Karl, and two co-authors, reported in 2016 that Alaska has considerable potential for undiscovered and known, but undeveloped critical mineral resources. Critical minerals are those for which the United States imports more than half of its total supply and which are largely derived from nations that cannot be considered reliable trading partners. In this report, estimated mineral resource potential and certainty for the state of Alaska are analyzed and mapped for the following six selected groups of mineral deposit types that may each contain one or more critical minerals:
1) rare earth element (REE) deposits with or without thorium (Th), yttrium (Y), niobium (Nb), uranium (U), and zirconium (Zr), associated with peralkaline to carbonatitic intrusive rocks;
2) placer and paleoplacer gold (Au) deposits that in some places might also produce platinum group elements (PGE), chromium (Cr), tin (Sn), tungsten (W), silver (Ag), or titanium (Ti);
3) platinum group element (PGE) deposits with or without cobalt (Co), chromium (Cr), nickel (Ni), titanium (Ti), and vanadium (V), associated with mafic to ultramafic intrusive rocks; (
4) carbonate-hosted copper (Cu) deposits with Ag, and possibly cobalt (Co), germanium (Ge), and gallium (Ga);
5) sandstone-hosted uranium (U) deposits that in some deposits might also produce V or Cu; and (
6) tin (Sn)-tungsten (W)-molybdenum (Mo) deposits, possibly with indium (In), and (or) fluorspar associated with specialized granites.
This study used a data-driven, geographic information system (GIS)-implemented method to identify areas with mineral resource potential in Alaska. This method systematically and simultaneously analyzes geoscience data from multiple geospatially referenced datasets and uses individual sub-watersheds (12-digit hydrologic units [or HUCs]) as the spatial unit of classification. The final map output uses a red, yellow, and green color scheme to portray estimated relative potential (High, Medium, Low, Unknown [shown in gray]) for each of the six groups of mineral deposit types, and it indicates the certainty (High, Medium, Low,) of that estimate for any given HUC using dark, medium, and light shades of those colors, respectively. Accompanying tables describe the data layers employed to the score favorability for the presence of each mineral deposit group, the values assigned for specific analysis parameters, and the relative weighting of each data layer that contributes to estimated measures of potential and certainty.
Core datasets used include the Alaska Geochemical Database, Version 2.0 (AGDB2), the Alaska Division of Geological & Geophysical Surveys (ADGGS) web-based geochemical database, the digital “Geologic Map of Alaska” (Wilson and others, 2015), and the Alaska Resource Data File (ARDF). Maps accompanying this report illustrate the scores for estimated mineral resource potential for the six deposit groups on a statewide basis. Numerous areas in Alaska, some of them large, have high potential for one or more of the selected groups of deposit types within Alaska; selected examples of areas with high potential for critical minerals are described in the following paragraphs.
Areas in Alaska that have noteworthy potential for associated REE deposits include the Darby-Hogatza igneous belt and parts of the Ruby batholith in the Kokrines-Hodzana belt. Other areas that have high potential for REE deposits include the area surrounding a known carbonatite occurrence near
Tofty in the Hot Springs placer district, the Roy Creek area in east-central Alaska, and the Bokan Mountain to Dora Bay area in the southern part of southeastern Alaska. REE prospects at Bokan Mountain may have the greatest immediate potential for development of an REE resource in the state.
Placer gold potential is relatively high in many drainages widely scattered across the state. Prospecting for, and production from, placer gold deposits such as the Fairbanks, Circle, and Fortymile districts in east-central Alaska, the Aniak and Innoko districts in southwestern Alaska, the Nome district
on the Seward Peninsula, and the Valdez Creek district in south-central Alaska, were significant in the past, and continue at a reduced level in the present.
The western part of the Alaska Range might have additional untested and undeveloped placer gold potential. Areas that have high potential for PGE deposits associated with mafic to ultramafic intrusive rocks include the Angayucham terrane in the northwestern and south-central Brooks Range, the
Goodnews Bay area of southwestern Alaska; the Border Ranges belt in south-central Alaska, and the belt that includes Klukwan to Duke Island in southeastern Alaska.
The areas most deserving of additional investigation appear to be on the north flank of the central Brooks Range. Discrete belts that have high potential for carbonate-hosted copper deposits include the northeastern Brooks Range, the northern foreland of the Brooks Range, the Cosmos and Jade Hills areas,
the central Brooks Range, the central Seward Peninsula, and the Kennecott district in the Wrangell Mountains in south-central Alaska. Prospects in the northeastern Brooks Range are within the Arctic National Wildlife Refuge where prospecting and development are prohibited, but areas in east-central
Alaska and on the north flank of the Alaska Range are worthy of additional investigation to potentially distinguish them as basalt- or hydrothermal-related Cu deposits rather than Cu-skarn deposits.
Areas that have the highest potential for sandstone-hosted uranium potential are widely scattered across the state. Sandstone-hosted uranium in Alaska is mostly associated with areas proximal to felsic plutonic complexes, such as the Death Valley deposit adjacent to the Darby pluton on the eastern Seward Peninsula. Other areas in Alaska that show high potential for sandstone-hosted uranium deposits tend to be located in areas that also have potential for REE deposits associated with alkaline igneous rocks, such as in the Darby-Hogatza and Kokrines-Hodzana belts in central Alaska, and felsic plutons in the western Alaska Range.
In addition to the known tin province in the Lost River–Kougarok belt of intrusive rocks on the Seward Peninsula, areas that have high potential for tin-tungsten-molybdenum-tantalum-indium fluorspar deposits associated with ‘specialized’ granites also are widely scattered across Alaska. The main additional areas recognized for tin-tungsten and molybdenum potential include the central and northeastern Brooks Range, the Porcupine area southeast of the Brooks Range, the Hogatza and Kokrines-Hodzana intrusive belts that rim the Yukon-Koyukuk basin in central Alaska, the White Mountains and Yukon-Tanana uplands area in east-central Alaska, and the Coast Mountains, which include Groundhog Basin (Sn) and Quartz Hill (Mo) in southeastern Alaska. Felsic intrusive rocks in the western Alaska Range appear to have greater tin and molybdenum potential than has previously been recognized. Few areas in the United States have greater potential than Alaska for tin, tungsten, and molybdenum deposits, with or without indium and fluorspar, in association with specialized, highly fractionated granitic intrusions.
The authors concluded that a data-driven GIS process was used to screen and investigate the mineral resource potential for six deposit-type groups in Alaska. One or more examples of each deposit group, which may contain resources of one or more critical minerals, had previously been identified in Alaska. The six deposit type groups investigated include:
1) REE-Th-Y-Nb(-U-Zr) deposits associated with peralkaline to carbonatitic intrusive rocks,
2) placer and paleoplacer Au deposits,
3) PGE(-Co-Cr-Cu-Ni-Ti-V) deposits associated with mafic to ultramafic intrusive rocks,
4) carbonate-hosted Cu(-Co-Ag-Ge-Ga) deposits,
5) sandstone-hosted U(-V-Cu) deposits, and
6) Sn-W-Mo(-Ta-In-fluorspar) deposits associated with specialized granites.
For each of these groups, newly assembled datasets were used to refine an appropriate favorability scoring scheme so that known high potential areas (in particular, those that contain ARDF-record deposits and occurrences of the type under consideration) were identified.
The method was tested and then applied statewide. For each of the six deposit groups, some areas newly recognized as having high potential for additional deposits were identified and described. The resource-estimation process employed provides the means to assemble and analyze disparate datasets in a geospatial environment in order to quantify mineral-resource prospectivity across broad areas, in this case, the state of Alaska. Some data types are better adapted for use in this kind of analysis than others. For example, with regard to scoring schemes that favored certain types of igneous rocks, individual samples that have whole-rock and trace-element geochemical data constitute better data than those for generalized geologic map units, and scoring schemes could be weighted to address the robustness of datasets with both potential and certainty scores.
The data on which this resource potential estimate is based are not systematically or continuously distributed across the area that was evaluated. In particular, the number and spatial distribution of geochemistry sample localities differ significantly among different parts of the state (Granitto and
others, 2013). In some areas, ARDF records cluster around known occurrences, prospects, or mines, and (or) they cluster in areas that have the best access, such as along roads or rivers. In order to develop consistent resource-potential scores, methods were employed that were designed to equalize potentially skewed scores resulting from uneven sampling, such as choosing the single highest element value in HUCs that contain multiple stream-sediment samples.
The ability and opportunity to analyze and query the scoring results has been preserved by providing all of the data used to develop individual scoring components and score results for each deposit group (appendix E). For those datasets such as the AGDB2, ADGGS geochemical data, and ARDF that are publicly available, individual users can easily recompile the applied data and evaluate sampling trends for comparison to our results. Similarly, interested parties can integrate additional data with the datasets and apply different scoring methods to evaluate other types of inquiries for resource potential.
Each of the scoring schemes reported here was developed by trial and error. The time required to change the scoring schema and run a different estimate is relatively minor. Our approach promotes iterative data evaluation, which should result in ever-improving resource estimates. Each of the data layers used here consists of data that had been collected and reported previously. Industry and agency geologists have had access to much of these data for years, although not in a digital format suitable for geospatial manipulation. Although coverage for Alaska is spotty and incomplete, a huge amount of data is available to synthesize.
This study provides a new way to analyze large spatial datasets in combination and may result in identification of new areas for mineral exploration, new discoveries, and new insights into trends of mineral belts, tectonic settings, and ore-forming processes. Augmenting the available data with new information will undoubtedly contribute to a sharper focus for future exploration and resource assessment. Identification of areas that deserve additional study is an important outgrowth of this analytical process. In particular, HUCs rated as having High potential and Low or Medium certainty are places that merit additional sampling and investigation.
Future geologic investigations should prioritize areas that either have relatively high potential based on limited available data or are adjacent to such areas.
These areas might also benefit from the use of new exploration methods and types of data acquisition that would complement the currently available datasets. Land-use-planning decisions will benefit most from acquisition of new data in areas that have low degrees of certainty because the lack of relevant data precludes accurate assessment of mineral potential. Mineral-exploration targets are most confidently pursued in areas of known potential, but underexplored areas that lack data collected with previously unavailable technology and analytical tools may hold the greatest rewards. Although true for all six groups of deposit-types, such an approach is particularly pertinent to REE and PGE exploration, which include important commodities that were not previously targets for exploration.
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