Residual mineral deposits
Residual mineral deposits are natural accumulations of valuable minerals formed through a process of weathering that selectively removes unwanted constituents from rock. This process, known as residual concentration, leaves behind a concentrated residue of minerals that are insoluble or altered into insoluble forms, allowing these deposits to accumulate over time. Common minerals found in residual deposits include iron, aluminum (notably in the form of bauxite), manganese, nickel, phosphate, and various clays. These deposits typically form in regions with humid, tropical, or subtropical climates, where chemical weathering is enhanced and physical erosion is minimized.
The formation of residual deposits is usually a slow process, often taking millions of years to yield commercially significant concentrations. Notably, laterite is a specific type of residual deposit found in tropical regions, characterized by high concentrations of iron and aluminum oxides. Residual deposits can vary widely in their composition and geographical distribution, with significant occurrences in countries like Brazil, India, and the United States. The diverse environmental conditions conducive to their formation highlight the intricate relationship between geological processes and climate.
Residual mineral deposits
Residual mineral deposits are formed by chemical weathering processes that dissolve and remove undesired constituents of rocks, leaving behind valuable deposits of insoluble minerals. Examples include bauxite and residual iron, manganese, nickel, phosphate, and clays.
Background
Residual mineral deposits are the result of residual concentration, a process whereby weathering removes undesired constituents from rock to leave behind a concentration of valuable minerals. This residue, which is able to withstand further chemical weathering, can accumulate to form commercially significant deposits. Important deposits of iron, aluminum, manganese, nickel, phosphate, clays, and other economic minerals have been formed by residual concentration.
Formation
Residual mineral deposits form from rocks that contain valuable minerals that are either insoluble or alter to form insoluble compounds upon weathering; the undesired components of the rock are relatively soluble. As the rocks undergo chemical weathering, the unwanted materials are gradually dissolved and carried away. If the outcrop surface has low relief (so that physical weathering processes cannot remove significant amounts of the insoluble residues) and if the terrain remains stable over a period of time long enough to allow the residues to accumulate, a residual deposit can form. Because chemical weathering is a slow process and the materials being removed (such as calcite, feldspar, clay, and quartz) are often only slightly soluble, it may take millions of years for a residual accumulation to develop that is of sufficient purity and volume to be of commercial importance.
Warm, humid conditions are most conducive to the formation of residual deposits. While ores can develop through residual concentration in a temperate climate, tropical and subtropical environments host a greater variety of residual deposits.
Characteristics
Residual mineral deposits frequently occur in regions where the climate is or was humid, subtropical, or tropical. They form in relatively level depositional environments, where physical weathering processes exert a minimal influence. The presence of iron oxides concentrated in the weathering zone typically imparts a deep red or brown color to deposits formed by residual concentration. The removal of soluble material generally leaves the deposits with a porous texture or with a consistency resembling that of loose soil. Residual deposits are usually underlain by the rock from which they were derived.
A particular form of residual deposit, laterite, is characteristic of the hot, humid tropics. laterite is a highly weathered red soil or surface material that is rich in iron and aluminum oxides and hydroxides. A lateritic deposit forms after intense chemical weathering has leached the parent rock of most of its silica. Alternating wet and dry seasons, high drainage rates, and minimal physical erosion all contribute to laterite formation.
Residual Iron
Iron is present in most rocks, and residual iron deposits, including laterites, are widely distributed within nonglaciated regions. Residual concentration of iron is particularly likely where limestone or extremely iron-rich silicate rocks are exposed to warm, humid conditions. Significant residual-iron deposits include those found in the southeastern United States, Brazil, Venezuela, the West Indies, southern Europe, Africa, and India.
Residual Aluminum (Bauxite)
Bauxite, the chief ore of aluminum, is a lateritic deposit formed by residual concentration in tropical or subtropical regions. A mixture of several hydrated aluminum oxides, bauxite deposits result from the decomposition of aluminum silicate rocks that are high in aluminum silicates and low in iron and free quartz. Warm rain water, groundwater, oxygen, carbon dioxide, and humic acid interact to break down the parent rock. The French deposits at Baux, from which bauxite derives its name, formed from limestones or clays in limestones. In Arkansas, Brazil, and French Guiana, the source rock is nepheline syenite, an intrusive igneous rock composed largely of alkali feldspars and feldspathoids (minerals similar to feldspars but containing less silica). Deposits in India formed from basalt; those in Georgia, Alabama, Jamaica, and Guyana derived from clays; those in Ghana originated from clay shales and other aluminum-rich rocks; and those in Thailand derived from clay alluvium. Most bauxites were formed between the middle Cretaceous and middle Eocene times.
Residual Manganese, Nickel, Phosphate, and Clays
Residual deposits of manganese form under conditions similar to those that produce residual iron. Residual manganese deposits are commonly derived from crystalline schists, limestones previously enriched with manganese minerals, or primary deposits of manganese minerals. Important residual manganese deposits include those found in Brazil, Romania, Morocco, Egypt, Ghana, India, Japan, Malaysia, and the Philippines.
Under tropical and subtropical conditions, some low-silica igneous rocks decompose to produce hydrous silicates of nickel and magnesium. Nickel laterite derived from serpentinized peridotite is found in New Caledonia, Cuba, Brazil, and Venezuela. In Florida, the leading phosphate-producing state, residual concentrations of “land-pebble phosphate” occur. Weathering and solution of phosphate-containing Miocene limestones left behind this loose, easily mined residue of calcium phosphate pebbles and boulders.
In a humid, temperate climate, the chemical weathering of aluminum-bearing rocks produces clay. Weathering does not proceed far enough to remove silica and produce a laterite, as in tropical regions; instead, the aluminum and silica combine to form hydrous aluminum silicates—clay minerals. Crystalline rocks that contain abundant feldspars and little iron, such as granite and gneiss, are the primary source rocks for clay formation. High-grade residual clays occur in the southern and western United States, England, France, Germany, eastern Europe, and China.
Other Residual Deposits
Other products of residual concentration includetripoli, an earthy material composed almost entirely of silica and derived from weathered chert or silica-rich limestone; the residual kyanite deposits of India and the eastern United States; the residual barite deposits of Missouri; the nodular zinc ores of Virginia and Tennessee; and the residual gold accumulations in the United States, Brazil, Madagascar, and Australia.
Bibliography
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