Rare earth elements plus yttrium (REY) in phosphorites from the Tebessa region (Eastern Algeria): Abundance, geochemical distribution through grain size fractions, and economic significance

Rare earth elements and yttrium (REY) have gained greater attention for being largely used in various high-tech applications and green energies. Recently, supply shortage and high demand on REY led to target secondary resources such as phosphorites. Algerian sedimentary phosphorites, which are located mainly in the Tebessa region, Eastern Saharan Atlas, are relatively enriched in REY according to recent whole-rock analyses. The aim of this study is to explore variation of REY contents and other geochemical features with respect to particle size fractions. In addition, the economic significance of the P-deposits was assessed using new indicators, such as the outlook coefficient of REY composition (C-outl) and the percentage of critical element in total Sigma REY (REYdef). Three friable whole-rock samples from two P-deposits (Djebel El Kouif and Kef Essenoun) were sieved and retained fractions (f) of <45 mu m, 45-125 mu m, 125-250 mu m, 250-500 mu m and >500 mu m were analyzed for their major and REY contents using ICP-MS techniques. Principal Component Analysis (PCA) was applied on centred log-transformation (clr) data, which are adapted to such compositional dataset, as well as Variance Analysis (one-way ANOVA) technique. The results show that the samples yield P2O5 grades ranging from 20 to 36 wt% and total REY contents in selected 15 samples vary in the considered grain size fractions as follows: f(<45 mu m) = 325 ppm-719 ppm; f(45-125 mu m) = 309 ppm-893 ppm; f(125-250 mu m) = 314 ppm-1029 ppm, f(250-500 mu m) = 354 ppm-809 ppm, f(>500 mu m) = 308 ppm-652 ppm. The one-way ANOVA reveals that there is no significant difference of REY concentrations between the grain size fractions. However, the REY grades strongly based on both the studied deposits and the stratigraphic position of the hosting samples (p-value <= 0.01); this is also confirmed by PCA. REY geochemical signatures (normalized REY distribution patterns and Ce, Eu, and Y anomalies) are similar in the grain size fractions from the same hosting samples indicating synchronized evolution of all rock components, where REY distribution is mostly controlled by the depositional environment, irrespective of grain size in the rock. The C-outl coefficient shows values between 1.89 and 6.85 and REYdef varies from 47.44 % to 64.90 %. However, the upper sub-layer of Kef Essenoun phosphorites that yielded the lowest Sigma REY contents, shows markedly the highest C-outl (6.61-6.85) and REYdef (64.31-64.90), pointing to more promising source for extracting individual critical REY elements, even in low P-grade phosphorite ores.