Quantitative records of paleotemperature in Qarhan Salt Lake, Qaidam Basin and its relationship with potassium deposits.

Qarhan Salt Lake, located in the Qaidam Basin on the Qinghai-Tibet Plateau in northwestern China, is the largest potash production base in China. The main potash deposit in Qarhan Salt Lake is a comprehensive deposit in which the liquid potassium resources dominated by potassium-rich and lithium-rich brine coexist with the low-grade solid potassium salt deposit composed of solid potash minerals (polyhalite, carnallite, sylvite). Polyhalite is one of the representative potassium minerals, widely distributed in the Bieletan area of Qarhan Salt Lake, coexisting with halite, and its deposition indicates that the paleo-temperature of brine in this area once fluctuated obviously. Twenty core samples from two boreholes in the Bieletan area of the Qarhan Salt Lake were subjected to fluid inclusions thermometry, and homogenization temperatures reflecting the brine temperatures during the precipitation of evaporite minerals in the Salt Lake were obtained, ranging from 12.4 to 28.9 °C and 9.2 to 19.8 °C, respectively. By scanning electron microscopy, acicular polyhalite and pompom-like aggregate of polyhalite have been identified in the sampling layer, indicating that evaporite deposition have reached the stage of potassium salt deposition. The temperature fluctuations of the fluid inclusions and the elemental chemical data are compared vertically, and the sections with significant temperature fluctuations are close to the layers where polyhalite were deposited. It shows that the paleo-temperature fluctuation impact the sedimentary environment of brine, and then affected the polyhalite deposition of Qarhan Salt Lake.