Evaluation of the thermal potential of the Amiata geothermal field: Geochemical, Petrologic and Petrophysics data.

The Monte Amiata volcano consists of a succession of lavas and domes, with composition from trachyte/trachydacite to olivine-latite, emerged between 305 and 231 ka during the middle-Pleistocene. The volcanic edifice resulted in an emission of magma from an eruptive fissure aligned along the NNE-SSW. Key characteristics encompass abundant rounded magmatic enclaves, flat and elongated crustal meta-sedimentary xenoliths, and the prevalence of sanidine megacrysts. The area around the volcano underwent a regional uplift of about 2 km, extending from Monte Amiata to Radicofani, covering an area of 35 x 50 km caused by an unspecified magma intrusion at a depth of 5-7 km. Geological, geochemical, and geophysical studies revealed two significant geothermal reservoirs with high temperatures (> 250 °C) and a marked surface CO2-flux. Despite extensive research, debates persist regarding the stratigraphic relationships among lava flows and domes, the petrogenesis of silicic end-member magma, the magmatic chamber architecture, the petrophysics characteristic of xenoliths and the thermo-chemical interaction with magma. The main objective of this thesis is to evaluate the thermal energy emitted by the magmatic source and how this propagates in the crustal basement as a function of the host rock type. To achieve this, several objectives were achieved, such as, the investigation of lavas and emplaced domes, the characterisation of the composition of minerals within igneous and meta-sedimentary rocks, the characterisation of the volatile phase content trapped by melt inclusions of sanidine, the determination of a general T-P range, the characterisation of the water content and redox conditions of phlogopite crystals from magmatic rocks and meta-sedimentary xenoliths, and the development of a model outlining the architecture of the magma chamber. This thesis employs a multidisciplinary approach encompassing the stratigraphic definition, petrographic investigations, geochemistry analyses, mineral chemistry and isotope chemistry investigations, X-ray diffraction and crystal chemistry of phlogopite, microthermometry and thermobarometry experiments and calculations, melt inclusion analyses, seismic velocities and thermal conductivity. Whole-rocks chemistry reveal the high-K-calc-alkaline affinity of the basal complex magmas highlighting the same Sr-isotope values (87Sr/86Sr = 0.713019) of lava tongues all around the volcano, changing toward summit domes (0.712905) and the final lavas (0.71161). The “Pozzaroni” Formation is placed temporally and spatially between the basal complex (“Bagnore” Synthem) and summit domes (“Monte Amiata” Synthem), representing an intermediate outcrop with similar features between these two. Mineral chemistry investigations reveal several compositions reflecting the change of the geochemical signatures from different magmas, passing from sub-alkaline to alkaline affinity, and the mutual elements exchange among a triplet of components represented by magma – xenolith – magmatic enclaves. µSr-isotope investigation reveals a high-radiogenic sanidine core (0.71338) and lower rim (0.71305) indicating the nucleation from an ancient K-alkaline sub-crustal magma. Also, phlogopites, plagioclase and sanidine record the presence of peraluminous component released from magmatic-fluids by the near-solidus zone. Temperature, pressure and water content investigated, by experimental and empirical approaches, reveals mainly a shallow hypabyssal magmatic reservoir with deep mafic roots, characterised by a T–P range of 990-1170 °C and 1-9 kbars. The huge amount of carbon dioxide observed within sanidine melt inclusion aligns with the low water measured indirectly by phlogopite crystal-chemistry. Petrophysics investigations on meta-sedimentary xenoliths present a brand new data about the thermal and seismic propriety, as well as on petrography and mineral chemistry, highlighting the thermal-metamorphic transition between each samples indicating the presence of a contact aureola around the magma chamber. Geothermal exploitation simulation is conducted on a theoretical volume of rock located in the deep-Tuscany crustal basement near to the magmatic chamber using Hot Dry Rock technology. In conclusion, the results of isotopic and mineralogical investigations indicate a single extrusive eruption involved in the setting of lavas and domes. The petrogenesis of the high-silica magmas could be explained by the intrusion of HKCA/SHO magmas, derived from the sub-crustal mantle, which undergoes a process of RxMFC due to the progressive injection of hot mafic magmas similar in composition to the Roman type. The compositional variability observed in the groundmass glasses represents the last stage in the magmatic differentiation of high-silica rocks, and models indicate the potential presence of micro-assimilation processes of peraluminous melts/fluids release from pelitic xenoliths undergoing pyrometamorphic processes. The large amount of CO2 indicates that mantle processes are the main players, also influencing the amount of water dissolved in magma, and enrichments of carbon-rich fluids from crustal components cannot be excluded. The architecture of the magma chamber of Monte Amiata can be idealised with a batholith-shape divided into two zones with different thermo-chemical characteristics, with a contact aureole and a very differentiated rock in the most marginal part that could simulate the physical characteristics of a granite. The calculated present-day temperature emitted by the heat source is about 820 °C, quantifiable by 1024 calories emitted per km3, and a maximum theoretical energy production of 3277 MW/year or 0.33 GWyt.