Introduction

Critical minerals are mineral commodities “essential to the economic and national security of the United States” for which “the supply chain…is vulnerable to disruption” (Executive Office of the President, 2017; U.S. Department of Commerce, 2019). The term “critical minerals” refers to elements and minerals that are predominantly imported from various countries, some of which may discontinue exports to the United States because of political disputes or restricted availability of supplies. Presidential Executive Order No. 13817 (Executive Office of the President, 2017) and Secretarial Order No. 3359 (U.S. Department of the Interior, 2017) requested the U.S. Geological Survey (USGS) to determine possible domestic sources of critical minerals that could offset or eliminate import reliance. In 2018 a list of 35 critical minerals was released by the U.S. Department of the Interior (2018) (Fortier and others, 2018); 31 were designated import-reliant because imports provide more than 50 percent of annual domestic consumption. In 2021, zinc and nickel were added to the list, platinum group elements (PGE) and rare earth elements (REE) were individually listed, and several commodities were delisted (Nassar and Fortier, 2021), bringing the current list to 50 critical minerals. In response to the administration orders and first critical minerals list the USGS initiated and reorganized several projects, including the Earth Mineral Resources Initiative; the U.S. Mineral Deposit Database (USMIN) project; and the Systems Approach to Critical Minerals Inventory, Research, and Assessment project, to address domestic critical mineral supplies.

Domestic critical minerals covered in this report have been concentrated into deposits by magmatic and hydrothermal processes associated with plate subduction at convergent and transform plate margins of the Western United States. Subduction-related magmatism has been episodically active since the Early Triassic and is represented by granitic intrusions and volcanic fields in Arizona, California, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, and Washington. These igneous rocks mostly reflect partial melting of the mantle wedge by devolatilization of subducted oceanic crust and lesser pelagic sediments during convergent plate subduction. Basalt magmas generated by partial melting of mantle rocks intruded and partially melted lower and middle crust to form hybridized magmas. Hybridized magmas differentiated, buoyantly ascended, and erupted as andesites and felsic volcanic rocks or crystallized as granitic and porphyritic rocks in the upper crust. Production, resources, and inventories of metals and critical mineral commodities in the Western States were largely derived from, or occur in, deposits in granitic and porphyritic intrusions, in sedimentary rocks adjacent to intrusions, and in volcanic fields thought to overlie intrusions. Most deposits formed during the Late Cretaceous and Cenozoic. Those in westernmost States formed near the subduction zone during high-angle subduction and slab rollback whereas deposits in the Rocky Mountains States (Colorado, Montana, Wyoming, New Mexico, and parts of Utah) are thought to have formed when oceanic crust was subducted at low angles, thereby heating, and melting by devolatilization lithospheric mantle and lower crust hundreds of kilometers east of the subduction zone. This “flat slab” magmatism generally did not extend further east than the longitude of the Colorado Front Range, thus limiting critical minerals covered in this report to Western States (fig. 1; tables 1, 2).

Other smaller volume magmatism indirectly linked to plate tectonism includes partial melting of (1) lower crust rocks in the central and eastern Great Basin thickened by contraction and in part heated by oceanic crust during flat-slab subduction, (2) depressurized mantle in slab windows of the southern Cascades magmatic arc that were created by late Cenozoic impingement of the Pacific spreading center on the long-lived convergent subduction zone, and (3) depressurized mantle beneath crust in the Great Basin thinned by extension during the late Cenozoic. The small to moderately sized magmatic-hydrothermal deposits (less than 1 to several million metric tons [Mt]) formed during partial melting of tectonically thickened crust contain small masses of critical minerals. None of the deposits in the mostly small-volume volcanic fields of andesite, dacite, and rhyolite, and lesser basalt that represent magmatism of slab windows and extension younger than 17 mega-annum (Ma) contains appreciable quantities of critical minerals relative to recent domestic consumption.

Known and suspected subduction-related magmatic-hydrothermal deposits that contain critical minerals can be classified under four mineral systems and seven deposit types distinguished largely by characteristics of associated igneous rocks, hydrothermal mineral associations, and proportions of mineral commodities (table 1; Hofstra and Kreiner, 2020). Mineral systems evaluated in this report include porphyry copper-molybdenum-gold (Cu-Mo-Au), alkalic porphyry, porphyry tin (granite related), and reduced intrusion-related. Within these mineral systems deposit types represented include porphyry/skarn copper-gold (Cu-Au), skarn-replacement-vein (S-R-V) tungsten, polymetallic sulfide S-R-V intermediate sulfidation (IS), high-sulfidation gold-silver, low-sulfidation gold-silver, and lithocap alunite (aluminum, potash). Assignment of some deposits containing critical minerals to mineral systems and deposit types is problematic because of incomplete descriptions of deposits, especially small deposits, no known spatial or temporal association of deposits with subduction-related igneous rocks, and (or) deposit characteristics that do not easily fit the classification scheme but contain mineral commodities thought to have been concentrated by subduction-related magmatism. Some deposits containing critical minerals are therefore described in the “Unclassified Magmatic-Hydrothermal Deposits” sections.

Porphyry Cu-Mo-Au mineral systems consist of numerous deposit types including Cu-Mo-Au deposits in intrusions (often porphyritic and mostly calc-alkaline) and polymetallic sulfide S-R-V deposits and lithocap alunite deposits in adjacent and overlying sedimentary and volcanic rocks; some include high- and intermediate-sulfidation deposits (Hofstra and Kreiner, 2020). Porphyry Cu-Mo-Au systems occur within Phanerozoic magmatic arcs adjacent to subduction zones, and supply more than one-half of copper consumed annually worldwide. Critical minerals in porphyry Cu-Mo-Au systems in the Western United States include Al, Sb, As, Bi, Ga, Ge, In, Mn, Ni (designated a critical mineral in 2021), potash (KCl, K₂SO₄ salts; a critical mineral prior to 2021), Re (a critical mineral prior to 2021), Te, Ti, W, and Zn (designated a critical mineral in 2021) that occur in several deposit types. Critical minerals in other deposit types associated subduction-related magmatism include Al, Sb, Be, Bi, fluorite (CaF₂), Li, potash, Ta, Te and Sn in polymetallic sulfide S-R-V-IS deposits, high-sulfidation gold-silver deposits, low-sulfidation gold-silver deposits, lithocap alunite deposits, and in deposits with unclear association with subduction-related magmatism (fig. 2).

Alkalic porphyry mineral systems include Cu-Mo-Au deposits and other deposit types that form in alkalic intrusions, in adjacent sedimentary and volcanic rocks, and in veins in intrusions and in adjacent rocks; they also occur within Phanerozoic magmatic arcs and continental rifts. Deposit types of this system in Western States include low-sulfidation gold deposits that are mined for gold and silver and enriched in tellurium, bismuth, and vanadium and fluorspar deposits (table 1).

Porphyry tin (granite-related) mineral systems include deposits that contain the critical minerals beryllium, lithium, niobium, tin, and tantalum. These deposits occur in or near granitic rocks and in pegmatites that reflect partial melting of continental crust to form peraluminous magmas. Deposit types represented in Western States are mostly Precambrian pegmatite deposits that have been mined for beryllium, lithium, niobium, REE, tantalum, and tin (table 1). They are not clearly related to subduction magmatism, contain minor amounts of critical minerals relative to recent domestic consumption, and are not covered in this report.

Reduced intrusion-related mineral systems form in continental magmatic arcs in and adjacent to intrusions that assimilated sedimentary strata containing hydrocarbons and pyrite. Deposit types include gold, skarn copper-molybdenum-tungsten (Cu-Mo-W), polymetallic sulfide S-R-V-IS, disseminated gold-silver, intermediate sulfidation, and graphite. In Western States, few unequivocal reduced intrusion-related deposits are known; several have been mined and explored for gold and lesser silver. Critical mineral concentrations and forms in them are largely unquantified.

The 35 critical minerals initially listed by the Department of the Interior, and the recent additions zinc and nickel, are mineral commodities that are produced and marketed as elements (for example, Al, In, Ni, Te, and Zn), compounds and alloys (for example, GaAs, Si_1−xGe_x, SbPb, InSb, WC, FeV, and FeMn), oxides and carbonates (for example, SbO₃, GeO₂, Li₂CO₃, and WO₃), and minerals (for example, barite, fluorite, and potash). Critical minerals in Western States that have been produced as primary products, coproducts, and byproducts, and that are known to occur in elevated concentrations in unmined deposits, in interim products of copper concentrators, smelters, and refineries (tailings, slags, slimes, and electrolyte), and in mine dumps, include Al, Sb, As, Be, Bi, fluorite, Ga, Ge, In, Li, Mn, Ni, Nb, Pd, Pt, potash, Re, Ta, Te, Sn, W, V, and Zn.

Descriptions of deposits and resources of the former critical mineral commodities potash and rhenium were completed prior to their delisting and are retained in this report. More recently listed zinc is not covered because of the anticipated time required for evaluation of the large amount of information available. Nickel occurrences in Western States are uncommon, insignificant relative to consumption, and (although in part related to subduction) not related to subduction magmatism. However, nickel in interim products of processing streams of porphyry Cu-Mo-Au ores is potentially recoverable in significant quantities and is correspondingly included where processing information is available.

Elevated concentrations of Al, Sb, As, Bi, Be, Ga, Ge, Mn, potash, and W in some deposits in Western States enabled their production as primary products and coproducts, although most production was small relative to domestic consumption, short lived, and subsidized. Deposits in which critical minerals are primary products are often not distinguishable from coproduct critical minerals because invariably there is insufficient published information to separate them, fluctuating commodity prices can change the relative values of commodities in ore and redefine ore tonnages and grades, and some production was subsidized by guaranteed prices and other federal government policies (for example, War Production Board, 1942; cover image). As used herein, a primary product is a mineral commodity that is essential to the economic viability of a mine (profitability), the basis for exploration, development, and sustained production. A coproduct is one or more mineral commodities produced with other mineral commodities because their combined value is essential for profitable mining. A byproduct is a mineral commodity that is recovered because it adds production value but is not essential to mine profitability.

Production of the primary commodities Cu, Mo, Pb, Zn, Au, and Ag in Western States enabled recovery of most critical minerals, including Sb, As, Bi, Mn, Nb, Pd, Pt, Re, Ta, Te, Sn, W, and V, mostly as byproducts, and in a few deposits as coproducts and primary products. However, many critical minerals in mined deposits occur in low concentrations and small quantities and were never recovered because of unprofitability, and in some cases, no world markets. The porphyry copper-molybdenum (Cu-Mo) deposits in Arizona, New Mexico, Utah, Montana, and Nevada contain small concentrations of critical minerals in ores; however, processing of large tonnages of ore annually for copper and molybdenum recovery concentrates some chalcophile and siderophile critical minerals (Sb, As, Ge, In, Ni, Re, and Te), several of which are episodically recovered and marketed (Re and Te). Unmined porphyry Cu-Mo resources in Alaska, Idaho, and Arizona similarly contain large inventories of these and other chalcophile and lithophile critical minerals (for example, Al, Ti, and Zr) that theoretically could be recovered during ore processing for copper and molybdenum recovery.

In this report, deposits containing critical minerals are grouped by mined deposits in which critical minerals were primary products and coproducts (chap. A) and byproducts (chap. B), by unmined deposits and interim products of copper production with large critical mineral inventories relative to recent domestic consumption (chap. C), and by deposits with elevated concentrations of critical minerals (greater than crustal abundances) in archival specimens and collection samples (chap. D). Within each chapter, deposits, unmined resources, interim products, and specimens and samples are organized by mineral system and deposit type.

With the exception of the Pebble porphyry Cu-Mo and Margerie Glacier tungsten deposits in Alaska, all critical mineral occurrences described in this report are in conterminous Western States. Locations and descriptions of deposits and resources in Alaska that contain critical minerals are available at the USMIN web site (https://mrdata.usgs.gov/deposit/). Deposits and resources containing the critical minerals Be, Li, Nb, PGE, REE, Sn, and Ta as primary products and coproducts, although mostly related to intrusions (for example, Elk Creek niobium, Nebraska; Mountain Pass REE, Calif.; Round Top REE, Tex.; Stillwater Mountains PGE, Mont.; Be-Li-Nb-Sn-Ta pegmatites, N. Mex. and S. Dak.) (USGS and others, 1965, 1975; Bellora and others, 2019; Karl and others, 2021), include deposits in Midwestern States that are not clearly subduction-related and therefore not described below.

This report (1) provides a historical perspective of critical minerals produced from subduction-related magmatic-hydrothermal systems in the Western United States, (2) describes significant inventories of critical minerals in unmined deposits and mine reserves, and (3) tracks critical minerals in interim products of copper refineries where they comprise significant inventories but mostly are not recovered. The report is intended to inform policy makers of the approximate masses of domestic critical minerals that conceivably could become supplies for consumption if economic incentives for their mining and recovery are enabled.

Brief History of Critical Minerals

The concept of critical minerals arose in the early 20th century with the expanded application of alloys and compounds of C, Cr, Fe, Mn, Ni, and W that greatly improved metal durability, machining efficacy, and armament effectiveness (Lovering, 1944; Andrews, 1955; Limbaugh, 2006, 2010; Schmidt, 2012). These ferroalloys and compounds became essential to national defense and industrial competitiveness. Their components, variously termed critical or strategic minerals, faced supply uncertainty because domestic production since World War I has been insufficient for consumption. Concerns over importation security of tungsten and other commodities crucial for machining and armaments escalated in the 1930s and during World War II. Price guarantees (for example, Sb, As, Mn, and W) (War Production Board, 1942; cover image) and other forms of subsidization by the federal government increased domestic production prior to and during World War II and the Korean War; they were episodically extended into the 1960s through much of the Cold War era. However, peak tungsten production, for example, from domestic mines never exceeded 50 percent of required World War II supplies (approximately equivalent to consumption) (fig. 3; Morgan, 1983); then, as now, importation with trade route protection, as required, secured necessary imports.

Domestic critical mineral deposits, reserves, resources, and inventories described in this report attest to small low-grade deposits and resources of tungsten and other critical minerals, non-recovery of critical minerals at operating and shuttered or demolished refineries, and disincentives to explore for and produce critical minerals in the United States. Many of the largest and highest-grade domestic metal deposits were discovered and mined in the 19th and 20th centuries when there were no or small world markets for commodities subsequently deemed critical or strategic minerals. Comprehensive descriptions of these major deposits and districts (for example, Ridge, 1968) seldom include critical mineral commodities because they had no bearing on production economics and there was no to small demand for many. Imported supplies were generally deemed secure until the vulnerability of oceanic shipping was revealed by World Wars I and II.

Before, during, and after World War II and during the Cold War era, numerous investigations of domestic sources of critical mineral commodities were conducted by the U.S. Bureau of Mines (USBM) and USGS to address wartime shortages and importation vulnerability (USBM, 1941a; Moon, 1950; Foster, 1988). Several “underground stockpiles” of unmined, marginal “ore” were blocked out (measured, indicated, and inferred reserves) by drill holes, surface excavations, underground development, and metallurgical test work. These marginal ores were assumed recoverable, if needed, whereas price guarantees were used to encourage domestic critical mineral production by the mining industry. Some cumulative tonnages of marginal ores defined by USBM investigations are likely too small or scattered or have grades that are too low to mine profitably today without subsidization; for example, 1.380 million short tons (Mst) (1.25 Mt) grading 33 to 53.5 percent fluorite are distributed among 36 deposits, approximately 18 Mst (16.4 Mt) grading 9 to 23.2 percent manganese are distributed among 39 deposits, and 10.6 Mst (9.6 Mt) of alunite in two deposits grading 21 to 31 percent Al₂O₃ (table 1 in Moon, 1950).

Occurrences of many critical minerals associated with subduction-related igneous rocks in Western States are variably described in War Mineral Reports (USBM, 1942–1945), in collaborative reports by the USGS and state geological surveys (for example, USGS and Montana Bureau of Mines and Geology, 1963; USGS and Nevada Bureau of Mines, 1964; USGS and others, 1964, 1965, 1966a, 1969, 1975), and in topical publications by the USGS, most of which stem from World War II and Cold War era supply concerns.

Limitations and Assumptions

Evaluation of critical minerals in this report was conditioned by the quality of published concentrations and brevity of deposit descriptions, by incomplete access to unpublished drill hole records and reserves of mined and unmined deposits, and by general disinterest in critical mineral exploration and recovery. These limitations lead to uncertainty in classification of some deposits and necessitated application of a mass threshold to distinguish significant domestic inventories of critical minerals based on their current consumption.

Quantification of critical minerals is limited by accuracy of published concentrations, especially low concentrations reported in pre-2000 publications. Recovery (free market and subsidized) of some chalcophile, siderophile, and lithophile critical mineral commodities was usually enabled by average grades and grade ranges of percents to tens of percents (for example, Sb, fluorite, Mn, and W). Production at these grades obtained from published sources is therefore considered reasonably accurate. However, multi-element analyses of critical minerals and other commodities in ores and rocks used in mineral deposit investigations before approximately 2000 had insufficient precision and detection limits to accurately quantify chalcophile and siderophile critical minerals that mostly occur in low concentrations (less than 1 percent; for example, Bi, PGE, and Te) in deposits described in this report. Over the past two decades multi-element analyses with lower detection limits have been increasingly used in mineral deposit exploration programs and reserve delineation of unmined deposits. Concentrations of critical minerals in exploration and deposit delineation drill holes that represent large masses of mineralized rocks are used in this report, as available, to evaluate critical mineral resources and inventories in unmined deposits and resources, in addition to those inventories determined from production and refining records (tables 3–15). Multi-element analyses of archival specimens and collection samples of mineralized rocks also reveal elevated concentrations of critical minerals that portend possibly significant inventories in numerous mining districts in Western States (tables 16–19). Known and suspected sites of critical minerals in deposits, resources, and interim recovery products are listed in table 20 and described in the text.