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to enlarge<< |
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| white
cliffs township |
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| sunset |
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| underground
motel |
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| bun-like
concretions |
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| opalized remains of marine organisms |
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| 'opal
pineapple' |
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OPAL IN AUSTRALIA |
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| 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 |
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| 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:
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:
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.
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More
recent and authoritative accounts
of the geology of this field are those
of Kenny (R1511)
and Relph (R0361).
Relph states, in brief:
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:
This layer
was described as: Relph
further states that: |
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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.
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