Development of a genetic model for targeting gold mineralisation in the Dalradian Supergroup, Scotland

The mid-Neoproterozoic to mid-Cambrian Dalradian Supergroup metasedimentary rocks of Scotland and Ireland contain vein-hosted gold mineralisation, including the Cononish deposit, Tyndrum, with a resource of 163,200 oz Au. Through identifying new mineralised occurrences and understanding the genesis of these and the Cononish deposit, the characteristic features of the hydrothermal system are identified. This study aims to improve exploration strategy by defining a specific genetic model. Newly identified veins, in the Tyndrum area, are classified into three types: early molybdenite-bordered fractures, later gold-bearing poly-metallic veins and quartz-only veins. High-grade (>10 ppm) gold mineralisation at the Cononish deposit has distinct high Te (104 ppm) and Ag (119 ppm) with low Au/Ag (0.25). Other gold-bearing poly-metallic veins exhibit lower Te (8 ppm), higher As (234 ppm) and Au/Ag (1.3) at high grade (>10 ppm Au). Gold occurs with sulphides and δ34S data (average +6.7±2.8 ‰) indicate a significant portion of the sulphur is sourced from the metasedimentary succession (≥32-100%) with a small, but genetically significant, magmatic-sourced component (≤68%). The source of sedimentary sulphur is not the immediate host rock and is inferred to be stratigraphically overlying, but structurally underlying, Easdale Subgroup lithologies. 40Ar/39Ar, Re-Os and U-Pb geochronology on muscovite, K-feldspar, molybdenite and rutile from veins and altered wall rock records two metallogenic events; metamorphic molybdenite at 477-439 Ma and post-metamorphic gold at 408-407 Ma. All gold-bearing veins are coeval with the gold-bearing Rhynie Chert (c. 407 Ma), suggesting a regional system of hydrothermal fluid flow. This is coincident with the last stage of emplacement of the post-orogenic granite suite (c. 430-408 Ma) and fault movement on the Great Glen and subsidiary faults (c. 430-390 Ma) interpreted to be as a result of the change of the stress field to sinistral transtension (c. 420-400 Ma).