Obstructing or deflecting barriers allow faster moving waters to carry away the suspended load of light and fine grained material while trapping the more dense and coarse particles, which are moving along the bottom by rolling or by partial suspension. Placers may form wherever moving water occurs, though they are most commonly associated with streams.
The amount of gold present in placer ores is usually low compared with the associated primary hard rock deposit from which they were formed. However, due the easy of operation and low costs, placers are often commercially significant and may be the forerunner to further underground mining. The capital and operating cost of placer operations can be very low, allowing economic mining of ores containing as little as 0.2 gr/t Au. Nevertheless, the contribution of placer gold (excluding palaleo placers) to annual gold production is small.In several notable cases the proportion of gold present in placers exceeds that in parent deposit. These deposits are called giant placers and have many millions of ounces of gold. These deposits were particularly significant in the 19th century gold rushes.The gold mineralization in placer differs from all the other classes because the ore is in a particulate or loosely consolidated form and the gold has been liberated to a large extent by natural processes. Consequently, the savings in grinding costs compared to other types allows very low grade ores to be treated economically.Gold grains of several centimeters in diameter occasionally occur, although sizes of below 50-100 µm are more normal. There is usually an inverse relationship between the gold particle size and the distance from the parent deposit. In the case of Snake River (Montana, USA), very fine gold has been mined up to 400 km from the source, following periods of flooding.
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