Introduction

The Earth Mapping Resources Initiative (Earth MRI) was launched by the U.S. Geological Survey (USGS) to bolster the national geologic and geophysical datasets required to document the potential for domestic sources of critical minerals (Day, 2019). This report describes the background data, sources, and methodology used to define broad focus areas for future data collection (geologic mapping, aeromagnetic and radiometric geophysical acquisition, and geochemical characterization) in Alaska (fig. 1). Geologic and geophysical data generated from this effort will improve the understanding of the mineral resources and framework geology throughout the state that are suspected or known to contain nonfuel mineral systems. This report focuses on mineral systems that have potential for phase 3 (Sb, barite [BaSO₄], Be, Cr, fluorspar [CaF₂], Hf, Mg, Mn, U, V, Zr) critical mineral enrichments (tables 1 and 2). Many of these mineral systems also have potential for phase 2 (Al, Co, graphite [C], Li, Nb, platinum group elements [PGEs], rare earth elements [REEs], Ta, Sn, Ti, and W) and (or) phase 1 (REE) critical mineral enrichments (table 2; Kreiner and Jones, 2020). Furthermore, select mineral systems also have potential for future phase (As, Bi, Cs, Ga, Ge, In, Re, Rb, Sc, Sr, Te) critical mineral enrichments (table 2). Thus, the focus areas delineated herein and in Kreiner and Jones (2020) represent all mineral systems that have potential to contain critical minerals and are known or suspected to exist in Alaska.

The Alaska focus areas developed in this report were defined using a mineral systems framework (Hofstra and Kreiner, 2020). Datasets that contribute to the delineation of the focus area include the published and ongoing statewide geospatial prospectivity mapping (Karl and others, 2016, 2021; Kelley and others, 2021) and other relevant statewide datasets and publications (for example, Wilson and others, 2015; Granitto and others, 2019; Kreiner and others, 2020). Focus areas in Alaska are necessarily broad, due to substantial gaps in modern data coverage and quality across such a geologically complicated, large, and remote region. Where possible, mineral deposits that contain critical mineral enrichments identified in the Alaska Resource Data File (U.S. Geological Survey, 2008; ARDF; https://mrdata.usgs.gov/ardf/) are included within focus areas. Broad focus areas are drawn across regions of the state where no known mineral enrichments occur and include areas that exhibit key geological characteristics that are identified as important features of critical-mineral-bearing systems. Limited bedrock exposure across many regions of the state (for example, the North Slope and the Yukon-Kuskokwim delta, fig. 1) hampers the ability to collect appropriate data. Additionally, major fault systems (for example, Kaltag, Tintina, and Denali faults, fig. 1) juxtapose disparate geologic belts, which have been mapped at varying levels of detail, making interpretations across major faults difficult. The goal of Earth MRI is to acquire new geophysical, geologic, and geochemical data through mapping in these focus areas across Alaska to enable the USGS to evaluate the formation of and prospectivity for critical mineral deposits.

Criteria used to define the focus areas for the three regions of the conterminous United States are described in a companion report (Hammarstrom and others, 2022), which used a slightly different methodology and relied on additional datasets. These datasets include the USMIN Mineral Deposit Database, which is more complete and accurate for these regions than for Alaska (for example, Hammarstrom and others, 2022). Additionally, much of the conterminous United States has been studied in greater detail and experienced a rich history of geologic work leading to far greater information on the presence of known mineral occurrences, deposits, or mines that have current or past production. Thus, more detailed focus areas are drawn than can be done in Alaska, where a lack of knowledge and detailed data precludes such detailed analysis. In some cases, though, broader focus areas were developed to encompass regions containing mineral systems that have potential for phase 3 critical mineral enrichments in the absence of known deposits.

A related USGS data release (Dicken and others, 2021) provides geospatial data for all phase 3 focus areas in the U.S. and an associated data table that (1) summarizes what is known about the critical mineral potential of each focus area, (2) provides information on the extent and quality of the available geophysical and topographic coverages in each focus area, and (3) contains brief descriptions of data gaps that could be addressed through Earth MRI.

Mineral Systems Approach

The following two paragraphs are repeated and summarized for the benefit of the reader from Kreiner and Jones (2020, p. 4). Mineral systems provide the framework that considers all geologic features that can influence or control the formation and preservation of a mineral deposit. Ore deposits, where potential economic concentrations of critical commodities could be, are the culmination of the geologic processes that constitute the mineral system. Mineral systems require the following: (1) an energy driver (topography, geothermal gradient in the crust, or magma); (2) a source of components (for example, metals) and fluid (melts, aqueous fluids, petroleum), and ligands (to complex components); (3) transport pathways (faults, fractures, or permeable lithologic units); and (4) a physical and (or) chemical trap (mixing of fluids, reduced host rocks, or boiling) (Hofstra and Kreiner, 2020). A productive mineral system must incorporate each of these criteria to generate a mineral deposit (Hofstra and Kreiner, 2020).

Mineral systems generally are characteristic of a single episode that typically occurs relatively late in an otherwise broad, potentially long-lived geotectonic setting. These systems can be evaluated on larger scales and commonly exhibit larger spatial footprints than a single mineral deposit. Due to this larger spatial and temporal scale, a single mineral system can result in multiple genetically related deposits that are mineralogically distinct. Furthermore, within a single mineral system, subtle variations in the fluid chemistry, source rocks, ligands, and lithologic setting can result in unique differences in the types of metals that can be transported or trapped in otherwise similar geotectonic settings. These subtle differences are responsible for the presence or absence of critical mineral enrichments, as byproducts in a system. Critical minerals are rarely the primary mineral commodity being explored and (or) produced in a mineral system, although exceptions include some deposits where the critical mineral is the principal commodity (REE, PGE, graphite, or uranium; Hofstra and Kreiner, 2020). Thus, understanding where, how, and why critical minerals are enriched in mineral systems is essential for most effectively predicting where undiscovered critical mineral resources are more likely to be (Hofstra and Kreiner, 2020).

Table 2 lists the mineral systems that have potential critical minerals and are known or suspected to be in Alaska. Some phase 2 mineral systems, delineated by Kreiner and Jones (2020), also contain phase 3 critical mineral commodities and are updated accordingly in table 2. Some mineral systems that contain critical commodities that are planned to be the focus of future Earth MRI phases are included in table 2. Table 2 also lists the deposit types for which statewide data-driven prospectivity analyses have been published or are in progress. Some mineral systems listed in table 2 have not yet been included in Alaska mineral prospectivity mapping (Karl and others, 2016), so focus areas for these mineral systems were identified through synthesis of published geological data and (or) ongoing geological research.

Data Sources

The development and delineation of focus areas in Alaska for phase 3 critical minerals relies on publicly available geological, geochemical, geophysical, and mineral occurrence datasets (U.S. Geological Survey, 2008; Wilson and others, 2015; Granitto and others, 2019). The description of the datasets and an overview of data-driven, GIS-based assessment methods can be found in Kreiner and Jones (2020). More detailed discussion of data-driven mineral prospectivity mapping in Alaska can be found in Karl and others (2016, 2021) and Jones and others (2015).

Delineation of Focus Areas

Data-driven, GIS-based analyses that map out mineral resource prospectivity across Alaska have already been published for numerous ore system types known or suspected to contain critical minerals (Jones and others, 2015; Karl and others, 2016, 2021; Kelley and others, 2021). These studies query characteristics relevant to the mineral system of interest in the geospatial geologic and geochemical datasets. A score is generated for each subwatershed (that is, watersheds that have 12-digit hydrologic unit codes and cover approximately 100 km²), and subwatersheds are classified based on scores that indicate high, medium, low, or unknown potential for that mineral system. Areas are drawn around regions containing high- to medium-potential subwatersheds for the mineral system of interest, and these areas are interpreted to be prospective for the associated mineral commodities. For mineral systems covered by published prospectivity models (Karl and others, 2016, 2021; Kelley and others, 2021), the phase 2 and phase 3 focus areas directly correspond to the areas outlining elevated prospectivity in the reports. For mineral systems that have not yet been analyzed utilizing the data-driven approach previously outlined, we qualitatively assessed available data to delineate the focus areas. For example, relevant mineral occurrences from the ARDF were combined with queries of key lithologic units in the Alaska geological map database (Wilson and others, 2015) to help identify possible volcanogenic seafloor, Carlin-type, and marine evaporite mineral systems. The mineral occurrences and prospective lithologies were overlain geospatially, and polygons were drawn around areas that contained permissive geological environments and mineral systems. Relevant data including information about the mineral systems, rationale for delineating each focus area, mineral occurrences and (or) prospects, any known production, potential for future critical mineral discovery, and key references were compiled in a database using the template outlined in table 3. The phase 3 focus area database is published separately as a USGS data release, which also contains geospatial data for each focus area (Dicken and others, 2021). The preliminary phase 3 focus areas were then reviewed by collaborators at the Alaska Division of Geology & Geophysical Surveys.

Mineral Systems

The following sections provide the background data describing the mineral systems that contain phase 3 critical minerals and were not previously described in Kreiner and Jones (2020). For each mineral system, we discuss the rationale for its inclusion and the general locations and geological characteristics of the associated focus areas in Alaska. The focus areas for each mineral system described herein are shown in figure 2.

Discussion

Statewide publicly available geologic and geochemical datasets combined with published data-driven geospatial mineral prospectivity analyses form the basis of Alaska Earth MRI focus areas (fig. 2A–D) defined for phase 3 critical minerals. A mineral systems approach is adopted to identify focus areas where they contain the necessary geological ingredients required to generate deposits that may contain critical minerals. Current and historical production of base, precious, and critical minerals occurs in some focus areas (for example, Red Dog), and others contain identified resources known to contain critical minerals (for example, Arctic), as delineated in Dicken and others (2021). Focus areas are broad and reflect gaps in existing or modern data coverages and quality, which are needed to refine mineral system outlines in a large, remote, and geologically complex state.

This report delineates 22 new Alaska Earth MRI phase 3 focus areas. The focus areas are listed in Dicken and others (2021) for the four identified mineral systems (fig. 2). An additional 55 focus areas that were previously defined for 10 mineral systems in Phase 2 and discussed in Dicken and Hammarstrom (2020) and Kreiner and Jones (2020) are also prospective for phase 3 critical minerals. As a result, these mineral systems are included in the following discussion and accompanying data release (Dicken and others, 2021). Collectively, phase 3 focus areas span large regions of the state and occur in multiple, diverse geological belts. To help prioritize new geological, geophysical, and geochemical data collection in Alaska, the authors mapped the number of phase 3 focus areas that occur within each 1:63,360-scale quadrangle in the state. Each 1:63,360-scale quadrangle contains at least one focus area to a maximum of nine (fig. 3). The resulting data (table 4) indicate that 1,979 (~66 percent) of the 3,011 quadrangles at a scale of 1:63,360 in the state contain at least one focus area that has potential for phase 3 critical mineral commodities. More than 400 quadrangles have six or more overlapping focus areas that are permissive or prospective for phase 3 critical minerals together with other primary and secondary commodities (table 2). We also considered all focus areas and associated mineral systems that have been developed through Earth MRI in Alaska (Dicken and others, 2019, 2021; Dicken and Hammarstrom, 2020; Kreiner and Jones, 2020) and mapped the number that occur within each 1:63,360 quadrangle in the state. In the state, 17 known or suspected mineral systems have the potential to contain one or more critical minerals (table 2). However, no quadrangles in the state contain an overlap of more than 12 of the 17 mineral systems. In Alaska, 2,081 quadrangles (1:63,360 scale) have evidence for at least one mineral system of interest, and 32 quadrangles exhibit evidence for as many as 12 overlapping mineral systems (fig. 4; table 5).

The mineral systems described herein and in Kreiner and Jones (2020) were identified in support of Earth MRI phase 1, 2, and 3 activities. They cover a broad range of critical mineral commodities listed in Fortier and others (2018). We also note that these same mineral systems account for all other nonfuel-related critical commodities that might be present in Alaska (table 2). Thus, we expect the focus areas delineated to date to suffice for any additional phases of Earth MRI planning and prioritization. Regions of the state where the greatest mineral system overlap occurs are the Yukon-Tanana upland and environs in east-central Alaska, the central and eastern Alaska Range, and parts of the Seward Peninsula (fig. 4). These and other areas, which have the most overlap of mineral systems containing critical minerals, are expected to have the highest potential for new discoveries. Prioritizing new data collection in these regions could be an efficient and effective way to develop a more complete and modern understanding of the deposit types and styles that are present, and how associated critical minerals are mobilized and concentrated in a variety of geologic environments. Note that some regions that have less overlap also contain significant known prospects or deposits (for example, the Pebble deposit, western Alaska Range and Bokan Mountain in southeast Alaska; Kreiner and Jones, 2020). Thus, there are areas with potential for a smaller number of mineral systems, but the mineral system(s) that are present were particularly productive given the geologic environment and history. In these cases, known but undeveloped and (or) undiscovered deposits or prospects might have more significant potential for new critical minerals either as primary products, coproducts, or byproducts despite occurring in regions with fewer overlapping systems. Therefore, considering the overlap of mineral systems together with the state of knowledge and modern data in any region is important when prioritizing areas for new mapping and data collection.

Summary

Alaska focus areas for phase 3 of Earth MRI, which include potential for antimony, barite, beryllium, chromium, fluorspar, hafnium, magnesium, manganese, uranium, vanadium, and zirconium, have been defined based on a data-driven, mineral systems approach that uses publicly available statewide datasets to map prospectivity for a variety of mineral deposit groups. The prospectivity maps and associated data delineate regions that are prospective for mineral systems that could contain critical minerals of current interest. In addition, the statewide prospectivity analyses identify key gaps in existing datasets that highlight the need for new data collection throughout Alaska. Prioritization of data acquisition through the Earth MRI program is informed by the data gaps highlighted in the prospectivity analyses.

Four additional mineral systems discussed here and 10 previously discussed mineral systems contain high prospectivity for Earth MRI phase 3 critical minerals in Alaska. Evaluation of phase 3 critical minerals has resulted in 22 new and 55 previous focus areas identified in regions that are favorable for mineral systems that could contain the critical minerals. In total, 102 focus areas and 17 mineral systems containing high prospectivity for critical minerals have been identified in Alaska. Evaluating the amount of spatial overlap at the quadrangle scale provides a useful tool for prioritizing regions for new Earth MRI geologic, geophysical, and geochemical data acquisition. These regions exhibit evidence for the greatest variety of prospective mineral systems.