Interaction between low-angle normal faults and hydrothermal circulation during Early Permian extension in the central Southern Alps (Northern Italy)

Starting from the late Carboniferous, several episodes of crustal thinning affected the Variscan orogen, leading to the development of intracontinental basins at the Europe-Adria boundary, now-day preserved in the central Southern Alps (cSA). In this period, a megashear zone with dextral kinematics led to the transition from Pangea A to Pangea B configuration. During the later Alpine compression, the favourably oriented normal faults inherited from Permian tectonics played an important role, being frequently inverted as S-verging thrusts. Nevertheless, several Permian structures along the northern border of the Permian Orobic Basin still preserve their original features, since they exceptionally escaped the Alpine deformation (Zanchi et al., 2019). They are Low-Angle Normal Faults (LANFs) mainly developed at the interface between the Lower Permian sedimentary cover and the Variscan basement. Two major Permian LANFs are exposed at the head of the Brembana Valley (BG) and are characterized by cataclastic bands sealed by centimetric layers of dark aphanitic tourmalinites (Zanchi et al., 2019) and locally by U mineralizations. These rocks derive from rich in B fluids, preferentially channelled along high permeability shear zones and are associated to further exposures of Permian LANFs in other sectors of the cSA, testifying to the regional importance of this fluids circulation. Intrusive bodies of 285 Ma occur close to the Trompia Valley (BS), where microcrystalline tourmalinitic breccias cutting the basement have been studied from the mineralogical point of view (De Capitani et al., 1999). We performed whole rock analysis both on tourmalinites and granitoids of this area, and it came out that they are geochemically related, demonstrating that the rich in B fluids are a product of the Permian magmatism. Several authors genetically linked the cSA tourmalinites with the U mineralization of Novazza-Vedello district (De Capitani et al., 1999) but this correlation has not been proved so far. Bulk analysis of tourmalinites from different sectors of cSA indicate low U concentration, however new observations on rocks nearby the U mineralization district indicate the presence of tourmaline crystals combined with minerals likely relatable to the metallogenic event. Our main goal is providing a better characterization of the regional hydrothermal event and to relate it with the structural setting, which influenced the fluids circulation in this intracontinental extensional configuration.