Interaction between Early Permian low angle normal fault and hydrothermal activity in the central Southern Alps (N Italy)

The development of intracontinental basins at the Europe-Adria boundary is the result of several episodes of crustal thinning occurring along the Variscan orogen starting from the late Carboniferous. In this period, a megashear zone with dextral kinematics led to the transition from Pangea A to Pangea B configuration. Some of these basins are now-day preserved in the central Southern Alps (cSA, N Italy), but during the later Alpine compression, the favourably oriented Permian normal faults have been frequently reactivated and inverted as Sverging thrusts. Along the northern border of the Permian Orobic Basin some Permian structures 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 Variscan basement-sedimentary cover interface and some fragments of LANF are exposed at the head of the Brembana Valley (BG), where the fault zone is characterized by cataclastic bands sealed by centimetric layers of dark aphanitic tourmalinites (Zanchi et al., 2019) and locally by U mineralizations. The precipitation of tourmaline derives from rich in B fluids channelled along high permeability fault zones and the fluids circulation is of regional importance since tourmalinites are associated to further exposures of Permian LANFs in other sectors of the cSA. Microcrystalline tourmalinitic breccias cutting the basement have been studied from the mineralogical point of view close to the Trompia Valley area (De Capitani et al., 1999), where intrusive bodies of 285 Ma occur. Whole rock analyses were performed both on tourmalinites and granitoids of this area, which look geochemically related, demonstrating that the rich in B fluids are a product of the Permian magmatism. Tourmalinites genesis has been related to the U mineralization of Novazza-Vedello district by several authors (De Capitani et al., 1999), even though this correlation is not clearly demonstrated so far. Low U concentration was detected with bulk analysis of tourmalinites from different sectors of cSA, 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.