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Geothermal Resources Council, TRANSACTIONS Vol. 7, October 1983
STRUCTURAL CONTROL OF THE BALTAZOR HOT SPRINGS
GEOTHERMAL SYSTEM, HUMBOLDT COUNTY, NEVADA
J e f f r e y B. Hulen
Earth Science LaboratorylUni v e r s i t y of Utah Research I n s t i t u t e
420 Chipeta Way, S u i t e 120
S a l t Lake C i t y , Utah 84108
ABSTRACT
and major new photo-1 inears. Groundwater sampl ng
and ( i n p a r t i c u l a r ) geothermometry ( K l e i n and
Koenig, 1977) t a r g e t e d t h e immediate Bal t a z o r Hot
s p r i n g s area as t h e r e g i o n ' s most promising geothermal prospect. Shal low- and intermedi ate-depth
thermal g r a d i e n t d r i l l i n g (Langenkamp, 1978; EPPC
w e l l l o g d a t a ) d e l i n e a t e d strong heat flow anomal i e s a t B a l t a z o r and t h e Painted H i l l s prospect
(Hulen, 1979), about 11 miles/17.7 km t o t h e
southwest
A r e g i o n a l aeromagnetic survey
( S c i n t r e x , 1972) had shown only broad r e g i o n a l
basement c o n f i g u r a t i o n s , o f l i t t l e use i n chara c t e r i z i ng geothermal features.
Senturion Sci
ences (1977) demonstrated contemporary m i c r o e a r t h quake a c t i v i t y along, among o t h e r s t r u c t u r e s , t h e
range f r o n t f a u l t system passing west o f B a l t a z o r
Hot Springs, thus enhancing chances f o r discovery
there
of
adequate
subsurface
permeabi lity.
Edqui sJ, (1981) compl eted d e t a i 1ed g r a v i t y p r o f i1
ing, extremely u s e f u l i n s t r u c t u r a l i n t e r p r e t a t i o n
o f t h e s u r p r i s i n g l y deep graben beneath B a l t a z o r
Hot Spri ngs
D i pole-di p o l e r e s i s t i v i t y and s e l f p o t e n t i a l data gathered by Mining Geophysical
Surveys, Inc. (1981) f o r EPPC were a l s o i n t e r p r e t ed by Edquist (1981).
He c o r r e l a t e d shallow,
near-surface r e s i s t i v i t y lows and r a p i d l y changing
s e l f - p o t e n t i a l values a t B a l t a z o r w i t h mapped and
g r a v i t y - i n f e r r e d f a u l t zones b e l i e v e d t o be t h e
prime c o n d u i t s f o r deep thermal f l u i d convection.
A mercury-arsenic ' s o i l survey (Truex, 1980) o f t h e
B a l t a z o r area revealed no d i a g n o s t i c anomalies
e x c l u s i v e l y r e l a t e d t o t h e a c t i v e thermal system.
Hul en (1979) produced p r e l i m i nary geol ogy and
hydrothermal a l t e r a t i o n maps o f B a l t a z o r and t h e
Painted H i l l s .
The B a l t a z o r map, t o g e t h e r w i t h
r e s u l t s from EPPC's r e c e n t l y completed deep w e l l
near B a l t a z o r , form t h e b a s i s f o r t h e d e t a i l e d
surface and subsurface geologic i n t e r p r e t a t i o n s
presented i n t h i s r e p o r t .
The B a l t a z o r Hot Springs KGRA i s centered on
a low-yield,
(4-28 lpm),
sodium-sulfate,
7698"C/167-208"F
h o t spring, surrounded by small
o p a l i n e s i n t e r mounds and b e l i e v e d t o be p r e s e n t l y
p r e c i p i t a t i n g s i l i c a , t h u s suggesting a deep r e s e r v o i r temperature o f a t l e a s t 18OoC/356"F.
The
p o s i t i o n o f t h e springs, associated c a l c i t e - s i l i c a
stockworks, and presumably a deep thermal r e s e r v o i r a t Baltazor, i s c l e a r l y c o n t r o l l e d by t h e i n t e r s e c t i o n o f a n o r t h e a s t e r l y - t r e n d i ng Basin and
Range f a u l t trough w i t h an older, northwest-trendi n g normal f a u l t system.
O f numerous e x p l o r a t i o n
techniques appl l e d a t Baltazor, thermal g r a d i e n t
d r i l l i n g , mapping of geology and a l t e r a t i o n , geothermometry, d e t a i l e d g r a v i t y and d i p o l e - d i p o l e
r e s i s t i v i t y have been most u s e f u l i n a p p r a i s i n g
t h e geothermal resource p o t e n t i a l .
Baltazor i s
i n d i c a t e d by these techniques, -- and by probable
t o remain an enactive s i l i c a precipitation
couragi ng prospect: A t l e a s t one a d d i t i o n a l deep
d r i l l h o l e i s warranted. The t a r g e t f o r t h i s h o l e
i s a f r a c t u r e - c o n t r o l l e d r e s e r v o i r i n Miocene v o l canics and p r e - T e r t i a r y metamorphic rocks a t
depths g r e a t e r than 2000 f t (610 m) beneath
B a l t a z o r Hot Springs.
.
-
-
--
.
INTRODUCTION
The B a l t a z o r Hot Springs KGRA, near Denio,
Humboldt County, Neiada (Fig. l ) , has been i n t e n s i v e l y explored since 1977 by Earth Power Product i o n Company (EPPC)., under t h e terms of t h e
Department o f Energy's I n d u s t r y Coupled Program.
These e x p l o r a t i o n e f f o r t s have c l e a r l y demonstrat e d B a l t a z o r t o be a s t r u c t u r a l l y - c o n t r o l l e d system heated by deep c i r c u l a t i o n along permeable
f a u l t zones.
This paper w i l l discuss t h e n a t u r e
o f these s t r u c t u r a l c o n t r o l s , as revealed by v a r i o u s e x p l o r a t i o n techniques and deep d r i l l i n g , and
w i l l assess t h e p r o s p e c t ' s remaining resource
p o t e n t i a1
GEOLOGIC SETTING
.
A t t h e northwestern margin o f t h e Basin and
Range province, t h e B a l t a z o r Hot Springs KGRA and
v i c i n i t y exemplify t h e r e g i o n a l geology o f n o r t h western Nevada.
The Pueblo Mountains, west o f
B a l t a z o r (Fig.
l ) , are a typical, northerlytrending, t i l t e d f a u l t block range.
East of t h e
Pueblos, Continental Valley, which hosts B a l t a z o r
Hot Springs, i s an e q u a l l y t y p i c a l , deeply a l l u v i a t e d graben v a l l e y , i n t h i s case s u r f i c i a l l y b l a n keted w i t h l a c u s t r i n e sediments.
The Pueblo
PREVIOUS WORK
D i v e r s e e x p l o r a t i o n methods have been a p p l i e d
w i t h v a r y i n g success i n appraisal of t h e geotherm a l p o t e n t i a l o f t h e B a l t a t o r Hot Springs KGRA and
v i c i n i ty.
Photogeol o g i c mappi ng by Gardner and
Koenig (1978) d e l i n e a t e d p r e v i o u s l y mapped f a u l t s
157
Hul en
I
a
a
Ia
EXPLANATION
I
(Permian - C r e t a c e o u s )
7
Alluvium and older alluvium (Quaternary)
Qlo
contact:
definite,approximate
FZI,Fa
Lacustrine and older lacustrine deposits
(Quaternary)
0'
Landslide debris (Quaternary)
a F e l s i c ash Flow tuff a n d tuffaceous
sediments ( M i o c e n e )
u It s : d e f i nit e, a ppr o x ima t e,
in f er red, c o n c e a l e d
Calcareous a n d opaline sinter
%Calcite
@Pervasive
.~
--
Rhyolite and rhyolite flow breccia (Miocene)
Figure 1.
YAttltudes
-/-
- chalcedony
chalcedony
Intermediate
d e p t h drill hole
0
vein
-
calcite stockworks
D e e p test
well
Geologic map of t h e B a l t a t o r Hot Springs KGRA and v i c i n i t y , Humboldt County, Nevada.
158
.)
Hulen
Mountains are formed of a deeply dissected, h i g h l y
v a r i a b l e Pliocene-Pliocene v o l c a n i c and v o l c a n i
c l a s t i c sequence r e s t i n g w i t h profound unconformi t y on Permian-Triassic ( ? ) metavolcanic and metasedimentary rocks i n t r u d e d by intennedia t e compos i t i o n p l u t o n s o f Jurassic-Cretaceous age (Fig. 1;
Willden, 1964; Rowe, 1970; Burnham, 1971).
Immed i a t e l y above t h i s eroded basenlent near B a l t a z o r
i s a t h i c k sequence o f Miocene p o r p h y r i t i c b a s a l t
flows, flow-breccia, and tephra beds Tocally i n t e r r u p t e d by v o l c a n i c l a s t i c sediments, a i r - f a l l
t u f f s and ash-flow t u f f s .
I n t h e upper p o r t i o n o f
t h i s sequence, a t h i n d a c i t e (?) ash-flow t u f f and
a dense hornblende a n d e s i t e flow, along w i t h an
i n t e r v e n i n g b a s a l t i c a n d e s i t e flow, form an e a s i l y
mappable marker in t e r v a l (Figs. 1-3).
The basal t
sequence i s disconformably o v e r l a i n by a c l i f f forming r h y o l i t e , a l s o Miocene, which i n t u r n i s
o v e r l a i n by Miocene f e l s i c ash-flow t u f f s and v o l c a n i c l a s t i c sediment s.
-
'
The Pueblo Mountains a r e t i l t e d g e n t l y westward and bounded on t h e east by a major normal
range-front f a u l t system. Maximum displacement i n
t h i s system i s i n t e r p r e t e d by Edquist (1981) t o
have'occurred along a f a u l t (F3) concealed by a l luvium and t r e n d i n g n o r t h e a s t j u s t west o f
B a l t a z o r Hot Springs (Figs. 1, 3). A l l u v i a l depth
east o f f a u l t F3 i s modeled on t h e b a s i s o f g r a v i t y data by Edquist t o be as much as 2000 f t (610
m).
Low- t o moderate-angle f a u l t s F 1 and F6 i n
t h e range west o f f a u l t F3 probably were caused by
r a n g e - f r o n t oversteepeni ng r e s u l t i n g from F1
f a u l t i n g . The complexity o f F 1 and F6 immediately
n o r t h o f w e l l 45-14 (Fig. 1) i s almost c e r t a i n l y
due t o i n t e r a c t i o n o f these f a u l t s w i t h f a u l t F2,
a northwest-southeast-trending s t r u c t u r e o f minor
normal d i spl aceinent which, p r o j e c t e d southeastward,
passes d i r e c t l y
beneath B a l t a z o r Hot
Springs.
THERMAL PHENOMENA AND HYDROTHERMAL ALTERATION
Bal t a z o r Hot Springs, w i t h measured temperat ures va ryi ng between 76OC/167OF and 98OC/208"F,
produces, e s s e n t i a l l y from a s i n g l e o r i f i c e , s o d i um s u l f a t e waters a t r a t e s f l u c t u a t i n g between 4
and 128 lpm ( K l e i n a?d Koenig, 1977). S i l i c a and
Na-K-Ca geothermometers i n d i c a t e present subsurface temperatures o f a t l e a s t 16OoC/32O0F ( K l e i n
and Koenig, i b i d . ) .
Ooze c o l l e c t e d from t h e edge
of t h e B a l t a z o r Hot Springs pool i s revealed by Xr a y d i f f r a c t i o n (XRD) a n a l y s i s t o c o n s i s t l a r g e l y
o f opal w i t h minor c r i s t o b a l i t e and quartz, sugg e s t i ng r e s e r v o i r temperatures o f a t l e a s t 18OOC
1971).
Frothy-appearing o p a l i n e
(White e t a1
s i n t e r mounds s c a t t e r e d i n t h e immediate v i c i n i t y
o f t h e s p r i n g s (Fig. 1 ) a t t e s t t o past r e s e r v o i r
temperatures a t l e a s t t h i s high.
.,
Range-front normal f a u l t s and t h e s o l e of a
recent l a n d s l i d e west o f B a l t a z o r (Fig. 1 ) a r e
p e r v a s i v e l y a1t e r e d by s i 1 i c a - b e a r i n g (chalcedony,
c r i s t o b a l i t e and minor t r i d y m i t e as shown by XRD)
c a l c i t e veins and v e i n l e t s , l o c a l l y accompanied by
bleaching .of t h e i r h o s t rocks.
The veins, from
l e s s than 4 i n / l O cm t o a t l e a s t 10 f t / 3 m i n
t h i c k n e s s (mapped v e i n i n Fig. l),
range i n t e x -
t u r e from massive t o vuggy and d e l i c a t e l y banded.
C r o s s - c u t t i n g r e l a t i o n s h i p s and b r e c c i a t i o n
i n t h e v e i n s suggest several a l t e r n a t i n g episodes
o f carbonate d e p o s i t i o n and s i l i c i f i c a t i o n . A few
small (<6 f t / 1 . 8 m) surface mats and mounds of
mixed s i 1iceous and calcareous s i n t e r a r e s c a t t e r ed across t h e s u r f a c e o f t h e l a n d s l i d e , probably
i n d i c a t i n g t h e associated c a l c i t e
s i l i c a veins
t o have been former feeder channels f o r thermal
s p r i ngs.
-
Other a l t e r a t i o n i n t h e B a l t a z o r area cannot
be a t t r i b u t e d t o t h e a c t i v e geothermal system.
Base metal s u l f i d e m i n e r a l i z a t i o n and associated
a l t e r a t i o n i n Permian
Cretaceous basement rocks
i s c l e a r l y Pre-Tertiary.
F a u l t s F 1 and F6, p a r t i c u l a r l y near t h e h o t springs, commonly contain, as
i d e n t i f i e d by XRD, c a l c i t i c and h e m a t i t i c j a s p e r o i d (chalcedony) , l o c a l l y bearing small (<2 in/5
cm) pods o f b r i g h t yellow-green pure smectite,
almost c e r t a i n l y n o n t r o n i t e .
-
DEEP DRILLING RESULTS
Well 45-14, c o l l a r e d about one m i l e l l . 7 km
southwest o f B a l t a z o r Hot Springs (Figs. 1-3) pene t r a t e d a.lluvium t o a depth o f 160 f t / 4 9 m, a
t h i c k T e r t i a r y v o l c a n i c sequence t o 2180 f t / 6 6 4 m,
then p r e - T e r t i a r y i n t e r s t r a t i f i e d c h l o r i t e - s e r i c i t e s c h i s t and i n t e r m e d i a t e metavolcanic rock t o
t h e base o f t h e sampled i n t e r v a l a t 3590 ft/1094
m; t o t a l depth i s 3640 f t / 1 1 0 9 m. The rocks penet r a t e d i n t h e w e l l , most w i t h d i s t i n c t i v e gammar a y s i g n a t u r e s (Fig. 2) can be r e a d i l y c o r r e l a t e d
w i t h s u r f a c e rocks.
A prominent f a u l t between
1780 and 1790 f t . (543 and 546 m) separates mass i v e b a s a l t from u n d e r l y i n g interbedded t u f f s ,
sediments and t h i n b a s a l t flows.
This f a u l t i s
i n t e r p r e t e d t o be F1. A major f a u l t (F7; Fig. 3)
p r e - d a t i n g F 1 (and F6) i s i n f e r r e d from t h e f o r e shortening o f t h e b a s a l t sequence i n 45-14 (1700
f t / 5 1 8 m) r e l a t i v e t o i t s thickness j u s t a m i l e
(1.6 km) t o t h e n o r t h and two m i l e s (3.2 km) e a s t
(2700 f t / 8 2 3 m).
A l t e r a t d o n o f t h e rocks i n w e l l 45-14 i s minimal.
Trace t o minor amounts o f t h e yellow-green
smecti t e described e a r l i e r a r e e r r a t i c a l l y scat
t e r e d i n t h e i n t e r v a l between 400 f t / 1 2 2 m and
1000 f t / 3 2 8 m, along w i t h t r a c e s of c a l c i t e , goet h i t e and chalcedony.
The b a s a l t sequence i s
e s s e n t i a1 l y una1tered, except f o r 1oca1 t r a c e s of
c e l a d o n i t e , smectite, c a l c i t e , and various t e o 1i t e s .
Local q u a r t z - p y r i t e c h a l c o p y r i t e v e i n l e t s
i n t h e Permian-Triassic ( ? ) metamorphic sequence
a r e undoubtedly p r e - T e r t i a r y i n age.
-
An equi 1 ib r i um temperature prof il e f o r 45-14
(Fig. 3) shows t h r e e d i s t i n c t i v e segments -- an
isothermal i n t e r v a l a t about 43OC/109OF between
100 ft/30.5 m and 640 f t / 1 9 5 m; an i n t e r v a l w i t h a
temperature g r a d i e n t of about 130°C/km t o a depth
of 2100 f t / 6 4 0 m; and a conductive, low-gradient
(38OC/km) i n t e r v a l from 2100 f t / 6 4 0 m t o t h e b o t tom o f t h e w e l l , a t which t h e maximum temperature
o f 119.67°C/247.400F was recorded. The deep, cond u c t i v e , low g r a d i e n t i n t e r v a l i s c o n f i n e d almost
e n t i r e l y t o p r e - T e r t i a r y metamorphic rocks, and
159
.
Hul en
It,
CY
nL
E x P L A N ATION
A-
zg
LITHOLOGY
,
G A M M A RAY.
API UNITS
TEMPERATURE. OC
3 120
'0
2
I
Rhyolite and rhyolite
flow breccia
-
W
55
F
A
a H o r n b l e n d e andesite
O D a c i t e
(?>ash-flow
tuff
Olivine-augite basalt (flows,
_-
-250
a
In
W
IW
2
-500
-
200
I
I-
flow-breccias, tephra beds)
!
lnterbedded basalt and
!
tuffaceous sediments
!
Intermediate metavolcanic
rocks
':@Chlorite
++
!
NS
F i g u r e 2.
!
I
-serici te schist
Fault zone
I
I
I
!
I
I
*VA..VA
!
.
I
100
No sample
I
20
TEMPERATURE OF
L i t h o l o g y , gamma r a y and temperature l o g s f o r deep t e s t w e l l 8245-14, B a l t a z o r Hot Springs
KGRA, Humboldt County, Nevada. Temperature logs f o r 821500-1 and 821500-2 a r e shown f o r
comDari son. Gamma-ra.y l o q- b.y
- Gearhart (1983) Temperature data from Southwest D r i 11ing and
E x p l o r a t i o n Co. (198f).
.
probably i n d i c a t e s these rocks t o be impermeable.
The i n t e r m e d i a t e l e v e l h i g h e r g r a d i e n t
i n t e r v a l c o u l d r e f l e c t g e n e r a l l y lower thermal
c o n d u c t i v i t i e s o f associated b a s a l t s .
The i s o thermal p o r t i o n of t h e p r o f i l e i n d i c a t e s cool
water i n f l u x i n f r a c t u r e d h i g h - l e v e l v o l c a n i c
units.
t r a t e d by 45-14 (Fig. 3) apparently do n o t p r e s e n t l y form p a r t of t h e B a l t a z o r geothermal r e s e r voir.
This may be due t o h i g h - l e v e l s e a l i n g along
f a u l t s west o f F3 along t h e western margin o f t h e
deep f a u l t t r o u g h discussed above.
Evidence o f
such s e a l i n g i s t h e pervasive, recently-devel oped
c a l c i t e - s i 1 i c a stockwork a t t h e range f r o n t j u s t
northwest and west o f t h e h o t s p r i n g s (Fig; 1).
DISCUSSION AND CONCLUSIONS
The B a l t a z o r Hot Springs area remains an
encouraging moderate- t o high-temperature geotherIf, as i n d i c a t e d by XRD,
mal e x p l o r a t i o n t a r g e t .
B a l t a z o r Hot Springs i s p r e s e n t l y p r e c i p i t a t i n g
a t least
silica,
r e s e r v o i r temperatures
of
180°C/3560F can be reasonably a n t i c i p a t e d .
Adequate p e r m e a b i l i t y can be expected i n f r a c t u r e d
volcanics and metavolcanics, and perhaps i n overl y i n g rock rubble, beneath about 2000 f t / 6 1 0 m.
F a u l t F3 (Figs. 1, 3), concealed by alluvium, i s
probably t h e p r i n c i p a l c o n d u i t along which thermal
f l u i d s now ascend.
The high-temperature " s p i k e "
i n d r i l l h o l e BZ1500-1 (Fig. 2) may represent
eastward channeling of these f l u i d s i n t o a subsurface a l l u v i a l aquifer, o r may i n d i c a t e a concealed
The temperature decrease
f a u l t s u b s i d i a r y t o F3.
beneath t h e '%pike'', and associated low thermal
gradient, probably i n d i c a t e s t h e t r o u g h - f ill i n g
sediments below t h e " s p i k e " t o be r e l a t i v e l y
impermeable.
The l o c a t i o n o f t h e B a l t a z o r Hot Springs
thermal area i s almost c e r t a i n l y c o n t r o l l e d by t h e
intersection of
n o r t h e a s t e r l y - t r e n d i ng normal
f a u l t s w i t h an o l der, n o r t hwest-trendi ng h i g h angle f a u l t system. The main f a u l t o f t h i s system
(F2; Fig. l ) , i n f a c t , p r o j e c t s southeastward, beneath younger f a u l t blocks and alluvium, t o a pos i t i o n almost d i r e c t l y beneath t h e present h o t
spring location.
The i n t e r a c t i o n o f these two
f a u l t systems would c r e a t e a zone o f h i g h f r a c t u r e
pemieabi 1 it y , a1 l o w i ng deep c i r c u l a t i on and r e s u l t a n t h e a t i n g o f f l u i d s ( f a c i l i t a t e d by t h e abnorm a l l y high r e g i o n a l thermal g r a d i e n t s o f t h e
B a t t l e Mountain h e a t - f l o w h i g h ; Sass e t al.,
1971), f o l l o w e d by r a p i d convection o f these
heated f l u i d s t o shallow depths.
Additional res e r v o i r P e r m e a b i l i t y could be developed i n rock
r u b b l e accumulated i n t h e deep f a u l t t r o u g h
beneath B a l t a z o r Hot Springs, from t h e breakage,
i n t o t h e trough, o f h i g h - l e v e l low-angle f a u l t
b l o c k s exposed n o r t h o f w e l l 45-14 (Figs. 1,3).
The depths a t which commercially a t t r a c t i v e
h i g h e r temperatures might be encountered i s conjectural.
Simple downward p r o j e c t i o n of t h e
apparently conductive g r a d i e n t s (35-38OC/km)
As demonstrated by d e e p - d r i l l i n g , these low
angle f a u l t blocks, t h e lower of which was pene-
160
Hul en
F i g u r e 3.
I n t e r p r e t i v e 3-D geologic s e c t i o n AA'-BB'
encountered i n t h e lower p o r t i o n s o f 8245-14 and
821500-1, however, y i e l d s temperatures o f
15O0C/3O2"F a t about 5800 f t / 1 7 6 8 m and
18OoC/356"F a t about 8500 f t / 2 9 9 1 m.
Presumably,
convection would r e s u l t i n these temperatures
p r e v a i l i n g a t shallower depths.
ACKNOWLEDGEMENTS
This study was funded by t h e Department of
Energy,
D i v i s i o n o f Geothermal Energy under
Contract No. DE-AC07-80ID12079.
Discussions w i t h
Bruce S i b b e t t and Dennis Nielson were p i v o t a l i n
structural interpretations.
11l u s t r a t i o n s were
prepared by Connie Pixton.
The manuscript was
prepared by H o l l y Baker.
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