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Opal mining

Opal mining
The prize: gem-quality opal.
The prize: gem-quality opal.
Opal mining is a life of stamina and hard physical work, skill and luck, hope and disappointment...and - every now and again - the reward of discovering one of the earth's most rare and enchanting treasures as light touches it for the first time in eternity.

The technology and technological history of opal mining is fascinating. Every Australian opal field has produced innovations in mining machinery and methods, adapted to its particular geological, technological and social setting.

In time, we will post information and photographs from opal mining areas around Australia...but let's start with Lightning Ridge.


History
Opal was first discovered at Lightning Ridge in the 1880s, and the first recorded mine shaft was sunk in 1901, probably by boundary rider Jack Murray.

Miners and windlass, pre-1920. Photo courtesy of Lightning Ridge Historical Society.
Miners and windlass, pre-1920. Photo courtesy of Lightning Ridge Historical Society.
For years, mines were dug by hand using picks and shovels. It was incredibly hard work, in an isolated place distinguished by extreme heat, lack of water, and scant facilities. The miners dug square-sided shafts, which they climbed with their backs and legs braced against the walls. Initially, tunnels were no higher than required for a miner to crawl through, and dirt was hauled to the surface in buckets made of hide, tied to ropes. A windlass could make the job a bit easier.

No major advances in technology occurred until the 1960s, when local miner Eric Catterall, crippled by childhood polio, invented an automatic hoist to carry his dirt to the surface. In that decade, artesian water became available to wash the opal dirt, making separation of opal from the dirt more efficient. Other significant developments have included the invention of the blower and agitator (see below) and introduction of the Calweld drill for sinking shafts and prospecting drills for locating potentially opal-bearing ground.

The town of Lightning Ridge sprang up around the opal fields in the first decade of the twentieth century, but it took a while for black opal to achieve the worldwide acclaim it enjoys today - because nobody understood the dark, brilliant form of opal mined at Lightning Ridge. Determined efforts won a lucrative market for black opal, which in 2008 was declared the State Gemstone Emblem of New South Wales.

For more about the history of Lightning Ridge, see the Lightning Ridge Historical Society.


Where to look for opal: prospecting
An auger drill in use for opal prospecting.
An auger drill in use for opal prospecting.
It isn't easy to find opal. A miner may work for weeks, months or years without finding enough gem opal even to cover expenses. So deciding where to mine is one of the most important decisions an opal miner can make.

Some miners use a prospecting drill to sink a series of exploratory holes into the ground. Each hole is about 230mm diameter. Samples are brought to the surface and examined to see whether the sediments appear likely to contain opal and if so, at what depth.

There are also surface indications of opal - opal tends to form near fault lines and trees often grow on fault lines because the faults hold water. Wild Orange trees, in particular, are often said to be indicators of opal. Aerial photographs can be useful for identifying lines of trees along fault lines, or lineaments.

Once miners are underground, they learn to 'read' the geological indicators in the mine face to identify the ground that is most likely to contain opal. For more on opal geology, click here.

There are plenty of other, less scientific ways of deciding where to look for opal - for example, some old timers thought that if there were flies underground, it was because the flies could smell opal!


Pegging a claim and getting started
Claim pegs at the junction of two mining claims.
Claim pegs at the junction of two mining claims.
At Lightning Ridge each opal mining claim measures a maximum of 50 metres by 50 metres. Each person may hold a maximum of two mineral claims at any one time. As at January 2008, there were more than 4,500 mineral claims in the Lightning Ridge Mineral Claims District.

Before registering a claim or commencing mining, the miner must complete a Mine Safety Course conducted by NSW Department of Primary Industries. Mine Management and Environment courses are also required in some circumstances.


Underground mining
At Lightning Ridge, most opal is mined by shaft-and-tunnel underground mining.

First, a Calweld drill is used to sink a vertical shaft of about a metre in diameter down to the level where the miner hopes to find opal.

The miner hangs a set of steel ladders down the shaft; this is the miner's route in and out of the mine every day. All of the machinery and equipment used in the mine, and everything that comes out of the mine, enters and exits through the shaft. A second shaft may be sunk after a time to assist with movement of miners and opal dirt, and/or to improve ventilation.

From the bottom of the shaft, the miner digs out horizontally to make tunnels called drives, looking for indications of opal on the way. The miner uses a jackhammer to start the drive; once the drive has been opened out, some miners introduce a machine called a digger, which has a hydraulic arm that pulls down the opal dirt. The digger, lights and anything else requiring electricity are powered by a generator up on the surface.

A rickshaw is used to transport dirt from the mineface to the bottom of the shaft.
A rickshaw is used to transport dirt from the mineface to the bottom of the shaft. There, it is loaded into the bucket of an automatic hoist and sent on its way to the surface.


To get the dirt out of the mine, a miner may shovel it into a rickshaw (a specialised wheelbarrow), push the rickshaw to the bottom of the shaft and tip the dirt into a bucket attached to an automatic hoist, which carries the bucket up the shaft via a cable, then dumps the dirt into a truck waiting on the surface.

Blowers are used to suck dirt from underground then drop it into a waiting truck.
Blowers are used to suck dirt from underground then drop it into a waiting truck.
An alternative is a machine called a blower, which is like a giant vacuum cleaner with a pipe that goes down the shaft, then along the drive to the mine face, where it sucks up the dirt the miner has dug out, carries it to the surface then drops it into the truck. It's a far cry from earlier days, when miners hauled dirt to the surface in hide buckets tied to a rope.

The dirt is then tipped from the truck into a modified cement mixer called an agitator, in which it is tumbled with water for anywhere between a couple of hours to a couple of days, depending on how hard the dirt is, to wash away the clay and leave only hard tailings. The miner then sorts through the tailings and may - or may not - find some opal. This is called 'tailing out'.

Before the advent of the agitator, miners 'puddled' their opal dirt to isolate the opal. Dry puddling consisted of 'rumbling' the dirt in a mesh-walled barrel, promoting mechanical breakdown of the clay and sand. Wet puddling following a similar principle, with the addition of water.

An agitator site.
An agitator site. Dirt from the mine is loaded onto a conveyor belt that tips it into the agitator - a cement mixing barrel modified for opal processing (left). The agitator turns, churning the opal dirt with water to cause fine sediments to break up and exit through a mesh window on the side of the barrel (centre). Several hours later, the hard tailings that remain (right) are sorted in the hope of finding opal.


Open cut mining
Lunatic Hill Open Cut.
Lunatic Hill Open Cut.
The scarcity and unpredictability of gem opal at Lightning Ridge means it is not usually viable to mine by open-cut methods, which involve excavating millions of tonnes of earth to reach the level which may - or may not - contain the motherlode that could make the exercise worthwhile. Where open cutting does occur, the area has usually first been mined by underground methods that proved it to be unusually rich in opal.

The largest open cut opal mine ever worked in New South Wales is Lunatic Hill Open Cut, at the Three Mile field, Lightning Ridge, neighbouring the site on which the Australian Opal Centre is to be built. Lunatic Hill has seen a century of opal mining and the open cut mine has been preserved for its scientific, aesthetic, historic, educational and economic values.


Want to know more?
For more information about mining for opal at Lightning Ridge, see www.wj.com.au/mining/index.html or consult the Lightning Ridge page on the web site of NSW Department of Primary Industries (Mineral Resources), at www.dpi.nsw.gov.au/minerals/lightning-ridge.


 





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