Macraes Mine, Macraes (Macraes Flat), Otago Region, New Zealandi

Regional Level Types
Macraes Mine	Mine
Macraes (Macraes Flat)	- not defined -
Otago Region	Region
New Zealand	Country

Open-cast gold mine working gold-scheelite deposits in lode shears, stockworks and disseminated stockworks within the gently dipping Hyde-Macraes Shear Zone (although scheelite is not being currently extracted due to metallurgical difficulties). Mined from a series of open pits extending 6km.

Located close to the settlement of Macraes Flat, 40 kilometres west north-west of Palmerston, and 80 km north of Dunedin, eastern Otago. A heritage precinct, art park, and trout hatchery are planned for the site once the mine closes.

The Macraes Mine started in 1990, and in 2003 produced about 5,000 kilograms of gold. Production from 1990-2013 ~3 Moz of gold.

Owned by Oceana Gold Corporation. The Frasers underground mine was commissioned in January 2008. Oceana Gold Corporation states in 2016 that the mine was 740 metre below the surface and 200 metres below sea level, with over 48 kilometres of developed tunnel drives.

Mineralisation is part of a duplex thrust system called the Hyde-Macraes Shear Zone, around 25 kilometres long, north-west to south-east trending, dipping 10-20 degrees north-east, of variable thickness up to 150 metres, hosted by the Otago schist.

The duplex system is dominated by steep dipping thrust faults, below well defined shallow upper and lower bounding thrusts. Mineralisation forms mainly in dilational jogs, or local extension zones. This occurs in relatively shallow parts of the intrashear thrust faults, cross-cut by late stage shallow, south dipping extensional veins. Gold was deposited about 130-140 million years ago, when the site was part of a thrust fault. The site is unusual as most gold deposits in Otago are found in steeper locations, in well defined quartz veins.

There are four types of gold and scheelite mineralisation 1) schist with disseminated sulphides; 2) black sheared schist consisting of fine grained graphite and sulphide bearing shears, proximal to the Hanging Wall; 3) sub-parallel quartz veins within and/or adjacent to the black sheared schist, locally cross-cutting schist foliation, containing disseminated sulphides in quartz veins; 4) local swarms of stockwork 100-2000 metre square, containing 10-100 sub-parallel quartz veins each, filling fractures, dipping almost vertical to the shallow east dipping schist.

The shear zone has a complex history from overprinting of numerous alteration and deformation episodes. It contains micaceous and feldspathic schist. The micaceous schist contains abundant muscovite, chlorite, abundant pyrite cubes up to 5mms in diameter, massive pyrite, arsenopyrite and gold. The feldspathic schist contains abundant albite, with thin black microshears, contain graphite, pyrite, arsenopyrite, gold, and rutile.

White massive quartz veins form a stockwork, discontinuous shallow dipping 10-20 degrees, and steeper veins 50-80 degrees. The quartz veins contain pyrite, arsenopyrite, and scheelite. Some native gold can be seen as free in quartz veins, but over 90% is microscopic enclosed within pyrite and arsenopyrite. Late pyrite veinlets contain microscopic sphalerite, chalcopyrite, and galena. Pyrite is the most dominant sulphide, formed and recrystallised throughout the deposit's history. Arsenopyrite may be found intergrown with boulangerite.

Some hydrothermal alteration has occurred in the shear rocks, with albite partial altered to muscovite and titanite, and epidote to rutile, quartz, calcite, kaolinite, chlorite and widespread siderite (Craw et. al., 1995). Titanite is replaced by rutile, and epidote by siderite, chlorite, muscovite, and calcite (Craw et. al., 1999).

This alteration is most evident in the black micro shears. These shears also contain very fine hydrothermal graphite, with pyrite, arsenopyrite, and widespread rutile. Gold is found here without quartz veins, precipitated during the reduction of the fluids by hydrothermal graphite. The sequence was later cut by mesothermal quartz veins, containing gold, scheelite, rutile, pyrite, and arsenopyrite.

Oxidation is up to 50 metres below the surface, and some coarse gold that was able to escape from the sulphides was found here, in the initial alluvial mining of the 1860's to 1940's.

The mine is the largest gold producer in New Zealand. Most gold mines in New Zealand, accessed historically quartz veins of a limited extent, whereas the Hyde-Macaes Shear Zone contains a package up to 150 metres thick of gold found in quartz veins, and outside this within the zone, along a 25 kilometre front. Satellite pits have been developed at the richest points. However the gold grade is low, ranging usually 1 to 1.5 g/t, and is only economic due to the large scale at which it is mined.

Specimens are not usually seen from the location, common with industrial scale mines. However, these types of mine often contain a process yet to be explained by science, where rocks of a non-monetary value end up in boxes under the beds of geologists and mine workers, only to see the light of day years after the mine has closed.

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