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Comptes Rendus

About shape and development of sterile bodies in phosphatic deposits
Comptes Rendus. Géoscience, Volume 334 (2002) no. 16, pp. 1113-1114.
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DOI: 10.1016/S1631-0713(02)01858-8
Armand Boujo 1

1 Universidade de São Paulo, Brazil
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     title = {About shape and development of sterile bodies in phosphatic deposits},
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Armand Boujo. About shape and development of sterile bodies in phosphatic deposits. Comptes Rendus. Géoscience, Volume 334 (2002) no. 16, pp. 1113-1114. doi : 10.1016/S1631-0713(02)01858-8. https://comptes-rendus.academie-sciences.fr/geoscience/articles/10.1016/S1631-0713(02)01858-8/

Version originale du texte intégral


In their interesting paper [5], Kchikach et al. intended to map the sterile bodies under the Quaternary cover, the latter causing serious problems in the exploitation of open cast phosphatic deposits of Sidi Chennane (northeast to the Oulad Abdoun, large sedimentary basin called ‘Plateau des Phosphates’). These problems have been mentioned since the first prospection works [10].

The geophysical method used (electrical resistivity) brought encouraging results and its calibration close to wells and diggings ensures the quality and reliability of this approach. The latter was deliberately restricted to a purely applied aspect, but the excellent study presented here leads to a very interesting geological reflection. Why are these sedimentary ground features isolated the ones from the others? What kind of guide does govern their distribution? What part can be ascribed to weathering?

In fact, the shape of the sterile bodies in which is observed the lithological succession of the adjacent unweathered deposit evokes a weathering phenomenon, a partial dissolution, even a leaching of carbonates, clays, etc. This results in a compaction of the different layers. This compaction is more accentuated in the middle of the affected zone and more restricted on the border, which would explain the ‘small flexures’ pointed out by the authors. In the uppermost part of the series, above layer B, the dissimilar shape of the sterile body still quite reflects that of the unweathered deposit, being characterised by a strong decrease of the calcareous beds, which is counterbalanced by an increase of much softer layers.

In further works, maybe would it be judicious to try to highlight the possible occurrence, in the basement of the ‘Plateau des Phosphates’, of vertical ground features related to episodes of Alpine orogenesis; the latter could perhaps also reflect back (although very damped by Senonian marls) on the Cretaceous–Eocene cover.

It is also to be noted that this ‘case history’ is very important for the understanding of the post-phosphatic history of the Oulad Abdoun deposit. The latter exhibits specific and unknown features for the last transformations affecting this type of deposits in the world. These transformations, whose origin is often supergene, even pedological, results in a favourable way such as deposits of residual concentration, for example, in Morocco (Sidi Daoui) [3,7]; in Brazil (Olinda) [11], (Congaçari) [9]. In Senegal, the result is favourable in the Taı̈ba deposit, where weathering generates a sometimes complete leaching of carbonates [6,11], but quite unfavourable when is observed a development of iron and aluminium oxi-hydroxides, which is very detrimental to the ore exploitation (border of the Taı̈ba deposit and Thiès [4], or the settlement of sand-filled palaeochannels within the phosphatic layer [1,2,10]. The different features of the weathering of phosphatic series are synthesised in the paper by Lucas et al. [8].

References


[1] M. Atger, Étude du gisement de Keur Morr Fall, Rapport inédit, Compagnie sénégalaise des phosphates de Taı̈ba, 1964–1969

[2] A. Boujo Contribution à l'étude géologique du gisement de phosphates des Ganntour, Maroc occidental, Notes Mém. Serv. Géol. Maroc 262, 1976 (227 p. Sci. Géol. Mém. Strasbourg 43, 1976 227 p)

[3] M. El Mountassir, La zone rubéfiée de Sidi Daoui, altération météorique du phosphate de chaux des Oulad Abdoun (Maroc), thèse 3e cycle, université Louis-Pasteur, Strasbourg, 1977, 126 p

[4] R. Flicoteaux Genèse des phosphates alumineux du Sénégal occidental. Étapes et guides de l'altération, thèse d'État, université Aix–Marseille-III, Sci. Géol. Mém. Strasbourg, Volume 67 (1980), p. 229

[5] A. Kchikach; M. Jaffal; T. Aı̈fa; L. Bahi Cartographie de corps stériles sous couverture quaternaire par méthode de résistivités électriques dans le gisement phosphaté de Sidi Chennane, Maroc, C. R. Geoscience, Volume 334 (2002), pp. 379-386

[6] J. Lucas; E.A. Menor; L. Prévôt Le gisement de phosphate de chaux de Taı̈ba (Sénégal). Un exemple d'enrichissement par altération, Sci. Géol. Bull. Strasbourg, Volume 32 (1979), pp. 35-37

[7] J. Lucas; L. Prévôt; M. El Mountassir Les phosphorites rubéfiées de Sidi Daoui. Transformation météorique locale du gisement de phosphate des Oulad Abdoun (Maroc), Sci. Géol. Bull. Strasbourg, Volume 32 (1979), pp. 21-37

[8] J. Lucas; R. Flicoteaux; Y. Nathan; L. Prévôt; Y. Shahar Different aspects of phosphorite weathering, SEPM Spec. Publ., Volume 29 (1980), pp. 41-51

[9] P. Magat; A.J. Melfi O fosfato sedimentar de Congaçari (Bacia Pernambuco-Paraı̈ba): uma jazida de concentraçào residual, Univ. Săo Paulo, NUPEGEL, 1998

[10] D. Mariton Rapports internes inédits OCP, Service d'étude des gisements, 1966–1969

[11] E.A. Menor, La sédimentation phosphatée, pétrographie, minéralogie et géochimie des gisements de Taı̈ba (Sénégal) et Olinda (Brésil), PhD thesis, Louis-Pasteur University, Strasbourg, France, 1975, 153 p

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