Possibility of Mixed Origin of Rare Earth Elements in Sedimentary Marine Apatites: A Case Study from Phosphorites in the Cretaceous (Campanian-Maastrichtian) Duwi Formation, Abu-Tartur Plateau, Egypt

It is well known that the rare earth elements (REEs) incorporated in the crystal lattice of sedimentary apatite are commonly derived from ambient seawater. This study documents, for the first time, the possibility of mixed origin of apatite REEs present in the Egyptian Western Desert phosphorites, known as the Abu-Tartur phosphorites, one of the most important accumulations of sedimentary phosphorites in the Middle East and North Africa. Shale-like patterns of REEs, negative Ce anomalies, and a (La/Sm)N−(La/Yb)N binary diagram of the studied phosphorites indicate that the incorporation of REEs into apatite crystal lattices has occurred from the ambient seawater by substitution during late-stage diagenesis. The second origin is attributed to REE-bearing supergene fluids, which resulted in the occurrence of sedimentary britholite as green rims and patches containing about 14.93 wt% total REEs in places where the black phosphorites are gradually oxidized into the brown variety. For instance, the intensive chemical weathering induces the crystal lattice of carbonate fluorapatite to preferentially release Ca2+ and CO32− ion species to solution, resulting in the formation of a carbonate-depleted layer in which REEs, particularly heavy REEs, are incorporated into the preferential Ca2+ sites inside the altered apatite lattice, leading to britholite formation.