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

Abstract

Abstract. Active fumarolic solfataric zones represent important structures of dormant volcanoes, but unlike emitted fluids, their mineralizations are omitted in the usual monitoring activity. This is the case of the Campi Flegrei caldera in Italy, among the most hazardous and best-monitored explosive volcanoes in the world, where the landscape of Puteoli is characterized by an acid sulfate alteration that has been active at least since Roman time. This paper provides temperature, mineralogical, textural, compositional and stable isotope data for those solfataric terrains sampled at the crater and Pisciarelli slope of the Solfatara volcano between 2013 and 2019. Temperatures vary between 40 and 95 ∘C. Minerals include alunite with grain sizes generally larger than 20 µm, alunogen, native sulfur, well-ordered kaolinite, and, common at Pisciarelli, pyrite, illite and NH4 sulfates. Sulfate terrains have higher contents of Ti, Ba, Au, As, Hg and Tl relative to their parent substrate. The Pisciarelli slope is anomalous in terms of the presence of NH4. δ34S values for sulfides and native S range between −3.00 ‰ and 0.49 ‰ and from −4.42 ‰ to 0.80 ‰, respectively. Sulfates show δ34S and δ18O 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 isotope geochemistry do produce complex and not completely consistent classifications and genetic constraints. We merge our data with volcanological information, data from exploration drillings and geophysical results. With the conceptual model, we suggest a series of shallow and deep aquifers interconnected like “communicating vessels” through a main fault system that downthrows Solfatara with respect to Pisciarelli. Fluid outflow from the different discrete aquifers hosted in sediments – and possibly bearing organic imprints – is the main dataset that allows determination of the steam-heated environment with a supergene setting superimposed. Supergene conditions and high-sulfidation relicts, together with the narrow sulfate alteration zone buried under the youngest volcanic deposits, point to the existence of an evolving paleo-conduit. The data will contribute to monitoring and evaluating the volcanic hazards.

Identification of a New Heavy-Metal-Resistant Strain of Geobacillus stearothermophilus Isolated from a Hydrothermally Active Volcanic Area in Southern Italy

“…Temperature and pH values were 94.3 • C and 4.8 in the bubbling mud pool, respectively, while in the marginal water-poorer portion they were 55.3 • C and 6.0, respectively. The local levels of arsenic are in the 10-20 ppm range [39]. Enrichment cultures were set in 50 mL tubes with 20 mL of Luria-Bertani (LB) medium [42] and inoculated with 1 g of soil sample at 37, 50, and 75 °C for 24 h. Then serial dilutions of these culture samples were streaked on LB-agar plates (1.5%) at the same temperature values for 24 h. Bacterial colonies were found in the mud sample incubated at 50 °C and the isolation of a pure strain was carried out by repeated streak plating on solid medium prepared with the LB-agar incubated at 50 °C.…”

“…Like similar volcanic systems worldwide, the Solfatara volcano hosts an acidic sulfate environment determined by the hot circulation of aggressive sulfurous fluids deriving from mixing between deeply infiltrating meteoric waters and ascending magmatic gases [39][40][41]. This phenomenon causes intense rock alteration and concentration of certain elements, such as As [39,[49][50][51][52][53][54]. However, differently from the diffuse and fumarolic outgassing characterizing the Solfatara crater, the Pisciarelli site is a water-dominant environment, showing the formation of boiling pools and water springs and the opening of low-energetic geyser-type vents.…”

“…These values are in the range known for the area, although lower temperatures were also measured in the mud pool (approximately 70 • C). Furthermore, the mineralogical and chemical features of the sampled materials [39] are those usually determined. Indeed, the mud was typically gray in color and essentially enclosed sulfates (i.e., K-and Al-bearing alunite), sulfides (i.e., Fe-plus S-bearing pyrite), and silica-phases; dried water samples crystallized NH 4 According to Aiuppa et al [54], arsenate is the As-compost under equilibrium in the water solutions.…”

“…In this work, we describe the isolation and characterization of a new thermophilic heavy-metal-resistant microorganism from the solfataric mud pool of Pisciarelli in the Campi Flegrei volcano located near Naples in Italy. The site has extreme environmental conditions in terms of temperature, acidity, and As-rich geochemistry due to an intense hydrothermal activity [39,40].…”

The Pisciarelli main fumarole mechanisms reconstructed by electrical resistivity and induced polarization imaging

“…In addition, SP anomalies can be sourced by electrochemical effects, which can be attributed to several phenomena, such as the diffusion of ions from a concentration gradient between two regions, the separation of the ionic charges occurring across a concentration gradient, and the so-called “membrane potential” linked to the presence of clayey materials, in which the counterions are attracted to the mineral surface while the co-ions are electrostatically blocked. In the case of Pisciarelli, and generally in the fumarolic areas, geochemical observations emphasize the existence of different ion concentrations in gas and water discharges 34 and the likely presence of structures submitted to hydrothermal alteration and clay production 35 . Then, a chemical contribution to SP anomalies could be present.…”