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2 GRC Transactions, Vol. 35, 2011 Magmatic-Hydrothermal Systems Associated to Planchón-Peteroa and Descabezado Grande-Quizapu-Cerro Azul Volcanic Complexes, VII Region, Chile Oscar Benavente and Francisco Gutiérrez Departamento de Geología, Universidad de Chile, Santiago, Chile Keywords Fluids geochemistry, volcanisms, geothermal exploration, south volcanic zone, fault thrust and belt ABSTRACT Thermal manifestation associated to Planchón-Peteroa and Descabezado Grande-Quizapu-Cerro Azul volcanic complexes can be explained by a close relationship between a magmatohydrothermal systems and fault from the Malargue Fold Thrust and Belt. Two possible reservoir can be distinguish at different depths. The deeper one has temperatures near 350ºC and is controlled mainly by a mixture of volcanic and hydrothermal fluids. Instead the shallow one has a temperature range between ºC. thermal manifestation (i.e. hot springs, bubbling pool and fumaroles) with faults associated to the collapse and resurgence of the caldera complex. Along the thermal manifestation two principal groups can be distinguish: (i) the ones associated with the fault that control the volcanism and, (ii) the ones associated with the caldera s edges structures. For both groups a maximum temperature of 250ºC has been estimated by quartz geothermometers and the enthalpy-chlorine diagrams. The aim of this study is to understand the contribution of volcanism in the origin of the active thermal manifestation in the zone, Introduction Calabozos Caldera Complex (CCC) and Planchón-Peteroa (PPVC) and Descabezado Grande-Quizapu-Azul (DGQAVC) volcanic complexes, are located between 35-36ºS, at Maule s region, Chile (Figure 1). This complexes belong to the volcanic arc of the transitional south volcanic zone (TSVZ; Figure 1a and 1b), which correspond to a sector of 300 km ( ºS) where the arc has a width of 150 km and the crust has a depth of km (Hildreth & Moorbath, 1988). The time-spatial control of the PPVC-DGQAVC-CCC and the active hydrothermal system associated are defined by NW- SE and NE-SW structures (Cembrano and Lara, 2009; Figure 1) that develop along the Malargue fold thrust and belt (MFTB). The MFTB is a thick-skinned fold thrust and belt in this zone, composed by a series of basement blocks that limit the zone with thin-skinned internal deformation (Dicarlo and Cristallini, 2007). In this way PPVC and DGQAVC lie over inverse fault that put in contact, with NE-SW striking, Meso-Cenozoic unit (Cembrano and Lara, 2009; Figure 1c), whereas that CCC, is associated with NW-SE and NE-SW structures. Previews studies of the active hydrothermal systems associated with CCC (Thompson et al., 1983; Grunder et al., 1987) relate the Figure 1. a: Andean volcanic zone (modified from Parada et al., 2007). b: South volcanic zone (modified from Parada et al., 2007). c: Geological map from the study zone. Samples 1, 2 and 3 aguas calientes spring; 6 tigre naciente spring; 7 potrerillo spring; 8 pellejo spring; 9 tierra humeante del colorado spring; 10 quebrada de los colores spring; 12 azufre fumaroles; 14 Llolli fumaroles; 15 Baños de la Yegua spring; 17 san pedro spring; 19 romeral spring; 30 descabezado grande spring; 32, 33 y 34 valle del estero del volcán fumaroles; 35 medano spring; 36 campanario spring; 39 panimávida spring; 40 quinamávidas spring. 699

3 Benavente and Gutiérrez Table 1. Water composition of the spring samples. Latitud, longitude and altitude is in meters. Concentration of ions are in mg/l. Sample lon Lat Alt Temp ph CO3 HCO3 F CL SO4 SiO2 Mg Li Fe Ca Na K Figure 2. δ 2 H versus δ 18 O diagrams for water sample. based on the geological, structural and geophysics superficial information, and gas-water geochemistry, considering a bigger area than the latter authors. For this purpose we collected a total of 24 water and 6 gas samples from hot springs, bubbling pools, fumaroles and meteoric water. We analyzed cations, anions and stable isotope ( 18 O- 2 H) in water samples, and soluble and insoluble gas species (Table 1). Results and Discussions Here we outline the main chemical results of this work: (i) The origin of the fluids from the fumaroles, hot springs, bubbling pools are deep circulation of meteoric waters that are heated by shallows magmatic chambers (4 km). This can be seen by δ 2 H-δ 18 O ratio in water samples (Figure 2) and from relative content of N 2 -He-CO 2 in gas samples. Figure 3. a: Sulphate, chloride and bicarbonate diagram. b: Pearce element ratio diagram for thermal water samples. 700

4 Benavente and Gutiérrez the study zone, as a typical waters from volcanic-hydrothermal system do (chloride water on peripherical zone outflow zone ; sulphate water above fault trace that control the emplacement of magmatic chamber upflow zone ; bicarbonate water between this two members). (iii) Q u a r t z, c h a l c e d o n y a n d multimineral equilibrium geothermometers show equilibrium temperatures that range between 100º-140ºC, instead the geothermometers with slow kinetic reaction like Na-K-Ca and He-CH 4 for water and gases samples respectively, estimates maximum temperatures of 350ºC for some samples. Figure 4. Water geothermometer for all thermal water. Figure 5. a. Chloride-enthalpy diagrams for water samples. b. is the zoom of the red rectangle on Figura 2.a. On green triangles chloride waters; yellow triangles sulfate-chloride waters; blue triangles bicarbonate waters; red asterisks acid-sulfate waters, yellow circles meteoric waters. c: Possible reservoir and superficial thermal anomalies. Due to the different temperatures estimated by the geothermometers and the geology, it is possible to infer the existence of two reservoir at different depth. The deeper one would have temperature near to 350ºC, where water would rise threw a permeable zone associated with faults of the MFTB. In its rise, thermal water would mix with meteoric water and interact with the host rock changing the equilibration temperatures of the faster kinetic reaction geothermometers (i.e. quartz, chalcedony, Na-K, multimineral equilibrium). Therefore, on sectors where faults reach the surface it is possible to find active thermal manifestation with evidence of equilibrium from the deeper reservoir (samples 9, 15, 17, 19, 35). Meanwhile where faults do not reach the surface due to the impermeable volcanic cover, water would flow laterally having time sufficient to also reequilibrate the slower kinetic reaction geothermometers like Na-K-Ca and He-CH4, erasing all evidence from the deeper reservoir (samples 1, 2, 3, 6, 7, 8, 10, 30 y 39). (ii) Conceptual Model The origin of the water dissolved components can be explained by water-rock interaction based on molar ratios showing dissolution trends of the main minerals present in the zone (i.e. gypsum, calcite, dolomite, feldspars). Despite this, relative concentration of main anions are distributed along The conceptual model proposed can be represented in a better way on enthalpy-chloride diagrams (Figure 5). The upflow fluids (samples 9, 15 and 35) represent water arising directly 701

5 Benavente and Gutiérrez from fault, and can be modeled by mixing of parent water at 350ºC and meteoric water. Instead, the outflow fluids (samples 36, 17 and 19) also represent water arising from fault zone, but the parent water comes from the boiling of the 350ºC water and are located in the peripheral part of the systems. On the other hand, samples 1, 2, 3, 6, 7, 8, 10, 30, 39 are waters that flow laterally due to impermeable volcanic rocks and do not follow the mixing lines. Those can be affected by heat transfer from the crust, increasing water temperature but without mass transfer (chlorine). Acknowledgement Authors would like to thank Universidad de Chile for the necessary facilities and support to carry out this work, and CONICYT PBCT proyect PDA-07 for the financial support. References Cembrano J. and Lara L., The link between volcanism and tectonics in the southern volcanic zone of the Chilean Andes: A review, Tectonophysics, Volume 471, Issues 1-2, Pages Dicarlo, D.J. and Cristallini, E Estructura de la margen norte del río Grande, Bardas Blancas, Provincia de Mendoza. Revista de la Asociación Geológica Argentina 62: Grunder A., Thompson J., Hildreth W., The hydrothermal system of the Calabozos caldera, central Chilean Andes, Journal of Volcanology and Geothermal Research, Volume 32, Issue 4, Pages Hildreth W. and Moorbath S., Crustal contributions to arc magmatism in the Andes of Central Chile. Contributions to Mineralogy and Petrology 98: Parada M, et al., Andean Magmatism The Geology of Chile. T. Moreno and W. Gibbons. 1. Thompson, J.M., Selecting and collecting termal springs for chemical analysis: A method for field personnel.u.s. Geol. Surv., Open-File Rep , 12 pp. 702