Summary:
The Macraes gold deposits are in a major, low-angle (~15-20°) structure known as the Hyde Macraes Shear Zone (HMSZ). This regionally continuous, late metamorphic deformation zone cuts greenschist facies metasedimentary rocks of the Otago Schist. The Otago Schist is a moderately high-pressure metamorphic belt (Yardley, 1982) that formed by collisional amalgamation (“Rangitata” Orogeny) of the Caples and Torlesse terranes in the Early-Middle Jurassic (Coombs et al.,1976; Bishop et al., 1985; Little et al., 1999).
The Macraes mining area is centred on the Hyde-Macraes Shear Zone (HMSZ), the largest gold-bearing feature within the Otago Schist. Striking north (Mine Grid) and dipping shallowly (15-20 °) towards the east, 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.
Mineralisation
The 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 Types
The following four types of mineralization occur within the HMSZ at Macraes (Mitchell et al., 2006):
• Mineralised schist. This style of mineralisation involved hydrothermal replacement of schist minerals with sulphides and microcrystalline quartz. Mineralisation was accompanied by only minor deformation;
• Black sheared schist. This type of schist is pervaded by cm to mm scale anastomosing fine graphite and sulphide bearing micro shears. This type of mineralisation is typically proximal to the Hangingwall Shear. Scheelite mineralisation occurs in the silicified cataclastic shears;
• Shear-parallel quartz veins. These veins lie within and/or adjacent to the black sheared schist and have generally been deformed with the associated shears. The veins locally crosscut the foliation in the host schist at low to moderate angles. Veins are mainly massive quartz, with some internal lamination and localized brecciation. Sulphide minerals are scattered through the quartz, aligned along laminae and stylolitic seams. These veins range from 1 cm to > 2 m. Scheelite mineralisation is associated with quartz veining in some areas; and
• Sheeted veins (laminated veins) locally known as ‘stockwork veins’. These veins occur in the Intrashear Schist and can consist of numerous steeply dipping veins. Stockwork veins are typically traceable for 1-5 m vertically with most filling fractures that are 5 – 10 cm thick but can be up to 1 m thick. These veins generally display evidence of incremental opening.
Gold is associated with pyrite and arsenopyrite in all the above styles of mineralisation. Rarely free gold up to 300 µm occurs in quartz veins, but mostly presents as 1-10 µm scale blebs hosted in and near sulphide grains (Angus, 1993).
Tungsten as scheelite is found predominantly within mineralised quartz veins, although a subordinate phase of disseminated scheelite and a mineralisation phase are also observed (Farmer, 2016). The main phase of tungsten mineralisation occurred early in the development of the deposit and typically occur in the same lode and vein structures as gold mineralisation. However, tungsten mineralisation is not genetically related to gold mineralisation. MacKenzie (2015) recognised 5 types of scheelite. Types 1,3,4,5 are fine grained and disseminated varieties. Type 2 scheelite is the coarse grained to massive creamy coloured scheelite that was mined in the past.
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.
Deposit Type
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. Orogenic deposits typically formed on retrograde portions of pressure- temperature time paths during the last increments of crustal shortening and thus postdate regional metamorphism of the host rocks (Powell et al., 1991 and references therein). Orogenic deposits can be subdivided into epizonal, mesozonal, and hypozonal based on pressure-temperature conditions of ore formation. The Macraes deposit falls into the mesozonal category with mineralisation having occurred near to the brittle-ductile transition at about 300°C.
In orogenic deposits the association between gold and greenschist grade rocks is commonly thought to be related to: 1) the large fluid volume created during the amphibolite and/or greenschist transition and released into the greenschist zone; 2) the structurally favourable brittle-ductile zone that lies just above this transition; 3) fluid focusing and phase separation that are most likely to occur as fluids ascend into the greenschist pressure-temperature regime and/or gold solubility shows a sharp drop under greenschist facies temperatures (Phillips, 1991).