| Abstract |
Hydrological balance calculations suggested a potential, average annual infiltration rate of 446 mm/m2 for the Los Azufres area, which corresponds to a total of 82.2 million m3 per year. Due to the highly fractured and faulted structure of the volcanic underground, a considerable potential for the infiltration of meteoric water into deeper sections of the volcanic formations can be assumed. Isotopic data indicate the minor importance of recent meteoric water for the recharge of the geothermal reservoir. Very negative ‰13C values can be explained by the input of organic carbon from the surface, but the lack of 14C in the deep fluids reflects a pre-historic age for the infiltration event. The dilution of the meteoric water by 14C-free CO2 gas from a shallow magma chamber complicates the exact age determination of the infiltration event, which probably occurred during Late Pleistocene or Early Holocene glacial periods. Additionally, the positive ‰D and ‰18O trend of the geothermal brines towards an andesitic water composition indicates the influence of andesitic water, derived from the subduction of the Pacific Plate below the North American Plate. Due to its heterogeneous isotopic composition, the geothermal reservoir in the southern reservoir zone can be divided into four vertical zones: a) a rhyolitic sealing cap without geothermal fluids, b) a shallow infiltration zone from 300 to 600 m, which is characterised by a temperature dependent oxygen isotopic exchange of the fluids, c) the major geothermal reservoir zone from 800 to 1250m characterised by high water-rock interaction and, d) a paleozone at >1250m depth with restricted flow circulation, as indicated by constant ‰D-values. A preliminary hydrological model of the reservoir is as follows: the fossil hydrodynamic system was characterised by the infiltration of meteoric water and mixing with andesitic and/or magmatic water. High water-rock interaction processes in the main part of the production zone indicate the existence of a former active fluid circulation system. Due to changes in pressure and temperature, the rising fluids get separated into a liquid and vapour phase at a depth of 1,500 m. After cooling, main portions of both phases remain within the convective reservoir cycle. Isotope analyses of hot spring waters indicate the connection of the reservoir with the surface at some local outcrops. A recent reactivation of the hydrostatic system is caused by geothermal production, as indicated by the detection of natural tracers in production wells. |