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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.

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.

Proofs of synsedimentary tectonics during the Early Permian in the central Southern Alps (cSA, N Italy) are recorded in the volcanic and terrigenous successions of the Laghi Gemelli Group, which are characterized by the occurrence of abrupt facies variations often associated with coarse-grained deposits. These features are generally attributed to syn-sedimentary tectonic activity demonstrated by the local occurrence of sediments deformation such as liquefaction or slumping due to seismic shaking. Detailed fieldwork allowed us to recognize dewatering structures and sedimentary dikes, ball and pillars and small slumps, occurring along hundreds of mesoscopic faults showing meter-scale displacement in correspondence of high-angle conjugate systems as well as domino-style faults, often accompanied by growth structures. These structures are mainly concentrated in the fine-grained sediments of the Pizzo del Diavolo Formation, which were deposited on top of the volcaniclastic succession of the Ca’ Bianca Volcanite and crossed by seismogenic synsedimentary faults. The Permian synsedimentary structures of cSA are mostly associated with high-angle Andersonian normal faults which are combined with low-angle normal faults (LANFs) that developed along the interface between the Permian sedimentary cover and the Variscan basement. This LANFs system is relevant for the Permian hydrothermal circulation, resulting in widespread tourmalinites deposition along fault zones, and locally in U mineralization. According to our structural analysis the Permian tectonic setting is characterized by pure extension, dominated by ENE-WSW striking normal faults inverted during the Alpine shortening as high-angle reverse faults.

Durante il Permiano Inferiore, l’evoluzione post-Varisica dell’attuale regione Alpina è stata caratterizzata da una importante estensione crostale combinata con intensa attività magmatica, che ha favorito lo sviluppo di bacini estensionali intra-continentali con deposizione di sedimenti vulcanoclastici (e.g. Bacino Orobico). Nel settore centrale delle Alpi Meridionali, l’apertura di questi bacini era controllata da complessi sistemi di faglia, attivi contemporaneamente con l’intrusione di plutoni e l’attività vulcanica in superficie. Il seguente progetto di dottorato si focalizza sullo studio di queste caratteristiche, con l’intento di approfondire i processi geologici attivi durante il Permiano. Diverse faglie di età Permiana sono state indagate nel Bacino Orobico, con particolare enfasi sul riconoscimento dei loro tratti originali, poiché esse sono eccezionalmente sfuggite alla deformazione Alpina. Nel passato, l’architettura dei bacini Permiani era stata descritta con strutture a horst-graben, formatesi in risposta ad una tettonica transtensiva controllata da una zona di taglio che ha portato alla trasformazione da Pangea B a Pangea A. Tuttavia, studi più recenti hanno indicato un diverso setting di faglie che hanno controllato l’apertura dei bacini Permiani: si tratta di una combinazione di faglie normali a basso e alto angolo. Le faglie normali a basso angolo identificate lungo il Bacino Orobico rappresentano il miglior luogo in cui osservare l’interazione tra attività tettonica e magmatismo, poiché i loro nuclei di faglia sono caratterizzati da cataclasiti sigillate da livelli di tormaliniti. Queste ultime testimoniano la circolazione di fluidi arricchiti in boro e incanalati lungo importanti zone di faglia correlate all’apertura del Bacino Orobico. Simili brecce a tormalina affiorano anche in Val Trompia (BS): molti autori hanno suggerito che le tormaliniti delle Alpi Orobie, quelle della Val Trompia unitamente alla mineralizzazione di uranio del distretto di Novazza-Val Vedello potrebbero essere geneticamente correlate. Esse potrebbero essere correlate ad un sistema idrotermale di alta temperatura su ampia scala, coevo con l’attività tettono-magmatica del Permiano Inferiore che è responsabile anche della messa in posto di svariati depositi di minerali metalliferi nelle Alpi Meridionali. Tuttavia, la genesi delle tormaliniti non è mai stata dettagliatamente caratterizzata e la loro connessione con i giacimenti di U finora non è mai stata investigata. Le faglie tormalinizzate sono state riconosciute in molte località del settore centrale delle Alpi Meridionali quando ancora non era nota l’importanza delle faglie normali a basso angolo. In questa tesi di dottorato, vengono caratterizzate tutte le aree con i ritrovamenti di tormaliniti, ricercando la causa dell’idrotermalismo regionale nel contesto di estensione durante il Permiano Inferiore. Inoltre, per via del possibile link con i giacimenti di uranio, lo studio dei borosilicati viene proposto come strumento per meglio caratterizzare la genesi delle mineralizzazioni. Nuove osservazioni di terreno su tormaliniti sono combinate con lo studio della geochimica dei minerali e della roccia totale, insieme alla geocronologia, a studi microstrutturali e analisi isotopiche del B, con lo scopo finale di definire l’origine di questi fluidi arricchiti in boro. I risultati ottenuti da questa ricerca dimostrano che le tormaliniti e il magmatismo Permiano sono in stretta correlazione temporale e genetica. Inoltre, i dati geochimici sugli elementi in tracce forniscono maggiori indizi su una diretta connessione tra tormaliniti e la mineralizzazione a U dell’area di Novazza-Val Vedello. Tutti questi dati, per concludere, sono discussi nel contesto di interazione di processi tettonici, magmatici e di formazione di giacimenti che hanno interessato l’attuale settore centrale delle Alpi Meridionali durante il Permiano Inferiore. During the Early Permian, the post-Variscan evolution of the present-day Alpine region was characterized by crustal extension combined with strong magmatic activity at different crustal levels, which finally led to the development of intracontinental extensional basins filled with volcanoclastic sediments (e. g. the Orobic Basin, N Italy). In the central Southern Alps (cSA), the opening of these basins was controlled by complex fault system that were active at the same time of plutons intrusion and volcanic activity at the surface. Relationships among magmatism, tectonics and hydrothermal activity related to the formation of ore deposits in the Early Permian so far have been only briefly addressed. This Ph.D. research project focuses on the investigations of the above described features, trying to consider the geological processes active in the Early Permian in an integrated scenario. Several Early Permian faults of the Orobic Basin have been investigated with special emphasis on the recognition of their original features, as they have exceptionally escaped most of the Alpine deformation. In the past, the architecture of Permian basins was described as horst-and-graben structures, formed in response to wrench tectonics developed during the activity of a megashear zone that led to the Pangea B to Pangea A transformation after the collapse of the Variscan orogen. However, thanks recent studies, a different fault architecture has been suggested to had controlled the opening of the Permian basins: a combination of Low-Angle Normal faults and High-Angle Normal Faults. The identified LANFs of the Orobic Basin represent the best site to study the interplay among tectonics and magmatism, as they are characterized by cataclastic bands sealed with cm to dm thick layers of dark, aphanitic tourmalinites. These latter are proof of fluids circulation channelled along higher permeability fault zones related to opening of the Orobic Basin. Such tourmaline breccias also crop out in the Trompia Valley (BS): several authors suggested that tourmalinites from Orobic Alps, tourmalinites from Trompia Valley together with uranium mineralization of Novazza - Vedello Valley are genetically linked. They are seen as products of a large-scale high-temperature hydrothermal system coeval with the Early Permian plutonic-volcanic activity and tectonism, which was also responsible for the emplacement of various types of magmatic-hydrothermal ore deposits in the Southern Alps (Sn-W, U-Mo-Zn, Fe carbonates, sulphides, quartz). However, their genesis has never been fully characterized and the connection between tourmalinites and U ore bodies has also not been deeply investigated so far. The tourmalinized faults were first noted in various sites of the cSA during the 90’s, when the tectonic importance of LANFs was not yet recognized. In this thesis, all the occurrences of tourmalinites are characterized, looking for the cause of the regional hydrothermalism in the context of intracontinental extension during the Early Permian. Furthermore, due to the likely connection with U ore deposits, the borosilicate study is proposed as tool for better understanding the genesis of the mineralizations. New field based structural analysis are combined with mineral and whole-rock geochemistry, geochronology, microstructural studies and boron- isotopic analysis of tourmalinites from different sectors of the study area, in order to evaluate the origin of these fluids. Results coming out from this study demonstrate, together with B isotope ratios, a temporal and genetical relationship between tourmalinites and Early Permian magmatism in the cSA. Furthermore, the geochemical data on trace elements provide more clues on a direct connection between tourmalinites and the U-mineralization. All these results are finally discussed in the frame of the interplay between tectonic, magmatic and ore generation processes that interested the present day cSA area in the Early Permian

The occurrence of synsedimentary tectonics during the beginning of the Permian has been largely documented all cross the present-day region of the central Southern Alps. Evidence of active faults has been generally established based on facies variations often associated to coarse-grained deposits, a characteristic feature of the Laghi Gemelli Group, which was deposited during the Early Permian. Nevertheless, poor attention has been devoted to the reconnaissance and description of the mesoscopic fault record developed during the deposition of the Lower Permian successions, except for a few works (Berra et al., 2011) describing local synsedimentary features such as liquefaction or slumping due to seismic shaking.Working across the Orobic Alps, we identified several key areas where the occurrence of dewatering structures testify to the activity of synsedimenary faults together with sedimentary dikes, ball and pillars, and small slumps occurring along hundreds of mesoscopic faults showing meter-scale displacement along high-angle conjugate systems as well as domino-style faults, often accompanied by growth structures. These faults mainly affect the Pizzo del Diavolo Formation, which was deposited on top of the volcaniclastic succession of the Ca’ Bianca Volcanite.According to our structural observations, these high-angle Andersonian normal faults are often associated with low-angle normal faults, which developed along the interface between the Permian cover and the Variscan basement (Bloom & Passchier, 1997; Zanchi et al., 2019). LANF systems are responsible for significant tectonic elision of the volcaniclastic lower successions and for diffuse hydrothermal circulation, resulting in widespread tourmaline deposition along the fault surfaces.Our analyses point to the definition of tectonic setting characterized by pure extension dominated by ENE-WSW striking normal faults all across the central Southern Alps, which were later inverted during the Alpine shortening as high-angle reverse faults (Zanchetta et al., 2015). It is important to stress that in the considered area the strikes of the Early Permian structure are at odds with the Early Jurassic normal faults which generally show a N-S strike and were reactivated as strike-slip faults, pointing to an independent tectonic extensional event occurring 80 My after the Permian extension. Berra F. et al. (2011). Sedimentary Geology, 235, 249-263Blom, J. C., & Passchier, C. W. (1997). Geologische Rundschau, 86, 627-636.Zanchetta et al. (2015). Lithosphere, 7, 662-681.Zanchi A. et al. (2019). Italian Journal of Geosciences, 138, 184-201.

At the end of the Variscan orogeny, several episodes of crustal extension starting in the Early Permian occurred in central Southern Alps (cSA), affecting the Adria passive margin (Handy et al., 1999). During this period, a megashear zone with dextral kinematics led to the transition from Pangea A to Pangea B configuration (Muttoni et al., 2003). The transtensional to extensional deformation regime led to the development of intra-continental basins infilled by Upper Carboniferous to Lower Permian sedimentary successions (Cadel et al., 1996). Crustal shortening related to Alpine compression was responsible for a partial or complete inversion of favourably oriented normal faults inherited from the Permian tectonics (Blom & Passchier, 1997). Despite this, SSE-dipping Early Permian Low-Angle Normal Faults (LANFs) are well-preserved because they exceptionally escaped most of the Alpine deformations. Their surfaces are within the Lower Permian sedimentary cover, or at the interface between the sedimentary cover and the Variscan basement, passing to intra-basement shear zones.Two major Permian LANFs (Aga-Vedello and Masoni faults) are recorded in the Pizzo del Diavolo Fm. along the northern border of the Permian Orobic Basin (N Italy). They are “non-Andersonian” normal faults whose surfaces are characterized by cataclastic bands usually sealed by centimetric to metric layers of dark grey to black aphanitic tourmalinites (Zanchi et al., 2019). Tourmalinites indicate fluids circulation channelled along high permeability fault zones and are related to magmatic-hydrothermal fluids that produced metasomatic tourmalines with different compositions at different distances from the fluid source, i.e. the crystallizing intrusive bodies. In addition to Aga-Vedello and Masoni faults, further exposures of Permian LANFs occur in other sectors of the cSA and they are always associated with the presence of tourmalinites. Several authors (De Capitani et al., 1999; Slack et al., 1996; Cadel et al., 1996) link the cSA tourmalinites with the U mineralization of Novazza - Vedello district but this correlation could not be so direct and clear, due to the low concentration of Uranium in tourmalinites coming out from our whole-rock analyses.The main purpose of this research is to better characterize the entity and the genesis of this regional hydrothermal event and relate it to the role played by the structural setting on hydrothermal circulation in intracontinental extensional settings. Fieldwork and observations combined with microstructural and geochemical analyses of tourmalinites coming from different sectors of the cSA have been performed to reach this goal. Blom, J. C., & Passchier, C. W. (1997). Geologische Rundschau, 86, 627-636.Cadel, G., et al. (1996). Memorie di Scienze Geologiche, 48, 1-53.De Capitani, L., et al. (1999). Periodico di Mineralogia, 68, 185-212.Handy, M., R., et al. (1999). Tectonics 18, 1154-1177.Muttoni, G., et al. (2003). Earth Planet Science Letters, 215, 379–394.Slack, J., F., et al. (1996). Schweiz. Mineral. Petrogr. Mitt., 76, 193-207.Zanchi A. et al. (2019). Italian Journal of Geosciences, 138, 184-201

Low-Angle Normal Faults (LANFs) represent in the central Southern Alps area (N Italy) the main structures along which the Variscan basement is in contact with the Upper Carboniferous-Permian volcanic-sedimentary succession. Tourmalinites frequently occur along LANFs, usually replacing former cataclasites. The mineralogy and chemical composition of tourmalinites point to a metasomatic origin. LANFs, together with high-angle faults, controlled the opening of the Permian Orobic Basin and likely acted as a preferred pathway for hydrothermal fluids that triggered the Boron-metasomatism. Along the Aga-Vedello LANF, tourmalinites appear to have formed after the cessation of fault activity, as no brittle post-metasomatism deformation overprint has been observed. These relationships suggest that the circulation of B-rich fluids occurred after the opening of the Orobic Basin that is broadly constrained to the Early Permian. At the same time, ca. 285–270 Ma, a strong magmatic activity affected all the Southern Alps, ranging in composition from mafic to acidic rocks and from intrusions at deep crustal levels to effusive volcanic products. The Early Permian magmatism was likely the source of the late-stage hydrothermal fluids that formed the tourmalinites. The same fluids could also have played a significant role in the formation of the Uranium ore deposit of the Novazza-Vedello mining district, as the ore bodies in the Vedello valley are concentrated along the basement-cover contact.