Deposits related to climate

It has already been stated that the formation of hydrocarbon and coal deposits is dependent upon both climate and special conditions of sedimentation. There are certain deposits, however, which appear to be related solely to climate. Since climate is largely dependent upon latitude, north-south plate motion can be considered as controlling the formation of such deposits. They include laterites and evaporites.

The most important lateritic deposit is nickel laterite, which results from the extreme weathering of the ultramafic parts of ophiolite bodies under tropical conditions. The original nickel content of fresh peridotite becomes enriched by a factor of about seven under the influence of such weathering by percolating ground water. These deposits are becoming increasingly important sources of nickel, and are exploited in the southwestern Pacific and the northern Caribbean.

A similar deposit is bauxite, a residual deposit enriched in aluminum hydroxide, which provides the vast majority of the world's aluminum. This forms by the in situ weathering of aluminosilicate minerals on stable peneplaned topography in a wet tropical climate by the intense leaching of alkalis and silica. Bauxite only forms within 30° of the equator and requires high rainfall and high ambient temperatures. It is mined in Jamaica, northern Australia, and China.

Evaporites form in an arid climate by the evaporation of seawater in semi-isolated basins which receive periodic marine influxes. They cannot develop by the evaporation of a single isolated body of water, as this could not explain the vast observed thicknesses of evaporite deposits. The sequence of minerals precipitated is calcium carbonate and sulfate, sodium chloride, and finally magnesium or potassium minerals. Evapo-rites are important commercially in the chemical industry, particularly for the potash salts. They are also important in the generation of hydrocarbon traps because, being of low density, they mobilize after burial and rise through the sedimentary layers. Such haloki-nesis provides fault traps along the sides of rising salt masses and anticlinal traps in the layers above the masses which are folded during the ascent. This is an important process in the North Sea and Gulf of México, for example, which are underlain by salt deposits of Permian age.

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