The giant quartz-breccia veins of the Tyndrum–Dalmally area, Grampian Highlands, Scotland: their geometry, origin and relationship to the Cononish gold–silver deposit

Abstract

ABSTRACTThe area lies within a ∼15 km-wide compartment of polyphase-deformed Dalradian (Neoproterozoic) rocks, bounded by the NE-trending Tyndrum and Ericht–Laidon transcurrent faults. Sinistral movement on these faults caused a periclinal structure, the Orchy Dome, to develop from flat-lying Dalradian rocks. This dome controlled the spatial distribution of lamprophyre intrusions and explosion breccia pipes, before being cross-cut by a network of near-vertical faults. Some of these faults are host to giant, segmented, quartz-breccia veins up to 5 km long and 19 m thick, formed by cyclic injection of over-pressured Si-rich fluid into newly-formed faults. The quartz-breccia bodies consist of a plexus of quartz veins with cockade and vuggy textures, indicative of open-space, high-level crystallisation. The faults comprise a NE-trending set of mineralised veins, including the Cononish Au–Ag deposit, and two pairs of conjugate [NW- and NE-trending] and [NNW- and NNE-trending], generally non-mineralised, faults. Their geometry is that predicted by the Coulomb model for Riedel R and R′ shear fractures, modified by variations in pore fluid pressure. They were active c. 430–425 Ma ago, coincident with emplacement of the Lochaber Batholith, whose buried extension, together with the mantle, probably provided the bulk of the fluid needed to form the veins.