Gold, any miner will tell you, is where you find it – as the Cornish miners said, ‘Where it be, there it be’. Many geologists would agree, but they would also add that gold is likely to be found only in certain areas. In New Zealand these areas are Otago, Southland, the West Coast, Golden Bay and Marlborough, Coromandel Peninsula and a few other localised places.
Gold is a rare element. It is inert, very heavy, shiny and malleable. It is gold’s aesthetic properties and rarity that command a high price. Only 10% of global production is used for industrial purposes. Its main use is in making jewellery. It rarely occurs in a pure form, and is usually mixed with silver and other metals. In New Zealand, Otago and West Coast gold is purer (93–98% gold, with 2–7% silver or silver plus mercury), while on the Coromandel Peninsula it occurs as the gold–silver alloy electrum (typically 65% gold and 35% silver in the Martha mine at Waihī) along with separate sulfide minerals.
In New Zealand silver mainly occurs in association with gold on the Coromandel Peninsula, where about 1,440 tonnes of silver have been recovered from mining of the gold–silver deposits (1,212 tonnes of this has come from the Martha mine). Early reports of silver finds near Collingwood in Golden Bay and other localities amounted to nothing of economic value.
Gold occurs in fissures and fracture zones, where it has been deposited in quartz veins. It is almost always in very fine flecks in the quartz veins, usually too small to be visible. Gold can be deposited near the earth’s surface (epithermal gold) and at greater depths (mesothermal gold).
Epithermal gold can be found with silver in quartz veins on the Coromandel Peninsula. It has been deposited at depths of up to 1,500 metres by hot spring fluids at temperatures of 180–300°C.
Mesothermal gold occurs in the schist rocks of Otago and Marlborough. Here, gold has been deposited at depths of 3–12 kilometres by hot fluids at temperatures of 200–400°C. Greywacke and argillite rocks in the Aorere valley in Golden Bay, Lyell, Reefton and Mt Greenland on the West Coast, and at Preservation Inlet (Rakituma) in Fiordland, also contain mesothermal gold in faulted areas known as shear zones.
‘Chinaman’ and ‘Maoris’
While washing gravel with millions of litres of fresh water, the alluvial miners working in Central Otago named things as they saw them. Large quartz boulders that had been stained yellow by leaching minerals were called ‘Chinaman’, while dark, heavy boulders rich in iron were ‘Maoris’.
Alluvial or placer gold comes from eroded hard-rock sources. Rivers and glaciers flowing over gravels have washed and sorted them, concentrating the heavy gold in certain layers. This often makes placer gold deposits much richer than their hard-rock sources.
Alluvial gold can be mined by digging it up with earth-moving equipment, sluicing, dredging, or by hand with a gold pan and shovel. There are localised deposits in Nelson and Marlborough. Little placer gold has been found on the Coromandel Peninsula. It is most widely distributed in Otago and the West Coast, including West Coast beaches such as Gillespies Beach, and at Orepuki in Southland. Offshore gold deposits exist off the West Coast and Otago, although prospecting has been limited by the difficulty of sampling the sea floor.
The gold in river gravels convinced many miners in Otago that there must be a mother lode – a hard-rock source upriver. In a few places rich veins were found and worked – at Skippers, Bendigo and Macetown. Over thousands of years, glaciers and rivers had ground away Otago’s schist, which contained gold-bearing quartz reefs. So the gravels beneath glaciers and rivers often contained higher gold concentrations than the existing hard rock. There was no mother lode; or rather, the mother lode was the alluvial gold itself.