The δ 34S composition of sulfates and sulfides at the Los Humeros geothermal system, Mexico and...

ELSEVIER Journal of Volcanology and Geothermal Research 73 (1996) 99-l 18

Jounlalofvokanology audgeothcmlalreseafih

The 634S composition of sulfates and sulfides at the Los Humeros geothermal system, Mexico and their application to

physicochemical fluid evolution

Raymundo G. Martinez Serrano a*b** ,‘, Bertrand Jacquier a, Michel Arnold a a Centre de Recherches Pktrographiyues et Giochimiques, Vandoeuure 12s Nancy, France

h hstituto de Geojbica, UNAM, Mexico, Mexico D.F.

Received 18 July 1994; accepted 7 November 1995

Abstract

The 634S isotopic composition of sulfur was determined in more than 105 pyrite samples found in volcanic formations as

well as in the sulfates and sulfides dissolved in the present-day geothermal fluids in the Los Humeros system, Mexico. Analysis of the isotopic values demonstrated that the sulfur compounds of the geothermal system were derived from a magmatic source (634S,s - I%& The calculation of the different pH-fO,-(fS,) diagrams showed that the sulfates and

sulfides dissolved in the present-day fluids from well Hl do not show chemical equilibrium conditions as was indicated previuosly by Arnold and Gonzalez-P. (1987). The reason for this is that the physicochemical characteristics of the system

have been evolving almost continuously as a result of the exploration and exploitation of the thermal fluids from the system.

The residence time of the fluids in the geothermal reservoir is now reduced and the chemical and isotopic reactions that

occur between fluids and minerals are not carried out completely. Due to the thermodynamic evolution of the fluids, equilibrium among the sulfur phases dissolved in the fluids could not be demonstrated. The 634S values of pyrite sampled at

different depths in the geothermal system display evidence for different isotopic fractionation produced by boiling, fluid

mixing, and vapor condensation in meteoric waters. The 634S values of sulfates in the present-day fluids suggest that these were derived from the oxidation of H,S at relatively shallow depths ( < 600 m). In fact, the isotopic compositions of these

sulfates trend towards 634S values of sulfides found in the steam phase.

Keywords: Geothermal systems; Sulfur isotopic composition; Chemical equilibrium

1. Introduction equilibrium conditions between mineral phases and

Most geothermal systems studied during the first stages of exploration have particular geochemical characteristics which are representative of isotopic

* Corresponding author.

’ Present address: Instituto de Geofisica, UNAM, Del. Coyoa-

can, 04510, Mexico D.F., Mexico.

co-existing fluids (Sakai et al., 1980; Arnold, 1981).

However, economic development of such geothermal

systems can cause changes in the geochemical char-

acteristics of the circulating fluids and result in departures from equilibrium.-

Several studies have been focused towards under- standing the different parameters that rule equilib-

rium conditions among sulfur compounds. Igumnov

0377-0273/96/$15.00 Copyright 0 1996 Elsevier Science XX All rights reserved

SSDI 0377-0273(95)00083-6

100 R.G.M. Serrano et al. / Journal of Volcanology and Geothermal Research 73 (1996) 99- I18

(1976) and Igumnov et al. (1977) demonstrated that

the kinetics of the oxidation-reduction processes of

some of the sulfur phases depends on temperature,

pH, ionic strength and on the overall molality of the

sulfur dissolved in the solution. These authors showed

that chemical equilibrium between sulfates and sul-

fides can be attained by hydrothermal fluids at tem-

peratures above 350°C and with an almost neutral

pH. Later, the evaluation of experimental data on

sulfur isotope exchange rates between aqueous sul-

fate and sulfide led Ohmoto and Lasaga (1982) to

establish that the rate of sulfate - sulfide equilibra-

tion depends on temperature, pH, and total sulfur

content of the fluid. The rate increases with increas-

ing temperature and total sulfur content (mZ:S 2

10e2 moles), and decreasing pH.

Sulfur isotopic studies of active continental

geothermal systems have been found in which iso-

topic equilibrium exists in systems with temperatures

> 250°C low salinity ( < 4,000 mg/kg of dissolved

salts) and low sulfur concentrations (2.S = 10-j

moles) (Sakai et al., 1980; Arnold and Gonzalez-P.,

1987). McKibben and Eldridge (1989) also determined that isotopic equilibrium conditions in the

Salton Sea geothermal system exist between H,S

and SO, at 300°C with low sulfur concentrations

(2s = 10e3 moles), but at high total salinity (>

300,000 mg/kg).

Application of sulfur isotope geochemistry to ore

deposits and geothermal systems research has showed

that more than one process may produce the same

isotopic characteristics in a geothermal system, and

the same geochemical process may produce different

isotopic characteristics under different conditions

(Ohmoto, 1986). Therefore, the task of distinguish- ing the sulfur isotopic features formed during equi-

librium events in a geothermal system from those

overprinted during subsequent development has not always been easy. For example, different generation

of sulfate and sulfide minerals can show similar sulfur isotopic compositions, or sulfur minerals formed under the same equilibrium conditions can present slightly different 634S values. Until now, few sulfur isotopic studies have examined the isotopic changes in the geothermal fluids during both pre- and syn-production of a geothermal system.

The purpose of this work is to establish the evolution of the physicochemical characteristics of

the fluids found at the Los Humeros geothermal

system, Mexico during economic development using

an isotopic study of oxidized and reduced sulfur

species dissolved in hydrothermal fluids. This was

done by comparing the results obtained in this work

with similar data presented by Arnold and Gonzalez-

P. (1987). The present study also includes a system-

atic investigation of the sulfur isotopic variations in

pyrite from the host volcanic rocks and veins at

different depths in five exploration wells. These data

have been used to determine the contributions of

different potential sulfur sources to vein sulfide min-

eralization and isotopic equilibrium conditions be-

tween H,S and SO, in the present fluids.

I. 1. Geologic setting

The Los Humeros geothermal system is located in

a complex volcanic caldera system of less than

500,000 years old, which is at the eastern end of the

Plio-Pleistocene Trans-Mexican Volcanic Belt (Fig. 1) (Yaiiez-G., 1980; Ferriz, 1982; Ferriz and Ma-

hood, 1984; Martinez-S., 1993; Negendank et al., 198%. Thermal manifestations as well as most of the

exploration bore-holes are located in a small area

(5 X 7 km) called Colapso Central-Xalapazco, in a

system of coalesced calderas (Fig. 2). Studies of core and drill cutting samples and surface geology from

the system have shown that the basement of the

region is formed of Late Cretaceous limestone se-

quences (Viniegra-O., 1965; Yafiez-G., 1980). Most bore-holes penetrate a hydrothermally altered se-

quence of more than 2200 m consisting of andesites, dacites, rhyodacites, rhyolitic tuffs, rhyolites and

some basaltic rocks deposited on the basement less than 500,000 years ago (Ferriz and Mahood, 1984; Martinez-S., 1993). The most recent event was the eruption of an olivine basalt lava flow less than

20,000 years ago. The subsurface structure at the Los Humeros sys-

tem has been inferred from geophysical studies by Campos-E. and Arredondo-F. (1992). This consists of sequences of blocks bounded by fractures and faults forming grabens and horsts associated with the process of collapse caldera formation.

Petrographic and geochemical studies carried out at the Los Humeros system (Viggiano-G. and Rob- les, 1988; Prol-L., 1990; Martinez-S., 1993; Mar-

R.G.M. Serrano et al./Journal of Volcanology and Geothermal Research 73 (1996) 99-118 101

tinez-S. and Alibert, 1994) show that volcanic rock sequences were affected by the circulation of hot water ( > 29O”C), with less than 2500 mg/kg of total dissolved salts, transforming the primary rock forming minerals into stable phases at new physicochemical conditions. Geochemical analysis of the present-day fluids discharged from the bore-holes of the geothermal system suggest that these are probably the result of geothermal fluid mixing with meteoric water, plus the addition of a high percentage of steam (between 30 and 80%) at shallow depths (Barragan et al., 1991; Martinez-S., 1993). The geothermal system presently behaves as a mixed

system at high enthalpy with two fluid phases (vapor and liquid).

Hydrothermal alteration found in the core and drill cutting samples consists of a shallow argillic zone (O-600 m) with mostly zeolites, calcite and oxides, whereas at intermediate zones (600-1700 m), alteration style is propylitic (epidote, chlorite, calcite, quartz and sulfides). Calc-silicate alteration containing amphibole, garnet, clinopyroxene and bi- otite is found in deep zones of the system (> 1700 m) where temperatures exceed 320°C. Pyrite is the most ubiquitous sulfide mineral throughout the altered volcanic sequences. Andesitic and dacitic vol-

U. S. A.

MEXICO Gulf of Mexico

Ocean Mexico City 9

Trans-Mexican

-

Fig. 1. Location map of the Los Humeros geothermal system, Mexico.

102 R.G.M. Serrano et al. /Journal of Volcanology and Geothermal Research 73 (1996) 99- 118

0 1 2km 1 # I

0 alluvium and pyroclastics m

m olivine basalt

andesite and rhyodacite flows 7 B Xalapaco andesjtes

-. I”-“-^-1 Xalapazco basalts

l m basaltic-andesites (0.04 ma.)

m andesites (< 0.06 m.a.) -2C00’

Zaragoza ignimbrite and lithic tuffs (0.1 m.a.)

unconsolided rhyolite tuffs

fault

fracture

bore hole

contour line (in m)

Fig. 2. Geologic map of the Los Humeros caldera system, Mexico (modified after YaSez-G., 1980 and Ferriz and Mahood, 1984).

R. GM. Serrano er al. /Journal of Volcanology and Geothermal Research 73 (1996) 99-l 18 103

canic rocks located at depths between 1000 and 1900 m generally form the hydrothermal reservoir of the Los Humeros geothermal system.

Previous isotopic studies carried out by Gonzalez- P. (1985), Arnold et al. (1986), and Arnold and Gonzalez-P. (1987) showed that the aqueous sulfates and sulfides were in chemical and isotopic equilibrium at a temperature of 300°C even though the reservoir fluids have a low sulfur concentration (7.7 r&L/kg) and an alkaline pH at reservoir conditions. These authors found that the 634S values of the pyrite sampled in well Hl is O%O, and the sulfide (H,S) dissolved in the vapor phase of fluids has average 634S values of about - 12%0, whereas the sulfate (SO,) dissolved in the liquid phase of the system has 634S values of about + 14%0. The isotopic composition of the bulk sulfur (634Szs) calculated by Arnold and Gonzalez-P. (1987) was - 11%~ with a H,S/SO, ratio of 10 and these workers concluded that the pyrite sulfur was derived from a magmatic source, while the hydrothermal sulfur compounds, H,S and SO, of fluids collected in 1984, come from the regional Cretaceous limestone basement. These observations will be discussed later.

-2oj/r

2. Analytical material and methods

More than 150 drill-cutting samples, located at different depths from wells H12, H15, H16, H17 and H29 (Fig. 2), were selected for study on the basis of high sulfide content in the geothermal system. Exam- ination of polished samples in reflected light reveals mostly disseminated and vein pyrite mineralization with minor pyrrhotite and chalcopyrite. The pyrite occurs as fine-grained cubes (0.5 mm) disseminated in volcanic sequences, as variable-sized cubes or as porous compact masses in veins and cavities. Pyrrhotite is very rare and is sometimes associated with pyrite and magnetite at depths > 1700 m. Most of the pyrite in the five wells was found in rhyolitic rocks at depths ranging from 140 to 600 m, however the modal percentage of this sulfide decreases with increasing depth and increasing temperature gradient. Total sulfur in the cutting samples of the five wells ranges from < 0.02 to 4.0 wt.% (Martinez-S., 19931, reflecting a behavior similar to that of the modal percentage of pyrite observed in same sequences.

A total of 105 samples of pyrite (average weight of 15 mg), separated by hand-picking from drill-cut-

I 8 I . I ,

-30 -20 -10 0 10 20 30 40

6 CDT %o

Laboratory 6 2010 6 2olO standard measured CDT

217.2 -15.83 0.03 -5.85 0.01 GAV 18 -1.84 0.02 9.80 0.01 CdS 3.63 0.04 15.90 0.01 277.6 9.44 0.04 22.30 0.01 277.7 33.96 0.04 47.62 0.01 Minita -23.46 0.03 -13.69 0.01

y = (11.624 + 1.0741 X) R = 1.00

Fig. 3. Regression line obtained from laboratory standards used to calculate the 634S values of the samples with respect to the CDT

International standard (Canyon Diable Troilite), after Ault and Jensen (1963).

I04 R.G.M. Serrano et al./Joumal of Volcanology and Geothermal Research 73 (1996) 99-118

tings, were chosen for conventional sulfur isotopic 67 in very fine cubes and aggregates from veins and analysis. Of these pyrite samples, 38 were found in cavities. Pyrrhotite and chalcopyrite crystals were fine cubes disseminated in volcanic sequences, and not analyzed because of their very low concentration.

Table 1

Isotopic composition of pyrite from well cuttings, Los Humeros geothermal system

Depth Sample

Cm)

Sample weight 6’4s I o

hg) CM

Well HI2 Xalapazco area 800 pyrite in very fine cubes and aggreg. from veins

940 pyrite in very fine cubes and aggreg. from veins




I360 fine pyrite (< 0.6 mm) in disseminated cubes

1380 fine pyrite ( < 0.6 mm) in disseminated cubes

1480 tine pyrite ( < 0.6 mm) in disseminated cubes





1760 fine pyrite (< 0.6 mm) in disseminated cubes










Well HI5 Colapso Central area 40 pyrite in very fine cubes and aggreg. from veins

100(a) fine pyrite ( i 0.6 mm) in disseminated cubes

100(b) pyrite in very fine cubes and aggreg. from veins






800(a) pyrite in very fine cubes and aggreg. from veins

800(b) fine pyrite (< 0.6 mm) in disseminated cubes






I160 pyrite in very tine cubes and aggreg. from veins

1260 fine pyrite ( < 0.6mm) in disseminated cubes






2.80 I .44

5.70 2.37

2.40 3.34

7.10 I.41

3.00 I .57

6.20 1.90

7.20 2.34

6.30 2.20

6.70 2.47

6.40 I .69

6.20 4.36

6.70 2.05

6.30 I .98

7.20 I.16

6.30 2.13

6.20 1.32

6.60 I .46

6.30 2.08

6.70 I .58

7.20 I.61

7.90 I .75

6.20 I .28

2.7

2.8

4.1

4.1

3.9

3.9

4.1

3

4.1

4.3

4.7

4.2

4.1

4.6 4.4

3.6

3.4

5.4

4.9

5.7

2.3

- 0.75

- 1.79

- 2.09

- 1.32

- 0.37

0.19

-0.86

0.03

- 0.76

0.24

-0.90

I .23

- 0.85

- 2.47

- I .23

- 1.31

- 2.00

1.17

0.49

-0.41

-0.13

- 0.99

0.21

0.20

0.21

0.20

0.20

0.21

0.21

0.20

0.21

0.21

0.20

0.21

0.2 I 0.20

0.20

0.20

0.21

0.20

0.2 I

0.21

0.20

0.20

0.20

0.2 I 0.21

0.21

0.20

0.20

0.20

0.21

0.21

0.2 1

0.20

0.21

0.20

0.20

0.20

0.20

0.20

0.20

0.2 I

0.20

0.20

0.21

R.G.M. Serrano et al./ Journal of Volcanology and Geothermal Research 73 (1996) 99-1 I8

Table 1 (continued)

Depth Sample Sample weight Ps

(m) (mg) (%oc)

105

10

Well HI6 Colapso Central area 120 pyrite in very fine cubes and aggreg. from veins




















Well HI7 Colapso Central area 360 fine pyrite ( < 0.6 mm) in disseminated cubes






620 pyrite in very tine cubes and aggreg. from veins





1040 tine pyrite ( < 0.6 mm) in disseminated cubes





1760 pyrite in very tine cubes and aggreg. from veins

7.5

6.8

7.4

6.8

7.4

7.5

7.1

7.3

5.4

7.6

7.7

7.3

7. I 6.7

5.8

6.4

6.9

7.1

7.3

6.5 0.03

6.9 - 1.67

5 - 2.07

5.5 -0.73

6.1 - 0.57

5.7 -0.87

6.7 -0.19

5.4 0.37

4.5 0.92

4.9 0.35

3.2 -0.63

6.8 - 0.85

3.2 -0.57

2.2 0.16

5.3 0.70

1.3 1.71

2.5 - 1.40

-0.58

-5.41

2.92

-0.71

- 2.25

1.79

-0.29

-2.46

- 0.87

- 0.53

-0.19

-0.88

- 0.35

- 0.50

- 0.67

- 0.52

0.75

1.82

-0.69

- 0.67

0.20

0.21

0.2 1

0.2 I 0.20

0.20

0.20

0.21

0.21

0.20

0.20

0.21

0.20

0.21

0.20

0.20

0.2 I 0.20

0.20

0.20

0.21

0.20

0.21

0.20

0.20

0.20

0.20

0.20

0.20

0.20

0.21

0.20

0.20

0.21

0.20

0.20

0.21

The isotopic composition of sulfur from the SO, ids from wells at the exploration stage is difficult and H, S dissolved in present hydrothermal fluids and requires installation of correctly located sample was analyzed in samples obtained from more than 20 points in the pipeline through which a two phase exploration wells during individual well flow tests. mixture from the well is discharged (Mahon, 1961; The sampling of these sulfur compounds was carried Henley et al., 1984). In this case a mini-separator out in collaboration with technicians from the Comi- was used for the collection of a steam sample from sion Federal de Electricidad and from the Instituto de the two-phase discharge and a water sample was Investigaciones Electricas in Mexico. Sampling flu- taken from the weir box.

106 R.G.M. Serrano et al. / Journal of Volcanology and Geothermal Research 73 11996) 99-1 IX

Table I (continued)

Depth

Cm)

Sample Sample weight

(mg)

Well H29 Colapso Central area 160 pyrite in very fine cubes and aggreg. from veins




260(b) fine pyrite ( < 0.6 mm) in disseminated cubes


320(a) fine pyrite (< 0.6 mm) in disseminated cubes


340(a) fine pyrite ( < 0.6 mm) in disseminated cubes


360(a) fine pyrite (i 0.6 mm) in disseminated cubes



380(b) tine pyrite (< 0.6 mm) in disseminated cubes











It is assumed that virtually all H,S was parti-

tioned into the steam phase and that the SO, re-

mained in the liquid phase at flash temperatures

commonly observed (130°C) for the Los Humeros

geothermal system. Steam and non-condensable gas

passed through two traps in series, each consisting of fritted glass dispersion tubes immersed in a cylinder

containing a Pb(CH,COO), solution. This arrange-

ment yielded tiny gas bubbles, causing the H,S to precipitate as PbS. Flashed brine was collected in

polyethylene bottles from the weir box. The brine was diluted and acidified in the field to retard hy- droxide and silica precipitation. The resulting brine was treated in the laboratory with BaCl, to precipitate sulfate dissolved in the liquid phase (SO:- ) into barium sulfate (BaSO,).

Sulfur isotope analyses of sulfides and sulfates from the Los Humeros geothermal system were per- formed on a VG 602D mass spectrometer that was calibrated using sulfate and sulfide standards which

4.8

6.1

5.3

3.3

5.3

6.2

5

5.5

4.9

6.3

6

6.6

6.1

6.6

5.1

6.5

4.2

3.1

I .9

6.3

4.7

4.5

3.6

-0.85 0.2

-4.41 0.2 1

- 1.00 0.2 I -2.81 0.2

- I .44 0.2

- 0.25 0.2

-0.75 0.21

0.90 0.2

- 2.01 0.21

- 0.38 0.21

-0.85 0.22

- 0.72 0.21

- 0.73 0.2

- 1.33 0.2

- 1.88 0.21

- 3.25 0.2

- 2.05 0.2

1.13 0.2

3.10 0.2

I .03 0.2

1.24 0.2

1.71 0.21

2.52 0.2

1.89 0.2 1

were analyzed in different laboratories (e.g., BRGM

laboratory, Orleans, France). Using the calibration results we constructed a regression line (Fig. 3) to

calculate the isotopic composition of each sample in

according to the CDT international standard (Ault

and Jensen, 1963).

Pure pyrite crystals were finely ground and from 6 to 10 mg of pyrite powder was mixed with 300 mg

of CuO. The sulfide-CuO mixture was put into a

quartz tube blocked by two quartz wool stoppers and melted for 10 minutes at 1100°C in a gas separator

line to purify the SO, which was then ready for isotopic analysis (Grinenko, 1962; Robinson and Kusakabe, 1975; Coleman and Moore, 1978).

The sulfide samples (H,S) from the fluids precipitated as PbS was converted into SO, by means of the same technique that was used for pyrite. The sulfate samples (BaSO,) were converted into SO, using the technique described by Holt and Engelke- meir (1970). In this technique sulfate is mixed with

R.G.M. Serrano et al. /Journal of Volcanology and Geothermal Research 73 (1996) 99-l 18 107

Table 2

Isotopic composition of sulfides and sulfates in the present-day

fluids from Los Humeros geothermal system

Well T a 634s 634S A so4 u2s Well location _

(“0 H,S SO,

(%o) (%0)

Hl 300 -2.95 11.93 14.88 Los Humeros Fault

H2 250 - 2.25 Xalapazco area

H8 290 0.34 12.56 12.22 Los Humeros Fault

H9 280 -2.13 1.69 3.82 Colapso Central a.

HlO 260 7.7 7.7 Los Humeros Fault

Hll 285 1.41 13.55 12.14 Los Humeros Fault

H12 290 0.5 8.96 8.46 Xalapazco area

H13 270 -0.32 5.8 6.12 Los Humeros Fault

H15 211 - 1.42 Colapso Central a.


H18 230 - 1.56 Maztaloya Fault

H20 240 0.16 Colapso Central a.

H22 203 0.28 Los Potreros

H23 150 0.46 Las truces Fault

H24 248 0.63 7.09 6.46 Los Potreros F

H27 142 1.27 Las Papas Fault

H28 199 1.21 7.7 6.49 Colapso Central a.

H29 2.75 Colapso Central a.

H30 217 0.21 Colapso Central a.

H31 269 0.71 11.02 10.31 Colapso Central a.



a Reservoir fluid temperatures based on adiabatic quartz and

Na-K-Ca geothennometers and downhole logging (Martinez-S.,

1993 and Barragan et al., 1991).

pure quartz powder and immediately put it into a quartz tube. This tube is fixed to a gas separator line where the mixture is melted by means of a blowtorch at a temperature above 15OO”C, producing SO, ready for isotopic measurement.

3. Results and discussion

Sulfur isotope data for pyrite crystals, and for dissolved sulfide and sulfate are presented in per mil relative to the Canyon Diable troilite standard in Tables 1 and 2, respectively, and summarized in Fig. 4. The accuracy of the measurements is better than * 0.20%0.

To study the difference between fine-grained disseminated cubes and massive pyrite from veins and cavities, we compared the 634S values of these two types of pyrite from a single specimen collected

from the 320 m level of a well H-29 in Colapso Central (Table 1). In this sample the 634S values of disseminated cubic pyrite is slightly more negative (a34 S = - 0.75100) than that of massive or xenomorphic pyrite (634S = +0.90%0). Disseminated and massive pyrite from other levels in same well show a similar difference in 634S values, nevertheless, the mechanical separation of cubic disseminated pyrite from massive pyrite is very difficult, so we were unable to test all our samples for this results. We consider that the difference observed in S34S may reflect the relative time and duration of crystalliza- tion of each crystal under slightly different physicochemical and isotopic conditions.

Fig. 4 shows the histograms of the 634S values of pyrite from the five wells of the geothermal field, the 634S values of sulfate and sulfide found in present- day fluids and previous data from Arnold and Gon- zalez-P. (1987). The 634S values of pyrite and the H,S from the fluids analyzed in this work are very similar and range from - 4.5 to + 4.5%0, averaging about +0.02%0. However, the 634S values of pyrite from wells H15, H16, H17 and H29 (Colapso Cen- tral area) are slightly more negative than those of the same phase at well H12 located near the Xalapazco area (Fig. 4). The average 634S values of pyrite from the Colapso Central area is -0.4%0 whereas in the Xalapazco area the average S34S values is + 2%0. This difference may reflect the higher temperatures and higher degree of thermal and chemical disequilibrium in the Colapso Central area than in the Xalapazco area.

The 634S values of sulfates dissolved in the liquid phase are variable, ranging from + 1.7%0 to + 13.5%0 (Fig. 4). These isotopic values of the sulfates are not directly related to the location of the different wells where the fluids were sampled. The variability of these results will be discussed later on.

The near - 0.02 k 4.5 per mil 634S values found for most of the pyrite in this study are similar to 634S values for H,S (- 0.2 + 3.0 per mil) in the present-day geothermal fluid, suggesting that most of the pyrite precipitated sulfur that is in isotopic equilibrium with similar geothermal fluids. Pyrite was probably precipitated by cooling or boiling of fluids, involving loss of aqueous H,S and H, to the steam phase, causing a pH rise and an increase in the oxidation state of the residual fluid. These changes

108 R.G.M. Serrano et al. / Journal of Volcanology and Geothermal Research 73 (1996) 99-118

Pyrite well H12

Pyrite well H15

Pyrite well H16

Pyrite well H17

Pyrite well H29

H2S gas all bore holes

I Sulfates all bore holes

I a a mm 111”1111111111111)~1’~~’

F

-8 +15

’ Pyrite , sulfates and sulfides from wells Hl and H4

%o 634s k?J H2S Gas W Pyrite n Dissolved sulfates

Fig. 4. Histograms of isotopic composition (S34S) of sulfates and sulfides from Los Humeros geothermal system. Results from bore holes

Hl and H4 are from Arnold and Gonzalez-P. (1987). The F34S-average values of pyrite are: well H12 = + 2%0, well HI5 = -0.7%a, well

H16 = -0.3%~ well H17 = -0.3%0 and well H29 = -0.5%0.

R.G.M. Serrano et al./Journal of Volcanology and Geothermal Research 73 (1996) 99-118 109

produced a sligh fractionation of sulfur isotopes and could also be recorded by minerals formed during boiling.

The average sulfur isotopic composition of pyrite determined in this work is very similar to that determined by Arnold and Gonzalez-P. (1987) for the same phases. However, the 634S values of H,S from the present-day fluids are completely different from the results obtained by these workers for the same sulfide solution (Fig. 4). Moreover, the 634S values for sulfates in present-day fluids span a wider range than the data of Arnold and Gonzalez-P. (19871, but there is also some overlap between the two data sets at high 634S values.

and Gonzalez-P. (1987). The isotopic composition of sulfur has not been determined in the calcareous sediments of the basement of the Los Humeros geothermal system. However, estimations and gen- eral data for sedimentary sulfur (Ohmoto and Rye, 1979) suggest that sulfides and sulfates of marine origin for the Late Cretaceous have 634S values of - 25 per mil and + 22 per mil, respectively.

3.1. Chemical equilibrium in the present-day fluids

Taking into account the differences between the S34S values of dissolved sulfates and sulfides and considering an isotopic fractionation factor of 22%0 between sulfates and sulfides at 300°C (average temperature of the geothermal reservoir), Arnold and Gonzalez-P. (1987) concluded that the isotopic ratios between these phases showed chemical and isotopic equilibrium conditions. These authors also stated that disseminated pyrite in the rock formations could not have formed from a hydrothermal fluid with a bulk sulfur source (634S,s) approaching - 11%0, but could be the result of the addition of sulfur removed from sedimentary rocks of the substratum.

Before proposing any interpretation of the isotopic results obtained in this work, it is necessary to verify the existence of chemical equilibrium between sulfates and sulfides dissolved in the present-day deep fluids of the geothermal reservoir. Therefore, we constructed different diagrams of the pH-fO,- (fS,> type in order to represent the stability fields of various iron oxides and sulfides present at deep levels of the geothermal system. For calculation and construction of pH-fO,-(fS,) diagrams we followed the methods proposed by Garrels and Christ (1965).

However, in geothermal systems hosted by volcanic rock sequences, it is unusual to have negative 634S values from the H,S vapor (McKibben and Eldridge, 1989). We believe that the alteration style at the Los Humeros geothermal system indicates that fluids have reacted and leached sulfur and metals from the host volcanic rocks, not from the underly- ing sedimentary basement rocks as stated by Arnold

The fluids discharged from most exploration wells have undergone important dilution phenomena and the addition of an important vapor mass rich in gas (excess vapor between 10 and 90%). Nevertheless, to determine the chemical conditions of the deep fluids, we used data from well Hl, sampled on October 1988. Chemical data of the fluids calculated at the reservoir conditions (Henley et al., 1984) from well Hl are shown in Table 3. According to these results we can state that the fluids coming out of the well have not been significantly affected by either introduction of excess vapor or by dilution phenomena. Hence, we consider that the chemical composition

Table 3

Concentration of major components in water discharged and gas concentration in steam separated from bore-hole Hl

Enthalpy pH 25°C Na+ K+ Caf+ Cl- Mg’+ SiO, SO,-- HCO; % gas/ CO, SH, NH, H, CH, N, steam

(J/g H,O) (m&g) (mM/mole total gases)

1280 7.9 269 49 1.2 214 0.01 799 114 361 8.772 963 29.5 3.7 1.3 0.59 2.32 Concentration of major componenents in the fluid feeding well Hl * or reservoir conditions (mMole/kg)

7.6 0.82 0.02 3.9 0.001 8.17 0.78 3.88 161 4.94 0.72 0.22 0.1 0.39

* For the calculations a mass balance equation of the following type was used: CreSerVOir = y C,,,,, + (1 - y) C,iquid (after Henley et al.,

1984) where Creservoir = concentration of the component in the fluid reservoir; Cliquid = concenaation of the component in the liquid phase;

C Steam = concentration of the component in the steam phase, and y = steam fraction, for bore-hole HI that is 40% steam and 60% liquid.

110 R.G.M. Serrano et al./ Journal of Volcanology and Geothermal Research 73 (1996) 99-118

calculated is representative of the initial reservoir,

and then we can assume that physicochemical ratios

between sulfates and sulfides for temperatures above 300°C show chemical and isotopic equilibrium con-

ditions.

On the other hand, the main alteration minerals

are distributed in bands parallel to the present

geothermal gradient in volcanic rocks of the Colapso

Central-Xalapazco area. The existence of hematite, pyrite and minor magnetite at depths between 150

and 600 m indicates strongly oxidizing fluids at this

depth. Below 1600 m, relatively large amounts of

pyrite, xenomorphic pyrrhotite and some well-crystallized magnetite occur. This assemblage indicates

relatively reducing conditions at depth. The pH at

reservoir conditions was estimated by means of a

computer program based on data from Henley et al. (1984) and Amorsson et al. (1982), and this is close

to 8. A summary of the conditions under which

pH-fO,-(fS,) diagrams were calculated for the flu-

ids in well HI, as well as the chemical reactions and thermodynamic constants used for this purpose, are

shown in Table 4.

Fig. 5 represents the pH-fO,-(~5,) diagram cal-

culated for data at well Hl. We can clearly observe

in this diagram that the chemical equilibrium be-

tween the measured mH,S/mSO~- ratios in the

present-day fluids and the combination pyrite-pyr-

rhotite-magnetite, at a pH = 8, is not attained (point

B in Fig. 5) as long as mH,S/mSOi- = 6 does not touch the theoretical triple point of coexistence of

these minerals (point A in Fig. 5). This same tech-

nique was used by Arnold and Gonzalez-P. (1987) for samples from well Hl. They determined that the fluids were in chemical equilibrium with the pyrite-

pyrrhotite-magnetite assemblage at a slightly alkaline

pH. It is important to mention that these authors

sampled the fluids in 1984. At that time, there were

only four exploration wells in the geothermal field

and the reservoir fluids had not been altered significantly. However, in 1988 when we sampled the fluids there were more than 20 exploration wells and, consequently, the physicochemical conditions of the fluids had been greatly modified. Physicochemical data of the geothermal fluids, at the present-day, confirm that chemical equilibrium does not exist between these fluids and secondary mineral phases at the reservoir conditions (Martinez-S., 1993).

Table 4

Chemical reactions and thermodynamic constants used for calcu-

lating the pH-fO,-(fS,) diagram for well HI from the Los

Humeros geothermal system

T = 300°C

I = 0.01

mXS = 0.006 moles/kg H,O

ratio (mH,S/mSO,) = 6

Chemical reactions for the following equilibrium lines:

H,S=SH-,H,S=HS0,-,HS0,-=S0,2-,SH-=S0,Z-2

tog K,

l)H,S,,=H+SH-

2) HS04- = H+ +SO;’

3) 2H+ +SO; ’ = HaO,, +20,

4) H& + ;O- = H,O,, + t/2&,

Mineral equilibrium reactions

5) 2Fe,O, + k_ = 3Fe,Oa

(magnetite-hematite)

-8.31 a

-7.06 a

- 48.55 a

- 13.75 a

- 14.35 1

6) FeSz = FeS + iS1 (pyrite-pyrrhotite)

7) 3FeS, + 202 = Fe,O, + 35,

(pyrite-magnetite)

-11.63”

- 33.93 a

8) 3FeS + 202 = Fe,O, + iS2

(pyrrhotite-magnetite)

-51.39 3

9) 2FeS, + iO1 = Fe,O, +2S,

(pyrite-hematite)

-28.19’

K, = equilibrium constant at 300°C.

a Helgeson ( 1969). I

Ionic strength: I = 5 xrn,Z: where: Zm, = molar concentration

of component (i) and Zi = charge of (il. Activity coefficient: Debye-Htickel equation: -log y, =

(Az,l”2/l +(i),B1”2). A and B are solvent parameters.

(L) = effective ionic diameter.

‘y = activity coefficient.

The development of the field must be the main reason for the lack of chemical equilibrium between the 1988 deep fluids and their mineral assemblages. If the residence time of fluids decreases due to an accelerated circulation of these fluids, then the reactions will be irreversible and equilibrium conditions

will not exist (Sakai, 1983). Ohmoto and Lasaga (1982) suggest that the minimal time required to attain 90% equilibrium between aqueous sulfates and

R.G.M. Serrano et al. / Journal of Volcanology and Geothermal Research 73 (1996) 99-118 111

sulfides in a hydrothermal fluid containing CS = 10e2 moles, at 300°C and a pH of 7, is 140 days. However, in the case of Los Humeros the total sulfur concentration is < 10m3 moles and the temperature range varies from 250 to 310°C producing a de- crease in the rate of sulfate-sulfide equilibration by several orders of magnitude, taking up to lo3 years. Bore-holes at Los Humeros discharge an average volume of 7 tons/h of liquid and 50 tons/h of a vapor and gas mixture. These values suggest that the residence time for the present-day fluids is short (< 1000 years) and chemical equilibrium between sulfates and sulfides is not attained.

No tritium data nor any other studies on fluid circulation exist, but the fluid discharge volume can be used as an indicator of fluid residence time.

log fO2

HSO4 -24

-32

-36

Tello-H. (1992) presented stable isotope data on over 16 vapor samples at the Los Humeros geothermal system. The oxygen and hydrogen isotopic results show that recharge to the geothermal reservoir comes from meteoric precipitation and slow infiltration of ground water. Deep reservoir waters showed I80 enrichment of 4 to 8 per mil, suggesting rock-water isotopic exchange in a high-temperature environment (Tello-H., 1992).

Although we demonstrated the lack of chemical equilibrium between the mineral phases and the fluids presently found in the system, we considered that primary fluids derived from the major reservoir, before 1984, were in chemical equilibrium under reducing conditions and at a pH that ranged from alkaline to neutral. This consideration is based upon

Sampling date: 1988

0 2 4 6 8 lb 1.2 Fig. 5. A pH-logfO,-(fS,) diagram for selected iron minerals that indicate the physicochemical conditions of the deep fluids for bore hole

Hl. The chemical reactions and thermodynamic constants used for this diagram are show in Table 4. Chemical equilibrium between the

measured mH,S/mSO:- ratios and the combination pyrite-pyrrhotite-magnetite is not attained because the mH, S/mSO~- measured at a

pH = 8 (Point B) does not touch the theoretical triple point of coexistence of these mineral phases (point A).

112 R.G.M. Serrano et al./Journal of Volcanology and Geothermal Research 73 (1996) 99-118

the fact that mineral phases of the pyrite-pynrhotite- magnetite type that exist at depths of over 1600m are formed under such physicochemical conditions.

3.2. Isotopic equilibrium among sulfur compounds

Fig. 4 shows that the average 634S value of pyrite sampled at different depths in the volcanic sequences at Los Humeros is -0.02%0 whereas that of H,S contained in the vapor phase from different wells is -0.2%0. Taking into account the value of the isotopic fractionation factor between H,S and pyrite at 300°C (isotopic fractionation factor = 10” In

@HZS-PY = - 0.1%0, according to Ohmoto and Rye, 1979) and the difference in isotopic composition observed among sulfur compounds found at the Los Humeros geothermal system (S34SH2s - S34Spyrite = -0.2%0), we consider that the pyrite crystals sampled were formed by the circulation of a hot fluid with a sulfur isotopic composition similar to the one we found in this study.

We calculated the bulk (634S,s) isotopic composition of the sulfur compounds (sulfates and sulfides) contained in present-day fluids of the system by means of Ohmoto’s (1972) equation which does not take into account the equilibrium or disequilibrium states of the fluids of the given system. The Ohmoto equation is written as:

634Szs = 634Ssoam ’ x,,j- + 834SH,S . XHzS (1)

where 634S and 634S

Zs = bulk isotopic composition; 634Sso;m n,s are measured sulfur isotopic compos-

tion of sulfates and sulfides, respectively; and X,,:-

and XHZS are mole fractions of sulfates and sulfides. Considering the results obtained at well H 1 (Table

3) the mole fraction of the sulfates and sulfides is calculated from equations:

x,,,- = mzm-

mCH,S + mCSO,2- 1 and

X mCH,S

H,S = mZH,S + mZSO,2- 1

(2)

(3)

where m = molality and, therefore, Xso:- = 0.137 and XHZS = 0.863.

The average isotopic compositions measured for the sulfates and sulfides found in the fluids of well Hl are S”4Sso;- = +12%0 and S34SHZS = -3%,,. Therefore, the isotopic composition of the bulk sulfur source of the system calculated from the above data (eq. 1) is - l%o without taking into account that the sulfur phases found in the fluids do not show chemical equilibrium conditions.

The above results suggest that sulfur at the Los Humeros system was derived from a magmatic source type, or that it may have been derived from sulfur leaching of volcanic rock sequences. We find no evidence for a sedimentary source of sulfur as sug- gested by Arnold and Gonzalez-P. (1987).

An attempt was made to determine whether the isotopic composition of the sulfur compounds dissolved in the fluid showed conditions approaching isotopic equilibrium. In order to do this, we used Arnold and Sheppard’s equation (198 1) to calculate the theoretical equilibrium isotopic composition of dissolved sulfates and sulfides which would result from isotopic fractionation of a sulfur source whose S34SIs = - 1%0. The equation provided by Arnold and Sheppard (1981) is:

Sx4Si = S34SZs + lo3 In ai_, ’ .CX, (4)

where Xj and lo3 In c~_, are the mole fraction of the j component and the isotopic fractionation factor between i and j components, respectively, S34Si is the isotopic composition calculated for component i and d34SSs is the bulk composition of the sulfur source of the system. Normally, at temperatures above 300°C and in neutral or slightly alkaline hydrothermal fluids, the dominant sulfur compounds are H,S, ZSOi-, and SH-. However, since the isotopic fractionation factor between H, S and SH- is small ( < 1%0) (Ohmoto and Rye, 1979) we can write the equation (4) as follows:

634S Zso;m = 6”4SZs - lo3 ln an+so: ‘Xn$ (5)

s34s =nZS = S34Szs + lo3 In oHH,s_so;- . Xs,:-

(6)

The isotopic fractionation factor (lo3 In CL H,s _ so:- ) at 300°C between sulfides and sulfates is - 22%0 according to Sakai (1968). Therefore, the

R.G.M. Serrano et al./Joumal of Volcanology and Geothermal Research 73 (1996) 99-118 113

theoretical isotopic composition of the sulfates and the present-day fluids or pyrite crystals. There are sulfides from well Hl is: several possible explanations of the 634S variability:

834SBso:- = - 1 + 22.0.863 = + 18%0

634S PH,S = -1 -22.0.137 = -4%0

The theoretical values of 834 SCH,s are very simi-

(1) The S34S variability of the sulfates might be due to the lack of representation of the collected samples due to their low concentration in the liquid phase.

lar to the measured values of - 3%0 from well Hl, this being an indication that the dominant sulfur source of the system is probably magmatic. How- ever, the theoretical 634Szso~- (18%0) are much larger than the measured values of 12%0 from fluids at the present-day in the same well. This suggests the absence of isotopic equilibrium for sulfur in the current system.

(2) The existence of complex fluid mixing phenomena may explain such isotopic variability. For example, the mixing of hot geothermal water with cold meteoric water containing some sulfate from different sources will produce variable isotopic values.

(3) Condensation of a vapor phase rich in H,S, at shallow depths, which is derived from a major reservoir, may also explain the observed 634S values.

We made similar calculations to determine the chemical behavior of the sulfate-sulfide ratio in the fluids of other wells, but because of the existence of excess enthalpy and vapors rich in H, S at the discharge of the wells, the conditions necessary to show the existence of chemical and isotopic equilibrium between the phases are not fulfilled.

Sulfate sampling in exploration wells was carried out carefully in fluids discharged at the weir box conditions so, even though there are low concentrations of sulfate in fluids from the Los Humeros geothermal system, we consider that sulfate samples are representative of sulfate dissolved in geothermal fluid. Hence, the first explanation was not considered.

3.3. Discussion of the S34S values of sulfates at Los

Humeros

Fractionation of sulfur isotopes in a hydrothermal fluid can occur in a number of situations such as: hydrothermal separation of a fluid from a magma, leaching of primary sulfur compounds, cooling of hydrothermal fluids, fluid boiling processes or during precipitation or replacement of primary minerals (Ohmoto, 1972; Ohmoto and Rye, 1979). In addition, isotopic fractionation of sulfur species is also a function of temperature, pH, the overall sulfur concentration, oxygen fugacity (fOJ, and of the residence time of the fluids in the system. At Los Humeros, we suggest that boiling and condensation of vapor rich gas into meteoric waters cause different isotopic fractionation among sulfur compounds.

Chemical data from the geothermal fluids suggest the existence of some mixing phenomena at Los Humeros, such as the mixing of geothermal fluids with cold meteoric water, nevertheless, low sulfate concentration in collected fluids from wells at the Los Humeros indicate a small amount of mixing of meteoric water sulfur and magmatic sulfur. The sec- ond explanation may not be important to explain the sulfate 634S variability,

Fig. 4 shows that the 634S values of sulfates dissolved in the fluids from different wells are variable, ranging from + 1.7 to + 13.5%0 and showing no relationship between isotopic values and the location of sample wells in the geothermal field. 634S values of sulfates of wells H9, H12, H13, H24 and H28 tend to have values similar to those of H,S in

The principal mixing phenomenon at the Los Humeros system is condensation of a vapor phase rich in H,S at shallow levels into meteoric water or geothermal water. Physicochemical characteristics in the present-day fluids suggest that part of the H?S in the vapor phase is rapidly oxidized when it comes into contact with relatively oxygenated meteoric water, thereby producing sulfate waters. When H,S condensation occurs, slightly acidic waters attack the surrounding rocks, producing several clay phases such as the kaolinite or montmorillonite-Ca type. Similar clay phases were observed in the volcanic sequences located at depths below 600 m at the Los Humeros system.

Since the oxidation of H,S occurs much faster kinetically than the reduction of sulfate to sulfide,

114 R.G.M. Serrano et al. / Journal of Volcanology and Geothemal Research 73 (1996) 99-118

the irreversible oxidation of H,S enriches the sul-

fates in 32S. Therefore, the 634S values of sulfates in

the present-day fluids may be due to mixing of sulfate derived from the oxidation of H, S and sul-

fate a reservoir at depth. The 634S values of sulfate

will be shifted towards those of H,S in function of

the degree of oxidation.

Ohmoto and Rye (1979) and Sakai (1983) demon-

strate that kinetic effects control the isotopic compo-

sition of a sulfur compound produced by the oxida-

tion of another sulfur compound. The molecules

containing the lightest isotopes will act faster than

molecules containing the heaviest isotopes, thereby enriching the reaction products with light isotopes in

the case of a simple chemical system. The oxidation

of sulfides to sulfates can be represented by the following reactions:

H32S !+ 32S02m 2 4

H34S 2 34so2- 2 4

The ratio between the equilibrium constants of the

above reactions (k,/k,) is called the isotopic kinetic

effect. For example, if the value of this ratio is 1.005

(Nakai and Jensen, 1964), this implies that the HP S species will undergo transformation 5 per mil faster

than the HFS species. Therefore, the resulting sul-

fate will have a s4S deficiency of 5%0 with respect to

that of the residual H,S. It is important to mention

that the k,/k, ratio is also considered the instanta-

neous isotopic enrichment factor which is indepen-

dent of the number of chemical reactions leading to

the final product. Fig. 6 shows the isotopic variations of the sulfur

compounds (H,S and SOi- ) as a function of the

degree of oxidation obtained by a hydrothermal fluid.

Ohmoto and Rye (1979) considered a fluid with the

following initial characteristics: I = 250°C initial isotopic and chemical equilibrium between sulfates and sulfides with 634S values of 25%0 and O%O, respectively. The value of the isotopic fractionation factor for both species at 250°C is 25%0. If H,S is partially oxidized to SO:-, we can observe that the 634S of the H,S will be very close to O%O a few moments before mineral formation occurs, whereas the 634S of the sulfates produced will show values

-2 -

H2S so;

-3 I -10 0 10 20 30

PS%o

Fig. 6. Isotopic relations between coexisting sulfate and sulfide

species as a function of SSO:-/ SH,S, for the non-equilibrium

oxidation conditions of H,S (after Ohmoto and Rye, 1979).

under 25%0, depending on the oxidation degree at-

tained by the initial fluid. Finally, we suggest that the 6”jS of the Los

Humeros sulfates is the result of relative oxidation of H,S to SO:-, and a minor addition of sulfate-bearing

fluids from other sources. Consequently, the third

hypothesis provides a satisfactory yet uncomplicated explanation for the isotopic data obtained from sul-

fates at Los Humeros.

3.4. 6.‘“S variations of pyrite with depth

The a3’S values of several pyrite samples were studied to determine whether there were variations

with depth. This analysis was carried out in order to

determine the correlation between the 634S of pyrite, lithological composition, temperature of the system and f0,. A similar study by Sakai et al. (1980) demonstrated the relations between the 634S values of different sulfur phases and the lithology of several geothermal systems in Iceland, the temperature of the deposit, and the introduction of marine sulfates.

Fig. 7 shows the 634S values of pyrite sampled in five wells at Los Humeros (H12, H15, H16, H17 and

R.G.M. Serrano et al./ Journal of Volcanology and Geothermal Research 73 (1996) 99-118 115

H29) and the lithological characteristics found in each one. The 634S values of pyrite are relatively constant with respect to depth (with a maximum variation of 2%0X This figure also shows that the lithological composition does not have an influence on the 634S values measured in pyrite, nor does an increase in the temperature gradient seem to modify the isotopic composition of pyrite at different depths.

Although we did not determine whether sulfur compounds in the present-day fluids are in chemical and isotopic equilibrium conditions at the Los Humeros geothermal system, we made the following observations:

(1) Certain irregular variations in the isotopic

composition of pyrite were observed at depths of 1000 to 1700 m in the volcanic rock sequences where the highest concentrations of high-temperature alteration minerals (amphiboles, clinopyroxene, gar- nets, micas and epidote) exist. In particular, the pyrite sampled in wells H17, H15 and H16, at the depths mentioned above, has a 634S value which ranges from - 2%0 to + 1.5%0. Such isotopic variations are likely to be the result of an irregular isotopic fractionation produced by the boiling of fluids at these depths (Fig. 7). Fluid inclusions studies carried out by Prol-L. (1988) and Gonzalez-P. et al. (1992) show biphasic inclusions (liquid and vapor) coexisting with vapor-rich inclusions in the

HI7 24s

HI5 634S HI6 S34S

H29 &34S H12 L534S

-2 0 +2 -2 0 +2 -3 0 +3 -5 0

unconsolidated tuff welded tuff

andesite

rbyolitic tuff

unconsolidated tuff

andesite

and&tic tuff gfanodiorite

Fig. 7. ?i”S values of several pyrite samples from five wells with respect to depth and lithological composition. Note the irregular variations

in the isotopic composition of pyrite where the highest concentrations of alteration minerals and highest concentration of pyrite are found.

116 R.G.M. Serrano et al./Journal of Volcanology and Geothermal Research 73 (1996) 99-118

same event. These two types of fluid inclusions were

observed in hydrothermal calcite and quartz samples

at depths of between 850 and 1750 m. These inclu-

sions may indicate the existence of boiling in the

hydrothermal reservoir.

(2) The highest percentage of pyrite in the

geothermal system was registered at depths of 100 to

600 m in wells H17, H15, H16 and H29. Abundant

clay minerals, calcite, some zeolites, some mag-

netite, and hematite were observed at these depths. The 634S of pyrite obtained at these depths varies

from - 3%0 to + 3%0. These 834 S variations can be

explained by the existence of deep fluid-meteoric water mixing or by boiling of geothermal fluids and

precipitation of pyrite at shallow depths. Ohmoto

(1986) and McKibben and Eldridge (1990) suggest that the isotopic composition of mineral sulfides is

strongly affected by fractionation processes such as

boiling in hydrothermal systems.

We suggest that the large S34S variations in pyrite

were controlled by local variations of pH, f02,

pressure, temperature, sulfur concentrations in the fluid, and duration of fractionation process (crystalli-

zation time). Since a pyrite-pyrrhotite-magnetite as-

sociation exists at > 1700 m in most exploration

wells, we also consider that the primary hydrothermal fluids showed a reducing character where chem-

ical and isotopic equilibrium conditions existed be-

tween the dissolved sulfur compounds and sulfide minerals (vein and disseminated pyrite). The S34S

values of these sulfides showed little variation. How-

ever, as the fluids rise they are oxidized by boiling

which causes isotopic fractionation and variation

among sulfur compounds, consequently, the 634S of the sulfides crystallized will show great variations. Mixing or condensation of vapor enriched with gas

in meteoric waters can also cause variations in the isotopic compositions of the sulfates and sulfides

formed.

4. Conclusions

S”4S isotope data for more than 105 pyrite sam-

ples suggest the existence of more than one pyrite formation event at the Los Humeros geothermal system. However, the isotopic variations among the different generations of pyrite are relatively small.

The greatest variations in sulfides were found at

depths where boiling or fluid mixing phenomena

probably control the fractionation of sulfur isotopes.

The unstable physicochemical conditions of fluids

(temperature, pressure and composition) at the Co-

lapso Central area, with respect to those at the

Xalapazco area have caused slight changes in the

isotopic composition of pyrite sampled at both sites

and result from chemical reactions and isotopic frac-

tionation being carried out in different ways.

The sulfur compounds contained in the vapor

phase (H 2 S), in the liquid phase (SO:- ), and in the crystallized pyrite of the different volcanic units at

Los Humeros were derived from a common sulfur

source whose calculated value indicates a magmatic

origin (S34SZs = - l%o>. No isotopic study of sulfur

in the Late Cretaceous calcareous basement exists, but we assume a sulfate isotopic composition of

+22 per mil.

The 6j4S values of sulfates and sulfides in the

present-day geothermal fluids suggest that the fluids

of the system have evolved from a state of chemical and isotopic equilibrium in 1984 (Arnold and Gonza-

lez-P., 1987) to one of disequilibrium in 1988. The

lack of chemical and isotopic equilibrium in the

present system may result from drilling and intensive exploitation of the Los Humeros geothermal system.

Due to this activity, the residence time of geothermal fluids decreases in the system and upsets the chemi-

cal and isotopic equilibria. In addition, secondary

processes of boiling and mixing contribute to the chemical and isotopic modifications of sulfur com-

pounds. The important variations in the isotopic composi-

tion of sulfates found at present at Los Humeros and their tendency to have values similar to those of the

sulfides suggest that those sulfates are formed by

shallow ( < 600 m) oxidation of H,S contained in the vapor phase. This oxidation causes irreversible

chemical reactions that fractionate kinetically the sulfur isotopes, producing sulfates that are enriched in 32S with respect to the reactants.

Acknowledgements

The authors wish to express their gratitude to the Comision Federal de Electricidad and the Instituto de

R.G.M. Serrano et al./ Journal of Volcanology and Geothermal Research 73 (1996) 99-l 18 117

Investigaciones Electricas, Mexico for providing cutting samples and fluid sulfur compounds from the Los Humeros system. We are very grateful to Bar- bara Martiny for constructive review of the manuscript. We are also grateful to Oscar Campos- Enriquez and Peter Schaaf for comments on an early draft of the manuscript. The final manuscript was improved substantially by two JVGR reviewers. Fi- nally, we thank the Consejo National de Ciencia y Tecnologia (CONACYT) for supporting research on the Los Humeros Geothermal System.

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The δ 34S composition of sulfates and sulfides at the Los Humeros geothermal system, Mexico and...

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