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

位于南非Kaapvaal克拉通中部的中太古代Witwatersrand超群中含有近40%的已知黄金。尽管具有巨大的经济意义，但这些黄金的起源仍未得到解决。目前可获得的数据和观测结果最好由一个修正的古沙土模型来解释。然而，推断出的大量碎屑金的最终来源的问题仍然没有答案，并形成了拟议研究的主要焦点。工作假设是，与以往的观点相反，碎屑金不是来自于腹地更古老的热液金-石英脉，而是来自于中太古代绿岩带中分散的岩浆金。之前对威特沃特斯兰德盆地的直接基底的研究主要集中在花岗岩类上，而这个项目的目标是基性岩，它们的构造环境以及它们将大量Au从地幔转移到大陆地壳的潜力。在该项目的第一阶段，研究了基底花岗岩下这种基性岩的极少数现有岩心交叉点之一的岩石学和地球化学。研究结果表明，基性侵入岩和长英质侵入岩都是共成的，反映了在超俯冲背景下富水、钙碱性、幔源熔体的分离结晶产物，这与现代一些最有效的将金从地幔转移和富集到大陆地壳的系统相比是很好的。在该项目的第二阶段，将通过独立的方法，特别是同位素方法，并与威特沃特斯兰德盆地腹地的其他基性/长英质岩石组合进行比较，对第一阶段的结果进行测试和验证（或其他方式），这些结果令人鼓舞，在某些方面令人惊讶。此外，通过评估石油在金向其地壳位置运输过程中的作用，将检验威特沃特斯兰德金矿来自岩浆金的潜在假设的强度。为此，将首次分析威特沃特斯兰德砾岩中广泛存在的焦沥青的PGE和Au浓度。总而言之，预计由此产生的新数据将有可能制定一个更好的模型来解释世界上最大的、经济上最重要的地球化学异常之一，即威特沃特斯兰德盆地的高金浓度。

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.