Petrography and geochemistry comparative phosphate pellets and their matrix in the phosphate deposit Ras Draa (Tunisia). Implications for the genesis of phosphate pellets

Paleocene-Early Eocene phosphate outcrop in the Ras-Draa deposit (Tunisia) comprises alternances of P-rich strata (P sub(2)O sub(5) greater than or equal to 18%) and P-poor ones (P sub(2)O sub(5) less than or equal to 2%). In phosphate rich strata, P is concentrated in rounded grains--so-called pellets--(28% less than or equal to P sub(2)O sub(5) less than or equal to 38%) embedded in a matrix--so-called exogangue--much poorer in P (P sub(2)O sub(5) 7%) than pellets. The study of pellets (whose size lies mainly between 100 and 500 mu m), of their surrounding matrix and of poorly phosphatic sediments, interlayered between the phosphatic strata, has been performed by optical microscopy and various chemical analyses, ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectroscopy) and punctual chemical analyses by EDS (Energy Dispersive Spectroscopy). Mineralogical and chemical studies of the Tunisian phosphatic pellets show that the basic mineralogical component of these grains is carbonate-fluor-apatite, or francolite. The structural formula of a representative specimen of francolite in the basin is as follows: (Ca sub(4.63)Mg sub(0.13)Na sub(0.22))(PO sub(4)) sub(2.51) (CO sub(3)) sub(0.48)(OH sub(0.77)F sub(0.23)). The mineralogy of the constituents of the sediment surrounding pellets comprises carbonates, clays, silica (opal, quartz). Microscopic observations of Ras-Draa phosphorites, added to geochemical results, establish that pellets are small bodies, allochtonous to their matrix and whose composition is independent of that of this matrix. Fecal cylindrical bodies and subspherical pellets, the latest being supposed to proceed from the fecal bodies by fragmentation, are considered to have been formed by fishes, as P-rich and organic matter-rich faeces. These faeces turn out to be resistant to mechanical dispersion and to chemical diffusion of soluble elements from and in direction of sea water. This closeness is responsible for the strong reducing conditions having prevailed inside and during the pellets diagenesis.Original Abstract: Les series phosphatees tunisiennes, d'age Paleocene-Eocene inferieur, sont constituees d'alternances de strates richement phosphatees (P sub(2)O sub(5) greater than or equal to 18%) et de strates intercalaires peu riches en P (P sub(2)O sub(5) less than or equal to 2%). Les strates phosphatees sont constituees d'un sediment argilo-carbonate servant de matrice (ou exogangue) a des grains phosphates, arrondis, de taille com