Alluvial Gold Placer Formation
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.
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 (palaeo-placers). 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, hydraulic king and dredging.
There are some general characteristics of alluvial gold placers that will serve as a basis for the descriptions. These include:
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.