Geochemical Characterization of Geothermal Spring Waters In Casiri-Kallpuma, Tacna, Peru

In the Casiri-Kallapuma geothermal zone (CKGZ), geothermal springs were studied using the chemical relationships between the major ions and selected trace elements to characterize their geochemistry, origin and to explore the influence of regional and local structural controls on permeability that play a role in bounding the systems and influencing fluid flow. Three types of geothermal waters were identified based on the physiochemical characteristics: Na-Cl waters, Ca+Mg-HCO3 waters, and acid-sulfate waters. Additionally, there is a type of cold non-thermal groundwater of Ca+Mg-SO4 composition that is product of the mixing between hot springs and cold water along the Chungara ravine and in the Juntopujo plain. The Na-Cl fluid is the most prevalent and is characterized by having temperatures between 40 and 86 °C, with high electrical conductivity (6700 μS/cm), and a pH of 6.83. The Na-Cl fluid is characterized by high concentrations in Cl, Na, Li, B, As and SiO2 and low levels in Mg. Acid-Sulfate waters are observed south of the Casiri lagoon, geographically associated with the volcanoes (< 3 Ma) and has a strong magmatic / volcanic component due to the absorption of magmatic vapors. These fluids are acidic (pH <4), with a temperature of 35.6 ° C and present high concentrations of SO4, Ca and Mg. The Ca+Mg-HCO3 waters form as a result of intense leaching of cations at shallow depth by due to the absorption of volcanic vapors rich in CO2 in the peripheral areas of the geothermal area. The distribution of the geothermal springs illustrates a spatial relationship within the fault system of NW-SE, NE-SW, N-S, NNW-SSE and W-E structures that control the permeability of the geothermal system. The Na-Cl waters are related to the high vertical permeability faults in the NE-SW direction between the Chungara ravine and the Juntopujo plain. Therefore, these waters tend to have high Cl/B ratios and strong correlations between traces of alkali metals and Cl due to deep, fast and efficient circulation pathways with upward outflow that interact with volcanic and sedimentary rocks. The isotopic evaluation indicates that the Na-Cl waters are characterized by presenting an average δ18O of -15 ‰ and δ2H of -117 ‰, with a slight enrichment in δ18O, which suggests that the fluid is a mixture of meteoric water and thermal waters that have undergone water-rock interaction at elevated temperatures. In addition, these Na-Cl fluids are partially equilibrated according to the Giggenbach Na-K-Mg geothermometer, with a maximum temperature of 230 °C. The equilibrium temperatures for quartz and Na-K geothermometers are 162 °C and 230 °C respectively, however it should be noted that the quartz temperatures are likely a minimum estimate due to the impacts of mixing on the silica geothermometers.