Pegmatites

Pegmatites host the world’s major supply of the rare metals lithium, beryllium, rubidium, cesium, niobium, and tantalum. They are also major sources of tin, uranium, thorium, boron, rare-earth elements, and certain types of gems.

Simple Pegmatites

Pegmatites are relatively small rock bodies of igneous appearance that are easily distinguished from all other rock types by their enormous range of grain size and textural variations. Typically, fine-grained margins of aplite are abruptly succeeded by discontinuous interior layers of coarse, inward-projecting crystals, which, in turn, give way to zones of graphically intergrown quartz and alkali feldspar (that is, angular intergrowths, so called because they resemble a form of script). Most pegmatites contain numerous isolated pockets, rafts, and radial clusters of abnormally large, even giant, crystals and are cut by fractures filled with late-stage products. The spatial inhomogeneity of mineral distribution, rock texture, and chemical elements exhibited in pegmatites is unequaled in any other igneous product.

The majority of pegmatites are narrow lens-shaped, tabular, or pod-shaped masses measuring from a few meters to several tens of meters in length. Even large, commercially exploited pegmatites rarely exceed 1 kilometer in length and 50 meters in width. Nearly all pegmatites have a bulk composition approximating that of granite. They are composed predominantly of quartz, alkali feldspar, and muscovite, with minor quantities of tourmaline and garnet. Those containing only these minerals are termed simple pegmatites—“simple” referring to their mineralogy, chemistry, and internal structural features collectively.

Rare-Mineral Pegmatites

Complex pegmatites are composed of the same major mineral assemblage as simple pegmatites but, in addition, contain a great variety of exotic accessory minerals that host rare metals, such as lithium, rubidium, cesium, beryllium, niobium, tantalum, and tin. These pegmatites are also called rare-element or rare-mineral pegmatites, and they are of major economic and strategic importance. In contrast to simple types, they are complex in terms of mineralogy, chemistry, and internal structure. Beyond their economic value, rare-element pegmatites are of interest because they crystallize from the most evolved, or fractionated, granitic magmas in nature. They are the extreme product of extended crystal fractionation and therefore occupy a unique position between normal igneous rocks and hydrothermal vein deposits. The remainder of this article deals exclusively with this important class of pegmatites.

Major rare-mineral pegmatites may exhibit systematic internal variations in mineralogy and texture that are termed zoning. The usual zonal pattern begins with a thin selvage, or rim, of aplite, a few centimeters thick at most, which typically grades into the country rock. The aplite selvage is abruptly succeeded by a coarse-grained, muscovite-rich zone that, by decrease in muscovite abundance, passes into a zone dominated by quartz and feldspar that may also carry abundant spodumene, a lithium-bearing pyroxene. The innermost zone is composed mainly or exclusively of quartz. Ideally, the zones are crudely parallel structures symmetrically disposed with respect to the center line of the host pegmatite. In a real pegmatite, however, individual zones usually vary in terms of width, continuity, and position with respect to the center line of the body. The zonal pattern records the inward growth of a pegmatite from the enclosing walls of country rock. The bulk chemical composition of each individual zone is an approximate indication of the composition of the parental magma at the time the zone formed. It follows that the inward zone sequence is an approximate record of compositional changes that occurred in the evolving parent magma as the pegmatite formed. The existence of a common zonal pattern means that most pegmatite magmas evolve in a broadly similar fashion.

Composition of Rare-Mineral Pegmatites

In spite of the general similarity in zone pattern shown by rare-mineral pegmatites as a class, no two bodies are ever exactly alike in terms of the size, shape, and spatial distribution of their zones. Yet the mineral assemblages and textures that compose individual zones are surprisingly consistent. Excluding the selvage, most zoned pegmatites are composed of five distinct mineral assemblages: (1) coarse microcline (potassium feldspar) with or without rare-earth minerals; (2) quartz with subordinate lithium minerals; (3) massive quartz (virtually monomineralic bodies); (4) albite (sodium feldspar) or fluorine-rich mica bodies that contain abundant tourmaline, apatite, beryl and a wide variety of other minerals; and (5) mixed zones of sequential deposition (composed of very coarse-grained microcline and lithium minerals within a finer-grained matrix of albite).

Gross chemical inhomogeneity and remarkable concentrations of exotic minerals have earned zoned rare-mineral pegmatites a reputation for complex chemistry. In fact, the overall bulk composition of most such rock bodies is a rather simple one, dominated by the common elements oxygen, silicon, aluminum, sodium, and potassium, which compose the major pegmatite mineral phases of quartz, albite, microcline, and muscovite. Water and lithium oxide are next in importance, water forming generally 0.5 to 1.0 weight percent and lithium oxide being present in amounts up to roughly 1.5 weight percent. Oxides of phosphorus and iron usually constitute several tenths of a percent each. The elements calcium, magnesium, and manganese are present in trace amounts only. The total concentration of rare elements (beryllium, boron, cesium, rubidium, tantalum, niobium, tin, tungsten, uranium, thorium, fluorine, and rare-earths), for which pegmatites are famous, is somewhat less than 1 percent by weight in almost every case. This composition, except for the high lithium component, is essentially that of normal alkali granite. The lithium concentration in normal granites, however, is only 1.5 percent of that in typical lithium-bearing pegmatites; that is, pegmatites have about seventy times as much lithium as normal granite. The bulk composition of a pegmatite body is often assumed to be approximately that of its parent magma. That does not apply to the water content, because most pegmatite magmas exsolve water during crystallization, which then diffuses outward into the surrounding country rock.

Formation of Pegmatites

Experiments have firmly established that water, as a dissolved constituent in magma, lowers its melting temperature significantly. Water also lowers the viscosity (resistance to flow) of the residual melt, which is a major factor in promoting the growth of the large crystals that distinguish pegmatites from normal igneous rocks. Traditionally, pegmatites have been thought to be the products of complex interactions between water-saturated silicate melt of low viscosity, a separate coexisting aqueous fluid, and the enclosing rocks. Several important experiments, however, have shown that water has relatively little ability to dissolve certain major and trace elements concentrated in rare-mineral pegmatites. Also, fluid inclusion data from pegmatite minerals indicate that the trapped fluids are relatively dense and fall between silicate melt and simple aqueous solutions in terms of character. These data are important because they imply that pegmatite minerals crystallize from highly evolved silicate magma rather than whatever aqueous solution may be present. If that is true, it is likely that incompatible elements in the magma are responsible for the extreme concentration of rare metals in pegmatite magma.

Several lines of evidence suggest that boron, phosphorus, and fluorine play key roles in this regard. On account of their incompatibility with common silicate minerals, these elements are highly concentrated, along with water, in residual melt. High levels of these elements will have two major effects in silicate magmas. First, they will delay water saturation, or “boiling,” by increasing the solubility of water in the residual melt, and second, they will promote concentration of such elements as lithium, sodium, potassium, rubidium, and cesium in that melt. Thus, the result will be a water-rich, sodium-aluminosilicate-rich, late-stage melt, from which albite, tourmaline, phosphate minerals, fluorine-rich micas, beryl, zircon, and niobium-tantalum-tin oxides can crystallize prior to boiling. Such a melt would possess the required low viscosity and low melting temperature (below 500 degrees Celsius) required to enable pegmatite magma to migrate significant distances from the parent pluton prior to crystallization.

High concentrations of boron, phosphorus, and fluorine in the late-stage melt can produce the additional important effect known as immiscibility. At some critical concentration, the single parent melt can split into two mutually insoluble “partner melts” with drastically contrasting chemical compositions. One partner melt will be very rich in silica and could form massive quartz zones enriched in lithium minerals. The remaining partner melt would be a strongly alkaline silicate melt capable of producing albite-dominated zones. The crystallization of tourmaline has been shown to be a very effective means of concentrating water in the residual melt and has been suggested to be a “triggering process” for rapid water saturation.

Mapping Pegmatites

In the past, studies of pegmatites have mainly concerned either the evaluation of their economic potential or their exotic mineralogy. Surprisingly few efforts have focused on pegmatites in a coherent and systematic fashion, and as a result, satisfying theories for their origin are poorly developed relative to other areas of igneous petrology.

A comprehensive study of pegmatites in a particular region would include not only the pegmatites but also the country rocks that enclose them and any bodies of exposed granitic rock. The study begins with preparation of a geologic map showing the distribution of pegmatite bodies, by type, throughout the area of interest. The map is compiled from firsthand, detailed observations of rock outcroppings obtained by numerous foot-traverses across the area. Additional useful information may be obtained from earlier geologic maps, aerial photographs, satellite imagery, and mine records. In most cases, the geologic map will show numerous, small, discontinuous pegmatite veins cutting older, high-grade metamorphic country rocks. These are known as external pegmatites. If any granite plutons are present in the area, they must be examined closely, as they are potential parent bodies of the pegmatites. Such granites may themselves host so-called internal pegmatites. External and internal pegmatites, even if derived from the same pegmatite magma (cogenetic pegmatites), generally will differ in mineralogy and zoning traits. It is a considerable achievement if a study can demonstrate that groups of external and internal pegmatites derive from the same parent body of granite. In such a case, the geologist has the rare opportunity to study the chemical and temporal relationships between parent rock, host rock, and an evolving, mobile pegmatite magma. This happy state of affairs is seldom realized because of the all-important “level of erosion.” Since external pegmatites form by migration of pegmatite magma upward and away from the parent body along fracture pathways, they will normally be destroyed by erosion before their deeper parent and internal “relatives” are exposed.

By examining a large pegmatite-bearing area rather than a single pegmatite body (as is often done), it is possible to determine if systematic differences exist in the exposed pegmatites relative to host-rock type, local or regional fracture patterns in the host rock, or distance from igneous plutons. Individual pegmatites can also be compared in terms of shape, size, orientation, mineralogy, and zone characteristics. Systematic variations of these parameters on the scale of a geological map constitute “regional zoning.” Increased recognition of such effects is needed to provide greater insight into the operation of the pegmatite-forming process.

After regional geologic relationships are determined, individual pegmatites are mapped in the greatest detail possible (often at scales as large as 1 inch per 10 feet). The objective is to determine the size, shape, and zone sequence of each pegmatite body, which, in turn, will provide a basis for sampling and determining the composition of the pegmatite as a whole. Every effort is made to establish the correct sequence of zone crystallization, because the bulk compositions and fluid inclusion data from each member of the zone sequence can then be used to trace chemical changes in the pegmatite magma during the emplacement process.

Commercial Value

In the past, large and favorably located simple pegmatites were often worked for mica, quartz, and feldspar by the glass and ceramic industries, but production of this type has all but ceased. The post-World War II electronics revolution and other technological advances, such as lithium batteries, have fueled ever-increasing demands for the rare elements found in complex pegmatites.

Rare-element pegmatites with commercial grades and reserves that are sufficiently close to the surface for low-cost open-pit mining are distinctly uncommon. The few that do exist are the only sources for the elements lithium, beryllium, cesium, rubidium, tantalum, and niobium; they are important sources for boron, tin, tungsten, uranium, thorium, fluorine, and rare-earth elements (lanthanides) as well. Because many of the pegmatites in current commercial production are located in developing countries, some industrialized countries (the United States among them) stockpile rare-element commodities of strategic importance in case of national emergencies or supply disruptions.

Complex pegmatites are valued for reasons other than the commercial production of rare elements. From ancient times, they have been mined for precious and semiprecious gems. The intermediate zones of certain pegmatites are the major source of gem-quality topaz, tourmaline, and beryl (including morganite, aquamarine, and emerald). Each year, hundreds of small, commercially subeconomic rare-element pegmatites are prospected by amateur and professional collectors of rare minerals and fine crystals. These pegmatites, in fact, are supporting a small but vigorously growing industry. Many (and perhaps even most) of the spectacular crystal specimens displayed in museums were obtained from subeconomic pegmatites found by amateur collectors.

Principal Terms

aplite: a light-colored, sugary-textured granitic rock generally found as small, late-stage veins in granites of normal texture; in pegmatites, aplites usually form thin marginal selvages against the country rock but may also occur as major lenses in the pegmatite interior

crystal-liquid fractionation: physical separation of crystals, precipitated from cooling magma, from the coexisting melt, enriching the melt in elements excluded from the crystals; this separation, or fractionation, leads to extreme concentration of incompatible elements in the case of pegmatite magma

exsolve: the process whereby an originally homogeneous solid solution separates into two or more minerals (or substances) of distinct composition upon cooling

fluid inclusions: microscopic drops of parental fluid trapped in a crystal during growth; inclusions persist indefinitely unless the host crystal is disturbed by deformation or recrystallization

incompatible elements: chemical elements characterized by odd ionic properties (size, charge, electronegativity) that tend to exclude them from the structures of common minerals during magmatic crystallization

solidus/liquidus temperature: the liquidus temperature marks the beginning of crystallization in magmas, and the solidus temperature marks the end; crystals and melt coexist only within the liquidus-solidus temperature interval

viscosity: a property of fluids that measures their internal resistance to flowage; the inverse of fluidity or mobility

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