Silurian heritage in black shales of North Gondwana: A deeper insight into mineralogy and geochemistry of Southeast Sardinia (Italy) vanadium-bearing shales

During Silurian, significant black shale successions, locally containing variable amounts of vanadium and other trace metals, were deposited under anoxic basinal conditions in the North Gondwana realm. With only few exceptions, there are no specific studies dedicated to vanadium mineralogy and geochemistry in the Silurian black shales of the Gondwana supercontinent, which is the aim of this research. Since Southeastern Sardinia (Gerrei, Italy) represents one of the northern Gondwanan regions in which Silurian black shales occur, the present study aimed at providing a comprehensive mineralogical and geochemical characterization of vanadium-bearing phases, together with other geochemically significant trace elements, in order to constrain their enrichment within this specific depositional setting and evaluate their potential economic relevance. In addition, this study seeks to place the Sardinian black shale successions within the broader framework of Silurian Gondwanan evolution and to investigate the origin of their distinctive geochemical features, with particular emphasis on vanadium enrichment. Vanadium in the Gerrei shales is mainly hosted in rutile, illite and organic matter. Maximum content in illite is ca. 3 wt% V 2 O 3 , and in rutile ca. 2 wt% V 2 O 3 . The current study confirms the positive relationship between organic carbon and vanadium, similarly to other cases of world-known black shales. In fact, most analyses have shown the occurrence of V (mean 0.70% V 2 O 3 ) and other trace elements (Cr, mean 0.26% Cr 2 O 3 ; Cu, mean 0.55% CuO; Ti, mean 2.00 % TiO 2 ; Zr, mean 0.31 % ZrO 2 ) in the carbonaceous matter, where sulphur has also been consistently detected. Combined mineralogical and geochemical analyses have demonstrated that vanadium, which was originally assimilated by organic matter, may have been partly released during diagenesis and Variscan metamorphism, and then migrated into phyllosilicates and Ti-oxides. Moreover, the content in vanadium and other metallic elements in the Silurian black shales, is variable throughout SE Sardinia. Maximum concentrations (up to 2,500 ppm V) occur in the shales where the anoxic signature is stronger, whereas lower amounts (up to a maximum of 650 ppm V) have been measured in areas dominated by dysoxic to oxic depositional conditions, where the carbonatic and detrital input was substantially higher. Given the growing global demand, among critical metals, for vanadium, the presented research has implications on definition of V concentration and deportment mechanisms in of V in these Silurian black shales, which could result in a promising, still unexploited resource. The study highlights the exploration potential of lower Paleozoic, anoxic sedimentary basins, that have so far been considered mainly as source rocks for hydrocarbons, but earned only a limited attention in vanadium prospecting programs.