Hydrothermal alteration and fluid inclusion geothermometry of los humeros geothermal field, Mexico

0375-6505/89 $3.00 + 0.00 Pergamon Press plc © 1989 CNR.

Seothermics, Vol. 18, No. 5/6, pp. 677-690, 1989. Printed in Great Britain.

H Y D R O T H E R M A L ALTERATION AND FLUID INCLUSION GEOTHERMOMETRY OF LOS HUMEROS GEOTHERMAL FIELD, MEXICO R. M. P R O L - L E D E S M A *

a n d P. R. L. B R O W N E t

*lnstituto de Geofisica and DEPFI, UNAM., Cd. Universitaria, Coyoacan, 04510, Mexico, D.F. and tGeothermal Institute and Geology Department, Auckland University, Private Bag, Auckland, New Zealand (Received June 1988; acceptedfor publication December 1988) Abstract--The Los Humeros geothermal field, located in Puebla State, Mexico, occurs in a caldera; driUholes to 3000 m depth encountered a sequence of Quaternary lavas and pyroclastic rocks that range in composition from rhyolite to basalt but are dominantly andesitic. These rest upon the local basement comprising limestone and siltstone of Cretaceous age, which was encountered below 2500 m in the northern part of the field and 1000 m in its southern part. Examination of 29 cores, mostly from below 900 m depth, from 14 wells show that the hydrothermal minerals that occur in the volcanic host rocks include quartz, calcite, epidote, amphibole, sericite, smectite, illite, chlorite, biotite, pyrite and hematite. Their distribution mainly reflects the prevailing hydrological and thermal regime where temperatures locally exceed 300°C. The limestone basement rocks, however, have altered to an assemblage that includes calcite, quartz, wairakite, garnet, wollastonite, parawollastonite, sericite and fluorite. The homogenization temperatures of 356 fluid inclusions were measured and the freezing temperatures of 200 determined. All except two sets of inclusions homogenized into the liquid phase and neither daughter minerals nor a clathrate phase were seen. The homogenization temperatures mostly match measured bore temperatures that range from 250 to 360°C and the apparent salinities are from 0.2 to 2.7 weight per cent NaC1 equivalent, but some contribution to freezing point depression by CO2 is likely. A preliminary model for the hydrology of the field based upon the hydrothermal alteration mineralogy and fluid inclusion data suggests that dilute hot water ascends via faults in the Central Caldera collapse area of the field and moves laterally outward to elsewhere within the caldera.

INTRODUCTION Los H u m e r o s is the s e c o n d g e o t h e r m a l field within the M e x i c a n V o l c a n i c Belt to be d e v e l o p e d . It is located at the e a s t e r n b o r d e r of the belt (Fig. 1) within a caldera structure with a r a t h e r c o m p l i c a t e d volcanic history (Ferriz a n d M a h o o d , 1984). V o l c a n i c activity p r o b a b l y started a b o u t 0.5 m i l l i o n years ago a n d c o n t i n u e d until a b o u t 20,000 years ago, a n d f o r m e d three caldera structures: Los H u m e r o s , Los P o t r e r o s a n d E1 X a l a p a s c o . Los H u m e r o s C a l d e r a c o n t a i n s the Los P o t r e r o s a n d El X a l a p a s c o C a l d e r a s a n d has a d i a m e t e r of over 12 km. T h e chemical t r e n d of the e r u p t e d m a g m a has b e e n from silicic to a m o r e mafic type t o w a r d s the last stages of activity. T h e gross lithological s t r u c t u r e of the field (Fig. 2) comprises a s e q u e n c e of pyroclastic a n d lava flows d e p o s i t e d o n l i m e s t o n e a n d siltstone layers that are c o n s i d e r e d to be the local b a s e m e n t ( G u t i e r r e z - N e g r i n , 1982a,c). T h e volcanic rocks have c o m p o s i t i o n s that range from rhyolite to basalt; h o w e v e r , andesitic rocks p r e d o m i n a t e in the area. T h e field lacks h y d r o t h e r m a l surface m a n i f e s t a t i o n s , except for a few k a o l i n i z e d areas a l o n g the m a i n faults. This a b s e n c e of surface a l t e r a t i o n is also e v i d e n t in the core a n d c u t t i n g s a m p l e s r e c o v e r e d f r o m the u p p e r 300 m w h e r e most p r i m a r y m i n e r a l s r e m a i n u n a l t e r e d . A p r e l i m i n a r y p e t r o g r a p h i c analysis of cores a n d cuttings from some e x p l o r a t i o n wells has b e e n m a d e by 677

R. M. Prol-Ledesma and P. R. L. Browne

678

114 °

108 °

'q -.

i

g6 °

10Z °

90 °

W

i

30 ° --

NORTH AMERICAN PLATE

25 ° --

GULF

OF

MEXtCO

PACIFIC PLATE

/

20 ° --

HumeroI

City

2 0 0 km. I

O~ n LU

I~ ° --

COCOS PLATE

N

I

Col4lr~

/

\1

I

Fig. I. Location of the Los Humeros geothermal field.

m.a.s.l.N-NW

3000

PH-22

PH-9

PH-8 PH-7

PH-1

PH-18 S - S E ~

PH-6 PH-12r

2000

1000 : '. ', : '. '. ~" '. ', '.'.',: :..~": ~, lllllll:J ::',':l\~ "

N': •

0

........

0.5

1 km

Units I

Andesite and basalt

TT

Ignimbrite

n'[

Andesite

-1v

Limestone

and s k a r n

Fig. 2. Lithology of the Los Humeros geothermal field, according to petrographic data from exploration wells (Viggiano and Robles, 1988a).

Hydrothermal Alteration and Fluid Inclusion

679

personnel from the Comision Federal de Electricidad (Gutierrez-Negrin, 1982a,b,c; Viggiano and Fierro, 1986; Viggiano and Robles, 1988a,b). Their results indicate the presence of eight different alteration zones, though these zones are not observed in samples from all wells. The most common hydrothermal minerals are calcite, quartz, epidote, amphibole, K-micas, pyrite, hematite, magnetite, sericite and chlorite. A cooling effect produced by the inflow of colder waters through the Los Humeros fault was inferred after the temperature logs from well PH-4 showed a decrease of the temperature below 1000 m depth. Aside from the hydrothermal minerals studies, some fluid inclusion analyses were also carried out (Gonzalez-Partida, 1985). These analyses included samples from 4 exploration wells: PH-1, PH-4, PH-5 and PH-7. Only liquid-rich inclusions were observed in the samples, and all gave homogenization temperatures below the appropriate boiling temperature except for some samples from well PH-4. In this paper, we report results of a detailed petrographic analysis and a fluid inclusion study using 29 cores from 14 wells (Fig. 3). Our aims were to determine the alteration patterns, to establish the relation between the rocks forming the reservoir and the observed hydrothermal minerals, and to identify the variations in the thermal regime of the field. H Y D R O T H E R M A L ALTERATION MINERALOGY The analyzed core samples are mostly from depths below 900 m, so the data characterize the deeper reservoir rather than the shallower layers. The observed hydrothermal minerals are listed in Tables 1-3; their identification was carried out using petrographic microscope, X-ray diffraction and scanning electron microscope techniques.

LOS Hurneros geothermaL field CentroL coLLapse

",-

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Fig. 3. Exploration wells drilled in Los H u m e r o s and main structures within the caldera. Filled circles indicate the wells sampled for this study. (After an unpublished m a p by Comision Federal de Electricidad).

680

R. M . P r o l - L e d e s m a

a n d P. R. L . B r o w n e

Table 1. Alteration minerals most irequently observed in the wells located in the central collapse area Well Depth (m)

PH-II) 1469

1825a

PH-17 1825b

PH-19

2227

981

PH-20

PH-21

1769

610

968

1403

1700

2059

900

X X

X

X

X

X X X

X X

X

X X X

X X X

X

X X

X X

X X X X

X X X

X X X X

X

X

215(I

Mineral

Qtz Ab K-Feld Ct Chl Sph Py Mg Hem M I-M

X X

X X X

X

X X

X X

X

X X

X X

X X

X

X

X

X

X

X

X

X

X

I

Zeol Ep Gar, Woll Prx Amph Micas

X

X X

X

X

X

X

X X

X

X X

X

X

X

X X X

X

X

'~

X

X

X

X

X

X

X

Abbreviations correspond to: Qtz---quartz, Ab~-albite, K-feld--potassium feldspar, Ct---calcite, Chl---chlorite, Sph--sphene, Py--pyrite, Mg--magnetite, Hem--hematite, M--montmorillonite, l-M--interlayered, illite-montmorillonite, l--illite, Zeol--zeolite, Ep----epidote, G a r ~ a r n e t , Woll--wollastonite, Prx--pyroxene, A m p h - amphibole

Table 2. Alteration minerals in the wells corresponding to Los Potreros collapse Well

PH-2

Depth (m)

616

PH-5 325

600

X

X X

X X X

X X X X

PH-9

PH-18

2075

769

PH-22 1010

1180

Mineral

Qtz Ab K-Feld Ct Chl Sph Py Mg Hem M I-M 1

X X X X X X

X X X

X X

X X X X X X X X X

X X X X X X X X X

X X X X X X X

X

X

X

Zeol Ep Gar Woll Prx Amph Micas Fir

X X

X

X

Abbreviations as in Table 1. Fir--fluorite.

X

681

Hydrothermal Alteration and Fluid Inclusion Table 3. Alteration minerals in the wells located in the vicinity of Los Humeros fault Well Depth (m)

PH-1

300

Mineral Qtz

Ab K-Feld Ct Chl Sph Py Mg Hem M I-M I Zeol Ep Gar Woll Prx Amph Micas

X X X X

PH-4

PH-8

700

1000

907

1410

1724

X X

X X

X X

X

X

X X X X X X

X X X X X X

X X X

X X

X X

X X X X X X

X

X

X

X

X

X X

2091

X X X

X

X

X X

X

Abbreviations as in Table 1.

The petrographic analyses of the cores and other data available yielded a characteristic pattern for the distribution of the hydrothermal alteration within the field. Four main zones can be distinguished: the central collapse area (wells PH- 10, PH- 17, PH- 19, PH-20 and PH-21), Los Potreros collapse area (wells PH-2, PH-5, PH-9, PH-18 and PH-22); Los Humeros fault area (wells PH-1, PH-4, PH-8); and the caldera rim area (well PH-14). The first three zones correspond to Tables 1-3. In the central collapse area (Table 1), the typical alteration products at depth are characteristic of very high temperatures (with stability ranges from 250°C to above 300°C): epidote, garnet, micas, pyroxene, and amphibole; biotite being most abundant in several samples (PH10, PH-17, PH-19, PH-20 and PH-21). Also common in the samples from these wells is K-mica; in samples from wells PH-10 and PH-20 this is well crystallized (sericite) but in the samples from PH-17, PH-19 and PH-21 it is poorly crystalline (illite). K-mica occurrence indicates a lowering in the fluid pH took place at some stage during the history of the field. Frequently the recrystallization of secondary micas at different stages depends on the intensity of the alteration. This is most evident in the core sample from 1825 m depth from well PH-10. This core was cut in an ignimbrite with a sharp variation in welding, with the bottom part of the core having lower intensity alteration. However, the micas in both parts of this core exemplify the degree of recrystallization that is related to the different intensities of hydrothermal alteration at similar temperatures. Mica abundance is, of course, related to rock permeability. In the 981 m deep core from well PH-19, the presence of mica together with an unidentified Na-bearing platey zeolite, K-feldspar and hydrothermal quartz (Fig. 4), implies that in this part of the field there are two stages of hydrothermal activity at different temperatures as zeolites (except wairakite) and K-mica have different stability ranges. However, it is not clear from the data if this part of the field is heating or cooling. The coexistence of high- and low-temperature alteration minerals was also observed in well PH-21 at a depth of 900 m; here garnet crystals occur in veins (Fig. 5)

682

R. M. Prol-Ledesma and P. R. L. Browne

Fig. 4. Electronmicrograph taken with S.I2.M. of sample from 981 m depth (well PH-19), where the coexistence of biotite and Na-zeolitc is observcd.

Fig. 5. Electronmicrograph of a sample from 900 m depth (well PH-21 ), which shows garnet crystals deposited in veins.

together with an unknown zeolite. Well PH-17 is also located within the central collapse area, and from the alteration minerals observed, it can be inferred that at least two high temperature alteration stages have been experienced by the rocks found at 2227 m depth. This sample is assumed to have been initially composed of calcite that is now completely altered to garnet, wollastonite, parawollastonite, diopside and micas. Figure 6(a,b) shows the relationship between the different minerals; primary calcite must have altered first to garnet, which conforms to the rock matrix, but is now being altered to wollastonite and diopside. In this sample all alteration minerals have stability temperatures above 300°C.

Hydrotherrnal Alteration and Fluid Inclusion

683

(a)

(b)

Fig. 6. Electronmicrographsof a samplefrom2227m depth (wellPH-17). Theyshowwollastonite(a) and diopside(b) growingat the expenseof grossularcrystals. The Los Potreros collapse area comprises the wells located outside the central collapse and inside of Los Potreros Caldera rim. They all show a rather stable thermal regime, and the observed minerals correspond to the present temperatures (Table 2). The most common hydrothermal minerals at depth are: quartz, calcite, sphene, epidote, magnetite, hematite, pyrite and chlorites; K-micas were present only in samples from well PH-9 and no garnet, wollastonite, pyroxene or amphibole were observed in the wells of this area. This indicates that equilibrium temperatures must be lower here than in the central collapse area. A cooling effect produced by inflow of cold waters through the Los Humeros fault was inferred as the temperature logs from well PH-4 show a decrease of the temperature with depth below 1000 m, at the assumed intersection of the fault by this well (Gutierrez-Negrin, 1982c).

684

R. M. Prol-Ledesma and P. R. L. Browne

For this reason all the wells located in the vicinity of the Los Humeros fault were classified together in Table 3. However, only for well PH-1 can the decrease of temperature with depth below 1000 m be inferred on the basis of hydrothermal minerals (Gutierrez-Negrin, 1982b). hi the results obtained for the core samples from PH-1, PH-4 and PH-8, no retrograde alteration pattern was observed. Furthermore, in the deepest sample from PH-8 (209l m) garnet and Kmicas were obseived, where the temperature is locally about 300°C; these high temperature alteration minerals indicate that the cooling effect is confined to the vicinity of the fault. The boundaries of the field are contained entirely within the caldera rim. Well PH-14 is located on the inner border of the rim, and yet it shows quite low measured temperature (123°C). The main alteration products observed here are calcite and quartz veins in the limestone and montmorillonite and calcite in the shallower samples. FLUID INCLUSION S T U D I E S For this study, the homogenization temperatures of 356 fluid inclusions were measured and the freezing temperatures of 200 of them were determined. They occur in quartz and calcite crystals obtained in samples from wells PH-1, PH-2, PH-4, PH-5, PH-8, PH-9, PH-14, PH-19, 0

L ithology

50

CHF 4

I

Welt no. PH-19 (temp. C) Ioo i50 200 250 300 Fluid inclusion I I I I I

350

400

I

200 400--

3 600 -8

4

800 -I000 12OO

~

-

-

9

8

961m Da n=9 T:234

1

m n= 16 a aT=240 981rn n:5 T:245

-

1400a 1600F

n=6 T-- :~3 1769 m n=32 T=333

1800 F

7I

,a

Ik Casingshoe --2 Cole pt no. V BLind driLLing

2400 1 E

Maior/minor permeeble zone []-- DownhoLetemp. 2600 i-Fluid inclusion 2800

t

+ Homogen.temp. H Contact

3000 TD: 2290 m (VD) Fig. 7. Homogenization temperatures of fluid inclusions in calcite and quartz crystals from well PH-19. Continuous line--boiling point for pure water. VD--vertical depth, CHF---casing head floor. Lithology: 1--andesite, 2--tuff, 3 - ignimbrite, 4--basalt, 5--limestone, 6---basaltic pyroclast, 7--andesite and basalt, 8---rhyolite.

Hydrothermal Alteration and Fluid Inclusion

685

PH-20, PH-21 and PH-22. All samples contain liquid-rich fluid inclusions except some from wells PH-14 and PH-19, and no daughter minerals or other phases (clathrate) were observed. Two samples from PH-19 (1769 m depth) contained inclusions that homogenized into the vapor phase. Temperatures of homogenization of the fluid inclusions for PH-19, 1769 m depth sample, were bimodal in their distribution (Fig. 7) with a difference in mean temperature of 30°C (363°C and 333°C). For this sample it is convenient to take the lowest homogenization temperature as the most representative, because the presence of vapor-rich inclusions indicates that boiling took place during their trapping, yielding homogenization temperatures much higher than those of trapping. Within the Los Potreros Caldera, the fluid inclusion data agree with the interpretation based upon the observed alteration mineralogy and the homogenization temperatures do not show evidence of multiple thermal events. As stated above, the wells located in the vicinity of the Los Humeros fault present a peculiar effect of inversion in the temperature gradient below a certain depth, related to the fault intersection. In well PH-8, the temperature logs do not show a decrease with depth, possibly due to a downflow within the well. However, fluid inclusion data clearly record a decrease of the mean homogenization temperature with depth (from 288°C at 1410 m to 254°C at 1724 m, Fig. 8). The freezing temperature of the fluid inclusions also indicated a dilution process taking place at depth. The results yielded a decrease of the apparent salinity of the fluid from 0.9% eq. wt. NaCI for 1410 m to about 0.3% eq. wt. NaCI for 1724 m depth. In well PH-1 a cooling effect of 30°C was observed for fluid inclusions in samples from 1000 m depth, from 273°C to 243°C, which is the approximate present temperature here. Also note that no relict higher-temperature alteration products were found in any samples from the three wells (Table 3). These data are therefore consistent with the hypothesis of cold water inflow into the system along the Los Humeros fault. The results obtained for the freezing temperatures indicate that the apparent salinity is rather low, the highest being 2.7% eq. wt. NaCI for a group of inclusions from PH-19 (1769 m). In samples from this well vapor-rich inclusions were observed and the lowest apparent salinity in the field (0.2% eq. wt. NaCI) was measured for liquid-rich inclusions present in the same sample. Samples with both highest and lowest apparent salinities coincide with homogenization temperatures of more than 300°C. Figure 9b shows that for the 1769 m sample, high apparent salinity is characteristic of the inclusions with the highest homogenization temperature, and fluid trapped in the low apparent salinity-low homogenization temperature inclusions must have undergone boiling and significant CO2 loss. Therefore the depression of the freezing temperature of the high temperature inclusions must be due mainly to the presence of carbon dioxide rather than dissolved salts (Hedenquist and Henley, 1985). A shallower sample (981 m) from the same well has fluid inclusions with lower homogenization temperature and records an increase in apparent salinity (Fig. 9a); this can be related to an increase in dissolved salt concentration due to vapor losses that did not greatly decrease the CO2 content as earlier boiling had already done this. The lowest value for the apparent salinity was observed also in samples from well PH-9; the homogenization temperature of these fluid inclusions is higher than 300°C and they all homogenized into the liquid phase. The second to highest apparent salinity values (2.0% eq. wt. NaCi) were measured for fluid inclusions with homogenization temperatures lower than 150°C in samples from wells PH-14 (1385 m) and PH-20 (900 m). In both cores the trapped fluid may have experienced several boiling processes that cooled the liquid and increased its salinity. In the other samples homogenization temperatures are always higher than 180°C and apparent salinity varies from 0.4% to 1.1% eq. wt. NaCI.

686

R. M. Prol-Ledesma and P. R. L. Browne 0

LithoLogy

50

CHF

-

I,

I

WeLL no. P H - 8 (temp. C ) i00 Lso 200 250 3oo FLuid inclusion I

I

I

I

350

I

4oo

I

2OO

2 i

400-

"\\N~N

600 -3

800 -

2 ~

iooo-

3

1200-

~7 5 1400

[,

e

121 1600

1724m n=17

2

1800 I ~

T = 248

- -

2000 -3 2 2 0 0 --

K -2 V

Cosing shoe

E

Mojor/minor

Core pt.. no BLind driLLing permeable

2 4 0 0 --

--13

2600 --

1724m ' * T= n=12 2~B2

I

zone

Downhote temp. FLuid i n c l u s i o n

q-- Homogen. temp. H Contact

2800 3OOO TD: 2 3 8 8 m (VD)

Fig. 8.

Homogenization temperatures of fluid inclusions in calcite crystals from well PH-8. Continuous line, V D , C H F and lithology as in Fig. 7.

The fluid inclusion data for several samples from the 1385 m core from well PH-14 record two groups of inclusions with mean homogenization temperatures of 100°C and 135°C (Fig. 10). These fluid inclusions occur in a quartz vein present in the limestone, which in this area is shallower than elsewhere (about 1300 m deep). Note that most inclusions from PH-14 (1385 m) contain two different fluids: water and a gas, though it was not possible to identify the gaseous phase trapped; this condenses at approximately - 9 0 ° C and freezes between - 1 6 0 ° C and - 170°C. These temperatures do not correspond to the condensation and freezing point of a pure substance known to us, therefore, either it is a mixture of at least two different compounds or the pressure effect is large. However, according to the temperatures recorded for its condensing and freezing, the gas trapped is likely to be a hydrocarbon mixture perhaps derived from some organic material present in the limestone. DISCUSSION AND CONCLUSIONS Correlation of the occurrence of the hydrothermal minerals and the fluid inclusion data indicates a zonation of the temperature regime, the highest temperatures being attained within the area of the central collapse. The different stages of hydrothermal activity in this area

Hydrothermal Alteration and Fluid Inclusion (a)

687

P H - 1 9 (981 m)

04 02

i.-1

0 -02 -0.4

n

n O

q)

03 O

-0.6 -0.8

O

O DO 0

0 O

0

O

-i0 ! -12 -14 -16

--

-18

-20 190

I

l

1

I

210

230

250

270

T h (*C)

(b) o,

°I -01,

0 nm

P H - 1 9 (1769 m) ,o D

D

0

0

-0.2

-03

--

-04

--

-0.5

--

-06

--

-07

--

~

-08

--

~

-09

--

E F--

-iO

--

-II

--

-12

--

-14

DO

-I.5

--

-16

--

-17

--

-L8

--

-19 2~

rn

0 0 []

I 3OO

1 32O Th (*C)

I 34O

[]

O

0

nl 36O

Fig. 9. Freezingtemperatures (Tm) vs homogenizationtemperaturesfor fluidinclusionsfrom PH-19 samples: (a) 981 m depth; (b) 1769 m depth. correspond to temperatures higher than 300°C. The wells located outside the central collapse area, which do not intersect the Los Humeros fault, encountered a more stable hydrothermal regime and the temperatures through their drilled depths are as high as those present in the central area. However, close to the central collapse area, a cooling effect is associated with the Los Humeros fault, though it seems to be restricted only to its immediate surroundings. Wells PH-4 and PH-8 are assumed to intersect this fault but they are located in the central collapse area, and the samples from them follow the high temperature alteration patterns of the other wells (Table 1). The cooling effect is observed only in rock units intersected by the fault. Cooling does not affect the upper or lower units and has not yet caused the appearance of retrograde alteration, possibly because cooling has resulted in temperatures within the stability range of the previous alteration products. Another possibility is that the cooling is not recent at all, and that the equilibrium at this lower temperature has been reached, transforming the previously formed hydrothermal minerals into those reflecting the new conditions.

688

R. M. Prol-l,edesma and P. R. L. Browne o LithoLogy [--

CHF - -

6 2oo

740O 4

6OO

~

IO00

WeLL P H - 1 4 (temp. C) eoo 150 200 25o FLuid inclusion I I I I

50

300

-"k I

350

I

400

I

\

\\ . 1385m

E 1400

13ssm

\

o *1111__. n = 5

*

==u T=I2 ~

5

a. 1600 O

\

1800

2200 ' 24OO 26OO

-

IL --2

Casing shoe Core p t . no

V

BLind driLLing

1£ L

Major/minor permeable zone

---

DownhoLe temp.

n

FLuid inclusion

Jr-

Homogen. temp.

H

Contact

\ \

/

280O 3000 TD: 1:388 m (VD)

Fig. 10. Homogenization temperatures of fluid inclusions in quartz and calcite crystals from well PH-14. Continuous line, V D , C H F and lithology as in Fig. 7.

Of special interest is the hydrothermal alteration observed in the limestone encountered at the bottom of some exploration wells. In most limestone cores there is a total absence of clay minerals and the alteration found is represented by calcite, quartz, wairakite, garnet, wollastonite, parawollastonite, mica and fluorite. Note that the wairakite, wollastonite, parawollastonite and fluorite have not been seen in cores from volcanic reservoir rocks. The presence of wollastonite, however, is not very common in geothermal fields and it has only been reported, so far as we know, in four: Larderello, Italy (Cavarretta et al., 1982); Krafla, Iceland (Kristmannsdottir, 1981); Tongonan, Philippines (Ferrer, 1983); and Latera, Italy (Cavarretta et al., 1985). One of the core samples (well PH-17, 2227 m depth) was totally composed of garnet, wollastonite, diopside and accessory apatite; this is a typical assemblage of minerals in skarns. However, scanning electron microscope examination shows that this rock had at least two stages of alteration. The first possibly transformed original calcite into garnet (grossular and grandite) and in the second the garnet matrix altered to wollastonite, parawollastonite and small diopside crystals (Fig. 6a,b). The stability range of these three minerals indicates that both hydrothermal episodes took place at about the same temperature (approximately 300°C or higher), with fluidrock interactions having different physical-chemical parameters (Taylor and Liou, 1978; Bird et

H y d r o t h e r m a l Alteration and Fluid Inclusion

689

al., 1984). T h e s e high t e m p e r a t u r e h y d r o t h e r m a l episodes m a y be identified also in cores f r o m o t h e r wells located in the central part of the caldera; i.e. PH-10, PH-19, P H - 2 0 and PH-21. Fluid inclusion data indicate that the highest h o m o g e n i z a t i o n t e m p e r a t u r e s coincide with the highest a p p a r e n t salinities (2.7% eq. wt. NaCI) in well PH-19, and also the lowest a p p a r e n t salinity (0.2% eq. wt. NaCI) in wells P H - 9 and PH-19. This fact can be interpreted as evidence that the freezing point depression for the t r a p p e d fluid is mainly due to high concentrations of CO2, as o b s e r v e d in s o m e o t h e r fields ( H e d e n q u i s t and H e n l e y , 1985). Since a large p r o p o r t i o n of CO2 is lost during the first boiling process, the freezing t e m p e r a t u r e of the fluid inclusions will increase (i.e. be less negative), and their a p p a r e n t salinity will be lower in spite of the higher salt c o n c e n t r a t i o n due to v a p o r losses (Fig. 9a,b). T h e results show that the area comprising the central collapse and its surrounds seems to be the most thermally unstable part of the field, showing the highest t e m p e r a t u r e (about 340°C). T h e r e f o r e , it seems reasonable that this area is that of the main outflow f r o m the reservoir. Nevertheless, the t e m p e r a t u r e s o b s e r v e d in the wells outside the central collapse area are also high e n o u g h for p o w e r generation. T h o u g h the isotherms seem to d e e p e n outside the central collapse, the fluid inclusions data for well P H - 1 4 (Fig. 10) are evidence for a recent heating in the s o u t h e r n part of the field, t h o u g h this hypothesis still needs support, p e r h a p s by m o r e data f r o m exploration wells PH-2, PH-12 and PH-18. In conclusion, our results provide a preliminary m o d e l for the characterization of the h y d r o t h e r m a l activity in Los H u m e r o s . T h e s e results have yet to be correlated with geochemical data to establish the p a r a m e t e r s of the interactions of the g e o t h e r m a l fluid and the reservoir rocks, and to c o m p a r e with the fluid inclusion interpretation m a d e a b o u t the chemical characteristics of the fluid. Acknowledgements--Thanks are due to the manager of the Gerencia de Proyectos Geotermoelectricos C.F.E. Ing. H. Alonso, for his cooperation through a joint research project with the Division de Estudios de Posgrado Facultad de Ingenieria, and also to the Gerencia staff for making available to us the core samples and several unpublished reports and data. We especially thank Ing. A. Gonzalez, Ing. A. Razo, Ing. L. Gutierrez-Negrin, Ing. A. Templos, Ing. O. Rivera, Ing. F. Segura and Ing. D. Velazquez. This study was carried out while R. M. Prol-Ledesma was a visiting lecturer at the Geothermal Institute and the Geology Department of the University of Auckland, New Zealand; we are also indebted to the personnel of these institutions for their help during the laboratory work. The assistance of Ms S. F. Courtney was most valuable during the analyses in the S.E.M. Figures 1-3 were drawn by Ms S. Campos.

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