The origin of boron in boron-rich salt lakes in the Tibetan Plateau is highly controversial. In this study, we carried out a detailed study on boron geochemistry and isotope composition of lake sediments collected in Zigetang Co, central Tibet. Evaporites had high boron concentrations of 172.3-418.6 g/g and delta~(11)B values of-8.2%o to -3.3,suggesting a non-marine origin for the saline lake. The boron isotopic fractionation factor, alpha, between evaporite and brackish water (alpha_(evaporite-brackish)) decreased systematically with depth, from 0.9942 at the top of the drill core to 0.9893 at the bottom; the linear variation between alpha_(evaporite-brackish) and depth reflects boron isotopic fractionation associated with progressive crystallization. The positive correlation between delta~(11)B versus [B] and delta~(11)B versus depth in the evaporite phase reflects pH and boron speciation in the solution control on the adsorption of boron, and B(OH)_3 species incorporated preferentially into Mg(OH)_2 precipitation at high pH.