Depositional Placer Accumulations in Coarse-Grained Alluvial Braided River Systems

Burton, Justin; Fralick, Philip

doi:10.2113/gsecongeo.98.5.985

Cited by 16 publications

(2 citation statements)

References 49 publications

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“…If a river channel is deep enough and if discrete and differing flow regimes both persist and vary with depth they will deposit vertically stacked facies defined by grain-size as gravel, sand, or clay beds. Flow regimes can produce concentrations based on specific gravity (Burton and Fralick, 2003). These natural river flow sorting mechanisms have counterparts in the technologies used to recover ilmenite and zircon sands; e.g.…”

Section: River Accumulation Placersmentioning

confidence: 99%

Ten placer deposit models from five sedimentary environments

Stanaway¹

2012

Applied Earth Science

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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.

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Section: River Accumulation Placersmentioning

confidence: 99%

Ten placer deposit models from five sedimentary environments

Stanaway¹

2012

Applied Earth Science

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“…This fluctuation allows for the liberation and erosion of diamonds during wetter periods and the concentration of diamonds in alluvial deposits during the drier periods. Burton and Fralick (2003) showed that long intervals of average flows interspersed with large-scale flooding events are the optimum conditions for the formation of longitudinal placer development. This scenario seems consistent with the depositional history of the CAR alluvial materials.…”

Section: Alluvial Diamond Depositsmentioning

confidence: 99%