<p>The formation processes and quality of natural zeolite deposits in the Guryong mine, located within the Miocene Janggi Basin of southeastern Korea, were investigated using a multi-technique approach. Bulk-rock geochemistry (ICP-OES/MS), mineralogical semi-quantification (XRD with Rietveld refinement), high-resolution microtextural analysis (FE-EPMA), mass-balance modeling (isocon method), and cation exchange capacity (CEC) measurements reveal that zeolitization of the dacitic Nultaeri Tuff occurred within an oscillatory diagenetic system characterized by alternating open- and closed-system conditions driven by rapid tectonic subsidence in a pull-apart basin. Clinoptilolite formed predominantly during semi-closed phases marked by restricted fluid exchange and K-Na accumulation, whereas mordenite crystallized during open-system episodes associated with the influx of Ca-rich external fluids. Microtextural evidence further indicates a late-stage alteration event that produced secondary clinoptilolite through interface-coupled dissolution-reprecipitation and generated smectite under conditions of enhanced meteoric flushing. Geochemical and mineralogical data demonstrate that zeolite quality is strongly controlled by both the initial heterogeneity of the volcanic precursor and the intensity of subsequent water-rock interactions. Notably, Na-dominant zeolites exhibit CEC values that are 6%–12% higher than those of Ca-dominant varieties. Furthermore, the systematic depletion of heavy rare earth elements (HREEs) serves as a sensitive indicator of flushing intensity, suggesting that stratigraphic intervals less affected by intense meteoric influx are more favorable targets for high-quality ore exploration. These findings provide a new framework for evaluating zeolite deposits formed in dynamic diagenetic environments.</p>