· Metamorphic processes profoundly alter pre-existing mineral deposits and form new ones.
· The chief agents of metamorphism are heat, pressure, time, and various solutions.
· The materials acted upon are either earlier formed mineral deposits or ordinary rocks.
· Valuable nonmetallic mineral deposits are formed from rocks mainly through recrystallization and/or recombination of rock making minerals.
Role of Temperature and Pressure
· Metamorphic processes occur to make adjustments between the chemical potential of any system and the changes in temperature and pressure.
· A particular chemical reaction that cannot occur in one environment may readily do so under different temperature and pressure conditions.
An increase in
pressure will cause a reaction to move in a direction in which the total
volume of the system decreases, for example increasing pressure results in
the following changes with a reduction in the total molar volume:
Olivine + anorthite à garnet
augite +anorthite à garnet + quartz
ilmenite + anorthite à sphene + hornblende
nephelene + albite à glaucophane or jadeite
anorthite + gehlenite + wollastonite à grossularite
andalusite à sillimanite à kyanite
· An increase in temperature normally results in endothermic reactions. A possible example is the conversion of pyroxene to hornblende during the metamorphism of diabase to amphibolite.
· In short, metamorphic reactions result from the tendency of mineral systems to adjust to their physicochemical environment of high temperatures and pressures in contrast to the low temperatures of weathering processes, both of which processes generally occur in the presence of water.
Metamorphism of Earlier Deposits
· When rocks are metamorphosed, enclosed mineral deposits may also be metamorphosed.
· Unlike rocks that undergo both textual and mineralogical changes, Ores undergo less mineral re-combinations.
· Textual changes, however, are pronounced. Schistose or gneissic textures are induced, particularly with sectile minerals, and flow structure is not uncommon.
· Galena, for example, becomes gneissic. It may also be rendered so fine grained that individual cleavage surfaces cannot be discerned with a hand lens.
· It "flows" around hard minerals, such as pyrite. Other minerals, such as chalcopyrite, bornite, covellite, or stibnite, behave similarly.
· The result is that ores may exhibit streaked, banded, smeared appearances with indistinct boundaries between minerals of different color.
· The original textures and structures may be so obscured that it is difficult to determine to which class the originally deposits belonged. Such deposits are then classified as "metamorphosed".
Formation of Mineral Deposits by Metamorphism
· Several kinds of nonmetallic mineral deposits are formed as a result of regional metamorphism.
· The source materials are rock constitutions that have undergone recrystallization or re-combination, or both.
· Rarely, water or carbon dioxide has been added, but other new constitutions are not introduced as they are in contact metasomatism deposits.
· The enclosing rocks are wholly or in part metamorphosed; it is the rock metamorphism that has given rise to the deposits.
The chief deposits
thus formed are asbestos, graphite, talc, soapstone, andalusite-kyanite-sillimanite,
dumortieritea, garnet, and possibly some emery.
a) Asbestos forms by the metamorphism (hydration) of ultrabasic igneous rocks – peridotites and dunites.
b) Graphite forms by regional metamorphism of organic matter, crystallization from igneous rocks, contact metamorphism and hydrothermal solutions.
c) Talc, soapstone and pyrophyllite form by a mild hydrothermal metamorphism of magnesian minerals eg tremolite, actinolite, olivine, epidote and mica. Talc also occurs in regionally metamorphosed limestones, altered ultrabasic igneous rocks, and contact metamorphic zones.
d) Andalusite-kyanite-sillimanite – these minerals are high grade refractories. Kyanite is formed by the dynamothermal metamorphism of aluminous silicate minerals. Andalusite is formed by the pneumatolytic action on aluminous silicates. Sillimanite results from high temperature metamorphism of aluminous crystalline rocks.
e) Garnet forms during the regional and contact metamorphism and is consequently found in schists and gneisses. It is also found as a constituent of igneous rocks.
f) Emery is a mixture of corundum and magnetite with hematite or spinel and is a product of contact metamorphism.
Bedded manganese deposits in parts of Madhya Pradesh and Maharashtra, primarily of sedimentary origin, have been subsequently effected by metamorphism. Braunite, a manganese silicate, is the important ore mineral besides several other oxide minerals.
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Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)