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

The 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 multi-variate data analyses, were employed to analyse high-resolution geochemical profiles (> 360 samples) of ther-mally 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 ana -lyses 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 = similar to 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 dis -solved 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.

Automated summary

This study investigates the syngenetic enrichment pathways of V in black shales from the Baltic Palaeobasin, revealing the essential role of the fine mineral fraction in V hyper-enrichment. The results suggest that V is captured via reduced V(IV)-organic complexes adsorbed to fine clay particles, with the specific surface area of clay particles and particle flux to the sea bottom being the primary factors defining the degree of V hyper-enrichment.