Recently discovered Au in boulder conglomerate between the Mesoarchean West Pilbara superterrane basement and the overlying volcano-sedimentary stratigraphy of the Neoarchean Fortescue Group in Western Australia has renewed comparisons with the Witwatersrand conglomerate Au deposits in South Africa. As such, this has reignited the question of the Pilbara and Kaapvaal cratons being linked as part of the postulated Vaalbara continent during the Archean. However, little is known about the origin of the Pilbara conglomerate Au and its host conglomerates, as they are hitherto unstudied, and their formation and/or source is uncertain.Here we present a detailed study on the textures, composition, and sedimentology of one newly discovered Pilbara conglomerate Au deposit at the base of the Neoarchean Fortescue Group in the northwestern Pilbara craton. The Pilbara conglomerate Au occurrences are characteristically Ag-bearing but Hg-poor polycrystalline discoid masses that are overgrown by Au-poor chloritic halos, which are further enveloped by a hydrothermal alteration halo of disseminated Au within chlorite. Both the discoids and the auriferous chlorite halo are Ag bearing, with up to ~9 wt % Ag, consistent with a hydrothermal (orogenic) origin. The discoids do not display any physical or chemical evidence for sedimentary transport; thus, their formation (placer versus hydrothermal) remains unclear. However, the position of the Au in the conglomerate, limited to the basal section of the conglomerate, is difficult to account for in a purely hydrothermal deposit model.We argue the Pilbara conglomerate Au represents a modified placer deposit from a primary orogenic Au source, with surface evidence for sedimentation removed by partial dissolution during later hydrothermal alteration in the host conglomerate and the crystalline basement. While the basal Fortescue Group conglomerate Au shares commonalities with the time equivalent (>~2.7 Ga) Venterspost Conglomerate Formation, which overlies the Witwatersrand Supergroup, inconsistencies remain, with different Au chemistries and tectonic, magmatic, sedimentary, and metamorphic-metallogenic histories of the Pilbara and Kaapvaal cratons prior to deposition of the >2.7 Ga conglomerate sequences. This collectively indicates the drivers of Au metallogenesis and ultimate Au deposition in conglomerate facies were fundamentally different in the Pilbara and Kaapvaal cratons.

中文翻译：

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西澳大利亚州新皮尔巴拉克拉顿的新旧联合企业集团托管金

最近在中古西西皮尔巴拉地区上层地下室与上覆的火山沉积地层之间的巨石砾岩中发现了金，该矿床与西澳大利亚的新archarean Fortescue集团形成了新的对比，与南非的Witwatersrand砾岩金矿床进行了比较。因此，这再次点燃了皮尔巴拉和Kaapvaal克拉通在太古代时期被假定为瓦尔巴拉大陆的一部分联系在一起的问题。然而，关于皮尔巴拉（Pilbara）砾岩集团及其宿主集团的起源知之甚少，因为迄今尚未对其进行研究，并且它们的形成和/或来源尚不确定。新发现的皮尔巴拉（Pilbara）砾岩集团金矿，位于西北部皮尔巴拉（Pilbara）克拉通的新奇特尔福特（Neoarchean）Fortescue集团基地。Pilbara砾岩中的Au矿床典型地是含银但汞含量低的多晶盘状团块，而Au贫化的氯环晕过度生长，而后者又被亚氯酸盐中散布的Au的热液蚀变晕所包裹。盘状和亚铁的亚氯酸盐卤素都含银，含银量高达〜9 wt％，这与水热（造山）成因一致。盘状物质没有显示出任何有关沉积物运输的物理或化学证据；因此，它们的形成（砂浆与热液的关系）仍不清楚。但是，在纯水热矿床模型中很难解释金在团块中的位置（仅限于团块的基础部分）。我们认为Pilbara团块Au代表了来自主要造山型金矿的改良砂矿床，表面证据表明，在随后的热液蚀变过程中，主体砾岩和结晶基底中的部分溶解消除了沉积。尽管基部的Fortescue集团砾岩与时间上等价的（>〜2.7 Ga）Venterspost砾岩形成有共同点，该构造覆盖了Witwatersrand超群，但仍然存在不一致之处，具有不同的Au化学和构造，岩浆，沉积和变质成矿历史Pilbara和Kaapvaal克拉通> 2.7 Ga砾岩层序沉积之前。这共同表明，在Pilbara和Kaapvaal克拉通中，砾岩相中金成矿作用和最终金沉积的驱动因素根本不同。