Geological mapping of carbonatites and related ores from the Oulad Dlim massif (Dakhla Province, Morocco) using remote sensing, portable X-ray fluorescence, and mineralogical data

Abstract and Poste

Petrogenesis and crustal evolution of the Tasiast TTG suite (SW Reguibat Shield, Mauritania). Implication for crustal growth in the West African craton

The Mauritanian Tasiast unit in the Southwestern Reguibat Archean Shield (North of the West African craton = WAC) consists mainly of gneiss dated between 3.07 and 2.91 Ga. We present new field and petrographic observations combined with whole-rock geochemical data on gneisses of Tasiast to understand their petrogenesis, tectonic setting, and the evolution of the continental crust of WAC. These data provide firm evidence of two distinct orthogneisses: trondhjemite and high-K granite affinities. Geochemical characters suggest that (1) trondhjemites magma source were originally derived from polybaric partial melting at the thickened crust occurred over a range of P–T conditions, covering the stability fields of garnet amphibolite and rutile eclogite. The trondhjemite composition is attributed to mixing of two major melts: one originating from eclogitic facies source region (high Nb/Ta) and the other from a garnet-amphibolite facies (low Nb/Ta) leaving garnet, amphibole, and rutile in the residue. (2) Compared to trondhjemite, the granites with high KO and Rb contents, and low NaO, AlO and Sr contents suggest that basaltic oceanic crust was not their source material. Moreover, field relationships suggest that the granites were derived from partial melting of trondhjemites. Hence, the trondhjemite and high-K granites marked together two distinct stages during the growth of the continental crust in the Tasiast area.This paper is a contribution part of the PhD project of M. Bouha funded and supported by the FP7-PEOPLE-2013-IRSES project MEDYNA: Maghreb-Eu research staff exchange on geodynamics, geohazards, and applied geology in Northwest Africa 2014–2017, WP3–Deep structures and mantle processes. We would like to thank Carlos J. Garrido, co-coordinator of MEDYNA, for using the analytical facilities of Petrology, Geochemistry, and Geochronology department (PGG), IACT-CSIC, Granada. We also thank all the PGG members for their help in sample preparation and assistance in laboratory work. We would like to thank François Chabaux for editorial handling, and reviewers for their corrections and constructive remarks

Alkali-Hydrothermal Treatment of K-Rich Igneous Rocks for Their Direct Use as Potassic Fertilizers

International audienceDue to the increasing demand for conventional sources of potassium (K) and their inaccessibility by African countries, K-rich igneous rocks are increasingly studied as potential alternative sources. In this study, six potassic igneous rocks (syenites and trachytes) from the Tamazeght, Jbel Boho, Ait Saoun, and El Glo’a regions (Morocco) were sampled and characterized. Then they were hydrothermally treated to enhance their K release for potential use as potassic fertilizers. The raw materials are mainly formed by microcline (up to 74%), orthoclase (20–68%), albite (36–57%), biotite-muscovite (15–23%), and titanite, calcite, hematite, and apatite as accessory minerals. These samples were crushed and milled to reach a particle size <150 µm and mixed with 4 N NaOH solution in an autoclave. The liquid/solid (L/S) ratio was about 44 mL/50 g. The powders were allowed to react with the solution at 170 °C for 7 h. For all tests, NaOH reacted completely with the powders and no liquid was observed after the treatment. X-ray diffraction (XRD), thermal gravimetric analysis (TGA), infrared spectroscopy (IRTF), and scanning electron microscopy (SEM-EDS) were carried out on treated samples to characterize the mineralogical and structural changes due to the alkali-hydrothermal treatment. Indeed, the treated samples revealed the presence of sodic neoformed phases such as thermonatrite, sodalite, analcime, and cancrinite. The treated material was leached for a week using deionized water and the elements released were measured using inductively coupled plasma–atomic emission spectroscopy (ICP-AES). The hydrothermal process showed a strong effect on structure breakdown as well as on the release of K and other nutrients such as P, Fe, Si, Mg, and Ca. Therefore, the alkali-hydrothermal treatment allowed the release of 50.5 wt% K. Moreover, the release of Mg, Ca, Fe, P, K, and Si were significantly increased. Mg, Ca, Fe, P, K, and Si release within raw materials was about (0.5–3.6), (3.5–31.4), (0.01–0.4), (0.01–0.3), (20–55), and (4.6–8) mg/kg, respectively, whereas treated samples showed a higher release of these elements. Quantitatively, Mg, Ca, Fe, P, K, and Si releases were about (10–11.8), (60–70), (7–20), (1.2–15), (218–1278), and (1119–2759) mg/kg, respectively. Consequently, the treated igneous rocks (syenite and trachyte) could be directly used as potassic fertilizers that would also be a source of other nutrients

Neogene basins in Eastern Rif of Morocco and their potential to host native sulphur

International audienceSediment-hosted sulphur deposits provide valuable information on sedimentary biogeochemical processes related to microbial activity and are paleoenvironmental proxy records. They are also used as markers for oil exploration and the largest ones may represent exploitable economic resources of sulphur. Biogenic sulphur occurrences associated with Neogene formations are found around the Western Mediterranean, in Southern Europe (e.g. Hellin, Lorca, and Teruel, Spain, and Sicily, Italy). In northern Morocco, similar geological settings are present where several sediment-hosted sulphur occurrences were reported by unpublished studies. In this study, we provide the first description of such occurrences selected from two Neogene basins in the Eastern Rif (Taza-Guercif and Boudinar) and studied using sedimentological facies analysis and mineralogical data. The studied facies occur as yellowish sub-spherical concretions, nodules, and laminated structures associated with gypsiferous marls or organic-matterrich marly clays and gypsum lenses. Mineralogical analysis revealed a mineralogical assemblage composed mainly jarosite, gypsum, and native sulphur. These geo-markers are indicative of bacterially mediated sulphate reduction and favourable conditions for the formation of sulphur, especially at the level of the Guercif basin. In contrast, Ras Tarf volcanism probably contributed to the observed facies in the Boudinar basin through different processes

An Integrated Approach for Rapid Delineation of K-Rich Syenites Suitable as Unconventional Potash Resources

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Eruption dynamics of pleistocene maars and tuff rings from the Azrou-Timahdite district (Middle Atlas, northern Morocco) and its relevance to environmental changes and ground water table characteristics

International audienceThe small (≈500 km2) Azrou-Timahdite basaltic area (Middle Atlas Magmatic Province, Northern Morocco) lies on a Lias (200-180 Ma) karst plateau of medium altitude (between 1,750 and 1,950 m). This area includes 23 small-sized Early Pleistocene (2.5–0.8 Ma) scoria cones and 30 Middle Pleistocene (around 0.6–0.5 Ma) hydrovolcanic edifices. The Pleistocene was a period of strong climatic instability in North Africa: three peaks of aridity would have occurred near 2.8 Ma, 1.7 Ma and 1.0 Ma (DeMenocal, 2004), but the period between 500 and 650 ka would belong to a predominantly wet stage (Jimenez et al., 2010). The scoria cones of the Azrou-Timahdite area would therefore formed when the climate was dry and the hydrovolcanic edifices when it was wet. The hydrovolcanic edifices of the Azrou-Timahdite area are peculiarly interesting because they display an exceptional diversity in terms of morphologies and ejecta deposits which can be only explained by interaction of magma with variable amounts of water, sometimes even during a single eruptive event. The karst regions like the Middle Atlas Plateau are very sensitive to fluctuations in rainfall and during a wet climatic episode the deep karst is permanently saturated with water while the epikarst has degrees of saturation that vary with the topography. Our study suggests that the diversity of hydrovolcanic dynamics in the Azrou-Timahdite area could be linked not only to the degree of saturation of the epikarst but also to the thickness of the transmission zone separating the deep water-saturated karst and the epikarst which also depends on the topographic position of the eruptive sites

Short wavelength lateral variability of lithospheric mantle beneath the Middle Atlas (Morocco) as recorded by mantle xenoliths

International audienceThe Middle Atlas is a region where xenolith-bearing volcanism roughly coincides with the maximum of lithospheric thinning beneath continental Morocco. It is therefore a key area to study the mechanisms of lithospheric thinning and constrain the component of mantle buoyancy that is required to explain the Moroccan topography. Samples from the two main xenolith localities, the Bou Ibalghatene and Tafraoute maars, have been investigated for their mineralogy, microstructures, crystallographic preferred orientation, and whole-rock and mineral compositions. While Bou Ibalghatene belongs to the main Middle Atlas volcanic field, in the ‘tabular’ Middle Atlas, Tafraoute is situated about 45 km away, on the North Middle Atlas Fault that separates the ‘folded’ Middle Atlas, to the South-East, from the ‘tabular’ Middle Atlas, to the North-West. Both xenolith suites record infiltration of sub-lithospheric melts that are akin to the Middle Atlas volcanism but were differentiated to variable degrees as a result of interactions with lithospheric mantle. However, while the Bou Ibalghatene mantle was densely traversed by high melt fractions, mostly focused in melt conduits, the Tafraoute suite records heterogeneous infiltration of smaller melt fractions that migrated diffusively, by intergranular porous flow. As a consequence the lithospheric mantle beneath Bou Ibalghaten was strongly modified by melt–rock interactions in the Cenozoic whereas the Tafraoute mantle preserves the record of extensional lithospheric thinning, most likely related to Mesozoic rifting. The two xenolith suites illustrate distinct mechanisms of lithospheric thinning: extensional thinning in Tafraoute, where hydrous incongruent melting triggered by decompression probably played a key role in favouring strain localisation, vs. thermal erosion in Bou Ibalghatene, favoured and guided by a dense network of melt conduits. Our results lend support to the suggestion that lithospheric thinning beneath the Atlas mountains results from the combination of different mechanisms and occurred in a piecewise fashion at a short wavelength scale

Petrological and geochemical constraints on the origin of apatite ores from Mesozoic alkaline intrusive complexes, Central High-Atlas, Morocco

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An integrated ASTER-based approach for mapping carbonatite and iron oxide-apatite deposits

Mapping of carbonatites and related mineral deposits has occupied prominent place in mineral resource exploration programs given their potential to host valuable concentrations of critical metals such as rare earth elements and niobium. Based on spectral characteristics of most indicative minerals for these rocks, a mapping approach was developed using Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data. The combination of band rationing outcomes with components from the principal component analysis and minimum noise fraction techniques highlighted the targeted rocks, with the excellent prospective zone representing ∼0.2% of the total investigated area. This approach was successfully applied to the Gleibat Lafhouda complex to rapidly delineate carbonatites and iron oxide-apatite ore outcrops. Results were validated through field observations and in-situ geochemical analysis using a portable X-ray fluorescence analyzer. Field data have also served as training data to perform a supervised classification, allowing further improvement of the mapping results