Euxinic condition and semi-restricted marine environment promote vanadium mineralization in the marine shale at East Kunlun

Vanadium accumulation in marine shale is significantly controlled by both redox conditions and the degree of basin restriction. However, the favorable conditions for vanadium mineralization in marine shales have not been well determined, limiting our understanding of mineralization mechanism of the redox-sensitive elements. The East Kunlun region was situated along the evolving Paleo-Tethys Ocean during Late Carboniferous, where the Haoteluowa Formation deposited abundant vanadium-rich marine shale, forming typical black shale-hosted vanadium deposits. This study investigates the Haoteluowa Formation in East Kunlun and the primary marine shale in the global to confirm the favorable redox conditions and degree of basin restriction for vanadium mineralization, using sedimentary sequence stratigraphy, petrography and mineralogy, framboidal pyrite morphology, and in-situ sulfur isotopes of pyrite. In the Haoteluowa Formation, vanadium is primarily concentrated in organic-rich shale as V-bearing muscovite. Morphological analysis of framboidal pyrite from the V-bearing shales at East Kunlun reveals a small average grain-size range (4.5–5.5 μm) with a concentrated distribution (standard deviation: 1.63–2.52), which is consistent with the global vanadium-rich shale characteristics. This suggests that vanadium mineralization preferentially occurs under euxinic condition. In-situ sulfur isotopes of pyrite exhibit a cyclical shift (average δ 34 S py from bottom to top: 13.76‰, 8.95‰, 8.16‰, 4.98‰, −3.25‰, 19.62‰, 6.97‰, 13.33‰) revealing that the ocean basin evolved from semi-restricted to open, and then back to semi-restricted conditions during the Late Carboniferous. The vanadium-rich intervals consistently correspond to semi-restricted basins. Moreover, global vanadium-rich marine shale shows isotopic compositions consistent with those of the Haoteluowa Formation, suggesting that vanadium preferentially accumulates under the semi-restriced basin condition. Semi-restricted basins, characterized by an efficient vanadium supply, internal sulfur cycling, Fe-S coupling, organic matter burial, and sustained euxinic conditions facilitate substantial vanadium fixation and accumulation. Under such conditions, vanadium in the East Kunlun marine shale is efficiently accumulated to form deposits through reductive precipitation, complexation with organic matter, and binding with reduced sulfur species.