Zeolitization of Miocene dacitic tuffs in the Janggi Basin, Korea: influence of depositional conditions and fluid system evolution

Abstract

Zeolites are sensitive indicators of post-depositional geochemical conditions, commonly formed through the diagenetic alteration of volcanic glass in volcaniclastic rocks. Despite their paleoenvironmental significance, zeolites remain under-utilized in reconstructing depositional systems. This study examines the mineralogical and geochemical characteristics of zeolites in Early Miocene dacitic tuffs from the fluviolacustrine Seongdongri Formation in the Janggi Basin, southeastern Korea. Based on petrographic, mineralogical, and geochemical analyses of samples from four boreholes, we have identi-fied three zeolite species-mordenite, clinoptilolite, and analcime-with distinct vertical and lateral distributions. In Unit 1, which formed under subaerial settings, mordenite developed early along vesicle margins. Subsequent diagenetic path-ways diverged, with clinoptilolite forming in the eastern boreholes under relatively open fluid systems, while the western boreholes experienced smectite-dominated alteration under more restricted conditions. In the overlying Units 3 and 4, deposited in subaqueous settings, clinoptilolite became the dominant zeolite, reflecting enhanced glass dissolution and fluid exchange under water-laden conditions. Analcime appears exclusively in the western boreholes and is interpreted as a late-stage product of Na-rich fluids, likely related to either hydrothermal input or direct precipitation from porewaters. These findings demonstrate that volcanic deposits of similar composition can develop markedly different zeolite assem-blages depending on local depositional environments and hydrological regimes, underscoring the potential of zeolites as paleoenvironmental indicators in altered volcaniclastic deposits.