SEARCH · Search USGS

Results for “The Alaska Miner”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,207 records · Page 67Linked to original sources

USGS1994

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

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

Alaska↗

USGS2010

Seasonal ice and hydrologic controls on dissolved organic carbon and nitrogen concentrations in a boreal-rich fen

[1] Boreal wetland carbon cycling is vulnerable to climate change in part because hydrology and the extent of frozen ground have strong influences on plant and microbial functions. We examined the response of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) across an experimental manipulation of water table position (both raised and lowered water table treatments) in a boreal-rich fen in interior Alaska. DOC and TDN responses to water table manipulation exhibited an interaction with seasonal ice dynamics. We observed consistently higher DOC and TDN concentrations in the lowered water table treatment (71.7 ± 6.5 and 3.0 ± 0.3 mg−L) than in both the control (55.6 ± 5.1 and 2.3 ± 0.2 mg−L) and raised (49.1 ± 4.3 and 1.9 ± 0.1 mg L−1, respectively) water table treatments. Across all plots, pore water DOC concentrations at 20 cm increased as the depth to water table increased (R2 = 0.43, p < 0.001). DOC concentrations also increased as the seasonal thaw depth increased, with solutes increasing most rapidly in the drained plot (R2 = 0.62, p < 0.001). About half of the TDN pool was composed of dissolved organic N (DON). Inorganic N and DON were both highly correlated with changes in DOC, and their respective constraints to mineralization are discussed. These results demonstrate that a declining water table position and dryer conditions affect thaw depth and peat temperatures, and interactions among these ecosystem properties will likely increase DOC and TDN loading and potential for export in these systems.

Alaska↗

USGS2007

Assessment of marine-derived nutrients in the Copper River Delta, Alaska, using natural abundance of the stable isotopes of nitrogen, sulfur, and carbon

We performed nitrogen, sulfur, and carbon stable isotope analysis (SIA) on maturing and juvenile anadromous sockeye and coho salmon, and periphyton in two Copper River delta watersheds of Alaska to trace salmonderived nutrients during 2003–2004. Maturing salmon were isotopically enriched relative to alternate freshwater N, S, and C sources as expected, with differences consistent with species trophic level differences, and minor system, sex, and year-to-year differences, enabling use of SIA to trace these salmon-derived nutrients. Periphyton naturally colonized, incubated, and collected using Wildco Periphtyon Samplers in and near spawning sites was 34 S- and 15 N-enriched, as expected, and at all freshwater sites was 13 C-depleted. At nonspawning and coho-only sites, periphyton 34 S and 15 N was generally low. However, 34 S was low enough at some sites to be suggestive of sulfate reduction, complicating the use of S isotopes. Juvenile salmon SIA ranged in values consistent with using production derived from re-mineralization as well as direct utilization, but only by a minority fraction of coho salmon. Dependency on salmon-derived nutrients ranged from relatively high to relatively low, suggesting a space-limited system. No one particular isotope was found to be superior for determining the relative importance of salmon-derived nutrients.

American Fisheries Society Symposium↗

USGS2009

Sulfur- and oxygen-isotopes in sediment-hosted stratiform barite deposits

Sulfur- and oxygen-isotope analyses have been obtained for sediment-hosted stratiform barite deposits in Alaska, Nevada, Mexico, and China to examine the environment of formation of this deposit type. The barite is contained in sedimentary sequences as old as Late Neoproterozoic and as young as Mississippian. If previously published data for other localities are considered, sulfur- and oxygen-isotope data are now available for deposits spanning a host-rock age range of Late Neoproterozoic to Triassic. On a ??34S versus ??18O diagram, many deposits show linear or concave-upward trends that project down toward the isotopic composition of seawater sulfate. The trends suggest that barite formed from seawater sulfate that had been isotopically modified to varying degrees. The ??34S versus ??18O patterns resemble patterns that have been observed in the modern oceans in pore water sulfate and water column sulfate in some anoxic basins. However, the closest isotopic analog is barite mineralization that occurs at fluid seeps on modern continental margins. Thus the data favor genetic models for the deposits in which barium was delivered by seafloor seeps over models in which barium was delivered by sedimentation of pelagic organisms. The isotopic variations within the deposits appear to reflect bacterial sulfate reduction operating at different rates and possibly with different electron donors, oxygen isotope exchange between reduction intermediates and H2O, and sulfate availability. Because they are isotopically heterogeneous, sediment-hosted stratiform barite deposits are of limited value in reconstructing the isotopic composition of ancient seawater sulfate.

Geochimica et Cosmochimica Acta↗

USGS2009

Geologic and Geochronologic Studies of the Early Proterozoic Kanektok Metamorphic Complex of Southwestern Alaska

The Kanektok complex of southwestern Alaska appears to be a rootless terrane of early Proterozoic sedimentary, volcanic, and intrusive rocks which were metamorphosed to amphibolite and granulite facies and later underwent a pervasive late Mesozoic thermal event accompanied by granitic plutonism and greenschist facies metamorphism of overlying sediments. The terrane is structurally complex and exhibits characteristics generally attributed to mantled gneiss domes. U-Th-Pb analyses of zircon and sphene from a core zone granitic orthogneiss indicate that the orthogneiss protolith crystallized about 2.05 b.y. ago and that the protolithic sedimentary, volcanic and granitic intrusive rocks of the core zone were metamorphosed to granulite and amphibolite facies about 1.77 b.y. ago. A Rb-Sr study of 13 whole-rock samples also suggests metamorphism of an early Proterozoic [Paleoproterozoic] protolith at 1.77 Ga, although the data are scattered and difficult to interpret. Seventy-seven conventional 40K/40Ar mineral ages were determined for 58 rocks distributed throughout the outcrop area of the complex. Analysis of the K-Ar data indicate that nearly all of these ages have been totally or partially reset by a pervasive late Mesozoic thermal event accompanied by granitic plutonism and greenschist facies metamorphism. Several biotites gave apparent K-Ar ages over 2 Ga. These ages appear to be controlled by excess radiogenic 40Ar produced by the degassing protolith during the 1.77 Ga metamorphism and incorporated by the biotites when they were at temperatures at which Ar could diffuse through the lattice. Five amphibolites yielded apparent Precambrian 40K/40Ar hornblende ages. There is no evidence that these hornblende ages have been increased by excess argon. The oldest 40K/40Ar hornblende age of 1.77 Ga is identical to the sphene 207Pb/206Pb orthogneiss age and to the Rb-Sr 'isochron' age for six of the 13 whole-rock samples. The younger hornblende ages are interpreted as having been partially reset during the late Mesozoic thermal event. 40Ar/39Ar incremental heating experiments suggest metamorphism occurred at least 1.2 b.y. ago but do not exhibit high temperature plateau ages significantly older than the 40Ar/39Ar total fusion ages of these samples. The age spectra are much more uniform than expected from a terrane with such a complex thermal history, perhaps caused by the small grain size of the samples which may possibly be less than the effective Ar diffusion radii of the analyzed hornblendes.

Open-File Report↗

USGS2014

Controls on methane released through ebullition in peatlands affected by permafrost degradation

Permafrost thaw in peat plateaus leads to the flooding of surface soils and the formation of collapse scar bogs, which have the potential to be large emitters of methane (CH 4 ) from surface peat as well as deeper, previously frozen, permafrost carbon (C). We used a network of bubble traps, permanently installed 20 cm and 60 cm beneath the moss surface, to examine controls on ebullition from three collapse bogs in interior Alaska. Overall, ebullition was dominated by episodic events that were associated with changes in atmospheric pressure, and ebullition was mainly a surface process regulated by both seasonal ice dynamics and plant phenology. The majority (>90%) of ebullition occurred in surface peat layers, with little bubble production in deeper peat. During periods of peak plant biomass, bubbles contained acetate-derived CH 4 dominated (>90%) by modern C fixed from the atmosphere following permafrost thaw. Post-senescence, the contribution of CH 4 derived from thawing permafrost C was more variable and accounted for up to 22% (on average 7%), in the most recently thawed site. Thus, the formation of thermokarst features resulting from permafrost thaw in peatlands stimulates ebullition and CH 4 release both by creating flooded surface conditions conducive to CH 4 production and bubbling as well as by exposing thawing permafrost C to mineralization.

Alaska↗

USGS2012

The Kanuti ophiolite, Alaska

The Kanuti ophiolite is a mafic-ultramafic thrust sheet of probable Jurassic age, formerly considered to be the upper part of the Yukon-Koyukuk ophiolite belt (Angayucham terrane). It is here called the Kanuti ophiolite after the Kanuti River region on the southeastern flank of the Yukon-Koyukuk Basin. The thrust sheet crops out discontinuously for a distance of more than 900 km along the northern and southeastern margins of the basin. It is probably correlative with similar ophiolite thrust sheets to the north in the western Brooks Range and to the south in the Ruby geanticline. Technically, the ophiolite is considered to be the Kanuti thrust panel of the Angayucham-Totzitna terrane. The Kanuti consistently overlies another extensive thrust sheet, consisting mostly of pillow basalt and radiolarian chert of Devonian to Jurassic age (Narvak thrust panel). This sheet is thrust over a third sheet consisting of probable Devonian phyllite and metagraywacke, which is in turn thrust over older metamorphic rocks (Slate Creek thrust panel). The Kanuti ophiolite is a partial ophiolite that consists of a lower residual mantle suite and an upper magmatic suite, but dikes, extrusives, and sediments are absent. The residual mantle suite is composed of harzburgite and dunite with refractory mineral compositions. The harzburgite is attributed to partial melting and extraction of basaltic magma; residual dunite is attributed to partial melting or to reaction of orthopyroxene out of harzburgite in contact with ascending melt diapirs. The magmatic suite consists of layered ultramafic and gabbroic rocks, containing minerals having limited iron enrichment. The absence of large volumes of magmatic rocks intermediate in composition between cumulus ultramafics and evolved gabbros favors periodic introduction of magma, rather than closed system fractional crystallization. The ultramafic rocks of both the residual mantle and magmatic suites are tectonites, which have undergone high-temperature deformation involving isoclinal folding on all scales and related syntectonic recrystallization. The olivine fabric is consistent with the glide system {0kl} [100], which has been produced experimentally at 800°–1190°C at 20 kbar. Olivine Z axes and subparallel isoclinal fold axes have consistent, northeast trends throughout the Kanuti region (>100 km NE-SW) and may be close to the original upper mantle flow direction, despite later low-angle thrust faulting. The order of crystallization in the cumulus ultramafic rocks of the magmatic suite is olivine, clinopyroxene, plagioclase, and orthopyroxene. The high Mg numbers of clinopyroxene (0.85–0.93) coexisting with olivine suggest that the cumulus ultramafic rocks crystallized at relatively high pressures (>10 kbar). The effects of parental magma composition cannot be evaluated, but the small difference in Mg numbers of coexisting olivine and clinopyroxene in the cumulus ultramafic rocks and in residual harzburgite suggests that regardless of absolute pressures, the pressure difference between the melting that produced the basalt magma and the initial fractional crystallization of the magma is small. Because of the limited range in rock types in the ophiolite, the tectonic environment cannot be interpreted unambiguously. However, the structural and petrological data are best reconciled with an origin in a volcanic arc tectonic setting.

Journal of Geophysical Research Solid Earth↗

USGS1963

High-alumina kaolinitic clay in the United States, exclusive of Alaska and Hawaii

The general distribution of high-alumina kaolinitic clay deposits in the United States (exclusive of Alaska and Hawaii) is shown on the accompanying map. The locations of mines, pits, and prospects are shown by appropriate symbols. Certain clay-bearing areas are shown by patterns that designate the geologic age of the deposits. The locations are numbered consecutively by states and are keyed to an index which lists the name of the deposit, county, and principal references. Only significant references on the size, quality, and geologic occurrence of deposits are listed. Some of the data for the map and index were compiled by John W. Hosterman and the late Leona Boardman.

Mineral Investigations Resource Map↗

USGS1977

Genesis of a zoned granite stock, Seward Peninsula, Alaska

A composite epizonal stock of biotite granite has intruded a diverse assemblage of metamorphic rocks in the Serpentine Hot Springs area of north-central Seward Peninsula, Alaska. The metamorphic rocks include amphibolite-facies orthogneiss and paragneiss, greenschist-facies fine-grained siliceous and graphitic metasediments, and a variety of carbonate rocks. Lithologic units within the metamorphic terrane trend generally north-northeast and dip moderately toward the southeast. Thrust faults locally juxtapose lithologic units in the metamorphic assemblage, and normal faults displace both the metamorphic rocks and some parts of the granite stock. The gneisses and graphitic metasediments are believed to be late Precambrian in age, but the carbonate rocks are in part Paleozoic. Dating by the potassium-argon method indicates that the granite stock is Late Cretaceous. The stock has sharp discordant contacts, beyond which is a well-developed thermal aureole with rocks of hornblende hornfels facies. The average mode of the granite is 29 percent plagioclase, 31 percent quartz, 36 percent K-feldspar, and 4 percent biotite. Accessory minerals include apatite, magnetite, sphene, allanite, and zircon. Late-stage or deuteric minerals include muscovite, fluorite, tourmaline, quartz, and albite. The stock is a zoned complex containing rocks with several textural facies that are present in four partly concentric zones. Zone 1 is a discontinuous border unit, containing fine- to coarse-grained biotite granite, that grades inward into zone 2. Zone 2 consists of porphyritic biotite granite with oriented phenocrysts of pinkish-gray microcline in a coarse-grained equigranular groundmass of plagioclase, quartz, and biotite. It is in sharp, concordant to discordant contact with rocks of zone 3. Zone 3 consists of seriate-textured biotite granite that has been intruded by bodies of porphyritic biotite granite containing phenocrysts of plagioclase, K-feldspar, quartz, and biotite in an aplitic groundmass. Flow structures, pegmatite and aplite segregations, and miarolitic cavities are common in the seriate-textured granite. Zone 4, which forms the central part of the complex, consists of fine- to medium-grained biotite granite and locally developed leucogranite. Small miarolitic cavities are common within it. Eight textural facies have been defined within the complex, and mineralogic, petrographic, modal, and chemical variations are broadly systematic within the facies sequence. Study of these variations shows that the gradational facies of zones l and 2 systematically shift toward more mafic compositions inward within the complex. Seriate-textured rocks of zone 3 are similar in composition to those of zone 2, but porphyritic rocks of zone 3 and rocks of zone 4 mark shifts to more felsic compositions. These late-crystallizing felsic rocks are products of an interior residual magma system. This system was enriched in water and certain trace elements including tin, lithium, niobium, lead, and zinc. The complex as a whole has higher concentrations of these elements than many other granites. The nature of this geochemical specialization is particularly well demonstrated by the trace-element composition of biotite. The crystallization history of the pluton was complex. The available data suggest that this history could have included: (1) chilling and metasomatic alteration adjacent to the contact, (2) in-situ crystallization in several marginal facies accompanied by some transfer of residual constituents toward interior parts of the pluton, (3) slight upward displacement of magma that was subjacent to the crystallized walls, accompanied by disequilibrium crystallization and local vapor saturation, (4) upward displacement of part of the residual water-rich interior magma, accompanied by rapid loss of a separated vapor phase, and (5) displacement of the margins of the pluton by normal faults, accompanied by loss of an exsolved vapor phase from th

Open-File Report↗

USGS1965

Geology and ore deposits of the central York Mountains, western Seward Peninsula, Alaska

In the central York Mountains, carbonate rocks of Lower and Middle Ordovician age and aggregating at least 8,000 feet thick are thrust northward over slate and argillaceous limestone of pre-Ordovician age which were intruded by gabbro in pre-Ordovician time. Normal faults of four distinct systems cut the thrust plates, and in Late Cretaceous time, stocks of biotite granite, abnormally rich in beryllium, tin, boron and certain other trace elements, pierced the thrust plates. In part following the intrusion of the granites, a strong set of normal faults developed striking N. 60°-85° E. through the central York Mountains and locally these faults cut the granites. Dikes of granite, rhyolite porphyry, and lamprophyre were injected into some of these faults, the lamprophyres being younger. Trace elements in the lamprophyres prove they are mafic rocks probably derived from the lama, and that they cannot be related genetically to granite. Shortly after the intrusion of lamprophyre dikes, ore deposits of tin, beryllium, and fluorite were formed from solutions probably derived from deeply-buried hot granite where the granite was ruptured by normal faults. Ore shoots were localized beneath thrust faults where the faults are intruded by dikes, and a major ore-bearing structure, the Rapid River fault, is mineralized for half its length for a distance of eight miles. The tin deposits contain cassiterite and stannite in topaz greisen with abundant sulfides of copper, lead, zinc, and iron, as well as wolframite. The beryllium deposits contain fluorite, chrysoberyl, diaspore, muscovite, and tourmaline, with trace to small amounts of euclase, bertrandite, helvite, phenikite(?), todorokite and hematite. Beryl occurs sparingly in late veins of quartz and fluorite. Chrysoberyl is the earliest and commonest beryllium mineral, followed by euclase and bertrandite, and then phenakite(?) and beryl. Helvite is restricted to banded skarns near granite, and which consist of magnetite and fluorite. Throughout the district, a strong zonation is displayed from tin deposits in greisen through transitional veins of sulfide minerals with fluorite and chrysoberyl to fluorite-beryllium deposits and thence to barren veins of silica and fluorite with trace amounts of beryllium. This zonal arrangement of deposits probably will be found elsewhere in the world where greisen tin deposits occur in carbonate rocks. The geochemical cycle of the trace elements Be, Sn, W, B, Li, Cu, Pb, Zn and Nb shows that these elements were enriched in the biotite granites and were strongly fractionated among the minerals of granites. Fran the granites, these rare elements moved outward into contact rocks and ore deposits. During the supergene cycle, clear geochemical anomalies were formed in stream sediments, soils and plants near ore deposits, and geochemical prospecting led to the discovery of the beryllium lodes. With the possible exception of zinc and niobium, the rare elements that are associated in the rocks and ores remain associated in the supergene processes. Fixation of zinc in clay soil and tundra plants may account for the relatively small amount of zinc in strew sediments. Datable Pleistocene events in the York Mountains begin with the Yarmouth Interglaciation when the York Terrace, a wide marine platform, was cut. In Illinoian time, the York Terrace was uplifted almost 400 feet, and during the Sangamon Interglaciation a second marine platform (Lost River Terrace) was cut and is not deformed. During Wisconsin time the widespread York Glaciation was followed by the more restricted Mint River Glaciation. Because uplift of the York Terrace extended into the Bering Strait, it is probable that prior to the uplift in Illinoian time the Bering Strait was a seaway and a barrier to land migration rather than a land bridge.

Alaska↗

USGS1979

Analyses of surficial deposits, central Brooks Range, Alaska

Seventy-nine sediment samples from the central Brooks Range were analyzed for grain-size distribution, shape and composition of grains, and other physical properties. Four statistical measures (sorting, mean diameter, skewness, and kurtosis) were then computed for the sand-to-clay size fraction of all samples. Fan sediments resemble the other alluvial-gravel deposits in consisting of sand and gravel from which very fine sand and the smaller size fractions have been removed by running water. Sorting and rounding of particles is best at the distal ends of large fans. Modern alluvium is better sorted than most fan deposits, with more rounded clasts and generally higher ratios of sand to gravel. Other alluvial deposits from terraces and heavily dissected erosion remnants generally are comparable to modern alluvium. All classes of alluvial gravel are characterized by removal of fines and by decreasing values of mean 0, kurtosis, and skewness as sorting increases. Most samples also have high clay/silt ratios that probably were caused by deflation on windswept bars and floodplains. Lacustrine deposits have clay percentages ranging from about 85 to 35, with silt predominant in more than one-half of the samples. Several samples contain sand and gravel that presumably were ice rafted. Clay-sized particles are unweathered mineral grains, implying that glacial abrasion was their primary source. Flow-slide deposits are very poorly sorted mixtures of gravel, sand, and silt, with clay content averaging only about one percent. Clasts consist of angular fragments of local bedrock, usually schist and phyllite, and matrix materials usually are highly micaceous. Although related flow types, a mud-flow and a debris-flow in till, are generally similar to flow-slide deposits, the mudflow has been modified by running water and the debris flow reflects the composition of its parent till. Other colluvial deposits commonly have high silt and low clay contents, but one solifluction deposit has abundant clay derived from till. Several of the flow-slides and other colluvial deposits are polygenetic, having undergone several episodes of flow that incorporated different types of sediment. Sand deposits include silty floodplain and basin-fill deposits as well as five relatively pure and well sorted dunal and river-bar sands. The silt-rich deposits probably contain large amounts of loess that fell into late-Pleistocene basin fillings and muskegs and later was redeposited on Holocene floodplains. The five dunal and bar sands are dominantly medium to fine sand; relatively well sorted, symmetric to coarse skewed, and leptokurtic. The dunal sands can be distinguished by the presence of very fine sand: wind apparently is less effective than flowing water in removing the finest sand fraction. The glacial deposits consist of till, ice-content stratified drift, and outwash. Till is a poorly sorted mixed sediment that resembles many flow de-posits but typically contains more clay. Two clay-deficient tills resemble fan deposits, implying effective washing by meltwater during glacial transport or deposition of these deposits. The ice-contact stratified drift varies in character from fan-like gravel deposits to sand accumulations nearly as well sorted as those of river bars. Almost all samples reflect some restriction in washing of fines by meltwater, probably owing to irregular topography and resulting poor drainage on and around stagnating glaciers. Most outwash deposits have less fine sand than modern alluvium, probably reflecting the generally high energy of glacial meltwater streams and the absence of vegetation from their floodplains. Samples from the southern Brooks Range are better sorted than those from northern Brooks Range valleys, and their statistical values lie close to those of modern alluvium. Comparisons between the different classes of sediments are facilitated by combining them into (1) gravel, (2) sand, silt, and clay, and (3) mixed deposits. Deposits of unknown or uncertain origin may then be compared directly against sediments from modern streams, dunes, flow-slides and other known sources, and alternative origins of aberrant samples in each of the sediment classes can also be examined. Several samples that initially were classed as fan, lacustrine, or glacial proved to be mixed deposits created by frost-churning, frost-lifting of stones, flowage down faces of river bluffs, and other postdepositional processes. Sorting and mean ϕ of matrix materials were useful in distinguishing the different sediment classes, separating them into contrasting subgroups, and identifying atypical samples. Skewness and kurtosis were generally less useful in this study.

Alaska↗

USGS1952

Geology of the Alaska-Juneau lode system, Alaska

The Alaska-Juneau lode system for many years was one of the worlds leading gold-producing areas. Total production from the years 1893 to 1946 has amounted to about 94 million dollars, with principal values in contained gold but with some silver and lead values. The principal mine is the Alaska-Juneau mine, from which the lode system takes its name. The lode system is a part of a larger gold-bearing belt, generally referred to as the Juneau gold belt, along the western border of the Coast Range batholith. The rocks of the Alaska-Juneau lode system consist of a monoclinal sequence of steeply northeasterly dipping volcanic, state, and schist rocks, all of which have been metamorphosed by dynamic and thermal processes attendant with the intrusion of the Coast Range batholith. The rocks form a series of belts that trend northwest parallel to the Coast Range. In addition to the Coast Range batholith lying a mile to the east of the lode system, there are numerous smaller intrusives, all of which are sill-like in form and are thus conformable to the regional structure. The bedded rocks are Mesozoic in age; the Coast Range batholith is Upper Jurassic and Lower Cretaceous in age. Some of the smaller intrusives pre-date the batholith, others post-date it. All of the rocks are cut by steeply dipping faults. The Alaska-Juneau lode system is confined exclusively to the footwall portion of the Perseverance slate band. The slate band is composed of black slate and black phyllite with lesser amounts of thin-bedded quartzite. Intrusive into the slate band are many sill-like bodies of rocks generally referred to as meta-gabbro. The gold deposits of the lode system are found both within the slate rocks and the meta-gabbro rocks, and particularly in those places where meta-gabbro bodies interfinger with slate. Thus the ore bodies are found in and near the terminations of meta-gabbro bodies. The ore bodies are quartz stringer-lodes composed of a great number of quartz veins from 6 inches to 3 feet wide and extending along their strike and dip for several tens to hundreds of feet. In addition to quartz, the only other vein gangue mineral is ankerite. It occurs in small amounts along the borders of the quartz veins. Metallic vein minerals, in addition to native gold, are, in order of decreasing abundance, pyrrhotite, galena, sphalerite, and arsenopyrite. In the aggregate the metallic minerals comprise only 1 to 2 percent of the total amount of vein material. The wall rock, particularly the meta-gabbro, was profoundly altered by the vein-forming processes. The principal effects on the meta-gabbro were the addition of large amounts of soda, potash, titanium, carbon dioxide, and phosphorous, and the removal of considerable quantities of iron, magnesia, lime, and combined water. Silica also may have been decreased. The mineralogical changes involved in the alteration were the development of biotite and ankerite at the expense of original hornblende and feldspar, resulting in a brown-colored biotite- and ankerite-rich rock. The slates are relatively unaffected by the vein-forming processes. Because of their small size, relatively low grade, and discontinuity, no attempt has been made to mine any individual vein. The prevailing practice has been to mine large blocks of ground by a system of modified block-caving, followed by hand sorting to remove the barren country rock from the gold-bearing quartz prior to milling.

Open-File Report↗

USGS2020

Design and operations of the Hydrate 01 Stratigraphic test well, Alaska North Slope

The National Energy Technology Laboratory, the Japan Oil, Gas and Metals National Corporation, and the U.S. Geological Survey are leading an effort to conduct an extended gas hydrate production test in northern Alaska. The proposed production test required the drilling of an initial stratigraphic test well (STW) to confirm the geologic conditions of the proposed test site. This well was completed in December 2018 in cooperation with the Prudhoe Bay Unit Interest Owners. With the success of the STW, the project leadership group is developing plans to drill a geologic data well and a production test well. Drilling plans for the STW were advanced in late 2018. The Prudhoe Bay Unit Hydrate-01 well was spudded on 10-December-2018. Downhole data acquisition was completed on 25-December-2018 and the rig was released on 01-January-2019. The STW was drilled in two sections. The surface hole was drilled to a depth of 2248 ft (MD, measured depth) and cased, and the “production hole section” was drilled to a depth of 3558 ft (MD) and also cased. A thermally chilled mineral-oilbased mud was used to maintain drillhole stability and quality of the borehole acquired data. The primary borehole data were acquired using a suite of Schlumberger logging-while-drilling tools. To gather grain size and other data needed to inform the design of the production test well, sidewall pressure cores were collected using Halliburton’s CoreVault tool. In addition to confirming the geologic conditions at the test site, the Hydrate-01 well was designed to serve as a monitoring well during future field operations. Therefore, two sets of fiber optic cables, each including bundled Distributed Acoustic Sensors (DAS) and Distributed Temperature Sensors (DTS), were clamped to the outside of the well casing and cemented in place. In March 2019, the project team worked with SAExploration to acquire 3D DAS Vertical Seismic Profiling (VSP) data in the Hydrate-01 well, which was the largest 3D DAS-VSP ever conducted. Additionally, since the December 2018 completion of the STW, several borehole temperature surveys have been acquired with the DTS deployed in the Hydrate-01 well.

Alaska↗

USGS2006

The giant Carlin gold province: A protracted interplay of orogenic, basinal, and hydrothermal processes above a lithospheric boundary

Northern Nevada hosts the only province that contains multiple world-class Carlin-type gold deposits. The first-order control on the uniqueness of this province is its anomalous far back-arc tectonic setting over the rifted North American paleocontinental margin that separates Precambrian from Phanerozoic subcontinental lithospheric mantle. Globally, most other significant gold provinces form in volcanic arcs and accreted terranes proximal to convergent margins. In northern Nevada, periodic reactivation of basement faults along this margin focused and amplified subsequent geological events. Early basement faults localized Devonian synsedimentary extension and normal faulting. These controlled the geometry of the Devonian sedimentary basin architecture and focused the discharge of basinal brines that deposited syngenetic gold along the basin margins. Inversion of these basins and faults during subsequent contraction produced the complex elongate structural culminations that characterize the anomalous mineral deposit "trends." Subsequently, these features localized repeated episodes of shallow magmatic and hydrothermal activity that also deposited some gold. During a pulse of Eocene extension, these faults focused advection of Carlin-type fluids, which had the opportunity to leach gold from gold-enriched sequences and deposit it in reactive miogeoclinal host rocks below the hydrologic seal at the Roberts Mountain thrust contact. Hence, the vast endowment of the Carlin province resulted from the conjunction of spatially superposed events localized by long-lived basement structures in a highly anomalous tectonic setting, rather than by the sole operation of special magmatic or fluid-related processes. An important indicator of the longevity of this basement control is the superposition of different gold deposit types (e.g., Sedex, porphyry, Carlin-type, epithermal, and hot spring deposits) that formed repeatedly between the Devonian and Miocene time along the trends. Interestingly, the large Cretaceous Alaska-Yukon intrusion-related gold deposits (e.g., Fort Knox) are associated with the northern extension of the same lithospheric margin in the Selwyn basin, which experienced an analogous series of geologic events. ?? Springer-Verlag 2006.

Mineralium Deposita↗

USGS1953

Reconnaissance for radioactive deposits in the Fairbanks and Livengood Quadrangles, east-central Alaska, 1949

Several mines and prospects in the Fairbanks and Livengood quadrangles, east-central Alaska, were examined for the possible presence of radioactive materials in the summer of 1949. Also tested were pre-Cambrian and Paleozoic metamorphic and sedimentary rocks crossed by the Elliott Highway, which extends from Fox, near Fairbanks, northward about 70 miles to the town of Livengood. None of the lodes tested exhibited radioactivity in excess of 0.003 percent equivalent uranium, although nuggets consisting chiefly of native bismuth and containing as much as 0.1 percent equivalent uranium had been found previously in a placer on Fish Creek several miles below the reported bismuth-bearing lode on Melba Creek. The greatest radioactivity found in the rocks along the Elliott Highway was in an iron-stained schist of pre-Cambrian age and in carbonaceous shale of Middle Devonian or Carboniferous age. Respective samples of these rocks contain 0.003 and 0.004 percent equivalent uranium. A possible local bedrock source for a euxenite-polycrase mineral found in a placer concentrate containing about 0.04 percent equivalent uranium was sought in the watershed of Goodluck Creek, near Livengood. The bedrock source of this mineral could not be located; it is believed that the source could be outside of the Goodluck watershed, as drainage changes in the area during Quaternary time might well have introduced gravels from nearby areas.

Alaska↗

USGS1954

Reconnaissance for radioactive deposits in east-central Alaska, 1949

In the summer of 1949, several mines and prospects in the Fairbanks and Livengood quadrangles, east-central Alaska, were examined for the possible presence of radioactive materials. Also tested were metamorphic and sedimentary rocks of pre-Cambrian and Paleozoic age crossed by the Elliott Highway, which extends from Fox, near Fairbanks, northwestward about 70 miles to the town of Livengood. Nuggets consisting chiefly of native bismuth and containing as much as 0.1 percent equivalent uranium had been found previously in a placer on Fish Creek several miles downstream from the reported bismuth-bearing lode on Melba Creek, but none of the lodes tested in 1949 exhibited radioactivity in excess of 0. 003 percent equivalent uranium. The greatest radioactivity found in the rocks along the Elliott Highway was in an iron-stained pre-Cambrian schist and in a carbonaceous(?) shale of Middle Devonian or Carboniferous age. Respective samples of these rocks contain 0. 003 and 0.004 percent equivalent uranium. A possible local bedrock source for the euxenite-polycrase mineral found in a placer concentrate containing about 0.04 percent equivalent uranium was sought in the watershed of Goodluck Creek, near Livengood. The bedrock source of this mineral could not be located and it is believed that the source could be outside of the Goodluck watershed because drainage changes during Quaternary time may well have introduced gravels from nearby areas.

Circular↗

USGS1962

Chromite in the United States, exclusive of Alaska and Hawaii

The chromite deposits in the United States (exclusive of Alaska and Hawaii) are shown on the accompanying map. In compiling the map, the deposits were classified in five size categories according to the estimated total quantity of chromite present before mining. The map shows, by different symbols, deposits of chromite in ultramafic rocks, beach sands in which chromite is an important constituent, and lateritic iron ores in which chromium has been concentrated by weathering of peridotite in situ. Although geologically significant amounts of chromium are known to occur in some gabbros and sedimentary rocks other than beach sands and laterite, the data are too fragmentary to show on the map.

Mineral Investigations Resource Map↗

USGS1967

Iron in the United States, exclusive of Alaska and Hawaii

The iron-ore deposits in the United States (exclusive of Alaska and Hawaii) are shown on the accompanying map; many iron-bearing deposits that are of lesser or only potential value are also shown. Because these deposits range widely in extent, two general categories of symbols are used, each of which is subdivided as explained below. The first category is for deposits of such restricted extent that they can be represented by a "spot": the second is for deposits which, although not necessarily extremely large in terms of volume or tonnage, are either continuous (or essentially so) or clustered in reasonably well-defined districts of numerous small occurrences.

Mineral Investigations Resource Map↗