New Zealand Journal of Geology & Geophysics abstracts
Structure and geochemistry of the Rise & Shine Shear Zone
mesothermal gold system, Otago Schist, New Zealand
L. Cox
D. J. Mackenzie
D. Craw*
R. J. Norris
R. Frew†
Geology Department
University of Otago
PO Box 56
Dunedin, New Zealand
*Author for correspondence.
†Chemistry Department, University of Otago
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 δ18OVSMOW from these
late-stage mineralised veins ranges from +7 to +15‰ and δ13CPDB
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.
Keywords gold; arsenopyrite; Macraes mine; stable
isotopes;
fluid inclusions; schist; structure
G05044; Online publication date 20 November 2006; Received 25 August
2005; accepted 3 November 2006
New Zealand Journal of Geology & Geophysics, 2006, Vol. 49:
429–442
0028–8306/06/4904–0429 © The Royal Society of New Zealand
2006
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