Hydrogeochemical and isotopic signatures elucidate deep subsurface hypersaline brine formation through radiolysis driven water-rock interaction

Geochemical and isotopic fluid signatures from a 2.9 – 3.2 km deep, 45 55 ℃ temperature, hypersaline brine Moab Khotsong gold uranium mine in the Witwatersrand Basin of South Africa were combined with radiolytic water-rock exchange models to delineate evolution over geologic time, explore conditions for habitability. The brines (Ca-Na-Cl) 215 246 g/L TDS, Cl - concentrations up 4 mol/L suggesting their position as end-member significantly more saline than any previously sampled fluids. revealed low DIC (∼0.266 ∼1.07 mmol/L) high (∼8.49 ∼23.6 DOC pools, several reduced gaseous species (up 46% by volume H 2 ) despite microoxic (Eh = 135 161 mV). Alpha particle radiolysis water , O along anhydrous-silicate-to-clay alteration reactions predicted concentration deuterium enrichment fracture waters period >1.00 Ga, consistent reported 40 Ar noble gas-derived residence times 1.20 Ga this system. In addition, production 7 26 nmol/(L x yr) 3 11 1 8 was 100 μ g/g 238 U dosage scenarios, supporting significant source oxidant deep time that are available biomass system (10 10 cells/mL). host rock lithology predominately Archaean quartzite, minerals exposed on surfaces included calcite, pyrite, chlorite. Signatures δ 18 calcite 13 C Δ 33 S 34 pyrite 87 Sr/ 86 Sr obtained secondary ion mass spectrometry (SIMS) microanalyses suggest discrete events basin cooled peak greenschist equilibrium present-day temperatures. physiochemistry, geochemistry, cellular abundances different those younger, shallower, salinity dolomitic same mine, both service water. These results indicate discovery one few long-isolated systems supports subsurface formation via extended interaction, an example where abiotic geochemistry may support microbial community.