Isotopic and chemical composition of water and steam discharges from volcanic-magmatic-hydrothermal systems of the Guanacaste Geothermal Province, Costa Rica

Abstract

The Guanacaste Geothermal Province encompasses three major geothermal systems, each centered on its respective volcanic structure: Rincón de la Vieja to the NW, Miravalles in the center and Tenorióto the SE. Each shows corresponding sets of surface manifestations: vapor discharges from fumaroles and steam-heated pools at altitudes > 500 m; lower temperature SO 4 Cl springs on the lower slopes of the respective volcano; and cooler neutral Cl springs to the S of the volcanic chain, at altitudes < 500 m. The production of HCO 3-rich waters is limited to a narrow belt stretching to the S of Miravalles volcano. Chemical and isotopic evidence suggests that the neutral Cl waters, also discharged from deep wells, are derived from a more primitive Cl SO 4 water formed by transfer of readily mobilised, originally magmatic constituents to deeply circulating groundwater. Isotopic evidence suggests that this groundwater is derived largely from areas to the north of the volcanic chain. The presence of immature waters within drillable depth is indicated by the incursion of an acid SO 4 water into a well at Miravalles after deepening to 2000 m. As the Cl waters rise, they start to boil. The separated vapors feed the fumaroles and steam-heated features at the surface, the residual water travels underground to be discharged some 5–25 km to the S. The uniformity in chemical and isotopic compositions of the neutral Cl waters is compatible with the assumption that the individual thermal systems are linked at depth to form large, contiguous geothermal reservoirs beneath the Guanacaste Geothermal Province.