Source, Timing and Genesis of Scottish Orogenic Gold Deposits

苏格兰造山金矿的来源、时代和成因

• 批准号: 1949537
• 金额: --
• 依托单位: University of St Andrews
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2017
• 资助国家: 英国
• 起止时间: 2017 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-1949537
• 关键词: Source; Timing; Genesis; Scottish; Orogenic

Orogenic gold deposits are commonly associated with quartz carbonate veins and varying amounts of electrum (Au-Ag alloy), pyrite, pyrrhotite, white mica, K-feldspar, chlorite, and telluride, depending on the composition and redox state of the host rock. The source of mineralising fluids has been variably attributed to metamorphic processes, felsic to intermediate magmatism, mantle degassing, and to meteoric waters. Deposits commonly occur in second and third order structures along crustal-scale compressional to transpressional fault zones and typically form during, or shortly after, peak metamorphism in greenschist to amphibolite facies terranes.At Cononish, the deposit is hosted in veins that crosscut the metasedimentary rocks of the Dalradian Supergroup. The mineralised veins contain native Au, hessite (Ag2Te), electrum, tetrahedrite (Cu, Sb sulphosalt), pyrite, chalcopyrite, arsenopyrite, galena and sphalerite. Ar-Ar dating of syn-mineralisation feldspar grains places the timing of mineralisation as early Devonian (407 Ma), within error of the emplacement of the nearby calc-alkaline, I-type Lochaber pluton (425-408 Ma). Intriguingly, this age postdates the timing of the main regional metamorphic event (470-465 Ma) by ~60 million years. Furthermore, these data indicate that mineralisation and the onset of regional transtension was concurrent. Therefore, this system provides a unique opportunity to research the role and contribution of the different reservoirs (magmatism, mantle degassing, meteoric waters, metamorphism) in the genesis of this gold deposit.The aim of the project is to characterise geochemically, isotopically, magnetically and hyperspectrally the Au-bearing and Au-barren veins of the mineralising system as well as encasing host rocks at a regional scale. This 'holistic' approach is critical to gain strong knowledge of mineralisation systems and to identify critical parameters capable of distinguishing between ore-forming and non-ore-forming processes. In addition, this project aims to generate an exploration model that can be used as an exploration guide in the Scottish Highlands and be adapted as a protocol for use worldwide.Research Focus I: Characterisation of geology and alteration halos. Field mapping and detailed sampling of the mineralised and barren quartz veins found within the Cononish district will be undertaken to delineate alteration halos.Research Focus II: Timing of mineralisation. Optical petrography, cathodoluminescence and electron microscopy of representative samples of all veins and encasing lithologies will be used to characterise mineral paragenetic sequences. These data will underpin Ar-Ar and Re-Os dating of specific alteration phases typified by white mica, K feldspar and molybdenite. Such data will allow answering the question if there is a key age for the precipitation of gold.Research Focus III: Source of mineralisation. Stable isotopes of H, C, O, N, and S will be used to assess the sources of the alteration and mineralisation phases. H and O isotopes will delineate primary and/or secondary fluid inputs whereas C-N-O-S isotopes will differentiate between mantle, magmatic and metamorphic reservoirs. This data will determine if one or several sources of fluids were responsible for the different alteration and mineralisation phases and enable testing precipitation mechanisms models for the auriferous phases.Research Focus IV: Exploration methods. A hyperspectral reflectance study of quartz veins and surrounding alteration halos will be conducted in the visible to short-wave infrared (350-2500 nm). Magnetic susceptibility data will be collect at multiple frequencies as well as under high temperature and cryogenic conditions to characterise grain scale magnetic mineralogical changes through the mineralised zone. Together these methods will characterise changes in the chemistry due to the hydrothermal alteration and will highlight processes responsible for Au deposition.

造山带金矿通常与石英碳酸盐脉和不同数量的银（金-银合金）、黄铁矿、磁黄铁矿、白云母、钾长石、绿泥石和碲有关，这取决于寄主岩石的成分和氧化还原状态。成矿流体的来源不同地归因于变质作用、长英质到中间岩浆作用、地幔脱气和大气水。矿床通常产于沿地壳尺度挤压-逆挤压断裂带的二级和三级构造中，通常形成于绿片岩-角闪岩相地体的变质高峰期间或之后不久。在Cononish，该矿床赋存于与Dalradian超群的变质沉积岩相交的脉体中。矿化矿脉含天然金、赤铁矿（Ag2Te）、银、四面体（Cu、Sb亚硫酸盐）、黄铁矿、黄铜矿、毒砂、方铅矿和闪锌矿。同矿化长石颗粒的Ar-Ar定年将矿化时间定在早泥盆世（407 Ma），与附近钙碱性i型Lochaber岩体（425-408 Ma）的侵位误差一致。有趣的是，这个年龄比主要的区域变质事件（470-465 Ma）晚了约6000万年。此外，这些数据表明，矿化和区域伸展的开始是同时发生的。因此，该体系为研究不同储层（岩浆作用、地幔脱气作用、大气水作用、变质作用）在该金矿床成因中的作用和贡献提供了独特的契机。该项目的目的是在区域范围内描述成矿系统中含金和无金矿脉的地球化学、同位素、磁性和高光谱特征，以及包裹寄主岩石。这种“整体”方法对于获得成矿系统的强大知识和识别能够区分成矿和非成矿过程的关键参数至关重要。此外，该项目旨在生成一个勘探模型，可以用作苏格兰高地的勘探指南，并改编为全球使用的协议。研究重点一：地质蚀变晕特征。将对Cononish地区发现的矿化和贫瘠石英脉进行现场测绘和详细采样，以描绘蚀变晕。研究重点二：矿化时间。光学岩石学，阴极发光和电子显微镜的代表性样品的所有静脉和包围岩性将用于表征矿物共生序列。这些数据将支持以白云母、钾长石和辉钼矿为代表的特定蚀变相的Ar-Ar和Re-Os定年。这些数据将有助于回答是否存在黄金沉淀的关键年龄这个问题。研究重点三：矿化来源。H、C、O、N和S的稳定同位素将用于评估蚀变和矿化阶段的来源。H和O同位素将描绘原生和/或次生流体输入，而C-N-O-S同位素将区分地幔、岩浆和变质岩储层。这些数据将确定一种或几种流体来源是否对不同的蚀变和矿化阶段负责，并能够测试含金阶段的沉淀机制模型。研究重点四：探索方法。在可见光到短波红外波段（350 ~ 2500 nm）对石英脉及其蚀变晕进行高光谱反射研究。磁化率数据将在多个频率以及高温和低温条件下收集，以表征矿化带中颗粒尺度的磁性矿物学变化。这些方法将共同描述由于热液蚀变而导致的化学变化，并将突出负责金沉积的过程。