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

Geochemical and isotopic fluid signatures from a 2.9-3.2 km deep, 45-55 & DEG;C temperature, hypersaline brine from Moab Khotsong gold and uranium mine in the Witwatersrand Basin of South Africa were combined with radiolytic and water-rock isotopic exchange models to delineate brine evolution over geologic time, and to explore brine conditions for habitability. The Moab Khotsong brines were hypersaline (CaNa-Cl) with 215-246 g/L TDS, and Cl- concentrations up to 4 mol/L suggesting their position as a hypersaline end-member significantly more saline than any previously sampled Witwatersrand Basin fluids. The brines revealed low DIC (-0.266-�1.07 mmol/L) with high (-8.49-�23.6 mmol/L) DOC pools, and several reduced gaseous species (up to 46 % by volume H2) despite microoxic conditions (Eh = 135-16 1 mV). Alpha particle radiolysis of water to H2, H2O2, and O2 along with anhydrous-silicate-to-clay alteration reactions predicted 4 mol/L Cl- brine concentration and deuterium enrichment in the fracture waters over a period > 1.00 Ga, consistent with previously reported 40Ar noble gas-derived residence times of 1.20 Ga for this system. In addition, radiolytic production of 7-26 nmol/(L x yr) H2, 3-11 nmo l/(L x yr) O2, and 1-8 nmol/(L x yr) H2O2 was predicted for 1-100l g/g 238U dosage scenarios, supporting radiolysis as a significant source of H2 and oxidant species to deep brines over time that are available to a low biomass system (102-103 cells/mL). The host rock lithology was predominately Archaean quartzite, with minerals exposed on fracture surfaces that included calcite, pyrite, and chlorite. Signatures of 618Ocalcite, 613Ccalcite, D33Spyrite, 634Spyrite and 87Sr/86Sr obtained from secondary ion mass spectrometry (SIMS) microanalyses suggest several discrete fluid events as the basin cooled from peak greenschist conditions to equilibrium with present-day brine temperatures. The brine physiochemistry, geochemistry, and cellular abundances were significantly different from those of a younger, shallower, low salinity dolomitic fluid in the same mine, and both were different from the mine service water. These results indicate the discovery of one of few long-isolated systems that supports subsurface brine formation via extended water-rock interaction, and an example of a subsurface brine system where abiotic geochemistry may support a low biomass microbial community. & COPY; 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Automated summary

Geochemical and isotopic analyses were performed on hypersaline brines from the Moab Khotsong gold and uranium mine in the Witwatersrand Basin of South Africa to investigate brine evolution over time and evaluate habitability conditions. The results showed that the brines were highly saline and exhibited low dissolved inorganic carbon and high dissolved organic carbon pools. Radiolysis and water-rock interactions were identified as significant contributors to the brine chemistry, with radiolytic production of H2 and oxidant species supporting a potentially low biomass microbial community. The findings also revealed distinct fluid events during the cooling of the basin, as evidenced by stable isotopic signatures.