- Mining Methods

SURFACE MINING

Open pits are suitable for large tonnage near surface deposits. Typically the floor of the pit is lowered in about 30 foot benches. Blasthole drill holes are filled with explosive and blasted, preparing the rock to be moved. Often rock from the drill holes are assayed and the bench's grade and chemistry are tested. After blasting, the broken rock is marked by the geologists as being either ore or waste. Additionally if the mill blends different types of ore or if higher grade mill and lower grade heap leach ore is produced, the truck drivers may have a number of possible destinations for a load of broken rock.

UNDERGROUND MINING

Below we list some of the main underground mining methods used.

Block caving

This is normally the lowest cash cost per ton underground mining method. It does require substantial upfront development costs as large excavations must be made to "undermine" the block that is to cave, and large milling infrastructure must be established to handle the big ore tonnages that a successful cave will generate.

After the ore block is undermined, the ore block is fractured over time by gravity and the pieces fall down the finger raise. The size of the pieces passing to the chutes for loading is controlled by heavy duty sieves or "grizzlys" on the grizzly level.

Long hole stoping

Where large blocks of ore can be identified and the surrounding rock is reasonably strong, then a long hole mining method is generally the lowest cost mining method. The result is not unlike an underground quarry. Access to the top and bottom of the ore block is established with drifts or tunnels. A vertical hole (slot raise) is created within the ore from the top of the block to the bottom. Long holes are drilled to blast vertical slabs off the ore block. Normally a loader will pick up the broken ore from the lower tunnel and take it away to an ore pass. For safety reasons, the loader is operated remotely by a radio control when it is inside the large open stope. Once the ore block has been blasted and extracted, the stope will normally be filled with waste rock to stabilize the void and make possible the extraction of adjoining ore blocks.

This mining method is very popular, and is almost the underground analogy to the advances achieved in mechanizing a large open pit. Where large blocks of ore can be identified in relatively strong rock this method is productive and has low costs.

Cut and fill

Where the rock surrounding the ore zone is too weak to use long hole stoping, or the sides of the ore-zone are irregular and drilling long holes would create too much dilution or miss too much ore, then cut and fill mining methods could be more suitable. In this method, top and bottom access is again created, but the ore is taken out in horizontal slices (or lifts) from the bottom. After a slice is blasted and the broken ore is carried away, the void must be backfilled to provide a platform for the equipment to stand and to support the side walls. This process is repeated until the block of ore is extracted. In wide ore zones, this method can be highly mechanized, but in narrow ore zones it can be very labour intensive, with a negative impact on cost per ton. Because of its higher costs, this method generally requires higher grade ore than that for long hole stoping.

Rarely, where the ore zone is especially weak, the cut and fill method can be carried out in an underhand configuration. Here mining proceeds downwards. The advantage is that the miners are working under a roof of cemented fill, which might sometimes be stronger than the in-situ ore.

Drift and fill

This is a relatively expensive mining method used where rock conditions are very weak. Rather than risk opening a large excavation which in weak rock conditions might be difficult to support, ore is extracted by a large number of tunnels or drifts that are mined through the ore zone and which are then subsequently packed with cemented fill. This has the advantage that the rock roof or span above the miner's heads that must be supported can be as little as six feet across.

Unusual configurations

In an underground mine the best shaped orebody is close to vertical, since this facilitates the use of gravity to collect ore for transport to surface, as shown by the orepass system in Figure 25. If the ore body is flat, and especially if it is also narrow, this imposes an extra burden on the mine since the ore must be mechanically moved at all stages. Breast stoping is a method used extensively in the flat dipping South African gold fields but is little seen elsewhere. This relatively labour-intensive method has seen some innovations in terms of support methods and drilling technology, but the basic methodology has changed little over the years since the narrow flat-lying nature of the reefs in South Africa make mechanization very difficult.