B–Li differential enrichment of geothermal systems in the Da Qaidam and Gonghe–Guide Basin, northeastern Tibetan Plateau: Evidence from water chemistry and H–O–B–Li isotopes

Abstract

Typical geothermal systems in the Da Qaidam (DQ) and Gonghe–Guide Basin (GGB) on the northeastern Tibetan Plateau (NETP) discharged different B Li contents. A widely accepted metallogenic model is that the salt–lake type B Li deposits in the TP are recharged by geothermal fluids with B–Li–rich, carried by rivers and enriched in the terminal salt lakes. The B–Li–rich geothermal water is the key source of mineralization in salt lakes, however, enrichment mechanism governing differential B Li contents in DQ and GGB geothermal systems remains ambiguous. This study systematically deciphers water chemistry and isotope characteristics (δD, δ 18 O, δ 11 B, δ 7 Li) of river waters, geothermal waters, sinters and surrounding rocks to discuss the enrichment process of B Li elements in the DQ and GGB geothermal systems. The δD and δ 18 O values of geothermal systems in the DQ (δD = −79.60 ∼ −82.40 ‰, δ 18 O = −10.97 ∼ −11.38 ‰) and GGB (δD = −64.00 ∼ −97.10 ‰, δ 18 O = −8.70 ∼ −13.00 ‰) are close to the GMWL and LMWL, indicating meteoric origin. The δD and high Cl − values (300–900 mg/L) of geothermal waters in the DQ and Qiabuqia, Qunaihai, Zhacangsi along Riyueshan of the GGB imply that these geothermal waters mixed by magmatic fluid and meteoric water. The hot springs in the DQ (B = 38.35–46.29 mg/L, Li = 3.11–3.72 mg/L) and GGB (B = 0.17–8.16 mg/L, Li = 0.08–10.49 mg/L) exhibit differential B Li geochemical characteristics. B and Li contents are higher in DQ hot springs and in hot springs along Riyueshan of the GGB, respectively. Comparison of the B Li contents and δ 11 B–δ 7 Li values of geothermal waters, sinters and host rocks reveals that B Li contents of geothermal waters are controlled by B–rich HP-UHP metamorphic rocks formed by metamorphism and Li–rich granites or pegmatites formed by magmatism in the Qilian Shan, respectively. Moreover, metamorphic and magmatic processes, combining with deep circulation reactivated by the thrust or strike–slip faults, create differential enrichment of B Li elements in the geothermal systems, such as DQ and GGB on the NETP. This study highlights into understanding the differential enrichment of B Li in the geothermal system on the TP. Furthermore, the resource elemental abundance of geothermal waters can be applied as an important prospecting indicator of endogenous B Li deposits.

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