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| lightning
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| nobbies |
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OPAL IN AUSTRALIA |
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| The
Lightning Ridge Field |
| Opal was recorded from
the Lightning Ridge area in the late 1880's,
but its commercial value was not immediately
recognised. The first commercial shaft was
sunk by Charlie Nettleton in 1903, but produced
little opal. However, he continued his prospecting
and found some good black opal about a year
later. This precipitated a migration of
some of the miners from the now established
White Cliffs field, and Lightning Ridge
became a major centre of opal production.
Miners fanned out from the original discoveries
and found opal in several outlying areas,
so that today opal mining activities cover
a wide
area in the vicinity.
Lightning Ridge township is located in northern
New South Wales, some 770 km north west
of Sydney. Nowadays access is easy by means
of good surfaced roads, although the outlying
fields are served by gravel or earth roads,
some of which may be almost impassable immediately
after wet weather. However, the climate
is dry, so that transport problems even
in the more remote fields is seldom a problem.
The
township itself is now well developed,
with sealed roads and most of the facilities
of an outback country town.
The country around Lightning Ridge is less
desolate than that around White Cliffs and
the South Australian fields. There is much
scattered
scrub made up of eucalyptus and acacia,
giving some shade and, particularly in earlier
days, a plentiful supply of firewood.
While opal is still being found near the
town itself and in outlying areas such as
the Grawin, Angledool and other areas, the
economy is less dependent on the gemstone
because of the influx of tourists. Large
numbers of the latter come to Lightning
Ridge area, many of them to fossick, or
'noodle', for remnants of opal on old
dumps. There are many places where this
can be done, although care must be taken
to keep off registered claims! It is not
uncommon for small gemstones to be found;
these are then often cut and polished for
the finder by local craftsmen. | |
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There are now several modern
motels offering excellent accommodation,
as well as the hotel where, in addition
to local cafes and restaurants, good meals
can be obtained. Also for the tourist are
old mines which have been restored so that
the visitor can experience what it is like
to be underground
in the mining environment. In some cases,
the owner of the establishment, which usually
includes a shop and dwelling, is still digging
for opal.
In outlying areas where conditions are still
rough, miners often live in simple shacks
or caravans, and the only local amenity
may be a small hotel, itself little more
than a corrugated
iron shed where liquor is sold. It is
in such places that the visitor, especially
if accompanied by a known personality, can
enjoy the company of a variety of local
identities, and, if he is lucky, see at
first hand some of the mining activities.
These outlying areas seem to be largely
self-governing. Tracks wind amongst the
trees, mineshafts and living quarters, cars
of ancient and occasionally modern vintage
are to be seen, the former sometimes unregistered.
In such areas the spirit of brotherhood
and self help is paramount, and many of
the laws of the city are irrelevant.
However, even in these outlying areas, modern
amenities are beginning to be supplied.
At Glengarry, for example, public telephones
using solar
power have been installed. |
| A
Place of Character & Characters |
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| Tales
of the Ridge abound, and many of the
best are to be found in a remarkable,
if not literary, book entitled 'The
Lightning Ridge Book', by Stuart Lloyd
(R1513).
This entertaining book is a wonderful
collection of anecdotes of people,
places and activities from this opal
field. Some of the most beautiful
photographs of opals from various
fields are reproduced in a series
of books (R1608)
by Len Cram, a former miner and opal
dealer. He has also carried out some
remarkable experiments on growing
opal from natural sediments. |
Geology |
| Much
of the following information is taken
from Watkins (R1609).
Lightning Ridge lies towards the southeastern
corner of the Great Artesian Basin.
The geology of the locality (including
its peripheral fields) is therefore
similar to that of the other major
opal areas. The dominant feature is
of flat lying sandy clay beds, pale
in colour, sometimes iron stained.
A generalised cross
section (modified from Watkins
(R1609)
of the geology of the Lightning Ridge
field is shown in.
In the Lightning Ridge area Cretaceous
sediments outcrop in the form of interbedded
sandstones, siltstones and sandy claystones,
with some coarser beds. Underlying
these beds are medium grained light
coloured clayey sandstones which consist
largely of detrital quartz grains
distributed through a kaolinitic matrix.
Evidence of opal is found in the so-called
'steel band', a hard silicified band,
generally about 10 cm thick, which
occurs as a discontinuous layer within
the sandy clay. The cement in this
hard layer is usually opal.
Also occurring in these sandy clays
are layers or lenses of clay, usually
a mixture of kaolinite, smectite and
illite; it is generally grey or buff
in colour. As in other opal fields,
the Cretaceous beds are overlain by
quartz-rich sands and gravels. They
are usually confined to the ridges
and higher levels, and are seldom
more than about one metre thick. All
of the Cretaceous beds are deeply
weathered.
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Watkins
states:
At depths of
20-50 m the earlier weathering phase
grades into less weathered material
which is considered by the miners
to be the limit of useful opal production.
The formation and deposition of opal
is assigned to this period of deep
weathering, and the most likely sites
for its formation appear to be in
the top layers of the clay lenses
referred to above.
This is comparable with the observations
that opal is frequently concentrated,
in other areas, at the interface between
overlying porous, and underlying impervious
beds. However, opal may also occur
in small joints and faults ('slides'),
and as isolated nodules, termed 'nobbies'.
The origin of the cavities in which
the nobbies form is still not fully
understood.
In addition, localised breccia zones,
sub-circular in horizontal cross section
are sometimes encountered; these,
probably because of their broken nature,
also appear to be favourable sites
for the deposition of opal.
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