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Journal of the Geological Society

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Role of settling, entrainment and dispersive equivalence and of interstice trapping in placer formation

Departments of Geography and Geology, Birkbeck College, University of London, Malet Street, London WClE 7HX, UK

Grain-scale placer mineral concentrating processes are reviewed. Early explanations of localized enrichment of heavy particles in terms of sorting associated with the settling equivalence of clasts of differing density have been superseded. Entrainment equivalence is shown to be a more appropriate concentrating factor in both fluvial and littoral sedimentary environments. The vertical shuffling of particles of different density that is associated with dispersive equivalence is an ancillary process that presents appropriate particles for entrainment sorting. However, the role of entrapment of comparatively small grains in the interstices of the stream bed or the beach face is shown to be more important that hitherto envisaged. This depositional enrichment is controlled by the relative sizes of the bed sediment pores and the ingressing particles. Local vertical changes in framework particle size dictate patterns of interstice clogging and hence the quantity of placer mineral rich matrix passing into the bed.

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