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Etudes génétiques de la minéralisation en fer de Nador (Maroc nord oriental)

Abstract : Abstract The Nador arose during the alpine tectogenesis. The biggest iron mineralizations of Marrocco are situated there; there are gentically linked to a granodioritic intrusion; originating from the mantle, its type being calcoalcaline in the magnetite series, this intrusion is characteristic of the volcanic belts, and potentially rich in valuable metal deposits. The formation temperature and pressure have been estimated respectively between 700°C and 750°C, and between 0,8 and 2,7 kb. The contact metamorphism has converted pelitic sandstone into hornfels, and limestone rocks into marbles, the 600 bars "thermal peaks" of which having been estimated at 580°C, and at more than 520°C, respectively. The presence of metasomatic minerals in the granodiorites, the pelitic sandstones and the marbles implies exchanges of constituents with a metasomatic fluid, by percolation and at constant volume, finally and respectively generating endoskarns, skarnoïds and true skarns. But all these matter transfer processes are characterized by a major iron intake, under the form of sesquioxide: the metasomatosis may be as oxidizing. Three stages may be considered in the metasomatism process: the first one takes place in the absence of water, at 550°C max, under a pressure of 600 bars, and generates garnets (Ad6o) and climopyroxenes (salite); the second stage is hydrothermal; the maximum temperature is about 450°C, and hydrous minerals are formed, such as amphiboles, micas, épidotes, scapolites and chlorites; the third stage, finally, generates especially calcite and quartz at 250°C maximum. The metasomatic transformations observed in Nador have been controlled mostly by temperature, oxygen fugacity, and chemical potentials of CaO, FeO and K20. The metasomatic fluid probably originated from the magma; its composition underwent deep modifications by blending first with a connate fluid, then, later, with a meteoric one, at about 250°C. The fluid was very concentrated (63% eq. NaCl) in soluble chlorides, principally NaCl, KCl and CaC12 and also, but in smaller amounts, FeCl2 and MgCl2. ' The accompanying variations of the fluid characteristics may be very different. From the primary till the hydrothermal stage, the sulfur fugacity and the ratio respectively decreased from 10-4 till10-12, and from 4,5-6,3 till 1-1,16. The variations of f02 and XC02 were rather irregular, but globally decreased from 10-23 till 10-32 and from 0,1 till less than 0,5 respectively. The XMg and the ratios, however, increased, from 0,11 till 0,26 and from 0,001-0,005 till 0,008-0,05. The iron-bearing mineralization in Nador has a skarn-type metasomatic origin, genetically linked with the silicated metasomatosis, which itself depended on the gradual evolution of the granodioritic magma.
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Raouf Jabrane. Etudes génétiques de la minéralisation en fer de Nador (Maroc nord oriental). Planète et Univers [physics]. UNIVERSITE LIBRE DE BRUXELLES FACULTE DES SCIENCES Département des Sciences de la Terre et de l’Environnement, 1993. Français. ⟨tel-02660683⟩



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