Geology and Mineral 

Deposits, East Flank of the 

Elkhorn Mountains, 

Broadwater County, 

Montana 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 665 
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Geology and Mineral 

Deposits, East Flank of the 

Elkhorn Mountains, 

Broadwater County, 

Montana 

By M. R. KLEPPER, E. T. RUPPEL, V. L. FREEMAN, 

and R. A. WEEKS 

GEOLOGICAL SURVEY PROFESSIONA~·L PAPER 665 

A descriptive report of the geology, mineral 

deposits, and mines in an area east of the 

Boulder batholith 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1971 

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UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

W. A. Radlinski, Acting Director 

) ., " 

Library of Con~ess catalog card No. 70-611921 

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

Washington, D.C. 20402 _ 

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CONTENTS 

Abstract __________________________________________ _ 

Introduction ______________________________________ _ 

Location and access ____________________________ _ 

Fieldwork and acknowledgments _________________ _ 

Previous work _________________________________ _ 

Sedimentary rocks _________________________________ _ 

Paleozoic rocks ________________________________ _ 

Park Shale ________________________________ _ 

Pilgrim Dolomite __________________________ _ 

Red Lion and Maywood Formations---------~ 

Jefferson Dolomite _______________________ ~ __ 

Three Forks Shale _________________________ _ 

Madison Group ___________________________ _ 

Big Snowy Group and Amsden Formation ____ _ 

Quadrant Formation __ ----------------- ____ _ 

Phosphoria Formation and related strata _____ _ Mesozoic rocks ________________________________ _ 

Swift Formation ___________________________ _ 

Morrison Formation _______________________ _ 

Kootenai Formation _______________________ _ 

Colorado Formation_-_-- __________________ _ 

Lower black shale unit _________________ _ 

Middle siliceous mudstone and sandstone unit ________________________________ _ 

Upper black shale unit _________________ _ 

Slim Sam Formation _______________________ _ 

Tertiary rocks _______ ------- ___________________ _ 

Oligocene sedimentary tuff __________________ _ 

Sedimentary tuff of uncertain age ____________ _ 

Oligocene tufa ________________ ---- _________ _ 

Unconsolidated Tertiary and Quaternary deposits __ ~ 

Fan deposits and mantle ____________________ _ 

Gravel deposits of uncertain age _____________ _ 

Quaternary deposits __________ ------- ___________ _ 

Glaciation and glacial deposits ______________ _ 

DepositEt resulting from mass wasting _________ _ Alluvium _________________________________ _ 

Igneous rocks _____________________________________ _ 

Elkhorn Mountains Volcanics ___________________ _ 

Intrusive rocks related to the Elkhorn Mountains 

Volcanics------------------------------------

Younger intrusive rocks ________________________ _ Mafic rocks _______________________________ _ 

Intermediate and felsic rocks ________________ _ 

Granodiorite and related rocks __________ _ 

Porphyritic quartz monzonite ___________ _ 

Quartz monzonite porphyry and related rocks _______________________________ _ 

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Igneous rocks-C_ontinued 

Composite intrusive bodies_______________________ 27 

Thermal metamorphism_________________________ 29 

Quartz latite porphyry dikes of Eocene(?) age______ 29 

Basalt flows____________________________________ 29 

Rhyolite flow__________________________________ 30 

Structural geology__________________________________ 30 

Folds__________________________________________ 31 

Faults_________________________________________ 31 

Faults associated with folding________________ 31 

Northwest- and northeast-trending faults______ 32 

Range-front faults-------------------------- 32 

Other faults ____ :____________________________ 32 

Summary of. Late Cretaceous and Tertiary tectonics_ 33 

Mineral deposits____ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 34 

History and production___________________________ 34 

Types of metallic mineral deposits________________ 36 

Oxidation and enrichment_______________________ 39 

Description of deposits__________________________ 39 

Iron Age Gulch and Weasel Creek areas (Winston and Beaver Creeks)___________________ 39 

Parkarea__________________________________ 42 

Hassel area________________________________ 43 

Radersburg area (Cedar Plains) _ _ _ _ _ _ _ _ _ _ _ _ _ _ 44 

Quartzite Ridge area________________________ 46 

Other areas and deposits_____________________ 48 

Nonmetallic mineral deposits_________________ 48 

Possible future discovery and development of mineral deposits_________________________________ 48 

Measured sections__________________________________ 49 

1. Morrison, Swift, Phosphoria (and related strata), 

Quadrant, and Amsden Formations near Indian Creek________________________________ 49 

2. Morrison and Swift Formations near Montana 

Silver Star mine____________________________ 52 

3. Kootenai Formation and parts of the Colorado and 

Morrison Formations_______________________ 53 

4. Colorado Formation near south end of mapped 

area______________________________________ 54 

5. Colorado Formation along and near the Indian 

Creekroad________________________________ 56 

6. Slim Sam Formation (part) and Colorado Formation near Iron Mask mine___________________ 58 

7. Lower part of middle member of the Elkhorn 

Mountains Volcanics________________________ 60 

References cited____________________________________ 62 

Index___________________________________________ 65 

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CONTENTS 

ILLUSTRATIONS 

[Plates are in pocket] 

Geologic map and structure sections of the east flank of the Elkhorn Mountains. 

Geologic maps of the Little Olga lower adit (Kleinschmidt mine), Edna No.2 adit, Silver Saddle adit, and January 

mine, Winston mining district. 

Geologic map of parts of the Marietta, Gold Dust, Switzerland, and Sitting Bull mines, Park mining district. 

Geologic maps of the Diamond Hill mine area, Blacksmith mine, and Little Giant mine, Hassel mining district, 

and Santa Anita and Ruby mines, Quartzite Ridge mining district. 

Index map showing area of this report and of recent reports on adjacent areas ______________________________ _ 

Diagrams showing nomenclature of coarser grained igneous rocks _________________________________________ _ 

Triangular diagrams showing modes of intrusive rocks, east flank of Elkhorn Mountains----------------------

Sketch map· of Clancy, Townsend, Devils Fence, and Radersburg quadrangles, showing principal mines and mining . 

areas-------------------------------------------------------------------------------------------

Map showing distribution of metallic mineral deposits along the east flank of the Elkhorn Mountains __ ----------- Geologic map of the Iron Cross magnetite mine _______________________ --: __ -------------------- _________ _ 

TABLES 

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TABLE 1. Chemical analyses, norms, and modes of igneous rocks--------------------------------------------------- 18 

2. Summary of mine production, east flank of Elkhorn Mountains-------------------------------------------- 35 

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GEOLOGY AND MINERAL DEPOSITS, EAST FLANK OF THE 

ELKHORN MOUNTAINS, BROADWATER;COUNTY, MONTANA 

By M. R. KLEPPER, E. T. RUPPEL, V. L. FREEMAN, and R. A. WEEKs 

ABSTRACT 

The Elkhorn Mountains are an oval-shaped mountain mass 

in west-central Montana that is located between the city of 

Helena and the valley of the Jefferson River. This report describes the geology and mineral deposits of the east flank of 

this mountain mass, a strip about 6 miles wide and 26 miles 

long. 

The stratified rocks include a thick and, for the most part, 

conformable sequence of marine and nonmarine sedimentary 

rocks, which are overlain by a volcanic pile of Late Cretaceous 

age and by consolidated and unconsolidated sediments of Cenozoic age. The prevolcanic sedimentary rocks range in age from 

Middle Cambrian to Late Cretaceous and include a sequence 

about 4,000 feet thick of marine limestone, dolomite, and subordinate clastic rocks of Paleozoic age and a sequence almost 

as thick of marine and nonmarine rocks of Mesozoic age that 

are dominantly shale, mudstone, siltstone, and fine-grained sandstone, but that also includes limestone and some coarser grained 

cla·stic rocks. The youngest of these sedimentary rock units is 

the Slim Sam Formation, a sequence that is mainly marine 

sandstone in the lower part and nonmarine tuff in the upper 

part.· 

The Slim Sam is gradationally overlain by the Elkh?rn 

Mountains Volcanics of Late Cretaceous age, which in this 

area comprises mainly rhyodacitic and trachyandesitic pyroclastic rocks of the lower member of the fO'rmation and rhyolitic welded tuff, trachyandesitic and rhyodactic tuff, and tuff 

breccia of the lower part of the middle member ; the upper 

part of the middle member and the upper member of the Elkhorn 

Mountains Volcanics have been eroded. 

The sedimentary and volcanic rocks are cut by many bodies 

of intrusive igneous rocks, including fine-grained rocks, mainly 

porphyries, related to and of the same age as the Elkhorn 

Mountains Volcanics that were emplaced before or at the outset 

of folding, and by younger generally coarser grained intrusive 

rocks that were emplaced subsequent to volcanism and folding. 

The intrusive equivalents of the Elkhorn Mountains Volcanics 

comprise porphyritic rhyodacite, trachyandesite, and trachyandesite in sills, dikes, and small partly concordant bodies ranging to syenodiorite porphyry and granodiorite porphyry in larger 

partly concordant bodies; the largest of these, the Rattlesnake 

intrusive, c.rops out over an area of 20 square miles and is imperfectly laccolithic in form. 

The younger intrusive rocks form small, commonly discordant stocks or bodies of unknown configuration, and they range 

in composition from gabbro to granite. These rocks span about 

the same compositional range and were emplaced during approximately the same time interval as .the Boulder batholith 

to the west. .A: few small stocks and dikes consist wholly of 

mafic rocks ranging from syenogabbro to calcic syenite ; many 

stocklike b:XUes, from a few acres to a few square miles in 

outcrop area, consist of felsic rocks ranging in composition 

from syenodiorite to granite ; in the central part of the area 

a few small intrusive bodies are composite and consist of 

one or more mafic rock types and one or more felsic types. 

Where age relations can be deter~ined the mafic rocks are 

older than the felsic rocks. For descriptive purposes the felsic 

rocks can be subdivided into three petrographic groups : ( 1) 

medium-ln'ained equigranular roc~s, mainly granodiorite but b • 

ranging from syenodiorite and monzonite to quartz monzomte, 

(2) porphyritic quartz monzonite characterized by conspicuous K-feldspar megacrysts, and (3) porphyries ranging from 

granodiorite ·to granite in composition. Age relations of rocks 

within the felsic group could nowhere be determined. Mediumgrained equigranular rocks are widespread and much more 

abundant than the other petrographic types which are restricted to small bodies in the north half of the area. 

The structure of the mapped area i!s simple in gross aspeet 

but is complex in detail. In late Cretaceous and early Tertiary 

time the layered rocks were folded into a broad syncline, 

which trends north through the central part of the ~rea, and 

is broken by many faults of simHar trend that formed contemporaneously with the folding ; the syncline is one of a series 

of north-trending folds between the Boulder batholith to the 

west and the Lombard thrust to the east. Later, the rocks 

were broken by northeast- and northwest-trending tear faults. 

The youngest faults ·are range-front faults that bound the 

momitain front in the north part of the area ; movement along 

the range-f:ront faults began before deposition of Cenozoic 

sediments and has been recurrent almost to 1the present time. 

The east flank of the Elkhorn Mountains has been a significant 

tSource of metallic minerals and has yielded metals valued at 

about $17 million. Gold mined from lode deposits in the Radersburg, Winston, Park, and Hassel districts and from placer 

deposits at Radersburg, along Indian Creek, and along Weasel 

Creek accounts for about three-fourths of the total value. 

Silver, lead, zinc, and copper account for most of the remainder. 

The metallic mineral deposits comprise auriferous pyrite veins 

that contain sparse quartz, quartz veins that contain auriferous 

pyrite, base- and precious-metal veins that typically have quartz 

gangue, silver-lead-zinc replacement deposits in carbonate rocks, 

skarn deposits at or near intrusive igneous contacts, a titaniferous magnetite deposit, and gold placers. Most of the deposits 

in the Radersburg and Hassel districts are auriferous pyrite 

veins; the deposits in the Winston area are quartz veins with 

auriferous pyrite and base- and precious-metal veins. Gold Placers 

were mined at Radersburg, along Indian Creek from Hassel to 

the mouth, and along Weasel Creek. The replacement deposits 

of silver, lead, and zinc are mainly in the Quartzite Ridge area. 

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2 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

The principal skarn deposits are west of Radersburg and at 

Diamond Hill; titaniferous magnetite has been mined west of 

Radersburg. Perhaps as much as half the ore mined from the 

several types of vein deposits has been oxidized or partly oxidized; only a few veins have been profitably mined below the 

zone of oxidation. 

Despite extensive mining and prospecting, the area is considered favorable for discovery of additional mi.peral deposits. 

For example, auriferous py.rite veins or ancient gold ·placers 

might be concealed beneath the relatively thin Cenozoic deposits 

south of the Keating and Ohio Keating mines near Radersburg, 

and the untested outwash gravels along and near Cold Springs 

Gulch would seem to warrant testing for gold. 

INTRODUCTION 

LOCATION AND ACCESS 

The oval-shaped mountain mass west of the Missouri 

River valley between Helena, Mont., and the J e:fferson 

River valley is generally referred to as the Elkhorn 

Mountains. This report describes the geology of the 

east flank of that range -and the adacent part of tJhe 

Townsend Valley, an area of about 150 square miles 

(fig. 1) encompassing the west hal£ of the Townsend 

15-minute quadrangle -and the northwest quarter of the 

Radersburg 15-minute quadrangle. 

U.S. Highway 287 and the N orthem Pacific Railway 

cross the northeast corner of the area. Townsend, the 

seat of Broadwater County, is on the highway and railroad about 6 miles east of the area and 34 miles southeast of Helena. Two villages, Radersburg and Winston, 

are within the area. Both were formerly centers of mining activity but are now inhabited mainly by ranohers. 

The area is uninhabited except for these communities, 

a few I'lanches near them, arrd, from time to time, small 

mining camps in the mountains. The former mining 

community of Hassel (also known as St. Louis) is 

uninhabited. 

Most parts of the area, except the headwaters of 

Beaver Creek, are accessible by mountain roads. A~titudes range from 4,200 feet in the Townsend Valley 

to 8,600 feet ~along ~the divide between Beaver and Crow 

Creeks. 

FIELDWORK AND ACKNOWLEDGMENTS 

The area described in this report is part of a large 

region in and around the BoUlder batholith that has 

been mapped by the authors and their associates. Most 

of the fieldwork on which the report is based was done 

between 1949 ·and 1953. The geology of the area was 

plotted directly on preliminary topographic maps prepared at a scale of 1 : 24,000 by Fairchild Aerial Surveys, Inc., except for a strip about 2 miles wi'de along 

the west edge of the Radersburg quadrangle, within 

which the geology was plotted on aeriaJl photographs 

in the field and subsequently transferred to the topo112° 

0 5 10 15 20 25 MILES I " I I I I I I I 

FIGURE 1.-Ind't:'.X map showing location of present 

report area and of adjacent areas described in recent 

reports. 

graphic base map by use of a Mahan Plotter. Principal ·responsibility for mapping different parts of the 

area is shown on plate 1. E. J. Lyons, H. W. Smedes, 

D. E~ Brambilla, D. M. Pinckney, H. B. Nickelson, A. 

W. Berg, and R. F. Gosman assisted in the fieldwork 

at various times. 

The authors are indebted to many individuaJls and 

mining companies for information and maps of mining properties. Particularly helpful were Albert Dance, 

Harry Anders, Palmer Engh, and Fred Sclme:i!der of 

Townsend, Arthur Berg of Radersburg, Arthur and 

Harold Hogan and Mrs. Charles Muffley of Winston, 

the late Carl Trauerman of Butte, Herb Carver of Helena, Harry Kleinschmidt of Lincoln, Al Nugent of 

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1SEDIMENTARY ROCKS 3 

Wallace, Idaho, Edmund E. Pohl of Seattle, Wash., 

and officials of the Anaconda Co. The authol'ls also ru-e 

indebted to W. A. Cobban and the late J. B. Reeside, 

Jr., both of the U.S. Geological Survey, for material 

assistance in interpretation and correlation of the · 

Cretaceous sedimentary rocks. 

PREVIOUS WORK 

Several1previous geo'logic studies and investigations 

of minel'lal deposi~ts included i)arts of the area. Stone 

(1911) e~amined the minel'lal deposits ,and mapped the 

geology of partt of the area in reconnaissance. Bard 

(1910) and Winchell (1914) :briefly described !the geology ~and some of the ore deposits of the Radersburg district, Condit, Finch, and P~ardee ( 1928,. p. 177-178) repo!1ted on occurrence of phospib.aJte rock in the Elkhorn 

Mountains, ~and Corry ( 1933) described some of the 

gold placer deposits in and adjacent to the mountains. 

Reed (1951) summarized available information, including :production data for mines in Broadwater CounJty. 

Reports iby l(nopf (1913), Billingsley (1916), Billingsley ~and Grimes (1918), and Pardee and Schmder 

(1933), provide va'luruble inform~ation on the regional 

st:vatigraphy, structure, peJtrology, and ore deposits. The 

following geologic maps ~and reports have ibeen pu1blished recently of the ~areas that surround the present 

mapped area: The Canyon Ferry .quadrangle Ito the 

north mapped by Mertie, Fischer, and Hobbs (1951), 

the Elkih.orn MountJains to the west mapped by Klepper, 

Weeks, and Ruppel (1957), Smedes (1966), and Kno.pf 

(1963), the Three Forks quad!'langle to the south 

mapped lby Robinson (1963), ~and the Townsend VraHey 

to the east mapped by Freeman, Ruppel, ~and Klepper 

(1958), Lorenz and McMurtrey (1956), and Nelson 

( 1963) . The ore deposits of the nor.th pant of the Pa:rk 

mining district have recently Jbeen discussed by Schell 

( 1963) , and those of the Winston mining district, by 

Earll (1964). GI'Iavity ~and aeromagnetic daJta have been 

published by ICinoshita, Davis, Smedes, ~and Nelson 

(1964) ~and Kinoshita, Davis, and Robinson (1965). 

SEDIMENTARY ROCKS 

A sequence of consolidated sedimentary rocks rubout 

13,000 feet thick and ranging in age from late Precamibri,an Ito Cretaceous is exposed in ~and near the Elkhorn 

Mountains. These rocks underlie a volcanic pile of La;te 

Cretaceous age that in pl~aces was ~art 'least 10,000 feeJt 

and perhaps as much as 15,000 feet thick. The Cretaceous and older rocks are strongly folded and faulted 

~and are cut by the Boulder ibath:olivh and related intrusive rocks of Late Cretaceous age. The m:ajor valleys 

that flank the mountains .are filled with ash-rich sediments of Tertiary 'age, most of which are only wea:kly 

consolidated. 

Most of the sedimentary and volcanic units 'are nvther 

uniform throughout :the Elkhorn Mountains ~and nearlby. They !have ibeen fully described by Klepper, Weeks, 

~and Rup~l ( 1957), Freeman, Ruppel, and Klepper 

(1958), Roibinson ;(1963), Nelson (1963), and Smedes 

(1966), and >accordingly most of them are treated only 

summ,arily in this report. New information, such as 

measured sections and lists of fossils, is included, however. 

PALEOZOIC ROCKS 

P~aleozoic sedime:rutary rocks are exposed in two 

north-trending ibelts, one about a mile wide along the 

· southwest margin of !the area, ~and the other !aibout half 

~a mile wide along lthe east margin, norlth of Crow Creek. 

Rocks in the western outcrop belt ~are on the east flank 

of a broa:d north-trending dome (Klepper ~and otJhers, 

1957). Those in rthe eastern outcrop belt ~are pant of !the 

east flank of the ibDoad syncline north of 'Crow 'Creek 

(Freeman and others, 1958). 

PARK SHALE 

The oldest formation exposed is the Park Shale of 

Middle Cambrian age, which crops out along Johnny 

Gulch at the west margin of the area. The formation 

consists mainly of olive-green, gray, and brown shale 

but contains a few thin beds of argillaceous limestone, 

siltstone, and silty sandstone. The formation is 215 

feet thick on the north slope of Johnny Gulch (Klepper and others, 1957, p. 10). 

PILGRIM DOLOMITE 

The Pilgrim Dolomite overlies the Park Shale with 

apparent conformity. It is 482 feet thick along the 

north slope o.f Johnny Gulch in SE* sec. 17, T. 5 N., 

R. 1 W. (Klepper and others, 1957, p. 11). The lower 

part of the formation; about 50 feet thick, is oolitic and 

pebbly bluish-gray limestone overlain by dolomite that 

weathers mottled light and dark gray. The middle part 

of the formation, about 200 feet thick, is mainly gray 

limestone with mottles ·and ribbons of yellowish-gray 

silty dolomite; it closely resembles the mottled limestone of the lower and upper parts of the Cambrian 

Meagher Limestone, not exposed in this area. The upper 

part, about 230 feet thick, is medium-bedded, partly 

oolitic, mottled light- and dark-gray dolomite overlain 

by very :resistant, rough-weathering, thick-bedded lightgray dolomite. 

The Pilgrim Dolomite is considered to be Late Cambrian. Fossils from 10 feet above the base, at the south 

end of the Limestone Hills, about a mile east of the 

mapped area (250ft northwest of an abandoned quarry 
4 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

in NE% sec. 4, T. 5 N., R. 1 E.), are· typical of the 

lower Oedaria subzone (A. R. Pahner, 'vritten commun., 

1951) ; the collection contains: Oedarina cordillerae 

(Howell and Duncan) , B olaspidella wellvillensw 

(Lochman and Duncan), Nixonello cf. N. montanemis 

Lochman. 

Fossils from the middle part of the formation about 

41h miles south west of the area are characteristic of the 

middle Oedaria subzone (Klepper and others, 1957, 

p. 11). 

RED LION AND MAYWOOD FORMATIONS 

Varicolored argillaceous rocks and impure carbonate 

rocks above the Pilgrim Dolomite are 134 feet thick 

on the north slope of Johnny Gulch in NE% sec. 17, 

T. 5 N., R. 1 W. (Klepper and others, 1957, p. 13). The 

lower part of the sequence is referred to the Red Lion 

Formation of Late Cambrian age, and the upper part, 

to the Maywood Formation of Middle and Late Devonian age (Klepper :and others, 1957). The Red Lion 

appears to lie conformably on the Pilgrim Dolomite. 

The Maywood is disconformable on the Red Lion; in 

fact, near the south end of the Limestone Hills about 

a mile east of the area, the Red Lion was largely or 

wholly removed by erosion before the Maywood was deposited. Because the two formations are similar, thin, 

and generally poorly exposed, they were mapped as a 

single unit. The unit is wholly or partly equivalent to 

the Dry Creek 'Shale of earlier literature (Deiss, 1936; 

Emmons and Calkins, 1913; Lochman-Balk, 1950; 

Sloss and Laird, 1947; Hanson, 1952, Robinson, 1963). 

Along Johnny Gulch the Red Lion Formation is 93 

feet thick and consists of varicolored siltstone, shale, 

and fine-grained sandstone; most of the rocks are calcareous ·and weather to shades of red and yellowish 

brown. The overlying Maywood Formation is 41 feet 

thick and consists mainly of silty dolomite and dolomitic or calcareous siltstone and shale; the rocks 

weather to shades of gray and yellowish brown. 

JEFFERSON DOLOMITE 

The Jefferson Dolomite of Late Devonian age consists mainly of fetid dolomite that weruthers to a darkgray nodular rock commonly with a brownish cast, but 

the formation also con~ains beds of gray limestone and 

dolomite, which weather lighter shades of gray, and a 

few thin beds of intraformational pebble conglomerate. 

Much of the rock is formed of clastic carbonate grains. 

Most beds are a few inches to a few feet thick, some 'are 

laminated, and some are cross-laminated; small channels are present locally. Sm·alllenses and discontinuous 

thin layers of chert and partings of yellowish-gray 

mudstone are present sparingly in the lower half of the 

formation. Small lenses and discontinuous thin layers 

of dark dolomite breccia, some of which do not conform 

to bedding and probably were formed by collapse, are 

present locally. The uppermost 25-40 feet of the formation is distinctive :faintly mottled light-gray dolomite 

in beds 1-3 feet thick. 

The formation appears to lie conformably on the 

Maywood Form,ation and to be overlain conformably 

by the Three Forks Shale. It is 478~727 feet thick in 

measured sections nearby (Klepper and others, 1957, 

p. 14-15; Freeman and others, 1958, p. 548-549) and 

appears to be 500-600 feet thick in this area. 

THREE FORKS SHALE 

The Three Forks Shale is poorly exposed in several 

narrow strike valleys along and near the west margin of 

the area southward from sec. 8, T. 5 N., R. 1 "'\V. It is 

about 300 feet thick and consists largely of dark-gray, 

greenish-gray, and brown shale. About 200 feet above 

the base, fossiliferous argillaceous gray limestone, 10-

25 feet thick, in places crops out as a low ledge. The 

upper part of ·the formation, about 50 feet thick, is dolomitic and calcareous siltstone and mudstone that 

weathers yellowish brown and yellowish gray. 

The Three Forks Shale is l·argely of Late Devonian 

age. The upper part is probably a correlative of the 

Sappington Sandstone Member as used by Berry (1943) 

of nearby areas and therefore probably contains beds of 

J\1:ississippian age near the top. Recently, Robinson 

(1963, p. 34-38) provided detailed new information and 

summarized the extensive literature on the age of the 

Sappington Member of the Three Forks. 

MADISON GROUP 

The Three Forks Shale is conformably overlain by a 

thick sequence of gray limestone assigned to the Madison Group of Mississippian age. The lower part of the 

sequence, assigned to the Lodgepole Limestone, is about 

700 feet thick and consists largely of thin-bedded limestone. The upper part of the sequence, about 1,000 feet 

of thick-bedded limestone, is assigned to the Mission 

Canyon Limestone. These formations crop out in a 

belt about a mile wide extending southward along the 

west edge of the mapped area from sec. 32, T. 6 N., 

R. 1 W., and in a narrower belt along the east edge of 

the area north of Crow Creek. 

The Lodgepole Limestone consists almost entirely 

of thin and distinctly bedded medium-gray limestone, 

typically with a distinct bluish cast on weathered surfruces. Much of it is lithi:fied carbonate sand. Many beds 

are fossiliferous and have a fetid odor; some consist 

largely of waterworn fossil debris. In the lower part of 

the formation the beds range in thickness from 1 inch 

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'SEDIMENTARY ROCKS 5 

to 1 foot and average 3-4 inches; thin interbeds and 

partings of yellowish- or reddish-gray calcareous shale 

or mudstone rure characteristic. In the upper part of 

the formation, beds 2-3 feet thick are interlayed with 

much thinner beds, mudstone partings are rare, fossiliferous beds are less abundant, and thin siliceous 

seams and chert lenses are sparingly present. 

The Lodgepole rests with abrupt but apparently conformable contact on calcareous or dolomitic siltstone 

of the Three Forks. It grades upward into the Mission 

Canyon through a thickness of 100-200 feet. The contact was placed at the top of the uppermost unit of 

thin-bedded limestone. 

The Mission Canyon Limestone consists largely of 

thick and indistinct beds of medium- to light-gray 

fine- to coarse-grained limestone; a few beds of darker 

gray limestone are present near the top and bottom of 

the formation. The upper half contains sparse nodules, 

lenses, and thin layers of gray chert. Beds containing 

abundant small twiglike bodies of white crystalline calcite are common in the middle part. 

The formation weathers light gray to medium gray 

and is locally mottled light olive gray. Weathered surfaces differ in detail; some rure rough and cuspate and 

others are smooth or only slightly rough. Thin, irregular, siliceous crusts coat some surfaoos and stand 

in slight relief. 

One or more zones of solution breccia approximately 

parallel to bedding are present in the upper part of the 

Mission Canyon at most places. The lowest and most 

conspicuous breccia zone is about 200 feet beJow the top 

of the formation and is 10-75 feet thick. The breccia is 

believed to have formed by collapse of solution caverns 

in the Mission Canyon before Big Snowy rocks were 

deposited (Klepper and others, 1957, p. 20). 

BIG SNOWY GROUP AND AMSDEN FORMATION 

The Mission Canyon Limestone is overlain by 'an assemblage that consists mainly of red siltstone and gray 

car.bonrute rock but includes beds of mudstone, shale, 

and sandstone. Along rthe east edge of the ~area rubout.~a 

mile north of Crow Creek, the· measured thickness of 

these rocks is 259 feet (Freeman 'and others, 1958, p. 

550) and in sec. 32, T. 7 N., R. 1 E., on the slopes northwest of Indian Creek, ]tis 313 feet. In both places, ~and 

generally throughout <bhe ~area, tthe lower .part is very 

poorly exposed but ~appears tto consist ni~ainly of red 

siltstone. The middle parrt, 40-50 feet thick, is mainly 

gray limestone and dolomite. The upper part, also gen~ 

erally poorly exposed, consists of red, reddish-brown, 

and yellowish-gray sHty, ~argillaceous, sandy ~and caribona;te rocks tha;t grade upward into alternating beds 

of light-colored quartzitic sandstone ~and dolomite of 

419-246--71----2 

the Quadrant Formrution. The assemblage is probably 

250-320 fe~ thick in most places, but it may ibe as thin 

1as 200 feet in ·a few a;>laces al,ong rthe western belt of 

outcrop ~and perhaps is as much !as 350 fe~ thick in the 

vicinity of 1ndi,an Creek; ·a;t Indi~an Creek, however, 

the parttern of .rock distribution is peculiar and rthe apparent thickness may be grea;ter than Jthe true thickness 

as a resulrt of rep~tition of beds iby unrecognized minor 

folds and faults of small displacement or of deposition 

on an irregular surface of moderate local relief, perhaps 

'a karst. 

Though rthe conta~t with lthe underlying Mission 

Canyon Limestone was nowhere observed, indirect evidence indicates Jthat it is a disconformity (IClep:per ~and 

others, 1957, p. 19-20). Lenses and l~ayers of limestone 

breccia 'are a;bundant in rthe upper part of Jthe Mission 

Oanyon Limestone. Some of the breccia is cemented or 

pa11tly cemented by red and yellow limy mud. In a few 

pl,aces tthe mud is laminated ~and inclined ·rut the same 

angle as rthe enclosing Hmestone. The observ,rutions indicate !that the ibrecda formed, probrubly by collapse of 

caverns, ~and was cemented 1prior ito regional folding. 

The similarity of some of rthe cave fillings lbo the overlying rocks suggests that near-surface oaves formed and 

coUapsed just before or while the Big .Snowy ~and Amsden rocks were being depos]ted. fn most places the surface on whi<Yh the Big Snowy and Amsden was deposited seems to have been without a ppreciruble relief. The 

peculiar pattern of rock distribution in the vicinity of 

Indian Creek mentioned ~rubove, however, might indicate deposition on an irregular karst surface with loc.al 

relief of 150-75 feet. 

Blake (1959) measured a section near Indian Creek 

(sec. 32, T. 7 N., R. 1 E.) and assigned the rocks at that 

locality to the Big Snowy Group. He correlated the 

lower unit of predominant red siltstone with the Kibbey 

Form,a;tion ~and assigned the overlying rocks to ·a newly 

described Lombard facies of the Big Snowy Group. 

Dutro and Sando (1963), measured the Indian Creek 

section described as the Amsden Formation by Klepper, Weeks, and Ruppel (1957, p. 20) and Freeman, 

Ruppel, and Klepper (1958, p. 498, 550) and found 

that the uppermost beds of the meldial gray limestonedolomite unit contain Oaninia and Spirifer brazenanus, an oocurrence indicruting middle Chester age. 

Beds near the base of the limestone-dolomite unit contained Striatifera brazeriana, an occurrence indicating 

early Chester age. These fossils strongly suggest that 

tfue lower red siltstone and the medial limestone-dolomite units are part of the Big Snowy Group, and that 

the lower red siltstone is a correlative of the Kibbey 

Formation as Blake (1959) suggested. The upper red 

siltstone probably is part of the Amsden Formation. 
6 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

In the Three Forks and ':Doston quadrangles, Robinson (1936, p. 44-49; 1967) 'a;ssigned a lithologically 

varied assemblage of rocks locally as much as 400 feet 

thick to the Big Snowy Formation. The thickest sequence, in the Hossfeldt Hills about 15 miles southeast 

of tJhe south margin of the area of the present report, 

consists of yellowish-gray arrd yellowish-brown siltstone, mudstone, and sandstone, dark shale, and olivegray and dark-gray limestone. No sequence similar to 

this is present in the Elkhorn Mountains. Overlying 

the rocks assigned to the Big Snowy Group are rocks 

similar in lithology and generally comparable in thickness to the sequence in the E1,khorn Mountains that tlhe 

authors refer to the Amsden Formation. Robinson also 

assigned these rocks to the Amsden Formation on both 

lithologic and faunaJI grounds. 

QUADRANT FORMATION 

The Amsden Formation grades upward into the 

Quadrant Formation, which consists of alternating beds 

of ligiht-colQreu quartzitic sandstone and light-gray 

dolomite. Dolomite predominates in the lower part of 

the formation; the number and thickness of beds of 

quartzitic sandstone increase upward, and quartzitic 

sandstone constitutes about 75 percent of the uppermost 

100 feet. Typically, one or two ledge-forming beds of 

massive quartzite as much as 25 feet thick are present 

in the upper 100 feet. Some of the quartzite b~ds are 

conspicuously crossbedded. The formation is 327 feet 

thick in NW% sec. 29, T. 6 N., R. 1 E., about a mile 

north of Crow Creek (Klepper and others, 1957, p. 20-

21) . Outcrops suggest that the thickness is about the 

same throughout the area. 

The Quadrant Formation is more resistant to weathering and erosion than the overlying and underlying 

rocks, and it typically forms ridges. The formation behaved as a competent but brittle unit during deformation, and therefore it formed broad folds and broke 

along numerous fractures. Many smal'l cross-faults displace the Quadrant but die out in overlying and underlying less competent rock, as is well illustrated south of 

Johnny Gulcll. Some of the faults shown on the map 

(pl. 1) an!d other parallel fractures along which displacement was negligible channeled ore-bearing solutions ; along or adjacent to many of these fractures lead 

and silver minerals and gold replaced favorable ddlomite beds in the Quadrant Formation. 

PHOSPHORIA FORMATION AND RELATED STRATA 

The Phosphoria Formation and related strata in the 

Elkhorn Mountains consist mainly of chert and quartzitic sandstone, but in places includes one or two thin 

beds of phosphate rock, dark -colored· shale or mudstone, 

and gray carbonate rock. Throughout the area they are 

poorly exposed and in some places have been mapped 

with the underlying Quadrant Formation. These rocks 

and the Quadrant Formation appear to intergrade over 

a few feet, and alternations of quartzite and dolomite 

give way upward to interbedded quartzite and chert. 

The Phospharia and related strata are about 125 feet 

thick a few miles south of the area (Freeman and 

others, 1958, p. 500) and about 50 feet thick where 

thickness could be measured or closely estimated within 

the mapped area. 

One or two beds of phosphate rock and a few beds 

of quartzite or sandstone containing pellets of phosphatic material crop out locally, but none of the phosphate rock is of minable grade and thickness. Near the 

top of the Phosphoria,· along the east edge of the area; 

a 4-foot-thick quartzite bed contains fragments and 

chips of phosphatic material, and a lower bed, as much 

as 9 inches thick, is phosphate rock. The lower bed is 

probably the source of a sample reported to contain 

25.35 percent tricalcium phosphate (Condit and others, 

1928, 'P· 177-178). 

McKelvey and others (1959, pl. 2) have suggested a 

revised nomenclature for rocks of Phosphoria age in 

the western phosphate field. According to their usage 

the Permian rocks of the area, and of the Elkhorn 

Mountains in general, would be referred to the Shedhorn Sandstone and to the Tosi Chert Member and 

the Retort Phosphatic Shale Member of the Phosphoria 

Formation (Mcl(elvey and others, 1959, pl. 3, South 

Boulder Creek Section and pl. 2, East Madison Section). In the thickest section of phosphatic rocks of 

Permian age in and near the area (Klepper and others, 

1957, p. 22-23), the lower quartzitic part, about 25 feet 

thick, would correlate with the lower member of the 

Shedhorn Sandstone of their usage, the overlying 28 

feet of chert and phosphate rocks represents the Retort 

and Tosi Members of the Phosphoria Formation, and 

the remainder of the formation is the upper member of 

the Shedhorn Sandstone. In thinner sections consisting 

almost entirely of quartzite and chert and devoid of 

phosphate rock, such as the one along Indian Creek, 

correlation with the formal and informal units proposed by McKelvey and others is uncertain. For this 

reason and because these rocks constitute at most a 

single cartographic unit at the scale of the present 

study, the authors chose to ~retain the familiar term 

Phosphoria Formation and related strata. 

The Phosphori·a Formation is overlain disconformably by the Swift Formation of Late Jurassic age. The 

disconformity between the Phosphoria and ~Swift Formations represents all of Triassic time, most of Jurassic 

time, and perhaps ·part of Permian time. Regionally, the 

J 

, 
l 

1SEDIMENTARY ROCKS 7 

Phosphoria Form·ation and its equivalents thin northward and eastward from southwestern Montana. Northward thinning of all units of Phosphoria age and the 

absence of phosphate rock or phosphatic mudstone 

from the middle part of the sequence north of Indian 

Creek indicate thB~t most of the ·thinning is due to nondeposition. Locally, however, part of the formation may 

have been eroded before the Swift Formation was 

deposited. 

MESOZOIC ROCKS 

SWIFT FORMATION 

Th~ Swift Formation is ·an inconspicuous 15- to 30-

foot-thick unit of brown and brownish-yellow friable 

calcareous marine sandstone speckled with grains of 

black chert and limonite. In places, one or more beds 

near the base are crowded with broken and specifically 

unidentifiable shells of Ewmiaroti8, Ostrea, and Oamptonectes (R. W. Imlay, oral commun.). A basal conglomerate as much as 3 feet thick, consisting of pebbles 

and cobbles of chert in a mB~trix of medium- to coarsegrained sandstone, is present at many localities, and 

small pebbles :are present in some higher beds. Imlay 

(oral commun.) examined these rocks in the field and 

referred them to the upper unit of the Swift Form-ation 

of the Ellis Group of Late Jurassic age (Imlay and 

others, 1948, chart). The rela;tively uniform, though 

slight, thickness of the formation in and near the area 

indicates that it was deposited on a nearly flat surface, 

and the disconformity between it and the underlying 

Phosphoria Formation represents a long time of nondeposition but .probably only slight erosion. 

The formation is too thin· to be mapped separately 

and is included with the Morrison Formation on the 

geologic map. 

MORRISON FOMATION 

The Morrison Formation, a continental deposit of 

Late J ur.assic ·age, overlies ttJhe Swift Formation wiJth 

apparent conformity. It consists of varicolored beds 

of shale, mudstone, siltstone, fine-grained sandstone, 

and limestone. At most places the formation is 425....J5·50 

fe~ !thick, but in a few ,pl,aces, perhaps ;as a result of 

deformation, the apparent thickness is as much ·as 7·50 

feet. 

The :formation· consists predominantly of mudstone 

and sHtstone ibult includes many thin beds of finegr·ained sandstone, shale, ·and dense limestone, many of 

which weather yellowish brown or yellowish gray. Red 

shale and mudstone ·are relatively :a!bundant in some 

places, as in the se~tion measured just soU/tJh of the area 

(p. 52-53), and are almost absent elsewhere, as in 

the vicinity of Indian Creek (p. 49-52). The proportions of other rock ty:pes 1also differ from place Ito 

place. In some places, the upper 40--100 feet of !the formation includes dark-gray shale •and one or ·a few thin 

lenticular beds of impure lignite overlain by alternating 

varicolored dominantly ·argillaceous rocks, and saltand-pepper sandstone, speckled with dark chert grains, 

simHar lbo that in !the overlying Kootenai Formation. 

The interbedding 10f sandstone of Koo!tenai aspect with 

argillaceous rock of Morrison aspect suggests that in 

·these places rbhe !two forma;tions intergrade. On the 

other hand, dark-gray shale 1and salt-·and-pepper sandstone were not recognized •and ·are [>ro!bably ,a;bsent in 

other places, which suggests thalt elsewhere there is a 

disconformity at the top of the Morrison Formation 

and !that locally ·as much •as 50 feet of beds of the Morrison was eroded before the Koo:tenai was deposited. The 

latter intterpretaJtion is in !harmony with tha;t of Cobban ·and Reeside ( 1952, cha11t) , who indicated that .rocks 

of earliest Cretaceous age •are probrubly not present in 

Montana. 

KOOTENAI FOMATION 

The Kootenai Formation of Early Cretaceous age, 

a heterogeneous ·assemJblage of continental rocks, crops 

out in two north -trending belts, one near the west edge 

of the southern part of !the ·area and the other near the 

east edge between Crow Creek and Whitehorse Gulch. 

The lower ,part of the formation consists chiefly of 

coarse- to medium-grained speckled quartz-chert (sallt-

·and-pepper) sandstone. ~he middle ~and !thickest part 

consists mainly of red shale and mudsbone or claystone 

·and green or greenish-gray siltstone. The upper part 

is gr·ay limestone interbedded w~th dark shale and siltstone. The formation is 445 fe~ ,thick ~along the north 

slope of Johnny Gulch (Klepper and others, 1957, p. 25) 

~and ·529 feet thick h:alf a mile soulth of Indian Creek (p. 

53-54). The thickness of the formation seems to be 

within this range throughout most of the ~area, but in a 

few places irt may be as great as 700 feeJt, judging from 

outcrop width and attitude of :bedding. 

The /base of the formation is marked iby fairly resistant crossbedded salt-and-pepper sandstone 15-50 

feet thick tha;t contains abundant chert grains and /that 

resembles ibut is generally !thicker tJhan the lenticular 

sandstones in the upper part ·of !the Morrison Formation. Similar sandstone is interlayered with shale, mudstone, and siltstone throughout the :formation, except 

near the to:p, but !the ibeds are generally lenticula-r •and 

thinner, and most •are finer grained ·and contain less 

che11t than those at the base of the formation. 

The salt-and-pepper sandstones are gray, medium to 

coarse grained, and locally conglomeratic and show 

scour markings and cross-lamination. The sand grains 

in different beds and in some single beds are angular 

to rounded and poorly sorted to well sorted. The dark-
8 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

colored grains are chert; most of the light-colored 

grains are quartz. Most sandstone beds contain inter-

_.stitial clay minerals, and some also contain sparse 

chlorite and sericite that appear to replace clay minerals. Small and discontinuous conglomerate lenses contain pebbles and cobbles of quartzite and chert as much 

as 3 inches across. A few lenses also contain pebbles and 

cobbles of limestone. Chert grains form as much as twothirds of some sandstone beds in the basal part of the 

formation but are much less abundant in the middle 

part of the formation and are absent or sparse in the 

upper part of the formation. Accessory minerals are 

leucoxene, iron oxides, tourmaline, zircon, and anatase. 

The grains are cemented by secondary quartz, mostly as 

enlargements of quartz grains, and by calcite. 

The middle part of the formation, generally 350-400 

feet thick, consists of varicolored beds of siltstone, 

argillaceous siltstone/ claystone/ shale, silty shale, and 

fine-grained sandstone. Olive gray to greenish gray, 

grayish red, and grayish purple are the most common 

colors. Most of the beds are calcareous, and some contain 

abundant irregular concretions of limestone. 

The upper part of the formation, generally about 50 

feet thick, is charar.terized by beds of dark-gray limestone that form low ridge$ and weather light gray. Some 

of the limestone beds contain abundant fresh-wwter fossils, principally gastropods, and some thin beds are composed almost entirely of fossil detritus. Dark claystone 

and siltstone are interbedded with the limestone, and in 

places dark-gray or olive argillaceous siltstone overlies 

the uppermost limestone bed. 

T. C. Yen (written commun., 1952) identified the 

following Prosobanchian gastropodE, species that are 

common in the Kootenai Formation elsewhere, in collections from limestone beds near the top of the formation about one-half of a mile south of Indian Creek: 

Lioplacodes cf. L. convewwul!us Yen, Lioplacodes cf. 

L. cretaceus (Stanton), Oharydrobia cf. 0. cretaccea 

Yen, .~ Reesidella sp. undet. 

The lithologies of the Kootenai and Morrison Formations are similar enough that it is difficult to distinguish between them in areas of poor exposure. 

Placement of the contact between the two formations 

is particularly difficult where lenticular salt-and-pepper 

sandstones are present and dark gray shale is absent 

or inconspicuous in the upper part of the Morrison 

Form·ation. In such places the contact was arbitrarily 

mapped at the bottoom of the first thick salt-and-pepper 

sandstone beneath predominantly red and green argillaceous rocks and may deviate by 50-100 feet from a consistent stratigraphic position. In most places, however, 

1 Included in the term "mudstone" in the measured section 

(p. 53-54). 

the lithologic sequence is distinctive and exposures are 

good enough to permit placement of the contact within 

10-20 feet. 

COLORADO FORMATION 

The Colorado Formation consisting of marine and 

nonmarine sandstone, shale, and siliceous mudstone, 

crops out along the east edge of the mapped area from 

Crow Creek to Whitehorse Creek and thence discontinuously northward along the range front to Beaver 

Creek. Another discontinuous belt of outcrop extends 

from near the south west corner of the area to the junction of Crow Creek and Muddy Lake Creek. The contact with the underlying Kootenai Formation is proba:bly a minor disconformity, because in·some places the 

basal sandstone of the Colorado Formation rests on 

limestone whereas in other places as much as 40-50 feet 

of dark argillaceous rocks lies between the limestone and 

the sandstone. 

The formation has been subdivided into three units 

(Klepper and others, 1957, p. 26-28), the lower and 

upper units consisting chiefly of dark shale and the 

middle unit~ chiefly of sandstone and siliceous mudstone. 

The formation is 1,542-1,639 feet thick near the south 

margin of :the area (p. 54-56), 1,140 feet thick ~long 

Indian Creek (p. 56-58), 1,211 feet thick near the 

Iron Mask mine (p. 58-60), rand 858 feet thick about 

4 miles north of the area ( Srriedes, 1968, p. 13). The 

greater pant of the nonthward decrease in thickness 

is due to thinning of the lenticular nonmarine sandstones in the middle ·!lnit of the formation, but the upper 

black shale unit also appears to thin northward from 

about 275 to 66 feet (Smedes, 1966, p. 12). Local inconsistencies in appare:1t thickness, as for example the 

great apparent thickness of the lower black shale unit in 

the vicinity of Slim Sam Gulch, are probably due to 

unrecognized deformation. 

LOWER BLACK SHALE UNIT 

The lower black sha.le unit comprises: (1) quartzitic 

sandstone and siltstone (bottom), (2) dark-gray to 

grayish-black shale and a :few thin interbeds of siltstone, 

and (3) dark-gray earbonaceous argillaceous finegrained sandstone and siltstone. The unit is 328 feet 

thick at the south edge of the area and 240-255 feet thick 

in the vicinity of Indian Creek and the Iron Mask mine. 

Its true thickness is probably within or near these limits 

throughout; local discrepancies are probably due to unrecognized faults or minor folds. The basal quartzitic 

sandstone may rest on an erosion surface, but the relief 

of this surface must ha V£1 been negligible, for the thickness of the sandstone is relatively constant. 

In all well-exposed sections the basal part of the formation is sandstone, a few feet to 20 feet thick, overlain 

, 
l_ 

. , 

1SEDIMENTARY ROCKS 9 

by siltstone and thin interbeds of sandstone and dark and chert that are interlocked by secondary outgrowths 

shale, aggregating 30-45 feet in thickness. Many bed- on quartz grains. 

ding surfaces are irregula.r, are coated by films of dark The basal sandstone and siltstone is probably equivaclay and micaceous minerals, and exhibit ropy markings lent to the Fall River Formation of the typical great 

resembling worm trails. Some beds are laminated and Plains section (Cobban and Reeside, 1952) and to the 

some are cross-laminated. The basal sandstone is fine to Flood Member of the Blackleaf Formation of the Sweetmedium grained and is in beds as much as 4 feet thick. grass Arch section (Cobban and others, 1959). The overIt typically weathers very pale orange or grayish yellow lying black shale yielded fossils determined by Cobban 

and crops out as discontinuous low ribs. The siltstone (written commun., 195~) as "bwoeramus oomanoheanus 

is generally greenish gray or gray, thinly bedded, and and/. bellvuensis, guide fossils to the Skull Creek shale 

argillaceous and carbonaceous. It weathers to blocky or of the Black Hills and the Purgatoire formation of 

sla;bby fragments. The sandy and silty rocks grade up- Colorado." The lower part of the dark sandstone ~above 

ward into dark shale. the shale also yielded an I nooeramus, probably /. bellIn thin section the basal sandstone is a clean ortho- vuensis ,at one locality (p. 56, unit 11 of section in 

quartzite, consisting of very fine to medium-sized well- sec. 4, T. 4 N., R. 1 W., USGS Mesozoic location 23028) 

sorted tightly interlocked grains, of which 70-95 per- and is accordingly also a probable equivalent of the 

cent are quartz, 3-25 percent are chert, and a trace to a . Skull Creek. 

few percent are chlorite, muscovite, clay minerals, tourmaline, zircon, and black iron minerals. Original pore 

space has been filled by quartz, which occurs partly as 

secondary enlargements of quartz grains and partly as 

cement unrelated to grains in the plane of the thin section. Original boundaries of a few rounded to subrounded quartz grains can be detected by lines of inclusions. The chert has largely recrystallized to a mosaic 

of very fine grained quartz. The boundaries of the recrystallized chert grains 'are serrate. 

The basal sandstone and siltstone is overlain by a sequence about 100 feet thick consisting mainly of fissile 

dark-gray to grayish-black shale and silty shale but containing many thin beds of olive and gray siltstone. 

Roughly spheroidal calcareous concretions as much as 

18 inches in diameter are sporadically present; they 

weather to a rusty color. The shale breaks down readily 

to thin chips or flakes that resist decomposition and 

form very little soil. 

Overlying the shale is a sequence consisting mainly 

of dark fine-grained argillaceous sandstone with interbeds of siltstone and shale. This sequence is about 165 

feet thick at the south end of the mapped area and 100-

110 feet thick farther north, between Indian Creek and 

Whitehorse Gulch. The dark color is due to carbonaceous 

and argillaceous material that comprises 15-25 percent 

of most of the rocks. The fine-grained sandstone is made 

up of angular· to subrounded grains of quartz and chert 

in roughly equal amounts, together with lesser amounts 

of muscovite, biotite, clay, and black opaque minerals. 

Locally, some of the sandstone beds are speckled with 

limonite and weather rusty. At a few places north of 

Crow Creek several thin beds of clean sandstone are exposed. These are composed of medium-grained wellsorted and probably originally rounded grains of quartz 

MIDDLE SILICEOUS MUDSTONE AND SANDSTONE UNIT 

The middle siliceous mudstone and sandstone unit of 

the Colorado Formation comprises: ( 1) fine-grained 

sandstone, siltstone, and mudstone (bottom), (2) mudstone and siliceous mudstone, and ( 3) sandstone. The 

unit is largely nonmarine. It ranges in thickness from 

930 feet near the south end of the area to about 600 feet 

in the vicinity of Indian Creek and the Iron Mask Mine . 

Four miles north of the area it is 578 feet thick ( Smedes, 

1966, p. 13). Part of this variation may be due to undetected small faults, but most of it is probably due to 

original depositional differences and perhaps to intraformational erosion, as suggested by lenticular coarsegrained conglomeratic sandstone beds in the upper part. 

The base of the unit has been mapped at the bottom of 

a distinctive flaggy greenish-gray to medium-light-gray 

fine-grained sandstone that crops out throughout the 

area. The top has been mapped at the bottom of a sequence that consists mainly of dark-gray shale. 

Except for a relatively clean basal sandstone, the 

rocks in the lower part of the unit are chiefly thinbedded olive-gray and gray siltstone and fine-grained 

sandstone that consist largely of quartz and chert, in 

proportions that range from 10: 1 to 1: 1, hut also contain a trace to 20 percent of feldspar and 20-30 percent 

interstitial clay. Secondary chlorite and calcite and very 

small amounts of mica, opaque iron minerals, and zircon 

also are present. The relative abundance of fresh feldspar, which was not observed in any of the underlying 

rocks, and the presence of many shardlike grains of 

quartz suggest admixture of material from a contemporaneous but probably distant volcanic source. The thickness of this sequence in which sandstone and siltstone 

predominate ranges from about 400 feet near. the south. 
10 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

end of the mapped area to about 200 feet in the vicinity 

of Indian Creek and the Iron Mask Mine. 

The overlying sequence, which consists of olive and 

dark- to light-gray silieeous mudstone interbedded with 

olive to gray mudstone, siltstone, and subordinate sandstone, ranges in thickness from 200 to 300 feet. The siliceous mudstone, described in detail by Klepper, Weeks, 

and Ruppel (1957, p. 27-28), is characterized by white, 

light-gray, and light-greenish-gray colors on weathered 

surfaces, by blocky, hackly, or splintery fracture, and by 

abnormal hardness. Subrounded :to shardlike silt-sized 

grains of quartz and feldspar are commonly present in a 

cryptogranular matrix that may be either devitrified 

glass (purr .. iceous dust) or a product of the decomposi· 

tion of siliceous ash by sea water with penecontemporaneous deposition of the released silica (Rubey, 1929, 

p. 168-169). The siltstone, sandstone, and nonsiliceous 

mudstone interbedded with the siliceous mudstone are 

thin-bedded olive ,and gray rocks that fracture to splintery or haclcly fragments. These rocks are composed of 

quartz and chert grains and lesser amounts of feldspar, 

mica, opaque minerals, and fine-grained rock fragments. 

Clay minerals and locaUy :abundant calcite are the 

cementing materials. Most grains are angular or shardlike. A large part of the material is probably from a 

contemporaneous but distant volcanic source. 

The upper pa,rt of the unit, 165'-240 feet t!hick, con· 

sists of sandstone and subordinrute siltstone, shale, and 

conglomerate. The sandstone beds are of two types: 

light-gray, medium-grained quartz-chert (salt-and-pepper) sandstone and olive-gray, very fine grained to finegrained thinly laminated sandstone. The salt-and-pep· 

per sandstone commonly contains scattered pebbles of 

chert and locally contains lenticmla.r beds of chertpebble conglomerate. Both types of sandstone are composed mainly of angular to subrounded quartz and 

chert grains and a small amount of interstitial clay 

minerals, secondary chlorite, and accessory plagioclase, 

muscovite, opaque i,ron minerals, zircon, and tourmaline 

and are locaUy cemented by calcite and iron oxide. The 

salt-and-pepper sandstone is similiar to the basal sandstone of the Kootenai Formation, except that it contains grains of fibrous chalcedony. The other sand· 

stones are characterized by small amounts of feldspar 

and by a diflty appea,rance due to interstitial clay minerals, chlorite, and calcite cement. Some of the argillaceous sandstones are characterized by undulose beds, 

ripple marks, 'and fossil-:worm trails. They are probably lagoon deposits. 

The lower and middle parts of the middle siliceous 

mudstone and sandstone unit are correlated with the 

Mowry Shale of the reference sequence for the Western 

Interior (Cobban and Reeside, 1952, chart lOb) on the 

basis of spairse fossils and. lithology, particularly the 

peculiar silic~ous mudstones of probable volcanic origin. 

Rocks 140-145 feet below the top of the measured section along ln. dian Creek (p. 56) conta·in fossils indicative of GreBnhorn or early ·carlile age, and rocks 30-

35 feet :below the top of the un~t (p. 56) contain fossils indicativ-e of middle Carlile age (W. A. Cobban, 

written corrunun., Sept. 15, 1958). The lower part of the 

overlying upper black shale unit in the same section 

contains fossils indicative of latest Carlile and earliest 

Niobrara age (W. A. Cobban, written commun., May 31, 

1951, and S€~pt. 15, 19·58; J. B. Reeside, Jr., written 

commun., Oct. 3, 1952) . 

Lists and age interpretations of fossils collected from 

measured S84)tions are incorporated in pages 56-58. 

Information on fossils collected elsewhere in the area 

are summari:?:ed below : 

A collection (USGS Mesozoic loc. 23833) from the 

middle part of the siliceous mudstone and sandstone 

unit in the CE•nter of sec. 27, T. 5 N., R. 1 W., contains 

Tempskya gr'J,ndis Read and Brown, "a widespread. and 

typical Mowry species" ( J. B. Reeside, Jr., written 

commun., Se:;>t. 24, 1952). 

A collection from poorly exposed rocks that are faulted and cut by igneous intrusions in the SE:l)J,NW:l)J, sec. 

35, T. 6 N., H 1 W., (USGS Mesozoic loc. 23885) contains: 

I noceranvus n. sp. aff. I. fragilis Hall and Meek 

I noceran~us sp. fragments of very large shells 

Lucina aff L. juvenis Stanton 

Oollignoniceras woollgari (Mantell) var. praecox 

Hass 

Another collection nearby, in the center of the NW 

1)J, sec. 35, (USGS Mesozoic loc. 23884) contains : 

Lucina j-Utvenis Stanton 

Pseudomelania ~ sp. 

H aminea ~ sp. 

0 ollignoniceras sp., fragments 

These collections indicate equivalence to the lower 

part of the Carlile Shale ( J. B. Reeside, Jr., written 

commun., Sept. 24, 1952). Accordingly, the rocks are 

probably equivalent to rocks in unit 24 of the Indian 

Creek section (p. 57). 

UPPER BLACK SHALE UNIT 

The upper black shale unit consists of d.ark-gray to 

black fissile shale and, particularly in the lower part, 

thin interbed~; of very fine grained sandstone and siltstone. Sphero:ldal, brown-weathering limestone concretions 8-12 inehes in diamocer are locally abundant in 

the shale, and thin layers rich in feldspar crystals are 

interbedded in the uppermost part. In small isolated ex-

) 

, 
•SEDIMENTARY ROCKS 11 

. posures, it is difficult or impossible to distinguish between the black shales of the lower and upper units. 

Thin beds of very fine grained sandstone and siltstone comprise most of the lower 50 feet of the unit 

and 5-10 percent of the rest. These beds consist of 

tightly packed angular grains of chert and quartz in 

a proportion of about 3 : 1 ; the matrix is comn1only 

argillaceous. Accessory minerals include plagioclase, 

muscovite, biotite, tourmaline, and secondary calcite. 

The unit is about 280 feet thick in the vicinity of 

Indian Creek and the Iron Mask Mine (pl. 1) and may 

be thicker at the south end of the mapped area, though 

the ·apparent ·thickness of 381 feet there (p. 55) may 

be greater than the true thickness due to repetition of 

beds by unrecognized strike faults similar to those 

mapped in sees. 19, 30, and 31, T. 9 N., R. 1 E. Four 

miles north of the area the unit is only 66 feet thick 

(Smedes, 1966, p. 13). 

The upper black shale unit contains a fauna similar 

to that of the lower two faunal zones of the Niobrara 

Formation of the reference sequence for the Western 

Interior (Cobban and Reeside, 1952). Lists and age 

assignments of fossils collected from a well-exposed 

section along the Indian Creek road and from a section 

near the south boundary of the area are included among 

the measured sections (p. 54-58). In the Colorado 

Shale of north-central Montana and in the Cody Shale 

of Wyoming, about four more Niobrara faunal zones 

are known above the zone of Scaphites ventricosis (W. 

A. Cobban, written commun.), the highest zone recognized in the east flank area. 

SLIM SAM FORMATION 

The Colorado Formation grades upward into a sequence that consists mainly of sandstone in the lower 

part and of tuff in the upper part. The name Slim Sam 

Formation was assigned to these rocks by Klepper, 

Weeks, and Ruppel (1957, p. 28-31), and the type locality was designated along the east margin of the 

Slim Sam Basin, in the area of the present report. The 

formation crops out along the eastern front of the Elkhorn Mountains from Staubach Creek southward to 

Crow Creek (pl. 1) and along the west edge of the area 

from Eureka Creek south ward to the Slim Sam Basin. 

Farther south it has been faulted and disrupted by large 

intrusive masses· and is present only discontinuously. 

The lower part of the formation, 250-450 feet thick, 

is chiefly gray, greenish-gray, and yellowish-gray thinbedded to medium-bedded sandstone, largely or wholly 

of nearshore marine origin. Most beds consist mainly 

of quartz and chert grains and contain some argillaceous matrix ; a few beds contain a little feldspar 

and mica. Partings and thin beds of dark shale, somo 

containing layers rich in feldspar crystals, and a few 

beds of volcanic sandstone also are present. The volcanic sandstone contains suhrounded to angular and 

shardlike grains of quartz, crystals of plagioclase, flakes 

of fresh biotite and of an altered mica, and small fragments of microcrystalline volcanic rock ; in some beds 

calcite cement is moderately abundant. Most of the 

material in the volcanic sandstone is probably slightly 

reworked debris of contemporaneous ash falls from a 

nearby source. The subrounded quartz grains are probably nonvolcanic. In general, quartz decreases and plagioclase increases in abundance from the bottom to the 

top of this part of the formation. 

At many places, ·and perhaps generally, one or a few 

·beds of sandstone consisting la.rgely or entirely of detrital titaniferous magnetite are present in a gradational zone between rocks with sparse volcanic material 

below and rocks with abundant volcanic material above. 

The magnetite-rich rocks probably accumulate as placers along bea:C:hes of a receding sea. 

The upper part of the formation, 400-700 feet thick 

in most places but perhaps as much as 1,000 feet thick 

locally, consists mainly of crystal tuff, crystail-lirthic tuff, 

and volcanic sand~tone but contains beds of la.pilli-tuff, 

sandstone, mudstone, and siliceous mudstone. The tuff 

and lapilli-tuff consist of ·angrilar grains of plagioclase 

and small fragments of fine-grained volcanic rock and 

less abundant grains of quartz, mica, hornblende, and 

opaque minerals in a m·atrix of indeterminate microcrystalline material, possibly devitrified glass, that is 

partly altered to chlorite and sercite. The sandstones are 

similar to the same rock types in the lower part of the 

formation. The mudstone is olive and dark gray; most 

beds contain ·rubund'ant sand-sized grains, and many are 

hard and siliceous and break to hacky fragments. In 

general, bedding is less distinct and ibeds are considerably thicker in the upper part of tJhe formation th1

an 

in the lower part. Some beds are cross-la;minated. 

The few :£ossils collected from the lower p~rt of the 

formation indicate that it is of Colorado age and mainly 

if not entirely of marine origin. Scaphites· cf. S. ventricosus Meek and Hayden, common in the Niobrara Formati•on, and probably of the faunal zone next above the 

basal zone of the formation (W. A. Cobban, written 

commun., 1952), was collected from ruternating hlruck 

shale and flaggy sandstone beds in the basal 50 feet of 

the formation on the east side of Eureka Greek, ·albout 

one half of a mile west of the. mapped area ( SW% 

SW14 sec. 20, T. 7 N., R. 1 W.). "M actra" arer~.aria 

Meek, "a pelecypod not lmown from rocks as young as 

the Montana group" (W. A. Cobban, written ~ommun., 

1952) was collected from beds near the top of the 

dominantly nonvolcanic lower part of tJhe formation, 
12 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

300 and 375 feet above the base in NE·lh sec. 4, T. 4, N., 

R. 1 W., and from ·about 200 feet above the base of the 

formation south of Kimber Gulch. 

The upper part of the formation contains carbonized 

wood in many places and a silicified tree stump, apparently in place, at one locality, but no identifiable 

fossils were found. Consequently, its age has not been 

fixed as closely as the a.ge of the lower part. It could be 

of late Nidbrara, Telegraph Creek, or early Eagle age 

of the reference sequence for the Western Interior 

(Cobban and Reeside·, 1962, pl. lOb), but it is no 

younger, £or the overlying Elkhorn Mountains Volcanics are older than rocks of early Claggett age ( equivalent to lower part of Pierre Shale of the reference 

sequence), as discussed elsewhere in this report. 

The titaniferous magnetite zone in rocks transitional 

from the lower to the upper parts of the formation is 

believed to have accumulated as placer deposits along 

beaches as the sea retreated eastward. A similar but 

thicker and more persistent zone of titaniferous magnetite farther north in Montana is at the top of the Virgelle l\iember of the Eagle Sandstone (W. A. Cobban, 

oral commun.) which Stebinger (1915, p. 62-63) considered to be a sandy beach or nearshore deposit laid 

down in a retreating sea. Though the magnetite-bearing rocks in these two areas are mineralogically similar 

and accumulated under similar conditions, they are not 

necessarily ·contemporaneous. The gradational relation 

between the lower and upper parts of the Slim Sam 

Formation in the present area suggests uninterrupted 

deposition of fossiliferous marine rocks, not more than 

50 feet below the magnetite zone, and the magnetiterich rocks. If this is true, the upper part of the Slim 

Sam Formation is more likely to be of late Niobra,ra 

and Telegraph Creek age than of early Eagle age. On 

the other hand, at a few places within the area the 

lower part of the Slim Sam is abnormally thin, and 

son1e.of it may have been removed by erosion before the 

upper part was deposited. A few miles north of the 

area the upper part of the Slim Sam is unconformable 

on rocks of the lower part and is overlain with angular 

unconformity by the Elkhorn l\1ountains Volcanics 

(Smedes, 1966, p. 16, 21 112). The magnetite zone has 

not been recognized here. Whether its absence is due to 

nondeposition or erosion is not known. Though these 

relations suggest a significant time lapse between the 

lower and upper pa·rts of the Slim Sam Formation, they 

seem to indicate a n1ore significant lapse between the 

Slim Sam and the Elkhorn Mountains Volcanics. Accordingly, the earlier tentative correlation of ·the upper 

part of the Slim Sam Formation with the Telegraph 

Creek Formation of the reference sequence for the 

Western Interior (Klepper and others, 1957, p. 26) is 

strengthened. 

At some places in and around the Elkhorn Mountains, including most of the east flank, the Slim Sam 

Formation appears to grade upward into the Elkhorn 

Mountains Volcanics. But 1in other places nearby, the 

Elkhorn Mountains Volcanics rest with angular discordance on Slim Sam and older rocks (Klepper and 

others, 1957, pl. 1, south-central part; Freeman and 

others, 1958 pl. 42, northwest part T. 4 N., R. 1 W.; 

Smedes, 1966:, p. 21). In these places Slim Sam and 

older rocks were folded and eroded before the Elkhorn 

l\1ountains Volcanics were extruded. The evidence is 

compelling that folding affected some parts of the area 

east of the Boulder batholith after the Slim Sam Formation was deposited and before the Elkhorn Mountains 

Volcanics were extruded, and a few miles north of the 

east flank area weak folding concurrent with deposition of the Slirn Sam Formation is inferred ( Smedes, 

1966, p. 21). In the area of the present report, however, the general apparent conformity within the Slim 

Sam Formation and between the Slim Sam Formation 

and the Elkhorn Mountains Volcanics indicates little 

prevolcanic deformation. 

TERTIARY ROCKS 

Ter.tiary sedi12entary rocks include two units of 

waterlaid !tuff and one of calcareous tuf·a. The !tuffs are 

part of tJhe ibasin fill in the Townsend ¥alley; one is of 

Oligocene ·age ·and one, of uncertain age, and both are 

small ~aDts of mo:re extensive uni.ts discussed iby P·ardee 

(1925), Merttie, Fischer, ·and Hobbs (1951), Lorenz and 

McMurtrey (1956), Freem·an, Ruppel, ~and Klepper 

(1958), Nelson (1963), Kinoshita, Davis, 1Smedes, ·and 

Nelson (1964), Ro:binson (1967), and White (1954). 

OLIGOCi~NE SEDIMENTARY TUFF 

Gently warped ;beds of sedimentary tuff are poorly 

exposed south rof the Limestone Hills and along Johnny 

:Gulch. They overlie the pre-Teritiary rocks with marked 

!angular unconformity ·and are overlain iby several units 

of Qualternary and :perhaps late Tertiary age. Beneath 

~the mantle of younger deposiJts the unit is probably continuous wi1th the Oligocene sedimenltary tuff that crops 

out to rthe east and :3outh (Freem,an and others, 1958; 

Dunbar Creek Form·.%tion in Rohinson,-1963, 1967) and 

is also considered equivalent to the Tertiary unit 2 described by Mertie, Fischer, ~and Hobbs (1951) in the 

north end of the Townsend Valley. 

The sedimentary tu:ff includes beds of shale, mudstone, 

sandstone, and small lenses of poorly cemented conglomerate and conglHmeratic sandstone ~that contain 

angul,ar to subrounded fragments ·as much as 10 inches 

J 
l 

.SEDIMENTARY ROCKS 13 

long of pre-Ter1tiary rocks. Bedding ranges from thin 

~and distinct 1to very ;thick ~and indistinct. The rocks are 

soft, porous, typically cream and buff colored, slightly 

consolidruted, poorly sorted, and contain granules of 

older rocks, angular grrains of feldspar and quantz, ·and 

glass shards in a light-yellowish-gray matrix of calcite 

~and :benton~tic clay. The content of volcanic material in 

different beds ranges from nearly 0 to 100 percent ·and 

typically is more than half. Some beds 1are composed 

largely of bentonite; grayish-brown compaot finegrained rocks when ·fresh, they swell and become porous and cream colored on weathering. The lenses of 

conglomerate nnd conglomerrutic sandstone typically 

con~ain little or no megascopically identifi.aJble volcanic 

mruterial. 

The greatest exposed thickness of Oliogocene sedimentary tuff in the mapped area is about 1150 feeJt, but 

the thickness probably ranges from 0 lbo ~as much as 300 

feet. The rocks !thicken southward ~possibly to about 

1,000 feet (Freeman •and others, 1958, p. ·508) 1and Ito 

rthe east where Robinson (1967) found the Durubar 

Creek Formation to be as much as 1,600 feet thick. 

Vertebrate fossils of early Oligocene age from the 

sedimentary tuff at four localities in and adjacent to 

the rna pped area were identified by the late M. Jean 

Hough (written commun., Oct. 25, 1950, and Dec. 5, 

1952) as follows ('See also F-reeman and others, 1958, 

p.509,510): 

NW1,4SE:LJ.t sec.18, T. 5 N., R.1 E. 

M esohippus hypostylus Osborn 

Leptomeryw esulcafirus Cope 

Peltosaurus ~ sp. 

NW~SW1,4sec.19,T.5N.,R.1 E. 

Titanotheriomys sp. 

Palaeolagus sp. 

Bathygenys sp. 

Slh sec.13, T. 4 N., R.1 W. 

Brontotherium sp. 

Titanotheriomys cf. T. veterior (Matthew) 

Palaeolagus cf. P. brachydon Matthew 

H yracodon sp. 

N E~NW:LJ.t sec. 130, T. 4 N., R. 1 W. 

Oylilndrodon cf. 0. fontis Douglass 

Leptomeryw esulcatus Cope 

Lithology and bedding suggest that some of the 

Oligocene sedimentary tuff was formed partly of airborne pyroclastic debris that was deposited directly in 

a body of standing water and prurtly of mixed volcanic 

and nonvolcanic material washed from the surrounding 

hills. A small part of the unit seems to have been deposited in a fluviatile environment, perhaps as a delta. 

SEDIMENTARY TUFF OF UNCERTAIN AGE 

In the northeast corner of the mapped area, poorly 

exposed rocks resembling the Oligocene sedimentary 

tuff have been mapped separately. The rocks include 

pale-orange to yellowish-gray silty tuff, some of which 

is bentonized, gray and white calcareous volcanic siltstone, and deeply weathered conglomerate. Bedding is 

obscure and poorly exposed, but in a few places the 

beds seem to be horizontal or to dip gently eastward. 

The lffu.ickness of the unit i'S unknown. 

Mertie, Fischer, and Hobbs (1951, p. 31) suggested 

that these rocks are probably a part of the Oligocene 

sequence. Freeman, Ruppel, and Klepper (1958, p. 512) 

favored the interpretation that they are younger and 

rest unconformably upon the Oligocene rocks. 

OLIGOCENE TUFA 

Calcareous tura and handed travertine, probably ancient spring deposits, are exposed along Johnny Gulch 

near the west edge of the mapped area and in an unnamed tributary south of Johnny Gulch. The larger 

exposure, in Johnny Gulch, is about 2,500 feet long, 500 

feet wide, and 200 foot thick. Most of the rock is very 

pale yellowish brown, gray-weathering, finely crystalline, porous calcareous tu:f.a; the rest is hard compact 

rtraver.tine. Fibrous, la;minated, ·and twigJike structures 

are present locally. 

The tufa contains leaf imprints or molds from which 

the late R. W. Brown (written oommun.) identified 

Sequoia affinis Lesquereaux and Alnus or Betula sp. 

indicative of Oligocene age. Relative ages of the 

Oligocene tufa and the Oligocene sedimentary tuff are 

indeterminate. 

UNCONSOLIDATED TERTIARY AND QUATERNARY 

DEPOSITS 

FAN DEPOSITS AND MANTLE 

A poorly exposed group of surficial deposits of diverse 

origin were mapped as fan deposits and mantle. Gravel 

that was deposited in alluvial fans along the east flank 

of the Elkhorn Mountains, especially north of Indian 

Creek, comprises most of the unit;· the rest consists of 

pediment gravel, eolian silt, poorly sorted hill wash, ·and 

local deposits of alluvial gravel and finer debris. Pediment gravel and eolian silt, the "mantle" of Freeman, 

Ruppel, and Klepper (1958), form a veneer on the Oligocene sedimentary tuff and older rocks south of Crow 

Creek. Deposits of hillwash and alluvium are scattered 

throughout the 'area. 

The unit includes. deposits that range in age from 

Pliocene ( ~) to Holocene. The pediment gravel was 

deposited during the Pliocene Old Valley cycle of 

Pardee (1950), and the oldest fan gravel may be as old. 
\ 

14 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

The youngest parts o:f the :fans as well as the hill wash 

and alluvium are Holocene. 

The :fan deposits 'and mantle are composed o:f gravel 

with a sand and clay matrix. The gravel is entirely o:f 

local origin, and its composition reflects the bedrock 

exposed at higher altitudes nearby. The source of the 

eolian silt is unknown, but its distribution, centered on 

a broad divide between the Townsend and Three Forks 

valleys, offers a clue. Robinson (1963, p. 85-86) suggested that the silt was removed :from Oligocene sedimentary tuffs by northerly winds 'and was deposited 

where the sweep o:f the wind was obstructed. 

GRAVEL DEPOSITS OF UNCERTAIN AGE 

Gravel deposits ,of uncertain but probruble late Pliocene or early Pleistocene ·age and origin were mapped in 

a :few pl,aces. A veneer o:f gravel covers three small areas 

near the front o:f the range east o:f Sheps Gulch. The 

gravel is o:f local origin ~and .pro:hrubly is contemporaneous wittJh the gr.avel rthat :forms the apron along the 

mountain fronlt. An elong3ite body o:f very coa·rse debris 

east o:f !the mouth ·o:f Oold Springs 'Gulch is composed o:f 

cobbles, boulders, and 'a few !blocks, ~as much as 15 feeJt 

across, o:f Quadrant quartzite and Madison limestone derived :from rthe Limestone Hills to the east. The debris 

appears to fiU a channel,and is older than gravel referred tlo older outwash £an deposits. A ~train o:f coarse 

locally derived debris in an unnamed tributary of 

Johnny Gulch south ·of the KeaJting Gulch stock appears 

rto grade into .and be of the same age ~as the older outwash :f.an deposit in Johnny Gulch. 

QUATERNARY DEPOSITS 

:Surficial deposits o:f Pleistocene age are .widespread 

1along lthe east flank of the Elkhorn Mountains. They 

include glacial deposits and relaJted outwash f~ans, deposits :formed iby mass wasting, ~and alluvial deposits. 

GLACIATION AND GLACIAL DEPOSITS 

Glaciers accumu1ruted on the high main mass of the 

Elkhorn Mountains, west .of the mapped ·area, 1at least 

three times during the Pleistocene (Klepper and others, 

1957, 1p. 43; Ruppel, 1962, ~p. 12), and ·at least twice !the 

glacial ice was so extensive rthaJt it spread into canyons 

in the easlt flank area. '.Dhe earliest g1adation in the 

·range, probaibly early Pleistocene, seems also to have 

been the most extensive, for its poorly preserved moraine deposits are present near the mouth of Beaver 

Creek ·at altitudes of aboult '5,000 feeJt and in Crow 

Creek 1at ,altitudes of ~rubourt 5,700 feeJt. The most striking deposit of tJhis early glaciation is rthe large older 

outwash fan of Beaver Creek near Winston. The older 

moraines rand the older outwash fans ·are similar in 

composirtion; both ~are characterized by deep weathering 

that destroye.d nearly all burt !the most resistant volcanic 

rocks. The moraines at the mouth of Beaver Creek consist of subrounded to subangular resistant :fragments 

of volcanic and intrusive rocks -as much as 18 inches 

~across in a closely packed heterogeneous maJtrix. A few 

exceptionally large blocks are preserved; one on a hill 

north o:f Be~~ver Creek is 30 :feet on ~a side. Some o:f rthe 

larger iblocks have .polished faces and are fainrtly 

grooved and. striated. The deposits in Crow Creek are 

generally sbni1ar to those in Beaver Creek hut ·are obscured lby Vt3getaJtion 1and soil. They are composed o:f 

cobbles 1and iboulders of Elkhorn Mountains Volcanics 

~and ;related intrusive. rocks in a matrix of sand ·and 

pe;hbles and have no ~apparent sorting or stratification. 

In a ,few places, rthe !boulders are 1as much ~as 25 :feet long, 

and many ~are distinctly hlocky or angular. On the hill 

north o:f !the Vosburg mine (sec. 34, T. 8 N., R. 1 W., 

pl. 1), ·a sm::~:ll deposit of large boulders, some o:f which 

~are deeply ·disintegrated and one o:f whi0h is striated, 

is believed !lX> have been deposited during this older period o:f glacia;tion. 

The largo older outwash :fan :forming the prominent 

hills east and west o:f Winston and extending eastward 

(Freeman and others, 1·958, p. 513-514) beyond the 

mapped area is made up o:f coarse bouldery and cobbly 

debris o:f volcan'ic and dioritic intrusive rocks from the 

Beaver CrHek drainage. The debris is unusually coarse 

and contains relatively abundant subangular pieces, 

some severftl :feet across. Though most o:f the debris is 

of little-weathered resistant rocks, other pieces .are deeply weathered. This deep weathering, the advanced dissection and ilie destruction of ~the surface :form o:f lthe 

fan and o:f related moraines, and the overlapping by 

younger :f2m deposits strongly support an early Pleistocene age for these deposits. 

Coarse gravel of the older outwash fan flanks the 

valley o:f Crow Creek from Radersburg almost to Power 

Gulch and extends up Cold Spring Gulch to Indian 

Creek. Th~ gravel consists o£ slightly weathered cobbles 

and bould,ers, mostly subrounded rto rounded and less 

than a foot across, o£ resistant fine-grained volcanic 

rocks and dioritic intrusive rocks.· South o:f Crow Creek 

the grave1 is more than 200 feet thick in many places, 

but north o:f Crow Creek it is probably much thinner, 

or only a -veneer. The deposit is thought to be a remnant 

o:f a larg£1 composite outwash :fan built by Crow Creek 

and Indian Creek during the early period of glaciation 

and is probably o:f about the same age as the old :fan at 

tJhe moutb. o:f Beaver Creek. The saddle 1at the head o:f 

Cold Spring Gulch clearly was a spillway for melt water 

from Indian Creek. The Indian Creek outwash :fan was 

built southward during the early period o:f glaciation 

to coalesj~e with the outwash :fan being built in Crow 

IJ 

, 
IGNEOUS ROCKS 15 

Creek, and thus there is no old fan at the present mouth 

of Indian Creek comparable to those at the mouths of 

Beaver Creek and Crow Creek. The present canyon of 

Indian Creek, below Hassel,and the Cold Spring ·Gulch 

spillway, probably was cut in later Pleistocene time by 

a headward eroding stream that pirated the ancestral 

Indian Creek. 

Relatively thick, coarse older outwash gravel deposits, 

composed mainly of sedimentary rocks from the middle 

and upper reaches of Johnny 'Gulch, are preserved along 

the lower reaches of Johnny Gulch. Though the head of 

Johnny Gulch was not occupied by glaciers, the coarse 

gravel probably was deposited during a period of 

greatly accelerated erosion that coincided with the early 

Pleistocene glaciation. Thus, the gravel on Johnny 

Gulch is probably a correlative of the old Beaver Creek 

and Crow Creek-Indian Creek fans. The fan-shaped 

part of the deposit has been much dissected, and it 

grades imperceptibly into gravelly mantle that may 

include old pediment veneer as well as material that 

was redistributed as the fan was dissected. 

The deposits of the two late Pleistocene (Wisconsin~) 

glacial episodes have not been mapped separately, although both episodes must be represented in the extensive younger outwash .fan deposits and in the few 

morainic deposits. All the deposits ·are characterized by 

fresh rock fragments and by fairly well preserved 

surface form. 

The best preserved moraine is near the junction of the 

South Fork, Badger Creek, and Beaver Creek, where a 

terminal moraine marks the maximum advance of the 

last valley glaciers to occupy these valleys; this last 

advance was probaibly in late Wisconsin time, for the 

moraine is not dissected and it contains little-weathered 

boulders as much as 15 feet across. Other boulde·r 

deposits that may be partly destroyed moraines are 

preserved in Weasel Creek. 

Younger outwash fans similar in rock content and 

degree of preservation to the late Pleistocene moraines 

in the Elkhorn Mountains (Klepper and others, 1957, 

p. 43; Ruppel, 1962, p. 12) are much more widespread 

in the mapped area than deposits of till. Before these 

fans were deposited, the older outwash fans were partly 

eroded. The younger fans occupy about the same sites 

as the older fans, but they were graded to a lower level ; 

their tops are 200-300 feet below adjacent remnants of 

older fans. 

The younger Beaver Creek fan consists of unweathered cobbles and boulders of volcanic rocks and 

dioritic intrusive rocks, mostly subrounded or rounded. 

The south edge of the fan distinctly overlaps the upper 

part of the mountain front gravel apron, but it grades 

into the lower part of this apron east of the mapped 

area. To the north of Beaver Creek, fans at the mouths 

of Staubach, ·Pole, and Antelope Creeks were deposited 

at about the same time. Beaver Creek is now entrenched 

into its fan. At the head of the fan the trench is 40-50 

feet deep, about 200 feet wide, and is mostly cut in 

bedrock. Downstream the· trench is progressively 

broader and shallower and is wholly in gravel. East 

of U.S. Highway 287 the fan is not trenched and is 

probably still accumulating. 

The younger Crow Creek fan differs from the 

younger Beaver ·Oreek fan in that it has not been incised 

by the creek but is still being constructed. It consists 

largely of rounded or subrounded boulders and cdbbles 

of volcanic rocks and dioritic intrusive rocks but also 

includes quartzite and other rocks from resistant Palezoic and Mesozoic sedimentary formations. 

Terrace deposits related to the glacial deposits of 

late Pleistocene age are present at several places along 

Beaver and Crow Creeks as gravel veneers on rock 

benches (Klepper, and otJhers, 1957, p. 62). The benches 

were probaJbly cut and veneered with gravel during the 

maximum outwaSh phase of the last glacial period. The 

deposits consist of· coarse rounded gravel containing 

boulders several feet long. The surface of the gravel deposits slope at about the same angle as the modern 

streams, which are entrenched from 20 feet to ·as much 

as 75 feet below them, and they appear to be graded to 

the surface of the younger fan deposits. 

DEPOSITS RESULTING FROM MASS WASTING 

Surficial deposits resulting from mass wasting include delbris originally deposited by glaciers, by streams 

as alluvial fans, and by weathering of the bedrock. 

These deposits have been so modified by postglacial 

mass-wasting processes, probably mainly soil creep, 

solifluction, and frost action, that near-surface features 

attributaJble to the original depositional process have 

been largely destroyed. Accumulations of rock waste in 

clearly recognizaJble landslide deposits were mapped 

separately from the other mass-waste deposits. 

ALLUVIUM 

Deposits mapped ·as alluvium include gravel, sand, 

and silt accumulating in the valleys of the present 

streams, and similar deposits in small fans now forming 

or recently formed at the mouths of many gulches. Many 

other such deposits are present but are too small to be 

shown. 

IGNEOUS ROCKS 

Extrusive and intrusive igneous rocks are abundant 

along the east flank of the Elkhorn Mountains. Extrusive rocks of the Ellmorn Mountains Volcanics of Late 

Cretaceous age cover about one-half of the area, and 
16 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

remnants of much younger, probably middle to late Tertiary, basalt and rhyolite flows 'C'rop out in the central 

part. The many bodies of intrusive rocks fall into two 

main groups: ( 1) intrusive equivalents of Elkhorn 

Mountains Volcanics, for convenience called volcanic 

intrusives, and (2) intrusives younger than Elkhorn 

Mountains Volcanics. The volcanic intrusives comprise 

mainly seriate syenodiorite or granodiorite porphyry in 

larger partly concordant bodies and hiatal rhyodacite 

and trachyandesite porpihyry in smaller sills and dikes 

that were emplaced before folding or during the earliest 

stages of folding. 

The nomenclature used in this report for igneous 

rocks that are coarse grained enough for thin -section 

modes to be determined is shown in figure 2. Finer 

grained volcanic rocks for which modes cannot be determined are classified on the basis of eihemical composition of representative samples, in accordance with the 

system proposed by Rittman (1952). Though the term 

"andesite" was applied to many rocks in the field, none 

of tJhe analyzed rocks are andesite in the Rittman classification. The terminology applied to fragmental volcanic rocks is that of W entwortJh and Williams ( 1932, 

p. 43-53) . The term "porphyry" is used for rocks of both 

5 

K-FELDSPAR 

35 

QUARTZ 

65 95 

PLAGIOCLASE 

ROCKS WITH PLAGIOCLASE OF 

COMPOSITION An o-so 

QUARTZ 

5 35 65 95 

K-FELDSPAR PLAGIOCLASE 

ROCKS WITH PLAGIOCLASE OF 

COMPOSITION An:;o-too 

FIGURE 2.-Nomenclature of coarser grained intrusive igneous 

rocks. 

hiatal and seriate texture if a substantial part of the 

groundmass is too fine grained to be identified by conventional microscopic methods. Medium- and ooarsegrained roc:k:s with conspicuous phenocrysts are referred to as :porphyritic. The term "phenocryst" is used 

without genetic connotation for crystals that stand out 

from a finer grained goundmass. 

The younl~er intrusive rocks form generally discordant stocks, plugs, or bodies of unknown subsurface configuration that were emplaced after the enclosing rocks 

had been fo1ded. Most of the bodies consist of medirnngrained and subequigranular rocks, but a few are made 

up of porphyry. Compositionally, the rocks can be conveniently asdgned to one of two groups: ( 1) mafic rocks 

and (2) intermediate to felsic rocks. Some bodies are 

simple and comprise only one rock type; some are composite and consist of one or more mafic and felsic types. 

Lead-alpha ::tge determinations of zircon from two postvolcanic int:rusives are 72-76 million years (Jaffe and 

others, 1959:, p. 80-81), determinations indicating Late 

Cretaceous ~Lge according to the time scale of Holmes 

( 1959) ; theBe age determinations are in the same age 

range as roc.ks of the Boulder batholith 10-20 miles to 

the west. 

Four thin dikes of quartz latite porphyry probably 

are equivalent to the Lowland Creek Volcanics of 

Eocene age (Smedes, 1962) nearby. 

ELKHORN MOUNTAINS VOLCANICS 

The Elkhorn Mountains Volcanics of Late Cretaceous 

age (KleppE:r and others, 1957, p. 31-41), comprise three 

members in the type locality : a lower member consisting 

mainly of rhyodacitic and trachyandesitic pyroclastic 

rocks, a m:~ddle member· characterized by rhyolitic 

welded ~tuffs alternating with andesitic and rhyodacitic 

tuff and tuff breccia, and an upper member composed mainly of airlaid tuff and of waterlaid tuff and 

conglomerate that are largely debris eroded from the 

lower members. Only the lower and middle members are 

present along the east flank of the Elkhorn lfountains. 

Rocks of the lower member underlie most of the 

eastern slope of the Elkhorn Mountains in the northern 

part of the Jnapped area and much of the mountainous 

central part. Farther south, they form part of the roo£ 

of the Rattlesnake intrusive body. Cenozoic sediments 

in the southeastern part of the mapped area are probably underlain at shallow depth by rocks of the lower 

member. The lower part of the middle member occupies 

much of the northwestern part of the mapped area, but 

south of Indian Creek it is present only in a small fault 

block in_~ec~ 2, T. 6 N., R. 1 W. Rocks of the upper part 

of the inidd1e member and the upper member are not 

J 

, 
·~ 

.... 

IGNEOUS ROCKS 17 

present probably due to erosion rather than to nondepo~ 

sition. 

North of Johnny Gulch the Slim Sam Formation 

grades upward into the Elkhorn Mountains Volcanics. 

South of Johnny Gulch, however, the volcanics truncate Slim Sam and older rocks that had been tilted or 

gently folded at the beginning of the volcanic episode 

(Freeman and others, 1958, pl. 42). The volcanics are 

unconformably overlain by Oligocene and younger 

rocks and by unconsolidated sediments. 

The volcanic pile is cut by several large irregular intrusive masses that are broadly concordant with but 

that locally sharply crosscut the enclosing rocks and by 

many dikes and sills; most, if not all, of these rocks are 

of about the same composition and age as theElkhorn 

Mountains Volcanics and are products of the same 

magma or magmas that yielded the volcanic rocks. The 

Rattlesnake mass, south of Crow Creek, is the largest · 

of these. 

LOWER MEMBER 

The lower member of the Elkhorn Mountains Volcanics ranges in thickness from 2,500 to 5,000 feet and 

consists mainly of tuff, tuff breccia, and breccia interlayered with a few lava flows and a few thin units of 

welded tuff. These rocks are mostly dark gray, greenish 

gray, and grayish green, but shades of brown, red, and 

purple are common. Chemical analyses of three composite samples indicate that the average composition of 

the lower member (table 1, samples 1-3) is in the trachyandesite-rhyodacite range. 

Tuffs form massive unstratifi~d units that probably 

accumulated from air falls of ash and stratified units of 

which at least some were deposited in water. Crystal 

tuffs and crystal-lithic tuffs are the dominant rock types; 

by an increase in abundance and size of lithic fragments 

these rocks grade into tuff breccias and breccias, which 

are very poorly sorted and commonly exhibit complete 

size gradation from the largest lithic fragments to cryptocrystalline matrix. J\iost of the finer grained tuff is 

well bedded and delicately laminated and is locally 

crossbedded, channeled, and mud cracked; these features indicate that the ash was deposited in shallow 

water. Thick, poorly bedded units of breccia and tuff 

breccia are probably mud flows. The fact that most of 

the crystal and lithic fragments are angular, even in the 

well-bedded rocks, indicates a n1inimum of transport 

and reworking. 

Fragments of intermediate plagioclase and of volcanic rocks are by far the predominant clastic components. The plagioclase is complexly twinned, commonly 

shows progressive and oscillatory zoning, and ranges in 

composition from An4o Ito An65• Other crystalJ~agments 

include quartz, magnetite, pyroxene, and hdrnblende 

(partly or completely replaced by aggregates of chlorite, epiodote, magnetite, quartz, and leucoxene) , sparse 

biotite (partly altered to chlorite and sphene), and rare 

K-feldspar. Most rock fragments are trachytic, pilotaxitic, or cryptofelsic trachyandesite and rhyodacite; some 

are amygdaloidal basalt and brown devitrified glass. 

Fragments of prevolcanic sedimentary rocks are extremely rare. 

Because of greater permeability the pyroclastic rocks 

are more altered than the associated flows, dikes, and 

sills. Hornblende and pyroxene are com.pletely replaced, 

and plagioclase is saussuritized in many of these rocks. 

Calcite and epidote are common in veinlets and irregular replacement masses. 

The lava flows in the lower member, as much as 100 

feet thick, consist of trachyandesite and amygdaloidal 

basalt. The flows are mineralogically and texturally 

closely similar to the related dikes and sills but are generally more altered. 

Thin units of welded tuff are interlayered with the 

pyroclastic rocks of the lower member. The welded tuff 

is gray to greenish gray and contains a!bundant flattened 

and bent pieces of dark chloritic devitrified pumice as 

much as an inch long, angular rock fragments, and 

broken crystals of labradorite surrounded by microcrystalline intergrowths of quartz, feldspars biotite, 

chlorite, sericite, magnetite, and indeterminate material. Mafic minerals are completely replaced by chlorite, 

calcite, epidote, and magnetite. Chemical analysis indicates that a representative sample (table 1, sample 

4) is quartz latite. 

MIDDLE MEMBER 

The middle member of the Elkhorn Mounta:ins Volcanics consists of rhyolitic welded tuff interlayered with 

crystal tuff and lapilli tuff and less abundant volcanic 

breccia. The fragmental rocks are similar to those in 

the lower member. A measured section of the lower 

2,500 feet of the middle member is given on pages 

60-62. The total exposed ~thickness is ·about 3,000 

feet. Farther west, the middle member is as much as 

7,500 feet thick (Smedes, 1966, p. 26). 

Welded tuff units range in thickness from a few feet 

to about 200 feet, but most are between 20 and 60 feet 

thick. They range in color from light to dark shades 

of gray, greenish gray, red, and purple. Only a few 

thin dark-greenish-gray units resemble the quartz latite 

welded tuff of the lower member. Some of the units 

appear nearly homogeneous and apparently are welded 

from bottom to preserved top ; others consist of a 

strongly welded base that grades upward into a poorly 

welded or unwe,lded zone. Most of the welded rocks 

contain abundant flattened fragments of devitrified 
18 

Sample 

No. 

Field 

No. 

GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

~ABLE I.-Chemical analyses, norms, and modes of igneous 

J,aboratory No. Rock type 1 Geologic occurrence Location 

Elkhorn Mountains Volcanics 

L--------- 52Wl42 ________ 53-81CW 8 _________ Trachyandesite _____________ Middle part of lower meml•er (composite of NEX sec. 25, T. 7 N., R.1 w ________________ _ chips). 2 __________ 52W143 ________ 53-82SCW. ________ Labradorite trachyandesite .• Lower member (composite or chips from basal Iron Mask mine crosscut, sec. 30, T. 7 N., R. 1 

1,000 ft). E. 

3 __________ 52KL _________ 53-71CW __ -------- Labradorite rhyodacite ...•.• Lower member (composite of chips across 1,200 SEX sec.16, and NEU sec. 21, T. 8 N., R. 1 W. 

ft). 4 __________ 52R65 _________ 53-77CW 8 _________ Quartz latite ________________ Welded tuff, lower member .. ·------------------ NEU sec.19, T. 7 N., R. 1 E ________________ _ 5---------- 52K7 __________ 53-72CW •--------- Rhyolite ____________________ Welded tuff, middlememba~ •• ________________ NEUSWU, sec.16, T. 8 N., R. 1 W __________ _ 

Intrusive rocks related to Elkhorn Mountains Volcanics 

6---------- 52R66 _________ 53-78SCW s ________ Syenodiorite porphyry ______ Small stock in lower member of Elkhorn NWUsec.19, T. 7 N., R.1 E. _________________ _ 

Mountains Volcanics. 

7 __________ 61K659 ________ 158775 & ____________ Calcic granodiorite __________ Thick sill east of Santa Anita mine ____________ NE>-:1' sec .• ~ T. 4 N., R. 1 W ------------------ 8 __________ 52K127 ________ 53-736W a __________ Calcic syenodiorite .. _________ Sill in middle member of Elkhorn Mountains NEUNWn sec. 9, T. 8 N., R.1 W. ___________ _ 

Volcanics. 

g __________ Winchell None given 7 ______ Andesite (Winchell's (1914) Rattlesnake mass. ____________________________ Rena mine, sec. 7, T. 5 N., R. 1 E-------------

No. 1 usage). 10 _________ 49K13 _________ 51-939CW 9 ________ Syenodiorite ________________ Dutchman intrusive, southeastern lobe of NEU sec. 32, T. 5 N., R.1 w ________________ _ body. 11--------- 51K15--------- 53-69SCW a ________ Syenodiorite porphyry ______ Small intrusive in lower member of Elkhorn NWUNW>-:1', sec. 30, T. 7 N., R. 1 E---------- . Mountains Volcanics. 

12--------- 49K12 _________ 51-938CW o ________ Syenodiorite_ _______________ Dutchmanintrusive600ftno::thwestofsample NE>-:1' sec, 32, T. 5 N., R.1 W-----------------

14locality. . 13 _________ 52R6 __________ 53-76SCW a ________ Syenodiorite porphyry ______ Stock near Iron Mask mine .... ---------------- Sec. 19 T. 3 N., R. 1 W ------------------------ 14 _________ 49K14 _________ 51-941CW 9 ________ Granodiorite ________________ Dutchman intrusive, central part_ ____________ Boundary between sees. 29 and 32, T. 5 N., R. 

1 w. 

Younger intrusive rocks 

l5 _________ 49K9 __________ 51-931CW 9 ________ Granodiorite ________________ Keating Gulch stock __________________________ NE>-:1' sec.16, T. 5 N., R.1 W ________________ _ 

16 _________ 60S599 _________ G-3167 9 ___________ Syenogabbro---------------- Orphan Boy mafic dike _______________________ NE>-:I'NE>-:1' sec. 4, T. 8 N., R.1 W ------------

17--------- 49K10 _________ 51-936CW 9 ________ Syenodiorite. --------------- Keating Gulch stock ________ ------------------ SE>-:1' sec. 9, T. 5 N., R. 1 W-- ---------------- 18 _________ 49Kll _________ 51-937CW 9 ________ Granodiorite ________________ Slim Sam stock _______________________________ NWUSE>-:1' sec. 20, T. 6 N., R.1 w ___________ _ 19 _________ 49K8 __________ 51-934CW o _____________ dO----------------------- Keating Gulch stock-------------------------- NE>-:1' sec.16, T. 5 N., R.1 W ________________ _ 20 _________ 52K184 ________ 53-64SCW a ____________ .dO----------------------- Antelope Creek stock _________________________ SW>-:I'SW>-:1' sec. 33, T. 9 N., R. 1 \V ------------

21. ________ 52K19L _______ 53-75SCW a ___________ .• dO----- __________________ Antelope Creek stock (composite of samples ___ -- __ ---- _-- __ -------------------------------- throughout stock). 22 _________ 61K66QO _______ 1587768 ____________ Quartzmonzonite ___________ LoneMountainstock __________________________ SW>-:I'NE>-:1' sec. 6, T. 4 N., R. 1 E ____________ _ 23 _________ 49K7 __________ 51-933CW 9 ________ Granodiorite ________________ Keating Gulch stock __________________________ NE>-:1' sec. 9, T. 5 N., R.1 W ------------------ 24 _________ 51K1a. ________ 53-68SCW •-------- Porphyritic quartz mon- Vosburg stock _________________________________ Kleinschmidt mine, SW>4SEUsec.34, T.8N., 

zonite. R. 1 W. 

Sample 

No. 

Field No. Laboratory No. Rock type 1 Geologic OCCUITI:nce Location 

Elkhorn Mountains Volcanics 

L--------- 52Wl42 ________ 53-81CW a _________ Trachyandesite _____________ Middle part of lower memb~r (composite of NE>-:1' sec. 25, T. 7 N., R. 1 w ________________ _ chips). 2 __________ 52W143 ________ 53-82SCW a ________ Labradorite trachyandesite .. Lower member (composite of chips from basal Iron Mask mine crosscut, sec. 30, T. 7 N., 

1,000 ft). R. 1 E. 

3 __________ 52KL _________ 53-71CW. --------- Labradoriterhyodacite ______ Lower member (composite of chips across SEX, sec.16, and NEU sec. 21, T. 8 N., R.1 W. 

1,200 ft). 4 __________ 52R65. ________ 53-77CW a _________ Quartz latite. --------------- Welded tuff, lower member __ , _________________ NE% sec. 19, T. 7 N., R. 1 E------------------

5 ....... --- 52K7 __________ 53-72CW •--------- Rhyolite ____________________ Welded tuff, middle member .. _________________ NEUSWX, sec. 16, T. 8 N., R. 1 W. 

Intrusive rocks related to Elkhorn Mountains Volcanics 

6 __________ 52R66 _________ 53-78SCW 3 _______ Syenodiorite porphyry ______ Small stock in lower memher of Elkhorn NWX sec.19, T. 7 N., R.1 E __________________ _ 

Mountains Volcanics. 

7 __________ 61K659 ________ 158775 5 __ --------- Calcic granodiorite .• -------- Thick sill east of Santa Anita mine. __ -------- NEX sec. 4, T. 4 N., R. 1 w _______ ------------ 8---------- 52K127 ________ 53-736W 3_. ------- Calcic syenodiorite __________ Sill in middle member of Elkhorn Mountains NEXNWU sec. 9, T. 8 N., R. 1 w ____________ _ Volcanics. 

9 __________ Winchell No. L None given 7 ______ Andesite (Winchell's (1914) Rattlesnake mass ____________ ----------------- Rena mine, sec. 7, T. 5 N., R. 1 E-------------

usage). 10 _________ 49Kl3 _________ 51-939CW 9 ________ Syenodiorite ________________ Dutchman intrusive, south•lastern lobe of NEU sec. 32, T. 5 N., R. 1 w _________________ _ body. n _________ 51Kl5 _________ 53-69SCW s _______ Syenodioriteporphyry ______ Small intrusive in lower member of Elkhorn NWXN, sec. 30, T. 7N., R. 1 E_ ______________ _ 

Mountains Volcanics. 12 _________ 49K12 _________ 51-938CW 9 ________ Syenodiorite. _______________ Dutchman intrusive 600 feet northwest of NEU sec., 32, T. 5 N., R.1 w ___ --------- ______ _ sample 49K14locality. 13 _________ 52R6 __________ 53-76SCW a _______ Syenodiorite porphyry ______ Stock near Iron Mask mine._----------------- Sec. 19 T. 3 N., R. 1 W __ ---------------------- 14 _________ 49Kl4 _________ 51-941CW 9 ________ Granodiorite ________________ Dutchman intrusive, central )art _____________ Boundary between sec. 29 and 32, T. 5N., R. 1 

w. 

See footnotes at end of table. 

"' 
\.,.I 

IGNEOUS ROCKS 19 

rocks from the east flank of the Elkhorn M otmtains 

Chemical analyses (weight percent) Norms (percent) -------------------------------

8!02 TiOa Ah03 Fe20a FeO MnO MgO CaO Na20 K20 P20a Loss on Total 

ignition2 q or ab an wo en fs mt 11 ap cc c hm 

54.8 0.76 17.0 4.5 3.4 

56.2 

59.6 

60.7 

69.4 

. 66 16.8 

. 72 17.4 

. 74 16.6 . 56 15.4 

4. 0 3.1 

3.3 3.2 

1. 7 3. 5 1. 7 0. 86 

53.4 1. 0 17.2 4. 2 5. 0 

54. 9 . 63 17. 0 4. 6 3. 6 55. 4 • 74 13. 4 3. 3 5. 5 

56. 61 . 71 17. 91 4. 22 2. 7 

57.3 

57.8 

58.1 

58.6 

60.2 

55.2 

55.46 

56.2 

60.5 

61.3 

61.6 

63.5 

63.5 

65.8 

66.4 

.66 17.1 2.8 3.6 

. 68 17.1 3. 8 3. 6 

.57 17.4 3.9 2.1 

• 44 16. 8 3. 6 2. 6 . 52 17. 2 3. 6 2. 2 

1.1 

. 81 

• 76 

.62 

.58 

. 53 

.50 

.55 

.50 

. 29 

16.4 

16.93 

18.9 

17.2 

18.1 

16.5 

15.7 

15.6 

16.2 

16.4 

4.1 

2.12 

4.6 

3.4 

3. 0 2. 5 

2.6 

3. 9 

3. 2 

1. 6 

5. 0 5. 63 3.0 

2. 5 

2. 9 3.2 

2.8 

1. 4 

1.0 

1.3 

Elkhorn Mountains Volcanics-Continued 

0.16 3. 6 

.18 2. 6 

.16 2. 6 

6. 6 

5.4 

5. 2 

.12 1. 8 4. 2 . 08 . 37 1. 6 

3.6 

2. 6 

2. 6 

2.1 

2. 6 

1.6 

3.0 

3.4 

5. 0 6.3 

0.31 

.38 

.34 

.32 

. 09 

3.0 99.3 9.5 9.5 30.4 25.5 2.2 9.0 1.6 6.5 1.4 0.7---------------

5.1 100.0 15.2 18.9 23.1 25.6 .0 6.8 1.9 6.8 1.4 1.0-----0.4-----

1.5 100.0 16.4 20.1 22.0 23.6 .0 6.5 2.3 4.8 1.4 .8----- .8-----

2.3 99.1 16.8 29.5 17.8 18.7 .0 4.5 4.0 2.5 1.4 .8----- .9----- .80 99.8 26.5 37.2 22.0 7.3 .0 .9 .o 1.4 1.1 .2 ..... 1.6 0.7 

Intrusive rocks related to Elkhorn Mountains Volcanics-Continued 

0. 22 3. 6 6. 0 3. 0 2.8 

. 20 3. 9 8. 1 2. 8 2. 1 . 68 6. 8 7. 8 2. 0 2. 1 

.a am a~ aro an 

.08 2.8 

.16 2.6 

.15 2.4 

.18 2. 5 .17 2. 0 

7.0 

6.0 

6.9 

6. 0 5. 6 

3.8 

3. 2 

3.4 

3.6 

3.5 

2. 6 

2.1 

2. 9 

2. 2 

2. 8 

0. 39 2.4 99.2 6.5 16.5 25.4· 25.2 0.8 9.0 4.5 6.1 1.9 0.9---------------

.42 ~2.0 100.0 9.8 12.4 23.7 27.6 4.0 9.7 2.1 6.7 1.2 1.0 0.1---------- .49 1.1 99.3 10.0 12.4 16.9 21.4 5.9 16.9 7.4 4.8 1.4 1.2 ---------------

.46 (8) 99.84 11.9 16.0 26.2 27.0 1.5 5.5 1.3 6.1 1.3 1.1 .3----------

.47 

.35 

.34 

. 26 

.32 

2.3 100.5 8.0 15.4 32.1 21.9 4.1 7.0 3.4 4.1 1.3 1.1 ---------------

1.9 99.3 14.5 12.4 27.1 26.1 .6 6.5 2.6 5.5 1.3 .8 ------·--------

1.8 101.0 11.6 17.1 28.8 23.7 3.5 6.0 .o 5.6 1.1 .8 -------------··-

2.8 99.6 13.8 13.0 30.4 23.2 2.0 6.2 1.4 5.2 .8 .6--------------- 1.6 99.7 15.4 16.5 29.6 23.0 1.1 5.0 .5 5.2 1.0 .8---------------

Younger intrusive rocks-Continued 

0.18 

.16 

.13 

.14 

.14 

.68 

.14 

.14 

.07 

.11 

4.1 

4. 01 2. 7 

2.4 

2.1 

2.6 

2.4 

2. 2 

1. 4 

. 76 

7.3 

7.22 

7. 0 

5.8 

5.8 

5. 2 

4.6 

3. 9 

4.4 

4.2 

3. 0 3. 01 3. 7 3.4 

3.6 

3.4 

3. 3 

4.2 

3. 3 

4. 2 

2.3 

3.18 

2.4 

2.6 

1. 7 3.0 

3.2 

3.6 

3.4 

3.6 

3.1 0. 66 . 36 0. 71-.13 • 42 1. 0 .30 1. 2 . 28 1.2 ,22 .88 • 21 1. 4 100.0 

99.7 

101.0 

100.0 

101.0 

100.0 

101.0 

.38 

.20 

.12 

0. 84 100.0 1. 0 101.0 1. 0 100.0 

8. 2 13. 6 25. 4 24. 5 4. 0 4. 2 18. 8 25. 5 23. 3 4. 1 8. 2 14. 2 31. 3 27. 9 1. 7 15. 8 15. 4 28. 8 24. 0 1. 2 18. 1 10. 0 30. 4 26. 9 . 0 15.3 17.7 28.8 20.9 1.4 18. 8 18. 9 27. 9 18. 6 1. 2 

15. 6 21. 3 35. 5 13. 1 1. 4 22. 9 20. 1 27. 9 19. 4 . 5 19. 1 21. 3 35. 5 15. 3 2. 0 

10.2 4. 3 10.0 7. 5 6. 7 0. 7 6.0 1. 0 5. 2 2.1 6.5 3.2 6.0 2.4 

5. 9 3.1 

6. 7 

4. 9 4. 3 3. 6 3.8 

2.1 

1. 5 1.4 

1.2 

1.1 

1.0 

1.0 

0. 7 --------------- .9 0.1 ---------- 1.0 --------------- .7 --------------- .7 ----- 0. 5 -----

.5 --------------- .5 ---------------

5.5 

3.5 

1. 9 

. 0 3. 4 1. 1 • 9 . 1 --.-- 1. 6 . 0 2. 0 1. 0 . 5 ---------- 1. 8 

.8 2.3 .6 .3 ---------------

Quantitative spectrographic analyses (parts per million) 10 u Mode (volume percent) 

Cu Pb Co Ni Ga Cr Sc La Zr Be Ba Plagioclase K·feldspar Quartz 

Elkhorn Mountains Volcanics-Continued 

0. 012 0. 0042 0. 0020 0. 0021 0. 0042 0. 011 0. 0015 0. 0 0. 0094 0. 0001 0. 014 

Intrusive rocks related to Elkhorn Mountains Volcanics-Continued 

·------------------·------·---····------------------------------------·-----------------50 (An12) 

•••. -----------------•.• --.. --••••...••••• --.••..••. --••.•••.. --------.--.. ------... --- 53 (An43-55) 

0. 0029 0. 0037 0. 0022 0. 0009 0. 0054 0. 0013 0. 0032 0. 0 0. 012 0. 0001 0. 21 45 (An1o·52) 

----------·--------- __ ----... ------------ _______ . _______ ----- _____ . ------- -----------·· 50 (An!o-oo) 

. 0012 . 0037 . 0018 . 0 . 0046 . 0009 . 0010 • 0 . 0085 . 0 . 14 60 (An•o) -·-----.• ---...•• ----- _ ----· .. ----••.•.••............. ______ .. _. __ .. ----.--..... ------- 52 (An45) 

24 

14 

15 

24 

19 

17 

Biotite Hornblende Augite Other 12 

3 10 chlorite 

4 ------------

4 ------------

2 

3 ------------ 10 ------------

23 

19 

9 

15 

8 

2 

12 

14 

6 

3 

4 

4 

5 

2 

3 

3 
20 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

TABLE I.-Chemical analyses, norms, and modes of igneous rocks 

Sample Field No. Laboratory No. Rock type 1 Geologic occurrence Location 

No. 

Younger intrusive rocks 

15 ......•.. 49K9 .......... 51-931CW o ________ Granodiorite .•.............. Keating Gulch stock __________________________ NE~ sec. 16, T. 5 N., R. 1 W ________________ _ 16 ....•.•.• 60S599 ......... G-3167 o ___________ Syenggabbro ________________ Orphan Boy mafic dike _______________________ NE~NE~ sec. 4, T. 8 N., R.1 w ___________ _ 17 ....•.... 49K10 ......... 51-936CWo-------- Syenodiorite ________________ Keating Gulch stock .•........................ SE~ sec. 9, T. 5 N., R. 1 W __________________ _ 18.-------- 49Kll _________ 51-937CWo ________ Granodiorite ________________ Slim Sam stock _______________________________ NW~SE~ sec. 20, T. 6 N., R. 1 w ___________ _ 19 ......•.• 49K8 .........• 51-934CW o ________ Granodiorite ... ------------- Keating Gulch stock __________________________ NE7;1: sec. 16, T. 5 N., R. 1 w ________________ _ 20 ......•.. 52K184 ........ 53-64SCW a _______ Granodiorite ... ------------- Antelope Creek stock .•... -------------------- SW~SW~ sec. 33, T. 9 N., R. 1 W ........... . 21. ........ 52K19L ....... 53-75SCW a _______ Granodiorite ________________ Antelope Creek stock (Composite of samples 

throughout stock). 22 •.....•.• 61K6600 ..•.... 158776 e ____________ Quartz monzonite ___________ Lone Mountain stock ____________ ------------- SW7;i:NE~ sec. 6, T. 4 N., R. 1 E.------------

23 ......... 48K7 __________ 51-933CW o ________ Granodiorite ________________ Keating Gulch stock __________________________ NE~ sec. 9, T. 5 N., R. 1 w _________________ _ 24 .... ----- 51K1a _________ 53-68SCW •- .• ____ Porphyritic Quartz Vosburg stock·-------------------------------- Kleinschmidt mine, SW~ SE7;1: sec. 34, T. 8 N ., monzonite. R. 1 W. 

1 Volcanic rocks (samples 1-5) are named according to Rittman (1952); intrusitive rocks (samples 6-11) are named on basis of their modes (fig. 2). 2 Includes gain due to oxidation of FeO. a Chemical analysis by rapid method by J. M. Dowd, Katherine White, and F. S. Borris. • Analysis by rapid method by J. M. Dowd. ~Analysis by rapid method by P. L. D. Elmore, I. H. Barlow, S.D. Botts, and G. W. Chloe. e Reported as HzO; <O. 05 C02. 

pumice. Chemical analysis (table 1, sample 5) indicates 

that a composite sample of typical welded tuff fron1 

this me1nber is rhyolite. 

The welded tuffs consist of plagioclase and biotite 

phenocrysts, rock fragments, and flattened chunks of 

devitrified pumice in a devitrified groundmass. Plagioclase in anhedral angular fragments that are generally 

zoned, twinned, and unaltered makes up less than 10 

percent of most rock, and biotite phenocrysts comprise 

less than 5 percent; magnetite grains, which are 

oxidized to hematite near the margins of the phenocrysts are abundant. Rock fragments compose 5-50 

percent of different units and are mainly of trachyandesite, basalt containing chlorite-filled vesicles, microcrystalline rhyodacite, and chunks of brown devitrified 

spherulitic glass. Collapsed pumice,, ranging from 5 to 

30 percent in different units, is crumpled and cut by 

perlitic cracks. Groundmass and pumice are devitrified 

and in some rocks are recrystallized to mosaics of very 

fine grained quartz, feldspars, chlorite, biotite, sericite-, 

magnetite, minor calcite, and abundant leucoxene dust. 

The reconstituted pumice typically contains more sericite than does the groundmass. 

INTRUSIVE ROCKS RELATED TO THE ELKHORN 

MOUNTAINS VOLCANICS 

Several large irregular partly concordant bodies consisting mainly of syenodiorite porphyry and granodriorite porphyry (fig. 3) and many dikes and sills of 

trachyandesite poTII>hyry ·and rhyodacite porphyry cut 

Elkhorn Monntains Volcanics and olders rocks. These 

intrusives ,are similar in composition [;o the associruted 

volcanics and are products of ithe same magm·a or 

magmas. Lithic fragments in .the tuffs ·and breccias, 

except for rare fragments of underlying sedimentary 

rocks, are indistinguishable from the principal types of 

intrusive rock~; most lava flows also closely resemble 

one or another of rbhe intrusive rocks, so much so rthat 

in -areas of poor exposures and struotural complexity 

flows cannot .hf, distinguished from intrusives with confidence. Similarly, plagioclrase, augite, and hornblende, 

in that order, are the most abundant large mineral 

grains in flow~, intrusives, and fragmental rocks, and 

their optical properties rare identical or closely similar. 

The inttrusivo bodies range in size from dikes ·and sills 

less than ·a foot rthick and a few tens of feet long to the 

large grossly concordant Rattlesnake mass that is exposed over about 20 square miles. Most of ·the sills are a 

few tens of feet rthick, but some are several hundred 

feet thick; several are more than 2 miles long. Sills are 

more ·abundant in shales of the l{ootenai Formation and 

Colorado Formation. Different bodies were emplaced ,at 

different times during rthe period of volcanic aotivi,ty as 

indicaJted by crosscutting and truncating relaJtions. All 

the concordant and quasi -concordant masses were almost certainly empl,aced before the major episode of 

folding. The texture of 1the rocks and the shapes of most 

bodies indicate •3m placement at relatively shallow depth. 

The larger intrusive masses consist 1nainly of seriate 

syenodiorite porphyry that grades either into granodiorite porphyry or toward diorite porphyry. Some of tlhe 

rocks in the Dutchman intrusive are medium grained 

subequingranular. Smaller dikes and sills, however, 

are mainly hiat~~l rhyodacite or trachyandesite porphyry 

with abundant indeterminate groundmass. The rocks 

are mainly gray, greenish gray, and purple and consist of plagioclase, augite, hornb'lende, magnetite, and 

rarely hypersthene and olivine. On plate 1, map symbols distinguish those rocks in which hornblende is the 

dominant mafic mineral as determined in the field and 

those in which augite or olivine is dominant; many intrusive bodies, particularly the larger ones, contain 
' 

IGNEOUS ROCKS 21 

from the east flank of the Elkhorn Mountains-Continued 

Quantitative spectrographic analyses (parts per million) 1o u Mode (volume percent) 

Cu Pb Co Ni Ga Cr Sc La Zr Be Ba Plagioclase K.feldspar Quartz Biotite Hornblende Augite Other 12 

Younger intrusive rocks-Continued 

----------------------------------------------.-.--- __ ----.. _- -- .. --.------------------ 51(An45-6o) 9 8 13 9 8 2 --------------------------------------------------------------------------------------- 49(AnaMo) 20 3 8 7 11 2 ___ ---------.------ ___ . __ . ---- ______ .. _-. _____ .. ---- -·--- __ .. __________ -- .. __ ----------- 53(An4HO) 18 7 _ ----------- 16 2 4 _ ----- ______________ . ________________ -------------- ____ . ________ ----- ______ ---- ___ ----- 55(An42) 18 14 3 7 ---------- 3 

---------------------------------------------------------------- ______ ----------------- 55(An4o) 12 13 5 13 ---------- 2 ... 0012 0.0029 0.0 0.006 0.0030 0.0013 0.0012--------0.0075 0.0001 0.15 56(Anao-•5) 18 10 6 9 -------- · 1 

0. 0014 • 0037 . 0 . 0011 . 0028 . 003 . 0012 . 0078 . 0096 . 0002 .13 -------------------.. -------------------.---------------.----.- _. _________ --------.----

_______________________________________________________________________________________ 45(An2s-a2) 30 

22 

28 

12 1 8 1 3 

2 

1 

-- _.- _.- --. __________________ . ____ . _ ... _. _. ____ . __ . ___________ . ___ . _ ------------------. 51(Ana5-:l6) 16 ------------ 15 ------------ 9 ----------

6 ---------- ______________ ---- ___________________ . _______________________________ .. ____ .. _________ . 60(An2o-aa) 

7 Analysis by Chase Palmer (Winchell, 1914, p. 176). s Other constituents reported include 0. 29 percent H20-, 1.16 percent H20+, 0.13 percent C02, 0. 03 percentS, and 0.14 percent BaO. g Analysis by rapid method by S.M. Merthold and E. A. Nygaard. 

1o Quantitative spectrographic analyses by E. L. Hufschmidt. n Elements not detected: B, Ag, Ge, Mo, W, Sn, As, Sb, Bi, Zn, Tl, In, Y, and Nb. ' 2 Includes apatite, epidote, opaque minerals, sphene, chlorite, calcite, and sericite. 

both hornblende and augite. For example, in some parts 

of the Ratt~esnake mass, hornblende needles are conspicuous, in other par~ts hornblende and augite appear 

to be roughly equal, and in some places neither is conspicuous. Groundmass textures in narrow dikes and 

sills generally are pilotaxitic or trachytic; in larger bodies such ~as the Rattlesnake and Dutchman intrusives the 

groundmass is typically a fine-grained mosaic of plagioclase, !{-feldspar, and magnetite, with lesser amounts 

of augite, quartz, chlorite, and an interstitial carbonate 

mineral. Accessory minerals include sphene, leucoxene, 

hematite after magnetite, apatite, and zircon. Small 

:vugs in smne rocks are filled by one or more of the following minerals : Quartz, epidote, actinolite, chlorite, 

and K-feldspar. 

Percentages of phenocrysts in samples of syenodiorite porphyry examined under a microscope range as 

follows (table 1, fig. 3) : plagioclase, 19-41; augite, 

3-11; !hornblende, 0-12; hypersthene, 0-6; and magnetite, 2-5 percent. Plagioclase phenocrysts are equant 

to lathlike euhedra1 crystals 5 mm (millimeters) to 

less than 0.5 mm long and are strongly zoned from An70 

cores to Anzo rims. Delicate oscillatory zoning is common, and 1nany grains have zones of "glass" inclusions. 

Albite ;and cmnbined albite-Carlsbad twinning are 

most coinmon; pericline twinning, less so. Augite phenocrysts occur either as single euhedral prismatic grains 

<0.5-3.5 mm long or in c~umps of smaller subhedral 

crystals. They are faintly pleochroic from pale green 

to colorless and have a 2V=51-58+2°. Most of the 

grains are zoned, and multiple twinning is common. 

I-Iornblende fonns euhedral LatJhs as much as 5 em 

(centimeter) long and is fresh or partly replaced along 

the margins by finely granular clinopyroxene and magnetite. It has distinct pleochroism (X= color less to pale 

brown; Y =olive green; Z =deep green) and an extinction angle of about 18°. Twinned and non twinned 

grains are about equally abundant. In rocks having 

augite as ibhe dominant mafic phenocryst, the hornblende typica:lly is altered to oxyhornblerrde with oxidized margins, small extinction angle, and the followingpleochroism:.X=medium brown; Y=mediumolive 

brown; Z=dark olive brown. Euhedral to subhedral 

equant crystals of magnetite as muelh as 0.5 mm across 

are scattere1

d in a groundmass or as inclusions in hornblende and augite. 

Many of these rocks are deuterically ·al·tered, and the 

original groundmass minerals and textures are replaced 

by fine-grained intergrowths of albite, epidote, calcite, 

sericite, chlorite, magnetite, and quartz. Sericite replaces the calcic cores of plagioclase phenocrysts, augite 

phenocrysts are altered to chlorite, hypersthene and 

olivine phenocrysts are changed to serpentine, and hornblende phenocryBts are partly replaced by chlorite, magnetite, epidote, and sphene. Thin veinlets of calcite and 

chlorite which cut phenocrysts and groundmass are common. Where cut by younger intrusives, the rocks have 

been thermally metamorphosed ·and the groundmass recrystallized to a mosaic of albite, orthoclase, quartz, 

epidote, and magnetite, which superficially resembles 

the original microgranitic texture, but is mineralogically different and is restricted to rocks in which the 

phenocrysts also are recrystallized. 

A few distinctive dikes and sills in the vicinity of 

Kimber Gulch and Whitehorse Gulch are olivine-rich 

syenodiorite porphyry that contains phenocrysts of 

olivine, 'augite, and a few of biotite in a groundmass 

consisting chiefly of cryptoperthite and plagioclase. 

These olivine-bearing rocks cut the Slim Sam Formation and are cut by hornblende- and augite-bearing 

syenodiorite porphyry. They are clearly older than most 

of ~he syenodiorite porphyry and probably are intru-
22 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

QUARTZ 

K-FELDSPAR 

EXPLANATION 

AA DD 

Quarry Gulch intrusive* Indian Creek intrusive* 

BB 

Power Gulch intrusive* 

c 

.Limestone Hills intrusive 

EE 

Mammoth intrusive* 

Diorite or 

gabbro 

Diorite 

K-FELDSPAR PLAGIOCLASE 

ROCKS IN COMPOSITE INTRUSIVE BODIES 

EXPLANATION 

A 

Orphan Boy intrusive 

~ 

Jenkins Ridge stock* 

c 

Aldrich GuiCh intrusive* 

QUARTZ 

Diorite 

K-FELDSPAR PLAGIOCLASE 

K-FELDSPAR 

MAFIC INTRUSIVE ROCKS 

EXPLANATION 

A C 

Vosburg stock Pole Creek stock 

B D 

Edna stock Eagle Basin stock 

QUARiZ 

iorite 

PLAGIOCLASE 

PORPHYRITIC QUARTZ MONZONITE, GRANODIORITE 

PORPHYRY, AND RELATED ROCKS 

EXPLANATION 

A F 

Antelope Creek stock Monte Cristo stock 

B G 

South Fork Stock Bluebird stock 

c~ H 

Keating Gulch stock* Silver Wave stock 

D I 

Diamond Hill stock Vulture stock 

Diorite 

E J 

K-FELDSPAR PLAGIOCLASE Slim Sam stock Lone Mountain stock 

GRANODIORITE AND RELATED ROCKS 

FIGURE 3.-Modes of intrmdve rocks, east flank of Elkhorn Mountains, Mont. Mafic minerals subtracted and mode recalculated 

to 100 percent. *,underscore indicates a calcic (An>Go) rock. 

• 

• 
~· 

t 

IGNEOUS ROCKS 23 

sive equivalents of some of· the lowest Elkhorn Mountains Volcanics. 

The Rattlesnake intrusive body, which is crudely laccolithic or sill-like, e:rtends 10 miles southward from 

Crow Creek to about a mile beyond the southern limit 

of· ·the area; it is 1-31h miles wide. Most o~ the rocks 

are syenodiorite or granodiorite porphyry consisting 

largely of hornblende, augite, and plagioclase in differing proportions. Quartz 'and K-feldspar are sparse or 

unrecognizable in thin section but are significant components of norms of similar rocks. The base of the mass, 

along its west side, though discordant in detail, is 

grossly concordant with rocks of the Colorado Formation, the Slim Sam Formation, and the lower part of 

the Elkhorn Mountains Volcanics. The contact with the 

Elkhorn Mountains Volcanics that forms the roof is 

irregular but appears to be broadly concordant. Remnants of older rocks within the mass are abundant, and 

some are large; most of the larger remnants are severely 

faulted and 'are locally folded. The mass is thought to 

have been emplaced as follows: sheets of magma, probably fed through a north-trending zone of steep fractures, were emplaced along bedding surfaces in the 

Colorado Formation and younger rocks. As the sheets 

grew they swelled and coalesced, disrupting septa of the 

host rocks and strongly deforming and embaying the 

roof rocks, which were at most a few thousand feet 

thick. Locally, the magma probably burst through to 

the surface as suggested by the abundance nearby of 

coarse debris of probable mudflow origin consisting of 

the same petrographic types that constitute the Rattlesnake mass. For the most part the roof was shouldered 

up and aside but not engulfed; and ·accordingly, the 

blocks of older rocks are not products of stopping but 

rather are remnants of septa that once separated sills. 

Large inclusions of older rocks are abundant, but small 

inclusions are rare; no features suggest 'assimilation. In 

several places zones of healed intrusion breccia separrute · 

rocks of slightly different lithology or texture. 

The Dutchman intrusive ·body crops out along the 

west margin of the Radersburg quadrangle (pl. 1 and 

section/-/') in sees. 29 and 32, T. 5 N., R.1 W. Its westward continuation has 'been mapped 'by Klepper, Weeks, 

and Ruppel (1957, p. 45). The body is strongly discordant along its north and east margins in the mapped area 

but is roughly concordant with the enclosing Devonian 

rocks along most of its west m·argin. Several sill-like 

offshoots extend southward from the body, and the outcrop pattern of the entire intrusive body suggests that 

it is crudely sill-like. Field study, ex·ruminrution of thin 

sections ( ta:ble 1, fig .. 3), and chemical analyses of three 

representative samples (table 1, samples 10, 12, 14) indicate that the Dutchman intrusive is relatively homogeneous and that it consists ·of rocks ranging from 

quwrtz-poor granodiorite to syenodiorite; most of the 

rock has the texture of a porphyry, but some is mediumgrained subequigranular. The Dutchman intrusive 

closely resembles the Rattlesnake mass in ~hemical and 

mineralogical composition but is generally coarser 

grained and contains little Uiiidentifiable groundmass, 

lacks large inclusions, is more regularly shaped, and has 

not st:vongly disrupted the enclosing rocks. These similarities and differences probrubly result from emplacement and crystallization 'of hatches of similar magma at 

significantly different depths beneath the surface, the 

Rattlesnake mass beneath a cover no more than a few 

thousand feet thick a.nd the Dutchman mass a:bout ·a mile 

deeper. 

YOUNGER INTRUSIVE ROCKS 

The Elkhorn Mountains Volcanics and related shal1ow intrusives are cut and tJlrermally metamorphosed by 

younger Upper Cretaceous and Paleocene generally 

coarser grained intrusive rocks, in stocks, dikes, plugs, 

and bodies of unknown subsurface configuration. The 

rocks range from gabbro to granite (table 1, fig. 3) ; most 

are medium- to coarse-grained equigranular rocks; a 

few of the smaller bodies are fine-grained porphyry. 

The rocks -can readily be assigned to either a mafic group 

or an intermediate and felsic group. The mafic rocks 

range from gabbro to calcic syenite. The more felsic 

rocks range from syenodiorite to granite; rocks in the 

larger bodies are mainly equigranular and range from 

syenodiorite to quartz monzonite. Three stocks of porphyritic quartz monzonite and several small bodies of 

granodiorite porphyry, quartz monzonite porphyry, and 

leucocratic granite porphyry cut the Elkhorn Mountains Volcanics and related intrusives in the north \half 

of tJhe area. In the central part of the area a few composite bodies similar to the composite in-trusives of the 

southern Elkhorn Mountains (Klepper and others, 1957, 

p. 51) consist of one or more mafic rock types and one or 

more felsic rock types. In each of the few examples 

where ·age relations can be determined, felsic intrusive 

rocks are younger than associated mafic intrusives. 

Relative age of the several compositional and textural 

varieties of felsic and intermediate intrusive rocks, how-. 

ever, could not be determined with certainty. In only 

one place, near Hassel, are rocks of two different masses 

of intermediate and felsic intrusive rocks in contact with 

one another. There, the Silver Wave and Diamond Hill 

stocks are in contact, but relatively intense hydrothermal alteration and poor exposures obscure the contact 

relations. The Silver Wave stock is finer grained and 

rather strongly hydrothermally altered. The Diamond 

Hill stock is porphyritic in places, is cut by aplite and 

quartz veins, and is less affected by hydrothermal altera-
24 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

tion in the vicinity of the contact with the Silver Wave 

stock. As shown on section 0-0, plate 1, the Silver Wave 

stock is interpreted as the older of the two. 

Differences in types of mineral deposits seem to be 

associated with different rock types of the intermediate 

and felsic group. South of the Diamond Hill mine, for 

example, the intrusives are dominantly intermediate in 

composition and are equigranular and fine to medium 

grained, and the veins are notably poor in quartz and 

have been valuable primarily for gold associated with 

abundant pyrite. North of the Diamond Hill mine, the 

intrusives are generally more felsic and richer in quartz, 

and some are porphyritic; in this area the veins are 

dominantly quartz and havev been valuable for their 

gold, silver, lead, and zinc content. 

MAFIC ROCKS 

The Aldrich Gulch intrusive, (pl. 1, section D-D) 

Jenkins Ridge stock, and two dikes, all west of Glendale 

Butte in the south -central part of the mapped area, the 

Orphan Boy intrusive (pl. 1, section A-A) in the 

northern part, and the mafic parts of several small composite intrusive bodies in the central part consist mainly 

of calcic syenodiorite and calcic monzonite but include 

rocks ranging from mafic syenite to gabbro. Most of the 

mafic rocks form part or all of small discordant stocks 

or plugs; a few form dikes. An intrusive just west of 

the mapped area similar to the Orphan Boy intrusive 

has been described by Smedes ( 1960; 1966, p. 64, 66) as 

a segment of a ring dike marginal to the Antelope Creek 

stock. 

Most of tihese rocks ~are dark gray, medium ,to coarse 

grained, equigranular to porphyritic, and are characterized especially by their content of mafic minerals, which 

in most rocks ranges from 30 to 55 percent, and by their 

relatively high potassium content in the form of Kfeldspar or biotite, or bdth. The rocks are hypidiomorphic granular to porphyritic aggregates of feldspar and 

mafic minerals, and they contain phenocrysts and phenocrystic clusters of mafic minerals. The mafic minerals 

are olivine, augite, and hypersthene, hornblende, and 

biotite in different combinations and proportions. Olivine is in part replaced by serpentine and in part rimmed 

by pyroxene and biotite; pyroxene is replaced and 

rimmed by hornblende and biotite, and hornblende and 

biotite are replaced in part by chlorite and sparse calcite. Magnetite and perh31ps other black opaque minerals 

are associated with the ·mafic minerals and probably formed as a result of release of iron during replacement reactions. Plagioclase, forming 25-50 percent 

of different rocks, ranges from labradorite ~to andesine, 

and typically is in slightly zoned subhedrallaths smaller 

than the associated mafic minerals. The cores of some 

crystals are slightly altered. K-feldspar in subhedral to 

anhedral interstitial grains commonly forms about 10 

percent of most of the rocks examined under the microscopic but reaches as much as 30 percent. The K-feldspar 

includes cryptoperthite, anorthoclase or sanidine, and 

cloudy microperth.ite thrut poikilirtically encloses plagioclase and in places is in micropegmatitic intergrowth 

with quartz. The K-feldspar is intergrown with the 

marginal part of plagioclase crystals in some rocks. A 

small amount of quartz, commonly less than 5 percent, 

is present in small anhedral interstitial grains. Accessory minerals include apatite, sphene, and zircon. 

The mafic rocks are younger than the Elkhorn Mountains Volcanics and related intrusive rocks and, wherever relations can be determined, they are older than 

felsic intrusive rocks. They are discordant and intrude 

rocks of the Colorado and Slim Sam Formations and 

the Elkhorn Mountains Volcanics. One of the dikes appears to be truncated by the South Fork stock. Rocks 

of the Orphan Boy intrusive (pl. 1, section A-A'), 

which strongly metamorphosed Elkhorn Mountains 

Volcanics, are in turn recrystallized by granodiorite of 

the Antelope Creek stock. The rocks are similar to the 

mafic facies of the composite intrusives of the southern 

Elkhorn Mountains (Klepper and others, 1957, p. 51-

52), and to.ga:bbroic to monzonitic rocks that are among 

the oldest rocks of the Boulder batholith (Knopf, 1957, 

p. 90-91). 

INTERMEDIATE AND FELSIC ROCKS 

Rocks ranging in composition from syenodiorite to 

granite (table 1, fig. 3) comprise many stocklike intrusive masses along the east flank of the Elkhorn Mountains. All these masses, which range in area from a few 

acres to a few square miles, are sharply discordant, and 

different masses cut folded rocks ranging in age from 

Devonian to Late Cretaceous. Most consist of mediumto coarse-grained subequigranular rocks, but a few small 

bodies consist of fine-grained porphyry and a few are 

coarse grained ~and porphyritic. Three groups are defined on the basis of composition and texture: ( 1) granodiorite and related rocks, (2) porphyritic quartz monzonite, and ( 3) granodiorite porphyry and related rocks. 

GRANODIORITE AND RELATED ROCKS 

Rocks clustering near the boundary between granodiorite, syenodiorite, quartz monzonite, and monzonite (table 1 ; fig. 3) are the most abundant and widespread of the younger intrusive sui1te and make up most 

of the larger masses of th31t suite. The rocks ~are mainly 

medium gray to greenish gray, fine Ito ·medium grained, 

1and subequigranular; seriate or porphyritic textures 

are rare. Mineral composition falls generally within the 

f 

•ll' 

' 

•• 
.• · 

' 

IGNEOUS ROCKS 25 

following lim~ts: plagioclase, 40-60 percent; K-feldspar, 15-35 percent; quavtz, 1-20 ,percent; and ma;fic 

minerals, 15-35 percent. Plagioclase ty:pically is in 

weakly 2ioned subhedral .to ·anhedral crystals !that range 

in composition from '3ibouit An55 at the core .to An35 'at 

the border; rthe highest and lowest An contents ·observed 

were An75 in a core an An25 in a rim. The centers of 

zoned crystals are replaced to some extent by clinozoisite or epidote, calcite, albite, sericite; or by mixtures 

of .these minerals. Most of the K-feldspar is orthoclase, 

but l·ocally, 'anorthoclase ·and (or) orthoclase cryptoperthi,te( ~) are ·present. K-feldspar forms anhedval interstitial g~airis, .which ·are generally smaller rbhan the 

accompanying grains of plagioclase, and larger irregularly shaped crystals that poikilitically enclose and replace plagioclase and mafic minerals. Par:t of the !{-

feldspar is in micro:pegmatiitic intergrowrth with quartz. 

The orthoclase typically is cloudy and speckled with 

brown, semi opaque mineral dust; anorthoc1ase and (or) 

orthodase cryptoper:thite ( ~) are clear. Quartz forms 

small interstitial anhedral grains ·and appears Ito have 

been the last mineral to crystallize. 

The m,afic minerals ·are hornblende, pyroxene, and 

biotiite. They range in size from grains a few hundredths 

of ,a millimeter long in the groundmass to phenocrysts 

as mubh as 20 mm long. The m·afic grains in most roclns 

are about rthe same size ·as the felsic ones, but hornblende 

and pyroxene crystals are generally larger !than biotite 

crystals .in the same rocks. Hornblende is .the dominant 

mafic mineral in ·most rocks, ~and it occurs as suhhedral 

to ragged anhedral crystals that 'are partly ·altered to 

bioti:te, chlorite, epidote, and calcite. Pyroxene is widely present burt is rarely dominant; it is in ragged subhedral crystals rthat typically ~are rimmed and partly 

replaced by uralitic hornblende rand brown biotite. 

Most pyroxene is colorless or faintly pleochroic from 

colorless to pale green, has an extinction angle (Z/\c) 

of 45-50°, a 2V of 40-60°, and is 1probably pigeonitic. 

Biotite typically is in small subhedral to anhedval flakes 

replacing hornblende ~and pyroxene or, less commonly, 

is disseminated through the rock; much of it is partly 

al.tered rto chlorite and epidote. 

Accessory minerals includes apatite, sphene, magnetite, and small amounts of hematite and ilmenite commonly 'associated with par:tly altered mafic minerals ·and 

very sparse small crystals of zircon. 

Lone Mountain stock.-The Lone Mountain stock, 

named and described by Freeman, Ruppel, and Klepper 

(1958, p. 520-521), underlies an ·area of several square 

miles in the south of the mapped area (pl. 1 and section 

F -F' -F"). The stock cuts sharply across gently dipping Elkhorn Mountains Volcanics and has thermally 

metamorphosed them in a zone at least several hundred 

feet wide. The stock consists of medium-grained subequigranular rocks of uniform appearance that cluster 

around the intersection of the monzonite, syenodiorite, 

quartz n1onzonite, and granodiorite fields (fig. 3 ; table 

1) ; locally, near the margins subequigranular rocks 

grade into rocks with porphyritic or seriate textures. A 

lead-alpha age of 76 million years was determined on 

zircon from rocks of the Lone Mountain stock (Jaffe 

and others, 1959, p. 81). 

Keating Gulch stock.-The Keating Gulch stock cuts 

sharply and irregularly across upper Paleozoic and 

Mesozoic rocks. Two small bodies west of the stock and 

one to the south consist of rocks thait resemble those 

in the stock ~and are probably connected with it at depth. 

The stock ~and its outliers consist of gray or greenishgray medium-grained, looailly porphyritic rocks; the 

main body of the stock consists of granodiorite and 

syenodiorite, and !two smaller outliers to the west are 

monzonite (fig. 3 ; table 1) . The marginal part of !the 

stock includes quartz-rich rocks of .peculirar !texture that 

are probably metasoma;tically replaced sedimentary 

rocks. 

Slim Sam stock.-The intrusive body cutting upper 

Paleozoic, Jurassic, and Lower Cretaceous sedimentary 

rocks in and west of sees. 20, 21, 28, and 29, T. 6 N., 

R. 1 W., (pl. 1, section D-D') was named the Slim Sam 

stock by Klepper, Weeks, and Ruppel (1957, p. 50}, 

who briefly described the rock and reported one chemical 

analysis. Though exposures are poor, the map pattern 

suggests that the stock is discordant in most places but 

may be concordant along its northeast margin. The 

stock consists of relatively homogeneous mediumgrained granodiorite and quartz monzonite (fig. 3). 

South Fork stock.-The South Fork stock, along the 

South Fork of Crow Creek, is a discordant mass of 

medium- to fine-grained subequigranular hornblendebiotite quartz monzonite near granodiorite (fig. 3) that 

cuts folded sedimentary rocks of the Colorado and Slim 

Sam Formations. The rocks resemble those of the Slim 

Sam stock about a mile to the south. 

Silver Wave stock.-The Silver Wave stock on Indian Creek cuts discordantly across the Elkhorn Mountains Volcanics and is inferred to be cut by the Diamond 

Hill stock (pl. 1, sections B-B', C-C'), though contact 

relations are obscured by hydrothermal alteration and 

poor exposure. The rocks of three large outcrop areas 

are believed to be connected at shallow depth. The 

northern part of the stock is separated from the central 

part by Holocene alluvium; the central and southern 

parts are separated by a plate of intensely recrystallized 

Elkhorn Mountains Volcanics. A small elongate body of 

intrusive rock along Cold Springs Gulch is similar to 

the stock and probably part of it. The stock consists 
26 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

mainly of syenodiorite but includes granodiodte, monzonite, and diorite (fig. 3). Most o£ the rocks are fine- to 

medium-grained subequigranular; some are seri3tte and 

a few are porphyritic. Grain size o£ rocks near the border of the stock averages 1 mm or less, ·and grain size of 

rocks from the central part of the stock averages 1-2 

mm. The rocks near the border of the stock contain a 

few small inclusions of country rock and are a little 

darker than the coarser grained rocks in the central 

part. In the vicinity of the Diamond Hill mine the rocks 

are hydrothermally altered. 

Diamond Hill stock.-The Diamond Hill stock, at the 

junction of Indian Creek and its West Fork (pl. 1, 

section C-C') , consists of biotite quartz monzonite of 

uniform composition (fig. 3). The dominant rock type 

is medium gray, fine to medium grained, sparsely porphyritic, and contains biotite and plagioclase crystals as 

much as 5 mm long in a xenomorphic intergrowth of 

smaller grains of K-feldspar, quartz, and plagioclase. In 

a few places the rock is conspicuously porphyritic, and 

it contains euhedral pink phenocrysts of K-feldspar as 

much as 4 em long and 2 em wide that enclose small 

biotite flakes. Locally, small pegmatite and aplite dikes 

and irregular veinlets of quartz, some o£ which contain a little pyrite, chalcopyrite, and galena, cut the 

stock. The surrounding volcanic rocks are thermally 

metamorphosed in a zone at least several hundred feet 

wide. Along the east margin of the stock, the contact 

metamorphic zone is thin or absent and is inferred to 

have been cut off by a fault (pl. 1). 

Monte Oris to stock.-The Monte Cristo stock, near 

the junction of Beaver Creek and its South Fork, discordantly cuts rocks of the middle member of the Elkhorn Mountains Volcanics and related intrusive rocks. 

The rocks of the stock are medium- to light-gray finegrained subequigranular quartz monzonite and granodiorite (fig. 3). Mafic minerals are slightly more abundant 

near the margins of the stock than in the central part. 

A. representative sample from the peripheral part is 

biotite granodiorite (fig. 3); one from the central 

part is quartz monzonite that contains fewer mafic minerals and has more hornblende than biotite. The central 

part o£ the stock is cut by a few small aplite dikes and 

many quartz veinlets, adjacent to which the quartz monzonite is bleached. 

The volcanic rocks adjacent to the stock are thermally 

metamorphosed and cut by a few dikelike apophyses of 

the stock in a zone several hundred feet wide. Several 

pyritic quartz veins have been explored within the stock 

and the thermally metamorphosed border rocks. 

.Antelope Oreek stook.-The Antelope Creek stock, 

in the northwest corner of the area (pl. 1, section .AA'), extends northward and westward beyond the 

mapped area. The part to the north was briefly described by Mertie, Fischer, and Hobbs (1951, p. 44-45), 

and that to the west, by Smedes (1966, p. 66-67). The 

stock is composed of medium- to fine-grained subequigranular granodiorite o£ uni£orm appearance and composition (fig. 3 ; table 1) . It cuts and strongly metamorphoses rocks o£ the middle member of the Elkhorn 

Mountains Volcanics. The Orphan Boy mafic intrusive 

is separated from the stock by a screen of basalt of the 

Elkhorn Mountains Volcanics; both are thermally 

metamorphosed. The stock is cut by thin dikes of aplite, 

by many stringers and veinlets of quartz., and by a few 

quartz latite dikes. 

Other stooks.-The Bluebird stock, near the Bluebird mine in· sees. 1 and 18, T. 6 N., R. 1 E., and sees. 

12 and 13, T. 6 N., R. 1 W., is composed o£ homogeneous 

medium- to fine-grained subequigranular to porphyritic hornblende granodiorite (fig. 3). It is a small, 

strongly discordant body that cuts the Elkhorn Mountains Vol<canics and related intrusive rocks. 

The Vulture stock, near the Vulture mine in sees. 8 

and 9, T. 1 N., R. 1 W., is a small discordant body of 

fine- to medium-grained subequigranular hornblendebiotite quartz monzonite cutting the Elkhorn Mountains Volcanics (fig. 3). 

PORPHYRITIC QUARTZ MONZONITE 

The Vosburg, January, and Freiburg stocks, along 

Weasel Creek in the north-central part of the mapped 

area, consist of porphyritic quartz monzonite (fig. 3) 

distinctly different in appearance from the subequigranular granodiorite and related rocks. The rock is a 

medium-grained aggregate of anhedral quartz, K-feldspar, subordinate plagioclase, arrd sparse biotite and 

hornblende studded with large tabular phenocrysts (as 

much as 6 em iong) of K-feldspar, somewhat smaller 

subhedral phenocrysts o£ plagioclase, and sparse phenocrysts of chloritized biotite and hornblende. The K-

£eldspar phenocrysts are euhedral in general outline but 

in detail are ragged and intergrown with granular 

quartz. Many K-feldspar crystals enclose small ragged 

grains o£ plagioclase, hornblende, and biotite. Some are 

faintly perthitic, and some have a faint grid twinning 

suggestive of microcline. In parts of the Vosburg stock 

the K-feldspar phenocrysts are strongly alined. Plagioclase is in zoned subhedral laths most of which have 

oligoclase rinds and are partly embayed by granular 

aggregates of finer grained K-feldspar and quartz. The 

maximum range of composition observed is A.n4s to.A.n2o· 

Plagioclase in the groundmass is in ragged to anhedral 

grains, some o£ which are remnants of larger crystals 

, 
\ 

• 

IGNEOUS ROCKS 27 

that have be·en replaced by grooular quartz and K-

:feldspar, and some o:f which are more sodic plagioclase 

that crystallized along with the K-:feldspar and quartz. 

Hornblende and biotite are in embayed euhedral crystals ana ragged grains; both minerals a.re largely replaced by intergrowths of cfulorite, epidote, calcite, iron 

oxide, apatite, and sphene. Accessory minerals are tourmaline, a;patite, s~hene, zircon, and possible allrunite. 

The Vosburg, January, and Freiburg stocks lie along 

the N. 30° E.-trending Weasel Creek :fault zone, which 

may have controlled their emplacement. They cut discordantly across and strongly metamorphose the Elkhorn Mountains Volcanics. The contact zone of the 

January stock is cut by narrow aplite dikes. Most of 

the exposed rock in the January and Freibtirg stocks is 

hydrothermally altered; the altered rocks have relict 

textures similar to rocks of the Vosburg stock. Some o:f 

the larger mineral deposits in the area are within and 

adjacent to these stocks. 

QUARTZ MONZONITE PORPHYRY AND RELATED ROCKS 

The rocks, characterized by abundrunt phenocrysts in 

a fine-grained groundmass, :forin :four small stocks 

in the central and northern parts of the mappeld area. 

They range in composition :from granodiorite porphyry 

to granite porphyry, but all contain comparatively large 

phenocrysts of plagioclase, K-:feldspar, and quartz in a 

fine-grained sugary-textured groundmass composed of 

K;.:feldspar, quartz, and biotite. Typically, phem.ocrysts 

and groundmass each make up about half the rock, 

although proportions o:f phenocrysts to gDoundmass anJd 

proportions of plagioclase, K-feldspar, and quartz 

phenocrysts to each other differ :from one rock to 

another. 

In most o:f the rocks the most abundant phenocrysts 

are plagioclase in zoned (An27_55) subhedral laths as 

much as 5 mm long; in the more silicic rocks o:f the 

group the plagioclase is sericitized or replaced by 

groundm3!Ss minevals. K-:feldspar phenocrysts generally 

are less abundant than plagioclase phenocrysts but are 

conspicuous because o:f their large size, as much as 1.5 

by 3 em, and their su'bhedral to euhedral shape. K-.:feldspar is perthitic to microperthitic, and in general the 

degree of sericitization or replacement of plagioclase 

by groundmass increases with increasing K-:feldspar 

content. Quartz phenocrysts are mostly subhedral to 

euhedral and are a :few millimeters across. Biotite, 

partly altered to chlorite, opaque iron oxides, and apatite, :forms small euhedral to subhedral phenocrysts 

that contain inclusions of zircon and rutile. 

The groundmass is a fine-grained ( 0.1 mm or less) 

sugary-.textured aggregate o:f K-:feldspar, quartz, and 

subordinate biotite; it typically irregularly embays the 

:feldspar and quartz phenocrysts. 

Eagle Basin a;nd Ooburg Basin stooks.-Two small 

stocks o:f quartz monzonite porphyry grading to granodiorite porphyry (fig. 3) cut Elkhorn Mountains Volcanics in Eagle Basin and Coburg Basin, in the westcentral part o:f the mapped area (pl. 1, section B-B'). 

Some of the rocks in each stock contain conspicuous 

large subhedral to euhedral phenocrysts of K-:feldspar, 

and the plagioclase in these rocks is more altered than 

that in rocks lacking large crystals of K-:feldspar. Small 

aplite dikes and quartz veins ·also are common in the 

· K-:feldspar-rich rocks. 

Edna and Pole Oreek stocks.-The rocks in the Edna 

and Pole Creek stocks in the vicinity of Beaver Creek 

are quartz monzonite porphyry and granite porphyry 

(fig. 3) and are more siliceous and · pota·ssic than the 

otherwise similar rocks in Eagle Basin and Coburg 

Basin. Both stocks intrude Elkhorn Mountains Vol-

·Canics. 

The Edna stock (pl. 1, section A-A') is composed 

of light-colored granite porphyry that contains large 

phenocrysts of K .::feldspar. The stock and the rocks 

around it are cut by many quartz veins, some o:f which 

have yielded gold and lesser amounts o:f silver, lead, 

zinc, and copper. The veins are bordered by zones of 

intense sericitic alteration. 

Rocks in the Pole Creek stock are similar to those 

in the Edna stock, but they contain less quartz and 

more plagioclase (table 1) and are quartz monzonite 

porphyry rather than granite porphyry. The rocks in 

these stocks are similar, and both are cut by many 

quartz veins, but many veins in the Edn·a stock are 

sulfide-bearing, whereas :few in the Pole Creek stock 

are. 

COMPOSITE INTRUSIVE BODIES 

Most o:f the composite intrusive bodies are in the central part o:f tJhe ma;pped area, south, west, and east o:f 

the Silver Wave stock. These bodies o:f Late Cretaceous 

and Paleocene ~ge are strongly discordant irregular 

plugs, small stocks, or parts of larger subjacent bodies 

and consist of rocks that are indistinguishable :from some 

of the mafic and :felsic rock types already described. 

Whenever possible, the mafic and :felsic components 

o:f the composite intrusives have been mapped separately and 'are identified on the geologic map (pl. 1) 

by the same symlbols used :for the same rock types in the 

more homogeneous intrusive masses. Oontacts between 

the different rock types in the composite intrusives are 

rarely exposed, but rthe ·absence o:f intermediate rock 

types suggests that the contacts a~re sharp rather than 
28 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

gradational. In each instance where unequivocal relations could be determined, felsic rocks cut mafic rocks. 

The composite intrusive bodies of the east flank of 

the Elkhorn Mountains are similar to the composite or 

hybrid intrusives in the southern part of the Elkhorn 

Mountains described by Klepper, Weeks, and Ruppel 

(1957, p. 51-52). 

Limestone Hills intrusive.-The Limestone Hills 

composite body, cutting upper Paleozoic sedimentary 

rocks in sec. 20, T. 6 N., R. 1 E. (pl. 1, section D-D'), 

consists of a central mass of syenodiorite rimmed by 

light- to medium-gray, medium- to fine-grained chlorite 

granite. The contact is nowhere exposed, but as intermediate rock types are lacking, it is inferred to be sharp 

rather than gradational. The bulk of the intrusive is a 

dark-gray porphyritic olivine-biotite-pigeonite syenodiorite (fig. 3), consisting of a:bout 30 percent pigeonite 

and 7 percent olivine, mostly as phenocrysts as much as 

3 mm across, in a fine-grained groundmass of plagioclase, K-feldspar, and quartz. The two small tongues at 

the south margin of the stock are medium-dark-gray 

porphyritic biotite-pigeonite syenodiorite; about 20 percent of the rock is phenocrysts of pigeonit.e as much as 

2.5 mm across; the groundmass is made up of grains 

less than 0.6 mm a;cross. 

The mineral composition of the single thin section 

of granite examined is 54 percent K-feldspar, 18 percent plagioclase, 20 percent quartz, 7 percent chlorite, 

and 1 percent calcite. Plagioclase ( sodic andesine) tablets 0.5-0.8 mm long are intergrown with cloudy anhedral to subhedral grains of K-feldspar. Some of the ta;blets have a distinctly ragged appearance and appear to 

have been emibayed along margins and twin planes of 

K-feldspar. Quartz is present in small anhedral interstitial grains and in micropegmatite. The mafic mineral 

was probably originally biotite, but it has been completely replaced by chlorite ( penninite ~) . 

Quarry Gulch intrusive.-About a mile west of the 

Limestone Hills intrusive the Quarry Gulch mass cuts 

sharply across the middle unit of the Colorado Formation in sec. 19, T. 6 N., R. 1 E (pl. 1, section D-D') and 

metamorphoses it in an aureole at least a few hundred 

yards wide, relations suggesting that the intrusive body 

enlarges in depth. Most of the rock is fine-grained subequigranular chloritic quartz monzonite (fig. 3), but 

mafic augite-biotite-hornblende syenodiorite crops out 

locally at the margin. The quartz monzonite is strongly 

altered; cores of zoned plagioclase consist largely of aggregates of sericite and clay minerals; the more sodic 

rims are less strongly altered. The original mafic mineral or minerals, probably mainly biotite and perhaps 

including some hornblende, are completely replaced by 

chlorite and opaque iron oxides. The mafic minerals in 

the syenodiorite tend to be clustered, and large crystals 

of augite are partly rimmed and replaced by smaller 

subhedra of horrrblende and biotite. 

Power Gulch composite body.-The Power Gluch 

·body cuts Elkhorn l\1ountains Volcanics in the central 

part of sec. 11, T. 6 N., R. 1 W., and consists of two 

mappable masses of olivine-bearing mafic rocks that 

partly rim a larger mass of quartz monzonite (pl. 1) ; 

similar mafic rocks in bodies too small to map crop out 

elsewhere along the margins. The mafic rocks are calcic 

syenodiorite and grubbro (fig. 3) , that contain 50-70 percent mafic minerals, including olivine, hypersthene, 

augite, and biotite, along with subordinate plagioclase 

and K-feldspar. Plagioclase is notably sparse in the 

olivine-rich rocks. The felsic rocks of the mass are remarkably uniform quartz monzonite and closely resemble the K-feldspar-rich rocks of the granodiorite and 

related-rocks suite. 

The typical grain size of all the rocks ranges from 1 to 

2 mm. Texture of the mafic rocks and most of the felsic 

rocks is subequigranular to seriate; some of the lighter 

colored rocks have an inconspicuous porphyritic appearance owing to a tendency toward clustering of mafic 

minerals. 

Mammoth composite body.-The Mammoth intrusive 

body, on the west side of Eagle Creek in the N1;2 sec. 28, 

T. 7 N., R. 1 W., cuts Elkhorn Mountains Volcanics and 

related older intrusive rocks and consists of monzonite 

and calcic syenodiorite. The monzonite is a subequigranular medium- to medium-fine-grained felsic rock. The 

calcic syenodiorite resembles medium- to coarse-grained 

mafic rocks in other mafic and composite intrusive 

bodies; it grades in places to a finer grained facies and 

appears to be chilled against the Elkhorn Mountains 

Volcanics and associated older intrusive rocks. 

Indian Oreek composite bodies.-Four small intrusive 

bodies of varied composition form a cluster in the 

Kootenai Formation north of Indian Creek in the S1V~ 

sec. 29 and NW% sec. 32, T. 7 N., R. 1 E. The intruded 

rocks are intensely thermally metamorphosed around 

and between these four bodies, and the metamorphism 

suggests that they are the exposed apices of a larger 

stock. The largest area of outcrop is syenodiorite ; the 

sill-like projection from the south margin of this mass 

is olivine-biotite-augite syenodiorite. A small dike of 

hornblende-quartz monzonite cuts the north westernmost 

small mass of syenodiorite and indicates that the felsic 

rocks are younger than the mafic rocks. 

The mafic rocks in these small intrusive bodies are 

similar to the mafic rocks elsewhere along the east flank 

of the Elkhorn Mountains. The felsic rocks, though 

)· 

' 

• 
IGNEOUS ROCKS 29 

severely altered, appear to have been subequigranular 

monzonite or quartz monzonite consisting of approximately equal amounts of plagioclase ( sodic andesine~) 

and }(-feldspar, subordinate quartz (and as much as 10 

percent each of epidote and opaque iron oxides. The 

plagioclase is strongly embayed and replaced by Kfeldspar. 

THERMAL METAMORPHISM 

Most intrusives associated with the Elkhorn Mountains Volcanics were emplaced at relatively shallow 

depth into volcanic rocks or older sedimentary rocks. 

Visible effects of metamorphism are generally negligible adjacent to small dikes and sills. Adjacent to larger 

bodies, host rocks typically are baked or hardened and 

perhaps slightly recrystallized in zones ranging from 

a few inches to several tens of feet across. The most 

conspicuous metamorphic effect is adjacent to the 

Dutchman intrusive, where Paleozoic limestone has 

been recrystallized and notably coarsened as much as 

a few hundred feet from the contact. 

Metamorphism adjacent to younger intrusives is 

much more severe, and the host rocks around some 

bodies have been metamorphosed in relatively broad 

aureoles. Adjacent to the Keating Gulch stock, for example, limestone, impure carbonate rocks, and shale 

have been recrystallized to marble, calc-silicate rocks, 

and hornfels as far as 500 feet from the nearest exposures of igneous rock. Adjacent to the Silver Wave and 

Lone Mountain stocks, Elkhorn Mountains Volcanics 

have been recrystallized to hornfels and, locally along 

fractures, to garnet-epidote-chlorite rock, in a zone sevPral hundred feet wide. Lenses of skarn, some of which 

contain pyrite 'and sparse chalcopyrite, have formed in 

carbonate rocks adjacent to several stocks. Volcanic 

rocks have been converted to spotted hornfels for several hundred feet adjacent to the Vosburg stock. 

The broadest metamorphic aureole and greatest intensity of metamorphism surround the Antelope Creek 

stock and Orphan Boy intrusive. The aureole extends 

at least 1,500 feet south and about a mile east of these 

intrusive masses. The breadth of the aureole suggests 

that intrusive rocks underlie a large area east of the exposed contact at relatively shallow depth. Immediately 

adjacent to the Orphan Boy mass, rocks of the middle 

member of the Elkhorn Mountains Volcanics have been 

converted to granular pyroxene-biotite-hornblendeplagioclase-quartz-K-feldspar rocks with an average 

grain size of about 0.2 mm. Farther to the east and 

south, Elkhorn Mountains Volcanics and Slim Sam 

Formation have been recrystallized to hornfels of varied 

cmnposition and appearance. Nearer the intrusives the 

rocks are wholly or largely recrystallized to fine-grained 

419-246---71----3 

hornfels having different combinations and proportions 

of hornblende, biotite, plagioclase, quartz, cordierite, 

muscovite, K-feldspar, and magnetite; the combinations 

and proportions probably depend largely on the original composition of the rock. Farther away only the 

finer grained components of the original rock are recrystallized; larger crystals of plagioclase and mafic 

minerals in the original rocks are unaffected or recrystallized only at their margins. In general, the average 

grain size of recrystallized rock increases toward the 

stock. Field Tel at ions indicate that rocks adjacent to 

the Orphan Boy intrusive were strongly meta.-

morphosed by it, but that the bulk of the metamorphic 

aureole was formed during emplacement of the Antelope Creek stock, which affected both the volcanic rocks 

and the Orphan Boy intrusive. 

QUARTZ LATITE PORPHYRY DIKES OF EOCENE(?) 

AGE 

North of Beaver Creek, a dike of Eocene(~) age 

cuts the Slim Sam Formation, the Elkhorn Mountains 

Volcanics and associated intrusive rocks, the Orphan 

Boy dike, and the Antelope Creek stock. A short distance to the west . similar dikes cut rocks of the Antelope Creek stock (H.W. Smedes, oral commun., 1966), 

and elsewhere in the region they cut the Boulder batholith. The dikes closely rresemble rund are probably related to the quartz latite Lowland Creek Volcanics 

( Smedes, 1962; Smedes and Thomas, 1965) of early 

Eocene age. The dike rocks are biotite-hornblendequartz latite porphyry and are similar to rocks elsewhere in the region described as dacite by 1\::nopf 

(1913) and by Pardee and Schrader (1933). They 

contain phenocrysts of plagioclase, biotite, hornblende, 

and subordinate quartz in a very fine grained indeterminate groundmass and are classified as quartz latite 

on the basis of chemical analyses of similar rocks reported by Becraft, Pinckney, and Rosenblum (1963, 

p. 26-28). 

BASALT FLOWS 

Remnants of basalt flows crop out along and near 

Crow Creek from Sagebrush Creek eastward. Near 

Cold Springs Gulch, the remnants are parts of at 

least two flows, for near the center of sec. 30, T. 6 N., 

R. 1 E., the vesicular top of one flow grades upward 

into a sublateritic weathered zone as much as 10 feet 

thick thrut is in turn overlain by a basalt flow having 

a chilled vesicular base. About a mile to the south 

(NWll.t,SW~ sec. 31, T. 6 N., R. 1 E.), Oligocene 

sedimentary tuff and an overlying basalt flow are 

separated by 5 feet of unconsolidated prebasalt mantle 

that contains fragments of sedimentary tuff and 
30 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

angular fragments, as much as 9 inches long, of older 

rocks ; the lower 10 feet of the flow consists of pumiceous basalt and dense glass in which are embedded many 

blocks of sedimentary and volcanic rocks as much as 

18 inches 1ong. The form and outcrop relations of the 

basalt flows suggest that they were confined to valleys 

that were as much as 150 feet deep. 

The basalt near Cold Springs Gulch is black to dark 

gray and weathers reddish brown. Some of it is dense 

and porphyritic, smne is amygdaloidal and porphyritic, and some is highly vesicular or pumiceous, glassy 

or finely crystalline, and conspicuously flow-banded. 

The dense porphyritic rocks have an intersertal groundmass of plagioclase, pyroxene, and opaque minerals, 

and phenocrysts of labradorite, pyroxene, opaque iron 

oxides (probably magnetite), and 10livine ( ~) that is 

almost completely replaced by ·a serpentinelike or 

fibrous mineral. In both the vesicular and amygdaloidal 

rocks, the openings are rimmed by mineral aggregates 

that include quartz, chlorite, calcite, leucoxene ( ~) , and 

one or more unidentified minerals, probably zeolites. 

Some specimens consist mainly of glass or devitrified 

glass in which are embedded microlites of plagioclase, 

pyroxene, and iron oxides. The basalt flows rest on 

Oligocene sedimentary tuff and appear to have been 

gently folded with the tuff. This episode of folding 

has not been precisely dated but probably is of late 

Pliocene age (Freeman and others, 1958, p. 527-528). 

It thus seems most probable that the basalt is late 

1\fiocene or ellirly Pliocene, but it could be as old as· 

Oligocene. 

RHYOLITE FLOW 

Scattered outcrops of rhyolite of 1\fiocene or Pliocene 

age along Crow Creek near Jenkins Gulch probably 

are remnants of ·a single flow. The rock is medium gray 

to black, dense to sparsely vesicular, locally scoriaceous, 

and is sparsely porphyritic, and much of it is conspicuously flow banded. Where the base is exposed, the 

underlying rocks and the lower few feet of' the flow 

are oxidized bright red. Columnar jointing is conspicuous, especially near the mouth of Weston Creek. Phenocrysts form 1 percent or less of the rock. Most of 

them are labradorite (An55) in euhedral to rounded, 

partly resorbed crystals; a few are small euhedral 

crystals of pyroxene and an opaque mineral, probably 

mllignetite. The groundmass ranges in texture from 

glassy to microgranular and · contains microlites of 

plagioclase of undetermined composition, a mafic mineral that is probably pyroxene, and opaque ·iron 

minerals. 

That the flow is a rhyolite is shown by the following 

analysis. 

Chem-ical analysis and norm, in percent, of a representative 

sample of rhyolite flow along Crow Oreelc 

[Analyzed by J. M. Dowd, U.S. Geological Survey. Field No. 51RW-C40, 

laboratory No. 53-70CW] 

Chemical analysis 

AlzOa ----------------~-----------------------------

Total Fe as Fe20a----------------------------------

~fgO -----------------------------------------------

CaO -----------------------------------------------

NazO -----------------------------------------------

1{20 ------------------------------------------------ ~'iOa 

P203------------------------------------------------

~n0 -----------------------------------------------

Ignition --------------------------------------------

69.1 

15.0 

3.0 

. 27 

1.4 

3.8 

5.0 

. 34 

. 09 

.10 

2.2 

Sum ------------------------------------------- 100.3 

Norm 

Quartz--------------------------------------------- 25.0 

Orthoclase--------------------~--------------------- 30.0 

Albite---------------------------------------------- 33.0 

Anorthite------------------------------------------- 6. 1 

Corundurnn ------------------------------------------ 1.1 

IIypersthene ---------------------------------------- 2.0 

~agnetite ------------------------------------------ 1.9 Ilmenite _________________ .:___________________________ . 6 

Apatite --------------------------------------------- . 3 

Sum ------------------------------------------- 100.0 

The plagioclase computed in the norm is oligoclase 

(An16), whereas the phenocrysts in the rock are labradorite, about An55• This discrepancy suggests that the 

:few phenocrysts were carried up from depth and were 

not in equilibrium with the lava that rapidly chilled 

to form the groundmass. The partly resorbed form of 

. the labradorite crystals supports this interpretation. 

The rhyolite flow and the basalt flows occupy similar 

settings along and near Crow Creek valley and are 

probably of about the same age. Their relative 'ages, 

however, are indeterminate. 

STRUCTURAL GEOLOGY 

The structure of the mapped area (pl. l) is simple in 

gross aspect but complex in detail. The layered rocks are 

folded into a broad syncline that trends north through 

the central pa·rt of the mapped area. North of Whitehorse Gulch dips are gentler, the rocks are more complexly faulted, and the fold loses its identity; south of 

Crow Creek the large Rattlesnake intrusive obscures 

and partly obliterates the :fold (pl. 1, sections D-D', 

E-E'). Folding in adjacent areas began before volcanism, perhaps before the ·end of Niobrara time and 

certainly before Judith River time (Klepper and others, 

1957, p. 40; Freeman and others, 1958, p. 523-524), and 

was recurrent during and after the period of volcanism. 

I 
• 

STRUCTURAL GEOLOGY 31 

In the east flank area, the contact between the Slim Sam 

Formation and the Elkhorn Mountains Vokanics 'appears to be gradational and there is no evidence of prevolcanic deformation. 

The folded rocks are cut by many north- and easttrending steep faults. Many of these faults displace 

Elkhorn Mountains Volcanics 'and related intrusive 

rocks and were formed following volcanism, during the 

principal episode of folding. Others, however, are occupied by dikes related to the Elkhorn Mountains Volcanics and were formed during volcanism, probably 

during the ea;rly stages of folding. Northwest-trending 

cross faults appear to have formed too, as brittle rocks 

failed to accommod·ate the folding. 

The compression that produced the north-trending 

folQ.s and associated faults culminated in eastward 

thrust faulting on the Lombard fault zone east of the 

mapped area (Freeman and others, 1958, p. 525), and 

the rocks in the rna pped area were broken by northwest- and northeast-trending tear faults at about the 

same time. Still younger are range-front faults, along 

which there has been recurrent movement from preOligocene time to the present (Freeman and others, 

1958, p. 526-529). 

FOLDS 

The pre-Tertiary layered rocks between the Boulder 

batholith and the Missouri River (fig. 4) are folded 

into a series of gently plunging anticlines and synclines 

that trend north to N. 20° E. The mapped area .(pl. 1) 

covers part of the flanks of the two well-defined anti- . 

clines and the intervening broad syncline. The anticline to the west is described by Klepper, Weeks, and 

Ruppel (1957, p. 56) and that to the east, by Freeman, 

Ruppel, and l(lepper (1958, p. 524). 

In the central part of the area, in Cold Springs 

Gulch, the syncline is well defined and simple. The east 

limb dips more steeply than the west limb, and both 

limbs are complicated by minor folds. North of Indian 

Creek the S.ilver Wave and Diamond Hill stocks occupy 

the approximate position of the trough (pl. 1, section 

0-0'). Between these stocks and the Whitehorse fault, 

the rocks are massive, ·almost structureless volcanic 

breccias, and. the position of the trough line cannot be 

recognized. South of Crow Creek the rocks have been 

invaded and disrupted by the Rattlesnake intrusive, 

and the synclinal trough has been largely obliterated. 

FAULTS 

FAULTS ASSOCIATED WITH FOLDING 

Faults that formed during folding are the oldest 

faults recognized in the mapped area. They include 

north-trending faults that parallel the axial traces 

of the major folds and east-trending faults normal to 

the trends of the folds. 

The north-trending faults are the oldest faults recognized in the mapped area, and some are contemporaneous with the early part of the folding. Many of them 

are very steep reverse faults, and in a general way they 

flank the synclinal trough, a location suggesting that 

they formed during later stages of folding as the trough 

area was squeezed between the flanking anticlines . .Some, 

however, must have formed during the early stages of 

folding for they are cut by younger faults or are cut 

or occupied by dikes related to the Elkhorn Mountains 

Volcanics tha't were intruded during early stages of 

folding (Freeman ·and others, 1958, p. 524). 

A noteworthy example of these old north-trending 

faults is the Whitehead Ranch fault, which trends 

north-northwest from .the vicinity of the East Pacific 

mine to the Antelope Creek stock. North of Beaver 

Creek it is mostly concealed by gravel flanking the 

range front. South of Beaver Creek it displaces a sill 

of older intrusive rock; to the north, however, it is truncated by a sheet of older intrusive rock. The displacement is greatest just north of Beaver Creek where rocks 

near the base of the middle 1nember of the Elkhorn 

Mountains Volcanics are faulted against rocks of the 

Slim Sam Formation, a stratigraphic displacement of 

about 3,000 feet. A mile south of Beaver Creek the 

displacement is not more than a few hundred feet. 

Though the fault bounds the range north of Beaver 

Creek, the topographic relief and the displacement are 

in opposite directions, and the front of the range is 

a fault-line scarp. 

The east-trending faults which are nearly vertical 

faults along the mountain front south of Winston and 

between Eureka Creek and Slim Sam Gulch, trend 

west-northwest to about east-west and displace Cretaceous sedimentary and volcanic rocks as much as 500 

feet. These faults cut and are cut by north-trending 

faults, and thus both sets formed during the same time 

interval. Both are cut by all the other fault systems. 

The faults most probably are tensional breaks produced 

duringtfolding. 

Faults of similar trend displace Paleozoic and 

Mesozoic rocks as much as 100 feet along the northtrending ridge in the southwest corner of the area. 

These faults are best developed in the Quadrant Formation and are part of a system that . radiates from the 

large dome west of the area (Klepper, and others, 1957, 

p. 57) and that formed during folding as brittle beds 

failed to accommodate. Two of these faults, just south 

of Johnny Gulch, displace a sill of diorite porphyry 

similar to the Rattlesnake intrusive mass. 
32 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

Sulfide minerals, in places in minable concentrations, 

have been deposited along and adjacent to many easttrending faults. 

NORTHWEST- AND NORTHEAST-TRENDING FAULTS 

Northwest- and northeast-trending faults and lineaments are among the most prominent structural features of the east flank of the Elkhorn Mountains, for 

they control parts of the valleys of some of the main 

streams. The Indian Creek fault clearly displaces Cretaceous and older rocks southeast from the Diamond Hill 

stock but is recognized only as a prominent lineament 

to the west. The fault is vertical or nearly so, and the 

movement along it appears to have been principally leftlateral, for ~ast of Hassel it displaces a steep northtrending fault about 2,000 feet. In the Limestone I-Iills, 

east of the mapped area, the fault has been described as 

a tear fault (Freeman, and others, 1958, p. 525). 

Crow Creek parallels Indian Creek and the Indian 

Creek fault but is not so clearly controlled by a fault. 

The valley of Crow Creek and of Wilson Creek, one of 

its principal tributaries west of the mapped area, is almost straight for 12 miles and forms a striking lineament. But if this exceptional valley is controlled by a 

fault, the displacement cannot exceed a few hundred 

feet. Most probably it is controlled by a sheared or 

closely jointed zone along which there has been little if 

any displacement. 

The Weasel Creek fault extends south-southwest from 

the northeast corner of the Elkhorn Mountains along 

upper Weasel Creek to the Kleinschmidt mine and n1ay 

be continuous with a fault in Eureka Creek. The fault 

splits north of the Freiburg stock and may be hinged, 

for north of the Freiburg stock the block east of the 

fault has been dropped, perhaps more than 1,000 feet 

near the front of the range, whereas the block .east of the 

fault in Eureka Creek has been uplifted at least a few 

hundred feet and perhaps as much as 1,000 feet. 

The northeast-trending Whitehorse fault occupies 

upper Whitehorse Gulch and cuts across the spur north 

of Whitehorse Gulch at the front of the range. The 

block north of the fault has dropped 500-1,000 feet. The 

fault may merge with the Weasel Creek fault to the 

west; to the east it is cut off by the range-front fault 

system. 

South of the Vosburg stock a steep fault that trends 

west-northwest and that parallels the Indian Creek 

fault separates rocks of the lower and middle members 

of the Elkhorn Mountains Volcanics. The block north 

of the fault has been dropped at least several hundred 

feet and perhaps much more. The fault terminates eastward against the Weasel Creek fault. West of the 

mapped area (South Fork fault of Smedes, 1966, p. 

97-98) the relative displacement is about 3,000 :feet, 

north side down, and the fault terminates westward 

against the Beaver Creek fault, which is subparallel 

with the Weasel Creek :fault. 

The northeast and northwest-trending faults are complementary sets that occupy theoretical shear positions 

for east-west compression. However, movement on 

many of the northeast-trending faults has been domi~ 

nantly vertical rather than horizontal as would be required in the proposed model. Thus it seems likely that 

many of the faults were subjected to different stresses 

that resulted in vertical displacements after they had 

formed by shearing during east-west compression. Some 

of thmn may also have acted a;s tears during the thrusting that culminated the compressive episode (Freeman 

and others, 1958, p. 525-526). A few small northeastand northwest-trending faults are cut by rocks related 

to the Elkhorn Mountains Volcanics, and thus they were 

formed early in the period of compression. Most or all 

the displacement along the larger faults, such as the Indian Creek and 1Veasel Creek faults, was later, however, 

for these faults displace even the youngest rocks of the 

Elkhorn Mountains Volcanics and related intrusions. 

RANGE-FRONT FAULTS 

A system of linked and branching, roughly northtrending steep normal faults cuts rocks along the front 

of the range between Indian Creek and the Beaver 

Creek fan. The principal fault (or faults) of this rangefront system is covered by fan gravel ; the block to the 

west has been uplifted at least 2,500 feet, for in sec. 8, T. 

7 N., R. 1 E., the middle part of the Kootenai Formation is faulted against rocks of the lower member of the 

Elkhorn Mountains Volcanics. Recurrent displacement 

along some of these faults was an important factor in 

outlining the basin (now the valley of the Missouri 

River) in which Oligocene and younger sediments were 

deposited, and the range was lifted again along some of 

there faults after the youngest rocks and unconsolidated 

sediments in the valley had been deposited (Freeman 

and others, 1958, p. 527-529). The youthful appearance 

of the range front suggests that much of the displacement occurred in late Pliocene or early Pleistocene time. 

In a few places there are faint suggestions of northtrending scarps in fan gravel of probable Pleistocene 

age east of the range front, but there is no indication 

that younger Pleistocene or Holocene fans have been 

displaced. 

OTHER FAULTS 

On the south side of Indian Creek west of Diamond 

Hill, faulted welded tuff and other rocks of the middle 

• 
STRUCTURAL GEOLOGY 33 

member of the Elkhorn Mountains Volcanics form a 

graben within rocks of the lower member. The displacement along the faults that bound the graben need be 

no more than 500 feet, for the surrounding rocks are 

probably within 500 feet of the top of the lower member 

of the formation and the rocks in the graben are the 

basal part of the middle member. The proximity of the 

graben to the Silver Wave stock and its outliers suggests 

that the gra:ben may have formed by collapse of part of 

the roof of a magma reservoir. 

A few other faults have no apparent relation to regional structures, and proba;bly are related to nearby 

intrusive bodies. The sedimentary and volcanic rocks 

within and next to ·the Rattlesnake intrusive body, for 

example, are cut by many short faults of diverse trend. 

These faults almost certainly were formed during the 

emplacement of the body, for they are restricted to the 

rocks near the body and are not part of the regional pattern. Most of these faults are truncated by the nearby 

intrusive, but a few of them displace the intrusive. Most 

complexly faulted are isolated remnants of sedimentary 

rocks surrounded by intrusive rocks, within which displacements of several hundred feet occur. Most of the 

blocks within the intrusive probably were septa between 

sill-like bodies during the early stage of emplacement 

and were severely disrupted during a later more forceful stage of emplacement; some may have been carried 

up by magma that rose along a north-trending feeder 

dike or dikes, and some may have foundered from the 

roof. 

Small faults near the mafic intrusive along the ridge 

south of Jenkins Gulch may be associated with the emplacement of the mafic intrusive or with the emplacement of the South Fork stock. 

SUMMARY OF LATE CRETACEOUS AND TERTIARY 

TECTONICS 

The Elkhorn Mountains, of which the present study 

area is a part, were the site of marine sediinentation 

during much of Paleozoic time, but beginning in Mesozoic time they began to emerge and be covered by nonmarine sediments. The margin of the seas advanced and 

withdrew across the area several times during the Cretaceous, as indicated by several alternations of marine 

and nonmarine rocks. 

Folding and volcanism within or near the area are 

first recorded in rocks of the Slim Sam Formation which 

contains shardlike quartz crystals and feldspar crystals 

and which locally was weakly folded before the Elkhorn Mountains Volcanics were erupted, mainly or entirely during Campanian time. F:aulting and locally 

weak folding accompanied volcanism, but the main episode of folding and most of the faulting followed volcanism. In the Elkhorn Mountains and nearby the major folds trend about north and presumably resulted 

from east-west compressive forces. A single broad 

troughlike fold covers most of the mapped 'area. It is 

cut by many faults that developed at several different 

stages, some during and some subsequent to folding. 

Intrusive masses are abundant in the trough of the 

fold; the earliest of these are hypabyssal bodies related 

to the Elkhorn Mountains Volcanics; many of them are 

concordant or partly concordant and were probably 

injected during the initial stages of folding. 

Many faults formed during folding as rocks in the 

trough were squeezed between adjacent anticlinal or 

do mal folds. Steep north -trending normal ~and reverse 

faults are occupied or cut by older intrusives, and these 

relations indicate that they formed during an early 

stage of folding. East-trending faults of about the same 

age appear to represent failure of brittle rocks in tension. Some north ,and east-trending faults displace intrusive rocks related to the volcanics; others appear 

to control the margins of such bodies; many contain 

concentrations of sulfide minerals. The evidence indicates that the faul:ts are of several different ages, or 

that if all were fonneJd at about the same time, different 

ones were later reactivated. 

The conjugate shear directions for eastward-directed 

compression is represented by ex,tensive northwest- and 

northeast-trending faults. The northwest-trending 

fau1ts ·are well developed; horizontal motion is hrdicated along a few, and vertical movement is dominant 

along most. One of the northwest set of fau}ts becomes 

a tear fault associated with eastward displacement on 

the Lombard fault, a major thrust east of the present 

map area. Dominant movement along the northeasttrending fault appears to have been vertical. ':Dhe pattern of faulting established during folding and the 

emp:lacement of the older intrusives establirshed struc-

~tural blocks which have responded individually to subsequent regional and local stresses. 

The younger intrusives were emplaced after the folding and related faulting. Some of them appear to occupy zones of weakness developed during the deformation. These show little evidence of forceful injection; 

directed textures are present only locally along the 

margins of a few of the larger bodies. 

'I'he east front of the Elkhorn l\{ountains is partly 

bounded by a system of north- to norlhwest-tren!ding 

linked faults, along which the mountains appear to have 

been elevated repeate<lly, and east of which a major 

basin developed during Tertiary time. This fault system is complex and may represent the integrations, at 

least in part, of a number of preexisting faults. 
34 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

MINERAL DEPOSITS 

HISTORY AND PRODUCTION 

The east flank of the Elkhorn Mountains (fig. 4) has 

been a significant, but intermittent, source of metallic 

minerals since about 1866. The total recorded value of 

metal produced there through 1956 is about $17 million 

15' 

0 5 

and is distributed as shown in table 2. Gold accounts for 

about 75 percent of the total value of the metals produced; perhU~ps as much as 10 percent of this was 

won from placers at Radersburg, along Indian Greek, 

and along Weasel Creek; the remainder was from lodes. 

:M;ore than 40 percent of the total value is accounted for 

by gold from the Radersburg (Cedar Plains) district, 

111"30' 

I . 

. 

10 MILES 

EXPLANATION 

D 

Quaternary and Tertiary rocks 

Younger Tertiary intrusive rocks 

.. 

Older Tertiary and Upper 

Cretaceous ·intrusive rocks 

Upper Cretaceous volcanic rocks 

Mesozoic sedimentary rocks 

Paleozoic sedimentary rocks 

Precambrian sedimentary rocks 

Contact 

Mine with recorded production in 

excess of $300,000 

FIGURE 4.-P.vincipal mines and mining areas in Clancy, Townsend, Devils Fence, and Rader~burg quadrangles. 

• 
' 

MINERAL DEPOSITS 35 

TABLE 2.-Summary of mine production, east flank of Elkhorn Mountains 

[Data from Reed (1951), Stone (1911), Winchell (1914), Pardee and Schrader (1933lr_. Corry (1933), Trauerman and Waldron (1940), Reyner and Trauerman (1950), U.S. Geol. 

Survey (1883-1923), and u.S. Bur. Mines (1924-31, 1932-53)] . 

District Period Gold Silver Lead Zinc Copper Total value 

(dollars) (ounces) (tons) (tons) (tons) 

Beaver Creek t __________________ 1864-1904 ______________ $1, 750, 000 730, 000 2, 050 0 ·o $2, 500, 000 1905-28 ________________ 1929-56 ________________ 

TotaL _______________ 

Cedar Plains a ___________________ 1905-28 1864-1904_~------------ ________________ 1929-56 ________________ 

TotaL _______________ 

Park and Hassel a ________________ 1864-1904 ______________ 1905-28 ________________ 1929-56 ________________ 

TotaL _______________ 

Grand totaL __________ 

1 Mainly mines in Iron Age Gulch and Weasel Creek areas. 2 Mainly mines in Radersburg area and Quartzite Ridge area. a Mainly mines in Park and Hassel areas. 

primarily from the !(eating mine. Mines in the Winston 

(Beaver Creek) district, principally those in Iron Age 

Gulch and Weasel Creek, contributed about 30 percent 

of the total value in gold, silver, lead, and subordinate 

amounts of zinc and copper. Most of the remaining 30 

percent is accounted for by gold and, to a lesser extent, 

by silver and lead from mines in the Park and Hassel 

districts, by silver and lead from mines in the Quartzite 

Ridge area, and by byproduct copper from the Keating 

gold mine at Radersburg. 

The initial mineral discoveries in the area were placer 

gold at Radersburg, Weasel Creek, and Indian Creek in 

1866 and 1867. Intensive placer mining in these three 

areas during the next 10-15 years, intermittent smallscale operations thereafter, and mechanized operations 

along Indian Creek during the 1940's yielded gold worth 

at least $1 million and perhaps as much as $2 million. 

Some of the large lode deposits-the !(eating, Diamond 

lEU, and East Pacific-were discovered shortly after 

the placers were found, and most of the more productive lode deposits were known before 1900. 

':Dhe !(eating mine west of Radersburg was long famous for its gold ores. Other principal gold mines along 

the east flank of the Elkhorn Mountains include the 

Ohio !(eating and Black Friday near Radersburg, the 

Diamond Hill, Giant group, and Blacksmith in the 

I-Iassel distrid, the Custer-Iron Age-Hyantha group in 

Iron Age Gulch, and the Marietta in the Park district. 

Some mines have yielded complex ore from which 

gold, silver, and lead, and small quantities of zinc and 

copper have been recovered. Perhaps the principal m·ine 

250, 000 175, 000 950 0 25 500,000 

750,000 300,000 2,400 500 75 1,500,000 

2,750,000 1,205,000 5,400 500 100 4,500, 000 

2, 200,000 No data 70 0 140 2, 310,000 

3, 000, 000 180,000 2,530 0 1, 170 3, 820,000 

2, 700,000 80, 000 650 250 50 3, 000,000 

7, 900, 000 260,000 3,250 250 1, 360 9,130,000 

850, 000 50, 000 0 0 0 900,000 

50, 000 25, 000 220 100 40 100,000 

1,250,000 175,000 2,000 1,250 10 2,200,000 

2,150,000 250,000 2,220 1, 350 50 3, 200,000 

12,800,000 1, 715, 000 10, 870 2, 100 1, 510 16,830,000 

of this type is the East Pacific on Weasel Creek, from 

which ore valued at more than $2 million has been mined 

(Reed, 1951). Other mines that have produced noteworthy quantities of a similar type of ore are the J anuary, Kleinschmidt (Little Olga), Stray Horse, and 

mines of the Custer-Iron Age-Hyantha group. 

Some mines have produced ore valuable mainly for 

silver and lead. Important among these are the Jo 

Dandy, North Home, and Santa Anita, in the Quartzite 

Ridge area. The Ruby n1ine in the same area has yielded 

oxidized zinc-lead ore. 

Different lode mines have been productive at different 

times since about 1870. The greater part of the production from the Beaver Creek district was before 1904, 

from the Custer-Iron Age-Hyantha group, the East Pacific mine, and the l{leinschmidt group; in recent years 

the January mine and the East Pacific mine dumps have 

yielded most of the ore reported from the district. Most 

of the production from the Park district before 1930 

was gold from the Diamond Hill, Blacksmith, and Giant 

group mines; since 1930 the Marietta mine has been 

the largest and the most consistent producer. The Radersburg (Cedar Plains) district is noteworthy for 

gold from mines at Radersburg, particularly from the 

Keating mine which has accounted for more than half 

the output of gold from the district and for most of 

the copper. Mines in the Quartzite Ridge area, the Jo 

Dandy, North Home, Santa Anita, and Ruby, have 

yielded most of the lead and silver from the district. In 

recent years a deposit of titaniferous magnetite near 

Radersburg has been 1nincd on a small scale. 
36 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

TYPES OF METALLIC MINERAL DEPOSITS 

'.Dhe metallic mineral deposits of the area are assigned 

to the groups listed below: 

Auriferous pyrite veirrs (quartz-poor). 

Quartz veins with auriferous pyrite. 

Quartz veins with base and precious metals. 

Silver-lead-zinc replacements in carbonate rock. 

Skarn deposits. 

Titaniferous iron ore. 

Gold placers. 

Most of the deposits (fig. 5) in the Radersburg and 

Hassel areas are auriferous pyrite veins with only minor 

quartz; the deposits in the Beaver Creek area are quartz 

veins with auriferous pyrite and quartz veins with base 

and precious metals. The silver-lead-zinc replacement 

deposits are m·ainly in the Quartzite Ridge area. Skarn 

deposits are small and scattered. Deposits of titaniferous iron deposits of economic interest are known only 

near Radersburg. Gold placers have been worked near 

Radersburg and along Indian and Weasel Creeks. 

Most of the important veins trend near north or near 

east and are steep, but some veins in the Park and Iron 

Age Gulch areas dip gently. Recurrent movement took 

place along most veins during and after mineralization; 

some veins have been brecciated and then healed by two 

or more generations of quartz and sulfide minerals, and 

in some veins brecciated ore is incorporated in gouge. 

Most of the vein deposits :are clustered in the vicinity of bodies of younger intrusive rock, and some veins 

cut these intrusives. Accordingly, a genetic Telationship 

between the intrusives and the veins is inferred. A 

further correlation between type of vein and type of intrusive rock is suggested. South of the Silver Wave 

and Diamond HiH stocks the younger intrusives are 

intermediate in composition, meldium-grained equigranular, virtually lacking in phenocrysts, and are 

cut by almost no aplite and pegmatite dikes or quartz 

veinlets; in this area most of the veins consist of altered 

rock with auriferous pyrite and very lit·tle quartz and 

3ire enclosed by strongly altered rocks. North of the 

Silver Wave stock the intrusives ten!d to be more felsic 

and porphyritic, contain large K-feldspar.phenocrysts, 

and are cut by many quartz veins :and vein~ets and by 

small 31plite arrd pegmatite dikes ; the mineralized veins 

in this area contain quartz as the principal gangue, and 

many contain g·alena, sphalerite, cili.alcopyrite; arseno~ 

pyrite, ·and tetraheJdrite in addition to pyrite. Alteration halos adjacent to veins generally are narrower than 

they are to the south. Genetic relationship of vein type 

to intrusive rock type is most strongly suggested in the 

Hassel area where the Silver Wave stock and surrounding volcanic rocks are cut by abundant quartz-poor auriferous pyrite veins, whereas the nearby more felsic 

Diamond Hill stock is cut by quartz veins with auriferous pyrite and by quartz veins with base and precious 

1netals, hut it lacks quartz-poor auriferous pyrite veins. 

AURIFEROUS PYRITE VEINS (QUARTZ POOR) 

Steep veins that contain auriferous pyrite, sparse 

marcasite, very sparse chalcopyrite, and traces of 

pyrrhotite, galena, sphalerite, and chalcocite have been 

the principal source of go~d in the Radersburg and Hassel areas. The veins are a few inches to several feet wide 

and a few hundred to at least 2,500 feet long. The Keating vein has been exp~ored to a depth of about 1,200 

foot. Simple veins consist of discontinuous bands of 

pyrite sheathed by altered rock along a single fracture. 

In more complex veins, stringers ·and pods of pyrite 

occur in a zone of sheared and strongly altered rock as 

mucih as 20 feet wide. Quartz a.nrd calcite, or other carbonate minerals, are sparse anrd erratically distributed 

as gangue. Rock within and adjacent to •the veins is 

silicified, sericitized, kaloinized, and pyritized. Replacement has been more important than fracture filling. 

The veins are associated with younger intrusive bodies 

' of nonporpihyritic granodiorite and related rocks that 

contain few or no aplite dikes or quartz veins. 

In the zone of oxidation, which in places is at least 

a few hundred feet deep, the pyrite is largely altered 

to i·ron oxides, and the veins are aggregates of varicolored iron oxi1

des, locrully with sparse manganese oxide 

and flecks or films of secondary copper minerals ; the 

pyritized wallrocks are bleached almost white. In the 

Radersburg district, where veins are abundant and oxidation intense, most of the rock in an area a mile long 

by one-half a mile wide is altered and bleached (pl. 1). 

Many veins were profitably mined in the oxidized 

or partly oxidized zone, but only a few veins, such as 

the Keating and Ohio Keating, were profitably worked 

beneath this zone. This strongly suggests that oxidation significantly improved ~the grade of ore, but available assay data are too few to support any quantitative 

estimate. In the Radersburg area, at least, much or most 

of the oxidation was pre-Oligocene, for Oligocene sedimentary rocks overlie the oxidized part of the !{eating 

vein ('\Vinchell, 1914, p. 182). This fact is of possible 

economic importance ; for if similar veins exist beneath 

the relatively thin blanket of Tertiary sediments south 

of the Radersburg district, .the chances are good that 

they are oxidized. 

QUARTZ VEINS WITH AURIFEROUS PYRITE 

Gold veins north of the Hassel area are distinctly 

different from those farther south. They consist mainly 

of quartz but contain auriferlous pyrite, locally in 
.. 

••• 

No. 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

20 

21 

22 

23 

24 

25 

26 

27 

28 

29 

30 

31 

32 

33 

34 

35 

36 

37 

38 

39 

40 

41 

42 

43 

44 

45 

46 

47 

48 

49 

50 

51 

52 

53 

54 

55 

56 

57 

58 

59 

60 

61 

62 

63 

64 

65 

66 

67 

68 

69 

70 

71 

72 

73 

74 

75 

76 

Mine 

Orphan Boy Gold Bug Custer 

Edna No.2 

Hyantha 

Iron Age Maine 

Sunshine 

Monte Cristo 

Freiburg 

Silver Saddle 

East Pacific 

January 

Sunrise 

Stray Horse 

Little Bonanza 

Vosburg 

Kleinschmidt (Little Olga) Champion 

~~1t~~~ef Marietta 

Bullion King Little Annie 

Jawbone 

Park-New Era-Venezuela 

Parker 

Shep 

Buckeye 

Phoenix 

Iron Mask 

JohnL 

Lookout 

Mammoth 

St. Louis 

Queen Bee 

Silver Wave 

Central 

Spring Hill 

Diamond Hill 

Little 'Giant 

Blacksmith 

Bluebird . Monarch 

Quartzite 

Horn Silver 

Rothfus 

Iron Cross 

Jebro 

Rena 

North Butte 

Black Bear 

Beauty 

Surprise 

Delome 

Hidden Treasure 

Greenhorn 

Ohio-Keating Keating 

Etta 

Hard Cash 

Kahoka 

North Home 

Congress 

Iron Age (Allen) Black Friday Bluebird 

Gopher 

Cyclone 

Jo Jo 

Jo Dandy Gold Cross 

Santa Anita 

Ruby (Ruby Silver) 

Monterey 

Ralls 

Quartzite 

Ridge 

district 

0 

MINERAL DEPOSITS 

111 °37'30" 

2 3 4 5 MILES 

EXPLANATION 

Quaternary and Tertiary rocks 

Younger Tertiary intrusive 

rocks 

.. 

Older Tertiary and Upper 

Cretaceous intrusive rocks' 

Upper Cretaceous volcanic 

rocks 

· Mesozoic sedimentary rocks 

Paleozoic sedimentary rocks 

Contact 

u 

D 

Fault 

Dashed where approximately 

located. U, upthrown side·; 

D, downthrown side 

----+----

Syncline, approximately located 

• Quartz vein with base and 

precious metals 

... Quartz vein with auriferous 

pyrite 

• 

Auriferous pyrite vein 

(quartz poor) 

• Silver-lead-zinc replacement 

deposit 

• 

Skarn deposit 

Titaniferou!J iron deposit 

Gold placers 

Size of symbol indicates relative 

size of deposit 

37 

FIGURE 5.-Distct:bution of metallic mineral deposits along the east :flank of the Elkhorn Mountains. Geology generalized from 

plate 1. 

419-246--71----4 
38 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

abundance, and in places ankerite or another carbonate 

mineral. Some veins contain a little molybdenite a:r:td 

chalcopyrite, and a few contain sparse arsenopyrite, 

galena, and sphalerite. Veins of this type are mainly 

and perhaps entirely fissure fillings, and most of them 

are frozen to their walls. Many veins are less than a 

foot thick, but a few veins locally are as much as 4 feet 

thick. They occupy fractures of diverse trend, most of 

which dip less than 30°; none were observed to dip more 

steeply than 45 o. In several places veins of this type 

are cut by steeply dipping base- and precious-metal 

veins; the reverse relationship has not been observed. 

Veins of this type have been important sources of gold 

in the Iron Age Gulch area and have yielded some gold 

in the Weasel Creek and Park mining areas; much of 

the gold has been from oxidized or partly oxidized 

parts of veins. Most of the veins of this type are associated with felsic intrusive rocks that are porphyritic 

·and that are cut by numerous quartz veins and 

· stringers. 

. QUARTZ VEINS WITH BASE AND 'PRECIOUS METALS 

Many veins in volcanic and intrusive rocks north of 

Indian Creek and a few south of Indian Creek contain 

gold and silver and most or all of the following metallic 

minerals : pyrite, galena, sphalerite, arsenopyrite, chalcopyrite, and tetrahedrite. Such veins characteristically 

are associated with felsic intrusive rocks that are cut 

by aplite dikes and quartz veins and stringers. The 

amount and proportions of the minerals differ from vein 

to vein: for example, veins of this type at the Marietta 

mine are valuable mainly for gold; at the Iron Mask, for 

zinc and lead; at the Kleinschmidt, for silver, lead, and 

zinc; and at the January and East Pacific, for lead, 

silver, gold, and zinc. Quartz is the common gangue 

mineral-and is abundant in some veins. Many veins contain small quantities of a carbonate mineral as well, 

most commonly probably ankerite, but in some veins it 

is siderite, in some veins, calcite, and in some veins, 

dolomite. The veins are mainly replacement deposits 

and partly banded fracture fillings in silicified, sericitized, argillized, and pyritized rock. Typically, they 

consist of broken altered rock laced with lenses and 

stringers of sulfides, gangue minerals, and gouge, but 

in places they are clean -cut fracture fillings accompanied by little or no gouge. Recurrent fracturing during and after mineralization has brecciated many of 

the veins. 

Veins of this type are as much as several feet thick 

and a few thousand feet long ; a few have been mined 

through a vertical distance of 600 or 700 feet. Most ore 

bodies were only a few feet thick and a few tens or a 

few hundred feet long, but in the ·East Pacific mine an 

ore body of this type was about 1,500 feet long and was 

mined to a depth of 700 feet (Reed, 1951, pl. 13). Some 

of the veins have been enriched by oxidation, but many 

have been profitably mined well beneath the zone of 

oxidation. 

SILVER-LEAD-ZINC REPLACEMENTS IN CARBONATE 

ROCKS 

Replacements in limestone and dolomite of the Quadrant Formation and Mission Canyon Limestone have 

been explored south of Johnny Gulch. The deposits are 

lenticular or pi pelike aggregates of gailena, sphalerite, 

and pyrite, or their oxidation productions, in a gangue 

of cryptocrystalline quartz (chalcedony and jasper) 

and calcite or dolomite. A few contain a little barite. 

Most of the deposits have been mined only in the zone 

of oxidation or partial oxidation where iron oxides, 

cerussite, smithsonite, and subordinate amounts of secondary minerals of lead and zinc (hemimorphite, anglesite, pyromorphite, wulfenite, and probably others) 

strongly predominate over sulfide minerals. 

The deposits are localized along steep east-trending 

fractures. In the Quadrant Formation most of the deposits are steeply plunging pipelike bodies in dolomite 

beds. The few ore bodies in the Mission Conyon Lin1estone are irregularly shaped lenses in limestone beds or 

in premineral collapse breccias adjacent to or between 

steep fractures. Pi pes or lenses are from a few square 

feet to a few hundred square feet in cross section and are 

as much as a few hundred feet in length. 

A few small deposits (Na:da, Buckhorn) in the 

Quadrant Formation are similar in habit and control 

to these replacement deposits, but the ore is porous iron 

oxide that contains free gold and little or no silver, lead, 

or zinc. 

SKARN DEPOSITS 

In a few places, particularly at the margin of the 

Keating Gulch stock, limestone and associated beds have 

been converted to masses of coarse-grained silicate minerals, mainly garnet, epidote, vesuvianite, quartz, and 

calcite. Some of these skarns contain magnetite, hematite, pyrite, pyrrhotite, chalcopyrite, and secondary iron 

and copper minerals. The largest known body of this 

type, at the Rothfus mine, yielded a few thousand tons 

of partly oxidized copper ore. 

Volcanic rocks in the lower part of the Elkhorn 

Mountains Volcanics adjacent to the Silver Wave stock 

have been hornfelsed and locally replaced by coarser 

textured aggregates of epidote, chlorite, quartz, garnet, 

and pyrite that resemble skarn and locally contain gold 

and scheelite. Most of the coarse-textured silicate rock 

is along fractures in a zone of intensely fractured rock 

near the intrusive contact. The distribution of these 

• 
\ 

... 

MINERAL DEPOSITS 39 

silicate rocks and the intimate intergrowth of pyrite 

and scheelite with the silicate minerals suggest that they 

and the associated metallic minerals are genetically 

related to the Silver Wave stock. The silicate rock and 

the stock are cut by many ~pyritic gold veins, some of 

which contain scattered crystals of scheelite, and by a 

few small base- and precious-metal veins. 

TIT ANIFEROUS IRON ORE 

Lenticular beds of titaniferous magnetite are locally 

present in the middle of the Slim Sam Formation. These 

beds are th~ught to be lithified beach concentrations 

deposited as the Late Cretaceous sea withdrew, probably 

in Niobrara time. In the vicinity of Radersburg, ore has 

been mined from a slightly metamorphosed deposit of 

this type near the margin of the Rattlesnake intrusive 

body; tlwse magnetite-bearing rocks are distinctly 

harder, tougher, and of higher grade than those at other 

localities. Though the magnetite grains appear to be 

detrital, some recrystallization and perhaps even some 

migration of iron probably took place during thermal 

metamorphism. Samples of the ore contained 43.1 and 

44.1 percent Fe and 9.6 and 9.3 percent Ti02 • Most of 

the titanium is probably in solid solution in the magnetite ( titanomagnetite), for neither exsolved ilmenite 

nor any other titanium minerals were recognized in the 

few polished and thin sections examined. 

GOLD PLACERS 

Gold placers have been mined at several places. South 

and west of Radersburg, a pediment of probable late 

Pliocene age is veneered by gravel and is cut by a 

network of shallow gravel-filled channels (pl. 1), many 

of which have yielded placer gold. The age of the goldbearing gravel. probably ranges from late Pliocene to 

Holocene. The source of the gold is the numerous oxidized veins that crop out in the low hills that border 

the placer area to the west. 

The placer gold ·along Indian Creek, in the mapped 

area, is mainly in remnants of gravel terraces of probable late Pleistocene age that border the present chan-

·nel of the creek, and partly in the present channel 

gravels. East of the area, gold is in gravel-filled channels on a pediment of probable late Pliocene age as well 

as in gravel terraces along the present stream. The 

source of the gold is veins in the Park and Hassel areas. 

Placer gol1

d also was recovered from postglacial gravel 

along the present stream channels of Weasel and Beaver 

Creeks. 

OXIDATION AND ENRICHMENT 

At least one-third and perhaps more than one-half of 

the total value of metal produced from the area has 

been fron1 oxidized or partly oxidized ore; oxidized 

veins yielded most of the ore 1nined before 1900 anJd 

part of the ore mined since. In the Radersburg district 

only the !(eating and Ohio Keating auriferous pyrite 

veins sustained profitable operations beneath the zone 

of oxidation. In the Park, Weasel Creek, and Iron Age 

Gulch areas the importance of oxidation on the quartz · 

veins with auriferous pyrite and the base- and preciousmetal veins was not so great, but nevertheless it seems 

certain th31t the most profitable mines (E·ast Pacific, 

Custer-Hyantha-Iron Age groups, Kleinschmidt) were 

mostly in oxiJdized ore. Though unoxidized ore bodies 

or parts of ore bodies were profitably mined from these 

and other mines, most of them were relatively small and 

the grade of ore was generally lower than that in the 

oxidized zone. 

Most deposits are oxidized or partly oxidized to a 

depth of at least 100 feet, and some are largely ox~dized 

to depths of several hundred feet. Winchell (1914, 

p.173) reported that the !(eating vein was oxidized to a· 

depth of 175 feet and partly oxidized for an additional 

59 feet. The East Pacific vein is reported to be oxidized 

to a depth of about 300 feet (Pardee and Schrader, 1933, 

p. 219). The importance of oxidation was probably twofold : gold was liberated from the pyrite with which it 

was associated in primary ore, and thus recovery was 

facilitated ; and the grade of ore was increased by 

removal of pyrite, sphalerite, and arsenopyrite. Furthermore, liberation of gold during oxidation was a 

necessary first step in the formation of placer deposits. 

Much of the ore in the silver-lead-zinc replacmnent 

bodies in the Quartzite Ridge area also is oxidized. 

For example, the unusually rich ore from the J o Dandy 

mine was mainly lead carbonate, a.nd virtually the entire 

output of the Ruby mine has been secondary minerals of 

zinc and leaJd. 

The history of oxidation in the ·area is complex and 

imperfectly known. In the Radersburg area, and perhaps ·elsewhere, the veins were oxidized in latest Cretaceous and early Tertiary time and subsequently oovered, 

at least in part, by Oligocene (and Miocene~) sediments. 

This cover has been erodeJd from the productive part of 

the Radersburg area in relatively recent time (late 

Pliocene~ to Holocene), but oxidized veins may lie 

beneath the Tertiary cover farther south. North of Indian Creek, some veins have probably been exposed to 

weathering and oxid31tion since l31test Cretaceous or 

early Tertiary time. 

DESCRIPTION OF DEPOSITS 

IRON AGE GULCH AND WEASEL CREEK AREAS 

(WINSTON AND BEAVER CREEKS) 

The Iron Age Gulch and Weasel Creek areas include 

most of the deposits that are referred to the Winston 

----· \ 
40 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

region or district (fig. 5) or the Beaver Creek district 

of earlier reports (Reed, 1951; Pardee and Schrader, 

1933; Stone, 1911; U.S. Geol. Survey, 1883-1927; U.S. 

Bur. Mines, 1925-32, 1932-56; Earll, 1964; Schell, 1963). 

As the principal ore deposits in these two areas have 

been described by Pardee and Schrader (1933, p. 211-

277), Earll (1964, p. 4-29), and Schell (1963), only a 

resume of pertinent features is given here. Earll (1964, 

p. 30-43) also described tJhe results of a geochemical 

survey of the Winston district. 

Most of the deposits in both areas are fissure veins 

adjacent to or within stocks of quartz-rich felsic intrusive rocks. The mines in the Iron Age Gulch area cluster 

around the Edna stock and have yielded ore from steep 

base- and precious-metal veins and from gently dipping 

gold -bearing quartz veins with auriferous pyrite. In 

the Weasel Creek area the greater part of the ore has 

been from steep east-trending base-and precious-metal 

veins, but some ore has also boon mined from quartz 

veins with auriferous pyrite; these mines are near several stocks, of which the Vosburg is the largest. 

Mineralogy of the veins is fairly uniform. The gently 

dipping veins cons'ist of limonite-stained quartz and a 

little pyrite at the surface and, where less affected by 

weathering, of clear glassy quartz with pyrite. The veins 

appear to heal fractures and contain little or no gouge. 

The steep veins contain pyrite, argentiferous galena, 

sphalerite, subordinate chalcopyrite and arsenopyrite, 

and sparse tetrahedrite in a gangue of quartz and locally 

carbonate minerals. Gold is present, probably mostly in 

pyrite. Much of the ore mined from these two areas was 

oxidized. Pardee and Schrader (1933, p. 214-215) reported that most of the oxidized ores were valuable 

mainly for gold and that in the unoxidized ores "gold 

appears to be carried mostly by pyrite, but its distribution is irregular" and that "silver is associated mainly 

with the galena." 

These veins have been explored over a vertical distance 

of 3,400 feet, from an altitude of 4,250 feet at the bottom 

of the Custer mine (Reed, 1951, fig. 12) to an altitude of 

7,650 feet along the outcrop of the Little Olga vein 

(ICleinschmidt mine) ; the deepest workings on a single 

vein, about 750 feet, are at the East Pacific mine. l\Iost 

ore shoots do not exceed a few hundred feet in length 

and depth and a few feet in thickness; a few are much 

longer and deeper, but, except for small local bulges, 

none has exceeded 5 feet in thickness. An ore shoot in 

the East Pacific mine was almost completely stoped for a 

length of 1,200-1,500 feet through a vertical distance of 

400 feet, mainly in the oxidized zone, and was partly 

stoped for 300 feet below this in the sulfide zone (Reed, 

1951, fig. 13). Vein thickness in most places did not exceed 2 feet and in some places was a foot or less. A longitudinal section of the Custer mine (Reed, 1951, fig. 12) 

suggests that an ore shoot, or several closely spaced ore 

shoots, in the Custer vein was of comparable size. 

Most of the mines are wholly or partly inaccessible, 

and consequently little information could be obtained on 

the characteristics of veins and ore shoots at depth except in the Kleinschmidt (Little Olga) mine. In this 

mine a long south-trending crosscut adit was driven to 

explore several east-trending steep veins at depth (Pardee and Schrader, 1933, pl. 31). The Little Olga vein, 

which yielded partly ·oxidized silver-lead-gold ore within 250 feet of the surface, was intersected at a depth of 

500-750 feet below the outcrop. A level was driven 400 

feet along the vein from the intersection, and a raise at 

least 180 feet long was driven on the vein. In these workings the vein was 2-4 feet thick and consisted of altered 

rock ·and gouge cut by stringers of pyrite, sphalerite, 

and galena. Only a few small pockets of ore were rich 

enough to be mined. Several other veins were intersected by the crosscut, but they were too narrow or of 

too low grade to be mined. None of them could be correlated with certainty with veins near the surface a:bove 

the crosscut (Irish Syndicate, Cynosure, Harmony, and 

two unnamed veins shown on pl. 31 of Pardee and 

Schrader, 1933). 

A map published by Reed (1951, fig.13) suggests that 

the East Pacific ore shoot pinched and became discontinuous below the No. 4 adit (about 400 ft. below the 

outcrop and 100 ft. below the bottom of the oxidized 

zone as reported by Pardee and Schrader, 1933) and 

was less than 200 feet long on the bottom level of the 

mine, about 300 feet below theN o. 4 adit. 

CUSTER MINE (3) 2 

The largest mine in the Iron Age Gulch area, the 

Custer, explored a steep base- and precious-metal vein 

that trends N. 30° E. Ore was mined from a single shoot 

2,400 feet in length (Mrs. Charles MufHy, oral commun.; 

see also Reed, 1951, fig. 12). Gold was the principal 

metal produced. The vein cuts the Edna stock and the 

Elkhorn Mountains Volcanics, but the ore shoot was 

confined to that part of the vein in the volcanic rocks. 

The mine was operated through two vertical shafts and 

an adit. It was largely inaccessible in 1952, and only a 

small part of the adit level· was examined. Where seen, 

the vein comprises two splits striking about N. 30° E. ; 

the east split is about vertical, and in a 12-inch width it 

contains chiefly pyrite and smaller amounts of galena, 

arsenopyrite, chalcopyrite, and sphalerite in brecciated 

quartz and a carbonate mineral (ankerite~) . Gouge 

along the vein walls contains brecciated sulfides and 

2 The number in parentheses following each mine name is the number 

of that mine on plate 1 and in figure 5. 
a. 

• 

MINERAL DEPOSITS 41 

fragments of altered andesite. The west split dips 

48°-65° mv. and consists of brecciated andesite, gouge, 

and, locally, of brecciated sulfides. 

EDNA NO. 2 MINE (4) 

Most of the shallow workings in the Iron Age Gulch 

area apparently explored gently dipping gold-bearing 

quartz veins that cut granite porphyry of the Edna 

stock. One of tJhe workings on the Edna No. 2 clailn 

(pl. 2) illustrates many of the features seen in the 

workings that were examined. In this adit, the continuity of any single gently dipping vein is difficult to 

prove because of the many steep faults. Displacements 

on the faults are small, but are enough to hinder systematic mining. Similar gently dipping ·veins somewhat less disrupted by faults were developed on the 

adjacent Edna claim. The veins typically are one-hal:£ 

to 8 inches thick, and. the gold content reportedly ranges 

:from a trace to a few ounces per ton. 

HYANTHA MINE ( 5) 

The Hyantha mine yielded lead, zinc, silver, and gold 

from a base- and precious-metal vein that trends N. 

50°-70° E. and dips 60° N. to vertical. The vein cuts 

several gently dipping quartz veins with auriferous pyrite. The vein is explored by a 340-:foot-deep vertical 

shaft and by two adits. At the shaft collar, the vein is 

as much as 4 feet wide, occupies a fault in the Elkhorn 

Mountains Volcanics, and consists of pyrite, galena, and 

sphalerite in brecciated quartz and aJbundant gouge. 

A. short distance west of the shaft, the vein cuts into the 

Edna stock and probably continues as one or more of 

the barren northeast-trending, steeply dipping faults 

in the Edna No.2 mine (pl. 2). 

EAST PACIFIC MINE ( 1 2) 

The East Pacific mine, the most productive mine in 

the vVeasel Creek area, yielded ore having an estimated 

value of more than $2 million (Reed, 1951, p. 38, fig. 

13). It was largely inaccessible in 1952. The mine exploited a vein that trends N. 65°-80° E., dips 65°-85° 

N., and cuts the Elkhorn Mountains Volcanics near the 

Freiburg stock of porphyritic quartz monzonite. The 

vein is a filled fracture that has banded structure and 

is accompanied by minor replacement in the adjacent 

wallrock. Vein width ranges from a few inches to several :feet. The vein contained several shoots of baseand precious-metal ore, within which were narrow 

chimneys of high-grade gold ore. The principal ore 

minerals were argenti:ferous galena, auriferous pyrite, 

sphalerite, and smaller amounts of chalcopyrite and 

tetrahedrite in a gangue of quartz and one or more carbonate minerals. 

JANUARY MINE ( 1 3) 

The January mine exploited a base- and precio1JSmetal vein for a length of 450 feet and over a vertical 

distance of about 400 feet. The vein trends N. 70°-90° 

W. and dips about 40°-70° N. (pl. 2). Ore has been 

mined from a discontinuous shoot that rakes to the 

east and is as much as 160 feet l'Ong. West of the ore 

shoot the January vein has not been identified with 

certainty, but a number of faults along the projected 

strike of the vein contain brecciated sulfides and altered 

rock. The ore ranges in thickness from a few inches 

to about 4 :feet and typically is 12-18 inches thick in a 

vein that is 1-15 feet thick and consists of altered a.nd 

broken :vock laced with strands of gouge and with lenses 

and stringers of sulfide and gangue minerals. Postmineralization movement along the vein is indicated 

by brecciated sulfides and thick gouge. Sulfide minerals 

are pyrite, galena, sphalerite, chalcopyrite, tetrahedrite, and arsenopyrite. The gangue is quartz and r-hodochrosite. Minerals in the oxidized ore include limonite, cerussite, pyromophite, pyrolusite, and gold. Reed 

(1951, p. 39) reported the average metal content of 

the ore to be: copper, 0.11 percent; lead, 5.0 percent; 

zinc, 3.4 percent; silver, 5.3 oz (ounces) per ton; gold, 

0.108 oz per ton. 

A. second vein, the Montana-Idaho, was explored for 

a short distance on the No. 1level. This vein trends N. 

70°-90° E., dips 60° N. to vertical, and appears to be 

offset by the January vein. It differs from the Januacyvein in containing abundant arsenopyrite and pyrite 

and little galena or sphalerite. Both veins cross the contact between the Elkhorn Mountains Volcanics and 

the January stock, and the January vein contains ore 

on both sides of the contact. 

KLEINSCHMIDT (LITTLE OLGA) MINE (18) 

,The l{leinschmidt (Little Olga) mine (pl. 2) 

(Prurdee and Schrader, 1933, p. 31; Reed, 1951, p. 40) 

explores two sets of veins (Little Olga and Quartette) 

in the porphyritic quartz monzanite of the Vosburg 

stock. Most of the production has been from the Little 

Olga vein, a steep east-trending base- and preciousmetal vein as much as several feet thick that contains 

pyrite, galena, sphalerite, chalcopyrite, stephanite, and 

tetrahedrite in a gangue of quartz, siderite, altered 

rock, am.d gouge; some ore was mined from other 

steeply dipping base- and precious-metal veins and 

from thin north-trending gently dipping quartz veins 

that contained pyrite and sparse ,arsenopyrite, molybdenite, and gpld. The gently dipping veins are cut and 

slightly displaced by the steep veins. The steep veins 

contain considerable broken rock and gouge, and the 

country rock bordering them is sericitized, pyritized, 
42 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

and argillized. The gently dipping veins are largely 

fillings that heal the fractures ; gouge and broken rock 

are uncommon along them and the adjacent altered zone 

is generally no more than a foot thick. 

A crosscut was driven 3,300 feet southward from the 

head of vVhi~tehorse Gulch to explore at a depth of 

wbout 500 feet beneath the Little 0 lga lower adit (pl. 2) . 

In this crosscut the Quartette vein was not identified 

and the Little Olga vein. consisted of altered broken 

rock and gouge cut by a few narrow stringers of sulfides. 

Only a few small pods of ore were mined above this 

crosscut and adjacent to a raise driven from it. 

OTHER MINES 

Several other mines near vV easel Creek yielded ore 

from veins similar to those described above. The Stray 

Horse {15) mine expored two parallel, east-trending, 

steep veins in tthe Elkhorn Mountains Volcanics, both 

typically a foot or less thick. The veins are displaced 

as much 'as 20 feet by steep north-trending faults. 

Mineralization is similar to that in the January vein. 

The Little Bonanza mine explored a small ore shoot on 

yielded small amounts of high-grade silver-lead ores. 

The Little Bonanza mine e~plored a smrull ore shoot on 

a vein trending N.75° E. and@ping 65° NW. in the 

Elkhorn Mountains Volcani9s The Silver Saddle mine 

explores a mineralized fracture zone th31t trends northeast and dips 30°-45° SE. (pl. 2) in the Elkhorn Mountains Volcanics. A few small pods of ore containing 

argentiferous galena, pyrite, aiid sparse sphalerite in a 

quartz gangue were mined. Between ore pods the vein 

consists of gouge seams in bleached, slightly silicified, 

and pyritized volcanic rock. 

PARK AREA 

The Park area at the head of Indian Creek centers 

around the ~1arietta mine in and near sec. 15, T. 7 N., 

R. 1 W. It is underlain by rocks of the Elkhorn Mountains Volcanics, mainly tuff, and a few small bodies of 

intrusive rocks related to the volcanics. Most of the 

ore mined has been from gently dipping lenticular 

quartz veins with base and precious metals, valuable 

mainly for gold, but in places containing important 

quantities of lead and silver. In some veins arsenopyrite 

is relatively abundant. The veins are mostly 2-3 feet 

thick with local swells to as much as 5 feet. Some of 

them have been mined for several hundred feet along 

strike and vertically 250-300 feet. Most veins dip 35° 

or less and in places roll to almost horizontal. In some 

places rolls appear to localize thicker parts of veins. 

The veins commonly are frozen to the walls. They 

contain a few thin gouge seams and small horses of 

altered rock. 

Granodiorite on the dump of a caved shaft at the 

Venezuela mine and thermally metamorphosed volcanic 

rocks in the vicinity suggest :that a sizable body of 

younger intrusive rock lies at shallow depth. 

MARIETTA MINE (22) 

The Marietta mine (pl. 3; Reed, 1951, p. 48; Schell, 

1963) explores several gently to moderately dipping 

north- ·and northeast-trending lenticular veins in the 

Elkhorn Mountains Volcanics and associated intrusives. 

Most of the ore has come from a north-trending zone 

that includes the Marietta and Blue veins (pl. 3). The 

veins contain pyrite, arsenopyrite, galena, sphalerite, 

and sparse chalcopyrite in a gangue of quartz and a 

manganiferous carbonate mineral. Limonite and manganese oxide stain .are prevalent in oxidized and partly 

oxidized ore. The veins are a few inches to rarely as 

much as 6 feet thick, are in part banded, and are mainly 

fracture fillings. Adjacent to the veins the volcanic 

rocks are argillized, pyritized, and locally sericitized. 

The dip of the veins is variable but generally low, and 

some of the thicker ore bodies are localized by rolls. 

Most of the ore is valuable mainly for gold, which apparently is in ·auriferous pyrite and arsenopyrite in 

unoxidized ore. Lead and silver are important locally, 

for example in a flat part of the Marietta vein in the 

Gold Dust intermediate level (pl. 3). Zinc and copper 

are present only in minor quantities. 

The north-trending vein zone is cut by faults or fault 

zones of several trends {pl. 3). Displacements along the 

faults are small and range from a few feet to rarely as 

much as 20 feet, even though in some places the rocks 

are intensely fractured in zones 10 feet or more across. 

Because of the fracturing and displacement of the veins 

and their lenticularity and varia:ble dip, correlation of 

veins in the north-trending zone is uncertain. The most 

persistent fauLt trends N. 30° W. along the course of the 

intermediate level, dips steeply, and has an a;pparent 

offset of 12-16 feet, southwest side down. On the southwest side of the fault the Blue vein trends nearly parallel to the fault but dips very gently. This vein has been 

stoped for a length of 200 feet over a vertical distance 

of at least 150 feet. Northeast of the fault, the northtrending Marietta vein or vein zone has been mined 

intermittently for about 700 feet through a vertical distance of more than 250 feet. 

OTHER MINES 

Gently to moderately dipping veins of diverse trend 

similar to those in the Marietta mine have yielded gold 

ore with different amounts of lead and silver at the 

Park {26), New Era {26), and Venezuela {26) mines, 

a short distance southwest of Marietta, at the L~ttle 
... 

'& 

MINERAL DEPOSITS 43 

Annie (24) mine about 1,000 feet east of the Marietta, 

and at the Vulture (21) mine at the head of Eureka 

Creek. Vuggy milky quartz characterizes the vein material at the Vulture mine. l\1assive and partly vuggy 

veins of white quartz and small amounts of a carbonate 

mineral, probably siderite, and pyrite yielded gold ore 

at the Ohampion (19) mine (NE~SElJ.., sec. 4, T. 7 

N., R. 1 W.) and at. the Bi~ Chief (20) mine near the 

head of 'Vhitehorse Creek. A northwest-trending steep 

vain on the Jawbone ('25) claim (NWlJ..,NW~ sec. 

17, T. 7 N., R. 1 W.) also yielded gold ore from the 

oxidized part of a pyritic vein that contained quartz, 

calcite, and ankerite or siderite. High-grade silver-lead 

ore is reported to have been mined from a nearly vertical pipe along a north-trending fault at the Bullion 

King (23) mine (Albert Dance, oral oommun., 1952), 

about 1,000 feet northeast of the Marietta mine. At the 

Parker (27) mine in the val'ley of Eureka Creek, baseand precious-metal ore containing pyrite, galena, sphalerite, and arsenopyrite was mined from a pipelike body 

in a steep vein of N. 70o E. trend. 

HASSEL AREA 

Veins in the Hassel area are around and in the Silver 

Wave and Diamond Hill stocks. North of Diamond 

Hill most of the veins are of the base- and preciousmetal type; the most productive, explored by the Iron 

Mask mine, yielded 23,000 tons of zinc-lead-silver ore 

(Reed, 1951, p. 46). South of Diamond Hill, the veins 

are mainly of the quartz-poor auriferous pyrite type, 

similar to those in the Radersburg area. Oxidized gold 

ore from auriferous pyrite veins at the Diamond Hill, 

Blacksmith, and Little Giant mines accounted for most 

of the production from the Hassel area. The Little Giant 

and Blacksmith mines explore the oxidized parts of 

narrow, vertical or steeply dipping east-trending veins. 

The Diamond Hill mine explored a north-trending belt 

of thermally metamorphosed and hydrothermally .altered (pyritized) volcanic rock cut by lenticular pyritic 

veins and stringers of diverse trend ; ore was mined 

from open pits and underground workings in the oxidized zone, and the mineralized rock beneath the zone of 

oxidation was explored by a long adit with appended 

short drifts. 

The Diamond Hill stock is cut by many aplite dikes 

and by small quartz veins which contain pyrite and 

sparse chalcopyrite, and which are similar to the quartz 

veins with auriferous pyrite in the Beaver Creek and 

Iron Age Gulch areas. The stock is also cut by a few 

thin base- and precious-metal veins. Though quartzpoor auriferous pyrite veins are the principal vein type 

in the volcanic rocks nearby and in the Silver Wave 

stock, none has been recognized cutting the Diamond 

Hill stock. 

IRON MASK MINE ( 31) 

The Iron Mask mine explored a north-trending steeply east-dipping base- and precious-metal vein that cuts 

the lower member of the Elkhorn Mountains volcanics. 

The mine yielded about 23,000 tons of ore of the :following average grades: 4 percent lead, 7.4 percent zinc, 

0.2 percent copper, 4.9 oz. per ton silver, and 0.005 oz. 

per ton gold (Reed, 1951, p. 46). The mine is developed 

by three vertical shafts and a 2,300-:foot-long crosscut 

adit at a depth of 570 feet beneath the outcrop (Reed, 

1951, p. 46). In and above this adit the vein was explored 

and partly stoped for a length of about 600 :feet. 

The only exposure o:f the vein at the time of this study 

was in a sublevel 100 :feet below the adit, where the 

thickness of the vein ranged :from 3 to 8 :feet, and averaged about 4 feet. In the north heading of this sublevel 

the vein consisted of a thin footwall gouge, 16 inches 

of altered :fractured tuff with disseminated sphalerite, 

galena, pyrite, and quartz, a 4-inch-thick band of sphalerite and pyrite and some galena, 36 inches of fractured 

and altered tuff laced with stringers of a manganiferous 

carbonate mineral and sulfides, and a 1-:foot-thick hanging-wall gouge containing sparse brecciated sulfides. 

Strands of gouge overlap and split within the vein 

zone. The ore shoot on this sublevel was about 90 feet 

long. The tuff adjacent to the vein is bleached and contains disseminated pyrite and sericite. 

Unoxidized ore contains pyrite, sphalerite, and galena, and sparse chalcopyrite and tetrahedrite in a 

gangue of quartz, calcite, and a manganiferous carbonate mineral. Samples of oxidized ore on dumps of workings along the outcrop of the vein contain limonite, jarosite, psilomelane, pyrolusite, cerussite, smithsonite, and 

sparse galena and sphalerite. 

A parallel vein, the Red Wing vein, is exposed in the 

crosscut adit and at the surface. This vein trends north, 

dips 80° E., is 3-4 feet thick, and contains abundant 

limonite and manganese oxide and sparse chalcopyrite 

in a gangue of quartz and two carbonate minerals. 

DIAMOND HILL MINE (40) 

The Diamond Hill mine is in a north-trending belt 

of metamorphosed tuff of the Elkhorn Mountains Volcanics adjacent to the Silver Wave stock (pl. 4). The 

mine explores one of the first lode deposits discovered 

on Indian Creek, and its workings include eight adits~ 

six open pits, and numerous prospect pits and shafts. 

Reed (1951, p. 45) stated that these workings explore 

and develop the deposit ·for a horizontal distance of 

2,700 :feet and a vertical distance of 250 feet. The mine 
44 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

has yielded about $550,000 worth of ore valued chiefly 

for its gold content, but it also contains minor scheelite 

and base-metal sulfides. 

Rocks in the north-trending metamorphosed zone 

are silicified and pyritized and locally contain epidote, 

chlorite, garnet, calcite, and sparse scheelite; in the zone 

of oxidation they are bleached. The zone is cut by erratically distributed but abundant seams and irregular 

stringers, pipelike shoots, and distinct veins of auriferous pyrite in a gangue of calcite, perhaps other carbonate minerals, and quartz. Lenses of massive auriferous 

pyrite are present in places. The pyrite veins and lenses 

contain native gold and sparse chalcopyrite and scheelite. Scheelite also is sparsely disseminated in epidotechlorite-garnet rock with no particular concentration 

near pyrite veins and lenses and probrubly is older than 

the pyritic mineralization. A few quartz-pyrite veins 

that contain galena and sphalerite appear to be younger 

tihan the more widespread pyritic mineralization. 

Rock alteration and metallization appears to have 

occurred in several episodes. During emplacement of 

tJhe Silver Wave stock, the invaded andesitic tuffs were 

hornfelsed and, locally along fractures, epidote-chloritegaTnet rock was formed; 'PYrite and prdbably most of 

the sparse scheelite was deposited at this time. Somewhat later, probably during the final stages of consolidation of the Silver Wave stock, rocks in the fractured 

zone were hydrothermally altered, and stringers and 

lenses of •pyritic gold ore were deposited, partly as fractur~ fillings and· partly ·as replacements of altered volcanic and intrusive rocks. Following emplacement and 

consolidation of the Diamond Hill stock, a few quartz 

veins containing pyrite, galena, and sphalerite were 

formed. The final stage of mineralization in the area is 

marked by the filling of fractures by calcite. 

LITTLE GIANT MINE ( 4 1 ) 

At least 14 na.rrow, east-trending steep pyritic gold 

veins in hydrothermally altered voleanic rocks south 

of tJhe Diamond Hill mine have been explored iby the 

Little Giant and nearby mines. The veins are explored 

by several adits, three shafts, and many prospect pits 

and trenches, all in the oxidized zone. A:. typically easttrending vein of this group is shown on plate 4. The estimated value of the ore from these mines is about $300,-

000 (Reed, 1951, p. 47), nearly all of it mined before 

1901. The veins occupy narrow but persistent fractures 

that range in thickness from 1 inch to rarely 5 feet, 

and in most places are less than 2 feet thick. The easttrending fractures are displaced a few feet by at least 

two steep faults that trend N. 25°-35° E.; locally these 

faults are weakly mineralized. The ore in the oxidized 

zone is native gold in ·a gangue of limonite and altered 

andesite with very little quartz. Below the zone of oxidation auriferous pyrite is the only sulfide present, and 

apparently the ore could not be profitably mined. T-he 

country rocks, lapilli tuff, tuff breccia, and crystal tuff 

of the Elkhorn Mountains Volcanics, are hydrothermally altered and contain ·abundant epidote, kaolinite, 

sericite, and finely disseminated pyrite. They are intensely fractured and cut by many quartz stringers. 

Weathered rocks are heavily seamed and stained by 

limonite. 

BLACKSMITH MINE ( 4 2) 

The Blacksmith mine (pl. 4), an adit with appended 

drifts, two shafts, and prospect pits southeast of the 

Little Giant mine, explores a series of narrow anastomosing north-trending steep veins that consist mainly 

of altered rock and limonite with a little quartz. The 

ore, mined for its gold content, was similar to that in 

the Little Giant veins, and as in the Little Giant mine 

it apparently was not profitable below the zone of oxidation. The mine reportedly yielded ore valued at about 

$250,000 (Reed, 1951, p. 44). The volcanic country rocks 

are bleached ·and limonite stained for several feet adjacent to the veins. 

OTHER MINES 

A number of mines and prospects in the Hassel area 

explore base- and precious-metal veins that contain 

pyrite, galena, sphalerite, and ·arsenopyrite in a gangue 

of quartz and ankerite or another carbonate mineral. 

In this group are the Central (38) and.Queen Bee (36) 

mines, which yielded ore from steep N. 50° W.-trending 

veins; the St. Louis (35), Spring Hill (39), and the 

Mammoth ( 34) mines, which explore steep N. 80° W.-

trending veins, ·and the Silver Wave mine (37), which 

developed an ore body on a N. 50° W.-trending vein at 

its intersection with a N. 70° E.-trending vein. 

RADERSBURG AREA (CEDAR PLAINS) 

Except for a deposit of t~taniferous iron ore and a 

few narrow base- and precious-metal veins, the ore deposits of the Radersburg area are auriferous pyrite 

veins (Winchell, 1914; Corry, 1933) in a gangue of 

strongly altered wallrock and relatively sparse quartz, 

calcite, and an iron-bearing carbona;te mineral. Most of 

these veins trend nearly north and dip 65 o W. to 

vertical. They range in thickness from less .than a foot 

to as much as 5 feet and in length from a few hundred 

feet to about 3,000 feet. The Keating vein has been explored to a depth of about 1,200 feet. 

Most of the veins are clustered in an area of llibout 

1 square mile in which the country rocks are intensely 

altered and include two gradational zones (pl. 1) : an 

inner zone (heavy stipple on pl. 1), which is almost en-
MINERAL DEPOSITS . 45 

tirely in the Rattlesnake intrusive and contains most 

of the veins, and an outer zone (light stipple) in both 

intrusive and extrusive rocks. Rocks :in the inner zone 

are highly fractured, pyritized, silicified, sericitized, 

and argillized. Most of the rock in this zone has been 

oxidized, bleached white or light gray, and is seamed 

and coated with limonite. The alteration is most intense 

adjacent to the major veins, but no distinct zonation 

related to specific veins was recognized. Rocks in the 

outer zone are green or greenish gray, and the effect of 

pervasive alteration seems to have been mainly the 

formation of epidote, chlorite, and locally pyrite; adjacent to individual veins in this zone, however, an envelope of wallrock is bleached and pyritized, and it 

resembles the strongly altered rock of the inner zone. 

The difference in intensity of alteration between the 

inner and outer zones appears to he· related to abundance of fractures; part of the inferred difference between zones, however, may be more apparent than real 

because of differences in original rock type and degree 

of oxidation, which tend to emphasize effects of alteration. In places the rocks are so intensely altered that 

diagnostic features have been obliterated and the contact between intrusive and extrusive rocks cannot be 

located with confidence. 

KEATING MINE (59) 

The Keating mine has been the most productive in 

the area and was for years one of the foremost gold 

producers in Montana. During the period 1901-48 it 

yielded 412,471 short tons of ore from which were recovered 157,538 oz of gold, 43,557 oz of silver, and 

1,064 tons of copper (Reed, 1951). The deposit was 

discovered ~n 1866 and was worked at intervals until 

1947, when underground operations were suspended. 

Since then, small shipments have been made from the 

dumps. Total recorded production of gold is about $4 

million (Trauermann and Waldron, 1940, p. 9), of 

whi'Ch a small part may have come from nearby small 

mines. The n1ine was flooded and inaccessible in 1952. 

The average recovered metal content of the ore was 0.38 

oz gold per ton, 0.25 percent copper (Reed, 1951), and 

a little silver. 

The Keating mine is aJong one of the most persistent 

of a number of steep north-trending auriferous pyrite 

veins in a large are•a of intensely altered rock that is 

partly tuff of the Elkhorn Mountains Volcanics and is 

partly porphyritic rocks of the Rattlesnake intrusive. 

The !(eating vein has been explored by three shafts 

and 10 levels over a length of 2,200 feet and for a depth 

of 1,100 feet (Reed, 1951, p. 25). The vein trends north 

to N. 5° E., dips aibout 70° W., and is about 1-5 feet 

thick. Typically, it was about 20 inches thick south of 

the main inclined shaft, and it formed a stringer zone 

over a 1- to 2-foot width north of the shaft. According 

to Winchell ( 1941, p. 132), the vein was followed southward on the fourth level into "broken ground" consisting of unstratified boulders as much as 18 inches across 

of altered and oxidized rock of local origin. Winchell 

correctly considered this material to be part of Tertiary 

deposit. To the north, the vein narrows and was not 

followed to its termination. The vein is displaced as 

much as 25 feet by a few faults (Winchell, 1914, p. 

179). 

The ore is auriferous pyrite in altered pyritic rock 

with a little quartz and calcite, and sparse chalcopyrite, 

marcasite, pyrrhotite, ·galena, sphalerite, and chalcocite (Stone, 1911, p. 93). Quartz and calcite are more 

abundant in parts of the vein that do not constitute ore. 

Near the surface, the oxidized part of the vein contains 

abundant limonite, a little malachite, and gold. The ore 

was cmnpletely oxidized to a depth of 175 feet and 

partly oxidized _for an additional 60 feet (Winchell, 

1914, p. 173). 

OHIO-KEATING MINE (58) 

The Ohio-Keating mine, formerly the second most 

productive in the area, explored an auriferous pyrite 

vein in altered rock of the Rattlesnake intrusive parallel to arid about half a mile west of the Keating vein. 

The mine, inaccessible in 1952, was opened by a 550-

foot-deep inclined shaft with five levels. The Ohio vein 

yielded about 200,000 tons of ore with an average grade 

of 0.31 oz gold per ton (Reed, 19.51, p. 27). The vein 

trends about N. 15° W. and dips about 80° W., it is 

about 1-5 feet wide and contains abundant auriferous 

pyrite and very sparse chalcopyrite and bornite. Quartz, 

calcite, and lenses of altered country rocks are conspicuous in parts of the vein that contain little or no ore 

(Winchell, 1914, p. 179) . The vein is offset a few feet 

by two cross-faults (Winchell, 1914, p.179). 

OTHER DEPOSITS 

Ore has been mined from a number of pyritic gold 

veins in the vicinity of the Keating and Ohio-Keating 

veins (fig. 2). Most of the veins trend nearly north 

and dip steeply. Perhaps the most productive has been 

the Black Friday (66), which trends about N. 5° W. 

and dips 70° W. The Cyclone (69) vein, which trends 

N. 15° E. and dips 60°-75° W., is 1-4 feet .thick and 

may be a continuation of the Black Frid:ay vein. A few 

veins in this vicinity contain base metals in addition 

to auriferous pyrite. Ore containing galena, cerussite, 

wulfenite, pyrolusite, quariz, limonite, and probably 

vanadinite (Winchell, 1914, p. 180), abundant pyrite 

and traces of calcite was mined from a vein trending 
46 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

N. 68° vV. and dipping 65° NE. at the Congress (64) 

mine, and lead-silver-gold ore was mined from a vein 

trending N. 60° vV. and dipping 80° NE. at the Gopher 

mine (68). 

.IRON CROSS MINE ( 4 8 ) 

At least two lenticular beds of titani:ferous magnetite 

in sandstone and sandy crystal tuff in the Slim Sam 

Formation have been exploited at the Iron Cross mine. 

The ore beds are :folded (fig. 6), and the principal ore 

bodies are along a fold crest .and a fold trough. The 

magnetite accumulated as placer sands, but the massive 

appearance of the ore and the localization of the richest 

and thickest ore bodies along the crests and troughs of 

folds suggest that some of it was redistributed, perhaps 

by hydrothermal solutions. 

Ore is mined from several small open pits and sold 

:for use in a special type of cement made at Trident, 

Mont. 

QUARTZITE RIDGE AREA 

Pipelike and lenticular bodies of silver-lead-zinc ore 

have replaced favorable rocks adjacent to east-trending 

steep faults of slight displacement in the Quartzite 

Ridge area, south of Johnny Gulch and the Rattlesnake 

intrusive. Most of the bodies are in thin dolomite beds 

between quartzite beds in the Quadrant Formation. At 

least two ore bodies :are in the Mission Canyon Limestone, where one of then1 is partly localized in a collapse breccia. 

JO DANDY MINE ( 71) 

The Jo Dandy mine, the most productive of the mines 

tha.t explore silver-lead-zinc replacement deposits in 

carbonate rocks, since 1920 has yielded about 9,300 tons 

of ore with an average grade of 34 percent lead and 

11.9 oz per ton silver. The deposit is opened by a 600 

foot-deep vertical shaft with six levels. The mine was 

inactive in 1952. 

The ore is in pi pelike .replacement bodies ·at intersections of east-trending nearly vertical fractures with 

some of rthe nor.th-·trending dolomite .beds in the Quadrant Formation; the main host beds •are immediately 

a;bove and ibelow the thick quavtzite bed vhat forms the 

ridge crest in this vicinity. The I.argest ore !body was 

beneaJth lthe thick quar.tzite ·and was mined from the 

surface Ito rthe 600-foot level. The cross-sectional dimensions of tlhis ore pipe near the surface were 2-3 feet 

·along the :f~adture •and 10-15 :feet along bedding; ron 

the 300-foort level they were about 16 feet along the 

fracture •and 4 feet along bedding (A. R. Berg, oral 

commun., 1950). The second l·argest ore pipe was along 

tJhe same :fracJture at its intersedtion with the car.bonate 

bed ·above the thick quartzite bed ; !this pipe is reported 

to have exrtended 1-3 feet ·along 1the fracture and 7-20 

feet along bedding and .to have had an average c~oss 

section of 2 by 12 :feet. A:t least three smaller pipes were 

mined, two in !these same dolomite beds along a fracture ahouJt 30 feet south of the main fracture, and the 

third in ,a higher dolomite bed along a fracture 35 feet 

north of ~the main fracture. The dolomite a;bove :the thick 

qua·rtziite was explored f.or a length of 350 feet at 'aJbout 

'the level of the collar of the vertical shaft and for about 

800 feet in ·a drif.t driven toward the Sanrta Anita mine 

( 73) , on the 500 level, but no other pipes were found. 

Most .of the ore was oxidized 'and consisted of cerussilte, .pyromorphi1te, limonite, and calcite along with 

galena, manganese oxides, and wulfenite. The galena 

content increased in depth, and ore from rthe bottom 

level of the shaft was a mixed sulfide-carbonate ore 

(Carl Trauerman, oral commun.). 

SANTA ANITA MINE ( 7 3) 

The Santa Anita. mine, a short distance sou:th of the 

Jo Dandy, consists of a 4715-:foot adit wi1th connooting 

stopes 'and inclined shafts (pl. 4) and explores a mineralized fault thaJt cuts the Quadrant Formation ~and 

1

the upper part of rthe Amsden Formation. The :fault 

trends N. 7·5° .to 85° W., dips from veriticallto 65° 8., 

and offsets the beds about 60 :feet. The ore was similar 

1to ·the lead-silver ore of rthe Jo Dandy but was mainly 

in lenticul,ar lba;bular hodies along the fault; a few small 

ore :bodies were in irregular replacements projecting 

from :bhe fault in1to :f.avor3ible dolomite beds. The oxidized ore was a mixrture of limonite, cerussite, smithsonite, ·and manganese oxides with minor galena, sphalerite, and pyrite. The mine has yielded ~rubout 1,600 tons 

of ore since 1910 with an average of 23 percent lea;d, 

5.7 oz. per ton silver, and 0.02 oz. per tton gold (Reed, 

19,51, p. 28). The rock in the upper part of the Amsden 

Formrution and a;t the ;base of the Quadrant Formation 

at 1the west end of the ·adit is st~ongly breocia;ted and 

ox:idized. This oxidized breccia zone is thought Ito have 

:formed by leaching of carbonaJte rocks along the fault 

and subsequent collapse during an early Teritiary episode of weathering. 

RUBY MINE (RUBY SILVER) (74) 

The Ruby mine (pl. 4), southwest of the Santa Anita, 

explores a replacement deposit of zinc-silver-lead ore in 

the upper part of the Mission Canyon Limestone adjacent to two N. 75° W. vertical small faults. The mine 

includes two inclined shafts with interconnecting stopes. 

The workings extend about 75 feet along the strike and 

300 :feet down the dip of beds that trend about N. 15° 

E. and dip 40°-55° SE. About 4,000 tons of ore with a 

recoverable grade of 4.4 percent lead, 8.8 percent zinc, 

:lt 

• 
• 

MINERAL DEPOSITS 

0 1000 2000 FEET 

CONTOUR INTERVAL 20 FEET 

DATUM IS MEAN SEA LEVEL 

Elkhorn Mountains Volcanics, 

lower member 

EXPLANATION 

Porphyritic pyroxenebearing dike rock 

Slim Sam Formation 

~ 

Magnetite ore 

28~ 

Syenodiorite 

porphyry· 

----- Axis of .syncline, 

showing plunge 

Low-grade magnetite-bearing 

tuffaceous sandstone 

Contact 

Dashed where approximately 

located 

__ 40 ...L __ 

Fauit, showing dip 

Approximately located 

Axis of anticline, 

showing. plunge 

Strike and dip of beds 

-:;_ 

Strike and dip of over- · 

turned beds 

Spring 

(/) 

:::> 

0 

LJJ 

u 

<( 

1-

w 

a:: 

u 

FIGURE 6.-Iron Cross magnetite mine, Broadwlllter County, Mont. Geology by V. L. Freeman 

and D. E. Brambilla, 1952. 

47 
48 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS', MONTANA 

and 5 oz. per ton silver has been mined (Reed, 1951, 

p. 27). 

The limestone beds in which the ore occurs have a 

maximum thickness of 35 feet. Most of the ore near the 

surface was in two distinct zones, each several feet thick 

and 5-10 feet apart; in the lower workings only the 

upper zone was followed, and the ore in it thinned. In 

the upper zone, from which most of the ore was obtained, several feet of high-grade oxidized lead ore was 

underlain gradationally by several feet of high-grade 

oxidized zinc ore (A. R. Berg, oral commun., 1949). 

The ore bodies consisted of cerussite, smithsonite, hemimorphite, limonite, drusy quartz, calcite, dolomite, and 

jasper and were sheathed by partly silicified limestone. 

Galena, sphalerite, and pyrite were sparsely present 

near the bottom of the mine. A little fluorite was seen 

in a few specimens of ore. 

NORTH HOME MINE ( 6 3) 

The North Home mine, north of Johnny Gulch, also 

explored replacement silver-lead ore bodies in the upper 

part of the Mission Canyon Limestone. Between 1901 

and 1950 it yielded about 4,000 tons of ore with an 

average grade of 2.3 percent lead, 17.4 oz. per ton silver, 

and 0.06 oz. per ton gold (Reed, 1951, p. 26). 

The ore bodies were developed by steep shafts, both 

of which were inaccessible in 1952. The main ore body, 

in beds immediately beneath a thick collapse breccia, 

was followed to a depth of about 400 feet. The shapes 

and trends of open stopes suggest that the main ore body 

was a lens that raked 60°-70° N. in beds that trend N. 

15° E., and dip 65° SE. It was 25 feet long and 4 feet 

thick at the surface and was about 70 feet long and as 

much as 5 feet thick at a depth of 40 feet. Below 40 feet 

the stope is filled. The other ore body in the basal part 

of the collapse breccia was pipelike and was followed 

to a depth of about 150 feet. Near the surface the cross 

section of the pipe was 10 by 6 f·eet. 

The ore bodies are partly sheathed by silicified limestone and limestone breccia and appear to have formed 

partly by cavity filling and partly by replacement. Most 

of the ore that was mined was oxidized. It consisted of 

limonite, cerussite, smithsonite, cerargyrite, pyrmorphite, and traces of galena in a gangue of dolomite, barite, calcite, aragonite, manganese oxides, brown jasper, 

gray quartz, clear drusy quartz, and traces of fluorite. 

Ore deposition was preceded by silicification and perhaps by the formation of caverns in massive limestone 

and in earlier formed collapse breccia. 

OTHER AREAS AND DEPOSITS 

Elsewhere in the mapped area small mineral deposits 

occur in and near most of the younger intrusive bodies, 

but few of these have been produotive. In and around 

the Antelope Oreek, Monte Cristo, Vulture, and Mammoth intrusives are narrow veins that contained gold; 

some of them also contained silver and lead. Near the 

Slim Sam and !{eating Gulch stocks are gold-bearing 

skarn (Monarch mine ( 44) ) , and gold (Quartzite mine 

(45)) and silver-lead (Horn Silver mine (46)) replacement bodies in carbonate rocks 

NONMETALLIC MINERAL DEPOSITS 

Potentially useful deposits of limestone, dolomite, 

and building stone are found in the different sedimentary formations, and abundant sand and gravel are present in the surficial deposits 

Low-grade bentonite, potentially useful as a sealing 

agent for dams and canals, is present along Beaver 

Creek a few miles northeast of the mapped area (Mertie, 

Fischer, and Hobbs, 1951, p. 89-90). The bentonitic 

strata probably extend southward into the mapped area 

but are concealed beneath yotmger deposits. Bentonitic 

strata are also present in the Oligocene sedimentary 

rocks near Johnny Gulch, but the deposits are probably 

too small and too impure to be exploited. Similarly, 

beds of phosphate rock in the Phosphoria Formation 

are too thin and too low grade to be of commercial 

interest. 

Lignitic shale near the top of the Morrison Formation 

has been prospected in the SW:lh sec. 32, T. 7 N., R. 1 E., 

a few hundred yards west of the Indian Creek road, but 

the deposit is to thin and too impure to be of economic 

interest. 

POSSIBLE FUTURE DISCOVERY AND DEVELOPMENT 

OF MINERAL DEPOSITS 

. ']}he area has long been readily accessible and has 

been explored and exploited for more than 100 years, 

so that veins and ore bodies that crop out almost certainly have been found. The possibilities for discovery 

and future development of ore in the area are of three 

kinds: (1) concealed deposits o~ the same kinds that 

have been mined in the past, ( 2) extensions of known 

deposits, and (3) development of low-grade deposits 

under more :favorable economic conditions or through 

development of cheaper mining and recovery methods. 

Concealed deposits or extensions of known deposits may 

be discovered as a result of detailed geological studies 

and geochemieal prospecting (:for example, see Earll, 

1964, p. 30-3). As technology improves, perhaps t'he 

low-grade gold deposits, such as those at the Diamond 

Hill and Keating mines, could be profitably mined. 

Parts of the area are judged worthy of more detailed 

geologic, geochemical, and perhaps geophysical study 

in the search :for concealed deposits or extensions of 

known deposits. 

• 
·.i. 

MEASURED SECTIONS 49 

AURIFEROUS PYRITE VEINS 

Auriferous pyrite veins in the Radersburg and Hassel 

areas have accounted for most of the value of metal 

produced from the mapped area. The oxidized and enriched parts of the known veins have been largely or 

wholly mined. Large tonna.ges of unoxidized vein material remain but apparently have not been profitable to 

mine. For example, the Keating mine was reopened during 1947 and about 18,000 tons of ore was mined (U.S. 

Bur. Mines, 1947, p. 1449); if the total output of lode 

gold from the Cedar Plains district during that year 

(2,472 oz, U.S. Bur. Mines, 1947, p. 1446) was from this 

ore, the indicated recovery was 0.138 oz per ton '($4.83 

per ton at $35) . A 2,800-foot-long adit in unoxidized 

material at the Diamond Hill mine is reported to be in 

"ore assaying from 80 cents to $2.50 a ton" (Stone, 1911, 

p. 82) based on $20.67 per oz price for gold; the equivalent value is $1.35 to $4.25 per ton at $35 per oz price 

of gold. The tonnage of gold -bearing material that may 

be of about this grade is larger, probably at }east several 

hundred thousand tons, of which a part might be amenable to open-pit mining. Some of this rock also contains 

small quantities of scheelite, but most of it almost certainly does not contain as much as 0.1 percent wo3. 

Undiscovered oxidized ore bodies in auriferous pyrite 

veins may exist beneath Cenozoic deposits south of the 

!(eating and Ohio-l(eating mines in the Radersburg 

area. The veins in this area were exposed to oxidation 

and enrichment in Tertiary time (p. 39), and the belt 

of mineralization west of Radersburg can reasonably be 

extended southward beneath the Cenozoic cover, which 

is not more than a few hundred feet thick. Oxidized 

pyritic gold veins also might be sought beneath the 

gravel-covered area in Cold Springs Gulch. 

QUARTZ VEINS WITH BASE AND PRECIOUS-METALS 

AND QUARTZ VEINS WITH AURIFEROUS PYRITE 

Small and intermittent production can be expected 

from quartz veins with base and precious metals and 

from quartz with auriferous pyrite veins. Almost without exception veins of both types are narrow, and few of 

the known ore shoots extend more than several hundred 

feet in length or depth. Furthermore, values beneath the 

oxidized zone tend to be erratic. From these considerations, it seems unlikely that new deposits or extensions· 

of known deposits will be found that will yield more 

than several thousand tons of direct shipping ore or that 

would yield enough ore of lower grade to sustain a mill 

of capacity larger than about 50 tons per day. 

REPLACEMENT BODIES IN CARBONATE ROCK 

More intensive exploration in the Quartzite Ridge 

area might lead to the discovery of high-grade but relatively small replacement bodies of silver-lead-zinc ore. 

Geochemical prospectJing in carbonate rocks known to 

be favorable host rocks would probably be a logical first 

step. 

GOLD PLACERS 

The gravel along Cold Springs Gulch (p. 14) was 

deposited by an ancestral Indian Creek that headed 

in the Park and Hassel areas, which are noted for their 

gold veins; these areas contributed the placer gold 

mined from younger gravels along the present channel 

of Indian Creek. The gravel along Cold Springs 'Gulch 

accordingly may contain placer concentrations of gold 

derived from the Park and Hassel areas. 

Placer deposits of gold may also be buried beneath 

the Oligocene sedimentary tuff in the vicinity of Radersburg. The vein deposits in the Radersburg area were 

exposed to erosion before deposition of the tuff 

(p. 39), and gold eroded from the veins could have 

been deposited with gravels in now-buried stream channels, or concentrated in the lower part of the Oligocene 

sedimentary unit itself. 

Neither the gravel in Cold Springs Gulch nor the 

base of Oligocene sedimentary tuff near Radersburg 

appears to have been prospected for placer gold, but 

both areas are considered promising. 

MEASURED SECTIONS 

SECTION 1.-M orris on, Swift, Phosphoria (and related st1·ata), 

Quadrant, and Amsden Formations near Indian Oreek 

[Measured by M. R. Klepper, 1948, along and near road on northwest 

side of Indian Creek (pl. 1) NW:!;.I, sec. 5, T. 6 N., R. 1 E., to approxi- mate center sec. 32, T. 7 N., R. 1 E.] 

Kootenai Formation: 

Sandstone, "salt-and-pepper," gray; speckled with 

darker chert grains ; light gray to medium light 

gray and medium coarse grained in lower 9 ft; 

greenish gray, finer grained, and less conspicuously 

speckled in upper 14.6 ft. Approximately 24 ft 

thick. 

Disconformity. 

Morrison Formation: Feet 

129. Concealed ---------------------------------- 11 

128. Sandstone, gray, speckled, "glassy," relatively 

fine grained-------------------------------- 1.5 

127. Concealed ----------------------------------- 2 

126. Limestone, olive-brown, very finely crystalline, 

sandy or silty; in beds 1 ft. thick; weathers 

brown ------------------------------------ 10 

125. Mudstone, olive and gray, fissile; interbeds of 

silky dolomite; weathers yellowish brown ---- 24 

124. Siltstone, light-olive-gray to yellowish-orange, 

dolomitic, blocky___________________________ 10 

123. Dolomite, gray, dense, siltY-------------------- 3 

122. Concealed ----------------------------------- 6 

121. Sandstone, gray, silty; speckled with dark chert 

fragments --------------------------------- 6 
50 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

SECTION 1.-M orris on, Swift, Phosphoria (and related strata), 

Quadrant, and Amsden Formations near Indian Oreek-Con. 

Morrison Formation-Continued Feet 

120. Sandstone, salt-and-pepper, medium-gray with 

faint greenish tint ; in beds about 0.5 ft. 

thick ; lower 2 ft. silty and :tine grained, 'upper 

6 ft. medium coarse grained_________________ B 

119. Concealed; mostly if not wholly underlain by 

dark carbonaceous mudstone and one or more 

lignite. layers______________________________ 20. 5 

118. Concea;led ----------------------------------- 10 

117. Mudstone, yellowish-orange, punky, blocky; two 

lignite beds, one 0.5 ft. thick and one 0.7 ft. 

thi<;!k, in upper 2 ft of unit__________________ 6 

116. Concealed ; soil in part reddish________________ 45. 5 

115. Shale, olive-gray and dark-greenish-gray, hackly to pencilly _____________ ..,;,___________________ 16 

114. Concealed ----------------------------------- 9. 5 

113. Mudstone, olive-gray and brownish-olive; in 

part calcareous____________________________ 8 

112. Concealed----------------------------------- 8. 5 

111. Mudstone, olive-brown; in beds 0.1 ft thick_____ 2. 5 

110. Concealed ----------------------------------- 4 

109. Mudstone, olive and dark-gray---------------- 13 

108. Limestone, siltstone, and sandstone; olive and 

olive-gray silty limestone, limy siltstone, and 

fine-grained sandstone in beds 0.2l>--0.5 ft 

thick; in part fissile ; abundant darl{-gray 

cherty fragments in the upper 6 ft o·f unit; 

weathers orange brown to drab brownish 

gray-------------------------------------- 27.5 

107. Limestone, olive and gray, silty; weathers dark 

yellowish orange to moderate yellowish brown or lighter __________________________________ 15.5 

106. Concealed ----------------------------------- 9 

105. Sandstone, light-brownish-olive, speckled, finegrained, calcareous; weathers to medium light 

gray -------------------------------------- 5 

104. Quartzite, light-olive, dense, calcareous________ 1 

103. Limestone, dolomite, and siltstone ; brownisholive-gray, pale-yellowish-brown, and yellowish-brown, silty and dense limestone and dolomite and limy or dolomitic siltstones with carbonate rocks predominant; siltstones commonly in beds less than 0.02 ft thick, carbonate rocks in beds as much as 0.7 ft thick; unit 

weathers dark yellowish orange, grayish orange, and light olive________________________ 31 

102. Concealed ----------------------------------- 86 

101. Siltstone, weathers orange brown______________ 2 

100. Siltstone, punky, calcareous, or dolomitic; in 

part very fine grained sandstone; unit weathers yellowish brown________________________ 3. 5 

99. Sandstone, pale-yellowish-brown, light-olive, and 

light-gray, speckled, fine-grained, som·ewhat 

crossbedded; in beds 0.2-1 ft thick; weathers 

pale yellowish brown_______________________ 19. 5 

98. Siltstone, orange-brown, calcareous, fissile ; upper half· of unit mostly concealed__________ 22. 5 

97. Siltstone, calcareous; weathers dull yellowish brown _________________________ ..,;, __________ 11.5 

SECTION 1.-Morrison, Swift, Phosphoria (and related strata), 

Quadrant, and Amsden Formations near Indian Oreek-Con. 

Morrison Formation-Continued 

96. Siltstone and limestone; hackly to fissile siltstone interlayered with thicker beds of silty 

limestone ; weathers dark yellowish orange __ _ 

95. Limestone, brownish-olive, silty; weathers 

brownish ---------------------------------

Q4. Siltstone and limestone; sim'ilar to unit 96, but 

siltstone is calcareous _____________________ _ 

93. Concealed; underlain at least in part by brownish-gray, olive, and grayish-orange calcareous 

siltstone that weathers nearly dark yellowish 

orange 

92. Siltstone, 1b~ownish-gray, olive, and grayishorange, calcareous, fissile to thin-bedded ; 

weathers nearly dark yello'wish orange _____ _ 

91. Limestone, olive-gray, silty, indistinctly ·bedded; 

weathers gray to brown ___________________ _ 

Total thickness of the Morrison FormaFeet 

15 

0.5 

11.5 

17 

16 

4 

tion -------------------------------- 523 

Swirt Formation: 

90. Concealed ----------------------------------- 7 

89. S-andstone, brownish-gray, speckled, finegrained ; consists of whitish to glassy. surb-

'angul,ar to subrounded quartz grains in finegrained br,own-stained sand matrix; unit 

weathers !brownish gray-------------------- 11 

88. San:dstone, moderate-dusky-yellow, speckled, 

fine-grained, with subrounded grains, very calcareous, frialble, locally pebbly-------------- 1 

Total thickness of S.wift Formation______ 19 

Disconformi ty. 

Phosphoria Formation ·and rel•ated strata: 

87. Chert, dark-gray, mottled dull red, ·brown, and 

white ------------------------------------- 1 

86. Quartzite, dark-gray, vitreous, very fine grained; 

contains opaque white-weathering ovoids 

(proba,bly ,phosphatic material) as much ·as 14 

in. long, :and near ~top a few pebbles of 

quartzite ---------------------------------- 16 

85. Qua·rtzite, light-grayish-brown, very fine grained, 

vitreous; one bed__________________________ 3 

84. Sandstone, dolomitic, lig.bt.,brownish gray, finegrained; in beds as much :as 1 rt :thick________ 10 

83 . .Sandstone, silty, orange.;brown, strongly limonitic, .punkY-------------------------------- 1 

82. Igneous rock; 3lh ft .thick. 

81. ,Concealed ----------------------------------- 1 

80. Chert and very fine-grained quartzite, light-gray 

and light-bro,wnish-gray; weathers to yellowish OI,ange, grayish orange, :and yellowish gray 

rough surface ; some beds are chert, some nre 

quartzite, and some contain both chert and 

quartzite ---------------------------------- 15 

Total 'thickness of Phosphori!a Formation 

and rel·ated strata____________________ 47 

~ 

).. 
• 

MEASURED SECTIONS 51 

SECTION 1.-M orris on, Swift, Phosphoria (ana related strata), 

Quadrant, ana Amsden Formations near Indian Greek-Con. 

Quadrant Formation: Feet 

79. Quartzite, pinkish-gl'ay to light-grayish-brown; 

in !beds 1-2 ft thick________________________ 7 

78. Concealed ----------------------------------- 5 

77. Quartzite, simila.r to unit 79__________________ 11 

76. Concealed ----------------------------------- 33 

75. Dolomite, sandy ; weathers light gray---------- 3 

74. Quartzite, pale-gray and brown, fine-grained 

vitreous ; weathered surface is brown ·and 

rough ------------------------------------ 3 

73. Dolomite, light-gray, thin-'bedded, sandy to fine 

crystalline ----------------.---------------- 2. 5 

72. S•andstone, dolomitic_. honey-colored, very fine 

grained; weathered surface very rough_______ 2 

71. Dolomite, sandy, light-gray ; contains irregular 

chert masses; 2 in. 1band of gray chert at top__ 2 

70. Dolomite, dense; weathers very light gray; in 

·beds as much .as 2 ft thick; contains thin sandy 

layer nea·r tOP------------------------------ 10 

69. Quartzite, white very fine grained, thin-bedded__ 4 

68. Dolomitie, cherty; weathers pale bluish gray; 

contains fossil fragments___________________ 1 

67. Concealed ----------------------------------- 3 

66. Quartzite, cherty matrix with quartz grains, 

yellowish-gray ----------------------------- 1 

65. Concealed ----------------------------------- 15 

64. Quartzite, white, very fine grained____________ 4 

63. Concealed ----------------------------------- 6 

62. Dolomite, dense; weathers light gray, contains 

3 beds of quartzite each 1 in. thick near top__ 2 

61. Quartzite, dolomitic, pale gray, dense; middle 

part weathers to pitted surface_____________ 4 

60. Quartzite; similar to unit 61, but weathered surface is ribbed______________________________ 4 

59. Concealed ----------------------------------- 11 

58. Dolomite, cherty, dense; weathers light gray; 

contains thin beds of quartzite in upper halL_ 15 

57. Quartzite, honey-c.olored; bedding thick and indistinct ----------------------------------- 18 

56. Concealed ----------------------------------- 11 

55. Sandstone, dolomitic, fria:ble__________________ 1 

54. Dolomitic, sandy; l-in. layer of gray chert near 

top--------------------------------------- 4.0 

53. Quartzite, dolomitic, honey-colored____________ 1 

52. Sandstone, calcareous, or limestone, sandy, almost white, punky_________________________ 5. 5 

51. Dolomite, dense, light gray ; contains chert laminae in lower half__________________________ 11. 5 

50. Concealed ----------------------------------- 8 

49. Sandstone, calcareous, or limestone, sandy, lightgray, friable _______________________________ 10.5 

48. Quartzite, light-gray to light-yellowish-gray; 

weathers pale brown ; upper 4 ft. vitreous ; 

lower 1 ft calcareous and friable____________ 5 

47. Sandstone, yellowish-gray, calcareous, finegrained ----------------------------------- 2 

46. Dolomite, dense; weathers white______________ 1 

45. Dolomite, very finely crystalline, light-gray ; 

· weathers darker gray----------------------- 1 

SECTION 1.-M orris on, Swift, Phosphoria (ana related strata), 

Quadrant, ana Amsden Formations near Indian Greek-Con. 

Quadrant Formation-Continued 

44. Dolomite, dense gray; weathers white ________ _ 

43. Concealed -----------------------------------

42. Dolomite, sandy, gray------------------------

41. Concealed -----------------------------------

40. Sandstone, dolomitic, yellowish-gray and very 

Feet 

1 

2 

5 

2 

light gray, fine-grained; upper part quartzitic_ 4 

39. Dolomite, sandy; weathers very light gray____ 3 

38. Dolomite, honey-colored, sandy---------------- 1 

37. Quartzite, very light gray and yellowish-gray, 

fine-grained------------------------------- 1.5 

36. Covered-------------------------------------

35. Sandstone, dolomitic, pale-yellowish-brown, thinbedded ------------------------------------ 2 

Total thickness of the Quadrant Formation_ 250 

Contact between Quadrant Formation and Amsden 

Formation is gradational and is placed at bed above 

uppermost red or pink rocks. 

Amsden Formation : 

34. Quartzite, dolomitic, pale~red; weathers pale 

brownish gray_____________________________ 1 

33. Siltstone and fine-grained sandstone, in part 

dolomitic; weathers light yellowish gray, pinkish gray, and white ; thin bedded ( :JA;-:14 in.), 

in part crossbedded and very fine grained 

sandstone --------------------------------- 10 

32. Siltstone, red; in part slightly dolomitic______ 3 

31 .. Conglomerate; weathers pale red; fragments 

are from underlying bed-------------------- 1 

30. Dolomite, silty, very pale gray, thin-bedded____ 0. 5 

29. Sandstone, pale-red, coarse and gritty, crossbedded ------------------------------------ 1 

28. Sandstone, slightly dolomitic, very light gray, 

very fine grained___________________________ 0.5 

27. Sandstone, grayish-red, very fine grained, 

oolitic? ----------------------------------- 1 

26. Concealed; float is grayish-red mudstone and 

very fine grained sandstone________________ 10 

25. Dolomite, silty, pale-red______________________ 1 

24. Siltstone, dolomitic, pale-reddish-gray--------- 0. 5 

23. Quartzite, dolomitic, pale-gray to pinkish-gray- 2 

22. Shale, papery, grayish-red____________________ 3 

21. Dolomite, pinkish-gray and blue-gray, dense___ 0. 5 

20. Quartzite, light-gray, slightly dolomitic; 

weathers yellowish gray and brown ; in beds 

about 3 in. thick__________________________ 1.5 

19. Concealed; float is red siltstone_______________ 7 

18. Dolomite, pale-bluish-gray, dense______________ 4 

17. Ooncealed; float is red siltstone_______________ 5 

16. Dolomite, reddish-gray to pinkish-gray, dense; 

in beds 6-8 in. thick_______________________ 3 

15. Concealed ---------------------~------------- 7 

14. Dolomite, gray, dense_________________________ 1 

13. Dolomite, dense, pale-olive-gray; weathers 

grayish orange_____________________________ 6 
52 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

SECTION 1.-Morrison, Swift, Phosphoria (and related strata), 

Quadrant, and Amsden Formations near Indian Greek--Con. 

Amsden Formation-Continued Feet 

12. Limestone, fine-grained, dark-g.ray ; weathers 

bluish gray to yellowish gray ; most beds in 

lower 10 ft are 4-10 in. thick and in upper 18 

ft are 12-18 in. thick; chert present as nodules 

and thin layers; 3 ft above base is bed with 

8Jbundant small horn corals overlain hy 4-in. 

bed of black calcareous paper shale; darkgray pencilly shale in beds as much as 1 in. 

thick present throughout; beds of fossiliferous, 

light-g.ray crystalline limestone 5.5-7.5 ft 

below tOP---------------------------------- 34 

11. Limestone, dolomitic, or dolomite, calcareous, 

dense, yellowish-gray; weathers light gray; in 

beds 8 in. to 2 ft. thick_______________________ 15 

10. Concealed ------------------------------------ 38 

9. Dolomite, calcareous in lower half of unit, pale 

gray locally stained pink, dense______________ 11 

8. Concealed ----------------------------------- 12 

7. Dolomite, dense, light-gray ; weathers with brownish hue; in beds 6-8 inches thick; interbedded 

with dusky-red siltstone_____________________ 3 

6. Siltstone, grayish-red to dusky-red on fresh and 

weather surfaces___________________________ 3.5 

5. Concealed ----------------------------------- 5 

4. Limestone, light-gray to almost white, finegrained; contains chips of red siltstone_______ 6 

3. Concealed ----------------------------------- 7 

2. Dolomite, light"'gray partly stained pink; in beds 

6 in. thick ; contains a few thin interbeds of 

grayish-red siltstone------------------------- 15 

1. Concealed; float is red siltstone, quartzite and 

dolomite; interval main1y or wholy underlain 

by red siltstone_____________________________ 96 

•rotal thickness of Amsden Formation _______ 315 

Mission Canyon Limestone : 

Limestone, massive, cliff-forming; weathers light gray. 

not measured. 

SECTION 2.-Morrison and Swift Formations near Montana 

Silver Star mine 

[Measured, by 1\f. R. Klepper, 1948, in SW~ sec. 16, T. 4 N., R. 1 W., 

about 2,000 ft east of Montana Silver Star mine (2 miles south of 

southwest corner of mapped area)] 

Kootenai Formation (basal part only) : Feet 

46. Sandstone, salt-and-pepper, light-gray speckled 

with darker chert grains, coarse-grained in 

beds as much as 1 ft thick; partly crossbedded; basal part gritty and locally pebbly___ 50 

Disconformity 

Morrison Formation : 

45. Concealed; some brown-weathering mudstone in 

float -------------------------------------- 23 

44. Sandstone, speckled, medium-grained, crossbedded ------------------------------------ 1 

43. Concealed ------------------------------------ 15 

42. Siltstone and rsandstone, fine-grained, yellowishgray, thin-bedded, friable____________________ 4 

SECTION 2.-Morrison and Swift Formations near Montana 

Silver Star mine-Continued 

Morrison Formation-Continued Feet 

41. Mudstone, medium-gray, blocky weathering, contains a few thin beds of yellowish-gray siltstone and fine-grained sandstone_____________ 4 

40. Sandstone, yellowish-gray, fine-grained, friable__ 1 

39. Shale, dark-gray, fissile to papery-------------- 5 

38. Concealed; mainly sandstone float_____________ 43 

37. Sandstone, light-gray to light-brownish-gray, in 

part color laminated; speckled with dark chert 

grains ; some beds very argillaceous ; some contain abundant chips of black shale; thin to 

medium bedded, in part crossbedded__________ 19 

36. Concealed; lower half of interval covered by dark 

shale and mudstone float_____________________ 52 

35. Mudstone, dull-red and olive, slightly calcereous; 

weathers to small chips; alternates with darkolive, very calcareous siltstone and fine-grained 

sandstone; unit weathers to plates and chunks 

averaging 1 in. on a side---------------------- 19 

34. Concealed; abundant hematite-stained light-gray 

sandstone float near middle of intervaL_______ 30 

33. Siltstone and very fine grained sandstones, olivegray, calcareous; interval contains a few beds 

of dull-grayish-red silty limestone-'----------- 15 

32. Shale, alternating dull-red and green; interval 

partly covered------------------------------ 21 

31. Concealed ------------------------------------ 27 

30. Limestone, greenish-gray, dense, sublithographic_ 4 

29. Concealed, red soiL____________________________ 21 

28. Limestone; similar to unit 30------------------ 5 

27. Concealed ------------------------------------ 2 

26. Limestone, greenish-gray, dense________________ 1 

25. Concealed; lower part probably underlain by olive 

and brownish siltstone and fine-grained sandstone and upper half 'by red shale------~----- 20 

24. Sandstone, calcareous, friable; alternating with 

sandy and silty limestone ; most beds are olive 

or brownish yellow on fresh and weathered 

surfaces ----------------------------------- 11 

23. Concealed ------------------------------------ 11 

22. Limestone, impure ; alternating with calcareous 

shale -------------------------------------- 5 

21. Limestone, dolomitic, crystalline, pale-yellowishbrown ------------------------------------- 1. 5 

20. Shale, red and grayish-yellow, flaggy, calcareous_ 4 

19. Mudstone and shale, mostly grayish-red, chippy 

to hackly; contains conspicuous pale-greenishgray to grayish-red limy concretions_________ 18 

18. Shale, grayish-red; few thin interbeds of dense 

brownish-gray limestone_____________________ 4. 5 

17. Siltstone, pale-yellow, calcareous; grades upward 

into thin-bedded dolomitic limestone__________ 1 

16. Shale, grayish-red, greenish-gray, and olive, 

partly mottled, very slightly calcareous, fissile 

to hackly ; contains concretions and at least one 

0.5-ft bed of dense greenish-gray limestone____ 14 

15. Mudstone, greenish-gray, calcareous------------ 1 

14. Limestone, pale-greenish-gray, dense, argillaceous 0. 5 

13. Siltstone, greenish-gray________________________ 1 

• 
_tl 

MEASURED SECTIONS 53 

SECTION 2.~Morrison and Swift Formations near Montana 

Silver Star mine-Continued 

Morrison Formation-Continued Feet 

12. Limestone, weathers light gray; blocky fracturing; contains a few small irregular cherty concretions------------------------------------ 0.5 

11. Sandstone, greenish-gray, very fine grained_____ 0. 5 

10. Limestone, pinkish-gray, dense, silty; weathers 

greenish-gray ------------------------------ 0. 5 

9. Concealed ----------------------------------- 1 

8. Dolomite, grayish-orange, calcareous, finegrained ------------------------------------ 4 

7. Concealed; pale-yellow clayey soiL_____________ 9 

6. Siltstone, dusky-yellow; weathers into blocks 

about 1 in. on a side_________________________ 2 

5. Concealed; float is mixed fine-grained sandstone, 

shale, and dense limestone__________________ 16 

Total thickness Morrison Formation ________ 438 

Swift Formation: 

4. Sandstone, grayish-orange, pale-yellowish-brown, 

and light-olive-gray, fine- to medium-grained, 

thin-bedded, locally crossbedded, friable; in 

part argillaceous and in part weakly calcareous ------------------------------------ 18 

3. Sandstone, gray, calcareous, . friable; contains 

abundant fragments of pelecypod shells______ 8. 5 

2. Concealed --~-------------------------------- 3.5 

Total thickness Swift Formation_____________ 30 

Disconformity. 

Phosphoria Formation : 

1. Chert -------------------------------- Not measured 

SECTION 3.-Kootenai Formation and pa1·ts of the Colorado and 

Morris on Formations 

[Measured by E. T. Ruppel and 1\L R. Klepper, 1948, in SW:IA, sec. 5, 

T. 6 N., R. 1 E.] · 

Colorado Formation (part of lower unit only) : Feet 

67. Shale, medium-dark-gray, fissile, papery to thin:ly 

platy; in part contains very small flakes of 

mica --------------~----------------------- 41 

66. Siltstone, dark-gray, hard, blocky; alternating 

beds of dark-gray and very dark gray, fissile, 

papery shale in. lower 20.5 ft of unit; two moderately thick beds of similar shale in the upper 

28.6 ft; weathers yellowish gray____________ 49 

65. Concealed ; probably underlain by olive and gray 

compact blocky siltstone____________________ 27. 5 

64. Siltstone, dark-gray, compact, blocky____________ 6 

63. Sandstone, light-olive-gray to yellowish-brown, 

ch~an, quartzitic; weathers pale yellowish 

brO\Vn ------------------------------------- 2. 5 

Total measured thickness of Colorado 

Formation ----------------------------- 126 

Kootenai Formation : 

62. Siltstone, dark-gray, coarse-grained, compact, 

blocky ; very calcareous in lower 2 ft________ 5. 5 

61. Concealed ; shale intervaL_____________________ 4 

60. Limestone, dark-gray, dense to fine-grained, 

lumpy-weathering -------------------------- 2 

59. Concea·led ------------------------------------ 12 

SECTION 3.-Kootenai Formation and parts of the Colorado and 

Morris on Formation-Continued 

Kootenai Formation-Continued Feet 

58. Limestone, medium-dark-gray, irregularly bedded ; 

weathers medium g-ray to medium light gray-- 2. 5 

57. Siltstone, micaceous; contains 0.5-ft-thick platy 

gray limestone bed in middle .of unit________ 3 

56. Limestone, gray, platY------------------------ 1 

55. Sill of Late Cretaceous and Paleocene •age, decomposed; contains abundant biotite phenocrysts; 

2ft .thick .. 

54. Limestone. dark-gray, very fine grained; contains 

·scattered pebbles ·and chips of limestone 

similar to that in un~t 53; weathers light gray 

to light olive gray-------------------------- 5 

53. Limestone, medium-dark-gray, granul·ar, fetid; 

fossil gastropods abundant; weathers medium 

gray rto da·rk yellowish brown_______________ 2. 5 

52. Limestone, medium-gray to medium-dark-gray, 

very fine to medium grained; contains a few 

g·astropod-rich layers; weathers medium light 

gray--------------------------------------- 5.5 

51. Limes·tone, similar to unit 53, but only slightly 

fetid--------------------------------------- 1.5 

50. Concealed------------------------------------ 10.5 

49. 1Shale, grayish-red to grayish-purple and greenishgray to light-olive-gray; grayish red predominant in lower 48ft, greenish gray predominant 

in upper 23.6 ft; fissile to thinly platy; con-

·tains irregular thin, lenticul·ar, and ovoidal 

dense limestone concretions from 4.3 to 14 ft 

a1bove base, and 0.2-rt-thick brown-weathering 

dense concretionary limestone tbed 48 :ft above 

base--------------------------------------- 71.5 

48. Shale, pale-red to grayish-red, and subordinate 

light-olive-gray to greenish-gray interbeds; 

fissile, chippy, rand hackly; zone of irregular 

ovoidal to spherical 'brown-weathering limestone 

concretions fr:om 12.1 -to 13.3 ft above base____ 17 

47. Mudstone, light-olive-gray to olive-gray, and 

subordinate grayish-red inter:beds, platy to 

fissile ------------------------------------- 3 

46. Limestone; pale-purple to pale-reddish-purple, 

locally with pale-green tint, very fine gvained, 

subconchoidal fracture; weathers dark yellowish brown_______________________________ 0. 5 

45. Mudstone, light-olive-gr:ay to olive-gray, and 

grayish-red interbeds in upper 6ft, platy to subfissile ; thin zone of limestone concretions 

20.6 ft ahove base is similar to those of unit 

49 ----------------------------------------~ 27.5 

44. Concealed ---.--------------------------------- 20 

43. Mudstone, light-olive-gray, platy----,----------- 4 

42. Limestone, dark-gray, dense; contains scattered 

small calcite crystals; very fossiliferous____ 0. 5 

41. Shale, yellowish-gray to light-olive-gray, fissile 

to chirppy___________________________________ 3 

40. Shale, dusky-'blue to grayish-purple, fissile to 

chippy ------------------------------------- 3 

39. Concealed ------------------------------------ 3 

38. Limestone. pale-purple to grayish-red-purple, 

very fine gr:ained; weathers dusky yellowish 

brown ------------------------------------- 1 
54 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

SECTION 3.-Kootenai Formation ana parts ot the Colorado ana 

Morris on Formatio11r-Continued 

Kootenai Formation-Continued Feet 

37. !Shale, gr·ayish-red, and grayish-olive interbeds 

in lower 6 ft, fissile ; a:bundant limestone concretions similar to those in unit 49 in upper 

1 ft________________________________________ 16 

36. Shale, greyish-olive, fissile, micaceous ; contains 

.thin, lenticular, and flattened grayish-olive 

dense limestone concretions in upper 4 ft_____ 15 

35. Mudstone, light-olive-gray to greenish-gray, fissile, pencilly, and chippy; contains .sparsely 

disseminated light-gray dense limestone concretions up to 2 in. in diameter ; upper 1 ft is principally medium-gray concretionary limestone 

that weathers to ir.regular rounded pieces up to 

t8 in. in diameter--------------------------- 16 

34. Mudstone, light-greenish-gray, very thin and i·rregularly bedded ; weathers very light gray____ 6 

33. Mudstone, greenish-gray ·and yellowish-gray, fissile to 'blocky; pencilly in lower 5 ft; weathers 

yel!lowish orange____________________________ 10 

32. Shale, grayish-purple tto very dusky puJ:l)le, very 

fi·ssile; conspicuous rounded su'blithographic 

limestone concretions up to 5 in. in diameter 

in zones 2 ft •thick near the top and ibottom 

of unit_____________________________________ 16 

31. Concealed; red shale float______________________ 19 

30. Siltstone, yellowish gray to grayish yellow in 

lower half and pale red to grayish red in upper 

half of unit; sparse limestone concretions 

throughout --------------------------------- 9 

29. Sandstone, white, clean, in beds 0.4 ft thick; 

a few interbeds of grayish-yellow mudstone, in 

part irregularly ripple marked ______________ _ 6 

28. Sandstone, pale-yellowisho~brown, limonitic; contains subordinate rounded black chert grains; 

medium-grained; in ·beds 0.4-1 ft thick______ 17 

27. Sandstone, yellowi~h-gray, fine-grained, compact; 

in ·beds 0.5-0.7 ft thick; contains interbeds 

of yellowish-orange, grayish, ·and grayish-red, 

rp1aty, fissile, irregularly lumpy sandstone ___ _ 

26. Concealed; float is red mudstone ______________ _ 

25. Mudstone, pale-red to grayish-red, fissile to platy, and lumpy _________________________________ _ 

24. Sandstone, medium-grained, limonitic _________ _ 

23. Mudstone, s'imilar to unit 25------------------

22. Mudstone, yellowiS'h...orange, p1aty to chippy ___ _ 

21. Concealed -----------------------------------

20. Conglomera·te, fine-grained sandstone matrix and 

pebbles of yellowish siltstone and light-gray 

:li:mestone ----------------------------------

19. Concealed -----------------------------------

9 

8.5 

2 

0.5 

13.5 

1.5 

4 

2 

6 

18. 'Sandstone, pale-reddish-brown, very fine grained_ 2 

17. Sandstone medium-gray, "salt-and-pepper" ruedium grruined, compact, quartzitic; in 1beds 0.25--

0.75 ft thick________________________________ 10 

16. Mudstone, intel'lbedded pale-red and grayish-yel-

. low, chippy to lumpy; 0.3-ft thicik ibed of reddish-brown fine-grained 'blocky sand!stone at 

base of unit-------------------------~------ 13 

SECTION 3.-Kootenai Formation ana parts ot the Colorado ant! 

Morris on Formatio11r-Continued 

Kootenai Formation-Continued Feet 

15. ·Sandstone, pale-reddish..ibrO'Wn, very fine grained, 

!blocky; contains a few thin ·beds of grayishyellow, fissile to lumpy mudstone____________ 4 

14. Conglomerate, •subrounded gray limestone and 

orange siltstone fragments up to 0.25 in. in 

diameter in a matrix of sand and mud________ 2 

13. Mudstone, intel'lbedded yellowi!sh-gray and palered, fissile to platy and ll];J]lpy ---------------- 15 

12. Concealed ------------------~---------------- 2 

11. Sandstone, pale-yellowish-~brown, slightly calcareous, very fine grained; contains sparsely 

disseminated ·black chert pebbles_____________ 1 

10. Concealed ----------------------------------- 7 

9. IJi.mes·tone, pale-yellowish-brown, dense, sulblitJhographic, and yellowish-gray mud!stone; 

edgewise pelblble conglomerate zone in upper 

0.5 ~t-------------------------------------- 7 

8. Mudstone, oUve-gray, somewhat hackly; contains 

.fmJ:aU limestone chips or flattened concretions_ 9 

7. Concealed ----------------------------------- 9 

6. Mudstone, similar to unit 8, •but contains small 

concretions of light-olive-gray dense limestone 

in upper pa1'1t------------------------------- 17 

5. Mudstone, dark-gray, platy; weathers light gray_ 1 

4. Concealed ------------------------·----------- 8 

3. Sand!stone, pale-yellowis'h.J,brown to light-gray, 

salt-and-pepper medium-grained, homogeneous, 

quartzitic, very indistinctly 1bedded to massive; 

weather.s to large irregular blocks up to 6 ft 

in diameter________________________________ 36 

Total ·thickness of ~the Kootenai Formation __ 529 

Morrison Formation : 

2. Concealed ----------------------------------- 35 

1. Sandstone, light-olive-gray, medium-fine- to medium-gra-ined, limonitic, in 'beds less than 1 ft 

thick, which are crossbedded on a small scale__ i$1 

SECTION 4.-Coloraao Formation near south end of mapped 

area 

[Measured by M. R. Klepper, 19:49, along shallow gully that approximately coincides with boundary between NE* and SE:J4 sec. 4, T. 4 

N., R.l W.] 

Slim Sam Formation: 

Measured section published in Klepper, Weeks, and 

Ruppel ( 1957, p 31). 

Basal unit is sandstone, light-olive-gray, thin-bedded, 

non-resistant; contains a few thin beds of siltstone in lower half. "Mactra" arenaria Meek, a 

pelecypod not known from rocks as young as the 

Montana Group was collected from beds about 300 

ft (USGS Mesozoic loc. 23039) and 375 ft (USGS 

Mesozoic loc. 23040) a·bove the bas·e of the formation ( W. A. Cobban, written commun. May 31, 

Feet 

1951) ------------------------------------·----- 55 

Gradational contact. 

' 
,,. 

l\1EASURED SECTIONS 55 

SECTION 4.-0olorado Formation near south end of mapped 

area-Continued 

Colorado Formation : Feet 

Upper black shale unit: 

66. Shale (80-90 percent), dark-gray to very 

dark-gray, chippy to fissile, and darkgray very fine grained micaceous sandstone 

(10-20 percent) ; shale contains sporadic 

moderate-yellowish-brown-weathering limestone concretions up to 6 in. in diameter. 

Collection (USGS Meso~oic loc. 23032) 

near top of unit contains Inoceramus cf. I. 

umbonatus Meek and Hayden, Scaphites 

ventricosus Meek and Hayden, ·and Scaphites cf. S. tetonensis Cobban. Another collection (USGS Mesozoic loc. 23031) near 

middle of unit contains Inoceramus umbonatus Meek and Hayden, Ostrea congesta 

1Conrad, and Baculites asper Morton. A collection ( USG'S Mesozoic loc. 23030) near 

the 'base of the unit contains Inoceramus cf. 

I. umbonatus Meek and Hayden, and Ostrea 

congesta Conrad. These collections are indicative of the next faunal zooe rubove the 

zone of Inoceramus deformis of earliest 

Niobrara age (W. A. Cobban, written 

commun., May 31, 1951)------------------ 254 

65. Concealed; probable duplication by faulting__ 97 

64. Shale, very dark gray, and olive-gray; mudstone ; contains conspicuous brownweathering limestone concretions________ 30 

Total thickness of upper black shale 

unit_ ________________ maximum 3 ____ 381 

minimum_____ 284 

Gradational contact. 

Middle ·siliceous mudstone and sandstone unit: 

63. Concealed; sandstone float________________ 22 

62. Sandstone, coarse-grained, crossbedded ; 

speckled with chert and feldspar(?) 

grains -------------------------------- 4 

61. Concealed; floa;t irs sandstone ·similar to 

unit 62-------------------------------- 24 

60. Sandstone, quartzitic, medium-light-gray, 

very fine grained, hard ·and 'blocky; contains scattered woody fragments_________ 4 

59. Concealed ------------------------------- 45 

58. Sandstone, quartzitic, medium-d.ark-gray____ 5 

57. Sandstone, lig:ht-gray and olive, thin- to medium-bedded; in pavt speckled with chel'lt 

and feldspar grains; contains a few inter-

'beds of 'blocky mudstone_________________ 88 

56. Concealed ------------------------------- 56 

55. 'Sandstone, light-gray, ·speckled, crossbedded ; 

contains a1bundant woody fr.agments near 

·barse and one bed of blocky quartzitic 

sandstone near tOP---------------------- 37 

54. Mudstone, med-ium-dark-gray, siliceous, 

blocky -------------------------------- 7 53. Concealed ______________ _:_______________ 87 

a About 100 ft. thicker than this unit at other localltles: some beds are probably duplicated by a concealed fault or faults in interval 65 ; true thickness is greater than 284 ft. and probably less than· 381 ft. 

SECTION 4.--,0olorado Formation near south end of mapped 

area-Continued 

Colorado Formation-Continued Feet 

Middle siliceous mudstone and sandstone unit-Con. 

·52. Mudstone, similar to unit 54-------------- 24 

51. Mudstone, yellow, olive, and gray, siliceous; 

in layers 4 in. to 1 :flt thick_____________ 19 

50. Concealed ------------------------------- 28 

49. Mudstone, 'Siliceous ; similar to unit 51 ; contains 4-ft-thick bed of quartzite near top__ 22 

48. Concealed ------------------------------- 21 

47. Mudstone, very light to dark gray and olivegray, siliceous, blocky ; contains a few thin 

layers of hard sandstone_________________ 18 

46. Concealed ------------------------------- 20 

45. Mudstone, medium-gray, siliceous__________ 2 

44. 'Sandstone, light-olive-gray, medium-grained, 

-speckled ------------------------------ 3 

43. Concealed; float is 'blocky siliceous mudstone --------------------------------- 22 

42. Mudstone, pale-greenish-yellow and mediumdark-gray, siliceous, blocky to subconchoi-

. dal !fracture___________________________ 10 

41. Quartzite, very light gray with thin brown 

layevs -------------------------------- 10 

40. Concealed ------------------------------ 17 

89. Mudstone, greenish-gray, si'liceous, •blocky __ 4 

38. Concealed ------------------------------- 12 

87. Sandstone, very li'ght gray, thi.on-'bedded, 

croSJsbedded, speckled ___________ ,_________ 8 

36. Mudstone; 'alternation of units from 1 to 

6 f.t thick colored grayish olive, lighrt gray, 

·and medium gray; contains 1 thin layer 

of siliceous mudstone and sever·al of 'lightgray standstone------------------------ 50 

85. Sandrstone, :fine-grad.ned___________________ 8 

84. Mudstone, grayish-olive, b:ackly to chippy; 

contain·s a few thin layers of fine-grained 

gray sandstone-------------------------- 14 

83. ;Sandstone, speckled very light gray .and 

medium-gray; one coar.se-grained 'bed art 

base ---------------------------------- 8 

32. Mudstone, :similar to uniJt 8!4-------------- 20 

81. !Sandstone, olive- ·and :medium-gray, finegrained; contains a few thin beds of darkgray shale-------~--------------------- 63 

30. Concealed ------------------------------- 10 

29. Mudstone or shale, olive-gray, medium-gray, 

and medrium-dark-gray, chippy to :firssile__ 85 

28. Sandstone, siltstone, shale, and a few limestone concretions, drab-colored___________ 39 

27. Mudstone and .shale, very dark . gray; contain·s 'brown-weathering limestone concretions near base------------------------- 6 

26. Sha'le and sandstone, very dark gray_____ 4 

25. ·Sandstone, similar to ·unit 23-------------- 7 

24. Mudstone, medium-dark-gray and olive-gray, 

blocky -------------------------------- 12 

23. Sandstone, medium-gray ; •speckled with chert 

and feldspar(?) grains__________________ 7 

22. Concealed ------------------------------- 6 

21. S.andstone, similar to unit 19______________ 8 

20. Concealed; sandstone float________________ 24 
56 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

SECTION 4.-..0olorado Formation near south end of mapped 

area-Continued 

Colorado Formation-Continued 

Middle siliceous mudstone and sandstone unit-Con. Feet 

19. Sandstone, medium- to Ught-gray, very fine 

grained; contains a zone of roundish calcareous sandstone concretions ne.ar middle 

of uni<t. Collections (USG'S Mesozoic loc. 

23029 and 23498) from this unit contain : 

Lingula cf. L. subspatula Hall ,and Meek, 

"Yoldia" sp.; Nemodon? sp.; Ostrea ano-

'lnioides Meek; VolSella n. sp.; .Anat-imya 

n. sp.; Gymella n. :sp.; Cardium (Nemocardium) n. sp. aff. G. kansasense Meek; 

Linearia n. sp. aff. L. cribelli Stephenson; 

Leptosolen n. sp., Mactra? aspenana 

Reeside and Weymouth; "Gorbula" sp.; 

Protodonam aff. P. stantoni Vokes ; 

.Anchura? sp.; "Cyrena" inflema Meek. A 

number of the species are known _a,t other 

localities only in the Mowry and A·spen 

Shales (W. A. Cobban, written commun., 

May 31, 1951, and Oct. 24, 1951) --------- 11 

18. Ooncealed; sandstone float________________ 11 

17. Sandstone, similar to unit 15______________ 2 

16. Concealed ------------------------------- 12 

15. Sandstone, medium-light-gray, crossbedded, 

speckled, cherty ,and feld,spathic( ?) ; in 

beds about lh in. thick ; calcareous sandrstone concretions (2-3 in. in diameter) are 

abundant in upper few feeL____________ 36 

14. s~andstone, similar to unit 15, with interbeds 

of medium-gray shale___________________ 23 

Total thickness orf middle smceous mudstone and sandstone unit____________ 930 

Gradational contact. 

Lower black ·shale unit : 

13. Concealed ------------------------------- 17 

12. Shale, very dark gray____________________ 9 

11. Sandstone and siltstone, medium-gray; !bedding is irrt>gular and wavy. Collection 

(USGS Mesozoic loc. 23028) from middle 

part of unit contains Inoceramus cf. I. 

bellvuensis Reeside; 1this fossil indicates 

a probable correlation with the Skull Creek 

Sh.ale of the Black HiHs (W. A. Cobban, 

written commun., May 31, 1951) ---------- 37 

10. Concealed; very dark gray shale float______ 34 

9. Sandstone, similar to unit 7--------------- 2 

Fine-grained diorite sill 40 ft thick 

8. Mostly covered; outcrops are sandstone similar to un'it 7--------------------------- 47 

7. Sandstone, medlium-dark-gray with 'li.mon!tic 

specks, medium-grained ; conrt;;lins finely 

dispersed organic matter 'and grains of 

black shale; ~bedding typically wavy and 

irregular ------------------------------ 19 

6. Concealed; very dark gray and dark-gray 

shale float is predominant ; sandstone float 

is subordinate_________________________ 106 

SECTION 4.-Golorado Formation near south end of mapped 

area--Continued 

Gradational contact-Continued 

Lower black shale unit-Continued Feet 

5. S'iltstone, dark-gray and olive, thin-bedded 

and almost fissile ; cont.a.ins irregular 

limonitic concretions in upper 4 ft________ 13 

4. 'Shale, very dark gray; a few 2-inAhick beds 

of rusty-weathering siltstone____________ 19 

3. Ooncealed; olive-gray siltstone float________ 10 

2. Sandstone, very light gray, calcareous, very 

fine grained; in beds lh-3 in. thick_______ 12 

1. Sandstone, light-olive-gray, fine-grained____ 3 

Total thickness of lower black shale unit__ 328 

Total thickness of Colorado Formation : 

Maximum 1,639 

Minimum --------------------------- 1, 542 

Kootenai Formation (Not measured) 

Limestone near top contains the following proso-

:brach:Uan gastropods: Lioplacodes cf. L. convexiculus Yen, Lioplacodes cf. L. cretaceus 

(Stanton), Gharydrobia cf. G. cretacea Yen, 

and ? Reesidella sp. undet. common species of 

the Kootenai Formation (T. C. Yen, written 

comm. Mar. 25, 1952). 

SECTION 5.-Golorado Formation along and near the Indian 

Greek road 

[Measured by M. R. Klepper and V. L. Freeman in sees. 5 and 6, T .. 6 N., R. lE.] 

Slim Sam Formation: 

Measured section published in Klepper, Weeks, 

and Ruppel (1957, p. 30-31). Basal part is sandstone, medium gray, argillaceous, feldspathic, 

thin bedded with few thin black shale interbeds, 

14 ft thick-----------------------------------

Poet 

Total thickness of Slim Sam Formation_____ 910 

Colorado Formation: 

Upper black shale unit : 

29. Shale, very dark gray to black. Fossil collection (USGS Mesozoic loc. 23037) 

about 20 ft below top of unit contains : 

Ostera congesta Conrad; Baculites asper 

Morton; and Scaphites ventricosus Meek 

and Hayden. Fossil collection (USGS 

Mesozoic loc. 23034) near base of unit 

contains : Pinna dolosoniensis McLean ; 

Inoceramus umbonatus Meek and Hayden; Ostrea congesta Conrad; Volsella n. 

sp. ; Pholadomya aff. P. papyracea Meek 

and Hayden ; Lucina sp. ; Legwmen cf. 

L. ellipticum Conrad; Gyrodes conradi 

Meek; Baculites asper Morton; Baculites 

cf. B. codyensis Reeside; Scaphites tetonesis Cobban. This collection is indicative of the next faunal zone above the 

zone of Inoceram~us deformis of earliest 

Niobrara age (W. A. Cobban, written 

commun., May 31, 1951) --------------- 56 
MEASURED SECTIONS 57 

SECTION 5.-0olO?·ado Formation along and near the Indian 

Greek road-Continued 

SECTION 5.-0olorado Formation along and near the Indian 

Greek road-Continued 

Colorado Formation-Continued 

Upper black shale unit-Continued 

28. Shale, very dark gray to black ; upp~r 36 

ft mostly covered and may contain diorite porphyry sill about 5 ft thick ____ _ 

27. Shale, very dark to black; mostly outcrop ; 

some interbeds of very fine grained black 

sandstone. Fossil collection (USGS mesozoic loc. 23033) near middle of unit 

contains Inoceramus umbonatus Meek 

and Hayden; Inoceram~ts n. sp. Ostrea 

congesta Conrad; Pholadomya cf. P. 

papyracea Meek and Hayden ; Veniella 

goniophora Meek; Legumen cf. L. ellipticum Conrad; Baculites asper Morton Idiohamites? sp. Scaphites sp. This 

collection is indicative of the next faunal 

zone above the zone of Inoce1·amus dejormis of earlest Niobrara age (W. A. 

Cobban, written comm., May 31, 1951) _ 

26. Sandstone and subordinate, dark gray ; 

foul-bottom type lithology and structure, 

with undulose bedding, ripple marks, and 

worm trails. Fossil collection (USGS 

Me~ozoic loc. 23035) from top of unit 

contains Inoceramus cf. I. dejormis 

Meek, indicative of earliest Niobrara age 

(W. A. Cobban, written commun., May 

31, 1951). Another collection ( 58K4), 

possibly from the same bed but probably from another bed a few feet lower 

in the section, contains Inoceramus cf. 

I. incert1.ts Jimbo, Ostrea sp., Baculites 

sp., and Scaphites sp. Inoceramus cf. I. 

incertus marks the very highest Carlile 

bed in the Western Interior region (W. 

A. Cobban, written comm., Sept.15, 1958) 

A collection (USGS Mesozoic loc. 24000) from 

10-25 ft above base of the unit (2~0 ft 

below collections listed above) contains Inoceramus dejormis Meek, Inoceramus, unnamed species resembling I. labiatus ( Schlotheim), PholJadomya papyracea Meek and 

Hayden, and Scaphites sp. cf. S. ventricosus 

group and is indicative of the basal Niobrara 

zone (J. B. Reeside, Jr., written commun., 

Oct. 3, 1952) . .Another collection ( 58K3) 

from about the same bed contains Inoceramus sp. and Cardium cf. 0. pauperculum 

Meek. (W. A. Cobban, written commun., 

Sept. 15, 1958) . 

The presence of rock's that are pr·o'bably o·f 

highest Oarlile age between rocks of earliest 

Niobrara age suggests that 10-20 ft of beds 

has 'been duplicated by an unrecognized fault 

wi,thin unit 26. In any event the contact !between rocks of Carlile 'and Niobrara age is 

within thi:s unit, for rocks a;t the top of the 

Colorado Formation-Continued 

Feet Upper black shale unit-Continued Feet 

underlying unit are proba!ble equivalents of 

the middle part of the Carlile and those in 

72.5 the overlying shale are equivalents of -the 

lower part of the Niobrara (next fa·un•al zone 

above zone of Inoceramus dejormis of earliest Niobrara age)----------------------- 50 

95 

Total thickness of upper black shale uniL 273. 5 

Middle siliceous mudstone and sandstone unit: 

25. Sandstone, tan, s.alt-and-pepper. Collection 

58K2 (USGS Me'Sozoic loc. D1749) fr.om 

top of unit contains: Inoceramus cf. I. 

perplemus Whitfield, M actra, 1sp., and TelUna sp. The. impressions of Inoceramus 

suggest the species perplemus of middle 

Carlile age (W. A. Co'b'ban, written commun., Sept. 15, 1958) ------------------

24. Sandstone, sHtstone, and thin inter:beds of 

grayi'sh-'bl.aok shale; ·sandstone is argillaceous foul-bottom lithology having 

scattered fossil fragments, undulose 1bedding, ripple marks and worm trails ; 

poorly exposed. Collection 58Kl (USGS 

Mesozoic loc. D1748) from 1base of unit 

contains Inoceram~ts f sp., Ostrea 'sp., and 

preserved fragments of gastropods. The 

fr.agmen ts of oysters suggest a small 

smooth species that is common in rocks 

of Greenhorn and early Oarlile age (W. 

A. Oo'bban, written commun., Sept. 15, 

37 

1958) ------------------------------- 109 

23. Sandstone, tan, salt-and-pepper, pebb'ly, 

thick-bedded ------------------------ 37 

Sill, diorite porphyry, not measured; section 

offset. 

22. Concealed; d-iorite porphyry float obscures 

bedrock, but proba1bly underlain by sandstone, siltstone, and mudstone ; may include some thin sills__________________ 92. 5 

21. Mudstone, siliceous, ·and some mudstone 

pebble conglomera,te ; poorly exposed, less 

than o~e-quarter of interval is oUJtcrop__ 21 

20. Concealed ; probably underlain by siliceous 

mudstone, mudstone, and siltstone______ 118 

Sill, dior:ite pol"phyry, 32ft thick. 

19. S•andstone, ·greenish-gray 'to pale-olive, medium-coarse-grained, feldspathic; interbedded with dark-gray shale___________ 70. 5 

18. Sandstone, greenish-gray to pale-olive, medium-coarse-grained, feldspathlc________ 12 

17. Concealed ; but probalbly underlain by 

sandstone simHar .to 18---------------- 14 

Sill, d·iorite porphyry, 14.5 f.t thick. 

16. Concealed; probably underla·in by sandstone similar to 18-------------------- 43 
58 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

SECTION 5.-0olorado Formation along and near the Indian 

Greek road-Continued 

Colorado Formation-Continued 

Middle siliceous mudstone and sandstone unitContinued F'eet 

15. ·Sandstone, greenish-gray, medium-finegrained, feldspathic, distinctly 'bedded 

and thin~bedded______________________ 4.5 

14. tConcea'led; .float similar to 1'5------------ 19 

13. Sandstone, greenish-gray, medium-fine-

•grained, feldspathic, slightly to moder8:tely calcareous______________________ 4. 5 

12. Concealed ; proba!bly underlain by sandstone ,similar to rthat in unit 13. Sandstone fi<>:at nearby from this unit or 

within 50 ft a1bove it (USGS Mesozoic 

loc. 23413) contains Lingula cf. L. subspatulata Hall and Meek, Ostrea anomioides 'Meek, Trigonia sp., and Mactra 

sp.; fossils th'at indicate a correlation 

with 'the Mowry ·and Aspen formations 

( W. A. Cobban, written commun., May 31, 

1951) -------------------------------- 29 

Total thickness of middle siliceous 

mudstone and sandstone unit ___ .___ 611 

Lower 1Mac'k shale unit: 

11. Concealed ------------------------------ 4 

10. 1Sandstone, fine-grained, argillaceous, .and 

dal"k-gray poorly eX!posed 'Siltstone_____ 83 

9. Sandstone, very fine-grained, and siltstone, 

inter;bedded with dark-gray papery 

,shale : all •are rusty weathering, in P.arrt 

mottled and irregularly laminated in 

shades of medium and dark gray ; three 

lbeds, each 6 in. thick, of light-gray 

coarse-gr.ained salt~and-pepper sandstone in upper 13 fL------------------ 72. 5 

8. Shale, very dark gray to grayish.Jblack, 

fis~le ------------------------------- 10.5 

Sill, diorite porphyry 15.5 ~t thick. 

7. Shale; similar to that in unit 8---------- 62 

·Sill, diorite porphyry 11 ft thic~. 

6. Shale; similar to that in unit 8---------- 23. 5 

5. Siltstone, olive-gray, blocky, and brownweathering, very fine grained sandstone ; 

poorly exposed; top 1-2 ft of interval is 

fine- to medium-grained light-gray 

quartzite, similar to unit 4------------- 35 

4. Quartzite, light-gray to olive-gray, salt-

·and-pepper fine- to medium-grained, massive bedded___________________________ 15 

Total thickness of lower black shale 

unit ---------------------------- 255. 5 

Total thickness of Colorado Formation 

Kootenai Formation: 

3. Concealed ----------------------------------

2. Mudstone, siltstone--------------------------

1. Limstone, light-gray-weathering; contains fos1,140 

19 

11 

sil gastropods----------------------- Not measured 

SECTION 6.-SZim Sam FormatiQn (part) and Colorado Formation near Iron Mask mine 

[Measured by M. R. Klepper and V. L. Freeman, 1~52, near Iron Mask 

Mine in SE% sec. 30, T. 7 N., R. 1 W., and adJacent parts of sees. 29, 31, 32] 

Slim Sam Formation (lower part only): 

71. Sandstone, medium-gray to light-olive-gray, medium-grained, feldspathic and argillaceous; 

beds *-4 in. thick with a few *-1 in.-thick 

medium-dark-gray shaly interbeds; one 8-in. 

crystal tuff bed 44ft above base---------'----

70. Sandstone, medium-gray, fine- to mediumgrained, feldspathic, thin-beddfd ( * -2 in.)-

69. Concealed ; except for 1 ft of sandstone similar 

to that in unit 70--------------------------

68. Sandstone, medium-gray, slightly mottled, 

argillaceous ------------------------------

67. Concealed --------------------------------- 66. Siltstone and very fine grained sandstone, medium-gray and olive; some- interstitial clay 

and thin lenses of crystal tuff; thin-bedded __ 

65. Concealed ---------------------------------- 64. Siltstone and very fine grained sandstone, like 

runit 66-----------------------------------

63. Siltstone, interbedded olive and medium- to 

dark-gray, hackly to rubbly fracturing _____ _ 

Feet 

71 

2 

15 

3 

6.5 

2 

J.3.5 

28.5 

3 

=== Measured thickness of Slim Sam Formation --------------------------------- Colorado Formation : 

Upper black shale unit : 

62. Shale, grayish-black, fissile ; local thin interbeds of siltstone and very fine grained 

sandstone form about 5 percent of interval ; weathers with some limonite 

staining -----------------------------

61. Shale, dark-gray to grayish-black, fissile, 

and thin blocky siltstone beds ; diorite 

dike 5-6 ft thick cuts middle part of 

unit; 1- to 3-in.-thick bed·s of darkgray, very fine grained sandstone common 28-40 ft above base; large brownweathering limestone concretions com- mon in upper 11 ft of unit ___________ _ 

60. Concealed ------------------------------ 59, Sandstone, very fine grained; interbedded 

with siltstone; dark-gray and olive ____ _ 

Sill, diorite porphyry with conspicuous hornblende phenocrysts, approximately 40 ft 

thick. 

· 58. Shale, dark-gray, fissile; upper part contains about equal amounts of shale and 

olive fine-grained laminated sandstone in 

%- to 3-in.-thick bedS------------------

57. Shale, grayish-black, fissile, ·and numerous 

very thin beds of dark-gray siltstone; 

limonite staining common _____________ _ 

56. Concealed ------------------------------ 55, Shale, dark-gray to grayish-black, fissile to 

finely chippy, few 1-in. thick beds of siltstone ; in part limonite stained along 

fractures -----------------------------

54, Concealed ------------------------------

144.5 

57 

45.5 

8.5 

3 

19 

32.5 

30.5 

11.5 

20 

,, •. 
MEASURED SECTIONS 59 

SECTION 6.-SUm Sam Formation (part) and OoZorado Forma- SECTION 6.-SUm Sam Formation (part) and OoZorado Formation near Iron Mask mine-Continued ,tion near Iron Mask mine-Continued 

Colorado Formation-Continued 

Upper black shale unit-Continued 

53. Sandstone, very fine grained, and siltstone, 

medium-dark-gray to dark-gray, micaceous, distinct bedding in 1- to 6-in.-thick 

beds ; weathers blocky near base to hackly or crumbly in upper two-thirds of unit52. Sandstone, dark-gray, fine grained; with 

one 3-ft-thick zone of olive siltstone; 

chippy weathering ___________________ _ 

51. Sandstone, medium- to dark-gray, finegrained, argillaceous, laminated, in part 

light gray streaked; subordinate very 

dark gray ~shale interbedS--------------

Total thickness of upper black shale 

unit -----------------------------

Middle siliceous mudstone and sandstone unit: 

50. Sandstone, medium-gray, coarse-grained 

salt-and-pepper; indistinctly bedded with 

irregular streaky laminae _____________ _ 

Sill, diorite porphyry about 6 ft thick. 

49. Sandstone, similar to that in unit 50 _____ _ 

48. Sandstone, pale-olive, medium-grained, 

thin- to medium-bedded; in part crosslaminated, slightly calcareous in part_ __ 

47. Shale, olive and dark-gray, fissile; siltstone ; a few thin beds of fine-grained olive sandstone ______________________ _ 

46. Sandstone, medium-gray to light-olive-gray 

and light-yellow-brown, limonite-stained, 

fine-grained; in beds 1-12 in. thick; subordinate dark-gray to very dark gray 

fissile shale in beds %,-6 in. thick ______ _ 

45. Sandstone, mottled medium-gray and darkgray, fine-grained, argillaceous; interbedded with dark-gray subfissile siltstone --------------------------------

44. Concealed ------------------------------ 43. Sandstone, very fine grained, and siltstone, 

medium-dark-gr.ay, blocky; some beds 

thinly laminated and cross-laminated; 

sUJbordiuate dark-gray !Shale __________ _ 

Sill, diorite porphyry, ·about 13ft thick. 

42. Sandstone, medium-light-gray to mediumgray, ·salt-and-pepper; coarse.-grained 

-wii1Jh peblble conglomerate near 1base a-nd 

scattered pebbies above; medium ·bedded 

and distinctly •bedded. Lower .halif is finer 

gra'ined, less friable _________________ _ 

41. ·Sandstone, light-olive-gray, fine- to medium-fine-grained, medium-bedded ____ _ 

·Sill, diorite porphyry, 15 ft thick. 

40. Mudstone, greenish-gr.ay to pale-olive, subfissile to rU!b'bly or hackly -weathering; 

a few thin ·blocky weathering 1siliceous 

mudstone beds-----------------------

39. Sandstone, medium-tight-gray, fine-

·grained, thin-bedded, calcareous _______ _ 

Jt'cct 

24 

13 

16.5 

281 

7 

3 

24.5 

15 

36 

12 

20 

9 

19.5 

18.5 

22 

8.5 

Colorado Formation-Continued 

Middle siliceous mudstone and sandstone unitContinued 

38. Mudstone, gray ·and olive, splill'tery and 

chippy ------------------------------ 37. ·Sandstone, light-gray, fine-grained, thinlbedded, calcareous--------------------

36. Siltstone, medi•um-gray, light-gray, and 

olive, splintery- and rubbly-weathering; 

middle part calcareous _______________ _ 

35. Mudstone, very light~gray, siliceous, 

blocky ; weathers white, a few· darkgray siltstone interbedd•s ______ ..; ______ _ 

Sill, diorite porphyry, 7 :Dt tllick. 

34. Siltstone, olive and dark-gray; interbedded siliceous and nonsiliceous beds and 

prominent 'beds of light-gray-weathering 

speckled coarse-grained sand·stone 7-9 ft 

•albove base __________________________ _ 

Concealed ; prolba•bly underlain lby diorite sill 

or dike, 8ft thick. 

33. Siltstone, light-olive and greenish-gr.ay, 

1spl·intery; medium- to dark-gray !blocky siliceous mudstone __________________ _ 

·Sill, diori'te porp'hyry 1.•5 rt thick. 

32. ;SUtstone, olive, light-gray, and dark-gray, 

blocky fracturing, chippy to rub'bly 

wea:thering ; some 'beds are siliceous ; 

diorite porphyry sill 2 ft thick cuts uuiL 

31. ·sandstone, light-·gray, medium-grained; ·and olive siltstone ____________________ _ 

30. Mudstone, dark-gray, siliceous __________ _ 

29. Concealed -----------------------------

28. •Siltstone, greeni1sh-gray ~o pale-olive, splintery; subordinate olive to d·ar:k-gray siliceous mudstone and light-gray sandstone ------------------------------- Sill, diorite porphyry, 7ft thick. 

27. Mudstone, dark-gray, siliceous, fi·ssile to 

hackly ------------------------------- 26. Siltstone, olive, :hackly fracture _________ _ 

25. Concealed ----------------------------

24. Mudstone, olive and gray, siliceous, blocky 

fracture ; intertbedded wit1Jh. irregularfracturing olive siltstone and very fine 

grained .sandstone ___________________ _ 

23. Mudstone, :siliceous, •blocky fracture; interbedded with gray ·and olive hackly- to 

splintery-fracturing siltstone to very 

fine grained sandstone ________________ _ 

22. Sandstone, speckled, medium- to coarsegrained ----------------------------- 21. Siltstone, pale-olive, olive-gray, and greenish-gray; similar to unit 17; two conspicuous medium-grained sandstone beds, 

one 3 f.t albove base, other 22 ft ·above 

lbase ; diorite porphyry sill. 2.5 ft tbick 

14.5 ft above base (thickness not included) ----------------------------- Sill, diorite POll>hyry. 4.5 ft thick. 

Peet 

19 

2.5 

29 

13 

31 

29 

22 

6.5 

5 

20 

22 

6 

7 

12 

22 

10.5 

1 

27 
60 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

SECTION 6.-Slim Sam Formation (part) and Colorado Forma-

,tion near Iron Mask mine-Continued 

Colorado Formation-Continued 

Middle siliceous mudstone and sandstone unitContinued 

20. ·Sandstone, medium-gr.ay, fine-grained, calcareous, thin-bedded and irregularly 

'bedded; weathers greenish-gray _______ _ 

19. Siltstone, similar to unit 17 ____________ _ 

Sill, diori1te portphyry, 3-4 ft thick. Se0tion off-

.set on top of sill. 

18. 'Siltstone, olive-gray, sandy, subhackly 

fracture ---------------------------- 17. Siltstone, pale-olive, olive-gray, and greeni,sh-gray; a few dusky-yellow 'blocky rto 

chippy fr.acturing beds, and common 

dark-gray chippy to hackly or pencd.lly 

fracturing 1beds ; a few 1beds 6-12 in. thick 

of medium .. grained ·argillaceous sandstone ; most of the coarser grained ;bed·s 

are calcareous-----------------------

16. Concealed ----------------------------- 15. Sandstone, olive-gray, medium- and finegrained, and su!bordinarte siltstone ; cut 

by a 3- to 4-ft-thick diorite sill (thickness not included)--------------------

14. Sandstone, medium-gray ~to medium-darkgray and some olive-gray beds, finegrained, t~in- to very thin-bedded ( lk1 in. bed·s) ; weathers olive gray ; medium-gray to dark-gray platy siltstone __ _ 

13. Concealed -----------------------------

12. 'Sandstone, medium- to greenish-gray, medium..fine-grained, well-'bedded; in beds 

1 in. thick with one 3.:f.t..,thick 1bed near 

middle o'f unit ; ·argillaceous and ·slightly 

calcareous --------------------------

Sill, pyroxene andesite, a1bout 10 ft thick. Section offset. 

Total thickness of middle siliceous mudstone and sandstone unit_ ____________ _ 

Lower black shale unit : 

11. Concealed ; probably underlain by gray 

sandstone and dark-gray siltstone _____ _ 

10. Sandstone, dark ... gray, and dark-gray ·snt-

·stone ------------------------------- Sill, diorite porphyry, 141ft thick. 

9. Sandstone, dark-gray, and dark-gray siltstone -------------------------------- 8. Sandstone, l·ighrt olive-gray, ~speckled, medium-grained ------------------------ 7. Mostly covered; underlain by gr.ay mottled 

argillaceous sandstone and dark-gray 

pencilly fracturing siltstone __________ _ 

6. Mostly concealed ; underlain by black to 

very dark gray papery shale _________ _ 

5. Ooncealed -----------------------------

4. Argillite, dra'b and olive, and some blocky 

:siLtstone and rfirre-grained sandstone ; irregular lumpy 'bedding, 'some ripple 

marks and worm ·trails on 'bedding 

planes ------------------------------

Feet 

5 

22.5 

12 

53.5 

5 

25 

29 

29 

30 

690.5 

11.5 

3 

12.5 

2.5 

85 

54.5 

26 

29 

SECTION 6.-Slim Sam Formation (part) and Colorado Formation near Iron Mask mine-Continued 

Colorado Formation-Continued Feet 

Lower black shale unit-Continued 

3. Quartzite, yellowish-gray to light olivegray, fine-grained____________________ 16 

Total,thickness lower black shale unit_ 240 

Total thickness Colorado Formation_ 1, 211. 5 

Kootenai Fol'IIIl.ation: 

2. Claystone and siltstone__________________ 36 

1. Limestone; contains abundant fossil 

,gastropods -------------------- Not measured 

SECTION 1.-Lower part of middle member· of the Elkhorn Mountains Volcanics 

[Measured by M. R. Klepper, H. W. Smedes, and R. A. Weeks, starting at road at line between F.ecs. 15 and 16, T. 8 N., R. 1 W., extending along spur through the NW%,NE* sec. 21, and ending along the line 

between sees. 17 and 20] 

Top of measured section: Feet 

63. Welded ash-flow tuff, dark-gray; weathers to medium gray or medium light gray; contains abundant flatJtened pumice ribbons, many of which 

are more than 18 in. long, and sparse rock 

fragments --------------------------------- > 100 

62. Covered-------------------------------~------ 17 

61. Crsst.al tuff and crystal lithic 1tuff, poorly ·bedded ; 

coutains sparse to a'bundant fragments up to 

8 in. in diameter; ma,trix is dominantly siltsize crystal tuff ; lithic fragments resemble 

fragments in unit 58------------------------ 33 

60. Crystal tuff ·and crystal lit,hic ·tuff, indistinctly 

to distinctly bedded ; most layers are fine 

grained, a few are coarse ·grained, and a few 

are lapilli tuff______________________________ 12 

59. Covered ------------------------------------- 10 

58. Tuff breccia; lithic fragments, which average less 

than 4 in. in diameter but are as much as 10 in., 

from half to two-thirds of the unit; most fragments are of same apparent composition as the 

crystal tuff matrix; some fragments are sharply 

bounded, others are vaguely outlined. Vague 

bedding at 60 ft above base and sparsely 

throughout upper 50ft indicates that transporting agent was able to accomplish some sorting ; 

probably of hot mudflow origin_______________ 160 

Probable erosional surface of slight relief. 

57. Crystal and crystal lithic tuff, poorly bedded; 

contains sparse lapilli throughout; number of 

lapilli increases upward _________________ ..;___ 35 

56. Covered ; probably underlain by rocks similar to 

those in unit above__________________________ 16 

55. Tuff breccia, similar to unit 58; largest fragment 

observed was 3 in. across____________________ 2 

54. Crystal· tuff, dark-gray to dark-greenish-gray, 

massive to well-bedded, dominantly finegrained; sparse 14- to lh-inch lapilli in beds 

near middle of unit------------------------- 65 

53. Ash flow tuff, poorly welded____________________ 3 

52. Crystal tuff, well-bedded; contains a few mudstone laminae and one thin mudstone dike_____ 2 
., 

MEASURED SECTIONS 61 

SECTION. 1.-Lower part of middle member of the Elkhorn Mountains Volcanics-Continued 

51. Tuff breccia, medium-gray with a greenish cast; 

weathers light olive gr:ay, grayish orange, and 

greenish gray; forms blocky to rubbly cliffs; 

s~ightly rounded fragments are ~-9 in. across 

Feet 

and average about 3 in. ; all are of the same 

lithology and strongly resemble the crystal tuff 

matrix. On fresh surface, rock is apparently 

homogeneous, but on weathering the breccia 

fragments become apparent. Massive, except for 

indistinct layering and one thin irregular finegrained crystal tuff lens at 70 ft a·bove base. 

Probably of hot mudflow origin______________ 125 

50. Ash-flow tuff, welded to partly welded, mediumdark-gray to dark-greenish-gray; weathers medium gray to light greenish gray; top few feet 

is of unwelded lapilli tuff, grades down to 

strongly welded tuff containing flattened pumice 

ribbons several inches long and abundant 

lithic fragments up to 3 in. in diameter, which 

in turn grades into lower ,50-ft-thick unit containing sparse flattened pumice about 1 in. in 

maximum diameter and relatively abundant 

lithic fragments less than lh-inch across. The 

strongly welded part is grayish black and 

dense; the rest of the unit is somewhat lighter 

colored and less dense_______________________ 100 

49. Crystal tuff, dark-greenish-gray, indistinctly 

bedded, medium-to coarse-grained; weathers 

greenish graY------------------------------- 80 

48. Vein zone, with limonite-stained crystal tuff and 

quartz ------------------------------------ 10 

47. Crystal tuff, dark-gray, and medium-gray to grayish-green; mostly massive, but a few thin zones 

are well bedded ; pal'lticles dominantly fine 

grained; a few beds are crystal-lithic tuff; 

upper half of unit contains common long hornblende prisms------------------------------ 120 

46. Concealed ; probably underlain by crystal tuff 

similar to unit 47-------------------------- 33 

Sill, dark-greenish-gray; weathers olive gray to 

brownish gray ; contains abundant plagioclase 

phenocrysts in a fine-grained matrix; 9 ft. thick. 

45. Crystal tuff; comprises' intel'lbedded relativeily 

massive siliceous volcanic mudstone and distinctly bedded units of sand-sized grains_____ 150 

44. Crystal tuff; relatively massive siliceous mudstone ---------------~---------------------- 20 

43. Crystal tuff, well-bedded, fine- to mediumgrained ------------------------------------ 15 

42. Crystal tuff; relatively massive siliceous mudstones ------------------------------------- 30 

41. Concealed; some welded tuff floatt______________ 5 

40. Crystal tuff, medium- to greenish-gray; weathers 

grayish orange, very light gray, and yellowish 

gray ; mostly fine grained ; local thin layers and 

lenses of coarser crystals and as much as ~ in. 

thick -------------------------------------- 46 

419-246--71----5 

SECTION 1.-Lower part of middle member of the Elkhat·n Mountains Volcanics-Continued 

39. ·Ash-flow tuff, welded, dark-gray, very dense; 

contains abundant collapsed pumice ribbons up 

to 6 in. long and abundant lithic fragments up 

Feet 

to 3 in. in diameter_________________________ 24 

38. Crystal tuff, grayish-'black to dark-gray; weathers 

pale yellowish green ; poorly bed<ied ; fine 

grained -----------------------~----------- 76 

37. Crystal tuff, well-'bedded, fine-grained_________ 22 

36. Ash-flow tuff, welded, medium-gray; weathers 

yellowiS!h green, pinkish gray, and yellow gray; 

contains abundant flattened pumice ribbons and 

wisps as much as 3 in. long and lJi in. thick 

and abundant lithic fragments. Basal 7ft. contains smaller .collapsed pumice fragments 

about 1 in. in maximum length______________ 30 

35. Crystal tuff, poorly bedded to massive, finegrained ------------~----------------------- 42 

34. Crystal tuff, medium-darl{-gray ; weathers light 

olive gray and greenish gray ; distinctly bedded 

to finely laminated with alternate layers of 

. ~oarse to fine sand, silt- and clay-sized particles ; characterized by layers and lenses of 

white plagioclase crystals; locally has small 

channel deposits. Thin layers of siliceous mudstone weather out as ri1bs. Almost certainly a 

water-laid deposit. Unit forms coarse blocky to 

thin slabby debris ___________ .________________ 153 

Sill, dark-greenish-gray pyroxene-bearing porphyry 

with abundant smaller plagiocl•ase phenocrysts, 

small reddish altered phenocrysts (hematite?), and 

abundant epidote-quartz-filled amygdules; . about 

300 ft thick; ooncealed zone from 45 to 75 ft above 

lbase rthat probably contains a fault of rel<atively 

small displacement. 

33. Crystal tuff, dark-purplish-gray .lJo med<ium-gray; 

upper 26 ft is distinctly bedded and coarse 

grained with many layers compo.sed chiefly of 

mafic minerals; next 14 ft is dominantly fine 

grained; lower 32 ft is indistinctly bedded ·and 

fine grained at base, grading upward to more 

·distinctly bed•ded and coarser grained tuff____ 72 

32. Ash-flow tuff, welded, black to dark-gray, dense; 

contains a moderate amount of small ooUapsed 

pumice fragments and small lithic fragments 

generally less than * in. across------------- 27 

31. Crystal tuff, distinctly thick-bedded, mediumcoarse-grained ----------------------------- 7 

30. Crystal tuff, distinctly thin-bedded, fine- to medium-grained -------------------------------- 25 

29. Crystal tuff, distinctly thin-bedded, mediumgrained ----------------------------------- 9 

28. Crystal tu1f, distinctly thin-bedded; fine grained 

with common layers of coarser grained white 

plagioclase crystals and a few thin clastic dikes 

of fine-grained materiaL_____________________ 43 

27. Crystal tuff, mostly massive; dominantly fine 

grained but some granule beds are present; 

some epidote-rich layers 35 ft above base______ 100 

26. Crystal tuff, dark-gray ; mostly mudstone but contains contorted crystal-bearing layers________ 6 
62 GEOLOGY AND MINERAL DEPOSITS, ELKHORN MOUNTAINS, MONTANA 

SECTION 7.-Lower part ot middle member of the Elkhorn Mountains Volcanics-Continued 

25. Crystal tuff, medium-grained __________________ _ 

24. Crysbal tu~, dark- to medium-gray; mostly mudstone with coarser grained plagioclase crystals ; 

some layers are contorted and brecciated; resembles welded tuff, but is probably a contemporaneously deformed mudflow deposit_ _______ _ 

23. Crystal tuff, particles of sand and silt size in 

distinct beds _______________________________ _ 

22. Crystal tuff, d~ark-gray; weathers olive gray; 

some layers contorted and locally brecciated; 

similar to unit 24 ____________________ .:_ ______ _ 

21. Crystal tuff, distinctly bedded, coarse-grained ; 

some channeling and some clm;tic dikes ______ _ 

20. Crystal tuff, obscurely bedded, fine-grained _____ _ 

19. Crystal tuff, distinctly bedded, fine-grained ; broad 

shallow channels on some bedding planes ____ _ 

18. Crystal tuff, medium-gray; weathers medium 

light gray to light brownish gray; monthly massive mudstone with laminae, layers, and lenses 

of coarser grained particles; predominantly 

coarse grained in upper part ________________ _ 

17. Ash-flow tuff, partly welded; top few feet contains abundant fragments as much as 3 in. 

across, lower and middle parts contain almndant slightly collapsed pumice fragments ___ _ 

16. Crystal tuff, bedded, medium-grained _________ _ 

15. Ash-flow tuff, welded, blackish-gray, dense; collapsed pumice abundant_ ___________________ _ 

14. Crystal tuff, dark-greenish-gray to medium-gray; 

dominantly massive with some thin~y bedded 

layers, dominantly fine grained ; locally con- tains sparse pyrite crystals _________________ _ 

13. Ash-flow tuff, welded to poorly welded; contains 

abundant partly collapsed pumice fragments 

and very abundant smaU lithic fragments ____ _ 

12. Ash-flow tuff, welded; contains strongly coJlapsed 

pumice fragments and a few large accessory 

lithic fragments as much as 9 in. across ______ _ 

11. Crystal tuff, thinly bedded, medium-grained, lenticular ___ _. _____________________________ _ 

10. Ash-flow tuff, welded; contains strongly collapsed 

pumice fragments, some with altered halos ___ _ 

9. Crystal tuff, medium- to medium-dark-gray ; 

weathers olive gray; well bedded, fine and 

medium grained with lenticular layers of coarser crystals at top _____________________ _ 

8. Ash-flow tuff, welded--------------------------

7. Concealed, probably underlain by same welded tuff 

as units 6 and 8-----------------------------

6. Ash-flow tuff, welded _________________________ _ 

5. Concealed; a few outcrops and all float suggest 

interval is underlain by welded tuff like· units 

above and below----------------------------

4. Ash-flow tuff, welded ; strongly collapsed pumice 

fragments many with alteration halos of epidote and cores of epidote and chlorite ________ _ 

3. Crystal tuff, dark-gray, to medium-greenishgray, fine-grained, massive; forms blocky debris ---------------------------------------

Peet 

43 

6 

23 

5 

60 

25 

37 

50 

34 

5 

13 

18 

62 

30 

31 

9 

23 

26 

6 

37 

30 

6 

SECTION 1.-Lowe1· part ot middle member ot the Elkhorn Mountains Volcanics-Continued 

2. Ash flow tuff, welded, grayish-black; weathers 

medium light gray; contains abundant flattened 

pumice fragments, many of which are altered, 

some with a bleached halo and some entirely 

Feet 

bleached ----------------------------------- 49 

1. Concealed; float is welded tuff__________________ 29 

Total thickness of measured section __________ 2, 483 

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A Pan 

Acknowledgments •. ___________ ----___________ 2 

Aldrich Gulch intrusive______________________ 24 

Alluvium_____________________________________ 15 

Amsden Formation ____________________ ·_______ 5 

measured section_________________________ 49 

Andesite, defined_____________________________ 16 

Anteiope Creek stock ... ---------------~ 24,26,29,31 Auriferous pyrite veins _______________________ 96,46 

B 

Barlow, I. H., analyst________________________ 20 

Basalt flows _______ -------------------------~- 29 

Beaver Creek area ____________________________ 15,99 

Bentonite. __________________ ---_----_________ 48 

Big Snowy Group____________________________ 5 

Black Friday mine. _________ --_______________ 35 

Black Friday vein .. ----~--------------------- 45 

Blacksmith mine .• _----------------- ______ 35, 43,44 

Blue vein •... --------------------------------- 42 

Bluebird stock_______________________________ 26 

Borris, F. S., analyst.------------------------ 20 

Botts, S.D., analyst_________________________ 20 

Boulder batholith __________ --------------- 16, 24,31 

Brown, R. W., fossil identification____________ 13 

Buckhorn mine_______________________________ 38 

c 

Campanian time_____________________________ 33 

Cedar Plains. See Radersburg. 

Central mine.-------------------------------- 44 

Champion mine______________________________ 43 

Chloe, G. W., analyst________________________ 20 

Claggett Formation__________________________ 12 

Cobban, W. A., cited _______________________ 9, 10,11 

Coburg Basin stock__________________________ 27 

Cody Shale._-------------------------------- 11 

Cold Springs Gulch, graveL----------------- 49 

Collapse breccias ___ -----------------_________ 38 

Colorado Formation __________________ 8, 20,23,24,28 

measured section ___________________ 53, M, 56,58 

Colorado Shale.______________________________ 11 

Congress mine._----------------------------- 46 

Copper •• ____________________ ----- ______ 35, 43, 42,45 

Cretaceous rocks. _____________ ---____________ 3 

Crow Creek fan.----------------------------- 14,15 

Custer-Iron Age-Hyantha group mine________ 35 

Custer mine •• __________ -----_________________ 40 

Cyclone vein. _________________ .______________ 45 

Cynosure vein. ____ ~------------------------- 40 

D 

Diamond Hill mine_---------------- 24, 26,35,49,48 

Diamond Hill stock ______________ 23, 26, 31, 32,36,49 

Dikes._--------------------·16, 20, 21, 23, 24, 26, 29,43 

Dowd, J.M., analyst.------------------------ 20,30 

Dry Creek Shale.---------------------------- 4 

Dunbar Creek Formation ••• ----------~------ 12 

Dutchman intrusive. __ ------- __ -------- _____ 20,23 

E 

Eagle Basin stock.--------------------------- 27 

Eagle Formation.____________________________ 12 

Early Pleistocene. ______ --------------------- 14 

East Pacific mine.--------------------- 35,38,40,41 

INDEX 

.[Jtallc page numbers indicate major references] 

Page 

East Paclflc vein ••• ·-------------------------- 39 

Edna No. 2mine.---------------------------" 41 

Edna stock ____________ ---------------- _______ 27,40 

Elkhorn Mountains Volcanics. _____ 12, 14, 16, 24,60 

Elmore, P. L. D., analyst____________________ 20 

F 

Fall River Formation________________________ 9 

Faults. __ ------------------------------------ 6, 91 

Felsic rocks~. ___ ------------ __ ----------_____ 23 

Fieldwork •. ---------------------------------- 2 

Folds.- ___ ---- ______ -·- ______________________ 30, 91 

Fossils. 

Alnus sp. _ ------------------------ _____ ·__ 13 

Bathygenys sp __ ------------------------- _ 13 

Betula sp. _ -- __ -- _. ----- _- _ -------. __ _ _ _ _ _ 13 

Bolaspidella wellvillensis. ----------------- 4 

Brontotherium SP------------------------- 13 Camptonectes ____________ -- ______________ _ 

Caninfa. ________ ------ _________________ _ .: 5 

Cedaria __ _______ . ___ ---- _ _ _ _ _ __ _ __ __ __ __ _ _ _ 4 

Charydrobia cretaccea __ __ - -- _ -------- ____ _ Collignoniceras woolgarL _____ • _ _ _ _ _ _ _ _ _ _ _ _ 10 

sp _________ ------------------- _____ --- 10 

Cylfndrodon fontis .• _ -------- ___ ----- __ ___ ·13 

EumicrotiB. __ ----------------------- _____ 7 

Haminea SP------------------------------ 10 

Hyracodon SP----------------------------- 13 

Inoceramus bellvuensis. _________________ _ 

comanchean us bellvuensis _____________ _ 

{ragUiB. _- ---------------------------- 10 

sp _____ ---------------------- "_ _ _ __ __ _ 10 

Leptomeryx esulcatus _- __ --- _ __ _ _ __ __ __ _ _ _ 13 

Lioplacodes convexfculuB •• _ -------------- _ 8 

cretaceus •• ___ ----------- _____ -_ __ __ ___ 8 

Lucina juvenis ___ ------- _- _ -- __ __ _ _ _ _ __ _ _ _ 10 

Mesohfppus hypostylus____________________ 13 

Mactra arenaria __ -------- _ --- _ _ _ _ _ _ __ _ ___ _ 11 

Nixonello montanensis. __ ----------------- 4 Ostrea •• _________ •• __ ------ _ _ _ _ __ _ _ _ _ __ __ _ 7 

Palaeolagus brachydon_____________________ 13 

sp. ___ --- __ ------------- _- _- _- _ _ _ __ __ _ 13 

Peltosaurus sp. ___ • ------- _ --- ___ -- __ __ _ __ 13 

Pseudomelania SP------------------------- 10 

Reesidella sp _______ --------- ______ -" _ _ _ __ _ 8 

Scaphites ventricosis. ------ _ ---- _ --- _ _ _ _ _ _ _ 11 

ventricosus. __ ------------------- __ _ __ _ 11 

Sequoia affinis. _ -------------- _ -·---------- 13 

Spirifer brazeriana _______________________ _ 

brazerianus ___________ --- ____________ _ 

Tempskya orandis_________________________ 10 

Titanotheriomys veterior _ ------------------ 13 

sp __ ----- __ -_ ------------------ ______ ~ 13 

Freiburg stock________________________________ 26 

G 

Giant group mine____________________________ 35 

Glacial deposits. ______________ ---- __ ----_____ 14 

Glaciation____________________________________ 14 

Gold ___________ 34, 36, 38, 40, 41, 42, 43, 44, 45, 46, 48,49 

Gold Dust leveL----------------------------- 42 

Gold placers._------------------------- ___ 36, 89,49 

Gopher mine.----- __ --------••• ____ ~_________ 46 

Granodiorite. _____ ------ __ ---- ____ ------_____ 24 

Gravel deposits ______________________ --------- 14,49 

H Page 

Harmony mine •• _____________________ ._______ 40 

Hassel area. ___ ------- ____________ ---_----_ 35, 36, 48 

. History and production ___ ------------------- 34 

Horn Silver mine_____________________________ 48 

H~!Jgh, M. Jean, fossil identification__________ 13 

Hufschmldt, E. L., analyst___________________ 21 

Hyantha mine _____________________ -_-_-- __ --- 41 

I 

Igneous rocks_________________________________ 15 

Indian Creek, composite bodies_______________ 28 

gold. ___________________________ ---- ___ 34, 35, 39 

fan. _________________________ --------_____ 14 

fault _____________________________ ------___ 32 

stock. ___ ------ __________________ --------_ 31 

Intrusive bodies, composite___________________ 27 

Intrusive rocks, related to tho Elkhorn Mountains Volcanics___________________ 20 

Irish Syndicate vein__________________________ 40 

Iron Age Gulch area _____ ---------------·------ 38,89 

Iron Cross mine. ____________ -- __ ---_-_--_____ 46 

Iron Mask mine •• -------------------- 8, 10, 11, 38,49 

J 

January mine ____ ------------------------- 35, 38,41 

January stock ___________ --------------------- 26,41 

Jawbone claim. _________________ -_----------_ 43 

Jefferson Dolomite____________________________ 4 

Jo Dandy mine •• ---:·------------------------ 35,46 

K 

Keating Gulch stock_________________________ 25 

metamorphism___________________________ 29 

Keating mine ••• --------------------------- 35, 45,48 

Keating vein __________ ---------------------~- 36,39 

Kibbey Formation. _______ ---- __ ------------- 5 

King mine _________________________ - ___ ------- 43 

Kleinschmidt mine ________________________ 3S,40,4/ 

Kootenai Formation ... --------~---- 7, 8, 10,20,32,53 

L 

Late Cretaceous and Tertiary tectonics_______ 33 

Lead _____________________________ 35, 38, 42, 43, 46,48 

Llgnitic shale ••• ------------------------------ 48 

Limestone Hills intrusive_____________________ 28 

Little Annie mine. ______________ ----------___ 42 

Little Bonanza mine__________________________ 42 

Little Giant mine.-- __ -- __ -------------_-- ___ 43,44 Little Olga mine ___________________________ 35, 40,41 

Little Olga vein------------------------------ 40 

Location _______________ ------ ____ ------ __ ----- 2 

Lode mine .•••• ------------------------------- 35 

Lodgepole Liln.estone_________________________ 4 

Lombard facies •• __ -----------_------------~-- 5 

Lombard fault •• ------------------------------ 33 

Lone Mountain stock •. -------~--------------- 25 

metamorphism. ___ -_________ ------------- 29 

Lower black shale unit_______________________ 8 

Lower member, Elkhorn Mountains Volcanics._---- _________ ----------_-- 17 

Lowland Creek Volcanics _____________________ 16,29 

M 

Madison GrOUP------------------------------- 4 

Madison limestone _____ - __ ----- __ -------- __ --_ 14 

65 
66 

Page 

Mafic rocks.---------------------------------- 23,24 

Mammoth composite body____________________ 28 

Mammoth mine______________________________ 44 

Marietta mine _____________________________ 35, 38,42 

Mass wasting_________________________________ 15 

Maywood Formation_________________________ 4 

Meagher Limestone .• ------------------------- 3 Merthold, S.M., analyst______________________ 21 

Mesozoic Rocks_______________________________ 7 

Metamorphism, thermaL_____________________ 29 

Middle member, Elkhorn Mountains Volcanics.---------------- ______________ ------- 17 

Middle siliceous mudstone and sandstone unit. 9 

Mineral deposits______________________________ 94 

future discovery and development________ 48 Mines ________________________________ ._----___ 39 

See also individual mine name. Mission Canyon Limestone ___________________ 4,38 

Monarch mine________________________________ 48 

Montana-Idaho vein__________________________ 41 

Monte Cristo stock___________________________ 26 

Moraines.-------------- _______________ ----.:_ 14, 15 

Morrison Formation__________________________ 7, 8 

measured section.--------------------- 49, 52,53 

Mowry Shale.-------------------------------- 10 

N 

Nada mine._------ ______________ ------------. 38 

New Era mine.______________________________ 42 · 

Niobrara Formation _____________ ------------- 11,12 

North Home mine.-------------------------- 3.'i, 48 

Nygaard, E. A., analyst______________________ 21 

0 

Ohio Keating mine ___________________________ 35,45 

Ohio Keating vein __ ------------------------- 36,39 

Oligocene sedimentary tuff___________________ 12 

Oligocene tufa •. _____ . _________________ --- ___ - 13 

Ore pipe ______________________________________ 46,48 

Orphan Boy intrusive _____________________ 24, 26,29 

Outwash fan. ___ . ___ -------- __________ -----·-- 14 

Oxidation and enrichment _____________ - ______ 39, 49 

Oxidation zone. _______________ 36, 40, 43, 44, 45, 46, 48 

p 

Paleozoic rocks.------------------------------ 3 

Palmer, A. R.. fossil identification. ___ ------- 4 

Palmer, Chase, analyst_______________________ 21 

Park area. ____________________________ - 35, 38, 39,42 

Park mine._--------------------------------- 42 Park Shale___________________________________ 3 

Parker mine _____________________ . ____ -------- 43 

Phenocryst, defined. ____________ - _____ ------- 16 

Phosphate rock. ________________________ ----- 48 

INDEX 

Page 

Phosphoria Formation.---------------------- 6, 7 

measured section. ___ ------ ________ --_____ 49 

Pierre Shale ___ -------------- __________ .---___ 12 

Pilgrim Dolomite_____________________________ 3 

Placer gold. _______________________________ 35, 39,49 

Pliocene Old Valley cycle._------------------ 13 Plugs. ___________________________________ . 16, 23,24 

Pole Creek stock.---------------------------- 27 

Porphyry, defined. __ . _______________ --._._.-- 16 

Power Gulch composite body________________ 28 

Precambrian rocks.-------------------------- 3 

Previous work.------------------------------ 3 

Purgatoire formation. _____ -- .. __ --_ ... -- .. ---

Q 

Quadrant Formation __ -------------- _______ 6, 31,38 

measured section._----------------------- 49 

Quadrant quartzite .. -- ____ ---- __ -_---------_- 14 

Quarry Gulch intrusive._---------------_____ 28 

Quartz latite porphyry_---------------------- 29 Quartz monzonite. __ --_______________________ 26 

Quartz monzonite porphyry__________________ 27 

Quartz veins, auriferous pyrite ________________ 36,49 

base and precious metals ______________ 36, 38, 49 

Quartzite mine_______________________________ 48 

Quartzite Ridge area •. -------------------- 35, 3!.1, 46 

Quaternary deposits .. ___ ------ __ ------------- 13, 14 

Queen Bee mine __________ --------------------- 44 

R 

Radersburg area _________ ------ ___ 34, 35, 36, 3!.1, 44. 4!! 

Range-front faults ___ -- ______ -- __________ ---_- 32 

Rattlesnake intrusive. ________ 20, 21, 23, 30, 31, 33, 3\J 

Red Lion Formation_________________________ 4 

Red Wing vein_______________________________ 43 

Reeside, J. B., Jr .• cited______________________ 10 

Replacement bodies, carbonate rock _____ ----- 49 

Retort Phosphatic Shale Member______________ 6 

Rhyolite flow. ___ ---------------------------- 30 

Roads ___________ ---------- __________ --------- 2 

Rothfus mine.---------- ____ -- __ ------------- 38 Ruby mine ___________________________________ 35,46 

Ruby Silver mine .•. ----------------~-------- 46 

s 

St. Louis mine_______________________________ 44 

Santa Anita mine _____________ --------------- 35,46 

Sappington Sandstone Member_______________ 4 

Scheelite. ___ --- __ ------------.-------- _ 38, 39, 44, 4!J 

Sections, measured. __ .-_-_---.-_-----.------- 49 

Sedimentary rocks ____ . ______ -.-- __ -_-_------- 3 

Shedhorn Sandstone ..•. _____ -. ______ -- ____ --- 6 

Sills .• ________ ·------------ ___________________ 20,21 

Silver. _____________ ----------· 35, 38, 4~. 43, 45, 46,48 

Page 

Silver-lead-zinc replacements _________________ 36,38 

Silver Saddle mine___________________________ 42 

Silver Wave mine_____________________________ 44 

Silver Wave stock __________ 23, 25, 27, 29, 31, 36, 38,43 

Skarn deposits ___________ -------- _____________ 36,38 

Skull Creek shale. ______________________ ----_ 9 

Slim Sam Formation ____ 11,12, 17, 21, 23, 24, 29, 31,39 

measured section_________________________ 58 

Slim Sam stock.----------------------------- 25 

Solution breccia. ______ --~- __________ ------___ 5 

South Fork stock __________________________ 24,25,33 

Spring Hill mine.---------------------------- 44 

Stray Horse mine ..• ------------------------- 35,42 

Structural geology____________________________ 30 

Sulfide minerals______________________________ 32 

Swift Formation______________________________ 6, 7 

measured section __________ ....• c .• ____ --- 49,52 

T 

Tectonics, Late Cretaceous and Tertiary---__ 33 

Telegraph Creek Formation. ___________ .---__ 12 

Terrace deposits .. ___ ------ __ ---- __ ---- __ ---_- 15 

Tertiary rocks _________ ------ ______ -- ____ ---__ 12 

Three Forks Shale._------------------------_ 4 

Titaniferous iron ore. _________ 11, 12, 35, 36, 39, 44,46 

•rosi Chert Member__________________________ 6 

u v 

Upper black shale unit----------···----·----- 10 Veins ____________ --·--- __________________ ----_ 23,36 

Venezuela mine_______________________________ 42 

Vosburg mine .. ------------------------------ 14 

Vosburg stock •. ---------------- __ ------·-- 26, 2\!, 32 Vulture mine ______________________ --- ___ ----- 43 

Vulture stock._---·-------------_------------ 26 

w 

Weasel Creek area .• ---------------·---- 34, 35, 38,89 

Weasel Creek fault._------------------------- 32 

White, Katherine, analyst____________________ 20 

Whitehead Ranch fault_______________________ 31 

Whitehorse fault_ ________ --·- ________ --------- 31,32 

Winston Creek.-----------~------------------ 39 Winston district. __________________________ -__ 35 

Wisconsin glaciation _____________________ ----- 15 

y z 

Younger intrusive rocks______________________ 2li 

Yen, T. C., fossil identification_______________ 8 

Zinc _________ •• ______ • _____ • _________ 35, 38, 42, 43, 46 

t