The origin of rare alkali metals in geothermal fluids of southern Tibet, China: A silicon isotope perspective

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. Δ 30 Si precipitate-solution < −0.1‰) is less than that due to water-rock interaction (i.e. Δ 30 Si solution-rocks at least as high as −0.47‰), which makes it possible to use the δ 30 Si 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.

Geochemical processes and mechanisms for cesium enrichment in a hot-spring system

“…The mineralization of stage V is closely associated with the primary geothermal vent of ~4 m diameter (Fig. 1d), and the primary geothermal vent fluids' Cs concentrations are up to ~5 μg/mL (Zheng et al, 1995;Wang et al, 2019).…”

“…The mineralization temperature is 40 °C for stage III (Zhao et al, 2008). The excellent correlation between Cs and Cl in geothermal fluids (Zhao et al, 2006b;Wang et al, 2019) implies that both elements migrate together as CsCl.…”

“…The contents of Cs (up to ~5 ppm), Li (~5 ppm), and HBO 4 (~100 ppm) in geothermal or ore-forming fluids of the Tibetan geothermal area are several to dozens of times the average abundance of the Earth's crust (Wang et al, 2019). Note the high Li and B contents in geothermal fluids compared to Cs (Wang et al, 2019), but Li and B are not in geyserite (Li, 0-44 ppm; B, 0-400 ppm, and Cs, Table A3). The enrichment factor of Li and B is several times higher than that of fluids (Li up to nine times; B up to four times).…”

“…The enrichment factor of Li and B is several times higher than that of fluids (Li up to nine times; B up to four times). The contents of Be (~0.1 ppm), Ga (~0.1 ppm), Rb (~0.6 ppm), and Sr (~0.2 ppm) (Wang et al, 2019) in geothermal fluids are negligible, but they can be enriched hundreds to thousands of times in ore (Be, 0.6-600 ppm, up to 6000 times; Ga, 0-585 ppm, up to 5850 times; Rb, 20-300 ppm, 33-500 times; Sr, 0-625 ppm, up to 3000 times). The results imply the property of adsorption selectivity of the ore, indicating the adsorption capacity of Be 2+ , Ga 3+ , Rb + , Sr 2+ , and Cs + and no adsorption capacity of Li + and B 3+ .…”

Active metal deposition in a giant geothermal system

“…The Tibetan Plateau is one of the most intensive geothermal regions globally (Nelson et al, 1996). Geothermal spring waters in South Tibet commonly have high concentrations of trace metals such as Li, Rb, Cs, B and Au (Grimaud et al, 1985; Wang et al, 2019; Wei et al, 2001). This area was selected for this study in order to trace fluid and metal fluxes through extensive regional sampling and geochemical analyses of the major‐ and trace‐element and O–H isotopic compositions of geothermal springs and surface waters in relation to regional fault zones.…”