Geochemistry of Bled El Hadba phosphorites (NE Algeria): Glauconitization process versus REE-enrichment

Rare earth element (REE) enrichment in Algerian phosphorites has been evoked recently by a number of authors and are considered among the richest Paleocene-Eocene phosphorites worldwide. The Bled El Hadba deposit belongs to the giant Djebel Onk phosphorite complex in northeast Algeria. The deposit was subject to many geological studies that were focused on P2O5 contents for commercial purposes. The upper Thanetian phosphorite layer is about 30 m thick, and subdivided into three sub-layers (lower, main and upper sub-layer), based on P2O5 contents, where the main sub-layer is considered to be the richest. Despite this, few detailed geochemical investigations were carried out on this deposit thus far. In this work, major, trace and REE analyses were conducted on phosphate particles (pellets, coprolites and glauconites) using ‘in situ’ LA-ICP MS, and on whole-rock samples using XRF technique. The results show that the main sub-layer displays the highest whole-rock P2O5 contents, varying between 19.65 and 21.32 wt% compared to the lower (10.47–16.87 wt%) and upper (9.43–13.87 wt%) sub-layers. Among the phosphate particles, glauconites display the lowest P2O5 contents in the three sub-layers (17.45–19.35 wt%) compared to pellets (21.14–24.33 wt%) and coprolites (21.75–24.12 wt%), and largely the highest Al2O3, SiO2, MgO and Fe2O3(t). Glauconites also show higher ΣREE contents (764–2050 ppm) compared to pellets (221–910 ppm) and coprolites (214–909 ppm). Within the glauconite particles, the ΣREE, along with Al2O3, SiO2, MgO, and Fe2O3(t) contents, increase from core to the rim, whereas P2O5 contents decrease, which suggests that glauconitization postdates phosphatization processes. Also, the glauconitization process increase from the lower to the upper sub-layer; this is shown by the positive correlations between Al2O3 contents and those of MgO and SiO2 in one hand, and negative correlations between Al2O3 and P2O5 on the other hand. Ce, Eu, Y anomalies, along with La/Nd ratios and Nd contents, all point towards phosphatization under oxic conditions as a result of warm water upwelling, whereas glauconitization started under more reduced (sub-oxic) conditions; i.e., during early diagenesis, the peak of REE uptake from porewater, and under slow sedimentation rates.