The origin and age of biogeochemical trends in deep fracture water of the Witwatersrand Basin, South Africa

Water residing within crustal fractures encountered during mining at depths greater than 500 meters in the Witwatersrand basin of South Africa represents a mixture of paleo-meteoric water and 2.0–2.3 Ga hydrothermal fluid. The hydrothermal fluid is highly saline, contains abiogenic CH 4 and hydrocarbon, occasionally N 2 , originally formed at ∼ 250–300°C and during cooling isotopically exchanged O and H with minerals and accrued H 2 , 4 He and other radiogenic gases. The paleo-meteoric water ranges in age from ∼ 10 Ka to > 1.5 Ma, is of low salinity, falls along the global meteoric water line (GMWL) and is CO 2 and atmospheric noble gas-rich. The hydrothermal fluid, which should be completely sterile, has probably been mixing with paleo-meteoric water for at least the past ∼100 Myr, a process which inoculates previously sterile environments at depths > 2.0 to 2.5 km. Free energy flux calculations suggest that sulfate reduction is the dominant electron acceptor microbial process for the high salinity fract...