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Karen D. Kelley

Publications and source records attributed to Karen D. Kelley.

At least 55 records · Page 3Linked to original sources
USGS2012

An exploration hydrogeochemical study at the giant Pebble porphyry Cu-Au-Mo deposit, Alaska, USA, using high-resolution ICP-MS

A hydrogeochemical study using high resolution ICP-MS was undertaken at the giant Pebble porphyry Cu-Au-Mo deposit and surrounding mineral occurrences. Surface water and groundwater samples from regional background and the deposit area were collected at 168 sites. Rigorous quality control reveals impressive results at low nanogram per litre (ng/l) levels. Sites with pH values below 5.1 are from ponds in the Pebble West area, where sulphide-bearing rubble crop is thinly covered. Relative to other study area waters, anomalous concentrations of Cu, Cd, K, Ni, Re, the REE, Tl, SO 4 2− and F − are present in water samples from Pebble West. Samples from circum-neutral waters at Pebble East and parts of Pebble West, where cover is much thicker, have anomalous concentrations of Ag, As, In, Mn, Mo, Sb, Th, U, V, and W. Low-level anomalous concentrations for most of these elements were also found in waters surrounding nearby porphyry and skarn mineral occurrences. Many of these elements are present in low ng/l concentration ranges and would not have been detected using traditional quadrupole ICP-MS. Hydrogeochemical exploration paired with high resolution ICP-MS is a powerful new tool in the search for concealed deposits.

Alaska↗
USGS2012

U.S. Geological Survey energy and minerals science strategy

The economy, national security, and standard of living of the United States depend heavily on adequate and reliable supplies of energy and mineral resources. Based on current population and consumption trends, the Nation's use of energy and minerals can be expected to grow, driving the demand for ever broader scientific understanding of resource formation, location, and availability. In addition, the increasing importance of environmental stewardship, human health, and sustainable growth place further emphasis on energy and mineral resources research and understanding. Collectively, these trends in resource demand and the interconnectedness among resources will lead to new challenges and, in turn, require cutting-edge science for the next generation of societal decisions. The contributions of the U.S. Geological Survey to energy and minerals research are well established. Based on five interrelated goals, this plan establishes a comprehensive science strategy. It provides a structure that identifies the most critical aspects of energy and mineral resources for the coming decade. * Goal 1. - Understand fundamental Earth processes that form energy and mineral resources. * Goal 2. - Understand the environmental behavior of energy and mineral resources and their waste products. * Goal 3. - Provide inventories and assessments of energy and mineral resources. * Goal 4. - Understand the effects of energy and mineral development on natural resources. * Goal 5. - Understand the availability and reliability of energy and mineral resource supplies. Within each goal, multiple, scalable actions are identified. The level of specificity and complexity of these actions varies, consistent with the reality that even a modest refocus can yield large payoffs in the near term whereas more ambitious plans may take years to reach fruition. As such, prioritization of actions is largely dependent on policy direction, available resources, and the sequencing of prerequisite steps that will lead up to the most visionary directions. The science strategy stresses early planning and places an emphasis on interdisciplinary collaboration and leveraging of expertise across the U.S. Geological Survey.

Open-File Report↗
USGS2011

Porphyry Cu indicator minerals in till as an exploration tool: Example from the giant Pebble porphyry Cu-Au-Mo deposit, Alaska, USA

Porphyry Cu indicator minerals are mineral species in clastic sediments that indicate the presence of mineralization and hydrothermal alteration associated with porphyry Cu and associated skarn deposits. Porphyry Cu indicator minerals recovered from shallow till samples near the giant Pebble Cu-Au-Mo porphyry deposit in SW Alaska, USA, include apatite, andradite garnet, Mn-epidote, visible gold, jarosite, pyrite, and cinnabar. Sulphide minerals other than pyrite are absent from till, most likely due to the oxidation of the till. The distribution of till samples with abundant apatite and cinnabar suggest sources other than the Pebble deposit. With three exceptions, all till samples up-ice of the Pebble deposit contain <10 grains/10kg of garnet (0.25-0.5 mm). Samples in the immediate vicinity of the Pebble deposit contain 10-20 grains, whereas samples with the most grains (>40grains/10kg) are in close proximity to smaller porphyry and skarn occurrences in the region. The distribution of Mn-epidote closely mimics the distribution of garnet in the till samples and further supports the interpretation that these minerals most likely reflect skarns associated with the porphyry deposits. All but two till samples, including those up-ice from the deposit, contain some gold grains. However, tills immediately west and down-ice of Pebble contain more abundant gold grains, and the overall number of grains decreases in the down-ice direction. Furthermore, all samples in the immediate vicinity of Pebble contain more than 65% pristine and modified grains compared to mostly re-shaped grains in distal samples. The pristine gold in till reflects short transport distances and/or liberation of gold during in-situ weathering of transported chalcopyrite grains. Jarosite is also abundant (1-2 500 grains/10kg) in samples adjacent to and up to 7 km down-ice from the deposit. Most jarosite grains are rounded and preliminary Ar/Ar dates suggest the jarosite formed prior to glaciation and it implies that a supergene cap existed over Pebble West. Assuming this interpretation is accurate, it suggests a shallow level of erosion of the Pebble deposit by glacial processes. Overall the results of this study indicate that porphyry Cu indicator minerals in till samples may be useful in the exploration for porphyry deposits in SW Alaska.

Alaska↗
USGS2011

Investigation of the potential for concealed base-metal mineralization at the Drenchwater Creek Zn-Pb-Ag occurrence, northern Alaska, using geology, reconnaissance geochemistry, and airborne electromagnetic geophysics

In 2005, the U.S. Geological Survey, Bureau of Land Management, and State of Alaska cooperated on an investigation of the mineral potential of a southern part of the National Petroleum Reserve in Alaska, Howard Pass quadrangle, to provide background information for future land-use decisions. The investigation incorporated an airborne electromagnetic (EM) survey covering 1,500 mi 2 (~3,900 km 2 ), including flight lines directly over the Drenchwater Creek sediment-hosted Zn-Pb-Ag occurrence, the largest known base-metal occurrence in the survey area. Samples from the mineralized outcrop and rubblecrop contain metal concentrations that can exceed 11 percent Zn+Pb, with appreciable amounts of Ag. Soil samples with anomalous Pb concentrations are distributed near the sulfide-bearing outcrops and along a >2.5 km zone comprising mudstone, shale, and volcanic rocks of the Kuna Formation. No drilling has taken place at the Drenchwater occurrence, so alternative data sources (for example, geophysics) are especially important in assessing possible indicators of mineralization. Data from the 2005 electromagnetic survey define the geophysical character of the rocks at Drenchwater and, in combination with geological and surface-geochemical data, can aid in assessing the possible shallow (up to about 50 m), subsurface lateral extent of base-metal sulfide accumulations at Drenchwater. A distinct >3-km-long electromagnetic conductive zone (observed in apparent resistivity maps) coincides with, and extends further westward than, mineralized shale outcrops and soils anomalously high in Pb concentrations within the Kuna Formation; this conductive zone may indicate sulfide-rich rock. Models of electrical resistivity with depth, generated from inversion of electromagnetic data, which provide alongflight-line conductivity-depth profiles to between 25 and 50 m below ground surface, show that the shallow subsurface conductive zone occurs in areas of known mineralized outcrops and thins to the east. Broader, more conductive rock along the western ~1 km of the geophysical anomaly does not reach ground surface. These data suggest that the Drenchwater deposit is more extensive than previously thought. The application of inversion modeling also was applied to another smaller geochemical anomaly in the Twistem Creek area. The results are inconclusive, but they suggest that there may be a local conductive zone, possibly due to sulfides.

Professional Paper↗
USGS2011

Exploration case study using indicator minerals in till at the giant Pebble porphyry Cu-Au-Mo deposit, southwest Alaska, USA

The Pebble deposit in southwest Alaska (Fig. 1) contains one of the largest resources of copper and gold in the world. It includes a measured and indicated resource of 5,942 million tonnes (Mt) at 0.42% Cu, 0.35 g/t Au, and 250 ppm Mo (0.30% copper equivalent, CuEQ, cut off) and contains significant concentrations of Ag, Pd, and Re (Northern Dynasty Minerals 2011). The deposit remains open at depth. The Pebble West zone was discovered in 1989 by Cominco American. In 2005, Northern Dynasty Minerals Ltd. (NDM) discovered Pebble East, and in July 2007, NDM partnered with Anglo American to form the Pebble Limited Partnership (PLP). The U.S. Geological Survey began collaborative investigations with PLP in 2007 to identify techniques that will improve mineral exploration in covered terranes. The Pebble deposit is an ideal location for such a study because the deposit is undisturbed (except for drilling), is almost entirely concealed by post-mineral volcanic rocks and glacial deposits, and because its distribution is well constrained in the subsurface by PLP&rsquo;s drill-hole geology and geochemistry. An exploration method developed by Averill (2007) that utilizes porphyry copper indicator minerals (PCIMR) in glacial till samples was applied at Pebble; samples were collected up- and down-ice (of former glaciers) from the deposit. The distribution of several PCIMs identifies the deposit, which suggests that PCIMs may be useful in exploration for other concealed porphyry deposits in the region. In this study, we compare the efficacy of PCIMs relative to that of pond and stream sediments also collected in the deposit area. The Pebble deposit is located 380 km southwest of Anchorage, in the Bristol Bay region of southwest Alaska. There is no road network and access to the study area is by helicopter. The deposit is situated in a broad glacially sculpted topographic low at the head of three drainages, Talarik Creek, North Fork Koktuli River, and the South Fork Koktuli River (Fig. 1). The study area is in a zone of discontinuous permafrost and is masked by lichen-rich tundra vegetation.

Alaska↗
USGS2010

USGS exploration geochemistry studies at the Pebble porphyry Cu-Au-Mo deposit, Alaska— Pdf of presentation

From 2007 through 2010, scientists in the U.S. Geological Survey (USGS) have been conducting exploration-oriented geochemical and geophysical studies in the region surrounding the giant Pebble porphyry Cu-Au-Mo deposit in southwestern Alaska. The Cretaceous Pebble deposit is concealed under tundra, glacial till, and Tertiary cover rocks, and is undisturbed except for numerous exploration drill holes. These USGS studies are part of a nation-wide research project on evaluating and detecting concealed mineral resources. This report focuses on exploration geochemistry and comprises illustrations and associated notes that were presented as a case study in a workshop on this topic. The workshop, organized by L.G. Closs and R. Glanzman, is called 'Geochemistry in Mineral Exploration and Development,' presented by the Society of Economic Geologists at a technical conference entitled 'The Challenge of Finding New Mineral Resources: Global Metallogeny, Integrative Exploration and New Discoveries,' held at Keystone, Colorado, October 2-5, 2010.

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↗
USGS2009

An orientation soil survey at the Pebble Cu-Au-Mo porphyry deposit, Alaska

Soil samples were collected in 2007 and 2008 along three traverses across the giant Pebble Cu-Au-Mo porphyry deposit. Within each soil pit, four subsamples were collected following recommended protocols for each of ten commonly-used and proprietary leach/digestion techniques. The significance of geochemical patterns generated by these techniques was classified by visual inspection of plots showing individual element concentration by each analytical method along the 2007 traverse. A simple matrix by element versus method, populated with a value based on the significance classification, provides a method for ranking the utility of methods and elements at this deposit. The interpretation of a complex multi-element dataset derived from multiple analytical techniques is challenging. An example of vanadium results from a single leach technique is used to illustrate the several possible interpretations of the data.

Alaska↗
USGS2008

Geochemical data for samples collected in 2007 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-molydenum (Cu-Au-Mo) deposit in southwest Alaska. The Pebble deposit is extremely large and is almost entirely concealed by tundra, glacial deposits, and post-Cretaceous volcanic and volcaniclastic rocks. The deposit is presently being explored by Northern Dynasty Minerals, Ltd., and Anglo-American LLC. The USGS undertakes unbiased, broad-scale mineral resource assessments of government lands to provide Congress and citizens with information on national mineral endowment. Research on known deposits is also done to refine and better constrain methods and deposit models for the mineral resource assessments. The Pebble deposit was chosen for this study because it is concealed by surficial cover rocks, it is relatively undisturbed (except for exploration company drill holes), it is a large mineral system, and it is fairly well constrained at depth by the drill hole geology and geochemistry. The goals of the USGS study are (1) to determine whether the concealed deposit can be detected with surface samples, (2) to better understand the processes of metal migration from the deposit to the surface, and (3) to test and develop methods for assessing mineral resources in similar concealed terrains. This report presents analytical results for geochemical samples collected in 2007 from the Pebble deposit and surrounding environs. The analytical data are presented digitally both as an integrated Microsoft 2003 Access? database and as Microsoft 2003 Excel? files. The Pebble deposit is located in southwestern Alaska on state lands about 30 km (18 mi) northwest of the village of Illiamna and 320 km (200 mi) southwest of Anchorage (fig. 1). Elevations in the Pebble area range from 287 m (940 ft) at Frying Pan Lake just south of the deposit to 1146 m (3760 ft) on Kaskanak Mountain about 5 km (5 mi) to the west. The deposit is in an area of relatively subdued topographic relief with an elevation of around 300 m (1000 ft). This portion of Alaska is part of the subarctic regime mountains division, Yukon intermontane plateaus-tayga-meadow province ecoregion, as defined by Bailey (U.S. Forest Service, 2007). Between June 28th and July 12th, 2007, scientists from the USGS collected soil, water, stream sediment, vegetation, heavy-mineral concentrate, till, and rock samples from the deposit area. This report contains analytical results for soil, water, stream sediment, and vegetation samples. Analyses for the heavy-mineral concentrate, till, and rock samples are still in progress. The sampling was undertaken during relatively dry and stable weather conditions. Only minor scattered rain showers occurred during the sampling period, so surface conditions were largely unaffected by weather. The predominant sample media collected were soils and surface waters. Soil and water (mostly from ponds and springs, some from small creeks) samples were collected along a single 7.8 km-long (4.8 mi) east-west traverse across the Pebble East and Pebble West zones and from more distal background areas around Koktuli and Kaskanak Mountains. Sample sites are shown on figure 2 and plate 1, and locality coordinates are provided in the accompanying Access and Excel files named FieldSite. Water samples were analyzed by USGS laboratories with one subset analyzed by Activation Laboratories (Actlabs), as indicated below. Soils and stream sediments were analyzed for their total content by SGS Minerals Services under a contract with the USGS. Soil samples were also leached by selected partial-extraction leaching procedures and then analyzed by several commercial laboratories, as described below. Vegetation samples were analyzed as indicated below.

Alaska↗
USGS2007

Regional Fluid Flow and Basin Modeling in Northern Alaska

The foothills of the Brooks Range contain an enormous accumulation of zinc (Zn) in the form of zinc sulfide and barium (Ba) in the form of barite in Carboniferous shale, chert, and mudstone. Most of the resources and reserves of Zn occur in the Red Dog deposit and others in the Red Dog district; these resources and reserves surpass those of most deposits worldwide in terms of size and grade. In addition to zinc and lead sulfides (which contain silver, Ag) and barite, correlative strata host phosphate deposits. Furthermore, prolific hydrocarbon source rocks of Carboniferous and Triassic to Early Jurassic age generated considerable amounts of petroleum that may have contributed to the world-class petroleum resources of the North Slope. Deposits of Zn-Pb-Ag or barite as large as those in the Brooks Range are very rare on a global basis and, accordingly, multiple coincident favorable factors must be invoked to explain their origins. To improve our understanding of these factors and to contribute to more effective assessments of resources in sedimentary basins of northern Alaska and throughout the world, the Mineral Resources Program and the Energy Resources Program of the U.S. Geological Survey (USGS) initiated a project that was aimed at understanding the petroleum maturation and mineralization history of parts of the Brooks Range that were previously poorly characterized. The project, titled “Regional Fluid Flow and Basin Modeling in Northern Alaska,” was undertaken in collaboration with industry, academia, and other government agencies. This Circular contains papers that describe the results of the recently completed project. The studies that are highlighted in these papers have led to a better understanding of the following: The complex sedimentary facies relationships and depositional settings and the geochemistry of the sedimentary rocks that host the deposits (sections 2 and 3). The factors responsible for formation of the barite and zinc deposits (sections 4 and 5). The geochemical indicators or exploration tools that might be used to locate other large deposits of similar character in the Red Dog district and elsewhere (section 6). The isotopic compositions of barite and sulfide deposits (sections 7 and 8) The distribution and nature of phosphate and metalliferous oil shale localities (sections 9 and 10). The architecture, kinematics, and timing of the complex thrust systems that disrupted and redistributed the Carboniferous and younger rocks; these studies are necessary in order to make a realistic palinspastic reconstruction of the basin (sections 11 and 12). The nature and extent of the petroleum system sourced from Mississippian rocks (section 13).

Circular↗
USGS2005

Sediment-hosted lead-zinc deposits: A global perspective

Sediment-hosted Pb-Zn deposits contain the world’s greatest lead and zinc resources and dominate worldproduction of these metals. They are a diverse group of ore deposits hosted by a wide variety of carbonate andsiliciclastic rocks that have no obvious genetic association with igneous activity. A range of ore-forming processes in a variety of geologic and tectonic environments created these deposits over at least two billion years of Earth history. The metals were precipitated by basinal brines in synsedimentary and early diagenetic to low-grade metamorphic environments. The deposits display a broad range of relationships to enclosing host rocks that includes stratiform, strata-bound, and discordant ores. These ores are divided into two broad subtypes: Mississippi Valley-type (MVT) and sedimentary exhalative (SEDEX). Despite the “exhalative” component inherent in the term “SEDEX,” in this manuscript, direct evidence of an exhalite in the ore or alteration component is not essential for a deposit to be classified as SEDEX. The presence of laminated sulfides parallel to bedding is assumed to be permissive evidence for exhalative ores. The distinction between some SEDEX and MVT deposits can be quite subjective because some SEDEX ores replaced carbonate, whereas some MVT deposits formed in an early diagenetic environment and display laminated ore textures. Geologic and resource information are presented for 248 deposits that provide a framework to describe and compare these deposits. Nine of the 10 largest sediment-hosted Pb-Zn deposits are SEDEX. Of the deposits that contain at least 2.5 million metric tons (Mt), there are 35 SEDEX (excluding Broken Hill-type) deposits and 15 MVT (excluding Irish-type) deposits. Despite the skewed distribution of the deposit size, the two deposits types have an excellent correlation between total tonnage and tonnage of contained metal (Pb + Zn), with a fairly consistent ratio of about 10/1, regardless of the size of the deposit or district. Zinc grades are approximately the same for both, whereas Pb and Ag grades are about 25 percent greater for SEDEX deposits. The largest difference between SEDEX and MVT deposits is their Cu content. Three times as many SEDEX deposits have reported Cu contents, and the median Cu value of SEDEX deposits is nearly double that of MVT deposits. Furthermore, grade-tonnage values for MVT deposits compared to a subset of SEDEX deposits hosted in carbonate rocks are virtually indistinguishable. The distribution of MVT deposits through geologic time shows that they are mainly a Phanerozoic phenomenon. The ages of SEDEX deposits are grouped into two major groups, one in the Proterozoic and another in the Phanerozoic. MVT deposits dominantly formed in platform carbonate sequences typically located within extensional zones inboard of orogenic belts, whereas SEDEX deposits formed in intracontinental or failed rifts, and rifted continental margins. The ages of MVT ores are generally tens of millions of years younger than their host rocks; however, a few are close (<~5 m.y.) to the age of their host rocks. In the absence of direct dates for SEDEX deposits, their age of formation is generally constrained by relationships to sedimentary or diagenetic features in the rocks. These studies suggest that deposition of SEDEX ores was coeval with sedimentation or early diagenesis, whereas some deposits formed at least 20 m.y. after sedimentation. Fluid inclusion, isotopic studies, and deposit modeling suggest that MVT and SEDEX deposits formed from basin brines with similar temperatures of mainly 90° to 200°C and 10 to 30 wt percent NaCl equiv. Lead isotope compositions for MVT and SEDEX deposits show that Pb was mainly derived from a variety of crustal sources. Lead isotope compositions do not provide criteria that distinguish MVT from SEDEX subtypes. However, sulfur isotope compositions for sphalerite and galena show an apparent difference. SEDEX and MVT sulfur isotope compositions extend over a large range; however, most data for SEDEX ores have mainly positive isotopic compositions from 0 to 20 per mil. Isotopic values for MVT ores extend over a wider range and include more data with negative isotopic values. Given that there are relatively small differences between the metal character of MVT and SEDEX deposits and the fluids that deposited them, perhaps the most significant difference between these deposits is their de-positional environment, which is determined by their respective tectonic settings. The contrasting tectonic setting also dictates the fundamental deposit attributes that generally set them apart, such as host-rock lithology, deposit morphology, and ore textures. Brief discussions are also presented on two controversial sets of deposits: Broken Hill-type deposits and a subset of deposits in the MVT group located in the Irish Midlands, considered by some authors to be a distinct ore type (Irish type). There are no significant differences in grade tonnage values between MVT deposits and the subset that is described as Irish type. Most features of the Irish deposits are not distinct from the family of MVT deposits; however, the age of mineralization that is the same as or close to the age of the host rocks and the anomalously high fluid inclusion temperatures (up to 250°C) stand out as distinctly different from typical MVT ores. The dominance of bacteriogenic sulfur in the Irish ores commonly ascribed as uniquely Irish type is in fact no different from several MVT deposits or districts. A comparison of SEDEX and Broken Hill-type deposits shows that the latter deposits contain significantly higher contents of Ag and Pb relative to SEDEX deposits. In terms of median values, Broken Hill-type deposits are almost three times more enriched in Ag and one and a half times more enriched in Pb compared to other SEDEX deposits. Metamorphism is a characteristic feature but not a prerequisite for inclusion in the Broken Hill-type category, and known Broken Hill-type examples appear to occur in Paleo- to Mesoprotero-zoic terranes. Broken Hill-type deposits remain an enigmatic grouping; however, there is sufficient evidence to support their inclusion as a separate category of SEDEX deposits.

Book chapter↗
USGS2004

Nature of hydrothermal fluids at the shale-hosted Red Dog Zn-Pb-Ag deposits, Brooks Range, Alaska

The Red Dog Zn-Pb-Ag district in the western Brooks Range, northern Alaska, contains numerous shale-hosted Zn-Pb sulfide and barite deposits in organic-rich siliceous mudstone and shale, chert, and carbonate rocks of the Carboniferous Kuna Formation. The giant Red Dog shale-hosted deposits consist of a cluster of four orebodies (Main, Qanaiyaq, Aqqaluk, and Paalaaq) that lie within distinct thrust panels that offset a single ore deposit during the Mesozoic Brookian orogeny. These Zn-Pb-Ag-barite orebodies contain one of the world's largest reserves and resources of zinc. Fluid inclusions in samples of vein sphalerite, which accounts for about 20 percent of the ore in the Main deposit, and quartz that composes the bulk of the extensive silicification in the ore deposit, were studied by microthermometry, Raman spectrometry, and ion chromatography. The study of fluid inclusions in the vein sphalerite was limited by the intense postore deformation of the ore deposits. However, four primary aqueous fluid inclusion assemblages in vein sphalerite yield temperatures of homogenization of 115&deg; to 120&deg;C, 123&deg; to 127&deg;C, 110&deg; to 120&deg;C and 175&deg; to 180&deg;C. More abundant final-melting temperatures indicate that the fluid inclusions in sphalerite have salinities of about 14 to 19 wt percent NaCl equiv. The fluid inclusion electrolyte data show that the ore fluid responsible for the vein sphalerite derived its salinity from the evaporation of seawater. Considering the salinity of the fluid inclusions together with the electrolyte data, it is possible that the evaporative brine was initially about 30 wt percent saline fluid and that it mixed with a more dilute fluid somewhere along its flow path. The temperature, salinity, and electrolyte composition of vein sphalerite in the Red Dog deposits are remarkably similar to those characteristics in sphalerite veins near the Century zinc deposit, Australia. Together, these data compose the majority of information on the temperature and composition of sphalerite in deposits of this type. On the basis of data describing fluid inclusions in sphalerite and the geologic setting of the ore deposits, a "reflux brine" model is suggested for the Red Dog deposits. In this model, brines were produced in evaporative environments in supratidal carbonate facies of the Lisburne Group less than 100 km from the Red Dog deposits. These reflux brines may have infiltrated the underlying rocks of Endicott Group or fractured metasedimentary basement rocks. In the absence of a local heat source at the Red Dog deposits, the temperature of the ore fluids (~100&deg; to <200&deg;C) requires that the fluids circulated at depths between ~ 2.4 and 7.4 km. In the Red Dog area, the metalliferous fluids ascended into the organic-rich rocks of the Kuna Formation, probably along zones of active extensional faults or breaches in the shale aquitards overlying the aquifers in the Endicott Group. Fluid inclusions were also studied in the abundant quartz that constitutes the majority of the silica rock in the ore deposits. This postore quartz extensively replaced barite and was traditionally thought to be part of the main ore event. Primary fluid inclusion assemblages contain two-phase aqueous inclusions, single-phase inclusions of dense methane, or both. Primary assemblages that contain single-phase, dense-methane inclusions together with two-phase aqueous inclusions yield consistent homogenization temperatures that provide unequivocal evidence for the coeval trapping of immiscible gas and aqueous fluids. The densities of the methane inclusions, together with the temperature of homogenization of coexisting aqueous fluid inclusions, show that these fluid inclusions were trapped between pressures of 800 and 3,400 bars and temperatures between 187&deg; and 214&deg;C. The pressures obtained provide unequivocal evidence that the quartz formed after ore deposition in the Carboniferous because such high fluid pressures could only have been produced from thrust loading during the Mesozoic Brookian orogeny. The observed large variation in pressure is best explained by transient fluid pressures from hydrostatic to lithostatic conditions during thrust loading. The 3,400 bars pressure corresponds with about 12 km of lithostatic burial, whereas the lower pressures (800 bars) correspond with about 8 km of hydrostatic pressure. Because of their low salinity (0-5 wt % NaCl equiv) the electrolyte compositions of the quartz fluid inclusions do not constrain their origin.

Alaska↗
USGS2002

Cripple Creek and other alkaline-related gold deposits in the Southern Rocky Mountains, USA: Influence of regional tectonics

Alkaline-related epithermal vein, breccia, disseminated, skarn, and porphyry gold deposits form a belt in the southern Rocky Mountains along the eastern edge of the North American Cordillera. Alkaline igneous rocks and associated hydrothermal deposits formed at two times. The first was during the Laramide orogeny (about 70–40 Ma), with deposits restricted spatially to the Colorado mineral belt (CMB). Other alkaline igneous rocks and associated gold deposits formed later, during the transition from a compressional to an extensional regime (about 35–27 Ma). These younger rocks and associated deposits are more widespread, following the Rocky Mountain front southward, from Cripple Creek in Colorado through New Mexico. All of these deposits are on the eastern margin of the Cordillera, with voluminous calc-alkaline rocks to the west. The largest deposits in the belt include Cripple Creek and those in the CMB. The most important factor in the formation of all of the gold deposits was the near-surface emplacement of relatively oxidized volatile-rich alkaline magmas. Strontium and lead isotope compositions suggest that the source of the magmas was subduction-modified subcontinental lithosphere. However, Cripple Creek alkaline rocks and older Laramide alkaline rocks in the CMB that were emplaced through hydrously altered LREE-enriched rocks of the Colorado (Yavapai) province have 208 Pb/ 204 Pb ratios that suggest these magmas assimilated and mixed with significant amounts of lower crust. The anomalously hot, thick, and light crust beneath Colorado may have been a catalyst for large-scale transfer of volatiles and crustal melting. Increased dissolved H 2 O (and CO 2 , F, Cl) of these magmas may have resulted in more productive gold deposits due to more efficient magmatic-hydrothermal systems. High volatile contents may also have promoted Te and V enrichment, explaining the presence of fluorite, roscoelite (vanadium-rich mica) and tellurides in the CMB deposits and Cripple Creek as opposed to deposits to the south. Deep-seated structures of regional extent that formed during the Proterozoic allowed the magmas to rise to shallow crustal levels. Proterozoic sites of intrusions at 1.65, 1.4, and 1.1 Ga were also important precursors to alkaline-related gold deposits. Many of the larger gold deposits are located at sites of Proterozoic intrusions, and are localized at the intersection of northeast-trending ductile shear zones formed during Mesoproterozoic deformation, and an important north-trending fault formed during 1.1 Ga rifting.

Colorado, New Mexico↗
USGS1995

The Alaska Mineral Resource Assessment Program: Background information to accompany geologic and mineral-resource maps of the Killik River 1°x3° quadrangle, northern Alaska

This report summarizes results of integrated geological, geochemical, and geophysical field and laboratory studies conducted by the U.S. Geological Survey in the Killik River 1°x3° quadrangle, Brooks Range, northern Alaska. These studies were designed to provide an assessment of the mineral resources of the quadrangle. The geological and geochemical data were the primary sources of information used to assess the mineral-resource potential of the quadrangle. The presence of permissive host rocks and favorable geochemical signatures in stream- and lake-sediment and (or) heavy-mineral-concentrate samples were used to infer the potential for undiscovered resources of minerals in deposits of different types. The Killik River quadrangle may contain undiscovered resources of preciousand base-metals (principally lead, silver, and zinc) in veins and breccias hosted by Devonian and Mississippian clastic sedimentary rocks or in stratiform massive sulfide deposits hosted primarily by Mississippian and Pennsylvanian sedimentary rocks. Resources of barium, phosphate, and manganese may be present in Mississippian to Jurassic sedimentary rocks. Cretaceous fluvial sedimentary rocks in the northern part of the quadrangle are permissive hosts for uranium deposits, as well as for placer deposits of heavy minerals such as gold and chromite; however, geochemical data suggest that the potential for deposits of this type is low. Numerous coal beds are present in the Cretaceous rocks throughout the northern part of the quadrangle, but the limited thickness and lateral extent of most of the beds reduce their resource potential. Although the southern third of the quadrangle has no potential for petroleum resources, there is potential in the northern two-thirds.

Alaska↗