ABSTRACT

Upper Paleocene to Lower Eocene phosphates of the Jebel Oum El Khecheb profile (Gafsa-Metlaoui Basin GMB, Tunisia) are mostly composed of brown phospharenites lithified or not, granular, with pellets, coproliths, bone clasts and fish teeth. The matrix in analyzed samples is reduced and constituted of various clay minerals (dominantly smectites, palygorskite, sepiolite and rare kaolinite), opal-CT and carbonates. Clinoptilolite is recognized along the series; very few amounts of quartz and feldspar grains are sparsely present. The phosphate mineral is a carbonate-fluorapatite (CFA) sulphated, sodic enriched in strontium and rare earth elements (REE). Statistical analyses of mineralogical and geochemical data highlight the associated elements in the main mineral lattice. In the CFA, the group is composed of CaO-P₂O₅-Na₂O-SO₃-F-Sr-Th-REE-U-Ba-Zr-Cr. In the silicates, the group contains SiO₂-Al₂O₃-Fe₂O₃-MgO-K₂O-Na₂O-TiO₂-Co-Cs-Rb-Sc-Ta-Sb-Mo-As and Cu. Sulphides are represented by Zn and Cd; iron oxides by mainly Fe and Ni. Dolomite and gypsum are present; SO₃ is contained in the latter, but CFA may also have some of this radical in its structure. The Upper Continental Crust (UCC) normalized REE patterns present a variably negative Ce-anomaly, suggesting that the imprint of the marine genesis environment is very well-preserved in the main phosphatic units. Positive-Eu and negative-Sm anomalies were detected in some samples. The fractionation of light-REE to heavy-REE, represented by the La/Yb ratio of untreated values varies between 8.28 and 12.94, somewhat less than UCC value (13.64). Enrichment of HREE is due to phosphates. Both ratios reflect the marine genesis environment wherein the formation of the CFA took place in a confined and oxygen poor environment. Then the CFA removed the REE and the trace elements present from the interstitial waters, as was proposed for the various phosphatic basins of Tunisia.