Geometry and petrography of stockwork vein swarms, Macraes mine, Otago Schist, New Zealand

Abstract

The Macraes gold deposit is located in a low‐angle northeast‐dipping (c. 15°) shear zone, the Hyde‐Macraes Shear Zone (HMSZ) cutting greenschist facies metasediments of the Otago Schist. The shear zone is host to large volumes of mineralised schist and relatively sparse quartz veins. During the development of this through‐going shear, the schist underwent episodes of hydrofracturing and fluid redistribution. As a result, parts of the HMSZ are pervaded by swarms of quartz‐filled pure extension and extensional‐shear fractures with gold and scheelite. The swarms comprise networks or “stockworks” of veins that strike northeast, are subvertical, and form subperpendicular to the shear zone fabric. Vein swarms are mostly restricted to more competent pods of massive schist within the Intrashear Schist. Vein frequency across strike within swarms is typically c. 1/m, with most veins 5–10 cm thick and vertically continuous for up to c. 20 m. Many of the veins show delicately laminated internal textures that are parallel to the vein margins. These laminae indicate incremental growth and are defined by a variation in grain size or by thin slivers of wall‐rock schist. Veins were initiated as brittle structures, but have been subsequently deformed in a ductile manner. Most veins are variably overprinted by plastic deformation, although many primary structures are preserved. This deformation has produced subgrain structures, migration of grain boundaries with sutured margins, and undulose extinction of grains. These microstructural observations indicate temperatures were between 300 and 400°C during stockwork vein deformation. The geometry of stockwork veins indicate that localised regions of extension occurred within the HMSZ during continuous shortening. Localised extension was mainly parallel to the structural trend; that is, it took place perpendicular to the direction of inferred thrusting. Transitions from shortening to local extension were most likely driven by changes in shear zone geometry (e.g., lateral or oblique ramps).

Keywords:

• stockwork veins
• gold
• lateral ramps
• Hyde‐Macraes Shear Zone
• Otago Schist