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Isotopic relationships of epigenetic Pb-Zn mineralisation in the Ventersdorp Supergroup near Douglas, Northern Cape Province

Department of Geology, University of the Free State, Box 339, Bloemfontein 9300, South Africa Present address: 2/1 12 Ruskin Terrace, Glasgow G12 8DY, Scotland, e-mail: hannah{at}horne.co.uk

H. de Bruiyn, A.E. Schoch and W.A. van der Westhuizen

Department of Geology, University of the Free State, Box 339, Bloemfontein 9300, South Africa, e-mail: dbruiynh.sci{at}mail.uovs.ac.za

F.J. Kruger

Economic Geology Research Institute-Hugh Allsopp Laboratory, School of Geosciences, University of the Witwatersrand, WITS 2050 South Africa., e-mail: krugerj{at}geosciences.wits.ac.za

Hydrothermally altered and mineralised lavas of the ~2.7 Ga Ventersdorp Supergroup were sampled at two localities near Douglas (Kalkdam and Katlani). Propylitic alteration has affected some of the lavas (the dark coloured amygdaloidal type), while others (the light coloured amygdaloidal type), show signs of potassium metasomatism as well, representing at least two mineralisation events. A Rb/Sr age of 2014±38 Ma was obtained on clinochlore from the amygdales and whole rock samples. This is interpreted to be the age of the first hydrothermal alteration and main mineralisation of the lavas. Variable isotopic values for Sr and Pb suggest a complex provenance for the hydrothermal fluids, whilst a narrow range of ³⁴S in galena (–3.4 to –6.3) indicates a specific source region. Four fluid inclusion populations are dominated by a brine population (T_m = –12.9°C) with a model density of 1.07g/cm³ (T_h = 117°C). The temperature of the fluid must have been ~270°C according to the chlorite geothermometer of Cathelineau. The temperature of entrapment implies a pressure of roughly 2 kb when the slope of the relevant isochore (1.07g/cm³) is taken into account. Fluids seem to have migrated along channelways that were provided during extensive fracturing such as along the Griquatown fault zone. This migration probably propagated from rocks of the Transvaal Supergroup (Griquatown and Kuruman Formations) into the underlying Ventersdorp lavas.