Fluvial placer deposits form when mechanical sorting causes concentration of heavy minerals of economic interest. The heavy minerals must be chemically stable to survive so placers usually consist of native metals such as gold (SG 15–19) and the platinum group (14–21); resistate minerals, for example, cassiterite (∼7), chromite (4–5), ilmenite (4.5–5), rutile (4.2), magnetite (5.2), zircon (4.7); and, diamonds (3.5) and other gemstones. Placer deposits can be of considerable economic importance—for example, the tin (cassiterite) deposits of South East Asia (Taylor, 1986) and the gold paleoplacers of the Witwatersrand (Smith and Minter, 1980).
Small accumulations of heavy minerals can form as planar laminae (Ljunggren and Sundborg, 1968); thin ripples on the upstream slopes of dunes (Brady and Jobson, 1973); or develop on topsets and foresets of dunes (Brady and Jobson, 1973). At intermediate scales heavy minerals accumulate at bar heads (Hanson, 1979; Day and Fletcher, 1991); in zones...
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© 1978 Dowden, Hutchinson & Ross, Inc.
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Fletcher, W.K. (1978). Placers, fluvial. In: Middleton, G.V., Church, M.J., Coniglio, M., Hardie, L.A., Longstaffe, F.J. (eds) Encyclopedia of Sediments and Sedimentary Rocks. Encyclopedia of Earth Sciences Series. Springer, Dordrecht. https://doi.org/10.1007/978-1-4020-3609-5_159
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