|Typical Open Cut mining in PNG.|
Most of the products we use every day, for example; mobile phones, televisions, computers and cars are made out of valuable minerals and metals found in rocks that were mined from near the earth’s surface. This rock material that contains sufficient valuable minerals and metals and is mined from the ground is called ore.
Open-cut mining or surface mining is a technique used to extract ore located close to the surface and is the most common method of mining. Open-cut mines are used when deposits of commercially useful minerals or rock are found near the surface and where the overburden (surface material covering the valuable deposit) is relatively thin or structurally unsuitable for tunneling. Open cut mines that produce building materials and dimension stone are commonly referred to as quarries.
The overburden in the form of surface vegetation, topsoil and often waste material several metres thick is removed first to reach the buried ore deposits. The open-cut pit is mined downwards in horizontal layers with the side walls of the pit cut into benches or steps which slope inwards to diminish the danger of rock falls. The deeper the minerals are from the surface the larger the pit area and therefore the more waste material that has to be removed. Waste material is rock that does not contain sufficient minerals and is therefore uneconomical and although mined is not transported to
the processing plant and is often dumped to one side. Explosives are used to break the rock
containing the ore, especially as the pit gets deeper and the rock harder. The broken rock that is classified as ore is scooped up by front end loaders or shovels and loaded onto large trucks and
hauled to the processing plant often sited near the pit in an area that will not be affected by the pit increasing in size. In some open cut mines the ore is crushed within the pit and then placed onto conveyors that transport the crushed ore to the processing plant. Waste rock is also broken and
transported in the same way, not to the processing plant, but rather to a waste dump. The ore and waste is transported on a haul road which is situated along the side of the pit forming
a ramp. Mining continues until as much of the ore is removed without making an economic loss. Mining costs increase as the pit is deepened. Often this is a function of the transport costs to
remove ore, waste and other materials, but also practical limitations, like flooding can slow progress until the mining costs exceed the value of the metals extracted, and then the mine will close.
Copper, gold, iron, nickel, silver and zinc are some of the metals recovered from open-cut mining.
Although not a metal, coal is a major commodity that is also extracted using open-cut mining.
Open-cut mining is used in PNG by Ok Tedi Mining, Porgera Gold Mine, Lihir Gold Mine, Hidden
Valley Gold Mine, Ramu Nickel and Simberi Gold Mine.