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

International audienceIn Northern Africa, Zn-Pb ore deposits are numerous, and Zn ores of supergene originhave been long extracted for local use using calcination to smelt the metals. While therenewal of interest in non-sulfide ores coincides with improvements in mineralprocessing, the recent opening of a Zn-oxides production plant in Morocco attests to themultipartite will to revive interest in the non-sulfide ores disseminated throughout theAtlas Mountains, as it represents a direct source of employment and an opportunity tocreate 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 theMoroccan High Atlas, to understand processes and timing of their formation. Oreoccurs either in veins associated with a faulted anticline, or in orthogonal fractures orstratabound horizons in tilted strata. A vertical zoning from deep Zn-Pb sulfidestratabound lenses to shallow non-sulfide deposits has been reported. Zn (Pb)carbonates, Zn silicates and hydrated associated minerals directly replace the sulfide orebodies 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 parageneticevolution for all deposits, as follows: 1) formation of the protore sulfides, 2) earlysupergene weathering with formation of Zn-Pb-bearing carbonates and ironoxi-hydroxides replacing protore sulfides or in the form of karst-related internalsediment and 3) late supergene weathering with deposition of Zn-carbonates,Zn-silicates and hydrated phases. Direct replacement of Zn-Pb sulfides stronglycontrolled by pyrite oxidation is accompanied by precipitation of zinc non-sulfideminerals in cavities or karst-related internal sediment filling.A complementary rock magnetism study from the Toulal and Aït Labbes ore depositshave been carried out. It shows that magnetite and minor hematite and goethite are themagnetic carriers. Measurement of the anisotropy of the magnetic susceptibility (AMS)from both ore and karstic internal sediments has revealed consistent directions andhorizontal magnetic fabrics, with the magnetic lineation usually paralleling the directionof the calamine veins. The attitude of the AMS parameters (PJ and T) suggests anabsence of significant deformation supporting the theory that the magnetic fabricoriginates from sedimentary dynamics within the veins. High temperature componentsof primary origin have been isolated from the localities. The reverse and normaldirections of this component are antipodal and similar within error for both calaminesand karst-related internal sediments, suggesting that non-sulfide ore deposits andkarst-related internal sediments are contemporaneous. We therefore calculated anaverage paleomagnetic direction for each locality. The corresponding poles areconsistent 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 Znsupergene deposits.The proposed three-step scenario can be placed within the tectonic evolution of theMoroccan High Atlas belt. Deposition of primary sulfides is contemporaneous withopening of the Tethyan and Atlantic oceans. During the Tertiary, intracontinentaldeformation 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 aridclimate (with short intermediate humid episodes facilitating karst development) to formkarst-related Zn-Pb non-sulfide ore bodies