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The Geology and Genesis of Gem Deposits

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CHAPTER 2: THE GEOLOGY AND GENESIS OF GEM CORUNDUM DEPOSITS

Gaston Giuliani1, 2, Daniel Ohnenstetter2, Anthony E. Fallick3, Lee Groat4, Andrew J. Fagan5,4

1 Université Paul Sabatier, GET, UMR 5563 CNRS-IRD-CNES-UPS, 14 avenue Edouard Belin, 31400- Toulouse, France; e-mail: [email protected]

2 Université de Lorraine, CRPG, UMR 7358 CNRS-UL, BP 20, 54501- Vandœuvre-lès-Nancy cedex, France

3 Scottish Universities Environmental Research Centre, Rankine Avenue, East Kilbride, Glasgow G75 0QF, Scotland, UK

4 Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada

5 True North Gems Inc., Vancouver, BC, Canada

INTRODUCTION The aim of this review is to present the state of ratnaraj, queen of precious stones and symbol of the art on the geology and genesis of gem corundum permanent internal fire. In Greco-Roman times, the deposits, including the main characteristics and reference to fire was common; in 400 CE classifications, to an audience of non-specialists, Theophraste (Eichholz 1965) related to anthrax academic students, professional geologists and (coal). Two thousand years ago Pliny (Littré 1850) gemologists. The careful detailed descriptions and called all the red stones carbunculus (diminutive of studies on the geology, and structures of carbo) which became escarboucle in old French. In the deposits carried out over the last century the 11th century, Marbode (Hernault 1890) separated provides a foundation to unravel their genesis "three times" three types of carbuncles that through new field and laboratory research including correspond to the three hues known for ruby petrography, mineralogy, , fluid (Myanmar, Thailand, and Sri Lanka), red inclusions, and stable and radiogenic isotopes. (balas, ruby–spinel, and pleonaste), and red Descriptive geology coupled with recent advances (pyrope, almandine, and spessartine). The exact in geology of gems permits decoding of how some origin of the term is unknown; it probably of these historical and fabulous deposits formed as comes from the Sanskrit sauriratna, became well as to estimate exploration potential in the sappheiros to the Greeks, then sapphirus to the future. New oxygen isotope data are presented here Romans. Up to the 13th century this term referred to for the first time. all blue , especially . Marbode used the term hyacinthe to refer to all colored HISTORY, GEOGRAPHIC DISTRIBUTION, sapphire. In the 18th century the contribution of the AND ECONOMIC SIGNIFICANCE French scientists to the characterization of the The term corundum likely originates from the corundum as a species was major. In 1784, Sanskrit "kurunvinda" meaning "hard stone", that Romé de l'Isle showed that ruby and sapphire were became kurund in Dravidian popular language similar minerals. In 1787, Brisson published the (corundum is kurund in German). This term might specific gravities, and in 1805 René-Just Haüy also have originated from the Tamil word kurmidam (Brard 1805) establishes the chemical formulae of (Anthony et al. 1997). The term "ruby" is derived both gemstones and formally united all varieties from the Latin word ruber. In Sanskrit, it was called under corundum (Haüy 1817). In the 19th century,

Mineralogical Association of Canada Short Course 44, Tucson AZ, February 2014, p. 29-112

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Chenevix in 1802 first established the chemical informative review of the corundum and spinel composition of ruby and sapphire. The work of deposits from Afghanistan in their Gemstones of Rose allowed the characterization of corundum as Afghanistan. The English edition of the book practically pure alumina (Rose 1843). Between Geology of Gems written by Kievlenko (2003) 1894 and 1921, the field studies in of offered an interesting overview on the geology of Lacroix and his team allowed the mineralogical gem corundum worldwide. The geology and genesis inventory of the island. In 1922, Lacroix published of gem corundum deposits was reviewed by his three-volume book on the Mineralogy of Giuliani et al. (2007a) in the first edition of the Madagascar, a fascinating treatise on Malagasian Mineralogical Association of Canada, Short Course mineralogy and especially on corundum, its 37; on the Geology of Gem Deposits (Groat 2007). occurrence, typology, and mineralogy. It was the Themelis (2008) wrote the first state of the art on first book to present a geological overview of the Gems and Mines of Mogok, the ruby-sapphire- worldwide corundum deposits. Today, blue gem spinel Mogok Stone Tract famous for centuries. (translucent or transparent) corundum is called Finally, the special issue edited by Ore Geology sapphire, and all other colors including yellow, Reviews (Graham et al. 2008a) on the Genesis of orange, violet, green, purple, peach-apricot, and Gem Deposits offered six papers on the topic of brown commonly called fancy are referred corundum deposits in contrasted geological to by color (e.g., yellow sapphire, orange sapphire, environments (Garnier et al. 2008; Graham et al. etc.). The valuable other colored sapphire is the 2008b; Rakotondrazafy et al. 2008; Simonet et al. orange-pink or pinky orange variety called 2008; Sutherland et al. 2008; Yui et al. 2008). In the padparadscha after the lotus blossom. European literature, in 1998 Weise published in Since 1990, gem corundum has been the subject Extra-Lapis Ruby, Sapphire, Corundum in German, of several reference books on the gemology, and Cesbron et al. (2002) detailed the mineralogy mineralogy, and geology of this fascinating and crystallography in Corundum and Spinel from . The following were written by Hughes: the French special issue of the Journal Minéraux & Corundum in 1990 gives an invaluable comprehen- Fossiles. More recently, the French Journal le Règne sive gemological and geological overview of gem Minéral published a review on the geology and corundum; Ruby and Sapphire in 1997 updated the classification of corundum deposits (Garnier et al. first edition and completed the story and lore of 2004a), a review on marble deposits from central ruby and sapphires, and Ruby & Sapphire: A and southeast Asia (Garnier et al. 2006a), a Collector's Guide by Hughes & Manorotkul in synthesis on the French corundum occurrences in 2014. Ward (1992) gave a general overview of gem the Saphirs et rubis de France (Giuliani et al. 2010), corundum in his book and Sapphires while and finally the state of the art on corundum deposits Themelis (1992) documented all the secrets of heat in the Neoproterozoic metamorphic Mozambique treatment applied to ruby and sapphire. Bowersox & belt (Giuliani 2013). Chamberlain (1995) provided a wonderful and very

Plate 1 A. Typical mineral association of purplish sapphire from the deposit ( region), Madagascar. The sapphire (1.6 x 1.4 cm) is associated with garnet (orange) and potash feldspar (white) in a biotitized feldspathic (black). Collection MNHN of Paris (N°108-1747). Photograph courtesy of L.-D. Bayle © le Règne Minéral. B. The ruby-bearing anyolite from Longido, Tanzania, is used for carving. The hexagonal prism of ruby (5 cm across) is contained in a gangue of green zoisite and black amphibole (tschermakite). Photograph courtesy of G. Giuliani. C. Polished slab of a colored zoned sapphire crystal from Garba Tula, Kenya. The blue and yellow sapphire (2.3 x 1.5 cm, 19.6 grams) was hosted by a monzonite dyke. The colors and chemistry are similar to those found for the green and yellow sapphires found in placers formed in basaltic environments. Collection J. Saul. Photograph courtesy of L.-D. Bayle © le Règne Minéral. D. Ruby crystal from Ampanihy, Madagascar. The ruby (6 x 4.8 x 1cm) was hosted by an amphibolite. Collection J. Béhier. Photograph courtesy of L.-D. Bayle © le Règne Minéral. E. Ruby in marble from the mine of Baw Padan, Mogok, Myanmar. The crystal is hosted in a gangue of calcite. Collection F. Barlocher. Photograph courtesy of L.-D. Bayle © le Règne Minéral. F. Transparent yellowish sapphire (2.3 x 0.7 x 0.5 cm) presenting blue zoning. Placer of Balangoda, southern Sri Lanka. Collection F. Lietard. Photograph courtesy of L.-D. Bayle © le Règne Minéral. G. Sapphires related to the Mont Coupet basaltic field in the French Massif Central, Saint-Eble, Haute-Loire. The highest crystal on the left: 12 x 8 x 8 mm. The cut stones: 5 x 4 mm. Collection and photograph courtesy of L.-D. Bayle © le Règne Minéral.

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Corundum is an industrial raw material, prized The main major and minor commercial, for its optical, mechanical and chemical qualities, industrial, and historical world sources of corundum and in jewelry for its gem varieties, ruby and are reported in Figure 2-1. The high-value gem ruby sapphire (Plates 1 and 2). Natural corundum was and sapphire are important mineral resources which mined in antiquity as emery, a mechanical mixture contribute significantly to the domestic product of of corundum and magnetite, used as a grinding and many countries. In 2005, Africa was responsible for polishing compound. The ancient Greeks called around 90% of world ruby production with 8,000 emery smyrnis after the harbor city of Smyrna, tons (Fig. 2-2) in addition to 42% of sapphire with which served as the main exportation for Naxos 10,700 tons (Yager et al. 2008). Highest quality emery in the Cyclades Islands (King 1865). The ruby crystals are from central and southeast Asia. Romans called emery naxium and Pliny in his Myanmar, with the Mogok Stone Tract, has Natural History (Littré 1850) reported the use of produced 'pigeon blood' rubies since 600 CE emery as an for and gem (Hughes 1997), and the Mong Hsu deposit, first engravers. Corundum was mined in North Carolina reported by Hlaing (1991), has ruby which was (USA) during two periods (Yurkovich 1985): the studied in detail by Peretti et al. (1995, 1996). Other main mining period, between 1871 and 1900, took Asian producers include Thailand (Chanthaburi-Trat place in the Clay and Macon counties accounting mines) and Cambodia (Pailin). Ruby deposits in for all of world production until 1893. An estimated Vietnam (Luc Yen, Yen Bai, Quy Chau), 3,720 to 7,000 tons of ore was produced and the Afghanistan (Jegdalek), Pakistan (Hunza valley), main site of extraction was the Corundum Hill Azad-Kashmir (Batakundi, Nangimali), and Mine. The second mining period, 1914–1919, Tajikistan (Kukurt, Turakoluma, Badakshan) have reactivated the mining operations at Corundum Hill. become significant producers as the traditional areas South Africa was also a main producing country in in Thailand have become depleted. Gem quality the early twentieth century (Kupferburger 1935). ruby is also extracted in Tanzania (Umba-Kalalani, The corundum was obtained from desilicated Morogoro, Mahenge, Winza), Kenya (Mangare), in Northeast Transvaal (Robb & Robb Mozambique (Montepuez, Ruambeze, M'sawize), 1986). Production fell from 3876 tons per year in and Madagascar (Vatomandry, Andilamena, Didy). 1918 to 74 tons per year in 1979, and the use of Blue sapphire from Indian Kashmir (Sumjam) is the synthetic hastened the closure of the source of the world's best quality, as well as those mines. Today, natural corundum abrasive is used in from Myanmar and Sri Lanka. Australia, Thailand, the fabrication of window glass for , and Cambodia, Nigeria, Laos, United States, manufacturing ceramics. Alumina powder is used as Madagascar, and China are also traditional sources a refractory mineral for traditional, composite for blue sapphire. Colored sapphire is found ceramics and for neoceramics due to its high worldwide, but Sri Lanka is thus far the only temperature of fusion (T > 2000°C). Alumina has a country to produce excellent quality crystals of all high resistance to chemical attack and abrasion, colors, including salmon colored padparadscha. high electric and mechanical resistance, is inert in Colored sapphire deposits were discovered in reductive and oxidative atmospheres, and has a low Tanzania (Tunduru and Songea) and in Madagascar dielectric loss. (Ilakaka, Ambondromifehy, Vatomandry). In 2005

Plate 2 A. Pink sapphire crystals (foreground) with layering in marble. Revelstoke ruby occurrence in British Columbia, Canada. Photograph courtesy L. Groat. B. Cut stones from the Revelstoke ruby and sapphire occurrence. From the bottom to the top the weight of the sapphires are respectively of 0.38, 0.30, 0.20, and 0.14 carats. Photograph courtesy of B.S. Wilson. C. Non-gem ruby in phlogopitite matrix, minor kyanite (blue) and plagioclase (white) also observable, hosted in leucogabbro subjected to metamorphic metasomatism. This material was extracted at surface from the main ore zone at the Aappaluttoq ruby deposit, southwest Greenland, Denmark. The weight of the different cut stones are respectively from left to right: on the bottom 2.37, 0.71 and 4.03 ct, in the middle 0.73, 0.42, and 0.31 ct, and on the top 2.53, 0.77, 5.70, 1.26, and 1.34 ct. Photograph courtesy of True North Gems Inc. D. Faceted rubies and pink sapphires from the Aappaluttoq deposit, Greenland, Denmark. Photograph courtesy of True North Gems Inc. E. Amphibolitic gneiss hosting ruby in the M'sawize deposit at Niassa, Mozambique. The ruby is associated with plagioclase (white), phlogopite (brownish-yellow) and amphibole (greenish). Photograph courtesy of V. Pardieu © GIA. F. A 40-grams silky ruby crystal from Montepuez, Mozambique. Photograph courtesy of V. Pardieu © GIA.

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FIG. 2-1. Main major and minor commercial, industrial, and historical world sources of corundum, ruby and sapphire. Madagascar: 1- Ambondromifehy, Anivorano, Nosy Be, Ambato peninsula; 2- Andilamena, Didy, Vatomandry; 3- Ankaratra (Antsirabe-Antanifotsy region, Soamiakatra); 4- Ilakaka-; 5- Tranomaro (Andranondambo). Ethiopia: 6- Kibremengist, Dilla. Kenya: 7- Turkana, 8- Garba Tula; 9- Kitui; 10- Mangare (John Saul mine). Tanzania: 11- Umba valley; 12- Longido; 13- Winza; 14- Morogoro, Mahenge; 15- Songea; 16- Tunduru. Malawi: 17- Chimwadzulu. Mozambique: 18- Montepuez (Namahumbire/Namahaca), M'sawize, Ruambeze. Zimbabwe: 19- O'Briens (verdites). South Africa: 20- Barberton (verdites); 21- Leydsdorp (Cobra pit). Cameroon: 22- Adamawa district. Nigeria: 23- Kaduna plateau. Brazil: 24- Indaia. Chile: 25- El Salvador. Colombia: 26: Mercaderes. United States: 27- Plumas; 28- North Carolina; 29- Montana. Canada: 30- Revelstoke; 31- Baffin Island. Denmark: 32- Greenland (Fiskenæsset district). Scotland: 33- Loch Roag. Germany: 34- Eiffel. Switzerland: 35- Campo Lungo. France: 36- French Massif central. Italy: 37- Piemont. Czech Republic: 38- Jizerska Louka. Slovakia: 39- Cerová Highlands. Norway: 40- Froland. Finland: 41- Kittilä. Macedonia: 42- Prilep. Greece: 43- Xanthi; 44: Naxos. Turkey: 45- Aidin province. Tajikistan: 46- Turakuloma, Badakhshan. Afghanistan: 47- Jegdalek. Pakistan: 48- Hunza valley; 49- Batakundi, Nangimali. India: 50- Sumjam; 51- Orissa, Kalahandi; 52- Karnakata (Mysore); 53- Andhra Pradesh (Salem district). Sri Lanka: 54-Ratnapura, Elahera, Kataragama. Nepal: 55- Chumar, Ruyil. China: 56- Qinghai; 57- Yuan Jiang; 58- Hainan island (Penglai); 59- Fujian (Mingxi); 60- Jiangsu; 61- Shandong (Changle). Vietnam: 62- Luc Yen, Yen Bai; 63- Quy Chau; 64- Da Lat, Ban Me Thuot, etc. Myanmar: 65- Mogok; 66- Mong Hsu. Thailand: 67- Kanchanaburi, Bo Phloi; 68- Chantaburi-Trat. Cambodia: 69- Pailin; 70- Den Chai-Phrae; Laos: 71- Ban Huai Sai. Japan: 72- Ida. Australia: 73- Rosville-Lakeland Downs; 74- Lava Plains; 75- Anakie fields; 76- West-Brisbane fields; 77- New England fields (Inverell); 78- Cudgegong, Barrington; 79- Tumbarumba; 80- Western Melbourne fields; 81- Tasmania; 82- Harts range. New Zealand: 83- Westland (Hokitika). Russia: 84- Podgelbanochny, Shkotovo plateau; 85- Primorye, Nezametnoye; 86- Polar Urals (Hit Island); 87- Ural mountains (Kuchin-Chuksin-Kootchinskoye); 88- Karelia. the Canadian company True North Gems Inc. Malagasy rubies are treated and cut in Bangkok and collected ruby and pink sapphire from the sold at Idar-Oberstein in Germany. Bangkok is also Aappaluttoq deposit near Fiskenæsset the main center of distribution for sapphires from (Qeqertarsuatsiaat), Greenland (Rohtert & Ritchie Australia, Thailand, and Cambodia. Sapphires from 2006, Fagan et al. 2011, Fagan 2012). Other Sri Lanka are sometimes negotiated in Colombo, but countries known to produce gem corundum the greater part is exported to Bangkok. Today, sporadically include Brazil (Malacacheta, Indaia), Hong Kong also remains an important trade center Canada (British Columbia, Baffin Island), Colombia for ruby and sapphire. (Mercaderes), Malawi (Chimwadzulu), Russia The ruby and sapphire industry is a very (Urals), and the United States (Idaho). lucrative business. Hughes (1997) devoted a chapter Rubies from southeast Asia (Vietnam, to the corundum gem market and pricing, ‘How to Myanmar) are cut and sold in Bangkok. African and judge the quality of fine ruby and sapphire and how

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FIG. 2-2. Global production of sapphire (A) and ruby (B) in 2005 with the percentages (%) for African countries (modified from Yager et al. 2008). Global BGY sapphires production by country (C) and by type of deposit comparing BGY sapphires in basaltic fields to other geological types (D). to define a price of sale?’ The term ‘fine’ may vary Sapphire has not been affected by such controversy from one store to another, and among different and the prices have been stable around 2500 USD/ct countries, and so too the price. When buying natural since 1991. In 2001 the input on the gem market of quality ruby and sapphire, the criteria are based on a new kind of heat-treated orange sapphire has the geographic origin, but also on the clarity, the caused much controversy and the issue of erroneous color, the cut, and the weight in carats of the gem certificates bearing the name of natural (Hughes 1997). Enhancements and treatments are padparadscha (McClure et al. 2002, Fritsch et al. also of importance in the final evaluation, and it is 2003). Natural sapphire from Songea, Ilakaka and very rare to find gem corundum that has not been even Burma and Thailand were heat-treated at heat-treated (Themelis 1992). Thailand has great 1800°C and in an oxidizing atmosphere in the expertise in heat treatment and today crystals of presence of Be. Beryllium diffused from the outside corundum are routinely heat-treated in Thailand and of the stones inward, adding a yellow and orange Australia. Drucker (in Shigley et al. 2000), at the component and more clarity to the color of the end of 1990, explained that the price of ruby fell natural stone. Nowadays this sapphire is certified after controversy over its treatments. The medium diffusion-treated sapphire not to be confused with price of sale went from 4000 to 6000 USD/ct natural untreated and heat-treated padparadscha. between 1990 and 1992, and the price remained The prices for fine untreated padparadscha stable up to 1997. The discovery of the Mong Hsu sapphires are quite demanding and only a few have deposit in 1991 provided a new global source of the opportunity to purchase them. Even heated excellent Burmese rubies, but the availability on the padparadscha sapphire is quite expensive if it is market of heated remnants of Mong Hsu rubies good quality. caused a fall in the price, to less than 4000 USD/ct, Ruby and sapphire are the most important between 1997 and 2000. Recent discoveries and colored gemstones in today’s world gem trade; exploitation of ruby associated with biotitite in together they account for over 50% of global amphibolite such as at the Aappaluttoq (Greenland) colored gem production (Hughes 1997). Ruby is and Montepuez (Mozambique) deposits will ensure perhaps the world’s most expensive gemstone; the the supply of gem ruby in the near future (Plate 2). best Burmese rubies are more valued than an They represent huge tonnage of ruby, approximately equivalent-sized awless colorless 400 million cts for the Aappaluttoq deposit, with (Walton 2004). The world record price at auction around 5% classed as gem grade (Fagan 2012). paid for a single ruby is USD 210,000 per ct in 2012

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for a 32.08 ct Burmese ruby (total USD 6.74 stacked atoms of O leave between them a tetrahedral million). The world record price paid at auction for site whereas six contiguous atoms of O leave a single blue sapphire is USD 3.84 million for a between them a bigger octahedral site occupied by spectacular 26.41 ct cushion shaped Kashmir Al3+. As there are two atoms of Al for three atoms sapphire ($145 342 per ct). Recently discovered of O, two octahedral sites in three are occupied in ruby and sapphire from the Didy mine in each layer of Al. The chromophorous elements of Madagascar (Pardieu 2012) also commands high corundum, such as Cr3+, enter the corundum lattice prices for natural unheated ruby. A set of eight in place of Al. faceted rubies ranging from 7 to more than 14 ct Euhedral crystals can present different faces exceeded an estimated market value of USD (Fig. 2-4) that correspond to seven crystalline forms 10 million (Peretti & Hahn 2013). (Cesbron et al. 2002): the pinacoid {00.1}, the first order hexagonal prism {10.1} and second order MINERALOGY, PHYSICAL PROPERTIES {11.0}, the hexagonal prism {kk.0}, the hexagonal AND GEOCHEMISTRY dipyramid {hh.l}, the ditrigonal scalenohedron Corundum belongs to the group {hk.l} and the rhombohedron {h0.l}. The first five (X2O3) of rhombohedral oxides comprising hematite crystalline general forms are also present in the (Fe2O3), corundum (Al2O3), eskolaite (Cr2O3), classes that belong to the hexagonal system. karelianite (V2O3), and tistarite (Ti2O3). There are Corundum can also crystallize in a particular texture no solid solutions between any of the five species called 'trapiche' (Fig. 2-5), formed by six skeletal but they have the same type of structure. Hematite arms which separate six sectors of growth group mineral structures are based upon hexagonal (Sunagawa et al. 1999, Garnier et al. 2002a). closest packing of O atoms, with cations in Corundum has a hardness of 9 on the Mohs scale octahedral coordination. Corundum crystallizes in and the specific gravity is between 3.98 and 4.06. the 3 2/m class of the rhombohedral system with, for The is adamantine to vitreous, transparent to the hexagonal multiple unit cell, a between 4.76 and translucent. The is denied in the literature 5.04 Å, and c between 12.99 and 14.01 Å (Cesbron but it is present in synthetic colorless sapphire (in et al. 2002). The smallest unit cell volume of the Hughes 1997). Cleavage is found along the 3+ group is for corundum (rAl = 0.54 Å) and the rhombohedron {h0.l} and the prism {kk.0}. Parting 3+ largest for hematite (rFe = 0.65 Å). The network of is identical to cleavage and is conchoidal to 2– 2– O ions (rO = 1.40 Å) is formed practically by a sub-conchoidal when the break does not follow the hexagonal closed packing formed by layers of ions cleavage. The melting point of corundum is 2044°C arranged successively in the order ABAB...etc. (Fig. and its boiling temperature as 3500°C. 2-3). All the layers are stacked two-two. Four

FIG. 2-3. A, Hexagonal close packing of O2– spheres. In the second layer, the spheres are located in the B position, at the contact with three spheres of the lower layer, the third one overlapping in the first one. In a cubic-centered faces, the spheres of the third layer would occupy the C position. B, Octahedral site formed by the superposition of three spheres in the position B on three others in position A. Metallic ions are placed in the C position and they are in octahedral coordination. At the left is a vacant tetrahedral site in the structure of corundum (Cesbron et al. 2002). C, The structure of corundum looking down the c axis (Hughes 1997). D, Perspective view of the corundum structure. Only two of three octahedral sites are occupied in each Al3+ layer.

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and phase equilibria are summarized by Bowles (2011). Pure corundum is colorless; the array of gem corundum colors is due to impurities (chromo- phores) that substitute for Al (in the form Al3+) in the internal structure. Hughes (1997), Fritsch & Rossman (1987, 1988), Emmett et al. (2003), Peretti et al. (2008), Schwarz et al. (2008) and Rossman (2009) provided excellent reviews and articles on current understanding of corundum coloration. Chromophorous elements are metallic ions from the FIG. 2-4. Crystalline forms of the 3 2/m class of the transition elements family of the periodic table. rhombohedral system (after Cesbron et al. 2002). A, When chromium (in the form Cr3+) replaces Al3+, a positive rhombohedron {10.1}; B, negative red color results in the crystal but only if the Cr O rhombohedron {01.1}; C, hexagonal dipyramid {hh.l}; 2 3 content is between 0.1 to 3.0 wt.% (Hughes 1997). D, pinacoid {00.1}; E, hexagonal prism of first order {10.1}; F, hexagonal prism of second order {11.0}; G, However concentrations of 9.4 wt.% were detected dihexagonal prism {hk.0}; H, ditrigonal scalenohedron in the ruby of Karelia (Bindeman & Serebryakov {hk.l}. 2011)) and up to 13 wt.% in the ruby hosted in a rock called ‘goodletite’ within serpentinite from Corundum is uniaxial negative. Pure corundum Westland in New Zealand (Grapes & Palmer 1996). has a nȦ = 1.7687 and nİ = 1.7606. The 'goodletite' is composed of ruby, and Colored corundum with Fe and/or Cr atoms has a green fuchsite and named for its discoverer variation respectively between 1.766 and 1.780 for William Goodletite. of nȦ and 1.758 and 1.772 for nİ. The birefringence The optical absorption spectrum of ruby is extremely consistent at 0.008–0.009. Additional consists of two large absorption bands (Fig. 2-6). optical and physical properties of corundum as well They are situated at highest energy with two as experimental work on its solubility, dissolution transmission windows at 480 nm centered in the

FIG. 2-5. Trapiche textures. A, Trapiche ruby from Mong Hsu. B, Texture in trapiche emerald (left) and trapiche ruby (right) following a basal section (Sunagawa et al. 1999). Longitudinal section parallel to the c axis in a trapiche ruby (bottom). C, Detail of the core of a trapiche ruby from Mong Hsu (Garnier et al. 2002a).

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FIG. 2-6. Typical polarized UV–Vis absorption spectra of sapphire and ruby. A, polarized spectra (E//c) of four sapphires of different deposits (Notari & Grobon 2002). The interpretation of color, indicated for iron (Fe3+, line at 388nm and Fe3+ pairs, lines at 450nm and 377nm), and Fe and Ti (Fe2+ Æ Ti4+, band at 565nm). B, polarized absorption spectra (E//c) of Winza ruby (Schwarz et al. 2008) and orangey-pink sapphire from Madagascar (Peretti & Günther 2002). The spectrum of ruby shows the well-known Cr3+ absorption bands at 405–410 and 560nm, and the Cr "doublet" at 694nm. The sapphire from Madagascar is reminiscent of padparadscha color (Fe + Cr), with for Cr (Cr3+, band centered at 405–410 nm and 554 nm), for Fe (Fe3+, line at 388 nm, and Fe3+ pairs, lines at 450 nm, and bands at 540 nm), and for Ti (Fe2+ Æ Ti4+ charge transfer band centered at 565 nm). blue, and at 610 nm centered in the red. As the of other elements such as Fe, Ti and Cr (Fritsch & human eye is more sensitive in the red just above Rossman 1987, Hughes 1997), and charge transfer 610 nm than in blue, ruby appears red. In the involving Fe2+, Fe3+, Ti4+ (Fig. 2-6), and color corundum structure, Cr3+ occupies a site where the centers (Fristch & Rossman 1988). Less than 0.01% distance between the metallic ion and O is short of Fe and Ti are necessary to obtain the blue color (1.913 Å), and in consequence the Cr3+ ions have of sapphire. high electrostatic repulsion and the absorption Color from fuchsia to reddish can be ascribed to feature is shifted to higher energies than for other sapphires (Caucia & Boiocchi 2005, Ralantoarison Cr-bearing minerals such as green Cr-zoisite where et al. 2006, Andriamamonjy et al. 2013) and they the average metal–O distance is 1.967 Å (Dollase then resemble ruby; however, the concentration of 1968), causing the red color. Chromium-related red Fe is always higher than Cr and the fluorescence is fluorescence under ultraviolet light and sometimes very low because Fe quenches the fluorescence even in daylight, combined with the red color of effect. In marble deposits, a crystal of ruby can be ruby, is the cause of the particular fire effect found made up of regions of ruby and regions of pink in Burmese and Vietnamese rubies. Substitution of sapphire but in each zone, the content of Cr2O3 will 3+ 2+ 3+ 4+ Al by iron (Fe or Fe ) and (Ti ) always be higher than the content of Fe2O3. The generates the blue color of sapphire by a mechanism definition of ruby (like emerald; Giuliani et al. of heteronuclear intervalence charge transfer 2004) has to take into account not only the color, 2+ 4+ between metal ions, i.e., Fe –O–Ti (Fig. 2-6) The but also the chemistry (Cr2O3 > Fe2O3), and the heat treatment of colorless ‘geuda’ sapphires absorption spectra features. The orange-pink to produces blue stones because during the dissolution pinkish-orange colored corundum variety is called of inclusions of rutile and spinel at high ‘padparadscha’ (Notari 1997), a term derived from temperatures, Fe and Ti are released into the crystal the Sanskrit padmaraga, signifying the color of the and form pairs of Fe2+–O–Ti4+, which then allow lotus flower. The color is due to the presence of Cr3+ intervalence charge transfer as Fe2+ ĺ Ti4+ in octahedral coordination (pink hue) and of Fe3+ in (Themelis 1992). Color in colored sapphires is due intervalence charge transfer with O2– (yellow hue; to the classical substitution of Al3+ by a combination Schmetzer & Bank 1981).

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THE GEOLOGY OF GEM CORUNDUM aluminous gneiss, mafic and ultramafic rocks (M– DEPOSITS UM), or juxtaposed felsic and silica-poor rocks Corundum forms in mafic and siliceous (limestone, marble, M–UM) affected by the geological environments, but it is always associated circulation of fluids at their contact and altered by with rocks depleted in silica and enriched in metasomatism (desilicated pegmatite, skarn, etc.); alumina. In the presence of silica, Al is and (2) alkaline basaltic volcanism in continental preferentially incorporated into rifting environments. minerals such as feldspars and micas. The mineral Marble is a silica- and alumina-depleted rock, association of corundum commonly consists of and under these conditions the transport of alumina plagioclase, , biotite, phlogopite, by a fluid phase seems necessary for the crystalliz- amphibole, pyroxene, and carbonate minerals ation of corundum; however, Al is usually (Garnier et al. 2004a). considered to be an immobile element. In meta- Gem corundum is rare because its requires an morphic rocks, corundum crystallizes at high environment enriched in alumina and impoverished temperatures and high to medium pressure such as in silica, but also the presence of Cr, Fe, and Ti to in garnet–clinopyroxene assemblages in clino- substitute for Al in the structure (Muhlmeister et al. pyroxenite and metagabbro from the lower crust at a 1998, Abduriyim & Kitawaki 2006), and temperature around 1100°C and a pressure about 20 thermobarometric conditions favorable for its kbar (Rakotosamizanany 2009). crystallization and stability (Kievlenko 2003, Corundum rarely occurs in granite; for Simonet et al. 2008). Corundum can be hosted in example, Wilson (1975) described blades of blue different rocks type but two major geological sapphire associated with pink in the environments are favorable for the presence of gems Land's End granite in Cornwall. The author (Simonet et al. 2008). These are (1) amphibolite- to suggested that the corundum and andalusite formed medium pressure -facies metamorphic in xenoliths of pelitic rocks assimilated by the belts. The gem corundum field corresponds to a granite. Sapphire is also described in Chilean skarns domain above 3 kbar, and temperature between 500 formed in the thermal anomaly accompanying the and 800°C (Fig. 2-7). The lithologies are alumina- porphyry intrusions in El Salvador and Los rich and/or silica-poor rocks such as marble, Prelambres mines (Singer et al. 2008).

FIG. 2-7. P–T conditions for the formation of corundum in metamorphic deposits (modified from Simonet et al. 2008). P–T fields of North Carolina (Tenthorey et al. 1996), Mangare (Mercier et al. 1999a), Morogoro (Altherr et al. 1982), southern Kenya (Key & Ochieng 1991, Simonet 2000), Hunza (Okrusch et al. 1976), Sri Lanka (De Maesschalk & Oen 1989), Greenland (Garde & Marker 1988), Kashmir (Peretti et al. 1990) with three P–T boxes corresponding to the evolution of the fluids in the sapphire crystals from the center (c), to intermediate (i) and outer (o) zones, Urals (Kissin 1994), and Mong Hsu (Peretti et al. 1996).

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During the past two last decades, our al. (2008), and Nissimboim & Harlow (2011). The knowledge of the formation of corundum deposits Jegdalek ruby deposit in Afghanistan, described by has improved significantly (see papers in Groat Al-Biruni in the 11th century, was only recently 2007, Graham et al. 2008a). However, there exists studied by Bowersox & Chamberlain (1995). little genetic data on primary gem corundum Garnier (2003) and Garnier et al. (2008) reported deposits, i.e., gem corundum that is hosted by a detailed geological and geochemical studies of ruby mother rock, certainly insufficient to understand the deposits from the Hunza valley (Pakistan), formation of all deposits and to elaborate precise Nangimali (Azad-Kashmir), and Luc Yen-Yen Bai genetic models. Magmatic deposits have been (northern Vietnam), and proposed a new genetic studied the most. The geology and mechanisms of model for marble-hosted ruby. Likewise, the formation of basalt deposits in Asia and Oceania economic interest in blue and padparadscha have been the focus of several studies (Coenraads et sapphires found in placer deposits from Sri Lanka al. 1990, 1995, Levinson & Cook 1994, O'Reilly & has stimulated local geologists to characterize the Zhang 1995, Guo et al. 1996a, 1996b; Sutherland & parental rocks to guide exploration in the Highlands Coenraads 1996, Sutherland et al. 1998a, 1998b, Precambrian series of the island (Rupasinghe & Limkatrun et al. 2001, Sutherland & Schwarz 2001, Dissanayake 1985). Sutthirat et al. 2001, Garnier et al. 2005, Graham et Further, the majority of geological studies are al. 2008b, Sutherland et al. 2009a, 2009b, performed with economic objectives and the reports Sutherland & Abduriyim 2009, and Abduriyim et al. remain confidential (Bassett 1984, Malik 1994). 2012), as well as their ages of formation (Coenraads Because of this most published research is focused et al. 1990, Guo et al. 1992, Coenraads et al. 1995, on the gemology and mineralogy of corundum, and Khin Zaw et al. 2006, Sutherland et al. 2008, details of the geological context, mechanisms of Graham et al. 2008b). Sapphire associated with formation, and the origin of the mineralizing fluids lamprophyre at Yogo Gulch in Montana (USA) was are scarce (Okrush et al. 1976, Peretti et al. 1990, studied by Voynic (1985), Meyer & Mitchell Mercier et al. 1999a, Giuliani et al. 2003, Garnier et (1988), Brownlow & Komorowski (1988), Gauthier al. 2008, Schwarz et al. 2008). Oxygen isotopic data et al. (1995) and Harlan (1996). Sapphire hosted in for corundum are now available (Gauthier et al. monchiquite at Loch Roag in Scotland was 1995, Pomian-Srzednicki 1997, Upton et al. 1999, described by Jackson (1984) and Upton et al. (1983, Yui et al. 2003, 2006, 2008, Giuliani et al. 2005, 1999, 2009). In Kenya, Simonet et al. (2004) 2007a, 2007b, 2009, 2012a, Khin Zaw et al. 2006, studied in detail the Garba Tula sapphire deposit Sutherland et al. 2009a, 2009b, Uher et al. 2012). hosted by dykes of syenite. Studies of albitite dikes The global distribution of corundum deposits is in serpentinized lherzolite in the Pyrenees, France, closely linked to collision, rift and subduction has shed new light on the origin of corundum and geodynamics. Three main periods of corundum associated xenoliths in alkali basalts (Monchoux et formation are recognized worldwide as described al. 2006, Pin et al. 2006). Finally, sapphire and ruby below (Giuliani et al. 2007a, Graham et al. 2008b, inclusions in diamond from Brazil were reported by Stern et al. 2013). Watt et al. (1994) and studied by Hutchinson et al. (2001, 2004). The Pan-African orogeny (750 – 450 Ma) Models of formation of the other types of Primary ruby and sapphire deposits found in the deposits, i.e., metamorphic, metasomatic–hydro- gemstone belt of eastern Africa, Madagascar, India thermal, have been insufficiently developed. Indeed, and Sri Lanka are linked to the collisional processes the most famous deposits are not necessarily between East and West Gondwana (Fig. 2-8), during accessible due to government policy or military Pan-African tectonometamorphic events (Kröner conflicts (e.g., Azad-Kashmir, Afghanistan, 1984). The metamorphic ruby and sapphire deposits Myanmar, Tajikistan, Colombia), and others (such from southern Madagascar have numerous as Madagascar) have not been systematically geological similarities with those from eastern investigated (Rakotondrazafy et al. 1996, Africa (Mercier et al. 1999a, 1999b, Giuliani et al. Rakotosamizanany et al. 2009a, 2009b, Peretti & 2007b Rakotondrazafy et al. 2008, Feneyrol et al. Hahn 2013). The Mogok Stone Track in Myanmar 2013), Sri Lanka (Rupasinghe & Dissanayake 1985, was mapped by Iyer (1953), but modern geological Silva & Siriwardena 1988, Dharmaratne et al. investigations were lacking until Harlow et al. 2012), and South India (Santosh & Collins 2003). (2006), Kyaw Thu (2007), Themelis (2008), Yui et Based on correlations between gem- and graphite-

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FIG. 2-8. Juxtaposition of Antarctica, Sri Lanka, India, Madagascar, and Eastern Africa in a tight-fit reconstruction of Gondwana with the location of the main gem corundum deposits (modified after Windley et al. 1994, Dissayanake & Chandrajith 1999, Mercier et al. 1999b, Collins & Windley 2002; Santosh & Collins 2003, Rakotondrazafy et al. 2008, Feneyrol et al. 2013). Eastern Africa: GT, Garba Tula; M, Mangare; T, Twiga; SN, Si Ndoto; LL, Longido and Lonsogonoi; MM, Mahenge and Morogoro; K, Kalalani and Umba; S, Songea; T, Tunduru. Mozambique: Mz, Montepuez; Madagascar: Ila, Ilakaka; ZS, Zazafotsy and ; Vo, all the deposits from the Vohibory region; Am, Ambatomena; An, Andranondambo; And, Andilamena; Dy, Didy. South India: My, Mysore; KK, Karur-Kangayam corundum belt; Or: Orissa. Sri Lanka: RE, Ratnapura and Elahera, WC, Wanni complex; HC, Highland complex. Antarctica: NC, Napier complex; RC, Rayner complex; LHC, Lützow-Holm complex; YBC, Yamoto-Belgique complex.

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bearing rocks Menon & Santosh (1995) and with sapphire and ruby from the deposits of Dissanayake & Chandrajith (1999) proposed the Ambatomena, Sahambano and Zazafotsy range existence of a Neoproterozoic–early Cambrian between 494 and 487 Ma (Giuliani et al. 2007b). gemstone province developed under granulite-facies These ages confirm a corundum mineralizing conditions in East Gondwana. The common episode during the late Pan-African orogenic cycle presence of ruby and pink sapphire in Kerala and (Cambrian), specifically corresponding to the southern Madagascar led Santosh & Collins (2003) Kuunga orogeny (Meert et al. 1995, Meert 2003). to support the view that the Achankovil shear zone in southern India extends into the Ranotsara fault The Cenozoic Himalayan orogeny (45 Ma – zone (Fig. 2-8) in Madagascar (Collins & Windley Quaternary) 2002). However, few studies have concentrated on The marble-hosted ruby deposits in central and the petrology, mineralogy and geochronology of the eastern Asia occur in metamorphic blocks that were extensive mineralization associated with the affected by major tectonic events during the formation of Gondwana. The ages of formation of Cenozoic Indo-Asian collision (Garnier et al. protolith ruby have been dated by U–Pb in 2006a). The ruby was indirectly dated by 40Ar/39Ar from the John Saul mine in Kenya, Longido in stepwise heating experiments performed on single Tanzania, and Vohibory deposits in Madagascar. grains of syngenetic phlogopite (Garnier et al. The respective ages of 612 ± 6 Ma (Simonet 2000), 2002b, Pêcher et al. 2002, Garnier et al. 2006b), and 610 ±0.6 Ma (Le Goff et al. 2010), and 612 ±5 Ma by ion probe U–Pb analyses of zircon inclusions in (Jöns & Schenk 2008) pinpoint a similar period of corundum (Garnier et al. 2004b, 2006b). All of the formation related to the East African orogeny. Oligocene to Pliocene ages (40–5 Ma) are consistent Moreover, U–Pb ages of zircon coeval with blue with extensional tectonic events that were active gem sapphire in the Andronandambo skarn deposit from Afghanistan to Vietnam, in the ruby-bearing range between 523 and 510 Ma (Paquette et al. metamorphic belt (Fig. 2-9). 1994), and the 40Ar/39Ar ages of biotite syngenetic

FIG. 2-9. Location of marble-hosted ruby deposits from central and southeast Asia. Ar–Ar dates on phlogopite syngenetic with ruby are from Garnier et al. (2006b).

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The Cenozoic alkali basalt extrusions (65 Ma – 1998), the genetic processes responsible for Quaternary) corundum formation (Simonet 2000, Simonet et al. Gem corundum occurs worldwide as xenocrysts 2008), the genetic type of deposits (Kievlenko or megacrysts either in xenoliths or enclaves 2003), the type of deposit and the nature of the incorporated in basaltic magmas during their ascent. corundum host rock (Garnier et al. 2004a; Walton Such sapphire and ruby deposits occur from 2004, Giuliani et al. 2007a), and the oxygen isotopic Tasmania, Australia, in the south, through eastern composition of corundum (Giuliani et al. 2005, Australia, Southeast Asia and eastern China to far 2012a). The geological classification schemes group eastern Russia (Graham et al. 2008b). They are also well-known deposits with lithological criteria as a found in Nigeria and Cameroon, in the Aïr and common basis (marble, gneiss, basalt, and others). Hoggar regions (Wright et al. 1985, Boaka à Koul et With this approach, the physical and chemical al. 2011), the French Massif Central in the Limagne mechanisms of corundum formation, the source of rift (Merle et al. 1998, Gaillou et al. 2010, Giuliani the fluids, and the origin of the elements are et al. 2009, 2010), and in Northern and Central generally not constrained by stable and radiogenic Madagascar (Rakotosamizanany 2009, Rakoto- isotopes, fluid inclusions, and geochemistry; and samizanany et al. 2009a, 2009b). therefore the genesis of the deposits remains In general, U–Pb isotope ages of zircon in uncertain. For example, for marble-hosted ruby sapphire-bearing xenoliths show ages that are deposits, the presence of granite in the proximity of similar to K–Ar and 40Ar/39Ar ages of the basalt the marble does not imply that the deposit is a skarn (Sutthirat et al. 1994, Sutherland et al. 2002, 2003). type related to the emplacement of the granite, as The zircon inclusions dated in magmatic sapphire suggested by Kievlenko (2003). Dating of the are considered to represent corundum crystallization mineralization, its host rock, and the intrusion is the ages. In the Australian and Asian West Pacific first step to take after fieldwork to create a true margins intraplate basaltic fields, the U–Pb ages on genetic story. The study of solid and fluid inclusions zircon are widespread between 60 and 1 Ma trapped by corundum during growth sometimes (Sutherland et al. 2002, Graham et al. 2008b). allows assessment of the following conditions of Limited U–Pb data gave ages of 150 and 180 Ma for corundum crystallization: temperature, pressure, and zircon from Cudgegong (Sutherland et al. 2003), composition of the fluids (e.g., Upton et al. 1999, and 400 Ma for those from Tumbarumba (Graham Sutthirat et al. 2001). Muhlmeister et al. (1998) et al. 2004). Generally, the sapphire fields’ gems classified ruby deposits into three categories by produced a simple eruptive event and zircon– analyzing the chemical composition of the ruby by sapphire crystallization age while others had in most X-ray fluorescence: basaltic, marble, and two zircon–sapphire ages and two eruptive events metasomatic deposits (including the deposits of (Graham et al. 2008b). Age correlation of Cenozoic Umba, Tanzania, and Kenya). The concentrations of gem corundum occurrences with associated trace elements (Fe, V, Ga, and Ti) are different magmatic and tectonic events in central and east among the three categories (Fig. 2-11). Ruby Asia is reported in Figure 2-10. It shows that the associated with basalt (Thailand and Cambodia) is magmatic–polygenetic corundum crystals within the Fe-rich, and V- and Ga-poor. Ruby in marble basaltic fields largely crystallized in a tectonic (Afghanistan, Myanmar, Nepal, and China) is V- regime of distributed E–W extension (with the rich, and Ga- and Fe-poor (excepting ruby from opening of the Tasman and Seas) whereas the Afghanistan and some analyses from China, Nepal metamorphic–metasomatic corundum (ruby in and Mogok mines in Myanmar). Metasomatic ruby marble) crystallized in a transpressional regime presents a great variation of trace element associated with the collision between Indian and concentrations and also high variability for its host Eurasian plates (Graham et al. 2008b). rocks. The combination of chemical fingerprinting of corundum with the nature of solid inclusions CLASSIFICATION OF CORUNDUM allows the gemologist to separate great families of DEPOSITS ruby, but gemological study should not be Several classification schemes have been disconnected from knowledge of the geological proposed based on different features such as the environment and formational processes. morphology of corundum (Ozerov 1945), the Corundum deposits can be classified into geological context of the deposits (Hughes 1990, primary and secondary deposits. They are 1997), the lithology of the host rocks (Schwarz considered primary where the corundum is either

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FIG. 2-10. Age correlation of Cenozoic gem corundum deposits with associated magmatic and tectonic events in central and east Asia (modified after Graham et al. 2008b). The dates are from: East Tibet–Indochina igneous rocks (Wang et al. 2001); Himalayan granite bodies (Singh & Jain 2003), tectonic activity (Yin et al. 2002); age of ruby deposits (Garnier et al. 2002b, 2004b, 2005, 2006b, Graham et al. 2006a, 2006b, Yui et al. 2006, Sutherland et al. 2002, Akinin et al. 2004, Graham et al. 2008b).

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FIG. 2-11. Chemical composition of natural and synthetic rubies (after Muhlmeister et al. 1998). Gallium (Ga)–vanadium (V)– iron (Fe) diagrams (in wt.%) of (A) natural ruby from different deposits, and (B) natural versus synthetic ruby. hosted in the rock where it crystallized, or in the nepheline syenite. Corundum is formed by direct rock that carried it from the zone of crystallization crystallization from the magmatic melt as an in the crust or mantle to the Earth’s surface (as accessory mineral phase. The classical examples are xenolith or enclave). Secondary deposits are the syenite pluton from Haliburton and Bancroft sedimentary and of detrital origin. They correspond (Ontario, Canada; Moyd, 1949), and the Kangayan to the accumulation, in basins of variable extent, of syenite (Coimbatore district, Madras, India, Hughes material eroded from primary deposits. This 1997). In addition, aluminous rocks carried as material has been primarily transported by rivers. xenoliths by plutonic rocks can lead to the formation The primary deposits are subdivided into two of corundum, as in the xenoliths of biotite sub-groups, according to the magmatic or carried by a monzonite at San Jacinto, in Southern metamorphic context of the corundum deposit (Fig. California (Murdoch & Webb 1942). Corundum is 2-12). Magmatic deposits include corundum in found also as an accessory mineral in porphyry Cu alkali basalt, sapphire in lamprophyre, sapphire in deposits (Bottril 1998). syenite, and corundum associated with porphyry In volcanic rocks, sapphire and less commonly copper mineralization and kimberlite (Fig. 2-12a). ruby are found in continental alkali basalt extrusions Metamorphic deposits are divided into metamorphic (Sutherland et al. 1998a). Corundum is found as deposits sensu stricto (marble, mafic and ultramafic xenocrysts in lava flows and plugs of subalkaline rocks, granulite, cordieritite, gneiss, and migmatite), olivine basalt, high alumina alkali basalt, and and metamorphic–metasomatic deposits (Fig. 2-12b) basanite. The sapphire is blue-green-yellow (BGY) characterized by high fluid–rock interaction and and the deposits only have economic importance metasomatism (plumasite [i.e., desilicated pegmatite because advanced weathering in tropical regions in mafic and ultramafic rocks] and marble, skarn concentrates sapphire in eluvial and especially large deposit, shear-zone related deposits in different alluvial placers. Sapphire also occurs in alkaline substrata, mainly corundum-bearing Mg-biotite basic lamprophyre (Brownlow & Komorowski schist but having experienced granulite-facies 1988) as mafic dikes of biotite monchiquite metamorphic conditions). (lamprophyre characterized by the abundance of phlogopite and brown amphibole, olivine, GEOLOGY AND GENESIS OF PRIMARY clinopyroxene, analcime), ouachitite (a variety of CORUNDUM DEPOSITS monchiquite) and diamond-bearing kimberlite. Magmatic deposits Corundum in magmatic deposits is found in Sapphire in lamprophyric dikes (xenocryst): plutonic and volcanic rocks. In plutonic rocks, sapphire was discovered in Montana in alluvial corundum is associated with rocks deficient in silica gravel by prospectors in 1894 (Voynic 1985, and their , especially syenite and Hughes 1997). The placer deposits occur associated

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sy of F. Fontan, 2 and 6 of L.-D. Bayle, , The magmatic deposits with their main types. For A metamorphic. Photographs respectively 1 courte 1 respectively Photographs metamorphic. versus based on the lithology of the corundum-host rocks. rocks. corundum-host the of lithology the on based : magmatic d . 2-12. Classification of primary corundum deposits IG each type the origin of ruby and/or sapphire is precise 3 and 5 G. Giuliani, and 4 of S. Rakotosamizanani. F

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GIULIANI ET AL. – GEM CORUNDUM , The metamorphic deposits with their main types. B based on the lithology of the corundum-host rocks. rocks. corundum-host the of lithology the on based of G. Giuliani, 5 and 6 of L.-D. Bayle. Bayle. L.-D. of 6 and 5 Giuliani, G. of . 2-12. Classification of primary corundum deposits IG Photographs respectively 1 to 4, and 6 courtesy F

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with the Missouri River, Dry Cottonwood Creek, they were rounded, corroded, and broken by the Rock Creek, and Yogo Gulch (Fig. 2-13). The magma (Clabaugh 1952, Dahy 1988, Gauthier et al. primary deposit of Yogo Gulch was intensely 1995). developed from 1897 to 1929. The sapphire is The sapphire host rock is composed of hosted by an Eocene ultramafic lamprophyre and megacrysts of phlogopite and dispersed in hydrothermal breccia known as the Yogo dike, a matrix of phlogopite, clinopyroxene, calcite, which is 8 km long and 2 m thick (Brownlow & analcime, magnetite, and apatite (Brownlow & Komorowski 1988, Meyer & Mitchell 1988). The Komorowski 1988). Accessory minerals include dike is a member of the Central Montana alkali spinel and sapphire crystals with a corona of dark province (Cade & Groat 2006) and cuts green spinel (Gauthier et al. 1995). Chemical Carboniferous limestone and shale, which analysis by Meyer & Mitchell (1988) showed that underwent contact . Underground the rock can be classified as a ouachitite, defined by mining took place at a depth of 100 m from the LeMaitre (1989) as an ultramafic lamprophyre surface and from 1 to 1.5 km in length. The sapphire containing olivine, phlogopite, amphibole, and/or grade is highly variable and can reach up to 70 ct clinopyroxene phenocrysts within a groundmass of per ton, and 5 ct per ton in hydrothermal breccia feldspathoid and carbonate minerals, first described with limestone fragments (Myachaluk 1995). from the Ouachita River in Arkansas (Kemp 1890). This sapphire is famous for its cornflower blue coloration, the rarity of inclusions and zoning, and its brilliancy under natural and artificial light (Myachaluk 1995). Among the sapphires produced, 97% are blue, ranging from pale to saturated color; the others are purple to violet. Solid inclusions of pyrite, phlogopite, calcite, rutile, spinel, zircon, and analcime are rare (Gübelin & Koivula 1986). Sapphire crystals are up to 1 ct, but the largest reported raw crystal weighed 19 ct. The crystals (Fig. 2-14) have an uncommon aspect due to the combination of a flattened rhombohedron (k{30.2}, FIG. 2-14. Crystal habits of sapphire from Yogo Gulch, d{01.2}, r{10.1}, b{10.7}) and pinacoid {0001} Montana (after Hughes 1997). r, rhombohedron {10.l}; (Hughes 1990). Nevertheless, the majority of the c, pinacoid {00.1}; d, rhombohedron {10.2}; n, crystals have not kept their original form because second-order hexagonal dipyramid {22.3}.

FIG. 2-13. Map of Montana sapphire localities (after Mychaluk 1995). 1, Yogo Gulch; 2, Missouri River; 3, Cotton Creek; 4, Dry Cottonwood.

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Within the lamprophyre Brownlow & Komorowski xenocrysts probably formed in mantle (but (1988) described xenoliths of cumulates, composed the į18O values indicate a crustal origin), and of megacrysts of olivine, Ti-augite, and hypersthene subsequently brought up by the lamprophyric dispersed in a fine matrix, with a chemical magma. composition between monchiquite and ouachitite. At Loch Roag in Scotland sapphire occurs in a The origin of corundum in lamprophyre bodies Paleozoic monchiquite dike crosscutting Archean is debated on the basis of a magmatic versus pyroxene gneiss (Jackson 1984). The monchiquite metamorphic origin. Clabaugh (1952) suggested an contains megacrysts of biotite, sanidine, excess of alumina in the magmatic melt by anorthoclase, apatite, and sapphire, and mantle assimilation and fusion of alumina-rich sedimentary xenoliths. The corundum crystals are up to 3 cm in or metamorphic rocks that favored the diameter and present hexagonal, pyramidal- crystallization in situ of the sapphire. Brownlow & dipyramidal or bladed habits. The smallest crystals Komorowski (1988) proposed the following are blue but the biggest are spotty blue and green, magmatic genesis: (1) formation of an aluminous some with yellow patches (Kievlenko 2003). magma by the partial fusion of mantle rocks Mineral inclusions include hematite and rutile. The enriched in alumina; (2) magmatic differentiation Loch Roag monchiquite is similar in geochemical with crystallization of sapphire and cumulates of composition to the Yogo Gulch dike. Jackson olivine, augite, and hypersthene; (3) injection of the (1984) noted a similarity in petrographic magma into the upper crust, with the corrosion of composition, and the presence of mantle xenoliths, sapphire by the residual liquid magma; (4) in lamprophyre dikes and sapphire-bearing lavas in fractionation of the residual liquid into a silicate Australia and Thailand. He suggested a genetic liquid and a gas phase; (5) explosive injection of the affinity between the two types of intrusive and dike with brecciation of the limestone and extrusive magmatism. hydrothermal alteration. Berg (2002) observed Upton et al. (1983) reported some 70 localities volcanic glass on two alluvial sapphire crystals from in northern Britain and Ireland where deep-seated western Montana and suggested dacitic volcanic xenoliths (spinel lherzolite and pyroxenite) and rocks as a possible source material (in Walton megacrysts have been brought up by basaltic 2004). Garland (2003) studied the trace element magma. In Scotland, megacrysts are abundant chemistry of sapphire and its solid inclusions and anorthoclase, biotite, clinopyroxene, magnetite, concluded that the sapphire might be of zircon, and apatite. These minerals occur as metamorphic origin. The xenocrysts were included megacrysts but also as polycrystalline syenite in the lamprophyric magma and brought up to the xenoliths. Composite xenoliths provide evidence surface. The sapphire formed between 580 and that the anorthoclasites may occur as pegmatitic 720°C at about 30 km depth. Cade & Groat (2006) veins crosscutting pyroxenitic wall rocks in the showed that reddish-orange garnet inclusions within shallow mantle (Upton et al. 1999). The geochem- the formed in the mantle in group II istry and oxygen isotope composition of sapphires eclogite (B), according to the classification of (4.6 < į18O < 5.25‰, n = 4) and feldspar indicate Schultze (2003). They therefore considered the crystallization of these syenitic facies from felsic sapphire to occur as xenocrysts in the melt, melts originating from partial melting of meta- originating from the mantle. Giuliani et al. (2005) somatized mantle rocks (Upton et al. 1999, 2009). have shown that colored and blue sapphires from Yogo Gulch have į18O-values between 5.4 and Sapphire in syenite (megacryst): sapphire is a 6.8‰ V–SMOW (mean 6.0 ±0.4‰, n = 16). For a typical mineral of pegmatite in syenitic rocks. The mean sapphire į18O of 6‰, the calculated oxygen pegmatite contains megacrysts of K-feldspar, isotope composition of water in equilibrium with the titanite, Nb–Ta minerals, and sapphire (Kievlenko corundum as a function of temperature (between 2003). Corundum is mainly opaque but in some 600 and 800°C) indicates an oxygen reservoir cases it is transparent and translucent with a characterized by crustal į18O (between 12.6 and dominant blue color. Its gemstone production is 14.5‰). small, although gems were extracted from syenite The genesis of the Montana sapphire is not pegmatites in Russia (Ilmeny-Urals, Khibiny-Kola resolved because the composition of garnet Peninsula), Canada (Haliburton, Bancroft, inclusions within the crystals is a strong argument Dungannon, all in Ontario), India (Coimbatore, for the hypothesis that sapphire crystals are Madras), and Norway (Seiland, Finmark). Sapphire

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is also found in a unique example of sapphire diagram of LeMaitre (2002) as an alkali-feldspar bearing-monzonite, the Dusi deposit, known as syenite. Zircon is included in feldspar or in Garba Tula syenite, in Kenya. The į18O values of corundum. U–Pb geochronology on zircon crystals sapphire in syenite define a range between 4.4 and allowed dating of the crystallization of the dike at 8.3‰ (n= 25; Giuliani et al. 2010). 579 ±6 Ma (Simonet 2000) which corresponds to The Ilmeny-Urals deposit was discovered in the one of the tectonic events of the Mozambique 19th century in the southern Urals. Sapphire-bearing metamorphic belt. Sr–Nd isotopes indicate that the pegmatite occurs as veins some tens of metres long, monzonite is of mantle origin, with little or no 1.5 to 2 m thick, and up to 200 to 300 m along participation of a crustal component (Simonet et al. strike, in a large biotite-nepheline syenite. It consists 2004). The corundum is barrel-shaped with a size dominantly of plagioclase (albite–oligoclase), with varying from a few millimetres to more than 20 cm. orthoclase, , biotite, and The gem material varies in color from dark blue to (Kievlenko 2003). Corundum is either disseminated golden yellow (Plate 1C), through various shades of or in pockets. It is intergrown with plagioclase and blue, green and yellow (Feneyrol et al. 2013). The garnet and commonly rimmed by biotite. The crystals contain significant amounts of iron (FeO crystals have barrel or bladed shapes and are between 0.95 and 1.13 wt.%) and are Ti-poor (TiO2 typically 3 to 4 cm long. The corundum is not < 0.04 wt.%). Simonet et al. (2004) compared the transparent and is bluish to brownish-gray; less Dusi sapphires with those found in the Turkana gem commonly it is pink, yellow or green. The field in northern Kenya, which are associated with corundum is associated with zircon, ferrocolumbite, Tertiary basalt. Indeed, they have the same range of aeschinite (Nb–Y oxide), , and molybdenite. color (identical to the BGY sapphire suite defined The same type of deposit is described in the Khibini by Sutherland et al. 1998a), the same habit, the syenitic massif, Kola Peninsula (Kievlenko 2003). same chemical composition, and similar inclusions. Transparent varieties of sapphire are very rare and Simonet et al. (2004) suggested that the similarities are cornflower blue to deep blue. between sapphire crystals and the association with In Ontario, Canada, corundum and limited gem zircon is a strong indication of a similar sapphire are found in nepheline syenite pegmatite crystallization in a peraluminous Zr-rich syenitic within an alkaline belt (Field 1951, Kievlenko melt. Yellow and green sapphires from Garba Tula 2003). Nepheline syenite from Hastings County have į18O values of 5.2‰ (Giuliani et al. 2005) contains orthoclase and albite feldspar. Nepheline is which clearly indicate a mantle derivation (į18O very abundant and is associated with cancrinite, mantle of 5.5 ± 0.4‰, Mattey et al. 1994). , scapolite, calcite, biotite, and hornblende. These silica-undersaturated rocks are formed by low degrees of partial melting in the Earth’s mantle. Bancroft sapphire crystals are greyish-blue to violet and are associated with microcline, titanite, and biotite. However, the į18O values of respectively 7.6, 7.8, 7.6, and 9.3‰ for the sapphires of Craigmont, Hastings-New Carlow, Craig Hill, and Dungannon indicate a crustal contamination of the magma oxygen reservoir. The Dusi (Garba Tula) deposit in Central Kenya was discovered by nomads in the 1960s (Fig. 2-15). The deposit is a subvertical dike several metres thick and with an inferred horizontal extension of several kilometres. It is in contact with biotite-hornblende gneiss, but the lack of outcrop does not permit study of the field relations with the host rocks nor appraisal of the continuity of the dike (Simonet et al. 2004). The dike is leucocratic monzonite, and is made of K-feldspar (microcline) and albite (An3–7) in equal proportions, biotite, FIG. 2-15. Major gem corundum deposits of Kenya zircon and magnetite, and is classified in the QAPF (modified from Simonet 2000).

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At Urdach and Espéchère in the western huge placers. They include the deposits of the Pyrenees, France, dikes of pegmatitic albitite have corundum belt of eastern Australia (Fig. 2-16) that intruded serpentinized lherzolite bodies. The albitite extends from Queensland in the north to Tasmania contains albite, muscovite, and biotite, with chlorite, in the south (Oakes et al. 1996, Sutherland 1996, epidote, zircon, titanite, thorite, pyrochlore, Sutherland & Coenraads 1996, Graham et al. 2008b, aeschinite, ferrocolumbite, rutile, ilmenite, and Abduriyim et al. 2012); in Asia (Fig. 2-1), the magnetite (Monchoux et al. 2006). The corundum- deposits of Mingxi, Fujian Province (Keller & bearing albitite also contains Nb-bearing rutile, Keller 1986), of Changle, Shandong Province (Guo samarskite, manganocolumbite, fersmite, and et al. 1992), and Penglai, Hainan Island in China fergusonite. In corundum-free albitite, the (Furui 1988); the provinces of Binh Thuan, Lam paragenesis is composed of allanite, chevkinite, Dong, Dong Nai, and Dak Lak in southern Vietnam apatite, and monazite. The texture of the rock is (Smith et al. 1995, Poirot 1997, Garnier et al. 2005); pegmatitic and corundum is barrel-shaped, dark Pailin, Cambodia (Lacombe 1970, Jobbins & blue, pale blue or gray. Geochemical and isotopic Berrangé 1981); the provinces of Chanthaburi-Trat, data of the albitite rocks document a mantle origin Kanchanaburi (Vichit et al. 1978, Sutthirat et al. for these rocks (Pin et al. 2006). They have very 2001), and Denchai in the province of Phrae in high contents of Sr (up to 1089 ppm), Ba (up to 860 Thailand (Limkatrun et al. 2001); Laos (Sutherland ppm), Nb (167–299 ppm), Ta (6–17 ppm), and an et al. 2002); in central and far eastern Russia with extreme enrichment of the light rare-earth elements the deposits of Nezametnoye, Podgelbanochny, and (Ce ~ 1020 ppm). İNd values range from 1.9 to 3.4 Shkotovo Plateau (Graham et al. 2008b); in Africa, and the 87Sr/86Sr initial ratios are between 0.7039 and 0.7068, reflecting a mantle source without any major contribution from continental crust. The į18O values of sapphire between 5.1 and 5.4‰ for the Espéchère deposit confirm the mantle source. Those from Urdach deposits between 6.1 and 6.6‰, show a į18O increase indicative of a hybridization of the mantle material (Giuliani et al. 2010).

Corundum in porphyry Cu deposits (accessory mineral): corundum is an accessory mineral found in several porphyry Cu deposits in Tasmania (Botrill 1998), British Columbia (Simandl et al. 1997), El Salvador and at Los Pelambres deposits in Chile (Singer et al. 2008). At the Empress Cu–Au–Mo deposit in southwestern British Columbia corundum is reported in association with an andalusite- rock free of . Corundum growth has been explained by a rapid heating of the cool meteoric water influx in fractures over the thermal anomaly of the porphyry Cu. The sapphire grains are on a millimetre scale, and can be transparent, colorless, or blue. Although corundum is theoretic- ally incompatible with quartz, a metastable associ- ation quartz–corundum–andalusite–pyrophyllite has been described in a zone of intense hydrothermal pervasive argillic alteration associated with the Bond Range Tasmanian porphyry Cu deposit (Botrill 1998). FIG. 2-16. Gem corundum in the basaltic terranes of Eastern Australia with the main sapphire mines of Corundum in alkali basalt (xenocryst and Australia. Most production is centered on the Lava xenolith): blue-green-yellow (BGY) sapphire makes Plains, Anakie, New England and Oberon fields up most of the gem trade and is concentrated in (modified after Sutherland & Schwarz 2001)

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the sapphire deposits of Cyangugu in southwest 1996a). Rwanda (Krzemnicki et al. 1996), of the Kivu Within a single gem province, variations of region in the Democratic Republic of Congo color observed for corundum found ~10 km apart (Frazier & Frazier 1990), the region of Turkana in suggest the existence of multiple sources for the the north of Kenya (Barot et al. 1989, Keller 1992, gemstones (Coenraads et al. 1990, Sutherland et al. Simonet 2000), the province of Kaduna in Nigeria 1998b). Indeed, Sutherland et al. (1998b) and (Irving & Price 1981, Kiefert & Schmetzer 1987), Sutherland & Schwarz (2001), studying the trace the regions of Mamfe and Adamawa in Cameroon element contents distribution in corundum from the (Lettermann & Schubnel 1970, Boaka à Koul et al. states of New South Wales and Victoria, found 2011), the volcanic massif of Atakor in the Algerian evidence for two contrasting geochemical fields, Sahara (Conquéré & Girod 1968), and the Aïr one for magmatic sapphire and one for metamorphic massif in Niger (Carbonel & Robin 1972); deposits crystals. The same geochemical behavior was in the region of Ambondromifehy, Antsiranana confirmed for corundum of the Kanchanaburi-Bo Province, and Antsirabe in Madagascar (Lacroix Rai and Nam Yuen deposits in Thailand, and Pailin 1922, Schwarz et al. 2000, Rakotosamizanany 2009, in Cambodia. The chemical variation diagram Fe/Ti Rakotosamizanany et al. 2009a, 2009b); in Europe, versus Cr/Ga proposed by Sutherland et al. (1998b) the Espaly deposit, near Puy-en-Velay, in France is commonly used for the separation of magmatic (Carbonel et al. 1973, Forestier 1993, Giuliani et al. and metamorphic blue sapphires from several 2009, 2010, Gaillou 2003, Gaillou et al. 2010), in deposits worldwide (Fig. 2-17). On the one hand, the Eifel massif in Germany (Hochleitner 1998), in the corundum crystals called "metamorphic" have the alluvial deposits of Jizerská Louka and pastel (blue, pink, orange) to ruby color, and are Trebivlice-Ceské in the Czech Republic (Hughes rich in Cr and poor in Ga (Cr2O3/Ga2O3 ratio > 3); 1997), the deposit of Wilcza PorĊba in Poland solid inclusions are chromiferous spinel, pleonaste, (Maliková 1999), in the alluvial deposits of Al-rich diopside and sapphirine. On the other hand, Hajnáþka and Gortva in southern Slovakia (Uher et the corundum crystals called "magmatic" are al. 1999, 2012), and in Loch Roag in Scotland xenocrysts of BGY sapphires with a Cr2O3/Ga2O3 (Upton et al. 1983, 1999, 2009), and lastly in South ratio < 1; the solid inclusions are Nb-bearing rutile, America, the Mercaderes–Rio Mayo deposit in Colombia (Keller et al. 1985, Sutherland et al. 2008). These deposits have a number of features in common. To begin with, they are generally associated with intra-plate alkali basalts containing ultramafic xenoliths, mainly mantle lherzolite (Coenraads et al. 1990). The corundum is commonly associated with pyrrhotite, clino- pyroxene, zircon, Fe-rich spinel, and in some cases sapphirine (Muhlmeister et al. 1998, Sutthirat et al. 2001, Giuliani et al. 2009, 2010). The xenocrysts of corundum contain numerous mineral inclusions (e.g., Nb-bearing rutile, ilmenite, ferrocolumbite, fersmite, samarskite, zircon, feldspars, apatite and, in some, sulfides (Simonet 2000, Graham et al.

2008a). These mineral inclusions are enriched in Zr, Nb, Hf, Ta, and lithophile elements such as U in FIG. 2-17. Fe2O3/TiO2 versus Cr2O3/Ga2O3 diagram of betafite and uranopyrochlore, and in Th such as blue sapphire from non-basalt-related and basalt- related origins (after Abduriyim & Kitawaki 2006). thorite. The metamorphic blue sapphires originate from the The corundum commonly exhibits corrosion placers of Ilakaka in Madagascar, Ratnapura in Sri features on the surface of the crystals due to Lanka, and Mogok in Myanmar. The magmatic blue transport by the magma (Coenraads et al. 1995, sapphires are from the placers of Bo Phloi and Sutherland & Coenraads 1996). In the placers, the Kanchanaburi in Thailand, and Changle in China. The crystals are broken but they keep their usual six- chemical field of the BGY sapphires defined by sided barrel or "keg of beer" shape (Guo et al. Sutherland & Schwarz (2001) is also reported.

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ilmenite, ferrocolumbite, pyrochlore, fersmite, the southern extension of the Limagne rift (Merle et samarskite, hercynite, magnetite, zircon and Fe al. 1998), also associated with a mantle plume. In oxides. Australia, it is generally accepted that the central Over the last decade use of laser ablation– volcanoes became younger towards the south inductively coupled plasma–mass spectrometry (Sutherland 1996). This variation, developed during (LA–ICP–MS) analysis applied to trace elements in the last 35 Ma, suggests that the migration towards gemstones (Günter & Kane 1999, Guilliong & the north of the Indo-Australian plate occurred Günther 2001) has brought new insights on above a mantle plume. The huge basaltic provinces geographic typing of gem corundum in basaltic (70–14 Ma) resulted from the rise from the mantle environments (Abduriyim & Kitawaki 2006, Peucat of a region of high geothermal gradient, related to et al. 2007, Sutherland & Abduriyim 2009). the extension provoked by the opening of the Generally, Fe and Ga reveal higher concentrations Tasman and Coral Seas (Graham et al. 2008b) in magmatic sapphire (a1800 to 11000 ppm Fe and and/or the separation of Gondwana at the end of the a70 to 570 ppm Ga) than in metamorphic Mesozoic (O'Reilly & Zhang 1995). The geological corundum, which have Fe and Ga contents less than context of formation of the basalts in Southeast Asia 3000 and 75 ppm, respectively (Sutherland et al. is debatable. Levinson & Cook (1994) thought that 2002, 2009a, Khin Zaw et al. 2006, Peucat et al. the alkali basalts in Thailand, Cambodia, and 2007). Low Cr and Mg contents are typical for Vietnam were associated with the subduction of the magmatic sapphire (both usually < 40 ppm), whereas Indian plate under the Eurasian plate. Barr & gem corundum of metamorphic origin is enriched in McDonald (1979) proposed crustal pinching with a these elements (both generally > 60 ppm). Therefore, mantle rise that would explain the volcanic the Ga/Mg ratio is usually > 6 in magmatic sapphire eruptions in Thailand. Barr & McDonald (1981), and < 3 in that of metamorphic origin. Conversely, Whitford-Stark (1987), and Hoang & Flower (1998) the Cr/Ga ratio is < 0.1 for magmatic and > 1 for considered that the Neogene to Quaternary metamorphic sapphire. The Fe/Ti ratio is generally intraplate volcanism is associated with the formation higher in magmatic than metamorphic sapphires. of "pull-apart" basins and rifts. This magmatic The Fe/Mg ratio is also significantly different in activity post-dated the collision between India and magmatic and metamorphic sapphires (Peucat et al. Eurasian plates (Fig. 2-9) and is linked to the 2007), very high for magmatic (Fe/Mg >> 100) and extrusion of the Indochina Peninsula. low for metamorphic and metasomatic sapphires Several mechanisms have been proposed for the (Fe/Mg < 100). The use of the different discrimin- formation of ruby and sapphire found within alkali ating binary and ternary diagrams and chemical basalt. Although the models differ, researchers now ratios (Fe/Ti, Ga/Mg, Fe/Mg) is somewhat agree that the gems formed at great depths, under debatable and the origin of sapphire crystals from metamorphic or magmatic conditions; subsequently the Hajnáþka placer and Gortva syenite in southern the parental rocks were intruded by the alkali basalt Slovakia is a good example (Uher et al. 2012). The magma, and mineralized xenoliths or sapphire different ratios of Fe/Ti, Ga/Mg, and Fe/Mg for blue xenocrysts were transported upwards to the surface sapphires indicate bimodal origins whereas the (Coenraads et al. 1990; Levinson & Cook 1994, mineral inclusions were magmatic. They signify that Sutherland & Coeenrads 1996, Guo et al. 1996a, the final composition of the magmatic sapphires Upton et al. 1999). depends of the behavior of the parent magma during Experimental studies have shown that crystallization which can sometimes generate corundum and mineral inclusions such as zircon, discrepancies with the use of the chemical diagrams ferrotantalite, pyrochlore-group minerals, ferro- for corundum classification. columbite, and samarskite cannot crystallize from a The deposits from Nigeria and Cameroon are basaltic magma (Levinson & Cook 1994; Guo et al. linked to a mantle plume located beneath the rift of 1996a). The sapphires must have grown in an the volcanic province of Guinea that extends environment rich in Th, Zr, Nb, Ta, V, alkali northward to the basaltic plateau of Aïr and Hoggar elements such as Na and K, and Fe, Al, and volatile (Wright et al. 1985, Déruelle et al. 2007). The components. The etching on the surface of deposits in Rwanda, Democratic Republic of corundum suggests that crystals were partially Congo, and of Baringo in Kenya (Simonet, 2000) absorbed into the entraining magma. Nevertheless, are associated with the East African rift. The Sutherland et al. (1998a) thought that such etching deposits in the French Massif Central are located on is not sufficient to conclude that crystals are

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xenocrysts; indeed, such etched surfaces are massif of Madagascar. Three stages were character- sometimes observed on olivine, pyroxene, and ized by: (1) the association of clinopyroxene 1 + plagioclase phenocrysts in the alkaline olivine pyrope + scapolite + ruby 1, and spinel + ruby 1 + basalt, as a result of modification of pressure and pyrope, indicated a P–T formation of 1100°C and 20 temperature. kbar, at the limit of the eclogite domain; (2) the Xenocrysts in alkali basalt may have several destabilization of scapolite in anorthite + calcite origins. Levinson & Cook (1994) proposed a two- related to the transformation of diopside in pargasite stage model of genesis. In the first stage, corundum (at 1100°C and P < 20 kbar), and by spinel + ruby 1 formed in the crust, at depths of 20 to 50 km, by + pyrope in sapphirine (at 1100°C and P ~ 15 kbar); metamorphism of aluminous rocks (such as shale or and (3) the association phlogopite + ruby 2 related mudstone) or by metamorphism of gibbsite. In the to the destabilization of spinel + ruby 1 + pyrope second stage the corundum was transported to the during decompression. Metagabbro went from surface. While ascending to the surface, the magma eclogite to granulite facies. incorporated xenoliths of corundum-bearing rocks. Among magmatic models, Irving (1986) argued These xenoliths were dissolved and the refractory that corundum crystallized from undersaturated minerals (corundum and zircon) were brought to the felsic magma at high pressure, and Aspen et al. surface. This model leads to the problem of the (1990) concurred that corundum originated from the existence of an aluminous protolith of bauxitic or crystallization of syenite generated in the crust or lateritic composition, commonly found at the the upper mantle. Coenraads et al. (1990) proposed surface, but not on subduction-prone ocean floors. A that corundum formed in magmas, enriched in subduction zone may have been active before the volatile and incompatible elements, derived from the crustal extension in Australia, but this is not the case fusion of mantle but with a low rate. This may either in eastern Africa or in Southeast Asia explain why corundum is not in equilibrium with (Saminpanya 2000), and the model seems dubious magmas generated from partial fusion, and why the here. surfaces of the crystals are commonly etched. The origin of ruby has long been debated but its Coenraads et al. (1995) proposed that sapphire from metamorphic origin is now accepted. Study of the Australia and Thailand crystallized from phonolitic trace element contents and mineral inclusions magma at pressures equivalent to those found at the (sapphirine, spinel) in Barrington rubies from mantle–crust boundary. The thermal events that Australia (Sutherland et al. 1998a) indicated a generated such magma bodies are directly linked to metamorphic origin and similarity to some the processes that have generated and caused Cambodian–Thai rubies from Southeast Asian eruption of the corundum-bearing alkali basalt basaltic gem placers. Sutthirat et al. (2001) reported magma. temperatures of crystallization for ruby between 800 Guo et al. (1996a) studied mineral inclusions in and 1150 ±100°C and pressures between 10 to 25 corundum from Australia, China, Thailand, United kbar, corresponding to depths of 35 to 90 km, for States, and Kenya placer deposits. They suggested ruby-bearing clinopyroxene xenocrysts with garnet that corundum forms by the mixing and/or and sapphirine inclusions from eastern Thailand. interaction between carbonatite and a magmatic They proposed a garnet clinopyroxenite and garnet system of alkaline or hyperalkaline magmas (Fig. granulite source in the lower crust or upper mantle 2-18), in the liquid or solid state (alkaline granite or (ultramafic rocks), as previously suggested by syenitic pegmatite). During an extensional event Sutherland et al. (1998a). The nature of the solid linked to the rise of the asthenosphere, carbonatite inclusions trapped by ruby is important for P–T bodies crystallized in the crust between 10 to 20 km thermobarometry determinations. Sutherland et al. depth; these carbonatite bodies are locally crosscut (2003) found Al-rich diopside, meionite and anatase by syenite and granitoid plutons. The mechanism of in a xenolith from the Cudgegong-Macquarie River hybridization between these two systems can in New South Wales. Clinopyroxene–corundum provoke the rapid crystallization of corundum, and thermometry suggested T > 1000–1300°C, but the the formation of lenses of rock carrying corundum presence of meionite (silvialite variety) indicated a with its characteristic mineral association. Later, a probable high-T skarn type source. rapid basaltic volcanic event occurred with included Rakotosamizanany (2009) found ruby in pyroxenite fragments of rock and lenses with corundum. and metagabbro enclaves in alkali basalt from the This genetic model is complex because it primary Soamiakatra deposit in the Ankaratra requires the coincidence of successive magmatic

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FIG. 2-18. Genetic model for the origin of corundum xenocrysts in alkali basalts based on a crustal growth model for eastern Australia (after Guo et al. 1996a). 1- Crystallization of carbonatite in crustal extensional zone, and locally crosscut by granitoid rocks. 2- Hybridization process between these two systems leads to the formation of corundum lenses. 3- Later in the history, alkali basalts intrude the corundum lenses and corundum is carried up to the surface.

events. It is not a satisfactory model that can be (1996) unlikely. Rubies originate from mafic and applied to all the deposits found elsewhere on the ultramafic rocks in the subcontinental lithosphere Earth. Sutherland et al. (1998a) rejected the model (Kornprobst et al. 1990). The range of į18O values because, in the African rift, corundum is generally defined for rubies found in placers from basaltic found in alkali basalt located at the periphery of the environments is between 1.3 and 5.9‰ (mean į18O central uplift and not in association with ultramafic = 3.1 ±1.1‰, n = 80, Giuliani et al. 2007a). The rocks and carbonatite, as suggested by the model. oxygen isotopic range fits within the range of rubies Sutherland (1996) proposed that corundum associated with mafic and ultramafic rocks (0.25 < formed during the crystallization of felsic alkali į18O < 6.8‰, n = 21). The į18O values of ruby magma produced by the fusion of metasomatized lower than 5‰ indicate a previous high-temperature lithosphere. Sutherland & Coenraads (1996), seawater interaction of the parental rocks (Yui et al. studying the assemblage ruby-sapphirine-spinel 2006). This hypothesis was already proposed by found in the placers from Barrington in Australia, Pearson et al. (1993) for the low į18O values of proposed that the fusion of ultramafic rocks at high clinopyroxene from the ruby-bearing pyroxenite temperatures and pressures by basanite can produce from the Beni Bousera peridotite complex in limited volumes of anatectic magma able to Morocco; moreover, the į18O value of this ruby is crystallize these assemblages with aluminous and 4.4‰. refractory minerals. The fusion of the accessory Sutherland et al. (1998a) proposed a genetic phases rich in Cr, such as chromiferous spinel or model in four stages for the formation of 'magmatic' chromite, would contribute to the local sapphire from eastern Australia (Fig. 2-19). The concentration of Cr, allowing the crystallization of lithosphere is displaced above a mantle plume. A ruby and sapphire in millimetre-size crystals. The low rate of initial fusion generated felsic magma discovery of ruby-bearing garnet pyroxenite and enriched in volatile elements in zones where the metagabbro enclaves in the Soamiakatra alkali lithosphere is rich in amphibole, allowing the basalt from Madagascar (Rakotosamizanany 2009) crystallization of corundum and zircon. This magma renders the hypothesis of Sutherland & Coenraads can also be derived from a mantle enriched in

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genesis of such "magmatic corundum", but the mechanism of generation of the Al-rich magma is not yet constrained. Pin et al. (2006) have shown that the albitite dikes of Espéchère and Urdach in the western Pyrenees (France), which intruded serpentinized lherzolite, are of mantle origin. The mineral accessory phases of these albitite dikes are roughly similar to those found in xenoliths associated with alkali basalt fields. Such xenoliths are generally composed of alkali feldspar ± zircon ± corundum and megacrysts. Pin et al. (2006) observed that the Si–Al–Na-dominated bulk composition is similar to that of certain glass inclusions included in peridotitic xenoliths in alkali basalt. In addition, the extreme enrichment of incompatible elements in the albitite implies premelting metasomatism by a fluid or a melt. These rocks are interpreted to be products of very low degree of partial melting of a harzburgite source previously enriched by carbonatite-related metasomatism. The presence of volatile phases such as H2O and CO2 may account for the variation of the solubility of SiO2 and Al2O3 in mantle fluids and the consequent precipitation of corundum in some batches of felsic magma. The geochemical composition of the albitite bodies plots in the field of syenite in the diagram of Cox et al. (1979) as does that from the sapphire- bearing syenite of Garba Tula (Simonet et al. 2004). A syenitic origin for sapphire is confirmed by the case studies of different examples of sapphire- bearing anorthoclasite from Scotland (Upton et al. 1999), France (Giuliani et al. 2010), Slovakia (Uher et al. 2012) and Madagascar (Giuliani et al. 2009). The sapphires plot in the geochemical field of FIG. 2-19. The successive stages for corundum formation syenite (Fig. 2-20A) and their solid inclusions are and eruptive alkali basalt transport for eastern Australia typical of magmatic BGY sapphire (Upton et al. gem fields (after Sutherland et al. 1998a). 1999, Gaillou et al. 2010, Uher et al. 2012). The 18 amphibole and mica, or from a mantle enriched in į O range of sapphires between 4.4 and 6.5‰ (Fig. 2-20B) fits in the isotopic range of sapphire in felsic components, at between 45 and 90 km depth. 18 When the lithosphere is located above the plume, a syenite (4.4 < į O < 8.3‰, n = 25; Giuliani et al. high rate of partial fusion produces alkali basaltic 2010). Nevertheless, the origin of the parent magma magma that extracts and carries assemblages with has not yet found a consensus: the mantle source corundum as xenocrysts and in xenoliths. When the model without crustal contamination (Pin et al. lithosphere moves away from the plume, the rates of 2006) is not in agreement with the models proposed fusion decrease and lead again to the crystallization by Guo et al. (1996a) and Sutherland et al. (1998a). of corundum and zircon. This model explains the We should add that the chemical composition of enrichment of Hf, Nb, and Ta, generally observed in magmatic glass inclusions trapped in sapphire minerals cogenetic with corundum, and in crystals from Menet, Coupet, and Sioulot in France amphibole veins found in the peridotite xenoliths (Gaillou et al. 2010), are trachytic with high Al2O3 (Sutherland et al. 1998b). (15 to 22 wt.%), K2O (5 to 6 wt.%) and Na2O (4 to The presence of a mantle plume under the 6 wt.%). Uher et al. (2012) proposed that the lithosphere is a main geodynamic feature for the anorthoclasite from Gortva in southern Slovakia was

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FIG. 2-20. The sapphire crystals associated with anorthoclasite xenoliths in alkali basalt (modified from Uher et al. 2012): the cases of Hajacka (Slovakia), Loch Roag (Scotland), Menet (France) and Kianjanakanga (Madagascar). A, FeO – Cr2O3 – MgO – V2O3 versus FeO + TiO2 + Ga2O3 diagram (in wt.%) used for the geological classification of corundum deposits (Giuliani et al. 2010). The main fields defined for the different types of gem corundum deposit are: for ruby, marble (R1); John Saul Ruby Mine (Kenya) type (R2); mafic and ultramafic rocks (R3); metasomatites (R4); for sapphire, syenitic rocks (S1); metasomatites (S2); xenocrysts in alkali basalt and lamprophyre (S3). The domains of R4 and S2 which correspond to metasomatic–metamorphic corundum are overlapping. The sapphires from the anorthoclasites of Loch Roag, Kianjanakanga and Menet plot in the field of syenite (S1). The sapphire of Hajnacka plots in the metasomatite-related field (S2). B, Oxygen isotope values (į18O; V–SMOW, Vienna Standard Mean Ocean Water) of the same sapphires, with reference to the worldwide į18O database (Giuliani et al. this work). The į18O values are between 4.4 to 5.2‰ which fall in the range of sapphire crystals in syenite. For comparison the į18O values of the sapphire in syenite from Garba Tula (Kenya), Urdach and Espéchère (France) are reported.

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derived from an igneous reservoir in the sub- the origin of ruby-bearing within the continental spinel lherzolitic mantle (į18O of lower mantle at ~770 km (Hutchinson et al. 2004); sapphire = 5.1 ‰). In the lithospheric mantle, felsic (2) crystals of ruby and sapphire are found as up to melts crystallized to form anorthoclasite, the most 1.5 cm xenocrysts in the Mbuyi-Mayi kimberlite in evolved peraluminous variant of the alkaline the Democratic Republic of Congo (Pivin et al. basaltic melt. The proposed model is consistent with 2013). The color of ruby varies from red to fuchsia an origin of analogous felsic syenitic xenoliths from and flashy-pink, and its Cr2O3 content reaches up to southern Slovakia where petrological study 5.6 wt.%. The į18O of 5.55 ± 0.05‰ (n = 3) indicated a pressure around 600 MPa (6 kbar, ~200 indicates for ruby clearly a mantle derivation with km depth) and liquidus temperature of inclusion no substantial variations (į18O mantle of 5.5 ±0.4‰; melts at ~1080°C (Huraiová et al. 1996). Mattey et al. 1994). (3) Cr-bearing sapphires of less than 100 µm size were recorded in kimberlite Corundum in kimberlite (accessory mineral, xenoliths in kyanite eclogite from the Roberts xenocryst and xenolith): corundum has been Victor mine (Hatton 1978), corganite xenoliths described as crystal inclusions in diamond, (Mazzone & Haggerty 1989) and corgaspinite xenocryst or megacryst in xenoliths from kimberlite xenoliths (Exley et al. 1983; Mazzone & Haggerty bodies: (1) ruby and Cr-bearing sapphire crystals 1989) from Jaggersfontein and Bellsbank in South have been reported as syngenetic inclusions in type Africa. Padovany & Tracey (1981) recorded a 150 II diamond from the alluvial deposits in Brazil (Watt µm long Cr-rich ruby (Cr2O3 content up to 3.2 et al. 1994, Hutchinson et al. 2001, Hutchinson et wt.%) in a pyrope-spinel xenolith of the kimberlite al. 2004), and in a cut diamond of unknown origin from Moses Rocks Dike, Utah. (Meyer & Gübelin, 1981). The size of the crystals in Juina diamonds is less than 200 µm and the color is Metamorphic deposits red or red-brown, blue vitreous or light blue, and Corundum is a high temperature mineral that white. The Cr2O3 contents of the ruby are between forms naturally by metamorphism of alumina-rich 0.3 and 9.2 wt.%. The association of a ruby crystal rocks under amphibolite- and granulite-facies with aluminous pyroxene and ferropericlase placed conditions (Fig. 2-21). Most corundum is formed as

FIG. 2-21. The different types of metamorphic corundum deposits in their geological and geodynamical environments such as plumasite, skarn, desilicated gneiss and granite, cordieritite, biotite schist in shear zone, mafic–ultramafic rocks, and marble. The geodynamic model shows also the location of the ruby and sapphire deposits associated with alkali basalt in continental extension domains. The vertical dimension is not to scale. The subcontinental lithosphere is not represented.

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emery during regional metamorphism of bauxite lenses in limestone, as fine aggregates of corundum with mica, hematite, magnetite, and sericite e.g., Naxos and Samos islands in Greece, and Smyrna in Turkey, or at the contact of aluminous schist and intrusive mafic and ultramafic rocks e.g., Peekskill in Winchester County, New York State, USA. Gem quality corundum is absent in emery deposits. Metamorphic corundum gem deposits are found in complexes formed by mafic and ultramafic rocks, marble, quartzite, gneiss and metapelite where heated either regionally or by the thermal anomaly generated by the intrusion of different plutonic rocks and crosscut by their dikes and intrusive bodies such as granite and pegmatite. The various formation mechanisms depend on either isochemical metamorphism i.e., regional metamorphism or metasomatic metamorphism i.e., contact and/or hydrothermal affecting the contrasting lithologies. Nevertheless, these mechanisms are not well understood for each type of deposit and they remain hypothetical. Moreover, in some cases, several episodes of metamorphism have affected the complexes, as in the Neoproterozoic metamorphic Mozambique Belt that runs from Somalia through Kenya, Tanzania, Malawi, Mozambique, Madagascar, South of India, Sri Lanka, and to East Antarctica (750–540 Ma). Each episode had different thermobarometric and chemical conditions, and the formation of corundum would have to be defined accurately on a pressure–temperature–time scale. Accordingly, the proposed classification for metamorphic corundum relies on the lithology, and FIG. 2-22. Types of metasomatic deposits located at the not on the chemistry of the corundum or the type of contact between two chemically different rocks (after metamorphism. Simonet et al. 2008). The metasomatic fluid circulated along the contact between two rocks of contrasting Metamorphic deposits associated with desilicated lithology such as ultramafic rock or marble and pegmatite and skarn-related granite or mafic– pegmatite or gneiss. The percolation inside the rocks ultramafic rocks: these corundum deposits formed induced metasomatic desilication of the silica-rich as the result of hydrothermal metasomatism rocks and the formation of metasomatic rocks such as occurring either at the contact of intrusive plagioclasite (desilicated pegmatite, i.e., the aluminosilicate plutonic rocks or mafic–ultramafic plumasite), and/or phyllosilicate-rich rocks (phlogopite rocks and host rocks depleted in silica (Fig. 2-22) schist, vermiculite schist, chloritite). such as marble, amphibolite, and ultramafic rocks, (1986) thought that corundum was the result of the or at the contact of a shear zone. interaction of invading pegmatitic fluids and vapor The origin of desilicated pegmatite erroneously with the host rock. Today, the hydrothermal– called "anorthosite" (Kievlenko 2003) has been metasomatic theory proposed by Korzhinsky (1964, debated since the 19th century. Pratt (1906) 1970), and revised by Kolesnik (1976) for advanced a magmatic origin. Du Toit (1918), Hall metasomatic corundum, is commonly accepted by (1923), and Barth (1928) proposed desilication of a geologists (Simonet et al. 2008). Corundum formed granite–pegmatitic melt at the contact with mafic during hydrothermal and/or bi-metasomatic rocks, leading to enrichment in alumina of the melt processes developed at the contact of two and the formation of corundum. Robb & Robb contrasting lithologies, i.e., granite or pegmatite,

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against mafic and ultramafic rocks, marble or gneiss pyrochlore and lower temperature minerals such as (Fig. 2-22). The bi-metasomatic reactions are related andalusite, , diaspore, chlorite, prehnite, and to the infiltration of post-magmatic solutions, which scapolite (Lasnier 1977). The sapphire crystals are originated from the same granite or from other barrel-shaped and elongate along the c axis. They magmatic or metamorphic events. The mechanism are grey to blue, in some cases with a pronounced of metasomatic desilication involves diffusion of asterism. silica, from a pegmatite to an ultramafic rock which World-class gem deposits in desilicated plays the role of SiO2 sink, at a rate more rapid than pegmatite bodies are described from the Umba the diffusion of alumina. The Al:Si ratio increases in River (Solesbury 1967) and Kalalani (Seifert & the pegmatite vein, and metasomatic plagioclase– Hyrsl 1999) in Tanzania, at the Sumjam deposit in corundum association in which the mass of alumina Indian Kashmir (La Touche 1890, Atkinson & per unit volume is much greater than in the initial Kothavala 1983), in southern Kenya with the rock, may develop if the volume of rock decreases deposits of the Mangare area (Key & Ochieng 1991, at the same time, and underlined by the zone of Mercier et al. 1999a, Simonet 2000), and from the quartz dissolution. Another case of bi-metasomatic Russian Polar Ural Mountains (Spiridonov 1998, metasomatism by fluid percolation is the intrusion Kievlenko 2003). of mafic to ultramafic rocks into marble, such as The famous blue sapphire from the States of those described for the ruby from the Mahenge Jammu and Kashmir in northern India was region in Tanzania (Le Goff et al. 2008, Feneyrol et discovered in 1881 by peasants after a landslide. al. 2013). This corundum deposit produced the blue sapphires Desilicated pegmatite bodies with the mineral most prized worldwide for their gorgeous blue color assemblage oligoclase (75%), corundum (23%), and and velvety lustre due to the presence of layers of 2% other minerals (margarite, mica, sericite) are microscopic liquid inclusions. Mallett (1882) and La called plumasite, a terminology derived from the Touche (1890) described the deposit formed by the locality of Plumas in California, where this small pit called the 'Old Mine'. The sapphire was association was first described (Lawson 1903). embedded in clay within an altered pegmatite. The Corundum deposits of this type are known from the geological structure and history of the deposit is northeastern Transvaal (Robb & Robb 1986), poorly investigated because the area is politically Yosemite National Park (Rose 1957), Kinyiki Hill unstable and at high altitude (4600 m above sea and Mangare in Kenya (Simonet 2000), and the level). upper Allier Valley in France (Forestier & Lasnier The deposit is located in metamorphic 1969). In the Allier Valley plumasite is hosted by formations of Cambrian to Silurian age in the spinel-bearing harzburgite. There, corundum is Zanskar range, within the Paddar region (Lydekker associated (Fig. 2-23) with plagioclase (oligoclase– 1883). The series (Fig. 2-24) comprises a succession andesine), spinel, biotite, apatite, zircon, uraninite, of marble, amphibolite, and garnet-graphite-biotite

FIG. 2-23. Mineralogical assemblages in corundum-bearing plumasite (after Lasnier 1977). The sapphire-bearing plumasite of Treignac (Haut Allier, France) with the different stages of transformation of spinel. Prism of spinel surrounded by a corona of blue sapphire (center) and a final transformation into corundum (left). On the right, presence of corundum altered by a later retrograde phase composed of andalusite, diaspore and chlorite. The plagioclase is transformed in scapolite and prehnite. Drawing from P. Brun (le Règne Minéral).

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FIG. 2-24. Geological cross-section of the Sumjam sapphire deposit, Kashmir (after Atkinson & Kothavala 1983). and amphibole-bearing gneiss which has been carbonic fluid inclusions trapped by the sapphire by intruded by pegmatite (Atkinson & Kothavala microthermometry. The pressure–temperature of the 1983). The sapphire is associated with pockets or fluid is estimated to have been 3.7 to 5.6 kbar and lenses of metamorphic rocks formed of olivine, talc, 680–700°C at the center of the crystal, 3.9 to 4.8 and spinel. The lenses range from 1 to 100 m long kbar and 590–600°C in the intermediate zone, and and up to 30 m thick. They are enveloped by a green 2.9 to 3.1 kbar and around 500°C in the outer zone aureole of tremolite, actinolite, and anthophyllite- (Fig. 2-7). This study showed the variability of the bearing rocks. Laterally, the lenses are surrounded P–T conditions of crystallization during the growth by garnet-bearing amphibolite, then amphibolite. of the crystals, suggesting either a slow growth or Pegmatite bodies are located at the contact between three distinct stages of growth. the lenses and the amphibolite. They have been Alluvial ruby and sapphires were discovered affected by an alkaline metasomatism that provoked associated with the Umba River in Tanzania in dissolution of quartz. The mineralogical association 1960, and field exploration exposed the primary consists of plagioclase, mica, tourmaline, and deposits in the Umba and Kalalani serpentinite sapphire. The rims of the desilicated pegmatite are massifs (Solesbury 1967; Seifert & Hyrsl 1999). composed of talc-biotite-carbonate and tourmaline- Mining ceased between about 1970 and 1980, but in bearing rocks (Peretti et al. 1990). The greatest 1989 an important discovery of 80 kg of transparent concentration of sapphire is in plumasite. Near the reddish orange (called 'Tanzanian padparadscha') surface, the veins are altered into , which and yellow sapphires stimulated new mining was derived at depth from fresh plagioclase rich- activity. In 1994, a primary tsavorite (V-rich green rocks. The sapphire is embedded in the plumasite grossular) deposit was discovered in the and sporadically in the tremolite-actinolite zone. surrounding graphitic gneiss, and pyrope-almandine The sapphire crystals have a spindle shape, with deposits in the serpentinite. hexagonal dipyramids and are partially corroded. The serpentinite massif is about 2 km long and Some crystals present small pinacoid faces that are 0.5 to 1 km wide, and surrounded by marble, flattened following one of the a axes (Cesbron et al. kyanite- gneiss, and quartzite. Gem- 2002). The common color is pale blue to deep quality sapphire and ruby are confined to desilicated cornflower blue, but other colored sapphires have pegmatite bodies that crosscut the serpentinite (Fig. been observed, such as violet, green, and yellow. 2-25A, B). They form vertical lenses, 5 to 10 m long Pink sapphire was recovered from the contact of the and 2 to 3 m thick. The pegmatite bodies pinch and graphite-biotite gneiss (Atkinson & Kothavala swell and the dimensions are quite variable. They 1983). Hänni (1990) identified rutile, green and red- have coarse-grained texture and display meta- brown tourmaline, pargasite, plagioclase, uraninite, somatic reaction zones at the contact with the allanite, diopside, and zircon as inclusions in the serpentinite. Two types of corundum-bearing veins sapphire. have been described (Fig. 2-25C): (1) veins with The velvety luster of the blue sapphire is caused calcic plagioclase, vermiculite-actinolite-chlorite, by minute two-phase (liquid and vapor) fluid and colored sapphire; and (2) veins with vermiculite inclusions. Peretti et al. (1990) studied the primary and orange and yellow-brown sapphire. In Kalalani,

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FIG. 2-25. The location and geology of the Kalalani gem corundum in Tanzania (modified after Seifert & Hyrsl 1999). A, Location of the Kalalani and Umba primary corundum deposits. B, The gem corundum deposits are associated with desilicated pegmatite bodies (plumasite) crosscutting serpentinite. C, Geological sketch map and cross-section of a gem corundum deposit. The sapphire is contained in argillitic nodules in the desilicated pegmatite. other major constituents are chlorite, spinel, and sapphire, and black in the ruby, is a common kyanite. Tabular ruby was found within a desilicated feature. Seifert & Hyrsl (1999) demonstrated that pegmatite associated with biotite. The inclusions desilicated pegmatite results from intense alkaline and color range of reddish to orange sapphire from metasomatism of the pegmatite and the serpentinite Umba and Kalalani are very similar to those of the at high fluid–rock ratio. orange sapphire from Tunduru (Tanzania) and Ruby was discovered and mining initiated in Chimwadzulu in Malawi (Henn & Milisenda 1997). the Mangare area of Kenya in 1973, but the mines The sapphire crystals are tabular with pinacoids were closed during most of the 1970s and 1980s due and prismatic hexagonal faces. Color zoning in the to litigation. Exploitation began anew in 1993–1994 form of a tube parallel to the c axis, colorless in blue and since then has provided a regular supply of

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facetable and cabochon material of high quality. In 2011 the production decreased and in 2012 the Rockland mine (formerly the John Saul mine) was in discontinuance of business, as a consequence of the discovery and marketing of ruby from Mozambique. In Mangare area the ruby contains low iron and consequently exhibits an intense fluorescence. The John Saul, Penny Lane and Hard Rock deposits occur in serpentinite intercalated in graphite-sillimanite gneiss (Mercier et al. 1999a, Simonet 2000). In the Hard Rock mine, the contact zone between the gneiss and the serpentinite is a tectonic shear zone. The serpentinite is considered to be an imbricate structure formed during the main tectonic Pan-African event (700–600 Ma) of the Mozambique belt. Corundum is found in two types of structures (Fig. 2-26): (1) micaceous lenses and pockets located at the contact between the serpentinite and the gneiss along the shear zone. They are usually composed of ruby ± spinel ± sapphirine and mica, but in some cases of ruby, phlogopite and magnesian chlorite; (2) veins that crosscut only the serpentinite. Corundum is rare and is associated with zoisite, plagioclase ± phlogopite ± muscovite. These veins resemble plumasite. Corundum formed by the following reaction (Mercier et al. 1999a): sapphirine + water ĺ corundum + chlorite + spinel (1) In the veins that crosscut the ultramafic rocks, the corundum is associated with anorthite and zoisite. The textures and the mineral associations suggested the following reaction: anorthite + water ĺ FIG. 2-26. Geological map of the Hard Rock mine in the corundum + zoisite + silica (2) Mangare gem corundum mining district, Kenya (after Mercier et al. 1999a). The serpentinite corresponds to Thus, two hydrothermal alteration phenomena, an imbricate structure in the graphite-sillimanite- hydration and desilication, are superposed on the bearing gneiss. Ruby is found either in lenses and shear zone system. The conditions of pressure and pockets at the contact of the shear zone or in veins that temperature for ruby formation are between 700– crosscut only the serpentinite. 750°C and 8 to 10.5 kbar (granulite facies). These A special case of desilication of pegmatite in P–T conditions contrast with those of the mafic and felsic gneiss is described for the ruby surrounding gneiss (P = 650°C and 4.5 < P < 6.5 deposits from Karelia in Russia (Kievlenko 2003, kbar, amphibolite facies; Fig. 2-27. In consequence, Terekhov 2007, Bindeman & Serebryakov 2011). Mercier et al. (1999a) suggested that the ruby and The Belomorian polymetamorphic complex located the ultramafic rocks are exotic. They correspond to between the Kola and Karelia Archean blocks fragments of deeper crust brought up by thrust and underwent the 1.9–1.8 Ga metamorphism and shear zone tectonics. Comparison of the regional intrusion of pegmatite. Ruby is hosted by thermobarometric and mineralogical data of twenty plagioclase veins crosscutting corundum-bearing deposits formed in ultramafic rocks in Kenya and kyanite--garnet-biotite-hornblende gneiss Tanzania show that they formed in the granulite and the overlying garnet amphibolite. The veins facies and that they are associated with shear zones.

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FIG. 2-27. Pressure–temperature conditions of the formation of gem corundum and surrounding metamorphic rocks in the Mangare area, Kenya (after Mercier et al. 1999a). An: anorthite, Anth: anthophyllite, Bt: biotite, Cd: corundum, En: enstatite, Gt: garnet, Kf: K-feldspar, Ky: kyanite, Mg-Chl: magnesian chlorite, Sil: sillimanite, Sp: spinel, Spr: serpentinite, Qz: quartz, V: H2O vapor, Zo: zoisite.

range from several centimetres to 10 m thick and up Now we will consider skarn-type deposits to 60 m long, parallel to the metamorphic foliation. where pegmatite and granite intruded marble, calc- Corundum crystals have prismatic and bladed silicate rocks and Ca-bearing gneiss. In the shapes, 5 cm long and up to 3 cm in diameter; ruby following examples, the mechanisms responsible for contains up to 9.4 wt.% Cr2O3. Metasomatism and corundum formation are quite similar to those fluid–rock interaction with the gneiss caused proposed for the genesis of corundum within desilication of the pegmatite and transformed it into desilicated pegmatite bodies that intruded mafic and plumasite with oligoclase-andesine, phlogopite, ultramafic rocks. Every post-magmatic process tourmaline, zoisite, epidote and chlorite. linked to the emplacement of granitic intrusions (or Phlogopitization of the border of the vein is equivalents) has a stage at which the solutions common. Low oxygen isotope ratio of minerals became acid and, therefore, constitute a source of including ruby is characteristic of this deposit with strong reactions with their wall rocks. As the the report of the lowest known į18O (–16 to –27‰) activity of K, Na, and Al increases, these elements for terrestrial silicate minerals, oxides and rocks enter into reactions with marble or dolomitic (Bindeman et al. 2010). The highly depleted į18O marble, increasing the concentration of Ca and Mg values are due to the circulation of meteoric glacial in solution. Such reactions produce feldspar, biotite, subwaters in a rift zone at around 2.4 Ga which and Ca-bearing minerals. The association of Ca- affected the initial isotopic composition of the bearing, usually Fe-rich silicate minerals (including protolith prior to ruby formation (Bindeman & amphibole, pyroxene, garnet, epidote, and zoisite), Serebryakov 2011). or Mg-rich silicate minerals (such as phlogopite,

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diopside, pargasite, and forsterite), defines the so- and tourmaline in a skarn-formed marble called skarns. The surrounding marble is (Nissimboim & Harlow 2011). Trace element transformed by exo-metamorphism i.e., exoskarn, compositions measured by LA–ICP–MS or by but sometimes complex phenomena of recrystalliz- EPMA on ruby from different types of environment, ation may affect the granite itself by endo- i.e., in marble, skarn, and alluvial deposits, show no metamorphism i.e., endoskarn. B or Zr enrichment despite the skarn-related Corundum-bearing desilicated pegmatite in paragenesis of some samples (Harlow & Bender mafic rocks and marble are typical endoskarns. 2013). The metamorphic versus skarn-related source Nevertheless, some authors believe, erroneously, of ruby cannot be discriminated and the Mogok that all the corundum deposits hosted in marble Stone Tract rubies record rather metamorphic– belongs to the endoskarn type (Kievlenko 2003). metasomatic processing. The metamorphic formations that contain deposits The Bakamuna exoskarn skarn deposit at of this type, such as in Myanmar (Iyer 1953, Harlow Elahera in Sri Lanka (Fig. 2-28) was described by et al. 2006, Kyaw Thu 2007, Themelis 2008) or Silva & Siriwardena (1988). The skarn body is 35 m Tajikistan (Terekhov et al. 1999), have been long, 12 m thick and occurs in a calcite-dolomite subjected to tectonomagmatic activity, but the effective participation of the magma is not constrained by isotopic studies. In consequence, the marble deposits cannot be classified as endoskarn types and further studies are necessary to characterize them completely. One of the more enigmatic regions is the Mogok Stone Tract in Myanmar. The Mogok metamorphic belt was intruded by syenitic to granitic magma in Cretaceous and Eocene times accompanied by subsequent metamorphism. Kyan Thu (2007) defined four ages for the emplacement of magmatic rocks based on radiometric dates of zircon by the U–Pb method: (1) 129 ±8.2 Ma (Early Cretaceous) for augite-biotite granite; (2) 32 ±1 Ma (Early Oligocene) for leucogranite; (3) 25 Ma (late Oligocene) for foliated syenite; and (4) 16 ±5 Ma (Middle Miocene) for painite-bearing skarn formed at the contact leucogranite–marble. Khin Zaw et al. (2008) obtained an age of 31–32 Ma for a zircon inclusion in the Mogok ruby by the LA–ICP–MS technique. Garnier et al. (2004b) obtained 40Ar/39Ar ages at 18.7 ±0.2 to 17.1 ±0.2 Ma on phlogopite syngenetic with metamorphic ruby. Themelis (2008) showed that the diversity of the paragenesis associated with primary ruby and sapphire deposits reflected multiple geological origins either with FIG. 2-28. Simplified geological map of Sri Lanka intrusions of granitoid rocks or regional (modified after Kriegsman 1995) with the corundum metamorphism. The assemblage of ruby, calcite, localities in their geological formations (after graphite, muscovite and pyrite is common, but Dissayanake & Chandrajith 1999, Dissanayake et al. adjacent silicate minerals typical of skarn and 2000). Main mining districts and mines of Sri Lanka: syenitic rocks, such as cancrinite, scapolite, sodalite, 1, Polonnaruwa; 2, Nalanda; 3, Elahera; 4, Kurunegala; nepheline, sodalite, datolite, and painite 5, Rangala; 6, Kandy; 7, Hanguranketa; 8, Nilgala; 9, (CaZrBAl9O18), are also found e.g., such as at Wet Avissawella; 10, Hatton; 11, Nuwara Eliya; 12, Loo (Harlow et al. 2006, Harlow & Bender 2013). Passara; 13, Panadura-Horana; 14, Ratnapura; 15, The radiometric data suggest that the Oligocene-age Haputale; 16, Buttala; 17, Alutgama; 18, Rakwana; 19, ruby grew either as disseminated crystals in marble Timbolketiya; 20, Kataragama; 21, Ambalangoda; 22, Morawaka; 23, Ambalantota; 24, Galle; 25, Matara. (Khin Zaw et al. 2008) or as overgrowths on painite

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marble. The skarn mineralization consists of three different mineralogically distinct zones (Fig. 2-29A): (1) the center zone is a coarse-grained orthoclase-quartz-bearing pegmatite; (2) the middle zone contains fine-grained spinel-scapolite aggregates with disseminations of corundum and phlogopite; and (3) the outer zone has a porphyritic texture made of large crystals of corundum and spinel. The substitution of corundum by spinel is common. The intermediate and outer zones are crosscut by a network of veinlets forming lenses up to several centimetres wide filled with spinel, magnesite, and phlogopite. The vein formation is associated with hydraulic fracturing, i.e., a fluid overpressure in the rock. The contact with the marble (Fig. 2-29B) shows a fringe of phlogopite and spinel, 10 to 20 cm thick, with 70% of spinel by volume, and a border where marble recrystallized into coarse-grained calcite. Gem corundum is represented by light greenish blue and blue sapphire, with crystals up to 1 cm. According to Silva & Siriwardena (1988), the Bakamuna skarn deposit formed by the interaction of pegmatitic fluids with dolomitic marble. The pegmatite is not desilicated and quartz is always present. Thus, the sapphire deposit is an exo-skarn deposit. Percolation of the fluid caused the metasomatic alterations. Three stages of fluid–rock interaction have been proposed: Fluid 1 + marble o scapolite + corundum + magnesite + CO2 (3) magnesite + corundumo spinel + CO2 (4) Fluid 1 + magnesite + spinel + scapolite o FIG. 2-29. The Bakamuna exoskarn skarn deposit at phlogopite + Fluid 2 (5) Elahera in Sri Lanka (after Silva & Sirawardena, 1988). The skarn is developed around the pegmatite Fluid 2, enriched in alumina, also reacted with the that intruded the marble. A, Schematic distribution of marble following reaction (3), and resulted in the the different mineral assemblages in the skarn zones: crystallization of large quantities of corundum. The (1) the center zone is a coarse-grained orthoclase- low Mg content of the marble allowed the quartz-bearing pegmatite; (2) the middle zone contains preservation of corundum whereas it was fine-grained spinel–scapolite aggregates with transformed into spinel following reaction (4). corundum and phlogopite, and (3) the outer zone has a In 1952, the French geologist Hibon reported porphyritic texture made of large crystals of corundum small eluvial sapphire grains associated with and spinel. B, Details of the outer zone with the concentrations of corundum and spinel at the contact of hibonite south of the village of Andranondambo in marble. the Tranomaro area in Madagascar. The first deposit was discovered at Esiva, near Tranomaro village in A number of studies have been published on the 1991 (Fig. 2-30). News of the discovery spread origin of the sapphire. Kiefert et al. (1996) com- quickly, and by 1996 around 10,000 miners were pared the Andranondambo deposits to the famous estimated to be working in pits up to 15 m deep and Kashmir sapphire desilicated pegmatite deposit, 50 to 80 cm wide, at Andranondambo, Antirimena, Milisenda & Henn (1996) advocated a high-grade Analalava, and Andranomitrohy (an area of more granulite-facies metamorphism for sapphire than 7000 km2). formation, and Rakotondrazafy et al. (1996) and

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intrusions from the Anosyan magmatism (Fig. 2-30) and a metasomatic skarn origin is proposed by Rakotondrazafy et al. (1996). Three stages of crystallization in the skarns have been defined (Fig. 2-31). In the first stage (metasomatism at T a 850°C

and P a 5 kbar), Ca-rich hyperaluminous pods of meionite, spinel, thorianite, and corundum crystallized in a titanite-bearing matrix consisting of scapolite and aluminous diopside. Corundum is found also in the diopsidite at the contact with the Ca-rich pods. U–Pb dating on zircon from a diopsidite gave an age of 565 r10 Ma (Andriamaro- fahatra & de La Boisse 1986), which is in agreement with Pan-African ages (540–580 Ma) obtained for the granulite-facies metamorphism and the syn-metamorphic emplacement of the Anosyan charnockite and granite (Paquette et al. 1994). In the second stage (metasomatism T a 800°C

and P a 3–3.5 kbar), diopside was partially transformed into F-rich pargasite and most calcian scapolite went into anorthite + calcite. Thorianite crystallized with F-rich phlogopite and hibonite at the expense of corundum and spinel. In the third stage (retrograde amphibolite metamorphism at T a 500°C and P a 2 kbar), lenses of phlogopite associated with calcite, diopside and anhydrite, and late-stage REE-rich calcite veins with zircon, titanite, and urano-thorianite crosscut the 'calc-magnesian complex'. U–Pb dating of zircon

from a metasomatic calcite vein gave ages of 516 FIG. 2-30. Geological map of the Tranomaro area with the r10 Ma (Andriamarofahatra & de La Boisse 1986) location of the corundum-hibonite skarn deposits in and 523 5 Ma (Paquette et al. 1994), which Madagascar (after Rakotondrazafy et al. 2008). r correspond to the latest Pan-African event in the Gübelin & Peretti (1997) suggested that the sapphire area. At this late stage, blue gem sapphire from the formed as a result of various metasomatic skarn Andronandambo area crystallized in K-feldspar stages. veins crosscutting marble and diopsidite at T a The sapphire deposits occur in the high-grade 500°C and P a 2 kb (Rakotondrazafy et al. 2008). granulite facies of the Proterozoic Tranomaro The veins are vertical with centimetre to decimetre Group, composed of metasedimentary rocks width. Sapphire is associated with K-feldspar, (metapelite, calc-silicate rock, and marble) inter- F-apatite, calcite, and phlogopite. The common layered with leucocratic gneiss (Rakotondrazafy et crystalline forms of sapphire are the dipyramids n al. 1996). During granulite-facies metamorphism {22.3}and z {22.1} and the basal pinacoid c {00.1}; and syn-metamorphic emplacement of the Anosyan the second-order prism a {11.0} and the charnockite and granite, marble and calc-silicate rhombohedron r {10.1} are also present (Schwarz et gneiss were transformed into skarns called the 'calc- al. 1996). Color zoning is very pronounced with magnesian complex' (Moine et al. 1985). This dark blue or dark brownish blue areas delineated by consists of impure calcitic marble, diopsidite with faces parallel to the basal pinacoid, the dipyramids n variable amounts of scapolite, spinel, thorianite, and and z. pargasite, and peraluminous rocks made of At the border of the veins, marble is plagioclase and/or scapolite with spinel, thorianite, feldspathized and the stability of the K-feldspar- hibonite (CaAl12O19), and/or blue to pink corundum. corundum-calcite assemblage is controlled by the Clinopyroxenite commonly occurs at the contact equilibrium equations: between marble and granitic or charnockitic

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FIG. 2-31. The U–Th-corundum skarn deposits from the granulite facies of the Tranomaro area (modified after Rakotondrazafy et al. 1996). A, Al–(Fe+Mg)–Ca showing the distribution of the different assemblages of the calc-magnesian and skarn rocks from stages I and II. B, The three stages of skarn formation with stage I meta- somatism (leucocratic segregation in clinopyroxenite; T ~ 850°C, P = 5kb), stage II (formation of hibonite and anorthite + calcite assemblages; T ~ 800°C, P = 3.3.5 kb), and stage III (retrograde amphibolite facies and fracturing stage with REE-bearing calcite veins and gem blue sapphire–K- feldspar bearing veins; T = 500°C, P = 2 kb).

muscovite œ solubility of silica is low and a H2O–NaCl-rich fluid K-feldspar + corundum + water (6) may possibly transport the metals (Ramambazafy et al. 1998). Unmixing would produce a CO2-rich anorthite + CO2 œ phase in equilibrium with a H2O–NaCl brine (Gibert calcite + corundum + H2O (7) et al. 1998). With such a high CO2 concentration in Fluid inclusions in different minerals from the fluid, the CO2-rich phase (XCO2 > 0.8 mole%; P gneiss, skarn, and sapphire-bearing vein are CO2- = 5 kb and T = 600°C) would have to coexist with rich (Ramambazafy et al. 1998). Fluids with high NaCl salt (Shmulovich & Graham 1999). PCO2 (XCO2 > 0.8 mol.%) and low PH2O are in Sapphire from the Andranondambo district 18 equilibrium with the mineral assemblages. The C yielded three sets of į O values (Giuliani et al. and O isotopic composition of Tranomaro marble 2007b): (1) 3.9‰ for a pink sapphire hosted in a has shown the crustal origin of these fluids and their pyroxenite (diopsidite) from stage 1 of skarn 18 probable relation with granitic magmatism metasomatism; (2) 10.1 < į O < 10.7‰ for blue (Boulvais et al. 1998). The infiltrated marble and and pink sapphire hosted by scapolitite and marble metasomatic diopsidite do not record any from stage 1. The important isotopic variation for contribution from a crustal C source and the sapphire of stage 1 is due to the variability in contribution of fluid infiltration is insignificant. The chemistry of the pre-metamorphic host rock. During formation of the skarn in marble, however, results stage 2, hibonite crystallized at the expense of 18 from the transport of Si, Mg, Th, U, Zr and REE by corundum and its į O of 10.6‰ falls within the the metasomatic fluid. CO2-rich fluids are not a isotopic range defined for corundum of stage 2. It good candidate for the mobilization and transport of confirms that in a closed system and at high such elements which are rather immobile temperature (T ~ 800°C), the oxygen isotopic (Rakotondrazafy et al. 2008). At high PCO2, the composition of a protolith or a mineral phase can

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control the final į18O of corundum; and (3) 14.0 < pargasite, garnet, spinel, scapolite, tourmaline, į18O < 15.6 (mean į18O = 14.8 r0.6‰; n = 4) for apatite, zircon, , and lapis lazuli. The blue gem sapphire hosted in K-feldspar veins from continental collision setting of southern Baffin is stage 3. The isotopic range overlaps the range analogous to gem-producing areas within the India– defined worldwide for sapphire hosted in desilicated Asia collision zone, i.e., from Afghanistan to pegmatite within marble (14.0 < į18O < 16.4 ‰; Vietnam. Giuliani et al. 2009), and the oxygen isotopic Most of the sapphire crystals in Kimmirut are composition of sapphire is buffered by the host deep blue in color but yellow, colorless, and light rock. The important isotopic variation for sapphire blue crystals have been found. The sapphire is between stages 1 and 3 might be related to the large hosted by calc-silicate lenses in a marble unit of the isotopic variation found in į18O of marble between metasedimentary Lake Harbour Group, near a major 6.5 and 19‰ (Boulvais et al. 1998). The lowest terrane boundary within the Paleoproterozoic Trans- į18O-values are found in dolomitic marble whereas Hudson Orogen (Lecheminant et al. 2004, 2005). calcic variety has higher į18O between 13.5 and At the main Beluga showing the sapphire crystals 19‰. occur with plagioclase, clinopyroxene, phlogopite, Metasomatic alteration by fluid percolation muscovite, calcite, graphite, nepheline, and related to the emplacement of granitic magma, scapolite. Apatite, rutile, titanite, and zircon are including desilication, can also affect rocks such as common in the host rock, and rare phases include gneiss or quartzofeldspathic and calc-silicate rocks chlorite, tourmaline (dravite), monazite, sanbornite (Lecheminant et al. 2004, 2005, Simonet et al. (BaSi2O5), thorianite, and uraninite. The rocks in the 2008, Zwaan & Zoysa 2008, Liu Shang-I & Zoysa Bowhead showing, 170 m south-southwest of the 2011, Dharmaratne et al. 2012). The primary Beluga showing, appear to be less altered. sapphire deposit discovered in 2012 at Petrographic studies suggest that this diverse Thammannawa, near Kataragama in southeastern mineral suite formed during retrograde Sri Lanka, is an example of the effect of the metamorphism accompanied by infiltration of CO2- circulation of pegmatitic fluid in gneiss bearing fluids. Syenitic or ijolitic magmas may have (Dharmaratne et al. 2012). The garnetiferous quartz- played a role in initial formation of the calc-silicate feldspar gneiss and charnockitic biotite gneiss were lenses. intruded by various granitoid and pegmatite bodies. The ruby and pink sapphire at the Greyson and The pegmatite consists of quartz-free coarse-grained Kitwalo mines in the Mahenge mining district in feldspar and mica surrounded by a micaceous layer Tanzania are found in marble crosscut by mafic which hosts blue sapphire. The metasomatic process dykes (Le Goff et al. 2008 in Feneyrol et al. 2013). affected both pegmatite and gneiss. The The crystallization of corundum is assisted by mineralization extended generally in the foliation chemical exchanges through the percolation of direction and measured about 60 cm thick. The fluids along the contact between the dyke and the crystals of sapphire display a combination of marble. Ruby and pink sapphire are located in hexagonal bipyramids with rhombohedral and basal metasomatic pargasite-phlogopite zones formed pinacoids, flat faces, sharp edges, and a vitreous along the contact zones. Ruby is associated with lustre. They have a good transparency and pure phlogopite, spinel, pargasite, sapphirine, rutile, Mg- color with cut gems up to 20 ct. The inclusions are chlorite and pyrite. The formation of sapphirine rutile, uraninite, graphite, zircon, and spinel originates from the destabilization of corundum- (Pardieu et al. 2012). Such metasomatic deposits are spinel-Mg-chlorite following the reaction: also described in other areas of Sri Lanka 6 corundum + 2 spinel + Mg-chlorite ĺ (Hapuarachchi 1989, Fernando et al. 2001). 2 sapphirine + 4HO (8) The Kimmirut sapphire occurrence (Fig. 2-1) 2 was discovered by Seemeega and Nowdla Aqpik in Metamorphic deposits associated with biotitite and 2002 (Lecheminant et al. 2004). The showing is cordieritite in gneiss: the deposits found in located approximately 1.5 km southwest of feldspathic gneiss and cordieritite represent new Kimmirut (formerly Lake Harbour), near the south types of metamorphic gem corundum deposits in coast of Baffin Island, Nunavut, Canada Madagascar: sapphire-bearing biotite schist in (62°49.7´N, 69°53´W). The area also hosts other feldspathic gneiss from Sahambano, Zazafotsy, and gem minerals in complexly deformed, high-grade Ionaivo deposits (Giuliani et al. 2007a, Uher et al. metamorphic rocks. These include diopside,

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2012), and ruby and polychrome sapphire in cordieritite intercalated within charnockite at Ambatomena and Iankaroka deposits (Giuliani et al. 2007b, Andriamamonjy et al. 2013). Both types of deposits are within shear zones and corundum is linked to fluid circulation through channels such as fractures, foliation planes, and lithological contacts. The Sahambano deposit is located 30 km east of Ihosy (Fig. 2-32). The deposit was discovered in 1999 and exploited sporadically. The sapphire crystals are multicolored, but rarely of gem quality and treatment is necessary to improve color and transparency. On average, 100 kg of corundum picked from washed material contains 24 kg of colored sapphires with 1 kg of translucent crystals, of which 50 g would be of gem quality. The division of the color is 15% brown to orange, 5% orange to pink, 40% pink to purple, 5% purple to fuchsia, and 35% violet to blue. The deposit occurs in the Tranomaro Group, characterized by a high abundance of calcic and magnesian paragneiss and leptynite. The deposit sits in the Ranotsara fault zone, a 30 km wide and 300 km long steep structure that had a long history of deformation and high temperature metamorphism dated between 600 and 500 Ma (Collins & Windley 2002). 40Ar/39Ar dating of biotite from a sapphire- bearing biotite schist gave a minimum age of formation at 492 r5 Ma (Giuliani et al. 2007a). Mylonitization and dextral shears are common in the Sahambano area. The sapphire occurs in FIG. 2-32. Structural and lithological sketch map of feldspathic gneiss lenses intercalated within southeast Madagascar with the location of the leptynite at Dominique, Nono, Momo, Jeanne d'Arc, corundum deposits of mixed origins (modified after and Ambinda Sud pits. Shearing was accompanied Martelat et al. 2000). Corundum deposits as 4: Ambinda (Betroka); 12: Miarinarivo, 13: Zazafotsy, by the opening of fissures and fluid circulation, 14: , 15: Ambinda (Ihosy), 16: Sahambano, which resulted in the biotitization of the host rock. 18: Vohidava, 19: Iankaroka, 20: Ambatomena, 21: Sapphire occurs in biotitite, which also contains Ianapera, 22: Fotadrevo, 23: Anavoha, 24: Maniry, 25: sillimanite and spinel, and in gneiss composed of Gogogogo, 26: Vohitany, 27: Ejeda, 28: Ilakaka, 29: K-feldspar, biotite, sillimanite, spinel, sapphirine, Sakaraha, 30: Andranondambo, 31: Sakeny. Major garnet, albite, and sapphire. Sapphire formed during shear zones referred as A: Ampanihy, B: Beraketa, C: the prograde metamorphism at T ~ 650°C and P ~ 5 Ranotsara fault zone, D: shear zone of the Phanerozoic kbar according to the reaction (Ralantoarison 2006): cover. Minor shear zones referred as (a) to (h). The Pressure (kbar) and Temperature (°C) of 3 hercynite + K-feldspar + H2O o metamorphism are from Moine et al. (1985), 3 corundum + annite (9) Ackermand et al. (1989) and Nicollet (1990). The size of the crystals varies from 1 mm to 5 cm. pinacoid {00.1}. Laminar habits are characterized Sapphires display different colors according to their by well-developed pinacoids, and cylinder habits are Cr and Fe contents: colorless, grey, greenish grey, formed by hexagonal dipyramids associated with orange, blue, dark pink, purple, brown, pink, and pinacoid {00.1} and/or hexagonal prism. Solid red to fuchsia. All crystals are euhedral and exhibit inclusions in sapphire are K-feldspar, zircon, barite, either short or long prismatic habits, which are often spinel, cheralite, sillimanite, diaspore, albite, and associated with the rhombohedron {10.1}, the pyrite. hexagonal prism {10.0} or {11.0}, and the basal The sapphire is contained in metasomatic zones

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and the peraluminous gneiss supplied Al and the African tectono-metamorphic event (Martelat et al. chromophore elements (Cr, Fe, and Ti) necessary 2000). The 40Ar/39Ar age at 494 r5 Ma obtained for sapphire coloration (Fig. 2-33). The distribution from a biotite crystal associated with sapphire of the color of the sapphire is controlled by confirmed that the mineralizing episode was the lithology: colorless to blue sapphires are found in latest Pan-African event in the area (Giuliani et al. the biotitite zone developed from feldspathic gneiss, 2007b). green to brown and greenish sapphires are in the Like at Sahambano, the mineralization occurs sillimanite-bearing feldspathic gneiss, and finally in several lenses of feldspathic gneiss intercalated red to fuchsia to pink and pinky orange within garnet-bearing leptynite and affected by fluid sapphires are located in the sapphirine-bearing circulation in shear zone fractures (Andriamamonjy feldspathic gneiss. The other colored crystals are 2006). The inner zone of the lens consists of distributed randomly at the interface of biotitite almandine and sapphire up to 10 cm associated with and biotitized sapphirine-bearing feldspathic biotite, plagioclase, spinel, and K-feldspar formed gneiss. around sapphire and garnet (Plate 1A). The outer The Zazafotsy deposit, also called zone is biotite schist with biotite, sapphire, spinel, "Amboarohy" (Pezzotta 2005), is located 35 km and very few crystals of grandidierite (boro- 2+ northeast of Ihosy on National Road 7 (Fig. 2-32). aluminosilicate of Fe and Mg, (Mg,Fe )Al3(BO3) The sapphire occurrence was discovered in 1950 (SiO4)O2) which passed into biotitized feldspathic and exploitation by local miners began in 1989. The gneiss. In one lens, the outer zone consists of fine- exploitation was sporadic until 2003, when it started grained metasomatic alternation of biotitite and again producing very beautiful euhedral sapphire black tourmalinite developed on a 20 cm-wide scale. crystals. The majority of the sapphires are not of In the same area, apatite and are found in gem quality and heat treatment is necessary to pegmatite veins. improve their transparency and color. All of the sapphire crystals are euhedral and, as The deposit is located in the Itremo Group at Sahambano, exhibit either short or long prismatic which is mainly composed of garnet-sillimanite- habits associated with the hexagonal prism and the cordierite leptynite and amphibole-clinopyroxene rhombohedron terminated by basal pinacoid. gneiss with minor intercalations of quartzite and Mineral inclusions found in sapphire are zircon, impure limestone. The sapphire deposit lies in the K-feldspar, plagioclase, sillimanite, spinel, and Zazafotsy shear zone system, also linked to the Pan- biotite. The sapphire occurs in different colors

FIG. 2-33. Schematic geological section of the Momo trench in the Sahambano sapphire deposit (modified after Ralantoarison 2006). The sapphire mineralization is contained in biotitized feldspathic gneiss and sapphire-bearing feldspathic gneiss. The respective contents in Al, Mg, Fe, Ti (in wt.%) and in Cr (in ppm) are reported for each different protolith and the metasomatite, i.e., biotitite. The colored sapphire distribution is reported for the different zones (br, brown; bl, blue; bg, blue-green; cl, colorless; p, pink; o, orange; f, fuschia; r, red; pu, purple.

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including dark blue, light blue, grey blue, fuchsia, of retrograde isochemical reactions mainly in a orange, pink, violet, mauve, and brown. closed system such as the ruby occurring as Deposits in cordieritite include the Iankaroka disseminated crystals in marble from central and occurrence, which was reported in 1984–85, and southeast Asia (Garnier et al. 2008); and (2) in described by Salerno (1992). It is located 35 km impure marble containing gneiss and silicate layers south of the city of Betroka (Fig. 2-32) in the where ruby crystallized at the peak of prograde province of . The deposit is characterized by metamorphism such as at Revelstoke in Canada polychrome sapphire displaying distinct color (Dzikowski 2013, Dzikowski et al. 2010, 2013) and bands. When observed in a plane parallel to the c Morogoro in Tanzania (Le Goff et al. 2008). axis, the crystal is uniformly pinkish to purple, but The first type of marble deposit forms the main in a direction perpendicular to the c axis thin layers sources for excellent quality ruby with intense color of green blue, orange, brown, and pink are visible ('pigeon blood') and high transparency in marble- (Koivula et al. 1992). The size of the crystals varies hosted ruby deposits from central and southeast between 1 to 10 mm. Euhedral crystals are elongate Asia (Fig. 2-9). These types of occurrences are also to tabular hexagonal prisms and bipyramids. The known in North America, Europe, and Africa: in sapphire is found in a cordieritite lens, ~ 7 m long Sussex county, New Jersey, USA (Dunn & Frondel and 4 m wide, intercalated concordantly within 1990), at Mercus and Arignac in Ariège, France leptynitic biotite-cordierite-bearing gneiss of the (Lacroix 1890), at the vicinity of Xanthi in Greece Androyan series. The cordieritite is affected by (Hughes 1997), at Prilep in Macedonia (Hunstiger shearing; on its border this results in the formation 1990), at Campolungo in Switzerland (Hochleitner of a biotitite developed on the gneiss, and in the 1998), and in the Urals (Kissin 1994). cordieritite in the development of sapphire-bearing These deposits share many common geological, fissures. The cordieritite is composed of phlogopite, structural and mineralogical features (Garnier et al. cordierite, plagioclase, green tourmaline, chlorite, 2008). They are hosted by metamorphosed platform pyrite, spinel, and sillimanite. carbonate assemblages associated generally with The Ambatomena ruby deposit, located 10 km intercalations of garnet-biotite-sillimanite- or northeast of the city of Isoanala (Fig. 2-32), was biotite-kyanite-bearing schist or gneiss (Kievlenko exploited by a private company from 2000 to 2001. 2003). The series commonly contain alternations of The rubies were of good quality consisting of quartzite and amphibolite. Dikes of granite and/or euhedral prismatic crystals up to 3 cm long and 1 to pegmatite intrude these metamorphosed sedimentary 2 cm in diameter. The deposit occurs in the rocks, but the ruby mineralization is not directly Androyan metamorphic series composed of linked with granitoid bodies, as observed for paragneiss, orthogneiss, marble, granite, clino- desilicated pegmatites in other protoliths. The pyroxenite, and quartzite (Andriamamonjy 2010). mineralization is generally stratiform, i.e., The ruby is contained in cordieritite layers or lenses conformable with the bedding of the marble, and intercalated within biotite-cordierite-sillimanite- occasionally disseminated within a particular level bearing charnockite. The mineralized zone has in the marble. The mineralized zones are between suffered intense metasomatism, transforming 0.5 and 10 m thick. A 'ruby zone' is formed by a pegmatite veins into plagioclasite, and in the succession of benches of centimetre- to metre-scale formation of corundum-free phlogopitite and marble, where ruby is located in veinlets, gash- sapphirine-anorthite-phlogopite-bearing rocks. The veins, lenses or disseminations in the carbonate ruby occurs in a cordieritite composed of cordierite, gangue. The estimated grade of the Nangimali rutile, K-feldspar, sapphirine, phlogopite ± diopside. deposit in Azad Kashmir (exploited by the Ruby crystals exhibit a spinel and sapphirine AKMDC, Azad Kashmir Mineral and Industrial coronitic texture. Spinel results from the Development Corporation) was estimated at 11 g/m3 destabilization of ruby during the retrograde phase, from a pilot study on the economic zone carried out and in some cases ruby is totally pseudomorphosed over three years (Malik 1994, Pêcher et al. 2002). into spinel. In general the ruby-bearing marble is composed of calcite and dolomite. It contains the oxide Metamorphic deposits associated with marble: the minerals spinel and corundum, and diopside, ruby and pink sapphire deposits hosted by meta- phlogopite, garnet, chlorite, margarite, tremolite, pure or impure limestone are of two types: (1) in pargasite, edenite, uvite, and forsterite. Graphite is marble where the corundum crystallized as a result sometimes associated with these minerals as in

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deposits of Afghanistan, Pakistan, Myanmar, Nepal, Nepal, northern and central Vietnam, and southern and Vietnam. Other minerals include anorthite, China (Hughes 1997). The mines in Afghanistan are anhydrite, aspidolite, titanite, fuchsite, scapolite, among the oldest in the world and were already zoisite, K-feldspar, epidote, pyrite, and pyrrhotite exploited seven hundred years ago (Rossovskiy (Iyer 1953, Harding & Scarratt 1986, Kissin 1994, 1980, Hughes 1997, Bowersox & Chamberlain Garnier et al. 2004c). 1995, Bowersox et al. 2000, Cesbron et al. 2002). The most common crystalline forms include the Ruby deposits from the Hunza Valley in Pakistan hexagonal prism or the hexagonal dipyramid were discovered in the late 1970s during the (Hughes 1990, Peretti et al. 1995, Smith et al. 1997, building of the Karakorum Highway which links Smith 1998, Cesbron et al. 2002). Ruby from Pakistan with China (Piat 1974, Okrush et al. 1976, Jegdalek is generally tabular and formed by the Gübelin 1982, Kazmi & O'Donogue 1990, Garnier hexagonal prism a {11.0} and the pinacoid c {00.1} 2003). The deposit from Nangimali in Azad- and the positive rhombohedron {10.1}. Ruby from Kashmir was discovered in 1979 by the AKMDC Mogok is tabular and prismatic with a development mining society (Malik 1994), and the geology and of the rhombohedron faces (Fig. 2-34). Ruby from the geochemistry of the deposit was studied by Mong Hsu has several hexagonal dipyramids Pêcher et al. (2002). The Tajik ruby deposits remain associated with the pinacoid and the hexagonal poorly known and were first reported by Henn & prism a (Fig. 2-34). Bank (1990). Kievlenko (2003) reported geological In general the color of ruby and sapphire from studies carried out by Russian geologists and Smith marble is variable: red to violet for Tanzanian (1998) described the standard gemological (Hänni & Schmetzer 1991) and Nepali (Smith et al. properties and internal features of the Tajik ruby. 1997); red, pink, brown, yellow, blue, violet or The first report of marble-hosted ruby deposits from colorless for corundum from the Urals (Kissin Nepal was published by Bassett (1984), followed by 1994). Additionally, ruby from Mong Hsu Harding & Scarratt (1986), and the mineralogy was (Myanmar) shows unusual features such as a blue studied by Smith et al. (1997). The Mogok ruby sapphire core and multiple zoning (Fig. 2-35) from deposits in Myanmar have been exploited since the ruby to violet sapphire in the outer zones of the sixth century (Chhibber 1934, Gübelin 1965, Kane crystal (Peretti et al. 1995). The rubies from Mogok & Kammerling 1992, Kammerling et al. 1994), appear to glow red or are like 'pigeon blood' and whereas the Mong Hsu deposits were first reported they are among the most expensive in the world for in 1991 (Hlaing 1991, Peretti et al. 1995). In their top quality. northern Vietnam, a peasant discovered the first The deposits occur (Fig. 2-9) in Afghanistan, ruby at Luc Yen in 1983 and exploitation of the Tajikistan, Pakistan, Azad-Kashmir, Myanmar, occurrences began in 1988 (Kane et al. 1991,

FIG. 2-34. Crystal habits of ruby from Mong Hsu and Mogok, Myanmar (after Smith & Surdez 1994, Hughes 1997).

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FIG. 2-35. Variations of habit and color during the growth of ruby from Mong Hsu, Myanmar (Peretti et al. 1995). The successive growth zones are red (R, R'), Violet (V, V') and intermediary (I, I') colors which are associated to a coeval variation of chromium (Cr2O3) and titanium (TiO2).

Garnier 2003, Pham et al. 2004, Nguyen et al. 2011, Karakorum batholith. The famous Mogok ruby Le et al. 2012). Galibert & Hughes (1995) reported deposits occur in the Mogok Metamorphic Belt. that ruby-bearing marble was discovered in Yunnan This belt accommodated a large part of the Indo- province, China, at the beginning of the 1980s. Asian collision and is marked by high temperature Detailed characteristics of gems from the Ailoshan ductile Oligocene stretching and post-Miocene structural belt were given by Zhang et al. (2003). brittle right-lateral faults, such as the Shan scarp Marble-hosted ruby deposits at Jegdalek in fault zone and the Sagaing fault (Bertrand & Rangin Afghanistan, at Nangimali in Azad-Kashmir, at the 2003). The ruby deposits in northern Vietnam are Hunza valley in Pakistan), at Chumar and Ruyil in located along the Red River shear zone that was Nepal, at Mogok and Mong Hsu in Myanmar), and active during Cenozoic times (Schärer et al. 1990, at Luc Yen-Yen Bai, and Quy Chau in Vietnam, Tapponnier et al. 1990, Garnier et al. 2004b). The share many common structural, mineralogical and deposits from Luc Yen (An Phu, Bai Da Lan-Mong geochronological features. To begin with, they all Son, Khoan Thong, Luc Yen, Minh Tien, and Nuoc occur in metamorphic blocks that were affected by Ngap mines) occur in weakly deformed marble units major tectonic events during the Cenozoic Indo- from the Lo Gam tectonic zone, located on the Asian collision (Fig. 2-9). The Jegdalek deposit is eastern flank of the Day Nui Con Voi belt within the located in the Western Nuristan Block, in the Indus Red River shear zone (Leloup et al. 2001). suture zone. The ruby-bearing marble is underlain In general these ruby deposits are hosted by by metamorphic basement of presumed Precambrian metamorphosed platform carbonate units associated age (Rossovskiy et al. 1982) intruded by Oligocene with garnet-biotite-sillimanite- or biotite-kyanite- (34–26 Ma) plutonic units that define the southern bearing gneiss and granite (Fig. 2-36). The marble part of an axial batholith. The Nangimali deposit units consist of discontinuous horizons up to 300 m lies in the High Crystalline Himalaya, in the in thickness, now oriented parallel to the main Southern part of Nanga Parbat. It is located within a regional foliations, thrusts or shear zones related to syncline formed during an upper Cretaceous to Cenozoic Himalayan orogenesis. Dikes of granite Miocene Himalayan tectono-metamorphic event and/or pegmatite intruded these metamorphosed (Pêcher et al. 2002). The Chumar and Ruyil sedimentary rocks. Ruby mineralization is generally deposits in Nepal are located on the southeastern stratiform and distributed within marble layers. In flank of the Ganesh Himal, in the Nawakot series, Luc Yen (Vietnam), Nangimali (Azad-Kashmir), just below the Main Central Thrust. These ruby and Hunza (Pakistan), amphibolite bodies alternate deposits occur within boudinaged marble lenses, 60 with calcitic and/or dolomitic marble. to 150 m wide and up to 1 km long, oriented parallel The marble parageneses typically consist of to the Main Central Thrust. The Hunza Valley calcite, dolomite, spinel, phlogopite, margarite, deposits (Okrush et al. 1976) are located in the amphibole, chlorite, forsterite, and titanite ± Baltit formation of the Karakorum metamorphic graphite ± garnet ± pyrite. The surrounding gneiss complex, north of the Nanga Parbat Massif, between or schist exhibits assemblages with biotite-garnet- the Main Karakorum Thrust and the intrusive sillimanite ± scapolite ± kyanite ± amphibole ±

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phlogopite associated with ruby are Oligocene (24.7 ±0.3 Ma) at Jegdalek, Miocene at Mogok (18.7 ±0.2 to 17.1 ±0.2 Ma), at Hunza (10.8 ±0.3 to 5.4 ±0.3 Ma) and Chumar (5.6 ±0.4 Ma), and Pliocene (4.6 ±0.1 Ma) at Ruyil (Fig. 2-8). In Vietnam, a zircon included in a ruby from Luc Yen yielded a 238U/206Pb age of 38.1 ±0.5 Ma, indicating that ruby formed around 40 Ma when ductile deformation was active under peak metamorphic conditions in the Red River shear zone. All these ages are consistent with extensional tectonic processes that were active from Afghanistan to Vietnam, in the ruby-bearing metamorphic belts, between the Oligocene and the Pliocene. The ruby deposits are contemporaneous with the intrusions spatially associated with them. Only a few detailed geological studies have been devoted to these ruby deposits but several hypotheses on their genesis have been advanced. One hypothesis suggests that ruby formation is a consequence of amphibolite-facies regional meta- morphism in the calcareous rocks enriched in Al relative to Si, Cr and Ti (Okrush et al. 1976). Such original bulk rock chemistry may result from lateritic weathering of an impure limestone, formed in a karst environment, before metamorphism. This FIG. 2-36. The Nangimali Formation showing the location of the ruby-bearing marble in the metamorphic pile model, proposed by Okrush et al. (1976) for (after Pêcher et al. 2000). deposits in the Hunza Valley, was also used by Rossovskiy et al. (1982) and Bowersox et al. (2000) clinopyroxene. At the Jegdalek, Nangimali, and to explain the genesis of the Jegdalek, Pamirs, and Ruyil-Chumar deposits pegmatites are absent (Smith Mogok deposits. Hunstiger (1990) explained that et al. 1997, Pêcher et al. 2002) and ruby occurs in corundum formed from diaspore as a result of marble lenses parallel to the main foliation and/or intensification of pressure and metamorphism of an within thin fractures (up to 3–4 cm wide) in marble initial impure protolith following the prograde horizons (Pêcher et al. 2002). transformation from hydrargillite, to boehmite and The ruby crystals typically occur: (1) diaspore, and finally corundum. Kissin (1994) disseminated within marble and associated with proposed a variant model with metasomatic trans- phlogopite, muscovite, scapolite, margarite, spinel, formations by metamorphic fluids of terrigenous titanite, pyrite, and graphite, as at Jegdalek, Chumar layers in marble. and Ruyil, Hunza, Mogok, Mong Hsu, Myanmar, A second hypothesis suggests that ruby has an and Luc Yen; (2) in veinlets or gash veins, as in indirect relation with granite and its formation is due some occurrences in northern Vietnam, and to contact metamorphism (skarn-type) developed associated with phlogopite, margarite, titanite, around granitic intrusions, as proposed by Iyer graphite, and pyrite, and sometimes related to (1953) and Harlow et al. (2006) for some of the micro-shear zones, as at Nangimali; or (3) in Mogok deposits where ruby-painite [CaZrAl9O15 pockets associated with orthoclase, phlogopite, (BO3)] and typical skarn assemblages are found. In margarite, graphite, and pyrite in some occurrences that case, the source of heat and some of the of northern Vietnam. chemical elements necessary for ruby formation is Ruby hosted in marble was indirectly dated by the granite. The hypothesis of alkaline meta 40 39 Ar/ Ar stepwise heating performed on single somatism of marble by the circulation of magmatic grains of phlogopite syngenetic with ruby (Garnier and/or pegmatitic fluids is retained by Nguy et al. et al. 2002b, Garnier et al. 2006b), and zircon (1994) for the genesis of Luc Yen deposits. inclusions in ruby were dated by U–Pb ion probe A third model suggests that the ruby has a 40 39 techniques (Garnier et al. 2004b). Ar/ Ar ages of direct genetic relation with a magma and formed

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during the emplacement of alkaline magmatism ments for ruby crystallization because Si, K and Na intrusions in marble for the Tajikistan deposits in activities are low, and above 400°C alumina is the Central Pamirs (Terekhov et al. 1999); transported within aqueous fluids. The model magmatic solutions carrying Al interacted with requires the presence of Cr and also a fluid- marble. The deep-seated source is reinforced by saturated environment. numerical modeling of fluid–rock interaction Kissin (1994) observed the formation of ruby in between marble and an alkaline fluid, performed by marble in the Urals from the destabilization of Kolstov (2002). Stability of the corundum-bearing spinel at a temperature between 620–660°C and assemblages in the modeling was described by the pressure around 2.5 kbar, according to the reaction: equilibrium equation: spinel + calcite + CO2 ļ 2 pargasite + corundum + 9 CO2 ļ corundum + dolomite (12) 3 anorthite + 2 albite + calcite + dolomite The formation of corundum by spinel breakdown is + 2 HO (10) 2 the main reaction observed for the ruby deposits The high concentration of CO2 appeared unusual to from southeast and central Asia (Garnier 2003). In Kolstov (2002) for most of the fluids in this ruby belt, the experimental reaction proposed by amphibolite-facies metamorphism and for fluids Spirodonov (1998) does not occur and magnesite degassing from magmatic chambers. Thus a deep- has never been identified: seated source which interacted with an initial rock package made of marble and kyanite-garnet-biotite spinel + CO2 o corundum + magnesite (13) schist was preferred. The metasomatic interaction of A fifth model suggests that during the the fluid with the composite protolith resulted in the metamorphism of evaporite lenses which reacted as formation of metasomatic columns where corundum molten salts with the marble and its impurities i.e., formed in Mg or Ca-rich marble zones. To the organic matter and phengite (Garnier 2003, Garnier contrary, based on isotopic and geochemical data on et al. 2008) to produce CO2–H2S–COS–S8– the corundum-bearing metasomatic rocks in the AlO(OH)-bearing fluids subsequently trapped by Central Pamirs, Dufour et al. (2007) demonstrated Vietnamese rubies (Giuliani et al. 2003). Pêcher et by numerical simulations of the fluid–rock alteration al. (2002) have shown that the formation of that ruby was the product of regional alkaline Nangimali ruby was a good example of fluid metasomatism during the closing stages of transfer during metamorphic processes. Parental Himalayan metamorphism. The terrigenous fluids of ruby are of metamorphic origin and CO2 is material hosted by the Phanerozoic carbonate derived from the decarbonation of dolomitic marble. reservoir was desilicated by the metasomatic fluids In marble, the presence of aspidolite and anhydrite and ruby formed at T = 600–650°C and P = 4–6 (Garnier et al. 2004c), together with anhydrite and kbar. salts in ruby, implies the presence of evaporite rocks A fourth model suggests that the ruby formed for the genetic formation of ruby. Such sulfates and during high-pressure metamorphism of originally salts in ruby were also described at Mogok (Smith impure limestone in evaporitic series. Spiridonov & Dunaigre 2001, Garnier et al. 2008), Luc Yen (1998) proposed this hypothesis for the ruby (Giuliani et al. 2003), Quy Chau, Afghanistan, deposits of Turakuloma, Pamirs, and the Uralian Nepal, Tadjikistan, and Campo Lungo in folded areas. Ruby occurs within a sequence of Switzerland (Garnier et al. 2008). Raman and dolomite, calcite and magnesite marble containing infrared spectroscopy, combined with microthermo- intercalated schist. Ruby is associated with metric investigations, on primary and secondary carbonate minerals, scapolite, and fuchsite. The fluid inclusions in gem ruby from all the ruby model is based on experiments and petrological deposits from central and southeast Asia provided observations which show that at a high CO2 fugacity evidence for CO2–H2S–COS–S8–AlO(OH)-bearing spinel breaks down into corundum according to the fluids with native sulfur and diaspore daughter reaction: minerals, without visible water (Giuliani et al. 2012b). Crush-leach analyses identified sulfates and MgAl O + CO o Al O + MgCO (11) 2 4 2 2 3 3 chlorides that are attributed to the presence of spinel + CO o corundum + magnesite 2 anhydrite and Na–Ca–Cl salts in the ruby crystal Spiridonov (1998) thought that both meta-evaporite (Giuliani et al. 2012b). The thermal reduction of and meta-ultrabasite rocks are favorable environ- evaporitic sulfates explains the fluid chemistry.

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As we have seen, marble-hosted ruby deposits evaporites. In the Mogok rubies, multi-solid fluid from central and southeast Asia share common inclusions (Fig. 2-37E) are coexisting with the features: (1) they are hosted by metamorphosed primary CO2–H2S–COS–S8–AlO(OH)-bearing marine carbonate assemblages; (2) salts and sulfate fluids (Fig. 2-37D). They are composed of molten minerals are found as solid inclusions in the ruby salts and solid inclusions which have been studied samples; (3) minerals associated with ruby are by micro-Raman spectrometry. The main molten enriched in Mg, Al, F and in some cases Cl; (4) they salt and sulfates are dawsonite (NaAl(CO3)(OH)2, have trapped CO2–H2S-bearing fluids with native barite, unnamed sulfates, Al-nitrates, Na–Ca–CO3 sulfur and diaspore daughter minerals; and (5) they mineral like shortite Na2Ca2(CO3)3, and complex formed during or directly after upper amphibolite- to carbonate minerals (REE-carbonates?). Halite cubes granulite-facies metamorphism. are sometimes present. The other minerals are (C–O–H) isotopic studies have shown that fluorite, calcite, dolomite, apatite, boehmite, marble units were not infiltrated by externally magnetite, graphite, and rutile. The presence of derived fluids (Garnier et al. 2008). CO2-rich fluids molten salts corroborates the model of formation for are of metamorphic origin and derived from ruby based on the thermal reduction of sulfates of devolatilization of marble. The second genetic evaporitic origin (Giuliani et al. 2012b). These model, based on a possible circulation of magmatic different features support models four and five and/or pegmatitic fluids, and the third model, based proposed for the formation of marble-hosted ruby on a direct genetic relation of ruby with magma or a deposits. deep seated source as proposed for the Tajikistan Elemental sulfur, hydrogen sulfide and COS in deposits (Terekhov et al. 1999), are not tenable. the CO2-inclusions originated from the partial Nevertheless, ruby is sometimes associated with dissolution and reduction of anhydrite by organic skarn at the contact granite–marble in some mines at carbon (Giuliani et al. 2003). Such chemical the Mogok district (Nissimboim & Harlow 2011). association, and especially the presence of COS in The presence of a high CO2 fugacity for ruby geological fluids, was previously described by formation is in agreement with the five proposed Grishina et al. (1992) for the Siberian platform, models for ruby formation and experimental results where dolerite intruded carbonate–anhydrite and – (Kolstov 2002). Such thermodynamic conditions are halite beds. Furthermore, nitrates as NO3 were favorable for spinel breakdown according to the identified in the crush leach and Giuliani et al. main reaction: (2012b) proposed a non-marine evaporite origin i.e., evaporitic lakes. spinel + calcite + CO2 o The high temperature conditions for ruby corundum + dolomite (14) formation imply a heat source, i.e., metamorphic or This reaction occurring during the retrograde magmatic. Contact metamorphism or regional metamorphic path has been commonly observed in metamorphism are both suitable. Control of the marble from Jegdalek, Afghanistan, Hunza and Luc mineralization is lithologic, with ruby formed in Yen in Vietnam (Fig. 2-37A; Garnier et al. 2008), marble intercalated with meta-evaporite lenses and is also described from the Urals (Kissin 1994). (Pêcher et al. 2002, Garnier 2003). As no externally In dolomitic marble, the presence of aspidolite and derived fluids are interpreted to have circulated anhydrite together with hypersaline fluids (salt and through the marble, the source of Al and Cr must sulfate) in rubies implies the necessary presence of have been deposited coevally with the carbonate evaporites for the generation of ruby (Fig. 2-37B, rocks on the platform. The Al was probably C). contained in clay minerals, which are produced in The petrographical and geochemical evidence is great quantities by weathering of continental rocks reinforced by S and B isotopic studies of anhydrite and transported by rivers to the sedimentary basins. and tourmaline respectively that have pointed to the A contribution from organic matter for Cr and Al is participation of non-marine and marine evaporites in a possibility, as is apparent in bitumen associated the formation of ruby (Garnier et al. 2008). At with Colombian in a black shale Hunza the presence of inclusions of anhydrite and environment with Cr 3–87 ppm, Al 700–2200 ppm spinel in ruby (Fig. 2-37B), inclusions of spinel in (Giuliani et al. 2000), but also from detrital minerals anhydrite, and restite of anhydrite in marble, enriched in Cr, and possibly Ti. Thus, ruby is suggests that the limestone was deposited in an present only in marble units that preferentially environment propitious for the deposition of contained these Cr–Al–bearing minerals, as

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FIG. 2-37. Petrography and fluid inclusions in ruby from marble (Garnier et al. 2008, Giuliani et al. 2012b). A, Ruby (Crd) in equilibrium with dolomite (Dol) forms from the reaction of spinel (Sp) with calcite (Cc). Spinel is a pre-ruby phase and it contains high Cr (up to 19 wt.%) and Zn contents (10 wt.%). Spinel contains also anhydrite (Anh). Hunza valley, Pakistan. B, Detail of anhydrite (Anh) and spinel (Sp) inclusions in this ruby. SEM photographs A and B are back-scattered electron (BSE) mode. C, Inclusions of anhydrite (Anh) and phlogopite (Phg) in ruby (Ru) from Nangimali in Azad Kashmir. SEM photograph. D, Ruby from Mong Hsu (Myanmar). Two pseudo-secondary biphase fluid inclusions containing a carbonic liquid (l) and vapor phases (v). The biggest FI contains diaspore crystals (Di) and one globule of native sulfur (S8). E, Polyphase fluid inclusions with molten salts in a Mogok ruby with respectively, calcite (Ca), graphite (C), dawsonite (Dw), apatite (Ap), halite (H), shortite (Sh), fluorite (F), corundum (Crd), and a liquid CO2–H2S–S8 fluid (l).

FIG. 2-38. Synthesis presenting the proposed model for genesis of ruby deposits in marble (Garnier 2003). The different stages of formation of these deposits are illustrated from sedimentation, i.e., Precambrian to Permo-Triassic, to the Himalayan metamorphism. Stage I: sedimentation of the protoliths in a continental platform with a detrital supply, and deposition of carbonate, dolomitic carbonate, organic matter-bearing shale, and marine and non-marine evaporite sediments i.e., lagoon, sabkha paleogeography as found in the representative lithostratigraphic section of Nangimali in Azad Kashmir. Stage 2: Cenozoic Himalayan metamorphism a consequence of the Indo-Asian collision with the metamorphism of the ruby protoliths as shown on the metamorphic unit cross-section of Nangimali. Corundum formed during the prograde metamorphic P–T path and then during the retrograde stage from the destabilization of different mineral assemblages as drawn in the P–T sketch: 1, from margarite, then 2, from micas, and 3, from spinel. At this stage, corundum is destabilized at lower temperature and pressure in margarite, and then diaspore. Gem ruby formed at a pressure between 2.6 and 3.3 kbar and temperature between 620 and 670°C (black rectangles). Abbreviations: An: anorthite; And: andalusite; Cc: calcite; Co: corundum; Do: dolomite; Dsp: diaspore; Kfs: K-feldspar; Ky: kyanite; Mrg: margarite; Ms: muscovite; Si: sillimanite; Sp: spinel; W: water.

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proposed by Okrush et al. (1976) and Hunstiger leading finally to the formation of ruby; (1990). The Al2O3 content of the marble is not a (2) thermochemical reduction of sulfates produced critical factor for ruby formation (Garnier et al. S8, COS and H2S. H2S combined with Fe to produce 2004c). At Nangimali the marble contains 0.1 wt.% pyrite; (3) the presence of lenses of evaporites in Al2O3, and in the marble from the Urals deposits, the some specific horizons explains why ruby is Al2O3 content is between 0.08 and 0.13 wt.% sporadic in a single homogeneous marble level, (Kissin 1994). even though Al is ubiquitous; and (4) this genetic The following model (Fig. 2-38) has been model for natural ruby in marble resembles the proposed for formation of the Indo-Asian ruby industrial process that uses molten salts (NaF–AlF– deposits (Garnier et al. 2008): (Stage I) deposition Al2O3) to produce Al (Lacassagne et al. 2002). Al of sedimentary protoliths on the Paleotethys fluoride and oxy-fluorine Al are major chemical platform with local formation of restricted species involved in dissolution of Al in a F-rich endoheric basins. The ages of sedimentation vary fluid. from Precambrian at Jegdalek (Terekhov et al. Guidelines for prospecting for new deposits 1999) to Permo-Triassic at Nangimali (Malik must take into account (1) the lithological control of 1994). Restriction of sedimentary basins may be due mineralization: the protolith deposited in a to tectonism, volcanism (e.g., intrusion of carbonate platform, in a lagoonal to lacustrine basaltic dikes at Nangimali) or sedimentation environment, isolated from open sea, and promoting dynamics with coral reefs, oolitic units. (Stage II) the deposition of marine and non-marine evaporites; during the Indo-Asian collision, the sediments and (2) the presence of index minerals such as Na- were metamorphosed in upper amphibolite to rich phlogopite (aspidolite) or pargasite and edenite, granulite facies, transforming carbonate into Mg-tourmaline, Cr-rich titanite (more abundant than marble. ruby itself), and spinel and anhydrite in marble (or Mg, Al, and Na enrichment of the minerals in alluvium), are all indicators of possible marble- associated with ruby in marble, the presence of hosted ruby mineralization in the area. tourmaline and anhydrite and salts, and the The second type of marble deposit concerns replacement of some of the hydroxyl groups by F ruby and pink sapphire in marble with intercalated and Cl are evidence for the participation of silicate and gneiss layers (Plate 2A, B). It is the case evaporites in the formation of ruby. Molten salts at the Revelstoke occurrence in British Columbia, (NaCl, KCl, CaSO4 and Na2CO3) enriched in F, Cl Canada, discovered in 2005 (Dzikowski et al. 2010, and B, mobilized (over short distances) Al and 2013). The occurrence is in the Monashee Complex transition metal elements contained in the marble. of the Omineca belt of the Canadian Cordillera. The Organic matter played a key role in the mineralizing corundum occurs in thin, folded and stretched layers process. Thermal reduction of evaporitic sulfate, with a predominant assemblage of green muscovite based on an initial assemblage of anhydrite, calcite + Ba-bearing K-feldspar + anorthite (An0.85–1) ± and graphite, is a convenient explanation for the phlogopite ± Na-poor scapolite. Other silicate layers fluid chemistry found in fluid inclusions but also for within the marble are: (1) diopside + tremolite ± the main paragenesis associated with ruby, i.e., quartz and (2) garnet (Alm0.7–0.5Grs0.2–0.4) + Na-rich carbonate and pyrite, with a reduction of sulfate and scapolite + diopside + tremolite + (Na,K)-amphibole oxidation of organic matter. The deposition of pyrite minerals. Non-silicate layers in marble are either occurs as follows: magnetite- or graphite-bearing. The pink sapphire

2+ 2– + and ruby crystals have elevated Cr2O3 (” 0.21 wt.%) 7H2S + 4Fe + SO4 o 4FeS2 + 4H2O + 6H (15) and low amounts of TiO2; rare blue rims contain Gem ruby formed during the retrograde higher amounts TiO2 (” 0.53 wt.%) and Fe2O3 (” metamorphism at temperatures of ca. 620–690 ºC 0.07 wt.%). The associated micas have elevated Cr, and pressures of approximatively 2.2–3.3 kbar V, Ti, and Ba. Petrography of the silicate layers (Garnier et al. 2008). Evaporites appear to be a key show that corundum formed from muscovite at the factor in the formation of ruby deposits: (1) Cl and F peak of metamorphism (T~ 650–700°C at P ~ 8.5–9 acted as activators as proposed by Peretti et al. kbar). The scapolite-bearing assemblages formed (1996) to mobilize Al in marble, which normally is during or after decompression of the rock at T~ 650 poorly mobile. F and Cl facilitate the mobilization °C and P~ 4–6 kbar. of Al from mica, formed by the metamorphism of Whole-rock geochemical data show that the clay minerals, organic matter, and spinel minerals, corundum-bearing silicate (mica–feldspar) layers

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formed by mechanical mixing of carbonate with the (Simonet 2000). Grew et al. (1989) also described host gneiss protolith; the bulk composition of the biotite-plagioclase-spinel-corundum in the gneiss of silicate layers was modified by Si and Fe the Aldan craton in eastern Siberia. depletion during prograde metamorphism. High In Sri Lanka, the rich alluvial gemstone element mobility is supported by homogenization of deposits are located in the Precambrian Highland- į18O and į13C values in carbonates and silicates for Southwestern Complex formed of granulitic rocks the marble and silicate layers. The silicate layers (Fig. 2-28). Gemstones are rarely discovered in situ and the gneiss contain elevated contents of Cr and V and the prospecting methods for primary deposits due to a volcanoclastic component of their are based on geology, geochemistry, and mineralogy protolith. (Wells 1956, Cooray & Kumarapeli 1960, Katz Such a deposit has been also described in 1972, Dahanayake 1980, Dissanayake et al. 2000, Morogoro, Tanzania (Le Goff et al. 2008). At the Walton 2004). The mother rocks of corundum may mine Ngong'Oro, the ruby-bearing horizon is include charnockite, gneiss with quartz, feldspar, formed by several centimetre-wide metasomatic garnet, sillimanite, graphite, and marble and calcitic zones parallel to the metamorphic foliation, and gneiss (Cooray & Kumarapeli 1960, Munasinghe & formed at the contact between marble and Dissanayake 1981; Dissanayake & Chandrajith intercalated biotite gneiss layers such as at 1999). The P–T conditions of formation of gem Revelstoke. Ruby is associated with spinel and corundum in the amphibolite facies was sapphirine. Hänni & Schmetzer (1991) described approached by De Maesschalk & Oen (1989) ruby from Morogoro. The crystals are composed of studying fluid inclusions trapped by corundum from the faces of the pinacoid c and the rhombohedron r, gem gravels. but in some crystals the faces a {11.0} are present Several studies have explored the modes of and thus the crystals look like octahedral spinel occurrence of gemstones and guides for exploration. (Fig. 2-39). Dahanayake (1980) studied gem-bearing gravels and suggested that gem corundum (Plate 1F) and Metamorphic deposits in granulite, charnockite spinel derived from both garnet-bearing gneiss and and gneiss: granulite is a with a skarn deposits. Munasinghe & Dissanayake (1981) high-pressure and high-temperature origin. It is fine- suggested that corundum formed at the contact grained with dominant lenticular quartz and between basic charnockitic intrusions and feldspar, and hypersthene and garnet, with aluminous metasedimentary rocks. Rupasinghe et accessory minerals such as sillimanite, kyanite, al. (1984) and Dissanayake & Weerasooriya (1986) rutile, cordierite, and spinel. Granulite is found in used Be and F geochemistry to locate the bedrock Precambrian terranes with other rocks such as sources. Rupasinghe & Dissanayake (1985) leptynite, kinzigite, charnockite, and gneiss. proposed that the intrusion of charnockite into Several gem corundum deposits are hosted by aluminous metasedimentary rocks was responsible biotite-sillimanite-K-feldspar gneiss in the granulite for desilication in the pelitic sedimentary rocks and facies, for example, those from Azov in Russia, the formation of corundum following the reaction: Froland in Norway, Mysore in India, Hida in Japan, spinel + sillimanite ļ and southern Madagascar (Schwarz 1998). Other cordierite + corundum (16) deposits are known from Indaia in Brazil (Epstein et al. 1994) and in the metamorphic Mozambique belt Mendis et al. (1993) applied structural geology in the exploration for residual gem deposits and found that they are apparently associated with structurally deformed areas where marble and granite occur (skarn-type deposit). Gamage et al. (1992) showed that high Rb/Sr ratios in stream sediments correspond to high gem concentrations. They suggested that depletion of Sr is a function of fractionation of the parental granite generated FIG. 2-39. Crystal habits of ruby from Morogoro, during granulite metamorphism. Tanzania (after Hänni & Schmetzer 1991). a, Ruby and sapphire associated with granulite- hexagonal prism of second order {11.0}; r, facies metamorphism from Sri Lanka show rhombohedron {10.l}; c, pinacoid {00.1}. numerous mineralogical and geological similarities

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with those from Eastern Africa (Mercier et al. and Ambatomena), amphibolite, and anorthosite 1999b, Simonet 2000), Madagascar (Lacroix 1922, (Ejeda, Fotadrevo, Vohitany, and Gogogogo) to Rakotondrazafy et al. 1996, 2008), and southern impure marble (skarns of Andranondambo, India (Santosh & Collins 2003, Giuliani et al. Tranomaro). The 'sakenite' described by Lacroix 2007a, 2007b). Based on the correlation of gem and (1941) is found in a metamorphic series made up of graphite fields, Menon & Santosh (1995), and paragneiss with intercalations of amphibolite, Dissanayake & Chandrajith (1999) proposed the clinopyroxenite, and impure marble (the Sakeny, existence of a Neoproterozoic–early Cambrian Vohidava, Ejeda-Anavoha, and Andranondambo gemstone province developed in East Gondwana occurrences) and consists of anorthite veins or under granulitic conditions (Fig. 2-8). Indeed, the segregations ± spinel ± corundum ± sapphirine ± gemstone areas are located in the Precambrian phlogopite and (± hibonite). Their original protolith basement, which includes remnants of early crust is either metapelite (Lacroix 1941, Raith et al. 2008) (de Wit 2003) which were intensely reworked or meta-anorthosite (Boulanger 1959, Ohnenstetter between 950 and 450 Ma, during Pan-African et al. 2008). tectonometamorphic events (Kröner 1984). The The diversity of bedrock sources of gem collision processes between East and West corundum explains the incredibly rich variety of Gondwana resulted in the formation of sapphires found in the Ilakaka placer, located on the Neoproterozoic mobile belts (~650 Ma), mostly western border of the granulitic domain (Garnier et metamorphosed to high-grade granulite facies. They al. 2004a, Giuliani et al. 2007a, Rakotondrazafy et include similar metamorphic rocks varying from al. 2008). The sources of ruby and colored sapphire amphibolite, feldspar gneiss, charnockite, granulite, are multiple, and the expression 'gem corundum in cordieritite, anorthosite, and mafic and ultramafic granulite' includes a variety of rocks that might rocks (Mercier et al. 1999a, 1999b; Feneyrol 2012; contain sapphire and/or ruby. The Malagasy Feneyrol et al. 2013). example illustrates the complexity in the search for Southern Madagascar is a key area for the primary deposits in Sri Lanka. characterization of primary corundum deposits formed under granulite-facies conditions Metamorphic deposits in migmatite: migmatite is a (Rakotondrazafy et al. 2008). The metamorphic mixture of rocks of granitic and gneiss types. The rocks are high- to medium-pressure granulite and gneiss is coarse-grained with a diffuse foliation. are well exposed throughout southeastern These rocks formed at the boundary of high-grade Madagascar (Fig. 2-32). The granulitic terranes are metamorphic and magmatic rocks. They formed by divided into four major lithostratigraphic groups partial melting during ultrametamorphism of an (Besairie 1967, de Wit 2003) corresponding to the initial rock called the paleosome; one part of the juxtaposition of tectonic blocks of different crustal rock melts and one part remains solid and is called a levels (Martelat et al. 2000). This patchwork is due restite. The whole range of migmatitic rocks is to the relative movements of major ductile shear grouped under the term anatexite. zones, reflecting a crustal scale strike-slip system. The corundum-bearing anatexite at the vicinity Rocks in all blocks underwent metamorphism of Morogoro in the Precambrian terranes of around 750°C. The pressure shows an E–W Tanzania is an anatectic gneiss (Altherr et al. 1982). decrease from 11 to 8 kbar in the west to 5 to 3 kbar It is composed of different assemblages: (1) a in the east (Nicollet 1990). Granitoid rocks are medium-grained gneiss with albite + muscovite + abundant in the eastern part of the country, whereas phlogopite + corundum and albite + kyanite or anorthosite and metabasite are abundant in the west. sillimanite + phlogopite, and accessory minerals as Corundum deposits are found in all the different rutile and baddeleyite; (2) a coarse-grained gneiss tectonic blocks, but are strongly associated with the with nests of corundum and antiperthite, with minor presence (Fig. 2-32) of major or minor shear zones phases of albite, muscovite, phlogopite, rutile, (Rakotondrazafy et al. 2008). These structures acted baddeleyite, and tourmaline. Assemblages with as preferential fluid pathways, and the corundum- albite + muscovite + phlogopite + corundum are bearing rocks have suffered intense fluid–rock restite, while those with kyanite or sillimanite are interaction resulting in huge amounts of recrystallized paleosome. The anatexite formed at metasomatic alteration. The nature of the parental PH2O = 7.7 kbar and T = 695°C. host rock varies from feldspathic gneiss (Zazafotsy Cartwright & Barnicoat (1986) described pale and Sahambano deposits), cordieritite (Iankaroka blue sapphire in kyanite-muscovite restite in

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anatexite from Scotland. The P–T conditions at the al. 1971), at Sittampundi in India (Janardharan & peak of metamorphism were estimated at 11 kbar Leake 1974), at Chantel in France (Forestier & and 900–925°C. Lasnier 1969), at Losongonoi in Tanzania (Dirlam et al. 1992), at Kitui in Kenya (Barot & Harding Metamorphic deposits in amphibolite and gabbro: 1994), at Chimwadzulu in Mozambique (Andreoli corundum in amphibolite originating from the 1984, Henn et al. 1990), in the Vohibory formations metamorphism of gabbroic and dunitic rocks is an in Madagascar (Nicollet 1986, Mercier et al. important source of ruby. Generally ruby is not of 1999b), in the Harts Range in Australia (McColl & gem quality but the crystals are used for cabochons Warren 1980), at Dir in Pakistan (Aboosally 1999), and ornamental uses, for example the famous and Hokkaido in Japan (Morishita & Kodera 1998). 'anyolite' from Longido in Tanzania (Plate 1B, These deposits display several common Dirlam et al. 1992, Le Goff et al. 2008, Feneyrol et features. The corundum-bearing amphibolite is al. 2013) or the 'smaragdite ruby' from North associated with mafic and ultramafic complexes Carolina in the USA (Hughes 1997). Three recent metamorphosed to granulite facies. The common ruby deposits discovered in the last decade are assemblage is composed of corundum, anorthite, highly economic for their quality and quantity such , and margarite (Tenthorey et al. 1996, as at Winza in Tanzania (Peretti et al. 2008, Nicollet 1986). Others minerals such as sapphirine Schwarz et al. 2008), at Aappaluttoq in Greenland (Forestier & Lasnier 1969, Herd et al. 1969, (Rohtert & Ritchie 2006, Fagan et al. 2011, Fagan Nicollet 1986, Tenthorey et al. 1996), garnet 2012), and at Montepuez, Ruambeze and M'sawize (Nicollet 1986), spinel (Forestier & Lasnier 1969, in Mozambique (Pardieu et al. 2009a, 2009b, 2009c, Morishita & Kodera 1998), kornerupine (Rohtert & Pardieu & Chauviré 2013). Ritchie 2006), phlogopite and zoisite (Dirlam et al. Such corundum deposits in amphibolite and 1992) may be present (Fig. 2-40A, B). The initial gabbro are known at Kittila in Finland (Haapala et basic composition of the protolith includes gabbro

FIG. 2-40. Mineralogical assemblages in corundum-bearing amphibolite and gabbro (after Lasnier 1977, in Giuliani et al. 2010). A, Ruby-bearing amphibolite from Chantel (Haut Allier, France). Successive transformation of spinel in sapphirine, and sapphirine in corundum. B, The ruby-bearing coronitic gabbro from Champtoceaux is amphibolitized. Hornblende (Hb) formed a corona around pargasite (Pr) and symplectites (S) with neoformed Na-rich plagioclase (Pl2) onto plagioclase of the first generation (Pl1). During the amphibolitization ruby is transformed in zoisite (Z). C, Ruby-bearing coronitic gabbro from Champtoceaux (Bretagne, France). The transformation of spinel (sp) in corundum. Spinel formed granules and 'vermicules' onto the ortho (opx)- and clino (cpx) pyroxenes which are transformed in pargasite (Pr). Ruby grows on spinel. Drawings from P. Brun (le Règne Minéral).

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(Fig. 2-40C, Morishita & Kodera 1998), but also resulted in an overlying soil horizon (eluvial troctolite (Nicollet 1986, 1990; Tenthorey et al. deposit) that contained gem corundum. 1996). The conditions of metamorphism are The ruby and sapphire crystals (Fig. 2-41A, B) granulite facies; for example P = 9–11.5 kbar and T are closely associated with cross-cutting dikes of a = 750–800°C for the ruby-bearing amphibolite from garnet-bearing pargasitite rock. Macroscopic Vohibory in Madagascar (Nicollet 1986) and P = 7 observations clearly showed that, according to its to 10 kbar and T = 800–850°C for those from Buck mineralogy, the dike-like body was mafic in Creek in North Carolina (Tenthorey et al. 1996). composition before it was metamorphosed. Possible At the Winza deposit in central Tanzania lithologies for the protolith include high-alumina (Peretti et al. 2008, Schwarz et al. 2008), gem layered gabbro or leucogabbro. The central part of corundum crystals are hosted by 1800–2000 Ma the dike is composed of garnet (pyrope–almandine) basement rocks of the Usagaran Belt (Sommer et al. + pargasite ± plagioclase ± corundum ± spinel ± 2005). The main rock types are migmatitic and well- apatite. Metamorphic conditions estimated by using foliated gneiss, indicative of upper amphibolite- to garnet–amphibole–corundum equilibria showed that granulite-facies conditions. Mining of the primary the metamorphic overprint occurred at 800 ±50°C deposits has revealed corundum crystals embedded and 8–10 kbar (Schwarz et al. 2008). Spinel that within dark-colored amphibolite. The corundum is overgrew or is included in the corundum is Fe- and locally associated with areas of brown garnet ± Mg-rich, whereas spinel included in the host feldspar, with accessory Cr-spinel, mica, kyanite, amphibole is Cr-rich with up to 32 wt.% Cr2O3. The and allanite. Weathering of the primary deposits presence of two chemically contrasting spinel

FIG. 2-41. The Winza corundum deposit in Tanzania (modified after Schwarz et al. 2008). A, The primary mineralization is contained in amphibolite dykes. The hand specimen consists of fine-grained amphibolite (Am) margins flanking orangey brown garnetite (Gt) and corundum (Crd). B, Detail of the dike showing the association of sapphire (Sa) and ruby (Ru) in a same crystal. C, Fe2O3 versus Cr2O3 diagrams showing (on the left) the Winza ruby composition compared with those from Mogok and Mong Hsu in Myanmar (marble type) and from Thailand and Cambodia (basaltic type). On the right, chemical comparison with some East African ruby deposits such as Chimwadzulu (Malawi), Mangare (Kenya), and Songea (Tanzania).

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compositions indicates that two different it lies between layers of ultramafic rock and generations are present. The Cr-spinel is probably leucocratic gabbro (Reggin & Chow 2011, Fagan relict from the initial magmatic rock before the 2012). In the early 1970s the Danish geological metamorphism. The Fe- and Mg-rich spinel survey located Aappaluttoq and several other ruby included in the corundum crystallized during the showings while conducting regional scale metamorphic stage. geological mapping in the Fiskenæsset Complex. Two types of ruby and sapphire crystals are The Aappaluttoq deposit was subsequently ‘lost’ in distinguished by their crystal morphology (Schwarz the archives until its rediscovery in 2005 by an et al. 2008): (type 1) prismatic-tabular-rhombo- exploration team from True North Gems Inc., the hedral, and (type 2) dipyramidal. In general, Canadian junior mining company who hold the medium-red and dark (orangey) red top-quality ruby current exploration rights to the deposits. To date are type 1 (rhombohedral). Pinkish and purplish red the company has discovered a total of more 50 ruby, as well as pinkish, purplish, and bluish individual corundum occurrences in the gemstone (commonly strongly color-zoned) sapphire crystals district, three of these show significant potential for are type 2 with tabular (very rare) and prismatic development as future gem corundum mines. At this forms, or dipyramidal. The inclusions in both types time, the company is focussing on pre-mining are pargasite, spinel, garnet, and rare apatite, development at the Aappaluttoq Deposit, but are xenotime, and zirconolite (Peretti et al. 2008). expecting to drill and define the resources at the The chemical compositions of Winza ruby and Siggartartulik and Kigutilik occurrences in the sapphire (Fig. 2-41C) indicate moderate contents of upcoming years. Cr (0.1–0.8 wt.% Cr2O3) and Fe (0.2–0.8 wt.% The geology of the Aappaluttoq deposit is Fe2O3), very low to low amounts of Ti (55–192 ppm dominated by a sequence of intrusive gabbro to TiO2) and V (up to 164 ppm V2O3), and low to leucogabbro with significant volumes of ultramafic moderate Ga (64–146 ppm Ga2O3). The relatively rocks. This sequence is intruded into and is high Fe content of Winza ruby separates it from structurally juxtaposed against the felsic gneiss most other natural counterparts. basement suite (Groves & Banfield 2009). The The oxygen isotope values of Winza corundum currently defined dimensions for the deposit are defined a consistent and restricted į18O range of 4.7 ~160 m long, ~100 m wide and 65 m deep; however ±0.15‰ (n = 3) which fits with the worldwide range corundum has been recovered in drill core below of ruby hosted in mafic and ultramafic rocks (0.25 200 m depth, and the geological structures remain to 6.8‰, n = 21). The Winza ruby crystals are of open along strike. The main corundum bearing ore metamorphic origin, with a high-alumina meta- is composed of three main rock types (Plate 2C): (leuco)gabbro protolith for the host rock. sapphirine-gedrite, leucogabbro and a phlogopitite; The gem corundum deposits of southwest the latter being the most important of these. The Greenland are another example of the ruby-hosted sapphirine-gedrite rocks at Aappaluttoq lie 30 km type in gabbroic rocks submitted to metamorphic from the world type locality for the mineral metasomatism (Fagan 2012). The deposits are sapphirine ([Mg,Al]8[Al,Si]6O20) located within the located within the Fiskenæsset Complex, a layered village of Qeqertarsuatsiat (formerly known as the cumulate igneous complex that occurs in the lower village Fiskenaesset). The Fiskenæsset igneous zone of the Bjørnesund structural block (Windley & Complex contains multiple areas rich in sapphirine- Smith 1971). It appears today as a sheet-like body, gedrite; many of these were described in detail by apparently concordant with the adjacent orthogneiss Herd et al. (1969). The presence of the sapphirine- and amphibolite. The complex is mid-Archean (2.86 gedrite rock at surface has been used by True North Ga), and individual layers in the Complex range in Gems Inc. as an exploration tool, as it acts as a key width from 2 km to less than a metre. The complex indicator for potential corundum mineralization. and the gemstone district are very large, extending This rock only forms along the boundary between over an area in excess of 400 km2 (Weston 2009). the ultramafic units and the Al-rich rocks of the The intrusive suite comprises layers of gabbro, gabbroic suite in fluid-metasomatized areas. It is ultramafic rocks, leucogabbro, and calcic believed that this unit represents a SiO2 sink anorthosite. The corundum mineralization is between the ultramafic units and the gabbro, which connected with a specific stratigraphic horizon in depleted the availability of SiO2 in the metasomatic the complex; this probably represents a unit with fluid, and relatively increased the amount of Al high Al content, low silica and a high fluid flux and present.

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The leucogabbro contains a small amount of 2007b, Boyd et al. 2010): (1) Ponta Messula gem corundum as ruby, but a large amount of pink complex, (2) Unango and Maruppa complexes, (3) sapphire. The gabbro is rich in Al, and is believed to Nampula complex, and (4) the east–west Pan- be the unit responsible for releasing the Al to form African overthrusts (e.g., Nairoto, Meluco, the corundum. Its structural location, distal from the Muaquia, M'sawize, Xixano, Lalamo, and ultramafic, is thought to be responsible for the lack Montepuez) which form the Cabo Delgado complex of ruby, as the Cr geochemical gradient was not overlying the Marrupa complex. Recent discovery sufficient to enrich the corundum within the of ruby in Mozambique within the Cabo Delgado gabbroic units sufficiently to create more than a complex concerns the deposits of Ruambeze and pink coloration. The phlogopitite is the host for the M'sawize in the Niassa province, and the deposits of majority of the ruby held within Aappaluttoq, and Namahumbire and Namahaca in the Delgado has little pink sapphire within it. This is due to the province (Fig. 2-40, Pardieu et al. 2009a, 2009b). proximity of this unit to the ultramafic Cr source- The Ruambeze deposit (or Luambeze) is rock and also reflects the relative mobility of Cr in a located between Marrupa and Meculo along the fluid-rich environment. This unit is a metasomatic Luambeze River (no precise location of the deposit). product, and comprises around 90% phlogopite, 5% It was discovered apparently in 1992 (Pardieu et al. biotite, and 5% corundum (Plate 2C). 2009a) and it produced dark red to brown cabochon The gemstones within the Greenland ruby grade material. The deposit is probably located in deposits are highly abundant and of a good quality. the Marrupa complex formed of felsic orthogneiss In the primary deposits, there is a large variation in associated with metasedimentary and migmatitic the quality of corundum material. It is estimated that paragneiss. Two episodes of metamorphism affected over 85% of the corundum is non-gem (opaque) the series in the amphibolite facies, and dated corundum, with around 5% classed as gem grade respectively at 953 Ma and 555 Ma (Norconsult, and 10% of cabochon quality (Mattinson 2007). The 2007a, 2007b). color of the stones range from a deep intense red, The M'sawize deposit (called also Niassa, through to a very light pink, and occasionally white. 12°40'53"S 36°49'20"E) is located in the Niassa At Aappaluttoq the majority of the stones are pink National Reserve, about 43 km southeast of to dark red (Plate 2D); they have 0.26–1.14 wt.% M'sawize village (Pardieu et al. 2009a). The Cr2O3 and 0.21–0.45 wt.% Fe2O3, and occasional M'sawize complex is formed of orthogneiss (granitic elevated TiO2 (0.34 wt.%). The stones can grow to a to amphibolitic in composition), and paragneiss. The substantial size (the largest recovered to date is a M'sawize ruby deposit is contained in metagabbro 440 ct opaque stone that was carved into the 303 ct and gabbroic gneiss (Pardieu et al. 2009b). Ruby in Kitaa Ruby). situ is associated with actinolite, anorthite, scapolite, The Aappaluttoq deposit is the world’s first epidote, and diopside, as well as in eluvial ruby-rich gem deposit to have its value calculated and defined soil located above the primary deposit (Pardieu et according to Canadian Securities Regulations (NI al. 2009b). Preliminary examination of ruby 43-101). It alone represents a total of approximately revealed the presence of many twinning planes with 405 million ct to a depth of 65 m, giving it a boehmite and silks formed by rutile-like needles. minimum mine life of 10 years. Since the Chemical analyses of ruby indicate Fe and Cr mineralization continues outside of this it would be contents between 0.28–0.33 and 0.12–0.48 wt.%, reasonable to conclude that this resource outlines respectively, which fall in the range of the Winza only a portion of the potential at this site. Together, ruby from Tanzania (Pardieu et al. 2009c). In 2009 Appaluttoq with other deposits such as around 10% of the gem material did not require any Siggartartulik and Kigutilik, represent the three type of heat treatment while 85% was treated using most promising gem concentrations in southwest lead glass and 5% by borax flux heat treatments Greenland; they also represent significant gem (Pardieu et al. 2009a). deposits that will be capable of producing sizable The Namahaca (13°03'32"S, 39°11'08E) and volumes of good ruby and sapphire gemstones for Namahumbire (13°05'18"S, 39°20'35E) deposits years to come. (also called Montepuez), discovered in May 2009, The Neoproterozoic metamorphic Mozambique are located at about 30 km from Montepuez in the Belt of Mozambique is subdivided in several direction of Pemba. The deposits are hosted in the complexes grouped in four geological entities which Pan-African Montepuez overthrust. The Montepuez are from West to East (Fig. 2-42, Norconsult 2007a, complex is made of Neoproterozoic orthogneiss

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FIG. 2-42. Geological map of northeast Mozambique (after Boyd et al. 2010) with the location of the M'sawize (Niassa), and Namahumbire and Nahamaca (Montepuez) ruby deposits (modified after Pardieu et al. 2009a, 2009b). The deposit of Ruambeze is not reported because the GPS location is not precisely described. The deposit may be located in the Marrupa complex or in the Xixano complex. Ruby deposits: 1, M'sawize; 2, Montepuez.

(granitic to amphibolitic in composition), and fractions have been noted in this unit. The layer paragneiss mainly formed by quartzite, meta-arkose, appears to be grain supported, with minor muddy marble, quartz-feldspar gneiss, and biotite gneiss matrix material and quartz grains in the 5–10 mm (Boyd et al. 2010). The regional deformation size range making up the bulk of the unit. This unit appears to be mainly ductile in nature, with large has extensive artisanal workings cut through it, and regional folding and localized tight isoclinal folds a minor amount of corundum does occur within its that make the highly covered sequences difficult to upper layer: however most of this material appears interpret. In general the rocks appear to have to be non-gem quality. The primary deposit appears experienced regional amphibolite-facies meta- to be a highly metamorphosed or metasomatized morphism. Paragneiss in the western part of the rock that has been completely altered from its complex was dated to 942 ±14 Ma with a meta- original primary nature (Plate 2E). The resulting morphic overprint at 599 ±10 Ma (Boyd et al. clay appears to be a complex Cr-rich smectite group 2010). Chemostratigraphic dating of marble in the clay, with at least 5% of the layer made up of Complex suggests their deposition between 1100 corundum, altered amphibole, and yellowish mica and 1050 Ma. (Pardieu et al. 2009b). Large gemmy stones The ruby area exploited by the Montepuez recovered from the artisanal workings have been Ruby Mining Ltd. hosts two distinct geological noted in the Montepuez village (Plate 2F), and these types of deposit (Pardieu & Chauviré 2013), alluvial pieces can be of significant gem quality with good and primary. The alluvial sedimentary sequence color and shape (around 5% of the stones). The with a thickness from 0.5 m to over 10 m ranges in majority of the production has a great color but with grain size from 10 cm cobbles through to fine quartz too many fissures; however, 70% of the production sand. The grains tend to be equidimensional with can be improved by Pb glass heat treatment and significant rounding along the grain edges. No 25% (such as the milky stones) by borax flux heat significant quantities of lithic fragments or clay treatment (Pardieu et al. 2009a). To date no mining

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operations have commenced; however commercial corundum and fuchsite in which the minerals are bulk sample operations are under way at the sequential alteration products of ultramafic rocks by Namahumbire site by Gemfields Inc. of London high temperature, low pH hydrothermal solutions (www.gemfields.co.uk); no record of geological related to a deep level acid epithermal system. work can be found for the Namahaca Deposit. The Grapes & Palmer (1996) proposed that the total thickness of the corundum-bearing layers, and fuchsite, margarite, tourmaline, and ruby-bearing thus the overall resource potential of the area, is boulders from Westland in New Zealand formed by unknown at this time. metasomatism of quartzofeldspathic enclaves located in serpentinite (Fig. 2-43). This rock so Metamorphic deposits in ultramafic rocks: the called 'goodletite' from Hokitika crystallized at T ~ ultramafic-hosted ruby deposits of South Africa, 450 ±20°C and P ~ 5–6 kbar, and the ruby contains Zimbabwe, and New Zealand have been the subject between 0.5 and 13.0 wt.% Cr2O3. of several detailed studies. The corundum-fuchsite rocks from South Africa occur in eastern Transvaal GEOLOGY AND GENESIS OF SECONDARY (Hall 1923) and in the Barberton greenstone belt CORUNDUM DEPOSITS (Anhaesseur 1978). In Zimbabwe, the 'boulder Placer deposits are of major commercial corundum' supplied four-fifths of the world importance and provide most of the gem corundum. production of corundum in the 1970s (Morrison Corundum from primary deposits is liberated from 1972). The ruby-fuchsite rocks located at O'Briens their parent rocks by weathering, transported by in the Archean greenstone belt of Harare occur in metavolcanic rocks, ferruginous quartzite, banded iron formation, siliceous limestone, and schist. They form lenses in ultramafic rocks (Schreyer et al. 1981, Kerrich et al. 1987). The lenses contain more than 89 wt.% of Al2O3, and Cr2O3 up to 2.8 wt.% (Schreyer et al. 1981). These rocks are so-called 'verdites'; generally the corundum is not of gem quality, but instead is used for carving. In Zimbabwe, the verdites contain andalusite, chlorite, and associations of margarite, tourmaline, diaspore, rutile, and gersdorfitte (NiAsS), and native Bi. In Transvaal, the verdites contain biotite, plagioclase, kyanite, and rutile with exsolution of Cr–Fe–Al oxides. The verdites are enriched in Al, Cr, B, V, and As (Schreyer et al. 1981). Schreyer et al. (1981) argued that the verdites from Zimbabwe have been metamorphosed at temperatures around 400°C and pressures lower than 3.5 kbar whereas the verdites from Transvaal at temperatures of 600°C and pressures higher than 5 kbar. A debate on the origin of these rocks ensued (Kerrich et al. 1987, Schreyer 1988). Schreyer et al. (1981) thought that the rocks formed by exhalative alteration of komatiite before metamorphism. The elements B, K, Rb, As, Sb, Bi, and Te were deposited from the solutions, while Al, Cr, Ni, and V were concentrated as immobile remainders from the original rock. The mineralogy of the alteration products was governed by aluminous sulfide FIG. 2-43. The gem corundum deposit of Westland in minerals such as alunite, which under regional New Zealand (after Grapes & Palmer 1996). A, Section metamorphism formed fuchsite and andalusite of a ruby-bearing pebble. B, Chemical distribution of (Schreyer et al. 1981). Kerrich et al. (1987) the ruby (filled circles) and sapphires (unfilled circles) proposed a hydrothermal model for the formation of in the (Cr3+)–(Fe3+)–(Ti4++Fe2+) triangular diagram.

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streams and rivers, and concentrated in hydro- weakly consolidated, but correspond to paleo- graphic traps. These deposits are considered placers (Garnier et al. 2004a). The erosion and sedimentary and corundum is either found as remobilization of these sedimentary rocks by rivers pebbles in gravel lenses or in lithified conglomerate results in the formation of newer present day (paleo-placer). placers. Two types of concentrations are recognized for Climate is a major factor for the formation of present day placers (Fig. 2-44): (1) eluvial and secondary deposits. In tropical areas rocks are colluvial deposits: residual eluvial concentrations exposed to meteoric alteration resulting in an are the product of chemical weathering of parent assemblage of clay minerals, Fe and Mn oxides, and rocks. The altered and decomposed primary deposit other supergene phases. Corundum and zircon are settled only vertically. Eluvial-colluvial deposits are resistant minerals and are found in soils, laterite, on the slopes and in karst cavities (marble type). and in gravelly levels overlying bedrock. In Colluvial deposits correspond to decomposed Myanmar, these gem-bearing levels enriched in primary deposits which have moved vertically and pebbles, sand, silt and clay, and iron oxides, are laterally downslope as the hillside eroded away; (2) called "byon" (Kane & Kammerling 1992). Other alluvial deposits result from the erosion of the host examples of such deposits are found in central and rock and transport of corundum by water in streams southeast Asia where the effect of the monsoon in and rivers. The concentration occurs where water low altitude environments is an effective mechanism velocity drops at a slope change in the hydrographic of erosion (Hughes 1997). In high-altitude areas profile of the river, at the base of a waterfall, broad such as in India, Nepal, Pakistan, and Azad- gullies, debris cones, meanders, and inflowing Kashmir the weathering alteration is mechanical and streams. corundum is found in situ in boulders accumulated The placer deposits of barrier sea beaches are on the lower part of the slopes (e.g., 'old mine' at the rare and cited examples include the blue sapphire Sumjam sapphire deposit, Kashmir). At Nangimali, occurrence from the Vendée region in France a ruby boulder was discovered in a creek in 1979, (Goujou 2002), and the BGY-sapphire deposits but due to difficult access and the short working from Madagascar at Befotaka in Nosy-Be Island, season, the primary occurrence was not found until and in the Andovokonko area in the Ambato 1988 (Malik 1994). Australia is an example of Peninsula (Ramdohr & Milisenda 2004). sapphire and ruby occurring in alkali basalt that is Paleo-placer deposits are characterized by subject to a tropical climate. The resulting alluvial different mineral phases which are cemented in a deposits are in eastern Australia, primarily central carbonate or silica-rich matrix. Paleo-placers of Queensland and northern New South Wales corundum are known from alkali basalt deposits in (Sutherland & Abduryim 2009, Abduryim et al. Madagascar: (1) in the deposits of the Antsiranana 2012). The main ruby production is located in province, the paleo-placer is formed by a carbonate Barrington Tops, Yarrowitch, and Tumbarumba. karst breccia contained in karst cavities in Jurassic The sapphire-bearing basaltic sediment, locally limestone (Schwarz et al. 2000, Giuliani et al. called 'wash', occurs in layers 1 to 3 m thick, 2007b); (2) in the Ilakaka area, gem corundum is underneath dark clayey soil some 1 to 3 m below the found in three gravel levels from alluvial terraces surface (Abduriyim et al. 2012). In the Inverell deposited on the Isalo sandstone. The terraces are district at the Mary Ann Gully mine, the brown

FIG. 2-44. Main types of mechanical and physical hydrographic traps for corundum in placer deposits (modified after Hughes 1997).

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basaltic sedimentary soils are extracted from an area origins of these gems are thus difficult to assess, of 10 to 40 km2. The basaltic flows came from a although the problem can be approached by the large eroded volcano and the fine-grained basalt study of the solid and fluid inclusions trapped in the contains large quantities of corundum, black spinel, individual crystals (Hughes 1997), and their zircon, ilmenite, hematite, olivine, pyroxene, chemical (Muhlmeister et al. 1998) and isotopic feldspar, and amphibole. composition (Giuliani et al. 2005; 2007a; Fig 2-45). Many methods are used for the exploitation of placers. In developing countries, the discovery of a Placer deposits in marble environments: the case new deposit provokes the migration of a huge of Vietnam: the placers formed in marble environ- population of poor people working with rudimentary ments such as those of Luc Yen and Yen Bai in tools. These independent miners, called garimpeiros Vietnam, and of Mogok and Mong Hsu in Myan- in Brazil and guaqueiros in Colombia, are equipped mar, are closely related to the formation of karst and with shovels and pickaxes to dig pits, often tens of chemical weathering of marble and associated rocks metres deep, to reach the mineralized zone. The gem i.e., schist, gneiss and granitoid rocks. The gravels are lifted by bags and buckets with the help corundum is trapped in eluvial, colluvial, alluvial, of a pulley, washed in sieves at the river, and finally and fracture-filling and cave deposits (Fig. 2-46). the gravels are hand-picked. Mechanization permits In Vietnam, the placers are proximal to the the intensive exploitation of placers, especially main primary deposits of ruby and colored sapphires those from southeast Asia, Australia, and North in marble. They are located in the Luc Yen and Yen America, more rarely in Africa and Madagascar. Bai provinces in the Red River shear zone area and Barren levels are displaced by mechanical shovel in the Nghe An province in the Bu Kang and bulldozer. The gem gravels are brought back to metamorphic dome in central Vietnam (Pham et al. the washing plant either by trucks or pumps after 2004). They are in Luc Yen, An Phu, Khoang water cannon extraction. A classical washing plant Thong, and Nuoc Ngap (Luc Yen district), in Truc contains one or several trommels, which facilitate a Lau, Yen Thai, Tan Huong, Tan Dong, Hoa Cuong, cut-off of the pebbles according to their size, and and Tran Yen (Yen Bai district), and Quy Hop and jigs, which sort the pebbles by their density. Finally, Quy Chau (Nghe An district). The oxygen isotopic the jigs are cleared out and the pebbles sorted by study of the ruby and sapphire crystals shows that hand. The exploitation of placer deposits in deep the majority of these corundum have a į18O range rivers is aided by mining dredges. This type of between 15.5 and 22.3‰ (Fig. 2-47), that overlaps exploitation is active in the Missouri River in the worldwide oxygen isotopic range defined for Montana for the recovery of sapphires (Hughes ruby hosted in marble (Giuliani et al. 2005; Fig. 1997). The washing plant is directly fixed to the 2-40). Four sapphires respectively of blue, grey and barge which dredges the river bottom. yellow color are in the field of desilicated and skarn Important secondary gem corundum deposits vein type in marble. are found in Australia (Queensland and New South Wales), Madagascar (Ilakaka, Ambondromifehy, Placer deposits in alkali basalt fields: the case of Vatomandry, Didy), Tanzania (Tunduru, Songea, the French Massif Central: gem blue-green-yellow Umba), Myanmar (Mogok and Mong Hsu districts), sapphire and ruby crystals are mostly recovered Vietnam (Luc Yen, Yen Bai, Da Lat, Ban Me from alluvial deposits in continental basaltic fields Thuot), the United States (Montana), and Sri Lanka (Hughes 1997, Sutherland 1996, Abduriyim et al. (Ratnapura, Elahera, Kataragama). The historical, 2012). The large corundum-bearing regions are geological, and mineralogical features of these located in Australia, the United States, Laos, deposits are described in details by Heilmann & Thailand, Cambodia and Madagascar. In France, Henn (1986), Silva & Siriwardena (1988), Hughes alluvial sapphire is located in the French Massif (1997), Sutherland et al. (1998a, 1998b), Schwarz et Central and has been known since the Middle Ages al. (2000), Garnier et al. (2004a), Pham et al. 2004, and exploited up to the 19th century (Forestier Themelis (2008), Pardieu et al. (2009a, 2012), 1993). The sapphire occurrences are located within Hughes et al. (2011), Abduriyim et al. (2012), the volcanic areas of the Chaîne de la Sioule, Coldham et al. (2012), Pardieu (2012), Pardieu & Chaîne des Puys, Mont Dore, Cantal, Devès and Chauviré (2013), and Peretti & Hahn (2013). Velay (Lacroix 1901, Gaillou et al. 2010). The Generally, placers contain gem corundum sapphires crystals are either transparent to originating from multiple sources. The geological dominantly pastel blue, pastel lilac and colorless, or

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FIG. 2-45. Oxygen isotopic ranges of corundum from the giant placer of Ilakaka (Madagascar) with a special focus on the origin of the blue sapphires. These sapphires are separated in two distinct groups: one group dominated by rutile needle inclusions (reported as needle in the figure), the other by bands and growth structures (reported as band in the figure). The oxygen isotopic composition of blue sapphire from Ilakaka are compared with the oxygen isotopic ranges defined for corundum types of deposit worldwide (Giuliani et al. 2005, 2007a, 2009, 2012a; Yui et al. 2003, 2006, 2008) and for colored sapphires of primary deposits from Madagascar. The oxygen isotopic data are reported in the conventional delta notation (į18O, expressed in per mil, ‰) relative to V–SMOW (Vienna Standard Mean Ocean Water). Color in diamonds represent color for ruby (in red) and colored sapphires (others). White diamonds represent colorless sapphires. blue, deep blue, greenish, greyish, yellowish, provides an additional argument for a magmatic pinkish, and colored but milky. The O origin. (2) A second group, with an isotopic range isotope composition of the sapphires ranges between 7.6 and 13.9‰ (n = 9), suggests a between 4.4 and 13.9‰ (Fig. 2-48). Two distinct metamorphic sapphire source such as biotite schist groups have been defined (Giuliani et al. 2009): (1) in gneiss or skarn. They are metamorphic sapphires the first with a restricted isotopic range between 4.4 from granulite which were transported to the surface 18 and 6.8‰ (n = 22; mean į O = 5.6 ±0.7‰) falls by the basaltic magma. within the worldwide range defined for blue-green- yellow sapphires related to basaltic gem fields (3.0 Placer deposits in sedimentary fields: the case of < į18O < 8.2‰, n = 150), and overlaps the range Ilakaka (Madagascar): the Ilakaka mining district defined for magmatic sapphires in syenite (4.4 < is located in the Isalo massif, between the cities of į18O < 8.3‰, n = 29; Fig 2-45). The presence of Sakaraha and Ilakaka (Fig. 2-32). Other districts are inclusions of columbite-group minerals, pyrochlore, found north of Ilakaka and near Bezaha, 120 km Nb-bearing rutile, and thorite in these sapphires southwest of Ilakaka. The alluvial giant deposit was

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FIG. 2-46. Main types of gem corundum deposits in the Mogok area (modified after Themelis 2008). The model represents the deposits of Dattaw (left) and Thurein-taung (right). The primary deposits are metamorphic or magmatic. The secondary deposits are residual, colluvial, alluvial, fracture-filling and cave deposits. Drawing courtesy of T. Themelis.

discovered in late 1998 on the border of Route Triassic sandstone. The deposit is not well National 7, and the rush of locals and immigrants consolidated and is composed of quartziferous sands resulted in the discovery of several mining zones which contain pebbles of ferruginous laterite, between Ilakaka and Sakaraha, including Sakalama, rounded blocks of Isalo sandstone and quartz, and Ampasimamitaka, Vohimena, Bekily, and quartzite and schist pebbles (Garnier et al. 2004a). Manombo Vaovao. The deposits are generally On the Benahy River, two levels of gemmiferous exploited by illegal miners using 1 m diameter terraces locally called "lalambato" are found (Fig. shafts reaching up to 20 m deep with windlass 2-49). The bottom terrace is the richest in systems, washing gravels in pans and removing the gemstones, with concentrations reaching up to 5 to 7 stones from the sieves by hand. The deposits are g/m3 for the mechanized exploitations. The sometimes exploited in open pits following benches concentration in some terraces exploited by washing to reach the mineralized zones. and hand recovery reached about 10 g/m3. They The deposits produce very fine blue, pink, correspond to deep potholes or sinuous meanders blue-violet, violet, purple, orange, yellow, and which are not accessible for mechanical translucent sapphire crystals along with zircon, exploitation. River sands, sampled in the stream alexandrite, , garnet, spinel, andalusite, and bed, contain between 0.2 and 2.1 g/m3 of sapphire. tourmaline. In 2002, test mining analysis by the This alluvial concentration results from the erosion Gem Mining Resources company during 38 days of of old terraces. production resulted in approximately 43 kg of The placers of Ilakaka represent accumulation gemstones, comprising 4% semi-precious stones, of ruby and colored sapphire from different origins volcanic glasses, and ruby, and 96% sapphire (Fig. 2-45). Microscopic examination of 30 ruby comprising 58% pink sapphire, 30% blue sapphire, and 58 blue sapphire crystals has shown that all and 8% other colored sapphire (e.g., yellow, orange, rubies have the same gemological features, i.e., 'padparadscha', green). The operators exploited the growth, twinning, color bands and solid inclusions, alluvial terraces of the Ilakaka and Benahy rivers, but that blue sapphires defined two different which lie on the Isalo sandstone. The terraces mineralogical groups. resulted from the erosion and stripping of the

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source for ruby in Ilakaka from the Vohibory region is likely. Inclusions and UV–Visible spectroscopy allowed for the differentiation of two distinct groups in the blue sapphire crystals from Ilakaka (Dunaigre & Giuliani, unpublished data). One group dominated by rutile needle inclusions, the other by bands and growth structures. Two samples from the "band" group with į18O values of respectively 7.4 and 10.1‰, and two from the "needle" group with į18O values of respectively 9.1 and 13.6‰, are compared to previous data obtained on blue sapphire (į18O value of 3.8‰; Giuliani et al. 2007b), and considering the different types of primary corundum deposit from southern Madagascar (Rakotondrazafy et al. 2008). The į18O value of 3.8‰ suggests two possible geological origins for sapphire (Fig 2-45): (1) BGY sapphire in alkali basalt; and (2) biotite schist in gneiss. The chemical composition of the sapphire has shown that it was incompatible with a basaltic origin, a type which has not been reported in southern Madagascar. Therefore the second hypothesis is highly probable as blue sapphire in biotite schist has been described in the Ionaivo deposit (with į18O = 3.3‰; Uher et al. 2012) located south of Sahambano deposit (Fig. 2-32). The band group contains inclusions of carbonate, phlogopite, graphite, and garnet. The į18O value of 7.4 and 9.1‰ suggests two possible FIG. 2-47. Oxygen isotopic composition of ruby and geological origins for sapphire (Fig. 2-45): biotite sapphire originating from placer deposits of the Luc schist deposit which results from the metasomatism Yen and Yen Bai mining district (north Vietnam). The of feldspathic granulitic gneiss (į18O between 3.3 oxygen isotopic range of these gem corundum samples and 9.0‰ for Ionaivo, Sahambano, Ambinda sud, overlaps that of ruby and sapphire hosted in marble and Zazafotsy deposits; see the section above: from the same area (Giuliani et al. 2007a). The data are 18 Metamorphic deposits, p. 58), and skarn in calc- reported in the conventional delta notation (į O, 18 expressed in per mil, ‰) relative to SMOW (Vienna silicate rocks and marble (į O between 7.7 and Standard Mean Ocean Water). 10.7‰ for Andranondambo deposit). Carbonate and phlogopite are found in sapphire from the 18 Ruby crystals are within the same į O range, Andronandambo skarn (Schwarz et al. 1996) but 18 between 2.6 and 3.8‰ (į Omean = 3.5 ±0.5‰, n = graphite and garnet have never been described. 4). The isotopic range corresponds to that defined Nevertheless, graphite is a mineral which is often for primary ruby from mafic and ultramafic rocks found in marble and gneiss from the Tranomaro from southern Madagascar (range 2.5 to 6.8‰; Formation which hosts the sapphire and U–Th Giuliani et al. 2007b). The Phanerozoic sedimentary mineralization (Pili et al. 1997; Rakotondrazafy et sequences and the Isalo Group underlie the northern al. 1996). Phlogopite, calcite and garnet are part of the Vohibory unit, where intercalations or described in the sapphire from the biotite schist but complexes of mafic and ultramafic rocks hosting graphite is lacking (Ralantoarison et al. 2006). ruby and pink sapphire are common in amphibolitic The needle-dominant group contains rutile and and aluminous gneiss (Rakotondrazafy et al. 2008). spinel which are also characteristic inclusions of The erosion of the Vohibory unit during the Late sapphire hosted in schist-type deposits such as Carboniferous to Middle Jurassic contributed to the Sahambano and Zazafotsy. Besides, the į18O of concentration of ruby in the basin. A proximal 9.1‰ for one sapphire fits with the isotopic range

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FIG. 2-48. Oxygen isotope values of colored sapphires found in placers within the volcanic fields of the French Massif Central (Giuliani et al. 2009), with reference to the worldwide oxygen isotopic database of corundum reported by Giuliani et al. (2005, 2007a, 2009). The data are reported in the conventional delta notation (į18O, expressed in per mil, ‰) relative to V– SMOW (Vienna Standard Mean Ocean Water). SUMMARY defined for the Zazafotsy deposit (Fig 2-45). The The primary deposits can be divided into two other crystal with į18O of 13.6‰ is close to the groups, depending on their geological setting, isotopic range defined for blue to colorless gem magmatic versus metamorphic (Fig. 2-12A, B). sapphires associated with skarn veins in marble Corundum in magmatic deposits is found in plutonic from Andronandambo (į18O between 14.0 to and volcanic rocks. In plutonic rocks, corundum is 15.6‰). Nevertheless, its origin cannot be associated with rocks deficient in silica and their accurately determined. pegmatites, especially syenite and nepheline syenite. The oxygen isotopic composition of blue Corundum is found also as an accessory mineral in sapphire from Ilakaka deposits shows the porphyry copper and kimberlite deposits. In complexity of the determination of origin for volcanic rocks, sapphire and ruby are found in corundum in placer. Nevertheless, the two continental alkali basalt extrusions. Corundum is gemological groups of blue sapphire from Ilakaka found as xenocrysts in lava flows and plugs of are metamorphic in origin. This result fits with the subalkaline olivine basalt, high alumina alkali basalt chemical fingerprinting and the types of solid and basanite. The sapphire is blue-green-yellow and inclusions found in the sapphire crystals. But at least the deposits only have economic importance three geological sources have probably contributed because advanced weathering in tropical regions to the blue sapphire concentration in the Ilakaka concentrates sapphire in eluvial and especially large deposit (biotite schist, skarn in marble or calc- alluvial placers. Sapphires also occur in alkaline silicate rocks, and skarn vein in marble). However basic lamprophyre dikes of biotite monchiquite. The there is currently no correlation between the ruby is metamorphic in origin and occurs as gemological groups and the isotopic composition xenocrysts or megacrysts in xenoliths in alkali and origin of the sapphire crystals. basalt.

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FIG. 2-49. Lithological section of the gem corundum terraces and aspect of the pebble levels exploited in the Vohimena Talo placer, Ilakaka district, Madagascar. L: lutite, A: arenite, R: rudite (Garnier et al. 2004a). The metamorphic deposits are hosted by is also linked to the intrusion of pegmatites and/or different kinds of rocks of variable grade of granitoid bodies into marble or mafic to ultramafic metamorphism. Among these deposits, marble rocks, like the famous Kashmir sapphire deposits of represents the most important source of ruby, such Sumjam. Gem corundum forms by fluid–rock as the famous pigeon blood-colored rubies from interaction and metasomatism of the host rock Mogok in Myanmar. They are found from (desilicated pegmatite and skarn). Afghanistan in central Asia to northern Vietnam in The secondary deposits are sedimentary and of southeast Asia. The other metamorphic deposits are detrital origin. Several types are distinguished: hosted by gneiss, anatexite, granulite (gneiss, colluvial, eluvial, deluvial and alluvial placers, and metashale, charnockite, cordieritite), and mafic to beach ridges. The minerals contained in eluvial ultramafic rocks. New discoveries of ruby hosted in placer deposits are little transported and the parental amphibolite (deposits of Greenland and rocks of the gemstones can thus be constrained. In Mozambique) will supply the demand on the market the other cases, the placer deposits can form by the in terms of quantity and quality in the near future. weathering of paleoplacers and transport of the Finally, the formation of ruby and sapphire deposits minerals. In some cases, gemstones coming from

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several types of deposits are mixed in a single basin ALTHERR, R., OKRUSCH, M. & BANK, H. (1982): (Tunduru, Songea, Ilakaka, Didy, Ratnapura, Corundum- and kyanite-bearing anatexites from Elahera, etc.). The study of gemological the Precambrian of Tanzania. Lithos 15, 191–197. characteristics, solid and fluid inclusions, as well as ANDREOLI, M.A.G. (1984): Petrochemistry, tectonic the geochemical and isotopic signatures of these evolution and metasomatic mineralizations of gemstones are useful techniques for determining Mozambique belt from Malawi and their geological origins. Tete (Mozambique). Precamb. Res. 25, 161–186.

ANDRIAMAMONJY, S.A. (2006): Les corindons ACKNOWLEDGEMENTS associés aux roches métamorphiques du Sud ouest The authors wish to thank the Mineralogical de Madagascar: le gisement de saphir de Association of Canada, GET UMR 5563 CNRS- Zazafotsy. Mémoire de DEA, Université IRD-CNES-UPS in Toulouse, CRPG UMR 7358 d'Antananarivo, Madagascar. CNRS-UL and GéoRessources UMR 7359 CNRS- ANDRIAMAMONJY, S.A. (2010): Importance des UL in Nancy, Geological Survey of Pakistan, fluides sur la métallogénie des gisements de Institute of Geological Sciences in Hanoi, saphir et rubis dans le domaine granulitique de University of Antananarivo, University of haute température du Sud de Madagascar: cas Kathmandu, SUERC Glasgow, School of Earth de Zazafotsy et d'Ambatomena. Mémoire de Sciences Leeds, University of Montpellier 2, Doctorat, Université d’Antananarivo, Mada- MNHN-UMR 7160 Paris, Ministère des Mines de gascar. Madagascar, and T. Themelis for the Mogok illustration. The authors thank very much Dr. David ANDRIAMAMONJY, A., GIULIANI, G., Turner for his very carefully and constructive OHNENSTETTER, D., RAKOTONDRAZAFY, A.F.M., review, and Dr. Rob Raeside and Dr. Lee Groat for RALANTOARISON, T., RANATSENHO, M.M., final reviews and editorial efforts. RAKOTOSAMIZANANY, S. & FALLICK, A.E. (2013): Les corindons d’âge néoprotérozoïque de REFERENCES Madagascar. le Règne Minéral, Cahier 2, "Les Gemmes du Gondwana", 109–117. ABDURIYIM, A. & KITAWAKI, H. (2006): Applications of laser ablation-inductively coupled ANDRIAMAROFAHATRA, J. & DE LA BOISSE, H. plasma-mass spectrometry (LA–ICP–MS) to (1986): Premières datations sur zircon du gemology. Gems & Gemology 42, 98–118. métamorphisme granulitique du sud-est de Madagascar. 11ème Réunion des Sciences de la ABDURIYIM, A., SUTHERLAND, F.L. & COLDHAM, T. Terre, Clermont-Ferrand, Résumés. (2012): Past, present and future of Australian gem corundum. Austr. Gemmol. 24, 234–242. ANHAESSEUR, C.R. (1978): On an Archean marundite occurrence (corundum margarite rock) ABOOSALLY, S. (1999): Update on production in in the Barberton Mountain Land, Eastern Pakistan, Afghanistan. News Asia 1, Transvaal. Trans. Geol. Soc. Afr. 81, 211–218. 60–64. ANTHONY, J.W., BIDEAUX, R.A., BLADH, K.W. & ACKERMAND, D., WINDLEY, B.F. & NICHOLS, M.C. (1997): Handbook of Mineralogy, RAZAFINIPARANY, A. (1989): The Precambrian vol. III, Halides, Hydroxides, Oxides. Mineral mobile belt of southern Madagascar. In Evolution Data Publishing Tucson, Arizona, U.S.A. of Metamorphic Belts (J.S. Daly et al. eds.). Geol. Soc. Spec. Pub. 43, 293–296. ASPEN, P., UPTON, B.G.J. & DICKIN, A.P. (1990): Anorthoclase, sanidine and associated megacrysts AKININ, V.V., VYSTOSTSKIY, S.V., MAZDAB, F.K, in Scottish alkali basalts: high-pressure syenitic MILLER, E. & WOODEN, J.L. (2004): SHRIMP debris from upper mantle sources? Eur. J. dating of zircon from the Podgelbanochny alkali Mineral. 2, 503–517. basalt volcano in Primorye, Russian Far-East: application to genesis of megacrysts. In ATKINSON, D. & KOTHAVALA, R.Z. (1983): Khanchul, A.I., Gonevchuk, G.A., Mitrokhin, Kashmir sapphire. Gems & Gemology 19, 64–76. A.N., Simanenko, L.F., Cook, N.J., Seltmann, R. BAROT, N. & HARDING, RR. (1994): Pink corundum (Eds), Metallogeny of the Pacific Northwest: from Kitui, Kenya. J. Gemmol. 24, 165–172. tectonics, magmatism and metallogeny of active BAROT, N., FLAMNIN, A., GRAZIANI, G. & GÜBELIN, continental margins. Dal'naka, Vladivostok, E.J. (1989): Star sapphire in Kenya. J. Gemmol. Russia, 323–326.

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