Clay minerals are widespread alteration products in most hydrothermal systems and are considered as efficient petrogenetic guides. Their distribution in active geothermal fields depends on the ability of the fluid to approach equilibrium with host rocks at any scale during the hydrothermal processes. Both illitization and chloritization reactions depend on factors such as time, water-rock ratio, deformation history... as well as the variables used to define equilibrium (i.e., temperature, composition). This paper is focussed on how the properties of clay minerals change with respect to the evolution of hydrogeological conditions during the life time of geothermal systems. The presented data are based on comparative studies performed in productive and non productive zones of several active geothermal systems related to subduction plate tectonics (Chipilapa, Milos and Bouillante). The temporal sequence of clay crystallizations can be summarized as follows. The opening stage of the geothermal system promotes a massive crystallization of metastable smectites in conditions of strong thermodynamic disequilibrium. As time and alteration progress, clay minerals of the newly sealed parts of the reservoir are transformed in more stable phases (via dissolution crystallization processes). Metastable smectites continue to precipitate in the permeable zones still subjected to strong physico-chemical disequilibrium such as boiling or mixing of fluids of contrasted composition and/or temperature, whereas clay minerals close to those predicted by thermodynamic calculations (illite, chlorite) crystallize in other zones in which the geothermal fluid can reach chemical equilibrium with the altered host rock. Downward infiltration of surficial water led to crystallization of montmorillonite in the outer part of geothermal fields. Collapsing of hydrothermal activity leads to a change in the heat flow regime with predominance of a depth dependant conductive thermal gradient. Even if such thermal conditions promote the transformation of the previously formed smectite rich metastable clay minerals and lead to a progressive emplacement of depth dependant conversion series of di- and trioctahedral smectites to respectively illite and chlorite, signatures of the ancient geothermal reservoirs are preserved in the textural or microstructural properties of the clay particles which sealed them.

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