Drilling out a shaft
Our mines are located in and around the town of Lightning Ridge in the state of New South Wales, Australia. Over the years we have held a number of leases across the extensive Lightning Ridge opal fields. Our predominant leases have been at the Grawin and Coocoran fields, the former producing Seam opal and the latter Nobby opal.
Lightning Ridge opal mining is predominantly an underground pursuit, with the opal layer or layers anywhere from 3 – 30m underground. It is not unusual to have multiple layers of potential opal-bearing clay in the one claim at different depths.
The trick is knowing which layer will produce gem quality opal, as not all of the ‘levels’ are productive. Local knowledge of the immediate area and lots of luck are involved in working this out!
Opal is thought to form over long periods of time by water seeping down through layers of sandstone, picking up absorbable silica before being deposited in cracks and crevices in the clay layer, which prevents further downward leaching. Over time, the water evaporates, leaving behind the hardened silica deposit. In the Grawin field, the opal has been deposited in bands, forming Seam opal which can be followed though the opal layer in the hope of finding spots where it has opened out to form pockets and intersections where gem grade material is more likely to be formed. The Coocoran fields have Nobby opal, which forms as nodules in isolated pockets in the clay layer, making them a lot harder to find, as there is no real telltale signs to follow. Nobbies are often located around fault lines and ‘blows’ in the sandstone roof layer, which are considered to have assisted in the travel and deposition of silica-rich water.
In typical Aussie fashion, the unique nature of mining in Lightning Ridge has seen the invention of numerous types of machines to help speed up the recovery of opal dirt. Very few pieces of machinery are the same, most being ‘bodged up’ with whatever spare parts are available at the time of a breakdown or even during the initial build, with may components often sourced from the local scrapheap!
We are currently using a hydraulic digger and bogger underground, which are driven by large generators on the surface. The digger is like a mini excavator arm that pushes into the wall with a pronged tool that scrapes downwards, dislodging the opal dirt from the face.
When a section of wall has been worked, the digger moves off to another section and the bogger comes in to pick up the debris. The bogger is a hydraulically driven wheelbarrow with a bucket on the front.
It drives into the dirt pile with the bucket down and scoops up the dirt and continues to pivot over the trailing wheelbarrow depositing the dirt in the tub. When the tub is full, the wheelbarrow is driven to the shaft where it is emptied into the hoist drum that is recessed into the floor. A cable is pulled and the hoist pulls the drum up the shaft where it pivots over a hemispherical track and dumps the dirt in a truck before automatically returning down the shaft.
Hoist dumping opal dirt into the truck
Filling the Agi with opal dirt for washing
When the truck is full, it gets driven to the wash-plant located around one of the many artesian bore dams in the fields. The opal dirt is deposited via conveyor belt into a modified cement mixer bowl, where it is mixed with a constant flow of water. This slurry is ‘washed’ for 6-24 hrs depending on the hardness of the dirt to clean away all the fine clay that adheres to any opal. When the process is complete, the remaining ‘tailings’ are emptied onto a sorting table and sorted though in the hopes of finding the big one. There is not often any rock material in the opal layer apart from pieces of the sandstone roof and hardened clay nodules, so a full truckload of dirt usually washes down to a couple of 20L buckets worth of tailings.