Here is an excellent article about alluvial placers - reprinted courtesy of MiningLinks.com
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:
(1) alluvial placer gold in pay streaks near its source is invariably coarse and is found in the lower layers of the alluvium either on bedrock, in a zone a few feet above the bedrock or in crevices, fractures, etc. in the bedrock within a few feet of the surface. An exception to this is where the so called 'false bottoms' or 'false bedrocks' occur in thick beds of alluvium. These false bottoms may be clay layers (hard pans) within the gravels, compacted sands or more rarely limonite-cemented sands and gravels (conglomerates). Alluvial placer gold far from its source is generally finely divided, and while part of the gold may be on or near bedrock or false bottoms most of it is dispersed throughout great thicknesses of the sediments. This is especially true of extensive flood plain and deltaic deposits. Two other features are characteristic of most alluvial gold placers:
A/. The further from the source the more finely divided the gold.
B/. Placer gold is finer in value than its source lode gold and that with increasing distance from the source the finer is the gold.
These two statements are general. The first is invariable, but there are exceptions to the second in some districts.
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.
Such gravels invariably occur where the primary deposits and wall rocks are rich in pyrite, siderite, chlorite and other iron and manganese-bearing minerals. Secondary siliceous and calcareous cements are rare in alluvial gravels except in the vicinity of siliceous springs and lime-bearing waters; some alluvial deposits may be cemented by caliche in arid regions. In permafrost zones the alluvial gravels are frozen solid and have to be thawed out before dredging or hydraulicking.
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.