Residual placer deposit

Synonym

Eluvial placer deposit

Required Geological Setting

Neogene and Quaternary sediments

Occurrence

Residual deposits formed by removal of dissolved components of host rocks by chemical weathering. Alkali metals and alkali earth metals such as Na, K, Ca, and sulphur are removed by rainwater as the result of oxidation and dissolution reactions. Aluminosilicates are removed as colloids in rainwater. Consequently, more insoluble and higher density metal components such as quartz, aluminum oxides, iron oxides, and titanium oxides are concentrated in weathered soils. These reactions are enhanced by the humid and high temperature conditions, and residual placer deposits are more oberved in tropical regions. The oldest residual placer deposits are reported in the Precambrian strata, and many residual placer deposits had been formed during Neogene to middle Pleistocene period as average temperature of the earth had been higher. Residual placer deposits has been less formed since middle Pleistocene as average temperature of the earth has been lower. Concentrating elements depend on rock types of hosts; Aluminum and iron are concentrated from granitoid rocks, and called as laterite. Aluminum-concetrated laterite is mined under the name of bauxite. Iron is concentrated from mafic igneous rocks. Nickel is concentrated from ultramafic rocks, and called as nickel-laterite. Gold and cinnabar are concentrated in weathered soils of gold and mercury deposits, respectively.

Mineral Assemblages (Laterite)

Goethite, Gibbsite, Diaspore

Mineral Assemblages (Nickel-laterite)

Garnierite