Residual and Eluvial Placers

Residual and eluvial placers often times contribute to stream placers but either can be an actual placer deposit on its own completely separate from a stream. Both of these often take place in the western part of the United States and Alaska. They are normally located in areas where there is water and often times worked on by carrying water to a sluice box at a good distance from the stream.

Residual placers are produced by weathering as well as chemical action. These consist of gold that has been formed in a mineral or rock that little by little disintegrates due to the forces of nature. The sun, storms, wind, winter and other types of natural events cause this disintegration as well as by the chemical decay of the carrying rock.

If we were to imagine that this process has been taking place over hundreds of years, the earth would be placer mined in a natural way by the water and the wind. The lighter material would be carried and blown away and would leave the heavier materials in the earth where they would be more concentrated by the rain storms that occur occasionally and the runoff from snow that melts. This is why is it called residual placer.

In order for this type of deposit to be preserved, a lot depends on the fact that sufficient water is naturally supplied to concentrate of gold of the residual placer but not too much volume or speed to where it carries it to a stream or eluvial placer, and because of this, this kind of placer does not occur very often and also difficult to find unless a person has a good amount of experience at it as well as good equipment and a good knowledge of geology.

One of the most important things to keep in mind about residual placer Is that unless the starting ore body has disintegrated completely due to weathering, a vein will be located at the deepness the chemical action has make a way into the outcrop of the deposit.

The seam placers of the days of the gold rush were residual placers that could be found in cracks or seams in between rocks. In order to produce a good amount of gold this kind of placer has to be contained in that kind of a seam or another type of barrier that restricts it and the weathering has to be able to penetrate through it a good depth.

An eluvial placer is usually only a continuation of a residual placer and is transitional amid it and the stream placer. In many cases they are very similar looking and sometimes only a professional is able to tell the difference. Rock that has been weathered or disintegrated is washed or blown down a hillside and lays in a depression or at the lower part of a hill.

In some cases, an eluvial placer can be on the whole face of a slope. In this area, due to more action of the weather and the downhill slide, minerals that have more weight are concentrated and carried over a good amount of time. Just like a residual placer, the area is usually dry placered and at times worked by carrying water to the area.

As opposed to residual, eluvial placers do not have lode deposits right underneath them, as the original source is located up the hill from the location of the placer. Eluvial placers are not usually located as the material is always moving and heavy rains are able to move them downhill very quickly and in great quantities. This kind of deposit is somewhat like a bajada placer and is also sometimes confused with them.