Download Gem Formation, Production, and Exploration: Why Gem Deposits

Gem Formation, Production, and
Exploration: Why Gem Deposits Are Rare
and What is Being Done to Find Them
Lee A. Groat1 and Brendan M. Laurs2
1811-5209/09/0005-0153$2.50 DOI: 10.2113/gselements.5.3.153
T
he geology of gem deposits is a relatively new area of research focused
on understanding the rare and exceptional geologic conditions that give
rise to gem-quality materials. These conditions may include the availability
of sometimes uncommon major constituents, the presence of adequate
chromophores, limited concentrations of undesirable elements, open space,
an environment conducive to forming crystals of sufficient size and transparency,
and a favorable environment for mining. Future research should aid exploration,
which until recently has been nonsystematic and nonexistent for many gem
minerals, with diamond as the notable exception.
Keywords : gems, diamond, emerald, sapphire, ruby, geochemistry, exploration
INTRODUCTION
Gems (defined by Fritsch and Rondeau 2009 this issue)
have been prized for thousands of years for their color,
luster, transparency, durability, and high value-to-volume
ratio. The value of a gem depends primarily on esthetic
and durability factors, but rarity is also significant. Some
gems are fashioned from minerals that are quite rare in
nature (e.g. benitoite, taaffeite, and brazilianite), while many
others are produced from common minerals (e.g. quartz,
feldspar, and garnet). Features that make a gemstone valuable, such as color, size, and transparency, can be extremely
elusive even if the mineral itself is common. When a
common mineral has certain features, such as an attractive
color, a relatively large size, and a high degree of transparency, it can be used as a gemstone (e.g. amethyst—the
purple variety of quartz, SiO2). It is not the mineral itself
that makes a gemstone; it is the characteristics of a specific
sample. For example, a corundum crystal is not a gemstone
(e.g. ruby, sapphire) unless it formed in an environment that
allowed it to attain a suitable size, transparency, and color.
Gem deposits are rare because the geologic conditions
necessary for the formation of gem-quality materials are
rarely attained. These conditions include some or all of the
following: (1) availability of major constituents, which in
some cases are uncommon in nature; (2) presence of adequate
chromophores (elements responsible for color in minerals),
which can be rare in certain environments; (3) limited
concentrations of undesirable elements, which may be
common either in nature or in a specific geologic environment (such elements can either impart an “off” color or
1 Department of Earth and Ocean Sciences
University of British Columbia
Vancouver, BC V6T 1Z4, Canada
E-mail: [email protected]
2 Gemological Institute of America (GIA)
5345 Armada Drive, Carlsbad, California 92008, USA
E-mail: [email protected]
E lements , V ol . 5,
pp.
153–158
impede crystal formation); (4)
open space for crystals to grow
unimpeded, which is rare in most
geologic environments; (5) an environment to form crystals of sufficient size and transparency; and (6)
a favorable environment for
mining. These exceptional requirements also make gem deposits
fascinating for scientific study. This
fascination, combined with economic
considerations, has stimulated an
increasing interest in the geology
of gem deposits in recent years (e.g.
Kievlenko 2003; Groat 2007).
GEM FORMATION
Ingredients
Most gems are minerals and therefore have a definite (but
not fixed) chemical composition. Examples include diamond
(C), ruby (red gem corundum, Al2O3), sapphire (any other
color of gem corundum), and emerald (green chromium/
vanadium-bearing gem beryl, Be3Al2Si6O18). One notable
exception is jade, a rock composed of either microcrystalline
jadeite (NaAlSi2O6), referred to as jadeite jade, or tremoliteactinolite [Ca2(Mg,Fe)5Si8O22(OH)2], referred to as nephrite.
The formation of most gems requires adequate concentrations of essential constituents (or essential structural
components; London 2008). For example, carbon is a trace
element in the mantle, and the formation of gem-quality
diamonds requires local enrichment of carbon (Stachel
2007). Most gems, diamond being an exception, also
require that their essential elements be brought into contact
with appropriate concentrations of a chromophore, often
a transition metal [e.g. chromium in ruby or vanadium in
tanzanite (violetish blue gem zoisite, Ca 2Al3Si3O12OH) and
tsavorite (green gem grossular, Ca3Al2Si3O12)].
A good example of the requirement for a major element
and a chromophore is emerald. Beryl is a relatively rare
mineral because there is very little beryllium in the upper
continental crust (2.1 ppm; Rudnick and Gao 2003), where
it tends to be concentrated after prolonged fractional crystallization of a magma (London 2008), in granites, pegmatites, and their metamorphic equivalents. Chromium and
vanadium are more common (92 and 97 ppm, respectively;
Rudnick and Gao 2003) but are concentrated in different
rocks: chromium in dunite, peridotite, basalt, and their
metamorphic equivalents, and vanadium in organic- and
iron-rich sediments and their metamorphic equivalents.
These are typically not found near beryllium-rich environments. Dynamic geologic and geochemical conditions are
required for beryllium and chromium or vanadium to
meet. In the classic model, beryllium-bearing pegmatites
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J une 2009
interact with chromium-bearing ultramafic or mafic rocks.
However, in the black shale–hosted Colombian deposits,
there is no evidence of magmatism, and it has been demonstrated that hydrothermal circulation processes associated
with tectonic activity were sufficient to form emerald (e.g.
Ottaway et al. 1994; Cheilletz and Giuliani 1996; Branquet
et al. 1999; see Fig. 1). In addition, some researchers have
suggested that regional metamorphism and tectonometamorphic processes, such as shear zone formation, have
played a significant role in certain emerald deposits (notably
Habachtal in Austria, Leydsdorp in South Africa, and
Franqueira in Spain; Grundmann and Morteani 1989; Nwe
and Morteani 1993; Franz et al. 1996).
It is interesting to note that chromium is the chromophore
in both ruby and (most) emerald. In ruby, the details of
the atomic environment and local charges around the chromium ion result in a strong interaction, equivalent to a
small “cage” around the ion. This induces absorption at
high energy, and hence most of the transmission is at low
energy, in the red part of the visible spectrum. The opposite
occurs in emerald, in which the local environment is more
relaxed, resulting in a looser “cage” around the chromium
ion. The absorption is at lower energy, which results in the
well-known emerald-green color (Burns 1993).
The formation of a gem deposit requires not only the presence of sometimes rare constituents, but also the exclusion
of undesirable elements. For example, corundum will form
only in the relative absence of silica, because in the presence of silica, aluminum is preferentially incorporated into
aluminosilicate minerals such as feldspars and micas.
Certain chromophores, such as iron, can hinder the forma-
tion of attractive, economically important gems by creating
undesirable colors in normal-sized facetable gems (e.g.
black in tourmaline, over-dark green in emerald, brownish
overtones in ruby).
The Recipe
Gem deposits also require specific thermobarometric conditions favorable for the crystallization and stability of the
specific mineral. For example, “Clifford’s Rule” formulates
the close association between diamondiferous kimberlite
and Archean cratons. Deep (up to ~200 km), relatively cool
lithospheric roots are believed to cause the graphite–
diamond transition to rise beneath cratons. The region
where the lithospheric mantle reaches into the diamond
stability field corresponds to a window of opportunity
where diamond may form and reside (see, for example,
Kirkley et al. 1991; Stachel et al. 2005). The diamonds are
later brought to the surface in rapidly ascending ultramafic
magmas, which commonly solidify as kimberlite diatremes
or “pipes,” or as small volcanic dikes and sills. Recent
research suggests that diamonds precipitate from oxidized
(i.e. carbonate-bearing) fluids that could be related to
devolatilization of subducting oceanic slabs (Stachel 2007),
and that they form at specific times in the Earth’s history
that can be correlated to major tectonic events in the lithosphere (e.g. Cartigny 2005).
In some cases, little is known about the thermobarometric
conditions required for gem formation. For example,
corundum occurs in magmatic, metamorphic, and hydrothermal environments. In magmatic deposits, it occurs as
xenocrysts or phenocrysts in alkali basalt, lamprophyre,
and syenite. In metamorphic deposits, it is hosted by
marble, mafic and ultramafic rocks, granulite, cordieritite,
gneiss (Fig. 2), migmatite, desilicated pegmatites, skarns,
and shear-related deposits (e.g. Garnier et al. 2004; Simonet
et al. 2008). Magmatic corundum is generally thought to
form in the lower crust or upper mantle, but a variety of
models have been proposed to explain the particular conditions and geology of the crystallization environment (e.g.
Giuliani et al. 2007; Simonet et al. 2008). Metamorphic
corundum crystallizes at high temperatures and high to
moderate pressures. However, as pointed out by Giuliani
et al. (2007), there exist few data on primary corundum
deposits—that is, where corundum is hosted by a parental
rock—and numerous questions remain. For example,
marble is depleted in silica and aluminum, and to form
corundum in such an environment, a fluid phase seems
necessary. However, aluminum is usually considered to be
an immobile element, and the fluid phase would have to
infiltrate the marble, which in general would have few
fractures and low porosity.
Growth Conditions
Gems need room to grow and thus are often found in
cavities or “pockets.” For example, in the final stages of
crystallization of complex pegmatites (zoned granitic
pegmatites with superimposed areas of metasomatic alteration or replacement zones), volatile-rich fluids may exsolve
and produce cavities lined with beautiful gem-quality crystals, most commonly beryl, topaz [Al 2 SiO4 (F,OH) 2 ], and
tourmaline (London 2008). The pockets tend to be centrally
located within and along the margins of the core zone.
Gem-bearing cavities also occur in hydrothermal veins and
in volcanic rocks (gas pockets).
Emerald from the Coscuez mine, Boyacá, Colombia.
The crystal is 1.3 cm high. Sample courtesy of
Mel Gortatowski ; photograph © Jeff Scovil
Figure 1
E lements
In some cases, gem crystals occur in solid rock and open
space is not critical. The best example is diamond in ultramafic rock (usually kimberlite). Emerald can occur in
schists as a result of metasomatism. Tsavorite rarely occurs
as well-formed crystals, but instead forms rounded “potato”
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J une 2009
nodules that are typically fractured. Peridot [olive-green
gem olivine, (Mg,Fe) 2SiO4] can form crystal aggregates in
ultramafic rocks. In eastern Zambia, rhodolite [rose-pink
to red pyrope, (Mg,Fe) 3Al2 (SiO4) 3] forms nodular crystals
up to 10 cm in diameter in plagioclase segregation veins
in mafic granulite (Seifert and Vrana 2003). Nodules of
tsavorite, peridot or rhodolite may occasionally contain
fragments of gem-quality material. Other examples of gem
minerals for which open space is not critical include zoisite
(tanzanite), cordierite (iolite), corundum, spinel, and zircon.
Size is important in that the rough material must be large
enough to permit faceting or carving. For example,
cordierite can occur in abundance in Al-rich metamorphic
rocks, but the grain size is generally too small for gem use.
Only in exceptional cases does cordierite form crystals of
sufficient size and transparency to qualify as gems. In most
cases transparency is also an issue; for example, beryl crystals can grow to huge size in pegmatites, but in general
these large crystals are not transparent. Gems must also
be preserved from mechanical fracturing, chemical etching,
metamorphism, and other postgrowth damage. Olivine
can be a common mineral in mafic and ultramafic rocks,
yet peridot is relatively rare in crustal rocks because, in the
presence of water, it is highly susceptible to chemical
attack. In a process analogous to diamond formation in
kimberlite, peridot can form at depth in the mantle and
be carried to the surface relatively quickly in alkali basalt,
minimizing the opportunity for chemical attack.
Ruby and pink sapphire from Greenland, together with
gneissic host rock. The largest cut stone weighs 5.69 ct.
Samples courtesy of True Nor th Gems Inc.; photograph © Robert Weldon /
Gemological Institute of A merica
Figure 2
E lements
Preservation from extensive fracturing can be seen in gems
that grow in cavities (e.g. hydrothermal veins and pegmatites) as opposed to within solid rock; this is particularly
evident in emeralds from veins as opposed to those from
schist-type deposits. At the Stewart mine in California,
gem-forming fluids followed fractures, resulting in the
formation of near-vertical “chimneys” with pockets
containing pristine gem crystals that precipitated from
late-stage volatile fluids (J. Blue Sheppard, pers. commun.
2004; Fig. 3). However, pegmatite gems may also experience fracturing (probably from pocket-rupture events), as
well as etching and, particularly, chemical reequilibration.
Some pegmatites from Brazil contain partly dissolved beryl
crystals, giving rise to spectacular etched specimens but
fewer gems. Gem beryl crystals from the Ukraine also often
show the effects of etching.
Closer to the Surface
Some gem deposits result from shallow subsurface processes.
Opal (SiO2.nH2O), for example, may be deposited by hot
springs at shallow depths, by meteoric waters, or by lowtemperature hypogene solutions (Gaillou et al. 2008). It is
most often found lining and filling cavities in rocks, but may
also replace fossils. The formation of the valuable play-ofcolor implies very stable conditions during the slow accumulation of silica spheres in a regular, light-diffracting,
three-dimensional network. Curiously, this gem is found in
Blue Sheppard next to a “chimney” in the Stewart
mine, Pala, California. The chimneys are composed of
sodic plagioclase (albite–oligoclase), black tourmaline, and muscovite.
An excavated gem pocket can be seen at the base of the chimney.
Photograph © Brendan L aurs /Gemological Institute of A merica
155
Figure 3
J une 2009
two contrasting geologic environments: volcanic rocks such
as rhyolitic tuff (mostly poorly crystallized opal-CT) and
sedimentary rocks within basins (mostly amorphous opal-A).
Turquoise [CuAl6 (PO4)4 (OH) 8 ·4H2O] is a secondary mineral
usually found in the form of small veins and stringers
traversing more or less decomposed volcanic rocks
(“porphyry copper”) in arid regions. Green malachite
[Cu 2CO3 (OH) 2 ] and blue azurite [Cu3 (CO3) 2 (OH) 2 ] (Fig. 4)
are widely distributed supergene copper minerals found,
for example, in the oxidized portions of copper deposits
associated with limestones.
It is important to recognize that most gems, like other
commodities, are subject to the forces of supply and
demand. Although it is difficult to obtain accurate figures
for many of the gem varieties, it looks as though demand
is at least staying constant while traditional sources are
becoming depleted. For example, peak world diamond
production may soon be passed, and it is speculated that
the Colombian emerald mines are becoming exhausted,
whereas commercial quantities of tanzanite are (so far)
found at only one place in the world.
Many gems come from poor countries, where the discovery
of a new deposit could lead to a major change, for better
or worse, in the standard of living in the immediate area.
For example, the discovery of diamonds in 1967 transformed Botswana from one of the world’s poorest countries
into an upper-middle-income economy. However, poverty
rates in Botswana are high, and the distribution of income
and resources is extremely unequal. Some countries have
seen very little exploration, especially for colored gems
(Canada, with its huge land mass and low population
density, would seem to be a logical place to look).
Irrespective of country, the stability of the political regime
and security of mineral tenure are important issues. In
some places, smuggling is a major problem, and the gem
trade has been used to fund civil wars, rebellions, and
terrorist activities. A foolproof technology to track individual stones seems desirable.
GEM EXPLORATION
Cabochons consisting of intergrown blue azurite and
green malachite, from the Milpillas mine in Sonora, Mexico.
The larger stone measures 4.9 by 3.5 cm. Samples courtesy of Palagems.
com ; photograph © Robert Weldon /Gemological Institute of America
Figure 4
It is important to note as well that many gems occur in
secondary deposits of sedimentary origin. These form by
the accumulation, in basins of variable extent, of material
eroded from primary deposits. This material is primarily
transported by rivers. Gems found in such placer deposits
must be dense enough to be concentrated by gravity and
durable enough to survive transport. Because cracked and
weathered specimens are more likely to be destroyed during
transport and cleaner pieces more likely to survive, there
tends to be a higher ratio of gem to non-gem material in
alluvial deposits compared to primary sources. Typical
examples include some diamond and corundum deposits,
in which the proportion of gem-quality crystals increases
with distance from the source, the more fractured material
having been progressively destroyed along the way.
GEM PRODUCTION
Because many gems are produced from relatively small,
low-cost operations in remote regions of developing countries, it is difficult to obtain accurate statistics regarding
production and value (Yager et al. 2008). However, diamond
production in 2007 was an estimated 173 million carats
(worth US$13.9 billion) from some 20 countries, with
Botswana, Russia, Canada, South Africa, and Angola being
the top five producers by value (Read 2008). In 2001, the
world colored-gem trade was estimated to be worth about
US$6 billion per year (Beard 2001).
E lements
Exploration protocols for gems range from highly developed (diamonds) to unsystematic or nonexistent (most
other gem materials). Techniques used for diamond exploration include heavy mineral sampling and processing
(often using dense-media separators), indicator mineral
chemistry, and geophysics. These techniques are becoming
increasingly sophisticated as diamond exploration activities evolve. In Canada, it is also necessary to consider
regional advance and retreat patterns of glacial ice. The
indicator mineral technique is based on the recognition of
distinctive minerals in glacial sediments (chromiumpyrope, chromium-diopside, magnesium-ilmenite, and
olivine) associated with the diamond source rocks, and
then tracing them back to the source (see http://atlas.
nrcan.gc.ca/site/english/maps/economic/diamondexploration). A large fraction of most diamond exploration
budgets is allocated to high-resolution geophysical techniques operating from a variety of platforms. Most notably,
Shore Gold Inc. and Newmont Mining have used airborne
magnetic surveys to delineate kimberlites buried under
100 m of glacial overburden in central Saskatchewan, and
De Beers has operated a gravity survey system from an
airship (Read 2008).
Prospecting guides exist for some of the other gem materials, for example, emerald (see Groat et al. 2008). These
guides point out that mineral associations are important.
For example, chrysoberyl and phenakite are obvious indicator minerals for “metamorphic-type” emerald occurrences. Geochemistry has proven to be useful in Colombia.
Escobar (1978) studied the geology and geochemistry of
the Gachalá area and found that enrichment of Na and
depletion of Li, K, Be, and Mo in the host rocks were good
indicators for locating mineralized areas. Beus (1979)
presented the results of a United Nations–sponsored
geochemical survey of the streams draining emerald
deposits in the Chivor and Muzo areas of Colombia. The
spatial distribution of areas with emerald mineralization
was linked, on a regional scale, to intersections of northnortheast- and northwest-trending fault zones. The black
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J une 2009
shales in tectonic blocks containing emerald mineralization were found to be enriched in CO2, Ca, Mg, Mn, and
Na and depleted in K, Si, and Al (Beus 1979). The results
of this study were tested with a stream-sediment sampling
program in the Muzo area, and samples collected from
emerald-bearing tectonic blocks had anomalously low K/
Na ratios. Subsequently it was discovered that the Na
content of the sediments was the best indicator of the
mineralized zones in the drainage basins. Several new
emerald occurrences were discovered by United Nations
teams using the results of this study. Also, Ringsrud (1986)
reported that Colombian geologists were analyzing soil
samples collected from altered tectonic blocks for Li, Na,
and Pb to delineate emerald mineralization. Cheilletz et
al. (1994) showed that the Be content of black shales outside
of the leached mineralized areas ranges from 3.4 to 4 ppm.
Beryllium concentrations in the leached areas were found
to range from 0.1 to 3.0 ppm (Beus 1979). Structural
geology is also important for emerald exploration in
Colombia (Branquet et al. 1999). In the western zone (Muzo
and Coscuez areas), deposits are linked by tear faults and
associated thrusts.
Other publications available include exploration guidelines
for environments favorable for gemstone formation. For
example, Simonet and Okundi (2003) described and evaluated prospecting methods adapted to gemstone prospecting, including geological mapping, systematic eluvial
test pitting, geophysical and geochemical prospecting, and
remote sensing. They also presented a case study from the
Kisoli rhodolite, tourmaline, and ruby prospect in southern
Kenya, in which resistivity mapping, radiospectroscopy,
and soil geochemistry helped to identify geological conditions favorable for gem deposits. In another example,
Turner and Groat (2007) listed criteria for distinguishing
granites that could be parental to highly evolved granitic
pegmatites in the Canadian Cordillera; these include size
(smaller than 30 km2 ), affinity (S-type), age (mid-Cretaceous), geochemistry (peraluminous; enrichment in largeion-lithophile and high-field-strength elements; initial
strontium isotope 87Sr/ 86Sr ratio greater than 0.7100; large
negative neodymium anomaly), and mineralogy (peraluminous, e.g. containing both muscovite and biotite). A
survey of geophysical techniques used in gem exploration
was published by Cook (1997).
A new interest in mining and exploring for colored gem
deposits appears to be dawning, in general by smaller
companies of which an increasing number are listed on
world stock exchanges, in particular the TSX Venture and
AIM (Alternative Investment Market) exchanges. In addition, there has recently been a trend toward vertical integration, whereby a single company conducts exploration,
mining, beneficiation, and marketing. One example is
Pallinghurst Resources, which in June 2008 announced a
reverse takeover of Gemfields Resources Plc by one of its
portfolio companies, Rox Limited (see www.pallinghurst.
com). Rox contributed a 75% interest in the Kagem mine
in Zambia, Africa’s largest emerald mine, and an option to
acquire a portfolio of licenses for gemstone exploration in
Madagascar. Pallinghurst also announced that Fabergé
Limited, another portfolio company, has granted Gemfields
an option to acquire a worldwide and exclusive 15-year
license to use the Fabergé brand name for its better-quality
gemstones (excluding diamonds). Pallinghurst and
Gemfields claim that these transactions are key steps in
their objective to create a leading colored-gemstone
company and to pursue consolidation and vertical integration in the sector. Other examples include advanced exploration projects by True North Gems Inc. in Canada (for
emerald and sapphire; see Fig. 5) and Greenland (for ruby;
E lements
Brad Wilson using a diamond-bladed chainsaw to
extract emerald-bearing rock from the Ghost Lake
occurrence in northwestern Ontario, Canada
Figure 5
Fig. 2), and Cluff Resources Pacific NL, which has been
producing pink sapphire and ruby from placer deposits in
New South Wales, Australia.
see
CONCLUSIONS
Many questions (and therefore opportunities for research)
remain regarding the geology of gem deposits. For example,
researchers are only now beginning to understand the
genesis of marble-hosted ruby and sapphire deposits
(Giuliani et al. 2007). As pointed out by Groat et al. (2008),
there is a paucity of modern electron microprobe and trace
element composition data for emerald, and a comprehensive library of such compositions would be a valuable asset
for future researchers. In addition, the role of metamorphism in the formation of some emerald deposits is controversial (see Zwaan 2006) and, thus, worthy of additional
study. There is also a need for an unambiguous classification scheme that would aid in our understanding of the
mechanisms and conditions leading to the formation of
emerald deposits (Zwaan 2006).
However, understanding how gem deposits form is of more
than academic interest because it can provide guidelines
for exploration. Because of this, the geology of gemstones
is developing into a specialization within economic
geology. Although existing exploration guidelines are
starting to generate new discoveries, most new mines are
found by chance, not by design, and further development
of exploration guidelines is desirable for many gem materials. This, combined with new technologies, should ensure
a healthy supply of gems for the future. In this sense, not
only diamonds, but all gems “are forever.”
ACKNOWLEDGMENTS
Funding was provided by the Natural Sciences and
Engineering Research Council of Canada in the form of a
Discovery Grant to LAG. The authors thank Jean-Jacques
Guillou, Mackenzie Parker, Cédric Simonet, Bradley S. Wilson,
an anonymous reviewer, and Guest Editors Emmanuel
Fritsch and Benjamin Rondeau for their constructive criticism of earlier versions of the manuscript. 157
J une 2009
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