A large number of magnesite deposits with reserves totaling several million tons occur in Eastern Iran. These deposits are found as veins within ultramafic rocks (Afzalabad vein-type) and as lenses overlying the surrounding flysch sedimentary strata (Torshak strata bound). The occurrence of both vein-type and strata-bound deposits is not common among magnesite deposits worldwide. The Afzalabad magnesite is texturally heterogeneous and characterized by higher Mg and lower Ca, Sr, As, Ba, and B contents than the Torshak magnesite. These compositional differences are also reflected by the unit-cell parameters of the magnesite. However, magnesites from both localities have similar carbon and oxygen isotope compositions that are characteristically heavy (δ13C values of 7.2 to 8.2‰, δ18O values of 27.5 to 33.6‰). Such high δ-values are unlike those for magnesite found near and within other ultramafic complexes worldwide but are similar to the isotopic compositions of magnesite found in lagoon or hypersaline basin environments. However, the settings of Eastern Iran magnesite deposits differ from those formed in lagoons or hypersaline environments. Instead, we infer that evaporating surface water containing dissolved atmospheric CO2 percolated through fractures in ultramafic rocks, acquiring Mg by hydrolysis of Mg-rich minerals. As the fluids subsequently ascended, magnesite precipitated in the fractures within the ultramafic complex and in the open channelways in the surrounding flysch strata.