Paragenesis and structure of high-grade, high-angle, gold veins at Macraes mine in the Hyde Macraes Shear Zone, east Otago

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.