Geochemistry and economic evaluation of REE + Y potential in the fine-grained matrix of sedimentary phosphorites from the Tebessa region, eastern Algeria

Abstract

The Algerian phosphorites, deposited during the Paleocene-Eocene, are part of the Tethyan phosphogenesis along the southern paleo-Tethys margin. Located primarily in the Tebessa region, these deposits hold reserves exceeding 2 billion metric tons. Recent interest has grown due to their enrichment in rare earth elements plus yttrium (REY). While previous studies have examined whole-rock and grain-size fractions, the fine-grained matrix (<45 μm) remains poorly explored. This study provides the first mineralogical and geochemical characterization of this fraction to assess its economic potential and paleoenvironmental significance. Twenty-two fine-fraction samples from four Tebessa localities were separated using humid grain-size classification. Mineralogical analysis was supported by X-Ray Powder Diffraction (XRD), whereas geochemical analyses were carried out using inductively coupled plasma optical emission spectroscopy (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS).

The XRD results show that the phosphorite fine-grained matrix is composed of calcite, dolomite, carbonate fluorapatite, glauconite, quartz, chlorite, and gypsum. Notably, glauconite occurs in higher amounts in southern deposits (Kef Essenoun), suggesting intensified glauconitization process. Geochemically, the fine fraction contains an average of 12.48 wt% P₂O₅, with REY concentrations ranging from 55 to 863 ppm. REY contents increase southward, with REEs ranging between 68 and 678 ppm (avg. 416 ± 198 ppm) and Y varies from 9 to 187 ppm (avg. 125 ± 56 ppm). The higher REY content is partly linked to glauconite phase abundance. Normalized REY contents indicate seawater-like patterns in the northern deposits, whereas the southern deposits show middle REE (MREE) enrichment patterns. High (La/Yb)_N ratios in the glauconite-rich samples suggest early-diagenetic adsorption under slow sedimentation rate and sub-reduced conditions. An enhanced glauconitization process occurred in the southern basin at the Paleocene–Eocene boundary, which is marked by global thermal event. These findings suggest that the fine-grained matrix, typically considered as waste during the treatment of raw phosphorites, holds economic potential due to its high REY content, presenting a promising resource for future exploitation.