Thermobarometry of hydrothermal alteration in the Los Azufres geothermal system (Michoacan, Mexico): Significance of fluid-inclusion data

Chemical Geology, 76 (1989) 229-238 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

229

Thermobarometry of hydrothermal alteration in the Los Azufres geothermal system (Michoacan, Mexico)" Significance of fluid-inclusion data M. CATHELINEAU 1, G. IZQUIERDO 2 and D. NIEVA 2 1CREGU et GS-CNRS-CREGU, F-54501 Vandoeuvre-l~s-Nancy Cddex (France) 2Instituto de Investigaciones Electricas, Cuernavaca, Morelos (Mexico) (Accepted for publication April 13, 1989)

Abstract Cathelineau, M., Izquierdo, G. and Nieva, D., 1989. Thermobarometry of hydrothermal alteration in the Los Azufres geothermal system (Michoacan, Mexico): Significance of fluid-inclusion data. In: K. GrSnvold (Guest-Editor), Water-Rock Interaction. Chem. Geol., 76: 229-238. Fluid inclusions were analysed systematically in samples from different wells and depths of the Los Azufres (Mexico) active hydrothermal system. Fluids were studied either in authigenic minerals (primary inclusions) in quartz, calcite or anhydrite, or in magmatic minerals (secondary inclusions in microcracks). The fluids are aqueous and have low salinity, ~ 1 eq wt.% NaCl. They were trapped in P - T conditions very close to that fixed by the liquid-vapor equilibrium curve. Fluid pressure is lower than the calculated hydrostatic one assuming a liquid-dominated system, because of the existence of a two-phase vapor-dominated zone at a depth between 500 and 1500 m. Temperature was estimated also by a good number of techniques. Present-day distribution of temperature is given by direct measurements in the wells by Kuster ® equipment. Downhole temperature may be estimated using different dissolved species geothermometers and the chemical composition of the produced fluids. The present-day measurements above the downhole give in general lower estimates than those obtained by other means. However, agreement between downhole temperature, chemical geothermometer estimates and fluid-inclusion data is relatively good.

1. Introduction

Quantitative estimation of P - V - T - X of fluids in an active geothermal field throughout the course of the hydrothermal activity is of considerable interest for the understanding and appropriate exploitation of geothermal resources. Present-day features of the fluids and physical properties of the reservoir are relatively well documented from data obtained by direct measurements in the wells, but several difficulties are inherent to these methods. For example, it takes a long time to reach thermal 0009-2541/89/$03.50

equilibrium in a drilled well. The composition of produced fluids may also give important information, especially an estimation of temperature using available chemical geothermometers (Fournier and Truesdell, 1973; Fournier and Potter, 1982). But these fluids may come from heterogeneous sources or from a relatively large section of the reservoir, thus giving an average estimation of compositions, not a local estimation. Different P - T - X estimates may also be deduced from preserved mineral assemblages that once equilibrated with the surrounding fluid. However, it may be difficult to estimate whether these assemblages have re-

© 1989 Elsevier Science Publishers B.V.

M. CATHEIANEAUET AL.

230

equilibrated to the present-day conditions or not. Fluid inclusions give local information on well-defined zones within the reservoir. Furthermore, they are the only direct information on paleofluids which have circulated through the rocks. Thus, they can give very useful data on the P - T evolution with time of a given part of the field. As stressed by Roedder (1984), relatively few studies have been performed on terrestrial geothermal systems. The geothermal data available concern fields from Japan (Taguchi et al., 1979; Taguchi and Hayashi, 1982), New Zealand (Browne et al., 1976; Hedenquist and Henley, 1985) and California, U.S.A. (Freckman and Olson, 1978; Sternfeld, 1981). The purpose of the present work was: (1) to test the range of temperature estimates and significance of fluid-inclusion data in definite zones of a geothermal field; and (2) to compare these data with data obtained by means of other geothermometric approaches.

2. Geological framework The Los Azufres geothermal field (Fig. 1) is located in the Sierra of San Andres (Michoacan, Mexico). It is related to a resurgent zone (Pradal and Robin, 1985) within a major volcanic caldera of the main recent volcanic belt of Mexico. The geological sequence in the geothermal area consists of an andesitic basement overlaid with acidic series (Gutierrez and Aumento, 1982; Cathelineau et al., 1987). Previous studies of hydrothermal alteration zones (Cathelineau and Nieva, 1985; Cathelineau et al., 1985) have emphasized the existence of a progressive metamorphism of volcanic rocks with increasing depth and temperature, from the zeolite to the amphibolite facies. Very little evidence has been found of marked discrepancies between mineral associations which could suggest retrograde metamorphism. Thus, the mineralogy of hydrothermal alteration in the Los Azufres field corresponds to that of a rela-

Fig. 1. Geological map of the Los Azufres geothermal field with the location of the wells mentioned in this study (slightly modified from Cathelineau et al., 1987 ). B = drilled well; C=surface emission; D = m a j o r fault; E = t u f f s ; F = v i t r e o u s rhyolite; G=flow rhyolite; H=andesite; I = dacite.

tively young system mostly subjected to prograde metamorphism.

3. Analytical techniques Fluid inclusions (F.I.) were analysed in doubly polished 200-~m-thick sections from drillcores. Microthermometric measurements (Weisbrod et al., 1976; Roedder, 1984) were performed using a Chaix Meca ® heating and cooling stage (Poty et al., 1976). The stage was calibrated with organic melting-point standards at T>~ 25°C, and natural and synthetic fluid inclusions at T~<0°C. The accuracy of measurements is considered to be +0.2°C at the freezing stage and + 1°C at the heating stage. As the fluid composition at Los Azufres is aqueous, only two parameters were determined for each fluid inclusion: (1) the melting temperature of ice (Tmi) which can be converted easily to eq wt.% NaC1 salinity (Potter

HYDROTHERMALALTERATIONIN THE LOSAZUFRESGEOTHERMALSYSTEM

et al., 1978); and (2) the bulk homogenization temperature (T h), which gives the density of the fluid and its minimum trapping temperature. As most of the fluid inclusions are of a very small size of ~ 5 - 1 0 ttm, Tmi was very frequently difficult to observe. Thus, the number of Th measurements is far more abundant.

4. Analytical data About 800 inclusions were analysed in several thick sections from cores of wells AI, A3, A4, A9, A12, A21 and A23. Table I presents the main microthermometric data. The degree of filling of the inclusion, for example, the volumetric fraction of the aqueous liquid in the inclusion (flw) was estimated by reference to the volumetric chart of Roedder (1972). Three types of inclusions were identified, based on microscopic and microthermometric data: type L1 (liquid-rich inclusion, and L'I,

231

with in addition, a trapped solid inclusion ) and, two types in minor abundance, L2 (no gas bubble) and V (vapor-rich inclusion). L1 inclusions (0.7 ~
TABLEI Microthermometric data (Llotype inclusions) Well Depth Corrected Host rock Host mineral No. (m) depth (m) 23

424

517

rhyolite

21 23

3 9

384 600 600 700 1,000 1,000 1,000 1,237 1,705

585 682 750 857 1,093 1,140 1,150 1,387 "1,764

andesite rhyolite andesite andesite rhyolite andesite andesite andesite andesite

magmatic quartz quartz vein quartz magmatic quartz calcite calcite magmatic quartz anhydrite calcite calcite quartz

3

1 , 8 7 4 2,024

andesite

quartz

1

1 , 8 8 5 2,029

andesite

calcite

3 12 23

4 19

Salinity Th (eqwt.% NaC1) (°C)

0.9 0.8 2.0 0.8 1.3

(0.2-1.7) (0.2-3.2) (1.4-2.4) (0.2-0.9) (0.9-2.8)

2.5 (0.5-3.2) 0.8 (0.2-1.4) 0.5 (0.2-0.9) 5.0 (4 -5.5) 0.7 (0.1-0.70) (0.2) 3.0 (2.5-4.0) 1.3 (0.4-2.7) (1.0) (0.4-2.2) 4.6 (3.3-8.5)

90-195 (_+ 5) 210-215 (_+5) 215 (_+5) 245250 ( - 5,+ 15) 250-255 (+-10) 235 (-+ 10) 250-255 (-+ 10) 255-260 (-+5) 260-275 {_+5) 280-290 (_+ 15) 305-310 (_+5) (275-280)*(_+15) 330 (_+ 10) 310 (_+10) (265)* (+_5) 32O ( _+10) (275)* (-+5)

d Number of (gcm -3) measured fluid inclusions 0.88 0.86 0.86 0.81 0.80

0.79 0.80 0.78 0.79 0.71 (0.76) 0.69 0.70 (0.79) 0.73 (0.81)

25 23 54 42 73 20 58 22 35 38 135 224 32

Depth is related to the well head and to a reference level at 3000 m above sea level. Range of salinity and the uncertainty on Th are given in brackets. Data from 2029°m depth are from Gonzales Partida (1985). *Less abundant fluids in a sample (cooler stage trapping).

232

studied only in authigenic minerals (calcite, anhydrite, quartz). In this case, primary inclusions trapped during mineral crystallization give the early hydrothermal conditions, whilst the secondary inclusions give information on the late evolution of geothermal fluids. In rhyolites, fluid inclusions were observed either in authigenic quartz (veinlets with primary inclusions), or in quartz of magmatic origin (trails of secondary fluid inclusions along the microcracks). Type-L'l inclusions were observed, for instance, in samples from well A9 and they give the same microthermometric data as type-L1 inclusions. Solids are in variable amounts and size and do not dissolve during microthermometric experiments. They are considered to have been trapped mechanically and were identiffed by comparison with solid inclusions trapped in the host mineral. Epidote and illite are most commonly trapped in such a way. A great number of inclusions were analysed in different samples from two wells (A3 and A23) in order to get the range of variation of microthermometric parameters. Wells A3 and A23 are the only ones where cores are available at three different depths. Fig. 2 shows histograms of Th and Tmi. Most of the fluid inclusions have high Tmi ( - 1 . 2 to - 0 . 2 ° C ) , which corresponds to very low salinities, 0.3-2.0 eq wt.% NaC1. The homogenization temperatures increase from the shallowest levels (600 m, Th_~250+ 10°C) to the deepest ones (1874 m, 300 °C ~
M. CATHELINEAUET AL. ,o F N 600m

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oo 200 N 1010 N

-1

o

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-3

o

-2

oof

-~

o

200

N-,40 N

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3oo

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2so

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~so

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; 7°f llo I 11o

- 30

i

so

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,

-1

io,O

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;:;iTh ['C) 3so

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Fig. 2. Melting temperature of ice (Tm~) and homogenization temperature (T~,, in the liquid phase) of fluid inclusions from A3 well samples (depths refer to the well-head altitude). Data from two different samples from the same core (1874 m) are superimposed (dotted area) in the same histogram.

ferent stages of the field activity. Discussion about such complex trapping is given, with the interpretation of the Th-Tmi plot (Fig. 4). Very similar results (Table I) were obtained in samples from well A23, which penetrates rhyolites from the surface to a depth of 1100 m. Inclusions were studied not only in hydrothermal quartz veins, but also in healed microcracks of magmatic quartz (secondary inclusions). Secondary inclusions studied in two samples from depths of 600 and 1000 m, respectively, give single-mode Th histograms, with modes at 245-250°C ( + 1 0 ° C ) and 255°C ( + 10 ° C ), respectively. Small Th range of these secondary inclusions can be considered in such cases as representative of a single trapping at the hydrothermal stage, which gives similar information as secondary inclusions trapped in authigenic minerals. A more complex history of

HYDROTHERMAL ALTERATION IN THE LOS AZUFRES GEOTHERMAL SYSTEM

trapping at the scale of a sample is clearly demonstrated by sample A23-424. Two stages of trapping are recorded by the secondary inclusions, one exemplified by Th of 210-215°C, similar to the Th of the primary inclusions in quartz veinlets, and the other of ~290°C. Hence, in spite of interpretation difficulties, a more complete record of fluids having circulated in a sample may be obtained from the study of secondary inclusions in such cases.

5. Results and interpretation 5.1. Microthermometric data interpretation In the literature, data on Th measurements on fluid inclusions from geothermal systems are presented without any pressure correction. This is correct for fluids trapped under boiling conditions. Thus, if boiling liquids and coexisting vapors are trapped separately in different inclusions, the Th of both inclusions must be the same, and represent the trapping temperature (Roedder, 1984). Considering the microscopic features of fluid inclusions at Los Azufres, there is no evidence of extensive boiling in the samples studied, except the few occurrences of anhydrite-rich samples. Inclusions have very similar volumetric fraction % of the vapor phase, which indicates a trapping of a homogeneous single-phase fluid. Thus, most of the inclusions homogenize in the liquid phase whilst vaporrich inclusions (type V) lack in most of samples. This indicates that no vapor coexisting with a liquid was trapped separately. As immiscible fluids have different capillary and moistening prol~erties they are not randomly distributed among vesicles and fractures. This could suggest that authigenic crystals formed from the liquid phase, and that most of the primary fluid inclusions represent only samples of this phase. In addition, the lack of vapor-rich inclusions may be explained by the lack of a major secondary fracturing-healing process which could favour gas trapping. Thus, by way of comparison, most of the vapor-rich inclu-

233

sions, in the Larderello field, Italy, are secondary and vapors are very often trapped separately from their coexisting liquids outside of the boiling zone (Cathelineau et al., 1986). This was interpreted as a consequence of differential migration speeds of vapor and liquid from the boiling zone. Thus, the immiscibility process was only partly recorded during fluid trapping. The lack of major vapor trapping does not exclude consequently the hypothesis about boiling and the existence of two-fluid phase zones in the field. It has been considered important when studying fluid inclusions to check whether the pressure in the field was different to that given by the liquid-vapor equilibrium or not. In the relatively faulted areas where wells have been drilled, pressure is mainly determined by the weight of the fluid column pressure. This means that the pressure is hydrostatic, and the lithostatic pressure, which represents a maximum estimate of pressure, is not applicable in the considered field. For instance, at 2-km depth, the hydrostatic pressure is ~ 180 bar, considering the thermal gradient of the field, and the decrease of the fluid density with increasing temperature (d--0.72 g cm -3 at 300°C, 1 eq wt.% NaC1; Khaibullin and Borisov, 1965). However, the present-day data (Iglesias et al., 1986) indicate that pressure is lower than the hydrostatic one (Fig. 3). Consequently, the fluid column is expected to be composed partly of a liquid, and partly of a vapour. Thus, there is considerable evidence of the existence of a twophase condition of the fluid in the Los Azufres reservoir. Shallow wells produce only steam, and even the intermediate-depth wells which produce a liquid-steam mixture show a high specific enthalpy in their total discharge, which indicates that they are fed from a two-phase mixture (Nieva et al., 1983). A detailed study of the pressure vs. depth profile in Los Azufres indicates that there are at least two distinguishable segments, indicating the existence of a compressed-liquid zone at depth ( ~ 1600 m),

234

M. CATHELINEAUET AL.

200 '

,

©

O0

100

/

"

,~c

/ 250

T (°C)

300

350

Fig. 3. Plot of pressure obtained by direct measurements vs. temperature: (1) derived from direct measurements (open circles); and (2) estimated from the fluid-inclusion data curve given in Fig. 6 {solid circles). C represents the maximum pressure correction for the fluid inclusions (H~O1 eq wt.% NaC1) which homogenize at 300°C in the liquid phase assuming an hydrostatic pressure (thermal gradient B ) in a liquid-dominated system. Present-day trapping PT conditions (circles) are, however, lower and very near of the liquid-vapor equilibrium curve (L/V). eq

wt.°/o

1800

6

NaCI

to

-1900

(3

samples)

m

shows that pressure is intermediate between vaporstatic and hydrostatic (Fig. 3). The P-T pairs are similar to those fixed by the liquidvapor equilibrium curve. The scattering of the values is due to uncertainties in the present-day temperature estimates, which are considerably lowered when using the measured pressures together with temperatures estimated from fluidinclusion data. Fig. 3 shows that maximum pressure correction does not exceed 10°C and that likely pressure corrections in some cases are very low, and are between 0 and 5 oC. This minimizes the difficulties in estimating the right pressure correction, which comes from the disagreement between the different P-1]-T data in the vicinity of the liquid-vapor equilibrium curve on one hand (Lemmlein and Klevtsov, 1961; Potter, 1977), and on the other hand the more recent and accurate data from Hilbert (1979), Gehrig (1980), and the equation of state of Bowers and Helgeson (1982), which are dif-

levels

•.

®

,e

-3

(~

Dilution

2~

-2

Cooling (~

Late

Fluids

+5

E •e oe

__ • e •

meemn

-%

•0 • Iwee~ •~•eeom

L. - - 2

I

I

I

200

250

300

O.

E

Th C'C)

200

250

~e

o

•

I

I

3O0

2

31~0

• ~...

~

•

b-

Fig. 4. Salinity-homogenization temperature diagram for fluid inclusions from the deepest levels {1800-1900 m depth) of the Los Azufres field.

and a two-phase zone at shallower levels {Iglesias et al., 1986; Nieva et al., 1986). The P - T plot of present-day measurements

eeoo •N

%'. m

•

%• •

1

••1

I

200 250 300 Th Homogenization temperature (°C)

Fig. 5. Salinity-homogenizationtemperature diagram for fluid inclusions from the upper levels: (a) 1000-1500-m depth; and (b) 500-1000-m depth.

235

HYDROTHERMAL ALTERATION IN THE LOS AZUFRES GEOTHERMAL SYSTEM

ficult to extrapolate to low pressures. Consequently, trapping temperature was considered to equal Th. The plot of salinity vs. Th for samples from the deepest part of the reservoir (Fig. 4) shows two different trends: (1) Varying salinities at almost constant Th may indicate a mixing process between a dilute fluid and a slightly saline fluid during the early stage of hydrothermal activity. The saline fluids may indicate an episodic recharge in saline fluids into the deep levels. T h e y may also represent the deepest fluids whilst dilute ones are fluids issued from vapor condensation. The mixing phenomenon may be put in relation with that demonstrated from isotopic (H, O) data obtained on actual fluids which come from deep levels. Thus Nieva et al. (1983) consider that the heterogeneity of the liquid phase suggests a mixing process which probably occurs only at depth and does not involve shallow meteoric water. (2) The flat trend of Th at constant salinity may represent the general cooling of the field but not in excess of 30-40 ° C. Fluids characterized by lower Th of ~ 280°C were observed in different samples, which range in depth from 1700 to 2100 m. T h e y may be considered either as fluids infiltrated from the upper levels, or as more recent fluids. Considering the hypothesis of recent trapping of cooler fluids, data given by the corresponding fluid inclusions would give a temperature estimate more in agreement with the present-day estimates, and the more abundant fluid inclusions the more precise the estimate of the early stage of the field is. However, as a first approximation, the low-Th inclusions were considered, to give a minimum estimate of the temperature in the field. Fig. 5 shows a diagram similar to that of Fig. 4 for the samples from shallower levels. Mixing trends are less clear in the upper part of the field, but higher salinities t h a n 1.5 eq wt.% NaCl are still observed. Only dilute fluids were encountered in the 500-1000-m depth range.

5.2. Other geothermometric data: a comparison Direct measurements in wells were obtained by the Comision Federal de Electricidad, using Kuster ® equipment. Some of these data are reported in Figs. 6 and 7. Fluid-inclusion data give clearly a higher temperature estimation than the measured values (Fig. 6). Such a difference could indicate that the present-day temperature is lower than that of fluid trapping, indicating a cooling process within the reservoir. However, an underestimation of the presentday measurements is far more probable. In most of the direct measurements, thermal equilibrium is not completely reached, because of too short a time of the experiments, and values are lowered by 30-90 oC. The projected plot of fluidinclusion data obtained on different wells was made, taking into account the differences in 100

200

300

T ('C)

500

1000

1500

Fluid- Inclusion

2000

data

"~

Direct Measurements

a

Depth

(m}

Fig. 6. Temperature ( Th)-depth projectedplot of F.I. data, and present-day measurementsfor different wells.The reference for well-head altitudes is 3000 m above sea level (depth=0). a is the temperature (Th)-depth curve drawn from fluid-inclusiondata and curve b refers to coolerfluids (stars), in minor abundance in the samples.

236

0

M. CATHELINEAUET AL.

100

200

300

200

300

T~(°C:)

1000

5 x

- --

Fl~,~-

t~cl~s~on

data

Chemical G e o l h e r m o m e l e r s J 0 Pre~enl -- day eslimales

"?

2000

( '~,

Depth. ( m l

• Depth Ira)

"1 • e\

Fig. 7. Temperature-depth projected plot of different temperature estimations: fluid-inclusion data, chemical geothermometers (a = K - N a - C a , b = CCG*~; Nieva et al., 1983 ) and present-day estimates (c=Fournier (1985); d = Rodriguez et al. (1984); e = downhole temperatures from CFE "2, in Nieva et al. ( 1983 ). Liquid-vapor curve (for H201 eq wt.% NaCl) is given for an hydrostatic pressure (2) and for the real present-day conditions (1). • ~CCG = Composicfon Cationica Geotermometra. • 2CFE = Comisfon Federal de Electricidad.

well-head altitudes (Fig. 6). These data served to draw an extrapolated curve, which gives a depth-temperature estimate from microthermometric data. The curve gives a maximum estimate of the thermal gradient in the field, and is coherent with the downhole temperatures for depths ranging from 500 to 2000 m. A minimum estimate of the present-day temperatures at greater depth is given by section b of the fluidinclusion curve, which refers to the cooler fluids trapped in the same samples. Chemical composition of produced fluids constitutes also one of the early important sources of information, which could be used for temperature estimation. The temperatures for different wells were calculated (Nieva et al., 1983) by means of the K - N a - C a geothermometer (Fournier and Truesdell, 1973) and of the K - N a - C a - M g g e o t h e r m o m e t e r (Nievaand Nieva, 1987). There is a relatively good agree-

ment between the values. Fig. 7 shows a projected plot of measured and calculated temperatures (Rodriguez et al., 1984), temperatures calculated from cationic geothermometers (Nieva et al., 1983), the fluid-inclusion data curve and the liquid-vapor equilibrium curve calculated from present-day pressure-depth pairs of data. Temperatures from Rodriguez et al. (1984) are consistent with the general thermal gradient, but exhibit unrelated variation at a given depth which cannot be interpreted as local horizontal gradient. Different values for a given well are proposed and suggest relatively high uncertainties. Contrarily, there is a considerable agreement between the fluid-inclusion data and the liquid-vapor equilibrium curve. This confirms that the trapping conditions of fluids are very close to those fixed by the liquid-vapor equilibrium without significant amount of gas (solution containing < 0.01 rn CO2 and negligible amounts of CH4, N2, etc. ). The boiling curve based on a hydrostatic pressure and a wholly liquid-dominated system, is not applicable to the Los Azufres field, but is given in the same figure as a reference. These results may be compared to those given by Taguchi and Hayashi (1982) for the Hatchubaru field, Japan, and Sternfeld (1981) for Geysers, California, U.S.A., where temperatures are close to those fixed by the liquid-vapor curve. Contrarily, evidence of boiling from the fluid-inclusion study is more clearly demonstrated at the Broadlands Field, New Zealand (Browne et al., 1976) and Larderello (Cathelineau et al., 1986) than at Los Azufres. 6. Conclusions

Fluid-inclusion data give useful information on the P - T - X evolution of a geothermal field. The information on the natural state of the system in the pre-exploitation stage is valuable in any attempt to monitor the changes occurring in the reservoir (initial point before exploitation). In the case of Los Azufres, data from fluid inclusions are consistent with the present-day ones, apart from slight cooling. This argues in

HYDROTHERMAL ALTERATION IN THE LOS AZUFRES GEOTHERMAL SYSTEM

favor of the hypothesis which considers the field as a relatively young system submitted mostly to prograde metamorphism. Such a study during drilling m ay give a relatively quick information compared to other methods (direct measurement, fluid analysis) which are time consuming. However, it requires t h a t a n u mb er of inclusions are analysed in order to be certain t h a t the variation range is not too large. In spite of the higher u n c e r t a i n t y of t e m p e r a t u r e estimation in some samples, results are in general consistent with the general t h e r mal gradient of the field. It is shown t h a t either primary or secondary fluid inclusions, for a given sample, give a relatively low range of Th-values, and may be considered as good records of the th er mal conditions in the field. Finally, this study of fluid inclusions in an active geothermal system gives a new example of the validity and accuracy of the microthermometric approach for geothermobarometric purposes. T h e r e is a good agreement between the present data and estimates from some cationic g eo th er mo m et er s ( K - N a - C a , CCG), and especially, a good agreement between fluid-inclusion t e m p e r a t u r e s and those given by the vapor-liquid equilibrium, when considering the p re sen t- d ay pressure. T hi s confirms the existence of pressures lower t h a n the hydrostatic one, and of a two-phase zone in the Los Azufres field. It is i m p o r t a n t to consider such low pressures in geological process, especially in the case of fossil h y d r o t h e r m a l systems.

Acknowledgements T h e authors wish to express their indebtedness to the personnel of the Los Azufres field and Departamento de Exploraci6n, Gerencia de Projectos Geothermoelectricos of ComisiSn Federal de Electricidad, for their general support and assistance during these studies, and to the Centre de Recherches sur la Gdologie de l'Uranium for technical, analytical and scientific assistance. P. Nehlig is t h a n k e d for his contribution to m i c r o t h e r m o m e t r i c measure-

237

m ent s and J. Dubessy for his very helpful comments.

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