The U.S. Geological Survey has completed a series of geologic, mineral resource, and environmental assessment studies in the Rocky Mountains of central Colorado, from Leadville eastward to the range front and from New Mexico to the Wyoming border. Regional stream-sediment geochemical maps, useful for assessing mineral resources and environmental effects of historical mining activities, were produced as part of the study. The data portrayed in this 56-parameter portfolio of landscape geochemical maps serve as a geochemical baseline for the region, indicate element abundances characteristic of various lithologic terranes, and identify gross anthropogenic effects of historical mining. However, although reanalyzed in this study by modern, sensitive methods, the majority of the stream-sediment samples were collected in the 1970s. Thus, metal concentrations portrayed in these maps represent stream-sediment geochemistry at the time of collection.

Under the World Bank-funded Second Projet de Renforcement Institutionnel du Secteur Minier de la Republique Islamique de Mauritanie (PRISM-II), this Phase V geochemistry report follows earlier Phase I and Phase II summary reports on geochemical data (U.S. Geological Survey, 2007 and Eppinger, 2007; respectively). All the reports are based on evaluations of geochemical data collected in 1999-2004 under an earlier World Bank program (PRISM-I) by the British Geological Survey (BGS) and the Bureau de Recherches Géologiques et Minières (BRGM) for the Government of Mauritania. There are no associated Phase III or IV reports. The geochemical sample media collected by the BGS and BRGM under the PRISM-I contract included rock, sediment, regolith, and soil samples. Details on sample collection procedures are in unpublished reports available from PRISM. These samples were analyzed under PRISM-I contract by ALS Chemex Laboratories using various combinations of modern methods including fire-assay inductively coupled plasma-atomic emission spectrometry (ICPAES) and ICP-mass spectrometry (ICP-MS) for Au; multi-acid digestion, atomic absorption spectroscopy (AAS) for Ag and As; 47-element, four-acid digestion, ICP-MS; 27-element, fouracid digestion, ICP-AES; special four-acid ICP-MS techniques for Pt and B; fire assay followed by ICP-AES for platinum-group elements; whole-rock analyses by wavelength dispersive X-ray fluorescence (XRF); special techniques for loss-on-ignition, inorganic C, and total S; and special ore-grade AAS techniques for Ag, Au, Cu, Ni, Pb, and Zn. Around 30,000 samples were analyzed by at least one technique. However, it is stressed here that: (1) there was no common sample medium collected at all sites, likely due to the vast geological and geomorphologic differences across the country, (2) the sample site distribution is very irregular, likely due in part to access constraints and sand dune cover, and (3) there was no common across-the-board trace element analytical package used for all samples. These three aspects fundamentally affect the ability to produce country-wide geochemical maps of Mauritania. Gold (Au), silver (Ag), and arsenic (As) were the three elements that were most commonly analyzed.

In 1996, at the request of the Government of the Islamic Republic of Mauritania, a team of U.S. Geological Survey (USGS) scientists produced a strategic plan for the acquisition, improvement and modernization of multidisciplinary sets of data to support the growth of the Mauritanian minerals sector and to highlight the geological and mineral exploration potential of the country. In 1999, the Ministry of Petroleum, Energy, and Mines of the Islamic Republic of Mauritania implemented a program for the acquisition of the recommended basic geoscientific information, termed the first Projet de Renforcement Institutionnel du Secteur Minier (Project for Institutional Capacity Building in the Mining Sector, PRISM-I). As a result of the PRISM-I efforts, a great deal of new geological, geophysical, geochemical, remote sensing, and hydrological data became available for evaluation and synthesis. However, the Ministry of Petroleum, Energy, and Mines recognized that additional work was required to extract the full benefit of the data before it could be of greatest use to the international community and of benefit to the Mauritanian minerals and development sector. To achieve this benefit, the Ministry of Petroleum, Energy, and Mines implemented a second Projet de Renforcement Institutionnel du Secteur Minier (PRISM-II) in 2006 to consolidate, synthesize, and interpret all of the existing data, create a new 1:1,000,000 scale geologic map, and define the mineral resource potential of the country. A consortium in which the USGS was the lead scientific agency carried out the majority of the PRISM-II work. In 2008, the USGS Mauritania Minerals Project was interrupted due to political changes in Mauritania. PRISM-II work resumed in 2011, and was completed in 2013 with the delivery of over 40 separate written reports and plates, an access file containing the Mauritanian National Mineral Deposits Database, and an interactive GIS containing all of the multi-disciplinary data and interpretive areas of mineral resource potential in Mauritania. This report contains the USGS results of the PRISM-II Mauritania Minerals Project and is presented in cooperation with the Ministry of Petroleum, Energy, and Mines of the Islamic Republic of Mauritania. The Report is composed of separate chapters consisting of multidisciplinary interpretive reports with accompanying plates on the geology, structure, geochronology, geophysics, hydrogeology, geochemistry, remote sensing (Landsat TM and ASTER), and SRTM and ASTER digital elevation models of Mauritania. The syntheses of these multidisciplinary data formed the basis for additional chapters containing interpretive reports on 12 different commodities and deposit types known to occur in Mauritania, accompanied by countrywide mineral resource potential maps of each commodity/deposit type. The commodities and deposit types represented include: (1) Ni, Cu, PGE, and Cr deposits hosted in ultramafic rocks; (2) orogenic, Carlin-like, and epithermal gold deposits; (3) polymetallic Pb-Zn-Cu vein deposits; (4) sediment-hosted Pb-Zn-Ag deposits of the SEDEX and Mississippi Valley-type; (5) sediment-hosted copper deposits; ( 6) volcanogenic massive sulfide deposits; (7) iron oxide copper-gold deposits; (8) uranium deposits; (9) Algoma-, Superior-, and oolitic-type iron deposits; (10) shoreline Ti-Zr placer deposits; (11) incompatible element deposits hosted in pegmatites, alkaline rocks, and carbonatites, and; (12) industrial mineral deposits. Additional chapters include the Mauritanian National Mineral Deposits Database are accompanied by an explanatory text and the Mauritania Minerals Project GIS that contains all of the interpretive layers created by USGS scientists. Raw data not in the public domain may be obtained from the Ministry of Petroleum, Energy, and Mines in Nouakchott, Mauritania.

The U.S. Geological Survey (USGS) has investigated the environmental geochemistry of a group of unmined volcanogenic massive sulfide (VMS) deposits in the Bonnifield mining district, Alaska Range, east-central Alaska. The spectacularly colored Red Mountain deposit is the best exposed of these and provides excellent baseline geochemical data for natural environmental impacts of acidic rock drainage, metal dissolution and transport, and acidic salt and metal precipitation from an exposed and undisturbed VMS deposit.

This report is a revised model for a specific type of cobalt-copper-gold (Co-Cu-Au) deposit that will be evaluated in the next U.S. Geological Survey (USGS) assessment of undiscovered mineral resources in the United States (see Ferrero and others, 2012). Emphasis is on providing an up-to-date deposit model that includes both geologic and geoenvironmental aspects. The new model presented here supersedes previous USGS models by Earhart (1986) and Evans and others (1995), which are based solely on deposits in the Blackbird mining district of central Idaho. This report is a broader synthesis of information on 19 Co-Cu-Au deposits occurring in predominantly metasedimentary successions worldwide (table 1–1) that generally share common geologic, mineralogical, and geochemical features; preliminary summary versions were presented in Slack and others (2010) and Slack and others (2011), which are superseded by this report. As defined herein, the individual Co-Cu-Au deposits are located more than 500 meters from similar deposits and contain 0.1 percent or more by weight of Co in ore or mineralized rock; some deposits included in the database lack reported average Co grades, but they contain high Co concentrations, at least locally. Most of the deposits also have high As contents, present in Co arsenide and sulfarsenide minerals. Type examples of the Co-Cu-Au deposits are those in the Blackbird district, Skuterud in Norway, and Kouvervarra and Juomasuo in Finland. Some deposits in the database have low grades for Cu (for example, NICO in Canada) or Au (for example, Lemmonlampi in Finland), but these deposits are included because their geological, mineralogical, and alteration features are similar to those of the type examples. Several deposits included in the model are partly hosted by metavolcanic or metaigneous rocks (including granite), but regionally these deposits are within metasedimentary successions; no deposits are wholly within granite or other plutonic igneous intrusions.Despite having a lower average Co grade, the Mt. Cobalt deposit in Australia is included here because it has past Co production from higher-grade ore zones (Nisbet and others, 1983). The Black Pine deposit in the Idaho cobalt belt is included because it contains mineable Co- and Au-rich lenses within Cu-rich mineralized zones (Formation Metals, Inc., 2012). Six deposits that lack data for average Co grades are also included because each reportedly contains abundant Co (>0.1 weight percent Co), at least locally. Many of the deposits are noteworthy as possible resources of Ag, Bi, W, Ni, Y, REE, and (or) U. Detailed data on the deposits listed in table 1–1, including references, are available in appendix 1. Significantly, the grouping in this report of Co-Cu-Au deposits in metasedimentary rocks into a single model includes deposits that other workers have previously classified in different ways. For background information, a global overview of different types of Co deposits worldwide is given in Smith (2001).Additional geologically and compositionally similar deposits are known, but have average Co grades less than 0.1 percent. Most of these deposits contain cobalt-rich pyrite and lack appreciable amounts of distinct Co sulfide and (or) sulfarsenide minerals. Such deposits are not discussed in detail in the following sections, but these deposits may be relevant to the descriptive and genetic models presented below. Examples include the Scadding Au-Co-Cu deposit in Ontario, Canada; the Vähäjoki Co-Cu-Au deposit in Finland; the Tuolugou Co-Au deposit in Qinghai Province, China; the Lala Co-Cu-UREE deposit in Sichuan Province, China; the Guelb Moghrein Cu-Au-Co deposit in Mauritania; and the Great Australia Co-Cu, Greenmount Cu-Au-Co, and Monakoff Cu-Au-Co-UAg deposits in Queensland, Australia. Detailed information on these deposits is presented in appendix 2.

The Cretaceous Pebble porphyry Cu-Au-Mo deposit is covered by tundra and glacigenic sediments. Pb-Sr-Nd measurements were done on sediments and soils to establish baseline conditions prior to the onset of mining operations and contribute to the development of exploration methods for concealed base metal deposits of this type. Pebble rocks have a moderate range for 206Pb/204Pb = 18.574 to 18.874, 207Pb/204Pb = 15.484 to 15.526, and 208Pb/204Pb = 38.053 to 38.266. Mineralized granodiorite shows a modest spread in 87Sr/86Sr (0.704354–0.707621) and 143Nd/144Nd (0.512639–0.512750). Age-corrected (89 Ma) values for the granodiorite yield relatively unradiogenic Pb (e.g., 207Pb/204Pb <15.52), low values of 87Sr/86Sr, and positive values of ɛ Nd (1.00–4.52) that attest to a major contribution of mantle-derived source rocks. Pond sediments and soils have similar Pb isotope signatures and 87Sr/86Sr and 143Nd/144Nd values that resemble the mineralized granodiorites. Glacial events have obscured the recognition of isotope signatures of mineralized rocks in the sediments and soils. Baseline radiogenic isotope compositions, prior to the onset of mining operations, reflect natural erosion, transport and deposition of heterogeneous till sheets that included debris from barren rocks, mineralized granodiorite and sulfides from the Pebble deposit, and other country rocks that pre- and postdate the mineralization events. Isotopic variations suggest that natural weathering of the deposit is generally reflected in these surficial materials. The isotope data provide geochemical constraints to glimpse through the extensive cover and together with other geochemical observations provide a vector to concealed mineralized rocks genetically linked with the Pebble deposit.

The Pebble deposit is located about 320 km southwest of and 27 km northwest of the village of Iliamna in Alaska (Fig. 1A). It is one of the largest porphyry deposits in terms of contained Cu (Fig. 2A) and it has the largest Au endowment of any porphyry deposit in the world (Fig. 2B). The deposit comprises the Pebble West and Pebble East zones that represent two coeval hydrothermal centers within a single system (Lang et al., 2013). Together the measured and indicated resources total 5,942 million metric tons (Mt) at 0.42% Cu, 0.35 g/t Au, and 250 ppm Mo with an inferred resource of 4,835 Mt at 0.24% Cu, 0.26 g/t Au, and 215 ppm Mo. In addition, the deposit contains significant concentrations of Ag, Pd, and Re (Northern Dynasty Minerals, 2011). Fig. 1 A. Map of southwestern Alaska showing extensive cover by Tertiary and Quaternary deposits and locations of known mineral deposits with outline of the Pebble district. Distribution of Tertiary and Quaternary deposits is after Wilson et al. (2006). B. Map of the Pebble district showing locations of deposits (all deposits are named as zones, for example the 37 zone). Regional mineral deposit locations are from U.S. Geological Survey (2008); descriptions of district deposits are in Hawley (2004) and Lang et al. (2013). Boundaries for the Pebble West and Pebble East zones are based on >0.3% and >0.6% Cu equiv, respectively. Fig. 2 The 25 largest porphyry copper deposits, based on (A) the tonnage of contained copper metal and (B) the tonnage of contained gold (modified from Cooke et al., 2005). The classifications of deposits into Cu ± Mo, Cu-Au-Mo, and Cu-Au types are described in Cooke et al. (2005). Unlike most porphyry deposits in southwest Alaska, which are <75 Ma (U.S. Geological Survey, 2008), Re-Os …

Exploration geochemical and mineralogical studies by the U.S. Geological Survey at the Pebble porphyry Cu-Au-Mo deposit were designed 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 existing methods for assessing concealed mineral resources and/or develop new ones. Surface water (ponds, streams, and springs), pond and stream sediment, soils subjected to various leaching techniques, and glacial till samples were collected. The tilted nature of the undisturbed orebody, varying depth of cover, and later glacial processes, strongly influence the geochemical responses and processes active on the various sample media. The multimedia approach aids in identifying possible processes that caused the significant geochemical variations within and among the various media. These processes include the following:1. In the Pebble West zone, thin cover and local exposure of the orebody have facilitated the oxidation of pyrite and other sulfides, and associated ferrous-ferric iron reactions, resulting in the local natural acidification of ponds observed in the West zone, and in associated metal anomalies in waters, sediments, and soils.2. In contrast, the East zone is concealed by both glacial deposits and underlying thick cover rocks, which precludes the oxidation of sulfides in the underlying orebody. Low-level geochemical anomalies in circumneutral spring and pond waters from the East zone are discernible only by using high resolution-inductively coupled plasma-mass spectrometry with lower limits of determination two and perhaps three orders of magnitude lower than traditional methods. A variety of partial leaches of soils over the East zone reveal geochemical anomalies in a similar suite of elements that may be related to upwelling waters from depth along graben-bounding faults.3. The indicator minerals gold, jarosite, and andradite in till reveal a displaced mineralogical anomaly to the west and south of the Pebble orebody, as ore-related minerals were scraped from the orebody and deposited in till downice of the deposit. Geochemical anomalies in pond water and sediment over the displaced till are attributed to the ore-related minerals in till. This orientation study demonstrates the strong control of local geologic and geochemical settings on the effectiveness of different traditional and newer reconnaissance geochemical exploration techniques and thus has important implications for exploration.

Research Article| October 01, 2012 Mauritania: A Greenfields Exploration Opportunity in Northwestern Africa C.D. Taylor; C.D. Taylor 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 †Corresponding author: e-mail, ctaylor@usgs.gov Search for other works by this author on: GSW Google Scholar E.D. Anderson; E.D. Anderson 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 Search for other works by this author on: GSW Google Scholar D.C. Bradley; D.C. Bradley 2U.S. Geological Survey, 4200 University Dr., Anchorage, AK 99709 Search for other works by this author on: GSW Google Scholar G. Beaudoin; G. Beaudoin 3Université Laval, 1065, Avenue de la Médecine, Quebec City, Quebec, Canada G1V 0A6 Search for other works by this author on: GSW Google Scholar M.A. Cosca; M.A. Cosca 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 Search for other works by this author on: GSW Google Scholar R.G. Eppinger; R.G. Eppinger 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 Search for other works by this author on: GSW Google Scholar G. L. Fernette; G. L. Fernette 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 Search for other works by this author on: GSW Google Scholar C.A. Finn; C.A. Finn 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 Search for other works by this author on: GSW Google Scholar M.J. Friedel; M.J. Friedel 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 Search for other works by this author on: GSW Google Scholar S.A. Giles; S.A. Giles 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 Search for other works by this author on: GSW Google Scholar R. J. Goldfarb; R. J. Goldfarb 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 Search for other works by this author on: GSW Google Scholar J.D. Horton; J.D. Horton 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 Search for other works by this author on: GSW Google Scholar G.K. Lee; G.K. Lee 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 Search for other works by this author on: GSW Google Scholar E. E. Marsh; E. E. Marsh 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 Search for other works by this author on: GSW Google Scholar J. L. Mauk; J. L. Mauk 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 Search for other works by this author on: GSW Google Scholar H.A. Motts; H.A. Motts 2U.S. Geological Survey, 4200 University Dr., Anchorage, AK 99709 Search for other works by this author on: GSW Google Scholar M.Y. Ould El Joud; M.Y. Ould El Joud 4Unité de Coordination du Projet Minier, B.P. 5430, Nouakchott, Mauritania Search for other works by this author on: GSW Google Scholar S. Ould Soueidatt; S. Ould Soueidatt 4Unité de Coordination du Projet Minier, B.P. 5430, Nouakchott, Mauritania Search for other works by this author on: GSW Google Scholar A. Ould Taleb Mohamed; A. Ould Taleb Mohamed 5Ministère du Pétrole, de l’Energie, et des Mines, B.P. 5430, Nouakchott, Mauritania Search for other works by this author on: GSW Google Scholar B.W. Rockwell B.W. Rockwell 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 Search for other works by this author on: GSW Google Scholar Author and Article Information C.D. Taylor 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 E.D. Anderson 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 D.C. Bradley 2U.S. Geological Survey, 4200 University Dr., Anchorage, AK 99709 G. Beaudoin 3Université Laval, 1065, Avenue de la Médecine, Quebec City, Quebec, Canada G1V 0A6 M.A. Cosca 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 R.G. Eppinger 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 G. L. Fernette 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 C.A. Finn 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 M.J. Friedel 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 S.A. Giles 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 R. J. Goldfarb 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 J.D. Horton 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 G.K. Lee 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 E. E. Marsh 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 J. L. Mauk 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 H.A. Motts 2U.S. Geological Survey, 4200 University Dr., Anchorage, AK 99709 M.Y. Ould El Joud 4Unité de Coordination du Projet Minier, B.P. 5430, Nouakchott, Mauritania S. Ould Soueidatt 4Unité de Coordination du Projet Minier, B.P. 5430, Nouakchott, Mauritania A. Ould Taleb Mohamed 5Ministère du Pétrole, de l’Energie, et des Mines, B.P. 5430, Nouakchott, Mauritania B.W. Rockwell 1U.S. Geological Survey, Box 25046 Federal Center, MS-973, Denver, CO 80225-0046 †Corresponding author: e-mail, ctaylor@usgs.gov Publisher: Society of Economic Geologists First Online: 13 Aug 2021 Online Issn: 1550-2961 Print Issn: 1550-297X © 2012 The Society of Economic Geologists, IncThe Society of Economic Geologists, Inc SEG Discovery (2012) (91): 1–17. https://doi.org/10.5382/SEGnews.2012-91.fea Article history First Online: 13 Aug 2021 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation C.D. Taylor, E.D. Anderson, D.C. Bradley, G. Beaudoin, M.A. Cosca, R.G. Eppinger, G. L. Fernette, C.A. Finn, M.J. Friedel, S.A. Giles, R. J. Goldfarb, J.D. Horton, G.K. Lee, E. E. Marsh, J. L. Mauk, H.A. Motts, M.Y. Ould El Joud, S. Ould Soueidatt, A. Ould Taleb Mohamed, B.W. Rockwell; Mauritania: A Greenfields Exploration Opportunity in Northwestern Africa. SEG Discovery 2012;; (91): 1–17. doi: https://doi.org/10.5382/SEGnews.2012-91.fea Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySEG Discovery Search Advanced Search With a land area of approx. 1,000,000 km2 along the western edge of the Sahara desert and 90% covered by gravel and sand dunes (Fig. 1, inset), the Islamic Republic of Mauritania may not be on the top of many exploration managers’ lists of places to expend their exploration budgets. The harsh desert climate and lack of water keep most of the country’s population of 3 million confined to four or five cities along the Atlantic coast and Senegal River and ensure that transportation corridors and infrastructure development in the interior of the country are limited. The... 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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.

The distribution of Cu, Co, As and Fe was studied downstream from mines and deposits in the Idaho Cobalt Belt (ICB), the largest Co resource in the USA. To evaluate potential contamination in ecosystems in the ICB, mine waste, stream sediment, soil, and water were collected and analyzed for Cu, Co, As and Fe in this area. Concentrations of Cu in mine waste and stream sediment collected proximal to mines in the ICB ranged from 390 to 19,000μg/g, exceeding the USEPA target clean-up level and the probable effect concentration (PEC) for Cu of 149μg/g in sediment; PEC is the concentration above which harmful effects are likely in sediment dwelling organisms. In addition concentrations of Cu in mine runoff and stream water collected proximal to mines were highly elevated in the ICB and exceeded the USEPA chronic criterion for aquatic organisms of 6.3μg/L (at a water hardness of 50mg/L) and an LC50 concentration for rainbow trout of 14μg/L for Cu in water. Concentrations of Co in mine waste and stream sediment collected proximal to mines varied from 14 to 7400μg/g and were highly elevated above regional background concentrations, and generally exceeded the USEPA target clean-up level of 80μg/g for Co in sediment. Concentrations of Co in water were as high as in 75,000μg/L in the ICB, exceeding an LC50 of 346μg/L for rainbow trout for Co in water by as much as two orders of magnitude, likely indicating an adverse effect on trout. Mine waste and stream sediment collected in the ICB also contained highly elevated As concentrations that varied from 26 to 17,000μg/g, most of which exceeded the PEC of 33μg/g and the USEPA target clean-up level of 35μg/g for As in sediment. Conversely, most water samples had As concentrations that were below the 150μg/L chronic criterion for protection of aquatic organisms and the USEPA target clean-up level of 14μg/L. There is abundant Fe oxide in streams in the ICB and several samples of mine runoff and stream water exceeded the chronic criterion for protection of aquatic organisms of 1000μg/L for Fe. There has been extensive remediation of mined areas in the ICB, but because some mine waste remaining in the area contains highly elevated Cu, Co, As and Fe, inhalation or ingestion of mine waste particulates may lead to human exposure to these elements.

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.

The porphyry copper indicator mineral (PCIM (R)) method was recently applied at the Pebble porphyry deposit. Andradite garnet, Mn-epidote, gold and jarosite are the most useful PCIMs in till samples. Samples near Pebble contain 10-20 grains/10 kg of garnet, whereas samples with the most grains (>40) occur southwest in close proximity to smaller deposits with associated skarn mineralization. The distribution of Mn-epidote mimics that of garnet. All samples collected contain some gold. However, tills immediately west and down-ice of Pebble contain 3-30 times more gold, and the overall number of grains decreases in the down-ice direction. Furthermore, samples near Pebble contain mostly pristine and modified grains whereas distal samples have primarily re-shaped grains. Most gold in the deposit is contained in chalcopyrite; therefore, the pristine grains likely reflect liberation during in situ weathering of transported chalcopyrite grains. Jarosite is abundant in samples adjacent to and down-ice of Pebble and near smaller occurrences to the southwest. Most jarosite grains are detrital and preliminary age dates (>5 Ma) suggest it formed prior to glaciation. Overall, the results indicate that PCIMs are useful for exploration of porphyry deposits in southwest Alaska.

Eppinger, R. G., Fuge, R. (2009). Natural Low-pH Environments Unaffected by Human Activity Preface. Applied Geochemistry, 24, (2), Special Issue, 189-190.

In 2007, the U.S. Geological Survey began a multidisciplinary study in southwest Alaska to investigate the setting and detectability of mineral deposits in concealed volcanic and glacial terranes. The study area hosts the world-class Pebble porphyry Cu-Au-Mo deposit, and through collaboration with the Pebble Limited Partnership, a range of geophysical and geochemical investigations was carried out in proximity to the deposit. The deposit is almost entirely concealed by tundra, glacial deposits, and post-mineralization volcanic rocks. The discovery of mineral resources beneath cover is becoming more important because most of the mineral resources at the surface have already been discovered. Research is needed to identify ways in which to assess for concealed mineral resources. This report presents the uninterpreted geophysical measurements and geochemical and mineralogical analytical data from samples collected during the summer field seasons from 2007 to 2010, and makes the data available in a single Geographic Information System (GIS) database.

A compilation of data on global Co-Cu-Au deposits in metasedimentary rocks refines previous descriptive models for their occurrence and provides important information for mineral resource assessments and exploration programs. This compilation forms the basis for a new classification of such deposits, which is speculative at this early stage of research. As defined herein, the Co-Cu-Au deposits contain 0.1 percent or more by weight of Co in ore or mineralized rock, comprising disseminated to semi-massive Co-bearing sulfide minerals with associated Fe- and Cu-bearing sulfides, and local gold, concentrated predominantly within rift-related, siliciclastic metasedimentary rocks of Proterozoic age. Some deposits have appreciable Ag ? Bi ? W ? Ni ? Y ? rare earth elements ? U. Deposit geometry includes stratabound and stratiform layers, lenses, and veins, and (or) discordant veins and breccias. The geometry of most deposits is controlled by stratigraphic layering, folds, axial-plane cleavage, shear zones, breccias, or faults. Ore minerals are mainly cobaltite, skutterudite, glaucodot, and chalcopyrite, with minor gold, arsenopyrite, pyrite, pyrrhotite, bismuthinite, and bismuth; some deposits have appreciable tetrahedrite, uraninite, monazite, allanite, xenotime, apatite, scheelite, or molybdenite. Magnetite can be abundant in breccias, veins, or stratabound lenses within ore or surrounding country rocks. Common gangue minerals include quartz, biotite, muscovite, K-feldspar, albite, chlorite, and scapolite; many deposits contain minor to major amounts of tourmaline. Altered wall rocks generally have abundant biotite or albite. Mesoproterozoic metasedimentary successions constitute the predominant geologic setting. Felsic and (or) mafic plutons are spatially associated with many deposits and at some localities may be contemporaneous with, and involved in, ore formation. Geoenvironmental data for the Blackbird mining district in central Idaho indicate that weathering of abundant Fe, S, As, Co, and Cu in sulfide minerals of the deposits produces acidic waters, especially in pyrite-rich deposits; mine runoff has high concentrations of Fe, Cu, and Mn that exceed U.S. drinking water or aquatic life standards.

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.

s of the Symposium of the geology and mineral deposits of the Ajo and Lukeville 1° by 2° quadrangle, Arizona Presented at Tucson, Arizona