Metasomatism

Metasomatism is produced by the circulation of aqueous solutions through rock undergoing metamorphic recrystallization. The solutions cause chemical losses and additions by dissolving and precipitating minerals along their flow paths. Metasomatic processes have produced the world’s major ore bodies of tin, tungsten, copper, and molybdenum, as well as smaller deposits of many other metals. Scientists believe that metasomatism even takes place on Mars, where it produces methane.

Processes of Chemical Change

Metasomatism is an inclusive term for processes that cause a change in the overall chemical composition of a rock during metamorphism. Such processes may be described as positive or negative depending upon whether a net gain or a loss is produced in the affected rock body. Where chemical changes are slight, the minerals in the original rock remain unchanged or register only very subtle changes. In contrast, intense metasomatism may result in total destruction of the original mineral assemblage and its replacement by new, and different, minerals. In these extreme cases, metasomatism is usually difficult to detect, particularly if large rock volumes are affected. Metasomatic effects are most obvious when the original rock texture and mineral assemblage is partially destroyed. In such cases, the resulting rock will exhibit an unusually large variety of minerals as well as microscopic evidence of incomplete chemical reactions.

In normal metamorphic reactions, chemical migration occurs on the scale of a single mineral grain (a few millimeters at most) during recrystallization. In contrast, metasomatism involves chemical transport on the scale of a few centimeters or more. In areas where metasomatism has been intense, it can often be demonstrated that the scale of chemical transport ranged from about 100 meters to as much as several kilometers. It is the movement of chemical components through rocks on this larger scale that distinguishes metasomatism from metamorphism.

Dehydration (water-releasing reaction) and decarbonation (carbon dioxide-releasing reaction) are the most common types of chemical reactions during metamorphism at the higher grades. These reactions certainly produce significant changes in rock composition and involve large-distance chemical transport, but because they typify normal prograde metamorphism, they do not constitute metasomatism. Conversely, if water and/or carbon dioxide were reintroduced into a rock that had previously experienced prograde metamorphism, this would constitute a fairly common type of metasomatism.

Diffusion and Infiltration Metasomatism

The reshuffling of the chemical components into new mineral assemblages during metamorphism, particularly when gaseous phases are lost, leads to major changes in rock volume (usually volume reductions) and a corresponding change in rock porosity and density. Metasomatic replacement of a metamorphic rock will induce additional changes in volume, porosity, and density. Although metasomatism is defined as a process of chemical change, physical changes in rock properties also occur, and these are an essential aspect of metasomatism. For convenience, introductory textbooks ignore volume changes in metamorphic and metasomatic reactions. This assumption may hold true in specific instances, but it has no general validity, particularly when volatile constituents are involved in the reactions. It has been demonstrated on theoretical grounds that chemical transport on the scale of a few centimeters or more requires the presence of an intergranular pore fluid that can effectively dissolve existing minerals and deposit others while the rock as a whole remains solid.

The relative mobility of this fluid phase leads to two theoretical types of metasomatism: diffusion metasomatism and infiltration metasomatism. In diffusion metasomatism, the chemical components move through a stationary aqueous pore fluid permeating the rock by the process of diffusion. The effects are limited to a distance of a few centimeters from the surface of contact between rocks of sharply contrasting composition. Because this process acts over small distances, it cannot produce large-scale metasomatic effects. Infiltration metasomatism involves a mobile aqueous fluid that circulates through pores and fractures of the enclosing rock and carries in it dissolved chemical components. This fluid actively dissolves some existing minerals and deposits new ones along its flow path. The scale of infiltration metasomatism is thus determined by the circulation pattern of the fluid, and rock compositions over distances of several kilometers can be easily altered. For convenience, the term “metasomatism” will be used in place of “infiltration metasomatism” and the effects of diffusion metasomatism will be ignored.

Conditions Favorable to Metasomatism

The degree of chemical change that accompanies recrystallization is closely related to the fluid/rock ratio prevailing during the process. Since regional metamorphism is a deep-seated process, a low fluid/rock ratio generally prevails, and the process is “rock-dominated” in the sense that minerals dominate the composition of the fluid circulating through the rocks. A significant degree of metasomatism under such conditions is unlikely unless the fluid is very corrosive. Small quantities of fluorine or chlorine can produce corrosive aqueous fluids, but these elements are rarely important in regional metamorphism. Contact metamorphism, however, occurs close to the earth’s surface, where large volumes of groundwater circulate in response to gravity. Groundwater will mix with any water given off by a high-level crystallizing pluton; it follows that contact metamorphism takes place under conditions of relatively high fluid/rock ratio (although the amount of fluid is always much less than the amount of solid rock). Such a process will be “fluid-dominated” in the sense that the circulating fluid will govern the compositions of the minerals formed by recrystallization. These conditions are highly favorable for metasomatism.

Intense, pervasive metasomatism will develop under the following conditions: when energy is available to provide temperature and pressure gradients to sustain fluid movement; when a generally high fluid/rock ratio prevails; when the aqueous fluid has the solvent capacity to dissolve minerals in its flow path; and when the enclosed rocks possess, or develop, sufficient permeability to permit fluid circulation. These conditions must be sustained for a sufficient time period, or recur with sufficient frequency, to produce metasomatism.

Greisenization and Fenitization

Such conditions are commonly attained in rocks adjacent to large bodies of intrusive igneous rock, or plutons, particularly those that expel large quantities of water. As an example of contact metasomatism, consider the effects produced in the fractured roof zone of a peraluminous, or S-type granite pluton (meaning “derived from the melting of sedimentary parent material”). Hot water vapor, concentrated below the roof by crystallization, is often enriched in corrosive fluorine accompanied by boron, lithium, arsenic, silicon, tungsten, and tin. This reactive fluid migrates up fractures, enlarging pathways by dissolving minerals and seeping into the adjacent rock. The result is a network of quartz-muscovite-topaz-fluorite replacement veins that may contain exploitable quantities of tin, tungsten, and base metal ore (copper, zinc, lead, and so on). Fluorine-dominated metasomatism is known as greisenization. Greisen effects may extend 5 to 10 meters into the wall rocks from vein margins. Within this zone, the original rock textures, minerals, and chemical composition will be profoundly modified by metasomatism. In many cases, the entire roof of the granite pluton is destroyed and replaced by greisen minerals. Some of the world’s major tin and tungsten deposits—such as those in Nigeria, Portugal, southwest England, Brazil, Malaysia, and Thailand—were formed by just such a process.

Spectacular sodium and potassium metasomatism develops adjacent to intrusions of ijolite and carbonatite plutons. Ijolite is an igneous rock extremely rich in sodium and potassium, and carbonatites are intrusions consisting of carbonate minerals rather than silicates. This intense alkali metasomatism is called fenitization and is developed on a regional scale in the vicinity of Lake Victoria in East Africa. In this region, ijolite-carbonatite complexes are plentiful, and aureoles, or ring-shaped zones, of fenite extend outward 1 to 3 kilometers from the individual plutons. The width of the fenite zone around a source pluton is largely determined by the fracture intensity in the surrounding rocks; where they are highly fractured, large-scale and even regional fenitization is present. Massive, unfractured rocks resist fenitization, and the resulting aureoles are narrow.

Metasomatism around ijolite intrusions is dominated by the outflow of sodium dissolved in an aqueous fluid expelled by the magma and an apparent back-flow migration of silicon to the source intrusion. A typical result would be an inner zone, 20 to 30 meters wide, of coarse-grained “syenite fenite” composed of aegirine, sodium feldspar, and sometimes nepheline. Beyond the outer limit of this zone, there would be a major aureole of shattered host rock veined by aegirine, sodium amphibole, albite, and orthoclase, which might extend well beyond 1 kilometer from the parent intrusion. Fenitization around carbonatite intrusions is dominated by potassium metasomatism, which converts the country rocks into a coarse-grained metasomatic rock composed almost exclusively of potassium feldspar called orthoclase. The resulting fenite aureole is typically less than 300 meters wide and is roughly proportional to the diameter of the parent carbonatite. The chemical composition of the country rocks appears to exert little influence on the progression of fenitization. The rocks adjacent to ijolite intrusions are driven toward a bulk chemical composition approaching that of ijolite, while those adjacent to carbonatite intrusions are driven toward orthoclase regardless of their initial composition. Not all fenites are associated with intrusions, although perhaps the source intrusions are deeper underground.

Metasomatic Mineral Zones

Intensely metasomatized rocks usually exhibit mineral zoning, which is more or less symmetrical around the passageways that controlled fluid migration. These passages may be networks of vein-filled fissures, major fault zones, shattered roof zones of igneous plutons, or the fractured country rocks adjacent to such plutons. Emphasis is placed on fracture permeability as a control of fluid migration, as most metamorphosed rocks have negligible porosity. The metasomatic mineral zones are often dominated by a single, coarse-grained mineral species, which is obviously “exotic” with respect to the original mineral assemblage. Inner zones often cut sharply across outer zones, and the resulting pattern reflects a systematic increase in metasomatic intensity as the controlling structure is approached. By means of foot traverses across the metasomatized terrain, geologists carefully map the mineral zoning pattern and its controlling fractures. Such maps provide insight into the fracture history of the area, show the distribution and volume of the metasomatic products, and indicate the relative susceptibility of the various rock types present to the metasomatic process. If ore bodies are present, geologic maps provide essential information for exploring the subsurface extent of the ores through drilling. They also provide a basis for systematically sampling the metasomatic zones as well as the country rock beyond the limit of metasomatism; the unaltered country rock is called the protolith.

The sample collection provides material for study in the laboratory after field studies are complete. A paper-thin slice is cut from the center of each rock sample and is mounted on a glass slide for microscopic examination. From such examinations, the scientist can identify both fine- and coarse-grained minerals and can determine the order of metasomatic replacement of protolith minerals.

Study and Sampling of Protoliths

The objectives in a study of metasomatism are to determine the chemical changes that have taken place in the altered rocks and to reconstruct the history of fluid-rock interaction. To this end, it is essential to determine the chemical compositions of the various protoliths affected by metasomatism so that additions and losses in the metasomatized rocks may be calculated. The ideal protolith is a rock unit that is both chemically uniform over distances comparable to the scale of zoning and highly susceptible to metasomatism. Considerable attention must, therefore, be devoted to the study and sampling of protoliths during the field stage of a project. Ideally, the samples should be collected just beyond the outer limit of metasomatism in order to avoid metasomatic contamination and to minimize the effect of a lack of protolith uniformity. Unfortunately, it is only possible to approximate the position of this outer limit in the field, because the decreasing metasomatic effects merge imperceptibly with the properties of the unaltered protolith. Sampling problems are further compounded in study areas where rock exposures are poor or where exposed rocks are deeply weathered. Rock weathering promotes chemical changes that are at times indistinguishable from metasomatism, so weathered samples cannot be accepted for chemical study. The sample requirements for metasomatic research are more stringent than for any other type of geological study, and meeting them always taxes the ingenuity of a geologist.

The samples, having been cut in half for the microscope slides, are then prepared for chemical analysis and for density measurement. One half is stored “as is” in a reference collection for future use. The other half is cleaned of all traces of surface weathering and plant material. The samples are then oven-dried, and bulk densities are obtained by weighing and coating them with molten paraffin (to seal pore spaces), followed by immersion in water to determine their displacement volumes. Next, the samples are crushed and ground to fine powder. The average mineral grain density is determined by weighing a small amount of rock powder and measuring its displacement volume in water. The porosity of each sample is determined, and the powdered samples (20-30 grams each) are then sent to a laboratory specializing in quantitative chemical analysis. Because it is desirable to study as many as sixty-five different chemical elements, the laboratory will use several modern instrumental techniques as well as the traditional “wet method” to determine their concentrations.

Modern studies use the mass balance approach, which relates the physical, volumetric, and chemical properties between the altered rocks and their protoliths. As an example, consider a particular element in a given volume of protolith. The mass of this element is the product of volume × bulk density × element concentration. If the element is totally insoluble in the circulating fluid, then its mass must remain constant during the metasomatism of the enclosing rock. It follows that for this immobile element, the product of volume × bulk density × element concentration in the altered rock must equal that of the parent protolith. This provides an objective test for determining the exact elements that were mobile and immobile during the metasomatic event. From this point, it is a simple matter to calculate the gains and losses of each element for the altered rocks.

Skarn Deposits

Metasomatic deposits of a wide range of metals and industrial minerals are commonly found at or near the contact between igneous plutons and preexisting sedimentary rocks. Ore deposits concentrated by contact metasomatic processes are collectively known as skarn deposits. Skarn deposits are major sources of tungsten, tin, copper, and molybdenum. Important quantities of iron, zinc, cobalt, gold, silver, lead, bismuth, beryllium, and boron are also mined from skarn deposits. Additionally, such deposits are a source of the industrial minerals fluorite, graphite, magnetite, asbestos, and talc. For the most part, skarn deposits are found in relatively young mobile belts that are not yet deeply eroded. The most productive skarns are generally those in which granitic magma has invaded sedimentary sequences dominated by layers of carbonate rocks.

The physical and chemical principles of metasomatism have been deduced by research geologists over many decades from thousands of individual field and laboratory studies. The knowledge derived from the studies is put to practical use by economic geologists who explore remote areas in search of new skarn deposits or who exploit known skarn ores at producing mines. These economic geologists test, on a daily working basis, the theoretical hypotheses and generalizations formulated by research geologists regarding metasomatism.

Principal Terms

aqueous solution, hydrothermal fluid, and intergranular fluid: synonymous terms for fluid mixtures that are hot and have a high solvent capacity, permitting them to dissolve and transport chemical constituents; they become saturated upon cooling and may precipitate metasomatic minerals

contact metasomatism: metasomatism in proximity to a large body of intrusive igneous rock, or pluton

density: the ratio of rock mass to total rock volume; usually measured in grams per cubic centimeter

diffusion: process whereby atoms move individually through a material

meteoric water: water that originally came from the atmosphere, perhaps in the form of rain or snow, as contrasted with water that has escaped from magma

permeability: the capacity to transmit fluid through pore spaces or along fractures; high-fracture permeability is generally requisite for metasomatism, as porosity is greatly reduced by metamorphic recrystallization

porosity: the ratio of pore volume to total rock volume; usually reported as a percentage

prograde metamorphism: recrystallization of solid rock masses induced by rising temperature; differs from metasomatism in that bulk rock composition is unchanged except for expelled fluids

recrystallization: a solid-state chemical reaction that eliminates unstable minerals in a rock and forms new stable minerals; the major process contributing to rock metamorphism

regional metasomatism: large-scale metasomatism related to regional metamorphism

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