MINERALOGICAL AND GEOCHEMICL STUDIES ON THE SHALES INTERCALATED WITH THE PHOSPHATE DEPOSITS IN THE NILE VALLEY, EGYPT.

Abstract

The phosphate-bearing (Duwi) Formation in the Nile Valley district that outcrops between Wadi Qena to the North and Idfu to the South is made up of three Members namely, Mahamid, Sebaiya and Adyma Phosphate Members. Each of which is composed of a number of phosphate and phosphatic beds intercalated with shales, marls, cherts, limestones, sandstones and oyster limestones. The shale and marl intercalations were subjected to field and detailed petrographic, size analysis, mineralogic and geochemical investigations. Petrographically these sediments are• coliiPosed essentially of argillaceous materials, quartz, feldspar, phosphatic pelloids and skeletal fragments. They are mainly fossiliferous. Calcite which is a main constituent of marls is mainly micritic and non ferroan in nature reflecting precipitation from relatively oxidizing water. Dolomite occurs as fine to very fine non ferroan rhombs indicating early diagenetic dolomitization in oxidizing shallow water conditions. The light fraction ofthe seperated sand fraction is composed of quartz, plagioclase and microcline while the heavy fraction is composed of the opaque minerals, zircon, tourmaline, epidote, garnet, rutile and phosphates. Grain size analysis reveals that the shales are dominated by the clay size while marls are coarser than shales and contain more sand and silt and Jess clay fraction content than the shales. Mineralogical analysis reveals that quartz, calcite, dolomite, francolite, gypsum, anhydrite, microcline, albite, halite, goethite and limonite are the non clay minerals present. The main clay minerals are montmorillonite, kaolinite, and illite in a decreasing order of abundance. Chemical analysis reveals that shales are generally rich in Si02, AhOJ, Ti02 and Fe20J compared with marls which have a higher CaO, MgO and L.O.I. content. The enrichment oftrace elements in the shale