Dolomites occur extensively in the lower Cretaceous along syn‐sedimentary fault zones of the Baiyinchagan Sag, westernmost Erlian Basin, within a predominantly fluvial‐lacustrine sedimentary sequence. Four types of dolomite are identified, associated with hydrothermal minerals such as natrolite, analcime and Fe‐bearing magnesite. The finely‐crystalline dolomites consist of anhedral to subhedral crystals (2 to 10 μm), evenly commixed with terrigenous sediments that occur either as matrix supporting grains (Fd1) or as massive argillaceous dolostone (Fd2). Medium‐crystalline (Md) dolomites are composed of subhedral to euhedral crystals aggregates (50 to 250 μm) and occur in syn‐sedimentary deformation laminae/bands. Coarse‐crystalline (Cd) dolomites consist of non‐planar crystals (mean size >1 mm), and occur as fracture infills crosscutting the other dolomite types. The Fd1, Md and Cd dolomites have similar values of δ18O (−20·5 to −11·0‰ Vienna PeeDee Belemnite) and δ13C (+1·4 to +4·5‰ Vienna PeeDee Belemnite), but Fd2 dolomites are isotopically distinct (δ18O −8·5 to −2·3‰ Vienna PeeDee Belemnite; δ13C +1·4 to +8·6‰ Vienna PeeDee Belemnite). Samples define three groups which differ in light rare‐earth elements versus high rare‐earth elements enrichment/depletion and significance of Tb, Yb and Dy anomalies. Medium‐crystalline dolomites have signatures that indicate formation from brines at very high temperature, with salinities of 11·8 to 23·2 eq. wt. % NaCl and Th values of 167 to 283°C. The calculated temperatures of Fd1 and Cd dolomites extend to slightly lower values (141 to 282°C), while Fd2 dolomites are distinctly cooler (81 to 124°C). These results suggest that the dolomites formed from hydrothermal fluid during and/or penecontemporaneous with sediment deposition. Faults and fractures bounding the basin were important conduits through which high‐temperature Mg‐rich fluids discharged, driven by an abnormally high heat flux associated with local volcanism. It is thought that differing amounts of cooling and degassing of these hydrothermal fluids, and of mixing with lake waters, facilitated the precipitation of dolomite and associated minerals, and resulted in the petrographic and geochemical differences between the dolomites.