Instituto de Investigaciones Eléctricas, Cuernavaca, Morelos, México 3 Comisión Federal de Electricidad, GPG. Morelia, Michoacan.

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1 GRC Transactions, Vol. 38, 2014 Significance of Deep Zones of Intense Bleaching and Silicification in the Los Humeros High-Temperature Geothermal Field, Mexico: Evidence of the Effects of Acid Alteration Wilfred A. Elders 1, Georgina Izquierdo-Montalvo 2, Alfonso Aragón Aguilar 2, Rigoberto Tovar Aguado 3, and Magaly Flores Armenta 3 1 Dept. of Earth Sciences, University of California, Riverside CA, USA 2 Instituto de Investigaciones Eléctricas, Cuernavaca, Morelos, México 3 Comisión Federal de Electricidad, GPG. Morelia, Michoacan gim@iie.org.mx Keywords Los Humeros geothermal field, acid alteration, bleaching, leaching, silicification, acid leaching, hydrothermal mineralogy ABSTRACT The Los Humeros geothermal field is a high-temperature, wells produce high steam fraction, with a large resource potential. However, despite the drilling of more than 40 deep wells, many of which encountered > 300 o C steam, it has an installed electrical generating capacity of 40 MWe. Low permeability reservoir rocks and acid fluids that cause accelerated corrosion have hampered development. It has been suggested that acid fluids occur throughout the reservoir. However, this is inconsistent with the marked absence of alteration minerals in the volcanic rocks typical of low ph fluids. Instead the mineralogical evidence indicates that the hydrothermal alteration of the reservoir rocks is typical of f neutral to alkaline ph waters. Thus it appears that since the reservoir has apparently transitioned from being waterdominated to being vapor-dominated in the geologically recent past. An example of local acid leaching that formed a bleached, intensely silicified, zone is found at a depths of about 2000 m in a non-productive but very hot (>370 o C) well. These rocks retain the textures of a lithic crystal tuff. The altered rock consists almost entirely of microcrystalline quartz, with sparse relic pseudomorphs of plagioclase phenocrysts and traces of chlorite and pyrite. This zone is approximately 160 m thick; and the deeper rocks lack this silicification. Smaller similarly bleached silicified zones in adjacent wells have been recognized in drill cuttings at different depths. These observations are interpreted to result from lateral flow of corrosive fluids. The origin of these acid fluids is still a subject of debate. They may represent emanations from a magma chamber or be due to post-exploitation processes (e.g. reaction of water and salts forming hydrogen chloride 497 by hydrolysis at high temperatures). The very high boron content of the fluids produced by the Los Humeros wells suggests that the ultimate source of the acid gases is most likely magmatic. These acid gases have not reacted widely with the rocks. We suggest that the silicified zones are forming locally where descending waters encounter superheating steam containing acid gas and form low ph liquids that react and leach the rocks. Introduction The Los Humeros geothermal field (LHGF) is situated within a large caldera in the western part of the state of Puebla, Mexico, about 280 km east of Mexico City (Figure 1). The Los Humeros Figure 1. Map of the Los Humeros geothermal field within the Los Potreros Collapse caldera in the southern part of the larger Los Humeros caldera, showing the smaller volcanic crater Xalapazco Mastaloya together with known and inferred faults. The numbered well locations are shown without the preceding letter H. (Lopez-Romero, 2006).

2 caldera lies in the north-eastern part of the calc-alkaline Trans- Mexican Volcanic Belt. It is associated with the subduction of the Cocos Plate along the Middle American Trench (Verma, 1983). The Los Humeros caldera has had a complex history (Ferriz and Mahood, 1984). Its dimensions are 21 by 15 km but its margins tend to be obscured by younger volcanic rocks, except in the northeast quadrant where the topographic rim is visible. The oldest rocks of the caldera are > 1.6 Ma porphyritic andesites and ferrobasalts of the Teziutlán Formation. These rocks have a volume of 60 km 3. At 0.46 Ma, main collapse of the Los Humeros caldera was initiated by the eruption of the mostly non-welded Xáltipan Ignimbrite, aphyric high-silica rhyolite; which has an estimated volume of 115 km 3. e. The ignimbrite is overlain by air-fall lapilli tuffs ranging from rhyodacite to andesite in composition. The amount of collapse, determined by the offset of the lower contact of this ignimbrite is about 450 m. The next pyroclastic eruption was the Faby Tuff with a volume of approximately 10 km 3, a thickness of 16 m, and age of 0.2 to 0.3 Ma This eruption was followed by a period of quiescence, erosion and the formation of lake deposits. The next pyroclastic eruption formed the 0.1 Ma Zaragoza Tuff, which has a volume of 12 km 3. It consists of non-welded ignimbrite and an overlying lithic-rich airfall tuff. The Zaragoza Tuff was associated with the collapse of the 10 km diameter Los Potreros caldera in the southern part of the Los Humeros caldera. This rhyodacite tuff contains pumice with phenocrysts of pyroxenes, plagioclase, and Fe-Ti oxides. Based on an apparent displacement of 375 m of the base of the Xáltipan Tuff between boreholes H2 outside the collapse structure and H3 and H4 within it, the estimated volume of the magma erupted during this collapse was 17 km 3. There was then a change to magmas of more variable composition. The next major eruptions produced 6 km 3 of andesite at Ma and 10 km 3 of rhyodacite at Ma. The most recent eruptions produced 0.25 km 3 of olivine basalt of an age of <0.02 Ma (Ferriz and Mahood, 1984). Los Humeros Geothermal Field The Los Humeros geothermal field (LHGF) lies entirely within the Los Potreros Collapse caldera (Figure 1). More than 40 exploration, production and injection wells have been drilled to depths between 1500 and 3100 m. These wells have encountered temperatures as high as 378 o C at less than 3,000 m depth. The subsurface geology of the field has been described by (Viggiano and Robles (1988) Cedillo (1997) and Gutiérrez-Negrín and Izquierdo-Montalvo (2010). These authors identified four basic units with variable thicknesses. However, not all of the units are observed in every well. Unit 1. Post-Caldera Volcanism; Quaternary, 0.1 Ma. This unit is composed of andesite basalt, dacite, rhyolite flows and tuffs, pumice, and ejecta from phreatic eruptions. Its thickness is between 90 and 1010 m, but averages 340 m. It contains shallow cold aquifers. Unit 2. Volcanic Caldera; Quaternary Ma. This unit consists of lithic and glassy ignimbrites, from two eruptive centers; Los Humeros and Los Potreros. It includes tuffs, andesitic lavas, and rhyolite domes. Its thickness is between 185 and 880 m, but averages 600 m. This unit forms a low permeability aquitard or caprock. 498 Unit 3. Pre-caldera Volcanism; Miocene Pliocene, Ma. This unit is composed of andesite flows, with interbedded tuffs, basalts, dacite, and rhyolite. In the upper andesite flows the characteristic mafic mineral is augite and in the deeper andesites, it is hornblende. The unit is between 90 and 2600 m thick, but averages 1200 m. Unit 3 is the host rock for the geothermal fluids, its permeability varies from low to medium. Unit 4. Basement; Jurassic-Oligocene, Ma. The basement complex is composed of limestone with subordinate shale and chert. It is intruded by stocks of, granite, granodiorite, tonalite younger (Miocene) dikes of diabasic composition. Contact metamorphism has altered the sedimentary rocks to marble, skarn, and hornfels.the total thickness of the unit is unknown. High temperatures and low permeabilities characterize this basement complex. Chemical Characteristics of the Los Humeros Geothermal Fluids Most of the wells in the LHGF produce high steam fraction with a small liquid fraction and enthalpies greater than 2600 J/g). The chemical composition of separated water indicates that they steam condensates. With the exception of boron and silica; the concentrations of the major cations are low. The total flow of geothermal fluids from the wells of this field is generally of sodium chloride type with lesser amounts of sodium sulfate. Representative fluids from the hottest producing wells are characterized by high boron concentrations (> 2,000 ppm), relatively high but variable concentrations of SiO2 ( ppm) and low concentrations of cations such as Na, K, Ca and Mg. Despite the acid nature of the condensed steam, in the majority of the total produced flows is reported to have ph s near neutral with low Cl concentrations (Tello, 1991). Table 1 presents the chemical analyses of separated water of six steam wells sampled in November The capital N, C and S, attached at the well number refers to the area of the field where the well is located, north, center, or south. The main anions are B and SiO 2. Table 1. Chemical analysis of separated water from 6 wells at Los Humeros. Concentration is given in mg/l. N is for north, C for center and S for south (unpublished analyses by Izquierdo). Well Cl HCO 3 SO 4 Na K Ca Mg SiO 2 B Fe Mn H-35/N H-37/N H-19/C H-45/C H-6/S H-39/S With time, the steam fractions of the wells typically increase, which favors the transport of volatile species. The origin of the high concentrations of boron is unknown but it is likely that it is a mobile magmatic species, as there are no boron-rich minerals found in the reservoir. We suggest the relatively high and variable concentrations of SiO 2 are due to interaction of the acid condensates with the rocks. Our studies of the hydrothermal alteration

3 indicate that locally acid condensates cause reactions with the reservoir rocks preferentially dissolving Na, K, Ca, and Mg from primary and secondary minerals leaving behind microcrystalline quartz and a fluid with higher ph. Hydrothermal Alteration in the Los Humeros Geothermal Field The results of fluid/rock interactions are ubiquitous in moderate to high-temperature geothermal systems. The nature and abundance of hydrothermal minerals are functions of temperature, fluid composition, ph, and water/rock ratio (Browne, 1978). Assemblages of hydrothermal minerals thus record the processes and conditions that have occurred in a geothermal reservoir (Elders, 1977). Two broadly different hydrothermal assemblages are commonly recognized (Browne, 1978; Henley and Ellis, 1983, Reyes, 1990; 1992). The most common is produced by interactions with neutral ph to basic sodium chloride fluids. Less commonly, acid argillic alteration is formed by reaction of the rocks with fluids of low ph, usually hydrochloric or sulfuric acids (argillic). Table 2 presents a summary of the hydrothermal minerals that are most typical of different ranges of temperature and ph. Table 2. Occurrence of common hydrothermal minerals as function of temperature and ph, (Compiled from Browne, 1978, Henley and Ellis, 1983 and Reyes, 1990; 1992). Acid environment ph < 100 C < 200 C > 250 C Cristobalite, Kaolinite, Halloysite, Alunite Kaolinite, Quartz, Alunite, Pyrite, Illite Near neutral Clay Minerals Interstratified clay minerals, Illite, Chlorite, Feldspars Neutral to alkaline environment Zeolites, Clay Minerals (Smectites) Zeolites, Prehnite, Chlorite, Quartz, Calcite Diaspore, Pyrophyllite, Quartz, Pyrite Illite, Actinolite, Epidote Wairakite, Epidote, Chlorite, Biotite, Prehnite, Amphiboles The reservoir rocks of LHGF contain abundant hydrothermal alteration minerals. Figure 2 is a compilation showing the temperature ranges of these alteration minerals as determined by Viggiano and Robles (1988) and Martínez and Alibert (1994). Our work indicates that the main alteration minerals are chlorite, epidote, quartz, calcite, and minor leucoxene and pyrite. Other alteration minerals we have identified include smectite, kaolinite, illite, anhydrite, amphiboles and garnet. As illustrated in Table 1, these hydrothermal mineral assemblages are typical of those produced by neutral or alkaline fluids. In spite of the chemistry of the fluids produced from the wells, mineral assemblages typical of acid alteration are not observed in the deep geothermal reservoir. The intensity of alteration of these rocks is variable, depending on rock permeability. Minerals that change as a function of temperature, rock and fluid composition, such as phyllosilicates and zeolites, which is common in other geothermal reservoirs, has not been observed in the LHGF reservoir. X-ray diffraction (XRD) analysis the clay fraction of samples from wells H14, H15, H17 and H29 (Libreros, 1991; Izquierdo, Figure 2. Temperature ranges of major hydrothermal alteration minerals observed in various wells in the LHGF (Modified from Martínez and Alibert, 1994). 1993) showed the presence of kaolinite but only as a replacement of volcanic glass in the ignimbrites at shallow depths. Superficial argillic alteration indicative of a low ph, oxidizing environment is only observed at shallow depths. This alteration is produced by H 2 SO 4 formed by oxidation of H 2 S at the surface. XRD analyses of surface samples from to the vicinity of wells H4, H7, H9 and close to the Xalapasco Mastaloya cratercontain alunite, kaolinite, gypsum, jarosite, alunogen and alum (Arellano et al., 1998). These zones of superficial argillic alteration are located along mapped faults. Results Deep Zones of Bleaching, Leaching, and Silicification As mentioned above, mineral assemblages characteristic of acid alteration have not been seen in the deeper reservoir rocks. However evidence of local interaction with acid fluids is present. The best example is seen in samples well H26, which was drilled in 1988 in the eastern part of the geothermal field near the central part of the Los Potreros caldera (Figure 1). It is located 1360 m from well H1, in an east-northeast direction; 950 m from well H23 in a west-southwest direction, and 100 m distant from well H26 in a north-northwest direction (Figure 1). H1 and H6 are in production, but because of very low permeabilities H23 and H26 are non-producers and currently shut-in observation wells. Well H26 is unique in that: (1) five drill cores were obtained from it; and (2) the initial well report by the field operator, the Comisión Federal de Electricidad, suggested that it had the highest static temperature observed in the field (378 C). However this temperature was not measured directly but was calculated using Horner plots from the rate of heating observed in repeated temperature surveys measured shortly after drilling and before the well reached thermal equilibrium (Figure 3). Mechanical problems 499

4 Figure 4. A cut surface of core 4 from well H26 ( m depth), is a bleached lithic crystal tuff showing relict eutaxitic (welded) textures. Examination of drill cuttings indicates that the thickness of this altered zone in well H26 is about 160 m. The results of different processes can be obfigure 3. Loss of circulation (m3/h) during drilling of well H26, with selected temperature served in this silicified zone. Firstly, the protolith of logs. T/30 temperature log 12 hours after circulation ceased, T/31 after 18 hours, T/32 after this rock is a welded tuff, which at this depth, is most 24 hours, T/36 after 490 hours. Stars = static temperatures estimated by the Horner method. likely part of Unit 3, the pre-caldera volcanism that The column on the right shows 1ithology:1 = pumiceous tuff, basalt and andesite; 2 = lithic has subsequently subsided and was buried by 2.1 km tuff; 3 = ignimbrite; 4 = augite andesite; 5 = vitric tuff, 6 = hornblende andesite (Arellano et thickness of younger rocks. The dominant feature of al., 1998). the altered rock is the absence of primary minerals except for relict plagioclase, and micro crystalline quartz, and the due to an obstruction at about 400 m depth evidently prevented usual hydrothermal alteration minerals that would be expected further temperature logging. at these temperatures, such as epidote, biotite, garnet, actinolite Figure 3 is summary of data for well H-26, taken from Areland diopside. It appears that silicification was not as an additive lano et al., 1998; it includes data from an internal unpublished process, but was caused by partial dissolution that leached out the report from the Comisión Federal de Electricidad. Near 2000 m more easily dissolved components and generated millimeter sized depth, it mentions an occurrence of a silicified andesite. The cavities, sometimes empty or partly mineralized with microcrystalreport also notes an unusually high concentration of boron in the line quartz and minute crystals of pyrite. returned drilling muds, with a maximum concentration of 862 ppm We can speculate about the timing and environment of this (by weight) at 2000 m depth, but diminishing at greater depths. bleaching and silicification. As indicated above, superficial althe best samples of this silicified andesite come from teration indicative of a low ph environment is locally observed a drill core taken at m depth in well H26. The at shallow depths in the LHGF, and similar processes could have altered rock is cream colored and consists almost entirely of microcrystalline quartz, relict pseudomorphs of primary plagioclase phenocrystals and traces of hydrothermal epidote, chlorite and pyrite. Our examination of drill cuttings indicates that the thickness of this altered zone is about 160 m. In spite of the high degree of alteration, primary pyroclastic textures are still discernible in this drill core, but are generally difficult to identify in the drill cuttings from this well. Figure 4 shows a fragment of this core from 2000 m in well Figure 5. Photomicrographs of the core in well H26 at m. It shows plagioclase crystals in a matrix of mih26 and photomicrographs are crocrystalline quartz. The foliation of the groundmass defines an augen-like texture that is a recrystallized, eutaxitic (welded tuff) texture. Left plane polarized light. Right crossed nicols. shown in Figure

5 operated on these rocks before they subsided. In contrast, rocks in Unit 2 and Unit 3 in well H26 have been propylitically altered and contain hydrothermal epidote, and chlorite. Propylitic alteration is common throughout the reservoir. Only minute remnants of these hydrothermal minerals are seen in the bleached silicified zone at 2000 m depth, and where present the hydrothermal chlorite and epidote are partially dissolved, indicating that the propyltic alteration occurred before the bleaching, leaching and silicification. The samples of core 4 of the well H26 are a witness to a bleaching and silicification process caused by lixiviation, a process of selective dissolution, leaving the less soluble residues enriched in silica relative to the parent rock. The restricted thickness of this bleached and silicified zone in well H26 shows that the trajectory of the ascent of these fluids was lateral. Core and drill cutting samples from above and below the bleached zone do not exhibit the effects of acid leaching, but do show hydrothermal alteration with quartz, epidote and chlorite due to interaction with neutral or basic fluids. Similar bleaching and relative increases in proportions of silica and leaving minute cavities, and open micro-fractures is in deep samples from wells H10, H19, H23 and H27 at different depths. Further studies are underway to better understand the processes of the acid lixiviation that occurred. These studies include more detailed petrography, investigation of fluid inclusions, and chemical and petrophysical analyses of the rocks to investigate the transfer of components between the fluids and rocks. A complementary experimental study of reaction of various mixtures and concentrations of acid solutions with the reservoir rocks of the LHGF has also been undertaken. The results of those on-going studies will be reported later. Discussion In spite of its large resource potential, development of the highenthalpy Los Humeros Geothermal Field is a challenge because of the low permeability of large sections of the field and the acid nature of the produced fluids. Another challenge is to understand the evolution of this complex system. It is now a vapor-dominated system and the production fluids are mostly mixtures of steam and acidic steam condensate with liquid fractions of about However, the hydrothermal alteration of the reservoir rocks at Los Humeros is typical of the interaction of volcanic rocks with waters of neutral to alkaline ph. Thus it appears that a transition from a neutral water-dominated reservoir to an acid vapor-dominated reservoir has been geologically recent and this transition was accompanied by, and due to, a reduction in permeability and a lowering of fluid pressures that induced widespread boiling in the reservoir. Perhaps in the future, the deep reservoir of the LHGF could be considered as a candidate for an Enhanced Geothermal System (EGS) in which hydrofracturing would create permeability and the acid chemistry may be mitigated by controlling the chemistry of the injected fluids. Locally the results of acid alteration can be seen in the reservoir rocks in the form of bleached silicified zones that indicate that lateral flow of corrosive fluids formed zones of bleaching, leaching and silicification. Because the locations and volumes of acid altered rocks in the reservoir are quite restricted, in spite of the acid nature of the fluids produced by the wells, these acid fluids could not have been widespread in the reservoir or could not have had a long residence time in the geothermal system. The origin of these acid fluids is also unclear. Acid conditions are fairly common in vapor-dominated systems such as Larderello and the Geysers due to the formation of acid condensates. Truesdell (1991) described a process whereby HCl can be generated in situ in deep reservoirs where superheated steam hydrolyzes NaCl by reaction with silicates. Acid liquids coming from great depth would have to move through the basement beneath the caldera and would tend to be neutralized as the basement contains a large proportion of marbles and limestones, unless the acid gases move in superheated steam or supercritical vapor, precluding the formation of acid condensates. Conclusions Drill hole samples from depths of about 2 km in a very hot (>370 o C) but low permeability part of the LHGF show evidence of local acid leaching that formed a bleached, intensely silicified, zones consisting almost entirely of microcrystalline quartz. Lateral flow of corrosive fluids formed these zones of bleaching and silicification, but the effects of these fluids are quite restricted in space so that they either formed only locally or they could not have had a long residence time in the geothermal system. The origin of these acid fluids is still a subject of debate. Acid gases may either be emanations from a cooling magma chamber or instead could form by some post-exploitation process (e.g. reaction of water and salts forming hydrogen chloride by hydrolysis at high temperatures). The very high boron content of the fluids produced by the Los Humeros wells is also puzzling as sources of boron, such as tourmaline, are not seen in the rocks drilled so far. This suggests that the source of the acid gases is most likely magmatic. As the deep reservoir has temperatures approaching 400 o C, at moderate pressures, the reservoir fluids at depth must be superheated or supercritical steam in which HCl gas is unreactive. We suggest that the localized acid leached zones observed could be formed where conduits permit colder liquid water can descend and encounters dry steam containing acid gases and forms low ph liquids. Acknowledgements The participation of W. Elders in this project is made possible by a grant from UCMexus and CONACYT. Funding from the Comision Federal de Electricidad (CFE) and the Instituto de Investigaciones Electricas (IIE) supports this ongoing study. We are grateful to the staff of CFE at Los Humeros by their cooperation in freely supplying samples, data and unpublished reports. Also the authors give thanks to Joseph Moore for its time to review this paper and his helpful comments. References Arellano V.M., A. García, R.M. Barragán, G. Izquierdo, A. Aragón, D. Nieva, E. Portugal, and I. Torres, Desarrollo de un modelo básico actualizado del yacimiento geotérmico de Los Humeros, Pue. Instituto de Investigaciones Eléctricas, Unpublished Report. IIE/11/11459/01/F. 501

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