Geochemical charachteristics and geological significance of Rare earth element (REE) in oil shale of the Ama Fatma Coastal Site in Southwest Morocco

The Ama Fatma oil shale area, located in the Tarfaya-Boujdour basin of southwestern Morocco, was investigated to analyze the content and vertical distribution of rare earth elements (REEs) in shale samples collected from multiple depths. REE concentrations were precisely determined using inductively coupled plasma mass spectrometry (ICP-MS) to better understand their geochemical behavior and origin. The total rare earth element content (ΣREE) ranged from 18 to 50.49 ppm. Average REE concentrations exceeded chondrite values but remained lower than those of the North American Shale Composite (NASC) and the upper continental crust (UCC). Chondrite-normalized patterns showed clear enrichment in light rare earth elements (LREEs) with an average LaN/YbN ratio of 6.4. Negative europium (Eu) anomalies (0.53 < Eu/Eu* < 0.59) and negligible cerium (Ce) anomalies (0.8 < Ce/Ce* < 0.88) were also observed. REE concentrations displayed strong positive correlations with major elements, suggesting a predominantly terrestrial detrital origin. A sequential extraction procedure fractionated REEs into five distinct fractions: exchangeable (F1), carbonate (F2), iron and manganese oxides (F3), organic matter (F4), and residual (F5). The majority of REEs were associated with the iron and manganese oxides fraction (F3), indicating their affinity to these mineral phases. These findings provide insights into the geochemical characteristics and sources of REEs in the Ama Fatma oil shale.