Mineralogy and Petrology (1999) 66:1-3

Mineralogy and Petrology © Springer-Verlag1999 Printed in Austria

Preface Most people, both inside and outside South Africa, associate the term "Witwatersrand" with gold. This is not surprising when one considers that the late Archaean Witwatersrand Basin in South Africa is the world's largest known gold depository. With an annual production of around 500t, about 25% of the world's gold supply come from the mines in the Witwatersrand. Since 1886, when the first discovery of gold was made there, some 47000t of gold have been recovered from the Witwatersrand Basin and there are still an estimated 20000 t in reserves. Plans to sink shafts as deep as 5000 m show that, in spite of a weak bullion price, gold mining in the Witwatersrand is far from being dead. The second term that features prominently in the title of this Special Issue of Mineralogy and Petrology is becoming equally famous: "Vredefort" relates to a circular, dome-like structure which, in recent years, has been acknowledged as the oldest and one of the largest preserved impact structures on Earth. Highlighting the significance and unique character of the Vredefort Dome is the current proposal to declare it a World Heritage site. Considering that the Vredefort Dome is located in the centre of the Witwatersrand Basin, it is tempting to speculate whether some of the unique features of this "basin", such as its perfect preservation and its richness in gold, may be linked to the meteorite impact which has struck the Archaean Kaapvaal Craton some 2020 Ma ago. As the origin of the Vredefort Dome was enigmatic for decades, so has been the genesis of the Witwatersrand gold. The debate over the question whether the gold deposits were formed by hydrothermal processes or whether they represent ancient placer deposits is almost as old as the mining of the Witwatersrand gold, but it gained momentum during the late 1980's and early 1990's, when a number of workers have established that metamorphism and hydrothermal alteration left a major imprint on the Witwatersrand sediments and associated ores. The exact nature of this metamorphism, its timing and tectonic cause(s) and its influence on the distribution of the gold remained, however, unresolved. This prompted a new wave of research projects, carried out largely, but not exclusively, by South African scientists over the last few years. The debate on the origin of the Witwatersrand gold is still on-going, but it appears as if some consensus is being reached. Our recently gained insights into the evolution of the Witwatersrand Basin let us realize not only the complexity of this evolution but also that the term "Witwatersrand Basin" is actually misleading as it refers to an erosional remnant of tectonically stacked sediments which were originally deposited in a variety of tectonic settings. The term is, however, so deeply entrenchedin the literature that it

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will be retained in this Special Issue with the understanding that it really refers to a successor basin. On 20 February 1998, most of the researchers from South Africa, who currently work on aspects of the post-depositional history of the Witwatersrand rocks, gathered at a symposium on "Witwatersrand and Vredefort - Metamorphism and Mineralization", held at the University of Cape Town under the auspices of the Mineralogical Association of South Africa. That meeting provided an opportunity for exchanging sometimes vastly different ideas and views on a hotly debated topic and aimed at helping in coming closer to a well-founded consensus. This Special Issue emanated from that meeting and offers a closer look at the recent research on the post-depositional history of this important basin, with particular emphasis on the possible role of the Vredefort metorite impact. In the first paper, Gibson and Reimold provide an overview of our current understanding of the link between the Vredefort Dome and the metamorphic alteration of the surrounding Witwatersrand strata. This is followed by the presentation of new data on impact-related pseudotachylite by Reimold, Koeberl, Fletcher, Killick and Wilson. Then follow three contributions, all of which deal with fluid inclusion studies: Frimmel, Hallbauer and Gartz present new compositional data for those fluids which were capable of mobilizing the gold. discuss their possible source(s), and show that the most crucial factor for the gold mobility was an elevated pH; Drennan. Boiron, Cathelineau and Robb provide new micro-thermometric and Raman spectroscopic data on a series of post-depositional fluid generations throughout the basin: and Klemd compares the fluids that have altered the Witwatersrand sedimentary rocks with those that have altered the surrounding granitoids. By integrating new data on the metamorphism of sections through the Witwatersrand strata that are far away from the goldfields with published data from the goldfields. &evens and Preston reconfirm that the low grade metamorphism throughout most parts of the basin is not directly related to the relatively high metamorphic grade found in the immediate vicinity of the Vredefort Dome. The final two contributions deal with new isotopic data: Zhao. Clauer, Robb. Zwingmann, Toulkeridis, and Meyer present K-Ar data based on which they propose a thermal overprint of the Witwatersrand strata during the Eburnian orogeny along the western margin of the Kaapvaal Craton. In the last paper, Zartman and Frimmel make a case for the gold-mobilising fluid having been highly radioactive and enriched in 222Rn. leaving behind sulphides extremely rich in radiogenic Pb. with metal transport over only short distances. Although the conclusions reached in the various contributions are not necessarily all consistent - thus highlighting thai we have by far not reached the end of Witwatersrand research - it is interesting to note that all contributors, though coming from different fields and different schools of thought, reached agreement on the detrital origin of the Witwatersrand gold. and thus on the model of hydrothermally modified placer deposits. Apart from the authors, the following referees contributed to the success of this Special Issue: J. Barton. Rand Afrikaans University, P. S. Fiske. Lawrence Livermore Nationa.] University of Laboratory, R. Frei, Kobenhavns University, M. Frey. University of Basel, J. Ggtze, Bergakademie Freiberg, D. K. Hallbauer, University of Stellenbosch, R. Hart, Council for Geoscience. Johannesburg, S. E. Kesler, University of Michigan, C. K6berl, University of Vienna, E Koller,

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University of Vienna, F. M. Meyer, RWTH Aachen, G. N. Phillips, Great Central Mines, A. M. Reid, University of Houston, S. Roberts, University of Southampton, J. G. Spray, University of New Brunswick, A. M. van den Kerkhof, University of G6ttingen, C. Veale, Robertson Research International, R. E. Zartman, University of Cape Town. Thanks to all of you! Last but not least I would like to thank the Editor of Mineralogy and Petrology for the possibility to publish this account of recent findings on one of the most fascinating geological enigmas. H. E. Frimmel, Cape Town Guest Editor