Composition and radioactivity of Oligocene fossil bones and their country rocks in Gabal Qatrani, Fayoum, Egypt

Abstract

Several pieces of fossil bones of scute (scutum), shells, teeth, ribs, and vertebra belonging to turtle, crocodile, hippo, and other unidentified mammals were collected from the Oligocene Qatrani Formation, Fayoum, Egypt. Natural radioactivity was measured to all samples of the country rocks (mainly sandstone, calcareous muddy sandstone and dolostone), in addition to the bone samples. Petrographic examination, XRD, SEM, and complete chemical analyses were done with an emphasis on the more radioactive samples that comprise both fossil bone samples and dolostone. The main mineral composition of these bones is carbonate fluorapatite (francolite), while dolostone consists of microcrystalline dolomite, which is cemented by gypsum and/or silica. The complete chemical analysis indicates that Ca and P are the main components of fossil bone. The high content of the redox-sensitive elements such as Cu, Zn, Se, Mo, Cd, Sn, and Pb is a diagnostic feature of the fossil bones. The total rare earth elements (REE) in fossil bones ranges between 635 and 4984 ppm but decreases to 90 ppm in the dolostone bed. The high content of the REE in fossil bones seems to be a result of interaction with fluids that were heated by the thermal gradients prevailed during the Oligocene volcanic eruptions. The available data indicate that the fossil bones are more enriched in their uranium contents (from 42 to 457 ppm), while the contents in the country rocks include the dolostone range from 1 to 31 ppm. The fraction of heavy minerals of the studied sandstone is generally enriched in magnetite, ilmenite, and leucoxene as opaques especially magnetite and ilmenite (> 50%) and contains a wide variety of non-opaques such as zircon, rutile, garnet, and kyanite among others. These non-opaque minerals have variable but insignificant impacts on the REE pattern and the natural radioactivity of the bone-bearing sediments.