Geothermal Reconnaissance of the Caribbean Flank of the Rincón de la Vieja Volcano, Costa Rica

Abstract

The Caribbean flank of the Rincón de la Vieja volcano is characterized by sequences of lavas and pyroclastic rocks of andesitic composition, most of them originated in the recent Rincón de la Vieja volcanic complex.
Sedimentary rocks of marine origin, which have been reported near from the study area by other authors, could be part of a regional geological basement. Hydrothermal alteration is scarce and limited to the hot springs areas and basically consists of clays and iron oxides associated with silica, manganese oxides and calcium carbonate (travertine). Geomorphologic and field studies suggest two main local structural lineaments affecting the area with NE-SW and N-S trending, some of them could be correlated with the distribution of hot springs, CO2 and CH4 (methane) anomalies. By the other hand, radar image analysis shows E-W regional structural lineaments which could be also important for the permeability.
Both cold and hot springs were classified in four main geochemical facies: Cl-SO4, SO4, HCO3 and mixed. Cl-SO4 water facies corresponds to the hottest springs and also with the highest electrical conductivities in the northern sector know as El Volcancito (T = 57-63°C, pH = 6.0, cond. = 5660-6020 µS/cm). The springs located in southern sector (Albergue Agroecológico) were also classified in the same facies but present lower temperatures, conductivities and pH values (T= 21- 37°C, cond. = 319-2430 µS/cm, pH = 3.0-6.0).
Both the springs located in El Volcancito and Albergue Agroecológico are associated with a common geothermal reservoir affected by the condensation of magmatic gases. According to the Na/K/Ca geothermometer of Fournier & Truesdel (1973) the reservoir temperature was estimated in 237°C; however this value must be taken with care since the possibility of a lack of water-rock equilibrium.