'

v... .. ̂ T v,.,.-. • t-t^.~

Journal of volcanologyand geothermal research

E LS E VIE R Journal of Volcanology and Geothermal Research 104 (2000) 277-296www.elsevier.nl/locale/jvolgeores

Evolution of the hydrothermal system at Los Azufres, México,based on petrologic, fluid inclusión and isotopic data

E. González-Partida3'*, P. Birkleb, LS. Torres-Alvarado0

"Unidad de Investigación en Ciencias de la Tierra, Campas UNAM, Junquilla, Oro., México^Instituto de Investigaciones Eléctricas. Unidad Geotermia, A.P.1-475, Cuernavaca, Mor., 62001 México

''Centro de Investigación en Energía, UNAM, A.P. 34, Temixco, Mor.. 62580 México

Received 20 íuly 1999; revised 23 May 2000; accepted 23 May 2000

Abstract

The Los Azufres geothermal reservoir, formed in a fractured, Upper Miocene to Pliocene andesite and basalt base complex, issealed to the surface by a silicic, mainly rhyolitic sequence of Pleistocene age. Almost the entire sequence is affected byhydrolhermal alteraüon to varying extent. Petrological and fluid inclusión studies conftrmed vertical zonaüon of the reservoirby secondary processes-. Hydrothermal alteration processes under low temperature conditions (<1709C) caused argillitizationof the shallow zone of the reservoir (depth <500 m). Smectite, zeolite, calcite, and chlorite predominate the mineral assemblagein the shallow zone,

At a depth of 1200-1500 m, the máximum ice melting temperatures (TmO valúes of -0.7 to -4°C and salinities of 6.4 wt%NaCl eq. indícate boiling conditions of the geothermal brine in a vapor-rich zone. Chlorite, calcite, quartz, zeolite, anhydrite,albite, sphene, pyrite, hematite, and illite form the hydrothermal mineral paragenesis with máximum (emperatures of 250°C andpressure conditions of 150 bar. Below 1500 m, Tmi reach máximum valúes of —0.1°C and low salimúes of 0.2 wt% NaCl eq.The minerals epidote, amphibole, prehnite, and garnet indícate temperatures above 250°C and pressure conditions between 150and 200 bar. The measured homogenization temperatures (Th) of fluid inclusions (FI) are consistently higher than the in-situmeasured temperatures, which indicates retrograde cooling of the Los Azufres geothermal reservoir since the time of thehydrothermal mineral formation.

Updoming temperature isotherms at the center of the geothermal field (below well Az-9, Az-23, and Az-25) indícate theheating up of the fluids by a shallow magma chamber. Stable isotope dala (S18O, 5D) of the geothermal brine indicates mixingprocesses between meteoric water and a minor magmatic component. Secondary reactions forming sericite may have caused afurther positive shift in 18O in the shallow part of the geothermal reservoir. © 2000 Elsevier Science B.V. A\l rights reserved.

Keywords'. hydrothermal system; isotopic data; fluid inclusión data i

1. Introduction well as their physico-chemical evolution throughtime, is valuable Information during exploration and

The chemical composition of geothermal fluids, as exploitation of an active geothermal field. Fluid inclu-sions (FI) in hydrothermal minerals provide importantdata, as they are the only direct information source ontemperature during mineral formation and paleofluidE-mail aadresses: eep@comn.umcii.unam.mx (E. González- D

Partida), birkle@üe.org.mx (P. Birkle). ita@mazatl.cie.unam.mx composition, which have circulated through the rocks.d.S. Torres-Alvarado). FI studies may help to assess the temperature

0377-0273/00/$ - see front matter © 2000 Elsevier Science B.V. AH rights reserved.P11-. 80377-0273(00)00211-0

278 E. González-Partida et al. /Journal of Volcanology and Geotltenua/ Research 104 (2000) 277-296

20° -

110° 100° 90° W

Fig. 1. Location inapof the Los Azufres geothermal field in the centra! part of Mexico and western part of the Mexican Volcanic Belt (MVB).Two other ¡mportant Mexican geothermal fields (Cerro Prieto and Los Humeros) are also shown.

evolution with time in the field, and estímate the cool-ing rate or the physical state of the fluids prior toexploitation. Furthermore, FI provide local informa-tion on well-defined zones within the reservoir,compared to the information given by geothermalfluids, which may origínate from heterogeneoussources or from a relatively large section of the reser-voir. Fluid inclusión data, together with petrologicand isotopic information, may provide completeinformation for knowing the P-T-X evolution of ageothermal field.

This paper presents results from petrologic, isotopicand fluid inclusión studies of the hydrothermal systemat the geothermal field of Los Azufres, Mexico.Mineralogical zoning, as well as temperature distribu-tion profiles and the D/1SO isotopic fluid compositionare used to characterize the formation and determinethe origin of geothermal fluids at the Los Azufresreservoir.

2. Geological setting

Los Azufres is one of several Pleistocene silicicvolcanic centers with active geothermal systems inthe Transmexican Volcanic Belt (TMVB) (Aguilar

et al., 1987). It is located approximately 200 kmNW of Mexico City and 80 km E of the Michoacánstate capital Morelia (Fig. 1). With an electricityproduction of 98 MW, it represents the second mostimportant geothermal field in Mexico (Quijano Leónand Gutiérrez Negrín, 1995).

The volcanic rocks at Los Azufres have beendescribed by different authors (De la Cruz et al.,1982; Dobson and Mahood, 1985; Huitrón Esquiveland Franco Serrano, 1986; Cathelineau et al., 1987;Razo Montiel et al., 1989; López Hernández, 1991).Geologically, two principal divisions can be distin-guished (Fig. 2):

(1) Silicic sequence of rhyodacites, rhyolites anddacites with ages between 1.0 and 0.15 m.y. andhaving a thickness up to lOOOm (Dobsonand Mahood, 1985). According to Razo Montiel etal. (1989), five different units can be differentiated:Agua Fría rhyolite, Tejamaniles dacite, Cerro Mozoand San Andrés dacites and Yerbabuena rhyolite. Thissequence seáis the geothermal aquifer from thesurface, allowing the geothermal system to pressurize.

(2) 2700 m thick interstratification of lava flows andpyroclastic rocks of andesitic to basaltic compositionwith ages between 18 and 1 m.y., forming the localbasement (Dobson and Mahood, 1985). This unit

^4«*-t/*—«^

100° 411 30

19° 50'

19° 49'

... 100° 39'100° 401

Alluvium&PyroclasticsYerbabuena

rri Tejamaniles

Agua FríaRhyolite

rrmTTl Andesite-\ UÜiLl Complex

Fautts

Fig. 2. Geological map of the Los Azufres geothermal rield (based on Razo Montiel et al.. 1989). the principal fault systems, and the profile

Unes for Figs. 3 and 4. Legend abbreviations-. (1) Quaternary-Pleistocene-. (2) Pleistocene; (3) PUocene-U, Miocene.

• -•—< ;n ,hP field (Garduño Mc"--^' 1988): NE-

telsl,-rT¿srsrs*Es¿ss^=5J-*i*'^^ .̂.eo-opc,,̂ " -̂:,»:̂unS'»S «e,, . «jS-JÜSSi ™eeodheLostoufceshel' «o secondary

Ssr.rsJí-s^1^

guished in the field (Garduño Monroy, 1988): NE-SW, E-W and N-S. The E-W system is the mostimportant for geothermal fluid circulation. Geother-mal manifestations (fumaroles, solfataras andmudpits), geophysical anomalies and importantenergy production zones are related to this faultsystem. The profile in Fig. 3 shows a lithologicalcross-section in a N-S direction, from the northernMantaro zone towards the southern Tejamaniles

2SO E. González-Partida et al. /Journal of Vo/canology and Geothennal Research 104 (2000) 277-296

I

NW SE

-500 -500

Pleistocene -

Pliocene -U. Miocene

S Y M B O L S

Agua Fría Rhyolite

Basalt

Pyroclastics

Andesite Complex

Andesite & Basalts

1000m

Normal fault

A Thermal manifestation

^ Geological contad

Fig. 3. N-S directed geological cross section from the northern geothermal production área (Marítaro) to the southern área (Tejamaniles). Alsoillustrated are the principal fault systems and thermal surface manifestations.

zone. Fig. 4 shows a cross-section in E-W directionthrough the Tejamaniles zone.

3. Hydrothermal aspects

Iglesias et al. (1985) suggest that the present hydro-thermal system in Los Azufres is related to a deepaquifer extending throughout the field. Nieva et al.(1986, 1987a) demonstrated the existence of apressurized, hot (330°C) liquid system at a depth of3500 m in the Los Azufres reservoir. Rising towardsthe surface along fractures, boiling related to the lossof pressure and temperature favors increasing vapor-ization and the formation of a two-phase system.Cióse to the surface, the dominating vapor phase

and the liquid phase are mixed with meteoric water.Giggenbach and Quijano (1981) proposed that thegeothermal fluid has a major magmatic contribution,whereas Birkle (1998) indicated the existence of ameteoric water component.

However, some importan! regional differences arenoted. In the northern part of the field (Marítaro zone),geothermal fluids are formed by a mixture of gasesand liquid with T around 300-320°C. In the south(Tejamaniles zone), the gas phase generally domi-nates over the liquid phase (wt%) and temperaturesare lower than in the north (260-280°C). Regionalpermeability and systematic pressure differences, aswell as different boiling rates may be the reason forthese divergences.

The thermal fluids are sodium chloride rich waters

*-•"—':y am/ Geothennal Research 104 (2000) 277-296

i l '

281

3000

3000

2500

o IS.L rPleistocene 4

^Tejamaniles T1̂1J Caerte^ U.Miocenel

pyroclastics

Andes'rteComplex

Pleistocene-i , . . . . , . . , .<£7«j5<x| Agua ruóL E%»&a Rhyolite L- n n u m . . - - .

Fig. 4. W-E directed, e;eolog¡cal cross section through the southern geothermal production área (Tejamaniles). Also illustrated are the principal

fauh systems and thermal manifestaüons at the surface. • • -- »^mn1e>.te hvdrothermal alteration o

with high CO2 and H2S contents (Moreno Ochoa,1989), with a pH between 5.5 and 7.4 under reservoirconditions (Nieva et al., 1987b). Average Cl~contents are 3100mg/kg and CO2 can represent asmuch as 90% of the total gas phase. Fluid tempera-tures can reach valúes as high as 320°C, although

240-280°C are normal for the field.

4.Petrologicalconsiderations

~p hvdrothermal ¡

a,out, among oRobles Camacho et al. (19«)

and Torres-AWarado (have

shown partial to complete hydrothermal alteration ofprimary rocks, consisting of propylitic mineral assem-blages at higher temperatures (deeper zones) andof important argillite assemblages within zones oflower temperatures, as well as on the surface.

Systematically mineralogical changes occur withincreasing temperature and pressure (increasingdepth). Most important alteration assemblages are,with increasing depth, argillitization/silicincation,zeolite/calcite formation, sericiüzation/chloritization,and chloritization/epidotization. Calcsilicates areimportant temperature indicators in the field. Threedifferent zones may be defined, based on the observed

hydrothermal minerals:Zone ¡: Smectites, zeolites (mostly stilbite),calcite and chlorites predomínate, bul amorphous

2S2 E. González-Punida el al. /Journal of Volcanology and Geothermal Research 104 (2000) 277-296

N

rs

ro

I-z.O

pUJ

LU

3000-

2500-

2000-

1500-

1000-

500-

0-

-500-

200 °Cr 200 °c

220 °C

. 250 °C270 °C

300 °C

•̂ 310 °C- 300 °C

320 °C335 °C

o-0.1 TO-0.5°C -0.5 TO -0.7 °C -0.7 TO -3 °C +0.5TO11°C

5 •

Studied sample

Th (Homogenization temperatures)

In situ measurements

Fig. 5. Based on microthermometric analysis of the well cuttings and core samples from Az-27-A, Az-44, Az-9, Az-23, and Az-16, four zoneswith different ice melting temperatures can be distinguished in the N-S oriented section. The continued lines represen! the measuredhomogenization temperatures, whereas the dashed lines give the in situ measured valúes.

silica, quartz, kaolinite, and ilute are also present(Izquierdo et al., 1995). This mineral paragenesismay be found from ground level to 500 m depth, attemperatures lower than 170°C and pressure up to50 bar.

Zone II: This zone is characterized by the presenceof chlorite, calcite, quartz, zeolites (heulandite andwairakite), anhydrite, albite, sphene, pyrite, hematiteand illite. Quartz, chlorite and calcite reach here theirmáximum abundance. Depth ranges between 500 and1500 m, where 250°C and 150 bar may be reached.

Zone III: The deepest zone in the field is distin-guished by the presence of epidote, amphibole (tremo-lite), prehnite and garnet. As well as for other zones,chlorite, quartz, calcite, pyrite and hematite are alsopresent. This mineral paragenesis may be found at

depths >1500m, where temperatures higher than250°C and pressures between 150 and 200 bar arenormally reached.

5. Samples and methodology

Microthermometric determinations were made onwell cuttings and core samples from wells Az-9,Az-16, A/-18, Az-23, Az-25, Az-26, Az-27A, Az-35, Az-44 and Az-50 (Figs. 5 and 6), containingveins which are filled with secondary minerals(epidote, quartz, wairakite, anhydrite and calcite).Core slices and cuttings were double polished. Themicrothermometric measurements were performedon a Chaix-Meca heating-cooling stage, calibrated

E. González-Partida el al. /Journal ofVolcanology and Geothermal Research 104 (2000) 277-296 283

II'

3000 T

2500 --

2000

V)toE,

O

Ul

335 °C

1 k—I *\7>'A 3(^21-0.1 TO -0.5 °C -0.5 TO -0.7 °C -0.7 TO -3 °C

. Th (Homogenization temperatures)In situ measurements

+0.5TO11°C

5 •

Studied sample

Fig. 6. The W-E orienled section of the Tejamaniles production zone Ulustrales the microthermometric zonation oí the geotheniial reservoir.The continued Unes represent the measured homogenization temperatures, whereas Ihe dashed Une gives the in situ measured valúes.Decreasing homogenization temperatures can be observed towards the well Az-50.

using the melting points of seveval analytical reagentsas standards. From the observation of each fluid inclu-sión, two parameters were deterrained: (1) ice meltingtemperatures to estimate fluid salinity in terms ofequivalen! weight percent of NaCl (using Pottefsequation; Potter et al., 1978); and (2) the homogeniza-tion temperatures to estimate minimum formationtemperature of the studied minerals. Furtherdetails on the analytical procedures and techniquesfollowed are described by González Partida et al.(1997).

6. Results

6./. FI studies

Fl in hydrothermal quartz, calcite, wairakite, andepidote were studied. Table 1 summarizes the infor-mation obtained from the fluid inclusión analyses. Theresults are shown in Figs. 5 and 6, with an orientationof the cross-sections identical to the geologicalprofiles of Figs. 3 and 4. Most measured Fl consistedof liquid + vapor at room temperature, with

284 E. González-Partida et al. /Journal of Volcanology and Geoíhermul Research 104 (2000) 277-296

Table 1Results ofthe microthennometríc studies of fluid inclusión in hydrothennal minerals from Los Azufres geothermal field. 7], = homogenizationtemperature (°C); Tm = ice melting temperature (°C); n = number of inclusions measured; depth in meters; salinity in wt% NaCl eq; Qz =quartz; Ep = epidote; Ce = calcite; Pre = prhenite

Depth

Az-924502450230023002127212719001900170017001500130010001000800500400400300300200

Az-162450210021001950195015501550140012501250950800550400

Az-181320122011601000900800

Az-23170012751100

Host min.

QzEpQzBpO/EpQzEpQzBpEpQzWaiQzQzQzWaiQzWaiQzQz

QzEpQzWaiEpQzWaiWaiWaiQzWaiWaiQzQz

EpWkQzQzQzCe

QzQzQz

Th range

317312303340324324307302282286285320248234223210203215195199180

328299284290302268285260252241241206220204

230324252236213190

285251210

347351346348340326311307305305287327272295273255208229203215188

346319330309318288291272268265265257225210

233324286274262237

315292282

Th av.

334335340345329325310309296297286322173285243240205220200205185

335310305301310286288264262256260240223208

231324272257232212

310276264

n

2332872632541112522211110106

9110712

81642256

1 1232137803235510

152049605018

1 1814

Tm range

-0.1-0.6-0.1-0.5-0.3-0.5-0.1-0.5-0.8-1.3-0.3-0.9-0.4-0.4-1.50.40.30.3

-0.20.30.5

-0.3-0.6-1.0-1.1-0.4-0.6-0.6-0.5-0.6-0.6-0.4-0.6-1.2-1.1

-0.6-0.4-1.2-0.2-1.0-0.5

-0.1-0.1-0.5

-0.3-0.6-0.6-0.6-0.5-0.5-0.3-0.3-0.7-1.3-0.3-0.8-0.8-0.8-1.50.60.60.4

-0.50.50.5

-0.7-2.0-1.8-1.5-0.4-2.2-0.6-0.7-0.6-0.6-1.1-1.3-1.2-1.1

-0.6-0.4-0.5-1.0-1.0-0.5

-0.2-0.6-1.6

rm¡ av.

-0.2-0.6-0.3-0.5-0.4-0.5-0.2-0.4-0.7-1.3-0.3-0.9-0.5-0.5-1.50.50.50.4

-0.40.40.5

-0.4-1.5-1.3-1.2-0.4-1.0-0.6-0.6-0.6-0.6-0.8-1.1-1.2-1.1

-0.6-0.4-0.7-0.6-1.0-0.5

-0.2-0.3-1.1

n

233287263254

1 112522211110106

9110712

816

42

256

1123213780

3235510

15

2049605018

11814

Salinity

0.31.00.5

0.80.70.80.30.71.22.20.51.40.80.82.4-—-0.7•-

0.72.52.22.00.71.71.0

1.01.01.01.41.92.01.9

1.00.71.21.01.70.8

0.30.51.9

28S

, ......... _ ..........-o— —

286 E. González-Partida er al. /Journal of Volcanology and Geothermal Research 104 (2000) 277-296

Table 1 (cnntiinied)

Depth

26802580238023202080198018201600140012001000800700500400

Az-3510501000900780700600

Az-40208020001900180017001600150014001300120011001000900800700120

Az-4442003300320031002880288026002400240024002200

Host min.

EpEpQzEpEpQzQzQzQzQzQzQzQzQzQz

QzQzQzQzQzQz

EpEpQzEpEpCeQzEpEpEpQzEPCeQzQzQz

QzQzQzQzQzEpHPAnhQzEpEP

7], range

287289287227273254261212223247202203158106104

252232205177169148

230226232206216226222222221218217217221166136132

250302300297293307298290301305289

310334308359283312298360350288290231198134139

274261277212244149

326267292260304334340231221287273290277242205149

276327310303321332325337335322353

TI, av.

301303300308280277272292295275236219172115126

264243220190204148

242243263228252230229227221274228232243212199141

260313302288303300310307320314309

n

101812181025406029304138201518

365040352018

5103211626401310726836262810

352314196292043¡72

20

rm¡ range

-1.0-0.4-0.8-2.1-1.0-2.4-1.7-3.6-1.0-1.4-1.5-0.7-0.7-0.40.6

-1.3-0.4-0.4-3.4-1.7-0.9

-0.7-0.8-0.5-1.0-1.2-0.8-0.8-1.2-1.9-1.1-1.7-0.6-1.1-0.3-0.46.0

-0.1-0.4-0.9-0.9-1.1-0.9-0.7-0.7-0.6-0.6-1.1

-1.0-0.4-0.8-1.0-1.0-0.8-1.7

-1.0-1.0-1.4-1.5-0.7-0.7

0.90.6

-1.3-2.4-1.2-4.0-3.9-1.0

-0.7-0.8-0.5-0.7-0.6-0.8-0.8-0.6-0.7-0.7-1.7-0.6-1.6-0.8-0.88.1

-1.5-1.2-2.1-1.6-1.1-0.9-0.7-1.4-1.3-0.6-1.1

rm¡ av.

-1.0-0.4-0.8-1.2-1.0-1.5-1.5-2.0

-1.0-1.4

-1.5-0.7-0.70.50.6

-1.3-1.5-0.7-3.2-2.0-0.9

-0.7-0.8-0.5-0.5-0.6-0.8-0.8-0.7-1.0-1.0-1.7-0.6-1.5-0.6-0.67.0

-0.7-0.6-1.0-1.2-1.1-0.9-0.7-0.8-0.8-0.6-1.1

H

10182i1810

25406029304138201518

365040352018

5103211626401310726836262810

352314196292043171

20

Sülinity

1.70.71,4

2.01.72.52.53.31.72.42.51.21.2--

2.22.51.25.23.31.5

1.21.40.80.81.0

1.41.41.21.71.72.91.02.51.0LO-

1.21.01.72.01.91.51.21.41.41.01.9

E. González-Part

287

2S8 E. González-Partida et ti!. /Journal of Volcanology and Geothermal Research ¡04 (2000) 277-296

Table 1 (continued)

Depth

500400300

Az-5320001900182017001600150014001300120011001000900KOI)

720600500

400

Host min.

CeCeQz

EpEp

EpQzQzQzQzQzCeQzCeQzCeCeCeQzCe

7~h rangí

144144140

307298311289289268256206198231184221149190104118127

246246257

380358335334333333375300355348255247249232202184163

rhav.

168168155

324335326315305285265261272267221241213215176357150

n

20108

108

1 128374046

50282660502028311210

rm¡ range

0.30.10.1

-0.8-0.8-0.6-0.6-0.4-0.6-1.0-0.6-0.1-0.6-0.1-0.2-0.3-0.6-0.20.30.2

000

-0-0-0-0-0... (,-1-0-0-0-0-0-0-0-000

31I

88664

6068632

36232

rm¡ av.

0.30.10.1

-0.8-0.8-0.6-0.6-0.4-0.6-1.0-0.6-0.1-0.6-0.1-0.2-0.3-0.6-0.20.30.2

n í

20108

108

1 12837 (40465028 (26 160 (50 C20 (28 131 C1210

alinity

.4

.4

.0

.0).7.0.7.01.8

.01.31.31.5.01.3

predominance of the liquid phase. The sizes of FIranged from 3 to 10 (Jim.

6.7.7. Ice melting temperaturesThe microthermometric analyses indicated a

geothermal fluid of relatively low concentration,since ice melting temperatures (7m¡) vary from —0.1to -3.0°C (0.2 to 4.9 wt% NaCl eq.). This concentra-tion is compatible with the composition of geothermalfluids in the present system (Moreno Ochoa, 1989).The ice melting temperatures in Los Azufres showconsistent changes in the fluid concentration withincreasing depth (Figs. 5 and 6). A first zone, observedin the deepest áreas of the geothermal wells, showsvery diluted fluids as the Tmí from this área rangedfrom -0.1 to -0.5°C (0.2-0.9 wt% NaCl eq.). Asecond zone was identified overlying the first one.Due to its higher salinity, Tm¡ ranges between —0.5to -0.7°C (0.9-1.2 wt% NaCl eq.). The highest fluidconcentrations were measured in FI in hydrothermalminerals, sampled between 1200 and 1500m depth.Their rmi yielded valúes between —0.7 and — 3°C,reaching salinities as high as 6.4 wt% NaCl eq. It isimportant to note that some samples presented FI with

only one liquid phase (vapor) in them, and that someinclusions homogenize into the vapor phase (and notinto the liquid phase, as normally seen in the field).These observations may indicate the influence of avapor rich zone in this área, e.g. the zone where thegeothermal brine is supposed to boil. The importantgain in fluid salinity may as well reflect the boilingprocess, as gas loss from the system changes the fluidcomposition significantly.

The shallowest áreas in the field presented system-atically positive ice melting temperatures (Tm\ from+0.4 to +7.5°C). This has been observed in otherfields and hydrothermal systems (Bodnar et al.,1985; Hedenquist and Henley, 1985; Sasada, 1985)and have attributed positive Tm[ to the presence ofC02 in the fluids. Moore et al. (1992) emphasizedthe existence of a CO2-rich liquid phase in the shal-lowest zones in Los Azufres, estimating a CO2

concentration up to 5.7 wt%. However, these observa-tions have not been confirrned by any analytical studies(e.g; Raman spectroscopy). Consequently, further studiesare necessary in Los Azufres to corrobórate the presenceof CO2 in the gas phase of the FI and to explain thepositive melting temperatures at shallow depths.

11

E. González-Partida et al. /Journal of Volcanology and Geothermal Research 104 (2000) 277-296

Temperature [°C]

O 50 100 150 200 250 300 350

289

250

O % NaCI- 5 % NaCI

10% NaCI— - Measured• Quarz

Caíate

500

a0)a

750

1000 --

1250

Fig. 7. Depth distribution of the homogenization temperatures (Th) and in situ measured temperaturas of the well Az-26 from the southern partof the geothermal field. The positions of the mineral symbols show the Th mean valúes.

6.1.2. Homogenization temperaturesMeasured homogenization temperatures (TV) of the

FI range from 122 to 345°C (Table 1). Using theseresults, isotherms were drawn for two cross sectionsthrough the entire field (Figs. 5 and 6). For compar-ison, in situ temperatures were also plotted on thesefigures. In Los Azufres, in situ temperatures aresystematically measured using geophysical methodsdirectly in the drilled well (shut-in), after reachingthermal recuperation (up to 24 h). Figs. 5 and 6show that homogenization temperatures are consis-tently higher than the present in situ temperatures.This may indícate a retrograde cooling of the LosAzufres geothermal fteld of about 50-70°C, sincethe time of widespread hydrothermal mineralformation.

Figs. 7 and 8 present the range of Th measured andin situ temperatures versus depth for the wells Az-26and Az-44 from the southern and northern part of thefield, respectively. Boiling curves for fluids with aconcentration (NaCl-equivalent) similar to that inLos Azufres are also shown in the figures. Figs. 7and 8 show that most of the observed Th ranges atdifferent depths do not reach the boiling curves ofwaters with composition similar to that of LosAzufres. Since boiling is an important process in theLos Azufres system (Iglesias et al., 1985; Nieva et al.,1986, 1987a), we conclude, that important physico-chemical parameters (in particular pressure) havechanged since the time of mineral formation and/orstart of field exploitation.

The cross section presented in Fig. 6 reveáis a

290 E. González-Partida el al. /Journal of Volcanology cuul Geothermal Research ¡04 (2000) 277-296

Temperature [°C]250 300 350

Fig. 8. Depth distribution of the homogenization temperatures (7,,) and in situ measured temperatures of the well Az-44 from the northern partof the geothermal ñeld. The positions of the mineral symbols show the 7], mean valúes.

decrement in the homogenization temperatures in thedeepest parts of well Az-50. In order to corrobóratethis lower temperatura gradient, two plan sections atan altitude of 1000 and 2000 m were drawn in Fig. 9.These figures show a group of isotherms distributedalmost concentrically, with the highest temperaturesin the vicinities of well Az-9. A región of lowertemperatures is observed to the SW, where Az-50 islocated. This may indícate the presence of cold fluidsrecharging the geothermal reservoir from the SWduring formation of the hydrothermal minerals.

6.2. Isotopic indications

6.2.1. Global hydrothermal systemsA variety of theories exist on the origin of fluids in

hydrothermal systems. Craig (1963) indicated thatgeothermal waters were composed mostly of localmeteoric origin with minor contributions from othersources. Deviations of the primary meteoric compo-sition in form of a positive l!íO-shift are explained bysecondary water/rock-interaction processes. On the

other hand, studies of Sakai and Matsubaya (1977),Taran et al. (1989) and Giggenbach (1992) indícatethe influence of a magmatic component in hydrother-mal systems hosted at sorne relatively young volca-noes. The most probable composition of primarymagmatic waters (PMW) is intermedíate betweenthat of degassed magma and subduction related vol-canic vapor with an estimated 8D of -60 ± 10%o(Giggenbach, 1992) (Fig. 10). Primary magmaticwater is considered to be identical to "residual"magmatic water, i.e. water left after crystallizationof the magma (Taylor, 1992). In subduction relatedtectonic systems, such as the Pacific Convergen! PíateBoundaries, seawater is transponed together withmarine clay sediments by the subducting slab todepth beneath the continental crust. The 8D valúesof this subducted sedimentary water, also called"convergent margin degassed magmatic waters"(D'Amore and Bolognesi, 1994) or "andesiticwaters", is assumed to be -30 ± 10%o (Giggenbach,1992). Most of the largely neutral, high Cl watersfrom the Pacific Conversen! Píate Boundaries are

-cwfc—'«'»"'2¡ 291

E. GonzáZ«-Pflrtíáfl <'

19-50'

19-48'

hvdrothermaTeraüon is

late-stage

WUh a range

6 2 2 ." ¿otopic data from spnn^ (1980-1981) and

tirm wells dunng the > ^ geothermalproduction^ } penod of gjintermedíate g3 Nieva et al, ' B -oroduction (Nieva et al, \ > Most ot the

on thel971;Hedenquist

water anu metec.posiüondepending

(Sheppard et al.,

1994).

292 E. González-Partida et al. /Journal of Volcanology and Geothermal Research 104 (2000) 277-296

-20

-40

o— -60

-80

-100

-120

METEORICCOMPONEN!

Andesiticwater

MAGMATICCOMPONEN!

Primarymagmaticwater

Low sulphidation (barren)

Los Azufres

A Coldsprings (Birkle, 1998)O Geothermal fluids (Nieva et al., 1983)

• Geothermal fluids (Nieva et al., 1987b)

! _L I

-20

-40

-60

-80

-100

-120

-15 -10 -5 +10 +15 +20

18Ó O

Fig. 10. 6D and 518O composition of primary magmatic and andesitic waters (Giggenbach, 1992), as well as secondary alteration processes.such as epithermal ore deposits (low sulphidation, high sulphidation) (O'Neil and Silberman, 1974; Taylor, 1979; Rye, 1993) or hydrothermalsystems (sericitation) (Hedenquist and Lowenstern, 1994). The Los Azufres brines show an intermedíate ¡sotopic position between meteoricwater and subduction-related andesitic water. Secondary sericitation processes caused probably a further shift from the meteoric water linetowards more positive isotopic valúes.

recent meteoric water, indícales the influence of both,water-rock interaction and/or mixing processesbetween local meteoric water and magmatic fluidswithin the geothermal reservoir. Similar mixingprocesses were described for hydrothermal fluidsfrom Mount St. Helens, USA (Shevenell and Goff,1993), and Larderello, Italy (D'Amore and Bolognesi,1994). Because of the very small fractionation factorsgoverning mineral-water isotope exchange atmagmatic temperatures (Friedman and O'Neil,1977), the 5I8O valúes of magmatic waters are likelyto be cióse to those of local parent magmas (D'Amoreand Bolognesi, 1994).

!herefore, a measured ¿>'8O valué of +8%o formafic rocks (Torres-Al varado, 1996) from the Los

Azufres reservoir and surface meteoric water with5D=-75%c and «5I8O = -10.7%c were used asmagmatic and meteoric end members, respectively.A mixing portion between 25:75 and 35:65 betweenmagmatic waters and meteoric water was achieved forthe Los Azufres fluids. The low SD-values of thegeothermal fluids can not be deduced exclusively tothe subducted sedimentary slab, but to a primarymagmatic water component. On the other hand, thelack of I4C in the geothermal fluids indícales a majorinfluence of magmatic CO2 gas and minor influence ofreceht meteoric water within the geothermal reservoir(Birkle, 1998). In addilion, the existence of hydrother-mal alteration, related to the formation of a variety ofsecondary minerals (see Petrology Chapter) supports

£. González-Partida tí al. /Journal of Volcanology and Geothermal Research 104 (2000) 277-296 293

the hypothesis of further isotopic modifkations byposterior water-rock interaction. The geothermalliquids from Los Azufres could also be related tohigh sulphidation or low temperature sericitic alter-ation processes (Fig. 10). Due to the abundance ofsericitic mineral paragenesis, the latter case isfavored. Thus, the positive 180- and D-shift of thegeothermal fluids can be explained by: (a) fossilmeteoric water, that subducted partially as part ofthe sedimentary slab ("andesitic waters"), or infil-trated during glacial period; and (b) was mixed withprimary magmatic water and c) modified by second-ary sericitation processes. Especially the latter processcamouflages the real proportion of the meteoric watercomponent.

The Broadlands geothermal system, New Zealand,is an isotopic similar case to the Los Azufres reser-voir. the H-shift indicates a 20% component of low-salinity magmatic vapor mixed with local meteoricwater. Both systems are characterized by elevatedgas contents (2.0 and 1.1 wt%, respectively)(Brown, 1986).

1. Discussion

Small differences in the chemical composition ofthe FI (differences in T^) indicate a distinctive zona-tion of the geothermal fluids with depth. Two zonesare important to distinguish: a deeper zone with lowconcentrated fluids and the overlying zone with ahigher salinity (Figs. 5 and 6). These chemical differ-ences may be interpreted as the effect of boilingprocesses, since the loss of the gas phase changesthe fluid composition. In fact, boiling is a frequentprocess in low pressure environments (such asgeothermal systems; Hedenquist and Henley, 1985).Its presence in Los Azufres is supported by someevidences such as: (a) the occurrence of some gasinclusions together with liquid ones; and (b) homoge-nization into the gas phase, as well as into the liquidphase in the same mineral sample; (c) elevated fluidconcentration; (d) lower pressure than the hydrostaticone (Figs. 1 and 8). In this sense, the FI results supportthe model of the physical state of the Los Azufresfluids presented by Iglesias et al. (1985), who inter-preted three distinguishable zones in the field: (1) acompressed-liquid zone under hydrostatic pressure

regime at increased depths (~1600m); (2) a zonewith two fluid phases (where boiling occur); and, (3)a shallow, vapor pressure-dominated zone. Thisobservation may have important implications forfield exploitaüon.

On the other hand, fluid inclusión results seem tosupport the hypothesis related to the origin of thegeothermal fluids, based on their isotopic characteris-tics (see above). Very little variation of the salinitywas observed at constant Th (see Table 1), indicating aprobable mixing process between diluted and slightlysaline fluids during hydrothermal mineral formation.Furthermore, samples with constant salinities andchanging Th (see Table 1) reflect the general cooling(50-70°C) taking place in the field.

Unfortunately, few studies of FI in hydrothermalminerals have been accomplished on active geother-mal systems (Cathelineau and Marignac, 1994). Twocase studies of two hydrothermal systems (LosHumeros, México, and Larderello, Italy) will bediscussed in the following section and compared tothe Los Azufres system.

The Los Humeros geothermal field resembles LosAzufres in several aspects. Both systems are present inthe Mexican Volcanic Belt and related to young, felsicvolcanism. However, in Los Humeros, the volcanicsequence rests upon a calcareous basement of Cretac-eous age. These older rocks provide special chemicalconditions to the occurring water-rock interactionprocesses, resulting in abundance of hydrothermalcalc-silicate minerals (especially garnet, wairakite,and wollastonite), besides the common propyliticalteration of the volcanic rocks. FI in secondary quartzand calcite from volcanic and calcareous cores,obtained from wells in Los Humeros, were studiedby Prol-Ledesma and Browne (1989). They observedmainly liquid-rich Fl and no clatharates or daughterminerals were detected. Freezing temperatures indi-cated the presence of a diluted fluid during mineralformation (0.2-2.7 wt% NaCl eq., slightly lessconcentrated than in Los Azufres) with some contri-bution to freezing point depression by a relativelyhigh COi concentration. Most Th-values match withpresent bore temperatures (contrasting the observa-tions made in Los Azufres). FI results in Los Humerospointed out the importance of geological structures forthe fluid circulation regime with a zonation of thetemperature pattern related to the central collapse

294 E. González-Partida et al. /Journal of Volcanology and Geothermal Research 104 (2000) 277-296

structures and some cooling effect due to the intersec-ción of the geothermal wells with major faults. Thiskind of correlation has not been encountered in LosAzufres due to its complex structural characteristics.

The Larderello geothermal field is one of the best-studied active geothermal systems in the world. It ischaracterized by productive horizons in permeablelayers of Triassic dolomite. Fluid inclusión studiesfrom Cathelineau et al. (1994) demonstrated thepresence of magmatic fluids in the very early stagesof the hydrothermal system. They observed severalgenerations of high temperature, high concentratedfluids trapped in several fluid inclusión planes, record-ing several hydrothermal circulation regimes. Theydifferentiated: (1) aqueous-carbonic fluids, interpretedas the result of reheating of the fluids in the meta-morphic basement series during contact metamorph-ism; and (2) Li-rich fluids considered to be expelledfrom an underlying Li-rich leucogranite and to havemigrated through the metamorphic series, having as aconsequence the mixing of both fluids. The character-ization of Li-rich fluids in different proportions of theobserved FI was possible by the identification of salthydrate (LiCl-5H2O) using Raman spectroscopy atvery low temperature. This study showed the impor-tance of precise geochemical analyses of fluidstrapped in FI and the necessity of following a similarattempt at Los Azufres, in order to prove the hypoth-esis of mixing as the origin of the geothermal fluids inthe field.

8. Conclusions

The Los Azufres geothermal field is one of severalhydrothermal systems related to young silicic volcan-ism in México. Hydrothermal alteration has affectedmost rocks in the field to varying extent. Argillitiza-tion, zeolite and calcite formation and propillitizationare the most importan! alteration processes occurringin the field. Studies of FI in hydrothermal mineralsshow that the geothermal fluid at the tirne of mineralformation presented low concentrations (up to—7 wt% NaCl eq.), very similar to present fluidscomposition. Homogenization temperatures show,however, that retrograde cooling and probablychanges in the pressure of the system have been takingplace since mineral formation. Lower homogenization

temperatures to the SW reflect a former recharge zonenear well Az-50 during the formation of the hydro-thermal system.

The high temperature conditions of the Los Azufresreservoir can be explained by conductive heat produc-tion of a shallow magma chamber, and additionally bythe rise of heated meteoric water mixed withmagmatic fluids. Mixing of the primary magmaticfluids with infiltrating meteoric water causes changesin the <5I8O and ¿>D ratios of the original meteoricfluids. Resides, more recent infiltration of consider-able components of meteoric water under lowtemperature conditions causes sericitic alterationprocesses within the more shallow part of the reser-voir.

Acknowledgements

CFE provided valuable information for this study.CONACyT (grant I-25814-T) and DGAPA-UNAM(IN-119798) partially supported this work. We thanktwo anonymous reviewers for their helpful comments.

References

Aguilar, Y., Vargas, V.H., Verma, S.P., 1987. Composición química(elementos mayores) de los magmas en el Cinturón VolcánicoMexicano. Geofís. Int. 26, 195-272.

Birkle, P., 1998. Herkunft und Umweltauswirkungen der Geother-rnalwasser von Los Azufres, Mexiko. Wissenschaftliche Mittei-lungen. Technische Universitá't Bergakademie Freiberg,Freiberg/Saxony, Germany, vol. 6, 226 pp.

Bodnar, R.J., Reynolds, T.J., Kuehn, C.A., 1985. Fluid inclusionssystematics in epithermal systems. In: Geology and Geochem-istry of Epithermal systems. Reviews in Economic Geology 2,73-97.

Brown, K.L., 1986. Gold deposition from geothermal discharges inNew Zealand. Econ. Geol. 81, 979-983.

Cathelineau, M., Marignac, Ch., 1994. Use of fluid inclusions for abetter understanding of intracontinental geothermal activities.In: De Vivo, B., Frezzotti, M.L. (Eds.), Fluid Inclusions inMinerals: Methods and Applications. Short Course Notes,Virginia Polytechnic Institute. pp. 309-326.

Cathelineau, M., Oliver, R., Garfias, A., Nieva, D., 1985. Mineral-ogy and distribution of hydrothermal mineral zones in the LosAzufres (México) geothermal field. Geothermics 14, 49-57.

Cathelineau, M.. Oliver. R., Nieva, D.. 1987. Geochemistry ofvolcanic series of the Los Azufres geothermal field. Geofís.Int. 26, 273-290.

Cathelineau, M., Marignac, C., Boiron, M.C.. Gianell i , G.,Puxeddu, M., 1994. Evidence of Li-rich brines and early

£. González-Partida et al. /Journal of Volcanology and Geothermal Researi'h 104 12000) 277-296 295

rnagmatic water-rock interaction in a geothermal field: the fluidinclusión data from the Larderello field. Geochira. Cosmochim.Acta 58, 1083-1099.

Craig, H.. 1963. The isotope geochemistry of water and carbón ingeothermal áreas, In: Nuclear Geology on Geothermal Áreas,CNR, Pisa (pp. 17-53).

D'Amore. F., Bolognesi, L., 1994. Isotopic evidence for a magmaticcontribution to fluids of the geothermal system of Lardarello,Haly, and The Geysers, California. Geothermics 23 (1), 21-32.

De La Cruz, V.M., Aguilar, 3.S., Ortega, D.G., Sandoval, J.M..1982. Estudio geológico-estructural a detalle del campo geotér-mico de Los Azufres, Mich. Comisión Federal de Electricidad,México. Unpublished Report.

Dobson, P.F., Mahood. G.A.. 1985. Volcanic stratigraphy of the LosAzufres Geothermal Área, México. ]. Volcanol. Geotherm. Res.25.273-287.

Friedman. I., O'Neil, J.R., 1977. Compilation of stable isotopefractionation factors of geochemical interest. In: Fleischer, M.(Ed.), Data of Geochemistry. Geological Survey ProfessionalPaper, 440-KK.

Garduño Monroy, V.H., 1988. La caldera de Los Azufres y surelación con el sistema regional E-W. Geotermia, Rev. Mex.Geoenergía 4, 49-61.

Giggenbach, W.F., 1992. Isotopic shifts in water from geothermaland volcanic systems along convergent píate boundaries andtheir origin. Earth Planet. Sci. Lett. 113. 495-510.

Giggenbach, W.F., Quijano León, J.L., 1981. Estudio isotópico delas aguas del campo geotérmico de Los Azufres. ComisiónFederal de Electricidad (CFE), México. Intemal Report.

González Partida, E.. Barragán, R.M., Nieva, D., 1989. Caracteriza-ción mineralógica en 10 pozos del campo geotérmico de LosAzufres, Mich. V. Minerales opacos. Geotermia, Rev. Mex.Geoenergía 5, 347-373.

González Partida. E., García Gutiérrez, A.. Torres Rodríguez, V.,1997. Thermal and petrologic study of the CH-A well from theChipilapa-Ahuachapan geothermal área, El Salvador. Geother-mics 26, 701-713.

Hedenquist, W.Í., Henley, R.W., 1985. The importance of COj onthe freezing point depression of fluid inclusions: Evidence fromactive geothermal systems and implications for epithermal oredeposition. Econ. Geol. 80, 1379-1406. •

Hedenquist, J.W., Lowenstern, J.B., 1994. The role of magmas inthe formation of hydrothermal ore deposits. Nature 370, 519-527.

Huitrón Esquivel, R., Franco Serrano, A.. 1986. Carta geológica delcampo geotérmico de Los Azufres, Mich., México. Geotermia,Rev. Mex. Geoenergía 2. 211-213.

Iglesias.E.R., Arellano, V.M., Garfias, A., Miranda, C., Aragón, A.,1985. A one-dimensional vertical rnodel of the Los Azufres,México, geothermal reservoir in its natural state. Geotherm.Resour. Council Trans. 9, 331-336.

Izquierdo, G., Cathelineau, M., García, A., 1995. Clay minerals,fluid inclusions and stabilized temperature estimation in twowells from Los Azufres geothermal field, México. In: Barbier,E., Frye, G., Iglesias. E., Palmasen, G. (Eds.), Proceedings ofthe World Geothermal Congress, Florence, Italy. InternationalGeothermal Association, pp. 1083-1086.

López Hernández. A., 1991. Geología de Los Azufres. Geotermia,Rev. Mex. Geoenergía 7, 265-275.

Moore, J.N.. Adams, M.C., Lemieux, M.M.. 1992. The formationand dislribution of COrenriched fluid inclusions in epilhermalenvironments. Geochim. Cosmochim. Acta 56, 121-135.

Moreno Ochoa, J., 1989. Hidrogeoquímica de la región de LosAzufres, Michoacán. Comisión Federal de Electricidad,México. Unpublished Report.

Nieva, D.. Quijano, L., Garfias, A., Barragán, R.M, Laredo, F.,1983. Helerogeneity of the liquid phase. and vapor separationin Los Azufres (México) geothermal reservoir. Ninth Workshopon Geothermal Reservoir Engineering. Stanford, California,pp. 253-260.

Nieva, D., Iglesias. E., Arellano. V.. Contreras, E., Cathelineau. M..Quijano, L., 1986. Development in geothermal energy inMéxico. Part Four: Evaluation of geothermal resources. Multi-disciplinary studies of the Los Azufres field. México. HeatRecovery System 6, 201 -207.

Nieva, D., Verma, M., Santoyo. E., Barragán. R.M.. Portugal. E.,Ortiz, L, Quijano, L., 1987a. Estructura hidrológica del yaci-miento de Los Azufres. International Symposium on Develop-ment and Exploitation of Geothermal Resources, Cuetnavaca,México, Proceedings. pp. 202-213.

Nieva, D., Barragán, R.M., Cathelineau, M.. González, E..Izquierdo, G., Oliver, R., Portugal, E., Santoyo, E., Verma,M.. 1987b. Estudio de la estructura hidrológica y de la distribuciónde parámetros físico-químicos en el yacimiento de Los Azufres,Mich. Fase U. Instituto de Investigaciones Eléctricas, Cuernavaca,México. Internal Report UE/U/2011/1. vol. 1, 159 pp.

O'Neil, J.R., Silberman, M.L.. 1974. Stable isotope relations inepithermal Au-Ag deposits. Econ. Geol. 69, 902-909.

Potter II, R.W., Clyne, M.A.. Brown, D.L., 1978. Freezing pointdepression of aqueous sodium chloride solutions. Econ. Geol.73, 284-285.

Prol-Ledesma. R.M., Browne, P.R.L., 1989. Hydrolhcrmal altera-tion and fluid inclusión geothermometry of Los Humerosgeothermal field. México. Geothermics 18, 677-690.

Quijano León, J.L., Gutiérrez Negrín, L.C.A.. 1995. Present situa-tion of geothcrmics in México. Proceedings of the WorldGeothermal Congress, Florence. Italy, pp. 245-250.

Razo Montiel, A.. Huitrón Ezquivel, R., Moore, J.N., 1989. Geolo-gía del campo geotérmico de Los Azufres. Mich. PreliminaryProceedings of the Final Symposium on the Agreement betweenUS Department of Energy and the Comisión Federal de Elec-tricidad de México in the field of Geothermal Energy, pp. 7-15.

Robles Camacho, J., Izquierdo Montalvo. G., Oliver Hernández, R.,1987. Alteración hidrotermal en el Módulo Marúaro. LosAzufres, Michoacán. Geotermia. Rev. Mex. Geoenergía 3,131-143.

Rye, O.O., 1993. The evolution of magmatic fluids in the epithermalenvironmenf. the stable isotope perspective. Econ. Geol. 88,733-753.

Sakai, H.. Matsubaya. O.. 1977. Stable isotope studies of Japanesegeothermal systems. Geothermics 5, 97-124.

Sasada. M., 1985. COZ bearing fluid inclusions from geothermalfields. Geotherm. Resour. Council Trans. 9, 351-352.

Sheppard, S.M.F., Nielsen, R.L., Taylor Jr., H.P., 1971. Hydrogen

296 E. González-Partida et al. / Journal of Volcaiwlogy and Geothermal Research 104 (2000) 277-296

and oxygen isotope ratios in minerals from porphyry copperdeposits. Econ. Geol. 66, 515-542.

Shevenell, L.. Goff, F., 1993. Addition of magmatic volátiles intothe hot spring waters of Loowit Canyon Mount St. Helens,Washington, USA. Bull. Volcanol. 55, 489-503.

Taran. Y.A., Pokrovsky, B.G., Esikov, A.D., 1989. Deuterium andoxygen-18 in fumarolic steam and amphiboles from someKamchatka volcanoes: andesitic water. Dokl. Akad. Nauk.USSR 304, 440-443.

Taylor Jr.. H.P., 1979. Oxygen and hydrogen isotope relationship inhydrothermal mineral deposits. In: Barnes; H.L. (Ed.),Geochemistry of Hydrothermal Ore Deposits. Wiley, NewYork, pp. 236-277.

Taylor. B.E.. 1992. Degassing of H:O from rhyolite magma duringeruption and shallow intrusión, and the isotopic composition ofmagmatic water in hydrothermal systéms. In: Hedenquist, J.W.(Ed.), Magmatic Contributions to Geothermal Systems. Reportno. 279. Geológica! Survey of Japan. Tsukuba. Japan, pp. 190-194.

Torres-Alvarado. I.S.. 1996. Wasser/Gesteins-Wechselwirkung imgeothermischen Feld von Los Azufres, Mexiko: Mineralo-gische. thermochemische und isotopengeochemische Untersu-chungen. Tuebinger Geowissenschaftliche Arbeiten, Reihe E,Nr. 2. 181 pp.