Study of the Upper Paleocene-Lower Ypresian phosphate succession in El Guettar out crop (Gafsa area, central Tunisia)

摘要

This research work corresponds to a sedimentological and geochemical investigation applied on the phosphate series cropping out in Oued Tarfa section in El Guettar area (eastern Gafsa) leading to the characterization of the phosphate deposits, as well as the reconstruction of the phosphatogenesis environment.It is worth mentioning that the phosphatic series in El Guettar area is elaborated and described for the first time in the present work. It corresponds to the Chouabine Formation (Upper Paleocene-Lower Eocene) which is underlain by Selja Formation (Lower Paleocene) and is overlain by Kef Eddour formation (Lower Eocene). The studied series includes nine phosphate layers alternating with limestones and marls. The latter intercalations are themselves interbedded with 5-10 cm-thick phosphate horizons. The deposit facies analysis discloses that the Chouabine formation includes an alternation of fossiliferous phosphates that developed within a relatively deep-water open-marine circatidal environment and sandy phosphates which developed within a relatively shollow-water infratidal environment. The total rock major element distribution reveals that the fossiliferous phosphates are in particular relatively rich in Na2O against low concentrations of SiO2, whereas the sandy phosphates are rather poor in Na2O against significant concentrations of SiO2.The deep-water phosphates are richer in P2O5 (30% in average) than the shallow-water phosphates (18% in average), which suggests that the phosphatogenesis in the El Guettar area was controlled by the depositional environment conditions.The chemical analysis reveals that The P2O5 contents of Guettar samples range from 11% to 31%; the ratio CaO/P2O5 varies between 1.58 and 2.16 for the studied layers with an average of 1.76. Samples contain high amounts of SiO2 with an average of 11.30% and may reach 34%. The amount of organic carbon ranges from 0.15% to 0.58%. The low Cd contents at El Guettar amounting to 8 ppm in the fossiliferous and 14 ppm in the sandy phosphates, probably suggest sedimentation in suboxic conditions. A Poor correlation is recorded between MgO and P2O5 (r(2) = 0.382) indicating that Mg might not have been incorporated in the apatite structure. Likewise, a poor correlation is reported between MgO with K2O and Al2O3.The antipathetic relationship between P2O5 and Fe2O3 (r(2) = 0.374) indicates the presence of absorbed phosphate onto Fe-oxy-hydroxides (Fig 9).XRD analysis confirms the presence of apatitic minerals (francolite), carbonates (Calcite, and dolomite), quartz, opal-CT and even sulphides as well as gypsum. Clay minerals in El Guettar deposits are dominated by smectite (48-80%) of the clay mineral assemblage, with subordinate quantities of palygorskite (typically 17-36%), kaolinite (about 18%) in the lowest beds only, and sepiolite (generally 10-15%).