Almost 230 years after it was first described, formation of the mineral dolomite remains enigmatic. Dolomitic rocks are abundant throughout the geological record, and most are thought to form by replacement of limestone. Recent work has identified substantial volumes of high-temperature primary dolomite deposited in Cretaceous lacustrine rift basins in northern China. Here we provide the missing conceptual understanding of the fundamental processes that may have formed such dolomitic sediments by combining an analysis of the physics and chemistry of sub-lacustrine hydrothermal systems with new data on the mineralogy and geochemistry of the dolomites. This novel mechanism accounts for systematic changes in the characteristics of the dolomite sequence that intimately link with the evolution of convection within the fault damage zone and venting of brine-rich mixtures to the lake basin, and provides a new model to be tested against dolomitic sediments in high-enthalpy settings such as rifting basins. The dataset is used to decrible geochemical characteristics of dolomites in the paper.