Mesozoic magmatic and hydrothermal uranium mineralization in the Huayangchuan carbonatite-hosted U-Nb-polymetallic deposit, North Qinling Orogen (Central China): Evidence from uraninite chemical and isotopic compositions

Abstract

The giant Huayangchuan deposit in the Qinling Orogen is a newly discovered U-Nb-polymetallic deposit mainly hosted in Triassic carbonatite. Petrographic, elemental, and isotopic composition analyses of uraninite were conducted to determine the formation and evolution of this deposit. At Huayangchuan, four types of uraninite are identified. Pristine Ur1 and Ur2 formed at 209 Ma and 131 Ma, representing two episodes of uranium mineralization. The early episode belongs to carbonatitic magmatic uranium mineralization. It was related to a mantle-derived, initially U-rich carbonatitic magma, which has been further enriched in U through a high degree of crystal fractionation. The late episode corresponds with a high-temperature hydrothermal U mineralization that is related to Early Cretaceous granitic intrusions in the region. This high-temperature fluid event was also responsible for the alteration of Ur1 into Ur1alt. The LREE/HREE ratio variation between Ur1 and Ur1alt indicates that this fluid may have been rich in F. The post ore alteration, characterized by the alteration of Ur2 into Ur2alt, was caused by a low-temperature F-free fluid. We propose that the early stage of U mineralization formed under a post-orogenic extensional environment following the collision between the Qinling and South China blocks, and the late stage of U mineralization corresponds to a long phase of lithospheric thinning due to westward subduction of the Paleo-Pacific Plate. Moreover, compared with other carbonatite-related uranium deposits in the world, the high U contents and low Th/U ratios inherited from the subducted oceanic crust may be critical factors that caused the formation of magmatic-origin uraninite in the Huayangchuan deposit.