Carlin-type mineral systems include antimony deposits as well as gold-silver-mercury deposits with potential byproduct antimony. An antimony district in Utah produced 105,000 short tons of antimony in the past; samples taken near old mines suggest subeconomic inferred resources on the order of 14 million metric tons (Mt) at an average grade of 0.75 percent antimony (Krahulec, 2018). Although antimony concentrations can be anomalously enriched in Carlin-type gold deposits, no antimony resource estimates are available. A small amount of antimony was previously produced from other Nevada gold and antimony deposits.

Coeur d’Alene-type antimony deposits in Idaho and Montana are hosted in Precambrian Belt sediments (Hofstra and others, 2013). Similar to deposits in orogenic systems, Coeur d’Alene-type antimony deposits are related to metamorphic dewatering during exhumation, but the source rocks are moderately oxidized rather than reduced siliciclastic sequences (Hofstra and Kreiner, 2020). Antimony and polymetallic silver-rich ores of the Coeur d’Alene district (Silver Valley antimony) represent the second-largest known antimony resource in the United States (fig. 3). The U.S. Antimony mine in Montana, also known as Stibnite Hill, closed in 1983 due to declining prices, with reported production and reserves of about 15.4 metric kilotons of antimony; the nearby Sunshine silver mine stopped recovering antimony in 2001 (Hofstra and others, 2013). Antimony in this district occurs in simple quartz-stibnite veins where antimony is recovered as the primary commodity. Antimony is also recovered as a byproduct from the silver mineral tetrahedrite in polymetallic silver-lead-zinc veins. Other examples of antimony in the Coeur d’Alene-type system include shallow mines developed in polymetallic, antimony-bearing fissure veins in Mississippian shale in the southwest Arkansas antimony district (fig. 3), which produced 5,400 short tons of antimony from 1873 to 1947 (Howard, 1979).

Antimony is a byproduct commodity associated with gold, silver, and mercury in some low-sulfidation epithermal Au-Ag and antimony deposits of the meteoric convection mineral system. The mined out and reclaimed McLaughlin hot spring gold-mercury deposit and other hot spring deposits in the Coast Ranges of California lie along faulted contacts between the Coast Range ophiolites and the Great Valley sequence (fig. 3). At the McLaughlin deposit, antimony occurs in stibnite and sulfosalts; the deposit also hosted various arsenic-bearing sulfosalts, arsenian pyrite, and native arsenic (Sherlock and others, 1995).

Processes that form simple quartz-antimony deposits are related to the metamorphic dewatering of different rock types, including sulfidic, carbonaceous, or calcareous siliciclastic rocks during exhumation, where fluid flow along dilatant structures leads to vein deposition (Hofstra and Kreiner, 2020). The Yellow Pine deposit in Idaho (fig. 3) represents the largest known antimony resource in the United States. In the Stibnite-Yellow Pine mining district in Idaho, antimony occurs in narrow (<1 foot wide) high-grade quartz-stibnite veins and disseminated stibnite in shear zones in the granitic rocks of the Idaho batholith (White, 1940). The district was mined intermittently for gold, silver, tungsten, and antimony over the past century (Gillerman and others, 2019). In 2021, exploration and permitting are in progress at the Stibnite Gold project with plans to develop an open-pit gold-antimony mine, produce gold, silver, and antimony on-site, reprocess historical mine tailings, and conduct reclamation and restoration on the effects of historical mining (Zimmerman and others, 2021).

As of December 2020, the proven and probable mineral reserves at the Stibnite Gold project were estimated to be 104 Mt grading 1.43 grams per ton (g/t) gold, 1.91 g/t silver, and 0.064 percent antimony. The project was estimated to contain 4.8 million ounces (Moz) of gold, 1.2 Moz of silver, as well as 148.6 million pounds (Mlb) of antimony, with an estimated mine life of 14 years (Zimmerman and others, 2021). In addition to orogenic antimony deposits, orogenic gold and mercury deposits commonly contain byproduct antimony.

Antimony occurs in polymetallic sulfide S-R-V-IS (skarn, replacement, vein, intermediate sulfidation epithermal) deposits in porphyry Cu-Mo-Au, reduced intrusion-related, and IOA-IOCG mineral systems. For example, a cluster of polymetallic antimony occurrences in the Lakeview mining district near the Coeur d’Alene district in northern Idaho includes the Weber mine, which has historical assays of 1 percent antimony and silver, gold, lead, zinc, and arsenic. Similarly, antimony can be enriched in high-sulfidation epithermal deposits in Porphyry Sn and Climax-type systems. While these deposit types and mineral systems can be enriched in antimony, they have not represented significant antimony resources historically.

Bedded-sedimentary barite deposits have been extensively mined in Nevada since the 1960s, where high-grade deposits requiring minimal processing meet specifications for use in drilling muds by the oil industry (Johnson and others, 2017). The Greystone and Argenta mines are active barite producers (Nevada in fig. 4). The Snake Mountains mining district in Nevada produced more than 1 million short tons of barite between 1974 and 1985 (LaPointe and others, 1991). In the Eastern United States, the Sweetwater barite district in Tennessee produced more than 1 million short tons of barite; districts in southeast Missouri, Alabama, and Virginia also historically produced barite from bedded-sedimentary and residual deposits.

In hybrid magmatic/basin brine mineral systems, CO₂- and HF-bearing magmatic volatiles condense into basinal brines that replace carbonate with fluorspar ± barite, REE, titanium, niobium, and beryllium. Examples include the Illinois-Kentucky Fluorspar district and the Hicks Dome in southern Illinois. These are primary fluorspar deposits with byproduct or coproduct barite.

Barite can occur as a principal commodity in carbonatites and peralkaline syenite assemblages in magmatic REE systems. For example, at Mountain Pass, California, barite comprises about 25 percent of the carbonatite (Johnson and others, 2017).

Bedded volcanic deposits form as volcanic seafloor deposits associated with copper, lead, zinc, or precious metal sulfide ores. The Barite Hill gold deposit in South Carolina is a Kuroko-type volcanogenic massive sulfide deposit in the Carolina slate belt with lenses of massive barite and quartz (Clark, 1999). Other examples of barite in this system include the Kings Creek barite district on the North Carolina-South Carolina border (table 5). Volcanic seafloor systems in other parts of the country are permissive for barite but unlikely to host significant resources.

Climax-type systems form in continental rift settings characterized by hydrous bimodal magmatism (Hofstra and Kreiner, 2020). A variety of deposit types can form as aqueous supercritical fluids exsolved from anorogenic topaz rhyolite plutons, and the apices of subvolcanic stocks move upward and outward, split into liquid and vapor phases, react with country rocks, and mix with groundwater. The large thermal and chemical gradients in these systems result in diverse deposit types, such as volcanogenic beryllium deposits or beryllium in greisen, skarn, and replacement deposits. At deep levels in these systems, late-stage niobium-yttrium-fluorine (NYF)-type pegmatites carrying beryllium emanate from plutons.

The volcanogenic beryllium deposit at Spor Mountain in Utah, which opened in 1968, provides most of the world’s beryllium. Beryllium ore deposits occur with topaz-bearing rhyolite flows, pyroclastic deposits, and fluorite-bearing pipes along the ring fracture zone of an Oligocene caldera (Foley and others, 2016). Spor Mountain contains sufficient reserves to meet current expected domestic demands with resources of 7,011,000 metric tons (t) of ore and a grade of 0.76 percent BeO (Brush Engineered Materials, Inc., 2009). Other known large deposits include Apache Warm Springs in New Mexico, which has a beryllium resource of 39,063 t of ore at a grade of 0.72 percent BeO (McLemore, 2010).

Greisen and skarn in a large, fluorspar-rich system at McCullough Butte, Nevada, produced beryllium in the past (fig. 5). The deposit has a resource of 175 Mt of ore at an average grade of 0.27 percent BeO (J. Muntean, Nevada Bureau of Mines and Geology, written commun., 2021). NYF-type pegmatites and greisens associated with the Redskin Granite at Boomer Lake in Colorado produced 3,000 t of high-grade ore (2.0–11.2 percent BeO) between 1948 and 1969 (Hawley, 1969; Piper, 2007).

Beryllium occurs in Magmatic REE systems in deposits grouped as Peralkaline syenite/granite/rhyolite/alaskite/pegmatites (Hofstra and Kreiner, 2020). Examples include the Hicks Dome deposit in Illinois, where the mineral bertrandite occurs in breccia bodies (Baxter and Bradbury, 1980) and the Round Top deposit in Texas. Round Top is being primarily developed as an REE deposit with the potential for byproduct recovery of both beryllium and lithium (Pingitore and others, 2016). Resource estimates are available for potential commodities at Round Top, but no reserves are reported; 364,000 t of measured and indicated resources have an average beryllium grade of 32.15 parts per million (Hulse and others, 2019).

Granite-related porphyry Sn systems form in back-arc or hinterland settings by similar processes from fluids exsolved from more crustally contaminated S-type peraluminous plutons and stocks. At deep levels, LCT pegmatites emanate from plutons (Hofstra and Kreiner, 2020). Beryllium occurs as a main or byproduct commodity with tin and tungsten in porphyry, skarn, and greisen deposits in this system and related LCT-type pegmatites. Examples of beryllium-rich LCT-type pegmatites include the famous pegmatite deposits in the Black Hills of South Dakota, as well as the pegmatite districts in Maine and Colorado (fig. 5).

The Stillwater Complex, Montana (fig. 6, table 7), hosts multiple types of mineral deposits. The Basal series of this layered igneous complex contains low-grade copper-nickel sulfide mineralization. The overlying Ultramafic series contains laterally extensive stratiform chromite seams ranging in thickness from <1 m to about 4 m. The Upper and Banded series host PGE sulfide deposits. Sibanye Stillwater operates the Stillwater and East Boulder mines to produce palladium and platinum from the J-M Reef in the Banded series; a third project along the J-M Reef, the Blitz project, is in development (Sibanye Stillwater, 2021).

Chromite exploration at the Stillwater Complex started before 1900. Mines were developed during the Second World War under a government subsidy. Chromite seams in the Stillwater Complex are referred to by letters, starting with “A” at the base. The “A” and “B” seams are enriched in PGE. Between 1956 and 1962, the “G” and “H” chromite seams produced 2 Mt of chromite ore, averaging 22.8 percent Cr₂O₃ (Courtney, 2000). Further exploration in the 1980s identified a drill-indicated reserve of 14.6 Mt at the same average grade as the earlier production (Courtney, 2000).

In a mineral resource assessment of the Custer and Gallatin National Forests that covers the Stillwater Complex and adjacent areas, Zientek (1993) compiled available resource data for all deposits, prospects, and occurrences in the Stillwater Complex. He noted that chromite resources in some areas are partially delimited by exploration; additional resources are likely within undiscovered deposits as fault-offset extensions of known deposits and prospects. Exploration for chromite, however, is unlikely unless low-cost options for ore processing become available.

Podiform chromite deposits, many containing <1,000 t of ore, are scattered along the Pacific coast from Alaska to southern California. The chromite deposits of the Sierra Nevada foothills, the Klamath Mountain districts, and the Coastal Ranges in California shipped nearly 600,000 t of chromite in the 1940s (Thayer and Lipin, 1979). Small, ultramafic bodies were discovered in the Red Lodge district of southwest Montana in 1916; total production from the area was about 61,600 t of chromite ore at an average grade of 24–40 percent Cr₂O₃ (Simons and others, 1979; Loferski, 1986). Most of the massive chromite at the Red Lodge district is mined out. The high iron content, alteration, and low concentrations of cobalt, nickel, and PGEs as potential byproducts should be considered negative factors for future mining (Loferski, 1986).

Podiform chromite deposits also occur in the Eastern United States in Maryland, Pennsylvania, and North Carolina. The State Line district in Maryland and Pennsylvania (table 7) was mined extensively in two periods between 1820 and the early 1870s. About 40 deposits were developed during that time with the production of 250,000–280,000 t of chromite ore; these included the Wood deposit, which was the largest massive chromite deposit in the United States at that time (Pearre and Heyl, 1960). The large part of the focus area for mafic magmatic systems in the Southeastern United States (shown as the hachure pattern in fig. 6) represents the extent of possible, buried, mafic intrusions in Triassic basins that could host chromite or other resources, based on geophysical anomalies.

Chromite-rich placer deposits are uncommon. However, a notable example is the terraced black-sand deposits in the Coos Bay area of southwestern Oregon (table 7), which were explored for chromite, garnet, and iron-rich ilmenite starting in 1989. Oregon Resources Corp., a subsidiary of the former Industrial Minerals Corp, Ltd. (Australia), started recovering chromite in 2011 and was the only domestic producer of foundry-grade chromite until the property became inactive in 2013 (Papp, 2013). As of 2011, the Oregon deposit had JORC¹1

Australasian Joint Ore Reserves Committee professional code of practice that sets minimum standards for Public Reporting of minerals Exploration Results, Mineral Resources and Ore Reserves.

Some chromite placers were related to the State Line podiform chromite deposits of Maryland and Pennsylvania. Pearre and Heyl (1960) suggested that a potential of at least 30,000 tons of chromite concentrates (30–54 percent Cr₂O₃) could remain in placers in the State Line and nearby Soldier’s Delight districts. However, these areas are unlikely as sites of future resources due to urban development.

Alkalic porphyry systems in the Western United States include historical fluorspar producing areas in Idaho, Montana, and Colorado. Tertiary-age fluorite deposits occur in a belt in south-central Idaho that includes the Bayhorse, Meyers Cove, Yankee Fork, and Stanley districts. In the 1950s, the Bayhorse mine and Meyers Cove area (fig. 7) produced about 650 t and 33,900 t of fluorite, respectively (Anderson and Van Alstine, 1964). Measured reserves of 3.2 million short tons of ore (averaging 36 percent CaF₂) were reported for the Bayhorse deposit in the Challis fluorspar area in Idaho (Snyder, 1978). Fluorspar occurs in low-sulfidation epithermal deposits associated with the Zortman syenite in Montana and the Jamestown and St. Peter’s Dome mining districts in Colorado.

In Climax-type systems, fluorite is a primary commodity in fluorspar deposits, greisens, and NYF-type pegmatites, commonly occurring with beryllium-bearing minerals. Examples include a relatively large fluorite deposit at the Daisy mine in Nevada containing more than 80 Mt of ore at an average grade of 10 percent CaF₂ and the Northgate district in Colorado (fig. 7). In the Northgate district, veins and faults in breccia zones associated with Precambrian quartz monzonite produced $25 million worth of fluorspar between 1952 and 1973 (Shawe, 1976; Schwochow and Hornbaker, 1985). The Spor Mountain area of Utah produced more than 350,000 t of fluorspar from 29 deposits starting in 1943 (Hughes, 2019). The deposits are fault-controlled breccias, pipes, and replacements associated with Paleozoic dolomites and Tertiary topaz- and beryllium-bearing rhyolite and rhyolitic tuff.

The Lost Sheep mine, the largest fluorspar producer in the area, was the subject of a 2019 technical report summarizing the historical mining, exploration, and sampling results indicating high-grade (70–89 percent CaF₂) fluorspar deposits, but no recent drilling results or resources have been reported (Hughes, 2019). In 2021, Ares Strategic Mining Inc. announced the results of a geophysical (IP) survey over the permitted mine area and plans for the construction of a plant that would produce metallurgical- and acid-grade fluorspar (Ares Strategic Mining Inc., 2021a, b).

Historically, fluorspar deposits associated with zinc-lead deposits represented the major source of domestic fluorspar production. The Illinois-Kentucky fluorspar district, for example, produced more than 8 Mt of fluorspar from the 1880s until the 1970s (Pinckney, 1976). A large vein at the Klondike II property in the Illinois-Kentucky fluorspar district (fig. 7) contains at least 1.6 Mt at a grade of 60 percent CaF₂ (Feytis, 2009). In the past, fluorspar was also produced in New Hampshire and northern New York.

Fluorspar deposits representing a hybrid of magmatic REE and basin brine systems occur at the Hicks Dome in Illinois (fig. 7). These types of fluorspar deposits were identified in New Mexico, where mines in the fluorite district along the western rim of the Mogollon Mountains provided production in the past.

In magmatic REE systems, the rare-earth mineral bastnaesite—found in carbonatite at Mountain Pass, California—contains about 7 percent fluorine and constitutes 5–15 percent of the rock. Based on an estimated 100 Mt of potential ore at Mountain Pass (Olson and others, 1954), about 1 Mt of fluorine is estimated as a potential byproduct of REE extraction.

Marine phosphate rock in Florida, North Carolina, Tennessee, Utah, Wyoming, Idaho, and Montana (fig. 7) was estimated to represent a potential fluorine resource of about 2 billion tons (Gt) or about 4 Gt of fluorspar (Worl and others, 1973). More than 75 percent of domestic mining of phosphate rock in 2020 came from Florida and North Carolina (U.S. Geological Survey, 2021a).

Helium occurs in oil and natural gas deposits in the Central United States (fig. 8). Focus areas represent basins with current and potential helium production and basins with historical helium production (table 9). The U.S. Government operates the helium production, refining, and distribution system. Since 1962, the Bureau of Land Management has maintained a long-term, large-scale storage facility within the Hugoton-Panhandle gas field complex spanning from southwest Kansas into northwest Oklahoma and the panhandle of Texas. However, this system is being sold off due to declining helium production. New data could revive this critical mineral production system.

Helium-bearing oil and natural gas deposits occur throughout Colorado (fig. 8), including the Piceance Basin, Sand Wash Basin (part of the Greater Green River Basin), Hugoton Embayment, Paradox Basin, San Juan Basin, San Juan Mountains (Sag), San Luis Basin, and Raton Basins. There is some current production at McElmo Dome in the Paradox Basin, and current exploration in Baca County identified 173 million cubic feet (MMcf) of marginal helium reserves (Gage and Driskill, 2001). Marginal reserves of 368 MMcf of helium are reported for the Douglas Creek Arch. Subeconomic and inferred resources are reported for other focus areas in Colorado.

In Wyoming, helium is recovered along with natural gas and carbon dioxide from wells in the northern part of the Moxa Arch (fig. 8) and processed and sold through ExxonMobil's LaBarge-Shute Creek Treating Facility. Production is primarily from the Mississippian Madison Limestone. The Moxa Arch focus area outlines a broad, general region around fields with known production on the northern Moxa Arch (Clark, 1981). Helium was measured in natural gas elsewhere in the State (Clark, 1981; De Bruin, 2004).

In New Mexico, helium has been extracted from produced gases since 1943. Permian Basin reservoirs in New Mexico have elevated helium concentrations associated with regional northeast-trending strike-slip faults that provide migration pathways for helium produced in the underlying Precambrian basement. The Redbed sandstone of the Abo Formation represents the trap with the overlying Yeso Formation acting as a seal (Broadhead, 2005).

Most domestic helium is extracted from the Hugoton-Panhandle (Kansas, Oklahoma, and Texas) and fields along the Moxa Arch in Wyoming (fig. 8). Most helium in Kansas is thought to come from the Precambrian basement, which is brought closer to the surface by the Central Kansas Uplift. Numerous “hot shales” that thicken into the Cherokee-Forest City Basin in eastern Kansas may contribute to shallower production. The two large focus areas in central and eastern Kansas (Kansas and High Plains) outline areas of potential helium resources in the Precambrian basement and shales (fig. 8).

Magnesium has been commercially produced from brines in the Great Salt Lake within the Bonneville Basin of Utah since 1972 (fig. 9). Naturally occurring magnesium chloride in the lake is concentrated by solar evaporation. The magnesium concentration in the lake is variable but averages 0.45 weight percent magnesium (Tripp, 2009).

Intracratonic marine basins contain magnesium salts associated with potash deposits (fig. 9). In the intracratonic Paradox Basin, the Paradox Member of the Middle Pennsylvanian Hermosa Formation contains halokinetic potash-bearing salt comprised of sylvite and carnallite along with halite, dolomite, and anhydrite. Carnallite and langbeinite occur as potash minerals in other marine basins, such as the Permian, High Plains, and Williston basins (Orris and others, 2014).

Sabkha dolomite deposits such as the high-Mg dolomite in the Florida Mountains of New Mexico represent another potential source of magnesium (McLemore and Austin, 2017). American Magnesium LLC is developing plans to quarry dolomite at the Foothills Dolomite deposit near Deming, New Mexico, and process it into magnesium metal at a local mill (Keeven and Torrez, 2020).

In a meteoric recharge system, dissolved carbon dioxide in meteoric groundwater can alter magnesium silicate minerals, such as olivine in ultramafic rocks, to form cryptocrystalline magnesite deposits. Focus areas for magnesite can thus be coincident with focus areas for mafic magmatic deposit types that host chromite because of their similar host rocks. Examples include the peridotite and serpentinite belts in California and the State Line district associated with the Baltimore Mafic Complex in Maryland and Pennsylvania (fig. 9). These types of deposits were mined for magnesite in the past but are much less important now that the technology is available for magnesium production from brines and seawater. In addition to the exposed deposits in the State Line district and Blue Ridge belts, geophysical anomalies indicate buried mafic rocks in Triassic basins in the southeastern United States (fig. 9) that could host ultramafic and mafic magmatic deposits, which could subsequently alter to magnesite.

The Premier mine at Gabbs, Nevada, is the only (2021) active magnesite mine in the United States. This area was explored and drilled in the 1940s for magnesia production during the Second World War (fig. 9). Both magnesite and brucite were produced in the 1940s. The deposit is in Mesozoic carbonate rocks in the Walker Lane terrain in western Nevada that surround a granodiorite intrusion. Skarn magnesite deposits formed at intrusion contacts with dolomite of the Triassic Luning Formation. Magnesite currently produced from the Premier mine is mainly used for animal feed supplements and acid neutralizers. As of 2018, Premier Magnesia, Inc., estimated a mine life of 70 years, with additional reserves and resources and the potential for re-mining waste piles (Harding, 2018).

Other skarn magnesite deposits that produced magnesium in the past include the Currant Creek mining district in Nevada and a 30-mile-long belt in Stevens County, Washington, that produced about 5 million short tons of magnesite between 1916 and 1954 (Campbell and Loofbourow, 1962).

Iron-manganese deposits also form where chemical gradients along ocean chemoclines result in the precipitation of manganese and iron oxides along with carbonates and silicates. A 50-kilometer (km)-long discontinuous belt of manganese deposits in Aroostook County, Maine, is an example of this type of deposit (fig. 10). The Maine deposits are large, with about 300 Mt of ore. However, an economic analysis showed that these low-grade ores (about 9 percent manganese) were not economically viable in the 1980s owing to the costs of mining, beneficiation, and transportation (Kilgore and Thomas, 1982). No further work has been done on these resources.

Other examples of marine chemocline iron-manganese deposits include Precambrian iron formations in the Lake Superior region, such as the Cuyuna Range in Minnesota that produced manganiferous iron ore until 1984 (Cannon and others, 2017).

Manganese oxide (layers, crusts, and nodules) deposits in the Sierra Nevada foothills, the Klamath Mountains of northern California and Oregon, the Blue Mountains island arc of western Idaho, and the Artillery Mountains of Arizona represent remnant seafloor deposits. Before 1957, approximately 175 locations in California produced 263,000 short tons of manganese (Davis, 1957). No recent mining or exploration activity for manganese, however, has occurred in these areas.

Manganese is a potential byproduct in polymetallic sulfide skarn, vein, replacement, and epithermal deposits in porphyry Cu-Mo-Au, Climax-type, and alkalic porphyry systems. The Emma manganese mine (fig. 10) near Butte, Montana, is an example of a relatively small (1 Mt) but high-grade (18 percent manganese) deposit associated with a porphyry system (Kilgore and Thomas, 1982). Active exploration and project development at the Hermosa project in Arizona (fig. 10) is currently being advanced by the South32 company with a combined measured, indicated, and inferred mineral resource of 65.3 Mt at an average grade of 2.2 percent zinc, 2.3 ounces per ton silver, and 9.5 percent manganese for an oxidized carbonate replacement deposit that overlies a zinc-lead-silver deposit (Methven and others, 2018). Some sandstone uranium deposits in meteoric recharge systems contain manganese, in addition to uranium and vanadium.

Lacustrine evaporite systems operate in closed drainage basins in arid to hyperarid climates where elements in meteoric surface, ground, and geothermal recharge waters are concentrated by evaporation. As salinity increases, evaporite minerals typically precipitate in the following sequence: gypsum or anhydrite, halite, sylvite, carnallite, borate. Residual brines enriched in lithium and other elements often accumulate in aquifers below dry lake beds (Hofstra and Kreiner, 2020). In these systems, potash can occur in evaporite minerals and residual brines.

Potash and magnesium are currently produced at the Great Salt Lake and the Bonneville Salt Flats in the Bonneville Basin of Utah (fig. 11). The Great Salt Lake Minerals Corporation uses solar evaporation ponds to produce more than 360,000 t of potassium sulfate annually from surface brines (Rupke, 2012). The Bonneville Salt Flats deposit is estimated to have a 30-year mine life (Mills and Rupke, 2020). The Sevier Lake playa has an estimated in-place resource of 36 Mt of potassium sulfate in shallow brine (Brebner and others, 2018). Evaporation ponds are used to precipitate potash minerals at all of Utah’s processing facilities.

Potash was produced in the early 1900s from the Searles Lake area in the California lithium and potash focus area (fig. 11). Resources at Searles Lake are estimated at 32 Mt of K₂O (British Sulphur Corporation Limited, 1984). The Salton Sea geothermal brines in the southernmost part of the California lithium and potash area have an average potassium concentration of 14,000 milligrams per liter (mg/L); however, no estimates of potassium resources have been made.

Marine evaporite systems operate in shallow, restricted, epicontinental basins in arid to hyperarid climatic zones where elements present in seawater are concentrated by evaporation. As salinity increases, evaporite minerals typically precipitate in the following sequence: gypsum or anhydrite, halite, sylvite. Potash deposits in marine evaporite basins represent significant identified and potential domestic resources. Examples include the Michigan Basin (fig. 11), which produced potash from brine from 1952 to 1970; solution mining at the Hersey mine produced up to 160,000 short tons of potash annually until the mine was decommissioned in 2014. As of 2020, plans were underway to operate additional solution mines in the basin. The southern part of the areally extensive Williston Basin extends from Canada southward into northern North Dakota (fig. 11). The currently productive part of the basin is in Canada; however, the Devonian Prairie Formation in North Dakota is estimated to contain 50 Gt of potash (Anderson and Swinehart, 1979). Potash occurs in six stratigraphic horizons of the Prairie Formation in North Dakota. Maps were made showing the distribution, thickness, and potassium contents estimated from gamma-ray intensities for these formations in northwestern North Dakota (Kruger, 2014; S. Box, USGS, written commun., 2020). Circa 2010, several companies were exploring for sylvinite and carnallite in North Dakota (Wetzel, 2012), but no development has occurred. The North Dakota potash deposits occur at depths that exceed 5,600 ft; therefore, solution mining rather than conventional underground mining is required (Kruger, 2014).

In the Paradox Basin in southeastern Utah and southwestern Colorado (fig. 11), sylvinite ore is mined using solution mining from deeply buried evaporite deposits having proven and probable reserves estimated to last 100 years (Mills and Rupke, 2020). The western part of the Permian Basin in southeastern New Mexico (fig. 11) produces most of the potash in the United States—from sylvinite and langbeinite—from the Permian Salado Formation (U.S. Geological Survey 2021a; Orris and others, 2014).

Other marine basins that host salt deposits are permissive for the occurrence of potash but have not produced potash (Orris and others, 2014). These include Silurian Salina Group strata in the High Plains area of Kansas (fig. 11), where several Permian formations host extensive salt beds containing minor amounts of K₂O; however, no potash production has occurred.

Climax-type systems occur in continental rifts with hydrous bimodal magmatism. Aqueous supercritical fluids exsolved from anorogenic topaz rhyolite plutons, and the apices of subvolcanic stocks, form a variety of deposit types as the supercritical fluids move upward and outward, split into liquid and vapor phases, react with country rocks, and mix with groundwater. Lithocap alunite deposits form in the advanced argillic alteration stages of epithermal activity in the uppermost parts of the system, typically above, or offset from, the causative intrusion. Blawn Mountain, Utah (fig. 11), is a Climax-type porphyry molybdenum system with a well-developed alunite lithocap and represents the largest known alunite resource in the United States. In addition to alunite resources, the deposit has an identified resource of 32 Mt of K₂SO₄ (Kerr and others, 2017; SOPerior Fertilizer Corp., 2019).

Potash-bearing lithocap alunite deposits also formed in some porphyry copper systems in the Southwestern United States. Drilling at the Patagonia alunite property in the Southwest Laramide porphyry belt (fig. 11) in Arizona in the 1970s resulted in a preliminary (noncompliant) resource estimate of 303 Mt of mineralized material, averaging 30 percent alunite, for potential recovery of alumina and potassium sulfate (North American Potash Developments, Inc., 2012). In Wyoming, the Leucite Hills ultrapotassic mafic volcanic rocks (lamproites) were mined during World War I for KCl to use in fertilizer (Thoenen, 1932; Hausel, 2006). These rocks represent an unusual example of deposits classified as “peralkaline syenite/granite/rhyolite/alaskite/pegmatites” in Magmatic REE systems.

In 2019, 0.17 Mlb of U₃O₈ concentrate was produced from U.S. uranium mines (EIA, 2020a). This quantity represents 76 percent less production than the previous year (2018) and is the lowest domestic production since at least 1950 (EIA, 2021b). In 2019, U.S. civilian nuclear power owners-operators purchased 48 Mlb of U₃O₈ averaging $35.59 per pound (EIA, 2020b). Most of this uranium was of foreign origin, primarily from Kazakhstan, Russia, Uzbekistan, Canada, and Australia. The EIA estimates the uranium requirements for civilian operated reactors over the next 10 years to be 388 Mlb of U₃O₈ (EIA, 2020b).

Reasonably assured uranium resources in the United States are estimated by the EIA as 31 Mlb in the forward-cost category²2

Forward costs include power and fuel, labor, materials, insurance, severance and advalorem taxes, and applicable administrative costs. The forward costs used to estimate U.S. uranium ore reserves are independent of the price at which uranium produced from the estimated reserves might be sold in the commercial market. Reserves values in forward-cost categories are cumulative; that is, the quantity at each level of forward cost includes all reserves at the lower cost in that category (EIA, 2021a).

Six countries accounted for 88 percent of world production in 2018 (IAEA–NEA, 2020). These countries produced uranium from sandstone-type deposits in Kazakhstan (41 percent), high-grade unconformity-type deposits in Canada (13 percent), as a byproduct of copper mining the large Olympic Dam IOCG-type deposit and some sandstone-type uranium production in Australia (12 percent), calcrete and intrusive-type deposits in Namibia (10 percent), sandstone-type uranium deposits in Uzbekistan (6 percent), and mostly volcanic-related and some sandstone-type deposits in Russia (5 percent [1 percent lost to rounding]) (IAEA–NEA, 2020).

Basin brine systems can host unconformity and breccia pipe uranium deposits. No unconformity-type uranium deposits have been recognized in the United States, although they are important in Canada and Australia. These systems can form sediment-hosted and replacement copper deposits that contain potential byproduct uranium and vanadium.

The focus area for Northwest Arizona uranium breccia pipes (fig. 12) outlines an area in the Grand Canyon region that includes hundreds of solution-collapse breccia pipes (Van Gosen and others, 2016). After six decades of exploration, however, only a small percentage that contains significant mineralization has been found. Thirteen breccia deposits were mined for uranium from the 1950s to the present; most are mined out and reclaimed (Alpine, 2010). Development of a copper-uranium-bearing breccia pipe at the Canyon Mine, including the construction of a mine shaft, was completed in 2018; mining is inactive pending higher uranium oxide prices (Van Gosen and others, 2020a, 2020b). Geochemical and mineralogical analyses of uranium ores from former mines confirmed previous data showing that the ores are enriched in uranium oxide as well as copper, arsenic, cobalt, lead, nickel, and zinc minerals (Wenrich, 1985; Van Gosen and others, 2020a, 2020b, 2020c). The largest production is from the Hack II deposit, which produced 7 Mlb of uranium oxide (Otton and Van Gosen, 2010). The mined deposits had production numbers that ranged from 428,000 lb of uranium oxide to the 7 Mlb of the Hack II deposit, with average grades ranging from 0.44 to 1.08 percent U₃O₈.

Chemical weathering systems operate in stable areas of low to moderate relief with sufficient rainfall, where the downward percolation of surface water in the unsaturated zone chemically dissolves and concentrates elements present in various rock types and mineral occurrences. Chemical gradients cause different elements to be concentrated at different positions in a weathering profile and at the water table. Dissolved uranium in this setting is reduced on carbonaceous material in lakes and swamps (Hofstra and Kreiner, 2020).

Uranium occurs in chemical weathering systems in surficial and lacustrine deposits and coal. In the southwestern part of the Williston Basin, uranium occurs in carbonaceous shale and lignite throughout multiple horizons of Upper Cretaceous, Paleocene, and Eocene rocks more than 2,500 ft thick (Denson and others, 1965). In North and South Dakota, uranium was produced from lignites in the Paleocene Fort Union Formation. In North Dakota, blanket-type mineralization ranges from 100 to 700 parts per million U, with irregular, higher grade pods from <0.1 to 0.29 percent U₃O₈, and in South Dakota, grades range from 0.1 to 0.4 percent U₃O₈ for mined lignite, with maximum values of up to 2.8 percent U₃O₈ (Dahlkamp, 2010).

The Fort Union Formation extends into eastern Montana, where Paleocene and late Cretaceous lignite and coal beds are widespread, but uranium resources are low-grade based on reconnaissance studies (Boberg, 1975). Uranium-bearing lignite beds 1.5–8 ft thick occur in the Fort Union Formation of the southern part of the Ekalaka Hills, where surface outcrops indicated about 16.5 Mt of subsurface uranium-bearing lignite. The uranium content of the lignite beds ranges from 0.001 to 0.034 percent uranium, the average being about 0.005 percent (Gill, 1959).

Volcanogenic uranium deposits in the Western United States produced uranium in the 1950s and in areas of Colorado until the 1980s. Orebodies at the Los Ochos mine in the Cochetopa areas in Colorado are in brecciated and silicified sandstones and mudstones of the Junction Creek and Morrison Formations and in Precambrian schist. The genesis of the deposits is unclear, but based on work by Olson (1988), who identified Oligocene volcanic rocks as the possible source of uranium, a tentative assignment of volcanogenic uranium (IAEA volcanic-related structure-bound deposit type) is assigned to this area. The Los Ochos mine produced about 1.25 Mlb of U₃O₈ between 1976 and 1981 (T.C. Pool, USGS volunteer with Central Energy Resources Science Center in Denver, written commun., 2017). Before 1971, the Los Ochos Group produced 448,685 t of ore at 0.14 percent U₃O₈, producing 1,253,513 lb of U₃O₈ (Nelson-Moore and others, 1978).

Arizona’s Date Creek focus area includes prospective lake deposits of the Artillery Peak-Date Creek Basin and encompasses the Artillery Peak and Anderson deposits. The Anderson deposit, hosted in Miocene tuffaceous lakebed sediments, had minor historical production (Lindblom and Young, 1958); recent exploration identified an NI 43–101 compliant indicated resource of 15.5 Mlb of U₃O₈ and an inferred resource of 2.5 Mlb of U₃O₈ (Davis and Sim, 2012).

Other examples of volcanogenic uranium deposits include structurally controlled deposits associated with Miocene volcanic rocks in the Lakeview focus area in Oregon and veins and tabular uranium orebodies associated with the McDermitt caldera in Nevada (fig. 12).

The Southeast Piedmont Rift Zones focus area (fig. 12) is delineated for an unusual type of uranium deposit, classified for this study as albitite uranium. The deposit type is based on a genetic model for the Coles Hill deposit in the Piedmont physiographic province of Virginia (Hall and others, 2022). The Coles Hill uranium deposit model indicates favorable areas for concealed mineralization along structural zones adjacent to Triassic basins in the Eastern United States. The type-deposit is the undeveloped Coles Hill deposit with an NI 43–101 compliant indicated resource of 132 Mlb of U₃O₈ (119 Mt at 0.056 percent eU₃O₈ using a 0.25 percent eU₃O₈ cutoff) and an inferred resource of 30 Mlb of U₃O₈ (36 Mt at 0.042 percent eU₃O₈ using a 0.025 percent eU₃O₈ cutoff) (Kyle and Beahm, 2013), making it the largest unmined uranium deposit in the United States.

Calcrete uranium deposits formed by regional groundwater evaporation occur in Pliocene to Pleistocene sediments in the Southern High Plains physiographic province (Hall and others, 2019). The Southern High Plains focus area (fig. 12) comprises the prospective and favorable assessment tracts of an undiscovered resource assessment of this region and includes 15 known occurrences, 2 of which have estimated historical in-place resources (Van Gosen and Hall, 2017). Two calcrete uranium deposits discovered within the focus area in Texas (Sulphur Springs Draw and Buzzard Draw) represent the first identified occurrences of this deposit type in the United States (Van Gosen and Hall, 2017). The deposits have historic non-NI 43–101 compliant drill-delineated resources of about 2.1 Mt of ore with an average grade of 0.037 percent U₃O₈ (Sulphur Springs Draw) and another deposit of about 0.93 Mt of ore averaging 0.047 percent U₃O₈ (Buzzard Draw) (Van Gosen and Hall, 2017).

Carbonate uranium occurrences and mines in the Todilto Limestone in the Grants uranium district, New Mexico (fig. 12), produced 6.6 Mlb of U₃O₈ between 1950 and 1981; many deposits remain undeveloped in the area (McLemore, 2011).

The most important uranium deposit type in the United States is the sandstone-hosted deposit. Forty-eight focus areas are delineated for sandstone uranium deposits, mostly in areas with historical uranium production of Utah, Colorado, Wyoming, and Texas. A few focus areas include currently (2021) productive uranium mines or have modern identified resources. Triassic, Jurassic, Cretaceous, and Tertiary clastic sediments in basins in Wyoming, Colorado, Nebraska, New Mexico, Utah, and Texas host significant roll-front and tabular sandstone deposits. More than 240 mines in the Uravan district in Colorado and Utah produced significant amounts of uranium and vanadium prior to the late 1940s (Chenoweth, 1981).

Currently (2021) the only active uranium mining is from in situ recovery mines in Wyoming. There is only one active, conventional uranium mill (2021)—the White Mesa mill in Blanding, Utah—which is in the central portion of the Colorado Plateau uranium region (Boberg, 2010). The most important regions, based on past production and potential resources, are: (1) the Colorado Plateau (fig. 12), in which mineralization is mostly as tabular sandstone deposits in the Jurassic Morrison Formation and Triassic Chinle Formation, (2) Wyoming Basins including portions of Nebraska and South Dakota where roll-front type mineralization is hosted mostly in Paleocene Fort Union and Eocene Wasatch, Wind River, and Battle Spring Formations and the Cretaceous Inyan Kara Group, (3) the Texas Coastal Plain, throughout which roll-front type uranium deposits form in Eocene to Pliocene sediments (the Claiborne and Jackson groups, Catahoula Formation, Oakville and Goliad Sands, and Beaumont, Lissie, and Willis Formations), (4) the Denver Basin, in which roll-front deposits have been identified in the Cretaceous Fox Hill and Laramie Formations, but remain unmined, and (5) the Tallahassee Creek district, in which mixed sandstone and volcanic type deposits are hosted in a Tertiary graben that developed in the Rocky Mountains (Boberg, 2010, Chenoweth, 1981, Dahlkamp, 2010, Hall and others, 2017).

Phosphate and black shale deposits in marine chemocline systems represent another important system for uranium. Focus areas for these deposits include the Miocene-Pliocene phosphates along the eastern coast of the United States and broad areas of Pennsylvanian phosphate and black shale extending from Texas to New York (fig. 12). See the discussion of these deposits in the vanadium section of this report.

Iron-titanium oxide deposits associated with mafic and ultramafic igneous rocks can host significant amounts of vanadium in vanadiferous titanomagnetite. Although these deposit types represent the largest global source of vanadium, few deposits in the United States have proven economic. The Sanford Lake district in New York produced titanium, iron, and vanadium from deposits hosted in anorthosite and gabbro from 1834 until 1982 (Tahawus, fig. 13). The Ossining mine in southeastern New York is another example. Before 1950, vanadium was produced as a byproduct of titanium mining in the San Gabriel Mountains anorthosite in California. Scattered occurrences of mafic and ultramafic rocks in the North-Central States are permissive for occurrences of these deposit types.

Vanadium is enriched in black shales in many marine chemocline systems, primarily in Proterozoic and Phanerozoic marine settings associated with phosphorite deposits and marine oil shales (Kelley and others, 2017). The Gibellini and Carlin vanadium focus area includes two deposits with identified vanadium resources (fig. 13). The Gibellini vanadium project in Fish Creek, Eureka County, Nevada, targets thin-bedded shales of the Devonian Woodruff Formation in an allochthonous fault wedge along a 21-km northeast-trending vanadium belt. The project is designed as an open-pit heap leach operation. The deposit has NI 43–101 compliant measured and indicated resources of 22.95 Mt of ore at an average grade of 0.286 percent V₂O₅, with additional inferred resources at Gibellini and Louie Hill (Hanson and others, 2018). Production decisions are waiting on a 2021 environmental impact statement “record of decision” for the project (Silver Elephant Mining Corp., 2021). First Vanadium Corporation’s Carlin vanadium project in north-central Nevada also targets shales in the Woodruff Formation. The deposit is partly exposed, although most of the mineralization lies at shallow depths (60 m). An NI 43–101 compliant measured (24.64 Mt at 0.615 percent V₂O₅) and indicated resource (7.19 Mt at 0.520 percent V₂O₅) has been defined for the Carlin project at a cutoff grade of 0.3 percent V₂O₅ (Stryhas and others, 2019).

Vanadium-enriched black shales occur in other areas of the country, such as a broad belt of Devonian and Pennsylvanian black shales (for example, Mecca Quarry, fig. 13) in the Eastern and Central United States, but no resources have been identified. Phosphorite deposits, such as the regionally extensive Phosphoria Formation in the Western United States, also contain vanadium-enriched black shales.

Carbonate uranium deposits occur as collapse breccia pipes within the Little Mountains district in Wyoming (Gregory, 2019) and deposits with a similar geology occur in the Pryor Mountains of Montana (fig. 13). In these deposits, uranium minerals with accompanying silica fill open space in solution collapse features in paleokarst developed in the Mississippian Madison Limestone (Van Gosen and others, 1996; Dahlkamp, 2010). The principal ore minerals of the Pryor Mountains deposits are the uranium-vanadium minerals tyuyamunite [Ca(UO₂)₂(VO₄)₂• 5-8H₂O] and metatyuyamunite [Ca(UO₂)₂(VO₄)₂• 3H₂O]. A quantitative mineral resource assessment by Van Gosen and others (1996) estimated that undiscovered carbonate uranium deposits in the Pryor Mountains area in Montana might contain a mean of 170 t of undiscovered uranium resources (U₃O₈) and 140 t of vanadium (V₂O₅). These amounts of potential undiscovered resources are comparable to the tonnages of uranium and vanadium produced in this area in the past. These deposits are small relative to many other uranium deposit types (Van Gosen and others, 1996).

Calcrete uranium deposits in the Southern High Plains physiographic province are permissive for the occurrence of vanadium (Hall and others, 2019). Although no vanadium resources are available for the known deposits, mineralogy indicates that vanadium occurs in the form of the minerals carnotite and finchite, a newly identified strontium-uranium-vanadium mineral (Spano and others, 2017; Van Gosen and Hall, 2017).

Many sandstone uranium deposits in the United States also produced vanadium in the past. The Shiprock area in northeastern New Mexico (Carrizo deposit, fig. 13) produced about 3.9 Mlb of U₃O₈ and 6,603 short tons of vanadium between 1948 and 1967 (McLemore, 2020). Thirty-two mines in the AEC Circle Cliffs ore-reserve area produced about 70,000 lb of U₃O₈ and 4 short tons of vanadium between 1951 and 1978 (T.C. Pool, USGS volunteer with Central Energy Resources Science Center in Denver, written commun., 2017). The Uravan district in Colorado and Utah includes more than 240 mines with significant historical production of both uranium and vanadium. Before 1947, about 1,700 t of U₃O₈ and 12,000 t of V₂O₅ were mined from the Uravan district (Chenoweth, 1981). Exploration activity continues in the Uravan focus area at the Wray Mesa uranium-vanadium project in southwestern Colorado (Hartman, 2019).

Zircon is a byproduct of mining heavy-mineral sands for titanium minerals from placer deposits. In the Atlantic Coastal Plain focus area, modern economic deposits that are located in Florida, Georgia, and Virginia include Trail Ridge, Mission, and Old Hickory (fig. 14, table 16). The Trail Ridge mine produces titanium minerals (ilmenite, rutile, leucoxene), zircon, and staurolite separated from coastal deposits of heavy-mineral sands. Projected potential mineral production for a proposed Trail Ridge South project for the period of 2021–2028 is 532,690 t of titanium minerals, 184,951 t of zircon, and 173,018 t of staurolite (Urbanomics, 2019). The Mission deposit area in Georgia, explored since the 1970s, is a series of ancient beach ridges, some of which are actively mined through dredging operations. Titanium minerals are the primary ore minerals, with zircon concentrations ranging from 9 to 25 percent (O’Driscoll, 2015). The deposit area produced 5,000 t of zircon in 2014 with an expected production life of 10–15 years. Exploration is ongoing in Virginia around the Old Hickory mine, which produced zircon through 2017. In North Carolina, the focus area includes numerous past-producing mines. The Tennessee Fall Line placer focus area delineates the Cretaceous McNairy Sand, where paleoplacers were prospected and drilled in the past, but no resources are reported.

Some areas in the Western United States produced zircon from stream and river placers, such as modern heavy-mineral sands in central Idaho, where sediments mainly eroded from the Idaho batholith are deposited in valleys (fig. 14). Paleoplacers, which represent ancient coastal deposits, occur in a belt of Cretaceous black sands extending from Colorado into Wyoming, Montana, North Dakota, and South Dakota. The belt traces the distribution of ancient shorelines marked by the Fox Hills Formation, which is permissive for zircon-bearing paleoplacer deposits. Exploration for mostly buried paleoplacers in the Fox Hills Sandstone in the Denver Basin resulted in an estimated 17.5 million short tons of heavy minerals comprised of ilmenite, rutile, zircon, and garnet (Wojcik, 2000). Further studies by the Colorado Geological Survey are underway (O’Keeffe and others, 2019). The resource potential of the Cretaceous black sands to the north of the Denver Basin has not been evaluated.

The Coos Bay placers along the Oregon coast are primarily a chromite resource but have reserves and resources of 18,217,009 t of ore with average grades of 0.16 percent zircon (Industrial Minerals Corp., Ltd., 2011).

Peralkaline syenite/granite/rhyolite/alaskite/pegmatite deposit types in magmatic REE systems have the potential to be enriched in zirconium and hafnium. However, no such deposits are known to occur in the conterminous United States, and these are unlikely to represent a significant source of zirconium. Eight focus areas outline areas broadly permissive for zirconium in igneous rocks, such as the Central Laramie Range focus area in Wyoming. Historically, some LCT-type pegmatites in granite-related porphyry tin systems produced zircon on a small scale; however, these deposits are of mineralogical interest and do not represent significant resources. The Zirconia district of North Carolina is an example of pegmatite deposits that produced large (up to 1.5 centimeters) zircons during mining in the early 1900s (Callahan and others, 2007).