The interplay between fault-fracture networks activity, fluid flow and mineralization in the Andes: A case study in the Tolhuaca geothermal system, southern Chile.

Abstract

The nature of the interplay between active tectonics and fluid flow is a key feature to better understand the chemical evolution offluids in geothermal and hydrothermal systems. The prominent hydrothermal, tectonic and volcanic activity of the Southern Andes volcanic zone (SVZ)makes it one of the best natural laboratories to address this issue. In the northern termination of the Liquiñe-Ofqui Fault System (LOFS), tectonic andvolcanic processes interact to define the geothermal field of Tolhuaca.The objective of our current research is to assess the nature of the interplay between brittle deformation and chemical evolution of fluids and mineralparagenesis. Tol-1 is a vertical 1.080 m deep borehole which could yield relevant information regarding the evolution of the Tolhuaca geothermalsystem. The methodology to achieve our objective includes the structural and geochemical analysis of oriented faults, fault-veins and veins -formerpathways- in the core. Structural mapping at the regional scale will help to identify the main structural system, which accommodates the regionalstresses, and promotes fluid migration, accumulation and arrest. Fluid inclusions analysis by microthermometry, LA-ICP-MS and Raman spectroscopywill allow a better understanding of the feedback between the fluid flow episodes and the mineralization.More than 120 structural measurements of faults, veins and fault-veins were performed (strike, dip, rake -when available-). Forty seven samples weretaken for thin & fluid inclusions sections. Detailed mapping of structures including dip and kinematic indicators from mineral sealing was synthesized in astructural log of Tol-1 core. Our preliminary results show that there is a strong correlation between abundance of structures and rock type. Lavaintervals exhibit more intense fracturing and veining than tuff and volcaniclastic intervals. In the upper 300 m of the core, structures are primarily steeplydipping with a dominant normal sense of displacement (some dextral component). Below a cataclastic zone at 300 m, structures are more variable in dipand sense of motion, with some reverse faults. Fluid inclusions petrography reveals the periodically feedback between fault-fractures networksactivation and mineral mineralization sealing the conduits for fluid flow.