Reconnaissance 

Geology of Western 

Mineral County 

Montana

By ROBERT E. WALLACE and JOHN W. HOSTERMAN

A CONTRIBUTION TO ECONOMIC GEOLOGY

GEOLOGICAL SURVEY BULLETIN 1027-M

The geologic structure and mineralogy 

of western Mineral County, Mont., as 

determined by a 'series of traverses, are 

similar to those of the rich Coeur 

d'Alene mining district

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

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

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

Washington 25, D. C. - Price »1.25 (paper cover)
CONTENTS

	Page 

Abstract_____________-_____-_-__------_--_-__-_-_-_-_______-_-_ 575

Introduction ______________________________________________________ 575

Geologic formations__________-____-__---____---_-__-_-______-_--_ 577

General characteristics._________________________________________ 577

Prichard formation.___-_________J_-_______________________ 578

 'Burke and Revett formations___-_-_-__-_______--_________ 580

St. Regis formation-_-_-____--___---_-____________________- 582

Wallace formation.________________________._.^____________ 584

Glacial deposits.__---_-______--___-__________^___________v 585

Lake Missoula deposits_____-__.-_-_________________ _._-__ 585

Diorite____._--- T - -.___---__---_________--___ __> _ i 587

Structural geology.__________-_____-_-____-__________________i_____ 588

General setting..._______________________________________j_____ 588

Faults___-.-_____._____.__*__.__.-____..----- 589

. Osburn fault zone---__-___-_-------__--_-----___---_ _--_ 589

Location______-_--___-_----_--_-____________L__ _-__ 589

Displacement on fault zone.____________________________ 590

Age of fault zone___________________________________ 592

Twelvemile fault--.----.-.---------____------______-__ 592

Mount Bushnell fault.________________________ 593

Goat Mountain fault_______-__-__-_.___-_-______________- 593

Tamarack Creek fault_______------__________.____________ 594

Silver Creek fault_____.__-___._______________ 594

Gilt Edge fault.. . ... .... .. 595

Crittenden Peak fault____.___-_-_---______-_-__-_______-___ 595

Placer Creek fault-------_------_---_--____-_-_-______---_- 596

The Hope and unnamed faults-__-----________-___________._ 596

Folds.. _. 597

Savenac syncline._________________________________________ 597

Flat Rock syncline_______________________________________ 597

Boyd Mountain anticline____----_-_--_-_-__-_-_-_________-_ 598

Eagle Peak anticline.______________________________________ 599

Lookout syncline and Lookout anticline ______________________ 599

Swamp Creek anticline_-__--__----_--_----_-_-_-______._.-- 599

Cleavage, lineation, and minor structures_-_____-___._____________ 599

Alteration ________________________________________________________ 601

Mineral deposits_________________________________________________ 602

General features_______-_-____-_--_-_-_-__.________.________._ 602

Deposits near the Osburn fault zone____-_-_-________________.__ 602

Silver Cable mine_________.__.__.__.____-______--_-_ 603

Last Chance mine__--_-__-_---_-_--___-_______-___-__-___- 604

Wabash, Meadow Mountain, and Tarbox mines.______________ 604

Rock Island mine____-_-_________-_________________________ 605

True Fissure mine._-_--____--__-_--_______-___-________--- 606

Deposits near the Wishards sill__-_-___-___________________---- 607

St. Lawrence mine_______________________________________ 607

 in
IV CONTENTS

Mineral deposits Continued

Deposits near the Silver Creek fault.__________________________ 607

Other deposits.___________________________________________ 608

Suggestions for prospecting.__________________________________ 608

Literature cited________________________________________________ 610

_-______ __________________________ 611

ILLUSTRATIONS

[Plates are in pocket]

PLATE 48. Reconnaissance geologic map and section of western Mineral County, 

Mont.

49. Geologic map of the Silver Cable mine.

50. Geologic maps of the Last Chance, Rock Island, and St. Lawrence 

mines.

51. Geologic map of the True Fissure mine.

Face

FIGURE 73. Index map showing location of western Mineral County,

Mont. __________ A_-_--_-__-__-_--_-_-_-__-_.__ __ 576

74. Block diagram of the Savenac and Flat Rock synclines__ _ 598

75. Map of underground workings of the Tarbox mine._--__ 606
A CONTRIBUTION TO ECONOMIC GEOLOGY

RECONNAISSANCE GEOLOGY OF WESTERN MINERAL 

COUNTY, MONTANA

By ROBERT B. WALLACE and JOHN W. HOSTERMAN ;

ABSTRACT ,

Rocks underlying western Mineral County, Mont., are principally metasediments.of the Precambrian Belt series consisting of quartettes, argillites, and 

ph'yllites. Dioritic instrusive rocks, principally sills, underlie a small part of 

the arfea. Pleistocene and Recent sediments, including lacustrine, glacial, and 

fluVial;deposits, overlie some of the area. :

A major structural grain that trends northwest and that is made up of subparallel faults and folds characterizes the area. The Osburn fault zone, one of 

the major faults of the Northern Rocky Mountains province, is the major structure in. the area. The fault zone is known to extend from a point near Coeur 

d'Alenej Idaho, to a point near Superior, Mont., across western Mineral County, 

Mont It may extend even farther to the east and to the west. North of the 

Osburn fault zone in western Mineral County intense structural; deformation 

has produced large overturned isoclinal folds and many large faults. South 

of; the Osburn fault zone folds are relatively open and dips rarely, exceed 50°. 

Faulting is notably less south of than north of the Osburn zone. Evidence suggests that there were at least three periods of deformation each represented 

by a different stress system: compressional stresses which produced overturned 

folds and! reverse faults, rotational stresses which produced strike-slip faults, 

and stresses which produced normal faults.

Mineral deposits are principally of lead, zinc, silver, and copper in the form 

of .replacement and fissure-filling veins containing galena, sphalerite, tetrahedrite, 

and chalcppyrite in a gangue of quartz, carbonate, and in a few places barite. 

The geOlpgy and mineralogy are similar to those in the Coeur d'Alene district, 

and the two areas should be considered parts of the same geologic district. 

Production from Mineral County, however, has been extremely small compared 

to production in the Coeur d'Alene district, but Mineral County has not been so 

Intensively explored as Shoshone County. The amount of vein material and 

favorable structural settings of several beltlike areas following major faults .In 

Mineral County suggests that much more exploration is warranted,

: INTRODUCTION

 This reconnaissance study was undertaken to determine the major 

geologic features of the western part of Mineral County, Mont., principally in the drainage basin of the St. Regis River (fig. 73). It 

seemed possible that geologic conditions similar to those of the adja-

. ,:-.:-. '. . .;--. .- .".' ,:« :-575
576 CONTRIBUTIONS TO ECONOMIC GEOLOGY

cent Coeur d'Alene mining district in northern Idaho, from which 

more than one billion dollars' worth of metals have been recovered, 

might extend into Mineral County.

Fieldwork was done in the summer of 1952. The writers spent 20 

days making joint traverses by jeep along accessible roads and then 

20 days making separate traverses afoot across significant areas. 

Twelve days were spent mapping accessible underground workings 

in the area. Most of the underground mapping was done by Hosterman, who was assisted by Earle Cressman.

These reconnaissance traverses formed the basis for the interpretation of the general geologic setting as shown on the map (pi. 48). As 

there is some uncertainty about the interpretation of the geology of

116

FIOUBB 73. Index map showing location of western Mineral County, Mont
RECONNAISSANCE GEOLOGY, WESTERN MINERAL COUNTY, MONT. 577

various parts of the area, an attempt has been made on the map to 

differentiate recorded data from interpretation by identifying traverse 

lines with bands of grayed color. Symbols have also been used to 

indicate different sources of information. For example, much information was derived from the interpretation of aerial photographs, 

and although many features observed on the photographs can be interpreted with a high degree of certainty, information thus derived 

is designated by symbols on the map.

The reconnaissance nature of the investigation is indicated by the 

fact that a geologic interpretation of about 350 square miles of country was made in 60 man-days of traversing, or at a rate of slightly less 

than 6 square miles per day. The map and report are incomplete in 

many ways and represent merely what was seen and recorded in a short 

time.

Because of the intricacy of the structure and the difficulty presented 

in delineation of geologic formations, many areas will require a more 

detailed study before an adequate geologic interpretation can be made.

The excellent cooperation of all the mining companies active in the 

area and the assistance of residents of the area on many occasions are 

appreciated,by the authors.

GEOLOGIC FORMATIONS

GENERAL CHARACTERISTICS

The western part of Mineral County, Mont., (fig. 73) is underlain 

predominantly by metasediments of the Precambrian Belt series. 

Rocks exposed in the area include parts of the following formations, 

listed from oldest to youngest: Prichard, Burke, Revett, St. Regis, 

and Wallace. Relatively recent unconsolidated sediments cap these 

older metasediments. The unconsolidated sediments include lacustrine and glaciofluvial deposits of Pleistocene age, and alluvium 

of Recent age. A small part of the area is underlain by diorite sills 

and dikes.

The presence of rock units in each formation of the Belt series which 

resemble rock units in every other formation of the Belt series makes 

classification of individual outcrop areas difficult. Only by judging 

the predominent rock type through a stratigraphic thickness of several hundred feet or over areas of several thousand square feet can an 

adequate determination be made.

The study of 25 thin sections of rocks, including typical specimens 

from each formation, shows the preponderance of quartzites in this 

area. Many of the fine-grained quartzites were identified in the field, 

as argillites, but extremely fine-grained quartz is the dominant ,constituent, and sericite and chlorite are present in conspicuous amounts. 

Many,of .the fine-grained quartzites are composed of grains ranging
578 CONTRIBUTIONS TO ECONOMIC GEOLOGY

from 0.01 to 0.03 mm in diameter (medium-silt size). The coarsest 

grained quartzites examined contain grains as much as 1 mm in-diameter, but these coarser sizes are relatively uncommon, and grains 

rarely exceed 0.3 mm in diameter. The quartz grains in most specimens have been recrystallized and their borders at least slightly 

sutured, but the angular shape of original grains is apparent in many. 

Some specimens examined are highly sericitic, and a few from the 

greenish St. Regis strata contain 50 percent or more pale-green 

chlorite.

In the specimens examined, sericite is the most abundant mineral 

associated with quartz in samples from the Wallace formation^ 

whereas sericite and chlorite are both abundant in samples from the 

St. Regis formation. Biotite is abundant in the fine-grained quartzites 

of the Prichard formation. Plagioclase, apparently as detrital grains, 

was identified in quartzites from both the St. Regis and Prichard 

formations. The few grains identified were oligodase. Magnetite is 

sparsely scattered through all the formations and is locally concentrated in greater amounts in some thin beds or laminae.

In the following descriptions colors are classified according to the 

system employed in the "Rock Color Chart" (Goddard and others, 

1948) distributed by the National Research Council.

PRICHARD FORMATION

The uppermost 5,000 feet of the Prichard formation is shown in the 

northeastern corner of the mapped area where it is exposed at the head 

of Swamp Creek in the overturned flank of the Swamp Creek anticline. No section was measured or studied in detail, but in general 

the formation is characterized by the dark argillites and fine-grained 

quartzites, as it is at its type locality. The contact between the Prichard and the Burke and Revett formations undifferentiated is gradational, and relatively light-colored massive quartzites are interbedded 

with dark thin-bedded argillites and fine-grained quartzites through a

Stratigraphic range of several hundred feet. Many of the beds m 

Swamp Creek Contain Considerable blotite in minute flakes; of random

orientation, Ripple marks and graded bedding were observed

wt a.re not common. Limomte staining On weathered surfaces is

Characteristic.

In addition, a thick Well-GXpOSGd SCCtlon of the Prichard forrr^io^

a few miles east of the mapped area was sWUed and is described 

briefly Here m w*<» to sWe tt ^ore comprehensive idea of what the 

Prichard formation is like in this region. From «~ 5 ^^ 0:f tKe 

Clark Fork and Flathead Rivers southwest to Paradise Ferry across 

the Clark Fork River, an air line distance of 5% HlileSj an almost
RECONNAISSANCE GEOLOGY, WESTERN MINERAL C&UNTY, MONT. 579

continuous section of the Prichard formation is exposed in the canyon 

walls of the Clark Fork River and in railroad and road cuts along 

the canyon. In this entire section, except for a minor flexure north 

of Quinns Hot Springs, dips are uniformly to the southwest and 

range between 40° and 60°. Discounting unidentified faults, this 

section is very nearly 17,000 feet thick. A few apparently minor 

faults were identified northeast of the ferry, and the top of the section 

is truncated by a major fault. This large fault is believed to be part 

of the Tamarack Creek-Thompson Pass fault system and is well 

exposed in road cuts near the mouth of Patrick Creek. The base of 

the exposed section is at the crest of a major breached anticline that 

is followed by the Clark Fork Eiver from its confluence with the Flathead River northwestward to the vicinity of Plains, Mont. More 

than 5,000 feet of section can be seen in one almost continuous exposure 

southwest of Quinns Hot Springs. The presence of the hot springs 

suggests a fault in the section, but no lineament suggestive of a fault 

was observed on the aerial photographs. No field study was made 

to confirm the presence or absence of such a structure.

Throughout the section the Prichard formation is most commonly 

characterized by very fine to fine-grained quartzites and by siliceous 

argillites, with beds from 2 inches to 2 feet thick; colors are medium 

gray (N3-N5) and greenish gray (5GY 5-7/0.2-1). Pyrite is abundant, and weathered surfaces are typically moderate brown (5 YR 

3-4/4). The dark, neutral-gray colors (N3-N5) are as characteristic as any other feature, because in other formations a slight chroma 

is generally present and darker values are generally absent. Compared to the Prichard formation near Kellogg, Idaho, this section is 

more uniformly siliceous, has fewer soft argillites, and has fewer 

thinly laminated argillite beds.

No subdivision of the formation is possible with present data, but 

two special units are described below.

Exposed near the top of the Clark Fork section are medium-gray 

homogeneous argillites whose excellent cleavage parallel to their 

bedding gives them the appearance of slate. The color is typically 

neutral gray (N3-N4). Pyrite is abundant along bedding planes. 

Near the middle of the section fine- to medium-grained vitreous 

quartzites of light greenish-gray (5 GY 7/0.2-1), in beds as thick 

as 5 feet, make up a unit about 100 feet thick. Thinner units of similar quartzite are present throughout the section. These quartzites 

near the middle of the section may correspond in a general way to the 

middle Prichard quartzites of the Kellogg, Idaho, area. This statement does not imply that any one quartzite bed is considered to be 

continuous over such distances.
BURKE AND REVETT FORMATIONS

In the western part of Mineral County, quartzites, which are presumed to be the equivalent of the Burke and Revett formations of the 

Coeur d'Alene district have not been differentiated on the niap. This 

was necessary because differences between these formations are much 

less distinct than in the Coeur d'Alene district where they have been 

fairly well delineated, and the reconnaissance study did not permit 

detailed scrutiny of subtle differences that might permit a differentiation of the units in all places. Only on the geologic maps of and 

in the discussions of the Silver Cable and Last Chance mines are the 

formations differentiated.

The writers believe that there is a gross Kthologic difference between 

the quartzites of the Burke and Revett formations north of the Osburn 

fault zone and those south of the fault zone. North of the fault zone, 

however, tight folding and much faulting make complete analysis of 

the stratigraphy very difficult. In only a few places was a normal 

contact found between the quartzites of the Burke and Revett formations and the adjacent formations, and all of these contacts are in the 

deformed flanks of tight folds. All other known contacts are fault 

contacts.

South of the Osburn fault zone the quartzites of the Burke and

Revett formations are typically fine grained, somewhat argillaceous

quartzites, generally a light gray of low chroma or greenish gray 

(5GY 5-6/1). Bedding thicknesses are usually more than l foot and 

range up to 8 or 10 feet. The most common thicknesses are between 

1 and 4 feet. White vitreous quartzites characteristic of the Revett 

quartzite in the Coeur d'Alene district are rare. Nowhere south of 

the OSblim fault Was the base of the quartzite section of the Burke

and Revett formations found, but the top is well exposed in many 

places. Because the quartzites of both the Revett and St. Regis formations in Mineral County, Mont., are shades of gray, the color distinction commonly used in Idaho between the white quartzites of 

the Revett and the overlying purplish-gray St. Regis formation is 

scarcely applicable. In Mineral County a contact between the formations has been chosen so that the principally argillaceous beds are 

above and the principally quartzitic beds are below the contact. The

authors believe that this use of rock type as the diagnostic feature

rather than color is appropriate. It should be recognized that on

many maps of the Coeur d'Alene district purplish-gray quartzites 

are classified and mapped as St. Regis formation, but that in Mineral 

County some distinctly purplish-gray quartzites are included in the

Burke and Revett formations.

The quartzites of the Burke and Eevett formations are well exposed 

in road cuts along a 4-mile section of U. S. Highway 10 immediately
RECONNAISSANCE GEOLOGY, WESTERN MINERAL COUNTY, MONT. 581

east of Saltese, Mont., and in road cuts on the same highway in the 

canyon of the St. Eegis Kiver between Henderson and St. Regis, 

Mont. In the cuts, lithologic units ranging from thick vitreous 

quartzites to gray argillites are found, but no characteristic gradation 

in rock type from top to bottom of the section exposed was recognized. At least 3,000 feet of quartzites of the Burke and Revett 

formations are present in the south flank of the Boyd Mountain 

anticline. Although 8,000 feet are shown in the structure section, 

there is almost certainly some faulting which could repeat part of 

the section.

In the cirques at the heads of Deer, Ward, and Newman Creeks are 

excellent exposures of the uppermost 1,500 feet of the quartzites of 

the Burke ,and Revett formations. The quartzites are predominantly 

gray and vitreous and are in beds from 1 to 10 feet thick. The contact 

with the argillites of the overlying St. Regis formation is excellently 

exposed but is difficult to map because of the gradational char: 

acter. Through a stratigraphic range of about 400 feet grayish 

vitreous quartzite beds as much as 1 or 2 feet thick are interbedded 

with greenish-gray thinly laminated argillites. More detailed study 

might make it possible to map this as a separate transition unit, but 

in .this reconnaissance the separation of Burke and Revett from St. 

Regis was made merely by differentiating the principally quartzitic 

part of the section from the principally argillaceous part of the 

section.

Hortb of the Osburn fault zone deformation has been so intense 

that continuous sections of the quartzites of the Burke and Revett 

formations- are rare. Very light-colored thick-bedded vitreous: quartzites similar to Revett quartzite in its type locality were found in the 

vicinity of Mullan Pass, Sawhill, the Silver Cable and the Bryan 

mines, near Camels Hump Peak, and near the mouth of Tamarack 

Creek. Greenish-gray (5GY 6/1) thin-bedded impure quartzite similar to quartzite of the Burke formation in its type locality was 

found in several places. North of the fault shown on the map at 

the Silver Cable mine the rocks appear to be typical of the quartzites 

of the Burke formation and at several other places float more characteristic of Burke than Revett was found.

On Mount Bushnell the quartzites include both impure, argillaceous 

quartzites and some vitreous quartzites. Beds range from 1 inch to 

4 feet in thickness, and have abundant mud cracks and ripple marks. 

They are pale gray to greenish gray. These rocks are probably near 

the top of the section of quartzites and grade upward into the 

argillites of the St. Regis formation. . ' ;
582 *' CONTRIBUTIONS TO ECONOMIC GEOLOGY " ' 

ST. REGIS FORMATION

Mapping of the St. Regis formation in Mineral County, Mont., is 

complicated by a marked f acies change within a distance of 4 or 5 miles 

near Lookout Pass. West of the State line in Idaho, the St. Regis 

formation is typically gray to purplish-gray argillite, about 1,200 to 

1,400 feet thick, and has an upper member of thinly laminated greenish argillite, 150 to 450 feet thick.

Half a mile southeast of Lookout Pass on TJ. S. Highway 10 road 

cuts expose interbedded greenish-gray (5G 6-7/1) and purplish-gray 

,(5P 5-6/0.2) argillites or reddish-gray (5R 6/1) argillites and finegrained quartzites. Two miles farther southeast of Lookout Pass 

purplish-gray colors are uncommon and are confined to the more 

quartzitic beds. The typical color of the argillaceous beds here is a 

greenish gray. Near Randolph Creek, 5 miles east of Lookout Pass, 

thinly laminated greenish-gray argillites, typical of the upper member 

of the St. Regis formation west of Lookout Pass, make up the major 

part of a stratigraphic section about 3,000 feet thick. Near Silver 

.Creek 3 miles farther east, the section is more than 5,000 feet thick. 

ISTear the mouth of Ward Creek, 15 miles east of Silver Creek, the 

greenish argillites are about 1,700 feet thick. It is difficult to interpret 

.the significance of this f acies change because there is no time marker 

.present. It is known that an argillaceous section overlain by banded 

argillites and limy quartzites and underlain by quartzites is relatively 

thin and typically purplish in Idaho, but is three times as thick and 

typically greenish in Montana. Several interpretations of the f acies 

change are possible. First, the arginite may represent an equivalent

time in Idaho and Montana, but with three tim6S RS glCRt a thickness 

Of argillite deposited in Montana as in Idaho, and under conditions

of oxidation in Idaho and of reduction in Montana. The purplish

color is produced By SpeCUlarit6j Which contains ferric iron, and the 

greenish color is produced principally by chlorite, which COntamS fer- 

.rous iron, Or perhaps bj iron in other configurations. Because the St.

RegiS formation is typically quartzitic farther tO the W6St Jiear Pinehurst, Idaho, the suggestion of a gradatlOn from nearshore f acies in 

the west to offshore f acies in the east might be a plausible interpretation. The thin purplish faCiOS then might represent deltaic top-set

beds which were subject to exposure and oxidation periodically, and

the thick greenish facies might represent fore-set beds OUt Of an 'oxidizing environment,

.Such an interpretation cannot be certain, hOWeVei*, teUSO Other 

evidence suggests different correlations, For example, the quartzite 

beds below the argillite are much more impure in Montana than in 

Idaho, and south of the Osburn fault zone no white quartzites like 

those of the Eevett quartzite were found. Thus the possibility exists
RECONNAISSANCE GEOLOGY, WESTERN MINERAL COUNTY, MONT. 583

that the Revett quartzite in Idaho grades eastward into a more argillaceous fades and parts of the rocks mapped as the St. Regis formation in Montana are the time equivalents of Revett rocks in Idaho, 

Likewise, greenish argillites are known in the Wallace formation in 

western Mineral County. Calkins (1908, p. 25) and Calkins and Jones 

(1914, p. 173) in reports on the Coeur d'Alene district included the 

greenish argillites at the top of the St. Regis in the Wallace formation ; so at least part of the rocks mapped as St. Regis in Montana 

may be the time equivalent of Wallace rocks in Idaho. The St. Regis 

formation in Montana, as here mapped, may include the contiguous 

parts of the adjacent formations as they have been mapped in the 

Coeur d'Alene district in Idaho.

The predominance of argillite was used as the principal criterion 

in distinguishing the St. Regis formation, although quartzitic beds a 

foot or more thick can be found almost everywhere in the section. 

The contact between the St. Regis and the overlying Wallace formation is defined here as being at the top of the predominantly argillaceous series. Above the argillaceous series lies a section composed predominantly of alternate quartzite beds and argillite beds ranging from. 

1/2 inch to 6 inches in thickness, and accompanied almost everywhere 

by limy quartzite beds. At least two units of greenish argillite several tens of feet thick are included within the lowermost 1,000 feet 

of the Wallace formation above the contact as defined. The lower 

contact of the St. Regis formation has been placed where the predominantly argillaceous beds grade into the predominantly quartzitic 

Burke and Revett formations below. Of course, the boundaries be1 

tween such units which intergrade are purely arbitrary.

All the preceding description of the St. Regis formation in 

Mineral County applies to the rocks south of the Osburn fault zone. 

North of the fault zone the St. Regis formation has the characteristics 

that it has in the Pottsville and Mullan quadrangles of Idaho. Exposures are extremely poor, however, and no continuous section is 

exposed. Isolated outcrops and float indicate that there is considerable purplish-gray argillite and thinly laminated green argillite. 

Purplish-gray argillites were found in two places between the north 

and south branches of the Osburn fault. This suggests a closer relation of these rocks to the rocks toward the north than to those toward the south. This relation in turn suggests that the major movement took place along the south branch of the Osburn fault.

In the Flat Rock syncline the St. Regis formation is predominantly 

greenish phyllite, with cleavage developed to a degree that obliterates 

bedding in many places. Exposures of the St. Regis are poor at the 

head of Twelvemile Creek but abundant float indicates a predominance of purplish-gray argillite with a relatively thin greenish zone 

at the top.
584 ' CONTRIBUTIONS TO ECONOMIC GEOLOGY 

WALLACE FORMATION

The Wallace formation crops out over a large part of Mineral 

County, Mont. The formation has not-been subdivided on the map, 

although four members are recognized in contiguous areas. The top 

of the Wallace formation was not found in this area; so, only a part 

of the formation is mapped.

The thickest section found is at the head of Savenac Creek near 

Mount Bushnell where about 6,500 feet of Wallace formation is exposed in the north flank of the Savenac syncline. Because the overturned Savenac syncline shows intense deformation, 6,500 feet may 

represent less than the original thickness. The base of this exposed 

section is a fault contact; nevertheless, the fault contact may be close 

to the true bottom of the section. The section can be divided approximately into the four units that follow:

Feet

-Argillite, thinly laminated, laminae commonly from paper thin to *4 

, inch thick, contrasting dark- and light-gray; many limy beds___ 2,000± 

Argillite, phyllitic, bedding obscure, light olive-gray____________ 1,500± 

Argillite, thinly bedded, dark-gray, relatively homogeneous_______ 500± 

Argillite and limy quartzite, banded and interbedded, bands commonly 

from 1 inch to 2 inches thick; limy parts etched on weathered sur- 

. faces and stained brown____________'._________________ 2,500±

6,500±

. At the head of Flat Hock Creek the Wallace formation is composed 

.principally of light olive-gray to brownish-gray phyllite. Some beds

of crystalline limestone were found, and thinly bedded limy argillites 

and quartzites are common. In this area and in Twelvemile Creek 

.valley the Wallace formation contains a myriad of small quartz lenses 

and veins. The veins are from one to several tens of feet long and 

are a fraction of an inch to a foot or more wide. They contain fragments of pnylllte and gome dark-green chlorite pods, but apparently 

,no sulfides. The abundance of the quartz veins may in some way be 

.related to the structural knot formed by the Flat Rock syncline crossing the Savenac syncline, but why the quartz is predominant in the

Wallace and not in the other formations is not understood.

South of the Osburn fault the "Wallace formation is -well exposed

.in road cuts on the North Fork of Little Joe Creek and in cirques

.that head against the State-line divide, as at the head of Silver Creek, 

the Middle Fork of Big Creek, and Copper Gulch. EXDOSUreS on Silver OreeK suggest about e,ooo feefc of v^u-oo

formation, but unrecognized faults may have repeated me section. 

-At the St. Lawrence mine, at the head of Silver Creek, the lithology 

is typical of the lower unit of the Wallace formation as known in the 

Coeur d'Alene district. The rocks consist of interbedded medium
RECONNAISSANCE GEOLOGY, WESTERN MINERAL COUNTY, MONT. 585

dark-gray (N4) argillite and light brownish-gray (5 YR 7/1) to 

yellowish-gray (5Y 7/1) limy quartzite. The beds of argillite and 

quartzite are most commonly 1 to 4 inches thick, and mudcracks are 

abundant.

The transition zone from the St. Regis formation to the Wallace 

formation is well shown on Silver Creek north of the mouth of Burke 

Creek and at the mouth of Dominion Creek. Like the boundaries 

between other formations of the Belt series, the boundary between the 

St. Regis and Wallace formations is not distinct. Units 10 to 50 feet 

thick of greenish-gray argillite typical of the St. Regis are separated 

from the main unit of St. Regis by 1,000 feet of interbedded argillites 

and quartzites typical of the Wallace.

GLACIAL DEPOSITS

Deposits of glacial and glaciofluvial origin mapped in this area 

are confined entirely to deposits formed by valley glaciers which 

occupied many of the larger valleys draining the north slopes of the 

major divides.

On the south side of the St. Regis River most of the tributaries 

which head against the State-line divide were occupied by glaciers for 

the upper third to upper half of their lengths, and cirques have been, 

formed at the heads of each. The Middle Fork of Big Creek is 

typical; it has alluvium in its lower part, a canyon section of steep 

gradient in its middle part, a morain-covered broad valley in the 

upper part, and a cirque at its head. In similar tributary valleys east 

of Henderson the lower alluviated section is missing, for the St. Regis 

River itself in that area is actively cutting a canyon and has not 

reached grade.

The Middle Fork of Packer Creek is rather unusual. The headwaters are in a broad valley, and glacial gravels that cover the floor 

extend up the gentle northeast valley side to the divide between 

Packer and Wilkes Creeks. The head of Wilkes Creek is a cirque 

wall forming a rock step typical of glaciated valleys and very likely 

some of the drainage from Packer Creek flowed into Wilkes Creek 

at one time. Now, all the Packer Creek drainage flows southward 

through an ungraded section with many cascades and falls just below 

the broad headwater region to the St. Regis River.

LAKE MISSOTTLA DEPOSITS

Lacustrine silts, fluvial gravels, and ice-rafted cobbles and boulders 

are evidence that glacial Lake Missoula (Pardee, 1910, p. 376-386) 

flooded many of the valleys in the area mapped. These deposits are 

not differentiated on the map, for to do so would require a much more
586 .'. CONTRIBUTIONS TO ECONOMIC GEOLOGY

thorough study of their character and distribution than was possible 

during this reconnaissance of bedrock geology in the region.

Cobbles and boulders of red and purple quartzite from the Missoula group have been distributed, presumably by ice rafting, throughout the area mapped, although the nearest outcrop of similar rocks 

is near Superior, Mont., about 14 miles southeast of St. Regis. Such 

erratics have been found on Camels Hump Pass, at an altitude of 

3,950 feet, near the northern branch of the Osburn fault on the 

Deborgia Cutoff Road at an altitude of 4,000 feet, and 1 mile southwest of Cabin City on a spur at an altitude of 3,950 feet. These points 

have the highest altitudes at which ice-rafted boulders were observed,, 

and they indicate that the highest level of Lake Missoula was about 

4,000 feet above sea level or perhaps slightly higher during at least 

part of its existence. The lake is known to have had a maximum 

altitude of 4,200 feet at Stevensville in the Bitteroot Valley (Campbell and others, 1915, p. 135) ; this corresponds closely to that suggested in the present area.

Three lines of evidence suggest that ice rafting has been the agency 

for distributing the erratics. First, isolated subrounded boulders of 

quartzite of the Missoula group as much as 2 feet in diameter are not 

uncommon on many hillsides otherwise covered with mantle from the 

underlying bedrock and with angular talus. At most such places 

there is no evidence of former terrace gravels of which the boulders 

might be remnants; their completely erratic occurrence is characteristic. Second, the distribution of these erratics can hardly be attributed

to stream action because the possibility of former drainage from the 

Superior source area westward through St. Kegis valley, Tamarack

Creek valley, and many other tributaries Seems ruled out by the general 

elevation Of the divide surrounding St. Kegis valley. Drainthat could have flowed westward over these high divides would

have been at a level of more than l,ooo feet above present 

drainage levels and could not have le:rt erraiicS 6n tuG WU"llS Of .YallCyS

not yet developed. Third, there seems to be no supporting evidence 

of valley glaciers moving westward into St. EegiS Yalley from the

Superior area; thus, this means of transport is ruled out.

in a few places, especially at the head of Tamarack Creek and on 

West Twin Creek) quartzite erratics of the Missoula group are clustered in areas only a few acres in extent. Throughout til6 ulinmiOCky

topography in the Osburn fault trough on Timber and Twin Creeks

erratiCS Were found on the crests of all Kills traversed.

In addition to ice-rafted erratics, laCUStrine Silt deposits are widespread. One Of the best exposures of silt is in Mullan Gulch from 1

to 3 miles west of St. Regis. Silt 20 feet thick is exposed in road cuts,, 

and terraces are apparently cut in silt at a height of 50 to 75 feet above
RECONNAISSANCE GEOLOGY, WESTERN MINERAL COUNTY, MONT. 587

the valley bottom. The silt is light buff, occurs in flat-lying beds 

which are varved in some places, and contains abundant concretions, 

having elliptical and irregular outlines. Many cuts in silt in Mull an 

Gulch and east of St. Regis on U. S. Highway 10 were searched for 

vertebrate fossils, but no bones-or bone fragments were found. Silt 

beds believed to be lacustrine are abundant, although not thick 

throughout the topographic trough following the Osburn fault zone 

west of Henderson. The bedrock is obscured and only a few pieces 

of bedrock float have worked their way through the silt veneer although 

bedrock-type topography is readily recognized on aerial photographs. 

Partly because of this veneer, there is great uncertainty about bedrock identification in the fault trough area. A symbol has been employed on the map to indicate where the identity of the bedrock is 

especially uncertain. The symbol should not be construed to indicate 

distribution of Lake Missoula sediments, because undoubtedly other 

factors, principally the shattered character of bedrock in the fault 

trough, add to the difficulty. .

DIORITB

Sills and dikes of dioritic composition make up a small proportion 

of the rocks of the region. The Wishards sill is the largest of these 

igneous bodies and crops out for a distance; of more than 10 miles 

along the Montana-Idaho State line in the southwestern part of the 

area mapped. The sill is more than 400 feet thick in some places but 

is less than 50 feet thick in other places. Calkins and Jones (1914, 

p. 174) report that the Wishards sill persists from the head of Placer 

Creek southwestward to St. Joe River, 30 miles away. Wallace has 

seen some of these outcrops of the Wishards sill along the St. Joe 

River and on traverses has crossed outcrops of what may be a westward extension of the sill southwest of Stevens Peak in Idaho, between Stevens Peak and the Little North Fork of the St. Joe River.

Calkins and Jones (1914, p. 175) give a good general description 

of the rock as follows:

The rock has the normal appearance of diabase. Its general color is black 

to dark gray with a tinge of green; its texture is coarse to fine. The minerals 

visible to the naked eye are white feldspar, dull greenish-black augite and 

amphibole, and black iron ore of metallic luster. The feldspar forms crystals 

that give many narrow oblong sections. No olivine is visible, even under the 

microscopic, which reveals considerable amounts of quartz, alkali feldspar, 

and biotite.

The diorite in places has a very high content of magnetite or ilmenite, in other places is nearly void of mafic minerals, and elsewhere 

has various amounts of magnetite, pyroxene, and amphibole.

359881 56 3
588 CONTRIBUTIONS TO ECONOMIC GEOLOGY

The sill intrudes argillites and impure quartzites of the Wallace 

formation and the rocks both above and below the sill are metamorphosed. Metamorphism extends only a few feet from the sill and 

the argillites and impure quartzites have been changed to fine-grained, 

pale-gray hornfels.

Calkins and Jones (1914, p. 174-175) suggest that the Wishards 

sill was intruded before the Algonkian rocks were deformed and that 

it has been folded and faulted with'them; In the paragraph describing the Crittenden Peak fault, the authors present evidence which, 

though uncertain, suggests that the Wishards sill may have succeeded 

a period of major faulting. This interpretation in turn suggests that 

the Wishards sill may have been intruded in Laramide or postLaramide time.

GENERAL SETTING .

The region has been subjected to intense folding and faulting. The 

major structural feature is the Osburn fault zone which approximately 

bisects the mapped region along a line trending N. 65°-70° W. All 

other major faults and fold axes roughly parallel this trend, although 

in gross pattern they trend slightly more north of west and appear to 

i>e truncated at a very low angle by the Osburn fault zone.

Deformation is much more intense in the structural block north of 

the Osburn fault zone than in the block to the south. On the north 

side of the fault zone overturned folds are the common structure, but 

on the south side overturning has occurred iii only a few places, Most 

of the western half of the area south of the Osburn fault zone is underlain by a faulted homocline dipping south. The eastern half of 

the homocline is complicated by two eastward-plunging anticlines, 

and the north flank of each is truncated by a fault. South of the 

Osburn fault zone near the extreme; western edge of the map are two 

overturned folds, and at the extreme eastern edge of the map the Wallace formation Kas been intricately folded. This alternation of intensely folded and relatively unfolded blocks south of the Osburn

fault zone is typical of the area between Kellogg, Idaho, and St. Regis,

Mont.

The Osburn fault zone itself is a complex of anastomosing faults, 

shears, and shattered zones ranging in composite width from 1,000

feet to 2 miles. A broad elongate topographic trough has been eroded

in the soft broken rock along the general line of the Osburn fault zone.

Pleistocene and Recent deposits occupy this trough and almost everywhere mask the fault zone, but gouge and intensely sheared rock are 

exposed in several places.
RECONNAISSANCE GEOLOGY, WESTERN MINERAL COUNTY, MONT. 589 

FAULTS ; 

OSBURN FAULT ZONE

LOCATION

The Osburn fault zone has been'traced continuously from Fernan 

Lake, near the city of Coeur d'Alene, Idaho, eastward through the 1 

Coeur d'Alene mining district to a point ,a few miles east of the Nancy 

Lee mine north of Superior, Mont. Its western extension is covered 

by Columbia River basalt (Anclerson, 19.40, pi. 2). ,A long-distance 

projection of its trend eastward suggests that a large fault zone a few 

miles north and northeast of Missoula, Mont., may possibly represent, 

the Osburn fault zone. The length, therefore, is at least 90 miles' 

and possibly more than 140 miles. The fault zone is marked throughout its entire length by such erosion features as deep saddles in ridgesand structurally controlled valleys. Alluvium in valleys masks long 

sections of the fault zone, but the zone is well exposed in a few road, 

cuts and river cutbanks and in several mine tunnels. To the west of 

the area studied, between the towns of Wallace and Mullan, Idaho, 

the fault zone can be seen in the Morning, Golconda, and Granada 

mine workings. Between Mullan, Idaho, and the Montana State line 

the fault zone lies under alluvium in the South Fork of the Coeur 

d'Alene Valley and crosses the Idaho-Montana State line through a 

saddle known as Mullan Pass.

Within the mapped area the fault zone lies about 2 to 3 miles north 

of the St. Regis River and is marked by a broad troughlike depression 

eroded in the crushed and sheared rocks of the fault zone. At Mullan 

Pass the fault zone seems to be more nearly one fault having a sheared 

and crushed zone several hundred feet wide. In contrast to this, to 

the east where the Osburn fault zone crosses Packer Creek, the fault 

zone is comprised of at least two distinct faults, one north and the 

other south of Meadow Mountain. In this western part of the area 

the fault zone crops out in several cuts along the Randolph Creek road, 

southeast of Silver Cable mine, and along one road cut southeast of 

Meadow Mountain. Its approximate position can be traced across 

Randolph Creek and along the West Fork of Packer Creek by a mantle composed principally of gouge and crushed quartzite. On aerial 

photographs, lineaments can be seen which coincide with these zones 

of gouge in the mantle and serve as corroborating evidence of the 

position of the fault zone.

Between Packer Creek and Twelvemile Creek the northern branch 

of the Osburn fault zone is clearly marked topographically by a southfacing escarpment that also appears as a prominent lineament on the 

aerial photographs. Intensely sheared rocks in this zone can be seen 

at the portal of the Saltese Consolidated mine. Near Twelvemile
590 CONTRIBUTIONS TO ECONOMIC GEOLOGY

Creek light-gray soil over the fault zone has proved by microscopic 

examination to be composed of fine-grained angular fragments, principally of quartz, evidently fault gouge. In this central segment of 

the Osburn fault zone most of the area south of the escarpment is 

obscured by Pleistocene (?) gravels, Lake Missoula sediments, and 

Quaternary alluvium, but the identification of several large faults 

suggests that more than two distinct faults here constitute the zone. 

Some of the southern faults seem to diverge from the main zone, as 

their eastern parts are farther from the main zone than their western 

parts.

. East of Twelvemile Creek, near Camels Hump Pass, the northern 

branch of the Osburn fault zone appears to be the largest of several 

faults and was visible in a road cut at Camels Springs. In road cuts 

West of Camels Hump Pass, numerous intensely shattered zones were 

found. About 1% miles'to the south the Boyd Mountain fault may 

represent the southernmost element of the Osburn fault zonm However, a conspicuous saddle near Sloway Camp, a short distance east 

of the map area, suggests that a large fault zone diverges from the 

Osburn fault zone, and the Boyd Mountain fault may represent a 

western segment of the diverging fault zone. In prospect trenches 

on the Monarch vein at Shoemaker ranch, gouge zones indicate a large

fault parallel to the Boyd Mountain fault.

An unusually sharp U-shaped meander of the Clark Pork River lifiS 

along the eastward projection of the Osburn fault zone north of St.

Regis. Probably this meander is at least indirectly controlled by the 

fault zone.

DISPLACEMENT ON FAULT ZONE

Undoubtedly the Osburn fault zone represents an extensive structural break, but the amount and type of displacement are little known.

If displacement of a fault is even crudely in proportion to its length

and to the width of gouge and crushed zones, the movement on the

Osburn fault must have been great, perhaps on the order of miles.

Apparent displacement is anomalous, for in the vicinity of Osburn, 

idaiio, the fault zon© Kas brought rocks of the Prichard formation

111 COntftCt With rOCks Of Wallace formation. This displacement indicates about 6,000 feet (Calkins, 1908, p. 62) of SfratigTapMC ttllW j 

yet east of Mullan, IdahO, aild in the western end of Mineral County, 

Mont.j locks of tHo same, formo.tioj\ are north and south of the fault.

Thus, rocks of Wallace formation SOUth Of the faillt Zone are in contact with other Wallace rocks north of the fault zone and also ;the St. 

BiOgis *<» «*io~ ia ixx oo^t^ct ^yitK St. Kegis rocks, and the Revett 

quartzite is in contact with Bevett quarizite along tuft laillt near .DUG 

Idaho-Montana line. These relations might be explained in three
RECONNAISSANCE GEOLOGY, WESTERN MINERAL COUNTY, MONT. 591

ways: by differential vertical displacement, by strike-slip displacement cutting diagonally across structures deformed at an earlier time, 

or by a combination of both types of displacement.

Different structural arrangements are generally found on opposite 

sides of the fault, indicating that the fault is a major structural element. For example, in the Wallace-Mullan area northward-trending 

faults and folds are prevalent north of the Osburn fault and eastward-trending folds and faults are prevalent south of the fault zone. 

In western Mineral County structural trends are similar north and 

south of the fault zone, but folds are predominantly tight and overturned north of the fault zone in contrast to broad and open folds 

south of the zone. A reverse situation exists near Osburn, Idaho, 

where overturning is characteristic of the silver belt south of the 

Osburn fault zone, and open folds typify the northern block. A block 

of overturned folds also lies south of the fault just west of the Montana-Idaho State line. The mechanics for producing this structural 

discontinuity along the fault are difficult to conceive. One explanation may be that strike-slip of about 16 miles has offset earlier tightly 

folded blocks.

Near the mouths of Twelvemile and Flat Rock Creeks the Flat Rock 

syncline has been formed in the overturned flank of the Savenac syncline, and strikes of beds in the Flat Rock syncline are approximately 

parallel to a sharp bend in the north branch of the Osburn fault 

zone. These flexures in the sediments and the fault are interpreted as 

large drag features developed by strike-slip on the south branch of the 

Osburn fault zone when the south side moved westward at least half 

a mile. The cross-fold relationship of the Flat Rock syncline to the 

Savenac syncline, further discussed under "Cleavage, lineation, and 

minor structures," suggests a time relation in which overturned folds 

north of the fault zone were produced first and then a drag fold 

was produced in the overturned flank of an earlier overturned fold by 

strike-slip stresses along the fault zone. It might be postulated that 

the stress which produced strike-slip and the stress which produced 

overturned folds were merely parts of the same stress system, and 

that the Flat Rock syncline represents merely a late phase of strain. 

If this were true, however, the early folds should trend more nearly 

west or southwest, rather than parallel to the Osburn fault zone or 

more northwest than the Osburn fault zone as they do. The existing pattern is believed to indicate a fairly distinct separation in time 

of the strike-slip and overturned-fold phases of distortion. As will 

be indicated later in a discussion of faults which cut the Wishards 

sill, there may be a third and later phase of deformation which probably produced a relatively minor amount of displacement.
592 CONTRIBUTIONS TO ECONOMIC GEOLOGY

AGE OF FAULT ZONE

The age of the fault zone can be determined only within broad limits, 

and it seems possible that the general line of the fault zone has been 

in existence for a long period of time. The present shear zone, of 

course, can be no older than the Precambrian Belt series rocks which 

it cuts. At the other extreme, basalts of presumed Pliocene or younger 

;age serve as a time marker. The basalts fill valleys incised in an old 

erosion surface, the Idaho peneplain. During the development of 

this peneplain all recognizable topographic expression of vertical displacement on the Osburn and related faults was removed. Thus major 

displacement must have taken place considerably earlier than the outflow of the basalts. Closer limits can be tentatively established by assuming a relation to other deformation in the Northern Rocky Mountains province, most of which took place in Late Cretaceous or early 

Tertiary time during what is generally called the Laramide revolution. The multiple deformation already suggested very likely was not

 confined to one simple period of deformation, but as yet no way has 

been found to determine dates of the different periods during which 

movement occurred.

Stratigraphic differences which have been recognized on opposite 

sides of the fault zone cannot yet be evaluated fully, but neither the 

possibility of large strike-slip displacement or primary differences in 

rsedimentary environment can be ruled out as possible explanations. 

Primary differences in sedimentation across the line of the fault 

would be evidence that the fault was active during or before deposition of the Belt series. The Osburn fault may be one manifestation 

of an ancient deep-seated lineament which crosses the northern Rocky 

Mountains. The existence of a lineament of this sort even as early 

as the Precambrian is suggested by an eastward-projecting prong of 

the area of deposition of the Belt series rocks (Eardley, 1951, fig. 

163, p. 287), and by a possible basement control of the localization

 of mineralized belts in the Wallace-Burke area of the Coeur d'Alene

district.

TWELVEMILE FAULT

The upper part of Twelvemile Creek appears to follow a fault 

named the Twelvemile fault, and the divide between the heads of

Twelvernile Creek and Knox Creek is a distinct saddle developed on

the fault. The exact position of the fault is known with fair certainty

only at the Twelvemile-Knox Creek saddle where float indicates a 

contact between the argillite of the St. Regis formation and the

Ilevett quartzite, A structural discontinuity is suggested, west of the

saddle where beds of the Wallace formation appear to dip soutK beneath Revett quartzite.
RECONNAISSANCE GEOLOGY, WESTERN MINERAL COUNTY, MONT. 593

The Twelvemile fault appears to be one of a group of faults forming 

a composite fault zone similar to but not of such magnitude as the 

Osburn fault zone. The Twelvemile fault may be the most important 

member of this fault zone. To the west, the shear zone projects approximately into the position of the Thompson Pass fault (Calkins, 

1908, pi. 2), and further work may confirm that they are segments of 

the same structure.

From available data, displacement cannot be determined on the 

Twelvemile fault, although apparent displacement of the south side 

relatively upward is indicated. If this fault is the same as the 

Thompson Pass fault, strike-slip of several miles would be a probability. In the Thompson Pass area to the west, segments of the 

Granite Peak syncline are separated as much as 3 miles by the Thompson Pass fault (Calkins, 1908, pi. 2), and the southern segment is 

displaced westerly.

MOUNT BUSHNELL FAULT

In the vicinity of Mount Bushnell (north-central part of the map) 

the Revett quartzite is in contact with the Wallace formation even 

though all the beds have a general southward dip. The absence of 

the argillite of the St. Regis formation indicates a fault which has 

been named the Mount Bushnell fault. The exact line of the fault 

has not been identified in the field. Most of the St. Regis is absent 

from surface exposure along the strike of the beds for more than 

3 miles; thus the fault appears to be almost parallel to the strike of 

the beds. South of Mount Bushnell about 1,000 feet, a few pieces of 

argillite float typical of the St. Regis suggest that at least some of 

the St. Regis formation is near the surface along the fault. Where 

the Deborgia Cutoff Road crosses the fault a small amount of float 

also suggests argillite of the St. Regis. The uppermost part of the 

section mapped as Burke and Revett formations also appears to represent beds which are transitional in character between Revett and St. 

Regis formations.

Lineaments that show on aerial photographs and the absence of 

strata in Twelvemile and Tamarack Creek valleys and in intervening 

areas are evidence of the eastward continuation of the Mount Bushnell fault, which is here called the Tamarack Creek fault. The net 

displacement on the fault is not known, but a stratigraphic throw of 

at least 1,000 feet is represented.

GOAT MOUNTAIN FAULT

Discontinuities of the stratigraphic section observed on the Deborgia 

Cutoff Road and on Savenac Creek suggest a large fault, but the position and trend are very poorly known, and it is possible that an entirely different interpretation may better explain the anomalies
594 -CONTRIBUTIONS TO ECONOMIC GEOLOGY

observed. The postulated fault is believed to pass a short distance 

north of Goat Mountain and is named after that point. Similar 

anomalies were found near Hawk Mountain and on Packer Creek, and 

the fault may be continuous to the west.

TAMARACK CHEEK FATTI/T

Lineaments appearing on aerial photographs, which are interpreted 

as fault traces and which have been verified by field examination in a 

few places, suggest at least three major parallel faults forming a milewide band through Tamarack valley. The most clearly defined of 

these three is the Tamarack Creek fault. Although direct continuity 

has not been established, there is a suggestion that this group of faults 

represents eastward convergence of the Mount Bushnell and Goat 

Mountain faults with the Twelvemile fault.

A lineament has been traced eastward on aerial photographs beyond 

the limits of the map and may represent a continuation of this fault. 

At the Clark Fork River a zone of sheared rock several hundred feet 

wide is exposed where this lineament intersects road cuts near the 

mouth of Patrick Creek, about 1% miles southwest of the Paradise 

Ferry over the Clark Fork River.

SILVER CREEK FAULT

A large fault extends from the upper part of the St. RegiS valley 

southeastward for more than 15 miles. It was first recognized in the 

Silver Creek valley and thus is referred to as the Silver Creek fault. 

Westward from the mouth of Rainy Creek near the town of Taft, 

movement on the Silver Creek fault appears to have been dissipated 

in several minor faults. However, a fault of opposite displacement 

and slightly divergent trend continues westward beyond the edge of

the map. On Ward Creek the quartzite of the Burke and Revett formations is faulted against phyllite of the Wallace formation and repreSentS the easternmost point at which the Silver Creek fault is known

with any certainty. From Ward Creek eastward the fault is obscure 

in a terrane composed entirely of Wallace rocks.

The Silver Creek fault can be seen as a body of sheared TOcks more 

tlian 20 f66t Wld6 9/t the SOUthem eild Of a railroad tunnel near Dominion Creek, and it is well marked by saddles and the" StrUCtUTillly 

controlled drainage in the vicinity Of BuTke and Silver Creeks. 

The fault is known with lair cert^i^ty « tK& Middle Fork of Big

Creek, on Deer Creek, and on Ward Creek; but OH the East Fork and

West Fork of Big Creek evidence Of its existence and its position is

obscure.
RECONNAISSANCE GEOLOGY, WESTERN MINERAL COUNTY, MONT. 595

The southern side of the Silver Creek fault apparently has been 

upthrown about 2,000 feet in the vicinity of Silver Creek, and in the 

valley of the Middle Fork of Big Creek a stratigraphic throw of 

several thousand feet is indicated.

At the head of Gilt Edge Creek the Wishards sill is offset about 250 

feet by a fault which shows distinctly as a lineament on aerial photographs. This fault is called the Gilt Edge fault. To the east, in 

other tributaries of Big Creek and in Deer Creek, repeated stratigraphic sections suggest that the Gilt Edge fault continues its known 

trend eastward for 7 miles and may join the Silver Creek fault near 

Deer Creek. A greater stratigraphic throw is indicated in the Middle 

Fork of Big Creek than where the fault cuts the Wishards sill. To the 

west the Gilt Edge fault appears to pass through the Stewart prospect 

south of the Monitor mine.

CRITTENDEN PEAK FAULT

A fault which cuts the Wishards sill near Crittenden Peak and 

which shows on aerial photographs as a definite lineament is called 

the Crittenden Peak fault. The Crittenden Peak fault strikes about 

N. 70°-80° W. and may be continuous with the fault that is accompanied by a sharp flexure at the Deer Creek mine. There is little evidence of the fault at the place it would presumably cross the Middle 

and East Forks of Big Creek. Float found in the Middle Fork suggests that argillite of the Wallace formation is in contact with quartzite of the Burke and Eevett formations and also suggests the presence 

of a fault. In addition, an extrapolation of trends of bedding from 

localities where evidence is clearer produces a structural pattern consistent with the interpretation of a fault. In the East Fork no stratigraphic break was identified. In both the East and West Forks 

glacial gravels in the valleys made reconnaissance traverses rather 

unproductive of evidence.

The direction of relative displacement of bedding in the Middle 

Fork of Big Creek is interpreted as being opposite from that of the 

offset of the Wishards sill; however, there is considerable uncertainty in the interpretation of the stratigraphic relations in the 

Middle Fork of Big Creek. If the interpretation is correct, a twofold history of movement on the fault is indicated, and a possible 

post-Precambrian age of the Wishards sill is strongly suggested. 

Furthermore, the largest movement would have preceded intrusion 

of the Wishards sill. If the major faulting is Laramide, the Wishards 

sill must be at least that young.
; ' : -CONTRIBUTIONS TO ECONOMIC GEOLOGY ' 

PLACER CREEK FAULT

The Placer Creek fault is well known in Idaho where many prospects Have been developed along it, and topographic features such 

as saddles and controlled drainage mark its position. The eastern end of this fault is shown in the western part of the area mapped. 

Its eastward extension has been recognized entirely by lineaments 

seen on aerial photographs. It is possible that the Placer Creek fault 

converges with the Silver Creek fault.

THE HOPE AND UNNAMED FAULTS

The Hope fault may cross the northeastern corner of the map, but 

it was not identified in the mapped area. It may, however, follow 

approximately the valley of Cherry Creek.

Many faults shown on the map wrere not named because some are 

relatively small, others are postulated on weak evidence, and little is 

known about still others.

Near Wards Peak is a group of faults, some of which are exceptionally well exposed in the cirques at the heads of Newman, Ward, 

and Deer Creeks. Particularly at the head of Newman Creek, where 

large dip slopes are bared by glacial erosion, the massive beds of 

Cjuartzite of the Burke and Revett formations can be seen to be sharply

flexed and broken. Diabase dikes appear to follow some of these

faults on the southeast flank of Wards Peak and at the Aladdin mine. 

About 2 miles upstream from the mouth of Twomile Creek wide 

sheared zones are exposed in road cuts, and lineaments on aerial photographs delineate three faults completely within the Wallace formation. None of these faults could be traced for any great distance,

A few hundred feet east of the mouth of Savenac Creek a fault 

surface is exposed; the fault zone, now silicified and stained with a 

Small amount Of limonite, stands out in brown faceted spurs. The

quartzites of the Burke and Kevett formations near the fault are 

shattered and bleached.

One-half mile east of the fault exposure at the mouth of Savenac

Creek another body of intensely distorted and faxilted rocks is exposed in a, i-oad cut on the main highway. Although these two fault 

ZOlieS probably are not the same fault, the two together do show

the presence of a disturbed zone which seems to treHo! mOrfi SOutlleasterly than the Osburn fault zone, as though diverging from it,

In TWelYGHlile Creek small faults are shown associate ^itix *Ke

Flat Rock syncline. Although these are very pOOrly (MMl, it IS 

obvious that there is faulting in this highly strained block of rocks,

Only dfttailad mapping ana

exactly.
RECONNAISSANCE GEOLOGY, WESTERN MINERAL COUNTY, MONT. 597

Near the junction of the East and West Forks of Big Creek two 

road cuts expose wide zones of sheared rocks. The southernmost 

exposure appears to represent a fault with a northerly strike and a 

relatively shallow dip to the east. The strike and dip, if interpreted 

correctly, represent an unusual trend in the area mapped. The northernmost exposure is thought to be part of the same fault, but it may 

strike northwesterly and be related to fault zones exposed in road 

cuts along U. S. Highway 10 east of Saltese.

FOLDS 

SAVENAC SYNCLINE

Between the Twelvemile fault and the Goat Mountain fault rocks 

of the Burke and Kevett formations, the St. Regis formation, and 

the Wallace formation are involved in a large, tight syncline, which 

appears to control the trellis-type drainage net of the headwaters of 

Savenac Creek. -The syncline is named the Savenac syncline after 

this locality.

The Savenac syncline apparently extends from the north-central 

part of the map for at least 12 miles southeastward to the vicinity of 

Flat Rock Creek. The axis has not been accurately identified along 

any part of it although foliated rocks near the axis are believed to 

control.the drainage of the headwaters of Savenac Creek, and near 

the DeBorgia Cutoff Road observed outcrops give fair evidence 

of the position of the axis. In the northwestern part the fold is 

nearly isoclinal, with flanks dipping 70° to 90°. In the southeastern 

part the fold not only is essentially isoclinal but is also overturned, 

so that the axial plane dips about 55° to the south.

FLAT ROCK SYNCLINE

A sharp flexure has been produced in the southern overturned limb 

of the Savenac syncline near Flat Rock Lookout and is referred to 

as the Flat Rock syncline (see fig. 74). The Flat Rock syncline is 

referred to as a "syncline" although by some definitions it might 

be considered an overturned anticline. As the fold is thought to 

have been produced by a downwarp in previously overturned beds, 

and as it now is made up of beds which dip toward each other, it 

seems preferable to use the designation "syncline. 1 ' Although small 

compared to the Savenac syncline, the Flat ROCK syncline is important because it provides one of the few pieces of evidence suggesting 

that the movement on the Osburn fault zone was in large part horizontal and that the north side moved relatively eastward. (In the 

paragraphs on the displacement on the Osburn fault the significance 

of the Flat Rock syncline is considered in more detail.) The fold
598.

Bedding

.Cleavage

FIGURE 74. Block diagram of the Savenac and?'Flat RoctTsyiiclines.

involves rocks underlying about 10 square miles of the mapped area. 

Its axis plunges about 35° to the southwest, and the two flanks form 

an angle slightly less than 90° with each other.

BOYD MOUNTAIN ANTICLINE

From Haugan eastward the St. Regis River approximately follows

the axis of an anticline which is best exposed on Boyd Mountain and 

which is called the Boyd Mountain anticline. Although attitudes

of bedding in the vicinity of Boyd Mountain indicate the anticline,

and the axis is drawn on the map only in this locality, the pattern of

outcrop of stratigrapKic units shows more clearly the gross nature 

and trend of the fold. From this pattern it appears that the Boyd 

Mountain anticline is truncated at a very low angle by the Osburn 

fault zone. The Boyd Mountain anticline may possibly be correlated 

with the Lookout anticline, in which case the north flank of the fold

has been sliced off by the Osburn fault in the vicinity of Mullan Pass.

The Boyd Mountain anticline has minor flexures, particularly near

its axis, and it is also complicated by associated faults. The south 

flank of the Boyd Mountain anticline dips at angles generally between 20° and so0, but the attitude of the north flank is poorly known

except near Boyd Mountain beacon where the beds are near the Osburn fault zone and are overturned, having dips as low locally as 10°.
RECONNAISSANCE GEOLOGY, WESTERN MINERAL COUNTY, MONT. 599 

EAGLE PEAK ANTICLINE

The axis of the Eagle Peak anticline is a few hundred feet north 

of Eagle Peak. This anticline is the largest of several folds exposed 

in the cirques at the heads of Newman, Ward, and Deer Creeks. As 

the pattern of outcrop of stratigraphic units shows, the anticline 

plunges southeastward, but its eastward extension was not traced 

beyond the Aladdin mine. To the west the anticline disappears in a 

homocline dipping generally southward, and the north flank is truncated by the Silver Creek fault.

LOOKOUT SYNCLINE AND LOOKOUT ANTICLINE

The axis of the Lookout syncline is about half a mile south of Lookout Pass and has an easterly trend. It is known as far as 2 miles 

west and 3 miles east of Lookout Pass. The syncline is almost parallel 

to the Lookout anticline, an anticline overturned to the north, the axis 

of which is about a mile north of the axis of the Lookout syncline.

To the east both of these folds die out in the homoclinal structure 

which underlies much of the area mapped south of the Osburn fault 

zone.

SWAMP CREEK ANTICLINE

In the northeastern corner of the area mapped, in Swamp Creek 

valley, an overturned stratigraphic section was found in which parts 

of all formations from Prichard through Wallace are present. Near 

the Miller mine, at the head of Swamp Creek, a thin stratigraphic 

section of argillites of the Prichard formation which is not overturned 

was identified, and it is believed the axis of a major anticline is represented by the flexure thus defined. The anticline is referred to as 

the Swamp Creek anticline. The axial plane dips steeply to the 

south, and the axis trends northwestward. A large anticline east of 

Thompson Falls is believed to be this same fold, but there, although 

asymmetrical, it is not overturned as on Swamp Creek. If the Thompson Falls anticline correlates with the Swamp Creek anticline, the 

structure must be more than 15 miles long. The fold in Swamp Creek 

may be overturned because of its nearness to the Hope fault, which may 

pass through the area.

CLEAVAGE, LINEATION, AND MINOR STRUCTURES

Two kinds of cleavage are recognized. One is related to folding 

and is termed "axial-plane cleavage" because it tends to subparallel 

the axial planes of the folds. The second type seems to be related to 

faulting. Axial-plane cleavage is useful in the identification of overturned beds, but care had to be exercised not to confuse cleavage related to faulting with axial-plane cleavage.
600 CONTRIBUTIONS TO ECONOMIC GEOLOGY

Cleavage most commonly ranges in strike between N. 45° W. and 

N. 70° W. and in most places has a steep dip to the southwest. Welldeveloped axial-plane cleavage nearly parallels bedding in the isoclinal Savenac syncline, and in many places where phyllites have been 

produced bedding is obliterated. Phyllitic axial-plane cleavage, related to the Savenac syncline, has been warped around the axial plane 

of the Flat Rock syncline just as bedding has been warped. At the 

axis of the Flat Rock syncline, for example, phyllitic cleavage is at 

right angles to the axial plane, and on the southeast flank cleavage 

strikes northeast and dips northwest. This relationship strongly suggests that the Flat Rock syncline is a crossfold produced after the 

Savenac syncline and cleavage related to the Savenac syncline had 

been formed. The Flat Rock syncline, therefore, appears to be good 

evidence of a period of deformation distinctly later than the period 

in which the overturned Savenac syncline was produced. As described 

in the discussion of the Osburn fault zone, this late period of deformation appears related to that fault zone. The Flat Rock syncline is believed to represent a large drag fold indicating a large horizontal 

component of net slip on the Osburn fault zone.

Lineation is present in many places, especially in bodies of phyllitic 

rocks, and appears as small ridges and troughs on the planes of folia 

and as streaks of mineral grains. Lineation of this same type, studied 

more carefully in the Pottsville quadrangle, Idaho, by Wallace, is believed to be parallel to the orientation of shearing movement. In the 

Pottsville quadrangle the lineation generally plunges down dip, or 

steeply to the west; but examples were found in Mineral County where 

lineation plunges to the west at angles as low as 20°. Thus, in Mineral County, this type of lineation suggests more of a strike-slip component of shearing, at least locally, than is suggested by lineation in

the Pottsville quadrangle to the west.

"Puckers" is a term coined to refer to secondary shear zones crossing

foliated rocks in which the folia have not been ruptured but merely 

bent sharply to form Z-like flexures. "Fracture cleavage" is used by 

some to refer to similar structures, which are much smaller or, even

microscopic. The axes of these small folds generally are nearly horizontal in Mineral County, as in the Pottsville quadrangle, and the

planes of the shear zones clip at low angles either to the north or south.

The fold axes of puckers produce a second type of lineation conspicuous on cleavage planes. In the Pottsville quadrangle it was found that

the lineations believed to be parallel to shearing movements generally

are at right angles to the pucker-ioia axes. TUB two types oi iine»-

tion. therefore? were interpreted as resulting from the Same movement

within the phyllitic bodies; namely, a near dip-slip orientation of 

shearing. In Mineral County the same relationship is generally true,
RECONNAISSANCE GEOLOGY, WESTERN MINERAL COUNTY, MONT. 601

but pucker axes were found which plunge 35° east, whereas shear lineations in the same outcrop plunge 25° west.

If there has been a large strike-slip component on the Osburn fault 

:zone, a shear component of steep dip as suggested by steep lineation 

and nearly horizontal puckers is anomalous. The shear component 

of steep dip possibly represents a period of deformation distinct from 

that represented by strike slip on the Osburn fault zone, but its place 

in the tectonic history is not understood.

Foliation, lineation, and puckers are all produced in areas of intense deformation, and are, therefore, much more common north of the 

Osburn fault zone than south of it.

Minor folding is far more common in the Wallace formation than 

in the other formations. This characteristic is especially well shown 

along the North Fork of Little Joe Creek. Flexures in even the most 

competent quartzites in the stratigraphic section, however, are common 

and attest to the relatively plastic behavior of the rocks of Belt series 

under proper conditions.

ALTERATION

So-called "bleaching" in the Coeur d'Alene mining district has 

been used as a guide to ore deposits, and it is considered by some to be 

related, at least indirectly, to the solutions which introduced the ore 

minerals. Mitcham (1952) has presented an excellent review of the 

ideas concerning "bleaching." The term "bleaching" refers to the 

change in color of the sediments, usually from darker to lighter shades, 

presumably by hydrothermal alteration. Sericitization, silification, 

pyritization, carbonatization, and chloritization are the principal 

types of alteration involved.

In Mineral County "bleaching" is not as widespread as in the Coeur 

d'Alene district. Hydrothermal alteration, or "bleaching," appears 

to be localized along major faults, but it pervades some of the unsheared country rock, principally in a belt approximately a mile 

wide along the north side of the Osburn fault zone. In the Osburn 

fault zone itself rocks are intensely bleached and altered, although 

hydrothermal alteration is difficult to identify with certainty where 

rocks are sheared and gougelike.

The green color of the upper part of the St. Kegis formation in 

Idaho has been considered by some to represent the introduction of 

material by one phase of hydrothermal alteration, but it is considered 

by the authors to represent either alteration of material in place or 

possibly an original characteristic of the sediments. The pale-green 

chlorite, which is responsible for the color, appears to have a particular affinity for argillite beds which are typically thinly laminated. In 

many places discrete units of the thinly laminated argillite are green,
602 CONTRIBUTIONS TQ ECONOMIC GEOLOGY

whereas the adjacent beds of other lithologic character are not green, 

and are not bleached. Such a correlation of the chlorite with lithology suggests that an original compositional difference is responsible 

for the chlorite, and that it has not been introduced by hydrothermal 

solutions. Further evidence that the chlorite is related to a factor of 

sedimentation rather than hydrothermal alteration is that in Mineral 

County, where the St. Kegis formation is more than twice as thick as in 

Idaho, chlorite imparts a greenish-gray color to the formation. Palegreen chlorite is not confined entirely to the St. Kegis formation but 

is found in almost all the other formations also. In the Wallace 

formation light olive-gray (5Y 6/1) colors can result from the presence of chlorite plus other pigment material. Other greenish hues of 

gray (5Y 6/1, 5 GY 6/1) which are so common in the Belt series rocks 

probably result in part from the presence of chlorite in the sediments. 

Some highly sericitic rocks are a hue of greenish gray, but they generally are a more yellowish hue. The pale-green chlorite as it now 

exists may or may not be an original constituent of the sediment even 

though the chlorite may have been derived from original constituents 

rather than from introduced material. There has been at least some 

recrystallization during which chlorite has enlarged locally to grains 

a millimeter or more in diameter.

Besides hydrothermal alteration, dynamic metamorphism has 

played an important role in Mineral County. North of the Osburn 

fault zone, where the beds have been tightly folded, many of the argillites have been changed to phyllites and schists, and bedding in a 

great many places has been obliterated. All the quartzites examined 

microscopically are metaquartzites having grain boundaries at least 

partially sutured.

MINERAL, DEPOSITS

GENERAL FEATURES

In western Mineral County certain mineral deposits are very distinctly related to prominent structural features and other deposits 

are SOmewhat leSS distinctly related to basic igneous rocks. The Osburn fault zone seems to be one of the major COntrol StrUCtUFeS,' tll6

Silver Creek fault and associated fractures also appear to have been

the locus for ore deposition, and many small veins are known to b0 associated with other fractures in the region, in addition, several

deposits are localized in or near the Wisnards sill and other

sills and dikes. .

The belt of deposits near the Osburn fault zone include the following mines and prospects from west to east: Silver Cable, Bryan, Last
RECONNAISSANCE GEOLOGY, WESTERN MINERAL COUNTY, MONT. 603

Chance-Ben Hur, Hemlock, Galligar, Tarbox, Wabash, Meadow 

Mountain, Black Traveler, Saltese Consolidated, East Coeur d'Alene, 

Copper Rock, True Fissure, Monarch, Wolf, and Keith. The Rock 

Island, Valentine, and Texas properties might be included in this 

.group except that they are about 2 miles north of the Osburn fault 

zone and perhaps represent a different structural setting.

The veins near the Osburn fault zone are similar to the veins of the 

Coeur d'Alene district; in fact, this mineralized belt can be considered 

a continuation of the belt in Idaho near the Osburn fault, which contains such mines and prospects as the Golconda, Alice, Lucky Friday, 

Vindicator, and Idaho Silver. Near the Osburn fault zone in Montana galena is the most common ore mineral, and sphalerite is abundant in some properties. Silver-rich tetrahedrite and chalcopyrite 

are relatively more common than in the Idaho deposits. Ankerite and 

quartz are the common gangue minerals; but massive barite is abundant in at least two veins, the Monarch and a prospect just north of 

the West Fork of Packer Creek. All the veins are of fissure type, 

filling cavities along fractures in part and replacing the wall rock in 

part. Most of the veins trend west or north of west and dip 45° or 

more to the south. However, the Silver Cable vein strikes northeast 

and dips southeast, and the True Fissure vein strikes north and dips 

west. In general, the ore shoots are shorter in strike length than in 

dip length, and most are not more than a few tens of feet long. A 

westward rake of these elongated ore shoots appears to be common.

The following descriptions are of the mines that were accessible to 

the authors.

SILVER CABLE MINE

The Silver Cable mine is near the head of Brimstone Creek about 

4*/£ miles north of Taft, Mont. The workings consist of three levels, 

of which only the lower level was completely accessible and the upper 

level partly accessible (pi. 49). The first 200 feet of the lower level 

exposes thick-bedded white Revett quartzite. A fault separates this 

quartzite from the light greenish-gray, impure, altered quartzites and 

argillites of the Burke formation which are exposed in the remaining 

workings of the lower level and in the upper accessible level. The 

beds strike approximately east and dip from 45° south to 50° north, 

with the north dips being overturned.

The vein, exposed in the lower level of the Silver Cable mine, strikes 

approximately N. 45° E. and dips 35° to 40° to the southeast. The 

vein is composed of tetrahedrite, galena, sphalerite, chalcopyrite, and 

a little pyrite and has a quartz and siderite gangue. According to 

Calkins and Jones (1914, p. 20), before the lower tunnel was driven, 

the best showing of ore minerals was in the upper level where the vein 

reportedly was about 4 feet thick. Unfortunately the vein on the
604 CONTRIBUTIONS TO ECONOMIC GEOLOGY

upper level was not accessible to the authors, but the stope above the 

lower level has about the same width; No data were available on the 

past production of the Silver Cable mine.

LA.ST CHANCE MINE

The Last Chance mine, (pi. 50), owned by the Day Mines, Incorporated, is 3% miles north of Saltese, Mont., on a tributary of the 

West Fork of Packer Creek. The workings consist of seven levels 

and the 2,000 foot caved Ben Hur tunnel; however, only three levels 

(No. 1 or U. S. Tunnel, and Nos. 3 and 6) were partly accessible. In 

the U. S. Tunnel (No. 1 level) which is the lowest of the three and 

the most recently worked, the Revett quartzite, consisting of white 

quartzite with an occasional bed of greenish-gray argillite, is exposed 

for 1,300 feet. Greenish-gray argillite, which probably belongs to 

the Wallace formation, is exposed in fault contact with the Revett 

quartzite near the caved portion of the U. S. Tunnel. The beds strike 

east and dip 85° south to 50° north, with the north dip probably overturned.

The Last Chance vein, where exposed in the U. S. Tunnel, is a pod 

along the fault between the Revett quartzite and the Wallace formation. The fault strikes northeast and dips 65° south. The vein consists of tetrahedrite, chalcopyrite, and some galena in a gangue of 

quartz and siderite.

The following table gives figures from company records indicating the amount and kind of ore in 4 shipments, 1 in 1940, 2 in 

1943, and! in 1951:

Year 

1Q40

1943__ __ _ . __

1943._ ___ ___ _

Amount 

produced 

(tons)

33. 63

19.25

2. 63

256. 00

Gold 

(ounces 

per ton)

0. 10

. 018

Silver 

(ounces 

per ton) 

102. 5

31.6

19. 4

2. 4

Lead 

(percent) 

1. 2

.2

12. 65

2. 7

Zinc 

(percent)

0. 3

.3

. 4

. 4

Copper 

(percent)

0.9&

The Ben Hur tunnel was completely caved at the portal when the

authors visited the property in 1952. The 2,000-foot Ben Hur tunnel

was driven to tap the two veins which, according to old company

reports, strike east and dip toward each other on the No. 2 and No. 8

levels (pi. 50). Although the veins were probably intersected, there is no record that they were mined at tins lower le^ei.

W ABASH, MEADOW MOUNTAIN, AND TARBOX MINES

The Wabash, Meadow Mountain, and Tarbox mines are owned by

the Mineral King Mining Company of Missoula, Mont, These mines

are about 3 miles north of Saltese, Mont, on the Middle Fork of Packer 

Creek.
RECONNAISSANCE GEOLOGY, WESTERN MINERAL COUNTY, MONT. 605

The Wabash mine consists of one tunnel which bears N. 55° E. and 

which was .completely caved 150 feet from the portal. The tunnel 

exposes altered and crumpled argillites and quartzites of the Wallace 

formation, which dips moderately southwest. Unfortunately, no information is available about the composition, position, or extent of 

the Wabash vein.

Two levels of the Meadow Mountain mine were partly accessible. 

The No. 2 level was driven N. 30° E., but it was caved 150 feet from the 

portal. The No. 1 level, about 125 feet above the No. 2 level, has a 

bearing of N. 40° E. and is caved at 140 feet. About 120 feet from 

the portal a 60-foot drift was driven in a S. 70° E. direction. The 

vein, exposed in a small stope at the junction of the drift and the adit, 

consists of 5 to 10 inches of galena in a quartz and siderite gangue. 

The vertical vein is along a bedding-plane fault zone of crushed and 

crumpled argillites and quartzites (Wallace formation) striking N. 

75° W. According to Calkins and Jones (1914, p. 194), the Meadow 

Mountain mine workings intersected two veins. The south vein contained considerable pyrite and the north vein contained appreciable 

amounts of galena.

The Tarbox mine (fig. 75) comprises an 800-foot shaft which collars near creek level and various levels of workings totaling nearly 

4,300 feet. The lowest level is 1,000 feet below the collar of the shaft. 

The workings were caved and flooded and thus inaccessible to the 

authors. According to old company reports, no ore has been produced 

from this mine, which was allowed to fill with water in December, 

1922. The vein material on the dump includes siderite, quartz, galena, 

pyrite, and limonite.

ROCK ISLAND MINE

The Eock Island mine (pi. 50) is near the head of Kock Creek about 

41/2 miles northeast of DeBorgia, Mont. The workings consist of 

three levels and a 90-foot raise connecting the upper two levels. The 

lower level, however, was inaccessible. Interbedded thinly laminated 

dark-gray argillites and limy brownish-gray quartzites of the Wallace formation are exposed at this mine. The beds strike northwest 

and are overturned, with dips ranging between 40° to 60° to the 

south.

The Kock Island vein strikes approximately east and dips 30° to 40° 

to the south. A shootlike vein, about 50 feet in strike length and at 

least 160 feet in dip length, is exposed in the raise between the 

middle and upper levels. The vein consists of irregular pods of quartz 

and sulfides along a fault zone. Galena, chalcopyrite, pyrite, and 

pyrrhotite make up the sulfides of the vein. The principal secondary 

minerals are cerussite, malachite, azurite, chrysocolla, and limonite. 

Reportedly, 1 or 2 car loads of low-grade ore have been shipped from 

the Rock Island mine.
606 CONTRIBUTIONS TO ECONOMIC GEOLOGY

After company maps

200 Feet

75. Map of underground workings of the Tarbox mine. 

TRUE FISSURE MINE

The True Fissure mine is about 2% miles northeast of DeBorgia 

on East Twin Creek. The accompanying map (pi. 51) is after one

prepared by Garth Crosby of Day Mines, Incorporated, of Wallace, 

Idaho, The mine is in quartzites of the Burke and Revette formations one-half mile north of the Osburn fault zone. The quartziteg of 

this block immediately north of the Osburn fault are greatly deformed and are in tight, overturned, nearly isoclinal folds. In One 

relatively large drag fold, beds have been rotated more than 180°.

Vein material exposed in both adits consists chiefly of siderite, 

hematite, quartz, and pyrite, with some galena and. a little chalcopyrite. Samples taken from the lower adit are reported to contain 

1 to 3 ounces of silver per ton ana o to 10 percent of lead per ton. o*

particular interest ia tKe northerly strike o£ tli© vein, TvkicK is unlike that of most other vein trends in the region, The relative positions Of the Veins exposed in the upper and lower adits, if they represent parts of the same vein, indicate a plunge of the long axis of the

vein of approximately 45 ° io tilQ SOUft W6St.
RECONNAISSANCE GEOLOGY, WESTERN MINERAL COUNTY, MONT. 607

DEPOSITS NEAR THE WISHAEDS SILL

The deposits in the vicinity of the Wishards sill, an intrusive mass 

of dioritic composition, are typically chalcopyrite-dolomite veins filling steeply dipping eastward-trending fractures. These include the 

Monitor, Stewart, St. Lawrence, Richmond, and Hansy deposits 

which occupy fractures from a few hundred to a thousand feet vertically above the Wishards sill. The Bullion vein and other veins 

farther west in Idaho occupy fractures related to the Placer Creek 

fault. Although the Bullion vein and other related veins are not 

in positions above the Wishards sill, they are generally no more than 

1,000 feet away from it.

The spatial relation of these copper veins to the dioritic rocks is 

rather striking. However, the present study has produced no other 

specific evidence to suggest a direct genetic connection between the 

copper-bearing veins and the dioritic rocks. Furthermore, there are 

several significant copper deposits, including the Snowstorm and the 

Copper King, in the area directly east of Mullan, Idaho. These copper deposits are on the opposite side of the Osburn fault from the 

Wishards sill and have no apparent proximity to any intrusive rocks. 

Much more detailed work will be necessary to establish the relation 

of the predominantly copper-bearing veins to the predominantly leadzinc-silver veins and to establish the relation of both types of veins 

to an ultimate source.

At the head of Gilt Edge Creek one massive dolomite vein was 

found occupying a position along the Gilt Edge fault where diorite 

is on the south and the Wallace formation is on the north. The Buffalo vein is reported to contain galena, chalcopyrite, and a few pods 

of free gold; but this property was not visited.

ST. LAWRENCE MINE

The St. Lawrence mine is south of and above Silver Lake, which is 

drained by Silver Creek. The mine is about 4^ miles south of Saltese, 

Mont. The workings consist of two partly accessible levels (pi. 50). 

The mine exposes gently dipping interbedded, medium dark-gray 

argillites and light brownish-gray to yellowish-gray limy quartzites 

of the Wallace formation. The vein occurs as pods along vertical 

fractures trending N. 70° E. and is composed of chalcopyrite in a 

dolomite and quartz gangue. Several raises and stopes have been 

made, but there is no record of the past production.

DEPOSITS NEAR THE SILVER CREEK FAULT

Deposits of chalcopyrite, quartz, and some galena are along the 

Silver Creek fault and its branches.
608 CONTRIBUTIONS TO ECONOMIC GEOLOGY '.. .

The Silver Strand property, near the St. Eegis Biver and about 2 

miles west of the mouth of Eandolph Creek, was examined, although 

only, a short section of one tunnel was accessible. Chalcopyrite was 

the principal mineral exposed but some malachite and azurite were 

also found; some galena is reported to be present.

The Agnes property, 2^ miles southwest of Saltese, was not visited. 

It is reported to contain copper and lead minerals, principally chalcopyrite and various products of oxidation, and galena.

The Silver Creek fault may be continuous with faults in the Atlas 

property near Mullan, Idaho, where 20-foot wide veins of ankerite 

containing chalcopyrite and some galena are known.

The workings of the Amazon Dixie mine, which is about 1 mile 

south of Lookout Pass on the south side of the St. Regis River, were 

inaccessible. Ankerite vein material was found in Copper Gulch on 

the dump of the upper workings of the mine, but the exact position 

of the vein could not be determined.

OTHER DEPOSITS

. At the Miller property at the head of Swamp Creek and at the Silver Bell property on the North Fork of Little Joe Creek, quartz veins 

in diorite dikes have been prospected. In both prospects quartz is by 

far the most abundant mineral, and pods of galena, chalcopyrite, and

sphalerite in the quartz are relatively rare. The Silver Bell veins

appear to occupy tension cracks in the diorite dike. There are many 

similar veins in other diorite bodies in northern Idaho and western

Montana, but none has produced a large amount of ore. 

The Deer Creek vein near Crystal Lake, 5 miles southwest of r>eborgia, Mont., was prospected and worked for its gold content as

was the Aladdin vein, 9 miles southwest of St. Regis, Mont. It is

reported that neither has produced much gold. The Aladdin 1111118

and a prospect hall a mile southeast of Wards Peak are on yeins occupying Shears in or adjacent to diorite. The veins are principally

quartz but contain a little chalcopyrite and pyrite,

MOSt Of the Creeks have been prospected for placer gold, and placer

ground on Deer Creek has been patented. Placer mining, principally by hand, was done on Deer Creek and Big Creek, and it is reported that although total production was not great some rich areas

Were fOUnd, Large piles Of hand-Sorted boulders in Deer Creek show

-what difficulties must have been encountered.

SUGGESTIONS FOE PROSPECTING

..By .far the most promising area for exploration is the mineralized 

belt following the Osburn fault, where it.is fllfBdy tHOWH Ml UIMJ
RECONNAISSANCE GEOLOGY, WESTERN MINERAL COUNTY, MONT. 609

veins do exist. Vegetation and a thick mantle obscure most of the 

bedrock and very possibly cover some unknown veins, but bulldozer 

trenching and diamond drilling can be used effectively to outline 

known veins more clearly and to reveal new veins. The area appears 

to be well suited to geochemical .prospecting (Kennedy, 1952) and 

this method well may be used to find gargets suitable for detailed physical exploration. Although no large ore bodies have been found in 

this belt along the Osburn fault zone, the similarity of structure and 

mineralogy to the structure and mineralogy of the Silver Belt of the 

Coeur d'Alene district and the general continuity of structure with 

that of the Golconda-Lucky Friday belt in Idaho suggest that exploration in this belt is warranted. Unless new methods of prospecing are developed, deep workings may be necessary to test adequately 

the potentialities of this block of ground.

. The Silver Creek fault may be an important ore-bearing structure. 

It is one of the largest faults in the area and is known to have a few 

scattered mineral deposits along and near it. Geochemical prospecting near the base of valley walls where they are crossed by the Silver 

Creek fault should provide a relatively inexpensive method of exploring the fault zone for significant mineralized bodies.

In the Tamarack Creek valley and in the head of the Flat Rock 

Creek valley there is an area underlain by intensely faulted and deformed rocks. In the Coeur d'Alene district of Idaho structural 

knots are thought to be favorable for the deposition of ore if ore-bearing solutions are present. Although no sulfides are known to have 

been found in this area, the area warrants attention.

It is likely that other veins similar to the Hansy and St. Lawrence 

can be found near the Wishards sill. Aerial photographs show clearly 

the limof fracture the-St. Lawrence vein follows and also the fractures 

other minor veins on the property follow. Many similar fractures, 

which very possibly contain copper minerals, can also be identified on 

the aerial photographs. For example, the Crittenden Peak and Gilt 

Edge faults show clearly on the photographs and may warrant prospecting, especially west of the outcrop of the Wishards sill. The 

Hansy and St. Lawrence veins probably are the sort of veins that 

might be expected along these fractures.

It is also likely that other veins similar to the Miller and Silver Bell 

can be found by prospecting areas underlain by diorite sills and dikes; 

but under present economic conditions, it will be a rare one that 

contains workable metalliferous deposits.
610 CONTRIBUTIONS TO ECONOMIC GEOLOGY 

LJTEKATURE CITED

Anderson, A. L., 1940, Geology and metalliferous deposits of Kootenai County,

Idaho: Idaho Bur. Mines and Geol., Pamph. no. 53. 

Calkins, F. C., 1908, in Ransome, L. R., and Calkins, F. C., The geology and ore

deposits of the Coeur d'Alene district, Idaho: U. S. Geol. Survey, Prof.

Paper 62. 

Calkins, F. C., and Jones, E. L., Jr., 1914, Economic geology of the region around

Mullan, Idaho, and Saltese, Mont.: U. S. Geol. Survey Bull. 540-E. 

Campbell, M. R., and others, 1915 Guidebook of the western United States part

A, The Northern Pacific route: U. S. Geol. Survey Bull. 611. 

Eardley, A. J., 1951, Structural geology of North America: New York, Harper

and Brothers Publishers. 

Goddard, E. N., chairman, and others, 1948, Rock-color chart, Natl. Research

Council. (Now distributed by Geol. Soc. America.) 

Kennedy, V. C., 1952, Geochemical studies in the Coeur d'Alene mining district.

Idaho: U. S. Geol. Survey Circ. 168. 

Mitcham, T. W., 1952, Indicator minerals, Coeur d'Alene Silver Belt: Econ.

Geology, v. 47, no. 4. 

Pardee, J. T.f 1910, The Glacial Lake Missoula: Jour. Geology, v. 18, p. 376-386.
INDEX

Page

Alteration. _ - 601 

Ankerite, Silver Creek fault..-................ 608

Anticlines, Boyd Mountain .. ..... 698

Eagle Peak._ .._ ....._..... 699

Lookout..... 599

Swamp Creek. ..... . . ' 699

Argillite, Last Chance mine.--.... .. . 604

Meadow Mountain and Rock Island

mines_____.........__... 605

Prichard formation_ . 679

St. Lawrence mine.. 607

St. Regis formation... . 682-583

Wallace formation. . 684-585

Axial-plane cleavage. 599

Azurite, Rock Island mine. . 605

Silver Strand property . 608

Biotite.- - .............. 678

Bleaching. . . 601

Cerussite, Rock Island mine 605 

Chalcopyrite, Buffalo vein and St. Lawrence

mine... ... 607

Last Chance mine____________ 604

Rock Island mine.... 605

Silver Cable mine.._ . 603

Silver Strand property ... ....... 608

True Fissure mine__ _......... 606

Chalcopyrite-dolomite veins, Wishards sill... 607 

Chlorite, St. Regis formation...... ...... 578,582

Chrysocolla, Rock Island mine. 605

Copper deposits 607

Copper production, Last Chance mine 604

Copper veins, relation to lead-zin c-silver veins. 607

Deposits, Last Chance mine. 604

Osburn fault zone..__ . 602

Rock Island mine..... 606

St. Lawrence mine 607

Silver Cable mine..__ . 603

Silver Creek fault. .................... 607

True Fissure mine..______ .... 606

 Wabash, Meadow Mountain, and Tarbox

mines.. _ . 604

Wishards sill.... ...................... 607

Diabase dikes . . 696

Diorite.. . 587

Dynamic metamorphism_ . 602

Fault system, Tamarack Creek-Thompson

Pass........ . . 679

Faults, Crittenden Peak . 695 

Gilt Edge..... . 595 

Goat Mountain.. .. . . 693

Page 

Faults, Crittenden Peak Continued

Hope and unnamed__ __ 696 

Mount Bushnell -__ __ .... 693

Osburn zone..... __. __ .. 589-592

Placer Creek...____.___ _. 696 

Silver Creek.._ __ 694 

Tamarack Creek.. .. 894 

Twelvemile. __ 692 

Ftcldwork... ...... 676

Fossils.... ...".. 687

Galena, Buffalo vein ... ..... 607

Last Chance mine___..________ 604 

Meadow Mountain, Rock Island, and

Tarbox mines... ...... 605

Osburn fault zone.._............. _. 603

Silver Strand property...... 608

True Fissure mine......... ... 606

Gangue minerals, Osburn fault zone. 603

Geochemical prospecting.. 609

Glaciers, effects________________ 685

Gold. Aladdin and Deer Creek veins. 608

Buffalo vein__._____________ 607

Last Chance mine 604

Hematite, True Fissure mine...-_ _. 606 

Hydrothermal alteration__ _ _. 601

Ice rafting, evidence 686

Lacustrine silt deposits, description . .. 586-587 

Lake Missoula deposits.._ . 585-587 

Lead, Last Chance mine. 604

True Fissure mine..__ 606 

Limestone, Wallace formation.. 584 

Limon ite, Prichard formation... __ . 578

Rock Island and Tarbox mines. 605

Magnetite...--.----.-. - - -- 578

Malachite. Rock Island mine _ 605

Silver Strand property... 608

OHgoclase... 678 

Osburn fault zone.... . 689-592

Phyllite, St. Regis and Wallace formations... 683-584 

Placer gold mining_ _ 608 

Plagioclase.. . 578 

Pucker-. 600-601

Pyrite, Meadow Mountain, Rock Island,

and Tarbox mines.. 605

Prichard formation. . . 679

Silver Cable mine...__ . . 603

True Fissure mine.,... 606

Pyrrhotite, Rock Island mine. 605

611
612 INDEX

Page 

Quartz, Little Joe and Swamp Creeks and

other deposits..__________ 608 

Tarbox mine.._______________ 605 

True Fissure mine. --- - 606 

Quartzites, Burke and Revett formations_ 580,581 

field identification.. -. . - 577 

Last Chance mine..- -- . _-. 604 

Meadow Mountain and Rock Island

mines..-______.______ 605 

Missoula group... 586 

Prichard formation, description of....- 579 

St. Lawrence mine.-.__.-__.---__. 607 

St. Regis formation__.__.._ _.. 582 

Silver Cable mine.____________ 603 

True Fissure mine. - 606 

Wallace formation.__._ . ... 584-585

Sericite...----.---.---.---------------------- 578

Siderite, Tarbox mine.- - ------ -- 605

True Fissure mine .--. 606

Page 

Silt deposits, description_....... __. 586-587

Silver, Last Chance mine____________ 604 

True Fissure mine..___--___-____ 604 

Specularite, St. Regis formation. __..____ 582 

Sphalerite_______.____________ 603 

Syncline, Flat Rock...--------________ 597

Lookout.. . .--.... - 599

Savenac... __.-.. _ -- - 597

Tetrahedrite, Last Chance mine_ -_---^_ 604 

SUver Cable mine-. .. - 603

Uriconsolidated sediments. _____.___ 677

Wishards sill, age _.- --- - - 595 

description_________._- .,.... 687-588

Zinc, Last Chance mine..__ .. ... 604

U. S. GOVERNMENT PRINTING OFFICE: 1956