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