SMITHSONIAN CONTRIBUTIONS TO THE

EARTH SCIENCES

NUMBER 7

William Q. Melson Geolog y of the

Lincoln Area,

Lewis and Clark

County, Montana

SMITHSONIAN INSTITUTION PRESS

CITY OF WASHINGTON

1971
ABSTRACT

Melson, William G. Geology of the Lincoln Area, Lewis and Clark County, Montana. Smithsonian Contributions to the Earth Sciences, number 7, 29 pages, 13 figures, 1971.— The Lincoln area (townships 13 and 14 N, ranges 7 and 8 W, about

thirty miles northwest of Helena, Montana) is underlain by Pre-Cambrian Belt sedimentary rocks intruded by late Cretaceous (?) granitic stocks with concomitant widespread contact metamorphism and mineralization. The granitic stocks are probably

related to the Boulder batholith. The pre-intrusion structure is characterized by high

angle faults and broad open folds of Cretaceous age (Laramide). Oligocene (?) volcanic rocks were extruded on an eroded surface of the Belt rocks and granitic stocks.

A second period of mineralization followed extrusion of the volcanic rocks.

Fracture cleavage which dips consistently to the southwest as well as the overall

structure show that a southeast plunging syncline which marks the north end of the

Boulder batholith continues into the Lincoln area. The syncline extends at least

twenty miles north of the batholith and dominates the structure over an area of about

350 square miles.

About forty square miles of middle Tertiary volcanic rocks are composed of a

lower series of andesitic to latitic flows and an upper series of rhyolitic welded ash

flows. The features of the welded ash flows suggest that they were deposited in part

by a vesiculating mass of rhyolitic magma (pumice froth flows). The volcanic rocks

are presumably about the same age as the Lowland Creek volcanics of the Butte area.

The area and the region several miles to the north are about the northern limit

of Boulder batholith activity, Tertiary volcanism, and associated mineral deposits.

The superposition of these two periods of igneous activity and their gross similarities

imply that they are genetically related.

Gold and silver have been produced from epithermal fissure veins. The scant

available data suggests that the veins are vertically zoned. There were probably at

least two periods of epithermal mineralization: one during the late stage cooling of

the stocks, and a second after extrusion of the lower volcanic series.

Remnants of Tertiary surfaces preserved under the volcanic rocks imply that

there have been topographic inversions since the middle Tertiary.

Glacial deposits suggest at least one early period of valley glaciation and later,

perhaps recent, periods of restricted mountain glaciation. Rich gold placer deposits,

such as in McClellan Gulch, accumulated after the earliest period of valley

glaciation.

Official publication date is handstamped in a limited number of initial copies and is recorded

in the Institution's annual report, Smithsonian Year.

UNITED STATES GOVERNMENT PRINTING OFFICE

WASHINGTON : 1971

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

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Contents

Page

Introductien 1

Geography 1

Summary of Previous Work 3

Stratigraphy of the Belt Series 7

Discussion 7

Stratigraphic Repetition of Similar Rock Types 7

Belt Formation Contacts 8

Structural Geology 8

Discussion 8

Relation Between Folding and Fracture Cleavage 9

Description of Fracture Cleavage 9

Relation to the Black Mountain Syncline 9

Laramide Faults 9

Igneous Rocks 12

Discussion 12

Pre-Cambrian (?) Dioritic Sills 12

Cretaceous (?) Pre-Laramide Quartz Gabbro Sills 12

Granitic Stocks 13

Comparison of Stocks in the Lincoln Area 14

Relation to the Boulder Batholith 14

Mode of Emplacement 15

Lack of Extrusive Equivalents 16

Middle Tertiary Volcanic Rocks 16

Lower Volcanic Series 17

Vents 17

Upper Rhyolitic Series 17

Vents 18

Mechanism of Ash Flow Deposition 18

Regional Correlation of Volcanic Rocks 21

Post-Laramide Dikes 21

Contact Metamorphosed Belt Rocks 21

Mineral Deposits 21

Introduction 21

High-Temperature Veins and Replacement Deposits 21

Epithermal Gold-Silver Fissure Veins 21

Structure 23

Mineralogy and Texture 24

Evidence 'of Two Periods of Epithermal Mineralization 25

Origin of Tension Fractures 25

Undiscovered Ore Bodies 26

Placer Deposits 26

Relation Between Placer Deposits and Glaciation 26

Summary of Post-Laramide Igneous Activity and Mineralization 26

Geomorphology 26

Middle Tertiary Surfaces 26

Pleistocene Glaciation 27

Recent Surface Changes 27

Acknowledgments 27

Literature Cited 28
William Q. Melson Geology of the

Lincoln Area,

Lewis and Clark

County, Montana

Introduction

The Lincoln area, which is in the east-central part

of the Rocky Mountains of Montana, contains features which are of interest to topical petrologic

studies and to the regional geology of western Montana. The location of the Lincoln area and some of

the major geologic features of western Montana are

shown in Figure 1.

Of particular topical interest in volcanology is a

series of welded ash flows which contain features

suggesting deposition by the pumice froth flow

mechanism (Smith 1960). Of these features, fluid

inclusions with uncommonly large vapor-to-glass

ratios are the most striking.

Contact aureoles around granitic stocks in the

area are developed in a wide variety of sedimentary

rocks. These aureoles have features which bear on

problems in metamorphic petrology and are the

subject of a separate study (Melson 1966).

On a regional geologic scale, the study of the

Lincoln area establishes some lithologic correlations

of the Pre-Cambrian Belt Series of western Montana,

describes part of the northernmost extent of the

Boulder intrusive activity and of middle Tertiary volcanism in the eastern Rocky Mountains, and points

out the large size of a complexly faulted syncline

which marks the north side of the Boulder batholith.

A collection of rocks in the National Museum of

Natural History from the Marysville mining district

(Barrell 1907) was of special interest to the followWilliam G. Melson, Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution,

Washington, D.C. 20560.

ing study. Other suites of rocks in the Museum collected during the early geologic surveys of the western United States and of particular interest to

development of geologic knowledge of Montana are

listed in Table 1.

The field work, which was carried out in 1962

and 1963, and much of the laboratory work involved in this study were done by the writer as part

of a Ph.D. thesis submitted to Princeton University

in 1964.

Geography

The Lincoln area is used here to refer to townships

13 and 14 north, ranges 7 and 8 west, about thirty

miles north of Helena, Montana (Figure 1). Figure

2 shows some of the major topographic features of

the region. The Continental Divide, the most prominent feature, trends northeast, and is at elevations

between 6,300 and 7,581 feet (Granite Butte). The

local relief is mainly between 1,000 and 2,000 feet.

The area is mountainous except for the Lincoln

Valley, which is a relatively flat, gravel-covered surface at about 4,600 feet. The mountains have forestcovered slopes, a few cliffs and rounded crests. Most

geologic information was gained from traverse along

ridge crests, where there are normally good outcrops.

The intersections of major valleys with the divides

generally are marked by steep slopes and, in places,

cliffs which are the headwalls of former glaciers.

The topography bears little relation to the structure of the underlying Belt rocks. The resistant contact rocks around granitic stocks, however, commonly

1
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

TABLE 1.—Rock collections from Montana in the collections of the National Museum of Natural History and published reports concerning some of them, or source

of the collections

BEARPAW MOUNTAINS. Geology. Weed and Pirsson (1896a).

BIG BELT MOUNTAINS. Geology. Weed, W.H.

BOULDER HOT SPRINGS. Mineral vein formation. Weed (1900).

BUTTE. Manganese deposits. Pardee (1919a).

BUTTE. Geology and ore deposits. Weed (1913).

BUTTE. Geology. Weed et al. (1897).

CASTLE MOUNTAIN DISTRICT. Geology. Weed and Pirsson (1896b).

CRAZY MOUNTAINS. Geology. Wolff, J.E.

DILLON. Geology and mineral deposits. Winchell (1914).

DUNKLEBERG DISTRICT. Geology and mineral deposits. Pardee (1918b).

ELKHORN DISTRICT. Geology and mineral deposits. Weed (1901).

ELLISTON. Phosphate deposits. Stone and Bonine (1915).

FLATHEAD INDIAN RESERVATION. Rocks and ores. Stone, R.W. U.S. Geological Survey.

FORT BENTON. Geology. Weed (1899).

GALLATIN COUNTY. Phosphates. U.S. Geological Survey of the Territories, 1871-1872.

GARNET RANGE. Mineral deposits. Pardee (1918a).

GARRISON AND PHILLIPSBURG. Phosphates. Pardee (1917).

HELENA MINING REGION. Mineral deposits. Pardee and Schrader (1933).

HELENA MINING REGION. Mineral deposits. Knopf (1913).

HIGH WOOD MOUNTAINS. Geology. Weed and Pirsson (1894).

JUDITH MOUNTAINS. Geology and mineral deposits. Weed and Pirsson (1897).

LITTLE BELT MOUNTAINS. Geology. Weed and Pirsson (1900).

LITTLE BELT MOUNTAINS. Geology. Weed (1899).

LITTLE ROCKY MOUNTAINS. Geology. Weed and Pirsson (1896c).

LIVINGSTON. Geology. Iddings, J.P. and Weed, W.H. (1894).

MADISON COUNTY. Manganese deposits. Pardee (1919b).

MADISON COUNTY. U. S. Geological Survey of Territories, 1871-1872.

NORTHERN PACIFIC LAND GRANT. Geology. Stone, R.W. U.S. Geological Survey.

PARK COUNTY. Geology. U.S. Geological Survey of Territories, 1871-1872.

PHILIPSBURG. Geology and mineral deposits. Emmons and Calkins (1913).

PHILIPSBURG. Geology and mineral deposits. Emmons and Calkins (1915).

THREE FORKS. Geology. Peale (1893).

THREE FORKS. Geology. Peale (1896).

TOWN SEND VALLEY. Geology. Pardee (1925).

VARIOUS LOCALITIES. Manganese deposits. Pardee (1922).

form natural amphitheaters around deeply eroded

stock interiors.

Principal access to the area is by the PoormanVirginia Creeks secondary dirt road. Montana Route

289 is near the northeastern part and Route 20 near

the northern part of the area. These roads lead to

and from the small town of Lincoln, which marks the

area's northwestern corner.

The Lincoln area is principally in the Helena

National Forest, and the Dalton Mountain, Continental Divide, and Humbug Creek foot-trails, which

are maintained by the United States Forest Service, provide access to the remote parts of the area.

Geologic data were plotted on United States Forest

Service 1:15840 aerial photographs (Project 102a,

Helmville-Lincoln area). There are no accurate topographic maps available at a scale of 1:31680. Data

were transferred to part of the United States Army

Map Service 1:125,000 Butte, Montana, topographic

sheet (1958) enlarged to 1:31680. There are, thus,

some inaccuracies in the base map used in preparing

the geologic map of the Lincoln area.

Most of the data on which the study of the Lincoln area is based are shown on the geologic map

(Figure 3), reproduced here in reduced form. Approximately four months were spent in the field

during the study.
NUMBER 7

116°

FIGURE 1.—Location of the Lincoln area with regard to some major regional geologic features.

Regional geology from the Tectonic Map of the United States. (U.S. Geological Survey 1962),

and Nelson and Dobell (1961, fig. 20).

Summary of Previous Work

Charles D. Walcott first described the Pre-Cambrian

rocks in the area north of Helena in 1899. Most of

the formation names proposed by him have been

retained with only minor modification.

The important mineral deposits in the Helena area

led to several early studies by the United States Geological Survey. Some of these which were useful in

the present study are by Barrell (1907), Knopf

(1913), and Pardee and Schrader (1933).

Barrell (1907), in his report on the Marysville

mining district, several miles southeast of the Lincoln

area, presented one of the earliest studies of magmatic stoping and of contact metamorphism.

Knopf's work on the Marysville mining district

(1950), the Boulder batholith (1957), and the mineral deposits of the Helena area (1913) has proven

helpful in relating the igneous rocks of the Lincoln

area to areas nearer the batholith.

Pardee and Schrader (1933) discussed some of the

mineral deposits of the Lincoln area and presented

a reconnaissance map. Although they mapped some

of the igneous rock contacts in the Lincoln area,

their map does not show structure or differentiate

between the formations of the Belt Series.

The Montana Geologic Map (1955) shows the

rock types of the Lincoln area, but the writer's map-
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

R.8W. 112°30' R.7W.

470oo'

T.14N

T.13N.

0 1 5 mi.

FIGURE 2.—Topographic features of the Lincoln area, reproduced from the Butte, Montana,

1:250,000 topographic map (U.S. Geological Survey 1962).

ping suggests that several important modifications

are needed. Figure 4 permits direct comparison of

the two maps. There is no Newland formation in

the Lincoln area, as indicated on the state map.

The area mapped as Newland formation is actually

Helena formation. The designation and contacts of

the igneous rocks of the Lincoln area should also

be modified, particularly the large areas of "Cretaceous Diorite-gabbro" shown in the western and northern parts. Some of these correspond roughly to areas

of middle Tertiary volcanic rocks.

FIGURE 3.—Summary geologic map of the Lincoln area.

Reduced from a 1:31680 geologic map (Melson 1964).

Formations, which are fully described in the text are as

follows, from youngest to oldest: Pg = mainly Pleistocene

gravels and recent alluvium; Tvwt = Tertiary, probably Oligocene, welded tuffs; Tvl = Tertiary volcanic rocks, probably

mainly latitic, but ranging from basalt to rhyolite; Tkb =

dioritic to granitic stocks of probable Boulder batholith age;

Kdg=Cretaceous "diorite and gabbro," mainly quartz gabbro in the Lincoln area; Pcm=Pre-Cambrian Marsh formation, with each of four members indicated; Pch = Pre-Cambrian Helena formation; Pee = Pre-Cambrian Empire formation; Pcs = Pre-Cambrian Spokane formation.
BIACKFOOT

RIVER '
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

Pen

14

13

B

FIGURE 4.—Comparison of new and old geologic maps of the Lincoln area. A. Geologic map of

Montana (1955). Symbols the same as in Figure 3, with the addition of Pen, the Pre-Cambrian

Newland formation, and Qal, Quaternary alluvium, B. New geologic map (this study), redrawn

to the same scale as A. Symbols same as in FIGURE 3, except for Qual, Quaternary alluvium.
NUMBER 7

Stratigraphy of the Belt Series

Discussion

The Lincoln area is underlain by about 80 square

miles of heretofore undescribed sedimentary rocks

of the Pre-Cambrian Belt Series. The rocks of the

Belt Series of western Montana form one of the

largest areas of unmetamorphosed Pre-Cambrian

rocks of the world, and thus are important to the

interpretation of Pre-Cambrian sedimentary environments, and have provided evidence of Pre-Cambrian life. Regional studies of the Belt Series, however, have been impeded by the many unmapped

areas in western Montana and consequent lack of

satisfactory lithologic correlations.

The Belt Series of the Helena area were first described by Walcott (1898) as part of a survey of the

"Algonkian" of Montana. Subsequent work has resulted in mapping of large parts of the Belt Series

and elaborate regional correlations and interpretation of sedimentary environments. Most of this work

has been reviewed recently by Ross (1963). Bierwagen (1964) and the writer have cooperated in

mapping of the Belt Series of the Black Mountain

and Lincoln areas. Suggested regional correlations

and probable local environment of deposition were

presented by Bierwagen and are not repeated in this

study.

The principal features of the Belt formations and

the features which mark their contacts in the Lincoln

area are summarized in Tables 2 and 3, respectively.

The formation names are essentially those proposed

by Walcott (1898) with but one modification. The

Marsh formation is thicker and more variegated in

the Lincoln area than in the Marysville and Helena

areas where it was first described. It was subdivided,

therefore, into four easily mappable members.

Stratigraphic Repetition of Similar Rock Types

The Belt Series of the Lincoln area has a remarkable repetition of rock types. Note that in Table 1 the

sequence of grayish-red siltstones grading upward to

grayish-green siltstones is repeated twice: (1) Spokane to Empire formations, and (2) member B to

member C of the Marsh formation.

Of these units, the Spokane formation and member B of the Marsh formation are particularly similar. Both consist mainly of thinly bedded grayish-red

(5R4/2, Geologic Society of America Color Chart)

TABLE 2.—Diagnostic features of the Pre-Cambrian

Belt Series formations in the Lincoln area (from

oldest to youngest, with thickness in feet)

SPOKANE. Over 5000, bottom not exposed. Dominantly thinbedded noncalcareous red siltstone with argillaceous

mud-cracked partings.1 Some thin white fine-grained

quartzite layers. Lower portion composed of green massive noncalcareous mudstone commonly showing intense

fracture cleavage.

EMPIRE. About 1000. Lithologically transitional to the Spokane and Helena formations. Dominantly grayish-green

to dark grayish-red, commonly calcareous or dolomitic,

mudstones. Distinguished from Spokane formation by

finer grain size, predominant color (green) and common calcareous to dolomitic beds. Distinguished from

Helena formation (gray) by green color and common

grayish-red layers.

HELENA. 5000—7500. Calcareous to dolomitic gray siltstones

and mudstones. Buffweathering dolomitic beds common.

Several thin Collenia beds. "Molar tooth" beds abundant. Thickest carbonate-rich formation in area. Siliceous

to locally cherty noncalcareous mudstone beds common

near top.

MARSH.

Member A. 600-1000. Calcareous or dolomitic green to

gray siltstones and sandstones. Commonly platy (thinbedded) and buff weathering. Distinguished from Helena formation by platy beds (Helena more massive)

and by speckled sandstone beds composed of green

(chlorite after glauconite?), pink (microcline) and

white-to-red quartz grains in a green calcareous matrix.

Member B. 2500. Reddish-gray siltstones with interbedded

dark reddish-gray mudflakes in white fine-grained

quartzites locally common. In isloated outcrops indistinguishable from the Spokane formation. Lower portion

contains interbedded red and greenish-gray noncalcareous siltstones and fine-grained quartzites.

Member C. 2500. Calcareous and dolomitic siltstone and

mudstone. Platy and dominantly greenish-gray beds

which weather buff. Thin interbeds of grayish-red calcareous siltstone common. In isolated outcrops may be

mistaken for Empire formation. One thin "molar tooth"

and one Collenia bed (Continental Divide section)

noted.

Member D. Highly viariable. Thick-bedded, light pinkishgray, arkosic quartzites. Restricted to southwest corner

of area. Wedge-shaped beds up to 300 feet thick intertongued with the upper part of member C.

1

 Siltstone and mudstone are used here in preference to

the term "argillite" to specify more clearly the grain size.

All the Belt rocks, as a whole, are intensely indurated.
8 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

TABLE 3.—Definition of contacts of Belt formations

in the Lincoln area (from oldest to youngest)

Contact Character

Spokane-Empire

Empire-Helena

Helena-Member A of

the Marsh formation

Member A-Member B

Member B-Member C

Member C-Member D

First occurrence of grayish-green,

calcareous to dolomitic (buff

weathering) siltstones to fine-grained sandstones.

First occurrence of gray carbonaterich siltstones and mudstones. "Molar tooth" beds are particularly

distinctive.

Buff weathering grayish green to

very light gray fine-grained dolomitic to calcareous sandstones,

mudstones, or siltstones. Mottled

pink and green sand-sized grains

in a calcareous matrix are distinctive. Contact is apparently gradational over about 50 feet.

Occurrence of hard, grayish-green,

fine-grained quartzites and siltstones which grade upward into

grayish-red, fine-grained quartzites

and siltstones.

Marked by occurrence of grayishgreen, buff weathering siltstones

and fine-grained, calcareous siltstones.

Occurrence of massive, commonly

cross-bedded, arkosic sandstones.

Contact varies greatly stratigraphically because of rapid facies

changes common in the arkosic

sand near the top of the Marsh

formation.

siltstones and fine-grained quartzites with thin, mudcraked, shaly partings. White quartzites about 3 to 5

inches thick and with dark reddish-gray mudflake

inclusions are more common in member B of the

Marsh formation but have also been noted in the

Spokane formation.

The repetition of these rock types may lead to

confusion in correlations as well as in determining

structure. In the Lincoln area, the Helena formation, which separates the repeated lithologies, and

the arkosic quartzites of the upper part of the

Marsh formation are distinctive and led to eventual recognition of the repeated sequence. In areas

of isolated outcrops or of complicated structure,

however, the distinction between repeated rock types

may be impossible. This should be kept in mind during further work on the Belt rocks near the Lincoln

area, particularly just to the north in the unmapped

Belt rocks of the Bob Marshall Wilderness Area.

Belt Formation Contacts

An attempt was made to use the descriptions of

Knopf (1950) for the Marysville area to identify

the contacts of the Spokane, Empire, and Helena

formations in the Lincoln area. The contacts as listed

by him, however, were not distinctive in the Lincoln

area and thus were redefined (Table 2). Although

the formations are grossly similar in the two areas,

facies changes are sufficient to make precise definitions of the contacts of only local applicability.

Structural Geology

Discussion

The Lincoln area is characterized by broad areas of

Belt sedimentary rocks which rarely dip more than

30 degrees. They show evidence of at least four

periods of deformation: (1) gentle regional PreCambrian tilting, (2) "Laramide" (Cretaceous)

folding and faulting, (3) late Cretaceous (?) faulting

associated with intrusion of granitic stocks related to

the Boulder batholith, and (4) faulting related to

Oligocene (?) volcanism. Table 4 summarizes the

structures produced during each of these periods of

deformation. Although extremely complicated on a

small scale because of these superimposed structures,

the main structural features are simple and dominated by northwest to north trending high-angle

faults, open folds and fracture cleavage developed

during the "Laramide orogeny."

The Lincoln area and Black Mountain area

(Bierwagen 1964) are dominated structurally by a

broad southeast plunging syncline, referred to hereafter as the Black Mountain syncline. The gross

structure of the Lincoln area shows that it lies entirely on the northeast limb of this syncline. Although

there are many subordinate folds, successively younger

rocks from Spokane to Marsh occur toward the axis

of the Black Mountain syncline (Figure 3).
NUMBER 7

TABLE 4.—Periods of deformation (from oldest to

youngest)

Age Structures

Late Pre-Cambrian Gentle tilting and erosion of the

Pre-Cambrian Belt Series, as seen

in the Black Mountain area. (Bierwagen 1964).

Cretaceous (Laramide) North to northwest trending folds,

faults, and fracture cleavage.

Faults in contact zones of stocks.

"Collapse" faults along eastern

margin of welded ash flow sheet

and small faults in lower volcanic

series. High angle, small displacement faults which cut epithermal

fissure veins.

Late Cretaceous ( ?)

Middle to late Tertiary

Relation Between Folding and Fracture Cleavage

Fracture cleavage is developed in argillaceous beds

throughout the area. Although the Pre-Cambrian

section is about 15,000 feet thick, fracture cleavage

shows no stratigraphic restrictions. It has proven

useful in defining the regional trend of fold axes

from bedding-cleavage intersections and in showing

that the Black Mountain syncline is the dominant

structure.

DESCRIPTION OF FRACTURE CLEAVAGE.—Cleavage

is excellently developed in some outcrops but absent

in others. Commonly it is inclined at between 5

and 40 degrees to bedding and may be strongly refracted in a sequence of interbedded argillaceous

and quartzite beds. The spacing between cleavage

fractures varies from microscopic in argillaceous beds

to more than an inch in quartzites.

RELATION TO THE BLACK MOUNTAIN SYNCLINE.—

Throughout the southern two townships (Poorman

and Virginia Creek drainage basins) the fracture

cleavage strikes mainly northwest and dips southwest

regardless of the attitude of bedding.

Figure 5 shows the idealized relationship of bedding and fracture cleavage in folds. If the cleavage

is formed by stresses generated within a given fold,

it will dip toward the axial plane of the fold. The

fracture cleavage in the area as a whole, however,

dips west regardless of its position in the limbs of

more local folds. Thus, as shown in Figure 5b, the

dominant stress within the beds during folding was

probably due to the development of the regional

B

FIGURE 5.—Idealized relation between fracture cleavage and

folds (A), and relationship in the Lincoln and Black Mountain areas (B), showing control of fracture cleavage by the

Black Mountain syncline.

syncline and not to stresses generated by smaller folds.

Figures 6 and la show that on a still larger scale

fracture cleavage fans about the axis of the Black

Mountain syncline. The cleavage dips consistently

east in the Washington Creek area, west of the syncline axis.

Some fifteen to twenty miles northeast of the

syncline axis (near Flesher Pass), fracture cleavage

fans on minor folds and thus in places dips east.

Here there is little indication of the regional Black

Mountain syncline.

Cleavage-bedding intersections dip mainly to the

south throughout the southern half of the area.

Figure 76 shows that they dip most consistently

southeast in the McClellan Creek area, just east of

the axis of the Black Mountain syncline. The intersections thus reliably reflect the larger scale structures in the area.

Farther northeast of the axis of the Black Mountain syncline, cleavage-bedding intersections dip more

irregularly and commonly dip and strike north (Figure 7b). They, thus, suggest another regional syncline beginning immediately north or northwest of

Lincoln in the Bob Marshall Wilderness.

Laramide Faults

The two major faults noted in the area, the Fool

Hen Ridge and Rochester Gulch faults, are probably
10

451 R9W R8W

SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

H3°30' R7W R6W

TI4N

TI3N

TI2N

FIGURE 6.—Index map to areas of cleavage and bedding measurements used in Figure 7.

high-angle faults. Their actual orientation is uncertain because of poor exposures. They appear, however, to be relatively straight regardless of topography.

The Fool Hen Ridge fault separates the Spokane

and Helena formations in the southeastern part of

the area. Because of lack of distinctive units in these

formations, and their great thickness, only very rough

limits may be placed on the amount of stratigraphic

displacement. Where the fault parallels Gould Creek,

estimated stratigraphic displacement is between 1,000

feet (thickness of Empire formation) and 10,000 feet.

A rather peculiar, and as yet unexplained, observation is that the beds dip toward the fault on both

sides of the Fool Hen Ridge fault.

The Rochester Gulch fault is one of the major

north-south structures in the area. Because of the

difficulties in determining the magnitude of displacements in Belt rocks, precise throw is again uncertain.

There probably is, however, a minimum of 1,000 feet

of stratigraphic displacement.

The Rochester Gulch fault parallels cleavage and

minor fold axes on the west side of the fault and,

thus, probably was connected with strain developed

during Laramide folding. Contact metamorphism

extends across the fault, clearly showing that it

formed before intrusion of the Silver Bell stock.

The relative age of the Fool Hen fault is more

difficult to determine. On the basis of the inferred

large displacement, it is assumed that the fault

developed during Laramide folding and faulting,

the major period of deformation.

The Empire-Helena contact is repeated by faulting three times between the Silver Bell stock and

Fool Hen Ridge. The faults are assumed to be of

Laramide age because they parallel fracture cleavage. They occur between broad areas of contact-
NUMBER 7 11

FIGURE 7.—A. Fracture cleavage fans about the axis of the Black Mountain syncline. Squares:

Washington Creek area. Solid circles: Wilborn area. Open circles: McClellan Creek area.

Open circles with solid centers: Gould Creek area. Dip "vectors" plotted on lower half of

equal area stereographic projections, B. Most cleavage-bedding intersections dip southeast in

the southern part of the area, which is parallel the Black Mountain synclinal axis. Cleavagebedding intersections dip more divergently in the northern part of the area. Open circles:

southeastern part of the area, mainly T7W, R13N. Squares: northern part of area. Solid

circles: McClellan and Washington Creek areas, c. Intrusion of the Silver Bell stock resulted

in random rotations of the contact rocks. Solid circles: fracture cleavage orientation in the

contact aureole of the Silver Bell stock. Open circles: fracture cleavage in the McClellan

Creek area for comparison.
12 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

metamorphosed rocks, however, and are perhaps

related to faulting during the intrusion of the Silver

Bell and Granite Peak stocks. These stocks may well

be connected at a shallow depth under the faults.

The faults are not exposed and, therefore, their orientation is uncertain. Stratigraphic displacement is

probably between 500 and 1,000 feet and the west

sides are consistently down, relative to the east sides.

The consistent downward movement of the west

sides of faults relative to the east sides contributed

substantially to the occurrence of the youngest rocks

in the western part of the area.

Igneous Rocks

Discussion

At least four periods of igneous activity are evident

in the Lincoln area (Table 5). Two pre-Laramide

periods of intrusion gave rise to an early series of

dioritic sills and a later series of thick quartz gabbro

sills. The most pronounced periods of igneous activity in the area were (1) post-folding, late Cretaceous (?) intrusion of three large granitic stocks

and concomitant mineralization and widespread

contact metamorphism, and (2) middle Tertiary

volcanism and mineralization.

TABLE 5.-—Principal igneous rock types (from oldest

to youngest)

Name Comments

1. Dioritic sills Very common in the Helena and

Empire formations. Commonly altered mainly to chlorite and calcite. Mainly less than ten feet

thick.

2. Quartz gabbro sills Noted only in the Spokane formation. Mainly unaltered plagioclase

and clinopyroxene with minor

quartz. Up to several hundred feet

thick.

3. Granite stocks and Granite Peak, Silver Bell, and Dalassociated dikes and ton Mountain granodioritic stocks,

sills Probably of same age as Boulder

batholith.

4. Tertiary extrusives Rhyolitic (mainly welded tuffs) to

and intrusives andesitic flows. Clearly post-folding. Lie unconformably on Belt

Series as well as granite stocks.

Pre-Cambrian (?) Dioritic Sills

Sills less than ten feet thick but laterally extensive

occur throughout the Empire and Helena formations. They are rare in the Spokane and Marsh

formations. In most outcrops, the sills are clearly

cut by fracture cleavage and, thus, were intruded

before Laramide folding.

Chlorite and calcite locally have replaced the

original hornblende and plagioclase of the sills. This

replacement of the dioritic sills by chlorite and calcite

is probably a result of very low-grade, regional metamorphism. These rocks are typically grayish green

because of the abundance of chlorite and lesser

amounts of amphibole.

The diorite sills, in the few places where they are

unaltered, are petrographically similar to the "microdiorites" exposed near Bald Butte in the Marysville

district (Barrell 1907:12) and to the numerous sills

in the Helena formation several miles west of Lincoln along Montana Route 20.

Cretaceous (?) Pre-Laramide Quartz Gabbro Sills

Younger sills, also pre-Laramide folding, occur in

the Spokane formation. In contrast to the Pre-Cambrian (?) sills, they are much thicker (up to 300 feet

thick), unaltered, and contain clinopyroxene as the

principal mafic mineral. Intermediate weakly zoned

plagioclase (about Ang ) composes about 40 to 60

percent of the rock. Small amounts of interstitial

quartz and myrmekite usually are present. The sills

characteristically contain late-magmatic granophyric

seams and dikelets composed mainly of quartz and

plagioclase. Locally, specimens of the sills show malachite stains, suggesting that they contain small

amounts of disseminated copper sulfides.

The Stemple Creek sill ("Kdg." Figure 3) is the

only large quartz gabbro intrusion in the area, although several large bodies occur east of the map

area and one is particularly well exposed near Flesher

Pass on Route 285. These sills show a remarkable

stratigraphic restriction. They were noted only in

the Spokane formation.

The age of the sills is uncertain, and a Cretaceous age is assumed from their designation on the

Montana geologic map. The reason for this designation as of Cretaceous age is not clear. They are,
NUMBER 7 13

however, probably younger than the altered dioritic

sills.

Petrographically, the quartz gabbro sills are identical to gabbro sills described from the northeastern

part of the Marysville mining district (Barrell 1907:

13) and to quartz gabbro sills described by Knopf

(1963:6) from the immediate Helena area. Knopf

(op. cit.) concludes that they are Pre-Cambrian because they occur only in Belt rocks. This is not conclusive proof of their age, however, because sills

commonly show stratigraphic restriction. The western end of the Stemple Creek sill is cut by postLaramide porphyritic dikes.

Granitic Stocks

Three granitic stocks and their contact aureoles are

prominent features of the geology of the Lincoln

area. The Granite Peak and Silver Bell stocks are

about two square miles in surface area. The Dalton

Mountain stock, which continues southwest of the

Lincoln area and was not mapped entirely, is probably about three square miles in surface area. Several areas of hornfelsed Helena formation, such as

between Mead Gulch and Prickly Gulch, occur

without central exposures of granitic rocks, suggesting that an intrusive underlies them at shallow depth.

Although the Granite Peak stock is a single continuous exposure, the Silver Bell stock is but the

largest exposure of many granitic exposures within

a continuous contact aureole. Thus, the present level

of exposure appears to be the roof zone of a single

pluton.

The stocks in the Lincoln area have many features

in common: (1) strongly discordant contacts, intensely and brittlely deformed without plastic shouldV.

\

FIGURE 8.—Petrographic sketch of hornblende pyroxene granodiorite from the inner portion

of the Granite Peak Stock. Hornblende (lightly stippled, high relief) has clinopyroxene cores

(heavily stippled). Orthoclase shows carlsbad twins and commonly vein and patch perthite.

Plagioclase (about Ar3o) is weakly zoned. Plagioclase and orthoclase in parallel intergrowth in

lower central crystal. Quartz in anhedral interstitial grains. Sphene is most common accessory

(upper left, heavily stippled).
14 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

ering aside; (2) large contact aureoles, in general

exceeding the area of exposed granitic rock; (3) lack

of chilled contacts; (4) mainly massive, unfoliated,

and unlineated granodiorite to quartz monzonite, although small amounts of more mafic contaminated

rocks may occur along contacts with calc-silicate

rocks; (5) mainly composite (Table 6) without

chilled contacts between separate phases; and (6) associated with mineral deposits.

TABLE 6.—Modes of specimens from the Granite

Peak stock

Mineral

Plagioclase .

Orthoclase

Quartz

Hornblende

Biotite

Clinopyroxene

Accessories3

Sketch in

a

.. 52.3

.. 2.3

6.4

7.0

17.3

: 12.9

1.8

Figure 8

2

 Aplite dike.

3

 Accessory

epidote.

minerals:

Abundance in volume percent

b

51.4

17.1

15.4

11.5

1.0

1.2

2.4

Fe-Ti

c

44.4

24.0

2.6

19.5

1.0

1.7

6.8

d

39.6

26.8

6.0

9.3

15.8

1.4

1.1

e

34.6

25.9

8.4

18.0

4.8

7.3

1.0

f

1

16.8

50.7

27.6

1.1

0.0

3.6

0.2

g'

1.9

53.5

44.0

0.0

0.2

0.0

0.4

oxide, sphene, pyrite, apatite,

Stratigraphic reconstruction suggests that the stocks

were emplaced at a maximum depth of about three

to four miles. The stocks are thus, in view of the

above features and shallow depth of intrusion,

typical epizonal plutons as described by Buddington

(1959:676).

COMPARISON OF STOCKS IN THE LINCOLN AREA.—

The mineralogy of the Silver Bell and Granite Peak

stocks suggests that they are compositionally similar

and are mainly granodiorite and quartz monzonite.

The Silver Bell stock, however, in contrast to the

Granite Peak stock, shows considerable evidence of

late magmatic alteration and mineralization by hydrothermal solutions.

Specimens from the Silver Bell stock characteristically contain considerable quantities of epidote dispersed as a deuteric alteration of plagioclase. Epidote

also occurs along joints, commonly with hematite,

within the stock and in the contact rocks nearest the

granite. Prospect pits show that much of the Silver

Bell stock contains considerable disseminated pyrite

and in places contains seams and disseminated grains

of chalcopyrite. One prospect pit reveals a small

irregular body containing libethenite, a copper hydroxy phosphate, and fluorite.

These features suggest that late magmatic solutions were largely retained during cooling. Alternatively, the present level of exposure of the Silver

Bell stock is at the roor*o£ a much larger body, and

was permeated by solutions derived during cooling

of the underlying magma. This latter idea is supported by the broad contact aureole and common

shallow dips of the contact. If these features are

restricted largely to the roof zones of the stocks,

erosion has tripped much of thfe roof zone of the

Granite Peak stock.

The Silver Bell stock also differs texturally from

the Granite Peak stock. Most of the Granite Peak

stock shows a well-developed, even-granular texture,

although it locally contains large crystals of orthoclase near its northern margin (see mode /, Table 6).

In contrast, the Silver Bell stock characteristically

is porphyritic and contains large crystals of strongly

zoned plagioclase and, more rarely, orthoclase.

Only a small portion of the Dalton Mountain

stock was mapped and thus it cannot be compared

satisfactorily with the Silver Bell and Granite Peak

stocks. The few specimens collected, however, suggest that the stock is slightly more mafic than the

Silver Bell stock and Granite Peak stock, contains

about 20 to 40 percent hornblende, and is probably

mainly quartz diorite with, perhaps, some granodiorite. Where mapped, it has an even-granular texture, and no porphyritic phases were noted.

The most mafic stock, hornblende gabbro, is near

the head of Washington Creek south of the Lincoln

area (R. 8 W, T. 12 N, Figure 3). It contains oval,

corroded clinopyroxene crystals which are up to

one-half inch in diameter and are surrounded by a

thin rim of uralitic hornblende. The matrix consists

of plagioclase which is deuterically altered to calcite,

epidote, and micaceous minerals.

RELATION TO THE BOULDER BATHOLITH.—Barrell

(1907:81) suggested that the "great Boulder Batholith" is but the largest exposure of a widespread intrusive mass which underlies a considerable region

in western Montana, and that stocks such as at

Marysville and Granite Peak are upward prolongations or cupolas in its roof.
NUMBER 7 15

Although radiometric dates have not been made

on the stocks in the Lincoln area, their age is clearly

bracketed between formation of fracture cleavage

(presumably middle-to-late Cretaceous), and extrusion of volcanic rocks (probably Oligocene, page 38).

The fracture cleavage is randomly rotated in the

inner contact zones (Figure 7c) and must have

formed before the differing competency of beds was

destroyed by hornfelsing. The stocks, thus, are presumably the same age as the Boulder batholith,

which gives radiometric ages on five samples of 61

to 72 million years by the lead-alpha method (Chapman, et al. 1955) and of 87 million years by the

potassium-argon method (Knopf 1956). Numerous

new, more reliable K-Ar ages on biotite and hornblende indicate that the emplacement occurred over

a long time span, between 78 to 68 million years ago

(Tilling, etal. 1968).

Other evidences of genetic relationship to the

Boulder batholith are proximity of stocks to the

batholith and similar modes of emplacement.

Two field trips permitted brief comparison of the

stocks of the Lincoln area with parts of the Boulder

batholith near Butte, Montana. Although the batholith is slightly coarser grained than the stocks, they

are essentially similar. Hornblende and biotite are

the principal mafics in the batholith, although small

amounts of clinopyroxene may occur within the

hornblende. The stocks in the Lincoln area, neglecting the small mafic intrusion near the Dalton Mountain stock, similarly show clinopyroxene only as relics

in the core of hornblende crystals (Figure 9).

Both the batholith and the stocks are composite.

As pointed out by Knopf (1957), the batholith contains a wide variety of rock types. The variations in

composition, however, are probably no greater than

variations within and between the stocks in the

Lincoln area.

Aplite dikes in the batholith and stocks are common. Simple pegmatites occur in both and are composed mainly of orthoclase and quartz. Such pegmatites are common in the Granite Peak and Dalton

Mountain stocks, but are rare in the Silver Bell

stock.

The stocks and associated mineral deposits apparently end at, or just north of, the northern limit of

the Lincoln area. The area, thus, is perhaps near the

northernmost limit of batholith activity.

MODE OF EMPLACEMENT.—Barrell (1907) in his

SURFACE

+ + +• + + + +

j. + + + + + +

+ + + + ++ +

0 100 200 300 FT

FIGURE 9.—"Frozen-in" stoping of the Marysville stock.

Cross-section of the Drumlummon mine, simplified from

Barrell (1907).

study of the Marysville stock, located several miles

southeast of Granite Peak, presented one of the first

completely documented arguments for granite emplacement by magmatic stoping. Although the details of the granite-hornfels contacts at depth are

not known in the Lincoln area, the surface exposures

of the contact are very similar to those about the

Marysville stock.

For example, detailed mapping about the Granite

Peak stock shows that within the inner contact zone

intricate brittle deformation has occurred. Structural

trends are not deflected by the stock except in the

inner contact zone. Thus, because of the lack of

doming or other evidence of shouldering aside of the

country rock, and abundant faulting and random

rotations in the inner contact zone, the Granite Peak

stock is inferred to have been emplaced by magmatic stoping. Barrell's evidence (1907, such as plate

X, summarized in Figure 9), however, is a much
16 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

better case for emplacement by magmatic stoping

than can be given by surface mapping. The extensive

mining of epithermal veins which parallel the contact of Marysville stock at the Drumlummon mine

(Figure 9) clearly provides evidence of magmatic

stoping which has been "frozen" in process.

The general lack of extensive contamination of the

granites, even where they intrude carbonate rocks,

as well as the lack of abundant inclusions, shows that

the stoped blocks were removed without assimilation

at the present level of exposure. The Silver Bell stock

is free of inclusions and uncontaminated. If it is the

roof zone of a large stock as the field evidence indicates, the fragments of the roof presumably were

removed downward.

Diopside-rich rocks which characterize the contact hornfels of the Silver Bell and Granite Peak

stock have densities between about 2.9 and 3.1, and

thus would readily sink in granitic magma. The

Dalton Mountain stock, however, which also shows

evidence of emplacement by magmatic stoping, is

largely in quartzites of the Marsh formation, which

were perhaps slightly less dense than dioritic-togranodioritic magma. Thus, the subsidence of stoped

blocks regardless of density implies that there were

downward movements of magma in the stocks. The

evidence of emplacement by stoping, the lack of

chilled contacts, and the large contact aureoles suggest that the stocks were not emplaced by a single

injection of granitic magma but rather were intruded over a long period of time. Perhaps largescale convection occurred during emplacement and

during the initial stages of cooling of these stocks.

LACK OF EXTRUSIVE EQUIVALENTS.—It is difficult

to reconcile the shallow depths of intrusion of the

stocks in the Lincoln area with the lack of extrusive

equivalents. Barrell (1907:172-173) shows that this

problem exists for the Marysville stock and for the

Boulder batholith as a whole. He points out that the

Boulder batholith, which also lacks extrusive equivalents, may have maintained a thin cover, perhaps

a half mile to a mile thick, over some thousands of

square miles although it too was emplaced by sinking of blocks from its roof:

The greatest theoretical difficulty in the way of accepting

magmatic stoping as one method of batholithic invasion, a

perhaps dominant method in the zone of fracture, is the fact

that it apparently ceases without breaking through to the

surface with crustal foundering over the limits of the batholithic invasion. Such a caving in of the surface would presumably lead to enormous outpourings which find no suggestion in nature.

The middle Tertiary volcanic rocks, on the other

hand, as discussed in the following section, reached

the surface from probably numerous vents and are

associated with some small intrusive bodies. These

contrasting observations, although clearly presented

by Barrell in 1907 for the Boulder batholith region

as a whole, still lack adequate explanation.

Recent radiometric ages on the Elkhorn Mountain

volcanics show that they are but slightly younger

than the Boulder batholith. Thus, it appears that

during an early stage the Boulder batholith did

break through to the surface, only to later intrude

its early eruptives.

Middle Tertiary Volcanic Rocks

The area contains about forty square miles of middle

Tertiary volcanic rocks which occur mainly in the

northern two townships. The volcanic rocks are

particularly well exposed at and to the west of Crater

Mountain.

The volcanic rocks are mainly acid-to-intermediate

flows, flow breccias, and welded ash flows. They

reach a maximum of about 1,000 to 1,500 feet thick

in the north-central part of the area. Here they consist of a thick series of latitic-to-andesitic rocks and

an upper rhyolitic series. The rhyolitic rocks are between 50 and 500 feet thick and are composed

mainly of welded ash flows. The welded ash flows

show several features which suggest they were deposited as pumice froth flows (Smith 1960:809).

Rhyolite domes occur along a minor unconformity

which separates the two series.

As noted by Knopf (1963:8-9), the immediate

Helena area contains flows which range from rhyolite to olivine basalt. Thus, regionally, as well as in

the Lincoln area, the volcanic rocks show a wide

variation in composition and fit well into the

postorogenic basalt-andesite-rhyolite association of

Turner and Verhoogen (1960:282) :

Surface eruption of basalts, andesite, and rhyolite during

and following elevation of the folded (land) mass. This

phase is typically separated by a lengthy period of time from

the main phase of folding and plutonic activity.

The precise age of the volcanic rocks is uncertain.

They clearly followed unroofing of the major stocks.

Near the Dalton Mountain and Silver Bell stocks
NUMBER 7 17

they rest unconformably on the contact aureoles.

Volcanic rocks lie directly on the eroded surface of

the Blackfoot City stock in the Black Mountain area

(Bierwagen 1964: pi. 1). Except along later mineralized zones, the flows have remained essentially unaltered since their final cooling. Fresh glass is particularly common at the bottom of welded tuff sheets

in the vicinity of Crater Mountain.

Unidentifiable plant fragments were collected near

Crater Mountain in thin tuff beds. A few plants,

however, of probable Oligocene age (Erling Dorf,

oral communication) were collected by the writer in

siltstone beds in lavas near Little Prickly Pear Creek,

which is on the north side of the Marysville district.

These volcanic rocks are lithologically similar to the

lower latitic series in the Lincoln area and are perhaps about the same age. The Lincoln area volcanic

rocks, thus, are referred to as of Oligocene age, but

confirmation by additional fossil finds or radiometric

dates is needed. The unconformity within the volcanic rocks, as well as evidence of considerable erosion between periods of volcanism, suggest that eruptions occurred intermittently over a long period of

time.

The lower volcanic series are cut by gold- and

silver-bearing fissure veins. Such veins were not noted

in the overlying welded ash flows.

Lower Volcanic Series

The lower volcanic series is characterized by dark

gray to dark reddish-gray massive flows. In general,

they are aphanitic and thus their composition is uncertain. Where finely porphyritic, however, the dominant phenocrysts are plagioclase and hornblende or

biotite.

The flows characteristically show considerable

deuteric alteration and are locally vesiculated. Vesicles near the Seven-Up Mine and in volcanic rock

fragments in the Crater Mountain landslide are filled

by euhedral calcite crystals. The vesiculation as well

as the abundance of deutric alterations suggest

that the lavas were, as a whole, water-rich on

eruption.

A well-developed platy structure is commonly

present. Generally, the platy structure is nearly perpendicular or at high angles to the flow contacts.

Isolated areas of volcanic rocks similar to the

lower volcanic series to the north occur along the

Dalton Mountain-Continental Divide trail which

marks the southern limit to the area.

Flows also similar to the lower volcanic series are

exposed on the north side of the Lincoln Valley and

along the dirt road leading south from the valley to

the Dalton Mountain lookout. The full extent of

rocks similar to the lower series remains uncertain,

although reconnaissance mapping suggests that they

probably do not extend more than several miles

north of the Lincoln Valley.

VENTS.—The many isolated patches of volcanic

rocks of varying composition suggest that there were

numerous local centers of eruption. Dikes and irregular intrusive breccias composed largely of volcanic

rock fragments occur in several places in the lower

volcanic series. These are particularly well exposed

along the Blackfoot River and Willow Creek in and

just outside the northeastern part of the area, and

along the Poorman Creek road just before it emerges

into Lincoln Valley. The intrusive breccias perhaps

are associated with vents in the lower volcanic series.

The volcanics along the Dalton Mountain Trail

are associated with latitic (?) dikes and fine-grained

porphyritic pluglike intrusives commonly containing

plagioclase and hornblende phenocrysts. Both dikes

and plugs may have served as vents. These are well

exposed at the head of Mead Gulch and in a cirque

at the head of Prickly Gulch.

Upper Rhyolitic Series

Welded ash flows form a topographically prominent

high, flat, steep-sided area which is at an elevation

of about 7,000 feet along the northeastern part of

the Continental Divide. The ash flows are well exposed along escarpments which everywhere mark

the edge of the flat area. As evidenced by isolated

erosional remnants on Crater Mountain and on an

unnamed mountain about one-half mile due east of

it, the ash flows initially covered a much larger area.

Typically, the contact of the ash flows with the

underlying Spokane formation in the eastern and

northeastern part of the sheet, and with the lower

volcanic series, is irregular. The lower contact ranges

from an elevation of about 6,300 to 6,700 feet, suggesting a minimum relief of about 400 feet on the

surface on which the ash flows were deposited.

A poorly consolidated conglomerate is commonly

exposed at the base of the ash flows. The conglom-
18 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

erate on the south and northwest side of the sheet

is composed largely of pebbles and cobbles of the

older volcanic rocks; and on the east and northeast

side, of the Spokane formation. Locally on the northwest side blocky pyroclastic beds as much as 10 feet

thick occur at the base of the sheet. No interbedded

sedimentary rocks or soil zones were noted within

the main ash flows, suggesting that they were deposited over a relatively short period of time.

The ash flows are remarkably similar to one

another in hand specimens. They are light gray, and

contain about 10 to 30 percent crystals (sanidine and

dark gray quartz in varying ratios) and less than 5

percent accidental lithic fragments. The lithic fragments are mainly silstones derived mostly from the

Spokane formation. The ash flows contain no mafic

minerals except for rare flakes of biotite.

The ash flows lack well-developed contacts where

examined carefully at two places along the escarpment. The presence, however, of several ash flows

which individually may be up to 50 to 100 feet

thick is suggested by varying degrees of welding.

Some specimens presumably from the upper parts of

ash flows are not welded and are more properly

termed crystal tuffs.

Characteristically, the more intensely welded ash

flows contain beautifully opalescent ("moonstone")

sanidine crystals. Opalescent (or chatoyant) sanidine

crystals are due to cryptoperthitic exsolution and are

most common in the slowly cooled parts of ash flows

(Smith 1960:829; Boyd 1961:399). Thus, their occurrence at specific horizons in vertical sections of

the main sheet also suggests the presence of a number of individual ash flows.

Two quantitatively minor but perhaps significant

variations occur in the ash flows. Along the Continental Divide at the head of Specimen Creek, unusually large sanidine crystals up to an inch and a

half long occur. The significance of this coarser grain

size is uncertain, but it is clear that such large crystals crystallized before eruption.

The second textural variation occurs in thin,

welded ash flows near Crater Mountain. Although

only about 10 feet thick, the ash flows are commonly welded to a black glass at the bottom. The

black glass portion of the flows grades rapidly upward into a partially welded top. The index of

refraction of the glass is about 1.50 (±.01) suggesting a silica content of around 73 percent. The glass

is rhyolitic and has the following composition based

on electron probe analysis: SiO2 = 75.9, A^Oa ^

12.4, total Fe as FeQ = 0.93, MgO = <0.01, MnO =

0.08, CaO = 0.21, Na2O = 2.9, K2O = 3.9, TiO2 =

0.06, total = 96.4.

Such intense welding of thin ash flows implies high

temperatures of emplacement, because there is little

load pressure to aid welding and compaction (Smith

1960:830). Rapid cooling of the thin ash flows is

suggested by the lack of opalescent sanidine crystals,

which are common in the thicker flows.

VENTS.—Several rhyolitic "domes" are associated

with the main welded ash-flow sheet. One such body

is excellently exposed at the upper part of a landslide scar at Crater Mountain (Figure 10), and

consists of strongly flow-banded spherulitic rhyolite.

A thin hydrothermally altered zone and a coarse,

perhaps intrusive, pyroclastic body occur between

the dome and the nearest welded ash flow. The contact between the pyroclastics and the welded ash

flows and "dome" are, however, gradational. Thus,

the plug which gave rise to the rhyolite dome perhaps earlier served as a vent for the welded ash flows.

At least two other rhyolitic "domes" are exposed

near the welded ash flows. The largest of these

bodies occurs about one mile west of Crater Mountain and at about the same altitude. Here the flowbanded rhyolite contains spherulites up to four inches

in diameter. The "dome" here, however, is separated

from the nearest welded tuff by about one mile of

exposures of the lower volcanic series (Figure 2).

Although plugs were perhaps the source of welded

ash flows, fissure eruptions may well have been involved. Domes or intrusives were not noted in the

northern portion of the ash flows.

MECHANISM OF ASH FLOW DEPOSITION.—Welded

ash flows have not been noted in recent "nuee ardente" deposits (Smith 1960:802), and thus their

origin has been controversial. For several reasons it

is clear that welded ash flows do not represent normal volcanic flows or more typical tuffs. Although

of rhyolitic composition, which implies high viscosity,

the ash flows were apparently very fluid and flowed

considerable distances. In spite of these distances,

sufficient heat was conserved to give intense welding

after cessation of flow. Smith (1960) has shown that

ash flows must be at a minimum temperature of

600° C to weld and for thin flows perhaps at 800° C

at the time of deposition. The occurrence of pheno-
RHYOLITE DOME

Sphtrulitic

MYOROTHERHAUY

ALTERED ZONE

FIGURE 10.—Rhyolite dome and environs exposed in the landslide scar on the southwest side

of Crater Mountain.

crysts of quartz and sanidine at the time of eruption

suggests that the temperature could not have been

much above 900° C at eruption and certainly was

less than 1000° C (Smith 1960). Thus, the temperature decrease between eruption and deposition could

not have been greater than about 300° C.

A principal problem then lies in proposing a

mechanism for rapid emplacement of the flows with

sufficient conservation of heat to cause welding.

Smith (1960:802-809) has summarized the possible

modes of eruptions of ash flows and favors emplacement by "avalanche or flowage of fragmental material and hot gas." He considers "particulate or foam

(froth) flows" as a less likely mode of emplacement

and summarizes some problems presented by the

"froth flow mechanism":

The supporters of the particulate flow mechanism generally agreed that the driving force of the ash flow or avalanche is gravity aided by expanding gases. Herein lies a

problem that needs solution, namely, what is the true nature

of the gas emission process, and how does it vary in different ash flows? Is total vesiculation confined to the vent,

conduit, or chamber, or does some of it take place in the

flow, and if so, how much?

The welded ash flows in the Lincoln area show

several features which support the "froth flow mechanism" and suggest that although much vesiculation

occurred in the vent, small amounts of gases were

emitted even during final welding.

Figure 11 shows fluid inclusions which commonly

occur in quartz phenocrysts in the ash flows. Only

two or three inclusions occur in thin sections of each

phenocryst, and are widely separated. The walls of

the inclusions commonly "mimic" the beta quartz

form (dipyramids) of the quartz crystals at the time

of initial crystallization.

The inclusions are commonly filled by brown glass,

with the volatile phase confined to a bubble which

segregated before solidification of the glass. Several

inclusions show what may be a liquid phase within

the gas phase. The size of the bubble suggests that

the magmas trapped by growth of the crystals were

rich in volatiles, which, along with fragmentation,

would greatly lower the viscosity of the ash flow.

With release of pressure during eruption, an immense quantity of volatiles probably were evolved

from the magma.

Figure 12 suggests that the lavas were supersaturated with volatiles even during final welding and

compaction. Thus, continuous emission of volatiles

may have occurred after, as well as during, flow.

Smith (1960:810) suggests that only small amounts

of gases liberated by diffusion would be sufficient

to "lubricate" ash flows:
20 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

.02. mm.

FIGURE 11.—Petrographic sketch of glass inclusions in quartz phenocrysts from the welded ash

flows. Dipyramidal shape of the inclusions reflect beta quartz habit during crystallization. Glass

normally contains a large gas bubble, trichites, and is light brown and in places fractured.

FIGURE 12.—Petrographic sketch of completely welded bottom of thin ash flow near Crater

Mountain. Note "draping" of collapsed vesicular glass about quartz phenocryst. Tension zones

in the collapsed and stretched glass are defined by numerous minute bubbles, presumably mainly

of exsolved water vapor.

Vesiculation, especially in the vent, probably does not reduce the glass to a state of saturation or undersaturation

and the glass may continue to lose gas even after some cooling. If the amount given off by diffusion were only a few

hundredths of a per cent by weight, the volume at eruption

would be impressive and would provide an adequate lubricent.

In summary, it is likely that the welded ash flows

in the Lincoln area, particularly the thin but intensely welded flows in the vicinity of Grater Mountain, were deposited as a rapidly moving vesiculating

mass of rhyolitic lava (froth flow).
NUMBER 7 21

Although the source for the flows as a whole is

uncertain, plugs near Crater Mountain may mark

the vent areas.

Regional Correlation of Volcanic Rocks

Volcanic rocks of the Lincoln area are mineralogically similar to some of the small areas of volcanic

rocks in the Black Mountain locality described by

Bierwagen (1964:89-102). Welded ash flows, however, were not noted in the Black Mountain area.

The Lowland Creek volcanic rocks (Smedes 1962)

are probably about the same age as the volcanic

rocks in the Lincoln area. They cover a much larger

area, however, and their welded ash flows apparently are less indurated.

Post-Laramide Dikes

Porphyritic latitic dikes occur throughout the Lincoln area, but are particularly abundant between

the Silver Bell and Granite Peak stocks and at the

head of Mead Gulch in the southern part of the

area. The dikes are clearly later than the formation

of fracture cleavage.

In general, the dike rocks consist of about 10 to

30 percent plagioclase phenocrysts which rarely are

rimmed by potassium feldspar. The plagioclase phenocrysts were not noted in the chilled contacts, and

commonly are largest in the center of the dikes,

suggesting formation of the phenocrysts after intrusion. Small phenocrysts of hornblende and biotite

compose about 10 percent of most dikes.

Characteristically the phenocrysts and aphanitic

matrix are altered deuterically to epidote, calcite,

and mica minerals. Deuteric alteration is usually

most intense at the center of the dikes. This abundance of deuteric alterations suggests that water was

abundant in the intruded magmas and only partially

escaped during cooling.

The dikes generally fill high-angle, en echelon

tension fractures which individually are rarely more

than 200 feet long and 10 feet wide. This arrangement is particularly well shown along ridge crests

just south of Poorman Creek and the Silver Bell

stock. No consistent regional orientation of the dikes

was noted.

The relative age of the dikes to the stocks and

Tertiary volcanic rocks is uncertain where the dikes

are isolated in Belt rocks. Porphyritic dikes are common in the Stemple Pass mining area, and are cut

by the epithermal fissure veins. They may be related,

therefore, either to intrusion of the stocks or to

Tertiary volcanism. Dikes in the contact aureole of

the Silver Bell stock, however, characteristically lack

a chilled contact, showing that they were intruded

during or immediately after emplacement of the

stock, at a time when the contact rocks were still hot.

Dikes and epithermal fissure veins are commonly

associated, and may be parallel in the Silver Bell

stock area. The dikes in general are earlier than the

associated epithermal veins.

Contact Metamorphosed Belt Rocks

Most of the contact metamorphic rocks occur around

the granitic stocks, although the Spokane formation

is metamorphosed as much as fifty feet from the

contact of the Stemple Creek quartz gabbro sill.

Table 7 summarizes some of the principal changes

produced by contact metamorphism of the Belt

formations.

In general, formations may be identified even

where they are contact metamorphosed. Around the

Silver Bell stock, however, hornfelsed Helena and

Empire formations apparently are intricately faulted

and, with only isolated exposures, it is difficult to

distinguish between the two. Both contain dolomitic

siltstone beds which react to form tremolitic and/or

diopsidic hornfels. Areas where the hornfels are predominately dark gray are mapped as Empire formation. Hornfels derived from the Helena formation

are mainly lighter colored and as a whole contain

considerably more diopside. Hornfelsed "molar tooth"

beds are most useful in recognition of metamorphosed

Helena formation.

In the few places, it has been seen, the transition

from unmetamorphosed Helena or Empire formation

to hornfels of the aureole occurs over a distance of

several feet. Within the aureole itself, few megascopic

changes occur in the hornfels on approach to the

Granite except within 100 to 200 feet of the contact,

where skarns and other metasomatic assemblages are

common.

Broad contact aureoles are developed only in "reactive rocks," such as the impure carbonate rocks

of member C of the Marsh formation and of the

Helena and Empire formations. The quartzites of
22 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

TABLE 7.—Hornfelsed Belt formations

Formation

Spokane

Empire

Helena

Marsh

Member A

Member B

Member C

Member D

Intrusive

Quartz gabbro

(Stemple Creek

sill)

Silver Bell stock

Granite Peak and

Silver Bell stocks

Nowhere metamor

phosed

Dalton Mountain

stock

Dalton Mountain

stock

Dalton Mountain

stock

Hornfels description

Dark gray due to recrystallization of red iron oxides to specular hematite. Detrital

muscovite recrystallized and coarser grained.

Dark brown and gray to light green to gray layered to massive hornfels. Dark

colored varieties contain some recrystallized iron oxides (specular hematite)

and/or biotite. Light green layers contain tremolite and/or epidote; light gray

layers contain considerable diopside.

Mainly light-colored, layered hornfels; diopside and/or tremolite main minerals.

Hornfelsed "molar tooth" beds and massive white diopside, quartz, and calcite

beds distinctive. Minor brown, biotitic layers.

Dark gray; colored largely by specular hematite.

Mainly green layered hornfels composed largely of quartz, micas, and actinolitic amphibole. Some dark gray massive beds. Locally "spotted hornfels" occur

showing spots of unreacted rock surrounded mainly by actinolitic amphibole,

muscovite, and quartz.

Massive quartzites, metamorphism suggested only by intense induration (conchoidal fracture) and some metamorphic minerals in rare impure beds.

the upper Marsh formation show little field evidence

of contact metamorphism around the Dalton Mountain stock, presumably because they are composed

mainly of quartz and feldspar, an assemblage stable

at high temperatures.

Mineral Deposits

Introduction

The mineral deposits in the Lincoln area were

formed after Laramide folding, and are probably

genetically related to the late Cretaceous (?) granitic

stocks and middle Tertiary volcanism. The deposits

may be divided into (1) high-temperature veins and

replacement deposits within the stocks and their inner contact zones, (2) epithermal gold- and silverbearing fissure veins, and (3) gold placer deposits.

High-Temperature Veins and Replacement Deposits

Milky quartz veins, such as the Prize and Hubbard

veins (Pardee and Schrader 1933), occur very near

and within the Granite Peak stock and locally contain minable gold. The mineralogic details of the

veins are uncertain, although the dumps show mainly

milky quartz with considerable pyrite. These differ

from the epithermal veins in their low calcite content, massive milky quartz, and lack of epithermal

textures. Small milky quartz veins contain sulfides,

mainly pyrite, chalcopyrite, and galena. Chalcopyrite

and andraditic garnet-bearing skarns occur locally at

the contact of the Granite Peak stock and more rarely

at the contact of the Silver Bell stock. The skarn

deposits are not large, and have not proven of economic value.

Pyrite and chalcopyrite locally occur in disseminated grains and in small veins in many prospect

pits within the Silver Bell stock. The full extent of

such areas and their copper content are unknown.

Scheelite has been found in the inner contact zone

of the Marysville stock (Knopf 1950), but it was

not noted in the few specimens of skarns and hornfels collected around the Granite Peak and Silver

Bell stocks and examined in ultraviolet light.

Andraditic garnet fissure veins occur locally in the

inner contact zone of the Granite Peak stock. The

veins commonly contain small amounts of chalcopyrite, calcite, and epidote. In one well-exposed garnet

vein in a prospect pit about a quarter of a mile

east of the Granite Peak lookout, a thin but distinct

seam of chalcedonic quartz occurs at the center.

Thus, the last filling occurred at a much lower

temperature than the initial deposition of andraditic

garnet.
NUMBER 7 23

Epithermal Gold-Silver Fissure Veins

Most of the gold and silver deposits of the Lincoln

area are discussed briefly by Pardee and Schrader

(1933:77-87; 117-19) as part of the greater Helena

mining region. Their descriptions are of particular

value because many of the mines are now caved.

Gold and silver valued at a minimum of $10 million were produced in the area mainly between 1870

and 1930 (Pardee and Schrader 1933). Most of the

gold was from placer deposits in McClellan Gulch

(about $7 million) and from the Jay Gould mine

(about $2.5 million).

STRUCTURE.—The fissure veins are in tension fractures which dip between 60 and 90 degrees and may

be traced without interruption for distances up to a

thousand feet although they are generally much

shorter. The width varies, but rarely exceeds twenty

feet, and in most places is less than five feet. The

veins contain breccia which is mainly an open aggregate of fragments of their wallrocks. The breccias

were probably formed by local collapse of parts of

the vein walls during opening of the tension fractures. Fault gouge occurs locally along post-mineralization faults. Gold noted in a prospect pit in the

Silver Bell area is deep yellow and occurs as films

which coat amethystine quartz crystals. The only

gold assays available to the writer for mines in the

Lincoln area are those listed by Pardee and Schrader

(1933:77-87).

As shown by stope maps of mines in the Lincoln

area (such as for the Gould Mine, plate 10, Pardee

and Schrader 1933), the ore distribution is restricted

both vertically and horizontally. Intervening areas,

though essentially barren of silver and gold, show

gangue minerals and textures identical to those in

ore. Figure 13 shows the location and orientation of

some productive and nonproductive fissure veins.

The scant available data show that most gold and

silver was produced less than five hundred feet below

the ore-bearing outcrop. This restricted vertical extent of ore in the fissure veins is the only available

evidence of vertical zoning in the area. As indicated

by Pardee and Schrader (1933:79), however, the

Marysville Drumlummon Mine, which shows welldeveloped vertical zoning, is similar to the Gould

Mine. Thus, the zoning in the Drumlummon Mine,

which corresponds to zoning in most of the epithermal veins in the Marysville area (William R.

Wade, oral communication), is of interest as a guide

to possible zoning in the Lincoln area. The zoning

as summarized by Wade (Table 8) is of value to

future studies of the epithermal veins, as well as

an important aid to exploration and development of

fissure veins in the Lincoln area.

TABLE 8.—Zoning of minerals in the Marysville epithermal gold deposits (from the top down)

1. TOP OF VEIN. Vein begins as narrow fissure which may be only a few inches wide. May contain calcite but no quartz.

Vertical range about 500 feet and may begin about 1500 feet below surface at time of formation.

2. UPPER CALCITE ZONE. Zone most often exposed at surface in Marysville district. Vertical extent about 100 to 200 feet.

No gold but maybe a trace of silver. Calcite and siderite or ankeritic calcite are most abundant gangue minerals. No

quartz. Some breccia and perhaps fault gouge. Vein wider than at top.

3. MAIN CALCITE ZONE. Mainly ankeritic calcite with little or no quartz. Traces of manganese oxides. Traces of gold and

about 0.1 to 0.2 ounces of silver per ton. Vertical range about 200 to 300 feet.

4. TOP OF ORE ZONE. Gold and silver content increases abruptly. Gold commonly about 2 to 3 ounces per ton and silver

about 10 to 20 ounces per ton. Quartz more abundant and commonly chalcedonic. Manganese, iron oxides, and adularia

commonly present. Sulfides become abundant and are dominantly pyrite and tetrahedrite. Vein generally about 8 to 12

feet wide. Vertical range 100 to 200 feet.

5. MAIN ORE ZONE. Gold about 0.5 ounces and silver about 6 ounces per ton. Vein commonly about 25 to 35 feet wide.

Quartz often replaces platy calcite crystals giving "lamellar" quartz. Very little calcite present. Sulfides about 1 weight

percent of ore and are mainly pyrite, tetrahedrite, chalcopyrite, galena, and sphalerite. Sulfides are mainly concentrated

about fragments of hornfels breccia ("wedding ring" ore). Vertical range 500 to 800 feet.

6. NEAR BOTTOM OF ORE ZONE. Ore may stop abruptly along a horizontal plane or may decrease abruptly to 0.1 to 0.2

ounce gold per ton and less than 1.0 ounce per ton of silver. Vein may commonly divide into two veins. Sulfides continue as in main ore zone and may even increase to 2 or 3 weight percent. Chalcopyrite and galena are dominant

sulfides. Vertical range about 100 to 200 feet.

7. BARREN ZONE. NO gold or silver. Minerals and textures, however, similar to zones 4 and 5. "Milky" quartz common.

Vein may grade downward into uncemented hornfels breccia.
24 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

7

FIGURE 13.—Location and orientation of some fissure veins in the Lincoln area. Stars indicate

mine locations. Most of these were abandoned prior to the period 1962-1963, and none were

in major production. Heavy dashed lines indicate orientation of veins; both the length and

width of the veins are exaggerated. Question mark indicates that the orientation of the vein

was not obtained. Country rock is indicated as follows: Tvl = Tertiary volcanic rocks; Tkb =

granitic stocks; Pch = Helena formation, Pce = Empire formation, and Pcs = Spokane formation.

MINERALOGY AND TEXTURE.—Quartz and calcite,

the main gangue minerals, characteristically occur

as plates of calcite in a matrix locally composed of

anhedral quartz. In some specimens, quartz has

replaced calcite plates. This platy calcite-quartz intergrowth is characteristic of epithermal deposits

and has been discussed by Lindgren (1933:449) and

Pardee and Schrader (1933:82-83). Calcite and
NUMBER 7 25

quartz commonly fill interstitial areas in vein breccias, and quartz has in places completely replaced

breccia fragments. Calcite, as shown by its common

yellowish-to-dark-brown color where weathered, normally contains some iron (ankeritic).

Amethystine quartz, although common in epithermal veins (Lindgren 1933:446), was noted only

in the veins in the vicinity of the Silver Bell stock.

Here, quartz crystals show delicate "phantoms" of

amethystine quartz. Colorless chalcedonic quartz also

occurs in most veins.

Pyrite is the principal sulfide mineral, although

small amounts of chalcopyrite were noted in the

fissure veins in the Silver Bell and Seven-Up areas.

Gold occurs mainly as the native metal alloyed

with some silver. Native gold from the Gould Mine

is very light yellow, suggesting a high silver content, and commonly occurs as thin films and fillings

around granular, clear quartz. Assays from the Jay

Gould Mine suggest that such gold may contain

about 5 percent silver (Pardee and Schrader 1933:

81). Robert Clearman of Helena provided the writer

with a sample of such silver-rich gold from the

Gould Mine. Joseph Nelen of the Smithsonian Institution analyzed this gold on the electron microprobe and found that it contains about 40 percent

silver, and a minor amount (less than 1 percent) of

copper. This gold alloy is thus electrum, a silver-rich

alloy containing 20 or more percent silver.

EVIDENCE OF TWO PERIODS OF EPITHERMAL MINERALIZATION.—The age of the gold-silver deposits

and their possible genetic relation to the granitic

stocks and middle Tertiary volcanism in the greater

Helena mining district have been discussed by several authors. Barrell (1907) suggested that there were

at least two periods of mineralization: one following

but genetically related to the intrusion of the Marysville stock, and a second later period after extrusion

of middle or late Tertiary volcanic rocks. Knopf

(1950:843) suggested that there was a single period

of mineralization and that it occurred after extrusion of the middle Tertiary volcanic rocks. The

study of the mineral deposits in the Lincoln area

supports Barrell's conclusion that there were at least

two periods of epithermal mineralization.

The gold and silver veins in the Seven-Up area

formed after extrusion of the lower volcanic series

and are probably the youngest mineral deposits in

the area. Epithermal veins were not noted in the

welded ash flows and perhaps formed before their

extrusion.

The principal problem lies in establishing that

there was an earlier period of mineralization associated with the granitic stocks. The evidence of lowtemperature deposition of the gold-silver veins, such

as their common content of chalcedonic-to-opaline

quartz, suggests that they did not form immediately

after intrusion of the stocks. Tremolite and diopside

hornfels probably formed at least at 300° to 400° C,

considerably higher than temperatures estimated for

the formation of epithermal veins (100° to 200° C,

Lindgren 1933:454). Thus, the epithermal veins in

the contact aureoles of the Granite Peak and Silver

Bell stocks undoubtedly formed after considerable

cooling of the stocks and their contact aureoles.

This, however, does not prove that the veins are

unrelated to the stocks, as has been suggested for

the veins about the Marysville stock (Knopf 1950:

843). They may well have been deposited from

solutions from deeper parts of the cooling stocks or

batholith, which were injected into the already

cooled upper parts of the stocks and contact aureoles.

Such a genetic relationship between the epithermal

veins and associated stocks is indicated by (1) the

proximity of most epithermal veins to the stocks,

and (2) the possible cooling sequence of the stocks

shown by chalcedonic quartz fillings in the center

of andraditic garnet veins. The presence of epithermal veins which parallel porphyritic dikes in

the aureole of the Silver Bell stock (section 13, T. 12

N., R. 8 W.) is an additional line of evidence

suggesting that the deposits are genetically related

to the stocks. As shown by the lack of chilled contacts on such dikes, they formed during or immediately after intrusion of the stocks. It is likely that

tension fractures which formed due to the same

stress conditions during the cooling of the stocks were

filled by magma and later, after considerable cooling,

by hydrothermal solutions.

ORIGIN OF TENSION FRACTURES.—Barrell (1907:

105) suggested that the tensile stress which resulted

in formation of the Marysville fissure veins was developed during cooling and contraction of the stock

and contact aureole.

Barrell's explanation cannot apply to veins in the

unmetamorphosed Belt formations such as in the

Stemple area. Similarly, fissure veins in the volcanic

rocks are very linear, although they may cut many
26 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

different flows. These areas suggest that the tension

fractures may have developed in response to more

regional, horizontally directed tensile stress—perhaps

in connection with the fissuring which allowed lavas

to reach the surface.

UNDISCOVERED ORE BODIES.—Exploration for gold

and silver fissure veins was carried out mainly by

shallow prospect pits. If these produced no workable

ore, the site was abandoned. The restriction of ore

bodies only to certain portions of the veins, both

laterally and vertically (Table 8), makes this a most

unsatisfactory method of prospecting. Once a sizable quartz-calcite vein has been discovered at the

surface, only lateral tracing and drilling allow a

realistic appraisal of the vein's value. Table 8, if

correct, provides a valuable guide to the location of

a prospect in regard to ore bodies. The scheme,

however, is in need of additional verification.

Numerous unprospected and prospected fissure

veins were noted during the mapping. These have

not been adequately explored, and it is reasonable to

expect that some of these contain sizable ore bodies.

Placer Deposits

Gravels, which locally contain gold, veneer the valley

floors. According to Pardee and Schrader (1933),

most placer deposits were largely exhausted before

1900. Placer deposits are along streams which drain

mining areas, but the gold production of the placers

bears no relation to production in known fissure

veins. For example, the richest placer deposits occurred in McClellan Gulch, although fissure veins

at the head of the Gulch have not been highly

productive.

RELATION BETWEEN PLACER DEPOSITS AND GLACIATION.—As discussed under glaciation, many valleys

were probably at times filled by glaciers. Such glaciation results in the scouring of the bed rock along

valley bottoms and, thus, in the removal of placer

deposits.

Small cliffs and abrupt steep slopes mark the head

of McClellan Gulch as well as the adjacent head

of Washington Gulch, and suggest perhaps an early

period of valley glaciation in these gulches. Thus,

the rich placer deposits of these creeks have probably

accumulated since the earliest Pleistocene, or during

a maximum period of about three million years. More

likely, they have accumulated within the last several

thousand years.

Summary of Post-Laramide Igneous Activity and

Mineralization

The evidence presented here suggests that there were

two major post-Laramide periods of igneous activity

in the Lincoln area. Both were probably accompanied by approximately contemporaneous periods

of mineralization and both were characterized by

intrusion or extrusion of magmas of diverse composition, although granitic rocks predominated. The

principal difference between the two periods is that

the earlier or Boulder batholith period was mainly

intrusive, whereas the magma involved in the Tertiary intrusions made their way to the surface.

Also, Tertiary volcanism, Boulder batholith activity, and mineralization all appear to end at, or

just north of, the Lincoln area. Thus, there are not

only gross similarities between the periods of igneous

activity, but also spatial superposition.

Geomorphology

Middle Tertiary Surfaces

The area contains remnants of middle Tertiary surfaces under the volcanic rocks. Probably the earliest

surface is that preserved under the lower volcanic

series in the northwest part of the area. Beginning

along the Continental Divide near Crater Mountain this contact is at about 6500 feet. West, toward

the Lincoln Valley, the contact is exposed at about

4500 feet. The volcanic rocks show little evidence

of extensive faulting and, thus, it is likely that the

surface had a relief of about 2000 feet at the time of

earliest eruptions. This surface, therefore, was probably similar in relief and slope direction to the present surface.

Several features, however, suggest some differences

between the Tertiary and recent drainage patterns.

Silicified conglomerates interbedded with flows cap

several ridges south of Poorman Creek. The conglomerates are composed mainly of rounded, arkosic

quartzite boulders and gravels derived mainly from

the Marsh formation far to the south. Thus they

probably came from south of the Dalton MountainGranite Peak Divide during a period when the drain-
NUMBER 7 27

age system was different from that at present. The

boulders, which show no evidence of glacial origin,

are up to a few feet in diameter and suggest very

steep stream gradients and high relief at the time of

deposition.

After eruption of the lower volcanic series a period

of erosion and alluviation occurred, as shown by the

soil zone and conglomerates between the welded ash

flows and the lower volcanic series.

The ash flows presumably flowed into the lower

areas. Thus, the present area below the ash flows of

the Continental Divide may have been a flat or

perhaps gently rolling valley bottom at the time of

eruption. This shows that the volcanic field to the

west must have been quite thick, perhaps with

volcanoes towering to over 8,000 feet, and reaching

more than 3,500 feet thick. Since their deposition,

the compact, durable welded ash flows have resisted

erosion, resulting in a reversal of topography. The

steep canyons which begin near the unconformity

under the ash flow have probably been eroded a

minimum of 3,000 feet since the ash flow eruptions.

Pleistocene Glaciation

There were at least two periods of glaciation in the

area. The earliest was marked by large-scale valley

glaciation evidenced by the widespread glacial features of the Lincoln Valley. Later periods are indicated by local moraines near the heads of most

valleys.

The western half of the Lincoln Valley is marked

by numerous small lakes and glacially derived deposits. Near the eastern end of the glacial deposits,

large boulders—many with glacial striae—occur up

to about 500 feet above the valley floor. The boulders

which veneer the northern part of the valley between

Landers Fork and Keep Cool Creek (Figure 2) are

mainly fragments of welded ash flows, and thus were

derived near the Continental Divide about seven

miles to the west. The glacial debris also contains

abundant fragments of brightly colored chalcedony.

The source of these is uncertain.

The abundant and, in general, high-level glacial

deposits in and around the Lincoln Valley are perhaps a result of the narrow Blackfoot River canyon

on the west side of the valley. Because of this constriction, valley glaciers at times may have accumulated rapidly, becoming over 500 feet thick around

the valley margins. Flat, flood-plainlike areas with

small abrupt gravel-covered hills mark the entrance

of Humbug Creek into the Lincoln Valley. This

surface perhaps formed during late periods of glaciation, when rapid additions of sediments from the

head of Humbug Creek were deposited on the valley

floor. These deposits partially bury older valley

moraines.

The very steep slopes and cliffs which commonly

mark the intersection of principal valleys and the

Continental Divide and Poorman-Nevada Creek

Divide are the head walls of former valley glaciers.

Additional evidence of extensive valley glaciation is

shown by moraines and boulder trains composed

mainly of welded ash flow fragments near the ridge

crests about the intersection of Trout and Virginia

creeks.

In spite of evidence of glaciation, the valleys are

characteristically V-shaped. This is perhaps a result

of post-glaciation erosion or very short periods of

valley glaciation, or both.

Recent Surface Changes

A large landslide formed by the collapse of the west

side of Crater Mountain into the upper valley of

Humbug Creek. The slide stretches over a mile down

the Creek and is composed mainly of blocks of the

lower volcanic series as much as thirty feet in diameter. A small lake has formed in a side canyon

dammed by the slide.

The sparse growth of pines on the slide and the

lack of weathering of the blocks suggest that the

landslide took place recently.

Acknowledgments

I am indebted to Dr. George Switzer of the Department of Mineral Sciences for editing the manuscript,

and to the faculty of the Geology Department of

Princeton University, particularly to Professor John

C. Maxwell and Professor Robert B. Hargraves, for

much helpful advice during the study. The field work

and laboratory study were financed in part by the

National Science Foundation.

Residents of the Lincoln area were most hospitable. Special thanks are owing William R. Wade, a

retired mining engineer, who took me on several

field trips through the Lincoln and Marysville areas
28 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

at the start of the study, and provided much unpublished data on the epithermal mineral deposits.

M. R. Klepper, H. Smedes, and R. Tilling of the

United States Geological Survey led two field trips

in the vicinity of the Boulder batholith and visited

me in the field during the summer of 1962. I benefited greatly from these trips.

Edward Grady, Jr., Edward Grady, Sr., and

Robert Clearman, Jr., provided housing and other

support during the field work. Robert Clearman, Sr.,

kindly provided a sample of rich ore from the

Gould mine and information on the Gould district.

George Phillips provided information on the Stemple

district.

Judith Melson assisted with the drafting and numerous tasks involved in preparing this report, and

Helen Lanier typed the final copy. All this assistance

is gratefully acknowledged.

Literature Cited

Barrell, J.

1907. Geology of the Marysville Mining District, Montana. U.S. Geological Survey Professional Paper

57.

Bierwagen, E.

1964. Geology of the Black Mountain Area, Montana.

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Boyd, F.R.

1961. Welded Tuffs and Flows in the Phyolite Plateau

of Yellowstone Park, Wyoming. Geological Society

America, Bulletin 72:387-426.

Buddington, A.F.

1959. Granite Emplacement with Special Reference to

North America. Geological Survey America, Bulletin 70:671-747.

Chapman, R.W. et al.

1955. Age Determinations of Some Rocks from the

Boulder Batholith and Other Batholiths of Western Montana. Geological Society America, Bulletin

66:607-609.

Emmons, W.H., and F.C. Calkins

1913. Geology and Ore Deposits of the Philipsburg

Quadrangle, Montana. U.S. Geological Survey

Professional Paper 78.

1915. Geology and Ore Deposits of the Philipsburg

Quadrangle, Montana. U.S. Geological Survey

Folio 196.

Iddings, J.P., and W.H. Weed

1894. Livingston, Montana. U.S. Geological Survey

Folio 1.

Knopf, A.

1913. Ore Deposits of the Helena Mining Region, Montana. U.S. Geological Survey Bulletin 527.

1950. Marysville Grandiorite Stock. American Mineralogist 35:834-844.

1956. Argon-Potassium Determinations of the Age of the

Boulder Bathylith, Montana. American Journal of

Science 254:744-745.

1957. Boulder Bathylith of Montana. American Mineralogist 255:81-103.

1963. Geology of the Northern Part of the Boulder Bathylith and Adjacent Area, Montana. U.S. Geological Survey, Miscellaneous Geological Investigations

Map 1-381.

Lindgren, W.

1933. Mineral Deposits. Fourth edition, xvii + 930 pages.

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1919b. Some Manganese Deposits in Madison County,

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1922. Deposits of Manganese Ore in Montana, Utah,

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1925. Geology and Ground-Water Resources of Townsend Valley, Montana. U.S. Geological Survey

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1896. Three Forks, Montana. U.S. Geological Survey

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1963. The Belt Series in Montana, with a Geologic Map

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29

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The Bearpaw Mountains, Montana. American

Journal of Science 1.

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Montana. U.S. Geological Survey Bulletin 139.

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of Geology 4.

Geology and Mining Resources of the Judith

Mountains, Montana. 18th Annual Report, U.S.

Geological Survey.

Geology of the Little Belt Mountains, Montana,

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A.N.

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U.S. GOVERNMENT PRINTING OFFICE: 1971 O 437-338/5