Sedimentary- and vein-type magnesites were deposited within and on ultramafic rocks of the Kop Mountain region in Bayburt province. In the field, magnesites are exposed along NE-SW trending normal faults and in fractures in the ultramafic rocks. Petrographic studies reveal that magnesite is predominantly micrite, but also occurs as microsparite formed by recrystallization of micrite. The ultramafic rocks hosting the magnesites consist of serpentinized olivine, hypersthene and diopside. Ni, Co and Ti contents of magnesites suggest precipitation from percolating water through the serpentinized ultramafic rocks. The sedimentary-and vein-type magnesites have different delta O-18 and delta C-13 values, characterizing formation under different conditions. Temperature estimates using the average delta O-18 values reveal precipitation from water at similar to 24.5 degrees C for sedimentary magnesite and similar to 37.0 degrees C for vein-type magnesite. The delta C-13 values of vein-type magnesites are distinctly more negative than those of sedimentary magnesites, indicating carbon isotopes derived from predominantly decarboxylation of organic sediments in shales and carbonate dissolution. Less negative delta C-13 values in the sedimentary magnesite are mainly due to outgassing of mineralizing water. Our data suggest a petrogenetic model in which the surface water percolates through the ultramafic and sedimentary rocks becoming heated by volcanics at depth and enriched in Mg+2 and light carbon isotopes, followed by migration upward to form magnesite near the surface in ultramafic rocks as fracture-fill and as sediment at the surface.