Abstract
Fluid–rock interaction experiments between a biotite-rich schist (from Mt. Calamita Formation, Elba Island, Italy) and B-bearing aqueous fluids were carried out at 500–600 °C and 100–130 MPa. The experiments have been carried out in order to reproduce the reaction, which would have produced tourmalinisation of the biotite schist, supposedly by circulation of magmatic fluids issued from leucogranitic dykes. The reacting fluids were either NaCl-free or NaCl-bearing (20 wt %) aqueous solutions, with variable concentration of H3BO3 (0.01–3.2 M). The experimental results show that tourmaline (belonging to the alkali group) crystallise under high-temperature and upper crustal conditions (500–600 °C, 100–130 MPa) when H3BO3 concentration in the system is greater than 1.6 M. The composition of tourmaline is either dravitic (Mg-rich) or schorlitic (Fe-rich), depending if an NaCl-bearing or NaCl-free aqueous solution is used. In the first case, a significant amount of Fe released from biotite dissolution remains in the Cl-rich solution resulting from the experiment. By contrast, when pure water is used, Na/K exchange in feldspars makes Na available for tourmaline crystallisation. The high concentration of Fe in the residual fluid has an important metallogenic implication because it indicates that the interaction between the saline B-rich fluid of magmatic derivation and biotite-rich schists, besides producing tourmalinisation, is capable of mobilising significant amounts of Fe. This process could have produced, in part or totally, the Fe deposits located close to the quartz–tourmaline veins and metasomatic bodies of the Mt. Calamita Formation. Moreover, the super-hot reservoir that likely occurs in the deepest part of the Larderello–Travale geothermal field would also be the site of an extensive reaction between the B-rich fluid and biotite-bearing rocks producing tourmaline. Thus, tourmaline occurrence can be a useful guide during deep drilling toward a super-hot reservoir.
Highlights
Tourmaline is a common accessory mineral in granitoids, mainly in peraluminous leucogranites, in related pegmatites, and in hydrothermal quartz vein and breccia systems that frequently develop around granitoids [1,2,3,4,5,6]
The super-hot reservoir that likely occurs in the deepest part of the Larderello–Travale geothermal field would be the site of an extensive reaction between the B-rich fluid and biotite-bearing rocks producing tourmaline
A further starting material was prepared by enriching biotite crystals by means of Franz isodynamic magnetic separator. This was done with the perspective to favour the expected tourmaline crystallisation since biotite was thought to be the major supplier to the crystallisation of this phase
Summary
Tourmaline is a common accessory mineral in granitoids, mainly in peraluminous leucogranites, in related pegmatites, and in hydrothermal quartz vein and breccia systems that frequently develop around granitoids [1,2,3,4,5,6]. Tourmaline crystallises either as a magmatic mineral, or during the magmatic/hydrothermal transition and as a late post-magmatic hydrothermal mineral. During the crystallisation of peraluminous melts, Fe and Mg are initially incorporated in phases such as biotite and cordierite, whereas B increases in the residual magma [7]. When tourmaline saturation is reached, B-rich residual melts eventually react with earlier Fe-Mg phases, causing tourmaline. Minerals 2017, 7, 155; doi:10.3390/min7090155 www.mdpi.com/journal/minerals formation. A significant amount of B originally present in magma can be released to wall formation.
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
