Residual aeolian placers (RAPs) are formed by the wind, by preferential entrainment and removal of small and low-density detritus
from a pre-existing, typically sedimentary sequence containing heavy minerals. Heavy minerals that are resistant to chemical alteration
and either soft and malleable (gold), or extremely hard (diamond, sapphire, ruby, etc), are concentrated as a residual on the wind deflation
surface, typically in the interstices between larger resistant clasts. RAPs form preferentially in arid or semi arid areas, where the vegetation
is sparse or absent, and where topography and surface gradients are low, but may also form on beaches and in periglacial environments. RAPs
consist of a continuous or discontinuous, planar or gently undulating horizon of heavy minerals in a single-clast-thick pavement of resistant pebbles,
cobbles, or boulders, which mantles the present topography or palaeo-topography. Extreme wind erosion and surface deflation may condense the full
pre-existing sequence into a layer of heavy minerals only. The grade of RAPs is substantially higher than in the precursor deposit, and commonly much
higher than those typically found in most other placer types. Aside from their general occurrence as a thin pavement of resistant clasts, the best indicators
of residual aeolian gold placers are the presence of wind-eroded stones (ventifacts) on the pavement, and the distinctive and unique shapes of aeolian gold
grains themselves. The shape of the gold grains is progressively transformed by sand blasting during placer development, from grains with thickened rims, to
toroidal grains on which the rims are split' and peened inward toward the grain centre on one or both of the grain sides, to hemispherical or sub-spherical
grains that are fully encapsulated by the in-turned rims. Gold that has undergone aeolian shape transformation is difficult to entrain and transport away from
the site of aeolian concentration by anything other than mass movement or glacial processes. It is also easily retained by commercial recovery plants of any configuration.