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December
2001
OPAL
HUNTERS TAKE TO THE AIR FOR ELECTROMAGNETIC TRIALS AT LIGHTNING RIDGE
OPAL FIELDS
New South
Wales Department of Mineral Resources has conducted an $80,000 electromagnetic
aerial survey at Lightning Ridge, Australia, as part of an attempt to
improve prospecting methods aimed at finding the rare and elusive opal
gemstone.
Lightning Ridge produces the world's most valuable opal, including the
bulk of world black opal supply. Opal contributes tens of millions of
dollars to the economy of New South Wales each year.
Opal Miner
Michael Larkin beside a classic 'Z-fault', one of the geological structures
sought by underground opal miners at Lightning Ridge as an indicator of
a potentially opal-bearing area.
The smooth, yellow-brown rock above Michael's head and within the Z-fault
is 110-million-year-old sandstone, which forms the roof of underground
opal mines at Lightning Ridge. Below the sandstone is the clay 'opal level',
in which most opal is found.
Vertical lines through the sandstone and clay are gouge marks from a 'digger',
a hydraulic digging machine used
by some Lightning Ridge opal miners. Others use hand-held jackhammers
or air drills to dig through the sandstone and opal dirt.
But the
opal is located in small pockets that are distributed irregularly within
the clay 'opal layer', up to 25 metres underground - making opal mining
a chancy and expensive venture, and opal miners eager for better ways
to locate their prize. One thing that appears certain is that opal in
the Lightning Ridge area is usually found near fault lines, probably
because these faults conducted water deep underground to take part in
opal formation.
Opal miners look for surface indications of underground faults - indications
such as visible changes in the rocks and sediments; or tree species
that survive only where their roots can seek moisture trapped in the
faults.
Once miners have chosen a prospective area, many drill narrow test holes
from the surface down to the opal level. If the drill brings up promising
signs of opal, the miners will go ahead with the expensive process of
drilling a 3-foot-wide shaft down to the opal level, then digging horizontal
passages called 'drives' through the clay and sandstone in the hope
of coming across opal.
Still,
most test holes prove fruitless; and all too often, the miner will
be lucky to find enough opal even to pay for fuel and machinery.
Of course, it is the ever-present chance of a 'big find' - together
with the addictive qualities of opal and the opal-mining lifestyle
- that keep people in the mining game.
After comparing their electromagnetic data with the underground
knowledge accumulated by opal miners, geologists found that their
results clearly showed up the clay 'opal level' and overlying sandstone,
down to a depth of 10-15 metres below the ground's surface. The
electromagnetic maps also appear to show underground faults. The
geologists now plan to drill holes in the areas they surveyed, to
test how accurate their underground maps are, and how useful they
might be in searching for opal. |
HOW
OPAL FORMS
Opal is made of tiny spheres of silica.
Opal forms within spaces in rock. Some scientists think opal
took thousands of years to form, at high temperatures
and under great pressure; others think opal formed relatively
quickly, at about 20 degrees celcius. It is often
suggested that silica in the rock spaces turned into a gel that
gradually hardened to form opal.
It seems that at least some opal formed when bacteria took
silica from the surrounding clay or stone, then deposited it in
spaces in the rock, where the silica became opal. Where the rock
cavities had once contained plant or animal remains, opalised
fossils were formed. |
Many opal
miners say the technology is too expensive to be useful to them, and
no more effective in locating opal than are their traditional, surface-based
methods; but the NSW Department of Mineral Resources says the electromagnetic
survey is a pilot study in a larger opal project.
"This is the first time anything like this has been done in an opal
field," says geologist John Watkins.
"We have to decide which technology works in the environment and what
specifications and parameter must be met.
"Then there will be a shakedown in technology and we will be able to
look at how to make the technology accessible to the prospector."
For the moment, though, miners are sticking to the old adage: "The only
way to find opal is to dig holes."
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