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Location: 55 km N from Dunedin, New Zealand
Golden Point Road, RD3, Macraes Flat 9483, East OtagoMacraesNew Zealand9483
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The Macraes deposit is the largest known orogenic style gold deposit in the South Island of New Zealand. This style of deposit is recognized to be broadly synchronous with deformation, metamorphism, and magmatism during lithospheric-scale continental-margin orogeny (Groves et al., 1998). Most orogenic gold deposits like Macraes occur in greenschist facies rocks.The Macraes gold deposits are in a major, low-angle (~15-20°) structure known as the Hyde Macraes Shear Zone (HMSZ). The shear zone can be traced 30 km along strike where schist is exposed at surface and only ends where it is covered by younger volcanic rocks in the north at Hyde and sedimentary rock cover in the south towards Palmerston. This regionally continuous, late metamorphic deformation zone cuts greenschist facies metasedimentary rocks of the Otago Schist.MineralisationThe mineralisation at Macraes is principally developed within the gently dipping HMSZ, though anomalous grades are also recorded in narrow, steeply dipping quartz veins locally occurring in the hanging wall schists, collectively known as the Eastern Lodes. Within the shear zone, mineralisation is generally constrained between the Hangingwall Shear and the Footwall Fault. Schists above the Hangingwall Shear and below the Footwall Fault are generally barren though there are exceptions to this rule, for example at Innes Mills and the Eastern Lodes. Economic mineralisation is typically restricted to the upper part of the HMSZ. The Hangingwall Shear, which varies from 1 m to >30 m in thickness contains the most continuous and consistent mineralisation. This zone is locally underlain by extensive but low grade stockwork zones which may be developed over a width of up to 100 m.Higher grade zones of mineralisation within the shear zone form tabular shoots that may have strike lengths of >300 m and extend up to 800 m down-dip (i.e. Frasers and Round Hill). In most cases these zones are observed to trend towards the north, oblique to the shear zone dip direction. This orientation is interpreted to be due to the interaction of the HMSZ with folds within the host schist units, creating a preferred lineation direction for mineralisation. The exception to this is the most recently discovered deposit Coronation North where the trend of the mineralisation is south-east.Mineralisation distribution is broadly consistent along the HMSZ but shows considerable variability in grade, width, continuity and geometry at mine-scale. This variability is attributed to the local development of the HMSZ structure during mineralisation and the influence of host rock lithology, particularly with respect to competency contrasts.There is a strong empirical correlation between gold, arsenic, scheelite, silicification and strain intensity within the HMSZ. Gold-scheelite-pyrite-arsenopyrite mineralisation is associated with replacement and fissure quartz veins within post-metamorphic shear zones. Shear parallel quartz veins and cataclastic shears contain the highest gold and scheelite grades (Lee et al. 1989).Mineralisation TypesThe following four types of mineralization are recognised within the HMSZ at Macraes:• Mineralized schist;• Black, sheared schist;• Shear-parallel quartz veins, ranging from 1 cm to 2 m; and• Stockworks (aka quartz vein arrays).Gold is associated with pyrite and arsenopyrite in all the above styles of mineralization.Tungsten as scheelite is found predominantly within mineralized quartz veins, although subordinate phases of disseminated scheelite within schist and remobilised stringer veins are also observed.Within the Macraes open-pits, gold mineralisation comprises a combination of Hangingwall, shear-parallel quartz veins (‘concordant lodes’), and ‘stockwork’ veins.Apart from Coronation, a large amount of irregular mineralisation occurs between the base of the Hangingwall and the Footwall Fault. This is stockwork mineralisation and generally appears in the drilling as clusters of elevated gold grades. Stockwork mineralisation refers to mixtures of steeply dipping narrow quartz veins and concordant lodes, which appear discontinuous at the resource drilling scale. The Footwall Fault lies between 80 m and 120 m below the Hangingwall Shear and is easily identified in drill holes as a distinctive light-grey fault gouge between 5 and 30 cm thick. To date, no economic mineralisation has been located below the Footwall Fault.Coronation depositThe Coronation deposit consists of a 15 to 20 degrees dipping HWS that is between three metres and ten metres thick. Unlike deposits to the south, there is very little development of stockwork mineralization beneath the HWS. Located one kilometre further north is the Coronation North deposit, which was discovered in 2015. Coronation North differs from previously mined ore bodies along the HMSZ and comprises a high-grade zone of ENE plunging mineralization associated with a left-hand lateral bend in the strike of the HWS.DeepdellThe HMSZ at Deepdell consists of a 50 m to 60 m thick pelite, constrained by the Hangingwall and Footwall shears. The geology of Deepdell North is comparatively simple. It comprises the Hangingwall shear, which has a planar geometry and dips 15° to 20° to the east. Beneath the Hangingwall shear, up to three subparallel shears have been identified. These shears are generally thin (less than 3 m thick), weakly mineralized, and do not have the continuity of the Hangingwall shear.At Deepdell South, the western portion of the Hangingwall dips at 20° to 25° to the southeast while the eastern section dips at 35° to 40° to the southwest. At both Deepdell North and Deepdell South quartz vein arrays and subsidiary shear development beneath the Hangingwall is relatively poor.Innes Mills open pitThe Innes Mills open pit is centred on mining the HWS and subparallel stacked lenses beneath. In outcrop, the shears typically dip 15 to 20 degrees to the east and are approximately five metres thick. Within the open pit, gold mineralization comprises mineralized schist and cataclasite, shear-parallel quartz veins and arrays of sub-vertical quartz veins.Erratic mineralization locally occurs between the base of the HWS and the Footwall Fault. At the resource drilling stage, this mineralization manifests as poorly developed clusters of elevated gold grades, which often appear discontinuous. During mining, however, these typically present as zones of quartz vein arrays and mineralized shears. The Footwall Fault lies between 80 metres and 120 metres below the HWS and is identified as a cataclastic zone up to ten metres thick. To date, no economic mineralization has been located below the Footwall Fault. Golden Point Underground mine (GPUG)The Golden Point Underground mine (“GPUG”) encompasses the down-dip continuation of the HWS mined in the Round Hill and Golden Point open pits. Current drilling has shown this to extend more than 700 metres beyond the limit of the open pit design. The thickest, most mineralized part is a series of stacked lodes proximal to the Golden Point open pit. The thickest, most mineralized part is a series of stacked lodes proximal to the Golden Point open pit. Mineralization continues as a single higher-grade lode down-dip to the northnortheast. Mineralization is contained within the Intra-shear Schist, which is generally 80 metres to 100 metres thick, with the higher gold grades confined to the upper part, which is dominated by cataclasite, lode schist and local stockwork pelite lithologies. The highest gold grades are contained within the strongly developed and visually distinguishable zone within the upper hanging wall, characterized by quartz cataclasite, and mineralized schist. This typically forms a well mineralized, continuous zone up to five metres to ten metres thick, with a gold grade of approximately 3 g/t.
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