The Middle Permian Lucaogou Formation in the Jimusar Sag,eastern Junggar Basin,NW China,was deposited in a salt lake within an intracontinental rift basin with intense hydrothermal activity.Hydrothermal-sedimentary dolomite in the form of three types of dolostones,namely,analcime-feldspar dolostone (AFD),silicic dolostone (SD) and buddingtonite-albite dolostone (BAD),related to syn-sedimentary hydrothermal activity at lake bottom was discovered.The characteristics and formation mechanism of the dolomite were studied based on micron-scale petrographic and isotopic geochemical research.The syn-depositional formation of these dolostones was indicated by their rock-mineral features and syn-sedimentary deformation stage.The dolomite was composed of relatively poorly ordered proto-dolomite crystals with micron-sized spherical or sub-spherical morphology and coexisted with hydrothermal minerals,including analcime,buddingtonite,albite and chalcedony.Albite clasts were replaced by the dolomite,indicating high-temperature conditions during formation.The remarkably low strontium isotopic compositions of the dolostones (87Sr/86Sr with an average of 0.705687) indicated that mantle-derived materials might have involved in the ore-forming fluid.The dolostones had positive δ13CPDB values (with an average of 6.94‰) and negative δ18OPDB values (with an average of-8.12‰).Based on the δ18OPDB values,the formation temperatures of the dolomite were at least ～25 ℃ higher than those of the penecontemporaneous dolomite in the Lucaogou Formation in the study area.It is concluded that the dolomite precipitated from hydrothermal fluid erupting at the lake bottom.The possible genetic models are described.We suggest that the hydrothermal-sedimentary dolomite is an important genetic type,and this study may help increase the awareness of this understudied type of dolomite.