The origin of Cenozoic dolomite, especially formed in lacustrine environments, remains poorly understood. Magnesium is a key element in the formation of dolomite, and can be a robust tool to determine the sources of Mg2+ and (semi-)quantitatively constrain the dolomitization process, but have not been fully explored for low-temperature dolomites. Here, we present a study of a stromatolitic dolomite deposited during the Oligocene in a saline to hypersaline lake in the northern Tibetan Plateau. There are many dolomites exhibiting sub-spherical and rod-shaped crystals that are 0.1-0.25 mu m in size. The S13C and S18O values of these dolomite display a negative relationship (r =- 0.71 and-0.93 in the stromatolitic and banded dolostone, respectively), which may indicate the result of microbe activities and input of organic carbon released as a product of sulfate reduction. The S18O and S26Mg values both display an upward increasing through a core, as well as a strong positive relationship (r = 0.78 and 0.86, respectively), indicating a strong evaporational environment. However, the negative S13C values, along with strikingly negative S18O values and a lack of any correlation among the S13C, S18O and S26Mg values for dolomite from the dolomitic-anhydrite-halite-black-shale suggest the effects of sulfate reducing bacteria. Rayleigh distillation models for simulated magnesium isotopes reveal the Oligocene dolomite formation at temperatures of 20 to 40 degrees C. These new data expand a new origin mechanism model and elucidates the "dolomite problem" under low-temperature microbial environments, which provides an effective and feasible research approach to study Cenozoic and modern dolomite formation.