History
In the district surrounding the Macrae's Mine, the first alluvial mining started at Murphy's Flat in 1862, with Macrae's Flat, Deepdell and parts of Horse Flat following soon after. The alluvial workings at Murphy’s Creek are reasonably well preserved and are considered to be of historic significance.
In the 1860's, lode quartz mining commenced but the scale of operations was very small. The Golden Point/Round Hill lode system was not discovered until 1889. Development of Golden Point commenced in 1889 and it soon became established as a significant producer of scheelite and gold.
From 1890 to 1933, it produced an estimated 13,000 ounces of gold and 800 tons of scheelite. Others areas mined included Maritana, Golden Bell and Deepdell but the quantities were small with a total reported of 8,463 tons of crushed ore for 1,630 ounces of gold and 50 tons scheelite.
Lodes were worked for either scheelite or gold depending on the price at the time. This was due to the fact that the fine grinding required to extract the gold resulted in poor recovery of scheelite.
Areas continued to be mined after 1929 as tungsten was in demand during the Second World War but proceeds from gold production declined markedly during this time. The scales of operations at this time were small and work was discontinuous, and as a result records of production of ore at this time were poor. Local miners suggest that less than 100 tons of scheelite was mined since 1939 but estimates were widely varied.
It was always a question of economics (due to the preferential recoverability of gold or tungsten) rather than ore availability that controlled the scheelite industry at Macrae's Flat.
The first lode worked in the Macrae's field was probably the Duke of Edinburgh, as documented in 1875. Also mentioned were the Golden Bar Reef and the Moonlight Reef, at the head of Macrae's Flat. In 1888, the Highlay Reef was discovered on the Mareburn, and the lode was soon traced to Golden Point, where it was opened out in 1889. Further prospecting soon resulted in the opening of other mines along the lode - some of them, however, being little more than surface workings.
The mines that have been worked, given in order of west to east, are Mount Highlay, New Zealand Gold and Tungsten, Coronation, Golden Bell, Maritana, Deepdell, Golden Point, Round Hill, Innes Mills, Griffins, Golden Ridge, Ounce and Golden Bar.
Present Day
Long gone are the days when gold nuggets could be simply picked up out of the gravel and the remaining gold at Macrae's Flat is firmly locked inside of solid rock, requiring much more innovation and high technology (not to mention perserverance) to extract it.
The present operations of the Macrae's Mine consists of the Macrae's Open Pit and the Frasers Underground operation, with each supplying, respectively, about 4.9 million tonnes and 0.9 million tonnes of ore for processing annually. Further stockpiles of ore are available for processing when weather inhibits mining operations.
A short distance from the Macrae's Open Pit is a processing facility that can process approximately 5.6 million tonnes of ore per year, and includes a pressure oxidation plant for the processing of sulphide ore (which is one of only three in the southern hemisphere). At the Macrae's processing plant, the ore is put through crushing, grinding flotation, fine grinding, pressure oxidation carbon in leach, elution, elctro winning and smelting.
Since the Macrae's Mine opened in 1990, it has produced over 3 million ounces of gold, averaging, over the last decade or so, around 180,000 ounces per year. In 2012, the mine produced 169,609 ounces of gold from 5,558,056 tonnes of gold bearing ore mined.
Geology
Generally, the gold deposits in the Otago area are centered on steep and well defined quartz veins in the rock. At Gelnorchy, Shotover and Barewood, for example, gold is found mostly in quartz and calcite veins in normal (steep/near vertical) faults.
The geology of the Macrae's Mine, however, is geologically very different. The Macrae's Mine is situated above the Hyde-Macrae's Shear Zone - a 10-20° sloping formation up to 150m thick in places. The shear zone slopes down towards the NE and ends abruptly where it intersects the Waihemo Fault Zone, approximately 12km from the Macrae's Mine.
The shear zone occurs in chlorite zone schist that was metamorphosed at about 350°C at 10-15 km depth. In the latter stages of the metamorphism, about 130-140 million years ago, flow of hot hydrothermal water brought gold up into the shear zone. At this time the shear zone was part of a thrust fault and was subsequently uplifted to the surface between 130 and 100 million years ago.
In this gold-bearing formation, gold is only present in the mineralised rock of the shear zone itself, which shows up as slightly darker rock than the surrounding Otago schist. The darker colour is caused by graphite (carbon) that was added to the rock by the hydrothermal processes that introduced the gold and also some recrystallisation and crushing of rocks caused by the hydrothermal water flow.
Because gold is only present in the shear zone rock itself, only the extracted shear zone rock ore is sent to the processing plant and rock removed from either side is waste rock. Mining of the shear zone only occurs in daylight hours as it is impossible to differentiate at night under lights.
While the gold bearing geological formation at Macrae's is unique in terms of commercial gold extraction in the Otago gold fields, there is strong scientific evidence to suggest that similar mineralised shear zones exist in the main divide of the Southern Alps, holding significant quantities of gold.
This Earthcache
This is an earthcache and like any earthcache, the purpose of it is to provide a geological lesson at the site you are visiting. This is not a virtual cache and you must visit the site and answer the questions below in order to claim you have 'found it'.
Before claiming your find, you must submit your answers for the questions below to the owner of this earthcache, by clicking on the link to the owner's profile at the top of the page, and sending the owner an email.
When emailing your answers, don't forget to include the name of this earthcache, and choose the option to include your email address so that we can reply to you. Do not include answers in your log on the cache page.
After you have emailed your answers, you can go ahead and log your find. Any problems with your answers, and we'll get back to you.
1. According to the information panel, what geological feature "runs several kilometres below your feet"? (Exact wording please)
2. According to the information panel, exactly how long (in kilometres) is the Hyde-Macrae's Shear Zone?
3. Explain how and why small amounts of gold can be found in some surface alluvial gravels outside the shear zone.
4. From the lookout, describe the rock you can see in the quarry (in terms of texture and colour) and give its name if you can. If you can see and distinguish the gold bearing shear zone rock, describe this too.
5. Take a photo of the mine (optional) and upload it with your 'found log'. Please include your GPS unit if you can.
Please note, this is an extremely easy earthcache to complete if you visit the site, and we will therefore reserve the right to delete logs if answers are incomplete, incorrect, or not provided at all.
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