Structural controls and origin of gold-silver mineralization in the Grampian Terrane of Scotland and Ireland

Gold-bearing mineral deposits occur over a strike distance of > 300 km within the Grampian Terrane of Scotland and Ireland. This terrane consists of Neoproterozoic-Lower Ordovician rocks of the Dalradian Supergroup that were polyphase deformed and metamorphosed during the c. 470 Ma Grampian Orogeny. Sulphide-rich Au-Ag deposits occur in Scotland at Calliachar-Urlar Burn, Tombuie, Tyndrum and Cononish, and in Ireland at Curraghinalt (Omagh), Cavanacaw, Croagh Patrick, Cregganbaun and Bohaun. They are hosted by 0.1-6 m thick quartz veins and have a similar overall mineralogy, including native gold, As, Cu, Fe, Pb and Sn sulphides, with hessite, tetrahedrite and electrum present in the first six localities above. The mineralized quartz veins, which are characterized by open-space textures, crystallized at c. 3-5 km depth in the crust. All of the deposits were structurally controlled and, apart from Curraghinalt, occur within second-order Riedel R, R' and T fractures resulting from a regional N-S-trending maximum principal stress. These deposits are of Upper Silurian to Lower Devonian (post-Scandian) age, and are inferred to have crystallized from hot, silica-rich metamorphic fluids derived from dehydration reactions at the greenschist/amphibolite-facies boundary. Curraghinalt is an older, Grampian, thrust-related deposit. Plutonic igneous rocks (mainly granitoid) contributed in part to the fluids, which were channelled into major orogen-parallel, strike-slip faults, to be injected by fault-valve pumping into the damage zones and fault breccias of newly formed Riedel fractures. Any residual fluid probably percolated to the ground surface to form Rhynie chert-type hot-springs.