Investigating fossil geothermal systems: the case of Elba Island (Tuscany, Italy)

In Eastern Elba Island, the Porto Azzurro granitoid (5.9 Ma) intruded in the mid-crust gave rise to a widespread boron-rich contact metamorphic aureole, intense hydrothermal circulation, and Fe-deposits. At Cala Stagnone (Calamita Peninsula), tourmaline-bearing metasomatic bodies, and a complex quartz-tourmaline hydrothermal system, are now exposed at surface, and evaluated as a possible exposed analogue of the deepest part of the present-day Larderello-Travale geothermal fields. The hydrothermal Fe deposits of the Rio Marina and Porto Azzurro areas (Valle Giove, Bacino, Topinetti, Terra Nera) can be indeed considered the expression of a paleo-geothermal reservoir developed at the depth of the present-day main reservoir at Larderello-Travale geothermal fields. As part of the EU FP7-funded Integrated Methods for Advanced Geothermal Exploration (IMAGE) project, fluid inclusions (FIs) were studied in quartz-tourmaline veins at Cala Stagnone and in hydrothermal quartz, adularia and calcite from Fe-deposits of Rio Marina and Porto Azzurro areas to derive: i) the characteristics of fluids circulating in a paleo-geothermal system similar to the Larderello-Travale ones, ii) the P-T conditions of the system, and iii) the reactions occurred between the host rocks and fluids. FIs were studied by optical microscopy and microthermometry. SEM-EDS and Raman spectroscopy were employed as complementary techniques. FIs were distinguished on the base of phase assemblage at room temperature, and on the homogenization mode: 1) S-type, or polyfasic, displaying 1-4 daughter minerals; 2) L-type, homogenizing in the liquid phase; 3) V-type, homogenizing in the vapor phase. Specifically, at Stagnone we found: a) hypersaline S-type FIs with high total Th (up to >600°C), and b) saline L- and V-type FIs with moderate to high Th (up to 600°C). Both a) and b) presented a composition dominated by Na, Cl, Fe and minor amount of Mn, K and Ca. At Rio Marina and Porto Azzurro areas, FIs are mainly of L-type and sporadically of S-type, and registered fluid with both variable salinity and Th values (up to ca. 400°C). At Stagnone, the hypersaline FIs record the circulation of magmatic derived fluids; the rather high Th and salinity of the saline FIs also suggest that this fluid is, at least in part, of magmatic derivation. The few L inclusions showing lower Th and salinity can record the presence of a partly meteoric-derived fluid in the geothermal system. On the other hand, FIs in hydrothermal Fe-deposits of Rio Marina and Porto Azzurro areas record the contemporaneous circulation and mixing of a low-salinity fluid of probable meteoric origin and saline fluid, sometimes halite-saturated at room temperature. The saline fluid can be a magmatic derived fluid (similar to that recorded by S-type at Stagnone) that undergone cooling and dilution before entering in the hydrothermal systems of Fe-mineralization or a fluid that interacted with evaporitic levels of the Tuscan Nappe.