Petrography and geochemistry of Eocene limestone from Khashm Al-Raqaba area, El-Galala El-Qibliya, Egypt

The Khashm Al-Raqaba area lies in the central part of El-Galala El-Qibliya Plateau (Southern Galala, Egypt). It contains a huge reserve of hard crystalline light-colored limestones. The studied limestones are differentiated into three lithofacies as follows: Nummulitic-biomicrudite facies (packstone), Nummulitic-bioclastic micrite facies (wackestone) and fossiliferous micrite facies (wackestone). The rocks were subjected to extensive diagenetic processes with varying intensities, the most important of which are compaction (chemical and mechanical), neomorphism, internal filling and stylolitization. X-ray diffraction analysis of the studied limestones revealed that calcite is the main carbonate mineral in the studied rocks and is considered as low-magnesian type. Quartz and clay minerals are the main non-carbonate minerals concentrated along the stylolite seams as insoluble residues, while kaolinite, montmorillonite, and illite are the main clay minerals forming the joint-filling materials. The chemical analysis of the limestone samples revealed high calcium content, low magnesian content, low content of acid insoluble residues, and consequently low SiO2 and Al2O3 contents. Very low contents of total Na2O and Cl indicate a contribution of freshwater. The low value of total Sr content in the selected rock samples may indicate depletion of Sr during diagenetic processes.