A new type of magnesite mineralization in Eskisehir - Erenkoy in northwestern Turkey is reported. The mineralization is located in the Izmir - Ankara suture zone, associated with ultramafic rocks which lie on top of metamorphites. The deposits are heavily deformed due to tectonic effects. Magnesites have been analyzed for their delta C-13 and delta O-18 stable isotope contents in order to determine the origin of mineralizing fluids. Results show mean values of -8.4 for delta C-13 and 31.9 for delta O-18. The CO2 responsible for the mineralization is postulated to be a mix of atmospheric origin and decarboxylation of organic sediments.