The Qom Formation, which hosts the Mazraeh celestite deposit, contains dominantly limestones, marl and subordinate evaporites deposited in shallow marine environments. Petrographic evidence indicates that the celestite deposits were formed by replacement of limestone. The presence of carbonate inclusions within celestite crystals as well as the pseudomorphic habits of some crystals suggests that celestite replaced preexisting carbonate. Based on petrographic evidences and celestite mineralization occurred as a late-diagenetic process during deposition of Qom Formation. Celestite occurs mostly at lower part of members of the Qom Formation as banded, rhythmic, massive, cavity and cave fillings. The layered, rhythmic and elongated lenticular celestite crystals are positioned parallel to the general bedding of the sequence and formed at early principal diagenetic stage. Relatively high strontium ion content of the Karadj Formation suggests a high probability of ion supply to form celestite from these formations.