Geochemical fingerprinting of Maastrichtian oil shales from the Central Eastern Desert, Egypt: Implications for provenance, tectonic setting, and source area weathering

Upper Cretaceous oil shales accumulated in intracratonic sedimentary basins along the Central Eastern Desert. Comprehensive geochemical analyses have been applied on 3 Maastrichtian oil shale horizons from the Duwi and Dakhla formations (El-Beida and El-Nakheil mines). We provide significant clues for chemical weathering, sediment recycling, petrogenesis, and tectonic setting of oil shales' source material. The mineralogical composition of the studied oil shales is generally characterized by carbonates, phyllosilicates, quartz, fluorapatite, and sulphides. The geochemistry is characterized by high Ca, P, Ni, U, and Cr values compared to standard post-Archean Australian shales (PAAS). Their chondrite-normalized patterns are distinguished by LREE enrichment, HREE depletion, negative Eu anomaly, and typical shale-like PAAS-normalized REE patterns. Furthermore, the total REE content is positively correlated with Si, K, Na, Fe, Al, Ti, and Mn, indicating that the REE of the investigated oil shales originated from terrigenous sources. Diverse proxies (e.g., CIA, CPA, and PIA) and the A-CN-K ternary diagram suppose that the source rock succession of the oil shales experienced moderate to intensive chemical weathering. Various geochemical parameters suggest that the oil shales were predominately derived from intermediate, with a minor contribution from felsic, igneous source rocks without any remarkable recycling signals. The average values of (Gd/Yb)(N) (1.39), Eu/Eu* (0.76), Th/U (0.29), and discrimination diagrams reveal that the oil shales were derived from Neoproterozoic island arc parent rocks. The geochemical results for oil shale provenance are in accordance with the composition and the palaeotectonic history of the igneous rocks in the Central Eastern Desert of Egypt.