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F u n 2 d 6 la serena octubre 2015 ada en 19 Tectonic control of the geothermal system at Mt. - Insights from geophysical and geochemical surveys

Sebastian Held1*, Eva Schill2, Maximiliano Pavez3,5, Daniel Diaz3,5, Diego Morata4,5 and Thomas Kohl1 1Institute of Applied Geosciences, Karlsruhe Institute of Technology (KIT), Kaiserstraße 12, 76131 Karlsruhe, Germany 2Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany 3Departament of Geophysics, Facultad de Ciencias Físicas y Matemáticas, Universidad de , Chile 4Departament of Geology, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Chile 5Centro de Excellencia en Geotermina de Los Andes (CEGA), Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile

*Contact email: [email protected]

Abstract. Geothermal systems are often related to active products are absent (Held et al., 2015). volcanic areas forming typical high-enthalpy geothermal The tectonic situation, including the over 1200 km long N- reservoirs. However at Mt. Villarrica the processes active in S aligned LOFS, may be responsible for the absence of a the subsurface do not favor the formation of high-enthalpy high-enthalpy geothermal reservoir in the vicinity of one of geothermal reservoirs. Geophysical and geochemical the most active volcanoes of Chile (Ortiz, 2003). However, investigations demonstrate the importance of major faults the origin and possible flow paths of the geothermal fluid governing fluid flow in the subsurface. Magnetotelluric are essential to understand processes in the reservoir and to measurements indicate the presence of deep fault zones. define optimal geothermal utilization. Therefore in 2013 MT results detect the Liquiñe-Ofqui fault system (LOFS), and 2014 geophysical and geochemical surveys were described as a 1000km long N-S aligned intra-arc fault zone, and a second NW-SE striking fault zone, that conducted to identify the mechanism in the subsurface. crosscuts the LOFS. Geochemical investigations Generally, investigations of water-rock interaction characterize the fluid pathways as a local small-scale contribute to describe the conditions of geothermal circulation. Reservoir temperatures are estimated to 130- reservoirs. First geochemical investigations are done by 140°C using sulfate geothermometry. However, springs Sanchez et al. (2013). The meteoric origin of fluids could located along the distinct run of the major fault zone in be determined by δ18O and δD isotopy. Furthermore a granitic batholith complex show lower equilibrium general low mineralization of the fluids at the hot springs temperatures of 80-90°C. As a conclusion the major fault was observed. The here presented surveys extend those zone, accompanied by secondary fault zones, causes high geochemical investigations by using additional analytical secondary permeability, hindering the formation of high- techniques. Sulfate Geothermometry is applied to constrain enthalpy reservoirs. reservoir temperatures as cation geothermometers to not yield explicit results. Strontium isotopy is used to identify possible reservoir rocks hence identify possible fluid Keywords: Magnetotelluric, Geochemical exploration, pathways. Structural control of geothermal systems, secondary The geochemical investigations are combined by permeability geophysical campaigns to determine reservoir locations and extension. The magnetotelluric method is applied 1 Introduction allowing the identification of high-enthalpy reservoirs, if present, and brine circulation in permeable fault zones.

The combination of geochemical and geophysical In contrast to most of the active volcanoes in northern and investigations enable the generation of an integrated model central Chile, high-temperature geothermal reservoirs are describing the processes in the geothermal reservoir at the not present in the subsurface of Mt. Villarica (Held et al., Mt. Villarrica. 2015); even though, a magma chamber is present at shallow depth (Hickey -Vargas et al., 1989). The Villarica area is part of the South Volcanic Zone of Chile and 2 Method and Results belongs to a NW-SE striking chain of three major stratovolcanoes (Villarrica-Quetrupillan-Lanín). This chain and a related parallel fault zone crosscut and offset the 2.1 Magnetoellurics major N-S striking Liquiñe-Ofqui fault system (LOFS) by few kilometers at the surface (Figure 1). In the Villarica Magnetotelluric campaigns measure secondary magnetic area, a large number of natural hot springs distributes over waves at various stations on a broad frequency range as an area of 2000 km3. Temperatures of thermal water at the penetration depth depend on wave frequency. outlet do not exceed 80°C. Hydrothermal alteration As this study focuses on the identification of geothermal 460 AT 2 geología económica y recursos naturales

reservoirs a frequency range of 1000Hz - 0.001Hz was 2013) are significantly lower than 200°C. As cation used to study features in shallow - intermediate depth. geothermometers are vulnerable for dilution by meteoric Magnetotelluric measurements are highly sensitive to clay water or kinematic effects during ascent, in this study, we minerals, having a high conductivity due to surface applied sulfate-isotope geothermometry. The temperature- charges, and saline brines, due to their ion content. dependent oxygen fractionation between water and sulfate Therefore identification of clayey cap rocks, generated species was first described by Hoering and Kennedy above geothermal reservoirs due to hydrothermal alteration (1957) and Lloyd (1968). Besides the reduced vulnerability at higher temperatures, and permeable fault zones with to dilution, its advantage over cation geothermometers is circulating brines is possible. As previous studies (Held et the slow reaction rate at lower temperatures, i.e. during al., 2015) doubt the presence of a high-enthalpy reservoir ascent, resulting in fluids representing the fractionation in at the research location the survey focuses the the deep reservoir (Chiba and Sakai, 1985). The dominant identification of possible major fault zones. In Nov.-Oct. sulfate species at reservoir conditions is governing the 2013 mainly two profiles were measured using 31 MT fractionation (Boschetti, 2013; Chiba and Sakai, 1985; stations, crosscutting the proposed run of the LOFS. A Sakai, 1977; Zeebe, 2010) and therefore, also the detailed description of the MT campaign is given in Pavez temperature estimation. After Boschetti (2013), at lower 2- et al. (2015). Within this publication the results of the temperatures and near neutral pH, SO4 is the dominate campaign presented as 2D inversion of magnetotelluric species resulting in the application of the formulas data are discussed. indicated below in Table 1. For the measurements sulfate The profile, generated by15 stations, presented in Figure 2 was precipitated completely with stoichiometric abundant 18 crosscuts the LOFS perpendicularly (Figure 1). Each BaCl2. Measurements of δ O were done by isotope-ratio station represents point data which can be combined to a mass spectrometry (IRMS) using a GV Instruments profile using inversion techniques. Therefore both apparent IsoPrime combined with a HTO Pyrolysis from resistivity and phase were inverted using 2-D nonlinear HEKAtech. conjugate gradients (NLCG) algorithm of Rodi and Table 1: Results of sulfate geothermometers calculated using the Mackie (2001) considering TM, TE and vertical magnetic equations of Halas & Pluta (2000) Zeebe (2010). field data. Modelling parameters were set to 5% error for T T phases compared to a higher error of 15% for apparent Reservoir Reservoir δ18O resistivity TM and 70% apparent resistivity TE. The higher Sample Equation I Equation II SO4 H2O errors for apparent resistivities are used to overcome static [‰] [‰] [°C] [°C] shift. The TE error floor was set higher than the TM error Carranco 3.19 -9.92 84 87 floor because of the sensitiveness of the TE mode to 3D Chihuío 2.11 -10.3 91 94 galvanic distortion effects (Unsworth, 2004; Wannamaker, Coñaripe 0.63 -8.09 134 134 1984). Smoothing parameter τ was determined to 7.5 using Liquine 3.78 -9.41 83 87 the L-curve technique (Hansen and O’Leary, 1993). Liucura 2.55 -9.07 99 101 Figure 2 shows the results of inversion containing three Los Pozones 0.68 -10.2 123 123 important electromagnetic features: 1) a shallow Menetue 1.55 -9.04 110 112 widespread resistive layer with a thickness of Palguin 0.74 -9.83 110 112 approximately 8km interrupted by 2) a narrow (2-3km) Panqui -1.99 -10.4 139 137 shallow conductor with almost vertical extension. In the Rincon 0.33 -9.74 116 117 deeper part a 3) conductive feature with resistivity of <20 Rinconanda 0.56 -11.1 98 101 Ωm is evident also observed by Brasse et al. (2009). Due Rio Blanco 0.85 -10.1 106 108 to the great depth (decreased data quality) of that feature San Luis 0.04 -9.28 126 126 and the overlaying “shielding” resistive feature the exact Toldeo -1.95 -10.4 138 137 size, orientation and resistivity value are debatable. Trancura 0.14 -9.24 125 125

I 3 2 Formulae after Halas & Pluta (2000): 10 ln α(SO4−H2O) = 2.41⋅106/T − 5.77 II 3 2 2.2 Geothermometry Formulae after Zeebe (2010): 10 ln α(SO4−H2O) = 2.68⋅106/T − 7.45

During fluid geochemical campaign, carried out in spring Sulfate-isotope geothermometry of the sampled springs in and autumn 2013, 15 samples of hot springs (Figure 1) and the Villarrica area reveals temperatures ranging from 83°C two meteoric water samples from Lake Pucon and rain fall in Liquiñe to 139°C in Panqui (Table 1). While cation were taken. Fluids were sampled closest to the discharge of geothermometers reveal a scattered and mostly higher the hot spring and in situ measurements such as pH, temperature estimated the temperature conditions electric conductivity, temperature and alkalinity were calculated from sulfate geothermometry can be divided carried out. Samples were analyzed for cations, anions, into two categories: 1) Springs located along the distinct trace elements, hydrogen/ oxygen isotopes, sulfate- run of LOFS in granitic rocks (Liquine, Carranco, Chihuio) geothermometry and chlorofluorocarbon (CFC) species. south of the volcanic chain possess estimated reservoir Reservoir temperatures, inferred from cation temperatures of 80-90°C in the range of measured spring geothermometers for Villarrica sources (Sánchez et al., temperature; 2) For the remaining springs mostly to the north of the volcanic chain the reservoir temperature is 461 SIM 4 SISTEMAS GEOTERMALES ANDINOS

determined to 130-140°C. Note that the temperature active strike-slip fault zone with high horizontal offset calculations show quite uniform distribution without high (Cembrano et al., 1996). The origin of higher conductivity scattering. in the lower part is debatable, partial melting could be a plausible explanation for reduced resistivity. 2.3 Strontium Isotopy The importance of major fault zones for fluid flow is confirmed by geochemical investigations. Meteoric fluids Strontium isotope signatures of geothermal fluids and circulate on small-scale fluid pathways as indicated by rocks are used in this work as a way to identify water-rock strontium isotopy. Lateral reservoirs do not develop. interaction. Inclusion of fluids in a closed reservoir combined with Besides the above mentioned geothermal fluids, 26 rock high temperatures is not observed. Medium temperatures samples were analyzed measuring strontium isotope are documented (130-140°C), which lower temperatures signatures. The high number of different rocks documents (80-90°C) occurring along the distinct run of the LOFS in the complex geological history of the Andean cordillera. granitic enviroment. The basement geology changes from Miocene and granites (North Patagonian batholith) south of the volcanic chain to compacted -sedimentary basin deposits (Cura-Mallín Fm.) to the N of the investigation area. A transition zone of mostly Miocene granites and the Cura-Mallín Fm. in-between, characterizes the northern part of the research area. MT"profile" For the purpose of this study, i.e. strontium isotopy, 50ml of geothermal fluid was taken. Rock samples were grinded with an agate mill and dissolved using aqua regia. Isotopes were measured with a Thermal Ionization Mass Spectrometer (TIMS) at the IsoAnalysis commercial laboratory (Berlin) for the fluids and at the University of Tübingen for the rock samples. The results of strontium isotope analysis of the fluid and rock samples are presented in ¡Error! No se encuentra el origen de la referencia.. Sr-isotope ratios between 0.7041 and 0.7174 have been identified for both fluid samples and possible reservoir rocks. Generally, we observe ratios < 0.7045 in the recent volcanic and the Cenozoic plutonic rocks. Higher values (> 0.7045) are observed in the and Paleozoic plutonic rocks that outcrop mostly south of the volcanic chain; with one exception, the location N of Palguín (Pal) revealing a ratio of 0.7047. Sr- ratios obtained from the fluid samples follow the pattern given by the outcropping rocks very closely (Figure 1) High consistency between Sr isotopy of fluids and adjacent rocks may be related to the occurrence of relatively local fluid circulation. This is supported by comparably low " reservoir temperatures observed from sulfate geothermometry. Figure 1: Map of the research area including volcanic centers, major fault zones and the sampled hot springs. Color code represents the Strontium isotope ratios.

3 Discussion Acknowledgements Dominant features detected by magnetotelluric measurements are the fault zones crosscutting each other The study is part of a collaborative research project in the research area. Inversion results identify two between the Karlsruhe Institute of Technology (KIT) and conductive features. An east dipping (60-70°) fault zone the Andean Geothermal Center of Excellence (CEGA). would connect both conductors. Note that the two The study is supported by the CONICYT-BMBF conductive parts are separated by a less conductive area in International Scientific Collaborative Research Program intermediate depth of 8-10 km. Higher conductivities in project PCCI130025/FKZ 01DN14033. the upper part could be explained by fluids circulating in The authors like to thank the University of Tübingen and an area of increased secondary permeability generated by the IsoAnalysis laboratory for the strontium isotope active shearing. The LOFS is described as an recently measurements. Furthermore, we like to thank Jochen 462 AT 2 geología económica y recursos naturales

Schneider from Hydrosion and Gerard Muñoz from Hoering, T.C., Kennedy, J.W., 1957. The Exchange of Oxygen between GeoForschungszentrum (GFZ) Potsdam for fruitful Sulfuric Acid and Water. J. Am. Chem. Soc. 79, 56–60. discussions. Lloyd, R.M., 1968. Oxygen Isotope Behavior in the Sulfate-Water System. J. Geochemical Explor. 73. References Lucassen, F., Trumbull, R., Franz, G., Creixell, C., Vásquez, P., Romer, Boschetti, T., 2013. Oxygen isotope equilibrium in sulfate–water systems: R.L., Figueroa, O., 2004. Distinguishing crustal recycling and juvenile A revision of geothermometric applications in low-enthalpy systems. J. additions at active continental margins: the Paleozoic to recent Geochemical Explor. 124, 92–100. doi:10.1016/j.gexplo.2012.08.011 compositional evolution of the Chilean Pacific margin (36–41°S). J. South Am. Earth Sci. 17, 103–119. doi:10.1016/j.jsames.2004.04.002 Brasse, H., Kapinos, G., Li, Y., Mütschard, L., Soyer, W., Eydam, D., 2009. Structural electrical anisotropy in the crust at the South-Central Ortiz, R., 2003. Villarrica volcano (Chile): characteristics of the volcanic Chilean continental margin as inferred from geomagnetic transfer tremor and forecasting of small explosions by means of a material failure functions. Phys. Earth Planet. Inter. 173, 7–16. method. J. Volcanol. Geotherm. Res. 128, 247–259. doi:10.1016/s0377- doi:10.1016/j.pepi.2008.10.017 0273(03)00258-0

Cembrano, J., Hervé, F., Lavenu, A., 1996. The Liquifie Ofqui fault zone: Pavez, M., Diaz, D., Held, S., Schill, E.,2015. Estudio de resistividad a long-lived intra-arc fault system in southern Chile. Tectonophysics 259, eléctrica mediante Magnetotelúrica, en la zona de falla Liquiñe-Ofqui 55–66. entorno al volcán Villarrica. Proceedings XIV Chilean Geological Congress 2015 Chiba, H., Sakai, H., 1985. Oxygen Isotope Exchange Rate between Dissolved Sulfate and Water at Hydrothermal Temperatures. Geochim. Rodi, W., Mackie, R.L., 2001. Nonlinear conjugate gradients algorithm Cosmochim. Acta 49, 993–1000. for 2-D magnetotelluric inversion. Geophysics 66, 174. doi:10.1190/1.1444893 Elderfield, H., Greaves, M.J., 1981. Strontium Isotope Geochemistry of Icelandic Geothermal Systems and Implications for Sea Water Chemistry. Sakai, H., 1977. Sulfate-Water Isotope Thermometry applied to Geochim. Cosmochim. Acta 45, 2201–2212. Geothermal Systems. Geothermics 5, 67–74.

Faure, G., Powell, J.L., 1972. Strontium Isotope Geology, Strontium Sánchez, P., Pérez-Flores, P., Arancibia, G., Cembrano, J., Reich, M., Isotope Geology. Springer Verlag, Berlin, Heidelberg, New York. 2013. Crustal deformation effects on the chemical evolution of geothermal systems: the intra-arc Liquiñe–Ofqui fault system, Southern Andes. Int. Geol. Rev. 55, 1384–1400. Hansen, P.C., O’Leary, D.P., 1993. The use of the L-Curve in the doi:10.1080/00206814.2013.775731 regularization of discrete ill-posed problems. SIAM J. Sci. Comput. 14, 1487–1503. Unsworth, M., 2004. Crustal and upper mantle structure of northern Tibet imaged with magnetotelluric data. J. Geophys. Res. 109, B02403. Held, S., Schill, E., Sanchez, P., Neumann, T., Emmerich, K., Morata, D., doi:10.1029/2002JB002305 Kohl, T., 2015. Geological and Tectonic Settings Preventing High- Temperature Geothermal Reservoir Development at Mt. Villarrica (Southern Volcanic Zone): Clay Mineralogy and Sulfate-Isotope Wannamaker, P.E., 1984. Magnetotelluric responses of three-dimensional Geothermometry, in: World Geothermal Congress 2015. bodies in layered earths. Geophysics 49, 1517. doi:10.1190/1.1441777

Hickey -Vargas, R., Moreno-Roa, H., Lopez-Escobar, L., Frey, F., 1989. Zeebe, R.E., 2010. A new value for the stable oxygen isotope Geochemical variations in Andean basaltic and silicic from the fractionation between dissolved sulfate ion and water. Geochim. Villarrica-Lanin volcanic chain (39.5° S): an evaluation of source Cosmochim. Acta 74, 818–828. doi:10.1016/j.gca.2009.10.034 heterogeneity, fractional crystallization and crustal assimilation. Contrib. to Mineral. Petrol. 103, 361–386. doi:10.1007/BF00402922

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Figure 2: Resistivity distribution of the subsurface from a magnetotelluric E-W profile crosscutting the major fault zone south of the Caburgua lake.

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