Multistage uranium mineralization in organic-rich mudstone: A case study of the Qianjiadian uranium deposit in the Songliao Basin, China

The two stages of uranium mineralization were identified in organic-rich mudstone. The early-stage uranium mineralization was related to supergene fluids, while the late-stage uranium mineralization was associated with hydrothermal fluids related to basic dikes. • Two stages of uranium mineralization were identified in organic-rich mudstone. • Early-stage uranium mineralization was related to supergene fluids. • Late-stage uranium mineralization was associated with hydrothermal fluids related to basic dikes. The Qianjiadian uranium deposit in the southern Songliao Basin provide a nice case to reveal uranium mineralization in organic-rich mudstones. Uranium mineralization occurs in the upper and lower parts of the mudstones which is adjacent to sandstone-hosted ores. The mineralized grey mudstones are primarily characterized by colloform pyrite, illite, kaolinite, ankerite, pitchblende, coffinite, and uranium-bearing heavy minerals, whereas the non-mineralized grey mudstones are distinguished by framboidal pyrite, and siderite. Formation of uranium minerals from early to late is as follows: pitchblende Ⅰ or coffinite Ⅰ → colloform pyrite → pitchblende Ⅱ or coffinite Ⅱ. P 2 O 5 contents of the mineralized grey mudstones are higher than that of the non-mineralized grey mudstones, indicating that phosphorus enrichment is accompanied by the uranium mineralization in organic-rich mudstones. Average ratio of V/(V + Ni) in mineralized grey mudstones is lower than that in non-mineralized grey mudstones, suggesting that the fluids of uranium mineralization in organic-rich mudstones were oxidized. Compared with non-mineralized grey mudstones, REE in mineralized grey mudstones is depleted, indicating that uranium mineralization in organic-rich mudstones may be affected by hydrothermal fluids related to basic dikes. The pitchblende Ⅰ and coffinite Ⅰ were formed in the supergene fluids, and the pitchblende Ⅱ and coffinite Ⅱ occurred in the hydrothermal fluids related to basic dikes. This study reveals that the uranium mineralization in organic-rich mudstone may be related to multi-stage ore-forming fluids.