Occurrence and enrichment mechanisms of helium in geothermal systems in continental China

Abstract

Helium (He) is an indispensable and scarce strategic resource. We compiled 682 published He concentration and He–Ne–C–N isotopes data points for geothermal volatiles in continental China to systematically investigate distribution and enrichment mechanisms of helium in geothermal systems by linking fluid geochemistry with crustal structures, lithologies and thermal regimes. Active volcanic centres can be regarded as lower-priority helium provinces (He < 0.05 vol%), with up to 79 % of He from depleted MORB-type mantle. Samples with He concentrations up to standards for commercial He resources (He > 0.1 vol%) are widely distributed in the nonvolcanic geothermal systems, and can be classified as He-rich N₂ type (81 %), He-rich CO₂ type (15 %) or He-rich CH₄ type (4 %) gases. Helium in nonvolcanic regions has a purely crustal or crustal–mantle complex origin, with subcontinental lithospheric mantle contributions of < 40 %. The widely distributed Proterozoic to Cenozoic granitic rocks are the dominant He source rocks, with average ⁴He production rates 2.0–7.2 times that of the average crust. He-rich gases in all tectonic units are characterized by ⁴He/N₂ ratios of 0.001 to 0.44, likely formed by variable mixing of a crustal component (radiogenic ⁴He and He-associated N₂) from granitic and metamorphic basements, and sedimentary (e.g., N₂ and CH₄) and atmospheric (i.e., N₂ and ²⁰Ne) components at shallow levels. In addition to these processes, in the Tibetan Plateau and eastern China, CO₂ derived from mantle and metamorphic decarbonation at depths > 10–11 km can strip He generated and accumulated in the crust, carrying it to shallow geothermal reservoirs along large-scale faults. Subsequent CO₂ loss (93–99 % on average) by calcite precipitation and dissolution processes can enrich He and N₂, forming He-rich N₂ type gases. Extremely He-rich geothermal fields (He ≥ 0.5 vol%) are distributed mainly along the Indus–Yarlung suture zone, southern Tibet detachment system and Anninghe fault in the Tibetan Plateau, the Weihe Basin and local regions in eastern China due to the existence of favorable crustal structures, and are potential sites for future helium exploration.