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

Concentrations and isotopic ratios of dissolved noble gases, Cl-36, deltaD and delta(18)O 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 20degreesC 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 4 He concentrations as high as 1.55 (.) 10(-1) cm(3)STP(4)He g(-1), Ar-40/Ar-36 ranging between 806 and 10331, and Xe-134/Xe-132 and Xe-136/Xe-132 ratios above 0.46 and 0.44, respectively. Corrected Xe-134(136)/Xe-132 and Xe-134(136)/He-4-ratios are consistent with their production ratios, whereas the nucleogenic He-4/Ar-40, and Xe-134(136)/Ar-40 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 Cl-36-ratios of (4-37) (.) 10(-15) and comparatively high Cl-36-concentrations of (8-350) (.) 10(-15) atoms Cl-36 l(-1) reflect subsurface production in secular equilibrium indicating an age in excess of 1.5 Ma or 5 times the half-life of Cl-36. 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. Copyright (C) 2003 Elsevier Ltd.