Hydrothermal alteration in Nevados de Chillán Geothermal System: constrains regarding the chemical and thermal evolution of an active geothermal system

Nevados de Chillán Geothermal System (NCGS), hosted in an active volcanic complex in the Southern Andes, is considered one of the most significant high-enthalpy geothermal areas in central Chile. Given NCGS's relevance and the existence of well Nieblas-1, located on the southern flank of the active volcano and covering a depth of 1000 m, we propose a study of the hydrothermal alteration in order to constrain the development of this active geothermal system. Coupling petrographic studies, X-ray diffraction (XRD) data (both bulk rock and clay fraction), and scanning electron microscopy (SEM), performed in thirty-eight samples, we characterized NCGS's hydrothermal alteration.

Regarding the clay minerals, we observed the presence of chlorite-smectite (C-S), chlorite, corrensite, illite-smectite (I-S) and illite. Both corrensite and C-S are recognized from the shallower parts of the well up to 678 m depth, were corrensite disappears and C-S is replaced by chlorite up to the well's end. I-S is present up to a depth of 719 m, being replaced by illite down to the bottom of the well. We report as well a vertical zoning with depth of the alteration mineralogy considering a shallow argillic zone up to 280 m, an intermediate sub-propylitic section and, from a depth of 320 m, an intense propylitic alteration. Besides the aforementioned, the cross-cutting relationships of the alteration mineralogy and the filling of veinlets and amygdules point out the existence of at least two alteration events, during which the deposition of silica and carbonates include jigsaw quartz and bladed calcite, correspondingly. The data collected allows us to stablish both a spatial and temporal evolution regarding the development of NCGS. The infiltration and mixing with shallow fluids as well as deep boiling entail a change in the thermal regime of the system, as well as a modification in the chemical signature of the hydrothermal fluids, which in turn translates into different conditions for water-rock interactions processes. Our research evidences the dynamic changes present during the evolution of an active volcano-hosted geothermal system and highlights how, the integration of a hydrothermal alteration model grants the bases to constrain the factors shaping the hydrothermal system and the specification of its current state.