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locality plan
suburb of coober pedy
dugouts
OPAL IN AUSTRALIA

The Coober Pedy Field

The name 'Coober Pedy' is generally believed to be a corruption of Australian aboriginal words meaning, approximately, 'white men's holes'. The area was broken, arid country with low ranges of hills which were called the Stuart Ranges, after the early explorer, John McDouall Stuart.

One of the earliest reports on this area was that by L. Keith Ward, Government Geologist to South Australia, in 1917 (R1535). The area was referred to as the Stuart's Range Opal Field, and it is interesting to note the locality plan published in this report.

Two main routes are illustrated for reaching the area. Both depended on railways for the greater part of the journey. One route was via the old north-south line from Adelaide to Oodnadatta as far as Williams Creek railway station, a journey of 985 km, which was narrow gauge (3 feet 6 inches or 1067 mm) from Marree, then by bush tracks some 160 km westwards.

The other route was to travel on the transcontinental railway to Tarcoola, a journey of 830 km, and then northwards, again by dirt roads and tracks, to the opal field, a distance of about 200 km. The country is dry and desolate, and the features marked along the tracks are mainly wells and waterholes, apart from a few isolated sheep stations.

Ward describes the geographical position as follows:

"Stuart's Range is the name given to the ridge that constitutes the divide between two systems of surface drainage. The divide extends in a direction having a bearing nearly N.W.-S.E., and separates the basin of Lake Cadibarrawirracanna from that containing the lakes Woorong, Phillipson and Wirrida. These so-called 'lakes' are typical 'playas' or 'clay-pans' of large size. They contain water only for a short time after heavy rain has fallen, and for the greater part of the year the silt-filled depressions are dry and smooth."

The difficulties of travel are highlighted in Ward's paper by the table listing in some detail the positions, and distances between, water holes. Nowadays Coober Pedy is readily accessible by a good sealed road, a journey of 935 km northwards from Adelaide.

The mining of opal in this area dates back to 1915, although there are suggestions that the actual discovery of opal may have been a little earlier. It began to produce significant amounts of opal about the time White Cliffs was becoming exhausted, and production from Lightning Ridge was decreasing.

The township of Coober Pedy is located in a very dry and desolate area. Even as late as the 1960's one approached the town across flat, featureless country along a gravel road.

The land surface was largely covered by small stones, several centimetres in size, termed 'gibbers', and there was little plant growth to be seen, most of the wood having been cut out in earlier days for firewood and for shoring up mine shafts and drives.

Around the present township area there are a number of low hills, and some of the earliest mines were dug into the hillsides, as the earliest finds showed that the opal level actually outcropped on the sides of some of the hills.

Most of these old mines are now underground dwellings, locally called 'dugouts'. The great advantage of this type of dwelling in this area is the even temperature maintained inside, a temperature which in the heat of summer, is 15°C or more cooler than outside. Many of these dugouts are quite large, the owners having dug further into the hillside to provide new rooms for their families.

An inner room may often have a small window which looks into the bottom of a shaft which may go several metres to the surface, providing a little natural light, and ventilation. The walls between the rooms are usually left at about a metre thick for support; at the same time this provides excellent sound insulation between rooms. The white sandy clay of the area is a very stable material, and allows flexibility in dwelling design. If you need a bookcase, you merely excavate a section of the wall.

By the 1960's settled conditions had been established in the township area. There was a hotel, general store, a few tourist shops and showrooms, a primary school and other facilities. Tourists were adding significantly to the economy of the town, with tourist buses becoming a regular feature, passing through on their way to Alice Springs and Darwin in the Northern Territory.

Sufficient families were living in the area to support a primary school, a frame structure surrounded by an earthen schoolyard. By the 1960's Aboriginal children were being educated at the same school but regrettably, at that time, most aboriginal families had poor living conditions with little prospect of local work. Some of the aborigines attempted to make a living by 'noodling', the process of searching mine dumps for small pieces of opal missed by the miners. With their keen powers of observation, aboriginals were good at this activity.

By the mid 1970's Coober Pedy had become a thriving outback town with, for Australia, the large population of 3000-4000.

Coober Pedy lies on a plateau. A few kilometres north one comes to the 'breakaway' country - an area where the plateau suddenly drops away to an extensive plain covered in gibbers with almost no vegetation. The scenery is dramatically beautiful. One looks out towards a distant horizon, the vista being broken here and there by hills, pointed or mesa-like.

Because of the vastness of the landscape, one is deceived by some of the dimensions. The impression of height is exaggerated, so that objects on the plain down below often seem to be larger than life.

For most of the time since its discovery in 1915, Coober Pedy has been the major producer of gem quality opal in Australia. White or milky opal has been produced in large quantities,
and much high quality 'crystal' opal showing fine colour flashes in a variety of patterns has been found.

Geology

The host rock for the opal is a white to off-white sandy clay of early Cretaceous age, which is underlain by older Mesozoic rocks of the Great Artesian Basin. These deposits are of varying thickness, but are rather well documented because of the large numbers of boreholes sunk in the search for water.

In the Coober Pedy area, according to Barnes et al (R0239), the basement early Permian rocks have been encountered at depths ranging from some 149 to 623 metres. The lowest Mesozoic rocks in this area are poorly consolidated arenaceous sediments, and considered to be of Jurassic to Lower Cretaceous in age. A geological cross-section - adapted, with permission, from the South Australian Department of Mines and Energy Handbook No. 5 (R0239) - is shown.

Overlying these sandy beds is a series of grey clays and silts with a basalt unit containing sand lenses and boulder beds. In erosion areas, these boulders may weather out of the beds and lie exposed on the surface. Limestone lenses also occur, and the fossils, both macro- and micro-, indicate an Early Cretaceous age. Overlying these beds, Barnes states:

"At a point about 40 m stratigraphically above the base of the Subgroup, near the foot of the Stuart Range escarpment, fresh grey muds change, rather abruptly, to vari-coloured bleached silty claystones, the kopi, or sandstone, as it is called at Coober Pedy".

The kopi, mainly off-white to buff, although not uncommonly stained red to purple by iron oxide, is a sandy clay, rather permeable to the passage of water. It is at the boundary between the sandy claystone and the underlying clay beds that the opal generally forms. This is termed the 'opal level', although variations can occur. This level is, in numerous places cut by localised small faults or 'slides', which are particularly favourable places for the deposition and occurrence of opal.

Barnes (R0239) actually lists five kinds of levels at which opal can be found; these are mainly characterised by associated mineralisation at the kopi-clay boundary.

These types of level are:

1. Red tubule levels: These may be vertical or horizontal tubules filled with iron-stained silt, and may contain opal. Their origin is uncertain, but some studies by Barker and co-workers (R1610) suggest that they may be the result of termite activity. It is known that modern termite mounds have subterranean passages at depths equivalent to several times their height above ground.

2. Alunite levels: These are formed where large masses of alunite, commonly white, yellowish or greenish in colour, with the typical chalky texture, impregnate or replace the kopi at the opal level. The alunite may occur as small nodules or large masses up to metres in size. It appears to be much more common in the Coober Pedy area than in other opal fields.

An analysis of the material given by Ward (R1535) shows a substantial replacement of potash by soda (K2O 5.3%, Na2O 4.9%). E.S. Larsen (quoted by Ward) records the association of opal with alunite in Colorado, U.S.A.

3. Gypsum levels In this type of level gypsum, in the form of the fibrous satin spar, forms veins of striking appearance, up to 0.5 m in thickness (pg63). Small fibres of gypsum sometimes penetrate, or have been incorporated in the opal itself.

4. Ironstone bands, or limonite levels: These are essentially opal levels at the sandy clay-clay interface, but iron oxide staining has occurred on both sides of the level and in cracks.

5. Unmarked level: These are areas where cracks occur in the kopi, generally near horizontal, perhaps related to the bedding of the sediments.

An important type of structure in the fields is that represented by the 'slides' mentioned above. These are alsolides' mentioned above. These are also termed 'verticals' by the miners. They are probably minor faults or sedimentary slump structures where the original sedimentary layers have been obliterated. They may contain any of the minerals encountered in the levels, but are an important source of opal because of the openings and cavities formed in these areas. These open spaces have often been optimum areas for the deposition of opal.

At Coober Pedy the opal occurs mainly in horizontal bands, or in patches in the 'verticals'. The miner usually encounters it as a band of potch, which is then followed carefully in the hope that it will show colour at some stage. There seems to be no way in which they can predict where to find gem quality material, except that the chances seem to improve when a slide is encountered.


The presence of abundant sulphates seems to be particularly characteristic of the Coober Pedy area, although gypsum in particular is encountered in most of the opal-bearing areas, as well as being widespread throughout the Great Artesian Basin.