Placer mining is a group of mining methods that use water to separate valuable minerals or ores from the surrounding sediment. It can be used to recover platinum, tin, and diamonds, but most commonly gold. A very crude form of placer mining was used by gold prospectors during the California gold rush in the mid 1800s. Since then, the technology has progressed to allow a larger volume of material to be processed. Modern mining methods include dredging and sluicing.
Placer mining is named after the deposits from which the material is mined. Placer deposits are concentrations of heavy minerals that form when minerals are washed, by weather or flooding, down slope into streams. The minerals settle in one place because the flow of the stream is not strong enough to move them any further. Placer deposits are usually found in streams, on beaches, and in gravel left behind by flowing water.
The most basic, and perhaps most iconic, method of placer mining is panning. The only equipment a miner needs is a pan, usually slightly larger than the average dinner plate. The miner puts a small amount of sediment in the bottom of the pan and then fills the pan with water. He or she then swirls the water around, allowing it to wash over the sides of the pan. In this way, common sediment is washed over the side while the gold or other valuable materials are left behind.
Placer mining works because certain precious metals tend to be more dense than the sediment surrounding them. When water is poured or sloshed over the sediment, the movement of the water is enough to wash the sediment away, but not strong enough to move the gold. This principle works even with very tiny flakes of gold.
Panning is still used by hobbyists and small-scale miners, but it is very labor intensive. More sophisticated methods must be used to mine large areas. Sluicing and dredging are often used both individually and in conjunction.
Sluicing requires a sluice box, which is basically just a rectangular box set on a slope. The top has an opening, and the bottom has a series of wood or metal bars that break up the current as water and dirt are dumped into the open top. The lighter sediment flows out of the bottom of the box, but the heavier gold is trapped by the breaks in the current caused by the bars. Dredging is really just a way of gathering up material to be sluiced. It usually uses suction to vacuum sediment from a stream bed or river.