Surface mining methods are used when a mineral resource is at or near the Earth's surface. There are several types of surface mining including open pit, strip mining, quarrying, placer mining, and evaporation ponds. In British Columbia today, open pit mining and quarrying are common, while placer mining for gold was common in the early days.
Open Pit Mining
The open pit method is used for large mineral resource deposits that are at or near the Earth's surface. It is commonly used for base metal deposits that are very large. Such deposits can be efficiently and profitably mined using very large equipment even if the deposit is low-grade. As mining proceeds from the surface downward, this type of mine forms a pit with stairstep-like walls made by the mined benches.
Open Pit Mining Slideshow
Quarrying generally refers to surface mining that does not require complex processing. Quarrying is used to mine sand and gravel, crushed stone aggregate, many industrial minerals (like limetone, gypsum and magnesite), and dimension stone for building material and construction.
Sand and gravel quarries extract sediment that is not cemented together into solid rock. Blasting is not required. Instead, the loose material is scooped up with a front end loader or scraped from the quarry floor with a scraper, and then hauled to a mechanical processing plant where the sediment is sorted by size.
Crushed stone aggregate and industrial mineral quarries are similar to open pit mines in that they are developed in hard rock that requires blasting to begin mining. After blasting, the broken material is hauled by a loader or loaded into a truck and taken to a crusher. Crushed stone aggregate is conveyed to a mechanical processing plant at the quarry, that sorts the crushed stone by size for different construction uses. Crushed industrial minerals that require finer grinding and additional processing, like making wallboard from gypsum or kitty litter from clay, will be conveyed or hauled by truck out of the quarry to a plant where those steps occur.
Dimension stone quarries use techniques that make it possible for very large blocks of rock to be wedged loose or sawed out of a deposit. In many cases the rock must be drilled and blasted or wedged to create sheet like fractures that define the blocks. Chains and pulleys, or powerful excavators are used shuffle and lift the blocks from the rock face.
Hardy Island Granite Dimension Stone Quarrying Slideshow
Granite Quarrying in Ontario
Strip mining is used to extract layers of coal that are near the Earth’s surface and either flat lying or very gently dipping. Soil must be stripped before mining begins. It is then stockpiled for reclamation.
A dragline or a bucket wheel excavator moves in a line across the mine, removing the sedimentary rock to expose a strip of underlying coal. The rock (waste) is dumped by the drag line into a pile behind the coal seam. The exposed coal is drilled and blasted, and the blasted coal is picked up by shovels or loaders and loaded into haul trucks that take the coal to the processing plant.
When the coal seam is mined out, the waste rock is moved back in place from where it was removed from and capped with the salvage soil. The strip is revegetated or made ready for agriculture.
Placer mining uses water to concentrate gold and other heavy minerals from sand and gravel. The most basic and popular form of placer mining is gold panning.
An industrial scale placer mine uses large equipment to process large volumes of sand and gravel. The loose material must be 1) excavated using heavy machinery and, 2) hauled to a wash plant by mine trucks where it is dumped onto a grizzly, a screen that removes all the large boulders. From there the material is generally converyed to a trommel or to other screens to 3) sort the sand sized grains from the pebbles and cobbles. 4) The sand is directed with water to a sluice where the gold and other dense metal grains settle out, and the lighter grains (tailings) are washed into the tailings pond.
Gold can also be mined from sand and gravel excavated from a dredge floating on a pond or lake. It can also be concentrated from sediment that is vacuumed off the sea floor and pumped to a wash plant on a barge.
Placer Mining the Cariboo