U.S. Department of the Interior

U.S. Geological Survey

Circular 1336

Thorium Deposits of the United States—

Energy Resources for the Future?
Cover photograph. View to the north of the Lucky Horseshoe prospect (middle 

left of photograph), which is one group of numerous exploration trenches, 

cuts, and short adits that explored thorium-rich vein systems of the Lemhi Pass 

thorium district in the 1950s to 1980s. More than 200 thorium-rich veins in the 

Beaverhead Mountains form the Lemhi Pass district, Idaho and Montana. This 

district is thought to contain the largest concentration of thorium resources in 

the United States and is the site of renewed thorium exploration activity.
Thorium Deposits of the United States—

Energy Resources for the Future?

By Bradley S. Van Gosen, Virginia S. Gillerman, and Theodore J. Armbrustmacher

Circular 1336

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

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copyright owners to reproduce any copyrighted material contained within this report.

Suggested citation:

Van Gosen, B.S., Gillerman, V.S., and Armbrustmacher, T.J., 2009, Thorium deposits of the United States—

Energy resources for the future?: U.S. Geological Survey Circular 1336, 21 p. 

[Only available at URL http://pubs.usgs.gov/circ/1336]
iii

Contents

Introduction.....................................................................................................................................................1

Thorium Deposits of the United States ......................................................................................................1

Epigenetic Vein Deposits.....................................................................................................................3

Lemhi Pass Thorium District, Montana-Idaho ........................................................................3

Wet Mountains Area, Colorado.................................................................................................6

Other Thorium Vein Districts in the United States..................................................................7

Environmental Considerations...................................................................................................8

Disseminated Low-Grade Thorium Deposits ...................................................................................9

Carbonatite Stocks ......................................................................................................................9

Alkaline Intrusions.....................................................................................................................10

Black Sand Placer Thorium Deposits..............................................................................................11

Placer Deposits of North and South Carolina.......................................................................11

Placer Deposits of Idaho ..........................................................................................................14

Florida-Georgia Beach Placers...............................................................................................14

Summary........................................................................................................................................................18 

References Cited..........................................................................................................................................18

Figures

1. Index map of thorium districts in United States ......................................................................2

2. View to west of Lemhi Pass, Beaverhead Mountains, Idaho-Montana..............................4

3. View of outcrop of Wonder vein, Lemhi Pass district.............................................................5

4. View of Sewell Ranch thorium vein, Wet Mountains, Colorado...........................................7

5. View to west of McClure Mountain, Fremont County, Colorado...........................................8

6. View to northwest of Iron Hill, Gunnison County, Colorado.................................................10

7. Closeup view of heavy-mineral layers in quartz beach sand, Chennai, India..................12

8. Generalized map of known monazite-bearing placer deposits in Piedmont 

region of North and South Carolina.........................................................................................13

9. Generalized map of known monazite placer districts in Idaho...........................................15

10. Photograph of Porter Brothers dredge in Bear Valley, Idaho, circa 1955–1960...............16

11. Photograph of heavy-mineral sands dredge in southeastern United States ...................17

Tables

1. Ideal compositions of thorium minerals described in this report.........................................1

2. Estimated reserves of ThO2 in significant thorium districts of United States.....................3
iv

Conversion Factors

Multiply By To obtain

kilometer (km) 0.6214 mile (mi)

meter (m) 3.281 foot (ft) 

square kilometer (km2

) 0.3861 square mile (mi2)

metric ton (t) 1.1023 ton, short (2,000 lbs.)
Introduction

Many nations are exploring new ways to meet their growing energy supply needs, with a particular focus upon methods 

that produce lower carbon dioxide emissions compared to 

traditional oil, natural gas, and coal power plants. As a result, 

thorium-based nuclear power has experienced renewed attention as a potential energy source (Branan, 2008).

India, in particular, has committed much research 

towards the development of thorium-based nuclear energy. 

An internet search of the terms “India” and “thorium” reveals 

considerable information on India’s recent efforts to bring a 

thorium-based reactor online in the near future. One of the 

main reasons for India’s interest in thorium-based nuclear 

energy (an interest they have pursued for several decades) is 

that although the country has relatively small uranium deposits, it does possess large indigenous thorium deposits. Easily 

exploitable thorium deposits include large accumulations of 

monazite in alluvial deposits along India’s southern coasts 

(Bhola and others, 1958; Mahadevan and others, 1958).

The potential use of thorium as a nuclear energy 

source was studied as early as 1946 at the Oak Ridge 

National Laboratory in Tennessee (Weinberg, 1994). However, in contrast to uranium, widespread use of thoriumbased energy has yet to be realized. Several test reactors 

designed in the United States, Europe, Japan, Russia, and 

India have successfully generated electricity using thorium 

fuel sources. Although successful, these technologies have 

not advanced beyond experimental reactors thus far. The 

world supply of uranium has been inexpensive and adequate 

to meet supply needs, so there has been less incentive for a 

new nuclear energy technology to replace or complement 

uranium-based reactors.

Thorium has several apparent advantages over uranium 

as a power source: (1) thorium is about three times more 

abundant in the Earth’s crust than uranium; (2) many thorium 

deposits contain other mineral resources of value, such as 

rare earth elements; (3) the spent-fuel waste products of thorium fission are not the types used in nuclear weaponry (such 

as plutonium, a byproduct of uranium power generation); and 

(4) the thorium-based spent fuels contain fewer radioactive 

elements and are smaller in volume and mass than conventional uranium-based nuclear wastes.

If India or another country can prove successful in generating electricity safely and efficiently from a thorium-based 

nuclear power plant, then considerable interest and activity will 

focus on thorium exploration across the globe. Recently, the 

U.S. Congress has expressed interest in thorium-based energy 

(Branan, 2008). Thus, it benefits the United States and other 

countries to identify and evaluate their indigenous thorium 

resources. This report describes the geology and resources of 

the principal thorium districts of the United States (fig. 1).

Thorium Deposits of the United States

Thorium minerals occur in alkaline igneous rocks and carbonatites, but the most concentrated deposits occur in epigenetic veins that surround alkaline igneous complexes. Thorium’s 

genetic association with alkaline igneous rocks also places 

thorium in close association with minerals that host other valuable elements, such as those containing rare earth elements 

(REE), titanium, niobium, and phosphorus. Large titanium 

deposits can exist in the ultramafic units of the alkaline igneous 

complex (Van Gosen and Lowers, 2007). In addition to metallic 

resources, many of the rock units of these alkaline complexes, 

such as nepheline syenite and carbonatite (an apatite source), 

can have industrial mineral uses in their raw crushed form. Several alkaline intrusions contain deposits of vermiculite, formed 

by the surficial weathering and alteration of mica minerals 

(biotite, phlogopite). Alluvial concentrations of thorium minerals (mainly monazite; table 1) occur with other heavy minerals 

of industrial value, such as ilmenite, rutile, magnetite, zircon, 

garnet, staurolite, tourmaline, kyanite, and sillimanite.

Table 1. Ideal compositions of the thorium minerals described 

in this report.

Mineral Ideal composition or range of composition

Monazite (Ce,La,Y,Th)PO4

Thorite (Th,U)SiO4

Brockite (Ca,Th,Ce)(PO4)∙H2O

Xenotime (Y,Th)PO4

Euxenite (Y,Ca,Ce,U,Th)(Nb,Ta,Ti)2O6

Thorium Deposits of the United States—

Energy Resources for the Future?

By Bradley S. Van Gosen, Virginia S. Gillerman, and Theodore J. Armbrustmacher
1 

2 3 4 5 6 78 9 10 11 12 13 14 16 15 1

1

EXPLANATION

 1. Hall Mountain, Idaho

 2. Diamond Creek area, Idaho

 3. Lemhi Pass district, Idaho-Montana

 5. Bear Lodge Mountains, Wyoming

 6. Monroe Canyon, Utah 7. Mountain Pass district, California

 8. Quartzite district, Arizona

 9. Cottonwood area, Arizona

10. Gold Hill district, New Mexico

11. Capitan Mountains, New Mexico

12. Laughlin Peak, New Mexico

13. Iron Hill, Colorado

14. Wet Mountains area, Colorado

15. Near Wausau, Wisconsin

19. Bokan Mountain, Alaska 

 4. Idah

17. Nor

18. Flor

Vein districts

Disseminated deposits

 5. Bear Lodge Mountains, Wyoming

16. Hicks Dome, Illinois 

 7. Mountain Pass carbonatite, California

13. Iron Hill carbonatite, Colorado 

Alkaline intrusions

Carbonatite stocks

B

19

ALASKA

MONTANA

IDAHO

WYOMING

COLORADO

UTAH

ARIZONA

NEW MEXICO

CALIFORNIA

GEORGIA

FLOR

WISCONSIN

ILLINOIS

Figure 1. Index map of thoriu
Carbonatite.—A rare, carbonate igneous rock 

formed by magmatic or metasomatic processes. Most 

carbonatites consist of 50 percent or more primary 

carbonate minerals, such as calcite, dolomite, and (or) 

ankerite. They are genetically associated with, and therefore typically occur near, alkaline igneous rocks. Thorough treatises on carbonatites are provided by Tuttle and 

Gittins (1966), Heinrich (1980), and Bell (1989).

Alkaline igneous rocks.—A series of igneous 

rocks that formed from magmas and fluids so enriched 

in alkali elements that sodium- and potassium-bearing 

minerals form constituents of the rock in much greater 

proportion than “normal” igneous rocks. Alkaline igneous rocks—sometimes referred to as “alkalic rocks”—

are characterized by feldspathoid minerals and (or) 

alkali pyroxenes and amphiboles. For detailed discussions about alkaline igneous rocks and their scattered 

geographic distribution, refer to Sorensen (1974) and 

Woolley (1987).

Thorium Deposits of the United States 3

The significant thorium deposits in the United States 

occur in:

•	 Epigenetic veins peripheral to alkaline intrusions,

•	 Alkaline igneous complexes or carbonatites, and

•	 Alluvial stream and beach deposits (placers) derived 

from the erosion of alkaline igneous terranes.

Lemhi Pass Thorium District, Montana-Idaho

The Lemhi Pass district contains numerous thoriumrich veins within a 140 km2

 core of a larger 400 km2 area in 

the central Beaverhead Mountains; the district straddles the 

Continental Divide on the Montana-Idaho border (fig. 2). 

This district is thought to represent the largest concentration 

of thorium resources in the United States (table 2). Earlier 

studies by the U.S. Geological Survey (USGS) estimated 

that the Lemhi Pass district contains total reserves of 64,000 

metric tons (t) of thorium oxide (ThO2) and probable potential resources of an additional 121,000 t (Staatz and others, 

1979). The 10 largest veins, with an average grade of 0.43 

percent ThO2, represent 95 percent of the district’s identified thorium resources. Using a compilation of surface, 

underground, and drilling assays, the Idaho Energy Resource 

Company reported a “quantitative proven” reserve of 176 t of 

ThO2 within the Last Chance vein with a possible resource of 

2,000 t of additional ThO2 (Idaho Energy Resource Company, written commun., 2008). The Last Chance vein and 

the Wonder vein (fig. 3) are the only deposits in the district 

that have been sampled by underground or drill hole access. 

Much exploration potential exists in the district.

Table 2. Estimated reserves of ThO2 in the significant thorium 

districts of the United States.

[Estimates from Armbrustmacher (1988) for the Wet Mountains area, Colorado, and from Staatz and others (1979, 1980) for the other deposits]

District Total ThO2

 reserves,

metric tons (t)

Vein deposits

Lemhi Pass district, Montana-Idaho 64,000

Wet Mountains area, Colorado 58,200

Hall Mountain, Idaho 4,150

Iron Hill, Colorado 1,700 (“thorium veins”)

690 (carbonatite dikes)

Massive carbonatites

Iron Hill, Colorado 28,200

Mountain Pass, California 8,850

Placer deposits

North and South Carolina

stream placers 4,800 

Idaho stream placers 9,130 

Florida beach placers 14,700

Epigenetic Vein Deposits

Thorium vein districts contain the largest tonnage highgrade thorium resources in the United States. Two thorium-vein 

districts in particular host the majority of the known high-grade 

thorium resources in the United States—the Lemhi Pass district 

along the border between southwestern Montana and Idaho, 

and the Wet Mountains area of south-central Colorado (fig. 1).
4 Thorium Deposits of the United States— Energy Resources for the Future?

Within the Lemhi Pass district, Staatz (1972a, 1979) 

mapped 219 veins enriched in thorium and REE. Most of these 

veins are quartz-hematite-thorite veins, which fill fractures, 

shears, and brecciated zones in Mesoproterozoic quartzite and 

siltite host rocks (fig. 3). Thorium also occurs in monazitethorite-apatite shears and replacements with specularite, 

biotite, and alkali feldspar. The thorium veins of the district 

range from 1 m to at least 1,325 m in length and from a few 

centimeters to as much as 12 m in width. The Last Chance 

vein (1,325 m long and 3–8 m wide over most of its length) is 

the longest and widest vein in the district; this vein also represents the largest individual thorium resource in the district. 

Fifteen thorium veins in the district exceed 300 m in length. 

Some of the veins contain carbonate minerals, such as calcite, 

siderite, and ankerite, and local fluorite. Rare-earth- and 

thorium-bearing allanite and monazite are locally abundant. 

Other reported ore minerals include brockite, xenotime, and 

thorite. The primary gangue minerals include quartz, hematite, 

limonite, apatite, potassium feldspar, biotite, albite, and barite. 

Most of the veins are extensively weathered and have abundant iron-oxide staining. The district also hosts small quartzcopper-gold-veins (and rare molybdenum), and some of the 

thorium veins contain sparse amounts of base metals, such as 

copper, iron, manganese, lead, and zinc.

The thorite veins of the Lemhi Pass district are approximately equally enriched in thorium and REE: the ratio of 

thorium to REE concentrations in the veins averages around 

1:1. Staatz (1972a) reported the REE analyses of 31 vein 

samples, which showed total REE-oxide contents ranging 

from 0.073 to 2.20 percent, with an average value of 0.428 

percent (very similar to the average thorium oxide content 

of 0.43 percent found in the 10 largest veins in the district). 

The thorium veins are most often enriched in the middle 

REE (especially neodymium), with some veins apparently 

enriched more in the heavy REE (Staatz, 1972a, p. 76–77). 

Using modern techniques, recent analytical work by the Idaho 

Lemhi Pass

Figure 2. View to west of Lemhi Pass. The ridge, a part of Beaverhead Mountains, forms a segment of 

the Continental Divide and the Idaho-Montana border, with Montana in foreground and Idaho in distance. 

More than 200 thorium-rich veins in this area form the Lemhi Pass district, thought to represent the largest 

concentration of thorium resources in the United States.
Thorium Deposits of the United States 5

Geological Survey and industry (Thorium Energy in 2008; 

Idaho Energy Resource Company in 1991) has confirmed the 

overall thorium and REE levels along with the unusual middle 

REE-group enrichment in the veins.

In many places the type of high-grade, thorium-REE vein 

system found in the Lemhi Pass district can be clearly linked 

to nearby alkaline magmatic features; however, no obvious 

features of alkaline magmatism were noted in the earlier geologic descriptions of the area. Thorium-bearing quartz veins can 

occur as far as several kilometers from the center of an alkaline 

intrusive system. Yet, no alkaline igneous body was known 

within tens of kilometers of the Lemhi Pass area, and the age 

of the mineralization was unknown, but thought to be Tertiary. 

However, recent mapping by the Idaho Geological Survey, 

supplemented by geochronology, has discovered copper or 

thorium-REE veins that are cut by structures likely Cretaceous 

in age, as well as thorium veins that terminate against preTertiary unconformities. The recent field studies also discovered 

a small outcrop of Cambrian (529 Ma U-Pb age on zircon) 

syenite (80 percent feldspar) that is cut by nonradioactive 

specular hematite veins in the central part of the district (Gillerman and others, 2008). In addition, 40Ar/39Ar geochronology on 

hornblende in mafic dikes, including lamprophyres, indicates 

Paleozoic or older igneous ages, and alteration-related feldspars 

and biotite in the thorium deposits have ages that appear to be 

older than Jurassic. Some evidence for the possible presence 

of an underlying carbonatite is provided by (1) the carbonate 

minerals present in many of the district’s veins, (2) one breccia pipe with a calcite matrix adjacent to a mafic dike, and (3) 

a few carbonate-bearing mafic dikes in the area. Identification 

of the syenite and numerous alkaline mafic rocks, coupled with 

carbonate alteration, provides a potential association between 

the quartz-hematite-thorite-REE veins of the Lemhi Pass district 

and alkaline magmatism. These observations suggest that the 

veins of the Lemhi Pass region could represent distal parts of a 

buried alkaline intrusive complex, which was later structurally 

Figure 3. Outcrop of Wonder vein (between red lines), Lemhi Pass district, exposed in a mined bench. Vein 

is heavily oxidized and consists mainly of silica, likely some carbonate, and iron oxide minerals with thorite 

and altered thorite. Host rock is Precambrian quartzite and siltite.
6 Thorium Deposits of the United States— Energy Resources for the Future?

disrupted by regional Cretaceous thrusting and Tertiary extensional faulting (Gillerman and others, 2003; Gillerman and 

others, 2008). Altered, thorium-enriched intrusives are known 

farther south in the Beaverhead Mountains (Staatz and others, 

1972), but their exact genetic and age relations to the Lemhi 

Pass veins are unknown. Veins in the Diamond Creek area, 

Lemhi County, Idaho, are very similar to those at Lemhi Pass 

and are most likely related. Many geologic questions about the 

Lemhi Pass area have yet to be answered.

Wet Mountains Area, Colorado

This thorium district, which is located in Fremont and 

Custer Counties of south-central Colorado, may be comparable 

in thorium resources to the Lemhi Pass district of Idaho-Montana (table 2). The thorium deposits occur over an area of about 

60 km north-to-south by 24 km west-to-east. Thorium occurs 

in veins, syenite dikes, fracture zones, and carbonatite dikes 

(Armbrustmacher, 1988) associated with three Cambrian-age 

alkaline complexes (Olson and others, 1977) that intruded the 

surrounding Precambrian terrane. These alkaline complexes are 

(1) the McClure Mountain Complex (Shawe and Parker, 1967; 

Armbrustmacher, 1984), (2) the Gem Park Complex (Parker 

and Sharp, 1970), and (3) the complex at Democrat Creek 

(Armbrustmacher, 1984). The thorium-mineralized veins and 

fracture zones, which are distal to the three alkaline intrusive 

complexes, have the highest economic potential for thorium 

resources. The USGS estimated that the vein and fracture zone 

deposits of the Wet Mountains area contain reserves of 58,200 

t of ThO2

 and probable potential resources of 145,600 t of 

ThO2; the seven largest carbonatite dikes in the district contain 

additional reserves of 119 t of ThO2 and probable potential 

resources of 683 t of ThO2 (Armbrustmacher, 1988).

The thorium veins and fracture zones are linear features, typically 1–2 m thick, but a few are as much as 15 m 

thick (fig. 4). Some individual thorium veins can be traced in 

outcrop for 1,500 m and some radioactive fracture zones as 

much as 13 km. Most of these vein- and fracture-zone deposits occur within a 57 km2

 tract of Precambrian gneiss and 

migmatite located south and southeast of the quartz syenite 

complex at Democrat Creek. Christman and others (1953, 

1959) mapped nearly 400 veins in this area. The major minerals forming these veins are smoky and clear quartz, microcline, barite, iron oxides, carbonates, and accessory rutile and 

sulfide minerals. Waxy, red thorite is the primary thorium 

mineral. When broken, the veins and surrounding fenite emit 

a strong fetid odor produced by a variety of hydrocarbon and 

sulfur-bearing compounds (Armbrustmacher, 1988), which 

Heinrich and Anderson (1965) attributed to hydrocarbonfluorine gas of magmatic origin. The odor-producing component in the thorite veins “may be the fugitive constituents 

of complexing agents that combined with thorium to produce 

a highly mobile species capable of being transported under 

pressure-temperature conditions that existed in the hydrothermal solutions” (Armbrustmacher, 1988, p. F5–F6). The veins, 

fracture zones, joints, and associated faults of the area trend 

mainly northwest-southeast and cut across the foliation of the 

host Precambrian metamorphic rocks at nearly right angles. 

On the basis of the analysis of 201 samples of veins and 

fractures, Armbrustmacher (1988) found an average grade of 

0.46 percent ThO2

, with an average thorium:total-REE ratio 

of about 2.2:1.

Thorium mineralization in the Wet Mountains district 

also took place in carbonatite dikes and small plugs. The 

carbonatite dikes are especially associated with the McClure 

Mountain complex (Staatz and Conklin, 1966). The carbonatites occur in a variety of forms, such as (1) composite dikes 

with two or more generations of carbonate side-by-side with 

lamprophyre (Heinrich and Salotti, 1975; Armbrustmacher and 

others, 1979); (2) phreatic explosion breccia pipes satellite to 

the McClure Mountain complex (the Pinon Peak breccia pipes 

of Heinrich and Dahlem, 1967); and (3) siliceous carbonate 

dikes associated with amethyst veining (the Amethyst carbonatites of Heinrich and Shappirio, 1966). Armbrustmacher 

(1979) separated the carbonatites into two groups: (1) replacement carbonatites and (2) primary magmatic carbonatites. 

Replacement carbonatites have microscopic textures that 

indicate the nearly complete pseudomorphous replacement 

of the relict igneous dike mineralogy by carbonate minerals. 

The replacement carbonatite dikes have ThO2

 contents of <0.1 

percent (Armbrustmacher and Brownfield, 1978). In contrast, 

the primary magmatic carbonatite dikes do not display mineral 

replacement textures and are enriched in elements and minerals typical of magmatic carbonatites, including enrichments in 

thorium, niobium, and REE residing in the minerals thorite, 

bastnasite, synchysite, ancylite, and monazite. Thorium concentrations in the primary magmatic carbonatite dikes often 

exceed 0.1 percent ThO2

. Armbrustmacher and Brownfield 

(1978) reported an average value of 0.17 percent ThO2

, with 

a high value of 2.0 percent ThO2

. However, thorium is more 

concentrated and occurs in more volume in the quartz-iron 

oxide-barite vein- and fracture-zone deposits of the district in 

comparison to the carbonatites (Armbrustmacher, 1988).

The Wet Mountains area also contains thin (≤2 m thick) 

red syenite dikes that contain anomalous thorium and REE, 

particularly where the dikes are located nearest the intrusive 

centers. The syenite dikes are composed primarily of alkali 

feldspar and ferric oxides, with trace amounts of thorite, 

barite, rutile, xenotime, bastnasite, and brockite (Armbrustmacher, 1988). Samples of red syenite veins analyzed by this 

study found only 30–40 ppm (parts per million) Th, equivalent 

to 0.0034–0.0046 percent ThO2

.

Sampling and geochemical analyses conducted during 

this study found that the syenite, mafic, and ultramafic rock 

units that form the core of the three intrusive complexes do not 

contain significant concentrations of thorium. Samples of the 

quartz syenite pluton of the complex at Democrat Creek had 

an average content of 62 ppm Th (0.007 percent ThO2

). Gabbro and pyroxenite units of the Gem Park complex showed no 

greater than 13 ppm Th. In the McClure Mountain complex, all 

samples of the plutonic units of hornblende-biotite syenite (fig. 

5), nepheline syenite, pyroxenite, and gabbro had <10 ppm Th.
Thorium Deposits of the United States 7

Other Thorium Vein Districts in the United States

Several other areas with thorium veins exist in the United 

States (Staatz, 1974; Staatz and Armbrustmacher, 1982), but 

they all appear to contain significantly less thorium reserves 

than the Lemhi Pass district of Idaho-Montana and the Wet 

Mountains area of Colorado. Other districts that contain thorium veins (fig. 1) include:

•	 Alaska: Bokan Mountain district, southern part of 

Prince of Wales Island (MacKevett, 1963; Staatz, 

1978; Staatz and others, 1980, p. 19–22; Thompson, 

1988)

•	 Arizona: Cottonwood area, Yavapai County; Quartzite 

district, Yuma County (Staatz, 1974)

•	 California: Mountain Pass district, San Bernardino 

County (Olson and others, 1954; Castor and Nason, 

2004)

•	 Colorado: Iron Hill carbonatite complex (Powderhorn 

district), Gunnison County (Hedlund and Olson, 1961; 

Temple and Grogan, 1965; Olson and Wallace, 1956; 

Olson and Hedlund, 1981)

•	 Idaho: Hall Mountain, Boundary County (Staatz, 

1972b); Diamond Creek area, Lemhi County (Anderson, 1958; Staatz, 1974)

•	 New Mexico: Capitan Mountains, Lincoln County; 

Gold Hill district, Grant County; Laughlin Peak, Colfax County; and other minor deposits (McLemore and 

others, 1988)

•	 Utah: Monroe Canyon, Sevier County (Staatz, 1974)

•	 Wisconsin: near Wausau, Marathon County (Vickers, 

1956)

•	 Wyoming: Bear Lodge Mountains, Crook County 

(Staatz, 1983)

0

0

1 FOOT

1 METER

Gneiss Quartz-barite-limonite-thorite vein Gneiss

Figure 4. Sewell Ranch thorium vein (between yellow lines), Wet Mountains area, Custer County, 

south-central Colorado. This northwest-southeast-trending Cambrian thorium-rich vein, 9 ft (2.7 m) wide 

here, cuts about perpendicular to foliation of Precambrian mafic gneiss country rock. Channel sampling 

of this vein by Christman and others (1953, p. 26–27) found as much as 0.60 percent equivalent ThO2

.
8 Thorium Deposits of the United States— Energy Resources for the Future?

Of these vein deposits, perhaps the most noteworthy are 

those of Hall Mountain, located just south of the Canadian 

border near the middle of the Idaho panhandle. Although these 

thorium-rich veins are exposed across an area of only about 

1,830 m by 300 m, they may be the highest grade thorium 

veins known in the United States. Select vein material from 

this district (Staatz, 1972b) contained as much as 24 percent 

ThO2 content. The primary vein minerals are quartz and 

calcite, and the principal thorium-bearing mineral is thorite. 

Staatz (1972b) described the mineralogy of the Hall Mountain 

veins and contrasted them to those of the Lemhi Pass district. 

The veins of Hall Mountain intruded Middle Proterozoic 

quartzite and quartz diorite sills (Purcell Lava) of the lower 

part of the Prichard Formation of the Belt Supergroup. The 

veins range in length from 2 to 270 m and in thickness from 

thin seams to 4 m (Staatz, 1972b). The larger veins of the district average about 4.0 percent ThO2

, but they are few in number. Thus, Staatz and others (1979) estimated that the veins of 

the Hall Mountain district contain total reserves of about 4,150 

t of ThO2 (table 2).

Environmental Considerations

In addition to the disturbance and impacts typically associated with the surface and underground mining of vein deposits, the development of thorium veins should also consider 

several other potential environmental aspects, including:

•	 The mine wastes and ore materials extracted from 

thorium vein deposits are radioactive materials that 

Figure 5. View to west of McClure Mountain, Fremont County, Colo., composed of hornblende-biotite syenite phase of 

McClure Mountain complex. This stock is genetically related to thorium deposits of Wet Mountains area. However, this 

rock unit, and other core units of three intrusive complexes in this area, contain only modest thorium concentrations of 

<100 parts per million. The thorium resources occur in epigenetic vein and fracture-zone deposits (fig. 4) distal to the 

alkaline intrusive complexes.
Thorium Deposits of the United States 9

thereby require special handling and transportation 

considerations.

•	 These vein types can have moderately high sulfur content, but this sulfur occurs primarily in sulfate form as 

barite, which is quite insoluble.

•	 Thorium-REE veins generally contain low sulfide mineral content, and therefore have little ability to generate 

acidic waters.

•	 The carbonatite veins and masses have an inherent 

capacity for acid neutralization due to their high carbonate content.

•	 The thorium-ore minerals are resistant to chemical breakdown, forming relatively insoluble detrital 

minerals. Thus, thorium minerals contribute only trace 

concentrations of thorium (about 1 part per billion) to 

natural waters (pH 5–9). Thorium is very insoluble in 

most natural waters.

A detailed discussion of the potential environmental 

aspects of thorium-REE vein deposits is provided by Armbrustmacher and others (1995).

Disseminated Low-Grade Thorium Deposits

Large-volume, low-grade deposits of thorium occur in 

carbonatite stocks and some alkaline intrusions in the United 

States. Carbonatites are commonly enriched in thorium and 

REE (Deans, 1966; Heinrich, 1980; Mariano, 1989). That the 

thorium in the majority of these intrusions would be developed 

as the primary commodity seems unlikely; however, thorium 

could be recovered as a byproduct along with the extraction 

of associated mineral resources. Examples of large-tonnage 

thorium-bearing carbonatites and alkaline intrusions in the 

United States are described in the following.

Carbonatite Stocks

The carbonatite stock at the center of the Iron Hill 

(Powderhorn) carbonatite complex in southwestern Colorado (figs. 1, 6) is the largest exposed carbonatite mass in 

the United States, cropping out over an area of about 3.7 

km long by 1.9–0.8 km wide (Olson, 1974; Hedlund and 

Olson, 1975; Olson and Hedlund, 1981). The carbonatite 

stock is the youngest unit of this classic carbonatite-alkaline 

rock intrusive complex, emplaced about 570 million years 

ago (Olson and others, 1977; Olson and Hedlund, 1981) 

in an area about 35 km south-southwest of Gunnison in 

Gunnison County, Colo. The primary rock units of the 

complex include (from oldest to youngest): pyroxenite, 

uncompahgrite, ijolite, nepheline syenite, and the carbonatite (Temple and Grogan, 1965; Nash, 1972; Olson, 1974; 

Hedlund and Olson, 1975; Olson and Hedlund, 1981; Armbrustmacher, 1983).

Much of the complex of Iron Hill has significant mineral 

resource potential owing to enrichments in titanium, REE, 

thorium, niobium (columbium), vanadium, and deposits of 

vermiculite and nepheline syenite (Van Gosen and Lowers, 

2007). The pyroxenite unit of the complex hosts the largest 

known titanium reserve in the United States (Thompson, 1987; 

Shaver and Lunceford, 1998; Van Gosen and Lowers, 2007). 

The carbonatite stock is enriched in REE, thorium, and niobium. The anomalous thorium is mainly due to concentrations 

of the thorium-rich mineral thorite, and at least partially due 

to monazite and pyrochlore (the niobium host mineral), which 

each contain lower thorium content than thorite. Staatz and 

others (1979) estimated that the carbonatite stock of Iron Hill 

consists of 655.6 million t of carbonatite, averaging 0.0043 

percent ThO2 (38 ppm Th), thereby representing a potential 

resource of 28,200 t of ThO2

 (table 2). Sampling of the Iron 

Hill carbonatite stock by Armbrustmacher (1980) found an 

average content of 0.0041 percent ThO2

 (36 ppm Th). Sampling of the carbonatite stock conducted for this study (13 

samples; Van Gosen, 2008) found the following median values: 1,630 ppm total REE (0.20 percent REE oxide), including 

680 ppm Ce; 31 ppm Th (0.0035 percent ThO2

); and 416 ppm 

Nb (0.06 percent Nb2

O5).

The highest thorium concentrations in the Iron Hill 

complex occur in discontinuous veins (“thorium veins”) and 

mineralized shear zones composed of quartz-plagioclasecalcite-iron oxide-thorite that cut the Precambrian granite 

and metamorphic rocks that surround the complex. The 

thorium veins and shear zone deposits range in width from a 

few centimeters to 5.5 m (mostly less than 0.3 m), and they 

range in length from a meter to the 1,067-m Little Johnnie 

vein, which occupies a mineralized fault (Olson and Wallace, 

1956; Hedlund and Olson, 1961). These vein and shear zone 

deposits are estimated to contain reserves of about 1,700 t of 

ThO2

 (Staatz and others, 1979). In addition, several hundred 

carbonatite dikes have been identified within 1.6 km of the 

complex, radiating outward from the carbonatite stock and 

cutting all of the earlier formed intrusive rocks of the complex 

and the Precambrian rocks bounding the complex. These dikes 

are usually 1–2 m wide, but one dike is about 46 m wide. The 

dikes are discontinuous, but can typically be traced for about 

90–150 m along strike. Staatz and others (1979) estimated that 

the 13 longest carbonatite dikes hold reserves of about 690 t of 

ThO2

 (table 2).

Another prominent, thorium- and REE-bearing carbonatite mass in the United States is the Mountain Pass deposit 

near the east edge of the Mojave Desert in the northeast 

corner of San Bernardino County, Calif. This carbonatite 

deposit has world-class REE reserves of more than 18 million 

t of ore with an average grade of 8.9 percent rare earth oxide 

(Castor and Nason, 2004). The typical ore contains about 

10–15 percent bastnasite (the ore mineral), 65 percent calcite 

and (or) dolomite, and 20–25 percent barite, plus other minor 

accessory minerals (Castor and Nason, 2004). The massive 

carbonatite core is called the Sulphide Queen body, which has 

an overall length of 730 m and average width of 120 m (Olson 
10 Thorium Deposits of the United States— Energy Resources for the Future?

and others, 1954). Projecting the Sulphide Queen body down 

dip for 152 m (500 ft), Staatz and others (1979) suggested 

that this mass comprises about 33.4 million t of carbonatite 

containing approximately 8,850 t of ThO2 (table 2). However, 

mining to date (2009) has not recovered the thorium and, furthermore, the thorium here has been considered an environmentally deleterious element because of its radioactivity.

Alkaline Intrusions

Many alkaline intrusions in the United States contain 

average ThO2 concentrations of several tens of ppm, but few 

exceed 100 ppm. As examples, the alkaline Conway Granite 

of New Hampshire (Adams and others, 1962) and alkaline 

granite of the Darby pluton of the Seward Peninsula, Alaska 

(Miller and Bunker, 1976), have similar average ThO2

 contents of approximately 65 ppm. Although plutons of such 

large volume represent very large thorium resources, thorium 

deposits of this low grade seem unlikely as primary targets 

for development. Two disseminated thorium deposits of large 

volume and somewhat higher grade—Hicks Dome in southern Illinois and the Bear Lodge Mountains deposit of northeastern Wyoming (fig. 1)—are summarized in the following.

Hicks Dome lies in Hardin County of southernmost 

Illinois. The dome-shaped structure, approximately 14.5 km 

in diameter, was formed by the displacement of sedimentary 

rocks at least 1,200 m upward above an alkaline intrusion at 

depth. More than 600 m of sedimentary rocks, mostly limestone, were pushed up by the explosive intrusion of magmatic 

fluids (Heyl and others, 1965). A hole drilled near the apex of 

the dome (Brown and others, 1954) intercepted a mineralized 

breccia at a depth of 490 m (1,607 ft) and remained in breccia 

to the bottom of the hole at 897 m (2,944 ft). Analysis of eight 

samples of this drill core, each 7.6–9.1 m (25–30 ft) long, had 

Figure 6. View to northwest of Iron Hill, Gunnison County, southwestern Colorado. Iron Hill is formed by a massive 

carbonatite stock, which forms the center of an alkaline intrusive complex. This complex hosts multiple mineral 

resources, including titanium, niobium, rare-earth elements (REE), and thorium (Van Gosen and Lowers, 2007). The 

carbonatite stock of Iron Hill is estimated to consist of 655.6 million metric tons (t) of carbonatite containing 2.6 million t

of REE oxides, 28,200 t of ThO2

, and 373,700 t of Nb2O5 (Staatz and others, 1979, p. 30).
Thorium Deposits of the United States 11

thorium concentrations ranging from 0.007 to 0.18 percent 

ThO2 (Brown and others, 1954). These samples represent 

only 64 m (210 ft) of a thorium-enriched breccia zone that 

could extend across the roof of Hicks Dome over a large area. 

Shallow diamond drilling and trenching sampled another area 

of radioactive breccia atop Hicks Dome, in which the radioactive mineral was tentatively identified as monazite, found in 

association with florencite, a cerium-aluminum phosphate 

(Trace, 1960). Using airborne gamma-ray data, Pitkin (1974) 

delineated the extent of the thorium anomaly at Hicks Dome. 

However, the considerable depth to this thorium deposit limits 

its resource potential. Much more surface and subsurface 

exploration is necessary to evaluate the extent and grade of this 

thorium deposit.

A disseminated thorium deposit lies in the southern Bear 

Lodge Mountains, about 8 km northwest of Sundance, Crook 

County, Wyo. The deposit is hosted by alkaline intrusions of 

middle Tertiary age, primarily trachyte and phonolite; they 

intruded Paleozoic and Mesozoic sedimentary rocks, forming a dome about 13 km long by 10 km wide. The alkaline 

igneous rock crops out in an oval pattern over an area of 9 

km long by 4 km wide (Staatz and others, 1979). The igneous 

core is microfractured and altered. Thorium mineralization in 

the thin fractures formed coatings and veinlets as much as 6 

mm thick. The coatings and veinlets consist predominantly of 

iron and manganese oxide minerals, along with feldspar and 

quartz. The thorium occurs in monazite, thorite, and brockite. 

Thorium concentrations vary across the alkaline intrusive 

core, ranging from 46 ppm ThO2

 in apparently unmineralized rock to 1,200 ppm (0.12 percent) in rock crisscrossed by 

many veinlets (Staatz and others, 1979). On the basis of 52 

samples collected within an area of 2.4 by 1.6 km, where the 

alkaline rock has numerous small veinlets, Staatz and others 

(1979, p. 27) delineated three subareas: “(1) The northern area 

has an average grade of 0.023 percent ThO2 and 0.75 percent 

combined rare-earth oxides, (2) the central area has an average 

grade of 0.042 percent ThO2 and 1.71 percent combined rareearth oxides, and (3) the southern area has an average grade 

of 0.035 percent ThO2 and 1.35 percent combined rare-earth 

oxides.” They also noted that drilling “indicates that the veining extends at least 1,200 ft [365 m] below the surface.”

Black Sand Placer Thorium Deposits

Globally and in the United States, alluvial accumulations of monazite are a significant type of thorium deposit. 

For example, the alluvial monazite deposits of the Coastal 

Belt of southernmost India are thought to represent one of 

the largest thorium resources in the world. The monazite 

deposits of southern India contain detrital heavy minerals 

and are found in piedmont lakes, in shallow seas, in parts of 

the beaches (fig. 7), in sand bars across the mouth of rivers, 

in deltas, and in sand dunes behind the beaches (Bhola and 

others, 1958). A study by Mahadevan and others (1958) estimated that the beach sands of the southwestern coast of India 

alone contain estimated reserves of 446,400 t (492,200 short 

tons) of monazite, in which the ThO2 content of the monazite 

varies from 7.5 to 9 percent.

Monazite is a REE- and thorium-bearing phosphate mineral [(Ce,La,Y,Th)PO4

] whose thorium content can range from 

a few percent to 20 percent. Monazite’s resistance to chemical 

weathering and high specific gravity accounts for its association with other resistant heavy minerals such as ilmenite, 

magnetite, rutile, and zircon. Monazite weathers from alkaline 

crystalline rocks of the surrounding region and is transported 

downstream and deposited by alluvial processes.

In the United States, alluvial deposits of monazite are 

known to occur in the intermontane valleys of Idaho, the 

Carolina Piedmont of North and South Carolina, and the beach 

deposits of northeastern Florida–southeastern Georgia. In 

the past, mining of these unconsolidated stream deposits was 

conducted by small-scale sluicing (Idaho and North and South 

Carolina) and dredges (Idaho); beach sands were mined by 

large shovels (Florida-Georgia). Sluicing instantly produces 

a heavy-mineral separate, but even in the case of the dredge 

and shovel operations, the heavy-mineral separation was 

still performed at the site. Thus, the mining of thorium from 

alluvial deposits has the advantages of relative ease of mining and rapid mineral separation, in comparison to hard-rock 

mining for thorium. Another benefit of placer thorium deposits 

is the potential for coproduct development. Coproducts can 

include REE obtained from monazite; titanium from ilmenite 

and rutile; iron from magnetite; zirconium and hafnium from 

zircon; and industrial-grade garnet, staurolite, tourmaline, 

kyanite, and sillimanite, which are used as abrasives and as 

refractory minerals.

As with any mining operation planned for a riparian 

environment, considerations beyond grade (thorium concentration) and reserves must be addressed, such as (1) assessing 

environmental issues associated with “in-stream mining;” and 

(2) obtaining access, mineral rights, and permits to work the 

stream channel and disturb adjacent lands as needed. Much 

information is available online regarding the many issues to be 

considered by in-stream mining operations.

The three monazite placer districts highlighted in the 

following—North and South Carolina stream deposits, Idaho 

stream deposits, and Florida-Georgia beaches—are the largest volume known alluvial thorium deposits in the United 

States (fig. 1). The geology and estimated thorium resources 

of these districts have been well described by Staatz and 

others (1979, the North and South Carolina placer deposits, 

p. 33–39) and Staatz and others (1980, the Idaho stream placers, p. 9-18, and the Florida beach deposits, p. 3–9); numerous references cited therein provide more detailed information on these deposits. Thus, we will only summarize the 

findings of these earlier studies.

Placer Deposits of North and South Carolina

In 1887, a few short tons of monazite were produced 

from stream deposits in the Piedmont region of North and 

South Carolina, giving this region the distinction of being the 
12 Thorium Deposits of the United States— Energy Resources for the Future?

world’s first supplier of thorium (Olson and Overstreet, 1964). 

Monazite-bearing placers of this region were worked by smallscale sluice operations from 1887 to 1911 and 1915 to 1917, 

producing a total of 4,973 t (5,483 short tons) of monazite 

(Overstreet and others, 1968). Monazite mining ended here 

in 1917, not because reserves had been exhausted, but rather 

because the beach deposits of India and Brazil were producing 

thorium at lower cost.

The high-grade monazite placers of the Piedmont 

of North Carolina and South Carolina occur between the 

Catawba River on the east and the Savannah River on the west 

(fig. 8). The stream sediment deposits across this region are 

generally consistent in character; the heavy-mineral concentrations are greatest in the headwaters areas. The alluvium is 

deposited in flat valleys, forming well-bedded, poorly graded 

layers of unconsolidated sediment. Stacked layers contain 

gravel, sand, clay, and clayey silt, at an average total thickness 

of about 4.5 m (Staatz and others, 1979). Monazite typically 

occurs in all units, but is generally most abundant in the basal 

gravel layers and least abundant in the clay layers. Dredging 

in this area between the summers of 1955 and 1958 (Williams, 

1967) found heavy-mineral contents of about 1 to 1.5 percent; 

monazite formed about 8 percent of this fraction (Mertie, 

1975). These dredging operations recovered monazite, ilmenite, rutile, zircon, and staurolite (Williams, 1967). 

The region is underlain by crystalline, high-grade metamorphic rocks intruded by quartz monzonite and pegmatite. 

The monzonite and pegmatite intrusions vary from monazite 

bearing to monazite free. Overstreet (1967) suggested that the 

primary source of the alluvial monazite was the high-grade 

metamorphic rocks, particularly sillimanite schist.

According to Staatz and others (1979), the overall heavymineral content of the placer deposits of this region ranges 

from 0.15 to 2.0 percent; monazite forming about 3.5–13 

Figure 7. Heavy-mineral layers (“black sand”) in a quartz beach sand, Chennai, India. Photograph taken by Mark A. Wilson, 

Department of Geology, The College of Wooster, Wooster, Ohio; Mr. Wilson has granted permission for use of this photograph. 

Penny for scale.
Thorium Deposits of the United States 13

EXPLANATION

 1 South Muddy Creek

 2 Silver Creek

 3 First Broad River and its tributaries—Hinton

 Creek, Duncans Creek, and Wards Creek

 4 Knob Creek

 5 Sandy Run Creek

 6 Buffalo Creek

 7 Broad River at its junction with

 Buffalo Creek

Placers mentioned in text

 8 Thicketty Creek

 9 North Tyger River at its junction with

 Middle Tyger River

10 North and South Rabon Creeks

11 Big Generostee Creek

12 Horse Creek

13 Hollow Creek 

General area containing placers

Figure 8. Generalized map of known monazite-bearing placer deposits in Piedmont region of North and 

South Carolina. From Staatz and others (1979).
14 Thorium Deposits of the United States— Energy Resources for the Future?

percent of the heavy minerals. They also stated that the other 

parts of the heavy-mineral fraction include ilmenite, 20–70 

percent; garnet, 2–50 percent; rutile, 0.3–7 percent; zircon, 

trace to 14 percent; and sillimanite and kyanite together, trace 

to 20 percent. Other heavy minerals present in some placers 

include epidote, magnetite, xenotime, tourmaline, sphene, 

staurolite, andalusite, and an unidentified black radioactive 

mineral (Staatz and others, 1979). Analysis of 52 samples of 

alluvial monazite from this region (Mertie, 1975) found that 

the monazite contains 60-63 percent total rare-earth oxides 

and 2.5–7.8 percent ThO2 content, with a mean value of 5.67 

percent ThO2.

For the 13 largest placer deposits of the Piedmont region 

of North and South Carolina, Staatz and others (1979) estimated total reserves of about 4,800 t of ThO2

 (table 2), with 

potential resources seven times higher. Their estimate was 

based on regional thorium resource studies by Overstreet and 

others (1959) and Overstreet (1967), and the studies of individual drainage basins by several others (see Staatz and others, 

1979, p. 37). Future exploration for monazite placer deposits 

in the mid-Atlantic region can be aided by regional stream 

sediment geochemistry datasets assembled by the USGS 

(Grosz, 1993).

Placer Deposits of Idaho

A number of monazite-bearing placer districts exist in 

the valleys of a region that extends north of Boise, Idaho, and 

along the western flank of the Idaho batholith (fig. 9). Monazite was first recognized here in 1896, as the heavy, yellow 

to brownish-yellow mineral that collected with other heavy 

minerals and gold within the sluice boxes of gold placer 

operations in the Boise Basin near Idaho City, Centerville, 

and Placerville (Lindgren, 1897). In 1909, a mill designed to 

capture the monazite was built by the Centerville Mining and 

Milling Co. Only a small amount of monazite concentrate 

was produced before the mill burned down in a forest fire in 

1910.

In the 1950s, two areas of west-central Idaho were 

mined by dredges for monazite recovery—Long Valley and 

Bear Valley (figs. 9, 10). Beginning in September 1950, 

Long Valley was worked by three dredges that were earlier 

used to recover gold, but were converted to recover monazite. 

The dredges were redesigned for monazite recovery with 

assistance from the U.S. Bureau of Mines under the sponsorship of the U.S. Atomic Energy Commission. The history 

of these dredging operations is described by Argall (1954) 

and Staatz and others (1980, p. 9–16, and references cited 

therein). The heavy minerals recovered in the Long Valley 

district were dominated by ilmenite (84 percent of heavies), 

followed by monazite (8 percent), garnet (5 percent), and 

zircon (3 percent). During this 5-year period, Staatz and others (1980) estimated that the three dredges recovered 6,430 

t (7,085 short tons) of monazite containing 269 t (297 short 

tons) of ThO2

. The dredging ended here in mid-1955, when 

the government stockpile order was fulfilled.

Thorium was also unintentionally recovered within the 

minerals euxenite and monazite from the Bear Valley placers (fig. 10). The Bear Valley placers were worked by first 

one dredge in 1955, then a second in 1956, with the intent to 

recover niobium and tantalum for another Federal government 

contract. According to Staatz and others (1980, p. 10), “from 

alluvium of Bear Valley, 2,049 short tons [1,858 t] of euxenite, 

83.5 tons [75.7 t] of columbite, and 54,862 tons [49,760 t] of 

ilmenite were recovered.” No records of the monazite recovery 

were kept.

Most of the Idaho thorium resource data used by Staatz 

and others (1980) came from a 1950s program in which the 

U.S. Atomic Energy Commission funded the U.S. Bureau of 

Mines to identify new domestic monazite reserves (Storch and 

Holt, 1963). The USGS (Staatz and others, 1980) conducted 

an extensive review of the results of this program, including 

a review of the U.S. Bureau of Mines literature, interviews 

with former dredge companies, and field reconnaissance and 

sampling of the major thorium placer districts in Idaho. They 

determined that the five most important thorium districts 

(fig. 9) are Long Valley, Bear Valley, the Boise Basin, the 

Burgdorf-Warren area, and the Elk City–Newsome area. The 

reported ThO2 contents of monazite in the Idaho placer deposits ranges from 2.2 to 6.24 percent. Staatz and others (1980) 

calculated thorium reserves for each of the five major placer 

districts individually; they estimated that the five districts have 

total reserves of about 9,130 t of ThO2 (table 2).

The primary source of the resistant thorium-bearing minerals in the Idaho placers is thought to be the Idaho batholith, 

in particular the quartz monzonite and pegmatite phases of 

the batholith (Mackin and Schmidt, 1957). The most common 

heavy minerals in the alluvial deposits (in generally decreasing 

amounts) are ilmenite, magnetite, sphene, garnet, monazite, 

euxenite, zircon, and uranothorite (uranium-rich thorite). In 

addition to REE and thorium from monazite and euxenite, 

the Idaho placer deposits (and the abandoned historic dredge 

waste piles) contain coproducts of titanium (in ilmenite), and 

niobium and tantalum (from euxenite).

Florida-Georgia Beach Placers

The modern and raised Pleistocene and Pliocene beach 

deposits of northeastern Florida and southeastern Georgia host 

low-grade but persistent concentrations of thorium. Heavy minerals constitute a small part of the beach sands, and monazite 

forms a small part of the heavy minerals. However, due to the 

large tonnage of suitable beach sand deposits and the ease of 

mining and processing this material, these beach deposits represent a very large thorium resource. Staatz and others (1980) 

estimated that the beach placer deposits of this region contain 

total reserves of about 14,700 t of ThO2

 (table 2) and 198,000 t

of rare-earth oxides, which occur in 330,000 t of monazite.

Some of the heavy-mineral operations in beach deposits of 

Florida were once domestic suppliers of monazite. Staatz and 

others (1980, p. 3) noted “During 1978 monazite was produced 

from two of the three operating heavy-mineral deposits in 
42°

43°

44°

45°

46°

47°

48°

49°

117° 116°

115°

114°

113° 112°

8

5

6

4

1

3

11

10

2 7

9

IDAHO

EXPLANATION

 1 Long Valley

 2 Bear Valley

 3 Boise Basin

 4 Burgdorf Warren area

 5 Elk City–Newsome area

 6 Dixie-Florence area

 7 Stanley Basin

 8 Clearwater River

 9 Salmon River

10 Rock Creek–Camp Creek area

11 Dismal Swamp 

Monazite placer districts

3 Placer locality and number

Approximate boundary of

Idaho batholith

0

0 50 100 150 KILOMETERS

50 100 MILES

Thorium Deposits of the United States 15

Figure 9. Generalized map of known monazite placer districts in Idaho. 

From Staatz and others (1980).
Figure 10. Porter Brothers dredge in Bear Valley (Idaho) columbium-tantalum placer deposits. This dredge operated from 1955 to 1960; exact date of Monazite was produced as a byproduct from these placer deposits. Photograph from Savage (1961, fig. 13); used with permission of Idaho Geological 
Thorium Deposits of the United States 17

Florida: Titanium Enterprises at Green Cove Springs and Humphrey Mining Corporation at Boulogne recovered monazite as 

a byproduct.” These deposits were mined primarily for titanium 

(in ilmenite and rutile), which was used by the pigment industry. Other minerals sold from these deposits include kyanite, 

sillimanite, staurolite, garnet, and the host sand itself. Staatz 

and others (1980, p. 3–4, and references cited therein) described 

the mining history of the heavy-mineral beach placers of the 

northeastern Florida–southeastern Georgia area. Mining ceased 

in this area in late 1978: increasing environmental regulations 

made mining operations more costly, which was compounded 

by steep increases in coastal real estate values. Thus, heavymineral concentrations in the modern beaches became much 

less valuable than the real estate they occupy.

In addition to the modern beach deposits, monazite-bearing 

sands occur in the raised Pleistocene and Pliocene beach 

deposits that lie as much as 80 km inland; they are deposits of 

former shorelines. Relict shorelines, which lie 3–33 m above the 

sea level, occur in the outer coastal plain region from Maryland 

to Florida. Thorium exploration in the Eastern United States 

coastal plain areas can benefit from several aeroradiometric 

maps that were compiled and interpreted by the USGS (Force 

and others, 1982; Grosz, 1983; Grosz and others, 1989; Owens 

and others, 1989).

The monazite and associated heavy minerals in the 

relict shoreline deposits of the southeastern United States 

were eroded from crystalline rocks of the Piedmont province 

(Mertie, 1953), carried towards the Atlantic Ocean by streams 

and rivers, and eventually redeposited by coastal processes. 

The natural concentration of heavy minerals in the shoreline 

area is a multistage process, involving transport by longshore 

drift, gravity separation by specific gravity and particle size 

and shape, differential chemical weathering (Neiheisel, 1962), 

wave action, and, in some parts of the coastal environment, the 

actions of tides. All of these forces rework the sediments in the 

shoreline environments through time and naturally concentrate 

the heavy minerals (see Force, 1991, p. 73–84). The most 

abundant heavy mineral in the southeastern United States 

shoreline deposits is ilmenite, in many places forming more 

than 50 percent of the heavy-mineral fraction. Monazite forms 

a minor part of the heavy-mineral fraction, usually less than 1 

percent. Despite the low concentrations of monazite (and thus, 

thorium and REE) in the typical coastal placer deposit of the 

southeastern United States, these deposits have a few distinct advantages as potential sources of thorium: (1) they are 

relatively easy deposits to excavate (fig. 11); (2) it is relatively 

easy to separate the heavy-mineral fraction on-site; and (3) 

these deposits contain multiple salable mineral products. 

Figure 11. Heavy-mineral sands dredge in southeastern United States.
18 Thorium Deposits of the United States— Energy Resources for the Future?

Summary

Thorium as a potential source fuel for energy generation 

has gained new attention in the United States, including consideration in the U.S. Congress (Branan, 2008). Thus, it benefits 

the United States to recognize and reevaluate its domestic thorium resources (fig. 1). This report summarizes the significant 

known thorium districts in the United States, and, more importantly, points to the considerable amount of published geologic 

and resource information that is available from previous studies 

of these districts. In particular, past studies by the USGS and 

the former U.S. Bureau of Mines have provided a notable 

legacy of information on the significant thorium districts and 

deposits in the United States.

Vein deposits host the largest volume known high-grade 

thorium resources in the United States. Two thorium vein 

districts—the Lemhi Pass district of Montana-Idaho and the 

Wet Mountains area of Colorado (fig. 1)—dominate the known 

high-grade thorium reserves in the United States (table 2). In 

recent years, exploration and claim staking of the thorium veins 

in the Lemhi Pass district has been underway, straddling both 

sides of the Continental Divide in the district. Average grades 

of 0.43 percent ThO2 content in the Lemhi Pass veins, along 

with similar concentrations of rare-earth oxides, has sparked 

renewed interest in these vein deposits. Veins and mineralized 

fractures of similar grade occur in the Wet Mountains area of 

south-central Colorado; the analysis of 201 vein and fracture 

samples from this area by Armbrustmacher (1988) revealed 

average values of 0.46 percent ThO2, with an average thorium 

to total-REE ratio of about 2.2:1.

Massive carbonatites, which are rare in the United 

States, also contain thorium resources (table 1). Examples 

are the Mountain Pass REE deposit in California and the Iron 

Hill (Powderhorn) carbonatite complex in Colorado (fig. 1). 

Because thorium concentrations in these intrusions are low, 

development most likely would be as a byproduct of the other 

mineral resources, such as redevelopment of the enormous 

REE reserves of the Mountain Pass carbonatite deposit (Castor 

and Nason, 2004). Niobium and REE resources are commingled with the thorium mineralization products in the Iron Hill 

carbonatite (Van Gosen and Lowers, 2007).

Resistant and dense thorium-bearing minerals, chiefly 

the mineral monazite, occur in the black sand heavy-mineral 

concentrations of some stream and beach deposits. These alluvial deposits have been mined in the past by placer methods 

(sluicing, dredging). Notable examples occur in intermontane 

stream valleys of Idaho, in stream and river deposits of the 

Piedmont region of North and South Carolina, and in beach 

deposits of northeastern Florida–southeastern Georgia (fig. 1; 

table 1). Whereas alluvial deposits have the lowest thorium 

concentrations of the thorium deposit types, they do have 

practical advantages favoring their development, such as relatively low mining costs, easier separation of the heavy-mineral 

concentrates, and the potential for coproduction of other mineral commodities. Coproducts may include REE in monazite; 

titanium in ilmenite and rutile; iron in magnetite; zirconium 

and hafnium in zircon; and industrial-grade garnet, staurolite, 

tourmaline, kyanite, and sillimanite, salable as abrasives and 

as refractory minerals.

As interest increases towards thorium as an alternative 

nuclear energy fuel source, exploration interest will become 

renewed on the three principal types of domestic thorium 

deposits—veins, disseminated, and alluvial (placer). The 

intent of this report is to provide an overview of the significant thorium deposits of the United States, and highlight the 

breadth of preexisting published information that can be used 

as a framework for the next generation of thorium exploration 

in the United States.

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Manuscript approved for publication March 10, 2009

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Van Gosen and others—Thorium Deposits of the United States—Energy Resources for the Future?—U.S. Geological Survey Circular 1336