Surface exploration and petrological applications in high enthalpy geothermal areas: a multidisciplinary approach for the Cerro Pabellón project (northern Chile)

The present thesis was carried out in the frameworks of a collaboration with Enel Green Power and of a Memorandum of Understanding between the Department of Pure and Applied Sciences (DiSPeA) of the University of Urbino and the Faculty of Physical and Mathematical Sciences of the Universidad de Chile (Santiago). The Cerro Pabellón geothermal system, located in Northern Chile, which is the main object of this work, is classified as a blind (or hidden) geothermal field. In fact, the only hydrothermal manifestations are represented by two fumaroles located on the top of the Apacheta volcano, set to the west and about 600 m higher than the Cerro Pabellón geothermal area. The aim of this PhD research project was to deepen the knowledge concerning the heat source and the evolution of the related magmatic plumbing system, define the clay-cap units and constrain its role and importance in the circulation of fluids toward the surface, producing an update conceptual model. A multidisciplinary approach was involved and focused on three complementary scientific studies: • A petrological, geochemical and isotopic study on the Apacheta-Aguilucho Volcanic Complex and on the most recent volcanic products of the Altiplano-Puna Volcanic Complex, represented by the dacitic domes of Chanka, Chac-Inca and Pabellón. The main purpose was, after the identification of the amphibole crystallization depth (i.e. the depth of the heat source/s), to define the evolution of the magmatic plumbing system, and the mixing/mingling processes of the different magmas. • A mineralogical study through X-Ray Powder Diffraction analysis of samples from a production geothermal well and an exploration drill core, besides clay-rich samples from the Cerro Apacheta fumarolic field (adjacent to the project area). The purpose of this work was to examine the vertical and superficial distribution of the clay minerals forming the hydrothermal system of the Cerro Pabellón active geothermal field, in order to identify the minerals associations, define the clay-cap units and characterize the type of hydrothermal alteration. • A geochemical study of the diffuse CO2 soil flux and temperatures measurements, coupled with soil temperature gradients carried out at the fumarolic field, in order to define the sealing capacity of the clay-cap units and their importance in preventing the rise of the fluids toward the surface. The final goal of this investigations was to give a multidisciplinary concept of geothermal exploration combining geological, volcanological, mineralogical, petrological and geochemical investigations, applying it to a blind (or hidden) geothermal system. The primary characteristic of a blind (or hidden) geothermal system is related to the lack of classic high-enthalpy-related superficial manifestations, such as geysers, fumaroles, sinter deposits, and hot springs. Obviously, blind or hidden geothermal systems have not been extensively developed, as they are difficult to confirm in absence of drilling programs that can confirm the productivity of the reservoirs. Thus, the presence of the first geothermal power plant of South America, related to such a blind geothermal system, gave us a great opportunity to study and deepen the main features characterizing this system. A major comprehension of the processes involved in the formation of these geothermal areas is essential toextend geothermal exploration to other feasible areas where no surface manifestations are recognized.