Fine clay shuttle as a key mechanism for V hyper-enrichment in shallow water Tremadocian black shale from Baltica

he essential role of the fine mineral fraction in V hyper-enrichment in black shales has been suggested in previous studies; however, ‘the operational details’ of such metallogenic systems remain poorly understood. This study addresses the syngenetic enrichment pathways of V in black shales from the Lower Ordovician in the inner shelf of the Baltic Palaeobasin, showing very high content of redox-sensitive elements, including that of V. X-ray fluorescence spectroscopy (XRF) and organic element analysis (CHNS-O), combined with bivariate and multivariate data analyses, were employed to analyse high-resolution geochemical profiles (> 360 samples) of thermally immature black and grey shales, as well as black shale containing siltstones, from two drill cores from NE Estonia. The detected V (up to 2300 ppm), Mo (up to 4500 ppm), and U (up to 500 ppm) enrichment lacked well-defined co-variance, indicating a complex redox environment and element-specific enrichment pathways. While the maximum V and Mo concentrations were confined to the basal part of the black shale–black shale complex, V hyper-enrichment was also observed in the Mo-U-poor black shale interbeds in the siltstones. Co-variance analyses in metalliferous horizons revealed strong positive relationships between V and organic matter, Al, Ti, and K. Cr, characterised by a high affinity for natural sorbents, demonstrated an almost perfect correlation with V (r = ∼95), despite its low concentrations. These patterns are hypothesised to reflect V capture via reduced V(IV)-organic complexes adsorbed to fine clay particles after reduction of V(V) in the seawater column rich in dissolved organic matter. These data suggest that the specific surface area of clay particles acted as the primary limiting parameter for V capture, and the particle flux to the shallow sea bottom defined the degree of V hyper-enrichment in the studied settings.