Mafic basement of the Mesoarchaean Witwatersrand basin fill: Possible source of the world's largest known accumulation of gold?

点击翻译按钮获取中文摘要

The siliciclastic Mesoarchaen Witwatersrand Supergroup in the central Kaapvaal Craton (South Africa) hosts close to 40 % of all known gold. In spite of its enormous economic significance, the genesis of this gold remains unresolved. The currently available data and observations are best explained by a modified palaeoplacer model. The question of the ultimate source of the huge amounts of inferred detrital gold remains, however, unanswered and forms the main focus of the proposed research. The working hypothesis is that the detrital gold is, contrary to previous ideas, not derived from older, hydrothermal gold-quartz veins in the hinterland, but from finely dispersed magmatic gold in Mesoarchaean greenstone belts. Previous work on the immediate basement of the Witwatersrand Basin focused on granitoids, whereas this project is aimed at mafic rocks, their tectonic setting and their potential to transfer large amounts of Au from the mantle into the continental crust. In the first stage of this project, one of very few existing drill core intersections of such mafic rocks underneath basement granite were investigated petrologically and geochemically. The obtained results suggest that both mafic and felsic intrusives are cogenetic and reflect the products of fractional crystallization of an H2O-rich, calcalkaline, mantle-derived melt in a supra-subduction setting that compares well with some of the most effective modern systems for the transfer and concentration of gold from the mantle into the continental crust. In this second phase of the project, the encouraging, and in certain respects surprising, results from the first phase will be tested, verified (or otherwise) by independent methods, specifically isotopic ones, and by comparison with other mafic/felsic rock associations in the close hinterland of the Witwatersrand Basin. In addition, the strength of the underlying hypothesis of the Witwatersrand gold being derived from magmatic gold will be tested by assessing the role of petroleum in the transport of Au into its crustal position. To that effect, widespread pyrobitumen in the Witwatersrand conglomerates will be analyzed, for the first time, for its PGE and Au concentrations. All in all, it is expected that the resulting new data will make it possible to formulate a better model to explain one of the world’s largest, and economically most significant, geochemical anomalies, i.e. the huge concentration of Au in the Witwatersrand Basin.