Chemical composition of granitic rocks and constituent minerals in the Eumseong Basin and adjacent areas, South Korea: Implications for weathering and groundwater chemistry

Abstract

Abstract The chemical composition of bedrock and rock-forming minerals can influence both rock weathering characteristics and groundwater chemistry through water–rock interaction. Although plagioclase and biotite have both been widely studied in relation to these processes, detailed quantitative assessments based on their chemical composition have been limited. Therefore, this study investigated the influence of mineral chemistry on groundwater and weathering processes using whole-rock and mineral chemical analysis. The Eumseong Basin and adjacent areas in South Korea, where granitic rocks are widely distributed, were selected as the study area. Rock samples were analyzed using microscopic observations, modal analysis, and whole-rock and mineral chemical analyses. Granitic rocks in the study area have low levels of plagioclase, which itself exhibits a higher Na/Ca molar ratio, indicating that these rocks originate from highly differentiated magmas and have greater weathering resistance. However, variation in the chemical composition of biotite indicates that some rocks have experienced advanced weathering and that the characteristics of the source magma are heterogeneous within the study area. The results also suggest that mineralogical characteristics of granitic bedrock aquifers may exert major controls on groundwater chemistry and fluoride concentration through long-term water-rock interaction. This study may provide foundational data for understanding changes in mineral-controlled groundwater in similar lithological settings and may have potential applications in petrography, rock engineering, and atmospheric CO 2 reduction through enhanced rock weathering. Further research incorporating reaction experiments and geochemical modeling is required to quantitatively constrain the chemical change of groundwater and rock weathering processes.