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USGS1952

Preliminary report on the Nelson and Radovan copper prospects, Nizina district, Alaska

Renewed copper exploration by Alaska Copper Mines, Incorporated, at the Nelson and Radovan prospects, Nizina district, Alaska, led the Geological Survey in 1951 to map in detail the Nelson fault block, and to re-examine the old workings. In addition, two new prospects were studied. The Nelson fault block is cut by many dominantly strike-slip faults of small displacement, and by bedding faults. Slickensided chalcocite shows post-mineral movement, and chalcocite veinlet in a filled solution cavity indicates that some of the chalcocite is secondary, perhaps very recent. Structural relations indicate two overthrust faults cut the block. The Radovan Greenstone prospect shows massive chalcocite, up to 3 feet wide, in a silicified, epidotized fault zone in the Nikolai greenstone. Ore indicated by surface exposures may amount to 450 tons of chalcocite. The Radovan Low-Contact prospect is on a continuation of the same fault approximately 3 miles southwest of the Greenstone prospect, and 150 feet above the contact of the Nikolai greenstone and the overlying Chitistone limestone. Limonite staining is widespread in bedding planes and small faults near the fault zone; mineralization in the fault zone consists of pyrite, chalcocite, bornite, malachite, realgar, orpiment and stibnite. The sulphides in the fault zone, plus the widespread silicification and epidotization indicate a strong zone of hydrothermal activity which merits extensive prospecting.

Open-File Report↗
USGS1980

Thorium resources of selected regions in the United States

Thorium resources have been assessed in a previous report entitled 'Principal thorium resources in the United States' (Staatz and others, 1979) for (1) veins in the larger districts, {2) massive carbonatites, {3) disseminated deposits, and {4) stream placers of North and South Carolina. This report is a sequel to that report and assesses thorium resources in {1) Florida beach placers, (2) Idaho stream placers, (3) veins and pipes in the Bokan Mountain district, Alaska, (4) carbonatite dikes, and {5) apatite-bearing iron deposits near Mineville, New York. Thorium resources for each of these categories are divided into reserves and probable potential resources. When data are available, each of these is then divided into the following cost categories: (1) the amount of ThO2 producible at a cost of less than $15/lb (per pound), (2) the amount producible at a cost of between $15 and $30/lb, and (3) the amount producible at a cost of between $30 and $50/1b. Beach placers of northern Florida have reserves of 16,200 short tons of ThO2 and probable potential resources of 5,120 tons of ThO2. These deposits are heavy-mineral placers that are mined for a variety of minerals--principally titanium minerals and zircon. The thorium-bearing mineral in these placers, monazite, makes up only a minor part of the heavy minerals. Therefore, production of ThO2 from these placers is dependent on the markets for other heavy minerals. Assuming the market for other heavy minerals to be the same as in 1978, then 98 percent of the ThO2 could be produced for less than $15/lb. If, however, no other coproducts were produced, then the cost of producing ThO2 would be greater than $50/1b. Stream placers containing thorium are found along many streams that drain the Idaho batholith, but most are too small to add significantly to the thorium resources. The resources of the five largest districts, each of which consists of at least several individual placers, have been tabulated. These districts are (1) Long Valley, (2) Bear Valley, (3) Burgdorf-Warren area, (4) Boise Basin, and (5) Ell City-Newsome area. These five areas have reserves of 10,100 short tons of ThO2 and probable potential resources of 10,300 tons. Long Valley contains about half the reserves--5,680 tons of ThO2--and all the probable potential resources. Monazite is the most important heavy mineral in all except the Bear Valley deposit. Here euxenite, although not quite as abundant as monazite, is a more important mineral, because it contains approximately 14.5 percent U3O8 in addition to 5 percent ThO2. Reserves in this placer amount to 1,605 short tons of ThO2 and 1,475 tons of U3O8. Eighty-two percent of the reserves and all of the probable potential resources can be produced at less than $30/1b of ThO2. The lower cost reserves are concentrated in the Long and Bear Valley areas. Here 64 percent of the ThO2 can be produced for less than $15/1b and another 29 percent of the ThO2 at between $15 and $30/lb. Sixteen veins and pipelike bodies are evaluated in the Bokan Mountain area of southeastern Alaska. The district contains other deposits that are too poorly exposed to make meaningful resource estimates. Reserves estimated in this district are 1,440 short tons of ThO2; probable potential resources amount to 2,320 tons of ThO2. About 99 percent of these resources are in deposits whose grade is at least 0.2 percent ThO2. In addition, these deposits contain reserves of 420 tons of U3O8 and probable potential resources of 820 tons of U3O8. Eighty-two percent of the reserves and probable potential resources can be produced at less than $15/lb. The average grade of this ore is 0.54 percent ThO2 and 0 15 percent U3O8. Some carbonatite dikes, although generally not as high grade as the veins, contain resources of thorium. Carbonatite dikes in the following six districts were investigated: (1) Wet Mountains, Colo.; (2) Powderhorn district, Colorado; (3) Mountain Pass area, California; (4) Bearpaw

Circular↗
USGS1990

A comparison of geochemical exploration techniques and sample media within accretionary continental margins: an example from the Pacific Border Ranges, Southern Alaska, U.S.A.

The Pacific Border Ranges of the southern Alaskan Cordillera are composed of a number of allochthonous tectonostratigraphic terranes. Within these terranes are widespread volcanogenic, massive sulfide deposits in and adjacent to portions of accreted ophiolite complexes, bands and disseminations of chromite in accreted island-arc ultramafic rocks, and epigenetic, gold-bearing quartz veins in metamorphosed turbidite sequences. A geochemical pilot study was undertaken to determine the most efficient exploration strategy for locating these types of mineral deposits within the Pacific Border Ranges and other typical convergent continental margin environments. High-density sediment sampling was carried out in first- and second-order stream channels surrounding typical gold, chromite and massive sulfide occurrences. At each site, a stream-sediment and a panned-concentrate sample were collected. In the laboratory, the stream sediments were sieved into coarse-sand, fine- to medium-sand, and silt- to clay-size fractions prior to analysis. One split of the panned concentrates was retained for analysis; a second split was further concentrated by gravity separation in heavy liquids and then divided into magnetic, weakly magnetic and nonmagnetic fractions for analysis. A number of different techniques including atomic absorption spectrometry, inductively coupled plasma atomic emission spectrometry and semi-quantitative emission spectrography were used to analyze the various sample media. Comparison of the various types of sample media shows that in this tectonic environment it is most efficient to include a silt- to clay-size sediment fraction and a panned-concentrate sample. Even with the relatively low detection limits for many elements by plasma spectrometry and atomic absorption spectrometry, anomalies reflecting the presence of gold veins could not be identified in any of the stream-sediment fractions. Unseparated panned-concentrate samples should be analyzed by emission spectroscopy and atomic absorption spectrometry for Ag and Au. If, however, magnetic and nonmagnetic concentrate fractions are used in a reconnaissance program, semiquantitative emission spectrography is adequate for all analytical work.

Journal of Geochemical Exploration↗
USGS1917

Thirty-eighth annual report of the Director of the United States Geological Survey

The appropriations for the work of the United States Geological Survey for the fiscal year 1916-17 comprised items amounting to $1,605,520. The plan of operations as approved by the Secretary of the Interior included geologic surveys in the United States and Alaska, reconnaissance and detailed, of 40,937 square miles, topographic surveys of 32,245 square miles, stream gaging at 1,197 stations, the classification of public lands to an amount of more than 18,000,000 acres, and the collection of statistics of production and consumption from more than 90,000 producers, covering more than 75 mineral products. During the year 203 scientific and economic reports were published, and at the end of the year the Survey members holding appointments from the Secretary numbered 934, an increase of 62.

Annual Report↗
USGS2004

Structure of the Red Dog District, western Brooks Range, Alaska

The Red Dog district of the western Brooks Range of northern Alaska, which includes the sediment-hosted Zn-Pb-Ag ± Ba deposits at Red Dog, Su-Lik, and Anarraaq, contains one of the world's largest reserves of zinc. This paper presents a new model for the structural development of the area and shows that understanding the structure is crucial for future exploration efforts and new mineral discoveries in the district. In the Red Dog district, a telescoped Late Devonian through Jurassic continental passive margin is exposed in a series of subhorizontally stacked, internally imbricated, and regionally folded thrust sheets. These sheets were emplaced during the Middle Jurassic to Late Cretaceous Brookian orogeny and subsequently were uplifted by late tectonic activity in the Tertiary. The thrust sheet stack comprises seven tectonostratigraphically distinct allochthonous sheets, three of which have been subject to regional and detailed structural analysis. The lowermost of these is the Endicott Mountains allochthon, which is overlain by the structurally higher Picnic Creek and Kelly River allochthons. Each individual allochthon is itself internally imbricated into a series of tectonostratigraphically coherent and distinct thrust plates and subplates. This structural style gives rise to duplex development and imbrication at a range of scales, from a few meters to tens of kilometers. The variable mechanical properties of the lithologic units of the ancient passive margin resulted in changes in structural styles and scales of structures across allochthon boundaries. Structural mapping and analysis of the district indicate a dominant northwest to west-northwest direction of regional tectonic transport. Local north to north-northeast transport of thrust sheets is interpreted to reflect the influence of underlying lateral and/or oblique ramps, which may have been controlled by inherited basin margin structures. Some thrust-sheet stacking patterns suggest out-of-sequence thrusting. The west-northwest-east-southeast-trending Wrench Creek and Sivukat Mountain faults were previously interpreted to be strike-slip faults, but this study shows that they are Tertiary (Eocene?) late extensional faults with little or no lateral displacement.

Alaska↗
USGS1947

Preliminary report on the stratigraphy and structure of the area of the Utukok River with notes on the Corwin-Cape Beaufort region, Alaska

U.S. Geological Survey Party 3 studied the geology of the Utukok River area and conducted a short reconnaissance of the Corwin-Cape Beaufort Region during the period May 12 to August 31, 1947. The upper part of the Utukok River was examined for several miles on either side while the lower, or Coastal Plain-portion, was necessarily limited to the few outcrops along the river. During this study, the major emphasis was placed upon rocks of Upper Cretaceous age and their relationships to the underlying Lower Cretaceous rocks. All macrofossils, and numerous samples for porosity, heavy mineral, and microfossil determinations were collected. A triangulation net was established by theodolite from Camp. 1 near the headwaters of the river to the mouth of Carbon Creek. The survey started and ended on measured base lines, and stations were determined by intersection and resection. Stations were selected, insofar as possible, on points that could be pin pricked on aerial photographs where adequate coverage was available, Horizontal control was computed for cartographic purposes. Vertical differences were computed from an elevation at Camp 1 assumed at 2,000 feet above sea level based upon two different airplane altimeter readings. Most of the country is too rough for wheel landings with the exception of several gravel bars in the river between Camp 6 and the coast. Pontoon landings with a Cub plane can be made at most places on the river. Driftwood, Adventure, Disappointment, Carbon, and Elusive Creeks are the main tributaries to the Utukok. At various localities, terrace gravels are common on divides of these streams. A terrace north of Driftwood Creek and about ten miles east of Camp 2 is covered with a deposit of gravel which is apparently, thick and is spread over an irregular area about two miles long and a half mile wide. A thin tundra cover obscures what is probably a much larger continuation of the gravel deposit. With favorable conditions plane landings could be made, but with a little construction work a commercial size airfield could be made.

Alaska↗
USGS2012

Using rocks to reveal the inner workings of magma chambers below volcanoes in Alaska’s National Parks

Alaska is one of the most vigorously volcanic regions on the planet, and Alaska’s national parks are home to many of the state’s most active volcanoes. These pose both local and more distant hazards in the form of lava and pyroclastic flows, lahars (mudflows), ash clouds, and ash fall. Alaska’s volcanoes lie along the arc of the Aleutian-Alaskan subduction zone, caused as the oceanic Pacific plate moves northward and dips below the North American plate. These volcanoes form as water-rich fluid from the down-going Pacific plate is released, lowering the melting temperature of rock in the overlying mantle and enabling it to partially melt. The melted rock (magma) migrates upward, collecting at the base of the approximately 25 mile (40 km) thick crust, occasionally ascending into the shallow crust, and sometimes erupting at the earth’s surface. During volcanic unrest, scientists use geophysical signals to remotely visualize volcanic processes, such as movement of magma in the upper crust. In addition, erupted volcanic rocks, which are quenched samples of magmas, can tell us about subsurface magma characteris-tics, history, and the processes that drive eruptions. The chemical compositions of and the minerals present in the erupted magmas can reveal conditions under which these magmas were stored in crustal “chambers”. Studies of the products of recent eruptions of Novarupta (1912), Aniakchak (1931), Trident (1953-74), and Redoubt (2009) volcanoes reveal the depths and temperatures of magma storage, and tell of complex interactions between magmas of different compositions. One goal of volcanology is to determine the processes that drive or trigger eruptions. Information recorded in the rocks tells us about these processes. Here, we demonstrate how geologists gain these insights through case studies from four recent eruptions of volcanoes in Alaska national parks.

Alaska↗
USGS2015

Differences in ecosystem carbon distribution and nutrient cycling linked to forest tree species composition in a mid-successional boreal forest

In the boreal forest of Alaska, increased fire severity associated with climate change is expanding deciduous forest cover in areas previously dominated by black spruce ( Picea mariana ). Needle-leaf conifer and broad-leaf deciduous species are commonly associated with differences in tree growth, carbon (C) and nutrient cycling, and C accumulation in soils. Although this suggests that changes in tree species composition in Alaska could impact C and nutrient pools and fluxes, few studies have measured these linkages. We quantified C, nitrogen, phosphorus, and base cation pools and fluxes in three stands of black spruce and Alaska paper birch ( Betula neoalaskana ) that established following a single fire event in 1958. Paper birch consistently displayed characteristics of more rapid C and nutrient cycling, including greater aboveground net primary productivity, higher live foliage and litter nutrient concentrations, and larger ammonium and nitrate pools in the soil organic layer (SOL). Ecosystem C stocks (aboveground + SOL + 0–10 cm mineral soil) were similar for the two species; however, in black spruce, 78% of measured C was found in soil pools, primarily in the SOL, whereas aboveground biomass dominated ecosystem C pools in birch forest. Radiocarbon analysis indicated that approximately one-quarter of the black spruce SOL C accumulated prior to the 1958 fire, whereas no pre-fire C was observed in birch soils. Our findings suggest that tree species exert a strong influence over C and nutrient cycling in boreal forest and forest compositional shifts may have long-term implications for ecosystem C and nutrient dynamics.

Ecosystems↗
USGS1989

Evolution of the western part of the Coast plutonic–metamorphic complex, South-Eastern Alaska, USA: A summary

The western Cordillera of North America extends for over 6000 km from the tip of Baja California to the Alaska Range. It includes a wide variety of metamorphic and plutonic terrains, but none is more spectacular scenically or geologically than the Coast plutonic-metamorphic complex (Brew & Ford 1984) of western Canada and south-eastern Alaska. This report briefly describes the evolution of the western part of the complex, integrating information from the deformational, plutonic and metamorphic events. Most of the original studies are reported by the authors in U.S. Geological Survey Circular numbers 733, 751, 823-B, 868, 939, 945, 967 and 978, and are not cited specifically here. This summary does not contain either a comprehensive bibliography or a comparison of the metamorphic histories of south-eastern Alaska with the adjacent parts of British Columbia. The Coast plutonic-metamorphic complex is here divided into three major elements: the western metamorphic, the central granitic and the eastern metamorphic zones (Fig. 1). The western metamorphic belt is extremely long (900 km), and narrow (7–25 km). It consists of regional dynamothermally and regional thermally metamorphosed rocks with mineral assemblages ranging from prehnite-pumpellyite to upper amphibolite facies, scattered mesozonal to epizonal granitic bodies, and a few concentrically zoned mafic-ultramafic masses. The metamorphic grade and the amount of deformation increase from south-west to north-east, culminating at, or slightly to the north-east of, the ‘great tonalite sill’: a remarkable 700-km-long, 3- to 25-km-wide vertical to northeast-dipping belt of mostly syntectonic plutons of approximately the same age, composition and structural

Alaska↗
USGS1992

Map, tables, and summary of fossil and isotopic age data, Mount Hayes Quadrangle, eastern Alaska range, Alaska

This report describes, summarizes, and interprets all known bedrock fossil and isotopic age studies for the Mount Hayes quadrangle, eastern Alaska Range, Alaska. The accompanying map shows the location of all known bedrock fossil and isotopic sample localities in the quadrangle on a generalized geologic base map. These fossil and isotopic age data are obtained from new studies, unpublished data of the U.S. Geological Survey, contributed unpublished data, and published data. This report is one result of a five-year mineral resource assessment of the quadrangle that was done during the summers of 1978 through 1982, with additional topical studiesin 1985 and 1986. This report is one part of a folio on the geological, geochemical, geophysical, and mineral-resource assessment studies of the quadrangle prepared as part of the Alaskan Mineral Resource Assessment Program (AMRAP) of the U.S. Geological Survey.

Alaska↗
USGS2012

Migrated hydrocarbons in exposure of Maastrichtian nonmarine strata near Saddle Mountain, lower Cook Inlet, Alaska

Magoon and others (1980) described an 83-meter- (272-foot-) thick succession of Maastrichtian (Upper Cretaceous) conglomerate, sandstone, mudstone, and coal exposed on the south side of an unnamed drainage, approximately 3 kilometers (1.8 miles) east of Saddle Mountain in lower Cook Inlet (figs. 1 and 2). The initial significance of this exposure was that it was the first reported occurrence of nonmarine rocks of this age in outcrop in lower Cook Inlet, which helped constrain the Late Cretaceous paleogeography of the area and provided important information on the composition of latest Mesozoic sandstones in the basin. The Saddle Mountain section is thought to be an outcrop analog for Upper Cretaceous nonmarine strata penetrated in the OCS Y-0097 #1 (Raven) well, located approximately 40 kilometers (25 miles) to the south–southeast in Federal waters (fig. 1). Atlantic Richfield Company (ARCO) drilled the Raven well in 1980 and encountered oil-stained rocks and moveable liquid hydrocarbons between the depths of 1,760 and 3,700 feet. Completion reports on file with the Bureau of Ocean Energy Management (BOEM; formerly Bureau of Ocean Energy Management, Regulation and Enforcement, and prior to 2010, U.S. Minerals Management Service) either show flow rates of zero or do not mention flow rates. A fluid analysis report on file with BOEM suggests that a wireline tool sampled some oil beneath a 2,010-foot diesel cushion during the fl ow test of the 3,145–3,175 foot interval, but the recorded fl ow rate was still zero (Kirk Sherwood, written commun., January 9, 2012). Further delineation and evaluation of the apparent accumulation was never performed and the well was plugged and abandoned. As part of a 5-year comprehensive evaluation of the geology and petroleum systems of the Cook Inlet forearc basin, the Alaska Division of Geological & Geophysical Surveys obtained a research permit from the National Park Service to access the relatively poorly understood ‘Saddle Mountain exposure’ that is located in the Lake Clark National Park and Preserve. This work was done in cooperation with the Alaska Division of Oil & Gas and U.S. Geological Survey (USGS) research geologists. This report expands on Magoon and others’ (1980) description of the exposure, presents new data on sandstone composition and reservoir quality, presents new geochemical data on petroleum extracted from the outcropping sandstone, and describes oil-bearing correlative strata penetrated by the Raven well. Although the exposure is more than a kilometer (0.6 mile) east of Saddle Mountain (fig. 2), in this report we variously refer to it as the Saddle Mountain succession, Saddle Mountain section, or the rocks at Saddle Mountain underlain by Upper Jurassic strata of the Naknek Formation.

Alaska↗
USGS2009

Geochemical data for samples collected in 2008 near the concealed pebble porphyry Cu-Au-Mo deposit, Southwest Alaska

In the summer of 2007, the U.S. Geological Survey (USGS) began an exploration geochemical research study over the Pebble porphyry copper-gold-molybdenum deposit. This report presents the analytical data collected in 2008. The Pebble deposit is world class in size, and is almost entirely concealed by tundra, glacial deposits, and post-Cretaceous volcanic rocks. The Pebble deposit was chosen for this study because it is concealed by surficial cover rocks, is relatively undisturbed (except for exploration company drill holes), is a large mineral system, and is fairly well-constrained at depth by the drill hole geology and geochemistry. The goals of this study are to 1) determine whether the concealed deposit can be detected with surface samples, 2) better understand the processes of metal migration from the deposit to the surface, and 3) test and develop methods for assessing mineral resources in similar concealed terrains. The analytical data are presented as an integrated Microsoft Access 2003 database and as separate Excel files.

Alaska↗
USGS1954

Magnetite deposits at Tuxedni Bay, Alaska

The magnetite deposits on an island in Tuxedni Bay occur in contact-metamorphosed volcanic and sedimentary rocks near their contact with a quartz diorite batholith which underlies large areas of the adjacent Aleutian Range. The deposits are pyrometasomatic in origin and are localized along northeast-trending fractures. The eastern deposit is a low-grade disseminated deposit occurring in hornfels. The western deposit occurs as a massive lens of magnetite and garnet with a marble hanging wall and a hornfels footwall. The tenor of mineral rock in the western deposit may average 75 percent of magnetite. The deposit is lenticular and can be followed along strike for 55 feet in outcrop and, where concealed beneath a cover of soil and vegetation, can be followed for an additional 55 feet with a dip needle. Where exposed in a sea cliff the deposit is 30 to 35 feet thick.

Alaska↗
USGS1981

Gravity anomaly and interpretation map of the Chignik and Sutwik Island quadrangles, Alaska

The gravity field of the Chignik and Sutwik Island quadrangles near the center of the Alaska Peninsula represents a complex series of transitions between probable continental crust on the north, probable oceanic crust on the south, sedimentary basins on each side of the peninsula, and a central structural high and volcanic arc. The resulting gravity field may be generalized as a central southwest- to northeast-trending gravity high bordered on both sides by flanking gravity lows over sedimentary basins underlying the lowlands and continental shelf. All three gravitational features show discontinuities and magnitude variations that reflect the complex geologic setting. Data concerning this complex gravity field have accumulated slowly and intermittently since an initial measurement was made at Port Heiden more than 25 years go (Thiel and others, 1958, 1959). A few reconnaissance measurements by R. V. Allen and an offshore traverse conducted by the U.S. Coast and Geodetic Survey R/V Surveyor were made in the early 1960's (Barnes and others, 1966; Barnes, 1967). At about the same time, Gulf Oil Corp. contracted for a relatively detailed survey of the lowlands on the north shore of the peninsula; the map prepared from this survey is now available (Gulf Oil Corp., Port Moller Development Contract Report, available at U.S. Geological Survey, Anchorage, Alaska). In the early 1970's, a few new land measurements were made by P. L. Dobey, of the Alaska Division of Geological and Geophysical Surveys, and an offshore traverse was completed by the R/V C. Greene on contract to the U.S. Geological Survey (Fisher, 1979, and M. A. Fisher, written commun., 1977). The most recent measurements, the basis for this map, were made by M. E. Yount, D. L. Detra, D. R. Jefferis, and J. E. Case during the mineral assessment of the two quadrangles. D. F. Barnes, R. L. Morin, D. R. Jefferis, and R. F. Sikora have shared in the data compilation. Geologic background for the text was provided by R. L. Detterman, T. P. Miller, E. Young, and F. H. Wilson (1979, 1981).

Alaska↗
USGS2020

Evaluation of the analytical methods used to determine the elemental concentrations found in the stream geochemical dataset compiled for Alaska

A recent U.S. Geological Survey data compilation of stream-sediment geochemistry for Alaska contains decades of analyses collected under numerous Federal and State programs. The compiled data were determined by various analytical methods. Some samples were reanalyzed by a different analytical method than the original, resulting in some elements having concentrations reported by multiple analytical methods. Consideration of the analytical methods used to determine the elemental concentrations is an important step in a mineral prospectivity analysis. We used the compiled data to compare concentrations of barium (Ba), cobalt (Co), copper (Cu), chromium (Cr), nickel (Ni), lead (Pb), and zinc (Zn) determined by different analytical methods to show how simple data comparisons can identify bias and provide a general sense of the comparability of different analytical methods. The elements were selected because they have a range of geochemical properties that may affect the performance of different analytical procedures. Generally, agreement between Ba, Co, Cu, Cr, Ni, Pb, and Zn concentrations is good for most quantitative methods that use a total decomposition of the sample. However, Cr concentrations typically were lower for methods using quantitative-instrumental analysis following a multi-acid dissolution technique that included hydrofluoric acid compared to those using sinter decomposition. Additionally, low- to middle-range concentrations for Co, Cr, Cu, Ni, Pb, and Zn by instrumental neutron activation (NA) and energy-dispersive x-ray spectroscopy (EDX) analyzed by the National Uranium Resource Evaluation (NURE) program have high uncertainty. Concentrations determined by methods that use partial decomposition of the sample generally correspond well to concentrations determined by methods that use a total decomposition technique, except for Ba and Cr. For Ba and Cr, partial decomposition techniques yield lower concentrations than those determined by methods that use a total decomposition technique. Comparison of Ba, Co, Cr, Cu, Ni, Pb, and Zn concentrations determined by semiquantitative visual six-step direct-current arc emission spectrography (ES_SQ) to those determined by quantitative methods using either a total or partial decomposition technique consistently show scatter that exceeds the values expected based on the range represented by the semiquantitative concentration. The data compilation includes a best-value determination that was selected based on the analytical method from the all concentration data for that sample. Ba, Cr, Co, and Zn concentrations determined by NA usually are selected as the best-value determination. However, the NURE-NA method was designed for high throughput and the uncertainty associated with low- and mid-range concentrations is greater than that of the multi-acid method used to reanalyze many samples. Selection of the multi-acid method over the NURE-NA method for Ba, Co, and Zn could be warranted. Additionally, concentrations determined by ES_SQ usually are selected as the best-value determination over all methods that use a partial decomposition of the sample. Substitution of concentrations determined by methods that use a partial decomposition for those of ES_SQ may be warranted for Co, Cu, Ni, Pb, and Zn. Regardless of the selection of the best-value determination, the dataset remains a mixed method dataset and the uncertainty due to differences in analytical methodology must be considered when using the dataset.

Alaska↗
USGS1962

Borates in the United States, exclusive of Alaska and Hawaii

This map shows the location of borate deposits in the United States (exclusive of Alaska and Hawaii). Because all the known borate deposits of the United States of any historic or present economic importance occur in California, western Nevada, and southern Oregon, this map covers only the western part of the country.

Mineral Investigations Resource Map↗
USGS1946

Preliminary report on the stratigraphy and structure of the Kurupa, Colville, and Oolamnagavik Rivers, Alaska

U. S. Geological Survey Party No. 4 covered the area between 68° 30' and 69° 08' N. latitude and between 154° and 155°20' W. longitude during the period May 18 to September 2. Traverses were confined mainly to the valleys of the Kurupa, Colville, and Oolamnagavik Rivers inasmuch as very little rock is exposed in the interstream areas. This report is limited to the Colville River valley area between the Kura and Killik Rivers, and to a stratigraphic section exposed. on the Kurupa 7-12 miles south of the Colville. A final report will be prepared by April 1, 1947, on the total area and will include discussions of the stratigraphy and structure of Upper Cretaceous formations A through D on the Oolamnagavik and Kurupa, the stratigraphy of Formation D on the Colville, and the Lower Cretaceous and Triassic rocks. Separation and identification of heavy minerals from the sandstones and conglomerates, binocular and petrographic microscope examinations of specimens, identification of megafossils, and separation and identification of microfossils, if any, will be undertaken. Detailed examination of all aerial photos will be made to trace out structures and formations in the adjacent areas to the east, north, and west. It seems probable that close correlation between this area, the Killik River area, and Maybe Creek area will be possible by extrapolating field and aerial photo data. It is possible also to trace structures and some formation westward from the Kurupa River.

Alaska↗
USGS2003

Experimental and textural constraints on mafic enclave formation in volcanic rocks

We have used experiments and textural analysis to investigate the process of enclave formation during magma mixing at Southwest Trident volcano, Alaska. Andesite enclaves are present throughout the four dacite lava flows produced by the eruption, and resemble mafic enclaves commonly found in other volcanic rocks. Our experiments replicate the pressure–temperature–time path taken by enclave-forming andesite magma as it is engulfed in dacite during magma mixing. Pressure and temperature information for the andesite and dacite are from [Coombs et al., Contrib. Mineral. Petrol. 140 (2000) 99–118]. The andesite was annealed at 1000°C, and then cooled to 890°C at rates of 110°C h −1 , 10°C h −1 , and 2°C h −1 . Once cooled to 890°C, andesite was held at this lower temperature from times ranging from 1 to 40 h. The andesite that was cooled at the slower rates of 2°C h −1 and 10°C h −1 most resembles enclave groundmass texturally and compositionally. Based on simple thermal calculations, these rates are more consistent with cooling of the andesite groundmass below an andesite–dacite interface than with cooling of enclave-sized spheres. If enclaves do crystallize as spheres, post-crystallization disaggregation must occur. Calculations using the MELTS algorithm [Ghiorso and Sack, Contrib. Mineral. Petrol. 119 (1995) 197–212] show that for incoming andesite to become less dense than the dacite ∼34 volume % of its groundmass must crystallize to undergo ∼18 volume % vesiculation; these values are similar to those determined for Southwest Trident enclaves. Thus such crystallization may lead to ‘flotation’ of enclaves and be a viable mechanism for enclave formation and dispersal. The residual melt in the cooling experiments did not evolve to rhyolitic compositions such as seen in natural enclaves due to a lack of a decompression step in the experiments. Decompression experiments on Southwest Trident dacite suggest an average ascent rate for the eruption of ∼2–3 MPa h −1 . An andesite experiment that was cooled and then decompressed at this rate contains melt that matches that of the natural enclaves. It is apparent that decompression (ascent)-induced crystallization occurs in enclaves, but not in the form of microlites as happens in the dacite host, due either to insufficient residence time at chamber temperatures or to the pre-existing microphenocrysts which act as sites for new growth.

Alaska↗