Boron isotope geochemistry of salt sediments from the Dongtai salt lake in Qaidam Basin: Boron budget and sources

Abstract

Dongtai salt lake in Qaidam Basin contains abundant boron and lithium resources, and has the highest reserve of lithium found in the world, but the origin of these resources is highly controversial. In this study, we carried out a detailed study on chemical compositions and boron isotope characteristics of salt sediments collected from a drill core in the Dongtai salt lake. The evaporite, carbonate and silicate phases of the sediments show δ11B values of −6.4‰ to +2.2‰, −17.3‰ to +0.5‰ and −14.6‰ to −2.8‰, respectively, which suggest a non-marine origin for the salt lake. The variation of δ11B vs. K% identified the evolution process of the salt lake. The geochemical/hydrochemical signatures and the reserve estimation of boron resources approved the dominant sources of saline resources containing abundant B, K and Li from the inflowing Nalenggele River. The boron isotopic fractionation factors between evaporite and brine (i.e. αevaporite–brine) vary from 0.9869 to 0.9955, and the linear variation of αevaporite–brine vs. Li/Mg molar ratios reflects the boron isotopic fractionation during crystallization of the salts. The positive correlation between δ11B and [B] in the carbonate phase reflects the pH control on the incorporation of boron into the carbonates, and the equilibrium isotope fractionation follows the Rayleigh isotope fractionation rule in a closed system. The variation of δ11B values in the silicate phase behaved as an oscillating pattern, and the more negative shift of δ11B values corresponds to the mudstone strata deposited under warm–humid climate conditions, indicating the influence of intensive weathering.