<p>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 (~ 5–40&#xa0;m), whereas bentonite layer and lapilli tuff samples were collected from outcrops where prolonged water–rock interactions may have influenced mineralogical alteration. In the &lt; 75&#xa0;µm 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 &lt; 2&#xa0;µm size-fraction (group C), montmorillonite from B0 and B1 had Si-dominant tetrahedral sheets and octahedral sheets with Al &gt;&gt; Mg &gt; Fe. In montmorillonite from B2, Al partially substituted Si in tetrahedral sheets and octahedral sheets contained Al &gt;&gt; Fe &gt; 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 <i>δ</i>¹⁸O and <i>δ</i>²H values of montmorillonite indicate formation from freshwater at low temperatures. Montmorillonite in B0 and B1 showed slight enrichment in ¹⁸O and ²H 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.</p>