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