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Friday, April 27 2018

A good rule of thumb that most prospectors agree on is to look for placer gold on bedrock and within crevices in the bedrock. This simple principle makes sense— placer gold is heavy and dense and therefore settles at the lowest point as it is pushed around by flowing water. However, gold isn’t always going to make it all the way down to true solid bedrock at the bottom of all gravels. Instead, in certain conditions, it will be found ABOVE bedrock. When these conditions exist, gold will collect on “false bedrock.” For example, a clay layer in streams can act like bedrock and yield more gold than the true bedrock below it. 

bedrock goldAnother situation where gold cannot make it to bedrock is where the streams and gullies have “caliche.” Caliche is a form of cemented gravel often found in desert areas. The gravel is deposited by water flows, just as it would be in any other stream. The difference is that the water contains dissolved calcium salts, and as the water dries up in the streambed, the calcium salts turn into calcium carbonate or calcite. This material gets deposited on the gravel a little at a time and eventually the calcite glues it all together into a solid mass. The solid mass then acts like false bedrock and any new gold flowing across it is stopped in its downward movement by the caliche.

Residual placer gold is formed near the source of the gold and doesn’t really have a chance to work its way downward. Hillside placers are residual placers that are sliding and making their way down the side of a hill into a stream drainage. Gold generally is distributed all through these types of gravels and is not concentrated in any one place. A hillside placer often will be located up above a stream, making it difficult to determine it from a bench placer. Old benches of river gravel can be good places to prospect because they are virgin ground, but because the river used to flow at this location, it’s highly likely that the best gold will be right on bedrock or in crevices of the bedrock. The best way to tell the difference between hillside and bench placers is the shape of the rocks in the gravel. If the gravels and rocks are well-rounded and smoothed, then it’s probably an old river bench. If the gravel is angular, pointy, and sharp, it’s probably a hillside placer that hasn’t yet made its way into a stream.

Another type of placer—the windblown placer— concentrates on the surface, just exactly the opposite of what you would find in a river or stream. The lightest materials such as sand and silt are blown away by the wind. Heavy materials like gold remain behind on the surface. These types of placers are marked by desert pavement, which is a concentration of small to medium-sized rocks that cover the surface and prevent continuing wind erosion. They occur in dry regions with flat terrain, and are often favorite locations of metal detectorists who find nice nuggets at shallow depths.

The next time you find yourself in an environment with well-washed river gravels, you know that the gold will most likely be on the bedrock. But if you find yourself in other types of environments, especially in the desert, consider the possibility that the best gold may actually collect ABOVE it.  Good luck!

Posted by: Denise AT 06:43 pm   |  Permalink   |  Email
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Nugget of News Blog