The term mineral deposit has been explained in the opening page of this chapter. A mineral deposit is simply a deposit of minerals, not necessarily commercially workable; the viability will vary with the price which fluctuates with time. Thus a mineral deposit, considered unprofitable at one time, may be economically ex tractable from a mine with increase in its market price.
Magma is the original source of most of the minerals. The magma which is chemically very reactive due to its pressure, temperature and composition of various minerals dissolves adjacent rocks through which it travels, giving rise to new minerals. The constituent minerals, mostly rock-forming silicates and oxides are deposited at various stages as the magma cools down during its passage. Minerals having nearly similar fusion points segregate and concentrate together resulting in magmatic segregation. Important deposits of metallic oxides such as magnetite and ilmentite, and sulphides such as pyrrhotite and chalcopyrite are formed in this way. Magmatic segregation may take place at different depths during the travel of the magma and at different temperatures. Most of the ferromagnesium silicates and other oxides are formed at depth by magmatic segregation.
After deposition of minerals by the process of magmatic segregation the magma is fluid and has a concentration of volatile constituents, i.e. various gases and vapours. Cooling of the liquid portion results in the formation of pegmatites. Such pegmatites often contain a concentration of minerals which occur only as accessory minerals in the main body of the igneous rock. The fluid magma that forms pegmatites intrudes in the pre-existing rocks forming dykes and veins. Economic deposits of minerals like felspar, quartz, mica, beryl and apatite are formed in this way e.g. in Giridih, Hazaribag (Bihar), Bhilwara (Rajasthan), etc.
The magmatic segregation and formation of pegmatites leaves the residual magma very fluid and it contains heated gases of great chemical activity. These gases penetrate the adjacent country rock and by their reaction with the latter form mineral deposits. Such deposits are known as pneumatolytic ore deposits. Examples are cassiterite deposits.
During the final stage of consolidation of magma, its aqueous solutions which consist of heated waters of great chemical activity deposit their mineral load. These aqueous solutions, because of their fluidity, are capable of travelling long distances from their parent source. The ore deposits formed by such aqueous but highly fluid solutions of magma are known as hydrothermal ore deposits. The term also covers deposits formed by descending surface waters which sometimes leach away valuable constituents of existing rocks and precipitate their load of minerals in the cracks, fissures and cavities in the earth's crust.
Surface waters passing down into the fissures and cracks in the earth sometimes carry minerals in solution or suspension derived during their passage over a variety of rocks. The heat beneath the earth's surface renders such descending circulating waters chemically active and sometimes the minerals of prexisting rocks are replaced, partially or completely, by minerals of the circulating waters, particle by patricle. The structure of the pre-existing rock may remain unaltered. The ore deposits so formed are called metasomatic ore deposits. The term metasomatism includes the alterations arising in rocks by the passage through them of heated waters from igneous sources. Some deposits of chlorite, serpentine, and chalcopyrite have been formed in this way.
The process of metamorphism which results in the formation of metamorphic rocks may generate enough heat and pressure to alter existing mineral deposits of impure or low-grade ores into comparatively more pure and valuable minerals. Some banded hematite formations have changed to banded magnetite-quartzite rocks in Salem and Tiruchirapalli districts by metamorphism, Another example of heat changing pre-existing mineral into a more pure mineral is offered by the conversion of bituminous coal into anthracite in the vicinity of dykes and sills in some cases, Sillimanite (Al2O3, SiO2) in Assam and eastern Maharashtra (Bhandara district) and kyanite (Al2O3, SiO2) are formed by metamorphism. Talc, hydrated magnesium silicate, is also a product of metamorphism of magnesium bearing rocks like dolomite, e.g. near Jaipur in Rajasthan.
Some mineral depostis are of sedimentary origin and the deposits of sediment may be formed organically as in the case of coal deposits, or chemically, as in the case of some limestone or chalk deposits. Such deposits are always bedded and stratified.
Alluvial, detrital or placer deposits are formed by breaking up of the parent rock and subsequent transportation of the mineral particles by stream or wave action. The minerals are found in sizeable concentration where the velocity, and hence the carrying power of the currents, is decreased. In such deposits the minerals are concentrated into fractions according to their specific gravities and two or more minerals of similar sp. gr. may be found together. Examples of such placer deposits are gold placers, with the gold being associated magnetite, chromite, etc. Alluvial, gem deposits, platinum, tin and wolfram are some other examples of alluvial or placer deposits.
Laterite deposits are formed by the leaching away of soluble minerals leaving behind in the laterite a valuable ore such as nickel or bauxite. Thus these are normally surface deposits.
Ore deposits which outcrop at the surface undergo weathering in the outcrop zone and may decompose. The weathered upper part of the deposits is known as gossan. The gossan is usally an oxidised zone which may sometimes change into carbonates. Thus a vein of galena at depth may consist of cerussite (PbCO3) in the gossan. Copper sulphide of chalcopyrite (Cu2S, Fe2S3,) which may occur in a vein at depth changes into malachite, CuCO3, Cu(OH)2 in the gossan, e.g. at Khetri in Rajasthan. Concentration of minerals takes place in the gossan as the lighter or less stable minerals arc washed away by percolating waters during weathering. Rich mineral deposits of economic value, therefore, occur as a cap over low-grade ore.
Evaporation of water from solutions containing minerals is a familiar example of one process of mineral formation, e.g. common salt produced by evaporation of enclosed sea water.
Formation of the mineral coal is explained in the next chapter.
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