Mineralogical and geochemical composition variations related to facies changes in Paleocene-Eocene phosphorites, Gasfa Basin, southern Tunisia

A detailed multidisciplinary study of the Oued El Khasfa phosphate series in southern Tunisia was carried out together with facies analysis and geochemical, mineralogical and textural investigations of whole-rock phosphorite samples and separated apatite grains using XRD, XRF, FTIR, and SEM. The results provide a better understanding of the causes of chemical composition variability in phosphorites related to facies change by discussing the role played by the basin configuration (subsidence and paleobathymetry), the facies change between pristine (autochtonous) and reworked phosphorites and subsequent compositional trends (variation of apatite lattice parameters, co-variation of CO 3 2− and F contents) of carbonate fluorapatite CFA between both depositional facies. In Oued El Khasfa, located in the southeastern part of the Gafsa Basin, the lower Eocene phosphate series is characterized by two major facies separated by erosional truncations marking the onset of a forced regressive phase. The lower pristine ( in situ ) facies occurs within the Chouabine Formation, in which the phosphorites are of potential economic value. The upper reworked facies is mainly dominated by detrital quartz-rich and calciclastic discharges mixed with reworked phosphorites derived from pristine lithofacies. The shift from anoxic conditions in pristine facies to suboxic to oxic conditions in reworked facies influenced the composition of carbonate-fluorapatite (CFA) between both facies demonstrated in the reworked facies by decreases in (1) F contents (from 1.96 to 1.55%) as a result of leaching, (2) cadmium Cd (from 102 to 5 μg/g) as a result of sulphide oxidation and (3) structural CO 3 2− (from 6.583 to 4.61) (more decarbonated fluorapatite) and an increasing in P 2 O 5 (from 34.56 to 36.95%) as a result of alteration and weathering. Even though the transition from pristine facies to reworked facies is associated with a drop in the whole-rock phosphate content (dilution by the detrital discharge), the CFA compositional trends between the two facies are regarded as natural beneficiation processes.