Geology, Geochemistry, and

Mineral Resources of the Lower Part of the 

Middle Fork Boise River Drainage Basin,

Boise and Elmore Counties, Idaho

By Thor H. Kiilsgaard and Cole L. Smith

U.S. Geological Survey Bulletin 2064-AA

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-aa/

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 March 22, 2001

Graphics by Joe and Alice Springfield

Photocomposition by Gayle M. Dumonceaux
III

Contents

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

Geology........................................................................................................................................... 1

Plutonic and Hypabyssal Rocks ...................................................................................... 1

Plutonic Rocks of Cretaceous Age ........................................................................ 1

Plutonic Rocks of Eocene Age ............................................................................... 2

Dioritic Suite ...................................................................................................... 2

Biotite (Pink) Granite ........................................................................................ 2

Hypabyssal Rocks of Probable Eocene Age ........................................................ 3

Sedimentary Rocks............................................................................................................ 4

Payette Formation..................................................................................................... 4

Unconsolidated Sediments ..................................................................................... 4

Glacial Moraine................................................................................................. 4

High Gravel ........................................................................................................ 4

Terrace Gravel .................................................................................................. 4

Alluvium.............................................................................................................. 4

Structure.............................................................................................................................. 4

Geochemical Studies ................................................................................................................... 5

Data Handling ..................................................................................................................... 5

Evolved Granites ................................................................................................................ 6

Geochemical Study Areas................................................................................................ 6

Mt. Heinen Area........................................................................................................ 6

Grand Mountain Area .............................................................................................. 7

Sheep Creek Area..................................................................................................... 9

Swanholm Antimony District............................................................................................ 13

Hermada Antimony Mine......................................................................................... 15

Svensson Antimony Prospect................................................................................. 16

Weatherby Prospect ................................................................................................ 16

Anderson-Bida Property.......................................................................................... 17

Discussion.................................................................................................................. 17

Middle Fork Boise River Placers ..................................................................................... 17

Mineral Resources ....................................................................................................................... 18

Placer Resources .............................................................................................................. 18

Potential Lode-Deposit Resources ................................................................................ 20

References .................................................................................................................................... 21

Figures

1-3. Maps showing:

1. Lower part of the Middle Fork Boise River drainage basin,

Boise and Elmore Counties, Idaho ............................................................................ 2

2. Simplified geology of the Hailey 1°×2° quadrangle ................................................ 3

3. Location of geochemical study areas and Swanholm antimony district ............ 5
IV

4-5. Bar graphs showing average rank of:

4. Selected elements in stream sediment samples from geochemical study

areas and from the regional geochemical survey of the

Hailey 1°×2° quadrangle .......................................................................................... 6

5. Selected elements in panned-concentrate samples from study areas ............ 6

6-8. Maps showing geochemical study areas showing locations of

stream-sediment and panned-concentrate samples:

6. Mt. Heinen.................................................................................................................... 7

7. Grand Mountain and Swanholm antimony district ............................................... 9

8. Sheep Creek ................................................................................................................ 12

9. Geologic sketch map of the Swanholm antimony district ......................................... 13

10. Sketch showing geology of the northern part of the upper bench,

Hermada pit....................................................................................................................... 15

11. Photographs of the Twin Springs placer district ........................................................ 20

1. Analytical determinations of nonmagnetic fraction of panned-concentrate

from the Mt. Heinen study area ..................................................................................... 8

2. Analytical determinations of stream-sediment samples and nonmagnetic

fraction of panned concentrate samples from Grand Mountain

geochemical study area and Swanholm antimony .................................................... 10

3. Analytical determinations of nonmagnetic fraction of panned-concentrate

samples from Sheep Creek geochemical study area ................................................ 14

4. Gold and silver production from placers along or near the Middle Fork

Boise River from 1901–1947 ............................................................................................ 18

5. Gold values of placers mined along Middle Fork Boise River .................................. 19

6. Fineness of placer gold mined along Middle Fork Boise River................................. 19

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

Tables
1

Geology, Geochemistry, and Mineral 

Resources of the Lower Part of the 

Middle Fork Boise River Drainage Basin, 

Boise and Elmore Counties, Idaho

 

Introduction

Geologic mapping of the western part of the Hailey 1°×2° 

quadrangle and geochemical sampling of sediments from various streams that drain into the Middle Fork of the Boise River 

indicate that three areas in the drainage basin contain anomalous 

amounts of one or more elements, but no mines. The three areas 

are in the vicinity of Mt. Heinen, south of Grand Mountain, and 

south and west of Sheep Mountain. A fourth area, in the vicinity 

of Swanholm Creek, contains anomalous amounts of antimony, 

and one mine, the Hermada, which has produced significant 

amounts of antimony. Placer gold has been mined at a number 

of sites along the Middle Fork Boise River.

The study area covers approximately 385 mi2 in the lower 

part of the Middle Fork of the Boise River drainage basin, in 

Boise and Elmore Counties, Idaho. It extends about 43 mi 

upstream along the Middle Fork from near the Arrowrock Dam 

to the junction of the Middle Fork and the Yuba Rivers (Worl 

and others, 1991). The approximate boundary of the study area 

is shown on figure 1. 

Geology

The oldest plutonic rocks in the study area are principally 

biotite granodiorite and muscovite-biotite (two-mica) granite 

(fig. 2, units Kgd, Kg) of the Idaho batholith of Cretaceous age. 

These rocks are intruded by younger (Eocene) dioritic rocks and 

biotite granite units (Td, Tg). Intruding these plutonic rocks is a 

myriad of hypabyssal dikes of Tertiary age that range in composition from basalt to rhyolite. Cutting all of the rocks are steep 

faults, most of which strike northeast or northwest. Patches of 

unconsolidated sand, gravel, and cobbles are present locally. 

Some exposures of these unconsolidated rocks cap ridges, but 

most are along or near the Middle Fork Boise River and at the 

mouths of various tributary streams that empty into the Middle 

Fork. Many of the gravel exposures contain placer gold.

Plutonic and Hypabyssal Rocks

Plutonic Rocks of Cretaceous Age

Light-gray, medium- to coarse-grained, equigranular to 

porphyritic biotite granodiorite (fig. 2, Kgd) (Worl and others, 

1991) is widespread in the study area. Excellent exposures of 

this rock crop out along the Middle Fork Boise River, from 

near the mouth of the North Fork Boise River upstream to 

beyond Atlanta. Mineralogic and chemical characteristics of 

the biotite granodiorite are presented in Kiilsgaard and others 

(in press).

Along the ridge that extends southwest from Mt. Heinen 

are a number of exposures of altered biotite granodiorite that 

were mapped as silicified granodiorite by Scanlan (1986, unit 

Ksi) and as altered granodiorite by Worl and others (1991, unit 

Kai). Feldspar in the rock is altered to sericite and clay minerals. Biotite is altered to chlorite or to various iron oxide minerals that stain the rock a rusty color. Locally, the altered rock is 

moderately to intensively silicified, either by disseminated silica flooding or by countless veinlets of quartz that commonly 

are 2 mm or less thick. Exposures of the rock also are present 

south of Mt. Heinen and to the southeast as far as 2.5 mi southeast of Cottonwood Creek. All mapped exposures of the rock 

are near younger intrusive rocks of Eocene age. The field relationships and alteration features indicate that the alteration is 

the result of hydrothermal processes associated with Eocene 

intrusive activity.

Muscovite-biotite (two-mica) granite (fig. 2, unit Kg) is 

exposed in the vicinity of Grand Mountain and Swanholm 

Creek (Worl and others, 1991) (fig. 2). The rock is similar to 

biotite granodiorite (unit Kgd), the major differences being the 

presence of laminated plates or books of primary muscovite that 

are large enough to recognize in hand specimen. Books of 

primary muscovite are a field criterion that was used both in the 

Hailey 1°×2° quadrangle and in the Challis 1°×2° quadrangle to 

the north (Fisher and others, 1992) to differentiate two-mica 

granite from biotite granodiorite.

By Thor H. Kiilsgaard Cole L. Smith and 
2 Geology, Geochemistry, and Mineral Resources, Middle Fork Boise River Drainage Basin, Idaho

Mt Heinen

Grand Mtn

Atlanta

Swanholm Peak

Steel Mtn

Sheep Mtn

Grape Mtn

Arrowrock

Dam

Approximate boundary

of study area Black Warr oi r Cr

Willow

Cr

North Fork Boise River

Middle

Fork Boise

River Eagle Cr

Queens River

River

Yuba

Phifer Cr

Hot Cr

Big

Five Cr

Swanholm

Creek

Browns

Cr

Devils Cr

Sheep Cr

Cottonwood Cr

Placer locality

0 5 MILES EXPLANATION

43 ̊

30'

44 ̊

116 ̊ 115 ̊

Plutonic Rocks of Eocene Age

Two categories of Eocene plutonic rocks are recognized in 

the study area: a dioritic suite of rocks and biotite granite (commonly called pink granite). Both categories of plutonic rocks 

intrude the more widely exposed Cretaceous rocks (fig. 2). 

Mineralogic and chemical characteristics of the Eocene rocks 

are discussed in Kiilsgaard and others (in press).

Dioritic Suite

The dioritic suite of rocks (fig. 2, unit Td) is composed of 

gabbro, diorite, quartz monzodiorite, and hornblende-biotite 

granodiorite, all of Eocene age. The large exposure of these 

rocks in the western part of the study area has a pronounced 

northeast trend (fig. 2). Three small stocks of gabbro (shown as 

diorite (unit Td) on Worl and others, 1991) crop out on ridges 

northeast of Mt. Heinen and north of Cottonwood Creek. Near 

the western border of the study area, southwest of Mt. Heinen, 

are a number of exposures of diorite, most of them near or 

peripheral to larger exposures of quartz monzodiorite or hornblende-biotite granodiorite (Scanlan, 1986). Quartz monzodiorite, a rock containing less than 20 percent quartz, makes up 

most of the western part of the block shown as dioritic suite in 

figure 2. From the vicinity of Mt. Heinen and to the northeast, 

the quartz content of the rock increases to more than 20 percent; this increase in quartz content changes the classification 

of the rock from quartz monzodiorite to hornblende-biotite 

granodiorite (Streckeisen, 1973).

Biotite (Pink) Granite

Biotite (pink) granite is the principal rock of the northeasttrending Sheep Creek batholith and the Sawtooth batholith (fig. 

2, unit Tg). Coarse crystals of potassium feldspar account for 

the pink color that is characteristic of the rock, although at some 

localities finer grained and medium-grained varieties of the rock 

are not pink but light gray.

Stocks and batholiths of pink Eocene granite are widely 

exposed in central Idaho, and these rocks have geochemical 

Figure 1. Map showing lower part of the Middle Fork Boise River drainage basin, Boise and Elmore Counties, Idaho. Location of placer mine sites 

(diamonds) and approximate boundary of study area identified.
3

Biotite granodiorite

Dioritic suite

Muscovite-biotite granite

Biotite granodiorite—Includes pegmatite-rich areas

Hornblende-biotite tonalite

Potassium-rich hornblende-biotite granodiorite

Quartz diorite

Contact

Fault

EXPLANATION

Kgdk

Kqd

Kgd

Kt

Tg

Td

Kg

0 10 20 MILES

0 10 20 30 KILOMETERS

43 ̊

43 ̊

30'

44 ̊ 114 ̊ 115 ̊ 116 ̊

SHEEP CREEK 

BATHOLITH

SAWTOOTH BATHOLITH

Mountain Home

Atlanta

Idaho

City

Hailey

Fairfield

Approximate

boundary

of study area

affinities that are typical of A-type (evolved) granite (Lewis and 

Kiilsgaard, 1991). Relative to other granitic rocks, evolved 

granite such as that of the Sheep Creek batholith is enriched in F 

and in incompatible elements including U, Th, Li, Be, Rb, Cs, 

Nb, and Y and is depleted in compatible elements such as Ca, 

Mg, Ba, and Sr. Concentrations of incompatible and other elements in the Sheep Creek batholith are discussed in a later section of this report entitled “Sheep Creek area.”

A distinctive characteristic of Eocene pink granite in the 

central part of Idaho is its content of radioactive elements 

(Swanberg and Blackwell, 1973; Bennett, 1980a, b; Bennett and 

Knowles, 1985; Kiilsgaard and others, 1970). Commonly, pink 

granite is two to four times more radioactive than other Eocene 

or Cretaceous plutonic rocks of the region. Unpublished 

National Uranium Resource evaluation (NURE) airborne 

gamma-ray spectrometry surveys for uranium and thorium 

clearly outline the Sheep Creek batholith (Bennett, 1980b).

Hypabyssal Rocks of Probable Eocene Age

Countless dikes that range from andesite to rhyolite in 

composition occur in the Cretaceous and Eocene plutonic rocks 

of the study area (Kiilsgaard and others, in press). The dikes 

may be classified as a rhyolitic suite of light to red-tinted felsic 

dikes and a suite of dark-green to grayish intermediate to mafic 

dikes. Rhyolitic dikes are most numerous in or near Eocene 

Geology

Figure 2. Simplified geologic map of the Hailey 1°×2° quadrangle, Idaho, showing location of study area (solid heavy line).
4 Geology, Geochemistry, and Mineral Resources, Middle Fork Boise River Drainage Basin, Idaho

biotite granite. Excellent examples of northeast-trending rhyolite dikes crop out in the Eocene biotite granite along the southfacing canyon wall of the Middle Fork Boise River near Twin

Springs and in the canyon wall to the west. The similarity in

chemical composition between the rhyolite and the intruded

biotite granite suggests a genetic relationship. Intermediate to

mafic dikes tend to be concentrated in or near exposures of the

Eocene dioritic plutonic rocks, and similarities in chemical composition between the dikes and the dioritic plutonic rocks suggest a similar genetic relationship.

The crosscutting dikes strike in diverse directions, but the

majority trend northeast, more or less in the same direction as

the Eocene plutonic rocks. Emplacement of the dikes and the

Eocene plutonic rocks probably was guided by the northeaststriking pattern of regional faults and associated shears that are

widespread throughout central Idaho.

Sedimentary Rocks

Payette Formation

On the divide at the head of Swanholm Creek, at an altitude

of 5,640 ft, beginning about 500 ft east of the road to Atlanta and

continuing as a ridge cap to the northeast for about 2,000 ft, is an

exposure of stratified sandstone and interbedded gravel that is

correlated with the Miocene Payette Formation. The sandstone

is loosely consolidated, and the grain size ranges from fine to

coarse. Some thin beds of very fine grained material are probably volcanic ash.

Bedding of the sandstone strikes N. 40°–50° W. and dips

10° SW. The exposed unit is about 250 ft thick. It is in a downdropped fault block that is bounded on the east by the Deer Park

fault and on the west by the Swanholm Creek fault, and it may

be a remnant of a far larger expanse of the formation.

Unconsolidated Sediments

For purposes of field geologic mapping, unconsolidated

sediments of the study area were classified as glacial moraine,

high gravel, terrace gravel, and alluvium.

Glacial Moraine

Glacial moraine consisting of unsorted boulders, some 5 ft

or more in long dimension, cobbles, pebbles, and sand extends

along the upper reaches of Hot Creek, which drains the northern

side of Steel Mountain. Morainal debris is scattered along Hot

Creek for a distance of about 3 mi, extending from an altitude of

about 5,800 ft at its lower point to 7,600 ft at its upper end. The

moraine was formed by Pleistocene alpine glaciation that

scoured the northern and northeastern sides of Steel Mountain.

High Gravel

High gravel of probable Pleistocene age is present as isolated patches of boulders, cobbles, gravel, and sand high on the

valley walls of the Middle Fork Boise River. Deposits of high

gravel also are present at the heads of tributaries leading to the

river and as caps on some ridges. The well-rounded nature of

the boulders and cobbles indicates that they were deposited by

rapidly eroding, high-energy streams, such as were active during Pleistocene time when abundant water was derived from

melted glacial ice.

A patch of ridge-capping high gravel about 800 ft long is

on the divide at the head of Swanholm Creek, near the foot of

Swanholm Peak. The base of this patch is at an altitude of 6,200

ft, which is 1,840 ft above the Middle Fork Boise River at the

mouth of Swanholm Creek. Some of the high gravel deposits

along the Middle Fork Boise River, and north of the study area

in the vicinity of Idaho City, are gold bearing and have been

placer mined at localities where miners could get water to the

deposit. At some high-gravel deposits, old ditches indicate that

the water for placer mining was derived from melting snow.

Terrace Gravel

Terrace gravel is present in terraces along the Middle Fork

Boise River. Some terraces are as much as 60 ft above river

level. The terraces consist of boulders, coarse to medium

gravel, and sand. Most of the boulders are of resistant rhyolite

or fine-grained aplite.

Much of the terrace gravel probably is reworked high

gravel, the tops of the terraces marking the former valley floor.

Erosion into the old valley floor by the river removed most of

the old gravel but left behind the present remnant terraces.

Terrace gravel along the river walls is gold bearing and has been

placer mined at many locations.

Alluvium

Alluvial sand, gravel, and boulders cover most of the

Middle Fork valley floor. At the mouths of some of the larger

tributary streams, the alluvium forms fans that extend out onto

the valley floor of the river. Locally, the alluvium is only a few

feet thick, but at some places it extends to unknown depths. Valley alluvium along the Middle Fork is gold bearing, particularly

at its contact with underlying bedrock.

Structure

Major faults in the study area strike either northeast or

northwest (Worl and others, 1991). Northeast-striking faults

probably are part of the Trans-Challis fault system (Bennett

and Knowles, 1985; Kiilsgaard and Lewis, 1985; Kiilsgaard

and others, 1989; Fisher and others, 1992). The age of faulting is unknown but could be as old as Precambrian (Lopez,

1981). Field evidence both in the Hailey 1° 2° quadrangle and

in the Challis 1° 2° quadrangle to the north (Worl and others,

1991; Fisher and others, 1992) indicates that northeast-trending faults guided emplacement of most Eocene plutonic rocks

and dikes. Subsequent left-lateral movement along some
5

Approximate

boundary

of study area

Outcrop of Eocene granite

Boundary of geochemical study area

Boundary of Swanholm antimony district

EXPLANATION

0 5 MILES

43 ̊

30'

44 ̊

116 ̊ 115 ̊

Mt Heinen

Sheep Mtn

Grand Mtn

northeast-trending faults in the Challis quadrangle has displaced some Eocene plutonic rocks.

The most conspicuous northeast-striking fault in the study 

area is marked by a number of sheared and altered rock exposures along a 19-mi stretch of the Middle Fork Boise River. The 

exposures extend from near the mouth of Cottonwood Creek to 

a localities about 1.5 mi west of the mouth of Browns Creek, 

where surface traces of the fault are lost on the weathered, 

south-facing slopes of the ridge that forms the north wall of the 

Middle Fork canyon (fig. 1) (Worl and others, 1991). The best 

exposure of the fault zone is in the north bank of the Middle 

Fork road about 1 mi northeast of the mouth of Willow Creek; 

here, the Eocene biotite granite host rock has been sheared and 

altered in a zone more than 400 ft wide. Parts of the sheared 

zone consist, essentially, of white, altered material that is so 

pulverized that the original rock type cannot be identified. Elongation of northeast-trending Eocene plutonic rocks along the 

fault zone and alignment of swarms of Tertiary dikes with the 

fault zone suggest that initial fault movement was pre-Eocene; 

however, the above-described exposure of sheared Eocene 

biotite granite denotes definite post-Eocene fault adjustment. 

Eocene biotite granite in apparent fault contact with Cretaceous 

biotite granodiorite, east of the mouth of Big Five Creek, also 

indicates post-Eocene adjustment along the northeast-striking 

fault. The direction of movement along the fault could not be 

determined. A number of hot springs, including those at Twin 

Springs, at the mouth of Sheep Creek, and at other locations 

along the Middle Fork Boise River from above the mouth of the 

North Fork Boise River to the area west of the mouth of Browns 

Creek, are along or near the fault.

Several major northwest-striking faults cross the study area 

(Worl and others, 1991). Of these, the Deer Park fault may be 

the longest. It has been mapped for a strike length of about 50 

mi. A splay of this fault follows Swanholm Creek (fig. 9) and is 

referred to as the Swanholm fault. The northeast end of the 

study are is crossed by the Montezuma fault, another major 

northwest-striking fault (Anderson, 1939; Reid, 1963; Kiilsgaard and others, 1970; Kiilsgaard and Lewis, 1985; Kiilsgaard 

and Bacon, in press). The Deer Park and Montezuma faults are 

similar in that rocks on the southwest side of each are downdropped and both display prominent scarps that are conspicuous 

topographic features. 

Geochemical Studies

The following three areas (fig. 3) within the study area are 

anomalous in one or more of the suites of elements that were used 

to interpret the geochemistry of the Hailey 1°×2° quadrangle 

(Smith, 1995): the Mt. Heinen vicinity; south of Grand Mountain; and south and west of Sheep Mountain. There is no mining 

activity in any of these areas and little evidence of extensive mineral exploration. Geochemical anomalies extend well beyond 

any of the known prospects. These three areas are referred to 

herein as the “geochemical study areas.” A fourth area, the 

Swanholm antimony district (figs. 3, 9), contains the Hermada 

antimony mine and several antimony prospects. Many streamsediment samples from this area were anomalous in antimony.

Each of the geochemical study areas is dominated by different groups of elements. To better characterize the geochemistry of these areas, samples of both stream sediments and the 

heavy-mineral fraction of stream sediments (panned concentrates) were collected, but little attempt was made to find the 

sources that contributed anomalous concentrations of elements 

to the stream sediments. Gold, tungsten, and tin tend to be concentrated in the heavy-mineral fraction of stream sediments; 

thus, panned concentrates were collected to check for these elements. The samples were analyzed at U.S. Geological Survey 

laboratories in Denver, Colorado. Methods of sample handling 

and analysis and the geochemical data are given in Malcolm and 

Smith (1992).

As used in this report, the terms “anomalous concentration” or “anomalous sample” refer to exceptionally high concentrations of elements of interest in stream-sediment or pannedconcentrate samples. Generally, an analytical value that is two 

standard deviations above the regional mean for that element in 

stream sediments is considered anomalous. For some elements 

that are rarely detected in stream sediments, such as gold, silver, 

tin, or bismuth, the mere detection of the element in the sample 

is sufficient to consider the sample anomalous.

Data Handling

To characterize the geochemistry of the geochemical study 

areas, two data sets were formed. Panned concentrate data for 

each area were combined into one data set, and stream-sediment sample data from each area were combined with streamsediment data for the western part of the Hailey 1°×2°quadrangle to form the other data set. Because elements have different 

Geochemical Studies

Figure 3. Map showing location of geochemical study areas, 

Swanholm antimony district, and outcrops of Eocene granite.
6 Geology, Geochemistry, and Mineral Resources, Middle Fork Boise River Drainage Basin, Idaho

Grand Mountain geochemical study area

Mount Heinen geochemical study area

Sheep Mountain geochemical study area

Regional geochemical survey

EXPLANATION

AVERAGE RANK

SELECTED ELEMENTS IN STREAM SEDIMENTS

1400

1200

1000

800

600

400

200

0

Be Li Th Y

Grand Mountain geochemical study area

Mount Heinen geochemical study area

Sheep Mountain geochemical study area

EXPLANATION

AVERAGE RANK

SELECTED ELEMENTS IN PANNED CONCENTRATES

80

70

60

50

40

30

20

10

0

Sb Bi Pb Mo W Sn Be Au Ag

concentration ranges in different geologic environments, a ranking system was designed that display, on a common scale, elemental concentrations for geologically different areas. The rank 

of a particular element was determined as described in the following example: If a sample set consisting of concentrations of 

silver contains 20 samples, the sample that has the highest silver 

concentration is assigned a rank of 20, the sample that has the 

next highest concentration of silver is assigned a rank of 19, and 

so forth. The average rank of each element in each geochemical 

study area was calculated for each data set. The average ranks 

of selected elements in each of the areas are shown in figures 4 

and 5. This ranking procedure is explained in more detail in 

Smith (1995).

Evolved Granites

Evolved igneous rocks can be associated with economic 

deposits of some elements, and it is important to recognize these 

rocks for the purposes of resource assessment. Lewis and 

Kiilsgaard (1991) indicated that Eocene biotite granite in the 

study area has geochemical affinities of A-type (evolved) granite. As summarized by Hannah and Stein (1990), evolved granite and rhyolite have characteristics related to their tectonic 

setting, the chemistry and mineralogy of their source, timing and 

mechanism of fluid release, and differentiation processes. These 

rocks also are distinguished by their distinctive chemical composition. Relative to other granite and rhyolite, these rocks are 

enriched in F and in incompatible elements such as U, Th, Li, 

Be, Rb, Cs, Nb, and Y and are depleted in compatible elements 

such as Ca, Mg, Ba, and Sr. In the Hailey 1°×2° quadrangle, 

study of the enrichment and depletion of these elements in 

stream sediments, soils, and concentrates helps to identify associated granite and rhyolite as evolved rocks. As shown in figure 

4, relative to samples collected for the regional geochemical survey, samples from the three study areas are enriched in some of 

the elements that characterize evolved rocks.

Geochemical Study Areas

Mt. Heinen Area

Mt. Heinen is near the center of the Mt. Heinen geochemical area (fig. 3). The eastern side of the area is drained by 

Cottonwood Creek and its various tributaries (fig. 6), whereas 

the western side is drained by Smith, Dunnigan, and Volquelin 

Creeks. Geologically, the area is underlain chiefly by Eocene 

dioritic rocks that include diorite, quartz monzodiorite, and 

hornblende-biotite granodiorite. Eocene biotite granite crops 

out on the southeast side of Mt. Heinen. All of the Eocene rocks 

are components of the large, northeast-trending block of plutonic rocks that forms a striking geologic feature of the area (fig. 

2) (Worl and others, 1991). The Eocene rocks intrude Cretaceous biotite granodiorite of the Idaho batholith, and west and 

south of Mt. Heinen the intrusive action of the younger Eocene 

rocks has had a profound alteration affect on the intruded Cretaceous rocks (Kiilsgaard and others, in press). There are a few 

prospect pits in the area, but no evidence of mineral production.

Stream-sediment samples from the Mt. Heinen area contain anomalous concentrations of bismuth, molybdenum, silver, 

and lead, as shown in the regional set of stream-sediment 

sample data (Smith, 1995). Panned-concentrate samples from 

Figure 4. Bar graphs showing average rank of selected elements in 

stream-sediment samples from three geochemical study areas and from 

the regional geochemical survey of the Hailey 1°×2° quadrangle, excluding areas that might contain Paleozoic rocks.

Figure 5. Bar graph showing average rank of selected elements in 

panned concentrate samples from the three geochemical study 

areas.
7

0 1 MILE

43 ̊

37'

30"

43 ̊

42'

115 ̊52'30" 115 ̊47'

Mt Heinen

1KS

2KS

3KS

4KS

5KS

6KS

7KS

3CS

4CS

5CS

6CS

7CS

8CS

9CS

1CB

2CB

3CB

4CB

5CB

6CB

Creek

Cottonwood

North

Fork

Virgil

Gulch

Wood Gulch

Garden Gulch

Ranger Creek

Creek Boren

the area are enriched in silver, bismuth, lead, molybdenum, 

tungsten and to a lesser degree, tin (fig. 5, table 1).

Some stream-sediment samples, from streams flowing west 

from Mt. Heinen, contain anomalous concentrations of one or 

more of the elements silver, molybdenum, copper, lead, cadmium, and bismuth (Scanlan, 1986). Some of the anomalous 

values in stream-sediment samples from Voquelin Gulch may 

have been derived from northwest-striking veins that crop out on 

the ridge at the head of Voquelin Gulch, a ridge that extends 

southwest from Mt. Heinen. In the headwaters of Smith Creek, 

a stream that flows northwest from Mt. Heinen, no vein exposures were found that could account for anomalous samples collected along that stream. A stream-sediment sample collected by 

Scanlan (1986) in the upper stretches of Deer Creek, southwest 

of Mt. Heinen, contained 2 parts per million (ppm) Bi. Steamsediment samples collected east of Mt. Heinen, in the upper 

stretch of North Fork Cottonwood Creek, and from the mouths 

of Wood, Garden, and Toms Gulches, contain anomalous concentrations of one or more of the elements silver, molybdenum, 

cadmium, and bismuth.

The initial collection of stream-sediment samples was 

followed by a more intensive effort to collect panned-concentrate samples (fig. 6, table 1). Of particular significance in the 

panned-concentrate samples are the high concentrations of 

tungsten (table 1). Scheelite was recognized in the pannedconcentrate samples but was not identified in any outcrops. No 

mineralized exposure extensive enough to account for the anomalous metal content in stream sediments from Virgil, Wood, 

Garden, and Toms Gulches and from Ranger and Boren Creeks 

were found.

Grand Mountain Area

The Grand Mountain geochemical study area (fig. 3) 

extends from Grand Mountain south to the Middle Fork Boise 

River and east to beyond the East Fork of Swanholm Creek. It is 

underlain by biotite granodiorite and muscovite-biotite (twomica) granite of Cretaceous age. These rocks are intruded by 

many porphyritic dikes of Tertiary age, most of which strike 

northeast (Worl and others, 1991). Principal streams of the area 

are Browns, Swanholm, Granite, and Dutch Creeks. Comparison of stream-sediment sample data from the Grand Mountain 

area (table 2) with data obtained from the regional study of the 

Hailey 1°×2° quadrangle shows that the Grand Mountain samples are enriched in beryllium and lithium (fig. 4), elements that 

are associated with the margins of evolved plutons. Eocene 

granite does not, however, crop out in the Grand Mountain area, 

and the source of the beryllium and lithium could be pegmatites 

associated with near-surface Eocene intrusive rocks. Initial 

stream sediment sampling in the Grand Mountain area also indicated anomalous concentrations of arsenic and antimony. 

Because the elements arsenic and antimony can serve as pathfinders to precious-metal deposits, the Grand Mountain area and 

the nearby Swanholm Creek antimony area were sampled in 

more detail. Stream-sediment and panned-concentrate sample 

localities are shown on figure 7, and analytical results are shown 

in table 2.

A number of gold-bearing panned-concentrate and streamsediment samples were collected in the Grand Mountain area 

(fig. 7). Placer gold has been mined extensively in nearby areas, 

along the Middle Fork Boise River and near Phifer and Swanholm Creeks (see section on Middle Fork Boise River placers, 

this report), and some placer gravels extend hundreds of feet 

above river level. The high gravel deposits that have been mined 

are remnants of extensive high gravel deposits that probably 

extended up lower reaches of the various nearby Middle Fork 

tributary streams. The highest elevation of nearby placer deposits is 5,000 ft; thus, it is reasonable to assume that ancestral high 

gravel did not exceed that elevation in any stream valleys in the 

Grand Mountain study area.

At present, there is no evidence of high gravel in any of the 

Middle Fork tributaries in the Grand Mountain area. All such 

gravel has been removed by erosion. Placer gold that formerly 

Geochemical Studies

Figure 6. Map showing Mt. Heinen geochemical study area and 

stream-sediment and panned concentrate sample localities (solid 

circles).
8 Geology, Geochemistry, and Mineral Resources, Middle Fork Boise River Drainage Basin, Idaho

was in the high gravel conceivably could have lagged behind 

during the course of erosion and thereby account for the gold in 

stream gravel, but this is unlikely along upper reaches of the 

streams because the tributary streams have steep gradients and 

are undergoing rapid erosion. Stream-bed gravel along the tributary streams is shallow to nonexistent and is the product of 

recent or current erosion. If placer gold in the panned-concentrate samples is lag gold derived from erosion of high gravel, it 

would be reasonable to expect placer gold to be present in all 

tributaries because all tributaries, to an elevation of 5,000 ft, 

likely once contained high gravel. Samples from some tributaries do not, however, contain placer gold (fig. 7), and thus most of 

the gold in the panned-concentrate samples probably was 

derived from local sources.

Of particular interest are panned-concentrate sample nos. 

12KS and 13KS, from upper reaches of Browns Creek (fig. 7), 

(sec. 19, T. 6 N., R. 9 E.), sample no. 09CB, from the left fork of 

Granite Creek (sec. 20, T. 6 N., R. 9 E.), sample no. 25KS, from 

the northeast fork of Dutch Frank Creek (W1/2, sec. 32, T. 6 N., 

R. 9 E.), and sample no. 36KS, from the mouth of the left fork of 

Dutch Creek, at an altitude of 5,000 ft. The first four samples 

were collected above the altitude of 5,000 ft; thus the gold content in each (table 2) could not have been derived from ancestral 

high gravel. Sample no. 36KS was collected at an altitude of 

5,000 ft, and it is possible that the contained gold originated in 

the high gravel, although for reasons given above, it is unlikely. 

Sample nos. 12KS and 13KS were collected on the west side of 

a long ridge that extends south from Granite Mountain and the 

gold in the samples presumably was derived from one or more 

deposits that crop out on the ridge. Sample no. 09CB was collected near the headwaters of the west fork of Granite Creek, 

within half a mile of the ridge crest. The source of the gold presumably is on the east side of the ridge, within the small drainage area above the sample site. The high gold value (300 ppm), 

for sample no. 25KS, is of special interest because, although the 

sample site is less than a quarter of a mile from the crest of the 

ridge, no contributing source was observed upstream in that 

short interval. Sample no. 36KS (700 ppm Au) from the mouth 

of the left fork of Dutch Creek is also interesting. The gold in 

the sample probably originated from erosion of an unknown 

deposit somewhere above the sample site, in the drainage area 

of the headwaters of that stream.

The gold values of panned-concentrate samples listed in 

table 2 should be viewed with caution. Sample handling in the 

field and laboratory can greatly enrich gold in the concentrate 

(Nash, 1988). In addition, the small size of the sample used for 

analysis (about 15 mg), coupled with the tendency of gold to be 

present as coarse grains or small nuggets, can have a profound 

effect on sample evaluation. A single grain of gold in a small 

sample gives a distorted value, which, if applied to a large 

Table 1. Analytical determinations of nonmagnetic fraction of panned concentrate samples from Mt. Heinen geochemical 

study area.

[Concentrations shown in parts per million. Sample localities shown on fig. 6]

Field 

No.

Location Ag Bi Cu Mo Pb Sn Th W

1CB South Fork, Garden Gulch 2 700 10 30 150 500 2,000 1,000

2CB North fork, Garden Gulch 5 300 100 150 1,000 500 >5,000 1,000

3CB Mouth, Garden Gulch <1 100 10 150 500 500 >5,000 300

4CB North fork, Wood Gulch 2 300 100 300 1,000 500 >5,000 1,000

5CB Mouth Wood Gulch 5 700 15 1,500 5,000 300 >5,000 1,000

6CB Mouth, Virgil gulch <1 <20 15 300 2,000 200 2,000 100

3CS West part, Boren Creek 2 500 <10 10 100 150 >5,000 500

4CS North fork, Ranger Creek <1 700 10 50 700 200 >5,000 150

5CS South fork, Ranger Creek 5 1,500 70 200 700 200 5,000 15,000

6CS Ranger Creek <1 500 <10 200 150 200 >5,000 1,000

7CS North fork near mouth,

Ranger Creek

<1 200 <10 <10 100 150 >5,000 <50

8CS Mouth, Boren Creek <1 200 <10 10 500 200 >5,000 500

9CS Mouth, Ranger Creek <1 300 15 10 150 300 >5,000 1,000

1KS North fork, Garden Gulch 10 1,500 100 300 1,000 200 >5,000 700

2KS South fork, Garden Gulch 10 2,000 20 30 500 700 >5,000 10,000

3KS Garden Gulch <1 500 10 150 700 300 >5,000 1,000

4KS Mouth, Toms Gulch 2 1,500 15 30 200 500 >5,000 300

5KS South fork, Wood Gulch 7 <20 10 100 300 200 >5,000 300

6KS North fork, Wood Gulch <1 <20 <10 10 30 500 2,000 <50

7KS Wood Gulch 5 700 <10 100 500 300 >5,000 2,000
9

0 1 MILE

43 ̊42'30"

115 ̊30' 115 ̊19'

GRAND MOUNTAIN

07CB

08CB

09CB

10CB

12CB

13CB

14CB

15CB

16CB

17CB

18CB

19CB

20CB

21CB

22CB

23CB

24CB

25CB

33KS

12KS

13KS 14KS

15KS

16KS

17KS

19KS

20KS

21KS

22KS

23KS

24KS

25KS 26KS

27KS

28KS

29KS

30KS

31KS

32KS

34KS

35KS

36KS

37KS

38KS

Middle Fork Boise River

Middle Fork Boise River

Browns

Creek

Swanholm

Creek

volume of gravel yields highly erroneous values. Nevertheless, 

the fact that placer gold was present in all of the streams that 

drain south from Grand Mountain raises speculation as to the 

source of the gold because no gold-bearing veins or mineralized 

zones were seen in the drainage area.

Sheep Creek Area

The Sheep Creek geochemical study area (fig. 3) is southwest of Sheep Mountain. It includes the upper part of Sheep 

Creek, the East Fork of Sheep Creek, the lower part of Devils 

Creek, and various tributaries of these streams and ridges 

between the streams (fig. 8). Sheep Creek is a major tributary to 

the Middle Fork Boise River. The entire Sheep Creek drainage 

basin is within the Sheep Creek batholith, which consists of 

Eocene biotite (pink) granite (Worl and others, 1991; Kiilsgaard 

and others, in press). Many northeast-striking rhyolitic dikes cut 

the Eocene granite.

Anomalous concentrations of various incompatible elements such as beryllium, lithium, thorium, niobium, and yttrium 

are characteristic of an evolved granite such as the Sheep Creek 

batholith. Anomalous concentrations of other elements such as 

fluorine, uranium, tin, and molybdenum have been reported 

from various exposures of Eocene granite in central Idaho 

(Kiilsgaard and others, 1970; Bennett, 1980b; Kiilsgaard, 1983). 

Because the entire Sheep Creek drainage is underlain by Eocene 

granite, it was decided to test for these elements by collecting a 

number of panned-concentrate samples from the principal 

streams and their tributaries (fig. 8) and by reviewing published 

and unpublished literature on the area.

The Blue Cloud beryl deposit is the only mineralized 

deposit in the area that has been explored in any way; the 

exploration consists of a few surface pits and bulldozer cuts. 

The deposit is west of the mouth of Devils Creek and south of 

Sheep Creek, in the NW1/4 sec. 14, T. 4 N., R. 7 E. In two 

mineralized zones, about 88 ft apart, are narrow 1-2 inch-wide 

northeast-striking quartz veinlets that contain small blue-green 

crystals of beryl, the maximum size of which is 0.2 by 0.75 in. 

Geochemical Studies

Figure 7. Map showing Grand Mountain geochemical study area and the Swanholm antimony district and stream sediment and panned 

concentrate sample localities. Circles indicate sample localities; solid circles indicate localities where anomalous amounts of gold was 

detected.
10 Geology, Geochemistry, and Mineral Resources, Middle Fork Boise River Drainage Basin, Idaho Table 2. Analytical determinations of stream-sediment samples and nonmagnetic fractions of panned concentra

geochemical study area and the Swanholm antimony area. [Concentrations in parts per million. Sample localities shown on figure 7] Field No. Au (AA) Ag Au Be Bi La Mo Nb Pb Sb 

07CB <0.05 <1 <20 <2 <20 >2,000 <10 2,000 100 <200 08CB <0.05 <1 <20 3 100 >2,000 <10 3,000 150 <200 09CB <0.05 20 20 3 <20 >2,000 <10 2,000 100 <200 <10CB <0.05 5 20 <2 100 >2,000 50 300 300 <200 <12CB <0.05 200 100 <2 150 1,500 <10 300 50 <200 13CB <0.05 <1 <20 <2 30 1,000 <10 300 150 500 314CB <0.05 2 <20 2 <20 >2,000 <10 700 100 <200 <15CB <0.05 5 <20 <2 150 2,000 <10 200 150 700 16CB <0.05 <1 <20 <2 500 >2,000 <10 500 70 <200 17CB <0.05 <1 <20 2 <20 >2,000 <10 500 100 <200 18CB <0.05 <1 <20 <2 <20 2,000 <10 70 30 <200 <19CB <0.05 <1 <20 <2 <20 >2,000 <10 150 50 <200 <20CB 1.8 <1 <20 <2 <20 >2,000 50 1,000 300 <200 21CB <0.05 <1 <20 <2 <20 >2,000 <10 200 70 <200 <22CB <0.05 <1 <20 <2 50 1,500 <10 300 100 <200 23CB <0.05 50 50 <2 <20 >2,000 <10 1,000 700 <200 <24CB 0.05 <1 <20 <2 50 >2,000 <10 500 200 <200 25CB 0.30 <1 <20 <2 <20 1,500 30 300 50 <200 

12KS 0.06 <1 <20 <2 <20 >2,000 <10 700 30 <200 13KS <0.05 150 200 2 500 >2,000 <10 3,000 200 <200 <14KS <0.05 <1 <20 3 150 >2,000 <10 2,000 150 <200 15KS <0.05 <1 <20 3 <20 >2,000 20 5,000 150 <200 <16KS <0.05 20 <20 5 100 >2,000 <10 1,500 100 <200 17KS <0.05 <1 <20 2 <20 >2,000 <10 2,000 100 <200 <18KS <0.05 <1 <20 3 700 >2,000 <10 3,000 700 <200 19KS <0.05 15 <20 3 1,000 >2,000 <10 1,500 300 <200 <20KS <0.05 <1 <20 3 <20 >2,000 <10 2,000 200 <200 
11

21KS <0.05 2 <20 2 300 >2,000 <10 1,500 200 <200 <22KS <0.05 <1 <20 2 200 >2,000 <10 1,500 150 500 <23KS <0.05 5 <20 2 500 >2,000 <10 3,000 70 <200 <24KS <0.05 <1 <20 15 150 1,500 <10 200 100 <200 25KS 0.13 100 300 2 300 >2,000 <10 150 50 <200 26KS <0.05 <1 <20 <2 <20 700 <10 100 30 <200 <27KS <0.05 <1 <20 <2 <20 2,000 <10 500 100 <200 28KS <0.05 15 <20 <2 300 2,000 <10 150 70 <200 <29KS 0.15 2 <20 <2 100 >2,000 30 1,000 50 <200 30KS <0.05 <1 <20 <2 <20 >2,000 <10 200 30 <200 <31KS <0.05 2 <20 <2 200 700 <10 70 20 <200 <32KS 0.05 2 <20 <2 <20 >2,000 <10 200 200 <200 <33KS <0.05 <1 <20 2 100 >2,000 <10 1,000 150 <200 34KS <0.05 <1 <20 <2 150 >2,000 <10 500 150 <200 35KS <0.05 2 <20 <2 <20 2,000 <10 70 30 <200 <36KS <0.05 300 700 <2 100 2,000 <10 200 50 <200 <37KS <0.05 70 100 <2 <20 >2,000 <10 200 70 <200 38KS <0.05 <1 <20 <2 150 1,500 <10 100 30 <200 <

Table 2—Continued. Analytical determinations of stream-sediment samples and nonmagnetic fractions of pannMountain geochemical study area and the Swanholm antimony area.

[Concentrations in parts per million. Sample localities shown on figure 7]

Field No. Au 

(AA)

Ag Au Be Bi La Mo Nb Pb Sb 

Geochemical Studies
12 Geology, Geochemistry, and Mineral Resources, Middle Fork Boise River Drainage Basin, Idaho

East Fork

0 1 MILE

43 ̊42'

43 ̊38'

115 ̊38' 115 ̊28'

SHEEP

MTN

1SJ

2SJ 3SJ

4SJ

5SJ

6SJ 

7SJ

8SJ

40KS

39KS

30CB

34CB

26CB

27CB 28CB

29CB

31CB

41KS

42KS 43KS

45KS

46KS

47KS

48KS

49KS

44KS

50KS

51KS

52KS

Devils

Creek

Sheep Creek

Sample localities where tin detected,

in parts per million

> 2,000

< 2,000

Outline of drainage basins where 

anomalous tin detected

EXPLANATION

(Pattee and others, 1968). Random chip samples of the veins 

contain less than 650 ppm Be. Thirteen chip and grab samples 

taken from the deposit by the Inspiration Development Company in 1981 also contained low beryllium values; the highest 

value was 239 ppm Be (Patrick DeWilliams, written commun., 

1991).

An airborne gamma-ray and magnetic survey of the Hailey 

1°×2° quadrangle by EG&G Geometrics (1979), as part of the 

National Uranium Resource Evaluation (NURE) program, 

shows that the Sheep Creek batholith is unusually radioactive 

(Bennett, 1980b). In an extensive stream-sediment sampling 

program in Trinity Mountain-Steel Mountain area undertaken by 

the Idaho Bureau of Mines and Geology in 1976-77, several 

stream-sediment samples collected from Sheep Creek, Devils 

Creek, and some of their tributaries were anomalous in one or 

more of the metals molybdenum, copper, and silver (Bennett, 

1980a).

In 1981, Inspiration Development Company staked 135 

lode claims on the ridge between Devils Creek and Sheep Creek, 

in secs. 13 and 14, T. 4 N., R. 7 E., and secs. 18, 19, and 20, T. 4 

N., R. 8 E.; the claims were quitclaimed to Pat DeWilliams in 

1983. The geology of the ridge area was mapped by geologists 

from Inspiration Development and several hundred rock and soil 

samples collected. Silicified, brecciated, and mineralized zones 

were observed on the northeast-trending ridge top in the SW1/4 

sec. 19, T. 4 N., R. 8 E., and on the northwest-trending ridge top 

in secs. 13 and 14, T. 4, R. 7 E. Molybdenite, fluorite, and some 

beryl were seen in some of the stockwork quartz veins in the 

silicified zones. A suite of 105 soil samples taken on a grid 

across the ridge averaged 405 ppm F. Chip samples of the silicified and mineralized zones contained as much as 465 ppm 

MoS2, 25 ppm Ag, 570 ppm Sn, and >2,000 ppm WO3. Two 

hundred and five chip samples of pink granite averaged 829 

ppm F, 24 ppm Sn, and 17 ppm Mo (Patrick DeWilliams, written commun., 1991). The elemental average of these elements 

is much greater than that in reported for unaltered Eocene or 

Cretaceous granitic rocks in central Idaho. For example, in 17 

samples of unaltered granitic rock from the Sawtooth Mountains (presumably Eocene in age), two samples contained 30 

and 100 ppm Sn but the others less than 10 ppm, and one sample 

contained 5 ppm Mo but the others less than 5 ppm (Kiilsgaard 

and others, 1970, p. D158).

Figure 8. Map showing Sheep Creek geochemical study area and stream sediment and panned concentrate sample localities.
13

High gravels

Payette Formation (Miocene)

Muscovite-biotite granite (Cretaceous)

Biotite granodiorite (Cretaceous)

Fault

EXPLANATION

Kgd

Kg

Middle Fork Boise River

East Fork

Swanholm Cr

0 1 MILE

43 ̊

56'

43 ̊

47'

115 ̊28' 115 ̊17'

GRAND MOUNTAIN

1

2

3

5

4

Swanholm Creek Fault

Deer Park Fault

Sample localities of the panned concentrates collected as 

part of the present study are shown on figure 8, and analytical 

values for the samples are given in table 3. The values obtained 

for tin are particularly high for panned concentrates. Ten of the 

29 panned-concentrate samples contained more than 2,000 ppm 

Sn. Unusually high values also were obtained for niobium, thorium, yttrium, and lanthanum (table 3). The average ranks for 

the incompatible elements beryllium, thorium, and yttrium are 

higher in the Sheep Creek are than in any of the other terranes 

that were sampled (fig. 4).

Swanholm Antimony District

The Swanholm antimony district contains the only mine in 

the Hailey 1°×2° quadrangle that has produced a significant 

amount of antimony. It also contains several prospects that have 

been explored for antimony. None of the properties are known 

to contain significant quantities of other metals.

The Swanholm antimony district is located between Swanholm Creek and the East Fork of Swanholm Creek (fig. 9), in T. 

6 N., R. 9 E., Elmore County, Idaho. The district is about 70 mi 

northeast of Boise, Idaho, via the road along the Middle Fork of 

the Boise River leading to Atlanta. From Idaho City, the district 

can be reached by following State Highway 21 northeast to the 

Edna Creek turnoff and then by various well-marked U.S. Forest 

Service roads leading to Atlanta.

The Swanholm antimony district is in a downdropped 

fault blocked that is bounded on the east by the Deer Park fault 

and on the west by the Swanholm Creek fault (fig. 9). The 

northeast part of the district is underlain by Cretaceous biotite 

granodiorite (fig. 9, unit Kgd), and the rest of the district is 

underlain by Cretaceous muscovite-biotite granite (unit Kg). 

Crosscutting these Cretaceous intrusive rocks are numerous 

dikes, most of andesitic to dacitic composition. Most of the 

dikes strike northeast, more or less aligned with the regional 

strike of widespread Tertiary dikes in this part of Idaho (Worl 

and others, 1991). Some small, discontinuous andesitic dikes 

in the area trend north to northwest. These north-trending 

andesitic dikes probably are older than the northeast-trending 

dikes because in at last three locations they are cut by the 

northeast-striking Tertiary dikes.

An unusual feature of the Swanholm district is the occurrence of several separate exposures of gravel and one exposure 

of sandstone. The exposures are all at or near the tops of ridges 

and are considered to be remnants of much more extensive 

exposures of sedimentary rocks that formerly covered parts of 

the region. The sandstone is correlated with the Payette Formation (fig. 9), the nearest known exposure of which is near the 

mouth of Thorn Creek, about 5 mi south of Idaho City (fig. 2) 

and about 28 mi southwest of the Swanholm area. The gravel 

exposures are remnants of more extensive Quaternary gravel 

that formerly covered much of this part of Idaho. In the Swanholm area, the sedimentary rock exposures probably were preserved because they are in a downdropped block west of the 

Deer Park fault (fig. 9). Judging from changes in elevation of 

ridge tops on opposite sides of the Deer Park fault, there could 

be as much as 2,600 ft of vertical displacement on the fault. The 

sedimentary rocks have been eroded from nearby ridges that are 

higher in altitude and outside of the fault block.

Stibnite, the principal antimony-bearing mineral of the district, is present in quartz veins as scattered crystals, as small discontinuous aggregates of crystals, and as masses and ore shoots. 

The quartz veins are small; they range from a fraction of an inch 

to 5 ft in thickness and are as long as 115 ft. Mineralized parts 

of the veins rarely are more than 30 ft in strike length, and most 

range from a few inches to 2 ft in thickness. Granitic rock near 

the veins commonly has undergone intense hydrothermal alteration and now contains sericite, clay minerals, and, locally, 

silica. Most of the veins strike north-northwest and are located 

near or contiguous with the strike of the northerly striking 

andesitic dikes. Small inclusions of stibnite and native antimony are present sporadically in the northerly striking andesitic 

dikes, which could be of Cretaceous age. Some antimony-bearing veins, however, strike northeast, more or less parallel with 

the northeast-striking dikes, and stibnite and native antimony are 

present in northeast-striking dacite dikes, which are believed to 

Figure 9. Simplified geologic map of the Swanholm antimony district. 

Mines and properties described in the Swanholm antimony district are 

as follows: 1, Svensson antimony prospect; 2, Weatherby antimony 

prospect; 3, Anderson-Bida property; 4, North pit of the Hermada 

antimony mine; and 5, Hermada antimony mine.

Geochemical Studies
14 Geology, Geochemistry, and Mineral Resources, Middle Fork Boise River Drainage Basin, Idaho Table 3. Analytical determinations of nonmagnetic fraction of panned-concentrate samples of the Sheep Creek geochemical study area. [Concentrations shown in parts per million] Field No. Location Ag Au Be Bi La Mo Nb Pb 

26CB West tributary, Sheep Creek <1 <20 7 150 >2,000 20 2,000 150 27CB Sheep Creek 5 <20 7 <20 500 20 500 100 28CB East Fork Sheep Creek <1 <20 20 <20 >2,000 <10 5,000 100 29CB North tributary, Sheep Creek <1 <20 100 500 >2,000 <10 5,000 150 30CB North tributary, Sheep Creek <1 <20 30 <20 500 50 500 300 31CB Devils Creek <1 <20 7 500 >2,000 <10 2,000 50 34CB North Sheep Creek <1 <20 300 <20 2,000 <10 1,500 150 39KS Sheep Creek <1 <20 7 <20 >2,000 <10 3,000 500 40KS West tributary, Sheep Creek <1 <20 10 100 1,500 <10 1,500 50 41KS East tributary, Sheep Creek <1 <20 5 200 1,500 15 3,000 30 42KS Sheep Creek <1 <20 7 100 2,000 <10 1,500 70 43KS East tributary, Sheep Creek <1 <20 20 700 2,000 100 3,000 300 44KS South tributary, Sheep Creek <1 <20 200 <20 1,500 20 2,000 100 45KS North tributary, East Fork Sheep Creek 2 <20 70 <20 2,000 10 2,000 500 46KS East Fork, Sheep Creek <1 <20 15 300 700 <10 700 50 47KS North tributary, East Fork Sheep Creek <1 <20 100 500 2,000 <10 1,500 100 48KS East Fork Sheep Creek <1 <20 50 300 500 <10 700 100 49KS North tributary, East Fork Sheep Creek <1 <20 150 100 >2,000 <10 >5,000 100 50KS South tributary, East Fork Sheep Creek <1 <20 15 <20 >2,000 <10 >5,000 200 51KS North tributary, East Fork Sheep Creek 10 20 50 700 1,000 10 500 100 52KS South tributary, Sheep Creek 20 <20 200 1,000 >2,000 300 5,000 1,000 01SJ South tributary, Sheep Creek <1 <20 5 <20 1,500 <10 1,500 100 02SJ South tributary, Sheep Creek <1 <20 7 150 1,500 <10 700 100 03SJ South tributary, Sheep Creek <1 <20 5 <20 2,000 100 1,000 30 04SJ South tributary, Sheep Creek <1 <20 10 150 500 20 1,000 50 05SJ Devils Creek <1 <20 7 <20 2,000 <10 1,500 70 06SJ East tributary, Devils Creek <1 <20 7 <20 1,500 <10 1,000 30 07SJ Devils Creek <1 <20 7 <20 1,000 <10 700 30 08SJ East tributary, Devils Creek <1 <20 7 <20 2,000 <10 700 100 
15

72

75

N

Dump

Elevation 5438 ft

Biotite granodiorite (unit Kgd)

Three ft of altered, silicified biotite

granodiorite. Stibnite in quartz

stringer on dike hanging wall

Native antimony in andesite dike

Iron-stained and altered biotite

granodiorite along dike footwall

Zone of altered biotite

granodiorite 15 ft wide

along dike hanging wall

containing parallel to subparallel quartz-stibnite

veinlets 0.5-2 in. thick 

Stibnite lens 14 in. thick

and 40 ft long

Zone of biotite granodiorite 18

in. to 3 ft wide between

stibnite lens and dike

Andesite dike, altered

0 50 FEET

be of Tertiary age. The presence of primary antimony minerals 

in the Tertiary dikes indicates that the antimony minerals are of 

Tertiary, not Cretaceous, age.

Thirty-three sediment samples were collected from streams 

that drain the Swanholm antimony district during three different 

sampling programs. In 1986, five stream-sediment samples 

were collected and analyzed by the standard U.S. Geological 

Survey semiquantitative spectrographic method. No detectable 

antimony, or anomalous concentrations of any other metals, was 

determined in these samples even though one sample was collected only 500 ft downstream from the North pit of the Hermada antimony mine. These disappointing findings probably 

resulted from the way in which the samples were collected or 

from the analytical method used. Semiquantitative spectrographic analysis has a lower limit of detection of 100 ppm for 

antimony and is not recommended for analysis of antimony in 

stream sediments.

As part of the second sampling program, stream-sediment 

and panned-concentrate samples were collected at 8 different 

sites in 1988. The stream-sediment samples were analyzed by 

an inductively coupled plasma spectrometry (ICP) method, 

which has a lower detection limit of 1 ppm for antimony. The 

panned-concentrate samples were analyzed by the semiquantitative spectrographic method, using an analytical procedure that 

has a detection limit of 200 ppm Sb for this type of concentrated 

sample (Malcolm and Smith, 1992). To check the analytical 

results obtained for these eight samples, a third set of twelve 

panned-concentrate and stream-sediment samples was collected 

in 1989 and analyzed by the same methods as the samples collected in 1988.

The sampling programs show clearly that analyzing 

panned-concentrate samples by semiquantitative spectrographic 

analysis and stream-sediment samples by an ICP analytical 

method are both successful ways to detect antimony in stream 

sediments. In the Swanholm antimony district, panned-concentrate samples yielded anomalous concentrations of antimony at 

most sites that were downstream from the Hermada mine or 

below other known antimony prospects.

Hermada Antimony Mine

The Hermada antimony mine (fig. 9, loc. 5) is in the western half of sec. 13, T. 6 N., R. 9 E. Mine workings consist 

chiefly of the Hermada pit and the North pit, which is about 

2,500 ft north of the Hermada pit. Both workings are along the 

west side of the East Fork of Swanholm Creek, about 3 mi 

northeast of the junction of Swanholm Creek and the Middle 

Fork Boise River. In addition to the Hermada and North pits 

(fig. 9, loc. 4), several thousand feet of bulldozer trenches, many 

of them more like roads than trenches, extend along hillsides 

west of the Hermada pit. In 1992, the Hermada property consisted of 13 unpatented mining claims and 5 patented claims 

(Hermada and Hermada Nos. 1, 2, 4, and 6).

The Hermada claims were staked in 1947. The most active 

period of mining operations at the property was from 1947 to 

1950. Original exploration was by adits, but these were obliterated by subsequent development of the Hermada open pit, which 

is about 250 ft long and as wide as 125 ft. Mining in the Hermada pit was by bulldozer and handsorting of selected pieces of 

ore. According to Popoff (1953), in 1947–48 the Hermada Mining Company mined and shipped 207.65 tons of crude ore that 

averaged 32.05 percent Sb. A high-grade ore sample collected 

in 1948 contained 35.3 percent Sb, 12.4 percent S, 0.05 percent 

As, 0.11 percent Pb, 46.0 percent SiO2, 0.30 percent CaO, 1.30 

percent FeO, 4.7 percent Al2O3, and nil gold and silver (Popoff, 

1953). From 1947 to 1950, 4,600 tons of ore averaging 11.7 

percent Sb was produced from the mine and trucked to the 

Talache mill at Atlanta, Idaho. From these shipments, the mill 

produced 776 tons of concentrate that averaged 59.92 percent 

Sb. Antimony ore also was produced from the Hermada mine in 

1951 and 1952 and concentrated at the Atlanta mill. As of 1992, 

total output from the mine was about 1,502,000 pounds of antimony according to unpublished U.S. Bureau of Mines records. 

Almost all of this antimony was mined from the Hermada pit.

The Hermada deposit is in biotite granodiorite (fig. 10, unit 

Kgd), and consists of a broad zone of north-northwest-striking 

faults and narrow andesitic dikes along which are quartz-stibnite 

veins that are irregular in strike length and thickness. The 

Figure 10. Geologic map of the northern part of the upper bench of the 

Hermada pit. Mapped by T.H. Kiilsgaard in 1951.

Geochemical Studies
16 Geology, Geochemistry, and Mineral Resources, Middle Fork Boise River Drainage Basin, Idaho

longest vein explored to 1950 was 115 ft (Popoff, 1953); it commonly was only a few inches thick, but some shoots were as 

thick as about 5 ft. The overall width of the mineralized zone 

was about 100 ft. Stibnite was the principal ore mineral, 

although other antimony minerals, including kermesite, stibiconite, cervantite, and valentinite, were also present. At the Hermada pit, the northerly trending Hermada zone is intersected and 

crosscut in at least three locations by younger northeast-striking 

dacitic dikes.

Biotite granodiorite along dikes in the Hermada zone is 

hydrothermally altered to sericite and clay minerals. Locally, 

the alteration is so intense that identification of the original rock 

is difficult. The andesitic dikes also are altered and decomposed 

locally to a brownish, earthy material. The quartz-stibnite veins 

are in the granodiorite, contiguous with or near the northerly 

trending dikes (fig. 10). Mineralized lenses in the veins are in 

granodiorite that has undergone sericitic and silicic alteration. 

Near mineralized shoots, the granodiorite has been mostly 

replaced by quartz, much of which has been broken and granulated by postmineral faulting.

The Hermada deposit was explored under a Defense Minerals Administration (DMA) contract in 1951 and 1952. The 

exploration work consisted of extensive bulldozer trenching in 

the North pit, several thousand feet of bulldozer trenching west 

of the Hermada pit and driven west to intersect north-striking 

veins projected south from the Hermada pit (Kiilsgaard and 

Zoldak, 1955, unpublished DMA report available in Spokane 

field office). Exploration in the North pit exposed a north-striking quartz-stibnite vein a few inches thick that was cut by a 

large, northeast-striking shear zone that postdates quartz-stibnite 

mineralization. Pulverized stibnite was found in the shear zone. 

A quartz-stibnite vein about 10 in. thick was exposed in a bulldozed cut about 2,400 ft S. 60° W. from the Hermada pit. Native 

antimony was found in a northeast-trending dacite dike, about 

1,100 ft south of the Hermada pit. Sparse pyrite and flecks of 

stibnite also were found in altered biotite granodiorite intruded 

by a northeast-striking dacite dike that was intersected in the adit 

about 260 ft from the portal. The final 60 ft of adit intersected 

highly altered biotite granodiorite in which there was numerous 

north-striking quartz stringers as thick as 5 in. and much pyrite 

but no stibnite.

Svensson Antimony Prospect

The Svensson antimony prospect (fig. 9, loc. 1) is along the 

eastern side of sec. 10, T. 6 N., R. 9 E., about 0.5 mi east of 

Swanholm Creek and about 4 mi north of the intersection of 

Swanholm Creek and the Middle Fork Boise River. Mine workings at this prospect explore several small, discontinuous stibnite-bearing quartz veins that cut Cretaceous biotite 

granodiorite.

Mineral exploration at the prospect has been concentrated 

at two locations (1) a caved adit, the portal of which is at an altitude of 5,800 ft, and that, according to Bill Weatherby (oral commun., 1992) extended about 120 ft southeast into the hillside, 

and (2) a large bulldozed pit about 140 ft long, 70 ft wide, and 20 

ft deep, the bottom of which is at an altitude of 5,930 ft and 400 

ft southeast of the portal of the adit. Several other bulldozed 

cuts are northeast and east of the principal workings. All of the 

workings are on the Hope Nos. 1 and 2 unpatented claims.

The large bulldozed pit was excavated in 1952 as part of a 

Defense Minerals Exploration Administration (DMEA) contract 

let to Oscar V. Svensson, Boise, Idaho, for the purpose of 

exploring three parallel stibnite-bearing quartz veins that strike 

N. 15 ̊ W. and dip 50°–60° NE. The veins range from 2 to 12 in. 

in thickness, are about 5 ft apart, and consist chiefly of a hanging-wall gouge zone along which are quartz veins that contain 

disseminated, fine-grained pyrite and minor amounts of stibnite. 

Biotite granodiorite within the vein zone is strongly altered to 

sericite and clay minerals and, locally, stained by limonite 

derived from oxidized pyrite. Samples of the veins collected in 

1952 ranged from nil to 7.5 percent Sb. Atomic absorption analysis of two samples of the vein material collected in 1986 

showed 0.08 and 0.09 ounces of gold per ton. Semiquantitative 

spectrographic analysis of the two samples showed 200 and 

more than 1,000 ppm As and 100 and more than 1,000 ppm Sb. 

Svensson reported that prior to 1951, several tons of quartz, 

some of it containing 6–8 percent Sb, was mined from the pit, 

production that was a justifying reason for the subsequent 

DMEA contract. No significant amount of stibnite, however, 

was discovered by the DMEA exploration program.

The lower, caved adit was driven by the Weatherby family 

during the 1960’s to test the veins explored by the large open 

pit. A few tons of quartz and stibnite are reported to have been 

mined from the adit, but the work did not find a significant 

amount of antimony (Bill Weatherby, oral commun., 1992).

About 570 ft N. 20° E. of the large open pit is a veinlet of 

quartz containing disseminated stibnite. The vein strikes N. 25° 

W., dips 50° NE., is about 4 in. thick, and was explored in 1952 

over a strike length of 70 ft by bulldozer trenching. Two samples collected from the vein in 1952 contained 0.30 and 0.06 

percent Sb. One of the samples contained 0.44 percent As and 

0.10 percent Pb (Popoff, 1953).

About 850 ft east of the large open pit, on top of the hill, 

two parallel quartz veins about 5 ft apart strike N. 19° W., dip 

60° NE., and range from 2 to 12 in. in thickness. The veins were 

explored by bulldozer trenching over a strike length of 140 ft in 

1952. Stibnite was found in the veins but not in an amount sufficient to warrant additional exploration.

Another quartz vein tested in 1952 is 950 ft N. 50° E. of the 

large open pit. The vein strikes N. 12° W., is as thick as 12 in., 

contains trace quantities of stibnite, and was explored by a 

crosscutting bulldozer trench.

Stibnite-bearing quartz veins at the Svensson prospect are 

in a northwest-striking zone about 850 ft wide. The veins are 

small and discontinuous. Locally, they contain concentrations 

of stibnite, but the overall antimony grade is low.

Weatherby Prospect

The Weatherby antimony prospect (fig. 9, loc. 2) is in the 

north-central part of sec. 14, T. 6 N., R. 9 E., along a southwestflowing tributary of Swanholm Creek, about 3.25 mi north of the 

junction of Swanholm Creek and the Middle Fork Boise River.
17

The prospect consists of a sloughed bulldozed cut and a 

caved adit a few feet below, which is reported to be about 150 ft 

long and to have been driven in the 1960’s (Bill Weatherby, oral 

commun., 1992). Both the bulldozed cut and the adit trend S. 

20° E. Several tens of tons of hand sorted quartz, stibnite, and 

altered biotite granodiorite were observed on the adit dump in 

1986. The handsorted material probably was stockpiled in 

anticipation of acquiring a volume large enough to warrant shipping, but apparently that volume was not collected. Some of the 

pieces of ore are too large to have come from the adit and probably are from the bulldozed cut. Quartz of the hand sorted material contains fine-grained pyrite and stibnite. Many pieces of ore 

have slickensides and are coated with gouge; they may have 

been mined from a fault zone. Atomic absorption analysis of a 

selected sample of the dump ore yielded 0.35 ounces of gold per 

ton. Semiquantitative spectrographic analysis of the same sample yielded more than 10,000 ppm (1 percent) Sb, 1,000 ppm As, 

and 5 ppm Ag. No vein or fault was seen in the sloughed workings, but the trend of the workings suggests that the workings 

were driven on a vein that strikes about N. 20° W.

Anderson-Bida Property

The Anderson-Bida property (fig. 9, loc. 3) is in SW1/4 sec. 

14. T. 6 N., R. 9 E., about 1 mi southwest of the Hermada mine, 

and about 2.25 mi north of the junction of Swanholm Creek and 

the Middle Fork Boise River. The property was staked in 1948 

as the Reno Nos. 1–5 claims.

According to Popoff (1953), a number of pits and trenches 

on the property expose a north-northwest trending zone of dacitic dikes and hydrothermally altered and erratically mineralized 

granitic rocks that more or less parallels the trend of the Hermada mineralized zone. The hydrothermally altered granitic 

rocks are in two parallel bands that range from 2 to 10 ft in 

thickness, strike north-northwest, and were traced by Popoff on 

strike for about 700 ft. A quartz-stibnite vein that ranges from 1 

to 14 in. in thickness is at the northern end of the hydrothermally 

altered rocks. About 30 tons of low-grade mineralized material 

was mined from the vein prior to 1949 but was not shipped. At 

the southern end of the 700-foot-long belt of hydrothermally 

altered rocks, north-striking quartz-stibnite veinlets as thick as 2 

in. were observed in the silicified granitic rock (Popoff, 1953).

In 1986, surface workings at the property were sloughed to 

the point where no trace of the veins or altered bands could be 

seen. Fragments of quartz on a northern dump contained finegrained but sparse stibnite. The property is in rough alignment 

with the Weatherby and Svensson properties and probably is 

part of the mineralized trend that extends through the three 

properties.

Discussion

In epithermal gold-silver-bearing veins, concentrations of 

antimony and arsenic minerals commonly are highest in the 

upper parts of the veins, gold and silver values are highest in the 

middle parts, and base-metal contents are highest at deeper 

levels. The Swanholm district is in a downdropped fault block, 

the structural location of which has protected the block from 

erosion. The presence of stibnite as the primary antimony ore 

mineral and the concentration of arsenic in the quartz-stibnite 

veins suggest that only the upper parts of the veins may have 

been eroded. Gold has not been recovered from Swanholm district ore, but the two samples of vein material from the Svensson 

property that contained 0.08 and 0.09 ounces of gold per ton, the 

dump sample from the Weatherby property that contained 0.35 

ounces of Au per ton and 5 ppm Ag, and panned sample nos. 10 

and 12CB (fig. 7) that contained 20 and 100 ppm Au (table 2) 

indicate the presence of gold in the district. Known veins are 

small in size, and vein samples indicate low gold values, but the 

mineralized zone in which the veins are present is large. The 

possibility of higher gold and silver contents at depth can be 

determined only through further exploration.

Middle Fork Boise River Placers

Placer gold was discovered at Atlanta (fig. 1) in August 

1863 (Wells, 1983) and in the vicinity of Phifer and Swanholm 

Creeks in the spring of 1864. River placers in the vicinity of 

Lucky Peak dam (west of the study area) were being worked in 

1864, and placer mining near Twin Springs (fig. 1) was underway by 1877. Little is known about the history or early production of placer gold at various sites along the Middle Fork Boise 

River because production records were kept only by the County 

and State, and there is little or no information on individual mine 

sites. There is, however, considerable evidence of early placer 

mining, including large evergreen trees growing from piles of 

placered boulders at different sites along the river. Examples of 

early placer mining are near the mouths of Phifer and Swanholm 

Creeks. South of the mouth of Phifer Creek, extending into the 

NW1/4, sec. 2, T. 5 N., R. 9 E. (fig. 9), is a placered area that is 

unique in that it is as much as 640 ft above the Middle Fork 

Boise River. The placer is in high gravel deposits of Pleistocene 

age. In upper parts of the placered area, spring runoff of melted 

snow apparently was the source of water for mining. Similar 

evidence of early placer mining is on the north side of Middle 

Fork Boise River, east of the mouth of Swanholm Creek, in 

NW1/4 sec. 36 and SW1/4 sec. 25, T. 6 N., R. 9 E. (fig. 9). 

Placer mining at this location extended more than 500 ft above 

the Middle Fork Boise River and, similar to placer mining on the 

south side of the river, was confined to high gravel deposits.

Piles of boulders and other evidence of early placer mining 

are present at many locations along the Middle Fork Boise 

River, but examination of U.S. Geological Survey and U.S. 

Bureau of Mines placer production records dating back to 1901 

indicates five areas have been the principal producers (table 4). 

Of these, a stretch of river-valley alluvium along the Middle 

Fork Boise River, above the mouth of Phifer Creek, has been, by 

far, the most productive. Worked on a small scale in 1908 and 

1909, the alluvial deposit was brought into major production by 

Boise King Placers in 1941 when 7.5-ft3 bucket-line dredge was 

placed in operation. During the years 1941, 1942, and 1946, the 

dredge processed 2,801,894 yd3 of alluvium from which was 

recovered 15,278 ounces of gold.

Geochemical Studies
18 Geology, Geochemistry, and Mineral Resources, Middle Fork Boise River Drainage Basin, Idaho

The Twin Springs mining district was the most consistent 

producer of placer gold from 1901 to 1940. The gold was mined 

primarily by hydraulic operations. Water for hydraulic mining 

came from Sheep Creek, was siphoned across the Middle Fork 

Boise River (fig. 11A), and then flowed chiefly by flume (fig. 

11B) about 2.5 mi to a placer site in high gravel near Twin 

Springs (fig. 11C). Smaller scale hydraulic placer mining was 

conducted in terrace gravel on the east side of Middle Fork 

Boise River, across the river from Twin Springs, using water 

brought by ditch from nearby Alder Creek.

Gold values varied at different placer sites (table 5). Deposition of placer gold is affected by many conditions, including 

changes in stream gradient, obstacles in the stream bed such as 

large boulders or protruding outcrops of bedrock, size of gold 

particles, and so on. Different gold-bearing channels of gravel 

usually represent deposition at different times and under different conditions. As would be expected, such channels commonly 

contain different amounts of gold. The range of gold values 

shown in table 5, however, may be in part because of incorrect 

estimates of the cubic yards of gravel mined or because of the 

recovery procedures. The most reliable gold values of mined 

placer material probably are those from the Boise King placers, 

where the bucket-line dredge provided a means of accurately 

measuring the cubic yards mined. Gold in alluvium mined at the 

Boise King placers location averaged 0.0054 oz per cubic yard 

(table 5).

Most of the placer gold in high gravel, terrace gravel, and 

alluvium along the Middle Fork Boise River originated from 

erosion of lode deposits that crop out near Atlanta. This conclusion is supported by the fact that there are no significant 

gold placer deposits along the Middle Fork upstream from 

Atlanta. Considerable placer gold, however, has been transported by the Queens and Little Queens Rivers, (Worl and others, 1991); the gold probably was derived from lode deposits 

that crop out upstream along the Little Queens River. Another 

source of placer gold has been Black Warrior Creek, where 

known lode deposits have been eroded both by Black Warrior 

Creek and its principal tributary, West Warrior Creek (Worl and 

others, 1991).

The high gravel deposits along the valley walls of the Middle Fork Boise River are the oldest gold-bearing gravel deposits 

in the area. These deposits formed from Pleistocene erosion by 

the ancestral Middle Fork Boise River. A larger volume of 

water, derived from melting glaciers in the Sawtooth Mountains, 

probably flowed in the river at that time. Uplift of the region 

increased the stream gradient and the rate of erosion. Erosion 

exposed the lode deposits in the vicinity of Atlanta and at other 

locations, and the gold eroded from the lodes was transported 

downstream by the flowing water and deposited in the gravel. 

Continued uplift of the mountainous region caused deeper 

stream erosion, thereby transporting most of the high gravel 

deposits and the gold to lower elevations and forming what are 

now referred to as the terrace gravels. Further uplift of the 

region and erosional deepening of the stream valleys transported 

most of the terrace gravels downstream, leaving heavier materials in the present stream alluvium.

Placer gold in the transported gravel was subjected to 

stream-transport abrasion, to the forces of “nature’s ball mill.” 

The milling action dislodged adhering and contaminating mineral and rock particles from the gold grains and flakes. It also 

resulted in smaller gold particles; gold is coarser nearer the 

eroded source. Stream-transport abrasion also probably 

affected the fineness (degree of purity) of the transported gold. 

A sampling of placer gold mined along the Middle Fork supports the conclusion that placer gold mined farther from its originating source, as in the Twin Springs district, tends to be of 

higher purity (table 6). Pure gold (1000 fine) rarely is found in 

placer deposits.

Mineral Resources

Placer Resources

A large but unmeasured yardage of unmined auriferous 

gravel remains along the Middle Fork Boise River, most of it in 

high and terrace gravel of the Twin Springs district (fig. 1) (Worl 

and others, 1991). Other areas that contain substantial amounts 

Table 4. Gold and silver production from placers along or near Middle Fork Boise River from 1901 to 1947.

[Concentrations shown in ounces. Data compiled from the U.S. Geological Survey Mineral Resources of the United States, volumes 1908–1916, 

U.S. Bureau of Mines Yearbooks, 1941–1946, and unpublished U.S. Bureau of Mines reports]

Placer locality Gold Silver

Boise King placers—Alluvium along 2.65 mi of Middle Fork Boise River, 

from 0.3 mi east of mouth of Phifer Creek to 0.35 mi east of mouth of Smith Creek

15,361 5,202

Twin Springs district— Terrace and high gravel deposits near Middle Fork Boise River, from Ada 

County line northeast to mouth of North Fork Boise River

1,496 409

Terrace gravel along Middle Fork Boise River near the mouth of Phifer Creek 268 91

Terrace gravel along Middle Fork Boise River, near the mouth of Eagle Creek 118 36

Terrace gravel, Queens River, at and near junction with Middle Fork Boise River 141 42

Various small placers of terrace gravel and alluvium at or near the mouths of different streams and their 

junctions with Middle Fork Boise River

313 90

Total........................................................................................................ 17,697 5,870
19

Table 5. Gold values of placers mined along Middle Fork Boise River.

[From unpublished U.S. Bureau of Mines sources. Values calculated from a gold price of $35.00 per ounce]

1

 Total yardage mined in 3-year period.

2

 Total ounces of gold produced in 3-year period.

3

 Average grade.

Placer locality Year Cubic yards

of

gravel mined

Ounces of

gold 

produced

Ounces of 

gold 

per yd3

Values 

per yd3

Near mouth of Eagle Creek 1935 500 13 0.0260 $0.91

Near mouth of Eagle Creek 1937 300 6.5 0.0218 0.76

Mouth of Buck Creek 1939 5,000 13 0.0026 0.09

Mouth of Yuba River 1939 1,400 8 0.0057 0.20

Mouth of James Creek 1940 8,000 27 0.0033 0.12

Mouth of Queens River 1940 500 4 0.0080 0.28

Boise King Placers, along

Middle Fork Boise River

1941

1942

1946

2,801,8941 15,2782 0.00543 0.19

Near mouth Phifer Creek 1938 7,524 68 0.0090 0.32

Twin Springs district 1934 1,000 15.4 0.0150 0.53

Twin Springs district 1938 1,000 18 0.0180 0.63

Twin Springs district 1938 1,500 19 0.0063 0.22

Mouth of Birch Creek, Twin Springs district 1940 2,000 11 0.0055 0.19

Twin Springs district, mouth of Birch Creek 1947 1,000 5 0.0050 0.18

Twin Springs district 1939 1,800 13 0.0072 0.25

Twin Springs district, near Slide Gulch bridge 1940 500 3 0.0060 0.21

Table 6. Fineness of placer gold mined along Middle Fork Boise River.

[Compiled from U.S. Geological Survey Mineral Resources of the United States, volumes 1908–1916, 

U.S. Bureau of Mines Yearbooks, and unpublished U.S. Bureau of Mines report]

Placer location Ounces of 

mined gold

Fineness of gold

 (weighted average)

Junction, Yuba and Middle Fork Boise Rivers 17 743

Mouth, Queens River 86 734

Near mouth of Eagle Creek 93 753

Twin Springs district 273 789

of auriferous gravel are terrace gravels along the north side of 

the Middle Fork Boise River between the mouths of Phifer and 

Smith Creeks and terrace gravels near the mouth of Eagle Creek. 

The Black Warrior Creek valley, beginning about 1.7 mi from 

the mouth of the stream and continuing upstream for about 1.8 

mi is a placer resource, as is a tributary, West Warrior Creek, 

from the stream mouth upstream for about 0.5 mi. From the 

Black Warrior-West Warrior stream junction, about 1.3 mi 

upstream along Black Warrior Creek and 0.6 mi downstream, the 

alluvium-floored valley of the creek ranges from 200 to 500 ft in 

width and the alluvium is as deep as 10–15 ft (Ballard, 1928). In 

1925 attempts were made to mine the alluvium using hydraulic 

methods. Coarse gold was recovered, but apparently the low 

stream gradient created a tailings-dump problem that hampered 

mining. Little evidence of extensive placer mining was seen at 

the site in 1987.

Placer mining probably occurred at Alexander Flats, along 

the Middle Fork about 7 mi upstream from the junction of the 

North Fork Boise River and the Middle Fork. A small placered 

area in river-valley alluvium at the upstream end of the flats and 

another small placered area of terrace gravel on the northeast 

side of Big Five Creek, near the mouth of the stream, at the 

down-stream end of the flats attest to previous placer efforts in 

the area. Alexander Flats consists of about 315 acres of alluvium, most of which extends along the southeast side of the 

Middle Fork. Part of the flats was explored in 1951 by the U.S. 

Bureau of Mines as part of a project designed to search for 

radioactive materials. Ten holes were churn drilled through the 

Mineral Resources
20 Geology, Geochemistry, and Mineral Resources, Middle Fork Boise River Drainage Basin, Idaho

A

B

C

alluvium to depths ranging from 10 to 23 ft before bedrock was 

reached (Storch and Holt, 1963). Material from the holes averaged 1.1 pounds of ilmenite, 0.08 pounds of monazite, and a few 

cents in gold values per cubic yard, low metal values for a placer 

deposit. A significant yardage of alluvium is present at Alexander Flats, but the low metal contents in the drill-hole samples 

indicate that the alluvium could not be profitably mined at 1992 

costs and metal prices.

A commodity in the alluvial gravels that has received little 

attention but which warrants testing is tin. Biotite granite of the 

Sheep Creek batholith contains unusually high concentrations 

of tin, most if not all in the mineral cassiterite. Cassiterite is a 

heavy mineral, which, as it was eroded from the biotite granite 

by the tributaries of the Middle Fork Boise River, has become 

concentrated in the black-sand or heavy mineral fraction of the 

stream sediments. Cassiterite, by itself, probably would not be 

minable, but it and concentrations of heavy rare-earth minerals 

that are likely to be associated with it might be recovered as 

byproducts from a gold-placer operation.

As of April 1992, most of the Federal land along the Middle Fork, upstream from the Arrowrock Dam to the junction of 

the North Fork and Middle Fork Boise River, had been withdrawn from the mineral entry. From the river junction, most of 

the Federal land along the Middle Fork to the vicinity of the 

Middle Fork-Roaring River junction also has been withdrawn 

from mineral entry, as had Federal land along the North Fork 

from the river junction to the vicinity of the North Fork-Lost 

Owl Creek junction. In March 1993, information on the withdrawn status of these Federal lands could be obtained from the 

U.S. Bureau of Land Management, Idaho State Office, Boise, 

Idaho.

Potential Lode-Deposit Resources

Stibnite is present at several separate localities in the 

Swanholm antimony district, although estimates of the amount 

of antimony that might be present cannot be made without more 

exploration. Unknown but probably minor amounts of lowgrade gold- and silver-bearing minerals are likely to be associated with stibnite at the mineralized localities.

There are no known mineral deposits in any of the three 

geochemical study areas that could be considered minable in 

March 1993. Several brecciated and silicified exposures of 

Eocene granite in the Sheep Creek area contain molybdenite, 

Figure 11. Former placer operations in the Twin Springs placer district. 

A, Siphon used to transport water across the Middle Fork Boise River. 

Water was brought from Sheep Creek by ditch and flume to a point on 

the southeast side of the river, across the river and discharged into a 

flume that extended from the ridge crest on the south side of Logging 

Gulch, through a ridge saddle that is about 2,200 ft southwest of the 

mouth of Logging Gulch, and southwest to the placer site near Twin 

Springs. Copyrighted photograph from Idaho State Historical Society 

#74-165.17, used with permission. B, Long wooden river built to carry 

water from Sheep Creek to a placer site near Twin Springs. Copyrighted 

photograph from Idaho State Historical Society #76-81.48, used with permission. C, Placer mine 1,500 ft north of Twin Springs and about 400 ft 

above river level in a deposit of high gravel. Note the hydraulic giant that 

washed the north bank of the pit and the horizontal riffles below the pit 

that trapped the gold. Beneath the riffles, the water was recaptured and 

passed through a flume to a point about 2,100 ft upstream from Twin 

Springs and about 100 ft above river level, where it was reused to mine 

placer gold from a deposit of terrace gravel. Photograph, probably taken 

in the early 1900’s, courtesy of George C. Castle, Twin Springs, Idaho. 
21

but additional exploration is needed. Possible byproduct 

minerals, including beryl, fluorite, scheelite, cassiterite, and, 

possibly, unidentified tin-bearing minerals and rare-earthelement minerals, also are present at the molybdenite-bearing 

localities.

Geologic and geochemical studies of this project have 

identified the possibility of presently unknown gold deposits in 

the Granite Mountain area. Deposits containing bismuth, silver, 

and tungsten, and other metals probably are present in the Mt. 

Heinen area, but additional exploration is needed.

References

Anderson, A.L., 1939, Geology and ore deposits of the Atlanta District, 

Elmore County, Idaho: Idaho Bureau of Mines and Geology Pamphlet 49, 71 p.

Ballard, S.M., 1928, Geology and ore deposits of the Rocky Bar quadrangle: Idaho Bureau of Mines and Geology Pamphlet 26, 41 p.

Bennett, E.H., 1980a, Reconnaissance geology and geochemistry of the 

Trinity Mountain-Steel Mountain area, Elmore County, Idaho: Idaho 

Bureau of Mines and Geology Technical Report 80-11, 44 p.

Bennett, E.H., 1980bGranitic rocks of Tertiary age in the Idaho batholith 

and their relation to mineralization: Economic Geology, v. 75, p. 278–

288.

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, p. 81–95.

EG&G Geometrics, 1979, Aerial gamma ray and magnetic survey, Idaho 

project, Hailey quadrangle of Idaho: Final report of Bendix Corporation for the U.S. Department of Energy, subcontract 79-323-S and 

Bendix contract DE-AC13-76GJ01664, Bendix Index No. GJBX-1080, 

131 p.

Fisher, F.S., McIntyre, D.H., and Johnson, K.M., 1992, Geologic map of the 

Challis 1°×2° quadrangle, Idaho: U.S. Geological Survey Miscellaneous Investigation Series Map I-1819, scale 1:250,000.

Hannah, J.L., and Stein, H.J., 1990, Magmatic and hydrothermal processes in ore-bearing systems, in Stein, H.J., and Hannah, J.L., eds., Orebearing granite systems; petrogenesis and mineralizing processes: 

Geological Society of America Special Paper 246, p. 1–10.

Kiilsgaard, T.H., 1983, Geochemical map of the Ten Mile West Roadless Area, Boise and Elmore Counties, Idaho: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-1500-B, scale 

1:62,500.

Kiilsgaard, T.H., and Bacon, L.D., in press, Geology and mineral deposits 

of the Atlanta Hill area, Elmore County, Idaho: U.S. Geological 

Survey Bulletin 2064-Z.

Kiilsgaard, T.H., Fisher, F.S., and Bennett, E.H., 1989, Gold-silver deposits 

associated the trans-Challis fault system, Idaho: U.S. Geological 

Survey Bulletin 1857-B, p. B22–B44.

Kiilsgaard, T.H., Freeman, V.L., and Coffman, J.S., 1970, Mineral resources of the Sawtooth Primitive Area, Idaho: U.S. Geological Survey 

Bulletin 1319-D, 174 p.

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

and faults in the Atlanta lobe of the Idaho batholith, Challis 1°×2° 

quadrangle, 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-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, south-central Idaho: U.S. Geological Survey Professional Paper 2064-U.

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.

Lopez, D.A., 1981, Stratigraphy of the Yellow Jacket Formation of eastcentral Idaho: U.S. Geological Survey Open-File Report 81-1088, 

219 p.

Malcolm, M.J., and Smith, C.L., 1992, Analytical results and sample locality maps of USGS and NURE samples from the Hailey 1°×2° quadrangle, Idaho: U.S. Geological Survey Open-File Report 92-24, 267 p.

Nash, J.T., 1988, Interpretation of the regional geochemistry of the 

Tonopah 1°×2° quadrangle, Nevada, based on analytical results 

from stream-sediment and nonmagnetic heavy-mineral-concentrate samples: U.S. Geological Survey Bulletin 1849, 28 p.

Pattee, E.C., Van Noy, R.M., and Weldin, R.D., 1968, Beryllium resources 

of Idaho, Washington, Montana, and Oregon: U.S. Bureau of Mines 

Report of Investigations 7148, p. 63–85

Popoff, C.C., 1953, Hermada antimony deposit, Elmore County, Idaho: U.S. 

Bureau of Mines Report of Investigations 4950, 21 p.

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

Bureau of Mines and Geology Pamphlet 129, 37 p.

Scanlan, T.M., 1986, Geology and landslide hazards of the Dunnigan 

Creek 7 1/2 minute quadrangle, Boise County, Idaho: Moscow, University of Idaho, M.S. thesis, 68 p.

Smith, C.L., 1995, Interpretation of regional geochemistry of the Hailey 

1°×2° quadrangle, south-central Idaho: U.S. Geological Survey Bulletin 2064-L, 22 p.

Storch, R.H., and Holt, D.C., 1963, Titanium placer deposits of Idaho: U.S. 

Bureau of Mines Report of Investigations 6319, 69 p.

Streckeisen, A.L., 1973, Plutonic rocks—Classification nomenclature 

recommended by the IUGS Subcommission on the systematics of 

igneous rocks: Geotimes, v. 18, p. 26–30.

Swanberg, C.A., and Blackwell, B.D., 1973, Areal distribution and geophysical significance of heat generation in the Idaho batholith and 

adjacent intrusions in eastern Oregon and western Montana: Geological Society of America Bulletin, v. 84, p. 1261–1282.

Wells, M.W., 1983, Gold camps and silver cities: Idaho Bureau of Mines 

and Geology Bulletin 22, 16 p.

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