Sources of helium and associated heat in hydrothermal fluids from the central western branch of the East African Rift System (Democratic Republic of Congo and Burundi)

The area around Lake Kivu, situated in the western branch of the East African Rift System (WBEARS), which spans the Democratic Republic of Congo, Rwanda, and Burundi, is characterized in its southern part by numerous hot springs. Chemistry and isotopic signatures of hydrothermal fluids have been studied in the northern part of Lake Kivu due to the presence of one of the world's active volcanoes, Nyiragongo. However, the areas along the coast of Lake Kivu, extending south to the shores of Lake Tanganyika, where active volcanism is currently absent, have been largely overlooked, despite the presence of several tens of hydrothermal manifestations with temperatures ranging from 25 to 86 °C, located in a highly faulted region characterized by low to moderate seismicity. This study aimed to fill the gap by collecting samples from 15 hot springs, four lake waters, and one cold spring, located in the Democratic Republic of Congo and Burundi. Stable isotopes of water (δ 2 H and δ 18 O), noble gas isotopes of He, Ne, and Ar, Sr isotopic ratio ( 87 Sr/ 86 Sr), and major ions and trace elements were measured. Most water samples are classified as Na-K-HCO 3 or Ca-Mg-HCO 3 types. Stable isotopes indicate that the waters are of meteoric origin. The helium isotopic ratios ( 3 He/ 4 He or R), normalized to the atmospheric ratio (Ra = 1.384 × 10 −6 ), range from 0.058 to 1.304, indicating a dominant crustal helium source and a maximum of 13 % of mantle helium. The 87 Sr/ 86 Sr ratios range from 0.70362 in the Lac Vert sample, typical of mafic volcanic products, to 0.77520 in hot springs of the Ruzizi basin, a very radiogenic value possibly indicating water-rock interaction with the Proterozoic silicate basement of Central Africa. The 87 Sr/ 86 Sr shows a rough trend with the 40 Ar/ 36 Ar ratios, indicating a slightly detectable terrigenic 40 Ar* excess in hot spring water. These results contrast with those on the northern shore of Lake Kivu, which are characterized by magmatic helium, suggesting that moving further south, the heat is controlled by the local geothermal gradient, with fluids likely circulating deeper into the crust and warming up. This circulation is facilitated by the extensional faults of the African rift, where most of the hot springs are located. Calculated low crustal fluxes of helium in the Lake Kivu region are insufficient to create economically valuable helium reserves, as found in the southern termination of the WBEARS, in the Rukwa Rift Basin. • Thermal springs and crater lake water south of Lake Kivu were sampled for chemistry and isotopes. • Stable isotopes indicate a meteoric origin, and chemistry suggests immature waters. • The presence of little radiogenic 40 Ar* and radiogenic 87 Sr/ 86 Sr ratios suggests that fluids may circulate deep in basement rocks, heated by the local geothermal gradient. • Geothermal potential is likely restricted to low- to medium-enthalpy hydrothermal systems. • Low crustal helium fluxes in the region cannot create economically exploitable helium reserves, such as those found in the Tanzanian termination of the African Rift.