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| suburb of coober pedy |
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OPAL IN AUSTRALIA |
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| The
Coober Pedy Field |
The name 'Coober Pedy' is
generally believed to be a corruption of
Australian aboriginal words meaning, approximately,
'white men's holes'. The area was broken,
arid country with low ranges of hills which
were called the Stuart Ranges, after the
early explorer, John McDouall Stuart.
One of the earliest reports on this area
was that by L. Keith Ward, Government Geologist
to South Australia, in 1917 (R1535).
The area was referred to as the Stuart's
Range Opal Field, and it is interesting
to note the locality
plan published in this report.
Two main routes are illustrated for reaching
the area. Both depended on railways for
the greater part of the journey. One route
was via the old north-south line from Adelaide
to Oodnadatta as far as Williams Creek railway
station, a journey of 985 km, which was
narrow gauge (3 feet 6 inches or 1067 mm)
from Marree, then by bush tracks some 160
km westwards.
The other route was to travel on the transcontinental
railway to Tarcoola, a journey of 830 km,
and then northwards, again by dirt roads
and tracks, to the opal field, a distance
of about 200 km. The country is dry and
desolate, and the features marked along
the tracks are mainly wells and waterholes,
apart from a few isolated sheep stations.
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| Ward describes the geographical
position as follows:
The difficulties of travel are highlighted
in Ward's paper by the table listing in
some detail the positions, and distances
between, water holes. Nowadays Coober Pedy
is readily accessible by a good sealed road,
a journey of 935 km northwards from Adelaide.
The mining of opal in this area dates back
to 1915, although there are suggestions
that the actual discovery of opal may have
been a little earlier. It began to produce
significant amounts of opal about the time
White Cliffs was becoming exhausted, and
production from Lightning Ridge was decreasing.
The township of Coober Pedy is located in
a very dry and desolate area. Even as late
as the 1960's one approached the town across
flat, featureless
country along a gravel road.
The land surface was largely covered by
small stones, several centimetres in size,
termed 'gibbers', and there was little plant
growth to be seen, most of the wood having
been cut out in earlier days for firewood
and for shoring up mine shafts and drives.
Around the present township area there are
a number of low hills, and some of the earliest
mines were dug into the hillsides, as the
earliest finds showed that the opal level
actually outcropped on the sides of some
of the hills.
Most of these old mines are now underground
dwellings, locally called 'dugouts'.
The great advantage of this type of dwelling
in this area is the even temperature maintained
inside, a temperature which in the heat
of summer, is 15°C or more cooler than
outside. Many of these dugouts are quite
large, the owners having dug further into
the hillside to provide new rooms for their
families.
An inner room may often have a small window
which looks into the bottom of a shaft which
may go several metres to the surface, providing
a little natural light, and ventilation.
The walls between the rooms are usually
left at about a metre thick for support;
at the same time this provides excellent
sound insulation between rooms. The white
sandy clay of the area is a very stable
material, and allows flexibility in dwelling
design. If you need a bookcase, you merely
excavate a section of the wall. |
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| By the 1960's settled conditions
had been established in the township area.
There was a hotel, general store, a few
tourist shops and showrooms, a primary school
and other facilities. Tourists were adding
significantly to the economy of the town,
with tourist buses becoming a regular feature,
passing through on their way to Alice Springs
and Darwin in the Northern Territory.
Sufficient families were living in the area
to support a primary school, a frame structure
surrounded by an earthen schoolyard. By
the 1960's Aboriginal children were being
educated at the same school but regrettably,
at that time, most aboriginal families had
poor living conditions with little prospect
of local work. Some of the aborigines attempted
to make a living by 'noodling', the process
of searching mine
dumps for small pieces of opal missed
by the miners. With their keen powers of
observation, aboriginals were good at this
activity.
By the mid 1970's Coober Pedy had become
a thriving outback town with, for Australia,
the large population of 3000-4000.
Coober Pedy lies on a plateau. A few kilometres
north one comes to the 'breakaway' country
- an area where the plateau suddenly drops
away to an extensive plain covered in gibbers
with almost no vegetation. The scenery is
dramatically beautiful. One looks out towards
a distant horizon, the vista
being broken here and there by hills, pointed
or mesa-like.
Because of the vastness of the landscape,
one is deceived by some of the dimensions.
The impression of height is exaggerated,
so that objects on the plain down below
often seem to be larger than life.
For most of the time since its discovery
in 1915, Coober Pedy has been the major
producer of gem quality opal in Australia.
White or milky opal has been produced in large quantities,
and much high quality 'crystal' opal showing fine colour flashes in a variety of patterns has been found. |
| Geology |
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| The
host rock for the opal is a white
to off-white sandy clay of early Cretaceous
age, which is underlain by older Mesozoic
rocks of the Great Artesian Basin.
These deposits are of varying thickness,
but are rather well documented because
of the large numbers of boreholes
sunk in the search for water.
In the Coober Pedy area, according
to Barnes et al (R0239),
the basement early Permian rocks have
been encountered at depths ranging
from some 149 to 623 metres. The lowest
Mesozoic rocks in this area are poorly
consolidated arenaceous sediments,
and considered to be of Jurassic to
Lower Cretaceous in age. A geological
cross-section
- adapted, with permission, from the
South Australian Department of Mines
and Energy Handbook No. 5 (R0239)
- is shown.
Overlying these sandy beds is a series
of grey clays and silts with a basalt
unit containing sand lenses and boulder
beds. In erosion areas, these boulders
may weather out of the beds and lie
exposed on the surface. Limestone
lenses also occur, and the fossils,
both macro- and micro-, indicate an
Early Cretaceous age. Overlying these
beds, Barnes states: |
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The
kopi, mainly off-white to buff, although
not uncommonly stained red to purple
by iron oxide, is a sandy clay, rather
permeable to the passage of water.
It is at the boundary between the
sandy claystone and the underlying
clay beds that the opal generally
forms. This is termed the 'opal level',
although variations can occur. This
level is, in numerous places cut by
localised small faults or 'slides',
which are particularly favourable
places for the deposition and occurrence
of opal.
Barnes (R0239)
actually lists five kinds of levels
at which opal can be found; these
are mainly characterised by associated
mineralisation at the kopi-clay boundary.
These types
of level are:
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1.
Red tubule levels: These
may be vertical or horizontal
tubules filled with iron-stained
silt, and may contain opal. Their
origin is uncertain, but some
studies by Barker and co-workers
(R1610)
suggest that they may be the result
of termite activity. It is known
that modern termite mounds have
subterranean passages at depths
equivalent to several times their
height above ground.
2.
Alunite levels: These
are formed where large masses
of alunite, commonly white,
yellowish or greenish in colour,
with the typical chalky texture,
impregnate or replace the kopi
at the opal level. The alunite
may occur as small nodules or
large masses up to metres in
size. It appears to be much
more common in the Coober Pedy
area than in other opal fields. |
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| An
analysis of the material given by
Ward (R1535)
shows a substantial replacement of
potash by soda (K2O 5.3%,
Na2O 4.9%). E.S. Larsen
(quoted by Ward) records the association
of opal with alunite in Colorado,
U.S.A.
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3.
Gypsum levels
In this type of level gypsum,
in the form of the fibrous satin
spar, forms veins of striking
appearance, up to 0.5 m in thickness
(pg63). Small fibres of gypsum
sometimes penetrate, or have
been incorporated in the opal
itself.
4. Ironstone bands,
or limonite levels:
These are essentially opal levels
at the sandy clay-clay interface,
but iron oxide staining has
occurred on both sides of the
level and in cracks.
5. Unmarked level:
These are areas where cracks
occur in the kopi, generally
near horizontal, perhaps related
to the bedding of the sediments.
| An important
type of structure in the fields is
that represented by the 'slides' mentioned
above. These are alsolides' mentioned
above. These are also termed 'verticals'
by the miners. They are probably minor
faults or sedimentary slump structures
where the original sedimentary layers
have been obliterated. They may contain
any of the minerals encountered in
the levels, but are an important source
of opal because of the openings and
cavities formed in these areas. These
open spaces have often been optimum
areas for the deposition of opal.
At Coober Pedy the opal occurs mainly
in horizontal bands, or in patches
in the 'verticals'. The miner usually
encounters it as a band
of potch, which is then followed
carefully in the hope that it will
show colour at some stage. There seems
to be no way in which they can predict
where to find gem quality material,
except that the chances seem to improve
when a slide is encountered.
The presence
of abundant sulphates seems to be
particularly characteristic of the
Coober Pedy area, although gypsum
in particular is encountered in most
of the opal-bearing areas, as well
as being widespread throughout the
Great Artesian Basin.
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