Native sulfur, sulfates and sulfides from the active Campi Flegrei volcano (southern Italy): Genetic environments and degassing dynamics revealed by mineralogy and isotope geochemistry

Textural and isotopic data constrain the genesis of alunite at the expense of K-feldspars through rock alteration by hydrothermal fluids. We suggest that the caldera is a low-sulfidation system hosting acid-sulfate deposits in its active degassing area. The acid-sulfate environment developed on an argillitic facies that thins outwards and is characteristic for steam-heated and magmatic-steam environments. These environments developed in relation to the fractured settings that facilitates the gas emissions from deeper levels and the meteoric infiltration from surface to depth. Newly determined δ34S data are lower than previously published values, suggesting a progressive decrease in the sulfur isotopic composition through time at least since 1956. The isotopic variation is associated with a lowering of the SO2/H2S ratio and the widening of the surface of CO2 degassing, in the absence of significant changes in the average CO2 flux values and in the carbon and helium isotope compositions. Native sulfur and sulfates move from sulfur isotope equilibrium to disequilibrium. We ascribe this transition to enhanced SO2 scrubbing due to a high rock permeability that facilitates a faster gas ascent rate in the shallow crust.