Thorium and Rare-Earth 

Minerals in 

Powderhorn District, 

Gunnison County, 

Colorado

By J. C. OLSON and S. R. WALLACE

A CONTRIBUTION TO ECONOMIC GEOLOGY

GEOLOGICAL SURVEY BULLETIN 1027-0

This report concerns work done on 

behalf of the U. S. Atomic Energy 

Commission and is published with 

the permission of the Commission

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON ; 1956
UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

Thomas P. Nolan, Director

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

Washington 25, D. C. - Price 60 cents (paper cover)
CONTENTS

	Page 

Abstract__--___.--_-__._..-_______-__._.-.___.__-..__._...___.__ 693

Introduction. _______________--___-_--_-_-_-___________-__-__._____ 694

Geographic setting___________________________________________ 694

Previous work______________________________________________ 695

Field work and acknowledgments.-____-_______----______-__.--__ 695

General geology.____________________________________________ 696

Pre-Cambrian metamorphic and igneous complex._________________ 696

Intrusive rocks._______________________________________________ 696

Carbonate rock of Iron Hill___________________________________ 697

Sedimentary and volcanic rocks__---_-__---__-_-----------_----_ 698

Structure____..____..__________________________________ 698

Mineral deposits_____---__-___-_---__-_-____________.._-_____-___ 699

Thorium deposits____________________________________________ 701

Structural features_____-_________ -________-__________-_-_ 701

Mineralogy-_ _.___________________________________________ 702

Relation to alkalic igneous rocks___________________________ 703

Age____________-__-____________-__---_____-_____-_-- 704

Effects of weathering and alteration________________________ 704

Distribution of rare earths_____-__-____--___--_--__--_---.______---_ 706

Little Johnnieclaims__-__-___-____-____-___-_____----___.-_______ 708

General geology.____--__-______-____-___-_-_--__-__--_____---- 708

Pre-Cambrian rocks_________-____-___-__-____--_-__-_________ 708

Quartz-biotite schist and quartzite.___--____-_--_-____--_-_-_ 708

Metarhyolite porphyry.________-___----___--__.-___-___--__ 708

Amphibolite. ___-_____________-_-__--____---_--_-___-____- 710

Granite. ___________________-____--_-_-__..__._-_ 710

Quartz veins-_------_------__------------------_-__------- 710

Rocks relatedJ;o the Iron Hill composite stock ____________________ 711

Gabbro__"-__._..---__----_---__-__----_--------_-_--__---- 711

Syenite-__-_----_-------------------_---------_-_-------- 712

Carbonate veins.-___________________-_-_--_.--__-_____-___ 713

Breccia veins____________________________________________ 713

Sodic amphibole rock_________-____-__--_---_-_-__---__---- 714

Sedimentary and volcanic rocks.__-_-_____-------_-_-_---_-_---- 714

Structure.__________________________________________________ 714

Thorium deposit______________________________________________ 715

Outlook for future exploration__-_______________----_---_--____-_-_ 717

Literature cited,__________________________________________________ 721

Index________________________.-____.-_______--_-__.--._-_-_-- 723

ILLUSTRATIONS

Page

PLATE 55. Generalized geologic map of Powderhorn district showing localities examined....____________________________ In pocket

56. Geologic map and sections of the Little Johnnie thorium

vein_____________________________________________ In pocket

FIGURE 86. Index map of Colorado, showing location of Powderhorn

district, Gunnison County, Colo_-__-__--_-_-_----_-_--_ 694

in
IV CONTENTS

TABLES

Page 

TABLE 1. Niobium content of 11 samples of veins and carbonate rocks,

Powderhorn district____________________________________ 700

2. Abundance of certain minor elements in spectrographic analyses

of mineralized shear zones and carbonate veins.-------_-_-_ 704

3. Analyses, in percent, of carbonate rock containing rare-earthbearing apatite.______________________________________ 707

4. Deposits showing abnormal radioactivity, Powderhorn district 718
A CONTRIBUTION TO ECONOMIC GEOLOGY

THORIUM AND RARE-EARTH MINERALS IN THE

POWDERHORN DISTRICT, GUNNISON COUNTY,

COLORADO

By JERRY C. OLSON and STEWART R. WALLACE

ABSTRACT

Thorium has been found since 1949 in at least 33 deposits in an area 6 miles 

wide and 20 miles long in the Powderhorn district, Gunnison County, Colo. The 

district is underlain largely by pre-Jurassic metamorphic and igneous rocks, most 

of which, if not all, are pre-Canibrian in age. These rocks are overlain by sandstone of the Morrison formation of Jurassic age, and by volcanic rocks of the 

Alboroto group and the Hinsdale formation of Miocene and of Pliocene (?) age, 

respectively.

The thorium deposits occur in or near alkalic igneous rocks in which such 

elements as titanium, the rare-earth mtetals, barium, strontium, and niobium are 

present in greater-than-average amounts. The greatest mass of the alkalic 

igneous rocks the Iron Hill composite stock occupies an area of 12 square 

miles in the southeastern part of the district. The thorium deposits, like the 

alkalic igneous rocks, have not been dated more precisely than pre-Jurassic.

The thorium deposits are veins and mineralized shear zones that range from 

a few inches to 18 feet wide and from a few feet to 3,500 feet long. The veins 

are composed of calcite, dolomite, siderite, ankerite, quartz, barite, pyrite, sphalerite, galena, goethite, apatite, alkali feldspar, and many other minerals. At 

least part of the thorium occurs as thorite or thorogummite. A small amount 

of what has been tentatively identified as xenotime is found in one deposit. 

Several minerals containing the rarerearth metals of the cerium group as major 

constituents are found in carbonate veins near Iron Hill. Bastnaesite and synchisite have been identified by X-ray methods, and cerite is probably present 

also. The fluorapatite in some veins and in parts of the mass of carbonate rock 

that occupies 2 square miles in the central part of the Iron Hill composite stock 

contains rare-earth elements of the cerium group, generally in amounts of a 

fraction of a percent of the rock.

The radioactivity of the deposits appears to be due almost entirely to thorium 

and its daughter products. The thorium oxide content of selected high-grade 

samjples from the Little Johnnie vein is as much as 4 percent, but in most veins 

is generally less than 1 percent, and is only from 0.05 to 0.1 percent in many of 

the veins studied.

The Little Johnnie vein, which was mapped in detail, is a mineralized fault 

zone that ranges from less than 6 inches to 5 feet in width and can be traced 

discontinuously for a distance of more than 3,500 feet. The thorium-bearing

693
694 CONTRIBUTIONS TO ECONOMIC GEOLOGY

material occurs as irregular veinlets and thin films introduced along the fault 

zone. Near its west end the vein is broken by many faults in a zone that marks 

the edge of a roughly circular fault block, 1% miles in diameter, that has dropped 

1,000 feet or more since the deposition of Miocene volcanic rocks that now floor 

the Milkranch Gulch basin.

In 1949 thorium deposits were discovered in the Powderhorn district of Gunnison County, Colo. The district has long been known 

for its alkalic igneous rocks, of which the best known are those of the 

Iron Hill composite stock (Larsen, 1942). Some of these alkalic 

rocks contain titanium, barium, strontium, rare-earth metals, thorium, 

and niobium in greater percentages than in most igneous rocks. A 

study of the thorium deposits was made to learn more about the 

geologic relation of minor elements, particularly the rare-earth metals 

and thorium, to alkalic igneous rocks, and to determine the potentialities of the thorium deposits.

GEOGRAPHIC SETTING

The Powderhorn district is an elongate area about 20 miles long 

and 6 miles wide that extends northwestward from the vicinity of 

iron Hill to the Lake Fork of the Gunnison River, in the southwestern part of Gunnison County, Colo. (fig. 86). Powderhorn, a small

106

FIGDHE 86. Index map showing location of the Powderhorn district; shaded part indicates

area shown on plate 55.
THORIUM AND RARE-EARTH MINERALS, POWDERHORN DISTRICT 695

settlement in the south-central part of the district, is about 30 miles 

by road southwest of Gunnison, the nearest town.

The district is drained by three northward-flowing tributaries of the 

Gunnison River: Willow Creek, Cebolla Creek, and the Lake Fork of 

the Gunnison. These streams and their tributaries are deeply incised 

and in places have cut canyons as much as 1,000 feet deep in the preCambrian rocks which underlie most of the district. The interstream 

areas consist of dissected mesas that slope gently northward toward 

the Gunnison River. Many of these are capped by remnants of nearly 

flat-lying Mesozoic sedimentary rocks and Tertiary volcanic rocks. 

The altitude ranges from 7,500 feet on the Lake Fork of the Gunnison 

to more than 10,000 feet in the southeastern part of the district.

PREVIOUS WORK

The first comprehensive geologic study of the area that includes the 

Powderhorn district was made by Hunter (1925), who studied the 

pre-Cambrian rocks of the Gunnison River region in 1911-13. In 

part of this work Hunter was associated with E. S. Larsen, Jr., and 

Whitman Cross (Cross and Larsen, 1935). Larsen also studied the 

area subsequently, and his Professional Paper (Larsen, 1942) on the 

Iron Hill complex contains a wealth of data on the petrography, mineralogy, and geochemistry of the alkalic rocks.

In 1950, soon after the discovery of thorium deposits in the district, 

J. W. Adams and F. B. Moore of the U. S. Geological Survey examined 

the Little Johnnie, Jeanie, and Red Rock claims. In 1951, during 

reconnaissance for radioactive materials in the San Juan Mountain 

region, W. S. Burbank examined thorium deposits in the Lake ForkDubois area and veins in the vicinity of Iron Hill (Burbank and Pierson, 1953).

FIELD WORK AND ACKNOWLEDGMENTS

This report is the result of a brief reconnaissance for thorium-bearing deposits by J. C. Olson during the period August 11-25,1952, and 

planetable mapping of the Little Johnnie thorium deposit by S. R. 

Wallace and J. E. Roadifer from July 1 to August 6, 1953, with the 

assistance of J. C. Olson for 8 days. Notes, sketch maps, and other 

unpublished data obtained by W. S. Burbank in 1951 and by J. W. 

Adams and F. B. Moore in 1950 contributed much to the field study 

and the preparation of the report. In the laboratory, samples from 

the Powderhorn district were studied by chemical, spectrographic, and 

x-ray methods.

The investigations of the Powderhorn thorium deposits were made 

by the U. S. Geological Survey on behalf of the U. S. Atomic Energy 

Commission.
696 CONTRIBUTIONS TO ECONOMIC GEOLOGY 

GENERAL GEOLOGY

Thorium deposits have been found in an area 6 miles wide and 20 

miles long, extending from the Iron Hill complex to the Lake Fork of 

the Gunnison River (pi. 55). The area is underlain chiefly by preCambrian metamorphic and igneous rocks. The metamorphic rock 

complex is intruded by pre-Jurassic alkalic igneous rocks. Flat to 

gently dipping strata of the Morrison formation, of Jurassic age, 

overlie the complex of metamorphic and igneous rocks. Tertiary 

volcanic rocks were deposited on an early Tertiary erosion surface that 

in places nearly coincides with the late Paleozoic or early Mesozoic 

erosion surface on which the Morrison formation was deposited.

PRE-CA3VTBRIAN METAMORPHIC AND IGNEOUS COMPLEX

The pre-Cambrian rocks, as mapped by Hunter (1925), are divided 

into the Black Canyon schist, the Dubois greenstone, and the Powderhorn granite group. In the Powderhorn district (pi. 55) the Black 

Canyon schist of Hunter (1925) is dominantly biotite schist, quartzmica schist, and micaceous quartzite. Granite gneiss and amphibole 

schist make up a small part of the Black Canyon schist.

The Dubois greenstone occurs in an east-trending band from 1 

to 4 miles wide in the north-central part of the district. The darkgreen rocks of this formation consist chiefly of hornblende schist, 

hornblende gneiss, amphibolite, metadiorite, and chlorite schist.

The Black Canyon schist underlies most of the area of pre-Cambrian 

rocks north of the belt of Dubois greenstone, whereas the Powderhorn 

granite predominates to the south.

The Powderhorn granite group of Hunter (1925) comprises granitic rocks of relatively uniform composition but includes such diversified teXtUral Varieties as schistose metarhyolite porphyry, coarsely

porphyritic granite, and aplite. Granites of the Powderhorn group 

cut the Dubois greenstone, and therefore are younger than it, but 

some foliated amphibolite appears to form dikes that cut Powderhorn

granite. TKe porpnyritic texte and quartz plienOCiygtS Of the metarhyolite porphyry suggest that this variety was either extrusive or 

intruded. «,t roicctWoi^ si^iio^ depth. Other facies of the Powderhorn granite are almost certainly plutonic,

Except for Some facies of the Powderhorn granite, all the rocks 

described above are well foliated and have been metamorphosed to a

much greater degree than any of tne raieozoic ^^^7 «**« -*

central Colorado and the San Juan Mountains.

INTBUSIVE BOCKS

Many dikes and small irregular bodies of only poorly foliated or 

massive igneous rocks intrude the pre-Cambrian metamorphic and
THORIUM AND RARE-EARTH MINERALS, POWDERHORN DISTRICT 697

igneous complex. These igneous rocks include diorite, quartz diorite, 

gabbro, syenitic rocks ranging from leucosyenite to shonkinite, and 

the alkalic rocks of the Iron Hill composite stock, which occupies an 

area of about 12 square miles in the southeastern part of the district. 

In composition, about 70 percent of the stock is pyroxenite, 17 percent 

is a dolomitic carbonate rock, and the rest is uncompahgrite (melilite 

rock that contains pyroxene and magnetite), ijolite (nephelinepyroxene-garnet rock), soda syenite, nepheline syenite, nepheline 

gabbro, and quartz gabbro. The mineralogic and chemical features 

of these uncommon alkalic igneous rocks have been described by 

Larsen (1942, p. 35-36).

The Iron Hill stock has been dated no more precisely than preJurassic (Larsen, 1942, p. 2), and that dating is based on the fact that 

certain heavy minerals in the Iron Hill alkalic rocks are found in 

the basal Morrison beds. The age of the outlying dikes and irregular 

bodies of diorite, gabbro, and the variety of syenitic rocks ranging 

from leucosyenite to shonkinite relative to the Iron Hill stock is 

unknown. The chemical and structural features of these rocks, however, are generally similar to those of the Iron Hill rocks and indicate 

a probable genetic relationship. According to Hunter (1925, p. 78), 

quartz diorite and augite syenite in Wildcat Gulch are cut by dikes 

of pegmatite. If the pegmatite is pre-Cambrian in age, the age assigned to most of the pegmatite in this region, the quartz diorite and 

the augite syenite would also be pre-Cambrian. The age is speculative, however, for intrusion of pegmatite in the district may have 

occurred in more than one period.

CARBONATE BOCK OF IBON HILL

A mass of carbonate rock, the marble of Iron Hill (Larsen, 1942), 

occupies about 2 square miles near the center of the Iron Hill composite 

stock. Most of the carbonate of the mass is dolomite, but some calcite 

is present (Larsen, 1942, p. 5). Apatite, which contains small 

amounts of rare earths, makes up several percent of the carbonate 

rock; other accessory minerals locally present include phlogopite, 

magnetite, aegirite, and soda-amphibole.

The carbonate rock is not bedded, but generally is massive, although 

locally it has a conspicuous steeply plunging to vertical lineation 

formed mainly by streaks of apatite along shear planes; the lineation 

is especially well developed near the margins of the mass. In a 

few places, as at locality 13 (pi. 55) and in the isolated mass (locality 

12) south of the mouth of Beaver Creek, it has a layering or banding 

resulting from streaks of such minerals as phlogopite and magnetite.

Numerous carbonate veins similar in composition to the large carbonate-rock mass of Iron Hill cut the pyroxenite and other intru367125 56 2
698 CONTRIBUTIONS TO ECONOMIC GEOLOGY

sive rocks of the Iron Hill stock. Several small isolated bodies of 

carbonate rock, generally similar to the carbonate rock of Iron Hill, 

occur both in the pyroxenite and in the pre- Cambrian rocks surrounding the Iron Hill stock, a mass of white carbonate rock at least 200 

feet in diameter is enclosed in Powderhorn granite at the point indicated by a prospect pit 1 mile N. 45° E. of Powderhorn on plate 55.

Locally, the carbonate rock is both older and younger than the 

alkalic dike rocks. The numerous carbonate veins that cut the pyroxenite and other igneous rocks clearly are younger than the main mass 

of the stock. The opposite of this age relationship is seen at a 

prospect pit in Sammons Gulch 1.3 miles east of Powderhorn, where 

a body of carbonate rock about 20 feet wide is cut by pyroxenite dikes 

1 to 8 inches wide (Larsen, 1942, p. 4-5). At locality 12 just south 

of the mouth of Beaver Creek, an outlying body of carbonate rock 

and the pyroxenite adjacent to it are cut by thin micaceous dikes. 

The relations at these two localities show that at least some dikes are 

younger than some carbonate rock.

The age relationships of the large Iron Hill mass of carbonate rock 

to the rest of the stock are not known because contacts are poorly exposed, and the mode of origin is uncertain. Larsen favored the concept that the main mass is a large hydrothermal deposit formed in the 

throat of a volcano, though it may have been intruded as a carbonate 

magma or it may be an inclusion of pre-Cambrian or Paleozoic marble 

(Larsen, 1942, p. 6-9) .

SEDIMENTARY AND VOLCANIC BOCKS

The metamorphic and igneous rock complex is overlain unconf ormably by thin remnants of sandstone and siltstone of the Morrison 

formation of Jurassic age, which were deposited on a nearly flat surface. The Mancos shale of Cretaceous age occurs in two small areas 

on the Lake Fork of the Gunnison Kiver. Volcanic rocks of the 

Alboroto group of the Potosi volcanic series of Miocene age lie unCOnformablj On the Morrison formation, and in turn are overlain 

unconf ormably by rhyolite of the Hinsdale formation of Pliocene ( ?) 

age (Cross and Larsen, 1935). The sedimentary and volcanic rocks 

are found chiefly as erosional remnants on mesas and ridges, and 

have been removed from most of the district by erosion.

STRUCTURE

The foliation and individual layers of the pre-Cambrian metaorphic rocks strike generally within 45 degrees of east and dip

thTOU£llOUt inOSt Of tlie ^Strict. Many of the dikes and veins

in the district were emplaced parallel to this foliation and the layers. 

The Mesozoic and Cenozoic sedimentary and volcanic rocks dip 

gently northward except locally where disturbed by faults.
THORIUM AND RARE-EARTH MINERALS, POWDERHORN DISTRICT 699

The largest fault known in the region (pi. 55), the Ciniarron fault 

of Hunter (1925, p. 88), is a southward-dipping normal fault that has 

a throw of more than 1,500 feet in the Powderhorn district (Hunter, 

1925, p. 90). It extends at least 35 miles northwestward from 

Powderhorn but has not been mapped southeast of Powderhorn. Its 

probable location in the valley of Cebolla Creek is marked by hot 

springs and spring deposits. The movement on this fault occurred 

chiefly in late Cretaceous or early Tertiary time but continued after 

the volcanic rocks were formed. Several other northwest trending 

faults in the district were mapped by Hunter (1925).

MINERAL DEPOSITS

Mineral deposits in the Powderhorn district are related to three or 

more distinct periods of mineralization. There are sulfide-bearing 

quartz veins and massive-sulfide replacement deposits associated with 

pre-Cambrian rocks, various types of veins and segregations associated with the alkalic rocks of the Iron Hill composite stock and 

with the related intrusive rocks in other parts of the district, and 

small veins associated with the Tertiary volcanic rocks.

Sulfide-bearing quartz veins in pre-Cambrian rocks near Dubois, 

Spencer, Midway, and Vulcan, were mined or prospected principally 

during the period 1870-1900. Similar veins in the Cochetopa district to the east are considered to be pre-Cambrian by Hill (1909, p. 

29); therefore the quartz veins of the Powderhorn district are 

probably of pre-Cambrian age. The veins are composed chiefly of 

quartz and chlorite, but tourmaline is present in some. The dominant metallic minerals are pyrite, chalcopyrite, and sphalerite, but 

the deposits were worked for the gold and minor amount of silver 

they contained. A small quantity of sulfur was produced from 

massive-sulfide (pyrite, chalcopyrite, sphalerite) bodies which replace chloritic schist near Vulcan.

Iron and manganese oxides, vermiculite, and minerals containing 

titanium, rare earths, thorium, and niobium are found locally in veins 

or segregations in the alkalic igneous rocks and carbonate rock of the 

Iron Hill stock and are present in sufficient quantity to encourage 

some prospecting. Except for a small production of vermiculite and 

manganese, the output of these materials in the Powderhorn district 

has been insignificant.

The vermiculite, an alteration product of biotite, has been mined at 

several places from biotite-rich facies of the pyroxenite. Iron occurs 

in veins and irregular bodies of hematite and limonite in the carbonate 

rock of Iron Hill, and in lenticular bodies of magnetite and very finegrained perovskite in the pyroxenite which are commonly from 20
700 CONTRIBUTIONS TO ECONOMIC GEOLOGY

feet to as much as 75 feet wide. The iron-bearing veins in the carbonate rock are relatively small and contain abundant phosphorus in 

the form of apatite.

Titanium minerals are abundant in some of the pyroxenite of the 

Iron Hill stock, and considerable exploratory work and investigation 

of recovery methods has been done, chiefly by the Humphreys Gold 

Corp., during the past three years. Many cuts have been excavated 

by bulldozer, and large samples have been obtained for study of 

beneficiation of the titanium ore.

Titanium oxide averages 7A percent in the pyroxenite and exceeds 

35 percent in some of the bodies of perovskite-magnetite rock (Larsen, 

1942, p. 57-58). The bulk of the titanium occurs as the calcium 

titanate, perovskite, but some is present as sphene and ilmenite, and 

small amounts are contained in melanite, biotite, pyroxene, and other 

minerals.

Titanium-rich alkalic rocks commonly contain a relatively high percentage of niobium. The rocks of the Iron Hill stock contain niobium 

in amounts greatly exceeding those of average igneous rocks but 

analyses to date indicate a somewhat lower content of niobium than 

has been found in certain other alkalic-rock complexes (see Fleischer 

and others, 1952). Perovskite concentrates from the pyroxenite of 

Iron Hill generally contain 0. X to 0.0 X percent niobium. Spectrographic analyses of 11 samples of veins and carbonate rocks are given 

in table 1.

TABLE 1. Niobium content of 11 samples of veins and carbonate rocks, 

Powderhorn district, Colorado

[SpeCtrographiC analyses by E. G. Havens and P. R. Barnett U. S. Geological Survey]

Type of rock

i-utio JotnnJo thorium-bearing vein - ....-.-

DO... .... - -- : - - - 

CalClte-apaUte-martite vein in Iron Sill carbonate roeit"~ Do """" """ "" -------

Carbonate vein in pyroxenite east of Iron Hill. 

Do................................. ....... 

Number of 

samples

1

2

1

1 

3

1

Nb (percent)

o.x

.ox 

 >^

.OX . X 

- ox 

.OX

.OCX

Sulfide-bearing ca.x-bona.te veins in the Iron Hill stock have been 

PrOSP6Ct6Cl fOr Silver) aild Small amounts of manganese are found 

both in the Iron Hill composite stock and in unrelated QGpOSltS 111

the Tertiary volcanic rocks (Harder, 1910),

A feW Of the gOM-bearing quartz-chlorite veins in pre-Csxwibrian

metamorphic rocks have been tested lor radioactivity, bllt With 11111101 

exceptions radioactivity appears to be negligible (Burbank and Pierson, 1953, p. 2). Essentially all significant radioactivity known ill 

the district seems to be associated with the Iron Hill composite stock 

and its related intrusive rocks and veins.
THORIUM AND RAKE-EARTH MINERALS, POWDERHORN DISTRICT 701 

THOBIUM DEPOSITS

The known radioactive deposits in the Powderhorn district are of 

three types: (1) mineralized shear zones in pre-Cambrian foliated 

rocks; (2) carbonate-rich, tabular veins formed along other fractures, especially abundant in rocks of and adjacent to the Iron Hill 

intrusive complex; and (3) weakly radioactive zones in nontabular 

masses of carbonate rock, such as the marble of Iron Hill.

The radioactivity in the deposits sampled appears to be due almost 

entirely to thorium and its daughter products, mostly in the form of 

thorite and thorogummite (hydrothorite). Of 33 samples analyzed 

chemically for uranium, the maximum uranium content found in any 

sample is 0.005 percent; most contain from 0.001 to 0.002 percent. 

Radium has been determined in four samples from thorium-bearing 

veins, and the highest radium content obtained is 2.7 x 10"11 percent. 

The thorium oxide content of selected high-grade samples from the 

Little Johnnie vein is as much as 4 percent. The Th02 content of the 

veins is generally less than 1 percent, however, and is only 0.05 to 0.1 

percent in many of the veins indicated on the geologic map (pi. 55) 

as abnormally radioactive.

The greatest radioactivity in the district comes from material found 

in mineralized shear zones, a foot to several feet wide, in which the 

introduced material forms numerous closely-spaced films and veinlets 

in fractures in the country rock. Near the veinlets the wall rock 

commonly is partly replaced and greatly altered. Masses of wall rock, 

generally iron-stained, are found in many places within the mineralized shear zones, which are commonly discontinuous and variable in 

width.

The large mass of carbonate rock at Iron Hill is radioactive in at 

least two localities (12 and 13, pi. 55), where readings on a scintillation-type counter were locally as much as 0.3 and 0.14 mr per hr,1 

respectively (background 0.03 mr per hr). At both these localities 

the carbonate rock, unlike most of that at Iron Hill, is banded because of streaks of minerals such as magnetite and phlogopite. In 

general, the carbonate rock of Iron Hill and in several smaller masses, 

such as the ones in Sammons Gulch, which is 1.3 miles east of Powderhorn, and on the ridge south of Milkranch Gulch, shows negligible 

radioactivity.

STRUCTURAL FEATURES

The thorium-bearing veins are a few inches to 18 feet wide and a few 

feet to more than 3,500 feet long. Most of the veins dip essentially 

vertical; few dip less than 60 degrees. The strikes of the veins are not 

consistent throughout the district. In general, the veins near Iron

1 Mllllroentgens per hour.
702 CONTRIBUTIONS TO ECONOMIC GEOLOGY

Hill have a crude radial arrangement with respect to the central mass 

of Iron Hill carbonate rock. The veins and mineralized shear zones 

near the Little Johnnie claims generally strike northeast to east, 

and those in the Dubois-Lake Fork area strike variably northwest to 

northeast.

The thorium deposits have not been exposed at depth, with the 

exception of a few exposures underground in adits at Dubois. The 

length and tabular form suggest that some thorium veins may persist 

to depths of hundreds of feet. Thorium-bearing veins are exposed 

in the bottom of Lake Fork Canyon, 1,000 feet below exposures of 

similar veins on the mesa to the east; the Little Johnnie vein is exposed 

for a vertical distance of more than TOO feet.

Well-defined carbonate veins are particularly common in the Iron 

Hill complex. Locally these veins contain fragments of wall rock. 

The great lengths, 3,500 feet or more, of some of these veins and the 

rather constant widths suggest that they were emplaced in fractures. 

Many of the carbonate veins have a foliation or banding, shown by the 

concentration of various minerals in streaks parallel to the walls of 

the vein.

Among the deposits in the rocks surrounding the Iron Hill complex, the few veins or mineralized shear zones in relatively massive 

granite, such as deposit 31 (pi. 55), appear to be shorter and less persistent than veins in foliated or layered rocks, such as amphibole and 

chlorite schists and gneisses near Dubois and the interlayered quartzite, schist, and metarhyolite porphyry in the vicinity of the Little 

Johnnie claims.

MINERALOGY

The thorium-bearing deposits are composed of calcite, dolomite, 

siderite, ankerite, quartz, barite, pyrite, sphalerite, galena, hematite, 

goethite, apatite, thorite or thorogummite, xenotime(?), and alkali 

feldspar. In addition, rare-earth minerals, including bastnaesite, 

cerite(?), and synchisite, were found in carbonate veins near Iron

Hill. The carbonate veins in the Iron Hill complex also contain

several silicate minerals; among them are aegirite, sodic amphibole, 

phlogopite, zeolites, idocrase, melilite, diopside, garnet, and other

minerals (Larsen, 1942, p, 31),

Thorium is known to occur as thorite or thorogummite in some of 

tKe deposits, but many of the radioactive deposits have not been studied sufficiently to identify the thorium-bearing mmerai or mm*

erals. Among the carbonate-rich veins near Iron Hill, the more 

radioactive deposits appear to be those containing unweathered sulfide

minerals, such as pyrite, but such unoxidized material is not common 

in surface exposures. The more oxidized veins and mineralized shear 

zones in near-surface exposures, characterized by abundant powdery
THORIUM AND RARE-EARTH MINERALS, POWDERHORN DISTRICT 703

goethite, also are appreciably radioactive, and the thorium is apparently chiefly associated wtih the iron oxides, perhaps in part as thorite 

or thorogummite.

Several minerals containing the rare-earth metals of the cerium 

group occur in the Iron Hill carbonate rock and in veins east of Iron 

Hill. The fluorapatite in parts of the large mass of Iron Hill carbonate rock and in many of the carbonate veins contains rare-earth 

metals of the cerium group. In addition to the widespread rare-earthbearing apatite, X-ray patterns by A. J. Gude, U. S. Geological Survey, show that bastnaesite is one of the rare-earth minerals present in 

a sample from a prospect 0.2 mile southwest of locality 10 (pi. 1). 

Another mineral in this same sample has an X-ray pattern that 

matches the pattern of cerite( ?) from Jamestown, Colo., although the 

identity of the Jamestown cerite(?) used as a reference standard is 

not certain. A third mineral containing abundant rare-earth metals 

of the cerium group was found in samples from localities 8 and 10 

(fig. 2). This mineral is provisionally considered to be synchisite by 

Fred A. Hildebrand of the U. S. Geological Survey, on the basis of 

X-ray powder patterns. It seems likely that several other minerals 

containing rare earths as major constituents, such as monazite and 

rare-earth carbonates and fluocarbonates, may be found by additional 

detailed mineralogical studies.

Hematite and goethite are sufficiently abundant in some deposits, 

particularly in mineralized shear zones, such as the Little Johnnie 

deposit, to impart a red- to yellow-brown color to them that is an aid 

in prospecting. This color is not an infallible guide to thorium, 

however, for some zones relatively rich in hematite and goethite have 

no radioactive minerals.

RELATION TO ALKALIC IGNEOUS ROCKS

The map of the district (pi. 55) indicates a spatial relation between 

thorium-bearing deposits and alkalic igneous rocks. Most of the 

known thorium deposits are in three areas: near Dubois, near the Little Johnnie claims, and in the Iron Hill composite stock. In each of 

these areas alkalic igneous rocks are present.

Among the elements that characterize the mineralized shear zones, 

the carbonate veins, and the alkalic igneous rocks are titanium, 

barium, strontium, niobium, phosphorus, and the rare-earth metals. 

The abundance of these elements in the mineralized shear zones and 

the carbonate veins is indicated in table 2. These elements are also 

present in greater-than-average amounts in the alkalic igneous rocks, 

and the geographic and geochemical relations of the thorium-bearing 

veins and the alkalic rocks strongly suggest a genetic tie between them.
704 CONTRIBUTIONS TO ECONOMIC GEOLOGY

TABLE 2. Abundance of certain minor elements in spectrographic analyses 

samples from mineralized shear zones and caroonate veins

[R. Q. Havens and P. R. Barnett, U. S. Geological Survey, analysts]

Element

Ba._... ....... .......

Ti... .................

8r........ ............

Th...... .............

La....................

Ce....... .............

Nd.......... .........

Number of samples in 

which element exceeds 0.1 

percent

Mineralized 

shear zones 

(6 samples 

analyzed)

6 

5 

0 

4 

0 

0 

0

Carbonate 

veins (8 

samples 

analyzed)

8 

4 

4 

0 

4 

4 

4

Element

Sm.. .................

Y- .. ....... .... ...

Nb._ .................

P.............. .......

V........... ........ ..

ZT.. .......... ........

Number of samples in 

which element exceeds 0.1 

percent

Mineralized 

shear zones 

(6 samples 

analyzed)

1 

3 

1 

0 

0 

0

Carbonate 

veins (8 

samples 

analyzed)

0 

0 

1 

3 

2 

2

Larsen (1942, p. 29) has pointed out the abundance of potash in 

many of the alkalic rocks of the areas surrounding the Iron Hill complex, in contrast to the relative abundance of soda and lime in the 

igneous rocks within the complex. However, some sodic dikes, such 

as those composed of soda syenite, have also been found beyond the 

Iron Hill composite stock. Thorium appears to be associated with 

alkalic rocks of both the potassic and sodic types, and this suggests 

that other thorium deposits may be found by prospecting in the vicinity of the syenitic rocks in Wolf Gulch, Wildcat Creek, and Willow 

Creek.

AGE

The thorium deposits are known to be older than the Alboroto 

rhyolite of the Potosi volcanic series of Miocene age. ThorillHl-bearing veins out tke pyroxenite and other iglieoUS rocks in the Iron Hill

complex. The association of the veins with the rocks of the Iron Hill 

composite stock and with other alkalic igneous rocks in the district

suggests that the thorium-bearing veins formed during the late stages 

of the iron Hill igneous activity. The vein on the Little Johnnie 

claims, however, may be cut by a gabbro dike. The contacts between

the Little Johnnie vein, and the dike are not exposed, but no abnormal 

TclCliOJlCtivity Wa5 detected across a 100-foot width of the dike along

the projected strike of the vein. The gabbro contains anataSG and

brookite along fractures, and the presence of these titanium minerals

Suggests the dike is related to tKe Iron Hill complex. Accordingly

<he thorium deposits may be older than some of the alKalic rocks,

have not been dated other than pre- Jurassic.

The rocks of the district have been altered through weathering 

during pre-Jurassic, pre-Miocene, and Recent erosion cycles. Oxidation, leaching, and jasperization of the vein material during one or
THORIUM AND RARE-EARTH MINERALS, POWDERHORN DISTRICT 705

more of the three erosion cycles may have dispersed some of the radioactive material in parts of deposits that are near the erosion surfaces.

The veins on the mesa between Dubois and Lake Fork are altered 

in places to a jaspery material, which has chiefly replaced the carbonate gangue, locally forming pseudomorphs of carbonate minerals 

of the veins (Burbank and Pierson, 1953, p. 3). The jasper retains 

some of the radioactivity of the original vein matter. In some places 

where the veins are exposed beneath the volcanic capping of the mesas, 

jasperization extends to depths of 50 to 100 feet beneath the volcanic 

rocks; veins exposed on the lower slopes of recently cut valleys are 

not jasperized where examined. The volcanic rocks rest on an early 

Tertiary erosion surface that is closely coincident with the late Paleozoic or early Mesozoic erosion surface on which the Jurassic sedimentary rocks were deposited (Burbank and Pierson, 1953, p. 2). 

Thus, the jasperization of the veins probably results from weathering 

and leaching during one or both of the earlier of the three cycles of 

erosion; it may be partly related to hot-spring activity.

Several deposits show some evidence of increase in amount of radioactivity, and presumably an increase in thorium content, with depth. 

The apparent variation may be due to oxidation and leaching, as well 

as to jasperization of veins near the pre-Miocene erosion surface. At 

locality 20 (pi. 55), much of the vein outcrop shows radioactivity of 

0.07 to 0.10 mr per hr (background 0.03 mr per hr), whereas less 

altered rock from the vein, found on the dump of a 30- to 40-foot shaft, 

has a maximum radioactivity of 0.5 mr per hr. Locality 20 is just 

north of and below the exposure of the pre-Miocene erosion surface on 

which the volcanic rocks rest; hence this vein has been subjected to both 

pre-Miocene and the Kecent cycles of erosion. At locality 10 (pi. 55), 

a carbonate vein 1,500 feet or more long shows radioactivity of about 

0.07-0.10 mr per hr (background 0.3 mr per hr) along much of 

the oxidized surface outcrop, but unweathered carbonate material 

from this vein containing abundant siderite and pyrite, on the dump 

of a shaft slightly more than 25 feet deep, shows radioactivity of 

0.15-0.20 mr per hr. The greatest radioactivity measured by Burbank 

in the Dubois area was found 500 feet underground in one of the adits 

at Dubois, in a relatively unoxidized carbonate vein containing sulfide 

minerals. Vein exposures in the adit near the portal were more 

limonitic and showed less radioactivity, suggesting that oxidation of 

the veins and dispersal of radioactive material are related to the 

present erosion surface.

The jasper that has replaced part of the carbonate vein material on 

mesas is locally radioactive, but the most radioactive material in the 

district is not jasperized, and it is possible that some of the radioactive material in the jasperized veins was removed at the time of

367125 C6 3
706 CONTRIBUTIONS TO ECONOMIC GEOLOGY

jasperization. At locality 25 (pi. 55), greater radioactivity was noted 

in vein matter containing some carbonate mineral than in more 

thoroughly jasperized rock.

Although the high radioactivity of the deposits examined in the 

district is due to thorium, it is conceivable that some uranium was 

leached from the surficial parts of veins, especially those near the 

erosion surfaces on which the Mesozoic and Tertiary rocks were deposited. Although there are indications that some radioactive material may have been removed near the surface, and that less weathered 

rock at depth may contain a somewhat greater concentration of radioactive material, the fact that relatively unweathered radioactive rocks 

that have been analyzed contain no more than 0.005 percent uranium 

suggests that the variations in radioactivity probably result from 

variations in thorium content.

DISTRIBUTION OF RARE EARTHS

Eare-earth-bearing minerals have been found in the Powderhorn 

district in carbonate rocks, pyroxenite, and thorium-bearing veins. 

Rare-earth metals of the cerium group predominate and were detected 

by field observation of absorption bands, due to neodymium, in a hand 

spectroscope.

The most widespread rare-earth-bearing mineral in the carbonate 

rock is apatite, which constitutes several percent of the carbonate rock. 

The hand spectroscope indicates that the rare-earth content of the 

apatite is not uniform in different parts of the Iron Hill carbonate

mass. In some areas several hundred feet Wide tll6 apatite Contains 

fare earthS, but in Other areas the apatite apparently contains little or 

no rare earths, on the basis of tests with the hand SpectrOSCOpe.

The apatite is pale green to White and occurs in linear streaks along

shear planes in the carbonate rock. The apatite grains COmmOnly 

Stand OUt in relief On weathered surfaces. Apatite in apatite-martite

veins from a few inches to 3 feet wide that cut the carbonate body also

contains rare earths. Near the top of Iron HIII, 4 Pits have been dug 

in a zone of carbonate rock at least 10 feet wide and 200 feet long that

is richer in rare-earth-bearing- apatite and more iron-Stained than the

common type of carbonate rock, Eare-earth-bearing apatite also

occurs in some of the carbonate veins both in the pyroxenite and in preCambrian rocks surrounding the Iron Hill complex. Ka3re-ecvrtn metais nave teen found in small percentages in apatite

from many localities, probably substituting for calcium in the apatite 

Structure. Specimens of apatite having a range in rare-earth content

from a trace to 3,18 percent nave been reported by StarynkevichBorneman (1924, p. 41). The rare-earth content ranges from a trace 

to about 5 percent in apatite samples from various types of occurrences 

tabulated by Fersman (1924, p. 43-44).
THORIUM AND RARE-EARTH MINERALS, POWDERHORN DISTRICT 707

Spectrographic, chemical, and radioactivity analyses of carbonate 

rock containing rare-earth-bearing apatite are summarized in table 3.

In addition to the apatite, five samples of perovskite concentrates 

from the pyroxenite all have lanthanum, cerium, and neodymium in 

amounts of tenths of a percent, and yttrium in the hundredths, on the 

basis of spectrographic analyses by A. T. Myers, P. J. Dunton, and 

J. D. Fletcher of the U. S. Geological Survey. Samarium and 

praseodymium were determined in three of these perovskite samples 

in amounts of hundredths of a percent. All the samples from the 

vicinity of Iron Hill, whether of apatite-bearing carbonate rock, 

perovskite concentrates from the pyroxenite, or carbonate veins, show 

a high ratio of cerium group to yttrium group of rare-earth metals.

TABLE 3. Analyses, in percent, of carbonate rock containmff rare-earth-bearing 

apatite, Iron Hill, Powderhorn district

Location of sample

I. West side, carbonate 

mass, Cebolla Creek ...

2. South side, carbonate

3. Northeast side, carbonate

4. Area at least 100 by 300 

feet, south side, carbonate mass, Beaver

6. Apatite-rich zone at least 

10 by 300 feet, on ridge 

near top of Iron Hill 

G.rApatite-martite vein 2 to 

3 feet thick, adit on 

west slope of Iron Hill. 

7.f Apatite-martite vein 150

8. Lens 4 by 18 inches, 

largely coarse apatite, 

on ridge west of top of 

Iron Hill. ... . 

Equivalent 

uranium '

0.001

.001

.014

.001

.002

.005 

.002

.001

Combined 

rare-earth 

and 

thorium 

oxide >

(<)

.07

.07

.10 

.09

.09

s La

o.ox

.X

 X

.X 

.X

*Ce

O.OX

 X 

.X

3Nd

o.ox

 X 

.X

JY

o.oox

ov

nv

.oox 

.oox

J PiOj

3.6

7.4 

13.8

1O A

35.1

1 Determined by radlometric analysis by S. P. Furman, U. S. Geological Survey. 

> Determined by chemical analysis by J. W. Meadows, W. Mountjoy, and J. P. Schuch, U. S. Geological 

Survey.

»Determined by spectrographic analysis by P. R. Barnett, B. G. Havens, U. S. Geological Survey. 

* Dashes indicate that element was not looked for.

The results of 6 spectrographic analyses indicate that mineralized 

shear zones at localities 18,17,16, and 15 (pi. 55) several miles northwest of Iron Hill contain more yttrium than cerium, lanthanum, and 

other metals of the cerium group of rare earth metals. Of the 6 

samples, 2 have O.X percent yttrium, 1 has O.OX percent, and 3 have 

0.00 X percent. These values may be compared with the lanthanum 

content which is 0.0 X percent in 3 of the samples, 0.00 X percent in one, 

and only a trace in the other two. Yttrium also constitutes 0. X percent 

of one sample collected by Burbank (Burbank and Pierson, 1953, p. 8) 

from the vein at locality 21 (pi. 55).
708 CONTRIBUTIONS TO ECONOMIC GEOLOGY

Although these analyses do not indicate that concentrations of rareearth minerals are of commercial grade, they are of geologic interest 

for they show, together with the large area in which the rare-earthbearing apatite and perovskite occur, that the Iron Hill composite 

stock as a whole contains a relatively large quantity of rare-earth 

metals. The presence of rare-earth metals is reflected also by the 

presence of bastnaesite, cerite(?), and synchisite in carbonate veins 

in the pyroxenite east of Iron Hill.

LITTLE JOHNNIE CLAIMS

The Little Johnnie group of claims (loc. 18, pi. 55) *is in sees. 14, 15, 

and 22, T. 47 N"., E. 2 W., near the northeastern margin of the Milkranch Gulch drainage basin ; the west end of the area shown in plate 

56 is about a quarter of a mile east of the Lot mine. Radioactivity 

was detected on the Little Johnnie claims in 1949 by Neil Foreman and 

J. A. McGregor, of Powderhorn. Analyses of samples by the U. S. 

Geological Survey in 1950 indicate that the radioactivity is due chiefly 

to thorium and its daughter products.

GKENEBAL GEOLOGY

Most of the Little Johnnie area is underlain by pre-Cambrian metasedimentary and meta-igneous rocks. The foliation of these rocks 

strikes N". 50°-65° E. and dips steeply north or south, commonly within 

15 degres of vertical. The rocks are cut by carbonate-rich veins, 

small bodies of alkalic intrusive rock, and a large gabbro dike of

alkalic affinities that are believed to be related to the Iron Hill composite StOCk, WlliCh is about 4 miles to the south, The Alboroto rhyolite of the Potosi volcanic series (Tertiary) and the sandstone of the 

Morrison formation are exposed in the southwestern part of the Little

Johnnie area. These rocks are in fault contact With one another and 

with the pre-Cambrian rocks.

The thorium deposit is a mineralized, fault that strikes at a Small angle to the foliation of the enclosing metamorpnic rocks, and is offset

at Several places by faults younger than the Alboroto rhyolite.

The pre-Cambrian rocks on the Little Johnnie claims include quartzbiotite schist, quartzite, and amphibolite belonging to the Black canyon SChiSt 0± Hunter, and. metarhyolite and granite of nis Powderhorn granite group (Hunter, 1925) .

QTTARTZ-BIOTITB SCHIST AND QUAETZITE

The oldest and most abundant rock exposed in trie area is a quartzitic metasedimentary rock that includes several closely related f acies*
THORIUM AND RARE-EARTH MINERALS, POWDERHORN DISTRICT 709

The most common facies is a medium-gray, fine- and even-grained, 

biotitic quartzite which contains an estimated 5 percent biotite. With 

increasing biotite this rock grades into a quartz-biotite schist and, with 

decreasing biotite, into an almost pure, massive quartzite. Some specimens appear to be f eldspathic, and fine-grained feldspar is probably a 

constituent of most of the quartz-biotite rocks. A few specimens 

contain a small quantity of amphibole, and these are difficult to distinguish from some specimens of f eldspathic varieties of amphibolite. 

Outcrops are scarce or lacking in many parts of of the area, and some 

metarhyolite, syenite, and various types of amphibolite occur in the 

area mapped as underlain by this quartz-biotite rock unit shown on the 

map (pi. 56).

METARHYOLITE PORPHYRY

Metarhyolite porphyry is one of the lithologic types included by 

Hunter (1925, p. 41-44) in the granite porphyry of the Powderhorn 

granite group. Within the map area (pi. 56) it forms dikes that generally parallel the foliation and layering of the quartzitic metasediments. Pinch-and-swell features are common; in a few places the 

dikes bifurcate. At the eastern end of the area a metarhyolite body 

of unknown size and shape cuts sharply across the foliation of the 

quartzose metamorphic rocks. A similar discordant mass occurs on 

the southern edge of the area about 800 feet to the west.

The metarhyolite porphyry is readily distinguished from the quartzbiotite schist and quartzite by the presence of oval masses of quartz 

from 1 millimeter to more than 1 centimeter in diameter which commonly have a characteristic grayish-blue cast. Under the microscope 

many of these masses of quartz appear to be distorted phenocrysts, 

and the degree of distortion appears to vary with the size of the original crystals. Some of the smaller crystals have the outline of perfect 

bipyramids. Medium-sized crystals commonly have broken and displaced crystal faces, and some of the larger masses exhibit strong 

granulation.

The porphyritic texture of the rock varies with the size and abundance of the quartz masses. Individual phenocrysts are as small as 

1 millimeter but commonly range from 3 to 6 millimeters; at the Lot 

mine they attain a maximum length of nearly 2 centimeters. The 

quartz phenocrysts constitute from 10 to 15 percent of the typical 

metarhyolite porphyry.

The quartz crystals are set in a fine granular aggregate of quartz, 

alkali feldspar, and biotite. A thin section of a specimen collected by 

J. W. Adams from the sheared and silicified wall of a vein on the 

Jeanie No. 6 claim (loc. 16, pi. 55) shows orthoclase; albite; quartz,
710 CONTRIBUTIONS TO ECONOMIC GEOLOGY

in part secondary; biotite, partly altered to chlorite; and sericite as 

the principal constituents. Apatite, zircon, pyrite, iron oxides, and 

allophane are present in minor amounts.

AMPHIBOLITE

Amphibolite forms dikes and irregular bodies that intrude both the 

quartz-biotite rocks and the metarhyolite porphyry. Most of the dikes 

parallel the foliation of the country rocks, but several discordant dikes 

strike nearly east.

The amphibolite is typically a dark-green hornblende-rich rock 

containing as much as 85 percent hornblende in crystals from 1 to 5 

millimeters long. Less common, lighter colored types contain plagioclase feldspar in varying amounts. A dark-gray amphibolite from 

the west end of the area contains an estimated 40 percent hornblende, 

25 percent oligoclase, 20 percent quartz, and 10 percent biotite, with 

minor amounts of sericite, clinozoisite, apatite, and magnetite. Both 

the feldspar and the mafic minerals are corroded and embayed by 

quartz, which may have been introduced into the rock. Much of the 

amphibolite at the west end of the area is light to medium gray and 

contains small crystals of amphibole in a quartz-feldspar matrix. 

These rocks are transitional in appearance to some of the amphibolebearing quartz-biotite schist. Schistosity is poorly developed in the 

amphibolites.

The darker amphibolites probably represent metamorphosed intrusive rocks of basic to intermediate composition. In a few places

the light- to medium-gray amphibolites appear to be cut by metarhyolite, These older amphibolites are of unknown origin but may

be metasedimentary.

GRANITE

In the Southwestern part of the area (pi. 56) the rocks designated

as undivided pre-Cambrian include the metarhyolite porphyry, the 

quartz-biotite metasedimentary rocks, and a few patches of pink,

coarse-grained, equigranular and porphyritic granite. The granitic

rOCks are Similar to those described and mapped by Hunter (1925,

p. 44r-46) as the porphyritic biotite granite of the PowcUrkorn group.

QTJA.TtTZ VEINS

Quartz veins ranging from a few inches to slightly more than a

fOOt in Width, OCCUr in the pre-Cambrian rocks in and adjacent to the 

map area, (pi. 56). The veins cut the amphibolite, the quartz-biotite

rocks, and the metarhyolite. The quartz veins are probably of preCambrian age and are chiefly of ,two kinds: quartz-chlorite and

quartz-tourmaline.
THORIUM AND RARE-EARTH MINERALS, POWDERHORN DISTRICT 711 

ROCKS RELATED TO THE IRON HILL COMPOSITE STOCK

The rocks described below occur as dikes and veins that cut the preCambrian metamorphic rocks. The composition of these rocks is not 

uniform, but all show some chemical or mineralogic affinity titanium 

minerals, sodic amphibole, sodic pyroxene, abundant alkali feldspar, 

and abundant carbonate with one or more of the rock types of ,the 

Iron Hill composite stock, and they are probably the same age as the 

Iron Hill rocks. The age relationships of the different rock types in 

the Little Johnnie area (pi. 56) could not be determined because of 

poor exposures.

GABBBO

A gabbro dike cuts the quartzose metasedimentary rocks, the metarhyolite porphyry, and the amphibolite near the east end of the area. 

The dike is about 75 feet thick and strikes about N. 60° W. At one 

place the gabbro appears to cut a linear concentration of fragments 

of breccia that probably marks the location of a breccia vein.

The most common facies of the gabbro is a medium-gray equigranular rock that contains from 30 to 40 percent labradorite as lathshaped crystals about 2 millimeters long. The feldspar laths are 

randomly oriented and intergrown with pyroxene, giving ,the rock 

a coarse ophitic texture. Magnetite is abundant and makes up an 

estimated 8 percent or more of the rock. This medium-gray facies 

grades into a dark-gray rock that probably contains only from 10 

to 15 percent plagioclase, bujb which is otherwise similar to the more 

feldspathic varieties.

The pyroxene in all specimens examined with the microscope has 

a 2V of 20-30 degrees and probably is pigeonite. Primary accessory 

minerals include abundant prisms of apatite, magnetite, and a few 

small scattered phenocrysts of orthoclase. Several grains of greenishbrown hornblende, probably uralite, are present. Both ,the hornblende and the pigeonite are replaced locally by a fibrous, dark-blue, 

pleochroic soda-amphibole. Other alteration products include sericite, epidote, allophane, iron oxides, and carbonate. Except for the 

lack of quartz, this rock is similar to the quartz gabbro .that Larsen 

(1942, p. 28-29) has described from the Iron Hill area.

In contrast to the equigranular rocks described above, a few specimens from the dike area are porphyritic. These rocks contain an 

estimated 10 percent labradorite as phenocrysts approximately 1 

millimeter long, which are set in dark-gray aphanitic groundmass.

The distribution of the textural and compositional varieties within 

the dike is unknown, but the equigranular types are believed to be 

differentiates of approximately the same age. The association of the 

equigranular gabbro and ,the fine-grained porphyritic facies, and the
712 CONTRIBUTIONS TO ECONOMIC GEOLOGY

similar composition of the plagioclase in these rocks, indicate a genetic relationship between them. The texture and relative scarcity 

of the porphyritic rocks suggest that they occur as small dikes within 

the main gabbro body.

Thin films of brookite and anatase coat fracture surfaces in parts 

of the gabbro dike where it crosses the ridge. Associated with these 

titanium minerals on the fracture surfaces are minor amounts of 

aegirine, quartz, albite, and potash feldspar ( ? ) . Both the quartz and 

the feldspar appear reddish brown when examined with the hand lens 

because of small amounts of included iron oxides ( ? ) . The aegirine 

ranges from medium green to very light green ; under the microscope 

fragments as much as 0.04 millimeter thick are almost colorless with 

very little pleochroism. The quartz is biaxial and has a 2V estimated 

at from 15 to 30 degrees. The occurrence of anatase is similar to 

that described by Larsen and Hunter (1914, p. 470) from a locality 

about half a mile northeast of the Lot mine.

Samples of the anatase and brookite contain 0.06 percent niobium 

and 2 percent niobium respectively, on the basis of spectrographic 

analyses by Paul R. Barnett of the U. S. Geological Survey. The 

striking difference in niobium content of these polymorphs from the 

same geologic environment may be due in part to a difference in crystal 

Structure. Anatase is tetragonal, whereas brookite and columbite are 

orthorhombic dipyramidal. The X-ray powder pattern of brookite 

matches that of columbite much more closely than does the anatase 

pattern. These facts suggest that niobium may svibstitute for titanium

in the brookite structure more readily tliail It d06S 111 the a

structure.

SYENITE

Pink tO reddish-Orange, fine-grained, porphyritic syenite 

intrude the pre-Cambrian metamorphic rocks at S6V6rill pkCeS IB tile 

ai'ea. and probably Correspond to some of the syenite described by 

Hunter (1925, p. Tfi). No OUtcrOpS Of SyCnlte W6r6 f Guild, but the

linear pattern of syenite fragments in the noat and a Aveii-de^eloped

primary foliation in many of the fragments indicate that the Syenite

occurs as dikeS. PotaSu feldspar, as plienocrysts from i to 

meters long, IS the only identifiable mineral in most hand

in some rocks these phenocrysts are closely packed ; in others they are

Sparselj disseminated. TJnder tKe microscope, almost all material

from the fine-grained groUlldmaSS Is 86611 tO be Cloudy alkali feldspar. 

Specimens from the Syenite dikes near the west end of the area are 

altered, poroxis, and contain an unidentified mafic mineral.

CARBONATE VEINS

At three localities in the area there are linear concentrations of finegrained carbonate rock in the float. These probably represent veins
THORIUM AND RARE-EARTH MINERALS, POWDERHORN DISTRICT 713

similar to those in the vicinity of Iron Hill. The vein material is 

predominantly fine-grained carbonate in grains less than 0.5 millimeter in diameter, mixed with and stained by iron oxides which color 

the specimens yellowish-brown. In thin section, sparse euhedral 

crystals of potash feldspar are seen to be surrounded and embayed 

by the carbonate, which is irregularly mottled in various shades of 

yellowy and brown. Apatite is abundant, both as anhedral masses 

which in part replace the feldspar, and as euhedral crystals surrounded 

and in places embayed by the carbonate matrix. Small nests of quartz 

enclosing needles of apatite are sparingly present and appear to be 

older than the carbonate. A single immersion mount revealed the 

presence of a green pleochroic pyroxene and a little chlorite in addition to the minerals noted above.

BRECCIA VEINS

Several breccia veins occur in the east-central part of the area 

(pi. 56). The veins contain angular to subangular fragments, as much 

as 3 inches in diameter, of pre-Cambrian rocks in a matrix composed 

chiefly of carbonate. The most abundant fragments are pink coarsegrained granite similar to the porphyritic biotite granite of Hunter 

(1925, p. 44-46), but amphibolite and quartz-biotite (?) rock have both 

been observed. The fragments commonly constitute from 40 to 50 

percent, rarely as much as 80 percent, of the rock.

The fine-grained, reddish- to chocolate-brown matrix contains a 

large percentage of carbonate, effervesces violently in acid, and is 

altered to a soft, brown, porous coating on weathered surfaces. The 

groundmass contains pink feldspar in euhedral tabular crystals' and 

irregular fragments, many of which were probably derived from the 

granite inclusions. Fine-grained pyrite and galena(?) are sparsely 

disseminated in the groundmass of a few specimens.

In thin section the breccia veins are similar to the fine-grained 

carbonate veins except for the presence of the included fragments. 

Most of the fragments are quartz, potash feldspar, or aggregates of 

these two minerals. The minerals and rock fragments commonly are 

embayed and partially replaced by the carbonate matrix. Feldspar 

is the most strongly attacked, but the quartz grains in an inclusion of 

quartzite are also rimmed and partially embayed by the carbonate.

The abundant coarse-grained granite inclusions are unlike any of 

the rocks exposed near the breccia veins and have probably been transported hundreds of feet by the vein-forming fluids.

SODIC AMPHIBOLJB ROCK

A small vein or dike about 2 inches thick, containing abundant blue 

amphibole, cuts the quartz-biotite schist in the north-central part of
714 CONTRIBUTIONS TO ECONOMIC GEOLOGY

the area. Specimens from unaltered rock are bluish gray; altered-rock 

specimens are bluish gray, chalky gray, and light brown. Crushed 

fragments examined in index liquids show a fibrous pleochroic amphibole intergrown with a fine granular substance, either isotropic or very 

faintly birefringent. Other minerals include potash feldspar, apatite, 

chlorite, carbonate, soda-pyroxene, clay minerals, and iron oxides.

A small filled vug about 1 inch long in the sodic amphibole rock 

contains aegirite, dahllite, brookite, iron oxides, thompsonite(?) as 

fine botryoidal films, and probably other zeolites. The dahllite occurs 

as tiny hexagonal crystals about 0.3 millimeters across, with the following optical properties: biaxial negative, 2V about 15 degrees, nY= 

1.632, nZ-nX=0.008.

SEDIMENTARY AND VOLCANIC ROCKS

Sandstone and a little siltstone of the Morrison formation of Jurassic age crop out in the southwestern part of the Little Johnnie area. 

The sandstone is medium to coarse grained, well cemented, and commonly reddish brown and dark-reddish purple; some specimens are 

light gray. Banded and mottled rocks are common.

Light-gray to buff Alboroto rhyolite crops out along the southwestern margin of the area.

STRUCTURE

The drainage basin of Milkranch Gulch is a roughly circular depression about 11/2 miles in diameter floored with Alboroto rhyolite and 

surrounded by hills composed of pre-Cambrian rocks. The east end 

of the map area (pi. 56) is on the divide which forms the northeast side 

of the basin); the southwestern part of the map area extends into the 

basin and includes the northeastern edge of the central area of volcanic 

rocks. Here, sandstone of the Morrison formation Of Jurassic age is in 

fault contact with both the volcanic rocks and the pre-Cambrian rocks. 

Beyond the map area (pi. 56) the periphery of the central area of 

volcanic rocks was examined at four places, and discontinuous outcrops of Jurassic sandstone were found at three places along the IlOlth 

contact of the volcanic rocks and at one place on the south contact.

The nearest exposures of the sandstone of Jurassic age beyond the 

Milkranch Gulch basin are on Huntsman Mesa (see pi. 55), about \

mil© east Of the Little Johnnie claims at an altitude of nearly 10,000 

feet. Here the Sandstone IS alniOSt hOfiZOntcal and extends along'the

top of the mesa for about 4 miles. This position probably represents

the common attitude ^of the Jurassic rocks in the diStriCt, and the

sandstone in the basin is thought to mark the edge of a roughly circular 

fault block iy2 miles in diameter, in which beds of the Morrison formation and the overlying volcanic rocks have dropped at least 1,000 feet. 

Where mapped, the margin of the subsided block is marked by a
THORIUM AND RARE-EARTH MINERALS, POWDERHORN DISTRICT 715

complex fault zone. In a few places the presence of individual faults 

is indicated by silicified breccia, altered and pyritized rock, and zones 

of sheared rock and gouge exposed in some of the bulldozed cuts. In 

general, the faults are poorly exposed; therefore they are only approximately located, and probably the true pattern is even more complex than that shown.

The overall structure in the fault zone is a jumble of tilted blocks 

bounded by faults which are irregular in strike, dip, and displacement, 

and the resultant pattern is difficult to interpret, except in a general 

way. Probably the major breaks are normal faults that dip toward 

the center of the basin. The net displacement of the zone is down on 

the south (basinward) side, but along many of the faults the northern 

block was downthrown. Many of the subsidiary faults probably dip 

to the north and some are thought to be reverse faults.

The full width of the fault zone is not shown on the map (pi. 56). 

South of the southernmost outcrops of pre-Cambrian rock, the Alboroto rhyolite contains several well-defined zones of altered and ironstained rock. These zones are roughly parallel to the faults shown on 

the map and are believed to be related faults. The few faults that are 

shown in the central and eastern parts of plate 56 are also probably 

related to the fault zone between the volcanic rocks and the preCambrian rocks.

THORIUM DEPOSIT

The Little Johnnie vein is a steeply-dipping mineralized fault at 

least 3,500 feet long. Irregular veinlets and seams containing quartz, 

hematite, goethite, alkali feldspar, thorite or thorogummite, and other 

minerals have been introduced along the fault in a mineralized zone 

ranging from less than 6 inches to 5 feet wide.

The Little Johnnie vein strikes N. 65° E., dips commonly within 10 

degrees of vertical, and is exposed through a vertical range of 700 feet 

in a distance of 3,500 feet. At the bulldozed cut near the west end of 

the area the vein dips 38° N., but this departure from the generally 

steep dip is probably due to displacement along faults younger than 

the Alboroto rhyolite. The vein is nearly parallel with the foliation 

of the enclosing pre-Cambrian rocks, but in general it strikes a little 

more easterly than the strike of the foliation, and in a few places it 

cuts across the foliation of the country rock at an angle of as much 

as 15 degrees.

The vein is poorly exposed and in most places it was located by 

scintillation counter. Points where radioactivity is above normal are 

shown on the map (pi. 56). Headings are given in milliroentgens per 

hour; background radiation for the area was approximately 0.025 

milliroentgens per hour on the counter used. The best exposure of 

the vein is in a 35-foot drift on the vein in the central part of the area;
716 CONTRIBUTIONS TO ECONOMIC GEOLOGY

here a small dike of altered and bleached amphibolite on the north 

wall of the drift is cut off by the vein, but the insufficient evidence 

does not indicate the direction or amount of movement. In places 

the vein is brecciated, indicating movement during or after mineralization.

Where exposed in trenches, the Little Johnnie vein ranges from 

less than 6 inches to about 5 feet in width; the average width along 

a length of 3,500 feet probably does not exceed 2 feet. At the east 

end of the map area (pi. 56) the vein either pinches out or contains 

no thorium. At the top of the ridge just northeast of the map area, 

the projection of the vein passes through an exposure of bedrock in 

which the vein does not crop out and the radioactivity does not appear 

to be abnormal. East of the ridge the bedrock is covered, but several 

abnormal radioactivity readings along the presumed projection of 

the vein suggest that the vein probably persists. About 1,100 feet 

eastward along strike from the east end of the vein as shown on the 

map (pi. 56), a narrow vein is exposed in a bulldozed cut (no. 19, 

pi. 55). This is probably a continuation of the Little Johnnie vein, 

although it possibly is a separate but related vein. Another vein 

about 6 inches wide and of unknown length strikes about N. 35° E. 

at the east end of the map area (pi. 55).

Exposures of the vein are commonly of a reddish- to yellowishbrown color that contrasts sharply with the dull hue of the preCambrian wall rocks. The vein material is heterogeneous in composition and grain size. Three thin sections, provided by J. W. Adams 

of the U. S. Geological Survey, show potash feldspar, albite, and 

quartz as the dominant constituents. Typically these are intimately 

mixed in a fine-grained aggregate clouded with fine hematite, goethite,

and some allophane; the percentages of quartz and the alkali feldspar

vary, and parts of two thin sections contain little quartz. Euhedral 

crystals of hematite, as much as 0.5 millimeters across, are common. 

Many are partly altered to goethite. Associated with the hematite 

and goethite are small subhedral to anhedral masses of thorite or 

thorogximmite. Apatite, as a felted mat of needles in the feldspar,

JS ftbllHdaJlt IE 1 °^ the 3 thin Sections examined, but is rare in the

other 2. Coarse-grained barite and carbonate m flbUJlcL'int ID 9i feW 

places in the vein, but were not identified in thin section. Some of 

the barite fluoresces pink.

Accessory minerals in the vein include clinozoisite(?), tourmaline,

fluorite, and rutile. Small euhedral crystals of a uniaxial positive

mineral are tentatively identified as xeixotime n^d probably account

for much of the yttrium content of the veins. An Unidentified DlWIlish-yellow mineral having high indices, moderate birefringence, 

probably biaxial negative A^7 ith moderate 2V, is present in small 

anhedral grains.
THORIUM AND RARE-EARTH MINERALS, POWDERHORN DISTRICT 717

The Th02 content of chip samples ranges from less than 0.03 percent to nearly 5 percent. Veinlets and thin stringers of thoriumbearing material are irregularly distributed in the mineralized fault 

zone. In the drift in the central part of the area of plate 56, several 

small thorium-bearing stringers branch from the main vein into the 

south wall. A similar subsidiary veinlet may account for the anomalous radioactivity about 18 feet south of the main vein 365 feet 

N. 70° E. of the portal of the drift.

Bedrock is not exposed where the vein intersects the gabbro, and 

the relative ages of vein and gabbro are unknown. Inasmuch as no 

anomalous radioactivity was detected within the gabbro along the line 

of strike of the vein, the thorium mineralization may have occurred 

before intrusion of the gabbro, but this is uncertain because of poor 

exposures.

The vein is offset at many places by faults that formed during the 

subsidence of the fault block that constitutes the Milkranch Gulch 

basin. Most of these faults are near the west end of the Little Johnnie 

map area.

OUTLOOK FOR FUTURE EXPLORATION

Except for the Little Johnnie claims, the district has been studied 

only in a brief reconnaissance. The conclusion seems warranted, however, that the district is a promising one in which to search for veins 

containing thorite and thorogummite. Many of the 33 radioactive 

deposits listed in table 4 appear to contain about 0.1 percent or less 

Th02 , but some higher grade samples contain from 0.5 to 4.9 percent 

Th02 . The finding of the thorium deposits in a large area through 

reconnaissance examination indicates that other veins, possibly containing on the order of 0.5 percent ThO2 , might be found by more 

detailed examination. Thorium deposits of this type have not been 

mined because of the limited market for thorium.

Because the known thorium deposits in the district are related 

spatially to the alkalic igneous rocks, the areas in or near the bodies 

of alkalic igneous rocks are considered the most favorable in which to 

prospect for thorium. Concentrations of other minor elements, such 

as the rare-earth metals and niobium, are commonly found in the same 

geologic environment. Although commercial deposits of rare earths 

are not known in the district, the discovery of rare-earth minerals 

suggests that such minerals should also be looked for in future 

exploration.

Although significant amounts of uranium were not found in the 

analyses of samples given in this report, uranium has recently been 

found in the Cochetopa district 20 miles east of Powderhorn, in preCambrian and Mesozoic rocks similar to those in the Powderhorn 

district, indicating that uranium should also be sought in future 

exploration.
718 CONTRIBUTIONS TO ECONOMIC GEOLOGY

*"* a. a

111

Sfl2

 ~* £§23 

3'«T3

ag.a din i>»

i|l a§3

§1?

03 .« >"

Ml- 0

o 5^3

1 O feCD-W 03 i- ^ et«

O r0 <g 03

o sfa o ^S-d

s 111 n1!

^ 3 -a cfl *-*

H « * «££t-* Q3

o $ >

^ ftt. 

s~ S> t, 3

I y;

I &=>.2

0 «?g> ^ §1.1

* gS°-

 a «5*" 2 m

-*2 «2 03 tD

§ *§S 

o ^"5

S 9-2 >, 1 11?

1 i^i

1 «9fe 1 a^S

^ Sfe 1^

 § ^^s 

e «^« . §> as-ss ? t-"M-t g-J^ £ d d?^ L, 1 ££ 5.°

1 IsSs

95 Ifl^l »* g^ft

I isSi

| !?i!

s l-°-l»

'. ^faSf_

^ +j fl^ B

a -g ^^ ^

M S S'SSB

5 lll°

S:3 S>S

*5.S3 £!l«

|||i

§£ ^.1

§sjl

sill

3

1

"3

^

1L

8

0 

03

03 ^

P03 ta

S15"

11

 c g

iadioacti adings (i

ffig

c

Cj c^

p03

,

03 3

OHM

1

§

1

S

J

V) do

a

1

0 

d

a

ft

S

0

'S

1

hi

a

 H

^

1

OS 

4-3

lapo 2

ft

X E-i

i«i

3ls

a

!«§ |»

 §§^§.ogo ® do d

§|feo3 O 0 t fi O C

5 o/g ^ fc n S.

S CO fe > f^

g

o

o 

o 

o

i .-£

"tn *

w <

§'-§rC

ft tr 5S* ! §!

o c.

10

c

t>

tr:

1?

i -

1

-2SV- <U !« «

JQ1 O ^J yj

 * «5 * Tf IM 00 CD CO T 

(Ni-HOOOOCOO ,-

5 R O

5J:§

O

CO CD ^- OO CO b- ^ O irtfH 1^ 

SC^» lOOi-HCOO C4 CO i t 

ooooooo ooo

o ' ' ' ...

ir

t

^(Ej O^

Ifll §

ea »ca fe 

O O

» 

o

I

&

1

1 g

1 1

C

. -c c T:

*"I 9

O (N IM O 

O . . iH

« 

-<J *"

1.5 £

 S3 c

03 0 6 « o

.. bJO 

If ^

r* o-^fl

' ^* "^ ^5 S M

S»§ £"3

'I'lrf^

 J§1||^1

S S rr-^ ^ +J '£ OJffl flfl't 

Q3 Q1 *O O**^ S ,£3 O* O O r

W So c

^

1 f

a K

P*

J2J pc

17

f |

S +:

o' fc

£ >

p«

OC

^

 S

1

w

 i

1

' T s

c

a

1, 

)

&

GO

^

t~

!?

us

t M- *? H S - 3 P5 -H 0 f

a £+> PI a»i «s g.aa s |

.2 it 3g :iS ||a 1^1 1

fill s! iy i

llll^li ||I^> g>3 I^^|

OO*O D 0"^ 1"" g- 00 -^

d ^ ^ rl

ct, -"a. S-^S.S

^j t5 ~ ,g M

s It SlsS H H fc t>*^> =1 <» a Sfl S 1* 0 -0

0 to

rHN«^»0 « NW
THORIUM AND RARE-EARTH MINERALS, POWDERHORN DISTRICT 719

 SJj'b iq

+3 o« "9 9

js cr CD £* cd

£-2o t5 ** ft 1

is

§£

o o

P 0

o S

fi -a

c3 2 . CD ,q

2 3o:SftB 2 O O C? O

.2

I O

115 pill

8

« cc

S

Ao

. ?? 

a ft

®"OS

'C ,

^»S i

.JJ <B f

«a*f c 

««a c C CD o j: S o 5 "t S w>« «

CO V.

R

o

CN

^

CO 21 2

S o en 

 H 0

S

111 1 s- \ I

Ojg o * S tJ c

ffl^s ffl« § *

-£» 03 rt » -2s hD D* ^

c3 9$ C3 fl JT |il |ia>« t

 P 03 ft "P SS ,"t2 -^H C g >- ca gxS-ufl 5 

O O D O

i "2

1 ^ ®

g Sa

» fe

J

«

1

H

i

I 1

o cs "3 

§ § - §fc 1

C3

T? | c

8

lr~ O "3 »O CO CO CO »O * - b- ^ O O> CN ( - Tft IO 00 tf3 CN 

O "H -! O --I O t~ CN CO CO CO O r-l rH i-H rH 00 O O O O

 * -H O

f--0<OOtOgC>t- « 0^ rt OJ-3-05 g

Soooooo2o22Scoo8SoiHoo8 8

o

1 I CN >fi CN C

i > O V

I O C3 O>

1 eX O O

I rH

'O"Xl .(^^-iS tljQ "*'ScD

" rf a . s'co «<sf I a -S tsi«: 1 11 ?! I|l| i saMs B g-3 gg ^6 <= a 9 &.g «^

ll 11 1.1 ill! II lilll ? & & <y & <y o o

i | i

i ?? ii j 1 1 1

CQ Kg Jz; ra > w

: w > W H ^ ^ P

S »O kO P ^ 

U3 tO l-» lo

^ - Z ti Z Z Z Z z %

45 oxi " ^o ' t3

| g. SM g8.|§°-.9 | §

11 g

i

cc

Iz

a

^ll «« °l|'-S^|o- I *.!

o *^ g"" 8^ a ^ ^ |

i w ^ -w qj

1 «*> g

i > « S 

I 10 >

S . S ^ 1 F1 '1 ^

x|g .2 |g.s g| -3 s| «a"|s^ §| . %$ |.|«

M

T*

fls | S|L-9f .9 5^ ^2 M «| M °^ .S 5 |||

I-9« 5 -aj"l SflSSgg -818 §« 1|| ipi ]i|i p p i «

8

^o

S

1

Sa

,11

,

&

£

{ 

C

-c
720 CONTRIBUTIONS TO ECONOMIC GEOLOGY

,x

o

 l>1

"5

03 1! s> M

p03

CO

^

"3tS

||

lao fl

« £

6

H

P03

ii

ag 

oa

Minerals

!

?

^

s.

o's

Sa

1

s

M

a 

S

a)

S

,d

H

^

s

 s

 «

*O !lp. sg 

s>> E->

III

« 0 jj) 03 "2 ^

S'S ® "^

"Q. <13 ^ § « .

rt -r) 60 w en 03 73 

H f) bJO '2*J fl

"^n-l ° S-S5 'S § ® ® '5» M t? ^--tS

OH^ W Hci >^o W W '-- ^3 -2 ^

33s oaofllo d :" o* -^B

S WO 0 § S

>r

ro <N n ri

ill l§ i » « ""- e<

Quartz, jasper, h tite, goethite, rite(?), minor 

rite. 

Jasper, hematite thite,horie(?: 

bonate.

I >>

& CQ

m &

i i

0 0

f) "3

Kr* ^7

.'

i

 S

s

w

w

to"

ffl

1

3

00 P

d, <M J.

i

i .j

! 'a 

i §

to fl i3 w

S. g 

TO ft

.a .a o 

.a .a ^

goethite.

cq ^aj § oococons cooo

r*

O <NO O OOOO OO

c

C

C 

X

c;

c.

i 

«

o

a

K

*

ft)

a-l

B S"^

Pv

P

«S

^ ft

a ig t!

Carbonate adits and

...do

 o * co eq cq

oi

<N W

°-

1 || | ff d |js |

fl .a ,a xi u- § o- ° r ^ -^ w ca & ^ "-^ S JS3-S ^

O.MP aSiOfl 3:3 g " ^'S-S s 

C? c? 0? W 0

a

trtft

C

g' £ fe

t» 88 g

!> >.W t-' t>

Q '?f *O n n

0 _§ * 00 IT

fc £ !?' fc

I'-g, ^

*t 3

fi"0 2

i *i|e g|

f

^ "" ^M^

g

gafe sphalerite, 

-i~.a^2S

I fl S^«O-Plp4ct5

M CQ N O

1 IO M+*

i H 03*" M

^j "Ov (T^

o d ® ^ +5

 S .J3 a) Ji s> bo -9

.g ag B|§ .s ^ o1"" '^ N " >

S <38&8S8?.S a

# # * ~
THORIUM AND RARE-EARTH MINERALS, POWDERHORN DISTRICT 721 

LITERATURE CITED

Burbank, W. S., and Piersoii, C. T., 1953, Preliminary results of radiometric

reconnaissance of parts of the northwestern San Juan Mountains, Colo.:

U. S. Geol. Survey Oirc. 236, 11 p. 

Cross, Whitman, and Larsen, E. S., 1935, A brief review of the geology of the

San Juan region of southwestern Colorado: U. S. Geol. Survey Bull. 843,

13S p. 

Fersman, A., 1924, Sur la presence des terres rares dans les apatites des gisernents divers : Coruptes rendus Acad. Sci. Russe, p. 42-45. 

Fleischer, M., Murata, K. J., Fletcher, J. D., and Narten, P. F., 1952, Geochemical

association of niobium (columbium) and titanium and its geological and

economic significance: U. S. Geol. Survey Circ. 225,13 p. 

Harder, E. C., 1910, Manganese deposits of the United States: U. S. Geol. Survey

Bull. 427, 298 p. 

Hill, J. M., 1909, Gold and silver. Notes on the economic geology of southeastern

Gunnison County, Colo.: U. S. Geol. Survey Bull. 380-A, p. 21-40. 

Hunter, J. F., 1925, Pre-Cambrian rocks of Gunnison River, Colo.: U. S. Geol.

Survey Bull. 777, 94 p. 

Larseu, E. S., 1942, Alkalic rocks of Iron Hill, Gunnison County, Colo.: U. S.

Geol. Survey Prof. Paper 197-A, p. 1-64. 

Larsen, E. S., and Hunter, J. F., 1914, Melilite and other minerals from Gunnison

County, Colo.: Washington Acad. Sci. Jour., v. 4, no. 16, p. 473-479. 

Starynkevich-Borneman, J., 1924, Sur la presence des terres rares dans les

apatites: Comptes rendus Acad. Sci. Russe, p. 39-41.

INDEX

Alboroto group__ .. 698

Amphibolite . ........ 718

Analyses, spectrographlc.. _________ 700,707 

Apatite . .. . 697,703,706, 713,716

Black Canyon schist- 696

Carbonate rock, Iron Hill stock............. 697,701

veins......... . .. . 697-698,702

Cerium group, rare-earth metals._____ 702,707 

Cimarron fault.--....-.--..--....-....-.-...- 699

Cochetopa district........-..-.-.......-.... 699,717

Dikes.......... .. . 696-697,698

Dubois greenstone-.-..-.--------.----..-.. 696

Erosion cycles__ . . ___._. 704-705

Faults 699,715

Feldspar-..-..-.......-------.--.- 711-712,716

Foliation . . 699,702,708,715

Oude, A. J., cited. 702

Hematite....__ .. ............ 699,703,716

Hildebrand, F. A., cited. --.-.. .... 703

Hunter, J. F., cited..------....-.--...... 697

Iron Hill composite stock.............. 697-698,700

age. .. 697

composition.... ----------_-___... 697

related rocks. _ - _. 711-714

Jasper___._- -. -.. 705

Magnetite. . . 699,711

Page 

Metarhyollte 709-710

Milkranch Gulch fault zone................ 714-715

Morrison formation .......---.-..-... 696,698

Niobium.............. ....... 700,712

Perovskite.... .. ...... 698,699,707

Pigeonite .. .. 711

Powderhorn district, description....____ 694-695

geology, general....._.__________ 696

mining.___________________ 697 

Powderhorn granite group_..__..___ 696,710

Quartz. . . 709-710,712 

Quartzlte __ ....... ........ 709

Radioactivity. . . 700,701 

affected by weathering. _________ 706 

measurements______________ 718-720

Rare-earth minerals............. _ 702,703,707-708

distribution... 706

Shear zones..... 701,702,703,707

Thorium deposits, age....._.__._..._ 704

depth...._______.....____...... 702

future prospecting....-. __...___... 717

Little Johnnle vein.. . . .. 715-717 

minerals_______.___.___..... 702

Titanium minerals.-. _.. .... 700,712

Veins, carbonate-___-...-__...___ 712-713 

jasperization...__...__._.__.__ 705 

quartz..____.________._ 699,700,710 

tabular . .. 701-702

Vermicullte ___ 699

723

U. 5. GOVERNMENT PRINTING OFFICE: 1986