Controls on the genesis of gold mineralisation in the Brucejack epithermal deposit, British Columbia

一些浅成热液矿床的特征是金和银的金属浓度极高，但尽管它们具有明显的吸引力，但由于超富集带（富矿带）通常与贫瘠带相邻，因此难以勘探和开发。Pretium Resources Inc.于2017年开始开采的不列颠哥伦比亚省北部的Brucejack矿床就是此类矿床的一个例子。在那里，含有千克金/吨的矿脉与缺乏金（银）的同一代矿脉相伴随。为什么这个矿床和其他一些矿床含有这些局部超富集的金银矿脉还没有令人满意的解释。事实上，很难理解那些被认为是造成浅成热液矿床形成的流体是如何通过简单的沉淀把几米宽的裂缝（布鲁斯杰克的情况）填满黄金的，因为这些流体的含金量已显示为10 ppb左右。另一种假设是，通过对蓝jack样品中金银纳米颗粒（直径3-7 nm）的初步高分辨率透射电子显微镜（HRTEM）图像提出的，金/银超饱和为纳米电子颗粒，随后絮凝、积聚，然后机械输送到选定的裂缝中，可能是主要流体管道的附属。我们将通过进一步的HRTEM成像、eds辅助TEM分析和旨在产生胶体并确定其絮凝控制的实验来验证这一假设。在Brucejack金矿化之前，富含金的砷黄铁矿在5公里长的叶状蚀变走廊内局部发育，可能反映了深部的斑岩系统，对资源有重要贡献，并与electrum密切相关。本研究将建立一个综合的布鲁斯杰克矿床成因模型，解释植物蚀变、砷黄铁矿和富矿型银（以及它们之间的联系），考虑套缩式浅热斑岩系统的可能性，并有助于指导正在进行的勘探和矿山开发。

Some epithermal deposits are characterised by extraordinarily high metal concentrations of gold and silver, but despite their obvious attraction, they are difficult to explore and develop because zones of hyper-enrichment (bonanza zones) are commonly adjacent to barren zones. The Brucejack deposit in northern British Columbia, where Pretium Resources Inc. initiated mining in 2017, is an example of such a deposit. There, veins containing kilograms of Au/ton are accompanied by veins of the same generation that are devoid of gold (electrum). The reason why this and some other deposits contain these locally hyper-enriched gold-silver veins has not been satisfactorily explained. Indeed, it is difficult to understand how the fluids considered to be responsible for the formation of epithermal ore deposits could fill cm-wide fractures with gold for distances of several metres (the case at Brucejack) by simple precipitation, given that the gold content of these fluids has been shown to be on the order of 10's of ppb. An alternative hypothesis, which is suggested by preliminary high resolution transmitted electron microscope (HRTEM) images of gold-silver nano-particles (3-7 nm diameter) in samples from Brucejack, is that the gold/silver super-saturates as electrum nano-particles which subsequently flocculate, accumulate and then are mechanically transported into selected fractures, perhaps subsidiary to the major fluid conduits. We will test this hypothesis with further HRTEM imaging, EDS-assisted TEM analyses, and experiments designed to produce colloids and determine the controls on their flocculation. The bonanza gold mineralisation at Brucejack was preceded by auriferous arsenian pyrite that is developed locally within a five km-long corridor of phyllic alteration, possibly reflecting a porphyry system at depth, and contributes significantly to the resource and is closely associated with the electrum. The proposed research will develop a comprehensive model for the genesis of the Brucejack deposit that will explain the phyllic alteration, arsenian pyrite and bonanza-type electrum (and the links between them), consider the possibility of a telescoped epithermal-porphyry system, and help guide the ongoing exploration and mine development.