Origin of magnetite veins in serpentinite from the Late Proterozoic Bou-Azzer ophiolite, Anti-Atlas, Morocco: An implication for mobility of iron during serpentinization

Vein-like deposits of low-TiO2 (<0.03 wt%) magnetite are present in the mantle section of Bou Azzer ophiolite, Anti-Atlas, Morocco. Two types of magnetite veins, I and II, can be recognized: magnetite is fibrous in Type I, but is stout or idiomorphic in Type II. Mode of occurrence and petrography indicate they had formed filling the open space of cracks. Magnetite is Ni-bearing in Type I, and is Ni-free and sometimes includes Mn-rich chromian spinel in Type II. Magnetite is commonly accompanied by serpentine mainly and magnesite in Type I, and by chlorite (clinochlore), serpentine, talc and lesser amount of garnet (andradite) in Type II. In situ trace-element analysis shows depletion with Al, Si, V, Cr and Zn in the vein magnetite relative to the disseminated one in wall serpentinite. Positive slope of PGE distribution pattern and Au enrichment were the result of hydrothermal activity. The formation of magnetite veins was apparently associated with enrichment of magnetite component in chromian spinel. The iron was supplied from olivine upon serpentinization which accompanied the obduction of the Bou-Azzer complex. The iron mobility may have been enhanced by high water/rock ratio of the serpentinization.