Natural hydrogen can be generated through fluid–rock interactions, particularly via the reduction of water by oxidation of Fe²⁺-bearing minerals. Among these reactions, the serpentinization of Fe-rich olivine in ultramafic rocks is the most extensively studied. However, a less explored but potentially significant source of natural H2 is the hydrothermal alteration of peralkaline granites. In such systems, H2 may form through the oxidation of Fe2+-rich biotite, accompanied by the neoformation of Fe3+-bearing minerals such as chlorite or magnetite. Additionally, at lower temperatures, the hydrothermal alteration of magnetite within peralkaline granites may produce H2 via oxidation of H2S under acidic conditions, leading to magnetite pyritization, or through direct oxidation of magnetite to hematite. This study, which is part of NHEAT (Natural Hydrogen for Energy trAnsiTion) project, funded by the Italian Ministry of University and Research (MIUR) under the PRIN PNRR 2022 program, investigates the potential for natural H2 generation from peralkaline granites in Tuscany, where hydrothermal alteration is active. Tuscany is characterized by a high heat-flow anomaly linked to Late Miocene-Pliocene granite intrusion associated with the opening of the Tyrrhenian back-arc basin, which is currently exploited in the Larderello geothermal field. Combining multi-scale structural, mineralogical, and geochemical analyses, we examine the hydrothermal alteration of peralkaline granites exposed in the Gavorrano and Botro ai Marmi areas. Micro-Raman spectroscopy of fluid inclusions within quartz grains—present in fault-related sulfide mineralization and chloritized granites—reveals the trapping of molecular hydrogen (H₂). These findings demonstrate that hydrothermal alteration of peralkaline granites can generate natural hydrogen. Future perspectives required to quantify rate and volume of hydrogen production will be discussed with the final goal of assessing the potential for economic exploitation of H2 in this region.
Genesis of Natural Hydrogen by Hydrothermal Alteration of Tuscan Peralkaline Granites / Schirripa Spagnolo, G.; Marchesini, B.; Aldega, L.; Ruggieri, G.; Novella, D.; Caracausi, A.; Billi, A.; Boschi, C.; Carminati, E.. - (2025). (Intervento presentato al convegno Società geologica Italiana tenutosi a Padova).
Genesis of Natural Hydrogen by Hydrothermal Alteration of Tuscan Peralkaline Granites
Schirripa Spagnolo G.
;Marchesini B.;Aldega L.;Ruggieri G.;Billi A.;Carminati E.
2025
Abstract
Natural hydrogen can be generated through fluid–rock interactions, particularly via the reduction of water by oxidation of Fe²⁺-bearing minerals. Among these reactions, the serpentinization of Fe-rich olivine in ultramafic rocks is the most extensively studied. However, a less explored but potentially significant source of natural H2 is the hydrothermal alteration of peralkaline granites. In such systems, H2 may form through the oxidation of Fe2+-rich biotite, accompanied by the neoformation of Fe3+-bearing minerals such as chlorite or magnetite. Additionally, at lower temperatures, the hydrothermal alteration of magnetite within peralkaline granites may produce H2 via oxidation of H2S under acidic conditions, leading to magnetite pyritization, or through direct oxidation of magnetite to hematite. This study, which is part of NHEAT (Natural Hydrogen for Energy trAnsiTion) project, funded by the Italian Ministry of University and Research (MIUR) under the PRIN PNRR 2022 program, investigates the potential for natural H2 generation from peralkaline granites in Tuscany, where hydrothermal alteration is active. Tuscany is characterized by a high heat-flow anomaly linked to Late Miocene-Pliocene granite intrusion associated with the opening of the Tyrrhenian back-arc basin, which is currently exploited in the Larderello geothermal field. Combining multi-scale structural, mineralogical, and geochemical analyses, we examine the hydrothermal alteration of peralkaline granites exposed in the Gavorrano and Botro ai Marmi areas. Micro-Raman spectroscopy of fluid inclusions within quartz grains—present in fault-related sulfide mineralization and chloritized granites—reveals the trapping of molecular hydrogen (H₂). These findings demonstrate that hydrothermal alteration of peralkaline granites can generate natural hydrogen. Future perspectives required to quantify rate and volume of hydrogen production will be discussed with the final goal of assessing the potential for economic exploitation of H2 in this region.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
