Chemical, isotopic and geotectonic relations of the warm and cold waters of the Cotronei (Ponte Coniglio), Bruciarello and Repole thermal areas, (Calabria - Southern Italy)

Abstract

Geochemical, geological, and hydrological data suggest, for the Cotronei-Caccuri thermal area (Northern Calabria – Southern Italy), the presence of a deep primary geothermal aquifer hosted in the crystalline basement (at Ponte Coniglio) and two secondary aquifers (at Bruciarello and Repole) developed in the Miocene sedimentary succession. Despite the spatial proximity of the three thermal sites, the waters have different compositions: Na(Ca)-Cl for Ponte Coniglio-Cotronei, Na(Ca)-Cl(SO4) for Bruciarello and Na-SO4(Cl) for Repole.The composition of the Cotronei-Ponte Coniglio end-member was reconstructed based on conservative constituents and resulted in a Cl concentration of about 1700 mg kg−1. The thermal waters of Bruciarello and Repole are derived from this primary fluid. Geothermometric calculations support a possible dilution affecting the waters studied to different degrees. For the pure (undiluted) thermal end-member, hosted in the primary granitic aquifer, comparable temperature estimations are provided by cationic geothermometers (67 ± 4 °C) and geothermometric simulations (65 ± 4 °C). However, Bruciarello and Repole waters are in equilibrium with anhydrite and fluorite at a temperature (68.5 ± 4 °C) very similar to that obtained for the granitic aquifer suggesting slight cooling during the travel from the primary granitic aquifer and fast equilibration with anhydrite and fluorite favoured by the rapid dissolution kinetics of these solid phases. From the isotopic point of view, the secondary aquifers have δ34SS(6+) values (+23.01‰ and +22.77‰ ± 0.23‰) comparable with those of the Messinian gypsum (average of +21.8‰). The thermal waters of Cotronei, on the other hand, show enriched δ34SS(6+) values (+38.60‰ ± 0.78‰) due to bacterial reduction of sulfate probably also derived from by Messinian evaporates.Furthermore, the δ18O and δ2H values indicate a meteoric origin for all three thermal sites with an average infiltration altitude of about 1650 – 1850 m asl, which agree with the Sila plateau elevations. Based on the results obtained in this work, assuming a geothermal gradient of about 33 °C/km, a temperature of the deep thermal reservoir of about 65 °C and an average predicted atmospheric temperature of 6 °C in the recharge area (Sila plateau), it follows that the meteoric waters descend to depths of ∼1800 m, where the thermal aquifer of interest is presumably located. This depth is consistent with the location of the primary geothermal reservoir within the crystalline basement. The reconstruction of this conceptual model has been made possible thanks to adopting a "local" approach integrating previously existing and new geological, hydrogeological, and geochemical data. Furthermore, this study has allowed us to identify the presence of an important regional geothermal aquifer hosted in the crystalline basement that, in addition to the carbonate and Upper Triassic evaporite reservoirs (well characterized in previous works), make the Calabria region of high geothermal interest.