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The origin of Middle Jurassic evaporites in the Qamdo Basin is still controversial because palaeontological studies have reported that they have both marine and continental characteristics. The ⁸⁷Sr/⁸⁶Sr ratios of the gypsum in the Middle Jurassic Dongdaqiao Formation in the Qamdo Basin range from 0.707602 to 0.708163, which are higher than that of contemporaneous seawater. Model calculations suggest that continental water prevailed over seawater during the precipitation of these evaporites. However, the majority of the gypsum samples have δ³⁴S values of 15.3‰ to 16.3‰, which are consistent with that of contemporaneous seawater. This range of values (15.3‰ vs. 16.3‰) was likely caused by S isotope fractionation during evaporation because the δ³⁴S values and Sr contents are negatively correlated. The δ³⁴S values of the other three gypsum samples are 20.0‰, 20.5‰, and 20.8‰, which are significantly higher than that of Middle Jurassic seawater. The trace element compositions and scanning electron microscopy (SEM) observations indicate that these elevated δ³⁴S values were caused by bacterial sulphate reduction (BSR). The Sr and S isotope systematics of the gypsums from the Dongdaqiao Formation demonstrate that the parent brines from which the evaporites precipitated were marine based with a large quantity of continental input. A comparison of the lithologies and Sr isotope compositions of the Middle Jurassic sequences in the Qamdo and Qiangtang Basins revealed that the Qiangtang Basin was mainly recharged by Jurassic seawater, while the Qamdo Basin was primarily recharged by continental water with some seawater-derived overflow from the Qiangtang Basin. Full article

This paper discusses the origin and evolution of saline springs in north and central Laos, based on chemical and stable isotopes (δD, δ¹⁸O, δ¹¹B, and δ³⁷Cl). All the saline springs in this study are of the Na–Cl geochemical type. The geochemical and water isotope values suggest that the saline springs in this study are mainly derived from meteoric water and/or ice and snow melt from the surrounding mountains and that they also experienced strong evaporation and intense rock–water interactions. The ionic ratios, characteristic coefficients, ternary Ca–SO₄–HCO₃ phase diagrams, and saturation indices of minerals show that the dissolution of halite, sulfate, and carbonate rocks may be the solute sources for saline springs in this study, whereas the underground brines in the Thakhek potash mining area are geochemically influenced by the dissolution of carnallite and sylvite. The global geothermal δ¹¹B–Cl/B relationship and δ¹¹B values (5.50 to 36.01‰) of saline springs suggest a continental origin of B. This B is most likely derived from marine carbonate rocks and marine evaporates (gypsum and halite) of the late Cretaceous, which is similar to the saline springs of the Nangqen–Qamdo–Simao Salt Basin. The δ³⁷Cl value (−0.12 to +0.79) and the Cl/Br ratio (4076 to 9853) show that dissolution of late cretaceous marine halite layers, atmospheric precipitation, and water–rock interactions between volcanic rocks, mudstones, and sandstone can restrict the δ³⁷Cl values in saline springs. Results from silica geothermometry and multi–mineral equilibrium diagrams indicate that the reservoir temperatures for the saline springs range from 87–137 °C and experience deep circulation. Hydrochemical characteristic coefficients suggest that saline springs in the Muang Say basin may have leached sylvinite and carnallite and that the potash exploration prospect in this area is relatively good. Full article

Qamdo basin is located between the suture zone of Jinsha River (Ailao Mountains) and that of Ban Gong Lake (Nujiang) in the eastern Tethys. Part of the Jingxing Formation is deposited in the southwest of the basin. In this study, two profiles were investigated from the north and south of Qamdo basin. The characteristics of detrital zircon LA-ICP-MS U-Pb age, and the main and trace elements of sandstone were analyzed. The characteristics of major and trace elements showed that the tectonic setting of the study area is mainly composed of a relatively stable active continental margin and a passive continental margin, showing characteristics of a continental island arc. The weathering degree of Jingxing Formation in the Qamdo area is lower than that in the Lanping-Simao area, which may be closer to the origin. The age distribution characteristics of detrital zircon grains indicate that the Qiangtang Block, Youjiang basin, and Yangtze area jointly constitute the provenance of the Qamdo-Lanping-Simao basin. Both basins may be part of a large marine basin with unified water conservancy connection before evaporite deposition. Metamorphic seawater from the Qamdo basin may migrate to the Lanping-Simao basin and even the Khorat basin, where evaporite was deposited. Full article