Key factors controlling late Mesozoic uranium mineralization potential in the Luzong basin, South China

The late Mesozoic lithologic associations—comprising syenite, quartz syenite, and K-feldspar granites—exhibit significant uranium (U) enrichment in the Luzong basin of the Middle−Lower Yangtze River metallogenic belt, South China. Some of these lithologic associations are the primary contributors to several economic sandstone-hosted U deposits and >10 geochemical anomalies, while others are barren or host only subeconomic high-temperature mineralization in the region. Elucidating their petrogenesis and source characteristics is critical for identifying the key factors that control the U-mineralization potential of granites within this backarc extensional setting. Integrated geochronological and geochemical analyses delineate two spatially and geochemically distinct intrusive suites: (1) 133−128 Ma highly fractionated I-type granites, associated with U-barren to subeconomic U-mineralization; and (2) 129−123 Ma A-type granites, exhibiting U-barren to U-bearing characteristics. U-bearing A-type granites are characterized by more pronounced Ba, Sr, and Eu negative anomalies; higher εHf(t) (−7.8 to +5.0); and younger TDMC ages (1.7−0.9 Ga) compared to U-mineralized syenites [εHf(t) = −11.9 to −6.5; TDMC = 2.0−1.3 Ga]. These contrasting features reflect distinct crustal sources: The syenites originated from an enriched mantle that was metasomatized by Neoproterozoic crustal components, whereas the A-type granites were derived from a mixture of the metasomatized lithospheric mantle and juvenile lower crust. U-bearing A-type granites are geochemically distinct, showing higher silicic and alkali contents, with lower whole-rock Sr/Y ratios, V/Sc ratios, and zircon ΔFMQ (deviation from the fayalite-magnetite-quartz oxygen-fugacity buffer) values compared to U-mineralized syenites. These signatures collectively indicate that the U-bearing A-type granites underwent extensive magmatic differentiation under relatively low oxygen fugacity, a condition that promoted U partitioning into late-stage, low-temperature hydrothermal fluids. In contrast, syenites show limited U enrichment potential, which is attributed to their distinct magma sources and incomplete fractional crystallization. The geochemical features in the Luzong basin systematically record the transition from a transtensional to an extensional tectonic regime, and this crustal-extension dynamic serves as a fundamental control on the spatial heterogeneity of U-mineralization patterns. Nevertheless, some syenite intrusions host subeconomic high-temperature U mineralization, likely facilitated by their relatively higher magmatic water contents and oxygen fugacities. Despite sharing overlapping Sr-Nd-Hf isotopes, whole-rock V/Sc ratios, and zircon ΔFMQ signatures with fertile systems, some A-type intrusions remain U-barren, likely due to their small magma volumes and poorly developed fracture systems that inhibit U mobilization. We therefore propose that the primary enrichment of U is governed by the combination of a favorable extensional setting, partial melting at the mantle-crust interface, extreme fractional crystallization, and low oxygen fugacity. This integrated model not only explains the observed U mineralization potential in the Luzong basin but also establishes a predictive framework for U exploration in analogous extensional settings.