CONTROLS ON THE FERRARIA THERMAL WATER COMPOSITION, S. MIGUEL ISLAND, AZORES

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. 60ºC. It is of sodium chloride type with significant dissolved CO2(g) resulting from a 50 % mixing between an acid brine, at ≈100°C, and seawater. The thermal Na-Cl water is strongly enriched in Sr and Mn and, comparatively, has low concentrations in Al, Fe and As. Wells logging show fracture planes and pores fully/partly filled up with poly-phase 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 afected by pH and redox variations of the brine, due to volcanic gases pressure alterations, and fluid pressure or flow-velocity oscillation in the fractured aquifer