Rare earth elements in phosphorite from Maknassy-Mezzouna basin (Central Tunisia): Geochemical features, paleoenvironmental indicators and implications for economic resource potential

Sedimentary phosphorites represent key archives of marine geochemical evolution and are increasingly investigated as unconventional sources of rare earth elements (REEs). This study presents an integrated geochemical and economic assessment of Paleocene–Eocene phosphorites from the Maknassy–Mezzouna Basin in central Tunisia. Trace element geochemistry, REE systematics, and Sr–Nd isotopes are combined to reconstruct depositional conditions and to evaluate the feasibility of REEs co-recovery from phosphate ores. Traceelement proxies indicate deposition under predominantly oxic to suboxic bottom-water conditions within a shallow, semi-restricted marine environment, marked by episodic redox fluctuations and the absence of persistent euxinia. Post-Archean Australian Shale (PAAS) normalized REEs patterns show systematic middle REEs enrichment, negative Eu anomalies, and moderate negative Ce anomalies, reflecting a dominantly marine REEs source and strong early diagenetic control on REEs incorporation into apatite. Sr–Nd isotopic compositions constrain deposition to the Late Paleocene–Early Eocene and indicate a continental-margin setting influenced by felsic detrital input. REEs concentrations exhibit pronounced spatial and stratigraphic variability. The highest enrichments occur at sites T1 and T2, whereas site T4 displays consistently lower values. Sample-scale economic indicators identify Nd, Pr, Dy, and Tb as the dominant contributors to in-situ REE value. Deposit-scale assessment, restricted to sectors with documented geological reserves, indicates that the Jbel Jebbes (T1–T2) and Abdallah (T3) deposits together contain approximately 72.4 kt of ΣREE, corresponding to 85.4 kt of rare earth oxides (REO). Although T3 shows lower REE grades, its larger tonnage accounts for most of the basin’s REO inventory, while T1 and T2 host higher-grade horizons that dominate economic value per tonne. The results demonstrate that REEs recovery at Maknassy is best considered as a co-product integrated into wet-process phosphoric acid production rather than as a standalone REE operation. This study provides the first combined paleoenvironmental and economic framework for the Maknassy–Mezzouna phosphorites and is consistent with geochemical characteristics reported from other Tethyan phosphogenic settings as both, archives and potential contributors to diversified REEs supply.