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