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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, delta(2)H, delta(18)O and H-3 of water, delta(34)S and delta(18)O of dissolved sulfate, delta(13)C of dissolved total carbonate, and Sr-87/Sr-86 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 degrees 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 submodern (pre-1952). A chloride content of 5,400 mg/l is estimated in the main reservoir, for which delta(2)H and delta(18)O values, respectively of -78 parts per thousand and -6.9 parts per thousand, are calculated applying the multistage-steam separation isotopic effects between liquid and vapor. From these data, the meteoric recharge (Cl approximate to 0 mg/l) of the main reservoir should approach a composition of 107 parts per thousand in delta(2)H and -14.6 parts per thousand in delta(18)O, when a magmatic water of delta(2)H = -20 parts per thousand, delta(18)O = +10 parts per thousand and Cl = 17,500 mg/l is assumed. The Sr-87/Sr-86 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 delta(18)O-Sr-87/Sr-86 model was developed for the main geothermal reservoir, by which a meteoric-magmatic composition of the fluids is not excluded. The uniform delta(34)S (SO42-) values of +1.4 to +2.6 parts per thousand, 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 delta(34)S (SO42-) value in steam-heated water sulfate (-9.8 parts per thousand) and mass-balance calculation, which exclude leaching at depth of igneous iron-sulfides with delta(34)S near zero per mill. All the delta(13)C values of total carbonate in the chloride waters are negative, w...

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The paper deals with the major chemistry and stable isotopes (hydrogen, oxygen, carbon, sulfur, strontium) of waters and solutes from the Salar de Atacama basin (Rio Pedro, Honar Creek and Laguna Chaxa) and Andean Altiplano (Laguna Minique and Laguna Miscanti). The water inflows of the Salar are chemically quite different, the Rio San Pedro being of Na-Cl type and the Honar Creek of Na-HCO3 type, in keeping with the sedimentary-evaporitic and volcanic nature of the catchment rocks respectively. The delta S-34 and delta O-18 values of sulfate and the Sr-87/Sr-86 ratio of strontium in the streams match those of drained rocks, whereas the delta C-13 values of dissolved carbonate are largely controlled by vegetation. The lagoons are evaporated meteoric water bodies, and the relative air humidity estimated from the slope of the isotopic evaporation line is in accordance with historical data on air humidity in the area. The Laguna Chaxa is Na-Cl rich, and its isotopic composition are consistent with a mixed sedimentary-volcanic provenance of sulfur and strontium solutes. The Laguna Minique is Na-SO4 rich, and its sulfate delta S-34 is nearly identical to that of Laguna Chaxa. The delta C-13(HCO3) values are quite different in the Laguna Chaxa and Laguna Minique, with the former being notably enriched in C-13 probably because of preferential uptake of C-12 by the high biological productivity occurring in the lagoon. The limited set of new data is interpreted in the context of a much larger literature database. In particular, previous chemical data on inflows and brines in the Salar de Atacama were revisited, and compared with evaporation path models and mineral stability diagrams (boron, lithium and Mg-minerals) computed using updated software and thermodynamic databases. The modeling shows that the removal of boron and lithium from sulfate-rich brines possibly occurs, respectively, as ulexite and sulfate salts, and carnallite should be the final magnesium phase of the brine evolution