Geochemistry and uranium mineralogy of the Black slate in the Okcheon Metamorphic Belt, South Korea

Black slate in the Okcheon Metamorphic Belt (OMB) has been considered the perspective uranium ore in South Korea. In this study, three representative areas were selected for better understanding on the uranium-mineralogy and geochemistry of black slate in the OMB. It is found that the OMB black slate is highly enriched with metals such as U (∼0.03%), V (∼0.3%), Ba (∼1.4%) and Mo (∼0.04%). It is also notable that an average content of total organic carbon (TOC) is as high as 21.2%. Upon relationships between metals and TOC, Ba shows a weak correlation with TOC whereas the correlation is not clear for U, Mo, Cu, P, and V. In the comparison of the study areas, the average concentrations of metals such as Ba, U, and Mo and organic carbon decrease in the order of Goesan > Geumsan > Miwon. Four uraniumbearing minerals in the OMB black slate are identified. Tetravalent uranium (U⁴⁺) minerals such as uraninite and brannerite are found, whereas francevillite and Ba-bearing torbernite containing hexavalent-uranium (U⁶⁺) are observed. It is shown that uraninite and brannerite are less than 100 μm in dimension. In contrast, the size of francevillite or torbernite is larger, and the scaly texture with cracks is shown in their morphology. We note that uraninite is the most common uranium mineral throughout the black slate bed in OMB. It is demonstrated that the formation of U-bearing minerals in the bed is greatly affected by diversity of geochemical environments in the OMB, Korea.