Post-collisional tectono-magmatic evolution, crustal fertilization, and ore genesis: the late Variscan metallogenic stages in Sardinia

The Variscan post-collisional tectonic and magmatic events were critical for ore deposits in the Paleozoic massifs of Europe. In Sardinia, two main types of Variscan ore deposits can be recognized: (1) “orogenic”, structurally-controlled deposits associated with shear zones and extensional structures; (2) granite intrusion hosted/related deposits. Both types are unevenly distributed in the Paleozoic basement and in the Sardinian batholith, being much more abundant in Southern Sardinia. “Orogenic” deposits include As-Au±Sb ± W and Zn-Cu-Pb-Ag vein systems hosted in greenschist facies metamorphics. They result from multiple mineralization stages from fluids of mixed (but dominantly metamorphic) origin at different structural levels in the nappe stack of the Variscan front, during the post-collisional extension. Large-scale antiformal structures were traps for fluids; base metal and As-Au-rich deposits prevail in deeper structural zones, whereas Sb-W (Au) deposits are related with shallower extensional structures. Granite-hosted/related deposits consist of Mo ± Cu greisens, Pb-Zn-Cu-Fe±Sn± W±Bi±F and Pb-Zn-Cu- Fe±REE skarns, and W-Sn-Mo-Bi (±Te±Au) and Pb-Zn-F-Ba veins, also associated in magmatic-hydrothermal systems. Most intrusion-hosted/related ores may be bracketed into the latest and narrow (290-285) Ma growth stages of Sardinian Batholith. They are more abundant in southern Sardinia, associated to multiple suites of F-rich granites, including a suite of Sn-W-Mo (Bi, In)-bearing, “rare metals” granites belonging to an ilmenite rock-series (Naitza et al., 2017). As evidenced in recent literature (Cocco et al., 2018; Secchi et al., 2022), the Paleozoic basement of Sardinia is made of micro-terranes assembled by large-scale shear zones in the post-collisional Variscan stages, when both “orogenic” and granite-related ores resulted from a wide metallogenic fertilization of the Sardinian crustal segment. These events may be tentatively framed in a context of lithospheric delamination, which triggered production and crustal-scale migration of magmas and fluids along lithospheric shear zones, widely redistributing ore-forming elements, including (a) new contributions linked to partial melting of different crustal levels and to crust/mantle interactions, and (b) widespread recycling of elements present in the pre-Variscan successions (e.g., Pb, Zn, Ba from Cambrian MVT ores of SW Sardinia; REE from Upper Ordovician paleoplacers in Eastern Sardinia). Variscan crustal fertilization was essential for the post-Variscan metallogenic stages (Middle Permian-Triassic?), during which new and widespread fluid circulation in the basement produced, in the context of the breakup of Pangea and the opening of the Tethys (Burisch et al., 2022), different kinds of low-temperature polymetallic hydrothermal ores (Pb-Zn ± Cu±Ag; Ni-Co-As-Bi-Ag; Ba-F ± Pb ± Ag), often directly remobilizing previous magmatic-hosted/related ones.