Secondary hydrothermal minerals in buried rocks at the campi flegrei caldera, Italy: a possible tool to understand the rock-physics and to assess the state of the volcanic system

The distribution of the alteration assemblages and the related physico-chemical changes
induced in the rocks with depth, may provide useful information on the state of the system.
Drillholes are the only way to define hydrothermal alteration depth-profiles in variable
geological contexts. Deep drillings exploiting programs were conducted since the 1970’s by
the Agip-Enel Joint Venture in the Quaternary Campi Flegrei caldera (southern Italy), where a
geothermal system has been active since at least historical times. New macroscopic and
microscopic investigations were performed on selected samples made available by Agip in
order to: 1) define the precursor lithology, 2) describe the relationships among texture,
mineralogy and depth of the studied core samples and 3) examine the character of the secondary
minerals and their distribution with depth and temperature. The new data are integrated with
physical properties and elastic parameters of cored rocks, as well as structural information and
field data, all available from the physical, seismological, geodetical and volcanological literature.
The depth-related multi-parameters profiles provide evidence on the different behavior of the
buried rocks beneath the Licola 1, Mofete and San Vito 1 areas, sited in three structurally
different sectors of the caldera. The features of the hydrothermally altered rocks are a key to
interpret the heterogeneities of the Campi Flegrei substratum, as deduced by velocity, attenuation
and scattering P- and S- waves tomography. The time and space distribution of both the eruptive
vents and the extruded magma volumes are consistent with the results of our analysis. Therefore,
we interpret the observed Campi Flegrei geothermal system as a response to the distribution of
volcanic activity in two structurally distinct sectors of the caldera. The central-eastern sector,
where the San Vito 1 well was drilled, represents the preferential pathways for both gas escape
and magma ascent at least since 8 kyrs, in contrast with the other sites of the caldera where
eruptions occurred with minor frequency and magnitude.