Geochemical Study of Thermal Waters in the Tutupaca Geothermal Zone, Tacna, South of Peru

Abstract

In this study, representative samples from hot springs were chemically analyzed. Geothermometers were used to calculate the deep temperatures of geothermal reservoirs on the basis of fluid mineral equilibrium. In all cases, the chemical components are not in equilibrium with the minerals in the reservoir, due to the fact that most of these waters are acidic. Geothermal manifestations in the Tutupaca zone are evidenced by the presence of hot springs, mud pools and fumaroles with temperatures up to 90°C. The pH values range from 2.90 to 6.9 and conductivity from 800 µS/cm to 2900 µS/cm. Geochemical interpretation according to Cl-SO4-HCO3 ternary diagram shows that thermal waters are classified as sulphate, sulphate-chloride and bicarbonate waters. In B-Cl binary diagram the waters of the Tutupaca area are reacting at deep levels with marine sedimentary rocks, which probably have high porosity and permeability and with abundant fractures. The Na-K-Mg triangular diagram was also used to evaluate equilibrium between water and reservoir rocks, showing a linear trend and pointing to an equilibrium temperature of 200°C (Na/K) in the reservoir and fluid dilution or mixing. The d2H and d18O isotopes diagram also show that the thermal waters originate mainly from meteoric water. According to the geological and geochemical exploration results a conceptual hydro-geochemical model has been proposed for the Tutupaca zone. This model shows a geothermal system associated with magmatic sources from where the geothermal fluids emerge. There is a structural trend which allows the deep circulation of the waters between 2 and 3 km. The chemistry of these thermal waters could be explained by the interaction between the thermal fluid with sedimentary and volcanic rocks.