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Alastair King

Alastair King

New Zealand

MSc topic: An investigation into the paragenesis and structure of high-grade, high angle, gold veins at Macraes in the Hyde Macraes Shear Zone
Supervisor: Doug MacKenzie
Email: kinal762[at]student.otago.ac.nz

Project description

The Hyde-Macraes Shear Zone (HMSZ) in east Otago is a 25km long regional-scale structure that hosts New Zealand’s largest gold mine. Oceanagold’s gold 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 and thrust fault system hosted in TZ3 Chlorite zone Otago Schist (Craw and MacKenzie, 2016). 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 (Mortensen etal 2010). The best mineralised rocks and the bulk of material mined at Macraes include graphite-bearing shears and mineralised schist with disseminated sulphides. Most of the gold is refractory and contained within the sulphides, although 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 at Frasers. This vein is relatively narrow, generally less than 1m wide, contains visible gold and dips steeply, cutting the shear foliation at 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 the 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 (e.g. Begbie & Craw 2006;  Upton et al. 2008).  In addtion, detailed SEM work on polished sections will be used to determine the paragenetic sequence of vein textures and ore minerals, such as pyrite, arsenopyrite, scheelite, gold and other accessory minerals within the vein. This will help place the vein in the established paragenetic sequence for the Macraes deposits (MacKenzie et al. 2016)