The metasomatic nature of magnesite formation, sequence and timing of geological processes, and solution sources have been established by comprehensive geological and geochemical study of the typical Satka and Ismakaevo deposits of sparry magnesite in the South Ural province. The hydrothermal metasomatic formation of magnesite is related to injection of high-Mg evaporite brine into heated carbonate rocks within permeable rift zones. The numerical physicochemical simulation of solution–rock interaction allowed us to determine the necessary prerequisites for sparry magnesite formation: the occurrence of marine salt solutions with a high Mg/Ca ratio and heating of solutions before or during their interaction with host carbonate rocks. The contribution of compositionally various solution sources, the temperature variation regime, proportions of CO2 and H2S concentrations in solution created specific features of particular deposits.