Links between orogenic and placer gold on the Old Man Range, Central Otago, New Zealand

Abstract

Gold-bearing normal faults of inferred Late Cretaceous age cut both Caples Terrane and Torlesse Terrane schists on the Old Man Range in Central Otago. Mineralised fault rocks are dominated by brittle-formed breccias and fault gouges, with minor hydrothermal silicification and quartz vein formation, and scattered pyrite and arsenopyrite. Gold occurs replacing silicates, encapsulated in sulphide minerals, and as free particles in vein quartz. The orientations of mineralised faults were controlled by pre-existing joints in the host rocks, with west strike in Torlesse schists and northwest strike in Caples schists. A major component of placer gold on the slopes of the Old Man Range has been derived from nearby mineralised faults, and detrital gold particles are equant and angular rather than flaky. Gold in a Late Pleistocene Clutha River channel is flaky and has been transported from outside the area, but there is little or no evidence of externally derived detrital gold along the course of a Middle Pleistocene Clutha River channel.