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Journal of Petrology Advance Access published online on November 4, 2009
Journal of Petrology, doi:10.1093/petrology/egp073
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© The Author 2009. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Geochemical and Geochronological Constraints on the Nature of the Immediate Basement next to the Mesoarchaean Auriferous Witwatersrand Basin, South Africa

H. E. Frimmel1,2,*, A. Zeh1, B. Lehrmann1, D. Hallbauer3 and W. Frank4

1Geodynamics & Geomaterials Research Division, Institute of Geography, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany
2Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa
3Ortsstrasse 36, D-07429 Döschnitz, Germany
4Ceal Laboratory, Slovak Academy of Sciences, Dubravska Cesta 9, 84104 Bratislava 4, Slovakia

Received February 17, 2009; Revised typescript accepted October 6, 2009


  Abstract

A combined petrological, geochemical, and geochronological (Rb–Sr and Sm–Nd whole-rock, U–Pb and Lu–Hf zircon, and Ar–Ar hornblende) study on a section of pre-Witwatersrand basement drilled at the northwestern margin of the Witwatersrand Basin has revealed new insights into the nature and tectonic setting of the likely source area for some of the Mesoarchaean auriferous Witwatersrand sediments. The protoliths of intersected altered granite and hornblende metagabbro are of indistinguishable age (3062 ± 5 Ma) and have very similar geochemical signatures. Trace element characteristics typical of calc-alkaline magmatism and evidence of variable contamination with older crust (subchondritic {epsilon}Nd and {epsilon}Hf in zircon) point to an active continental margin setting. The Ar–Ar hornblende ages are within error of the magmatic crystallization age or slightly older. Alteration of presumably primary magmatic hornblende to magnesio-hornblende immediately after gabbro emplacement during late magmatic autometasomatism is suggested. The presence of hydrous melts (>4 wt % H2O), comparable with fertile Au-bearing magmatic–hydrothermal mineralizing systems in Phanerozoic volcanic arcs, is inferred. Thus, a kind of hinterland is proposed for the Witwatersrand that compares favourably with the tectonic domains that are known to host the majority of post-Archaean gold deposits. Later retrograde hydrothermal alteration at c. 2720 and 2630 Ma led to variable Pb loss in zircon and the resetting of the whole-rock Rb–Sr isotope system whereas the Ar–Ar and Lu–Hf isotope systems in the hornblende and zircon grains, respectively, were not significantly affected. Comparison with published data suggests that these alteration events are the same as those that affected the Witwatersrand Basin fill associated with major early Ventersdorp flood basalt volcanism and possibly a pre-Transvaal thrusting event in response to contractional deformation in the Limpopo Belt.

KEY WORDS: active continental margin; Archaean crust; U–Pb; Lu–Hf; Witwatersrand hinterland; gold genesis

*Corresponding author. Telephone: +49-931-3185420. Fax: +49-931-3184620. E-mail: hartwig.frimmel{at}uni-wuerzburg.de


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