Vanadium and other critical elements in North Gondwana Silurian black shales: The SE Sardinia (Italy) case

Vanadium currently represents a critical metal for the global economy, due to its increasing application in low CO2-emission technologies such as vanadium redox flow batteries. Black shale-hosted deposits cover about 80 % of the potential global V resources, although only <2 % of them is economic. During Silurian, significant black shale successions were deposited under strong anoxic conditions in the North Gondwana. This study aims at investigating the deportment of V in association with other trace elements in Silurian black shales of SE Sardinia (Italy), for understanding the nature of the depositional environment and the metallogenetic potential of these rocks at regional scale. Sardinian black shales have variable V amounts, ranging from hundreds to thousands ppm and organic carbon contents from 0.7 to 10.1 wt%. These values, together with observed Th/U, Ni/Co, V/Cr ratios indicate that the initial sedimentary environment was clearly anoxic. The positive correlation between V and total organic carbon indicates that organic matter retains most of V. Local V concentrations in rutile and illite may derive from metal remobilization during diagenesis and low-grade metamorphism. Compared to other coeval Silurian successions of North Gondwana, Sardinian black shales have anomalous values of Sb, Ba, Se, Mo, As, V, and Ag, this suggesting that, in the frame of a peculiar sedimentary process occurred in a similar way at the global scale, distinct controls existed in the various basins.