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There is an increasing interest in the distribution of rare earth elements (REEs) within soils, primarily as these elements can be used to identify pedogenetic processes and because soils may be future sources for REE extraction, despite much attention should be paid to the protection and preservation of present soils. Here, we evaluate the processes that control the distribution of REEs in subsoil horizons developed over differing lithologies in an area of low anthropogenic contamination, allowing estimates of the importance of source rocks and weathering. Specifically, this study presents new data on the distribution of REEs and other trace elements, including transition and high-field-strength elements, in subsoils developed on both Quaternary silica-undersaturated volcanic rocks and Pliocene siliciclastic sedimentary rocks within the Mt. Vulture area of the southern Apennines in Italy. The subsoils in the Mt. Vulture area formed during moderate weathering (as classified using the chemical index of alteration) and contain an assemblage of secondary minerals that is dominated by trioctahedral illite with minor vermiculite. The REEs, high-field-strength elements, and transition metals have higher abundances in subsoils that developed from volcanic rocks, and pedogenesis caused the Mt. Vulture subsoils to have REE concentrations that are an order of magnitude higher than typical values for the upper continental crust. This result indicates that the distribution of REEs in soils is a valuable tool for mineral exploration. A statistical analysis of inter-elemental relationships indicates that REEs are concentrated in clay-rich fractions that also contain significant amounts of low-solubility elements such as Zr and Th, regardless of the parent rock. This suggests that low-solubility refractory minerals, such as zircon, play a significant role in controlling the distribution of REEs in soils. The values of (La/Yb) and (Gd/Yb) fractionation indices are dependent on the intensity of pedogenesis; soils in the study area have values that are higher than typical upper continental crust ratios, suggesting that soils, especially those that formed during interaction with near neutral to acidic organic-rich surface waters, may represent an important source of both light REEs and medium REEs (MREEs). In comparison, MREE/heavy REE fractionation in soils that form during moderate weathering may be affected by variations in parent rock lithologies, primarily as MREE-hosting minerals, such as pyroxenes, may control (La/Sm) index values. Eu anomalies are thought to be the most effective provenance index for sediments, although the anomalies within the soils studied here are not related to the alteration of primary minerals, including feldspars, to clay phases. In some cases, Eu/Eu* values may have a weak correlation with elements hosted by heavy minerals, such as Zr; this indicates that the influence of mechanical sorting of clastic particles during sedimentary transport on the Eu/Eu* values of siliciclastic sediments needs to be considered carefully. [ABSTRACT FROM AUTHOR]

A review of the compositional features of Tunisia, Algeria, and Morocco phosphorites is proposed in order to assess and compare the paleoenvironmental conditions that promoted the deposit formation as well as provide information about their economic perspective in light of growing worldwide demand. Since these deposits share a very similar chemical and mineralogical composition, the attention was focused on the geochemistry of rare earth elements (REEs) and mostly on ΣREEs, Ce and Eu anomalies, and (La/Yb) and (La/Gd) normalized ratios. The REEs distributions reveal several differences between deposits from different locations, suggesting mostly that the Tunisian and Algerian phosphorites probably were part of the same depositional system. There, sub-reducing to sub-oxic conditions and a major REEs adsorption by early diagenesis were recorded. Conversely, in the Moroccan basins, sub-oxic to oxic environments and a minor diagenetic alteration occurred, which was likely due to a different seawater supply. Moreover, the drastic environmental changes associated to the Paleocene–Eocene Thermal Maximum event probably influenced the composition of Northern African phosphorites that accumulated the highest REEs amounts during that span of time. Based on the REEs concentrations, and considering the outlook coefficient of REE composition (Koutl) and the percentage of critical elements in ΣREEs (REEdef), the studied deposits can be considered as promising to highly promising REE ores and could represent a profitable alternative source for critical REEs. [ABSTRACT FROM AUTHOR]

• The Chah-Gheib Nickel laterites is formed on the Neyriz serpentinized ultramafic rocks. • Iron oxy-hydroxides dominantly is hosted Ni, and other transition metals. • Alkaline pH condition control enrichment of transition metals and depletion of copper. • Alternating wet-dry climate allows lateritization during the Late Cretaceous-Late Paleocene period. Nickel laterites are supergene deposits derived from the weathering of ± serpentinized ultramafic rocks and they commonly accounts for over 70% of world land-based Ni resources. Serpentinized ultramafic rocks outcrop extensively in Iran as result of collision between Afro-Arabian and Iranian micro-plate during the Late Cretaceous. Two Ni-bearing laterite belts developed on the ultramafic rocks of the Neyriz ophiolite have been recently discovered in the Bavanat region (South Iran). The Chah-Gheib deposit, as the biggest Ni-bearing laterite in the Bavanat area, hosts a total estimated reserve mineralization of about 102,000 tons of ore at 0.92% Ni and 180,000 tons at 1.63% Cr. This study focuses on elemental fractionation and mineralogical control of major, trace and rare earth elements in a selected profile from the Chah-Gheib Ni-laterite, in order to assess the conditions of ore formation and its paleo-environmental significance. Four distinct horizons, including saprolite, lower limonite, upper limonite, and lateritic duricrust, have been recognized from the bottom to the top of the studied profile. Iron oxy-hydroxides are the dominant phases found in the laterite profile. Overall, Fe and other first raw transition metals (i.e. Cr, Ni, V, Co, Cu, and Zn) show abundances higher than those of the bedrock. The main Fe and Cr carriers in the laterite deposit are Fe oxy-hydroxides and chromite, respectively, although authigenic Ni- and Cr-bearing palygorskite is also common throughout the profile. Elemental mobility, estimated using Th as immobile element, showed that the major oxides are largely depleted with the exception of Fe 2 O 3 and CaO. Among trace elements, Cr is the element resulting strongly enriched in those levels where large chromite crystals occurs while V, Co, Ni, and Zn are enriched only in some levels of the upper limonite horizon and in the lateritic duricrust, likely as adsorbed cations onto iron oxy-hydroxides and as a consequence of an alkaline pH environment. Alkaline pH conditions can also explain the Cu depletion observed throughout the profile. Such a condition during lateritization appears further supported by both the occurrence of carbonates and palygorskite and by the distribution of the Ba/Sr ratio. Carbonates and palygorskite, as well as secondary quartz, require dry climate to form and alternating wet-dry climate may allow lateritization. This study suggests that the Chah-Gheib laterite formed in a subtropical climate, possibly during the Late Cretaceous-Late Paleocene period, in a roughly 25°–35° latitude range, promoting Si and Mg mobilization during the wet season and carbonates, quartz, and Mg-rich silicate precipitation during the drier periods. [ABSTRACT FROM AUTHOR]

Bauxite is a residual rock, consisting mainly of a mixture of aluminium hydroxides, whose industrial significance is primarily due to its profitably exploitable alumina contents. In the last decades, bauxite, mainly karst bauxite, has been also considered as possible resource of a great number of economically interesting elements including some critical metals such as rare earth elements, Sc, Co, Ni, and Nb. In this study, we present results of univariate and multivariate (R-mode factor analysis) statistics performed on a large data set including chemical composition of the principal Cretaceous karst bauxites from central and southern Italy with the twofold aim to evaluate the inter-elemental relationship among major oxides and critical metals, and describe factors affecting their distribution. Univariate statistics reveal that some critical metals Co, Ni, and LREE, have a significant number of outliers. The Co-Ni relationship associated to the outliers dataset suggests that Co is likely hosted in Ni-rich phases whereas Ce accumulation in authigenic minerals, carbonate-fluoride and phosphate, is at the origin of LREE outliers. Multivariate R-mode statistics, applied to the outliers-devoid database, instead demonstrate that in absence of specific mineralization events, the distribution of most critical metals is controlled by Al-, Fe-, and Ti-oxi-hydroxides and, to a lesser extent, by detrital phases. Among the critical metals, Cr and Y are the exceptions. Their geochemical behaviour seems to be influenced primarily by their own chemical features that are responsible for the mobility of Cr during bauxitization and for the decoupling of Y from the REEs. [ABSTRACT FROM AUTHOR]

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 (C outl) and the percentage of critical element in total ΣREY (REY def). 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 P 2 O 5 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; f 45 – 125μm = 309 ppm–893 ppm; f 125 – 250μm = 314 ppm–1029 ppm, f 250 – 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 C outl coefficient shows values between 1.89 and 6.85 and REY def 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 C outl (6.61–6.85) and REY def (64.31–64.90), pointing to more promising source for extracting individual critical REY elements, even in low P-grade phosphorite ores. • Grain-size fractions of Algerian sedimentary phosphorites were analyzed for their REY contents. • Geochemical characteristics were investigated with respect to particle size fractions. • REY contents range from 308 ppm to1029 ppm (average of 621 ± 244 ppm). • Local depositional environments control REY distribution rather than grain size classes. • C outl and REY def suggest that P-ores are promising to highly promising potential resources. [ABSTRACT FROM AUTHOR]

Rare earth elements (REEs) distribution in groundwater is an important tool for investigating water-rock interaction processes as REEs content in aqueous systems strongly depends on the degree of rock weathering. However, subsequent to REEs' dissolution from the aquifers rocks, REEs' removal from solution by sorption or coprecipitation can occur. In this work, the concentrations of dissolved REEs were determined in groundwater of Mount Vulture volcanic aquifer. Mt. Vulture is a Pleistocene composite volcano located at the easternmost border of the Apennine compressive front whose aquifer core is mainly composed of pyroclastic and subordinate lava flow of different permeability, which give rise to distinct aquifer layers. The collected water samples were cold and slightly acidic water with electrical conductivity (EC) that varies over a wide range of values; in addition, Eh and dissolved oxygen (DO) of groundwater were also measured. The water samples, due to the high variability of some physical-chemical parameters such as EC and DO, were the object of a cluster analysis, which showed the presence of two main subset having substantial differences in the measured parameters. Thus the dataset was divided into a high salinity subset (EC > 800 µs/cm) with low Eh and OD values and a low salinity subset (EC < 800 µs/cm) characterized by higher OD and Eh values. The concentration of dissolved REEs in groundwater fluctuates over several order of magnitude and shows a slightly positive correlation with water temperature and a slightly negative correlation with the dissolved oxygen (DO); relationships with EC, pH and Eh are not observed. The Post-Archean Australian Shales (PAAS) normalized pattern of the high salinity subset showed an important fractionation between LREE and HREE and moderate CeN* and EuN* anomaly. Instead, the low salinity subset showed a minor fractionation between LREE and HREE and an important EuN* anomaly compared to the previous group. The EuN* anomaly of the whole dataset is in a positive correlation with the Eh whereas the CeN* is in weak negative correlation. These preliminary results confirm the usefulness of REEs' distribution in groundwater for assess water-rock interaction processes. [ABSTRACT FROM AUTHOR]