Abstract: Magnesite occurrences in Turkey are generally formed as veins and stockworks in fractures and fissuresof sedimentary rocks and altered ultramafic rocks. Ophiolitic units and magnesite formations crop out in the CentralAnatolia part of the Anatolian Block within the İzmir-Ankara-Erzincan Suture Belt and around Yakacık village,located 15 km northwest of Ankara.  The genetical investigation of the ore-forming fluid(s) was carried out using mineralogy and petrography, XRDcharacteristics, geochemical investigations and C-O isotope studies of ophiolitic units and magnesite formationsin this study. Magnesite mineralizations were observed in two different forms, as quartz magnesite veins and asvein-stockworks, within the serpentinites of the Kapaklı member, which consists of olistostromal layers containingserpentinite, gabbro, diabase, basalt, radiolarite, and limestones around Yakacık. The vein-type magnesites showcryptocrystalline and microcrystalline structures and are accompanied by quartz, chlorite, dolomite, and serpentineminerals. Quartz, serpentine, olivine, calcite, and dolomite were determined in stockwork-type magnesite. The clayminerals were chlorite, smectite, and kaolinite.In mineralogical and Raman spectroscopic studies, both the vein- and stockwork-type mineralizations,alterations from olivine to kammererite and kammererite minerals were detected. The values obtained from the resultsof the geochemical analysis indicate that magnesites are associated with ultramafic rocks. In order to determine theorigin of magnesites, δ13C (VPDB) and δ18O (VSMOW) stable isotope studies were carried out on magnesite andlimestone. The δ13C and δ18O values are -3.07 to 9.67‰ and 23.05 to 27.49‰ for magnesites and 0.20 to 3.74‰ and27.05 to 28.96‰ for limestones, respectively. The most important rocks that could be the main source for the C andO of magnesite formations are Jurassic limestones.In conclusion, the movement of surface waters towards the depths, the heating role of volcanism during theearly-middle Miocene, the decarbonation of limestones due to the warming of the deep-flowing fluids, and theincorporation of CO2, partly from a magmatic contribution, are the main formation mechanisms of magnesite formedin the fractures of serpentinized ultramafic rocks.