Characterization and evaluation of hydrothermally influenced clayey sediments from Caldeiras da Ribeira Grande fumarolic field (Azores Archipelago, Portugal) used for aesthetic and pelotherapy purposes

Caldeiras da Ribeira Grande fumarolic field is located in the Fogo Volcano complex, São Miguel Island (Azores Archipelago, Portugal). These are associated with a geothermal system of high enthalpy that allows the existence of singular thermal muds or peloids that are continuously mixed with water inside a manmade pool. The peloids produced by the maturation (mixture) in situ are extracted from this boiling-mud pool and used directly in a local thermal centre (the “Banhos da Coroa” centre), which exists since 1811, for the relief of pain associated with rheumatic diseases and skin disorders treatment. Until this investigation, the application of sediments from Ribeira Grande for pelotherapy ends was only evaluated by human perception. Thus, this work aims the physical, mineralogical and geochemical characterization of the peloids collected inside the boiling-mud pool and the evaluation of their potential to be used for aesthetic and therapeutic purposes. The assessment of the volume of clays available for exploitation is also estimated. Sediments from Caldeiras da Ribeira Grande are naturally heated (above 36 °C in most of the cases) and present pH up to 5. The X-ray fluorescence analyses revealed a high content on Si and S, which are useful for therapeutic purposes and the risk inherent to potential toxic elements’ concentrations was evaluated. The X-ray diffraction data showed the presence of clay minerals, sulphates (alunite) and feldspars. Additionally to these features, silty-clay fraction also exhibits interesting features, such as suitable abrasivity and high plasticity. A detailed field survey was carried out at the Caldeiras da Ribeira Grande boiling-mud pool, including the measurement of the sediments thickness at 45 points and the determination of the ratio wet to dry sediments weight. This allowed to estimate a volume of sediments available for exploitation of 84 m³, or about 36 ton of dry sediments.