Genesis of Co-Ni mineralizations during serpentinisation process in Bou Azzer ore deposits, the Neoproterozoic ophiolite of Bou Azzer El Graara (Central Anti-Atlas, Morocco)

English Abstract:

The neoproterozoic Bou Azzer inlier central Anti Atlas Mountains, southern Morocco represents a geological window into the Precambrian basement. It’s surrounded by a discordant Phanerozoic cover. It is interpreted as the boundary between the northern margin of West African Craton (WAC) and dismembered parts of the Anti-Atlas Pan-African orogenic belt commonly known as Bou Azzer ophilite. In Bou Azzer two main phases of serpentinization at Bou Azzer was pointed out (Wafik, 2001) : i) an oceanic syn-rifting pseudomorphic serpentinisation stage with antigorite and two continental tectonic serpentinzation stages during tectonic setting of ophiolite with ii) syn-collision extensional stage lizardite and iii) post-collision extensional stage chrysotile filling veins and faults. After this author, Serpentinite is initial reserve of Ni-Co mineralization in Bou Azzer, where is reconcentrated by tectono-magmatic events that took place later.

The genesis of Co-Ni arsenides resulted from hydrothermal and tectono-magmatic processes. A primary paragenesis of magmatic origin with pentlandite, chromospinelle, and cobaltiferous and nickeliferous magnetite has been highlighted in Ambed massive where the serpentinization of ultramafic rocks is partial, and it is probably associated with Fe sulphides. However, the mineralogical assemblages identified in Aghbar and Bou Azzer East massive where serpentinization is total are composed on secondary minerals of polydymite cobaltiferous, millerite cobaltiferous, orcelite and magnetite cobalti-nickeleferous. These mineralizations could come during serpentinization from mutation of a primary mineral pargenesis formed on olivine and pentlandite+pyrrhotite aggregates. The genesis of cobaltiferous hydrothermal mineralizations is basically related to the serpentinization process. In tectonical term, serpentinization modify serpentine plasticity and provide doming phenomenon and has important contribution on genesis and structural context of Bou Azzer mineralized corps. In chemical level, serpentinization control the mobility of nickel and cobalt exist in primary ultramafic minerals which marked in arsenides, sulfides and iron oxides.

KEYWORDS: mineralizations, hydrothermal, cobalt, nickel, arsenides.