Opals Born with a Boom?
June 10, 2004
EXPLODING artesian springs are the birthplace of opals, according to Canberra chemist, Byron Deveson. Mr Deveson has been testing a new theory on the formation of opals in Lightning Ridge this week.
If the theory, to be published by Australian Gemmologist, is right then opal is still being created. To test his ideas he is asking for "anyone who may have witnessed or heard an unexplained explosion" around artesian springs to get in touch with him. "There is both anecdotal and field evidence that some Australian mound springs occasionally erupt explosively, ejecting gravel and boulders 600mm in diameter," Mr Deveson said.
For instance in 1828 along the Darling River, explorer Charles Sturt recorded that when he was on Mount Oxley he heard "a rolling barrage like heavy artillery," he said. "He sent his tracker up a tree, but never did find the cause. "There are Aboriginal legends in the Northern Territory about 'desert noises' and in Bangladesh they call their exploding mound springs, 'Barisal guns'. At Eulo near Cunnamulla there were reports of noises every few years, while pebbles and boulders of exotic rock have been noted on many of the opal fields. Patches of gravel, some 10m thick which miners find at The Ridge "originated from unconsolidated gravel at the base of the Great Artesian Basin (GAB) that have been brought to the surface by the long term action of mound spings," he said.
Breccia pipes were evidence of sometimes repeated disturbances occuring in the clays. Gravel could block the springs causing huge water pressures to build up behind it, he said. They may block the spring completely or it may clear the blockage with periodic explosions. Backing his claim was evidence of gold, tungsten, silver and platinum found in Ridge opals and the presence of topaz crystals in the gravels, Mr Deveson said. There are two nearby sources of topaz - one of them in the New England area. "But topaz is a soft mineral of tin, that doesn't travel well," he said. The other source is the Gilmore Suture, a tin, tungsten and gold mineralisation belt within the basement of the GAB near Lightning Ridge.
Mound springs have all the ingredients for opal, Mr Deveson said. They are silica-rich, alkaline and high in minerals - and are found, both active and extinct near continental fault lines at Lightning Ridge, Mehi, Lila Springs, Nymagee, White Cliffs, Anda-mooka, Coober Pedy and Eulo/Yowah. Mr Deveson isn't the first to make the connection with artesian springs, fault lines and opals but what he adds is the fireworks.
"Pockets of silica sol (silica spheres and chains of spheres) are intermittently injected by hydraulic pumping caused by seismic or other events, such as geyser activity or mud volcano activity, into a system of localised cracks and voids."
The voids, for example could be fossilised animal impressions so perhaps, as recent carbon dating found, opal should be younger than the fossils they look like. Mr Deveson describes the chemical conditions needed to create opal and says they are all available in nature. Even the claystone in which the opals are found near Lightning Ridge is important. Clays not only filtered the silica sol and purified the water but as the clay absorbed water and swelled up, the pressure it generated "could act as the driving force for the ultra-filtration of the fluid." Making opal's uniform sized silica spheres was a matter of chemistry, he said.
"Mixing warm, alkaline mound spring water with cool, slightly acid ground water with low total dissolved salt content would decrease the pH, lower the temperature, and lower the ionic strength of the mound spring water," he writes. "All three changes facilitate the formation of silica spheres." The spheres line up to make shapes around "nucleating agents" and artesian waters include zirconium, titanium and thorium, all of which are found "at anomalously high concentrations" in opal. Other theories for the formation of opal incude weathering, mineral water alone, or by microbial action of Cretaceous-era insects.
Source: The Black Opal Advocate