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barren country
coober pedy
great artesian basin
hand windlass
miner working
MINING & EXPLORATION

Mining and exploration

Prospecting for opal has always been an uncertain activity. Most of the known deposits have been found because of the occurrence of small pieces of weathered opal on the ground surface, indicating the probable presence of the gem nearby. Because of the extreme dryness of the climate in the Australian opal fields and the lack of regular runoff of water, the opal weathering out has tended to stay on the surface near the matrix sediments from which it has weathered. This is in contrast with the Brazilian deposits, where higher rainfall and more regular streams have resulted in some opal being recovered from alluvial deposits.
In most cases, opal has been found by local residents of the areas; not by the professional prospector. Once the first deposits have been exploited, the prospector and miner then fan out from the initial discovery area, sinking shafts in the hope of cutting an opal horizon. This has been the history of all of the major opal fields of Australia.

Once the initial areas of discovery have been pegged out and tested, with some claims being profitable, others barren, prospectors may sink shafts in nearby areas. Many extensions of the original fields, have been discovered in this way, especially in the Lightning Ridge and Coober Pedy areas.

The decision as to where to sink a shaft is the most difficult as there are usually few direct surface indications. However, the experienced miner has been able to improve the odds by his knowledge of surface indicators. Aracic, in his book 'Discover Opals' (R1774) describes how he does this, after relying on the type of plant growth to define sub-surface conditions.

It is well known that the main opal fields occur around the edge of the Great Artesian Basin, so that this zone, an enormous area in itself, is the logical area for exploration. There have been numerous minor discoveries of fields which have yielded a little commercial production, especially in South Australia,. In general, these small fields have been located from surface exposures.

Systematic large scale exploration for opal in completely new areas has seldom been undertaken because of the highly irregular distribution of the mineral. The South Australian Mines Department has undertaken exploration at Andamooka and Coober Pedy, and a few companies have been formed from time to time to systematically explore for new deposits. Geologists have been employed to study the geology and the nature of occurrence of the opal to develop a more systematic programme of exploration. However, the work still tends to be carried out at the margins of known fields.

Systematic exploration is usually carried out by a drilling program using calyx drills of about 30 cm diameter. The shafts are put down to the opal level, which can usually be recognised by the occurrence of potch, hard bands, or other changes in lithology. Because of the speed with which the holes can be drilled, together with the technique of spreading the base of the holes at the opal level, it is not uncommon for small amounts of opal to be recovered in the neighbourhood of known fields, thus helping to finance the exploration.

The earliest opal deposits to be worked in Australia were close to the surface, but as these were mined out, it was realised that the 'opal levels' could be found at greater depths. Shafts were usually rectangular in shape and about one metre by 0.7 metre. These were laboriously dug by hand to depths of up to 30 metres and ladders, or merely ropes, were used to climb in and out of the mines. Two or three men usually mined each lease, with the surface man using a hand windlass to haul buckets of barren or opal-bearing ore to the surface. This mullock was dumped around the mouth of the shaft, which was timbered above ground level, resulting in the patterns of small, white 'craters' so strikingly seen from the air.

The shafts were sunk to about 1.5 metres below the opal level. These levels could be recognised by certain characteristics in each field. At Andamooka, for example, the opal was mainly confined to a band of silicified conglomerate some 30-40 cm thick.

At Coober Pedy the main opal level appears at a change in the nature of the sediments, often verified by a seam of potch. From the base of the shaft horizontal drives were excavated, ideally allowing the miner to stand upright with the opal level approximately at eye level.

The good miner would leave continuous walls of the sandy clay rock to support the roof; timbering is seldom necessary. A few miners take out so much material that dangerous conditions can be created, such as those shown in this linked image. Under such conditions, it is a miracle that more accidents have not happened!

At first, all work was done with pick and shovel and lighting was by means of candles. Opal was located sometimes by ear as much as by sight in fields like Coober Pedy, where the opal occurs in softer rock. The miner could hear the scraping of his pick on the harder, gritty opal. The opal band, usually potch, was then followed very carefully to avoid breakage of any valuable material which might be encountered.

Later, the manual hoists were replaced by electric hoists such as the Yorke hoist. Rails were introduced into the drives to transport loaded drums to automatic hoists and dumpers for the mullock. A further development in shifting the mullock and 'opal dirt' to the surface has been the use of 'blowers', a type of giant vacuum cleaner which forced the dirt to the surface with a high pressure blast of air through a large, flexible tube about 0.5 metre diameter.

In recent years the shafts have been sunk by large calyx drills, about a metre in diameter, and drives excavated with large boring machines.

There is also a tendency nowadays, especially in Lightning Ridge, where much opal occurs as 'nobbies' (individual stones sparsely scattered in the sandy clay matrix) to treat the mixed opal dirt in bulk. This began by swirling the dirt with a jet of water pumped up from the 'puddling' dam in what appeared to be the barrel of an old domestic washing machine (minus the agitator). The clay and fine sand were washed away into the surrounding bushland, the stony material being retained in the drum.

Today, the washing machine has graduated to the use of the large mixers of the type used for transporting concrete. The cleaned gravel is then screened, sorted and examined by hand to pick out any nobbies or other opal fragments.

The clay which is washed away has been tested for use as a pottery material. It a smooth clay, suitable for throwing on a wheel, and, using the appropriate techniques with regard to its firing characteristics, has potential as a pottery raw material. Such clay is already being used on a small scale, and handmade ceramics have been made and marketed in White Cliffs and Lightning Ridge.

In some areas, especially in the boulder opal fields of south western Queensland, and at Mintabie, where the host rocks are much harder, bulldozers are used to remove the overburden. When the opal level is reached, thin layers are scraped away, with men following the machine looking for opal. This system often yields opal quickly, although significant amounts may be lost.