Zgounder (Ag)

Geological and Structural Framework

The Zgounder silver deposit sits within the Proterozoic Siroua “boutonnière” or massif that occupies a transitional position between the northern mobile Panafrican belt and the southern Eburnean domain comprised in the stable West-African Craton. The Siroua massif is composed of geological assemblages belonging to the Precambrian I, II and III, each separated by major discontinuities. The oldest rocks of the Siroua Massif (P1) consist of gneisses and amphibolites unconformably overlain by ophiolitic complexes, volcanosedimentary units, alternating schist-sandstones and limestones, quartzites and turbidites (PII). The Zgounder deposit occurs in the PIII assemblage (Late Neoproterozoic) characterized by a felsic calc-alkaline/alkaline volcanic activity corresponding to the initiation of rifting process announcing the Infracambrian–Cambrian transgression.

The Zgounder volcanosedimentary assemblage forms a large EW-oriented monoclinal structure with a general southerly tilt. To the north, the assemblage rests on an andesitic basement, to the west it is intruded by the Askaoun granodioritic massif (PIII), whereas to the east it is overlain by volcanosedimentary rocks of the Ouarzazate series (PII) and Neogene phonolites.
The Zgounder series is divided into three units, which are in stratigraphic order:
1) The Blue Formation (300-400m) consisting of sandstone, greywacke and shale with layers of tuff and quartz keratophyre followed by an orange rhyolitic unit;
2) The Brown Formation composed of 350 to 450m of micaceous schistose sandstone overlain by a 45m thick dolerite sill/dyke; and
3), The 900m-thick Black Formation containing at its base a felsic volcanic complex (ignimbrite, rhyolitic breccia, divitrivied rhyolite) and forming the hanging rock of the Ag-mineralization over the Brown Formation. To the south, the Black Formation culminates in sandstone, greywacke and conglomerate.