Hydromagnesite (Mg5(CO3)4(OH)2 4H2O) is a common hydrated magnesium carbonate mineral found in alkaline lakes on Earth, potentially present on Mars, and is also a key mineral for carbon capture and storage. However, the mechanisms governing its formation in alkaline lakes remain enigmatic. Extensive hydromagnesite formed during the Holocene in the alkaline Dujiali Lake, central Qinghai-Tibetan Plateau, making it an ideal field site to constrain the process of hydromagnesite formation in a modern environmental context. In this study, we report a set of magnesium isotope ratios (26Mg/24Mg expressed as δ26Mg) data from Dujiali Lake on abiotic hydromagnesite (mean=−1.35 ± 0.14‰) modern lake waters (−0.07 to +0.46‰), rivers and groundwater (−0.53 to −1.46‰). These differences in δ26Mg (and also Mg/Ca) are most likely caused by low-Mg carbonate precipitation, a process which fractionates Mg/Ca and δ26Mg. A semi-quantitative box model of the lake chemistry was developed based on carbonate equilibria to investigate the behavior of Mg isotopes during the evolution of the lake chemistry. The modeling results indicate that evaporation concentrates solutes in the lake driving saturation of multiple minerals. Aragonite reaches saturation before hydromagnesite, preferentially removing Ca relative to Mg via aragonite precipitation. This process elevates the Mg/Ca of the lake and the saturation index of hydromagnesite increasing the likelihood of hydromagnesite formation. Given that the Mg/Ca ratio of many alkaline lakes is far below that required for the formation of hydromagnesite, our findings suggest that low-Mg carbonate precipitation may be a common precursor process for abiotic hydromagnesite precipitation in evaporative environments in addition to a high Mg source likely derived from ultramafic rocks.