Multidisciplinary exploration for geofluids in small islands, the case study of the Formiche di Grosseto islets, Italy

The Formiche di Grosseto are three islets, of about 1,500 m2, included in the Tuscan Archipelago, Italy. The islets rise abruptly from a seabed approximately 100 meters deep in correspondence of a structural high separating Neogene basins of the Tuscan Shelf. Onshore it is possible to observe hydrothermal breccias and speleothems. Offshore, CO2-rich submarine thermal fluids discharge from several springs with temperatures as high as 41°C. These submarine springs are mainly located along the Northern scarp at depths ranging from 6 to 32 m below the sea level.

To better understand the past and present nature of these fluid-driven systems extending offshore, a multidisciplinary exploration campaign was undertaken in the framework the TEMPEST and MIGRATE projects.

The main islet, offers a superb continuous outcrop of the Liassic carbonate rocks forming the backbone of the island.  Fault and fracture systems have been characterized across different scales to create a representative model. Multiple drone-flights have been performed to obtain an optical image for faults and fractures mapped over the islands. Drone-derived observations have been verified in the field and integrated in the structural model. Fractures and structural key geometries have been captured by means of a 2-D fracture network analysis with a circular sampling window. Connectivity within the fracture network have been parameterized by characterizing the different types of fracture terminations and intersections, which have been used to understand the structural architecture controlling the fluid flow. Rocks, hydrothermal breccias and calcitic precipitates have been analysed with cathodoluminescence and stable isotopes. Furthermore, a surface electrical resistivity (ERT) survey was carried out on the islet of Formica Grande di Grosseto - the main inslet - for the characterization of the subsoil up to 20-25 meters deep. The inversion shows a possible upflow of fluids, centred where the hydrothermal breccia has been found. Finally, CO2 diffuse-degassing soil flux measurements were also performed along these alignments.

The integrated study of the collected data pointed out that the northern part of the major islet is marked by upwelling of hot hydrothermal fluids. This was confirmed by a manual excavation that revealed rising thermal fluids - temperature of about 38°C- which was previously unknown on the island. Geochemical investigations have been conducted on the island's fluids, on the already known submarine thermal discharges as well as on the known thermal springs located inland, in the proximity of the Formiche di Grosseto area. 

The chemical compositions of the Formiche water samples are similar to those of seawater, although marked by significant enrichments of SO4, Ca and B and Mg depletion. These compositional characteristics are likely to be ascribed to the mixing of seawater with hydrothermal fluids. Our preliminary results aim to assess the origin of the discharged fluids and their relation with local and regional tectonics, providing an example of how multidisciplinary integrated exploration can be effectively conducted even in challenging contexts.