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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.
Coal geology of the Northeast Circle area, McCone and Dawson counties, Montana
The Northeast Circle area is in central McCone and northern Dawson Counties, northeastern Montana. The area encompasses about 940 square miles and has a maximum relief of greater than 1,000 feet. The lowest point (altitude of less than 2,200 feet) is in the Redwater River Valley in the northern part of the area. The highest point (altitude of less than 3,200 feet) is on the Yellowstone-Missouri divide, located in parts of Tps. 18 and 19 N., Rso 49, 50, and 51E., and T. 20 N., Rs. 52 and 53 E. Surface exposures in most of the area consist of yellowish or light-colored sandy shales of the Tongue River Member of the Paleocene Fort Union Formation. The coal beds in the Northeast Circle area occur in the lower 500 feet of this member and are persistent in the field. The predominant structural features of the area are the northeast end of the Weldon monocline-fault in Tps. 21 and 22 N., R. 47 E., and a small steep depression near Circle in sec. 10, T. 19 N., R. 48 E. Otherwise, the structure of the area is nearly flat. Coal in the Northeast Circle area is composed of six main beds: the S, L1, R, Q, P, and Pust beds, in ascending stratigraphic order. Of these, the S, Pust, and P beds are the thickest. The S bed is more than 20 feet thick in some places, but usually ranges in thickness from 6 to 10 feet. The Pust bed, which is about 430-490 feet above the S bed, is more than 18 feet thick in the upper bench and as much as 9 feet thick in the lower bench. Coal thickness in the P bed varies from 0 to 10.5 feet. The other coal seams in the area are generally less than 5 feet thick. All coals are lignite in rank.
The circle creek rhyolite, a volcanic complex in northern Elko county, Nevada
The Circle Creek Rhyolite is a multiple-source mass of fayalite-ferro-augite rhyolite, forming an extensive complex in northern Elko County, Nevada, with a diameter of about 8 miles and an exposed area of about 36 square miles. From the structure of the flow layering and the complex sequence of microbrecciation structures, it seems to be a relatively thick mass that rose passively through many fissures to flood a sag basin in the older rocks, which are as young as the early Pliocene Idavada Volcanics, a sequence of ignimbrites and tuffs. Chemical analyses, thin-sections, and X-ray diffractometer studies show that the Circle Creek Rhyolite is a two-feldspar rhyolite. The interrelations of the pyroxene phases are unusually complex. The Circle Creek is partly concealed by later tuffs, gravels, and olivine basalt and is cut by two sequences of high-angle faults of different ages.
Hydraulic characteristics of an underdrained irrigation circle, Muskegon County wastewater disposal system, Michigan
Muskegon County, Michigan, disposes of wastewater by spray irrigating farmland on its waste-disposal site. Buried drains in the highly permeable unconfined aquifer at the site control the level of the water table. Hydraulic conductivity of the aquifer and drain-leakance, the reciprocal of resistance to flow into the drains, was determined at a representative irrigation circle while calibrating a model of the groundwater flow system. Hydraulic conductivity is 0.00055 m/sec, in the north zone of the circle, and 0.00039 m/sec in the south zone. Drain leakance -6 -6 is low in both zones: 2.9 x 10m/sec in the north and 9.5 x 10 m/sec in the south. Low drain leakance is responsible for waterlogging when irrigation rates are maintained at design levels. The capacity of the study circle to accept wastewater is 35 percent less than design capacity.
Botanical prospecting for uranium in the Circle Cliffs area, Garfield County, Utah
Astragalus pattersoni and Stanleya pinnata broadly define some uraniferous localities adjacent to the contact of the Moenkopi formation and the Shinarump member of the Chinle formation, but the general paucity of Astragalus in the Circle Cliffs area limits the usefulness of this genus. Astragalus pattersoni , Stanleya pinnata , and Aster venustus (?) may serve as guides to mineralized parts of the Salt Wash sandstone member of the Morrison formation in the Circle Cliffs area. Thick and thin sandstones of the Shinarump member generally can be distinguished by pinyon-juniper ration studies. These studies may supplement drilling to define channel-fill sandstones which are associated with ore deposits in the Circle Cliffs area. Ratio studies appear to be applicable to other areas throughout the Colorado Plateau where similar geological and ecological conditions exist.
Hydraulic characteristics of an underdrained irrigation circle, Muskegon County, wastewater disposal system, Michigan
Muskegon County, Michigan, disposes of waste water by spray irrigating farmland on its waste-disposal site. Buried drains in the highly permeable unconfined aquifer at the site control the level of the water table. Hydraulic conductivity of the aquifer and drain-leakance, the reciprocal of resistance to flow into the drains, was determined at a representative irrigation circle while calibrating a model of the ground-water flow system. Hydraulic conductivity is 0.00055 meter per second in the north zone of the circle and 0.00039 meter per second in the south zone. Drain leakance is low in both zones: 2.9 x 10-6 meters per second in the north and 9.5 x 10-6 meters per second in the south. Low drain leakance is responsible for waterlogging when irrigation rates are maintained at design levels. The capacity of the study circle to accept waste water is 35 percent less than design capacity.
The use of the two-circle contact goniometer in teaching crystallography
The two-circle contact goniometer was devised to supplement the two-circle reflecting goniometer, by furnishing a means for the measurement of medium-sized crystals not provided with sufficiently good reflecting faces for the successful use of the latter instrument. The contact goniometer has however proved itself adapted, in a greater degree even than the reflecting goniometer, to the demonstration of the principles of two-circle measurements, and their relation to geographic and astronomical measurements, and of the relations of normals to crystal faces and to projections, both stereographic and gnomonic. This simple and inexpensive instrument, a description of which is given on the following page, is thus fitted to serve a very important purpose in instructional work in crystallography.
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.
Potential effects of surface coal mining on the hydrology of the Circle West coal tracts, McCone County, eastern Montana
The Circle West coal tracts in McCone County, Montana, contain about 460 million tons of recoverable coal reserves. Estimates of coal reserves for the tract are based predominantly on the S coal bed, which averages about 16 ft in thickness. About 175 million tons, or 38%, of the recoverable coal is Federally owned and has been identified for potential lease sale. A hydrologic study has been conducted in the potential lease area to describe existing hydrologic systems and to assess potential effects of surface coal mining on local water resources. Geohydrologic data collected from wells and drill holes indicate that shallow aquifers exist in sandstone and coal beds of the Tongue River Member of the Fort Union Formation (Paleocene age). These shallow aquifers generally have small values of hydraulic conductivity (0.1 to 380 ft/day) and typically yield from 2 to 20 gal/min to stock and domestic wells. Where coal is extremely fractured or the thickness of saturated sandstone is large, some wells can yield in excess of 70 gal/min. Chemical analyses indicate that most shallow aquifers contain a sodium sulfate bicarbonate type water. Surface water resources of the area consist of intermittent streamflow in parts of the Nelson and Timber Creek basins plus a large network of reservoirs. The reservoirs provide a large part of the water supply for area livestock and irrigation. Water quality data for Nelson and Timber Creeks indicate that the water generally is a sodium sulfate type and has a large concentration (181 to 6,960 mg/L) of dissolved solids. Mining of the S coal bed in the Circle West coal tracts would permanently remove shallow coal and sandstone aquifers, resulting in the loss of shallow stock wells. Mining would destroy livestock reservoirs, alter runoff characteristics of Nelson Creek, and temporarily lower water levels in shallow aquifers near the mine. Leaching of soluble constituents from mine spoils may cause a long-term degradation of the quality of water in shallow aquifers in and near the coal tracts. Some of the effects on local water supplies could be mitigated by development of alternative water resources in deeper aquifers such as the Tullock aquifer of Paleocene age and the Fox Hills-lower Hell Creek aquifer of Late Cretaceous age. (Author 's abstract)
Oil and gas resource potential north of the Arctic Circle
The US Geological Survey recently assessed the potential for undiscovered conventional petroleum in the Arctic. Using a new map compilation of sedimentary elements, the area north of the Arctic Circle was subdivided into 70 assessment units, 48 of which were quantitatively assessed. The Circum-Arctic Resource Appraisal (CARA) was a geologically based, probabilistic study that relied mainly on burial history analysis and analogue modelling to estimate sizes and numbers of undiscovered oil and gas accumulations. The results of the CARA suggest the Arctic is gas-prone with an estimated 770–2990 trillion cubic feet of undiscovered conventional natural gas, most of which is in Russian territory. On an energy-equivalent basis, the quantity of natural gas is more than three times the quantity of oil and the largest undiscovered gas field is expected to be about 10 times the size of the largest undiscovered oil field. In addition to gas, the gas accumulations may contain an estimated 39 billion barrels of liquids. The South Kara Sea is the most prospective gas assessment unit, but giant gas fields containing more than 6 trillion cubic feet of recoverable gas are possible at a 50% chance in 10 assessment units. Sixty per cent of the estimated undiscovered oil resource is in just six assessment units, of which the Alaska Platform, with 31% of the resource, is the most prospective. Overall, the Arctic is estimated to contain between 44 and 157 billion barrels of recoverable oil. Billion barrel oil fields are possible at a 50% chance in seven assessment units. Undiscovered oil resources could be significant to the Arctic nations, but are probably not sufficient to shift the world oil balance away from the Middle East.
Circum-Arctic resource appraisal: Estimates of undiscovered oil and gas north of the Arctic Circle
The U.S. Geological Survey (USGS) has completed an assessment of undiscovered conventional oil and gas resources in all areas north of the Arctic Circle. Using a geology-based probabilistic methodology, the USGS estimated the occurrence of undiscovered oil and gas in 33 geologic provinces thought to be prospective for petroleum. The sum of the mean estimates for each province indicates that 90 billion barrels of oil, 1,669 trillion cubic feet of natural gas, and 44 billion barrels of natural gas liquids may remain to be found in the Arctic, of which approximately 84 percent is expected to occur in offshore areas.
Bathymetric and hydraulic survey of the Matanuska River near Circle View Estates, Alaska
An acoustic Doppler current profiler interfaced with a differentially corrected global positioning system was used to map bathymetry and multi-dimensional velocities on the Matanuska River near Circle View Estates, Alaska. Data were collected along four spur dikes and a bend in the river during a period of active bank erosion. These data were collected as part of a larger investigation into channel processes being conducted to aid land managers with development of a long-term management plan for land near the river. The banks and streambed are composed of readily erodible material and the braided channels frequently scour and migrate. Lateral channel migration has resulted in the periodic loss of properties and structures along the river for decades. For most of the survey, discharge of the Matanuska River was less than the 25th percentile of long-term streamflow. Despite this relatively low flow, measured water velocities were as high as 15 feet per second. The survey required a unique deployment of the acoustic Doppler current profiler in a tethered boat that was towed by a small inflatable raft. Data were collected along cross sections and longitudinal profiles. The bathymetric and velocity data document river conditions before the installation of an additional spur dike in 2006 and during a period of bank erosion. Data were collected along 1,700 feet of river in front of the spur dikes and along 1,500 feet of an eroding bank. Data collected at the nose of spur dikes 2, 3, and 4 were selected to quantify the flow hydraulics at the locations subject to the highest velocities. The measured velocities and flow depths were greatest at the nose of the downstream-most spur dike. The maximum point velocity at the spur dike nose was 13.3 feet per second and the maximum depth-averaged velocity was 11.6 feet per second. The maximum measured depth was 12.0 feet at the nose of spur dike 4 and velocities greater than 10 feet per second were measured to a depth of 10 feet. Data collected along an eroding bank provided details of the spatial distribution and variability in magnitude of velocities and flow depths while erosion was taking place. Erosion was concentrated in an area just downstream of the apex of a river bend. Measured velocities and flow depths were greater in the apex of the bend than in the area of maximum bank erosion. The maximum measured velocity was 12.9 feet per second at the apex and 11.2 feet per second in front of the eroding bank. The maximum measured depth was 10.2 feet at the apex and 5.2 feet in front of the eroding bank.
Summary of references to mineral occurrences (other than mineral fuels and construction materials) in the Circle Quadrangle, Alaska
These summaries of references are designed to aid in library research on metallic and nonmetallic (other than mineral fuels and construction materials) mineral occurrences in the Circle quadrangle, Alaska. All references to reports of the Geological Survey, to most reports of the U.S. Bureau of Mines, and to most reports of the State of Alaska Division of Geological and Geophysical Surveys and its predecessor State and Territorial agencies released before January 1, 1976, are summarized. Certain, mainly statistical, reports such as the annual Minerals Yearbook of the U.S. Bureau of Mines and the biennial and annual reports of the State of Alaska Division of Geological and Geophysical Surveys and its predecessor State and Territorial agencies are not included. This report is divided into three parts: a section made up of summaries of references arranged alphabetically by occurrence name; a section that lists synonyms for names in the first section, claim names, and the names of operators and owners of mines and prospects; and a section that lists, by author, all references summarized in the first section.
Moving forward in circles: Challenges and opportunities in modeling population cycles
Population cycling is a widespread phenomenon, observed across a multitude of taxa in both laboratory and natural conditions. Historically, the theory associated with population cycles was tightly linked to pairwise consumer–resource interactions and studied via deterministic models, but current empirical and theoretical research reveals a much richer basis for ecological cycles. Stochasticity and seasonality can modulate or create cyclic behaviour in non-intuitive ways, the high-dimensionality in ecological systems can profoundly influence cycling, and so can demographic structure and eco-evolutionary dynamics. An inclusive theory for population cycles, ranging from ecosystem-level to demographic modelling, grounded in observational or experimental data, is therefore necessary to better understand observed cyclical patterns. In turn, by gaining better insight into the drivers of population cycles, we can begin to understand the causes of cycle gain and loss, how biodiversity interacts with population cycling, and how to effectively manage wildly fluctuating populations, all of which are growing domains of ecological research.
Botanical prospecting for uranium in the Circle Cliffs area, Garfield County, Utah
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Geology of the Circle Cliffs area, Garfield and Kane Counties, Utah
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Newly located occurrences of lode gold near Table Mountain, Circle Quadrangle, Alaska
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