The Ain Dibba and Ain Kissa phosphorites, Tebessa (NE Algeria): REE depletion versus shallow, open depositional environment during the Paleocene-Eocene phosphogenesis in North Africa

The Ain Dibba and Ain Kissa phosphorites, located north of Tebessa town, NE Algeria, belong to the Upper Cretaceous-Paleocene-Eocene Afro-Arabian phosphatic province that extends from Morrocco to the Middle East. These two phosphorite showings, which were partly exploited at the beginning of the 20th century, were deposited during the Late Paleocene (Thanetian) times on the northern basin of the Kasserine Paleo-Island. Although they are not yet economically very important, the present petrological and geochemical study contribute highly to the knowledge of their critical trace and REE contents as well as the understanding of their paleo-depositional environment. Petrographic and XRD data show that these phosphorites are mainly composed of pellets, coprolites, bioclasts and rare glauconite, gypsum, quartz grains and zeolites. These constituents are cemented by calcareous, siliceous or rarely clayey matrix. The pellets and coprolites are composed mainly of cryptocrystalline carbonate fluor-apatite (CFA). The XRF and ICP-MS chemical analyses of these phosphorites, show they are not as rich as those from Kef Essenoun and Bled El Hadba southern basin, neither in P₂O₅ nor in REE contents. P₂O₅ concentrations range from 14.16 to 26.57 wt% (average = 20.24 ± 4.33 wt%, n = 15), with only one sample having 31.89 wt% P₂O₅, whereas ΣREE contents range from 171 to 344 ppm (average = 252 ± 69 ppm, n = 9) and, therefore, considered as moderately REE-enriched phosphorites. Ce anomaly values display a decrease from the lower to the upper phosphorite sub-layers (from −0.57 down to −0.72), suggesting an increase from relatively sub-oxic to more oxic conditions. The Eu anomalies range from 0.89 to 1.35 for Ain Dibba and from 0.93 to 1.35 for Ain Kissa phosphorites. The highest Ce/Ce∗ and lowest Eu/Eu∗ anomalies are recorded in the lower layers, often enriched in both REE and glauconite contents, whereas the lowest Ce/Ce∗ and highest Eu/Eu∗ anomalies, reflecting more oxic conditions, are characteristics of the glauconite-free and REE-poor upper layers. The gradual timewise transition from relatively sub-oxic to oxic conditions (from the lower to the upper layers) is also recorded in the redox-sensitive (Cr, Ni, V, U) trace element data. This implies that during early diagenesis, the sedimentary environment became slightly reduced, which enabled intensive uptake of both REE and some trace element (e.g., Cr, V, Ni, U) mainly from pore-water through substitution and adsorption mechanisms onto apatite and glauconite minerals. This is noticed especially in the lower phosphorite layers of both Ain Dibba and Ain Kissa deposits. In a larger scale, the Ain Dibba and Ain Kissa phosphorites show lower REE contents, lower Ce and Eu anomalies, and lower redox-sensitive trace element contents than those of the southern (Kef Essenoun and Bled El Hadba) and eastern (Tunisian) basins of the Kasserine paleo-Island. These geochemical features indicate that phosphorites from the northern basin were deposited, through upwelling current, in a more open, shallower, oxygenated and agitated environment during the Paleocene-Eocene phosphogenesis in North Africa and Middle East.