Zeolite Minerals from Wat Ocheng, Ta Ang, Ratanakiri Province, Cambodia – Occurrence, Composition, and Paragenesis

Abstract

Located in the southwest corner of the Ratanakiri Volcanic Province, the Wat Ocheng basalt is the first known zeolite locality in Cambodia. The basalt is a fine-grained, vesicular to amygdaloidal, subalkaline to transitional alkaline intraplate tholeiite comprised of 30% lath-like plagioclase (average: Ab51An45Or4), 35% interstitial augite (average: Wo44En35Fs21), 25–30% zeolites after plagioclase and volcanic glass, and minor skeletal ulvöspinel. It contains mineralized amygdales ranging in size from 0.5 × 0.5 cm to 5 × 9 cm. Nine zeolite species occur at Wat Ocheng, including analcime, chabazite-Ca, gonnardite, natrolite, phillipsite-Ca, and thomsonite-Ca, along with clays, aragonite, calcite, and pyrite. All the zeolite species are being described from Cambodia for the first time. The zeolite and secondary mineral assemblages observed at Wat Ocheng are similar to those reported from other alkaline basalt localities, including those in neighboring Vietnam, and are a product of alteration of the primary Ca-Na minerals and volcanic glass as a result of burial metamorphism and infiltration of heated meteoric waters. The mineral assemblage is not homogeneous across amygdales within the exposed lava flow, suggesting localized closed systems, likely the result of early precipitation of clay minerals and fine-grained zeolites. Decreased porosity and differences in fluid geochemistry would account for the diversity in the observed assemblages. Four stages of hydrothermal alteration and zeolitization have been defined based on mineral textures and chemistry. Zeolite formation began with fine-grained Ca- (chabazite-Ca and phillipsite-Ca) and Na- (analcime) dominant, high TSi (Si/Si+Al) species in Stage II following deposition of clay minerals in Stage I. Stage III is characterized by increasing Na+K contents and decreasing TSi. Crystallization of coarse-grained chabazite-Ca and phillipsite-Ca with increasing Na+K contents in the rims of the crystals followed the development of natrolite with a later-stage epitaxial overgrowth of thomsonite-Ca. The final stage of mineralization (Stage IV) included late-stage calcite, pyrite, and termination of growth of acicular sprays of thomsonite-Ca. Post-magmatic cooling and circulation of meteoric water and fluids derived from alluvial sediments overlying the basalts were involved in zeolitization. Thermal sources include an underlying basaltic andesite flow as well as regional deep-seated, extensional pull-apart structures, the result of a thinned lithosphere and injection of fertile mantle following the collision of the Eurasian and Indochina plates during the Himalayan Orogeny.