Placers deposits are now known from five sedimentary environments; washout, river, aeolian, beach, and continental shelf. In each environment, the concentration of mineral grains, or sorting, takes place either by removal of gangue grains (denudation) or by addition of valuable grains (accumulation). Any given deposit will result from both processes, but one will usually predominate. Denudation placers all sit on or just above erosive scour surfaces.
They arise from a two-step process; initial particle deposition followed by selective removal of gangue particles. For example, deposits from a waning flood-stage river will include many different size, shape and density particles but a subsequent lower energy normal river flow might remove only the smaller, flatter or the less dense particles. The second fluid flow can be quite different from the first as, for example, when the wind selectively removes sand grains deposited by waves. Repeating these two steps, transportation from source and selective entrainment of grains results in a high placer mineral flux allowing denudation placers to achieve high concentrations of particular minerals. Denudation placers have a small thicknesses or vertical dimension, and so they are essentially condensed sections. To be economic, they must have a high value mineral, a large surface area, a long linear dimension, or exceptional grade, and preferably several of these features. Accumulation placer formation does not involve later partial rework and selective grain removal.
Concentration grade depends on maximum availability of a valuable mineral and minimal availability of gangue grains capable of being carried with, and deposited from, a given fluid flow condition. Such placer deposits tend to be lower grade compared to denudation placers because the placer mineral flux does not focus on a single two-dimensional surface. Instead repeated favourable flow energy episodes superimpose placer grain enriched-sediment in situations of accumulation with minimal scour. Their large volume makes them economically valuable.
© 2012 Institute of Materials, Minerals and Mining and The AusIMM.