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Metasomatism













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METASOMATISM (Gr. Aera, change, QW / .(a, body), in petrology, a process of alteration of rocks by which their chemical composition is modified, new substances being introduced while those originally present are partly or wholly removed in solution. For example a limestone may be converted into a siliceous chert, a dolomite, an ironstone, or a mass of metalliferous ores by metasomatic alteration. The process is usually incomplete, greater or smaller portions of the original rock remaining. The agencies of metasomatism are in nearly all cases aqueous solutions; probably they were often at a high temperature, as metasomatic changes are especially liable to occur in the vicinity of igneous intrusions (laccolites, dikes and necks) where large quantities of water were given off by the volcanic magma at a time when it had solidified but was not yet cold. Metasomatism also usually goes on at some depth, so that we may readily believe that it is favoured by increase of pressure. On the other hand, there are many instances in which these processes cannot be shown to have taken place at temperatures or pressures above those which normally exist in the upper part of the earth's crust (e.g. dolomitization and silicification of many limestones). There are also cases of metasomatism in which steam and other vapours are supposed to have been operative; the temperatures were probably above the critical temperature of water. Changes of this sort are described as pneumatolytic, being induced by gases (see Pneumatolysis) .

By metasomatism new minerals replace the primitive ones; at the same time the original rock-structures may be completely obliterated. An igneous rock for example may be entirely replaced by crystalline massive quartz, a fossiliferous limestone by granular crystalline dolomite. It is equally common, however, to find that the structure of the original rock is preserved though its substance has been entirely altered. An oolitic limestone may become an oolitic ironstone or chert (see Petrology, Pl. IV. fig. 5.) and casts of the fossils which the limestone contained may be formed of siderite or of chalcedony. In this case the rock resembles a pseudomorph, which is the term applied to a mineral which has been entirely replaced by another mineral without losing its original crystalline form. As a result of metasomatism rocks usually become more crystalline, especially those which have been in large part built up of fossil organic remains; this is a consequence of the new substances having been deposited by purely inorganic processes from solution in water.

The chemical change is often complete, as when a limestone is replaced by chert or otherwise silicified, but it is probably more usually incomplete, part of the substance of the original rock having been retained though possibly in new mineral combinations. When a limestone is replaced by ironstone (e.g. carbonate of iron or siderite) part at least of the carbonic acid may be that of the limestone. A dolomite, formed from a limestone, contains more than one-half of its weight of carbonate of lime presumably derived from the limestone itself; yet in this case the mineral transformation may be perfect, as the dolomite retains none of the calcite of which the limestone was formed; it is all present as the double carbonate of lime and magnesia (or dolomite). When a granite is converted by emanations containing fluorine and boron into a quartz-tourmaline rock (schorl rock, q.v.) or a quartz mica rock (greisen, q.v.) it can be proved by analysis that there has been very little modification of the chemical composition of the original mass. This resembles paramorphism in minerals, in which one mineral is substituted for another having the same chemical composition (e.g. kyanite for andalusite).

The relations between metamorphism and metasomatism are ", very close; in fact some authors regard metasomatism as a variety of metamorphism. It is generally true, however, that in metamorphic changes there is little chemical alteration; sandstones pass into quartzites, clays into mica-schists and gneisses, limestones into marbles without any essential modification in chemical composition, for the original minerals new ones being substituted and new structures being produced at the same time. In metasomatism, on the other hand, chemical alteration is supposed by most geologists to be an essential feature; new minerals appear, but the original structures are sometimes retained.

The facility with which a rock undergoes metasomatism depends partly on its nature, and partly on the circumstances in which it is placed. Limestones, being readily soluble under natural conditions, are especially liable. The Cleveland iron ores of Yorkshire are limestones replaced by siderite and limonite; the Whitehaven iron ores are metasomatic replacements of limestone by haematite. The former are of Mesozoic, the latter of Palaeozoic age, but both have been changed in very much the same way by percolating solutions containing salts of iron. In some cases limonite and magnetite are the principal ores. Often the changes have taken place very irregularly, along bedding planes, faults and fractures. An ironstone may in many places be traced laterally into a limestone, the amount of iron in the rock gradually diminishing. Some ironstones (Carboniferous, Jurassic, &c.) retain the oolitic structures of the original limestone; others show corals, shells and other calcareous fossils replaced by iron ores. When beds of shale or sandstone are intercalated among the limestones they usually show little change, a fact which indicates that the ready solubility of the calcareous rocks was a dominating factor in determining the metasomatic deposits. It is believed that the Whitehaven iron ores may be derived from the ironstones of the Coal-Measures which once covered the limestone districts.

Dolomitization of limestones is even more common than replacement by iron ores. That it is going on at the present day is evident from the fact that cores obtained by boring in recent coral reefs have shown that these may be extensively dolomitized in their deeper parts, and the older limestones such as the Triassic of the Alps, the Carboniferous Limestone of England and the Cambrian Limestone of Scotland are sometimes converted into dolomite over wide areas. There has been an introduction of magnesia, with sometimes a little silica and iron; the rock recrystallizes owing to the formation of small rhombohedra of dolomite; it frequently becomes porous and full of drusy cavities, owing to the contraction in volume which takes place, and the fossils and other organic structures of the original rock disappear. The change proceeds outwards from fissures and bedding planes and spreads gradually through the mass of the limestone; often the transformation is complete and no unaltered rock remains. Silicification or the replacement of limestone by chalcedony, chert or quartz, is often exhibited on a large scale. The formation of flint nodules and chert bands is of this nature; the silica is not really an introduction from without, but is merely the material of the fine siliceous skeletons of sponges, radiolaria and other organisms, which at first were widely scattered through the limestone and at a later time were dissolved by percolating waters, percolated through the rock and were deposited in certain situations as bands, nodules and tabular masses of cryptocrystalline silica. In limestones extensive deposits of zinc ore may occur, usually calamine. These are important sources of the metal and there is little room for doubt that they have formed by a process of metasomatic replacement, e.g. Carthagena, Raibl (in Carinthia) and Belgium. In many parts of western North America (Nevada, Arizona, &c.) great deposits of copper, lead and silver ores are worked in crystalline limestones. They are often highly silicified, and associated with them are intrusive igneous rocks such as granite, dacite, porphyry and diabase. The ores occur not only in veins and shoots, but also in great masses replacing the limestone, and the geologists who have examined these mining districts are nearly unanimous as to the metasomatic nature of a large part of these deposits. Other rocks such as tuff, volcanic breccia, shale, porphyry and granite may also be impregnated with metalliferous ores, but the largest ore bodies are found in the limestones. Secondary enrichment has often taken place on a considerable scale. The constant presence of igneous rocks indicates that they are connected with the introduction of the metals, and the deposits are often of such a kind as to show that post-volcanic discharges of magmatic gases and water have been the actual mineralizing agents. Bisbee, Clifton and the Globe district in Arizona, Flagstaff in Utah, and the Eureka district in Nevada are good examples of the deposits in question.

As indicated above, shales, sandstones and igneous rocks may be silicified and mineralized under suitable conditions. Rhyolites and rhyolitic tuffs are often impregnated with silica to such an extent that they become almost massive quartz, and the fluidal, porphyritic, spheroidal and other igneous structures of the original rock may be retained in the siliceous pseudomorph. There are many examples of this in North Wales and the Pentland Hills. In andesites, serpentines and trachytes silicification is frequently found in circumstances indicating that the changes are not due to weathering but are the effect of post-volcanic emanations of heated waters. Silicified shales may accompany mineral deposits, e.g. in the Cornish tin mines the killas or grey slate may be converted into quartz and brown tourmaline and contains small quantities of tin stone. In the copper mines of Parys Mountain, Anglesey, formerly of great importance as producers of this metal, there are large areas of silicified slate and silicified porphyry. White mica, kaolin, gilbertite, chlorite and epidote are frequently present in silicified igneous rocks. As a further instance of mineral deposition in metasomatized igneous rocks we may quote the Cripple Creek goldfield in Colorado, where syenites, latites, phonolites, breccias, &c., have been filled with pyrite, dolomite, fluorite, calaverite and other new minerals together with quartz.

Another type of metasomatic alteration is phosphatization. This is most common in limestones, and many of the most important bedded phosphate deposits are of this origin. Trachytes and other igneous rocks are occasionally phosphatized. The source of the phosphate is for the frost part the skeletons of animals, vertebrate bones and teeth, shells of certain brachiopods, trilobites and other organisms. Guano, the excreta of birds, is rich in phosphates and these are washed downwards by rain producing metasomatic changes in the underlying rocks. Phosphatized limestones are obtained in great quantities in Christmas Island, Sombrero, Curacoa and other uninhabited limestone islands. (J. S. F.)



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