The Ni-Co-As-Ag-Bi-rich hydrothermal vein ores of the Arburese district (SW Sardinia, Italy): mineralogical and geochemical characterization of possible sources for critical metals

The southern Arburese region (SW Sardinia) hosts a wide variety of ore deposits of historical importance but sofar rather poorly known. The deposits range from Pb-Zn skarns to Fe-As, Sn, Pb-Zn and Ni-Co-As-bearing hydrothermal veins. They are emplaced in an Ordovician/Devonian very low grade metasedimentary sequence (pelites, black shales and limestones) belonging to the Variscan Foreland, along a mineralized corridor close to the contacts with the late-Variscan Arbus and Monte Linas igneous complexes. Among them, a Ni-Co-As-Ag-Bi-rich vein system may be traced for over 6 km in E-W direction, across the Sa Menga, Acqua Is Prunas, Fenugu Sibiri and Pira Inferida abandoned mines. The Ni-Co system crosscuts the As-rich Punta Santa Vittoria vein system. Ore petrography and microchemical analyses were carried out to define assemblages and main geochemical signatures of the different orebodies. The Ni-Co-As veins consist of dominant Ni-Co sulfarsenides and arsenides with varying amounts of Pb-Cu-Zn sulfides and sulfosalts (galena, transparent Cd-rich sphalerite, tetrahedrite, chalcopyrite, bournonite) in quartz-siderite gangue cementing pelitic/black shale breccia. Mineralization is polyphasic and displays textures with centimetric spheroidal nodules composed of niccolite-breithauptite solid solution or sphalerite cores including droplets of native Bi, and overgrown by concentric crusts of sulfarsenides, diarsenides or sulfides. Ag is enriched in tetrahedrite and acanthite, while Au blebs are disseminated in the external sulfarsenide crusts. Major sulfides contain detectable amounts of selenium. The Punta Santa Vittoria As-rich vein system consists of arsenopyrite in highly strained quartz gangue with minor chalcopyrite, Fe- and Cd-rich sphalerite and inclusions of accessory native Bi, pyrrhotite, cubanite, bismuthite, Se-rich galena and Pb-Ag-Bi-Se sulfosalts. As, Se, Bi and Ag contribute to link these two ore systems distinguished by mineralogical features and geothermometry. Textures and temperature estimates based on sphalerite and arsenopyrite-pyrrhotite assemblages suggest rather high crystallization temperatures, up to 450 °C, for the Fe-As vein. A lower range, close to 250 °C, is suggested for Ni-Co-As mineralization, based on cockade textures, fluctuating Co/Ni ratios in the (Ni,Co)As nodules and low-Fe sphalerites, compatible with preliminary microthermometric analyses. C-O stable isotope analyses on siderite gangue confirm this thermality and suggest deposition in relatively reducing conditions. Ni-Co-As ores of SW Sardinia are analogous to the uncommon and strategically interesting Five Elements Veins, Ni-Co-As/ Sb-Ag-Bi, type deposits. Possible sources of metals, especially Ni and Co, are discussed in reference to the Variscan igneous complexes. Moreover, the occurrence of Se-bearing minerals in this part of Sardinia may represent a direct link with late-Variscan Se-rich ores of SE Sardinia (e.g. Baccu Locci mine, Gerrei district).