Geochemical and microbial indicators of smectite and zeolite formation in volcanic tuff from Pohang, South Korea

Smectite- and zeolite-dominated assemblages occur at different depths within the same volcanic tuff sequence at the study site, indicating variability in post-depositional alteration conditions. The present study investigated the geological and environmental factors associated with the vertical mineralogical differences using outcrop and core samples from Pohang, South Korea. Mineralogical, geochemical, thermal, spectroscopic, and microbial analyses were conducted on representative samples. The outcrop samples contain Ca-smectite, cristobalite, and amorphous aluminosilicates, whereas the core samples contain zeolite (clinoptilolite and mordenite), quartz, and feldspar. (Na,Ca)-smectite occurs only at specific depths within the core. Major- and trace-element geochemistry indicates that the outcrop and core samples were derived from rhyolitic and andesitic precursors, respectively. Chondrite-normalized rare earth element patterns show no evidence of hydrothermal enrichment or depletion, suggesting diagenesis as the dominant alteration process. Bacterial community compositions, used as environmental indicators, indicate contrasting formation environments: the outcrop samples represent anaerobic, freshwater conditions, whereas the core samples reflect aerobic and saline conditions. Mössbauer spectra independently support these redox differences, showing structurally bound Fe within smectite in the outcrop sample and hematite-magnetite assemblages in the core samples. These results indicate that variations in precursor composition, salinity, and redox conditions were closely associated with the development of contrasting smectite- and zeolite-bearing assemblages within the same volcanic sequence.