The Geothermal System of Copahue-Caviahue Volcanic Complex (Argentina): New Insights from Self-Potential

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F u n 2 d 6 la serena octubre 2015 ada en 19 The geothermal system of Copahue-Caviahue volcanic complex (Argentina): New insights from self-potential,

CO2 and temperature measurements, with structural and fluid circulation implications.

Emilie Roulleau*, Marcela Pizarro, Francisco Bravo, Carlos Muñoz, Juan Sanchez CEGA, Departamento de Geología, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile

Federico de la Cal and Carlos Esteban Centro Administrativo Ministerial, Antártida Argentina 1245 - Edificio 4 - Piso 3, Neuquén, Argentina

*Contact email: [email protected]

Abstract. Geothermal systems represent natural heat resources occur in close spatial relationships with active transfer engines in a confined volume of rock which are volcanism along the Cordillera which is primarily strongly controlled by the regional stress field and the local controlled by the ~1000km long, NNE-trending Liquiñe- faults/fracture network. In Chile, there is still a lack of Ofqui Fault Zone (LOFZ), an intra-arc dextral strike-slip information on how fault network and lithology control the fault system associated with second-order anisotropy of fluid circulation. Here, we propose to study the geothermal overall NE-SW (extensional) and NW-SE (compressional) system of Caviahue-Copahue Volcanic Complex coupling orientation (Fig.1). The Caviahue–Copahue volcanic

dense self-potential (SP), CO2 and temperature (T) complex (CCVC) is located at the border between measurements. We demonstrate that all geothermal zones Argentina and Chile, and hosts in its northeastern flank

are characterized by a combination of SP maxima and CO2 five geothermal areas with surface manifestations and T peaks, related to ascent of fluid flows. These active including thermal springs, bubbling pools and fumaroles.

zones are interspersed by SP minima and no peaks in CO2 and T, which represent self-sealed zones (e.g. altered rocks) at depth, thus creating a barrier to gases and fluids. Outside the geothermal systems, the abrupt SP maxima and minima reveal new structural and lithological limits that may allow either ascent of small amount of deep fluids, or infiltration of meteoric water.

Keywords: Geothermal system, Copahue, Self-potential, CO2 concentration and diffuse flux, Temperature.

1 Introduction

The hydrothermal and geothermal systems represent natural heat transfer engines in a confined volume of rock. In volcanic regions, a deep magmatic body provides the heat source, and water circulation and convection exert a first-order control on this activity (Hochstein and Browne, 2000). Thus, water incorporation inside the system is responsible for permanent energy and mass transfer from depth to the surface. In geothermal systems, fluid circulation may range from few hundreds of years to more Figure 1. Plate tectonic setting of the Southern Andes, showing than 10,000 years (Arnorsson et al., 2007), and is strongly the Liquiñe-Ofqui Fault Zone (LOFZ), the Southern Volcanic controlled by the regional stress field and the local Zone (SVZ), Northern Volcanic Zone (NVZ), Central Volcanic faults/fracture network. Therefore, the study of fluid flow Zone (CVZ) and Copahue–Caviahue Volcanic Complex (CCVC). Modified from Velez et al., 2011. in volcanic edifices is a powerful approach to investigate the influence of the regional and local tectonic setting on Although there is consensus that the regional-scale tectonic the uprising of fluids (Arnorsson et al., 2007). In the stress field largely controls the volcanism and geothermal Southern Andes of Chile and Argentina, geothermal activity (Alam et al., 2010; Lahsen et al., 2010), there is 469 SIM 4 SISTEMAS GEOTERMALES ANDINOS

still a lack of information on how the fracture and fault anomalies are linked to the shallow hydrothermal or network and the lithology control the fluid circulation and geothermal system associated to the active volcano. In the evolution of geothermal systems at CCVC. For this absence of fumarolic activity, the increase of ground purpose, we present the first high-density data of self- temperature can be related to the condensation of water potential (SP), CO2 and temperature (T) measurements steam at depth, which releases large quantities of heat from achieved in Copahue geothermal areas. Ten profiles were convective cells. done for a total of 12800 m of measurements (Fig. 2). 2.2 Methods

Coupled measurements of SP, T and CO2 soil gas concentration and flux were carried out during a field campaign in March 2015, along various profiles representing a total of ~13 km (Fig. 2). SP measurements were acquired every 20 m for each profile. We used a pair of Cu/CuSO2 non-polarizing electrodes and an insulated electric cable. The difference of electric potential between the reference electrode and the moving electrode was measured with a high-impedance voltmeter. The electric contact with the ground was good (≤ 500 kΩ) in the geothermal areas, since moisture was always found a few centimeters below the surface. Outside the geothermal Figure 2. Satellite map (from Google Earth) showing the study areas, the electric contact with the ground gave high values area at NE of Copahue volcano. The lines in colors represent the (sometimes > 2 MΩ) resulting in a high background noise. profiles done for SP, T and CO2 measurements. The black lines Temperature was measured at ~30 cm depth every 20 m in are the main structural faults. the geothermal areas and every 40 m elsewhere. Thermal probes and digital thermometer were used for the ground temperature measurements. The temperature reading was 2 Background and Methods taken after 15 min, the necessary time to reach thermal equilibrium. CO2 data were measured every 20 m in the 2.1 Background geothermal areas and every 40 m elsewhere. The soil CO2 concentration was obtained by pumping the gas through a The interpretation of the SP anomalies in volcanic or copper tube of 4 mm diameter, inserted into the soil to a geothermal areas depends on two mechanisms: depth of ~30 cm, as a difference of potential after electrokinetic and thermoelectric coupling (Corwin and calibration of the gascheck infrared gas sensor (from Hoover, 1979). However theorical considerations and Edinburgh Sensors). Two spectrometers with different observations suggest that the electrokinetic potential is concentration sensitivities (0-3000 ppm and 0-10 %) were significantly larger than the thermoelectric potential. used to avoid dilution issues. The diffuse soil CO2 flux Electrokinetic potentials are created by fluid flow in measurements followed the methodology of Chiodini et al. porous systems. In the absence of a hydrothermal or (1998). The instrumentation consisted of an infrared geothermal system, fluid flow is restricted to the mostly spectrometer (LICOR-820) with a range of 0-20000 ppm, vertical flow of vadose water towards a water table. The an accumulation chamber (volume of 37cm3) and a laptop linear reverse relationship between SP and altitude to plot the cumulative CO2 concentration as a function of commonly observed on volcanoes is thought to be related time. The increase of concentration in the chamber through to an increase in thickness of the vadose zone with the time allows determining the flux of CO2 from the soil. altitude (Finizola et al., 2003, and references therein). Thus, the lower SP represents the maximum thickness of the unsatured zone. The presence of a hydrothermal or 3 Results geothermal area produces positive SP anomalies (Aubert, 1999). However, an ambiguity may alter the interpretation Our preliminary results show that SP maxima occur in of SP maxima. If the system is purely hydrogeological (i.e. each geothermal area of the CCVC, in association with no connection to the hydrothermal system), a SP maximum CO2 and T peaks, as it is observed in Anfiteatro and can be considered as due to the rise of the water table. In Copahue city (e.g. domains I, II and III in Fig. 3). They are contrast, in a hydrothermal or geothermal area, a SP in general interspersed by SP minima and the absence of maximum will correspond to the convective upward flow CO2 and T peaks (D1 and D2 in Fig. 3). The CO2 and T of fluids. This ambiguity can be resolved by coupling SP peaks observed for Anfiteatro are smaller than those for measurements with, for example, temperature and CO2 Copahue and other geothermal areas. Elsewhere, abrupt SP measurements. On active volcanoes, CO2 anomalies are variations (maxima and minima) seem structurally generally associated with a highly permeable zone, which controlled (F1 to F5 in Fig. 3), and the corresponding CO2 may also drain heat and other fluids. Surface temperature 470 AT 2 geología económica y recursos naturales

concentrations and fluxes are close to the CO2 atmospheric value (400 ppm), with some exceptions. Low temperatures (ambient temperature: ~13-15°C) have always been measured in these areas.

4 Interpretation and Discussion

4.1 Source of SP, CO2 and T anomalies in the geothermal systems

The geothermal activity in CCVC is mainly seen in the field as fumarole activity and thermal pools in Anfiteatro, Copahue, Las Maquinas, Cabañita and Las Maquinitas (Fig. 2). The signal expected from a simple geothermal system is an increase of SP values and possibly an increase of temperature and CO2 degassing. This is indeed observed in Anfiteatro and Copahue (I, II, and III geothermal subsystems; Fig. 3) and in other geothermal areas, but also associated with prominent low SP, CO2 and T values (D1 and D2; Fig. 3). The sharpness of the geothermal areas suggests that the whole system contains a number of geological barriers or more impermeable layers (Finizola et al., 2003). Altered rocks are usually considered as natural geological barriers (Barde-Cabusson et al., 2009, and references therein). The altered zones can become self- sealed at depth, thus creating a barrier to gases and fluids. The SP minimum D2 in Anfiteatro (Fig. 3), associated with absence of peaks of CO2 and T, supports the idea of meteoritic water infiltration. The SP maximum M1 in Anfiteatro (Fig. 3), associated with absence of peaks of CO2 and T, is ambiguous but can reflect the presence of an old structural limit which is totally impermeable, producing a reserve polarity of SP (Guichet et al., 2006). The presence of lower CO2 emanations (up to 60,000 ppm) and lower temperatures (up to 50°C) in Anfiteatro in comparison to Copahue (up to 400,000 ppm and 90°C, respectively) supports the low uprising of fluid flow. We also observed that each geothermal field (e.g. Copahue; Anfiteatro) is surrounded by an increase of SP, in general coupled with CO2 and T peaks, as due to the main NE- Figure 3. Comparison between SP, T and CO2 measurements trending faults (e.g. CF1 and CF2) that allow the deep fluid along the profile Anfiteatro-Copahue-Las Maquinitas. The largely spaced dotted lines are the main structural faults (named supply. The variability of the CO2 and T peaks in these zones is the result of permeability changes of the fault AF and CF for Anfiteatro fault and Copahue fault, respectively). The closely spaced dotted lines represent the secondary faults (F1 system. In the case of Anfiteatro, the main NW-trending to F5). The red dotted line F2 marks a structural limit between faults suggest a less efficient ascent of fluids. two aquifers at different altitudes. The grey-shaded rectangles represent the geothermal areas in Anfiteatro and Copahue. SP maxima and T and CO2 peaks are numbered for each geothermal 4.2 Source of SP, CO2 and T anomalies elsewhere area: I, II and III as distinct geothermal subsystems. D1 and D2 represent SP minima in relation with a geological barrier. M1 Outside the geothermal areas, the SP signals is highliths the positive increase of SP without CO2 or T peaks in characterized by strong SP noise variations (Fig. 3) due to Anfiteatro. The empty symbols of the SP profile represent the low permeability (Rowland and Simmons, 2012) of the measurements with high resistance (> 2 MΩ). host rocks (ignimbrites from Las Mellizas; Melnick et al., 2006). It is however possible to detect SP minima Figure 3, the red dotted line F2 separates two SP landings associated with absence of CO2 or T peaks (F3, F4, F5), or plateaus: a first one at ~100mV and a second one at 0- which are interpreted as the result of infiltration of 20mV. This change of mean SP supports the presence of meteoric water in secondary NNW-trending fault zones. In two aquifers at different altitudes/depths (Finizola et al.,

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2004; Bennati et al., 2011); the water table appears higher Barde-Cabusson, S., Finizola, A., Revil, A., Ricci, T., et al, 2009. between F1-F2 compared to F2-F4. New geological insights and structural control on fluid circulation in La Fossa cone (Vulcano,Aeolian Islands, Italy). J. Volcanol. Geotherm. Res. 185, 231–245. 4.3 Regional tectonic setting: implication for fluid Bennati, L.; Finizola, A.; James A. Walker, Dina L.et al. (2011). circulations Fluid circulation in a complex volcano-tectonic setting, inferred from self-potential and soil CO2 flux surveys: The Santa Maria- CCVC is located where the LOFZ bends eastward and Cerro Quemado-Zunil volcanoes and Xela caldera (Northwestern decomposes into a series of NNW- to NE-striking Guatemala) Journal of Volcanology and Geothermal Research extensional and transtensional fault splays that form an 199 (3-4). arrangement with a horsetail-like geometry (Rosenau et al., 2006). These structures, due to their high permeability, Corwin RF, Hoover DB (1979) The self-potential method in promote vertical fluid circulation (Rowland and Simmons, geothermal exploration. Geophysics 44: 226–245 2012) in favor of geothermal system appearance. SP maxima and CO2 and T peaks observed in the CCVC Chiodini, G., R. Cioni, M. Guidi, B. Raco, and L. Marini (1998), Soil geothermal areas might be related to the presence of NE- CO 2 flux measurements in volcanic and geothermal areas. trending main faults, with the exception of Anfiteatro Applied Geochemistry: 13: 543-552. where all signals are related to NNW-trending faults producing a pull-apart structure. Anfiteatro appears as a Finizola, A., Sortino, F., Lénat, J.-F., Aubert, M., Ripepe, M., place where both the upward fluid flow and infiltration of Valenza, M. (2003). The summit hydrothermal system of meteoric water coexist. This is consistent with the Stromboli. New insights from self-potential, temperature, CO2 observed moderate 3He/4He ratios (~5Ra; Roulleau et al., and fumarolic fluid measurements, with structural and monitoring implications. Bull. Volcanol. 65 (7), 486–504. 2015), CO2 emanations and temperatures. The secondary NNW-trending faults F3, F4 and F5 only allow infiltration Finizola, A., Lénat, J.-F., Macedo, O., Ramos, D., Thouret, J.-C., of water whereas the secondary NE-trending fault F1 Sortino, F. (2004). Fluid circulation and structural discontinuities permits ascent of deep fluids. Importantly, we conclude inside Misti volcano (Peru) inferred from self-potential that the NE-striking faults may control the upward flow of measurements. J. Volcanol. Geotherm. Res. 135 (4), 343–360. deep fluids. This is also consistent with high 3He/4He (7- 8Ra; Roulleau et al., 2015) observed in Copahue, Las Hochstein, M.P. and Browne, P.R.L. Surface manifestations of Maquinas, Las Maquinitas and Cabañita geothermal zones. geothermal systems with volcanic heat sources. In: Sigurdsson, H. (Ed.), Encyclopedia of Volcanoes. Academic Press, San Francisco, 835-855, 2000. Acknowledgements Guichet, X., Jauniaux, L. and Catel, N. (2006) Modification of This work was funded by E. Roulleau's research grant streaming potential by precipitation of calcite in a sand–water through FONDECYT project 11130351 and at CEGA by system: laboratory measurements in the pH range from 4 to 12. Geophysical journal Int. 166, 445-460. FONDAP project 15090013. M.Pizarro was funded by a PhD grant from CONICYT. Thanks to Dr. Finizola and Dr. Lahsen, A., Muñoz, N., Parada, M.A. (2010) Geothermal Barde-Cabusson for their help with the dissection of the development in Chile. Proceedings World Geothermal Congress, dataset, and to Vicente Solar for supplying diffusive CO2 Bali, Indonesia, Paper Nº 25. flux equipment. Melnick, D., Folguera, A., Ramos, V. A. (2006). Structural control on arc volcanism: The Caviahue-Copahue complex, Central to References Patagonian Andes transition (38°S). JSAES. 22. 66-88.

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