Petrology, mineralogy, and geochemistry of phosphorites from southern Djebel Onk (case study of the Tarfaya–Kef Essenoun section, Tébessa)

Abstract

Phosphate deposits in the Tarfaya region, on the southern flank of the Djebel Onk basin (northeastern Algeria), belong to a Maastrichtian–Quaternary sedimentary sequence and are subdivided into three sub-layers (basal, main, and upper) based on lithological, mineralogical, and geochemical features. A multidisciplinary study involving petrography, XRD, and grain size analysis reveals that phosphate grains mainly consist of pellets and coprolites in a matrix of carbonate fluorapatite, dolomite, and quartz, with minor glauconite and fish teeth. Grain size data indicate predominantly fine, well-sorted sands, especially in the main sub-layer, the most economically significant. Geochemical profiles highlight vertical variability: the basal layer shows moderate P₂O₅ (22.13–24.52%), low MgO (avg = 1.99%), and CO₂ (avg = 8.47%); the main layer is more heterogeneous (P₂O₅: 19.61–31.07%, MgO: 0.76–3.79%, CO₂: 4.32–14.11%); and the upper layer presents intermediate values (P₂O₅: 26.19%, MgO: 2.66%, CO₂: 8.97%). Strong negative correlations between P₂O₅ and both MgO (r = -0.66) and CO₂ (r = -0.68), along with a positive correlation between MgO and CO₂ (r = 0.98), suggest mineral segregation between phosphate and dolomitic-carbonate phases, reflecting the influence of depositional conditions on phosphate quality.