Abstract
The Rise & Shine Shear Zone is a late metamorphic deformation zone developed in biotite zone Textural Zone 4 schist in Central Otago. The shear zone has been hydrothermally altered, with addition of gold associated with replacement of schist minerals by pyrite and arsenopyrite. Hydrothermal alteration of schist during mineralisation involved replacement of titanite by rutile, recrystallisation of metamorphic quartz, muscovite and chlorite, and addition of ankerite. Mineralised schist has abundant microshears that have developed parallel and subparallel to the pervasive schist foliation, and these microshears contain much of the hydrothermal sulfides and gold. Microshears have been deformed locally by upright syn-mineralisation brittle reverse faults and angular folds that have a southerly axial trend. These more brittle deformation zones are confined to the Rise & Shine Shear Zone. Gold-bearing veins and mineralised breccias, made up of quartz, albite, pyrite, arsenopyrite, calcite, and chlorite, fill extensional sites associated with upright fold zones. Calcite δ
18
O
VSMOW
from these late-stage mineralised veins ranges from +7 to +15‰ and δ
13
C
PDB
from -5.3 to -6.6‰, are similar to many other gold-bearing vein systems in Otago, but are distinctly different from the Macraes deposit. Mineralisation occurred near to the brittle/ductile transition, at 200-400°C. The upper part of the shear zone was truncated by a shallow northeast-dipping normal fault, the Thomsons Gorge Fault, which juxtaposed shear zone rocks against unmineralised Textural Zone 3 chlorite zone rocks in the middle Cretaceous. The Rise & Shine Shear Zone has some structural and geochemical features in common with the Hyde-Macraes Shear Zone, but also some important differences, and is not a simple strike-extension of that structure.