DEPARTMENT OF THE INTERIOR 

UNITED STATES GEOLOGICAL SURVEY

GEORGE OTIS SMITH, DIRECTOR

BULLETIN 527

ORE DEPOSITS

OF THE

HELENA MINING REGION

MONTANA

BY

ADOLPH KNOPF

WASHINGTON

GOVERNMENT PRINTING OFFICE

1913

CONTENTS.

Page.

Outline of report.... '. ..................................................... 9

Geography.............................................'................... 12

Location of the region................................................. 12

Physical features...................................................... 13

Climate.............................................................. 14

Historical sketch of mining................................................. 15

Production................................................................ 7

Field work and acknowledgments............................................ 17

Literature.................................................................. 18

General geology............................................... ............ 19

Character and distribution of the rocks.................................. 19

Proterozoic rocks...................................................... 20

Algonkian system.................................................. 20

Belt series.................................................... 20

Paleozoic rocks....................................................... 2.1

Cambrian and Devonian systems.................................... 21

Carboniferous system. .............................................. 21

Madison limestone............................................ 21

Quadrant quartzite............................................ 22

Mesozoic rocks........................................................ 22

Jurassic and Cretaceous sedimentary rocks.......................... 22

Igneous rocks..................................................... ' 23

Andesites and latites.......................................... 23

Occurrence................................................ 23

General character......................................... 24

Petrography.............................................. 25

Contact-metamorphic alteration of the andesites and latites.. 26

Age of the andesites ....................................... 28

Quartz monzonite.............................................. 29

General character......................................... . 29

Petrography.............................................. 29

Contact metamorphism..................................... 31

Age....................................................... 33

Aplite........................................................ 34

Occurrence and character..................;............... 34

Origin of the aplites................................'........ 35

Cenozoic rocks........................................................ 35

Tertiary sedimentary rocks........................................ 35

Igneous rocks..................................................... 36

Dacites....................................................... 36

General character and distribution.......'.................. 36

Petrography................................................ 37

Age....................................................... 39

Rhyolites...................................................... 39

General character and distribution......................... 39

Petrography....'.......................................... 40

Age........................................................ 41

Quaternary system.................................................. 41

3 "
4 CONTENTS.

Page

The ore bodies.............................. '.............................. 42

General character...................................................... 42

Older ore deposits (late Cretaceous?)................................... 43

Classification...................................................... 43

Magmatic deposits................................................. 44

Contact-metamorphic deposits...................................... 44

Tourmalinic lodes.................................................. 45

Varieties of ores..................'.............................. 45

Tourmalinic silver-lead deposits................................ 46

Description................................................ 46

Chemical processes.. '. ..................................... 48

Tourmalinic silver-copper deposits............................. 50

Tourmalinic gold depoits ..................................... 51

Origin of the older ore deposits...................................... 51

Systematic importance of tourmalinic silver-lead deposits:.......... 53

Younger ore deposits (post-Miocene)..................................... 54

General character................................................. 54

Metasomatic processes.............................................. 55

Age and origin of the younger ore deposits........................... 58

Comparison of the metasomatic processes of the two periods of mineralization.. .............................................................. 60

Descriptions of special districts.......'............,......................... 61

Method of treatment................................................... 61

Marysville district..................................................... 61

Location and history............................................... 61

Geology.......................................................... 62

.Ore deposits...................................................... 64

General features............................................... 64

Origin........................................................ 65

Descriptions of mines............................................. 68

Drumlummon mine........................................... 68

Cruse mine.......:........................................... 72

Belmont mine................................................. 73

Bald Butte mine............................................... 73

Penobscot mine................................................ 74

Empire mine.................................................. 75

Piegan-Gloster mine........................................... 75

Strawberry mine. .1........................................... 76

Elliston district....................................................... 76

Location........................................................... 76

Geology.......................................................... 76

Descriptions of mines............................................. 77

Julia mine..................................................... 77

Evening Star mine............................................ 78

Twin. City Mining & Milling Co.'s mine.......................... 78

Ontario mine.................................................. ' 79

Monarch mine................................................. 79

Blue Bell mine................................................. 79

Rimini district................... .-7.................................. 80

Location.......................................................... 80

Geology.......................................................... 80

Ore deposits...................................................... 81
CONTENTS. 5

Descriptions of special districts Continued.

Rimini district Continued." , page.

Descriptions of mines.............................................. 82

Valley Forge mine.-...................... ^...................... 82

Lee Mountain mine................. ^.......................... 83

John McGrew prospect........................................ 84

Peerless Jennie mine........................................... 84

Porphyry Dike................................................ 84

Helena district.........................................^............. 85

Historical sketch.................................................... 85

Geology.......................................................... 86

Sedimentary rocks............................................. 86

General features. ...................... r. ^. ................ 86

Algonkian system......................................... 87

Distribution and subdivision........................... 87

Spokane shale........................................ 88

Empire shale.............:... .\ ....................... 88

Helena limestone..................................... 88

Marsh shale........................................... 89

Cambrian system......................................... 89

Subdivisions and character............................ 89

Flathead quartzite..................................... 89

Wolsey shale................... r ..................... 90

Meagher limestone......................:.............. 90

Park shale...............:........................,.... 90

Pilgrim limestone................................ 1..... 91

Dry Creek shale...................................... 91

Yogo limestone....................................... 91

Devonian system........................................... . 91

Jefferson limestone:................................... 92

Threeforks shale................................:..... 92

. Carboniferous system....................................... '92

Distribution and character.............................. . 92

Madison limestone...................................... 92

Quadrant quartzite.................................... 93

Cretaceous system..........7.............................. 93

Tertiary system........................................... 93

Lake beds............................................. 93

Hot spring deposits................................... 94

Auriferous gravels...................................... 94

Quaternary system............................'............. 94

Igneous rocks................................................. 94

Distribution and character.................................. 94

Basic dikes and sheets..................................... 95

Acidic sheets.............................................. 95

Meta-andesite............................................. 95

Granitic rocks............... J............................. 95

Rhyolites.................;................................ 96

Structure...................................................... 97

Faults........................................................ 98

Ore deposits...................................................... 99

Location...................................................... 99
6 CONTENTS.

Descriptions of special districts Continued.

Helena district Continued. Page.

Descriptions of mines ............................................. 99

Whitlatch-Union mine......................................... 99

Big Indian mine..............................................." 100

Spring Hill mine.............................................. 101

Dutro mine.................................................... 101

Clancy district......................................................... 102

Geology.............................................................. 102

Ore deposits...................................................... 108

Descriptions of mines ............................................. 104

King Solomon mint;............................................ 104

 Little Nell mine............................................... 105

Yellowstone prospect........................................... 105

Kennedy or Jackson Creek mine................................ 106

Legal Tender mine............................................ 106

Wickes district......................................................... 107

' Location and history................................................ 107

Geology.......................................................... 108

Descriptions of mines............................................... 109

Alta mine..................................................... 109

Bertha mine.................................................. 110

Blizzard mine................................................ Ill

Blue Bird mine............................................... 11]

Genera] features........................................... Ill

The ore............................................'....... 113

Comet mine.................................................. 114

Corbin Copper Co. 'a property.................................. 115

Corbin Metals Mining Co.'s property ............................ 116

Gregory mine................................................. 11.7

Horseshoe prospect............................................. 117

Minah mine.................................................. 118

Minnesota mine.........................."...................... 118

Northern Pacific mine.......................................... 119

Wickes-Corbin Copper Co.'s property. .......................... 119

Boulder and Basin districts..../...................................... 120

Location.......................................................... 120

Descriptions of mines. ............................................. 120

Robert Emmet mine........................................... 120

Amazon mine................................................. 120

Baltimore mine.............................................. 120

Gray Eagle mine................................................ 121

Copper King prospect..........................'................ 122

Quartz mass near Basin ...........................'............. 122

Eva May mine....'........................................... 122

Uncle Sam mine............................................. 123

Western Reserve Mining Co. 's property................:........ 124

Butte and Philadelphia prospect.............................. 124

Bullion mine.................................................. 124

Morning Star mine............................................ 125

Custer mine.................................................. 125

Allport mine................................................. 125

Ruby mine.................................................... 125
CONTENTS. 7

Descriptions of special districts Continued. 

Boulder and Basin districts Continued.

Descriptions of mines Continued. Page.

Kit Carson, mine. .............................................. 1 27

Memphis prospect. ............................................. 128

Elkhorn district. ..................................................... 128

Location and history. .............................................. 128

Stratigraphic geology. .............................................. 128

Intrusive igneous rocks . . ......................................... 131

Contact metamorphism ........................................... 1 31

Ore deposits. ..................................................... 133

Descriptions of mines. ............................................ 135

Elkhorn mine. . ............................................... 135

Golden Curry mine ........................................... 136

General features .......................................... 136

The pyrrhotite ore body .................................. 137

Queen mine ........................................ '. .......... 139

Index.

ILLUSTRATIONS.

Page. 

PLATE I. Geologic map of the Helena mining region....................... 20

II. A, Stratified andesite breccias forming scarp on Cliff Mountain;

B, Jointing in-granite south of Boulder....................... 24

III. Lamellar calcite ore, Belmont mine, Marysville.................. 54

IV. Lamellar calcite ore, Belmont mine, Marysville.................. 55

V. Lamellar quartz pseudomorphic after calcite, Belmont mine, Marysville. ...................................................... 56

VI. Lamellar quartz pseudomorphic after calcite, Belmont mine, Marysville ....................................................... 57

VII. Geologic map of Helena and vicinity............................ 86

FIGURE 1. Index map showing the location of the Helena mining region..... 12

2. Contact-altered rocks in Colorado Gulch......................... 32

3. Tourmalinized zone in granite, Rimini.........................; 48

4. Drumlummon vein system on the level of the Maskelyne adit...... 70

ORE DEPOSITS OF THE HELENA MINING REGION,

MONTANA.

ByADOLPH KNOPF.

OUTLINE OF REPORT.

The dominant geologic feature of the Helena mining region is 

the granite mass which forms the northern extension of a great 

intrusion in southwestern Montana, known as the Boulder batholith.

This body of granitic rock represents an invasion of the upper zones 

of the earth's crust by an enormous volume of fluid magma in late 

Cretaceous time. The cover under which the granite cooled was 

largely stripped off by the prolonged erosion of the Tertiary period 

and the granite was laid bare and now forms the predominant surface 

rock of the region.

As a consequence of the granite invasion a series of ore deposits, 

the 'principal source of the metallic wealth of the region, was formed 

around the margin of the granite and in the roof rocks overlying it.

The rocks of the region may be broadly subdivided with reference 

to the great granite mass as (1) those formed prior to its intrusion, 

and (2) those formed subsequent to its intrusion in short, as prebatholithic and as postbatholithic rocks.. Of these two groups the 

prebatholithic rocks are the main repositories of the ore bodies of the 

region.

The prebatholithic rocks consist principally of sedimentary rocks 

which range in age from Algonkian to Cretaceous. They include 

mainly limestone, shale, and quartzite and lie in angular conformity 

from the lowermost member to the top of the series. These rocks 

are present chiefly in the northern part of the region. They are overlain by, andesite and latite lavas and breccias which are definitely 

known to be post-Jurassic and are probably of late Cretaceous age. 

The andesites and associated latites are the youngest rocks intruded 

by the granite, save perhaps a small area of thermally metamorphosed 

sandstones overlying them west of Helena.

After the andesites had been erupted the rocks were folded; subsequently the granite magma came to place beneath the region. The 

granite broke irregularly across the folds of the stratified rocks, the 

most notable example being the truncation of the southern limb of 

 : -, . , 9
10 QBE DEPOSITS OF HELENA MINING EEGION, MONTANA.

the Prickly Pear dome. This is shown to best advantage south of 

Helena, where the strata dip. steeply toward the batholith. At 

other localities, however, the covering rocks of the batholith principally andesites dip away from the granite.

The granitic rock of the Boulder batholith is termed a quartz monzonite because of its high plagioclase content. It is composed of 

plagioclase (near labradorite), orthoclase, quartz, biotite, and hornblende. As a whole, the mineral and chemical composition is 

remarkably uniform and homogeneous.

Large intrusions of aplite in irregular masses and dikes are common 

throughout the area of the batholith and constitute 5 per cent of its 

superficies.

The covering rocks, forming a roof of unknown thickness under 

which the granite came to place, crystallized, and cooled, consisted 

principally of andesites and latites, remnants of which still remain 

within the central portion of the batholith. The most extensive of 

these roof remnants is the large andesitic area west of Wickes, and 

this has been the great repository of the mineral wealth of the region.

A series of dacites, comprising lavas, breccias, and tuffs, locally 

2,400 feet thick, rest on the deeply eroded .surface of the granite and 

andesite. In the southwest corner of the region they form part of an 

extensive area extending to Butte and forming Big Butte at that city. 

They were formed contemporaneously with the lake beds west of 

Butte and are of upper Miocene age.

Rhyolites also rest on the eroded surface of the granite and the 

older rocks. Their relation to the dacites was not determinable. 

They were erupted on a surf ace whose relief was much like that of the 

present topography, and they are apparently the youngest bedrock 

formation in the region. At the head of Tenmile Creek, in the 

Rimini district, they have been mineralized and constitute low-grade

gold ore.

The ore deposits fall into two distinct groups, widely separated 

in time of origin. The older are late Cretaceous or early Tertiary in 

age, the younger are post-Miocene.

The older ore bodies are mainly silver-lead and gold-silver deposits. 

In metallic content they contrast strongly with the ores of the near-by 

Butte district, which are predominantly copper ores, although formerly silver ores were of importance. The scarcity of copper deposits 

in the Helena region is a notable feature, doubly so from the fact that 

the deposits belong to the same metallogenetic province as those of 

Butte, being formed at approximately the same time and presumably 

from the same magma.

The ore bodies of the older group have furnished the greater part of 

the production of the region; in fact, the value of their output has 

been roughly three times that of the post-Miocene deposits.
OUTLINE OP REPORT. 11

The silver-lead deposits constitute the prevailing type of ore body of 

the Cretaceous deposits. They are commonly situated near the contact 

of 'the granite and the rocks invaded by it. Hence, as a rule, they are 

inclosed in andesite or granite. The ore bodies are replacementfissure lodes containing galena, sphalerite, pyrite, and arsenopyrite.

The older ore bodies are commonly tourmaline bearing. In certain 

deposits, as at Rimini, tourmaline is extremely abundant; in fact, it 

is there developed in the same abundance that characterizes the tin 

lodes of Cornwall. The premier ore deposit of the region the Alta, 

credited with an output of $32,000,000 in silver-lead ore shows 

notable tourmalinization. Three types of tourmalinic lodes with 

transitions between them are recognized lead-silver, copper-silver, 

and gold. The predominant type is the tourmalinic lead-silver, a 

type that is unique, as far as shown by the literature of ore deposits.

The extreme depth to which any of these deposits has been worked 

is 1,200 feet. At Rimini, where the extreme depth attained below 

the present surface is only 600 feet, it is probable that this depth is 

2,000 feet below the intrusive contact surface of the granite.

The ores were formed at high temperatures, and it is regarded as 

probable that the ore-forming solutions were derived from a final 

differentiate of the quartz monzonite magma.

The post-Miocene ore bodies are essentially precious-metal deposits. They are characterized by the tendency of the quartz gangue 

to display a cryptocrystalline development, either flinty, chalcedonic, 

or densely saccharoidal, resembling porcelain. EquaUy characteristic is the thinly lamellar calcite of the gangue and its pseudomorphic 

replacement by quartz, forming a type of ore common in so many of 

the late Tertiary gold fields of the West. These .deposits are typically developed in the upper Miocene dacites of Lowland Creek, but 

their analogues at Marysville, which have yielded $30,000,000, have 

furnished the bulk of the output.

The extreme depth attained on veins of this class is 1,600 feet, at 

the Drumlummon mine, where the lode was found to be barren below 

the 1,000-foot level, and the greater part of the production has come 

from above the 500-foot level.

In early days the ...ines of the district furnished extremely rich 

surface ores. The evidence, although inadequate, suggests that the 

richness of the ores was due largely to mechanical concentration and 

to the elimination of worthless and objectionable constituents. In 

depth an objectionable amount of zinc in the form of sphalerite appears in many mines, but in the carbonate ores formerly worked the 

sphalerite had been oxidized and removed in solution. Secondary 

enrichment, especially such as has led to the production of great 

bodies of copper ore, has taken place nowhere in the region. The 

Marysville district is possibly an exception, for the ore bodies there
12 ORE DEPOSITS OP HELENA MINING REGION, MONTANA.

may have been enriched in gold by descending solutions; the evidence, 

however, is not conclusive, and .the indications are that the impoverishment in depth was due, in part at least, to failure in the primary

-metallization.

GEOGRAPHY.

LOCATION OF THE REGION.

The Helena mining region, as it is termed in this report, is an area 

of 1,300 square miles in southwestern Montana. A large number of 

mining camps are included in the confines of this area, among which,

FIGURE 1. Index map showing the location of the Helena mining region, Montana.

in point of production, Elkhorn and Marysville are the most important. Helena, however, is by far the largest and best known city 

in the region, and although it long ago yielded preeminence to other 

localities as a mining center, it is perhaps still appropriate to speak 

of this as the Helena mining region, for it was the discovery of the extraordinarily rich placers at Last Chance, where Helena now stands, 

that led to the development of the mineral wealth of the surrounding 

territory.

The Marysville mining district is 'situated in the northwest corner 

of the region, Elkhorn lies in the southeast corner, and the southwest 

corner is 14 miles north of Butte.
GEOGRAPHY. 13

The area lies within the rectangle formed by parallels 46° 12' and 

46° 48' north latitude and meridians 111° 50' and 112 p -30' west 

longitude. It includes portions of the Helena and the adjoining Fort 

Logan quadrangles, and accordingly does not form a regular quadrangle of the Geological Survey. "

A large part of the area lies within Jefferson County, a part within 

Lewis and Clark County, and that part west of the Continental Divide 

in Powell County. A considerable number of organized and unorganized mining districts are included in the region, but these subdivisions are no longer well known nor employed frequently in local 

usage, so they will not be used in this report.

The region is traversed from east to west by the main line of the 

Northern Pacific Railway and from north to south by the HavreButte branch of the Great Northern Railway; branch lines of the 

Northern Pacific extend to the mining camps of Marysville, Rimini, 

and Elkhorn, so that hi comparison with most mining regions it is 

unusually well -favored with transportation facilities. Good wagon 

roads render nearly every part of the area readily accessible.

PHYSICAL FEATURES.

The region, broadly considered, is mountainous and has a considerable and abrupt relief. The western portion constitutes the broad 

back of the Continental Divide, which here averages 7,000 feet in 

altitude. The topography, however, is neither rugged nor alpine, 

but is characterized by smoothly rounded profiles, due to the prevalence of domelike summits.

Red Mountain, 14 miles southwest of Helena, is the highest mountain in the northern part of the region; it attains an altitude of 8,200 

feet, and its broad, rounded summit looms up above its fellows for 

many miles around. The Elkhorn Mountains, on the eastern side 

of the region, are separated from the mountainous country on the 

west by the broad Boulder Valley. They are the highest mountains 

in the region, Elkhorn and Crow peaks reaching a height of 9,300 feet. 

On the northeast flanks of these peaks are a series of finely developed 

glacial amphitheaters, whose sheer walls and heavy talus slopes make 

this the most rugged part of the area.

The largest stream is Boulder River, flowing across the southern 

part of the region from west to east in a narrow valley deeply sunk 

between the surrounding mountains. On the east this valley opens 

out abruptly into the broad valley at Boulder; on the west beyond 

Bernice it again broadens and shows a fine series of terraces, the 

highest 250 feet above the stream. The irregular mountainous area 

through which Boulder River flows, and which forms the larger 

part of the area concerned in this report, is some tunes known as the 

Boulder Mountains.
14 OEE DEPOSITS OF HELENA MINING EEGION, MONTANA.

Missouri River crosses the extreme northeast corner of the region. 

Its principal tributaries are Prickly Pear, Teiunile, and Silver creeks

The most important stream west of the Continental Divide is Little 

Blackfoot River, which flows westward beyond the confines of the 

region, joining Clark Fork at Garrison. The Northern Pacific Railway in crossing the Continental Divide eastward ascends the valley 

of the Little Blackfoot to Elliston, and thence parallels an affluent 

from the northeast. The ascent is gradual through a comparatively 

broad and open- valley bordered by smooth, rounded hills. Near 

Mullan Pass, at the mouth of Dog Creek, there is a wide expanse of 

flat, so that it is difficult to realize that this is the summit of the 

Rocky Mountains.

CLIMATE.

The climate is semiarid, the annual precipitation ranging from 10 to 

13 inches. The summers are warm and pleasant; the winters are 

comparatively mild, despite the altitude and inland position of the 

region. The winter weather is ameliorated by the warm chinook 

wind that blows from the northwest; snow does not cover the ground 

in the open valleys during long periods, but disappears before the 

chinook.

The important climatic elements are given"in the subjoined tables. 1

Monthly and annual precipitation, in inches, in the Helena region, Montana.

Helena............

Jan.

1.00

.65

Feb.

0.66

.43

Mar.

0.83

.50

Apr.

1.06

.86

May.

2.13

1.66

June.

2.26

1.95

July.

1.13

1.02

Aug.

0.69

.76

Sept.

1.13

.92

Oct.

0.77

.41

Nov.

0.76

.38

Dec.

0.79-

.46

Annual.

'13.21

10.00

Mean temperature, in degrees Fahrenheit, in the Helena region, Montana.

Jan.

20.0

20.0

20.9

Feb.

22.7

21.7

21.9

Mar.

31.7

23.2

25.7

Apr.

42.1

41.5

40.4

May.

51.8

49.5

47.4

June.

59.1

55.9

55.0

July.

66.9

62.9

62.9

Aug.

66.2

62.8

62.1

Sept.

56.2

52.2

51.5

Oct.

45.8

42.9

43.0

Nov.

32.3

31.1

29.8

Dec.

26.3

24.7

25.9

Annual.

<M fi

41.1

40.5

Average number of days with 0.01 inch or more of precipitation in the Helena region,

Montana.

Jan.

9

5

8

Feb.

7

4

7

Mar.

8

6

8

Apr.

8

6

7

May.

12

8

14

June.

12

8

10

July.

8

6

8

Aug.

5

4

6

Sept.

7

6

5

Oct.

6

3

5

Nov.

7

4

7

Dec.

8

4

9

Annual.

97

R4

94

1 These tables are taken from more extended data given by R. F. Young, U. S. Weather Bureau, in the 

Twelfth Kept. Montana Bur. Agr., Labor, and Industry for 1909 and 1910, pt. 1, pp. 82-91.
HISTOBIOAL SKETCH OF MINING. 

Average snowfall, in indies, in the Helena region, Montana.

15

Jan.

11.5

6.7

16.2

Feb.

8,0

4 8

11 ^

Mar.

9.6

4.9

18.8

Apr.

5.8

4,?,

8.1

May.

1.4

1.1

5.9

June.

Tr.

Tr,

.5

July.

0.0

.0

Tr.

Aug.

0.0

.0

Sept.

O.R

1.1

1.6

Oct.

3.1

.9

5.4

Nov.

6.6

2.3

14 9

Dec.

8.1

4.4

15.0

Annual.

54.7

30.4

100.1

HISTORICAL SKETCH OF MINING.

The first important find of metallic wealth in Montana was the discovery of placer gold at Bannock in 1862. In the following year 

the far greater discovery at Alder Gulch was made, and from this 

stream was taken within the next three years $30,000,000. The 

extraordinary richness of these placers immediately attracted a large 

population, and so strong was this influx that within 18 months a 

town of 10,000 people sprang up, which was named Virginia City. 

Alder Creek was the longest and most productive creek ever discovered in Montana; 20 miles of stream bed, comprising about 1,000 

claims, were worked and yielded a profit at a time when wages were 

from $10 to $14 a day.

Montana was soon overspread by eager prospectors, and in the next 

few years all the important placers were discovered. Last Chance, 

on the present site of Helena, was located in the summer of 1864. This 

was the first important discovery in the region that is the subject of 

this report. The Whitlatch-Union vein, carrying rich gold ore, was 

found a few miles south of Helena in. September of the same year. 

Lode and placer discovery were thus contemporaneous. The finding 

of gold in place caused a quartz excitement which stimulated prospecting all over the Territory.

Last Chance yielded $16,000,000, the bulk of which was taken out 

before 1868. Many streams in the vicinity of Helena were also found 

to be auriferous Prickly Pear Creek, Nelson Gulch, Silver Creek, and 

others. In fact some of these prior to the discovery of Last Chance 

had been found to contain placer gold. Prickly Pear Bars, a few 

miles east of the site of Helena, were found to be auriferous in August, 

1862, and led within a few months to the discovery of gold on Silver 

Creek. Here Silver City was founded, which became, before Last 

Chance "was organized, the political and commercial center of Lewis 

and Clark County.

West of the Continental Divide placers were found in 1865 at Ophir 

Creek, Carpenter Bar, and Snowshoe, small affluents of Little Blackfoot River from the north. Here sprang up Blackfoot City (now 

called Ophir), which has long ago lapsed into decay. By 1875 the 

diggings had been abandoned to Chinese. In 1911, however, the 

gravels at the mouth of Snowshoe Gulch were tested for their possibility as dredging ground. 
16 QBE DEPOSITS OF HELENA MINING REGION, MONTANA.

The silver-lead ores and true silver ores in the vicinity of Wickes, 

Jefferson, and Clancy were discovered simultaneously with the finding 

of the placers. The Gregory lode, one of the earliest finds, was located 

in 1864 by a party going from Alder Gulch to Helena. An American 

hearth was erected here hi 1867 to smelt the ore; this was the second 

smelter established in Montana, the first being that built at Argenta.

The great need of the Territory at this time was an adequate transportation system, with rail connection with the centers of civilization. 

By 1870 the placers had been largely exhausted and a period of stagnation set in, for lode mining was not likely to flourish when high 

freight charges consumed the profits. According to the historian 

Bancroft, "Freights during the first decade were enormous, costing 

the country between a million and a half and two millions annually, 

even after the population had shrunk to 18,000." The chief overland transportation route was Missouri Kiver, by which steamers 

could reach Fort Benton, 150 miles from Helena, during high-water 

stages. But this period of high water lasted only 4 to 6 weeks, and 

steamers were often forced by low water to stop at Fort Union, at the 

mouth of the Yellowstone. On the completion of the Union Pacific 

Railroad in 1869 much of the traffic was diverted to this route, 

Corinne in Utah, being the initial point for freight bound for Montana. 

In addition to the handicap of high transportation charges most of the 

ores were of the kind that required expensive metallurgic treatment. 

Under these adverse conditions only the richest ores could be worked. 

The ores were either hauled to Fort Benton, thence sent to the seaports, and shipped to Swansea, Wales, or were hauled by wagons to 

Corinne on the Union Pacific Railroad, a distance of 450 miles, sent to 

San Francisco, and shipped to Europe.

The Northern Pacific Railway reached Helena in 1883, the first 

train crossing the Continental Divide west of Helena on August 7, 

1883, and the golden spike signalizing the completion of the overland 

road was driven on September 8, 1883, near Garrison.

The advent of the railroad exerted a strong stimulus on the mining 

industry. During the latter part of 1883 the Helena & Jefferson Railroad was built. This is now part of the Great Falls branch of the 

Great Northern Railway and connected Helena and Wickes, a distance 

of 20 miles. In the same year the Helena Mining & Reduction Co. 

acquired by purchase from the Alta Montana Mining Co. the Alta, 

Comet, and other well-known mines near Wickes. The smelter at 

Wickes was rebuilt and enlarged, so that it was for some years the 

most extensive reduction plant in Montana and drew ores from a large 

radius, even as far as the Cosur d'Alene district. In 1893 it was shut 

down and dismantled. The same fate has overtaken the many small 

smelters built at the various mines throughout the region, so that now 

the only smelter in operation is the East Helena plant of the American 

Smelting & Refining Co.
PRODUCTION. 17

The period from 1883 to 1893 comprises the years during which the 

bulk of the silver-lead ore was produced. The lode gold of the region 

has come mainly from the Marysville district. Mining flourished 

there principally from 1880 to 1900, although during the latter part 

of the period the yield came largely from the cyanidation of tailings 

piles.

PRODUCTION.

The total production of the region, including the yield from silverlead ore, placer gold, and lode gold, is not accurately ascertainable 

but is roughly in the neighborhood of $150,000,000. The estimates 

of placer production are particularly unsatisfactory, and for many of 

the streams no data are available. The output of Last Chance, the 

most productive of the placers, is estimated^ in widely different 

amounts by different writers, ranging from $10,000,000 to $35,000,000. 

Bancroft's estimate of $16,000,000 is accepted here. ^

The total output is distributed as follows : Silver-lead, $80,000,000* 

lode gold, $40,000,000 ; and placer gold, $30,000,000. Of these figures 

probably that for lode gold is entitled to the most confidence.

During 1910 the total production from the deep mines of the 

region, as compiled from Mineral Resources of the United States for 

1910, was $997,359. Of this amount gold represented $275,134, and 

silver, lead, and copper the remainder. The Elkhorn district furnished the preponderant part of the production $652,738. Placer 

mining in the region is now insignificant, and in 1910 yielded only

The total production for 1911 was $864,050, of which gold constituted $267,914.

The f ollowing table shows that in recent years the production of the 

region has fluctuated around $1,000,000.

Production of the Helena mining region, Montana. 

1906. . ............... ...................................... $917, 165

1907 .................................................... i 1, 617, 379

1908 ..................................................... 1, 118, 884

1909 ..................................................... 908, 044

1910 ..................................................... 997, 359

1911 ..................................'.......:............ 864, 050

FIELD WORK AND ACKNOWLEDGMENTS.

Field work was commenced at Helena on June 20 and completed 

September 24, 1911. While this work was carried on, the writer 

was efficiently assisted by Henry G. Ferguson, to whom fell most of 

the task of mapping the areal geology of the region. The examination of the mines and prospects was .undertaken mainly by the 

writer.

70935° Bull. 527 13 2
18 "ORE DEPOSITS OF HELENA MINING REGION, MONTANA.

During the preparation of this report in the office the writer has 

availed himself of certain collections of rocks ores, and thin sections from Elkhorn, Marysville, and the Boulder quadrangle, 

which had been gathered by W. H. Weed and Joseph Barrell for the 

United States Geological Survey and are now deposited in the United : 

States National Museum. Some of the mines from which they had 

collected specimens are no longer accessible, and such specimens were 

often found to be of much value for comparative purposes.

LITERATURE.

The following list includes the principal papers bearing on the 

geology and ore deposits of the region. The literature concerning 

the ore deposits is scanty and is limited to papers on the Elkhorn and 

Marysville districts.

BANCROFT, H. H., History of Washington, Idaho, and Montana, 1848-1889, San Francisco, 1890.

Gives an interesting account of the history of Montana and much valuable 

information concerning the growth of the mining industry.

BARRELL, JOSEPH, Microscopical petrography of the Elkhorn mining district, Jefferson 

County, Mont.: Twenty-second Ann. Kept. U. S. Geol. Survey, pt. 2, 1901, pp. 

511-549.

 The physical effects of contact metamorphism: Am. Jour. Sci., 4th ser., vol. 

13, 1902, pp. 279-296.

A theoretical discussion of contact metamorphism, based largely on the phenomena at Elkhorn, Mont.

 Geology of the Marysville mining district, Mont.; a study of igneous intrusion 

and contact metamorphism: Prof. Paper U. S. Geol. Survey No. 57, 1907, 178 ppAs indicated by the subtitle, this report deals mainly with the phenomena- of 

igneous intrusion and contact metamorphism exhibited at Marysville; it presents 

the strongest detailed evidence in favor of the magmatic sloping hypothesis yet 

published.

Relative geological importance of continental, littoral, and marine sedimentation: Jour. Geology, vol. 14-, 1906, pp. 553-560.

It is argued that the Belt series of Montana is largely of terrestrial origin. 

CLAYTON, J. E., The Drumlummon group of veins and their mode of formation: Eng. 

and Min. Jour., vol. 46, 1888, pp. 85-86, 106-108.

An able study of the structural features of the Drumlummon lode. 

GRISWOLD, L. S., The geology of Helena, Mont., and Vicinity: Jour. Assoc. Eng. Soc., 

vol. 20, 1898, pp. 51-68.

A careful description of the character and distribution of the rocks at Helena, 

accompanied by a geologic map.

KEYES, W. S., Mineral resources of the Territory of Montana: Mineral Resources of the 

States and Territories west of the Rocky Mountains, 1868, Appendix, pp. 38-56.

Gives many valuable facts concerning the early mining history. 

LEESON, M. A., History of Montana, 1739-1885. Chicago, 1885, 1367 pp.

Gives much detailed information concerning the settlement and history of the 

mining camps.

LINDGREN, WALDEMAR, Relation of the coal of Montana to the older rocks; Appendix B, Eruptive rocks: Tenth Census, vol. 15, 1886, pp. 733-734. 

Concludes that the intrusive granite at Mullan Pass is of Jurassic age.
CHAEACTEE AND DISTEIBUTION OF THE BOOKS. 19

STONE, R. W., Geologic relation of ore deposits in the Elkhorn Mountains, Mont.: 

Bull. U. S. Geol. Survey No. 470, 1911, pp. 75-98.

Deals principally with the area east of that concerned in this report. 

WALCOTT, C. D., Pre-Cambrian fossiliferous formations: Bull. Geol. Soc. America, vol. 

10, 1899, pp. 199-244.

In this paper are given the type descriptions of certain divisions of the Belt 

series. " WEED, W. H., Granite rocks of Butte, Mont., and vicinity: Jour. Geology, vol. 7, 

1899, pp. 737-750. . The Boulder batholith is defined in this paper, and chemical analyses of the 

rock from different localities are given.

 Mineral vein formation at Boulder Hot Springs, Mont.: Twenty-first Ann. 

Kept. U. S. Geol. Survey, pt. 2, 1900, pp. 227-255.

It is shown that the hot waters: are forming veins filled with quartz; stilbite, and 

calcite, and containing small but appreciable amounts of gold and silver.

 Geology and ore deposits of the Elkhorn mining district, Jefferson County, 

Mont., with an appendix on the microscopical petrography of the district, by 

Joseph Barrell: Twenty-second Ann. Kept. U. S. Geol. Survey, pt. 2, 1901, pp. 

399-549.

 Gold mines of the Marysville district, Mont.: Bull. U. S. Geol. Survey No. 

213, 1903, pp. 88-89.

Outlines briefly some of the salient features of the gold veins at Marysville. 

Geology and ore deposits of the Butte district, Mont.: Prof. Paper U. S. Geol.

Survey No. 74, 1912, pp. 26-30.

Gives a geologic reconnaissance map of the area underlain by the Boulder batholith and a brief description of the general geology.

WINCHELL, H. V. and A. N., Notes on the Blue Bird mine: Econ. Geology, vol. 7, 

1912, pp. 287-294.

Attention is called to the tourmaliniferous character of the ore, and various 

rocks occurring at the mine are described petrographically.

GENERAL GEOLOGY.

CHARACTER AND DISTRIBUTION OF THE ROCKS.

Sedimentary rocks occur in the main only in the northern part of 

the region. They consist for the most part of an apparently conformable succession of limestone, shale, sandstone, and quartzite, 

ranging in age from Algonkian to Cretaceous. At certain localities 

deposits of poorly consolidated clays, gravels, and tuffs of Tertiary 

age so-called lake beds rest unconformably on the older rocks.

The prevailing rocks of the region are igneous andesite and latite, 

quartz monzonite and aplite, dacite, and rhyolite. Of these the 

quartz monzonite predominates; it is of intrusive origin and invades 

all the rocks older in age than the lake beds. The other igneous 

rocks, except the aplite, which is closely associated with the quartz 

monzonite, are of extrusive origin and form thick stratiform series 

of lavas, tuffs, and breccias. The andesites and latites are older than 

the quartz monzonite, and the dacites and rhyolites are younger.
20 ORE DEPOSITS OF HELENA MINING KEGION, MONTANA.

The distribution of these rocks is shown on the geologic map forming Plate I of this report. The small scale of the map 1:250,000 or 

approximately 4 miles to 1 inch allows the broader features only 

to be shown. It is believed that for the greater part of the area the 

boundaries of the geologic formations as given are accurate to scale. 

An earlier geologic map of a portion of this region, based on reconnaissance work, appeared as Plate I in the report on the Butte district. 1 In that map, while in press, certain new data obtained during 

the present investigation were incorporated, \ with Mr. Weed's concurrence, but the older map was not completely revised.

Plate VII (p. 86) of this report shows the areal geology of Helena 

and vicinity on a scale of 1:62,500. This map was prepared by Mr. 

Weed and his assistants, and is published for the first time in this 

report in order to show the detailed distribution of the rocks in a 

district which is highly interesting geologically details necessarily 

lost on the small-scale map of Plate I.

PBOTEBOZOIC ROCKS.

ALGONKIAN SYSTEM. 

, BELT' SERIES.

The oldest rocks of the region are a series of limestones and shales, 

known collectively as the Belt series. Walcott recognizes eight 

formations, aggregating at the maximum 12,000 feet in thickness. 

The rocks occupy a large area in the northern part of the region of 

this report, but only the four uppermost formations are present. 

These in ascending order are known as the Spokane shale, Empire 

shale, Helena limestone, and Marsh shale. At Helena these attain 

a thickness of 3,300 feet, of which the Helena limestone makes up 

2,400 feet, increasing to 4,000 feet at Marysville, according to 

Barrel!.

In the present mapping no attempt was made to differentiate the 

various formations of the Belt series, and all rocks below the Flathead quartzite were mapped under the same symbol. They are the 

rocks termed by W. M. Davis "the barren slates, shales, and sandstones," encountered in the area between Helena and Mullan Pass 

on the Continental Divide and regarded as of probable Cambrian age. 2 

They were assigned to the Algonkian system by Walcott, who believes 

that the unconformity at the base of the Flathead quartzite of 

Middle Cambrian age is of sufficient magnitude to justify the placing 

of the Belt series in the pre-Cambrian.

1 Weed, W. H., Geology and ore deposits of the Butte district, Mont.: Prof. Paper U. S. Geol. Survey 

No. 74,1912.

2 Tenth Census, vol. 15,1886, p. 703.
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LATE CRETACEOUS (?) TERTIARY ALGONKIAN CAMBRIAN 

AND DEVONIAN

CARBONIFEROUS JURASSIC AND 

CRETACEOUS

TERTIARY QUATERNARY
ORE DEPOSITS OF HELENA MINING REGION, MONTANA. 21

PALEOZOIC BOCKS. 

CAMBEIAN AND DEVONIAN SYSTEMS.

The Paleozoic rocks below the Madison limestone (Mississippian; 

or lower Carboniferous) comprise a conformable series, consisting 

dominantly of limestones, with some shale and quartzite. The 

aggregate thickness is estimated to be 2,300 feet and includes rocks 

of Cambrian and Devonian ages. Owing to the fact that the formations, except the Flathead quartzite, are not readily discriminable 

throughout the region the Paleozoic rocks below the" Madison limestone have been mapped as a unit.

The lowest formation of the Paleozoic is a persistent and easily 

recognizable series of quartzites, known as the Flathead quartzite. 

It consists of a highly lithified, vitreous, coarse siliceous quartzite,

- stratified in beds commonly exceeding 1 foot in thickness. It is 

normally white, but in weathering takes on characteristically a red 

tint and in places displays cross-bedding. The thickness of the quartzite ranges from 80 to 180 feet.

The Flathead quartzite forms prominent outcrops and is well 

exposed. The contact with the underlying Belt series can be traced 

mile after mile northwest from Helena; it was found that the quartzite rests on the subjacent rocks in strict angular accordance. In some 

places .it overlies the Helena limestone, in others the Marsh shale. 

The basal portion of the quartzite, nowhere more than a few feet 

thick, is slightly conglomeratic, the kind of pebbles composing it 

varying with the character of the underlying rocks. ' It is believed by 

Walcott that several thousand feet of strata were removed before the 

Flathead was laid down, and this is held to prove that a profound 

unconformity separates the Flathead from the underlying Belt series.

Above the quartzite, which is so prominent a horizon marker, 'is 

a great thickness of limestone strata, with interstratified shales of 

Cambrian and Devonian age. Details concerning these rocks are 

given in the description of the local geology of Helena (pp. -86-98).

CARBONIFEEOUS SYSTEM. . . 

s MADISON LIMESTONE.

The Madison limestone is prominently developed in the northern

- part of the region. Its basal member is a dense dark gray-blue limestone, which contains crinoid remains and other fossils and is pierced 

by innumerable tremolite needles. This member seems to range in 

thickness from 100 to 200 feet. Above this comes a heavy succession of thickly bedded, coarsely crystalline limestone, which commonly forms bold' outcrops of dazzling whiteness.

The total thickness at Helena, according to Weed, is 2,600 feet. 

The age is Mississippian (lower Carboniferous).
22 ORE DEPOSITS OF HELENA MINING REGION, MONTANA.

QUADRANT QUARTZITE.

The Quadrant quartzite overlies the Madison limestone conformably. It consists predominantly of quartzite and ranges in thickness from 190 to 500 feet in the Helena area. The quartzite has a 

characteristic appearance that distinguishes it readily from the. other 

quartzites of the region. It is a light-gray rock of dense, almost 

cherty texture and outcrops show typical rough hackly surfaces. As 

a rule the bedding is neither clearly nor positively recognizable. The 

age of the formation here correlated with the typical Quadrant is 

probably Pennsylvanian (upper Carboniferous). To the west, in 

the Philipsburg quadrangle, Calkins obtained Pennsylvanian fossils . 

from the Quadrant formation. In other areas to the east, however, ' 

rocks referred to the Quadrant formation have yielded Mississippian 

fossils.

Phosphate rock of good grade was found in 1911 by J. T. Pardee, 

of the Geological Survey, near the top of the Quadrant quartzite in 

the town of Elliston.

MESOZOIC BOCKS. 

JURASSIC AND CRETACEOUS SEDIMENTARY ROCKS.

Rocks definitely determined as of Jurassic age occur at Elliston, 

where they overlie the Quadrant quartzite in angular concordance.

The beds of determined Jurassic age are buff-weathering arenaceous limestones and some purer limestones, aggregating 50 feet in 

thickness, referable to the Ellis formation. At the base, as exposed 

at Elliston, is a sill of diorite porphyry 25 feet in thickness. The 

basal beds are highly fossiliferous; above them, and possibly a part 

of the same formation, rests a banded sandstone, as a rule remarkably cross-bedded, with numerous unconformities between the different sets of cross-bedded laminae. The sandstone is a coarse 

quartzose rock, carrying numerous minute black chert particles, 

which impart a speckled appearance to the rock. In places fragments 

of lignitized vegetal matter, some over a foot long, were found. The 

section immediately at Elliston shows a thickness of several hundred 

feet of this sandstone, although there are a few intercalated beds of 

limestone and of a soft, greenish, conchoidally breaking shale.

The fossils were submitted to T. W. Stanton, who reports:

7242, No. 1. Elliston, Helena quadrangle, Mont. 

Pentacrinus asteriscus Meek and Hayden. 

Camptonectes pertenuistriatus Hall and Whitfield. 

Pseudomonotis (Eumicrotis) curta (Hall). 

Undetermined gastropod casts. 

Cardioceras ? sp. (small fragment). 

This lot is from the marine Jurassic Ellis formation.
MESOZOIC ROCKS. 23

Fossils were found, apparently at the same locality, by Lindgren 

in 1883 and were referred also to the Jurassic by Whitfield. 1

There is no evidence of unconformity between the Ellis formation 

and the underlying Carboniferous rocks at Elliston. At Philipsburg, 75 miles to the west, however, the Ellis formation, although 

resting in angular concordance on the underlying Carboniferous, has 

a basal conglomerate holding pebbles of Carboniferous and older 

rocks. An erosion interval is therefore proved.2 The maximum 

thickness of the Ellis formation at Philips >urg is 430 feet.

In the region surrounding Elliston ther; is an extensive development of coarse-grained speckled sandstone lying conformably above 

the known Jurassic rocks. The sandstone is commonly cross-bedded 

and semi-quartzitic in appearance. A specimen of dark-gray crossbedded sandstone from the summit of the 7,000-foot ridge southwest 

of Elliston was found microscopically to be composed of particles of 

quartz, black chert, and shale. Considerable secondary quartz in 

optical continuity with the original grains had grown around the 

quartz particles. Some few of the quartz grains (0.5 millimeter in 

diameter) are remarkably perfectly rounded; the chert fragments 

are subangular.

Limestone, shale, and conglomerate are associated with the crossbedded sandstone in small amount. Toward the top the rocks apparently grade 'into the andesite and latite series. Limestones of like 

appearance and fossil content (obscure fresh-water shells) are interstratified with the volcanic rocks above and the sedimentary rocks 

below.

These rocks are referred to the 'Cretaceous, but the line between 

them and the underlying Jurassic was not determined, and the entire 

assemblage, Jurassic and Cretaceous, is shown under one symbol on 

the geologic map.

Rocks of probable Cretaceous age occur at other localities around 

the margin of the Boulder batholith and even within it, as roof 

remnants. They occur at Elkhorn, where they are termed by. Weed 3 

the Crow Ridge series, north of Rimini, south of Helena, and at the 

head of Basin Creek, where they form part of the cover of the batholith.

IGNEOUS ROCKS.

ANDESITES AND LATITES.

OCCURRENCE.

The remnants of the roof of the Boulder batholith consist prin- 

(ipally of andesitic rocks, comprising a bedded series of lavas, breccias, 

and tuffs, with some interstratified quartzite, shale, and limestone. 

At most localities the truly volcanic portion predominates.

i Tenth Census, vol. 15, 1886, p. 703.

"Emmons, W. H., and Calkins, F. C., Prof. Paper U. S. Geol. Survey No. 78, 1913, p. 75.

8 Twenty-second Ann. Kept. U. S. Geol. Survey, pt. 2,1901, p. 440.
24 ORE DEPOSITS OF HELENA MINING REGION, MONTANA.

Rocks of andesitic character are developed also in great volume on 

both the eastern and western borders of the batholith. As a rule 

breccias and tuffs form the bulk of the andesite and latite series; they 

predominate at Elkhorn on the east and form practically the entire 

series on the west. The breccias are in places rather coarse; at Cliff 

Mountain, which displays a scarp of stratified breccias (PI. II J.),some 

of the included blocks of andesite are 3 feet long. At this locality beds 

of hard green and black shales are interstratified with the breccias. 

Sedimentary rocks are found at other places, as east of Thunderbolt 

Mountain, where andesite, breccia, banded tuff, and hard shale are 

interstratified. Fossiliferous limestone carrying fresh-water forms 

occurs southwest of Elliston.

Where the underlying rocks are exposed the andesites rest on them in 

angular accordance. At one locality only, at the mouth of the canyon 

of Tenmile Creek, do sedimentary rocks overlie the andesites. These 

rocks are thermally metamorphosed sediments, mainly of original 

arenaceous character. Quartzites, some of great purity, are common, 

but most are partly micaceous. These rocks, as a whole, appear to 

correspond to Weed's Cretaceous of the Helena district. Near the 

contact of the andesitic rocks and the overlying sedimentary rocks 

there is apparently an interstratification of both kinds.

On the eastern side of the batholith, at Elkhorn, the andesitic 

rocks dip eastward at a gentle angle away from the granite; on the 

west, at Cliff Mountain, they dip westward at angles of 15° to 20° 

away from the batholith, passing beneath the soft Tertiary deposits 

of Deer Lodge Valley. In the areas lying within the batholith 

steeper dips occur, some reaching 45°, and the strike differs greatly 

from place to place.

The andesite and latite series is of considerable economic interest, 

because it forms the country rock at some of the important mines. 

In fact, the largest producer, the Alta, credited with an output of 

$32,000,000, is inclosed in rocks of. this kind. The favorable factor, 

however, in the formation of .these deposits was certainly not the 

chemical character of the rock but its position with respect to the 

sources of mineralization.

GENERAL CHARACTER.

i

On the steeper declivities the andesites generally form bold outcrops, below which extend long talus slopes of angular fragments. In 

a broad way they are of similar appearance, fine grain, and dark 

color. Where, however, the latites prevail, the outcrops weather to a 

light gray or white, and the exposures resemble those of the rhyolites 

of the region. On examination at any one locality in detail, the andesites are found to comprise a considerable number of varieties. For 

example, the andesites at the mouth of the canyon of Tenmile Creek
U. S. GEOLOGICAL. SURVEY BULLETIN 52? PLATE it

.1, STRATIFIED ANDESITE BRECCIAS FORMING SCARP ON CLIFF MOUNTAIN.

B. JOINTING IN GRANITE SOUTH OF BOULDER.
MESOZOIC EOCKS. 25

show the following distinct varieties: (1) With numerous conspicuous 

feldspar phenocrysts; (2) with pyroxene phenocrysts alone prominent; (3) intermediate between (1) and (2); (4) amygdaloidal phases; 

(5) flow-banded phases; and (6) breccias, in which highly porphyritic 

fragments are inclosed in aphyric varieties; also other breccias, 

probably flow breccias in the main. On the summit of Thunderbolt 

Mountain thick sheets of lava, strikingly different in appearance, are 

found in superposition. One variety, a dark bluish black aphanitic 

rock, carrying small obscure phenocrysts of hornblende and plagioclase, is a fresh-looking lava and shows flow banding but only on the 

weathered surface. The adjoining lava sheet is of altered appearance 

and is a bluish-gray rock, carrying numerous conspicuous tabular 

plagioclase feldspars. It presents, therefore, a strong contrast to the 

former rock. Glassy porphyries also are prominently developed here. 

These rocks are all of typical andesitic appearance. With them, 

however, are associated other varieties, which exhibit an obvious 

divergence from the andesitic type. These show small scattered 

phenocrysts of plagioclase and biotite in a groundmass of flinty texture and commonly display flow streaking. Conspicuously flowbanded varieties, nearly devoid of phenocrysts and of red flinty 

appearance, are found east of Boulder. North of Thunderbolt Mountain and west of Thunderbolt Creek these rocks, which chemical 

analysis shows to belong to the latite group, are present in great 

abundance. Flow banding and streaking is developed in these 

latites in an extraordinary degree, so that they rival in this respect 

the Tertiary rhyolites of the region.

PETROGRAPHY.

An andesite from the entrance of the canyon of Tenmile Creek is a 

dark heavy rock, in which the pyroxene phenocrysts are prominent 

because weathering in relief. In thin section it is found to contain 

numerous phenocrysts of calcic plagioclase, ranging up to Ab30An70 , 

and of augite, which are surrounded by rims of brown-green amphibole. The groundmass consists of tabular plagioclase and granular 

amphibole, with some biotite and accessory magnetite bordered by 

titanite (?) rims. The rock has very probably been modified to some 

extent by contact metamorphism exerted by the near-by intrusion of 

quartz monzonite. It corresponds in appearance and mineralogy 

to the hornblende andesite porphyry, altered from an augite andesite and approaching a basalt in composition, from the southwestern 

part of the Elkhorn district, which has been described by Barrell 

and of which an analysis is available. 1

The bluish-black aphanitic andesite from Thunderbolt Mountain 

shows a porphyritic hyalopilitic texture. The andesite with the

i Twenty-second Ann. Rept. U. S. Geol. Survey, pt. 2,1901, p. 529.
26

prominent tabular feldspar crystals contains phenocrysts of plagioclase near AfyAnj and of chloritized and epidotized femic minerals 

in a cryptocrystalline matrix. Both are much poorer in femic minerals than the first-described andesite.

The latites are characterized by the occurrence of plagioclase and 

biotite phenocrysts in a cryptocrystalline groundmass of salic appearance. Blue, red, and flint-gray are common colors. Streakiness and 

flow banding are nearly universal.

A specimen from the top of the ridge on the west side of Thunderbolt Creek, near its head, is a porphyry carrying small vitreous plagioclase phenocrysts and bronzy plates of biotite embedded in a darkred glassy groundmass of dull appearance. It displays a strongly 

marked eutaxitic structure, especially conspicuous on weathered 

surfaces, which are of light-gray color. Under the microscope the 

feldspars, which are well preserved, are found to be andesines near 

AbjAnj, and the biotite plates to be chloritized. The groundmass is 

glassy, thcrugh partly devitrified; flow banding is finely developed, 

and swirls encircling the feldspars are common, together with a certain 

amount of breakage of the phenocrysts thus enveloped. Apatite and 

ilmenite, now altered to leucoxene, are present as accessory minerals.

Analysis of latite from Thunderbolt Creek, Mont. 

[J. G. Fail-child, analyst]

SiOa .. ..............

Al O

FeO

MgO. ...............

CaO................

TT o

HoO....... .........

............. 64.45

............. 17.69

............. 1. 33

.,.......'.... 1.93

............. .57

............. 3.73

............. 3. 85

............. 3. 68

............. .59

H 0+

TiO

C02 . ..............

P206 ... .............

s...... .....

MnO..............

BaO. ...............

.............. 0. 80

.............. .69

.............. .29

.............. .16

.............. .04

.............. .05

.............. .19

100. 04

The low magnesia and the relatively high potash, together with the 

general andesitic composition, are noteworthy. The potash, which, 

as shown by the mineralogic make-up of the rock, is restricted largely 

to the groundmass, evidently caused the flowing lava to become stiff 

and viscous, so that most of these rocks show a highly flow-banded 

structure.

CONTACT-METAMORPHIC ALTERATION OP THE ANDESITES AND LATITES.

The andesites and latites in proximity to the quartz monzonite 

intrusion have undergone contact-metamorphic alteration. This 

has taken oue of two forms, either simple recrystallization or recrystallization with addition of material. The former is the more common 

occurrence.
MBSOZOIC EOCKS. 27

The contact-metamorphic alteration seems to be most apparent to 

the eye in the latites or andesites high in potash. Near the contact 

these rocks lose their dark color and become nearly white; the groundmass takes on obviously a microcrystalline character. Where the 

latites are flow banded this structure is retained, and the increased 

light color of the rocks enhances their resemblance to rhyolites. Under 

the microscope the main change is found to have taken place in the 

groundmass; whereas before metamorphism it was of cryptocrystalline texture, it is now a clear, well-individualized aggregate, consisting essentially of quartz and orthoclase, with biotite dispersed 

through it in small flakes. In places tourmaline appears as an accessory.

The most extensive metamorphism of the andesites has taken place 

on the north side of Elkhorn Peak, extending down to Prickly Pear 

Creek at 8,000 feet altitude. The rocks are mainly breccias and have 

been remarkably thoroughly recrystallized. The matrix of the breccias is commonly more coarsely crystalline than the included fragments, and in fact in many localities resembles a fine-grained diorite. 

The rocks are extensively injected with aplite dikes, many of which 

carry tourmaline, probably pyrogenetic, and are most thoroughly 

recrystallized where pierced by numerous aplitic dikelets. In the 

porphyries the augite phenocrysts have generally been converted 

to fibrous amphibole, which along with disseminated flakes of biotite, 

is also common in the groundmass.

The most notable pneumatolytic metamorphism observed in the 

region is that which has aifected the thick stratum of andesitic breccia 

overlying the large limestone bed on the southwest flank of Elkhorn 

Peak. The breccia is composed of angular fragments of andesite, the 

largest 4 inches long; it has obviously been metamorphosed, showing 

patches of garnet and in places discrete areas composed solidly of 

fibrous tremolite, with fibers up to 2 inches in length. Thin sections 

cut from different places naturally show different textures and proportions of the component minerals. Garnet, pyroxene, scapolite, plagioclase, and accessory titanite and apatite, are the minerals present. 

Remnants of pyroclastic structure remain, as indicated by the presence of unobliterated porphyry fragments; plagioclase phenocrysts 

or pyroclastic fragments tend to remain intact, but as a rule the new 

growth of lime silicates has spread throughout the rock. The garnet 

is not grossularite, but very probably andradite, from the fact that 

its -refractive index is near that of the minimum index of titanite 

intergrown with it. The pyroxene is a monoclinic variety, suggestive 

of hedenbergite in part. Scapolite occurs locally in broad plates poikilitically inclosing the other minerals. The plagioclase is mainly 

residual and is commonly scapolitized. All these features, together
28 ORE DEPOSITS OF HELENA MINING REGION, MONTANA.

with the macroscopic patches of fibrous tremolite, indicate that the 

breccia was originally calcareous, and was subsequently metamorphosed under the influence of chlorine-bearing solutions.

Northwest of Mullan Pass andesite sills lying between thick beds of 

coarsely garnetized sedimentary rocks are traversed by small veins 

carrying calcite and dodecahedral garnet.

AGE OP THE ANDESITES.

The age of the andesites is a matter of considerable interest, inasmuch as they are the youngest rocks into which the Boulder batholith 

is intrusive. Weed states that an Eocene age is indicated, but the 

evidence on which this is based is not given.1 That they are certainly 

of pre-Oligocene age at least is proved by the fact that Oligocene sediments overlie them west of Cliff Mountain,2 and at Pipestone Springs, 

near the southern border of the batholith.3

Southwest of Elliston the andesite breccias at the base of the andesite series were found interstratified with the sedimentary rocks, principally cross-bedded semiquartzitic sandstone, that overlie the fossiliferous strata of the marine Jurassic. A certain amount of conglomerate and limestone occurs with the sandstone, and the conglomerate contains some andesite fragments. These facts tend to 

show that the andesitic eruptions went on contemporaneously with 

the deposition of the late Mesozoic sediments. In the andesites 

occur thin intercalated beds of fossiliferous limestone, in appearance 

like those underlying the andesites. T. W. Stanton reports on the 

fossils:

7243. From a 7,000-foot mountain southwest of Elliston, Mont. This lot consists 

of drab fresh-water limestone containing poorly preserved undetermined ostracods 

and gastropods. The age is probably Cretaceous.

It is known that in eastern Montana volcanic activity commenced 

in the Montana epoch of the Cretaceous. Formerly it was believed 

that the eruptions began only after the Laramide orogenic revolution, 

but this has been disproved both by paleozoologic and paleobotanic 

evidence.4

It will therefore be held, pending the collection of more satisfactory material from the limestones interbedded with the andepites 

overlying the Boulder batholith, that the andesites are of late Cretaceous age.

The original thickness of the andesitic rocks is unknown. Southeast of Elkhorn, where they are regularly bedded and dip at a low

> Weed, W. H., Geology and ore deposits of the Butte district, Montana: Prof. Paper U. S. Geol. Survey 

No. 74,1912, p. 29.

* Douglass, Earl, Annals Carnegie Mus., Pittsburgh, vol. 5,1909, p. 263.

s Matthew, W. D., Bull. Am. Mus. Nat. Hist., vol. 19,1903, p. 197.

4 Stone, R. W., and Calvert, W. K., Stratigraphic relations of the Liviirjstone formation of Montana: 

Econ. Geology, vol. 6,1910, pp. 551-557, 652-669, 741-764.
MESOZOIC BOOKS. 29

angle, a thickness of 2,000 feet is indicated, 1 but this figure is obviously 

a minimum estimate. The high ridges in which the andesites are 

exposed at Elkhorn have been carved from an area that has been 

subjected to prolonged and intense erosion ever since the beginning 

of the Tertiary period. During the same time other parts of the 

surrounding region, equally subjected to erosion, have been denuded 

of several thousand feet of rock covering, so that it is highly probable 

that the andesitic rocks had originally a far greater thickness.

QUARTZ MONZONITE. 

GENERAL CHARACTER.

Granite is the predominant rock exposed throughout the region. 

It is, however, not a true granite, but is accurately designated a 

quartz monzonite. In local usage the term granite is employed 

universally and where adopted in this report it is employed in the 

popular sense. No true granite was found anywhere in the region; 

such modifications of the quartz monzonite as were noted are towarjd 

dioritic rocks.

The granite area of this region is the northern part of the great, 

intrusive mass of granitic rock extending uninterruptedly from the 

Highland Mountains, 16 miles south of Butte, to Mullan Pass on the 

north. This large intrusion was named by Weed the Boulder batholith, from the name of the mountains throughout which it is 

exposed.2 It occupies an area of" about 1,100 square miles, extending 60 miles from north to south and averaging 18 miles in width.

In the mountainous areas the granite weathers in characteristic 

fashion, forming huge bowldery outcrops and making the country 

exceedingly rough and nearly impassable.

Locally the granite is traversed by pronounced systems of jointing, 

as at Rimini and also at Boulder (PL II, B, p. 24).

The granite has been quarried within the area considered in this 

report mainly at two points on the Rimini road 8 miles west of 

Helena and at a locality north of Clancy. The stone from Clancy is 

used in the construction of the wings of the State capitol at Helena.

PETROGRAPHY.

The prevailing rock of the Boulder batholith is a coarse granitoid 

composed in the order named essentially of plagioclase, orthoclase, 

quartz, biotite, and hornblende. It is remarkably homogeneous in 

composition over a large area. A widespread feature, extending 

more or less persistently from Mullan Pass to Butte, is a rough 

porphyritic habit due to the development of large imperfect phenocrysts of orthoclase.

1 Personal communication by R. W. Stone.

* Weed, W. H., Granite rocks of Butte, Mont, and vicinity: Jour. Geology, vol. 7,1899, p. 737.
30 OEB DEPOSITS OF HELENA MINING REGION, MONTANA.

Toward the margins of the batholith the rock becomes somewhat 

finer grained and equigranular, as noted south of Helena, east of 

Boulder, and other localities. The rock apparently becomes more 

basic and the eye is at a loss to surely distinguish quartz monzonite 

from such modifications as granodiorite and quartz diorite. At 

certain localities, as verified by the microscope, the rock becomes 

dioritic in character.

A considerable number of chemical analyses of rock from widely 

separated localities on the northern part of the batholith are available, 

arid emphasize the chemical homogeneity suggested by the mineral 

composition. Two of these analyses were made during the course 

of the present investigation; the others have been published by 

Weed. 1

Analyses of quartz monzonite from northern part of the Boulder batholith.

SiOj.. ....................................

AlsOa. ....................................

Fe208 . ....................................

FeO. .....................................

MgO. ............................. ........

CaO.. .....................................

Na20.....................................

KS0. .....................................

H20.....................................

H20 + . ...................................

TiOj.-... ............ ......................

PsOs.. ................. ...................

MnO. .....................................

SrO.......................................

BaO... ...................................

LijO. .....................................

S..........................................

S0«.......................................

Cl........... ..............................

YeSt... ...................................

1

65.91

15.32

2.28

2.02

1.52

3.28

3.08

 4.80 ~.60

.60

.59

.18

.10

.02

100. 51

2

64.49

15.49

1.19

2.71

1.89

4.32

3.53

4.04

.16

.48

.56

.19

.07

.06

.13

99.80

3

64.17

15.25

2.16

2.98

2.60

4.24

' 2.62

4.34

.16

.65

.67

.16

.04

Trace.

.07

Trace.

.07

100. 18

4

67.12

15 00

1.62

2.23

1.74

3.43

2.76

4.52^

.09

.58

.48

.is

.06

.03

.07

99.88

5

64.31

15.44

2.43

2.58

2.21

4.22

2.71

4.09

.19

.79

.71

.22

Trace.

.07

Trace.

99.97

6

64.03

15.58

1.96

2.83

2.15

4.20

2.76

4.11

.23

.76

.59

.18

.11

.04

.07

99. 86

1. Valley Forge mine, Rimini. J. G. Fairchild, analyst.

2. King Solomon mine, Clancy. J. G. Fairchild, analyst.

3. Frohner mine. H. N. Stokes, analyst.

4. Boulder. H. N. Stokes, analyst. '

5. Elkhorn. H. N. Stokes, analyst.

6. Butte, average of four closely similar analyses. H. N. Stokes, analyst.

The average analysis- of the quartz monzonite at Butte has been 

added to show that it does not differ essentially from those of rocks 

from other parts of the batholith; it is, in fact, practically identical 

with that of the rock from Elkhorn, which lies on the eastern margin 

of the batholith, 33 miles northeast of Butte.

Under the microscope rock from different parts of the batholith 

shows broadly similar features. A zoned plagioclase near labradorite 

(Ab50An50) is found to be the dominant feldspar; the most calcic 

variety, from Elkhorn, as measured on the core is Ab45An55 and the 

least calcic Ab59An41 a remarkably narrow range in composition. 

Orthoclase and quartz are present in abundance and lie interstitially

» Op. cit., pp. 737-750.
MESOZOIC EOCKS. 31

between the idiomorphic plagioclase crystals. Biotite and hornblende are the ferromagnesian minerals, and the biotite commonly 

predominates. At Elkhorn a small amount of augite intergrown 

with the hornblende is found but none occurs elsewhere. The 

accessory minerals are magnetite, titanite, apatite, and zircon. The 

computed mineral composition of the quartz monzonite from Clancy 

(see p. 56) is 41.58 per cent plagioclase, 18.90 per cent orthoclase, 

19.86 per cent quartz, 8.94 per cent biotite, and 7.26 per cent hornblende. At Elkhorn, according to Barrell, 1 it contains by volume 34 

pe.r cent labradorite, 22 per cent orthoclase, 25 per cent quartz, 7 

per cent biotite, 9 per cent hornblende, and 2 per cent augite.

On Red Rock Creek and extending to Thunderbolt Mountain is a 

large body of gray diorite of medium grain. Rock from the mouth 

of the tunnel on the Calendar prospect under the microscope is seen 

to consist predominantly of plagioclase (Ab55 An45 ),. with interstitial 

quartz and orthoclase, each probably under 10 per cent in amount. 

Hornblende and biotite occur in minor quantities. Whether this 

diorite is a marginal phase or an earlier or later intrusion than the 

main batholith was not determined.

CONTACT METAMORPHISM.

The granite batholith and its outliers have invaded a great number 

of rocks of widely different character. The contact-metamorphic 

alteration produced by the invasion is correspondingly diverse from 

place to place, dependent on the original chemical composition of the 

rocks and on the local accession of material during metamorphism.

The simplest form of the contact metamorphism is the marmarization of the limestones hi proximity to the granite contact. This is 

well shown south of Helena, where the Madison limestone has been 

coarsely recrystallized, so that it forms conspicuous snow-white outcrops. It is worthy of note that the basal member of the Madison 

limestone contains a multitude of minute tremolite and tourmaline 

needles, although the matrix inclosing them has not recrystallized. 

The tourmaline-tremolite content extends for over a mile from the 

visible contact. The content of tourmaline and tremolite differs in 

different beds; some contain much delicately fibrous or bladed 

tremolite and no tourmaline; others contain much tourmaline, and 

again others contain both minerals together. Tremolite, however, is 

by far the more abundant.

Along the contact south of Helena the rocks overlying the Quadrant 

quartzite have been extensively recrystallized, forming various kinds 

of biotite hornfels and micaceous quartzite. The metamorphism of 

the andesites has already been described on pages 26-28.

twenty-second Ann, Kept. U. S. Geol. Survey, pt. 2,1901, p. 538.
32 ORE DEPOSITS OF HELENA MINING REGION, MONTANA.

Garnetization has taken place on an extensive scale west and 

northwest of Mullan Pass at distances of over a mile from the exposed 

contact. Thick beds of solid garnet rock-have been produced, the 

garnet crystals attaining a maximum diameter of 1 inch. Associated 

sills of andesite, as already pointed out, are traversed by veinlets 

carrying calcite and crystalline garnet.

At other localities in the region garnet and other silicate minerals 

traverse limestones in veins. They occur along the contact west of 

Nelson Gulch. Here the pure snow-white coarse marble is traversed 

by masses of lime-silicate minerals breaking across the bedding in

200 FEET

FIGURE 2. Contact-altered rocks in Colorado Gulch. 1, Quartz monzonite; 2, biotitic quartzite; 

3, garnet rock; 4, d topside hornstone; 5, marble stratum.

veinlike fashion. Similarly at the Spring Hill mine metamorphic 

minerals are developed locally and abruptly across the bedding of 

thinly stratified limestone.

These facts prove, without the necessity of chemical analyses, that 

an accession of material has taken place during contact metamorphism.

Near Colorado Gulch, west of Helena, contact-metamorphic rocks 

are exposed at the granite contact. Garnetization has proceeded 

continuously to a distance of 350 feet from the granite. At this 

distance the garnet rock merges into a series of thin-bedded dense 

diopside hornstones, aggregating 12 feet in thickness. The hornstones are inclosed between a footwall of black micaceous quartzitQ
MESOZOIC ROCKS. 33

' \

and a hanging wall of coarse white marble 4 to 5 feet wide. The 

strike can readily be followed, as the marble on the hanging wall 

forms a continuous outcrop. Near the granite contact, a,s shown in 

figure 2, the garnetization has spread into the marble, forming a mass of 

brown garnet rock 25 feet thick. The portion of garnet rock that has 

replaced the limestone' is irregularly rilled with discontinuous veinlets 

or blebs of quartz holding large euhedral garnets. A small amount 

of galena and brilliant black sphalerite is intergrown with the garnet 

rock, but on the whole seems to favor the quartzose portions of the 

rock. Enough sulphides are present to have, encouraged the prospector to sink an exploratory shaft.

Under the microscope the garnet rock is found to consist essentially 

of garnet, intergrown with small amounts of diopside, hornblende, 

calcite, and quartz. The cores of the garnet are generally isotropic, 

but the margins show zonal structure and optical anomalies. By the 

immersion method the index of the garnet is found to be 1.77, indicating a variety intermediate between grossularite and andradite. 

The hornstone into which the garnet rock grades along the strike is 

seen microscopically to consist of an intergrowth of a finely granular 

diopside and calcite, with accessory pyrrhotite.

Other examples of contact-metamorphic rocks that carry sulphides 

and oxides in quantities of economic importance are described under 

the contact-metamorphic ore bodies.

AGE OF THE QUARTZ MONZONITE.

The first description of the granitic rock from what is now known as 

the Boulder batholith is given by S. F. Emmons,1 based on reports 

and specimens brought in by special agents of the Census. He recognized its unusual character and pointed out that it "proves to be a 

diorite of somewhat singular character, possessing certain marked 

characteristics. " It was provisionally designated a diorite-granite 

and was thought to represent an eruptive body of Archean age.

Lindgren, who was a member of the Transcontinental Survey of 

1883, termed this intrusive mass the Jefferson granite field, and 

regarded the great eruption as possibly of Jurassic age. He gave a 

brief petrographic description ,of the granite and recognized its 

resemblance to Zirkel's Jurassic granite from Nevada. The coalbearing sediments resting on the eroded surface of the granite at 

Mullan Pass were believed to be probably of Laramie age.2

The later work on this problem has been done mainly by Weed, 

who assigned a post-Cretaceous age to the batholith in 1899.3 His 

final view is that the granite is Miocene in age.4

i Tenth Census, vol. 13,1885, p. 95. 

*Idem, vol. 15,188G,p. 733.

» Weed, W. H., Granite'rocks of Butte, Mont., and vicinity: Jour. Geology, vol. 7,1899, p. 737. 

< Weed, AV. H., Geology and ore deposits of the Butte district, Mont.: Prof. Paper U. S. Geol. Survey 

No. 74,1912, p. 29.

70935° Bull. 527 13 3
34 ORE DEPOSITS OF HELENA MINING BEGION, MONTANA.

The evidence at hand is still not as complete or conclusive as is 

desirable. The youngest rocks into which the granite is intrusive 

are the andesites and latites, which, as shown earlier in this report, 

are probably of Cretaceous age and are definitely of later age than 

the Ellis formation. On the eroded surface of the granite west of 

Mullan Pass rests a series of unconsolidated lignite-bearing sediments, 

possibly of White River (lower Oligocene) age. At other localities 

in this part of Montana beds assigned by Douglass to the White River 

have been found by him to be slightly lignitiferous. If, as seems 

most probable, the rocks at Mullan Pass should prove to be the equivalents of either the Lance or the Fort Union, which are common coalbearing formations in eastern Montana, the period of intrusion of the 

batholith would be certainly of pre-Tertiary age.

The Boulder batholith, covering an area of 1,100 square miles is in 

all probability an outlier of the far greater Idaho batholith, which is 

situated 150 miles to the west and covers an area of 20,000 square 

miles.1 Between the two are many similar batholiths, such as those 

of the Philipsburg quadrangle. These have been found by F. C. 

Calkins 2 to cut rocks of Colorado (late Cretaceous) age. This fixes 

the inferior limit, but the superior limit in that area is not known. 

In view of the foregoing considerations it is concluded that the 

available evidence points to a late Cretaceous age for the Boulder 

batholith. APLITE.

OCCURRENCE AND CHARACTER.

Aplite is a common rock.within the area of the Boulder batholith 

and around its margin. It forms dikes and irregular masses, some 

of. which are measured in square miles. The largest of these masses, 

east of Corbin, is shown on the geologic map (PL I), but other extensive areas of aplite were noted west of Jefferson and on the ridge 

south of Red Mountain. As estimated roughly, the aplite forms 5 

per cent of the surface exposure of the batholith. Although aplite 

is so abundant, pegmatite is rare.

The border of the granite at many places is fringed with aplite, 

usually tourmaliniferous, which lies between the normal quartz monzonite and the invaded rock. Dikes of aplite also penetrate the 

rocks that inclose the batholitri. They are particularly abundant 

near Wickes, where they occur as persistent dikes, up to 200 feet 

in thickness, traversing the andesites.

The aplites are fine-grained granular white rocks composed essentially of orthoclase and quartz, with minor amounts of biotite. Certain coarsely granular masses show a normal granitic texture and 

should be designated alaskite. Under the microscope the typical

i Umpleby, J. B., An old erosion surface in Idaho: Jour. Geology, vol. 20,1912, p. 145. 

> Persona} communication.
CENOZOIC BOCKS. 35

aplites show a. panidiomorphic aggregate of orthoclase, somewhat 

perthitic, arid quartz, with subordinate plagioclase and biotite and 

accessory magnetite, titanite, and apatite. Two analyses of aplites 

from Elkhorn are available.

The' aplites are commonly tourmaliniferous, but it is usually difficult to determine whether the tourmaline is an original constituent 

or was subsequently introduced. At a number of localities, however, 

the tourmaline seems to be clearly of pyrogenetic origin arid not 

connected with fractures or jointing. South of Montana City there 

occurs a considerable body of aplite carrying nodules composed of 

tourmaline and quartz, which are scattered throughout the rock 

with some regularity. The nodules range from a fraction of an inch 

up to several inches in diameter, and from regular spheroids to most 

irregular blebs. The aplite is a snow-white rock, completely devoid 

of biotite, which as a rule is present to the extent of 1 or 2 per cent in 

the other aplites of the region. Under the microscope the aplite is 

seen to be of normal composition and texture; in the tourmaliniferous portion the tourmaline, which is an iron-rich variety, is intergrown in poikilitic fashion, mainly with quartz, but also with some 

orthoclase.

Similar tourmalinic nodules occur in the aplite at Elkhorn and have 

been described by Barrel!.

ORIGIN OF THE APLITES.

Some aplites show intrusive contacts with the incloshig quartz 

monzonite, but others, especially those occurring along the borders of 

the batholith as marginal facies, show blended contact. The intrusive contacts, moreover, are welded and the aplite exhibits little 

evidence of chilling, so that it is clear tnat the aplite was intruded 

while the quartz monzonite was still hot.

Despite the great abundance of aplite in the region, there is a noteworthy scarcity of other sateUitic intrusions, especially of larnprophyric or so-called complementary rocks, which are practically 

absent. This fact leads to the conclusion that the aplite was not 

derived from the splitting up of a single magma into two magmas, 

one salic and the other femic, but that it originated from the quartz 

monzonite magma by a process of fractionation during'the cooling 

and consolidation of that magma. During this process tourmaline 

was concentrated in the aplite magma; by further segregation within 

the aplite magma the tourmaline formed the tourmaline-quartz 

nodules.

CENOZOIC BOCKS. 

TERTIARY SEDIMENTARY ROCKS.

Stratified deposits which are here referred to the Tertiary period 

occur at three localities in the region the Continental Divide west
36 f ORE DEPOSITS OF HELENA MINING REGION, MONTANA.

of Mullan Pass, at Helena, and at Elliston. Those deposits exposed 

at Helena and Elliston belong to the so-called lake beds of western 

Montana.

The deposit near Mullan Pass consists mainly of clays and poorly 

indurated shales carrying some thin beds of low-grade lignite. 

Coarse conglomerate containing limestone and quartzite pebbles 

occurs, but as natural exposures are poor its relation to the rest of 

the deposit could not be determined. To the east the sedimentary 

rocks are apparently overlain by rhyolite. They rest on the eroded 

surface of the northern end of the Boulder batholith and contrast 

strongly with the thermally metamorphosed and garnetized older 

rocks surrounding them.

Some poorly preserved and indeterminable leaves were found near 

the coal exposures. These rocks were referred to the Laramie by 

Lindgren,1 probably on the basis of their lignite content. Coalbearing rocks of as late age as Fort Union (lower Eocene) having been 

formerly assigned to the Laramie, it is possible that the rocks at 

Mullan Pass are of Fort Union age. There is, moreover, a possibility 

that they may be even younger, inasmuch as Douglass has found beds 

of White River (lower Oligocene) age to be slightly lignitiferous, 

although this is uncommon in the so-called lake beds of western 

Montana.

The deposits near Helena consist of a bedded series of conglomerate, 

sands, clays, and tuffs, supposedly of Miocene age. West of Elliston 

is an extensive area of similar rocks, which according to Douglass are 

very probably of lower Oligocene age.2 These beds are usually 

spoken of as lake beds. It is noteworthy that there are two very 

similar sets of lake beds in western Montana one of lower Oligocene 

age and the other upper Miocene lithologically alike and discriminable only by their mammalian fauna.

IGNEOUS ROCKS. 

DACITES.

GENERAL CHARACTER AND DISTRIBUTION.

Dacites, consisting of a bedded succession of lavas, tuffs, and breccias, form a notable accumulation of volcanic rocks in the area immediately west of Wickes, and more especially in the area west of Basin. 

They are present here in great volume and extend southwestward to 

Butte, where they form Big Butte, from which that city takes it 

name. They occur also east of Silver City, in the northern part of the 

region considered in this report.

Although the dacites, when examined in detail, comprise a number of varieties, they display in general a remarkable uniformity.

1 Tenth Census, vol. 15,1886, p. 733.

2 Douglass, Earl, Fossil Mammalia of the White Riverbeds of Montana: Trans. Am. Philos. Soc., new ser. 

vol. 20,1902, p. 244. ' x
CENOZOIC IGNEOUS ROCKS. 37

The commonest variety of dacite is a porous ash-gray rock carrying 

a multitude of plagioclase feldspar arid quartz phenocrysts interspersed with small brilliant black flakes of biotite. The size and 

abundance of the feldspar and quartz phenocrysts is the characteristic feature of the dacites. The feldspars, many of which are an 

inch long, together with the quartzes make up, as a rule, half the 

bulk of the rock.

The dacites are unusually well exposed along the sides of Boulder 

Valley west of Bernice to the mouth of Lowland Creek. Tuffs exposed on both sides of the valley prove that the bedding is horizontal. 

This is the more noteworthy because the flow banding in the lava 

sheets is commonly vertical and certain lavas show a platy structure 

standing at a high angle. Swirl structures are Common. Lava, otherwise massive, in places contains large irregular inclusions of breccia, 

which are enwreathed by a platy parting in the lava.

The section west of Bernice shows that here the dacitic rocks 

attained a thickness of at least 2,400 feet. The surface on which they 

were erupted was one of considerable relief carved in the granite and 

andesite. A section of this .old surface is well exposed in a railroad 

cutting near the Great Northern tunnel, south of Wickes. Here at the 

base of the dacite series occurs a stratum of dacite tuff resting on an 

old stony soil derived from the underlying andesite. The surface is 

of moderate but irregular relief, and the tuff bed conforms completely 

to the underlying topography, proving that the tuff here is a windborne deposit. Locally, as near the Minah mine, conglomerate 

carrying well-rounded cobbles up to 12 inches long, some of them 

dacite but most of them andesite and aplite, is associated with tuff 

and lava sheets and doubtless indicates the operation of fluviatile 

agencies during the eruption of the dacites.

Dikes of dacite traverse the quartz monzonite and the andesitelatite series. At some localities these dikes are remote from areas of 

the corresponding surface rocks. Because of their highly porphyritic 

character such dikes may easily be mistaken for granite porphyry or 

quartz diorite porphyry dikes. The porphyries that followed closely 

the great granite irruption, however, are far more granular than 

the dacites, and they do not resemble the dacites of the region, which 

have their own characteristic appearance. Certain of the dacite 

dikes have chilled margins showing a ropy structure, a feature which 

indicates intrusion comparatively near the surface.

PETROGRAPHY.

As already stated, the dacites are light-colored porphyries inclosing 

phenocrysts of plagioclase, quartz, and biotite embedded in an 

aphanitic groundmass. The feldspar is clear and glassy and is visibly 

striated, so that its plagioclase character is obvious to the eye. The
38 OltE DEPOSITS OP HELENA MINING REGION, MONTANA.

quartz is colorless, differing in this respect from the prevailingly 

smoky character of the quartz in the rhyolites of the region. It varies 

in amount in different rocks, being abundant in some and absent in 

others. Those devoid of quartz would be termed biotite andesites. 

Although such andesitic phases occur at a number of places, they are 

comparatively rare. Biotite is universally present, commonly as 

small thin hexagonal plates, but no other ferromagnesian mineral is 

found. Under the microscope the rocks, which are ideally fresh, 

are found to show essentially similar features in specimens from widely 

separated localities. The ratio of groundmass to phenocrysts approximates 1 to 1. The porphyritic feldspars are andesine, as a rule 

slightly more calcic than Ab^An^. Sanidine is absent from the 

phenocrysts, as shown by the Becke test on all unstriated feldspar 

sections. In one specimen only, from Sugarloaf Mountain (examined because of all the dacites it exhibits the nearest resemblance to 

the rhyolites of the region), were sanidine phenocrysts associated 

with the andesines. The quartz is generally much fractured and 

shows corrosion and embayment. Biotite is present in idiomorphic 

crystals and may be fluidally arranged around the plagioclase phenocrysts. The groundmass ranges in different specimens from glassy 

to, microcrystalline. The microcrystalline form is best developed in 

those rocks occurring as dikes. Where the constituents of the groundmass are determinable, they appear to consist largely of sanidine and 

quartz. Apatite and zircon are present as accessory minerals, but 

magnetite is absent in all specimens examined.

According to the prevailing qualitative classification, the rocks are 

clearly biotite dacites. It has been pointed out by Rosenbusch 1 

that the dacites fall into two rather distinct groups the liparitic 

(rhyolitic) dacites and the andesitic dacites. Mineralogically and 

,texturally the first group is closely related to the rhyolites and is 

characterized by the dominance of biotite and amphibole among the 

femic phenocrysts. The second is in an analogous way related to the 

andesites and is characterized by the. dominance of pyroxene among 

the porphyritic minerals. The dacites of the region are rhyolitic in 

affinity. It is noteworthy perhaps that despite their great development biotite is uniformly the sole ferromagnesian mineral to appear 

as a porphyritic constituent. In the Butte folio these rocks at 

Butte were called rhyolites, but subsequently they have been termed 

rhyolite-dacites.2 The rock at Big Butte is stated to grade from 

rhyolite through rhyolite-dacite to dacite. Apparently there is a 

greater mixture of varieties at Butte than there is farther north. 

The analysis cited is that of a typical dacite.3

1 Eosenbusch, H., Mikroskopische Physiographic der massigen Gesteine,4th ed., vol. 2,pt. 2,1908,p. 999.

2 Weed, W. H., Geology and ore deposits of the Butte district, Mont.: Prof. Paper U. S. Geol. Survey No. 

74, 1912, p. 43. 

8 Idem, p. 45.
CENOZOIC IGNEOUS ROCKS. OV)

AGE OP THE DACITES.

No evidence closely fixing the age of the dacites was found within 

the region. Their general Tertiary age is indicated by the fact that 

they rest on the deeply eroded surface of the Boulder batholith. 

Weed, however, has found that the eruptions were contemporaneous 

with the deposition of lake beds west of Butte. These 'are of upper 

Miocene age.1

RHYOLITES.

GENERAL CHARACTER AND DISTRIBUTION.

A series of rhyolites rests on the eroded surface of the'older rocks. 

Rhyolites are conspicuously developed near Rimini, where they form 

the capping of Red Mountain, whose commanding summit is a prominent landmark for many miles around. They crown the summits 

of many of the neighboring mountains and form, the bedrock of the 

well-known "porphyry dike" country. They occur at Elliston, and 

on the Continental Divide north of Mullan Pass; they form small 

scattered patches east of Helena and' occur in some abundance in the 

mountains east of Clancy.

The relief of the surface on which they were erupted has an extreme 

range of 3,600 feet from an altitude of 7,600 feet on Red Mountain 

down to 4,000 feet in Prickly Pear Valley. At Rirnini the relief was 

2,000 feet in a horizontal distance of less than a mile. Thus it appears 

that the rkyolites were, poured out on a surface as highly accidented 

as the present surface and venose configuration, as shown by the distribution of the rhyolites, was broadly similar to the existing topography. ^ - ,

The rhyolites are mainly lithoidal lava flows, but include some 

breccias and obsidians. They display a great variety of textures 

and colors, including grayish blue, red, pink, and white. Nearly 

universal features are pronounced streakiness and flow banding, 

abundant quartz phenocrysts, commonly smoky, and clear glassy 

sanidine phenocrysts. Dark minerals, such as biotite and hornblende, are not present in most of the rhyolites; some few show a 

scattering development of biotite. Lithophysae occur locally in 

notable abundance and perfection.

A platy or laminated structure parallel to the flow banding is 

common,, causing the rhyolites to break in thin slabs. In places, as 

west of Elliston and on Minnehaha Creek near Rimini, "there is a pronounced columnar structure. The columns are irregular four and 

five sided prisms up to 6 feet in length. Single columns may show 

different sets of flow bands, making widely different angles with the 

same prism edge, a feature proving conclusively that the flow banding is no criterion of the original attitude of the surface on which the

' Weed, W. H., op. cit., p. 46.
4Q ORE DEPOSITS OF HELENA MINING REGION, MONTANA.

rhyelites were extruded. In fact, flowage lamination standing at 

angles ranging from. 0° to 90° was found at various places throughout 

the region.

The thickness of the rhyolite series is not well known, because the 

bedding or order of superposition of the lava flows is not easily recognizable. On Red Mountain they aggregate at least 600 feet and may 

be as much as 1,200 feet.

The rhyolite series consists of rhyolites only; neither dacites, 

quartz latites, nor other transitional rocks are associated with them. 

They differ notably in appearance from the dacites of the region, which 

are of general rhyolitic habit, and in fact have been termed rhyolites 

at Butte. They lack the extraordinarily prominent porphyritic 

development of the dacites, which is expressed by the abundance of 

unusually large phenocrysts of quartz and plagioclase. They lack 

also the biotite, so common in the dacites. Lithophysae, smoky 

quartz crystals, and streakiness are restricted to the rhyolites. 

There is some possibility that the far older latites, which are light 

colored and show pronounced streakiness, especially those in the 

country north of Thunderbolt Mountain and at the headwaters of 

Little Blackfoot River, may be mistaken for rhyolites. However, 

.the latites show no quartz phenocrysts nor do they have the fresh, 

unaltered appearance that characterizes the rhyolites. West of 

Elliston some black glistening basalt carrying large amber-colored 

phenocrysts of plagioclase is found, and this is one of the, few exceptions to the rule that the rhyolite series is free from rocks of other 

types.

PETROGRAPHY.

The rhyolites are light-colored porphyries carrying phenocrysts of 

quartz and sanidine. The quartz phenocrysts average one-tenth of an 

inch in length and the sanidine phenocrysts are perhaps twice that 

size; together they make up between 10 and 20 per cent of the rock. 

The groundmass of the lithoidal rhyolites ranges from material of 

rough fracture to that of enamel-like appearance.

Under the microscope the phenocrysts are found to consist of 

corroded and embayed quartz and idiomorphic sanidine crystals; no 

plagioclase phenocrysts have been detected. The groundmass 

commonly shows a microcrystalline development; flow streaking is 

generally present.

A chemical analysis of a rhyolite from the summit of Red Mountain 

near Rimini is available 1 and is -doubtless typical of the rhyolites 

generally. This rhyolite shows numerous dark smoky quartz and

. ' Cited by F. W. Clarke in Analyses of rocks and minerals from the laboratory of the United States 

Geological Survey, 1880-1908: Bull. U. S. Geol. Survey No. 419,1910, p. 80; the rock has not hitherto been 

described.
CENOZOIC IGNEOUS ROCKS. 41

clear vitreous sanidine phenocrysts inclosed in a groundmass remarkably mottled and streaked. The general color of the rock is gray, 

diversified, by mottlings and bands of various colors, among which 

dark red is especially prominent. Along some of the flow bands the 

rock is porous. Microscopically the quartz phenocrysts are seen to be 

corroded and embayed, whereas the sanidine crystals are comparatively intact. Some of the crystals show that they were broken by 

flowage movement with the cooling lava. Many of the quartz 

crystals are surrounded in optical continuity by aureoles of quartz 

inclosing feldspar. The quartz crystals are commonly much cracked. 

The sanidine phenocrysts are beautifully clear and fresh; optical 

tests show that they are nearly uniaxial. The groundmass is holocrystalline'and eutaxitic, and in places is considerably. pigmented 

with hydrated iron oxide, which is in part a later infiltration along the 

flow bands. Mineralogically the groundmass consists of feldspar 

and quartz; the fabric is different in different flow streaks but is commonly microgranitic and micropoikilitic, with sporadic spherulitic 

growths. Doubtless the groundmass contains much albite, but it 

can not be easily differentiated. Zircon is present as a rare accessory.

 Analysis of rhyolite from top of Red Mountain, Rimini. 

[H. N. Stokes, analyst]

H2 0 above 110°.................. 0.61

Ti02 ..........:................... .17

P205 , MnO, BaO, Li20, Cl....... ..Trace.

SrO, C02 ........................« None.

S03 .............................. .44

Organic matter................... .45

100.17

AGE OF THE RHYOLITES.

The rhyolite series seems, because of its general petrographic homogeneity, to represent an outburst of volcanic activity distinct from 

that of the dacitic eruptions which are known to have taken place 

in upper Miocene time. However, as the rhyolites have not been found 

in contact with the dacites, their relation to the dacites is not known. 

If they were 'not contemporaneous with the dacites, they probably 

represent a somewhat younger period of volcanism, as'they were 

erupted upon a surface broadly similar to that of .the present.

QUATERNARY SYSTEM.

The Quaternary deposits consist of alluvial gravels, sands, and 

silts. Patches of high bench gravels occur at many localities, but on

O;A

A12 0V .............

FeO

MgO.. .............

CaO. ........ ......

Na20... ...........

K,0.. ............

HoO at 110°. .......

.............. 75.30

..........;... 11.95

| 2. 17

.............. .05

.............. .62

.............. 3.09

.............. 4.96

.............. .36
42 ORE DEPOSITS OF HELENA MINING REGION, MONTANA.

account of their small size are not shown on the map (PI. I). Glacial 

deposits were noted at Elkhorn, at Red Rock Creek, and on Little 

Blackfoot River, but because they are of small extent and no special 

study of them was made they have also not been differentiated.

x THE ORE BODIES.

GENERAL CHARACTER.

-The ore bodies of the region, classified according to metallic contents, are principally, silver-lead and gold-silver deposits. These two 

classes include 'practically all the productive deposits of any importance, both past or present.

The silver-lead ore body of the Alta mine, having an accredited 

output of $32,000,000, has been the largest producer; next in order 

comes the Drumlummon, the premier gold-silver mine, with an 

output of $15,000,000.

The deposits, classified according to age of origin, fall into two ( 

distinct groups formed during widely separated periods of mineralization. Both groups, based on this age distinction, include silver-lead 

and gold-silver ores, but heavy lead ores are not common in the lodes 

of the younger set, which are mainly valuable for their content of the 

precious metals.

The. older group of metalliferous deposits was formed subsequent to 

the intrusion of the Boulder batholith and prior to the eruption of 

the rhyolitic and dacitic lavas; the younger group was formed after 

the extrusion of the volcanic rocks. The older deposits are probably 

of late Cretaceous or early Eocene age, and the younger are surely of 

post-Miocene age. A number of the older deposits have been shattered and broken and subsequently recemented by quartz of the 

second period of mineralization, so that composite ore bodies were 

produced. Whether any important addition to the metallic contents 

of the older deposits accompanied the,new mineralization could not 

be determined.

That a long period of time intervened between the formation of the 

older and the younger deposits is indicated by the geologic history of 

the region. The rocks covering the granite were largely removed 

by erosion, the granite was deeply dissected to an uneven surface, 

and on this surface there was erupted a considerable mass of volcanic 

rocks dacitic lavas, tuffs, and breccias to the thickness, locally at 

least, of 2,400 feet. Following this volcanic outburst came the 

second period of mineralization.
OLDER ORE DEPOSITS. 

Primary minerals of the ore deposits.

43

-

Apatite.............................................................

A rgentite. ..........................................................

Calcite.. .............................................................

Chlorite.......................................................;....

Epidote ............................................................

Fluorite....... .....................................................

Gold...............................................................

Opal...............................................................

Pyrite..............................................................

Khodochrosite. .....................................................

Siderite ............................................................

Tetrahedrite........................................................

Tourmaline.........................................................

Earlier deposits (Cretaceous).

X 

X 

X

X 

X 

X

X '

X

X 

X 

X

X 

X 

X 

X 

X 

X

X 

X 

X 

X

X

X 

X 

X 

X

Later deposits (postMiocene).

X

X

'X

X 

X 

X

X

X

X

X 

X 

X

X

X 

X 

X

X

Doubtful.

X

X

X

X

According to the classification of Lindgren, 1 it will be seen that 

many minerals of the earlier group are characteristic of ore deposits 

formed under deep-seated conditions, whereas those of the later group 

belong- to those of shallow-zone origin.

OLDER ORE DEPOSITS (LATE CRETACEOUS P). 

CLASSIFICATION.

The ore deposits of the first period of mineralization are mainly 

silver-lead lodes, but include some gold and some copper deposits. 

They are here classified on a-genetic basis, although this has certain 

practical disadvantages. They are grouped as (1) magmatic deposits, 

(2) contact-metamorphic deposits, and (3) lodes and veins.

The third group embraces most of "the ore bodies of the region. 

This group may be further subdivided on the basis of the predominant 

metasomatic process, in accordance with the classification of Lindgren, as tourmalinic lodes and sericitic lodes. The separation of the 

tourmalinic from the sericitic lodes is somewhat arbitrary, inasmuch

i Lindgren, AValdemar, The relation of ore deposition to physical condition: Econ. Geology, vol. 2,1907, 

p. 122.
44 ORE DEPOSITS OF HELENA MINING REGION, MONTANA.

as the tourmalinic lodes are also sericitic and transitional forms occur 

from highly tourmalinic to nontourmalinic. This subdivision has 

the disadvantage of separating into different groups' ore bodies 

having similar metallic contents. In the following pages, therefore, 

the sericitic veins are not accorded extended treatment, because the 

evidence seems clear that they represent a slightly less energetic 

phase of the tourmalinic mineralization.

MAGMATIC DEPOSITS.

The single representative of this class found within the region is 

in the Golden Curry mine at Elkhorn. The ore body, consisting of 

an intergrowth of pyrrhotite and chalcopyrite with augite, is inclosed 

in quartz monzonite and represents a local differentiation and segregation in the magma.

CONTACT-METAMORPHIC DEPOSITS.

A number of contact-metamorphic deposits occur throughout the 

region but as a rule are of small economic importance. They are 

invariably situated near the contact of limestone with quartz monzonite or a related granitic rock. They consist of a simultaneous 

intergrowth of the ore minerals with various silicates, among which 

an iron-bearing garnet, probably andradite, is usually present. In 

some ore bodies, however, the generation of the ore minerals seems 

to have persisted somewhat longer than that of the silicate minerals. 

Deposits valuable for gold, for copper, and for iron have been found.

The largest ore body of this type is the auriferous deposit of the 

Spring Hill mine, near Helena, which has been worked on an extensive scale. This deposit lies at the contact of a small mass of diorite 

intrusive into the Madison limestone. The ore minerals are pyrite 

and pyrrhotite; in places the pyrrhotite forms solid masses several 

feet in length and width. They are usually associated with a finegrained aggregate of lime-silicate minerals, forming an exceedingly 

hard and tough rock, but are also disseminated in grains and blebs 

through the pure limestone.

Another auriferous deposit of contact-metamorphic origin is the 

Dolcoath ore stratum at Elkhorn, carrying telluride and sulphide of 

bismuth and chalcopyrite intergrown with an iron-bearing garnet, 

diopside, and calcite.

Contact-metamorphic copper ore has been exploited to some extent 

near Elliston, where an oxidized copper ore intergrown with garnet 

and calcite was mined.

Bodies of magnetite ore, associated with andradite and, lesser 

amounts of axinite, have been mined as fluxing material at Elkhorn; 

they carry also minor quantities of copper and gold.
OLDER QBE DEPOSITS. 45

$

An aberrant type of ore deposit is represented by the-Blue Bell 

mine, near Mullan Pass. This deposit consists of a lode of garnet 

rock traversing the -inonzonite and carrying locally chalcopyrite, 

molybdenite, and pyrite. The contact of the garnet rock with the 

inclosing monzonite is blurred and shows gradual transition from 

garnet rock to monzonite. Branch veinlets of garnet break off at 

right angles to the larger veins. The adjoining monzonite shows 

unusual metasomatic alteration, apparent even macroscopically. 

When examined microscopically the most intensely altered monzonite is found to consist of scapolite in broad plates poikiliticallyinclosing garnet. Other details are given under Elliston.

The deposit is in many respects analogous to that at Mackay, 

Idaho, described by Kemp,1 which consists of pipes of cupriferous 

garnet rock inclosed in quartz porphyry. Whatever explanation is 

adopted for the Blue Bell deposit, at least the conclusion is incontrovertible that the garnet molecule was able to migrate and form veinlike deposits under the influence of solutions that carried chlorine, 

as shown by the accompanying scapolitization.

TOUKMALINIC LODES. 

VARIETIES OF ORES.

A striking feature of many of the ore deposits of the earlier group 

is their abundant content of tourmaline. This mineral, as is well 

known, is of comparatively rare occurrence in ore deposits other than 

tin lodes. It is found associated, to some extent, with gold ores at 

various localities and with gold-copper ore, notably in Chile. It is 

therefore a matter of considerable interest that tourmaline is a common gangue and metasomatic mineral in the ores of this region.

The ores include varieties differing widely in mineralogic character 

and metallic content. On this basis three types can be distinguished (1) tourmalinic silver-lead; (2) tourmalinic silver-copper; 

(3) tourmalinic gold.

In connection with the prevalence of tourmaliniferous ore deposits 

in the region it is worthy of note that cassiterite was frequently found 

in the gold placers in early days in Tenmile Creek, in Prickly Pear, 

and in many others. It has nowhere yet been found in its bedrock 

source. The cassiterite was commonly in the form of wood tin, a 

form recognized by German authors as of secondary origin. It is 

probable therefore that the cassiterite was derived from the erosion 

of the oxidized croppings of lodes carrying slightly stanniferous 

sulphides. . 

i Kemp, J. F., The White Knob copper deposits, Mackay, Idaho: Trans. Am. lust. Min. Eng., vol. 38, 

1907, pp. 269-296.
46 OBE DEPOSITS OF HELENA MINING REGION, MONTANA.

TOURMALINIC SILVER-LEAD DEPOSITS. 

DESCRIPTION.

The silver-lead deposits are the most important group of the tourmalinic lodes; they have given rise to the largest number of mines and 

have yielded the greatest metallic output. This is the more noteworthy in that they constitute a group of ore deposits that is unique 

so far. as shown by the literature of economic geology.

They are developed most strikingly at Rimini, where tourmaline 

forms the dominant metasomatic mineral of the ore at certain mines, 

notably the Lee Mountain and the Valley Forge. At Wickes' the 

Alta lode, which has been by far the most productive ore body in 

the region, shows pronounced tourmalinization. Other lodes in the 

Wickes district, such as the Comet and Gregory, are probably end 

members, of the same type of deposit, but in which the tourmalinization was feeble. Finally, the tourmalinic silver-lead type is represented at Elkhorn by the Queen mine.

The deposits at Rimini afforded a better opportunity for examination than those at other localities, so their characteristics will be 

presented in some detail.

The lodes are inclosed in quartz monzonite of uniform composition, 

which is quite similar to that prevailing throughout the Boulder 

batholith. A pronounced system of jointing that trends N. 85° E. 

and dips 80° S. traverses the quartz-monzonite. The joints as a rule 

are widely spaced and are remarkably plane and parallel. Locally 

they are closely spaced, as at the falls of Beaver Creek, above the 

town of Rimini, and the joint faces are coated black with a thin felt 

of tourmaline fibers.

The ore bodies conform in direction to this system of jointing. 

The ore occurs in chambers or shoots scattered through a zone which 

in a few places attains an extreme width of 50 feet. In this zone 

the granite is profoundly altered by sericitization and is strongly 

impregnated with metallic sulphides. Commercial ore consists of a 

heavy sulphide aggregate, composed principally of galena associated 

with sphalerite and pyrite. Arsenopyrite in small amount is a 

ubiquitous constituent; tetrahedrite and chalcopyrite occur rarely. 

The ore is more or less mixed with metasomatically altered granite, 

but not with vein quartz or other gangue material common in fissurefilled veins.

The ores carry approximately 2 ounces of silver to each 1 per cent 

of lead, and gold running as high as $11 a ton in high-grade ore.

The characteristic feature that distinguishes these deposits from 

the usual types of ore body is that they are accompanied along one 

wall or the other by what is termed locally a ledge of black quartz. 

This ledge consists of a coal-black rock composed essentially of an 

intergrowth of quartz and black tourmaline. In places the grain is
OLDER QBE DEPOSITS. 47

so dense that the tourmaline is not discriminable and the rock 

resembles a black jasperoid; in other places the grain is coarser and 

the radial fibrous structure of the tourmaline is readily apparent. 

Another phase of the toiirmalinic rock shows a mottling due to 

small patches of vitreous quartz, and is perhaps of interest because 

of its resemblance to the so-called capel accompanying the cassiterite 

lodes of Cornwall, as determined by direct comparison with a specimen of capel a quartz-tourmaline rock from the 2,800-foot level 

of the Dolcoath mine.

In places the quartz-tourmaline rock is roughly banded, owing to 

the presence of intercalated, slabs of sericitized granite. This banded 

phase indicates clearly how the tourmalinic ledge originated. The 

black tourmalinic bands merge, by diminution in the amount of 

tourmaline, into sericitized granite. The tourmalinization evidently 

proceeded from joint planes in the granite; where the jointing was 

closely spaced or the tourmalinization was intense the granite was 

altered completely and solidly to a quartz-tourmaline rock. In this 

way black tourmalinic ledges were formed, extremely dissimilar to 

the granite from which they were derived, and attained a maximum 

thickness, so far as observed, of 8 feet. It is noteworthy that the 

most productive lodes are accompanied by the thickest ledges, 

although, on the other hand, ore does not occur continuously along 

these ledges. The tourmalinic rock carries a varied amount of 

metallic sulphides, among which arsenopyrite predominates, accompanied by pyrite, sphalerite, and galena. Locally it is brecciated 

and traversed by thin veinlets of quartz and arsenopyrite.

Black jasperoidal tourmaline rock from the Valley Forge mine, as 

shown by- microscopic examination, is composed of quartz and 

tourmaline in approximately equal amounts. The tourmaline is a 

brown variety, and is present in granular, columnar, and acicular 

forms. Other minerals are arsenopyrite, pyrite, sphalerite, galena, 

sericite, and apatite.

The manner in which the tourmalinization proceeded is clearly 

indicated in an open cut on the probable eastern extension qf the 

Lee Mountain lode, in the town of Rimini. A sketch of this is given 

in figure 3. Twenty feet west of 'the open cut a prospect shaft 

encountered some heavy galena ore on this tourmalinized zone.

The ore bodies occur in shoots alongside the tourmalinic ledge 

matter. There is, however, complete though abrupt gradation 

between the tourmaline rock and the ore material. The ore favors 

the sericitically altered granite. A certain definite tendency of the 

sulphides to segregate during deposition is therefore evident, galenarich ore tending to develop most abundantly in the sericitic phase of 

alteration. That the development of the sulphides and the tourmalinization were contemporaneous is proved by the intergrowth of
48 OEE DEPOSITS OF HELENA MINING REGION, MONTANA.

the sulphides with the tourmaline. Another line of evidence is the 

fa.ct that the granite, where traversed by quartz-tourmaline veinlets 

devoid of metallic sulphides, is tourmalinized, sericitized, and impregnated with arsenopyrite, pyrite, sphalerite, and galena. This indicates beyond question that the same solutions that carried the 

tourmaline-producing elements carried also the metallic sulphides; 

it indicates further that there was probably a tendency of the tourmaline to develop slightly ahead of the sulphides.

CHEMICAL PROCESSES.

The chemical processes accompanying the formation of the tourmalinic silver-lead deposits were investigated by a series of analyses 

of rocks from the Valley Forge mine at Rimini.

The normal country rock, taken at 30 feet from the footwall of the 

lode at the portal of the main tunnel, is a comparatively fresh quartz 

mo'nzonite composed of zoned plagioclase near Ab51An49 , orthoclase,

FIGURE 3. Tourmalinized zone in granite, Rimini. 1, Quartz monzonite; 2, pyritized and sericitized 

quartz monzonite; 3, black tourmaline-quartz rock.

quartz, micropegmatite, biotite, and hornblende, with accessory 

magnetite, titanite, apatite, and zircon. Sericite, chlorite, and iron 

oxide occur in small amounts as secondary minerals. Altered quartz 

monzonite, taken from rock adjoining the quartz-tourmaline ledge 

on the hanging wall and selected so as to contain a minimum of tourmaline, is a coarse quartzose rock containing considerable glossy 

sericite in fine scales and pyrite in deeply striated cubes. Arsenopyrite, sphalerite, and galena are present in lesser quantities. Under 

the microscope quartz in large optically homogeneous anhedrons is 

seen to be the dominant constituent; sericite and sulphides are common. A deep-brown iron-rich variety of tourmaline is present in 

small amount. Arsenopyrite is as a rule sharply crystalline; pyrite 

is also well crystallized, but the galena and sphalerite are anhedral. 

Apatite and zircon appear as accessory minerals.

Rock taken from the footwall, and therefore several feet from the 

tourmalinic ledge, consists of highly sericitized quartz monzonite 

impregnated with pyrite, galena, arsenopyrite, and sphalerite. The
OLDER ORE DEPOSITS. 49

sericite forms translucent waxy patches of light yellowish-green 

color. The microscope shows that the component minerals are, in 

order of abundance, quartz, sericite, pyrite, galena, arsenopyrite, 

arid sphalerite and accessory rutile, apatite, and zircon.

Analyses of fresh and altered wall rocks, from Rimini, Mont., and St. Michaels Mount,

Cornwall.

S102.. ......................... .........^.. ......................

AW3 . ........... ..............................................

FeO. ...........................................................

MgO........................ .............. ....................

CaO.. ..........................................................

NasO.. ................:........................................

K2 0.. ..........................................................

H2 0-. .........................................................

H2 0+.. ........................................................

TiOj... ........................................................

C03 . ...........................................................

PsOs-- .........................................................

MnO...........................................................

BaO.................................................... .......

LiaO ...........................................................

BjOa................................................ ...........

F... ...........................................................

PbS.. ..........................................................

ZnS. ...........................................................

FeS2 ...........................................................

Less 0 for F.... ...............................................

1

65.91 

15.32 

2.28 

2.02 

1.52 

3.28 

3.08 

4.80 

.60 

.60 

.59 

.21 

.18 

Trace. 

.10

100. 51

2.651

2

66.70 

13.25 

1.34 

.51 

.30 

.05 

.39 

4.03 

.08 

.37 

.20 

.21 

.12 

Trace. 

None.

.59 

1.86 

5.09 

4.75

99.84

3

66. 02 

14.14 

1.53 

.37 

.67 

.26 

.39 

4.63 

.10 

.48 

.56 

.25 

.17 

Trace. 

.04

.02 

2.02 

.74 

'.72 

6.73

99.84

2.893

4 '

69.42 

15.65 

1.25 

3.30 

1.02 

.63 

.27 

4.06 

.06 

.54

Trace.

.40 

.39

.81

3.36

101. 75 

1.41

100. 34

1. Quartz monzonite, fresh, from Rimini, Mont. J. G. Fairchild, analyst. 

2. Quartz monzonite, altered, from hanging wall, Valley Forge mine, Rimini, Mont. J. G. Fairchild, 

analyst.

it. Quartz monzonite, altered, from footwall, Valley Forge mine, Rimini, Mont. J. G. Fairchild, analyst. 

4. Greisen (with tourmaline and topaz), from St. Michaels Mount, Cornwall. W. Pollard, analyst.

It is apparent from the analyses that the course of alteration has 

been essentially similar in both wall rocks; perhaps the fact is noteworthy, however, that the high arsenopyrite and low galena go with 

the tourmaliniferous phase of alteration. The pronounced removal 

of all bases other than potash and the low combined water lower 

than in the unaltered rock are striking features. The potash is all 

present in sericite,'but the amount of water is far too low to compute 

the sericite as K20.2H2 0.3Al2 03 .6Si02 , the formula usually employed. 

In this respect the alteration of these wall rocks departs from that 

of most sericitized wall rocks, it being the general experience that 

water is commonly in excess of the amount required by the formula 

K2 0.2H2 0.3AJ2 03 .6Si02 . For purposes of comparison an analysis of 

a greisen from Cornwall has been added, which shows a similar 

deficiency of water with regard to the content of potash. 1 But such

examples are rare.

- - .

' Reid, Clement, and Flett, J. S., Geology of the Land's End district: Mem. Geol. Survey England 

and Wales, 1907, p. 59.

70935° Bull. 527 13 4
50 ORE DEPOSITS OF HELENA MINING REGION, MONTANA.

To trace the course of alteration more closely the following computations have been made, based upon the assumption of constant 

volume. They bring out clearly that the net increase of mass, 

amounting to 9 per cent, has been due chiefly to the introduction of 

the metallic sulphides; that there has been a notable gain in iron (as 

pyrite and arsenopyrite), sulphur, lead, arsenic, zinc, and silica; and 

that there has been auheavy loss in all bases except potash, which has 

remained practically unchanged in amount.

Alteration of quartz monzonite from Rimini, Mont.

SiOj...........................

AljOs...... .....................

Fej03 ..........................

FeO............................

MgO...........................

CaO........ ............. .......

NteO............ ...............

KzQ............. ...............

H20.................. .........

H20+..........................

TiO».... .......................

COS -. ..........................

Ps06 ... ........................

BaO... .........................

PbS............................

ZnS. ...........................

VeSi...:. ......................

1

65.91

15.32

2.28

2.02

1.52

3.28

3.08

4.80

.60

.60

.59

.21

.18

.10

100. 51

2.651

2

66.02

14.14

1.53

.37

.67

.26

.39

4.63

.10

.48

.56

.25

.17

2.02

.74

.72

6.73

99.82

0 RQ1

la

174.80

 40.60

6.05

5.36

4.03

8.70

8 17

10 70

1 TO

1.56

.56

dX

.26

264.88

2a

191. 00

40.90

4.43

1.07

1.94

.75

1.13

13.38

1 3"

.72

.49

5.84

2.14

2.08

19.45

288.45

3

+16.2

+ .3

- 1.62

 4 9Q

- 2.09

- 7.95

- 7.04

- + .66

- .20

+ .06

+ .16

+ .01

4

+ 9.28

+ .74

-26.8

-80.1

-51.9

-90.4

-86.2

- i t 1Q

5

+6.12

+ .10

- .61.

-1.62

- .79

-3.00

-2.65

+ .20

+2.20

+ .81

+ .78

+7.33

+8.87

1. Chemical composition of unaltered quartz monzonite.

2. Chemical composition of altered quartz monzonite.

la. Constituents in grams of 100 cubic centimeters of unaltered rock.

2a. Constituents in grams of 100 cubic centimeters of altered rock.

3. Gain or lossln grams of each constituent during alteration.

4. Gain or loss in percentage of original mass of each constituent.

5. Gain or loss in percentage of original mass of the unaltered rock.' o

TOTTRMALINIC SILVER-COPPER DEPOSITS.

The principal representative of the tourmalinic silver-copper type 

is the Blue Bird mine in the Wickes district. The ore at this mine 

consists essentially of tetrahedrite and pyrite intergrown with tourmaline. Sphalerite and galena are rare constituents. The ore contains from 10 to 25 ounces of silver, 3 to 5 per cent of copper, and' 

small amounts of gold.

Pockets of ore, composed of columnar arsenopyrite in a matrix of 

sphalerite and pyrite and carrying 2 ounces of gold to the ton, have 

been found in highly tourmalinized diorite porphyry near the lode. 

A shoot of ore formerly. worked carried 0.56 ounce of gold and 65 

ounces silver a ton, but it probably contained more/ galena and 

arsenopyrite than the ore-now being extracted, which is an argentiferous tetrahedrite intergrown with tournialine.

The bearing of these facts is that the tourmalinic silver-copper 

type of deposit is very probably connected by transitions with the
OLDER QBE DEPOSITS. 51

other types of tourmalinic lodes of the region. That the tourmalinic 

silver-copper deposit of the Blue Bird mine differs from previously 

described tourmalinic ore bodies, notably from the tourmalinic goldcopper type, has recently been pointed out by the Winchells.1

Tourmalinic ore bodies, such as the Eva May and the Bullion, 

which' carry pyrite, tetrahedrite, galena, sphalerite,, chalcopyrite,. 

and arsenopyrite in variable amounts and are both argentiferous and 

auriferous, form transitions between all three types of tourmalinic 

deposits recognized in the region.

TOURMALINIC GOLD DEPOSITS.

The only tourmalinic deposit valuable solely for its gold content 

is the Big Indian, situated in Jefferson County, 4 miles south of 

Helena. Most of the other tourmalinic deposits, however, are 

auriferous to some extent, and in many the gold is an important 

constituent. In the Evening Star ore, a tourmalinic gold-silver-lead 

ore, the gold is stated to have run as high as 3 ounces to the ton.

The Big Indian mine is situated in the quartz monzonite one-half 

mile from the intrusive contact with the sedimentary rocks lying to 

the north. The ore was taken out from a large pit. In the bottom 

of this pit it can be seen that the quartz monzonite is. in places 

extremely sheeted by a system of vertical joint cracks. These sheeted 

zones were loci of tourmalinization and pyritization, and probably of 

auriferous impregnation. The tourmalinized rock is a mediumgrained gray granitoid, which contains about as much tourmaline as 

the unaltered rock contains ferromagnesian minerals approximately 

10 per cent. The tourmaline aggregates are somewhat porous and 

contain some yellowish material of undetermined character. Under 

the microscope the rock is found to consist of plagioclase, orthoclase, 

micropegmatite, tourmaline in irregular granular aggregates, brown 

mica in fan-shaped groups, magnetite apparently in part epigenetic, 

titanite, and apatite.

According to Mr. James Winscott, the discoverer of the deposit, 

approximately SI 10,000 in gold was extracted during five years' 

operations of the old 10-stamp mill formerly on "the property. The 

ore averaged $5 a ton and some clean-ups ran 985 fine. Subsequently 

the property was sold and a 6Q-stamp mill was erected, but this is 

now idle.

ORIGIN OF THE OLDER ORE DEPOSITS.

I

The ore deposits of>the first period of mineralization are closely 

related to the intrusion of the quartz monzonite, as shown by three 

features: (1) Their geologic environment, namely, their restriction

' Winchell, H. V. and A. N., Notes on the Blue Bird mine: Econ. Geology, vol. 7,1912, p. 289,
52 QBE DEPOSITS OF HELENA MINING KEGION, MONTANA.

to the vicinity of intrusive contacts of the quartz monzonite; (2) 

their mineralogy; and (3) the fact that a number of them are cut 

by Tertiary dacite dikes.

They occur either at the margins of the batholith or in remnants 

of the roof under which the quartz monzonite was intruded. The 

largest fragment of the roof that has escaped the erosion and general 

removal of the rocks formerly covering the batholith the large 

remnant of the andesitic roof west of Wickes has been the great 

treasure vault of the region. The most productive mines, to name 

only the Alta, Comet, Gregory, and Minah,. were grouped in this 

fragment of the roof or along its margin.

This geographic restriction of the. ore bodies to the intrusive contacts is not of itself conclusive proof that the deposits are genetically 

related to the intrusion of the quartz monzonite. This is illustrated 

by the Marysville district, where a series of lodes is massed along the 

periphery of a boss of quartz diorite but is not connected in origin 

with the intrusion of the plutonic rock. It is obviously of more 

importance to show that a, close sequence in time exists between 

intrusion and metallization, but that such a sequence exists can be 

shown in the broadest sort of a way only. After the granite had 

consolidated to a considerable depth it was fractured; the ore deposits 

were formed; subsequently the covering rocks, of unknown thickness, 

were stripped off and the granite was deeply eroded; and finally in 

upper Miocene time the dacite lavas were erupted on this surface.

THe geographic restriction of the ore deposits considered in connection with their mineralogy is, however, conclusive proof of their 

genetic dependence on the quartz monzonite intrusion. The third 

criterion, the intersection of an ore deposit by dacite dikes, is not of 

great moment, but it establishes the fact that ore deposits so intersected were formed prior to the second period of mineralization.

The ore deposits, as shown by their abundant content of tourmaline, were formed at a high temperature a condition obviously obtaining near the intrusive contacts at a time shortly after the coming 

to place of the quartz monzonite magma. That tourmaline is an 

index of formation at high temperature is regarded as established so 

firmly that argument in support of the fact is unnecessary.

Although it seems clear, therefore, that the ore deposits are closely 

linked in origin with the intrusion of the quartz monzonite, this 

causal connection may ultimately be due either to (1) the heat furnished by the magma, thus stimulating the circulation of meteoric 

waters and increasing their solvent powers, or (2) to the release of 

metalliferous solutions from the cooling magma, or (3) to combinations of (1) and .(2). The current theory of ore deposits holds that 

the presence of tourmaline, because of its content of boron and fluorine, 

is proof that the deposits in which it occurs were formed by direct
OLDER QBE DEPOSITS. 53

exhalations from a cooling magma. In the ore deposits here considered, as previously shown, the solutions that carried the elements 

of the tourmaline carried also those of the metallic sulphides, and the 

sulphides were formed contemporaneously with the tourmaline.- It 

is therefore probable that the ore-depositing solutions were of magmatic derivation and represent the final manifestation of the intrusive 

energy of the great quartz monzonite invasion.

The tourmalinization and introduction of the ore took place, however, distinctly later than the intrusion of the aplites, some of which 

were profoundly altered by the ore-forming solutions. The quartz 

monzonite magma was therefore not directly the "ore bringer." 

The release of the metalliferous solutions took place at a more advanced state of magmatic differentiation; in fact, at a stage succeeding the intrusion of the aplite.

The formation of the aplite magma, as pointed out already, was 

accompanied, as inferred from undoubted facts observed in the field, 

by an increase in its content of tourmaline to a proportion considerably greater than that in the original quartz monzonite magma. 

The facts observed in the field suggest a tentative speculation. The 

concentration of the tourmaline in the aplite magma brought about 

a further differentiation of which the ultimate product was a highly 

mobile solution, endowed with great migratory powers. The tourmaline-quartz-orthoclase segregations are regarded'as imprisoned and 

congealed globules of this final differentiate. It is noteworthy that 

the composition of these tourmalinic segregations is essentially similar 

to that indicated for the ore-forming solutions by the study of their 

metasomatic activities. Both contained boron, silicon, aluminum, 

and potassium. Presumably the increase of tourmaline in the aplite 

magma was accompanied by a. concurrent increase in the concentration of the metallic constituents. But on this point the field evidence 

fails, for at only one locality was a tourmaline segregation noted to 

carry a sulphide mineral (pyrifce). The phenomena of the ore deposits 

show that the tourmaline and the metallic sulphides traveled together, 

but the field evidence does not show how this association was originally brought about.

SYSTEMATIC IMPORTANCE OF TOURMALINIC SILVER-LEAD DEPOSITS.

As is well known, a sharp distinction was drawn by Frenchxgeologists between the cassiteri.te-bearing veins and the ordinary sulphidebearing veins, the so-called "filons plombife'res." This division into 

two classes is recognized by Vogt 1 as valid, only that here, as elsewhere in nature, transitional forms exist. Certain sych gradations 

are pointed out by him, but in conclusion he states that "galenasilver ore veins carrying tourmaline in abundance are not known."

' Genesis of ore deposits: Special publication Am. Inst. Min. Eng., 1902, p. 665.
54 ORE DEPOSITS OF HELENA MINING REGION, MONTANA.

A review by Stutzer in 1906 of all known tourmaline-bearing ore 

deposits confirms this statement. Eight classes are defined, the 

limits of which are somewhat arbitrary, owing to transitional forms, 

but tourmalinic silver-lead ore deposits do not appear among them. 1 

It is therefore a matter of some theoretic interest to find a series of 

highly tourmalinic /galena-silver lodes genetically related to the 

intrusion of the Boulder batholith. Tourmaline is the most characteristic mineral of the stanniferous deposits; here in Montana it is 

developed in extreme abundance as an accompaniment of silver-lead 

deposits. Moreover, the lodes have been important producers; the 

largest of these, the Alta, credited with a yield of $32,000,000, appears; 

if the stanniferous lodes are excluded, to outrank in value of output 

any known ore body of the tourmalinic class of deposits.

YOUNGER ORE DEPOSITS (POST-MIOCENE). 

GENERAL CHARACTER.

The ore deposits of the postvolcanic mineralization are essentially 

precious metal deposits. They include gold, gold-silver, and silver 

deposits.

Unlike the ore bodies of the older group, the younger ore deposits 

show no significant geologic distribution; They occur generally 

throughout the region and are inclosed in rocks of all ages in the 

dacites andv rhyelites, in the quartz monzonite, and in the sedimentary 

rocks. At certain localities a considerable number of productive 

veins are massed together, giving rise to mining districts of economic 

prominence. Districts whose importance has depended on veins of 

late Tertiary age are Marysville, Clancy, and Lowland Creek. Besides 

the veins of these districts there are a large number of widely scattered 

deposits.

The cause of the localization of the deposits in certain districts is 

not clearly apparent. The most productive portion of the Clancy 

district, that of Lump Gulch, is situated in the heart of the quartz 

monzonite area and at a considerable distance from the nearest 

accumulation of rhyolites; Lowland Creek, which is 14 miles north of 

Butte, is situated in the middle of the great dacite area extending 

southward to Butte; the geologic features do not indicate why there 

should be a grouping of deposits in these localities. On the other 

hand, the localization of the deposits at Marysville seems plausibly 

explicable as due in part to the favoring influence of the contact of 

dissimilar rocks, inasmuch as the deposits are grouped around the 

margin of a quartz diorite stock in the Belt series.

Each of the different districts shows a certain degree of individuality, 

but the general features in common will be pointed out here; the 

detailed descriptions must be sought under the various districts.

i Stutzer, 0., Tunnalin fuhrende Kobalterzgange: Zeitschr. prakt. Qeologie, vol. 14, 1906, pp. 294-299.
U. S. GEOLOGICAL SURVEY

LAMELLAR CALCITE ORE, BELMONT MINE, MARYSVILLE. 

Thin section, parallel light; magnified 30 diameters.
BULLETIN 527 PLATE IV

LAMELLAR CALCITE ORE, BELMONT MINE, MARYSVILLE. 

Thin section, crossed IMicols; magnified 30 diameters.
YOUNGER ORE DEPOSITS. 55

The ore bodies are mainly fissure veins of branching and irregular 

character. At Rimini, however, there are some extensive deposits of 

disseminated low-grade gold ore in the rhyolite. As a rule, the ore 

bodies carry subordinate quantities of sulphides and are worked for 

their content of precious metal alone. A prominent feature of the 

deposits is the prevalence of cryptocrystalline quartz in the gangue 

material. The cryptocrystalline quartz appears in three varieties: 

(1) A dark gray-blue flinty variety, (2) a dense-grained white variety 

resembling porcelain, and .(3) chalcedony. These three modifications, 

however, are likely to be present together in the same deposit, confusedly intergrown and each variety grading into the other. It is not 

uncommon to find veinlets lined with chalcedonic quartz which toward 

the central portion of the veinlets grades into drusy crystalline quartz.

In some deposits, notably in some of those inclosed in the dacites on 

Lowland Creek, the quartz is pseudomorphic after a thinly lamellar 

calcite. This has given rise to a gangue of characteristic appearance. 

The arrangement of the quartz plates is commonly such as to inclose 

irregular pyramidal spaces, which are lined with innumerable small 

quartz Crystals. The plates are also arranged in radiating groups. 

On fractured surfaces the partial overlapping of thin superposed 

plates produces an imbricating structure.

'the pseudomorphic lamellar quartz is developed in extraordinary 

perfection in the veins of Marysville, where in fact considerable 

quantities of unreplaced lamellar calcite still remain. Plates III 

and IV are photomicrographs of a thin section of calcite ore from the 

second level of the Belmont mine, Marysville. The, lamellar habit 

of the calcite and the extreme thinness which the plates attain 

some under 0.001 of an inch are well displayed. Plates V and VI 

show high-grade ore from the same mine. The ore consists wholly 

of quartz, much of which shows a lamellar structure' in the hand 

specimen; even in parallel light the replacement structure is brought 

out by the arrangement of inclusions and is confirmed by the behavior 

between crossed nicols. . ,

METASOMATIC PROCESSES.

The metasomatic alteration of the wall rocks of the late Tertiary 

veins is principally in the nature of a thorough sericitization, accompanied by the introduction of carbonates and locally by the development of chlorite. The resulting product is as a rule, widely different 

in appearance to the eye from the altered wall rocks of the older veins. 

In the wall rocks of the younger veins the feldspars have, as a rule, 

been reduced to chalky-white spots suggesting kaolin aggregates, 

but which prove microscopically to consist of microcrystalline sericite; the wall rock of the older veins are of green glossy appearance,, 

due to the development of sericite flakes sufficiently coarse to be 

distinctly recognizable by the eye.
56 ORE DEPOSITS OF HELENA MINING REGION, MONTANA,

In order to determine the chemical processes involved in the alterations produced by the late Tertiary vein-forming solutions, analyses 

of altered and unaltered rocks adjoining the King Solomon lode 

were obtained. This lode traverses quartz monzonite and consists of 

parallel veinlets of galena, sphalerite, and tetrahedrite in a chalcedonic gangue, locally carrying some siderite. The late Tertiary age 

of the vein is proven by the fact that it is younger than the dacites, a 

dike of which forms the hanging wall of the lode, as described on 

pages 104-105. As the quartz monzonite is similar to that at Rimini, 

the alterations produced during the two contrasted periods of 

mineralization are directly comparable.

The quartz monzonite is remarkably homogeneous within a radius 

of several miles of the mine. It is a coarse-grained granitoid of 

subporphyritic habit, due to the rude development of large sporadic 

phenocrysts of orthoclase, and consists of plagioclase, orthoclase, 

quartz, biotite, and hornblende, with a minute amount of finely 

scattered pyrite. Under the microscope the plagioclase is found to 

be a zoned andesine near Ab59 An41 , although the periphery is more 

sodic, than this; the orthoclase to be coarse anhedral and somewhat 

clouded or "watered" from admixed albite; and magnetite, titanite, 

and apatite to occur as accessories, with a small amount of sericite 

as secondary mineral.

The altered rock, taken from the lode on the 300-foot level, still 

shows the original texture of the subporphyritic quartz monzonite; 

quartz is easily recognizable; the plagioclase is represented by dull 

waxy aggregates of ligfyt yellowish-green color; the dark minerals 

have altered to silvery sericite, and pyrite is present in small amount. 

In thin section it can be seen that the rock has undergone profound changes. Quartz is 'the most abundant constituent. Sericite

forms exceedingly fine grained 'dense aggregates, mainly localized

in the former plagioclase feldspars, which have disappeared entirely. 

Microcline appears as large sporadic phenocrysts, representing 

dynamically altered orthoclase phenocrysts, and microcline and 

orthoclase as recrystallization products are present in abundance. 

In the original rock the orthoclase was mainly in the form of large 

anhedral plates; here it is finer grained and subhedral. Pyrite in 

small crystals has developed in the biotite and hornblende and is 

intimately associated with siderite. Another carbonate, which is 

present as a minor constituent, is probably magnesite, as indicated 

by the chemical analysis. Apatite and zircon appear as accessories. 

The notable mineralogic feature of this wall-rock alteration is the 

selective sericitization that has taken place; the soda-lime feldspar 

has been completely transformed to sericite, whereas the orthoclase,
U. S. GEOLOGICAL SURVEY BULLETIN 527 PLATE V

.*! ' :-.">"

LAMELLAR QUARTZ PSEUDOMORPHIC AFTER CALCITE, BELMONT MINE, MARY5VILLE. 

Thin section, parallel light; magnified 30 diameters.
U. 8. GEOLOGICAL SURVEY BULLETIN 527 PLATE VI

LAMELLAR QUARTZ PSEUDOMORPHIC AFTER CALCITE, BELMONT MINE, MARYSVILLE. 

Thin section, crossed Nicols; magnified 30 diameters.
YOUNGER ORE DEPOSITS. 57

essentially a potassium feldspar, has remained largely intact, or has 

been in part recrystallized, although the admixed soda feldspar has 

been eliminated during the recrystallization.

The mineral composition of the altered and unaltered rocks, computed from the chemical analyses, is as follows:

Mineral composition of fresh and altered quartz monzonite.

Wster (excess) * '

Fresh.

19.86

18.90

28.80

12. 78

8.94

7.26

.70

.59

.31

.13

1.00

.38

99.65

Altered.

42.42

25.58

.31

.45

2.44

.67

26.80

.29

99.02

The specific gravity of the fresh quartz monzonite as determined 

on a rock specimen is 2.714; calculated from the computed mineral 

composition it is approximately 2.712. The porosity is therefore 

probably very small.

The specific gravity of the altered rock as determined on a rock 

specimen is 2.599; calculated from the computed mineral composition 

it is approximately 2.695. The computed porosity is accordingly 

3.6 per cent. The alteration of the wall rock by the vein-forming 

solution has therefore been accompanied by a small increase of 

porosity. .

In the preceding computation the conventional formula of sericite, 

K20.2H2O.3Al2O3 .6Si02 , has been used. The result conflicts apparently with the microscopic diagnosis, which indicates a considerably 

larger quantity of sericite than 26.86 per cent. Probably the use of 

the formula of a less aluminous mineral, corresponding to Sandberger's lepidomorphite, K20.2H2 0.2Al208 .7£Si02 , or of an admixture 

of this mineral would yield a more harmonious result.

The chemical analyses and computations to show the percentage 

gain or loss of the different constituents during metasomatic alteration follow. These computations are based on the assumption that 

the volume changes are zero; that this assumption is fully justified is proved by the fact that the altered wall rock still retains 

the subporphyritic' habit and granitic structure of the fresh quartz 

monzonite. >
58 ORE DEPOSITS OF HELENA MINING REGION, MONTANA.

Comparison of fresh and altered quartz monzonite from the King Solomon mine, Clancy,

Mont.

SIOi... .........................

AlsOg.. .........................

FesOs

FeO... .........................

MgO.. .........................

CaO............................

NajO.. .........................

KjO... .........................

HsO-. .........................

HS0+... .......................

TiOj............................

COS .. ..........................

PS06 ... ......... ................

MnO. ..........................

BaO... .........................

FeSj............................

1

64.49

15149

1.28

2.71

1.89

4.32

4.04

.16

.48

.56

.49

.19

 .07

.06

11

99.80

2.714

2

71.22

15.05

1.54

.33

Trace.

.42

6.99

- .32

1.52

.31

1.50

.08

.05

45

99.78

2.599

la

175.00

42.00

3.47

7.35

5.13

11.72

9.58

10.96

1.30

1.52

1.33

i52

.19

.16

.35

270.58

2a

185.00

39.10

4 00

86

1 09

18.16

3.95

.81

3.90

.28

.13

1.17

258.45

3

+10.0

- 2.9

_ Q A7

- 3.35

4 97

-11.72

- 8.49

+ 7.20

+ 2.65

- .71

+ 2.57

+ .82

4

+ 6.68

- 6.90

-100.0

- 45.5

- 83.3

-100.0

88.6

+ 65.7

+203.8

- 46. 7 

+ 193.2

5

+3. 69

-1.07

1.28

-1.23

1 ^7

-4.32 i 11"°"

+2.65

+ .98

- .26

4. 1(1

-4.23

1. Chemical composition of fresh quartz monzonite. J. G. Fairchild, analyst.

2. Chemical composition of altered quartz monzonite. J. G. Fairchild, analyst, 

la. Constituents in 100 cubic centimeters of fresh rock. 

2a. Constituents in 100 cubic centimeters of altered rock.

3. Gain or loss in grams of each constituent during alteration.

4. Gain or loss in percentage of original mass of each constituent.

5. Gain or loss in percentage of original mass of the fresh rock.

The computations show that there has been a net loss of material 

amounting to 4.23 per cent. There have been heavy gains in silica 

and potash and additions of water and carbon dioxide, but these 

gains are unable to offset the heavy losses in alumina, ferric and 

ferrous iron, magnesia, lime, and soda.

AGE AND ORIGIN OF THE YOUNGER ORE DEPOSITS.

The ore deposits of this group are post-Miocene in age. This 

determination is based on the fact that veins of this group are inclosed 

in dacites on Lowland Creek which are known to be of upper Miocene

age. It is a fairly safe presumption that they were formed relatively 

near the surface, although it is to be noted that if they were formed 

immediately after the cessation of volcanic activity they may possibly 

have originated at a depth of 2,400 feet.

The cryptocrystalline and chalcedonic character of the quartz 

gangue, 'the content of adularia, and the lamellar calcite and pseudomorphic quartz ally these veins on Lowland Creek with those 

common in the late Tertiary lavas in the western mining States. 

Many, such as those at De Lamar, Idaho, 1 and Jarbidge, Nev.,2 are 

identical in mineralogy and structure. Furthermore it is perhaps 

significant that the calcite in the veins from which hot water is 

trickling at Boulder Hot Springs has a lamellar structure; the silica 

in the gangue is cryptocrystalline, chalcedonic, and opaline.

' Lindgren, Waldemar, Twentiett, Ann. Kept. U. S. Geol. Survey, pt. 3, 1900, p. 106. 

«Schrader, F. C., A reconnnlssauce of the Jarbidge, Contact, and Elk Mountain mining districts, 

Elko County, Nev.: Bull. U. S. Geol. Survey No. 497,1912, pp.
YOUNGER ORE DEPOSITS. , 59

Partly because of the striking analogy between the Marysville 

veins and those of Lowland Creek and other late Tertiary veins 

throughout the Western States and partly because of other reasons, 

as detailed under the head of Marysville district, the deposits at 

Marysville are regarded as of post-Miocene origin, although the 

'evidence from a purely local study of the geology at Marysville 

strongly favors the idea that the deposits are genetically related 

to the intrusion of an outlier of the Boulder batholith. It is believed 

in this connection that one of the more important results of the 

present investigation is that it shows clearly the necessity of a broad 

regional study in approaching the problems of ore genesis at any 

one locality.

The true silver veins of the Clancy and Lump Gulch districts 

differ somewhat from the preceding deposits, but their late Tertiary 

age is proved by the fact that they were formed after-the intrusion 

of dacitic dikes. The chalcedonic character of the gangue material 

is in harmony with their post-Miocene age; in fact this feature is 

alone sufficient to differentiate them from the older (late Cretaceous) 

deposits. The study of the wall-rock alterations produced by the 

ore-forming solutions of this group of deposits shows that the solutions were particularly rich in potassium and silica, and carried carbon 

dioxide, sulphur, antimony, lead, zinc, copper, and silver. They 

were strongly reducing and were deficient in iron, calcium, sodium, 

magnesium, and aluminum. The chemical work which they accomplished shows that they were ascending thermal solutions.

As to the source of these solutions the evidence is as obscure as it 

is correspondingly clear for the older or batholithic deposits. These 

deposits were formed from metalliferous solutions expelled from the 

final differentiate of the quartz monzonite magma. But that the 

post-Miocene veins are related to the intrusion or extrusion of any 

igneous rocks is not clear;. That they followed the extravasation 

of the dacites is obvious, but how close this sequence was can not be 

determined. The distribution of the veins shows no significant 

relation to the dacites; if' the veins, are genetically related to the 

dacites it might confidently be expected that an unusual number 

of veins would be grouped around old volcanic vents, where communication was most ready with the underlying-sources of supply. 

So far as the geology of the region is now known the Big Butte at 

Butte is the only recognized vent from which dacites were erupted. 

Nevertheless the ore deposits at Butte were all formed prior to the 

eruption of the dacitic lavas, which are said to cover and conceal 

them. 1 , .

1 Weed, W. H., Geology and ore deposits of the Butte dlstrlct,,Montana: Prof. Paper U. S.'Geol. Survey 

No. 74, 1912, p. 43. ' . '
60 QBE DEPOSITS OP HELENA MINING REGION, MONTANA.

COMPARISON OF THE METASOMATIC PROCESSES OF THE TWO 

PERIODS OF MINERALIZATION.

The ore deposits of the first period of mineralization are clearly 

of high-temperature origin and belong to the so-called deep-zone 

group of deposits; those of the second period the post-Miocene 

deposits belong to the shallow-zone group. As the older deposits 

at Rimini and the younger deposits at Clancy are inclosed in quartz 

monzonite of nearly, similar composition direct comparisons can be 

instituted between the metasomatic processes operative during the 

two periods of mineralization.

In the wall-rock alteration of the first period the quartz monzonite 

has been transformed completely to a quartz-sericite rock carrying 

abundant metallic sulphides; in extreme phases of alteration the 

transformed rock is analogous to a fine-grained greisen carrying 

accessory tourmaline. In the wall-rock alteration of the second 

period the sericitization has been selective; the plagioclase has been 

completely altered to sericite, but the orth.oclase has been unaffected 

by this change. In further contrast to the earlier mineralization 

the introduction of metallic sulphides into the wall rock has been 

insignificant, but water and carbonates have been introduced in 

noteworthy amounts.

In order to facilitate the comparison, certain of the analyses and 

computations are assembled in the table below.

Comparison of fresh and altered quartz monzonite from Rimini and Clancy.

Si02 . .....................................

A1S08 . ....................................

FejOj.....................................

FeO. .....................................

MgO......................................

CaO............................... ..

Na«0................. .....................

K20. .....................................

H20+.. ...................................

H20-. ....................................

TiOj......................................

COt................ .......................

PiOt .... ......... .........................

BaO......................................

MnO. .....................................

PbS.......................................

ZnS.......................................

FeAsS....................................

FeSs......................................

1

65.91

15.32

2.28

2.02

1.52

3. 28

3.08

4.80

.00

.00

.59

.21

.18

.10

100.51

2

04.49

15.49

1.28

2.71

1.89

4.32

3.53

4.03

.16

.48

.56

.49

.19

06

.07

.13

99.80

la

66.02

14.14

1.53

.37

.67

.26

.39

4.63

.10

.48

.56

.25

.17

.04

2.02

.74

.72

6.73

99.82

2a

71.22

15.05

Trace.

1.54

.33

Trace.

.42

6.99

.32

1.52

.31

1.50

.08

.06

.45

99.78

lb

+6.12

+ .10

- .61

-1.62

- .79

-3.00

-2.65

+ .20

+2.20

+ .81

+ .78

+7.33

+8.87

2b

+3.69

-1.07

-1.28

 -1.23

1.57

-4.32

_ O 10

+2.65

+ QO

- .20

+ .95

a. in

-4.23

1.> Fresh quartz monzonite, Rimini.

2. Fresh quartz monzonite, Clancy. -

la. Altered quartz monzonite, Rimini.

2a. Altered quartz monzonite, Clancy.

lb. Gain or loss in percentage of original mass of the unaltered rock, Rimini.

2b. Gain or loss in percentage of original mass of the unaltered rock, Clancy.

The difference between the alterations produced during the contrasted periods of mineralization is readily apparent, but its inter-
MARYSVILLE DISTRICT. 61

pretation is an open question, for part of the difference may be due 

to difference of temperature and part due to difference in the composition of the solutions.

The most striking contrast is that a net increase in mass of 9 per 

cent has been produced by the earlier mineralization and a net loss 

of 4 per cent has taken place during the post-Miocene mineralization. So far as the metasomatic record indicates the early or hightemperature alteration might have been produced by anhydrous 

solutions, for the altered wall rock shows an actual decrease of 

water content, whereas the late Tertiary, a low-temperature alteration, shows a noteworthy increase. Moreover, during the hightemperature alteration no carbonates were formed, but only during 

the low-temperature mineralization.

DESCRIPTIONS OF SPECIAL DISTRICTS,

METHOD OF TREATMENT.

In the preceding pages the region has been treated as a unit. The 

different mining districts, however, differ somewhat among themselves in their geology and in the character of their ore deposits, so 

that for the sake of definiteness and explicitness descriptions of the 

special districts are added. These descriptions are generally brief 

and the broader relations of the geology at any locality and the detailed characterization of the formations must be sought under the 

preceding systematic treatment.

' The districts employed here conform to'no political subdivisions. 

They include the areas easily reached from the natural supply points 

of the region and their boundaries are accordingly somewhat indefinite. In this respect the scheme accords essentially with local usage..

MARYSVILLE DISTRICT. 

LOCATION AND HISTORY.

Marysville is situated in Lewis and Clark County at an elevation of 

5,600 feet, 17 miles northwest of Helena. It is the terminal point 

of a branch from the main line of the Northern Pacific Railway. A 

number of mining camps now idle, including Bald Butte, Empire, 

and Gloster, are tributary to Marysville.

The prosperity of Marysville has hinged largely on the fortunes of 

the Drumlummon mine, the oldest, most steadily operated, and most 

productive property of the district. The Drumlummon lode was discovered in 1876 by Thomas Cruse, who had been placer mining in 

Silver Creek, below the present site of Marysville, and the mine was 

gradually developed by him until 1880, when a 5-stamp mill was 

erected. In 1882 the property was sold to an English company, 

known as the Montana Mining Co. (Ltd.), for $1,500,000. During the
62 QBE DEPOSITS OF HELENA MINING REGION, MONTANA.

operations of this property SI5,000,000 was extracted. In the early 

nineties the property became involved in protracted litigation, which 

extended over some 18 years, and the mine in recent years has been 

worked only spasmodically. Finally the property was sold in default 

of payment to satisfy a judgment of $240,000 rendered against the 

Montana Mining Co. (Ltd.), and was bought in 1911 by the successful 

litigant, the St. Louis Mining & Milling Co. The new owner has 

commenced to .rehabilitate the milling plant, to open the old workings, which had caved badly, and to search for new bodies of ore.

Other notable mines in the district are the Belmont, Cruse, Penobscot, Bald Butte, Empire, and Piegan-Gloster, but all of these were 

idle during 1911 except the,Cruse or Bald Mountain mine.

The total production of the district has been approximately 

$30,000,000 in gold and silver. 1 In 1910 the production was $145,663 

and in 1911 it was $165,832, distributed as follows: Gold, $143,918, 

and silver, $21,914.

GEOLOGY.2

Geologically, the Marysville district is situated on and around a 

small stock of quartz diorite intrusive into limestones and shales of the, 

Belt series. This intrusive mass represents without question an outlier of the Boulder batholith, the main body of which appears at 

Mullan Pass, 7 miles south of Marysville.

- The sedimentary rocks surrounding the quartz diorite have been 

thoroughly metamorphosed' to * exceedingly hard, tough, and densetextured rocks termed horns tones by Barrell. These are made up 

mainly of various silicates, such as diopside, wollastonite, tremolite, 

and biotite, but these minerals as a rule are not visible or discriminable 

by the eye. In local parlance the rocks are somewhat vaguely known 

as slates. They form a zone around the quartz diorite, ranging in

width from one-half mile to two miles, and grade outward from the 

diorite stock into the unaltered siliceous shales and limestones of the 

Belt series.

The quartz diorite is a medium-grained granitic rock composed 

essentially of andesine, quartz, orthoclase, biotite, and hornblende. 

Younger than the quartz diorite are a variety of dikes, some of 

which seem from their petrographic characters to belong to the 

intrusive aftereffects ~of the quartz diorite magma. These minor 

intrusions comprise aplite, pegmatite, and the Belmont diorite 

porphyry, so named because of its noteworthy abundance on the 

slopes of Mount Belmont. The aplites are not important quantitatively and the pegmatites are nearly insignificant, but the diorite 

porphyry occurs in considerable abundance as persistent dikes.

i Weed, W. H., Gold mines of the Marysville district, Mont.: Bull. U. S. Geol. Survey No. 213,190X, 

p. 88.

8 The geology of the district has been carefully described in great detail by Joseph Barrell in Geology 

Of the Marysville mining district, Mont.: Prof. Paper U. S, Geol. Survey No, 57,1907,
MARYSVILLE DISTRICT. 63

Among the more prominent of these is the long dike at the Bald 

Butte mine. The diorite porphyries are rocks of conspicuously 

pprphyritic texture, containing prominent crystals-of andesine feldspar, together with prisms of hornblende and flakes of biotite in lesser 

amounts, in a dark-gray groundmass. The feldspar phenocrysts are 

more numerous and the groundmass more crystalline in those dikes 

near the main mass of quartz diorite; in fact, in some of them the 

feldspar phenocrysts are so thickly massed and the matrix so visibly 

crystalline that the rock closely resembles the quartz diorite. .This 

assumption of granitoid habit by the dikes within the inner metamorphic zone is believed by the writer to be significant and to indicate 

that the diorite porphyry dikes were injected soon after the main 

intrusion of the quartz diorite.

Other dikes whose genetic relations are less obvious are the Drumlummon porphyry and the porphyry sheets of Piegan Gulch. That

there is a possibility that these various intrusives, including the 

diorite porphyries, do not all belong to the same stage of igneous 

activity was recognized by Barrell, 1 but not much importance was 

attached to it. The determination of the age of these dikes, however, would afford an independent line of evidence on the question 

concerning the origin of the Marysville ore deposits.

The'Drumlummon porphyry is principally encountered in the 

workings of the- Drumlummon mine, which lies at the contact of the 

quartz diorite and hornstones. The rock is a grayish porphyry carrying small scattered phenocrysts of plagioclase feldspar inclosed in an 

aphanitic groundmass. These prove under the microscope to be near 

andesine in composition; the dark minerals once present are found 

to have been destroyed by vein-forming solutions, and the groundmass is seen to consist of feldspar and quartz.

The porphyry of Piegan Gulch, so far as is now known, occurs in 

the Marysville area in three sills, a mile north of Gloster, but is 

found in great abundance in other parts of the region, as shown 

in previous pages of this report, where it is termed dacite. The 

porphyry of Piegan Gulch shows phenocrysts of chalky feldspar, 

biotite, and quartz in a dense gray groundmass. Under the microscope the feldspars are found to be a plagioclase near andesine, which 

has been largely altered to calcite. Possibly some sanidine is present. 

The quartz crystals, which are comparatively rare, show strong magmatic corrosion; the biotite phenocrysts are largely calcitized and 

sericitized; the groundmass in which the phenocrysts rest is of 

cryptocrystalline texture. The rock accordingly resembles the dacites 

from surrounding parts of the general region.

The nearest important development of dacites is east of Silver 

City, 7 miles east of Marysville; rhyolites, which are closely related

i Op. cit., p. 49. \
64 OEE DEPOSITS OF HELENA MINING REGION, MONTANA.

to the dacites, occur south and southwest of Marysville, an especially 

large area extending north from Mullan Pass, and dacites or rhyolites 

are associated with the extrusive andesites of Little Prickly Pear 

Creek, a few miles, north of Marysville. It is therefore not surprising 

that isolated dikes of this character should occur within the district, 

although, as pointed out elsewhere, it is difficult to distinguish isolated 

dacite dikes from quartz diorite porphyry dikes, and the study of the 

local geology would afford no clue to their origin.

The dacites found at Marysville show the hydrothermal alteration 

characteristically associated with the late Tertiary veins, and this 

fact suggests, although it does not prove, that the principal mineralization took place in late Tertiary time.

ORE DEPOSITS. 

GENERAL FEATURES.

The ore deposits are steeply dipping fissure veins carrying gold and 

silver. They occur around the border of the quartz diorite, and 

although some are situated in the marginal portion of the diorite, 

most of the veins are inclosed in the metamorphic hornstones. None 

occur in the central part of the diorite'area. This massing of a considerable number of productive ore bodies around the diorite is the 

cause of the economic importance of the Marysville .district.

The veins average near 1,000 feet in length; the Drumlummon, 

which is the longest, was developed for a distance of 3,000 feet along 

the strike. The-maximum width of the ore bodies is 40 feet, but 

the average is considerably less than this, in most of the veins being 

probably less than 6 feet. The ore occurred in shoots in which the 

high-grade ore was found above the 200-foot level; none of the mines,

except the Drumlummon, are developed deeper than 500 feet below 

the surface. Gold is the predominantly valuable metal and the ores 

range between $10 and $20 a ton.

The gold is finely divided and not visible. In the ore of many 

mines the Belmont, Empire, and others it is unaccompanied by 

sulphides or by insignificant quantities only. In the Drumlummon 

it is accompanied mainly by tetrahedrite and chalcopyrite, and in 

the Bald Butte mine by pyrite, sphalerite, and galena. The gangue 

material is of highly characteristic composition and structure. It 

consists of quartz, commonly of lamellar habit, and calcite in broad; 

thin lamellae. At the Bald Butte mine fluorite occurs, but this is exceptional, for fluorite has not been noted at any other mine in the district. In many of the typical ores of Marysville the calcite lamellae 

attain great breadth and extraordinary thinness, at the Empire mine, 

for example, the extreme diameter being 6 inches and the thickness 

only one-fiftieth of an inch, The calcite plates are common!}?
.MARYSVILLE DISTBICT. 65

arranged so as to form rude tetrahedral or other irregular pyramidal 

forms. The quartz is a sugar-textured white variety, commonly 

pseudomorphic after lamellar calcite. This gives rise to hollow 

tetrahedral spaces, which are lined with small clear glassy pyramids 

of quartz. In places the quartz is pseudomorphic after thin curved 

lamellae of calcite, and a foliated or schistose structure results. 

Rarely the quartz shows a delicate amethystine tint.

The calcite carries some iron and manganese and in consequence 

the outcrops of some of the veins are highly manganiferous. Probably this prevalence of manganese is in part responsible for the enrichment in gold of the upper zone of the Marysville veins. Impoverishment of the outcrop, however, as demanded by Emmons's theory,1 

has not taken place.

The lamellar quartz and calcite form a striking gangue material. 

It is identical with the gangue of the gold-silver veins in the Miocene 

dacites on Lowland Creek and resembles the calcite gangue in the 

veins from which hot water is oozing at Boulder Hot Springs. Further, it resembles the lamellar quartz ore of the De Lamar mine, 

Idaho, described by Lindgren in his paper on the gold and silver 

veins in Idaho, and well illustrated in Plate XXVIII of that report.2 

Similar ore occurs also in many of the late Tertiary veins in Nevada, 

Utah, Arizona, New Mexico, and Washington.

ORIGIN.

The study of the ore deposits in the area of the Boulder batholith 

has shown that they fall into two groups, whose age with reference 

to the rhyolites and dacites resting on the eroded surface' of the granite has been determined absolutely. The older group is of prevolcanic origin and is genetically related to the intrusion of the granite; 

the younger group is of postvolcanic origin. Each group carries a 

distinctive set of features generally sufficient for the determination 

of the relative age of a particular ore deposit even where the relation 

of the ore deposit is not absolutely determinable owing to the 

absence of the volcanic rocks.

So far as the geology of the Marysville district is now known, the 

age of the productive ore deposits can not be conclusively established with reference to the Tertiary volcanic rocks. Possibly the 

productive deposits may prove to be the equivalents of the barren 

veins north of Little Prickly Pear Creek, which are known from undisputable geologic evidence to be of late Tertiary age,3 but the matter 

has not been investigated adequately with this point in view. On the

1 Emmons, W. H., The agency of manganese in the superficial alteration and secondary enrichment of 

gold deposits in the United States: Trans. Am. Inst. Min. Eng., vol. 41,1910.

2 Lindgren, Waldemar, Twentieth Ann. Rept. U. S. Geol. Survey, pt. 3,1900.

8 Barrel!, Joseph, Geology of the Marysville mining district, Mont.: Prof. Paper U. S. Geol. Survey 

No. 57,1907, pp. 112-113.

70935° Bull. 527 13 5
66 ORE DEPOSITS OF HELENA MINING EEGION, MONTANA.

other hand, the criteria for the recognition and discrimination of 

the younger deposits from the older group point unmistakably to the 

conclusion that the Marysville veins belong to the younger or late 

Tertiary deposits. This conclusion, following inevitably from a 

regional study of the ore deposits of the Boulder batholith, is opposed 

to the conclusion arrived at by Barrell from an intensive study of the 

local geology.

According to Barrell 1 the Marys ville veins 

as shown by their structural and mineralogical features, are due to contraction effects 

on the margins of the granite mass, at that time recently .solidified and cooling.

The structural evidence consists of the relation of the veins to the vicinity of the 

contact, or at least to the metamorphic zone; the fact that their courses are either 

approximately parallel or at high angles to this contact surface; the parallel and 

branching character of the fissures; the generally shattered and infiltrated character 

of the walls; the dying out of the vein fissures, many of which are continued in parallel 

lines off set to one side; and, lastly, the absence of marked throw along the fissure planes.

The mineralogical evidence is fpund most clearly in the study of the vein fillings 

and alteration-of the walls in the Bald Butte mine, since it is believed that fluorite, 

which is there a characteristic mineral, is indicative of magmatic emanations, 

though the deposition may take place under hydrothermal as well as under pneumatolytic conditions. Fluorite is often found in the contact aureole of granitic masses 

and is presumably given off as acidic vapors in water solutions from solidifying magmas. The probability of this view of the origin of the vein fissures is seen when the 

effects of such contraction from cooling are considered in some detail. The central 

portions of a granite intrusion the size of the Marysville batholith would cool slowly 

and as a whole; the contraction would be equal throughout; and there would be little 

or no tendency to form wide contraction cracks in any direction. This agrees with 

the absence of vein fissures in the center of the intrusion. The cooling of the zone 

of contact metamorphism and of the marginal portions of the batholith, however, 

would result in a shrinkage away of those portions from the surrounding rocks, producing fissures parallel to the contact; while the lessened circumference of the outer 

zone, caused by its cooling before the center, would tend to set up another set of 

fractures more or less radial in nature.

That the veins are massed around the intrusive contact of the quartz 

diorite is a strong argument in favor of their genetic dependence on 

the intrusion of the igneous rock. But the argument of itself is not 

conclusive. The opening of the fissures may have been long posterior 

to the consolidation of the diorite; it is well known that the contacts 

of dissimilar rocks are favorable loci for dynamic action and therefore 

the deposits may have been localized in the brittle hornstones because 

of their superior frangibility. "The parallel and branching characT 

ter of the fissures; the generally shattered and infiltrated character of' 

the walls; the dying out of the vein fissures, many of which are continued in parallel lines offset to one side; and, lastly, the absence of 

marked throw along the fissures planes," can hardly be accepted as 

proof of ,the contractional origin of the Marysville fissures. These 

structural features are of common occurrence in late Tertiary veins 

which cut surface lava flows and are therefore as indicative of fraclOpiCit-.p. 105.
MABYSVILLE DISTRICT. 67

turing under light load as they are of contraction during the cooling 

of plutonic rocks. In the region considered in this report all these 

structural features are found in the post-Miocene veins on Lowland 

Creek, as is well exemplified in the Ruby mine, described on pages 

125-127. 

Says S. F. Emmons: 1

Some writers on ore deposits speak of vein fissures as sometimes resulting from contraction, but I have yet to learn of a well-authenticated instance. I regard a certain 

amount of movement as necessary to break the cohesion between the respective walls 

of a joint or fissure sufficiently to make a water channel. This I hold to be true also 

of eruptive contacts. * * * It is to be noted that a contraction fissure could not 

extend from one rock into another.

Yet the continuity of the Marysville veins across intrusive contacts, 

as exemplified in the Belmont, Cruse, and Drumlummon mines, is a 

salient characteristic of many of the ore bodies. Further, numerous 

mechanical features, such as brecciation and faulting all prior to the 

filling of the veins indicate considerable movement, even if only 

of oscillatory character, during the opening of the fissures. These 

facts would seem to place the burden of proof on one who maintains 

the contractional origin of the Marysville fissures.

The mineralogic evidence adduced by Barrell seems unfortunately 

chosen. The Bald Butte mine, the only mine in which fluorite is 

found, is not characteristic of the mines that have yielded 95 per cent 

of the production of the Marysville district. Nor is fluorite necessarily indicative of magmatic emanations, and its presence assuredly 

does not prove that veins' containing it are genetically related to 

plutonic igneous rocks. It occurs in the late Tertiary veins of New 

Mexico and is even now being deposited by the hot springs at Ojo 

Caliente.2 According to Lindgren, fluorite is a persistent mineral 

and is of little value in determining the genesis of a deposit. Its 

presence is therefore not incompatible with the late Tertiary origin 

of the veins at Marysville; nevertheless it can be conceded that the 

Bald Butte ore is of magmatic derivation without affecting the argument in regard to the origin of the majority of veins of the district.

The characteristic veins of Marysville, comprising those that 

yielded 95 per cent of the production, have a gangue identical in structure and composition with that of the veins in the dacites of Lowland 

Creek; that is with the veins of proved post-Miocene age. This striking gangue, the lamellar calcite and pseudomorphic lamellar quartz, 

is of wide occurrence in the late Tertiary deposits of the Western 

States, and this circumstance indicates that peculiar conditions were 

responsible for this type of ore. The widespread condition common to 

all the deposits was probably deposition near the surface. For De 

Lamar, Idaho, Lindgren has estimated that the deepest ore bodies

i Suggestions for field observations of ore deposits: Min. and Sci. Press, vol. 95,1907, p. 18. 

 Lindgren, Waldemar, Graton, L. C., and Gordon, C. H., The ore deposits of New Mexico: Prof. Papev 

U. S. Geol Survey No. 68,1910, pp. 70-73.
68 QBE DEPOSITS OF HELENA MINING KEGION, MONTANA.

were formed at a depth of 700 feet.1 Further confirmatory evidence 

is the fact that the Marysville quartz is in general exceedingly dense 

or porcelainoid in texture, agreeing thus with the younger veins 

rather than the older, which are characterized by coarse-grained 

quartz. Then, too, the metasomatically altered wall rocks, when directly comparable, resemble those accompanying the younger deposits. Tourmaline, which is so conspicuously developed in most of 

the older deposits of the Boulder batholith, is entirely absent; so also; 

is that coarse sericitization which yields a macroscopic sericite of 

glossy habit. On the other hand, the feldspars have been reduced 

to chalky spots of cryptocrystalline sericite, and chloritization and 

calcitization, both of which are absent from the older deposits, are 

common alterations. In short, the Marysville deposits are in no way 

like those of the older group; on the other hand, they closely resemble 

those of the younger group.

It may, perhaps, be argued that the Marysville veins are nevertheless genetically related to the intrusion of the quartz diorite, but represent a. later phase of the same mineralization to which the older 

group.of deposits belongs. A strong objection to this argument from 

a theoretical standpoint is the fact that the Marysville veins were 

first filled with lamellar calcite and that this was subsequently 

replaced by quartz. This sequence of mineralization is in direct 

contravention of the sequence of magmatic emanations, according to 

which the carbonate stage should come last and not first.? The 

veins show internal evidence of two phases of mineralization, both 

similar to those shown by the late Tertiary veins, but neither is analogous to any phase of mineralization of the older deposits, and this 

objection, based upon field evidence, seems conclusive against the 

argument that the Marysville veins represent a final stage of the older 

mineralization.

DESCRIPTIONS OF MINES.

DKTJMLtJMMON MINE.

The Drumlummon mine is worked principally through the Maskelyne tunnel, a 1,200-foot crosscut intersecting the lode at a depth of 

400 feet. At the intersection a shaft was sunk to the 1,600-foot level, 

but it is now filled with water. The vein has been developed along 

the strike for a distance of 3,000 feet, but because of caved stopes 

most of this part of the vein was inaccessible during 1911.

Much of the material filling the caved stopes is said to constitute*lowgrade milling ore, and it is now being drawn off and sent to the mill. 

Twenty stamps of the old 60-stamp mill have been put in working 

condition by the new owner, the St. Louis Mining & Milling Co., and

i Twentieth Ann. Kept. U. S. Geol. Survey, pt. 3,1900, p. 165.

sClarke, F. W., The data of geochemistry, 2d ed.: Bull. U. S. Geol. Survey No. 491,1911, p. 275.
MAKYSVILLE DISTElOT. 69

the cruslilng of ore was commenced again in July, 1911. In addition 

to the ore taken out through the Maskelyne tunnel, surface ore Was 

being mined through the Roadside tunnel on the Nine-Hour claim, 

which is situated on the southwest end of the Drumlummon vein. 

This ore" is stated to carry from $35 to $80 a ton and constitutes, 

therefore, shipping ore, which is sent to the smelter at East Helena.

The total production of the mine has been $15,000,000, of which 60 

per cent was in gold. The surface ores were of high grade, probably 

comparable to that now taken from the Nine-Hour claim, but the ore 

mined during the later years of the operations of the Montana Mining 

Co. was of medium grade only, containing one-half ounce of gold and 

from 7 to 12 ounces of silver a ton.1

The Drumlummon vein parallels the contact of the granite and 

hornstones, having a general trend of N. 15° E. and steep easterly dip. 

Extensive pits due to caving mark the course of the vein on the surface and show clearly that the wall rocks consist mainly of densegrained banded hornstones, which are cut irregularly by granitic 

dikes and by small aplite dikes. The aplite dikes are shattered and 

in places are mixed with the general country rock. The hornstones 

are the metamorphic equivalents of the banded phase of the Helena 

limestone of Algonkian age, and when examined microscopically are 

found to consist essentially of diopside and wollastonite, which explains why these so-called slates prove so extraordinarily difficult to 

drill. Porphyry dikes are also encountered in the mine, as. discussed 

by Barrell.2

As seen on the 400-foot level, the Drumlummon vein has a welldefined footwall, strongly wavy and heavily corrugated. The cprrugations trend vertically, and the striae and surface protuberances 

show that the hanging wall moved downward on the footwall. In 

places a layer of attrition product 1 to 1^- feet thick rests on the footwall; in places there are a number of superposed false walls above 

what may be termed the real or ultimate footwall. The hanging 

wall of the vein consists of a well-defined plane, but the country rock, 

as shown by a short crosscut south of the shaft, is more or less interlaced with quartz-calcite veinlets to a distance of at least 10 feet. 

In places the fissure continues, but the filling of the vein consists of 

barren detritus from the wall rocks. In the south end of the vein, 

according to J. E. Clay ton,3 "the quartz gangue gives out entirely 

and the filling between the walls is nothing but crushed shale and 

abraded material from the walls of the fissure." This feature is particularly well shown on the Nine-Hour claim, where numerous open 

cuts have been made during the progress of litigation to show the

1 Bayliss, R. T., Trans. Am. Inst. Min. Eng., vol. 26,1897, p. 33.

2 Geology of the Marysville mining district, Mont.: Prof. Paper U. S. Geol. Survey No. 57,1907, p. 108. 

»Eng. and Min. Jour., vol. 46,1888, p. 106.
70 OEE DEPOSITS OF HELENA MINING KEGION, MONTANA.

continuity of the vein. The fissure, which is some 25 feet wide, is 

barren here that is, devoid of quartz and the filling consists of 

broken country rock, more, or less stained with iron oxide. The footwall is marked by a strongly slickensided plane. All these features 

indicate that powerful forces were operative during the formation of

the Drumlummon fissure and prove beyond question that its opening 

was not due to contraction produced by the cooling of the surrounding rocks after the intrusion of the granite.

The New Castletown and Old Castletown veins are branches of 

the Drumlummon vein, as shown in figure 4. The North Star is 

an intersecting vein. According to Clayton it is older than the 

Drumlummon; according to Barrell it is younger.
MARYSVILLE DISTRICT. 71

The ore occurred in pay shoots, which in places expanded to a 

maximum width of 40 feet, but on the whole averaged 20 feet in width. 

The shoots pitched obliquely "to the south within the plane of the 

vein, the direction being determined by the structural features of the 

vein. The wavy character of the walls causes the vein to swell and 

pinch along the strike and on the dip, and the expanded portions of the 

vein are found farther south on successively lower levels. The cause 

of this southward pitch of the ore shoots is clearly explained by 

Clay ton i 1

The shifting of the hanging wall has been nearly straight down the elope of the underlie, and the curvatures, or wave lines of the fissure, pitch down obliquely to the south, 

corresponding generally with the pitch of the ore shoots in that direction.

The rich ore of the mine occurred mainly above the 200-foot level, 

and an impoverishment of the ore shoots began to be noticed below 

the 400-foot level. Good ore, however, was found down to the 1,000- 

foot level, but none, according to Weed, occurred below that level. 

The data at hand are insufficient-to prove whether this decrease in the 

grade of the ore in depth is due to secondary enrichment or to primary 

vertical distribution; the facts seem to point to a combination of 

both processes.

The surface ore now being taken out on the Nine Hour claim is 

thoroughly oxidized. It shows considerable azurite, together with 

oxides of iron and manganese in a quartz gangue, and is rich in fine 

gold, although this is not visible. The primary sulphides in the Drumlummon ore are tetrahedrite and chalcopyrite in rather sparse 

amounts. Pyrite,, galena, and sphalerite are rare. The bonanza 

ore formerly worked doubtless contained silver sulphantimonides, 

such as pyrargyrite, but tetrahedrite (variety freibergite) appears 

to have been the most important constituent. The gangue material 

is a fine-grained saccharoidal white quartz, in places so dense as to 

resemble porcelain. The texture is rather drusy and vuggy, and the 

vugs are lined with quartz, calcite, and dolomite crystals. Fragments of altered wall rock are common in the quartz. For example, 

good ore from the 1,000-foot level consists essentially of angular 

fragments of wall rock cemented by calcitic quartz of drusy structure; sparse chalcopyrite is the only visible metallic mineral. Calcite, somewhat ferriferous and manganiferous, is prominent in various 

parts of the mine, and shows the lamellar habit and tetrahedral 

arrangement of the lamellae so characteristic of the Marysville ores. 

Quartz that is pseudomorphic after calcite is found, but appears not 

to be as common as in other mines of the district.

Primary ore was seen in place on the 400-foot level, 50 feet south 

of the shaft. This ore is a somewhat calcitic quartz carrying tetrahedrite and chalcopyrite; the adjoining hanging-wall rock is a firm,

i Op. cit, p. 107.
72 QBE DEPOSITS OF HELENA MINING BEGION, MONTANA.

hard granite (or, accurately, quartz monzonite), which under the 

microscope proves to be highly calcitized and chloritized. At 150 feet 

north of the shaft the wall rock, which is here the banded horns tone, 

is extensively shattered by postmineral fracturing and leached by 

sulphate solutions. So thorough has this leaching been in places that 

all the bases have been removed, leaving a white rock of low specific 

gravity which consists essentially of silica. These altered wall rocks 

show an abundant development of manganese dendrites.

The calcitic nature of the primary- ore of the Drumlummon vein 

precludes the possibility that secondary enrichment, whether of gold, 

silver, or copper, has extended downward to any considerable depth.

CRTTSE MINE.

The Cruse or Bald Mountain mine, as it is known locally, together 

with the West Belmont mine, which is worked through the Bald 

Mountain adit, was the most active property at Marysville in 1911. 

Seventy men were employed until July lj when a considerable reduction of the force took place. A 20-stamp mill is on the property; 70 

per cent of the gold and silver is recovered on the plates, and the 

tailings go directly to a cyanide plant.

The mine is situated at the contact of the granite and hornstones, 

which form a capping resting horizontally on the intrusive granite. 

It is developed by three tunnels drifted on the vein; tunnel No. 1 is 

the working adit of the mine. The maximum depth reached anywhere in the mine is 500 feet. The vein trends east and west at right 

angles to the contact and dips 70° S. The two tunnels above No. 1 

are in the hornstones; No. 1 commences in granite but encounters 

solid hornstone at 1,000 feet from the portal, although some minor 

masses of hornstones are inclosed in granite at some distance from the 

main contact. The 100-foot level below tunnel No. 1 is entirely in 

granite. The main ore shoot of the mine was in granite and was from 

5 to 30 feet wide and 500 feet long. Along the strike, as seen in 

tunnel No. 1, the vein 'becomes abruptly narrow on entering the hornstone and tends to split and fork. The upper portion of the vein was 

inclosed in hornstones, which show pronounced evidence of oxidation 

and leaching by downward-moving solutions. The ore shoot did 

not come to the surface through this highly altered zone, and this 

fact, together with the decrease in value of the ore in depth, suggests 

that a secondary enrichment of gold has taken place.

The West Belmont vein, which is worked through the Bald Mountain adit, resembles the Bald Mountain vein. The ore consists of 

quartz intergrown with manganiferous calcite in large thin plates. 

The quartz in part is pseudomorphic after lamellar calcite; black oxide 

of manganese is more or less common, and the ore is therefore like 

that characteristic of the Marysville district.
MAKYSVILLE DISTRICT. 73 

BELMONT MINE.

The Belmont mine, situated on the east slope of Mount Belmont, 

has been one of the largest producers, but was idle-during 1911 and 

not accessible. According to Barrel!,1 the mine 

has been developed on a system of veins running, in general at a high angle to the contact surface. What is known as the main or south vein, running from west to northwest, forks to the west into two main branches which diverge at a slight angle, and the 

southern of these branches splits again. The north vein begins at a point about 100 

feet north of the main vein, bearing at first northwest and then nearly north. This 

vein, as shown on the map of the second level, tapers out into a fork at the east end 

and passes into a barren fault to the northwest. The dip of the vein is toward the 

main vein at an angle of 70° to 75° The veins are very short, and the ores are found 

only~from the fourth level to the surface. They do not continue across the large dike 

or porphyritic hornblende-quartz diorite which separates this arm of hornstones from 

the main sedimentary body, but, as mentioned above, show a disposition to branch 

a,nd taper out. Where the branches .separate the country rock near the junction, 

shows considerable brecciation with infiltration of quartz veinlets, and many such 

parallel quartz seams may be noted in the granite in association with the main vein, 

even where the latter does not branch. In the hornstones the vein matter is more 

sharply limited to the principal fissures, but these have an aggregate width as great 

as in the granite, indicating no greater opening of one than of the .other. Within the 

mineralized zone the average aggregate width of vein quartz is estimated at about 3 

feet per 100 feet of granite. On the fifth level the vein quartz shows widths of 3 to 

5 feet but contains many opening vugs and is almost barren.

A series of east-west normal faults with dips to the south, later than the veins, in 

some places follow the strike and in others cut across the veins. These ore deposits 

have probably received considerable secondary enrichment in which the fault fissures 

have taken part.

Exceptional ore from the main level consists of fine-grained quartz 

which shows traces of lamellar structure and is traversed by manganiferous films. Some gangue material consists almost entirely of 

white lamellar calcite; other specimens show an equal admixture of 

calcite and quartz, and such material under the microscope reveals 

the remarkably perfect replacement of calcite by quartz. Comby 

amethystine quartz is also found.

BALD BTTTTE MINE.

The Bald Butte mine is on the'northwest slope of Bald Butte near 

the head of Dog Creek. A mill of 20 stamps belongs to the property, 

but is situated 1 mile downstream from the mine, necessitating 

haulage of the ore by teams.

The mine was operated continuously from 1890 to 1902, intermittently since that time, and was idle during 1911. The total production 

is two and three-fourths million dollars, of which approximately 90' 

per cent was recovered in. the bullion and 10 per cent in the concentrates. The average yield on the plates was $10.11 a ton and the

1 Geology of the Marysville mining district, Mont: Prof. Paper TJ. S. Geol. Survey No. 57,1907, p. 109.
74 ORE DEPOSITS OF HELENA MINING EEGION, MONTANA.

average fineness of the bullion was 753. The concentrates carried 

about 4 ounces of silver to each ounce of gold and averaged roughly 

10 per cent of lead. v

The workings comprised a number of surface pits and a series of 

tunnels, of which No. 2 has been the main working level in recent 

years. The extreme depth thus attained is 500 feet. Many of the 

drifts are inaccessible and no ore in place was seen. The workings 

are associated with a diorite porphyry dike, nearly vertical in attitude, 

and 60 feet wide, as shown in crosscuts. -In places the dike is cut 

by a closely spaced system of veinlets, but this mineralized porphyry 

does not constitute ore, at least upon the lowermost level, where it 

assays under $1. The veinlets are dense-textured aggregates of 

quartz and fluorite, showing a faintly banded structure, and commonly 

containing a small amount of fine flaky molybdenite. Shoots of ore 

were found alongside the dike in places and in the adjoining horns tone 

country rock on its northeast side. Here was found a highly irregular system of short narrow fissure veins. The "Knifeblade," so 

named because of its extreme narrowness, was extraordinarily rich 

in free gold. Associated metallic minerals were pyrite, galena, and 

sphalerite.

PENOBSCOT MINE.

The Penobscot mine, which is one of the best-known mines in the 

history of Montana on account of the extraordinary richness of its 

surface ore, lies near the head of Penobscot Gulch 2 miles southwest 

of the town of Marysville. The Penobscot vein as seen from the surface had a remarkably well defined hanging wall; the ore body, which 

swells and pinches, averaged 3 to 4 feet in width. The country rock 

inclosing the vein consists of the metamorphic hornstones of the 

Helena limestone cut by intrusions of diorite porphyry. Two parallel 

veins, known as the Eagle and the Leopard veins, which like the 

Penobscot dip 65°-80° NW., lie in the footwall of the. Penobscot 

vein; the Eagle at 250 feet, and the Leopard at 600 feet.

As shown on the stope map, ore was extracted from thePenobscot 

vein for 1,000 feet along the strike at the surface. The mine was 

developed to a depth of 500 feet, but the ore came principally from 

above the 300-foot level and largely from above the 200-foot level. 

The production was one and one-fourth million dollars. The ore 

rock is a porous manganiferous quartz, which in places carries lamellar 

calcite.

On the Eagle vein an incline was sunk 70 feet; the ore shoot 

yielded $10,000, but gave out at 50 feet depth.

The present attempts to find ore in the Penobscot vein, which have 

continued in a small way during the last 10 years, consist of a tunnel 

300 feet long (mainly along the vein) attaining a depth of 90 feet.
MAKYSVILLE DISTKICT. 75

The face of this tunnel shows a lode vof highly manganiferous and ironstained porous quartz, which is said on competent authority to 

resemble the ore formerly found in the productive part of the Penobscot vein, but this material contains no gold. Another tunnel, which 

like the preceding is situated on the west end of the property, aims 

to tap the vein on the 300-foot level. The evidence from the history 

of the mine seems unfavorable to the probability of finding ore in 

depth; the geologic evidence, based on the presence of a highly manganiferous leached quartz, suggests that the gold may have been 

dissolved and carried to a lower level, and that therefore there is a 

possibility, at least worth testing, that ore may be found on the 

downward extension of the vein.

EMPIRE MINE.

The Empire mine and mill of 60'stamps, now idle, are situated on 

the west slope of Mount Belmont. The vein is said to be stoped out 

above the principal haulage level for a length of 1,500 feet, the extreme 

depth thus attained being approximately 400 feet. The country 

rock consists of the metamorphic phase-of the Empire shale, which 

dips here 10° W. The vein trends N. 60° W. and stands vertical. 

The Whippoorwill is a parallel vein several hundred feet northeast of 

the Empire.

The ore now available is a sugar-t'extured white quartz full of 

vugs and devoid of metallic sulphides. It contains as a rule much 

lamellar calcite, which commonly forms plates of extraordinary 

thinness and diameter, some plates attaining a diameter of 6 inches.

PIEGAN-GLOSTER MINE. !

The Piegan-Gloster, a consolidation of two adjoining properties, is 

situated 3 miles northwest of Marysville. The mine is opened by 

two tunnels on the vein, the upper one of which is now inaccessible. 

The lower tunnel, trending N. 25° E., is 1,200 feet long.

The mine is situated near the northwestern margin of the Marysville stock' and the country rock is quartz diorite, but the contactmetamorphic hornstones are found near by. The vein as seen in the 

lower tunnel clips 60° E. and averages 3 feet in width. Some postmineral movement has taken place along the walls, but the ore is 

generally frozen to the quartz diorite. The gangue is mainly quartz 

with some calcite. The quartz, which in places is amethystine, commonly shows large vugs lined with coarse crystals; in places also it 

displays the lamellar structure and tetrahedral habit so characteristic 

of the replacement of the calcite of the district. At the face of the 

tunnel the vein narrows to 1 foot of massive quartz containing small 

streaks of pyrite. Fragments of the diorite wall rock up to 6 inches 

in diameter are common in the vein.
76 OEE DEPOSITS OF HELENA MINING BEGION, MONTANA.

The Gloster is stated to have yielded a considerable output in 

former years, the ore having been derived from a shoot 800 feet long, 

600 -feet deep, and from 3 to 12 feet wide. 1 The Piegan part of the 

property was systematically explored during 1906, and a winze 750 

feet from the portal of the lower tunnel was sunk to a depth of 750 

feet, but the mine was idle during 1911.

STRAWBERRY MINE.

The Strawberry mine lies 3 miles southeast of the town of Bald 

Butte and near the head of the gulch on the east side of Greenhorn 

Mountain. The developments consist of an inclined shaft sunk on 

the vein, a drift at 120 feet vertical depth, and a crosscut tunnel from 

the surface on the mill level. A 10-stamp mill stands on the property.

The vein, which strikes north and south and dips 60 ? W., is situated 

along the faulted contact of diorite and dark Marsh shale. It ranges 

in width from 3 to 15 inches. On the hanging-wall side it is separated 

from the diorite by a narrow seam of gouge, but is frozen to the footwall. The vein filling consists of quartz and calcite, both of which 

are lamellar in habit, forming a gangue material entirely similar to 

that of the veins at Marysville. The quartz is commonly of a delicate 

amethystine tint, and the lamellae are so disposed as to form hollow 

tetrahedrons, which are incrusted internally with small quartz 

crystals. Small pieces of slate inclosed in the vein are surrounded by 

a radiating growth of quartz crystals.

The ore is said to yield upon the plates between $30-and $70 a ton 

in gold.

ELLISTON DISTRICT. 

LOCATION.

Elliston is the first town on the Northern Pacific Railway west of 

the Continental Divide and is situated on Little Blackfoot River at 

an altitude of 5,000 feet. It is the shipping point of a large territory, 

but during 1911 little mining was in progress throughout this region.

The lime plant of the Elliston Lime Co. is situated 1 mile east of 

the town, the kilns standing alongside the railroad right of way. The 

raw material is quarried from the Madison limestone, which outcrops 

prominently here. The seasonal output of lime is reported to be 

125,000 bushels of 80' pounds.

GEOLOGY. . "

The rocks in the immediate vicinity of Elliston consist of an apparently conformable succession of limestone, quartzite, and sandstone, 

striking north and south and dipping westward at an angle of 20°. 

They range in age from Carboniferous to Cretaceous. The lowest

1 Rept. Montana State Inspector of Mines, for 1905-6, p. 109.
ELLISTON DISTRICT. 77

formation is the Madison limestone, well exposed 1 mile east of Elliston. This is overlain by the Quadrant quartzite, consisting predominantly of a dense massive quartzite, near the top of which occurs 

a bed of high-grade phosphate rock. The Quadrant quartzite is 

succeeded concordantly by the Ellis formation of Jurassic age, which 

is highly fossiliferous through a thickness of 50 feet. Above these 

rocks come several hundred, possibly a thousand, feet of crossbedded sandstone, the upper portion of which is. doubtless of Cretaceous age. Near the west end of the town the quartzite of the 

Quadrant formation is brought up again by a fault, but disappears 

within a short distance benea'th the rhyolites that are prominent 

along the valley of Little Blackfoot River west of Elliston.

Rhyolites are present in considerable volume southwest of Elliston and persist on the flanks of the mountains up to, an altitude of 

6,200'feet. Some of these rhyolites differ considerably from the 

rhyolites common throughout the rest of the region; one of these is 

especially noteworthy because of its marked characteristics. It is 

a snow-white porphyry carrying as its sole porphyritic constituent 

large scattered phenocrysts of smoky quartz. Breccias of this rock 

are common. With the rhyolites are associated various basaltic 

rocks, among which a black glistening variety carrying large ambercolored phenocrysts of plagioclase is most prominent.

Tertiary lake beds underlie an extensive area west and northwest 

of Elliston. According to Douglass the lake beds north of Avon, 

which is 9 miles west of Elliston, are very probably of lower Oligocene 

age. At the mouth of Snowshoe Creek auriferous gravels rest on an 

eroded surface of lake beds.

Well-rounded gravels, presumably of Pleistocene age, occur 600 

feet above the river in the vicinity of Elliston, but their origin was 

not investigated. A few miles southeast of the town extensive 

morainal' deposits, as indicated by the pitted plain structure, are 

spread over the broad valley of Little Blackfoot River. These 

deposits, lying at an altitude of 5,000 feet, are the lowest glacial 

deposits found anywhere in the region considered in this report.

DESCRIPTIONS OF MINES. 

JULIA MINE.

The Julia mine, the property of the Montana-Clinton Copper Mining Co., is situated 8 miles south of EUiston at an altitude of 6,600 

feet on the west side of Telegraph Creek. This is a comparatively 

recently discovered property. It is developed by a shaft 210 feet 

deep. The vein is inclosed in granite and as a rule lies between 

exceedingly well defined walls, although the ore does not, except in a 

few places, fill the entire vein. The course of the vein is slightly 

south of east and the dip is 80° S. As seen on the 200-foot level,
78 OEE DEPOSITS OF HELENA MINING BEGION, MONTANA.

which is 150 feet long, the vein swells and constricts abruptly, 

ranging in thickness from 6 inches to a maximum of 5 feet. The 

sulphides reach 2. or 3 feet in thickness. The granite or quartz 

monzonite inclosing the vein is firm and unaltered up to the walls, 

being only slightly pyritized for about 1 foot from the vein; that 

within the vein is greatly altered and along the walls there is considerable soft sticky gouge due to postmineral movement.

The ore consists of a heavy sulphide mixture of galena, sphalerite, pyrite, and tetrahedrite, associated with subordinate quartz. 

The texture in places is rather drusy, vugs lined with quartz crystals 

being common. Tetrahedrite indicates a high content in silver. 

Sulphides consisting predominantly of pyrite, which commonly 

occurs in large well-crystallized pentagonal dodecahedrons, are discarded as valueless. The ore is stated to carry 60 ounces. in silver 

with small amounts of lead and copper.

The surface ore is said to carry much gold, but the amount has 

decreased with increasing depth. The silver is said to have increased. 

As a rule, however, there is little evidence of oxidation or enrichment.

EVENING STAB, MINE.

The Evening Star mine, better known as the Big Dick, lies 6 miles 

due south of Elliston on the summit of the mountain on the south 

side of Little Blackfoot River. The altitude is 7,000 feet. This 

property was idle during 1911.

The country rock is a coarse andesitic breccia, apparently bedded; 

the strike is N. 50° E. and the dip 20° NW. It is much jointed so 

that determinations of strike and dip of bedding are likely to be 

unsafe. The ore deposit is said to be a blanket vein developed by a 

shaft 300 feet deep and is said to dip north at a low angle. The ore 

carries galena, pyrite, sphalerite, and arsenopyrite in a quarts 

gangue; some of the quartz contains numerous large columns of black 

tourmaline intergrown with pyrite. The ore is reported to have 

been high grade, carrying as much as 3 ounces of gold a ton. During 

1906 a gold-silver-lead ore was shipped to the East Helena smelter 

and a production was maintained until 1910. The deposit was 

worked out to the west end line; during 1911 the lessees were engaged 

in sinking a shaft near this end line on the adjoining claim, which 

is one of a group known as the Finnish or Weston group. It was 

expected that the Big Dick vein would be struck at a depth of 150 

feet.

TWIN CITY MINING & MILLING CO.'S MINE.

The Twin City Mining & Milling Co.'s property is situated 8 miles 

south of Elliston, at an altitude of 6,800 feet, on the east side of Telegraph Creek. It is developed by a shaft 85 feet deep with a crosscut 

at the bottom 21 feet long, but these were under water at the time of
ELUSION DISTRICT. ' 79

visit. The vein is inclosed in granite, trends N. 70° E., and is 18 

feet wide. It is reported that 8 feet of the lode adjoining the footwall 

averages $3,60 a ton, the product being gold, silver, copper, and lead. 

The material on the dump resembles in part the ore at Rimini, being a 

black tourmaline-bearing quartz ore. Some strong tourmalinic lodes 

have been uncovered in the vicinity of the shaft, but they have not 

been found to carry any valuable constituents.

ONTARIO MINE.

The Ontario mine, which has the reputation of having been one of 

the largest producers in the Rimini district, is situated near the head 

of Ontario Creek, a branch of Little Blackfoot River. The main 

developments consist of a crosscut tunnel 800 feet long, from which 

is sunk a shaft 320 feet deep. These workings were inaccessible during 

1911. The country rock is granite; and the ore, as shown ori the 

dumps, consists of pyrite, sphalerite, and galena, inclosed in quartz.

. MONARCH MINE.

The Monarch mine lies on the north flank of Bison Mountain at an 

elevation of 7,250 feet. Geologically it is situated near the intrusive 

contact of the quartz monzonite and the overlying andesites. The 

ore minerals are galena, blende, pyrite, and arsenopyrite in quartz.

BLUE BELL MINE.

The Blue Bell mine, although long idle, is worthy of description 

here on account of its unusually interesting geologic features. The 

mine is situated 1 mile southeast of the west portal of the Mullan 

tunnel. The developments consist of a shaft, a number of surface 

cuts, and a tunnel about 175 feet below the collar of the shaft. It is 

reported that some rich copper ore was shipped from the mine.

The country rock is a monzonite containing numerous large porphyritic orthoclase crystals. The nearest intruded rock is a mile distant 

and consists of Madison limestone. The ore material is mainly finegrained garnet rock carrying disseminated molybdenite and pyrite. 

There is also much porous rock composed of large euhedral garnets 

coated with secondary minerals. An open cut 200 feet from the 

shaft shows clearly the geologic relations of the ore to the inclosing 

country rock. In this open cut the lode is about 12 feet wide, and 

the filling consists of garnet rock inclosing a few irregular thin masses 

of monzonite. These horses are' in fact ungarnetized residuals of 

monzonite. Veinlets of garnet traverse the monzonite exactly as 

veinlets of quartz and tourmaline do in the ordinary forms of mineralization throughout the region. In the vicinity of fine cracks near the 

garnetized zone the plagioclase feldspars of the monzonite have been 

converted solidly to brilliant yellowish-green aggregates of epidote,
80 OBE DEPOSITS OF HELENA MINING KEGION, MONTANA.

and the rock is thus rendered strikingly porphyritic. The thin veinlets of garnet are commonly bordered by a white band of altered 

monzonite, contrasting markedly with the normal dark-gray color 

of the monzonite. When examined microscopically, the white rock 

adjoining the garnet veins is found to consist essentially of garnet and 

scapolite, with subordinate amounts of epidote, orthoclase, plagioclase, pyroxene, quartz, and apatite. Some of the massive garnet 

rock is found under the microscope to consist predominantly of garnet, 

with which is associated some monoclinic .pyroxene, suggestive of 

hedenbergite, and a small amount of sericite. The garnet is a darkbrown variety, whose index of refraction exceeds 1.79.

All the features of this deposit suggest that it is a vein of garnet 

in the monzonite. It is possible that it represents a highly metamorphosed inclusion of limestone from the roof of the batholith, but the 

field evidence seems to be against this supposition. Whatever explanation is adopted, the phenomena here show conclusively that the 

garnet molecule is able to migrate and form veinlike deposits under 

the influence of chlorine-bearing solutions.-

RIMINI DISTRICT. 

LOCATION.

Rimini lies at an altitude of 5,200 feet, 14 miles by air line southwest of Helena. It is situated on Tenmile Creek, which flows in a 

deep gorge just above the town, between Red Mountain on the east 

and Lee Mountain on the west. The surrounding country is of high 

relief, so that mine development is favored to an unusual extent by the 

ease and advantage with which adit tunnels can be constructed.

Rimini is the terminal point of a branch of the Northern Pacific 

Railway from Helena; owing to the present low state of the mining 

industry a semiweekly service only is maintained. A considerable 

mining territory is tributary to Rimini; this was formerly, and is 

occasionally now, known as the Vaughn mining district.

GEOLOGY.

The prevailing country rock is coarse-grained granite, technically 

a quartz monzonite, composed of plagioclase, orthoclase, quartz, 

biotite, and hornblende. It is of uniform appearance and, as shown 

by chemical analysis of material from the Frohner and Valley Forge 

mines, coincides in composition with the general rock-of the Boulder 

batholith. A considerable body of aplite extends along the ridge 

from the Peerless Jennie mine to Red Mountain, where it is covered 

by rhyolite.

Pregranitic rocks, represented by the andesite-latite series, occur 

2 miles below Rimini and extend westward- across the Continental
BIMINI DISTRICT. . 81

Divide. On the summit a flow-streaked biotite latite is well exposed, 

and masses of diorite porphyry are common. Farther south on the 

divide is a small patch of profoundly tourmalinized andesite. The 

distribution of these pregranitic rocks suggests that although bare 

granite is the dominant surface rock in the Rimini district, the present 

surface is neither far removed nor much below the under surface of 

the cover beneath which the granite was intruded. The greatest 

depth is indicated at Rimini, possibly 2,000.feet.

Rhyolites are a conspicuous element in the geology of the district. 

They form the capping of Red Mountain and of Lee Mountain, 

extend southward along the high summits, and form the bedrock 

of the well-known "porphyry dike" country. Those of Red Mountain display a great variety of color and texture, but nearly universal 

features are flow streaking, abundant phenocrysts of smoky quartz, 

and phenocrysts of sanidine, commonly opalescent. The rhyolites are 

mainly lithoidal porphyries but include some obsidian and some 

breccias.

ORE DEPOSITS.

Two periods of mineralization are recognizable at Rimini: An 

older or late Cretaceous and a younger or late Tertiary period. The 

ore bodies of the first period are auriferous silver-lead deposits 

inclosed in granite. The principal ore mineral is galena, accompanied by sphalerite and pyrite* Most of the lodes are notably 

tourmaline-bearing, and the characteristics of these deposits have 

been described on pages 46-51 under the heading "Tourmalinic silverlead deposits." It is to be noted that at Rimini tourmaline is usuaUy 

called hornblende.

Among the deposits of this kind may be enumerated the Lee 

Mountain, Valley Forge, East Pacific, Lady Washington, John 

McGrew, and Armstrong on Minnehaha Creek. Others, such as the 

Election, show only feeble tourmalinization, and still others, such as 

the Eureka, show no tourmalinization, so far as this study was able 

to determine. These probably represent the nontourmalinic phase 

of the same mineralization, comparable in its effects to that which 

has altered the footwall of the Valley Forge mine.

Sulphide ore is encountered at shallow depths beneath the car-, 

bonate zone; in the Valley Forge mine, for example, at 40 feet. 

Although the relief of the district is large and abrupt, water stands 

near the surface. At an altitude of 6,250 feet water stands in the 

East Pacific shaft at 50 feet depth, although the bed of Tenmile 

Creek, less than a mile away, is 1,000 feet lower.

Mining activity was at a low ebb during 1911. Yet it seems probable that at least some.of the mines which have been important producers in the past may again become productive upon systematic

70935° Bull. 527 13 6
82 OEB DEPOSITS OF HELENA MINING REGION, MONTANA.

exploration and development. There seems to be no reason against 

the possibility that as good bodies of ore may be encountered in 

depth as have been found on the upper levels. Further, as has 

already been pointed out, the ore occurs in chambers scattered 

through zones of tourmalinized and sericitized granite; it follows, 

therefore, that any such zone of profound alteration should be 

explored thoroughly along the strike and equally thoroughly across 

the strike by crosscuts driven across the full width of the ore-bearing 

zone.

The Tertiary deposits consist of low-grade disseminated gold ore 

inclosed in the rhyolites resting on the eroded surface of the granite. 

The -most prominent is the "porphyry dike" property, near the 

head of Tenmile Creek. The "dike," however, is not a dike but a 

series of surface flows of rhyolite lying on the granite. The practical 

importance of this distinction is of course obvious: A dike has an 

indefinite extension downward, whereas the rhyolite flows form a 

capping of definite thickness, being terminated in depth by the. 

granite floor on which they rest. The rhyolites are altered and 

impregnated with gold along small fractures, the whole mass constituting a low-grade ore said to carry several dollars to the ton. More 

details are given in the special descriptions.

DESCRIPTIONS OF MINES. 

VALLEY FORGE MINE.

The Valley Forge mine is situated on the flank of the mountain 

just east of the town of Rimini. It is developed by a series of drift 

tunnels, the lowest of which is at an altitude of 5,800 feet; in this a 

winze was sunk 100 feet deep. The ore bins at the mouth of the lowest 

tunnel are connected with the Northern Pacific Railway by an 

aerial tramway. The extreme depth attained below the surface is 

approximately 325 feet. A tunnel was commenced from the level 

of the town of Rimini which will give an additional depth of 600 feet 

on the lode; the length required is estimated to be 1,385 feet, but as 

yet only 300 feet have been completed.

All ore above the main tunnel has been stoped out. During the 

early part of 1911 three men were employed getting out carbonate 

ore from the outcrops. This was shipped to the East Helena smelter 

and is stated to have carried $6 in gold, 12 ounces in silver, and 7 per 

cent lead, but this is low-grade ore. Higher-grade material is said 

to carry as much as $11 in gold.

The total production of the mine is about $200,000, the ore ranging 

in value from $15 to $30 a ton in gold, silver, and lead. The gold 

exceeded the. silver in value and the lead ran as high as 40 per cent.

The general country rock at the mine is granite, although just to 

the north across the gulch is a small patch of flow-streaked rhyolite.
EIMINI DISTEICT. 83

The lode trends east and west and dips 80° S., thus conforming with 

the general trend of the veins at Rimini. It averages 4 feet in width. 

The footwall is commonly well defined and consists of sericitized granite 

impregnated with pyrite, galena, sphalerite, and arsenopyrite, as 

described elsewhere in detail in this report. The hanging wall consists of quartz-tourmaline rock or so-called black quartz, which in 

places reaches 6 feet in thickness. This carries a small amount of 

pyrite, some sphalerite, and galena; locally all three of these sulphides are intergrown.

The commercial ore consists of an aggregate of galena and pyrite 

which is associated with some sphalerite and rarely some chalcopyrite. The ore occurs in shoots lying beneath the hanging wall of 

black quartz-tourmaline rock. The shoot on the lowermost tunnel, 

No. 3, was 350 feet long, but for 450 feet from the portal the lode or 

mineralized zone, although well denned, was barren.

Two hundred feet south of the Valley Forge lode is a vein of heavy 

jasperoid which has been opened by prospect pits to some extent. 

It is said to carry gold in places. The ledge is formed along a brecciated zone of tourmalinized. granite. Analysis by J. G. Fairchild 

of the heavy red jasperoid material shows that it contains 43.60 per 

cent ferric oxide, representing total iron, 49.06 per cent silica, and 

6.38 per cent water.

LEE MOUNTAIN MINE.

The Lee Mountain mine is situated on the west bank of Tenmile 

Creek at Rimini. The lode was discovered in 1864 by John Caplice 

and the claim was staked under the old Montana law that allowed 

the preemption of claims 2,200 feet long and 100 feet wide: The mine 

was opened by a series of drift tunnels, the lowest of which, driven 30 

years ago, is approximately 850 feet long and attains a depth of 600 

feet. The present lessees contemplate sinking a shaft to a depth of 200 

feet below the level of this tunnel and drifting eastward under the bed 

of the creek, where, according to report, a good body of ore was exposed before the stream bed was covered with rock debris.

The production of the Lee Mountain mine is conservatively estimated as $1,500,000, of which $750,000 is authenticated by smelter 

returns.

The ore occurs in chambers distributed through a zone about 50 

feet wide. The general trend of the ore-bearing zone is N. 85° E. 

and the dip 80° S. A ledge of black tourmaline-quartz rock, ranging 

up to 8 feet in thickness, accompanies the ore-bearing zone. The ore 

makes in the adjoining highly sericitized granite and consists of a 

mixture of galena, sphalerite, pyrite, and subordinate arsenopyrite, 

with some tetrahedrite. More or less altered granite is mixed with 

the sulphides. Considerable postmineral movement has affected the 

ore bodies and in consequence much soft sericitic gouge is present.
84 OEE DEPOSITS OP HELENA MINING REGION, MONTANA.

Ore recently developed at the face of the lowest tunnel yielded, as 

shipped, $2 to $4 in gold, 20 ounces of silver, and 10 per cent lead; it 

also carried 8 per cent zinc. This ore is a mixture of primary sulphides 

and indicates that the unoxidized ore of the deeper workings is of 

payable grade. It is probable that the ore worked formerly consisted 

largely of oxidized material, from which, as shown by some sacked ore 

still remaining, the zinc had been entirely leached out.

JOHN McGREW PROSPECT.

The John McGrew claim was being developed under bond during the 

summer of 1911. It is an old property, having been located over 30 

years ago.' Developments consist of a drift tunnel within which a 

winze was sunk 35 feet; at the bottom of this a drift along the vein 

was being driven.

The lode is inclosed in granite and the ore makes abruptly and capriciously along a ledge of black quartz-tourmaline rock. In the bottom 

drift the lode is 18 inches wide and shows a banded structure. Along 

the hanging wall there is 3 inches of barren black tourmaline; this 

merges toward the footwall into a low-grade pyrite galena ore and 

becomes quartzose on the footwall. Barren gray quartz, 3 inches wide, 

rests directly on the footwall. A small amount of iron-stained gouge 

is found along the walls.

( PEERLESS JENNIE MINE.

The Peerless Jennie mine is situated near the divide at the head of 

Tenmile Creek at an altitude of 7,500 feet. It was formerly a considerable producer of silver-lead ore, but has been idle for many 

years. The vein consists of a crushed zone in granite about 6 feet 

wide and dipping 70° N. The underground workings comprise a 

crosscut tunnel 240 feet long and several hundred feet of drift along 

the vein. The adit level attains a depth of approximately 200 feet 

from the surface and the ore is stoped out to the shaft level above. 

At the face the vein consists of crushed granite, containing locally 

small seams of quartz. The granite adjoining the vein is thoroughly 

sericitized and impregnated with cubical pyrite." The ore on the 

dump is chiefly pyrite embedded in quartz, associated with sphalerite 

and in places with cerusite. The richest ore is said to have occurred 

in pockets of galena.

According to K. W. Kaymond 1 the surface ores were extraordinarily rich; 50 tons averaged 900 ounces of silver to the ton and 200 

tons averaged nearly 500 ounces.

PORPHYRY DIKE.

The summits of the mountains between Ruby and Monitor creeks 

at the head of Tenmile are capped by a series of rhyolite flows and

i Statistics of mines and mining in the States and Territories west of the Rocky Mountains, 1874, p. 362.
HELENA DISTRICT. 85

interbedded tuffs and breccias. This rhyolite capping is known as 

the porphyry dike, and the whole area throughout which it occurs 

has been covered by mining claims. That this rhyolite is auriferous 

is indicated by the fact that old placer workings extend to the very 

summit of the range, where the streams flowed on a rhyolite bedrock.

At 7,500 feet altitude is a large pit about 50 feet in diameter and 

averaging 40 feet deep, from which ore was taken out and treated in 

a 10-stamp mill until the work was stopped by the water company 

that furnishes the municipal supply for Helena. A tunnel has been 

driven approximately 125 feet below the pit and is said to be 1,400 

feet long. It commences in the granite on which the rhyolites rest.

The country rock as exposed at the pit from which the one was 

quarried is a strongly flow-banded and laminated rhyolite; the flow 

bands are steeply tilted at angles ranging from 60° to 90° and are 

convoluted and irregular in direction. The rhyolite is remarkably 

full of lithophysse (stone roses), ranging in size from a fraction -of 

an inch to several inches. At one place there is a width of 20 feet 

of lithophysal rhyolite. The walls of the lithophysae are coated 

with quartz crystals, as are also the pores and 'cavities in the flow 

bands of the rhyolites. In places the quartz has grown exceedingly 

coarse, some prisms an inch in diameter having been formed. The 

rhyolites are generally of white chalky appearance and in places are 

interlaced' with quartz veinlets one-tenth of an inch in thickness. 

They are considerably less porphyritic than the varieties of rhyolite 

prevailing in the surrounding region, but carry sporadic phenocrysts 

of quartz. Under the microscope they exhibit the usual features of 

rhyolites, except that they have been considerably sericitized and 

that much kaolin is present.

The rhyolites show some limonite derived from the oxidation of 

pyrite crystals. They are traversed also by limonite-stained fractures, which are said to be particularly favorable places for the occurrence of gold. As a rule, however, gold is not visible to the eye-. 

The- deposits have been extensively sampled, both by surface work 

and by diamond drilling, and are said to show a gold tenor of several 

dollars to the ton.

In addition to the ore taken at the pit described above, considerable 

ore was mined at the pits on the property known as the Pauper'sDream. The rock exposed in these pits consists of rhyolite tuffbreccia of kaolinized appearance; this.ore is reported to have milled 

$2 a ton in gold.

HELENA DISTRICT. 

HISTORICAL SKETCH.

Helena, the capital of Montana, is a city of 12,515 population, 

situated in Lewis and Clark County at an elevation of 4,100 feet. 

It was founded in 1864 as a result of the discovery of the Last Chance
86 OEE DEPOSITS OF HELENA MINING REGION, MONTANA.

placer by a party of prospectors who had turned back from the 

Kootenai stampede. In the fall of the same year lode gold was 

discovered on the divide between Oro Fino and Grizzly gulches, 5 

miles south of Helena. This was the famous Whitlatch^Union lode 

which, according to report, has produced $6,000,000 to date.

The gold placers were soon exhausted, but a considerable quartz 

mining industry sprang up south of Helena, at Park and Unionville. 

The earliest estimate on record concerning the output of the Last 

Chance placer is that by Vom Eath in 1883, who gives the output as 

over $10,000,000 j 1 succeeding estimates, by Bancroft in 1889, place 

the output at $16,000,000 ; 2 by Swallow in 1890, at $30,000,000, 3 and 

by recent writers at $35,000,000. The estimates have thus increased 

enormously, although placer mining had long ceased to exist previous to 1883.

During 1911 little mining of any kind was in progress. The production of precious metals,, mainly gold, in 1910 was about $15,000.

GEOLOGY.

The distribution of the geologic formations in the vicinity of Helena 

is shown on the accompanying map (PI. VII) on the scale of approximately 1 mile to the inch. This map was prepared by W. H. Weed 

and his assistants and shows the geology of the area in far greater 

detail than is shown on Plate I.4 The following text (pp. 86 to 98) 

is taken, with few changes, from an unpublished report by Mr. Weed.

SEDIMENTARY ROCKS.

GENERAL FEATURES.

The various sedimentary formations recognized in the Little Belt 

Mountains and in the-ranges near Livingston can in a general way be 

correlated with the sedimentary series at Helena. " The series differs, 

however, somewhat in detail and no sharp discrimination of the formations by means of fossil remains is as yet possible, owing to lack 

.of evidence. Fossils are not abundant. The Cambrian and the 

Carboniferous limestones usually carry shell remains, but in general 

the rocks are barren and over a considerable area are altered, the 

limestones being marmarized by contact metamorphism due to the 

granite intrusion. This contact metamorphism is especially notable 

in the Mesozoic rocks, whose determination as such is based on

1 Zeitschr. Deutsch. geol. Gesell., vol. 36,1884, p. 630.

2 History of Washington, Idaho, and Montana, 1890, p. 721.

a Kept. Montana State Inspector of Mines for 1S90, Helena, 1891, p. 37.

< In preparing the drawing of this map for publication here, the areas originally shown as quartz monzonite, granite, granite porphyry, and diorite have been given the general symbol of quartz monzonite; 

the area of igneous rock east of Lenox. originally shown on the map as granite porphyry and described 

In the text as rhyolite porphyry, has been marked with the dacite symbol, in conformity with the results 

of the present investigation. '
U. S. GEOLOGICAL SURVEY 

GEORGE OTIS SMITH. DIRECTOR BULLETIN 527 PLATE VII

LEGEND

SEDIMENTARY ROCKS

Qal

Alluvium

Tau

Auriferous gravels

Hot-spring deposits

LEJ

Lake beds

I Ku

1___ __ 

Contact-metainorphosed 

sandstone and shale

O V) 

LJ en 

u< 

< o:

Quadrant quartzite

Lfll

Madison limestone

Three 

CD

Forks shale and 

Jefferson limestone

2: *

z

O

I -eis

Yogo limestone, Dry Creek shale,

Pilgrim limestone. Park shale, \ K. 

Meagher limestone, and Wolsey shale r & ..i____________ ' *r

Flathead quartzite

Am

Marsh shale 

Ah

Helena limestone

Ae I 

_._____J 

Empire shale

As

Spokane shale 

IGNEOUS ROCKS

Tbr 

Rhyolite breccia

Rhyolite

[ Td_ | 

Dacite

Aplite

Quartz monzonite

DETAILED GEOLOGIC MAP OF HELENA, MONT., AND VICINITY

Scale 62500

aMiles

Geology by Walter Harvey Weed 

Assisted by Joseph Barrell 

and L. S. Griswold

Surveyed in 1901-1906

Basic dikes

3 House Doc. ; 62d Cong., 3d Sess.

Fault

Contain- interval 50 feet. 

i.y jneaji sesi le.vel,- 

1913
GEOLOGY OF HELENA DISTRICT. 87

stratigraphic sequence and character fortunately satisfactory evidence in this case. The formations are mapped according to the 

detailed tracing of distinctive and easily recognizable beds by 

L. S. Griswold. The boundaries adopted for intervening formations 

are somewhat arbitrary but have been checked up or connected at 

several points and are substantially correct.

Great difficulty was experienced in attempting to define the subdivisions of the Algonkian. The various formations of the Belt 

series are as a whole individualized, but they are extremely difficult 

to define in mapping and the boundaries on the map are not closely 

accurate.

Thicknesses were determined by two carefully measured sections, 

one on Mount Helena and hi its vicinity and the other near Unionville, many partial sections being measured elsewhere. No place was 

found where the whole sequence of beds was satisfactorily exposed, 

but the section is correct so far as determinable along the line measured, additional information being derived from work near by.

For the reasons already stated no distinction could be made in the 

Mesozoic. The Jurassic is believed to be present, as it is well developed at various points not far distant on every side, but careful 

examination furnished no satisfactory proof of its presence here. The 

same reason explains the lack of subdivision in the Cretaceous.

ALGONKIAN SYSTEM.

Distribution and subdivision. The oldest rocks of the Helena district are those of the Belt series, of Algonkian age. The series, 

where fully developed, consists of eight formations, each well defined 

as a whole, but grading into one another, so that the subdivisions are 

not sharp and can not be adequately separated in mapping in this 

region. The series is named from its occurrence in the Big Belt 

Mountains, where it is typically developed and forms the core of the 

range. The rocks cover a large area in northwest Montana, extending northward into Canada. The following formations have been 

established by Walcott, who gives the thicknesses here stated:

Feet. 

Marsh shale................................................... 300

Helena limestone............................................... 2,400

Empire shale...................................../..........".. 600

Spokane shale.................................................. 1,500

Greyson shale.................................................. 3, 000

Newland limestone............................................ 2,000

Chamberlain shale.............................................. 1, 500

Neihart quartzite........................"....................... 700

In the Helena district only the four uppermost formations are 

found, ,and the top of the Marsh shale has been eroded before the 

deposition of the Cambrian, so that the upper limit of .the formation 

and its entire thickness are not known.
88 QBE DEPOSITS OF HELENA MINING BEGION, MONTANA.

Spokane shale. The oldest rock of the Helena district is the Spokane shale. It consists of massive and thinly bedded siliceous shales, 

usually of a deep-red color but passing in places into green and gray 

rocks, containing arenaceous beds which merge at times into sandstone. The formation is named from the Spokane Hills, 15 miles 

east of Helena, where it is finely developed. The rocks form low 

hills bordering the Prickly Pear Valley on the north and west.

Empire shale. The Empire shale is a formation composed of massive-bedded, greenish-gray, well-banded, siliceous shales, showing 

color bandings of light and dark material and locally a marked 

knotty structure. The thickness is estimated by Walcott at 600 

feet. The rocks are seen in the hills near the railroad from 2 to 3 

miles west of East Helena. At this locality they consist of pale 

greenish-gray slates, with characteristic purple spots.

Helena limestone. The Helena limestone is .a formation composed predominantly of impure bluish-gray or gray noncrystalline 

limestone. The limestones occur in beds 1 foot to 6 feet thick and 

contain thin interbedded bands of gray siliceous shale, more rarely 

of green to purple clay shales. The limestones are ordinarily dark 

blue on fresh fracture but show a characteristic buff-colored velvety 

appearing surface on weathering.- The upper beds have a rough 

surface, with a pale or blue-gray color, and. resemble Cambrian rocks. 

These beds alternate with shale and form the ridges on the northwest, foot slopes of Mount Helena.

The formation has no distinctive physiographic expression within 

the district, but its relatively massive bedding gives the limestone 

prominence on the slopes about the city.

The name was given the formation by Walcott from its typical 

occurrence about the city of Helena.

. The formation is barren of fossils, though the oolitic character and 

the local presence of carbonaceous markings has led to the belief that 

they will ultimately be found. The estimated thickness of 2,400 

feet in this vicinity is based on rough measurements, as it is impossible to find a satisfactory exposure of the entire formation for exact 

measurement. The formation covers a large part of the district, 

especially about the borders of the Prickly Pear Valley. So far as 

known the rocks are conformable to the formations above and below, 

and grade into them by intercalations of shale. The upper and 

lower limits are therefore not sharply definable. About a mile east 

of Helena the Marsh shale is wanting and the Cambrian quartzite 

rests directly on the eroded surface of the Helena limestone. Good 

outcrops are seen near the high school within the city limits.

On the north face of Mount Helena the Cambrian quartzite rests 

on dark-blue and dense limestones, weathering buff, and these rocks 

in turn rest on pink and buff-colored shales, which appear red in
GEOLOGY OP HELENA DISTRICT. 89

most exposures and are included as part of the Helena limestone. 

These reddish shales contain numerous massive beds of white cherty 

limestone, forming reefs running obliquely across the slopes and 

extending downward within a few yards of the streets of Kenwood. 

The beds of limestone are 6 to 10 feet thick and in weathering and 

character resemble those of Paleozoic rocks. The same pink and 

buff-colored shales, with interbedded limestones, are seen in the gulch 

east of Lenox, the 4,350-foot knoll at that locality showing light-gray 

fine oolitic limestone with black grains in a white matrix.

Marsh shale. The Marsh shale consists of red and yellowish-green 

shales and thin-bedded sandstones. According to Walcott, it is 250 

feet thick east of Helena; on the north slope of Mount Helena it is 

reduced to 75 feet; and to the northwest it increases in thickness to 

300 feet. The formation is cut out 1 mile southeast of Helena, but 

the shales reappear at a point 2 miles southeast of the city. The 

name is derived from Marsh Creek, in the vicinity of Marysville, 

Mont., where the formation is typically developed. It is also the 

predominant formation on the slopes of Greenhorn Mountain, near 

Marysville. It is seen underlying the Flathead quartzite on'upper 

Main Street in the city of Helena, 165 feet of beds being exposed

CAMBRIAN SYSTEM.

Subdivisions and character. The Cambrian rocks of the Helena 

region, as in the Little Belt Mountain region to the east, are divisible 

into seven formations. All these formations except the uppermost 

the Yogo limestone are of Middle Cambrian age; the Yogo limestone 

is of Middle Cambrian and Upper Cambrian age, according to Walcott. 

The Helena region shows the same alternations of quartzite, limestone, and shale recognized in the mountains to the east of the Little 

Belt region and southward to the Yellowstone Park; the formation 

names adopted for the Little Belt region will therefore be used in 

this report. These names are, from the basal formation upward, 

Flathead quartzite, Wolsey shale, Meagher limestone, Park shale, 

Pilgrim limestone, Dry Creek shale, and Yogo limestone. On the 

map the Flathead quartzite is discriminated and the rest of the 

Cambrian formations are shown undivided.

The line between the Cambrian and Devonian rocks was originally supposed to be the upper limit of a well-marked shale belt. 

This shale belt is persistent and well recognized and was in mapping 

regarded as the upper limit of the Cambrian. Later work, however, 

showed that the limestones lying above this shale also belong to the 

Cambrian, and it became necessary to draw a new line, based on 

stratigraphic evidence and the topographic relief but not checked 

by field observations; this line is.not, therefore, free<from error.

Flathead quartzite. The Flathead quartzite, the lowest formation 

of the Cambrian system, consists of a hard, fine-grained, massive
90 O&E DEPOSITS OF HELENA MINING KEGlON, MONTANA.

quartzite varying to grayish-yellow to gray sandstone. The lowest 

stratum in places is pebbly, grading into a conglomerate at the base. 

The bedding planes range from a few inches to a few feet apart and 

faint lines of sedimentation are seen. The rock is jointed by sharply 

cut planes. Higher up in the formation thin beds of gray-brownmottled and green micaceous shale are found locally, increasing in 

thickness toward the top. The pebbles in the basal bed vary in 

character from place to place and consist predominantly of the material derived from the immediately underlying beds. As mentioned 

elsewhere, this' quartzite is in most places apparently conformable 

to the Algonkian, but there is a slight angular unconformity observable east of Helena, and the Marsh shale is in places cut out, so that 

the quartzite rests directly upon the Helena limestone. The only 

fossils recognized are scolithus borings. The total thickness'of the 

Flathead quartzite in the Plelena district is 300 feet. The formation 

is easily recognized in the topography of the area, as the resistant 

nature of the beds causes them to form low foothill ridges, which are 

prominent on the slopes of Mount Helena, and as the cap reef on the 

mountain ridge running southward from that peak.

Wolsey shale. The Wolsey shale consists of micaceous and calcareous gray to greenish shales, which contain small oval and flat 

concretions of limestone, grading in places into thin and very irregular plates of limestone. Trilobite and shell remains of Cambrian 

types occur abundantly along the contact between these shales and 

limestones. The rocks are in few places well exposed, owing to their 

soft and crumbly nature, but their position is recognizable by the 

ravines cut in them or, on the mountain slopes, by their forming a 

more gentle angle between the limestone bluffs above and the quartzite ridges below. They have a thickness of about 420 feet.

MeagJier limestone. The Meagher limestone is composed of lightgray to bluish limestones, which are shaly near the base but grade 

into alternating beds of massive dark-colored and flaggy white limestones, and these into thinly-bedded dark-purple to blue fossiliferous 

limestones," forming the top of the series. In other regions the rocks 

are pebbly, but this character is not conspicuous in the Helena district. The rocks have an estimated thickness of 400 feet. They 

form the characteristic bluffs on the, north face of Mount Helena, 

extending from the gentle slopes formed by the Wolsey shale upward 

almost to the very summit of the mountain. The rocks are also 

seen in the bold cliffs below the east side reservoir. Fossil remains 

occur, but no collections were made.

Park shale. The Park shale consists of earthy and micaceous 

dark-gray to green or purple shales. The rocks are not well indurated and crumble readily, so that very few good exposures are seen. 

A partial section is exposed in the quarry near the upper part of the
GEOLOGY OF HELENA DISTEICT. 91

city of Helena, and shows the formation to contain lavender or 

pinkish beds, grading through green shales to a grayish earthy shale 

carrying an abundance of small fossil shells, identified as Obolella. 

The upper portion contains limestone lenses in a jaspery shale, which 

grades downward into a dense cherty rock resembling hornstone. 

This shale has an estimated thickness of 150 feet. It forms the flat 

bench on the summit of Mount Helena, between the apex and the 

northern cliffs, and covers the ridge followed by the trail.

Pilgrim limestone. The Pilgrim limestone consists of massive beds 

of bluish to dark-gray limestones. The lowest bed is a dark-colored 

crystalline rock, mottled with yellow and dark-gray spots; its peculiar 

coloration and massive character are characteristic of the limestone 

throughout Montana. This bed of mottled limestone is overlain 

by light-gray to white noncrystalline limestone, used for making quicklime in the Grizzly Gulch kilns. No fossils have been found in the 

mottled limestone, but its position and lithologic character correlate 

it with the "Mottled" limestone of the Yellowstone Park folio. 

This formation occurs on the very summit of Mount Helena, where 

it forms the uppermost bed of the gentle syncline sweeping down 

the southeastern side of the mountain. It is also seen in bluffs 

above the East Side reservoir,.and forms a low cliff extending up 

Oro Fino Gulch for 2 miles above the city, the relief being due to the 

crumbly nature of the Park shale in which the gulch is being eroded. 

The mottled beds are 150 feet thick and are overlain by white limestone, which is included in the formation. The total thickness of 

the formation is 317 feet.

Dry Creelc shale. The Dry Creek shale consists of light-colored 

brownish-yellow, red, and pink shales and calcareous sandstones. 

The formation is well exposed in few places but can be recognized by 

its topographic relief,.as it forms sags in the high ridges and ravines 

on the mountain flanks. No fossils have been found in the shale.- 

It is correlated on the basis of lithology and stratigraphic position 

with the Dry Creek shale of the Threeforks and Little Belt regions. 

The thickness is estimated at 40 feet.

Yogo limestone. The Yogo limestone consists of light-colored, 

thin-bedded limestones, with crinkly bands and films of jasper, in 

many places composed of limestone pebbles held in a glauconitic 

matrix. The formation corresponds to the so-called "Pebbly? 

limestone of the Threeforks folio. It has a thickness of 175 to 450 

feet. The jaspery flaggy limestone forms prominent buttress exposures along the east side of Oro Fino Gulch above the city.

DEVONIAN SYSTEM.

The Devonian period is represented by two formations, the Jefferson limestone at the base and the Threeforks shale at the top. On 

the map these formations are not separated.
92 ORE DEPOSITS OF HELENA MUTING' REGION, MONTANA.

Jefferson limestone. The Jefferson limestone, the lower of the Devonian formations, consists of a series of limestones, characteristically dark colored, but includes also some light-colored beds. The 

dark beds have a characteristic granular structure, are commonly 

mottled by light-colored patches due to metamorphosed corals, and 

are quite fetid when struck with a hammer. The assignment to the 

Devonian is based on fossil remains found in other areas, as the rocks 

within the Helena district have not yielded any identifiable fossils. 

The lowest beds are dark colored, almost steely in luster and texture, 

and usually form a bluff or ledge that rises abruptly above the more 

fissile Yogo limestone. The buff-colored argillaceous shales which 

overlie the limestones have been included in the Jefferson, but they 

may belong to the Threeforks shale. The total estimated thickness 

of the Jefferson limestone is 243 feet.

TTireeforks sliale. The Threeforks shale varies somewhat in the different parts of the Helena district. It is essentially composed of 

black shale and alternating beds of limestone; it contains brachiopod 

remains, which correspond to those identifiable elsewhere as Devonian 

species. It corresponds in position to the formation in the Threeforks quadrangle known by this name. The uppermost bed of the 

formation is 15 feet thick, consisting of fine-grained black carbonaceous shale. This rests on light-colored fossiliferous calcareous 

shales that grade downward into earthy shales with interbedded bands 

of quartzite, the total thickness being 270 feet. The rocks are seen 

on the ridge west of Grizzly Gulch about 1£ miles from Park. At this 

locality there is a basal bed of shale 40 feet thick, capped by 186 feet 

of calcareous argillite overlain by 56 feet of black shale. The formation is well exposed only on the high ridges, where it forms low saddles between limestone knobs. In the gulches cut in these rocks 

exposures are poor and more than a few feet of the beds can be found 

at but few places.

CAEBONIPEROUS SYSTEM.

Distribution and character. The Carboniferous system is represented 

in this region by a basal formation consisting of a mass of limestones 

characterized by lower Carboniferous or Mississippian fossils, to which 

the name Madison limestone has been applied. This limestone is 

overlain by a formation, essentially quartzitic, which is correlated 

with the Quadrant formation of the Yellowstone Park, Threeforks 

quadrangle, and other areas in Montana. The rocks are well exposed and cover a considerable portion of the mountain area in the 

southern part of the district. As a rule they are readily distinguished 

from the soft Mesozoic rocks which overlie them, but in this region 

contact metamorphism has so altered the Mesozoic rocks that they 

are as hard and resistant as those of the Carboniferous period.

Madison limestone. The Madison limestone in the eastern ranges 

of the State has been subdivided by Weed into ~three members, to
GEOLOGY OF HELENA DISTKICT. 93

the lowest of which he gave the name Paine shale, to the middle the 

name Woodhurst limestone, and to the upper the name Castle limestone. All these subdivisions can be recognized hi a general way in the 

Helena district. The lowest member consists of thin-bedded impure bluish limestones, which carry crinoid stems and other fragmentary fossil remains. A very persistent bed is characterized by black 

needles of tourmaline, and as this bed occurs near the base of the 

formation it is quite useful in mapping the limits of the Madison. 

This member grades upward into the well-bedded limestones of the 

middle member, which forms the steep slopes and well-marked cliffs 

throughout the southern part of the district. The rock is light gray 

to dark gray and the beds are separated by partings of impure shaly 

material, which break into angular splintery fragments; much of it is 

cherty.

The upper member consists of very massive white or light-colored 

limestones showing no bedding planes, and in the Helena region they 

are commonly altered to white marble. The total thickness of the 

Madison is 2,600 feet.

Quadrant quartzite. The Quadrant quartzite consists, as its name 

implies, chiefly of quartzite with sandstones and some interbedded 

white limestone. Throughout the area the rocks are much altered 

by contact metamorphism and their distinctive characters obliterated. 

The formation is named from Quadrant Mountain, in the Yellowstone 

Park, with which the rocks in this area are correlated. The formation 

composes the high hills adjoining the granite contact in the south part 

of the district. The thickness is estimated at from 190 to 500 feet. 

To the west, in the Philipsburg quadrangle, Calkins obtained from the 

Quadrant formation fossils which were pronounced by Girty to be 

of Pennsylvanian age. In other areas to the east in Montana, however, rocks referred to the Quadrant formation have yielded Mississippian fossils.

CRETACEOUS SYSTEM.

Rocks which are referred to the Cretaceous-because of their stratigraphic position, and which possibly include some of Jurassic age, 

occur in the southern part of the area. They form a belt lying along 

the great granite mass which has invaded them and which in a number of places has broken the continuity of the belt. They consisted 

originally of sandstones and shales but have been subjected to intense 

contact metamorphism, so that now they consist of biotite hornfels 

and allied varieties of rocks.

TERTIARY SYSTEM.

Lake leds.-^-The Tertiary lake beds cover a far greater area than 

any other formation within the Helena district. The rocks vary in 

character from point to point according to the composition of. the
94 . OEE DEPOSITS OF HELENA MINING EEGION, MONTANA.

underlying formation. In general they consist of well-bedded sands, 

gravels, and conglomerates composed of the underlying country rock, 

together with a large amount of rhyolite. In the center of Prickly 

Pear Valley, where the rocks have been penetrated by boring, several 

sheets of rhyolite have been found. In the hills to the east the beds 

are well exposed, and the indurated tuffs forming a part of the series 

are quarried in the vicinity of Lenox. As a rule, however, the rocks 

crumble on weathering, and good exposures are rare. These lake 

beds undoubtedly underlie the entire Prickly Pear Valley, but have 

been covered by detritus washed from "the surrounding slopes and 

the alluvium brought in by the streams, so that although a thickness 

of over 1,200 feet is known from the bore holes put down in seeking 

artesian waters, the bottom of the formation has not yet been found. 

The rocks are nearly everywhere distinctly bedded, but as a rule they 

.show little persistence in character. The pebbles are normally 

rounded but in part subangular, and as the material is mostly of 

local origin the color and composition vary from point to point. The 

beds are regarded as of Miocene or Pliocene age.

Hot spring deposits. The hot spring deposits shown on the map 

(PI. VII, p. 86) consist mainly of large masses of chalcedony.

Auriferous gravels. The aurif erous, gravels of the Helena district 

are regarded as of Pliocene age, as the remains of the mammoth and 

other vertebrates believed to be of that age have been found in the 

vicinity of Helena and of Montana City. The gravels are composed 

of the harder, more resistant material from the drainage basins of 

the streams and naturally vary at different points. In the Montana 

City area there is a large proportion of andesite brought down from 

the headwaters of Prickly Pear Creek, together with much vein 

quartz and granite. In the Last Chance placer gravels, which underlie the city of Helena, the material has been derived from the various 

tributary gulches draining the granite contact and therefore consists 

of pebbles, cobbles, and well-bedded sands, composed largely of 

quartz monzonite, with varying proportions of limestone/ Invariably 

the gravels .are auriferous when the streams drain the contact region, 

and the source of the-gold is to be found in the veins which follo.w the 

granite contact.

QUATERNARY SYSTEM.

The Quaternary deposits of the district consist of alluvial silts and 

gravels and are developed most extensively in the great plain north 

of the city of Helena.

IGNEOUS ROCKS.

Distribution and character. Igneous rocks cover about one-fourth 

of the area of the district, forming high mountains to the south and 

the isolated peaks known as the Scratch Gravel Hills, as well as 

numerous lesser elevations. The oldest rocks occur only as intruded
GEOLOGY OF HELENA DISTRICT. 95

. sheets or sills in the Cambrian sediments and have been folded with 

them. They are of unknown age but are certainly pre-Tertiary. 

The most abundant rock is granitic in character and forms part of 

the Boulder batholith. The Miocene rocks occur as intrusive dikes 

and masses of rhyolite and as lava flows and tuff beds of the same 

rock. They also enter into the composition of the lake beds.

Basic dikes and sheets. The basic dikes and sheets shown on Plate 

VII consist mainly of dark-greenish to steely-gray rocks of dense 

texture and commonly marked by rusty spots or the holes resulting 

from the alteration of augite and olivine crystals. Sometimes fresh 

augite is recognizable in the specimens. The commonest rock may 

be called a greenstone of diabasic character. It occurs intrusive in 

the Wolsey shale on the west side of Mount Helena and at the same 

horizon about a mile south of Lenox. Another dike of this material' 

cuts the Flathead quartzite in the northwest portion of the city.

An augite porphyry, a tough and dense green rock containing 

white porphyritic crystals of plagioclase, occurs as a sheet 40 feet 

thick intrusive in a belt of black shale in the upper part of the Cambrian. This rock is exposed on the slopes east of Oro Fino Gulch, a 

short distance above the city, and it follows this shaly belt for a distance of several miles, outcropping at intervals.

A hornblende porphyry, a dark-green, fine-grained rock carrying 

stellate clusters of hornblende, occurs intrusive in the Helena limestone east of the Broadwater hotel. It forms a sheet 6 feet thick and 

outcrops for several hundred yards.

Acidic sheets.^-A. porphyry that contains altered feldspar phenocrysts but that is too thoroughly decomposed for specific identification occurs as a persistent sheet from 10 to 20 feet thick intrusive in 

the Wolsey shale close to the Flathead quartzite. It is exposed on 

the northeast side of Mount Helena and is traceable for several miles 

westward. A second sheet of similar nature occurs 30 to 40 feet 

higher in the shale.

Meta-andesite. This name is applied to andesites which have 

undergone distinct alteration but whose original structure and composition are still recognizable. Patches of these rocks occur east of 

Montana City and at one or two other places in the Helena district. 

They are mostly fragmental in origin, being old volcanic tuffs and 

breccias baked and altered by contact metamorphism. A few intrusives are doubtfully referred to this type. They are older than the 

granitic rocks and in the region south of Helena form the cover to 

the granite. Fragments torn off by the granite intrusion occur in 

places in the later rock near its contact with the andesite.

Granitic rocks. The coarse-grained rocks composed of feldspar 

and quartz, with lesser amounts of mica and hornblende, are conveniently called granite, as it is not usually possible to determine the 

species of feldspar in the field. A more precise definition, based on
96

a study of thin sections of the rock under the microscope, enables us 

to classify the material more exactly. In the granitic region extending southward from Helena to Butte the prevailing form of rock is a 

coarse-grained quartz monzonite, called the Butte quartz monzonite, 

which under the new quantitative system of classification is designated amiatose. A rock corresponding to this is seen in the Scratch 

Gravel Hills, at the south end, and also occurs in a small area near 

Holmes Gulch. The greater part of the granite of the Helena district is, however, a finer grained rock of slightly more basic composition.

Rhyolites. The rhyolites are all of Miocene age and occur as 

intrusives, extrusives, and fragmentary rocks.

The rhyolite breccia covers a considerable area in the southeast 

part of the district. It is a mass that varies in character irom place 

to place but is composed essentially of both rounded and angular 

fragments of rhyolite, lying without stratification and at all angles 

in a fine-grained stony matrix which is speckled with minute flakes 

of biotite and carries a few fragments of feldspar and quartz. The 

rock is generally light colored, varying from pale red and purple to 

buflLcolor and white. Much of the fragmentary material composing 

the rock is a white, porous, pumice-like rhyolite. A good exposure 

is found in the quarry on Holmes Gulch 1£ miles west of Montana 

City. At this place the breccia is a light, porous, somewhat friable 

rock, made up mainly of white rhyolite pumice, with small rounded 

pebbles of granite, jasper, and other rocks. It is regarded as a mud 

flow composed essentially of volcanic dust and ejectamenta which 

fell on the granitic slopes and was washed down to fill the hollows of 

the lower ground.

The rhyolite flows are readily distinguished from the intrusive 

rocks by their porosity and flow structure and as readily from the 

breccias by their compact nature. The rocks are prevailingly reddish 

to purple colored and contain phenocrysts of quartz, biotite, and in 

some specimens feldspar in a dense lithoidal groundmass. The chief 

exposures of this rock are in the vicinity of Montana City, "one flow 

forming the cap to the highest hill in the region. The rock on this 

summit has a platy parting with a dip of 45° and forms extensive 

areas of slide rock on the west side of the hill but none on the east. 

It is evident that it was a lava flow covering a surface which has been 

much tilted since the consolidation of the lava.

Rhyolite intrusives are exposed in-the vicinity of Lenox, where a 

patch one-fourth of a mile across and three-fourths of a mile long 

is intrusive in the Algonkian slates and has broken up and thrown over 

the Flathead quartzite. The rock is a normal rhyolite porphyry, showing phenocrysts of biotite, feldspar, and quartz in a stony groundmass.1

1 The present investigation shows that the rock is more accurately termed a dacite, and it is therefore 

co designated on Plate VII.
GEOLOGY OF HELENA DISTRICT. 97 

STRUCTURE. -

The Helena district lies on the south side of a great dome-shaped 

uplift some 25 miles in diameter, whose center lies north of the Scratch 

Gravel Hills. This dome extends from the mountain ridge west of 

Marysville eastward to York, on the west side of the Big Belt Mountains. The city of Helena and the mountains south of it lie on the 

south side of the dome. Although the general structure is that of a 

simple anticline, the dome is not perfectly regular but shows secondary 

crumpling. Within the Helena district the dome shape is well shown. 

Subsequent to the folding which formed this great dome the granitic 

rocks broke through and faulted the south flank of this anticline.

Broadly considered, Prickly Pear Valley is a basin deeply eroded 

in the dome-shaped uplift noted above, an arch whose summit has 

been worn away and cut down into the soft shales of the Belt series 

that form its nucleus, so that the sheets of white limestone and other

rocks that once covered it are now only seen on Mount Helena and the 

hills south of the city, in the Spokane Hills to the east, and the flanks 

of the Belt Mountains north of the district. This broad arch involved the entire sedimentary series of the region from Algonkian 

to Cretaceous. The simplicity of structure has been, however, modified by the granite intrusion and faulting.

The granite area in the southern part of the district (see PI. VII), 

which forms a rugged mountainous tract, mostly unfit for agriculture and largely denuded of valuable timber, is the extreme northern 

part of a granite area extending 75 miles southward, a great mass of 

intrusive igneous rock designated the Boulder batholith. A part of 

this batholith extends northward under the sedimentary rocks, and, 

as already stated, the latter are highly altered and commonly crystalline, as a result of the heat and vapor given off by this once molten 

material. The upper contact between the granite mass and the bedded rocks is uneven, and arms and offshoots of the former rock extend 

upward. Some of these, bared by erosion, are seen as dikes and 

bosses, as, for example, the granite area near tha Broadwater Hot 

Springs, that north of Helena, and a smaller area east of the city.

In the adjustment of the rocks following the violent intrusion of 

this great body of granitic rock, with its initial uplifting force and subsequent settling as cooling progressed, faulting occurred and the 

irregularly triangular blocks of the lim.es.tone series and their accoinpanying rocks were dislocated. These faults are not of very great 

magnitude. To the east, however, the bedded rocks were not 

clearly, cut off, but broken into small blocks by the force of the intrusion, and a very complicated mosaic of sediments and igneous rock is 

shown on the map.

70935° Bull. 527 13 7
98 QBE DEPOSITS OP HELENA MINING KEGION, MONTANA.

FAULTS.

The Helena area has a number of faults, which cross ths stratified 

rocks at approximately right angles to their strike and which maybe 

regarded as radial faults of the great anticline. Owing to the fact 

that the faults are marked by more or less crushed matter, which is 

easily eroded, they are followed by the stream gulches.

The West Side Keservoir fault shows an upthrow on the east with 

a horizontal offset of 5QO feet and 290 feet vertical. This fault is 

indistinguishable where it passes through the massive beds of the 

Madison limestone, and in the Algonkian rocks the exposures are too 

few to determine it correctly. There are two small subsidiary parallel faults near-by, at the north base of Mount Helena. These faults 

have offshoots of 60 and 40 feet respectively, corresponding to 30 and 

20 feet vertical, and have a downthrow on the east. On Montana 

Avenue, the north-south street separating sections 19 and 20, and 29 

and 30, where the Flathead quartzite crosses, there are two small 

faults; one has a throw of 30 feet and the other, 100 yards farther 

east, has a throw of 15 feet.

A long northeast fault passing through Lenox splits at the north 

end into two faults 300 feet apart. The upthrow is on the west, and 

the fault displacements are 110 feet and 230 feet.

One-half mile farther east occurs the Mount Ascension fault. 

This is a big fault, having an offset of 850 feet (as the strata dips 40° 

this corresponds to 765 feet vertical). A half mile farther east is 

another fault of like magnitude.- The wedge between these faults 

has sunk at least 765 feet, forming a trough or "graben."

These faults have been actually observed and measured, but it is 

probable that there are many faults of small displacement which have 

not been recognized, for the great block of altered sediments that 

rested OH the granite while it was cooling must have suffered considerable contraction, with consequent fracturing and readjustment.

Strike faults due to slipping of the beds on one another probably 

accompanied the uplifting of the sediments. One was recognized 

southeast of Lena, where the Meagher limestone rests against Pilgrim limestone, the intervening shale being cut out.

In "general the faults just noted can not be recognized in the relief 

of the region. A gulch marks the line of the Lenox fault (northeastsouthwest) toward Unionville. The north-south fault on the east 

side of the depressed triangular block already noted seems to break 

into smaller faults and is strongly marked in the landscape. Several 

mine and quarry openings disclose its nature. The fault walls are 

smooth or polished .and show well-marked striae. Between these 

vertical walls, which vary from 5 to 40 feet apart, the fissure is filled 

by friction breccia, ground up wall rock, showing angular and partly 

rounded rock fragments in a clayey matrix. 1

1 Up to this point the text follows the manuscript of Mr. Weed.
OKE DEPOSITS OF HELENA MININQ REGION, MONTANA. 99

ORE DEPOSITS OF HELENA DISTRICT. 

LOCATION.

The ore deposits of the Helena district, using this term in its broad 

sense, are mainly located south of Helena along the heads of Oro Fino, 

Grizzly, and Nelson Gulches. Geologically they are situated mainly 

along the contact of the granite with the invaded sedimentary rocks 

hornfels, quartzite, and limestone.

Opportunities for examination of the ore deposits were unfavorable 

during 1911. The properties were idle and most were inaccessible. 

The systematic position of certain of the deposits has already been 

pointed out the Big Indian on page 51 and the Spring Hill on page 

44 so that repetition is unnecessary here.

DESCRIPTIONS OF MINES. 

WHITLATCH-UNION MINE.

The Whitlatch-Union lode, discovered by James W. Whitlatch in 

September, 1864, is the oldest quartz discovery in the region. The 

lode was rapidly developed by several different mining companies, 

each of which owned but a short length, the longest piece being the 

500 feet held by the Whitlatch-Union Mining Co. By 1872 the mine 

had produced $3,500,000, when it was shut down because of litigation. It has been operated intermittently since that time and is 

stated to. have yielded to date $6,000,000 in gold.

The mine was originally opened by a number of inclines, but with 

the exception of the Owyhee and Mclntyre they are all caved near 

the surface. Later a vertical shaft was put down to a depth of 500 

feet. During 1911 some of the upper levels were accessible through 

the Owyhee incline, from which in recent years lessees have been 

getting out small shipments of ore.

The .lode is situated 4 miles south of Helena on the summit of the 

low divide between Grizzly and Oro Fino gulches. It lies in the 

granite just south of the contact-with the Quadrant quartzite. There 

occur scattered in the granite near the contact some huge blocks of 

highly metamorphosed sedimentary rocks, probably belonging to the 

Cretaceous formation shown on Weed's map.

The general course of the vein is N. 84° W.; the dip averages 45° 

N., but is subject to remarkable and abrupt changes, ranging from 

nearly horizontal to vertical. In width the vein ranged from a thin 

seam up to 15 feet, averaging about 4 feet. The ore taken out in early 

days averaged from $20 to $25 a ton in gold.1

As seen in the Owyhee incline the Whitlatch-Union vein is partly 

inclosed in hornfels and partly in granite. Abrupt changes in dip 

accompany the passage of the vein from the one rock to the other.

Statistics of mines and mining in the States and Territories west of the Rocky Mountains, 1869, p. 147.
100 OKE DEPOSITS OF HELENA MINING KEGION, MONTANA.

A specimen taken from the footwall of the vein in the end of a drift 

driven west from the Owyhee incline is a dark heavy rock showing 

numerous small lustrous black flakes of biotite. When examined 

microscopically it is found to consist of a confused intergrowth of 

brown-green amphibole, biotite, and andesine, with accessory quartz. 

It is evidently a thermally metamorphosed rock, but further than this 

its origin is not apparent. It is possibly a recrystallized igneous rock. 

The locality from which this rock was taken is in the newest workings of the mine. The vein here is narrow, averaging 4 inches, and 

dips at a low angle to the north. The ore consists of pyrite and some 

quartz and is slightly oxidized. A small shipment running $70 a ton 

in gold was taken out; unmixed with waste the ore runs SI30 a ton.

BIG INDIAN MINE.

The Big Indian mine is situated 4 miles south of Helena. The 

two main claims, the Alabama and the Gold Hill, were located in 

1875. The ore was taken out from a large quarry, and a 10-stamp 

mill was operated at a profit for a number of years. During the 

operation of this mill $110,000 was produced, the ore averaging $5 

a ton in gold. According to James Winscott, the original locator of 

the property, some clean-ups ran 985 fine. Later the property was 

taken over by a corporation and a 60-stamp mill was erected and put 

in commission in 1902. This mill was operated by electric power 

from the Missouri River Power Co.'s plant at Canyon Ferry on Missouri 

River, and it was estimated that the ore could be mined and milled 

at a cost of 60 cents a ton. During 1903 and 1904 the Big Indian 

mine was the leading gold producer in Jefferson County, but soon 

afterwards it was shut down and has remained idle since that time.

The mine is situated half a mile south of the main contact of the 

granite with the sedimentary rocks on the north. The country rock 

is a gray medium-grained quartz monzonite. Under the last management the mine was worked by the glory-hole method, the glory 

hole being at the level of the top of the mill. The pit is several hundred feet in length #nd breadth and 50 to 75 feet deep. Near the 

surface the rock is soft and incoherent and crumbles easily, but 

toward the bottom of the pit it apparently grows hard. Near the 

bottom the granite is traversed by a close-spaced system of vertical 

jointing, along which considerable tourmalinization has taken place. 

About as much tourmaline was developed in the granite as the biotite 

and hornblende originally present. In addition to the tourmalinization there was a slight introduction of pyrite. Noteworthy is the 

fact that there are no quartz veinlets anywhere in the ore body.

The granite is cut by a few aplite dikes and also by some darkcolored diorite porphyry dikes. The tourmalinization ensued after 

the intrusion of the aplite dikes and probably after the intrusion of 

the porphyry dikes, which are somewhat pyritized.
QBE DEPOSITS OF HELENA DISTRICT. 101 

SPRING HILL MINE.

The Spring Hill mine is situated 4 miles northwest of Helena in 

Grizzly Gulch. It is developed mainly by an adit tunnel and by 

levels below this tunnel. A 3,000-foot crosscut was driven, to connect with the 500-foot level of the Whitlatch-Union mine in order 

to give the ore of the Spring Hill mine an easy outlet to the mill at 

Unionville.

The general country rock is the Madison limestone, which strikes 

N. 5° E. and dips 67° E., as shown on the surface near the large 

open cut. The ore deposit is located near the contact of a dark 

fine-grained gray granitoid, in which biotite is the most prominently 

recognizable constituent. As determined microscopically this rock 

is a diorite composed essentially of plagioclase,. hornblende, and biotite, with subordinate interstitial quartz and orthoclase. The limestone is a white dense saccharoidal variety and shows selective metamorphism, for certain narrow bands from 2 to 3 inches wide show 

silication; that is, the development of pyroxene and garnet. At 

50 feet from the contact some of these silicate bands fray out gradually into unaltered white limestone. Another open cut shows even 

more clearly that the development of silicate minerals is independent 

of the composition of the limestone, for metamorphic minerals are 

formed locally and abruptly across the bedding of thinly stratified 

limestone.

As seen on the main level the deposit consisted of a large body of 

contact-metamorphic ore which was stoped out in large high galleries, 

thick pillars of ore being left as supports. The ore minerals are 

pyrite and pyrrhotite; in places the pyrrhotite forms solid masses 

several feet in length and width. The sulphides are usually associated 

with a fine-grained aggregate of lime-silicate minerals that form an 

exceedingly hard and tough rock. They are also disseminated in 

grains and blebs through pure limestone. A notable feature of the 

deposit is the lack of oxidation exhibited, contrasting in this respect 

with other deposits in the region, The deposit is, as it were, doubly 

protected against the effects of enrichment, namely, by calcite 1 and 

by pyrrhotite, 2 both of which are effective precipitants for downward-moving solutions. The gold tenor is stated to run as high as 

$30 a ton, but the average value makes this a low-grade ore.

DTTTRO MINE. -

The Old Dominion or Dutro mine, as it is known locally, is situate'd 

7 miles west of Helena and 1 mile north of the Rimini road. During 

1911 two men were employed here in getting out ore by means of a 

small gasoline hoist from an incline extending down 150 feet.

i Bard, D. C., Econ. Geology, vol. 5,1910, pp. 59-61.

5 Emmons, W. H., Public presentation before Geological Society of Washington, April, 1912.
102 OEE DEPOSITS OF HELENA MINING EEGION, MONTANA.

The general country rock consists of a series of Paleozoic limestone and dolomite standing on edge and trending north and south. 

The mine is situated near the end of an intrusion of fine-grained 

diorite, whose longer axis, about one-fourth of a mile long, lies parallel 

to the strike of the inclosing rocks. The limestones are generally 

saccharoidal in texture and in places show abundant needles of 

tremolite. At the main working is a small boss of black fine-grained 

diorite about 150 feet in diameter.

The ore, occurring in irregular masses in the dolomite, consists of 

jasper and opal and usually contains much iron oxide. On account 

of the oxidized condition the original character of the ore is rather 

obscure. The prevailing ore is a jaspery quartz commonly showing 

specks of free gold. Locally it contains an iron-gray metallic mineral, 

which has been determined in the laboratory of the Geological Survey 

to be bismuth sulphide. The oxidation of this mineral seems to be 

the cause of the chrome-yellow color apparent in some of the ore. 

Under the microscope the ore is seen to consist of microcrystalline 

and cryptocrystalline silica, which forms a replacement of the dolomite. 

In addition to the minerals already noted, some cassiterite was 

detected. This is the only occurrence of cassiterite in its bedrock 

source yet discovered here, although it has been known to occur in 

the placer deposits of the region; the quantity is very small and the 

mineral could not be detected in hand specimens of the ore.

The.ore is shipped to the East Helena smelter and is reported to 

average $75 a ton in gold.

CLANCY DISTRICT.

Clancy is a small town in Prickly Pear Valley, 14 miles southeast 

of Helena. It was settled in 1865 and was formerly a thriving mining 

center, but is now chiefly dependent on agriculture and on the railroad, 

being a division point on the .Havre-Butte branch of the Great 

Northern Railway.

GEOLOGY.

The prevailing rock is the typical quartz monzonite of the Boulder 

batholith. The abundance of aplite in dikes and large masses is 

noteworthy; some of the larger masses attain an area of several 

square miles, but they were not separately mapped. Intrusions of 

pegmatite" and granite porphyry also are common.

Of the younger rocks, rhyolites are present in considerable quantity 

in the area east and northeast of Clancy. They were erupted on a surface whose relief was much like that of the present topography. Those 

east of Clancy are light-colored porphyries and include flow-banded, 

lithophysal, and breccia varieties. They are true rhyolites, consisting essentially of phenocrysts of quartz and sanidine inclosed in a 

macrocrystalline groundmass of quartz and sanidine. Most of'them
CLANCY DISTBICT. 103

are devoid of dark minerals, but some contain scattered hexagonal 

plates of biotite. The rhyolite forming the butte at the mouth of 

Clark Gulch, north of Clancy, is strongly flow-banded; in places the 

banding, as a result of movement in the following lava, is contorted 

and stands vertical. Locally the rhyolite is filled with lithophysse 

or "stone roses." The walls or petals of the lithophysse are extremely 

thin and delicate and weather out readily, so that such rhyolites 

yield honeycombed outcrops.

Some dikes of rhyolite cutting the quartz monzonite occur locally; 

noteworthy among them are those paralleling the veins at the head 

of Warm Spring Creek. They commonly show margins several feet 

thick consisting of dark obsidian glass. West of Clancy, in the 

valley of Clancy Creek, a few dikes of dacite have been noted.

ORE DEPOSITS.

The mines in the vicinity of Clancy were founded on true silver 

ores; that is, ores valuable principally for their silver content. A 

considerable number of such mines were grouped in Lump Gulch, a 

few miles northwest of Clancy, and gave rise to a once flourishing 

mining camp called Lump Gulch City. All, however, are now idle. 

Of the mines of the Lump Gulch district, the Liverpool, credited 

with a production of over a million dollars, was the deepest and most 

productive. It attained a depth of 750 feet.

The veins were inclosed in quartz monzonite and carried rich" 

argentiferous sulphides galena, sphalerite, and tetrahedrite in a 

chalcedonic quartz gangue. The only mine operating in 1911 on a 

vein of this type was the King Solomon mine, a description of which is 

given in some detail on pages 104-105. In its essentials this description doubtless applies to all the other mines. Dikes of dacite porphyry like that accompanying the King Solomon lode, however, were 

not noted at the Little Nell, Free Coinage, and Legal Tender mines.

The ore at the King Solomon mine is clearly of primary origin and 

is unaffected by enrichment; possibly, as suggested by the following 

description of the Legal Tender vein, the surface ores of the veins of 

this type were enriched to 'a certain extent by oxioTation and the 

removal, by leaching, of some of the constituents.

A feature of the Clancy district is the abundance of iron-stained 

chalcedonic reefs, which form large, prominent outcrops. Some of 

these reefs carry a small amount of tetrahedrite. Their relation to 

the productive veins is not entirely clear; they are possibly somewhat 

younger or they may be merely the large barren equivalents.

The ore deposits at the head of Warm Spring Creek, a tributary 

entering Prickly Pear at Alhambra, 1 mile above Clancy, are of a 

different type than the foregoing and probably belong to the older 

mineralization. They are quartz veins, carrying mainly pyrite with
104 ORE DEPOSITS OF HELENA MINING REGION, MONTANA.

subordinate galena, sphalerite, and arsenopyrite, and yield a smelting 

ore running from $15 to $30 a ton, mainly in gold. They were 

inaccessible during 1911, although preparations were being made to 

reopen the Fleming, or Bell, mine, which has been the principal producer. These mines have recently been described in brief by R. W. 

Stone.1

DESCRIPTIONS OF MINES. 

KING SOLOMON MINE.

The King Solomon mine is situated on the north side of the valley 

of Clancy Creek, 2 miles west of Clancy. The property was located in 

1889. but has been worked only intermittently since that time. The 

output has been $100,000, mainly by lessees. In 1901 the mine was 

bought by Mr. I. S. Moreland, who organized the King Solomon 

Mining Co. During 1911 siliceous silver ore was being extracted and 

shipped to the smelter at Butte.

The mine is opened by an inclined shaft sunk on the lode to a depth 

of 300 feet. Development and exploitation is at present proceeding 

on the 300-foot level, west of the shaft.

The country rock at the mine is a coarse granite, the chemical and 

mineralogic features of which are described in detail on pages 56-58. 

Some dikes of fine-grained white aplite cut the granite but are of small 

importance. Of considerably more importance is a thick porphyry 

dike, .which forms the hanging wall of the King Solomon lode. The 

dike trends slightly south of west and dips 60° to 70° S.; as shown by a 

crosscut on the 200-foot level it is 85 feet wide and the walls are rather 

wavy. In appearance the dike strongly resembles a granite porphyry, 

but familiarity with the rocks of the region suggests that it is allied 

to the highly porphyritic dacites so abundant at Wickes and elsewhere. 

It is an ash-gray porphyry, holding abundant crystals of feldspar 

and quartz, and numberless small brilliant black flakes of biotite, 

the phenocrysts forming half of the rock. Under the microscope 

the feldspar phenocrysts are found, to be crystals of glassy andesine 

(Ab56An44) and are seen to be embedded in a groundmass of cryptocrystalline texture. The identity of the rock as a dacite is thus established. The ore body, being younger than the dike, is therefore 

of late Tertiary age, a fact immediately suggested by the prevalence 

of chalcedonic quartz in the gangue matter of the lode.

The lode consists of a wide shear zone, traversing mainly granite 

but in places aplite also. It trends parallel to the dacite dike, which 

forms, as it were, a hanging waU to the ore deposit, the course of the 

dike, as already pointed out, being somewhat wavy, but the lode pursues a straighter course than the dike and touches the footwall of

1 Geologic relation of ore deposits in the Elkhora Mountains, Mont.: Bull. U. S. Geol. Survey No. 470, 

1911, pp. 87-88.
CLANCY DISTEICT. . 105'

the dike at the crests of the waves only. At such points the dike is 

generally much sheared and altered. The granite adjoining the 

lode for a distance of at least 20 feet is softened and has been greatly 

altered through the action of the ore-forming solutions, as described 

in detail on pages 56-58.

The lode, or ore-bearing zone, measured normal to the dike, is 25 

feet or more wide, but of this width the ore aggregates a few feet at 

most. The ore consists of narrow slabs of high-grade material trending parallel to the structure. In rich places along the lode the ore 

forms a series of imbricating slabs of nearly solid sulphides 1^ to 3 

inches thick, constituting short ore shoots aggregating from 1 to 2 

feet in thickness. The metallic minerals are essentially galena, sphalerite, and tetrahedrite; pyrite is rare, but molybdenite has locally 

been found in some abundance. The sulphides are irregularly intergrown and are of contemporaneous and primary origin. Ore high 

in tetrahedrite is rich in silver. The sphalerite is mainly the resincolored variety. The gangue of the ore-bearing veinlets is principally 

chalcedonic quartz. Some of the ore, however, is highly brecciated 

and angular fragments of chalcedony and of sulphide aggregates have 

been recemented by fine-grained curved siderite.

The ore is carefully sorted, and a product high in silver is obtained, 

carrying between 100 and 200 ounces a ton.

On the 200-foot level, east, the dacite dike was crosscut and much 

gouge arid shattered granite, in places 4 feet wide, was found on the 

hanging wall of the dike. Streaks of rich ore were encountered here, 

and a ribbon of ore,l£ to 8 inches wide, consisting essentially of resinous sphalerite intergrown with tetrahedrite in a quartz gangue, was 

found; this constituted 400-ounce rock.

LITTLE NELL MINE.

The Little Nell was the only mine of the Lump Gulch mines at which 

there was some activity during 1911. The main operations consisted 

in reworking the old dumps, but some underground work was also 

done. The mine was partly unwatered and a tunnel was run westward 

on the 150-foot level in the expectancy of reaching an ore shoot.

The old workings attained an extreme depth of 500 feet and smelter 

returns show a production of $400,000. The geologic and mineralogic 

features are similar to those of the King Solomon lode, namely, an 

argentiferous galena-sphalerite ore in a chalcedonic quartz gangue. 

As a rule, ore high in zinc is also high in silver.

YELLOWSTONE PROSPECT.

The Yellowstonevgroup, comprising 10 or 11 claims, is situated on a 

small stream tributary to Clancy Creek, 5 miles west of Clancy. A 

90-foot shaft had been sunk on.the prospect by former owners and had
106 ORE DEPOSITS OF HELENA MINING REGION, MONTANA.

been abandoned, but the property was relocated in 1908 by the present owners. A drift tunnel, which is now 200 feet long, was commenced 

by the new owners 150 feet below the collar of the shaft.

The country rock is quartz monzonite, carrying large porphyritie 

orthoclase crystals and abundant biotite in hexagonal plates. The 

lode is considered to be 25 feet wide; the footwall is not. well defined, 

but toward the hanging wall there are a number of good walls which 

show considerable gouge ,and horizontal corrugation. The 25-foot 

ore-bearing zone contains a number of thin and widely spaced veinlets; the ore minerals are principally chalcopyrite and galena, with 

minor amounts of sphalerite and pyrite. The gangue is mainly 

quartz, grading in places into chalcedony. Azurite in small quantity 

and a little malachite are found on the surface. The lode, which 

strikes east and west and dips 65° S., apparently cuts across a dacite 

dike 90 feet wide and trending nearly north and south.

KENNEDY OR JACKSON CREEK MINE.

The Kennedy mine is one of the old mines of the district that has 

been recently reopened. The former workings are said to have 

attained a depth of 250 feet. The ore consisted of rich silver sulphides, which, in 1872, were hand sorted and hauled to Fort Benton 

by ox teams and shipped to Swansea, Wales. The old workings are 

no longer accessible. An incline of 69° has recently been, sunk to a 

depth of 115 feet and a crosscut back to the ledge on the 100-foot 

level was commenced.

The country rock is a coarse granite, consisting of orthoclase, 

plagioclase, quartz, and biotite. At the surface this has been dis-v 

integrated to a sand through which project a few hard fresh aplite 

dikes. Three parallel ledges occur on the property, trending east 

and west. One that is being prospected by pits shows an overburden 

of disintegrated granite 10 feet thick. . In places the ledge is represented by oxidized sheared granite.

LEGAL TENDER MINE.

The Legal Tender mine is situated a short distance east of Clancy. 

According to Raymond,1 writing in 1873:

This lode is a true fissure vein, cutting through granite, and is considered the richest 

and best developed silver property in the Territory. The ore and vein matter varies 

in thickness from 1 to 3 feet, increasing gradually in width as depth is obtained. 

A portion of the ore in the vein is a very rich argentiferous galena, carrying a larger or 

smaller quantity of dark-brown zinc blende, sulphurets, and carbonates of lead, oxide 

of manganese, iron, copper, and antimony, with streaks in which native wire, flake, 

sheet, and ruby silver occur abundantly. The heaviest galena ores contain black and 

gray sulphurets of silver, and yield from $2,000 to $4,000 per ton. A fine blue quartzose ore predominates, in which occurs very frequently native and ruby silver. A

1 Statistics of mines and mining in the States and Territories west of the Rocky Mountains, 1873, p. 230.
WICKES DISTRICT. 107

email portion of these ores can be successfully reduced by the smelting process, as has 

been demonstrated in the works at Helena and Jefferson City; but the larger portion 

of the ore from this mine can be most profitably reduced by the milling (roasting,1 

chloridizing, and amalgamating) process. The lode was first discovered by Joseph 

Fultz in 1866, but no work of development was accomplished until 1872, under the 

management of its present owners, who came into possession in the winter of 1871-72. 

Up to May 1,1872, a shaft was sunk only to the depth of 60 feet, which, however, demonstrated the existence of a true vein and a body of very rich ore. Since that time 

work has been prosecuted with vigor, and large and commodious whim, shaft, and ore 

houses have been erected, with company office, assay office, and other necessary 

buildings.

The mine is developed to the depth of 160 feet, showing an increasing width of vein 

with depth. At the depth of 80 feet levels were run 140 feet east and 90 feet west, 

and at the depth of 160 feet the east level is out 130 feet. Some stoping has been done 

east of the main shaft. An air shaft 60 feet west of main shaft extends down to the 

80-foot level. -

From the commencement of work by the present owners up to January 1, 1873, the 

mine has produced 7 tons of ore yielding $6,090 coin value, or at the rate of $870 per 

ton; also 130 tons of ore yielding $54,000 coin value, or at the rate of $417.78^ per ton; 

and 240 tons yielding $31,607 coin value, in lots, at rates varying from $128.42 to $202.49 

per ton. 

*******

About 175 tons of second-class ore are now on hand.

The first-class ores have mostly been shipped out of the Territory in w.agons to 

Corinne, thence to San Francisco and Europe.

The mine has long been idle and dismantled. The preceding description has been thus fully quoted because the facts .therein presented, taken together with the present condition of the mine, give 

an accurate picture of what has happened to so many other mines 

throughout this same region.

From what can now be seen on the surface, the vein ranged from 

4 to 6 feet in width, trended N. 80° E., and dipped vertically. It 

belonged to the same type of ore body as the King Solomon. The 

rine blue quartzose ore of Raymond's report is the fine-grained flinty 

or chalcedonic quartz of this report. Ninety feet south of the Legal 

Tender shaft are huge iron-stained outcrops of one of the chalcedonic 

reefs so common in this part of the district. The Legal Tender vein 

is either cut off by this reef or converges with it at a narrow angle 

approximately 200 feet west of the shaft.

WICKES DISTRICT. 

LOCATION AND HISTORY.

The Wickes district has been the largest producer of silver-lead ore 

in Jefferson County. Within the district as designated in this report, 

are included the towns of Wickes, Corbin, and Jefferson, and the 

deserted mining camps of Gregory and Comet. Jefferson was settled 

in 1864 and Wickes in 1877. Corbin is considerably more recent.

Wickes is situated 20 miles south of Helena on the Great Northern 

Railway. Formerly it was served also by a branch of the Northern
108 OEE DEPOSITS OF HELENA MINING REGION, MONTANA.

Pacific Railway, extending from Helena, but this line was abandoned 

about 1900. The large smelting plant of the Helena Mining & Reduction Co. was situated here and was operated until 1893, when it was 

shut down. It has never been started again and is now a wreck. 

This blow, together with the cessation of mining at many of the properties in the vicinity, destroyed the prosperity of Wickes. Corbin, 

situated 2 miles below Wickes, has in recent years enjoyed a copper 

boom on account of the large bodies of ore believed to exist there, but 

since the panic of 1907 the boom has largely collapsed.

Although the district contains an array of mines noteworthy for 

their output the yield of the Alta, Comet, Gregory, and Minah 

mines aggregating over $50,000,000 only one property was shipping 

ore in 1911. This was the Blue Bird mine, which produces a coppersilver ore. Extensive development work, however, was in progress 

at the properties of the Boston & Corbin Gold & Silver Mining Co., 

Corbin Copper Co., and Corbin Metals Mining Co.

GEOLOGY.

The oldest rocks in the Wickes district belong to the series of anclesites and latites. They form the largest remnant of the cover under 

which the granite of the Boulder batholith cooled. Massive lavas, 

breccias, and an unusual proportion of fine-grained tuff and argillaceous rocks are found. In places the andesites, or more probably 

latites, display conspicuous flow streaking, as near the summit outcrops of the Alta lode and along the road from Wickes to Gregory. 

The prevalence of banded tuffs allows the dip and strike to be determined more readily than is generally possible in the andesite-latite 

series. Measurements show that, although flat dips are of general 

occurrence, steep dips up to 45° are common. These dips are steeper 

than those obtaining along the margins of the batholith; furthermore, 

the strike of the beds is inconstant in direction. This structural 

complexity suggests a possibility that the roof-remnant has suffered 

partial foundering and collapse during the intrusion of the quartz 

monzonite.

The granitic rock of the Wickes area is the normal quartz monzonite 

of the Boulder batholith. It is intruded, however, by unusually 

large masses of aplite, the largest of which is shown on the geologic 

map (PI. I, p. 20). Another large area, probably approaching in size 

that east of Corbin and Wickes, lies west and southwest of Jefferson. 

Dikes of aplite also are common in the quartz monzonite and in the 

andesites.

West of Wickes is a considerable area of dacites that form a series 

of prominent white and light-gray porphyries. These rocks rest on 

the eroded surface of the andesites and granite and are therefore 

considerably younger. Dikes of dacite pierce the older rocks; some
WICKES DISTRICT. 109

of these that carry few or no quartz phenocrysts are, strictly speaking, biotite andesites and are not easily distinguishable" from diorite 

porphyries.

DESCRIPTIONS OF MINES. 

ALTA MUTE.

The Alta mine, whose output of .$32,000,000 is by far the largest of 

the mines in the region, is situated on Alta Mountain, 2 miles southwest of Corbin. It was developed by a "series of tunnels, the lowest 

of which is known as the 800-foot tunnel, although it is considerably 

less than 800 feet vertically below the summit of Alta Mountain. The 

property, together with many others in the vicinity of Wickes, was 

acquired in 1883 by the Helena Mining & Reduction Co. A smelter, 

which was for some years the largest reduction plant in Montana, 

was built at Wickes, and regular mining operations continued until 

1893. Since then an intermittent production has been maintained 

by lessees. In 1909, however, the property was taken over by the 

Boston & Alta Mining £o. and a shaft, situated near the mouth of 

tunnel No. 8, was sunk to a depth of 600 feet. Drifting was commenced to intercept the vein below the old levels, but operations were 

suspended in August, 1910. Lessees were engaged during 1911 in 

extracting carbonate ore from shallow surface workings and grassroot tunnels near the summit of Alta Mountain.

The production of the mine previous to 1893 is stated to have been 

1,250,000 tons of ore, aggregating $32,000,000, mainly in lead and 

silver, and up to that time only the highest grade ore was shipped.1

The prevailing rock in the vicinity of the Alta mine is a latitic 

andesite. The rock at the summit of the mountain shows a pronounced streaky and flow-banded structure. The intrusive granite 

.occurs several hundred yards east of tunnel No. 8; on the wagon road 

below the mine a dike of coarse aplite and two dikes of dacite 50 or 

100 feet wide are exposed, penetrating the andesites.

The underground workings being inaccessible, only surface observations could be made. These show that the ore body, trending 

slightly south of west, was inclosed in the andesites and that the ore 

minerals consisted of galena, sphalerite, and pyrite. The introduction of the ore, as shown by material on the dumps at 6,100 feet altitude, was accompanied by intense tourmalinization, so that radial 

aggregates of black tourmaline are common in the altered andesite. 

In places the andesite was fractured and netted with quartz tourmaline 

veinlets and impregnated with pyrite. Such rock examined microscopically proves to be composed of quartz and sericite in nearly equal 

abundance, tourmaline, and pyrite.

i Mineral Resources U.'s. for 1909, U. B. Geol. Survey, 1910, p. 374.
110 QBE DEPOSITS OF HELENA MINING EEGION, MONTANA.

The output of $32,000,000 of lead-silver ore entitles the Alta lode 

to rank among the world's most notable deposits of lead-silver ore 

occurring in noncalcareous rocks. Further, this great yield would 

seem to give it foremost place in the class of tourmalinic ore deposits.

BERTHA MINE.

The Bertha mine, the property of the Boston & Corbin Copper & 

Silver Mining Co., is situated 1 mile west of Corbin. The company 

was organized in 1907 and the main work done on the property has 

been accomplished since that time. A 2-compartment shaft, operated 

by a balanced electric hoist, was sunk to a depth of 900 feet, which was 

the bottom level of the mine at the time of visit. This level, however, is called the 1,200-foot level, owing to the fact that the discovery 

cropping^ are situated 300 feet above the collar of the shaft. It is 

planned to erect a 300-ton concentrator on the property.

The country rock is a coarse gray granite or quartz monzonite. 

The vein trends northeast-southwest and dips steeply northwest, 

commonly in excess of 80°. Drifts have Ijeen run along the vein 

southwest from the shaft on various levels and a considerable tonnage 

of ore has been blocked out. The vein ranges in thickness from a few 

inches to a maximum of 10 feet and on the 700-foot level averages 

about 4 feet; on the 900 and 1,200 foot levels it is somewhat narrower. 

The vein on the 700-foot level has a well-marked ribbon structure, 

where undisturbed, but much postmineral movement has taken place 

and the quartz is brecciated along the footwall. This postmineral 

movement was mainly horizontal, as shown by corrugations on the 

walls, and took place chiefly along the footwall, which is overlain by 

a layer of gouge in places 1£ feet thick. Locally this gouge is somewhat stained with iron oxide. The granite adjoining the vein on the 

700-foot level shows white chalky feldspars, which give it a porphyritic 

appearance; it slacks and crumbles readily. In this phase the biotite 

plates have remained unattached and lustrous; in other phases both 

feldspar and biotite are sericitized and the rock is more highly 

pyritized.

The ore is a coarse white quartz, carrying chalcopyrite and pyrite, 

and is stated to average from 2 to 3 per cent copper, with small 

values in silver and gold. Some of the ore carries small amounts of 

a lead-gray mineral, locally known as gray copper, but which, on qualitative chemical analysis, proves to be,a sulphide of lead and bismuth 

answering to the description of cosalite. It would probably be a 

matter of some metallurgical interest to determine whether this 

mineral carries any silver, as it will undoubtedly slime during concentration. Some bornite occurs in small amount on the 700-foot 

level, but is not found above or below this level. It may represent
WICKES DISTKICT. Ill

local enrichment, but this is doubtful. Other than the occurrence of 

this bornite, there is no evidence of sulphide enrichment, and the. 

bulk of the ore is clearly of primary origin.

According to a recent report,1 a carload shipment of average ore 

for testing purposes sampled 3.6 per centxcopper, 7.1 ounces of silver, 

and 20 cents in gold to the ton.

BLIZZARD MIKE.

The Blizzard mine is situated 3 miles west of Wickes. It was 

operated by a company from 1888 to 1896 and by lessees from 1899 

to 1906. In 1911 three men were engaged in exploring the property 

with a view to opening it up again. The old workings produced 

about $150,000, smelter returns showing that the ore carried from 

0.1 to 0;8 ounce of gold and 10 to 60 ounces of silver to the ton .and 

6 to 20 per cent of lead. There are two veins on the property, and 

of these only the northern vein carries ore. The northern vein occurs 

in a shear zone in andesite and ranges in thickness from a few inches 

to 6 feet. The joint planes in the andesite near the vein are at 

many places heavily pyritized; rarely slight tourmalinization occurs 

along the joints in the andesite, but occurs nowhere in connection with 

the ore. The ore consists principally of pyrite and. arsenopyrite, in 

places with small bands and patches of galena, sphalerite, and 

blende, rarely chalcopyrite. Practically no quartz is present.

The smelter, returns show that this ore carries 0.3 ounce of gold 

and 14 ounces of silver to the ton, 8.5 per cent of lead, and 1.3 per 

cent of copper. The southern vein, which is said to fault the northern 

vein, occupies a fault plane in the andesite and dips 45° N., except at 

the east end near the fault, where it dips 85°. The filling of this vein 

consists of pyrite, rhodochrosite, pyrolusite, calcite, and quartz.

BLUE BIRD MINE.

 GENERAL FEATURES.

The Blue Bird mine is situated at an altitude of 7,000 feet, 4 miles 

west of Wickes. It is one of the old mines of the district, and the 

property, recently enlarged by the purchase of the adjoining Penn 

Yan claim on the west, was taken over in 1911 by a new company, 

which is proceeding to develop the mine systematically.

The authenticated production is $175,000, but the amounts taken 

out by lessees from time to time are believed to bring the total up to 

$250,000.

The main haulage way, whose portal is situated 400 feet below the 

highest point on the outcrop of the lode, consists of a crosscut tunnel 

648 feet long bearing S. 45° W., and a tunnel following the trend of

i Eng. and Min. Jour. vol. 93,1912, p. 668.
112 ORE DEPOSITS OF HELENA MINING EEGION, MONTANA.

the lode for 1,600 feet in the general direction N. 80° W. The tunnel, 

which is of somewhat devious course, though considerably straightened 

by the present management, will eventually undercut the lode 

beneath a shoot of ore known to have been exploited in the summit 

workings. This shoot was 350 feet long and has been stoped out 

from the 120-foot level to the surface. The ore averaged 0.56 

ounce hi gold and 65 ounces in silver to the ton,1 being considerably 

higher in precious metals than the ore now mined.

Geologically the Blue Bird mine is situated on the western margin 

of a large andesitic area that constitutes a remnant of the roof 

under which the quartz monzonite was intruded. It lies at the head 

of a long embayment of the quartz monzonite extending into the 

andesites, an embayment sufficiently pronounced to be apparent on 

the geologic map, even with the small scale employed. On account 

of this proximity to the intrusive contact, various granitic dikes, 

comprising quartz monzonite, aplite, and diorite porphyry, penetrate 

the general country rock at the mine. Contact-metamorphic alteration also has affected the rocks, especially those on the west end of 

the property.

The rocks at the Blue Bird mine consist principally of andesitic 

tuffs grading into hard shales. They are known locally as "slates/' 2 

but none show slaty cleavage. Such local terms are apt to be misleading in a general report, so the term will'not be used here; it need 

but be pointed out that the "slates" at the Blue Bird mine are in no 

respect similar to the rocks locally called "slates" at Marysville. A 

specimen taken 100 feet south of the'lode line at the summit of the 

hill is a massive fine-grained greenish-gray rock of andesitic appearance. Under the microscope its tuffaceous character becomes 

apparent. It is .seen to be composed of plagioclase fragments with 

numerous epidote aggregates and chlorite scales embedded in a 

cryptocrystalline matrix. Some of the coarser-grained rocks are 

clearly of pyroclastic origin, but the finer-grained rocks are less easily 

discriminable, and it is of course impossible to draw any hard and 

fast line between argillaceous tuffs and pure shales. Here, as elsewhere in the region, as shown under the systematic treatment of the 

geology (pp. 23-29), lava, tuff, and argillaceous rocks occur interstratified. Undoubted massive porphyritic andesite, possibly latitic 

like that on Alta Mountain, is found in the mine; it was specially 

noted in drift No. 812. The bedded andesitic tuffs, as shown in 

a cutting near the superintendent's house, strike north and south 

and dip 40° E. '

Diorite porphyry forms extensive outcrops 800 feet north of the 

lode line, and similar rock has been encountered on the main haulage

J Winchell, H. V. and A. N., Notes on the Blue Bird mine: Econ. Geology, vol. 7,1912, p. 287. 

* They are so termed by H. V. and A. N. Winchell, op. clt, p. 292.
WICKES DISTRICT. 113

level of the mine. The relation of this rock to the surrounding rocks 

has not been determined, and it is not known whether the diqrite 

porphyry preceded or followed the quartz monzonite intrusion. 

Diorite porphyry from crosscut No. 605, locally termed granite, is a 

highly pprphyritic rock containing numerous plagioclase phenocrysts 

and hornblende inclosed in a microcrystalline matrix. Under the 

microscope the plagioclase phenocrysts are found to be labradorite 

near A^Arij and remarkably clear and well preserved, in view of the 

altered condition of the rest of the rock. The porphyritic hornblendes 

are largely changed to confused intergrowths of actinolite. The 

groundmass is composed essentially of a microgranitic aggregate of 

orthoclase and quartz, with accessory magnetite. Common secondary 

minerals are chlorite, epidote, and pyrite. The diorite porphyry mass 

in drift No. 605 shows various phases; where pyritized and sericitized 

it is light colored, and where mainly tourmalinized it is nearly black. 

Such kinds of alteration took place concurrently with the formation 

of the lode, and a small pocket of 2-ounce gold ore, consisting of 

columnar arsenopyrite embedded in a matrix of sphalerite and 

pyrite, was encountered in this altered diorite porphyry, which is 

therefore clearly older than the ore body.

Dacite is the youngest igneous rock at the mine. It forms a 50-foot 

dike lying in the footwall of the lode and dipping 70° S. The dike 

is apparently several thousand feet long, having been noted at the 

Penn Yan shaft on the west and at the Abe tunnel on the east. The 

dacite is a light-gray porphyry holding phenocrysts of quartz, plagioclase, and biotite in numerous small hexagonal plates. The characteristic features of this rock are sufficient to establish its identity 

with the extrusive dacites of the region, which are so well developed 

a few miles east of the mine. The dacite dike was therefore injected 

long after the main mineralization at the Blue Bird mine had taken 

place. Some brecciation of the footwall of the dike has occurred, 

followed by a.slight introduction of pyrite, but this mineralization 

was of no economic importance.

THE ORE.

The ore occurs in shoots along a profoundly tourmalinized zone 

trending approximately east and west. On the 200-foot level of the 

Blue Bird shaft a crosscut into the hanging wall shows 40 feet of 

tourmalinic rock. This rock is of striking appearance and shows,. 

contrasted against a light-colored matrix, a great number of radial 

groups of black columnar tourmaline, the largest several inches 

in diameter. Arborescent growths are common. Microscopically 

the rock is seen to be composed of tourmaline, quartz, sericite, 

pyrite, and apatite. -The tourmaline ranges from broad columns 

of brown-green color" to long colorless ultramicroscopic needles 

70935° Bull. 527 13 8
114 OKE DEPOSITS OF HELENA MINING KEGION, MONTANA.

traversing a succession of consecutively adjoining quartz grains. 

Apatite, although ranking as an accessory mineral only, was 

probably introduced with the tourmaline and pyrite. The rock as 

a whole has been so thoroughly recrystaUized that its original character is no longer recognizable. The ore is found in rock of this kind. 

Along the lode some remarkably thorough postmineral brecciation 

and crushing has taken place, producing rounded fragments of ore 

embedded in a clayey matrix.

In the workings on the main haulage way and in the two levels 

below it, the features of the ore-bearing zone are somewhat different. 

A tourmalinic breccia occurs on both sides of a granitic dike, which 

is 10 to 15 feet thick and dips 70° S. In places this granitic dike is 

intensely shattered. The tourmalinic breccia is not continuously 

developed along the course of the dike. It is composed of angular 

fragments of various rocks cemented together by a black fine-grained 

aggregate of tourmaline and quartz. This cement is similar in composition and appearance to the "black quartz" at Rimini. Among 

the rock fragments inclosed in the breccia, granite is most easily 

recognizable, but many light-colored fragments of fine texture are 

present, which are probably the highly altered equivalents of the 

andesitic tuffs. The granite has undergone extreme metamorphism 

and now consists essentially of quartz, sericite (unusually well developed in fan-shaped groups), tourmaline, and cubical pyrite. Arsenopyrite is found locally.

Isolated bodies of ore-bearing tourmaliniferous rock are found 

parallel to the tourmalinic breccia. Along the strike, where the 

tourmalinization was feeble, the tourmaline rock is represented, as in 

drift No. 812, by a series of vertical stringers of tourmaline 1 inch wide, 

which consist of a succession of radial groups centered along a crack 

commonly lined with pyrite.

, The ore shipped to the smelter during 1911 was composed principally 

of iron-black tetrahedrite and pyrite intergrown with perfect radial 

groups of tourmaline, commonly under half an inch in diameter. 

The pyrite has a tendency to. occur as large irregular or distorted 

cubes. Sphalerite and galena are rare constituents. Under microscopic examination such ore shows tourmaline, quartz, pyrite, 

tetrahedrite, sphalerite, and galena. The thin delicate prisms of 

tourmaline pierce the various sulphides and all the minerals appear 

to be essentially contemporaneous.

The ore is stated to carry 10 to 25 ounces to the ton in silver, 3 to 5 

per cent of copper, and small amounts of gold.

COMET MINE.

The Comet mine, the property of the Montana Consolidated Copper 

Co., lies 7 miles northwest of Boulder. It has been one of the largest producers in the region and is popularly credited with a produc-
WICKES DISTBICT. 115

0

tion of $13,000,000. For some years no systematic work has been 

in progress; during 1911 four different parties of lessees were at work 

getting out ore above the tunnel levels.

The mine lies near the contact of the quartz monzonite and the 

andesite. On the summit of the ridge northeast of Comet the andesite, which is of massive porphyritic character, is tourmalinized, 

showing numerous well-developed radial groups of tourmaline. Far- 

.ther northeast on this same ridge considerable coarse andesitic breccia 

is exposed and shows much tourmalinization accompanied by development of sericite. The country rock at the Comet mine itself is 

a fairly coarse grained quartz monzonite. A large pit over 100 feet 

long and 30 feet wide," on the summit of the hill east of the shaft, 

contains a few horses of quartz mixed with altered granite, which is 

intensely sericitized and shows a small development of tourmaline. 

'This feature indicates that the Comet ore body belonged to the older 

tourmaline-bearing set of deposits. In confirmation of this is the 

fact that the ore body is cut by a dacite dike 35 feet wide; much rock 

from this dike can be seen on the mine dumps where, because of 

its striking appearance, it is quite conspicuous. It shows large 

quartz phenocrysts and altered feldspars embedded in a gray matrix 

commonly showing fluxion structure.

The old mine workings are generally inaccessible; the timbers are 

crushed and the tunnels caved. At one place the lessees are taking 

out ore from three parallel lenticular masses trending east and west 

and dipping nearly vertical. The ore is a solid mass of sulphi es, 

more or less oxidized, and is usually accompanied by a vein of quartz 

against one or other of the walls. The granite is much shattered and 

slickensided and reduced to gouge. The ore is commonly kneaded 

into fragments embedded in vein matter. The ore minerals are 

essentially galena and sphalerite, with which are associated pyrite and 

chalcopyrite and rarely arsenopyrite.

CORBIN COPPER CO.'S PROPERTY.

The property of the Corbin Copper Co., comprising about 40 

patented claims, is situated at the head of Ciancy Creek, a few miles 

west of Corbin. A large amount of development work has been done 

here in recent years and a concentrator has been built on the property. 

At the present time most of the work is being done on the Dewey and 

Bonanza tunnels.

The country rock consists mainly of andesitic tuffs and interstratified argillaceous beds. The tunnels above the Dewey tunnel show a 

zone of shattered and leached rocks at least from 60 to 100 feet wide. 

Near the mouth of the Bonanza tunnel are large reef-like croppings 

of red-weathering tourmalinized andesite. The rock exposed in the 

Bonanza tunnel is generally soft and of granitic appearance. It is a 

gray medium-grained granitoid, in which biotite is the only mineral
116 OEE DEPOSITS OP HELENA MINING KEGION, MONTANA.

o

certainly recognizable, and is evidently an intrusive dike in the 

andesites. Under'the microscope'it shows an aplitic texture and is 

seen to consist of plagioclase, orthoclase, micropegmatite, and biotite. 

In places in the tunnel this rock has been highly altered, so that it 

is now made up largely of tourmaline and forms exceedingly hard 

and resistant material. It is composed, as determined microscopically, of quartz, brown tourmaline, sericite, and pyrite, accompanied 

by accessory apatite and zircon. Locally masses of ore, composed 

of galena and chalcopyrite associated with pyrite and sphalerite, 

occur in the tourmalinic rock. At the time of the writer's visit a raise 

was being put through on a mass of ore of this kind from the Bonanza 

tunnel to the Dewey tunnel, 207 feet above.

The Rosalie tunnel, situated 2,000 feet southeast of the preceding 

tunnels, shows dense-grained argillaceous rocks lying horizontally 

or inclined at low angles to the south. A drift was run on a narrow vein more than 800 feet. The vein shows a remarkably fine and 

continuous hanging wall. It is faulted in places, the greatest displacement encountered being 35 feet. The ore was an oxidized 

pyritic ore,, showing considerable tourmalinization. It was of good 

grade, but on account of the small quantity found work has been 

suspended on this vein.

In the Montana tunnels a mineralization of an entirely different 

character and origin from that of the Rosalie tunnel is exposed. 

It consists mainly of a pyritized conglomerate associated with dacitic 

tuff and massive dacite, showing flow banding. In the upper tunnel 

pyritization ceases against the massive dacite, which forms a sort of 

hanging wall. A winze to the lower tunnel, 35 feet below, shows that 

the rock is considerably impregnated with pyrite, perhaps to the 

extent of 3 or 4 per cent. Galena occurs rarely. The rock along the 

upper tunnel is said to average 8 ounces silver for a distance of 130 

feet.

. The tuffaceous rock is soft and incoherent in the tunnels but hardens 

on exposure to .the atmosphere. The conglomerate carries wellrounded cobbles, the largest 12 inches long, some being dacite, some 

alaskite, but the most andesite. It holds irregular lenses of white 

material, probably tuff, abruptly delimited from the inclosing rock. 

In the lower tunnel the conglomerate is overlain by a' flow of dacite 

dipping 20° N. At other places massive dacite abuts against the 

conglomerate along fault contacts.

CORBIN METALS MINING CO.'S PROPERTY.

The Corbin Metals Mining Co., whose.property is situated about 

2 miles southeast of Corbin, is developing the Silver King and other 

lodes. An incline has been sunk to a depth of 400 feet and considerable drifting and cross-cutting has been done. Five parallel
- WICKES DISTRICT. - 117

veins, inclosed in the granite country rock and trending N. 30°-45° E., 

have been crosscut. The veins are strong shear zones, ranging in 

thickness from a few niches to 8 feet. They are filled with crushed 

granite, which has been sheared into small irregular lenticles. Some 

- of the veins are tight and dry, but others let in considerable water. 

As seen on the lowermost level the gangue material of the veins is 

somewhat altered by vein-forming solutions and slightly pyritized. 

There is no evidence of oxidation at this depth, nor that enrichment 

has taken place, and it is clear that the zone of primary sulphides 

has been encountered. The metallic minerals noted in material on 

the dump are pyrite, galena, subordinate sphalerite, and rarely 

molybdenite.

GREGORY MINE.

The Gregory lode was located in 1864 and is therefore one of the 

oldest properties in the region. The workings are now inaccessible, 

the surface plant destroyed, and the smelter is represented by only 

the stumps of the stacks. The production, according to popular 

report, was $8,000,000 in lead and silver.

The mine lies near the contact of-the andesite and a tongue of 

diorite intrusive from the main mass of quartz monzonite. The upper 

Miocene dacites, as shown on the geologic map (PL I), occur a few 

hundred feet south of the mine but are of later origin than the ore 

bodies. The dumps show mainly andesite, which is commonly 

pyritized and in places slightly tourmalinized. Diorite is present in 

smaller amounts and is as a rule pyritized and sericitized. The ore 

consists of galena, sphalerite, and pyrite. According to Mr. Lindgren, 

who was assayer at the mine in 1883, the ore worked at that time was 

extremely high in zinc, and ore carrying as much as 15 per cent zinc 

was smelted. 

HORSESHOE PROSPECT.

The Horseshoe prospect lies 2 miles west of Corbin. Several hundred feet of tunnel have been drifted along an irregular intrusion 

contact of aplite and andesite. The aplite is highly tourmalinized, 

pyritized, and sericitized. The andesite also shows a similar set of 

alterations, but a further alteration has been superposed upon the 

earlier by the action of downward-moving sulphate solutions. Exposed surfaces in the tunnels are covered with needles of gypsum, and 

the andesite when broken open is found to be permeated with flat 

crystals of gypsum. In addition to the aplite and andesite, masses of 

dacite were encountered in the tunnels and complicate the geology. 

The contacts of the dacite with the other rocks are sheared, and this 

shearing is probably a measure of the amount of postmineral movement, as the dacite itself is unmineralized and shows neither the 

intense tourmalinization nor sericitization that the older rocks do.
118 ORE DEPOSITS OF HELENA MINING REGION, MONTANA.

The primary mineralization, in which apparently the principal metallic 

sulphide,, introduced was pyrite, therefore took place after the intrusion of the aplite and before the intrusion of the dacite. Pyrite has 

possibly been introduced after the dacite came into place, but in 

comparatively small amount.

MINAH MINE.

The Minah, one of the old and well-known mines in the Wickes dis- \ . . . trict, is situated 1£ miles northwest of Wickes. According to popular

report the production has been $2,000,000, but the property is now idle 

and the smelter dismantled. During 1901 the output aggregated 

3,000 tons of shipping ore, which carried 20 ounces of silver and $12 

in gold to the ton.1

The country rock is a highly porphyritic dacite, showing large 

phenocrysts of striated feldspar, numerous small quartzes, and small 

black flakes of biptite. The ore bodies, however, seem to have been 

not in the dacite, but in the underlying andesite. The mine was 

developed by a series of tunnels, the lower of which extends 300 feet 

through dacite to reach the vein. The upper tunnel is situated at 

the contact of the dacite with the andesite, which is here a fault contact. Such ore as can now be seen contains large amounts of arsenopyrite in columnar crystals, and also pyrite, galena, sphalerite, and 

minor chalcopyrite, but it is not known whether this ore is representative of what was .the productive part of the deposit.

MINNESOTA MINE.

The Minnesota mine is situated on Clancy Creek, a short distance 

south of the old town of Gregory. It is one of the oldest located 

properties in the region; some development work has been done on it

in recent years, but it was idle during. 1911. The property is developed by a number of tunnels and an open cut. At the upper end of 

the open cut a small vein of copper-stained quartz is inclosed in muchaltered andesite. The strike is east and west, and" the dip is steep 

to the north. On the south wall, near the east end, there is altered 

granitic rock which contains small veins of tourmaline with quartz 

and pyrite. The upper tunnel is in altered granitic rock. The lower 

tunnel shows an irregular intrusion into the andesite of granitic rock, 

which where fresh appears to be a diorite. Slipping is shown along 

most of the contacts. The ore occurs in small veins, which cut both 

rocks but do not appear to follow the contact. They are of workable 

width in the granite only.

The ore minerals, consisting of galena, blende, pyrite, a minor 

amount of arsenopyrite, and rare specks of chalcopyrite, are inclosed 

in a small amount of quartz gangue.

i Fourteenth Ann. Kept. Inspector of Mines, Montana, 1902, p. 67.
WICKES DISTRICT. 119 

NORTHERN PACIFIC MINE.

The Northern Pacific mine lies on the west side of Alta Mountain. 

The country rock is a highly flow banded andesite. The ore contains 

galena, pyrite, and sphalerite, in a gangue of manganiferous calcite 

holding fragments of brecciated andesite.

WICKES-CORBIN COPPER CO.'S PROPERTY.

The property of the Wickes-Corbin Copper Co. is situated a short 

distance southeast of Wickes in a small gulch known locally as Picnic 

Gulch. The principal piece of development work is the Bunker Hill 

tunnel No. 4, approximately 900 feet long. The prevailing country 

rock at the workings is a dark gray-blue andesite, well exposed on the 

west side of Picnic Gulch. On the east side of the gulch the andesite 

is intruded by granitic rocks belonging to the Boulder batholith and 

shows conspicuous evidence of thermal metamorphism. The summit 

of the ridge on the east of the gulch is occupied by an intrusive mass 

of aplite or alaskite, which in places is prominently porphyritic, owing 

to the presence of large phenocrysts of quartz and feldspar. As a rule 

it is thoroughly sericitized, approaching a fine-grained greisen in appearance, and it is almost impossible to obtain unaltered rock.

The Bunker Hill tunnel No. 4 is being driven to undercut an extensive outcrop .of leached andesites exposed approximately 300 feet 

above the portal. A tunnel 275 feet above the Bunker Hill tunnel 

is 200 feet long and traverses leached andesites; drifts extending 50 

to 100 feet in a north-south direction are more or less coated with chalcanthite crystallized copper sulphate and with gypsum needles. 

The rocks contain disseminated pyrite, evidently cupriferous, veinlets of solid pyrite, and also some that are galena bearing. The rocks 

traversed in the lower or Bunker Hill tunnel are exceedingly jointed 

andesites, which are pierced by a vertical aplite dike, approximately 

200 feet wide as exposed along the tunnel. The andesites are more or 

less pyritized throughout the length of the tunnel and are traversed by 

pyritic seams. Some weak and poorly defined irregular shear zones 

are encountered at various places and are veined with discontinuous 

quartz stringers carrying galena, sphalerite, chalcopyrite, and pyrite. 

The main lode, lying on the inner side of the aplite contact, is 4 feet 

wide but does not constitute pay ore. It is believed that the deposit 

exposed on the surface, if the dip is vertical, will be intersected in 

tunnel No. 4 at about 900 feet from the portal. If there is any secondary enrichment it can be predicted that it has taken place a short 

distance below the upper tunnel level, as all ore in the main crosscut 

is of primary character.

In the Tulare tunnel, on the east side of the gulch, considerable 

drifting has been done along irregular shear zones in the andesites.
120 ORE DEPOSITS OP HELENA MINING EEGION, MONTANA.

In these workings there were encountered some masses of biotite 

andesite, a rock related to the dacites that occur in great volume west 

of Wickes.

BOULDER AND BASIN DISTRICTS. 

LOCATION.

Boulder and Basin are situated on Boulder River. Boulder, a 

town of 800 population, is the county seat of Jefferson County. Little 

mining is in progress in the district adjoining it. Basin, founded in 

1880, lies 8 miles west of Boulder; a concentrator for the treatment 

of Butte ores was built here because of the abundance of water. 

Cataract and Basin creeks enter Boulder River near Basin from the 

north and northwest; the roads that ascend the valleys of these 

streams render a large part of the Boulder Mountains tributary to 

Basin.

The Boulder Hot Springs are situated 2 miles south of Boulder. 

They have been carefully described in detail by Weed,1 who has 

shown that the hot waters are in process of forming mineral veins 

filled with quartz, stilbite, and calcite, which contain small but 

appreciable amounts of gold and silver.

DESCRIPTIONS OF MINES. 

ROBERT EMMET MINE.

The Robert Emmet mine lies at an elevation of 5,600 feet on the 

south side of the Wickes-Boulder divide near the road between those 

towns. It is electrically equipped and a shaft has been sunk to a 

depth of 470 feet, with levels at 200 feet and 350 feet, but these were 

not accessible at the time of visit. The vein occurs in the granite 

several hundred yards east of the contact of the granite and andesites, 

which are well exposed upon the divide; it is said to strike east and 

west and to average 5 feet in width. The ore in the upper level as 

seen on the dump consists of sphalerite, pyrite, chalcopyrite, and 

galena in a quartz gangue. The valuable constituents are silver and 

copper, together with a small amount of gold (12 to $5).

AMAZON MINE.

The Amazon mine is situated on the west side of Boulder Valley, 

4 miles north of Boulder. The mine was closed down during 1911, 

but its owners intend to reopen it. The country rock is a granite. 

The ore contains much sphalerite and galena, with a small amount of 

chalcopyrite and pyrite in a quartz gangue.

BALTIMORE MINE.

The Baltimore mine is situated on Boomerang Creek, 4£ miles 

northwest of Boulder. A small crew of men were employed on the 

property during 1911.

1 Weed, W. H., Twenty-first Ann. Kept. U. S. Geol. Survey, pt. 2,1900, pp. 227-255.
BOULDER AND BASIN DISTRICTS. - 121

Two separate ore bodies have been exploited. The upper one is 

opened by tunnels 4, 5, and 6; the lower one by tunnels 1, 2, and 3. 

Tunnel 1, which enters the hill at an elevation slightly above the 

bed of Boomerang Creek, is the only one in which work was being 

done at the tune of visit.

The general country rock in the vicinity of the mine is granite, but 

the principal workings are in intrusive masses of aplite, a lightcolored granular rock consisting of orthoclase, quartz, and a minor 

amount of biotite. There have been encountered also some porphyry 

dikes of andesitic character, related to the dacites capping the granite 

on Sugarloaf Mountain, which lies a short, distance southwest of the 

mine. The ore bodies that have been mined were short, irregular 

veins with numerous branches, but locally swelling to considerable 

dimensions. In tunnel 5 there remains a large body of ore, consisting 

mainly of black sphalerite inclosed in a coarse white quartz gangue.

Galena, pyrite, and chalcopyrite are associated sulphides. Sphalerite transformed to covellite is found as a rarity. This ore body 

was too zincky to work formerly, and only the portions containing 

considerable lead were extracted.

The ore body developed in the three lower tunnels has been largely 

stoped out to the surface from above tunnel 2. . In this tunnel the 

vein is approximately 130 feet long and is terminated at both ends 

by faults. The first 65 feet has been stoped out; the remainder of 

the vein consists of flinty gray and chalcedonic quartz, evidently too 

low grade to pay for extraction. In the winze between tunnels 2 and 

1, 86 feet below, the vein is well shown. It stands vertical and 

ranges in width from 6 to 8 feet. The hanging wall is sharply defined, but the footwall is less well marked, the vein inclosing some 

aplite, reticulated with fine-grained siliceous veinlets. The gangue is a 

coarse white quartz irregularly mingled with dark flinty quartz and 

carrying a sparse amount of metallic sulphides. Pyrite is the principal sulphide, and sphalerite, galena, and chalcopyrite occur in 

minor amounts. The ore is stated to contain $7 a ton in gold.

GRAY EAGLE MINE.

The Gray Eagle mine is situated at the head of Bishop Creek, a 

tributary of High Ore Creek. The lower or main working tunnel is 

1,800 feet long, but only 1,000 feet are now accessible; it traverses 

both granite and aplite. The pit on the surface, however, is in 

andesite. The course of the vein is N. 75° W. The lower tunnel is 

approximately 200 feet below the pit, and a shaft inside the tunnel 

was sunk another 200 feet. Apparently there was one principal ore 

shoot, which was .stoped out to the surface. The estimates of production show a considerable range, but half a million dollars' worth 

of lead-silver ore seems to be a moderate figure.
122 QBE DEPOSITS OF HELENA MINING EEGION, MONTANA. 

COPPER KING PROSPECT.

The Copper King prospect is situated 3 miles northeast of Basin in 

Hiawatha Gulch. The principal development consists of a tunnel 

drifted 125 feet along the vein. During 1911 a shaft was being sunk 

on the vein and attained a depth of 35 feet. Two men were employed.

The ore body is a narrow vein a foot thick at most, trending east 

and west, inclosed in granite walls. The ore is a quartz carrying 

tetrahedrite and a small amount of pyrite. The quartz is of coarse 

glassy character mingled with cryptocrystalline and chalcedonic 

phases. By sorting, a product containing 24 ounces of silver and 

$1.60 in gold to the ton and running 94 per cent of silica is obtained. 

The ore is sorted so as to run high in silica, because it is shipped to the 

copper smelter for use as converter lining, and a bonus of 15 cents is 

paid for each unit in excess of 75 per cent of silica.

QUARTZ MASS NEAR BASIN.

A great mass of quartz is inclosed in granite 1 mile east of Basin 

alongside the track of the Great Northern Railway. This quartz 

mass was formerly of some ecomomic importance; it was quarried 

and shipped to Butte as converter lining for the copper smelters.

The quartz mass is 200 feet in altitude from its top.to the floor of 

the pit; most of it forms a sheer face of coarse solid white quartz 

absolutely devoid of any metallic sulphides; on the level of the pit it 

is 350 feet wide. Coarse quartz monzonite surrounds the quartz mass, 

but the actual contact is covered everywhere by loose material. The 

monzonite shows megascopically no noticeable alteration. In 'the 

tunnel, which is 100 feet below the pit, the main rock is a coarsely 

granular alaskite porphyry slightly pyritized. Only an insignificant 

amount of quartz is exposed in the tunnel. A noteworthy feature is

that small quartz stringers, branching off from the larger masses and 

penetrating the porphyry, inclose numerous euhedral crystals of 

orthoclase up to three-fourths of an inch in length, forming a rock 

resembling a quartzose pegmatite.

EVA MAY MINE.

The Eva May mine is situated on Cataract Creek, 8 miles north of 

Basin. The mine is developed by a shaft 1,200 feet deep, with levels 

at every 100 feet down to the 600-foot level, and also at 800 feet and 

1,200 feet. The general rock in the vicinity of the mine is a coarse 

gray quartz monzonite cut by intrusions of a tourmaline-bearing 

white aplite. The ore is a coarse white quartz carrying pyrite, chalcopyrite, galena, sphalerite, and tetrahedrite. Further, it is noteworthy that some of the ore contains considerable black tourmaline. 

An interesting fact is that the mine dump shows also considerable 

blue-gray cryptocrystalline quartz, but the significance of this or its
BOULDEK AND BASIN DISTEICTS. 123

relation to the tourmaline-bearing quartz ore could not be determined on account of the inaccessible condition of the underground 

workings. 

Concerning this mine Weed 1 says:

An example of the economic necessity of carefully observing secondary fractures 

and accompanying enrichment is shown by the Eva May mine, on Cataract Creek, 

near Boulder, Montana. In the early history of the mine much high-grade ore was 

found, consisting of pyrite. together with more or less galena, blende, and chalcopyrite, 

the whole impregnated with scattered bunches of rich antimonial sulphides of silver. 

The vein is a large one and shows thick ore shoots of pyritic ore, but the bulk of this, 

away from the enriching fracture, is too poor to work.

An analysis of a concentrate of the pyritic ore, cited by Weed, shows. 

9.83 per cent of lead, 6 per cent of zinc, 4.56 per cent of copper, 0.1 

ounce of gold, and 1,85 ounces of silver.

The King tunnel, which is being driven to hold some unpatented 

ground, is 450 feet in length and is drifted along a highly tourmalinic 

lode 2£ to 3 feet wide.. The principal sulphide is pyrite, with arsenopyrite, sphalerite, and galena in minor amounts. . Sulphides are contemporaneous with the tourmaline and it is noteworthy that in 

comby veinlets of quartz the tourmaline fills the central portion of 

the veins.

TINGLE SAM MINE.

The Uncle Sam mine is an old property, from which in early days 

some 12,000 tons of ore were taken out and hauled to Wickes. This 

was said to carry from $30 to $90 a "ton. Operations have recently 

been commenced, to reopen the mine. A crosscut tunnel is being 

driven to intersect the lode at a depth of about 175 feet; the estimated 

length is 450 feet, of which 250 feet had been completed at the time of 

visit.

The country rock is aplite, forming a border between the main body 

of granite, which is encountered in-the tunnel, and a capping composed of andesites. An aplite dike, 10 to 15 feet wide, with gouge on 

both contacts was crosscut in the tunnel. The vein is near the con-' 

tact of the aplite with the overlying andesites; in fact the pits on the 

extension of the vein are in andesit'e. The andesites, or probably 

more accurately latites, are flow-banded varieties; they have been 

considerably metamorphosed by the intrusion of the granitic rocks 

and in consequence are not easily recognizable. They are light colored 

and because of their light color and highly developed flow banding 

have been regarded as rhyolites and phonolites. 

The old workings took out a shoot of ore extending 80 feet east of 

the incline shaft and reaching a depth on the incline of 130 feet. 

The ledge is said to be 12 feet wide and the ore minerals are galena 

and sphalerite with subordinate tetrahedrite.

» Weed, W. H., Genesis of ore deposits, special publication Am. Inst. Min. Eng., 1902, p. 495.
124 ORE DEPOSITS OF HELENA MINING REGION, MONTANA. 

WESTERN RESERVE MINING CO.'S MINE.

The Western Reserve Mining Co. owns the Hattie Ferguson mine 

on Cataract Creek, 6 miles from Basin. The mine was formerly 

worked by a shaft 140 feet deep, but a crosscut, now over 1,400 feet 

long, is being driven and is expected to cut the vein at a depth of 362 

feet below the collar of the shaft. The country rock is aplite, which 

extends northward to the Sirius and Klondike mines, although a 

small amount of normal granite is encountered in this area. The 

aplite is somewhat porphyritic, and near the vein this texture is 

accentuated by the chalky appearance of the feldspar. The ore 

minerals are galena, pyrite, and sphalerite, and 40 per cent of the 

value is said to be in gold.

BUTTE AND PHILADELPHIA PROSPECT.

The Butte and Philadelphia prospect is situated on the flank of 

the group of peaks known as the Three Brothers and lies northwest 

of the south peak at an elevation of 7,800 feet. The property is 

reached from the south by the road extending up Basin Creek from 

Basin. Some machinery was installed and a small amount of development work was in progress during 1911. A series of three parallel 

veins traverses the granite, one of which is developed by 250 feet of 

tunnel. The strike is N. 80° W. and the dip 65° N. The ore occupies 

a 7 to 10 foot crushed zone in the granite. The metallic minerals 

are chiefly pyrite, with small amounts of galena and sphalerite, 

associated with very little quartz. Between the pyrite and the 

heavy zone of gouge that occurs along the hanging wall, there are 

in places slabs of galena 1 inch thick and from 1 to 2 feet in diameter, 

showing a banded structure.

BULLION MINE.

The Bullion mine lies at an elevation of 7,400 feet on the northwest 

side of Jack Mountain, 10 miles by road from Basin. A concentrator 

and smelter of 200 tons daily capacity was constructed and several 

thousand feet of drifts and tunnels run, but the property is now idle.

The country rock at the mine is mainly quartz monzonite, intruded, 

however, by small dikes arid masses of tourmaline-bearing aplite. 

Near the ore the rock is much altered by sericitization and pyritization and is cut by irregular veins, carrying black columnar tourmaline, quartz, and pyrite. The ore minerals in order of decreasing 

abundance are pyrite, tetrahedrite, galena, sphalerite, chalcopyrite, 

and arsenopyrite, and are inclosed in a gangue of coarse white quartz. 

A second and later mineralization has affected the veins and is 

represented by a gray, flinty quartz carrying minor amounts of 

metallic sulphides. The ore therefore is of composite origin.
BOULDEK AND BASIN DISTBICTS. 125 

MORNING STAR MINE.

The Morning Star mine is situated on the west side of the valley 

of Basin Creek, half a\ mile west of Winters Camp. The ore body 

strikes east and west and consists of a quartz vein inclosed in a 

much-altered andesite. The metallic minerals are galena, sphalerite, 

pyrite, and chalcopyrite in small amount. The richest ore occurs 

in a 2^-inch streak, consisting principally of galena, and is reported 

to run 40 ounces in silver and S3.80 in gold to the ton and 50 per 

cent of lead.

OUSTER MINE.

The Custer mine is situated 1 mile southeast of Basin at an elevation of 6,000 feet. The ore body is a wide quartz vein, in places 

attaining a width of 30 feet, traversing sericitized granite. The 

strike of the vein is east and west, but the pay streaks, which are 

generally under 1 foot in width, are very irregular in width and 

direction. The ore is a high-grade silver-lead carbonate, which is 

stated to carry 40 ounces in silver and $18 in gold to the ton.

ALLPOKT MINE.

The Allport Mining Co.'s property, consisting of 5 end-on-end 

claims extending northeast and southwest, is situated 4 miles south 

of Basin, at an altitude of 7,500 feet. The main development work 

consists of a shaft 110 feet deep and a number of short levels. During 1911 work was in progress on the 55-foot level, but the vein 

below this level is flooded with water.

The country rock is a coarse gray granite. On the 55-foot level 

the vein, which trends N. 36° E. and dips nearly vertical, is from 5 

to 6 feet wide and consists' partly of quartz and partly of granite. 

The wall rocks are much altered by sericitization and impregnation 

with pyrite. The vein on the 100-foot level is said to be 12 feet 

wide and to assay $16 a ton in silver and gold across the face. The 

ore is a bluish-gray cryptocrystalline quartz, carrying mainly pyrite 

in small amount and some sphalerite.

On the northwest extension of the course of the vein some coarsegrained white quartz veinlets were seen that carried considerable 

tourmaline, but these were said to be valueless. These quartztourmaline veinlets have been brecci'ated and recemented by chalcedonic quartz.

RUBY MINE.

The Ruby mine is situated on Lowland Creek, 4 miles from its 

junction with Boulder River. The 10-stamp mill on the property isbuilt near the stream at 6,000 feet altitude, but the mine lies half a 

mile southeast of the mill, at an altitude of 6,750 feet. By air line 

the Ruby mine is 14 miles north of Butte. The property was located 

in the seventies and surveyed for patent in 1888.
126 OBE DEPOSITS OF HELENA MINING REGION, MONTANA.

The production is estimated to be $1,250,000 70 per cent in gold 

and the remainder in silver. Rich ore was sent to the smelter, and 

netted from $17 to $302 a ton in carload lots. The mine has been 

operated only intermittently, having several times been,considered 

"as worked out. In September, 1911, the mill was again started and 

commenced crushing a dump of some 1,200 tons of second-class ore 

which had accumulated during recent exploratory work. 

1 The main shaft is situated at 6,750 feet altitude and was sunk to a 

depth of 400 feet. A crosscut tunnel intersects the shaft at 150 feet 

below the surface. The mine is now flooded with water below the 

150-foot level.

The general country rock at the Ruby mine is dacite, or rhyolite, 

as it is better known locally. It lies in the heart of the great area of 

dacites that extends southward to Butte and forms Big Butte at that 

city. At the Ruby mine the original thickness of the superposed succession of dacite lavas and breccias was at least 2,500 feet. The dacite 

at the mine is a markedly porphyritic rock, showing numerous crystals 

of quartz and glassy striated feldspar embedded in a light-grayish 

groundmass of dense texture. In the vicinity of the mill the rock is 

prevailingly a dacite breccia.

The ore-bearing zone extends from the main shaft to the Columbia 

claim, a distance of several thousand feet, the general trend being 

S. 20° E. Within this zone the ore occurs in shoots, or more properly 

in parallel veins, dipping steeply westward and apparently disposed 

roughly in Echelon fashion. The Ruby shaft was sunk on a shoot of 

ore which, as shown on the stope map, was 260 feet deep, 40 feet long, 

and 20 fee't wide. The extreme length, which was 70 feet, was on 

the 200-foot level. This shoot of ore, or pipe, as it is termed locally, 

yielded $600,000. The country rock in the vicinity of this shoot is 

extremely sheared and shattered. To the north the ore body is terminated by a southward-dipping fault zone, which is brecciated and 

mineralized through a width of 40 feet or more.

South of the Ruby shoot another vein was found, in which the 

productive portion was 170 feet long and 4 feet wide. The walls of 

this vein are generally well denned. A marked vertical undulation 

is a characteristic feature of the hanging wall. In places the vein is 

filled with fault breccia, some fragments of which are 20 niches long. 

Toward the north the fissure is apparently cut off by a cross fault 

dipping 45° S., although the fissure is here so small as hardly to be 

recognizable. The fissure is traceable by a well-defined footwall for a 

distance of 60 feet south of the productive portion, where it is terminated by a cross fault dipping south. It is not impossible that some 

of the cross faults are fractures contemporaneous with the major 

fissuring. The ore consists of angular dacite fragments cemented by 

quartz, calcite, and minor adularia. The adularia, where embedded
BOULDEK AND BASIN DISTRICTS. 127

in quartz and calcite, is not easily distinguishable, but in places forms 

crystals up to one-fourth inch hi diameter. Its identity was established chemically and optically. The quartz is commonly clear, 

glassy, and drusy, but where it is solid it is of compact saccharoidal 

texture. Some of the siliceous veinlets show a porcelain-like texture,, 

but such cryptocrystalline quartz is far less common at the Ruby 

mine than at the surrounding properties. The sulphides, which are 

confined to the gangue material that cements, the dacite fragments, 

comprise pyrite, argentite, and possibly others that because of their 

fine subdivision are not readily recognizable. Native silver is common locally. The grade of the ore decreases with increase in size 

of the dacite fragments filling the fissure.

The workings extend on the southeast into the Columbia claim, 

which was recently acquired by the owner of the Ruby mine. The 

Columbia vein is developed by a number of drift tunnels, one ,of which

now connects with the Ruby workings. A pronounced joint system, 

trending N. 20° W., occurs in the Columbia tunnels and appears to 

have governed the trend of the ore body. The Columbia vein represents a zone of brecciation 12 feet or more wide. The individual 

fragments of brecciated dacite show little movement or deorientation, 

and the filling of the vein consists for the most part of angular fragments of dacite coated with a crystalline crust of quartz. In places, 

however, there is considerable white quartz unmixed with country 

rock. Some of this quartz is identical in appearance with that of the 

Empire and other veins of Marysville; that is, it is of lamellar habit 

 and shows irregular pyramidal hollows lined with small glassy quartz 

crystals. Such ore is of low grade, carrying $4 to $5 a ton in gold. 

The former owner took out only a rich streak along the hanging wall.

KIT CARSON MUTE.

The Kit Carson mine is situated on Lowland Creek near the stamp 

mill of .the Ruby mine. The production is said to have been $100,000 

in gold and silver. The property has in recent years been worked by 

lessees, but was idle during 1911.

The main development is a drift tunnel entering the hill from the 

level of the stream bed. It is approximately 325 feet long and trends 

N. 10° E. The country rock is porphyritic dacite. At the face of 

the tunnel is seen 5 feet of highly brecciated porphyry, in which- the 

fragments are irregularly traversed by chalcedonic and porcelainoid 

veinlets. The west wall (footwall ?) is marked by a zone of soft, white 

clay. Locally, as shown along the.tunnel, the hanging wall seems to 

have been determined by a platy structure parallel to the flow banding 

of the dacite.

Material on the dump shows that the dacite was netted with veinlets of silica carrying pyrite. Drusy crystalline, chalcedonic, and 

porcelainoid varieties of silica are intimately associated together.
128 OKE DEPOSITS OF HELENA MINING REGION,' MONTANA.

The feldspars of the dacite have been altered to chalky spots, in 

appearance strongly suggestive of kaolin but which prove under the 

microscope to be cryptocrystalline aggregates of sericite. The quartz 

veinlets when examined microscopically are found to contain minute 

crystals of adularia.

MEMPHIS PROSPECT.

The Memphis prospect is on Lowland Creek, half a mile upstream 

from the stamp mill of the Ruby mine. The geologic features are 

similar to those of the Kit Carson mine. The dacite is cut by irregular veinlets of cryptocrystalline silica, carrying a little pyrite in 

cubes and pentagonal dodecahedrons. In addition to the dark grayblue quartz the porcelainic variety is common.

ELKHORN DISTRICT. 

LOCATION AND HISTORY.

The Elkhorn district forms the southeast corner of the region 

considered in this report. The economic importance of this area 

has depended almost wholly on the Elkhorn mine, around which 

grew the town of Elkhorn.

The town of Elkhorn, situated at an altitude of 6,500 feet, lies 12 

miles northeast of Boulder, which is on the Butte branch of the 

Great Northern Railway. It is connected with Boulder by a branch 

line operated by the Northern Pacific Railway, which runs three 

trains a week.

The district was prospected before 1870 but did not attain prominence until after the discovery of the Holter lode in 1875, on which 

was founded the great mine subsequently known as the Elkhorn 

mine. This property was worked nearly continuously until 1900, 

when it was regarded as worked out and sold for a small sum, practically for the value of the machinery and equipment. The new 

owners commenced to rework the old dumps and to reopen the mine 

in 1901; since 1906 the mine has again been in continuous operation.

STRATIGRAPHIC GEOLOGY.

Elkhorn lies on the eastern margin of the Boulder batholith. The 

geology here is far more complex than at any other district in the 

region, owing to the number of sedimentary formations and to the 

abundance and variety of igneous intrusions. The details, however, 

have been ably worked out by Weed and must be sought in his report 

on the district. 1 The purpose of the present report is to show the 

relation of the ore, deposits at Elkhorn to the mineralization of the 

province as a whole.

1 Weed, W. H., Geology and ore deposits of the Elkhorn mining district, Jefferson County, Mont., with 

an appendix on the microscopical petrography of the district by Joseph Barrell: Twenty-second Ann. 

Kept. U. S. Geol. Survey, pt. 2,1901, pp. 399-549.
ELKHOKN DISTRICT. 129

The sedimentary rocks comprise mainly a series of Paleozoic limestones, shales, and quartzites, overlain in angular accordance by a 

series of Mesozoic sandstones, shales, and impure limestones, all 

highly altered by contact metamorphism. Overlying the Mesozoic 

rocks are a bedded series of andesitic breccias, tuffs, and lavas, which 

make up the summits of Elkhorn and Crow peaks, the culminating 

points of the district.

The oldest rock of the district is the metamorphosed shale to which 

Weed has applied the name Turnley hornstone. This is regarded 

as the local metamorphic equivalent of the Spokane shale of Algonkian age. The lowest formation of the Paleozoic is a white quartzite, 

125 feet thick, which is correlated with the Flathead quartzite of 

Middle Cambrian age. Above this formation is a succession of limestones and shales, ranging in age from Cambrian to Carboniferous, 

to- which various local names have been applied by Weed. Above 

these rests the Madison limestone of lower Carboniferous age, a 

massive-bedded white limestone aggregating 1,900 feet in thickness. 

At Elkhorn it is coarsely crystalline, owing to the thermal metamorphism exerted by the various igneous intrusions. The Quadrant 

quartzite (of probable Pennsylvanian age) overlies the Madison and 

consists of quartzites and lime-silicate hornstones totalling 382 feet 

in thickness.

The Mesozoic rocks overlie the Paleozoic, as already stated, in 

angular accordance. They are best exposed on the southwest spur 

of Crow Peak, whence they were termed by Weed 1 the Crow Ridge 

series and were regarded as including rocks of Jurassic and Cretaceous age, with which Stone agrees. 7 Weed's measured section 

shows a thickness of 1,680 feet between the underlying Quadrant 

quartzite and the overlying andesites.

The andesitic rocks at Elkhorn Peak are predominantly tuffs and 

breccias. The breccia bed at the base of the series contains angular 

blocks up to a foot in length. According to Weed,3 the andesitic 

lava flows, with tuffs and breccias at the bottom, lie on an erosion 

surface and terminate the series of Mesozoic sediments on Crow 

Ridge. At the point where the section was measured the actual 

contact, however, is hidden by talus, so that the existence of an 

erosion surface and its position in the stratigraphic column are 

matters of inference and not of observational fact. Certain it is, 

however, that the andesitic breccias and tuffs rest in angular accordance on the sedimentary rocks below them. It is probable that here 

as elsewhere in the region the andesites were folded at the same time 

as the underlying sedimentary rocks.

i Twenty-second Ann. Kept. U. S. Geol. Survey, pt. 2, 1901, p. 440.

-2 Stone, R. W., Geologic relation of ore deposits in the Elkhorn Mountains, Mont.: BulK U. S. Geol. 

Survey No. 470,1911, p. 82. . 

3 Op. cit.,p. 441.

70935° Bull. 527 13 9
130 ORE DEPOSITS OF HELENA MINING REGION, MONTANA.

Conspicuous features of the geology of Elkhorn are the marble 

cliffs on the western -flank of Elkhorn Peak. They are masses of - 

coarsely crystalline marble intercalated in the andesites and form 

bold cliffs of brilliant whiteness at the head of Elkhorn Gulch. Weed 1 

says: "These inclusions are clearly blocks torn off from the underlying Madison limestone and borne into their present position by the 

force of the ascending magma." As the inclusions are not fossiliferous the reference to the Madison limestone was probably based on 

lithologic similarity. Two marble masses are known, the larger of * > »> i o

which is 125 feet thick and 3,000 feet long. When this immense 

size is considered, together with the facts that the andesites below 

the limestone are predominantly tuffs and breccias and that the 

limestone is at least 1,000 feet hypsometrically above the nearest 

outcrop of the Madison limestone, this explanation seems inherently improbable. An alternative explanation, involving less 

catastrophic processes, is suggested by the fact that fossiliferous 

fresh-water limestone was found interstratified with the andesites 

southwest of Elliston. It therefore seemed probable that the supposed inclusions at Elkhorn were merely highly metamorphosed 

lenses of limestone interstratified with the andesitic series. To test 

this hypothesis the upper and lower contacts of the marble masses 

were carefully examined. The larger marble stratum was found to 

be overlain by a heavy stratum of coarse andesitic breccia and to be 

underlain by a thin bed of what appeared to be a banded tuff approximately 1\ feet thick. All the rocks are obviously highly metamorphosed owing to the proximity of quartz monzonite and aplite intrusions. . The rock beneath the limestone is found in thin section to be 

composed of andalusite, biotite, plagioclase, orthoclase, and quartz. 

The texture is typical of hornfels, and the andalusite, which constitutes 25 per cent of the rock, is developed in large irregular prismatic 

individuals of spongiform or poikilitic habit. Certain of the larger 

plagioclase particles appear to be of fragmental origin. The abundance of andalusite seems to bear directly on the point at issue; it 

proves that the material directly below the limestone mass was highly 

aluminous and indicates that it was probably an argillaceous tuff.

Certain small masses of limestone intercalated in the andesitic 

series afford even stronger evidence that they are merely interstratified lenses in the andesitic series. Some are underlain by undoubted 

tujff, which is found to consist under the microscope of irregular 

angular particles of plagioclase and fragments of various porphyritic 

andesites embedded in an extremely fine grained matrix. At the 

altitude of .8)750. feet, near the head of Elkhorn Gulch, a band of 

finely crystallinei limestone 8 feet thick rests oh a stratum of coarse 

breccia 20 feet thick. It is to be noted that if the limestone masses

1 Op. cit., p. 449.
ELKHORN DISTRICT. 131

were floated up by the magma they would be underlain by massive 

aridesite, which would in all probability show some small intrusive 

tongues extending into the limestone.

INTRUSIVE IGNEOUS BOCKS.

The intrusive igneous rocks include a series of granular rocks and 

porphyries that were injected prior to the invasion of the region 

by the quartz monzonite of the Boulder batholith. They include 

the gabbro of Black Butte (which is of interest because Weed sees in 

it the source of the metals in the Elkhorn deposit), diorite, diorite 

porphyry, and quartz diorite porphyry. All are distinct and easily 

recognizable rocks. They were followed by the granite or quartz 

monzonite, which is remarkably similar in appearance and composition to the rock at Butte on the western margin of the batholith. 

Succeeding this came extensive injections of aplite.

CONTACT METAMOEPHISM.

The intensity of contact metamorphism and the variety of minerals 

produced have been greater at Elkhorn than at any other locality 

in the region. Moreover, the phenomena are of such a character 

as to appeal to the eye, and the mineralogic changes that have ensued 

during metamorphism can be determined, in part, at least, without 

recourse to microscopic analysis. In this respect the district presents a strong contrast to the Marysville district, where the metamorphosed rocks consist predominantly of hornstones of aphanitic 

texture and of correspondingly obscure character to the eye.

The scapolitic and andraditic metamorphism of tha calcareous (?) 

andesite breccia above the marble stratum on Elkhorn Peak, the 

andalusite hornfels below it, and the recrystallization of the andesites have already been described (pp. 27-28).

Barrell concluded, as a result of his study at Elkhorn, that "even 

under the most intense metamorphism the carbonic acid is not 

expelled from the limestone except by the presence of silica and the 

other impurities which accompany it." l It was shown mathematically 

that during metamorphism a notable reduction in volume, or increase 

in porosity, must result. For the Dolcoath stratum a shrinkage of 

50 per cent was computed. But these mathematical results were 

unsupported by chemical analyses and'the computations were based 

on the unverified assumption that the garnet produced during metamorphism is invariably a pure grossularite. Iron-bearing garnets, 

however,, seem, < according to the present hasty 'investigation, to be 

the prevalent variety at Ejfcndrn. Soriie are certainly andradite, 

especially those occurring in the cpntact-metamorphic ore deposits, 

but others are probably intermediate varieties.

i Twenty-second Ann. Kept. U. S. Geol. Survey, pt. 2,1901, p. 549.
132 ORE DEPOSITS OF HELENA MINING REGION, MONTANA.

The following analyses made for Weed in 1906 * show the actual 

chemical composition of gafnetized limestone occurring 2,300 feet 

north of Black Butte at Elkhorn. No. 135 (analysis I) is the analysis 

of a rock composed of a closely packed assemblage of subhedral 

brown garnet grains, interspersed with sporadic patches of calcite 

containing euhedral garnets. Under the microscope the rock is 

found to consist predominantly .of brown garnet. Zonal structure 

and optical anomalies are common in the garnet, and a dodecahedral 

parting is well developed. The cores of many of the garnets have 

apparently shrunk slightly away from the peripheral zones. By the 

immersion method the refractive index is found to exceed considerably 1.79 (that of grossularite is 1.744 and andradite 1.85). Calcite, 

quartz, chlorite, and amphibole occur interstitially. No. 147 (analysis G) is the analysis of a garnet rock of dense massive texture. 

Microscopically the rock proves to consist largely of garnet poikilitically inclosing cliopside, calcite, feldspar, titanite, and apatite.

Analyses of garnet rock, Elkhorn, Mont.a

Si02...........................................................'... ...................

AU03 - ..............................................................................

FpoOo

FeO.................................................................................

MgO. ...............................................................................

CaO.... .............................................................................

Na20.... ...........................................................................

K2O........... ......................................... \... .........................

H2 0...... ...... ....................................................................

H«0+....................... ..........................................................

Ti02 :.. .............................................................................

OO2 .................................................................................

P205 . ...............................................................................

No. 135.

36.91

6.54

19.43

.67

1.07

31.09

.18

.70

1.57

1.91

Undet.

100. 07

No. 147.

40.31

12.11

8.67

.40

2.65

33.61

79

.12

.23

.22

.78

.69

100.58

o Analysis No. 135 by E.C. Sullivan: No. 147 by W. T. Schallcr.

The approximate mineral composition, calculated from the chemical analysis in accordance with microscopic diagnosis, is as 

follows:

Approximate mineral composition oj garntt rock, Elkhorn, Mont.

Calcite......'........................................................................

Feldspar............................................................................

No. 135.

61.50 

22. 50

4.30

4.92

4.20

97.42

No. 147.

27. 43 

52.70 

15.75

1.08 

1.96 

1.55

100. 47

i Listed by F. W. Clarke, as analyses G and I, in Bull. U. S. Geol. Survey No. 419,1910, p. 92; the descrip. 

tion of the rocks is hitherto unpublished.
ELKHORN DISTRICT. 133

These analyses and computations show clearly enough the considerable range in composition of the garnet rocks produced.during 

metamorphism; nevertheless, in the two rocks under discussion, 

there are no macroscopic features that would" serve to distinguish 

surely the-iron-rich from the lime-rich garnet rock.

ORE DEPOSITS.

A wide variety of ore deposits is found within the small area of the 

Elkhorn district. Several genetically distinct types are included: 

The magmatic sulphide ore body at the Golden Curry mine, the 

auriferous contact-metamorphic lode at the Dolcoath mine, the contact-metamorphic bodies of magnetite ore, the tourmaline-bearing 

galena-quartz ore at the Queen mine, and the lead-silver replacement 

deposit at the Elkhorn mine.

The great ore body of the Elkhorn mine has overshadowed all 

others in economic importance, and Weed's report on the district is, 

as he himself states, essentially a report.on the Elkhorn mine. From 

the present writer's point of view, however, other of the deposits are 

also of much interest.

The magmatic sulphide body, the details of which are given under 

the description of the Golden Curry mine, is an ore mass consisting of 

a mixture of pyrrhotite and chalcopyrite intergrown with augite. 

Surrounding the sulphide body and grading into it is a zone of dark 

heavy rock of fresh appearance and of even-grained granitic texture, 

which proves on microscopic examination to be a pyrrhotitic augite 

diorite. This rock in turn grades laterally outward into quartz 

monzonite, composed of plagioclase, orthoclase, quartz, biotite, and 

hornblende, which is the normal country rock. The sulphide mass 

has yielded 2,000 tons of ore, carrying $4 hi gold, 35 per cent excess 

iron, and 2 per cent copper.

The ore body at the Dolcoath mine is a stratum 12 to 18 inches 

thick, composed essentially of garnet (andradite), diopside, calcite, 

and epidote. The reck is commonly of dense massive texture, but in 

proportion as the amount of calcite increases, the garnet and epidote 

show more complete crystallographic development1. Sulphide and 

telluricle of bismuth, carrying gold, together with chalcopyrite, constitute the ore minerals and occur in small amount, intergrown both 

with the silicates and the calcite. That the garnetization and the. 

recrystallization of the calcite took place nearly simultaneously is 

proved by the fact that perfect crystals of garnet occur completely 

isolated in the midst of large crystalline patches of calcite.

According to Weed, the Dolcoath ore stratum is composed of nearly 

equal parts of diopside and grossularite with calcite. The garnet, 

however, according to the present examination, is not grossularite 

but is an andraditic variety. Weed, following Barrell, has suggested
134 OBE DEPOSITS OF HELENA MINING EEGION, MONTANA.

that the Dolcoath ore stratum represents a bed of limestone rendered 

porous by contact metamorphism and subsequently impregnated by 

metalliferous solutions, 1 which also brought in the calcite. This is 

an exceedingly plausible a priori conception, but the field evidence, 

as shown by the simultaneous crystallization of the garnet and the 

calcite, does not support it. Furthermore, if the garnet is largely 

andradite instead of grossularite, the accuracy of the mathematics 

employed in calculating the hypothetical porosity produced during 

contact metamorphism is seriously impaired.

On the north side of Elkhorn Peak occur bodies of iron ore near the 

contact of the large, limestone, stratum and the underlying andesite. 

The iron ore 'consists principally of magnetite associated with andradite. It has been mined to some extent for a flux.' The magnetite is 

mainly intergrown with the garnet but is also present as thin strings 

and quartz-magnetite veinlets traversing the garnet rock. The garnet, 

as determined by the immersion method, has a refractive index considerably higher than 1.79, indicating its andraditic character. 

Locally axinite is found in coarse-bladed masses, some as large as a 

fist; some of it incloses euhedral crystals of garnet. Other minerals 

occurring in the ore bodies in lesser amounts are specular hematite, 

pyrite, chalcopyrite, and epidote. The deposits are extensively oxidized, and much soft iron oxide, in part derived from the andradite, is 

present, obscuring the geologic relations. The ore bodies are doubtless largely replacements of the limestone, but it is not improbable 

that the underlying andesite is also considerably garnetized or otherwise metamorphosed in a manner similar to that of the andesite 

breccia overlying the great limestone stratum of Elkhorn Peak. 

. The ore body at the Queen mine is the only representative of the 

tourmalinic class of deposits in the Elkhorn area. It is an argentiferous galena-quartz ore carrying tourmaline. Arsenopyrite, which is 

a constant associate of tourmaline the world over, occurs in this 

deposit but is found nowhere else in the Elkhorn area.

The premier ore deposit of the district has been that of the Elkhorn 

mine. By 1900 it had produced 8,902,000 ounces of silver, 8,500 

ounces of gold, and 4,000,000 pounds of lead. The ore body consists 

essentially of argentiferous lead ore replacing dolomite beneath the 

arches of a hanging wall of hornstone. In comparison with the other 

ore bodies, one of the most interesting features of the Elkhorn deposit 

is the complete absence of metasomatic gangue minerals. The other 

deposits of the district show such characteristic minerals as andradite, 

diopside, epidote, axinite, and tourmaline all indicative of hightemperature origin but this, the largest and most productive ore 

body of the Elkhorn district, is devoid of any evidence that it was 

formed under conditions of high temperature.

i Twenty-second Ann. Kept. U. S. Geol. Survey, pt. 2,1901, p. 458.
ELKHORN' DISTRICT. 135

DESCRIPTIONS OF MINES. 

ELKHORN MINE.

The Elkliorn mine is opened by an incline 2,300 feet long, attaining 

a vertical depth of 1,439 feet. During 1911 work was in progress on 

a large number of levels, from those near the surfac down to the 

2,300-foot level. About 175 men were on the pa.y roll, but a large 

number of these were engaged in surface work. All ore from the 

mine is washed and sorted into milling, shipping, and waste rock. 

The old dumps are being reworked and yield 5 per cent of their content 

in milling and shipping ore. ,

The geologic features of the Elkhorn mine have been described in 

detail by Weed, so that a brief outline is all that is necessary here. 

According to Weed: 1

The ore deposit does not constitute a true vein, though commonly spoken of as a 

vein or lode. The ore occurs in two principal shoots lying on the under side of folds - 

i. e., in the saddle of the folds and conforming to the dip of the stratified beds in 

which they occur. These ore bodies are found along a bedding plane between indurated 

shale (hornstone) and dolomitic marble. This contact plane was followed in mining 

and has commonly been spoken of as the lode. Although there has been some slipping 

and movement along this plane, the contact is not mineralized, nor does it show any 

vein quartz or other lode material except in the steeply pitching arches underneath 

which the ore bodies occur. The ore deposits consist of more or less irregular bodies of 

rudely lenticular cross section lying against the hanging-wall hornstone, and as isolated 

masses forming so-called "chamber" deposits in the underlying dolomite.

The hanging-wall ore bodies consist of solid quartz 2 carrying galena, 

tetrahedrite, pyrite; and blende; the footwall bodies consist mainly 

of replacement masses of galena, sphalerite/ and pyrite in the dolomite beneath the hanging-wall hornstone.

At the time of Weed's examination the ore deposit was supposed 

to have a definite footwall a thin stratum of argillaceous limestone 

occurring about 35 to 50 feet from the hanging wall, as measured 

along crosscuts into the footwall country. This is now known as the 

"40-foot contact.'' It is particularly well shown on the 350-rfoot 

level south; where unsheared, it consists of a somewhat argillaceous 

stratum averaging 3 inches in thickness. It swells and constricts 

and has evidently been a locus of movement. The dolomite above 

and that below this stratum seem to be alike. The new owners of 

the Elkliorn mine, who acquired the property in 1899 after it had been 

regarded as worked out, have explored the footwall country below 

the 40-foot contact and have discovered other ore bodies. They 

have found two other "contacts," known as the 80-foot and 160-foot 

contacts. In other 'words, the thickness of the ore-bearing zone 

has been found to be 160 feet instead of 40 feet (or 80 and 20 feet, if

1 Twenty-second Ann. Kept. U. S. Geol. Survey, pt. 2, 1901', pp. 470-471.

2 Aragonite in fibrous aggregates over an inch long was noted (and verified chemically) by the writer to 

be intergrown with quartz from the 1,750-foot level.
136 ORE DEPOSITS OF HELENA MINING REGION, MONTANA.

measured normal to the dip of the stratification). The 80-foot 

contact is apparently a stratification plane and is not particularly 

well marked. The dolomite shows no recognizable change above or 

below this plane. The 160-foot contact is even more feebly marked 

than -the 80-foot contact.

The ore masses, which after extraction are represented by extremely' 

irregular caverns and crooked pipes, are known as first, second, or 

third contact bodies, according to the contact on which they were 

discovered. Man^ of them extend by exceedingly devious courses 

back to the hanging-wall hornstone, or slate, as it is known locally. 

Great ore masses have been found whose major dimensions cut 

across the bedding of the dolomite. In this respect the Elkhorn 

deposit departs strongly from the saddle-reef type of ore body, with 

which Weed has identified it, but in the broader feature the localization of the ore shoots beneath anticlinal arches, so clearly shown 

on Plate LVlt of Weed's report it is in striking accord with the 

saddle-reef type.

GOLDEN CURRY MINE. 

GENERAL FEATURES.

The Golden Curry, or Jacquemin mine, as it is shown on the Elkhorn map, 1 is located near the intrusive contact of the quartz monzonite and a series of thin-bedded limestones, to which Weed gave 

the name -Starrnount limestone. The limestones have been highly 

m'etamorphosed into various lime silicates, forming dark-colored 

banded hornstones of dense texture. Locally heavy garnet rocks 

have been produced and in places the garnet (probably andradite) 

is coarsely crystalline, especially near calcite patches; where the 

calcite has been partly leached out by weathering a porous rock is 

produced, showing druses lined with crystalline garnet. Some of 

the garnet rock contains a little intergrown chalcopyrite and is said 

to constitute ore. Above the main pit of the mine are found considerable amounts of a somewhat notable rock composed of garnet 

and opal. Some of it consists of garnets set in a matrix of conchoidally fracturing opal of light-yellowish color. Under the microscope a specimen of the garnet-opal rock was found to consist of 

garnet, diopside, calcite, opal, and cryptocrystalline silica, in part 

spherulitic. The evidence seems to show that the garnet had been 

* fractured, crushed, and recemented by opal and cryptocrystalline 

silica. It is to be noted that the cryptocrystalline silica mixed with 

yellowish opal produces a "hornstone" indistinguishable by the eye 

from the "hornstone" due to a mixture of contact-metamorphic 

silicates.

The main workings of the mine consist of a large open pit from 

which some 50,000 tons of iron ore have been mined, mainly as a

i Twenty-second Ann. Kept. U. S. Geol. Survey, pt. 2,1901, PI. XLV.
ELKHORN DISTRICT. 137

flux for the smelters. The ore averaged 40 per cent iron and carried 

from $4 to $5 a ton in gold. The pit is located at the immediate contact of the metamorphic rocks and the granite, and at first inspection 

it might appear that this deposit of iron ore was of the same type 

as those so common at many granite and limestone contacts in 

the Western States, namely, a deposit of magnetic iron ore developed 

in the limestone during contact metamorphism. In this deposit, 

however, the magnetite does not occur intergrown with the contactmetamorphic silicates but forms veins in the granite itself, some of the 

veins attaining a thickness of 3 feet. In some of the underground 

workings beneath the main pit, namely in tunnel No. 1, there are 

found soine lodes of heavy red jasper-like material which chemical 

analysis shows to be a hydrous ferric silicate.

Partial analysis of iron ore from the Golden Curry mine, Montana.

[J. G. Fairchild, analyst.]

Fe203 ................................'... .......................'59. 23 '

Si02 -......................................................... 31. 37

H20 (loss on ignition).................-......................... 8. 40

99. 30

This material is also mined as iron ore. It contains considerable 

martite and rarely some pyrite. The analysis above, however, 

represents only the homogeneous, conchoid ally fracturing portion.

In a raise above this lower tunnel a large chamber of ore inclosed 

in granite has been stoped out. This ore presents many features 

similar to the pyrrhotite body about to be described. It consists 

of pyrrhotite, magnetite, and subordinate chaicopyrite intergrown 

with brown-green aiigite. In places pyrrhot.ite predominates, in 

others magnetite predominates, arid locally they are commingled in 

equal proportions. This body of ore is bounded on one side by a 

thick zone of gouge, along which occurs iron-bearing jasperoid 

similar to that of which the analysis has been given.

These iron dep'osits are unconnected with the body of sulphide ore 

described below. The Golden Curry mine affords a striking illustration of the fact that the ore at a single mine may be of very diverse 

origin and mode of occurrence, at least four kinds being recognizable 

at this property: Magmatic, contact-metamorphic, magnetite veins 

in granite, and hydrous ferric silicate masses apparently formed by 

downward-moving waters that derived their iron from the oxidation 

of sulphide masses.

THE PYRRHOTITE ORE BODY.

A mass of sulphide ore was encountered during the underground 

exploration of the mine 250 feet southwest of the magnetite deposits, 

found at the contact. This body of sulphide was wholly inclosed hi 

the granite and isolated from the other ore deposits. It is elliptical
'138 ORE DEPOSITS OF HELENA MINING REGION, MONTANA.

in shape, its dimensions being: Length, 100 feet; maximum width, 

18 feet; and it has been stoped upward to a height of 10 or 12 feet. 

From it were extracted 2,000 tons of ore running $4 in gold, 35 per 

cent excess iron,1 and 2 per cent in copper. The ore consisted of a 

mixture of pyrrhotite and chalcopyrite in a gangue of pyroxene. 

Under the microscope it is found that the ore is composed essentially of the monoclinic pyroxene augite intergrown with the two 

sulphides. The augite is in places intergrown with a small amount 

of pleochroic brown hornblende of the kind usually found in deepseated igneous rocks. Other silicate minerals found more rarely are 

biotite, plagioclase feldspar, orthoclase, and quartz, but these constitute an insignificant proportion of the whole. Texturally the 

augite forms an allotriomorphic assemblage of anhedral grains. 

The sulphides, which are mutually intergrown, occur interstitially 

between the augite grains or irregularly intergrown with them/ It 

is a noteworthy fact, however, that although the augite is usually 

anhedral it shows, where adjoined or surrounded by sulphides, a 

closer approximation to its idiomorphic outlines. The contours are 

as a rule somewhat rounded and smoothed by corrosion, and the 

sulphides often form distinct embayments into the augite which 

resemble those so common in the magmatically resorbed quartz 

phenocrysts of rhyolitic rocks. On account, however, of the general 

tendency of the sulphides and the pyroxene to be intergrown in 

allotriomorphic fashion, exactly as in gabbroic rocks, the microscopical evidence based on the texture alone is perhaps not conclusive 

as to the primary igneous origin of the sulphides.

Surrounding the sulphide mass and grading into it is a body of 

dark, heavy rock of fresh appearance and even-grained granitic texture. Under the microscope this rock is found to consist of approximately equal amounts of pyrrhotite, augite, and plagioclase of the 

composition Ab65An35 . The texture is allotriomorphic granular and 

the rock may -be appropriately termed a pyrrhotite-augite diorite. 

This rock is, however, somewhat variable in composition, in places 

containing considerable primary quartz, but the essential feature in 

all places is the dominant amount of augite. The rock grades laterally outward from the ore deposit within a distance of 6 to 12 feet to 

a quartz monzonite of normal appearance. Such rock 12 feet from the 

ore lens consists of a hypidiomorphic granular assemblage of plagioclase, orthoclase (the two feldspars occurring in approximately equal 

amounts), quartz, biotite, and hornblende. A specimen taken 50 feet 

from the periphery of the ore lens shows essentially the same features, 

the only noticeable difference being that the plagioclase feldspar is 

slightly more calcic, having the composition corresponding to Ab55An45 . 

A chemical analysis of the quartz monzonite (No. 5, p. 30) is available;

i That is, iron in excess of silica; excess iron is the basis of the price paid by the smelter.
ELKHORN DISTRICT. 139

the specimen was taken 1 mile from the contact at this locality, and is 

typical of the general rock mass in which the ore body is inclosed.

To summarize briefly, the primary igneous origin of'the sulphides 

is indicated by the following facts:

1. All the rocks, including those composing the ore body and those 

surrounding it, show an entire lack of hydrothermal alteration, such 

as the development of sericite, chlorite, carbonates, or other secondary 

minerals. They are fresh, unaltered rocks in which the ferromagnesian minerals are notably lustrous and the feldspars are clear and 

vitreous.

2. The textural relation of the sulphides to the augite, as displayed 

by the tendency of the pyroxene to show idiomorphic boundaries 

against the sulphides. This is a feature not easily explainable other 

than by the hypothesis of an igneous origin.

3. The marked differentiation that has taken place in the magma 

concurrently with the segregation of the sulphides. This is expressed 

mineralogically by the decrease of orthoclase, quartz, and biotite and 

the concurrent increase in ferromagnesian mineral as the ore body is 

approached. Instead of hornblende or biotite, however, it is augite 

that appears in the ore body.

In conclusion, the important features of this deposit as a member 

of the class of magmatic ore bodies are: (1) That it has segregated 

from a comparatively acidic rock; other pyrrhotite-chalcopyrite bodies 

of this type have separated from magmas of gabbroic composition; 

(2) its relatively high gold content, which is unusual, as most magmatic 

ore bodies are poor hi gold.

QUEEN MINE.

The Queen mine lies 2 miles south of Elkhorn. It was worked to a 

depth of 500 feet by a shaft 300 feet deep and by winzes extending 

200 feet deeper. The property has furnished a considerable quantity 

of ore, but no work has been done on it during the last 10 years.

The ore body is situated at the contact of quartz diorite porphyry 

and limestone and formed, as seen from the surface, an elliptical 

chimney measuring approximately 25 by 50 feet. The ore is a coarse 

vitreous white quartz of drusy and vuggy texture. Galena is the 

principal sulphide and is the mineral that gave value to the ore. Other 

sulphides present are pyrite and black sphalerite, but they are not 

common. The quartz contains a fair amount of black tourmaline in 

nests and groups of delicately radial structure. In places the galena 

is pierced by acicular tourmaline. Some altered porphyry found 

on the dump is thickly studded with well-crystallized arsenopyrite; 

galena is locally intergrown with the arsenopyrite.

The ore of the Queen mine was a silver-lead ore and is stated to have 

averaged from $10 to $15 a ton.

INDEX

A. Page.

Acidic sheets in the Helena mining district,

description of................... 95'

Acknowledgments.......................... 17-18

Allport mine, description of................. 125

Alta mine, description of........."......... 109-110

Amazon mine' description of................ 120

Andesite breccias, plate showing............ 24

Andesites, age of............................ 28-29

alteration of............................. 26-28

character of............................. 24-25

fossils in................................. 28

occurrence of............................ 23-24

petrography of................... I...... 25-26

Aplite, occurrence tod charact er of........... 34-35

origin of................................. 35

B.

Bald Mountain mine, description of......... 72

Bald Butte mine, description of............. 73-74

Baltimore mine, description of............ 120-121

Barrell, Joseph, collections made by......... 18

quoted.............................. 66,73,131

Basic dikes and sheets in the Helena mining

district, description of............ 95

Basin mining district, location of............ 120

Belmont mine, description of................ 73

Belt series, description of.................... 20,87

Bertha mine, description of............... 110-111

Big Dick mine, description of............... 78

Big Indian mine, description of............. 100

ore body in, description of............... 51

Blizzard mine, description of................ Ill

Blue Bell mine, description of............... 79-80

Blue Bird mine, description of............ 111-113

silver-copper ore body of, description

of....................... 50-51.113-114

Boston & Corbin Copper & Silver Mining

Co., mine of............................. 110

Boulder batholith, formation of.............. 9

See also Quartz monzonite. 

Boulder mining district, location of.......... 120

Boulder River, description of................ 13

Bullion mine, description of................. 124

Butte and Philadelphia prospect, description

. of........................:...... 124

C. 

Calcite ore, lamellar, plates showing......... 54,55

Cambrian rocks, occurrence of............... 21,89

Carboniferous rocks, occurrence of..... 21-22,92-93

Clancy mining district, geology of......... 102-103

ore deposits of......................... 103-104

Claytori, J. E., quoted....................... 69,71

Climate, description of...................... 14-15

Page. 

Columbia claim, description of.............. 127

Comet mine, description of................ 114-115

Contact-altered rocks, figure showing........ 32

Contact metamorphismat' Elkhorn....... 131-133

of andesites and latites.................. 26-28

of sedimentary rocks.................... 31-33

ore deposits produced by....... 44,101,133-134

Continental Divide in Helena mining region. 13,14 

Copper, scarcity of.......................... 10

Copper King prospect, description of........ 122

Corbin Copper Co., property of, description

of............................. 115-116

Corbin Metals Mining Co., property of, description of.................... 116-117

Cretaceous rocks, occurrence of.............. 23,93

Cruse mine, description of................... 72

Custer mine, description of.................. 125

D. 

Dacites, age of............................... 39

character and distribution of............ 36-37

petrography of.......................... 37-38

Devonian rocks, occurrence of............ 21,91-92

Drumlummon mine, description of.......... 68-79

Drumlummon vein system, figure showing.. 70 

Dry Greek shale, description of.............. 91

Dutro mine, description of................ 101-102

E.

Elkhorn, ore bodies at, nature of............ 44

Elkhorn mine, description of.............. 135-136

Elkhorn mining district, contact metamorphism in...................... -131-133

geology of............................. 128-134

history of............................... 128

igneous rocks in......................... 131

location of............................... 12

ore deposits in......................... 133-134

Elkhorn Mountain, description of........... 13

Ellis formation, description of.............. 22-23

Elliston, ore bodies at, nature of............. 44,45

Elliston Lime Co., plant of.................. 76

Elliston mining district, geology of.......... 76-77

Emrnons, S. F., quoted..................... 33,67

Empire mine, description of...,............. 75

Empireshale, description of................. 88

Eva May mine, description of............. 122-123

Evening Star mine, description of........... 78

F.

Ferguson, H. G., field work by.............. 17

Flathead quartzite, description of........ 21,89-90

Fossils, absence of, from Helena limestone... 88 

Cretaceous, report of T. W. Stanton on.. 28 

Jurassic, report of T. W.. Stanton on..... 22

141
142 INDEX.

O. Page.

Garnet rock, analyses of..................... 132

mineral' composition of.................. 132

Garn'etization, localities of.......... 28,32-33,45,80

Geology, general............................. 19-42

of Helena and vicinity, plate showing.... 86

of Helena mining region, map showing... 20 

Gloster mine, description of................. 75-76

Gold, discoveries of.......................... 15-16

production of............................ 17

deposits, tourmalinic, description of..... 51

Golden Curry mine, description of......... 136-137

pyrrhotite ore body of................. 137-139

G ranite, jointing in, plate showing.......... 24

See also Quartz monzonite. 

Granitic rocks, description of................ 95-96

Gravels, auriferous, description of........... 94

Gray Eagle mine, description of............. 121

Gregory lode, location of..................... 16

Gregory mine, description of................. 117

H. 

Hattie Fergiison mine, desVriptioji of........ 124'.

Helena limestone, description of.......... 87,88-89

Helena mining district, faults in............. 98

geology of............................... 86-98

history of................................ 85-86

igneous rocks in......................... 94-95

.ore deposits of, location of............... 99

placers of ............................... 86

production of........................... 86

sedimentary rocks in.................... 86-87

structure of.............................. 97

Horseshoe prospect, description of......... 117-118

Hot Spring deposits, description of.......... 94

I. 

Iron ore, analysis of.......................... 137

' J. 

Jackson Creek mine, description, of........... 106,

Jacqueminmine, description of. ........... 136-137

Jefferson limestone, description of........... 92

John McGrew prospect, description of....... 84

Julia mine, description of.................... 77-78

Jurassic rocks, occurrence of................. 22-23

K.

Kennedy mine, description of............... 106

King Solomon mine, description of.'....... 104-105

Kit Carson mine, description of............ 127-128

L. 

Lake beds, Tertiary, description of.......... 93-94

Last Chance placer, discovery of............ 15

Latite, analysis of........................... 26

Latites, age of............................... 28-29

alteration of............................. 26-28

character of............... 1............... 24-25

occurrence of............................ 23-24

petrography of........................... 25-26

Lee Mountain mine, description of........... 83-84

Legal Tender mine, description of......... 106-107

 Limestone,.,occurrence of, among. Mesozoic

rocks............................. 23

Lindgren, W., Jurassic fossils discovered by . 23 

Literature,list of............................ 18-19

Page.

Little Blackfoot River, description of........ 14

Little Nell mine, description of!............. 105

Location of Helena mining region........... 12-13

map showing............................ 12

M.

Madison limestone, description of......... 21,92-93

Marsh shale, description of.................. 89

Marysville mining district, geology of........ 62-64

history of................................ 61-62

location of...............'................ 12

ore deposits of, description of............ 64-65

origin of............................. 65-68

Meagher limestone, description of............ 90

Memphis prospect, description of............ 128

Meta-andesite, descrip tion of................ 95

Metasomatism,,varieties of, comparison of... 60-61 

varieties of, descrip tions of......... 44-45,55-58

Minah mine, description of.................. 118

Minnesota mine, description of.............. 118

Missouri River, tributaries of................ 14

Monarch mine, description of................ 79

Morning Star mine, description of........... 125

N. , 

Northern Pacific mine, description of........ 119

0.

Old Dominion mine, description of........ 101-102

Ontario mine, description of................. 79

Ophir, founding of........................... 15

Ore bodies, contact-metamorphic, description

of............................... 44-45

depth of................................. 11

magmatic, description of................ 44

nature of................................ 10-12

older, divisions of........................ 43-44

origin of........-................ : ... 51-53

older and younger, comparison of metasomatic processes of............. 60-61

division between.................... 42

primary minerals of................. 43

tourmalinic, descriptions of..........'.:.. 45-51

younger, age and origin of............... 58-59

character of......................... 54-55

formation of, metasomatic processes

attending....................... 55-58

primary minerals of................. 43

P.

Park shale, description of.................... 90-91

Peerless Jennie mine, description of......... 84

Penobscot mine, description of.............. 74-75

Philadelphia prospect, description of........ 124

Phosphate rock in Quadrant quartzite....... 22

Piegan-Gloster mine, description of........... 75-76

Pilgrim limestone, description of............ 91

Porphyry dike, ores in...................... 84-85

QQuadrant quartzite, description of........... 22,93

Quartz, lamellar pseudomorphic, plates

showing......................... 56,57

mass of, near Basin, description of....... '122
INDEX. 143

Page. 

Quartz monzonite, age of.................... 33-34

analyses of..................... 3?-31,49,58,60

character of............................. 29

contact meta'morphism of............... 31-33

mineral composition of.................. t>~

petrography of........................... 29-31

Quaternary deposits, character of......... 41-42,94

Queen m ine, description of.................. 139

K.

Raymond, R. \V., quoted................. 106-107

Red Mountain, description of................ 13

llhyolite, analysis of........................ 41

Rhyolites, age of........,........;.......... 41

character and distribution ot............ 39-40

description of............................ 90

petrography of.......................... 40-41

Rimini,silver-lead depositsat,description of. 46-48 

Rimini mining district, geology of........... 80-81

location of............................... 80

ore deposits of........................... 81-82

Robert Emmet mine, description of......... 120

Rocks, character and distribution of......... 19-20

igneous, character and occurrence of..... 23-35 

of the. Cenozoic era, description of... 36-41

of Helena mining region, nature of....... 9-12

Ruby mine, description of................. 125-127

S,

Sandstone, occurrence of, amons; Mesozoic

rocks............................ 23

Silver City, founding of...................... 15

Silver-copper deposits, tourmalinic, description of........................... 50-51

Silver-lead, production of.................... 17

Silver-lead deposits, tourmalinic, chemical

processes involved in............ 4S-50

l.ourmalinic, description of............... 46-48

systematic importance of............ 53-54

Spokane shale, description of................ 87,88

Spring Hill mine, description of............. 101

ore body of, nature of.................... 44

Strawberry mine, description of............. 76

T. Page.

Tertiary deposits in the Helena mining district. ............................" 93-94

Tertiary sedimentary rocks, occurrence and

character of...................'.. 35-36

Threeforks shale, description of............'.. 92

Tin, occurrence of............................ 45

Topography of Helena mining region........ 13-14

Tourmaline, abundance of................ 11,45,54

Tourmalinic lodes, descriptions of........... 45-51

Tourmalinized zone in granite, figure showing.............................. 4i

Transportation, facilities for................. 13,16

Twin City Mining &' Milling Co:, mine of, ,

description of.................... 78-79

U.

Uncle Sam inlne, description of.............. 123

V.

Valley Forge mine, description of...........82-83

Vaughn m ining district, location of.......... 80

W.

Weed, W. H., collections made by........... 18

quoted...........................;.... 123,135

West Belmont mine, description of.......... 72

Western Reserve Mining Co., mine of, description of...................... 124

Whitlatch-Union mine, description of...... 99-100

Whitlatch-TJ nion vein, discovery of......... 15

Wickes-Corbin Copper Co., property of, -

description of.................. 119-120

Wickes mining district, geology of......... 108-109

history of.............................. 107-108

Wolsey shale, description of................. 90

Yellowstone prospect, description of....... 105-106

Yogo limestone, description of............... 91

o