (carbon isotopic composition of carbon) data (n=38) from the upper Siksikpuk Formation through the Otuk Formation and into the Kingak(?) Shale in DDH 927 shows a pattern of positive and negative excursions similar to those reported elsewhere in Triassic strata. In particular, a distinct negative excursion at the base of the Otuk (from ‒23.8 to ‒31.3‰ (permil, or parts per thousand)) likely correlates with a pronounced excursion that marks the Permian-Triassic boundary at many localities worldwide. Another feature of the Otuk δ 13 C org record that may correlate globally is a series of negative and positive excursions in the lower member. At the top of the Otuk in DDH 927, the δ 13 C org values are extremely low and may correlate with a negative excursion that is widely observed at the Triassic-Jurassic boundary.

Alaska↗

USGS1987

The Alaska Mineral Resource Assessment Program: Background information to accompany folio of geologic and mineral resource maps of the Circle quadrangle, Alaska

The geology, geochemistry, geophysics, and Landsat imagery of the Circle quadrangle were investigated by an interdisciplinary research team for the purpose of assessing the mineral potential of the area. The quadrangle covers approximately 15,765 km 2 in east-central Alaska; most of it is included in the mountainous Yukon-Tanana Upland physiographic division, but the northernmost part is in the low-lying Yukon Flats section. The Circle mining district, in the east-central part of the quadrangle, has been a major producing area of placer gold since its discovery in 1893. For descriptive purposes, the Circle quadrangle is divided into three areas: the northwest Circle quadrangle, the area north of the Tintina fault zone, and the area south of the Tintina fault zone. The Tintina fault zone extends northwesterly through the northern part of the quadrangle. The northwest Circle quadrangle contains mostly folded and faulted, slightly metamorphosed sedimentary rocks that are intruded by Tertiary granitic plutons. In the northern part of the area north of the Tintina fault zone (Little Crazy Mountains and northern east Crazy Mountains), the rocks consist primarily of the gabbro and basalt of the Circle Volcanics and minor associated chert, graywacke, and limestone. Elsewhere in this area (south of the Circle Volcanics and in the western Crazy Mountains), the rocks are mostly slightly metamorphosed Paleozoic sedimentary rocks that have been folded and faulted. Rocks in the largest part of the quadrangle, the area south of the Tintina fault zone, consist largely of pelitic rocks that are regionally metamorphosed to greenschist and amphibolite facies. Felsic plutons, mostly Tertiary in age, occur throughout the area. The metamorphic rocks are separated from sedimentary rocks on the northwest by thrust faulting. The aeromagnetic and gravity data show clear differences between the areas north and south of the Tintina fault zone. The metamorphic terrane to the south has low overall gravity and local gravity lows over exposed granitic plutons. It is hypothesized that magnetic chlorite schist infolded with nonmagnetic quartzite and schist account for east-northeast-trending magnetic highs that approximately parallel the regional strike of the most prominent foliation in the metamorphic rocks. North of the Tintina fault zone, the Circle Volcanics are characterized by high gravity and east-west-trending magnetic highs. The Tintina fault zone has an intense magnetic high near the western margin of the Circle quadrangle overlying the magnetic granodiorite of the Victoria Mountain pluton. A magnetic high near Circle Hot Springs is less intense, but broader, and could reflect a buried magnetic pluton similar to that of the Victoria Mountain pluton. Computer-enhanced Landsat images of the Circle quadrangle show trends and patterns of concentrations of linear features. Features trending northeast-southwest predominate throughout the quadrangle; northwest-southeast-trending linear features are found mostly south of the Tintina fault zone. High concentrations of linear features were not found to correspond to areas of known mineralization in any consistent or significant way that could presently be used in locating areas of mineralization. Geochemical and mineralogical studies of stream sediment and heavy-mineral concentrates from the Circle quadrangle identify areas of anomalous concentrations of metallic elements, including gold, silver, tin, tungsten, lead, antimony, zinc, thorium, uranium, and beryllium. The data delineate areas of known mineral occurrences and areas that may contain undiscovered mineral resources. To date, placer gold has been the only significant metallic mineral resource from the Circle quadrangle, but the general geologic setting, especially the presence of post-orogenic plutons, is similar to that of regions that contain tin greisen deposits, tungsten skarn deposits, lode gold deposits in metasedimentary rocks, and uranium vein deposits. Six areas or tracts were identified in which such deposits might occur, and two more tracts were delineated as possible for the occurrence of shale-hosted Lead-zinc deposits. The discovery of two diamonds in the gravels of Cooked Creek point to the slight possibility of finding placer or lode diamond deposits. Although most of the past and present gold mining has taken place in four areas in the quadrangle, a sedimentary basin near the town of Central was identified as possibly containing buried placer gold deposits or sedimentary uranium deposits.

Alaska↗

USGS2009

Petrogenesis of basaltic volcanic rocks from the Pribilof Islands, Alaska, by melting of metasomatically enriched depleted lithosphere, crystallization differentiation, and magma mixing

The Pribilof Islands, Alaska, are located in the Bering Sea in a continental intraplate setting. In this study we examine the petrology and geochemistry of volcanic rocks from St. Paul (0??54-0??003 Ma) and St. George (2??8-1??4 Ma) Islands, the two largest Pribilof Islands. Rocks from St. George can be divided into three groups: group 1 is a high-MgO, low-SiO. 2 suite composed primarily of basanites; group 2 is a high-MgO, high-SiO 2 suite consisting predominantly of alkali basalts; group 3 is an intermediate- to low-MgO suite that includes plagioclase-phyric subalkali basalts and hawaiites. Major and trace element geochemistry suggests that groups 1 and 2 formed by small-degree partial melting of amphibole-bearing to amphibole-free garnet peridotite. Group 1 rocks were the earliest melts produced from the most hydrous parts of the mantle, as they show the strongest geochemical signature of amphibole in their source. The suite of rocks from St. Paul ranges from 14??4 to 4??2 wt % MgO at relatively constant SiO 2 contents (43??1-47??3 wt %). The most primitive St. Paul rocks are modeled as mixtures between magmas with compositions similar to groups 1 and 2 from St. George Island, which subsequently fractionated olivine, clinopyroxene, and spinel to form more evolved rocks. Plagioclase-phyric group 3 rocks from St. George are modeled as mixtures between an evolved melt similar to the evolved magmas on St. Paul and a fractionated group 2 end-member from St. George. Mantle potential temperatures estimated for primitive basanites and alkali basalts are ???1400??C and are similar to those of mid-ocean ridge basalts (MORB). Similarly, 87 Sr/. 86 Sr and 143 Nd/. 144 Nd values for all rocks are MORB-like, in the range of 0??702704-0??703035 and 0??513026-0??513109, respectively. 208 Pb/. 204 Pb vs 206 Pb/. 204 Pb values lie near the MORB end-member but show a linear trend towards HIMU (high time-integrated 238 U/. 204 Pb). Despite isotopic similarities to MORB, many of the major and trace element characteristics are similar to those of ocean island basalts (OIB), including enrichment in alkalis and incompatible trace elements. These characteristics are interpreted to indicate that their mantle source experienced an ancient melt-removal event that is reflected in depleted radiogenic isotopic compositions and was then re-enriched by metasomatism that elevated incompatible trace element contents, but was too young to produce a time-integrated change in radiogenic isotopic ratios. Evidence suggests that the Pribilof Island basalts did not form in either a plume or a back-arc basin tectonic setting. Rather, they were produced by melting of metasomatically hydrated upper mantle peridotite at relatively low temperatures and were able to erupt at the surface through extensional or transtensional faults that served as conduits for the magmas. ?? The Author 2009. Published by Oxford University Press.

Journal of Petrology↗