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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 [email protected]

Keywords thermal manifestation (i.e. hot springs, bubbling pool and fuma- Fluids geochemistry, volcanisms, geothermal exploration, roles) with faults associated to the collapse and resurgence of the south volcanic zone, fault thrust and belt 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 ABSTRACT caldera’s edges structures. For both groups a maximum temperature of 250ºC has been estimated by quartz geothermometers and Thermal manifestation associated to Planchón-Peteroa and the enthalpy-chlorine diagrams. Descabezado Grande-Quizapu-Cerro Azul volcanic complexes The aim of this study is to understand the contribution of volca- can be explained by a close relationship between a magmato- nism in the origin of the active thermal manifestation in the zone, hydrothermal 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 100-140ºC.

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 (34.4-37ºS) where the arc has a width of 150 km and the crust has a depth of 35-40 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 Figure 1. a: Andean volcanic zone (modified from Parada et al., 2007). b: thin-skinned internal deformation (Dicarlo and Cristallini, 2007). South volcanic zone (modified from Parada et al., 2007). c: Geological In this way PPVC and DGQAVC lie over inverse fault that put in map from the study zone. Samples 1, 2 and 3 aguas calientes spring; 6 contact, with NE-SW striking, Meso-Cenozoic unit (Cembrano tigre naciente spring; 7 potrerillo spring; 8 pellejo spring; 9 tierra hu- meante del colorado spring; 10 quebrada de los colores spring; 12 azufre and Lara, 2009; Figure 1c), whereas that CCC, is associated with fumaroles; 14 Llolli fumaroles; 15 Baños de la Yegua spring; 17 san pedro NW-SE and NE-SW structures. spring; 19 romeral spring; 30 descabezado grande spring; 32, 33 y 34 Previews studies of the active hydrothermal systems associated valle del estero del volcán fumaroles; 35 medano spring; 36 campanario with CCC (Thompson et al., 1983; Grunder et al., 1987) relate the spring; 39 panimávida spring; 40 quinamávidas spring.

699 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 1 360650 6070051 2573 36.4 7.9 0.00 244.67 1.28 38.32 51.52 35.30 3.90 0.30 0.02 28.59 74.63 3.40 2 360200 6069989 2569 38.9 6.9 0.00 262.37 2.11 61.26 80.28 42.79 5.89 0.36 0.01 43.79 113.80 3.98 3 361680 6070203 2563 37.9 7 0.00 314.84 1.66 70.98 84.02 45.14 5.48 0.31 0.04 44.91 98.01 3.70 4 362042 6070118 2573 12.3 8.3 0.00 97.02 0.46 45.30 62.47 24.17 8.36 0.22 0.02 11.66 53.06 1.64 5 362946 6071897 2580 12.6 9.1 0.00 3.05 0.00 1.80 2.68 3.40 0.23 0.20 0.01 0.41 5.75 0.58 6 362754 6073577 2563 64.9 7.2 0.00 275.18 4.58 95.28 110.36 49.20 1.06 0.44 0.02 20.80 146.30 3.10 7 362259 6082717 2343 46 7.4 0.00 295.32 2.98 52.80 265.27 47.71 5.45 0.27 0.01 65.03 133.15 5.42 8 360694 6081288 2129 48.9 6.7 0.00 812.73 2.62 195.28 134.75 116.59 32.46 0.45 3.37 125.20 194.35 20.62 9 361881 6080029 2154 75 7.4 0.00 244.67 1.74 1243.69 106.23 117.45 6.35 1.20 0.03 80.43 556.40 67.20 10 361386 6080040 2185 30.2 6.8 0.00 1434.49 1.43 39.73 374.81 69.31 93.70 0.68 9.21 227.00 176.75 8.47 12 360011 6090191 2738 70 6.7 0.00 485.08 0.25 3.66 2.50 170.50 23.81 0.15 0.47 62.63 60.37 10.95 14 356168 6084260 1996 94.5 2 0.00 0.00 0.17 1.67 1656.94 267.41 27.72 0.16 37.20 19.10 36.95 14.64 15 357295 6084620 1995 54 6.7 0.00 680.33 2.06 723.68 497.96 136.06 15.30 1.21 1.95 314.10 392.25 52.70 17 365777 6109783 1732 24 7.1 0.00 564.40 0.00 8389.90 1177.20 32.09 25.00 3.15 6.17 767.90 3698.00 728.70 18 365777 6109783 1732 7.4 9.72 0.00 70.17 0.04 20.78 429.50 13.97 5.75 0.17 0.08 200.20 29.95 4.87 19 363400 6116454 1433 6 8.6 0.00 96.41 0.12 910.92 118.64 11.51 7.29 0.75 0.03 141.20 464.10 25.47 30 334606 6059316 1880 29.8 5.8 0.00 611.99 0.35 100.99 137.32 99.91 101.30 0.25 0.04 53.50 96.18 23.84 31 334574 6059459 1879 12.2 6.4 0.00 186.71 0.09 10.20 25.93 51.56 22.09 0.22 0.03 19.77 32.96 7.94 32 340045 6070274 1945 94 0 0.00 69.56 0.01 3.14 79.81 26.53 9.68 0.23 0.47 13.71 21.39 4.67 33 339996 6070451 1962 84.5 5.8 0.00 49.42 0.21 3.17 1015.07 102.26 31.69 0.17 0.04 191.00 142.30 8.32 34 339969 6070449 1964 79.5 6 0.00 497.28 0.00 2.36 65.31 152.53 23.10 0.17 0.07 81.10 75.21 3.90 35 340794 6034171 988 29.5 6.6 0.00 36.61 0.67 330.90 266.20 41.07 5.14 0.56 0.02 103.90 134.05 7.91 36 356866 6022183 1546 54.5 5.4 0.00 518.03 0.00 11960.50 628.70 71.45 98.28 6.72 9.19 1818.00 5391.00 328.30 37 359787 6013471 2159 15 7.8 0.00 23.80 0.00 22.24 7.88 20.26 2.21 0.15 0.05 8.54 16.32 2.57 38 357009 6022221 1554 16.9 7.7 0.00 31.12 0.00 14.88 37.71 29.31 3.80 0.13 0.07 13.96 13.73 3.20 39 281522 6039846 970 33.7 9.3 3.61 5.49 0.34 53.09 151.53 29.95 0.03 0.09 0.02 29.82 57.25 1.31 40 280303 6035896 168 23.3 9.2 12.62 156.21 0.22 3.49 8.70 34.66 0.15 0.15 0.31 1.19 53.88 1.51

Figure 2. δ2H versus δ18O 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 (18O-2H) 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 δ2H-δ18O ratio in water samples (Figure 2) and from relative content of N2-He-CO2 in gas samples.

Figure 3. a: Sulphate, chloride and bicarbonate diagram. b: Pearce element ratio diagram for thermal water samples.

700 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) Quartz, chalcedony and multimineral equilibrium geothermometers show equilibrium temperatures that range Figure 4. Water geothermometer for all thermal water. between 100º-140ºC, instead the geothermometers with slow kinetic reaction like Na-K-Ca and He-CH4 for water and gases samples respectively, es- timates maximum temperatures of 350ºC for some samples.

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 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; He-CH4, erasing all evidence from red asterisks acid-sulfate waters, yellow circles meteoric waters. c: Possible reservoir and superficial thermal the deeper reservoir (samples 1, 2, anomalies. 3, 6, 7, 8, 10, 30 y 39).

(ii) The origin of the water dissolved components can be ex- Conceptual Model plained by water-rock interaction based on molar ratios showing dissolution trends of the main minerals present in the The conceptual model proposed can be represented in a bet- zone (i.e. gypsum, calcite, dolomite, feldspars). Despite this, ter way on enthalpy-chloride diagrams (Figure 5). The upflow relative concentration of main anions are distributed along fluids (samples 9, 15 and 35) represent water arising directly

701 Benavente and Gutiérrez from fault, and can be modeled by mixing of parent water at References 350ºC and meteoric water. Instead, the outflow fluids (samples Cembrano J. and Lara L., 2009. The link between volcanism and tectonics in 36, 17 and 19) also represent water arising from fault zone, but the southern volcanic zone of the Chilean Andes: A review, Tectonophys- the parent water comes from the boiling of the 350ºC water ics, Volume 471, Issues 1-2, Pages 96-113. and are located in the peripheral part of the systems. On the Dicarlo, D.J. and Cristallini, E. 2007. Estructura de la margen norte del río other hand, samples 1, 2, 3, 6, 7, 8, 10, 30, 39 are waters that Grande, Bardas Blancas, Provincia de Mendoza. Revista de la Asociación flow laterally due to impermeable volcanic rocks and do not Geológica Argentina 62: 187-199. follow the mixing lines. Those can be affected by heat transfer Grunder A., Thompson J., Hildreth W., 1987. The hydrothermal system of the Calabozos caldera, central Chilean Andes, Journal of Volcanology and from the crust, increasing water temperature but without mass Geothermal Research, Volume 32, Issue 4, Pages 287-298. transfer (chlorine). Hildreth W. and Moorbath S., 1988. Crustal contributions to arc magmatism in the Andes of Central Chile. Contributions to Mineralogy and Petrol- ogy 98: 455 - 489. Acknowledgement Parada M, et al., 2007. Andean Magmatism The Geology of Chile. T. Moreno and W. Gibbons. 1. Authors would like to thank Universidad de Chile for the nec- Thompson, J.M., 1975. Selecting and collecting termal springs for chemi- essary facilities and support to carry out this work, and CONICYT cal analysis: A method for field personneL.U.S. Geol. Surv., Open-File PBCT proyect PDA-07 for the financial support. Rep. 75-68, 12 pp.

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