Geology of the Vienna Mineralized Area, Blaine and Camas …

 

 

Geology of the Vienna Mineralized Area,
Blaine and Camas Counties, Idaho

By J. Brian Mahoney and Michael C. Horn

Prepared in cooperation with the Idaho Geological Survey,
Idaho State University, and the University of Idaho

Bulletin 2064-DD

U.S. Department of the Interior
U.S. Geological Survey

U.S. Department of the Interior
Gale A. Norton, Secretary

U.S. Geological Survey
Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2005

Posted online April 2005, version 1.0

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Suggested citation:

Mahoney, J.B., and Horn, M.C., 2005, Geology of the Vienna mineralized area, Blaine and Camas Counties, Idaho: U.S.

Geology Survey Bulletin 2064–DD, 9 p.

iii

Contents

Figures

Table

Abstract ……………………………………………………………………………………… 1
Introduction …………………………………………………………………………………… 1
Historical Perspective ………………………………………………………………………… 2
Previous Work ………………………………………………………………………………… 2
Geologic Setting ……………………………………………………………………………… 2
Lithology ………………………………………………………………………………… 2
Structure ………………………………………………………………………………… 4
Mineral Deposits ……………………………………………………………………………… 4
Quartz-Silver-Sulfide Ribbon Veins ……………………………………………………… 6
Quartz-Sericite-Pyrite-Galena Veins …………………………………………………… 6
Genesis of Mineral Deposits …………………………………………………………… 7
Acknowledgments …………………………………………………………………………… 8
References Cited ……………………………………………………………………………… 8

1. Map showing location of Vienna mineralized area, Blaine and Camas Counties,

Idaho ……………………………………………………………………………… 1

2. Generalized geologic map of the Vienna mineralized area, Blaine and Camas

Counties, Idaho …………………………………………………………………… 3
3. Photograph of silver-sulfide ribbon ore, Vienna mineralized area ………………… 6
4. Schematic diagram of mineralized shear zone typical of Vienna mineralized area … 6
5. Photograph of mineralized shear zone in the Pilgrim mine, Vienna mineralized area
7
6. Photograph of brecciated quartz and silver-sulfide ribbon veins in an

oxidized quartz-sericite-pyrite-galena matrix ……………………………………… 7

1. Analytical data from selected samples of the Vienna mineralized area, Blaine

and Camas Counties, Idaho ………………………………………………………… 5

iv

Metric Conversion Factors

Multiply
tons
short tons
troy ounces
ounces

By
1.016
0.907
31.103
28.35

To obtain
metric tons
metric tons
grams
grams

Geology of the Vienna Mineralized Area, Blaine and

Camas Counties, Idaho

By J. Brian Mahoney and Michael C. Horn

Abstract

The Vienna mineralized area of south-central Idaho was

an important silver-lead-producing district in the late 1800s
and has intermittently produced lead, silver, zinc, copper,
and gold since that time. The district is underlain by biotite
granodiorite of the Cretaceous Idaho batholith, and all mineral
deposits are hosted by the biotite granodiorite. The granodio-
rite intrudes Paleozoic sedimentary rocks of the Sun Valley
Group, is overlain by rocks of the Eocene Challis Volcanic
Group, and is cut by numerous northeast-trending Eocene
faults and dikes.

Two mineralogically and texturally distinct vein types are
present in a northwest- and east-trending conjugate shear-zone
system. The shear zones postdate granodiorite emplacement
and joint formation, but predate Eocene fault and dike forma-
tion. Ribbon veins consist of alternating bands of massive
vein quartz and silver-sulfide (proustite and pyrargyrite)
mineral stringers. The ribbon veins were sheared and brecci-
ated during multiple phases of injection of mineralizing fluids.
A quartz-sericite-pyrite-galena vein system was subsequently
emplaced in the brecciated shear zones. Both vein systems are
believed to be the product of mesothermal, multiphase miner-
alization. K-Ar dating of shear-zone sericite indicates that ser-
icitization occurred at 80.7±2.8 Ma; thus mineralization in the
Vienna mineralized area probably is Late Cretaceous in age.

Introduction

The Vienna mineralized area of south-central Idaho is at
the southwest end of the Stanley Basin, approximately 60 km
north of Ketchum, Idaho (fig. 1). The area is in the northern
Smoky Mountains, at the south end of the Sawtooth Moun-
tains, in Blaine and Camas Counties, Idaho.
drainages of Smiley, Frenchman, Johnson, Emma, and Beaver
Creeks and part of the headwaters of Alturas Lake Creek (Jake
and Eureka Gulches).

It includes the

The Vienna mineralized area, as described in this paper,
encompasses parts of the Vienna, Sawtooth, Skeleton Creek,
and Big Smoky mining districts (Van Noy and others, 1986;

Federspiel and others, 1987, 1992). The majority of mining
activity and ore production has been from the headwaters of
Smiley and Beaver Creeks, in the Vienna and Sawtooth mining
districts, although a number of small mines and prospects are
in the surrounding drainages (Umpleby, 1915; Van Noy and
others, 1986; Federspiel and others, 1987). The level of ero-
sion may be an important factor to the discovery of economic
deposits because most successful workings are in deep glacial
cirques that face northeast.

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Figure 1. Location of Vienna mineralized area, Blaine and
Camas Counties, Idaho.

2

Geology of the Vienna Mineralized Area, Idaho

Historical Perspective

Geologic Setting

The Vienna and Sawtooth mining districts were discov-

Lithology

ered in 1879, and the majority of mining activity occurred
between 1880 and 1888. The largest producers were the Web-
foot, Vienna, Silver King, and Pilgrim mines (fig. 2). Approxi-
mately two million dollars (historical prices) of silver ore was
removed from the districts during this initial period of activity
(Umpleby, 1915); significant production has not occurred
since that time. Exploration has been intermittent since the
late 1800s, including brief flurries of activity in the 1930s
and early 1940s and in the late 1960s to early 1970s, when 79
claims were filed in the Vienna mining district (Van Noy and
others, 1986). A small amount of production occurred in the
early 1980s when material from existing mine dumps was pro-
cessed by a custom mill along Warm Springs Creek, approxi-
mately 70 km south of the Vienna mining district. The Vienna
and Sawtooth mining districts currently (1992) contain 55
patented claims and 170 current claims (Federspiel and others,
1992). Exploration, including a drilling program, was being
conducted in 1992 at the Webfoot property in the headwaters
of Smiley Creek; production is anticipated but is contingent on
precious-metal prices.

Previous Work

The Vienna mining district and adjoining districts were

important metal-producing areas in south-central Idaho during
the late 1800s and have been the subject of numerous geologic
investigations. Umpleby (1915) compiled a brief reconnais-
sance report of the district during mapping of the Sawtooth
30-minute quadrangle. Ballard (1922) discussed the district in
a detailed report on the ore deposits of the Alturas 30-minute
quadrangle. Ross (1927) conducted a detailed investigation
of the surficial and subsurface geology of the Vienna mining
district. Shannon (1971) mapped the district in detail, and
conducted an extensive geochemical survey (stream sediment)
in order to evaluate the use of geochemistry as an exploration
tool. There are also numerous unpublished mining company
reports of the area. The Vienna mineralized area was discussed
as part of the northern addition to the South Boise Yuba study
area (Federspiel and others, 1987). Mahoney and Horn (1989)
reviewed the geologic setting and mineral deposits of the Vienna
mineralized area, and the present report is a reevaluation of
findings presented in that report. Federspiel and others (1992)
evaluated mines and prospects in the area during a mineral
resource investigation of the Smoky Mountains.

The Vienna mineralized area is on the southeast edge of

the Atlanta lobe of the Cretaceous Idaho batholith (Kiils-
gaard, Lewis, and Bennett, 2001). Biotite granodiorite to
quartz monzonite of the batholith underlies most of the area.
Mineral deposits in the area, with the exception of two small
sediment-hosted skarn deposits west of Smiley Creek (Ura
group and P&D claims) and one sediment-hosted replace-
ment and polymetallic vein deposit (Mountain King claim),
are hosted by the biotite granodiorite (fig. 2).

The biotite granodiorite is medium to coarse grained and
composed of quartz, plagioclase, microcline, and biotite, and
accessory magnetite, zircon and sphene. It is locally porphy-
ritic, containing alkali feldspar phenocrysts as long as 8 cm;
the porphyritic nature of the biotite granodiorite is believed to
be the result of potassium metasomatism (Johnson and others,
1988). Cataclastic textures are visible in thin section, and in
outcrop the biotite granodiorite locally displays an incipient
gneissic texture, particularly northwest of Beaver Creek (fig.
2). Thin (2–20 cm), randomly oriented aplite and pegmatite
dikes locally cut the granodiorite. The origin of the aplite and
pegmatite dikes is uncertain: they may be a late-stage differ-
entiate of the Cretaceous batholith. It is also possible that they
are related to the Tertiary biotite (pink) granite of the Saw-
tooth batholith, exposed 8 km to the north, or to the Prairie
Creek stock, a smaller Tertiary biotite (pink) granite intrusion,
exposed 5 km to the south.

The biotite granodiorite intrudes micritic sandstone,

siltstone, and sandy limestone of the Middle Pennsylvanian
to Lower Permian Grand Prize Formation of the Sun Valley
Group in the east half of the Vienna mineralized area (Mahoney
and others, 1991) (fig. 2). The Grand Prize Formation in this
area has been contact metamorphosed to calc-silicate horn-
fels and locally contains abundant wollastonite, diopside, and
tremolite. The metamorphosed Grand Prize Formation hosts
antimony-, silver-, and tungsten-rich skarn deposits (Ura group
and P&D claims) on the north edge of the Vienna mineralized
area, west of Smiley Creek, and silver-lead polymetallic veins
and replacement deposits on Mule Creek, in the southern part
of the area (fig. 2). Small roof pendants of sedimentary rock
are exposed along ridges in the northeastern part of the miner-
alized area.

Andesitic to dacitic lavas and associated volcaniclastic

sedimentary rocks of the Eocene Challis Volcanic Group
unconformably overlie both the Cretaceous granodiorite and
the Pennsylvanian-Permian Grand Prize Formation in the
eastern part of the mineralized area (fig. 2). The volcanic
rocks comprise a section more than 500 m thick of andes-
itic to dacitic flow rocks, tuff breccias, and volcanogenic

Figure 2 (following page). Generalized geology of the Vienna
mineralized area, Blaine and Camas Counties, Idaho.

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Geologic Setting

3

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4

Geology of the Vienna Mineralized Area, Idaho

sandstone and siltstone. The volcanic rocks are locally in
fault contact with the granodiorite where they are preserved
in downthrown fault blocks adjacent to north- and northeast-
trending normal faults.

Northeast-trending Eocene dacite porphyry dikes cut the
biotite granodiorite throughout the Vienna mineralized area.
The pervasive northeast trend and the spatial relationship
between northeast-trending faults and dacite porphyry dikes
suggest that dike emplacement was structurally controlled.
Chilled contacts within the dikes and minor alteration zones in
the country rock characterize the intrusive contact between the
dike rocks and the granodiorite, but no evidence of mineraliza-
tion associated with Eocene dike emplacement was identified.
Eocene dacite porphyry dikes reportedly cut mineralized veins
at depth, although we could not confirm this.

Thin (0.2–0.4 m) lamprophyric dikes of uncertain age
locally cut the biotite granodiorite. These dikes are inferred
to be Cretaceous in age elsewhere in the region, although
no absolute dates exist. Lamprophyric dikes apparently cut
mineralized veins in the Webfoot mine (“diabase” of Ballard,
1922; Ross, 1927), but on the surface the highly altered nature
of the dikes makes the exact relationship between these dikes
and mineralized structures ambiguous.

Structure

The Cretaceous biotite granodiorite contains a promi-
nent joint system that is exposed in the headwaters of Beaver
Creek. The dominant joint set strikes N. 35°–50° W. and
dips steeply to the north. This orientation roughly parallels
the east edge of the batholith, suggesting that the dominant
joint system represents contraction joints parallel with the
batholith margin. The geometry of the joint set varies mark-
edly, however, from planar, parallel joint sets to curvilinear
joint patterns. Previous workers have suggested that mineral
veins in the Vienna mineralized area are confined to the joint
sets (for example, Ballard, 1922), but it is now recognized that
the vein system is subparallel with the joint sets and clearly
postdates them. Joint sets are locally mineralized adjacent to
crosscutting subparallel mineralized shear zones.

Mineralized rock in the Vienna mineralized area occurs
along shear zones that cut the granodiorite. The shear zones
approximate a conjugate fracture set in which one set trends
about N. 60° W. and the second set trends east-west. The
shear zones are generally near vertical or dip steeply to the
north. On the surface, the shear zones are iron oxide stained
(limonite) and locally brecciated; below ground, breccia is
common within shear zones, and intense chloritic and sericitic
alteration extends for 1–3 m from the shear zones. Evidence
of shearing and brecciation is more pronounced along the east-
trending shear zones (Ballard, 1922). Both sets of shear zones
are mineralized, although the east-trending set is normally of
higher grade. Ore shoots are present locally at shear zone-
joint and shear zone–shear zone intersections. Vein material is
commonly sheared and brecciated, and small offsets from one

to tens of meters of veins attest to syntectonic vein emplace-
ment (figs. 4, 5).

The most pronounced structures in the Vienna mineral-

ized area are northeast-trending (N. 50°–75° E.) normal
faults that have displacements ranging from tens to hundreds
of meters. These high-angle normal faults cut the Paleozoic
Grand Prize Formation, the Cretaceous biotite granodio-
rite, and the Eocene Challis Volcanic Group (fig. 2) and are
believed to be associated with the Eocene trans-Challis fault
system (Bennett, 1986). The faults apparently controlled
emplacement of the Eocene dacite porphyry dikes and must
therefore be coeval with or slightly older than the Eocene
dikes. The brittle nature, prominent surficial expression, large
offset, lack of hydrothermal alteration, and lateral extent of
these normal faults make them different from the more sub-
dued mineralized shear zones previously discussed. There
is a strong correlation between the trend of the normal faults
and the location of mines and prospects in the Vienna mineral-
ized area, and the faults may structurally control the location
of mineral deposits. Three large economic deposits in the
Vienna mineralized area (Webfoot, Solace, and Vienna mines)
are apparently in a horst block bounded by Eocene normal
faults at the head of Smiley Creek; deeper, more highly miner-
alized levels of the biotite granodiorite may have been uplifted
along these Eocene structures. The relationship at depth
between the northeast-trending Eocene faults and mineralized
structures is unknown.

A prominent north-trending normal fault downdrops
andesitic flow rocks of the Challis Volcanic Group against
Cretaceous biotite granodiorite west of Smiley Creek (fig.
2). The fault is almost vertical and has an offset of least 500
m, based on the thickness of the volcanic rocks on the down-
thrown eastern block of the fault. The fault apparently cuts
shear zones in the granodiorite and is believed to be associated
with basin and range extension. This north-trending normal
fault is subparallel with the Sawtooth fault to the north, which
is the eastern bounding fault of the Sawtooth batholith horst
block, and may be an extension or splay to the Sawtooth fault
(Tschanz and others, 1986).

Mineral Deposits

Two distinct types of mineralized veins are present in the
Vienna mineralized area: shear-zone-hosted silver-sulfide rib-
bon veins and shear-zone-hosted quartz-sericite-pyrite-galena
veins. The veins are spatially associated but are mineralogi-
cally and texturally distinct. Crosscutting relations indicate
that formation of the ribbon veins preceded emplacement of
the quartz-sericite-pyrite-galena veins. Samples obtained from
the principal mines in the mineralized area (Pilgrim, Webfoot,
Solace, Vienna) were assayed; results are listed in table 1.

Both types of veins in the Vienna mineralized area are

classified in the regional descriptive model of Worl and
Johnson (1995) as polymetallic quartz veins and lodes; the