This is from an old doc. that I collected years ago.
1. RESIDUAL PLACERS
A residual placer is, in effect, a concentration of gold (or other heavy
mineral) at or near its point of release from the parent rock. In this type of
placer the enrichment results from the elimination of valueless material
rather than from concentration of values brought in from an outside source.
Residual placers may be rich but they are not likely to be large and as a
class, they have been relatively unimportant. The "seam diggings" in EI
Dorado County, Calif., (Clark and Carlson, 1956, p. 435) offer an example
of residual gold placers.
2. ELUVIAL PLACERS
Eluvial placers usually represent a transitional stage between a residual
placer and a stream placer. Where one type merges into another, they cannot
be clearly distinguished. They are characteristically found in the form of
irregular sheets of surface detritus and soil mantling a hillside below a vein
or other source of valuable mineral. It should be noted that the parent vein
or lode may or may not outcrop at the actual ground surface. Eluvial placers
differ from residual placers in that surface creep slowly moves the gold and
weathered detritus down hill, allowing the lighter portions to be removed by
rain wash and wind. As the detrital mass gravitates downhill, a rough
stratification or concentration of values may develop but this is rarely
perfected to the degree found in stream placers. Eluvial placers are typically
limited in extent but there have been cases such as at Round Mountain,
Nevada, (Vanderburg, 1 936, pp. 1 33-(45) where this type of placer
supported large-scale mining operations.