Source and Formation of Boron Deposits in Mahai Basin on the Northern Qinghai-Tibet Plateau: Clues from Hydrochemistry and Boron Isotopes

There are typical salt lake-type borate deposits in the northern Qaidam Basin, which are mainly distributed in Da Qaidam Lake (DQL), Xiao Qaidam Lake, and Mahai Basin (MHB). DQL has deposited famous solid borates and enriched a large number of brine boron deposits. It is the earliest industrial production base in China. Nanbaxian (NBX) to the west of DQL is a unique area where solid borates are deposited in MHB. Although there are three salt lakes in the MHB, borate deposits were only deposited in the salt pits of NBX, and the formation process of these borate deposits remains to be clarified. In this study, the major elements, boron contents, and d11B values in the water and sediments of NBX were investigated in conjunction with the B-Na-Mg equivalence diagrams and relevant data from other salt lakes to elucidate the source of boron in MHB and the depositional conditions of borate minerals in NBX. The results are as follows: (1) The source of boron in NBX differs from that in three salt lakes in MHB. The source in NBX is mainly constrained by the weathering and fluid-rock (Boron-bearing ultra-high pressure metamorphic belt) interaction, while that in Dezongmahai and Niulangzhinv–Balunmahai lakes are primarily controlled by river water and anticlinal brine, respectively. (2) The high boron content (0.28 to 41.38 mg/L) and low d11B values (- 34.71‰ to - 6.14‰) of the water-soluble phase of sediments in NBX are consistent with geochemical characteristics (d11B: - 23.67‰ to - 3.0‰) of borates in DQL, demonstrating that the re-dissolution of borate deposits in NBX. (3) Deposition of borate minerals in the MHB requires ionic equivalents of Mg, Na, and B to 0.02 to 0.4, 0.25 to 0.75, and 0.2 to 0.7, respectively. Additionally, the brine hydrochemistry in which the borate are deposited must be of the carbonate or sulfate type, and the brine water should be greater than 8 in pH and 400 mg/L in boron content. This study provides a theoretical basis for exploring and exploiting salt lake-type borate deposits.