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

• DOI: 10.1016/j.gca.2022.11.015
• Publication Date: 2022-11-17T04:59:45Z
• Authors: D.M. Nisson; T.L. Kieft; H. Drake; O. Warr; B. Sherwood Lollar; H. Ogasawara; S.M. Perl; B.M. Friefeld; J. Castillo; M.J. Whitehouse; E. Kooijman; T.C. Onstott
• Keywords: Brine

Geochemical and isotopic fluid signatures from a 2.9–3.2 km deep, 45–55 °C 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 H2) despite microoxic (Eh = 135–161 mV). Alpha particle radiolysis water H2, H2O2, O2 along anhydrous-silicate-to-clay alteration reactions predicted concentration deuterium enrichment fracture waters period > 1.00 Ga, consistent reported 40Ar noble gas-derived residence times 1.20 Ga this system. In addition, production 7–26 nmol/(L × yr) 3–11 O2, 1–8 H2O2 was 1–100μ g/g 238U dosage scenarios, supporting significant source H2 oxidant deep time that are available biomass system (102–103 cells/mL). host rock lithology predominately Archaean quartzite, minerals exposed on surfaces included calcite, pyrite, chlorite. Signatures δ18Ocalcite, δ13Ccalcite, Δ33Spyrite, δ34Spyrite 87Sr/86Sr 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.