Multi-isotopes (H, O, Sr, and Li) and element geochemistry constrain the formation of Kongchutso helium-rich geothermal field in western Tibet, China

Abstract

The Kongchutso geothermal field hosts helium-rich hot springs with helium contents (up to 1.54 vol.%) higher than industrial grade of He resources (0.05 – 0.1 vol.%). However, the geochemical signature of helium-rich geothermal fluids, reservoir feature, and their genesis remain poorly understood. Here, we present element geochemistry (major and trace elements) and multi-isotopes (H, O, Sr, and Li) compositions of hot and cold springs, lake, calcareous sinters, and rocks from Kongchutso to determine the formation mechanism of helium-rich geothermal system. The major elements studies of water samples show that most helium-rich hot spring waters belong to the Na-HCO3-Cl type, possibly affected by water-rock interaction in the deep and mixing of geothermal fluids with groundwater in the shallow. Based on chalcedony geothermometers and silica enthalpy mixing model, the reservoir temperature has been calculated to be 106 – 118 °C. The H and O isotopic compositions of water samples manifest that the Kongchutso geothermal system was recharged by snow-melt water and meteoric precipitation from the surrounding mountains at an elevation of 5366 m. The widespread Cretaceous granites in the region featured by high enrichment of uranium (up to 34.15 ppm) and thorium (up to 37.49 ppm) are considered as helium and heat sources, and also are reservoir rocks. The interpretation is supported by consistent signatures of rare earth elements and Sr-Li isotopes for waters, calcareous sinters, and granites. Therefore, we suggest that the helium was initially generated via radioactive decay by U-Th in granites and then dissolved in the deep circulated and heated meteoric waters, and finally ascended to surface through the main fault systems to form the helium-rich hot springs at Kongchutso. Some other hydrothermal manifestations, such as calcareous sinters, formed accompanying the fluids migrated to the surface. Based on these new findings, we further proposed that high radioactive granite, together with tectonic system and surface hydrothermal manifestations can be used as indicators for exploration of helium-rich geothermal resources in Tibet and elsewhere in the world.