Starting to mine
Locating prospective ground
A 9” auger drill for opal prospecting Coocoran opal fields Lightning Ridge NSW
Prospective ground is often indicated by surface features such as faults and blows. These features can be seen by locating large trees, such as box trees or from aerial photographs.
Preliminary drilling is usually done with a four inch (4” or 100mm) auger drilling rig.
Drilling a core is made to determine the existence of or depth of the opal level which is indicated by the change of rock strata from sandstone to claystone.
Some mining exploration is performed with a 9” Auger with small trommel
Developing prospective ground into a working mine
Once prospective ground for opal bearing “dirt” has been located miners determine the most appropriate place to drill (sink) a “three foot” (1 meter) shaft. This drilling operation is done with a “Calweld Drilling Rig”. These machines were adapted from machinery in California USA (the California Welding Company) that was used for drilling footings for pillars and piers for buildings. Similar equipment is used in South Australia. It is usual to sink two three foot holes on a mining lease to provide circulation of air and hence ventilation. One hole is used for access down a ladder and the other for “blower” access to remove the opal dirt.
Mining the opal dirt
The extraction of the “ore” or “opal dirt” as it is called by the miners in more modern opal mining is done by using the “underground digger”. “Diggers” are pneumatically operated and allow the removal of substantial amounts of opal dirt. It needs to be remembered that these diggers need to be “loaded down a “three foot hole” and reassembled for operation in the mine at a depth of some 10 – 20 meters.
Underground digging Carters Rush opal field Grawin precious stones field Lightning Ridge District NSW.
Opal dirt being loaded into a truck after being extracted by a blower at Lightning Ridge.
As the opal dirt is mined form the wall of the opal mined it can be directed into the pipe of the “blower”. This is an 8” plastic (PVC) pipe or tube, more often used by plumbers, and is attached to the modern extraction device known as a blower. The blower is some what of a misnomer; it gets its name from the exhausting of air that is used in the extraction of the opal dirt to the surface. In fact the blower acts as a giant vacuum cleaner and actually “sucks” the opal dirt to the surface.
Processing the opal dirt
Once the opal dirt has been extracted from the mine and loaded into trucks it is transported (in Lightning Ridge) to the “puddling tanks” Puddling tanks are essentially man made “dams” of water. Apart form saving some water from local rain fall water for the puddling tanks is obtained from bores into the Great Artesian Basin (GAB). These bores also supply the local “town water” for Lightning Ridge, and most other many outback towns. Miners pay for a “sight” to set up there processing equipment known as “agitators” (aggie for short). An agitator is simply a modified “cement mixer” barrel attached to a diesel motor. Water from the puddling tank is pumped in, the opal dirt fed into it by a conveyor belt, and the mixture washed for several hours to “break down” and wash out the “gangue” or sandstone and clay and so separate the opal from the dirt.
Unloading opal dirt onto a conveyor for loading into an agitator. Lightning Ridge NSW.
Showing opal dirt being conveyed into the agitator for processing.
Puddling tank Lightning Ridge NSW.
Line of agitators on sites at the puddling tank.
Retrieving the opal from the agitator
After the opal dirt has been “washed” in order to break down the opal dirt and leave the “opal nobbies” behind, the operation of the agitator is reversed so as to bring the remaining material out onto a table so it may be inspected and hopefully reveal its opal content. This process is known as “Tailing out”.