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

中太古代 Witwatersrand 盆地充填的镁铁质基底：世界上已知最大的金矿堆积的可能来源？

• 批准号: 35326172
• 负责人: Professor Dr. Hartwig E. Frimmel
• 金额: --
• 依托单位: Lehrstuhl für Geodynamik und Geomaterialforschung
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2006
• 资助国家: 德国
• 起止时间: 2005-12-31 至 2011-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/35326172?language=en
• 关键词: Mafic; basement; Mesoarchaean; Witwatersrand; basin

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.

Kaapvaal Craton（南非）的硅化学中间型Witwatersrand超级团体占所有已知黄金的近40％。尽管具有巨大的经济意义，但这种黄金的起源仍未解决。当前可用的数据和观察结果最好通过修改的古扫描模型来解释。然而，大量推断的碎屑黄金的最终来源的问题仍未得到答复，并构成了拟议研究的主要重点。有效的假设是，与以前的思想形成鲜明对比的是，杂种中的较旧的热液金夸尔兹静脉，而是来自中介中绿色绿石带中的磁性金。先前在Witwatersrand盆地的直接地下室的工作重点是花岗岩，而该项目的目的是针对镁铁质岩石，它们的构造环境以及它们将大量AU从地幔转移到连续地壳中的潜力。在该项目的第一阶段，在岩石学和地球化学上研究了地下室花岗岩下面的此类宏观岩石的现有钻芯交叉点之一。获得的结果表明，健康和长英质的侵入剂都是造成生物遗迹的，并且反映了富含H2O的富含H2O的含量，甲性甲性甲性，地幔衍生的融化的产物，它与某些最有效的现代系统可以很好地相比，这些系统可以很好地将金从地面转移到持续的壳中的金色转移和浓度。在项目的第二阶段中，将通过独立方法（特别是同位素的方法）进行测试，对第一阶段的鼓励和某些方面的结果，并与Witwatersrand盆地紧密的混合体中的其他Mafic/Felsic Rock关联进行比较。此外，将通过评估石油在AU运输到地壳位置的作用来测试源自岩浆黄金的Witwatersrand黄金的基本假设的强度。为此，将首次分析Witwatersrand砾岩中的宽度拟南芥，以其PGE和AU的浓度进行分析。总而言之，预计由此产生的新数据将有可能形成一个更好的模型，以解释世界上最大，最重要的地球化学异常之一，即Witwatersrand Basin中众多的AU集中度。