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Conference Papers Year : 2014

Non-sulfide zinc deposits of Morocco: from mineralogy to age characterization


In Northern Africa, Zn-Pb ore deposits are numerous, and Zn ores of supergene origin have been long extracted for local use using calcination to smelt the metals. While the renewal of interest in non-sulfide ores coincides with improvements in mineral processing, the recent opening of a Zn-oxides production plant in Morocco attests to the multipartite will to revive interest in the non-sulfide ores disseminated throughout the Atlas Mountains, as it represents a direct source of employment and an opportunity to create infrastructure in remote places. Six non-sulfide Zn-Pb ore deposits, namely Toulal, Aït Labbès, Tadaghast, Beni Tajite, Bou Arhous, Tizi n'Firest, (representing an overall production of ca. 1Mt at 24% Zn) hosted in the Lower Jurassic reef to para-reef limestone have been investigated in the Moroccan High Atlas, to understand processes and timing of their formation. Ore occurs either in veins associated with a faulted anticline, or in orthogonal fractures or stratabound horizons in tilted strata. A vertical zoning from deep Zn-Pb sulfide stratabound lenses to shallow non-sulfide deposits has been reported. Zn (Pb) carbonates, Zn silicates and hydrated associated minerals directly replace the sulfide ore bodies or filled cavities along fractures related to the Atlasic events. Field observation has been complemented by a multidisciplinary approach (Raman, SEM, EPMA) for mineralogical characterization, revealing a similar paragenetic evolution for all deposits, as follows: 1) formation of the protore sulfides, 2) early supergene weathering with formation of Zn-Pb-bearing carbonates and iron oxi-hydroxides replacing protore sulfides or in the form of karst-related internal sediment and 3) late supergene weathering with deposition of Zn-carbonates, Zn-silicates and hydrated phases. Direct replacement of Zn-Pb sulfides strongly controlled by pyrite oxidation is accompanied by precipitation of zinc non-sulfide minerals in cavities or karst-related internal sediment filling. A complementary rock magnetism study from the Toulal and Aït Labbes ore deposits have been carried out. It shows that magnetite and minor hematite and goethite are the magnetic carriers. Measurement of the anisotropy of the magnetic susceptibility (AMS) from both ore and karstic internal sediments has revealed consistent directions and horizontal magnetic fabrics, with the magnetic lineation usually paralleling the direction of the calamine veins. The attitude of the AMS parameters (PJ and T) suggests an absence of significant deformation supporting the theory that the magnetic fabric originates from sedimentary dynamics within the veins. High temperature components of primary origin have been isolated from the localities. The reverse and normal directions of this component are antipodal and similar within error for both calamines and karst-related internal sediments, suggesting that non-sulfide ore deposits and karst-related internal sediments are contemporaneous. We therefore calculated an average paleomagnetic direction for each locality. The corresponding poles are consistent with the last 30 Ma of the Apparent Polar Wander Path (APWP) of Africa. These promising results pave the way for an efficient method to constrain the age of Zn supergene deposits. The proposed three-step scenario can be placed within the tectonic evolution of the Moroccan High Atlas belt. Deposition of primary sulfides is contemporaneous with opening of the Tethyan and Atlantic oceans. During the Tertiary, intracontinental deformation has given rise to the High Atlas fold-and- thrust belt and to regional uplift. As a result, Zn-Pb sulfides, hosted in carbonates experienced oxidation under an arid climate (with short intermediate humid episodes facilitating karst development) to form karst-related Zn-Pb non-sulfide ore bodies.
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insu-01143082 , version 1 (16-04-2015)


  • HAL Id : insu-01143082 , version 1


Flavien Choulet, Nicolas Charles, Luc Barbanson, Yannick Branquet, Stanislas Sizaret, et al.. Non-sulfide zinc deposits of Morocco: from mineralogy to age characterization. International Mineralogical Association IMA 2014, Sep 2014, Johannesburg, South Africa. pp.97. ⟨insu-01143082⟩
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