Hydrogeochemical and Isotopic Signatures Elucidate Deep Subsurface Hypersaline Brine Formation through Radiolysis Driven Water-Rock Interaction

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Journal Article

Geochemical and isotopic fluid signatures from a 2.9 – 3.2 km deep, 45 – 55 ℃ 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 (Ca-Na-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 – 161 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 nmol/(L x yr) O2, and 1 – 8 nmol/(L x yr) H2O2 was predicted for 1 – 100 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 18Ocalcite, 13Ccalcite, Δ33Spyrite, 34Spyrite 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.

Geochimica et Cosmochimica Acta
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Open Access