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white cliffs township
sunset
underground motel
bun-like concretions
opalized remains of marine organisms
'opal pineapple'
OPAL IN AUSTRALIA

The White Cliffs Field

There is some uncertainty as to which of the two major New South Wales opal fields is the older in terms of commercial production. The first opal from White Cliffs was collected in 1889 by a couple of kangaroo hunters who took it to Wilcannia for identification.

News travelled slowly in those days, so it was two or three years before the rush began. By 1897, according to Leechman (R0384), there were 3500 people at the field. Over the next few years some of the best opal the world had ever seen came out of this area, but it was largely mined out by 1905, a small number of miners continuing for a few years after this.

Now it is difficult to imagine that several thousand people once strove for a living at White Cliffs. The area around the township is very barren and shows little relief except for a few low hills. There is little growth apart from a few stunted trees and low growing saltbush and bluebush.

At first sight it appears a most inhospitable country, but as one become accustomed to it, the region takes on a strange beauty of its own, especially at sunrise and sunset, when the redder rays of the sun enhance the redness of the panorama. Immediately around the town the land is pockmarked with scores of old shafts surrounded by their white mullock heaps; from the air the surrounding country can be likened to a lunar landscape. In earlier days it was not a place to go walking at night unless one carried a good light.

There is now plenty of accommodation for tourists and other visitors. The most remarkable is the Underground Motel. It is constructed almost entirely underground, passages having been driven into the hillside.

Even mining has taken on a new lease of life, and there are many miners searching for the precious opal in the district. The older areas in particular are being worked over by bulldozer excavation.

Geology

Two of the earliest reports on the White Cliffs opal fields, one by de v. Gipps in 1894 (R1489), the other by Curran in 1896, (R1606) recognised that the opal bearing horizons lay in the sandy Cretaceous beds. Gipps noted:

"The western termination of this portion of the cretaceous basin is situated about twenty miles (32 km) from the present opal field, at which the slates, with (probably) Devonian sandstones and quartzites resting uncomfortably thereon, commence to appear."

Gipps also made a curious observation related to the occurrence of sulphate minerals, which we know occur in all of the opal fields. He states:

"Lead, in the form of a mixed carbonate and sulphate with some gypsum, exists in many places in the field, occurring in small bun-shaped lumps of from 1/4 to 1 lb. (about 100 to 400 gm) in weight all through the clays and kaolins, but I have been unable to trace any relation between this and the opal, and can offer no explanation of its occurrence in this form. These lumps of lead have always a small depression on the lower side, and a small cavity in the centre containing minute crystals of lead. The form is always the same, that of a bun with the flat or rather slightly concave side downwards."

The occurrence of lead minerals in this form seems strange; it is possible that this was a misidentification of barite, which is a much heavier mineral than the other sulphates, and is common as fine-grained bun-like concretions in the recent sediments on the banks of the River Murray to the south.

More recent and authoritative accounts of the geology of this field are those of Kenny (R1511) and Relph (R0361). Relph states, in brief:

"Sediments of Lower Cretaceous age, having a general slight dip to the east, rest upon a basement of highly folded Proterozoic strata which have been termed the Torrowangee Series by Kenny (1934). Tertiary deposits have been developed over the Cretaceous beds."

The main opal bearing horizons lie within the Cretaceous beds, which are, as in most of the fields, sandy clays overlying claystones. The general area has been eroded extensively, so that these white beds are exposed on the sides of low hills. Overlying the Cretaceous beds are Tertiary gravels and sands up to about three metres in thickness. The actual opal horizon appears to be less well defined at White Cliffs than in other areas. Relph describes the section down to a depth of 37 feet (about 12 m) and states that:

"It will be noticed here that the Cretaceous sediments exposed here are all claystones, with only one horizon, between 16ft. 6 in. (about 5m) and 27 feet (about 8 m) being typified as sandy. The horizon below the 27 feet level has been worked for opal, and is termed opal-dirt"."

This layer was described as:

"Claystone, silty, containing 15% quartz in a clayey matrix. A little chloritic material and some secondary silica. Slight iron staining. Opal occurs in vertical and horizontal joints."

Relph further states that:

"Any of the claystone members that have so far been observed within the Lower Cretaceous at White Cliffs have proved to be opal-bearing, and small but uneconomical amounts have also been recorded from the beds of Tertiary conglomerate, locally termed 'geyser', which immediately overlays these beds."

It has also been noted by Jackson (R1335) in one or two instances in the overlying porcelanite (greybilly) in Queensland. This suggests that the opalisation of these sediments took place either during the late Tertiary or a post-Tertiary period. A feature of the White Cliffs area is the relative abundance of fossils, including gastropods, pelecypods, belemnites, crinoids, brachiopods and opalised woods.

The most spectacular fossils, however, have been the opalised skeletons of pliosaurs, large marine reptiles, a few of which have been recovered intact. One is on display at the Australian Museum in Sydney, and another in the showrooms of Andrew Cody Opals in Melbourne.

Many of the other fossil remains are also opalised by high grade gem material. The opalised wood in some cases shows a most remarkably detailed retention of the cellular structure of the original wood (pb37, pb39).

Another striking type of specimen found at White Cliffs is the so-called 'opal pineapple'. This is a pseudomorph, usually being described as after glauberite, a sodium calcium sulphate. However, Dr. A. Pring, of the South Australian Museum, has recently obtained specimens which appear identical in shape, which are pseudomorphs of calcite after ikaite, a rare hydrated calcium carbonate (CaCO36H2O). If this is the case, it suggests that the environment in which the ikaite formed was very cold, as the known occurrences of the mineral are from places such as Greenland and the Kola Peninsula in Russia.

The opal from White Cliffs was usually of the lighter coloured varieties, varying from clear ('crystal') to milky. At its bestvarieties, varying from clear ('crystal') to milky. At its best it rivalled the finest light coloured opals from the South Australian fields.