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

Abstract

We investigated sulfur-bearing minerals from the Campi Flegrei caldera, southern Italy, in relation to the increase of hydrothermal activity phenomena since 2006, aimed at providing insights into the volcanic system dynamics. Mineral encrustations and muds were sampled between 2013 and 2015 at the long-standing degassing crater of the Solfatara tuff cone and its recently restless north-eastern Pisciarelli slope. Deep-seated sulfides were further separated from two drill cores (AGIP's Mofete boreholes: 1500m and 2695m depth).The mineral assemblage and texture of sampled encrustations were determined by X-ray diffraction, optical and scanning electron microscopy and X-ray microanalysis by energy dispersive spectrometry. Native sulfur and alunite dominate among the newly formed mineral phases. Other minerals are mostly alunogen, and locally pickeringite, potassium alum, hematite and pyrite. Mereiterite and amarillite sporadically occur. The mud pools are rich in gypsum, potassium alum and pyrite. Quartz and argillic phases, locally with analcime, are dispersed in the outcropping rocks.δ34S values were determined for shallow subsurface native sulfur (−5.5 to 0.0‰) and alunite (−1.7 to −0.2‰), as well as for the deep-seated pyrite (3.3 to 7.4‰ in the depth range:1500–2695m). δ18O values were measured for shallow native alunite (4.2 to 7.0‰). Pisciarelli alunite was finally analyzed for its 87Sr/86Sr ratio and 143Nd/144Nd ratios (0.707517±6 and 0.512459±6, respectively).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.