The investigated license area Yildiz Maden lies 30 km west of the town Eskişehir in Turkey. It is located in the Kinik ophiolite that is part of the Dağküplü ophiolite. These allochtonous ophiolites belong to the North Anatolian Ophiolite belt, that were obducted onto the Anatolian platform during the late Cretaceous closure of the Neotethyian ocean. During the Neogene terrestrial sediments were deposited over parts of the ophiolite. In the Miocene tectonic activity along the Eskişehir Fault Zone caused strike-slip faults and extensions. In the north of the investigated area is a large tectonic shear zone, consisting of a carbonatic serpentinite breccia (CSB). In this zone multiple NNE-SSW striking dolomitic faults can be found, that indicate possible magnesite mineralizations. In the south there is serpentinte and some Neogene sediments. Between the serpentinite and the CSB there is a small occurrence of amphibolite. The serpentinite consists mainly of lizardite and weathered serpentinite contains carbonate mineralizations. Spinel compositions indicate an alpine type peridotite formed in a suprasubduction Zone setting. The cryptocristalline magnesite mineralizations are hosted by serpentinite and controlled by tectonic structures that can be linked with the Eskişehir Fault Zone. There are four main types of magnesite mineralizations: 1) massive, 2) stockwork, 3) Zebra stockwork and 4) Karnebahar stockwork. The main bodies of mineralizations in the investigated area are seperated by faults. The main strike direction of faults associated with the mineralizations is NNE-SSW and NW-SE. A silicified fault plane cuts the main mineralizations at the western side. It strikes NE-SW and dips steeply towards NW. The best magnesites regarding SiO2, Fe2O3 and CaO contents are in the south of the mining site “Birinçi Oçak”, in the massive magnesite and in the unweathered Zebra stockwork. The δ18O (V-SMOW) values of the magnesite range from 24.9 to 28.9 ‰ and the δ13C (V-PDB) range from -12.3 to -8.6‰ . Formation temperatures of magnesite are 47-74 °C , depending on the method of calculation. The source of carbon is most likely decarboxylation of sediments in the deeper crust. Recommendations for further exploration are trenching on the hill to the south and drilling in “Birinçi Oçak”, as well as drilling and trenching in the northern area.