Environmental Geochemistry of P-bearings from The Tebessa deposits

Abstract

Sedimentary phosphorite deposits from the Kef Essenoun sector of the Djebel Onk Basin constitute an important phosphate resource formed during the Paleocene–Eocene interval in northeastern Algeria. This study presents an integrated petrographic, granulometric, mineralogical, geochemical, and environmental assessment of these phosphorites in order to characterize their depositional evolution, mineralogical composition, trace-element distribution, and associated ecological implications. Fifteen (15) phosphorite samples representing basal, main, and upper stratigraphic sub-layers were investigated using petrographic observations, grain-size analysis, X-ray diffraction (XRD), and ICP-MS geochemistry approaches. Petrographic investigations reveal that the phosphorites are mainly composed of pellets and coprolites, accompanied by glauconite, phosphoclasts, and biogenic fragments embedded within a micritic to dolomitic matrix. Evidence of reworking, silicification, and dolomitization reflects intense depositional and early diagenetic processes. Grain-size analyses indicate polymodal to monomodal distributions with clear vertical variations between stratigraphic sub-layers. The basal layer is characterized by poorly sorted coarse sediments deposited under relatively high-energy conditions, whereas the upper layer displays finer and better-sorted grains indicative of low-energy and more stable marine environments. XRD analyses demonstrate the predominance of carbonate-fluorapatite (CFA), associated with carbonate-hydroxylapatite, calcite, dolomite, quartz, gypsum, clinoptilolite, and glauconite. Fine fractions are enriched in clay minerals and glauconite, suggesting reducing depositional conditions and significant diagenetic alteration. Geochemistry confirms high CaO (>40 wt.%) and P₂O₅ contents (up to 34.25 wt.%), together with substantial enrichment in trace metal elements (TMEs), particularly Sr, Y, Cr, Zn, Cd, and U. Environmental assessment indices reveal extremely high enrichment factors for Cd and U, with Cd reaching EF values up to 49,600. Geo-accumulation index (I-geo) and Potential Ecological Risk Index (PERI) results indicate that the phosphorites represent a considerable to very high ecological risk, particularly near the Paleocene-Eocene boundary, where intensified redox fluctuations, enhanced marine productivity, and climatic perturbations promoted trace-element accumulation. These findings demonstrate that the Kef Essenoun phosphorites preserve strong sedimentological and geochemical signatures of depositional and diagenetic evolution while also constituting a potential environmental concern due to the enrichment of toxic trace elements. The study emphasizes the importance of selective exploitation and sustainable waste-management strategies to minimize ecological and human-health risks associated with phosphorite mining and processing.