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Journ al of Volcanology and Geothermal Research 32 (1987) 287-298 287
Elsevier Science Publishers B.V., Amsterdam - - Print ed in The Net herlands
T HE H Y D R O T H E R M A L S Y S T E M O F T HE C A L A B O Z O S
C A L D E R A C E N T R A L C H IL E AN A N D E S
ANITA L. G RUNDER 1, J. MICHAEL THOM PSO N 2 and W. HILD RETH 2
1Geology De par tme nt Oregon Sta te University Corvallis OR 97331 U.S.A.
zU.S. Geological Surv ey 345 Mid dlefield Road Me nlo Park CA 94025 U.S.A.
(Rece ived August 4, 1986; revised and accepted January 14, 1987)
b s t r a c t
Grunder, A.L., Thompson, J.M. and Hildreth, W., 1987. The hydrothermal system of the Calabozos caldera, central
Chilean Andes. J. Volcanol. Geotherm. Res. 32: 287-298.
Active thermal springs associated with the late Pleistocene Calabozos caldera complex occur in two groups: the
Colorado group which issues along structures related to caldera collapse and resurgence, and the Puesto Calabozos
group, a nearby cluster that is chemically distinct and probably unrelated to the Colorado springs. Most of the Colorado
group can be related to a hypothetic al pare nt water co ntaining ~ 400 ppm C1 at ~ 250 C by dilution with > 50% of
cold meteoric water. The th ermal springs in the most deeply eroded part of the caldera were derived from the same
parent water by boiling.
The hydrothermal system has probably been active for at least as long as 300,000 years, based on geologic evidence
and calculations ofpa leo-he at flow. There is no evidence for economic mineralization at shallow depth. The Calabozos
hydrothermal system would be an attra ctive geothermal prospect were its location not so remote.
I n t r o d u c t i o n
Hot springs and steam vents occur along
faults associated with collapse and resurgence
of the Calabozos caldera, a late Pleistocene
structure on the crest of the central Chilean
Andes. This active hydrothermal system is of
interes t for four reasons:
1) It is the prese nt surface manif estati on of
a major hydrothe rmal system that has probably
persi sted for 300,000 yea rs or more.
2) Similar hot-spring systems are impor-
tan t targe ts for geothermal exploration.
3) Sinter deposits associated with hydro-
therm al systems can bear economic Au and Hg
mineralization.
4) It may be the surficial analog of hydro-
thermal systems that produce Cu-porphyry
mineralization.
Chemical analyses of hot waters and associ-
ated h ydrot hermal deposits from the Calabozos
region are used to calculate subsurface water
temperatures and to compare the Calabozos
system to other active hydrothermal systems.
Although there is no surficial evidence that eco-
nomic mineralization is present at shallow lev-
els in the Calabozos system, the dilute waters
and high calculated subsurface temperat ures of
the reservoir make it an attractive geothermal
exploration target.
0377-0273/87/ 03.50 1987 Elsevie r Science Publ ishers B.V.
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288
E X P L A N A T I O N
I t Holocene and Pleistocene
andesi te to rhyodaci te
lavas and tu f ts
Loma Seca Tuf f
Basement of Mesozoic
sed imentary and vo l -
canic rocks and late
Tert iary andesi tes
dac i tes and p lu tons
Gypsi ferous Mesozoic
rocks
Hot-sp r ing c lus ter
0
S t e a m v e n t s
Cold spr ing
-~ H o l ocene vo l can i c ven t
A Pleistocene eruptive
center
/ J c o n t a c t
. 4 / f a u l t b a r o n d o w n -
thrown s ide
. 4 ca lde ra h inge zone
. . . . - . r i ver o r s t ream
2 6 0 0 e l e v a t i o n in m e t e rs
Fig. 1. Simpli fied geologic map of the Calabozos caldera. RC Rio Colorado; VDC Volc~in Descabezado Chico; CDM Cerro
del Medio; CN Cerro Negro. Llolli N and Llolli S refer to thermal vents on the north and south banks respectively of the
Rio Colorado.
e o l o g ic s e t t i n g
T h e C a l a b o z o s c a l d e r a i s l o c a t e d a t 3 5 3 0 S
a t a n e l e v a t i o n o f ~ 2 6 0 0 m ( F i g . 1 ) . I t h a s b e e n
t h e l o cu s o f v o l u m i n o u s i n t e r m e d i a t e t o s il ic ic
v o l c a n i s m d u r i n g t h e l a te P l e i s t o c e n e . T h e
g e o lo g y , p r e s e n t e d b y H i l d r e t h e t a l. ( 1 9 8 4 ) , i s
s u m m a r i z e d b e l o w .
R e g i o n a l b a s e m e n t r o c k s , e x p o s e d o n t h e
n o r t h e r n a n d e a s t e r n s id e o f t h e c a l d e r a, a r e
c o m p o s e d o f l a t e M e s o z o i c v o l c a n i c l a s t i c se d i -
m e n t a r y r o c k s i n t e r b e d d e d w i t h l i m e s t o n e s ,
g y p s u m , a n d t h i n l a v as a n d t u f fs ; a g y p s u m d i a -
p i r s e v e r a l 1 0 0 m e t e r s i n d i a m e t e r , i s e x p o s e d
a t L l o l l i ( F i g . 1 ) . T h e M e s o z o i c r o c k s a r e
u n c o n f o r m a b l y o v e r l a in b y f l a t - ly i n g t o g e n t l y
w e s t - d i p p i n g , P l e i s t o c e n e a n d e s i t e l a v as . B a s e -
m e n t r o c k s e x p o s e d t o t h e s o u t h a n d w e s t o f t h e
c a l d e r a a r e c h i e f l y l a t e T e r t i a r y l a v a s a n d t u f f s
o f t h e C a m p a n a r i o f o r m a t i o n ( D r a k e , 1 9 7 6 ) ,
a l s o c a p p e d b y s u b h o r i z o n t a l a n d e s i t e l a v a s .
L a t e T e r t i a r y g r a n i t o i d i n t r u s i o n s a r e e x p o s e d
l o ca ll y t h r o u g h o u t t h e r e g io n .
A t l e a s t t w o m a j o r a s h - f l o w s h e e t s e r u p t e d
f r o m t h e C a l a b o z o s c a l d e r a 0 . 3 0 a n d 0 . 1 5 M a
a g o, c o m p o s i n g U n i t V a n d U n i t S , r es p e c -
t i v el y , o f t h e L o m a S e c a T u f f . A 0 . 8 - M a - o l d a s h -
f lo w , U n i t L , u n d e r l i e s U n i t V n e a r t h e s o u t h -
e r n a n d e a s t e r n c a l d e r a m a r g i n a n d i s c o m p o -
s i t io n a l l y a n d m i n e r a l o g i c a ll y s im i l a r t o U n i t s
V a n d S , b u t i t h a s n o t b e e n c o n c l u s iv e l y l i n k e d
t o t h e C a l ab o z o s c al d er a . E a c h u n i t o f t h e L o m a
S e c a T u f f r e p r e s e n t s 2 0 0 - 3 5 0 k m 3 o f c o m p o s i -
t io n a l ly z o n e d m a g m a r a n g i n g f r o m r h y o d a c it e
t o da c i t e .
T h e p r e s e n t 8 - b y - 2 6 - k m C a l a b o zo s c a l d e r a is
a c o m p o s i t e t r a p - d o o r c o l la p s e f e a tu r e c a u s e d
b y e r u p t i o n o f U n i t s V a n d S . O n t h e n o r t h e r n
a n d e a s t e r n s i d e s o f t h e c a l d e r a ( F i g . 1 ) , c o l -
l a pse i s de f i ne d by a w e l l - e xpose d se r i e s o f ne a r -
v e r t ic a l f a u l ts w i t h c o m b i n e d d i s p l a c e m e n t o f
a t l e a s t 5 0 0 m , w h i c h l e s s e n s s o u t h w a r d . T h e
w e s t e r n m a r g i n o f t h e c a l d e r a a p p e a r s t o b e a
f l e x u r a l h i n g e z o n e a n d i s l a r g e ly c o v e r e d b y
H o l o c e n e l a v a s a n d c o n e s . B r o a d s t e p - f a u l t e d
r e g i o n s c h a r a c t e r i z e t h e e a s t e r n a n d s o u t h e r n
m a r g i n s o f t h e c a ld e r a .
A f t e r c a l d e r a c o l la p s e , m a g m a t i c r e s u r g e n c e
d o m e d t h e n o r t h e r n h a l f o f t h e c a l d er a. L i k e
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c o l l a p s e , c a l d e r a r e s u r g e n c e w a s a s y m m e t r i c ,
w i t h m a x i m u m u p l if t o n t h e e a s t er n s id e o f t h e
d o m e a n d a n a s y m m e t r ic g r a b e n o n t h e w e s t-
e r n , h i n g e d s i d e. A c o m p l e x w e b o f n o r m a l
f a u l t s , m a n y w i t h o f f s e t s a n t i t h e t i c t o u p l i f t ,
c h a r a c t e r i z e t h e r e s u r g e n t s t r u c t u r e F i g . 1 ) .
T h e o u t f l o w fa c ie s o f t h e L o m a S e c a T u f f
o c c u r s a s d e n s e l y w e l d e d , p l a t e a u - c o v e r i n g
s h e e t s a n d a s t h i c k r e m n a n t s o f f lo w s t h a t o n c e
f i l l e d d e e p g l a c i a l g o r g e s . W i t h i n t h e n o r t h e r n
p o r t i o n o f t h e c a l d e r a, U n i t V i s as m u c h a s 5 0 0
m t h i c k a n d i t s b a s e i s n o w h e r e e x p o s e d . I t i s
c h i e f ly p o o r l y w e l d e d a n d i s o v e r l a i n b y n o m o r e
t h a n ~ 1 00 m o f d e n s e l y w e l d e d U n i t S o n t o p
o f t h e r e s u r g e n t d o m e . A t d e p t h , w i t h i n t h e
c a l d e r a , U n i t V i s a l m o s t c e r t a i n l y d e n s e l y
w e l d e d , b a s e d o n c o m p a r i s o n t o d e e p l y e r o d e d
a s h - f l o w s y s t e m s o f s i m i l a r c o m p o s i t i o n a n d
d i m e n s i o n e .g ., F r i d r i c h a n d M a h o o d , 1 98 4;
L i p m a n , 1 9 8 4 ; B o d e n , 1 9 8 6 ) .
T h e h o t s p r i n g s a r e g e o g r a p h i c a l l y d i v i s a b l e
i n t o t w o g r o u p s : 1 ) t h e C o l o r a d o h o t s p r i n g s
t h a t li e a l o n g a n o r t h - t r e n d i n g a r c , c o n c a v e t o
t h e w e s t, s k i r t i n g t h e b a s e o f t h e r e s u r g e n t
d o m e ; a n d 2 ) t h e P u e s t o C a la b o z o s g r o u p t h a t
l ie s a t t h e h e a d o f C a j o n l o s C a l a b o z o s n e a r t h e
h i n g e d m a r g i n o f t h e c a l d e r a F i g . 1 ). S o l fa -
t a r ic a c t i v i t y o c c u r s o n l y in t h e n o r t h e r n , m o s t
d e e p l y e r o d e d p o r t i o n o f t h e a r e a , a t L l o l li a n d
a t Pe l le jo Fig. 1 ) .
g e o f t h e h y d r o th e r m a l s y s t e m
N o d i r e c t e v i d e n c e , s u c h a s K - A r a g e s o f m i n -
e r a l s w i t h i n a l t e r a t i o n a s s e m b l a g e s , is a v a il a b l e
t o c o n s t r a i n t h e a g e o f th e C a l ab o z o s h y d r o -
t h e r m a l s y s t e m . S e v e r a l i n d i r e c t l i n e s o f e v i-
d e n c e s u g g e s t t h a t i t m a y b e a t l e a s t a s o l d a s
U n i t V , n a m e l y 3 0 0 , 0 0 0 y e a r s . F o r i n s t a n c e ,
m o s t o f t h e i n t r a c a l d e r a s e c t io n o f U n i t V i s
c h a l k y o w i n g t o p e r v a s i v e a r g i ll ic a l t e r a t i o n b y
a c i d l e a c h i n g . T h e h y d r o t h e r m a l a c t i v i t y t h a t
c a u s e d t h e a l t e r a t i o n m u s t h a v e e x i s t e d i m m e -
d i a te l y a f te r e r u p t i o n o f U n i t V , b e c a u s e a n
o v e r l y i n g i n t r a c a l d e r a l a v a , w h i c h h a s a n i n d i s -
t i n g u i s h a b l e K - A r a g e i s l o c a l l y u n a l t e r e d .
289
E x t e n s i v e l y a l t e r e d r o c k s , i n c l u d i n g U n i t S , a n d
f o s s i l s i n t e r d e p o s i t s a s s o c i a t e d w i t h t h e c a l d -
e r a s t r u c t u r e a t t e s t t o t h e p r e s e n c e o f h y d r o t h -
e r m a l a c t i v i ty s i n c e e r u p t i o n o f U n i t S .
W h e t h e r o r n o t h y d r o t h e r m a l a c t i v i t y w a s
c o n t i n u o u s b e t w e e n 0 . 3 a n d 0 .1 5 M a i s u n c e r -
t a i n , b u t t h e e x i s t e n c e o f a l o n g - l i v e d , d e e p ,
m e t e o r i c h y d r o t h e r m a l s y s t e m a t t h e C a l a b o -
z o s c a l d e r a i s i n d i c a t e d b y u n u s u a l l y l o w J l s O
v a l u e s o f p la g i o c l a s e f r o m f r e s h s a m p l e s o f U n i t
S 5 . 2 - 5 . 8 ) . T h e s e v a lu e s a r e l o w e r t h a n t h o s e
t y p i c a l o f s il ic i c v o l c a n i c r o c k s T a y l o r , 1 9 6 8 )
a n d i n d i c at e t h a t t h e U n i t S m a g m a i n t e r a c t e d
w i t h l o w - j 1 8 0 r o c k s t h a t h a d b e e n p r e v i o u s l y
a l t e r e d b y m e t e o r i c w a t e r G r u n d e r , 1 9 8 7 ) .
T h e s e a l t e r e d r o c k s w e re p r o b a b l y f o r m e d d u r -
i n g v i g o r o u s h y d r o t h e r m a l a c t i v i t y w i t h i n t h e
s h a t t e r e d , p e r m e a b l e c a l d e r a - f l o o r r o c k s , a f t e r
e r u p t i o n o f U n i t V .
S a m p l i n g a n d a n a l y ti c a l m e t h o d s
A t o t a l o f 29 h o t a n d c o l d w a t e r s a m p l e s w e r e
c o l l e c t e d f r o m s p r i n g c l u s t e r s i n t h e C a l a b o z o s
r e g i o n d u r i n g a u s t r a l s u m m e r 1 9 81 , 1 98 2 , a n d
1 9 84 T a b l e 1 ) . T h e h o t t e s t s p r i n g i n a n y g i v e n
c l u s t e r w a s s a m p l e d i n o r d e r t o r e p r e s e n t m o s t
c l o se l y t h e t h e r m a l w a t e r s a t d e p t h . A l l b u t t h e
1 98 1 s a m p l e s w e r e c o l l e c t e d u s i n g t h e b a c k -
p a c k i n g m e t h o d d e s c r i b e d b y T h o m p s o n
1 9 7 5) . T e m p e r a t u r e a n d p H w e r e m e a s u r e d i n
t h e f ie l d , a n d s p r i n g d i s c h a r g e r a t e s w e r e e s ti -
m a t e d . T e m p e r a t u r e s w e r e m e a s u r e d w i t h a
m a x i m u m r e a d i n g m e r cu r y - in - g l as s th e r m o m -
e t e r . S p r i n g w a t e r w a s f i l t e r e d t h r o u g h a 0 . 4 5 -
]~ m m e m b r a n e ; a 1 2 5 - m L a l i q u o t w a s a c i d i f ie d
w i t h h y d r o c h l o r i c a c i d f o r c a ti o n a n a l y si s , a n d
a n o t h e r 2 5 0 - m L p o r t i o n w a s l e ft u n a c i d i fi e d f o r
a n i o n a n a l y si s . B u b b l e s w e r e o b s e r v e d a t m o s t
o f t h e t h e r m a l s p r i n g s a n d p r e s u m a b l y r e p r e -
s e n t C O2 s a t u r a t i o n . O n e c o l d s p r i n g i n t h e C a l -
a b o z o s g r o u p is q u i t e s tr o n g l y c a r b o n a t e d a n d
i s r e p u t e d b y l o c a l t r a d i t i o n t o b e a t h e r a p e u t i c
b e v e r a g e . C o n c e n t r a t i o n s o f N H 3 a n d H 2 S i n
s a m p l e s c o l l e c t e d d u r i n g 1 9 8 2 w e r e m e a s u r e d
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2 9 0
T A B L E 1
R e p r e s e n t a t i v e a n a l y s e s o f s p ri n g s o f t h e C a l a b o z o s c a ld e r a c o m p l e x
L o c a t i o n : L l o l l i P e l l e jo P o t r e r i l l o s T r o n c o s C a s a P i e d r a
S o u t h N o r t h
S p r in g N o .: L S W 2 L S W 2 0 L S W l 0 L S W l L S W 1 7 L S W 7
D i s c h a r g e , L / m i n 1 5 1 0 2 0 0 0 2 0 3 0 0 0 3 0 0
T , C 9 8 . 5 9 5 . 5 5 0 . 5 6 4 5 0 6 8
F i e l d , p H 8 . 5 4 . 5 6 . 5 6 . 8 6 . 2 7 . 1
L a b , p H 9 . 3 0 2 . 9 7 7 . 1 5 7 .7 9 8 . 0 3 8 .3 2
A 1 ( p p b ) 0 . 2 * b l a c k ~ o l i v e ~ 1 . 2
S i O 2 5 8 2 * 2 3 9 1 3 3 8 4 5 6 4 0
F e < 0 . 0 5 2 . 8 3 . 8 0 . 8 0 . 1 1 . 3
M n 0 . 0 3 0 . 8 0 0 . 8 8 0 . 2 2 0 . 0 2 < 0 . 0 5
C a 9 2 6 1 1 0 9 9 4 3 1 7
M g < 0 . 0 2 1 8 .1 2 5 . 6 1 0 . 5 2 . 5 0 . 8
Sr 0 .2 - 1 . 0 2 .2 0 .4 0 .2
B a 0 .6 - 6 . 4 6 .9 - 1 . 4
N a 3 8 1 3 4 2 3 5 2 0 7 1 3 1 1 9 1
K 56 9 .3 16 .2 6 .6 4 .5 1 .9
L i 3 . 5 < 0 . 0 2 1 . 0 0 . 5 0 . 2 0 .4
R b 0 . 3 0 . 1 0 . 8 < 0 . 0 2 < 0 . 0 2 0 . 8
C s 0 . 5 0 . 1 0 . 6 < 0 . 0 2 0 . 2 0 . 5
H z S 0 . 2 - 0 . 0 4 0 . 1 - 0 . 0 5
NH ~ 1 .7 - 3 . 3 1 .8 - 0 . 5
H C 0 3 9 8 5 . 3 H 5 6 1 2 1 1 2 8 8 2 5 6
S O 4 1 4 4 6 3 0 1 1 1 3 9 0 6 0 6 6
C 1 5 4 9 4 1 9 6 1 0 9 4 9 1 0 7
F 3 .8 0 .1 2 .3 2 .9 3 .1 1 .0
B 14 0 .9 1 .5 0 .6 0 .3 0 .8
~ D - 89 - - 96 - 96 - - 96
eilSOH~ o --9 .8 - - - 13.5 -- 13.3 - - - 13.2
is Oso4 -- 4.5 - - - 0.2 - - - 2.8
N a / K 6 . 1 3 . 7 1 4 . 5 3 1 . 4 2 9 . 5 8 9 . 1
K / L i 1 6 . 0 - 1 6 . 2 1 3 . 2 1 8 . 9 4 . 8
N a / C a 4 0 . 6 1 . 3 2 . 1 2 . 1 3 . 1 1 0 . 1
C I / H C O 3 5 . 6 - 0 . 3 0 . 5 0 . 2 0 . 4
C I / S O 4 3 . 8 0 . 0 1 . 8 0 . 3 0 . 8 1 . 6
C l / B 3 9 4 1 3 1 1 8 2 1 6 3 1 3 4
C 1 / F 1 4 5 3 5 8 5 3 8 1 6 1 0 7
A n a l y t i c a l d a t a i n p p m e x c e p t A1 ( i n p p b ) . I s o t o p i c d a t a a r e i n s t a n d a r d d e l t a n o t a t i o n r e l a ti v e t o S M O W .
C o l d s p r i n g s i d e n t i f ie d b y n a m e o f n e a r e s t t h e r m a l s p r i n g ( F ig . 1 ) . R i o C . = R i o C o l o r a d o s a m p l e d n e a r P o t r e r il l o s .
S e e T h o m p s o n ( 1 9 8 5 ) f o r a n a l y t ic a l m e t h o d s : C a , M g , M n , a n d F e b y a t o m i c a b s o r p t i o n s p e c tr o s c o p y ; N a , K , L i, C s , R b ,
S t , a n d B a b y f l a m e e m i s s i o n s p e c t r o s c o p y ; S i O 2 , A 1, a n d B b y s p e c t r o p h o t o m e t r i c m e t h o d s . F b y F - s p e c i f i c el e c t r o d e ; S O 4
b y i o n c h r o m a t o g r a p h y ; C 1 a n d H C O 3 b y t i t r a m e t r i c m e t h o d s a n d f i e l d C 1 c o n c e n t r a t i o n s ( n o t s h o w n h e r e ) b y u s e o f
Q u a n t a b i n d i c a t o r s. L a b o r a t o r y a n d f i e ld C I d e t e r m i n a t i o n s a r e i n c lo s e a g r e e m e n t .
R i n d i c a t e s t h a t t h e v a l u e c i t e d i s f o r a c i d i t y .
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Alga Aguas Calabozos Cold springs Est imat ed
Blanca Calientes error
Pellej o Lloll i Rio C. _+
LSW8 LSWl2 LSW23 LSWl8 LSW ll LSW6
5000 4000 30 75 60 300 60,000 20
39 40 78 80 22 4 10 0.5
6.3 6.2 5.9 5.9 5.9 6.0 5.7 0.1
7.63 8.22 8.47 7.97 8.29 7.67 7.54 0.05
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TABLE 2
Chemical geothermometry of Calabozos waters
293
Location: Llolli Pellejo
Sample: LSW2 LSW20 LSWl0
Potre rillos Tronco s Casa Alga Aguas Calabozos
Piedra Blanca Calientes
LSW l LSWl7 LSW7 LSW8 LSWl2 LSW4 LSW23
Orifice temp. C 98 95 50 64 50 68 39 40 78 80
SiO2
(1) cond.-qtz 240 192 154 128 107 92 107 95 160 155
(2) max. st m-qt z 221 178 147 125 107 94 107 97 152 147
(3) chalcedony 237 173 129 100 78 61 77 65 137 130
Na/K
Fournier 252 322* 187 {136) (139) (77) (108) (102) (132) 158
Ellis 233 329* 151 (91) (95) (27) (61) (54) (86) (117)
Arn6r sson 238 * 160 101 107 - 72 60 98 126
Na-K-Ca
fl = 4/3 235 79 87 59 60 57 53 54 88 97
fl = 1/3 233 205 153 118 120 85 102 98 125 142
61s0 S04 H20
(4) conductive 280 133 144 172 148
(5) adiabati c 238 122 139 156 140
(6) con tin uous 250 124 139 160 141
x/'Mg-Li 351 59 67 57 65 58 47 48 83 94
Sample numbe rs as in Table 1.
The silica geothermometer applied according to Fournier and Rowe (1966); (1) is the value if cooling is conductive (for
springs well below boiling) a nd the water is in equi libriumwith quartz, ( 2 ) cooling is with max imum steam loss (for springs
near boiling), and (3) the water is in equilibrium with chalcedony.
The Na /K geothermometers are after Fourni er (1981), Ellis (1970), and Arn6rsson et al. (1983). *indicates 'not applicable'
because the water is acidic. Values in paren theses are below the cited limit s of applicab ility (150 ~C). ' -' indicates the
calculated temperature < measured orifice temperature.
The Na-K-Ca geothermometer is from Fournier and Truesdell (1973).
The oxygen isotope geothermometer is taken from McKenzie and Truesdell (1977) ; (4) is the value if cooling is conductive
(best for springs below boiling, viz., all but Llolli), (5) if there is one episode of steam separation (best for springs near
boiling), and (6) if there is continuous steam loss (best for springs associated with fumaroles).
The ~- Li geothermometer is taken from Kharaka and Mariner (1987).
P u e s t o C a l a b o z o s a n d L l o ll i w a t e r s a r e th e m o s t
c o n c e n t r a t e d a n d f o r m d i st i n c t c o m p o s i t io n a l
c l us t er s . O n e L lo ll i w a t e r S a m p l e L S W 2 0 ) ,
w h i c h w a s c o l l ec t e d a m i d s t a b a n k o f s t e a m
v e n t s , h a s v e r y l o w c o n c e n t r a t i o n s o f C1, B , K ,
a n d N a T a b l e 1 ) , s u g g e s t i n g t h e w a t e r c o n -
d e n s e d f r o m s t e a m . I t s a c i d p H a n d r e l a t i v e l y
e l e v a t e d S O4 c o n c e n t r a t i o n a r e c o n s i s t e n t w i t h
s t e a m c o n d e n s a t i o n a c c o m p a n i e d b y o x i d a t io n
o f H 2 0 S t o H2SO4.
eothermometry
B o t h m e a s u r e d o r i fi ce t e m p e r a t u r e s a n d c a l -
c u l a te d s u b s u r f a c e t e m p e r a t u r e s o f t h e C o l o-
r a d o s u i t e d e c r e a s e s o u t h w a r d a s e l e v a t i o n
i n c r e a s e s F i g . 4 ) . I t a p p e a r s th a t e r o s i o n h a s
a f f o r d e d a c r o s s s e c t i o n o f a v e r t i c a l ly , t h e r -
m a l l y z o n e d h y d r o t h e r m a l s y s t e m . E s t i m a t e d
s u b s u r f a c e t e m p e r a t u r e s r a n g e f r o m ~ 2 5 0 t o
< 10 0 C T a b l e 2 ). SiO 2, N a / K , N a - K - C a , a n d
S O 4 - H e O o x y g e n i s o t o p e g e o t h e r m o m e t e r s
a g r e e w e l l f o r t h e L l o ll i s p r in g , L S W - 2 T a b l e
2 ) a n d i n d i c a t e t h e w a t e r c a m e f r o m a r e s e r v o i r
a t ~ 2 5 0 C , w h i c h c o r r e s p o n d s t o t h e l o w e r r e s-
e r v o ir t e m p e r a t u r e e s t i m a t e d p r e v i o u s l y b y
T h o m p s o n e t a l. 1 9 8 3 ). T h i s te m p e r a t u r e i s
i n t e r m e d i a t e t o v a l u e s p r e d i c t e d i f c o o l in g o f th e
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294
w a t e r d u r i n g as c e n t is a d ia b a ti c ( m a x i m u m
s t e a m l o s s ) o r c o n d u c t i v e ( T a b l e 2 a n d F i g . 4 ) .
H i g h t e m p e r a t u r e s c a l cu l a te d w i th t h e N a / K
g e o t h e r m o m e t e r f o r s p ri n g L S W - 2 0 m a y r e fl ec t
a c i d l e a c h i n g o f w a l l r o c k s , w h i c h m a k e s t h e
g e o t h e r m o m e t e r u n r e l ia b l e .
T h e r e m a i n i n g s p r i n g s r e c o r d lo w e r t e m p e r -
a t u r e s, s u g g e s t i n g t h a t t h e y h a v e b e e n d i l u t e d
w i t h c o ld w a t e r ( F ig . 4 ) . T h e N a - K - C a g e o-
t h e r m o m e t e r u s i n g f l = 1 / 3 w a s u s e d , a s r e c-
o m m e n d e d f o r m i x e d w a t e rs b y F o u r n i e r
( 1 9 8 1 ) . F o r t h e C o l o r a d o su i te , th e t h e r m o m e -
t e r s b a s e d o n S iO 2 c o n t e n t , m o d e l l e d w i t h o u t
s t e a m l o s s , a n d N a - K - C a ( fl = 1 / 3 ) a r e i n g o o d
a g r e e m e n t . R e s e r v o i r t e m p e r a t u r e s f o r t h e
P u e s t o C a l a b o z o s g r o u p c l u s t e r b e t w e e n 1 4 0 a n d
1 5 0 C a n d d o n o t f o ll o w t h e t e m p e r a t u r e v e r -
s u s e l e v a t i o n t r e n d d e f i n e d b y t h e C o l o r a d o
g r o up . T h e ~ M - g - L i g e o t h e r m o m e t e r o f K h a r -
a k a a n d M a r i n e r ( 1 98 7 ) y i e ld s t e m p e r a t u r e s
s i m i l a r t o m e a s u r e d o r i f i c e t e m p e r a t u r e s
( e x c e p t f o r L l o l li w a t e r ) ( T a b l e 2 ) a n d s u g -
g e s t s t h a t M g c o n t e n t i n t h e s p r i n g s i s c o n -
t r o l l e d b y c h e m i c a l r e a c t i o n s n e a r t h e s u r f a c e .
m o d e l
A s i m p l e m o d e l c a n b e d e r i v e d f o r th e C o lo -
r a d o s p r i n g s u s i n g a p l o t o f e n t h a l p y v e r s u s
c h l o r i d e ( F i g . 5 ) . F l u i d f r o m a p a r e n t r e s e r v o i r
w i t h e n t h a l p y o f ~ 1 10 0 J g - 1 a n d C 1 c o n t e n t
o f ~ 4 0 0 p p m e v o lv e s b y b o il i n g ( e v a p o r a t i v e
c o n c e n t r a t i o n ) t o y i el d t h e L l o ll i h o t s p r i n g s
a n d b y d i l u t io n w i t h m e t e o r i c w a t e r t o p r o d u c e
t h e t h e r m a l s p r i n g s b e t w e e n A g u a s C a l i e n t e s
a n d P o t r e r il l o s . S u c h a m o d e l is s u p p o r t e d b y
t h e f a c t t h a t t h e m a i n L l ol li s p r i n g ( L S W - 2 a n d
2 2, T a b l e 2 ) y i e ld s a s u b s u r f a c e t e m p e r a t u r e
e s t i m a t e o f a p p r o x i m a t e l y 2 5 0 C , t h e t e m p e r -
a t u r e i n d i c a t e d b y t h e i n t e r s e c t i o n o f th e d i lu -
t i o n a n d b o i l i n g t r e n d s ( F ig . 5 ) . M o r e o v e r ,
L l o l li w a t e r i s e n r i c h e d i n 1 8 0 r e l a t iv e t o m e t e o r -
i c w a t e r ( F i g . 2 ) , c o n s i s t e n t w i t h a h i s t o r y o f
s t e a m l o ss .
I f t h e L l o l l i w a t e r i s i n f a c t d e r i v e d b y b o i l i n g
f r o m a r e s e r v o i r a t 2 5 0 C , t h e n a m i n i m u m
d e p t h o f 50 0 m t o t h e r e s e r v o i r c a n b e e s t i m a t e d
t steg h y p o t h e t i c o l
' h~
I
~ o l l i I o l l i
c_[ _ / a i l o t i o % - , - - - d
l H I I~ I J ~ ~ ~ ~ ~ C a l a b o z o s
4 To.~,, ' --~:. ~,
J ~ - ~ -
e l le }o
o
o ,5o zSo sSo 4 6 0 sSo 6bo
CI ppm)
Fig. 5. Enthalpy-chloride plot of the Calabozos hydrother-
mal system, constructed according o Fournier 1979). Open
circles = cold waters, filled circles = thermal waters. Data
from Tables 1 and 2 and sources referenced in Fig. 3.
Enthalpy-te mperature conversions are taken from steam
tables Keenan et al., 1969).
f r o m t h e b o i l in g c u rv e o f W h i t e ( 1 9 6 8 ) , b y
a s s u m i n g t h a t t h e w a t e r s a r e s u f f i c ie n t l y d i l u t e
t o b e h a v e l i k e p u r e w a t e r .
T h e P e l le j o s p r i n g d o e s n o t f a l l o n t h e s a m e
d i l u t i o n t r e n d a s t h e o t h e r C o l o r a d o s p r i n g s
( F ig . 5 ) . W e i n f e r t h a t i ts d e p a r t u r e f r o m t h e
d i l u t i o n t r e n d i n F i g . 5 i s t h e r e s u l t o f c o n d u c -
t i v e co o l i n g . I t i s a l t e r n a t i v e l y p o s s i b l e t h a t t h e
P e l le j o w a t e r e v o l v e d b y m i x i n g b e t w e e n a b o i l e d
w a t e r , l i k e t h e L l o l li w a t e r , a n d m e t e o r i c w a t e r
( F i g . 5 ) , b u t b e c a u s e i t s c h e m i c a l s i g n a t u r e
( F ig . 3 ) i s c o n s i s t e n t w i t h t h e d i l u t io n t r e n d
d e f i n e d b y th e r e m a i n i n g C o l o r a d o s p r i n g s th e
h y p o t h e s i s o f c o n d u c t i v e c o o l in g i s p r e f e r re d .
T h e C a l ab o z o s t h e r m a l s p r i n g s , if d e r i v e d f ro m
t h e s a m e o r a s i m i l a r r e s e rv o i r , r e p r e s e n t w a t e r s
e v o l v e d la r g e ly b y b o i l in g w i t h o n l y a m i n o r
c o m p o n e n t o f d i l u ti o n o r c o n d u c t i v e c o ol in g .
T h e c o n s t a n t r a t i o o f L i to C 1 o f th e C o l o r a d o
g r o u p b e a r s o u t t h e d i l u t i o n a n d b o i l i n g m o d e l
( F i g . 3 a ) . L i a n d C1 a r e h i g h l y s o lu b l e e l e m e n t s
a n d t h e i r p r o p o r t i o n s h o u l d n o t b e a f f e c te d b y
r e m o v a l o f s t e a m o r a d d i t i o n o f c o ld w a t e r l o w
i n C1 a n d L i . B a s e d o n t h e L i /C 1 t r e n d s t h e
s o u t h e r n C o l o r ad o w a t e rs r e p r e s e n t m i x t u r e s o f
a s m u c h a s 5 0 ( P e l l e j o ) t o 3 0 ( P o t r e r i l l o s )
a n d t o a s l i tt l e a s 10 o f h o t , c o n c e n t r a t e d r e s -
e r v o i r w a t e r w i t h c o l d w a t e r o f t h e r e g io n . T h e
L i c o n t e n t o f t h e p a r e n t r e s e r v o ir w o u l d b e 1 .9
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p p m a c c o r d in g to t h i s m o d e l. S u c h a h i g h L i
c o n t e n t i s c o n s i s t e n t w i t h t h e r e s er v o i r w a t e r
h a v i n g i n t e r a c t e d w i t h , o r b e i n g h o s t e d i n , si li -
c ic v o l c a n i c r o c k s ( E l l is a n d M a h o n , 1 9 7 7 ) .
C o m p o s i t i o n a l d a t a o f t h e C a l a bo z o s t h e r m a l
w a t e r s c l u s t e r o f f o f t h e L i- C1 t r e n d , w h i c h i n
a d d i t i o n t o t h e i r lo w e r J l s O a n d J D v a l u e s , a n d
d e v i a t i o n f r o m t h e e n t h a l p y - C 1 t r e n d s ( F ig . 5 ) ,
s u g g e s t s t h a t t h e y a r e f r o m a d i f f e r e n t g e o -
t h e r m a l r e s e r v o i r t h a n t h e C o l o r a d o t h e r m a l
s p r i n g s .
T h e c o n t e n t s o f B a n d S 0 4 o f t h e C o l o r a d o
s p r i n g s d o n o t f o l lo w t h e s i m p l e t r e n d d e f i n e d
b y L i v e r s u s C 1 (F i g . 3 ) . T h e L l o l l i s p r i n g s d a t a
a r e h i g h i n B w i t h r e s p e c t t o t h e C o l o r a d o d i lu -
t i o n t r e n d , w h i c h m a y r e f l e c t t h a t t h e w a t e r
p a s s e d t h r o u g h s e d i m e n t a r y ro c k s ( E l li s a n d
M a h o n , 1 9 7 7, p . 2 1 9 ) ; o f a ll t h e s p r i n g s , L l o l l i i s
n e a r e s t t o e x p o s e d M e s o z o i c s e d i m e n t a r y r o ck s .
T h e p r e s e n c e o f g y p s u m n e a r t h e L l o ll i sp r i n g s
a p p e a r s t o h a v e n o m a j o r e ff e c t o n t h e c h e m i c a l
c o m p o s i t i o n o f s p r i n g L S W 2 , o n t h e s o u t h e r n
s i d e o f t h e R i o C o l o r a d o . I n c o n t r a s t , S p r i n g
L S W 2 0 o n t h e n o r t h b a n k o f t h e r i v e r h a s r e l-
a t i v e l y e l e v a t e d M g a n d C a , i n a d d i t i o n t o e l e -
v a t e d S 0 4 , w h i c h m a y r e f l e c t t h e l e a c h i n g o f
g y p s u m a t s h a l l o w l e v e ls b y a c i d w a t er . P o t r e r -
i l l o s w a t e r i s s i m i l a r l y e l e v a t e d i n S 0 4 f r o m
w h i c h i t m i g h t b e i n f e r r e d t h a t t h e w a t e r i s
i n t e r a c t i n g w i t h g y p s i f e r o u s d e p o s i t s a t d e p t h .
e p o s i t s a s s o c i a t e d w i t h t h e s p r i n g s
M o s t o f t h e a c t i v e s p r i n g s i n t h e C a l a b o z o s
s y s t e m i s s u e i n t o s m a l l p o o l s o r r i v u l e t s t h a t
h a v e b a n k s a n d b o t t o m s c o a t e d w i t h o ra n g e F e -
o x id e m u d . A l t e r n a t i n g l a m i n a e o f m u d a n d
c a r b o n a c e o u s m a t e r i a l i n d i c a t e t h a t m u d a n d
a l ga e , w h i c h a r e c o m m o n i n t h e s p r i n g s , re p e a t -
e d l y c o v e r e a c h o t h e r . C a l c a r e o u s s i n t e r i s
a c t i v e l y d e p o s i t i n g a t T r o n c o s ( F i g . 1 ) , a n d s il -
i c e o u s s i n t e r a n d i n c r u s t a t i o n s , l o c a ll y w i t h
y e l lo w s p e c k s o f n a t i v e s u l f u r, a r e f o r m i n g a t
L l o ll i. A c t i v e s in t e r s o f b o t h k i n d s o c c u r i n t h e
R i o C o l o r a d o f a u l t z o n e a n d t e r r a c e s a r e t y p i -
c a l ly 1 00 m 2 a n d 0 . 5 - 2 m t h i c k . E x t e n s i v e b a n k s
295
o f F e - r i c h c a l c a re o u s t e r r a c e s w i t h a l t e r n a t i n g
l i m o n i t i c a n d h e m a t i t i c l a m i n a e a r e f o u n d a l o n g
f a u l t s n e a r P e l le j o ( F i g . 1 ) . T h e m o s t c o m m o n
d e p o s i t s r e l a t e d t o p a s t a n d p r e s e n t h y d r o -
t h e r m a l a c t i v i t y a r e s i l i c i f i e d a n d F e - o x i d e
e n c r u s t e d v o l c a n i c l a s t i c b r e c c ia s .
R e p r e s e n t a t i v e s a m p l e s o f i n c r u s t a t i o n s ,
s i n t e r s a n d v a r i o u s l y a l t e r e d r o c k s w e r e a n a -
l y z ed fo r p r e c i o u s - a n d b a s e - m e t a l s . T h e h i g h -
e s t A u , A g , a n d H g v a l u e s ( 0 . 0 7 , 0 . 0 2 , a n d 8 . 7
p p m , r e s p e c t i v e l y ) o c c u r i n s i l ic e o u s i n c r u s t a -
t i o n s a t L l o l l i n e a r L S W - 2 0 . C a l c i u m - c a r b o n -
a t e - c o a te d c r u s t s o f F e - ox i d e m u d f r o m t h e
C a l a b o z o s g r o u p y ie l d e d > 1 0 00 p p m A s a n d 6 8
p p m S b , b u t n o o t h e r s i g n i f i c a n t a n o m a l i e s w e r e
d e te c te d . T h e c o n c e n t r a ti o n o f t h e c o m m o n
m e t a l - c o m p l e x - f o r m i n g sp e c ie s C l - a n d S 2 -
( B a r n e s , 1 9 79 ) ( < 0 . 0 8 % N aC 1 , a n d < 1 0 - 6
m o l e s k g - 1 , r e s p e c t i v e l y ) a r e l o w e r t h a n t y p i -
c a l fo r a c ti v e m e t a l - r i c h h y d r o t h e r m a l s y s t e m s
s u c h a s B r o a d l a n d s , N e w Z e a l a n d ( s a l i n i t y :
> 0 . 3 7 % N a C1 a n d 2 - : 3 1 0 - 5 t o
5X10 4;
W e i s s b e r g, 1 9 6 9 ) , a n d s a l i n it i e s a re l o w e r t h a n
t h o s e i n f e r r e d f o r o r e f l u i d s i n o r e d e p o s i t s
p r o b a b l y r e l a t e d t o h o t - s p r i n g s y s t e m s i n t h e
G r e a t B a s i n ( 0 .2 % t o 7 .3 % ; W h i t e a n d H e r o -
p o u l o s , 19 8 3 ) . T h u s t h e C a l a b o z o s s y s t e m i s
l i k e ly to b e b a r r e n a t s h a l l o w d e p t h . T h e
a b s e n c e o f h i g h m e t a l c o n c e n t r a t i o n s a t t h e
s u r f a c e d o e s n o t p r e c l u d e t h e e x i s t e n c e o f ec o -
n o m i c m i n e r a l i z a ti o n a t g r e a t e r d e p t h b e c a u s e
h y d r o t h e r m a l o r e d e p o s i ts a re c o m m o n l y
s t r o n g l y v e r t ic a l l y z o n e d ( W h i t e , 1 98 1 ) .
G e o t h e r m a l p o t e n t ia l a n d h e a t f lo w
T h e C a l a bo z o s h y d r o t h e r m a l s y s te m i s a n
a t t r a c t i v e t a r g e t f o r g e o t h e r m a l e x p l o r a t i o n f o r
t h e f o l lo w i n g r e a s o n s:
( 1) T h e c a l c u l a te d r e s er v o ir t e m p e r a t u r e
c o m p a r e s f a v o r a b ly w i t h t e m p e r a t u r e s o f t h e
C o s o g e o t h e r m a l p r o j e c t in C a l i f o r n ia
( 2 4 0 - 2 5 0 C ; F o u r n i e r e t al ., 1 98 0 ) a n d t h e
R o o s e v e l t h o t s p r i n g s g e o t h e r m a l p r o s p e c t i n
U t a h ( 2 7 0 ; C a p u a n o a n d C o l e , 1 9 8 2 ) .
( 2 ) T h e d i l u te n a t u r e o f t h e w a t e r , t h e v e ry
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296
m gm chombe~
T = 8 o o o c ~ q l ~ : L
i ~ c . ~ ', : 5 ,, o S p o i ~ e ~ :
t
l T L l O 0 0 C f q 5 / 7 2 = 0 5 p i s e
F ig . 6 . L a y e re d mo d e l o f t h e C a la b o z o s ma g ma c h a mb e r
p r i o r t o e r u p t i o n o f b o t h U n i t V 0 .3 0 M a ) a n d U n i t S
0 .1 5 M a ) s i m p l i f i e d f r o m t h e d a t a o f G r u n d e r 1 9 8 6 ).
V e r t i c a l s c al e i s a rb i t r a ry .
reason why it is a poor prospect for mineral
exploration, would facilitate geothermal
exploitation because conduits and turbines
would not be encumbered by excessive mineral
deposition.
3) The Calabozos caldera has collapsed at
least twice an d the subsided block is likely to be
severely fractured, and is probably capped by
relatively impermeable densely welded tuff,
thereby providing a good reservoir for the
hydro thermal fluid, as at Los Humeros, Mexico
Ferriz, 1982).
4) Silicic magmat ism and hydro thermal
activity at the Calabozos center are long-lived.
Paleo-heat flow of ~ 10 -5 cal
c m - 2 s
1 at the
Calabozos caldera complex can be est imated for
the time immediately preceding eruption of
Units V and S based on a layered model of the
compositionally zoned magma chamber Fig.
6). Composition, viscosity, density, and tem-
perature are simplified from the data of Grun-
der 1986). Assuming tha t heat transfe r is only
vertical, and no heat is generated within a layer,
then, at steady state, heat flow at the surface
must equal heat flow across each horizontal
boundary. Heat transfer within the magma is
domina ted by convection because the Rayleigh
number of the magma is far greater than criti-
cal if the thickness of a layer is greater than 10
m. In fact, the thickness of the magma layers is
probably on the order of 1 km, based on esti-
mated eruptive volumes of several hundred km 3.
When heat tra nsfe r within the layer is by con-
vection, hea t flow can be expressed as:
\Rc}
where ~; =t he rm al conduc tivity 5 X 10 -3 cal
cm -1 s -1 K -l ), AT= the temperature gra-
dient across the layer, z=layer thickness,
R = Rayleigh number, an d Rc = the critical Ray-
leigh nu mb er = 2000 Sleep and Langen, 1981;
Christensen, 1985). Substitution of the Ray-
leigh number:
R_z ATo~pg
P
from Bartlett, 1969; a= 5 10 5 OK-l,
p = density, v = viscosity, g = gravi tationa l
acceleration) results in removal of thickness as
a variable. Calculated heat flows are:
qi=4X10 -Sa ndq2 =7 10 5calcm ~s - l for
densities and viscosities summarized in Fig. 6.
Comparable heat flows, i.e. between 10 -4 and
10 5 cal c m - 2 s 1 have been measured in the
hydro thermall y active Lassen and Medicine
Lake volcanic regions Mase and Sass, 1980),
in the Long Valley caldera region Sorey et al.,
1978), and have been estimated by Fournier and
Pit t 1985) for the Yellowstone caldera system.
In spite of the many factors that make the
Calabozos caldera system an interesting geo-
thermal target, geothermal exploitation of the
Calabozos system is unlikely in the nea r futu re
owing to the inaccessibility of the Calabozos
caldera 40 km to the nearest road) and the
existing hydroelectric potential of the centra l
Chilean Andes.
u m m a r y a n d c o n c lu s io n
The Calabozos hydrot hermal system is asso-
ciated with voluminous, late Pleistocene, cal-
dera-related magmatism. It has been active,
perhaps intermittently, for at least as long as
300,000 years and is presently expressed as a
suite of thermal springs and steam vents issu-
ing along caldera-rel ated structures in the Rio
Colorado fault zone. A nearby cluster of hot
springs, the Puesto Calabozos group, are chem-
ically distinct and are not directly related to the
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C o l o r a d o t h e r m a l w a t e r s . T h e y m a y b e r e l a t ed
t o V o lc ~i n D e s c a b e z a d o C h i c o ( F ig . 1 ) , t h e m o s t
r e c e n t l y a c t i v e c o n e in th e i m m e d i a t e v i c i n i t y
o f t h e C a l a b o z o s c a l d e r a .
T h e R i o C o l o r a d o s p r i n g s c a n a l l b e r e l a t e d
t o o n e r e s e r v o i r w a t e r c o n t a i n i n g ~ 4 00 p p m C 1
n e a r 2 5 0 C a n d a t a m i n i m u m d e p t h o f 5 0 0 m .
H o t s p r in g s w a t e r i n th e n o r t h e r n , m o s t d e e p l y
e r o d e d p a r t o f t h e c a l d e r a s y s t e m , w e r e e v o l v e d
f r o m t h e p a r e n t a l r e s e r v o i r b y bo i l i n g ; r e s e rv o i r
w a t e r d i l u te d w i t h > 5 0 r e g io n a l m e t e o r i c
w a t e r i s s u e s a s s p r i n g s a l o n g th e e a s t e r n m a r -
g i n o f t h e r e s u r g e n t d o m e .
N o n e o f t h e t h e r m a l w a t e r s n o r r e l a te d s i n -
t e r s a n d i n c r u s t a t i o n s i n t h e C a l a b o z o s r e g io n
h a v e s i g n i f i c a n t l y e l e v a t e d m e t a l c o n c e n t r a -
t i o n s , so th e a r e a i s n o t a p r o m i s i n g m i n e r a l
p r o s p e c t a t s h a l l o w d e p t h s . T h e d i l u t e w a t e r
c o m p o s i t i o n s a n d t h e h ig h c a l c u l a t e d r e s e r v o i r
t e m p e r a t u r e , c o u p l e d w i t h t h e l o n g e v i ty o f t h e
s y s t e m , e a r m a r k t h e C a l a b o z o s h y d r o t h e r m a l
s y s t e m a s a n i n t e r e s t i n g g e o t h e r m a l t a r g e t t h a t
i s , h o w e v e r , u n l i k e l y t o b e e x p l o i t e d g i v e n i t s
r e m o t e l o c a t i o n .
c know l e d gmen t s
W e t h a n k D . S m i t h o f A S A R C O I n c. fo r t r ac e
m e t a l a n a l y s e s o f h o t s p r i n g s d e p o s i t s a n d Y .
K h a r a k a f o r h e l p in m o d e l l i n g t h e w a t e r c h e m -
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f r o m r e v i e w s b y C . B a c o n , H . F e r r iz a n d G .
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