Origin and chemical nature of the thermal fluids at Caldeiras da Ribeira Grande (Fogo Volcano, S. Miguel Island, Azores)

The Caldeiras da Ribeira Grande (CRG) site is characterised by low-temperature fumaroles, boiling mud-pools and gas ground-emissions, some of them artificially confined to a tank. Cooling and oxidation of deep hydrothermal fluids under boiling control the formation of acid condensates and strong hydrothermal alteration of the hosting volcanic rocks. The latter feature is locally reinforced at shallow levels through flow of mixtures of acid condensates with surface and shallow groundwater triggering the development of thick clay deposits used in mud-therapy purposes at an old Thermal Bath facility. The emerging aqueous fluids are acidic (pH < 3), hot (≈70–90 °C) and display variable degree of mineralisation, as a result of mixing of deep vapours/gases with surface and shallow groundwater and of the progression of hydrothermal alteration processes in hosting rocks. The host trachyte rocks show severe leaching and the long-lasting circulation of an acid condensate generates an argillic-type alteration facies dominated by kaolinite and alunite; this alteration reflects moderate acidity (pH 3–5) and oxidising conditions, low temperature (<150 °C) and high fluid/rock ratios. Hydrated sulphates belonging to the alunite–jarosite series prevail in the major mud-pool; kaolinite predominates in sediments accumulated in tank, often occurring along with alunite and other Al-bearing sulphates. The neo-formation of these minerals also indicates that oxidised sulphur species prevail in the shallow component of the geothermal system. Based on mineralogical and geochemical criteria, the CRG site is envisaged as part of a large fluid-dominant geothermal system that is sustained by magmatic/hydrothermal processes related to degassing of the Fogo Volcano magma chamber. The formation of clay deposits for mud-therapy applications is assured only if several natural conditions are preserved, such as (1) contribution of deep geothermal steam enriched in volcanic gases, forming a strongly acidic condensate (pH 2–3); (2) maintenance of shallow groundwater flow to dissolve the hydrothermal condensate and to prevent very high sulphur oxidation, allowing condensate pH >3; and (3) rain, runoff, and subsurface water contributions must be balanced to confine the pH of thermal fluids between 3 and 5. Any change in conditions of geothermal fluid boiling at depth or in environmental and hydrological characteristics of the site can cause significant changes in leaching processes and sediment formation.