An experimental study of heavy-mineral segregation under alluvial-flow conditions

Brady, Lawrence L.; Jobson, Harvey E.

doi:10.3133/pp562k

Cited by 21 publications

(26 citation statements)

References 2 publications

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“…Sediment transport rates demonstrated that the bed erosion resistance increased when the amounts of heavy particles were increased. This is consistent with the findings of Brady & Jobson (), Li & Komar () and (Komar, ), which produced different threshold curves for various heavy mineral‐treated bed compositions similar to those of Shields (). However, a change in the overall erodibility of the entire bed leading to higher bed stability with increasing heavy mineral content, as shown by the model, could not be obtained in these prior studies.…”

Section: Discussionmentioning

confidence: 99%

Understanding heavy mineral enrichment using a three‐dimensional numerical model

2017

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Layered deposits of relatively light and heavy minerals can be found in many aquatic environments. Quantification of the physical processes which lead to the fine‐scale layering of these deposits is often limited with flumes or in situ field experiments. Therefore, the following research questions were addressed: (i) how can selective grain entrainment be numerically simulated and quantified; (ii) how does a mixed bed turn into a fully layered bed; and (iii) is there any relation between heavy mineral content and bed stability? Herein, a three‐dimensional numerical model was used as an alternative measure to study the fine‐scale process of density segregation during transport. The three‐dimensional model simulates particle transport in water by combining a turbulence‐resolving large eddy simulation with a discrete element model prescribing the motion of individual grains. The granular bed of 0·004 m in height consisted of 200 000 spherical particles (D50 = 500 μm). Five suites of experiments were designed in which the concentration ratio of heavy (5000 kg m−3) to light particles (i.e. 2560 kg m−3) was increased from 6%, 15%, 35%, 60% to 80%. All beds were tested for 10 sec at a predefined flow speed of 0·3 m sec−1. Analysis of the particle behaviour in the interior of the beds showed that the lighter particles segregated from the heavy particles with increasing time. The latter accumulated at the bottom of the domain, forming a layer, whereas the lighter particles were transported over the layer forming sweeps. Particles below the heavy particle layer indicated that the layer was able to armour the particles below. Consequentially, enrichment of heavy minerals in a layer is controlled by the segregation of a heavy mineral fraction from the light counterpart, which enhances current understanding of heavy mineral placer formation.

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Section: Discussionmentioning

confidence: 99%

Understanding heavy mineral enrichment using a three‐dimensional numerical model

2017

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“…In contrast to the upper plane bed runs, during the Brady & Jobson () runs with dunes, the heavy mineral concentration in the load did not significantly decrease in time. The reason for this difference may be associated with another segregation effect.…”

Section: Background Information On Transport and Deposition Of Sedimementioning

confidence: 97%

Downstream lightening and upward heavying: Experiments with sediments differing in density

2015

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Sorting and selective transport of particles by material density is important for understanding a wide range of processes, including the formation of mineral placers, deposition of mine tailings and routing of tracers and contaminants. This article describes an experimental study of the transport of mixtures of particles of differing density in a sediment-feed flume. During the runs, a downstream prograding wedge-shaped deposit was formed. Results show two sorting processes: (i) longitudinal sorting characterized by preferential deposition of heavy particles in the upstream part of the deposit -downstream lightening; and (ii) vertical sorting with less dense particles preferentially deposited in the lowermost portion of the migrating frontupward heavying. Downstream lightening is the analogue of the well-known downstream fining observed in the more studied case of mixtures of heterogeneous size with the same density. In both cases, the lighter particles are carried further downstream than the heavier particles. Upward heavying is unexpected when compared with deposits of heterogeneous size and samedensity particles, where the heaviest (i.e. coarsest) particles are deposited in the lowermost part of the front. The physical mechanism underlying this upward heavying might be related to the physics of gravity-driven granular flows; the front of the deposit acts like a dense granular flow down an inclined plane. In this case, the denser particles settle away from the free surface and at the top of the heap, while the lighter particles flow to the bottom. As the front of the deposit advances, this progressively gives rise to an upward heavying pattern. The application of classical surface-based fractional bedload transport models suggests that equal mobility is not approached in the case of mixtures of particles with uniform size and different densities. This study hypothesizes that other mechanics related to the physics of the segregation processes in these systems contribute to these results.

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“…Flume experiments using Rio Grande river sands containing 0?38% magnetite (median diameter 144 mm) show that magnetite concentrates from transition and upper flow regimes in sands characterised by washed out current bedding and horizontal lamination (Brady and Jobson, 1973).…”

Section: River Accumulation Placersmentioning

confidence: 99%

“…As the storm wanes planar-laminated fine sand deposits first, this gives way to hummocky cross stratified fine sand which initially may be anisotropic, but as the unidirectional geostrophic flow component weakens this will become oscillatory eventually declining to two-dimensional ripples. Current velocities over 0?5 m s 21 give the flat bed condition favourable for higher density mineral concentration (Brady and Jobson, 1973). According to Plint (2010), each storm unit would be centimetres to decimetres thick with most of the deposition taking place with the initial flat bed and anisotropic hummocky bed structure.…”

Section: Subsiding Shelf 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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An experimental study of heavy-mineral segregation under alluvial-flow conditions

Cited by 21 publications

References 2 publications

Understanding heavy mineral enrichment using a three‐dimensional numerical model

Understanding heavy mineral enrichment using a three‐dimensional numerical model

Downstream lightening and upward heavying: Experiments with sediments differing in density

Ten placer deposit models from five sedimentary environments

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