Geology and Uranium 

Deposits of the Caribou 

Area, Boulder County 

Colorado

GEOLOGICAL SURVEY BULLETIN 1030-N

This report concerns work done partly on 

behalf of the U. S. Atomic Energy Commission and is published with the permission of the Commission

Geology and Uranium 

Deposits of the Caribou 

Area, Boulder County 

Colorado

By F. B. MOORE, W. S. CAVENDER, and' E. P. KAISER

CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

GEOLOGICAL SURVEY BULLETIN 1030-N

This report concerns work done partly on 

behalf of the U. S. Atomic Energy Commission and is published with the permission of the Commission

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON i 1957
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

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

-~C. - Price1$i:25*CpapercoverT"'"""" -=
CONTENTS

	Page 

Abstract_______________________________________ 517

Introduction. ________--_-____-_ ______________________ ___ 517

Location and general features.________________________________ 518

History and production __ _ __ ___ ___ 521

Geology____________------_________________________________ . 522

Rock units_____ - - _________________ . - 522

Idaho Springs formation --______________ 523

Boulder Creek granite____..________________ _ 523

Caribou stock____________________________ _ 523

Structure_________---___--_________________________ _____ . 524

Ore deposits_______________________________________________ 526

Gold veins. _ __ 527

Lead-silver veins_____________________________________ 528

Uranium-bearing lead-silver veins___________________ 536

Mine descriptions_____________________________________________ 540

Silver mines______________ ____________________ _ 541

Caribou mine,_____--_--_____________________________ 541

No Name mine_______________________________________ 541

Poorman mine____ ______________ ________ . 541

Sherman mine.. _____ ______ \. 544

Columbia, Spencer, and Socorro mines__ ___ 544

Radium, Elmer, and Nelson veins____ _ _ 545

Belcher mine __ __ ___ _ _ 545

Comstock mine___ __,____________ 546

Native Silver mine. ____ ______________ 546

Seven-Thirty mine____ _____ ________________ _ 546

Isabel mine------ ___- ___________________ _ 547

Wigwam mine__ _ ___ __________,j _ ,. 547

Potosi and Cross mines__________________ 547

Great Northern mine___________________ 547

Gold-silver mines.. ___ ________________ ____ __________ 548

Silver Point mine______-________-___ ________________ _ 548

Idaho mine___________ ___________________ 548

Gold mine_____ 548

Literature cited_________________________________________ 549

Index._______--...___ ...__________________ 551

m
IV CONTENTS

ILLUSTKATIONS

[All plates are in pocket]

Plate 35. Geologic map of the Caribou area, Boulder County, Colo.

36. Composite map of the Caribou mine, showing vertical section through

the Caribou and Poorman veins. 

_ 37. Geologic maps of the 360, 920, 1040, and 1140 levels, Caribou mine.

38. Geologic map of the 500 level, Caribou mine and the Idaho tunnel.

39. Geologic map of the 300 level, Caribou mine.

40. Geologic maps of the 380,470, 530, 600,740, 800, and 860 levels, Caribou 

" mine.

176. Maps showing location of the Caribou area, Boulder County, Paee 

Colo..______________________________ 519

177. Geologic map of the Grand Island-Caribou mining district 520

178. Generalized structure of the Caribou group vein system_ 525

179. Generalized section of Caribou Hill____ _ _____ 530

180. Diagrammatic cross section of vein, showing alteration___ 531

181. Vertical longitudinal projection of the Radium and Elmer 

veins workings, showing location of pitchblende-bearing ore 

Shoots. _________ _ 537

182. Vertical longitudinal projection of the Caribou vein, Caribou

mine.._____...________-__--___-__.... 542

TABLE

Analyses of samples from the Radium vein, Caribou mine ______ 536
GEOLOGY AND URANIUM DEPOSITS OF THE CARIBOU 

AREA, BOULDER COUNTY, COLORADO

By F. B. MOOKE, W. S. CAVENDEK, and E. PThe Caribpu silver-mining area is underlain b/y sph^ste ap4 gnejgfNBS of, the 

Idaho Springs formation and the Boulder Creek granite, both units of Br^cambrian 

age, and monzonite of Tertiary (?) age. Most of .the uranium occurs in mesothermal lead-silver veins which cut the monzonite in an area less than a half 

mile square; a few lead-silver veins and a few gold veins occur i» the Idaho Springs 

formation but are relatively unimportani. The total value j£ metals pro.duc.ed 

from the area is estimated to be ab(?ut $6,000,000.

The lead-silver deposits pceur in, two sets of interconnect;^ fractures; P-) a 

northeast-striking series of shear fractures which parallel the regional trend, and 

(2) a west-striking series of tension fractures. The deposits occur in shoots 

localized chiefly at junctions of veins occupying the fractures trafcto & lesser extent 

where strike and dip of the fractures change. Few of the veins have bean worked 

to depths of more than 300 feet. !

In the Caribou area pitchblende in quantities of more than tra/;e amounts has 

been found only in the Radium vein in two ore shoots at depths between 900 and 

1,140 feet. The larger shoot has a horizontal dimension of 75 feet along the vein 

and extends from the 920 level to the 1140 level; the smaller shoot has a horizontal 

dimension of 35 feet along the vein and extends upwards 70 feet from the 1040 

level. In both shoots pitchblende occurs in a % to {Mw?h-rwid$ streak fctettB *h$ 

foptwall side of the vein. Most of the pitchblende jg soft and sooty, but in 

places sooty pitchblende encloses small quantities of a hard bptryoidaj pitchblende. 

Silver, lead, zinc, and carbonate minerals are in contact with but are nqt, intermixed with the pitchblende. There is no apparent difference in mineralogy, 

other than the presence of pitchblende, between the uraniunvbearing veins and 

other lead-silver veins at Caribou, :.

INTRODUCTION ' "'

Pitchblende was discovered in the Caribou mine of the Consolidated 

Caribou Silver Mines, Inc., in Boulder County, Colo., during the 

reopening of the 1040 level in 1948. Since then a small quantity of 

pitchblende has been produced from the Radium vein in this mine 

during exploratory work that was done under contract with the U. $. 

Atomic Energy Commission. The underground workings of the 

present Caribou mine, primarily a silver mine, expose five veins, the 

Caribou, No Name, Poorman, Sherman, and Silver Dollar, that 

formerly were worked through separate shafts. Three Other veins, 

the Radium, Elmer, and Nelson, are exposed only in the Caribou 

mine at and below the 920 level.

517
518 CONTRIBUTIONS TO THE GEOLOGY OF UEANTOM

The general geology of the Caribou area has been discussed by Bastin 

(1917) and more recently by Levering and Goddard (1950). In 1937, 

a detailed study of the petrography of the Caribou stock was made by 

Smith (1938). In 1948 and 1949, R. U. King (1952) of the U. S. 

Geological Survey examined the Caribou mine to evaluate the uranium 

occurrence. In 1951 the U. S. Geological Survey began a detailed 

investigation of the Caribou area on behalf of the Division of Raw 

Materials of the U. S. Atomic Energy Commission, the principal purpose of which was to evaluate the uranium occurrences in the area. 

The study was begun by E. P. Kaiser and W. S. Cavender, who 

established a triangulation net, using a base line in Caribou Park for 

horizontal control and a TJ. S. Geological Survey bench mark on the 

top of Caribou Hill for vertical control. Kaiser left the project inAugust 

1951, and the investigation was continued until November 1951 by 

F. B. Moore and W. S. Cavender, During the study of the district, 

a geologic map was prepared at a scale of 1:1,200 of an area covering 

about one square mile. The mapped area includes most of the mines 

in the Caribou district and embraces the eastern half of Caribou Hill 

and adjacent areas to the east and north. In addition, all accessible 

underground workings were mapped at a scale of 1:480. Surface 

mapping was done by planetable methods. For underground mapping, a compass and tape were used in conjunction with base maps furnished by the Consolidated Caribou Silver Mines, Inc.

The writers wish to acknowledge the cooperation of the staff of the 

Consolidated Caribou Silver Mines, Inc., who made the Caribou mine 

available for examination at all times and who furnished maps and 

suggestions that were of great assistance. Mr. A. E. Blakesley, 

owner of the Comstock mine, was also most cooperative hi making 

possible the examination of his mine. Thanks are due Dr. E. E. 

Wahlstrom of the University of Colorado and to the Boulder Daily 

Camera for the use of their files, which contain information on the 

Caribou mine.

LOCATION AND GENERAL, FEATURES

The Caribou area is in sec. 8, T. 1 S., R. 73 W., sixth principal 

meridian, in the Grand Island mining district, in the southwest part 

of Boulder County, Colo. (fig. 176). The now almost completely 

abandoned town of Caribou, 17 miles west of Boulder and 4 miles 

northwest of Nederland, is readily accessible by means of an improved 

gravel road from Nederland.

The Caribou lead-silver mines are at an altitude of about 10,200 feet 

on the northeast side of Caribou Hill. Klondike Mountain is on the 

west and Boulder County Hill on the east (fig. 177). Although the 

relief in the area mapped is nearly 1,000 feet, both Caribou and
GEOLOGY OF THE; CARIBOU AREA, COLORADO , 519

108° 106°

BOULDER COUNT

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FIGUBE 176. Index maps showing location of Caribou area, Boulder County, Colo.
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GEOLOGY OF THE CARIBOU AREA, COLOMBO 521

Boulder County Hills are well rounded, with moderate slopes. 

North and south of Caribou Hill are broad, flat, mountain meadows.

HISTORY AND PRODUCTION

The ores of the Caribou Hill "silver-belt" were discovered in 1869 

by a party of prospectors led by Samuel Conger, who had prospected 

Caribou Hill several years earlier without realizing the value of the 

minerals he found. Claims on the Caribou and the Poorman lodes, 

two of the richest in the area, were located in 1869 and in the following 

year the No Name, Native Silver, and Seven-Thirty claims were 

staked. By the end of 1871, most of the richer lodes on the hill had 

been found and in 1874, $330,000 worth of ore was shipped from the 

Caribou, No Name, Poorman, Sherman, Seven-Thirty, and Native 

Silver mines (Raymond, 1875).

The ore in the upper levels of many of the mines on Caribou Hill 

was exceedingly rich, as shown by reports in issues of the Rocky 

Mountain News for the period 1871-1872 and the Mining Review for 

1873-1874. Assays of more than 1,000 ounces of silver per ton were 

not uncommon. The ores mined at that time were divided into four 

classes: first-class ore, more than $300 per ton; second-class ore, 

$100-$300 per ton; third-class ore, $50-$ 100 per ton; and fourth-class 

ore, less than $50 per ton, which was either discarded or stockpiled 

to await construction of a mill.

The high value of much of the near-surface ore apparently resulted 

from supergene enrichment. Native silver and horn silver (cerargyrite), both secondary minerals, are frequently mentioned in early 

newspaper reports. Gold content was low in the lead-silver veins, 

and Henderson (1926) records no lead production for Boulder County 

until 1887, when 593 pounds were produced. Raymond (1875) noted 

a decrease by 1874 in the grade of the ore from the Caribou mine at 

a depth of 430 feet. The value of ore from the mine that year totaled 

$130,000, with an average value of $72 a ton. Many of the mines, 

from which ore valued at $300 to $1,000 per ton was reported early 

in the camp's history, failed to yield a large production. This apparent inconsistency could be the result, in part, of the over-enthusiastic reports of individual owners and, in part, to the inability 

of the owners to work the narrower veins profitably below the oxidized 

zone. Scattered reports in issues of the Mining Review and the 

Rocky Mountain News, for the 70's, and Corbett (1879), Fossett 

(1876), Burchard (1882), and Raymond (1872, 1873, 1874, 1875), 

when integrated show there was a definite, gradual decline in grade 

of ore as the mines became deeper.

Ore was produced from the Caribou area until 1893, when a drop 

in the price of silver forced most of the mines to close. Some of

427373 57 2
522 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

the richer of the gold mines, whose initial production had been completely over-shadowed during the early silver boom, resumed production in 1898. However, from 1900 to 1948, activity in the area 

was limited to sporadic attempts by individual operators to reactivate 

certain mines or to mill the material from some of the larger dumps. 

The production for this period is believed to have been small. In 

1948, after the discovery of pitchblende on the dump, the Caribou 

mine was reopened by the Consolidated Caribou Silver Mines, Inc. 

This mine, which has workings also intersecting the No Name, 

Poorman, Sherman, and Silver Dollar veins, was the only one operating in the Caribou Hill area at the time of the investigation in 1951.

The total value of lead and silver produced from the Caribou area 

before 1924 is estimated, based in part on figures compiled by 

Henderson (1926) for production from Boulder County, to have been 

approximately $6,000,000. Of this total, the larger part was furnished 

by five mines: the Caribou, No Name, Poorman, Native Silver, and 

Seven-Thirty. No figures are available from which to estimate the 

value of gold produced, but it is believed to have been small.

The ore currently produced from the Caribou mine is concentrated 

at the company mill at Lakewood, about five miles east of the mine. 

From the mill the concentrate is sent by truck to a smelter at Leadville.

GEOLOGY

The Caribou area, which is part of the Front Range mineral belt, 

is underlain by Precambrian igneous and metamorphic rocks and, 

with the exception of unconsolidated Quaternary glacial and stream 

deposits, is devoid of sedimentary rocks. The Precambrian rocks in 

the Caribou area and in the adjoining areas to the north and south 

are intruded by Tertiary (?) igneous rocks which form several small 

stocks (Levering and Goddard, 1950, plate 2). The dominant structural feature of the Front Range mineral belt, a series of northeasttrending folds and faults, is reflected in the Caribou area chiefly by 

northeast-trending mineralized shear fractures, several of which were 

quite productive of lead and silver. The lead-silver deposits at Caribou, with the exception of one mineral, are similar to many other 

lead-silver deposits throughout the Front Range mineral belt; the 

presence of pitchblende in some of the lead-silver ores at Caribou 

distinguishes these deposits from all but a few in the Front Range.

BOCK UNITS

The three principal rock units in the Caribou area are the Idaho 

Springs formation and the Boulder Creek granite, both of Precambrian 

age, and the monzonite of the Caribou stock (fig. 177), of Tertiary (?) 

age. The Caribou stock also contains small bodies of diorite, gabbro, 

and ultrabasic rocks.
GEOLOGY OF THE CARIBOU AREA, COLORADO, 523

Minor units include Precambrian quartz monzonite gneiss and 

pegmatite (fig. 177). All the Precambrian rocks are cut by Eocene(?) 

dikes of diorite and andesite in the area southeast of Caribou. Because 

none of the minor rock units occur in the area mapped (pi. 35)* they 

are not described below. Most of the lead-silver veins of the area 

are in the Tertiary (?) monzonite of the Caribou stock but all the gold 

veins are in Precambrian rocks.

IDAHO SPRINGS FORMATION

Schists and gneisses of the Idaho Springs formation occupy a large 

area in the Front Range and are described by Bastin and Hill (1917,, 

p. 26) as follows:

The predominant rocks of the Idaho Springs formation are light- to dark-grey 

quartz-biotite schists, in places carrying some hornblende or muscovite. With 

these are associated lesser amounts of biotite-sillimanite schist, quartzitic gneiss, 

dark-green hornblende schist and gneiss, and lime-silicate rocks that represent 

metamorphosed limestones.

In the Caribou area the Idaho Springs formation occupies a narrow 

belt, about a half mile wide, along the eastern edge of the area mapped 

(pi. 35); it is bordered on the west by monzonite and on the east by 

granite. Within this belt, the Idaho Springs formation consists mostly 

of quartz-biotite schist and injection gneiss and, near its contact with 

monzonite on Idaho Hill, includes a large amount of pegmatitic rock. 

This formation is the host rock for most of the known gold veins in 

the area.

BOUUMBR CREEK GRANITE

The Precambrian Boulder Creek granite does not crop out in the 

mapped area, but is mentioned here because it occupies an area of 

several square miles adjoining the mapped area to the north and east 

(fig. 177). According to Lovering and Goddard (1950, p. 25): "the 

Boulder Creek is commonly a dark-gray faintly banded rock that 

ranges in composition from a quartz monzonite to a sodic granite." 

In the area near Caribou it contains but few ore-bearing veins.

CAREBOTT STOCK

The Caribou stock underlies most of the area mapped (pi. 35). 

It is not uniform in composition and is described by Smith (1938, 

p. 161-196) as: "a composite Tertiary(?) intrusive mass composed 

chiefly of monzonite but with masses of titaniferous magnetite and 

bodies of ultramafic and gabbroic rocks." Lovering and Goddard 

(1950) mapped bodies of quartz monzonite, gabbro, and ultramafic 

rocks within it as part of the stock.

The monzonite, which constitutes about 90 percent of the stock, 

is a bluish-gray, medium-grained rock composed essentially of biotite,
524 CONTRIBUTIONS TO THE GBOLOGT OF URANIUM

augite, orthoclase, and andesine; locally, fine-grained and porphyritic 

facies are common. At many places, hornblende forms thin seams 

along joint planes; in a few places the fresh monzonite shows faint 

layering which is not apparent on weathered outcrops. Outcrops of 

monzonite are few, but fresh rock can be found in many prospect pits 

and on the mine dumps.

Bodies of ultramafic and gabbroic rocks, as much as several hundred 

feet in diameter, occur at many places hi the stock, but Smith (1938, 

p. 171) points out that "the distribution of the ultramafic and gabbroic 

bodies is irregular." The ultramafic bodies are chiefly pyroxenite and 

contain masses of titaniferous magnetite hi interlacing veins as much 

as 5 inches thick. One of the larger ultramafic bodies, known locally 

as the "Iron Dike," forms the eastern margin of the Caribou Hill 

lead-silver belt (pi. 35).

STRUCTURE

The dominant structure of the Caribou region is a large northnortheast-trending anticline of Precambrian age which appears to 

have controlled the intrusion of the Caribou and other nearby stocks. 

Lovering and Goddard (1950, p. 54) describe this anticline as extending 

south-southwest from Caribou to Empire, a distance of 12 miles. 

The Caribou stock and a monzonite stock two miles to the south form 

an elongate, discontinuous body that coincides with the axis of the 

anticline.

In the area mapped (pi. 35) the foliation in the Idaho Springs 

formation wraps around the north end of the anticline and, therefore, 

in many places the contact with the monzonite is discordant locally. 

The general distribution of the monzonite bodies in the region, however, is nearly concordant with the regional trend of the Precambrian 

structures. Such preexisting structures in the Precambrian rocks may 

have controlled the localization of the stocks in two ways: (1) directly, 

by forming zones of weakness or layers along which ease-of-access of 

intruding magmas would be greater than normal, and (2) indirectly, by 

influencing the trend of later (Laramide) structures, which in turn 

would control localization of the stocks. No evidence was found 

during mapping with which to determine which type of control was 

dominant.

Jointing in the monzonite is common but is not conspicuous. The 

most prominent set of joints ranges in strike from N. 40° to 60° 

E. and in dip from 70° to 90° NW., with an average strike of N. 50° 

E. and an average dip of 85° NW. Much less prominent are sets of 

steeply dipping north-trending and east-trending joints.

To the east of Caribou, in Boulder County, are a series of northwesttrending major faults (Lovering and Goddard, 1050, pi. 2), and to the
GEOLOGY OF THE CARIBOU ABBA, COLORADO 525

south, in the northern parts of Clear Greek and Gilpin Counties are 

several large northeast-trending faults. Although these faults are not 

apparent in the Caribou area, they can be traced to within a few miles 

of Caribou and, if projected along their strikes, would intersect in the 

Caribou district. These faults may be present at depth, even though 

no surface indication of the faults exist. Lovering (1932) believes 

that the northwest-trending faults of Boulder County localized most 

of the ore deposits in Boulder County.

Three sets of steeply dipping veins northeast-trending, east-? 

trending, and northwest-trending occur in the Caribou area. All the 

veins dip north at angles of 70° or more. The northeast and east' 

trending veins on Caribou HiE are chiefly in monzonite of the Caribou 

stock and form an interconnecting vein system (%. 178 and pi. 36); 

the east-trending veins are at an angle of about 40° to the more persistent northeast veins. To the east, on Idaho and Boulder County 

Hills, a few west- and northwest-trending veins, which contain chiefly 

pyrite, gold, and quartz, form a separate system in the Idaho Springs 

formation. These veins may be older than the Caribou stock.

The northeast-trending veins on Caribou Hill are interpreted as 

occupying shear zones and the east- and west-trending veins are interpreted as occupying tension fractures branching from the shear zones. 

Although such features as offset geologic contacts were not in evidence 

to furnish conclusive proof, this interpretation is compatible with most 

of the geologic data. A northeast-trending shear fracture could be 

produced either by a compressional force acting nearly in an east or 

west direction or by a northeast-southwest shearing couple. The

100 0 L 200 Feet

FIGURE 178. Generalized structure of the Caribou group vein system, Boulder County, Colo.
526 CONTKEBUTIONS TO THE GEOLOGY OF URANIUM

regional forces producing fracturing in the Front Range mineral belt 

of Colorado are interpreted by Levering and Goddard (1950, fig. 81) 

to have acted in a northeast-southwest direction. If the component 

of a couple on the northwest side of such a shear fracture moved 

northeast with respect to the southeast side, any tension fractures 

produced should trend about east-west, as do the Caribou, Poorman, 

and Radium veins (fig. 178).

The direction of relative movement of the vein walls is difficult to 

determine by underground examination. Slickensides are nearly 

horizontal as seen in the mine workings but the evidence presented 

by chatter marks is inconclusive. Horizontal movement along the 

veins was small. There is no apparent displacement of the ultrabasic 

body locally called the "Iron Dike" that is only 100 feet beyond 

the northernmost workings on the No Name vein (along the strike). 

What appears to be a displacement of the Radium-Elmer vein (fig. 

178) is the fortuitous branching of two separate veins from the No 

Name vein at nearly the same place. The Radium vein leaves the 

No Name vein in a smooth curve and is not cut off sharply, as would 

be true were it displaced by later faulting. Van Diest (1875) stated 

that the Caribou vein was cut and offset by the No Name vein, with 

the eastern extension of the Caribou vein being displaced about 10 

feet to the southwest. This interpretation seems unlikely, for no ore 

has been found along such eastern extensions of the Caribou vein, 

even on the levels where it contained good ore a few feet west of the 

No Name vein.

Although movement along the No Name vein may have been small, 

the brecciation and fracturing both of the vein walls and the vein filling 

show that movement did take place, some of it after the fractures were 

mineralized. If the walls along the No Name vein moved as indicated 

in figure 178, ore bodies would be expected in places where the strike 

changes to a more easterly direction. As intersections with the Caribou 

vein probably had an equal or greater effect in localizing ore bodies 

than did changes in strike along the veins, the effects of changes in 

strike alone will be most noticeable on the levels below the 860-level. 

On the 920-level and less obviously, on the 1040-level, ore bodies are 

found along the No Name vein in places where the strike changes to a 

more easterly direction. The movement along the northeast-trending 

Nelson vein apparently is in the opposite direction, because the vein 

widens where the strike is more northerly.

ORE DEPOSITS

Two types of ore deposits lead-silver veins and gold veins are 

present in the Caribou mining district. The lead-silver deposits have
GEOLOGY OF OHE CARIBOU ABBA, COLOBADO 527

been the most important economically, with silver being the most 

valuable metal recovered. Since 1948 small quantities of uranium 

have been produced from the lead-silver-bearing Radium vein.

Most of the lead-silver veins are within an area of about one-half 

square mile on Caribou Hill (pi. 35); a few are on Boulder County 

and Idaho Hills, half a mile to the east. Almost all are hi the 

Tertiary (?) monzonite but a few occur in the Precambrian rocks. 

The gold veins occur predominantly in gneisses of the Idaho Springs 

formation, and none are known in the monzonite, which suggests that 

the gold veins may have been formed before the fracturing of the Caribou monzonite stock or even before its emplacement, whereas the 

lead-silver veins were formed after its consolidation. This interpretation is in harmony with Bastin and Hill's (1917) conclusion that in 

the Central City district the gold mineralization was earlier than the 

lead-silver mineralization.

GOLD VEINS

The gold veins of the Caribou area are relatively unimportant, both 

quantitatively and economically. A few were worked in the 1890's 

(as reported in issues of Rocky Mountain News for 1890-1900), after 

the silver mines were closed, but then: total production is believed to 

be small. A small group of veins, including the Silver Point, Idaho, 

Elephant, and Windy Point (pi. 35), which contained both gold and 

silver minerals (Levering and Goddard, 1950, p. 202) are discussed in 

this report with the gold veins, but perhaps these should be classed as 

composite veins. None of the gold mines were accessible at the tune 

of this survey.

The gold veins are on the west side of Idaho Hill, less than half a 

mile east of the Caribou mine (pi. 35). Although the shafts of several 

of these mines are only a few hundred feet east of the monzonite stock, 

and the veins if extended westward would cut the monzonite, no 

monzonite was seen on the dumps. Therefore, these veins probably 

do not extend into the monzonite. The country rock cut by the gold 

veins is mostly schist and gneiss of the Idaho Springs formation; 

however, near the contact with the monzonite, large amounts of 

pegmatitic rock are present.

Although none of the gold veins could be examined, the character 

of the ore can be determined from a study of the dump material. 

Samples from the dump of the largest gold mine in the area, the St. 

Louis, contained quartz, pyrite, chalcopyrite, covellite(?), and minor 

amounts of galena and sphalerite. Some carbonates (mostly dolomite) and quartz are found in vugs and presumably were deposited 

later than the ore minerals.
528 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

Production figures for the Idaho Hill gold mines are not available 

and no estimate of the average value of the ore can be made. Bastin 

and Hill (1917, p. 181), in describing the St. Louis mine, state:

The ore treated was free milling and the value was mainly in gold. Sixteen 

tons shipped in 1905 are said to have shown an average content of gold, 3.28 

ounces; silver, 9 ounces; and silica, 17 percent. Two tons shipped in 1904 are 

said to have assayed gold, 5.81 ounces; silver 8.5 ounces; silica, 41.7 percent.

UEAJD-SILiVER VEINS

In all the veins in the Caribou monzonite stock, silver is the most 

valuable metal and lead the most abundant. The Radium vein also 

contains pitchblende and therefore is discussed separately. A small 

quantity of pitchblende was found on the dump of the Great Northern 

mine, but as this mine now is inaccessible, the vein could not be 

examined. The vein systems exposed in the Caribou mine and the 

Comstock mine were the only ones accessible for study in the Caribou 

area in 1951.

The lead-silver veins of the Caribou area are concentrated in an 

area about half a mile square, on the east and northeast slope of Caribou Hill (pi. 35); a few veins are present about one mile away to the 

east, north, and northwest of Caribou Hill. Only the Boulder 

County vein to the east is believed to have produced more than a 

small amount of ore and none of these outlying veins is included in 

the area mapped.

The description of mineralogy and paragenesis of the Caribou leadsilver ores is based largely on the study of samples from the Caribou 

mine, although samples from the dumps of most of the larger mines 

in the area also were examined. Listed in approximate sequence of 

deposition, the primary minerals in the lead-silver ores are quartz, 

pyrite, sphalerite, galena, chalcopyrite, argentite, ruby silver (pyrargyrite), and carbonates. Secondary minerals include native silver, 

azurite, malachite, and limonite. Reports by early workers (Mining 

Review, 1873, Endlich, 1874, Raymond, 1875) indicate that tetrahedrite, cerussite, "brittle silver" (stephanite(?)), "horn silver" 

(cerargyrite), and barite were present in small amounts in the upper 

workings.

Quartz, although one of the earliest minerals deposited, was deposited also in minor amounts as a late mineral. It formed either 

during several stages of mineralization or throughout the entire 

period of mineralization. The early quartz is massive white "vein 

quartz," commonly with small amounts of pyrite. Late quartz, in 

the form of clear crystals, fills vugs and forms veinlets cutting all 

other vein minerals except the carbonates. Cryptocrystalline quartz 

with fine-grained pyrite replaces the wall rock near the veins. Finegrained pyrite is common in all the lead-silver ore's but is not abundant.
GEOLOGY OF THE CARIBOU AREA, COLORADO 529

Galena and sphalerite are closely associated, although sphalerite is 

sparse at shallow depths, either because of zonal deposition or leaching 

by ground waters. Both dark sphalerite, "black jack," and yellowishgreen sphalerite, "rosin jack," are present in the Caribou mine. 

Except in one place along the Radium vein at the 1040 level, chalcopyrite occurs only in small amounts. For the most part it appears to 

be contemporaneous with sphalerite and galena. Carbonates (dolomite and calcite) were the last minerals formed, and carbonate veinlets 

cut all other minerals.

Distribution of the minerals along the veins shows some variation. 

Quartz is more abundant in the Caribou vein than in other veins of 

the Caribou mine. The highly silicified Caribou-vein material formed 

what was known to the miners as a "hard ore." Sphalerite appears 

to increase with depth. Chalcopyrite is plentiful only on the 1040 

level of the Radium vein. Carbonates are present in all the veins 

but are more abundant than average along the No Name vein and 

less so along the Caribou vein. Massive pink carbonate more than 

a foot in width occurs along the No Name vein on the 500 level.

The upper parts of the veins at the Caribou mine have been greatly 

enriched in silver in the highly oxidized zone above the 300 level. 

The workings are inaccessible, but Endlich (1874) reported cerussite, 

malachite, and native silver from the 210 level and Raymond (1875) 

reports "horn silver" from the 200 level. The writers found abundant 

native silver in a veinlet on the 300 level and saw small amounts on 

the 360 level. The operators of the mine report that some native 

silver was found in a stope on the 920 level. Raymond (1875) noted 

a decrease in the silver content of the Caribou ore at a depth of 420 

feet.

The presence of the secondary minerals cerussite and "horn silver" 

in only the oxidized parts of veins,,at depths of 300 feet or less; the 

abundance of native silver at depths of less than 300 feet; and the 

rapid decreases in grade of ore between 300 and 420 feet indicate that 

the oxidized zone of secondary enrichment did not extend much below 

300 feet. In the oxidized zone, the enrichment was from 3 to 10 

fold. Below the oxidized zone, in the zone of secondary sulfides, 

enrichment is much less pronounced. From the few production 

figures available, it is estimated that the enrichment in the secondary 

sulfide zone may have increased the grade of the ore values by as 

much as 50 percent. The base of the zone of secondary sulfides is 

believed to be at about 740 feet, where there is a noticeable decrease 

in iron oxide.

The lower limits of the oxidized and the secondary-sulfide zones are 

related to position of the water table, which is controlled primarily by 

the topography (fig. 179). The water table is normally somewhat

427373 57 3
530 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

oo-1
Silica decreases

I Width of zone ranges from a few inches to severall 

I feet, is proportional to thickness of vein ~l

EXPLANATION

Fresh rock, Chloritic zone; texture Silicified sericitized zone, 

unaltered preserved; slightly altered hard, bleached; texture

not preserved; carbonates, 

highly altered

Vein 

FIGURE 180. Diagrammatic cross section of vein, showing alteration, Carib
532 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

higher than indicated in figure 179 and only during excessively dry 

periods would be at the level shown. The lowest position of the 

fluctuating water table marks the base of the oxidized zone and of the 

highly enriched ore. In the oxidized zone, ground water would move 

relatively rapidly downward, but in the secondary-sulfide zone it 

would move much more slowly, because the hydraulic gradient would 

be controlled by Coon Trail Creek at the same altitude as the 740 

level in the Caribou mine. Below the 740 level, in the zone of primary ore, the ground water is nearly static and therefore the ore has 

not been altered. The boundaries of the oxidized, secondary-sulfide, 

and primary-ore zones are gradational. The absence or scarcity of 

pyrrhotite, chalcocite, and similar very active precipitants of silver 

in the mine make possible the easy transportation of the silver by the 

ground water.

The effects of supergene enrichment on ore minerals other than those 

of silver are not well known at the Caribou district but appear to be 

of minor importance. Because zinc sulfide, a relatively soluble ore 

mineral, is rarely mentioned in the early accounts of the district, 

presumably it was leached from the near-surface parts of the veins. 

The Mining Review (1873, v. 1, no. 6, p. 11) states that the ores of the 

Caribou mine then exposed to a depth of 320 feet contained "but 

little zinc, arsenic or antimony." Zinc is fairly abundant on the 1040 

level but there is no evidence of secondary deposition either here or 

on the upper levels. Galena is plentiful throughout the veins and, 

being only moderately soluble, was apparently little affected by 

ground-water action. The pitchblende in the Radium vein occurs 

below the oxidized zone and probably was not exposed to secondary 

processes. If it was present in the oxidized part of the vein, it probably was removed for it is readily soluble in acid solutions.

Most of the wall rock of the Caribou lead-silver veins is mediumgrained, gray to dark-gray monzonite. Small masses of diorite, 

gabbro, pyroxenite, and biotite pyroxenite are exposed in the Caribou 

mine, but no comparative study of the alteration products of the monzonite and those of the more mafic rocks was made.

The wall rock of the lead-silver veins of the Caribou area has been 

progressively silicified, argillized, and chloritized for as much as 3 feet 

outward from the vein. Silica and chlorite occur along all veins. 

The products of argillization, possibly once extensive, have been 

partly obliterated by later sericitic alteration; they form a distinct 

zone at only a few places. The monzonite adjacent to the veins is 

intensely altered to form an inner zone of hard, bleached, silicified 

rock in which the original texture has been destroyed. The zone 

ranges from a few inches to 2 feet in thickness and is characterized 

by disseminated cryptocrystalline quartz, sericite or illite, or both,
GEOLOGY OF THE CAEIBOTJ AREA, COLORADO 533

and carbonate minerals, as well as by numerous veinlets of finegrained quartz and carbonate minerals. In most places this intensely 

altered zone grades into a zone of slightly altered rock characterized 

by chlorite. This outer, or chloritic, zone ranges from about 6 inches 

to 2 feet in thickness, grading outward into fresh rock. A middle, or 

argillized, zone occurs in a few places between the chloritic and silicified zone (fig. 180). Composed of a green-gray, friable rock, the 

argillized zone contains abundant minerals of the montmorillonite 

group. This argillized, or montmorillonite, zone is only 6-12 inches 

thick along the Radium vein, but is several feet thick along the No 

Name vein on the 500 level. The contacts between the montmorillonite and adjacent zones are sharp.

Assignment of alteration minerals to definite zones is not meant 

to imply that these minerals are found only in a particular zone or 

that other alteration minerals are absent from this zone. The zones 

are named for the most abundant or characteristic alteration minerals, 

which were originally identified in thin section under a microscope. 

Recent X-ray studies by E. W. Tooker of the U. S. Geological Survey 

on the clay minerals of the inner and middle zones have largely confirmed the original identifications. Altered material in the inner or 

silicified zone believed to be sericite was shown by its X-ray pattern 

to be a mixture of sericite and illite. Although no determination of 

the relative distribution of sericite and illite within the zone could 

be made with the samples available, Tooker (1953, oral communication) states that, in general, the ratio of sericite to illite decreases 

with an increase in distance from the vein. The distribution of 

alteration minerals in the three zones are given below, listed according 

to means of identification:

SUicifie'd (inner) zone Montmorillonite (middle) zone Chloritic (outer) zone

Microscope Quartz, sericite, car- Montmorillonite, some Chlorite.

bonate, some kaoli- kaolinite, beidellite,

nite. and illite. 

X-ray Mostly sericite-illite, Montmorillonite, Not deter10-15 percent kaoli- 10-15 percent kaoli- mined.

nite, 5-10 percent nite, some illitemontmorillonite, sericite.

5-10 percent halloysite.

H. D. Wright (1950, written communication) in studying the major 

pitchblende ore shoot in the Radium vein, recognized four zones of 

alteration which he numbered in order outward from the vein as (1) 

a 2- to 6-inch hard, compact bleached zone containing sericite, carbonate, and fine-grained quartz, (2) a 2- to 3-inch dark green-gray, 

friable zone containing iron-stained montmorillonite, some kaolinite, 

and a little sericite, (3) a discontinuous zone resembling zone 1, but
534 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

with little silica and containing abundant sericite and small amounts 

of kaolinite and montmorillonite, which mark the outer limit of 

argillic alteration; (4) a zone of propylitized rock in which the pyroxenes have altered to chlorite, calcite, and a little epidote. Zones 

1 and 3 of Wright correspond to the inner zone of this report. Except 

where separated by a montmorillonite layer (zone 2 of Wright), they 

form a single gradational zone.

The most persistent vein in the Caribou area the No Name vein 

(pi. 36 and fig. 178) strikes northeast and has been worked for a 

horizontal distance of at least 1,000 feet and to a depth of 1,100 feet. 

This vein marks the eastern boundary of the main productive area 

on Caribou Hill (pi. 35). Most of the other veins are short 300-400 

feet in length and few have been worked to depths of more than 300 

feet. The Caribou vein, which can be traced for 300 feet on the 

300 level of the Caribou mine, appears to become shorter as the depth 

increases. Below the 860 level, it cannot be recognized. Several 

veins which are known only on the deeper levels the Elmer, Radium, 

and Nelson may actually be deeper parts of outcropping veins that 

were worked only to shallow depths.

The lead-silver ore bodies of the Caribou mine are vertical or 

steeply-plunging ore shoots. The larger shoots extend more than 

200 feet along the vein and have been followed to depths of more than 

700 feet. The veins are as much as 7 feet thick in places but the "pay 

streak" is rarely more than 18 inches wide.

The veins are mostly fissure fillings, but in places the wall rock has 

been replaced. Some veins have one well-defined wall; the other is 

gradational. The ore bodies are almost entirely in monzonite; and 

where the veins cut mafic rocks, they are weakly mineralized. There 

are no mines in the mafic rocks.

The ore shoots of Caribou Hill are localized by changes in dip or 

strike of the vein and by vein junctions. A series of tension fractures 

that trend west from the No Name vein contain ore shoots near 

their junction with the No Name vein. The concentration of ore 

minerals in the tension fractures such as the Caribou vein was in 

many places greater than in the stronger shear fractures, such as 

the No Name vein, that trend northeast. Prior to mineralization 

highly fractured rock partly filled the openings along the No Name 

vein and locally was cemented by vein minerals. Such brecciation 

is absent along the subsidiary west-trending tension fractures. Postmineral movement, indicated by crushed vein minerals, is apparent 

along the northeast-trending shear fractures. No postmineral movement is apparent in vein minerals of the west-trending tension fractures.
GEOLOGY OF THE CARIBOU AREA, COLORADO 535

The ore shoots in the Caribou vein are thicker in the less steeply 

dipping parts of the vein. This is especially evident on the 360- 

level where the vein decreases in dip from about 80° to 50° and 

increases to 10 feet in thickness (pi. 37, section A-A'). The Caribou 

vein steepens at the 800-level and here is too thin to be worth mining; 

it was not explored below the 860-level. Although the other veins 

in the Caribou group are not sufficiently well exposed to determine 

the structure of their ore shoots, it is believed that a similar structural 

relationship may also control, in part, the localization of ore in other 

shoots in the Caribou area.

Changes in strike, especially along the northeasterly-striking shear 

fracture veins, apparently helped influence the localization of ore. 

The best ore exposed in the Nelson vein is along that portion of the 

vein in which the strike is more northerly. Along the No Name vein, 

the junction of that vein with the Caribou vein was the dominant 

influence in localization of ore, but on the 1040- and 920-levels (pi. 37), 

the Caribou vein is absent and slight changes in strike to a more 

easterly direction coincide with the stoped areas. In general, veins 

occupying the westerly trending tension fractures are straight because 

the wall rock is homogeneous and the horizontal movement, relative 

to that of the shear fractures, is small. Accordingly, changes in strike 

were not important factors in the control of the ore deposition in the 

tension-fracture veins. The ore shoot localized by a change in strike 

along the Radium vein (tension-fracture) is small.

Silver is by far the most valuable metal in the lead-silver ores of 

Caribou Hill area. Gold rarely exceeds 0.1 or 0.2 ounce per ton and 

gold was not reported present in the lead-silver veins by early newspaper accounts or by Fossett (1879) or Raymond (1872, 1873, 1874, 

1875). Lead, which currently is being produced from the Caribou 

mine, is believed to account for only a small percentage of the total 

production from the mining district. The earliest recorded production 

of lead from Boulder County, as reported by Henderson (1926), was 

in 1887, a time at which the production of ore from the Caribou area 

was declining rapidly.

The ore, especially in the oxidized zone, was exceedingly rich; assay 

values of 200 to 300 ounces of silver per ton were not uncommon. 

However, the bulk of the production came from ore of much lower 

grade. The Caribou vein, the richest and most productive in the area 

(about $1,500,000 to 1883), yielded $334,000 worth of ore of an average 

grade of $70 per ton during 1874 and 1875.

Since the Caribou mine was reopened in 1947, a total of $519,642.19 

worth of lead-silver ore has been produced. 1 Most of the ore came

i Data furnished by Consolidated Caribou Silver Mines, Inc. Published with permission of the company.
536 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

from the 920- and 1040-levels but a small amount came from the 

500-level. The largest ore bodies were found along the Nelson vein 

but the Elmer, No Name, and Radium veins also yielded ore. Some 

of the ore contains as much as 100 ounces of silver and 10 percent lead. 

Zinc is less abundant than lead. Analyses of 17 samples taken from 

the uranium-bearing areas of the Radium vein are shown below.

Analyses of samples from the Radium vein, Caribou mine, Boulder County, Colo.

[Analysts: W. D. Goss and D. L. Skinner for samples CAA-1 to -10; E. C. Mallory and D. L. Skinner for

samples FM-5-48 to -56.]

Sample No.

Location, 

distance 

from No 

Name 

vein (feet)

Length 

ofsample 

(inches)

U Pb Zn Cu

Percent

Ag Au

Ounces per ton.

920 level

CCA- I.....

2..___

5 ._

6... ..

8. _

9...._

10._.__

75

80

111

116

121

126

131

8

8

9

2^<T

10

9

0 08

22

70

18

23

1 85

11

5. 85

2.29

4.28

8.29

3.01

5.20

3.96

3.94

3. 78

5. 27

4 07

3.07

2.77

2. 82

0 02

04

02

11

36

39

24

72.07

10. 14

15. 12

76.77

75.84

217. 64

38.28

0.03

.03

.04

. 10

Tr.

.14

.08

1040 level

CCA- 3 ..

FM5-48 ._

49.....

50...._

51.....

52.....

53...-.

54

55.....

56.....

51

27ltf

276

280tf

285

285

293#

302

309%

315

3

6

7

12

13

12

13

12

8

8

36

49

04

36

08

01

15

10J. U

28

02

22. 99

3.73

1. 96

10. 13

15.82

11. 84

7.74

12. 23

6. 16

.46

16.73

5.33

1. 46

480

7.68

17.06

3. 98

2. 78

6. 61

. 22

01

04

02

01

02

03

05

16

02

< 01

3. 20

22.31

2. 76

8.87

1. 64

4.30

14.05

108. 40

7.84

.72

Tr.

.05

None

.03

None

.04

.02

.07

.02

None

URANIUM-BEARING LEAD-SILVER VEINS

Except for the presence of pitchblende, the uranium-bearing veins 

in the Caribou area do not differ in mineralogy from other lead-silver 

veins of the area; the two types of veins form one interconnecting vein 

system. However, the uranium-bearing veins are discussed separately 

in this section, not because they are considered a separate type of vein, 

but because they warrant special attention, in view of the strategic 

importance of uranium.

The only known pitchblende deposits in the Caribou area are in the 

Caribou mine, but pitchblende is believed to occur in the Great 

Northern mine. The pitchblende occurs in the Caribou mine along
GEOLOGY OF THE CARIBOU AREA, COLORADO 537

Mine workings' dashed 

where projected

,0001

Diamond-drill hole, where drill 

hole intersects vein Assay 

is in percent uranium

Pitchblende'ore shoot

Compiled by F B Moore and W S Cavender, 1952

FIGURE 181. Vertical longitudinal projection of the Radium and Elmer vein workings, showing locations 

of pitchblende ore shoots, Caribou mine, Boulder County, Colo.

the Radium vein at depths between 900 and 1,140 feet (pi. 37 and 

fig. 181) and along an unnamed vein, possibly the Nelson vein, on the 

500 level (pi. 38). Pitchblende was found on the dump of the Great 

Northern mine, but because the mine is inaccessible and there is no 

record of pitchblende having been found in the mine, this occurrence 

cannot be evaluated.

In the Caribou mine, pitchblende occurs beside galena, sphalerite, 

and silver minerals along the footwall of the Radium vein. Some of 

the pitchblende is hard, but most is soft and sooty and coats vugs and 

fractures. The hard pitchblende occurs as a band, generally less than 

an inch thick, between sulfides and gouge along the footwall of the vein. 

In a few places the band is as much as 6 inches thick and locally splits 

into stringers that form a braided network.

The paragenesis of the minerals constituting the Radium vein was 

determined by a study of polished sections. Quartz and pyrite (a 

minor amount) were the earliest minerals. Most of the quartz is white 

and massive; however, some occurs in vugs as crystals. Sphalerite
538 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

and galena, apparently contemporaneous, fill vugs and fractures in 

the quartz. Chalcbpyrite, in part contemporaneous with the sphalerite and galena, is also probably younger in part. One specimen shows 

a chalcopyrite veinlet cutting sphalerite but not galena. A younger 

age for part of the chalcopyrite is indicated by its occurrence along 

cleavages in galena and as crystals growing along a sphalerite-galena 

contact.

The position of the pitchblende in the paragenetic sequence is not 

definitely known because only small amounts of the hard, lustrous 

pitchblende are present along the veins, and because the samples that 

were collected did not show definite age relationships with other 

minerals. One specimen contains an irregular veinlet of hard pitchblende that cuts pyrite. On the basis of negative evidence, it is 

believed that the hard pitchblende also is younger than the sphalerite 

and galena. If the pitchblende were earlier than the galena, evidence 

that galena cuts pitchblende should be present, owing to the relative 

abundance of galena in the pitchblende zone. If the pitchblende were 

contemporaneous with the galena and sphalerite, some of the pitchblende should be intergrown with those minerals. None of the pitchblende contains any appreciable quantity of sulfides nor do the adjacent 

sulfides contain any visible pitchblende. Autoradiographs of two 

specimens of sooty pitchblende show that the sooty variety is confined 

to vugs and fractures and is not intermixed with the sulfides.

H. D. Wright (1950, written communication) in discussing the 

mineralogy of the Radium vein states:

The ore minerals recognized in the polished surface are uraninite, 2 pyrite, 

chalcopyrite, sphalerite, galena, argentite, ruby silver, and native silver. Gangue 

minerals represented are quartz, carbonate and barite. The existence of two 

characteristic assemblages of ore minerals suggests two stages of vein formation. 

In Stage A, which is thought to be earlier, quartz, sphalerite and galena were 

deposited. Uraninite was deposited early in Stage B. The other minerals of this 

stage are chalcedony, chalcopyrite, sphalerite, argentite, ruby silver, and native 

silver.

The hard pitchblende at Caribou occurs as "inclusions" in the soft, 

sooty variety, suggesting that the sooty pitchblende was deposited by 

hypogene solutions after the hard pitchblende or that it resulted from 

the supergene alteration of it. Sooty pitchblende deposited in vugs 

after deposition of colloform quartz and fine oolitic pyrite in the vugs 

indicates a late and probably low-temperature stage of deposition.

Although age determinations for the comparison of the relative 

ages of the soft and hard pitchblende are not available, the field relations show that the soft variety is the younger. Kerr and Kulp

* Pitchblende is generally considered to be a noncrystalline form of uraninite.
GEOLOGY OF THE CARIBOU AREA, COLORADO 539

(1952) give an age of 23 million years (±10 million) for the pitchblende at the Caribou mine. This age determination was probably 

made on pitchblende of the hard variety and was not corrected for 

common lead. Phair (1952) recalculated the age of the Caribou 

pitchblende and determined a possible maximum age of 44 million 

years. The pitchblende from the nearby Central City district, 

Colorado, which contains a much higher ratio of hard to soft material 

and which Bastin (1917, p. 124) believed was contemporaneous with 

the gold ores, was determined by Nier, Thompson, and Murphey 

(1941) to be from 57 to 59 million years old.

Kerr (1951, p. 91-92), after studying samples of "uraninite" from 

Colorado, Canada, and the Belgian Congo, states: "A number of 

observations indicate that the sooty uraninite may be a later form 

high in UO3 that has originated at the expense of earlier hard uraninite, 

high in UO2". It is possible that the soft pitchblende is a first step 

in the chemical alteration of pitchblende to secondary minerals. The 

absence of any of the typical alteration products (gummite, torbernite, 

etc.) at Caribou may be due to the limiting factors of the environment. 

Kerr adds that "although the sooty mineral is later, it is found at such 

depths that it apparently does not represent a typical surface-weathering product." Two separate stages of primary uranium deposition 

seem unlikely because, were this true, in some places only hard pitchblende should occur and in others only soft pitchblende. However, 

the two varieties appear to be coextensive.

Pitchblende has been exposed along the Radium vein on the 920, 

1040, and 1140 levels, the only levels that have explored the Radium 

vein (pi. 37). Two ore shoots containing pitchblende occur about 170 

feet apart in the Radium vein on the 1040 level. The larger shoot 

extends from the 920 level downward to the 1140 level and is more 

than 70 feet long (fig. 181). The smaller ore shoot extends for 35 feet 

along the drift and vertically for 70 feet in a raise from the 1040 level. 

The larger ore shoot plunges steeply to the west; although the smaller 

one appears to be vertical, it also may plunge steeply to the west.

Within the ore shoots the pitchblende is erratically distributed. 

Assays vary by as much as 100 to 1 in a distance of 2% feet. At 

places where pitchblende occurs, minerals typical of the lead-silver 

veins are invariably present, commonly in considerable abundance. 

Some of the pitchblende-bearing ore is worth mining for lead and 

silver. The uranium ore, a part containing as much as 7 percent 

uranium, has been separated from the lead-silver ore and from the 

gangue by hand sorting. Except for a small shipment for metallurgical purposes, all the uranium ore from the Caribou mine has been 

stockpiled by the company.
540 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

MINE DESCRIPTIONS

The Boulder County claim map for sec. 8, T. 1 S., R. 73 W., shows 

100 patented claims in the area mapped; 46 of these claims have been 

identified with surface workings, and names also have been assigned 

to an additional 38 unpatented properties. The names of mines and 

veins shown on plate 35 were determined in large part from a map 

published in the Engineering and Mining Journal (1877) and in part 

from information given by Mr. Elmer Hetzer of Boulder, Colo. 

Many of the smaller mines or prospects could not be identified. 

About 30 mines were worked to a depth of 100 feet or more, but production figures are available for only a few of the larger properties. 

The mines described below are believed to be significant because of 

their production, size of workings, location, or mineralogy. One, 

the Great Northern, is a few hundred feet beyond the area mapped 

but its location is shown on figure 177. The Caribou and the Cornstock mines were the only ones accessible during this study.

SILVER MINES

Of the silver deposits in the Caribou area, those worked by the 

Caribou group of mines are the most important; only one other mine 

is accessible and only one other deposit yields evidence of pitchblende.

The Caribou group consists of eight mines, the Caribou, No Name, 

Poorman, Silver Dollar, Sherman, Columbia, Spencer, and the 

Socorro. The Caribou, No Name, Poorman, Silver Dollar, and 

Sherman veins can be identified on the Caribou mine's 300 level and 

form an interconnecting vein system. The Columbia and Spencer 

veins, which parallel the No Name vein, were incorporated in the 

holdings of the Caribou company as early as 1883. The Socorro 

vein is parallel to the Caribou vein on the south and may be identical 

with the South Caribou vein of this report. Recently three more 

veins, the Radium, Elmer, and Nelson, have been exposed on the 

920- and 1040-levels of the Caribou mine but have not been definitely 

correlated with any of the veins exposed at the surface.

The Caribou group is owned by the Consolidated Caribou Silver 

Mines, Inc., who reopened the mines in 1948 by means of an adit, the 

Idaho tunnel, 3,700 feet long (pi. 38) that intersects the Caribou 

shaft at a depth of 500 feet. In 1951 the ore was being exploited 

from the No Name, Nelson, Elmer, and Radium veins on the 920 

and 1040 levels.

CARIBOU MINE

The Caribou vein, discovered in 1869, is the richest silver vein in 

the Caribou area. The value of silver ore shipped from the mine 

before 1880 totaled more than $1,000,000 (Fossett, 1879). Corregan 

and Lingane (1883) gave the total production of the Caribou and No
GEOLOGY OP THE CARIBOU AREA, COLORADO 541

Name veins as $2,500,000. Since 1883, the Caribou mine has been 

operated intermittently. Some of the ore was very rich; assay values 

of $300 to $500 a ton were common.

A shaft 1,040 feet deep gives access to levels at 50, 100, 200, 300, 

360, 500, 530, 600, 670, 740, 800, and 860 feet below the collar (pis. 

37, 38, 39, 40, and fig. 182). Shafts 100 feet apart along the vein, 

1 east of the main shaft and 5 to the west are now covered by the 

main dump.

The Caribou vein strikes approximately west and has an average 

dip of 75° to the north. The vein is nearly 300 feet long on the 

300 level (pi. 39) but shortens at depth and feathers out 860 feet 

below the collar (pi. 36). The width of the vein averages about 2 

feet, increasing to a maximum of 10 feet on the less steeply dipping 

parts (flats) of the vein. The silver ore is localized in well-defined 

shoots, mostly hi the eastern part of the vein near its junction with 

the No Name vein (fig. 182). The ore is highly siliceous and shows 

no evidence of postmineral crushing. Carbonate minerals are sparse 

in the Caribou vein, probably owing to nearly complete filling of 

openings by earlier vein material.

Ore minerals are galena, sphalerite, argentite, pyrargyrite, and 

native silver. Finely disseminated pyrite replaces the wall rock near 

the vein, but only a small amount of pyrite is present in the ore. 

Some of the ore contains a minor amount of chalcopyrite.

The main Caribou vein has been stoped to a depth of nearly 800 

feet and westward from its junction with the No Name vein for 

distances ranging from 300 feet on the 300-1 evel to 150 feet on the 

740-level. Because the main Caribou vein feathers out a short 

distance below and to the west of the stoped areas and because no 

ore-bearing extension of the Caribou vein has been found east of the 

No Name vein, it is doubtful that the vein still contains workable ore.

NO NAME MINE

The No Name vein is the most persistent vein in the Caribou area, 

having been worked for more than 1,000 feet both horizontally and 

vertically. According to Raymond (1875, p. 370), the total value of 

ore produced before 1875 was estimated by the owner to be $400,000. 

Total production from the vein to date is believed to be about 

$1,000,000.

The No Name vein is developed by a shaft more than 500 feet deep; 

the levels correspond to those of the Caribou mine to a depth of 860 

feet. In addition there are three deeper levels at 920-, 1040-, and 1140 

feet (pi. 37). The vein was not being worked in 1951.

The No Name vein is not a single fissure filling like the Caribou 

vein but rather a mineralized shear zone, as much as 7 feet wide, that
542 CONTRIBUTIONS :TO THE GEOLOGY OF tJBANIUM

WEST

CARIBOU SHAFT

EAST 

CARIBOU SHAFT

860'

Stopcs

Dashed sfopes after 

Fossett, 1879

 

Drift

By Moore and Cavendec(95l

200 FEET

FIGTJBE 182. Vertical longitudinal projection of Caribou vein, Caribou mine, Boulder County, Colo
GEOLOGY OF THE CARIBOU AREA, COLORADO 543

strikes N. 55° E. and dips an average of 70° NW. (pi. 37). The ore is 

disseminated in stringers through the shear zone, cutting and surrounding brecciated fragments of country rock. The ore minerals are similar 

to those of the Caribou vein: galena, sphalerite, argentite, pyrargyrite, 

and native silver. Postmineral movement fractured the early vein 

minerals and formed openings in which late carbonates were deposited. 

The carbonate minerals, mostly dolomite and calcite, occur as veinlets 

and as tabular masses as much as one foot thick along the vein.

The shape and distribution of ore shoots along the No Name vein 

cannot be accurately determined because large parts of the No Name 

workings are inaccessible. One ore shoot extends from the 600 to the 

920 level at and near the junction of the No Name and Caribou veins 

and plunges steeply to the northeast. The lateral and upward limits 

of this shoot are not known, but it is believed to be a downward continuation of the main No Name ore shoot. A large stope to the 

southwest on the 600 level (pi. 40) indicates the presence of a second 

ore shoot in that direction.

Keserves of lead-silver ore in the No Name vein are believed to be 

small.

POORMAJST MINE

The Poorman shaft is 190 feet north-northeast of the main Caribou 

shaft (pi. 35). The mine worked the Poorman and probably the Silver 

Dollar veins, both of which were owned by the Poorman Mining 

Company. Although the Poorman vein was one of the first to be 

discovered in the area (1869), the mine was not one of the early producers. In 1879, when the shaft was 220 feet deep, the production to 

that time was estimated to be only $35,000 (Corbett, 1879). A 

prospectus written by the Poorman Mining Company about 1885 

states that mill certificates show a total production (Poorman and 

Silver Dollar veins) of nearly $200,000. The mine was operated until 

1893, and consists of the Poorman shaft, 600 feet deep, with acccessible levels corresponding to the 300,500, and 670 levels of the Caribou 

mine.

The Poorman vein strikes N. 78° W. and dips steeply to the north. 

On the 500 level, stope outlines indicate that the ore bodies were 

localized along the Poorman vein near its junctions with the No Name 

vein on the east and with an unnamed vein (possibly the Nelson) on 

the west (pi. 38). The eastern ore body extends for more than 120 

feet along the vein. The vertical dimensions of the ore bodies are not 

known.

The Poorman vein contains highly siliceous ore resembling the ore 

in the Caribou vein, although not as rich. A sample from the 670 level 

contains sphalerite, galena, pyrite, and chalcopyrite in silicified 

monzonite. The sulfides occur in interwoven veinlets and dissemi-
544 COKTRIFPTIOtfS TO TEE GEOfcO^Y OF URANIUM

nations with sphalerite the most abundant. No silver minerals are 

visible.

The nearly vertical Silver Dollar vein strikes northeast and crosses 

the Poorman vein without displacement (pi. 39). Where exposed on 

the 300 level of the Caribou mine, it is a 1 -foot-wide brecciated, ironstained shear fracture with no visible sulfides. The absence of stopes 

on this vein indicates that the production was probably small.

SHERMAN MINE

The Sherman mine, about 100 feet north of the Caribou mine, is 

developed by a shaft 265 feet deep. The workings are cut by the 

Caribou tunnel at a depth of 210 feet (pi. 39). The mine is on the 

Sherman vein which strikes east and dips 85° to the north. Where 

cut by the Caribou tunnel the vein is timbered and could not be 

examined. The ore apparently did not extend to depth and no evidence of the vein can be found on the 500 level of the Caribou mine.

According to Raymond (1875), $40,000 worth of silver was produced 

in 1874 from ore valued at $180 per ton. Fossett (1879) states that 

300 tons of ore produced in 1896 had an average value of $270 per ton.

COLUMBIA, SPENCER, AND SOCOBRO MINES

The Columbia, Spencer, and Socorro mines are on veins closely 

related to the Caribou-No Name vein system. About 1881, the 

Columbia and Spencer mines were incorporated with the Caribou and 

No Name mines, largely in order to avoid litigation. The Columbia 

is apparently on a southwestern extension of the No Name vein. The 

Columbia workings are undercut by the No Name workings. The 

mine produced little ore. The Spencer vein is adjacent and parallel 

to the No Name vein on the southeast. Developed by a shaft 420 

feet deep, it is credited with a total production of $10,000 before 1879 

(Corbett, 1879). The Socorro claim adjoins the Caribou claim on 

the south. No evidence remains of the Socorro mine but its reported 

location suggests that it probably was on the South Caribou vein 

which is exposed in the Caribou mine workings. No production is 

recorded from the Socorro mine.

RADIUM, ELMER, AND NELSON VEINS

The Radium, Elmer, and Nelson veins are exposed on the 920 and 

1040 levels of the Caribou mine (pi. 37), but have not been correlated 

with veins visible at the surface. The Radium vein branches to the 

east from the No Name vein near the point where the Elmer vein 

joins the No Name from the west. The Radium vein strikes east and 

the Elmer vein slightly north of west; both dip about 85° north on the 

1040 level but the dip of the Elmer vein decreases to about 60° above 

the 920 level. The Nelson vein strikes northeast and is nearly vertical.
GEOLOGY OF THE CARIBOU AREA, COLORADO 545

Lead-silver ore averaging at least $40 a ton has been produced 

recently from both the Elmer and the Nelson veins. The Radium 

vein also contains pitchblende, and mining of the vein has been confined to the uranium-bearing ore shoots. Except for pitchblende, the 

mineral suites of the Radium, Elmer, and Nelson veins generally are 

similar. Galena, sphalerite, argentite, and ruby silver are the ore 

minerals, with galena as the most abundant. Sphalerite, both dark 

and rosin colored, is intermixed with the galena. The gangue consists of quartz, carbonates, chalcopyrite and pyrite, with quartz as 

the chief gangue mineral, although, locally, carbonates and chalcopyrite are abundant. On the 1040 level, near the northeast face of 

the Nelson vein, more than half the vein filling is carbonate (calcite 

and dolomite). Chalcopyrite is the most abundant sulfide at a point 

30 feet east of the winze along the Radium vein on the 1040 level. In 

general, however, carbonates, chalcopyrite, and pyrite are present in 

only minor amounts. Pitchblende occurs with the lead-silver ore on 

the 920 and the 1040 levels of the Radium vein (pi. 37). The pitchblende forms a streak %-6 inches wide on the footwall side of the vein 

and is not intermixed with the sulfides.

The Radium and Elmer veins occupy tension fractures produced by 

the No Name shear fracture (pi. 37). Ore bodies occur along both the 

Radium and Elmer veins near their intersections with the No Name 

vein (pi. 37). The eastern ore body of the Radium vein and the ore 

body in the Nelson vein were localized apparently by changes in 

strike. The ore body west of the crosscut to the Elmer vein at and 

above the 920 level was controlled by a decrease in dip.

BELCHER MINE

The Belcher mine, 350 feet west of the Poorman mine, is on a 

northeast-striking vein. Corbett (1879) reports that the vein is 4 feet 

thick with a 6- to 18-inch "pay vein" that averages 50 ounces of silver 

per ton. The shaft was 110 feet deep in 1879 and 120 feet deep in 

1883. Most of the ore from this mine is believed to have been produced in the late 1890's, when most of the camp was closed. Seeley 

(1906) reports that the shaft was 300 feet deep and the total production was $75,000.

COMSTOCK MINE

The Comstock mine (pi. 37) is the only mine, other than the Caribou 

Group, that was accessible in 1951. Although not hi operation, the 

mine is being maintained in a working condition. The workings 

consist of a shaft 450 feet deep, accessible levels at 200 and 450 feet, 

and an inaccessible level at 45 feet.

The Comstock vein strikes nearly east and dips 88° to the north* 

The vein cuts gabbroic rocks about 50 feet east of the shaft but con-
546 CONTRIBUTIONS TO THE GEOLOGY OF -URANIUM

tains few ore minerals. On the 200 level west of the shaft, the vein 

consists of heavily iron-stained quartz and carbonate. One hundred 

and fifty feet west of the shaft on the 450 level, the vein splits; the 

south branch strikes S. 67° W., and is almost certainly the No Name 

vein. The Comstock workings on this level apparently joined with 

the 600 level of the No Name mine; but the workings are caved about 

35 feet, horizontally and vertically, from the presumed junction. 

The ore along the south (No Name) branch of the Comstock vein is 

similar to the ore in the No Name mine. Galena, sphalerite, quartz, 

and massive carbonates are abundant as in the No Name vein. 

Bastin and Hill (1917) report that the owners of the Comstock mine 

recognized the No Name vein on the 45 level, 80 feet west of the shaft.

NATIVE SILVER MINE

The Native Silver mine is 700 feet west of the Caribou shaft and 

nearly on strike with the Caribou vein. Although the Kocky Mountain News for July 21, 1877, states that the Native Silver vein is 

really a western extension of the Caribou lode, recent workings on the 

500 level hi the Caribou mine indicate that the two veins are not 

continuous.

The Native Silver vein strikes nearly east and dips steeply to the 

north. Minerals reported (Corregan and Langane, 1883) as present 

in the mine are galena, gray copper (tetrahedrite?), yellow copper 

sulphurets (chalcopyrite?), and quartz.

The vein reportedly averages 5% feet in width, with a pay streak 

as much as 4 feet wide (Corbett, 1879). Workings consist of a shaft, 

at least 580 feet deep, and levels aggregating 1,700 feet in length. 

The total production from the mine to 1883, the last year for which 

figures are available, is estimated to be $1,000,000 (Corregan and 

Lingane, 1883).

SEVEN-THLRTT MINE

The Seven-Thirty mine is 100 feet north of the Native Silver mine 

and like the latter was considered a westward extension of the 

Caribou lode. However, there is no evidence in the present Caribou 

mine workings to indicate a westward extension of the Caribou vein.

The Seven-Thirty vein strikes nearly east and dips about 80° to 

the north. Corbett (1879) reports a "pay vein" as much as 22 inches 

wide with ore averaging $143 per ton in silver. Unlike most of the 

mines on Caribou Hill, the near-surface ore was very low grade. The 

Kocky Mountain News (October 19, 1880) reports that the mine 

"having passed through 200 feet of 'Cap' it opens up with a rich pay 

streak." The main shaft was 185 feet deep in 1879, but the size of 

the dump indicates further development. Production of the mine 

before 1879 was $25,000 (Corbett, 1879).
GEOLOGY OF THE CARIBOU AREA, COLORADO 547

Samples of vein material found on the dump contain quartz, galena, 

sphalerite, chalcopyrite, and carbonate. In one sample from the 

dump only pyrite and quartz are visible; this sample bears much 

more resemblance to the ore from the gold mines than to the typical 

lead-silver ore of the Caribou Hill mines.

ISABEL MINX:

The Isabel mine is 400 feet southeast of the Native [Silver shaft. 

Corbett (1879) stated that the main shaft was 60 feet deep and production from the mine totalled $10,000. The size of the dump indicates that the mine is considerably deeper than 60 feet.

The Isabel vein appears to strike S. 78° E. and to have a vertical 

dip. The relationship of the vein to the southwest extension of the 

No Name vein is similar to that of the Caribou vein to the central 

part of the No Name vein.

WIGWAM MINE

The Wigwam mine is on the south side of the Caribou area and 

marks the southeastern limit of the mineralized area in the Caribou 

stock (pi. 35). The mine consists of a shaft 215 feet deep and levels 

at 75, 125, and 200 feet (Bastin and Hill, 1917). Production. from 

the mine in 1874 was valued at $40,000 (Lovering and Goddard, 

1950).

The Wigwam vein strikes about N. 75° E. and changes in dip from 

steeply north at the surface to steeply south at depth. Primary 

minerals reported by Bastin and Hi]] (1917) are galena, sphalerite 

(in part resinous), very minor amounts of chalcopyrite and pyrite, 

quartz, calcite and barite.

POTOSI AND CROSS MINES

Both the Potosi and Cross are fairly large mines on the south side 

of Idaho Hill near the contact of the Caribou stock. The material of 

the dump of the Potosi mine is mostly monzonite and that of the 

Cross mostly gneiss. Vein material on the dumps is too sparse to 

determine the average nature of the ore. It is believed that the 

Cross mine was worked for gold and silver during the late 1890's. 

[They are only briefly mentioned by the Kocky Mountain News 

(1895-1900) and the Mining Reporter (1898).] The Engineering 

and Mining Journal (1877) states that the Potosi vein is similar to 

the Caribou and is credited with a production of $18,000 to that time.

GREAT NORTHERN MINE

The Great Northern mine (fig. 177) is on the east side of Caribou Park, 

3,000 feet northeast of the Caribou mine. Although it is on the 

contact of the Caribou stock, most of the dump material is monzonite.
548 coNTBiBtmoNS TO 'THE GEOLOOT OF

The vein is believed to strike east, and on the basis of the size of the 

dump, to have been developed to a depth of at least 200 feet. Bastin 

and Hill (1917, p. 182), describe the ore as consisting of quartz, 

pyrite, and chalcopyrite with subordinate, possibly younger, sphalerite 

and galena. A small amount of pitchblende, not associated with any 

sulfide minerals, was found on the dump. Other than the veins in 

the Caribou mine, this is the only known evidence of pitchblende in 

the Caribou area.

GOLD-SILVER MINES

A small group of mines on Idaho Hill, 2,500 feet east of the Caribou 

shaft (pi. 35) was worked for both gold and silver. These mines are 

hi the gneissic and pegmatitic rocks east of the contact with the 

Caribou stock.

SILVER POINT MINE

The Silver Point mine, about 800 feet east-northeast of Caribou, is 

on a vein that strikes east and dips about 80° north. Lovering and 

Goddard (1950) state that the vein has been worked to a depth of 

250 feet and that in the oxidized zone the ore averaged about 2 ounces 

of gold and 100 to 200 ounces of silver per ton. Some of the pegmatitic 

material on prospect pits just east of the dump is slightly radioactive.

IDAHO MESTE

The Idaho mine is 400 feet east-southeast of the Silver Point mine. 

The Idaho vein strikes N. 75° E. and is vertical. In a crosscut at a 

depth of about 300 feet hi the Idaho Tunnel, it is a 2- to 3-inch ironand copper-stained shear zone containing no visible sulfides. Corregan and Lingane (1883) report that the vein contained galena and 

"sulphuret" ore which, when sorted, was valued at from $50 to $500 

per ton. The Idaho shaft in 1883 was 180 feet deep and total production was estimated at $7,000. Because this mine was operated hi 

1898, when most of the silver mines were closed as a result of the low 

price of silver, it is believed that ore from the vein contained considerably more gold than most mines on Caribou Hill.

GOLD MINE

The St. Louis mine on the west side of Idaho Hill, 1,800 feet east of 

the Idaho mine (pi. 37), is the only large mine in the area which was 

worked primarily for gold. Although no figures are available, production apparently was large enough to warrant the erection of a mill 

and an aerial tram to it. Bastin and Hill (1917) report the shaft to 

be 335 feet deep.

The vein strikes N. 63° W. and dips 78° NE. Ore found on the 

dump contained quartz, pyrite, chalcopyrite, and minor amounts of 

sphalerite and galena. Some crystals of late carbonate minerals and 

quartz line vugs.
540

LITERATURE CITED

Bastin, E. S., and Hill, J. M,, 1917, Economic geology of Gilpin , County and

adjacent parts of Clear Creek and Boulder Counties, Cdlo.: U. S. Geol. 

, .Survey Prof. Paper 94, 379 p. 

Burchard, H. G., 1882, Report of the Director of the Mint, on the production of

precious metals in the United States during the calendar year 1882, p.

397-398. 

Corbett, T. B., 1879, Colorado Directory of Mines, containing a description of

the mines and mills, and the mining and milling corporations of Colorado,

arranged alphabetically by counties, and a history of Colorado from its

early settlement to the present time; Denver Colo., Rocky Mountain News

Printing Co. 

Corregan, R. A., and Lingane, D. F., 1883, Colorado Mining Directory: containing an accurate description of the mines, mining properties and mills,

and the mining, milling, smelting, reducing and refining companies and

corporations of Colorado, arranged alphabetically by counties; Denver,

Colo., The Colorado Mining Directory Co. 

Endlich, F. M., 1874, Report of F. M. Endlich, in F. V. Hayden, Annual Report

of the United States Geological and Geographical Survey of the territories

embracing Colorado, being a report of progress of the exploration for the

year 1873, p. 300-301.

Eng. and Min. Jour., 1877, v. 24, no. 6, p. 105-107. 

Fossett, Frank, 1876, Colorado; a historical, descriptive, and statistical work on

the Rocky Mountain gold and silver region: Denver, Colo. Daily Tribune

Steam Printing House. 

 1879, Colorado; its gold and silver mines, farms and stock ranges, and

health and pleasure resorts. Also 2d ed., Tourists' guide to the Rocky Mountains: New York, C. G. Crawford. 

Henderson, C. W., 1926, Mining in Colorado, a history of discovery, development,

and production: U. S. Geol. Survey Prof. Paper 138. 

Kerr, P. F., 1951, Natural black uranium powder: Science, v. 114, no. 2952, p.

91-92. 

Kerr, P. F., and Kulp, J. L., 1952, Precambrian uraninite, Sunshine mine, Idaho:

Science, v. 115, no. 2978, p. 86-87. 

Lovering, T. S., 1932, Preliminary map showing the relations of ore deposits to

geologic structure in Boulder County, Colo.: Colo. Sci. Soc. Proc., v. 13,

no. 3, p. 77-88. 

Lovering, T. S., and Goddard, E. N., 1950, Geology and ore deposits of the Front

Range, Colo.: U. S. Geol. Survey Prof. Paper 223. 

Min. Rev., 1873, 1874, January, February, March, April, May, 1873. February,

May, July, August, December, 1874. 

Nier, A. D., Thompson, R. W., and Murphey, B. F., 1941, The isotopic constitution of lead and the measurement of geological time, pt. Ill: Phys.

Rev., v. 60, no. 2, p. 113. 

Phair, George, 1952, Radioactive Tertiary porphyries in the Central City district,

Colorado, and their bearing on pitchblende deposition: U. S. Geol. Survey

TEI-274, U. S. Atomic Energy Comm., Tech. Inf. Service, Oak Ridge, Tenn. 

Raymond, R. W., 1872, 1873, 1874, 1875, Statistics of mines and mining in the

States and Territories west of the Rocky Mountains: 3rd Ann. Rept., 566 p.,

(1872); 4th Ann. Rept., 550 p. (1873); 5th Ann. Rept., 585 p. (1874); 6th

Ann. Rept., 540 p. (1875). 

Rocky Mountain News, 1871, 1872, 1874, 1877, 1880, 1890-1900, Denver, Colo.
550 COHTBIBUTIQjre TO

Seeley, W. L., 1906, Report of mines and minerals of Boulder County, on line of

the Colorado and Northwestern Railroad. 

Smith, Ward C., 1938, Geology of the Caribou stock in the Front Range, Colo.t

Am. Jour. Sci., v. 36, no. 213, p. 161-196. 

Van Diest, P. H., 1875, The crossing of the No Name and Caribou veins: Min.

Rev., v. 6, p. 5.
INDEX

Page

Acknowledgments _____ .. ______ 518 

Alteration _________________ 532-534 

Analyses.. . _ __ 536 

Argentite __________________ 528 

Argillite zone 533

Bastin, E. S,, and Hill, J. M., cited _ ... 527, 528 

Belcher mine ____ ____ 645 

Boulder Creek granite __ . _____ 520, 522, 523 

Brecciation. ............... 526, 534, 541, 543

Carbonate minerals _ ... 527, 528, 529, 541, 543, 545 

Caribou area, location. ___ 518, 519 

relief ________________ . _ 518 

Caribou stock _________ 522, 523-524, 527, 528 

Cerargyrite _________________ 528 

Cerusslte . . 529 

Chalcopyrite. ... ....... . 529, 538, 541, 545

Chlorite ..... ...... .... ___ .... 532, 533

Chlorite zone ___ . _ .... __ 533

Crushing, postmineral ________ 541, 543

Displacement of veins ____________ 526

Faults. __ ...... 524-525

Foliation.. _ . _ ... ____ . _______ 524 

Front Range mineral belt _____ .. 522

Oabbroic rocks _____ .... _____ 524

Oalena _____________ 529, 532, 537, 538, 545 

Gneiss ____________ ... ______ 523, 527 

Goddard, E. N., and Levering, T. S., cited. 523,

526 

Gold............ .......... ............... 522, 535

Gold mines.... __ . ___ .. ___ 522, 527, 548

Gold veins ___________ - _ 523,525,527,528 

Granite ______ . ___ ______ 523 

Great Northern mine ____ - ___ 536, 547-548

Hfll, J. M., and Bastin, E. S., cited ___ 527, 528 

Horn silver __ . 528, 529

Idaho Springs formation ___ .. ___ 522, 523, 527 

niite _- ..-..--.- . 532, 533 

Intrusive rocks _ 524-525 

Iron dike 526

Joints _________ .... _ . __ . __ 524

Loverlng, T. S., cited..

Page

. 525

Kerr, P. F., quoted..... __ 

Kerr, P. F., and Kulp, J. L., cited

539

539

Lead _ .. ___ . __________ 521, 535 

Lead-silver deposits _ . ___ 522, 526-527, 528-536 

uranium-bearing __ . _ . __ 536-539 

Localization of ore bodies __ - _____ 534 

Levering, T. S., and Goddard, E. N., cited. 523,

Malachite._________________ 529 

Mine dumps, as indicators of vein material. 527,

537,547,548

Mineral assemblages, altered zones.. 533 

gold veins..__..___.__...... . 627

lead-silver veins_____ 633,543,545,646,647 

Mines, Caribou mine_________ 517,521, 

528,532,534,535-536,537-538,640-541 

Columbia mine________,__,_ 5 

Comstock mine_________ 528,545-546 

Cross mine . 647 

Idaho mine__. ___________ 548 

Isabel mine. _._ 547 

Native Silver mine_-__ __ 521,546 

Potosimine_________-__ 547 

St. Louis gold mine_______-__ 548 

Seven-thirty mine_________ 546-547 

Sherman mine. 544 

Silver Dollar mine... __ 540 

Silver Point mine______-___-_ 548 

Spence mine. . 544 

Socorro mine____-__ _ 540,544 

Wigwam mine .._________.... 547

Mining, history... .. . .. 521 

Montmorillonlte. -__ 533-534 

Monzonite....... 522,528,524,527,532,548

Movement along veins... _ . 526 

postmineral__ ... 534

Ore, enriched __..________ 521,529,532 

high-grade________________ 521,535 

silicified ..................... ..... 529

value 521,535,545 

Ore shoots___.___________ 534,535,541

Paragenesis, Caribou lead-silver deposits. 528-529,

537-538

Radium vein. ... 537 

Pegmatite 523,527

Pitchblende... __............ 517,532,536-38

age._________....__ . 538-539

distribution in Radium vein. 539,545 

hard. 537,538,539

paragenesis______.__.... 538

soft, sooty 537,538,538,539 

Precambrian rocks____________.. 522,523 

Production- . 522,535,540,545,546 

Pyrargyrite _________.. _ 528 

Pyrite __...... .... .... 528-529,537

Quartz- 528,529,532,537

551
552 INDEX

Page 

Raymond, R. W., cited.....__........ 521,529

Schist. .._.....__....__........ 523,527

Sericitization________________ 632 

Shear zones________________ 525,541 

Shear fractures...__........... 522,525,534,535

Siliciflcation______________._.. £32

Silver_________.________ 522,532,535

minerals........_________ 521,528,529

native.__ ___._________ 529 

Sphalerite...____________ 529,537-538,545 

Strike, changes and localization of ore shoots, 535 

Structure, regional...... ..__.... 522,524-525

Veins, Caribou vein_____ 526, 529,534,535,64i 

east-treMing.._____________ 628 

Elephant veto._____________ 527 

Elmer veto.. .__ -_...._ 640,644-*iS

fissure filling_______________ 534 

Idaho vein ________________ 527 

lead-silver......__.._.... 639-636,544,645

uranium-bearing.....___.___ 536-539

Nelson vein __________ 640,544-545

No Name veto .1__________ 526,

529,533, 534,536, 541, 543, 546

Page 

Veins Continued

northeast-trending. _ 525 

northwest-trending. ... 525 

Poorman vein _ 543-544 

Radium vein 529, 

533,534, 535, 536-537, 538, 539, 541, 544-545 

replacement.___ .. 534 

Seven-Thirty vein 546 

Silver Dollar vein 544 

Silver Point vein_______.. __. 527 

Spencer vein 540,544 

Will rock. .........__ ..... 532-533

Windy Point vein .... .1 527

Vein systems.. 526 

Vag filling........ ................ 527,537,588

Wrigbt, H. D.f quoted....... . 638

Zine .-. .............. ............... 632,536

Zone, of oxidation __________ 629,632,635 

of secondary sulfides . . 529,533 

of supergene enrichment........ ... 529

Zones, of alteration 531-534
Contributions to the 

Geology of Uranium 

1955

GEOLOGICAL SURVEY BULLETIN 1030

This bulletin was printed as separate chapters A-N

These reports concern work done on behalf 

of the U. S. Atomic Energy Commission 

and are published with the permission of 

of the Commission

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON ; 1957

CONTENTS

Tbe letters in parentheses preceding the titles are those used to designate the separate chapters.

Page

(A) Search for uranium in the United States, by V. E. McKelvey______ 1

(B) Stra|£ggap,hy of parts of De Soto and Hardee i Counties, >. Florida, by

MfH. Bergendahl..___________________________ 65

(C) Uranium deposits at base of the Shinarump conglomerate, Monument

Valley, Arizona, by I. J. Witkind___________________ _ 99

(D) Accuracy of ore-reserve estimates for uranium-vanadium deposits on

the Colorado Plateau, by A. L. Bush and H. K. Stager_________ 131

(E) Study of radioactivity in modern stream gravels as a method of prospecting, by R. T. Chew, III________________________ 149

(F) Geology and monazite content of the Goodrich quartzite, Palmer area,

Marquette County, Michigan, by R. C. Vickers____ ___ 171

(G) Wall-rock control of certain pitchblende deposits in Golden Gate 

Canyon, Jefferson County, Colorado, by John W. Adams and 

Frederick Stugard, Jr_.___________________ 187

(H) Uranium in black shale deposits, northern Rocky Mountains and

Great Plains, by W. J. Mapel_______________________ 211

(I) Reconnaissance study of uranium deposits in the Red Desert, Sweetwater County, Wyoming, by Donald G. Wyant, William N. Sharp, 

and Douglas M. Sheridan_________________ 237

(J) Stratigraphy of the Morrison formation in part of northwestern New

Mexico, by V. L. Freeman and L. S. Hilpert_____ 309

(K) Uranium deposits in oolitic limestone near Mayoworth, Johnson

County, Wyoming, by R. R. Guilinger and P. K. Theobald, Jr__ 335

(L) Uranium and copper deposits, Coyote district, Mora County, New

Mexico, by C. M. Tschanz_________________________ 343

(M) Description of indicator plants and methods of botanical prospecting

for uranium deposits on the Colorado Plateau, by Helen L. Cannon. 399

(N) Geology and uranium deposits of the Caribou area, Boulder County,

Colorado, by F. B. Moore, W. S. Cavender, and E. P. Kaiser 517