Relationships between uranium occurrence and pyrite in Yimin Formation in the Hailar Basin

Abstract The sandstone uranium deposits in the Kelulun Depression are the first commercially viable uranium deposits discovered in the Hailar Basin and the ore-bearing strata corresponding to the Lower Cretaceous Yimin Formation. To elucidate the origins and formative mechanisms of pyrites and investigate their relation to uranium mineralization, both the characteristics of pyrites and uranium minerals and the S isotope and the trace element composition of pyrites were investigated. Results indicated that coffinites (as cements and star-like clusters) are the most common uranium mineral type, followed by pitchblendes and U-Ti oxides. Pyrites are mainly framboidal, cement, or euhedral. S isotope fractionation in the pyrites varies due to different S sources (biogenic or abiotic), with framboidal, cement, and euhedral pyrites showing δ 34 S values of -63.70 to -3.38‰, -30.26 to 9.52‰, and 3.32 to 8.27‰, respectively. As and Tl enrichment indicates formation in a low-temperature environment. High Ca and P levels in the uranium minerals are indicative of microbial participation. The carbonaceous clasts, pyrites, and microbes of the Yimin Formation in the Kelulun Depression played synergistic roles in uranium mineralization, which resulted in highly diverse uranium ore occurrences.