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This study characterizes the effects of fluid migration into a predominantly shale cover which seals the active geothermal system of Mt. Amiata (Tuscany, Italy). During Alpine orogenesis the shale unit was affected by regional metamorphism at the limit of the diagenesisanchizone. Subsequently, the phyllosilicate clay minerals of the shales underwent significant alteration at diagenetic temperatures (175±25°C as determined by the geochemical model) by the pervasive circulation of fluids activated by the geothermal field. The overall mineralogical assemblages indicate that the main transformations consisted mostly of destabilization of illite and formation of kaolinite together with large amounts of I-S mixed layers, with higher smectite content and decreased Reichweite I-S ordering (from R3 to R1) with respect to the original, unaltered phases. Application of computer modelling indicates that the circulation of CO2-rich geothermal fluids into the shale unit was responsible for the observed phyllosilicate clay mineral transformations. [ABSTRACT FROM AUTHOR]

We present a geochemical model for the sequestration of anthropogenic CO 2 (from hypothetical hydrocarbon-based H 2 production) consequent to the theoretical injection of gas beneath the River Arno plain, near the city of Pisa, Italy. The potential reservoirs for gas disposal are of two different types: dolostones or sandstones. The model predicts the sandstones to be the best reservoirs because CO 2 sequestration in these rocks can occur via the deposition of solid carbonates. On the contrary, the injection of gas into the dolostone could generate gas accumulation and high gas pressures (50–100 atm) that could possibly promote rock fracturing and gas leakage in these shallow (500–1000 m depth) reservoirs. The best sites for gas disposal are therefore located west of the town of Pisa, along the coastal plain. [ABSTRACT FROM AUTHOR]

Abstract: Water–rock interaction experiments have been performed to investigate the production of CO2-rich gas in the deep reservoir of the geothermal system of Mt Amiata (Italy) through the interaction between a natural phyllite sample (coming from 1115m below ground level and considered representative of the deep reservoir) and a coexisting fluid phase. Experiments were conducted with an externally heated pressure vessel, a chamber furnace and a stirred micro-reactor at 300°C and at P up to 35MPa, using different solid/liquid ratios, and also under nominally anhydrous conditions. Experimental results show that CO2-rich gases containing variable amount of CO, CH4 and other aliphatic and aromatic hydrocarbons were produced by fluid–rock interaction. Compositional data of the gas, coupled with solid residua characterization, show that thermal decomposition of magnesian siderite in phyllite is the process responsible for the gas genesis. This process is particularly effective when aqueous solutions interact with phyllite and it may represent the thermometamorphic reaction mainly responsible of the production of CO2-rich gas under hydrothermal conditions similar to those of the main reservoir in the Mt Amiata geothermal area. [Copyright &y& Elsevier]

Abstract: The magnesite deposits of Malentrata (Tuscany, Italy) were derived from serpentinite silicification–carbonation of the Ligurian ophiolite, and represent a natural analogue of in situ CO2 mineral sequestration. Carbonation of magnesium silicate minerals (e.g. serpentine, olivine) at temperatures below 200 °C is an exothermal process, involving incorporation of carbon dioxide into stable carbonates (e.g. magnesite, dolomite). Serpentinites at Malentrata were transformed to a brownish friable rock characterized by the occurrence of opal, chromian montmorillonite, Fe-rich magnesite and minor iron sulfides and oxides. Widespread lenses of cohesive rocks occur within the altered serpentines, resulting from the complete silicification of the protolith. The pervasive alteration of serpentinite was accompanied by the formation of a network of magnesite and dolomite veinlets, and large magnesite–dolomite veins along major structures. Field observations, petrography and mineral chemistry define the following crystallization sequence in the major veins: i) early microgranular Fe-poor magnesite, ii) comb-textured Fe-rich magnesite and dolomite cementing the early brecciated magnesite vein infill, and iii) late quartz, chalcedony, and rare opal in the cavities. The mineral assemblage observed both in veins and in host rock is indicative of low-temperature hydrothermal alteration driven by Si- and CO2-rich fluids under relatively low pH conditions. Pervasive and cyclic hydraulic fracturing maintained a high structural permeability during the whole hydrothermal event, creating conduits for the input and output fluids. The concomitant presence of CO2 and silica in the fluid enhanced the congruent dissolution of serpentine and hampered immediate carbonate deposition. The Mg-bearing solutions were focused out of the reactor zone, and promoted massive carbonate veining along the main structural discontinuities. This two-steps reaction (dissolution of serpentinite followed by carbonate precipitation outside the reacting volume) seems to be very efficient and provide new insights on in situ and ex situ induced mineral carbonation and its industrial applications. [Copyright &y& Elsevier]