Abundance a«id Thorium

f

Prepared in cooperation

with the 

Department of Energy
COVER PHOTOGRAPHS

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5

9

11

2

7

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14

1. Asbestos ore

2. Lead ore, Balmat mine, N. Y.

3. Chromite, chromium ore, Washington

4. Zinc ore, Friedensville, Pa.

5. Banded iron-formation. Palmer, 

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6. Ribbon asbestos ore, Quebec, Canada

7. Manganese ore, banded 

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8. Aluminum ore, bauxite, Georgia

9. Native copper ore, Keweenawan 

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10. Porphyry molybdenum ore, Colorado

11. Zinc ore, Edwards, N.Y.

12. Manganese nodules, ocean floor

13. Botryoidal f luorite ore, 

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14. Tungsten ore. North Carolina
Abundance and Distribution of Thorium 

in the Carbonatite Stock 

at Iron Hill, Powderhorn District, 

Gunnison County, Colorado

By THEODORE J. ARMBRUSTMACHER

GEOLOGY AND RESOURCES OF THORIUM IN THE UNITED STATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 104 9-B

Prepared in cooperation with the 

Department of Energy

An evaluation of the thorium resources 

of the carbonatite stock at Iron Hill

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1980
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data

Armbrustmacher, Theodore J. 1946-

Abundance and distribution of thorium in the carbonatite stock at Iron Hill,

Powderhorn District, Gunnison County, Colorado.

(Geology and resources of thorium in the United States) (Geological

Survey professional paper ; 1049-B)

Bibliography: p. fill.

Supt.ofDocs.no.: I 19.16:1049-6

1. Thorium. 2. Carbonatites-Colorado-Gunnison Co. I. United States.

Dept. of Energy. II. Title. III. Series. IV. Series: United States.

Geological Survey. Professional paper ; 1049-B. 

QE516.T5A74 553.4'94 79-607145

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402

Stock number 024-001-03296-1
CONTENTS

Page

Abstract _______________________________________________________________________ Bl

Introduction ____________________________________________________________________ 1

Acknowledgments ____________________________________________________ 1

Geologic setting ___________________________________________________ 1

The carbonatite stock at Iron Hill _________________________________________ 3

Methods of study __________________________________________________________ 4

Mineralogy and geochemistry of carbonatite ____________________________________ 4

Abundance and distribution of thorium.__________________________________ 5

Resource assessment ______________________________________________________ 10

Summary __________________________________________________________________________ 10

References cited __________________________________________________________________ 11

ILLUSTRATIONS

Page

FIGURE 1. Geologic map of the alkalic complex at Iron Hill, Gunnison County, Colo __________________________ B2

2. Histograms showing frequency distribution of elements in 28 samples of carbonatite at Iron Hill _____________ __ 6

3. Histogram showing frequency distribution of 173 thorium values determined by field gamma-ray spectrometry from the

Iron Hill carbonatite ___________________________________________________________________ ______ 8

4. Contour map of thorium distribution in the carbonatite at Iron Hill ____________________________ ________ 9

TABLE

TABLE 1. Abundance of elements in the carbonatite at Iron Hill, the igneous rocks of the upper continental crust, and the

average carbonatite __________________________________________________________________________-___ B5

III

GEOLOGY AND RESOURCES OF THORIUM IN THE UNITED STATES

ABUNDANCE AND DISTRIBUTION OF THORIUM IN THE

CARBONATITE STOCK AT IRON HILL, POWDERHORN

DISTRICT, GUNNISON COUNTY, COLORADO

By THEODORE J. ARMBRUSTMACHER

ABSTRACT

The carbonatite stock at Iron Hill is part of an alkalic complex of 

Cambrian or latest Precambrian age containing pyroxenite, uncompahgrite, ijolite, nepheline syenite, and related rocks characterized 

by their subsilicic composition and by their lack of calcic plagioclase. 

Thorium averages 36.2 parts per million (ppm) Th (0.0041 percent 

ThO2) in the carbonatite, which also contains considerably more Ba, 

Ce, Nd, La, Nb, P, Mn, Mo, Sr, U, and rare-earth elements than 

average igneous rocks. Thorium content of the carbonatite ranges 

from 6 ppm Th (0.0007 percent ThO2) to 150 ppm Th (0.017 percent 

ThO2 ), and several areas of anomalous concentration are apparent. 

The distribution of thorium is not the same as the distribution of 

niobium and rare-earth elements. Thorium abundances of the level 

found in the carbonatite at Iron Hill are of little economic interest 

unless considered as a byproduct of potential niobium, rare-earth, 

and uranium exploitation.

INTRODUCTION

The alkalic rocks and especially the carbonatite 

stock at Iron Hill, near Powderhorn, Colo., have been 

intensely studied since 1912 (Larsen, 1942) not only 

because of their unusual lithologies but also because of 

the economically significant mineral concentrations. 

Economic minerals associated spatially and genetically with these rocks include vermiculite and minerals that contain thorium, iron, titanium, niobium, and 

rare-earth elements. Several reports by government 

agencies and mining company representatives have 

discussed various aspects of the economic geology of 

the complex of alkalic rocks, but the abundance and 

distribution of thorium in the carbonatite stock have 

not been investigated in detail. The interest in the 

thorium content of the stock results mainly from the 

fact that thorium is a potential byproduct of possible 

future niobium and rare-earth exploitation.

This study was conducted in order to provide resource data on one of the principal thorium deposits in 

the United States (Staatz and others, 1979) for a Department of Energy contract with the U.S. Geological 

Survey.

ACKNOWLEDGMENTS

Robert R. Norman of Buttes Gas and Oil Co. arranged for permission to investigate the thorium resource potential of their land holdings at Iron Hill. 

James V. Thompson of Kaiser Engineers, consultant to 

Buttes, visited us in the field and discussed the geology 

of the area. Jerry C. Olson of the U.S. Geological Survey provided unpublished data and also discussed various aspects of the geology of the area. Semiquantitative spectrographic analyses were performed by L. A. 

Bradley, and laboratory gamma-ray spectrometer 

analyses were performed by C. M. Bunker and C. A. 

Bush, all of the U.S. Geological Survey. The cost 

analyses of mine and mill models were provided by J. 

F. Lemons, Jr., L. V. Coppa, and B. V. Clingan of the 

U.S. Bureau of Mines. Mineral separations and other 

supporting laboratory work were conducted by Isabelle 

K. Brownfield and William F. Robinson IV. Brownfield 

also constructed the cross sections across the Iron Hill 

carbonatite stock and determined the volume of carbonatite used in the resource calculations. Michael B. 

Sawyer calculated the correlation coefficients used to 

test the strength of association of various element 

pairs.

GEOLOGIC SETTING

The carbonatite stock (fig. 1) is part of the complex of 

alkalic rocks at Iron Hill originally described by Larsen (1942). The complex, which occupies an area of 

about 30 km2 , consists chiefly of pyroxenite, 

magnetite-ilmenite-perovskite segregations, uncompahgrite, ijolite, hybrid pyroxenite-syenite rocks, 

nepheline syenite, and carbonatite, listed oldest to 

youngest (Hedlund and Olson, 1961). Rocks of the complex were emplaced about 570 million years ago (Olson 

and others, 1977) into Precambrian X Powderhorn 

Granite and metamorphic rocks, which are locally

Bl
107°05 107°00

WbO

38" 15' hEXPLANATION . Alluvium (Holocene) Talus (Holocene) Ash-flow tuff and volcaniclastic rocks (Tertiary) 10 " Outline of figure 4 Diabase dikes (Ordovician or Cambrian)

Carbonatite (Cambrian)

Alkalic rocks of Iron Hill stock 

(Cambrian or upper Precambrian)

Powderhorn Granite and 

other Precambrian rocks

Fault. Dashed where approximately located, dotted where covered

2 KILOMETER

R. 2 W. R. 1% W. R.1 W.

ow 

O5

o

FIGURE 1 Geologic map of the alkalic complex at Iron Hill, Gunnison County, Colo. Modified from Hedlund and Olson

(1975) and Olson (1974).
THORIUM IN CARBONATITE STOCK AT IRON HILL, COLORADO B3

fenitized adjacent to the complex. Parts of the complex 

are covered by ash-flow tuffs, welded tuffs, and colluvium mainly of Oligocene age and by colluvium and 

alluvium of Quaternary age, mainly along the drainages of Cebolla, Deldorado, and Beaver Creeks.

The complex is bisected by the Cimarron fault. Hedlund and Olson (1975) indicate that relative movement 

on the fault is such that a deeper level of the alkalic 

complex is exposed on the northeast side of the fault. 

The structural interpretation of Temple and Grogan 

(1965), however, suggests that a deeper level is exposed 

on the southwest side of the fault. Regardless of the 

interpretation, nearly all of the uncompahgrite, most 

of the ijolite, and the carbonatite stock are found 

southwest of the fault; the magnetite-ilmeniteperovskite segregations and the nepheline syenite 

seem to be restricted to the northeast side of the fault. 

Diabase dikes of Ordovician or Cambrian age mainly 

intrude rocks of the complex on the northeast side of 

the Cimarron fault, although one diabase dike does cut 

fenitized Powderhorn Granite just southwest of the 

fault (Olson, 1974).

Carbonatite dikes, probably similar in age to the 

carbonatite stock, intrude all rocks of the complex except the carbonatite stock and the Precambrian host 

rocks, especially those within the fenitized aureole. 

Martite-fluorapatite veins and jasper-rich veins, the 

latter probably representing silicified fracture zones, 

are found intersecting the carbonatite stock. Rocks of 

the complex and the surrounding area have been mapped in detail by Hedlund and Olson (1968, 1975) and 

Olson (1974).

Many aspects of the alkalic complex at Iron Hill suggest that it is a representative of the Ijolite Series as 

defined by Bailey (1974). Rocks of this series are 

characteristically undersaturated with respect to silica 

and contain fairly abundant alkalies. The complex at 

Iron Hill is also representative of the carbonatitic type 

of alkalic complex as defined by Rock (1976). These 

complexes are characterized by the occurrence of intrusive rocks such as carbonatite, ijolite, pyroxenite, 

melilitic rocks, and nepheline syenite, all lacking calcic plagioclase, and are also found occupying the root 

zones of nephelinitic volcanoes. King (1965) suggested 

that rocks of this type formed by differentiation from 

an immediate parental nephelinite-melteigite magma, 

which maintained silica undersaturation with a trend 

toward enrichment of lime, alkalies, and volatiles. 

With increasing concentration of these components, a 

point was reached whereupon a highly mobile fraction 

of alkali and calcium carbonates separated from the 

crystallizing silicate phase, forming a carbonatite 

melt. Initially the melt contained abundant alkali carbonates, which allowed the carbonatite to remain as a

liquid at low temperatures and pressures, according to 

King. The presence of fenitized country rocks reflects 

the alkali content of the melt and shows that alkalies 

were removed from the melt through reaction with the 

host. The final product was a carbonatite probably not 

unlike that found at Iron Hill.

Potentially economic deposits of a number of mineral 

commodities are known to be associated spatially and 

genetically with the alkalic complex at Iron Hill. Olson 

and Wallace (1956) discuss the occurrence of thorium 

and rare-earth minerals, the thorium mainly concentrated in veins and shear zones outside of the complex, 

the rare earths mainly in carbonatite dikes and in the 

Iron Hill carbonatite stock. Hedlund and Olson (1961) 

identify four environments containing thorium, 

niobium, and rare-earth minerals in the Powderhorn 

district: (a) carbonatite, (b) magnetite-ilmeniteperovskite bodies, (c) thorite veins, and (d) trachyte 

porphyry dikes. The magnetite-ilmenite-perovskite 

segregations were discussed as early as 1912 

(Singewald, 1912) as a source of iron and titanium. 

Rose and Shannon (1960) reported an average grade of 

6.5 percent titania and 11.7 percent iron and a possible 

tonnage in excess of 100 million tons in pyroxenites 

containing the segregations. In the February 25, 1976, 

issue of the Denver Post newspaper, Buttes Gas and 

Oil Co. announced results of a study that indicated 419 

million tons of reserves averaging about 12 percent 

TiO2 occurred at Powderhorn. Earlier studies of the 

carbonatite stock by E. I. DuPont de Nemours and 

Company (Temple and Grogan, 1965) indicated a 

niobium reserve of over 100,000 tons of Nb2O5 in rocks 

averaging at least 0.25 percent Nb2O5 . Vermiculite deposits are found in altered pyroxenite, but data on reserves are not available.

THE CARBONATITE STOCK AT IRON HILL

Various modes of origin have been proposed for the 

carbonatite stock at Iron Hill, the largest exposed massive carbonatite in the United States. Larsen (1942) 

considered the "marble" of Iron Hill to be the oldest 

rock of the alkalic complex. He felt that there were no 

data that suggested an igneous origin for the carbonate 

rocks and preferred a hydrothermal origin with deposition occurring in a volcanic throat and later modification due to subsequent intrusion of other rocks of the 

complex. The first application of the term "carbonatite" 

to the Iron Hill carbonate rocks appears to have been 

by Wallace and Olson (1956); the term "stock" for the 

form of the massive carbonatite, implying an igneous 

intrusive origin, was first used by Hedlund and Olson 

(1961). Temple and Grogan (1965) ascribed a mode of 

origin based on metasomatic modification of country
B4 GEOLOGY AND RESOURCES OF THORIUM IN THE UNITED STATES

rock by a CO2 -rich phase associated at depth with volcanic activity. Carbon isotopic data from the Iron Hill 

carbonatite (Taylor and others, 1967) are similar to 

values found in diamond, indicating that the CO2 in 

the carbonatite had a deep-seated igneous source. The 

carbon isotopic data and oxygen isotopic data from the 

Iron Hill carbonatite both fall within the field of primary igneous carbonatite defined by Taylor and others 

(1967). Strontium isotopic data from various 

lithologies within the alkalic complex, including carbonatite (Powell and others, 1966), indicate that the 

rocks within the complex are comagmatic. Strontium 

isotopic values also suggest that carbonatite-alkalic 

rock associations, such as that observed at Iron Hill, 

are mantle derived. Analyses of calcite-dolomite and 

pyrite-pyrrhotite mineral pairs by Nash (1972) indicate crystallization temperatures between 435°C and 

290°C for late-stage carbonatites. Samoylov (1975) determined formation temperatures of Iron Hill carbonatite at 600°C using biotite-pyroxene geothermometry 

and at 400°C using amphibole-pyroxene geothermometry.

METHODS OF STUDY

Distribution of thorium in the Iron Hill carbonatite 

stock was determined by field measurements of 

thorium abundances at 173 stations within or immediately adjacent to the carbonatite. Measurements 

were made with a Geometries 1 model DISA-400A 

portable differential four-channel gamma-ray spectrometer. This instrument uses a 7.6-cm-diameter by 

7.6-cm-thick sodium iodide crystal as a detector. 

Counting times at each station were 2 minutes duration. The instrument was calibrated using an external 

137Cs source, and the detector was held 0.5 m above the 

ground surface.

In order to relate the instrument count rate on the 

thorium channel of the spectrometer to thorium abundance, the following formula was used:

eTh = 0.094 (CTh - bTh ),

where CTh represents counts per minute on the thorium 

channel and bTh represents counts per minute (cpm) 

background. The bTh was measured over standing 

water and mainly represents cosmic-ray counts, which 

for this study is taken as 40 cpm. The value 0.094 is a 

sensitivity constant derived from a least-squares fitting procedure of measurements made over an area of 

known thorium abundance by J. S. Duval (written 

commun., 1976). Drift was checked by periodically re-

'Use of a brand name in this report is for descriptive purposes only and does not constitute 

endorsement by the U.S. Geological Survey.

turning to a base station and repeating the counting 

operation at a fixed location. Precision was determined 

by repeated measurements, after correction for drift, at 

the base station during the course of the field study. A 

total of 84 measurements, each of 2 minutes duration, 

averaged 769±67 counts. An attempt was made to 

evaluate accuracy by analyzing samples of carbonatite 

from 28 stations by laboratory gamma-ray 

spectrometric techniques. The laboratory analyses averaged 47 ppm Th, whereas field measurements at the 

same localities yielded analyses averaging 38 ppm Th. 

According to Hansen (1975), the gamma-ray spectrum 

observed in the laboratory is different from that observed in the field, the latter being strongly influenced 

by the interaction of gamma rays emitted within the 

rocks. Inconsistencies also result from the differences 

in the geometry of the field and laboratory environments. A much greater volume of rock is scanned in the 

field than in the laboratory. Thus, the accuracy of the 

field method is difficult to evaluate by comparison with 

laboratory data.

In addition to being analyzed for potassium, 

thorium, and uranium by laboratory gamma-ray 

spectrometric techniques, the samples of carbonatite 

collected from 28 localities were also analyzed for 61 

elements by semiquantitative spectrographic techniques. Splits of the same samples were processed by 

routine heavy-liquid and magnetic-separation 

methods, and the component minerals were identified 

either by physical characteristics or by X-ray powder 

diffraction methods. Thin sections of the rocks were 

also studied by routine petrographic techniques.

MINERALOGY AND GEOCHEMISTRY OF 

CARBONATITE

The mineralogy of the samples collected from the 

carbonatite stock during this study is not complex. The 

variety of minerals is somewhat less than that found in 

the neighboring carbonatite dikes. X-ray and optical 

studies show the presence of the following minerals, 

listed approximately in the order of their frequency of 

occurrence: dolomite, barite, goethite, hematite, calcite, quartz, fluorapatite, pyrochlore, pyrite, magnetite, biotite, rutile, fluorite, bastnaesite, aegirine, 

anatase, sphalerite, synchisite, zircon, magnesite(?), 

and manganese oxide minerals. The dolomite occurs as 

clear, colorless to milky-white grains and as brown, 

ferric oxide-rich grains often described as ankeritic or 

sideritic by Temple and Grogan (1965), Hedlund and 

Olson (1975), and Olson (1974). Neither ankerite nor 

siderite was identified in the current study, although 

siderite occurs in carbonatite dikes adjacent to Iron 

Hill. Barite occurs as disseminated colorless grains
THORIUM IN CARBONATITE STOCK AT IRON HILL, COLORADO B5

scattered throughout the dolomite matrix in highly 

variable amounts. Goethite most commonly occurs as 

shiny, dark-reddish-brown, conchoidally fractured 

grains entirely or partly replacing pyrite and forming 

pseudomorphs after pyrite. Hematite may be associated with goethite but also appears to form 

pseudomorphs after magnetite. This pattern of occurrence is especially apparent in the martite-fluorapatite 

veins that cut the carbonatite stock. Calcite is found in 

the carbonatite, but its paragenesis is difficult to determine because of its similar appearance to dolomite. 

Small rounded quartz grains are seen within some 

dolomite grains. Fluorapatite occurs as clear, colorless 

grains in widely variable amounts. Although chemical 

analyses are not available, the fluorapatite is suspected to contain abundant rare-earth elements. 

Pyrochlore occurs in a variety of colors including light 

gray, purplish gray, pink, and pale lemon yellow to 

greenish yellow. Pyrochlores analyzed by Nash (1972) 

contain up to several percent ThO2 . However, alpha 

counter measurements of pyrochlore separates show 

that not all are radioactive and that there is no correlation between color and radiation. Additional studies of 

pyrochlore are in progress. Pyrite is a common constituent but is frequently at least partly replaced by 

goethite. Biotite occurs in streaks at places within the 

earbonatite, helping to define a foliated variety, and 

magnetite occurs disseminated throughout the carbonatite. The remainder of the minerals listed earlier 

are sparse and are seldom seen either in thin sections 

or in mineral separates. The minerals that have been 

found in each sample are listed in Armbrustmacher 

and Brownfield (1979).

The abundance of a variety of elements in 28 samples of Iron Hill carbonatite, the average element content of igneous rocks of the upper continental crust 

(Wedepohl, 1971, table 7.3), and the average composition of carbonatites (Gold, 1963) are given in table 1. 

Histograms showing the frequency distributions of 

elements are presented in figure 2. In comparison with 

average igeneous rocks, the Iron Hill carbonatite contains greater than 20 times more Ba, Ce, and Nd, 

15-20 times more La, Nb, P, and SREE, and nearly 10 

or more times Mn, Mo, and Sr. There is 4.2 times more 

Th and 3.1 times more RaelJ. According to Heinrich 

(1966, p. 222) characteristic elements found in carbonatites and associated alkalic rocks in general include Ti, Nb, Zr, rare earths of the Ce subgroup (La, Ce, 

Nd, Pr, Sm, and Eu), P, F, Ba, Sr, and Th. All these 

elements except Ti and Zr occur in large average concentrations in the Iron Hill carbonatite relative to average igneous rocks. However, abundant Ti occurs in 

the magnetite-ilmenite-perovskite segregations that 

are found in the pyroxenite. The Iron Hill rocks were

Table 1. Abundance of elements in the carbonatite at Iron

Hill, the igneous rocks of the upper continental

crust, and the average carbonatite

[Values are in percent. Dashes signify that data are not 

available]

Element

Average

Iron Hill

carbonatite

Average igneous 

rocks of upper 

continental crust 

(Wedepohl. 1971)

Average chemical 

composition of 

carbonatites 

(Gold, 1963)

Fe. 

Mg. 

Ca. 

Ti.

Mn.

Ba. 

Be. 

Co. 

Cr. 

Cu.

La. 

Mo. 

Nb. 

Ni. 

Pb.

Sc. 

Sr. 

V.. 

Y.. 

Zr.

Si. 

Al. 

Na. 

K.. 

P. .

Ce. 

Ga. 

Yb.

Nd. 

Th.

RaeU

5.4 

4.3 

10. 

.3 

.58

1.56

.0007

.0018

.0065

.0050

.085

.0013

.040

.0053

.0018

.0015

.27

.0050

.0037

.0064

4.4

.60

.23 

1.2 

1.2

. 17

.0004

.00014

.067

.0047

.0011

.34

5.54

..39

!.87 

.47 

.069

.059

.0002

.0012

.007

.003

.0044

.0001

.002

.0044

.0015

.0014

.029

.0095

.0034

.016

30.54

7.83

2.45

2.82

.081

.0075

.0017

.00034

.003

.0011

.00035

.0225

5.12

3.40

25.10

.48

.47

.23

.0017

.0048

.00025

.0516

.0042

.1951

.0008

.001 

.34

.0096

.112

j.66 

..88

.31 

..24

.90

.15

.0001

Radium-equivalent uranium. 

Total rare-earth elements.

not analyzed for fluorine, but fluorite grains appear in 

a few mineral separates.

In comparison with the average composition of carbonatites in general (Gold, 1963), the Iron Hill carbonatite contains at least four times more Ba, Cu, Ni, 

and Ga. It also contains at least three times less Mo, 

Nb, Y, Zr, and Al. The remaining elements occur in 

amounts similar to Gold's averages. The analytical 

data for each sample are given in Armbrustmacher and 

Brownfield (1979).

ABUNDANCE AND DISTRIBUTION OF THORIUM

Although many carbonatite occurrences around the 

world are anomalously radioactive, available data
B6 GEOLOGY AND RESOURCES OF THORIUM IN THE UNITED STATES

15

10

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15

10

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Fe

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Mg Ca Ti

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(£ 

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Z

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CONCENTRATION, IN PERCENT

p p p p p O p 

Pb

If) ix O IT)

o o «- >- CM co in

o o o o o o o

p o o o o o p

Sc

FIGURE 2.—Histograms showing frequency distribution of elements in 28 samples of carbonatite at Iron Hill.

suggest that the overall abundance of thorium in carbonatites is not great and that there are probably few 

carbonatite bodies in which thorium would be of principal economic interest. Thorium, however, could be an 

important byproduct in those carbonatites being 

exploited chiefly for niobium or rare-earth elements.
THORIUM IN CARBONATITE STOCK AT IRON HILL, COLORADO B7

10

FT3

o«-«-cMcoinr»oin

OOOOOOOt-T-CMHini^O 

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If 111 ra

Z r* qi in

'«-' r^ CM co in r

K

Z CM w in r-; qi in

''-'«- CM co in rP

CONCENTRATION, IN PERCENT

EXPLANATION

Number of samples 

not tastad

Number of samples with 

detectable content below 

limit of determination

/Sr 

Element

Number of samples with 

content greater then the 

upper detection limit

Approximate value of 

midpoint of eech 

reporting class (percent)

Number of samples with 

no detectable content

oooo 7* o o o o o c

oooo z o o o o o t

• • • • o o o o o (

Ga Yb 

CONCENTRATION, IN PERCENT

FIGURE 2.—Continued.
B8 GEOLOGY AND RESOURCES OF THORIUM IN THE UNITED STATES

Thorium appears to have a wide variation in carbonatites in general. Gerasimovskiy (1973) reports 1.8 

ppm Th (0.0002 percent ThO2 ) for the Kerimasi carbonatite and 46.8 ppm Th (0.0053 percent ThO2 ) for the 

Fort Portal carbonatite; these values represent average compositions for East Africa rift-zone carbonatites. 

That part of the carbonatite at Palabora, Transvaal, 

that constitutes the copper ore body contains about 

0.033 percent Th (0.038 percent ThO2 ) (Hanekom and 

others, 1965). The Sulfide Queen carbonatite at 

Mountain Pass, California, contains an average grade 

of 0.026 percent ThO2 (Shawe, 1953). Primary magmatic carbonatite dikes in the Wet Mountains area, Colorado, average 0.18 percent Th (0.20 percent ThO2 ) 

(Armbrustmacher, 1979). The thorium content of carbonatite at Magnet Cove, Arkansas, appears to be very 

low (Erickson and Blade, 1963) and does not exceed 

about 35 ppm Th (0.004 percent ThO2 ) in any rock 

within the entire alkalic complex. Data on several 

other carbonatite localities are given by Olson and 

Overstreet (1964).

Occurrences of anomalous amounts of thorium in 

rocks of the Powderhorn district have been known 

since 1949 (Olson and Wallace, 1956). Wallace and 

Olson (1956) point out the presence of at least one locality within the Iron Hill carbonatite stock characterized by anomalous radioactivity, and Hedlund and 

Olson (1961) indicate that the carbonatite stock has a 

maximum radioactivity five times greater than background. Temple and Grogan (1965) indicate that "typical foliated carbonatite" in the carbonatite stock averages 0.01-0.02 percent Th (0.011-0.023 percent 

Th02 ).

Data gathered in the current study are summarized 

in figure 3. This histogram shows the variation in 173 

thorium analyses of carbonatite in the Iron Hill stock. 

The average thorium value is 36.2 ppm Th (0.0041 percent ThO2 ); the median value is 31 ppm Th (0.0035 

percent ThO2 ); the minimum value is 6 ppm (0.0007 

percent ThO2 ); and the maximum value is 150 ppm 

(0.017 percent ThO2 ).

The distribution and variation of thorium in the Iron 

Hill stock is portrayed by the contour map in figure 4. 

Using a contour interval of 10 ppm Th, several areas of 

stronger concentration of Th are outlined. Especially 

noteworthy are the areas just west of the summit of 

Iron Hill and at the southwest corner of the stock.

The areas containing stronger concentrations of 

thorium do not correspond to areas known to contain 

more abundant niobium, such as the area around the 

DuPont adit and the area immediately north of Beaver 

Creek (James V. Thompson, oral commun., 1977). 

Temple and Grogan (1965, p. 680) also point out the 

lack of general correlation between radioactivity and 

niobium occurrence. Using analyses of the 28 samples

55

50

45

40

35

30

25

20

15

10 —

5 —

I I I I I

inLOLOLOLOLOLOLOLOLOLOLOLOLOLO r-CNWtj-incorsoomOT-CNMtj' 

THORIUM, IN PARTS PER MILLION

FIGURE 3.—Histogram showing frequency distribution of 173 thorium values, in parts per million, 

determined by field gamma-ray spectrometry from 

the Iron Hill carbonatite. Thorium concentration 

values, on the abscissa, represent the midpoints of 

intervals 10 ppm wide.
THORIUM IN CARBONATITE STOCK AT IRON HILL, COLORADO B9

EXPLANATION

X Locality of field gamma-ray spectrometer

measurements

A Geochemical and mineralogical sample locality 

and field gamma-ray spectrometric reading 

_— Contact between carbonatite (including carbonatiterich talus) and alkalic rocks (including alluvium) 

^loll Contours of equal thorium content; contour interval 

10 ppm Th

0.5 KILOMETER

FIGURE 4.—Contour map of thorium distribution in the carbonatite at Iron Hill. See figure 1 for location of this area.

of carbonatite, correlation coefficients were calculated 

in order to test the strength of associations among 

thorium, niobium, total rare earths, and uranium. A 

perfect correlation would be 1.0, a perfectly random 

association would be 0.0. Results give Nb-REE, 0.41; 

Nb-U, 0.18; Nb-Th, 0.10; U-Th, 0.59; REE-Th, 0.50; 

and REE-U, 0.19. Although based on data insufficient

to support generalizations, it is interesting to note that 

the niobium-thorium association shows the weakest 

correlation. Alpha-counter measurements of mineral 

separates show moderate amounts of radioactivity, 

5-10 counts per minute above background, in some of 

the pyrochlore concentrates. None of the other minerals in the carbonatite appears to be radioactive.
BIO GEOLOGY AND RESOURCES OF THORIUM IN THE UNITED STATES

RESOURCE ASSESSMENT

As part of a program to assess the thorium reserves 

of the United States, funded by the U.S. Department of 

Energy, the Minerals Availability Field Office of the 

U.S. Bureau of Mines was requested to design cost 

analyses of mine and mill models for the recovery of 

thorium from massive carbonatite deposits. The details 

of the procedures, which are only outlined here, can be 

found in Staatz and others (1979).

The cost analyses provide for an open-pit mining 

procedure with recovery of thorium based on flotation 

of pyrochlore, followed by chlorination, acid leach, and 

a solvent extraction procedure. -Byproducts of the procedure are uranium, niobium, and rare-earth elements. It is assumed that thorium, uranium, niobium, 

and rare-earth elements occur together in the 

pyrochlore. Mining costs are subdivided into capital 

costs, which include mine development, mine structures, and mine equipment; and operating costs, which 

include materials, utilities, labor, and maintenance, as 

well as taxes and insurance. Milling costs are also subdivided into capital costs and operating costs. These 

four items are represented by curves that fit a range of 

1,000-10,000 tons per day mining capacity with the 

maximum value of 10,000 tons per day used in the cost 

calculations. A maximum mill life or mine life of 15 

years is assumed, and so the maximum tonnage that 

can be handled is 52,500,000 tons (10,000 tons per day 

times 350 days per year times 15 years).

Various methods can be used to determine the 

amount of carbonatite at Iron Hill that might be potentially economically important. The method used by 

Staatz and others (1979) is presented here.

Staatz and others (1979) determined the amount of 

carbonatite that projects above the surrounding land 

surface. Seven evenly spaced cross sections were constructed across the carbonatite stock, the area was 

measured in each cross section, and the average area in 

each pair of sections was multiplied by the distance 

between them. Using this method, Iron Hill was found 

to contain 722,700,000 short tons of carbonatite. 2 

Based on the field gamma-ray spectrometric readings, 

the average thorium concentration is 0.0041 percent 

ThO2 , which yields 29,775 tons of ThO2 in the carbonatite. Assuming the abundance of elements in the 28 

analyzed samples to be representative of the entire 

carbonatite stock, the stock would contain 9,180 tons of 

U3O8 (avg. 0.00127 percent), 2,865,500 tons total rareearth oxides (avg. 0.3965 percent), and 412,000 tons 

Nb2O5 (avg. 0.057 percent). Higher grades of thorium 

and the coproduct elements could be obtained by selec2 All calculations were made in U.S. customary units as required by the U.S. Department of 

Energy contract.

tive mining because the distribution of each element is 

not consistent within the stock. Staatz and others 

(1979) define these values as reserves. Carbonatite defined as probable potential resources by Staatz and 

others (1979) is considered to extend an additional 800 

feet in depth. With a map area of 38,070,000 ft2 (square 

feet), 2,672,000,000 short tons of carbonatite containing 110,090 tons of ThO2 , 33,930 tons of U3O8 , 

10,590,000 tons of total rare-earth oxides, and 

1,523,000 tons of Nb2O5 would fall into that category.

A more generalized way to arrive at tonnage and 

grade figures would be to calculate the amount of coproducts contained in each vertical foot of carbonatite. 

With those values one could select any depth and any 

mining configuration that seemed appropriate. The 

map area covered by the stock is 38,070,000 ft2 . With a 

depth of 1 ft, there is 38,070,000 ft3 (cubic feet) or 

3,339,470 tons of carbonatite, and thus 138 tons of 

ThO2 , 42 tons of U3O8 , 13,240 tons of total rare-earth 

oxides, and 1,900 tons of Nb2O3 per foot of depth.

The cost of producing ThO2 from the carbonatite at 

Iron Hill was estimated in two ways: (1) the cost if 

ThO2 is the only product obtained from the carbonatite, 

and (2) the cost if Nb2O5 , RE2O3 (rare-earth oxides), and 

U3O8 are products in addition to ThO2 . The final cost to 

mine and mill ThO2 alone would be $68 per pound. If 

Nb2O3 , RE2O3, and U3O8 were all marketable at current 

prices, the combined value of these commodities would 

be more than twice that of ThO2 , assuming ThO2 could 

be sold for $50 per pound. It thus appears certain that 

the economics of ThO2 contained in the carbonatite 

stock at Iron Hill are inseparably linked to those of 

Nb2O5 , RE2O3 , and U3O8 .

SUMMARY

Studies of the occurrence of thorium in the carbonatite at Iron Hill show a fairly inhomogeneous distribution with values ranging between 6 ppm Th (0.0007 

percent ThO2 ) and 150 ppm Th (0.017 percent ThO2 ); 

several local anomalies occur. The average thorium 

content of the carbonatite, the value used in the resource calculations, is 36.2 ppm Th (0.0041 percent 

ThO2 ). Comparisons with other potential sources of 

thorium, such as the vein deposits in the Lemhi Pass 

district, Idaho and Montana (Staatz, 1972), and in the 

Wet Mountains, Colo. (Armbrustmacher, 1976; 

Christman and others, 1959), and the disseminated deposits at Hicks Dome, 111. (Brown and others, 1954), 

and Bear Lodge Mountains, Wyo. (M. H. Staatz, written commun., 1978), show that the carbonatite at Iron 

Hill contains considerably less total reserves (Staatz 

and others, 1979, table 1). Therefore, the economic future of thorium at Iron Hill appears to be closely tied to 

the economics of niobium and rare earths.
THORIUM IN CARBONATITE STOCK AT IRON HILL, COLORADO Bll

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