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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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291

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 -

i s t ry . C a t h y J a n i k k i n d l y d i d t h e o x y g e n i s o -

t o p i c a n a l y s e s . C . G r u n d e r p r o v e d a n a b l e

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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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