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mintabie opal
OPAL IN AUSTRALIA

The Mintabie Field

The Mintabie opal field lies west of the main north-south road between Adelaide and Alice Springs, just south of the Northern Territory border. It is about 340 km by road from Coober Pedy. Barnes et al (R0239) state that:

"The workings are clustered together on the edge of a northeasterly facing scarp. In contrast to the other opal fields the area is thickly vegetated with mulga and mallee trees growing in a predominantly sandy soil; this greatly reduces the dust problem common to most other fields."

Aerial photographs of the area depict country not unlike that around Lightning Ridge. Aboriginals are reported to have sold black opal from an unknown source in Coober Pedy as early as 1919 (Keeling, R1629). It took local miners over 10 years to discover floaters along the Mintabie escarpment, but mining was intermittent because of the isolation of the field and the harder rocks in which the opal occurred.

In 1976, heavy equipment was moved into the area. Despite the advent of the bulldozers and backhoes, blasting is at times necessary because of the hardness of the overlying rocks, before the heavy equipment can be used. However this has resulted in the production of much opal, often of excellent quality. In recent years the production has rivalled, and even exceeded that of Coober Pedy.

By 1980, a small settlement had grown at Mintabie, but miners still had to travel some 400 km to Coober Pedy or 500 km to Alice Springs for their main supplies. Now the situation has eased because of the completion of the standard gauge railway to Alice Springs from the south.

The railway passes within some 50 km of the field, and supplies brought in by train can be obtained from the small, newly developed town of Marla. The township of Mintabie has also grown to the extent that there is a restaurant and other facilities, as well as a number of permanent homes to supplement the temporary houses and caravans.

There is also a good airstrip so that there is rapid access to the field by light aircraft from the cities, as well as making available the services of the Royal Flying Doctor Service.

Geology

The geology of the opal bearing horizons at Mintabie differs from that of the other Australian opal fields. Although Mintabie is still at the very edge of the Great Artesian Basin, the opal is found in more consolidated beds underlying the Lower Cretaceous.

The opal occurs mainly in these Paleozoic horizons, which have yet to be dated accurately, but may be Devonian or as old as Ordovician. The rocks in which the opal occurs are cross-bedded sandstones, dipping slightly to the southwest. The opal occurs in horizontal bands parallel to the bedding, or at times in vertical or steeply dipping fissures. Opal has also been found at deeper levels in an underlying somewhat less consolidated sandstone.

The difference in the nature of the opal bearing horizon at Mintabie does not necessarily indicate a difference in age for the formation of the gemstone, despite its occurrence in older rocks. The overlying physical conditions are related, in that soluble silica, formed from the decompositional weathering of complex silicates, could be transported downwards until the appropriate openings were encountered for the entrapment of the colloidal particles.

Somewhat similar conditions seem to apply for the formation of precious opal in the Brazilian fields in the area of Pedro Secundo, where the opal has been deposited, apparently by weathering processes, in cavities in broken basalt.

The Mintabie opal itself is generally of good quality. Keeling (R1629) states that:

"The opal varies from milky white to almost black, much being only potch or potch with traces of colour. The best material, which fetches top prices for South Australian opal, is precious associated with black potch which can be cut 'en cabochon' to produce a black-backed natural doublet. Some opal, found below or close to the water table, has a tendency to crack, but most opal found above the water table is stable."