Geochemical, mineralogical, and isotopic characteristics of bentonite and related rocks under near-surface conditions in the Naah mine, South Korea

초록

This study investigated the geochemical, mineralogical, and oxygen- and hydrogen-isotope characteristics of bentonite and dacitic lapilli tuff from the Naah mine, South Korea, focusing on the extent of bentonitization under near-surface conditions. A bentonite ore sample was obtained from greater depths (similar to 5-40 m), whereas bentonite layer and lapilli tuff samples were collected from outcrops where prolonged water-rock interactions may have influenced mineralogical alteration. In the < 75 mu m size-fraction (group B), montmorillonite dominated in the bentonite ore (B0) and near-surface bentonite (B1), whereas the lapilli tuff (B2) mainly contained plagioclase and opal-CT with minor montmorillonite. In the < 2 mu m size-fraction (group C), montmorillonite from B0 and B1 had Si-dominant tetrahedral sheets and octahedral sheets with Al >> Mg > Fe. In montmorillonite from B2, Al partially substituted Si in tetrahedral sheets and octahedral sheets contained Al >> Fe > Mg. Major element ratios, the Mineralogical Index of Alteration, and rare earth element distributions suggest that B1 experienced a higher degree of bentonization than either the B0 or B2. The delta O-18 and delta H-2 values of montmorillonite indicate formation from freshwater at low temperatures. Montmorillonite in B0 and B1 showed slight enrichment in O-18 and H-2 compared to modern spring and shallow groundwater, suggesting early diagenetic formation and/or partial isotopic re-equilibration. The results indicate that the inherent properties of montmorillonite did not change significantly, despite prolonged exposure to the near-surface environment at this locality. The absence of secondary weathering minerals in the B1 further suggests that continued, albeit limited smectitization rather than weathering-driven mineral decomposition was the dominant alteration process in the near-surface environment at this locality.

키워드