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New Zealand Goldmining
Introduction and Techniques
These pages brings together our information on New Zealand Gold, Goldmining Techniques and Goldfields gathered whilst touring over a number of years.
It would be easy to write a complete book about gold mining but I will attempt to give a brief flavour in a few pages. In the Coromandel in North Island the Bullion (Gold and Silver) is mostly concentrated in "reefs" of Quartz which were followed underground and largely mined by hand during the important initial years. The Quartz was then broken up to cm size pieces in jaw crushers then Stamper Batteries were used to pound the quartz to a fine powder releasing particles of Gold. The Stampers crushed the ore by lifting and dropping huge iron stamps onto the ore. The stamps were raised and dropped using cams driven by water wheels, Pelton water turbines or steam engines. Once reduced to a fine powder the initial separation was on vibrating water covered tables or amalgamating tables covered with a thin layer of mercury on the surface which trapped and amalgamated the gold.
A lot was missed and the heavy 'tailings' were then further treated in Berdans - inclined, revolving cast-iron basins containing a heavy iron block. The slow revolving action ground the sludge even finer enabling even tiny particles of Gold to be freed and recovered by amalgamation with mercury which was in the bottom of the Berdan. When the Mercury got thick the amalgam was separated by squeezing the paste through a chamois leather and distilling off the mercury from the Gold and Silver which was melted and cast into bars. These techniques only extracted about 50% of the Bullion and were latter augmented by a Cyanide treatment which increase the extraction to about 90%. The whole process from hand mining underground to use of mercury and cyanide was not the most healthy way of life!
The Goldfields in South Island in Otago are quite different to those in the Coromandel where the Gold is within Quartz veins. In the Otago most of the Gold is Alluvial Gold - that is Gold is contained in alluvial gravels, deposited by the action of water and ice. The gravel does not need to be crushed and the separation is by various techniques of "washing" to separate the Gold which is very dense (density 19) from the much less dense rocks and sand. The ratio of density of Gold to Gravel of about 7 allows simple panning or various sluicing techniques of increasing complexity to separate most of the Gold from the gravel.
Only at the earliest stages of a Gold Rush did one find loose gravel "washdirt" perhaps lying on a harder substrate at the bottom of a dry stream with Gold concentrated at the bottom which could be worked and panned by hand. This was quickly exhausted and the Alluvial Gold had then to be extracted from a solid mix of mud, gravel and Gold which had been deposited and hardened into something almost as solid as rock, sometimes refereed to as "cement".
It was at this point that the availability of water became critical for the separation of the Gold from other materials. There were three main techniques used in Otago for the separation - Ground Sluicing, Hydraulic Sluicing and Hydraulic Elevating - all depended on the availability of plentiful supplies of water. Requirements quickly exceeding supplies in the vicinity and the mining areas were characterised by dams and "Races", channels brings water from streams sometimes up to 40 kms away.
Ground Sluicing was the simplest technique and relied on water being discharged over the face and conducted into a channel running away from the base. Miners shovelled and pickaxed out extra gravel to add to that washed from the face into a channel where it was carried by the current into sluice boxes. Riffle bars in the sluice boxes trapped the Gold which sank to the bottom whilst the waste materials were washed over and into a Tail Race to a nearby creek. This method was dependent on a suitable height differential to carry away the tailings.
Hydraulic Sluicing was a more effective technique and used powerful jets of water to break down the faces and wash the loose gravel into the Sluice Boxes as before. The jets used were very strong and needed a head of water of 100-300 feet. The nozzle on what was called the Monitor could be anything from 1 inch diameter up to 10 inches - anything bigger than 3 inches was exceptional and the water requirements for even a 3 inch monitor were huge and involved large storage dams and pipes. Hydraulic sluicing required faces above the level of the surrounding land and routes for the tailings to be removed after passing through the Sluices. In many cases the ground level was considerably raised (sometimes tens of meters) by the build up of the tailings and long deep channels had to be cut to the nearest rivers to get rid of them.
Hydraulic Elevating was a subsidiary technique to extend the regions which could be worked by Hydraulic Sluicing. It involved the additional raising of the gravels from one level to another using water under very high pressure utilising the Venturi principle. The Elevator had a delivery pipe to a pit into which the gravel and water from the sluicing collected with a nozzle pointing up into another pipe leading to the upper level. The gravel and water slurry was sucked into the gap and blasted upwards by the water jet.These could raise the gravel 30 feet or more to sluice boxes often above the surface level to allow the tailings to carried away. By the 1890s these elevators were raising material up to 150 feet at some workings, usually in several stages but single lifts were built of up to 92 feet. The disposal of the tailings was still a problem and extra elevators were sometimes used to get rid of them as an alternative to digging ever bigger and deeper tail races. The immensity of these operations can be seen in early photographs and in the remains left at the major sites.
Dredging was a technique used extensively on the Clutha and other areas near Cromwell and, in particular round Alexandra. As the river banks were exhausted of readily accessible gold miners sought ways to reach the gold on the river bed itself with potentially huge pickings.It started with simple spoon dredges - a leather bag attached to an iron ring on a long pole taken out on a boat.Developments which followed included pneumatic dredges, diving bells and suction dredges. The greatest success came with the development of steam driven bucket dredges which quickly came to dominate dredging. They were expensive to build but changed the face of mining -and the countryside - as they worked rivers, swamp land and flats.
Dredging did not reach its peak till after 1900 when over a hundred companies were in operation, after which it steadily declined to one or two, with a brief resurgence whenever gold prices rose such as in the depressions of the 30s. The museum in Cromwell has models and pictures of some of the more famous dredgers and some of the huge buckets on display. The main separation was done on board the steam dredges so the problems of obtaining water and in particular the disposal of tailings, which plagued any working of alluvial gold on land, disappeared.
| Copyright © Peter and Pauline Curtis
Layout revised: 21th May, 2015