A Preliminary Conceptual Model of Azufral Geothermal System, Colombia

Abstract

Azufral volcano (number 351090 according to the Smithsonian Institution ) hosts a geothermal system which surface manifestations such as hydrothermal alteration zones, hot springs, fumaroles and hydrothermal eruption craters, have arose interest of the geothermal community since the publication of the national geothermal reconnaissance study, in 1982. The volcano is located southwest of Colombia, in Nariño Province, nearby the Republic of Ecuador, within the area of influence of five indigenous reserves. The volcano and the caldera lake (Laguna Verde) in particular, are sacred places for the native community. Progress in the prefeasibility studies carried out by the Colombian Geological Survey (SGC, for its acronym in Spanish) include the geological and structural map, magnetic and gravimetric studies, geoelectric surveys (VES), surface hydrothermal alteration and fluid geochemical analyses. Based on the integration of the existing studies and including the information of the volcanic activity surveillance program, particularly seismic, a conceptual model was formulated: A high temperature geothermal system (about 225°C) dominates the current activity of Azufral volcano. The geothermal surface manifestations such as hot springs and hydrothermal alteration zones are mainly controlled by faults and their intersections. The geothermal gradient, estimated from the magnetic anomaly (Curie depth), is higher than 120°C/km. The expected geothermal reservoir would be located about 2 to 2.5 km deep. The geothermal fluid gets a significant contribution of shallower water sources, along the upflow, which confer the local isotopic signature of the precipitation to the water discharged by the hot springs. The boiling zone, identified from the advanced argillic alteration related to current acidic sulphate fluid discharges, in fumaroles (steam vents) and hot springs, points out the upflow zone, at the volcano summit, by Laguna Verde. The outflow, favored by NW-SE structures, goes from the center of the volcanic edifice to 6-8 km east, where it is accumulated. Guachucal fault, a major SW-NE structure, seems to bound the lateral outflow extension just overflowed to the east in the junction between Guachucal and south line of El Diviso-Túquerres fault.