Placer Mining

Placer mining is the mining of alluvial deposits for minerals. In most cases this is open-cast mining.

Rocker Box, Placer Mining. Public Domain.
Park County, Colorado, 1870s, Placer Mining. Public Domain.
Flushing alluvium out, Placer Mining. Public Domain.
Steam and permafrost, Placer Mining. Public Domain.

Placer mining is frequently used for gold mining in loose sediments, the whole gold rush in Alaska worked this way. Gold is not dissolved by rain water, very difficult to weather and quite heavy. The placeer deposit forms when most of the roch is cracked down by weathering and the residulas are relocated (washed away) by erosion. This is the simple reason why there are so many gold placers. And it is quite convenient, using a dredge and a wash to find gold is much easier than digging an underground mine. Nature already did the hard job, separating the rock from the resource. Of course there are other forms of placers but gold placers are by far the most common. In Germany, for example, the mining of tin placers in the Ore Mountains was of considerable importance during the Middle Ages.

Placer mining is also called hydraulic mining, because water is used to transport the loose sediment and to separate the resource from the slag. It is also called dredge mining, if a dredge is used to transport the sediment into the processing machinery. However, it's possible to do placer mining with a simple pan or a sluice box. The typical methods used for placer mining are, dependent on the used equipment:

  • Panning Mined alluvium is placed in a large metal or plastic pan with a generous amount of water. By rotating the pan the lighter material is washed over the side of the pan, the ore remains.
  • Rocker A rocker box or cradle does the same as a pan, but it is capable of greater volume. And it requires less skill, just brute force.
  • Sluice box Barriers along the bottom called riffles trap the heavier gold particles as water washes them and the other material along the box.
  • Dry washing If there is not enough water air is used to blow away the lighter stuff.
  • Trommel A slightly-inclined rotating metal tube with lifter bars at the isate is rotated with water.
  • Gold dredge A mechanical dredges digs massive amounts of sediment and puts it on a conveyor to the separator. They operate by sucking water and gravel up through long hoses. For the use on rivers and lakes they are mounted on a flat bottom boat.
  • Fire Alluvial deposits in Siberia and Alaska are often frozen, so-called permafrost, which makes them too hard to mine. The solution is underground mining with heat throug fire setting or steam. After the ice is molten the sediments is loose again.

Many people think panning is a technique invented during the gold rush, but actually it is most likely as old as the metal ages. That's how the first metals were found. And it's the reason why early civilization had an abundance of the rather useless metall gold, which was poor for making weapons or household items. Too heavy and too soft, though easy to work and does not oxidise. So they used it for religious things instead.

Modern mining methods include the total mining of the sediment over huge areas by using machinery or water pressure. Then the gold is caught by using chemical substances. Those substances are often poisonous, most common it is mercury. Most of the substance is recycled during the process, but a certain amount is released and pollutes the surrounding. Spectacular are accidents were huge amounts of the poisonous 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.