Evidence of regional-scale hydrothermalism and polymetallic mineral deposition across northern Italy: the Early Permian to Early Triassic siderite-rich deposits in the southern Alps

The southern Alps in northern Italy host a wealth of siderite-rich orebodies forming a mineralized belt from western Lombardy to the Venetian Alps. Mineralization occurs as veins rooted in the greisenized cupolas of late Variscan granites as well as crosscutting the Variscan basement and the overlying Lower Permian volcano-sedimentary cover (Collio Fm); and as both veins and conformable stratabound orebodies in the Upper Permian (Verrucano Lombardo and Bellerophon Fms) and Lower Triassic sedimentary sequences (Servino and Werfen Fms) of the Lombardian and Venetian Alps. Accessory siderite is common also in the fault-controlled uranium mineralization hosted by the Collio volcanoclastic units in the Orobic Alps (Novazza-Val Vedello deposits). In the deposits siderite is a major component, often associated with dolomite and calcite, in highly variable assemblages encompassing quartz, baryte, fluorite, bitumen, oxides (wolframates, cassiterite), Cu-Zn-Pb-Fe-Mo-Hg sulfides, Cu-Bi-Ag-Co-Fe-As-Sb sulfosalts and native Bi and Au. Local geological settings, assemblages and ore textures are characteristic of polyphase hydrothermal deposition from hypo- to epithermal conditions. When applicable, geothermometry based on coexisting siderite, Fe-rich dolomite and calcite provided crystallization temperatures ≥ 250°C. However, all types of deposits show similar majorelement and REE patterns, suggesting a common mineralizing event. New and previously published δ13C and δ18O isotope data on southern Alpine siderite display signatures according to local lower to higher temperature conditions. But geochemical modelling, supported by REE and stable isotope analyses and mineral chemistry, also suggests that the hydrothermal fluids depositing Fe-carbonates in the basement and in the overlying Lower Permian cover were primarily derived from freshwater, which leached Fe and C from volcanic rocks and organic carbon-bearing continental sediments. U-Th-Pb microchemical dating of uraninite associated with siderite in the Novazza-Val Vedello deposits constrains the onset of hydrothermalism to 275 ± 13 Ma (Early–Mid Permian), coeval to the plutonism and the volcanic-sedimentary cycle occurring in the same area (Orobic Basin). In the youngest Fe-mineralizing stage, represented by orebodies in Lower Triassic shallow-marine limestones, the siderite-forming fluids contained a seawater component, interacted with the underlying Permian units and eventually replaced the marine carbonates at temperatures ≥ 250°C. The absence of siderite in younger rocks suggests an Early Triassic upper limit for the iron pulse in the southern Alps, which thus predates the Middle Triassic magmatism. Based on the overlap between hydrothermalism, extensional tectonics and, in part, magmatism, the genesis of siderite in the southern Alps may be related to plutonic activity and/or magmatic underplating occurring since the Permian in a geodynamic scenario preluding the opening of the Neo-Tethys ocean.