ABSTRACT

The formation of lake terrace and salar-type lake-bed borate deposit at Nie`er Co Lake (hereafter Nie`er Co), northwestern Tibet, remain debated. We performed a detailed study of the hydrogeochemistry, geochemical evolution, and enrichment of Nie`er Co waters using the chemical compositions and stable isotopes (δD and δ<sup>18</sup>O) of hot springs, stream water, ice, seep water, and lake water. Ion concentrations and the total dissolved solids (TDS) were in the order: lake water ≥ stream water > seep water > ice. We classified waters into three types: T1 (hot springs), T2 (seep water, stream water, and ice), and T3 (lake water). Hydrogeochemistry evolved from Na-HCO<sub>3</sub> in headwaters to Ca-Mg-SO<sub>4</sub>/Cl in the lake, passing through an intermediate Ca-Mg-HCO<sub>3</sub> transitional stage in-between. The main processes controlling water chemistry were found to be carbonate dissolution and continuous evapoconcentration. This was confirmed using the Hardie and Eugster model, which demonstrated that the evolution of lake water during evaporation should reach a final composition of Na-Mg-SO<sub>4</sub>-Cl. Using the δD and δ<sup>18</sup>O of snow, stream water, ice, and seep water, we determined the Nie`er Co meteoric water line (NMWL) to be δD = 8.18δ¹⁸O + 30.27 (γ = 0.97). Stream water, ice, and seep water originate from the melting of mountain snow or ice. Lake water undergoes strong evaporation, and hot springs originate from local meteoric water and belong to a low-medium temperature hydrothermal system. Three main steps control the formation of the Nie`er Co borate deposits: (a) meteoric waters filter through deep, B-rich volcanic strata; (b) water flows upwards into hot springs, carrying B and other minerals into the lake recharge waters; and (c) lake water is concentrated owing to the arid and cold climate, allowing kurnakovite and other borate minerals to precipitate. The integrated hydrogeochemical approach developed in this study furthers our understanding of Qinghai-Xizang (Tibet) plateau borate deposits.