FORMATION OF MINERAL DEPOSITS
FORMATION OF MINERAL DEPOSITS
The term mineral deposit has been explained in the opening page of this chapter. A mineral deposit is simply a deposit of minerals, * " I 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 extractable from a mine with increase in its market price. Magma isthe originalsource of most of the minerals. The magma which is chemically very reactive due to pressure, temperature and compostion 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 resuling in magmatic segregation. Important deposits of metallic oxides such as magnetite and limentite, and sulphides such as pyrrohtite 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, ' fost of the ferromagnesium silicates and other oxides are formed at depth magmatic segregation
Aft6r deP° s> tion ofrniiiZ^ rT ~ — U9 segregation the magma is fluid and has a proce« ^^ d constituents, t.e.various gases and vapoUr of voiatfle' results in the formation of pegmatites t S l i q u i d portion a concentration of minerals which occufonT^° ften « ain pegmatites introdes in the pre-existing rocks f " Economics deposits of minralslike felspar quan "T 8 ^ and v< * is. are formed in this way e.g. in Giridih ' ll ,; m,ca ' apute (Rajasthan), etc.
The magmatic segregation and formation of pegmatites leaves the residual magma very fuild and it contains heated gases of great chemical activitiy. 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 consists of heated waters of great chemical activity deposit their mineral load. These aqueous solutions, because of their fluidity, are capable of travelling long distancesfrom their parentsource. The ore deposits formed by such aqueous but highly fluid soluitons 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 orsuspension derived during their passageover a variety of rocks.The heat beneath the earth’ssurface renders such descending circulating waters chemically active and sometimes the minerals of pre-existing rocks are replaced, partially or completely, by minerals of the circulating waters, particle by particle. 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, seipentine, and chalcopyrite have been formed in this way.
The process of metamosphism which results in the formation 0 metamorphic rocks may generate enough heat and pressure to alter existing mineral deposits of impure orlow-grade oresinto comparatively more pure and valuable minerals. Some banded hematite formations have changed to banded magnetite-quarzite rocks in Salem and Tiruchirapalli districs by metamorphism. Another example of heat changing pre-existing mineral into a more pure mineral is offered by the conversion of bituminous coal into anthracit in the vicinity of dykes and sills in some cases. Sillimanite (A1203, Si02) in Assam and eastern Maharashtra (Bhadara district)and kyanite (Al203, Si02) 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 deposits 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 depositer are formed by breaking up of the parent rock and subsequent tranporation of the mineral particles by stream or wave action. The minerals are found in sizeable concentraion 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 worfram 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 the deposits is known as gossan. The gossan is usually an oxidised zone which may sometimes change into carbonates. Thus a vein of galena at depth may consist of cerussite (PbC03) in the gossan. Copper sulhipde of chalcopyrite (Cu2S, Fe2S3 ) which may occur in a vein at depth changes into malachite, CuC03, 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 are 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 soTT ~ a familiar example of one process of mineraH ^conta |ning minerals is salt produced by evaporation of enclosed sea ° ^ atlon ’ ^ common r< Formation of the mineral coal is explained in the next chapter
The term mineral deposit has been explained in the opening page of this chapter. A mineral deposit is simply a deposit of minerals, * " I 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 extractable from a mine with increase in its market price. Magma isthe originalsource of most of the minerals. The magma which is chemically very reactive due to pressure, temperature and compostion 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 resuling in magmatic segregation. Important deposits of metallic oxides such as magnetite and limentite, and sulphides such as pyrrohtite 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, ' fost of the ferromagnesium silicates and other oxides are formed at depth magmatic segregation
Aft6r deP° s> tion ofrniiiZ^ rT ~ — U9 segregation the magma is fluid and has a proce« ^^ d constituents, t.e.various gases and vapoUr of voiatfle' results in the formation of pegmatites t S l i q u i d portion a concentration of minerals which occufonT^° ften « ain pegmatites introdes in the pre-existing rocks f " Economics deposits of minralslike felspar quan "T 8 ^ and v< * is. are formed in this way e.g. in Giridih ' ll ,; m,ca ' apute (Rajasthan), etc.
The magmatic segregation and formation of pegmatites leaves the residual magma very fuild and it contains heated gases of great chemical activitiy. 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 consists of heated waters of great chemical activity deposit their mineral load. These aqueous solutions, because of their fluidity, are capable of travelling long distancesfrom their parentsource. The ore deposits formed by such aqueous but highly fluid soluitons 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 orsuspension derived during their passageover a variety of rocks.The heat beneath the earth’ssurface renders such descending circulating waters chemically active and sometimes the minerals of pre-existing rocks are replaced, partially or completely, by minerals of the circulating waters, particle by particle. 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, seipentine, and chalcopyrite have been formed in this way.
The process of metamosphism which results in the formation 0 metamorphic rocks may generate enough heat and pressure to alter existing mineral deposits of impure orlow-grade oresinto comparatively more pure and valuable minerals. Some banded hematite formations have changed to banded magnetite-quarzite rocks in Salem and Tiruchirapalli districs by metamorphism. Another example of heat changing pre-existing mineral into a more pure mineral is offered by the conversion of bituminous coal into anthracit in the vicinity of dykes and sills in some cases. Sillimanite (A1203, Si02) in Assam and eastern Maharashtra (Bhadara district)and kyanite (Al203, Si02) 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 deposits 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 depositer are formed by breaking up of the parent rock and subsequent tranporation of the mineral particles by stream or wave action. The minerals are found in sizeable concentraion 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 worfram 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 the deposits is known as gossan. The gossan is usually an oxidised zone which may sometimes change into carbonates. Thus a vein of galena at depth may consist of cerussite (PbC03) in the gossan. Copper sulhipde of chalcopyrite (Cu2S, Fe2S3 ) which may occur in a vein at depth changes into malachite, CuC03, 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 are 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 soTT ~ a familiar example of one process of mineraH ^conta |ning minerals is salt produced by evaporation of enclosed sea ° ^ atlon ’ ^ common r< Formation of the mineral coal is explained in the next chapter
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