Pétrographie et géochimie comparées des pellets phosphatés et de leur gangue dans le gisement phosphaté de Ras-Draâ (Tunisie). Implications sur la genèse des pellets phosphatés

Abstract

The Paleocene-Early Eocene phosphate outcrop in the Ras-Draâ deposit (Tunisia; Fig.1) comprises alternances of P-rich strata (P 2 O 5 ≥ 18%) and P-poor ones (P 2 O 5 ≤ 2%). In phosphate rich strata, P is concentrated in rounded grains -so-called pellets -( 28 % ≤ P 2 O 5 ≤ 38 %) embedded in a matrix -so-called exogangue -much poorer in P (P 2 O 5 # 7%) than pellets. The study of pellets (whose size lies mainly between 100 and 500 µm), of their surrounding matrix and of poorly phosphatic sediments interlayered between the phosphatic strata, has been Microscopic observations of Ras-Draâ 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.-3 - ABRIDGED ENGLISH VERSION

Paleoenvironmental conditions and strontium isotope stratigraphy in the Paleogene Gafsa Basin (Tunisia) deduced from geochemical analyses of phosphatic fossils

“…Reworking processes and possible mixing between layers by active bioturbation make biostratigraphic work further complicated. The exact position of the P/E therefore is not accurately known, and discrepancies referring to the age of the main phosphate series are apparent in the literature (Ben Abdessalam 1978;Chaabani 1995;Bolle et al 1999;Henchiri 2007;Ben Hassen et al 2010;Galfati et al 2010;Zili 2010).…”

Geological and Mineralogical Analysis of Phosphorites in the Jebel Dhyr Syncline, Eastern Algerian Atlas

“…They displayed a structure-less, sub-to wellrounded shape, often oval, and were relatively uniform in size, with diameters ranging from 200 to 250 μm. The particles exhibited polished and smooth surfaces due to remobilization from the original depositional environment (Chabou-Mostefaioui 1987;Ben Hassen et al 2011). The phosphatic ooids within the pellets sometimes contained multiple cores, and these cores were also coated ooids.…”

Petro-Mineralogical Analysis of Sedimentary Phosphate of Marine Origin, Case of the Locality of El Kouif (Algerian-Tunisian Confines)

“…Pellets without nuclei: distribution of organic matter is the major criterion of differentiation [20] [21]: -Organic matter collected in the center, leaving the rest of lighter grain.…”