Lithologic characterization of the phosphorite-bearing Duwi Formation (Campanian), South Esna, West Nile Valley, Egypt

Abstract

The Lower phosphatic member of the least Upper Cretaceous (Campanian) Duwi Formation, south Esna, at the western side of the Nile Valley is important rock unit for its relatively high enrichment of heavy metals and U content. The petrographic, mineralogical, and geochemical characteristics of the studied formation are determined using Polarizing Microscopy, X-ray diffractometry, and X-ray Fluorescence. Petrographically, the studied phosphatic grains are composed of phosphatic mudclasts and bioclasts embedded in varies matrix. Spherulitic dahllite, black opaque minerals, and framboidal pyrite are scattered in the matrix. Dahllite in the studied samples are authigenic in origin and formed as a secondary mineral by the re-crystallization of the original francolite. Esna Phosphatic rocks (Duwi Formation, Campanian) were deposited under reducing condition and were affected by chemical weathering. Mineralogically, they are composed of five mineral phases. Apatite (flour-apatite) and dahllite are the dominance; as well as, quartz, pyrite, and anhydrate. The studied phosphatic rocks samples are compared with the average of the Upper Continental Crust (UCC) and the Post-Archean Australian Shale. The trace and some rare earth element content: Mo, Cr, Sr, Co, La, and U are more enriched than those in the UCC. The trace elements in the study phosphatic rocks may be associated with each other or with the heavy minerals rather than with apatite. U contents in the study phosphatic rocks are associated with the heavy metals and trace elements during the chemical weathering under oxic conditions. The ratios Mo/Al and the high concentrations of the redox sensitive elements revealed that the studied phosphatic rocks were deposited under anoxic reducing marine environments coupled with hydrothermal solutions.