Source, Timing and Genesis of Scottish Orogenic Gold Deposits

造山金矿床通常与石英碳酸盐脉和不同数量的金银合金、黄铁矿、磁黄铁矿、白云母、钾长石、绿泥石和碲化物共生，这取决于容矿岩石的成分和氧化还原状态。成矿流体的来源多种多样地被归因于变质作用、长英质岩浆作用、中级岩浆作用、地幔脱气和大气水。矿床通常赋存于沿地壳规模的挤压-扭压断裂带的二级和三级构造中，通常形成于绿片岩至角闪岩相地体的峰期变质作用期间或之后不久。矿化矿脉包括自然金、闪锌矿(Ag2Te)、银矿、四方铜矿(铜、锑硫酸盐)、黄铁矿、黄铜矿、毒砂、方铅矿和闪锌矿。对同矿化长石颗粒的Ar-Ar测年表明，成矿的时间为早泥盆世(407 Ma)，与附近钙碱性I型洛哈伯岩体(425-408 Ma)的侵位存在误差。有趣的是，这个年龄比主要区域变质事件(470-465 Ma)的时间晚了约6000万年。此外，这些数据表明，成矿作用和区域伸展的开始是同步的。因此，该系统提供了一个独特的机会来研究不同的储集层(岩浆作用、地幔脱气作用、大气水、变质作用)在该金矿床成因中的作用和贡献。该项目的目的是从地球化学、同位素、磁学和高光谱等方面描述成矿系统的含金和贫金矿脉以及区域尺度上的围岩。这一“整体”方法对于深入了解矿化系统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，该模型可用作苏格兰高地的勘探指南，并可作为协议在世界范围内使用。研究重点I：地质特征和蚀变晕。将对Cononish区内发现的矿化和贫瘠的石英脉进行野外测绘和详细采样，以圈定蚀变晕。研究重点II：矿化的时间。所有矿脉和包裹体岩性的代表性样品的光学岩相学、阴极发光和电子显微镜将被用来描述矿物共生序列。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阶段的Ar-Ar和Re-Os测年。这些数据将有助于回答是否存在金矿沉淀的关键年龄的问题。研究焦点III：矿化来源。将使用H、C、O、N和S的稳定同位素来评估蚀变和矿化阶段的来源。H和O同位素将圈定原生和/或次生流体输入，而C-N-O-S同位素将区分地幔、岩浆和变质岩储集层。这些数据将确定是一种还是几种流体来源对不同的蚀变和矿化阶段负责，并使测试含金相的降水机制模型成为可能。研究重点IV：勘探方法。石英脉及其周围蚀变晕的高光谱反射率研究将在可见光至短波红外(350-2500 nm)下进行。将在多个频率以及高温和低温条件下收集磁化率数据，以描述矿化带中颗粒尺度的磁性矿物学变化。总之，这些方法将表征热液蚀变引起的化学变化，并将突出负责Au沉积的过程。

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.