ABSTRACT

The El Tatio geothermal field is located at an height of 4200-4300 m on the Cordillera de los Andes (Altiplano). Geysers, hot pools and mudpots in the geothermal field and local meteoric waters were sampled in April 2002 and analyzed for major and trace elements, δ²H, δ¹⁸O and ³H of water, δ³⁴S and δ¹⁸O of dissolved sulfate, δ¹³C of dissolved total carbonate, and ⁸⁷Sr/⁸⁶Sr ratio of aqueous strontium. There are two different types of thermal springs throughout the field, that are chloride-rich water and sulfate-rich water. The chemical composition of chloride springs is controlled by magma degassing and by water-rock interaction processes. Sulfate springs are fed by shallow meteoric water heated by ascending gases. In keeping with the geodynamic setting and nature of the reservoir rocks, chloride water is rich in As, B, Cs, Li; on the other hand, sulfate water is enriched only in B relative to local meteoric water. Alternatively to a merely meteoric model, chloride waters can be interpreted as admixtures of meteoric and magmatic (circa andesitic) water, which moderately exchanges oxygen isotopes with rocks at a chemical Na/K temperature of about 270°C in the main reservoir, and then undergoes loss of vapor (and eventually mixing with shallow water) and related isotopic effects during ascent to the surface. These chloride waters do not present tritium and can be classified as sub-modern (pre-1952). A chloride content of 5,400 mg/l is estimated in the main reservoir, for which δ²H and δ¹⁸O values, respectively of -78‰ and -6.9‰, are calculated applying the multistage-steam separation isotopic effects between liquid and vapor. From these data, the meteoric recharge (Cl≈0 mg/l) of the main reservoir should approach a composition of -107‰ in δ²H and -14.6‰ in δ¹⁸O, when a magmatic water of δ²H = -20‰, δ¹⁸O = +10‰ and Cl = 17,500 mg/l is assumed. The ⁸⁷Sr/⁸⁶Sr ratios of the hot springs are quite uniform (0.70876 to 0.70896), with values within the range observed for dacites of the Andean central volcanic zone. A water δ¹⁸O-⁸⁷Sr/⁸⁶ Sr model was developed for the main geothermal reservoir, by which a meteoric-magmatic composition of the fluids is not excluded. The uniform δ³⁴S(SO₄^2-) values of +1.4 to +2.6‰ in the chloride waters agree with a major deep-seated source for sulfur, possibly via hydrolysis in the geothermal reservoir of sulfur dioxide provided by magma degassing, followed by isotopic exchange between sulfate and sulfide in the main reservoir. This interpretation is supported by the largely negative δ³⁴S(SO₄^2-) value in steam-heated water sulfate (-9.8‰) and mass-balance calculation, which exclude leaching at depth of igneous iron-sulfides with δ³⁴S near zero per mill. All the δ¹³C values of total carbonate in the chloride waters are negative, with variable values from -9.2 to -20.1‰, pointing to an important proportion of biogenic carbon in the fluids. The interpretation of these data is problematic, and a number of alternative explanations are reported in the text.