The Hammer Down gold deposit is one of the most significant mesothermal vein-type gold deposits in the Canadian Appalachians. It is located within a complex sequence of Ordovician, mafic-dominated tholeiitic and calc-alkalic and arc-related volcanic rocks, which was intruded by Silurian felsic porphyry dykes. The host rocks have undergone complex polyphase deformation. At least three deformational events influenced vein emplacement and overall geometry of the deposit. A Taconian deformation (D1–2) was responsible for the development of a 250 m wide zone of high-strain deformation (HSZ1) at the interface between two blocks of Ordovician rocks: the Catcher's Pond Group and the Lush's Bight Group. Rocks included within the HSZ1, represent "exotic" slabs of volcanic rocks that were tectonically juxtaposed, intensively foliated (S1), and folded (F2). Gold occurs in high-grade, sulfide-rich, fault-fill quartz veins that occur within the HSZ1. At the outcrop scale, these veins are hosted by discrete centimetre- to metre-wide ductile–brittle D3 high-strain zones (HSZ3) of Silurian or younger age. The development of the gold-hosting structures (HSZ3) is genetically related to layer anisotropy induced by intrafolial F2 folds, and most importantly by the presence of felsic porphyry dykes, which were competent compared to the intensively foliated and incompetent mafic volcanic rock sequence. A postmineralization D4–5 deformation, which included two generations of folds (F4 and F5) and late brittle faulting, is responsible for the actual geometry of the deposit.