Geochemical Study of the Casiri-Kallapuma Geothermal Zone, Tacna, Peru

摘要

In this study, geochemical methods are used to reliably analyze and understand the Casiri-Kallapuma geothermal system, located at ～4500 m above sea level in the Western Cordillera of the Andes of southern Peru in the Tacna region. The geothermal system includes two different types of thermal waters: Na-Cl and Ca.Mg-HCO_3 and a cold non-thermal groundwater (Ca.Mg-SO_4) that mixes with these thermal waters along the Chungara Ravine, Juntopujo plain, Kallapuma and Maure rivers. The Na-Cl fluid is characterized by temperatures 40 °C to 86 °C, electrical conductivity 6700 μS/cm, pH of 6.83, is rich in Na, Cl, B, As and SiO_2, and low in Mg. These constituents indicate that these geothermal waters have deep circulation paths that interacted with volcanic and sedimentary rocks (for example the Maure Group). Isotopic evaluation indicates that the alkali chloride water is characterized by average δ~(18) O of -15 ‰ and δ~2H of -117 ‰, with a slight ~(18)O enrichment that suggests that fluid is a mixture of meteoric and magmatic water. The sulfate fluid (SO_4: 1423 ppm, Cl: 0.6 ppm) dominates discharge in the southern Casiri lagoon with pH of 4 and decrease in concentration down-gradient with SW-NE direction in the Chungara Ravine and Juntopujo plain, while Cl concentration increases (SO_4: 98 ppm, Cl: 2040 ppm). According to the Giggenbach Na-K-Mg geothermometer, the Na-Cl water includes partially equilibrated and immature waters with maximum equilibration temperatures of approximately 230-240 °C. Maximum equilibration temperatures for the quartz and Na-K geothermometers are 162 °C and 230 °C respectively. The mature Na-Cl waters discharging in the Chungara ravine and Juntopujo plain reflects water interaction processes with volcanic rocks of the Tacaza and Maure Group, also the Sencca and Capillune formations partially covered with Barroso Group rocks. Upflow of these fluids produces high convective heat flux and suggests that promising magmatic-driven geothermal reservoirs exist at depth.