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boulder opal
boulder opal matrix
yowah nuts
OPAL IN AUSTRALIA

The Queensland Fields

Mining activity in Queensland began in the latter part of the 19th century. The area was very difficult of access, water was scarce, and supplies had to be brought long distances.

The difficulties of reaching these outlying areas in the late nineteenth century was dramatically described by T.C. Wollaston in his book Opal, Gem of the Never Never (R1775). He took seven weeks to travel overland from Adelaide to southwest Queensland by train and camel, a journey which can be accomplished comfortably today by car in three days.

Ingram (R0263) stated that:

"One of the oldest mines in the area is the Aladdin mine which dates from 1872. Many others date from the mid 1880's. The Little Wonder mine which was working in 1891 was one of the richest, and at one time 50 men worked there. Another well known mine, the Hayricks, is the largest mine in the area and the lease is still held although it apparently has not been worked for some years."

Ingram found at that time the few mines being worked were old ones; little exploration was being done. Mining was done by sinking shafts, generally about seven metres deep, to the opal level; sometimes horizontal drives were made from the base of the shaft.

In some areas, where erosion had produced mesa formations with steep, almost vertical sides, it was possible to find the opal level outcropping, so that mining could be done by driving an adit into the hillside. Nowadays, particularly in the boulder opal areas, heavy equipment is used; extensive areas are stripped and deep and wide cuts are made down to the opal level.

Opal appears to be more widely distributed in Queensland than in the other producing States. In spite of, or perhaps because of this, no township specifically depending on the production of opal, has been established.

The mines, most of which produced only small amounts of opal, are widely scattered, although still lying within the Great Artesian Basin. The mining area extends from near the New South Wales border in the south, to Kynuna, nearly 1000 km to the north. The localities follow a zone through the north-south centre of the basin.

Opal occurs in two main forms in the Queensland fields. In some areas it forms thin seam opal, usually associated with a thin ironstone horizon. In many areas, however, it forms in cracks and cavities in hard ironstone (goethite) concretions. These concretions can be up to 30 cm or more in size, but the presence of opal cannot be determined for sure until they are broken or cut open. The latter material is termed 'boulder opal'.

The quality of boulder opal is often very good, but the layers are often thin. At times the goethite is criss-crossed with very thin veinlets of fine quality opal, so that the ironstone itself is cut and polished for the decorative effect of the opal veinlets; such material is known as 'boulder matrix'. When the opal is thicker it may still be cut with a backing of the ironstone to form a natural doublet; these are often of great beauty.

In the southwest corner of the State, small ironstone concretions up to several cm in size are found. These are sometimes found to contain a core of opal when cut or broken; they are termed 'Yowah nuts', named after the locality where they are found. Once again, the quality of the opal is often high, but even the poorer Yowah nuts can make good specimens for collectors.

Sometimes opal may occur in vertical or horizontal 'pipes'; these are irregular tubules lined with iron oxide. They may be hollow, or may be filled at times with opal. Their origin is uncertain, but some appear to be formed by the replacement of wood by goethite, with opal filling small veinlets. Less commonly, the opal may occur as veins in porcelanite, or as the cement in sandstone.

As in New South Wales, precious opal has also been recorded from volcanic rocks in Queensland, although not in commercial quantities. Transparent opal with diffraction colours similar to that found at Tintenbar, New South Wales, has been recorded from Springsure in central Queensland, where a small area of Tertiary trachyte appears on the geological map. The latter rock is presumably the origin of the opal.

A further occurrence is that near Maleny, some 110 km north of Brisbane. The opal here occurs in small cavities, usually less than five mm in size, in a highly altered vesicular volcanic rock. The opal is completely colourless and transparent, and sometimes shows diffraction colours. On occasions, only one flash of colour occurs across the vug, indicating a high degree of regularity in the packing of the silica spheres in the opal. This and other curious phenomena are illustrated in scanning electron micrographs of the section Transmission and scanning electron microscopy.