The Ribeira Grande geothermal field is located on the northern flank of Fogo Volcano (S. Miguel Island, Azores) and it is characterised by the presence of several active hydrothermal manifestations. An analysis of the RG5 geothermal well has been carried out by the recognition of neoformation minerals with depth and the estimation of equilibrium temperatures. The formation of hydrothermal minerals depends not only on temperature but also on subsurface geology. The mineral assemblage consisting of chlorite + quartz + haematite ± calcite ± anatase ± titanite + albite ± adularia (± dolomite) shows temperature above 235 °C, and the main alteration is divided into a smectite zone followed by a chlorite zone at depth. The adularia appearance evidences a boiling zone at the top of the reservoir and illite and chlorite also suggest fluid level fluctuations in the past. The H2/Ar gas geothermometry applied to the gases emitted by fumaroles points to a temperature of 256 °C for the reservoir feeding a fumarolic field located close to the RG5, which sustains the mineralogical research.