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.
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.
The evaluation of ore grade in placer is difficult because of the low grade and unusually coarse gold grain size. Extremely large samples of several hundreds of tonnes must be treated in a mineral concentration sampling plant for the evaluation of a placer ore grade and for flowsheet design. Commonly, this is a gravity concentration plant comprised of spirals, jigs and centrifugal concentrators. Estimates of gold grade by this method may differ greatly from the eventual average gold concentration recovered from operations with ratios of recovered to expect gold varying from 32 to 149% in one study.
The fineness of gold in young placers depends on the original source and varies from 600 to 900. Placer gold grains have been found to have an outer rim which has a higher fineness. This has been attributed to silver dissolution, as silver sulphate or silver carbonate, and is supported by evidence that fineness increases with distance downstream. The lower surface silver content gives placer gold a deeper/orange appearance than gold in hard rock ores.
The degree of gold liberation and the surface chemical properties of placer gold are important in the effectiveness of gravity concentration and amalgamation. As most gangue minerals are lighter than gold, unliberated gold grains are recovered less efficiently by gravity concentration. Detailed surface chemical data on gold grains in placer deposits are not abundant, however, it is known that sulphur and hydrocarbon adsorption can occur and the presence of impurities in the gold significantly affects amalgamation.