A way to hydrothermal paroxysm, Colli Albani volcano, Italy

The main issue addressed in this work is the process leading to fluid subsurface entrapment and pressure increase up to hydrofracturing and, possibly, to paroxysm in a hydrothermal setting, in order to envisage such processes and mitigate their effects in the volcanically active study area and elsewhere. A field and laboratory multidisciplinary approach is used in the fossil (late Pleistocene) portion of an active hydrothermal system (Colli Albani volcano, Rome, Italy). In this area, sulfate and sulfide mineralizations and strongly altered ignimbrites are exposed. The alteration acme occurs on top of a buried normal fault, where abundant degassing is still active, and fades away in 2–3 km. Based on pervasive versus discrete alteration styles, mineral assemblages, and further evidence, proximal and distal alteration domains are recognized. Both domains underwent steam-heated advanced argillic alteration with likely temperatures up to ~400 °C in the proximal domain and less than 150 °C in the distal domain. The process of hydrothermal alteration progressively and severely depleted many elements from the most permeable rock units, whereas the lowest-permeable unit (Tufo Lionato) underwent fracture and porosity healing accompanied by both mass and volume gain. In the proximal domain, the advanced argillic hydrothermal alteration eventually formed a substantial barrier to fluids. The hydrothermal fluids accumulated in and below this barrier, which was then suddenly hydrofractured when heat-driven hydraulic pressure overcame the effective stress, thus possibly leading to hydrothermal paroxysm. The decompression associated with hydrofracturing enhanced gas exsolution and mineral precipitation from the entrapped overpressured fluids. Mineral precipitation contributed, in turn, to fracture healing and to reinitiation of a new cycle of hydrothermal fluid entrapment. The key preconditions for the occurrence of the inferred processes are the contrasting compositions of K-alkaline host rocks and acidic alteration fluids, as also previously documented in other similar settings elsewhere.