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Several comprehensive, data-driven geographic information system (GIS) analyses were conducted to assess prospectivity for lode gold in Alaska. These analyses use available geospatial datasets of lithologic, geochemical, mineral occurrence, and geophysical data to build models for recognizing different types of gold deposits within physiographic units defined by stream drainage basins that are approximately 100 square kilometers in area. The analytical methods successfully delineated areas in the State that contain known lode gold deposits and occurrences, providing some measure of confidence in their ability to predict gold prospectivity in areas of unknown lode gold potential. The results of our analyses indicate high prospectivity in a few areas scattered around the State that are not known to contain lode gold deposits. In addition to assessing the potential for lode gold deposits in Alaska, we designed analyses to distinguish different lode gold deposit types, including orogenic, reduced-intrusion-related, epithermal, and gold-bearing porphyry. These can primarily be differentiated using their unique trace element geochemical fingerprints and elemental enrichments, which reflect the characteristics of the geologic environment and chemistry of the ore-forming fluids. We identified multiple parameters that would discriminate the different types of gold deposits, but owing to the limits of available data, the compositional similarity of ore-forming fluids among some types of lode gold deposits, and overlapping geologic environments, distinguishing deposit types at the state scale in Alaska remains problematic. These limitations resulted in overlapping areas of prospectivity for different deposit types, highlighting the challenges for targeted gold exploration in Alaska. Adjustment of some scoring parameters and recharacterization at smaller scales to highlight individual mineral systems for application of prospectivity analyses may be helpful at a district scale. At a regional scale, the aerial overlap of individual deposit type analyses reinforces confidence in prospectivity for a lode gold resource in a drainage basin. Our analysis for undivided lode gold deposits will be the most practical analysis for landuse decisions in which delineation of areas that have confident potential for gold deposits in general is the primary goal. Data-driven GIS analysis for lode gold potential in Alaska, although limited by the size and uneven coverage of available datasets, objectively indicates prospectivity in areas where exposure is good as well as in areas under cover. The results of our analyses show medium to high prospectivity in areas that surround known deposits, indicating an overall expansion of areas that have the potential to contain gold deposits. Exploration in these areas may help improve the balance between the volume of gold produced in placer districts statewide and the relatively low volume of identified lode resources that contribute to these placer deposits. The results of our analyses can help focus future investigations in areas that show prospectivity but are not known to contain gold deposits, as well as in areas where data are lacking and the geology is poorly understood, and acquisition of additional data may help better define and constrain gold prospectivity.
Since 1898 the United States Geological Survey has been carrying on systematic topographic and geologic surveys in Alaska under an appropriation made for the investigation of the mineral resources of the Territory. This work has included not only areal surveys of regions already being developed by the miner and prospector, but also explorations and investigations of regions that are little known or entirely unexplored. As a result of these explorations a network of reconnaissance traverses has been extended over a large part of Alaska, where route surveys of this character must necessarily precede more detailed topographic and geologic mapping. They serve to outline the main geographic features of the country and afford the pioneer or prospector a guide for his journeys as well as help him to select his field of operations. The present report and maps are the results of such an investigation.
The U.S. Geological Survey (USGS) has initiated a multi-disciplinary study investigating the applicability of remote sensing technologies for geologic mapping and identification of prospective areas for base and precious metal deposits in remote parts of Alaska. The Kahiltna terrane in southwestern Alaska was selected for investigation because of its known mineral deposits and potential for additional mineral resources. An assortment of technologies is being investigated to aid in remote analysis of terrain, and includes imaging spectroscopy (hyperspectral remote sensing), high spatial resolution electro-optical imagery, and Synthetic Aperture Radar (SAR). However, there are significant challenges to applying imaging spectroscopy and electro-optical imagery technologies in this area because of the low solar angle for parts of the year, seasonal periods of darkness and snow cover, and the frequently cloudy weather that characterizes Alaska. Synthetic Aperture Radar (SAR) was selected because this technology does not rely on solar illumination and has all-weather capability. The USGS has compiled a continuous, cloud-free 12.5-meter resolution radar mosaic of SAR data of approximately 212,000 square kilometers to examine the suitability of this technology for geologic mapping. This mosaic was created from Advanced Land Observing Satellite (ALOS) Phased Array type L-band Synthetic Aperture Radar (PALSAR) data collected from 2007 to 2010 spanning the Kahiltna terrane and the surrounding area. Interpretation of these data may help geologists understand past geologic processes and identify areas with potential for near-surface mineral resources for further ground-based geological and geochemical investigations.
New 1:63,360-scale geologic mapping of the Big Delta B-2 quadrangle provides important data on the structural setting and age of geologic units, as well as on the timing of gold mineralization plutonism within the Yukon-Tanana Upland of east-central Alaska. Gold exploration has remained active throughout the region in response to the discovery of the Pogo gold deposit, which lies within the northwestern part of the quadrangle near the south bank of the Goodpaster River. Geologic mapping and associated geochronological and geochemical studies by the U.S. Geological Survey (USGS) and the Alaska Department of Natural Resources, Division of Mining and Water Management, provide baseline data to help understand the regional geologic framework. Teck Cominco Limited geologists have provided the geologic mapping for the area that overlies the Pogo gold deposit as well as logistical support, which has lead to a much improved and informative product. The Yukon-Tanana Upland lies within the Tintina province in Alaska and consists of Paleozoic and possibly older(?) supracrustal rocks intruded by Paleozoic (Devonian to Mississippian) and Cretaceous plutons. The oldest rocks in the Big Delta B-2 quadrangle are Paleozoic gneisses of both plutonic and sedimentary origin. Paleozoic deformation, potentially associated with plutonism, was obscured by intense Mesozoic deformation and metamorphism. At least some of the rocks in the quadrangle underwent tectonism during the Middle Jurassic (about 188 Ma), and were subsequently deformed in an Early Cretaceous contractional event between about 130 and 116 Ma. New U-Pb SHRIMP data presented here on zircons from the Paleozoic biotite gneisses record inherited cores that range from 363 Ma to about 2,130 Ma and have rims of euhedral Early Cretaceous metamorphic overgrowths (116 +/- 4 Ma), interpreted to record recrystallization during Cretaceous west-northwest-directed thrusting and folding. U-Pb SHRIMP dating of monazite from a Paleozoic gneiss sample yields an age of 112 +/- 2 Ma; the monazite presumably grew during the waning stages of the intense regional Cretaceous ductile deformation. The Cretaceous ductile deformation was followed closely by granite plutonism and gold mineralization. The main pulse of gold mineralization is temporally and spatially associated with the Cretaceous granitic dikes and plutons and occurred during regional uplift and extension.
Geochemical data are presented for more than 1,500 archived stream-sediment samples and accompanying quality control samples. The archived sediments were reanalyzed to improve the stream geochemical dataset for Alaska and to support ongoing U.S. Geological Survey (USGS) studies. Sediment samples were primarily from the USGS Mineral Resources Program’s sample archive in Denver, Colorado, but a few were from the Alaska Geological & Geophysical Surveys’ Geologic Materials Center in Anchorage, Alaska. All samples were submitted to the USGS contract laboratory, AGAT Laboratories, for analysis. All samples were analyzed using a 60-element analytical method involving fusion of the sample by sodium peroxide, dissolution of the fusion cake by nitric acid, and elemental analysis by inductively coupled plasma-optical emission spectroscopy and inductively coupled plasma-mass spectroscopy. Additionally, 106 samples from the Nixon Fork area were analyzed by a second multi-element method involving decomposition by a mixture of hydrochloric, nitric, perchloric, and hydrofluoric acids and the elemental analysis of the resulting solution by inductively coupled plasma-optical emission spectroscopy and inductively coupled plasma-mass spectroscopy. The latter method was used because the detection limit is lower for several elements including As, Cd, Pb, and Sb. Mercury concentrations in 296 samples from southeast Alaska were determined using a cold-vapor atomic absorption spectrometry method. The concentration data from the archived samples are presented along with concentration data from the standard reference material that was submitted with the samples.
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The Yukon River Basin, which encompasses 330,000 square miles in northwestern Canada and central Alaska (Fig. 1), is one of the largest and most diverse ecosystems in North America. The Yukon River is also fundamental to the ecosystems of the eastern Bering Sea and Chukchi Sea, providing most of the freshwater runoff, sediments, and dissolved solutes. Despite its remoteness and perceived invulnerability, the Yukon River Basin is changing. For example, records of air temperature during 1961-1990 indicate a warming trend of about 0.75 deg C per decade at latitudes where the Yukon River is located. Increases in temperature will have wide-ranging effects on permafrost distribution, glacial runoff and the movement of carbon and nutrients within and from the basin. In addition, Alaska has many natural resources such as timber, minerals, gas, and oil that may be developed in future years. As a consequence of these changes, several issues of scientific and cultural concern have come to the forefront. At present, water quality data for the Yukon River Basin are very limited. This fact sheet describes a program to provide the data that are needed to address these issues.
This remote 700 square mile area in the Brooks Range is topographically rugged and geologically diverse; it contains a granitic pluton, low-grade metamorphic rocks, sedimentary rocks, and mafic igneous rocks, as well as glacial features. Rocks of sedimentary origin include from oldest to youngest: 1.Neruokpuk Formation Middle and Upper Devonian(?), more than 4000 feet thick, a variety of units which represent the greenschist facies, including quartzitic- and schistose-feldspathic graywacke; phyllite, argillite, and slate, as well as dark limestone, sandy limestone, and silicified carbonate rocks. The succession of units in parts of the area is uncertain. Correlations between these units and with others in the eastern Brooks Range are provisional. 2.Kekiktuk Conglomerate and Kayak(?) Shale (Upper Devonian(?) to Upper Mississippian), a single map unit, from absent(?) to 400+ feet thick, containing dark shale Kayak(?) in its uppermost part and quartzite, interbedded dark shale, and some pebble- to boulder-conglomerate in the locally absent lower part (Kekiktuk). The unit overlies the Neruokpuk with angular unconformity, which may reflect either a pre-Kayak(?) or pre-Kekiktuk hiatus or both. 3.Lisburne Group, almost entirely carbonate rocks, and relatively thin in this area, 600 to 800 feet thick. Alapah Limestone (Upper Mississippian), to 560 feet thick, includes gray sandy, crystalline, and cherty limestone; minor dark shale; and dark cherty carbonate rocks in the upper part. The lower contact is gradational with the Kayak(?). Wahoo(?) Limestone (Pennsylvanian(?) to Permian) conformably overlies the Alapah, is absent to 200+ feet thick, and is characterized by light-gray crinoidal limestones in its upper part. 4. Sadlerochit Formation, consisting of three intraconformable units: ferruginous sandstone member (Permian) of ironstained orthoquartzite and dark slate, 175 to 240 feet thick which unconformably overlies the Wahoo(?) and Alapah Limestones; shale member of dark shale, slate, and minor quartzite averaging 400 feet in thickness; and quartzite member (Lower(?) Triassic), 700 feet thick, mostly orthoquartzite with minor shale and conglomerate. The basal clastics were probably shed from the north. 5.Shublik Formation (Middle(?) and Upper Triassic), 600 to 700 feet thick, with the thin phosphatic sandstone member overlain by dark phosphatic limestones and limy shales of the limestone member. 6.Kingak Formation (Jurassic), more than 1000 feet thick. The siltstone member, resistant sandstone and siltstone 75 to 150 feet thick, is overlain by an undetermined thickness of black shale. The basal part contrasts sharply with the underlying Shublik, indicating possible disconformity. 7.Ignek(?) Formation (Cretaceous), represented in the foothills where lithic graywacke, shale, and coaly shale constitute the few exposures examined. 8.Glacial and glaciofluvial materials of five advances recognized on the basis of morphology and position, which are tentatively correlated with five glaciations 15 miles west of the area. 9. Recent alluvial and colluvial deposits including fans which appear to represent at least three stages of encroachment. The Ramanzof granite, exposed in the Okpilak batholith and Jago stock, is mostly light-gray quartz monzonite to granite, and contains essential quartz, perthitic microcline, albite-oligoclase, and partly chloritized biotite. Limited modal and chemical data are presented. Three textural facies are: 1) porphyritic (marginal), with abundant large microcline megacrysts; 2) variable (middle to marginal), which exhibits textural and mineralogical banding; and 3) coarse (inner to marginal), which is gneissoid to equigranular. Facies relationships appear to be mostly gradational but may be locally intrusive. Some schistose metasedimentary(?) rock occurs in the granite. Aplite dikes, inclusions, tourmaline veins and replacements, and chlorite and quartz veins are locally common, as well as quartz monzonite and mafic igneous dikes. Contacts with Neruokpuk Formation rocks are mostly abrupt, concordant to cross-cutting, and locally adjoin tactite and hornfels of the albite-epidote-hornfels and hornblende-hornfels facies. Contacts with Kekiktuk Conglomerate are apparently gradational through a schistoze zone. Both primary and secondary structural elements are present in the Romanzof in granite. Textural and mineralogical banding and, in general, feldspar foliation are considered to be primary in origin; biotite foliation, gneissic and schistose foliation, and schistose zones are considered secondary. Lead-alpha age of zircons appears to be Late Devonian, K-Ar age of biotite is Cretaceous, possibly indicating updating by later reheating. Field age relationships are inconclusive but suggest pre-Kayak(?) (Upper Devonian) granite emplacement. The pluton is interpreted to be essentially the product of melt crystallization, synorogenically emplaced by forceful injection with minor stoping, and may include marginally granitized rock. Mafic igneous rocks of altered basaltic composition (greenstones) include dikes in granitic and Neruokpuk Formation rocks, and volcanics(?). A late Paleozoic age is suggested for them. Structural grain strikes east-northeast; south-dipping elements are common. Structures include the major positive nature of the area (first order), relatively broad folds (second order) which contain small tight folds (third order). Related south-dipping cleavage, schistosity, and biotite foliation in granite in the northern part of the area are cut by prominent sets of transverse joints and faults. Other features are longitudinal normal and reverse faults, at least one large-scale overthrust fault, and sheared zones in granite with possible attendant retrograde metamorphism. Although Mesozoic and Tertiary deformational features are dominant in northern Alaska, the Romanzof area may have been part of a Late Devonian orogenic belt continuous with one in northern Canada. Three alternate trends of such a belt in northern Alaska are discussed, but evidence is inconclusive. The mineral potential of the area is largely unknown. Minor amounts of metallic sulfides and oxides are present in granite and Neruokpuk Formation rocks. Analyses of stream silt samples suggest the possibility of tin and beryllium potential. The Shublik Formation contains rock phosphate.
An aeromagnetic survey covering portions of the Yakutat Feveland region in south-central Alaska was made to delineate concentrations of magnetic minerals possibly associated with heavy mineral placers in beach sands and raised beach ridges. However, the larger magnetic anomalies do not correlate with the beach placers in the surveyed area. The dominant magnetic feature, a strong linear positive anomaly almost parallel to the coastline, is thought to reflect a bedrock feature, probably a mafic igneous body enclosed within nonmagnetic rocks of the Yakutat group.
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Antimony is an important mineral commodity used widely in modern industrialized societies. The element imparts strength, hardness, and corrosion resistance to alloys that are used in many areas of industry, including in lead-acid storage batteries. Antimony’s leading use is as a fire retardant in safety equipment and in household goods, such as mattresses. The U.S. Government has considered antimony to be a critical mineral mainly because of its use in military applications. The great majority of the world’s antimony comes from China, and much of the remainder is shipped to China for smelting. Antimony resources are unevenly distributed around the world. China has the bulk of the world’s identified resources; other countries that have identified antimony resources include Bolivia, Canada, Mexico, Russia, South Africa, Tajikistan, and Turkey. Resources in the United States are located mainly in Alaska, Idaho, Montana, and Nevada. The most significant antimony mineral deposits occur in geologic environments with a thick sequence of siliciclastic sedimentary rocks in areas with significant fault and fracture systems. The most common antimony ore mineral is stibnite (Sb2 S3 ), but more than 100 other minerals also contain antimony. The presence of antimony in surface waters and groundwaters results primarily from rock weathering, soil runoff, and anthropogenic sources. Global emissions of antimony to the atmosphere average 6,100 metric tons per year. Empirical data suggest that the acid-generating potential of antimony mine waste is low.
Aeromagnetic data are used to better understand the geology and mineral resources near the Late Cretaceous Pebble porphyry Cu-Au-Mo deposit in southwestern Alaska. The reduced-to-pole (RTP) transformation of regional-scale aeromagnetic data shows that the Pebble deposit is within a cluster of magnetic anomaly highs. Similar to Pebble, the Iliamna, Kijik, and Neacola porphyry copper occurrences are in magnetic highs that trend northeast along the crustal-scale Lake Clark fault. A high-amplitude, short- to moderate-wavelength anomaly is centered over the Kemuk occurrence, an Alaska-type ultramafic complex. Similar anomalies are found west and north of Kemuk. A moderate-amplitude, moderate-wavelength magnetic low surrounded by a moderate-amplitude, short-wavelength magnetic high is associated with the gold-bearing Shotgun intrusive complex. The RTP transformation of the district-scale aeromagnetic data acquired over Pebble permits differentiation of a variety of Jurassic to Tertiary magmatic rock suites. Jurassic-Cretaceous basalt and gabbro units and Late Cretaceous biotite pyroxenite and granodiorite rocks produce magnetic highs. Tertiary basalt units also produce magnetic highs, but appear to be volumetrically minor. Eocene monzonite units have associated magnetic lows. The RTP data do not suggest a magnetite-rich hydrothermal system at the Pebble deposit. The 10-km upward continuation transformation of the regional-scale data shows a linear northeast trend of magnetic anomaly highs. These anomalies are spatially correlated with Late Cretaceous igneous rocks and in the Pebble district are centered over the granodiorite rocks genetically related to porphyry copper systems. The spacing of these anomalies is similar to patterns shown by the numerous porphyry copper deposits in northern Chile. These anomalies are interpreted to reflect a Late Cretaceous magmatic arc that is favorable for additional discoveries of Late Cretaceous porphyry copper systems in southwestern Alaska.
From 1983 to 1987, the U.S. Geological Survey conducted research to develop a national resource data base of Federal lands under the auspices of the Federal Land Information System (FLIS) program. The program's goal was to develop the capability to provide information to national mineral-use policymakers. Prototype spatial data bases containing mineral, land status, and base cartographic data were developed for the Medford, Oreg., area, the State of Alaska, and the Silver City, N. Mex., area. Other accomplishments included (1) the preparation of a digital format for U.S. Geological Survey mineral assessment data and (2) the development of a procedure for integrating parcel-level tabular Alaska land status data into a section-level geographic information system. Overall findings indicated that both vector and raster capabilities are required for a FLIS and that nationwide data availability is a limiting factor in FLIS development. As a result of a 1986 interbureau (U.S. Geological Survey, Bureau of Land Management, and Bureau of Mines) review of the FLIS program, activities were redirected to undertake research on large-area geographic information system techniques. Land use and land cover data generalization strategies were tested, and areafiltering software was found to be the optimum type. In addition, a procedure was developed for transferring tabular land status data of surveyed areas in the contiguous 48 States to spatial data for use in geographic information systems. The U.S. Geological Survey FLIS program, as an administrative unit, ended in 1987, but FLIS-related research on large-area geographic information systems continues.
Epidote is a major magmatic mineral in tonalite and granodiorite in a belt coextensive with the Mesozoic accreted terranes between northern California and southeastern Alaska. Textural and chemical evidence indicates that epidote crystallized as a relatively late but magmatic mineral that formed through reaction with hornblende in the presence of a melt phase. The observed relations concur with experimental data on crystallization of epidote from synthetic granodiorite at 8 kbar total pressure. Plutonic rocks bearing magmatic epidote must have formed under moderately high pressures, corresponding to lower crustal depths, under fairly oxidizing conditions.