Abstract: Since dolomite cannot be synthetized in inorganic conditions, the cause of formation of dolomite has long been a disputed topic. A 938 m-long borehole, located in Western Qaidam Basin, contains plenty of dolomite and ankerite, which were found in the intervals of 938-438 m and 938-520 m, respectively. This study is focused on investigating the formation of dolomite and ankerite based on the evolution of the paleolake. As an evaporative mineral, dolomite cannot be synthetized without microbial activities. Therefore, evaporation and microbial catalysis are necessary for the formation of dolomite. Evaporation makes Mg2+ become highly concentrated and microbial catalysis helps Mg2+ enter into the structure of CaCO3 to form dolomite crystals. As carbonate minerals are generally precipitated in the early stage of a lake’s evolution, the contents of dolomite in the Core SG-1 in brackish conditions are higher than those in salty conditions. However, ankerite, which is a type of dolomite, has different forming conditions. Fe2+ can be from deep hydrothermal fluid and was released during the transformation process between clays. Fe2+ is participated into the structure of dolomite to form ankerite, which is mainly finished in high temperatures and inorganic conditions with minor or without microbial activities.

Key words: Qaidam Basin, dolomite, ankerite, formation, hydrothermal fluid