--__. .................................... &

GEOLOGY, HYDROLOGY AND GEOCHEMISTRY OF THE

GEOTHERMAL AREA EAST OF LOWMAN, IDAHO

by

Brad E. Dingee

A thesis submitted in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE IN GEOLOGY

WASHINGTON STATE UNIVERSITY Department of Geology

May, 1987

.... __. .... ----------------------------------... & ~

To the Faculty of Washington State University:

The members of the Committee appointed to

examine the thesis of BRAD E. DINGEE find it

satisfactory and reccommend that it be accepted.

ii

iii

ACKNOWLEDGMENTS

I wish to thank Dr. Philip E. Rosenberg for suggesting

this study and for his guidance as the project developed. Dr.

A. John Watkinson is thanked for critically reviewing the

manuscript. Dr. Richard L. Thiessen is also thanked for

reviewing the manuscript and for providing computer time. I

am grateful to Dr. Peter R. Hooper for graciously supplying

whole rock chemical analyses. The Idaho Department of Water

Resources is also thanked for granting access to their

computer facilities. I am grateful to Sigma Xi for providing

partial financial support for field expenses. Discussions

with John Reed during the course of this investigation were

quite helpful and are much appreciated.

.... _.------------------------------------------&

h

iv

THE GEOLOGY, HYDROLOGY AND GEOCHEMISTRY OF THE

GEOTHERMAL AREA EAST OF LOWMAN, IDAHO

Abstract

by Brad E. Dingee, MS Washington State University

May, 1987

Chair: Philip E. Rosenberg

An investigation was made of three hot spring areas-­

Kirkham, Bonneville and Sacajawea Hot Springs-- located east

of Lowman, Boise County, Idaho along the South Fork Boise

River. The objectives of this study were to determine the

detailed geologic, hydrologic and geochemical setting of these

hot spring areas.

Most of the study area is underlain by Cretaceous granite

and granodiorite of the Idaho batholith. A Tertiary granite

pluton occurs in the eastern portion of the area. The

predominant structural feature of the area is the recently

recognized trans-Challis fault system, a northeast striking

series of high-angle faults and fault zones. This major

structure has strongly influenced topographic and structural

featur~s in the region. Northwest trending Basin-Range style

faults are also present and terminate against the trans-

Challis fault system.

The Sio2 , Na/K and Na:K:Ca geothermometers were applied

to hot spring waters from each of the areas. Estimated

b

v

aquifer temperatures for Bonneville and Sacajawea Hot Springs

are 130-150°C, while those of Kirkham Hot Springs are 70-90°C.

using the silica heat flow method, an average geothermal

gradient of 50°Cjkm was calculated. Thus, the Bonneville and

sacajawea Hot Springs areas have an estimated aquifer source

about 2-3 km below the surface while the Kirkham Hot Springs

reservoir is about 1-2 km deep.

Hot spring vents in all areas are located along faults

and fault zones and discharge from fractures in

granite/granodiorite; they are frequently associated with

dikes.

On a regional basis, each geothermal area occurs where

northwest trending Basin-Range style faults terminate against

the trans-Challis fault system. Recharge is thought to occur

along Basin-Range faults; thermal waters migrate in a

northerly direction along these faults and ascend to the

surface when the trans-Challis fault system is encountered.

I -

vi

TABLE OF CONTENTS

ACKN'OWLEDGMENTS ••••••••••.••••••••••••••••••••••••••••••• page iii

ABSTRACT •...•.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

LIST OF TABLES . ............................................. ix

LIST OF ILLUSTRATIONS ........................................ X

Chapter 1. INTRODUCTION . ........................................ 1

Purpose and Scope. Location ...•..•. Methods .. Previous Investigations ..

... 1 • •• 2

• 2 • •• 4

2. REGIONAL GEOLOGY ....•.........•.•............•....... 6

The"Idaho Batholith. Tectonic Regional

Setting ...•. Structures ..

. ... 6 .• 8

..10

3. LOCAL GEOLOGY . •........•........•................•.. 14

Introduction ..••••. Roadside Geology ... Kirkham Hot Springs .. Bonneville Hot Springs .. Sacajawea Hot Springs ..•

. .14 • ••• 14

.15 . .. 18

. . 21

4 . PETROLOGY • •..•.••.........•.....•...••.............. 2 4

Introduction .. Igneous Rocks.

Rocks ...... . Plutonic Hypabyssal (Dike)

Metamorphic Rocks •.•.. Whole Rock Chemistry .. Regional Comparison •.

Rocks.

.24 • ••• 2 5

• ••••••• 2 5 .26

. ... 2 8 .28 .34

5 · STRUCTURAL GEOLOGY .........................•..•..... 3 6

Introduction ... Structural

Faults Features .. and Shear

• ••• 3 6 .37

Zones .. • ••• 3 7

chapter

vii

page Dikes .. . . ............. 46 Joints . ................. . • •• 48 Slickensided Surfaces ..

Structural Analysis .• Fault Planes. Dikes ....... . Joints ...... .

Regional Interpretation of

. ... 52 . .............. 52

structures ..

.55 ..57

. ... 57

...• 59

6. AQUEOUS GEOCHEMISTRY .....•.......................... 62

Introduction ...•..•.......... .62 Previous Investigations ..... . . ........ 62 Methods ................... 6 2 Chemical and Physical Character of Waters .. • •• 64 Chemical Geothermometers ... . ............... 6 6

Silica Geothermometers. The Na/K Geothermometer .. The Na-K-Ca Geothermometer ..

.68

.71 . ....... 72

Isotope Geothermometers ...... . . . . . . . . . . . . . . . 72 Mixing Models .. . . . . . . . . . . . ...... 73

Resu;t.ts .... . .74 Discussion .......... . . .... 81

6. HEAT FLOW .....•..........••...... ................... 83

Introduction .•. Source of Heat .. Estimation of Heat Flow. Silica Heat Flow ...••.... Calculation of Geothermal Gradient .• Conclusions ....................... .

• • 8 3 . ....... 83

. . 86 . ....... 88

• ••• 8 9 . ..... 91

7 • HYDROGEOLOGY • ••••••••••••••••••••••••••••••••••••••• 9 3

Porosity and Permeability. Source of Thermal Waters ..

Origin •••.•..•. Age • ••..•••••••.•.•. Recharge .......• Depth of Circulation ..

Conceptual Models. Introduction. Local Models .. Regional Models ..

E~tent ~f Geothermal Aquifer .. D1scuss1on .......•...........•

. ........ 93 • ••• 9 5

.95 . ..... 96

• • 96 . .... 96

. ....... 97 .97

..100 • •••. 105

..113 .114 .116 Comparison With Other

Economic Potential of Geothermal Areas. the Geothermal Area ...... 117

viii

chapter page 8. SUMMARY AND RECOMMENDATIONS FOR FUTURE STUDIES ..... 12 0

REFERENCES CITED . .......................................... 12 3

APPENDIX A. GEOLOGIC ROAD LOG FROM LOWMAN, IDAHO TO THE

SACAJAWEA HOT SPRINGS AREA .•...•.•.•....••.......... l33

B. LOWER HEMISPHERE EQUAL-AREA SCHMIDT NETS OF POLES TO JOINT PLANES .•.•.....•.....•.•.......... 13 6

PLATE (IN BACK POCKET) 1. THE GEOTHERMAL AREA EAST OF LOWMAN, IDAHO:

ROADSIDE GEOLOGY

2. GEOLOGY OF THE KIRKHAM HOT SPRINGS AREA

3. THE KIRKHAM HOT SPRINGS AREA: OUTCROP GEOLOGY AND LOCATION OF SPRING VENTS

4. GEOLOGY OF THE BONNEVILLE HOT SPRINGS AREA

5. THE BONNEVILLE HOT SPRINGS AREA (BHS-1): OUTCROP GEOLOGY AND LOCATION OF SPRING VENTS

6. GEOLOGY OF THE SACAJAWEA HOT SPRINGS AREA

7 •. THE SACAJAWEA HOT SPRINGS AREA: LOCATION OF SPRING VENTS

ix

LIST OF TABLES

Table Page

1. Major element oxide abundances as determined by X-ray fluorescence ........•.......•................... 30

2. Physical and chemical character of waters ............. 65

3. Chemical geothermometer equations ..................... 67

4. Calculated thermal aquifer temperatures ............... 75

b

X

LIST OF ILLUSTRATIONS

Figure Page

1. Location of study area ............................... J

2. Geologic setting of the Idaho batholith .............. 7

3. Major structures in the southern Atlanta lobe of the Idaho batholith ......................... 11

4. Location of the Great Falls Tectonic Zone ........... 12

s. The Kirkham Hot Springs area ........................ 17

6. View of the Bonneville Hot Springs area (BHS-1) from across Warm Spring Creek ....................... 19

7. Photo of vents B-1-1 and B-1-2 flowing from northeast oriented fractures ........................ 20

8. The Sacajawea Hot Springs area ...................... 23

9. C~PW normative analyses for plutonic rocks shown on a Q-A-P diagram .................................. . 31

10. K2o versus sio2 diagram showing dike rock analyses . ...................................... 32

11. Basalt dike from the Kirkham Hot Springs area ....... 33

12. Fractured granite along the south Fork Payette River (Kirkham Hot Springs area) ............ 39

13. Fractured granite along the Warm Spring creek shear zone (Bonneville Hot Springs area) ............ 40

14. Synform of foliated, granulated igneous rock along the Warm Spring Creek shear zone .............. 42

15. Normal fault plane near Wapiti Creek (Sacajawea Hot Springs area) ........................ 43

16. Lower hemisphere Schmidt equal-area nets of poles to fault planes ............................... 45

17. Lower hemisphere Schmidt equal-area nets of poles to dike orientation ........................... 47

~----------~---------------------------

xi

Figure page

18. Contoured lower hemisphere Schmidt equal-area nets and rose diagrams of poles to joint planes .......... so

19. Contoured lower hemisphere Schmidt equal-area nets and rose diagrams of poles to joint planes .......... 51

20. Lower hemisphere Schmidt equal-area nets of of fault striae orientations ........................ 53

21. The relation of thrust, normal and strike-slip faults to the principal stress directions ........... 54

22. Lower hemisphere Schmidt equal-area nets of faults and fault striae ............................. 56

23. Lower hemisphere Schmidt equal-area nets of great circles representing contoured joint maxima and possible s 1 orientations ............•............... 58

24. The so~ubility of various forms of silica in water at saturated water vapor pressures ............ 69

25. Equal temperature graph showing quartz (no steam loss) versus Na-K-ca geothermometer temperatures .... 77

26. Equal temperature graph showing quartz (after steam loss) versus Na-K-Ca geothermometer temperatures .... 78

27. · Equal temperature graph showing chalcedony versus Na-K-Ca geothermometer temperatures .......... 79

28. Heat flow contours for the western United States .... 87

29. Simplified hot spring model showing the migration of water through the system ....•.......... 98

30. Location of major faults and hot spring areas ...... 108

1

INTRODUCTION

Rurpose and Scope

A series of hot springs occur along and near the South

pork Payette River east of Lowman, Idaho. A detailed

investigation of the area--Kirkham Hot Springs (including

' Haven Lodge Hot Spring), Bonneville Hot Springs, and Sacajawea

Hot springs--was initiated to assess the geology, hydrology

and geochemistry of this geothermal system. The objectives of

this study are:

1) To determine the detailed structural geology and petrology

of the hot springs areas;

2) To determine temperature and characteristics of the

thermal reservoirs; and

3) To determine the local and regional hydrogeologic factors

controlling hot sprin·g locations.

Although the hydrochemistry of springs in the area of

investigation has been studied (e.g. Lewis and Young, 1980),

no detailed assessments of the local geology as related to the

thermal springs have been conducted. Comprehensive geologic

investigations are necessary to achieve a complete

Understanding of geothermal resourc_es. This investigation is

a continuation of recent geologic studies of hot springs in

the northern Idaho batholith (Kuhns, 1980; Youngs, 1981;

Vance, 1986), and it was carried out in conjunction with a

2

similar study of the geothermal area west of Lowman, Idaho

(Reed, 1986).

The results of this investigation will hopefully further

our understanding of geothermal systems in the Idaho batholith

and should be applicable to other geothermal areas occurring

in faulted and fractured granitic rocks.

Location

The area under investigation is located east of Lowman,

Idaho along the South Fork-Payette River in Boise County

(Figure 1). Sacajawea Hot Springs, the furthest east of the

three geothermal areas, is approximately 48 km from the town

of Lowman. The study area lies within the Atlanta lobe of

the Idaho batholith in central Idaho. Access to the area is

by Idaho State Highway 21 from the southwest (Boise area) or

northeast (Stanley area), or from the west by State Highway 17

from the Banks turnoff on Idaho State Highway 55.

Topography of the area is characterized by rugged

mountains and narrow drainages. The area mainly consists of

undeveloped Boise National Forest (U.S. Forest Service) lands;

the lands are predominantly used for logging and recreation.

Methods

Geologic mapping was conducted using enlarged 1:24000

U.s. Geological Survey topographic maps (Lowman, Eightmile

Mtn., and Grandjean 7.5 minute quadrangles) as base maps.

____i. _______ _

... .. ;>

Idaho City / R7E

-

RBE

BONNEVILLE HOT SPRINGS

AREA

River

miles 3 A S 6

:;= -r; ----r-:- :-;- r-1 ~ kllometers

R9£ RlOE

Stanley

SACAJAWEA / HOT SPRINGS

AREA

t

Figure 1. Location of the study area showing the three areas mapped.

T1

T9N

T8N

R11E

w

4

Geologic, petrologic and structural features were measured

using a Brunton compass and tape measure. Aerial photographs

of the area were examined to provide additional information

concerning the local geology. Areas of 4-10 km surrounding the

three hot springs complexes examined in this study were mapped

in detail.

Previous Investigations

The study area is located within the Challis 1 °X2°

quadrangle. This area was recently mapped as part of the

CUSMAP (Conterminous United States Mineral Assessment Program)

project (Fisher et al., 1983). Although this work constitutes

a vast addition to prior knowledge of the area, it is still

regional in nature due to the large area covered. Detailed

maps of smaller areas have been prepared as part of the

present investigation.

Geologic investigations of the region include Anderson's

(1947) assessment of the economic geology of the Boise Basin.

Olson (1968) investigated the Idaho porphry belt which is a

north-northeast trending belt of porphry dikes intersecting

the South Fork Payette River west of Lowman, Idaho. Reid

(1963) conducted a reconnaisance study of the Sawtooth

batholith, a Tertiary granite pluton southeast of the study

area, while Ross (1934) studied the economic geology of the

Casto pluton (north of the field area). Taubeneck (1971) and

Hyndman (1983) provide comprehensive overviews of the Idaho

~-----------------

·L

5

batholith.

The presence of thermal springs along the South Fork

payette River was reported by Waring (1965) who noted

temperatures and described locations of hot springs in the

study area. Young and Mitchell (1973) provide chemical

analyses and geothermometry data for Kirkham and Bonneville

Hot Springs, but no information is given for Sacajawea Hot

springs. Lewis and Young (1980) present chemical (including

isotopic) and geothermometry data for Kirkham, Bonneville and

sacajawea Hot Springs. Young (1985) summarizes recent

investigations of the hydrochemistry of thermal springs of the

Idaho batholith.

Kuhns (198.0) and Youngs (1981) studied geothermal areas

in the Bitterroot lobe of the batholith and integrated

geologic and hydrologic investigations in a manner similar to

this report; Vance (1986) conducted a similar study of hot

springs situated near the southeast margin of the Bitterroot

lobe. Reed (1986) provides a detailed geologic assessment of

hot aprings occurring along the south Fork Payette River

between Banks and Lowman, Idaho. No other detailed geologic,

hydrologic or structural investigations of thermal springs

along the South Fork Payette River (or any other portion of

the Atlanta lobe) have been published to the author's

knowledge.

~ .... ____________________________________________________ ___

6

REGIONAL GEOLOGY

~ Idaho Batholith

The study area is located within the Idaho batholith of

central Idaho and adjacent Montana (Figure 2). The batholith

is a large (about 40,000 sq km) composite mass of numerous

calc-alkaline plutons and is comparable in size to the Sierra

Nevada batholith in California. Geological studies of the

batholith are few and focus upon its border zones. The Idaho

batholith is probably the least known of the batholiths found

in the western United States.

The batholith is divided into two distinct lobes-- the

Atlanta lobe in the south and the Bitterroot lobe in the

north. These two lobes of the batholith are separated by the

Salmon River Arch which consists of Precambrian granites and

gneisses (Armstrong, 1975). Although Late Triassic to Late

Jurrassic plutons occur in western Idaho and adjacent Oregon,

the Idaho batholith itself appears to be entirely of Middle to

Late Cretaceous age (Anderson, 1952; Armstrong et al., 1977;

Hyndman, 1983). The Atlanta lobe (70 to 100 m.y.) is slightly

older than the Bitterroot lobe (about 80 m.y.) which is

approximately the same age as the Boulder batholith to the

east.

The interior of the batholith is predominantly composed

Of granite and granodiorite plutons. A discontinuous margin

---- --- -------~--- ... ------------

I I I I I I I

Salmon River Arch j

' I ________ ,

Atlanta Lobe

I I

' '

I I I I I I \

'

Snake

' ' '

Bitterroot Lobe

\

" ' I

~ Boulder Batholith

----1

I I I I I

-- -------------------- -------1 1 I

I I I I ____ _

7

Figure 2. Geologic setting of the Idaho batholith (modified from Schuster and Bickford, 1985), showing the location of the study area.

~------------------~----------~

8

1o to 30 kilometers wide consisting of granodiorite and

tonalite borders much of the batholith (Ross, 1963; Hyndman,

1983). This border zone is often gneissic with foliation

oriented parallel to country rock contacts. Country rocks of

the Idaho batholith are Proterozoic Belt metasediments and

pre-Belt basement orthogneisses.

Numerous Tertiary (Eocene) intrusives occur within the

Idaho batholith, many of which are of batholith size

themselves (Reid, 1963: Bennett, 1980). These epizonal

plutons form a belt 100 to 150 kilometers wide extending from

the eastern part of the Atlanta lobe north through the

Bitterroot lobe. These rocks consist of a distinctive bimodal

suite of granite and quartz monzodiorite. The Tertiary

intrusives are closely related to units in the Challis

volcanics which cover much of the eastern margin of the

Atlanta lobe (Bennett, 1980). The shallow emplacement of

these plutons caused widespread rock alteration due to the

generation of giant meteoric-hydrothermal systems (Taylor and

Magaritz, 1978: Criss and Taylor, 1983: Criss and Champion,

1984). A Tertiary granite pluton is present in the northern

half of the Sacajawea Hot Springs area.

Tectonic Setting

The Idaho batholith was intruded along the western edge

of a continental margin subduction zone which was active

during the Mesozoic (Talbot, 1977). This subduction process

9

t ed basement rocks to the east; developing magma rose be a

isostatically and incorporated basement rock, volcani-

clastics, limestones and metamorphosed Belt equivalents.

A period of magmatic inactivity, which occurred in Idaho

during the Paleocene, is attributed to a shallow subduction

angle that prevented the generation of magma (Armstrong,

1974). This period of time is marked by Laramide deformation

and only minor plutonism. A decrease in subduction rate

during the Eocene is thought to have caused a return to a

steeper subduction angle resulting in the eruption of the

Challis volcanics and the emplacement of Eocene plutons in the

Idaho batholith and adjacent areas. Recent work by Bennett and

Knowles (1985) ~ndicates that Tertiary plutons of the Idaho

batholith have many of the characteristics of anorogenic (A­

type) granite; they are probably genetically related to intra­

continental rifting or extension.

The Idaho batholith is emplaced directly east of the

apparently allocthanous terranes of western Idaho and eastern

Oregon, including the seven Devils volcanic arc and the

Wallowa-Blue Mountains (Hamilton, 1976). The stratigraphy and

structure of the Triassic Seven Devils complex is identical to

that of the "Wrangellia" terrane of southern Alaska and

western Canada, suggesting a similar origin (Jones et al.,

1977) .

10

Regional Structures -Basin-range faulting extends into the southern portion of

the Atlanta lobe. Northwest trending mountain ranges in

south-central Idaho (e.g. the Lemhi and Lost River Ranges) are

examples of these basin-range structures (Ruppel, 1982).

Locally, the most important structure is the recently

recognized trans-Challis fault system (Figure 3), a major

structural feature found during the Challis CUSMAP project

(Bennett, 1984; 1986) 0 The three geothermal areas

investigated in this study lie within or near this significant

structure. The trans-Challis fault system is a northeast

trending series of high angle faults and shear zones

(Kiilsgaard and ·Lewis, 1985; Bennett, 1986), approximately 15

miles wide. It apparently is a continuation of the Great

Falls Tectonic Zone (O'Neill and Lopez, 1985), a conspicuous

zone of northeast trending faults, Tertiary dike s~arms and

topographic lineaments in western Montana and east-central

Idaho that is thought to extend from Boise, Idaho into

Saskatchewan, Canada (Figure 4).

Northwest-trending basin-range faults terminate against

the southern edge of the trans-Challis fault system (Bennett,

1986). Examples of such faults are the Deer Park fault, which

terminates near the Kirkham Hot Springs area, and the

Montezuma fault, which has probable extensions that terminate

in the Bonneville and Sacajawea Hot Springs areas (Figure 3;

also, see Figure 30). Mineralization occurs along northeast

~·.

0

N

r

Figure 3. Major structures the Idaho batholith Knowles, 1985).

115"

0

txP!,..WTION

- Tertiary plutons

.1'~ Tertiary dikes

.,........ Fault

11

Generalized outline of Atlanta lobe of Idaho Satholith

10 20

K110Mters

in the southern Atlanta lobe of (modified from Bennett and

\

J I

---

I

I I I

f

I

I I

I I

....

I

\ I I \ \

' '

I I

-- I I -------------\ I I ' I I I I I I I 1 I I "'/ f 1 II ,.

I .,., I I -'// I I ? : 1/ I II I I \

\

' I ', I

1-;-' I~L..

I

/ /

I I \ I I -- -CJ

I I ---_I /- I I I ...,_ -- I'-

'\ / t, /R/.~ --;--...l. ____ i __ '-----------~ ~~__... ,.,., i \

. / ~ '--.. f.?. "' GREAT FALLS \

)i;!/', __ "-,,-!~~~~~-Z~NE -------\ I I I I I

r--- I 1 •---/ -- I I I -~----,' ~----------,~ I I ,' I \ I I '- 1 \ I I -~---------, \ ' I 1

I ' \ I I r----- -- '-...

\ I I I \ I I I

\ ~- I I ' I ---- I

\ I -f------ --,--------\ / I ,-----,

Figure 4. Location of the Great Falls Tectonic Zone (modified from O'Neill and Lopez, 1985).

12

J 13

trending faults within the trans-Challis fault system and

northwest oriented basin-range faults. Mineral deposits

commonly occur along faults that are near Tertiary plutons

(Kiilsgaard and Bennett, 1985). The presence of

mineralization indicates that these faults have acted as

conduits for fluid circulation in the past. Currently active

geothermal systems may represent a continuation of this

process to the present.

The trans-Challis fault system appears to mark the

transformation from northwest extension (Eocene) to northeast

extension (Oligocene) (Bennett, 1984; 1986). Structural

evidence indicates that the trans-Challis fault system/Great

Falls Tectonic Zone represents a major northeast-trending zone

of crustal discontinuity that has been active since the

Proterozoic.

14

LOCAL GEOLOGY

Introduction

Areas of 4-10 sq km surrounding the three hot spring

complexes were mapped. Geologic outcrop maps showing the

detailed geology in the immediate vicinity of each hot spring

area were also prepared.

The roadside geology of the area between Lowman and

Grandjean was examined to integrate geologic information

between the three areas in order to provide continuity.

outcrops along and near the road were examined, and

information concerning lithologies and structures was

recorded. This information was augmented with geology from

the Challis 1°x2° map (Fisher et al., 1983). The results of

this survey are shown in Plate 1, and a detailed log can be

found in Appendix A.

The lithologies and structures of the study area are

described fully in later chapters of this thesis.

Roadside Geology

Most of the study area is underlain by Cretaceous rocks

of the Idaho batholith (Plate 1). These rocks .are primarily

biotite granite and granodiorite (Kgd), but small plutons of

leucocratic granite (Klg) also occur in the area. The

northern half of the Sacajawea Hot Springs area is underlain

~-

15

bY a Tertiary granite (Tg) pluton which extends to the north

and the northwest. Dike rocks are of Tertiary age (Fisher et

al., 1983) and range in composition from basalt to rhyolite.

Dikes in the study area exhibit a strong northeast

orientation.

Plate 1 shows the predominant northeast trending faults

which are elements of the trans-Challis fault system. The

general westerly flow direction of the South Fork Payette

River is commonly altered to a northeast-southwest course when

these structures are encountered, as are minor drainages.

Northwest trending faults occur in the study area to a

lesser extent (Plate 1). These structures are Basin-Range

style faults and terminate against the trans-Challis fault

system. Although drainages are locally parallel to these

northwest oriented faults, the trans-Challis fault system

exhibits a much stronger topographic expression.

Kirkham Hot Springs

The Kirkham Hot Springs area (Plate 2) is entirely

underlain by Cretaceous biotite granite and granodiorite

(Kgd). Dikes of basalt and dacite composition occur in this

area. Although rhyolite dikes were not mapped at Kirkham Hot

Springs, they do occur in adjacent areas (see Plate 1 and

Appendix A).

A northeast trending normal fault occurs in the northern

Portion of the area (Plate 2); the I< i rkham Creek drainage is

16

10cated along this structure. A fault zone composed of east­

~est striking fractures dipping about 50° to the south was

mapped along the South Fork Payette River in the central

portion of the area. Dikes usually strike northeast, but

east-west oriented dikes are also present.

Hot spring vents are arranged in a curvilinear manner

roughly parallel to the South Fork Payette River (Plate 3 and

Figure 5). The majority of vents at Kirkham Hot Springs

discharge from alluvium. One major vent (K-6) is located in a

small Kgd outcrop and discharges from fractures. Minor vents

and seeps occur in outcrop near the South Fork Payette River

and discharge from east-west trending joints.

Two dikes of basaltic composition were mapped in the

immediate vicinity of Kirkham Hot Springs (Plate 3). An east­

west oriented dike is located near the western edge of Plate 3

and extends across the river. This dike can be projected to

the east through hot spring vents that discharge from

alluvium. Hot spring vents are located on both the north and

south sides of the projection of this dike. A second dike is

located in the eastern portion of the area and terminates

against the South Fork Payette River. This basaltic dike also

crops out on the slope above and to the south of the hot

spring vents (see Plate 2).

Haven Lodge Hot Spring is located along the western

margin of the mapped area (Plate 2) and consists of only one

vent. Hot water from this spring is piped to a swimming pool

Figure 5. The Kirkham Hot Springs area. Springs are located on the far side of the South Fork Payette River at the base of the slope.

17

18

at Haven Lodge. No outcrop exists in the vicinity of this hot

spring.

Bonneville Hot Springs

The Bonneville Hot Springs area is composed of thermal

springs located along Warm Spring Creek (Plate 4). The

country rock of this area is primarily biotite granite and

granodiorite (Kgd), but a pluton of leucocratic granite (Klg)

is also present. Dike rocks range from basalt to rhyolite in

composition and exhibit a marked northeast orientation.

Two faults are present in the area. A left-lateral shear

zone was mapped along Warm Spring Creek, and the South Fork

Payette River flows roughly parallel to a right-lateral fault

(Plate 4).

The four hot spring complexes located in the Bonneville

Hot Springs area (BHS-1, -2, -3, and -4) are arranged in a

relatively straight alignment along the Warm Spring Creek

shear zone {Plate 4). Plate 5 is a detailed geologic outcrop

map of the BHS-1 area, and Figure 6 is a view of this area

from across Warm Spring creek. BHS-1 contains the most vents

as well as vents with the highest discharge and temperature.

All vents in this area discharge from fractures within Kgd

outcrop; Figure 7 is a photo of vents B-1-1 and B-1-2 which

issue from well-developed, steeply dipping joints that strike

northeast. Hot water has enlarged these fractures and the

thermal vents are up to 5 em in diameter. Although no hot

19

Figure 6. View of the Bonneville Hot Springs area (BHS-1) from across Warm Spring Creek.

Figure 7. Major hot spring vents discharging from northeast oriented joints at the Bonneville Hot Springs area (BHS-1).

20

21

springs discharge from dikes, nearly half of the vents are

located within several meters of dikes (Plate 5).

BHS-2 is located upstream from BHS-1 on the west bank of

warm Spring Creek (Plate 4). This hot spring complex consists

of two vents and several seeps that discharge primarily from

unconsolidated river gravels. Seeps were observed flowing

from minor, steeply dipping shear planes striking northwest

and showing left-lateral displacement. Several dikes were

mapped in the vicinity including two dacite dikes that are

adjacent to BHS-2.

Hot springs BHS-3 and BHS-4 are located in the northern

portion of the mapped area along Warm Spring Creek (Plate 4).

Both hot springs· discharge from unconsolidated gravels. BHS-3

has only one vent and is located adjacent to a rhyolite dike

swarm that has been left-laterally displaced by the Warm

Spring Creek shear zone. BHS-4 consists of six vents located

on the east side of Warm Spring Creek and several seeps on the

west side of the creek. No dikes were mapped near BHS-4.

Sacajawea Hot Springs

The Sacajawea Hot Springs area is the furthest east of

the three geothermal areas. The south Fork Payette River

marks an inferred fault boundary between a Tertiary granite

(Tg) pluton to the north and Cretaceous granite and

granodiorite (Kgd) to the south (Plate 6). Leucocratic

granite (Klg) is also present in the southeastern portion of

22

the mapped area. Much of the area along the South Fork

payette River contains no outcrop and has been mapped as

undifferentiated Quaternary deposits (Qd). Numerous dikes

were mapped in this area and they range in composition from

basalt to rhyolite.

An east-west striking fault is thought to be present

along the South Fork Payette River, and a left-lateral shear

zone was mapped along Bear Creek (Plate 6). A steeply

dipping, northwest trending normal fault is present in the

southern portion of the area and terminates near the South

Fork Payette River. A fault is also believed to be present

along the eastern margin of the mapped area near Grandjean

Creek. Most dikes show a northeast trend, but several dikes

with an east-west .orientation are also present.

Hot spring vents in this area occur in a relatively

straight alignment along the north bank of the South Fork

Payette River (Plate 7 and Figure 8). All hot spring vents

issue from unconsolidated Quaternary deposits; no outcrop

exists near these springs. A shallow hand dug well supplies

hot water for a swimming pool at the Sawtooth Lodge in

Grandjean.

~~----------------------~

Figure 8. The Sacajawea Hot Springs area. Springs are located between the trail and the South Fork Payette River.

l\J w

' I

24

PETROLOGY

Introduction

The rocks of the study area are predominantly granites of

the Idaho batholith. Other rocks occurring in the field area

include leucocratic granite; dacite, basalt and rhyolite dike

rocks; leucocratic dikes; and metamorphic rocks.

A total of 41 thin sections were examined to determine

mineralogy and textural features present. Sixteen whole rock

(X-Ray fluorescence) analyses were made at the Washington

State University Basalt Research Laboratory to determine major

element oxide abundances. These data were used to calculate

CIPW normative mineralogy.

The following lithologic desc~iptions are based on field

and hand sample descriptions augmented with petrographic and

chemical analyses. Modal percent compositions were determined

by visual estimation. In most cases, rock locations and

descriptions are in agreement with the Challis 1°X2° geologic

map (Fisher et al., 1983). Terminology and lithologic symbols

assigned below are consistent with the Challis map when

possible. Many more dikes were mapped in the field area than

are found on the Challis map.

~-----------

25

19neous Rocks

Plutonic Rocks

Biotit~ §_ranite an~ §_ra!!_2di_2_!:it~ (Kgd)--Biotite granite

is by far the most common lithology in the study area (see

plates 1-7). This rock is light to medium gray, equigranular

and fine- to medium-grained. Plagioclase (An25 _35) is the the

dominant constituent (40 to 50%). Other minerals present are

orthoclase (30 to 35%), quartz (20 to 25%), pleochroic green

and brown biotite (up to 5%), and muscovite (up to 5%, but

usually only minor amounts). Accessory minerals include

sphene, epidote, zircon, apatite, allanite and opaque

minerals. Myrmekite and granophyric texture are common in

this lithology. Plagioclase and orthoclase are usually

altered to sericite, and both micas commonly exhibit chloritic

alteration. The biotite granite is Late Cretaceous in age

(Hyndman, 1983} and is found in each of the three geothermal

areas examined.

Leucocrati~ Granit~ (Klgj_--Leucocratic granite was

mapped in the Bonneville Hot Springs area (see Plate 4}. It

also occurs in the southeast corner of the Sacajawea Hot

Springs area (Plate 6), but this area was not mapped in this

investigation and its location is based on the Challis 1 °x2°

map (Fisher et al., 1983}; It is fine- to medium-grained,

light gray and has a granular texture. The rock consists of

plagioclase (An15 _30 , 40 to 50%), orthoclase (30 to 40%},

quartz (15%) and pleochroic brown and green biotite (up to

~--------------------------------

26

1%)· Accessory minerals are epidote, muscovite, sphene and

opaque minerals. Leucocratic granite intrudes (and is

therefore younger than) biotite granite (Kgd) (Fisher et al.,

1983) .

Granit~ 1!91--The southern margin of a Tertiary pluton

was mapped in the Sacajawea Hot Springs area (see Plate 6).

Tertiary granite is medium-grained, equigranular and medium

gray. It is composed of plagioclase (An 25 _ 30 , 45 to 55%),

orthoclase (20 to 30%), quartz (20 to 30%), biotite (5 to

10%), and muscovite (2 to 3%). Accessory components include

apatite, sphene, epidote and opaque minerals. Pink feldspar,

smoky quartz and miarolitic cavities are characteristic of

this lithology in the field area. Bennett (1980} states that

these features are diagnostic of Tertiary granites of Eocene

age in .the Idaho batholith.

Hypabyssal (Dike) Rocks

General-- Dikes of basalt to rhyolite composition are

common in each of the three geothermal areas, and they are of

Tertiary age (Fisher et al., 1983). These dikes are several

centimeters to 12 meters in width and display a marked

northeast trend in all areas. They are usually moderately to

highly altered; secondary calcite is common in dike vugs and

fractures.

Leucocratic dikes, composed predominantly of quartz and

feldspar, also occur in the area of investigation. They are

not mappable because of their narrow (rarely more than several

? ;

27

centimeters) width and limited occurrence. These quartzo­

feldspathic dikes are older than the Tertiary dikes and are

probably related to the leucocratic granite described above

(Fisher et al., 1983).

Dacite ~--Dacite is the most common dike lithology in

the study area (see Plates 1-7). It is medium grayjgreen and

is predominantly porphyritic, containing phenocrysts of

plagioclase, orthoclase, quartz, biotite and sparse

hornblende. Matrices are composed of plagioclase, orthoclase

and quartz. The rock as a whole contains plagioclase

(approximately An 30 _40 , 40 to 50%), orthoclase (20 to 30%)~

quartz (15 to 20%), opaque minerals (10 to 15%), biotite (5 to

10%), and hornblende (less than 3%). Minor amounts of

epidote, allanite and sphene also occur in dacite dikes.

Feldspars are largely altered to sericite, and mafic

phenocrysts are altered to chlorite. Dacite dikes rarely

exhibit faint flow banding.

Rh,Y£li_!~ .IT!:l-- Rhyolite dikes were mapped in the

Bonneville and Sacajawea Hot Springs areas (Plates 4 and 6),

but none were mapped in the Kirkham Hot Springs area (Plate

2). However, rhyolite dikes were mapped adjacent to the

Kirkham Hot Springs area (see Plate 1). Rhyolite dikes are

light gray or pinkish gray and are always relatively coarse

and porphyritic. They are composed of plagioclase (An25 _35 , 35

to 50%), orthoclase (30 to 50%), quartz (10 to 20%), and

biotite (up to 5%). Minor amounts of apatite, epidote, sphene

~-----------------

. !

28

and opaque minerals also are present in rhyolite dikes.

Feldspars are strongly altered to sericite as is biotite to

chlorite.

Basalt and Basaltic-Andesites ~--Basalt and basaltic­

andesite dikes were mapped in all three areas but they are the

least common of the dike rocks. Plagioclase laths (An35 _55 ,

35 to 50%), clinopyroxene (30 to 40%), quartz (5 to 10%) and

opaque minerals (5 to 10%) are the chief components. This

rock can be porphyritic; plagioclase and clinopyroxene occur

as euhedral to subhedral phenocrysts and are subophitic.

Basalt and basaltic-andesite dikes are generally less altered

than the rhyolite and dacite dike rocks and they are probably

younger (Fisher et al, 1983).

Metamorphic Rocks

Remnant Metamorphic Xenoliths l£El--Xenoliths of remnant

metamorphic rock were mapped in the Sacajawea Hot Springs area

and occur locally within the border zone between the Tertiary

granite pluton to the north and the Cretaceous granite to the

south (see Plate 6). These are banded schists, quartzites and

calc-silicate rocks of uncertain Precambrian-Paleozoic age.

~hole Rock Chemistry

Whole rock X-Ray fluorescence analyses were made on 16

samples from the study area (8 plutonic rock, 6 dike rock, and

2 metamorphic rock samples). All analyses are volatile free

~---------

29

and normalized to 100%; Fe2o3;Fe0 ratios are set at a constant

value of 0.87. The results of these analyses are found in

Table 1.

CIPW norm calculations for the plutonic rocks were

plotted on a Quartz--Alkali Feldspar--Plagioclase triangle

(Figure 9). Plagioclase values for this diagram are the

combined anorthite (An) and albite (Ab) CIPW norm values from

Table 1. All of these samples plot within the granite or

granodiorite fields of this diagram. Sample P is a Tertiary

granite sample and appears to be less alkalic than the

cretaceous granites.

The six dike rock samples were plotted on a K2o against

sio2 diagram (Figure 10). These samples fall within the

basalt, basaltic-andesite, dacite and rhyolite fields,

indicating that a wide range of dike rock compositions are

found ·in the study area.

It is interesting to note that one dike sample (sample K-

12, symbol H on Figure 10; see Figure 11) from the Kirkham Hot

Springs area has a whole rock chemistry that falls within the

ranges given for Imnaha basalts from the Weiser embayment

(Fitzgerald, 1982). The Weiser embayment is the southernmost

extension of t.he Columbia Plateau and is made up of rocks of

the Columbia River Basalt Group. A dike recently analyzed

from an area west of Lowman, Idaho (within the Idaho

batholith) also has a composition similar to Imnaha basalt

(John Reed, personal communication, 1986). Although Columbia

SYM8Q. • S I Ill 10. 96,

Al201 15.40 FE20 1 I. 40 FED lobi MGO 1.2'1 CAD 1.89 1'4A20 z .88 K20 1. •2 TI02 0.41 P205 0.14 MNO 0.07 TOTAl qq.,qq

Q n.on t 3. 319 OR 21.167 A8 24.372 AN 8. 462 wo EN 1.213 FS 1.zzo M7 2.030 HH ll ~.,.11

RU

•• 0.112 TUTAl 1 oo.ooA SAllC qz .. 3'16 FE" JC 7.hl2

01 01~0 OlfN orr s

HY 4.413 flY: Pf 3.213 HY< S 1.220

WOl

U203fil02 o. 21 7 FEOfFE 203 I. 150

0.1. "o. 61 5

~ '>

Table 1. Major element oxide abundances as determined by whole rock X-Ray fluorescence, and the results of CIPW norm calculations.

8 c 0 E F G H J K l " N 0

73.21 11. eo 78.05 66. 21 49.61 75.H 50.61 75.48 54.18 61.46 77.51 8I.JH 78. Yo 16.56 I 1. 52 I 2. 58 15.09 14.17 15.?1 15.76 15.03 15.6) 16.27 I<. II 4.43 13.48 o. 38 0.05 O.tO 2.42 5.47 0.26 5 • .66 0.59 4.50 2.76 0.11 o. 43 o. 14 0.4) 0.06 0.11 2. 77 6. 26 0.30 6.48 0.68 5.t6 ).17 0.15 o.so o. 16

0.08 2.M •• 36 0.07 6.2-'t •• 07 2. 54 3.zq 1.2} o. 74 0.40 3. 25 8.67 0.92 9.92 I. 75 6.)9 3.95 o. 74 8.os o. 35 4.12 2. 52 o. 59 2. 7T 2. 77 1.54 2.46 ).PO 2. 83 2.•6 J. 38 1.zo J.B 1.82 5.11 7. 93 3.4~ 2.n 4.16 O.l'J 2.n 2.17 1.50 ).86 4. II o.u 0.0'9 o. 09 o. 78 2. 55 0.12 2.02 0.22 I. 55 o.•1 o.o9 o. 23 o. 04 0.04 0.04 0.04 o. 26 2.17 0.02 0.}0 0.05 o. 78 0.32 0.02 0.()9 o. 02 0.0} o. 02 o.o2 0.10 0.15 0.0} 0.11 0.04 0.14 0.12 0.07 0.02

100.00 I 00.01 qq.qq 9'1.<19 100.01 '19.99 100.01 100.01 100.00 9Q.qft 9'1.<J9 100.00 ~.qq

31.'J97 41.880 43.408 2 5. 415 1. 464 36. 7Z5 6. 734 40.1>79 6.050 20.491 41.5" 5~.433 42. 4"~ 1.507 2. 52'1 2.3'15 1. 480 3.2H 1.151 1.1<>7 3.074 2. 965

22. 573 1 o. 548 46. 865 10.566 l6oll1 24.585 2.}0'> 14.004 16.369 20.687 22.812 24. 290 34. qh2 21.121 4.993 23.441 23.4H 29 •• 57 20.814 JZ.I51 23.9H 25.052 28.603 10. p; .. 28. 180

5 .... 3.409 I. 723 1<1. 426 18.439 4.•'>14 }0. 805 8.3H 21.764 17.509 }.541 6. 701 1.606 4. 340 6.866 2.021 14.q)6

0.199 7.0H 13. 348 o.IH 15.539 15.117 6.3:!7 8.lqlt 0.234 o.ao8 1.985 }.044 O.lqlt 4. 203 O.H2 3.4~'9 2.229 o.o1• o. 11 3 0.!49 0.5~1 o. 145 3. 509 7.'130 0.377 8.206 0.855 6.525 4.003 0.189 o.•21 0. ZOJ

o.oso o. 342 0.169 0.171 1.482 4. f'4l o.2ze 3. 836 0.418 2.944 1.767 o.111 0.437 0.076

0.001 0.0'>5 o. 095 o. 095 0.616 5.13'1 0.047 o.n1 0.118 1.a41 o. 758 0.041 0.213 0. Oft 1

100 .ooz I 00.002 100 .oo2 100.015 100.114 100.001 100.018 I 00.003 100.042 I 00.018 100.001 I oo.oos 100.001 9R.7Pl <q<t.,t.B7 qq., 3A-\ 85.349 61.4 70 9ft.981 60.657 98.13'1 68.12q 84.935 .... 575 75.288 99.525

1.222 0.315 0.618 14.666 3B.61t3 1.020 3Q. 360 1.864 11.913 15.08) 0.426 24.717 0.4 76

•• 264 13.118 3. 850 !8.264 4.340 6.8M z. 021 9. 757 3ol96 4. 921 1.488 8.1CJ1t o. 72'1 1.331 0.340 0.311

0.234 o. 207 '1. 059 12.467 0.368 13.491 0.472 16.748 A.5~6 o.DI9 0.149 0.199 7.0H 10.152 0.174 10.618 13.630 6.3?7

0.234 o.oos 1. 9A5 2.115 0.194 z. 872 0.472 3ol18 2.229 0.019 0.149 5.180

o.n. 0.174 0.161 o. 22 8 o.2A8 0.202 o. 311 0.1'19 o.?A8 0.256 0.182 0.055 0.112 1.1J 2 1.200 1.100 1.145 ).144 1.154 1.145 1.153 1.147 1.149 1-154 1.16] 1.143

Rq., 433 cn.71tCJ 95.266 69.443 43.032 91.268 29.852 86.634 46.365 66.22'1 92.959 68.5H CJ4.951t

68.90 16.19

1. 62

1. ~· 1.1.? 3. 78 3. 24 z. 38 1).,6'5

o. 20 o. 06

100.00

31.2H I. 890

14.064 2 7. 416 l1.1tltb

2. 789 1. 115 2. 349

I. 234

0.474 100.011 qz .o49 7.962

3. 905 2. 789 1.115

o. 235 1.148

72.113

74.17 15. 17 0.52 0.60 0.22 1.90 2.91 4. 23 0.19 0.06 0.03

100.00

}6.617 2. 4"4

2 .... Q96 24.624 9.0}4

o. 548 0.414 o. 754

o. 361

0.142 100.003 97.784

2.219

0.'162 0.548 0.414

o.zos 1.154

86.25 7

w 0

Q

quartz

granitoids

c granite

syenite monzonite

monzodiorite

and

monzogabbro

31

Figure 9. CIPW normative analyses for plutonic rocks shown on a Quartz (Q) --Alkali Feldspar (A) --Plagioclase (P) diagram (rock classification from Streckeisen, 1976). Reference letters refer to Table 1.

0 ... ::::..:

~ . -+-' 3

5.0 I J 4. 0 ~ I

Banakite High-K Dacite Rhyolite (High-K)

A L

I I Shoshonite ~ I 3.0

F

2.0 l Absarokite /'I Basaltic I --------- I

Dacite I Rhyolite (Calc-alkali)

~R.3co:.lt- I ................ ~- ~ Andesite I (Calc-alkali)

(Calc-alkali)

l.O ~ (Calc-alkali) ,....,n,..__-.J' ...,,_

0 r= 45

I ' .............. -. ··- .. ,

Low-K I I I Basaltic Low-K Andesite Low-K Dacite Low-K Rhyolite

Low-K Th~leiite 1 Andesite

50 55 60 65 70

Wt. % SiO.

Figure 10. K2o versus Sio2 diagram showing dike rock analyses (rock classification after Ewart, 1982). Reference letters refer to Table 1.

75

w 1\.)

33

Figure 11. Basalt (Tb) dike from the Kirkham Hot Springs area.

!

L

34

River Basalts have not been reported from the Idaho batholith

(P. R. Hooper, personal communication, 1985), it seems that

some dike rocks within the batholith may at least be related

to the Columbia River Basalt Group. Additional, more detailed

petrochemical investigations are necessary before any

conclusions can be drawn.

Regional Comparison

The maps resulting from this investigation (Plates 1-7)

indicate that dike lithologies differ in each area. Basalt

and basaltic-andesite dikes are most common in the Kirkham Hot

Springs area, while rhyolitic dikes are most common in the

Bonneville Hot $prings area. Dacite dikes are predominant in

the Sacajawea Hot Springs area. Alteration is equally common

in all areas. Both plutonic and dike rocks are similar

throughout the entire study area (i.e. basalt dikes have

similar characteristics in each area, for example); no major

differences for a given lithology were observed in the field

or in thin sections between hot spring areas.

Kiilsgaard and Lewis (1985) describe six rock types in

their study of Cretaceous plutonic rocks in the Atlanta lobe.

The rock types are tonalite, hornblende-biotite granodiorite,

porphyritic granodiorite, biotite granodiorite, muscovite­

biotite granite, and leucocratic granite. Only biotite

granodiorite and leucocratic granite were observed in the

present study. Descriptions of these two rock types are

35

consistent with those of Fisher et al., (1983); Hyndman,

(1983); and Kiilsgaard and Lewis, (1985).

Bennett (1980) and Bennett and Knowles (1985)

characterize Tertiary plutonic rocks as a bimodal suite of

granite and quartz monzodiorite. A Tertiary granite pluton

was mapped in the Sacajawea Hot Springs area (Plate 3); this

rock is part of the North Sawtooth batholith (Bennett and

Lewis, 1985). Field and thin section descriptions, as well as

whole rock data, are quite similar to those in the literature

(Bennett, 1980; Fisher et al, 1983; Bennett and Knowles,

1985) .

Very little information exists in the literature

concerning dike ~ocks in the Atlanta lobe. Bennett and Lewis

(1985) state that rhyolite and dacite-rhyodacite dikes are the

hypabyssal equivalents of Tertiary granite and monzodiorite,

respectively. The Challis 1°X2° map shows both rhyodacite and

diabase dikes mapped in the study area and the surrounding

terrain, but only brief descriptions with no modal

normative compositions are given (Fisher et al., 1983).

or

~--------------~n-

36

STRUCTURAL GEOLOGY

Introduction

Previous investigations concerning the geologic setting

of geothermal areas have established that thermal spring

locations are essentially a function of the structural

features of an area. In the Bitterroot lobe of the Idaho

batholith, Kuhns (1980) found that hot spring vents are

located where northeast trending dikes intersect north or

northwest trending shear zones in the Lochsa geothermal

system. In the Running Springs geothermal area, Youngs (1981)

observed that major hot spring vents are located in rhyodacite

dikes due to the preferential development of fracture-induced

permeability. Vance (1986) found that hot springs occur in

fracture zones and at fault intersections. Working in the

Atlanta lobe of the batholith, Reed (1986) found that fault

intersections and areas with relatively high fracture

densities contain thermal springs. Additional studies in the

Idaho batholith (Foley and Street, 1985; Young, 1985) and

other areas (e.g Whit~, 1973; McLaughlin ~nd Stanley, 1976;

Muffler, 1976; Wan,

association of hot

1984) have revealed an ubiquitous

spring vents with major geologic

structures. Therefore, a detailed structural analysis is a

necessary aspect of an evaluation of a geothermal resource.

Geologic mapping provided much information concerning the

.._ _____ _

37

structural geology of the area. The orientations of major

structures (faults, dikes) and minor structures (joints, fault

striae) were recorded. A total of 773 orientations were

plotted on lower hemisphere Schmidt equal-area nets. Each of

these structures are considered individually in the following

sections. An initial attempt at dividing each hot spring area

into structural domains bounded by faults was abandoned since

differences in structural trends could not be discerned. The

distribution of major structures and rock types is shown in

Plates 2, 4 and 6.

Structural Features

Faults and Shear Zones

Evidence for both shear zones and discrete faults occur

in each hot spring area. Shear zones in accordance with the

definition of Ramsay (1980) exhibit both ductile and brittle

deformation, but brittle deformation appears to be more common

and occurs as pervasively faulted rock. Ductile deformation

is expressed locally as small areas of foliated rock and, in

one location, a fold (see Figure 10).

Fault zones are common in the study area. Faults, fault

zones and shear zones were identified by slickensided

surfaces, brecciated and granulated rock, and displaced dikes

and veins (rare).

Kirkham Hot Springs

The most obvious faulting in the Kirkham Hot Springs area

~-------------------

38

has occurred along the South Fork Payette River (Plate 2;

Figure 12). In most areas, fracture surfaces could be found

with fault polish, grooves and striae. Fault surfaces were

oriented predominantly east-west with a dip of 40 to 50

degrees south. Indications of normal motion were prevalent

(i.e. the north side moved up); however, strike-slip and

oblique movement have also occurred (based on slickensides

striae and chattermarks) .

A fault that strikes east-northeast and dips 75 degrees

southeast was also mapped along Kirkham Creek. Normal motion

is indicated by slickensides striae and chattermarks.

Bonneville Hot Springs

There are two major faults in the Bonneville Hot Springs

area (Plate 4). Warm Springs Creek closely follows a north­

northwest trending shear zone. Both granitic and dike rocks

are highly fractured and jointed (Figure 13), and fault striae

occur locally along the entire length of the shear. Displaced

dikes (mafic and leucocratic), and fault striae and

chattermarks suggest predominantly left-lateral strike-slip

motion; however, slickensides striae indicate that both normal

and right-lateral motion have also occurred. Duc~ile

deformation is also found along Warm Springs Creek. Narrow

ductilely deformed shear planes are visible in the areas of

BHS-1 and BHS-2. Folding occurs in several areas; the best

developed example is a synform of foliated, granulated igneous

material with a fold axis orientation of trend (T) 305°;

Figure 12. Fractured granite along the South Fork Payette River (Kirkham Hot Springs area). Note thermal seeps (brown areas).

~-----~

39

Figure 13. Fractured granite along the Warm Spring Creek shear zone (Bonneville Hot Springs area).

40

~---------------

,1 ~

41

plunge (P) 25° (Figure 14).

Faulting has also occurred along the South Fork Payette

River in this area. Both granitic and coarse-grained mafic

rocks are highly brecciated and display fault grooves. Fault

striae and chatter marks suggest both strike-slip and normal

motion; right lateral strike-slip indicators are prevalent.

Both of these faults have been previously mapped by

Fisher et al., 1983.

Sacajawea Hot Springs

Faults are an important structural feature in the

Sacajawea Hot Springs area. Evidence for both shear zones and

discrete normal faulting was found in this area. A shear zone

occurs along Bear Creek (northeast section of mapped area,

Plate 6). The granitic rock in this area is highly jointed;

joints are curviplanar and discontinuous. Fault striae (T

85°, P 15°) and chatter marks suggest left-lateral strike-slip

motion. Numerous cold springs and seeps occur along and near

the drainage.

Fault planes were found on the slopes east of Wapiti

Creek (located along the southwest margin of the map area,

Plate 3; Figure 15). The·se fault planes are northwest

trending and dip approximately 75 degrees northeast. Striae

and grooves indicate normal/oblique motion with the northeast

block as the hanging wall. Brecciated, altered basalts within

a quartz matrix were found in this area. Displaced rocks in

many cases were brecciated andjor had striated surfaces in