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Alluvial Deposit


Sediments are deposits of debris of all sizes, which is formed by erosion. All kinds of geologic structures are eroded and transported, to be resedimented somewhere else. After some time the sediments may become sedimentary rocks by a process called diagenesis. The deposits are generally not altered by diagenesis, so the following also applies to sedimentary rocks.

Many important and valuable minerals and ores are resistant against erosion, e.g. gold and diamonds. So the surrounding rock is eroded, but the valuable minerals or ores remain. They are accumulated and resedimented with the residuals of the surrounding rocks. During the transport, depending on the relative weight of the mineral, it is deposited in typicals structures. Because of its weight, gold is transported by water only if the water has a very high energy, for example during floods. As soon as the energy of the water lowers, the heavy minerals are deposited first. The result is a graded layering, where the gold is concentrated at the bottom. This may happen various times, with the result of various goldbearing layers at the bottom of each sequence.

Another typical structure is a placer. If there are depressions, they become traps for heavy components. The water is able to transport the mineral on flat ground, but not to lift it over the rim of the depression. The mineral accumulates in the depression. Such placers are highly concentrated, but generally rather small. Unfortunately there is no overall structure so it is not easy to find them.

However, in the case of gold, placers are often mined. Historically the mining started by finding placers accidentially. Then the sediments were searched more or less shallow by washing the gold. This process is rather similar to the way the placers were formed.

Modern mining methods include the total mining of the sediment over huge areas by using machinery or water pressure. Then the gold is catched by using chemical substances. Those substances are often poisonous, most common is mercury. Most of the substance is recycled during the process, but a certain amount is release and pollutes the surrounding. Sepectacular are accidents were huge amount of the poison substances are released. Such an accident happened a few years ago in Romania, where a tributary and the lower part of the Danube have been polluted.


See also


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