Geochemistry of some deep gold mine waters from the western portion of the Witswatersrand Basin, South Africa

A suite of 12 mine water samples within the Witwatersrand Basin (South Africa) were analysed for trace element concentrations, strontium isotopic composition and stable isotopes (O and H). Chemical profiles for four Au mines (Western Deep Levels, Vaal Reefs, Freddies and President Steyn) are used to infer origin, chemical and isotopic evolution of the brines and for comparison with basement brines from other Precambrian areas. Systematic relationships are observed between Sr-87/Sr-86 and 1/Sr : two major mixing trends are required to explain the range of very radiogenic end-members. Possible end-members for the two components could be : (1) Sr-87/Sr-86 = 0.7251, Sr concentration = 0.12 mg l(-1) from a Vaal Reefs compartment and Sr-87/Sr-86 = 0.7694, Sr = 39.3 mg I-1 from a Freddies compartment. (2) Sr-87/ Sr-86 = 0.7251, Sr = 0.12 mg I-1 (as above) with a Sr-87/Sr-86 = 0.7404, Sr = 745 mg I-1 from Western Deep Levels. Strontium isotope ratios range above those expected for present-day seawater and are even higher than some locally-derived low Rb minerals from the Precambrian basement. The brines acquired radiogenic Sr-87 through interaction with granitic basement (Kaapvaal Craton), shales of the Central Rand Group (in particular Ventersdorp Contact Reef), siliclastics within the West Rand Group and particularly with dolomites from the overlying Transvaal Supergroup. In order to obtain their multielement and radiogenic isotope signatures the meteoric waters descended through fractured rocks between the main dykes and faults (example: Oberholzer and Bank Dyke at Western Deep Levels). They slowly interacted with Au bearing conglomerates and quartzites of the West Rand Group. The inverse relationship between Sr isotope composition and precious/heavy metal concentration suggests fluid/rock interaction within the shales and volcanics of the Ventersdsorp Contact Reef (high in Au and total organic carbon [TOC], less radiogenic strontium composition by comparison with other waters). Oxygen and hydrogen isotopes reveal a pattern that classifies the samples as meteoric but with modifications. The samples show extreme values especially for Western Deep Levels and President Steyn. (C) 1997 Published by Elsevier Science Limited.