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.

位于南非卡阿普瓦尔克拉通中部的硅质碎屑中古岩群Witwater srand超级群拥有近40%的已知黄金。尽管它具有巨大的经济意义，但这种黄金的成因仍然悬而未决。目前可用的数据和观测数据可以用修正的古砂岩模型来解释。然而，推断出的大量碎屑金的最终来源的问题仍然没有得到回答，这构成了拟议研究的主要焦点。工作假说是，与以前的想法相反，碎屑金不是来自腹地较古老的热液金石英脉，而是来自中太古代绿岩带中分散的岩浆金。以前关于威特沃特斯兰德盆地直接基底的工作集中在花岗岩类上，而这个项目的目标是镁铁质岩石，它们的构造背景及其将大量Au从地幔转移到大陆地壳的潜力。在该项目的第一阶段，对基岩花岗岩下为数不多的此类镁铁质岩石的钻芯交汇处进行了岩石学和地球化学学研究。研究结果表明，镁铁质和长英质侵入岩都是同源的，反映了在超俯冲环境下富含H2O的钙质地幔熔体的分离结晶产物，这与一些最有效的现代金从地幔向大陆地壳转移和聚集的系统相媲美。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，并通过与威特沃特斯兰德盆地封闭腹地的其他镁铁质/长英质岩石组合进行比较，对第一阶段取得的令人鼓舞的、在某些方面令人惊讶的结果进行测试、验证(或其他)。此外，威特沃特斯兰德金矿来自岩浆金的基本假设将通过评估石油在将金输送到其地壳位置中的作用来检验。为此，将首次分析威特沃特斯兰德集团中广泛存在的焦沥青，以确定其PGE和Au的浓度。总而言之，预计由此产生的新数据将使制定一个更好的模型来解释世界上最大、经济上最重要的地球化学异常之一，即维特沃特斯兰德盆地中金的巨大集中成为可能。