geologyPlatanares-heiken1991

7/24/2019 geologyPlatanares-heiken1991

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Journa l o f Vo lcanology and Geothermal Research

45 (1991) 41 - 58

Elsevier Science Publishers B.V., Amsterda m 41

G eology o f the Platanares geotherm al area D epartam ento de

C o p m H o n d u r a s

G r a nt H e i k e n a N a p o l e o n R a m o s c W e n d e l l D u f f i e ld b J o h n M u s g ra ve a K e n n e t h W o h l e t za Sue Pr ie s t b

J a m e s A l d r i c h a W i l m e r F l o r es c A l e x a n d e r R i t c h i ed F r a s e r G o f f a D e a n E p p l e r a a n d C a r l o s E s c o b a r c

a L os A la m o s N a t io n a l L a b o r a tor y M S 9 6 4 L o s A la m o s N M 8 7 5 4 5 U . S . A .

bU.S . Geolog ica l Survey 2255 N. Gem in i Dr . F lags ta f f A Z 86001 U.S .A .

CEmpresa Na ciona l de Energia Electrica Tegucigalpa Ho ndu ras

aCollege o f Charleston 66 George St. Charleston SC 29424 U .S.A .

(Accepted June 26, 1990)

ABSTRACT

Heiken, G., Ramos, N., Duffield, W., Musgrave, J., Wohletz, K., Priest, S., Aldrich, J., Flores, W., Ritchie, A., Goff, F.,

Eppler, D. a nd Escobar, C., 1991. Geology of the Platanares geothermal area, Depa rtamento de Cop~in, Honduras. In: F. Gof f

(Editor), Honduras - A Geothermal Investigation. J. Volcanol. Geotherm. Res. 45:41 -58.

The Platanares geothermal area, Depar tamento de Cop~in, Honduras, is located within a graben that is complexly faulted. The

graben is bounded on the north by a highland composed of Paleozoic (?) metamorphic rocks in contact with Cretaceous - Tertiary

redbeds of unknown thickness. These are uncon formably overlain by Tertiary andesitic lavas, rhyolitic ignimbrites, and associated

sedimentary rocks. The volcanic rocks are mostly older than 14 Ma, and thus are too old to represent the surface expression of

an active crustal magma body. Thermal fluids that discharge in the area are heated during deep circulation of meteoric water along

faults in a region of somewhat elevated heat flow. Geot hermometry based upon the chemical composition of thermal fluids from

hot springs and from geothermal gradient coreholes suggests that the reservoir equilibrated at temperatures as high as 225 to

240C, within the Cre taceous redbed sequence. Three c, atinuously cored geothermal gradien t holes have been drilled; fluids of

abou t 165C have been produced from two drilled along a NW-trending fault zone, from depths of 250 to 680 m. A conductive

thermal gradient of 139C/km, at a depth of 400 m, was determined from the third well, drilled 0.6 km west of that fault zone.

These data indicate that the Platanares geothermal area holds considerable promise for electrical generation by moderate- to high-

temperature geothermal fluids.

I n t r o d u c t i o n

M o s t h y d r o t h e r m a l s y s t e m s i n C e n t r a l

A m e r ic a s u c h a s t h o se at M o m o t o m b o

N i c a r a g u a ; M i r a v a l le s C o s t a R ic a ; a n d

A h u a c h a p ~ i n E 1 S a l v a d o r a re a s s o c i a te d w i t h

R e c e n t v o l c a n i c f ie l d s w i t h i n te r m e d i a t e t o

s i l i c i c l a v a s a n d p y r o c l a s t i c r o c k s . T h e

n u m e r o u s g e o t h e r m a l m a n i f e s ta t i o n s s c at te r ed

t h r o u g h o u t c e n tr a l a n d w e s t e r n H o n d u r a s a r e

n o t h o w e v e r a s s o c i a te d w i t h s i m i la r y o u n g

v o l c a n o e s . T h e y o u n g e s t r o c k s o f th e s e c o m -

p o s i t i o n s i n H o n d u r a s a r e m i d - to l a te T e r t ia r y .

C e n t r a l H o n d u r a s i s c r o s s e d b y a d i s c o n -

t i n u o u s l i n e o f Q u a t e r n a r y b a s a l t i c c i n d e r

c o n e s a n d la v a f l o w s b u t n o n e o f t h e se y o u n g

v o l c a n i c a r e a s o f f e r t h e p r o m i s e o f h y d r o t h e r -

m a l s y s te m s o f c o m m e r c i a l q u a l i t y.

S u r f a c e m a n i f e s t a t io n s o f t h e h y d r o t h e rm a l

s y s t em s o f H o n d u r a s a re a s s o c i a t e d w i t h la t e

C e n o z o i c f a u lt s a n d c r u s ta l e x t e n s i o n m u c h

l ik e t h o s e o f th e B a s i n - a n d - R a n g e P r o v i n c e o f

0377-0273/91/ 03.50 ( 1991 Elsevier Science Publishers B.V.


2/18

4 G HEIKEN ET AL

t h e w e s t e r n U n i t e d S t a t e s . M e t e o r i c w a t e r s ,

w h i c h c i r c u l a t e d e e p l y a l o n g f a u l t s y s t e m s , f i ll

i s o l a t e d g e o t h e r m a l r e s e r v o i r s t h a t f e e d h o t

sp r ings and bo i l i ng sp r ings , whe re s i l i c a s i n t e r

a n d c a r b o n a t e t r a v e r ti n e a re d e p o s it e d F l o re s ,

1980; Goff e t a l . , 1987b) .

T e n g e o t h e r m a l s i t e s w e r e i n v e s t i g a t e d i n

H o n d u r a s a s p a r t o f a r e c o n n a i s s a n c e s t u d y b y

L o s A l a m o s , E m p r e s a N a c i o n a l d e E n e r g i a

E l e c t r i c a d e H o n d u r a s , a n d t h e U . S . G e o l o g -

i c al S u r v e y ; t h i s s t u d y i n c l u d e d i n v e s t i g a ti o n s

o f t h e g e o lo g y o f A z a c u a l p a E p p l e r et a l .,

1987b), Pa va na Epp le r e t a l . , 1987a) , P l a t a -

na re s H e ike n e t a l . , 1986, 1987) , an d San Ig -

n a c i o A l d r i c h et a l . , 1 9 87 ), a n d o f t h e h y d r o -

geo che m is t ry o f a l l t en s it es G of f e t a l . , 1987b)

F i g . 1 ). P l a t a n a r e s w a s c h o s e n f o r d e t a i l e d i n -

v e s t i g a t i o n o n t h e b a s i s o f p r e l i m i n a r y g e o l o g i c

m a p p i n g a n d h y d r o g e o c h e m i c a l s t ud i e s, w h i c h

i n d i c a t e d r e s e r v o i r t e m p e r a t u r e s o f 2 25 t o

2 4 0 C , t h e h i g h e s t i n H o n d u r a s . S i n c e 1 9 8 6 ,

a d d i t i o n a l g e o l o g i c a n d g e o c h e m i c a l s t u d i e s

and g rav i t y , s e l f -po t en t i a l , and r e s i s t i v i t y su r -

v e y s H o o v e r a n d P i e r c e , 1 98 8; A n d e r e t a l . ,

1991 - th i s vo lume ; A iken e t a l . , 1991 - th i s

v o l u m e ) h a v e b e e n c o m p l e t e d .

T h e P l a t a n a r e s g e o t h e r m a l a r e a i s l o c a t e d i n

w e s t - c en t r al H o n d u r a s i n t h e D e p a r t a m e n t o d e

C o p ~ n , 3 0 k m e a s t o f t h e b o r d e r w i t h

G u a t e m a l a a n d 16 k m w e s t o f S a n t a R o s a d e

C o p ~ n , w h i c h i s t h e p r o v i n c i a l c a p i t a l . T h e

v i ll a g e o f P l a t a n a r e s is l o c a t e d o n a g r a v e l te r -

r a c e , a f ew te n s o f m e t e r s a b o v e t h e Q u e b r a d a

d e l A g u a C a l ie n t e , a c a n y o n w h o s e n a m e

r e f le c t s t h e p r e s e n c e o f n u m e r o u s h o t s p r i n g s ,

b o i l i n g s p r i n g s , a n d f u m a r o l e s t h a t i s s u e f r o m

t h e b a n k s a n d b e d o f th e q u e b r a d a o v e r a

d i s t a n c e o f a b o u t 3 k m . T h e l o c a l t e r r a i n i s

r u g g e d a n d r a n g e s i n e l e v a t i o n f r o m 7 0 0 t o 1 30 0

m . T h e q u e b r a d a f o l l o w s a f a u l t z o n e , a l o n g

w h i c h t h e f a u l t e d a n d b r e c c i a t e d r o c k s a r e e a s i-

l y e r o d e d .

T h i s w o r k , f u n d e d b y th e U . S . A g e n c y f o r

I n t e r n a t i o n a l D e v e l o p m e n t , f o l l o w e d e a r l i e r

c o u n t r y w i d e r e c o n n a i s s a n c e s t u d i e s b y t h e

U n i t e d N a t i o n s D e v e l o p m e n t P r o g r a m G is -

l a s o n , 1 9 80 ) a n d G e o t h e r m E x , I n c . 1 9 80 ).

g

I s la s d ~ l a B a h ia

::::.::::.:::

' " : ' ~ : : g : ~ , , ~. . . . ' " N

1 k m ]

Fig. 1. Map showinggeothermalareas investigated during the countryw idesurvey by L os Alam os, the U.S. G eological

Survey, and Em presa Nacional de E nergia El6ctrica.


3/18

GEOLOGY OF THE PLATANARES GEOTHERMAL AREA HONDURAS

4

Regional geologic setting

W e s te r n H o n d u r a s is a h i g h l an d b o u n d e d b y

ma jo r l e f t - l a t e r a l , s t r i ke - s l i p f au l t s on t he

n o r t h , i n c l u d in g t h e M o t a g u a f a u lt z o n e o f

G u a t e m a l a a n d H o n d u r a s , a n d t h e J o c o t a n -

C h a m e l e c 6 n f a u l t z o n e a n d t h e c o n v e r g e n t

p l a te m a r g i n o f t h e C e n t r a l A m e r i c a n v o l c a n ic

f r o n t o n t h e s o u t h w e s t . T h e s e f a u l ts d e f i n e th e

n o r t h w e s t c o r n e r o f t h e C a r i b b e a n p l a t e a n d

f o r m t h e n o r t h e r n b o u n d a r y o f t h e C h o r t i s

B l o c k , a b l o c k o f P a l e o z o i c a n d y o u n g e r c o n -

t i n e n t a l c r u s t ( H o r n e a n d F i n c h , 1 99 0). T o t h e

s o u t h , t h e C h o r t i s b l o c k i s i n c o n t a c t w i t h

y o u n g e r c r u s t a s s o c i a t e d w i t h s u b d u c t i o n o f

t h e C o c o s p l a t e a l o n g t h e M i d d l e A m e r i c a

t r e n c h ( B u r k a r t a n d S e l f , 19 85 ). E x t e n s i o n a n d

c r u s t a l t h i n n i n g w i t h i n t h e C h o r t i s b l o c k a r e

m a n i f e s t e d a s N W - t r e n d i n g g r a b e n s a n d

rh om b g ra ben s ( c f . M i ll s e t a l . , 1967 ; W i l l i ams

a n d M c B i r n e y , 1 9 6 9 ) . T h i s e x t e n s i o n m a y b e

r e l a t e d to

( 1 ) e a s t w a r d m o v e m e n t o f t h e C a r i b b e a n

p l a te w h i l e t h e N W c o r n e r ( w e s t er n H o n d u r a s )

i s p i n n e d b e t w e e n t h e C o c o s a n d A m e r i c a

p l a t e s (P l a fke r , 1976 ) ;

(2 ) r o t a t i o n o f t h e t r a i l in g e d g e o f t h e C a r i b -

b e a n p l a t e a r o u n d a b e n d i n t h e M o t a g u a a n d

J o c o t a n - C h a m e l e c 6 n f a u l t s ( r o t a t i o n o f t h e

Chor t i s b lock ) (Burka r t and Se l f , 1985 ) ; o r

(3 ) e x t e n s i o n o f l i t h o s p h e r e , f o r m i n g N - S

g r a b e n s a n d E N E - t r e n d i n g f r a c tu r e s w i t h l ef t-

l a t e r a l s t ri k e -s l ip m o t i o n t h a t f o r m e d p u l l -

a p a r t b a s i n s ( M a n t o n , 1 98 7) .

T h e P l a t a n a r e s g e o t h e r m a l a r e a is in t h e m i d -

d l e o f t h is t e c t o n i c a l l y c o m p l e x b l o c k t h a t

/ / , ; o = / . k . / - / . '~ . .. ~ t / / / I

I f > < - -

t / 3 , - q - ~ - J /-' -

O n-

o 7 -

< l v -

g ~

~ e

~o,

Subinal Formation. Red-stained sandstones, sil tstones

sandstones, and conglomerates.

Matagalpa Formation; highly al tered andesi te f lows and

some intarbaddad pyroclastic rocks.

Val le de Angeles Group redbeds, consist ing of hemati te-

stained si l tstones, immature sandstones, pebbly sand-

stones, and conglomerates. Minimum thickness is 1 km.

Total thickness is unknown.

Metamorphic sequence, including highly deformed and

sheared metaquartzi tes, schists, and phyl l ites. This

sequence is only in faul t contact wi th younger rock uni ts.

I t forms the high r idges north of the geothermal a r e a

STRATIGRAPHIC C OLUM N FOR THE PLATANARES GEOT HERM AL AREA

DEPARTAMENTO DE COPAN HOND URAS

F i g 4 C o m p o s i t e s t r a t i g r a p h i c s e c t io n o f l it h o l o g i c u n i t s i n th e P l a t a n a r e s g e o t h e r m a l a r e a


6/18

6

6 H E I K E N E T A L

m i d - C r e t a c e o u s Y o j o a G r o u p l im e s t o n e s,

s h a l e s , a n d r e d b e d s ) , J u r a s s i c t o E a r l y C r e t a -

c e o us H o n d u r a s G r o u p s a n d st o ne s a n d co n -

g l o m e r a t e s) , a n d J u r a ss i c E1 P l a n F o r m a t i o n

b l a c k s h a l e a n d t h i n - b e d d e d s a n d s t o n e )

F i n c h , 1 9 8 1; H o r n e a n d F i n c h , 1 9 9 0) . N o n e o f

t h e s e o l d e r M e s o z o i c u n i ts c r o p s o u t i n th e a r e a

o f P l a t a n a r e s .

V a l l e d e A n g e l e s r e d b e d s a r e d i f f i c u l t t o

d i s t i n g u i s h f r o m t h e y o u n g e r r e d b e d s o f t h e

S u b i n a l F o r m a t i o n d e s c ri b e d b e lo w ) . B o t h

u n i t s c o n t a i n i m m a t u r e s a n d s t o n e : l i t h a r e n i t e

a s w e l l a s p e b b l y i m m a t u r e s a n d s t o n e s , c o n -

g l o m e r a t e s , a n d s i lt s to n e s . S o m e o f t h e c o a r s e r

c l a s t i c b e d s i n b o t h c o n t a i n c a l c i t e c e m e n t .

T h u s , i n d r i ll c o r e s a n d i n t h e f i e l d , i t is n e a r l y

i m p o s s i b l e t o i d e n t i f y a r e d b e d s e q u e n c e a s

e i th e r V a l l e d e A n g e l e s o r S u b i n a l w i t h o u t i n-

d e p e n d e n t s tr a t ig r a p h i c c o n t r o l . H o w e v e r ,

m o d a l a b u n d a n c e s o f l i t hi c c la s ts t h a t m a k e u p

m o s t o f th e s e c la s ti c r o c k s p r o v i d e a m e a n s o f

d i s c r i m i n a t i o n . T h e V a l l e d e A n g e l e s s a n d -

s t o n e s a n d c o n g l o m e r a t e s c o n t a i n 20-/o t o 9270

b a s a l t c l as ts F ig . 5) . T h e S u b i n a l F o r m a t i o n

c o n t a i n s s a n d s t o n e s c o n s i s t i n g o f 65070 t o 9 37o

m e t a q u a r t z i t e a n d s c h is t c la s ts , w i t h o n l y t r a c e s

o f b a s a l t F i g . 5 ) .

Matagalpa ormation

W i t h t h e e x c e p t io n o f a f e w in t e r b e d d e d , t h i n

t u f f b e d s a n d s a n d s t o n e s , th e M a t a g a l p a F o r -

m a t i o n a t P l a t a n a r e s c o n s i s t s o f m a s s i v e ,

s i li c if i ed a n d e s i t i c l a v a s . T h e s e l a v a s c r o p o u t

a l on g t h e b o t t o m o f t h e u p p e r Q u e b r a d a d e l

A g u a C a l i e n te F i g. 3 ), w h e r e t h e y a r e c u t b y

q u a r t z - a n d c a l ci te - f il le d a n d p a r t l y o p e n f r a c-

t u r e s . T h e a n d e s i t e s w e r e p e n e t r a t e d b y

g e o t h e r m a l g r a d i e n t w e ll s P L T G - 1 a n d - 3 ,

w h e r e t h e y a r e 5 5 6 m a n d 2 6 3 m t h i c k , r e s p e c -

t iv e ly . P L T G - 3 is o n l y 6 8 0 m s o u t h o f P L T G - 1.

T h e l av a s w e r e n o t e n c o u n t e r e d in w e ll P L T G -

2 , w h i c h is a b o u t 1 ,3 6 0 m s o u t h o f P L T G - 1 .

T h i s a b r u p t s o u t h w a r d t h i n n i n g o f t h e la v a s is

r e f l e c t e d i n a s m a l l p o s i t i v e g r a v i t y a n o m a l y

Basalt and

Andes i te

\ \

M e t a q u a r t z i t e / ^

and Schist / LA L3

A

50

LITHICCLASTTYPES,

SANDSTONESlNREDBED

SEQUENCES

~;~ 0/o

/~ SubinalFormation

O Vallede AngelesGroup

\

\

~ Sandstone,

Mudstone

and Che~

Fig. 5. Triangular diagram of l i thic clas t composit ions from sandstone and conglomerate redbe ds w ithin the

Cre taceo us-T ertiary Valle de A ngeles Group and Tert iary Subinal Formation. B ased on 300-point modal analyses

with a spacing of 0.4 m m) of polished thin sections.


7/18

GEOLOGY OF THE PLATANARES GEOTHERMAL AREA HONDURAS 7

(A iken e t a l . , 1991 - th i s vo lum e) , w h ich i s i n t e r -

p r e t e d a s a s m a l l s t a c k o f l a v a f l o w s ( se e i n t e r -

p r e t i ve c ro s s - sec t i on i n F ig . 3 ) .

Th e l avas a r e po r ph y r i t i c , w i th 1570 t o 25 % ,

0 . 2 - t o 3 - m m - l o n g p h e n o c r y s t s a n d g l o m e r o -

c r y s t s o f p l a g i o c l a s e a n d m a f i c m i n e r a l s i n a

c o a r s e t r a c h y t ic g r o u n d m a s s . A l l o f t h e l a v as

s e e n in o u t c r o p a n d s a m p l e d f r o m d r i ll c o r e a r e

m o d e r a t e l y t o i n t en s e l y h y d r o t h e r m a l l y a l te r e d

a n d t h e r e f o r e d e s c r i p t i o n s o f i g n e o u s t e x t u r e s

a r e i n t e r p r e t a t i v e , b a s e d u p o n r e l i c t t e x t u r e s .

F e l d s p a r s h a v e b e e n r e p l a c e d b y s m e c t i t e ,

a u t h i g e n i c q u a r t z , c h l o r i t e , a n d t r a c e s o f

c a l c i t e ; m a f i c m i n e r a l s h a v e b e e n a l t e r e d t o

c h l o r i t e a n d m a g n e t i t e . V e i n s a r e f i l l e d w i t h

a u t h i g e n i c q u a r t z a n d b i o t i t e , c h l o r i t e , p y r i te ,

s t i b n i t e , c a l c i t e , a n d o t h e r m i n e r a l s ( B a r g a r ,

1991 - th i s vo lume) .

O u r i n t e r p r e t a t i o n t h a t t h e s e a n d e s i t i c l a v a s

a r e p a rt o f t h e m i d - T e r t i a ry M a t a g a l p a F o r m a -

t i o n i s b a s e d o n s i m i l a r i ty t o a d e s c r i p t i o n o f t h e

f o r m a t i o n i n n e a r b y G u a t e m a l a b y W i l l ia m s e t

a l . ( 1964 ) , whe re i t cons i s t s o f h igh ly a l t e r ed

p y r o x e n e - a n d e s i t e f l o w s a n d l a h a r i c b r e c c i a s ,

and l e s se r f l ows o f p l a ty dac i t e . Such a s e -

q u e n c e h as b e e n m a p p e d i n n o rt h w e s t e rn H o n -

d u r a s , w h e r e i t o v e r l i e s t h e V a l l e d e A n g e l e s

G r o u p a n d i s , i n t u r n , o v e r l a i n b y t h e tu f f s o f

t h e P a d r e M i g u e l G r o u p ( W i l l i a m s a n d

M c B i r n e y , 1 9 69 ). I n H o n d u r a s , l o c a l r e l i e f o n

t h e s e r o c k s r a n g e s f r o m 1 00 m t o 3 0 0 m , f o r m -

i n g k n o b b y r i d g e s. N e a r Z a c a p a , 8 0 k m e a st o f

P l a t a n a r e s , t h e M a t a g a l p a F o r m a t i o n i s u p t o

500 m th ick (Finch , 1972) .

Subinal ormation

A n 8 0 - t o 1 2 0 - m - t h i c k r e d b e d s e q u e n c e ,

w h i c h i s n e a r l y i d e n t i c a l i n a p p e a r a n c e t o t h e

o l d e r r e d b e d s o f t h e V a l le d e A n g e l e s G r o u p ,

u n c o n f o r m a b l y o v e rl ie s a n d e s it i c la v a s o f t h e

M a t a g a l p a F o r m a t i o n . T h e r e d b e d s c o n s i s t o f

r e d a n d g r e e n s a n d s t o n e s , s i lt s to n e s , a n d c o n -

g l o m e r a t e s . A s d i s c u s s e d e a r l i e r , t h e s a n d -

s t on e s a n d c o n g l o m e r a t e s m o s t l y c o ns i st o f

m e t a q u a r t z i t e a n d s c h i s t c l as t s (F i g . 5). M o s t o f

t h e c o a r s e r c l a s t i c r o c k s a r e i m m a t u r e , w i t h

p o o r l y s o r t e d c l a s t s i n a m a t r i x o f g r a y i s h -

p u r p l e s il t. T h i s c l a s t ic r o c k s e q u e n c e , u n d e r l y -

i n g t h e t u f ts o f t h e P a d r e M i g u e l G r o u p , c r o p s

o u t a l o n g t h e w e s t w a l l o f t h e u p p e r Q u e b r a d a

d e A g u a C a l i e n t e a n d i s p e n e t r a t e d b y w e l l s

P L T G - 2 a n d - 3 .

T h e S u b i n a l F o r m a t i o n w a s d e f i n e d fi rs t b y

H i r s c h m a n n (1 96 3) i n t h e M o t a g u a V a l le y o f

G u a t e m a l a a s r e d c l a s t i c s t r a t a b e t w e e n

P a l e o z o i c r o c k s a n d T e r t i a r y v o l c a n i c r o c k s .

W i l l i a m s a n d M c B i r n e y ( 1 9 6 9 ) a p p l i e d t h e

n a m e t o r e d b e d s u n d e r l y i n g t h e P a d r e M i g u e l

G r o u p i n H o n d u r a s ; t h e s e r o c k s a r e c o n t e m -

p o r a n e o u s w i t h t h e t y p e S u b i n a l s e c t i o n , a n d

w e r e d e p o s i te d a f t e r t h e e r u p t i o n s o f

M a t a g a l p a a n d e s it ic la v a s. U s e o f t h e n a m e

S u b i n a l is b a s e d o n st r at i g ra p h i c p o s i t io n

b e t w e e n M a t a g a l p a a n d e s i t i c l a v a s a n d t h e

P a d r e M i g u e l G r o u p t u f t s . A t P l a t a n a r e s ,

r o c k s a s s ig n e d t o t h e S u b i n a l F o r m a t i o n a r e in -

t e r p r e t e d a s a l l u v i a l f a n s d e r i v e d f r o m a

m e t a m o r p h i c h i g h l a n d , w h i c h p a r t l y b u r i e d

d o w n f a u l t e d p i le s o f M a t a g a l p a a n d e s it e s a n d

o l d e r r o ck s . C o r r e l a t i o n o f t h e se r e d b e d s w i th

o t h e rs d e s c ri b e d i n H o n d u r a s a n d G u a t e m a l a ,

on t he ba s i s o f c l a s t t ypes , i s d i f f i cu l t because

e a c h o f t h e s e f a n g l o m e r a t e s o r s m a l l b a s i n -

f i ll in g d e p o s i t s h a s a u n i q u e s o u r c e .

Padre Miguel Group

T h e P a d r e M i g u e l G r o u p c o n s is ts o f m o s t l y

t u f t s a n d t u f f a c e o u s s e d i m e n t a r y r o c k s t h a t

c r o p o u t o v e r m u c h o f w e s te r n H o n d u r a s a n d

s o u t h w e s t e r n G u a t e m a l a ; W i ll ia m s a n d M c B i r-

n e y ( 19 6 9) e s t i m a t e d t h e t h i c k n e s s o f t h e g r o u p

t o b e b e t w e e n 3 0 0 a n d 1 00 0 m . M o s t o f t h e t u f f s

a r e r h y o l i t i c . T h e g r o u p d e s i g n a t i o n r e f e r s t o

i g n i m b r i t e s e q u e n c e s v i s i b l e o v e r m u c h o f

w e s t e r n H o n d u r a s , w h i c h m a y c o m e f r o m

s e v e r a l c a l d e r a s o u r c e s a n d w h o s e d e p o s i t i o n a l

h i s t o r y s p a n s a c o n s i d e r a b l e le n g t h o f t i m e ; K -

A r a g e s f o r t h e P a d r e M i g u e l G r o u p r a n g e f r o m

5 M a t o 3 0 M a ( F . M c D o w e l l , U n i v e r s i t y o f

T e x a s , p e r s. c o m m u n . , 1 9 88 ; W i l l ia m s a n d

M cB i rney , 1969 ; Ep p le r e t a l . , 1987a) . Tu f f s

a n d a s s o c i a t e d m i n o r l a v a s a n d s e d i m e n t a r y


8/18

48

G H E 1 K E N E T A L

r o c k s o f t h e P a d r e M i g u e l G r o u p c r o p o u t o v e r

m o s t o f t h e P l a ta n a r e s g e o t h e r m a l ar e a . A t

P l a t a n a r e s t h e t h i c k n e s s o f t h i s g r o u p v a r i e s ,

r e a c h in g a m a x i m u m o f 3 10 m .

A l t h o u g h m o s t o f t h e tu f f s a t P l a ta n a r e s a r e

n o n w e l d e d i g n i m b r i t e s , o n e c o n s i s ts o f 1 5/0

w e l d e d , f l a t t e n e d p u m i c e c l a s ts , 5 7o q u a r t z ,

b i o ti te , a n d h o r n b l e n d e p h e n o c r y s t s , a n d 5 %

l i t h i c c l a s t s ( a n d e s i t i c l a v a ) i n a m a t r i x o f

d e v i t r i f i e d s h a r d s . S i m i l a r w e l d e d t u f t s ,

p o s s i b l y c o r r e l a t i v e , w e r e s a m p l e d f o r K - A r

a g e d e t e r m i n a t i o n s f r o m o u t c r o p s a l o n g t h e

Q u e b r a d a d e l as J u n i a p a s a n d a t a d e p t h o f 2 86

m i n w e l l P L T G - 2 . T h e d a t e s , d e t e r m i n e d b y E .

M c K e e , o f t h e U . S . G e o l o g i c a l S u r v e y ar e ,

res pe c t ive ly , 14 .2 +_ 0 .5 M a an d 14 .7 +_ 0 .5

M a ( T a b l e 1 ) .

A b o v e t h e w e ld e d t u f f s a m p l e d i n P L T G - 2

a r e a t l e a s t e i g h t t u f f u n i t s , m o s t o f w h i c h a r e

m a s s i v e , n o n w e l d e d , w h i t e , c r e a m - c o l o r e d , o r

g r e y , f i n e - g r a i n e d i g n i m b r i t e s ; t h e s e u n i t s

r a n g e , i n t h i c k n e s s , f r o m a b o u t 5 m t o 7 0 m .

M o s t a r e v i t r i c o r v i t r i c - l it h i c t u f t s , w i t h 5070 t o

2007o p h e n o c r y s t s o f q u a r t z , s a n i d i n e a n d l e ss e r

p l a g i o c la s e , b i o t it e , h o r n b l e n d e , a n d i l m e n i te ;

p u m i c e la p il li u p t o 1 c m l o n g a r e i n a m a t r i x o f

s h a r d s ( 20 # m t o 5 0 0 ~ m l o n g ) . T h e s e t u f t s a l s o

co n t a i n a t r a ce t o 2 00 70 l i t h ic c l a s t s (m o s t l y

b a s a l t a n d a n d e s i t e l a v a a n d c l a s t s o f o l d e r

t u f f s ) . A c c r e t i o n a r y l a p il li , u p t o 1 c m i n

d i a m e t e r , a r e c o m m o n i n s o m e o f t h e f in e r -

g r a i n e d i g n i m b r i t e s .

B e d s o f n o n w e l d e d t u f f s , w i t h t h ic k n e s s e s o f

0 .5 t o 1 0 m , a r e e x p o s e d a l o n g t h e R i o L a r a ( t o p

o f t h e s e c ti o n s o u t h o f t h e R i o L a r a , m i d d l e o f

t h e s e c t i o n t o t h e n o r t h o f t h e R i o ; F i g . 3 ) a n d

n e a r t h e t o p o f t h e s e c t i o n p e n e t r a t e d b y w e l l

P L T G - 2 . M o s t a re n o r m a l l y g r a d e d , w i t h l a v a

l i t h i c c l a s t s a n d c o a r s e r p u m i c e c l a s t s n e a r t h e

b a s e , g r a d i n g u p in t o f i n e a s h . I n t e r b e d d e d

w i t h t h e s e t u f t s a r e c o n c e n t r a t i o n s o f l i t h i c

c l a s t s a n d m a t r i x - s u p p o r t e d b r e c c i a s , w h i c h

m a y b e v o lc a n ic m u d f l o w d e p o s i ts ; s o m e o f t h e

b r e c c ia s a r e c o n c e n t r a t e d i n 1 - t o 3 - m - d e e p

c h a n n e l s . W i t h i n t h e t u f t s p e n e t r a t e d d u r i n g

d r il li n g , t h e r e a r e i n t e r b e d d e d m u d d y s a n d -

s t o n e s a s s o c i a t e d w i t h s h e a r e d r o c k . I n o u t -

c r o p s a l o n g th e Q u e b r a d a d e A g u a C a l ie n t e ,

s u c h s a n d s t o n e s a r e s e e n t o b e c la s ti c d i k e s

t h a t f o r m e d a l o n g n o r m a l f a u l ts i n th e t u f f s .

P u m i c e f a l l d e p o s i t s a r e r a r e a n d a r e i n -

t e r b e d d e d w i th n o n w e l d e d , f i n e - g r a in e d i g n im -

b r i t e s a l o n g t h e Q u e b r a d a d e A g u a C a l i e n t e .

A l s o i n c lu d e d i n t h e P a d r e M i g u e l G r o u p a r e

p o o r l y b e d d e d , r e d d i s h f a n g l o m e r a t e s a f e w

m e t e r s t h i c k ; t h e s e a r e f o u n d o n l y n e a r f a u l t

c o n t a c t s w i t h t h e m e t a m o r p h i c s e q u e n c e a n d

n o t s o u t h o f t h e R i o L a r a .

T A B L E 1

P o t a s s i u m - a r g o n a g e s o f w e l d e d t u f ts f r o m t h e P l a t a n a r e s g e o th e r m a l a r e a , D e p a r t a m e n t o d e C o p ~ n , H o n d u r a s

S a m p l e R o c k M a t e r i a l K 2 0 4 0 Ar ra d 4 0 Ar ra d A g e

n u m b e r t y p e d a t e d ( w t. ) ( t o o l / g ) ( ) ( M a )

P T L G - 2 R h y o l i t i c w e l d e d t u f f B i o t i t e 8 . 1 9 1 . 73 6 9 1 0 - 1 6 5 . 0 1 4 .7 _+ 0 . 5

J U N I A P A R h y o l i t i c w e l d e d t u f f B i o t i t e 7 .3 1 1 .5 0 17 9 1 0 - 1 4 0 . 5 1 4 .2 _+ 0 . 5

N o t e : a l l s a m p l e p r e p a r a t i o n a n d a n a l y t i c a l w o r k w a s d o n e a t t h e U . S . G e o l o g i c a l S u r v e y l a b o r a t o r i e s i n F l a g s t a f f , A r i z o n a

a n d M e n l o P a r k , u s i n g t h e t e c h n i q u e s d e s c r i b e d i n D a l r y m p l e a n d L a n p h e r e ( 19 6 9 ). W e a n a l y z e d p o t a s s i u m a f t e r l i t h i u m

m e t a b o r a t e f u s i o n , u s i n g a f l a m e p h o t o m e t e r , w i t h t h e l i t h i u m s e r v in g a s th e i n t e r n a l s t a n d a r d ( I n g a m e l l s , 1 97 0). E r r o r ,

s h o w n a s t h e _+ v a l u e , i s t h e e s t i m a t e o f t h e s t a n d a r d d e v i a t i o n o f a n a l y t i c a l p r e c i s io n . K - A r a g e w a s c a l c u l a te d u s i n g t h e

c o n s t a n t s f o r t h e r a d i o a c t i v e d e c a y a n d a b u n d a n c e o f 4 A r r e c o m m e n d e d b y th e I n t e r n a t i o n a l U n i o n o f G e o l o g i c a l

S c i e n c e s S u b c o m m i s i o n o n G e o c h r o n o l o g y ( S t e i g e r a n d J ~ g e r , 1 9 77 ). T h e s e c o n s t a n t s a r e : Y t = 0 . 58 1 1 0 - l 0 y r - 1 ,

Y t = 4 . 9 6 2 1 0 - 10 y r - l , a n d 4 0 K / K t ot a I = 1 .1 6 7 x 1 0 - 4 m o l / m o l . A g e d e t e r m i n a t i o n p r o v i d e d b y E . H . M c K e e , U . S .

G e o l o g i c a l S u r v e y .


9/18


M a n y o f t h e t u f f s a r e s i li ci fi ed a lo n g m a j o r

f a u l t s . T h e s e r o c k s a r e b r i t t l e a n d c r o s s e d b y

a b u n d a n t o p e n a n d c a l ci te - f il le d f r a c t u r e s .

S i l i c i f i e d t u f f s l o c a l l y f o r m e r o s i o n - r e s i s t a n t

r id g e s. M a n y o f t h e t u f f s p e n e t r a t e d d u r i n g

d r il li n g o f w e ll s P L T G - 2 a n d P L T G - 3 a r e

s i l i c i f i e d , a n d o n l y r e l i c t t e x t u r e s r e m a i n .

A f e w l a v a f l o w s h a v e b e e n i d e n t i f i e d a t

P l a t a n a r e s w i th i n t h e P a d r e M i g u e l G r o u p ;

r h y o l i ti c la v a f l o w b r e c c i a c r o p s o u t a l o n g a n d

n e a r t h e Q u e b r a d a d e la s J u n i a p a s a n d b a s a l ti c

l a v a c r o p s o u t a l o n g t h e R i o L a r a . N o v e n t

a r e a s h a v e b e e n i d e n t i f ie d f o r t h e s e la v a s .

S o u r c e s f o r t h e t u f fs o f t h e P a d r e M i g u e l

G r o u p a t P la t a n a r e s a r e n o t k n o w n . H o w e v e r ,

t h e m a s s i v e , n o n w e l d e d i g n i m b r i t e s e x t e n d

s o u t h w a r d t o t h e C o r d i l le r a d e C e l a q u e , a p r o -

p a b l e c a l d er a c o m p l e x 2 0 k m s o u t h o f P l a t a n -

a r e s . T h e t u f f s a t C e l a q u e a r e l i t h o l o g i c a l l y

s i m i la r t o t h o s e a t P l a t a n a r e s , s u g g e s t in g th a t

t h is c a l d e r a c o m p l e x m a y b e t h e s o u r c e f o r t h e

i g n i m b r i t e s a t P l a t a n a r e s . I f th i s i n t e r p r e t a t i o n

is c o r r e c t, t h e n p y r o c l a s t i c fl o w s f r o m t h e C e l a-

q u e a r e a f l o w e d n o r t h , b u r y i n g a c o m p l e x t er -

r a i n a t P l a t a n a r e s t h a t c o n s i s t e d o f a h i g h l a n d

o f P a l e o z o ic m e t a m o r p h i c r o ck s a n d M e s o z o ic

r e d b e d s o v e r l a i n b y m i d - T e r t i a r y a n d e s i t e

f l o w s a n d a l l u v ia l f a n s. A s m e n t i o n e d a b o v e ,

t h e P a d r e M i g u el G r o u p i n w e st er n H o n d u r a s

a n d e a s t e rn G u a t e m a l a r a n g e s in a g e f r o m 5 t o

3 0 M a ; t h e r a n g e o f a g es a t P l a t a n a r e s is

u n k n o w n , b u t t u f f s w e d a t e d n e a r th e b a s e o f

t h e s e c t io n h a v e ag e s o f a b o u t 1 4.5 M a . T h e r e

m a y b e m u l t i p l e so u r c es f o r t h e P a d r e M i g u e l

t u f f s , s u c h a s a n e a r b y s i l i c i c l a v a f i e l d o f

u n k n o w n l o c a t io n , b u t f i e ld w o r k is n e e d e d to

i d e n t i f y a ll p o s s i b l e s o u r c e s , i n c l u d i n g c a l d e r a s

o t h e r t h a n t h a t i n f e r r e d a t t h e C o r d i l l e r a d e

C e l a qu e . I m p o r t a n t t o g e o t h e r m a l m o d e l s o f

P l a t a n a r e s , h o w e v e r , is t h e f a c t t h a t t h e s e

v o l c a n i c r o c k s a r e r e l a ti v e ly o l d a n d t h u s t h e i r

s o u r c e m a g m a b o d i e s h a v e s o l i d i f i e d a n d

p r e s u m a b l y c o o l e d t o b a c k g r o u n d c r u s t a l

t e m p e r a t u r e s , w h e t h e r t h e y a r e l o c a te d b e n e a t h

P l a t a n a r e s o r s o m e d i s ta n c e a w a y S m i t h a n d

S h aw , 1 9 7 5 ) .

Terrace gravels

T h e v i ll a g e o f P l a t a n a r e s is l o c a t e d o n a 1 .0 -

b y 1 .2 5 k m s t r e a m t e r r a c e , w h i c h f o r m s t h e u p -

p e r p a r t o f t h e s o u t h w e s t e r n s l o p e s o f t h e

Q u e b r a d a d e l A g u a C a l ie n t e. T h e t e rr a c e

s l o p es a b o u t 3 .5 t o t h e s o u t h e a s t f r o m a n

e l e v a t i o n o f 8 0 0 m t o 7 4 0 m . T e r r a c e g r a v e l s

f o r m a n i r r e g u l a r d e p o s i t , u p t o 6 0 m t h i c k ,

w h i c h o v e r l i e s a s u r f a c e o f m o d e r a t e r e l i e f

e r o d e d i n t o tu f f s o f t h e P a d r e M i g u e l G r o u p .

T h e g r av e ls a r e p o o r l y b e d d e d a n d c o n s is t o f

a n g u l a r t o s u b a n g u l a r c o b b l e s a n d p e b b l e s ,

m o s t l y q u a r t z it e , s c h i s t, a n d e s i t e a n d t u f f . T h e

u p p e r t h i r d o f t h e d e p o s i t i s s t a i n e d r e d b y

h e m a t i t e .

A l o w e r - e l e v a t i o n t e r r a c e i s p r e s e n t i n t h e

s o u t h e a s t p a r t o f t h e m a p p e d a r e a, n e a r t h e

c o n f l u e n c e o f t h e Q u e b r a d a d e l A g u a C a l ie n t e

w i t h t h e R i o H i g u i t o . T h i s t e r r a c e s l o p e s g e n t ly

t o t h e s o u t h e a s t a t a n a v e r a g e e l e v a t i o n o f

a b o u t 7 0 0 m .

Silica sinter dep osits

D e p o s i t s o f s i l i c e o u s s i n t e r a n d c o n g l o m e r -

a t e s c e m e n t e d b y si li ca si n te r u n c o n f o r m a b l y

o v e r li e t u f f s a n d l a v a s a lo n g t h e Q u e b r a d a d e l

A g u a C a l i e n t e . T h e c o n g l o m e r a t e s a re l it h o g i-

c a ll y s i m i la r t o m o d e r n s t r e a m g r a v e l s a n d c o n -

s is t o f c o b b le s a n d p e b b le s f r o m m e t a m o r p h i c ,

v o l c a n i c l a v a ), a n d r e d b e d c l a s ti c s o u r c e s . T h e

s i n t e r d e p o s i t s f o r m o v e r h a n g i n g l e d g e s a n d

l o c a l l y f o r m i m p e r m e a b l e c a p r o c k s o v e r h o t

s p r in g s . M o d e r n s t r ea m g r av e ls i n th e m a i n h o t

s p r i n g a r e a a r e n o w b e i n g c e m e n t e d b y s i li ca

f r o m h o t s p r i n g s . O n e o f th e s i n t e r d e p o s i t s is

c u t b y a f a u l t in th e u p p e r Q u e b r a d a d e l A g u a

C a l i e n t e .

ot spr ings and fuma roles

M o s t h o t s p r i n g s a n d a l l o f t h e b o i l i n g

s p r i n g s a t P l a t a n a r e s l i e w i t h i n a 0 . 5 - 1 . 0 - k m -

w i d e , N W - t r e n d i n g f a u l t a n d f r a c t u r e z o n e

w h o s e p r i n c i p a l p h y s i o g r a p h i c e x p r e s s i o n i s

Q u e b r a d a d e A g u a C a l i e n te . A f ew w a r m

s p r i n g s d i s c h a r g e f r o m s i l i c i f i e d , f r a c t u r e d


10/18

50 G HEIKEN ET AL

t u f f s a l o n g t h e R i o L a r a . M o s t s p r i n g s v e n t a t

t h e b o t t o m o f t h e q u e b r a d a b u t s o m e d i s c ha r ge

o c c u r s f r o m f r a c t u r e d t u f f s a s h i g h a s 3 5 m

a b o v e s t r e a m l e v e l. A t a w a t e r f a l l , w h i c h f l o w s

o v e r a f a u l t s c a r p , h o t s p r i n g s f lo w a l o n g b e d -

d i n g p l a n e s i n s i li c if ie d t u f f . F o r m a p s a n d

d e t a i l e d d e s c r i p t i o n s o f t h e s p r i n g s , s e e H e i k e n

e t a l. ( 1 9 8 6 ) a n d G o f f e t a l. ( 1 9 8 7 b ) ; ( J a n i k e t

a l . , 1 9 9 1 - t h is v o l u m e ) .

tructural framework

M o s t s t r a t i f i c a t io n i n th e t u f f s o f t h e P a d r e

M i g u e l G r o u p d i p s 1 5 t o 3 0 t o t h e s o u t h ; a t -

t i t u d e s d i v e r g e f r o m t h i s r e g i o n a l d i p n e a r

f a u l t s a n d l a n d s l id e s . B e d d i n g w i t h in t h e V a l l e

d e A n g e l e s r e d b e d s d i p s g e n e r a l ly t o t h e s o u t h

o r s o u t h w e s t , a t 15 t o n e a r l y v e r t i c a l .

A n a r e a o f a b o u t 5 k m 2, b o u n d e d b y L a

B u f a f a u l t o n t h e n o r t h w e s t , a p o s s i b l e f a u l t

p a r a ll e l t o th e R i o L a r a o n t h e s o u t h , u n n a m e d

f a u lt s o n t h e w e s t, a n d t h e Q u e b r a d a d e l A g u a

C a l i e n t e f a u l t o n t h e e a s t is h i g h l y d i s r u p t e d b y

n o r m a l f a u l t s ( F ig . 3 ); s li c k e n s i d e o r i e n t a t i o n s

i n d i c a t e t h a t a n u m b e r o f h i g h - a n g l e f a u l t s

h a v e p r e d o m i n a n t l y s t r i k e - s l i p m o t i o n . T h e

T e r t i a r y v o l c a n i c r o c k s , i n c l u d i n g t h e a n d e s i t ic

l av a s o f th e M a t a g a l p a F o r m a t i o n a n d t h e tu f f s

o f th e P a d r e M i g u e l G r o u p , a r e d o w n f a u l t e d

a g a i n s t C r e t a c e o u s - T e r t ia r y r e d b e d s o f t h e

V a ll e d e A n g e l e s G r o u p , d e f i n in g a N W -

t r e n d i n g g r a b e n . T h e g r a b e n is t e r m i n a t e d o n

t h e n o r t h b y t h e E - W t o N E - t r e n d i n g L a B u f a

f a u l t . A l l h o t s p r i n g s o f t h e P l a t a n a r e s a r e a l ie

s o u t h o f th i s fa u l t . A d e t a i l e d s tr u c t u r a l m a p o f

t h e c e n t e r o f t h i s a r e a h a s b e e n p r e p a r e d b y

A l d r i c h e t a l . ( 1 9 9 1 - t h i s v o l u m e ) .

A l t h o u g h p o o r l y e x p o s e d i n o u t c r o p , a m a -

j o r E - t r e n d i n g f a u l t m a y p a r a l le l th e R i o L a r a

( R i o L a r a F a u l t o n F i g. 3 ). N o r t h o f t h e R i o

L a r a , t h e r e a r e n u m e r o u s f a u l t s a n d h o t

s p r i n g s , w h e r e a s t o t h e s o u t h t h e r e i s l i t t l e

d e f o r m a t i o n o t h e r t h a n t h e g e n t le r e g io n a l d i p

a n d n o h o t s p r i n g s a r e v i s i b l e . E a s t w a r d f r o m

t h e m o u t h o f t h e R i o L a r a , t h e R i o H i g u i t o

t a k e s a 9 0 b e n d a n d f o l lo w s t h e e x t e n s i o n o f

t h e a p p a r e n t R i o L a r a f a u l t ( F i g . 2 ) . W e l l

P L T G - 2 , d r il le d 0 .5 k m n o r t h o f th e R i o L a r a ,

p e n e t r a t e s a s e q u e n c e o f h e a v i ly f r a c t u r e d a n d

f a u l t e d r o c k s a n d c l a s t i c d i k e s .

T h e m a i n tr a c e o f t h e Q u e b r a d a d e l A g u a

C a l i e n t e f a u l t is p a ra l le l t o t h e q u e b r a d a o f t h e

s a m e n a m e a n d h a s a s t r o n g p h y s i o g r a p h i c e x-

p r e s s i o n , i n c l u d i n g e s c a r p m e n t s a l o n g a l l u v i al

t er r ac e s a t t h e m o u t h o f th e q u e b r a d a . T w e n t y -

e i g h t g r o u p s o f h o t a n d b o i l i n g s p r i n g s r i s e

a l o n g N W - t r e n d i n g f a u l t s a n d f r a c t u r e s o f t h is

f a u l t z o n e .

N e a r l y a l l o f t h e r o c k u n i t s s a m p l e d b y d r i l-

l in g a r e f r a c t u r e d a n d f a u l t e d . T h e c h a r a c t e r i s -

t ic s o f a l l f r a c t u r e s e t s w i t h i n r o c k u n i t s s a m p l -

e d b y d r i l li n g a r e c o n t r o l l e d b y l it h o l o g y . S t e e p -

l y d i p p i n g f r a c t u r e s ( d i p = 6 0 t o 9 0 ) a r e

f o u n d w i th i n h y d r o t h e r m a l l y a lt e re d l a va s a n d

w e l d e d t u f f o n l y , w h e r e a s t h o s e w i th l o w d i p s

a r e f o u n d i n a ll r o c k t y p e s , a n d a r e m o s t a b u n -

d a n t i n s i lt s t o n e s . F r a c t u r e s f i ll e d w i t h

a u t h ig e n i c m i n er a l s a r e m o s t c o m m o n i n s u c h

c o m p e t e n t r o c k u n i t s a s t h e a n d e s i t ic l a v as .

ydrotherm al alteration

T h r e e e x p l o r a t i o n c o r e h o l e s ( P L T G - 1 ,

P L T G - 2 , a n d P L T G - 3 ) w e r e d r il le d t o d e p t h s

o f 6 5 0 m , 4 2 8 m , a n d 6 7 9 m , r e s p e c ti v e ly a t

P l a t a n a r e s t o p r o v i d e d a t a o n l i th o l o g i e s ,

t e m p e r a t u r e g r a d i e n t s , p h y s i c a l p r o p e r t i e s ,

a n d c h e m i c a l c o m p o s i t i o n o f f l u i d s (F ig s . 3 a n d

6 ) (S . G o f f e t a l . , 1 9 8 7 , 1 9 8 8 ; G o f f e t a l . , 1 9 9 1 -

t h is v o l u m e ) .

P L T G - 1 a n d P L T G - 3 a r e lo c a t e d w i t h in t h e

m a i n N W - t r e n d i n g f a u l t z o n e c l o s e t o t h e

Q u e b r a d a d e A g u a C a l ie n t e (F ig . 3 ). P L T G - 1

e n c o u n t e r e d h o t w a t e r a t 2 5 2 m a n d f r o m 6 25

m t o 6 4 4 m . P L T G - 3 , l o c a t e d 65 0 m S E o f

P L T G - 1 , e n c o u n t e r e d h o t w a t e r at d e p t h s o f

4 5 9 m a n d 6 2 2 m t o 6 3 5 m ( F i g . 7 ). R o c k u n i t s

s a m p l e d b y c o r i n g i n b o t h w e ll s 1 a n d 3 a re

h y d r o t h e r m a l l y a l te r e d .

P L T G - 2 , l o c a te d 1 k m s o u t h o f P L T G - 1 a n d

a w a y f r o m t h e m a i n h o t s p r i n g a re a , s h o w s a

c o n d u c t i v e g e o t h e r m a l g r a d i e n t o f 13 9 C / k m


11/18

G E O L O G Y O F T H E P L A T A N A R E S G E O T H E R M A L A R E A , H O N D U RA S 5

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S E C T I O N , 1 0C 1 3 0 C \ ~ ~ - ' O ~ 2 '

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F i g . 6 . F e n c e d i a g r a m o f s t r a t i g r a p h y o f t h e P l a t a n a r e s g e o t h e r m a l a r e a , b a s e d u p o n c o m p o s i t e s t r a ti g r a p h i c s e c t io n s

o u t c r o p s ) a n d t h r e e g e o t h e r m a l g r a d i e n t c o r eh o l e s . P a t t e r n s w i t h in t h e M a t a g a l p a F o r m a t i o n : w h i t e = l a v a s a n d

s t i p p l e d = i n t e r b e d d e d t u f t s . P a t t e r n s w i t h i n th e P a d r e M i g u e l G r o u p ; w a v y p a t t e r n = w e l d e d t u f f, l i g h t s ti p -

p l e = m a s s i v e , n o n w e l d e d t u f t s , a n d c o a r s e s t i p p le = b e d d e d t u f t s .

a t a d e p t h o f 4 0 0 m ( l i n e a r g r a d i e n t s o f

2 3 9 C / k m f r o m 0 t o 1 50 m a n d 1 3 9 C f r o m

1 7 0 t o 4 0 0 m ; M e e r t a n d S m i t h , 1 9 9 1 - t h i s

v o l u m e ) ( F ig . 7 ). T h e r o c k u n i t s p e n e t r a t e d b y

t h is w e l l s h o w l it tl e h y d r o t h e r m a l a l t e r a t i o n .

ethods and procedure

C o r e s f r o m P L T G - 1 w e r e sa m p l e d e v e ry 1 0

m a n d t h o se f r o m P L T G - 2 a n d P L T G - 3 w e r e

s a m p l e d e v e r y 20 m ( m o s t w e r e r e p r e s e n t a t iv e

o f t h e w h o l e r o c k a n d s o m e s a m p l e s a re o f

v e i ns ) . T h i n s e c t i o n s w e r e c u t f r o m e a c h s a m p l e

a n d i o n - e tc h e d b e f o r e p e t r o g r a p h i c , E D S

a n a l y s e s o n a s c a n n i n g e l e c t r o n m i c r o s c o p e a n d

e l e c t r o n m i c r o p r o b e a n a l y s e s . B u l k r o c k

s a m p l e s f r o m e a c h i n te r v a l w e r e c r u s h e d a n d

t h e p o w d e r s g r o u n d to 5 m b e f o r e an a ly s is o n

a S i e m e n s D 5 0 0 a u t o m a t e d X - r a y d i ff r a c -

t o m e t e r a t a s t e p o f 0 .0 2 , a r a t e o f 1 .0 s / s t e p ,

s c a n n e d f r o m 2 t o 3 7 2 -t h e t a , u n d e r C u K a

r a d i a t i o n . I n s t r u m e n t s e t t i n g s w e r e 5 0 k V a c -

c e l e r a t i n g v o l t a g e a n d 4 0 m A t u b e c u r r e n t .

M i n e r a l p h a s e s w e r e i d e n t i f i e d b y o p t i c a l

m e t h o d s , X R D , a n d e n e r g y -d i sp e r s iv e X - r a y

s p e c t r o m e t r y d u r i n g e x a m i n a t i o n w i th a s c an -

n i n g e l e c t r o n m i c r o s c o p e . T h e w e i g h t p e r c e n -

t a g e s o f m i n e r a l p h a s e s i n F i g u r e 7 a r e s e m i -

q u a n t i t a t i v e a n d w e r e c a l c u l a t e d u s i n g a p r o -

g r a m ( Q u a n t v ) t h a t u s e s p e a k i n te n s it ie s o f t h e

s a m p l e , c o m p a r e d w i t h t h o s e o f s t a n d a r d s .


12/18

5 2

G. HEI KEN ET AL.

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F i g . 7 . V a r i a t io n o f m i n e r a l s f r o m b u l k r oc k s a m p l e s w i t h d e p t h in th r e e g e o t h e r m a l g r a d i e n t c o r e h o l e s ( h o s t r o c k

s a m p l e s ) . Q t z = q u a r t z , F e l d = f e l d s p a r , S m = s m e c t i t e , C c - - c a l c i t e , K a = k a o l i n i t e , I1 = i l l i te , C h = c h l o r i t e ,

C 1 = c l i n o p t i lo l i t e , C r = c r i s to b a l i te , a n d O p = o p a l C - T . V a l u e s a re i n w e i g h t p e r c e n t . T e m p e r a t u r e m e a s u r e m e n t s

( f r o m D . S m i t h , U n i v e r s i t y o f F l o r i d a a n d o n s i t e d r i ll in g te a m s , u n p u b l i s h e d r e p o r ts ) w e r e m a d e a t 3 0 - m i n t e r v al s , b o t h

d u r i n g d e s c e n t a n d a s c e n t o f t h e p r o b e . S a m p l e s r e p r e s e n t a ti v e o f t h e m a j o r l i t h o i o g i c u n i t s w e r e c o ll e c t ed f o r t h e r m a l

c o n d u c t i v i t y v a l u e s .


13/18


S e l e c te d , h o m o g e n i z e d r o c k p o w d e r s ( 17 7 t o 4 0

m ) o f v o l c a n i c r o c k s w e r e a n a l y z e d b y X - r a y

f l u o r e s c e n c e , f o l l o w i n g t h e p r o c e d u r e s o f

V a l e n t i n e ( 1 98 3 ); t h e d a t a a p p e a r i n T a b l e 2 .

H y d r o t h e r m a l m i n e r a lo g y P L T G 1

P r i n c i p a l h y d r o t h e r m a l m i n e r a l s i n c l u d e

q u a r t z , s m e c t i t e , m i x e d - l a y e r i l li t e -s m e c t i t e , il-

l i t e , k ao l i n i t e , p y r i t e , m a rca s i t e , c a l c i t e ,

f l u o ri te , a n d m i n o r to t r a ce a m o u n t s o f

l a u m o n t i t e , r u t i l e , s t i b n i t e , a n d s p h a l e r i t e .

P h e n o c r y s t s w i t h i n a n d e s it i c la v a s a r e p a r t l y t o

c o m p l e t e l y a lt e r e d t o s m e c t it e , m i x e d - l a y e r

i l l i t e - s m e c t i t e , i l l i t e , a n d t r a c e a m o u n t s o f

k a o l i n i t e . T h e g r o u n d m a s s m i n e r a l s o f l a v a

s a m p l e s a r e a l t e r e d t o q u a r t z , s m e c t i t e , i l l i t e ,

a n d k a o l i n i t e ( a l s o i d e n t i f i e d b y B a r g a r , t h i s

v o l u m e ) . S m e c t i t e i s m o r e o r l e s s e v e n l y

d i s t r i b u t e d t h r o u g h o u t t h e g r o u n d m a s s ,

w h e r e a s k a o l i n i t e a n d i l l i t e o c c u r a s s m a l l

c l u m p s . C h l o r i t e a n d b a r i t e a r e p r e s e n t a s

d i s c o n t i n u o u s v e in l e ts , a n d r e p l a ce s m a f i c

m i n e r a l s ; i t i s m o s t a b u n d a n t f r o m d e p t h s o f

3 9 0 m t o 5 4 0 m (F i g . 7 ) . T h e re i s a w ea k v e r t i c a l

z o n a t i o n o f a l t e r a t i o n m i n e r a l s , w i t h d e c r e a s -

i n g s m e c t i t e a n d i n c r e a s i n g c h l o r it e w i t h d e p t h .

A l l o p h a n e i s p r e s e n t at a ll d e p t h s a n d m a k e s u p

a s m u c h a s 5 v o l u m e p e r ce n t. N e a r a m a j o r

t h e r m a l - w a t e r e n t r y a t a d e p t h o f 2 5 2 m , t h e r e

is a n i n c r e a s e i n t h e v o l u m e o f k a o l i n i te a n d il-

l it e (1 5 a n d 2 0 w t . 70 ). I n s u f f i c i e n t t i m e f o r

w o r k o n a l t e r a t i o n a g e s p r e c l u d e s a n y d i s c u s -

s i o n o n w h i c h o f t h e a l t e r a t i o n p r o d u c t s a r e

r e la t e d to a n o l d e r o r p r e s e n t - d a y h y d r o t h e r -

m a l s y s t e m . T h e k a o l i n i t e - i l l i t e i n t h e

2 0 0 - 3 0 0 - m i n t e r v a l s u g g e s t s t h a t a t le a s t s o m e

o f t h e a l t e r a t i o n w a s c a u s e d b y t h e p r e s e n t

h y d r o t h e r m a l s y s t e m . T h e s e v e r i t y o f a l t e r a -

t i o n i s e v i d e n t i n t h e c h e m i c a l a n a l y s e s

p r e s e n t e d i n T a b l e 2 . A l t h o u g h t h e e n t i r e

v o l c a n i c s e q u e n c e in P L T G - 1 is a n d e s i t ic , c o n -

t e n ts o f d i v a le n t a n d m o n o v a l e n t m e t a ls a r e

v a r i ab l e an d t h e t o t a l s a r e < 9 4 w t . 70 , r e f l e c -

t i n g a l t e r a t i o n w i t h h y d r o u s m i n e r a l s .

P y r i t e o c c u r s t h r o u g h o u t t h e e n ti re 6 5 0 - m -

t h i c k s e c t i o n , p r i n c i p a l ly a s v e in l e ts u p t o 1 m m

w i d e . T y p i c a l l y i t f o r m s l o o s e , i r r e g u l a r a g -

g r e g a t e s o f f i n e - g r a i n e d , e u h e d r a l c r y s t a l s ; i n

t h e u p p e r m o s t 1 00 m o f t h e s e c t io n , p y r i t e is in -

t e r g r o w n w i t h m a r c a s i t e . O p e n f r a c t u r e s a t a

d e p t h o f 7 0 m a r e l i n e d w i th s m e c t it e , q u a r t z ,

p y r i t e , m a r c a s i t e , a n d s t i b n i t e . T h e m a r c a s i t e

o c c u r s a s c lu s t er s o f c o c k s c o m b c r y s ta l s a n d t h e

s t i b n i t e f o r m s s p r a y s o f a c i c u l a r c r y s t a ls u p t o

1 8 m m l o n g .

V e i n s o f q u a r t z , c a l c i te , a n d f l u o r i t e a r e p r e -

s e n t a t a ll d e p t h s u p t o 3 c m w i d e . A 2 - c m - t h i c k

q u a r t z v e i n a t a d e p t h o f 1 50 m c o n t a i n s v u g s

0

1 0 0

PLTG 1

0 5 10

N U M B E R O F F R A C T U R E S

P E R m O F E P T H

PL T G 3

P L T G 2

15 0 5 10 15 0 10 20

30 40

2 0 0

3 0 0

t~ 400

500

6 0 0

7 0 0

F i g . 8 . V a r i a t i o n o f o p e n f r a c t u r e d e n s i t y , w i t h d e p t h , i n t h r e e g e o t h e r m a l g r a d i e n t c o r e h o l e s , P l a t a n a r e s G e o t h e r m a l

S i te . F r a c t u r e s w e r e m e a s u r e d a l o n g a l l o f t h e c o r e s c o l le c t e d c o n t i n u o u s c o r i n g ) a n d a v e r a g e d a t 2 - m i n t e r v a l s .


14/18

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15/18

GEOLOGY OF THE PLATANARES GEOTHERMA L AREA HONDURAS 55

l i n e d w i t h f l u o r i t e c r y s t a l s . O t h e r v e i n s c o n -

t a i n i n g c a l c i t e a n d f l u o r i t e a r e t y p i c a l l y n a r -

r o w e r , le ss t h a n 5 m m w i d e .

A s a m p l e o f a n d e s i ti c w a l l r o c k a d j a c e n t t o

o p e n f r a c t u r e s p a r t l y f i l l e d w i t h s t i b n i t e ,

p y r i t e , a n d m a r c a s i t e ( d e p t h = 75 m ) c o n -

t a in s < 6 p p b A u , 1 8.3 p p m Sb , 91 p p m Z n , 4 2

p p m A s a n d < 3 p p m A g . V e i n q u a r t z f r o m a

d e p t h o f 2 52 m ( a m a j o r w a t e r e n t r y ) c o n t a i n s

3 .1 p p m A u , 3 7 4 p p m S b , 2 6 p p m Z n , 2 9 0 0 p p m

A s , 8 .5 p p m A g , 1 . 8 B a , 3 00 0 p p m M o , a n d

1 7 p p m S e ( i n s t r u m e n t a l n e u t r o n a c t i v a t i o n

a n a l y s e s b y S . G a r c i a , L o s A l a m o s ) . A

l i m o n i t i c j a s p e r o i d f r o m a f a u l t S E o f t h e

Q u e b r a d a d e la s J u n i a p a s c o n t a i n s < 6 p p b

A u b u t 47 p p m A s a n d 11 p p m S b .

H y d r o t h e r m a l m i n er a lo g y P L T G 2

H y d r o t h e r m a l m i n e r a l s i d e n t i f i e d i n t h e

r o c k s o f P L T G - 2 c o n t a i n s m e c t it e , m i x e d - la y e r

i l l i t e - s m e c t i t e , q u a r t z , c r i s t o b a l i t e , o p a l - C T ,

c l i n o p t i l o l i t e , a n d m i n o r c a l c i t e , k a o l i n i t e , i l -

l it e, a n d c h l o r i te . T h e u p p e r m o s t 1 50 m o f t h e

s e c ti o n c o n t a i n s o p a l C - T a n d v a r y in g a m o u n t s

o f c r i s t o b a l it e a n d c l in o p t i lo l i te . F r o m d e p t h s

o f 1 5 0 m t o 3 1 0 m , t h e r e i s a n a r g i l l i c

a s s e m b l a g e c o n s i s t in g o f s m e c t i te , q u a r t z , a n d

s p a r s e k a o l i n i t e . T h e a r g i l l i c a s s e m b l a g e i s

b e t t e r- d e v e l o p e d w it h d e p t h a n d p r e d o m i n a t e s

b e l o w 1 80 m . A t a d e p t h o f 2 71 m t h e r e is m i n o r

c h l o r i t e , a n d b e l o w 3 2 0 m i l l i t e i s m o r e a b u n -

d a n t , s u g g e s t in g a t r a n s i t io n t o a m o r e p h y l l ic

a s s e m b l ag e . C h e m i c a l ly , t h e tu f f s o f P L T G - 2

s h o w w e a k t o m o d e r a t e a l t e r a t i o n . T h e l e a s t

a l t e r e d s a m p l e ( d e p t h = 1 50 m ) m i g h t b e c la s -

s e d a s a r h y o d a c i t e ( 6 7 S i O 2 ). T h e o t h e r

s a m p l e s s h o w s i l i c if i c a t io n a n d p o t a s s i c a l t e r a -

t i o n ( T a b l e 2 ) .

H y d r o t h e r m a l m i n e r a lo g y P L T G 3

W i t h i n t h e t h i r d a n d d e e p e s t c o r e h o l e t h e

p r i n c i p a l h y d r o t h e r m a l a l t e r a t io n m i n e r a l s a r e

q u a r t z , s m e c t i t e , m i x e d - l a y e r i l li t e -s m e c t i t e ,

a n d m i n o r t o t r ac e a m o u n t s o f k a o l in i te ,

c a l c i t e , i l l i t e , a n d c h l o r i t e . T h e u p p e r m o s t 2 1 0

m i s s i li c if i e d a n d w e a k l y a l t e r e d t o a n a r g i ll ic

a s s e m b l a g e. A t u f f s a m p l e f r o m t h e b o t t o m o f

t h e P a d r e M i g u e l s e c t i o n s h o w s l i tt le a l t e r a t io n

a n d h a s a r h y o l i t ic c o m p o s i t i o n ( T a b l e 2) .

B e l o w t h i s d e p t h , q u a r t z , s m e c t i t e , i l l i t e , a n d

r a r e k a o l i n i t e a r e t h e d o m i n a n t p h a s e s ;

h o w e v e r , t h i s a s s e m b l a g e c o n t i n u e s o n l y t o 2 5 0

m w h e r e s m e c t i t e a n d m i n o r k a o l i n i t e a r e o n c e

a g a i n d o m i n a n t to t h e b o t t o m o f t h e h o le a t 6 79

m . T h e r e is a tr a c e o f c h l o r i t e a t a d e p t h o f 4 0 0

m . T h e a n d e s i t e s o f P L T G - 3 s h o w l es s a l te r a -

t i o n t h a n t h o s e i n P L T G - 1 .

P y r i t e i s l a c k i n g i n t h e t u f t s ( s u r f a c e t o a

d e p t h o f 2 71 m ) b u t i s p r e s e n t i n a n d e s i t i c la v a s

( d e p t h = 3 6 2 m t o 6 22 m ) , w h e r e i t o c c u r s as

d i s s e m i n a t e d c r y s ta l s a n d d i s c o n t i n u o u s

v e i n l e t s . A t a d e p t h o f 3 8 0 . 5 m , a q u a r t z v e i n

c o n t a i n e d a c i c u l a r c r y s t a l s o f b e r t h i e r i t e

( F e S b z S 4 ) . T h e w a l l r o c k a d j a c e n t t o t h i s v e i n

c o n t a i n s 49 0 p p b A u , 5 50 p p m A s , 17 p p m S b ,

8 00 p p m B a , 13 0 p p m Z n , a n d 2 p p m A g ( I N -

A A a n a l y s i s ) .

V e i n l e ts o f a n h y d r i t e a r e p r e s e n t i n t h e u p p e r

3 0 m o f P L T G - 3 , b u t t h e r e m a i n d e r o f th e t u f f

s e c t i o n (3 0 - 2 71 m ) l a c k s a n h y d r i t e v e i n s . T h e

a n d e s i t ic la v a s ( 36 4 - 6 2 2 m ) c o n t a i n n u m e r o u s

v e i n s, u p t o 5 m m w i d e , o f q u a r t z , c a lc i te , a n d

c l a y . C a l c i t e v e i n s , u p t o 2 c m w i d e , o c c u r a t

d e p t h s o f 5 65 a n d 6 4 5 m .

S u r f a c e h y d r o t h e r m a l a l te r a ti o n

A r e a s o f s u r f a c e a l t e r a ti o n a r e s i m i la r to t h e

a l t e r a t io n e n c o u n t e r e d i n t h e c o r e h o l e s ; m o s t l y

s i l ic i f i c a t io n a n d a r g i ll ic in c h a r a c t e r . T h e r e a r e

t w o m i n e s n e a r th e P l a t a n a r e s g e o t h e r m a l a re a ;

t h e S a n A n d r 6 s g o l d m i n e a n d t h e E l Q u e t za l

a n t i m o n y m i n e (R o b e r t s a n d I r v i n g , 1 9 57 ). T h e

S a n A n d r 6 s m i n e is p r o d u c i n g f r o m o r e h o s t e d

i n q u a r t z v e i n s t h a t a r e f o u n d i n la v a s a n d t u f t s

o f th e P a d r e M i g u el G r o u p a n d M a t a g a l p a F o r -

m a t i o n . A s a m p l e f r o m t h e o r e h o r i z o n c o n -

t a i n e d q u a r t z , c i n n a b a r , a n d p y r i t e ( X R D

an a l y s i s ) a s w e l l a s v i s i b l e g o l d f l ak e s .

T h e E 1 Q u e t z a l m i n e is l o c a t e d a b o u t 3 . 2 k m

N N W o f t h e v il la g e o f S a n A n d r 6 s . T h e o r e i s

h o s t e d i n c a lc i te a n d q u a r t z v e i n s , w h i c h a p p e a r

t o b e a s s o c i a t e d w i t h T e r t i a r y ( ? ) d i o r i t e s i l l s


16/18

56

G H E I K E N E T A L

and dikes that intrude the Paleozoic schists

Gof f et al., 1991-this volume). The hydro the r-

mal system responsible for these deposits ap-

pears to have been volca nogenic and related to

the Matagalpa andesitic composite cones; it

does not appear to be related in any way to the

present-day hydrothermal system, which is

related to extensional and strike-slip faults.

Di s cu s s i on

The Platanares geothermal area is located

near the intersection of major structural

elements of western Honduras, a group of

ENE-trending, sinistral strike-slip faults that

form the northern boundary of the Chortis

block and NW-trending normal faults formed

during the extension that has thinned this block

dur ing the past 10 m.y . Mills et al., 1967).

Cretaceous and Tertiary redbeds of unknown

thickness overlie metamorphic basement and

appear to serve as the major geothe rmal reser-

voir rock. They, in turn, are overlain by a se-

quence o f Oligocene to Miocene volcanic and

sedimen tary rocks, including andesitic lavas, a

thin fanglomerate, and a group of nonwelded

and welded ignimbrites. Normal faults offset

all of the Tertiary rocks and appear to offset

terrace gravels of probable Quaternary age.

Boiling springs are located along fractures of

the NW-trending fault system. Most hot

springs, surface patches of intense hydro ther-

mal alt eration, and siliceous sinter deposits oc-

cur within an area of abo ut 5 km 2. Beneath this

area, a geothermal reservoir is inferred to be

located in redbeds of the Valle de Angeles

Group, at a depth of 1.2 to 1.5 km, and at a

temp erat ure o f 225 oC Goff et al. , 1987a, 1991-

this issue). Two of the three coreholes en-

countered a 165C reservoir at shallower

depths.

None of the volcanic rocks within this area

appear to be Quaternary; two welded tuffs

from the lower part of the Padre Miguel Group

were dated at 14.2 and 14.7 Ma and the

andesitic lavas are older. There is no evidence

for a magmati c heat source for this geothermal

system. The volcanic rocks, including the

rhyolitic ignimbrites, are all over 14 m.y. old,

precluding a magmatic heat source interpreta-

tion based on thermal models by Smith and

Shaw, 1975). All thermal springs are located

along normal faults in a region of extensional

tectonics, suggesting that the system is similar

to those of the Basin-and-Range province of

the United States. The geothermal gradient

hole drilling at Pla tanares indicates that there is

a considerable geothermal resource there and

that it should be developed by production dril-

ling.

c know l e d g emen t s

The work within Hondur as has been funded

by the United States Agency for International

Development, through the Regional Office,

Central American Project and the Honduran

office and was done under the auspices of the

U.S. De part ment of Energy. We wish to tha nk

Lynn Sheldon and Carl Duisberg, USAID and

Rod Saubers, Inter Ameri can Geodetic Survey,

for their continued support. The paper was

greatly improved during reviews by T. Donnel-

ly State University of New York) and G.

Horn e Wesleyan University).

References

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Ritchie, A., 1987. San Ignacio La Tembladera)

geothermal site , Departamento de Francisco

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National Laboratory Rpt. LA-10696-MS, 27 pp.

Aldrich, M.J., Adams,A.I. and Escobar, C., 1991. Struc-

tural geology and stress history of the Platanares

geothermal site, Honduras: implications on the tec-

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Bargar, K.E., 1991. Fluid inclusions and preliminary


17/18


studies of hydrothermal alteration in core hole PLTG-

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