The Co, Ni, Cr and S mineralisations during serpentinization process in the Bou Azzer ore deposits (Anti-Atlas, Morocco)

The serpentinization of the harzburgitic mantle section of the Bou Azzer Neoproterozoic ophiolite is studied by field mapping, structural observations, optical micrography, SEM and X-ray diffractometry, and relationships with Co, Ni, Cr and sulphides minerals are discussed. Serpentinization occurred in two major steps with, i) intra-oceanic pseudomorphic serpentinization materialized by the isotropic serpophite often associated with antigorite fibres preserving the primary form of olivine crystals, and ii) tectonic serpentinization with lizardite and chrysotile crystallisation during ophiolite obduction and deformation in relation with synkinematic granitoid intrusions (ca. 650-635 Ma). Brucite is fairly frequent as inclusions in the serpentine aggregates. The serpentinization processes led to the remobilization of S, Fe, Ni and Co included in the primary magmatic assemblage. The sulfide assemblage varies with the degree of serpentinization, with from low to high degrees: pentlandite + pyrrhotite+ pentlandite + heazlewoodite (godlevskite); pentlandite + polydymite (or violarite); pentlandite +millerite, and polydymite + millerite. The presence of orcelite, the only stable Ni arsenide during serpentinization, reflects conditions of high activity of Ni and low activity of Fe in the fluids involved in serpentinization. The relative frequency of maucherite underlines the importance of arsenic in mantle-sourced fluids. The values of the arsenic activity could locally be as high as that of sulfur to form an immiscible As-rich liquid, which is the origin of As-rich paragenesis of high temperature.