The San Diego Area, a New Geothermal Prospect on the Colombian Andes

Abstract

Exploration on a new area of geothermal interest on the Central Cordillera of the Colombian Andes has been initiated by the Geological Survey of Colombia (SGC). The area, located 95 km north of Nevado del Ruiz Volcanic Complex, comprises a volcanic structure with a ~2,2 km diameter crater caused by phreatomagmatic eruptions known as San Diego Maar and a newly discovered volcano dubbed here as “Escondido de Florencia”, to which are associated distinctive pyroclastic deposits, volcanic bombs and the presence of rhyolitic – dacitic domes. The diverse volcanic deposits scattered around the three hot springs locations, namely of San Diego, Florencia and El Espíritu Santo, suggest different heat sources for the hydrothermal manifestations. A mayor structure known as Palestina fault cuts on a NNE trend the northern end of Central cordillera and its trace relates the San Diego and Escondido volcanoes and the Nevado del Ruiz volcano. Location of the volcanic structures and hot springs occurrences are related to transtension structures on this fault as well as with lithological contacts of intrusive bodies. Although the hot springs discharge temperatures are low (between 33 and 43°C) probably due to dilution, their chemical features suggests the probable existence of a medium to high temperature system. The chemical composition of hot springs at Florencia location is characterized by neutral sodium chloride water with relatively high lithium contents and oxygen-18 enrichment. A high magnesium concentration suggests a mixing process with shallow water. Based on a relatively high SiO2 contents the minimum temperature of the deep geothermal fluid is estimated in about 160°C. A well differentiated chemical composition characterizes the hot springs of the other locations: neutral sodium bicarbonate water at San Diego and neutral sodium chloride-bicarbonate mixed. Recently acquired geological and geochemical data, bring new insights on geothermal system model aided by upcoming geophysical surveys.