Sampling and detailed structural mapping of veins, fault-veins and faults from Tolhuaca Geothermal System, Southern Chile. Pamela Pérez* 1,2, Pablo Sanchez 2,3, Gloria Arancibia 1,2, José Cembrano 1,2, Eugenio E. Veloso 4, Silke Lohmar 5, Jim Stimac 6, Martin Reich 2,3 y Juan Rubilar 2.

(1) Departamento de Ingeniería Estructural y Geotecnia, Pontificia Universidad Católica, Av. Vicuña Mackenna 4860, Santiago, Chile (2) Centro de Excelencia en Geotermia de los Andes (CEGA), Universidad de Chile, Santiago, Chile. (3) Departamento Geología, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile. (4) Facultad de Ingeniería y Ciencias Geológicas, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile (5) GeoGlobal Energy Chile Limitada, Carmencita 25, Office 52, Las Condes, Santiago, Chile. (6) GeoGlobal Energy LLC, 115 4th Street, Suite B, Santa Rosa, CA 95401, USA *E-mail: pvperez1@uc.cl Abstract. In the northern termination of the Liquiñe-Ofqui Fault System (LOFS), tectonic and volcanic processes interact to define the geothermal field of Tolhuaca. The structural analysis of veins, fault-veins and faults of the Tol-1 drilled core give insights regarding the role of faults and fractures networks in the chemical evolution and migration pattern of fluids. In this work we present the methodology of mapping and sampling, the preliminary results and the future challenges of this study. Key words: Geothermal, Tolhuaca, LOFS, fluid flow, fault and fracture network 1 Introduction The geothermal field of Tolhuaca (Melosh et al., 2009, Lohmar et al., this meeting) is located in a tectonically active area (Cembrano and Lara, 2009; Rosenau et al., 2006; Melnick et al., 2006); where deformation processes govern fluid migration and accumulation (e.g. Zhang, et al., 2008). Tectonic activity controls the dynamics of deformation and defines the nature, geometry and kinematics of fault fractures networks (e.g. Sibson, 1996). These networks may act as conduits or baffles controlling geothermal fluid flow (e.g. Rowland and Sibson, 2004) during the chemical evolution of the system. Thus, a better understanding of the structural pattern and its link with the chemical evolution of fluids may give significant insights into the processes governing the dynamics of the geothermal system (e.g. Nemcok et al., 2007, Yamaguchi et al., 2010). These processes control the hydrology of the recharge and perched aquifer, location of the deep reservoir and existence of a deep magmatic feeder of heat and mass (e.g. Nemcok et al., 2007). The objective of our current research is to assess the nature of the interplay between brittle deformation and chemical evolution of fluids and mineral paragenesis.

Tol‐1 is a vertical 1.080 m deep borehole which could yield relevant information regarding the evolution of the Tolhuaca geothermal system. The methodology to achieve our objective includes the structural and

geochemical analysis of oriented faults, fault‐veins and

veins ‐former pathways‐ in the core. Structural mapping at the regional scale will help to identify the main structural system, which accommodates the regional stresses, and promotes fluid migration, accumulation and arrest. In this work we present the methodology of vein and faults mapping, sampling, some preliminary results and the challenges ahead.

Figure 1. Handmade Goniometer for false strike and real dip measurements.

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2 Sampling methodology The work was divided into 2 basic steps: Identification and definition of certain key core sections of interest followed by a detailed characterization and measurement of the structural systems within these sections. 2.1 Definition of interesting zones The first step was to analyze the whole core qualitatively, in order to divide it into different key zones or segments according to the presence/absence and changes in the structural elements found. Those structural elements are the following: • Veinlets: Veinlets are interpreted as pathways for fluid flow, which have been sealed (partially or completely) by the precipitation of hydrothermal minerals. Their geometry, kinematics and cross-cutting relationships should represent the stress field at the time of fluid transport and mineral precipitation. Mineralogy in veinlets was controlled by fluid chemistry and thermodynamic conditions (P-T-X) of fluid-rock interaction. • Faults: Fault surfaces, with their associated frictional striae, reflect tectonic events at depth. Displacement patterns, which are syntectonic with mineralization, reflect the regional stress state promoting fluid flow and/or hydrothermal precipitation. Crosscutting relationships among different fault and joint systems allowed us to obtain the relative timing of events. • Hydrothermal Breccia: Hydrofracturing events are the result of particular stress conditions. Mineralogic indicators, such as bladed calcite may indicate boiling conditions during the hydrofracturing event (Cox, 2010).

Figure 2. Single section with the reference system (drawn yellow line); the top direction is also marked (half arrow). 2.2 Detailed characterization and

measurements of structural elements. The second step was to quantitatively characterize certain areas of interest within the core. To obtain the absolute orientation of each structural element, an arbitrary reference system was defined. We

defined a single section as a continuous core segment which can be rigorously joined together, with a unique (yet arbitrary) reference system consisting of a false North (Figure 2). For each single section, at least one sample (7 cm-long) was set aside for paleomagnetic studies, which will give the actual orientation of the core. A handmade goniometer was constructed and used for orientation of the structural elements by measuring the following parameters strike with respect to the false North and real dip (Figure 1). These measurements were carried out for each structural element assuming the core was absolutely vertical. Therefore, the actual orientation of the structural elements measured and/or sampled will be obtained after the paleomagnetic studies. Also, lava flow outcrops around Tol-1 corehole were oriented and sampled, to check that the structural block has not been rotated significantly. 3 Results More than 120 structural measurements of faults, veins and fault-veins were performed (strike, dip, rake -when available-). Forty seven samples were taken for thin & fluid inclusions sections. Detailed mapping of structures including dip & kinematic indicators from mineral sealing was synthesized in a structural log of Tol-1 core. There is a strong correlation between abundance of structures and rock type. Lava intervals exhibit more intense fracturing and veining than tuff and volcaniclastic intervals. In the upper 300 m of the core, structures are primarily steeply dipping with a dominant normal sense of displacement (some dextral component). Below a cataclastic zone at 300 m, structures are more variable in dip and sense of motion, with some reverse faults. 4 Further work The necessary pending work is the reorientation of the already defined core segments by using paleomagnetic techniques in order to obtain the actual strike of the structures. This, in turn, will allow assessing the significance of structures in the core within the regional deformation context. Oriented thin sections will be analyzed by using optical and scanning electron microscopes, in order to define the nature and role of structural pathways on hydrothermal fluid migration, mineral precipitation and healing. Microthermometric and chemical (Raman; LA-ICP-MS)

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analysis will be conducted on fluid inclusion sections to constrain P-T-X condition of the fluids. Acknowledgments CONICYT-FONDAP 15090013 and GeoGlobal Energy are funding this research. PP and PS thank CONICYT and MECESUP grants, respectively.

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