Mining Methods

The following is an explanation of the two most important gold mining methods which consist of surface and underground mining.

Surface Mining: Open pits are appropriate for bigger tonnage close to surface deposits. In most cases the floor of the pit is lowered in around thirty foot benches. Blast hole drill holes are packed with explosive and are then blasted as this prepares the rock to be moved. Often times the rock from the drill holes are then assayed as well as the grade of the bench and chemistry is also tested. After blasting, the broken rock is marked by the geologists and determined as either ore or waste.

In addition if the mill has different kinds of ore or if higher grade mill and lower grade heap leach ore is produced, there will possibly the truck drivers may have be a number of places in which the broken rock will need to be delivered to.

Underground Mining: In the following we list some of the main underground mining methods that are used:

Block caving: Block caving most commonly has the least expensive cash cost per ton in the underground mining method. It does have need of considerable upfront development costs seeing that big excavations need to be made to undermine the block that is going to be caved, and big milling infrastructure needs to be set up to handle the big ore tonnages that a thriving cave will produce.

Once the ore block is undermined, it is then fractured over time by gravity and the pieces fall down into the finger raise. The dimension and sized of the pieces that are going to the chutes for loading are restricted by tough sieves or what are more commonly known as grizzlys on the grizzly level.  

Long Hole Stopping
Wherever big blocks of ore can be recognized and the immediate rock is convincingly sturdy, then a long hole mining method is in most cases the most inexpensive cost mining method. The outcome is not different from an underground excavation. Access to the top and bottom of the ore block is set up with drifts or tunnels. A vertical hole is then set up inside the ore from the top of the block to the bottom. Long holes are then drilled to blow up vertical slabs off the ore block. In most cases a loader takes care of picking up the broken ore from the lower tunnel and takes it away to an ore pass. For safety purposes, the loader is operated somewhat by a radio control when it is inside the big open stope. As soon as the ore block has been blasted and extracted, the stope in most cases is filled with waste rock to steady the empty space and to make possible the extraction of adjoining ore blocks.

This technique of mining is very well known, and is approximately the underground analogy to the advances that have been done in mechanizing a big open pit. Where large blocks of ore are able to be identified in moderately strong rock this method is useful and the great thing about it is that it is more cost efficient than other methods.

Cut and fill: In places where the rock that is surrounding the ore zone is not strong enough to use long hole stopping, or the sides of the ore-zone are not regular and drilling long holes would cause too much dilution or miss too much ore, then cut and fill mining methods could be more appropriate. In this technique, the top and bottom access is again formed, but the ore is removed in horizontal slices from the bottom. After a wedge is blasted and the broken ore is taken away, the void needs to be backfilled to offer a platform for the equipment to rest and in order to provide support to the side walls. This procedure is done over and over until the block of ore is extracted. In wide ore areas, this technique can be very mechanized, but in narrow ore zones it can require a great amount of work and have a negative impact on cost per ton. Due to its higher costs, this process in most cases requires higher grade ore than that for long hole stopping.

Infrequently, where the ore area is particularly weak, the cut and fill method can be done in an underhand configuration. In this case the mining process is done downwards. The benefit is that the miners are working under a roof of cemented fill and this may in some cases be stronger than the in-situ ore. 

Drift and fill: This is a moderately costly mining method that is done where the rock conditions are especially weak. Instead of taking the risk of opening a big excavation that in weak rock conditions may be complicated to support, the ore is taken or extracted by a big number of tunnels or drifts that are mined through the ore area and which are then consequently crammed with cemented fill. This has the benefit that the rock roof or span that is on top of the miner's head can then be supported which is important because in some cases it can be as little as six feet across.