The Algerian Glauconite-bearing Phosphorites: REE Variation and Insights on the Depositional Environment

International audienceNew data on rare earth element (REE) analyses are reported here on northeastern Algerian glauconite-bearing phosphorites. These rocks of Paleocene-Eocene age are located in the Eastern part of the Saharan Atlas, where two localities were investigated: the Djebel El Kouif in the north and the Kef Essenoun in the south. The latter belongs to the world-class Djebel Onk mining deposit. Hand-picked glauconite grains and other phosphate particles (pellets, coprolites, enamel and dentine of marine fish teeth) were embedded in epoxy resin and polished. They were imaged by SEM and major, trace and REE element contents were analyzed “in-situ” by Electron Microprobe and LA-ICP-MS techniques respectively. For comparison, bulk phosphorite samples were also analyzed by solution ICP-MS. The results show that phosphorite from the main layer of the Kef Essenoun deposit has a significant abundance in glauconite grains, while their occurrence in the Djebel El Kouif is restricted to the basal levels. These glauconite grains exhibit higher ∑REE contents than the co-existing particles (Kef Essenoun: min = 654 ppm, max = 1760, average = 1146 ppm; El Kouif: min = 543 ppm, max = 623 ppm, average = 584 ppm). The overall whole rock REE content also shows substantial differences between the two deposits. Therefore, whole-rock REE enrichment in the Kef Essenoun main sub-layer can be linked to the abundance of glauconite grains. PAAS normalized-REE patterns of the glauconite grains display signatures similar to those of co-existing particles and whole-rocks in each locality indicating a similar origin for the REE. It is noteworthy that phosphorites grains from the northern deposit show a REE uptake source from oxic-modern seawater, while those from Kef Essenoun deposit exhibit bell-shaped patterns with weak Ce anomaly suggesting a different environment of deposition (i.e. more reduced conditions). These geochemical results along with previous petrographic studies, also confirm the allochthonous character of the main phosphorite sub-layer in Kef Essenoun, i.e., winnowed, moved and re-deposited phosphorites as proposed by some authors