The study of active and fossil hydrothermal systems shows clay minerals to be a fundamental tool for the identification and characterization of hydrothermal alteration facies. The occurrence and composition of hydrothermal alteration facies could provide useful information on the physicochemical conditions of the hydrothermal activity affecting a rock volume. In particular, clay minerals (i.e., smectite group, chlorite, illite, kaoline group, pyrophyllite, biotite) are pivotal for extrapolating important parameters that strongly affect the development of water/rock interaction processes such as the temperature and pH of the hydrothermal environment. This work aims to give a general reference scheme concerning the occurrence of clay minerals in hydrothermal alteration paragenesis, their significance, and the information that can be deduced by their presence and chemical composition, with some examples from active and fossil hydrothermal systems around the world. The main mineralogical geothermometers based on chlorite and illite composition are presented, together with the use of hydrogen and oxygen isotope investigation of clay minerals in hydrothermal systems. These techniques provide a useful tool for the reconstruction of the origin and evolution of fluids involved in hydrothermal alteration. Finally, a list of oxygen and hydrogen fractionation factor equations between the main clay minerals and water is also provided.
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