The Geology of Opal Country
All the original finds throughout Australia were ‘floaters’ – pieces of opal, which had been eroded out from the horizontal opal levels within the desert sandstones. Weathered out from the high grounds on to the slopes and flats by wind, rain and ground movements over eons of time. Faults are essential to the process of opalisation as they provided channels for the transport of water as well as barriers against water loss after mineralisation commenced.
In periods of rising groundwater levels, the sediments increased in volume as the clay minerals swelled, causing considerable micro-faulting or ‘slides’ as the miners call them. The relationship or intersection of slides and large scale faulting associated with major lineaments, coupled with strong sandstones interspersed with claystone lenses are essential clues to the puzzle of finding opal.
Opal can be mined from the surface down to a depth of up to thirty meters, depending upon the height of the surrounding undulating country and the deposit of numerous opal levels.
Getting stuck into it!
To this day the Opal mining industry consists of numerous small businesses, partnerships and private individuals, invariably large corporations have failed in their Opal ventures.
The mineral rights in Australia belong to the Crown, in order to commence digging for Opal a mining claim or lease 50mx50m or larger is pegged and registered. These days, application fees, annual rent, pastoral compensation, native title compensation and environmental bonds may add up to considerably more than $2000. Not so long ago you could have a go at mining for $50, however that still gets you a prospecting right for fossicking only, no digging allowed, enough to infect you with the opal-bug.
Most Opal mines are initiated by sinking shafts; this once took several days by hand, but now takes only an hour or so using Auger drilling rigs.
Having ‘bottomed’ on a promising level, a drive should be made to connect with a second shaft. This creates airflow and ventilation to avoid the very real threat of carbon monoxide poisoning.
All the while the miner is interpreting changes in the ground; dipping levels or opal profiles which could contain roof or toe levels or both; trying to drive across faults and looking to find 'traces' of Opal preferably leading up to ‘pockets’ (patches of opal less than 1m wide) or ‘runs’ (tens of meters wide) of Opal.
Underground miners now use rotary tunnelling machines, diggers, bobcats and to a lessening degree jackhammers to remove the prospective ‘mullock’ or opal dirt. Mechanical hoists and ‘blowers’ (large vacuum extractors) now haul the mullock to the surface, having replaced the tedious hand windlass.
Most Opal is found by hand, gouging and checking the level, except at Lightning Ridge where the majority is found after the opal-dirt has been washed by an ‘agitator’. One in a row of large concrete mixers which sluice the sticky clumps of opal dirt at numerous tailings dam sites around the ‘Ridge’.
Professional Opal miners in South Australia employ heavy machinery and ‘noodling plants’ to process discarded opal dirt and their own mullock; sieving and collecting the fines by conveyer into a dark room for UV inspection – as White opal fluoresces under Ultraviolet light. At all of the fields there are ‘noodlers’, tourists and locals, who fossick discarded mullock dumps.
In Queensland open-cut mining is preferred, employing bulldozers and excavators to cut wide trenches in the ground. Miners known as ‘checkers’ are required to walk behind and inspect the wake of the bulldozer or the excavator bucketfuls as the opal levels begin to surface.
Once Boulders appear they are checked to look for traces of opal and may be cracked open using a small pick or geo-hammer. Hundreds of Boulders come up empty in search of that elusive fully loaded screamer!!!!!
Despite modern mechanisation, conditions are harsh in the remote deserts of Queensland. Working under the scorching sun with temperatures soaring above 50 degrees Celsius puts stress on the machinery as well as the courageous individuals who toil there, often hundreds of kilometres away from other settlements. Not to mention the blinding, suffocating dust made from tilling the dry earth and the swarms of flies so fierce that they patina ones shirt and find their way into every orifice.
By Peter Evans, February 2012.
Optimum depth of the Opal level is around 138 to 143m above sea level at the Lightning Ridge and associated fields – Opal found above or below these levels may be faulty (cracky) e.g. Mehi field Opal level is 148 m above sea level.
All other Opal fields Australia-wide will have an optimal altitude for opal very close to that of Lightning Ridge. Opal will never be found in the paleochannel, but in the ancient lagoons, billabongs or other areas of still water fed by flood conditions which brought the silt and organic matter that created conditions condusive to Opal formation.
Opal deposits will be close to the paleochannels and roughly parallel to it. Unstable Opal is usually the result of Opal formation under less than ideal conditions. Ideal conditions only occur at the optimal elevation, like a cake in the oven the middle shelf is optimal for correct cooking, top shelf results in overcooking, the bottom, undercooked.
The 'Deep 3-mile' at Lightning Ridge has a surface elevation of 160+ metres above sea level but the opal level is constant at 140 to 142 meters above sea level so depths of shaft sinking is 60 feet plus. The '3-mile Flat' is 140 to 146 m asl, the opal level is surface to 10 feet.
Over the best producing areas on all fields, Grawin, Glengarry, Sheepyard, Coocoran and Lightning Ridge, this level is relatively constant. Outside these parameters plenty of Opal will be found but will usually be potch or if gem it will be relatively less stable.
The clay lenses containing Opal will replicate the way Opal forms within a nobby, they are like a gigantic nobby. The gem Opal will be found at the optimal level of 140 meters above sea level. See diagram
Likewise Boulder - each whole boulder can be likened to an Opal nobby, gem colour, if present, will always be at the top of the boulder. See diagram
The boulder must be oriented correctly to be able to read it correctly. As most have been moved by Geological or mechanical action, this is not easy.
The amount of precious opal contained is dependent entirely on its position and orientation during Opal formation.
The King Boulder
This is a boulder containing only gem colour throughout, it is very rare and only one or two will be found in the very best mines. Usually football to grapefruit size with a fairly regular shape, the diagram below shows where the king boulder will be found if it exists at all in the mine.
Due to water, geological, or mechanical action the king boulder is usually mixed up with other displaced boulders. However there are other indicators, it is usually small, usually quite rounded or elliptical and when held it usually feels lighter in weight than a similar sized normal boulder, this is due to the quantity of opal contained within - as Opal is lighter than ironstone. These characteristics do not apply to a Winton pancake boulder which will be discussed later.
Opal formation within boulders depends mainly on the area of origin, each area produces a distinctive type of boulder. A Boulder Opal expert can, in most cases, identify the origins of both cut stones and rough to the point that they can nominate the exact mine of origin. This may seem unimportant to the average person, but a past history of each mine can show up possible and potential problems with the Opal; sandy or weak boulder, delamination (a tendency in some instances for the Opal to separate from the ironstone), prevalent inclusions, jelly Opals etc.
Conversely rough boulder from certain areas has a history of high-quality gem production and if offered for sale in the rough, may have a high possibility of producing gem quality opal.
Most notable among these are ‘Bull Creek’ near Quilpie and Winton boulder pancakes. They will be described later.
I'm sure there are many persons in the Opal industry far more conversant with boulder than I am, but having assisted and supervised the cutting of over 60 tons of good to gem quality and handled or sold in excess of 20,000 good-quality cut stones from all Queensland fields I have a good grasp of the subject. Dealers have the best overall knowledge of any local variety due to their purchases from and travels to the areas where mining occurs. How much is produced, what the average quality of the stones is, if there are inherent problems associated with the Opal from a particular miner, what characteristics they have, both detrimental and advantageous.
If the miner is reluctant to divulge where he is mining an experienced buyer can still by appearance and the ironstone back of the stone usually identify the area of origin with remarkable accuracy.
On the other hand the miner has, by far the best knowledge of the Opal from his own mine and when offering them for sale will saw them in a form that reduces their shortcomings or enhances the best qualities.
Opal Mining, Buying & Selling Scenarios:
Miner #1, is mining in an area which is producing good colour, but it is shot through with patches of yellow potch. So when seams are opened it will reveal large and small patches within the colour bar, virtually reducing the quality to near zero value, it may be possible to cut small good-quality stones from the colour areas but yield will be very low. The miner will present his Opals in blocked out form only. Blocked is the term for sawing out sections of the boulder showing a vein of colour all around the block but not exposing the face cover, only the edge of the vein will be visible showing as much of the gem colour as possible and eliminating the evidence of the yellow potch. This is not dishonest it is good marketing, an experienced buyer will shy away from a parcel like this but a ‘new chum’ buyer may fall for it, as the price relative to the colour showing will be very attractive.
Miner #2, has a variety of Boulder which shows beautiful colour on the edge of the vein (colour bar) but when opened the colour will not look at you, it will only be visible at an acute angle to the surface. What appeared to be a 500 p/ct red colour on the edge of the vein can be $50 p/ct green face up with red showing only at a very acute angle, often this type can show nice green at the edge of the bar but be virtually blank when opened and viewed 90° to the face.
He will present his Opal for sale in the rough or blocked out state. The miner knows far more about his Opal than you do. So, if he is selling rough or blocked out there is a reason, maybe it is the aforementioned, but sometimes he just needs the money quickly, he doesn't want to wait for the cutting. Knowing exactly where the Opal comes from in this situation can give an experienced buyer the information required to make an informed decision about value.
Sources & Image Credits
AUSTRALIAN PRECIOUS OPAL, Archie Kalokerinos, 1971. (Diagram: Levels ,Slides & opal Formation)
A JOURNEY WITH COLOUR Vol I&II, Len Cram (Photos. 3,4: Oldtimers, Drilling Rig)
OPALINE Pty Ltd (Photos.1,2,7,8) (Photo.2; 'Open Cut' mining operation: Showing Boulder Opal Levels behind Fault' - compliments the Kalokerinos diagram)
The Geology of Opal Country - By Peter Christianos, April 2009.
- (next) Opal Miners Hall of Fame