Tectonic Control of the Geothermal System at Mt. Villarrica - Insights from Geophysical and Geochemical Surveys

O EOL GIC G A D D A E D C E I H C I L E O S F u n 2 d 6 la serena octubre 2015 ada en 19 Tectonic control of the geothermal system at Mt. Villarrica - 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, 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

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