Download Granitic Pegmatites

Granitic Pegmatites:
Scientific Wonders
and Economic Bonanzas
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
David London1 and Daniel J. Kontak 2
1811-5209/12/0008-0257$2.50 DOI: 10.2113/gselements.8.4.257
G
ranitic 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.
synonymous with a granitic
composition. The bulk compositions of pegmatites plot close to
the thermal-minimum composiKeywords : pegmatite, granite, rare metals, industrial minerals, gemstones
tion in the granite system, which
includes rocks with nearly equal
INTRODUCTION
proportions of quartz, sodic plagioclase, and potassic
Pegmatites are texturally distinct variants of the more
(alkali) feldspar. Only a small proportion of pegmatites
common and more voluminous plutonic igneous rocks,
(<1%) possess assemblages that contain uncommon
including gabbros, granites, syenites, etc. Whereas common
minerals, e.g. those with essential lithium, beryllium,
plutonic bodies tend to be mineralogically and texturally
cesium, boron, phosphorus, and tantalum. These exotic
homogeneous throughout large volumes of rock, pegma- rocks are termed rare-element pegmatites (not to be
tites are precisely the opposite. Most pegmatite bodies are confused with rare earth element pegmatites, which are a
small, with dimensions on the scale of meters rather than subset of rare-element pegmatites). Gem-quality crystals
kilometers, and display internally complex fabrics. They
for the jewelry industry are found in a small number of
occur as segregations within granites (Fig. 1a) and as
these already sparse rare-element pegmatites. Most gemsharply discordant dikes intruding igneous and metamor- bearing pegmatites are classified as miarolitic, which refers
phic rocks (Fig. 1b). Exceedingly coarse crystal size is a
to the presence of clay-filled or open, crystal-lined
hallmark of pegmatites for most geoscientists (Fig. 2), but cavities.
gigantic crystal size is not the sole or even a necessary
defining factor. Other fabrics that qualify as pegmatitic
THE PEGMATITE PUZZLE
include systematic coarsening in crystal size from the
margins to the centers of bodies (Fig. 3); sharp mineral- Pegmatites have long been viewed as essentially igneous
rocks because of their bulk compositions. The origin of
ogical zonation from margin to center (Fig. 4); anisotropic
pegmatitic rock fabrics, however, has intrigued and baffled
fabrics, including layering or highly oriented crystalpetrologists. By the end of the 19th century, virtually every
growth directions; and graphic (skeletal) intergrowths of
quartz and feldspar, termed “graphic granite” (Fig. 5). conceivable process had been proffered to explain the
complex textures and the assemblages of uncommon
Pegmatites and hydrothermal vein deposits share all of
these textural attributes but one: that of graphic granite, minerals found in some pegmatites. Because pegmatites
and hydrothermal veins share common textural features,
which is not only unique to pegmatites but was the texture
many petrologists have called upon an aqueous fluid, alone
for which the term pegmatite (from phgnumi, to make stout
or acting in concert with a coexisting silicate melt, to
by binding together) was coined.
generate the complexities of grain size and mineral zonaPegmatitic textures can be found in igneous rocks of all
tion that are diagnostic of pegmatites.
compositions. However, pegmatitic textures are so prevaTwo concepts of pegmatite formation have dominated
lent in granitic compositions that the term pegmatite
implies a granitic composition to many geoscientists. For scientific thought for a century. A model now associated
with Cameron et al. (1949) attributed the chemical evoluthe sake of brevity, most of the authors of the articles in
tion of pegmatites (among and within individual bodies)
this issue use the term pegmatite, without a modifier, as
to the fractional crystallization of melt inward from the
margins of bodies. Through this process, rare elements (e.g.
1 ConocoPhillips School of Geology and Geophysics
Li, Be, and Ta), fluxes (e.g. B, P, and F), and other volatile
University of Oklahoma, Norman, OK 73019, USA
components (e.g. H 2O and Cl) that are excluded by the
E-mail: [email protected]
initial crystallization of quartz and feldspars become
2 Department of Earth Sciences, Laurentian University
concentrated inward into a diminishing fraction of residual
Sudbury, ON P3E 2C6, Canada
E-mail: [email protected]
E lements , V ol . 8,
pp.
257–261
257
A ugus t 2012
A
B
pegmatite
granite
(A) A pegmatitic segregation within granite,
Middletown, Connecticut (USA). The scale measures
9 cm. The yellow dashed line indicates the margins of the pegmatite. (B) Geologists ponder a set of parallel pegmatite dikes that
cut amphibolite and gneiss, Haddam, Connecticut (USA).
FIGURE 1
melt; eventually, this melt becomes saturated in minerals
containing these exotic components. In the model identified 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 crystallization from an aqueous fluid 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 pegmatites 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 –H 2O). Even the most chemically evolved pegmatites contain only a few weight percent
of viscosity-reducing components like H 2O, 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 ~10 8 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 pegmatites, 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 undercooling below the liquidus temperature and high supersaturation of very viscous melt in quartz- and
feldspar-forming components.
E LEMENTS
Yet somehow, within this state of high viscosity and rapidly
dwindling thermal energy, giant crystals manage to grow.
The fluxing components cited above are regarded as essential to the crystallization of gigantic crystals of silicates in
pegmatites. Hence, one conundrum in the puzzle of pegmatites is this: how can the need for high concentrations of
fluxing 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 BODIES
Pegmatites host an exceptionally diverse range of economic
commodities, and academic interest in granitic pegmatites
has stemmed in large measure from the scientific 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 efficient mechanisms 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 pegmatites 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 markedly with other ore-forming systems, for example, Cu- and
Mo-mineralized felsic porphyries, that originate from interactions between large volumes of magmatic rocks and
hydrothermal fluids 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 hightechnology 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,
258
A UGUS T 2012
m
Average size of crystals
(long dimension)
Any mineral,
largest crystal
Spodumene
average
1.0
Perthite average
0.5
0
wall
20
40
60
80
center
Proportion (%) of wall-to-center distance
in pegmatite bodies
Systematics of crystal size variation from the margin
to the center of pegmatite dikes. Modifi ed from Jahns
(1953), this figure shows the average dimension of crystals along
their longest axis of growth versus their location within the pegmatite as a percentage of the distance from the margin (wall) to the
dike center. Two curves are specific 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).
FIGURE 3
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 : M ILAN NOVAK
FIGURE 2
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 ultrahighpurity quartz from pegmatite is a foundational material in
the electronics industry. Because pegmatites consist chiefly
of quartz and feldspars, the ore grade of some of the most
important deposits approaches 100% of the minable rock,
a benefit that is rare in the mining industry. Even the clay
minerals that are produced from weathered or hydrothermally altered pegmatites now fulfi ll a significant 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 pegmatites. 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 Metals
In this issue, Linnen et al. (2012) assess the likely fluid
media and mechanisms that lead to the ore-grade concentrations 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-
E LEMENTS
nate. These significant 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 sufficient to reach saturation of the melt at the
temperature of the liquidus (the silicate liquid–crystal field
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 sufficient 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 Gemstones
Pegmatites are sources of some of the finest 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, tourmaline, 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 prolific
and spectacular occurrences. Although historically important 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 “artisanal” mining activity at a small, local scale. Most gemquality minerals come from open or clay-fi lled “miarolitic
cavities” in pegmatite. The smooth, shiny faces of these
259
A UGUS T 2012
border zone
3 cm
(LCT) as a diagnostic signature, and pegmatites that carry
niobium, yttrium, and fluorine (NYF) as a trace element
signature. Pegmatites of the LCT family are strongly correlated 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 associated 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 sedimentary material. Č erný et al. (2012) propose that the
associations between specific granite types and their
tendency to form pegmatites hinge upon the availability
of fluxing components, such as B, P, and F, in the source
regions of those granites.
granitic
graphic
plagioclase
-quartz
intermediate
zones
oriented
microcline
core margin
beryl
core
pure quartz
core margin
oriented
muscovite
intermediate
zones
GRANITIC PEGMATITES AS COMPLEX
ISOTOPIC SYSTEMS
graphic
plagioclasequartz
Pegmatites 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 information about sources of pegmatites. Significantly, 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.
garnet
layered aplite
skeletal
K-feldspar
granitic
wall zone
border zone
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 4
minerals and an abundance of hydrous minerals, including
clays and zeolites, in association with gem-quality minerals
signify that aqueous fluid 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 fluid from the pegmatite-forming
melt and to the transfer of melt-derived components via
aqueous fluid to the surfaces of growing crystals.
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 pegmatites 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 fields, and such work will
provide important data relevant to addressing melt sources.
PEGMATITES AS REFLECTIONS
OF THEIR SOURCES
Most 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 influences 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 Černý
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
E LEMENTS
260
FIGURE 5
Graphic granite: quartz (gray) in microcline (white),
Colorado (USA). This texture is unique to pegmatites.
A UGUS T 2012
The systematic fractionation of isotopes (18O/16O, D/H)
provides a means to assess equilibrium and, hence, temperatures of crystallization (Walker et al. 1986). However,
many studies indicate widespread ingress of externally
derived aqueous fluids, 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 7 Li/ 6Li by in situ
methods are beginning to document the differential fractionation of these isotopes among mineral phases, melt,
and fluids due to environmentally induced changes in the
coordination number of each element ( IIIB versus IV B, and
IV Li versus VI Li) (Marschall and Jiang 2011). Thus, these
isotopes provide a means to detect the presence of a fluid
phase and to test when fluids appear in the evolution of
pegmatite systems. For example, Trumbull et al. (2008)
used boron isotopes to show that a hydrothermal fluid
played a role during the growth of tourmaline in a latestage 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 scientific reason enough to want to understand
the underlying processes of formation. However, it is the
textural features of pegmatites that have generated the
most scientific debate and have intrigued scientists from
the inception of field petrology in the 19th century. Nothing
that geoscientists learn as students prepares them for interpreting 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.
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Petrology and chemistry of the Moose II
lithium-tantalum pegmatite deposit,
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LR (1949) Internal Structure of Granitic
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TK (2004) An integrated fluid–mineral
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Glover AS, Rogers WZ, Barton JE (2012)
Granitic pegmatites: Storehouses of
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E LEMENTS
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 significance of the assemblage spodumene + quartz. PHOTO: JOE BUDD, COURTESY OF THE ARKENSTONE
FIGURE 6
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