Noble Gas Study On Deep Mine Waters, South Africa
Abstract
We analyzed the dissolved noble gases of 16 water samples taken in the deep gold mines in the Witwatersrand Basin, South Africa. The fissure and borehole waters originate from 0.98 to 3.3 km depth. The noble gas data (He, Ne, Ar, Kr, Xe concentration and isotope ratios), in combination with 36Cl, δ dD, δ 18O and 14C data, are used to characterize - ideally - pristine formation water pockets in the deep subsurface. They also provide information of hydrodynamic relevance. A sample taken from the lower parts of the local dolomite aquifer is fresh water of meteoric origin, with a 14C-age of about 5.8k years and a noble gas temperature of about 18 +/- 1° C. This is the shallowest (0.98 km b.s.l.) and youngest sample of the data set. In comparison, all other samples show a noble gas abundance pattern indicating losses of the non atmospheric noble gases up to 80%. This undersaturation is most likely the result of a significant pressure release of the water, either in the formation due to the mining activity or during sampling. In the latter case, the losses of the noble gases are an artifact and should be corrected for. 4He concentrations range between 10-4 to 10-3 cm3STP g-1, the Ar40/36 ratios range from some 300 to above 10,000, Xe134/132- and Xe136/132- ratios up to 0.42 and 0.37, respectively. Model results rule out the possibility that these high ratios are mainly caused by fractionation by diffusion during the degassing of the water. All results so far indicate very long subsurface residence times; for selected samples we calculate minimum ages of the order of some ten million years.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.H22C0370L
- Keywords:
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- 1040 Isotopic composition/chemistry;
- 1829 Groundwater hydrology;
- 1832 Groundwater transport;
- 1894 Instruments and techniques;
- 5104 Fracture and flow