Hydrothermal alteration environments and recent dynamics of the Ischia volcanic island (southern Italy): Insights from repeated field, mineralogical and geochemical surveys before and after the 2017 Casamicciola earthquake

Abstract

This study presents the results of repeated surveys conducted in hydrothermally altered areas on the active volcanic island of Ischia (in the Gulf of Naples, southern Italy). Data were obtained from field work, in situ temperature measurements, X-ray diffraction, optical and scanning electron microscopy, EDS-SEM micro-analysis, infrared spectroscopy and whole-rock geochemistry analyses on samples collected in October 2016, September 2017,March 2018 and November 2018, both prior to and following the damaging seismic event that occurred in August 2017 at Casamicciola.Mineralogical results point to the evolution of acid sulfate zones along the northern andwestern flanks of a resurgent block located in the central part of the island, in strict relationwith endogenous fluid outgassing indicated in the literature.Mineralogy and whole-rock geochemistry of these acid sulfate zones revealmagmatic-hydrothermal environments in association with major structures that enable fluid circulation. It is interpreted that rare coarser alunites within dominant steam-heated and supergene alteration mineral assemblages are associated with environments that are no longer active, but which are supplied by magmatic vapours. In addition, collected data indicate slowendogenous degassing fromhydrothermal reservoirs. Data fromSeptember 2017 show variations in the sulfate assemblage within the acidic sulfate zone located near the earthquake's epicentre, and acidification and the transitory appearance of alum-(K) is apparent under a nearly unchanged endogenous supply. Meteorological conditions of late summer could, in principle, justify the appearance of alum- (K), but its coexistence with alunogen and soil temperatures refute this possibility. Alum-(K) is interpreted to be the product of alunite decomposition at a local temperature of ca. 100 °C. Assuming a correct space-time relationship between the appearance of alum-(K) and the earthquake, it is thus inferred thatmechanical activation by tectonic stress is the possible cause of alunite decomposition.