Occurrence, Chemical, and Pb-isotopic Composition of Uraninite: A Verification of the Effective Source of Uranium in a Granite Aquifer of the Daejeon Area, South Korea

Daejeon is well-known in South Korea for having the highest concentrations of uranium in the Jurassic granite aquifer. This study reports the texture and chemistry of uraninite and discusses the potential of uraninite as an effective source mineral of U in granite aquifers based on uraninite dissolution and U-migration. Its texture and mineral chemistry were examined using FE-SEM and EPMA, respectively, to investigate the occurrence of uraninite. The Pb isotope ratios of the minerals in granite were tested to verify their potential as geochemical tracers for elemental migration. The Pb isotope ratios were obtained using a SHRIMP IIe/MC ion microprobe. Uraninite occurred with muscovite and pyrite as large grains of up to 500 μm in length in the altered granite. Coffinite, a secondary U-mineral, occurs in the cracks within uraninite. The uraninite U content ranged from 79.12 to 89.05 wt. % and uraninites have major impurities of Th, and Pb ranged from 1.82 to 3.49 and 2.13 to 2.42 wt. %, respectively. The breakdown of uraninite may result in the redistribution of U and Pb and facilitate U-migration during surface weathering. The Eh–pH conditions of groundwater are suitable for the dissolution of uraninite, and the uranyl carbonate complex ion (UO₂(CO₃)₂²⁻) is the dominant U-species in the study area. The Pb isotope ratios indicated that uraninite contained radiogenic Pb, feldspar contained common Pb, and pyrite contained a mixture of both components. These Pb isotopic signatures indicate that the radiogenic Pb in uraninite has migrated from uraninite to pyrite. Considering the relatively high mobility of U compared to Pb, the migration of radiogenic Pb from uraninite to pyrite suggests that U can also migrate from uraninite to its surroundings when a reaction with groundwater occurs. Based on hydrological conditions and the Pb isotope signature, uraninite is considered a substantial and compelling host mineral for interpreting a source of uranium in the granite aquifer of the study area.