Abstract
The Hyde-Macraes Shear Zone (HMSZ) in east Otago is a 25 km long regionalscale
structure that hosts New Zealand’s largest gold mine. OceanaGold (NZ) Ltd.’s
mine at Macraes has produced over 4 Moz gold from a total resource of > 10 Moz.
The HMSZ is a low-angle northeast dipping compressional shear zone hosted in
TZ3, Chlorite zone Otago Schist. Gold-scheelite mineralisation along the shear
zone initiated in the early Cretaceous during the latter stages of metamorphism and
during exhumation of the schist host rocks through the brittle-ductile transition. The
bulk of material mined at Macraes includes graphite-bearing shears and mineralised
schist with disseminated sulphides. Most of the gold is refractory and contained
within the sulphides, however, some free gold occurs in quartz veins. The 3 Black
vein is one such gold-bearing vein that is currently being mined in the underground
workings below Frasers Pit. This vein is relatively narrow, generally less than 1m
wide, contains visible gold and dips steeply, cutting the shear foliation at a high
angle. The structural setting and relative timing of this vein is not well constrained
and it is unknown what structural kinematics controlled vein emplacement and how
it is related to the main low angle shear. This study aims to place this vein (and
others like it) in structural context with the rest of the shear zone. By mapping
host lithologies, metamorphic structures and vein orientations underground, it is
planned to model the principal stress directions that controlled the vein’s formation
and determine whether it is purely an extensional vein or hybrid extensional-shear
vein related to lateral ramps. In addition, detailed SEM work on polished sections
will be used to determine the paragenetic sequence of vein textures and ore minerals
including sulphides, scheelite, gold and other accessories within the vein. This
will help place the vein in the established paragenetic sequence for the Macraes
deposits.