A Recently Discovered Granitoid Rich Lithic Basal Horizon in the Liberia Formation and its Possible Implications on Source Vent Location and the Evolution History of a Geothermal Reservoir at the Las Pailas Geothermal Field, Guanacaste, Costa Rica

Abstract

The Liberia Formation is a Pleistocene rhyolitic biotite and quartz bearing pumiceous tuff that crops out in the periphery of the Las Pailas Geothermal Field, located on the Pacific slope of the Quaternary Rincón de la Vieja volcanic complex in northwestern Costa Rica and is used as a marker bed in the Las Pailas borehole field. A recently discovered outcrop at the base of the Liberia Formation, 1.5 to 2 kilometers east southeast of the wellfield, is a granitoid bearing horizon restricted to a 2 meter thick bed and spanning approximately 75 meters. The size and shape of these granitoid blocks may provide evidence for proximity to the source vent that produced the Liberia Formation as well as the implications that the explosive eruption may have had during the evolution of the Pailas geothermal reservoir. Although the actual source vent of the Liberia Formation has not been fully identified, careful mapping of the spatial and modal distribution of these granitoid lithic fragments provides useful clues as to its location. This paper presents field data and geostatistical analysis of these granitoid blocks, conclusions on the possible location of the source vent of the Liberia Formation and the implications that this may have with regard to the evolution of the geothermal reservoir at the Pailas Geothermal Field. This report presents the findings of this investigation and conclusions on the possible location of the source vent of the Liberia Formation and the implications that this may have with regard to the evolution of the geothermal reservoir at the Las Pailas Geothermal Field.