Source, Timing and Genesis of Scottish Orogenic Gold Deposits

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.