>>click to enlarge<<
lightning ridge
township
underground mine
cross section
nobbies
OPAL IN AUSTRALIA

The Lightning Ridge Field

Opal was recorded from the Lightning Ridge area in the late 1880's, but its commercial value was not immediately recognised. The first commercial shaft was sunk by Charlie Nettleton in 1903, but produced little opal. However, he continued his prospecting and found some good black opal about a year later. This precipitated a migration of some of the miners from the now established White Cliffs field, and Lightning Ridge became a major centre of opal production. Miners fanned out from the original discoveries and found opal in several outlying areas, so that today opal mining activities cover a wide area in the vicinity.

Lightning Ridge township is located in northern New South Wales, some 770 km north west of Sydney. Nowadays access is easy by means of good surfaced roads, although the outlying fields are served by gravel or earth roads, some of which may be almost impassable immediately after wet weather. However, the climate is dry, so that transport problems even in the more remote fields is seldom a problem. The township itself is now well developed, with sealed roads and most of the facilities of an outback country town.

The country around Lightning Ridge is less desolate than that around White Cliffs and the South Australian fields. There is much scattered scrub made up of eucalyptus and acacia, giving some shade and, particularly in earlier days, a plentiful supply of firewood.

While opal is still being found near the town itself and in outlying areas such as the Grawin, Angledool and other areas, the economy is less dependent on the gemstone because of the influx of tourists. Large numbers of the latter come to Lightning Ridge area, many of them to fossick, or 'noodle', for remnants of opal on old dumps. There are many places where this can be done, although care must be taken to keep off registered claims! It is not uncommon for small gemstones to be found; these are then often cut and polished for the finder by local craftsmen.

There are now several modern motels offering excellent accommodation, as well as the hotel where, in addition to local cafes and restaurants, good meals can be obtained. Also for the tourist are old mines which have been restored so that the visitor can experience what it is like to be underground in the mining environment. In some cases, the owner of the establishment, which usually includes a shop and dwelling, is still digging for opal.

In outlying areas where conditions are still rough, miners often live in simple shacks or caravans, and the only local amenity may be a small hotel, itself little more than a corrugated iron shed where liquor is sold. It is in such places that the visitor, especially if accompanied by a known personality, can enjoy the company of a variety of local identities, and, if he is lucky, see at first hand some of the mining activities. These outlying areas seem to be largely self-governing. Tracks wind amongst the trees, mineshafts and living quarters, cars of ancient and occasionally modern vintage are to be seen, the former sometimes unregistered. In such areas the spirit of brotherhood and self help is paramount, and many of the laws of the city are irrelevant.

However, even in these outlying areas, modern amenities are beginning to be supplied. At Glengarry, for example, public telephones using solar power have been installed.

A Place of Character & Characters

Tales of the Ridge abound, and many of the best are to be found in a remarkable, if not literary, book entitled 'The Lightning Ridge Book', by Stuart Lloyd (R1513). This entertaining book is a wonderful collection of anecdotes of people, places and activities from this opal field. Some of the most beautiful photographs of opals from various fields are reproduced in a series of books (R1608) by Len Cram, a former miner and opal dealer. He has also carried out some remarkable experiments on growing opal from natural sediments.

Geology

Much of the following information is taken from Watkins (R1609). Lightning Ridge lies towards the southeastern corner of the Great Artesian Basin. The geology of the locality (including its peripheral fields) is therefore similar to that of the other major opal areas. The dominant feature is of flat lying sandy clay beds, pale in colour, sometimes iron stained. A generalised cross section (modified from Watkins (R1609) of the geology of the Lightning Ridge field is shown in.

In the Lightning Ridge area Cretaceous sediments outcrop in the form of interbedded sandstones, siltstones and sandy claystones, with some coarser beds. Underlying these beds are medium grained light coloured clayey sandstones which consist largely of detrital quartz grains distributed through a kaolinitic matrix. Evidence of opal is found in the so-called 'steel band', a hard silicified band, generally about 10 cm thick, which occurs as a discontinuous layer within the sandy clay. The cement in this hard layer is usually opal.

Also occurring in these sandy clays are layers or lenses of clay, usually a mixture of kaolinite, smectite and illite; it is generally grey or buff in colour. As in other opal fields, the Cretaceous beds are overlain by quartz-rich sands and gravels. They are usually confined to the ridges and higher levels, and are seldom more than about one metre thick. All of the Cretaceous beds are deeply weathered.

Watkins states:

"In the Lightning Ridge area two main periods of silicification are postulated: Late Cretaceous to early Tertiary and late Tertiary. The two periods of silicification are associated with different lithologies. The first period of silicification appears to be related to a major weathering phase."

At depths of 20-50 m the earlier weathering phase grades into less weathered material which is considered by the miners to be the limit of useful opal production. The formation and deposition of opal is assigned to this period of deep weathering, and the most likely sites for its formation appear to be in the top layers of the clay lenses referred to above.

This is comparable with the observations that opal is frequently concentrated, in other areas, at the interface between overlying porous, and underlying impervious beds. However, opal may also occur in small joints and faults ('slides'), and as isolated nodules, termed 'nobbies'. The origin of the cavities in which the nobbies form is still not fully understood.

In addition, localised breccia zones, sub-circular in horizontal cross section are sometimes encountered; these, probably because of their broken nature, also appear to be favourable sites for the deposition of opal.