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Procedia Earth and Planetary Science 7 ( 2013 ) 859 – 862

Water Rock Interaction [WRI 14] Geochemical characterization of the Calacoa geothermal zone

Vicentina Cruza, Victor Vargasb*, Koji Matsudac, Yoshio Soedac

aInstituto Geologico Minero y Metalurgico, INGEMMET, Av. Canadá # 1470 San Borja, Lima, Lima 41, Perú bHot Rock Perú S. A. Calle Mariscal Sucre # 183 – Of. 101, Miraflores, Lima, Lima 18, Perú. formerly: INGEMMET. cWest Japan Engineering Consultants, INC, Space cube Bldg., 2F, 7-11,1-Chome, Haruyoshi Chuo-ku, Fukuoka, Japan

Abstract

The Calacoa geothermal zone (CGZ) is located in the western Cordillera of the in southern Peru, approximately 10 km on the western slope of Ticsani Volcano. In Calacoa there are mainly volcanic rocks that have been deposited over a sedimentary Cretaceous basement. The geochemical interpretation of the results shows that thermal waters are an alkaline-chloride-sulphate water type. The Cl-SO4-HCO3 diagram shows that most of the geothermal waters plot close to the mature water portion, which is typical of geothermal deep fluids. High B concentrations lead to a relatively high B/Cl ratios as shown on a B-Cl binary diagram. This can be used to elucidate the reactions of waters with sedimentary marine rocks at deep depths. The δ18O vs δD plot indicates that the geothermal water originates by a mixing of meteoric water with magmatic water. The results for chemical geothermometry allowed us to estimate temperatures between 170 and 230 °C at the depth of the geothermal resources.

©© 2013 2012 The The Authors. Authors. Published Publis byhed Elsevier by Elsevier B.V. B.V. SelectionSelection and/or and/or peer-review peer-review under under responsibility responsibility of the Organizing of Organizing and Scientific and Scientific Committee Committee of WRI 14 of – 2013WRI 14 – 2013.

Keywords: Calacoa; geochemical caracterization; chloride waters.

1. Introduction

The Calacoa geothermal zone (CGZ) is located approximately 10 km on the western part of Ticsani volcano (5408 masl) in the western Cordillera of the Andes in southern Peru. The CGZ is located in Mariscal Nieto province in Moquegua Region. The surface evidence indicates that the heat source in the CGZ is associated with the hydrothermal system of the Ticsani volcano, which has some surface evidence of thermal activity, including fumaroles observed close to the top of the volcano. The main thermal manifestations are located in three areas; first: Secolaque located 3 km to the NW of Calacoa town;

* Corresponding author. Tel.: + 51 979361531. E-mail address: [email protected].

1878-5220 © 2013 The Authors. Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Organizing and Scientific Committee of WRI 14 – 2013 doi: 10.1016/j.proeps.2013.03.210 860 Vicentina Cruz et al. / Procedia Earth and Planetary Science 7 ( 2013 ) 859 – 862

second: called Putina located 1.5 km to SW of the same town, and the third: called Soquesane and located 6 km to the south of the Ticsani volcano.

2. Geological setting

In the CGZ there are outcrops of sedimentary, volcanic and intrusive rocks, from to age. The oldest rocks are the sedimentary rocks composed mainly of limestone interbedded with a sequence of sandstones and lutites, on the top limestones are interbedded with sandstones, thickness > 1100 m and belonging to Yura group [1]. The oldest volcanic rocks that form the substratum over which were deposited the and pyroclastic flows from Ticsani events are constituted by and (Matalque formation and Toquepala group), continental clastic sediments (Pichu formation) and rhyolitic (Huaylillas formation) [2] and [3]. Regionally associated with the Ticsani and volcanos, two regional trends have been observed, one with Andean direction NW- SE, NNW-SSE and WNW-ESE, and the other with NNE-SSW and NE-SW direction

3. Discussion

In the CGZ the main geothermal manifestations are thermal springs located in the Putina River, between Sicolaque and the San Cristonal Bridge, in a range of altitude between 2900 and 3400 masl. The spring temperatures range from 50°C to 91°C, the pH ranges between 6 to 8 and the electrical conductivity is between 2.7 and 3.3 mS/cm. Table 1 shows the physical-chemical parameters and the results of the chemical analysis. The ionic balance is between ± 1 to 2 %, which is considered acceptable.

Table 1. Main hot springs in CGZ and the chemical composition (mg/L).

Spring Temp Li Na K Ca Mg Cl SO4 HCO3 B SiO2 T°C T°C T°C Na/K Na/Li °C mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L Quartz [6, 7, 4] [8] [9] Putina I 88.0 3 552 51 71 7 634 366 197 18 179 179-226 249 173 Putina II 91.8 3 658 55 64 5 730 423 181 21 178 169-218 237 172 Putina 1 90.2 4 617 54 60 5 746 379 158 21 187 173-221 263 175 Putina 2 89.3 536 49 72 12 649 328 198 18 182 177-225 174 Secolaque 67.4 304 36 50 5 439 130 93 206-246 Cuchumbaya 51.6 640 54 140 24 746 482 219 170-219

3.1. Classification of the thermal waters

According to the ternary diagram [4] the thermal waters are divided based in the composition of Cl- SO4-HCO3 (Fig. 1) where the waters plot in the chloride-sulphate field. We interpreted the water to be at the beginning of the boiling because the depressurization, the sulfide goes to the vapor phase, while the Cl tends to stay in the liquid phase. This reaction increases the pH, generating chloride-neutral waters, which move in the SO4-Cl axis going to Cl corner. At deep levels and high enough pressure the magmatic CO2 will stay in the hydrothermal deep fluid as CO2 (aq) [5].

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3.2. Chemical tracers

The Cl-Li-B ternary diagram (Fig. 2) [10] indicates a common origin for the Calacoa´s waters where Cl is the dominant ion, then B and relatively low in Li. Also these waters are located close to the Cl-B line, and according with the relative concentration of these ions the waters are clearly associated with volcanic magmatic system, evidenced also by the presence of recent volcanic activity of, the Ticsani volcano.

Fig. 1. Cl-SO4-HCO3 ternary diagram. Fig. 2. Cl-Li-B ternary diagram.

The concentration of B in the thermal waters in CGZ has a range between 18 to 21 mg/L, and the B/Cl mole ratio is 0.09. Figure 3 shows the thermal waters are sourced in the sedimentary rocks; the B/Cl ratios indicate that these waters are reacting with sedimentary rocks at deep levels, which presents high porosity and permeability with fractures [11] and [12].

3.3. Na-K-Mg Geoindicators

Figure 4 shows the Na-K-Mg geoindicator proposed by Giggenbach, where it is observed that the waters are plotted inside the region of immature waters, but trending to the partial equilibrium line; this could indicate that the thermal water have been mixed with the surface water from the Putina River. The diagram shows also a linear trend towards the 220°C Na/K equilibrium temperature of the reservoir, resulting either from water-rock interaction or mixing, but due to the high Mg concentration of the samples this value could be uncertain. Mg could come from superficial waters and Putina River where the thermal waters emerge. These waters suffer dilution and mixing leading to the Mg increase and decrease of the other ions The Na/K geothermometer estimates temperatures between 170 and 263 °C [4, 6, 7, and 8]. According to the fact that thermal waters are still in the immature field, temperature calculated from Na/K geothermometer could be overestimated. However calculated temperatures with other geothermometers (Na-Li or quartz) are also in the same range of temperature as it can be seen in the Table 1.

3.4. Isotopic results

δ18O values are in the range -9.73 to -14.87‰ and δ2H in the range -66.5 to -113.6‰. The cold springs are located close to the Local Meteoric Line (LML), which indicates that this water is being diluted mainly by meteoric water. The hot springs located in the S-SW are plotted on the LML while the hot springs located in the NW and N, are plotted besides the LML (graph not shown). This means that the 862 Vicentina Cruz et al. / Procedia Earth and Planetary Science 7 ( 2013 ) 859 – 862

thermal waters associated with magmatic fluids (15%) are mixed with meteoric water (85%). This value was estimated according to the water chloride content.

1000

s a nic á 7 rina c 0 100 . ma vol -0 s ia 0 r .0 0.02 ta 1 oca l= n 7- R /C .0 B ime 0 ed l= Granito: 10 s /C B B/Cl~0.01 oca R B (mg/L) 1 B/Cl=1

0.1 B/Cl=0.1

B/Cl=0.01 B/Cl=0.001 0.01

1 10 100 1000 10000 100000 Cl (mg/L)

Fig. 3. Boron vs. chloride diagram. Fig. 4. Na-K-Mg ternary diagram.

4. Concluding Remarks

The results of geological and structural analysis in CGZ have identified some favorable geological formations for development of a geothermal reservoir. The geological structure is very important because it improves the porosity and permeability in these rocks. Also the main hot springs are controlled by NW- SE, WNW-ESE and NE-SW faults. The results of δ18O vs. δ2H, Cl concentration, relatively high B concentration and B/Cl mole ratio, indicate that the geothermal water originates by mixing of mainly meteoric water and magmatic water, which are circulating in sedimentary deep rocks. The geothermometry shows a range of temperatures from 170 to 230 °C for the geothermal reservoir.

References

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