Hydro-geochemical and isotopic fluid evolution of the Los Azufres geothermal field, Central Mexico

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1 Applied Geochemistry Applied Geochemistry 20 (2005) Hydro-geochemical and isotopic fluid evolution of the Los Azufres geothermal field, Central Mexico E. González-Partida a, *, A. Carrillo-Chávez a, G. Levresse a, E. Tello-Hinojosa b, S. Venegas-Salgado b, G. Ramirez-Silva b, M. Pal-Verma c, J. Tritlla a, A. Camprubi a a Centro de Geociencias, UNAM, Campus Juriquilla, A.P. 15, Juriquilla, Qro., México b Comisión Federal de Electricidad, A.P. 31-7, C.P Morelia, Mich., México c Instituto de Investigaciones Electricas, A.P , C.P Cuernavaca, Morelos, México Received 30 August 2002; accepted 19 July 2004 Editorial handling by H. Ármansson Abstract Hydrothermal alteration at Los Azufres geothermal field is mostly propylitic with a progressive dehydration with depth and temperature increase. Argillic and advanced argillic zones overlie the propylitic zone owing to the activity of gases in the system. The deepest fluid inclusions (proto-fluid) are liquid-rich with low salinity, with NaCl dominant fluid type and ice melting temperatures (T mi ) near zero (0 C), and salinities of 0.8 wt% NaCl equivalent. The homogenization temperature (T h ) = 325 ± 5 C. The boiling zone shows T h = ±300 C and apparent salinities between 1 and 4.9 wt% NaCl equivalent, implying a vaporization process and a very important participation of non-condensable gases (NCGs), mostly CO 2. Positive clathrate melting temperatures (fusion) with T h = 150 C are observed in the upper part of the geothermal reservoir (from 0 to 700 m depth). These could well be the evidence of a high gas concentration. The current water produced at the geothermal wells is NaCl rich (geothermal brine) and is fully equilibrated with the host rock at temperatures between T = 300 and 340 C. The hot spring waters are acid-sulfate, indicating that they are derived from meteoric water heated by geothermal steam. The NCGs related to the steam dominant zone are composed mostly of CO 2 (80 98% of all the gases). The gases represent between 2 and 9 wt% of the total mass of the fluid of the reservoir. The authors interpret the evolution of this system as deep liquid water boiling when ascending through fractures connected to the surface. Boiling is caused by a drop of pressure, which favors an increase in the steam phase within the brine ascending towards the surface. During this ascent, the fluid becomes steam-dominant in the shallowest zone, and mixes with meteoric water in perched aquifers. Stable isotope compositions (d 18 O dd) of the geothermal brine indicate mixing between meteoric water and a minor magmatic component. The enrichment in d 18 O is due to the rock water interaction at relatively high temperatures. d 13 C stable isotope data show a magmatic source with a minor meteoric contribution for CO 2. The initial isotopic value d 34 S RES = 2.3&, which implies a magmatic source. More negative values are observed for shallow pyrite and range from d 34 S (FeS 2 )= 4& to 4.9&, indicating boiling. The same fractionation tendencies are observed for fluids in the reservoir from results for d 18 O. Ó 2004 Elsevier Ltd. All rights reserved. * Corresponding author. Fax: address: egp@geociencias.unam.mx (E. González-Partida) /$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi: /j.apgeochem

2 24 E. González-Partida et al. / Applied Geochemistry 20 (2005) Introduction The geothermal reservoir of Los Azufres, situated in the Trans-Mexican Volcanic belt (Fig. 1) was first discovered in It is located in the eastern central portion of the Michoacan state, central Mexico. Currently, the geothermal field has an installed capacity of 88 MW within an area of 35 km 2. There are 67 drilled wells (37 are producers) with an average production of 39 t/h of steam. The depth of the production wells ranges from 627 to 3544 m. The 88 MW are generated with 7 units of 5 MW each, a condensation unit of 50 MW and two units of binary cycle of 1.5 MW each. There are plans to expand the capacity of the geothermal field to 128 MW with the installation of two more units of 20 MW each. Los Azufres is the first geothermal field producing electricity from saturated fluids in volcanic rocks in Mexico, and the second in importance after Cerro Prieto, Baja California. Los Azufres field is a typical high-enthalpy hydrothermal system related to a volcanic caldera and intense fracturing (Ferrari et al., 1991; Pradal and Robin, 1994). The Los Azufres geothermal field is classified as a low sulfidation system (González, 1999) based on its geochemical and petrologic characteristics, the hydrothermal mineral alteration and the fluid composition, where S is mostly in the form of H 2 S. 2. Geological setting 2.1. Surface geology Fig. 1 shows the regional and local geologic and tectonic framework and a local geologic map of the Los Azufres region. Fig. 2(a) shows a W E cross-section indicated in Fig. 1, based on data from 6 geothermal wells in the northern sector of Los Azufres area. The local basement constitutes a group of andesitic rocks with some paleo-soil layers, basaltic rocks and volcanic agglomerates interstratified with the andesites. Overlying this sequence (unconformably) is a sequence of silicic volcanic rocks (Agua Fría rhyolite), lake sediments, the San Andres dacite, and ignimbrite deposits (Yerbabuena rhyolite), the last one forming dome structures. Mafic volcanic rocks (basalts) represent the last volcanic stage at Los Azufres, with outcrops and some cinder cones in the east and west of the geothermal field. The whole rock geochemistry from the different units shows a calc-alkaline trend (Cathelineau et al., 1987). Structural geology studies by Garduño (1988) and Ferrari et al. (1991) indicate the existence of three, sub-vertical, main fault systems. The main one trends E W and cross-cuts the other two (minors) with a NE SW trend. These faults are the major cause of the secondary porosity in the geothermal reservoir. The E W fault system has a lateral displacement, with regional relation to the Acambay fault, affecting mostly the south of the Los Azufres Caldera. The main surface hydrothermal activity in the caldera is associated with this E W faulting. These faults allow fluids to ascend and form an alteration zone, siliceous and clay-rich sinters, hot soils, hot springs, mud volcanoes and fumaroles at the surface Recent mineralogy (hydrothermal alteration) The deepest well in the Los Azufres reservoir reaches 3,600 m depth (Well Az-44). The host rock is mostly andesites and minor rhyolites. Hydrothermal alteration is evidenced by calc-silicates, which define paragenetic propylitic zones. Fig. 2(a) shows the boundaries of these alteration zones. The paragenetic mineral assemblages have been divided into three zones based on the thermal ranges defined by the fluid inclusion analyses within the newly formed minerals (González, 2000). From bottom to surface, these zones are named zone III, zone II and zone I. Zone III (deepest) is situated in the C range, it is characterized by the assemblage epidote + clinozoisite + actinolite + tremolite ± garnet and smectite + illite + chlorite in the clay fraction (more chlorite than illite and smectite). Zone II is situated in the C range, with the following mineral assemblage, epidote + laumontite + wairakite, and within the clay fraction illite + smectite + chlorite. Zone I is situated in the C range and is defined by the predominance of laumontite. Clay minerals dominate the zone characterized by a temperature between 100 and 150 C. The assemblage chlorite + quartz + calcite is present in all the three zones in variable quantities, with significantly abundant calcite in well Az-40. In surface sinters, there are elemental S + alunite + kaolinite + quartz + pyrite. A generalized zoning of the calc-silicate assemblage can be observed in the alteration zones, with zeolites in the upper part and epidote clinozoisite in the deepest levels. An argillic alteration zone overlies the calc-silicate zone and is the dominant surface manifestation of the hydrothermal alteration. In some parts, there is a mineral assemblage composed of kaolinite alunite S quartz (advanced argillic zone) formed by the interaction of steam and shallow groundwater. Chalcopyrite, bornite, covellite, digenite and idaite (in order of abundance) represent Cu sulfides at Los Azufres. Other metal sulfides present are pyrite, pyrrhotite and marcasite. Hematite is the only metal oxide present in the opaque mineral assemblages. A redox process at depth controls mostly the paragenetic assemblage succession of the opaque minerals. Boiling of ascending fluids seems to control the precipitation of hematite and pyrite in an oxidizing medium, associated with a considerable rate of generation of steam. Marcasite is associated with low permeability zones and acid

3 E. González-Partida et al. / Applied Geochemistry 20 (2005) North America Plate (a) (b) 'W 'W 'W ACAMBARO N 20 00'N Pacific Plate 0 km 500 Coco Plate La Primavera Los Azufres Los Humeros Acoculo 'N MORELIA Cuitzeo 10 km Lac Sierra los azufres Los Azufres volcanic complex C HIDALGO ' 30'' ' 30'' ' 30'' ' (c) 19 50' W Az-40 Rio Agrio Az-52 Nopalito Az-27A 30'' Maritaro Az-53 E 19 49' 30'' 19 48' Zone South Zone North Quaternary Pleistocene Pleistocene Pliocene U. Miocene Well El Chino Laguna Larga Alluvium & Pyroclastics Yerbabuena Rhyolite Tejamaniles Dacite Agua Fria Rhyolite Andesite Complex Fault Az-44 Az-29 Az-49 La Cumbre Laguna Verde Az-9 Laguna larga San Alejo Az-23 Az-25 Az-35 Az-18 Az-26 Los Azufres Az-16 Az km Fig. 1. (a) Regional tectonic framework of Mexico and distribution map of the main geothermal zones. (b) General structural map of the Los Azufres caldera and geothermal field. (c) Detailed geologic map of Los Azufres, location of the geothermal wells and location of the cross-section W E (wells Az-40, Az-52, Az-27A, Az-53 and Az-29).

4 26 E. González-Partida et al. / Applied Geochemistry 20 (2005) Fig. 2. (a) W E geologic cross-section (see Fig. 1(c) for location), showing the depth of the wells from which fluid was analyzed in the northern sector of Los Azufres geothermal field. The paragenetic zones I, II and III were determined with regard to petrologic considerations (see text for detailed discussion). Geologic symbols are the same as for Fig. 1(c). (b) W E cross-section indicating isoconcentration zones of the paleo-brine (fluid inclusions) and isotherms obtained from microthermometric analyses of fluid inclusions. Continued lines correspond to average homogenization temperatures zones, where (1) clathrate positive fusion temperatures; (2) fusion temperatures from 0.7 to 2.4 C; (3) fusion temperatures from 0.5 to 0.7 C; (4) microthermometric analysis of quartz (d); (5) microthermometric analysis of epidote (m); (6) microthermometric analysis of calcite (j); (7) microthermometric analysis of prehenite ( * ).

5 E. González-Partida et al. / Applied Geochemistry 20 (2005) ph, related to rhyolite domes (González et al., 2000; González, 2001). 3. Fluid in fluid inclusions At Los Azufres, fluid trapped in inclusions during the growth of hydrothermal minerals in veins (calcite, prehnite, wairakite, anhydrite, quartz and epidote) give a record of the earliest known fluids in this active hydrothermal system. Fluid inclusions are common in mineral veins occurring in samples from drill cuttings and core slices (Lemieux et al., 1989; Moore et al., 1992 and González, 2000). Complete data for samples from 15 wells are presented by González (2000), who presents and discusses the microthermometric behavior of fluid inclusions from the southern and central portions of the Los Azufres field. Data from González (2000) were used to plot the microthermometric results from the northern and NW portion of the Los Azufres area (Fig. 2(b)). The iso-concentrations of the proto-fluid and the average T h are indicated in Fig. 2(b), which is a crosssection based on 6 geothermal wells. The fluid inclusions have a first ice melting temperature (eutectic temperature, T e ), T e = 21.5 ± 0.5 C, which implies a NaClrich brine. The final ice melting temperatures (T mi ) reveals a zoning distribution: (a) deepest zone, close to the base line 0, zone 3 in Fig. 2(b), where T f = 0.5 to 0.7 C, with salinities between 0.88 and 1.23 wt% equivalent NaCl; (b) intermediate zone, located between 1000 and 2500 m. a.s.l. (zone 2 in Fig. 2(b)), with a mixture of L + V and V fluid inclusions, and T mi = 0.7 to 2.4 C, which corresponds to apparent salinities of ap wt% NaCl; and (c) shallowest zone located between the surface and the steam-rich zone (steam cap or clathrate cap zone 1 in Fig. 2(b)), with positive values for the fusion temperature, indicating a strong chemical change in the paleo-brine (proto-fluid). The fusion temperatures in this zone are T mi = +0.1 to +7 C, which implies an invasion of CO 2 with some CH 4,N 2, CO, H 2 and H 2 S. A high concentration of CO 2 in this sector of Los Azufres (Suárez et al., 1997) could affect the cryometric behavior of the fluid inclusions. The measured homogenization temperatures, plotted as isotherms (Fig. 2(b)), show an ascending zone between wells Az-29 and Az-27A, and a descending zone in well Az-40 (this well has a thermal inversion at depth). Boiling processes start at the T h = ±300 C isotherm, which coincides with an increase in the apparent salinity determined, and an increase of gas concentrations in the fluid (Suárez et al., 1997). Fig. 3 shows a depth vs. temperature diagram, in which the boiling curve for a fluid with 2 wt% NaCl equivalent has been plotted (considering the water table to be at the surface since the Agria River discharges water to the local aquifer). The same plot (Fig. 3) shows the data for the end member wells (Az-29, the hottest, and Az-40, the coolest), data for S stable isotope S from the geothermal fluids, data on the distribution of wt% of non condensable gases NCGs and alteration types for the different zones. The deepest location from which data were obtained are in the liquid field and the data show small dispersion. At a depth of 2300 m, where boiling has been estimated to occur, the data show a wider dispersion and the liquid steam field is entered. At 700 m depth, where the geothermal fluid is interpreted to mix with meteoric water, the data suggest liquid phase, but with positive fusion temperatures, indicating the presence of NCGs. The average data for the two end member wells show that the fluids in well Az-40 are cooler than those in well Az-29. In order to understand the vertical evolution of the geothermal system of Los Azufres, data from Potter (1977), Haas (1971) and the average homogenization temperatures (T h ), corrected for pressure effects, have been taken into consideration to plot 3 points (a, b and c), where important thermodynamic processes have affected the paleo-fluids (proto-fluid). At a depth of 2900 m in well Az-29 (point a), the characteristics of the paleo-fluid were: homogenization temperature, T h = 325 C; salinity, S = 0.7 wt% NaCl equivalent, pressure, P = 128 bar; fluid density = g/cm 3 ; corrected homogenization temperature (trapping temperature, T A ), T A = 334 C (T A = T h +8 C); enthalpy, E = 1499 kj/mol. Boiling would then occur at 2300 m depth (point b) with T h = 300 C; apparent salinity S = 1.4 wt% NaCl; density = g/cm 3 ; pressure, P = 85 bar; enthalpy, E = 2749 kj/mol; and +8 C for pressure correction (T A = 308 C). At 700 m depth (point c) the conditions would be: T h = 172 C; apparent salinity, S = 1.2 wt% NaCl equivalent; density = g/cm 3 ; pressure, P =8 bars; and T h = T A. Currently, the Los Azufres reservoir is composed of two sectors with different thermodynamic characteristics (Suárez et al., 1997). The northern sector is a singlephase liquid reservoir. It is characterized by a hydrostatic vertical profile at an average pressure of 90 bar and average temperature of 300 C. On the other hand, the southern sector constitutes three zones (Fig. 3): (1) a shallow 2-phase steam dominated (2FSD) zone, at initial average conditions of 55 bar and 270 C; (2) an intermediate 2-phase liquid dominated zone (2FLD), at 100 bar and 300 C; (3) and a deep compressed liquid zone (CLS), at 180 bar and 350 C. Noncondensable gases (NCGs in Fig. 3) that accompany steam extraction from this geothermal field are typically composed of a mixture of CO 2 ± 90 wt% of total NCGs. In the same way, the distribution of the NCGs constitutes a vertical zonation. At the deepest levels (CLS zone in Fig. 3), the NGCs wt% varies from 0.1 to 0.3; in the intermediate zone (2FLD) the wt% varies from 2.8 to ±0.3; and in the

6 28 E. González-Partida et al. / Applied Geochemistry 20 (2005) Fig. 3. Temperature vs. depth diagram showing the boiling curve for water with 2 wt% NaCl equivalent salinity (BPC). The water table level is considered to coincide with the surface. Temperature data for well Az-29 is in whole lines. Temperature data for well Az- 40 are shown as dashed lines; where FI, fluid inclusions; (1) positive fusion temperature; (2) fusion temperatures from 0.7 to 2.4 C; (3) fusion temperatures from 0.5 to 0.7 C; (a) (c) are points (samples), where the thermodynamic data were calculated. NGC = distribution in wt% of the non condensable gases. Isotopic behavior of 34 S&, 18 O& and D&. Types of hydrothermal alteration with respect to depth. uppermost zone (2FSD) the wt% varies between 6 and 8. Since boiling is an important process in the Los Azufres system (González, 2000), it is concluded that important physical chemical parameters (in particular pressure) have changed since the time of mineral formation and/ or starting of field exploitation. According to Moore et al. (1992), boiling and segregation of steam is the most likely mechanism for concentrating vertically transferred CO 2. The transfer of steam and gas to the upper parts of the Los Azufres geothermal system is demonstrated by the existence of an advanced argillic alteration zone formed by the interaction of steam and shallow groundwater (González, 2000), and by the current distribution of the NCGs. The original fluid (proto-fluid) phase starts boiling during its ascent through the fractured host rock. The deepest fluid has relatively low salinity (0.7 eq. wt% NaCl). The salinity recorded in the boiling zone is 2 5 times higher than the original salinity (from 1.0 to 4.9 ap. eq. wt% NaCl). The salinity increase implies an important vaporization process. It is also known that the high content of some gases (mostly CO 2 ) can extensively contribute to lower the melting temperature of ice within fluid inclusions. Boiling causes separation of CO 2 and other volatiles, initially dissolved in the liquid phase, causing the residual fluid to be more oxidizing than that in the deepest reservoir. Boiling is a common process in geothermal systems (Browne, 1984), as well as in epithermal systems during one or more stages of mineral deposition (Bodnar et al., 1985). This is due to the fact that, in both systems the hydrothermal fluids circulate at relatively high temperatures and low pressures. Boiling phenomena are characterized by a significant steam phase increase, during which there is considerable release of gases (H 2,CO 2, CH 4,N 2, CO, H 2 S). The gas release leads to a concentration of the brine (higher apparent salinity). The volatile components in geothermal fields and most epithermal systems are probably similar. The important role played by these gases in the geochemical control of the solubility of the fluids has been demonstrated. More important for the present discussion is the significant way in which gases affect pressure in the reservoir, and their direct control of boiling depth. In most active geothermal fields pressure and depth of formation can be calculated directly from fluid inclusion data. The results of these calculations can then be compared with direct field measurements, in order to test the accuracy of the data interpretation. This is not the case for fossil hydrothermal systems, for which there is a need for pressure corrections to calculate the depth of boiling. Boiling is the key process controlling of the precipitation of metallic and non-metallic minerals in an ore deposit system. The interpretation of the microthermometric data allows the understanding the general thermodynamic evolution of the Los Azufres geothermal field and the vertical evolution of its geothermal fluids. The conceptual model of Nieva et al. (1987) shows that in the deepest zone (3500 m), the system is controlled by a pressurized hot fluid (330 C). According to these authors, boiling is accompanied by a pressure drop,

7 E. González-Partida et al. / Applied Geochemistry 20 (2005) which in turns favors an increase in steam phase as the brine ascends towards the surface. During this ascent, the fluid changes to two phase in the shallowest zone with steam-dominated and then steam phases, where steam mixes with water in perched aquifers and with meteoric water. This model proposed by Nieva et al. (1987), suggests the existence of ascent and partial condensation of steam in the reservoir, during the ascent, non-volatile species (Cl) concentrate in the liquid in the deepest portion of the reservoir. According to Moore et al. (1992), liquid-rich fluid inclusions with CO 2 contents between 4 and 6 wt% are characterized by a CO 2 clathrate dissociation temperature greater than 0.0 C. Collins (1979), and Hedenquist and Henley (1985), have shown that dissolved CO 2 within the Na Cl brine could lower the ice melting temperature within an inclusion 2 C below the normal range. Thus, variations in ice melting temperature of inclusions in Los Azufres reservoir could be explained in terms of the variation in concentration of trapped brine within the fluid inclusions. 4. Geochemistry of present day fluids 4.1. Hydrogeochemistry Rodriguez (1984), Quijano (1987) and Tello (1991, 1996) have studied the hydrogeochemistry of the Los Azufres geothermal field. Table 1 shows the chemical composition of water produced from the different geothermal wells. Table 2 shows the chemical composition of water from hot springs. The geothermal brine from Los Azufres is dominated by Cl and Na. The Cl concentration ranges between 1485 and 7297 mg/l, and Na concentration varies between 914 and 4442 mg/l. The next highest dissolved species are SiO 2 and K. Boron concentration is relatively high but the Ca and Mg concentrations are relatively low (Table 1). Fig. 4 shows a triangular diagram (SO 4 Cl HCO 3, Giggenbach, 1989) for groundwater from geothermal wells and hot-springs water. In this diagram, the water can be classified into 4 types: (a) mature type or geothermal water (Cl-rich); (b) steam heated water (SO 4 -rich); (c) peripheral water (HCO 3 -rich); and (d) volcanic water (Cl SO 4 -rich). Fig. 4 shows that the water from all the geothermal wells is mature water type (Cl-rich), which is characteristic of deep geothermal water. Water from a third of the hot springs (Table 2), is heated by geothermal steam (SO 4 - rich), another third is peripheral water (meteoric water with little or no geothermal influence). Samples from a couple of hot springs fall in the Cl-rich corner, indicating that this geothermal water reaches the surface through faults and fractures. The rest of the samples are scattered in the diagram between steam heated water meteoric water and geothermal water, likely reflecting water mixing processes. The acid SO 4 -rich water is meteoric water that interacts with geothermal steam. The SO 4 -rich waters are present in geothermal and volcanic systems where steam condensate interacts with shallow meteoric water causing oxidation of H 2 StoSO 4 (Ellis and Mahon, 1977). The high concentrations of B in this type of water (B = 0.08 and 7.97 mg/l; Table 2) and the relatively low concentrations of Mg (Mg = 0.03 and mg/l; Table 2), suggest a deep circulation. Furthermore, the water/rock interaction must have taken place at relatively high temperature because chlorite solubility decreases with increase in temperature, thus lowering the concentration of Mg. All other waters from the springs fit a HCO 3 -Cl and HCO 3 Cl-poor trend (<12 mg/l), with a low B concentration (<1 mg/l). However, the Ca and Mg concentrations of these intermediate waters are high, suggesting that they are meteoric waters with shallow circulation through volcanic rocks Water rock equilibrium state Solute geothermometers are a powerful tool for evaluating the conditions at depth in geothermal systems. However, this method should be applied to samples with the following basic assumptions: (1) the system should be in equilibrium; and (2) water rock interaction should take place at depth and at a high temperature. The equilibrium state and the temperatures of the Los Azufres geothermal system can be deduced from Fig. 5, where all the samples from the geothermal wells plot closely indicating an equilibrium between water and rock. The temperature of this equilibrium state is from 300 to 340 C. Some water samples from surface springs are close to this equilibrium state. Fig. 5 shows a ternary diagram for Na K Mg. The samples at equilibrium (geothermal wells) plot along the Na K lines. The bicarbonate-na springs are displaced towards the Mg corner and indicate a temperature <100 C from the K/Mg geothermometer. This confirms that the bicarbonate-na springs discharge shallow waters equilibrated with the host rock at a relatively low temperature. On the other hand, the acid-sulfate springs also fall in the region of shallow waters, but the K/Mg geothemometer yields temperatures >100 C. These are waters of relatively recent infiltration heated by geothermal steam. The calculation of hydrological balance resulted in a potential, average annual infiltration rate of 446 mm/m 2 for the Los Azufres geothermal area, which corresponds to a total of m 3 /a. Due to the highly fractured and faulted structure of the volcanic formations, a considerable potential for the infiltration of recent meteoric water into deeper sections of the volcanic formations can be assumed. Isotopic data indicate the minor importance of recent meteoric water for the recharge of the geothermal reservoir (Birkle et al., 2001).

8 Table 1 Chemical composition of the water produced from the geothermal wells from Los Azufres Well Ho (kj/kg) Pc (Bars) ph CE (mhos/cm) Cl B HCO 3 SiO 2 SO 4 Na K Li Rb Cs Ca Mg As Concentrations are in mg/l AZ , AZ , AZ AZ AZ , AZ AZ AZ , AZ , AZ AZ , AZ , AZ AZ AZ AZ AZ , AZ AZ AZ , AZ AZ , AZ AZ AZ AZ , AZ , AZ AZ AZ AZ AZ , AZ AZ AZ , Data refers to atmospheric conditions. R indicates that the well is currently used for injection. 30 E. González-Partida et al. / Applied Geochemistry 20 (2005) 23 39

9 Table 2 Chemical composition of hot spring water located in and around the Los Azufres geothermal field Locality No Temp. sup. ph EC (mhos /cm) Cl B HCO 3 SiO 2 SO 4 Na K Li Rb Cs Ca Mg As Fe Al Concentrations are in mg/l Agua Fria II M Agua Fria III M AGUA FRIA IV M AZUFRES I M CASA LAZARO M CARDENAS CUMBRES I M CUMBRES II M CURRUTACO M CHIFLADOR M EL BOSQUE M STA. ROSA EL CARCAMO M FABRICA LA M VIRGEN JARIPEO M LAS ADJUNTAS M LA HERRADURA M LA TRASQUILA M LAGUNA VERDE M LOS HERVIDEROS M MARITARO M NOPALITO I M POZO AZ-24 M PUENTECILLAS M SANTA ROSA I M TEJAMANILES II M ZIMIRAO M TEMP SUP indicates surface temperature in C. E. González-Partida et al. / Applied Geochemistry 20 (2005)

10 32 E. González-Partida et al. / Applied Geochemistry 20 (2005) C Well Hot spring Na/ Well Hot spring Volcanic waters SO 4 5. Geochemistry of gases Mature waters C1 HCO 3 Peripheral waters Steam Heat Waters SO Noncondensable gases in the steam The results of the chemical analyses of gases produced from the geothermal wells are presented in Table 3. The gases in the steam dominant zone are mostly composed of CO 2 (80 98% of all the gases). Other gases present are: H 2 S (1.16%), H 2 (0.026%), N 2 (0.77%), and CH 4 (0.01%). The average concentration of the gases is between 2 and 9 wt% of the total mass of the fluid in the reservoir (Suárez et al., 1997). Giggenbach (1980) proposed a ternary diagram for He Ar N 2 to identify the relative proportion of the 3 components: (1) atmospheric, (2) crustal and (3) magmatic, in the geothermal fluids. Fig. 6 is a ternary diagram for N 2, He and Ar. Samples from geothermal wells (Az-9, Az-46, Az-17) plot on the mixing line between crustal and meteoric saturated water. The high He content detected in some of the geothermal well fluids suggests a relatively slow circulation of the geothermal fluids through the host rock. It is important to mention the two trends shown by the gases of deep circulation (geothermal samples) and the atmospheric-rich gases (fumaroles and smokers). Some geothermal wells are used for deep injection, and this process produces an equilibration of N 2 /Ar with atmospheric conditions Noncondensable gases in the smokers HCO 3 Fig. 4. Ternary diagram showing the relative Cl SO 4 HCO 3 content in the Los Azufres thermal waters. The digram shows the fields of the different types of water: mature water, volcanic water, steam-heated water and peripheral water. See text for discussion (Giggenbach, 1989). In order to understand the genesis of the gases in the surface geothermal manifestations (fumaroles), several samples were collected. The results of chemical analyses t K/Na Immature waters K/ Mg of gases from smokers are shown in Fig. 6. Two groups of samples are indicated, the northern and southern groups. The smokers from the north are derived from saturated water and gases of magmatic genesis. The samples from the Agua Fría site are most representative for this group. However, fumarole samples from the southern zone plot in the region of deep circulation in the crust. This suggests a longer residence time of the fluids in the host rock, or relatively lower permeability in the southern zone. Based on the local geology of this geothermal field, there are rhyolites with low permeability in the southern zone. These rhyolites act as seal rock, which causes a longer time for the fluids to reach the surface through the smokers. This process causes the formation of He by radioactive decay of 3 H. The Cl analyses show that boiling has been more intense in the southern than in the northern sector. Initial studies of the composition of phases in the reservoir (Nieva et al., 1987) showed that concentration of fluid volatile components such as CO 2 and 2 H will decrease with depth, while concentration of non-volatile components such as 18 O and Cl will increase (Nieva et al., 1987; Suárez et al., 1997) Water gas equilibrium Full equilibrium The chemical composition of the gases produced by the geothermal wells and smokers can be used to calculate the equilibrium state between water and gases and the equilibrium temperature as well. Fig. 7 is a plot of log H 2 /Ar vs. log CO 2 /Ar. The maximum temperature for the geothermal wells is close to 350 C, which is consistent with estimations by liquid geothermometers (González, 2000). The water gas equilibrium conditions Partial equilibrium Fig. 5. Ternary diagram Na K Mg showing the evolution of the Los Azufres thermal waters from wells and srpings. The diagram shows the fields for the different equilibrium states (inmature-partial equilibrium-full equilibrium), and isotherms (Giggenbach, 1989). t K/Mg 60 80

11 E. González-Partida et al. / Applied Geochemistry 20 (2005) Table 3 Chemical composition of the gases produced by geothermal wells at Los Azufres Well Ho kj/kg Pc Bars Ps Bars Xg wt% He H 2 Ar N 2 O 2 CH 4 CO 2 H 2 S NH 3 Data in wt% (dry) AZ AZ AZ AZ-006* AZ AZ AZ AZ AZ AZ-016D AZ-017* AZ AZ Az AZ AZ AZ AZ AZ AZ AZ-034* AZ AZ AZ-038* AZ-041* AZ AZ AZ AZ AZ AZ AZ AZ AZ AZ AZ AZ Data is in wt% (dry), Xg indicates total content of the gas Ho, enthalpy of the mixture; Pc, pressure at the head of the well; Ps, pressure at the separation. Note: the wells* indicate dry steam. are determined from Fig. 7. It can be said that there is equilibrium with the liquid phase in water dominated geothermal wells, whereas in vapor dominated wells there is equilibrium with the steam phase. This also indicates a water-steam mixing process at relatively high temperatures, and/or Ar loss Steam excess Two phases are considered to exist in the system if the enthalpy of the production wells exceeds that of the saturated liquid at the reservoir temperature. The production enthalpy of the wells at Los Azufres varies between 1235 and 2716 kj/kg. A high discharge enthalpy reflects two phases in the reservoir (water and steam). The discharge enthalpy for well Az-2 corresponds to a saturated liquid at a steam temperature of 280 C, hence the steam excess at reservoir conditions is small. For the rest of the wells the enthalpy is higher, but they produce from a liquid-rich zone. Some wells produce dry steam with an enthalpy of 2700 kj/kg, these are mostly located within the southern sector of the reservoir. One of the factors that causes steam excess or enthalpy excess in wells is different mechanisms of steam and liquid phase movement within the rock formations (different relative viscosities). On the other hand, is it

12 34 E. González-Partida et al. / Applied Geochemistry 20 (2005) N 2 / Magmatic Well Fumerole 60 in explaining the chemical evolution of the water in the geothermal reservoir with regard to steam excess data. If the reservoir contains water only, it would be easy to relate the water chemistry to the reservoir physico-chemical conditions (Arnorsson, 1982). On the other hand, if there is excess steam, it is not possible to relate the chemistry of the fluid at the surface with reservoir conditions. Thus, it is only possible to apply this method to wells with enthalpy below 2400 kj/kg, which is the case for most of the wells at Los Azufres. This method is not applicable to dry-steam wells. 20 Crustal Meteoric He Ar Fig. 6. Relative N 2 He Ar content in Los Azufres geothermal gas discharges. The diagram shows the field for the differnet sources of gases (magmatic, crustal, meteoric, Giggenbach, 1980). also possible that the steam excess is due to vaporization during contact of steam with conductively heated host rocks, and a drop in pressure during extraction of fluid by the wells. The combined processes cool the fluids (by boiling). There are, however, some complications Stable isotopes Stable isotopes have proven to be a very helpful tool for understanding the complexity of hydrothermal systems (recent-geothermal, or fossil-ore deposits; Valley et al., 1986). The use of stable isotopes for solutions of geologic problems depends on the assumption of some basic physico-chemical conditions. The most basic assumption is chemical and isotopic equilibrium between the different species considered (Henley et al., 1984). The definition of the original isotopic source for the different species and their later evolution has been a major research topic in earth sciences. Among the different environments, the modern-day hydrothermal 4 Equilibrium steam Well Log (X H2 / X Ar ) Equilibrium liquid 200 CO 2 / Ar -temp -2 H2 / Ar -temp Log (X CO2 / X Ar ) Fig. 7. Evolution of H 2 /Ar vs. CO 2 /Ar at equilibrium conditions in the Los Azufres reservoir. See text for further discussion.

13 E. González-Partida et al. / Applied Geochemistry 20 (2005) Table 4 Stable isotope data for geothermal brine and spring water Reservoir/Brine Springs Well dd& d 18 O& Locality dd& d 18 O& AZ AGUA FRIA AZ ARARO AZ AZUFRES AZ AZUFRES AZ-16AD CURRUTACO AZ CHIFLADOR AZ CHINO AZ ESPINAZO DEL DIABLO AZ LAGO CUITZEO AZ LAGUNA AZUFRES AZ LAGUNA LARGA AZ LAGUNA LLANO GRANDE AZ LAGUNA SECA AZ LAGUNA VERDE AZ MARITARO AZ NOPALITO AZ NOPALITO POZO HEDIONDO TEJAMANILES systems provide a great deal of information on physicochemical processes that are not observable in fossil hydrothermal systems, especially those related to the chemical controls of the hydrothermal brines Isotopic characteristics of the liquid phase Table 4 and Fig. 8 show the corrected d 18 O& dd& data for discharge water from the wells and hot springs at Los Azufres. In Fig. 8 well samples show a positive d 18 O& compared to the meteoric line, which is a characteristic of geothermal fluids. This enrichment in d 18 Ois due to water rock interaction at relatively high temperatures. Most of the spring samples plot close to the global meteoric water line. However, other spring samples, especially those with SO 4 -rich chemistry, indicate a modified isotopic composition and plot along a vaporization line at temperatures above 100 C. The isotopic composition of surface water samples (lakes) present an isotopic composition characteristic of highly evaporated water at surface temperature. The regression line for boiling is shown in Fig. 8 and is: D& = 3.11d 18 O The isotopic composition of the meteoric precipitation in the zone can be obtained from this expression and is 10.7 for d 18 O&, and 75 for dd&. On other hand, a value of d 18 O=+8& as been recorded for mafic rocks from the Los Azufres reservoir (Torres, 1996) d 34 S&, and d 13 C& Values for d 13 CinCO 2 from the reservoir at Los Azufres range between 13 C= 5& and 11& (Tabaco et al., 1991). According to Hoefs (1980) CO 2 of magmatic origin ranges between 5& and 8&. The d 13 C of the CO 2 in meteoric waters varies from d 13 C= 7& to 30&, and the d 13 C in carbonate rocks is between 2& to +2&. The influence of an atmospheric component in Los Azufres is reflected by the abundance of negative d 13 C values, which indicate some organic input into the reservoir by meteoric water. Another process taking place in the reservoir is the dilution of paleo-meteoric water with ascending magmatic CO 2 (Birkle et al., 2001). The isotopic composition of the dissolved CO 2 - (d 13 C &) in water is representative of its ultimate source (magmatic, carbonate rocks, organic matter or meteoric water atmospheric). Thus, the composition of d 13 C& in water can indicate the fractionation processes like boiling or condensation of the hydrothermal fluid (Henley and Ellis, 1983; Panichi and Tongiorgi, 1976). The isotopic composition of hydrothermal fluids depends on: (1) geochemistry of the source; (2) physico-chemical processes within the reservoir; and (3) water rock equilibrium temperature (Mahon et al., 1980). The detailed studies on d 13 C& in Los Azufres by Tabaco et al. (1991) show a range between 5& and 11&, in which the isotopic fractionation observed is explained by a boiling condensation process of the hydrothermal fluid within the productive zone of the reservoir. Fig. 9 shows the results for d 34 S& in pyrites, and H 2 S and SO 4 from the hydrothermal fluid (see Fig. 3 for special distribution within the reservoir). The isotopic composition of the pyrite falls into two distinctive groups, the first

14 36 E. González-Partida et al. / Applied Geochemistry 20 (2005) SMOW δd / Global Meteoric Water Line Evaporation δd = 3.11 δ18o Magmatic Component -96 Geothermals fluids Cold Springs δ18o / Fig. 8. Stable isotopes (dd& vs. d18o&) plot for samples of the geothermal brine and surface manifestations (hot springs) of Los Azufres. (SMOW, standard mean oceanic water). Aditional plotted data from Tello (1996). group (deepest zones without boiling) with values from d 34 S FeS2 ¼ 1:57 to 2:97, and the second group (shallowest zones with dominant steam) with values between d 34 S FeS2 ¼ 4 to 4:83. The average value for FeS 2 ðd 34 S FeS2 ¼ 2:5 Þ is very similar to the average value for H 2 S ðd 34 S H2 S ¼ 2 Þ. The aqueous SO 4 has d 34 S SO4 ¼þ21:47. The sulfide sulfate system in aqueous and crystalline media plays a very important role in geochemical cycles. It is widely known that sulfides oxidize easily, and that sulfates are difficult to reduce at relatively low temperatures (<200 C, or near-surface environments). Sulfate is reduced at temperatures >200 C only when the ph is low (acidic conditions) and the S concentration is high. At Los Azufres, the ph is near neutral at depth and the total concentration of S is low, but the temperature is ±340 C. González (1985) shows that the isotopic composition for the source in the reservoir is d 34 S RES.= 2.3& (average). The results of a calculation by Arnold and Gonzales-Partida (1985) indicate that there is equilibrium between sulfates and sulfides at 300 C at the Los Azufres geothermal field. It is accepted that the chemical and isotopic equilibrium between sulfate and sulfides is reached when the difference d 34 S SO4 d 34 S H2 S ¼ 10 3 Lna SO4 H 2 S is obtained (Truesdell, 1984). At the reservoir temperature of Los Azufres (T = C), the numeric value of 10 3 Lna SO4 H 2 S ¼ 23:5 (Truesdell, 1984), which is very similar to measured the ratio d 34 S SO4 d 34 S H2 S ¼ 23:47 for the reservoir. These data confirm the chemical and isotopic equilibrium state at Los Azufres. The initial isotopic value for S in the reservoir is d 34 S RES.= 2.3&, which implies a magmatic source. Values that are more negative have been observed in pyrites from shallower zones ðd 34 S FeS2 ¼ 4 to 4:9 Þ. On the other hand, the liquid phase from the reservoir has the following isotopic values (see Fig. 3): (a) deepest zone, d 18 O= 2.4&; (b) shallowest zone d 18 O= 5.54&, with an average of dd = 69& from both zones (Nieva et al., 1987). Boiling (extensively documented for the reservoir) seems to control the sulfide precipitation. González (2000), indicates that these processes promote a massive separation of sulfide from CO 2, H 2, and other volatiles initially dissolved in the liquid phase. The ascending meteoric water is more oxidizing than the deepest water from the reservoir, and more oxidizing than the volcanic rock environment which hosts the fluid. This situation is responsible for the evolution of paragenetic assemblages of Fe sulfides and Fe oxides, from pyrrhotite to pyrite to marcasite and pyrite hematite in the Los Azufres reservoir (González, 2000).