Ultramafic rocks are essentially constituted by silicates of magnesium: forsterite, and/or Diopside-enstatite; who’s the most common hydrothermal alteration are the serpentinization. To Bou Azzer, the serpentinisation affected almost all of the ultrabasic rocks. Among the phenomena bound to the serpentinisation observed on the ground, we distinguish the magnesium deposits (Carbonated serpentinite). These magnesian deposits correspond to veins taken in serpentinized peridotites either realizes the contact between serpentinites and ultrabasic and basic cumulats. These veins of some centimeters a few meters away from power concentrate essentially in the regions of Ait Ahmane, Ingujjem and Ambed and are formed by massive or brecciated magnesite or in cauliflower. The petrography ic and mineralogical studies of the deposits of magnesite of Bou Azzer, reveal mainly two different mechanisms: The replacement metasomatic in situ of the serpentine and the filling of fracture by circulation of a hydrothermal mineralizing fluid. A geochemical study of magnesites and serpentinites of Bou Azzer showed magnesite to be enriched in SiO2, Al2O3, Fe2O3, MgO, MnO and Sr and depleted in Na2O, P2O5, K2O and CaO relative to the hosted serpentinite. It is suggested that the ascent of hydrothermal fluids rich in CO2 pulls the change of the serpentine and the streaming of meteoric waters are at the origin of the magnesite. The latter gradually and partially replace the original serpentine, but occasionally total replacement occurs. Given the geological, structural, petrographic, and geochemical characteristics, the magnesite of Bou Azzer is of type Kraubath.