ElEmEnts, Vol. 8, pp. 257–261 August 2012257

1811-5209/12/0008-0257$2.50 DOI: 10.2113/gselements.8.4.257

Granitic Pegmatites: Scientific Wonders and Economic Bonanzas

INTRODUCTIONPegmatites are texturally distinct variants of the more common and more voluminous plutonic igneous rocks, including gabbros, granites, syenites, etc. Whereas common plutonic bodies tend to be mineralogically and texturally homogeneous throughout large volumes of rock, pegma-tites are precisely the opposite. Most pegmatite bodies are small, with dimensions on the scale of meters rather than kilometers, and display internally complex fabrics. They occur as segregations within granites (Fig. 1a) and as sharply discordant dikes intruding igneous and metamor-phic rocks (Fig. 1b). Exceedingly coarse crystal size is a hallmark of pegmatites for most geoscientists (Fig. 2), but gigantic crystal size is not the sole or even a necessary defining factor. Other fabrics that qualify as pegmatitic include systematic coarsening in crystal size from the margins to the centers of bodies (Fig. 3); sharp mineral-ogical zonation from margin to center (Fig. 4); anisotropic fabrics, including layering or highly oriented crystal-growth directions; and graphic (skeletal) intergrowths of quartz and feldspar, termed “graphic granite” (Fig. 5). Pegmatites and hydrothermal vein deposits share all of these textural attributes but one: that of graphic granite, which is not only unique to pegmatites but was the texture for which the term pegmatite (from phgnumi, to make stout by binding together) was coined.

Pegmatitic textures can be found in igneous rocks of all compositions. However, pegmatitic textures are so preva-lent in granitic compositions that the term pegmatite implies a granitic composition to many geoscientists. For the sake of brevity, most of the authors of the articles in this issue use the term pegmatite, without a modifier, as

synonymous with a granitic composition. The bulk composi-tions of pegmatites plot close to the thermal-minimum composi-tion in the granite system, which includes rocks with nearly equal

proportions of quartz, sodic plagioclase, and potassic (alkali) feldspar. Only a small proportion of pegmatites (<1%) possess assemblages that contain uncommon minerals, e.g. those with essential lithium, beryllium, cesium, boron, phosphorus, and tantalum. These exotic rocks are termed rare-element pegmatites (not to be confused with rare earth element pegmatites, which are a subset of rare-element pegmatites). Gem-quality crystals for the jewelry industry are found in a small number of these already sparse rare-element pegmatites. Most gem-bearing pegmatites are classified as miarolitic, which refers to the presence of clay-filled or open, crystal-lined cavities.

THE PEGMATITE PUZZLEPegmatites have long been viewed as essentially igneous rocks because of their bulk compositions. The origin of pegmatitic rock fabrics, however, has intrigued and baffled petrologists. By the end of the 19th century, virtually every conceivable process had been proffered to explain the complex textures and the assemblages of uncommon minerals found in some pegmatites. Because pegmatites and hydrothermal veins share common textural features, many petrologists have called upon an aqueous fluid, alone or acting in concert with a coexisting silicate melt, to generate the complexities of grain size and mineral zona-tion that are diagnostic of pegmatites.

Two concepts of pegmatite formation have dominated scientific thought for a century. A model now associated with Cameron et al. (1949) attributed the chemical evolu-tion of pegmatites (among and within individual bodies) to the fractional crystallization of melt inward from the margins of bodies. Through this process, rare elements (e.g. Li, Be, and Ta), fluxes (e.g. B, P, and F), and other volatile components (e.g. H2O and Cl) that are excluded by the initial crystallization of quartz and feldspars become concentrated inward into a diminishing fraction of residual

Granitic pegmatites have been a focal point of research by petrologists and mineralogists for over a century. Mineralogical interest stems from the diversity of rare minerals that some pegmatites contain.

Petrologic efforts are aimed at resolving the processes or agents that produce the complex textures and spatial heterogeneity that distinguish pegmatites from granites. Much of the scientific study of pegmatites has been motivated by exploration for the economic commodities they provide. Pegmatites yield quartz, feldspars, and micas for industrial uses; strategic rare metals for electronic, aerospace, and energy applications; and many of the world’s finest gem and mineral specimens.

Keywords: pegmatite, granite, rare metals, industrial minerals, gemstones

David London1 and Daniel J. Kontak2

1 ConocoPhillips School of Geology and Geophysics University of Oklahoma, Norman, OK 73019, USA E-mail: dlondon@ou.edu

2 Department of Earth Sciences, Laurentian University Sudbury, ON P3E 2C6, Canada E-mail: dkontak@laurentian.ca

Pink pezzottaite crystal, 1.2 cm long, from

Ambatovita, Fianarantsoa Province,

Madagascar. The rim of the crystal is white beryl with minor microcrystals of milarite and bavenite.

Named in honor of Federico Pezzotta. Photo

Matteo Chinellato

ELEMENTS AUGUST 2012258

melt; eventually, this melt becomes saturated in minerals containing these exotic components. In the model identi-fi ed with Jahns and Burnham (1969), the silicate melt was the source of constituents, and the defi ning textures and mineralogical zonation of pegmatites were ascribed to crys-tallization from an aqueous fl uid that “scoured” certain elements from the melt and redistributed them to growing crystals in all parts of the pegmatite body.

Since these models were introduced, our knowledge of the bulk compositions, depths, and cooling histories of pegma-tites has improved. The more we have learned, the more problematic some aspects of pegmatite geology (and prior conceptual models) have become. For example, pegmatite compositions lie close to the bulk composition of the minimum-temperature melt in the hydrous granite system (NaAlSi3O8–KAlSi3O8–SiO2–H2O). Even the most chemi-cally evolved pegmatites contain only a few weight percent of viscosity-reducing components like H2O, B, P, and F. Most pegmatites form thin dikes injected into cooler, brittle host rocks (FIGS. 1B, 4). Evidence from mineral compositions and thermal models indicates that crystallization within pegmatites commences at ~450 °C, which is ~200–250 °C below the liquidus temperature at which crystallization should commence. The viscosity of hydrous granitic liquid at this temperature is ~108 Pa⋅s, similar to the viscosity of asphaltic pitch at 25 °C. Such high viscosity severely impedes the diffusion of components through a melt and commensurately diminishes the transfer of nutrient components to growing crystal surfaces. In their review of the principal models for the internal evolution of pegma-tites, London and Morgan (2012 this issue) call attention to graphic granite, the defi ning texture that is unique to pegmatites. From what we understand about the origin of graphic granite (Fenn 1986), this texture represents prima facie evidence of the conditions of pronounced under-cooling below the liquidus temperature and high super-saturation of very viscous melt in quartz- and feldspar-forming components.

Yet somehow, within this state of high viscosity and rapidly dwindling thermal energy, giant crystals manage to grow. The fl uxing components cited above are regarded as essen-tial to the crystallization of gigantic crystals of silicates in pegmatites. Hence, one conundrum in the puzzle of pegma-tites is this: how can the need for high concentrations of fl uxing components be reconciled with their manifestly low abundance in all but a very few pegmatites? London and Morgan (2012) address this problem and, in so doing, reconcile the disparities between the models of Cameron et al. (1949) and Jahns and Burnham (1969).

PEGMATITES AS ORE BODIESPegmatites host an exceptionally diverse range of economic commodities, and academic interest in granitic pegmatites has stemmed in large measure from the scientifi c quest to understand ore-forming processes. The same factors that make pegmatites so exceptional in terms of textures are also likely responsible for the exceedingly effi cient mecha-nisms that concentrate trace elements as chemically diverse as Li, B, Cs, Ta, and Bi to values that are thousands of times their average crustal abundances. Element pairs that behave in a chemically coherent fashion, such as Zr–Hf and Nb–Ta, are extensively fractionated among pegmatites and within individual bodies, leading to the formation of such exotic mineral species as hafnon (HfSiO4) and tantite (Ta2O5). The process of rare-element enrichment in pegma-tites appears to proceed, in an essentially closed system, from a small fraction of residual silicate liquid derived from a much larger magma body. This process contrasts mark-edly with other ore-forming systems, for example, Cu- and Mo-mineralized felsic porphyries, that originate from inter-actions between large volumes of magmatic rocks and hydrothermal fl uids in chemically open systems.

Pegmatites have always been sought for minerals and metals that have specialty uses. In the 1940s, that search was for sheet muscovite, a mica, which was employed as grid separators in electronic vacuum tubes (a high-technology application of that time); for beryllium as a component of copper alloys used mostly for bearings and gears; and for tantalum as the optimal dielectric oxide for electrolytic capacitors. Today, niobium, tantalum, tin,

granite

pegmatite

A B

FIGURE 1 (A) A pegmatitic segregation within granite, Middletown, Connecticut (USA). The scale measures

9 cm. The yellow dashed line indicates the margins of the pegma-tite. (B) Geologists ponder a set of parallel pegmatite dikes that cut amphibolite and gneiss, Haddam, Connecticut (USA).

ELEMENTS AUGUST 2012259

beryllium, lithium, cesium, rare earths, and other normally rare elements are mined from pegmatites for applications in electronics, nuclear energy, aerospace, deep drilling, and other specialized industries.

Granitic Pegmatites –Storehouses of Industrial Minerals Not all pegmatitic ores consist of rare elements or rare minerals. Pegmatites are the primary sources of feldspar for the glass and ceramics industries. The low iron and calcium contents of feldspars in pegmatite make these materials most desirable for these applications. Quartz is used primarily in the manufacture of glasses, but ultrahigh-purity quartz from pegmatite is a foundational material in the electronics industry. Because pegmatites consist chiefl y of quartz and feldspars, the ore grade of some of the most important deposits approaches 100% of the minable rock, a benefi t that is rare in the mining industry. Even the clay minerals that are produced from weathered or hydrother-mally altered pegmatites now fulfi ll a signifi cant role in the fabrication of microprocessors. Glover et al. (2012 this issue) describe the myriad other uses of quartz, feldspars, clays, and other industrial minerals derived from pegma-tites. They make the case that pegmatite-derived industrial minerals play some part in the daily lives of most people who live in modern societies.

Granitic Pegmatites as Sources of Strategic MetalsIn this issue, Linnen et al. (2012) assess the likely fl uid media and mechanisms that lead to the ore-grade concen-trations of these normally trace elements and observe that pegmatitic ores are endogenic, meaning that they are deposited within the igneous body from which they origi-

nate. These signifi cant rare-element ores precipitate from a silicate liquid, and hydrothermal processes exert only a minor role in the internal redistribution of the ore-forming elements. As chemically evolved as these ore-producing pegmatites are, their concentrations of most rare metals are not suffi cient to reach saturation of the melt at the temperature of the liquidus (the silicate liquid–crystal fi eld boundary at equilibrium). Linnen et al. (2012), therefore, account for the primary deposition of rare-element ores mostly by the crystallization of melt at temperatures well below the liquidus. Pegmatite-forming melts contain suffi -cient concentrations of rare elements to achieve saturation in rare-element minerals (such as beryl, tantalite, pollucite, etc.) at temperatures that are mostly 100–200 °C below the liquidus temperature.

Granitic Pegmatites as Sources of Colored GemstonesPegmatites are sources of some of the fi nest and most prized mineral specimens in the collections of museums and private individuals (FIG. 6). Many of the colored stones on the gem market today—varieties of beryl, topaz, tourma-line, and others—are produced mainly or solely from pegmatites. Simmons et al. (2012 this issue) provide an overview of the gem materials mined worldwide from pegmatites, with examples from some of the most prolifi c and spectacular occurrences. Although historically impor-tant sources in Brazil, Russia, Madagascar, and the United States continue to supply much of the gem materials derived from pegmatites, these regions are now joined by countries in southern Africa and by Afghanistan and Pakistan in southern Asia. Mining gems from pegmatites is labor-intensive and suitable for what is known as “arti-sanal” mining activity at a small, local scale. Most gem-quality minerals come from open or clay-fi lled “miarolitic cavities” in pegmatite. The smooth, shiny faces of these

FIGURE 2 A gigantic skeletal crystal of tourmaline radiating into a pegmatite from the upper contact, from the Água

Santa pegmatite, Coronel Murta, Jequitinhonha valley, Minas Gerais, Brazil (Robert F. Martin for scale). PHOTO: MILAN NOVAK

FIGURE 3 Systematics of crystal size variation from the margin to the center of pegmatite dikes. Modifi ed from Jahns

(1953), this fi gure shows the average dimension of crystals along their longest axis of growth versus their location within the pegma-tite as a percentage of the distance from the margin (wall) to the dike center. Two curves are specifi c to perthitic K-feldspar and spodumene; the curve for “any mineral” represents an average that may involve more than one mineral species. The data are based on measurements from 27 large, zoned pegmatites in the Hualapai, Bagdad, and White Picacho pegmatite districts of western Arizona (USA).

Proportion (%) of wall-to-center distance in pegmatite bodies

Ave

rag

e si

ze o

f cr

ysta

ls

(lo

ng

dim

ensi

on

)

wall 20 40 60 80 center

m

1.0

0.5

0

Any mineral, largest crystal

Perthite average

Spodumene average

ELEMENTS AUGUST 2012260

minerals and an abundance of hydrous minerals, including clays and zeolites, in association with gem-quality minerals signify that aqueous fl uid plays a major role in the fi nal stages of consolidation of these very rare pegmatite bodies. Simmons et al. (2012) ascribe miarolitic cavities to the exsolution of aqueous fl uid from the pegmatite-forming melt and to the transfer of melt-derived components via aqueous fl uid to the surfaces of growing crystals.

PEGMATITES AS REFLECTIONS OF THEIR SOURCESMost geoscientists would agree that pegmatites represent the terminal stage in the fractionation of granitic magmas. That process begins with the redistribution of elements between parental rocks deep within the Earth’s continental crust (with mantle infl uences in some cases) and their partial melts. It continues as crystal fractionation proceeds toward completion in upwardly mobile magma bodies, with variable degrees of interaction with other rock types along the way. Considering the protracted history of granitic magmas, one might not expect their culmination as pegmatites to preserve a record of their origins at the source. In fact, they do, and to a surprising extent—the affi liations of granitic pegmatites with certain source rocks and particular tectonic environments are evident in a majority of instances. The chemical and tectonic links between pegmatites at one end of the magmatic spectrum and their source rocks at the other is considered by Cerný et al. (2012 this issue). The distinctive signatures of normally trace elements, but with elevated concentrations in pegmatites through fractional crystallization of their source granitic magmas, fall into two chemical families: pegmatites enriched in lithium, cesium, and tantalum

(LCT) as a diagnostic signature, and pegmatites that carry niobium, yttrium, and fl uorine (NYF) as a trace element signature. Pegmatites of the LCT family are strongly corre-lated with S-type granites, whose ultimate protoliths can be traced to chemically mature sedimentary sources, such as marine shales. Pegmatites of the NYF family are associ-ated with A-type granites that form within intracontinental rifts. The origins of A-type granites are more complex than the origins of S-type granites and involve varying degrees of crustal or mantle input. Pegmatites are notably sparse among the subduction-related I-type granites, except where such plutons have inherited a small component of sedi-mentary material. Cerný et al. (2012) propose that the associations between specifi c granite types and their tendency to form pegmatites hinge upon the availability of fl uxing components, such as B, P, and F, in the source regions of those granites.

GRANITIC PEGMATITES AS COMPLEX ISOTOPIC SYSTEMSPegmatites have been the focus of mineralogical and geochemical studies but little isotopic work. Despite this relative lack of data, valuable insight is provided by isotopes, indicating that this is an area worthy of future effort. Many radiometric isotope systems (e.g. U–Pb, Rb–Sr, Nd–Sm, K–Ar) have been employed to obtain ages or infor-mation about sources of pegmatites. Signifi cantly, some studies indicate that the ages for pegmatites are younger than known granites that might represent their sources (Tomascak et al. 1998; Kontak et al. 2005), thus raising important questions about temporal and source relations between granites and pegmatites.

Stable isotope systems provide a means of unraveling the provenance of pegmatite-forming melts. For example, the fact that the oxygen isotope ratios for the LCT-type pegma-tites can vary by several per mil (e.g. Longstaffe et al. 1981 versus Anderson et al. 2011) suggests distinctly different source materials. Comprehensive studies using multiple isotopic tracers (O, Pb, Sr, B, Li) remain to be done for single pegmatite bodies or pegmatite fi elds, and such work will provide important data relevant to addressing melt sources.

border zone

intermediate zones

core margin

core

core margin

intermediate zones

layered aplite

wall zone

border zone

granitic

graphic plagioclase -quartz

oriented microcline

beryl

pure quartz

oriented muscovite

graphic plagioclase- quartz

garnet

skeletal K-feldspar

granitic

3 cm

FIGURE 4 Textural and zonal attributes of pegmatites. The image shows a complete section of a pegmatite dike,

about 28 cm thick, located near Palomar Mountain, San Diego County, California (USA).

FIGURE 5 Graphic granite: quartz (gray) in microcline (white), Colorado (USA). This texture is unique to pegmatites.

ELEMENTS AUGUST 2012261

REFERENCESAnderson MO, Lentz D, Falck H (2011)

Petrology and chemistry of the Moose II lithium-tantalum pegmatite deposit, NWT. Geological Association of Canada-Mineralogical Association of Canada Annual Meeting, Ottawa, Program with Abstracts

Cameron EN, Jahns RH, McNair AH, Page LR (1949) Internal Structure of Granitic Pegmatites. Economic Geology Monograph 2, 115 pp

Carruzzo S, Kontak DJ, Clarke DB, Kyser TK (2004) An integrated fl uid–mineral stable isotope study of the granite-hosted mineral deposits of the New Ross area, South Mountain Batholith, Nova Scotia, Canada: Evidence for multiple reservoirs. Canadian Mineralogist 42: 1425-1441

Cerný P, London D, Novak M (2012) Granitic pegmatites as refl ections of their sources. Elements 8: 289-294

Fenn PM (1986) On the origin of graphic granite. American Mineralogist 71: 325-330

Glover AS, Rogers WZ, Barton JE (2012) Granitic pegmatites: Storehouses of industrial minerals. Elements 8: 269-273

Jahns RH (1953) The genesis of pegma-tites. I. Occurrence and origin of giant crystals. American Mineralogist 38: 563-598

Jahns RH, Burnham CW (1969) Experimental studies of pegmatite genesis: I. A model for the derivation and crystallization of granitic pegma-tites. Economic Geology 64: 843-864

Kontak DJ, Creaser R, Heaman L, Archibald DA (2005) U-Pb tantalite, Re-Os molybdenite, and 40Ar/39Ar muscovite dating of the Brazil Lake pegmatite, Nova Scotia: A possible shear-zone related origin for an LCT-type pegmatite. Atlantic Geology 41: 17-30

Linnen RL, Van Lichtervelde M, Cerný P (2012) Granitic pegmatites as sources of strategic metals. Elements 8: 275-280

London D, Morgan G VI (2012) The pegmatite puzzle. Elements 8: 263-268

Longstaffe FJ, Cerný P, Muehlenbachs, K (1981) Oxygen isotope geochemistry of the granitoid rocks in the Winnipeg River pegmatite district, southeastern Manitoba. Canadian Mineralogist 19: 195-204

Marschall HR, Jiang S-Y (2011) Tourmaline isotopes: No element left behind. Elements 7: 313-319

Simmons WB, Pezzotta F, Shigley JE, Beurlen H (2012) Granitic pegmatites as sources of colored gemstones. Elements 8: 281-287

Tomascak PB, Krogstad EJ, Walker RJ (1998) Sm-Nd isotope systematics and the derivation of granitic pegmatites in southwestern Maine. Canadian Mineralogist 36: 327-337

Trumbull RB, Krienitz M-S, Gottesmann B, Wiedenbeck M (2008) Chemical and boron-isotope variations in tourmalines from an S-type granite and its source rocks: the Erongo granite and tourma-linites in the Damara Belt, Namibia. Contributions to Mineralogy and Petrology 155: 1-18

Walker RJ, Hanson GN, Papike JJ, O’Neil JR, Laul JC (1986) Internal evolution of the Tin Mountain pegmatite, Black Hills, South Dakota. American Mineralogist 71: 440-459

The systematic fractionation of isotopes (18O/16O, D/H) provides a means to assess equilibrium and, hence, temper-atures of crystallization (Walker et al. 1986). However, many studies indicate widespread ingress of externally derived aqueous fl uids, which results in resetting or disequilibrium among the mineral reservoirs of stable isotopes, especially where pegmatites occur within large granite plutons (Carruzzo et al. 2004). The application of high-resolution analytical techniques provides a means to explore equilibrium in these systems and investigate the role of undercooling in pegmatites.

Relatively new studies of 11B/10B and 7Li/6Li by in situ methods are beginning to document the differential frac-tionation of these isotopes among mineral phases, melt, and fl uids due to environmentally induced changes in the coordination number of each element (IIIB versus IVB, and IVLi versus VILi) (Marschall and Jiang 2011). Thus, these isotopes provide a means to detect the presence of a fl uid phase and to test when fl uids appear in the evolution of pegmatite systems. For example, Trumbull et al. (2008) used boron isotopes to show that a hydrothermal fl uid played a role during the growth of tourmaline in a late-stage quartz–tourmaline intergrowth (i.e. orbicule) in a granite from Namibia.

WHY STUDY PEGMATITES?The ore-forming processes that lead to unparalleled element fractionation and rare-element enrichment in pegmatites would be scientifi c reason enough to want to understand the underlying processes of formation. However, it is the textural features of pegmatites that have generated the most scientifi c debate and have intrigued scientists from the inception of fi eld petrology in the 19th century. Nothing that geoscientists learn as students prepares them for inter-preting rock textures as complex as those in pegmatites. Understanding the textures and mineral zonation of granitic pegmatites is tantamount to understanding the fundamental process of crystallization. It is a challenge to our ability to discern, beyond reasonable doubt, what is igneous and what is hydrothermal. This is the context that has drawn many professional geoscientists to pegmatites for all or part of their careers.

FIGURE 6 Spodumene (var. purple kunzite), beryl (var. pink morganite), quartz, albite, and lepidolite from Kunar

Valley, Nuristan Province, Afghanistan (26 × 17 × 13 cm). This is a common late-stage assemblage in lithium-rich pegmatites, but it is rarely so beautifully crystallized in a miarolitic cavity. See Fig. 3 of Linnen et al. (2012) for the petrologic signifi cance of the assem-blage spodumene + quartz. PHOTO: JOE BUDD, COURTESY OF THE ARKENSTONE