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COVER PHOTOGRAPHS

1

5

9

11

2

7

12

3

6

10

13

4

8

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, 

Mich.

6. Ribbon asbestos ore, Quebec, Canada

7. Manganese ore, banded 

rhodochrosite

8. Aluminum ore, bauxite, Georgia

9. Native copper ore, Keweenawan 

Peninsula, Mich.

10. Porphyry molybdenum ore, Colorado

11. Zinc ore, Edwards, N.Y.

12. Manganese nodules, ocean floor

13. Botryoidal f luorite ore, 

Poncha Springs, Colo.

14. Tungsten ore. North Carolina
Alkalic Rocks and Resources of 

Thorium and Associated Elements 

in the Powderhorn District, 

Gunnison County, Colorado

By JERRY C. OLSON and DAVID C. HEDLUND

GEOLOGY AND MINERAL RESOURCES OF THORIUM IN THE UNITED STATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 104 9-C

An evaluation of thorium resources, 

their distribution, and 

geologic relationships

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1981
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY 

Doyle G. Frederick, Acting Director

Library of Congress Cataloging in Publication Data

Olson, Jerry Chipman, 1917-

AJkalic rocks and resources of thorium and associated elements in the Powderhorn district, Gunnison County, Colorado.

(Geology and resources of thorium in the United States)

(Geological Survey Professional Paper 1049-C)

Bibliography: p. 33

Supt. of Docs, no.: I 19.16:1049-C

1. Alkalic igneous rocks. 2. Intrusions (Geology)—Colorado—Gunnison Co. 3.

I. Hedlund, David Carl, 1924-joint author. II. Title. HI. Series. IV. Series:

Professional Paper 1049-C. 

QE462.A4044 552'.! 80-607811

Thorium ores—Colorado—Gunnison Co. 

United States Geological Survey

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

Washington, D.C. 20402
CONTENTS

Pl1TTVWf> ITI /I Qf*rmfk f\f TTtmrart' -T--I _ T...

Fenite —————————————————————————

Page

—— 1

......... 9

o

o 

——— 4

——— 5

1 p

——— 14

——— 14

1 C

1 C

Igneous rocks and veins outside the complex of Iron Hill —

Faults ————————————————————————————————

JvflTG'Cflrtil elements ----- ————————————— • ...........

Page

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30

32

33

ILLUSTRATIONS

FIGURE 1. Index map of Colorado showing location of the Powderhorn district •

2. Photomicrograph of augite syenite from intrusive body near Spencer, Powderhorn quadrangle •

3. Photograph showing view southeastward of Cebolla Creek valley and Iron Hill —————————

4. Geologic map of the complex of alkalic rocks at Iron Hill •

5. Photograph showing fenitized granite, northeast margin of complex of Iron Hill •

6. Photograph showing a series of parallel micaceous shear planes in pyroxenite •

7. Graphs showing percentages of principal minerals in 94 modal analyses of altered pyroxenite ——

8. Sketches of textures indicative of paragenetic sequence in pyroxenite, uncompahgrite, and ijolite •

9. Photograph showing vermiculite-rich lens or dike in pyroxenite of the complex of Iron Hill ————

10. Photograph showing two subparallel dikes of magnetite-perovskite rock cutting pyroxenite ———

11. Photograph showing outcrop of uncompahgrite •

12. Photomicrograph of uncompahgrite, complex of Iron Hill ——————————————

13. Photomicrograph of nepheline syenite, complex of Iron Hill —————————————

14. Photograph showing mixed pyroxenite-syenite rock from the complex of Iron Hill •

15. Photograph showing Iron Hill from the southeast •

16. Photograph showing carbonatite, strongly foliated, Iron Hill ————

17. Photomicrograph of pyrochlore and apatite in carbonatite, Iron Hill •

18. Photograph showing inclusion of felsite in carbonatite dike —————

19. Sketch of felsite xenoliths in carbonatite dikes ————————————

20. Map showing distribution and trend of thorium deposits, Powderhorn district ——————————————————————————

21. Curve showing cumulative total of estimated ThO2 resources at various grades ——————————————————————————

22. Map showing areas of principal thorium resources estimated in veins, magnetite-perovskite rock, carbonatite, and 

thorium-bearing granite ————————————————————————————————————————————————————————

23. Map showing proportions of ThO2, cerium-group rare-earth oxides, and yttrium-group rare-earth oxides in samples from 

31 thorium deposits ———————————————————————————————————————————————————————————

Page

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31

ill
IV CONTENTS

Page

FIGURE 24. Graph showing relation of total cerium-group rare-earth metals to ThO2 content in 59 samples of thorium deposits ————— 32 

25. Graph showing relation of total yttrium-group rare-earth metals to ThO2 content in 59 samples of thorium deposits ————— 33

TABLES

Page

TABLE 1. Chemical analyses of Powderhorn Granite and fenite—————————————————————————————————————————— 5

2. Semiquantitative spectrographic analyses of melanite, diopside, perovskite, and magnetite from pyroxenite, complex of

Iron Hill——————————————————————————————————————————————————————————————————— 11

3. Semiquantitative spectrographic, X-ray fluorescent, fluorometric, and radiochemical analyses of pyrochlore and apatite

from carbonatite, Iron Hill ———————————————————————————————————————————————————————— 18

4. ThO2 resources ———————————————————————————————————————————————————————————————— 27

5. Indicated and inferred reserves of ThO2 in veins of more than 500 ppm ThO2 average grade ———————————————————— 27

6. ThO2 content of magnetite-ilmenite-perovskite rock, complex of Iron Hill —————————————————————————————— 29

7. ThCyNb2O5 ratios in veins in the Powderhorn district —————————————————————————————————————————— 29

8. Ratio of cerium-group rare-earth oxides to ThO2 to yttrium-group rare-earth oxides in various thorium deposits ———————— 32
GEOLOGY AND MINERAL RESOURCES OF THORIUM IN THE UNITED STATES

ALKALIC ROCKS AND RESOURCES OF THORIUM AND ASSOCIATED 

ELEMENTS IN THE POWDERHORN DISTRICT, GUNNISON

COUNTY, COLORADO

By JERRY C. OLSON and DAVID C. HEDLUND

ABSTRACT

Alkalic igneous rocks and related concentrations of thorium, 

niobium, rare-earth elements, titanium, and other elements have 

long been known in the Powderhorn mining district and have been 

explored intermittently for several decades. The deposits formed 

chiefly about 570 m.y. (million years) ago in latest Precambrian or 

Early Cambrian time. They were emplaced in lower Proterozoic 

(Proterozoic X) metasedimentary, metavolcanic, and plutonic rocks.

The complex of alkalic rocks of Iron Hill occupies 31 km2 (square 

kilometers) and is composed of pyroxenite, uncompahgrite, ijolite, 

nepheline syenite, and carbonatite, in order of generally decreasing 

age. Fenite occurs in a zone, in places more than 0.6 km (kilometer) 

wide, around a large part of the margin of the complex and adjacent 

to alkalic dikes intruding Precambrian host rock. The alkalic rocks 

have a radioactivity, chiefly due to thorium, greater than that of the 

surrounding Powderhorn Granite (Proterozoic X) and metamorphic 

rocks. The pyroxenite, uncompahgrite, ijolite, and nepheline 

syenite, which form more than 80 percent of the complex, have fairly uniform radioactivity. Radioactivity in the carbonatite stock, carbonatite dikes, and the carbonatite-pyroxenite mixed rock zone, 

however, generally exceeds that in the other rocks of the complex.

The thorium concentrations in the Powderhorn district occur in 

six types of deposits: thorite veins, a large massive carbonatite 

body, carbonatite dikes, trachyte dikes, magnetite-ilmeniteperovskite dikes or segregations, and disseminations in small, 

anomalously radioactive plutons chiefly of granite or quartz syenite 

that are older than rocks of the alkalic complex.

The highest grade thorium concentrations in the district are in 

veins that commonly occur in steeply dipping, crosscutting shear or 

breccia zones in the Precambrian rocks. They range in thickness 

from a centimeter or less to 5 m (meters) and are as much as 1 km 

long. The thorite veins are composed chiefly of potassic feldspar, 

white to smoky quartz, calcite, barite, goethite, and hematite, and 

also contain thorite, jasper, magnetite, pyrite, galena, chalcopyrite, 

sphalerite, synchysite, apatite, fluorite, biotite, sodic amphibole, 

rutile, monazite, bastnaesite, and vanadinite. The ThO2 content of 

the thorite veins ranges from less than 0.01 percent to as much as 

4.9 percent in high-grade samples. The ThO2 content is generally 

less than 1 percent, however, and is only 0.05 to 0.1 percent in many 

of the veins examined in the district.

Samples of the dolomitic carbonatite of Iron Hill mostly range 

from 3 to 145 ppm (parts per million) thorium. Thirty samples of the

carbonatite dikes, the most radioactive rocks within the complex of 

Iron Hill, contain about 30 to 3,200 ppm thorium and a trace to 

about 1.5 percent rare-earth oxides.

The magnetite-ilmenite-perovskite rocks have a radioactivity of 2 

to 12 times the background of Precambrian granite that is attributable chiefly to thorium substitution for calcium in the perovskite. In two analyses the perovskite contains 0.12 and 0.15 percent 

ThO2.

Trachyte dikes as much as 25 m thick cut the Precambrian rocks; 

their radioactivity is generally about two to four times the 

background of typical Precambrian granite, is locally higher, but is 

low relative to other types of thorium concentrations. A finegrained granite that is anomalously radioactive occurs in thick, 

dikelike plutons as much as 1.2 km wide, or more. The thorium content varies widely within the granite bodies. Eight samples of the 

granite contain 32 to 281 ppm thorium (averaging 115 ppm).

The economic potential of thorium in the Powderhorn district is 

related in part to other elements such as niobium, titanium, iron, 

and rare earths. The proportions of niobium and rare earths to 

thorium vary in different parts of the district. Within the carbonatite body of Iron Hill, the Nb2O5 content greatly exceeds ThO2, 

but the ThO2-Nb2O5 ratio increases markedly northwestward to as 

high as 10.7 in vein deposits near the northwest end of the district.

The ratio of cerium-group rare-earth oxides to ThO2 to yttriumgroup rare-earth oxides in the district is roughly 3.6 to 1 to 0.5, but 

this ratio varies considerably in different parts of the district. For 

example, the proportion of the cerium group is greatest in and near 

the complex of Iron Hill, and is least in an area of thorite veins 

about 6 km to the north. The proportion of the yttrium group in 

these two areas varies approximately inversely to that of the cerium 

group.

Thorium deposits shown on the geologic quadrangle maps of the 

Powderhorn district number at least 261. Most are small or low in 

grade, hence only a minority contribute significantly to total 

resources. Indicated and inferred reserves of ThO2 total about 

10,000 short tons in rock greater than 0.1 percent ThO2 in grade. 

This estimated tonnage is in 15 deposits including 13 veins and two 

carbonatite dikes. Larger amounts of thorium are present in lower 

grade materials. Additional quantities are probably present in 

deposits as yet undiscovered, which could add significantly to the 

total resources.
GEOLOGY AND MINERAL RESOURCES OF THORIUM IN THE UNITED STATES

INTRODUCTION

The Powderhorn district is a 650 km2 area in Gunnison County, Colo., that has long been known for its 

occurrences of alkalic igneous rocks and related concentrations of thorium, niobium, rare-earth elements, 

titanium, and other metals. This mapping and geologic 

study of the district was undertaken primarily to learn 

more about the geologic occurrence and origin of the 

mineral deposits listed above and to evaluate their 

economic potential and resources.

Mining activity in the district began in 1872 and has 

continued intermittently to the present. The early 

mining was chiefly for gold, silver, copper, lead, zinc, 

and arsenic from veins and volcanogenic massive sulfide deposits. In more recent years mining and prospecting has been aimed toward thorium, rare-earth 

elements, niobium, and titanium as well. Various 

attempts have been made to mine and process vermiculite, particularly about 1935-45, by the Black 

Mica Mining Co., Allied Minerals Co., and the Alexite 

Co.

Thorium veins were discovered in the Powderhorn 

district in 1949, and about 250 veins were prospected 

for thorium during 1949-56. In order to assess niobium resources, the pyrochlore-bearing carbonatite 

and related rocks of the complex of Iron Hill were 

drilled and an adit was driven by the DuPont Co. in 

1957-58. Titaniferous iron ores in the complex have 

been prospected at various times since their discovery 

in the 1880's. The Humphreys Gold Corp. explored the 

titanium deposits by digging test pits in the early fifties, and in 1957 the complex was investigated by the 

U.S. Bureau of Mines by drilling and surface sampling. 

In the mid-1970 's the titanium deposits were being explored by Buttes Gas and Oil Co.

GENERAL GEOLOGY

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 m.y. ago, the Precambrian rocks were intruded by alkalic igneous rocks of 

Iron Hill and by slightly younger tholeiitic diabase 

dikes (Olson and others, 1977). Mesozoic sedimentary 

rocks and Tertiary volcanic rocks originally covered 

much of the area and now remain as scattered mesas 

and ridges where they cover the older rocks.

The geology of the complex of Iron Hill has been 

described by Larsen (1942), Temple and Grogan (1965), 

Rose and Shannon (1960), and Nash (1972). The thorium deposits in the Powderhorn district have been described by Burbank and Pierson (1953), Hedlund and

Olson (1961), and by Olson and Wallace (1956), who included a detailed map and description of one of the 

largest thorium deposits, the Little Johnnie vein. The 

thorium content of the Iron Hill carbonatite stock has 

been studied by Armbrustmacher (1980). The geology 

of the district and location of thorium deposits are 

shown on geologic quadrangle maps by Hedlund and 

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

Olson (1974).

PURPOSE AND SCOPE OF REPORT

This report focuses primarily on the distribution of 

thorium and associated elements in various rock types, 

the evaluation of resources of thorium in different 

areas and types of deposits, and the relation of the 

mineral deposits to geologic history and structural 

features. We have tried to avoid excessive duplication 

of data already published in the extensive literature on 

the district, listed in references at the end of the report. 

Accordingly, the reader is referred to these reports for 

additional information on rock descriptions, mineralogy, geologic maps, analyses of rocks and minerals, 

descriptions of selected deposits, and other geologic 

features not specifically treated herein.

^___—Cochetopa 

X-^-Kazor Creek 

(Area shown on r:

SAGUACHE

37° J- —

0 50 100 KILOMETERS

FIGURE 1.—Index map of Colorado showing location of Powderhorn 

district, Gunnison County.
THORIUM AND ASSOCIATED ELEMENTS, POWDERHORN DISTRICT, GUNNISON COUNTY, COLORADO

FIELDWORK AND ACKNOWLEDGMENTS

This report is based chiefly on the geologic mapping 

of the Powderhorn, Rudolph Hill, Gateview, Carpenter 

Ridge, and Big Mesa IVz minute quadrangles by 

J. C. Olson and D. C. Hedlund during 1956-63. During 

the mapping, 261 thorium deposits and numerous 

other mines and prospects were examined briefly with 

a scintillation counter, and some resources were 

estimated. In 1976 and 1977, Olson reexamined 67 of 

the deposits with the aid of a portable four-channel 

gamma-ray spectrometer and collected additional 

samples. These 101 samples were analyzed in the 

laboratory by C. M. Bunker and C. A. Bush by gammaray spectrometry to provide an approximate calibration of thorium content and the field readings.

Laboratory work has included petrographic study of 

many thin sections, chemical and spectrographic 

analyses of rock samples, and mineralogical studies. 

The analytical work is summarized briefly in this 

report, but the 178 analyses of rock and thorium vein 

samples are available in Hedlund and Olson (1980).

Mineral exploration companies and prospectors in 

the district gave friendly cooperation during the work. 

R. M. Grogan and A. K. Temple of the DuPont Co. provided information on drilling and access to drill-core 

samples. S. S. Shannon of the U.S. Bureau of Mines 

made available thin sections and drill-core data from 

various parts of the pyroxenite of the complex of Iron 

Hill. T. J. Armbrustmacher provided radiometric and 

mineralogic information on the carbonatite body. 

M. H. Staatz identified minerals in 23 samples of thorium veins. I. K. Brownfield made heavy-liquid separations and X-ray diffractograms of mineral concentrates from some of the thorium-bearing rocks and 

veins.

The estimation of costs to produce thorium from 

various types of deposits used a method of cost 

analysis designed by Lemons and Coppa (1979).

PRECAMBRIAN ROCKS

The oldest rocks in the Powderhorn district are 

metasedimentary and metavolcanic rocks that were 

deposited to great thickness largely in a trough 

beneath the sea. These rocks were metamorphosed, 

mostly to amphibolite facies, about 1.7 billion years 

ago, probably shortly after their deposition. This 

regional metamorphism has not been dated in the 

Powderhorn area but is probably part of the widespread metamorphism known in many places in Colorado to be of about this age. The grade of metamorphism increases generally northwestward in the

district to as high as the sillimanite subfacies. 

Migmatite and small dikes and lenses of pegmatite 

were formed locally in the northwestern part of the 

district. The metamorphic rocks throughout the 

district were intruded by plutons of granitic to quartz 

dioritic composition and by small, more mafic intrusive bodies. None of the concentrations of thorium 

and related minor elements appear to be related in 

origin to this older, 1.7-1.8 billion-year chapter of 

Precambrian history.

VOLCANOGENIC MINERAL DEPOSITS

Massive sulfide deposits and veins containing gold, 

silver, copper, lead, zinc, and locally telluride minerals 

occur in the metavolcanic rocks of the Dubois Greenstone of Proterozoic X age in an east to northeasttrending zone termed the Gunnison gold belt. These 

volcanogenic copper- and gold-bearing deposits differ 

markedly from the quartz-orthoclase-carbonate-baritethorite veins. The Gunnison gold belt deposits, which 

show no anomalous radioactivity, commonly strike 

northeast to east, generally parallel with the overall 

trend of the metavolcanic and metasedimentary formations, whereas the thorite veins are in younger fractures that cut discordantly across the country rocks. 

In the volcanogenic deposits pyrite is the principal 

sulfide, with local concentrations of chalcopyrite, 

arsenopyrite, sphalerite, galena, or native gold. The 

quartz of these veins differs from that of the thorite 

veins, being generally finer grained and chloritic. 

Spatial relations to the sequence of metavolcanic rocks 

indicate that the massive sulfide deposits of the Gunnison gold belt were formed at least 1,700 m.y. ago at 

about the time the enclosing volcanic and sedimentary 

rocks were deposited. The gold-bearing quartz-chlorite 

veins were formed slightly later, as they are discordant 

to their host rocks, but are also Precambrian in age.

SYENITES AND RELATED ROCKS

The older Precambrian rocks were intruded by small 

plutons of leucosyenite and melasyenite about 1,400 

m.y. ago (Olson and others, 1977). About 20 of these 

small stocks or plugs, the largest about 0.6 by 1.3 km, 

and numerous smaller dikes, occur in the district. They 

are mostly in the north half of the Gateview and 

Powderhorn quadrangles and the southeastern part of 

the Carpenter Ridge quadrangle (Hedlund and Olson, 

1973). The individual plutons are generally composite, 

consisting of several textural and compositional 

varieties of leucosyenite and melasyenite. Typical 

augite syenite, shown in figure 2, contains augite and
GEOLOGY AND MINERAL RESOURCES OF THORIUM IN THE UNITED STATES

FIGURE 2.—Augite syenite from intrusive body near Spencer, 

Powderhorn quadrangle. Augite (A) and biotite (B) phenocrysts 

are set in a fine-grained groundmass consisting chiefly of 

microcline laths.

biotite phenocrysts in a fine-grained groundmass consisting chiefly of microcline laths. The biotite phenocrysts are highly poikilitic and locally segmented. The 

augite phenocrysts show various stages of alteration 

to uralitic hornblende, chlorite, ferric oxides, and 

biotite. About 65 chemical, spectrographic, and modal 

analyses have been given by Hedlund and Olson 

(1980).

The l,400-m.y.-old (Olson and others, 1977) syenitic 

rocks contain thorium in amounts above the crustal 

average of about 11 ppm but not in exceptionally high 

amounts for syenitic rocks. Five samples of biotite 

melasyenite and one of leucosyenite had eU of 

0.003-0.004 percent and uranium contents of less than 

0.001 percent, suggesting thorium as the principal 

radioactive element. Three other samples were analyzed by gamma-ray spectrometry and found to contain 19.78, 14.5, and 18.3 ppm thorium, respectively. 

The thorium content of the syenites and their proximity to some thorium veins suggest a possible relationship of some thorium veins to the syenites. A relationship of thorium veins to the 570-m.y.-old rocks of 

the complex of Iron Hill, however, is more likely and is 

discussed in connection with those rocks.

Three pluglike bodies of intrusive breccia, discovered 

during mapping of the Carpenter Ridge quadrangle 

(Hedlund and Olson, 1973), are spatially and perhaps 

genetically related to the l,400-m.y.-old mafic syenites. The largest body of intrusive breccia is elliptical 

in general plan and is about 900 m long and 240 m 

wide. The intrusive breccia consists of angular fragments of diverse Precambrian rocks set in a light-gray 

or pale-green groundmass of comminuted rock fragments, ankerite or dolomite, and opaque minerals. 

Parts of these bodies of intrusive breccia have radioactivities of two to three times the background of the 

surrounding quartz-mica schist, and the one sample 

analyzed by gamma ray spectrometry contains 38 ppm 

thorium. Three semiquantitative spectrographic 

analyses showed (in percent) barium and strontium 

generally about 0.15, zirconium 0.03-0.07, cerium 0.03, 

lanthanum 0.015, neodymium 0.015-0.03, yttrium 

0.003-0.015, niobium 0.007, and vanadium 0.015-0.03 

(Hedlund and Olson, 1980, table 1). These concentrations are several times the crustal averages for these 

elements.

UPPER PROTEROZOIC OR LOWER 

PALEOZOIC IGNEOUS ROCKS

Rocks of Late Proterozoic or Cambrian age comprise 

the complex of Iron Hill; related outlying dikes of 

trachyte, carbonatite, nepheline syenite, pyroxenite, 

melasyenite, and lamprophyre; thorium-bearing veins; 

and diabase dikes. The alkalic rocks of Iron Hill have 

been dated at about 570 m.y. and the diabase dikes are 

somewhat younger, probably about 510 m.y. old 

(Olson and others, 1977).

The thorium concentrations in the district are probably all related to this episode of igneous activity. 

These concentrations include thorite veins, the Iron 

Hill carbonatite body, carbonatite dikes, trachyte 

dikes, magnetite-ilmenite-perovskite segregations, and 

the disseminated thorium-bearing minerals in small, 

anomalously radioactive plutons chiefly of granite or 

quartz syenite. The latter plutons are of uncertain age 

and may be older than the other deposits listed. The 

genetic relation of the thorium deposits to the 

570-m.y.-old igneous activity is suggested by the suite 

of minor elements such as thorium, rare-earth
THORIUM AND ASSOCIATED ELEMENTS, POWDERHORN DISTRICT, GUNNISON COUNTY, COLORADO

TABLE 1.—Chemical analyses of Powderhorn Granite and fenite

Powderhorn Granite Fenite

Sample No.—G2300a G4500 P9820a U1206a 1H-71

Si02

A1 203

Fe|o|

FeO

MgO

CaO

Na20

K96

TiO?

P2°5

MnO

H ?0+ 

HpOc62

Sum

76.3

13.8

.8

.27

.14

.36

3.3

4.8

.12

0.00

.02

/ .48

.07

100.46

73.1

13.2

1.2

.96

.45

1.2

3.5

4.7

.22

.10

.04

:&e

.14

99

73.15

13.45

1.25

1.19

.57

1.86

3.73

3.43

.16

.10

.07

:!?

.21

99.89

65.1

16.6

2.0

1.1

.2

1.5

7.3

5.1

.2

.0

.1

:3

.0

99.8

62.55

15.07

2.42

2.03

1.33

4.09

5.80

5.68

.15

.14

.29

105=

.03

99.8

Sample descriptions

G2300a. Powderhorn Granite, medium grained, light pink. NE1/4 

sec 3, T. 47 N., R. 3 W, at 8,680 ft elevation, east side of 

gully, 110 m northwest of road fork marked 8694. Analysis by 

P. L. D. Elmore and S. D. Botts.

G4500. Powderhorn Granite. NE1/4 sec. 8, T. 47 N., R. 3 W. Above 

road on east side of Lake Fork of Gunnison River. Analysis by 

N. G. Skinner.

P9820a. Powderhorn Granite, coarse grained, biotitic, pink 

perthitic microcline phenocrysts 2.5 cm across. SW1/4 sec. 36, 

T. 47 N., R. 2 W. At 9,320 ft elevation, north side of Huntsman 

Gulch, in tributary gully northeast of spring. Analysis by 

E. L. Munson.

U1206a. Soda syenite (Larsen, 1942, p. 26), considered to be 

fenite. SW1/4 sec. 7, T. 46 N., R. 1 W. South of gully, near 

northwest base of hill above 9,000 ft contour. Analysis by 

George Steiger.

IH-71. Fenite, gray, gneissic, medium grained, composed chiefly 

of microcline perthite and elongate, alined aggregates of 

aegirine-augite grains. NE1/4 sec. 12, T. 46 N., R. 1 1/2 W. 

Analysis by E. L. Munson.

elements, niobium, titanium, barium, and strontium 

characteristic of the rocks and deposits; the alkali 

metasomatism (fenitization) of the wall rocks adjacent 

to the thorium-bearing veins and the alkalic rock complex; and the fact that veins as well as alkalic rocks are 

cut by diabase dikes. Furthermore, the veins and the 

trachyte dikes are characterized by a fetid odor that is 

also found in places in the fenite surrounding the 

alkalic complex. Such a fetid odor has been described 

in other areas where thorium concentrations are 

related to alkalic rocks (e.g. Olson and others, 1954; 

Heinrich and Anderson, 1965).

Although the alkalic rock complex occupies 31 km2, 

thorium-bearing veins are present in at least a 

650-km2 area (fig. 1). They have been found from the 

Black Canyon of the Gunnison River southeastward to 

Cochetopa Creek and Razor Creek.

ALKALIC ROCK COMPLEX OF IRON HILL

The complex of Iron Hill is the best example of the 

carbonatite-alkalic rock association in the United 

States and is one of the outstanding occurrences in the 

world, comparable to many of the classic areas in 

Africa and other continents. This heterogeneous 

assemblage of alkalic igneous rocks was intruded, 

probably from a source in the mantle, about 570 m.y. 

ago. The complex of Iron Hill lacks a clearly defined 

ring or cone sheet structure typical of some of the 

African complexes, but it has a radial carbonatite dike 

system that partly surrounds an eccentrically placed 

carbonatite stock. This stock was emplaced as a carbonatitic magma, and it is suggested that forcible 

emplacement was accompanied by radial fracturing of 

the surrounding pyroxenite. The depth of the complex 

below the surface at the time of intrusion is not known. 

Possibly the complex breached the surface, but erosion 

during long periods of Paleozoic, Mesozoic, and 

Cenozoic time has removed the upper part.

The alkalic rock complex of Iron Hill occupies an 

elliptical area (fig. 3) of about 31 km2 along the east 

side of Cebolla Creek. The complex is about 10 km 

long, has a maximum width of about 5 km, and the 

long axis strikes N 55°-60°W. The dolomitic carbonatite (rauhaugite) stock that forms Iron Hill is also 

elliptical in outcrop, the long axis striking northwest.

The alkaline complex forms a distinctive topography 

that is largely controlled by lithology and climate. In 

this semiarid region the carbonatite of Iron Hill is

FIGURE 3.—View southeastward of Cebolla Creek valley and Iron 

Hill. Approximate outline of the complex of Iron Hill is indicated by the line.
GEOLOGY AND MINERAL RESOURCES OF THORIUM IN THE UNITED STATES

more resistant to weathering than the surrounding 

basic alkaline rocks; hence the carbonatite forms a hill 

surrounded by a horseshoe-shaped depression of low 

relief. Older Precambrian granite forms an outer, arcuate ridge about the inner outcrop belt of basic alkalic 

rocks.

The rocks in the complex (fig. 4) from oldest to 

youngest are pyroxenite, uncompahgrite, ijolite, 

nepheline syenite, and carbonatite; the last two 

rock types locally show overlapping age relations. 

Magnetite-ilmenite-perovskite dikes and segregations 

are chiefly confined to the pyroxenite. Fenite occurs 

commonly at the borders of the complex. Carbonatite 

dikes radiate outward from the dolomitic carbonatite 

stock and cut all the other rock types of the complex.

The distribution of radioactivity in the alkalic rocks 

of Iron Hill was studied by taking 1,258 random 

scintillation-counter readings distributed among the 

various rock types approximately in proportion to 

their area. These readings, which were made on the 

outcrop with a scintillation counter, are given in the 

description of the various rock types that follow. The 

total radioactivity of the alkalic rocks is well above 

that of the surrounding Powderhorn Granite and metamorphic rocks that commonly show a background 

count of 0.02-0.025 mr/hr (milliroentgens per hour). 

The radioactivity of pyroxenite, uncompahgrite, 

ijolite, and nepheline syenite, which, combined make 

up more than 80 percent of the complex, is fairly 

uniformly distributed among the rock types and 

averages about 0.035 mr/hr or about IVz times that of 

the surrounding granite.

Within the complex, carbonatite is the most radioactive rock. The carbonatite stock averages 0.055 mr/hr 

for 226 readings, or more than twice the background of 

typical Powderhorn Granite. Carbonatite dikes vary 

considerably in their radioactivity from one dike to 

another and within individual dikes, hence a representative average figure was difficult to obtain. On the 

average, however, they are somewhat more radioactive 

than the carbonatite stock. Furthermore, the radioactivity of pyroxenite increases markedly in those areas 

where the pyroxenite is permeated by many small 

dikes and seams of carbonatite, including the mixed 

rock zone near the carbonatite stock.

FENITE

Fenite occurs in a zone, in places more than 0.6 km 

wide, around a large part of the margin of the complex. 

It is a syenitic rock formed from various country rocks, 

chiefly granite and felsite, by metasomatic alteration 

associated with the intrusion of alkalic or carbonatitic

magmatic fluids. The fenite zone is generally wider 

along the south margin of the complex and is present 

but narrower along the north side. Although the fenite 

may not have all formed at one time, it is cut by and 

apparently predates some dikes and apophyses of the 

pyroxenite. In places the fenite is intruded by dikes of 

nepheline syenite, carbonatite, and by diabase. The 

fenite is essentially the same as the "soda syenite" 

described by Larsen (1942, p. 25-26).

The most abundant fenite in the Powderhorn area is 

gray, is medium to coarse grained, and contains lenticular aggregates of aegirine-augite that impart a 

schlieric texture to the fenite (fig. 5). The highly perthitic microcline grains are anhedral to euhedral and 

commonly have dusty inclusions of ferric oxides and 

thin, marginal selvages of fine-grained albite. Accessory minerals include calcite, muscovite, apatite, 

and sphene.

The gradation from granite country rock to fenite is 

characterized by the following changes: the microcline 

phenocrysts become grayer rather than red, more perthitic, less euhedral, and the porphyritic texture of the 

granite becomes less conspicuous. The quartz content 

diminishes, aegirine-augite replaces the biotite, and a 

gneissic or schlieric texture may develop or intensify. 

In advanced stages the fenite probably became rheomorphic locally, as suggested by the schlieric texture. 

Fracture fillings of blue-green sodic amphiboles, such 

as riebeckite(?) and richterite, and sodic pyroxenes are 

common in granite or felsite in places in the fenite 

zone. These represent a less intense form of fenitization and may be found as much as several kilometers 

from the complex. Some veinlets of sodic amphibole 

have slightly anomalous radioactivity due to thorium.

Fenite varies with the type of original country rock. 

Fenite derived from hornblende gneiss is likely to be 

rich in sodic pyroxene. Generally the felsitic rocks 

northwest of the complex, which are siliceous metavolcanic rocks such as metarhyolite, show less conspicious fenitization than other rocks close to margins. 

Fenite is present, however, in felsite at the northwest 

end of the complex, for example at a point 1 km north 

of the Powderhorn store. Here the schistose felsite 

near the contact with pyroxenite has been albitized, 

and the normally gray, fine-grained felsite grades into 

white, medium-grained, equigranular, albite-rich rock 

containing sparse wispy clots of biotite. The microcline 

has been albitized, and vein, patch, and string perthite 

is abundant.

Table 1 illustrates the chemical changes that occur 

as granite is altered to fenite near the alkalic complex. 

Two analyses of Powderhorn Granite from the Gateview quadrangle and one from about 1.1 km north of
THORIUM AND ASSOCIATED ELEMENTS, POWDERHORN DISTRICT, GUNNISON COUNTY, COLORADO 7

107°00'

38°15' -

EXPLANATION

Surficial deposits and volcanic rocks 

Diabase and gabbro dikes 

Carbonatite I

Ijolite

i

Uncompahgnte

Pyroxenite

Fenite

Granite and metamorphic rocks

X QUATERNARY

AND TERTIARY 

ORDOVICIAN 

OR CAMBRIAN

Nepheline syenite

Melasyenite Complex of 

alkalic rocks 

of Iron Hill

CAMBRIAN 

OR UPPER 

PROTEROZOIC

PROTEROZOIC

Contact, dotted where concealed 

Fault, dotted where concealed

FIGURE 4.—Generalized geologic map of the complex of alkalic rocks at Iron Hill, Powderhorn district.
8 GEOLOGY AND MINERAL RESOURCES OF THORIUM IN THE UNITED STATES

the pyroxenite contact are representative of the 

Powderhorn Granite. Development of fenite from the 

granite involves a loss of silica and a gain of alkalies, 

iron, and alumina.

Fenite was developed near several of the rock types 

of the complex and probably formed at different times 

in several places. It was formed, for example, extensively near pyroxenite and uncompahgrite of the complex, near isolated bodies of carbonatite outside the 

complex, and near the nepheline syenite stock and 

dikes at the east end of the complex.

PYROXENITE

Pyroxenite underlies about 70 percent of the outcrop 

area in the complex of Iron Hill and is about 10 km in 

maximum dimension. It is locally capped by remnants 

of Oligocene ash-flow tuffs and Quaternary and Tertiary fan deposits. The pyroxenite weathers readily, 

forming a terrane of low relief between the surrounding 

hills of Precambrian granite and felsite and the resistant Iron Hill carbonatite stock.

The pyroxenite has steep, sharp, discordant contacts 

with the surrounding Precambrian granite and felsite. 

A few apophyses and dikes of pyroxenite locally extend into the country rocks. Most of the pyroxenite 

is medium grained and equigranular. Generally the 

pyroxenite lacks a fine-grained, chilled border facies 

and shows medium- to coarse-grained texture near the

FIGURE 5.—Fenitized granite, with schlieric texture; northeast 

margin of the complex of Iron Hill. Lenticular clots of aegirine 

augite are dark. Microcline perthite is the dominant leucocratic 

mineral. Width of rock shown is about 1 m.

contact with the country rocks, indicating that the 

wall-rock temperature was close to that of the intrusive magma Primary internal structures within the 

pyroxenite consist locally of a strong layering, coarsegrained pyroxene-rich layers alternating with biotiteor vermiculite-rich layers. The pyroxenite is locally cut 

by numerous dikes and veins consisting of vermiculite 

and apatite or of magnetite. In some pegmatitic, 

vermiculite-rich dikes the vermiculite plates are 

coarsest where the intergrown pyroxene also coarsens. 

Prismatic diopside crystals as much as 15 cm (centimeters) long in the dikes have their c axes alined 

perpendicular to the walls of the dikes. Some thin 

vermiculite-diopside dikes are zoned. Coarse vermiculite plates are near and roughly perpendicular to 

the walls, whereas the core of the dike consists of 

decussate, fine-grained vermiculite intergrown with 

fine-grained diopside and magnetite.

The heterogeneity of the internal structures within 

the pyroxenite suggests that it was a multiple intrusion. Dilatant fractures in the earlier crystallized 

pyroxenite were filled with younger fractions of the 

magma, and considerable replacement of early formed 

minerals occurred in the later magmatic and deuteric 

stages. Secondary internal structures are chiefly due 

to shearing. Light gray, fine grained stringers and 

veinlets of calcite and biotite locally pervade the 

pyroxenite along shear zones and cut across the primary structures of the pyroxenite. Where subparallel, 

the micaceous shear planes resemble primary layering 

in the pyroxenite (fig. 6). The micaceous shear planes 

generally strike N. 60°-70° E. and N. 60°-70° W. and 

dip 70°-80° N. This orientation approximates that of 

the carbonatite dikes in the complex; hence the shearFIGURE 6.—Series of parallel micaceous shear planes in pyroxenite. 

Many of the micaceous shears follow joints and cut pre-existing 

structures in the pyroxenite.
THORIUM AND ASSOCIATED ELEMENTS, POWDERHORN DISTRICT, GUNNISON COUNTY, COLORADO

ing may have been related to the period of carbonatite 

intrusion. This relation also is suggested by the high 

carbonate content in the shear planes.

Several textural varieties occur in pyroxenite and include (1) pegmatitic, typically developed in coarsegrained clinopyroxene-biotite rock; (2) fine-to coarsegrained hypidiomorphic, shown by pyroxenite that 

contains euhedral pyroxene, sphene, and melanite with 

anhedral interstitial magnetite and ilmenite; (3) 

poikilitic, shown by inclusions of sphene in melanite, 

perovskite in magnetite, and diopside in melilite; (4) 

schlieric, shown by lenticular aggregates of melanite or 

magnetite in the pyroxenite; (5) brecciated, shown by 

fractures in brecciated pyroxenite filled by nepheline 

syenite, at the east end of the complex.

The pyroxenite is composed chiefly of diopsidic 

augite, biotite, magnetite, melanite, sphene, apatite, 

and perovskite. Proportions of these minerals vary 

considerably, making it difficult to assess the average 

mineral composition. About 94 modal analyses were 

made of the pyroxenite. These values were plotted on 

volume percent-frequency curves (fig. 7). In thin section the following minerals were observed: pyroxene 

(chiefly diopsidic augite), magnetite, ilmenite, perovskite, leucoxene, melanite, sphene, phlogopite, vermiculite, biotite, apatite, pyrite, chalcopyrite, pyrrhotite, orthoclase, brown amphibole, chlorite, sodic 

amphibole, idocrase, calcite, sericite, quartz, and 

hematite.

Pyroxene and its alteration products make up as 

much as 92 percent of the rock but commonly constitute 55 to 70 percent by volume. The pyroxene 

varies in different rocks of the complex; it is mostly 

augite but includes also diopside, aegirine-augite, and 

aegirine (Larsen, 1942, p. 47). The pyroxene shows a 

wide range in crystal size from very fine grained augite 

to grains of diopside as much as 15 cm long. Much of 

the augite of the pyroxenite has moderate pleochroism: 

green to yellowish green; Z to c, 40 °-45 °; 2 V, 60 °-70 °; 

NX, 1.68-1.72, Ny, 1.69-1.73, Nz, 1.71-1.75. Some 

grains are replaced along their margins by brown amphibole, biotite (fig. SA), magnetite, chlorite, sodic amphibole, and calcite; such replacement is sporadic and 

is most intense near carbonatite bodies or dikes. Spectrographic analyses of two diopside samples are shown 

in table 2.

Amphiboles are present in amounts of as much as 5 

percent. The amphibole is quite variable in composition (Larsen, 1942, p. 48-52) but commonly is dark 

brown to black and has moderate pleochroism light 

brown to greenish brown. The common variety in the 

pyroxenite was considered to be hastingsite by Larsen 

(1942, p. 52). It commonly mantles or replaces the 

pyroxene.

Melanite is present in many of the thin sections of 

pyroxenite to a maximum of about 25 percent by 

volume. The melanite is isotropic and is zoned; lighter 

brown zones alternate with darker brown. The 

melanite is anhedral to euhedral and is commonly 

poikilitic; inclusions of sphene (fig. 8H) and perovskite 

(fig. 8D) are common. Some melanite forms thin 

mantles about pyroxene and magnetite-ilmenite 

grains. The index of refraction ranges from 1.85 to 1.98 

and is commonly about 1.88. The specific gravity is 

3.71±0.05. The unit cell is 12.051±0.002 angstroms. 

Spectrographic analysis of one melanite sample is 

shown in table 2.

Magnetite and ilmenite are ubiquitous in the pyroxenite as dikes, veins, and segregations, and as discrete 

grains intergrown with melanite and other silicates. 

Discrete grains of magnetite and ilmenite constitute as 

much as 40 percent of the pyroxenite but commonly 

range from 10 to 15 percent by volume. They are commonly intergrown with each other and with perovskite 

to form composite grains. Some of the ilmenite forms 

bladed, exsolution-type intergrowths along the (111) 

direction in the magnetite. Spectrographic analysis of 

one magnetite sample from pyroxenite is shown in 

table 2.

Perovskite is present in about 53 percent of the thin 

sections of pyroxenite, making up as much as 20 

percent by volume. The amounts of perovskite and 

magnetite tend to correlate with one another and are 

comparable in most rocks. Larsen (1942, p. 36) 

estimated the perovskite to make up about 7 percent of 

the pyroxenite and 6 percent of the entire complex, and 

magnetite 11 percent of the pyroxenite and 10 percent 

of the entire complex. The perovskite commonly occurs 

as euhedral crystals intergrown with magnetite and ilmenite but also occurs as discrete grains. The perovskite is dusky purple, slightly anisotropic, and has 

complex penetration twinning. Semiquantitative spectrographic analyses of two perovskite samples are 

shown in table 2.

Leucoxene is present in 28 percent of the thin 

sections, as an alteration product of perovskite, and 

composes as much as 18 percent by volume of the 

pyroxenite.

Sphene is present in about 34 percent of the thin sections of pyroxenite and constitutes a maximum of 

about 19 percent by volume of the rock. The sphene 

commonly occurs as relatively coarse, dark, honeycolored euhedral crystals with excellent twinning. The 

sphene occurs as inclusions in melanite (fig. 8H), as 

discrete grains intergrown with silicates, and as aggregated crystals associated with magnetite. Sphenebearing pyroxenite is most common on the ridge in the 

NE1/* sec. 2, T. 46 N., R. 2 W.
10

30-i

GEOLOGY AND MINERAL RESOURCES OF THORIUM IN THE UNITED STATES

30n

BIOTITE

T

20 30 40 50 

VOLUME PERCENT

70 10 20 30 40 

VOLUME PERCENT

70- 70-

60-

50-

40-

30-

20-

10- 

0-

0

60 -| 60 -

50-

1

AMPHIBOLES => 40 -

a

QC

30-

"

I^ n

1 1 u 

10 20 0

50-

>- 

u

MELANITE => 40-

a

or

30-

20-

10-

^^T^ — °-

10 20 30 0

I BOH BOH 

II

50-

|

CHLORITE §40- 

o

30-

20- L -

r — i o10 20 0

50-

SPHENE =40- 

o

SE

30-

20-

1 10- 

* n

I I u 

10 20 C

LEUCOXENE

K „ i i

10 20

VOLUME PERCENT VOLUME PERCENT VOLUME PERCENT VOLUME PERCENT VOLUME PERCENT

ORTHOCLASE

30 40 50 

VOLUME PERCENT

60 70

CARBONATE

10 20 30 

VOLUME PERCENT

I I T 

10 20 3(

VOLUME PERCENT

FIGURE 7. Graphs showing percentages of principal minerals in 94 modal analyses of altered pyroxenite. Frequency is 

the percentage of thin sections that have the determined volume percent of a mineral. Low values for pyroxenes in 

some thin sections indicate degree of alteration or presence of variants related to the pyroxenite body but not strictly 

classifiable as pyroxenite. Minor accessory minerals include pyrite, chalcopyrite, pyrrhotite, sericite, quartz, zoisite, 

limonite, barite, and idocrase.
THORIUM AND ASSOCIATED ELEMENTS, POWDERHORN DISTRICT, GUNNISON COUNTY, COLORADO 11

TABLE 2. Semiquantitative spectrographic analyses of melanite, 

diopside, perovskite, and magnetite from pyroxenite, complex 

of Iron Hill

[Analysts: J. C. Hamilton, and R. G. Havens. >, greater than; <, less than; leaders (—), 

analyses did not show any elements in these concentrations]

Sample No.

Amount 

(percent)

>10

7

5

3

2

1.5

1

.7

.5

.3

.2

.15

.1

.07

.05

.03

.02

.015

.01

.007

.005

.003

.002

.0015

.001

.0007

.0003

<.05

<.005

<.002

,— P8085a

Melanite

Si, Fe,

Ca

—

-__

—

—

Al

Ti

Mr

—

Mg

V

Na

Ba, Sr

Y, Zr, Nd

—

___

La

—

—

—

__-

—

___

Cr

Sc

Cu

___

Ce

—

Ga

P301

Diopside

Si, Fe,

Ca

Mg

Na

-—

--.

—

Mn

Al

.--

___

Ti, Ba

Sr, V

—

—

—

Zr

Cu

Co

La

Sc, Y

—

Ni

Be

Ce

—

Ga

P8811

Diopside

Si, Fe,

Mg, Ca

—

—

—

_-_

___

Al, Na,

Ti

—

Mn

—

Sr

V

_

——

Zr

—

Cu

Ba

Co

Pb, Sc

Y

Cr

_

Ce

—

Ga

P8326

Perovskite

Ca, Ti

—

___

Fe, Mg

—

Ce

La, Nd

...

Si, Al,

Nb, Sr

___

Mn, Pr

___

Sm

—

V, Y, Zr

—

Ba

—

Cu

___

Pb

—

Yb

—

___

__.

—

Co, Dy,

Eu, Gd

—

P8326a

Perovskite

Ca, Ti

—

Fe

—

Mg, Ce

___

La, Nd

—

Si, Al,

Nb, Sr

_

Mn, Pr

Sm

—

V, Y, Zr

—

Ba

—

Cu

___

Pb

___

Yb

—

___

—

Er, Co, Dy,

Eu, Gd

——

P8922

Magnetite

Fe, Ti

—

Mg

—

Ca

Al

K

—

__.

Si, Mn

Zn

Na, V

___

Ce

La, Nd

Co

—

Ba, Nb,

Sr

Ni, Zr

Cu

--_

Mo

——

—

Cr

--.

Er, —

——

Apatite is ubiquitous in pyroxenite and makes up as 

much as 48 percent by volume of the rock but is most 

commonly 5 to 10 percent. The apatite occurs as colorless to pale green prismatic grains, rarely as much as 

5cm long and 7 mm (millimeters) thick, commonly 

associated with magnetite (fig. SB, C), phlogopite, 

biotite (fig. SB, C, D), and vermiculite. Apatite inclusions occur in melanite (fig. 8C, D), orthoclase, pyroxene, and other minerals. The apatite is generally a 

fluorapatite with N0 1.63-1.64. Some grains of apatite 

enclose minute rutile(?) prisms alined along the c axes 

of the apatite. The P2O5 content of the pyroxenite 

averages 1.3 percent in seven analyses weighted 

according to the proportions of the types present in the 

total pyroxenite mass (Larsen, 1942, p. 20).

Biotite ranges from 0 to 52 percent by volume of the 

pyroxenite and averages about 10 percent. The biotite 

is fine to coarse grained and is closely associated with 

magnetite (fig. 8A, B) in the more homogeneous 

varieties of pyroxenite. It is moderately to strongly 

pleochroic, dusky dark brown to pale yellowish brown, 

Nz 1.63. Vermiculite, a near-surface weathering product of biotite and phlogopite, is sporadically distributed in altered pyroxenite as segregations and in

dikelike bodies (fig. 9). The vermiculite occurs in plates 

as much as 10 cm across and is commonly associated 

with coarse-grained diopside, apatite, and magnetite. 

Chlorite is present but not abundant in about 17 percent of the thin sections of pyroxenite, in amounts as 

much as 12 percent. It commonly replaces biotite or 

pyroxene and is most abundant where the pyroxenite 

has been altered adjacent to carbonatite dikes and 

veinlets.

Orthoclase and microcline are sporadically distributed but uncommon in the pyroxenite. Potassic feldspar is present in about 30 percent of the thin sections 

from the pyroxenite body, and the maximum amount 

is about 77 percent. The orthoclase is a late stage 

mineral that generally forms veins in the pyroxenite. It 

poikilitically encloses some pyroxene and apatite 

grains, and it shows varying degrees of alteration to 

sericite.

Calcite and dolomite generally average 2 to 5 percent 

by volume of the pyroxenite but may constitute as 

much as 35 percent. Some of the carbonate minerals 

that are considered primary are generally coarse 

grained and occur interstitially between grains of pyroxene, melanite, and magnetite, which show little or 

no alteration. Ocellar textures were not observed. 

Secondary carbonate minerals that are probably 

hydrothermal are generally associated with 

anastomosing carbonate veins or are adjacent to carbonatite dikes that cut the pyroxenite.

A melanite-orthoclase-apatite rock, characterized by 

abundant, euhedral melanite crystals, occurs in the 

pyroxenite in one place at the head of the north fork 

of Huntsman Gulch (NE^SW1^, sec. 35, T. 47 N., 

R. 2 W.). The melanite-orthoclase-apatite rock ranges 

from coarse to fine grained; in places the dark-brown, 

zoned melanite crystals are as much as 2 cm across and 

compose 75 percent of the rock. Between the melanite 

crystals, interstitial orthoclase makes up about 18 percent of the rock. Pale-green fluorapatite, composing 

about 5 percent of the rock, is intergrown with orthoclase and fills fractures in the melanite and orthoclase. 

Pyroxene in amounts of generally 1 percent or less occurs as inclusions in orthoclase and melanite and as 

clustered aggregates about the melanite grains. The 

accessory minerals include sphene and magnetite.

Pyroxenite is generally more radioactive than the 

surrounding Precambrian rocks because of its higher 

thorium content. It also is more radioactive than the 

mafic rocks of some other alkalic complexes, such as 

the mafic-ultramafic rocks in the northeastern part of 

the Middle Cambrian McClure Mountain Complex in 

the Wet Mountains. The pyroxenite of the complex of 

Iron Hill ranges from 0.01 to 0.08 mr/hr for 596 random 

readings with a scintillation counter, averaging 0.035 

or about IVt times the background of surrounding
12 GEOLOGY AND MINERAL RESOURCES OF THORIUM IN THE UNITED STATES

FIGURE 8.—Sketches of textures indicative of paragenetic sequence in pyroxenite, uncompahgrite, and ijolite. Minerals are A, apatite; B, 

biotite; C, calcite; M, melanite; Me, melilite; Ne, nepheline; P, pyroxene; Pv, perovskite; S, sphene; black areas, magnetite-ilmenite. A, 

pyroxenite; biotite replacing diopside (P); abundant magnetite associated with the biotite. B, pyroxenite; biotite reaction rims around 

magnetite-ilmenite grains. C, pyroxenite; cusp-cary textures and veinlets of magnetite-ilmenite in apparently older melanite. D, pyroxenite; melanite interstitial and younger than pyroxene. E, ijolite; nepheline interstitial to and replacing pyroxene. F, uncompahgrite; 

melanite along intergrain boundaries of pyroxenes; melilite inclusion in melanite. G, ijolite; caries of nepheline in melanite; calcite 

replaces nepheline and pyroxene. H, ijolite; nepheline interstitial to melanite; sphene and apatite intergrown with and included in 

melanite. /, uncompahgrite; strongly twinned, primary calcite interstitial to pyroxene and melilite; melilite interstitial to pyroxene.
THORIUM AND ASSOCIATED ELEMENTS, POWDERHORN DISTRICT, GUNNISON COUNTY, COLORADO 13

FIGURE 9.—Vermiculite-rich lens or dike in pyroxenite of the complex of Iron Hill. Coarse vermiculite plates are intergrown with 

sparse amounts of coarse diopside, magnetite, and green 

apatite.

Precambrian granite. Where carbonatite pervades the 

pyroxenite as small dikes, seams, and veinlets, the 

pyroxenite has a higher level of radioactivity. When 

such areas are added to the previous readings, the 

combined total of 775 random scintillation-counter 

readings averages 0.045 mr/hr or about twice the 

Precambrian granite background.

MAGNETITE-ILMENITE-PEROVSKITEROCK

Dikelike bodies (fig. 10) and segregations of 

magnetite-ilmenite-perovskite rock constitute a small 

part of the complex of Iron Hill; they are most abundant in pyroxenite along a west-northwest-striking 

belt north of Deldorado Creek and at the northwest 

end of the complex. Most of the dikes are less than 

60 cm thick and are traceable for less than 30 m, but 

some are 25 to 45 m thick. One of the larger magnetiteilmenite-perovskite bodies, about 45 m thick, occurs 

about 300 m southeast of the Powderhorn store and 

strikes about N 15° E. Its south end is covered by the 

alluvium of Cebolla Creek valley, and to the north it extends a short distance beneath the overlying Oligocene 

Sapinero Mesa Tuff.

The magnetite-ilmenite-perovskite rock commonly 

contains apatite and sparse biotite or phlogopite, 

augite, and idocrase. The magnetite and ilmenite occur 

in a granular intergrowth, as seen in polished sections, 

and some of the magnetite grains contain exsolution 

blades of ilmenite along (111) directions of the 

magnetite. The perovskite is very fine grained, generally less than 1 mm across, and occurs in variable 

amounts in the magnetite-ilmenite-pervoskite rock, 

locally constituting as much as 50 percent of the rock. 

The perovskite is commonly present as octahedra and 

less commonly as anhedral grains, and shows varying 

degrees of alteration to leucoxene. The perovskite is 

dusky purple, has complex penetration twinning, is 

weakly anisotropic, and has a low reflectivity and index of refraction of 2.34. Augite and phlogopite or 

biotite locally are present in isolated pods in the 

magnetite-ilmenite-perovskite rock.

A spectrographic analysis by J. C. Hamilton of 

magnetite from a magnetite-ilmenite-perovskite dike 

indicated iron as a major constituent, and amounts, in 

percent, of 3 calcium and titanium (3.9 percent TiO2 by 

chemical analysis), 1.5 silicon and aluminum, 1 magnesium and potassium, 0.5 manganese, 0.3 sodium, 0.15 

zinc and vanadium (0.14 percent V2O5 by chemical 

analysis), and, in parts per million, 500 strontium, 200

FIGURE 10.—Two subparallel dikes of magnetite-ilmenite-perovskite 

rock cutting pyroxenite.
14 GEOLOGY AND MINERAL RESOURCES OF THORIUM IN THE UNITED STATES

cerium, 150 lanthanum, barium, and neodymium, 100 

niobium, 70 copper and cobalt, 50 nickel and zirconium, 30 molybdenum, and 15 chromium.

The radioactivity of the magnetite-ilmeniteperovskite rocks at 128 localities ranges from 0.05 to 

0.25 mr/hr, or two to 12 times the background of typical Powderhorn Granite, and is attributable chiefly to 

thorium substitution for calcium in the perovskite. In 

two radiochemical analyses the perovskite contains 

0.12 and 0.15 percent ThO2. In addition to thorium, 

rare earths are present in the perovskite, and niobium 

is present in amounts of 0.17, 0.2, and 0.7 percent in 

three semiquantitative spectrographic analyses of 

perovskite concentrates by E. L. Hufschmidt. Rare 

earths, like thorium, probably substitute for calcium in 

the perovskite and apatite, whereas niobium probably 

substitutes for titanium in the perovskite lattice. 

Because of the radioactivity of the perovskite and its 

common association with magnetite and ilmenite, 

radioactivity and magnetic surveys can give an approximate indication of perovskite concentration in 

the pyroxenite areas. Ground magnetic traverses show 

magnetic highs associated with many small magnetite 

concentrations in the pyroxenite, and belts of magnetic 

anomalies can be distinguished. The weathering of the 

magnetite-ilmenite-perovskite rocks and the pyroxenite, uncompahgrite, and ijolite has left numerous 

residual accumulations of granular magnetite.

Some of the magnetite and perovskite that is disseminated in the pyroxenite evidently coprecipitated with 

the silicate phases of the pyroxenite. The magnetiteilmenite-perovskite dikes and segregations, however, 

formed chiefly after or during the late stages of pyroxenite crystallization from iron-and titanium-enriched 

fluid. They are cut by nepheline syenite dikes, and at 

least one carbonatite dike about 12 cm thick cuts the 

magnetite-ilmenite-perovskite rock, which is reddened 

adjacent to the dike. Veins of phlogopite, chlorite, and 

carbonate mineral also cut the magnetite-ilmeniteperovskite rocks in places.

UNCOMPAHGRITE

Uncompahgrite forms irregular bodies and 

apophyses in the pyroxenite, chiefly south of Road 

Beaver Creek. The uncompahgrite is a light-gray, 

white-weathering, medium- to very coarse grained rock 

(fig. 11) composed of melilite and its alteration products, variable amounts of diopside (about 15 percent), 

and small amounts of magnetite, apatite, phlogopite, 

melanite, biotite, calcite, and perovskite. Texturally 

the uncompahgrite varies from poikilitic to hypidiomorphic granular. The melilite occurs both interstitialFIGURE 11.—Outcrop of uncompahgrite, composed chiefly of 

melilite and its alteration products (light colored); dark minerals 

are magnetite, pyroxene, and melanite.

ly between diopside grains (fig. 87) and poikilitically 

enclosing diopside, magnetite, and apatite. Where the 

rock is pegmatitic the lustrous, smoky-gray melilite 

crystals are rarely as large as 50 cm in diameter. In 

thin section the fresh melilite commonly has a mosaic 

pattern with subindividuals of different optical orientation. The melilite is commonly uniaxial negative, N0, 

1.63 and Ne, 1.62. The cell dimensions of the tetragonal 

melilite are: a, 7.796 A; and c, 5.038 A±0.004 A with 

c/a, 0.646. Using the reference lattice spacings for the 

synthetic akermanite-gehlenite solid-solution series 

(Ervin and Osborn, 1949), the melilite contains about 

70 percent akermanite. According to Nash (1972, 

p. 1368-1369), microprobe analyses of several melilite 

samples indicate they consist of roughly 50 percent 

sodium melilite and 50 percent akermanite.

In thin section, myrmekitic intergrowths of melanite 

and melilite are present but rare. Melanite also forms 

thin marginal selvages about diopside (fig. 12) in some 

varieties of the uncompahgrite. Biotite commonly 

mantles the magnetite.

The radioactivity of the uncompahgrite is generally 

high. Random readings with scintillation counter at 93 

places showed a range of 0.01-0.1 mr/hr, averaging 

0.033 or about 1 V* times the background of surrounding Precambrian granite.

IJOLITE

Several small bodies of ijolite in the pyroxenite and 

uncompahgrite form an arcuate zone suggestive of a 

ring dike. Contacts of ijolite with pyroxenite are commonly gradational. The ijolite is a mafic nephelinepyroxene rock (fig. 8E, G, H) consisting of nepheline,
THORIUM AND ASSOCIATED ELEMENTS, POWDERHORN DISTRICT, GUNNISON COUNTY, COLORADO

NEPHELINE SYENITE

15

FIGURE 12.—Uncompahgrite, complex of Iron Hill. Melanite (M) 

rims on diopside (P) with interstitial melilite (Me).

aegirine-augite, melanite, and accessory calcite, 

apatite, magnetite, ilmenite, and sphene. The nepheline content varies, and locally the ijolite grades into 

urtite containing about 80 percent nepheline. In most 

of the rock the nepheline is altered to very fine-grained 

sericite, cancrinite, natrolite, and analcite, chiefly 

along fractures and grain boundaries of the nepheline. 

The ijolite is fine to coarse grained and hypidiomorphic 

granular in texture. Locally, small pyroxene-rich 

layers 2 to 5 cm thick alternate with nepheline-rich 

layers. This layering is discontinuous and does not 

appear to reflect the shape of the intrusion.

The radioactivity readings at 25 points in ijolite 

range from 0.01 to 0.06 mr/hr, averaging 0.033 or 

nearly the same as pyroxenite, uncompahgrite, and 

nepheline syenite. High radioactivity was found on the 

hill of ijolite just north of North Beaver Creek, 2.3 km 

east of Iron Hill, but is associated mainly with small 

carbonatite dikes that cut ijolite and pyroxenite near 

their contact.

Nepheline syenite intricately intrudes granite, 

fenite, and pyroxenite near the eastern end of the complex of Iron Hill. In the vicinity of the nepheline 

syenite stock and dikes, the fenite aureole is more extensively developed than at other places along the 

north side of the complex. Immediately south of the 

nepheline syenite stock, the pyroxenite is brecciated 

and veined by nepheline syenite to form a mixed 

pyroxenite-syenite rock. The nepheline syenite is cut 

by carbonatite and diabase dikes.

The nepheline syenite is light gray, weathers pinkish 

gray, and is medium to coarse grained. Orthoclase 

microperthite laths compose about 75 percent of the 

nepheline syenite and locally impart a strong trachytic 

texture to the rock (fig. 13). Aegirine-augite, melanite, 

nepheline, and the alteration products of nepheline

FIGURE 13.—Nepheline syenite, complex of Iron Hill, composed 

chiefly of lath-shaped microcline perthite (white). Melanite 

(black), aegirine-augite, and altered nepheline occur interstitially to the microcline laths.
16 GEOLOGY AND MINERAL RESOURCES OF THORIUM IN THE UNITED STATES

make up much of the remainder of the rock. Accessory 

minerals include magnetite, sphene, apatite, musco 

vite, calcite, and an unidentified zeolite. The orthoclase 

microperthite is Carlsbad twinned and shows string 

and patch perthite. The aegirine-augite, composing 

about 5 percent of the rock, is deep bluish green and 

weakly pleochroic; Z c, 14° and Nz, 1.76; NX, 1.73; 2V 

is large. In some of the nepheline syenite dikes, 

aegirine-augite crystals are oriented perpendicular to 

the walls of the dike. Nepheline is largely altered to a 

mixture of muscovite, calcite, natrolite, and cancrinite. 

The nepheline and its alteration products are interstitial to the orthoclase and make up about 10 percent of 

the rock. The brown melanite is weakly zoned and commonly has inclusions of aegirine-augite, sphene, and 

apatite.

The nepheline syenite has a radioactivity that 

averages about \Vz times the background reading of 

the Precambrian granite surrounding the complex. A 

range of 0.02-0.08 mr/hr, averaging 0.035, was obtained in 39 random readings in nepheline syenite with 

a scintillation counter.

At the east end of the complex, near the head of 

North Beaver Creek in sections 7 and 12, the pyroxenite is brecciated and cut by thin fracture fillings and 

small dikelike bodies of nepheline syenite (fig. 14) to 

form a mixed rock. Nepheline syenite forms a small, irregular stock north of this area of mixed rock and was 

probably forcibly intruded into the pyroxenite to cause 

brecciation and some mixing. The pyroxenite part of 

the mixed rock consists chiefly of anhedral pyroxene, 

zoned melanite, sphene, apatite, and sparse amounts of 

altered melilite. The nepheline syenite is leucocratic

FIGURE 14.—Mixed pyroxenite-syenite rock from the complex of 

Iron Hill. The pyroxenite is brecciated, and syenite fills the fractures and locally replaces pyroxenite.

and consists chiefly of Carlsbad-twinned orthoclase 

and minor amounts of melanite, aegirine-augite, and 

altered nepheline.

CARBONATITE OF IRON HILL

Dolomitic carbonatite (rauhaugite) is the youngest 

member in the intrusive series of the alkalic complex of 

Iron Hill. It occurs as a stock or plug and as numerous 

dikes both within and outside the complex. The carbonatite stock is roughly elliptical in outcrop, the long 

axis striking N. 35° W., and its 4-km2 area is about 12 

percent of the outcrop area of the entire complex.

Carbonatite resists weathering more than the surrounding alkalic rocks of the complex, and Iron Hill 

rises about 300 m above the floor of the valley of 

Cebolla Creek or Road Beaver Creek (fig. 15) and above 

the adjacent pyroxenite. The resistance of the carbonatite to weathering is attributable to the semiarid 

climate; the jaspery, ferruginous capping or mantle 

that formed locally on the dolomite by weathering 

processes; and the cementation of fractures and other 

zones of weakness in the carbonatite by secondary 

quartz, carbonates, and iron and manganese oxides.

The carbonatite is generally well exposed except for 

the more highly timbered east- and north-facing slopes 

where talus has been stabilized and has accumulated to 

thicknesses of 6 m or more. Contacts of the carbonatite 

stock with the Precambrian country rocks and the 

alkalic intrusives of the complex are poorly exposed. 

The best exposures of the contact are near the north 

end of the stock, along the south side of Huntsman 

Gulch and along the east side of the valley of Cebolla 

Creek about midway between Deldorado Creek and 

Huntsman Gulch. The felsite and granite at these localities are generally sheared, reddened, and fenitized, 

and the contacts are sharp and discordant. South of 

Road Beaver Creek the carbonatite-pyroxenite contact 

is partly concealed by alluvium. Locally along the 

southern margin of the carbonatite stock there are

^S5E—«fc_

FIGURE 15.—Iron Hill from the southeast.
THORIUM AND ASSOCIATED ELEMENTS, POWDERHORN DISTRICT, GUNNISON COUNTY, COLORADO 17

phlogopite-rich zones in the carbonatite that may represent xenoliths of pyroxenite that have been partly 

replaced by carbonate minerals and phlogopite owing 

to reaction with the carbonatitic fluid. Near the carbonatite, the pyroxenite is highly veined by carbonates, and the pyroxene is highly altered to biotite, 

chlorite, or sodic amphiboles in a mixed zone about 

100-300 m thick of thin carbonatite and pyroxenite 

layers (Temple and Grogan, 1965, p. 681-682).

The west margin of the complex is largely buried by 

the alluvium of Cebolla Creek, but at one place on the 

hillside east of Cebolla Creek the contact is with 

sheared granite and felsite, indicating that the carbonatite stock of Iron Hill was emplaced between 

pyroxenite on the northeast and southeast and sheared 

granite and felsite on the southwest. Numerous radial 

dikes of carbonatite, which cut all the other rock types 

of the complex, are lithologically similar to the carbonatite stock of Iron Hill and increase in number 

toward the stock, indicating a genetic relationship.

The carbonatite stock is complex in its internal 

structures and textural features. These structures and 

textures include grain sizes ranging from fine to 

pegmatitic; varying degrees of development of foliation, drag folds, and shear zones; and the presence of 

partially replaced xenoliths of other rocks near the 

margins of the body. Most of the internal structures 

are considered primary, formed during or shortly after

FIGURE 16.—Strongly foliated carbonatite from Iron Hill. 

Phlogopite and apatite-rich lenticles are locally jasperized and 

are etched into relief by weathering.

intrusion; examples are the foliation surfaces or shear 

planes along which apatite, phlogopite, or barite have 

been deposited. Iron or manganese oxides and jasper 

deposited along joints or other structural features, 

however, are considered secondary.

The foliation in the carbonatite is well developed, 

and dips are generally steep to vertical. The foliation is 

commonly imparted by the presence of apatite-rich 

seams or segregations, phlogopite-biotite seams, or by 

strained and deformed carbonate crystals (fig. 16). Between Road Beaver and Deldorado Creeks the foliation 

surfaces strike dominantly northeast, but a few strike 

north-northwest along the western margin of the carbonatite. North of Deldorado Creek the strike of the 

foliation is dominantly northwest but is locally northeast. The pattern of foliation is generally arcuate and 

convex to the northeast. At the south end of the carbonatite stock the foliation is discordant to the inferred contact of the carbonatite.

Folds within the carbonatite are not common. At one 

place along Deldorado Creek, a well-exposed 8-cm layer of coarsely crystalline carbonatite is folded into a 

small drag fold, indicating some deformation after 

emplacement.

Texturally the carbonatite is heterogeneous; coarsegrained and pegmatitic varieties are in sharp contact 

with finer grained carbonatite. In some places this 

sharp change in texture can be attributed to dikes of 

coarsely crystalline carbonatite cutting finer textured 

carbonatite. Generally the coarse-grained carbonatite 

contains less silicate minerals, sulfides, apatite, and 

pyrochlore than the finer textured varieties.

Mineralogically, the carbonatite stock consists chiefly of ankeritic dolomite and minor amounts of apatite, 

phlogopite, biotite, barite, pyrite, calcite, magnetite, ilmenite, pyrochlore, and sodic amphiboles. Hematite, 

manganese oxide, and reddish-brown chalcedonic 

jasper or quartz occur as secondary minerals. Goethite 

and hematite pseudomorphs after pyrite and magnetite are common. Pyrolusite forms coatings on joints 

or vugs in the carbonatite. Apatite, which makes up a 

few percent to locally 10 percent or more of the carbonatite, is generally a colorless or pale green fluorapatite with N0,1.63-1.64. The apatite varies from stubby 

prisms to strongly oriented prismatic aggregates that 

tend to be localized along foliation surfaces. Much of 

the apatite appears to be late, as veins of apatite cut 

the carbonatite or are localized along intergranular or 

cleavage surfaces of the dolomite crystals. Eight 

chemical analyses of carbonatite showed a P2O5 content from 0.13 to 4.34 percent (Hedlund and Olson, 

1980, table 8A). Spectrographic analyses of two 

apatite concentrates from carbonatite are shown in 

table 3.
18 GEOLOGY AND MINERAL RESOURCES OF THORIUM IN THE UNITED STATES

TABLE 3.—Semiquantitative spectographic, X-ray fluorescent, 

fluorometric, and radiochemical analyses of pyrochlore and 

apatite from carbonatite, Iron Hill

[Analysts: J. C. Hamilton. N. M. Conklin, E. J. Fennelly, and A. P. Marranzino. >, 

greater than; <, less than; leaders (—). analyses did not show any elements in these 

concentrations]

Amount 

(percent) Pyrochlore Apatite Apatite

7

3

2

1.5

.7

.5 

.3 

.15

.07

.05

.03

.015

.007

Nb(33) 1 

Ca

^Th(1.32), Fe, Ti 

Na, Ba, Sr

Fe, Zr

Si, La, Nd

Pr, Sm

Ca, P

Mg

Si, Fe, Ce, Nb

Na, Ti, Sr, Nd 

Al, Mn, La

Ca, P 

Nb 

Si, Mg

Fe, Na, Ce, Sr

Ti, La, Nd

Mn

Mg, Pb, 1 Ta(0.085) —

Mn, 3 U(0.055)

Eu Ba Ba, Y

Zr, Eu, Dy, Gd, Y Zr, Eu, Dy, Gd 

Er Er

.005 Ag

<.005

.003

.0015

.0007 Be

—

V

Cu,

Cr,

Co

Ho

Ni, Sn, Yb

—

V

Pb,

Ni,

Cr,

Cu,

Yb

Co

Ho, Sn

By X-ray fluorescent analysis. 

jrBy radiochemical analysis. 

By fluorometric analysis.

Pyrite occurs widely in the carbonatite as disseminated cubes as much as 1.2 cm across, rare pyritohedrons, and as vein fillings. Some of the pyrite grains 

are poikilitic, containing inclusions of apatite. Other 

pyrite grains are fractured and veined by secondary 

quartz.

Barite is disseminated in minor amounts throughout 

the carbonatite stock and more abundantly in the carbonatite dikes. The barite grains are commonly interstitial to the dolomite, but some barite occurs as 

veinlets, generally having formed later than the 

dolomite. In nine analyses of carbonatite the barium 

content ranges from 0.037 to 4.9 percent; the strontium content ranges from 0.007 to 0.02 percent 

(Hedlund and Olson, 1980, table 8B).

Phlogopite and sparse amounts of bluish-green 

hydrobiotite(?) are disseminated sporadically within 

the carbonatite and also occur as lenticular aggregates. Along the south edge of the carbonatite the 

phlogopite and associated carbonates locally appear to 

replace a pyroxenite xenolith. Much of the phlogopite 

shows strong reverse pleochroism, one or two outer 

zones showing reversed positions for the pleochroic 

colors from that of the cores. Generally the cores of the 

phlogopite plates have a stronger birefringence than 

the margins. Around individual dolomite inclusions in 

the phlogopite there is a similar zone of reversed 

pleochroism. The Nz value of the cores is 1.613,

whereas that of the marginal resorption zones is 1.614 

(Larsen, 1942, p. 53). Such reversed pleochroism has 

been noted in the dark mica of other alkalic rock complexes and was first described at Iron Hill by Larsen 

(1942, p. 53). The reversed pleochroism of the dark 

mica was discussed by Nash (1972, p. 1365), who found 

that the significant chemical change between the two 

types of micas is the decrease of aluminum in the 

margin with a concomitant increase of iron.

Magnetite is present in small amounts in the carbonatite stock and is partly replaced by hematite to 

form martite. In one place at the southwest margin of 

the stock, 2- to 5-cm layers of carbonatite containing 

abundant magnetite octahedra alternate with biotitic 

carbonatite in an outcrop area of only a few square 

meters. The magnetite grains are as much as 1 cm 

across and are intergrown with ilmenite.

A computation of the normative carbonate minerals 

from five analyses (Hedlund and Olson, 1980, table 8A) 

of carbonatite from the Iron Hill stock indicates that 

the CaCO^MgCOg ratio is 1.32 whereas the ratio for 

pure dolomite is 1.185. This difference is thought to 

reflect the substitution of FeCO3 and MnCO3 for 

MgCO3 in the dolomite lattice, which is also indicated 

by the presence of ferric and manganese oxides along 

the margins and cleavage surfaces of individual 

dolomite grains in the weathered carbonatite. About 

30 X-ray diffraction patterns of carbonatite material 

from the Iron Hill stock showed predominantly dolomite with very little calcite or other carbonates.

Green, pink, purple, lemon-yellow, and gray pyrochlore is disseminated in minor amounts throughout 

the dolomitic stock and dikes as tiny octahedral 

crystals generally less than 3 mm across (fig. 17). 

Under the microscope the pyrochlore shows faint 

anisotropism, and the crystals are weakly zoned. 

Numerous opaque inclusions related to certain zones 

are present in some of the grains. The unit cell of the 

pyrochlore is 10.435±0.002 A. The pyrochlore is 

radioactive but not metamict. The radioactivity varies 

considerably. In one analyzed sample the pyrochlore 

contains 0.055 percent uranium, 1.32 percent thorium, 

33 percent niobium, and only 0.085 percent tantalum 

(table 3). Microprobe analyses of two pyrochlore 

samples by Nash (1972, p. 1369) indicate 62.5 and 65.4 

percent Nb2O5, 2.36 and 0.31 percent ThO2, and negligible UO2.

The dolomitic carbonatite (rauhaugite) stock of Iron 

Hill is generally more radioactive than other igneous 

rocks of the complex but is less radioactive than the 

carbonatite dikes. Field readings of radioactivity in 

places exceed 0.1 mr/hr or five times the background 

of nearby Powderhorn Granite, and samples mostly 

range from 3 to 63 ppm thorium. An average of 173 

gamma ray spectrometric measurements made by
THORIUM AND ASSOCIATED ELEMENTS, POWDERHORN DISTRICT, GUNNISON COUNTY, COLORADO 19

FIGURE 17.—Pyrochlore and apatite in carbonatite, Iron Hill. 

Euhedral to subhedral pyrochlore (black) in dolomitic carbonatite containing abundant small, rounded grains (high relief) of 

accessory apatite.

Armbrustmacher (1980; Armbrustmacher and Brownfield, 1979) was 41 ppm ThO2. The radioactivity of the 

carbonatite stock is probably due chiefly to thoriumbearing pyrochlore (table 3), although other thoriumbearing minerals may also be present.

CARBONATITE DIKES

Several hundred carbonatite dikes were mapped in 

pyroxenite and other rocks in and just outside of the 

complex. They are distributed radially about the carbonatite stock of Iron Hill although there is a great 

preponderance of N. 45 °-85 ° W. strikes parallel to the 

long dimension of the complex. Dikes and plugs of carbonatite cut all other alkalic rocks of the complex and 

also less commonly cut the surrounding Precambrian 

rocks. The dike orientations show a similarity to the 

directions of strong jointing in the adjacent alkalic 

rocks. This similarity suggests that fractures that 

developed in the complex prior to or during the intrusion of the Iron Hill carbonatite stock influenced the 

orientation of the dikes.

The carbonatite dikes commonly have sharp contacts with the wall rocks. Many are about 1 to 1.5 m 

thick, but some are as much as 45 m thick. Most are 

fairly homogeneous, but layered and brecciated dikes 

are also present. The layering parallels the walls of 

some dikes and is commonly due to the alternation of 

sideritic layers with calcitic or dolomitic layers or to 

the alternation of silicated carbonatite layers with 

nonsilicated carbonatite. The siderite-rich layers commonly weather dark gray or black, which sharply contrasts with the light brown color of the weathered dolomite. The silicated carbonatite commonly contains 

abundant phlogopite and biotite, and some of the 

micas are altered to a bluish-green hyclrobiotite(?). The 

brecciated texture of some carbonatite dikes is probably related to the forcible injection of carbonatitic 

fluids. Some fragments consist of partially replaced 

pyroxenite inclusions characterized by carbonate 

pseudomorphs after pyroxene and thin selvages of 

phlogopite about the margins of the former pyroxene 

crystal.

Carbonatite dikes also cut the Precambrian rocks 

outside the complex of Iron Hill. At one locality, about 

0.5 km N. 46° W. from the Powderhorn Post Office, 

there are inclusions of felsite within a carbonatite dike. 

Foliation or flow structure within the carbonatite 

wraps about the largest inclusion (figs. 18 and 19), 

which suggests flowage about the inclusion after the 

carbonatite was largely crystallized.

Most carbonatite dikes in Precambrian wall rocks 

are within 1.2 km of the margin of the pyroxenite of the

FIGURE 18.—Inclusion of felsite in carbonatite dike. Position of inclusion and foliation hi carbonatite are shown hi figure 19.
20 GEOLOGY AND MINERAL RESOURCES OF THORIUM IN THE UNITED STATES

FOLIATED DOLOMITIC 

CARBONATITE DIKE

XENOLITH OF

SERICITIZED FELSITE

SHEARS^ \

SE

SERICITIZED FELSITE 

PORPHYRYx

Approximately 14 m

FIGURE 19.—Sketch of felsite xenoliths in carbonatite dike (fig. 18), 

0.5 km west-northwest of Powderhorn Post Office. Foliae within 

the carbonatite wrap about the xenoliths.

complex of Iron Hill. In the Gateview quadrangle, only 

two small carbonatite dikes were found, 0.3 and 1.2 m 

thick. Of these the most distant carbonatite dike from 

the complex of Iron Hill was noted on the east side of 

Goose Creek near the old Dubois mine. It is calcitic 

and coarse grained, contains accessory orthoclase and 

pyrite, and is slightly radioactive. A semiquantitative 

spectrographic analysis of the carbonatite (Hedlund 

and Olson, 1980, table 3, G2630) shows a suite of 

elements similar to dikes in or near the complex.

In decreasing order of abundance, the minerals in the 

carbonatite dikes are dolomite, calcite, siderite, biotite, 

phlogopite, pyrite, barite, apatite, orthoclase, pyrochlore, monazite, chalcopyrite, sphalerite, galena, 

fluorite, synchysite, and parisite. The siderite occurs in 

a few dikes as distinct layers or segregations. The 

micas are disseminated and also form lenticular aggregates. The sulfide grains, especially pyrite, tend to 

be in clusters and are most abundant in the sideritic 

carbonatite dikes. Pyrite is the most common sulfide 

and is in cubes that show varying degrees of oxidation 

to goethite or hematite. The sphalerite contains exsolution blebs of chalcopyrite, and a few chalcopyrite 

grains occur outside the sphalerite host. Galena is 

present in very minor amounts associated with the 

sphalerite.

Eight partial chemical and 14 spectrographic 

analyses of carbonatite samples from various occurrences, tabulated by Hedlund and Olson (1980, table 

8A-8B), show a CaCOs/MgCO3 ratio ranging from 1.79 

to 5.18. These include samples from eight localities in 

the carbonatite stock of Iron Hill, three from carbonatite dikes, and one from a small pluglike body. 

The three carbonatite dike samples are atypical 

variants of carbonatite containing calcite, siderite, and 

FeCO3-rich dolomite. They differ considerably in composition from the other samples, having higher FeCO3 

content and lower M CO3 content, and this difference 

probably indicates fractionation of the carbonatitic 

fluids.

The carbonatite dikes are the most radioactive rocks 

in the alkalic complex of Iron Hill. Of 214 dikes in the

Powderhorn quadrangle, for example, 132 have radioactivities ranging from 0.05 to 0.79 mr/hr or two to 35 

times the background of typical Powderhorn Granite. 

Thirty samples of the dikes contain about 30 to 3,200 

ppm thorium. The more radioactive dikes commonly :| 

have a crudely layered structure, weather chocolate 

brown, and contain dolomite, siderite, calcite, barite, 

pyrite, apatite, hydrobiotite(?), and small amounts of 

monazite, pyrochlore, bastnaesite, and synchysite. The 

weakly radioactive carbonatite dikes are relatively 

homogeneous, weather buff or rusty yellow, and contain dolomite, calcite, apatite, pyrite, hydrobiotite(?), 

and sparse barite.

No mineral in which thorium is a major constituent 

is known in the carbonatite dikes. Some thorium is 

present in the sparse monazite, which contains about 

0.15 percent thorium, and in pyrochlore. Monazite is 

more common in the carbonatite dikes than in the carbonatite stock. The monazite is very fine grained, reddish brown, resinous, and where identified is mostly 

associated with barite. Rare earths in the carbonatites 

range from a trace to about 1.5 percent and occur in 

monazite, in the fluocarbonates synchysite and bastnaesite, and also substitute for calcium in apatite and 

pyrochlore.

A few localities for uncommon minerals identified by 

X-ray are: bastnaesite and synchysite in a carbonatite 

dike just east of Iron Hill in SE1^, sec. 12, T. 46 N., R. 

2 W.; monazite in a carbonatite dike from NVfe, sec. 11, 

T. 46 N., R. P/2 W., and rutile from one in SE1^, sec. 12, 

T. 46 N., R. IVfc W.; galena in a carbonatite dike in NVfe, 

sec. 11, T. 46 N., R. IVfc W., and sphalerite in a carbonatite dike in SE1/^ sec. 12, T. 46 N., R. 2 W.

IGNEOUS ROCKS AND VEINS OUTSIDE THE 

COMPLEX OF IRON HILL

TRACHYTE PORPHYRY DIKES

Pink or red, porphyritic to aphanitic trachyte dikes 

occur chiefly in the Powderhorn and Rudolph Hill 

quadrangles. They cut most Precambrian rocks but do 

not cut any alkalic rocks, diabase, or rocks younger 

than the Precambrian. The trachyte dikes are commonly 1.5 to 3 m thick but may be as much as 25 m 

thick. The dikes show some branching and sharp 

changes in strike, follow the local joint pattern, and are 

clearly structurally controlled. Many have low dips 

toward the complex of Iron Hill.

The trachyte is composed of scattered, alined, stubby to lath-shaped red orthoclase phenocrysts commonly 3 to 5 mm long set in a trachytic, aphanitic 

groundmass consisting of alined orthoclase microlites. 

The orthoclase contains abundant dusty ferric oxide 

inclusions. Some of the trachyte dikes are autobrec-
THORIUM AND ASSOCIATED ELEMENTS, POWDERHORN DISTRICT, GUNNISON COUNTY, COLORADO 21

elated, as angular fragments of coarsely textured 

trachyte occur in a fine-grained trachytic groundmass. 

The fragments generally consist of anhedral aggregates of coarse-grained orthoclase. Granite 

fragments have been observed as rare inclusions in 

some of the trachyte. Mafic minerals are rare in the 

trachyte and are chiefly interstitial iron oxides.

Some of the trachyte dikes are radioactive, generally 

about four times that (0.02 mr/hr) of nearby felsic 

metavolcanics; readings of 0.04 to 0.08 mr/hr are common. Most trachyte dikes are relatively low grade; 

hence, their contribution to ThO2 resources (table 5) is 

small. The trachyte dikes resemble some of the thorite 

veins in that they contain abundant red orthoclase, but 

the igneous origin of the dikes is indicated by the porphyritic texture. Locally the wall rocks are replaced by 

orthoclase in a thin zone resembling fenite along the 

margins of some of the trachyte dikes. Freshly broken 

trachyte dike rock commonly has a fetid odor resembling garlic that also characterizes the thorium veins 

and some fenite.

FINE-GRAINED GRANITE

Fine-grained granite of uncertain age occurs within 2 

km south and west of the complex of Iron Hill and is 

anomalously radioactive. The granite occurs in numerous dikelike bodies, irregular small plutons about 

150-750 m wide, and two larger bodies 3-4 km by 

600-1,200 m. The granite is mostly fine grained, some 

is medium grained, and locally it is porphyritic. It is 

composed of microcline microperthite, quartz, plagioclase, and biotite. Associated with the granite near 

Road Beaver Creek are two bodies of melasyenite 

60-120 m wide.

The fine-grained granite was considered by Hunter 

(1925) to be a variety of the Powderhorn Granite of 

Precambrian age. On the geologic maps of the Rudolph 

Hill (Olson, 1974) and Powderhorn (Hedlund and 

Olson, 1975) quadrangles, however, it is shown as a 

separate Precambrian(?) unit. It appears relatively 

little deformed in comparison with adjoining gneissic 

coarse-grained Powderhorn Granite. The granite is 

fenitized in places, as shown by feldspathization and 

blue-green amphibole veinlets, and it is cut by carbonatite, lamprophyre, and thorium veins that are 

probably related to the complex of Iron Hill. Some of 

the fine-grained granite occurs in crudely arcuate 

plutons around the southern part of the complex of 

Iron Hill. These forms, which are suggestive of ring 

dikes, together with the locally high radioactivity, may 

indicate a genetic link between fine-grained granite 

bodies and the complex.

The fine-grained body west of Cebolla Creek near the 

north edge of the Rudolph Hill quadrangle (Olson, 

1974) is at least 4 km long but is covered by Tertiary 

welded tuff at the south end. This granite is cut by 

many thorium-bearing veins but is not in itself highly 

radioactive where studied. Another granite pluton that 

crosses Cebolla Creek 5 km south of Powderhorn is 

about 3.3 km long and 1.2 km wide. It is strongly 

fenitized near the complex of Iron Hill. The radioactivity has been measured in only a few places. In the roadcut at the south end of the valley of Cebolla Creek, 

measurements with a gamma-ray spectrometer in 

slightly fenitized granite suggest a possible range of 

40 to nearly 300 ppm ThO2. This thorium-bearing part 

of the granite, which may average on the order of 100 

ppm ThO2, appears at least 120 m wide, but it has not 

been delineated and may be only a minor part of the entire granite body.

The fine-grained granite bodies on the canyon sides 

of Road Beaver Creek, 0.5 to 2 km south of the margin 

of the complex of Iron Hill, have been explored by 

numerous bulldozer trenches. These granites are in 

sees. 24 and 25, T. 46 N., R. 1V4 W., and are shown on 

the map of the Rudolph Hill quadrangle (Olson, 1974). 

In this area five granite bodies are 300 to 1,000 m in 

exposed length and 100 to 500 m wide. Within the 

plutons the radioactivity of the fine-grained granite 

varies considerably. Samples from some of the more 

radioactive, small, local concentrations exceed 1,000 

ppm in thorium. Considerable parts of the granites, 

however, have thorium contents on the order of 100 to 

225 ppm, as judged by eight samples and hundreds of 

field gamma-ray spectrometer readings. The eight 

samples of the granite analyzed in the laboratory range 

from 32 to 281 ppm thorium and average 115.1 ppm 

(131 ppm ThO2). Areas that have the most radioactivity are typically characterized by red and yellow-brown 

limonitic staining and by narrow, black, siliceous, 

rusty-weathering veinlets along joint or shear planes, 

suggesting transport and deposition of thorium by 

iron-bearing solutions.

A spectrographic analysis of a sample of the granite 

containing abundant limonite veinlets showed, in 

weight percent, 0.7 phosphorus, manganese, barium, 

cerium, and lanthanum; 0.3 neodymium and strontium; 0.07 titanium and praseodymium; 0.03 gadolinium, samarium, and yttrium; 0.015 niobium, lead, 

and vanadium; and 0.007 dysprosium and zirconium. 

Thorium was determined radiochemically to be 0.08 

percent in this sample. The rare earths are unusually 

abundant, particularly the light or cerium group of 

rare earths; the niobium content is also anomalous 

relative to other granites in the area but is not as high 

as in some of the alkalic rocks.
22 GEOLOGY AND MINERAL RESOURCES OF THORIUM IN THE UNITED STATES

THORITE VEINS

About 250 thorium veins are shown on geologic 

quadrangle maps of the Powderhorn district and are 

illustrated on figure 20. Of these about 200 are in the 

Gateview (Olson and Hedlund, 1973) and Powderhorn 

(Hedlund and Olson, 1975) quadrangles and the 

remainder in the Rudolph Hill (Olson, 1974) and Carpenter Ridge (Hedlund and Olson, 1973) quadrangles.

The thorite veins contain the highest grade thorium 

concentrations in the district. The ThO2 content of the 

thorite veins ranges from less than 0.01 percent to as 

much as 4.9 percent in high-grade samples. The ThO2 

content is generally less than 1 percent, however, and 

is only 0.05 to 0.1 percent in many of the veins examined in the district. The veins are as much as 1 km 

in length, but most cannot be traced for more than 

100 m. They range in thickness from 1 cm or less to 

5 m but most are less than 0.6 m thick. They commonly occur in steeply dipping shear or breccia zones 

within the Precambrian rocks. In one of the principal 

areas, near the north end of Huntsman Mesa, 3 to 8 km 

north of the complex of Iron Hill, most of the veins 

strike slightly north of east. In the Sapinero MesaGoose Creek area, in the northwestern part of the 

district, however, the veins generally strike northwest. 

The veins are generally discordant to the foliation of 

hornblende schist, felsite, quartz-biotite schist, 

granite, syenite, and other rocks, the type of host rock 

apparently having little effect on the localization of the 

veins.

The thorium veins are composed of thorite, white to 

smoky quartz, jasper, pink potassic feldspar, calcite, 

specular and earthy hematite, goethite, magnetite, 

barite, and pyrite, and sporadic galena, chalcopyrite, 

sphalerite, synchysite, apatite, fluorite, biotite, 

sodic amphibole, rutile, monazite, vanadinite, and 

bastnaesite.

Pink to red, poorly twinned orthoclase is common in 

the veins as large laths or as clustered subhedral to 

anhedral grains. Minor amounts of microcline are present in some veins. Adjacent to most veins the wall 

rocks have been partly replaced by pink to red orthoclase, and this fenitelike replacement is conspicuous 

where hornblende schist wall rocks are reddened and 

altered to biotitic and chloritic rocks alongside the 

veins. Some breccia fragments of feldspathized wall 

rocks have been incorporated in the veins. Thin feldspathic fracture fillings 1 to 3 cm thick in Precambrian 

melasyenite or other rocks are locally radioactive, 

giving readings of as much as 0.06 mr/hr or three times 

background.

The chief thorium mineral is thorite, much of which 

is partly metamict but shows an X-ray pattern for 

thorite (or thorogummite). The thorite is very fine 

grained and is intergrown with ferric oxides. A

thorium-bearing phosphate with rhabdophanelike 

structure has been found locally in the district (Dooley 

and Hathaway, 1961, p. 340). Monazite has been found 

in several veins.

Pink to white barite is present in small amounts in 

many of the thorium veins and is a major constituent 

in a few of them. At one locality on the ridge west of 

Goose Creek a discontinuous vein of massive, light 

pink barite 60 cm thick cuts the hornblende schist. 

Barite occurs in at least two generations in relation to 

orthoclase. Some early formed barite is intergrown 

with quartz. Other grains occur as inclusions in orthoclase and are partly replaced by orthoclase. Most 

barite appears later than the potassium feldspar, however, and veinlets of barite in orthoclase are common, 

especially along cleavage surfaces in the feldspar. The 

pink to red color of some barite may to be due to minute inclusions of ferric oxide observed in thin section.

Orthoclase and quartz formed early as primary 

minerals in the veins. Quartz occurs in at least two 

generations. The early quartz, some of which is smoky, 

occurs as anhedral and euhedral grains intergrown 

with orthoclase or barite. The later generation of 

quartz is commonly chalcedonic quartz that occurs as 

pseudomorphs after orthoclase or other silicates and as 

vug fillings. Much of this secondary quartz is associated with ferric oxides.

Carbonate minerals, chiefly calcite or ankeritic 

dolomite, are present in many of the thorite veins. 

They commonly appear to replace silicates, quartz, and 

barite, and are typically associated with ferric oxides. 

In some veins the carbonate minerals are present as 

pseudomorphs after prismatic silicate minerals. 

Coarsely crystalline calcite fills vugs in some veins. 

Some rare carbonate veinlets have been observed to 

cut the jasper and are believed to have formed at the 

time of Oligocene volcanism, unlike the bulk of the carbonate, which is of probable Cambrian or late Precambrian age.

Apatite and fluorite are present as rare constituents 

in a few veins. Apatite was noted as minute inclusions 

in orthoclase.

Specular hematite is not abundant but is conspicuous in some veins. It forms veinlets that cut early 

formed orthoclase, barite, and quartz. Earthy hematite 

and goethite are weathering products of iron-bearing 

minerals such as pyrite and specular hematite. In some 

small areas, numerous dispersed, thin, red hematitic or 

limonitic feldspathized veinlets and seams occur, locally causing a radioactivity of two to three times 

background. Red staining due to hematite accompanies some thorium mineralization and has encouraged some exploration of red rocks, but hematitic 

veinlets are also abundant in some places that are low 

in radioactivity.
THORIUM AND ASSOCIATED ELEMENTS, POWDERHORN DISTRICT, GUNNISON COUNTY, COLORADO

107°15'00" 7 -30"

23

38°22'30"

38°15'00"r

Carpenter Ridge 

quadrangle

"Povs/derhorn 

/•qtTadrangle

—— Deposit containing probably more than 1 ton Th02- Dotted where 

low grade (generally less than 250 ppm). Placement shows strike

- Deposit containing less than 1 ton Th02 or unevaluated. Dotted 

where low grade (generally less than 250 ppm). Placement 

shows strike

o Deposit of uncertain trend containing probably more than 1 ton 

Th02 and of low grade (generally less than 250 ppm)

• Deposit of uncertain trend containing less than 1 ton Th02 or 

unevaluated. Open circle where low grade (generally less than 

250 ppm)

0 4 MILES 

1

1

6 KILOMETERS

FIGURE 20.—Map showing distribution and trend of thorium deposits, Powderhorn district.

The thorite is so fine grained and sparsely dis- I difficult to establish. Because thorite is commonly 

tributed that its place in the paragenetic sequence is I associated with earthy hematite and goethite, it may
24 GEOLOGY AND MINERAL RESOURCES OF THORIUM IN THE UNITED STATES

be about the same age as the specular hematite and 

formed fairly late in the paragenetic sequence.

Biotite and sodic amphiboles are sparsely 

distributed in many thorite veins. Some of the biotite 

represents detached parts of biotitic wall rocks or 

formed by replacement of hornblendic wall-rock 

fragments. The sodic amphiboles, which have nearly 

parallel extinction and high birefringence, occur as 

radiating clusters or as aggregates of prisms in the 

veins.

Pyrite, chalcopyrite, galena, and minor sphalerite 

are present but not generally abundant in the thorite 

veins. The sulfide minerals are generally later and occur as veinlets filling fractures in orthoclase and barite 

and occupying interstices between barite grains. The 

chalcopyrite forms exsolution blebs in sphalerite and 

also occurs as separate grains. Galena is intergrown 

with sphalerite.

Varicolored jasper, mostly yellowish to reddish 

brown, has generally replaced the carbonate gangue of 

the veins on Sapinero Mesa and similar areas where 

the veins were near the surface that was covered by 

volcanic rocks in Oligocene time. The jasper is found to 

at least 30 m below the mesa level, and oxidation extends locally to at least 60-m depth. Within the deep 

canyons of the Lake Fork of the Gunnison River and 

Cebolla Creek, however, jasperization and oxidation 

have been less intense, and unaltered sulfide minerals 

persist in a few of the veins.

The veins have been altered and leached during 

several cycles. An earlier one is related to the surface 

on which the Junction Creek Sandstone of Jurassic age 

was deposited and may have resulted in some shallow 

jasperization of carbonate mineral in the vein. Later 

periods of oxidation and leaching are related to the 

Oligocene surface, on which the volcanic rocks were 

deposited, and also to the present cycle of weathering. 

Most of the silicification is probably a hydrothermal 

replacement localized near the ancient Oligocene surface just beneath the base of the flows.

The effects of oxidation, leaching, and jasperization 

of the veins on the original thorium content are not 

clearly known because few workings have penetrated 

more than 10 m in depth. Jasperized veins on the mesa 

surfaces show radioactivity in both jasper and carbonate gangue. Locally the jasper forms pseudomorphic replacements of the carbonate and has apparently 

retained some dispersed thorium. Oxidation, leaching, 

and jasperization might result in some enrichment or 

diminution in grade of near-surface vein material, but 

without sufficient subsurface data on possible changes 

in grade, the surface exposures are assumed to be 

reasonably representative of the grade of vein material 

at depth.

The following minerals probably occur in numerous 

other veins, but a few localities are listed. Bastnaesite 

and synchysite, for instance, have been identified by 

X-ray analysis from a vein in SE1^ sec. 2, T. 47 N., R. 3 

W. Bastnaesite occurs in a vein near Razor Creek in 

NWV4 sec. 27, T. 47 N., R. 3 E. and synchysite in a vein 

in NE1/* sec. 20, T. 47 N., R. 2 W. Monazite and vanadinite were identified in a vein in NWV£ sec. 2, T. 47 N., 

R. 3 W., and rutile in veins in SEVi sec. 2, T. 47 N., R. 3 

W., and NWV4 sec. 8, T. 47 N., R. 2 W. Galena and 

cerussite were found in a vein in NEV£ sec. 20, T. 47 N., 

R. 2 W., and galena in veins in NWx/4 and SEV* sec. 2, 

T. 47 N., R. 3 W.

DIABASE AND GABBRO DIKES

Several northwest-trending gabbro and diabase 

dikes occur in a narrow zone that cuts across the 

alkalic-rock complex. Similar dikes also occur in 

Precambrian rock elsewhere in the district. The gabbro 

and diabase dikes have the lowest radioactivity of the 

major rock types in the immediate area. They are as 

much as 150 m thick and 6.7 km in length. They are 

predominantly tholeiitic, but within the complex, one 

nepheline-bearing gabbro also occurs (Larsen, 1942, 

p. 28-29).

Most of the dikes have a diabasic texture and are 

referred to as diabase in this discussion. The diabase 

dikes are black, commonly weather rusty brown, locally show spheroidal weathering, and are medium 

grained with an intergranular hypidiomorphic to subophitic texture. Augite grains partly mantle the labradorite (An52_55) laths and partly occupy interstices between labradorite grains. The labradorite laths show 

oscillatory-normal zoning. The augite commonly is 

very pale purple and has only faint pleochroism; Z c, 

47°; Nz, 1.715; NX, 1.690; 2V = 65°±5°. Augite constitutes about 25 percent by volume of the diabase; 

some augite grains show incipient alteration to chlorite 

and biotite. Magnetite and ilmenite(?) occur as aggregated granules and skeletal crystals that may be as 

much as 25 percent of the diabase.

Three diabase dikes crop out in the complex of Iron 

Hill and are clearly younger than the rocks of the complex. These dikes resemble in mineralogy, texture, and 

attitude the other, genetically related, west-northweststriking diabase dikes of the region, but they are 

generally thicker, coarser grained, and locally have a 

strong trachytic texture. The dikes cut pyroxenite, 

nepheline syenite, and fenite of the complex but were 

not observed to cut the carbonatite stock. Carbonatite 

dikes and veins do not cut the diabase.

The diabase is generally coarse grained, ophitic, and 

locally trachytic in the thicker parts of the dikes.
THORIUM AND ASSOCIATED ELEMENTS, POWDERHORN DISTRICT, GUNNISON COUNTY, COLORADO 25

Where the dikes are thinner the texture changes to fine 

to medium grained and a hypidiomorphic-granular texture is common. Marginal chill zones, present along 

some of the diabase dikes, are porphyritic with phenocrysts of labradorite and augite. The groundmass 

makes up about 80 percent of the rock and consists of 

plagioclase microlites, minute grains of augite, and 

magnetite. Some of the augite phenocrysts have 

mantles of a later generation of pyroxene.

FAULTS

The Powderhorn district is on the south side of the 

Gunnison uplift, a northwest-trending block roughly 

30 by 100 km in size, composed of Precambrian rocks 

overlain by Mesozoic sediments, which was elevated in 

Laramide time. Within the district, Precambrian 

metamorphic foliation and elongate metamorphic and 

igneous bodies trend mostly northeast to east. This 

older trend is cut by many younger Precambrian 

faults, shear zones, and Cambrian or Ordovician 

diabase dikes that strike predominantly northwest 

(Olson and others, 1977). Many of the thorium veins 

are in northwest-trending faults and shear zones, 

although trends vary in different parts of the district.

The Gunnison uplift is bounded on the south by the 

northwest-trending Cimarron fault, which passes near 

Powderhorn (fig. 4). The concealed Cimarron fault is indicated by a series of six springs and travertine spring 

deposits that strike approximately N. 70 W. along the 

southwest side of Cebolla Creek at Powderhorn. Six 

travertine spring deposits can be mapped, although 

they coalesce in places. Movement on the Cimarron 

fault took place in Laramide time, although earlier 

movement may have occurred in late Precambrian 

time (Olson and others, 1977). Northwest of Powderhorn, the position of the fault at the beginning 

ofOligocene volcanism was marked by a long, 

southward-facing scarp, against which the early, 

intermediate-composition flows and volcanic breccias 

accumulated. Diabase dikes of Cambrian or Ordovician 

age parallel the Cimarron fault zone and are thus 

related to it in trend, but considerable postdike fault 

movement has occurred as some dikes are cut by the 

fault.

The Cimarron fault cuts the complex of Iron Hill and 

can be traced northwestward along strike at least 

65 km. Southeast of the complex the fault is buried by 

Oligocene volcanic rocks. For most of its length, the 

fault shows a displacement, northeast side upthrown, 

of 1,000 m or more. Much of this movement occurred 

after the Mancos Shale of Cretaceous age was 

deposited. A fault block of Junction Creek Sandstone 

of Jurassic age crops out along the east side of

Powderhorn Creek about 500 m south of the old 

Powderhorn Post Office. This sandstone block is 

slightly tilted and the base is at an elevation of about 

2,470 m. This elevation represents a difference of 

about 520 m from the elevation of the Junction Creek 

Sandstone outcrop on Huntsman Mesa to the northeast and 400 m difference from the outcrop of sandstone on Tolvar Peak. The different levels are accounted for by fault displacement, but the fault or 

faults are concealed.

Where the fault cuts the eastern part of the complex 

of Iron Hill, the displacement, as shown by offset of 

the Junction Creek Sandstone of Jurassic age, is very 

small and is probably no more than 50-120 m, with the 

south side upthrown. Post-Jurassic movement was 

greater, however, on a different but probably related 

east-trending fault that crosses the south end of 

Huntsman Mesa and cuts granite just north of the 

complex, because the Junction Creek Sandstone is 

displaced about 250 to 300 m in this area, south side 

upthrown. These displacements of south side up 

relative to north differ from the relative displacements 

of north side upthrown, shown along the Cimarron 

fault from Powderhorn northwestward. They are, 

however, compatible with the sense of displacement interpreted to have occurred between the two sides of the 

fault within the complex of Iron Hill. The varying 

amounts of displacement at different points along the 

Cimarron fault are taken up in part by northeast- to 

east-trending cross faults that divide the major blocks 

into smaller ones of varying throw.

Erosion since the fault movement has exposed two 

different levels in the alkalic stock. Whereas pyroxenite and ijolite are found on both sides of the fault, the 

block southwest of the fault contains the Iron Hill carbonatite body, a greater number of thin carbonatite 

dikes, and abundant uncompahgrite; it also has a generally wider fenite zone. The block northeast of the 

fault contains an abundance of nepheline syenite and 

magnetite-ilmenite-perovskite dikes, far more than 

found in the southwest block. The carbonatite and uncompahgrite southwest of the fault may well represent 

the deeper seated rocks, as interpreted by Temple and 

Grogan (1965, p. 689), in which case the fault movement at this point was the reverse of that farther 

northwest.

To account for the great difference between rocks of 

the complex of Iron Hill on the two sides of the fault, 

more displacement would seem to be required than is 

shown by the offset of Mesozoic sediments. Some preJurassic movement is therefore inferred. The Cimarron 

fault was apparently active over a long period of time 

and may also have been active in late Precambrian 

time and even have been a factor in the localization
26 GEOLOGY AND MINERAL RESOURCES OF THORIUM IN THE UNITED STATES

of the complex of Iron Hill (Olson and others, 1977, 

p. 686).

RESOURCES1 OF THORIUM AND 

ASSOCIATED ELEMENTS

THORIUM

Thorium exploration has been aimed at finding 

thorium concentrations of several tenths of a percent 

ThO2 or more, such as are found in the thorite veins, in 

a few of the carbonatite dikes, and in some trachyte. 

Lower grade material, containing 0.02 to 0.1 percent 

ThO2, adds significantly to total thorium resources in 

the district (Olson and Overstreet, 1964, p. 23) and includes, in addition to the thorite veins, concentrations 

in carbonatite, magnetite-ilmenite-perovskite bodies, 

trachyte porphyry dikes, and small plutons of granite.

Thorium deposits shown on maps in the Powderhorn 

district number at least 261, which are concentrated 

in the Gateview and Powderhorn quadrangles. The majority of the deposits are small or low in grade; hence, 

it is only a minority that contribute significantly to 

total resources. Sixty-seven localities, including most 

of the richer deposits, were examined with the aid of a 

portable four-channel gamma-ray spectrometer, and 66 

samples were collected. An additional 35 samples were 

collected from outside the Powderhorn district to provide additional data for calibration purposes. The 

samples were analyzed by C. M. Bunker and C. A. 

Bush for thorium, RaeU, and potassium by laboratory 

gamma-ray spectrometry. 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.

The gamma-ray spectrometer readings, coupled with 

sample analytical data, provide a reasonable approximation of the thorium content of the surficial part of 

the deposit. Projecting these readings to infer reserves 

at depth requires consideration of the amount of cover 

that might locally obscure the vein; the dispersal of 

radioactive elements by slope wash, soil creep, or 

leaching; the possible enrichment of surficial material 

by selective removal of constituents other than 

thorium; as well as the known variability of thorium 

mineralization from place to place, particularly in the 

thorite veins. Generally, the surficial thorium content 

was assumed to be fairly representative of that at 

depth except where radioactivity may be obscured by 

ground cover.

'The mining industry presently uses the inch-pound system of measurements.

For indicated ore, a depth was assumed of from onethird to one times the surface length of the vein, to a 

maximum of 1,000 feet; for inferred ore a maximum 

depth of 1 1A to two times the surface length was used, 

to a maximum of 3,000 feet.

Resource estimates of thorium veins in the Powderhorn district, compiled as cumulative totals, according 

to average grade of deposit, are shown in table 4, and 

the minimum amount of ThO2 estimated to be present 

at various grades is shown graphically on figure 21. As 

the curve is for minimum amounts, the actual amount 

of ThO2 present is somewhere to the right of the curve 

shown, perhaps two or three times as much.

In order to relate the reserves to the cost of producing the thorium, a method of cost analysis was designed by the U.S. Bureau of Mines for mine and mill 

costs to recover thorium from thorite vein deposits 

(Lemons and Coppa, 1979). The cost analyses were 

made on the veins that contain more than 35,000 short 

tons of indicated and inferred reserves. In testing the 

Powderhorn veins by cost analysis, it was found that 

ThO2 might be obtained for less than $30 per Ib (pound) 

in those having a grade of at least 0.15 percent ThQ2 

(1,500 ppm). Very few, if any, veins averaging less than 

700 ppm ThO2 could be expected to produce ThO2 at a

10*

O 

su. 

O 102

10* 10s 

ThO,, IN SHORT TONS

10s

FIGURE 21.—Curve showing cumulative total of ThO2 resources at 

various grades. Thorite veins (tonnages at several grades of 

ThO2 are plotted along VT; minimum of 100 ppm plotted at T). 

Carbonatite dikes (several grades along CT); magnetiteperovskite rock (MT); thorium-rich granite (GT); carbonatite 

stock of Iron Hill (H).
THORIUM AND ASSOCIATED ELEMENTS, POWDERHORN DISTRICT, GUNNISON COUNTY, COLORADO 27

TABLE 4. ThO2 resources in the Powderhorn district

[Values in short tons. Figures for carbonatite dikes are for only 10 dikes that have been estimated. The total, including other carbonatite dikes, is probably at least several times these 

amounts. Undiscovered resources in all categories are not included, but they would probably at least double the resource estimates for veins, carbonatite dikes, and granite. The total 

number of veins estimated is 56, and the number of carbonatite dikes is actually 10. These numbers are less than the total shown because several deposits are repeated by dividing them 

into more than one grade category. >, greater than]

Reserves Cumulative totals

Grade

range No. of

(ppm) deposits Indicated Inferred Total

No. of

deposits Indicated Inferred Total

Veins.............. >1,000

Carbonatite dikes.. >1,000

Veins.............. 500-1,000

Carbonatite dikes.. 500-1,000

Veins.............. 100-500

Carbonatite dikes.. 100-500 

Magnetite-perovskite

rock............. ±250

Granite............ >100

Carbonatite stock.. 41

13 1,800 7,900 9,700 13 1,800 7,900 9,700

2 100 200 300 15 1,900 8,100 10,000

15 200 600 800 30 2,100 8,700 10,800

1 500 1,000 1,500 31 2,600 9,700 12,300

38 1,400 3,800 5,200 69 4,000 13,500 17,500

9 700 1,000 1,700 78 4,700 14,500 19,200

1? None 25,000 25,000 79 4,700 39,500 44,200

5 5,000 9,000 14,000 84 9,700 48,500 58,200

1 30,000 110,000 140,000 85 39,700 158,500 198,200

cost of less than $50 per Ib. The approximate cost 

categories of the reserves (in veins only) are shown in 

table 5.

Indicated and inferred reserves in veins exceeding 

500 ppm ThO2 in average grade amount to 10,400 

short tons, as shown in the two columns on the left side 

of the table. Of this total, 95 percent is estimated to be 

in only seven deposits. The other four categories on the 

right side of the table are made up of veins having 

more than 700 ppm ThO2.

The larger and higher grade veins, which contain a 

large proportion of the resources estimated in veins exceeding 500 ppm ThO2 (table 4), are found in sees. 14, 

15, 16, and 20, T. 47 N., R. 2 W.; sees. 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. 47 N., R. 2 W.; and sees. 23 and 24, T. 46 N., 

R. ll/2 W. These areas are shown on figure 22 along 

with the areas of thorium-bearing magnetiteperovskite rock, carbonatite, and thorium-bearing 

granite.

Five deposits account for the 9,300 tons of 

reserves estimated, from the U.S. Bureau of Mines 

cost analysis, to be producible at less than $30 per Ib. 

The $30-50 per Ib category is made up of ore in seven 

deposits.

Analysis of production costs likely for low-grade 

disseminated deposits minable by open-cut methods 

suggests that some of the lower grade thorium 

resources in the fine-grained granite plutons near 

Powderhorn may be minable by open-cut methods at a 

cost on the order of $50-100 per Ib ThO2 (Staatz and 

others, 1979). Parts of the granites of four small areas 

in the Road Beaver Creek drainage and one near 

Cebolla Creek, judging from gamma-ray spectrometer

measurements and eight sample analyses, range mostly from 100 to 200 ppm ThO2 in average grade, probably averaging a little more than 100 ppm ThO2. Parts 

of the granite contain 200 ppm or more, and the highest sample analysis of 1,600 ppm ThO2 was from part 

of the granite that contains abundant limonitic 

material as small veins. The granites of the five areas 

are estimated to have a combined tonnage of about 125 

million tons of rock. At an assumed overall average 

grade of 112 ppm ThO2, this amount of granite is inferred to contain at least 14,000 tons ThO2 (at least 5,000 

tons indicated and 9,000 tons inferred), and additional 

low-grade resources probably exist in other parts of 

the granite bodies not estimated.

Thorium oxide content of the carbonatite stock of 

Iron Hill has been estimated by Armbmstmacher 

(1980) to average 41 ppm; thus, to a depth of 245 m the 

3.4 billion tons of carbonatite would contain about 

140,000 tons ThO2. Other elements in the same amount 

of rock were estimated (Armbmstmacher, 1980) as 

1,900,000 tons Nb2O5 (570 ppm); 13,500,000 tons combined rare-earth oxides (3,970 ppm), and 43,000 tons 

U3O8 (12.7 ppm). Inasmuch as the content of thorium 

and the other elements is not uniform, some large parts

TABLE 5.—Indicated and inferred reserves of ThO2 in veins of more 

than 500 ppm ThO2 average grade in the Powderhorn district

[Values are in short tons]

Total Total 

indicated inferred

1,900 8,500

Producible at less 

than $30/1 b

Indicated Inferred

1,600 7,700

Producible at between 

$30 and $50/1 b

Indicated Inferred

200 700
GEOLOGY AND MINERAL RESOURCES OF THORIUM IN THE UNITED STATES

107°07'30"

Blue Mesa Reservoir V^

36 31

EXPLANATION

Sections containing more than 95 percent o 

the thorium resources estimated for the 

district in grades exceeding 500 ppm

Complex of alkalic rocks of Iron Hill, in which 

thorium-bearing magnetite-perovskite 

rock carbonatite occur locally

Area in which thorium-bearing granite 

occurs locally

Area of

Powde'rhorn

district

R.3W. R. 2 W.

FIGURE 22.—Map showing areas of principal thorium resources estimated in veins,

bearing granite.

R. 1 1/2 W. R. 1 W. 

magnetite-perovskite rock, carbonatite, and thorium-
THORIUM AND ASSOCIATED ELEMENTS, POWDERHORN DISTRICT, GUNNISON COUNTY, COLORADO 29

of the carbonatite are higher or lower than the average 

grade. For example, Temple and Grogan (1965, p. 673) 

estimated a reserve of more than 100,000 tons of Nb2O5 

in rock averaging at least 0.25 percent Nb2O5; some 

parts of this tonnage, large enough for open-pit mining, were 0.35 percent Nb2O5. Similarly, higher ThO2 

concentrations of 60 to 100 ppm probably occur locally 

in parts of the carbonatite body large enough to be 

quarried.

The ThO2 content of the magnetite-ilmeniteperovskite rock has been approximated from only a few 

available analyses, which are summarized in table 6.

These data suggest a ThO2 content in the magnetiteilmenite-perovskite rock of perhaps 200-500 ppm; the 

radiochemical analyses, compared with TiO2 in the 

same samples, suggest an approximate TiO2/ThO2 

ratio of about 380. The grade of TiO2 in this rock is 

discussed in the section on "Titanium." From these approximate figures, 100 million tons of such rock, if 

mined and processed for titanium at 12 percent TiO2 

grade, might be expected to contain on the order of 

25,000-50,000 tons ThO2; the actual tonnage available 

may be several times this amount.

ASSOCIATED ELEMENTS

The economic potential of thorium in the Powderhorn district is related in part to other elements and 

potential mineral commodities that have been prospected in the area The complex of Iron Hill contains a 

minor-element assemblage characteristic of many 

alkalic rocks, including iron, titanium, phosphorus, 

strontium, barium, niobium, thorium, rare earths, and 

vanadium. Potential economic products include 

niobium, titanium, iron, and rare-earth elements.

TABLE 6.—ThO2 content of magnetite-ilmenite-perovskite rock, complex of Iron Hill

[>, greater than; leaders (—), not analyzed for]

Content in percent

Material Th0 2 or eTh02 Ti0 2 Nb Rare earths

Magnetite-ilmenite-perovskite 

rock NJX-1, spectrographic

Two radiochemical analyses

Magnetite-ilmenite-perovskite

Magnetite-ilmenite-perovskite rock 

Temple and Grogan, 1965, p. 688).

0.025 >10 0.07

12- 15 51 34

.01

.05 — - ---

0.772

.003

Magnetite-ilmenite-perovskite 

rock (0.05-0.25 mr/hr field 

readings, approximate)........... .0076-.0680

TABLE 7.—TAO2 arad//V62O5 ratios in veins in the 

Powderhorn district

Quadrangle

No. of 

analyses

Rudolph Hill......... 6 2.55

Powderhorn........... 16 5.50

Gateview............. 18 15.81

Carpenter Ridge...... _6____19.70

Total and average.. 46_____6.79

NIOBIUM

Niobium occurs in pyrochlore disseminated 

throughout much of the dolomitic carbonatite stock. 

Drilling by the DuPont Co. in 1957 and 1958 indicated 

a reserve of more than 100,000 tons of Nb2OB in carbonatite averaging at least 0.25 percent Nb2O5, a 

substantial amount of which averaged 0.35 percent. 

This reserve estimate was subsequently increased by 

underground work and additional drilling (Temple and 

Grogan, 1965, p. 673).

The pyrochlore is most abundant in the finer textured dolomitic carbonatite and also associated with 

fluorapatite along shear planes in the carbonatite. 

Small amounts of niobium also occur in several other 

minerals in the complex of Iron Hill. Semiquantitative 

spectrographic analyses of three perovskite concentrates from the magnetite-ilmenite-perovskite rock 

show niobium values of 0.17, 0.2, and 0.7 percent. 

Niobium is also present in trace amounts in the ilmenite, sphene, magnetite, and melanite.

The thorite veins commonly contain anomalous 

amounts of niobium; some of it is in columbite, which 

has been identified in a black siliceous vein in the Road 

Beaver Creek area (Temple and Grogan, 1965, p. 684). 

Rough calculations from semiquantitative spectrographic analyses of 46 thorite vein samples show 

niobium in amounts generally from 30 to 1,500 ppm, 

averaging 172 ppm (246 ppm Nb2OB). This amount may 

be compared to an average of 1,669 ppm for ThO2 in 

the same samples, a ThO2-Nb2OB ratio of 6.79. The 

ThO2-Nb2O5 ratio varies in veins in different parts of 

the districts, however, as shown in table 7. The ThO2- 

Nb2O5 ratio in the thorite veins thus increases, from 

southeast to northwest, with increasing distance from 

the complex of Iron Hill. Within the carbonatite body 

of the complex of Iron Hill, the Nb2O6 content greatly 

exceeds ThO2.

TITANIUM

Titanium-bearing minerals are abundant in the 

pyroxenite of the complex of Iron Hill. These include 

perovskite, ilmenite, leucoxene, sphene, melanite 

garnet, and titaniferous magnetite; minor amounts are 

present in diopside (0.1-0.7 percent) and biotite. The
30 GEOLOGY AND MINERAL RESOURCES OF THORIUM IN THE UNITED STATES

magnetite, ilmenite, and perovskite are commonly intergrown in a titanium-rich rock that forms segregations and dikes, and they also are disseminated as 

discrete grains in the pyroxenite. The perovskite is 

partly replaced by leucoxene, but the other titaniumbearing minerals show this alteration less commonly. 

Sphene and melanite are commonly intergrown with 

the silicates of the pyroxenite and occur less commonly 

as granular aggregates. The TiO2 content of perovskite 

concentrates is about 51 percent, of the magnetiteilmenite composite grains about 6 to 19 percent, and of 

melanite concentrates about 5 percent. Some titanium 

is in solid solution in the magnetite, and the TiO2 

content of two samples of magnetite is 3.90 and 15.0 

percent.

Modal analyses of 102 thin sections of pyroxenite indicate an average weight percentage of the following 

minerals in the pyroxenite: magnetite-ilmenite, 18.4; 

perovskite, 2.4; leucoxene, 1.1; melanite, 3.3; sphene, 

0.5 percent.

According to Larsen (1942, p. 57), the TiO2 content 

averages 7.4 percent in the pyroxenite and is more 

than 35 percent in some of the magnetite-ilmeniteperovskite bodies. Partial chemical analyses of pyroxenite drill-core samples have been published by Rose 

and Shannon (1960). Their data from 441 analyses, derived from three drill cores totaling 569.5 m of mostly 

pyroxenite, indicate that within the pyroxenite the 

TiO2 content ranges from 0.3 to 11.9 percent and the 

total iron content from 7.0 to 22.5 percent. Rose and 

Shannon (1960, p. 19) estimated from surface- and coresample data that the average grade of the pyroxenite is 

11.7 percent iron and 6.5 percent titanium, and that 

the deposit may contain more than 100 million tons. 

The same figures are given by Berkenkotter and Hazen 

(1963), who made statistical studies of the drill-core 

sample data supplied by Rose and Shannon. As the 

titanium content is highest in the magnetite-ilmeniteperovskite dikes and segregations, the parts of the 

pyroxenite containing large amounts of such rocks exceed the average titanium content and offer the most 

promise for titanium exploration.

The pyroxenite has been explored for titanium 

during the mid 1970's by Buttes Gas and Oil Co. A 

newspaper account (Denver Post, February 25, 1976, 

p. 31) reported company officials as saying that 

studies had indicated 419 million tons of reserves of 

rock averaging about 12 percent TiO2.

IRON

Iron deposits were discovered in the Powderhorn 

district before 1880. The iron content of the material 

mined has not been sufficient to attract mining for iron

alone, and the total production has been less than 

4,000 tons (Harrer and Tesch, 1959, p. 33). The iron 

occurs chiefly as magnetite-ilmenite-perovskite concentrations in pyroxenite. Thirteen samples of the 

titaniferous magnetite rock range from 8.6 to 30.8 percent iron and 3.8 to 32.7 percent TiO2 (Harrer and 

Tesch, 1959, p. 34). An average of 11.7 percent iron for 

more than 100 million tons of magnetite-bearing pyroxenite was given by Rose and Shannon (1960, p. 19). 

Small iron deposits in the carbonatite of Iron Hill have 

also been prospected; these consist of small martiteapatite veins and small bodies of cellular and siliceous 

limonite and hematite.

VANADIUM

Vanadium is present in the iron- and titaniumbearing minerals of the complex. Magnetite in one 

sample of uncompahgrite contains 0.21 percent V2O5 , 

and a magnetite sample from a magnetite-ilmeniteperovskite dike contains 0.14 percent V2O5 . Other 

mineral samples with the greatest concentration are 

melanite garnet, which contains 0.2 percent V2O5 ; 

perovskite 0.06 percent V2O5 ; and diopside, 0.05-0.07 

percent V2O5 .

Vanadium is commonly reported in spectrographic 

analyses of thorite veins. Vanadinite was identified in 

a sample from the Sand Rock No. 5 Lode 1 km northwest of the old Dubois mine by M. H. Staatz. In 45 

semiquantitative spectrographic analyses (Hedlund 

and Olson, 1980) of vein samples, vanadium ranges 

from 0 to 1.5 percent, averaging 0.069 percent. This is 

a higher concentration than is found in most of the 

alkalic igneous rocks in the district. Probably the 

highest concentration is in the titaniferous magnetiteilmenite-perovskite rocks, containing on the order of 

0.1 percent V2O5 . Fischer (1975, p. 5) has estimated 

that about 50,000 tons of vanadium may occur in 100 

million tons of magnetite-ilmenite-perovskite rock in 

the complex of Iron Hill.

RARE-EARTH ELEMENTS

All the various types of thorium deposits in the 

district contain an appreciable amount of rare-earth 

elements, generally considerably in excess of the thorium. In the thorite veins the ratio of total rare-earth 

oxides to ThO2 averages about 2.4. The rare earths 

occur at least partly in such minerals as monazite, thorite, rhabdophane, synchysite, and bastnaesite. Rough 

calculations based on semiquantitative spectrographic 

analyses, mostly of veins but including other rock 

types as well, indicate that in the entire district the 

ratio of light (cerium-group) rare-earth oxides to tho-
THORIUM AND ASSOCIATED ELEMENTS, POWDERHORN DISTRICT, GUNNISON COUNTY, COLORADO

107°15'00" 7'30"

38°22'30" -

38°15'00"

Carpenter Ridge 

quadrangle

2,572 110.

Rudolph Hill \

^quadrangle «134

01234 MILES

I I I I

(

I I I I I I 

) 1 23456 KILOMETERS

FIGURE 23.—Map showing proportions of cerium-group rare-earth oxides, yttrium-group rare-earth oxides, and thorium oxide in 

samples from 31 thorium deposits. Numbers show ratios of cerium-group rare-earth oxides (light type) and yttrium-group rare-earth 

oxides (bold type) to thorium oxide taken as 100. Amounts are approximate only, as they are based upon semiquantitative spectrographic analyses.
32 GEOLOGY AND MINERAL RESOURCES OF THORIUM IN THE UNITED STATES

TABLE 8.—Ratio of cerium-group rare-earth oxides to ThOt to 

yttrium-group rare-earth oxides in various thorium deposits

Cerium-group rareearth oxides:Th0 2 :

Area yttrium-group rare

earth oxides

1. Thorite veins in Gateview and Carpenter 

Ridge quadrangles.....................

2. Thorite veins in Powderhorn quadrangle excluding vicinity of complex of Iron Hill........

Thorite veins near south and west sides 

of complex of Iron Hill...............

4. Carbonatite, magnetite-perovskite rock, 

granite, and thorite veins of no. 3, 

in or near complex of Iron Hill.......

1.8:1:0.4 

0.6:1:0.78 

4.6:1:0.45

6.0:1:0.31

rium oxide to heavy (yttrium-group) rare-earth oxides 

is about 3.6 to 1 to 0.5. Restricting the ratios to thorite 

veins containing more than 100 ppm ThO2 only, these 

ratios are about 1.75 to 1 to 0.65. Like the niobiumthorium oxide ratios, the proportions of these three 

groups of constituents vary considerably in different 

parts of the district (fig. 23). These variations are summarized in table 8. Individual deposits differ greatly in 

rare-earth-thorium ratios from nearby deposits, suggesting that fractionation of rare-earth metals and 

thorium—and probably niobium as well—has taken 

place on a local as well as a district-wide scale. The 

variation in proportions of rare-earth metals, thorium, 

and niobium may be due partly to fractionation 

associated with changes in the composition of the 

fluids with distance from the source. In part the variations correlate very roughly with carbonate mineral 

content, for the deposits richer in carbonate minerals, 

as near the complex of Iron Hill, generally have a 

higher proportion of cerium in relation to the other 

elements.

The relation of cerium- and yttrium-group metals to 

ThO2 content in 59 samples of thorium deposits is illustrated in figures 24 and 25.

URANIUM

The ratio of thorium to uranium was determined for 

74 samples of thorite veins. Uranium content is generally quite low and exceeds 10 ppm in only three of the 

samples, in which it consists of 11.78, 20, and 41 ppm. 

Of the 74 samples, the thorium-uranium ratio ranged 

from 1 to 10 in six, 10 to 30 in eight, 30 to 70 in 20, 70

105

10"

103

102

EXPLANATION

A Carpenter Ridge quadrangle

• Gateview quadrangle

A Powderhorn quadrangle

o Rudolph Hill quadrangle

x Carbonate-rich veins or dikes in 

vicinity of Iron Hill carbonate

A

A

A

A

• •

• •

• A

A

A

•

A A A A 

A A

A

A • 

A.

A

10 102 103 

X

10" 105

TOTAL CERIUM-GROUP RARE-EARTH METALS, IN PARTS PER MILLION 

FIGURE 24.—Graph showing relation of total cerium-group rare-earth metals to ThOz content in 59 samples of thorium deposits.
THORIUM AND ASSOCIATED ELEMENTS, POWDERHORN DISTRICT, GUNNISON COUNTY, COLORADO 33

105

104

EXPLANATION

* Carpenter Ridge quadrangle

• Gateview quadrangle

A Powderhorn quadrangle

o Rudolph Hill quadrangle

X Carbonate-rich veins or dikes in 

vicinity of Iron Hill carbonate

A

103

A 

A

• A

A A °A A 

A A

• x .

A

A

X

10 A 102 103 

TOTAL YTTRIUM-GROUP RARE-EARTH METALS, IN PARTS PER MILLION

104

FIGURE 25.—Relation of total yttrium-group rare-earth metals to ThO2 content in 59 samples of thorium deposits. 

Rare-earth values approximated from semiquantitative spectrographic analyses.

to 130 in 10,130 to 1,000 or more in 14, and in 16 of the 

samples the thorium-uranium ratio is probably high 

but the uranium content is very low or not determined. 

The ratio of thorium to uranium is also generally high 

in rocks other than the thorite veins, such as carbonatite dikes and thorium-bearing granite. In 23 

samples of carbonatite or alkalic rocks, uranium exceeds 10 ppm in only one, a carbonatite dike sample in 

which it is 93.4 ppm, exceeding thorium. Significant 

uranium resources therefore seem unlikely to be associated with the thorium deposits and alkalic igneous 

rocks in the district.

REFERENCES

Armbrustmacher, T. J., 1980, Abundance and distribution of 

thorium in the carbonatite stock at Iron Hill, Powderhorn 

district, Gunnison County, Colorado: U.S. Geological Survey 

Professional Paper 1049-B, p. B1-B11.

Armbrustmacher, T. J., and Brownfield, I. K., 1979, The carbonatite stock at Iron Hill, Gunnison County, Colorado—Chemical 

and mineralogical data: U.S. Geological Survey Open-File 

Report 79-537,10 p.

Berkenkotter, R. D., and Hazen, S. W., 1963, Statistical analysis of 

diamond-drill sample data from the Cebolla Creek titaniferous 

iron deposit, Gunnison County, Colo.: U.S. Bureau of Mines 

Report of Investigations 6234, 58 p.

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radiometric reconnaissance of parts of the northwestern San 

Juan Mountains, Colorado: U.S. Geological Survey Circular 

236, lip.

Dooley, J. R., Jr., and Hathaway, J. C., 1961, Two occurrences of 

thorium-bearing minerals with rhabdophane-like structure: U.S. 

Geological Survey Professional Paper 424-C, p. C339-C341.

Ervin, Guy, Jr., and Osborn, E. F., 1949, X-ray data on synthetic 

melilites: American Mineralogist, v. 34, nos. 9-10, p. 717-722.

Fischer, R. P., 1975, Vanadium resources in titaniferous magnetite 

deposits: U.S. Geological Survey Professional Paper 926-B, 

10 p.

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occurrences in Colorado: U.S. Bureau of Mines Information Circular 7918, 82 p.

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_____1973 [1974], Geologic map of the Carpenter Ridge 

quadrangle, Gunnison County, Colorado: U.S. Geological 

Survey Geologic Quadrangle Map GQ-1070, scale 1:24,000.

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34 GEOLOGY AND MINERAL RESOURCES OF THORIUM IN THE UNITED STATES

and Saguache Counties, Colorado: U.S. Geological Survey 

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Colorado: U.S. Geological Survey Professional Paper 197-A, 

p. 1-64.

Lemons, J. F., Jr., and Coppa, L. V., 1979, Mining and processing 

methods and cost models for the recovery of thorium from 

domestic occurrences: U.S. Department of Energy report 

GJBX-9K79), 68 p.

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Olson, J. C., 1974 [1975], Geologic map of the Rudolph Hill 

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Geological Survey Geologic Quadrangle Map GQ-1071, scale 

1:24,000.

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Age and tectonic setting of lower Palezoic alkalic and mafic 

rocks, carbonatites, and thorium veins in south-central Colorado: U.S. Geological Survey Journal of Research, v. 5, no. 6, 

p. 673-687.

Olson, J. C., and Overstreet, W. C., 1964, Geologic distribution and 

resources of thorium: U.S. Geological Survey Bulletin 1204, 

61 p.

Olson, J. C., Shawe, D. R., Pray, L. C., and Sharp, W. N., 1954, Rareearth mineral deposits of the Mountain Pass district, San Bernardino County, California: U.S. Geological Survey Professional Paper 261, 75 p.

Olson, J. C., and Wallace, S. R., 1956, Thorium and rare-earth 

minerals in the Powderhorn district, Gunnison County, Colorado: U.S. Geological Survey Bulletin 1027-0, p. 693-723.

Rose, C. K., and Shannon, S. S., 1960, Cebolla Creek titaniferous 

iron deposits, Gunnison County, Colorado: U.S. Bureau of 

Mines Report of Investigations 5679,30 p.

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I. K., Brock, M. R., Lemons, J. F., Jr., Coppa, L. V., and 

Clingan, B. V., 1979, Principal thorium resources in the United 

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v. 60, no. 4, p. 672-692.

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