The magnesite deposit Kömürlük, located 30 km west of the city Eskişehir in Anatolia/Turkey, and its surroundings in the territory of the company MAŞ were investigated in detail in respect to geology, tectonic structure and geochemistry. The deposit Kömürlük is located in the Kinik ophiolite, which is a part of the mediterranean ophiolite belt. The obduction of these ophiolites during middle to late Cretaceous was associated with the closure of the Neotethys. The ophiolite body in the working area is made up of strongly tectonized and serpentinized harzburgite. In the footwall there is the Inönü marble and the ophiolite itself is covered by clastic Neogene sediments. The cryptocrystalline magnesite mineralization in the ophiolite is controlled by the tectonic structure. Kraubath type mineralizations occur as veins and network mineralization within and along shear zones. The mineralization is normally massive or cauliflower shaped within the shear zones. The strike direction of the mineralized tectonic structures is predominantly NE - SW. The magnesite of Kömürlük has very low values of Fe2O3 and Al2O3, SiO2 and CaO are in the middle of the quality limits. The magnesia to form magnesite derives from the serpentinized harzburgite. The CO2 is deduced from the atmosphere or from decarboxylation of organic rich sediments. Calcalkaline volcanism favours the convection of the mineralized fluids. The values of δ13C are between -11.2 and -8.1‰. Based on the values of δ18O between 23.6 and 28.7‰ the temperature for magnesite formation is calculated between 55 and 70°C. In this thesis a descendent model for magnesite formation in the area around Kömürlük is presented. For further exploration some trenches and drilling operations are recommended.