Dating ultra-deep mine waters with noble gases and ³⁶Cl, Witwatersrand Basin, South Africa

Concentrations and isotopic ratios of dissolved noble gases, 36Cl, δD and δ18O in water samples from the ultra-deep gold mines (0.718 to 3.3 km below the surface) in the Witwatersrand Basin, South Africa, were investigated to quantify the dynamics of these ultra deep crustal fluids. The mining activity has a significant impact on the concentrations of dissolved gases, as the associated pressure release causes the degassing of the fissure water. The observed under saturation of the atmospheric noble gases in the fissure water samples (70-98%, normalized to ASW at 20°C and 1013 mbar) is reproduced by a model that considers diffusive degassing and solubility equilibration with a gas phase at sampling temperature. Corrections for degassing result in 4He concentrations as high as 1.55 · 10-1cm3STP4He g-1, 40Ar/36Ar ranging between 806 and 10331, and 134Xe/132Xe and 136Xe/132Xe ratios above 0.46 and 0.44, respectively. Corrected 134(136)Xe/132Xe and 134(136)Xe/4He-ratios are consistent with their production ratios, whereas the nucleogenic 4He/40 Ar, and 134(136)Xe/40Ar ratios generally indicate that these gases are produced in an environment with an average [U + Th]/K-content 2-3 times above that of crustal average. In two scenarios, one considering only accumulation of in situ produced noble gases, the other additionally crustal flux components, the model ages for 14 individual water samples range from 13 to 168 Ma and from 1 to 23 Ma, respectively. The low 36Cl-ratios of (4-37) · 10-15 and comparatively high 36Cl-concentrations of (8-350) · 10-15 atoms 36Cl l-1 reflect subsurface production in secular equilibrium indicating an age in excess of 1.5 Ma or 5 time the half-life of 36Cl.In combination, the results suggest residence times of the fluids in fissures in this region (up to 3.3 km depth) are of the order of 1-100 Ma. We cannot exclude the possibility of mixing and that small quantities of younger water have been mixed with the very old bulk. © 2003 Elsevier Ltd.