ZEOLITE CRYSTAL GROWTH GENERATIONS DURING DIAGENETIC AND HYDROTHERMAL PROCESSES- CASE STUDY OF LACUSTRINE VOLCANICLASTICS, ABU TREIFIYA AREA, CAIRO-SUEZ ROAD, EGYPT

Abstract

ZEOLITES and clay minerals are common authigenic minerals in volcano-sedimentary rock association in the Abu Treifiya basin. They are formed by hydrolysis alteration of basic volcanic glass by reaction with percolating water in a partly closed basin of salinealkaline lacustrine environment. The zeolites and associated clay minerals and calcite are studied and described in detail microscopically and by X-ray diffraction, also occasionally scanning electron microprobe and Raman techniques are applied. The zeolite mineral species of phillipsite, thomsonite, tobermorite, chabazite, natrolite and analcime are recorded and described. Palagonitization and alteration of volcanic glass result in three stages of authigenic mineral formation; 1) clay minerals as smectite, chlorite/smectite (mixed layer) and chlorite, 2) zeolite minerals, and 3) finally calcite. These associations are found to form in two paragenetic phases: A) diagenetic phase, represented by clay minerals, followed by phillipsite or thomsonite that may be overgrown by phillipsite, filling interstitial spaces, vesicles and amygdales in mostly all facies types of the volcano-sedimentary succession. These resulted from volcanic glass alteration during increasing pH, alkalinity and salinity of the environment. Analcime granular crystals form in lapilli tuff and hyaloclastite tuff in higher Na rich saline -alkaline environment at extreme palagonitization, and B) hydrothermal phase represented by open space filling of cross cutting fractures and/or manifested by formation of tobermorite spherules along basalt lacustrine carbonate contacts. Large vugs exhibit sequential crystallization of Ca zeolite thomsonite spherules followed by calcsodic zeolite Cachabazite; with decreasing temperature and increase in Na in the fluid Na rich fibrous natrolite crystallize. Hydrothermal veins are filled by large euhedral Nachabazite associated with chalcedony, Feoxyhydroxides and clays.