The acid sulfate zone and the mineral alteration styles of the Roman Puteoli (Neapolitan area, Italy): clues on fluid fracturing progression at the Campi Flegrei volcano

摘要

Active fumarolic solfataric zones represent importantstructures of dormant volcanoes, but unlike emitted fluids, theirmineralizations are omitted in the usual monitoring activity. This is thecase of the Campi Flegrei caldera in Italy, among the most hazardous andbest-monitored explosive volcanoes in the world, where the landscape ofPuteoli is characterized by an acid sulfate alteration that has been active atleast since Roman time. This paper provides temperature, mineralogical,textural, compositional and stable isotope data for those solfataricterrains sampled at the crater and Pisciarelli slope of the Solfataravolcano between 2013 and 2019. Temperatures vary between 40 and95?~(°)C. Minerals include alunite with grain sizes generally largerthan 20?μm, alunogen, native sulfur, well-ordered kaolinite, and,common at Pisciarelli, pyrite, illite and NH_(4) sulfates. Sulfateterrains have higher contents of Ti, Ba, Au, As, Hg and Tl relative to theirparent substrate. The Pisciarelli slope is anomalous in terms of thepresence of NH_(4). δ ~(34)S values for sulfides and native Srange between ?3.00?‰ and 0.49?‰ and from ?4.42?‰ to 0.80?‰, respectively. Sulfates show δ ~(34)S andδ ~(18)O values in the range of ?2.78?‰ to 2.09?‰ and between 4.60?‰ and 31.33?‰,respectively. The style of mineralization and the stable isotopegeochemistry do produce complex and not completely consistentclassifications and genetic constraints. We merge our data withvolcanological information, data from exploration drillings and geophysicalresults. With the conceptual model, we suggest a series of shallow and deepaquifers interconnected like “communicating vessels” through a main faultsystem that downthrows Solfatara with respect to Pisciarelli. Fluid outflowfrom the different discrete aquifers hosted in sediments – and possiblybearing organic imprints – is the main dataset that allows determination ofthe steam-heated environment with a supergene setting superimposed.Supergene conditions and high-sulfidation relicts, together with the narrowsulfate alteration zone buried under the youngest volcanic deposits, pointto the existence of an evolving paleo-conduit. The data will contribute tomonitoring and evaluating the volcanic hazards.