The ground is blue in the volcanic cones. Two modes of mining exist: the open-cast (or open-pit) and the underground mine.
An average of 250 tonnes of ore must be mined in order to produce a 1 carat gem quality polished diamond.
This type of miningn is very much used, especially in South Africa where there is the famous Kimberley mine called « Big Hole ».
The mining is done by extracting the subsoil from the pipe using large hydraulic shovels.
Hard rock is drilled and blasted with explosives so the broken material can be removed.
The pipe is dug in tiers from which the materials to treat is put into trucks.
The underground mine can today reach depths of more than 1,000 meters.
The content of diamonds is decreasing with the depth.
Underground mining is more expensive than open-pit mining and requires more complex management and machinery.
Two techniques of extractions are used:
Block Caving: block caving is an inexpensive method of mining in which large blocks of ore are blasted, causing the ore to break or cave under its own weight. This method is largely depending upon the action of gravity and is considerably less expensive than the traditional method of mining copper porphyry deposits.
Underground mining in tiers: it is a mixed method which is used at the Premier Mine in South Africa.
Eluvial deposit: the ground is yellow and near to the primary deposit.
Alluvial deposit: this type of deposit is down in river beds over long distance.
Several types of mining exist: artisanal mining until it becomes industrial mining.
The extraction is generally done in the terraces of rivers where rough diamonds are extracted from the sediments which cover them.
They are then separated from the gravel which was collected from the river beds.
Dredges are used and also powerful pumps sucking sediments and gravels.
The industrial mining uses large machines (hydraulic shovels, dragline, etc...) or explosives to reach the diamantiferous deposits.
For this type of mining powerful equipemnts and explosives are used to reach the diamantiferous gravel.
The removal of the diamonds from the beaches requires the removal of a very large quantity of sand before being able to reach the diamantiferous gravel, and it is not rare to dig more than 20 meters below the sea level before reaching the diamantiferous gravels.
There are enormous problems of watertightness. People work day and night.
For the marine mining, the operations are rather delicate and require specific equipements.
De Beers is the leader in this field and has boats and equipments unequalled, they have concessions in Angola, Namibia, etc...
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