Anthraxolite Evolution and Vanadium Enrichment Mechanism in the Tanjianshan Group, Upper Ordovician in the Northern Qaidam Basin

Abstract

Earlier researchers have examined the interaction between organic matter (OM) and metals, but the paragenesis of anthraxolite and mechanism enrichment of vanadium (V) need further study. Our recent geological field survey discovered that anthraxolite contains a substantial amount of V in the Member “a” of the Tanjianshan Group, Upper Ordovician in the northern margin of the Qaidam Basin. Different organic geochemical studies (fluid inclusions, zircon U–Pb dating, and petrochemistry) were conducted to examine the origin and evolution of anthraxolite, as well as the enrichment of V. The extractable organic matter (EOM) content of anthraxolite is low, while it contains a high amount of non-hydrocarbon and asphaltene. Anthraxolite is over-mature with its equivalent vitrinite reflectance being up to 4.9%. Anthraxolite is sourced from mudstone of Member “a” of the Tanjianshan Group. There were two hydrocarbon charging phases: (i) Late Devonian; and (ii) Carboniferous. Multiple magmatic thermal events took place after the Late Devonian period, resulting in hydrocarbon fluid cracking, metamorphism, and finally the formation of anthraxolite. Marine hot water deposition was irrelevant to enrichment of V. It is inferred that V originated during biological and biochemical processes, then migrated and accumulated with hydrocarbon formation and evolution, and finally enriched in anthraxolite.