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Journal of the Geological Society

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Beach orientation, bar morphology and the concentration of metalliferous placer deposits: a case study, Lake Turkana, N Kenya

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

The concentration of metalliferous particles in migrating swash bars is shown to depend upon processes controlled by the cross-sectional shape of the bar. A sand-spit provides two shorelines, each with a different pattern of wind-waves and each characterized consistently by bars of different shape. Where the wave period is low the bar is dune-like and its crest is set asymmetrically landward of the centre line. Where the wave period is high the bar is tabular, with a crest set just at the swash limit and on the lakeward side of the centre line. These differences in shape are important to the concentrating process. It is shown that swash-dominated dune-like forms produce concentrations of metalliferous grains that average only 13%. The tabular bar form, on the other hand, effects a significantly higher concentration of 21% through a hydrodynamic sorting process that is governed by limited swash overspill onto the landward slope. Concentrations of 90% are reached at specific positions. Confirmation of the concentrating process is obtained from detailed microstratigraphic interpretation of the contemporary bar, while the ancient bars of abandoned shorelines add supporting evidence. Clear-cut differences in placer concentration allow several diagnostic parameters to be identified for reconnaissance exploration. These include shore-line orientation, wave pattern, and bar cross-sectional morphology which can be combined with the reconstruction of abandoned shorelines using remote sensing imagery to provide a resource evaluation.

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