The discontinuous ultramafic belt in the western part of the Chalkidiki peninsula, northern Greece, is characterized by extensive cryptocrystalline magnesite deposits. Within this belt, in the Vavdos area, massive cryptocrystalline magnesite veins up to 2 m thick, lie in fresh dunite and brown serpentinite. Most magnesite veins in dunite are unbranched and are less than 60 cm thick. In contrast magnesite veins in brown serpentinite may be as thick as 2 m and most are surrounded by stockworks of smaller veins. The deposits in 1976 had been mined to depths of 60 to 70 m where the magnesite vein systems continue to be strong.The minerals in the veins include magnesite, dolomite, chalcedony, quartz and calcite, deposited in that order. A thin layer of massive serpentine containing small amounts of chlorite and occasionally traces of talc and tremolite lies between the magnesite veins and the dunite. Dolomite, quartz, and chalcedony in the magnesite veins average about 3 to 4 weight percent. The magnesite itself consists of 97 to 99 mole percent MgCO 3 with 1 to 3 mole percent of calcium and trace amounts of iron and manganese carbonate.Correlation of mineralogical, structural, and geochemical data indicates that the deposits were derived from ascending CO 2 -rich solutions that leached magnesia from the ultramafic rocks well below the zone of deposition. Magnesite was deposited in progressively opening fractures as CO 2 was lost from the solutions when they approached the surface. Mineralization probably occurred during the final stages of the emplacement of the ophiolites (Cretaceous).