Gamma 'glow' pinpoints opal deposits: study

A new technique capitalising on natural radiation emitted by opals could dramatically improve the detection process for successful opal mining.

The sale of opals contributes $500 million to the Australian economy each year, yet finding the buried treasures has largely been a question of luck and perseverance for miners.

Two scientists have spent nearly 20 years developing a way to ‘see’ underground areas of high opal concentration with a method that does away with the need for blind drilling, according to a study published in the journal Australian Gemmologist.

“Australia is home to the largest and most widespread deposits of precious opal and accounts for 95% of the world’s production,” Prof Lewis Chadderton from The Australian National University said.

“Opal deposits are found within deeply weathered sedimentary rocks in the Great Artesian Basin that occupies almost one-fifth of the continent’s centre and east. Geologists agree that there are vast areas within the Basin where opal is likely to occur, but until now there was no means of detecting this mineral other than by direct recovery of opal fragments during exploratory drilling.

“Opal sites tend to be discrete and sporadic, so even drilling within a rich deposit can be fruitless unless there is a direct hit,” he said.

Dr Brian Senior, formerly a geologist with the Bureau of Mineral Resources (now Geoscience Australia), discovered in 1990 that opals exhibit higher than background levels of radioactivity. The radioactivity is the result of an unusual combination of uranium, thorium and their decay products trapped inside the structure of the opals when they formed approximately 20 million years ago.

By lowering instruments capable of detecting natural gamma radiation into an existing drill hole, the researchers are able to guide drilling towards the centre of the opal deposit.

“We’ve discovered through extensive field tests in New South Wales and Queensland that we can create a detailed gamma ray map by taking measurements in just a small number of holes — even with as few as three readings,” Senior said.

Chadderton, an adjunct professor at the Research School of Physical Sciences and Engineering at ANU, said the new technique could be a major boon for the opal mining industry.

“These discoveries have the immediate potential to revolutionise the way that exploration for precious opal is undertaken, and could pave the way for more cost-effective exploration and mining and continuity of supply,” he said.