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 iso topic composition and stable isotopes (O and H). Chemical profiles for four Au mines (Western Deep Levels, Vaal Reefs, Freddies and Presiden t Steyn) are used to infer origin, chemical and isotopic evolution of the brines and for comparison with basement brines from other Precambr ian areas. Systematic relationships are observed between Sr-87/Sr-86 a nd 1/Sr : two major mixing trends are required to explain the range of very radiogenic end-members. Possible end-members for the two compone nts 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 expec ted for present-day seawater and are even higher than some locally-der ived low Rb minerals from the Precambrian basement. The brines acquire d radiogenic Sr-87 through interaction with granitic basement (Kaapvaa l Craton), shales of the Central Rand Group (in particular Ventersdorp Contact Reef), siliclastics within the West Rand Group and particular ly with dolomites from the overlying Transvaal Supergroup. In order to obtain their multielement and radiogenic isotope signatures the meteo ric waters descended through fractured rocks between the main dykes an d faults (example: Oberholzer and Bank Dyke at Western Deep Levels). T hey slowly interacted with Au bearing conglomerates and quartzites of the West Rand Group. The inverse relationship between Sr isotope compo sition and precious/heavy metal concentration suggests fluid/rock inte raction within the shales and volcanics of the Ventersdsorp Contact Re ef (high in Au and total organic carbon [TOC], less radiogenic stronti um composition by comparison with other waters). Oxygen and hydrogen i sotopes reveal a pattern that classifies the samples as meteoric but w ith modifications. The samples show extreme values especially for West ern Deep Levels and President Steyn. (C) 1997 Published by Elsevier Sc ience Limited.