Geochemistry and origin of the ophiolite hosted magnesite deposit at Derakht-Senjed, NE Iran

The Derakht-Senjed magnesite deposit, hosted by Torbat-e-Heydarieh ophiolite in NE Iran, is developed as veins, veinlets and stockwork type mineralization. While the veins and veinlets only contain magnesite, the stockwork mineralization in addition contains sparry dolomite interlayered with magnesite. Magnesite and dolomite are both poor in FeO and SiO 2 . The carbon and oxygen isotope compositions of magnesite (δ 13 C V-PDB = −3.9 ± 0.1 to −5.0 ± 0.1‰; δ 18 O V-SMOW = +25.2 ± 0.1 to +26.5 ± 0.1 ‰) can be explained by contribution of atmospheric CO 2 and/or an involvement by organic carbon. Dolomite typically shows slightly lower values of δ 13 C V-PDB -5.2 ± 0.1 to −5.5 ± 0.1‰ and δ 18 O V-SMOW + 23.8 ± 0.1 to +24.8 ± 0.1‰ compared to the magnesite. The formation of magnesite at Derakht-Senjed was structurally controlled by a fracture network in the ultramafic host rocks, which provided suitable fluid pathways for leaching of Mg from the host rocks and subsequent precipitation of magnesite from carbonated solutions. It is likely that dolomite formed due to precipitation from a fluid having lower XCO 2 and higher Ca 2+ /Mg 2+ activity ratio, rather than by replacement of magnesite.