Machine learning models to predict rare earth elements distribution in Tethyan phosphate ore deposits: Geochemical and depositional environment implications

The global market for rare earth elements (REE) is growing rapidly, driven by rising demand and limited production sources, prompting interest in recovering REE from secondary sources such as phosphate deposits. The Tethyan belt, extending across North Africa and the Middle East contains substantial Upper Cretaceous to Eocene REE-rich phosphorite deposits but with limited geochemical data available. This study provides a novel machine-learning (ML) method to predict REE contents in these deposits and verify a useful geochemical classification based on the concentrations of nine major element oxides. Four ML models are developed to achieve this: eXtreme Gradient Boosting (XGBoost), Random Forest (RF), Support Vector Regression (SVR), and Decision Tree (DT). The datasets are divided geochemically into oxic and sub-oxic patterns and these are evaluated with the ML models separately and in combination to accurately predict light REE (LREE), heavy REE (HREE), and total REE contents (∑REE). For the oxic pattern dataset, Fe2O3 and K2O exhibit the highest feature importance consistent with a glauconite influence. For the sub-oxic pattern dataset, MnO and SiO2 exhibit the highest feature importance consistent with high terrigenous inputs (MnO), and silicification. The ML results support the importance of the local deposition environment in determining REE distributions in these deposits. Paleogeography, ocean-margin tectonics, sea-level oscillations, and marine currents exert influence on the local depositional environments. The eXtreme Gradient Boosting model generates the lowest REE prediction errors for all the datasets evaluated

Data-centric approach for predicting critical metals distribution: Heavy rareearth elements in cretaceous Mediterranean-type karst bauxite deposits,southern Italy

In the last few years, many efforts have been devoted to the factors controlling the distribution of CMs in karst bauxites, residual deposits hosted in carbonate rocks. Most of these efforts regard Mediterranean-type karst bauxite deposits of Cretaceous age occurring in southern Italy. Further, there is an increasing interest in assessing the usefulness of machine learning applications devoted to geochemically based datasets. With this in mind, we explored a data-centric machine learning arrangement aiming to find the proper input, limited to Al2O3, Fe2O3, TiO2, and SiO2, the most abundant major oxides occurring in these ores, for predicting the HREE distribution in southern Italy karst bauxite deposits. Among the machine learning techniques used, Artificial Neural Network (ANN), Support Vector Machine (SVR), Random Forest (RF) and Extreme Gradient Boosting (XGBoost) are those that effectively predict HREE concentrations. A predictive model based on just Al2O3, Fe2O3, and SiO2, is one conducing at the worst performance impact suggesting that TiO2 is a relevant input variable in order to predict HREE concentrations in considered karst bauxite deposits. The XGBoost model was found to deliver the highest accuracy in predicting HREE for the validation data records (R2 ~ 0.830, RMSE~7.299, MAE ~ 5.091). Moreover, Fe2O3 is the highest correlated input variable with the output variable and is a significant predictor in our model suggesting iron oxyhydroxides play a relevant role in distributing HREE, likely through a scavenging mechanism at the expense of soil solutions. A further step of our research will involve comprehensive cross-validation studies across multiple areas where Mediterranean-type karst bauxite deposits occur, thus providing a thorough assessment of the model's performance. By addressing these tasks and exploring avenues for improvement, the data-centric approach can advance its potential as a cheap and fast technique to perform a preliminary economic evaluation of potentially HREE abundance, as well as other CMs, in karst bauxite ores benefiting applications reliant on these critical resources

Preliminary data of REE in Algerian phosphorites: a comparative study and paleo-redox insights

This study deals with the preliminary data of rare earth elements (REE) obtained on northeastern Algerian phosphorites from the Tébessa region. These phosphorites are located in two different basins: the northern basin represented by Dj. El Kouif, Dj. Dyr and Tazbant showings and the southern basin with the giant Dj. Onk phosphate deposit. The host sedimentary formation is Late Paleocene to Early Eocene. Twenty-six (26) samples from the four (4) localities were collected and analyzed for their REE contents using the ICP-MS technique. Phosphates from the southern basin (Dj. Onk deposit) show ∑REE contents ranging from 174.41 and 906.39 ppm (average ∑REE = 623.01 ppm), while the northern phosphorites have lower ∑REE contents (from 125.45 to 472.44 ppm; average = 265.57 ppm). PAAS-normalized REE patterns and binary Box plot of (Sm/Pr)N vs (Sm/Yb)N show HREE enrichments for samples from the northern localities while most samples from the southern basin are HREE depleted. Normalized (La/Yb)N vs (La/Sm)N plot shows that (La/Sm)N ratios of all samples are similar to those of modern seawater (from 0.83 to 1.55). However, the (La/Yb)N ratios of the two basins are significantly higher (from 0.67 to 1.18), which indicates an early diagenesis. The Ce/Ce* vs Pr/Pr* diagram shows that the observed Ce anomaly was not affected by diagenesis and more likely represents a proxy for redox conditions. The obtained results substantiate that the northern phosphorites were formed in more oxic environment with more pronounced negative Ce anomalies, whereas the southern phosphorites have lower Ce anomalies. Northern phosphorites are different from those from the south probably because the northern basin was more connected to an open sea as did the Sra Ouartan basin in northern Tunisia. These results have also been confirmed by statistical method studies, such as factorial discriminate analysis

Glauconite-bearing sedimentary phosphorites from the Tebessa region (eastern Algeria): Evidence of REE enrichment and geochemical constraints on their origin

International audienceRare earth element (REE) analyses are reported on glauconite-bearing phosphorites from northeastern Algeria. These rocks of Paleocene-Eocene age are located in the Eastern part of the Saharan Atlas, where two localities were investigated: the Djebel El Kouif in the north and the Kef Essenoun in the south. The latter belongs to the world-class Djebel Onk mining deposit. Petrographic examination indicates that phosphorite from the main layer of the Kef Essenoun deposit has a significant abundance in glauconite grains, while their occurrence in the Djebel El Kouif is restricted to the basal levels. In both deposits, glauconite grains are richer in REEs than other co-existing particles (pellets, coprolites, enameloid and dentine of marine fish teeth), but the glauconites of the Kef Essenoun deposit exhibit significantly higher REE concentration (min = 654 ppm, max = 1760 ppm, average = 1146 ppm) than those of Djebel El Kouif deposit (min = 543 ppm, max = 623 ppm, average = 584 ppm). The whole-rock REE concentrations also show substantial differences between the two deposits and the REE enrichment in the Kef Essenoun main sub-layer is more likely the result of the high glauconite content. PAAS normalized-REE patterns of the glauconite grains display similar patterns between the northern and southern localities with weak negative Ce anomalies and slight middle REE enrichments. On the other hand, the pellets and whole phosphorite grains from the northern deposit indicate a REE source from oxic-suboxic seawater, whereas those from the Kef Essenoun deposit exhibit slight middle REE enriched patterns with weak negative Ce anomaly suggesting a different environment of deposition (i.e. a tendency to reduced conditions). These geochemical results along with previous petrographic studies confirm the allochthonous character of the main phosphorite sub-layer in Kef Essenoun, where winnowing, transport and re-deposition of previously deposited phosphorites resulted in high accumulation and formation of glauconite grains. The glauconitization process of fecal pellets happened in two stages and was controlled by semi-confined micro-environments. This can explain the abundance of glauconite grains in the phosphorites from Kef Essenoun deposit and their restricted occurrence in the basal levels of the Djebel El Kouif outcrop

Rare earth elements plus yttrium (REY) in phosphorites from the Tébessa 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 Tébessa 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 (Coutl) and the percentage of critical element in total ΣREY (REYdef). Three friable whole-rock samples from two P-deposits (Djebel El Kouif and Kef Essenoun) were sieved and retained fractions (f) of <45 μm, 45–125 μm, 125–250 μm, 250–500 μm and >500 μm 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<45μm = 325 ppm–719 ppm; f45–125μm = 309 ppm–893 ppm; f125–250μm = 314 ppm–1029 ppm, f250–500μm = 354 ppm–809 ppm, f>500μm = 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 Coutl 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 ∑REY contents, shows markedly the highest Coutl (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

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