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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 153 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” 154 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 156 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 REFERENCES Beard M (2001) Under the table. Colored Stone 14: 500-503 Beus AA (1979) Sodium - a geochemical indicator of emerald mineralization in the Cordillera Oriental, Colombia. Journal of Geochemical Exploration 11: 195-208 Branquet Y, Laumonier B, Cheilletz A, Giuliani G (1999) Emeralds in the Eastern Cordillera of Colombia: Two tectonic settings for one mineralization. Geology 27: 597-600 Burns RG (1993) Mineralogical Applications of Crystal Field Theory (second edition). 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Mineralogical Association of Canada Short Course 37, pp 1-22 Stachel T, Brey GP, Harris JW (2005) Inclusions in sublithospheric diamonds: Glimpses of deep Earth. Elements 1: 73-78 Turner D, Groat LA (2007) Non-emerald gem beryl. In: Groat LA (ed) Geology of Gem Deposits. Mineralogical Association of Canada Short Course 37, pp 111-143 Yager TR, Menzie WD, Olson DW (2008) Weight of Production of Emeralds, Rubies, Sapphires, and Tanzanite from 1995 through 2005. US Geological Survey Open-File Report 2008–1013, 9 pp Zwaan JC (2006) Gemology, Geology and Origin of the Sandawana Emerald Deposits, Zimbabwe. Scripta Geologica 131, 211 pp Element displays for collection & science complete periodic table displays Living science - touch it, grasp it... Sensational ! Acrylic Element Cubes NEW! A novelty ! Stunning periodic element samples in generous amounts encapsulated in high-quality clear acrylic glass cubes 50x50x50mm. Labelled with element symbol, atomic number and element name. 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