Gafsa-Metlaoui Basin Research Articles

Abundance, distribution, and ecological/environmental risks of critical rare earth elements (REE) in phosphate ore, soil, tailings, and sediments: application of spectroscopic fingerprinting

PurposesThis research focuses on the characterization of phosphate ore, its solid effluents, and nearby contaminated soils in the southwest Tunisia (Gafsa-Metlaoui Basin). It aims also at evaluating the vertical distribution and abundance of critical rare earth elements (REE) in the different materials and their ecological and environmental risks.Materials and methodsThe sampled materials went through physical, chemical, and mineralogical characterization which involved XRF, XPS, XRD, and ICP-MS analyses. The REE anomalies and the environmental and ecological indices were calculated.Results and discussionsResults show relatively high concentrations of nine rare REEs, following the sequence La > Ce > Nd > Y > Gd > Eu > Sm > Yb > Tb and trace metal elements (TME) such as Cd, Cr, Mn, Zn, Co, Fe, Sr, Cu, Ni, Pb, Ba that surpass, in some cases, international standards. The vertical distribution of the studied elements within a sediment, tailings, and soil profiles (beyond 20 cm of depth) indicates their likely in-depth migration. TME- and REE-bearing phosphate samples reflect mostly oxic conditions in the southern area with high positive Eu anomalies signifying possible mixing of sources. The environmental assessment indicate no contamination and a moderate enrichment of REEs, except for Eu, which displays significant contamination and extreme enrichment. Whereas, a deficiency of Y has been detected in most of the studied samples except for sludge sample which was also found significantly enriched with REE.ConclusionsThere are fundamental similarities between the different studied samples with high carbonate mineral concentrations. Statistical analysis confirmed the spectroscopic fingerprints demonstrating that the different materials share a similar composition. All of these similarities are most likely linked to the impact of mining activities on sediments and soils. Overall, our findings highlight the global effect of ore processing in determining the geochemical and the mineralogical characteristics of the surrounding environments of mines.

Geochemical Assessment and Mobility of Undesired Elements in the Sludge of the Phosphate Industry of Gafsa-Metlaoui Basin, (Southern Tunisia)

The raw phosphates in the Gafsa-Metlaoui phosphate basin are valorized by wet processes that are performed in the laundries of the Gafsa Phosphates Company (CPG, Gafsa, Tunisia) to reach market grades (>28% P2O5). This enrichment process allows the increase of P2O5 content by the elimination of the coarse (>2 mm) and fine (<71 µm) fractions. Mineralogical analysis has shown that all the investigated materials (raw phosphate, marketable phosphate, coarse waste, and fine waste) from the laundries of M’Dhilla-Zone L and Redeyef are both composed of carbonate fluorapatite, carbonates, quartz, gypsum, clays, and clinoptilolite. Chemical analysis shows that Cr, Cd, Zn, Pb, and U are concentrated in the fine wastes and associated with the clay–phosphate fraction. The rare earth elements are more concentrated in both raw and marketable phosphates. Drilling and sludge-water analysis, along with leaching tests conducted on the fine wastes, showed that, due to phosphate industry, cadmium, fluorine, and sulfate contributing to the pollution of water resources in the region, pollution is more conspicuous at M’Dhilla.

Characterization, nomenclature and factors controlling the stability of quartz and opal-CT of Burdigalian and Ypresian siliceous rocks from Tunisia

The present work aimed to specify the nomenclature and factors controlling the stability of the Burdigalian siliceous rocks of the Babouchite series by comparison to the results of the Ypresian siliceous rocks of the Chouabine formation. The selected samples were investigated using optical microscopy, X-ray diffraction (XRD), chemical and infrared analyses, and Scanning Electron Microscopy (SEM). The data obtained shows that the Babouchite sample is mainly made up of microquartz (more than 70%). The detrital cryptocrystalline quartz represents a start for crystalline growth of microquartz as clusters in the dissolved edges of the relatively preserved radiolarian test pores enhancing chertification. The preservation of frustules is encouraged by the adsorption of clay minerals at their surfaces. However, the porcelanite rock of the Gafsa-Metlaoui basin is composed mainly of opal-CT associated with calcite, dolomite, smectite, sepiolite and palygorskite. Based on the percentage of SiO2, mineralogical composition (high microquartz content) and geochemical data (Al2O3 rich and CaO, MgO poor), the chert is the appropriate nomenclature for Babouchite sample. But, porcelanite is the nomenclature for the bedded silica-rich rock of Chouabine formation. In addition, the high Al/(Al + Fe + Mn) and Al2O3/TiO2 ratios and a low Fe2O3/TiO2 and MnO/TiO2 ratios and the presence of radiolarian tests indicate a non-hydrothermal environment of Babouchite cherts formation with relatively terrigenous detrital input.

Stream sediments geochemistry and the influence of flood phosphate mud in mining area, Metlaoui, Western south of Tunisia

Heavy metals in industrial wastes do worldwide significantly influence surface waters, soils, and human health. The relationships between geochemical stream sediment composition and industrial pollution by phosphate mud flood were examined to assess the severity level of the heavy metal contamination due to the mining activities in the Gafsa-Metlaoui basin (Tunisia). A statistical approach based on linear correlations and principal components analysis of 8 metal contents was used in stream sediment, which was applied to 21 samples of surveyed Metlaoui stream. Within the final model, four Eigen factors did explain almost 90% of the total variance matrix (F1: 42.25%, F2: 19.60%, F3: 15.50 and F4: 10.15%). Three main metal associations were found in the Metlaoui stream sediment. The first was concerned with Titanium (Ti) and Zirconium (Zr) as conservative elements. The second related to the important role of Manganese (Mn) with Iron (Fe) hydroxides in fixing metals. Finally, the third one consisting of Zinc (Zn), Nickel (Ni), Copper (Cu) and Molybdenum (Mo) is all known to be linked to local mining activities sources. The overall degree of contamination was in the following order (high to low): Ti > Zr > Zn > Mo > Mn > Ni > Fe > Cu, but the extent of the contamination did decrease downstream from the mining activity. Phosphate mining activity, especially washing phosphate, leads to an unwanted heavy metals contamination in the nearby surrounding environment.

The Beneficiation Processes of Low-Grade Sedimentary Phosphates of Tozeur-Nefta Deposit (Gafsa-Metlaoui Basin: South of Tunisia)

The enrichment of the low-grade-phosphate ore of the Tozeur-Nefta deposit was investigated using scrubbing-attrition, ball grinding and anionic/cationic reverse flotation in order to separate phosphate-rich particles from their gangue. The choice of the beneficiation process was based on the petrographic, mineralogical and chemical analyses. The petrographic and mineralogical studies have revealed the abundance of phosphatic (carbonate-fluorapatite-CFA) particles) coupled with carbonates (calcite, dolomite,) and silicates (quartz, illite, kaolonite) that constitute the (endo/exo) gangue of these ores. Chemical analysis has demonstrated that the raw phosphate sample contains low amounts of P2O5 (12.0%) and MgO (4.9%) and high amounts of CaO (40.7%) and SiO2 (20.5%). Microscopic observation/counting has shown that the release mesh occurs in the 71–315 μm size. Scrubbing-attrition, grinding and reverse flotation methods were applied to the +71μm fraction. Scrubbing-attrition tests of the 71–315 μm fraction have helped to improve the P2O5 grade to 15.5%. Ball-grinding tests were used to reduce the coarse fraction +315 μm. Grounded materials were sieved to 71–315 μm and combined with the scrubbed fraction in the flotation feed. Reverse-flotation tests of the phosphate-rich fraction (71–315 μm) have helped to improve the P2O5 grade to 27.1%, with a recovery rate of 92.4%.

Desorption of La3+ and Ce3+ from Treated ‘Chert’ a Siliceous Byproduct of the Phosphate Mining Industry of Gafsa-Metlaoui Basin (Southwestern Tunisia)

Chert, the most abundantby-product of the phosphate mining industry, collected from the Metlaoui Gafsa basin underwent purification treatments and was characterized by various analytical methods (XRD, XRF, SEM, FTIR, laser granulometry). The obtained treated chert is a siliceous phase SiO2 (~ 98%) mainly composed of opal-CT (trydimite phase and cristobalite) and traces of quartz. The chert morphology is granular. Treated chert has been successfully tested and proved to be an effective sorbent of rare earths. In this study, the desorption of La3+ and Ce3+ from ion-loaded chert was investigated and the reversibility of sorption reaction was verified. Several eluting agents at different concentrations were tested to desorb those lanthanides such as mineral acids (HCl and HNO3), chloride salt (CaCl2) and ultra pure water. Results show that, HCl and CaCl2 are efficient eluents and the adsorbed Ce3+and La3+ can be easily eluted by them. However, the highest desorption percentages were achieved using calcium chloride as eluting agent (Ce3+, 93.651%; La3+, 96.446%). Kinetics of desorption of La3+ and Ce3+ from treated chert were studied. The rate of desorption was initially fast in the first 30 min, but gradually declined with time. Among all tested kinetic models, the pseudo-second-order model fits the kinetic data best with a better correlation coefficient (R2 > 0.9) and the desorption isotherms fitted better to freundlich, which means that lanthanum and cerium release was dominated by multilayer desorption process from heterogeneous and independent surfaces.

Geochemistry and depositional environments of Paleocene–Eocene phosphorites: Metlaoui Group, Tunisia

The Late Paleocene–Early Eocene phosphorites of the Metlaoui Group in Tunisia are a world-class phosphate resource. We review the characteristics of phosphorites deposited in three areas: the Northern Basins; Eastern Basins; and Gafsa-Metlaoui Basin. Comprehensive new bulk rock elemental data are presented, together with complementary mineralogical and mineral chemical results. Carbonate fluorapatite (francolite) constitutes the dominant mineral phase in the deposits. Phosphorite samples are enriched in Cd, Sr, U, rare-earth elements and Y, together with environmentally diagnostic trace elements that provide detrital (Cr, Zr), productivity (Cu, Ni, Zn) and redox (Mo, V) proxies. Suboxic bottom-water conditions predominated, with suboxic to anoxic porewaters accompanying francolite precipitation. Phosphorite deposition occurred under increasingly arid climate conditions, accompanying global Paleocene–Eocene warming. The Northern Basins show the strongest Tethys Ocean influence, with surface seawater rare-earth element signatures consistently developed in the phosphorites. Bed-scale compositional variation indicates relatively unstable environmental conditions and episodes of sediment redeposition, with varying detrital supply and a relatively wet local climate. Glauconitic facies in the Northern Basins and the more isolated evaporite-associated phosphorites in the dryer Eastern Basins display the greatest diagenetic influences. The phosphorite – organic-rich marl – diatom-bearing porcelanite facies association in the Gafsa-Metlaoui Basin represents the classic coastal upwelling trinity. Modified Tethyan waters occurred within the Basin during phosphorite deposition, with decreasing marine productivity from NW to SE evidenced by systematically falling enrichment factors for Cu, Ni, Cd and Zn in the phosphorites. Productivity declined in concert with increasing basin isolation during the deposition of the commercial phosphorite beds in the latest Paleocene to earliest Eocene. This isolation trend was temporarily reversed during an episode of maximum flooding associated with the earliest Eocene Paleocene–Eocene Thermal Maximum (PETM).

Textural evidences for dissolution of silica-rich rocks of the Ypresian phosphatic series, Gafsa-Metlaoui basin, southwestern Tunisia: implication of biogenic silica supply on genesis of fibrous clays

The present work aimed to determine the mineralogical composition of Ypresian series and to clarify the influence of the dissolution of siliceous frustules on the genesis of fibrous clay minerals. The specimens sampled from CPG trench are mainly constituted of silica-rich rocks at Mides area located at the western part of Gafsa-Metlaoui basin. The samples were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM) in order to determine texture of constituents. The data obtained indicate that the bulk rock samples are mainly made up of opal CT and clay minerals. The latter consists of palygorskite-sepiolite minerals associated with smectite and few amount of illite. The trend of mineralogical composition indicates that fibrous clay minerals are more concentrated at the upper part. SEM observations indicated that palygorskite mineral appears as thread-like facies, which surround foliated texture of smectite in the lower part of the Mides section, although with the low Mg activity confirmed by the absence of dolomite. But, at the upper part of the Mides section, SEM observations revealed the occurrence of siliceous frustules, which have numerous dissolved areas and replacement of carbonate tests by silica. The dissolution saturated the depositional environment with silica which is required for the formation of palygorskite and sepiolite minerals, in addition to high Mg activity confirmed by the presence of dolomite in the bulk rock, which is required basically for the formation of sepiolite. Although the genesis mode of palygorskite and sepiolite is similar with very little difference, the genesis of sepiolite needs a high alkalinity than the formation of palygorskite.

The effect of heating on mineral composition and grain size distribution of flux calcined porcelanites from the Gafsa-Metlaoui basin, southwestern Tunisia

The porcelanite rock of Ypresian phosphatic series of the Gafsa-Metlaoui basin (south-western Tunisia), is composed mainly of opal CT, and presents a variable percentage of carbonates and fibrous clays. This rock is treated with flux calcination at different temperatures in order to prepare a specific filter aid for cleaning melting sulfur which can be used for the production of sulfuric acid. This work presents the effect of heating on the mineralogy and grain size distribution of carbonate-rich porcelanite (Tm1) and clay-rich porcelanite (Gh) compared to flux calcined silica-rich porcelanite (CHM3) and diatomaceous filtration aids. The porcelanite samples used in this work come from three localities of the Gafsa-Metlaoui basin: Kef El Ghis (Gh), Tamarza (Tm1) and Mides (CHM3). Flux calcination at 1000 °C provokes a mineralogical transformation on carbonate-rich porcelanite samples. The opal CT transforms to opal C and becomes neater and more stable. The Thermal treatment of porcelanite (Tm1) incites also the apparition of new peaks of wollastonite. However, the structural change of opal CT to opal C by heat treatment is blocked for flux calcination of clay-rich porcelanite. The opal CT of fluxing clay-rich porcelanite becomes more ordered without significant change to opal C. The difference between fluxing carbonate-rich porcelanite (Tm1) and fluxing clay-rich porcelanite (Gh) appears also with granulometric distribution histogram of the tow heated samples. All raw samples have unimodal granulometric distribution (1–100 μm). After calcination with alkaline flux at 1000 °C fluxing carbonate-rich porcelanite displays bimodal granulometric distribution and a new mode appears systematically, between 0.1 μm and 1 μm. This occurs for fluxing silica-riche porcelanite and diatomaceous filtration aids as well and corresponds to the opal C formed after heat treatment. Whereas fluxing clay-rich porcelanite present trimodal granulometric distribution and a third mode appears (100–300 μm), which due to silica glass phase. Since, the granulometric rearrangement of porcelanite during thermal treatment may due to mineralogical transformation of opal CT to opal C and crystal grow.

Mineral characterization of the Oum El Khacheb phosphorites (Gafsa-Metlaoui basin; S Tunisia)

The mineralogy of the Tunisian phosphatic series of the Jebel Oum El Khacheb deposit which belongs to the Gafsa-Metlaoui basin, located in the South of Tunisia, was studied. Chemical and mineralogical characterization of eight representative samples (2mm–70μm) were undertaken by X-ray diffraction, thermal analysis TG/DTA, Carbon and Sulfur analyzer and Atomic Absorption Spectroscopy. It was revealed that each phosphatic layer contains various silicates and carbonates. The X-ray diffraction results were supported by the Rietveld method to confirm the presence of certain minor phases (calcite, dolomite, bassanite and anorthoclase). The precision and accuracy of the quantitative-phase analysis, using the Rietveld method as a routine tool for process control, were highlighted.

Depositional environment and stability of the porcelanite within the Ypresian phosphatic series of the Gafsa-Metlaoui basin, southwestern Tunisia

Ypresian porcelanite levels outcrop from the central to the western part of Gafsa-Metlaoui basin are valuable for the geological environment reconstitutions and the industrial applications, as a siliceous industrial filtration aids like diatomite. The present work aims to determine the mineralogical and geochemical characteristics of porcelanites levels and provides some data about their depositional environment. Thus, 29 specimens are sampled from these levels. The XRD data show that all samples are composed mainly of opal CT, clay minerals (smectite, sepiolite, palygorskite), and carbonates (calcite and dolomite), which contain also trace amount of clinoptilolite, feldspar, francolite, quartz, and pyrite. The variations of opal CT and carbonates percentages are in the opposite trends. These results agree with chemical composition of major elements determined by XRF and inductively coupled plasma mass spectrometry, which indicates that the selected samples are characterized by high SiO2 (64 %) and relatively low CaO (11 %) contents. Plot of SiO2 vs. CaO and of SiO2 vs. MgO show negative linear correlations indicating that the depositional conditions favor carbonate dissolution but encourage the formation of opal CT and vice versa. Therefore, carbonate dissolution increases Mg activity of the depositional environment, privileged then the opal CT agglomeration as lepispheres coated by microcalcite. Since, increasing Mg content is required for the autogenesis of Mg-rich fibrous clays and encourages the stability of opal CT, although the high Mg content avoids chert formation.

Fossil diatoms from endogangue of the Ypresian phosphatic pellets of the Gafsa-Metlaoui basin: implication on the origin of biogenic silica and depositional environment

This work deals with mineral and diatom contents of the lower Eocene (Ypresian) phosphatic pellets of the Gafsa-Metlaoui basin, using XRD and SEM method. Since, mineral composition and fossil diatoms are best indicators of the depositional environments and diatom content leads to understand the role of diatoms in the formation of siliceous rocks especially porcelanite of the interbedded facies. This study indicates that phosphate minerals (fluorapatite) are associated with opal CT, carbonate mineral (dolomite and calcite), pyrite, sphalerite, and clinoptilolite. This association corroborates with deposition of the Ypresian sediments of the Gafsa-Metlaoui basin in alkaline and anoxic to suboxic marine environments under sulfate-reducing conditions. On the other hand, the exogangue is roughly free from diatoms except some highly dissolved diatom frustules. Endogangue displays some preserved diatom frustules, which allow the determination of three species: Paralia sulcata, Stephanopyxis sp. and Aulacoseira sp. Aulacoseira sp occurs at the eastern part of the Gafsa-Metlaoui basin (M’Zinda section) and it is known as a freshwater diatom. Stephanopyxis sp. and P. sulcata are both indicative of warm shallow marine water. They occur at Metlaoui Table and Kef Eddour sections, respectively.

Mineralogical study and properties of natural and flux calcined porcelanite from Gafsa-Metlaoui basin compared to diatomaceous filtration aids

The purification of melting sulfur by Tunisian Chemical Group factories was performed with filtration by siliceous earth filters such as French clarcel, Algerian kieselguhr, Spanish perlite and Spanish diatomite, i.e. calcined diatomites which have biogenic origin, except for perlite, which is volcanic glass. The porcelanite rocks of the Ypresian phosphatic series from the Gafsa-Metlaoui basin also have a biogenic origin and are formed in Si-rich depositional environment, at low diagenetic conditions. This work examines the thermal treatment of these porcelanite rocks to obtain calcined siliceous material with physical–chemical characteristics comparable to the diatomaceous earth filters. The porcelanite samples used in this work come from the Mides CPG trench, where the Ypresian porcelanite beds are 10m thick. The flux calcined porcelanite was obtained by heating the raw samples at 400, 800 and 1000°C with 5% Na2CO3 flux. The raw porcelanites from the Gafsa-Metlaoui basin had high SiO2 content, similar to the usual filtration aids. The natural porcelanite is dominated by opal-CT and minor clay minerals (smectite, palygorskite and sepiolite), hydroxylapatite, quartz, calcite and hematite. During heating up to 800°C, the opal-CT progressively turned into opal-C. After 1000°C, the calcined porcelanite samples had similar properties as the calcined industrial products derived from the Spanish and French diatomite raw materials. Hence, the short range ordering of opal CT increased significantly as cristobalite-like structure formed. The thermal treatment of porcelanite rocks up to 800°C decreased the specific surface area and increased the permeability coefficient of the calcined porcelanite samples to values comparable to calcined industrial filtration aids.

Application of LA-ICP-MS to sedimentary phosphatic particles from Tunisian phosphorite deposits: Insights from trace elements and REE into paleo-depositional environments

Laser ablation ICP-MS was used to determine trace (TE) and rare earth element (REE) contents of sedimentary phosphatic pellets and coprolites collected from late Paleocene-early Eocene phosphorite deposits in Tunisia. TE and REE contents in the investigated deposits exhibit a close relationship with grain types and paleogeography. Phosphatic pellets seem to concentrate more TE and REE than co-existing coprolites. Both particle types display modern oxic-suboxic seawater patterns. In most coprolites, ∑REE, Y, Cr, Ni, Th, and V show an increase from core to rim, whereas Ba, Cd, Cu, and U abundances decrease from core to rim, reflecting adsorption processes during early diagenesis consistent with changing redox and productivity conditions.The northern basins (Sra Ouertane) within open marine and platform settings tended to be more oxic-suboxic with relative high organic matter flux, while the eastern basins (Jebel Jebs) and Gafsa-Metlaoui basin are characterized by semi-restricted situation associated with suboxic-oxic conditions. This result is consistent with previous reconstructions which suggest that phosphorites of northern basins had a more direct connection to the open ocean than those to the south and east of the emerged area.This study highlights the potential for using laser ablation ICP-MS as a tool to reconstruct phosphorite depositional paleoenvironments.

Granulometric distribution of natural and flux calcined chert from Ypresian phosphatic series of Gafsa-Metlaoui basin compared to diatomite filter aid

The cherty rocks of the Chouabine Formation of the Gafsa-Metlaoui basin (south-western Tunisia), that is composed by biogenic silica, are treated using thermal treatment at 1000°C with flux calcination method in order to prepare a specific filter aids of melting sulfur filter used for the production of sulfuric acid. This work presents the effect of heating on the granulometry of chert. The mineralogical composition of natural starting chert is composed by opal CT (cristobalite/tridymite) and by the mineral mixture of quartz, smectite clay minerals, palygorskite-sepiolite fibrous clay minerals, calcite and hematite. After thermal treatment, at 1000°C, the crystallinity of chert increases significantly and the opal-CT, transforms mostly to opal-C. The comparison of infrared spectra of untreated and treated cherts shows systematic variation of the wavenumber and the intensity of the absorption bands, due to the apparition of 619, 795, 1094 and 1202 cm−1 absorption bands of cristobalite in the infrared spectrum of chert treated sample, which is similar to diatomite. Granulometric analysis show that the natural untreated chert sample displays unimodal distribution, whereas the treated chert sample display bimodal distribution, as same as diatomite. Thus, a new mode appears systematically, between 0.1 μn and 1 μm, for all thermal treated samples with alkaline flux as observed for diatomite.

Mineralogical and geochemical studies of Ypresian marly clays and silica rocks of phosphatic series, Gafsa-Metlaoui basin, southwestern Tunisia: implication for depositional environment

Thick Ypresian marly clays and silica rich rocks constitute the so-called interbedded facies of phosphatic series, in the Gafsa-Metlaoui basin, southwestern Tunisia. Mineralogical and geochemical investigations are carried out on selected samples from Kef Eddour section. The data obtained by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Energy Dispersive X-ray microanalysis (EDX) and chemical analysis and IR of clay fractions indicate that the bulk rock samples are composed of cristobalite-tridymite and dolomite. Observation by SEM showed the occurrence of dolomite as a well-crystallized and a weathered shape. Whereas, cristobalite-tridymite observed in the cherty bed appears as a spheres, filling the pore spaces. However, samples which contain moderately a significant amount of feldspars, clinoptilolite and pyrite occur in the upper part of Kef Eddour section. Clay minerals consist mainly of smectite, palygorskite, and sepiolite. In this clay assemblages, the N-alkane (m/z = 57) distributions, revealed that organic matter has phytoplankton and bacterial origin. The Pristane/Phytane ratio indicated that the organic matter is deposited in a reduced environment. In this latter, sulfate-reducing bacteria generate hydrogen sulfur, which is incorporated on the organic biomolecules that occurred at the lower part of Kef Eddour section. However, the high Fe activity on the depositional environment favored the occurrence of pyrite in the upper part.

Origin and Depositional Environment of Palygorskite and Sepiolite from the Ypresian Phosphatic Series, Southwestern Tunisia

AbstractThe Ypresian phosphatic series of the Gafsa-Metlaoui basin, southwestern Tunisia, is represented by an alternation of phosphatic levels and interbedded facies, which are composed of marly clay and silica-rich rocks. The present work aimed to clarify the genesis of palygorskite and sepiolite of the interbedded facies and to understand the depositional environment of the phosphatic series. The interbedded facies of the Stah and Jellabia mines were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Energy Dispersive X-ray microanalysis (EDX) of individual constituents and their aggregates. The data obtained indicate that samples are made up of francolite, calcite, dolomite, quartz, feldspars, and clay minerals; the latter consist of palygorskite-sepiolite minerals associated with smectite. Observations by SEM revealed the occurrence of palygorskite and sepiolite as fine and filamentous fibers with thread-like facies and coating dolomite, calcite, and a marly matrix. Such features can be considered as textural evidence of authigenic palygorskite-sepiolite. At the bottom of the Stah section, SEM observations revealed that the fine fibers are more abundant within silica-rich rocks. Silica is commonly available due to bacterial activity saturating its environment with the silicic acid required for the formation of palygorskite-sepiolite. In the interbedded facies of the Jellabia section, the moderate fibrous clay content and the presence of well crystallized dolomite revealed that the shallow-marine water was characterized by high-Mg and low-Si activities.

Geochemistry and mineralogy of Paleocene-Eocene Oum El Khecheb phosphorites (Gafsa-Metlaoui Basin) Tunisia

Upper Paleocene to Lower Eocene phosphates of the Jebel Oum El Khecheb profile (Gafsa-Metlaoui Basin GMB, Tunisia) are mostly composed of brown phospharenites lithified or not, granular, with pellets, coproliths, bone clasts and fish teeth. The matrix in analyzed samples is reduced and constituted of various clay minerals (dominantly smectites, palygorskite, sepiolite and rare kaolinite), opal-CT and carbonates. Clinoptilolite is recognized along the series; very few amounts of quartz and feldspar grains are sparsely present. The phosphate mineral is a carbonate-fluorapatite (CFA) sulphated, sodic enriched in strontium and rare earth elements (REE). Statistical analyses of mineralogical and geochemical data highlight the associated elements in the main mineral lattice. In the CFA, the group is composed of CaO-P2O5-Na2O-SO3-F-Sr-Th-REE-U-Ba-Zr-Cr. In the silicates, the group contains SiO2-Al2O3-Fe2O3-MgO-K2O-Na2O-TiO2-Co-Cs-Rb-Sc-Ta-Sb-Mo-As and Cu. Sulphides are represented by Zn and Cd; iron oxides by mainly Fe and Ni. Dolomite and gypsum are present; SO3 is contained in the latter, but CFA may also have some of this radical in its structure. The Upper Continental Crust (UCC) normalized REE patterns present a variably negative Ce-anomaly, suggesting that the imprint of the marine genesis environment is very well-preserved in the main phosphatic units. Positive-Eu and negative-Sm anomalies were detected in some samples. The fractionation of light-REE to heavy-REE, represented by the La/Yb ratio of untreated values varies between 8.28 and 12.94, somewhat less than UCC value (13.64). Enrichment of HREE is due to phosphates. Both ratios reflect the marine genesis environment wherein the formation of the CFA took place in a confined and oxygen poor environment. Then the CFA removed the REE and the trace elements present from the interstitial waters, as was proposed for the various phosphatic basins of Tunisia.

Distribution de l'uranium, du thorium, du zirconium, du hafnium et des terres rares (TR) dans des grains de phosphates sédimentaires

U, Th, Zr, Hf and REE contents have been determined in phosphate grains from central Tunisia, southern France (Montagne Noire) and the southeastern Pacific, and in organic fractions (humic acids, kerogens and stable residues) separated in Paleocene-Eocene phosphates of the Gafsa-Metlaoui Basin (central Tunisia). Due to adsorption, phosphatic pellets concentrate high Zr, REE and Th contents, and fish debris takes up considerable amounts of Zr but not of Hf. A separation of Zr from Hf, related to the adsorption of Zr preferred to Hf in apatite, and a clear positive correlation between Zr and REE contents are found. We also note a more or less pronounced fractionation between light and heavy REE, which is influenced mainly by organic complexes, as shown from the clear enrichment of the heavy REE in each of all of the organic fractions.

Paleooceanography of the Mesogean sea based on ostracods of the northern Tunisian continental shelf between the late Cretaceous and early paleogene

A new method analysis used in the study of the El Kef Section (north-western Tunisia) led to the paleooceanographic reconstruction of the Mesogean between-late Cretaceous and early Paleogene. Several changes observed are, in fact, worthy of particular mention: paleobathymetrically the area changed from an upper bathyal environment of about 400 m deep to an outer shelf domain of 100-50 m. Furthermore, the Oxygen Minimum Zone (O.M.Z.) became highly intense at (a) the Cretaceous-Tertiary transition (catastrophic event); (b) the P1-P2 boundary (intense thermic stratification); and (c) between P4 and P6 (very active paleoproductivity). In this last episode the continental shelf environment was subjected to intense upwelling responsible for the formation of a »prephosphatic« environment at water/sediment interface. The paleooceanographic history of phosphates in the Gafsa-Metlaoui basin is, in fact, in perfect stratigraphic and paleoenvironmental agreement with the paleobathymetric evolution of the Kef region. Finally, paleooceanographic communication between the Mesogean and the North Atlantic was, at this period, highly free because of lack of hydrologic relations between the North Atlantic and the Norwegian Sea.