GEOLOGICAL SURVEY CIRCULAR 277

PRELIMINARY REPORT 

ON THE COMET AREA 

JEFFERSON COUNTY

MONTANA
UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

GEOLOGICAL SURVEY CIRCULAR 277

PRELIMINARY REPORT ON THE COMET AREA 

JEFFERSON COUNTY, MONTANA

By George E. Becraft

This report concerns work done on behalf of the

U. 5. Atomic Energy Commission 

and is published with the permission of the Commission

Washington, D. C., 1953

Free on application to the Geological Survey, Washington 25, D. C.
PRELIMINARY REPORT ON THE COMET AREA 

JEFFERSON COUNTY, MONTANA

By George E. Becraft

CONTENTS

Page

Abstract..................................... 1

Introduction ................................ . 1

Geology...................................... 3

General features......................... 3

Igneous rocks ........................... 3

Prebatholith volcanic rocks........... 3

Quartz monzonite................... 3

Alaskite ........................... 4

Dacite and andesite.................. 4

Structural features ...................... 5

Mineral deposits ............................. 5

General features ...................... . 5

Page

Mineral deposits Continued

Older deposits .......................... 6

Comet mine ....................... 6

Gray Eagle mine ................... 6

Silver Hill mine.................... 7

Hope-Bullion ...................... 7

Boulder Chief mine................. 7

Younger deposits........................ 7

Free Enterprise mine............... 7

High Ore (Montana Consolidated) mine.. 7

Literature cited .............................. 8

Unpublished report........................... 8

ILLUSTRATIONS

Page

Plate 1. Geologic map of the Comet area, Jefferson County, showing radioactivity

anomalies ................................................................... Inside back cover

Figure 1. Index map showing Comet area, Jefferson County .............................................. 2

ABSTRACT

Several radioactivity anomalies and a few specimens of sooty pitchblende and other uranium minerals 

have been found on the mine dumps of formerly productive base- and precious-metal mines alongthe CometGray Eagle shear zone in the Comet area in southwestern 

Montana. The shear zoneisfrom 50 to 200feetwide and 

has been traced for at least 5i miles. It trends N. 80° W. 

across the northern part of the area and cuts the quartz 

monzonitic rocks of the Boulder batholith and younger 

silicic intrusive rocks, as well as prebatholithic volcanic rocks, and is in turn cut by dacite and andesite 

dikes.

The youngest period of mineralization is represented by chalcedonic vein zones comprising one or 

more discontinuous stringers and veins of cryptocrystalline silica in silicified quartz monzonite and in 

alaskite that has not been appreciably silicified. In 

some places these zones contain no distinct chalcedonic 

veins but are represented only by silicified quartz 

monzonite. These zones locally contain uranium in 

association with very small amounts of pyrite, galena, 

ruby silver, argentine, native silver, molybdenite, 

chalcopyrite, arsenopyrite, and barite. At the Free 

1 Enterprise mine, uranium has been produced from a 

narrow chalcedonic vein that contains disseminated 

secondary uranium minerals and local small pods of 

pitchblende and also from disseminated secondary 

uranium minerals in the adjacent quartz monzonite.

Undiscovered deposits of uranium ore may occur 

spatially associated with the base- and precious-metal 

deposits along the Comet-Gray Eagle shear zone and 

with chalcedonic vein zones similar to the Free Enterprise.

INTRODUCTION

The Comet area is a few miles northwest of 

Boulder and about 20 miles southwest of Helena,Mont, 

(fig. 1). U. S. Highway 91 between Butte and Helena 

crosses the southwestern corner of the area, and a 

branch of the Great Northern Railway crosses the 

southeastern and southwestern corners. The area is 

drained southward by tributaries of the Boulder River. 

-3&e maximum relief is about 3, 300 feet, with most of 

the ridges about 1,000 feet above the principal valleys.

The geology of about 2 square miles in the 

southeastern corner of the area was mapped in 

September 1950 (Roberts and Gude, 1952); the remainder of the area was mapped in July to October 

1951 by D. Y. Meschter, E. E. Gross,and the writer. 

Much of the petrographic work on which the rock de scrip - 

tions are based was done by G. J. Neuerburg and Gross. 

The work was carried out on behalf of the Division of 

Raw Materials of the U. S. Atomic Energy Commission.

Except for the detailed study in the vicinity 

of the Free Enterprise mine (Roberts and Gude, 1952),
112° 10' 112° 00'

M Of N T A N A

112° 2O 112° 10' 112C 00'

10 Miles

Figure 1. Index map showing Comet area, Jefferson County, Mont.

2
previous geologic work in this area has been of a reconnaissance nature in connection with studies of the 

mineral deposits in and near the Boulder batholith. 

The most complete studies are by Knopf (1913), 

Billingsley (1915), Billingsley and Grimes (1918),' 

and Pardee and Schrader (1933).

GEOLOGY 

General features

The Comet area is near the center of the Boulder 

batholith and is underlain principally by quartz monzonite. The batholith intrudes a series of volcanic 

rocks, remnants of which are exposed near the northern 

margin of the map. Both the batholith and the volcanic 

rocks have been intruded by silicic rocks, including 

aplite, alasMte, alasMte porphyry, and pegmatite. 

These silicic rocks'are designated alasMte on the map. 

The youngest C9nsolidated rocks are dacites and andesites 

that occur principally as steeply dipping dikes, cutting 

the alasMte series and older rocks. (See plate 1. )

The major structural feature is the Comet-Gray 

Eagle shear zone, which trends N. 80 W. across the 

northern part of the mapped area. In the southeastern 

part of the area the quartz monzonite and alasMte have 

been sheared and brecciated and recurrently silicified 

to form steeply dipping, chalcedonic vein zones that 

trend N. 60 E. The vein zones comprise one or more 

discontinuous stringers and veins of cryptocrystalline 

silica in silicified quartz monzonite and in alasMte that 

has not been appreciably silicified. In some places the 

zones contain no distinct chalcedonic veins but are represented only by silicified quartz monzonite. The trend 

of the chalcedonic vein zones gradually changes to east 

in the center of the mapped area. Faults are commonly 

observed in the mines, but, because of'poor exposures, 

only a few could be traced on the surface.

The only sediments are alluvial and colluvial 

material of Pleistocene(?) and Recent age. A veneer 

of hillwash and mantle covers much of the area.

Igneous rocks 

Prebatholith volcanic rocks

The prebatholith volcanic rocks are part of a 

large roof remnant that extends northward beyond the 

limits of the area mapped. The part of the remnant 

within the area consists of two flows that have been 

mapped as a single unit. The contact between the volcanic rocks and the batholith is almost horizontal, although local irregularities exist. Field relations within 

these volcanic rocks were not studied in detail.

The rocks are light- to dark-gray andesite, generally fine-grained, with small phenocrysts of plagioclase 

and hornblende. Flow banding, observed in a few isolated localities, is uncommon.

The few thin sections examined show porphyritic 

texture, in part seriate. Corroded plagioclase phenocrysts comprise from 25 to 30 percent of the rock and 

are normally zoned from An 40 to Anzo- Phenocrysts 

of hornblende, which may comprise as much as 2percent of the rock, are commonly altered in part to

biotite, in part to groundmass-material, and in some 

specimens to ilmenite, penninite, and tourmaline. 

The mosaic groundmass consists principally of very 

fine grained albite and quartz with accessory amounts 

of biotite, apatite, zircon, magnetite, and tourmaline.

Knopf (1913, p. 28) concluded that these rocks 

are of Late Cretaceous age; recent studies near the 

eastern margin of the batholith support this conclusion (M. R. KLepper, R. A. Weeks, andE. T. Ruppel, 

oral communication).

Quartz monzonite

The Comet area is underlain principally by the 

Boulder batholith, which has been divided on the basis 

of textural and minor compositional differences into 

five distinctive mappable units; all five are within the 

quartz monzonite range. The relative ages and structural relations of the several map units are not known, 

but the similarities in composition and to some extent 

in texture suggest a common source and probably a 

close age relationship.

The most common unit, designated "qm" on 

plate 1, is dominantly medium-grained biotite-hornblende 

quartz monzonite. The texture is hypautomorphie, locally modified by large irregular grains of potash 

feldspar. The rock comprises 30 to 35 percent plagioclase, 25 to 35 percent potash feldspar, 20 toSOpercent quartz, 2 to 7 percent biotite, 1 to 5 percent hornblende, and 1 percent or less of magnetite, sphene, 

apatite, rutile(7), and zircon.

The plagioclase ranges from thick tabular crystals to small irregular grains and is commonly zoned 

from An45 to Anzo. The large crystals are euhedral 

against quartz and irregular against potash feldspar. 

Slight sericitization of the plagioclase is common, and 

some crystals have zones that are almost completely 

replaced by sericite.

Orthoclase appears to be more common than 

microcline; however, some potash feldspar, in which 

grid twinning cannot be detected, has faintly uneven extinction that may indicate a triclinic structure. Microperthite is common, and many large irregular grains 

have inclusions of quartz and corroded plagioclase crystals, suggesting that microperthite has replaced part of 

the plagioclase. Slight kaolinization gives a faint cloudiness to all the potash feldspar. Allthequartz is anhedral; 

some of it contains many liquid inclusions, and some 

has a faintly detectable grid structure. Strain shadows 

are common.

Biotite is generally more abundant than hornblende 

and ranges from small flakes to thick tabular grains; 

its pleochroic colors are X, light yellow to yellowish 

green; Y and Z, brown to brownish green. Many biotite 

crystals are altered to penninite only part of which exhibits the characteristic anomalous blue interference 

color clinozoisite, sphene, and magnetite. Hornblende 

typically forms subhedral crystals, but corroded euhedral 

crystals are not uncommon. The pleochroic colors are 

X, yellow green; Y and Z, dark green. The alteration 

products are commonly epidote, penninite, and rarely 

clinozoisite.

In the northwestern part of the mapped area, a 

distinctive rock type, designated "qmi, " lies between
"qm" and the prebatholith volcanic rocks and extends 

in both directions beyond the limits of the area mapped. 

The significance of this spatial relationship is not known. 

The "qmi" differs significantly from "qm" in composition, containing less quartz and more potash feldspar. 

The approximate composition of "qmi" is 20 percent 

quartz, 45 percent potash feldspar, 30 percent plagioclase, and 4 percent biotite and hornblende. -The texture is hypautom orphic, almost xenomorphic. Plagioclase crystals, commonly zoned from An45 to Anao, 

have ragged edges. Some crystals are cut by small 

irregular stringers of calcite that seem to be of replacement origin. Large irregular orthoclase crystals, 

commonly clouded by kaolin, seem to have partly replaced plagioclase. Microperthite is common. Quartz 

occurs inter stitially. Hornblende is present as subhedral crystals, partly replaced by penninite, epidote, 

calcite, and magnetite. Biotite, in irregular grains, 

is pleochroic; X, pale fellow; Y and Z, dark brown. 

Sparse accessory minerals include apatite, sphene, 

zircon, and very rare bluish-gray tourmaline.

A type of quartz monzonite designated " 

was found in only one small area, on the northwestern 

slope of Mount Thompspn, where it is apparently in 

contact with alaskite, "qmi, " and prebatholith volcanic rocks although the contacts are not exposed. The 

texture is xenomorphic. The composition of the rock 

is 21 percent quartz, 25 percent plagioclase, 42 percent orthoclase, 6 percent biotite, 3 percent augite, 

1 percent hornblende, and less than 1 percent magnetite, 

apatite, and zircon.

Plagioclase crystals, corroded and embayed by 

orthoclase and quartz, are normally zoned from An4o 

to Anas. Orthoclase is slightly kaolinized and shows 

no evidence of triclinic structure. Microperthite is 

common in irregular grains and contains many inclusions of quartz, plagioclase, and biotite. The 

relative abundance of orthoclase and microperthite 

ranges widely within the specimens examined. Quartz 

occurs interstitially and as small spherical inclusions 

in feldspars. Ragged anhedral to subhedral biotite 

grains are pleochroic, with X, light yellow, and Y and Z, 

greenish brown to dark yellow brown. Some biotite is 

altered to brown penninite. Anhedral crystals of 

augite, pleochroic from light green to very pale yellow, 

have a maximum extinction angle of 45 . Light -green 

weakly pleochroic uralitic hornblende is present as 

thin discontinuous rims and as irregular replacements 

of the augite.

A fresh-appearing quartz monzonite on the west 

slope of Mount Thompson is designated "qma. " This 

rock has a hypautomorphic -granular texture and is 

composed of 42 percent plagioclase, 25 percent potash 

feldspar, 19 percent quartz, 5 percent biotite, 4 percent augite, 3 percent hornblende, and accessory 

amounts of magnetite, apatite, zircon, and sphene. 

Unzoned plagioclase crystals are Anas; some crystals 

are normally zoned from An4o to Anas. Sparse sericite 

has penetrated along fractures in most of the crystals. 

Orthoclase grains are anhedral and some have very 

slight wavy extinction. Microperthite has irregular 

boundaries with plagioclase and seems to have partly 

replaced it. The relative abundance of microperthite 

and orthoclase ranges widely within the specimens 

examined. The potash feldspar is in part kaolinized. 

Quartz is present as anhedral grains. Biotite, also 

typically in anhedral grains, is slightly altered to

penninite and clinozoisite. Ragged subhedral prisms 

of weakly pleochroic, light-green augite, with a maximum extinction angle of 40 , are partly replaced by 

uralitic hornblende.

On the eastern and southern slopes of Mount 

Thompson'a textural variation, "qnu, " occurs along 

the border of the batholith between the "qm" and the 

volcanic rocks and grades into the "qm. " Near the 

contact with the volcanic rocks it is fine-grained xenomorpMc 

and slightly porphyritic. The grain size increases 

gradually with distance from the contact, grading into 

the common medium-grained quartz monzonite. The 

composition of "qnu" does not differ significantly from 

"qm" even within a few feet of the volcanic contact. 

It is composed of about 25 to 30 percent plagioclase, 

35 to 40 percent potash feldspar, 25 to 30 percent 

quartz, 4 percent biotite, 2 percent hornblende, and 

sparse apatite, magnetite, and zircon.

Alaskite

A complex group of quartz-rich rocks comprising aplite, alaskite, alaskite porphyry, and pegmatite 

intrudes the prebatholith volcanic rocks and the quartz 

monzonite. The rocks occur principally as dikes; however, a few large bodies of irregular shape have been 

recognized. All the above quartz-rich rock types may 

be present in a single body, but more commonly only 

one or two are present, in the Comet area aplite predominates, alaskite is common, and alaskite porphyry 

and pegmatite are relatively rare. The coarser grained 

rocks, where present, are not necessarily restricted 

to the center of dikes and intrusive bodies but seem to 

be sporadically and irregularly distributed. Similarly, 

selvages of finer grained rock are not characteristic 

of the margins of dikes and other intrusive bodies.

In the area mapped, there is a concentration of 

alaskite near the southeastern corner. Because of few 

exposures, it could not be determined whether the 

alaskite is in a few large bodies, as shown on the map 

in this vicinity, or in many small dikes separated by 

quartz monzonite wall rock.

The texture of the rock is xenomorphic, in places 

modified by phenocrysts of quartz and potash feldspar. 

Microperthite and granophyre are common. The rocks 

are composed of potash feldspar, 40 to 60 percent;, 

quartz, 25 to 40 percent; plagioclase Ang to Anig, 

5 to 30 percent; biotite, 1 to 3 percent; and small 

amounts of apatite, tourmaline, zircon, magnetite, 

and rutile. Tourmaline also occurs as a later replacement along joints and fractures.

A detailed study of the alaskite is now in progress 

to determine, if possible, whether there is a genetic 

relationship between the alaskite and some of the mineral deposits in the area, as suggested by the spatial 

relationship of the alaskite to areas of intense alteration and abundant siliceous reefs in the southeastern 

corner of the map, and by the common presence of tourmaline in the alaskite and also in the altered quartz 

monzonite surrounding base metal veins.

Dacite and andesite

The youngest rocks are dacite and andesite. 

Because of the sporadic distribution of quartz,both 

dacite and andesite may occur in a single outcrop.
They form a group of steeply dipping dikes with a general northeasterly trend. The dikes are restricted to 

a relatively narrow area that also trends northeasterly 

through the center of the mapped area. Many of the 

dikes exhibit a platy jointing of diverse trend but invariably steep dip. Commonly, small biotite flakes 

are oriented parallel to the joints.

The rock is gray to white fine-grained porphyritic 

dacite and andesite. Glassy euhedral plagioclase and 

biotite phenocrysts are common; euhedral quartz crystals are common in some hand specimens and entirely 

lacking in others from the same outcrop.

The rock contains from 12 percent to 40 percent 

phenocrysts that consist of plagioclase, normally ranging from 55 to 75 percent and rarely as low as 10 percent of total phenocrysts; quartz, normally ranging from 

12 to 25 percent, but in some specimens completely 

absent and in a few, comprising as much as 70 to90percent; and biotite, ranging from 10 to 25 percent.

The plagioclase crystals are subhedral to euhedral 

and are commonly embayed by and contain inclusions of 

the groundmass. Most crystals are unzoned; the composition in different specimens ranges from An45 to 

Angs. Some crystals are partly to completely replaced 

by fine-grained sericite and others by kaolin. Biotite 

occurs as thin, platy crystals with pleocnroism X, 

light yellow; Y and Z, dark brown to dark greenish brown. 

Quartz occurs commonly as euhedral'prisms, some of 

which are rounded, and embayed by the groundmass. 

The groundmass is generally a very fine grained 

aggregate of plagioclase, biotite, possibly quartz, and 

glass. Some streaming of the groundmass around 

phenocrysts is evident from orientation of the biotite 

flakes and feldspar laths.

North of Sugarloaf Mountain, a black glassy 

dikelike body of andesite about 2 feet wide and 25 feet 

long lies between the dacite-andesite body and the 

quartz monzonite. This andesite probably forms a 

selvage of the larger normal dike, but the contact was 

completely covered and could not be observed.

The glassy andesite is porphyritic and typically 

contains about 20 percent of plagioclase as phenocrysts 

and as much as 5 percent of biotite as phenocrysts. 

The groundmass comprises crystallites of sodic 

plagioclase, biotite, and interstitial glass.

The plagioclase phenocrysts are clear tabular 

euhedrons; wormy intergrowths of groundmass occur 

in the central parts of some. Two types of zoning are 

present: normal zoning from Anao to Ango and reverse 

zoning.' The reverse-zoned crystals have sharply defined rounded Anis cores and Angs rims.

Near the junction of High Ore Creek and the 

Boulder River is an intensely brecciated pipelike body 

consisting of angular fragments, ranging in size from 

less than one-tenth of an inch to several inches, of 

normal porphyritic dacite and extremely fine grained 

banded dacite. The pipe is about 400 feet long and 

50 to 80 feet wide, and it stands about 10 to 50 feet 

above the surrounding quartz monzonite. The only 

contact observed was a sharp vertical contact between 

the breccia and a small alasMte dike at the northern 

margin of the pipe. Within a few inches of this contact, 

the average size of the fragments is much smaller than

the average size of the fragments throughout the remainder of the pipe. The alasMte is not brecciated 

or altered at the contact; however, several rounded 

disintegrated fragments of quartz monzonite were 

found in the pipe within several feet of the contact.

Most of the fragments are composed of moderately to intensely altered dacite or andesite. Some 

fragments are altered entirely to kaolin, limonite, 

and quartz. In others, the quartz and biotite are 

essentially unaltered, but laths of plagioclase have 

been replaced entirely by chalcedony. A few fragments contain large irregular grains of calcite. More 

than one period of brecciation is indicated by several 

of the included large angular fragments that are com - 

posed of smaller angular fragments.

The fragments-are cemented by very fine grained, 

flow-banded dacite. The cementing material is essentially unaltered and consists of irregular grains of quartz, 

plagioclase, subhedral flakes of biotite, and glass. The 

banding is apparently the result of a slightly greater amount of glass in some bands which imparts a darker 

shade of gray.

Structural features

The most conspicuous structural features in the 

Comet area are the Comet-Gray Eagle shear zone in 

the northern part and the many steep-dipping chalcedonic 

vein zones in the southeastern part. The vein zones have 

been offset by northwest- and northeast-trending faults. 

Numerous faults of diverse trends were observed underground, but only a few of the most obvious could be 

traced on the surface because of the lack of key units 

for correlation.

The steep-dipping Comet-Gray Eagle shear zone 

trends in general N. 80°W., is as much .as 200 feet 

wide, and extends from beyond the eastern margin of the 

map almost to the western margin, a distance of 

5| miles. It transects the older volcanic rocks, 

the quartz monzonite, and the alasMtic rocks, all of 

which are intensely sheared and brecciated in the zone, 

and it, in turn, is transected by the younger dacite and 

andesite. Two northeast-trending shear zones join the 

main zone about 2,000 feet east of the Comet mine; in 

the vicinity of the Bismark mine, movement along the 

easternmost split-from the main shear zone has offset 

the contact of the older volcanic rocks and the quartz 

monzonite. This is the only place along a shear zone 

where displacement can be demonstrated.

The general trend of the chalcedonic vein zones 

changes from N. 60 E. in the southeastern corner of 

the mapped area to east near the center. Brecciation 

has occurred in many of the vein zones, and gouge or 

microbreccia is common at the margin and within some 

vein zones.

MINERAL DEPOSITS 

General features

The area mapped contains a number of base- and 

precious -metal deposits that have been explored by means 

of numerous prospects and in which several mines have 

been developed. The most important deposits occur 

along the Comet-Gray Eagle shear zone. Different
deposits contain gold, silver, lead, zinc, and copper 

in varying ratios. These are the oldest mineral deposits 

in the area, and some at least may be assigned to the 

tourmalinic silver-lead group of Knopf (1913, p. 46).

A younger and distinctly different type of deposit 

comprises the chalcedonic vein zones that locally contain a little silver and gold but no important concentrations 

of base metals. Deposits of this type are common near 

Boulder and particularly near Clancey, north of the area 

mapped (Knopf, 1913, p. 54).

A third type of deposit that has some characteristics of both of the above types contains abundant lead, 

silver, and zinc in a gangue that is predominantly 

cryptocrystalline silica; no tourmaline occurs in these 

veins or in the altered quartz monzonite wall rock 

adjacent to them. These deposits may have beenformed 

in an intermediate period of mineralization, or the^ may 

be the result of mineralization in both of the periods previously described. No abnormal radioactivity was detected in deposits of this type.

Uranium minerals and high radioactivity anomalies 

(pi. 1) have been found in deposits of both the older and 

younger groups. Uranium may have been deposited during both periods of mineralization or only during the 

later period, when some of the older veins may have 

been reopened and reinvaded by ore-bearing solutions.

The general features of the mineral deposits in 

this area and some of the principal deposits have been 

described by Knopf (1913, p. 114-122), Eillingsley and 

Grimes (1918, p. 306-315"), and Pardee and Schrader 

(1933, p. 285-299). In this report only the deposits 

with abnormal radioactivity are described.

Older deposits

Comet mine

The Comet mine is in the Comet-Gray Eagle 

shear zone in quartz monzonite near the roof of the 

Boulder batholith. In the mine the shear zone is cut 

by two dacite dikes. Elsewhere, along the shear zone, 

are a number of other base- and precious-metal deposits. 

The Comet deposit is the largest, and the value of lead, 

silver, zinc, gold, and copper produced from the Comet 

mine is reported by company records and unpublished 

reports to exceed 20 million dollars. During the latest 

period of activity, from 1934 to 1941, the production 

was about 3 million dollars from ore that averaged about 

7 to 10 dollars per ton in combined lead, zinc, silver, 

gold, and copper.

All workings are inaccessible, but old maps indicate that the mine was developed to a depth of 960 feet 

and for a strike length of 2, 200 feet by a vertical shaft 

and eight principal levels. No ore was mined below the 

700-level, and most of the production came from above 

the 500-level.

Three principal veins follow the trend of the shear 

zone and dip steeply to the south (pi. 1), and several 

splits of these veins were explored. The major portion 

of the stoping was done on these veins within 500 feet 

east and west of the shaft. Company stope maps indicate that the bulk of the ore occurred in shoots in the 

three main veins and the several minor splits of these

veins, in a roughly defined block that was longest on 

the 300-level a.id pinched out at auout the 700-level. 

Below the 700-level the veins contained abundant 

pyrite, but only very small amounts of other sulfides, 

in a quartz gangue.

The wall rock is dominantly altered quartz 

monzonite with subordinate altered alaskite. Altered 

rock is of three types silicified, sericitized, and 

kaolinized. A detailed study now in progress may yield 

more information on the relationship between the different types of alteration and the deposition of ore 

minerals.

Ore minerals identified from dump samples are 

galena, sphalerite containing exsolved chalcopyrite, 

pyrite, chalcopyrite, and arsenopyrite. The gangue 

is chiefly altered wall rock and quartz, part of which 

is a clear crystalline variety and part a dark bluishgray fine-grained variety.

Several samples of radioactive material were 

collected from the Come: dump John Giulio, 

present lessee of the property, reports that the 

material probably came from the east drift on 

the 200-level. Moderate to high radioactivity anomalies 

were found at only two places on the dump, but higher 

than average radioactivity was detected at a few other 

places.

Two samples of dump material were submitted 

for chemical analysis and mineral identification to the 

Geochemistry and Petrology Branch of the U. S. Geological Survey in Washington, D. C. One sample of 

weathered soft black vein material, selected as the 

most radioactive sample found, contained 0. 52 percent uranium. An X-ray powder pattern of the sooty 

black material, ignited, matched the standard patterns 

of quartz and hematite. A spectroscopic analysis of the 

X-ray spindle showed major silicon, minor iron, and 

traces of calcium, magnesium, sodium, potassium, 

and manganese. The second sample submitted was a 

soft yellow mineral spatially associated with the sooty 

black mineral of the first sample. It contained 0.015 percent uranium, and the X-ray powder pattern matched 

the standard patterns of quartz, possibly'gypsum, and 

possibly clay. Spectroscopic analysis showed major 

aluminum, iron, and silicon-, minor calcium and potassium; and a trace of magnesium and manganese. 

Although no uranium minerals were identified, the 

amount of uranium in the samples is significant. The 

dump material has been exposed to weathering for at 

least 10 years and possibly for 35 years or more; 

therefore, the uranium minerals originally present in 

the ore were probably destroyed by weathering with 

the removal by surface waters of some of the uranium. 

Because the radioactivity is localized in a few small 

areas in the dump, there is a strong possibility that 

the uranium minerals came either from a single vein 

that was not extensively mined or from scattered pods 

in one or more veins that were extensively mined.

Gray Eagle mine

The Gray Eagle mine, in sec. 35, T. 7 N., 

R. 5 W., is about ! § miles west of the Comet 

mine along the Comet-Gray Eagle shear zone. It 

was last operated in 1937 but is now inaccessible. 

It has been describe! by Knopf (1913, p. 121) and
Pardee and Schrader (1933, p. 287-289). Sulfide 

minerals identified from dump samples are galena, 

sphalerite with exsolved chalcopyrite, pyrite, chalcopyrite, and arsenopyrite. Tetrahedrite is reported, 

but none was identified in the present survey. The 

value of gold, silver, lead, zinc, and copper produced 

is estimated to be about 1 million dollars.

Radioactivity of dump samples was first reported 

by geologists of the Atomic Energy Commission on 

June 4, 1951 (Croyell, 1951). Subsequently the part 

of the dump that had appreciable radioactivity was 

shipped to the East Helena smelter for the silver content, and only slight radioactivity was detected during 

the present survey. However, Mr. Giulio, who shipped 

this ore, collected several strongly radioactive samples 

from this part of the dump. One of these, a piece of 

weathered vein material that contained pyrite, a sooty 

black mineral, and a spatially associated soft yellow 

mineral, was submitted to the Geological Survey laboratory for chemical analysis and mineralogic study. 

The sample assayed 2. 2 percent uranium. An X-ray 

powder picture of the sooty black mineral showed a 

very poor, possibly cubic pattern. An X-ray powder 

pattern of the same mineral, ignited, matched the 

UaOs standard pattern, which indicates the mineral is 

possibly metamict pitchblende. Spectroscopic analysis 

of an impure sample of the yellow mineral showed 

major iron and silicon, and minor uranium, vanadium, 

calcium, magnesium, aluminum, and manganese, and 

a trace of zirconium. This mineral is unquestionably 

a secondary uranium mineral but could not be specifically identified.

A man who had worked in the mine reported that 

the part of the dump from which these samples were 

collected came from a subsidiary vein about 100 feet 

south of the main productive vein on the 600-level, 

which is 200 feet below the main adit level. Mine maps 

show that this subsidiary vein was followed for only 

about 300 feet, which may account for the localization 

of radioactive material in one small part of the dump.

Additional strongly radioactive samples were 

found by Mr. Frank Rowe, part owner of the mine, in 

May 1952 in excavations on the dump. A sample tested 

in the Geological Survey office in Spokane contained 

about 2 percent equivalent uranium. A second sample 

containing a black sooty mineral intimately intermixed 

with pyrite was separated from gray crystalline quartz 

and was found to contain about 40 percent equivalent 

uranium. This mineral was identified as uraninite by 

the Geochemistry and Petrology Branch of the Geological Survey.

Although there is no basis for any estimate of 

reserves of uranium ore, the high uranium content 

and the presence of uraninite in the samples indicate 

that the deposit should be further investigated. In 

May 1952 the owners reopened the mine in order to 

obtain more information on the distribution and grade 

of uranium in the mine.

Silver Hill mine

The Silver Hill mine adjoins the Comet mine on 

the west. It is developed by a shaft 165 feet deep from 

which several levels have been driven. The mine is 

inaccessible, but available mine maps indicate that

the north crosscut on the 65-level intersects extensions of the three main veins that were mined in the 

Comet; where the veins were intersected, they contained no significant sulfide mineralization.

Moderate radioactivity was detected on the dump. 

The radioactive material, which occurs along joints 

and fractures in the vein material, could not be identified.

Hope-Bullion

The Hope-Bullion prospect, which adjoins the 

Comet mine on the east, is approximately at the 

junction of the main Comet-Gray Eagle shear zone and 

two northeasterly splits of this shear zone (pi. 1). A 

few bulldozer trenches on this property expose intensely altered quartz monzonite that contains sparse 

cerussite, galena, pyrite, and sphalerite.

Geologists of the Atomic Energy Commission 

reported weak radioactivity in the trenches, but none 

was detected in the present survey. The owner is 

sinking a 100-foot shaft and will drift 200 feet on the 

vein.

Boulder Chief mine

The Boulder Chief mine, in sec. 27, T. 7 N., 

R. 5 W., is developed by a shaft and an adit, both of 

which are now inaccessible. These workings apparently explore a small east-trending vein. A small 

amount of lead, silver, and copper was produced between 1913 and 1917. The shaft is collared in 

prebatholith volcanic rocks, but most of the rock 

on the dump is intensely altered alaskite or quartz 

monzonite. Sulfide minerals identified from dump 

samples are galena, sphalerite, pyrite, and arsenopyrite. No radioactivity was detected on the large 

dump from the shaft; several small pits upslope east 

of the shaft showed weak radioactivity. A sample of 

the most radioactive material that could be found consisted mostly of brown limonite and contained0.016percent equivalent uranium and 0.010 percent uranium.

Younger deposits 

Free Enterprise mine

The area around the Free Enterprise mine was 

mapped by Roberts and Gude (1952) in 1950, and no 

additional work was done in the present study. At the 

Free Enterprise mine, uranium has been produced 

from a narrow chalcedonic vein that contains disseminated secondary uranium minerals and local small pods 

of pitchblende and also from disseminated secondary 

uranium minerals in the adjacent quartz monzonite. 

Associated with the uranium are very small amounts ' 

of pyrite, galena, ruby silver, argentite, native silver, 

molybdenite, chalcopyrite, arsenopyrite, andbarite.

High Ore (Montana Consolidated) mine

The High Ore mine, patented as the Montana 

Consolidated claim, is in sec. 2, T. 6 N., R. 5 W. 

No record of production is available. The mine consists
of an 1, 800-foot crosscut with a 200-foot drift, two 

stopes, and a winze on a vein near the face of the drift. 

The crosscut intersects several siliceous veins, some 

of which contain sparse pyrite, galena, and chalcopyrite. 

The veins consist of silicified quartz monzonite with 

stringers of white to bluish-gray quartz commonly a 

few inches wide. About 200 feet from the1 face,, the 

crosscut intersects a vein, at least 10 feet wide, of 

dark-blue quartz with white quartz stringers containing 

sparse pyrite and galena. The vein, which strikes 

approximately N. 60° E. and dips 65° to 70 SE., is 

probably the downward extension of the large siliceous 

structure on the top of the ridge west of the mine portal. 

The drift follows the vein about 200 feet to the southwest. On the surface the vein is intensely silicified 

fine-grained quartz monzonite cut by many light-gray 

chalcedonic stringers from 0. 1 inch to 12 inches wide; 

some of the stringers contain a little milky quartz, 

and a few are cut by small brown-weathering carbonate 

stringers. The rock in the vein zone is intensely 

brecciated and faulted; most of the fragments are 

blocky and less than a foot in largest dimension.

Anomalous radioactivity was detected in the mine 

during the present survey in October 1951. No abnormal radioactivity was found on the dump or along 

the outcrop of the vein. Radiometric traversing of the 

tunnel, however, disclosed a gradual increase in radioactivity from normal background at the portal to 10 times 

normal background a few tens of feet from the face; beyond this point the increase is more rapid; at the

face the reading is 30 times background. This radioactivity is probably due to radon gas, but the source 

of the gas could not be determined nor could any 

samples of radioactive rock be found.

LITERATURE CITED

Billingsley, Paul, 1915, The Boulder batholith of Montana: 

Am. Inst. Min. Eng. Trans., vol. 51, p. 31-56.

Billingsley, Paul, and Grimes, J. A., 1918, Ore 

deposits of the Boulder batholith of Montana: Am. 

Inst. Min. Eng. Trans., vol. 58, pp. 284-368.

Knopf, Adolph, 1913, Ore deposits of the Helena mining 

region, Montana: U. S. Geol. Survey Bull. 527.

Pardee, J. F., and Schrader, F. C., 1933, Metalliferous deposits of the greater Helena mining region, 

Montana: U. S. Geol. Survey Bull. 842.

Roberts, W. A., and Gude, A. J., 3d, 1952, Geology 

of the area adjacent to the Free Enterprise uraniumsilver mine, Boulder district, Jefferson County, Mont.: 

U. S. Geol. Survey, Trace Elements Memo. Rept. 140, 

U. S. Geol. Survey Bull. 988-F.

UNPUBLISHED REPORT

Coryell, K. C., 1951, Gray Eagle mine, Basin district, 

Jefferson County, Mont.: Prelim. Reconnaissance 

Rept. M-81. [ In files of U. S. Atomic Energy 

Comm., Denver, Colo. ]

INT.-DUP. SEC., WASH., .0. M5208
GEOLOGICAL SURVEY

IIZMS*

CIRCULAR 277 PLATE 1

If SO" itsw. M.4W. OT'10"

SECTION ALONG LINE A-A1

 \\ 5 6000L

fM

SECTION ALONG LINE B-ff

nt and Pleistocene (?) alluvium, fan deposits, and, near the Boulder

River, terrace gravel

bacite and Andesite

Dominantly intrusive, light to dark dacite and andesite. Texture is

aphanitic or porphyritic with abundant, small phenocrysts of biotite

and plagioclase. Quartz phenocrysts are locally present. Dacite

and andesite of Sugarloaf Mountain are probably partly extrusive

Alaskite and Alaskite dikes, showing dip

Undifferentiated alaskite, aplite, alaskite porphyry, and pegmatite;

alaskite andaplite predominate Gradations between two or more types

may be present in a single outcrop. Occurs as dikes and irregular

masses; many small dikes have been omitted or grouped as a unit

Dominantly medium-grained biotite hornblende quartz monzonite; locally 

texture is coarse, or fine, grained; fabric is even grained or porphyritic. In«jm,, potash feldspar is relatively abundant and biotite and 

hornblende are relatively sparse In qm,. potash feldspar is relatively 

abundant and accessory pyroxene is common In <jm,, plagioclase and 

ferromagnesian minerals are relatively abundant In ^<"4 , potash feldspar is relatively abundant; texture is fine grained. It is gradational 

into 91 and may be a local contact phase between v» and Kv. Other 

quartz monzonites which cannot be differentiated megasropirally from 

or which are found in areas too small to be shown at this scale are 

included in <j«i

Prebatholith volcanic rocks 

Light -to dark - gray aphanitic andesitic rocks, not mapped in detail

Contact, showing dip, dashed where approximately located 

Contact, indefinitely located

Low intensity anomaly

Concealed contact

Moderate intensity anomaly

Fault showing dip. dashed where approximately located

 fas" ~~

High intensity anomaly

M indicates radioactive vein 

in mine workings

D indicates sample from dump

R indicates radon gas in mine 

workings

U indicates reported radioactivity »,

not detected in this survey

Absence of letter indicates 

radioactivity in outcrop

Vein (includes sulfide-bearing and barren siliceous types), showing

dip (X.'smeasured dip, -<eo approximate dip), dashed where

approximately located.

Altered rock, showing intensity of alteration

Preliminary topography by Fairchild Aerial Surveys, Inc., 

for U. S. Bureau of Reclamation, 1947-48 Geology mapped in 1950-51, by.G.E. Becraft, D. Y. Meschter, E.B. Grass, and *W.A. Roberts

Adit

Caved adit

Shaft

Caved shaft

Mill tailings

Areas, at the Comet and Gray Eagle mines, of

caved shallow workings, slopes which extend

to the surfsce, dumps, and trenches.

Liat of mines identified by numbers

1. Comet

2. Gray Eagle

3. Silver Hill

4. Baltimore

5. Free Enterprise

6. Boulder Mercury

7. Australian

8. Bismark

9. Cleveland

10. Hope-Bullion

1 1 . Morning Glory

12. High Ore (MonUna Consolidated)

13. East Mint

14. Boulder Chief

1 5. Ella

Map of part of the 300-level, shading indicates major veins

Longitudinal section along north vein

Representative level map, longitudinal section, and 

cross section of the Comet mine adapted from 

company maps mcde by Blair W. Stewart, 1935-36

300 600 f..i

Cross section through shaft showing 

north, middle, and south vein ond south 

split of south vein. Below SOOHevel, 

middle and south veins, xind south split 

are projected from west ta plane of 

section. Stopes not shown/

APPROXIMATE MEAN 

DECLINATION, 1950

2000 

I

6000 FMft 

_1

Contour interval 20 feet 

Dttum is approximate IHMUI a«a /*v«/

GEOLOGIC MAP OF THE COMET AREA, JEFFERSON COUNTY, MONTANA, SHOWING RADIOACTIVITY ANOMALIES