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OPAL IN AUSTRALIA

The Andamooka Field

Andamooka is some 600 km north of Adelaide by road. The country surrounding the township consists mainly of small hills with rocky outcrops, sandhills and flat dry claypans. The hills are covered with a sparse growth of acacias, small eucalypts and low scrub and fodder plants such as saltbush.

Some 15 km east of the township is the northern part of Lake Torrens, a large, usually dry, salt lake bed which is up to 50 km in width and extends nearly 200 km from north to south; it is a featureless and uniformly flat area. In many areas of the lake, dry as it looks on the surface, it can be thick black salty mud underneath, difficult to negotiate even with a modern four-wheel drive vehicle.

Andamooka township has grown haphazardly along the dry sandy river bed now called Opal Creek, which winds its way between the low hills which have been formed by the weathering of the flat lying sediments of the area. In the earliest days of the field, wells dug in the creek bed were the main, if sparse, supply of water for the miners. As in the Lightning Ridge area, the scarcity of water apparently was aggravated by the competition between the miners and the graziers.

In one of the earliest reports (1936) from the area, the authors uncle, R.W. Segnit (R0437) wrote:

"The gougers on the field have experienced considerable difficulty in connection with water supplies. In the past supplies have been drawn from two wells sunk in Opal Creek adjacent to the opal field (before its discovery) by the Manager of Andamooka Station, and a shallow well sunk by the gougers in the same creek about a quarter of a mile (0.4 km) downstream from the former two. The gougers' well has only been sunk to water level, where the very limited supply of a few bucketfuls of water per day is obtained.

The other two wells mentioned each have a yield of about 200 galls. (approximately 450 litres) per day, all of which will be required by the Manager of the Station during the coming summer for the development of the northern part of the run. The gougers will in future have to search for their own supplies."

Twenty two years later, Nixon (R1591) was able to report that:

"Water supplies, which have been scarce for some time, have now been augmented by a flow of good potable water at the rate of approximately 4,000 gallons (18,000 litres) per 24 hours in a well sunk recently, downstream from the township. Water, if collected from the wells, is free, but a delivery charge of 7s 6d (75c, or, in 1994's values, approximately $10) per 100 gallons (450 litres) is made."

In later years the population fell to a few hundred as opal became harder to find, although some growth has taken place in recent times because of the development of the huge Olympic Dam copper/gold/uranium ore deposit nearby.

Unlike Coober Pedy, most of the houses and other buildings were built above ground; some were semi-dugouts, being dug into a sloping hillside, the front walls and roof built in conventional fashion, generally with corrugated iron. Later houses were wooden frames covered with corrugated iron or asbestos cement sheeting.

Many of the miners believed that opal could be divined in much the same manner as water is supposed to be found, by using some kind of divining rod. One miner who had a well equipped mining operation, including one of the earliest mechanical diggers underground, firmly believed in divining. He had sunk a shaft, and dug substantial drives in three directions from the foot of the shaft. He had no luck, but did not consider it worth while to dig in the remaining direction, because his divining rod told him there was no opal there.


On one occasion we visited a young miner who had numerous types of divining equipment. However, the most favoured appeared to be a piece of fencing wire about 3 mm thick, bent into a right angle. The shorter end was held in the hand, while the long end (about 30 cm) projected forward. This was quite a sensitive balance, and a slight movement of the hand would send the wire moving towards one side or the other.

While divining for minerals, and especially for water, is a widespread belief, it is likely that the wire or forked stick reacts with people who, because of their experience and keen powers of observation, have an instinctive knowledge of where they are most likely to find their objective. This particular miner had, in fact, some fine opal, and was also a polishing craftsman.

Andamooka has produced some of the best quality opal in Australia. It occurs largely in a hard, dry environment, and is generally regarded as being particularly stable material ; cracking is rarely a problem. The best material approached Lightning Ridge black opal in quality, with a dark grey background and highly coloured patterns superimposed. In its best years, the value of production rivalled that of the much bigger Coober Pedy field.

Excellent specimen material unique to Andamooka could be found in the form of thin veins through quartzite boulders; these split along the veins, leaving a coating of precious opal on the two exposed faces, the so-called 'painted ladies'.

A further type of material, mainly confined to Andamooka, is the so-called 'opal matrix', a material which when found, is whitish, showing only weak colour. The best examples, however, are characterised by a fine porosity.

These samples can absorb a solution of lactose (sugar of milk); the resultant material is then immersed in warm sulphuric acid, when the lactose is decomposed and carbon deposited in the pores. This results in the suppression of scattered white light, rendering the colour visible. Good examples of this material can make fine cabochons.

Geology

The first geologist to visit the Andamooka field after the discovery of opal in 1930 and the commencement of mining appears to have been the authors uncle, R.W. Segnit (R0437), at that time Assistant Government Geologist. The problems of reaching the locality even in the 1930's are indicated by my uncles description of the area:

"The opal field is situated about 140 miles (225 km) north of Port Augusta, on the western side of Lake Torrens, and about 8 miles (13 km) distant from the north-western corner of the Lake. The usual route to the field is by the east-west train to Pimba, 113 miles (180 km) from Port Augusta, then by mail truck for about 65 miles (104 km), as far as Andamooka homestead.

The field is about 30 miles (48 km) further on, and this last part of the journey must be covered by arrangement with the driver of a local mail and ration truck, which runs once a week between the field and the Homestead."

Nowadays the establishment of the huge Olympic Dam copper-uranium-gold mine, about 40 km west of Andamooka, ensures easy access to the opal field and has stimulated the declining population and encouraged the tourist industry. Barnes and Townsend (R0725) list some 27 individual fields in the area, and note that:

"In the ensuing 45 years, the workings have expanded over an area 13 km by 5 km. Until 1972, Andamooka was a major producer of precious opal with output equalling, and occasionally exceeding in value that from Coober Pedy."

The oldest rocks to outcrop in the precious stones area proper are of Precambrian age; these are overlain by early Cambrian strata, consisting of sandy and shaly dolomitic beds. The opal-bearing beds are those of the Lower Cretaceous, deposited in a localised embayment of the Great Artesian Basin in this area.

The earliest Mesozoic beds to outcrop in the area contain fossils indicating a Late Jurassic to Cretaceous age. The top of the Early Cretaceous beds are characterised by the sandy clays of the kopi type. This has been studied in some detail, and is described by Barnes et al (R0239) as follows:

"Kopi consists essentially of up to 35% detrital quartz grains scattered through a clay matrix. The predominantly angular to subangular quartz grains vary in size from silt to fine sand, while occasional larger grains up to 0.3 mm, are well rounded to subrounded. The matrix is composed of very fine, fibrous and felted clay flakes of muscovite/sericite and in places, concertina-like kaolinite crystals."

At the base of the kopi there is a cemented conglomerate containing abundant boulders of Precambrian quartzite. This latter band is the main opal-bearing horizon. R.W Segnit (R0437) stated:

"The precious opal is obtained principally from a narrow boulder bed, having a maximum thickness of 2 feet (approx. 600 mm), but generally only 12 ins. (approx. 300 mm). The water-worn boulders are often found with a thin covering of opal almost completely surrounding the stone."

The boulders occasionally are up to one metre in length and consist largely of a strongly cemented quartzite with a sub-glassy fracture; they are sometimes cracked, and the cracks filled with precious opal. When broken, the fracture surfaces may be covered with a thin layer of good quality precious opal, the 'painted ladies' of the Andamooka miners.

Overlying the Cretaceous beds are superficial late Pleistocene to Recent sediments which have been kaolinised in some areas and are similar to the weathered Cretaceous. Recent sandhills cover much of the country in the vicinity, with flat pans of reddish sandy soil forming small playas in between.