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

Regional Level Types
Frasers pit	Pit
Macraes Mine	Mine
Macraes (Macraes Flat)	Town
Otago Region	Region
New Zealand	Country

For more geology information see 'Macraes Mine' Mindat sub-heading.

This heading covers the Frasers open pit, Frasers underground mine deposit 1 and 2, and Frasers underground mine Deeps deposit.

Frasers is the main pit at Macraes with the other locations satellite pits producing relatively minor amounts of ore. The pit was opened in 1990, and Frasers underground mine from the pit in 2008.

Alluvial mining occurred in the area from the 1860's, then underground mining to the 1940's. At the time it was thought the site contained two parallel auriferous quartz veins, with numerous outlying small quartz veins. Historic mining was near surface in the oxidised zone, within 50 metres of the surface, where gold had been liberated from the underlying sulphides. Once un-oxidised ore was reached, mining ceased, as getting gold out of the sulphides (pyrite and arsenopyrite) was too difficult at the time.

Renewed exploration began in the 1980's, and much drilling determined gold was found both within, and outside of quartz veins, along a 25 kilometre duplex thrust structure, up to 150 metres thick. While low grade, mining was amenable to large scale open pit development along the structure.

Macraes is the largest active gold mine in New Zealand, producing between 1990 to 2009 2.5 Moz, and since 2000 between 162 to 184 Koz, at an average grade of 1.6 g/t Au. It is operated by Oceana Gold (N.Z.) Ltd. The grade is very low, but is economic due to the industrial scale of the operations.

As of 31st December 2009 Frasers open pit deposit held a measured resource of 12.14 Mt of ore at 1.53 g/t Au, indicated resource 28.36 Mt at 0.91 g/t Au, inferred resource of 9.41 Mt at 0.7 g/t Au and 0.21 Moz Au (0.5 g/t Au cut-off). Frasers underground combined deposit 1 and 2 a measured resource of 1.38 Mt at 3.10 g/t Au, indicated resource 4.88 Mt at 2.2 g/t Au, inferred resource at 4.62 Mt at 2.00 g/t Au and 0.30 Moz Au. Frasers Deeps has an indicated resource of 0.35 Mt at 3.56 g/t Au, inferred resource of 0.54 Mt at 3.7 g/t Au and 0.06 Moz Au. The cut-off for the open pit is 0.5 g/t Au, with no cut-off for the underground workings.

The Frasers open cut is defined by a Hanging Wall shear, of quartz fault altered rock and lode schist, dipping 15-20 degrees east, under 5 metres thick, but at depth dips 5-10 degrees east, and 20-30 metres thick. The Footwall fault is 80 to 120 metres below the Hanging Wall shear, as a 10 metre wide zone of shearing.

Gold is found in the Hanging Wall, shear parallel quartz splay veins, and stockworks. The stockworks between the Hanging Wall and Footwall fault form erratic mineralisation, with locally high areas of gold-scheelite-pyrite-arsenopyrite in quartz veins, with more scheelite towards the Hanging Wall.

The Frasers underground mine deposit 1 is a continuation of the open pit mineralisation, found in schist between the walls. The deposit thickens to the north-east, and thins to the west, averaging 80-100 metres thick, with higher grades of gold found in the upper parts, and near the Hanging Wall in a zone 10-15 metres thick.

Frasers underground mine deposit 2 is a separate zone to 1, trending north-east, dipping 15-20 degrees east, showing gold 1.0-4.1 g/t Au in a zone averaging 10-12 metres on the Hanging Wall. Gold grades are generally higher than deposit 1.

On the south-east margin of deposit 2, below the Hanging Wall is Fraser underground Deeps deposit, as a low angle concordant lode with strong stockwork.

Specimens are usually not seen from the mine. Gold is not usually visible, enclosed within pyrite and arsenopyrite, however relatively minor free gold is visible in some quartz veins. Pyrite, arsenopyrite, fine grained graphite, rutile, siderite, boulangerite, gold, albite, calcite, muscovite, chlorite, are the most common species likely to be seen should any specimens appear. The mine in places contains abundant pyrite cubes to 5 mms each in diameter.

The company operates tours of the mine.

Other Regions, Features and Areas containing this locality

References