Abstract
The Co-Ni district of Bou Azzer (Anti-Atlas, Morocco) is a unique and particular type of deposit because formed in a context carbonated serpentinite. Two types of mineralisation are exploited: massive orebodies and a system of veins.The massive mineralisations are metallic bodies, in the form of lenses, oriented N120°E and essentially enriched in Ni- and Co-arsenides. They are located at the contact between serpentinite, quartz diorite and Precambrian volcanic rocks. Their formation is realised under medium temperature conditions at ca. 220°C, high salinity at 38% wt eq. NaCl fluids and pressures bracketed between 67-2883 bars.Vein systems are controlled by faults that are systematically mineralised in Co- and Fe- arsenides when they intersect the massive mineralisations. These veins intersect all the lithologic units of the Bou Azzer inlier, except for the Cambrian sedimentary formation. They are trending between NS to N070°E and are systematically associated with normal movements in vertical planes. In the horizontal plane, the mineralised faults present sinistral or dextral motions coherent with a transtensive context controlled by a N030°E shortening direction. Associated fluid yields minimum temperatures of emplacement at 170°C in average, salinity between 32 and 41% wt eq. NaCl and pressures ranging from 24 to 1800 bars.All structures describe a same mineral paragenesis, a same textures and a same type of hosted gangues (quartz and carbonates). Fluids evolve since Ni-(Co-Fe) rich end-members to Co-(Ni-Fe) rich one since massive mineralisations to vein systems inducing a continuum in the formation of both types of mineralisation.Two types of textures are observed in massive mineralisation: a Brecciated Massive Mineralisation (BMM) texture and a Laminated Massive Mineralisation (LMM) one. BMM is characterised by i) Ni-/Co-arsenides fractured by serpentine; ii) residual fragments of serpentinite, iii) spinel relics and iv) fragments of Ni-arsenides isolated within the carbonated gangue. The texture of the gangue reflects the fact that these mineralised lenses are certainly previously formed brecciated bodies of an early gangue of serpentinite and spinel relics. The LMM is described as alternating Ni- arsenides and carbonated gangue layers. The combination of these two textures reflects the early architecture of the faulted contact, i.e. the BMM are ancient brecciated bodies whereas the LMM are witness of ancient mylonitic levels.Geochemical analyses carried out on arsenides, carbonate gangues, spinels and the liquid part of fluid inclusions show a common enrichment in Co, Ni, As, Zn, Bi, Cu, Ag and Au and the same for Na/K, Li/B, V/Cr, As/V, Zn/V, Co/Cr. The close relationship between these mineralisations and their host-rocks indicates that the leaching of pre-existing ultramafic fragments (spinels and Ni-arsenides) by a mineralising fluid and the in-situ metal precipitation are the processes at the origin of the cobalt- Bou Azzer nickel.The early formation of the serpentinite brecciated gangue is interpreted as a result of crustal thinning in a hyper-extensional context that occurred around 540-560 Ma and accompanied by an initiation of oceanic crust formation. The circulation of seawater in the mantle would be the main agent of its serpentinisation. The exhumation would be the vector of the rise and the brecciation of the serpentinite put in contact with a quartz diorite. Consequently, the formation of the Bou Azzer mineralisation would begin with the serpentinisation process, described during the early stages of the mineralisation process and ended by tectonically controlled vein formation.