Fossil Versus Active Geothermal Systems: Reconstructing Fluid Pathways and Building a Bridge from the Past to the Present – Preliminary Results

Abstract

At Spiaggia Barbarossa on the Italian island of Elba, a hydraulic cataclastic shear zone produced by hydrothermal fluid overpressure was investigated in order to study anisotropic structures and fracture networks from macroscopic to microscopic scale. Fluid filled fractures and the degree of fracture networks that functioned as fluid pathways in a fossil hydrothermal system were also of interest. Tourmaline dykes and veins crosscut the oldest Tuscan units, the Calamita schists, parallel and oblique to the foliation and the Porto Azzurro pluton. Both metasomatic and hydrothermal tourmaline were identified in these veins. Metasomatic tourmaline exhibits reverse zoning with schoerl-rich cores and dravite-rich rims that indicate progressive replacement of biotite. Hydrothermal tourmaline forms the matrix fluid of cataclastic shear zones embedding quartz clasts within the range of ~ 100 – 2000 µm size. Fractal geometry analyses of the shear zones indicate the concentration of deformation during the regional extensional tectonic regime that was active from the Miocene to the Pliocene-Pleistocene. Quantification of fragmentation patterns by a modified Cantor-dust method (AMOCADO) indicates one process of fragmentation, whereas fragmentation of tourmaline-filled fragmentation zones at the western Barbarossa outcrop point to coeval processes of regional deformation and boron-rich fluid infiltration. The analysis of pattern anisotropy provides indication for two subsequent processes: hydraulic fracturing caused by fluid overpressure and shearing of the already fragmented rocks caused by a regional stress field. Both processes are regarded as roughly coeval.