Minerals 2022, 12, 1244. https://doi.org/10.3390/min12101244 www.mdpi.com/journal/minerals Article Clay Minerals in the Deep Reservoir of the Cerro Pabellón Geothermal System (Northern Chile) Jeanne Vidal 1, *, Patricia Patrier 2 , Daniel Beaufort 2 , Santiago Maza 1 , Germain Rivera 3 , Gianni Volpi 3 and Diego Morata 1 1 Department of Geology, Andean Geothermal Center of Excellence (CEGA), Facultad de Ciencias Físicas y Matemáticas, University of Chile, Santiago 8370450, Chile 2 Department IC2MP, UMR 7285, University of Poitiers, 86073 Poitiers, France 3 ENEL Green Power Chile and Andean countries, Santiago 8330099, Chile * Correspondence: jeannevidal@ing.uchile.cl Abstract: This study focuses on hydrothermal alteration, and more specifically clays, in the geother- mal reservoir of Cerro Pabellón in Andean Cordillera, Northern Chile. Although it is the first com- mercial geothermal power plant in South America with 48 MWe installed and an additional 33 MWe during 2022, the alteration mineralogy of the deep reservoir remains poorly investigated. Cuttings from two deep wells drilled between 500 and 3000 m depth were sampled. The fine-grained clay fraction was analyzed using X-ray diffraction to observe the mineral distribution and variations of crystal structure of the clays. Scanning electron microscopy coupled with energy dispersive spec- troscopy allowed us to perform microanalysis of the clays. The results suggest a high heterogeneity of the reservoir in term of alteration: a pervasive alteration dominated by trioctahedral clays was observed in all the samples, whereas a fracture-controlled alteration was only observed locally, dominated by dioctahedral clays. Illite-smectite mixed layers with more than 90% illite typically occur in permeable fracture and fault as a typical clay signature. This study completed an integrated conceptual model of the high enthalpy reservoir (> 250 °C). Circulations of a neutral pH Na-K-Cl fluid occur in a wide fracture system connected to highly dipping and NW-striking faults and could extend beyond the Pabelloncito graben. Keywords: high-enthalpy system; deep fluids; permeable fracture; clay minerals; hydrothermal alteration; Cerro Pabellón; Chile; Andean Cordillera 1. Introduction The Central Andean Volcanic Zone (CAVZ) represents one of the most important and mostly undeveloped geothermal areas, with no less than 300 geothermal systems as- sociated with quaternary volcanism resulting from the subduction of the Nazca and Ant- arctic oceanic plates below South America [1,2]. Several active geothermal systems were identified such as El Tatio, La Torta, and Cerro Pabellon (CP) [1,3]. Discovered in 1999 (previously referred as the ‘Apacheta geothermal project’ by [4]), the CP geothermal sys- tem is located ~100 km northeast of Calama city (Antofagasta region, Northern Chile), near the border between Chile and Bolivia. Since 2017, Geotérmica del Norte, a joint ven- ture between Enel Green Power Chile and ENAP (Chilean National Oil Company), has exploited this system with 13 wells at 4500 m a.s.l, a 48 MWe installed capacity power plant (two binary 24 MWe each one units), and an additional 33 MWe for 2022 [5]. The deep reservoir of ~5 × 5 km 2 between 500 and 2000 m depth is defined as a high-enthalpy (>250 °C) two-phase reservoir [1]. Previous geological studies were mainly conducted on surficial alteration and the 300 m thick clay cap, whereas the deep reservoir (>500 m depth) remains poorly investigated [6,7]. Although the Andean geothermal potential re- mains one of the highest in the world, the CP power plant is the only one in exploitation Citation: Vidal, J.; Patrier, P.; Beaufort, D.; Maza, S.; Rivera, G.; Volpi, G.; Morata, D. Clay Minerals in the Deep Reservoir of the Cerro Pabellón Geothermal System (Northern Chile). Minerals 2022, 12, 1244. https://doi.org/10.3390/ min12101244 Academic Editors: Daniele Tardani, Marco Taussi and Jacopo Cabassi Received: 29 August 2022 Accepted: 23 September 2022 Published: 29 September 2022 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and institu- tional affiliations. Copyright: © 2022 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https://cre- ativecommons.org/licenses/by/4.0/).