Abstract The thick middle Miocene evaporites of the Great Kavir Basin, Central Iran, are classified as MgSO 4 -poor potash-bearing deposits. In the northern domain of the Great Kavir Basin these sediments occur as 50 very large salt diapirs, as well as outcrops in several areas, including the Melheh salt pit to the south of Semnan, north Central Iran. Based on the excellent preservation of layering in these sequences, along with well-preserved bottom nucleated primary textures within the halite and sylvite, they are interpreted as largely primary deposits. They also contain some secondary minerals such as langbeinite, d'ansite, polyhalite and anhydrite. The Br content in the halite and sylvinite layers falls within the range of marine evaporites. These MgSO 4 -poor potash-bearing evaporites deposits, along with other marine and continental sediments, appear to have been precipitated in the marginal marine basins around continental rifts. Seemingly, these evaporites formed in marginal marine rift-related basins which also received input from deep, circulating hydrothermal CaCl 2 -rich brines. These hydrothermal brines were also rich in Fe and Zn and apparently were driven upward along faults and fracture zones by the thermal gradient along the axis of the extensional trough.