Fluid chemistry evolution during the natural carbonation of the Tuscan serpentinites: insights for CO2 mineralogical sequestration.

The magnesite deposits of Malentrata (Tuscany, Italy) were derived from serpentinite silicification-carbonation of the Ligurian ophiolites, and represent a natural analogue of in situ CO2 mineral sequestration. Serpentinite host rocks were transformed to a brownish friable mineral assemblage of opal, chromian montmorillonite, Fe-rich magnesite and minor iron sulfides and oxides. 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 tectonic structures. The major veins are characterized by the following crystallization sequence: i) early Fe-poor magnesite, ii) Fe-rich magnesite and dolomite cementing the early brecciated magnesite vein infill, and iii) late quartz, chalcedony and opal. 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. Here, we summarize our previous results and we report trace elements on host rocks and vein samples. Transects of the major carbonate veins, together with unaltered, partially altered and totally altered host rocks have been selected in order to investigate on the evolution of the chemical species in the fluids. The knowledge of the chemical-physical evolution of the fluids, the saturation/precipitation of the mineral species and the pH value of the solution in function of the time is fundamental to reproduce this process in laboratory or for in situ and ex situ induced mineral carbonation.