Alunite and kaolinite as geochemical markers in active acid sulfate alterations of southern Italy (Panarea, La Solfatara and Ischia)

Abstract

Hydrothermal alteration is a common process in volcanic and geothermally active areas. Interactions between host rock and hot, often acidic, fluids induce significant changes in texture, mineralogy, and geochemistry of the protolith, potentially impacting volcanoes stability. Southern Italy's volcanic regions are geothermally active, exhibiting conspicuous hydrothermal activity. Here, we focus on active hydrothermal acid-sulfate alteration in calc-alkaline (Panarea) and alkaline (Ischia - Solfatara) volcanic products in order to (1) spatially identify alteration assemblages using XRD, (2) determine the mobility of major and trace elements during hydrothermal alteration processes using FUS and ICP-MS, (3) estimate and compare the degree of alteration using alteration indices and elemental ratios. Our data reveal sulfur and sulfates sublimate next to the fumarole fields. Nearby fumaroles, alunite and kaolinite are the most abundant mineral phases in the alteration assemblages, often associated with amorphous silica, iron(hydr)oxides, smectites and gypsum. Kaolinite is rarely observed in La Solfatara assemblage. Comparison between the protolith and the alteration products geochemistry indicates that alteration retains the calco-alkaline or alkaline heritage, which confirms the usefulness of some chemical indices (PIA vs AI; (La + Ce)/Y vs (Zr + Hf)/(Ta + Nb) for alteration products. Despite comparable mineralogies, various degrees of alteration are observed in La Solfatara, Panarea and Ischia, constraining the alunite or kaolinite formation and highlighting local vs. global geochemical behaviors. We conclude that the (im)mobility of chemicals elements within the hydrothermal alteration - such as HFSE, REE or alkali - is mainly controlled by the protolith composition, the alteration intensity and mineralogy, pH, ionic strength and possibly crystallinity of alteration minerals.