This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and

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1 This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier s archiving and manuscript policies are encouraged to visit:

2 Geothermics 38 (2009) Contents lists available at ScienceDirect Geothermics journal homepage: Hydrothermal activity in the Tulancingo Acoculco Caldera Complex, central Mexico: Exploratory studies Aída López-Hernández a,b,, Gerardo García-Estrada a, Gerardo Aguirre-Díaz b, Eduardo González-Partida b, Hugo Palma-Guzmán a, José L. Quijano-León a a Gerencia de Proyectos Geotermoeléctricos, CFE, Alejandro Volta 655, Morelia, Michoacán, Mexico b Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, Querétaro, Qro., 76230, Mexico article info abstract Article history: Received 19 May 2007 Accepted 4 May 2009 Available online 30 June 2009 Keywords: Geothermal Hidden system Cold gas emissions Low permeability High temperature Volcanic Caldera Acoculco Tulancingo Mexico Mineral alteration and fluid inclusion studies of drill cuttings and core samples indicate that the sedimentary basement rocks and the volcanic rocks associated with Tulancingo Acoculco Caldera Complex have been the site of two distinct and major hydrothermal events. The complex, located in the eastern portion of the Trans-Mexican Volcanic Belt, is formed by the Pliocene Tulancingo Caldera and the younger (Pleistocene) Acoculco Caldera, which developed within the older depression. The volcanic rocks are underlain by Cretaceous sedimentary rocks of the Sierra Madre Oriental. The earliest important hydrothermal event occurred during the emplacement of Mid-Tertiary granitic intrusions that metamorphosed the sedimentary rocks; these intrusives are not exposed at the surface. However, granitic rocks were encountered at the bottom of exploratory borehole EAC-1, drilled within the Caldera Complex. The second main event occurred during the formation of the Tulancingo and Acoculco Calderas. Both episodes lead to secondary mineralization that reduced the permeability of the reservoir rocks. A possible third hydrothermal event may be associated with the recent magmatic activity within the Acoculco Caldera.Thermal logs from well EAC-1 display a conductive thermal gradient with maximum temperatures exceeding 300 Cat 2000 m depth. Although there are no active thermal springs in the area, there is extensive fossil surface hydrothermal alteration and cold gas discharges with high He 3 /He 4 ratios Published by Elsevier Ltd. 1. Introduction There are more that 500 geothermal areas in the Trans-Mexican Volcanic Belt of central Mexico. Of these, two are presently under commercial exploitation, Los Azufres and Los Humeros (Fig. 1); 10 additional geothermal systems are currently being investigated to determine their potential to produce electricity (Gutiérrez-Negrín and Quijano-León, 2007). During a regional exploration program conducted by the Comisión Federal de Electricidad (CFE) of Mexico, the Acoculco area attracted attention because of the presence of extensive surface hydrothermal alteration, cold acid springs and gas discharges that sometimes killed animals when they approached to drink (Romero and Hernández, 1981). The geothermal interest was reinforced when high 3 He/ 4 He values (R/R air = 6.3) were measured Abbreviation: CFE-GPG, Comisión Federal de Electricidad, Gerencia de Proyectos Geotermoélectricos, Morelia, Michocán, México. Corresponding author. Present address: Facultad de Ingeniería Civil, Universidad Michoacana de San Nicolás de Hidalgo, Ciudad Universitaria, Morelia, Michoacán, Mexico. Tel.: ; fax: address: aidalopher@gmail.com (A. López-Hernández). by Polak et al. (1982), which suggested the presence of an active magmatic source. The Acoculco geothermal system is located 130 km northeast of Mexico City, in the eastern portion of the Trans-Mexican Volcanic Belt (Fig. 1). It lies within the Tulancingo Acoculco Caldera Complex, one of the major ones in that belt. Other Caldera Complexes include La Primavera (Mahood, 1980), Huichapan (Milán et al., 1993; Aguirre-Díaz et al., 1997), Amealco (Aguirre-Díaz, 1996; Aguirre-Díaz and McDowell, 2000), Los Azufres (Campos-Enríquez and Garduño-Monroy, 1995), and Los Humeros (Ferriz andmahood, 1987). The geology of the Acoculco region was studied by De la Cruz and Castillo-Hernández (1986), who suggested that the hydrothermal activity was related to magmatism occurring within the previously unrecognized Acoculco Caldera. Additional investigations were undertaken to find the best location for a deep exploration borehole. These studies included DC Schlumberger surveys (Palma, 1987) and geochemical analyses of spring fluids (Tello-Hinojosa, 1986, 1987). The first exploratory well (EAC-1) was drilled near the Los Azufres bubbling pond (Fig. 2), in an area with numerous gas vents. Temperatures above 300 C were measured at 2000 m depth (Gama et al., 1995) /$ see front matter 2009 Published by Elsevier Ltd. doi: /j.geothermics

3 280 A. López-Hernández et al. / Geothermics 38 (2009) Fig. 1. Schematic geologic map of central Mexico showing the Trans-Mexican Volcanic Belt, the Sierra Madre Oriental and the locations of the Tulacingo Acoculco area and of the main geothermal fields, Calderas, lakes and cities of the region. Calderas and geothermal fields: A, Amealco; H, Huichapan; LA, Los Azufres; LH, Los Humeros; LP, La Primavera; M, Mazahua. Cities: G, Guadalajara; MC, Mexico City; Mo, Morelia; Pa, Pachuca. Tectonic features: CP, Cocos Plate; EPR, East Pacific Rise; OFZ, Orozco Fracture Zone; PP, Pacific Plate; RFZ, Rivera Fracture Zone. Subsequent studies have included a detailed evaluation of the downhole measurements and samples (Tello-Hinojosa, 1994; Palma, 1995; García-Estrada, 1995), various geologic investigations (López-Hernández and Martínez, 1996), and an interpretation of regional gravity data (García-Estrada, 2000). Here we summarize the results of these unpublished studies (mainly CFE internal reports) on the Tulancingo Acoculco Caldera Complex, and review drilling data and results of post-drilling research. 2. Geological setting The east-west trending Trans-Mexican Volcanic Belt is characterized by Mid-Miocene to Quaternary volcanism, active faulting Fig. 2. Geologic map of the Tulancingo Acoculco Caldera Complex area, including the location of the geologic section as shown in Fig. 12.

4 A. López-Hernández et al. / Geothermics 38 (2009) (Demant, 1978; Nixon, 1982; Pardo and Suárez, 1995; Suter et al., 1995; Aguirre-Díaz, 2003), and numerous hydrothermal systems. This volcanic belt has formed in response to the northward subduction of the Cocos and Rivera Plates beneath the North American Plate (Nixon, 1982; Pardo and Suárez, 1995; Aguirre-Díaz, 2003). In its eastern portion the volcanic rocks overlie folded sedimentary rocks of the Sierra Madre Oriental, which host the deeper portions of active hydrothermal systems. The volcanic rocks in the Tulancingo Acoculco region are calc-alkaline in composition and range in age from Pliocene to Pleistocene. K Ar and 40 Ar 39 Ar dates of representative rocks (López-Hernández and Martínez, 1996) document three periods of volcanic activity. The first two volcanic episodes were accompanied by Caldera collapse. The oldest of the two Calderas, herein referred to as the Tulancingo Caldera, was last active between 3.0 and 2.7 Ma. It is the larger of the two, with a diameter of 32 km (Fig. 2). The second, the 18 km diameter Acoculco Caldera, lies entirely within the older structure; its volcanism lasted from 1.7 to 0.24 Ma. The third and youngest episode is related to the monogenetic volcanism of the Tezontepec Apan regional volcanic field ( Ma). Units from the first of the volcanic episodes include the Las Minas Rhyodacite, the Alcholoya Ignimbrite (Fig. 3), the Acaxochitlan Dacites (3.0 ± 0.3 Ma; K Ar age), basaltic lava flows (represented in the map under the same symbol as the Acaxochitlan Dacites) and the rhyolitic-to-dacitic Tulancingo domes that were accompanied by short-lived pyroclastic flows (2.7 ± 0.1 Ma, 40 Ar 39 Ar age). Eruption of the Alcholoya Ignimbrite resulted in the formation of the Tulancingo Caldera. The Tulancingo domes, representing the youngest volcanic phase related to the formation of the Caldera, were emplaced along the northern portions of the Caldera ring structure, the same as the Acaxochitlan Dacites (Fig. 2). Volcanic rocks of the Acoculco Caldera include the Acoculco Rhyolites (1.7 ± ± 0.04; K Ar age), the Cruz Colorada Dacite, the Cuautelolulco Basalts and the rhyolitic Acoculco Ignimbrite (1.4 ± 0.2 Ma; K Ar age), whose eruption resulted in the collapse of the Caldera. Rhyolitic activity continued with the emplacement of domes along the ring structure. One of these domes on the southern rim, named Tecoloquillo, was accompanied by small pyroclastic flow eruptions that formed the Tecoloquillo Tuff. The interior of the Caldera was partially filled with lacustrine sediments. Caballero et al. (1999) reported the presence of two air-fall tephras at Lake Tecocomulco, located SW of the Caldera Complex (Fig. 2), that are intercalated between silt layers whose C-14 ages are in the 42,000 31,000 year BP range. These authors suggested that the tephras could have been derived from the Tecoloquillo dome (Fig. 2). However, newly obtained dates indicate that the age of this volcano is 0.8 ± 0.1 Ma ( 40 Ar 39 Ar) (López-Hernández and Martínez, 1996); consequently, these air-fall tephras must have originated from another, much younger, volcano. At same time that the Acoculco Ignimbrite was deposited, the ring fault of the Tulancingo Caldera was reactivated with the eruption of the rhyolitic Piedras Encimadas Ignimbrite (1.3 ± 0.2 Ma; 40 Ar 39 Ar age) at El Rincón (Fig. 2). Subsequently, a dacitic dome was formed and basaltic flows covered these rhyolitic ignimbrites. The Acoculco volcanic activity ended with the 0.24 ± 0.04 Ma (K Ar age) La Paila formation (De la Cruz and Castillo-Hernández, 1986) consisting of scoria cones and lava flows of basaltic and basalt andesitic composition (López-Hernández and Martínez, 1996), that were emplaced over the NW and SE portions of the Acoculco Caldera ring fault. Around the Tulancingo Acoculco Caldera Complex the Acoculco products were intercalated with Ma Apan and Tecocomulco basalts and basaltic andesites (Nelson, 1997; López-Hernández and Martínez, 1996). The Apan and Tecocomulco volcanics pertain to the third volcanic event, and consist of scoria cones, some shield volcanoes and associated lava flows that formed extensive volcanic plateaus. The distribution of these volcanic rocks was controlled by NE SW-trending normal faults and correspond to the northeastern continuation of the Tezontepec Apan volcanic field, a regional monogenetic volcanic province characterized by hundreds of scoria cones and associated lavas of basaltic andesitic composition (De Cserna et al., 1987; García-Palomo et al., 2002). The earliermentioned La Paila basalts and basaltic andesites, the latest volcanic products of Acoculco Caldera, are probably genetically related to this regional volcanism. Although post-caldera volcanic and alluvial deposits have buried the Calderas, their geometry can be inferred from the circular arrangement of volcanic eruption centers (Fig. 2) and is reflected by the gravity survey data (see Section 3). Underlying the volcanic units is a sedimentary sequence of the Sierra Madre Oriental, which in the Acoculco region is composed mainly of folded Cretaceous limestone and shales (Morales and Garduño-Monroy, 1984); this stratigraphic sequence is exposed north of Tulancingo and east of Chignahuapan (Fig. 2). Structurally, the Tulancingo Acoculco Caldera Complex lies at the intersection of two regional fault systems, one trending NE and the other NW (Fig. 2). We named the one striking NE the Apan Piedras Encimadas fault system and suggest it represents the NE continuation of the Tenochtitlán Shear Zone (De Cserna et al., 1987). The NW-trending system is herein called the Tulancingo Tlaxco fault system; it represents the southeastern limit of the Mexican Basin and Range Province (Pasquaré et al., 1991; Suter, 1991; Aguirre-Díaz et al., 2005). In the study area, this province is characterized by NW-striking regional faults and associated grabens that post-date the Late-Cretaceous to Early-Tertiary compressive deformation that created the Sierra Madre Oriental fold belt. In the Tulancingo Acoculco area volcanic rocks cover the NW and NE-trending regional fault systems, but to the SW, a NE SW alignment of volcanic cones and medium-size composite volcanoes can be observed. These volcanoes could be related to the NEstriking Apan Piedras Encimadas Lineament. The NW SE-trending fault system is represented by subtle morphological lineaments between the Pachuca and Apan regions (NW of the Acoculco zone). The NW SE and NE SW-trending regional fault systems of central Mexico are evident in the regional gravity data interpretation presented by García-Estrada (2000), which is discussed in Section 3. García-Palomo et al. (2002) and Campos-Enríquez et al. (2003) described NE trends in the Apan area and in the Tecocomulco basin, both located SW of the Tulancingo Acoculco Caldera Complex (Fig. 2). The only faults exposed at the surface within the Caldera Complex are short E-trending faults located W and SW of the town of Acoculco (Fig. 2). These faults bound an apical graben related to the formation of a resurgent dome within the Acoculco Caldera. The absence of hydrothermal alteration along these faults suggests they have no direct connection with the deep hydrothermal system Sources of permeability The NW SE and NE SW-trending regional faults systems appear to have been permeable fluid flow paths for ancient hydrothermal systems that formed in both Calderas. Studies at the Pachuca-Real del Monte mineral district, located NW of the Tulancingo Acoculco complex, confirm the presence of such regional structures; mineralization occurred through veins with both orientations during the Pliocene (Geyne et al., 1963). At the surface these faults can be inferred by the alignment of volcanic cones. Gravity and magnetic contour maps (next section) show the intersection of NW SE and NE SW trends at the center of the Acoculco Caldera; however self-sealing by mineral deposition appears to have decreased fault permeability with time. This reduction, along with the absence

5 282 A. López-Hernández et al. / Geothermics 38 (2009) of more recent tectonic or magmatic-induced faulting at the Tulancingo Acoculco Caldera Complex, has resulted in the present low rock permeabilities. This situation is anomalous when compared to other Mexican geothermal fields (e.g. Los Azufres and La Primavera), where recent tectonic and magmatic deformation has resulted in high vertical permeabilities. In those areas, the high permeability is associated with intense fracturing generated by the superposition of magmatic and tectonic deformation events, particularly in zones of magmatic resurgence, where tensional stress is generated (López-Hernández, 1988, 1991, 1994). Similar cases have been reported in Philippine geothermal fields (Reyes, 1990). Dissolution channels in the limestones may significantly control permeability at depth. These permeable features may form networks that provide flow paths connecting the deep hydrothermal system at Acoculco and the hot springs of Chignahuapan, located 19 km to the SE (Fig. 2). In that place, hot water flows from cavernous limestones that are 790 m below the average elevation of the Acoculco cold springs (see Fig. 3 and Section 5). 3. Geophysical exploration Gravity, magnetic, DC-resistivity and magnetotelluric (MT) surveys have been conducted at Acoculco. The Bouguer anomaly calculated using a reference density of 2.4 g/cm 3 (García-Estrada, 1995, 2000) displays a dominant NW SE trend, corresponding to the NW orientation of the fold system of the Sierra Madre Oriental. The presence of high-density rocks can be inferred under the Acoculco Caldera by the distortion of the smooth trend of the gravity contours; this effect was isolated by making a regional residual separation using a second-degree polynomial fitting procedure (Fig. 4). The 5 mgal contour in the residual map defines a NW SEtrending low, bounded by two sectors of higher density located at the SW and NE corners of the study area. High and low gravity sectors are separated by elongated contours indicating a relatively steep gradient, perhaps a reflection of the Tulancingo Tlaxco fault system (Fig. 4). The NW SE elongated gravity low is disturbed locally by three high gravity, NE SW oriented anomalies. One of them is related to the Apan Piedras Encimadas Lineament (Fig. 2). We suggest that magmatic activity in the study area was focused at the intersection of these two trends (see below). The 10 mgal contour has an elliptical shape with its major axis oriented NW SE, perhaps reflecting the combined effects of the Tulancingo Caldera gravity low and the NW SE-trending low-gravity contours, associated with the Tulancingo Tlaxco fault system. Their combined effect is represented by the hachured contours in Fig. 4. The displacement of the low-gravity anomaly associated with the trace of the Caldera fault is caused by the effect that results from the superposition of the regional and local gravity anomalies. The Acoculco Caldera corresponds to a relative gravity high composed of several separate positive anomalies within the Tulancingo Caldera gravity minimum. The expected decrease of gravity associated with the low-density tuff deposits in the Acoculco Caldera is completely obliterated by the increase related to the presence of local high-density intrusive rocks. The most important of these (relative) gravity highs has a circular shape and is located near the Fig. 3. Relevant drilling data from exploratory well EAC-1. Qz, quartz; CT, calcite; Py, pyrite; B, biotite; EP, epidote; HM, hematite; ACM, all clay minerals; CH, chlorite; I, illite; SM, smectite; K, kaolinite.

6 A. López-Hernández et al. / Geothermics 38 (2009) Fig. 4. Residual Bouguer gravity anomaly for the Tulancingo Acoculco area (reference density: 2.4 g/cm 3 ) calculated using second-degree polynomial fitting (contours in mgal). The location of the gravity profile given in Fig. 12 is also shown. town of Atotonilco (Fig. 4). Density measurements made on rock samples from outcrops and cores from exploration well EAC-1 (see below), and 2D modeling (Talwani et al., 1959), suggest that this anomaly is associated with buried high-density rocks whose top is at 1000 m depth. These rocks may represent an intrusion that was not reached by EAC-1 and may be connected to the andesite dikes intersected by the well. The Apan Piedras Encimadas Lineament coincides with a series of aligned local high-gravity anomalies shown in Fig. 4. In the Tecocomulco area, Campos-Enríquez et al. (2003) interpreted a local anomaly associated with this trend as a structural block bounded by a NE-striking normal fault. Ground surface elevations vary from 2300 m above sea level (m asl) in the surrounding plateau to 2700 m asl in the study area. An aeromagnetic survey was flown at 3500 m asl. The total magnetic field data reduced to the pole (Baranov, 1957; Ervin, 1976) show a regional pattern similar to that of the residual gravity; that is, a regional NW SE-trending magnetic low parallel to the Sierra Madre Oriental (Fig. 5). We suggest that the low-gravity and low-magnetic susceptibility corridor represents a NW SE-trending tectonic depression related to the Tulancingo Tlaxco fault system (Fig. 4) that is filled with low-density sedimentary and volcanic rocks. The Acoculco Caldera coincides with a subcircular magnetic high. The source of this anomaly was modeled as an intrusive body with a magnetic susceptibility of S.I. This high-density and high-susceptibility anomaly may represent a vent associated with post Tulancingo Acoculco Caldera mafic magmatism. A second magnetic anomaly occurs near the El Rincón geothermal manifestations (Fig. 5), and coincides with the vent of the Piedras Encimadas Ignimbrite located on the ring structure. These two highs are vents that could be part of the NE-trending system of volcanic centers emplaced along the Apan Piedras Encimadas Lineament. Electrical resistivities are sensitive to the presence of hot saline waters and argillic hydrothermally altered rocks. A 5000-km 2 Schlumberger survey consisting of 61 vertical electric soundings (VES) was conducted as part of the exploration program. The survey had a density of 2 VES/km 2 (Palma, 1987, 1995).A1Dlayer interpretation and a smoothed 1D Occam s inversion of the data show the electrical structure down to an average depth of 400 m (Vedanti et al., 2005). This limited penetration is due to the presence of near-surface conductive rocks that cover all but a small region of high resistivity in the NW portion of the surveyed area (Fig. 6). The low-resistivity anomaly near the Los Azufres bubbling pond (Figs. 2 and 6) is related to the presence of argillic alteration observed at the surface, and down to 530 m depth in well EAC-

7 284 A. López-Hernández et al. / Geothermics 38 (2009) Fig. 5. Total magnetic field reduced to the pole for the Acoculco area. Contours in nt. Constant flying height: 3500 m asl. The location of the magnetic profile given in Fig. 12 is also shown. 1. Apparent resistivities of less than 10 m are found north of Cruz Colorada (Fig. 6) and extend toward the west, and include the southern border of the Acoculco Caldera rim. High resistivities were observed near Pueblo Nuevo (Fig. 6). A 2200-km 2 MT survey consisting of 63 soundings was performed to study the electrical structure of the Acoculco geothermal system at greater depth (Departamento de Exploración, 2000). Data were interpreted using a Bostick inversion of the TE mode (Bostick, 1986) that allowed mapping of subsurface resistivities at depths exceeding 1000 m. The results indicate that the geothermal area is located in a zone of relatively high resistivity, with values above 100 m in the vicinity of the Los Azufres pond (well EAC-1; Fig. 7). We consider that this is caused by the silica deposition identified in EAC-1 cores and cuttings, or by the presence of a horst in the granitic basement inside the inner Caldera border. As with the gravity and magnetic data, the resistivity contours display a general NW SE orientation consistent with that of the dominant geologic structures (i.e. the Tulancingo Tlaxco fault system); some minor local NE SW trends may be related to the NE-striking Apan Piedras Encimadas Lineament. Setting aside differences in the resistivities inferred from DC and MT studies because the two methodologies are based on dissimilar (i.e. galvanic versus electromagnetic) phenomena, the DC study at Acoculco has a more local coverage than the MT exploration, which a smaller density of soundings over the area. Therefore, the DC approach is more adequate to define the characteristics of the shallower rock formations, while the MT data are better to describe those of the deeper and probably older rocks. 4. Surface thermal manifestations At the Acoculco Caldera there are no active hot springs, only cold springs, broad regions of diffuse CO 2 and H 2 S discharge (Polak et al., 1982), and low permeability argillically altered areas. Temperatures measured at these locations are in the C range, which are slightly higher than the expected mean temperature of 12 Cfor an elevation of 2860 m asl. However, these surface features do not give any indications of the high temperatures measured in EAC-1 at relative shallow depths. The main regions of anomalous surface activity are at Los Azufres and Alcaparrosa (Fig. 2), where there are extensive areas of argillic alteration, some active cold gas vents and a few intermittent springs with water temperatures close to ambient. Frequently, and due to the accumulation of toxic gases, dead animals are found by the gas emission areas (Castillo-Hernández, 1986; De la Cruz and Castillo-Hernández, 1986), as well as dead ( burnt ) vegetation. Because of the presence of the relatively impermeable surface clay layer, rainwater may accumulate in shallow ponds. When these overlie active gas vents, active bubbling occurs and the ponded water becomes acidic.

8 A. López-Hernández et al. / Geothermics 38 (2009) Fig. 6. Apparent resistivity distribution (Schlumberger soundings) with AB/2 = 750 m in the Acoculco Caldera area. Contours in m. Darker areas indicate lower electric resistivities. Other anomalous features can be observed East and SE of the town of Acoculco (at a location called Las Minas) where strong argillic alteration of the dacitic lavas and pyroclastic deposits has produced commercial quantities of kaolin. Another place worthwhile mentioning is Cuadro de Fierro (Fig. 2), where gas vents are no longer active, but rust-colored Fe oxides coat the ground surface. Around the Los Azufres pond the smell of hydrogen sulfide indicates the presence of active gas vents. Similar discharges, Fig. 7. Apparent resistivity distribution in the Acoculco Caldera based on the magnetotelluric (MT) method at 5 s (corresponding to m depth). Contours in m.

9 286 A. López-Hernández et al. / Geothermics 38 (2009) referred to as kaipohans, have occurred in Philippine geothermal fields (Bogie et al., 1987; López-Hernández and Castillo-Hernández, 1997). When well EAC-1 was drilled in this area, many of the workers became disoriented or had trouble breathing because of the release of borehole gases. Thermal springs with moderate temperatures are found peripheral to the volcanic complex. Hot springs at Los Baños de Chignahuapan and the Quetzalapa (Fig. 2) have temperatures of 49 C and 30 C, respectively. Thermal (30 C) springs also occur at Jicolapa and El Rincón, 15 km northeast of the Los Azufres pond, near the Piedras Encimadas volcanic center (Fig. 2). The flow rates at Chignahuapan and Jicolapa are very high (>20 l/s). These springs are possibly related to a large network of underground caves and dissolution channels carved in the Sierra Madre Oriental limestones that underlie the study area. The present hot springs at Los Baños de Chignahuapan (Fig. 2) are about 2 km SE of several levels of terraces consisting of fossil hydrothermal travertine deposits, indicating that the focus of the thermal activity has shifted toward the southeast Fluid geochemistry Three types of waters can be distinguished in the Tulancingo Acoculco area (Fig. 8a). The first is an acid-so 4 water formed by the dissolution of gases (H 2 S) into pools of standing water. This water type occurs within the Acoculco Caldera at Alcaparrosa and Los Azufres. The second is a calcium-bicarbonate water discharged by the Chignahuapan and Quetzalapa hot springs located outside the Tulancingo Acoculco Caldera Complex (Fig. 2) The third type is a sodium-bicarbonate water (Table 1) from some springs to the north, on the periphery of the Caldera Complex, and two pools at Los Azufres and Agua Salada; the latter is located 4.2 km NNE of Alcaparrosa. Calcite saturation indices (calculated based on discharge temperatures) for the Quetzalapa, Chignahuapan and Jicolapa spring waters are 0.61, 1.14 and 0.70, respectively, indicating that they are saturated with respect to that mineral. This may reflect the effects of deep circulation through the underlying carbonate rocks, as calcium-bicarbonate waters are normally associated with groundwater systems flowing through limestones (Ellis, 1959; Henley and Ellis, 1983). Because of the anomalously high content of chloride (118 mg/l) and boron (3.2 mg/l) in the waters of the Chignahuapan hot spring when compared to those of surrounding springs and the presence of ancient system faults (Tulancingo Tlaxco) connecting both zones, this hot spring is interpreted as being the farthest SE discharge of the hydrothermal system. Its waters could be a mixture of deep geothermal fluids and shallow groundwaters. The Na K Mg ternary plots (Fig. 8b) developed by Giggenbach (1988) indicate that the thermal waters of the Tulancingo Acoculco area are immature, having not reached chemical equilibrium with the host rocks. Since geothermometric results based on the compositions of immature waters do not provide useful reservoir temperature data (Tello-Hinojosa, 1986), they are not presented here. Trace elements in the spring waters (As 3+,Hg 2+,Cu 2+,Cd 2+,Fe 2+, Pb 2+ and Mn 2+ ) were analyzed to determine the effects of natural thermal water discharges on the chemistry of local shallow groundwaters (Quinto et al., 1995). Values exceeding maximum environmentally allowable standards were only reported at Los Azufres and Alcaparrosa. These elevated concentrations were due in part to the acidic character of the waters and the dissolution of the country rocks. The ıd and ı 18 O contents in the waters fall along the meteoric water line (Fig. 9), regardless of their chemical character or location. The sample from Los Azufres is the only one that presents any Fig. 8. Chemical composition of surface water samples of Acoculco Caldera (a) Cl, SO 4, and HCO 3, (b) Na, K, Mg. Full equilibrium line and region of immature waters are from Giggenbach (1988). See Table 1 for more details. isotopic enrichment, which most likely reflects evaporation in the shallow pond. No representative downhole fluid samples were collected in EAC-1 because of the low permeability of the rock column cut by the well Gas composition Analysis of cold gas samples show that CO 2 is the most abundant gas dissolved in the thermal waters, ranging from to mmol/mol, followed by H 2 S (see Table 2). Helium and CH 4 occur in relatively high amounts, while H 2 is only present in small concentrations. The elevated CH 4 content could result from bacterial decomposition of bituminous material in the calcareous rocks found at the base of the volcanic sequence. The origin of the gases can be deduced from their relative N 2, He and Ar contents (Fig. 10). Alcaparrosa gases plots near the He apex suggesting a crustal source, which is corroborated by a molar He/Ar ratio of that is larger than the atmospheric value of (Mazor, 1977). Gases in the El Rincón, Jicolapa and Quetzalapa waters fall on the crustal-meteoric line, suggesting that the crustal gases are diluted by air-saturated groundwater. Data

10 A. López-Hernández et al. / Geothermics 38 (2009) Table 1 Chemical composition of water samples from the Tulancingo Acoculco area and the Philippines (all components given in mg/kg). Location Sample Height (m asl) Sampling date Temperature ( C) ph Na K Ca Mg B Cl Li SO 4 SiO 2 HCO 3 Reported by Quetzalapa a Q /06/ nd b El Rincón 7 a R /06/ b Cuadro Fierro 9 a F /06/ nd b Alcaparrosa 17 a A /06/ nd b Los Azufres 21 a Z /06/ nd b Los Azufres 22 a S /06/ b Chignahuapan 33 a G /07/ b Jicolapa 34 a J /07/ nd b Kaipohan 5 c K 08/08/ nd 12.1 nd nil d Kaipohan 3 c P 10/01/1982 Cold nd d nd: not detected. a Acoculco. b Tello-Hinojosa (1994). c Philippines. d Ruaya (1980), quoted in Bogie et al. (1987). Fig. 9. Oxygen-18 and deuterium contents in hot spring waters of the Acoculco area. See Table 1 for more details. from the Los Azufres bubbling pond plot near the Ar apex probably due to errors incurred during sampling. One should note that the gases from New Zealand s kaipohans (Bogie et al., 1987) havea greater magmatic contribution. Fig. 10. Relative Ar, N 2 and He content of gas samples at Acoculco area. See Tables 1 and 2 for more details. According Polak et al. (1982), the high 3 He/ 4 He ratio (R/R air = 6.3) measured in the gas emissions from the Los Azufres pond suggests a high geothermal potential for this zone. On the other hand, Tello-Hinojosa (1986), following Ellis (1957), propose that the high N 2 /Ar ratio (=238) measured in the Alcaparrosa pond, which is Table 2 Chemical composition of gas samples from cold and hot springs from the Tulancingo Acoculco area and New Zealand (all components given in molar % of non-condensable gases). Location Sample Date CO 2 H 2S NH 3 He H 2 Ar O 2 N 2 CH 4 Reported by Quetzalapa a Q 18/06/ b El Rincon a R 19/07/ b Alcaparrosa 17 a A 24/06/ nd b Los Azufres 21 a S 25/06/ nd nd b Jicolapa a J 03/07/ b Pool c L 23/05/ d Waitotara c W 29/06/ nd nd e nd: not detected. a Acoculco. b Tello-Hinojosa (1994) mmol/mol. c New Zealand. d Sheppard (1986), quoted in Bogie et al. (1987) mol% of total NCG. e Giggenbach and Lyon (1977), quoted in Bogie et al. (1987).

11 288 A. López-Hernández et al. / Geothermics 38 (2009) much higher than the one expected for shallow air-saturated water (N 2 /Ar = 38), indicates that the high N 2 content in this sample has a deep origin. 5. Exploratory drilling The decision of drilling an exploratory geothermal well at Tulancingo Acoculco was made based on interpretation of field data, i.e., emission of magmatic gases, outcropping of young volcanic rocks associated with the Caldera Complex, presence of extensive zones of hydrothermal alteration, and low subsurface electric resistivities. The existence of high temperatures at depth was considered to be a real possibility. The vertical well EAC-1, sited close to the Los Azufres pond and numerous gas vents (Fig. 2), was drilled by CFE to a total depth of 2000 m in The main objective of the well was to determine if high temperatures and permeabilities existed at depth. Three major lithologic units were penetrated by EAC-1 (Fig. 3). The uppermost consists of 790 m Pliocene-to-Quaternary volcanic rocks that are related to the evolution of the Tulancingo Acoculco Caldera Complex. They include, from bottom to top, the Las Minas Rhyodacite and the Alcholoya Ignimbrite (related to the Tulancingo Caldera), and the Cruz Colorada Dacite and the Acoculco Ignimbrite (associated with the Acoculco Caldera). The two deepest units are intensively silicified, which has destroyed the original rock textures. Both of these deep units are interpreted to represent the fill of the Tulancingo Caldera. In well EAC-1 the Pliocene-to-Quaternary volcanics overlie an 870-m thick column of metamorphic rocks (the intermediate unit) that is composed mainly of skarns that present two thin zones of marble. This sequence consists of folded Cretaceous sediments of the Sierra Madre Oriental locally metamorphosed by a hornblende granite, the third and deepest unit penetrated by the well. This granite was encountered from 1660 m to total depth (2000 m). This intrusive may be correlated with a regional episode of Mid-Miocene granitic intrusions (Yáñez-García, 1980). However, since we have not dated the granite because of the alteration of the collected samples, we cannot be sure of its age. Therefore it could be related to a more recent igneous event. No evidence of faulting was observed at the contact between the granite and the metamorphic rocks, suggesting that the emplacement of the granite resulted in contact metamorphism and the formation of skarns and marbles. Underlying the Cretaceous formation, the Jurassic sequence is inferred and could correspond to the regional basement; Morales and Garduño-Monroy (1984) described such stratigraphic relationship in an area north of the Tulancingo Acoculco Caldera Complex. Aplite dikes cut the skarn but not the overlying volcanics. Consequently, these dikes are interpreted as late-stage intrusions related to the emplacement of the granite. Younger basaltic andesite dikes intrude the sedimentary sequence and the base of the overlaying volcanic section. In this case, the dikes may be related to the basaltic andesitic lava flows, exposed elsewhere, that correspond to the most recent volcanic event in the study area. During drilling there were few circulation losses, all of them less than 10 m 3 /h. However, at shallow (70 m) depth, EAC-1 encountered a permeable zone within the Acoculco Ignimbrite that discharged a large quantity of gas (Gama et al., 1995). A series of four temperature and pressure logs were run at different depths. Excluding the shallow permeable zone, the temperature profiles are linear, indicative of a conductive thermal regime. The average gradient was about 11 C/100 m, approximately three times the normal gradient in the Trans-Mexican Volcanic Belt (Ziagos et al., 1985). The last temperature log, taken 288 h after drilling had stopped, shows a high constant gradient of 13.8 C/100 m and a maximum temperature of 307 C at bottom hole (Fig. 3). The temperature data and the fact that the well did not flow on its own, was interpreted as an indication that EAC-1 had been drilled through a column of low-permeability rocks. The data shown in Fig. 3 indicate that the most permeable zone in EAC-1 is located in the upper 400 m of the well. 6. Hydrothermal alteration Alteration minerals were identified based on thin sections of EAC-1 core and drill cutting samples, the latter collected at 10- m intervals. X-ray diffraction analyses were used to determine the mineralogy of the clay fraction in samples taken every 100 m between 100 and 1800 m depth; these were analyzed after being first saturated with ethylene glycol and then dried at room temperature (IIE, 1995). Samples of the upper 800 m of the well display the most intense hydrothermal alteration (from 40% to 100%), being particularly strong between m and m depth. In contrast, the alteration of the skarn and granite is relatively weak, being about 10% (Fig. 3). The main hydrothermal minerals in the EAC-1 samples are quartz, calcite, pyrite, clays (illite, smectite, kaolinite), zeolites (stilbite), and iron oxides (hematite), which are distributed throughout the lithologic column. Wollastonite, garnet, diopside and epidote, as well as copper sulfides, occur in the skarn. Based on hydrothermal mineral assemblages four zones have been identified. The two shallowest mineral zones are interpreted to have formed during the evolution of the Tulancingo and Acoculco Calderas, and the deepest during granite intrusion and skarn formation. In order of increasing depth, these zones are: (A) A shallow (<200 m depth) zone of acid-altered rocks containing kaolinite + pyrite + calcite; the carbonate may be associated with a different hydrothermal pulse, (B) An intermediate ( m depth) zone of argillic-altered rocks containing smectite + illite + quartz + calcite, which is typical of neutral ph waters, (C) A deep ( m depth) high-temperature mineral assemblage in the skarn consisting of wollastonite + garnet + diopside (Lentz, 1998), and (D) A retrograde assemblage within the skarn consisting of quartz + epidote + calcite + chlorite + pyrite, copper sulfides (chalcopyrite, pyrrotite, idaite). In other hydrothermal systems, the deposition of sulfide minerals is a late-stage feature of skarn deposits (Meinert, 1992). Secondary silica minerals are present throughout the well, being most abundant at depths of , and m. Amorphous silica is found in the Acoculco Ignimbrite and intense silicification is observed in the Alcholoya Ignimbrite and the Las Minas Rhyodacite. Much of this silica may be derived from the alteration of the rock groundmass. In the skarn there are veins filled with quartz + pyrite + calcite. Narrow veins in the granite are filled by quartz. Calcite appears mainly in the upper 800 m of the column (Fig. 3), within the volcanic sequence, replacing plagioclase and filling veins. The maximum abundance of hydrothermal calcite is observed in the top 550 m, gradually diminishes with depth; it is not found below 1500 m. Calcite is rare in the skarn, occurring locally as vein filling. Pyrite is the most prevalent sulfide alteration; it generally occurs with quartz and calcite. It is most abundant between 20 and 240 m depth, reaching a maximum of 25% of total alteration; below 240 m it is less abundant. In the granite, pyrite occurs disseminated throughout the rock and as a vug filling.

12 A. López-Hernández et al. / Geothermics 38 (2009) At shallow depths stilbite is found sporadically in the Acoculco Ignimbrite replacing volcanic glass. Chlorite is a minor secondary mineral appearing between 400 m depth and bottomhole (Fig. 3). Chlorite, illite, pyrite, biotite and hematite were the secondary minerals identified in the granite; all in trace amounts. Adularia, a mineral frequently associated with permeable zones and boiling fluids (Browne and Ellis, 1970), occurs only within an aplite dike near the contact between the carbonate rocks and the granite. Its absence is consistent with the inferred low permeabilities of the rocks penetrated by EAC-1. Epidote is a common alteration mineral in geothermal systems where temperatures exceed 240 C(Reyes, 1990). However, at Acoculco, despite measured temperatures above 300 C, epidote is very scarce. It only appears as disseminated grains in the altered skarn. Hematite was recognized in all samples. It is more abundant between 100 and 800 m depth, reaching 2% of total alteration; the percentage increased at the contact between the Acoculco Ignimbrite and the Cruz Colorada Dacite (at 140 m depth). Chalcopyrite, related with the formation of marbles, appears in two thin (20 m) intervals at 1210 and 1450 m depth. Pyrrotite was identified between 1450 and 1690 m in the skarn and disappeared at the contact with the granitic intrusion. Rare idaite, associated with pyrrotite, is found at 1460 and 1600 m depth Clay minerals Clay minerals, replacing volcanic glass and K-feldspars, are found throughout the well. However, the highest concentration is found in the volcanic sequence (i.e. above 800 m depth). In the volcanic rocks, clay minerals reach abundances of up to 52% with an average value of 17%. Below 800 m depth, their abundance drops to 1 2% (Fig. 3). Kaolinite and pyrite are common at shallow depths (<200 m; Fig. 3), suggesting the interaction of geothermal gases with shallow groundwaters. In deeper parts of some geothermal systems, at temperatures of about 230 C, illite is the common clay alteration mineral (Reyes, 1990); however, in EAC-1 it is observed throughout the well, even at the shallowest depths where present-day temperatures are much lower. Smectite also occurs along the entire well column. In Zone II (700 and 1200 m depth) the inclusion data show a positive temperature gradient with a few variations that could be the result of weak vertical convection in the surrounding rock formations. Homogenization temperatures increase to a maximum value of 282 ± 6 C at 1200 m depth. In Zone III (the deepest) there is a sudden decrease in homogenization temperatures, which vary from 223 ± 13 Cat1400mto 252 ± 9 C at 1600 m depth. All these inclusions are related to a metamorphic event as indicated by their high salinities (see next section). At greater depths, their homogenization temperatures again increase, reaching 284 ± 10 C at 1700 m depth and their salinities diminish (see next section). These inclusions are associated with a more recent hydrothermal event. In Zone III the effects of the two main hydrothermal events are superimposed Fluid inclusion salinities and gas contents The relationship between ice-melting temperature (Tmi) and Th is shown in Fig. 11. Based on Tmi differences (Table 3), two groups of fluid inclusions can be distinguished. Group 1, characterized by high-salinity inclusion fluids (and lower Tmi values), occurs within the skarn (between 1400 and 1600 m depth; crosses in Fig. 11). The lower salinity Group 2 fluid inclusions are found in both the metamorphic and volcanic rocks intersected by the well. The ice-melting temperatures of Group 1 inclusions range from 12 C to 15.8 C. Based on Bodnar (1993) these values correspond to salinities between 16 and 19.3 wt% NaCl equivalent (Table 3). Such high salinities are typical of magmatic systems and skarn deposits (Einaudi et al., 1981; Roedder, 1984; Meinert, 1992). In contrast, the inclusions in Group 2 have low salinities that are typical of active geothermal systems (e.g. Ahuachapán in El Salvador and Los Azufres in Mexico) (González-Partida et al., 1997, 2000, 2005). Group 2 inclusions display two types of melting behavior after being frozen. Samples taken in the upper 200 m of well EAC-1, yield 7. Fluid inclusions studies Microthermometric measurements were done on drill cuttings, and on calcite and quartz veins in core samples of well EAC-1. At room temperature most of fluid inclusions consisted of liquid + vapor, with the liquid phase predominating. The size of the studied inclusions was in the m range; Table 3 summarizes the results of the fluid inclusion studies. The relationships among homogenization temperature (Th), depth, lithology, mud circulation losses, alteration percentage, secondary mineralogy and measured temperature are shown in Fig Homogenization temperatures (Th) The geologic column cut by well EAC-1 can be divided into three zones (Fig. 3) on the basis of the average homogenization temperatures obtained for fluid inclusions in drill cuttings and cores. Zone I ( m depth) is characterized by a temperature increase from 131 ± 10 C at 100 m depth to a maximum of 178 ± 9 C at about 400 m, followed by a decrease to 155 ± 3 C. This behavior reflects the existence of a subsurface zone where volatile gases were trapped; this is confirmed by the measured positive (>0 C) melting temperatures indicating the presence of CO 2 in the fluid inclusions. Fig. 11. Homogenization temperature vs. ice melting temperature for fluid inclusions in quartz and calcite crystals from EAC-1 samples from volcanic and metamorphic rocks.

13 290 A. López-Hernández et al. / Geothermics 38 (2009) Table 3 Fluid inclusion microthermometric measurements on drill cuttings and core samples from well EAC-1. Depth (m) Well EAC-1 (Host) Th range ( C) Th average ( C) (n) Tmi range ( C) Tmi average ( C) (n) Salinity (wt% NaCl) 100 Ca (20) +2.6 to (20) N.C. 200 Ca (25) +1.8 to (25) N.C. 400 Ca (40) 1.4 to (30) Ca (15) 0.7 to (15) Ca (35) 0.1 to (29) Ca (28) 0.1 to (20) Ca (45) 0.5 to (35) Ca Skarn (20) 12 to (15) Ca Skarn (35) 13 to (22) Ca Skarn (49) 15.8 to (30) Qz (35) 0.2 to (28) 0.35 Ca, calcite; Qz, quartz; Tmi, ice-melting temperature; Th, homogenization temperature; n, number of samples; N.C., not calculated; Salinity was calculated for Tmi using Bodnar (1993) equation. positive dissociation temperatures ranging from +1.8 C to +5.6 C, suggesting the formation of CO 2 clathrates upon freezing and the trapping of fluids rich in CO 2. Fluid inclusions from 200 to 800 m depth give Tmi values of 0.1 Cto 1.4 C, corresponding to salinities of wt% NaCl equivalent. A similar, but narrower, range of Tmi values characterizes the fluid inclusions occurring between 800 and 1200 m depth. These inclusions yielded ice-melting temperatures of 0.1 Cto 0.5 C corresponding to salinities of wt% NaCl equivalent. At 1700 m depth fluid inclusions record conditions similar to those measured at 1200 m depth (i.e. Th = 284 ± 10 C; Tmi = 0.2 C or 0.4 wt% NaCl equivalent). In summary, fluid inclusion data suggest that two hydrothermal events have occurred at Tulancingo Acoculco. One was characterized by high salinity fluids and was related to the formation of the skarns, while the second, a younger and superimposed event, had fluids of lower salinity and was associated with the evolution of the Tulancingo Acoculco Caldera Complex. 8. Discussion and concluding remarks The intersection of two regional fault systems, one striking NW and the other NE, created an intensely fractured zone in the study area providing a pathway for ascending magmas; their eruption led to the formation of the Tulancingo Caldera, and later, of the Acoculco Caldera (López-Hernández and Castillo-Hernández, 1997). Eventually this resulted in the development of the Tulancingo Acoculco Caldera Complex and its associated geothermal systems. A similar explanation has been proposed for the origin of the geothermal fields at Los Humeros and La Primavera, Mexico (Garduño-Monroy and López-Hernández, 1987). The Caldera Complex is located in a NW SE-trending tectonic depression now filled with volcanic and sedimentary deposits. Inside this feature an elliptical low-gravity zone was observed in the Tulancingo Acoculco area indicating the presence of low-density Caldera fill and felsic rocks. Inside the Caldera Complex the gravity and magnetic susceptibility highs are associated with the most recent post-caldera magmatism. The evidence of this igneous episode includes the mafic dikes encountered in the well EAC-1, the cinder cones and associated basaltic andesitic lava flows found along the Caldera ring structure, and the low volcanoes and scoria cones of the same composition observed around the Caldera complex (Fig. 2). The hydrothermal system at Tulancingo Acoculco, associated with the evolution of the two superimposed Caldera structures, is located in the central part of the younger Acoculco Caldera. The lack of thermal springs inside the volcanic complex and the presence of extensive areas of argillic alteration (some of them fossil) seem to indicate that the hydrothermal system is not very active at the present time. However, surface gas emissions, the high temperatures (>300 C) measured in well EAC-1, and the presence of hot (49 C) springs at Chignahuapan, located about 19 km from the Los Azufres cold-gas emission zone, suggest that there is a hidden active hydrothermal system in the study area (Fig. 12). The presence of calcite and of the kaolinite pyrite assemblage at the same depth could be evidence of two different hydrothermal pulses, similar to what was reported in geothermal systems of the Philippines (Reyes, 1990). This hypothesis, however, would have to be confirmed by data collected from future wells. The intense hydrothermal alteration (and mineral deposition) observed in EAC-1 samples indicates the presence of a very lowpermeability caprock that prevents the ascent of high-temperature fluids to the surface. Only exsolved residual gases (CO 2 and H 2 S) from the deep reservoir can flow up through very low permeability fractures in the caprock. As these gases cool during their ascent (by thermal and mass transfer to the surrounding rocks), they are cold when discharged at the surface. The present low permeability of the rocks encountered by EAC-1 is due to: (a) The inherent low permeability of skarns and granites, which may have limited the development of the hydrothermal system from the very beginning, (b) The partial sealing of the NW- and NE-striking regional faults that may channel the ascent of magmatic and hydrothermal fluids because of the deposition of hydrothermal minerals, (c) The intense secondary mineral deposition in the pyroclastic deposits created a very effective caprock, and (d) The lack of recent significant tectonic and magmatic activity that has precluded fault reactivation. The hydrology of the Tulancingo Acoculco geothermal system has not been studied yet, and more exploratory drillholes are needed; however a first hypothesis suggests that below the caprock, the geothermal fluids ascend through fractures associated with the old NW SE and NE SW faults and mix with shallow groundwaters of meteoric origin. The mixed fluids move laterally towards the southeast through fractures in the limestones to finally discharge, highly diluted, at Los Baños de Chignahuapan, 19 km SE of Acoculco. Based on the high hot-spring flow rates the permeability of the limestones in these areas must be high. The presence of boron and the concentration of chloride above the normal content of meteoric waters in that area (Table 1) are consistent with this interpretation and confirm the mixing of shallow groundwaters with geothermal fluids. Other evidence is the presence of terraces, consisting of fossil hydrothermal travertine deposits, at a location between the alteration zone at Acoculco and the Chignahuapan springs, indicating that the thermal activity has shifted toward the southeast.