The
Otago Schist is crossed by numerous deformation zones that formed in the latter
stages of metamorphism and uplift of the metamorphic belt. These deformation
zones are long (tens of km) linear features at the regional scale, and are
generally narrow (km scale or less). Many of these structural zones separate
schist domains with different rock types and structural/metamorphic histories.
The Hyde-Macraes Shear Zone is one such deformation zone, and this zone hosts
the active Macraes gold mine. The Rise & Shine Shear Zone, 80 km NW of
Macraes, is also enriched in gold and has been mined historically.
Mineralisation in both these shear zones occurred during the transition from
ductile to brittle deformation as the schist belt was uplifted from greenschist
facies metamorphic conditions. Recrystallisation of quartz and micas in the
early stages was overprinted by cataclasis, with sulfide and gold precipitation
throughout. Many of the other late metamorphic deformation zones formed under
similar greenschist to sub-greenschist facies conditions, with similar
overprinting of recrystallisation textures and folds by more brittle textures
such as breccias. Under these conditions, originally regionally pervasive
deformation at the highest metamorphic grades was evolving towards more focussed
high strain zones at lower temperatures and pressures. Shear zones initially
focussed fluid flow along microshears and mineral grain boundaries, resulting in
1 to 100 m scale alteration and replacement in the host schist, with only local
development of quartz veins. Fluid flow was not controlled by open fractures,
and the rate of fluid movement was probably slow and pervasive. After uplift
into the brittle regime, the schist belt underwent tectonic extension. Extension
was accompanied by development of swarms of steeply dipping veins, with
localised mineralised normal faults. Hydrothermal fluid flow throughout this
uplift history was a normal consequence of late stage devolatilisation of the
metamorphic belt. Hence, all late metamorphic deformation zones should be
considered prospective for gold accumulation.