Coober Pedy - opal mine

Oyster Moon
Paul Foskett & Rhu Nash
Mon 20 May 2013 08:50

Cooper Pedy, along with small amount from Andamooka and Mintabie supply 85% of the world’s opal.  Opal was originally discovered by gold prospectors who camped in the area and whilst searching for water one of them – William Hutchinson, the 14 year son of Jim - discovered opals.  The pair were two of a four man syndicate.  The opal mines extend about 40km around Coober Pedy and the landscape is riddled with waste material, or mullock, heaps and mine holes.  According to Mines Department somewhere between 1 to 2 million holes in the ground.  Everywhere are warnings not to step backwards whilst taking photograph in case you fall down a hole.

 

 

We visited Tom’s supposedly working mine for a tour.  Turns out not worked anymore because would be too dangerous with tourists wandering around.

 

 

Silica solutions trickles down faults and fractures in the ground and deposited in cavities within the sediment or places left by shells and belemnites (ancient squid).  Millions of years later you get opal.  Ancient shell beds and sedimentary deposits occur in layers so opals tend to be concentrated in horizontal planes. 

 

 

Opalised shell layer

 

 

You get white opal, black opal and crystal opal.  White has creamy background, black a darker background colour and crystal you can almost see through.  This is white opal.

 

 

About 90% of the opal that is found doesn’t have any of the fiery colours and is called pooch – I keep wanting to say pooch.  In early days miners had to sink a shaft and then tunnel along the level using a pick and shovel to search out opals.  In exploration shafts they hunted for slips/faults and followed them up or down to find those horizontal surfaces where water may have accumulated and silica been deposited.

 

 

Here’s some horizontal layering – the yellow is sulphide

 

 

When they found opal they used a small handpick or screwdriver to extract – apparently opal breaks very easily, so you need to be careful.  Since the 1970s machines have been used.  Most of the prospecting shafts are dug with a Caldwell drill – looks like a circular hole saw with a bucket on the top.  A small shaft is dug, maximum of 1 m diameter,  just wide enough to get a man down it.  Waste material (mullock) from the digging was originally lifted out by hand by windlass, then by power winches.  Here’s Paul enjoying a trip up on of the mine shafts.

 

 

The red streaks in the claystone are worm burrows.

 

 

This is a tunnelling machines with revolving cutting head that is used to cut through the rock.  It has a pipe behind that removes the spoil.  Spotters follow the machine and with the driver look out for opal.  If an opal vein is spotted the miners still dig it out by hand. 

 

 

Nowadays some people have a truck mounted blower that acts like a vacuum cleaner. 

 

 

Here’s Paul shovelling waste into the blower (which is really a sucker).

 

 

Invented at Coober Pedy and means the miners could get rid of a lot of waste.  This is the top view of the blower which deposits the spoil on the surface in big mounds or mullocks. 

 

 

We have noticed that waste piles are of different sizes.  It turns out that a small pile is from a prospecting hole, medium from a access shaft and the large ones are from mine the blower.

 

 

Once the rough or raw opal has been mined it just looks like bits of dirty rock.  It’s placed in a small water tumbler which cleans off about 95% of the dirt.  The opals are them passed through a series of size graded sieves and classified by size, colour and quality.   Paul couldn’t resist going noodling – searching through the heaps of discarded mullock for pieces of opal missed by the miners.