The
Macraes gold deposit is located in a low angle NE dipping (~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. Early in the development of this through going
shear the schist experienced an episode of hydro fracturing and fluid
redistribution. As a result, presently exposed schists within the Intrashear
sequence are pervaded by localised swarms of quartz filled pure extension and
extensional shear fractures hosting goldscheelite mineralization. The swarms
comprise networks or stockworks' of veins that strike NE, are subvertical and
form subperpendicular to the shear zone fabric. Vein frequency across strike is
typically ~1 m-1, with most veins five to ten centimetres thick and
vertically continuous for up to ~20 m. Many of the veins show delicately banded
internal textures that are parallel to the vein margins. These bands/lamina
indicate incremental growth and are defined by a variation in grain size or
alternatively by thin slivers of wallrock schist, though typically uncommon. The
majority of veins are overprinted by a weak plastic deformation, although
primary structures are largely preserved. This deformation has produced subgrain
structures, migration of grain boundaries with sutured margins and undulose
extinction of grains. These microstructural observations possibly indicate that
temperatures were around 300C during stockwork vein formation. In general, the
mode of brittle failure that developed the stockwork veins can be directly
related to lithological variation. The geometry of stockwork veins indicate that
local vertical shortening and extension occurred in the Intrashear sequence of
the HMSZ synkinematically with thrusting. Localised extension was mainly
parallel to the structural trend; that is, it took place perpendicular to the
direction of inferred thrusting. The development of lateral ramps most likely
gave rise to extension in parts of the shear zone during continuous shortening
to form swarms of stockwork veins.