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ScienceDirect Procedia Engineering 83 (2014) 60 – 69

“SYMPHOS 2013”, 2nd International Symposium on Innovation and Technology in the Phosphate Industry

Siliceous forms of phosphate deposits of Cretaceous age in Oulad Abdoun basin (Central Morocco). Mineralogy, geochemistry and diagenetic phenomena Hamid El Haddia *, Abdelmajid Benbouzianea, Mustapha Moufliha†, Es-said Jouranib & M’barek Amaghzaz b a

Sedimentary Georesources and Environment Laboratory, Faculty of Sciences Ben M'sik, Hassan II University. Morocco b OCP SA. Morocco.

Abstract In Oulad Abdoun basin several occurrences of siliceous phosphate deposits have been located in Cretacous sedimentary rocks. All kind of siliceous rock is hosted by phosphate series.The aim of this research is to describe mineralogy and geochemistry particulary types of silica Macroscopic and microscopic observations from phosphate deposits in Ghar El Tajer and Halassa outcrops show silica minerals as: silica quartz grains in clastic or neoformed, silica opal and agate and chalcedony silica. To provide more details on these mineralogical phases, analyzes by xray diffractometer were tested on pure siliceous concretions (nodules and kidney), limestone and siliceous phosphates, marl and siliceous or silexites porcelanites. X-Ray diagrams show major peaks 3.34 Å and 4.26 Å which correspond to minerals crystalline forms, clastic quartz and microcrystalline or fibrous. In contrast, the opal silica type shows intense and broad peaks 4.10 Å, 4.33 Å and 2.50 Å. These peaks are due to an opal CT (opal-cristobalite Tridymite) and an opal T: tridymite. In the diffractograms of flint, quartz always appears with the opal with a line at 4.26 Å, which interferes with it to 4.33 Å. The microcrystalline quartz, chalcedony and opal CT are the main components silex and their patina. Dominant mineral phases of porcelanites and siliceous marls are opal CT / AG and calcite. We also note that the quartz recrystallization of opal is more abundant in porcelanites hosted in siliceous marls where calcite is not yet opalized. The diagenetic origin and development of these siliceous forms will be discussed and developed in relation with the dynamic of transgressive phosphate deposits and phosphatic marl filling.

© 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

© 2014 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Scientific Committee of SYMPHOS 2013.

Peer-review under responsibility of the Scientific Committee of SYMPHOS 2013

Keywords: Phosphate, Opal, agate,chalcedony,Silica quartz,Cretaceous,Morocco.

1. Introduction Morocco has more than three quarters of global reserves in sedimentary phosphate [1], [2], these huge deposits are located in four major basins. The largest of those is the Oulad Abdoun basin with a size of about 100 km long, out of 80 km wide. Others are smaller, but cover vast areas, relatively explored by the Office Cherifians des Phosphates (OCP). These basins are marked by diagenetic silicification phenomena, object of our study. The aim of this papaer is to explore all aspects of siliceous forms and

* Corresponding author. Tel.: +212-669-014-786; fax: +212-522-704-675. E-mail address: [email protected] ; [email protected].

1877-7058 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Scientific Committee of SYMPHOS 2013 doi:10.1016/j.proeng.2014.09.013

Hamid El Haddi et al. / Procedia Engineering 83 (2014) 60 – 69

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silicified facies of Oulad Abdoun phosphate series. The silicification in this phosphate series is various, abundant, misidentified, loss classified morphologically and genetically [2], [3], [4], [5], [6], [7], [8], [9]. The aim is to provide more information onmineralogy and geochemistry aspects. Our approach develops a systematic descriptive and analytical method on all siliceous and silicified facies. We seek not only the identification of minerals links but also the origin and history of the precursor silica diagenetic phenomena. The identification and recognition of these siliceous levels provide useful precisions for exploration mining. In this paper we mainly discuss the mineralogical aspect of siliceous levels, obtained through analysis by X-ray diffraction and macroscopic and microscopic observations. Analyzes concern over twenty different samples of siliceous forms and silicified facies of Maastrichtian-Lutetian age of Oulad Abdoun Basin (EL Halassa Ghar Tager deposit). Through this study, information has been obtained on the variations of the type of silica. Depositional environment could influence siliceous form during the deposition. In the same time diagenesis phenomenas were occurred sedimentation or at the time of the diagenesis of the strata which are now the richest in silicification in the Oulad Abdoun phosphate series. This will be beneficial also for the phosphate industry especially during the recovery and processing of phosphate both to seek active silica enters the mineral that gives high affinity for phosphoric acid and finished products, and the elimination of harmful silica. 1.1. Geographic and geological setting The Oulad Abdoun Basin extends over 10 000 km ² from Ben Ahmed-Zaouiat Cheikh to Settat-Khouribga at the north and extends beyond the Oued Oum Er-Rebia under the plain of Bni Amir on the south. It is bounded by the High Atlas to the east and the Rhamna massive to the west (Fig.1).

Fig. 1: Location map and geological setting of the Oulad Abdoun Basin, (Western Moroccan Meseta) [10].

Regarding the geological setting phosphate deposits is formed of a succession of centimetric to metric phosphatic levels and clay, marl and limestone intercalations over 60 m thick. It is deposited there over 65 million years on a wide continental shelf, with a backcountry terrain changed to the north and east higher than South [11], [12], [13], [14]. This platform had irregularities (shoals, wrinkles, corridors, areas of subsidence ...) that conditioned the genesis and development of phosphate basin. The depths of this Basin vary between 10 m and 100 m with a marked influence of eustatic and low subsidence [15], [16], [17] , [18].

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1.2. Materials and methods The studied section was chosen according to criteria determined as data well recutting the largest proportion of the series, siliceous and silicified samples was taken on average every 50 centimeters in the same lithological levels. Our methodological approach is based on petrographic and mineralogical observations. This analytic approach was fettered by macroscopic and microscopic study particulary on siliceous facies. X-ray diffraction (XRD) performed on selected samples is added to identify quantitative mineralogical composition. This physicochemical method analysis allowed recognition phases with the same chemical components. Different forms of crystallization by measuring the diffraction held on planes of atoms of the crystal studied. It allows us to distinguish the different silicas (which all have the same molecular formula SiO2: quartz, cristobalite ...). 2. Results 2.1. Macroscopic and microscopic observations 2.1.1. Macroscopic description Based on observations in the field and the detail description of the different facies encountered in the Oulad Abdoun Basin (Fig. 2) [19], we note that the lithological succession of the series is generally homogeneous phosphate, phosphate mineralization is horizontal layers, interspersed with levels of limestone, marl and clay, showing silicification and siliceous forms varied from the floor Ypresian. This series phosphate is protected from erosion by the famous slab Thersitées (Hemithersitea, Thersitea). Silicification terms phosphatic Silicification related terms phosphate occurs from the Ypresian form of kidney phosphate silicified centimetric to decimetric lens ovoid form, in places continuous facies commonly called black nougat. These forms are stored in two to three overlapping segments of kidneys siliceous phosphates (layer 1) (Fig. 2, Fig. 3-C) the degree of silicification is intense on the lower level. These nodules have a texture similar to the texture of the phosphates cashing. Flint phosphate exist at all levels of the series studied and the basin. Characterized by colored coprolites not silicified. Most often in continuous space, centimeter diameter. Marly Silicification terms Silicification related terms marl occurs from the Maastrichtian of the Oulad Abdoun phosphate series in white marl, flint lenticular form at the end of the stage (layer 3) Fig.2, brown, centimetric to decimeter with a zonation from center to periphery. In this Ypresian silicification occurs by flint ménilite kind of interlayer C0-C0 '(Fig. 3-D) and the top of the layer 1, which is a very apparent lithological marker to define the layer 1 to the pelvis Oulad Abdoun. This silicification occurs as nodules and spherical ellipsoids, centimetric brown, surrounded by thin whitish marl. Silicification Lutetian the manifested form of flint and marl epigenesis these geodes form spherical hummocky surface and puffy cauliflower-like (Fig. 3-B). The complex alternation of siliceous marl, clay and large flint nodules, forming the interlayer interlayer C0-SA and SA-SB (Fig. 3-A) called Boujniba roof. Silicified limestone terms Silicification linked under limestone is manifested in the form of partially silicified limestone Maastrichtian form of continuous bench Ypresian centimeter in decimeter diameter and silicified fossil Lutetian limestone (Thersitées). In these terms the silica is the most common reddish chalcedony or carnelian called. Of limestone porcelanites especially at the end of the Ypresian show gradual transitions in weakly silicified marls. Porcelanite these are associated with marl and limestone more consolidated, and occur as separate lenses or nodules with distinct boundaries.


Fig.2. Lithostratigraphic column of Oulad Abdoun phosphate Series. [19].

2.1.2. Microscopic description The optical microscopy study obviously has application in the standard for reporting mineralogical, structural and some textural characteristics, principle phosphates nomenclature adopted is the [17] for determining components specific of our microfacies siliceous and silicified and the linking phase. These observations on all facies silicified the series and particularly those processions silica (Fig. 4) were able to distinguish the mineralogically:

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- Clastic silica quartz grains or neoformed with different sizes (phosphates, carbonates and clays) (Fig. 4-A, B, C, H); -Opal and Agate silica consists from alternating layers and amorphous silica microspheres or gel (opal AG) or porcelanite complex (Fig. 4-D); - Chacedony silica consists on microcrystalline quartz and fibrous silicification cements in fringe spaces and inter-granular phosphate and carbonate facies (Fig. 4-E, F, G).

Fig. 3. Some forms of silicification of Oulad Abdoun Basin (Layer I, intermediate (C1-C2 SA-SB)). (A) Silicification in phosphates and phosphatic limestones (B) silicification shaped cauliflower silex in the phosphate level, (C) kidney ellipsoidal phosphate silicified different hardness (facies nougat) (D) hard silexlying within the marl and marl phosphate.


Fig. 4: Texture and microscopic appearance of siliceous microfacies Oulad Abdoun Basin in polarized light. A: Pelphospharénite cement and siliceous test bioclaste (silicified foraminiferal test). B: Biophospharénite to presence with a silicification, fragment bioclast lamellibranch, more tooth section completely silicified. C: Plephospharénite microcracks filled with pure silica grains affecting phosphate intact and partially silicified phosphatic grains secondarily. D: Liserer silicification around phosphatic grains (early silicification). E: coprophospharénite to test bioclast in a siliceous matrix. Silica grains phosphatic fringe around. F: Silica fringe around the grains phosphate (cement palisade crystal aligned certifying an environment supersaturated with dissolved silica). G: Cement siliceous fringe (palisade cement) and centripetal siliceous cement (cement drusique). H: Bioplephospharénite to test body bioclastic (red algae). Phosphatic grains of more or less rounded grains and rare carbonate.

2.2. Mineralogical analysis To provide more information on the mineralogical phases and siliceous mirofacies, a set of analysis by X-ray diffractometer were performed on pure siliceous concretions (nodules) from Al Halassa bore hole, there are: silex (Fig.5-A), siliceous limestone (Fig.5-B), siliceous phosphate rognon (Fig.5-C) and siliceous marls (Fig.5-D). The X-ray diffraction (XRD) allowed us to verify the nature of certain crystalline silica. We used XRD as the approach to determine the type of silica. XRD and it is the only method that allows the differentiation of varieties of the crystalline silica. The results of the XRD diffractograms of the spectra (Fig. 5) is obtained regarding the silica opal A, C, CT and quartz. Opal CT be found with strong reflections at the peaks 4.32-4.10 Å and lower at 2.5 Å. These reflections are characteristic of cristobalite and tridymite [20], [21]. The others peaks of Cristobalite are not easy to identified, the strong disorder of the structure of the opal is characterized by very low reflection of tridymite (2.06, 1.63, 1.45 and 1.20 Å) [22]. The peaks 4.10 and 2.51 Å correspond to an opal C wish is different with silica glass: 4.23 Å [22], [23]. These analysis show major peaks 3.34Å and 4.26Å, which agree with varieties of minerals crystalline forms, clastic quartz and microcrystalline or fibrous. In contrast, the opal silica type shows intense and broad peaks 4.10Å, 4.33Å and 2.50 Å which are either an opal CT (opal-Cristobalite Tridymite) where their structure is disordered or an opal T (Tridymite). In the diffractogram of silex (Fig.5-A) the dominant mineral phase is the quartz characterized by peaks 4.26 Å, 3.34 Å and 2.88 Å ; in siliceous limestone facies (Fig.5-B) the dominant minerals paragenesis are the quartz with peak areas 4.26 Å, 3.34 Å and calcite with peak 3.03 Å and the second phase found a fluorapatite carbonate characterized by peaks 2.79 Å, 2.69 Å and 2.62 Å ; for siliceous phosphate rognon (Fig.5-C) two phases are determined, the first is the quartz and the second of is the fluorapatite carbonate, the peaks of these phases is similar with the others diagrams. In diffractogram of siliceous marls (Fig.5-D) the dominant minerals phases are the quartz characterized by peaks 4.26 Å, 3.34 Å and the second phase found a calcite with peak 3.03 Å.

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B

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C

Fig. 5. Diagrams of X-ray diffraction of samples of the series siliceous phosphate of Oulad Abdoun Basin.

3. Discussion and conclusions Silicification occurs in nature from the deep sea to the surface [24], [25] emphasize that the silicification (chert, chalcedony...) is the product of crystal growth from solutions that contain natural silica solution. For [24], silicification associated with limestones are interpreted a bit tricky, because of the contrast between the conditions of solubility of carbonates and silica, opposition that promotes mutual replacements and thus erases the witnesses primary structures. On the other hand, pure water in equilibrium with the partial pressure of atmospheric C02 and calcite, has a pH = 8.4: this is the critical area for the solubility of silica. When the pH drops, the calcite is dissolved and silica deposit when the pH increase. Establishment of epigenisis and replacement is observed in silicified limestones [26], [27]. In general, layers of silicification are consistent with sedimentary levels and have regular and continuous scale. It might be considered as synsedimentary event. Some silex found in Errachidia-Boudnib-Erfoud basin in Morocco [28], are hollow. The void is filled with debris of organisms. This type of silex is provided by centripetal genesis [29]. Other nodules of silex, cauliflower, with traces of dissolution are deposited on evaporitic environment under arid climate [30], [31], [32], [33], [34]. The origin of the silica is less discused. Authors suggest a high proliferation of siliceous test organisms in subtidal environments, favoured by up-welling currents [35]. It is probably the case in the Oulad Abdoun where siliceous rocks are hosted by phosphate deposits.

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The silica of biogenic origin, once remobilized, would be the origin of the silicification of carbonates [36], [37], [38], [39], [40], [41], [42], [43]. As indicated by geological facies, mineralogical and geochemical data, the silicification was enhanced in several stages It allows to identify types and forms of silica. It does not bring direct response to the questions of the origin of the silicification because numbers of factors as digenesis process, exceptional deposits conditions, basin architecture hydrodynamic sea level were influenced deposition process of silica. Distribution of siliceous facies is well marqued from Ypresian age to Lutetianwish is more abondunt Led to the conclusion that the existence of this occurrences of siliceous forms are not unusable for the exploration regarding to the small size either for environment.It might be useful for industry domains. Acknowledgements We thank Professor M. Laadila for discussions about petrographic thin sections. We also thanks geological departemant of Khouribga OCP Group for their kind help in the field. We are grateful to the Hassan II University Mohammedia-Casablanca for financial support of this research. References [1] Lenoble, A., Salvan, H., Ziegler, V., 1952. Découverte de l’uranium dans les niveaux phosphatés du Maroc. C. R. Acad. Sci. Paris, t. 234, n°9, pp. 976-977. [2] Moutaouakil, D., 1990. Sédimentologie et Minéralisation des Phosphates Ceno Mésozoïque du Sud du Bassin des Ouled Abdoun (MAROC); Application a la Géochimie de L’Uranium. Soutenue à Perpigna. [3] Azmany, M., 1977. Evolution de faciès sur le gisement des OULAD ABDOUN. Rapport N°184. Inédit Service De Géologie Groupe O.C.P., KHOURIBGA, N°: 36p. [4] Azmany, M., 1979. Evolution des faciès sur le gisement des Oulad Abdoun, Mines, Géologie et Energie, Rabat, 44pp.35-38. [5] Belfkira, O., 1980. Evolution sédimentologiques et géochimiques de la série phosphatée du Maactrichtien des Ouled Abdoun (Maroc). Thèse Doct. spec. 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