The Macraes mine lies in the Otago Schist Belt of Mesozoic age, which forms the basement to South Island New Zealand. Macraes occurs north of the city of Dunedin in the SE corner of South Island, and is the country's largest gold mine.
The mesothermal style deposit is set in a regional scale mineralised structure up to 120 m thick and with a strike length of at least 30 km. The late metamorphic shear dips at approximately 15°, which is sub-parallel to regional foliation. Mineralisation took place under greenschist facies conditions (~ 350°C) during uplift of the Otago Schist in late Jurassic times. The shear zone is truncated by an underlying low-angle normal fault that juxtaposes mineralised lower greenschist facies rocks with unmineralised rocks of upper greenschist facies. The entire host shear zone has been hydrothermally altered to some degree, but mineralogical evidence for this alteration is subtle.
Altered rocks contain disseminated siderite, kaolinite, and rutile rather than calcite; epidote and titanite form the peak metamorphic assemblage. Alteration is most pronounced in the more strongly deformed rocks which anastomose around less-deformed lenses on a scale of 1-20 m. Quartz veins occur locally but are thin (typically <1 m) and discontinuous on the scale of 1-10 m.
Mining at Macraes focused initially on a zone of quartz veins exposed at surface in historic workings. Expansion of the mine along strike in the host structure has revealed large volumes of mineralised rocks without quartz veins. Gold is disseminated in schist (up to 5 ppm), accompanied by sulphide minerals (principally pyrite and arsenopyrite), with only minor silicification. Some of the disseminated mineralisation was accompanied by the addition of hydrothermal graphite (up to 3 wt %), and sericitic alteration of metamorphic albite. Graphitisation and sericitisation has controlled location of individual shears, and vice versa. Hydrothermal addition of As, W, Au, and S has occurred along shear zones and in associated schist via grain boundary fluid migration. There has also been minor addition of Bi, Sb, Hg, and Mo. This style of disseminated hydrothermal alteration is difficult to detect in surface outcrop where quartz veins are absent. Nevertheless, these mineralised rocks constitute an important large-tonnage, low-grade resource at the mine.
The mine extracts gold from mineralised schist in a large open cut operation which produces 175 000 ounces gold/year with a typical bulk grade of 1.6 g Au/t. The 2004 resource estimate was 87 m tonnes @ 1.4 g/tonne for a total of 3.9 m oz gold.
(Source: Craw et al., 2004)