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township |
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opal |
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OPAL IN AUSTRALIA |
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| The
Andamooka Field |
| Andamooka
is some 600 km north of Adelaide by road.
The country surrounding the township consists
mainly of small hills with rocky outcrops,
sandhills and flat dry claypans. The hills
are covered with a sparse growth of acacias,
small eucalypts and low scrub and fodder
plants such as saltbush.
Some 15 km east of the township is the northern
part of Lake Torrens, a large, usually dry,
salt lake bed which is up to 50 km in width
and extends nearly 200 km from north to
south; it is a featureless and uniformly
flat area. In many areas of the lake, dry
as it looks on the surface, it can be thick
black salty mud underneath, difficult to
negotiate even with a modern four-wheel
drive vehicle.
Andamooka township has grown haphazardly
along the dry sandy river bed now called
Opal Creek, which winds its way between
the low hills which have been formed by
the weathering of the flat lying sediments
of the area. In the earliest days of the
field, wells dug in the creek bed were the
main, if sparse, supply of water for the
miners. As in the Lightning Ridge area,
the scarcity of water apparently was aggravated
by the competition between the miners and
the graziers.
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In one of the earliest reports (1936) from
the area, the authors uncle, R.W. Segnit
(R0437)
wrote: |
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Twenty
two years later, Nixon (R1591)
was able to report that:
In later years the population fell to a
few hundred as opal became harder to find,
although some growth has taken place in
recent times because of the development
of the huge Olympic Dam copper/gold/uranium
ore deposit nearby.
Unlike Coober Pedy, most of the houses and
other buildings were built above
ground; some were semi-dugouts, being
dug into a sloping hillside, the front walls
and roof built in conventional fashion,
generally with corrugated iron. Later houses
were wooden frames covered with corrugated
iron or asbestos cement sheeting.
Many of the miners believed that opal could
be divined in much the same manner as water
is supposed to be found, by using some kind
of divining rod. One miner who had a well
equipped mining operation, including one
of the earliest mechanical diggers underground,
firmly believed in divining. He had sunk
a shaft, and dug substantial drives in three
directions from the foot of the shaft. He
had no luck, but did not consider it worth
while to dig in the remaining direction,
because his divining rod told him there
was no opal there.
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On one occasion we visited
a young miner who had numerous types of
divining equipment. However, the most favoured
appeared to be a piece of fencing wire about
3 mm thick, bent into a right angle. The
shorter end was held in the hand, while
the long end (about 30 cm) projected forward.
This was quite a sensitive balance, and
a slight movement of the hand would send
the wire moving towards one side or the
other.
While divining for minerals, and especially
for water, is a widespread belief, it is
likely that the wire or forked stick reacts
with people who, because of their experience
and keen powers of observation, have an
instinctive knowledge of where they are
most likely to find their objective. This
particular miner had, in fact, some fine
opal, and was also a polishing craftsman.
Andamooka has produced some of the best
quality opal in Australia. It occurs
largely in a hard, dry environment, and
is generally regarded as being particularly
stable material ; cracking is rarely a problem.
The best material approached Lightning Ridge
black opal in quality, with a dark grey
background and highly coloured patterns
superimposed. In its best years, the value
of production rivalled that of the much
bigger Coober Pedy field.
Excellent specimen material unique to Andamooka
could be found in the form of thin veins
through quartzite boulders; these split
along the veins, leaving a coating of precious
opal on the two exposed faces, the so-called
'painted
ladies'.
A further type of material, mainly confined
to Andamooka, is the so-called 'opal
matrix', a material which when found,
is whitish, showing only weak colour. The
best examples, however, are characterised
by a fine porosity.
These
samples can absorb a solution of lactose
(sugar of milk); the resultant material
is then immersed in warm sulphuric acid,
when the lactose is decomposed and carbon
deposited in the pores. This results in
the suppression of scattered white light,
rendering the colour visible. Good examples
of this material can make fine cabochons.
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| Geology |
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| The
first geologist to visit the Andamooka
field after the discovery of opal
in 1930 and the commencement of mining
appears to have been the authors uncle,
R.W. Segnit (R0437),
at that time Assistant Government
Geologist. The problems of reaching
the locality even in the 1930's are
indicated by my uncles description
of the area:
Nowadays the
establishment of the huge Olympic
Dam copper-uranium-gold mine, about
40 km west of Andamooka, ensures easy
access to the opal field and has stimulated
the declining population and encouraged
the tourist industry. Barnes and Townsend
(R0725)
list some 27 individual fields in
the area, and note that: |
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The
oldest rocks to outcrop in the precious
stones area proper are of Precambrian
age; these are overlain by early Cambrian
strata, consisting of sandy and shaly
dolomitic beds. The opal-bearing
beds are those of the Lower Cretaceous,
deposited in a localised embayment
of the Great Artesian Basin in this
area.
The earliest Mesozoic beds to outcrop
in the area contain fossils indicating
a Late Jurassic to Cretaceous age.
The top of the Early Cretaceous beds
are characterised by the sandy
clays of the kopi type. This has
been studied in some detail, and is
described by Barnes et al (R0239)
as follows:
At the base
of the kopi there is a cemented conglomerate
containing abundant boulders of Precambrian
quartzite. This latter band is the
main opal-bearing horizon. R.W Segnit
(R0437)
stated:
The boulders
occasionally are up to one metre in
length and consist largely of a strongly
cemented quartzite with a sub-glassy
fracture; they are sometimes cracked,
and the cracks filled with precious
opal. When broken, the fracture surfaces
may be covered with a thin layer of
good quality precious opal, the 'painted
ladies' of the Andamooka miners.
Overlying the Cretaceous beds are
superficial late Pleistocene to Recent
sediments which have been kaolinised
in some areas and are similar to the
weathered Cretaceous. Recent sandhills
cover much of the country in the vicinity,
with flat pans of reddish sandy soil
forming small playas in between.
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