Phosphate Deposits in North Africa: An Overview of Algerian-Tunisian Deposits, Geochemistry and Paleoenvironments

The Algerian-Tunisian phosphorites, from Paleocene-Eocene, were mainly deposited around “Kasserine Island” into three main basins as a result of the large Tethyan phosphogenesis. This chapter reviews the main characteristics of the most representative phosphorite deposits with the aim of comparing their lithostratigraphy, petrography, mineralogy, and geochemical features (major and trace elements, and isotopes), in order to summarize the current state of knowledge of their depositional environments. The phosphorites while sharing similar characteristics in terms of lithology and petrography, also show significant variations in thickness and vertical configuration mostly related to the depth of local basins and depositional facies. Phosphorites are often made up of a complex carbonate fluorapatite (CFA) mineral phase that arises from a microbial-driven phosphatization process of former particles (pellets, coprolites, and bioclasts). The mineral exo-gangue is mainly represented by dolomite, calcite, quartz, gypsum, heulandite-clinoptilolite, and Opal-CT in addition to some accessory minerals. P2O5 contents range from 17.97 wt% to 35.00 wt% (median = 26.5 ± 3.59 wt%), and consequently the values of other oxides fluctuate in different phosphorites facies. Contents of trace elements and rare earth elements (REE) vary significantly through the deposits (e.g. ~0.8 ppm < Cd >  ~172 ppm; ~101 ppm < Cr >  ~374 ppm; ~2 ppm < Cu >  ~44 ppm; ~16 ppm < U >  ~126 ppm; ~125 ppm < ∑REE >  ~1018 ppm). REE + Y-geochemistry shows that the northern deposits formed under oxic conditions, while the eastern and southern deposits formed in sub-reduced to sub-oxic environments. There, a little detrital input and slight paleoproductivity occurred, as revealed by detrital and paleoproductivity geochemical proxies. This realm, associated with the δ13C, δ18O, and 87Sr/86Sr isotopic records, contrasts with the occurrence of the Paleocene Eocene Thermal Maximum (PETM) global warming event during the phosphorite formation.