Abstract
Geothermal waters from the Semi, Dagejia and Kawu hot springs in the Shiquanhe-Yarlung Zangbo geothermal field of southern Tibet (China) are highly enriched in rare alkali metals (RAM). However, the enrichment mechanism is still hotly debated. Here, we report the first silicon isotope data of these geothermal waters to unravel the origin of the extreme RAM enrichments. Sinter precipitation in the spring vents and water-rock interaction in the deep reservoir controlled both the silicon budget and silicon isotope fractionation. The rates of water-rock interaction and sinter precipitation in three spring sites decrease in the sequences Semi > Kawu > Dagejia, and Dagejia > Kawu > Semi respectively. Silicon isotope fractionation during sinter precipitation (i.e. Δ30Siprecipitate-solution < −0.1‰) is less than that due to water-rock interaction (i.e. Δ30Sisolution-rocks at least as high as −0.47‰), which makes it possible to use the δ30Si signatures of springs to evaluate the intensity of water-rock interaction. Based on the available evidence, a conceptual model of RAM enrichment is proposed: (i) persistent magmatic activity in southern Tibet provided the initial enrichment of the RAM in host rocks and a heat sources for the deep reservoirs of geothermal systems; (ii) the high Cl− content and long residence time (thousands of years) promote the leaching of RAM from the silicate host rocks.
Highlights
The Mediterranean-Himalayan geothermal belt in southern Tibet is one of the most active geothermal regions in the world
The geothermal waters were classified into two types based on their chemical compositions (Table S1), namely a Na-HCO3− type with lower concentrations of SiO2, B, and rare alkali metals (RAM) and a Na-Cl− type with higher concentration of SiO2, B, and RAM
The δ30Si values of the geothermal water ranges from −0.67‰ to +0.25‰ (−0.37‰ to −0.21‰ at Kawu, +0.13‰ to +0.25‰ at Dagejia and −0.67‰ to −0.44‰ at Semi) and are generally more negative than those of geothermal water elsewhere
Summary
The Mediterranean-Himalayan geothermal belt in southern Tibet is one of the most active geothermal regions in the world In this region, precipitates from hydrothermal springs display extreme enrichments in rare alkali metals (RAM) and boron, exemplified by rare geyserite Cs-deposits at the Dagejia, Semi and Gulu sites[1]. The most intense geothermal activity occurs in the Lhasa Terrane near the Indus Yarlung Zangbo Suture Zone, on the southern margin of the Gangdise Magmatic Arc[6]. This part of the Terrane is dominated by the Cretaceous-Tertiary Gangdise Batholith and the Paleogene Linzizong volcanic succession which are accompanied by minor Triassic-Cretaceous volcano-sedimentary rocks[15]. The siliceous sinter is better developed than around the Kawu site, it is thinner (≤5 m in thickness) and less extensive than that the Dagejia site (Fig. 1e)[1]
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