Prospecting … where’s the opal?
Trying their Luck
At Lightning Ridge, opal is scattered intermittently in small pockets or patches. It may occur in seams or in ‘nobbies’, which are nodules or lumps of opal unique to Lightning Ridge. Nobbies are usually found near the top of a softer claystone layer (the ‘opal level’), which is sometimes overlain by a thin band of ‘steelband’ and usually by thick layers of sandstone. In the first decades of mining, there were no reliable guidelines which miners could use to locate their shafts.
To identify new locations where opal would be likely to occur underground, miners learned to look for surface features which could indicate the presence of ancient subsurface disturbances, such as lines of trees growing over faults in the rock below. Blows (columns of fractured rock) and faults (large cracks in the rock) have voids where tree roots can penetrate deeply and where, in the past, silicarich water could have collected and slowly transformed into opal.
Calweld Drill for Prospecting
The first of these drill rigs was brought to the opal fields in 1970 by a mining syndicate Lightning Ridge N. L. This machine was made available for hire and sank numerous 3-foot diameter shafts, some taking less than a day when conditions were favourable. Opal dirt brought up in the bucket was put through a small rumbler to break down the large lumps and reveal any potch or precious opal.
After drilling the shaft down to the claystone level, miners could fix ladders inside and climb down to investigate the walls for signs of opal. Many miners adopted a practice of drilling the shaft deeper, several metres into the claystone level.
When used in this way for exploratory drilling, the Calweld drill rigs, although expensive to hire, were effective as a tool for prospecting. However, some miners queried the overall benefits of prospecting by sinking many shafts.
In the 1980s Jim Begg, Steve Aracic and other miners brought truck-mounted auger drills to Lightning Ridge These rigs could drill nine-inch diameter exploratory holes down to the claystone levels in a few hours – they were quicker and less expensive to buy and operate than the Calweld drill rigs. After Martin Tishler discovered opal at the Wyoming field in 1986 using an auger drill, this prospecting method was soon adopted by most miners. Use of auger drill rigs led to the discovery of many new fields around Lightning Ridge.