The origin of magnesite deposits from the Polish Foresudetic Block ophiolites: preliminary δ13C and δ18O investigations

Twenty‐three magnesite samples from vein deposits, found in ultramafic rocks related to ophiolites in the north‐east Bohemian Massif, have been examined for carbon and oxygen isotope ratios. In these rocks two stages of antigorite serpentinization took place prior to magnesite precipitation. δ18OSMOW in magnesites ranges from 24.5 to 27.9%o (corrected for acid fractionation factor) which leads us to conclude a meteoric origin of the ore‐forming solution. This inference is further supported by δ13C values (−18.7 to −4.4%o) which point to a biogenic origin of carbon in the magnesites.