GEOLOGICAL SURVEY CIRCULAR 805 

Principal Thorium Resources 

in the United States 

"'-wi .... ......." 01". 

u.s. 0.,-11, .... 01 &.vr 
Principal Thorium Resources 

in the United States 

By M. H. Staatz, T. J. Armbrustmacher, J. C. Olson, 

I. K. Brownfield, and Maurice R. Brock, U.S. Geological 

Survey and J. F. Lemons, Jr. I L. V. Coppa, 

and B. V. Clingan, U.S. Bureau of Mines 

GEOLOGICAL SURVEY 

Prepared on behalf of the 

u.s. Department of Energy 

CIRCULAR 

Thorium resources are tabulated for the maior vein districts, 

desseminated deposits, massive carbonatite bodies, and the 

stream placers of North and South Carolina 

J979 

805 
United States Department of the Interior 

CECIL D. ANDRUS I Secretary 

Geological Survey 

H. William Menard, Director 

Free on application to Branch of Distribution, U. S. Geological Survey, 1200 South fads Street, Arlington, VA 22202 
CONTENTS 

Page 

Abstract ......... ---.--------................ --..................... --... - .. .. 1 Disseminated deposits -........... - .... - ......... - ................ . 

Introduction ............ -........... -.... -.... -............... -........... - 2 Hicks Dome, IlL .-.-..... - •• -••••••••....••.•. - ......... -..... . 

Summary .... - ...................... - .. - .................................... . 3 Bear Lodge Mountains, Wyo .............. - .. - .. -..... . 

Veins .--.--.-................... ---.... - ........... -.. --.... -............ - 4 Powderhorn, Colo ..... -.-......... -.-.......... -...... -.......... . 

Lemhi Pass district, Montana-Idaho .. - .......... -.. -.. - 9 Massive carbonatite bodies ..... - ... _ .. _ ..................... .. 

Wet Mountains district, Colorado .... - ................. . 12 Iron Hill body, Powderhorn district, Colorado --.. -

Powderhorn district, Colorado ... - •.. - ...... -.... - .. -.. 13 Sulphide Queen body, Mountain Pass, Calif ..... -.. -

Hall Mountain district, Idaho ....... -.. --......... -.-..... 15 North and South Carolina placer deposits .--.. _ ......... . 

Diamond Creek district, Idaho -.. --...................... - 16 Beach placer deposits in Florida and Georgia .-....... -... . 

Bear Lodge Mountain district, Wyoming .... -... , ..... . 17 References cited ................. - ............ - .. -.-.-.......... -... -

Mountain Pass district, California .... -... -........... -.. . 19 

ILLUSTRATIONS 

Page 

20 

21 

23 

27 

27 

29 

31 

33 

39 

40 

Page 

FIGURE L Locations of thorium deposits in the United States ............... ----.-.. - ............................. - .. -........... -_ .. _ .. - 5 

2. Plan of an acid·leach solvent·extraction mill for recovery of thorium and an alternate circuit for recovery of 

rare earths -............ -....... - .......... --... -.......... -.......... - ... - .. - ............. -.-.-.-... -.... - ........... - ... --.--.... -.. 8 

3. Geologic and aeroradiometric map of Hicks Dome, Ill ........... --...... -... --.--.. ----... -.-.---.. --... -----............. --------- 22 

4. Thorium·content and radiometric map of northern part of the alkalic intrusive core in the Bear Lodge 

Mountains, Wyo ...... -.-...... - ....................................... ---.. -..... -..... --.-.----.. ---.-.. - .. --... -.-.--.............. 25 

5. Relationship of thorium content to total radioactivity in samples from the Bear Lodge Mountains, Wyo.--- 26 

6. Generalized geologic map and section of the original surface of the Sulfide Queen carbonatite body .. -·--····- 32 

7. Known stream·placer areas in North and South Carolina .. - .. -..... -.-.--........ -----.--.... --.---......... - .. --.. -. 35 

TABLES 

Page 

TABLE 1. ThO. reserves and probable potential resources in short tons of selected types of thorium deposits in the 

United Sta tes .. -.---.--..... -.... ------.--.----.--.... --........... ----.... -.......... - ... - .................................. - ............ ------ 4 

2. Th02 reserves and probable potential resources in short tons from seven thorium vein districts in the 

United States .--.-..... --..... - ..................... - ... - ........ --.-.... -..... --.---..... ---.. -..... -........... - ... -.... - ..... - ... -_.... 10 

3. Th02 reserves and probable potential resources in short tons from two massive carbonatite bodies in the 

United States .. --......... -....................... -.... ----.. ---.-.. - .......... -...... --.............. -... ---...................... - .... -...... - 30 

III 
Principal Thorium Resources in the United States 

By Mortimer H. Staatz, Theodore J. Armbrustmacher, Jerry C. Olson, 

Isabelle K. Brownfield, and Maurice R. Brock, 

U. S. Geological Survey 

and 

Jim F. Lemons, Jr., Luis V. Coppa, and Benjamin V. Clingan, 

U.S. Bureau of Mines 

ABSTRACT 

Resources were assessed for thorium in the higher grade 

and better known deposits in the United States in: U) veins, 

12) massive carbonatites, (31 stream placers of North and 

South Carolina. and (4) disseminated deposits. Thorium 

resources for the first three categories were divided into 

reserves and probable potential resources. Each of these then 

were separated into the following cost categories: U) the 

amount of ThO. prodicible at less than $15 per pound. (2) the 

amount producible at between $15 and $30 per pound, and (3) 

the amount producible at more than $50 per pound. The type 

of mining and milling needed at each deposit determines the 

capital, operating, and fixed costs of both mining and milling. 

Costs start with the clearing of land and are carried through 

to the fmal product, which for all depOSits is ThO •. Capital 

costs of mining are affected most by the type of mining and 

the size of the mine. Those of milling are affected most by the 

kind of mill, its size, and whether or not extra circuits are 

needed for the separa tion of rare earths or some other 

byproduct. 

Veins, massive carbonatites, and stream placers of North 

and South Carolina have reserves of 188,000 short tons of 

ThO. and probable potential resources of 505,000 tons of 

ThO •. Approximately haH of the reserves and probable potential resources can be produced at less than $30 per pound of 

ThO •. Veins are the highest grade source in the United States 

and have total reserves of 142,000 tons of ThO. and probable 

potential resources of 343,000 tons. About 90 percent of the 

reserves and 91 percent of the probable potential resources 

can be produced at less than $15 per pound of ThO •. Seven 

vein districts were evaluated: (11 Lemhi Pass, Mont.-Idaho, 

(2) Wet Mountains. Colo .. (3) Powderhorn. Colo .• (4) Hall 

Mountain, Idaho, (5) Diamond Creek, Idaho, (61 Bear Lodge 

Mountains, Wyo. and (7) Mountain Pass, Calif. Eighty·seven 

1 

percent of the total reserves and probable potential resources 

are in the Lemhi Pass and Wet Mountains Districts. The first 

district has reserves of 68,000 tons of ThO. and probable 

potential resources of 124,000 tons that can be produced at 

less than $15 per pound; the second district has 54,000 tons 

of reserves and 141,000 tons of probable potential resources 

producible at less than $15 per pound. Rare earths are a common byproduct, and in many veins they are from one-haH to 

several times as abundant as thorium. 

Massive carbonatite bodies are large-tonnage low-grade 

deposits. Thorium in these deposits would be a byproduct 

either of rare earth or of niobium mining. The Iron Hill carhonatite body in the Powderhorn district, Colorado, and the 

Sulfide Queen carhonatite body in the Mountain Pass 

district, California, were evaluated. These two deposits contain 40,800 tons of ThO. in reserves and 125,000 tons of ThO. 

in probable potential resources. More than 80 percent of this 

total is in the Iron Hillcarbonatite. This thorium is entirely a 

byproduct and is producible at less than $15 per pound of 

ThO •. The Sulphide Queen massive carbonatite deposit was 

being mined in 1977 for rare earths. and thorium could be 

recovered by adding an extra circuit to the existing mill. 

Stream placers in North and South Carolina occur both in 

the Piedmont and just east of the Fall Line. The reserves of 

these deposits total 5,270 tons of ThO., and the probable 

potential resources are 36,800 tons of ThO •. The Piedmont 

placers are all too small to produce ThO. at a cost of less than 

$50 per pound. One placer on Hollow Creek, S.C., just east of 

the Fall Line had reserves of 2,040 tons of ThO. that is producible at between $15 and $30 per pound. Thorium occurs in 

monazite in these placers. Other heavy minerals that would 

be recovered with the monazite include rutile, zircon, and il· 

menite. In addition to thorium, monazite contains large 
amounts of rare earths and small amounts of uranium; both 

can be recovered during the process that separates thorium 

from the monazite. 

Thorium resources in the disseminated deposits are not as 

well known as those of other types of deposits, and they do 

not fit into the Department of Energy classification of 

reserves and probable potential resources. They are 

categorized here as other resources. Total other resources in 

three disseminated deposits is 2,460,000 short tons of ThO,. 

The three deposits evaluated are (1) a deposit made up of fractured and veined trachyte and phonolite in the Bear Lodge 

Mountains, Wyo., (2) fine-grained granite in the Powderhorn 

district, Colorado, and (3) a large erypyovolcanic structure at 

Hicks Dome, Ill. Any mining would be underground. as most 

of this deposit occurs at considerable depth. The Bear Lodge 

Mountains and Powderhorn deposits are exposed at the surface and could be mined in open pits. Rare earths are much 

more common than thorium in the Bear Lodge Mountains 

deposit and probably would be recovered as coproducts in 

any mining operation. 

Another source of thorium from monazite is the Pleistocene 

beach placers of Florida and Georgia. During 1967 to 1977 

these placers were the only source of monazite mined in the 

United States. They are being mined principally for their 

titanium minerals. although zircon, garnet, monazite, 

staurolite, sillimanite, and kyanite are also marketed where 

present. Monazite makes up only 0.3 to 1.5 percent of the 

heavy mineral concentrates. Increases in the amount of 

thorium produced from the beach placers most likely will be 

due to greater demand for titanium. The amount of Th02 in 

monazite recovered from heavy-mineral concentrates that 

will be produced between 1978 and 1988 is projected at between 250 and 350 tons. 

INTRODUCTION 

Thorium has been known for a long time to be a 

potential source material for nuclear reactors; its 

resources, however, have received little attention. 

This paper on thorium resources is the first 

definitive study of these resources in the United 

States, although some detailed work was done in 

the early 1950's on the placers of the western 

Piedomont by the U.S. Geological Survey and the 

U.S. Bureau of Mines under the auspices of the 

U.S. Atomic Energy Commission. This resource 

work was summarized in a paper on the thorium 

and uranium resources of North and South 

Carolina Piedmont by Overstreet, Theobald, and 

Whitlow ~1959). The only resource calculations 

made in a thorium-vein district were done at the 

Lemhi Pass· district, Idaho and Montana, by 

Sharp and Hetland (1968) in the mid-1960's for 

the U.S. Atomic Energy Commission. They 

calculated the thorium resources of this area to be 

100,000 tons (short tons) of Th02 (1968, p. 11). 

This figure, with a few minor additions, has been 

2 

widely cited as the amount of thorium reserves in 

the United States (Organization for Economic 

Cooperation and Development, 1965, p. 20; 

Sondermeyer, 1975, p. 173). 

United States thorium resources are not confined to one district or even to one type of deposit. 

Thorium occurs in this country in veins, 

disseminated deposits, carbonatite bodies, 

stream placers, beach placers, unconsolidated 

and consolidated sedimentary rocks, and granitic 

and alkalic igneous rocks; Grades, quantities, and 

cost of recovering thorium from these different 

types of resources vary considerably. 

Data on resources used in this report are mainly a byproduct of regional studies of thorium 

districts made by the U.S. Geological Survey during the past 15 yr (years). Armbrustmacher is 

responsible for the section on veins in the Wet 

Mountains and the massive carbonatite body at 

Iron Hill in the Powderhorn district. Olson put 

together the sections on veins and fine-grained 

granite in the Powderhorn district. Brock compiled the data on Hicks Dome. Staatz wrote the 

other sections either from his notes or from the 

literature. Brownfield calculated the costs of producing ore from various properties. The costs of 

different types of mining and milling were 

calculated by the U.S. Bureau of Mines' team of 

Lemon, Coppa, and Clingan. 

This report does not describe all the thorium 

deposits in the United States, but it does include 

most of the higher grade ones, as well as the 

greater part of the known resources that could be 

mined relatively cheaply. Specifically, it 

describes (1) most of the thorium vein districts, 

including all the larger ones; (2) disseminated 

deposits, (3) massive carbonatite bodies, and (4) 

stream placers of North and South Carolina. It 

also has an estimate of the amount of ThO:? present in the monazite that will be recovered in 

heavy mineral mining in Florida and Georgia in 

the next 10 yr. 

This study was limited by two factors: 

(1) economic boundaries, and (2) lack of sufficient 

data on some deposits to evaluate them adequately. Some thorium can be obtained as a byproduct 

at a very low cost, other thorium is in deposits 

that are high grade, but some thorium deposits 

are both low grade and expensive to mine. These 

latter deposits, which will not be of economic interest for many years, purposely are not covered 

in this report. They include most thorium-rich ig-
neous rocks. such as the Conway Granite of New 

Hampshire. as well as some thorium-rich consolidated sedimentary rocks. The other limiting 

factor is that sufficient data have not been collected to evaluate certain districts or types of 

deposits. These unevaluated thorium deposits include (1) stream placers in Idaho. (2) beach placers 

along the Atlantic coast. (3) unconsolidated 

sedimentary rocks. (4) ancient Precambrian conglomerate and sandstone. (5) carbonatite dikes. 

and (6) some of the smaller vein districts. At present these thorium deposits are being studied. 

This study was supported by a contract with 

the U.S. Department of Energy. The terms 

"reserves" and "probable potential resources" 

used in this report are those of the Department of 

Energy and are used so that the thorium 

resources can be compared more directly with 

uranium resources. This comparison is important, as both metals compete as fuel sources for 

nuclear reactors. "Reserves" as used by the 

Department of Energy. is equivalent. for the 

most part, to the term "measured and indicated 

reserves" as used by the U.S. Geological Survey 

and the U.S. Bureau of Mines. "Probable potential resources" is, likewise, equivalent to "inferred reserves" and some hypothetical resources. 

Resource figures for veins, massive carbonatites, 

and stream placers of North and South Carolina 

are r"'ported two ways. First, the reserves and the 

probable potential resources of each district are 

given. Then these figures are divided into cost 

categories-the amount of each that could be produced for less than $15/1b ($15 per pound) of 

Th02, the amount of each that could be produced 

at cost of between $15 and $30/lb for Th02, and 

the amount of each that could be produced at cost 

of between $30 and $50/lb for Th02• These cost 

boundaries were chosen so that they would be 

directly comparable to those used by the Department of Energy in reporting uranium resources. 

SUMMARY 

Four types of thorium deposits are discussed in 

this paper. Three of these. veins, massive carbonatites, and Carolina stream placers have their 

resources well enough known to be classified as 

reserves and probable potential resources. These 

three types of deposits have total reserves and 

probable potential resources of 693,000 tons of 

3 

Th02• This figure can be divided into 188,000 

tons of reserves and 505,000 tons of probable 

potential resources. If these deposits were only 

mined for thorium. then 68 percent of the 

reserves and 62 percent of the probable potential 

resources are producible at less than $15/1b of 

Th02• Another 4 percent of the reserves and 3 percent of the probable potential resources are producible between $15 and $301lb of Th02, and 29 

percent of the reserves and 6 percent of the probable potential resources are prodUCible at between $30 and $50/lb of Th02• The largest 

amount (70 percent) of the total reserves and probable potential resources is in the veins. The 

North and South Carolina placers, historically 

considered a large resource because of their past 

production, make up only 6 percent of the total 

reserves and probable potential resources. These 

placers were mined at the turn of the century 

principally for the thorium-bearing mineral 

monazite, using shovels and wooden sluices. but 

the only modern operation, using a dredge with a 

connecting mill. was a multiproduct operation in 

which monazite, rutile, zircon, ilmenite, 

staurolite, sand, and gravel were recovered 

(Williams, 1967, p. 25). Th02 could be produced 

from this placer at $30/lb without the aid of 

byproducts. Any thorium obtained from massive 

carbonatite bodies would be as a byproduct either 

of rare earths or of niobium. Details of the total 

amount of Th02 in the four types of deposits are 

given in table 1. What the table does not show is 

that many of these deposits have equally 

valuable coproducts. Vein deposits, in general, 

have a higher Th02 content and less byproducts 

or coproducts than do other types of deposits. 

The fourth type is disseminated deposits whose 

resources are not as well defined as the other 

three types of deposits. These resources are 

classified as other resources because present 

available data do not satisfy the definitional 

restraints of the formal reserve and probable 

potential resource categories. This group of 

resources is estimated to contain 2,200,000 short 

tons of Th02• These deposits, which are in general 

of a lower tenor than the veins, make up the bulk 

of all the Th02 resources (70 percent). Some of the 

disseminated deposits also contain rare earths 

and niobium. Although the disseminated 

deposits would not be the first deposits exploited, 

they would undoubtedly playa major role in any 

large-scale thorium exploitation. 
TABLE I.-ThO. reserves and probable potential resources in short tans of selected types of thorium deposits in the United 

States 

Total producible at Total producible at Total producible at 

less than $15/lb between $15 and $30/lb between $30 and $50/lb 

Type of Total Total probable 

depos it reserves potent ial Reserves 

resources 

Vei ns ••••••.•••••• 142,000 343,000 128,000 ~assive 

carbonatite 

bodies •..•••..•• 40,800 125,000 a 

Carolina placers •• 5,270 36,800 0 

Tota 1 •••••••••• 

VEINS 

Veins have the largest resource of relatively 

high-grade thorium in the United States. Most 

are quartz-feldspar-iron oxide-thorium-bearing 

veins, which are known from at least 17 widely 

scattered areas in the United States (fig. 1). These 

areas are: (1) Lemhi Pass, Mont.-Idaho; (2) Wet 

Mountains, Colo.; (3) Powderhorn district, Colorado; ~4) Hall Mountain, Idaho; (5) Diamond 

Creek, Idaho, (6) Bear Lodge Mountains, Wyo., 

(7) Mountain Pass. Calif.; (8) Laughlin Peak; 

N. Mex.; (9) Capitan Mountains, N. Mex.; (10) 

Gold Hill. N. Mex.; (11) Cottonwood area, 

Arizona; (12) Quartzsite area, Arizona; (13) 

Rawhide Mountains, Ariz.; (14) Granite Wash 

Mountains, Ariz.; (15) Monroe Canyon, Utah, (16) 

Wausau, Wis.; and (17~ Bokan Mountain, Alaska. 

In some of these districts only one small vein or 

breccia zone is known, but in some areas many 

veins are found. In both the Wet Mountains and 

the Lemhi Pass districts more than 100 veins 

have been found. Some districts are small-the 

veins at Hall Mountain are localized in an area of 

only 0.5 mi2 (square mile)-but in some districts, 

the veins are scattered over a large area. The 

veins in the Lemhi Pass district are scattered 

over an area of at least 160 mi2 and the veins of 

the Wet Mountains are scattered over an area of 

at least 350 mi2. 

The thorium-bearing veins generally are 

tabular bodies from several feet to at least 5,000 

ft (feet} long (Staatz, 1974, p. 496). Some are narrow lenses. Many veins are more than several 

hundred feet long and in the Lemhi Pass district, 

12 veins are more than 1,000 ft in length. Veins 

range in thickness from 1164 in. (inch) to 50 ft. Individual veins commonly are variable in 

Probable Probable Probable 

potent i al Reserves potent i al Reserves potential 

resources resources resources 

4 

311,000 4,610 15,100 8,000 11,500 

0 0 0 9,750 9,750 

0 2,040 0 ·0 0 

21,250 

thickness. Although most of the veins are 

tabular, some are lenticular, and in others, vein 

material is interstitial to brecciated unmineralized country rock. The mineralogy of the thorium 

veins generally is similar; quartz or feldspar is the 

principal gangue mineral, and both commonly are 

present. Iron oxide veinlets, which are in some 

places accompanied by manganese oxides, commonly cut the earlier gangue minerals. Thorite is 

the most common thorium mineral, although in 

individual veins, monazite. allanite, or brockite 

may be the principal thorium-bearing mineral. 

Lesser amounts of barite, calcite, muscovite, 

biotite, apatite, fluorite, rutile, and pyrite occur 

in many veins. In addition, minor amounts of 

many other minerals occur locally in some veins. 

For example, a total of 42 minerals have been 

identified in the veins of the Lemhi Pass district 

(Staatz. 1972b, p. 60). 

This report describes the resources of only the 

larger and better known vein districts. These are: 

(1) Lemhi Pass, (2) Wet Mountains, (3) 

Powderhorn, (4) Hall Mountain, (5) Diamond 

Creek, (6) Bear Lodge Mountains, and (7) Mountain Pass (fig. 1). The veins in other areas are, in 

general, small in size and few in number. In a few 

areas, such as the Bokan Mountain district, 

Alaska, and Laughlin Peak, N. Mex., insufficient 

data are available to evaluate them adequately. 

They are, however, not as large as either the 

Lemhi Pass or Wet Mountains districts. We feel 

that the greater part of the known reserves of 

thorium in veins are described in this paper. 

Undiscovered thorium resources in veins are 

likely to equal those already discovered. Thorium, 

unlike uranium, has been prospected little during 

the past 20 yr. The lack of economic incentive has 
Q1 

.3'0· 

Ntn/ADA 

...... 

KILOMETERS ;>0. ""- o 500 1000 ! ! J I 

60.~ 

\' 

.i"~ .. ..... "'. "_ ",.~ .. 

110· 100· 

MONTANA NORTH DAKOTA 

SOUTH DAKOTA 

NEBRASKA 

KANSAS 

OKLAHOMA 

90· 

MISSOURI 

80· 

EXPLANATION 

• Veins 

)( Disseminated deposits 

• Massive carbonates 

,,;', Placers .. ' 

o 200 400 600 KILOMETERS 

I I I I 

FIGUR E 1. - Location of thorium deposits in the United States. 

70· 
hampered exploration on most veins, and only a 

few have been traced their entire length on the 

surface or have been explored at depth. Thus, the 

size of many known deposits is likely to increase 

as future exploration increases, new deposits will 

be discovered in the known districts, and new 

districts will be found. 

The thorium resources for each district are 

reported in two ways: (1) total resources of all the 

veins, and (2) resources that can be produced at 

three sets of costs. The first category is divided 

into reserves and probable potential resources of 

veins as thin as 0.2 ft. Reserves are those 

resources that are, in part, directly measured and 

sampled. Reserves in these veins are calculated in 

the following manner: known length of the vein 

times average thickness times a depth equal to 

one-third the length of the vein. The depth of none 

of the veins, however, is carried to more than 

3,000 ft. Veins in districts such as the Lemhi Pass 

and the Powderhorn are exposed over considerable differences in elevation, and this limit 

appears well warranted. Probable potential 

resources are extensions of the reserves both 

laterally and at depth, and they form a block of 

ore that surrounds the reserves. To identify the 

probable potential resources, the vein was extended on the surface one-quarter of the known 

length in each direction. This distance was 

shortened where rock exposures indicated that 

the vein either pinched out or had ended by 

faulting. In a few places, this distance was 

lengthened where anomalous radioactivity in the 

soil indicated a continuation of the vein. The 

same average vein thickness was used as in 

calculating the reserves. The depth of the probable potential resources block is one and onehalf times the trace of the vein, except that the 

total depth of reserves plus the probable potential 

resources is limited to 3,000 ft. All veins 

regardless of length are included in the total 

resource calculation. If a vein is only exposed in 

one place, it was assumed to have a length of 50 

ft. In a few areas, such as in the Bear Lodge 

Mountains, the veins are short and tend to pinch 

out over short distances. Here the length of the 

vein exposed in only one place is assumed to be 25 

ft; these exceptions are noted later under the 

discussions of each district. Although the 

resources of even the shortest veins are included 

in the overall resource calculation, the total 

amount of resources of all veins having length of 

less than 200 ft is insignificant when compared to 

those of the entire district. A tonnage factor of 12 

ft 3/short ton (cubic feet per short ton) is used in 

converting vein volume to vein weight. 

The grade of the deposit was obtained by cutting samples across the width of the vein. In a few 

places. where exposures are poor, grab samples of 

previously dug-out material were collected. All 

samples were analyzed for thorium and some, for 

rare earths and other elements. The average 

grade of a vein was obtained by adding together 

the product of the grade of the sample times the 

length of the sample cut and dividing by the total 

width of the vein samples cut. In other words, the 

grade is an average proportioned so that samples 

taken where the vein is thick were weighted more 

than those taken where it is thin. 

To compare the resources of thorium with those 

of uranium, the other nuclear fuel, the former are 

also calculated according to their cost of production. These resources are divided here into 

reserves and probable potential resources that 

can be produced at less than $151lb of Th02, those 

that can be produced at between $15 and $30Ilb, 

and those that can be produced at between $30/lb 

and $50Ilb. To obtain a basis for calculating these 

costs, a team from the U.S. Bureau of Mines 

made cost analyses for both mine and mill 

models. The mine models used for making cost 

calculations is the cut-and-fill mining system, 

which was chosen because of its flexibility, grade 

control, and ground-support advantages. This 

method, which is designed to use mill tailings as 

backfill, would fit almost all thorium veins 

because of their similarity in shape, size. and attitude. Using this method, the veins would be accessed by vertical shafts and connecting drifts. 

Each deposit would be mined from successive 

equally spaced levels. After the initial shaft sinking was made to get the vein in production, each 

, shaft would be advanced concurrently with the 

mining. Along each level, a number of stopes 

would be made by a series of horizontal cuts 

above the level. As one cut is completed, mill tailings. consisting of approximately 60 percent 

solids, would be hydraulically pumped into the 

space to use as a working platform for further 

mining as well as for ground support. This mining 

cycle would be repeated until the level had been 

completely mined and filled; the next lower level 

would then be mined. The cut-and-fill system is 

6 
not only an excellent mining method, but because 

of the lack of surface dumps, there are no 

downstream silting problems. The ore would be 

drilled using air-powered jackleg drills and 

blasted using a breasting round. Slushing equipment and mucking machines would be used to 

remove the ore from the stopes. The ore would be 

hauled by rail to the shaft and hoisted to the surface. Cost calculations are based on a 3-ft mining 

width. If the vein is less than 3 ft thick, then the 

grade of the ore is diluted by the amount of waste 

necessary to maintain this mining width. Mine 

recovery of the ore would be assumed to be 90 

percent. Although the resources of all veins of 

more than a certain minimum thickness may be 

included in total reserve and resource calculations, some properties do not contain enough ore 

to be economically mined. For purposes of the 

present calculations, properties containing less 

than 35,000 tons of minable ore were considered 

uneconomical and are not included in the 

resources producible at less than $15/1b or between $15 and $30/1b, or $30 and $50/1b pound. 

Mining costs include the cost of mine development, mine plant, mine equipment, and operating 

costs. Mine-development cost includes surface 

clearing, roads, initial shaft sinking, drifting, and 

other preproduction development. Min..e-plant 

cost includes all buildings, hoists, and 

underground facilities needed during mining. 

Mine-equipment cost includes all the equipment 

and supplies needed to bring the property into 

production. Operating costs are, in turn, divided 

into direct, indirect, and fixed costs. Direct 

operating costs are those directly incurred during 

mining and include labor, supervision, material, 

utilities, and haulage. The haulage cost is for 

trucking the ore to the mill and varies with the 

distance hauled and the amount of ore carried. 

The indirect operating costs are for administrative and clerical help, plant facility, and 

general overhead. Fixed operating costs are local 

taxes and insurance. Fixed costs can be estimated 

as approximately 3 percent of the capital cost. 

Milling costs are much more difficult to 

calculate than mining costs, as a full-scale mill for 

recovering thorium from veins has not been built. 

Data used to obtain milling costs were derived 

from limited chemical, mineralogical, and 

metallurigical tests. These analyses indicate that 

because of the small grain size of the various 

7 

thorium-bearing minerals and their common inclusion in gangue minerals, different types of 

physical beneficiation including tabling, size 

analysis electromagnetic separation, flotation, 

and electrostatic separation would be ineffective. 

Thorium, however, can be effectively recovered 

from these ores by using acid-leach solventextraction techniques. Rare earths, which commonly are as abundant as thorium in these veins, 

also can be best recovered by leaching and 

precipitating from solution. The separation of the 

rare earths adds to the cost, because in order to 

have a good recovery of rare earths without loss 

of the thorium, a more concentrated sulfuric acid 

must be used and an added heating step is 

necessary. Therefore, if rare earths are to be 

recovered, the added cost of obtaining the rare 

earths must be balanced against the value of the 

rare earths obtained A mill plan was designed 

based both on available data from the literature 

and on discussions with various people who are 

familiar with thorium and rare earth recoveries. 

This mill plan has either a thorium circuit or both 

thorium and rare earth circuits, depending on the 

cost analysis of the ores for a particular area. A 

generalized flow sheet showing the principal 

steps of an acid-leach solvent extraction mill for 

recovery of thorium with alternate circuits for 

recovery of rare earths is shown in figure 2. 

Milling costs, like mining costs, are divided in 

to capital and operating costs. Capital costs are 

for building the mill and buying sufficient equipment to operate it. The principal factors that affect the original costs of the mill are its size and 

whether or not it has a circuit for recovery of rare 

earths. One large mill receiving ore from a 

number of small properties would be less expensive generally than several smaller mills, as the 

capital cost of one large mill probably will be less 

than that of the total capital cost of the several 

smaller mills, and the operating cost of the large 

mill will be lower due to its larger volume. For the 

most efficient use of equipment, it is best to plan 

for a mill life of 15 yr or longer. Mill operating 

costs can be divided into direct, indirect, and fixed costs. The direct costs include fuel, power, 

reagent consumption, ball mill consumption, 

labor, supervision, and supply maintenance. The 

indirect costs are for administrative and clerical 

help, facility maintenance, and general overhead. 

Fixed costs are local taxes and insurance. Three 
AO 

l..../' 

~ 

~ 

~ 

0 

r---

CRUSHING 

~ Thonte ore from mine 

L

S 

.... 

I 

.... 

e 

.... 

p

-'---.---1-ln-..... r-D-;-~IND-, 

~.-------------~ I 

11/ I Alternate I 

WET GRIND LSlep 2 --I -:6Q Me-;h 

Slep 2 

L..-':"'-_"'---a-s---JMeSh r-;;;D CURE-' 

I k---concentrate I Alternale I H2S0 4 

I!tep~ __ J ACID LEACH 

Step a 1""' __ 1 

__ 1 

I ACID BAKE 

200'C 1 

IlAlternate I .", ~I--J 

r ACID LEACH-' 

~--------------1 k-I Dilute 

I Alternate H2SO 4 

\II _Step"::: ___ J 

COUNTER 

CURRENT 

DECANTATION ~ Tails discard 

Slep 4 

AO 

SOLVENT 

EXTRACTION I ... iRONHYDROLYSIS 1 

AO AND FILTRATION I 1--------.... \lT-,- - - - - 3Ii ~ Fe20a discard 

Slep i) 

.. _~ Alternate I - Discard I Step 1 0 J 

0 

SCRUBBING 

-E--pilute H2SO4 

Step 6 

° NHa used in reagent makeup 

\~ ~ I 

STRIPPING 

AO '" 

Step 7 

Th0 2 recovery circuit 

STEAM 

DISTILLATION 

Slep B 

EXPLANA TlON 

---r--

1 .... -RARE EARTH-' 

PRECIPITATION, 1 RareNHa -?1 DRYING ~ earth 

1 

Alternate I . cake 

_S_te_p _'_1 ___ J 

FILTRATION 

AND 

IGNITION at 

gOODe 

Slep g 

AQ Aqueous solution j-, Rare-earth recovery circuit L_J o Organic amine 

FIGURE 2.-Plan of an acid-leach solvent-extraction mill for recovery of thorium and an alternate circuit (dashed 

lines) for recovery of rare earths. 

8 
variables that most aff~ct the operating cost of 

the mill are its size, the use of additional reagents 

and power in a rare-earth circuit, and acid consumption during leaching. Acid consumption is 

dependent on the composition of the ore leached. 

If the calcium carbonate content of the ore is 

greater than 10 percent, the amount of sulfuric 

acid needed will be sharply increased. If, however, 

the calcium carbonate content of the ore is less 

than 10 percent, the acid consumption appears to 

be related to the iron content. 

Calculating the milling cost also involves making several decisions on the size, number, and 

location of the mill or mills in a particular district. 

These decisions involve visualization of what will 

actually occur at a future date and so will vary according to the person making estimates for a particular district; however, they are based both on 

various economic aspects of the area and on what 

would be done by a prudent person. Certain 

economic limitations would be imposed on the 

size of a mill. An acid-leach solvent-extraction 

mill design could be built to handle from 100 to 

2,000 short tons per day for 350 days a year. In 

areas where there are many properties, the mill 

would be a custom mill that would handle ore 

from all properties up to its capacity. In the 

Lemhi Pass district, topography is a consideration in selecting mill sites. The Continental 

Divide separates this district into two parts, and 

roads over it are steep, crooked, and not usable 

during the winer months. Hence, mines on either 

side of the divide would truck ore to mills on their 

side of the divide. The position of the mill is important in individual mine-cost calculations, as it, 

in part, determined the cost of trucking the ore 

from the mine to the mill. Mill sites were chosen 

in most areas so that they would be close to the 

largest mine furnishing ore, have a good water 

supply, and be easily accessible to the principal 

roads of the district. 

The thorium veins in the seven districts 

discussed in this paper have total reserves and 

probable potential resources of 485,000 tons of 

Th02• Ninety-one percent of this total or 439,000 

tons are producible at less than $15/Ib; 4 percent 

or 19,700 tons are producible at between $15/1b 

and $30/Ib; and another 4 percent or 19,600 tons 

are producible at between $30 and $50/Ib. The 

amounts of ThOz and estimated cost of production for the different districts are shown in table 2. 

9 

LEMHI PASS DISTRICT, MONTANA-IDAHO 

The Lemhi Pass district lies astride the Continental Divide, which here separates Montana 

and Idaho. The altitude of this district ranges 

from 4,700 ft along the Lemhi River on the west 

side of the district to 9,400 ft on the Continental 

Divide. The district, which is in the rugged' 

Beaverhead Mountains, is approximately 13.5 mi 

north-south by 12 mi east-west. A total of 219 

thorium veins (Staatz, 1979) are scattered 

through this large area. The veins occupy shear 

zones in quartzite and siltite of Precambrian Y 

age. These rocks resemble the rocks of the Belt 

Supergroup farther to the north in that they are 

roughly the same age and their lithology is grossly similar. They differ in being somewhat coarser 

grained, in lacking calcareous beds, and in having 

been deposited in a separate basin. Except along 

the steeper ridges, outcrops are widely scattered. 

Few thorium veins are exposed. Those which crop 

out generally have a high quartz content or are on 

a ridge. Only where a vein consists mainly of 

quartz will it be more resistant than the surrounding country rock and protrude above it. 

Thorium-bearing minerals tend to be concentrated in the iron oxide-rich parts of the veins. 

Thus, parts of the vein that crop out are generally 

less radioactive than those portions that do not. 

Many areas that are under thick soil cover have 

been virtually unexplored. Parts of some veins 

are exposed at the surface, but the extensions of 

these veins as well as most other veins are hidden 

by overburden. Prospecting in this district is 

commonly done by making a bulldozer trench 

across a radioactive zone in the soil or across the 

probable extension of a known vein. Underground 

workings are not common, although two adits 

having a total of 1,328 ft of workings were driven 

on the Last Chance property in Montana, and two 

adits having 823 ft of workings plus some smaller 

tunnels are found on the Wonder property in 

Idaho. Several other properties, including the 

Shear Zone, Cago No. 12, Buffalo, and Lucky 

Horseshoe have small adits but none of these exceeds 150 ft in length. Many veins have been only 

partly explored owing to lack of a market for 

thorium. 

The veins in this area generally occupy small 

shears parallel to the major structures. Most 

have a northwesterly strike, although a few have 

a northeasterly strike. Most have a steep dip, 
TABLE 2.-ThO, reserves and probable potential resources in short tons from seven thorium vein districts in the United 

States 

Total producible at Total producible at Total producible at 

less thall $15/1b between $15 and $30/1b between $30 and $50/1b 

Total Total probable Probable Probable Probable 

District rese rves potential Reserves potent i a 1 Reserves potential Reserves potential 

resources resources resources resources 

Lemhi Pass, 

Idaho-Mont •.••• 70,500 133,000 68,000 

Wet Mountai ns, 

Colo ••••.•••••• 64,200 160,500 54,000 

Powderhorn, 

Colo ........... 1,900 8,500 1,300 

Hall Mountai n, 

Idaho •..••••••. 4,580 26,300 4,540 

Diamond Creek, 

Idaho •••••••••• 250 13,000 117 

Bear Lodge 

Mountains, 

Wyo •••••••••••• 55 275 0 

Mountain Pass, 

Cal if •••••.••.• 280 1,500 175 

Total ••••••••• 141,765 343,075 128,232 

with only 15 percent of them dipping less than 

40 0 • The veins range in length from a few feet to 

at least 4,350 ft. The vein of largest continuous 

known length is the Last Chance in Montana. The 

Shear Zone vein, also in Montana, has been mapped for 6,000 ft (Staatz, 1972b, pI. 1). It is not a 

continuous vein but a shear zone along which 

several closely spaced, branching, and en echelon 

veins occur. Fifteen veins in the district are more 

than 1,000 ft long. These veins are the Shear 

Zone, Last Chance, main vein at the ThOz, Cago 

No. 12, central vein at the lola, north vein of the 

Silver Queen 52B, Contact, northwest-trending 

vein on the Black Rock (Montana) property, 

north vein on the G&G Nos. 1, 2, and 5 claims, 

Viola, Lucky Horseshoe, vein on G&G Nos. 6 and 

8 claims, northeast vein on the Reactor, principal 

vein on the Deer, and southeast vein on the Frying Pan (Staatz, 1979). The first six veins of the 

above group have each been traced for more than 

2,000 ft. The thorium veins vary considerably in 

thickness and range from thin seams to 40 ft 

thick. Although some veins pinch and swell 

abruptly, many are fairly uniform for most of 

their length; changes in thickness are most common near the ends of the vein. The maximum 

thickness of 63 percent of the veins in the district 

is 1 ft or greater, and for 6 percent (16 veins) it is 

more than 10ft. 

The thorium veins here are generally brown to 

reddish brown due to a high iron oxide content. 

124,000 1,650 3,330 507 1,840 

141,000 2,540 10,100 7,280 8,880 

6,700 300 1,000 200 700 

26,100 0 0 0 0 

12,200 115 660 0 0 

0 0 0 15 85 

685 0 0 0 0 

310,685 4,605 15,090 8,002 11 ,505 

10 

The grain sizes of the gangue minerals rarely exceed 118 in (inch), and those of the thorium 

minerals are generally between 0.0008 and 0.004 

in. In addition, every vein has been granulated by 

shearing. The basic mineralogy of the veins is 

fairly simple and consists primarily of quartz or 

microcline gangue cut by numerous irregular 

limonite or hematite veinlets and masses that 

contain thorium minerals and small amounts of 

various other accessory minerals. Although many 

minerals occur throughout the veins, only a few 

of the minerals make up as much as 10 percent of 

any sample. In addition to quartz, microcline, 

hematite, and limonite, only calcite and siderite 

make up this amount of any sample. Calcite is the 

chief gangue mineral in seven veins; siderite occurs in substantial amount in part of only one. 

Where these two minerals occur in other places, 

they make up only minor amounts of the veins. 

Minerals such as thorite, barite, magnetite, 

rutile, pyrite, monazite, muscovite, and apatite 

are found fairly frequently in the samples examined but occur in only minor amounts in each 

sample. 

Thorite is the dominant thorium mineral in the 

Lemhi Pass veins. Monazite is the next most 

common thorium-bearing mineral and was found 

in about one-quarter of the samples examined. 

Although monazite generally is not as abundant 

as thorite, in seven veins it is considerably more 

plentiful. Brockite, a calcium thorium phosphate, 
is present in nine veins. At four of these localities, 

it is the principal thorium mineral. Other 

thorium-bearing and rare-earth-bearing minerals 

occur in only a few veins and generally in minor 

amounts. Xenotime was noted in four veins; 

allanite in three; synchysite, a rare-earth calcium 

fluorocarbonate in two; and bastnaesite, a rareearth fluorocarbonate, and doverite, an yttrium 

calcium fluorocarbonate, in one each. In addition, 

23 other accessory minerals have been found in 

minor amounts in one or more veins. These 

minerals include those of copper, lead, zinc, and 

molybdenum. A detailed description of the 

mineralogy of the thorium veins may be found in 

Staatz (1972b, p. 57-74) and Staatz (1979). 

The Th02 content of the thorium veins in the 

Lemhi Pass district is known from about 420 

samples. The values range from 0.0008 to 9.4 percent. The samples represent a wide variety of vein 

materials. Some represent thin seams in quartzite 

country rock, but most are entirely of vein 

material which ranges from almost barren quartz 

to thorium-rich parts of veins. Although the Th0 2 

content of individual parts of different veins may 

vary widely, the grade of large sections of a particular vein may be fairly constant. 

The uranium content of 33 of these samples 

varies from 0.00001 to 0.0355 percent, and the 

thorium-to-uranium ratios of these samples range 

from 4.7 to 2,600. The thorium-to-uranium ratio 

of most samples, however, falls in the range of 30 

to 300. 

The total rare-earth oxide content of these 

veins is about equal to the Th02 content. 

Although rare earths commonly occur in the 

same minerals as thorium, there is no apparent 

relationship between the amount of one in a particular vein and the amount of the other. In 80 

analyses of the thorium veins, the total rare earth 

oxide content ranges from 0.042 to 10.95 percent 

(Staatz, Shaw, and Wahlberg, 1972, p. 76-77, 

Staatz. 1979; Austin, 1968). Analyses of samples 

from two properties, the Lucky Horseshoe and 

the Buffalo, show as much as 4 percent total rareearth oxides. The relative amount of each lanthanide commonly varies from vein to vein. In 31 

samples, which were carefully analyzed for the individual lanthanides (Staatz, Shaw, and 

Wahlberg, 1972), cerium was most abundant in 

six, neodymium in 17, gadolinium in three, 

dysprosium in two, and two lanthanides were 

equally abundant in three samples. In most veins, 

11 

however, the cerium group of rare earths (lanthanum, cerium. praesodymium. neodymium. 

samarium, and europium) make up most of the 

total rare earths. In at least nine veins, the yttrium group of rare earths, especially yttrium, 

make up a significant part of the total rare earths. 

Reserves and probable potential resources were 

calculated for all veins that have an average 

thickness of 0.2 ft or more. By this criterion, 

resources data were calculated for 114 separate 

veins. Although most properties contain only a 

single vein, some have several. There are five 

closely' spaced veins on the Deer, and four each on 

the Buffalo, Silver Queen 52B, and Reactor properties. The total reserves and resources of the 

various veins vary widely, and range from as little as 0.7 ton to thousands of tons of Th02• Total 

reserves for this district are 70,700 short tons of 

Th02 ; total probable potential resources are 

133,000 short tons of Th02• Together they total 

203,700 tons of ThO:!. The average grade of these 

veins varies from 0.034 to 3.83 percent Th02, but 

99 percent of these reserves come from veins that 

have between 0.1 and 2.0 percent Th02• Although 

the total resources are the aggregate resources of 

114 veins, 95 percent of these resources are in the 

10 largest veins. These veins are the Beaverhead; 

Black Rock (Montana); Cago No. 12; Contact; 

G&G Nos. 1, 2, and 5;' G&G nos. 6 and 8; Last 

Chance; Lucky Horseshoe; Shear Zone; and Th02 

(Staatz, 1979). These 10 veins have an overall 

average grade of 0.43 percent Th02• 

The Lemhi Pass district also has substantial 

resources of rare earths. Many deposits, however, 

have not been analyzed for rare earths. The 

average total rare earth oxide grade for the 29 

veins on which some analytical data are available 

is 0.51 percent. If this average grade is interpreted to be that of the entire district, then there 

are approximately 65,500 tons of reserves and 

132,000 tons of probable potential resources of 

total rare earth oxides in the Lemhi Pass district. 

Cost calculations were made on all properties 

containing more than 35,000 tons of vein material 

in the total reserves and probable potential 

resources. Thirty-four veins on 30 properties were 

evaluated. Eighteen of the veins could produce 

Th02 at less than $15/1b, six at between $15 and 

$30/lb, three at between $30 and $50/lb, and the 

rest at more than $50/Ib. Ore was assumed to go 

to one of three hypothetical mills. One mill was to 

be on the west side of the Continental Divide and 
was to process ore from the Idaho mines; the 

other two were to be on the east side of the Divide 

and were to handle mainly ore from the mines in 

Montana. A total of 192,000 tons of Th02 is producible in the Lemhi Pass district at less than 

$15Ilb. This total can be divided into 68,000 tons 

of Th02 in reserves and 142,000 tons in probable 

potential resources. This product would be obtained from 43,000,000 tons of ore having an 

average grade of 0.53 percent Th02• As the grade 

of the ore decreases, the cost of recovering it increases, and the average grade of ore where Th02 

could be produced at prices between $30 and 

$501lb is 0.13 percent. An equal amount of rare 

earths occurs in the veins. 

WET MOUNTAINS DISTRICT, COLORADO 

Thorium deposits in the Wet Mountains 

district occur principally west of the Wet Mountains proper in an area referred to as the 

De Weese Plateau by Scott and Taylor (1975). 

These deposits are scattered over an area nearly 

38 mi long, from north of the Arkansas River 

south to the Custer-Huerfano County line, by 

nearly 15 mi wide, from the Wet Mountains 

Valley on the east to the Wet Mountains. Most of 

them occur in an area between the WestcliffeTexas Creek faults on the west and the Use fault 

on the east. 

The thorium deposits are spacially and 

genetically related to three alkalic complexes of 

Cambrian age (Olson and others, 1977), the 

McClure Mountain Complex that includes the 

mafic-ultramafic rocks at Iron Mountain (Shawe 

and Parker, 1967), the Gem Park Complex 

(Parker and Sharp. 1970), and the complex at 

Democrat Creek (Heinrich, 1966, p. 339). A variety of younger lamprophyre, carbonatite, and red 

syenite dikes, and thorium-bearing quartzfeldspar-barite veins intersect the rocks of the 

alkalic complexes as well as the Precambrian X 

and Y metasedimentary and igneous country 

rocks (Scott and others, 1976). Anomalous concentrations of thorium are found in the carbonatite bodies (Armbrustmacher, 1979) and red 

syenite dikes, and especially in the quartzfeldspar-barite veins, fracture zones, and fenites 

that envelop these deposits. The veins and fracture zones have the greatest economic potential 

and are discussed in the following pages. 

12 

The fracture zones, which are persistent linear 

features as much as 8 mi long, generally have a 

northwesterly strike and typically are 3 to 4 ft 

thick. These zones commonly cross the foliation 

of the Precambrian metasedimentary rocks at 

right angles. Fracturing apparently is better 

developed across the well-foliated metasedimentary rocks than across the poorly foliated igneous 

rocks. Most of the fracture zones and veins occur 

south of the alkalic complexes, which may reflect 

the greater abundance of well-foliated 

metasedimentary rocks in that area. Several 

types of dike and vein material may occur along a 

given fracture zone, and in a few places various 

types overlap along some fracture zones, carbonatite dikes may replace earlier lamprophyre or 

red syenite dikes; along others quartz-feldsparbarite veins intersect carbonatite dikes; or parts 

of fracture zones may have been invaded by 

mineralizing fluids without abundant gangue 

minerals. In a few places, fracturing is later than 

the dikes and veins. 

The veins and fracture zones vary in mineral 

content, but in many places they contain abundant amounts of smoky quartz and barite. 

Specular and earthy hematite, goethite, and 

microcline generally are common; calcite, thorite, 

and galena occur in variable amounts; and pyrite, 

chalcopyrite, dolomite. chlorite, siderite, 

cerussite, xenotime, brockite, sphalerite, rutile, 

and other minerals are sparse. Light stableisotope studies of minerals in the veins show that 

physical-chemical conditions for thorite deposition were remarkably uniform throughout the 

area. The 180 values of quartz range from 9.3 to 

12.1 permil and indicate that the hydrothermal 

fluids had a uniform thermal history throughout 

the area. The 34S values for coexisting barite and 

galena range from 5.1 to 8.1 permil and -14.1 to 

-24.3 permil, respectively, and suggest that the 

hydrothermal solutions derived their sulfur from 

igneous sources and that vein deposition occurred 

under a rather limited range of pH and oxygen 

fugacity conditions. 

Exploration of the quartz-feldspar-barite veins 

and fracture zones consists mainly of shallow prospect pits, many of which no longer expose 

bedrock. Samples commonly were collected from 

these pits or from dumps if the amount of radiation warranted it. Because of the lack of data between pits where the vein or fracture zone is 

covered, estimates of thorium reserves between 
workings are subject to error and probably tend 

to be somewhat overestimated. At a few locations, mainly the Beardsley, Pine Tree, Anna Lee, 

and Hardwick deposits, there are some 

underground workings, but the workings mostly 

have caved or filled with water. 

A total of 202 samples of veins and fracture 

zones were analyzed for thorium and uranium by 

gamma-ray spectrometer and for 31 additional 

elements by semiquantitative spectrographic 

analysis. The samples contain an average of 0.46 

percent Th02 , ranging from 0.00075 to 10.2 percent. The maximum uranium value of 0.036 percent but samples generally contain less than 

0.0005 percent. The average thorium-to-uranium 

ratio is 64. The thorium-to-total-rare-earth ratios 

are somewhat variable but average about 2.2, 

with maximum total rare-earth concentrations 

near 1 percent. 

The total Th02 reserves of the Wet Mountains 

district are 64,200 tons; in addition, there are 

160,500 tons of Th02 in probable potential 

resources. 

Cost analyses were done on 29 veins and fracture systems whose reserves and probable potential resources total at least 35,000 tons. Th02 

could be produced from 10 of these at a cost of 

less than $15/1b, from six of them at a cost of between $15 and $30/1b, from four of them at a cost 

of between $30 and $50/1b, and the remainder at a 

cost of greater than $50/Ib. The amount of ThO'2 

producible at less than $15/1b is 195,100 tons, 

which is divisible into 54,100 tons of ThO'2 in 

reserves and 141,000 tons in probable potential 

resources. The average grade of the veins containing Th0 2 that is producible at less than $15/1b is 

0.51 percent. The amount of Th0 2 producible at 

between $15/1b and $30/1b is 12,640 tons, which is 

divisible into 2,540 tons of reserves and 10,100 

tons of probable potential resources. 'rhe amount 

of Th02 producible at between $30/1b and $50/lb 

is 16,160 tons. This amount is divisible into 7,280 

tons of reserves and 8,880 tons of probable potential resources. The average grade of the ore producible at between $30/1b and $50/1b is only 0.11 

percent. 

POWDERHORN DISTRICT, COLORADO 

The Powderhorn district occupies 250 mi2 in 

southern Gunnison County, Colo. (fig. 1). About 

13 

250 veins were prospected for thorium, chiefly 

during 1950-56. The area is underlain largely by 

metasedimentary and metavolcanic rocks intruded by Precambrian granitic and other igneous 

rocks. In late Precambrian or Cambrian time, 

about 570 million years ago, the Precambrian 

rocks were intruded by alkalic igneous rocks and 

carbonatite of the complex at Iron Hill and by 

slightly younger diabase dikes (Olson and others, 

1977). Mesozoic sedimentary rocks and Tertiary 

volcanic rocks originally covered much of the 

area, but now are exposed only on scattered 

mesas and ridges where they cover the older 

rocks. Thorium deposits in the Powderhorn 

district have been described by Olson and 

Wallace (1956) and by Hedlund and Olson (1961). 

The geology of the district and location of 

thorium deposits are shown on maps by Olson 

(1975), Hedlund and Olson (1973, 1975), and 

Olson and Hedlund (1973). The geology of the 

alkalic igneous complex at Iron Hill has been 

described by Larsen (1942), Temple and Grogan 

(1965), Rose and Shannon (1960), and Nash 

(1972). 

Abnormal amounts of thorium are found in the 

following types of deposits in the Powderhorn 

district: (1) veins, (2) disseminations in several 

small granite plutons, (3) a large massive carbonatite body, (4) scattered carbonatite dikes, (5) 

magnetite-perovskite-apatite bodies, and (6) 

trachyte dikes. Only resources of the first type, 

veins, are described in this section of the report. 

The second and third types are described in other 

sections of this report. The last three types have 

been discussed briefly by Hedlund and Olson 

(1961). 

Veins contain the highest grade thorium concentrations in the district. They are as much as 

3,500 ft in length, but most cannot be traced for 

more than a few hundred feet. The veins range in 

thickness from a fraction of an inch to 15 ft, but 

most are less than 2 ft thick. They commonly occur in steeply dipping shear or breccia zones 

within the Precambrian rocks. In the area 2-5 mi 

north of the complex at Iron Hill, most of the 

veins strike slightly north of east, but in the 

northwestern part of the district they generally 

strike northwest. The veins cut and generally are 

discordant to the foliation of the metamorphic 

and igneous rocks. The type of host rock apparently has little effect on the localization of the 

veins. 
The thorium veins are composed of thorite, 

white to smoky quartz, jasper, pink potassium 

feldspar, calcite, specular and earthy hematite, 

goethite, magnetite, barite, pyrite, and sporadic 

synchysite, apatite, purple fluorite, biotite, sodic 

amphibole, rutile, monazite, vanadinite, 

bastnaesite and various sulfides. The chief 

thorium mineral, thorite, is very fine grained and 

is intergrown with ferric oxides minerals. Carbonate minerals are abundant in some of the 

veins, commonly replacing silicate minerals, 

quartz, and barite, and are associated with ferric 

oxide minerals. Vug fillings of coarsely 

crystalline calcite occur locally in some veins. 

Pink and white barite is present in small amounts 

in many of the thorium veins, and it is a major 

constituent in a few. Sulfide minerals are present 

but not abundant in the veins; they include 

pyrite, chalcopyrite, galena, and minor sphalerite. 

Varicolored jasper, mostly yellowish to reddish 

brown, has replaced much of the carbonate 

gangue in veins that lie just below the Tertiary 

volcanic capping. This type of replacement is well 

developed on Sapinero Mesa in the northwest 

part of the Powderhorn district. Jasper replacement of the carbonate gangue has been found in 

veins that lie within 100 ft of this volcanic capping. Locally the jasper forms pseudomorphic 

replacements of the carbonate and has apparently 

retained some of the dispersed thorium, as veins 

partly replaced by jasper show radioactivity in 

both the jasper and carbonate gangues. The effects of oxidation, leaching, and jasperization 

might result in some increase or decrease in the 

original thorium content of near-surface vein 

material, but without sufficient subsurface data 

on possible changes in grade, the surface exposures are assumed to be representative of vein 

grade at depth. The jasperization may have 

resulted from the action of hydrothermal fluids 

localized near an ancient Oligocene land surface 

just beneath the base of the flows. 

The ThO 2 content of samples from thorium 

veins ranges from less than 0.01 percent to as 

much as 4.9 percent. The Th02 content generally 

is less than 1 percent, however, and is 0.05 to 0.1 

percent in many of the veins examined in this 

district. 

The complex at Iron Hill contains a minor element assemblage characteristic of many alkalic 

rocks, including titanium, phosphorus, strontium, barium, rare earths, and vanadium. Among 

14 

these, the niobium and rare earths are sufficiently valuable and widespread in the thorium veins 

to be considered in discussing the economic 

potential. 

The thorium veins commonly contain 

anomalous amounts of niobium. Some of it is in 

columbite, which has been identified in the South 

Beaver Creek area. The Nb20 s content of 46 vein 

samples that were semiquantitative spectrographically analyzed ranges from 45 to 2,150 

ppm (parts per million); overall average is 245 

ppm. The Th0 2-to-Nb20 5 ratio in these veins increases generally northwestward from the complex at Iron Hill from 2.6 to 16.7. The average 

ratio of all samples is 6.8. 

The thorium veins contain an appreciable 

amount of rare earths, which is generally in excess of the amount of thorium. The amounts of 

various rare earths was determined by semiquantitative spectrographic analysis. The proportions 

of total rare earth oxides to Th02 varies considerably but the average is 2.4. The ratio of the 

two is greatest in veins near the south and west 

sides of the complex at Iron Hill where the total 

rare-earth oxides are about five times as abundant as the Th02 • The cerium group of rare earths 

is approximately 2.7 times more abundant than 

the yttrium group of rare earths; they also vary 

considerably in proportion to one another from 

sample to sample. the cerium group rare earths is 

proportionately greater near the south and west 

sides of the complex at Iron Hill where the ratio 

of the cerium group to the yttrium group is approximately 10 to 1. This ratio is least in an area 

within 5 mi of the northern side of the complex 

where most of the veins are somewhat richer in 

yttrium group rare earths than in cerium group 

rare earths. 

Uranium was determined in 58 vein samples. 

The amount is generally low and exceeds 10 ppm 

in only three samples. The highest uranium value 

is 41 ppm. The thorium-to-uranium ratios of these 

samples are between 1 and lOin six samples, 10 

to 30 in eight samples, 30 to 70 in 20 samples, 70 

to 130 in 10 samples, and 130 to more than 1,000 

in 14 samples. 

Thorium deposits shown on maps of the 

Powderhorn district (Olson, 1975; Hedlund and 

Olson, 1973, 1975; Olson and Hedlund, 1973) 

were examined briefly with a scintillation counter 

during geologic mapping of the area, and some 

reserves were estimated. The majority of the 
deposits are small or low in grade; hence, it is only 

a minority that contribute significantly to the 

total resources. In 1976 and 1977, J. C. Olson reexamined 67 of the deposits with the aid of a portable four-channel gamma-ray spectrometer and 

collected an additional 66 samples. The field 

gamma-ray spectrometer readings were 

calibrated with the sample analyses from the 

same stations by a linear regression analysis to 

provide a rough guide to approximate thorium 

content for field readings on unsampled veins. 

Isorad maps were drawn for several deposits 

showing contours of equal thorium content. The 

gamma-ray spectrometer readings, coupled with 

sample analytical data, provide a reasonable approximation of the thorium content of the surficial layer of the deposit. Inferences regarding 

the extent and thorium content of the deposit at 

depth require, in addition, consideration of: (1) 

the amount of cover that might locally obscure 

the vein, (2) the dispersal of radioactive elements 

by slope wash, soil creep, or leaching, and (3) the 

possible enrichment of surficial material by selective removal of constituents other than thorium. 

Generally, the surficial thorium content was 

assumed to be fairly representative of that at 

depth except where radioactivity may be 

obscured by ground cover. 

Resource estimates were made on a total of 54 

veins. The average tenor of 21 veins is between 

0.01 and 0.03 percent Th02, and that of another 

10 veins is between 0.03 and 0.05 percent Th02• 

Thirteen of the veins estimated have an average 

tenor greater than 0.1 percent Th02• Reserves in 

all 54 veins amount to 1,900 tons of Th02 of 

which 1,600 tons is in veins having an averagegrade greater than 0.1 percent. Probable potential resources total an additional 8,500 tons. The 

veins in this district having a grade of at least 

0.05 percent ThOz, which make up most of the 

total resources, are widely scattered and occur in 

secs. 14, 15, 16, and 20, T. 47 N., R. 2 W.; secs. 1, 

2,4,9,10,15, and 24, T. 47 N., R. 3 W.; sec. 34, T. 

48 N., R. 3 W.; sec. 10, T. 46 N., R. 2 W.; and secs. 

23 and 24, T. 46 N., R. 1 Y2 W. 

Cost analyses were made on all properties containing more than 35,000 tons of total reserves 

and probable potential resources. The Th02 

reserves that can be produced at less than $15/lb 

are 1,300 tons. In addition, 6,700 tons of probable 

potential resources fall in the same category. 

Reserves that can be produced between $15 and 

15 

$30/1b amount to 300 tons of ThOz• Probable 

potential resources in this category are 1,000 

tons. Reserves that can be produced at between 

$30/1b and $50/1b amount to 200 tons of Th02• 

The probable potential resources in this category 

are 700 tons. 

Analysis of production costs in veins large 

enough to mine indicates that Th02 might be obtained for less than $30/1b in those having a grade 

of at least 0.15 percent Th02; whereas, a cost between $30/1b and $50/1b is probable for those 

veins having an average grade of between 0.07 

and 0.15 percent Th02• 

HALL MOUNTAIN DISTRICT, IDAHO 

The Hall Mountain district lies just south of 

the Canadian boundary in the center of the Idaho 

panhandle. Thorium veins on the steep west side 

of the Hall Mountain intrude quartzite and 

diorite of Precambrian age. The quartzite belongs 

to the Prichard Formation of the Belt 

Supergroup, which is intruded by diorite sills of 

the Purcell Lava (Staatz, 1972a, p. 242). The 

known veins are found in a small area about 6,000 

ft long by 1,000 ft wide. This area is covered by 

brush and trees. Thick cover has limited the vein 

exposures, and the lack of a ready market for 

thorium has limited the amount of prospecting. 

Hence, although some of the thorium veins have 

been explored by bulldozer, some of them have 

been explored only by one or two small hand-dug 

pits. Even on the surface, none of the veins has 

been fully explored. thirteen deposits are known 

in this area. They include shear zones, parallel 

veinlets, veins, and a small irregular breccia 

body. These deposits are tabular and have been 

traced along strike for distances ranging from 6 

to 900 ft. They vary in thickness from thin seams 

to 13 ft. Individual veins may also vary in 

thickness, and one long vein ranges from 0.6 to 

10.8 ft in thickness. Some of the known veins are 

too small to mine and process economically. The 

principal reserves in this district are concentrated 

only in a few veins. 

The mineralogy of the Hall Mountain veins is 

simpler than that of veins in most other districts. 

The principal mineral in most Hall Mountain 

veins is quartz, although parts of some veins contain considerable granular calcite. Thorite is by 

far the most common thorium mineral. Allanite is 

also found in some places and is the second most 
abundant thorium-bearing min€rai. Monazite, a 

common mineral in many vein districts, is exceedingly rare. Minor amounts of chlorite, 

magnetite, limonite, and pyrite occur in many 

parts of the veins. In addition, 20 other minerals 

occur in miner amounts in some part of these 

veins. A mOre detailed account of the mineralogy 

and geology of the thorium veins has been 

reported previously by Staatz (1972a). The small 

amount of iron oxide minerals in these ores 

should make the milling of this are easier and 

somewhat less costly than thorium arc from 

Lemhi Pass or the Wet Mountains. 

The ThO, content of samples from these veins 

ranges from 0.014 to 24 percent; the high-grade 

sample is a selected specimen. Tenor of individual 

veins varies erratically, and even the richest veins 

have some parts which are of low tenor. The 

average of several of the larger veins is nearly 5 

percent ThO" making these some of the highest 

grade veins in the United States. The total rare 

earth oxide content of these veins is much lower 

than that of the veins in other districts discussed 

in this report. The total rare earth oxide content 

of 12 samples frem the Hall Mountain district 

ranged from 0.0011 to 0.197 percent (Staatz and 

others, 1974, p. 680). In most other districts, the 

ThOrto-total rare-earth oxide ratio ranges from 

about 'I, to 2. In the Hall Mountain district, 

however, this ratio, from veins that are large 

enough to consider mining, varies from about 6 to 

526. Another difference between the rare earths 

in other thorium vein districts and thOBe in the 

Hall :Mountain veins is that the former belong 

principally to the cerium rare-earths (lanthanum, 

cerium, neodymium, praesodymium, samarium, 

and europium) and the latter to the yttrium group 

of rare·earths (gadolinium, terbium, dysprosium, 

holmium, erbium, thulium, ytterbium, lutetium, 

and yttrium). The content of the yttrium group of 

rare-earth oxides found in the Hall Mountain 

veins is about the same as in the Lemhi Pass 

district, but differs from the latter in that most 

of cerium group of earths are missing at Hall 

Mountain. 

Reserves in the Hall Mountain district are 

4,580 tons of ThO, and the probable potential 

resources are 26,300 tons of ThO, (table 2). The 

'fhO, reserves in this district occur in 115,000 

tons of vein material averaging 4.0 percent 'l'hO,. 

The probable potential resources are in 660,000 

tons of rock of the same grade. A byproduct of 

16 

rare earths probably would not be economical in 

this district because of the low total rare earth 

content. The reserves producible in this district 

at less than $l5!lb are 4,540 tons of ThO,; the 

probable potential resources in this cost catagory 

are 26,100 tons of ThO,. Actually, all of the above 

resources, because of the high tenor of the ore, are 

producible at less than $511b of ThO,. 

DIAMOND CREEK DISTRICT, IDAHO 

The Diamond Creek district is approximately 

8 mi north-northwest of the town of Salmon on 

the steep eastward-dipping slope of the Salmon 

River Mountains. The area is· about 2,000 ft 

above the Salmon River, which flows along the 

eastern edge of the mountains. Diamond Creek 

crosses the district, and the principal veins occur 

on its drainage. Known veins in this small 

district. however, are found in an area that is approximately 2 \I, mi long by 0.5 mi wide. Thorium 

veins were emplaced in to quartzite and siltite of 

Precambrian Y age and into granodiorite of the 

Idaho batholith of Cretaceous age. Most of the 

veins occur in the sedimentary rocks, which are 

approximately the sarno age as and, in part, 

lithologically similar to the rocks of the Belt 

Supergroup that are found farther north. The two 

groups of rocks were depositad in separate 

basins. The sedimentary rocks appear to be more 

favorable hosts than the igneous batholithic 

rocks as veins as much as 25 ft thick have been 

found in the former, but they rarely exceed 2 ft in 

the latter. Mest of the prospecting consists of 

scattered bulldozer trenches and hand-dug pits. 

One of the larger veins was explored by a 25-ft 

adit, and a vein also is exposed in the road bed of 

the Diamond Creek road. As most of the Diamond Creek area is soil covered, veins are only exposed in workings. The lack of development has 

limited the tracing of veins, and only eight veins 

have been identified in this region. The vein that 

is exposed in the Diamond Creek road between 

two forks of Diamond Creek is interpreted 88 

being the southern extension of the larger vein exposed on the ridge on the north side of Diamond 

Creek. All the veins have similar attitudes, ranging in strike from about N. 10' E. to N. 60° E.; all 

have steep dips_ 'fhey follow irregular fractures in 

the country rock, and . variations in strike of as 

much as 20' have been noted on the sarno vein. 

The veins in this district show a general a1inement 
along a northward-trending zone that is more or 

less parallel to their strike, and they probably 

were formed along one or more parallel shear 

zones. The eight veins have been traced along the 

surface for distances ranging from 110 to 2,560 ft. 

Some of these veins lie along strike from one 

another and they could either be part of the same 

vein or separate veins along the same structure. 

Evidence as to how far these veins extend 

generally is not available due to the lack of exploration in the intervening areas. The thorium 

veins vary in thickness from 0.5 to 25 ft; the 

largest vein in the district varies from 1.5 to 25 ft. 

The veins of the Diamond Creek district are 

fine grained, limonitic, and similar in general appearance to many of those in the Lemhi Pass 

district. Quartz and limonite (generally goethite) 

make up a large part of these veins. Monazite is 

the principal thorium and rare-earth mineral. 

Thorium also occurs in minor amounts in some 

veins in brockite and thorite. In addition to these 

minerals, rare earths are found in minor amounts 

in . some places in xenotime and bastnaesite. 

Fluorite, potassium feldspar, hematite, and 

biotite are locally abundant in some veins. In addition, small amounts of rutile, muscovite, montmorillonite, calcite, magnetite, plagioclase, 

malachite, pyrite, and gold were identified in one 

or more veins (Staatz, 1974, p. 498). 

The ThO';! content of samples from these vehis 

ranged from 0.04 to 1.71 percent. Only one sample contained more than 1 percent Th0 2• The 

uranium content of seven samples was determined. Uranium ranged from 0.0003 to 0.01 percent. 

The thorium-to-uranium ratios of these seven 

samples vary from 6 to 150. The lowest ratio 

comes from a vein too small to be economically 

mined. Rare earths are more abundant in this 

district than is thorium. Resource calculations of 

both the total rare-earth oxides and ThO 2 indicate 

that the former is 5.8 times more abundant. 

Total rare-earth oxide content of eight samples 

from this district varied from 0.59 to 5.51 percent. The light cerium group of rare earths Uanthanum, cerium, praesodymium, neodymium, 

samarium, and europium) made up most of the 

rare earths in most of these samples although one 

sample had almost as much yttrium group of rare 

earths (gadolinium, terbium, dysprosium, 

holmium, erbium, thulium, lutetium, ytterbium, 

and yttrium) as light. The thorium veins here 

17 

have also been explored for gold,. and this metal 

has been panned from altered zones along one of 

the veins. Three samples cut across one of the 

larger veins were assayed for gold, and contained 

0.017, 0.07, and 0.348 oz/ton (ounce per ton). 

The veins for which resources were calculated 

in the Diamond Creek district range in average 

thickness from 0.5 to 10 ft. Their average grade is 

from 0.07 to 0.30 percent Th02 and from 0.59 to 

4.92 total rare-earth oxides. Total reserves are 

250 short tons of Th02 and 2,870 tons of total 

rare-earth oxides. Total probable potential 

resources are 13,000 tons of Th02 and 75,500 tons 

of total rare-earth oxides. The total of both 

categories amounts to 13,250 tons of Th02 and 

78,370 tons of total rare-earth oxides. In the 

calculations of the probable potential resources, 

two veins having similar strike and dip and lying 

along the same shear zone were connected across 

a small valley. The overall average grade of the· 

reserves and resources is 0.21 percent Th02 and 

1.22 percent total rare-earth oxides. 

Three veins in this district were considered 

large enough to be mined. They contain a total of 

12,230 tons of Th02 that can be processed f()r less 

than $15/1b of which 30 tons are reserves and 

12,200 tons are probable potential resources. This 

ore also contains 63,000 tons of rare earths. 

Another 775 tons of Th0 2 is producible at between $15/lb and $30/lb of which 115 tons is 

reserves and 660 tons probable potential 

resources. None of the ThOz is producible at between $30/1b and $50/lb. 

BEAR LODGE MOUNTAINS DISTRICT, WYOMING 

The Bear Lodge Mountains district is just 

north of the Warren Peaks in the southern Bear 

Lodge Mountains. Veins are scattered over an 

area of about 5 mi2 and about 5 mi northwest of 

Sundance, Crook County, Wyo. The Bear Lodge 

Mountains have steep flanks, but the central 

part, where the veins occur, is a series of hills having but moderate relief. The southern part of the 

Bear Lodge Mountains consist of a core of Tertiary intrusive rocks that has domed up the surrounding sedimentary rocks of Paleozoic and 

Mesozoic age. The central core of igneous rocks is 

formed by multiple intrusions of alkalic igneous 

rocks, mainly trachyte and phonolite. This complex igneous body also contains minor amounts of 
syenite, nepheline syenite, and carbonatite. These 

three rock types increase in abundance with 

depth. After most of the igneous rock was intruded, the rocks in the central part of the core were 

fractured, altered, and many of the fractures were 

filled with vein material. The greater part of this 

material occurs in thin veinlets, which are 

described in the section on disseminated deposits. 

Some larger veins also were formed and are 

described here. 

These veins are distinct tabular bodies having 

an average thickness of at least 0.2 ft. Twenty-six 

veins are known in this district and their exposed 

length is from a few feet to 450 ft. Exposures are 

generally poor and veins are seen only where exposed in workings. Thickening and thinning are 

common; one vein varies in thickness from 0.2 to 

2.5 ft. 

Average thicknesses of various veins in this 

district range from 0.2 to 5.5 ft. These veins tend 

to be irregular and lenticular. Their strike apparently is random. The only exception is that the 

two longest veins, which lie only several hundred 

feet apart, are parallel; none of the other veins, 

however, have a similar strike. Veins commonly 

change strike, as most of them were intruded 

along sinuous fractures. Dips are also highly 

variable, and dip measurements range from 250 

to 90°. All except one of the veins are in the Tertiary intrusive. The exception is a vein on the 

west side of the intrusive core that cuts the 

Pahasapa Limestone of Early Mississippian age. 

The Bear Lodge Mountains veins generally are 

brown or black in color, but a few are pale green. 

Grain size is generally fine to submicroscopic. 

Although the principal gangue minerals are 

potassium feldspar, quartz, or cristobalite, the 

most common of the three is feldspar. Goethite, 

hematite, and various black manganese oxide 

minerals vein and impregnate the gangue 

minerals in many, but not all. of the veins. The 

principal thorium-bearing minerals are the 

phosphates. monazite and brockite. These 

minerals also contain rare earths. A common rare 

earth mineral is bastnaesite. Barite, rutile, 

brookite, magnetite, and fluorite are moderately 

common in these veins and, in addition, some 12 

other minerals are sparsely present in a few veins. 

The Th02 content of 35 vein samples ranges 

from 0.013 to 1.37 percent. The average grade of 

these veins is considerably lower than those in 

the Lemhi Pass district, and more than a third of 

the samples contain less than 0.1 percent ThOz• 

18 

The uranium content of these veins is erratic but 

commonly is somewhat higher than that in most 

other vein districts. Uranium content of 27 vein 

samples ranges from 0.0005 to 0.027 percent. The 

thorium-to-uranium ratios of these samples vary 

from 1.6 to 183, but most fall between 3 and 65. 

Rare earths are more abundant in these veins 

than thorium. The total rare earth oxides of 19 

vein samples vary from 0.27 to 1.14 percent, and 

the average is almost one-half a percent. More 

than three-quarters of the samples contain more 

than 10 times as much rare earths as they do 

thorium. The relative amounts of the individual 

rare earths may be different in different veins. In 

this district, cerium is the most abundant lanthanide in some veins, neodymium in some, 

gadolinium in a few, and erbium in a few. Veins 

containing abundant monazite or bastnaesite 

have cerium or neodymium as their most common 

lanthanide. The presence of brockite in a vein indicates that it is gadolinium rich or erbium rich. 

Reserves and probable potential resources have 

been calculated for all the veins in the Bear Lodge 

Mountains. Veins exposed in only one place are 

considered to be only 25 ft long instead of the 50 

ft used in most other districts because of the 

overall tendency for veins to be short in the Bear 

Lodge Mountains. Reserves of the Bear Lodge 

veins total 55 tons of ThOz• The probable potential resources also equal 275 tons of ThO 2 . 

Reserves and resources of most veins are small, 

and those of 17 of the 26 individual veins do not 

exceed 10 tons of ThOz• The average grade of all 

veins ranges from 0.011 to 0.68 percent ThOz• 

Rare-earth resources are far more plentiful than 

those of thorium. 

Most veins in this district are too small to support a viable mining operation. In addition, the 

grade of Th02 is low when compared with that of 

most other vein districts. The ore resources of 

each of only two veins exceed 35,000 tons. The 

cost of producing 1 lb of Th02 at one of these properties exceeds $50; the cost of producing 1 Ib of 

ThOz at the other is just under $50. Total 

reserves for the Bear Lodge veins that could be 

produced at between $30/1b and $50/1b are 15 tons 

of Th02. The probable potential resources of veins 

in this category are 85 tons of Th02• This ThOz 

would come from 109.000 tons of ore having an 

average grade of 0.092 percent. In addition, this 

ore contains 6,780 tons of total rare-earth oxides. 

The rare-earth oxides make up 6.23 percent of the 

ore. 
MOUNTAIN PASS DISTRICT, CALIFORNIA 

The Mountain Pass district is centered near 

Mountain Pass on Interstate Highway 15 between Las Vegas, Nev., and Barstow, Calif. 

Mountain Pass is 60 mi southwest of Las Vegas 

in the northeast corner of San Bernardino County, Calif. The district is, in part a rolling upland 

about 5,000 ft in altitude with several peaks and 

ridges rising about 1,000 ft above the surface. 

Thorium and rare earths occur in this district in 

veins, carbonatite dikes, and a massive carbonatite body. Rare earths are much more abundant than thorium in all three types of deposits. 

The massive carbonatite, the Sulphide Queen 

body, is the site of the world's largest rare-earth 

mine, which is operated by Molybdenum Corp. of 

America. The possibility of producing thorium 

from the Sulphide Queen carbonatite body is 

discussed in a separate section of this report. The 

thorium and rare-earth potential of the veins 

discussed here is smaller than the thorium and 

rare-earth potential of the other two types of 

deposits. All three types of deposit occur in a 

Precambrian terrain consisting of a well-foliated 

metamorphic complex intruded by syenite, 

shonkinite, and granite (Olson and others, 1954, 

p. 5-27). The rocks occur in a northwest-trending 

block about 18 mi long by 4-5 mi wide that forms 

the east side of the I vanpah Mountains, and is 

separated on the west from limestone and 

dolomite of Palezoic age, and volcanic rocks of 

Tertiary age by the Clark Mountain fault. Most 

of the thorium and rare-earth mineralization occurs in the southern half of the Precambrian 

block. The largest area of potassium-rich igneous 

rocks (syenite and shonkinite) lies about 1 mi 

north of the highway that passes through Mountain Pass. Just south of these rocks is the large 

Sulphide Queen carbonatite body. Many of the 

carbonatite dikes also are concentrated in this 

general area. They become less common to the 

southeast. The veins occupy major and minor 

shear zones having a general northwesterly trend. 

The veins tend to be irregular and lenticular. 

They are more discontinuous here than in the 

Lemhi Pass or Wet Mountains districts. This 

discontinuous nature is well illustrated at the 

Windy group of claims where, along one shear 

zone, Shawe (1953, fig. 7) mapped 14 separate 

veins, and on the Ray-Welch-Willmore group 

where he mapped 11 veins along another shear 

19 

zone. More than 35 veins are known in this district, ranging in length from about 10 to 1,000 ft. 

Most are short, and only three veins were traced 

for more than 300 ft. In thickness. they vary from 

a fraction of an inch to 12 ft. Veins commonly 

change in thickness through short distances, and 

one 60-ft vein in the Windy group varied from 0.5 

to 12 ft in thickness. All the veins are steeply dipping. generally at an angle of 60° to 80°. 

The veins in this district are distinguished from 

the carbonatite dikes by their total mineral composition. The veins consist principally of quartz 

and feldspar, and the dikes of carbonate minerals, 

although either may contain silica or carbonate 

minerals. Although the principal gangue minerals 

of the veins are quartz and microcline. all veins 

contain some calcite. Thorite is the principal 

thorium mineral and bastnaesite is the principal 

rare earth mineral. Bastnaesite is much more 

abundant than thorite and is commonly the principal heavy mineral. Other thorium-bearing and 

rare-earth-bearing minerals are allanite and 

monazite; barite is a common mineral in some 

veins. The iron oxide minerals, hematite, 

limonite, and magnetite occur in many of the 

veins, but generally make up less than 5 percent 

of anyone. Ten other minerals were identified in 

minor amounts from these veins (Staatz, 1974, p. 

498), some of which probably were derived from 

the country rock. The Th02 content of samples 

from these veins ranged from 0.03 to 6 percent. 

The overall ThOz grade of this district is lower 

than that of most other vein districts and the 

ThOz content rarely exceeds 0.5 percent. The 

uranium content of 34 samples is from 0.0004 to 

0.026 percent, and the thorium-to-uranium ratios 

of these samples is from 6 to 413. The ratio of 

most samples falls between 25 and 175. Some of 

the lower thorium-to-uranium ratios come from 

samples in which the thorium content is quite 

low. Thus, a decrease in this ratio does not necessarily signify an increase in the uranium content. 

The Mountain Pass district is primarily a rareearth district. and the total rare-earth oxide content of 10 samples of vein material ranges from 

1.96 to 8.94 percent. The total rare-earth content 

of these 10 samples is from 6.5 to 33 times more 

than the thorium content. Most of the rare earths 

are in bastnaesite, which contains little thorium. 

This mineral is made up predominately of the 

cerium group of rare earths, and cerium and Ian-
thanum are the principal rare earths in all the 

veins. 

The total reserves of the Mountain Pass veins 

are 280 tons of ThO" and the total probable 

potential resources are 1,500 tons of ThO, for an 

aggregate total of 1,780 tons of ThO,. The 

average grade of the various veins is from 0.4 to 

1.26 percent ThO,; most of the reserves fall between 0.1 and 0.54 percent ThO,. Rare-earth 

analyses are not available for all of these veins. 

Assuming that the average ratio of rare-earth oxide to ThO, in the veins for which analyses are 

available is the same as in all veins, then the total 

of reserves and resources of rare-earth oxides for 

the above categories is approximately 29,100 

tons. 

Many of the veins in the Mountain Pass area 

are too small to support mining. Only four veins 

in this district were large enough to be 

economically evaluated. The ThO, reserves in this 

district that can be produced at less than $15Ilb 

amount to 175 short tons; this are also contains 

1,810 tons of total rare-earth oxides. In addition, 

another 685 tons of ThO, are classified as probable potential resources. These resources also 

contain 7,200 tons of total rare-earth oxides. 

None of the veins large enough to mine have 

resources producible between $15/1b and $50/1b of 

ThO,. . 

DISSEMINATED DEPOSITS 

Disseminated deposits tend to be large and of 

low grade when compared to the veins, and they 

can be of several types. They can be rock that is 

cut by many tiny thorium-rich vein lets, thoriumrich material disseminated in breccia pipes, or abnormally thorium-rich igneous bodies. Although 

large igneous bodies having a ThO, content as 

low as 40 ppm could be included under this 

heading, we have limited the present study to 

deposits having an average of 100 ppm ThO, or 

greater. This eliminates such large bodies of 

granite as the Conway Granite of New Hampshire (Adams and others, 1962, p. 1901-1902) and 

granite of the Darby pluton of the Seward Perrinsula, Alaska (Miller and Bunker, 1976, p. 373), 

both of which have an average ThO, content of 

approximately 65 ppm. Higher grade granite has 

been reported in the small stock near Bull Creek 

canyon, Idaho, where isolated samples as high as 

1,500 ppm have been reported (Staatz, Bunker, 

20 

and Bush, 1972, p. B54), but the overall average 

grade at Bull Creek canyon is only 35 ppm. 

Although granite bodies of this sort might be 

economic to mine at some time in the distant 

future, the relatively high cost of handling this 

low-grade material would not make mining 

economic until the still abundant high-grade 

sources are depleted. 

Disseminated deposits form some of the 

world's large deposits of copper, molybdenum, 

and gold. Being large and low grade, they need 

both to be operated on a large scale and to have 

an active market for the commodity that they 

contain. As thorium has not been actively sold on 

the open market through 1977, there has been little incentive to explore for disseminated type 

thorium deposits. In spite of the lack of general 

interest in them, disseminated deposits have been 

noted in three widely separated areas: II) Bear 

Lodge Mountains, Wyo., (2) Powderhorn district, 

Colorado, and (3) Hicks Dome, III. (fig. I). The 

Hicks Dome area is in the northern part of the Illinois fluorspar district and the other two areas 

are better known as thorium vein districts. 

The three listed disseminated deposits show 

differences in composition and depth of occurrence that would affect the type of mining and 

milling used. Most large disseminated deposits 

are mined by open pit, as this is the cheapest 

method of mining a large deposit. The deposits in 

the Bear Lodge Mountains and Powderhorn 

districts are, for the most part, exposed at the 

surface and could easily be mined with one or 

more open pits. The Hicks Dome deposit, 

however, is in a large cryptovolcanic structure, 

and little is exposed at the surface. 

The composition and character of the 

disseminated deposits also differ. The Bear 

Lodge Mountains deposit consists of a composite 

trachyte-phonolit'e host rock that has been 

altered and cut by numerous thin veinlets contairring both rare earths and thorium. At 

Powderhorn, the rock is a thorium-rich granite 

cut in places by veinlets. Thorium would be the 

principal product here. The Hicks Dome deposit 

is a breccia containing fragments of sedimentary 

rocks and having a mineralized matrix. Milling of 

material from the first two areas would be similar 

to that used for thorium veins. 

The open-pit method is proposed for mining 

those disseminated bodies that occur at or near 

the surface. Open-pit methods also are applicable 
to mining the massive carbonatite bodies described in. the following section of this report. In the 

open-pit method, rock is broken by drilling and 

blasting. It then is loaded using a front-end 

loader and hauled to the mill by truck. Sides of 

the pit are benched; bench height is 50 ft. We envision this type of mine operated by two shifts 

per day, 350 days a year. 

Milling of the disseminated deposits of the 

Bear Lodge Mountains and the Powderhorn 

district would use the same mill design as for 

separating thorium and rare, earths from the 

veins (p. 8~ fig. 2). 

The lands on which the Bear Lodge, Hicks 

Dome, and Powderhorn disseminated deposits oc· 

cur are controlled by from one to several owners, 

and it has been necessary to consolidate the 

resource estimates of all disseminated deposits 

into an overall estimate. Resource data on the 

disseminated deposits are not as abundant as 

those on other types of thorium deposits. More 

definite data need to be obtained on the size of the 

disseminated deposits and many more samples 

should be collected. Thus, the resources in these 

deposits are not sufficiently well known to be called reserves. They also could not be considered as 

probable potential resources, because according 

to the Department of Energy classification, the 

system used in this paper, if a deposit does not 

have reserves, it does not have probable potential 

resources, It is clear, however, that large 

amounts of thorium do exist in this type of 

deposit, and we have classified these as "other 

resources." As used here, it refers simply to 

discovered resources for which a strong data base 

is not yet established. 

The dis semina ted deposits in the Hicks Dome, 

Bear Lodge Mountains, and Powderhorn districts 

have a total of 2,200,000 tons of Th02 as "other 

resources. I t Some of these deposits would probably be developed as multiproduct operations. If 

Th02 , however, were the only product, then probably a third of the total resources could be produced competitively with uranium. 

HICKS DOME, ILLINOIS 

Hicks Dome is in Hardin County, Ill., about 35 

mi northeast of Paducah, Ky., and 8 mi. north of 

Rosiclare, Ill. (fig. 1). The deposit was found during drilling by the St. Joseph Lead Co. of a hole 

21 

near the apex of the dome (Brown and others, 

1954). During this drilling, a mineralized breccia 

was found beneath a thick sequence of sedimentary rocks at a depth of 1,607 ft. The hole was still 

in breccia at its bottom at a depth of 2,944 ft. 

Descriptions of the surface and subsurface 

character of the dome and its contained breccia 

are from Brown, Emery, and Myer (1954) and 

Heyl, Brock, Jolly, and Wells (1966). Identification and description of the mineralized breccia 

and the thorium and rare-earth minerals are from 

Trace (1960) and Brown, Emery, and Myer (1954). 

The aeroradiometric data used in delineating the 

periphery of the deposit are from Pitkin (1974). 

Hicks Dome comprises more than 2,000 ft of 

Ordovician, Silurian, Devonian, and Mississippian sedimentary rocks, most of which are 

limestone, that have been pushed upward by an 

explosive intrusion of fluids. The Hunton 

Limestone of Devonian age is at the surface in 

the center of the dome, and younger rocks of 

Mississippian age are exposed on the flanks 

(fig. 3). 

Total upward displacement of the sedimentary 

rocks over the crest of Hicks Dome is about 4,000 

ft and the dome's diameter is approximately 9 mi. 

The dome lies on the crest and near the northwest 

terminus of a large northwest-trending anticline. 

The anticline and dome are transected by many 

northwest-trending faults. 

Several small bodies of breccia are exposed on ' 

the top of the dome (fig. 3), These appear to be the 

uppermost extensions of the much larger body of 

explosion breccia found in the St. Joseph Lead 

Co. Hamp No. 1 drill hole. The center of the dome 

is outlined by a three- to four-fold increase in 

radioactivity that occurs over the apex area 

(Pitkin, 1974). The area of anomalous radioactivity at a flight level of about 400 ft above the surface is delineated on the geologic map by a conspicuous increase in density of the isorads over 

the apex area. 

The breccia consists mostly of limestone and 

dolomite fragments of the Paleozoic sedimentary 

rocks in a finer grained matrix of similar rock 

fragments, rounded quartz grains, and various 

hydrothermal minerals. The rounded quartz 

grains are believed to be derived from deeply 

buried St. Peter Sandstone of Ordovician age. 

The breccia body has been termed a diatreme 

(Brown and others, 1954; Heyl and Brock, 1961) 

and originated from an explosion associated with 
EXPLANATION 

Fort Payne 

~ Formation and 

Springville Shale 

New Albany 

~ Group 

I 9& I 

Hunton limestone 

Explosion breccia 

Approximate contact 

----12---

---14---

z 

<( 

c... 

c... 

"' "' "' "' 

~ 

z 

<( 

z 

0 

> 

UJ 

0 

Contours drawn on lines of equal 

gamma-ray intensity of the 2.62 

MeV (million electron volts) 

photopeaks of thallium -208 in 

the thorium -232 decay series. 

Contour interval 2 counts per 

second. Hatchured to indicate 

closed areas of lower 

gamma-ray intensity 

Fault, approximately located 

0 800 1600 2400 FEET 

0 200 400 600 800 METER S 

A 

~Vl22 .-o 

zz 18 

:::>0 14 

8~ 10 

RADIOACTIVITY PROFILE A ' 

j 

FIGURE 3.-Geologic and aeroradiometric map of Hicks Dome, Ill. Geology modified from Baxter and Desborough (1965, pl. 1), 

Baxter, Desborough, and Shaw (1967, pl. 1), and Brown, Emery, and Myer (1954, p. 892). Aeroradiometric contours from 

Pitkin (1974, pl. 1). 

a deep-seated alkalic intrusion. Except for the 

hydrothermal mineralization, the diatreme is 

similar in character and composition to many of 

the diatremes in the midcontinent region as 

described by Zartman, Brock, Heyl, and Thomas 

(1967). 

Brown, Emery, and Myer (1954) noted that the 

brecciated zone between the 1,607-ft depth and 

22 

the bottom of the drill hole was mineralized. The 

mineralization occurs in the matrix of the breccia, 

as incipient veinlets, and as replacement of 

limestone and dolomite fragments. Hydrothermal minerals exposed in this hole and in a nearby 

surface trench 260 ft long (fig. 3) have been 

reported on by Brown, Emery, and Myer (1954, 

p. 897-899) and Trace (1960). These minerals in-
elude fluorite, calcite, quartz, barite, pyrite, 

sphalerite, galena, apatite, florencite, and 

monazite. 

Sample data came from vein and brecia 

material exposed in the 260-ft long trench (Trace, 

1960, p. B641 and from the St. Joseph Hamp No.1 

hole. Many more samples from many drill holes 

are needed to give an accurate picture of distribution of the radioactive minerals and the grade of 

this possible deposit as well as to verify whether 

the size of the anomaly on the radiometric map 

(fig. 3) in fact is a reasonable indication of the 

presence of a large subsurface breccia mass. 

Radiation as indicated by spectrographic 

analyses of the drill core samples (Trace, 1960, 

p. B64) and from the radiometric study (Pitkin, 

1974) is principally due to thorium. Eight 

samples, 25-30 ft long, were taken of core from 

the Hamp No. 1 hole in the breccia. These 

samples, which represent only 210ft of the total 

2,337 ft of breccia cut, were analyzed both by a 

semiquantitative spectrographic method (Trace, 

1960, p. B64) and by a radiometric method 

(Brown and others, 1954, p. 899). The first 

method identifies various elements and gives a 

semiquantitative value of each. The second gives 

a quantitative value for the amount of uranium 

present if the radiation is due to uranium and if 

the uranium is in equilibrium. The spectrographic 

and aeroradiometric data, however, indicate that 

the radiation is due to thorium. The amount of 

thorium actually present can be calculated from 

the equivalent uranium values. Th02 of these 

eight samples ranges from 0.007 to 0.18 percent. 

The resource calculation included in the 

generalized figure for disseminated deposits is 

based on assumptions expressed above concerning the probable extent of the subsurface breccia 

mass and its probable content of thorium 

minerals. The interpreted breccia mass may not 

be uniformly developed in lateral extent or depth, 

and its content of thorium minerals may be extremely uneven; thus, the resource calculation is 

recognized as highly speculative but is regarded as embodying geologic interpretation and 

extrapolation. 

BEAR LODGE MOUNTAINS, WYO. 

The Bear Lodge Mountains disseminated 

deposit is in the southern Bear Lodge Mountains 

about 5 mi northwest of Sundance, Crook Coun23 

ty, Wyo. (fig. 1). This is the same area in which 

the thorium-bearing veins in the Bear Lodge 

Mountains occur. The host rock of the 

disseminated deposit consists of alkalic intrusive 

rocks of middle Tertiary age. These rocks, which 

are principally trachyte and phonolite, make up 

an igneous complex that was formed by a series 

of intrusions during a period of at least 20 million 

years. The rocks of the igneous complex intruded 

Paleozoic and Mesozoic sedimentary rocks that 

they not only partly replaced but also forced up 

into a dome. The dome itself underlies an area 

about 8 mi long and 6 mi wide. The alkalic igneous core of this dome has an oval outcrop 

pattern and is exposed over an area approximately 5% mi long and 2% mi wide. After most of the 

igneous rock was intruded, a period of stress and 

alteration occured during which some of the igneous rock was fractured and altered, and the 

numerous fractures were filled with vein material 

Most of the veinlets are thin, ranging from fracture coatings to veins 1I4-in: thick. Some are 

thicker, and some are even large enough to be 

mined as separate veins. The fractures and the 

emplaced veinlets are numerous, crisscrossing, 

and strike in many different directions. Dips are 

variable. 

The veinlets are gray, black, or brown. The color generally depends on the presence and amount 

of various iron and manganese oxides. The 

manganese oxide minerals, pyrolusite and cryptomelane, or finely disseminated specular 

hematite, give the veinlets a black color; the 

presence of limonite, generally goethite, tends to 

color the veinlets shades of brown. Only when 

these oxides occur in small amounts does the vein 

show the color of other minerals present. The 

mineralogy of the veinlets is similar to that of the 

veins. The minerals in the disseminated deposit 

are those of both the country rock and the 

veinlets. Sanidine is one of the commonest 

minerals present, as it is abundant both in 

phenocrysts in the host rock and in gangue in the 

veinlets. Another abundant material might be 

called "groundmass." This is the altered glassy 

or partly recrystallized groundmass of the host 

rock, which may originally have been glass but 

has since recrystallized into minute undistinguishable grains. In places the groundmass 

contains discrete mineral grains that have subsequently been altered to unrecognizable minerals. 
Although feldspar is the principal gangue mineral 

in many of the veinlets, quartz or cristobalite is 

more abundant in some. Monazite, thorite, and 

brockite are the identified thorium-bearing 

minerals. In addition, many veinlets contain 

small amounts of barite, rutile, brookite, and 

magnetite. Some veins also contain minor 

amounts of fluorite, xenotime, sphalerite, and 

pyrite. 

The amount of ThO, in the disseminated 

deposit varies from place to place. In part, this 

variation is because the outer edges contain fewer 

veinlets; in part, it is due to variations in the 

number of veinlets within the body. Completely 

unaltered trachyte and phonolite are not common, but in areas where they are least altered, as 

near the south end of the core of the igneous complex and in separate bodies peripheral to the core, 

these rocks contain from 19 to 46 ppm ThO,. The 

grade of rock containing disseminated veinlets increases from about 46 ppm ThO, for unmineralized rocks to about 1,200 ppm for rock containing 

many veinlets. These grade figures do not include 

solid vein material as described in the section on 

"Veins," which can be of much higher grade. Rare 

earths are much more abundant than thorium in 

the disseminated deposit. The total rare-earth oxide content of 52 samples of rock containing 

veinlets varies from 430 to 32,000 ppm and has 

an average of about 2,300 ppm. Most of the 

analyses, however, range from 1,200 to 18,000 

ppm total rare-earth oxides. 

The part of the intrusive complex that contains 

numerous small veinlets is an area that extends 

for about 1-1/2 mi to the north from the northernmost of the Warren Peaks, and is a litt.le more 

than 1 mi wide. In other parts of the intrusive 

complex, especially to the south, the fracturing 

and alteration diminish. The area of veining and 

thorium mineralization can be separated from the 

less mineralized areas by its radioactivity. The 

area of thorium mineralization was first outlined 

by making a radiometric map using a hand-held 

scintillation counter (fig. 4). Unfortunately, the 

measurements include not only those obtained 

from thorium radiation, but from that of 

potassium and uranium as well. All three have an 

appreciable effect on the total radioactivity. 

Potassium feldspar occurs both in the host rock 

and in the veinlets. I n the altered and veined part 

of the intrusive complex, the potassium content 

is more or less constant and its average is approximately 10 percent. Although the amount of 

radioactive K40 in potassium is small in these 

potassium-rich rocks, the radioactivity due to 

this element would be about equal to 270 ppm of 

ThO,. The uranium content of these veined rocks 

varies from 8 to 346 ppm, and the thorium-touranium ratios of various samples taken ranges 

from 0.4 to 30. The uranium content is greater 

than in most other vein districts. The amount of 

radioactivity due to uranium is approximately 5.7 

times that created by an equal amount of 

thorium. Hence, a sample containing 12 th;nes as 

much thorium as uranium would have about onethird of its radiation due to uranium. Although 

thorium is overall the most important radioactive 

element, the presence of relatively large amounts 

of radiation from potassium and uranium could 

mask its presence. To answer the question of 

whether the relative proportions of thorium Can 

be discerned on a radioactive map, a graph was 

made showing thorium content versus total 

radioactivity (fig. 5). Points shown on figure 5 

represent the thorium content of individual 

samples plotted against their total radioactivity. 

The thorium content of most samples increases in 

proportion to total radioactivity, and the graph 

shows a linear trend. Therefore, a counter can be 

used to outline the area of largest thorium content (fig. 4). Other samples of the intrusive complex were taken within this area and analyzed for 

thorium (fig. 5). From these data, a second contour map was made using a contour interval of 

100 ppm thorium. Examination of the shape of 

the 100-ppm contour shows that it resembles the 

600-counts-per-minute total-radiation contour 

measured with the scintillation counter (fig. 4). 

The thorium-contour map made from the 

analytical data has three areas where the quantity of this element exceeds 220 ppm. Each area is 

considered a possible site for an open-pit mine for 

mining thorium and rare earths. As only a few 

feet of soil covers most of the area, there is little 

overburden to remove. Although some sampling 

has been carried out in the three areas, many 

more analyses are needed to obtain a valuable 

average grade. Grades of these areas, however, 

24 
44° 

30' 

104°32' 

7 

18 

19 

30 

Thorium contour --Drawn on 

-

points 

of equal thorium content. Contour 

interval 100 ppm thorium. Hachured 

to indicate closed area of lower 

thori urn content 

8 

R. 63 W. 

EXPLANATION 

9 

r,\ 

/ ) ~ / , '-./ 'I' I 6 

...... / 

/ 

28 

...,..------.. ...... 

Radioactivity contour--Drawn on all 

points that have a total-gamma 

radioactivity plus high-energy beta 

radioactivity of 600 counts per 

second. Hachured to indicate 

closed area of lower radioactivity 

T. 

52 

N. 

FIGURE 4.-Thorium-content and radiometric map of the northern part of the alkalic intrusive core in the Bear Lodge, 

Mountains, Wyo. 

25 
1000~--------r---------~--------'---------~---------'--------' 

• 

900 

800 

700 

z 

0 

:J 

...J 600 

~ 

II: 

w 

a.. 

C/) 

I- 500 II: 

<C 

a.. • 

z 

~ 

:::> 400 a: 

0 

:c 

l-

• • • 

300 

• • 

• 

• • 

200 •• • 

• 

•• 

• • • 

100 I' • . • • 

•••••• 

• • • • • °0~--------4~0~0---------8~00~-------1~2LO~0--------1~60~0~~----2~0~0~0------~2~400 

TOTAL RADIOACTIVITY IN COUNTS PER SECOND 

FIGURE 5.-Relationship of thorium content to total radioactivity in samples from the Bear 

Lodge Mountains. Wyo. 

26 
based on available samples are as follows: (1) The 

northern area has an average grade of 0.023 percent Th02 and 0.75 percent combined rare-earth 

oxides, (2) the central area has an average grade 

of 0.042 percent ThOz and 1.71 percent combined 

rare-earth oxides, and (3) the southern area has an 

average grade of 0.035 percent Th02 and 1.35 percent combined rare-earth oxides. Drilling in secs. 

17 and 18 (fig. 4) indicates that the veining extends at least 1,200 ft below the surface. 

Although mineralization is known to this depth, 

grade data are scarce. 

POWDERHORN, COLO. 

A fine-grained granite that in places is abnormally radioactive occurs within 2 mi of the south 

and west margins of the alkalic igneous complex 

at Iron Hill and has been mapped by Olson (1975) 

and Hedlund and Olson (1975). The granite occurs in dikelike bodies and irregular small plutons 

as much as 1,600 ft wide and in two larger bodies 

2% mi long and 2,000-4,000 ft wide. The granite 

mostly is fine grained, some is medium grained, 

and locally it is porphyritic. It is composed of 

microperthitic microcline, quartz, plagioclase, 

and biotite. Associated with the granite are two 

bodies of melasyenite, 200-400 ft wide. The finegrained granite was considered by Hunter (1925) 

to be a variety of the Powderhorn Granite of 

Precambrian age, but the fine-grained granite intrudes the Powderhorn Granite (Olson, 1975). 

Furthermore, a considerable age difference between the two granite types is likely, as the finegrained granite is little deformed in comparison 

with the adjoining gneissic coarse-grained 

Powderhorn Granite. 

The fine-grained granite is fenitized in places, 

as shown by potassium feldspathization and bluegreen amphibole veinlets, and it is cut by carbonatite and lamprophyre dikes and by thorium 

veins that are probably related to the complex at 

Iron Hill. Some of the fine-grained granite occurs 

in crudely arcuate plutons around the southern 

part of the complex at Iron Hill. These forms, 

which are suggestive of ring dikes, may indicate a 

link between the fine-grained granite bodies and 

the complex. 

Not all the fine-grained granite bodies are abnormally radioactive. Those with abnormal 

radioactivity occur in two areas: (1) along the 

western edge of the complex at Iron Hill for about 

2 mi to its southern tip, and (2) in an area 2,000 to 

6,000 ft south of the central part of the southern 

edge of the complex. In the first area is a pluton 

about 2 mi long and 4,000 ft wide. This pluton has 

been strongly fenitized adjacent to the complex. 

Part, but not all, of this body is abnormally 

radioactive. The extent of abnormal radioactivity 

is not well known, but in an area adjacent to the 

road down Cebolla Creek, readings on a gammaray spectrometer indicated that the partly feni tized granite here contains from 40 to 300 ppm 

Th02• An average of this area would be about 100 

ppm. The thorium-rich part of the granite is at 

least 400 ft wide here but probably makes up only 

a small part of the entire granite body. 

In the second area are four plutons that range 

from 1,000 to 3,500 ft in length and are 300 to 

1,600 ft wide. These plutons have been explored 

by numerous bulldozer trenches, especially on the 

sides of the road up Beaver Creek, which cuts 

through the center of the area. The radioactivity 

of these granite bodies was measured in the field 

using a gamma-ray spectrometer, but eight 

samples also were analyzed using a laboratory 

gamma-ray spectrometer. The Th02 content of 

these rocks varies considerably, but large parts of 

these granite bodies contain from 115 to 225 ppm 

ThOz, although some of the most radioactive 

areas yield as much as 1,140 ppm ThOz. The eight 

samples analyzed in the laboratory had from 35 

to 320 ppm ThOz• The parts of the granite bodies 

having the most radioactivity are characterized 

by red and yellowish-brown limonitic staining 

and by rusty-weathering black siliceous veinlets 

and brown limonitic veinlets, which occur along 

joint and shear planes. 

The greater part of the known thorium 

resources occurs in the four small plutons directly 

south of the complex at Iron Hill where the 

largest abnormally radioactive area is at least 

600,000 ft2 in size. 

27 

MASSIVE CARBONATITE BODIES 

Massive carbonatites have not been prospected 

as sources of thorium. The principal reason is 

that the grade is low compared to nearby vein 

deposits. The size generally is large, however, and 

as these carbonatites commonly are rich sources 

of other metals such as niobium and rare earths, 
any thorium that might be recovered from 

massive carbonatites likely would be a 

byproduct. 

Carbonatites are not common, although they 

are known from at least 13 localities in the United 

States. Most of them are narrow dikes. ~'our 

massive carbonatites are known in the United 

States: (11 Iron Hill carbonatite in the 

Powderhorn district of Colorado, (2) Sulphide 

Queen carbonatite in the Mountain Pass district 

of California, (31 carbonatite in the Magnet Cove 

district, Arkansas, and (4) Elk Creek carbonatite 

in Nebraska. The Iron Hill carbonatite underlies 

an area of more than 1 mi'. The Sulphide Queen 

carbonatite is an irregular body a little less than 

0.5 mi long. It is the site of the largest rare-earth 

mine in the United States. These two massive car· 

bonatites are evaluated for thorium in this section of the report. 'The Magnet Cove carbonatite 

was not considered because it is small and has little thorium. Although thorium Was looked for in 

spectrographic analyses of 10 samples from this 

carbonatite. it was not found (Erickson and 

Blade, 1963, p. 35). The Elk Creek carbonatite 

was discovered by the Nebraska Geological 

Survey when they were drilling at a geophysical 

anomaly on the Nemaha anticline, It is overlain 

by more than 600 ft of sedimentary rocks 

\Brookins and others 1975, p. 80). This carbonatite also was not evaluated, as little data are 

available as to its size, mineralogy, and grade of 

thorium, 

The Iron Hill and Sulphide Queen carbonatites 

(fig, 1) differ in size, topographic exposure, and 

mineral content. Their vertical dimension probably is large; the Iron Hill carbonatite forms a 

hill having a vertical exposure of morc than 1,000 

ft, The two carbonatites are projected to different 

depths in the resource calculations. The depth 

used in the large Iron Hill carbonatite includes 

the hill plus an additional 800 ft below its lowest 

topographic exposure. The smaller Sulphide 

Queen carbonatite is projected a total distance of 

1,000 ft, of which half is calculated as reserves 

and half as probable potential resources. A tonnage factor of 11.4 Wlshort ton was used in converting carbonatite volume to carbonatite 

weight. 

Mining of the Iron Hill carbona tile would be by 

open pit, but would differ somewhat from the 

standard open pit procedure, as most of Iron Hill 

could be removed first by mining from the sides 

28 

of the hill before a standard benched pit was 

started downward. The Sulphide Queen carbonatite is being mined as an open-pit operation. 

The open·pit mining and costs entaJled would be 

sImilar to those previously described under the 

section on disseminated deposits. 

Milling of ore from these two properties would 

be somewhat different because the principal 

economic product at Iron Hill probably would be 

niobium and at the Sulphide Queen it is rare 

earths. Furthermore, the principal thoriumbearing minerals at the two localities are not the 

same. Milling costs also would be different 

because capital costs at Iron Hill would include 

the cost of a complete mill bullt for recovery of 

thorium and other metals. At the Sulphide 

Queen, on the other hand, a mill built to recover 

rare earths already exists, so capital costs would 

cover only the costs of a thorium circuit. 

Pyrochlore is known to contain thorium in the 

Iron Hill carbonatite, and at present it is the only 

known source here of this element. This mineral 

also contains both rare earths and uranium. 

Hence, in the mill design used for this property, it 

was assumed that pyrochlore was not only the 

major source of the niobium, but of the thorium, 

rare earths, and uranium as well. Pyrochiore in 

the carbonatite would be recovered by a flotation 

process similar to that presently being used to 

recover this mineral at Oka in Canada (Carbonneau and Caron, 1965, p. 75-78). Pyrochlore concentrates are treated with chlorine gas at high 

temperature, causing the niobium to be volatized. 

It then is precipitated by hydrolysis. The 

thorium, rare earths, and uranium are extracted 

from the residue by an acid leach-solvent method. 

This separation method of the various metals 

from the pyrochlore is based on limited 

laboratory tests. These tests indicate that mill 

recovery of Nb,O" ThO" U,O" and total rareearth oxides would be about 80 percent. Possibly 

a better recovery could be achieved in plant practice. Capital and opera ting costs were based on a 

mill having a maximum handling capacity of 

10,000 tons per day and having a minimum mill 

life of 15 yr. The actual size of any mill built 

would depend on the demand for both niobium 

and thorium. Using a mill operating at maximum 

capacity of 3,GOO,OOO tons a year (10,000 tons X 

350 days), the deposit would last for many years. 

I ncreased demand could be met by building more 
similar mills or larger mills. For larger mills, both 

the capital and operating cost per ton of Th0 2 

probably would be a little lower. 

Most of the thorium in the Sulphide Queen carbonatite apparently is in monazite, which is fairly 

common in some parts of this body. The mill 

operation here would separate out the monazite 

and then extract the Th0 2 from it. To do this, 

only the cost of adding a circuit to Molybdenum 

Corp. of America's present operation was 

calculated. In their mill, bastnaesite, the principal rare earth ore mineral, is separated from the 

carbonate gangue by flotation (Harrah, 1967; 

Johnson, 1966). Most of this concentrate is dried 

and then leached using 30-percent hydrochloricacid solution, which dissolves the rare earths 

from the bastnaesite. Monazite probably floats 

with the bastnaesite, as oleic acid, which is used 

by Molybdenum Corp. of America as the flotation 

promoter, and is also a flotation promoter for 

monazite. The monazite probably would not 

dissolve in the acid and could be recovered from 

the acid leach residue. Based on these assumptions, a circuit was designed to recover Th02 from 

the acid residue. In this circuit, monazite would 

be first electrostatically separated from the 

residue. The monazite would then be acid leached 

and Th02 and total rare earth oxides would be 

recovered from the leach solution by solvent extraction and precipitation. The mill recovery of 

Th0 2 in this circuit is estimated to be about 70 

percent. Capital cost for the extra mill circuit 

would be about $1,500,000. 

The two massive carbonatites evaluated in this 

paper Uron Hill and Sulphide Queen) have a total 

of 165,100 tons of Th02 in reserves and probable 

potential resources. If only Th0 2 is recovered, 

none of it could be produced at less than $30/lb 

and only about 12 percent fthat occurring in the 

Sulphide Queen carbonatite) would be producible 

at between $30 and $50/lb. If other economic products were recovered, then 83 percent of this total 

or 137,500 tons of Th02 would be producible at 

less than $30/lb. Little would be producible at 

between $30/1b and $50/1b of Th02• Thus, the 

Th02 produced from massive carbonatites should 

be considered principally as a byproduct, and it is 

dependent on the markets for rare earths and 

niobium. The amounts of Th0 2 and estimated 

cost of production for the two massive carbonatites is summarized in table 3. 

29 

IRON HILL BODY, POWDERHORN DISTRICT, 

COLORADO 

The Iron Hill carbonatite, the largest exposed 

massive carbonatite in the United States, is part 

of the 570 million-year-old alkalic complex at Iron 

Hill, near Powderhorn, Colo. (Olson and others, 

1977). The complex, which consists primarily of 

pyroxenite, uncompahgrite, ijolite, nepheline 

syenite, and carbonatite intrudes Precambrian X 

Powderhorn Granite that is strongly fenitized for 

at least 0.5 mi away from the contact. The complex is, in turn, intruded by carbonatite dikes, 

magnetite-perovskite segregations, and diabase 

dikes of Ordovician or Cambrian age. Tertiary 

ash-flow tuffs and a variety of Tertiary and 

Quaternary sedimentary deposits cover parts of 

the complex. 

The complex of alkalic rocks at Iron Hill and 

the surrounding area have been mapped by 

Larsen f1942), Hedlund and Olson (1975), and 

Olson (1975). The mineral-resource potential of 

the complex of alkalic rocks has been discussed 

by Olson and Wallace (1956), Rose and Shannon 

(1960), and Temple and Grogan (1965). 

Anomalous amounts of thorium, 5 to 10 times 

greater than background, were reported at two 

localities within the massive carbonatite by 

Olson and Wallace (1956). The massive carbonatite also contains known reserves of 

niobium-more than 100,000 tons of Nb20s in 

rocks averaging at least 0.25 percent Nb20 s (Temple and Grogan, 1965). Abundant rare earths also 

are found in the carbonatite. 

A study was made to determine the abundance 

and distribution of thorium in the massive carbonatite at Iron Hill. Field measurements of 

thorium abundance were made at 151 stations 

within the directly adjacent to the massive carbonatite using a portable differential fourchannel gamma-ray spectrometer. This instrument uses a 3-in. diameter by 3-in. thick NaI 

crystal as a detector. Counting times at each station were of 2-minute duration. These measurements were compared with those from 28 samples 

that were analyzed for thorium in the laboratories 

of the U.S. Geological Survey. The accuracy of 

the field method is difficult to evaluate but, as a 

guess, probably does not differ from the 

laboratory method by more than about 25 percent. 

The massive carbonatite at Iron Hill consists 

primarily of dolomitic carbonatite, but several 
TABLE 3.-Th02 reserves and probable potential resources in short tons from two massive carbonatite bodies in tke United 

States. 

Total producible at Total producible at Total producible at 

less than $15/1b between $15 and $30/1b between $30 and $50/1 b 

Total probable Probable Probable Probable 

District Total potent i al Reserves potential Reserves potential Reserves potential 

reserves reSources resources resources resources 

Where ThOZ is the sale product 

Iron Hill, 

Colo •.•.••••• 31,080 0 ° 0 0 0 ° 0 

Sulphide Queen, 

Ca 1 if ••••.••• 9,750 9,750 ° 0 0 ° 9,750 9,750 

Total •••••••• 40,830 124,650 ° ° 0 0 9,750 9,750 

Where other economic protiucts are recovered 

I ron Hi 11 , 

Colo •..•.•••• 31,080 114,900 31,080 

Sulphide Queen, 

Ca 1 if •.•••••• 9,750 9,750 9,750 

Total •••••••• 40,830 124,650 40 

calcitic variants occur in the intrusion. The fresh 

light-gray carbonatite typically weathers dark 

brown. Variations in grain size are common. The 

carbonatite is cut by martite-apatite veins, some 

of which are more strongly radioactive than the 

neighboring carbonatite. Linear zones occupied 

by weakly radioactive jasper appear to represent 

silicified fracture zones. 

The most abundant mineral in the carbonatite 

is dolomite; ankerite, siderite, and calcite also occur. Pyrochlore is the dominant niobium mineral 

and fluorapatite probably is the most important 

rare-earth-bearing mineral. Magnetite, ilmenite, 

hematite, goethite, pyrite, rutile, zircon, 

monazite, fluorite, biotite, and barite also are constituents of the carbonatite (Temple and Grogan; 

1965; Nash, 1972). 

The 28 samples of carbonatite analyzed for 

thorium also were analyzed for radium-equivalent 

uranium (RaeU) and potassium using a 

laboratory gamma-ray spectrometer and for 61 

elements by the six-step semiquantitative spectrographic method. Certain characteristics of the 

Iron Hill carbonatite become apparent when the 

average abundances of elements in the 28 

samples are compared with elemental abundances 

in igneous rocks of the upper continental crust 

(Wedepohl, 1971, p. 65). The following abundances were noted: barium, lanthanum, niobium, 

phosphorus, cerium, calcium, and total rare-earth 

elements are at least 10 times more abundant in 

87,300 0 0 ° 0 

9,750 0 0 0 ° 

97,050 0 0 0 0 

30 

the carbonatite; manganese and strontium 5-10 

times more abundant; and beryllium, magnesium, 

uranium, and thorium are 3-5 times more abundant. The other elements are only slightly more 

abundant or slightly depleted. These elements 

constitute the assemblage of minor and trace 

elements that is characteristic of carbonatites in 

general (Heinrich, 1966). 

Iron Hill is made up entirely of the massive carbonatite. Other rock types form the low areas 

around the hill. The high point on the hill is more 

than 1,000 ft above the west side of the carbonatite body where it crops out along Cebolla 

Creek. The amount of material in the hill that projects above the surrounding land surface was 

calculated by making seven evenly spaced cross 

sections across the hill, measuring the area in 

each, and multiplying the average area in pairs of 

sections by the distance between them. The hill 

was found to contain 722,700,000 tons of carbonatite. The average grade, based on 

measurements at 151 stations, is 0.0043 percent 

Th02• This amount of rock would contain 31,080 

tons of Th02• The average grade of various other 

metals in the carbonatite was obtained from 

analytical data on the 28 samples. These grades 

are 0.057 percent for Nb 20 5, 0.397 percent for 

total rare earth oxides, and 0.00127 percent for 

U308> The hill contains 411,900 tons of Nb20 s, 

2,865,000 tons of total rare-earth oxides, and 

9,180 tons of UsOs> Higher grades of thorium and 
other elements could be obtained by selective 

mining, as the distribution of the various 

elements within the massive carbonatite is erratic. The carbonatite exposed on this hill would 

all be in the category of reserves. Probable potential resources would lie below Iron Hill and could 

be extended to a depth of another 800 ft. The area 

underlain by carbonatite is 38,070,000 ft2. Carbonatite classified as probable potential 

resources amounts to 2,672,000,000 tons, which 

contains 114,900 tons of Th02, 1,523,000 tons of 

Nb205, 10,590,000 tons of total rare earth oxides, 

and 33,930 tons of U30 S• 

The reserves that form Iron Hill, itself, could be 

mined by removing the hill. The probable potential resources, which would be mined from an 

open pit, are less than the total resources because 

the walls of the pit must be terraced to an overall 

slope of approximately 45 0 • Therefore, if the carbonatite were to be mined to a depth of 800 ft 

below the hill that was stripped away, then either 

some of the carbonatite must be left in the sides 

of the pit or the cost of mining must be increased 

by widening the pit and removing an everincreasing amount of country rock. The method 

chosen would depend on the price of both thorium 

and the other coproducts. In this paper, we have 

calculated the probable potential resources 

assuming that the open pit was made entirely 

within the carbonatite: The open pit would have 

an overall slope of 1:1 effectively eliminating 

about half the carbonatite on the flanks. Minable 

carbonatite in an open pit having a depth of 800 ft 

would be 2,030,000,000 tons. This amount of carbonatite would contain approximately 87,300 

tons of Th02 producible at less than $15/1b if 

niobium, uranium, and rare earths are recovered, 

but more than $50/1b if they are not. In addition, 

this carbonatite would also contain 1,160,000 

tons of Nb20;, 8,050,000 tons of total rare-earth 

oxides, and 25,800 tons of U30a. 

As previously noted, the cost of producing 

Th02 from this massive carbonatite is calculated 

two ways: (1) its cost if Th02 is the sole product 

obtained from the carbonatite. and (2) its cost if 

Nb20 5, total rare-earth oxides, and U 30S are all 

recovered in addition to the Th02. The cost of producing Th02 if it is the sole product red overed is 

$68/Ib. The value of the Nb205, total rare earth 

oxides, and U 308 were obtained from recent price 

quotations. If a market could be obtained for all 

these metals at 1978 prices, they would pay for 

31 

the cost of mining and milling. The value of these 

three products is more than twice that of Th02 in 

the rock if the latter were to be sold for $50/Ib. 

Therefore, mining this body is dependent on a 

multi-element product with the production of 

Th02linked to the production of Nb205, total rare 

earth oxides, and U30 a• 

SULPHIDE QUEEN BODY, MOUNTAIN PASS, CALIF. 

The Mountain Pass district is in the 

northeastern corner of San Bernardino County, 

Calif. near the eastern edge of the Mojave Desert 

(fig. 1). The district is part of a rolling upland that 

lies several thousand feet above and to the west 

of Ivanpah Valley. The area is easily accessible, 

as the massive carbonatite lies only two-thirds of 

a mile north of U.S. Interstate Highway 15, 

which goes through Mountain Pass, about 170 mi 

northeast of San Bernardino, Calif., and 60 mi 

southwest of Las Vegas, Nev. 

The massive carbonatite called the Sulphide 

Queen body is Precambrian in age and has intruded a well-foliated metamorphic complex. The 

metamorphic rocks also are intruded by several 

kinds of potassium-rich rocks, also of Precambrian age. The intrusives include biotite 

shonkinite, hornblende and biotite syenite, and 

granite (Olson and others, 1954, p. 5-10). The 

Precambrian rocks form an elongate block bounded on the east by the alluvium of I vanpah Valley 

and on the west by Paleozoic and Mesozoic 

sedimentary and volcanic rocks. The Clark Mountain fault separates the Paleozoic and Mesozoic 

rocks from the Precambrian rocks (Olson and 

others, 1954, pI. 1). In addition to the main 

Sulphide Queen carbonatite, carbonatite dikes intrude the older Precambrian rocks; most of the 

carbonatite dikes lie in an area extending about 

3,000 ft northwest from the main carbonatite. 

The Sulphide Queen carbonatite, most of the carbonatite dikes, and the bulk of the alkalic igneous 

rocks occur in an area less than 1 mi2. Thoriumbearing and rare-earth-bearing veins occur mainly to the southeast of this area. The thoriumbearing veins are discussed in the section on 

"Veins." 

The Sulphide Queen massive carbonatite (fig. 6) 

is an irregularly shaped body elongate in a northsouth direction, having an overall length of 2,400 

ft and an average width of about 400 ft (Olson 

and others, 1954, p.41). It varies considerably in 
z 

I{ 

a: 

Q. 

FEET 

4900 

4800 

4700 

A 

o I ! I I I I 

o CONTOUR INTERVAL 

500 FEET ! I 

I 

150 METERS A' 

50 FEET METERS 

1580 

1520 

4800ll __ ~2flliill2l~~~ ________ ~1480 4500- 1440 

EXPLANA TION 

Carbonatite --- Fault·· Approximately 

located 

Metamorphic and _ Drill hole 

igneous rocks 

FIGURE 6.-Generalized geologic map and section of the original surface of the Sulphide Queen carbonatite body. Map 

simplified from Olson. Shawe, Pray, and Sharp (1954. p. 

8); section from Evans 11966, p. 25,. 

width; its maximum width in the southern part is 

700 ft, but in the northern part is less than 100 ft. 

I t varies in thickness from 160 ft on the north to 

250 ft on the south. Apophyses of carbonatite ex32 

tend from this body into the enclosing gneiss, and 

inclusions of gneiss and various igneous rocks as 

much as 150 ft long are found in the carbonatite. 

The carbonatite body dips 33° to 40° to the west 

(fig. 6) (Evans, 1966, p. 29). 

The massive carbonatite varies in composition 

from place to place. Some of these composition 

changes are large enough to define and Olson, 

Shawe, Pray, and Sharp (1954, pI. 8, p. 42-47) 

outlined and described three separate rock types 

within the massive carbonatite, gray calcitebarite rock, ferruginous dolomitic rock, and 

silicified carbonate rock. Local variations in composition are observed within these three rock 

types. The most common rock type is gray 

calcite-barite rock that contains 40 to 70 percent 

calcite, 15 to 50 percent barite, and 5 to 15 percent bastnaesite. Chlorite, crocidolite, and 

phlogopite commonly occur in this rock. Other 

minerals present locally in small quantities include quartz, biotite, apatite, thorite, allanite, 

goethite, hematite. zircon. galena. magnetite, 

pyrite, and leucoxene (Olson and others, 1954, p. 

43). Fine-grained ferruginous dolomitic carbonatite occurs in small bodies in the northern 

and southwestern parts of the main body. This 

rock can be distinguished from the calcite-barite 

carbonatite by its reddish or grayish-brown color 

on weathered surfaces. The principal carbonate 

mineral in this second type is dolomite. Like the 

gray calcite-barite rock, this second type also cQntains bastnaesite and barite. In addition, 

monazite is common in many places. and parisite, 

(Ce,LahCa(C03)3F2, and sahamalite, 

(Mg,Fe)Ce2(C0314• also have been noted in a few 

places. Silicified carbonate rock is found principally along faults and shear zones in the calcitebarite carbonatite. This type is a hard resistant 

rock in which quartz is much more common and 

calcite much less common than in the more 

widespread gray calcite-barite carbonatite. The 

silicified carbonatite is made up principally of 

quartz, barite, and bastnaesite, with small 

amounts of calcite. hematite. goethite, sericite. 

and galena. 

The Sulphide Queen carbonatite was sampled 

by D. R. Shawe and J. C. Olson during their study 

of this district in the early 1950's (Olson and 

others. 1954; Shawe, 1953). Sample data on this 

deposit come from this early work. Samples of the 

carbonatite, which did not contain any visible 

concentrations of thorium-bearing minerals, con-
tained from 0.01 to 0.09 percent Th02 • Five small 

areas containing monazite yielded 0.08 to 0.2 percent Th02• The overall average of the massive 

carbonatite was calculated to be 0.026 percent 

Th02• Uranium was chemically analyzed in only a 

few samples, and in these it ranged from 0.001 to 

0.007 percent. Little accurate data are available 

on the thorium-to-uranium ratios, but if these 

ratios are similar to those of the carbonatite dikes 

that occur just north of the massive carbonatite, 

then they are from about 10 to 80. The Sulphide 

Queen carbonatite is extremely rich in rare 

earths. Olson, Shawe, Pray, and Sharp (1954, p. 

68) gave the total rare-earth-oxide content of 59 

samples. These range from zero to 38.92 percent. 

Only eight of these samples contained less than 1 

percent total rare-earth oxides, and the average of 

these 59 samples is 6.9 percent. 

The Sulphide Queen carbonatite is one of the 

world's large resources of rare earths. This carbonatite, which was discovered by the U.S. 

Geological Survey in August 1950 (Evans, 1966, 

p. 23), has been mined by open-pit methods by the 

Molybdenum Corp. of America since the early 

1950's. A mill has been constructed on the site 

within a few hundred yards of the ore body. The 

mill not only separates bastnaesite but also 

several types of rare-earth products. Recovery of 

thorium from this deposit normally would be as a 

byproduct of the rare-earth mining. 

Reserves in the Sulphide Queen carbonatite 

body were projected down dip for a distance of 

500 ft. This mass of rock has 36,800,000 tons of 

carbonatite that contains approximately 9,750 

tons of Th02 and 2,540,000 tons of total rareearth oxides. Probable potential resources would 

be projected another 500 ft down dip and would 

contain an equal amount of Th02, and total rareearth oxides. The only cost that normally would 

be charged to thorium would be that of adding an 

extra circuit to an already operating mill to 

separate ThOz• The capital and operating costs of 

this circuit would be considerably less than $15/1b 

of produced Th02• Therefore. all the reserves and 

probable potential resources of Th02 produced as 

a byproduct of rare earths would fall in the less 

than $30/1b category. If the entire cost of 

operating this property was paid by selling ThOz, 

then the price of Th02 would be between $30 and 

$80Ilb. Costs of mining would be greater as the 

pit is deepened, because the carbonatite body 

dips moderately to the west under increasing 

amounts of country rock which would have to be 

removed. Other factors that may either slow 

down or stop this operation include a drop in the 

demand for rare earths, development of a market 

by a larger and cheaper source of rare earths such 

as could be developed at the Elliot Lake uranium 

deposits in Ontario, Canada, and various management aspects. 

NORTH AND SOUTH CAROLINA 

PLACER DEPOSITS 

Monazite-bearing stream placers in the North 

and South Carolina Piedmont were the world's 

first source of thorium, with production starting 

in 1887 (Olson and Overstreet, 1964. p. 3). These 

placers, which were mined in the years 1887-1911 

and 1915-1917, are at the headwaters of streams 

that drained the monazite-bearing crystalline 

rocks (Mertie, 1975, p. 7). Total production during 

this period amounted to 5,483 tons of monazite 

(Overstreet and others, 1968, p. 2). Mining ceased 

in 1917 because the cost of prodUCing Carolina 

monazite was greater than that of monazite produced from beach placers in India and Brazil and 

not because the reserves were depleted Most early mining in the Carolina Piedmont was by sluicing. which was done mainly by groups of one to 

three men. Few of these deposits were worked to 

depths in excess of 3 to 4 ft (Mertie, 1975. p. 8). 

Five geologic factors controlled the location of 

these early placers: (1) relation of stream to 

monazite-bearing rock, (2) thickness of monazitebearing sediment, (3) thickness of overburden, (4) 

flow of water in stream, and (5) height of water 

table in flood plain (Overstreet and others, 1968, 

p. 8-9). These factors tended to restrict mining to 

headwaters areas where stream channels are 

small and flood plains are narrow. Sufficient 

water for sluicing generally was present in the upper parts of most of the small streams, but too 

great a flow of water, as commonly occurred 

downstream, prevented mining there. The small 

placers commonly were formed in a period of only 

a few years, as the headwater stretches of many 

streams were mined repeatedly during a period of 

20 yr. 

33 

The source of the monazite. as well as the location of many of the placers. is in a northeasttrending belt of rocks that stretches for 600 mi 

from Virginia to Georgia (Mertie, 1953, p. 15). 

Most of the placers, and those of highest grade. 
lie in the Piedmont of North and South Carolina 

(figs. 1 and 7). This area from north to south lies 

within parts of the following drainage basins: (1) 

Catawba, (2) Broad, (3) Pacolet, (4) Tyger, (5) 

Enoree, (6) Saluda, and (7) Savannah Rivers. The 

area is underlain by high-grade metamorphic 

rocks intruded by quartz monzonite and 

pegmatite. Some of the quartz monzonite and 

pegmatite bodies contain monazite; others are 

free of monazite. Overstreet (1967, p. 184-189) 

noted that most of the monazite is derived from 

such high-grade metamorphic rocks as sillimanite 

schist. He believed that the monazite formed 

from constituents in the original shales or sandstones, and that monazite is more abundant and 

is richer in Th02 in metamorphic rocks of higher 

grade facies than those of lower grade facies. 

Although monazite is best known from the 

placers in the Piedmont, it and other heavy 

minerals have been carried eastward from there 

down various rivers. All formations in the 

Coastal Plain probably received varying amounts 

of heavy minerals. In particular, the Tuscaloosa 

Formation of Late Cretaceous age has received a 

widely dispersed but significant amount of 

monazite (Mertie, 1975, p. 12). The Tuscaloosa 

directly overlaps the crystalline rocks of the Piedmont, and along its old shoreline, this formation 

might be expected to have a higher tenor of the 

heavy minerals than other formations of the 

Coastal Plain. The Fall Line, shown on figure 7, 

extending across South Carolina, is a sharp 

topographic break caused by differential erosion 

between the hard crystalline rocks of the Piedmont and the soft sedimentary rocks of the 

Coastal Plain. The Tuscaloosa Formation crops 

out just southeast of this line. Streams cutting 

this area have reworked the sand from the 

Tuscaloosa Formation and, in places, have formed heavy-mineral placers containing monazite. 

This area has not been nearly as well explored as 

the Piedmont, but Williams (1967, p. 17-20, 25) 

reported heavy-mineral concentrations at 28 

localities. The best known of these is Horse 

Creek, southwest of Aiken (fig. 7), which was the 

site of the first large-scale mining of stream 

placers for monazite and other heavy minerals in 

the Carolinas. The alluvium here was mined by 

dredge by Marine Minerals, a Division of Heavy 

Mineral Co., between June 1955 and July 1958 

(Williams, 1967, p. 25). This company recovered 

monazite, ilmenite, rutile, zircon, and staurolite. 

34 

The heavy-mineral content of these sediments 

was about 1 to 1.5 percent, of which monazite 

made up about 8 percent of the total (Mertie, 

1975, p. 27). Sand and gravel also were sold. This 

deposit was reported to contain about 10,000,000 

yd3 (cubic yards) of alluvium of which approximately 4,000,000 yd3 were mined during the 

dredging operation (Williams, 1967, p. 25). 

Although the actual output of the Horse Creek 

placer has not been released, Overstreet (1967, 

p. 260), by comparing it with a nearby placer, 

estimated that it may have yielded about 

1,200-1,500 tons of monazite per year. 

The stream sediments containing the monazite 

placers in the Piedmont are remarkably similar 

from the Catawba River on the north to the 

Savannah River on the south. They were deposited on deeply weathered flat-floored valleys. They 

are generally well bedded, poorly graded, unconsolidated, and occur in successive layers of 

differing lithology (Overstreet and others, 1959, 

p. 712). The lowermost unit commonly is a quartz 

pebble gravel with a sandy clay matrix. Overlying the gravel, or where it is missing, lying directlyon bedrock, is a bed of dense gray clay. Above 

the clay is fine-grained to coarse-grained gray to 

buff sand that is overlain by clayey silt. The uppermost unit is red to brown sandy silt. Average 

thickness of the sediments is 14.6 ft. The layer of 

gravel at the bottom of the deposit averages 1.5 ft 

thick, the layer of clay about 3.6 ft, and the sand 

and silt about 9.5 ft thick. Monazite generally is 

found in all units, but commonly is most abundant in the gravel at the bottom of the deposit 

and least abundant in the clay. 

Heavy mineral concentrations are greatest near 

the headwaters of the streams. Here, where the 

streams start and the placer deposits are shallow, 

narrow, and short, is where mining was carried 

out at the turn of the century. Progressing 

downstream, the streams have more water and 

the placers commonly are deeper, wider, and 

longer. Unfortunately, the grade of the heavy 

minerals, including monazite, decreases as the 

distance fthe source increases. Placers, in order to 

be operated by modern methods, must be close 

enough to the source so that the grade of 

monazite is not too low but far enough 

downstreram so that the volume of sediment is 

large enough to pay for the cost of mechanically 

mining the placer. These conditions most likely 

are to be found in two areas along the streams in 
36° 

34° 

GEORGIA 

[.Fx~:~r::l General area containing ,.:}.::.<:;'(.: placers " . ~ ....... . 

Placers mentioned in text 

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 

EXPLANA TION 

81° 800 

.Charlotte 

..... ' 

~ NORTH 

I 'CAROLINA .... _------ SOUTH / 

CAROLINA ,.;:::: J Hr:'~ (-- '.:' 

t I I j 

6 Buffalo Creek 

7 Broad River at its junction 

with Buffalo Creek 

8 Thicketty Creek 

9 North Tyger River at its 

junction with Middle 

Tyger River 

10 North and South Rabon 

Creeks 

11 Big Generostee Creek 

1 2 Horse Creek 

13 Hollow Creek 

FIGURE 7.-Known stream-placer areas in North and South Carolina. Compiled principally from maps by Overstreet, Theobald, 

and Whitlow (1959, fig. 1) and Williams (1967, fig. 1). 

35 
the Carolinas: (1) adjacent to, but not in, the headwaters of monazite-bearing streams in the Piedmont, and (2) just below the Fall line on the 

Tuscaloosa Formation where a second generation 

of placers have formed from reworked sandstone. 

The heavy-mineral content of the placers is fairly low, and ranges from about 0.15 to 2.0 percent, 

or 4.1 to 54 lb of heavy minerals per cubic yard of 

alluvium. Monazite makes up from 3.5 to 13 percent of the heavy minerals. The proportions of 

various other heavy minerals vary widely between individual placers. Ilmenite and garnet are 

the commonest heavy minerals in the Piedmont 

placers, with the former making up from 20 to 70 

percent of the total and the latter from 2 to 50 

percent. Rutile may make up from 0.3 to 7 percent of the heavy minerals and zircon from practically nothing to 14 percent. Sillimanite and 

kyanite together range from trace amounts to 

about 20 percent. Epidote is important in a few 

placers. Magnetite and xenotime commonly are 

found in minor amounts. Other heavy minerals 

that are present in some placers include tourmaline, sphene, staurolite, andalusite, and a black 

radioactive mineral. 

The heavy mineral distribution at the two 

deposits east of the Fall Line for which information is available shows several differences from . 

that in the Piedmont. The largest difference i~ in 

the increase in staurolite, which here makes up 7 

and 38 percent of the two deposits (Mertie, 1975, 

p. 27; Kline and others, 1954, p. 27). Rutile and 

zircon also make up a greater proportion of the 

heavy minerals here than in the Piedmont, with 

zircon reported at 19 and 11 percent and rutile at 

15 and 10 percent of the heavy minerals. 

Monazite concentrations for the two areas are 

similar to those found in part of the Piedmont. 

The differences in heavy mineral content of the 

two areas is due to varied sources of sediment in 

the Tuscaloosa Formation. 

The monazite in the alluvium has been sampled 

in many placer in the North and South Carolina 

Piedmont. These samples not only were taken in 

each placer on each stream, but also were divided 

into type of material sampled (gravel, sand and 

silt, or clay). Average grades were obtained for 

each stream, and these figures were compiled for 

all streams within the Piedmont that belong to 

the Catawba, Broad, Pacolet, Tyger, Enoree, 

Saluda, and Savannah River basins. Summaries 

of this detailed sampling program are reported by 

36 

Overstreet, Theobald, and Whitlow (1959, 

p. 710-711). Overall variations in tenor were 

similar in the various river basins. The clay layers 

in the various streams contained from 0.01 to 0.6 

Ib/yd3 (pounds per cubic yard) of monazite. 

Average grade of clay samples in each of the 

seven river basins ranged from 0.1 to 0.3 Ib/yd3 • 

The sand and silt in the individual streams had 

from 0.2 to 2.4 Ib/yd3 of monazite. The average 

grade of sand and silt samples from each of the 

seven basins varied from 0.5 to 1.3 Ib/yd3• The 

gravel in the various streams had an average 

yield of from 0.2 to 4.1 Ib/yd3 of monazite. 

Average grade of the gravel samples from the 

seven river basins was from 1.0 to 2.4lb/yd3. The 

overall weighted average of all the sediments in 

each river basin ranged from 0.4 to 1.0 Ib/yd3 of 

monazite, for an average monazite content of 

0.015 to 0.037 percent of the alluvium. 

The monazite content of the alluvium east of 

the Fall Line is less well known, but the average 

grade of 19 individual placers tabulated by 

Williams (1967, p. 6) ranged from 0.1 to 2.7Ib/yd3 

of monazite, a monazite content of 0.0037 to 0.10 

percent of the alluvium. 

Monazite is principally a rare earth mineral, 

commonly containing 60 to 63 percent total rareearth oxides. The Th02 content can be highly 

variable, but the range is small for the alluvial 

monazite of the Carolinas. The Th02 content of 52 

samples of alluvial monazite from the Carolinas 

ranges from 2.5 to 7.8 percent and the mean is 

5.67 percent (Mertie, 1975, p. 7). The average 

Th02 content of the seven river basins in the 

Piedmont ranges from 4.21 to 5.96 percent 

(Overstreet and others, 1959, p. 712). A sample of 

alluvial monazite from Hollow Creek, the largest 

known placer east of the Fall Line, contains 5.08 

percent Th02 (Kline and others. 1954, p. 26.) 

Monazite also contains a small amount of 

uranium, and the U 30S content of 52 samples of 

alluvial monazite from the Carolinas ranges from 

0.18 to 0.98 percent (Mertie, 1975, p. 7). Average 

U30 S content of the seven river basins in the Piedmont ranges from 0.39 to 0.70 percent 

(Overstreet and others, 1959, p. 712). A sample of 

monazite from Hollow Creek yielded 0.54 percent 

U30S (Kline and others, 1954, p. 26). 

Two types of resources have been calculated for 

the placer deposits: reserves and probable potential resources. The reserves are in 16 of the larger 

placers, which have been sampled both at the sur-
face and at depth by at least one line of drill holes. 

Drilling on these placers was done with a churn 

drill by the U.S. Bureau of Mines between 1951 

and 1954 (Marshall, 1956, p. 30:31). A field 

laboratory was established at Shelby, N.C. where 

the amount of monazite and various other heavy 

minerals was determined. These placers were 

discussed in nine reports, eight of which were 

prepared jointly by the U.S. Bureau of Mines and 

the U.S. Geological Survey. Nine of these placers 

are in the Piedmont of North Carolina on: (1) 

Silver Creek in Burke County (Hanson and 

White, 1954), (2) South Muddy Creek in 

McDowell County (Hansen and White, 1954), (3) 

Sandy Run Creek in Rutherford County (Griffith 

and Overstreet, 1953a), (4) Buffalo Creek in 

Cleveland and Lincoln Counties (Griffith and 

Overstreet, 1953b), (5) Knob Creek in Cleveland 

County (Griffith and Overstreet 1953c), (6) First 

Broad River in Cleveland County (Hansen and 

Cuppels, 1954), (7) Hinton Creek in Cleveland 

County (Hansen and Cuppels, 1954), (8) Duncans 

Creek in Cleveland County (Hansen and Cuppels, 

1954), and (9) Wards Creek in Cleveland County 

(Hansen and Cuppels, 1954) (fig. 7). Six of these 

placers are in the Piedmont of South Carolina. 

They are on: (U Big Generostee Creek in Anderson County (Hansen and Caldwell, 1955), (2) 

North Rabon creek in Laurens County (Hansen 

and Caldwell, 1955), (3) South Rabon Creek in 

Laurens County (Hansen and Caldwell, 1955), (4) 

North Tyger River at its junction with the Middle 

Tyger River in Spartanburg County (Hansen and 

Cuppels, 1955), (5) Thicketty Creek in Cherokee 

County (Hansen and Theobald, 1955), and (6) 

Broad River at its junction with Buffalo Creek in 

Cherokee County (Hansen and Theobald, 1955). 

Another of these placers, just east of the Fall 

Line in South Carolina, is on Hollow Creek in 

Aiken County (Kline and others, 1954). In addition, reserves in South Carolina have been summarized by Williams (1967). The reserves in the 

Carolinas, which are in 15 placers in the Piedmont 

and one placer east of the Fall Line, contain 5,270 

tons of Th02• This Th02 would come from 99,500 

tons of monazite, which also would contain 507 

tons of U 30a and 62,300 tons of total rare-earth 

oxides. In addition to the monazite, amounts of 

other economically valuable minerals would include 668,000 tons of ilmenite, 84,100 tons of 

rutile, and 147,000 tons of zircon. . 

The probable potential resources that were 

summarized by Overstreet, Theobald, and 

Whitlow (1959, p. 713-714), were appraised by 

reconnaissance methods. The margins of individual flood plains were outlined stereoscopicallyon aerial photographs, which then were compiled into mosaics. After a field check as to accuracy 

of these measurements, the areas of the flood 

plains were outlined by polar planimeter. Accuracy of the planimetry was within 5 percent on 

an area having 200,000 yd2 (square yards) and 1 

percent on an area exceeding 1,000,000 yd2• 

Average depth of alluvium in each flood plain was 

estimated from stratigraphic measurements of 

exposed sediments and from holes made with a 

vehicle-mounted auger. The amount of monazite 

and other heavy minerals in the sediment was 

estimated from microscopic study of heavy 

minerals panned from several thousand samples. 

The panned concentrates were divided into three 

to five size fractions. A split of 100 to 150 grains 

was taken from each size fraction. These grains 

were identified and counted. Numerical frequencies of each mineral were converted to weight percent by multiplying by the appropriate specific 

gravity. 

37 

The summary of thorium resources by 

Overstreet, Theobald, and Whitlow (1959), is based on more detailed studies made by them and 

others of the U.S. Geological Survey monazite 

project. The most important areal studies were of 

the following drainage basins: (1) Savannah and 

Saluda Rivers (Caldwell and White, 1973), (2) 

Enoree, Tyger, and Pacolet Rivers (Cuppels and 

White, 1973), (3) Broad River (Theobald, 1962), 

and (4) Catawba River (White, 1962). Data on 

amounts of heavy minerals in many stream 

placers in the southeastern United States also are 

summarized by Overstreet and others (1968). The 

thorium, rare-earth, and uranium contents of 

Il)onazite in the Carolinas were obtained mainly 

from Overstreet (1967, p. 180-228, 231-262), who 

has compiled most of the available analyses of 

this mineral. The probable potential resources of 

the Carolinas total 36,800 tons of Th02• This 

Th02 comes from 725,000 tons of monazite. which 

also contains 3,210 tons ofU30s and 441,000 tons 

of total rare-earth oxides. In addition, 318,000 

tons of zircon are estimated in these deposits. 

Data are not available on the amounts of ilmenite 

and rutile present. 

Cost studies also were made on placer sands by 

the U.S. Bureau of Mines personnel. Cost calcula-
tions made by operators of monazite placers go 

only as far as the separation of the individual 

heavy mineral. We have carried this one step further and have included the separation of the Th02 

from the monazite in order to have a basis for 

comparing the cost of Th02 between different 

types of deposits. The cost calculation covers the 

mining of the sand and its processing through 

three separate mills. In the first mill, the heavy 

minerals would be separated from the stream 

alluvium; in the second, monazite and various 

other heavy minerals would be separated from 

each other; and in the third, thorium, uranium, 

and rare earths would be recovered from the 

monazite. 

Mining would be done by dredge. Thus, the 

first step would be to make a pond using a drag 

line into which to put the dredge and an adjacent 

floating mill. A hydraulic dredge having a 

rotating cutter head would be used. The rotating 

cutter head would loosen the alluvium, and the 

sediment would be pumped to the surface. From 

there it would be pumped through a pipe line, 

floating on pontoons, to a floating heavy-mineral 

mill. Not all placers contain enough sediment to 

support operation of a dredge and a mill. The 

minimum capacity for a gravity mill was assumed to be 1,000 yd3 of material per day; the 

minimum mill life would be 5 yrs. Therefore, a 

placer containing less material than would feed a 

mill at this rate for 5 yr, 1,750,000 yd\ was not 

considered in the cost calculations. 

The mining costs are divided into capital and 

operating costs. Capital costs include mine 

development, mining plant, and equipment. 

These costs do not include those for exploration, 

acquisition of land, or for obtaining working 

capital, because these costs have not been considered in comparative cost calculations for 

uranium. Most development costs include clearing trees and brush from the placer, building 

roads, and digging the initial pond with the 

dragline. Mining-plant costs include the dredge 

and any buildings needed for the operation. 

Equipment costs include all equipment and supplies necessary to bring the property into production. Operating costs are divided into direct, indirect, and fixed costs. Direct costs apply directly 

to the dredging operation and include labor, 

supervision, power and fuel consumption, cutter 

bits, and supplies. The indirect costs are for administrative and clerical help, facility supplies, 

38 

and general overhead. Fixed costs are for local 

taxes and insurance. These costs are a function of 

the capital costs and are estimated to be 3 percent 

of capital costs. 

Milling of placer sands is a fairly well-known 

process. In the first mill, which is attached to the 

dredge, the various heavy minerals are separated 

from the rest of the alluvium. This separation can 

be made on jigs, as has been done on many small 

placer operations, or with spiral gravity 

separators, the process used on the beach sands 

in Florida and Georgia. We selected the spiral 

separator, as it fits our overall mill plan better. 

The heavy sands go through three series of 

spirals-roughers, cleaners, and finishers-and 

the finished product is dewatered. The heavy concentrate makes up only about 0.1 to 1.5 percent of 

the alluvium mined. The rest of the alluvium is 

returned to the end of the pond through a moving 

pipe that spreads it evenly into the valley bottom 

so that it can quickly be returned to its original 

agricultural use. The heavy sands are then trucked to a district heavy mineral separating plant. 

There, magnetic and electrostatic separators 

separate monazite from the heavy sands. In addition, where present in sufficient amounts, ilmenite, rutile, and zircon also are recovered. 

Th02 is extracted form the monazite in a leachsolvent plant. The amount of monazite form each 

placer makes up from about 1 to 14 percent of the 

total heavy sands, so the monazite form several 

magnetic and electrostatic mills can supply a 

smaller leach-solvent plant. This plant extracts 

not only the thorium from monazite, but also 

recovers uranium and rare earths. 

The milling-cost calculations are complicated 

because material from each placer goes to three 

mills. The cost of building and operating a spiral 

gravity mill is paid for from the minerals 

recovered from one or possibly two placers, but 

the costs for a magnetic-electrostatic mill will be 

proportioned among a number of placers, and the 

cost for a leach-solvent plant will be proportioned 

among many placers. In other words, that part of 

the capital and operating costs of a particular mill 

that are paid for from the receipts of a particular 

placer is directly proportional to the ratio of the 

amount of material received from that placer to 

the total material processed. 

The milling costs for each mill are divided into 

capital and operating costs. Capital costs for each 

mill consist of the cost of building each mill and 
the cost of the equipment that is necessary to 

make it operational. The size of the mill is the 

principal factor that affects the original cost of 

the mill. The proposed gravity mill can be designed to handle from 1,000 to 22,000 tons of 

sediments per day for an operating life of at least 

5 yr. The magnetic-electrostatic mill proposed is 

designed to handle from 10 to 1,200 tons of heavy 

concentrates per day. The life of this type of mill, 

assuming the most efficient use of all the equipment, would be at least 15 yr. The leach-solvent 

mill is a much smaller plant, and the proposed 

design can be used for a plant having an imput of 

from 1 to 50 tons of monazite per day. This plant 

also is most efficient if it has a life of at least 

15 yr. 

The operating costs for milling can be divided 

into direct, indirect, and fixed costs. The direct 

costs include fuel and power consumption, 

reagents, labor, supervision, and supply 

maintenance. The indirect costs are those for administration, technical and clerical help, facility 

maintenance, office supplies, and general 

overhead. Fixed costs are for local taxes and insurance on the milling facilities. The principal 

variable in a mill's operating cost is its capacity. 

Cost calculations were made on 16 of the 

largest and best known deposits in the Piedmont 

and on the Hollow Creek deposit east of the Fall 

Line. These were the only deposits for which sufficient data were available to make a complete cost 

calculation. Enough data are present, however, 

on most of the other Piedmont deposits so that a 

general cost range can be estimated by comparing the size and grade of these deposits to those 

where more complete data are available. 

The cost calculations indicate that only the 

deposit on Hollow Creek east of the Fall Line (fig. 

7) can produce Th02 at less than $30/lb. This property has reserves of 2,040 tons of Th02 and 216 

tons of U30a in 40,100 tons of monazite (Kline and 

others 1954, p. 26). The monazite also contains 

approximately 25,500 tons of total rare-earth oxides. In addition, this deposit would yield 66,100 

tons of zircon, 55,400 tons of rutile, and 160,000 

tons of ilmenite. Although these byproducts 

would normally be recovered during the mining 

and milling of the Hollow Creek placer, the cost 

here of producing Th02 without any byproducts 

would still be less than S30/lb. 

N one of the Piedmont placers could be operated 

39 

at less than S50/lb of Th02, although the placer on 

Knob Creek was within a few dollars of this 

figure. The reasons that the placer at Hollow 

Creek can be operated at a much lower cost than 

any of the Piedmont placers are: (1) its large 

size-it is about twice as large as the largest Piedmont placer, (2) it has a high monazite content, 

and (3) it contains more zircon and rutile than any 

of the Piedmont placers. 

BEACH PLACER DEPOSITS IN FLORIDA 

AND GEORGIA 

Beach placers, as a source of thorium, present a 

paradoxical problem. On the one hand, for at least 

20 yr they have been the only place thorium has 

been mined in the United States, yet few deposits 

contain thorium at as low a grade as do beach 

placers. For example, the concentration of 

thorium in a beach placer commonly is only 1/250 

as much as that in a vein deposit. The Th02 content of beach placers in the United States from 

which this oxide has been recovered commonly 

ranges from 8 to 20 ppm. How can thorium be 

produced from such low-grade ore? The answer is 

twofold: (1) beach placers are extremely cheap to 

process compared to most other deposits, and (2) 

the thorium is a byproduct, and its presence has 

little to do with the overall success of the operation. Beach placers may occur either along 

modern beaches or inland along older beaches 

where they formed during Pleistocene time. In 

either place they can be mined by open-pit 

methods, and even the older sands rarely have 

more than 15 ft of overburden. As the groundwater level is high in Georgia and Florida, pits 

dug inland on the Pleistocene sands fill with 

water. These deposits are efficiently and 

economically mined by a dredge floating on these 

ponds. Sands have the advantage over hard-rock 

deposits not only in ease of mining but also in 

milling, as the material is not only already broken 

up but also sized. Sands are pumped from a 

dredge into a wet heavy-mineral concentrator, 

such as a Humphrey spiral, and the 4 percent or 

so of heavy minerals is separated from the lighter 

minerals. The heavy minerals are dried and divided into various mineral products by electromagnetic and electrostatic separators. 

Beach placers in the eastern United States are 

mined principally for their Ti02 content. Ti0 2 is 

used in making the white pigment in paints, and 
many of the companies mining beach placers are 

at least in part controlled by paint companies. 

Titanium occurs in beach sand, principally as 

partly altered ilmenite; leucoxene and rutile may 

also be present. Ti02 makes up a large part of the 

heavy minerals in beach placers, and it commonly 

makes up from 1.1 to 2.0 percent of sands that 

contain 4 percent heavy minerals. Other mineral 

products that are marketed form some deposits 

include zircon, garnet, staurolite, kyanite, 

sillimanite, and monazite. Zircon, although not as 

common as the titanium minerals, is an important economic product, and during the latter part 

of 1977 was selling for $150 per ton (Engineering 

and Mining Journal, 1977, p. 54). The types of 

marketable minerals vary from placer to placer. 

Monazite is a relatively unimportant byproduct 

in all of them, as it makes up from about 0.3 to 1 

percent of the heavy mineral concentrates in 

deposits where it has been recovered; it was not 

reported in some beach placers. Monazite from 

beach placers contains about 50 percent total rare 

earths (Kremers, 1958, p. 2) and about 5 percent 

Th02 (Calver, 1957, p. 25). At present, monazite is 

sold for its rare-earth content and thorium is a 

byproduct. In short, thorium is a byproduct of 

rare earths in monazite, which is a minor 

byproduct of titanium mining. Increases in the 

amount of thorium produced from beach placers 

will most likely be due to greater demand for 

titanium, although a sharp increase in the price of 

zircon might also affect somewhat the rate of production. Thus, the amount of thorium produced 

from beach placers in the next lO yr will continue 

to be relatively steady and little affected by rises 

in the price of thorium. Production also may fluctuate with shutdowns for equipment repair or the 

opening of new deposits. An overall estimate of 

the amount of Th02 that will be produced for the 

next 10 yr from this source is 250 to 350 tons. 

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___ 1979, Geology and mineral resources of the Lemhi 

42 

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'" u.s. GOVERNMENT PRINTING OFFICE, 1979-677-026/3