Alteration and Mineralization in the 

Eastern Part of the Soldier Mountains,

Camas County, Idaho

By Reed S. Lewis

U.S. Geological Survey Bulletin 2064-V

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the Idaho Geological Survey,

Idaho State University, and the University of Idaho
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

This publication is only available online at:

http://geology.cr.usgs.gov/pub/bulletins/b2064-v/

Any use of trade, product, or firm names in this publication

is for descriptive purposes only and

does not imply endorsement by the U.S. Government

Version 1.0 2001

Published in the Central Region, Denver, Colorado

Manuscript approved for publication January 5, 2001

Graphics by authors and Gayle M. Dumonceaux

Photocomposition by Gayle M. Dumonceaux
III

Contents

Abstract ......................................................................................................................................... 1

Introduction .................................................................................................................................. 1

Lithology ........................................................................................................................................ 2

Structure ....................................................................................................................................... 2

Hydrothermal Alteration ............................................................................................................. 4

Muscovite-Quartz Alteration......................................................................................... 4

Potassic Alteration ......................................................................................................... 5

Propylitic Alteration ....................................................................................................... 5

Turbidization of Eocene Feldspar ................................................................................. 6

Mineralization ............................................................................................................................... 7

Texas Star (D. Marie) Mine .......................................................................................... 7

Richard Allen Mine ......................................................................................................... 7

Five Points (Perseverance) Mine ................................................................................ 7

Idaho Tungsten Mine (Soldier Mountain Deposit) ................................................... 9

Unnamed Mines and Prospects in Cretaceous Intrusive Rocks............................. 9

Unnamed Mines and Prospects in Eocene Intrusive Rocks.................................... 10

Mineralization and Lithology ...................................................................................................... 10

Mineralization and Structure...................................................................................................... 10

Conclusions .................................................................................................................................. 10

References .................................................................................................................................... 12

1. Index map showing the location of the Soldier Mountains study area,

Camas County, south-central Idaho ................................................................................ 1

2. Simplified geologic map of the eastern part of the Soldier Mountains, 

Camas County, south-central Idaho ................................................................................ 3

3. Map of the uppermost adit of the Richard Allen mine .................................................. 9

4. Schematic cross sections illustrating characteristics of the hydrothermal 

alteration types of Cretaceous and Eocene age in the Soldier Mountains

study area ............................................................................................................................ 11

Figures

Tables

1. Characteristics of alteration types in the eastern part of the Soldier Mountains,

Camas County, Idaho ........................................................................................................ 4

2. Mineralized samples from the eastern part of the Soldier Mountains, 

Camas County, Idaho ......................................................................................................... 8
IV

Metric Conversion Factors

Multiply By To obtain

Miles 1.609 Kilometers

Feet 0.3048 Meters

Inches 2.54 Centimeters

Tons 1.016 Metric tons

Short tons 0.907 Metric tons

Troy ounces 31.103 Grams

Ounces 28.35 Grams
1

IDAHO 

BATHOLITH

STUDY

AREA

IDAHO

115° 114°45'

43°37'30"

43°22'30"

Big Peak

Dollarhide

Summit

Cannonball

Mountain

Couch

Smoky Summit

Dome

Fairfield

Camas Prairie

BOUNDARY OF

STUDY AREA

Mount Bennett Hills

0 5 KILOMETERS

Iron

Mountain

SOLDIER MOUNTAINS

Big Smoky C r eek

South Fork Boise River Little Smoky Cr

Soldier Creek

Abstract

The eastern part of the Soldier Mountains in Camas 

County, south-central Idaho, is underlain principally by plutonic 

rocks of Cretaceous and Eocene age that locally have undergone 

propylitic, potassic, and muscovite-quartz alteration. Muscovite-quartz alteration is Cretaceous in age and is localized along 

joints and fractures, some of which are filled with quartz. Associated veins have yielded minor amounts of gold. Potassic alteration is probably both Cretaceous and Eocene in age but is 

weakly developed and limited in extent. Propylitic alteration is 

Eocene in age and is pronounced around biotite granite plutons. 

Despite a clear association between plutons of biotite granite and 

widespread propylitic alteration, mineralization associated with 

these rocks was minimal. Mineralized areas within more mafic 

Eocene plutons are characterized by veins and (or) stockworks(?) enriched in copper, molybdenum, and silver, but these 

areas are restricted in size and have not been productive.

Introduction

This report describes hydrothermal alteration and related 

mineralization in the eastern part of the Soldier Mountains of 

Camas County, Idaho (fig. 1). Reconnaissance of the area in a 

two-day period during the summer of 1984 revealed a complex 

series of plutonic rocks overlain in places by younger volcanic 

rocks. The plutonic rocks are locally altered, as indicated by 

pink zones adjacent to fractures in otherwise gray granodiorite. 

The alteration was thought at the time to be potassic and to have 

potential for associated porphyry-type mineral deposits. Criss 

(1981) and Criss and Taylor (1983) have shown that feldspar in 

plutonic rocks in the study area was depleted in 18O, a depletion 

that they attributed to exchange with heated meteoric waters. 

Their reconnaissance study indicated that epizonal Eocene plutons, unmapped at that time, were probably responsible for circulation of the meteoric water and resultant feldspar alteration. 

Because of the complex intrusive relations and the associated 

alteration, it was clear that the area was well suited for a detailed 

investigation of hydrothermal alteration and mineralization 

within a granitic terrane.

This research originated as part of a Ph.D. study at Oregon 

State University (Lewis, 1990). Geologic mapping and 

sampling were conducted during the summers of 1985 and 1986, 

in conjunction with the Hailey CUSMAP (Conterminous United 

States Mineral Assessment Program) project undertaken by the 

U.S. Geological Survey. Mapping was followed by detailed petrographic study of samples of the plutons and their altered 

equivalents, chemical analysis of whole-rock samples, and determination of 18O/16O ratios of constituent minerals.

Previous geologic mapping in the study area was reconnaissance in nature. Piper (1924) studied ground water on Camas 

Prairie and distinguished between plutonic and volcanic rocks 

north of Fairfield. As part of a study of the geology and ore 

deposits of the Little Smoky and Willow Creek mining districts, 

which are mostly east and north of the study area, Ross (1930) 

Alteration and Mineralization in the 

Eastern Part of the Soldier Mountains, 

Camas County, Idaho

By Reed S. Lewis

Figure 1. Index map showing the location of the Soldier Mountains 

study area, Camas County, south-central Idaho.
2 Alteration and Mineralization, Eastern Part of the Soldier Mountains, Camas County, Idaho

constructed a geologic sketch map that includes the eastern part 

of the study area. More recently, Cluer and Cluer (1986) investigated the formation of Camas Prairie, which they concluded was 

a late Cenozoic rift (graben) developed as a result of downwarping of the Snake River Plain. Gehlen (1983) and Darling (1987, 

1988) mapped the Dollarhide Mountain 7.5-minute quadrangle, 

which adjoins the study area to the northeast, and Bennett (in 

press ) mapped the quadrangles to the west. The studies by Darling and Bennett were part of the Hailey CUSMAP project, and 

results of their mapping are included on the geologic map of 

Worl and others (1991). 

Lithology

The eastern part of the Soldier Mountains is underlain principally by plutonic rocks of Cretaceous and Eocene age (fig. 2). 

Reliable radiometric ages for the Cretaceous plutonic rocks have 

yet to be obtained. Samples dated by the 40Ar/39Ar method indicate partial to total argon loss during Eocene time (Lewis, 1990; 

L.W. Snee , unpub. data, 1992). Correlation with similar rocks 

elsewhere in the batholith (Lewis and others, 1987; L.W. Snee, 

unpub. data, 1992) indicates that an age of 95–75 Ma is likely.

The Cretaceous plutonic rocks, which are part of the Idaho 

batholith, can be divided into three phases: (1) potassium-rich 

hornblende-biotite granodiorite (fig. 2, unit Kgdk); (2) hornblende-biotite tonalite (fig. 2, unit Kt); and (3) biotite granodiorite (fig. 2, unit Kgd). The hornblende-biotite granodiorite 

contains large books of biotite and belongs to a potassic suite of 

plutons that is more widely exposed east of the study area 

(Lewis, 1989; Worl and others, 1991). The hornblende-biotite 

tonalite and biotite granodiorite contain small “shreddy” biotite 

books and belong to a sodic suite of plutons mostly north and 

west of the study area (Lewis, 1989; Schmidt, 1962, Worl and 

others, 1991).

Four phases of Eocene plutonic rocks are present in the 

eastern part of the Soldier Mountains. All are epizonal and are 

associated with dike swarms. The earliest three units are the 

most mafic; they range from gabbro to granite in composition 

and are collectively referred to as the quartz monzodiorite suite 

(fig. 2, unit Tqmd). Regional distribution of the three units is 

shown by Worl and others (1991, units Tqm, Tfgd, Tgb). This 

intrusive activity probably began about 48 or 47 Ma, based on 

40Ar/39Ar age spectra of biotite and potassium feldspar (Lewis, 

1990; L.W. Snee, unpub. data, 1992). The youngest phase is 

biotite granite (fig. 2, unit Tg), the plutons of which are referred 

to as the biotite granite suite or pink granite suite. Contacts 

between the biotite granite and the older Eocene plutonic rocks 

are sharp, and dikes of biotite granite locally crosscut the older 

Eocene rocks. Both suites are present throughout central Idaho 

and are the intrusive equivalents of the Challis Volcanic Group 

(Bennett and Knowles, 1985). More detailed descriptions of the 

plutonic rocks in the Soldier Mountains and the surrounding 

region are given in Lewis (1990, 1991), Lewis and Kiilsgaard 

(1991), and Kiilsgaard and others (in press).

The Challis Volcanic Group in the eastern part of the Soldier Mountains is characterized by lava flows of rhyodacite, dacite, and andesite. Overlying these flows is a series of Miocene 

volcanic rocks including rhyolite tuff that correlates with rhyolite tuff of the Gwin Spring Formation, peralkaline rhyolite tuff 

of Cannonball Mountain, and a series of porphyritic olivine 

basalt flows (Lewis, 1990).

Structure

Two types of structures are prominent in the eastern part of 

the Soldier Mountains: faults and swarms of dikes that follow 

zones of structural weakness. The distribution of major faults is 

shown in figure 2. Most faults dip steeply and were recognized 

by topographic expression, changes of rock type, or zones of 

cataclasis. The only low-angle fault that could be traced any 

distance is present in the extreme northeastern corner of the 

study area, but the sense of motion along this structure is 

unknown. Numerous other low-angle structures probably are 

present but were not recognized with certainty because of poor 

exposures and lack of marker units.

The Worswick Creek fault system is probably one of the 

oldest high-angle structures in the area. It consists of a series of 

northeast-trending faults characterized by easily eroded zones 

of sheared rock. Some of these faults may continue southwest 

of the Couch Summit fault, but none could be traced because of 

poor exposures. The northeast trend of the Worswick Creek 

fault system parallels the predominant trend of Eocene dikes in 

the area, and a majority of the dikes were intruded along or near 

this zone of weakness. Thus, the fault system probably was 

present during Eocene time. The northeast trend is the same as 

that of the much larger Trans-Challis fault system to the north, 

along which Eocene intrusive activity was also localized (Kiilsgaard and Lewis, 1985; Kiilsgaard and others, 1986). The two 

systems are probably contemporaneous. Direction of displacement along the Worswick Creek fault system is unknown, but 

there is no topographic break that would indicate recent dip-slip 

motion. Although rocks along the fault zone are sheared and 

brecciated, hydrothermal alteration is minimal.

Intense propylitic alteration is localized along some of the 

faults in the study area. This alteration is characterized by epidote veinlets, the replacement of biotite by chlorite, and the 

replacement of oligoclase and andesine by albite. Here and 

elsewhere in the region this type of alteration is spatially associated with Eocene plutons (Criss and Taylor, 1983; Lewis, 1990). 

Thus, faults that localize propylitic alteration were probably 

present in the Eocene. Examples include the east-trending fault 

2.5 km southwest of the U.S. Forest Service Big Smoky Guard 

Station, the Couch Summit fault, and the northwest-trending 

fault 2.5 km east of the guard station (fig. 2).

West-northwest- to north-northwest-trending faults such as 

the Couch Summit and Soldier Mountains faults are characterized by topographic breaks and probably have had dip-slip 

motion in the past several hundred thousand years. Some or all 

of these faults may still be active. Their west-northwest to 

north-northwest trends parallel basin and range structures in the 

region, and all are interpreted as steep normal faults. Hydrothermal alteration along these faults is variable. In some cases 

juxtaposed rocks are sheared but not visibly altered. Rocks 

along the Couch Summit fault are propylitized across a width of 
3

Qal

Qal

Qal

Tv

Tg

Tg

Tg

Tg

Tg

Tqmd

Tqmd

Tqmd

Tqmd

Tqmd

Tqmd

Tqmd

Tcv

Tcv

Tcv

Tcv

Tcv

Kgd

Kgd

Kgd Kgd

Kgd

Kgd

Kgd

Kgd

Kgd

Kgd

Kgd

Kgd

Kgd

Kgd

Kgd

Kt

Kt

Kt

Kt

Kgdk

Kgdk

Soldier

Creek

Alluvium (Quaternary)

Volcanic rocks (Miocene)

Biotite granite suite (Eocene)

Quartz monzodiorite suite (Eocene)

Challis Volcanic Group (Eocene)

Biotite granodiorite (Late Cretaceous)

Horneblende-biotite tonalite (Late Cretaceous)

Potassium-rich horneblende-biotite

granodiorite (Late Cretaceous)

Contact

Fault—Bar and ball on downthrown side

Mines or prospects—Sample number refers

to chemical analyses, table 2

Qal

Tcv

Kgd

Kt

Kgdk

EXPLANATION

25

RL35

Tv

Tg

Tqmd

43°22'30"

43°37'30"

115° 114°45'

0 1 2 KILOMETERS

Big Smoky Guard Station

Worswick

Hot Springs

WORSWICK CREEK FAULT SYSTEM

RL203

RL236

RL61

RL35

RL63

RL05

RL31

RL381

CS01

?

SOLDIER MOUNTAINS FAULT

COUCH

SUMMIT

FAULT

Smoky

Dome

Sydney

Butte

Camas Prairie

Low-angle fault

Texas Star

mine

Richard Allen

mine

Five Points

mine

Idaho Tungsten

mine

roughly 300 m. Because of the spatial relationship between propylitic alteration and Eocene plutonic rocks, this type of alteration is thought to be Eocene in age. Thus, the Couch Summit fault has probably been the site of recurrent movement during 

the past 50 m.y. Most of the other steep normal faults are probably younger, but age of initial movement is difficult to establish. 

Figure 2. Simplified geologic map of the eastern part of the Soldier Mountains, Idaho. Modified from Lewis 

(1991).

Structure
4 Alteration and Mineralization, Eastern Part of the Soldier Mountains, Camas County, Idaho

The Soldier Mountains fault is the southernmost part of a structure 80 km long that extends as far north as the South Fork of the 

Payette River, east of Lowman, Idaho. The northern part of the 

fault, recognized by Anderson (1934), is termed the Deer Park 

fault. Net vertical displacement on the Soldier Mountains fault, 

based on topographic relief, is roughly 700 m (2300 ft). Vertical 

displacement on the order of 500 m (1600 ft) is likely along the 

southern end of the Couch Summit fault.

The low-angle fault in the northeastern corner of the study 

area dips about 25°. It continues northeast into the Dollarhide 

Mountain quadrangle, where it forms a resistant ridge of propylitized (chlorite- and epidote-rich) Cretaceous biotite granodiorite. The map by Darling (1987) of the Dollarhide Mountain 

quadrangle shows this feature as a hydrothermally altered zone 

in the biotite granodiorite, not as a fault. The origin of this structure is uncertain, but it is probably a low-angle extensional fault 

of Eocene age. Propylitic alteration, such as in this fault zone, is 

commonly associated with Eocene plutonism elsewhere in the 

region (Criss and Taylor, 1983). Darling (1987) mapped a hornblende quartz monzonite of suspected Eocene age below the 

altered and sheared biotite granodiorite and inferred an Eocene 

age for alteration in this part of the Dollarhide Mountain quadrangle.

Low-angle shear zones were also noted during underground 

mapping of the Richard Allen mine west of Soldier Creek. 

Alteration along these shear zones is characterized by coarsely 

crystalline secondary muscovite rather than by a propylitic 

(chlorite-epidote) assemblage. Similar alteration elsewhere in 

the study area has been radiometrically dated as Cretaceous 

(approximately 70 Ma) (Lewis, 1990). Because of poor 

exposure, none of these muscovite-bearing shear zones is recognizable at the surface.

Hydrothermal Alteration

Cretaceous and Eocene plutonic rocks in the eastern part of 

the Soldier Mountains area have undergone muscovite-quartz, 

potassic, and propylitic alteration (table 1). The muscovitequartz alteration is equivalent to quartz-sericite (or phyllic) 

alteration. A fourth type of alteration involves the widespread 

formation of turbid potassium feldspar in the Eocene plutonic 

rocks. This turbidization is particularly pronounced in the 

biotite granite. The only hydrothermally altered volcanic rocks 

are those of the Challis Volcanic Group, which are characteristically propylitized. Because the focus of this study was alteration in plutonic rocks, the following discussion is restricted to 

these rock types.

 Muscovite-Quartz Alteration

Muscovite-quartz alteration is most pronounced in the 

northeastern part of the area, particularly east of the Worswick 

Creek fault system. It is localized along joints and fractures and 

is restricted to rocks of the Cretaceous batholith. Granitic rock 

adjacent to gold-bearing quartz veins in the area has also undergone muscovite-quartz alteration. Muscovite-quartz alteration 

differs from the weak sericitization that affects propylitized 

Table 1. Characteristics of alteration types in the eastern part of the Soldier Mountains, Idaho.

Muscovite-quartz Potassic Propylitic “Turbidization”

Distribution Localized along 

fractures in 

Cretaceous 

granodiorite; 

local quartz-vein 

association 

Locally present 

adjacent to veins, 

dikes, and Eocene 

granodiorite of 

quartz monzodiorite 

suite

Common along 

fractures and locally 

pervasive in 

Cretaceous and 

Eocene units; 

common in vicinity 

of Eocene biotite 

granite 

Pervasive in Eocene 

biotite granite and to a 

lesser extent in quartz 

monzodiorite suite

Mineralogy Coarse muscovite, 

quartz, and

pyrite

+ albite

+ calcite

+ potassium feldspar 

+ biotite

(new minerals or 

recrystallized 

preexisting 

minerals)

Epidote,

chlorite,

albite, and

sericite

+ magnetite

+ hematite(?)

+ potassium feldspar

+ leucoxene

+ calcite

+ quartz

+ fluorite

Hematite(?) imparts pink 

color, but turbidity is 

probably result of open 

spaces in feldspar

Age Cretaceous Cretaceous and (or) 

Eocene

Eocene Eocene
5

rocks of all ages in the study area in that the secondary mica is 

coarsely crystalline, typically 1–4 mm across. The mica has 

replaced plagioclase, potassium feldspar, and biotite. In addition to muscovite, primary quartz and fine-grained, secondary 

quartz are abundant. Secondary pale-pink albite is generally the 

only feldspar and, unlike albite in the propylitized rocks, it is not 

turbid. Euhedral secondary pyrite is common and protrudes into 

small cavities in altered rocks. Calcite is present locally. Much 

of the weakly altered Cretaceous granodiorite contains populations of both coarse- and fine-grained secondary muscovite 

replacing plagioclase feldspar. The coarse-grained muscovite is 

probably indicative of weak muscovite-quartz alteration, and 

later propylitic alteration formed the fine-grained mica.

Muscovite-quartz alteration in the Soldier Mountains area 

is characterized isotopically by relatively heavy muscovite 

(δ18O of 7.7–8.0 per mil) and by quartz that has a higher δ18O 

value (11.6 per mil) than its original value near 10.2 per mil 

(Lewis, 1990). The relatively high δ18O values are the result of 

interaction either with magmatic fluids or with meteoric waters 

in which δ18O values have been greatly increased as a result of 

low water to rock ratios. A δqtz-musc value of 3.9 per mil from 

one sample indicates an alteration temperature of about 425°C 

(Lewis, 1990) using the fractionation data of Bottinga and Javoy 

(1973). Assuming that 425°C is the correct temperature, the 

water in equilibrium with the quartz and muscovite had a δ18O 

value of about 7 per mil. Altered rocks were depleted in sodium 

and iron, and enriched in calcium and potassium (Lewis, 1990).

Coarse secondary muscovite is common around quartz 

veins throughout the southeastern part of the Idaho batholith. 

The fact that it is absent in Eocene plutonic rocks suggests that 

muscovite-quartz alteration is Cretaceous in age. A sample of 

this coarse muscovite from the study area was dated by the 40Ar/

39Ar method and yielded a stepwise release plateau of 70.3+0.4 

Ma (Lewis, 1990). This result confirms the Cretaceous age of 

muscovite-quartz alteration in the study area and implies that 

similar alteration in the region may also be of this age.

Potassic Alteration

Potassic alteration of Cretaceous granodiorite was recognized at three localities in the study area, and only as a result of 

petrographic study. Although other areas are undoubtedly 

affected, potassic alteration probably is areally limited and 

weakly developed. Temperatures of alteration are high (>400°C) 

because biotite and potassium feldspar are stable phases.

The most pronounced potassic alteration is at the Texas 

Star mine north of Camas Prairie (fig. 2). Here, potassium-rich 

hornblende-biotite granodiorite is crosscut by widely spaced 

quartz veins, adjacent to which fine-grained secondary potassium feldspar has formed as subgrains (0.1 mm across) along 

grain boundaries. A second area of potassic alteration is about 

250 m northeast of the Couch Summit fault, 1 km from its 

southern end. Biotite granodiorite at this locality contains both 

finely crystalline biotite that has replaced more coarsely crystalline primary biotite, and subgrains of potassium feldspar along 

grain boundaries. Alteration is fracture controlled and not extensive. Small dikes of fine-grained biotite granite and dacite in this 

area may have been the source of heat and (or) potassium-rich 

fluids. The third locality affected by potassic alteration is on the 

ridge at the head of Bowns Creek, 8 km southwest of Big Smoky 

Guard Station. Here, Cretaceous biotite granodiorite collected 

about 75 m from the contact with Eocene granodiorite of the 

quartz monzodiorite suite contains patches of secondary potassium feldspar partly replacing plagioclase feldspar and finegrained biotite that has replaced coarser primary biotite. 

Geochemical data indicate that alteration at this locality 

involved high-temperature recrystallization of existing minerals 

but without significant addition of potassium to the rock (Lewis, 

1990). Despite the lack of measurable potassium enrichment, 

the term “potassic alteration” is applied because the potassiumbearing minerals biotite and potassium feldspar were stable.

Oxygen-isotope ratios of feldspar from the Bowns Creek 

locality probably were not affected by potassic alteration. The 

altered Cretaceous granodiorite has δ18O values of 8.6 per mil 

for potassium feldspar and 8.5 per mil for plagioclase feldspar, 

similar to typical unaltered values of 8.0–9.0 per mil (Lewis, 

1990). The lack of a shift in δ18O values and the proximity of 

the Eocene pluton indicate that the water involved in this recrystallization process was likely at high temperature and of magmatic origin. Assuming a temperature of 500°C, and using the 

fractionation data of O’Neil and Taylor (1967), the composition 

of the water in equilibrium with feldspar of 8.5 per mil would be 

about 6 per mil.

The age of the alteration at these three localities is not well 

constrained. Alteration and associated mineralization at the 

Texas Star mine may be Cretaceous in age because Eocene plutonic rocks are some distance away; however, the possibility of a 

buried Eocene pluton cannot be discounted. Alteration at the 

other two localities is most likely Eocene in age, especially at 

the Bowns Creek locality, because the altered rock is about 75 m 

from a contact with an Eocene stock.

Propylitic Alteration

Propylitic alteration is by far the most prevalent alteration 

type in the eastern part of the Soldier Mountains. It is characterized by the replacement of biotite by chlorite and the replacement of oligoclase and andesine by albite. Epidote is also 

common and typically forms 1–2-mm-thick veinlets spaced a 

few centimeters to decimeters apart. The hydrothermally altered 

areas are only part of an areally more extensive altered region 

recognized by Taylor and Margaritz (1976, 1978) and Criss and 

Taylor (1983) that was defined on the basis of lowered δ18O values of feldspar (as much as -8 per mil) and lowered δD values of 

biotites (as much as -180 per mil). These investigators demonstrated that the propylitic alteration was related to large-scale 

circulation of heated meteoric waters around epizonal Eocene 

plutons.

Although Cretaceous biotite granodiorite unit (fig. 2, unit 

Kgd) is propylitized in many areas, the most widely altered 

rocks in the study area are those of the Eocene quartz monzodiorite suite (unit Tqmd). Eocene biotite granite (unit Tg) is 

propylitized only locally. Important alteration controls are proximity to Eocene intrusive rocks (particularly biotite granite), as 

Hydrothermal Alteration
6 Alteration and Mineralization, Eastern Part of the Soldier Mountains, Camas County, Idaho

well as to faults, epidote veinlets, and joint surfaces. The altered 

areas are typically restricted to within a few centimeters of the 

veinlets and joint surfaces, but in some cases the alteration is 

pervasive over several hundred square meters. Prophylitic alteration implies relatively low temperatures (160°C–325°C), based 

on measurements in active geothermal systems (McDowell and 

Elders, 1980) and on fluid inclusion studies (Kerrich and 

Kamineni, 1988). Propylitized areas are not known to be 

mineralized.

A characteristic feature of the propylitic alteration is albitization of plagioclase feldspar. The albite, which is generally 

pink and turbid, was initially mistaken for potassium feldspar; 

however, X-ray diffraction analysis indicated a composition of 

almost pure anorthite. Locally, the albitized plagioclase feldspar 

was itself partly replaced by potassium feldspar, although secondary potassium feldspar is generally confined to veinlets. 

Minor epidote, in addition to albite and sericite, has replaced the 

plagioclase feldspar.

Secondary albite has been reported elsewhere in the region. 

Reid (1963) described albite in altered zones of Cretaceous granodiorite several hundred yards wide adjacent to the biotite granite in the Sawtooth Range, 40 km north of the study area. In 

addition, clay, sericite, chlorite, and epidote were reported. 

Albite and epidote that postdate silicification and sericitization 

along quartz veins were noted by Allen (1952) in the Volcano 

mining district, 30 km southwest of the study area. The veins 

reportedly crosscut early Tertiary granophyric dikes.

Potassium feldspar appears to be less affected by the propylitic alteration than plagioclase feldspar. At a few localities, turbid altered potassium feldspar is present. Epidote and, more 

rarely, chlorite also formed as secondary minerals after potassium feldspar.

Biotite in the propylitized rocks has been replaced by chlorite and by lesser amounts of epidote, sphene, sericite, and leucoxene. As recognized by Criss (1981), chloritization of biotite 

was widespread in the southern lobe of the batholith. Primary 

magnetite was mostly unaffected by the propylitic alteration, 

except in some of the most strongly propylitized samples where 

it was replaced by hematite. Secondary magnetite is common in 

epidote veinlets. Hornblende was replaced by chlorite and epidote but was less affected by hydrothermal alteration than 

biotite. Sphene was altered to leucoxene and, in strongly propylitized rocks, to epidote, as was allanite.

Epidote veinlets, typically 2 mm or less wide, are another 

common feature of the propylitized rocks. Most veinlets are 

entirely filled, but some contain central cavities into which euhedral epidote has grown. Plagioclase feldspar has been preferentially albitized adjacent to these veinlets. Magnetite and 

potassium feldspar are subordinate minerals in the epidote veinlets. Sericite, albite, chlorite, quartz, and fluorite are also in the 

epidote veinlets but in negligible abundance. The presence of 

secondary magnetite within veinlets may in part explain the variability in magnetic susceptibility of altered rocks in the southern 

part of the Idaho batholith that has been documented by Criss 

and Champion (1984). Potassium feldspar in the veinlets is typically anhedral and has poorly developed grid twins. Rarely, it 

forms euhedral rhombs, characteristic of adularia. Calcite is 

present locally and is more common in rocks that do not contain 

appreciable epidote.

Unaltered feldspar in the biotite granodiorite unit (fig. 2, 

unit Kgd) has δ18O values of 8.0 to 9.0 per mil (Lewis, 1990). 

Propylitized rocks contain oligoclase that ranges from 8.4 per 

mil to -1.4 per mil and albitized plagioclase feldspar that ranges 

from -2.0 to -4.7 per mil; potassium feldspar values are as low 

as -3.5 per mil (Lewis, 1990). Criss analyzed feldspars in the 

study area that have δ18O values as low as -5.8 per mil. These 

data all indicate that the feldspars have exchanged with low -

δ18O fluids (meteoric water) during propylitic alteration. Water 

in equilibrium with the albitic feldspar would be about -10 per 

mil, using fractionation data of O’Neil and Taylor (1967) and 

assuming a temperature of 250°C. Criss and Taylor (1983) estimated a δ18O value of -15 per mil for meteoric water in the 

Atlanta lobe during Eocene time but thought that δ18O values 

for those waters had been shifted to about -9 to -3 by interaction 

with wallrock. The -10 per mil value based on albite-water equilibrium is considered a rough approximation of the composition 

of some of the isotopically least shifted hydrothermal waters in 

the study area during Eocene time. This low δ18O value is in 

contrast to the relatively high δ18O values of water (about 7 per 

mil) calculated for the muscovite-quartz alteration. The majorand trace-element compositions of most propylitized samples 

are similar to those of unaltered samples; however, in strongly 

propylitized rocks sodium and hydrogen were added and calcium was depleted (Lewis, 1990).

Turbidization of Eocene Feldspar

A characteristic feature of Eocene biotite granite (fig. 2, 

unit Tg) is the presence of turbid potassium feldspar. The process that clouds the feldspar is here referred to as turbidization. 

Except for local phenocrysts that have gray cores, all potassium 

feldspar in Eocene granite is pink and turbid; the gray cores of 

phenocrysts are less turbid than the pink rims. Potassium feldspar in rocks of the Eocene quartz monzodiorite suite (unit 

Tqmd) is either gray or pink. Plagioclase feldspar in all of the 

Eocene rocks is white and less turbid than potassium feldspar. 

Cretaceous granodiorite (units Kgd, Kgdk) contains gray to 

pale-pink potassium feldspar that is less turbid than potassium 

feldspar in Eocene rocks.

The turbidity and pink color of the potassium feldspar in 

the Eocene rocks, particularly biotite granite, is a pervasive feature not related to large-scale faults or joint surfaces. Martin 

and LaLonde (1979) suggested that turbidity in potassium feldspar results not from mineral inclusions but from open spaces 

produced by reconstitution of a feldspar in the presence of 

water. Microfractures in turbid areas of the feldspar suggest that 

alteration (turbidization) was enhanced in more permeable areas 

of the crystals and that fluids were an important part of that process. The gray cores of phenocrysts are interpreted as relict, 

unaltered areas. The less turbid (and more gray) potassium feldspar in Cretaceous granodiorite apparently did not undergo fluid 

interaction to the same extent as did the Eocene plutons. The 

higher level of emplacement of the Eocene plutons relative to 

their Cretaceous counterparts may have facilitated both 

increased porosity and greater availability of meteoric waters, 

thus promoting turbidization. The pink color of the feldspar 

presumably is the result of crystals of hematite. Guthrie and 
7

Veblen (1988) reported oxides of iron line minute (<5μm) open 

spaces in turbid alkali feldspars. The inferred presence of hematite is an indication that the fluids were oxidizing.

Mineralization

The study area includes parts of the Soldier, Rosetta, Big 

Smoky, Little Smoky, and Skeleton Creek mining districts. 

Small mines and prospects are present (fig. 2), but none have 

had significant production. The principal interest has been in 

gold, but minor amounts of lead and silver have also been 

produced. 

Mines and prospects in the eastern part of the Soldier 

Mountains were visited, and the mineralization at these localities was related to specific geologic features such as rock type 

and structure. In this way, structures and rock units favorable 

for exploration were identified. Sixteen mineralized rock-chip 

samples were analyzed to characterize the metal types present, 

and 188 stream-sediment samples were collected to identify 

metal-enriched areas that may not have been previously identified. Rock-chip analyses are listed in table 2 and results of the 

stream-sediment sampling are given in Lewis (1990, 1991).

Texas Star (D. Marie) Mine

The Texas Star mine is at the northern edge of Camas Prairie in the southeastern part of the study area (fig. 2). The underground workings were not extensive and are now inaccessible. 

According to R.E. Gibbons of Fairfield, Idaho (written commun., 1985), the property was located in 1891 by E.M. and H.S. 

Hego as the Texas Star. Ross (1930) reported that in 1892 1.75 

tons of ore shipped from the Texas Star yielded 1.53 ounces of 

gold, 49.5 ounces of silver, and 1,651 pounds of lead. Continued work in 1893 produced 17.19 tons of ore yielding 75.2 

ounces of gold, 132.3 ounces of silver, and 6,000 pounds of 

lead. According to R.E. Gibbons, a ten-stamp mill was subsequently installed on the property, and underground work indicated that oxidized ore extended to a depth of 10–20 m (30–60 

ft). The oxidized zones of all known veins are reported to have 

been worked out, but records of the production from the mill, 

which was eventually removed, are not available. Subsequent 

work on the property is well summarized by R.E. Gibbons:

“The ground stood idle for many years although the late 

Jack Wallace, a lifetime prospector of the local Soldier 

Mountain area, claimed, worked, and lived on the property for over twenty-five years, until his death in the late 

forties, apparently without success as his efforts didn’t 

alter the landscape greatly, nor did he die a rich man.”

The Texas Star mine and a number of prospect pits present 

west and north of the mine are in potassium-rich hornblendebiotite granodiorite (fig. 2, unit Kgdk), part of the potassic suite 

of the Idaho batholith (Lewis, 1989). The granodiorite is both 

texturally and mineralogically similar to the Hailey granodiorite 

unit of Schmidt (1962), which is exposed in the Hailey gold belt 

area to the east (unit Kgdk of Worl and others, 1991). The contact with the biotite granodiorite unit (unit Kgd) is poorly 

known but is thought to be immediately east of the Texas Star 

mine. Dump samples indicate that veins of quartz containing 

pyrite, galena, and magnetite crosscut the granodiorite. Molybdenite is sparingly present along quartz-free fractures. Alteration minerals include sericite, chlorite, calcite, and secondary 

potassium feldspar. A composite sample of granodiorite and 

molybdenite veinlets (table 2, sample RL36) contained 200 ppm 

Mo, and a composite sample of vein quartz containing visible 

sulfide minerals (table 2, RL333) contained 49 ppm Ag, 15 ppm 

Au, and 3.9 percent Pb. A composite sample of vein quartz and 

sulfide minerals from a prospect pit north of the Texas Star 

(table 2, sample RL35) contained 200 ppm Ag, 2.7 ppm Au, and 

2.0 percent Pb. Complete analyses and sample descriptions are 

given in table 2.

Exposures of potassium-rich hornblende-biotite granodiorite (unit Kgdk) west and north of the Texas Star mine contain an 

unusually large number of prospects (fig. 2). The characteristics 

of these prospects are similar to those of the mine.

Richard Allen Mine

The Richard Allen mine is north of the Texas Star mine on 

the west side of Soldier Creek (fig. 2). The property was developed with several adits and prospects pits, but only the uppermost adit was accessible at the time of this study. According to 

the State Mine Inspector’s report for the year 1939 (Cambell, 

1940), total development is on the order of 600 m (2,000 ft). 

The amount of production from the Richard Allen mine is 

unknown, but it is unlikely that it was substantial.

Bedrock at the Richard Allen mine is biotite granodiorite 

(fig. 2, unit Kgd) of the sodic suite of the Idaho batholith. The 

granodiorite is crosscut by dikes and sills of dacite that are probably Eocene in age. Exposures of quartz veins or other mineralized rock are not evident.

A geologic map of the uppermost adit at the Richard Allen 

mine is shown on figure 3. Both the granodiorite and dacite in 

the adit are highly sheared and altered to clayey gouge, secondary muscovite, and chlorite. Most of the shears are subhorizontal and the shearing is pervasive; discrete faults are lacking. 

Because the shearing involves the dacite sills and dikes, at least 

some of the movement is likely Eocene or younger in age. 

Three composite chip samples were collected along the length 

of the upper adit (samples RL151, RL152, and RL153; fig. 3, 

table 2); all three contained less than 0.05 ppm Au.

Five Points (Perseverance) Mine

The Five Points mine is north of the Richard Allen mine 

and southwest of Sydney Butte (fig. 2). All of the workings are 

presently inaccessible. According to Ross (1930), the mine was 

developed by four tunnels having a total length of about 300 m 

(1,000 ft). As of 1930, development had been carried on intermittently for several years. Recorded production from the Five 

Points mine is less than 1,000 tons of ore that yielded between 

100 and 500 ounces of gold and between 1,000 and 5,000 

ounces of silver (Mitchell and others, 1991).

Mineralization
8 Alteration and Mineralization, Eastern Part of the Soldier Mountains, Camas County, Idaho Table 2. Mineralized samples from the eastern part of the Soldier Mountains, Idaho. [Sample localities are shown on figure 2. Detailed locations are given in Lewis (1991). In parts per million. Methods of determination: Atomic-absorption spectroscopy and in

Ag, Au, Cu, Mo, Pb, Zn, As, Cd, Bi, and Sb; six-step semiquantitative emission spectroscopy for B, Ba, Be, Co, Cr, La, Nb, Ni, Sc, Sr, V, Y, and Zr. Analysts: J. Motooka, C. TayB. Bailey, O. Erlich, B. Roushey, and P.L. Hagemon] Description of mineralized samples RL05 Chip sample across silicified zone 1 m wide from prospect pit in biotite granodiorite northwest of Richard Allen mine. RL31 Composite sample of high-graded vein quartz from prospect pit in biotite granodiorite southeast of Richard Allen mine.

RL35 Composite sample of high-graded vein quartz from prospect pit in coarse-grained hornblende-biotite granodiorite north of Texas Star mine.RL36 Composite sample of high-graded coarse-grained hornblende-biotite granodiorite and sparse molybdenite veinlets from the mine dump at the Texas Star mine.RL61 Chip sample across quartz vein 0.5 m wide from prospect pit in biotite-hornblende granodiorite north of Smoky Dome.RL63 Composite sample of copper-stained biotite granodiorite and quartz veins 1-3 cm wide from prospect pit southwest of the Richard Allen mine.RL87 Composite sample of high-graded vein quartz from the dump at the Idaho Tungsten mine.RL151 Chip sample along 21 m (70 ft) of dacite dike in the upper adit of the Richard Allen mine.RL152 Chip sample along 14 m (47 ft) of biotite granodiorite in the upper adit of the Richard Allen mine.RL153 Chip sample along 12 m (40 ft) of biotite granodiorite in the upper adit of the Richard Allen mine.RL163 Composite sample of high-graded vein quartz from the dump at the Five Points mine.RL203 Chip sample across silicified zone and fault gouge 1 m wide above small caved adit in biotite granodiorite west of Big Smoky Guard Station.RL236 Chip sample across quartz vein 2 m wide from outcrop in biotite granodiorite northeast of Worswick Hot Springs.RL333 Composite sample of high-graded vein quartz from the dump at the Texas Star mine.RL381 Composite sample of high-graded vein quartz and biotite-hornblende granodiorite from prospect pit northwest of Texas Star mine.CS01 Composite sample of high-graded vein quartz and biotite-hornblende granodiorite from prospect pit northwest of Texas Star mine.

RL05 RL31 RL35 RL36 RL61 RL63 RL87 RL151 RL152 RL153 RL163 RL203 RL236 

Ag 3.0 29 200 0.55 35 37 1.0 0.15 0.17 0.09 37 0.9 <0.05 As 20 <10 100 <10 <10 10 260 <10 <1.0 <1.0 100 <1.0 100 Au 0.80 2.4 2.7 0.05 0.05 <0.05 0.05 <0.05 <0.05 <0.05 3.3 <0.05 <0.05 B 20 10 10 <10 10 10 10 10 10 10 10 15 15 Ba 1,000 70 50 700 500 1,000 500 1,500 700 700 50 500 70 Be 1 2 1.5 1 1 1 3 <1 <1 <1 1 <1 1 Bi <1 28 590 1 340 120 <1 <1 <1 <1 15 <1 <1 Cd 0.6 0.7 1.3 <0.1 0.4 2.9 <0.1 0.1 <0.05 <0.05 30 0.90 0.06 Co <5 <5 <5 10 15 <5 <5 7 <5 <5 5 <5 <5 Cr <10 <10 10 50 20 <10 <10 70 <10 <10 <10 <10 <10 Cu 50 80 450 70 5,000 20,000 50 5 9.6 4.2 1,500 130 2.5 Hg 0.02 0.08 0.10 0.02 0.02 0.02 0.22 0.18 0.12 0.18 0.02 0.04 0.10 La 50 <20 <20 100 <20 50 20 70 50 70 <20 50 50 Mo <0.5 12 20 200 3.0 5.0 67 0.5 0.18 <0.15 15 0.32 4.5 Nb <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20 Ni 5 5 <5 15 10 <5 5 15 <5 <5 <5 <5 <5 Pb 440 1,500 20,000 45 110 180 10 10 9.9 16 2,000 94 10 Sb <2 <2 18 <2 <2 <2 16 <2 <1.2 <1.2 <2 <1.2 <1.2 Sc <5 <5 <5 7 <5 <5 <5 10 <5 <5 <5 <5 <5 Sr 200 <100 <100 500 100 500 <100 1,000 500 300 <100 200 <100 V 20 10 30 70 30 20 30 70 10 <10 15 15 15 Y <10 <10 <10 10 <10 15 <10 15 10 10 <10 <10 <10 Zn 140 290 250 70 110 150 10 40 26 32 10,000 260 47 Zr 70 20 70 50 70 100 100 100 70 70 20 50 100 
9

N

Raise to additional

workings

Dacite in back

Dacite

in back

0 5 METERS

70

75

50

45

30

RL151

RL152

RL153

70

Dacite dikes and sills (Eocene)

Biotite granodiorite (Late Cretaceous)

Contact—Showing dip direction and

amount (in degrees)

EXPLANATION

Bedrock at the mine is Cretaceous biotite granodiorite (fig. 

2, unit Kgd) of the sodic suite of the Idaho batholith. A steeply 

dipping quartz vein at the mine that strikes north-south probably 

was emplaced along the southern end of a fault that continues to 

the north along Five Points Creek. The vein is as wide as 1 m 

and can be traced about 60 m (200 ft) up the slope above the 

mine where it abruptly ends, possibly as a result of displacement 

along a low-angle fault. Dump samples of granodiorite are 

altered to clay gouge, sericite, and chlorite. Galena and pyrite 

are present in dump samples of vein quartz. A composite sample of sulfide- and oxide-rich dump material (table 2, sample 

RL163) contained 3.3 ppm Au, 37 ppm Ag, 0.2 percent Pb, and 

1.0 percent Zn.

Idaho Tungsten Mine (Soldier Mountain Deposit)

The Idaho Tungsten mine is between Rough Creek and the 

East Fork of Corral Creek about 5 km (3 mi) south of Smoky 

Dome (fig. 2). The mine is hosted by Eocene biotite granite 

(unit Tg). Mineralized rock is confined to narrow, chalcedonic 

quartz veins that contain sparse amounts of tungsten. Tungsten 

minerals were not recognized during this investigation, but, 

according to Shannon (1926), ferberite is present in three narrow 

parallel veins 1–0 cm wide and about 50 cm apart. Cook (1956) 

reported that one of the veins strikes about N. 55° E. and dips 

50° N. and is within one of a set of fractures of similar attitude 

that cuts the granite in this area. This vein was explored with an 

adit, about 60 m (200 ft) long, that is now caved. Production 

was limited to about 6.6 tons of hand-sorted ore averaging about 

4.4 percent WO3 (Cook, 1956). A sample of vein material 

collected from the dump (table 2, sample RL87) contained less 

than 50 ppm W.

Unnamed Mines and Prospects in Cretaceous 

Intrusive Rocks

Several prospects pits and small caved adits are present 

within the biotite granodiorite unit (unit Kgd) of the sodic suite 

(fig. 2). Those northeast of Worswick Hot Springs have been 

referred to as the Rosetta claim group (Umpleby, 1915). Deposits hosted in the biotite granodiorite are characterized by veins 

of quartz containing minor amounts of sulfide minerals and 

gold. The granodiorite adjacent to the veins has undergone muscovite-quartz alteration. Chip samples were taken across four of 

these veins (table 2, fig. 2, samples RL05, RL31, RL203, and 

RL236); two contained detectable amounts of gold (0.80 and 2.4 

ppm) and two contained <0.05 ppm Au.

Copper mineralized rock is not common in the biotite granodiorite, but one prospect pit about 3.5 km (2 mi) southwest of 

the Richard Allen mine contains small amounts of malachite and 

azurite. The copper minerals coat fractures and have replaced 

potassium feldspar. Sparse veinlets of quartz crosscut the granodiorite; they are 1–3 cm wide and contain iron-oxide minerals 

and epidote. The feldspar and biotite have been locally replaced 

by secondary albite, sericite, and chlorite. This mineralization 

may be related to the intrusion of Eocene granodiorite of the 

quartz monzonite suite (fig. 2, unit Tqmd), which is exposed 1 

km to the west. A composite sample rich in quartz and secondary copper minerals (sample RL63, table 2) contained 37 ppm 

Ag, 5 ppm Mo, and 2.0 percent Cu.

Figure 3. Map of the uppermost adit of the Richard Allen mine. Location of mine shown on figure 2. Analyses 

for samples RL151, RL152, and RL153 are shown in table 2.

Mineralization
10 Alteration and Mineralization, Eastern Part of the Soldier Mountains, Camas County, Idaho

Unnamed Mines and Prospects in Eocene

Intrusive Rocks

Mineralization is less common in rocks of Eocene age than 

in rocks of Cretaceous age. A small prospect pit has exposed a 

quartz vein within the quartz monzodiorite suite (unit Tqmd) 

about 3 km (2 mi) north of Smoky Dome (fig. 2). Malachite 

coats fractures in the vein, which is 0.5 m wide and trends eastwest. A chip sample taken across the vein (sample RL61, table 

2) contained 0.05 ppm Au, 35 ppm Ag, and 0.5 percent Cu. A 

small prospect pit 700 m to the southeast exposes a quartz vein 

12 cm wide that is also stained with malachite.

One of the more promising prospects in the area is between 

McMahan Creek and the West Fork of Threemile Creek, roughly 

midway between the Idaho Tungsten mine and the Texas Star 

mine (fig. 2). A small prospect pit has exposed malachitestained biotite-hornblende granodiorite of the quartz monzodiorite suite that is crosscut by numerous veinlets of quartz that are 

suggestive of a stockwork deposit. Chlorite, epidote, and ironoxide minerals are abundant. Two collections of composite samples of the veined granodiorite were analyzed (samples RL381 

and CS01, table 2) and contained high concentrations of silver 

(340 and 1000 ppm), copper (1.1 and 1.0 percent), and molybdenum (480 and 1500 ppm). This prospect may to be along a 

northeast-trending fault, but exposures in the area are poor and 

the existence of a fault is uncertain.

Mineralization and Lithology

Mineralization in the eastern part of the Soldier Mountains 

is confined to plutonic rocks of Cretaceous and Eocene age (fig. 

2). Volcanic rocks of Eocene and Miocene age exposed in the 

eastern part of the area are unmineralized. The spatial relationship between mineralization and Cretaceous and Eocene plutons, and the lack of mineralization in Miocene rocks, indicates 

that mineralization in the eastern part of the Soldier Mountains is 

Eocene and older.

The one radiometric date on an alteration mineral is Cretaceous (40Ar/39Ar date of 70.3+0.4 Ma on secondary muscovite), 

and gold-bearing quartz veins that have quartz-muscovite alteration selvages are probably of similar age.

Copper-silver and tungsten mineralization in the Eocene 

plutons can be no older than Eocene. The lack of documented 

post-Eocene intrusive activity suggests that the mineralization is 

related to the latest stages of Eocene plutonism.

Most of the mines and prospects are in Cretaceous intrusive rocks, and the greatest density of mines and prospects is 

within the small exposure of potassium-rich hornblende-biotite 

granodiorite unit at the northern edge of Camas Prairie. This 

granodiorite unit belongs to the potassic suite of the Idaho 

batholith, which characteristically contains a greater number of 

mines and prospects than does the sodic suite. This greater tendency toward mineralization is evident in plutons of the potassic 

suite exposed to the east in the area of the Hailey gold belt 

(Lewis, 1989).

Mineralization and Structure

As a result of poor exposures, the relationship between 

mineralization and structure in the southern part of the area has 

not been well established. Most of the veins in this area are not 

obviously related to large through-going faults; instead, they 

probably were emplaced along small shears and joints within 

the granitic rocks; strike lengths of these structures are on the 

order of a few tens of meters. The faults recognized in the 

southern part of the area are relatively young and either offset 

Eocene and Miocene volcanic rocks or show topographic 

expression (for example, the Soldier Mountains fault, fig. 2). 

Because these faults offset Miocene rocks, it is likely that latest 

motion on these structures postdated mineralization in the area.

Mineralization in the northern part of the area is more 

clearly related to larger structures. The Five Points mine is on 

the southern end of a north-trending fault (fig. 2), the trace of 

which is marked by sheared biotite granodiorite along Five 

Points Creek. The Worswick Creek fault system is a pronounced set of northeast-trending faults in the northeastern part 

of the area (fig. 2). Scattered prospects, collectively referred to 

as the Rosetta claim group, are in the area of this fault system. 

The northeast trend of the Worswick Creek fault system is parallel to that of the Trans-Challis fault system to the north, which 

hosts several gold-bearing deposits (Kiilsgaard and others, 

1986).

Conclusions

The eastern part of the Soldier Mountains in Camas 

County, south-central Idaho, is underlain principally by plutonic 

rocks of Cretaceous and Eocene age. Eocene volcanic rocks, 

correlative with the Challis Volcanic Group, and Miocene volcanic rocks, in part correlative with the Idavada Volcanics, are 

locally preserved. Numerous steeply dipping faults are present; 

those that trend northeast probably are the oldest and localized 

the emplacement of Eocene dike swarms. North-northwest- to 

west-northwest-trending faults exhibit topographic expression 

and are interpreted as basin and range structures; however, some 

of these faults may have been the site of recurrent movement 

since Cretaceous time.

Plutonic rocks in the eastern part of the Soldier Mountains 

have undergone propylitic, potassic, and muscovite-quartz alteration. In addition, feldspar in the biotite granite has been turbidized. A schematic diagram summarizing the effects of this 

alteration is shown on figure 4. Muscovite-quartz alteration is 

Cretaceous in age (40Ar/39Ar plateau of 70.3+0.4 on muscovite) 

and is localized along joints and fractures, some of which are 

filled with vein quartz. This alteration type involved relatively 

high δ18O fluids (about 7 per mil) of either magmatic, or 

strongly shifted meteoric origin.

Following a period of uplift, hydrothermal activity 

resumed in Eocene time, concurrent with intrusion of epizonal 

plutons. Weak potassic alteration occurred locally, but propylitization was the predominant and widespread alteration type. 

Propylitic alteration around the youngest plutons (biotite 
11

0 1 2 KILOMETERS

METASEDIMENTARY

ROCKS

CRETACEOUS

GRANODIORITE

EOCENE GRANITE SUITE

EOCENE

QUARTZ

MONZODIORITE

SUITE

CRETACEOUS

PLUTONIC ROCKS

Propylitic alteration

Ca loss, Na, H gain

Meteoric water

160-325 C (?)

Turbidization

No chemical change (?)

Magmatic water (?)

350-400 C (?)

Weak potassic alteration

No chemical change

Magmatic water

500 C

Muscovite-quartz alteration

Na, Fe loss, Ca, K gain

Magmatic water (?)

425 C

A B

granite) was particularly strong, and both Eocene and Cretaceous plutonic rocks were affected. Faults and joint surfaces 

served as conduits for fluid circulation. Concurrent(?) movement of heated oxygenated fluids within the cooling plutons of 

biotite granite caused turbidization of potassium feldspar, 

imparting an overall pink color to these rocks. Unaltered feldspar has δ18O values of 8.0-9.0 per mil, but in propylitized rocks 

δ18O values for feldspar as low as -5.8 per mil indicate exchange 

with heated meteoric water.

Potassic alteration is probably both Cretaceous and Eocene 

in age but is limited in areal extent and is weakly developed. 

Potassic alteration adjacent to Eocene granodiorite of the quartz 

monzodiorite suite resulted in recrystallization of existing minerals but did not cause a shift in the oxygen isotope ratios of 

feldspar. The water involved in this process was probably magmatic in origin (δ18O about 6 per mil). Little, if any, change 

took place in major-element compositions of these recrystallized 

rocks.

Rock type and, to a lesser extent, structure have influenced 

the development of mineralized areas in the eastern part of the 

Soldier Mountains. Most mines and prospects are hosted by 

granodiorite of the Late Cretaceous Idaho batholith. Although 

the highest density of prospects is within potassium-rich hornblende-biotite granodiorite of the potassic suite, most mineralized areas are in the more widespread biotite granodiorite of the 

sodic suite.

Mineralized rock within biotite granodiorite of the sodic 

suite is characterized by gold-bearing quartz veins that contain 

minor amounts of pyrite, galena, and sphalerite. Quartz-muscovite alteration is common along side these veins. This mineralization is probably Late Cretaceous in age and related to 

the final stages of emplacement and cooling of the Idaho 

batholith.

Mineralized rock within granodiorite of the potassic suite is 

similar to that of the sodic suite; however, in addition to gold 

and galena, molybdenite is present, and host rocks have locally 

undergone potassic alteration. Similar to the sodic suite, this 

mineralization is probably Late Cretaceous in age.

Despite a clear association between Eocene biotite granite 

plutons and widespread propylitic alteration, mineralized rock in 

the Eocene biotite granite suite is limited to a small number of 

quartz veins and minor amounts of tungsten. None of the veins 

in the older rocks surrounding the biotite granite is known to be 

related to the intrusion of these plutons. Mineralized areas 

within rocks of the quartz monzodiorite suite are characterized 

by veins or stockworks(?) rich in copper, molybdenum, and silver. Although these mineralized areas are geochemically similar 

to porphyry copper deposits, they are not extensive, nor are they 

associated with widespread hydrothermal alteration, as is the 

case for the porphyry-type deposits.

The mineral potential of the eastern part of the Soldier 

Mountains is moderate to low relative to other areas in central 

Idaho; however, additional discoveries of vein-type gold deposits within the plutonic rocks of Cretaceous age are possible. The 

rocks of the potassic suite are more favorable than those of the 

sodic suite, but they are not extensively exposed. Favorable 

Figure 4. Schematic cross sections illustrating characteristics of hydrothermal alteration types of Cretaceous 

(A) and Eocene (B) age in the Soldier Mountains study area, Idaho. Erosion has removed most of the metasedimentary rocks shown on the left, although roof pendants, too small to show on figure 2, are still preserved (see 

Lewis, 1991).

Conclusions
12 Alteration and Mineralization, Eastern Part of the Soldier Mountains, Camas County, Idaho

structures for mineralization include the north trending fault 

north of the Five Points mine and the series of northeast-trending 

faults of the Worswick Creek fault system. Plutonic rocks of the 

Eocene biotite granite suite are not likely to contain substantial 

mineralized rock, but the possibility exists for porphyry-type 

copper-molybdenum-silver deposits is in rocks of the quartz 

monzodiorite suite. None of the known areas of mineralized 

rock is of the size or intensity, however, that is characteristic of 

major porphyry systems.

References

Allen, R.M., Jr., 1952, Geology and mineralization of the Volcano district, 

Elmore County, Idaho: Economic Geology, v. 47, no. 8, p. 815–821.

Anderson, A.L., 1934, A preliminary report on recent block faulting in Idaho: Northwest Science, v. 8, p. 17–28.

Bennett, E. H., in press, Geology and mineral deposits of part of the western half of the Hailey 1°×2° quadrangle, Idaho, with a section on the

Neal mining district by Thor H. Kiilsgaard and Earl H. Bennett and a 

section on the Dixie mining district by Thomas M. Jacob: U. S. Geological Survey Bulletin 2064-W.

Bennett, E.H., and Knowles, C.R., 1985, Tertiary plutons and related rocks 

in central Idaho, in McIntyre, D.H., ed., Symposium on the geology 

and mineral deposits of the Challis 1°×2° quadrangle, Idaho: U. S. 

Geological Survey Bulletin 1658-F, p. 81–96

Bottinga, Y., and Javoy, M., 1973, Comments on oxygen isotope geothermometry: Earth and Planetary Science Letters, v. 20, p. 250–265.

Cambell, A., 1940, Mining Industry of Idaho for the year 1939: Mine 

Inspectors Reports, State of Idaho, p. 184.

Cluer, J.K., and Cluer, B.L., 1986, The late Cenozoic Camas Prairie rift, 

south-central Idaho: Contributions to Geology, University of Wyoming, v. 24, no. 1, p. 91–101.

Cook, E.F., 1956, Tungsten deposits of south-central Idaho: Idaho Bureau 

of Mines and Geology Pamphlet 108, 40 p.

Criss, R.E., 1981, An 18O/16O, D/H and K-Ar study of the southern half of 

the Idaho batholith: Pasadena, California Institute of Technology, 

Ph.D. thesis, 401 p.

Criss, R.E., and Champion, D.E., 1984, Magnetic properties of granitic 

rocks from the southern half of the Idaho batholith—Influences of 

hydrothermal alteration and implications for aeromagnetic interpretation: Journal of Geophysical Research, v. 89, no. B8, p. 7061–

7076.

Criss, R.E., and Taylor, H.P., Jr., 1983, An 18O/16O and D/H study of Tertiary 

hydrothermal systems in the southern half of the Idaho batholith: 

Geological Society of America Bulletin, v. 94, p. 640–663.

Darling, R.S., 1987, The geology and ore deposits of the Carrietown silverlead-zinc district, Blaine and Camas Counties, Idaho: Pocatello, Idaho State University, M.S. thesis, 168 p.

Darling, R.S., 1988, Ore deposits of the Carrietown silver-lead-zinc district, Blaine and Camas Counties, Idaho, in Link, P.K., and Hackett, 

W.R., eds., Guidebook to the geology of central and southern Idaho: 

Idaho Geological Survey Bulletin 27, p. 193–200.

Gehlen, W.T., 1983, The geology and mineralization of the eastern part of 

the Little Smoky Creek mining district, Camas County, Idaho: Moscow, University of Idaho, M.S. thesis, 136 p.

Guthrie, G.D, and Veblen, D.R., 1988, Transmission/analytical electron 

microscopy of turbid alkali feldspars from the Isle of Skye, Scotland: 

Eos, v. 69, no. 16, p. 291.

Kerrich, R., and Kamineni, D.C., 1988, Characteristics and chronology of 

fracture-fluid infiltration in the Archean, Eye Dashwa Lakes pluton, 

Superior Province—Evidence from H, C, O-isotopes and fluid 

inclusions: Contributions to Mineralogy and Petrology, v. 99, p. 

430–445.

Kiilsgaard, T.H., Fisher, F.S. and Bennett, E. H., 1986, The trans-Challis 

fault system and associated precious metal deposits, Idaho: Economic Geology, v. 81, p. 721–724.

Kiilsgaard, T.H., and Lewis, R.S., 1985, Plutonic rocks of Cretaceous age 

and faults in the Atlanta lobe of the Idaho batholith, in McIntyre, D.H, 

ed., Symposium on the geology and mineral deposits of the Challis 

1°×2° degree quadrangle, Idaho: U.S. Geological Survey Bulletin 

1658-B, p. 29–42.

Kiilsgaard, T. H., Lewis, R. S., and Bennett, E. H., in press, Plutonic and 

hypabyssal rocks of the Hailey 1°×2° quadrangle, Idaho: U. S. Geological Survey Bulletin 2064-U.

Lewis, R.S., 1989, Plutonic rocks in the southeastern part of the Idaho 

batholith and their relationship to mineralization, in Winkler, G.R., 

Soulliere, S.J., Worl, R.G., and Johnson, K.M., eds., Geology and 

mineral deposits of the Hailey and western Idaho Falls 1°×2° quadrangles, Idaho: U.S. Geological Survey Open-File Report 89-639, p. 

33–34.

Lewis, R.S., 1990, Geology, geochemistry, and mineral potential of Cretaceous and Tertiary plutons in the eastern part of the Soldier 

Mountains, Idaho: Corvallis, Oregon State University, Ph.D thesis, 

214 p.

Lewis, R.S., 1991, Geologic map and geochemical data of the eastern 

part of the Soldier Mountains, Camas County, Idaho: Idaho Geological Survey Technical Report 91-1.

Lewis, R.S., and Kiilsgaard, T.H., 1991, Eocene plutonic rocks in southcentral Idaho: Journal of Geophysical Research, v. 96, no. B8, p. 

13,295–13,311.

Lewis, R.S., Kiilsgaard, T.H., Bennett, E.H., and Hall, W.E., 1987, Lithologic 

and chemical characteristics of the central and southeastern part 

of the southern lobe of the Idaho batholith, in Vallier, T.L., and 

Brooks, H.C., eds., Geology of the Blue Mountains region of Oregon, 

Idaho, and Washington: U.S. Geological Survey Professional Paper 

1436, p. 171–196.

Martin, R.F., and LaLonde, A., 1979, Turbidity in K-feldspars—Causes 

and implications: Geological Society of America Abstracts with Programs, v. 11, p. 472–473.

McDowell, S.D., and Elders, W.A., 1980, Authigenic layer silicate minerals in borehole Elmore 1, Salton Sea geothermal field, California, 

U.S.A.: Contributions to Mineralogy and Petrology, v. 74, p. 293–310.

Mitchell, V.E., Ott, M.H., Vance, R.E., and Bennett, E.H., 1991, Mines and 

prospects of the Hailey quadrangle, (2nd ed.): Idaho Geological Survey Mines and Prospects Map Series, scale 1:250,000.

O’Neil, J.R., and Taylor, H.P., Jr., 1967, The oxygen isotope and cation 

exchange chemistry of feldspars: American Mineralogist, v. 52, p. 

1414–1437.

Piper, A.M., 1924, Ground water for irrigation on Camas Prairie, Camas 

and Elmore Counties, Idaho: Idaho Bureau of Mines and Geology 

Pamphlet 15, 46 p.

Reid, R.R., 1963, Reconnaissance geology of the Sawtooth Range: Idaho 

Bureau of Mines and Geology Pamphlet 129, 37 p.
13

Ross, C.P., 1930, Geology and ore deposits of the Seafoam, Alder Creek, 

Little Smoky, and Willow Creek Mining Districts, Custer and Camas 

Counties, Idaho: Idaho Bureau of Mines and Geology Pamphlet 33, 

26 p.

Schmidt, D.L., 1962, Quaternary geology of the Bellevue area in Blaine 

and Camas Counties, Idaho: U.S. Geological Survey Open-File 

Report 62-120, 92 p.

Shannon, E.V., 1926, The minerals of Idaho: Smithsonian Institution, 

United States National Museum Bulletin 131, 483 p.

Snee, L.W., and Kunk, M.J., 1989, 40Ar/39Ar thermochronology of mineral 

deposits in the southern part of the Idaho batholith, in Winkler, G.R., 

Soulliere, S.J., Worl, R.G., and Johnson, K.J.., eds., Geology and 

mineral deposits of the Hailey and western Idaho Falls 1°×2°

quadrangles, Idaho: U.S. Geological Survey Open-File Report 

89-639, p. 37–38.

Taylor, H.P., Jr., and Margaritz, M., 1976, An oxygen and hydrogen 

isotope study of the Idaho batholith: Eos, v. 43, p. 57, p. 350.

Taylor, H.P., Jr., and Margaritz, M., 1978, Oxygen and hydrogen isotope 

studies of the cordilleran batholiths of western North America, in

Robinson, B.W., ed., Stable isotopes in the earth sciences: New 

Zealand Department of Scientific and Industrial Research Bulletin, 

v. 220, p. 151–173.

Umpleby, J.B., 1915, Ore deposits in the Sawtooth quadrangle, Blaine 

and Custer Counties, Idaho: U.S. Geological Survey Bulletin 580, p. 

221–249.

Worl, R.G., Kiilsgaard, T.H., Bennett, E.H., Link, P.K., Lewis, R.S., Mitchell, 

V.E., Johnson, K.M., and Snyder, L.D., 1991, Geologic map of the 

Hailey 1°×2° quadrangle, Idaho: U.S. Geological Survey Open-File 

Report 91-340, scale 1:250,000.

References