Geochemistry of U and Th of Mesozoic granites in South Korea: implications of occurrences of different U-host minerals and dissolved U and Rn between Jurassic and Cretaceous granite aquifers

Rn concentrations are generally high in the groundwaters of Mesozoic granite area in Korea. However, those groundwaters generally do not show a close relation between Rn and U concentrations even though Rn is a daughter nuclide of U. For example, both U and Rn concentrations are very high in the Jurassic granite of Daejeon-Cheongju area (Jgr-DJ), while only Rn concentrations are high in the Cretaceous granites of the Ogcheon belt (Cgr-OB). This study was initiated to explore the cause of this mismatch between these two different geological settings. For this study, we reviewed all data for the U and Rn concentrations in groundwaters, which had been reported since 1996 by the South Korean governmental authorities. The whole rock geochemical data for the Korean Mesozoic granites were also reviewed to discuss the U-enrichment processes in these two granites. This study found out that the difference in the U-host minerals is the essential reason for the differences in the occurrences of dissolved U and Rn in groundwaters of both geological settings. The geochemical data strongly suggest that Jgr-DJ is the major source of U and Rn in groundwater. However, Cgr-OB rarely provide U species to groundwater even though it acts as the substantial Rn source. This discrepancy is found to be the results of the differences in U-host minerals in each Mesozoic granite and their solubility. Monazite is the major U-bearing mineral in Cgr-OB. Its solubility is very low and, thus, cannot provide aqueous uranium species to groundwater even though it can emit radon gas to groundwater. In Jgr-DJ, uraninite, which can be easily oxidized to highly soluble ions and complexes, is the main U-host mineral; thus, it can cause high Rn and U concentrations. The cause of U-host mineral contrast between Cgr-OB and Jgr-DJ can be explained by the difference in their U-enrichment processes, such as primary magmatic crystallization and secondary hydrothermal alteration in each granite.