Characterization and origin of clays in the Bou Arhous (Morocco) non-sulfide Zn ore deposit
Résumé
Supergene non-sulfide Zn deposits offer high mineralogical diversity including clay
minerals that can incorporate significant amounts of Zn like sauconite. Although clayey
material is commonly observed in such ore deposits, its nature and origin have been
rarely studied, especially in carbonate-hosting environments. However the role of clay
minerals is essential for mineral processing because they usually have a negative impact
on zinc recovery.
In this work, we focus on the Bou Arhous Zn-Pb ore deposit in Eastern High Atlas,
Morocco. This deposit is located in an anticline formed by Lower Jurassic limestone
and marl, with Upper Triassic shale in the core of the anticline. Zn-rich clayey material
fills karstic cavities and two types of clay rich deposits have been identified field: 1)
white to ochre granular clay (kaolinite/smectite (K/S) irregular mixed layers, smectite
and illite) and 2) red smooth clay (kaolinite, chlorite, illite). The white to ochre granular
clays are closely associated with supergene willemite (zinc silicate), one of the main Zn
bearing mineral phases of the Bou Arhous deposit. The red smooth clays fill
karst-related cavities cutting across the non-sulfide mineralized bodies. The red smooth
clays present evidence of stratification that reflects internal sedimentation processes
during karst evolution. Bulk chemical analyses reveal that the Zn content is highly
variable in all clayey samples and even can reach several percent. This enrichment may
be due to fine inclusions of zinc silicate or carbonate in the clayey material, or to the
occurrence of Zn clay minerals (i.e. sauconite).
The mineralogy of clays and associated minerals in bulk powders and separated <2μm
oriented fractions from the two types of clay deposits and from surrounding Triassic to
Lower Jurassic rocks has been investigated. X-ray diffraction (XRD) analyses
complemented by Scanning Electron Microscope (SEM) observations, have allowed us
to identify the nature of the clay minerals and to characterize the textural relationships
between clays and other zinc-bearing minerals like willemite. In a few samples with
high zinc content (5%), bulk XRD analyses do not reveal the occurrence of zinc
sulphides, carbonates or silicates, suggesting that zinc is associated with clay minerals.
Preliminary XRD results indicate that white and ochre clays are composed of
kaolinite/smectite (K/S) irregular mixed layers with up to 20% of smectite in some
samples; smectite, illite and chlorite are also present. On the other hand, the red clays
contain kaolinite, chlorite, illite, and illite/smectite mixed layers (goethite is commonly
present in these samples). SEM observations demonstrated that willemite crystals are
partially dissolved and are surrounded by authigenic clay minerals. Microprobe
analyses and cation-exchange capacity on clay mineral fractions are currently
performed to highlight the localization of zinc within the structure of clay minerals, and
to provide crucial information on Zn recovery potential of the studied deposit. All these
new results will enable characterizing Zn speciation in these supergene deposits to
check the following hypotheses for clay origin: 1) interactions between Zn-rich solution
due to supergene oxidation and residual clays originated from dissolution of
limestone-dominated host rocks, 2) secondary alteration of Zn minerals like willemite
into clay minerals, and 3) reaction between detrital clays and Zn-rich solutions.