SMITHSONIAN CONTRIBUTIONS TO THE
EARTH SCIENCES
NUMBER 7
William Q. Melson Geology 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, Mon-
tana. Smithsonian Contributions to the Earth Sciences, number 7, 29 pages, 13 fig-
ures, 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 sedi-
mentary rocks intruded by late Cretaceous (?) granitic stocks with concomitant wide-
spread 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 (?) vol-
canic 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.
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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 fea-
tures which are of interest to topical petrologic
studies and to the regional geology of western Mon-
tana. 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 vol-
canism 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 follow-
William G. Melson, Department of Mineral Sciences, Na-
tional Museum of Natural History, Smithsonian Institution,
Washington, D.C. 20560.
ing study. Other suites of rocks in the Museum col-
lected during the early geologic surveys of the west-
ern 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 in-
volved 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 prom-
inent 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 sur-
face at about 4,600 feet. The mountains have forest-
covered 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 struc-
ture of the underlying Belt rocks. The resistant con-
tact 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 Mu-
seum 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 Poorman-
Virginia 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, Conti-
nental Divide, and Humbug Creek foot-trails, which
are maintained by the United States Forest Serv-
ice, 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 top-
ographic 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 Lin-
coln area is based are shown on the geologic map
(Figure 3), reproduced here in reduced form. Ap-
proximately 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 Geo-
logical 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 mag-
matic stoping and of contact metamorphism.
Knopf's work on the Marysville mining district
(1950), the Boulder batholith (1957), and the min-
eral 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 "Cretace-
ous Diorite-gabbro" shown in the western and north-
ern 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 Oli-
gocene, 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 gab-
bro in the Lincoln area; Pcm=Pre-Cambrian Marsh forma-
tion, with each of four members indicated; Pch = Pre-Cam-
brian Helena formation; Pee = Pre-Cambrian Empire forma-
tion; 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 envi-
ronments, and have provided evidence of Pre-Cam-
brian life. Regional studies of the Belt Series, how-
ever, 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 de-
scribed by Walcott (1898) as part of a survey of the
"Algonkian" of Montana. Subsequent work has re-
sulted in mapping of large parts of the Belt Series
and elaborate regional correlations and interpreta-
tion of sedimentary environments. Most of this work
has been reviewed recently by Ross (1963). Bier-
wagen (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 remark-
able 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) Spo-
kane to Empire formations, and (2) member B to
member C of the Marsh formation.
Of these units, the Spokane formation and mem-
ber B of the Marsh formation are particularly simi-
lar. 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 thin-
bedded noncalcareous red siltstone with argillaceous
mud-cracked partings.1 Some thin white fine-grained
quartzite layers. Lower portion composed of green mas-
sive noncalcareous mudstone commonly showing intense
fracture cleavage.
EMPIRE. About 1000. Lithologically transitional to the Spo-
kane 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 com-
mon 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 abund-
ant. 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 (thin-
bedded) and buff weathering. Distinguished from Hel-
ena 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 indistin-
guishable from the Spokane formation. Lower portion
contains interbedded red and greenish-gray noncalcare-
ous 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 cal-
careous 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 pinkish-
gray, arkosic quartzites. Restricted to southwest corner
of area. Wedge-shaped beds up to 300 feet thick inter-
tongued 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-grain-
ed sandstones.
First occurrence of gray carbonate-
rich siltstones and mudstones. "Mo-
lar tooth" beds are particularly
distinctive.
Buff weathering grayish green to
very light gray fine-grained dolo-
mitic to calcareous sandstones,
mudstones, or siltstones. Mottled
pink and green sand-sized grains
in a calcareous matrix are distinc-
tive. Contact is apparently grada-
tional over about 50 feet.
Occurrence of hard, grayish-green,
fine-grained quartzites and silt-
stones which grade upward into
grayish-red, fine-grained quartzites
and siltstones.
Marked by occurrence of grayish-
green, buff weathering siltstones
and fine-grained, calcareous silt-
stones.
Occurrence of massive, commonly
cross-bedded, arkosic sandstones.
Contact varies greatly stratigraphi-
cally because of rapid facies
changes common in the arkosic
sand near the top of the Marsh
formation.
siltstones and fine-grained quartzites with thin, mud-
craked, 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 forma-
tion, which separates the repeated lithologies, and
the arkosic quartzites of the upper part of the
Marsh formation are distinctive and led to even-
tual 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 dur-
ing 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 defini-
tions 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 Pre-
Cambrian 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 domin-
ated 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 here-
after as the Black Mountain syncline. The gross
structure of the Lincoln area shows that it lies en-
tirely 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. (Bier-
wagen 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 displace-
ment 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 re-
fracted 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 bed-
ding 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 Moun-
tain 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 syn-
cline 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 inter-
sections thus reliably reflect the larger scale struc-
tures in the area.
Farther northeast of the axis of the Black Moun-
tain syncline, cleavage-bedding intersections dip more
irregularly and commonly dip and strike north (Fig-
ure 7b). They, thus, suggest another regional syn-
cline 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 uncer-
tain because of poor exposures. They appear, how-
ever, 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, observa-
tion 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 displace-
ments 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 fault-
ing three times between the Silver Bell stock and
Fool Hen Ridge. The faults are assumed to be of
Laramide age because they parallel fracture cleav-
age. 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. Cleavage-
bedding 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 ori-
entation 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 ac-
tivity in the area were (1) post-folding, late Cre-
taceous (?) 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 al-
tered mainly to chlorite and cal-
cite. Mainly less than ten feet
thick.
2. Quartz gabbro sills Noted only in the Spokane forma-
tion. Mainly unaltered plagioclase
and clinopyroxene with minor
quartz. Up to several hundred feet
thick.
3. Granite stocks and Granite Peak, Silver Bell, and Dal-
associated 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-fold-
ing. 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 forma-
tions. 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 meta-
morphism. 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 "micro-
diorites" exposed near Bald Butte in the Marysville
district (Barrell 1907:12) and to the numerous sills
in the Helena formation several miles west of Lin-
coln 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-Cam-
brian (?) 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 mala-
chite 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, al-
though 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 Cretace-
ous age is assumed from their designation on the
Montana geologic map. The reason for this desig-
nation 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 iden-
tical 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 be-
cause they occur only in Belt rocks. This is not con-
clusive proof of their age, however, because sills
commonly show stratigraphic restriction. The west-
ern end of the Stemple Creek sill is cut by post-
Laramide 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 prob-
ably about three square miles in surface area. Sev-
eral areas of hornfelsed Helena formation, such as
between Mead Gulch and Prickly Gulch, occur
without central exposures of granitic rocks, suggest-
ing that an intrusive underlies them at shallow depth.
Although the Granite Peak stock is a single con-
tinuous 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, in-
tensely and brittlely deformed without plastic should-
V.\
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, al-
though 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) as-
sociated 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
f1
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 hy-
drothermal solutions.
Specimens from the Silver Bell stock characteris-
tically contain considerable quantities of epidote dis-
persed 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 hy-
droxy phosphate, and fluorite.
These features suggest that late magmatic solu-
tions were largely retained during cooling. Alter-
natively, 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 sup-
ported 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 ortho-
clase 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, sug-
gest 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 granodi-
orite. Where mapped, it has an even-granular tex-
ture, 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 Batho-
lith" is but the largest exposure of a widespread in-
trusive mass which underlies a considerable region
in western Montana, and that stocks such as at
Marysville and Granite Peak are upward prolonga-
tions 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 extru-
sion 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 pre-
sumably 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 (Chap-
man, 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 horn-
blende 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 batho-
lith 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, neglect-
ing the small mafic intrusion near the Dalton Moun-
tain 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 con-
tains 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 com-
mon. Simple pegmatites occur in both and are com-
posed mainly of orthoclase and quartz. Such peg-
matites are common in the Granite Peak and Dalton
Mountain stocks, but are rare in the Silver Bell
stock.
The stocks and associated mineral deposits appar-
ently 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 em-
placement by magmatic stoping. Although the de-
tails 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 mag-
matic 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 con-
tact 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 in-
dicates, the fragments of the roof presumably were
removed downward.
Diopside-rich rocks which characterize the con-
tact 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-to-
granodioritic 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 sug-
gest that the stocks were not emplaced by a single
injection of granitic magma but rather were in-
truded over a long period of time. Perhaps large-
scale 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 equiv-
alents, 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 sink-
ing 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 batho-
lithic invasion. Such a caving in of the surface would pre-
sumably lead to enormous outpourings which find no sug-
gestion 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 con-
sist of a thick series of latitic-to-andesitic rocks and
an upper rhyolitic series. The rhyolitic rocks are be-
tween 50 and 500 feet thick and are composed
mainly of welded ash flows. The welded ash flows
show several features which suggest they were de-
posited 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 rhyo-
lite 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 mineral-
ized zones, the flows have remained essentially un-
altered since their final cooling. Fresh glass is par-
ticularly 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 per-
haps 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 vol-
canic rocks, as well as evidence of considerable ero-
sion between periods of volcanism, suggest that erup-
tions 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 un-
certain. Where finely porphyritic, however, the dom-
inant phenocrysts are plagioclase and hornblende or
biotite.
The flows characteristically show considerable
deuteric alteration and are locally vesiculated. Vesi-
cles 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 per-
pendicular 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 irreg-
ular 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 ex-
posed 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, sug-
gesting 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 north-
west 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 de-
posited 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 frag-
ments 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 escarp-
ment. 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 oc-
currence at specific horizons in vertical sections of
the main sheet also suggests the presence of a num-
ber of individual ash flows.
Two quantitatively minor but perhaps significant
variations occur in the ash flows. Along the Con-
tinental Divide at the head of Specimen Creek, un-
usually 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 crys-
tals 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 com-
monly welded to a black glass at the bottom. The
black glass portion of the flows grades rapidly up-
ward into a partially welded top. The index of
refraction of the glass is about 1.50 (?.01) suggest-
ing 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 land-
slide 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 con-
tact between the pyroclastics and the welded ash
flows and "dome" are, however, gradational. Thus,
the plug which gave rise to the rhyolite dome per-
haps 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 Moun-
tain and at about the same altitude. Here the flow-
banded 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 in-
volved. 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 ar-
dente" deposits (Smith 1960:802), and thus their
origin has been controversial. For several reasons it
is clear that welded ash flows do not represent nor-
mal 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 tempera-
ture 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 emplace-
ment by "avalanche or flowage of fragmental mate-
rial 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 gen-
erally agreed that the driving force of the ash flow or ava-
lanche 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 differ-
ent 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 mech-
anism" 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 im-
mense quantity of volatiles probably were evolved
from the magma.
Figure 12 suggests that the lavas were supersat-
urated 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 re-
duce the glass to a state of saturation or undersaturation
and the glass may continue to lose gas even after some cool-
ing. 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 lubri-
cent.
In summary, it is likely that the welded ash flows
in the Lincoln area, particularly the thin but in-
tensely welded flows in the vicinity of Grater Moun-
tain, 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 mineralogi-
cally similar to some of the small areas of volcanic
rocks in the Black Mountain locality described by
Bierwagen (1964:89-102). Welded ash flows, how-
ever, 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 appar-
ently are less indurated.
Post-Laramide Dikes
Porphyritic latitic dikes occur throughout the Lin-
coln 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 phe-
nocrysts were not noted in the chilled contacts, and
commonly are largest in the center of the dikes,
suggesting formation of the phenocrysts after intru-
sion. 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 abund-
ance 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 arrange-
ment 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 com-
mon 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 pre-
dominately dark gray are mapped as Empire forma-
tion. 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 "re-
active 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 actino-
litic 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 (con-
choidal fracture) and some metamorphic minerals in rare impure beds.
the upper Marsh formation show little field evidence
of contact metamorphism around the Dalton Moun-
tain 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 in-
ner contact zones, (2) epithermal gold- and silver-
bearing 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 con-
tain 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 con-
tent, 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 eco-
nomic value.
Pyrite and chalcopyrite locally occur in dissemi-
nated 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 horn-
fels 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 chalcopy-
rite, 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 mil-
lion 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 frac-
tures 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 ag-
gregate of fragments of their wallrocks. The breccias
were probably formed by local collapse of parts of
the vein walls during opening of the tension frac-
tures. Fault gouge occurs locally along post-miner-
alization 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 ex-
tent 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 well-
developed vertical zoning, is similar to the Gould
Mine. Thus, the zoning in the Drumlummon Mine,
which corresponds to zoning in most of the epi-
thermal 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 con-
tinue 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 in-
tergrowth 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 brec-
cias, and quartz has in places completely replaced
breccia fragments. Calcite, as shown by its common
yellowish-to-dark-brown color where weathered, nor-
mally contains some iron (ankeritic).
Amethystine quartz, although common in epi-
thermal 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 con-
tent, 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 In-
stitution analyzed this gold on the electron micro-
probe 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 MIN-
ERALIZATION.?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 sev-
eral authors. Barrell (1907) suggested that there were
at least two periods of mineralization: one following
but genetically related to the intrusion of the Marys-
ville 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 extru-
sion 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 asso-
ciated with the granitic stocks. The evidence of low-
temperature 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 epi-
thermal 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 con-
tacts on such dikes, they formed during or immedi-
ately 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 de-
veloped 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 siz-
able 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 oc-
curred in McClellan Gulch, although fissure veins
at the head of the Gulch have not been highly
productive.
RELATION BETWEEN PLACER DEPOSITS AND GLACIA-
TION.?As discussed under glaciation, many valleys
were probably at times filled by glaciers. Such gla-
ciation 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 accom-
panied by approximately contemporaneous periods
of mineralization and both were characterized by
intrusion or extrusion of magmas of diverse com-
position, 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 Ter-
tiary intrusions made their way to the surface.
Also, Tertiary volcanism, Boulder batholith ac-
tivity, 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 sur-
faces 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 Moun-
tain 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 prob-
ably similar in relief and slope direction to the pres-
ent 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 conglom-
erates 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 Mountain-
Granite 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 fea-
tures of the Lincoln Valley. Later periods are indi-
cated 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 de-
posits. 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 per-
haps a result of the narrow Blackfoot River canyon
on the west side of the valley. Because of this con-
striction, valley glaciers at times may have accumu-
lated 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 glacia-
tion, 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 diam-
eter. 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 Depart-
ment 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 hospi-
table. 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 unpub-
lished 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 bene-
fited 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 num-
erous tasks involved in preparing this report, and
Helen Lanier typed the final copy. All this assistance
is gratefully acknowledged.
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