Quantifying formations and contributions to boron enrichment in a hot spring-river-salt lake system in the Qaidam Basin: Insights from boron isotopes and geochemical proxies

The northern margin of the East Kunlun Mountains is a distinctive basin and range transition zone characterized by diverse landforms and abundant salt resources. Within this region, the Nalenggele River Catchment is particularly notable for its rich supergene boron deposits and complete hydrological systems including hot springs, rivers, and salt lakes. This region serves as an ideal natural laboratory for studying the boron behavior during regional chemical weathering and migration processes, as well as for understanding boron ore-forming mechanisms in arid basins with alpine-cold environment. However, the sources of dissolved boron in the river and lake waters are still controversial. The relative hydrological and hydrochemical processes have not been quantitatively researched, hindering a comprehensive understanding of boron enriching processes. In this study, the geochemical compositions and boron isotopes of river waters and rock samples in the Nalenggele River Catchment were systematically analysed to understand the formation processes of dissolved boron in rivers and to quantify potential contributions. The results showed: (1) Chemical weathering plays an important role in the formation of dissolved boron in low-boron rivers (<1 mg/L), whereas the enriched boron in high-boron rivers is mainly derived from geothermal waters in the source areas. (2) Geothermal water inputs account for 86.2 % ∼ 91.4 % of dissolved boron and lithium in the Hongshui River, a tributary of the Nalenggele River. During the tributary confluence, over 70 % of dissolved boron in the Nalenggele River is contributed by the Hongshui River. (3) Boron isotopes in water are effective for source identification but less reliable for quantitative assessment of boron enrichment processes in the Nalenggele River Catchment due to its fractionation-prone property. This study presents quantitative results of boron sources, contributions, and accumulations and gives an insight into constraints of boron isotope as a geochemical signature. It is expected to provide an enhanced understandings of boron migration, enrichment, and ore-forming processes within “hot spring - river - salt lake” systems of the Tibetan Plateau.