X-Ray Diffraction Analysis of Hydrothermal Minerals from the Los Azufres Geothermal System, Mexico

Transcription

1 International Geology Review, Vol. 48, 2006, p Copyright 2006 by V. H. Winston & Son, Inc. All rights reserved. X-Ray Diffraction Analysis of Hydrothermal Minerals from the Los Azufres Geothermal System, Mexico K. PANDARINATH, 1 IGNACIO S. TORRES-ALVARADO, Centro de Investigación en Energía, Universidad Nacional Autónoma de México (UNAM), Priv. Xochicalco S/No., Col Centro, Apartado Postal 34, Temixco, Mor , Mexico D. ESTHER PUSHPARANI, Ocean Science and Technology Cell (Marine Geology and Geophysics), Mangalore University, Mangalagangotri, Mangalore, , India AND SURENDRA P. VERMA Centro de Investigación en Energía, Universidad Nacional Autónoma de México (UNAM), Priv. Xochicalco S/No., Col Centro, Apartado Postal 34, Temixco, Mor , Mexico Abstract Los Azufres is an active geothermal field located in the middle of the Mexican Volcanic Belt (MVB), a vast Miocene-Recent, E-W oriented volcanic province spanning central Mexico. Three wells Az-5, Az-28, and Az-31 from this geothermal field were selected to investigate the distribution, alteration sequence, and thermal stability of hydrothermal minerals. Maximum in situ measured temperatures are 280ºC at 1493 m, 265ºC at 1700 m, and 288ºC at 1300 m depths in Az-5, Az-28, and Az-31, respectively. The host rocks in these wells are dominantly andesite followed by dacite, rhyolite, and basalt. Rock cuttings from different depths were analyzed for clay and non-clay minerals by X-ray Diffraction (XRD) methods. Hydrothermal quartz, calcite, and pyrite, as well as other alteration mineral phases (e.g., chabazite and chlorite) that are difficult to identify by traditional petrography were identified and their abundances semi-quantitatively estimated by XRD. We show that these mineral data present a better perception of distribution trends of hydrothermal minerals in geothermal wells than the qualitative mineral identifications generally used for this purpose. Homogenization temperatures measured in fluid inclusions of hydrothermal minerals, in situ measured temperatures in the wells, and K + /H + vs. Mg 2+ /(H + ) 2 activity diagrams for the chemical characteristics of the present geothermal fluids were used to define the thermal regime and the resultant stability conditions of the clay minerals. Smectite, illite, and chlorite are present in the <2 µm size fraction. Gradual variations in relative abundances of clay minerals range from smectite dominant at shallow well depths to a combination of smectite, illite, and chlorite at intermediate depths, and to illite and chlorite in the deepest levels. Excellent crystallinity and lack of mixed-layered clay minerals support a model involving a discontinuous change from smectite to chlorite and/or illite, rather than that involving continuous mixed-layering of smectite-illite and/or smectite-chlorite. Mineralogical and fluid inclusion data suggest that mineral distribution trends documented for the Los Azufres geothermal system reflect the prevailing thermal regime. The mineral parageneses of the Los Azufres geothermal field are broadly comparable with those reported in other geothermal systems of the world. Introduction IN A GEOTHERMAL system, apart from the nature of the parent rock, the thermal regime and fluid chemistry of the field are the main factors that control the distribution of hydrothermal minerals (Elders et al., 1 Corresponding author; pk@cie.unam.mx 1984; Browne, 1984). Some primary minerals in rocks become unstable as a result of interactions between them and the geothermal fluids. Due to this instability, a new equilibrium is achieved by dissolving primary minerals and precipitating new hydrothermal phases. The types and amounts of secondary minerals are controlled by temperature, chemical compositions of primary minerals and /06/859/ $

2 HYDROTHERMAL MINERALS 175 fluids (especially ph), lithostatic and fluid pressures, rock texture and permeability, the duration of water-rock interactions, and kinetics of alteration processes (Browne, 1984). The presence or absence of calc-silicate alteration minerals in general and clay minerals in particular depends, among other factors, on geothermal fluid temperatures. Mixed-layer clay minerals have been used as indicators of temperature change in many geothermal systems around the world (Harvey and Browne, 2000). Smectite is typically stable below 140 C and illite above 220 C in active geothermal systems (Browne, 1984). Montmorillonite does not occur above 160 C, but chlorite and illite-chlorite form at temperatures exceeding 145 C. Illite is stable between 220 and 300 C and transforms to muscovite (sericite) above 300 C (Simmons and Browne, 1998). Some hydrothermal non-clay minerals, such as garnet and amphiboles, typically present above 320 C, are also important temperature indicators. Epidote is reported in several fields above C (e.g., Bird et al., 1984; Bird and Spieler, 2004). The Los Azufres geothermal field is among the four fields in Mexico presently under exploitation (see inset in Fig. 1). The aggregate installed geothermal electric power from these fields is 853 MW, constituting about 2.3% of the total power capacity (37,682 MW for all energy resources) in Mexico, and about 10% of the geothermal power installed worldwide (Hiriart and Gutiérrez-Negrín, 2003). Los Azufres is an active geothermal field located in the Mexican Volcanic Belt (MVB), 200 km northwest of Mexico City. With 93 MW installed capacity, it is the second important geothermal field in Mexico (after Cerro Prieto, Baja California) in terms of electricity production. The Los Azufres geothermal system has been previously studied by several authors (see González-Partida et al., 2005; Verma et al., 2005, and references therein). Hydrothermal minerals were identified using cores and cuttings from drill wells (Cathelineau et al., 1985; Cathelineau and Nieva, 1985; Cathelineau and Izquierdo, 1988; Torres-Alvarado, 2002); their fluid inclusions were studied for estimating homogenization temperatures (González-Partida et al., 2000). This system is an ideal field to study hydrothermal alteration processes for several reasons (Cathelineau and Nieva, 1985): (1) the chemical and mineralogical compositions of unaltered andesite are relatively homogeneous within a depth range of 2500 meters; rhyolite and dacite occur mainly within the top 500 meters; (2) primary and hydrothermal minerals can be readily distinguished; and (3) the temperatures estimated from well logs are in agreement with those estimated by microthermometry. Despite several mineralogical studies, questions regarding the influence of the original lithology and temperature on the formation of specific hydrothermal minerals are unanswered. The role of the chemical composition of geothermal fluids controlling the occurrence of some clay minerals is also not completely understood. Hydrothermal minerals from geothermal fields in Mexico as well as in other parts of the world have been simply identified through X-ray diffraction (XRD) methods and used as markers for temperature estimates, except for one report by Cathelineau and Izquierdo (1988), where XRD peak heights (rather than peak areas) were used for semi-quantification purposes. These studies were mainly based on the presence or absence of a particular temperature-dependent phase at a particular depth interval. This article reports a precise semi-quantitative study of hydrothermal mineral abundances (based on weighted peak areas), and establishes the importance of XRD for the study of low-abundance minerals that are difficult to be identified by traditional petrography methods. Geological and Hydrogeochemical Setting The geology of the Los Azufres area has been described by several workers (e.g., Gutierrez and Aumento, 1982; Dobson and Mahood, 1985). This geothermal field is distinguished by extensive Neogene volcanic rocks, dominated by andesitic and basaltic lavas (Fig. 1), which unconformably overlie metamorphic and sedimentary rocks of Late Mesozoic to Oligocene age. The pre-volcanic basement consists of gently folded shales, sandstones, and conglomerates. The oldest volcanic activity reported in this area began as andesite flows about 18 Ma (Dobson and Mahood, 1985). Approximately 2700 m thick interstratified lava flows and pyroclastic rocks of andesitic to basaltic composition (Fig. 1), with ages between 18 and 1 Ma, form the local basement. This massive andesitic unit comprises the main aquifer, in which the geothermal fluids flow mainly through fractures. Silicic volcanism began shortly after eruption of the last andesite, forming a sequence of rhyodacite, rhyolite and dacite with ages between 1.0 and 0.15

3 176 PANDARINATH ET AL. FIG. 1. Geological map of the Los Azufres geothermal field, showing the location of the studied wells. The shaded area in the insert shows the location of the Miocene Recent, E-W oriented, volcanic province in central Mexico (Mexican Volcanic Belt = MVB), in which this field is located.

4 HYDROTHERMAL MINERALS 177 Ma and a thickness of up to 1,000 m (Dobson and Mahood, 1985). Five different units can be distinguished: Agua Fría rhyolite, Tejamaniles dacite, Cerro Mozo and San Andrés dacites, and Yerbabuena rhyolite (Fig. 1). They built domes and short lava flows with glassy structures. Rocks from these units are generally fractured on the surface. Close to hydrothermal manifestations, they show a very intense alteration, characterized by strong kaolinitization and silicification (Fig. 1). Deep geothermal fluids in Los Azufres are sodium chloride rich waters with high CO 2 contents, and ph around 7.5. Although fluid temperatures can reach as high as 320ºC, 240 to 280ºC are commonly measured in the field. Geochemical studies have shown that chemical reactions between the volcanic rocks and geothermal fluids are close to equilibrium (Verma et al., 1989; Torres-Alvarado, 2002). In general, hydrothermal alteration has affected most rocks in the Los Azufres geothermal field. Comprehensive mineralogical studies (e.g., Cathelineau et al., 1985; Torres-Alvarado, 2002) have shown that hydrothermal alteration of primary minerals and of rock matrix, as well as vesicle and fracture fillings, can range from incipient to complete. Most important alteration assemblages, with increasing depth, are: argillitization/silicification, zeolite/calcite formation, sericitization/chloritization, and chloritization/epidotization. Mafic rocks show an alteration succession directly related to the temperature of crystallization of primary minerals (Torres-Alvarado, 2002). Olivine alters rapidly, followed by augite, hornblende, and biotite. These minerals are typically altered to antigorite, chlorite, calcite, hematite, quartz, and to a lesser extent, to amphibole (tremolite). Analytical Methods Representative rock cuttings from drilling operations of wells Az-5, Az-28, and Az-31 were selected from different depths (Fig. 1). Minerals, both clay and non-clay, were quantified using XRD. Bulk rock materials and separated clay fractions (<2 µm) of each sample were analyzed separately. Clay fractions were separated by ultrasonic treatment of rock cuttings, followed by sedimentation using Stoke's law. Oriented clay slides were prepared by pipetting equal volumes (1 ml) of the separated clay fraction onto glass slides and drying them at room temperature. For bulk mineralogical studies, rock cuttings were finely powdered using agate mortar and pestle. The bulk materials and clay slides were scanned respectively from 3 60 and θ by a Bruker D8 ADVANCE diffractometer at 1 2θ/minute. The XRD instrument used is equipped with computer-controlled continuous variable slits, divergence, and antiscatter, for constant sample area irradiation. XRD diffractograms were obtained using 40 kv, 30 ma, and Ni filtered CuKα radiation. Samples were saturated with ethylene glycol and scanned again from θ to confirm the presence of smectite, and slow scanned (0.5 2θ/minute) from θ to differentiate between kaolinite and chlorite peaks. Minerals in both bulk material and clay fraction were identified using the PDF-2 mineral database and the computer software Diffrac Plus. Minerals present in the bulk materials were quantified on the basis of their peak intensity values. It was assumed that the identified minerals constitute the entire mineral phase in the samples. Clay minerals were quantified using weighted peak areas following the semi-quantification method of Biscaye (1965). Thus the proportions of minerals in this work represent relative (not absolute) percentages. Lithology and in situ temperature records (measured with Kuster equipment) were provided for all the wells by the Comisión Federal de Electricidad, Mexico. The geothermal fluid composition data of the three wells (Az-5, Az-28, and Az-31) and also of nearby wells (Az-13, Az-18, and Az-26; Fig. 1) are from Izquierdo et al. (1988). Total discharge compositions were calculated using the methodology proposed by Henley et al. (1984). Ion activities for these fluids were calculated using the software package The Geochemist s Workbench (Bethke, 1992) with the Debye-Hückel model for calculating the activity coefficients. Using the same computer package, activity diagrams were obtained from the chemical characteristics of the fluids defined by quartz excess conditions. Results Well Az-5 This well was drilled into porphyritic andesite to 1450 m, and into porphyritic dacite from 1450 to 1493 m (Fig. 2). At the bottom of the well, the maximum temperature measured was 280 C. Plagioclase is the dominant primary phase (Fig. 2), showing a labradorite-bytownite composition

5 178 PANDARINATH ET AL. FIG. 2. Depth-wise variations in relative abundance (%) of minerals for well Az-5 measured from bulk-rock samples. Filled triangles and circles represent primary and hydrothermal minerals, respectively.

6 HYDROTHERMAL MINERALS 179 FIG. 3. Depth-wise variations in relative abundance (%) of clay minerals in the <2 µm size fraction for well Az-5. (diffraction peak positions at 3.18 or 3.20 Å, respectively). This result agrees with the feldspar composition (labradorite-bytownite, An ) reported by Torres-Alvarado (1996) using microprobe techniques. Relative abundances of plagioclase decreases with the depth of the well (Fig. 2), suggesting increasing intensity of alteration in the bulk rock as a result of increase in temperature. Magnetite, another primary mineral, generally ranges from 3 to 6%, except for a high content of 14% at 292 m depth. Quartz, calcite, smectite, chlorite, chabazite, and muscovite are hydrothermal minerals observed in bulk-rock samples (Fig. 2). Quartz is absent in the near-surface sample (49 m depth) and its abundance gradually increases with depth, reaching 50 wt% at 1108 m. Calcite content is higher at 49 and 718 m depths. Chabazite was identified in several samples (diffraction peak at 2.92 Å). This mineral is more abundant between about 292 to 718 m depth. Muscovite (perhaps in the form of sericite) is present only in the shallowest sample (49 m depth; relative % of ~4.5). The clay fraction consists of smectite, illite, and chlorite (Fig. 3). Among the clay minerals, smectite is the only phase present above 292 m (~60 C). Below this depth, its content reduces to ~50% (at 575 m, ~100 C). This mineral is absent from the clay fraction at 718 m depth (~120 C). Illite and chlorite are present below 292 and 575 m depth, respectively. As the smectite content decreases from 292 m to deeper levels, the contents of illite and

7 180 PANDARINATH ET AL. chlorite increase from 292 m and 575 m depth, respectively. The maximum content of Illite occurs at 575 m (50%; ~100 C). Below this depth, the content of illite gradually decreases and chlorite increases (Fig. 3). Kaolinite is not present in this well. Well Az-28 This well was mostly drilled in microcrystalline andesite alternating with thin horizons of vitreous rhyolite, pilotaxitic andesite, porphyritic andesite, and microlitic andesite (Fig. 4). The maximum in situ measured temperature at the bottom of the well (at ~1700 m depth) was 265 C. As in Az-5, plagioclase (labradorite-bytownite composition) is the most abundant primary mineral (Fig. 4). Magnetite is present in amounts ranging from 2 to 5% of the total. Quartz, calcite, chlorite, pyrite, and chabazite are present as hydrothermal phases. In general, quartz and pyrite contents decrease, whereas calcite abundance increases with depth (between 738 and 1167 m). Smectite, illite, chlorite, and kaolinite are present in the clay fraction in this well (Fig. 5). Kaolinite occurs only in the shallower sample (50 m depth; ~51 C). Smectite content decreases with depth, from 65% at 50 m (~51 C) to 5% at 490 m (~158 C). Below 661 m (~190 C), smectite is absent, except in minor quantities (4%) at 1364 m depth. Illite content gradually increases from 27% at 50 m to 69% at 738 m depth (~200 C). Illite is the dominant clay mineral between 190 and 200ºC. Below 738 m, its content gradually decreases to trace quantities, whereas chlorite content increases. Chlorite abundance increases downward, from 4% at the shallowest depth to ~100% of the clay fraction in deeper parts of the well (>~205 C). Well Az-31 This well was drilled in rhyolite (vitreous to microcrystalline) from the surface to 280 m, and below this into andesite (Fig. 6). The maximum temperature measured at bottom of the well (~1300 m depth) was 288 C. As in the other wells, plagioclase (labradorite-bytownite composition) is the principal primary mineral. Its abundance decreases with depth (Fig. 6). In general, magnetite ranges from 2 to 7% with the exception of higher concentrations at shallower depths (down to 400 m). Quartz, calcite, smectite, chabazite, chlorite, muscovite, and pyrite constitute the major hydrothermal minerals (Fig. 6). Quartz is not present in the shallowest samples (94 m depth). Below this depth, in general, its content increases. Muscovite (sericite?) is absent down to 404 m; below this depth it occurs in minor quantities (<4%), with the exception of 12% at 502 m depth. Calcite shows a very high content of 35% at 802 m depth. Chabazite is more abundant in this well (with 14% between 404 and 502 m depth) than in wells Az-5 and Az-28. Pyrite is absent above 598 m, but occurs in minor quantity (~5%) below this depth. Smectite, illite, chlorite, and kaolinite are present in the clay fractions (Fig. 7). Sharp variations in the relative abundances of clay minerals occur in this well. Illite is the only clay mineral present in the top 206 m depth (~128 C). Illite is suddenly absent at 302 m depth (~140ºC), where smectite just occurs. From 302 to 598 m (~140 to ~24 C), smectite is the only clay mineral. Again from 702 to 902 m (~270 C), smectite is absent and chlorite is the only clay mineral present. Illite appears once again at about 998 m depth (~275ºC), where chlorite abundance decreases. At greater depth, illite and chlorite constitute the major clay mineral fractions, with minor quantities of smectite. Unexpectedly, kaolinite, not present at shallower depths, occurs (~10%) in the deepest ( m; ~280ºC) samples. Discussion Lithological control on mineral distribution Wells Az-5 and Az-28 were drilled into andesite, whereas Az-31 penetrated rhyolite and andesite. The influence of lithology on the clay mineral distribution pattern is clearly reflected in Az-31. In this well, rhyolite is present down to 280 m depth, and andesite below to the bottom of the well (1300 m; Fig. 7). Illite constitutes 100% of the clay fractions in the rhyolite section. Abruptly, illite is completely absent and only smectite is present at 302 m depth, exactly where the andesite section starts. This suggests that variations in the abundances of clay minerals cannot be assigned entirely to an increase in temperature. It may be the result of differences in the rock types. Earlier, Cathelineau and Izquierdo (1988) also reported more than 90% illite in the clay fraction in rhyolite in well Az-23 (Fig. 1) of the Los Azufres geothermal system and concluded that there is a lithological control on the distribution of clay minerals in this well. They reported that the sudden appearance of chlorite is strictly related to the

8 HYDROTHERMAL MINERALS 181 FIG. 4. Depth-wise variations in relative abundance (%) of minerals for well Az-28 measured from bulk-rock samples. Filled triangles and circles represent primary and hydrothermal minerals, respectively.

9 182 PANDARINATH ET AL. FIG. 5. Depth-wise variations in relative abundance (%) of clay minerals in the <2 µm size fraction for well Az-28. rhyolite-andesite contact. Similarly, the observed variations in the abundances of non-clay minerals (Fig. 6), quartz, calcite, and chabasite, at this contact in well Az-31, may also result from lithological differences, and not solely from contrasts in thermal conditions. Browne (1978) reported that the influence of parent rock material on the nature of alteration product is mainly evident at lower temperatures (not above 280 C) as is the case of Los Azures (wells Az-31: this study and Az-23: Cathelineau and Izquierdo, 1988). However, a lack of lithological control on the distribution of clay minerals has been reported for the Chipilapa geothermal system, El Salvador (Patrier et al., 1996; Robinson and Santana de Zamora, 1999). Deeper in well Az-31, as well as in wells Az-5 and Az-28, only andesite is present. Because there are no lithological differences, the gradual variations in mineral distribution patterns in these wells are evidently due to physicochemical factors discussed in the following sections. Temperature control on the distribution of non-clay minerals Chabazite is consistently present in samples from all the three wells. In general, chabazite

10 HYDROTHERMAL MINERALS 183 FIG. 6. Depth-wise variations in relative abundance (%) of minerals for well Az-31 measured from bulk-rock samples. Filled triangles and circles represent primary and hydrothermal minerals, respectively.

11 184 PANDARINATH ET AL. FIG. 7. Depth-wise variations in relative abundance (%) of clay minerals in the <2 µm size fraction for well Az-31. contents range from about 2 to 5%, except at shallower depths in wells Az-5 (6%) and Az-31 (14%). Chabazite has been reported, among other zeolites, in Iceland geothermal fields (Kristmannsdóttir and Tómasson, 1978). Although this mineral normally occurs at temperatures below ~80º C, these authors reported chabazite in the Krafla geothermal field at temperatures as high as 230ºC. Quartz is the dominant hydrothermal mineral in all three wells. Its abundance gradually increases with depth in Az-5 but shows fluctuations in Az-28 and Az-31. Calcite, although present throughout, does not show any obvious trend. Quartz and calcite have been extensively reported as part of the hydrothermal alteration in Los Azufres (Torres-Alvarado, 2002), as filling fractures and vesicles, and also replacing the primary minerals. Temperature control on the distribution of clay minerals Besides the lithology as discussed above, the distribution of clay minerals in geothermal systems

12 HYDROTHERMAL MINERALS 185 depends on the thermal structure of the field and its fluid chemistry (Steiner, 1968; Kristmannsdóttir, 1975; Elders et al., 1984; Harvey and Browne, 1991). Clay mineral distribution trends in wells Az-5 (Fig. 3), Az-28 (Fig. 5), and Az-31 (excluding the shallower rhyolite section; Fig. 7) show gradual variations with depth (and thus with temperature) from smectite as the only or major clay constituent at shallower levels to a combination of smectite, illite, and chlorite in about equal abundances at intermediate depths, followed by the complete absence of smectite in the deeper (hotter) zone. In the present study, the upper limit of the thermal stability of smectite ranges from 120, 190, and 270 C for wells Az-5, Az-28, and Az-31, respectively. The presence of smectite at higher temperatures (such as in well Az-31) is not a rare phenomenon. It has been reported at a wide range of temperatures in active geothermal systems at lower temperatures (below ~140 C) in New Zealand (Browne, 1984), as well as at higher temperatures (up to ~200 C) in the Chipilapa geothermal system, El Salvador (Beaufort et al., 1995; Patrier et al., 1996; Robinson and Santana de Zamora, 1999) and in Newberry caldera of the Cascade Range (Keith and Bargar, 1988). Smectite was reported as transforming to chlorite at C, with chlorite becoming predominant from ~230 C in Icelandic geothermal systems (Kristmannsdóttir, 1979). The thermal stability for smectite is different in the three studied wells. Cathelineau and Izquierdo (1988) also reported that the temperatures of disappearance of smectite and appearance of chlorite in the Los Azufres geothermal system are variable and difficult to estimate. Chlorite is present at temperatures above 100, 65, and 245ºC in wells Az-5, Az-28, and Az-31, respectively. It is a major hydrothermal mineral at temperatures between ~120 and ~270 C. Chlorite content generally increases with depth. The presence of an extensive chlorite zone in this hydrothermal system was reported earlier (Gutierrez and Aumento, 1982; Cathelineau et al., 1985; Cathelineau and Nieva, 1985). Cathelineau and Nieva (1985) reported 130 and 300ºC as the formation temperatures of chlorites in the Los Azufres geothermal system. As in Los Azufres, chlorite occurs at low as well as at high temperatures in other geothermal fields. Chlorite was reported in <2 µm size fractions as a minor phase from 110ºC (Beaufort et al., 1995; Patrier et al., 1996), and as a major phase from 160ºC (Robinson and Santana de Zamora, 1999) and from 170ºC (Beaufort et al., 1995; Patrier et al., 1996) to the maximum recorded temperature of 230ºC in the Chipilapa geothermal system, El Salvador. In the geothermal fields of New Zealand, illite and chlorite are typical clay minerals present above 220ºC (Browne, 1978). In the deeper parts of all the studied wells (and higher temperatures), plagioclase contents gradually decrease, and concomitantly chlorite contents increase (Figs. 2, 4, and 6). This distribution trend indicates the alteration of plagioclase to chlorite at higher temperatures in these wells. This type of alteration has been commonly reported in other geothermal fields as well. For example, conversion of plagioclase to sericite, chlorite, and other clay minerals (at ºC), to sericite and chlorite (at ºC), and to sericite (at 200ºC) was reported in the Tuzla hydrothermal system, Canakkale, Turkey (Sener and Gevrek, 2000). The thermal stability ranges of clay minerals in the wells of the present study are compared with other geothermal systems in Figure 8. In general, smectite stability conditions in wells Az-5 and Az-28 are comparable to those reported in the literature. But, in well Az-31, smectite is present also at somewhat higher temperature (~270 C). Chlorite thermal stability conditions in the studied wells are also comparable to those in the literature (Fig. 8), inasmuch as chlorite has been observed over a wide range of temperatures. On the other hand, illite is observed at low as well as high temperatures in the wells of the Los Azures geothermal system. Although illite has been reported at temperatures higher than 220ºC (Browne, 1984; Simmons and Browne, 1998), illite-montmorillonite mixed layered clay has been reported at lower-temperature regions (Fig. 8). Our work shows that caution is required to use illite as a geothermometer (Essene and Peacor, 1995). Clay mineral transformation sequence Clay minerals in the three wells clearly show a gradual variation from only smectite (or smectite dominant) at shallower depths to illite and chlorite dominant in deeper parts (Figs. 3, 5, and 7). Clay mineral peaks on the X-ray diffractograms clearly show well-crystallized smectite, illite, and chlorite, with no mixed layering. These observations disagree with previous clay mineral studies reported for Los Azufres (Cathelineau and Nieva, 1985; Cathelineau and Izquierdo, 1988) and support the model involving a discontinuous variation from smectite to

13 186 PANDARINATH ET AL. FIG. 8. Sketch summarizing temperature-clay minerals relationships. Grey bars represent the temperature ranges for those clay minerals observed in this study. Legend: Ref. # 1 = Beaufort et al., 1995; Patrier et al., 1996; Robinson and Santana de Zamora, 1999; Keith and Bargar, 1988; Ref. # 2 = Robinson and Santana de Zamora, chlorite and/or illite, rather than one involving continuous mixed-layering of smectite-illite and/or smectite-chlorite. The discontinuous variations observed in present study represent equilibrium prograde sequence, such as those reported for other fossil or active geothermal systems (La Palma seamount, Canary Islands by Schiffman and Staudigel, 1995; Minnesota by Schmidt and Robinson, 1997; and Chipilapa, El Salvador by Robinson and Santana de Zamora, 1999). Kaolinite is not present in well Az-5. It occurs only at lower-temperature conditions (~65ºC) in well Az-28, but at higher temperatures (at ~275 to ~288ºC) in two samples of well Az-31. It has been reported that kaolinite does not persist above about 60ºC in geothermal fields of New Zealand (Browne, 1978). The presence of kaolinite in these samples is confirmed using the PDF-2 database, slow scanning, and thermal treatment tests. At present, we are unable to provide an explanation for the presence of

14 HYDROTHERMAL MINERALS 187 FIG. 9. Comparison of in situ measured temperatures (T in situ, ºC) to homogenization temperatures (T h, ºC) recorded in fluid inclusions from hydrothermal minerals in studied wells. Fluid inclusion data were taken from González-Partida et al. (2000). Homogenization temperature data are presented as a range (maximum and minimum), and the position of the respective symbols marks the average values. kaolinite (11-14 relative % of clay minerals) at higher-temperature conditions in well Az-31. The present thermal regime of the Los Azufres geothermal system and the thermal conditions at the time of mineral crystallization are evaluated by comparing in situ temperatures measured in the well to homogenization temperatures (T h ) measured in fluid inclusions of hydrothermal minerals. For this comparison we have taken published fluid inclusion data (T h ) of nearby wells (González-Partida et al., 2000; see also Fig. 1). In situ measured temperatures are generally comparable to T h data of the wells, except for the middle depth (700 to 1200 m) in well Az-5 (Fig. 9). These discrepancies may partly be due to the fact that the in situ measured temperatures are lower than the static formation temperatures operative for mineral paragenesis. From these considerations, we may conclude that

15 188 PANDARINATH ET AL. FIG. 10. K + /H + vs. Mg 2+ /(H + ) 2 activity diagrams for the chemical characteristics of fluids from the Los Azufres geothermal field. Quartz excess conditions were assumed in all diagrams. The fluid composition data were taken from Izquierdo et al. (1988). See the text for details. the transformation sequence of clay minerals from Los Azufres corresponds to the present thermal regime. Analyses of the chemical composition of modern geothermal fluids at Los Azufres may contribute to an understanding of the stability conditions for clay minerals and their relationship to temperature. Figure 10 (prepared for Los Azufres using the package The Geochemist s Workbench ) shows activity diagrams for the system SiO 2- MgO-K 2 O-Al 2 O 3 at 100, 200, and 300ºC. Mg 2+ and K + activities calculated for the geothermal fluids of studied as well as nearby wells are plotted. Due to the lack of appropriate thermochemical data for mineral solid solutions, we have considered Mg-bearing beidellite and clinochlore as representing their respective mineral groups in these diagrams. Geothermal fluids from most of the wells (except Az-28 and Az-31) are in equilibrium with illite at 100ºC (Fig. 10A). At this temperature, fluids from wells Az-28 and Az-31 are in equilibrium with beidellite and at the border between illite and beidellite, respectively. This reflects the lower K + concentrations of the fluids discharged from Az-28 and Az-31 than other wells. At 200ºC (Fig. 10B), geothermal fluids from all the wells are in equilibrium with illite. At 300ºC, clinochlore is in equilibrium with all geothermal fluids (Fig. 10C), except those from Az-5 that are in equilibrium with beidellite. These phase stability changes (from beidellite

16 HYDROTHERMAL MINERALS 189 to illite and then to clinochlore) are consistent with the clay mineral distribution trends and gradual variations tracking with temperature in the studied wells (Figs. 3, 5, and 7). Conclusions The semi-quantitative hydrothermal XRD mineral data provide better distribution trends of hydrothermal minerals in geothermal wells than simple microscopic and XRD mineral identification data. The mineral distribution trends in the Los Azufres geothermal system reveal a discontinuous change from smectite to chlorite and/or illite, rather than through continuous mixed-layering of smectite-illite and/or smectite-chlorite. The comparison between in situ measured temperatures in the wells and homogenization temperatures (T h ) estimated in fluid inclusions of hydrothermal minerals suggests that the observed transformation sequence of minerals is in response to the prevailing thermal regime. The present geothermal fluids in the field are in equilibrium with illite and beidellite, illite, and chlorite at 100, 200 and 300ºC, respectively, showing close agreement with the clay mineral distribution and gradual variations as a function of temperature. Acknowledgments The final version of the manuscript benefited from helpful comments from our collegue Edgar Santoyo Gutiérrez. This research work was partially supported by DGAPA-UNAM (PAPIIT projects IN and IN105502). We would like to thank C. Krishnaiah and K. S. Jayappa, Ocean Science and Technology Cell (Marine Geology and Geophysics), Mangalore University, India for providing the XRD laboratory facility. REFERENCES Beaufort, D., Papapanagiotou, P., Patrier, P., and Traineau, H., 1995, Les interstratifiés I-S et C-S dans les champs géothermiques actifs: Sont-ils comparables a ceux des séries diagénétiques?: Bulletin Centres Rech. Exploration-Production Elf Aquitaine, v. 19, p Bethke, C., 1992, The geochemist s workbench: A user s guide to Rxn, Act2, Tact, React, and Gtplot: Urbana, IL, University of Illinois. Bird, D. K., Schiffman, P., Elders, W. A., Williams, A. E., and McDowell, S. D., 1984, Calc-silicate mineralization in active geothermal systems: Economic Geology, v. 79, p Bird, D. K., and Spieler A. R., 2004, Epidote in geothermal systems: Reviews in Mineralogy and Geochemistry, v. 56, p Biscaye, P. E., 1965, Mineralogy and sedimentation of recent deep-sea clay: Geological Society of America Bulletin, v. 76, p Browne, P. R. L., 1978, Hydrothermal alteration in active geothermal fields: Annual Review Earth and Planetary Sciences, v. 6, p Browne, P. R. L., 1984, Lectures on geothermal geology and Petrology: Reykjavik, Iceland: National Energy Authority of Iceland and United Nations University Geothermal Training Programme, Iceland, 93 p. Cathelineau, M., and Izquierdo, G., 1988, Temperaturecomposition relationships of authigenic micaceous minerals in the Los Azufres geothermal system: Contributions to Mineralogy and Petrology, v. 100, p Cathelineau, M., and Nieva, D., 1985, A chlorite solid solution geothermometer The Los Azufres (Mexico) geothermal system: Contributions to Mineralogy and Petrology, v. 91, p Cathelineau, M., Oliver, R., Garfias, A., and Nieva, O., 1985, Mineralogy and distribution of hydrothermal mineral zones in the Los Azufres (Mexico) geothermal field: Geothermics, v. 14, p Dobson, P. F., and Mahood, G. A., 1985, Volcanic stratigraphy of the Los Azufres geothermal area, Mexico: Journal of Volcanology and Geothermal Research, v. 25, p Elders, W. A., Bird, D. K., Williams, A. E., and Schiffman, P., 1984, Hydrothermal flow regime and magmatic heat research of the Cerro Prieto geothermal system, Baja California, Mexico: Geothermics, v. 13, p Essene, E. J., and Peacor, D. R., 1995, Clay mineral thermometry a critical perspective: Clays and Clay Minerals, v. 43, p González-Partida, E., Birkle, P., and Torres-Alvarado, I. S., 2000, Evolution of the hydrothermal system at Los Azufres, Mexico, based on petrologic, fluid inclusion, and isotopic data: Journal of Volcanology and Geothermermal Research, v. 104, p González-Partida, E., Carrillo-Chávez, A., Levresse, G., Tello-Hinojosa, E., Venegas-Salgado, S., Ramirez-Silva, G., Pal-Verma, M., Tritlla, J., and Camprubi, A., 2005, Hydro-geochemical and isotopic fluid evolution of the Los Azufres geothermal field, Central Mexico: Applied Geochemistry, v. 20, p Gutierrez, N. A., and Aumento, F., 1982, The Los Azufres, Michoacán, Mexico, geothermal field: Journal of Hydrology, v. 56, p Harvey, C., and Browne, P., 1991, Mixed-layer clay geothermometry in the Wairakei geothermal field, New Zealand: Clays and Clay Minerals, v. 39, p

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