The magnesite veins are located in the Beni Bousera massif (internal Rif, Morocco), especially in Tafidest region about 103 km SW of the Tetouan city. They are hosted by perridotite and serpentinite formations. The aim of this study is to clarify the origin of this type of orre genesis. Geochimical study of magnesite ore reveals that is a cryptocrystalline magnesite, formed by the CO2-rich hydrothermal fluids filling factures. These solutions were brought from serpentinized area following tectonic fractures. The magnesite precipitation is due to the decrease of partial pressure of supersaturated carbon dioxide hydrothermal fluids, most likely at temperature below 300 ºC.