Mineralogy of Egyptian Bentonitic Clays II: Geologic Origin

Abstract

AbstractReconstructing the origin of bentonitic clays is often a challenging and rather complicated undertaking, but the analysis of certain predictor clay minerals is proving to be an excellent method to simplify this process. The goal of the present investigation was to use abundance changes of five X-ray diffraction (XRD) predictor minerals to determine the relative contributions of weathering and parent-rock changes to the origin of clay minerals in Egyptian bentonitic clays as the test case. The XRD predictor minerals, selected in an earlier discriminant function analysis of quantitative abundances of 14 minerals, provided a simpler approach to the interpretation of clay-mineral origins because they are the minerals that were most responsible for statistically significant differences among the samples. Changes in mineral composition were basically a function of parent-rock lithology, drainage, and climate interactions. A Paleo-Climate Index (CI; the ratio of coarsely crystalline kaolinite to Fe-rich smectite), and a Parent-Rock Index (PI; the ratio of the illitic phases and quartz abundances to pure smectite) were established to track the paleo-climate and parent-rock changes, respectively. Low CI values indicated that a long, seasonally dry climate prevailed during the Middle Eocene, uppermost Eocene, Lower Miocene, and Upper Pliocene bentonitic clay deposition. Lowermost Upper Eocene and the Middle Miocene bentonitic clays were produced when a wet climate prevailed throughout the year. Moderate to high PI values suggested derivation of the clays from the acidic basement crystalline rocks at Uweinat-Bir Safsaf uplift and Lower Paleogene shales during the Middle Eocene and lowermost Upper Eocene. The youngest Upper Eocene and Lower Miocene materials contained abundant Fe-smectite and low PIs indicating derivation from tholeiitic basalts. Diagenetic and sedimentary segregation modifications were not apparent. Direct evidence for in situ derivation from volcanic precursor materials was lacking in general, but volcanic eruptions were common in the region. The minerals in the Egyptian bentonitic clays formed as weathering products on land and have been transported by north-flowing streams and rivers to the sites of accumulation.