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

一些浅成热液矿床的特点是金和银的金属浓度非常高，但尽管它们具有明显的吸引力，但它们很难勘探和开发，因为超富集区（富矿区）通常与贫瘠区相邻。位于不列颠哥伦比亚省北方的Brucejack存款，Pretium Resources Inc.于2017年开始开采，是这种存款的一个例子。在那里，含有千克Au/吨的矿脉伴随着没有金（银金矿）的同代矿脉。为什么这个和其他一些矿床含有这些局部超富集的金银脉，还没有令人满意的解释。事实上，很难理解被认为是形成浅成热液矿床的原因的流体如何能够通过简单的沉淀作用使几厘米宽的裂缝充满几米长的金（Brucejack的情况），因为这些流体的金含量已被证明是十亿分之一。由来自Brucejack的样品中的金-银纳米颗粒（3-7 nm直径）的初步高分辨率透射电子显微镜（HRTEM）图像提出的另一种假设是，金/银作为银金矿纳米颗粒过饱和，其随后絮凝、积聚并且然后被机械地输送到选定的裂缝中，可能是主要流体管道的附属物。我们将测试这一假设进一步HRTEM成像，EDS辅助TEM分析，和实验设计，以产生胶体，并确定其絮凝控制。在Brucejack的富矿金矿化之前，含金砷黄铁矿在当地五公里长的千枚蚀变带内发育，可能反映了深部的斑岩系统，对资源有重大贡献，并与银金矿密切相关。拟议的研究将为Brucejack存款的成因建立一个综合模型，解释千枚岩蚀变、砷黄铁矿和富矿型银金矿（以及它们之间的联系），考虑套叠式浅成热斑岩系统的可能性，并帮助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.