Systematic review of geochemical data from thermal springs, gas vents and fumaroles of Southern Italy for geothermal favourability mapping

Abstract

In recent years, two research projects specifically conceived by Italian Institutions of Research to promote the implementation of the use of geothermal energy in Southern Italy has allowed the review of most data on chemical and isotopic compositions of natural thermal manifestations in the territory of Italy. Two large databases, one for thermal springs and CO2-rich springs, and a second one for fumarolic condensates and associated gas phase have been produced and are available on line, with data spanning in time from the early 70's to the present.

We have used those data, after careful evaluation of the quality and reliability of them, to produce correlation diagrams and isodistribution maps of some relevant geochemical/geothermal parameters, such as: pCO2 in thermal springs, %CO2 and δ13C in CO2 of gas phases, 3He/4He ratio and %He. In this way, we have been able to delimit the areal patterns of thermal anomalies potentially related with geothermal reservoirs. The cross correlation among the many parameters (>40) selected has allowed the overview on the circulation of fluids at shallow crust, in one of the most active tectonic boundary of the Earth between the African and the Eurasian continents.

Shallow circulation of hot fluids is particularly active in the Roman Comagmatic Province, the Neapolitan area and Sicily (both at Etna, Aeolian Archipelago and Pantelleria island in the Sicily Channel) where active geothermal systems are already known, whose areal extension is probably much larger than what envisaged at present.

The geothermometric evaluation of data has not allowed to clearly identity new areas apart from those already known but, nevertheless, some areas in the inner Apennines, as well as Sicily and Sardinia have shown anomalous 3He/4He values that point to the presence of mantle fluids located inside the crust.

Being most of active volcanic islands likely much smaller than the thermal anomaly they are associated with, a futuristic perspective of utilizing geothermal fluids off shore is suggested. Moreover, the database and the proposed maps can be a useful tool both scientific community and stakeholders to perform geothermal favourability maps and to identify potential new areas interesting from a geothermal perspective.