Non-sulfide zinc deposits of the Moroccan High Atlas: Multi-scale characterization and origin

Abstract

To understand the processes and timing of their formation, six non-sulfide Zn–Pb ore deposits were investigated in the Moroccan High Atlas. Sulfide and non-sulfide ores are hosted in Lower Jurassic reefal to para-reefal limestone. Zn (Pb) carbonates, Zn silicates and associated hydrated phases directly replace the stratabound primary ore bodies or fill cavities along fractures related to the Atlasic compression. Field observation has been complemented by a multidisciplinary approach (e.g. XRD, Raman, SEM, EPMA) for the mineralogical characterization. All six ore deposits present similar paragenesis revealing three successive stages for ore deposition: 1) formation of the protore sulfides, 2) early supergene weathering with formation of Zn–Pb-bearing carbonates and iron oxi-hydroxides and 3) late supergene weathering with deposition of Zn-carbonates, Zn-silicates and hydrated phases. Direct replacement of primary sulfides is accompanied by precipitation of zinc non-sulfide minerals in cavities or internal sediments filling. 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 to form karst-related Zn–Pb non-sulfide ore bodies.