Chemistry of the Ferraria thermal water, S. Miguel Island, Azores: mixing and precipitation processes

The Ferraria thermal water emerges at the sea level in the Ferraria lava delta (western edge of S. Miguel Island, Azores) with temperature of ca. 60°C and pH varying between 5.4 and 6.2. It is of sodium chloride type, resulting from ca. 50% seawater mixing with an acid brackish, at ≈100°C, denoting the presence of significant CO₂(g) and the progress of water–rock interactions in open system conditions. The thermal Na–Cl water is strongly enriched with Sr and Mn and, comparatively, has low concentrations in Al, Fe and As. These elements are removed from the solution as critical conditions for the formation of several neo-formed mineral phases are gradually attained. Thermodynamic equilibrium calculations are consistent with this interpretation, showing that the thermal fluid can precipitate Fe³⁺-(hydr-)oxides, kaolinite and non-crystalline silica. Wells logging show fracture planes and pores fully/partly filled up with polyphase botryoidal aggregates mostly composed of goethite + ferrihydrite and displaying variable adsorbed contents of Si, P and As. These neo-formed phases result from the pristine fluid oxidation due to seawater mixing; its precipitation is easily affected by pH and redox variations of the brackish, due to volcanic gases pressure alterations, and fluid pressure or flow-velocity oscillation in the fractured aquifer.