Alluvial placers are those formed in present and past watercourses in gulches, creeks, rivers, flood plains and deltas. Reworking of some of these deposits together with others formed as a result of sedimentation or glacial processes by wave action may yield beach placers, which are treated separately.
Alluvial placers have been worked since ancient times in practically every country and have produced probably about one-quarter of man's store of gold. If we include the Witwatersrand deposits as fossil alluvial placers, the amount of gold produced from these types of placers probably approaches two-thirds of man's store of the precious metal.
Alluvial placers can be classified into two general categories - modern and fossil. The distinction between the two is commonly difficult to make in the field. Placers formed in present day water courses and most of those of Pleistocene and Tertiary age fall into the modern category. Those of greater age, commonly buried deeply by superincumbent sediments or volcanics and generally lithified we shall call fossil (paleoplacers). Fossil placers occur throughout the geological column.
There is an enormous amount of literature on alluvial gold placers, dealing with descriptions of the placer fields, the principles of placer formation and the methods of panning, rocking, sluicing, hydraulicking and dredging.
There are some general characteristics of alluvial gold placers that will serve as a basis for the descriptions. These include:
Alluvial placers are composed of loose unconsolidated gravels and sands that are commonly relatively clean. The terms 'white channel gravels', 'white leads' and 'white bars' reflect the latter circumstances with respect to quartz. In places, however, the pebbles and the gold may be coated with limonite, wad and other precipitates. Some alluvial gravels and sands are heavily impregnated with limonite and wad forming cemented gravels or 'cangalli' of the alluvial miner.
The heavy mineral suite accompanying gold in alluvial placers differs depending on the host rocks and types of primary deposits. Magnetite and ilmenite are the most common, and these may be accompanied by varying amounts of monazite, pyrite, arsenopyrite, cassiterite, wolframite, scheelite, cinnabar, native bismuth, bismuthinite, galena, sulphosalts, platinoids, tourmaline, garnet, chromite, rutile, barite, corundum, zircon, wad and limonite.
Most alluvial deposits in gulches, streams and rivers are characterized by a lack of regular and persistent bedding or stratification, but pseudobedding, laminations, current or false bedding may be developed in some accumulations. In deltaic deposits bedding and stratification is poorly- to well-developed depending upon the rate of sedimentation. Fractures, fissures and even faults can be seen in some modern alluvial placers. The last may throw the pay streaks several tens of meter in places.
Gold placers can be classified in a number of ways based upon their location or genesis. Kartashov (1971) gives a classification that is current among some placer geologists in Russia. He distinguishes two types of alluvial placers - autochthonous and allochthonous. In the autochthonous variety he lists those placers formed essentially near the primary or more rarely the secondary source(s) of their gold; the allochthonous variety implies considerable transport of the gold and deposition far from the primary and/or secondary source(s). This classification is quite satisfactory but requires a considerable knowledge of the details of placer deposits and the dynamics of their formation before they can be adequately categorized. Kazakevich (1972) provides another classification of placers.
Maps of alluvial gold deposits in: California, Western Canada, Eastern Canada, Russia, World
Maps of primary gold deposits in: Precambrian, Paleozoic, Mesozoic, Cenozoic Rocks
Rafal Swiecki, geological engineer email contact
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