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Review Deposits: A Review and Geological Classification

Gaston Giuliani 1,*, Lee A. Groat 2, Anthony E. Fallick 3 , Isabella Pignatelli 4 and Vincent Pardieu 5

1 Géosciences Environnement, Université Paul Sabatier, GET/IRD et Université de Lorraine, C.R.P.G./C.N.R.S., 15 rue Notre-Dame-des-Pauvres, BP 20, 54501 Vandœuvre, France 2 Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, BC V6T 1Z4, Canada; [email protected] 3 Isotope Geosciences Unit, Scottish Universities Environmental Research Centre (S.U.E.R.C.), Rankine Avenue, East Kilbride, Glasgow G75 0QF, UK; [email protected] 4 Georessources, Université de Lorraine, UMR 7539 CNRS-UL, BP 70239 Vandœuvre, France; [email protected] 5 Field gemmologist & Consultant at VP Consulting, Manama, Bahrain; vince@fieldgemology.org * Correspondence: [email protected]; Tel.: +33-3-83594238

 Received: 30 March 2020; Accepted: 23 June 2020; Published: 30 June 2020 

Abstract: is not uncommon on Earth but the gem varieties of ruby and are relatively rare. Gem corundum deposits are classified as primary and secondary deposits. Primary deposits contain corundum either in the rocks where it crystallized or as xenocrysts and xenoliths carried by magmas to the Earth’s surface. Classification systems for corundum deposits are based on different mineralogical and geological features. An up-to-date classification scheme for ruby deposits is described in the present paper. Ruby forms in mafic or felsic geological environments, or in metamorphosed carbonate platforms but it is always associated with rocks depleted in silica and enriched in alumina. Two major geological environments are favorable for the presence of ruby: (1) amphibolite to medium pressure granulite facies metamorphic belts and (2) alkaline basaltic volcanism in continental rifting environments. Primary ruby deposits formed from the Archean (2.71 Ga) in to the Pliocene (5 Ma) in Nepal. Secondary ruby deposits have formed at various times from the erosion of metamorphic belts (since the Precambrian) and alkali basalts (from the Cenozoic to the Quaternary). Primary ruby deposits are subdivided into two types based on their geological environment of formation: (Type I) magmatic-related and (Type II) metamorphic-related. Type I is characterized by two sub-types, specifically Type IA where xenocrysts or xenoliths of gem ruby of metamorphic (sometimes magmatic) origin are hosted by alkali basalts ( and others), and Type IB corresponding to xenocrysts of ruby in kimberlite (Democratic Republic of Congo). Type II also has two sub-types; metamorphic deposits sensu stricto (Type IIA) that formed in amphibolite to granulite facies environments, and metamorphic-metasomatic deposits (Type IIB) formed via high fluid–rock interaction and metasomatism. Secondary ruby deposits, i.e., placers are termed sedimentary-related (Type III). These placers are hosted in sedimentary rocks (soil, rudite, arenite, and silt) that formed via erosion, gravity effect, mechanical transport, and sedimentation along slopes or basins related to neotectonic motions and deformation.

Keywords: ruby deposits; classification; typology; magmatism; metamorphism; sedimentary; metasomatism; fluids; stable and radiogenic isotopes; genetic models; exploration

1. Introduction Ruby is the gem variety of the corundum. The name corundum was introduced in the early 19th century by French mineralogist De Bournon when gemmological terminology was transformed

Minerals 2020, 10, 597; doi:10.3390/min10070597 www.mdpi.com/journal/minerals Minerals 2020, 10, 597 2 of 83 from a rather confusing system mainly based on and origin into a system based on a new science: mineralogy. The term “corundum” likely originated from the Sanskrit kurunvinda meaning “hard stone”, which became kurund in vernacular. It was the name used by the native people met during the field expedition sent by De Bournon to describe the stones they were mining in Southern India. If the name “corundum” originates from Sanskrit, in Sanskrit were called ratnaraj, queen of precious stones and symbol of permanent internal fire. In Greco-Roman times, the reference to fire was also common; in 400 before Christ Theophrastus [1] related ruby to anthrax (coal). Two thousand years ago Pliny [2] called all red stones carbunculus (diminutive of carbo), which became escarboucle in old French. In the 11th century, the Arab scholar Al Biruni separated different types of rubies based on specific gravity and hardness, but his work remained unknown in Europe for centuries except possibly for a few scholars like French bishop Marbode [3] who, a few years later, also separated carbuncles into three different types. The term “ruby” appeared in European languages during the Middles Ages. How and why it was introduced is not clear but it was used by [4]. It is derived from the Latin ruber, meaning red. It became popular and then, until the introduction of the word corundum in the 19th century, most red stones were just called rubies. Experts and authors commonly separated different types based on their origin: “Oriental Ruby” was used for stones mined from Burma or Ceylon (ruby), “Balas Ruby” for gems mined from the modern-day (), and “Bohemian ruby” was used for gems coming traditionally from the modern-day Czech Republic (“red ”, either , , and/or spessartite). However, as rubies, , and are often found in association and as new deposits were discovered, the system became very confusing. Ironically, the majority of the most famous rubies (like the “ Prince Ruby” and the “Timur Ruby” of the British Jewels or the “Cote de Bretagne Ruby” of the French ) that contributed significantly to the public recognition of the word “ruby” would actually, according to the new terminology based on mineralogy, be spinels. As gemmology is not simply about science, but also about history, art, and trade, these gems are still called by their historic “ruby” name, maintaining the taste for either curiosity, controversy, or confusion among many authors, particularly those with a non-scientific and/or historical background. Currently we know that ruby is the (Cr)-rich variety of corundum, but other such as vanadium (V) and iron (Fe) may be present in the structure of the mineral. The ideal formula of corundum is Al2O3 and it crystallizes in the trigonal system, with a hardness of 9 on the Mohs scale (while spinel and garnets crystallize in the cubic system and are softer with a hardness of 8). Unlike spinel and garnet, ruby is a dichroic mineral, which will show a different color (either purplish- or orangey-red) depending on which direction travels inside the gem. In gemmology this is of critical importance because , wishing to optimize the color from a rough stone by cutting it, can produce a red, a pinkish red, a purplish red, or an orangey-red stone, depending on how they decide to orientate and cut the gem. The ruby industry is very lucrative [5] but it is far more complex than many ore industries such as iron, , or because the values of ruby do not depend only on weight or purity. The value depends on a much more complex quality, with grading systems including many artistic, subjective, and cultural aspects. In addition to the well-known and rather comprehensive system established by the trade based on the 4Cs (i.e., color, clarity, cut, and weight of the gem) in the middle of the 20th century, with colored stones such as rubies additional factors have a critical importance for market value. First, it has to be established that the gem was actually mined and was not produced from a factory. Synthetic rubies created either by flux, flame fusion, or some hydrothermal process appeared at the end of the 19th century. If the arrival of synthetics created some initial scandals and panic, the trade answered by establishing gemmological laboratories that were able to identify natural gems versus synthetics and thus re-establish the necessary confidence for the trade to grow. Second, enhancements and treatments are also of importance to the final evaluation. To satisfy the strong demand for large clean rubies, some very efficient heat treatment techniques were developed in the 1990s in Thailand. These were different from the traditional blow-pipe technique used in Sri Minerals 2020, 10, 597 3 of 83

Lanka for centuries and described by many travellers beginning with Al Burini in the 10–11th centuries. Whilst the blow pipe was used to remove the purplish aspect in some rubies, the new technique went further: using borax as a flux additive it could heal fractures in rubies and thus enable the cutting of larger stones. Due to this new treatment technique and the ones that followed, new deposits, such as Mong Hsu in eastern , started to dominate the ruby trade in the early 1990s and Thailand increased its importance as the World’s main ruby trading centre [6]. In the early 1990s, in Bangkok, it was very rare to find gem corundum that had not been heat-treated [7]. Drucker in [8], at the end of 1990, explained that the average price of ruby fell after controversy over its treatments. However, if the average price for rubies fell, prices for fine untreated rubies have exploded, particularly since the early 2000s (as observed in the reports from auction houses, many available online). As the auction results show, not only did the prices for unheated rubies increase, but also the prices for unheated Myanmar rubies. Geographic origin is the last important factor to consider in establishing the value of a fine ruby (Figure 1). At this point it is critical to understand that, as fine rubies are both very rare but also very durable products, they usually survive for centuries after being mined, passing from one owner to another. As a consequence, the few very fine gems coming from existing mines are actually in competition with all the fine gems mined in the past, which are mainly from Myanmar (formerly known as Burma). Due to this, modern ruby traders are actually acting more like antique dealers than exploration geologists or Indiana Jones [9]. Public recognition and romance are very important factors contributing to demand for fine [10]. The assignment of a commercial value to ruby and other gems is carried out by gem merchants with the help of a few specialized gemmological laboratories providing independent third-party opinions. These labs provide answers for the following key questions [11]: (1) Is the gem the ruby variety of corundum? (2) Is it natural or synthetic ruby? (3) Has it been enhanced? (4) Where is it from (geographic origin)? If for most of these laboratories the answer to the first question is more or less routine work, the question about origin determination remains a great challenge [11]. The geographic origin is often controversial as in many cases it is based on expert opinion more than on provable facts [12–15]. Origin determination was introduced by Eduard Gübelin who worked on inclusions in gems using mainly microscopy [16,17], but also Raman and infrared spectroscopy. Such techniques are usually suitable for high value large rubies (that are usually never really free of mineral inclusions) but find their limits in the case of small clean stones or for heated stones (with inclusions that have been altered or even melted by heat treatment). Currently, these techniques are combined with the analysis of trace elements using energy dispersive X-ray (EDXRF) and more especially since the early 2000s ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS). To be able to produce origin reports, gemmological laboratories require sophisticated instrumentation, an experienced and qualified team, and a solid and very reliable reference collection. The pricing of natural ruby is unique in terms of color and weight in carats. Currently if factory-grown synthetic rubies are cheap, if carving- or bead-grade treated rubies of low transparency and either dark or light color can be worth only a few dollars per kilo, a natural untreated ruby is still perhaps the world’s most expensive . The best Myanmar rubies are more highly valued than an equivalent-sized flawless colorless diamond. The world record price for a single ruby sold at auction currently belongs to the Sunrise ruby, sold in 2015 for US$32.42 million (US$1.27 million per carat at 25.59 ct) [18]. In December 2015, the 15.04 ct Flame ruby was sold for UU$18.3 million (US$1.21 million per ct). Another notable ruby is the 10.05 ct Ratnaraj, sold for US$10.2 million (just over US$1 million per ct) in November 2017. Fine-quality unheated rubies from the recently discovered Didy mine in Madagascar also command high prices [19]. A set of eight faceted rubies, ranging from seven to more than 14 cts in weight, had an estimated market value of US$10 million [20]. The discovery of the Montepuez ruby deposits in in May 2009 [21–23] and their exploitation by various companies including Gemfields Group Ltd. [24,25] changed the international ruby market [6,26]. A recent Competent Persons Report by SRK Consulting forecast production of 432 million carats from Montepuez over 21 years [27]. The resource at Montepuez is Minerals 2020, 10, 597 4 of 83 divided into primary reserves of 253 million cts (projection of 115 cts per ton) and secondary (mainly colluvial) reserves of 179 million cts (projection of 7.07 cts per ton). Twelve Montepuez auctions held since June 2014 have generated US$512 million in aggregate revenue (see Gemfields website). The2014 auction have bygenerated Gemfields US$512 Group million Ltd. in inaggregate June 2018 revenue yielded (seea record sale website). of US$71.8 The auction million by with an averageGemfields price Group of US$122 Ltd. in per June ct. 2018 The yielded success a of record the Gemfields sale of US$71.8 ruby million auctions with are an due average to product price of from the US$122 per ct. The success of the Gemfields ruby auctions are due to product from the Mugloto Mugloto colluvial deposit, which provided clean and very high-quality red stones with typical sizes colluvial deposit, which provided clean and very high-quality red stones with typical sizes between betweenone and one 10 andcts, and 10 cts,occasionally and occasionally larger larger [24]. crystals [24].

FigureFigure 1. 1.Rubies Rubies worldwide. worldwide. ( A(A)) Ruby Ruby crystal (1.7 (1.7 cm cm ×1.7 1.7 cm cm ×0.5 0.5cm) cm) in in marble marble fromfrom Bawpadan,Bawpadan, , × × Myanmar.Mogok, Myanmar. Photo: Louis-Dominique Photo: Louis-Dominique Bayle © Baylele Rè gne© le MinRègneéral; Minéral; (B) rubies (B) rubies (from (from left to left right: to right: 2.1 cm 1.3 × cm2.1 1.3cm cm × 1.3 and cm 1.7 × cm 1.3 cm1cm and 11.7 cm) cm from × 1 Ambohimandrosoa,cm × 1 cm) from Ambohimandrosoa, Antsirabe area, Madagascar. Antsirabe area, Collection: × × × L. Thomas;Madagascar. Photo: Collection: Louis-Dominique L. Thomas; Photo: Bayle Louis-Dominique© le Règne Min éBayleral; ( C©) le rubies Règne from Minéral; the Montepuez(C) rubies mine, from the Montepuez mine, Mozambique. Vincent Pardieu © Gemmological Institute of America; (D) Mozambique. Vincent Pardieu © Gemmological Institute of America; (D) Barrington Tops alluvial ruby Barrington Tops alluvial ruby concentrate, crystals up to 9 mm in size, New South Wales, . concentrate, crystals up to 9 mm in size, New South Wales, Australia. Photo: Australian Museum Collection, Photo: Australian Museum Collection, courtesy Gayle Webb; (E) ruby crystal (6 cm × 4.8 cm × 1 cm) courtesy Gayle Webb; (E) ruby crystal (6 cm 4.8 cm 1 cm) extracted from amphibolite rocks at , extracted from amphibolite rocks at Ampanihy,× Southern× Madagascar. Collection: J. Béhier. Photo: SouthernLouis-Dominique Madagascar. Bayle Collection: © le Règne J. BMinéral;éhier. Photo: (F) rubies Louis-Dominique in phlogopite matrix Bayle with© le plagioclase Règne Min ()éral; (F ) rubies in phlogopiteand minor matrix () with hosted plagioclase in between (white) leucog andabbro minor and kyanite meta-ultramafic (blue) hosted rocks in affected between by leucogabbrofluid andcirculation meta-ultramafic and metamorphic rocks affected metasomatism, by fluid circulation Aappaluttoq and deposit, metamorphic southwest metasomatism, Greenland. Photo: Aappaluttoq deposit,Courtesy southwest True North Greenland. Gems Inc. Photo: (Vancouver, Courtesy BC, TrueCanada). North Gems Inc. (Vancouver, BC, Canada).

OverOver the the last last decade, decade, knowledgeknowledge on on the the formation formation of ruby of ruby deposits deposits has improved has improved significantly significantly as as theirtheir origin origin determination determination usingusing a a variety variety of of new new methods methods and and analytical analytical procedures. procedures. The present The present article article synthesizessynthesizes the the state state ofof knowledgeknowledge on on the the geology geology an andd genesis genesis of ruby of ruby deposits. deposits. The different The diff issueserent issues focused on their location on the planet, their physical and chemical properties, pressure–temperature focused on their location on the planet, their physical and chemical properties, pressure–temperature conditions of crystallization, the source of the constituent elements (viz. aluminum, chromium, vanadium, conditions of crystallization, the source of the constituent elements (viz. aluminum, chromium, and iron), their age of formation, and finally the different proposed genetic models. vanadium, and iron), their age of formation, and finally the different proposed genetic models. Minerals 2020, 10, 597 5 of 83

InIn this this review review we we propose propose a geologicala geological classification classification scheme scheme for for ruby ruby based based on on scientific scientific works works and ownand experience. own experience. The classificationThe classification typology typology should should open open a a new new frameworkframework for for mineral mineral exploration exploration guidelinesguidelines and and geographic geographic determination determination forfor gemmological laboratories. laboratories.

2.2. Worldwide Worldwide Ruby Ruby Deposits Deposits TheThe distribution distribution of majorof major and and minor minor commercial, commercial, industrial, industrial, scientific, scientific, and historicaland historical world world sources ofsources ruby is of shown ruby inis shown Figure in2. Figure 2.

FigureFigure 2. 2. MainMain major major and and minor minor world world sources sources of ruby of pl rubyotted plotted according according to their togeological their geological types. types.Madagascar: Madagascar: (1) Andilamena, (1) Andilamena, Ambodivoangy-Zahame Ambodivoangy-Zahamena,na, Didy, Vatomandry; Didy, Vatomandry; (2) Andriba; Ankaratra (2) Andriba; Ankaratra(Antsirabe-Antanifotsy (Antsirabe-Antanifotsy region, Soamiakatra-Ambo region, Soamiakatra-Ambohimandroso;himandroso; Ambohibary); Ambohibary); (3) ; (3) - Zazafotsy; Ilakaka-;Sakaraha; (4) Bekily-Vohibory (4) Bekily-Vohibory area (Ambatomena; area (Ambatomena; ; ; Ianapera; Anavoha; Fotadrevo; ; Anavoha; ; Maniry; Gogogogo;; Vohitany; ). Ethiopia: Ejeda). Ethiopia: (5) Kibre(5) mengist, Kibre mengist, Dilla. Kenya: Dilla. (6 Kenya:) West(6) Pokot; West (7) Pokot; Baringo; (7) Baringo; (8) Kitui; (8) (9) Kitui; Mangare (Rockland-formerly John Saul mine; Penny Lane; , Hard Rock). : (10) Umba (9) Mangare (Rockland-formerly John Saul mine; Penny Lane; Aqua, Hard Rock). Tanzania: (10) Umba valley; (11) Longido, Lossogonoi; (12) Winza; Loolera (Kibuko); (13) Morogoro (Mwalazi; Visakazi; valley; (11) Longido, Lossogonoi; (12) Winza; Loolera (Kibuko); (13) Morogoro (Mwalazi; Visakazi; Nyama, Kibuko, etc.); Mahenge (Lukande; Mayote; Kitwaro; Ipanko; (14) Songea; (15) Tunduru. Nyama, Kibuko, etc.); Mahenge (Lukande; Mayote; Kitwaro; Ipanko; (14) Songea; (15) Tunduru. Malawi: Malawi: (16) Chimwadzulu. Mozambique: (17) Montepuez (Namahumbire/Namahaca), M’sawize, (16) Chimwadzulu. Mozambique: (17) Montepuez (Namahumbire/Namahaca), M’sawize, Ruambeze. Ruambeze. Zimbabwe: (18) O’Briens (verdites). South Africa: (19) Barberton (verdites). Democratic Zimbabwe: (18) O’Briens (verdites). South Africa: (19) Barberton (verdites). Democratic Republic of Congo: Republic of Congo: (20) Mbuji-Mayi. : (21) São Luis-Juina River. Colombia: (22) Mercaderes-Rio (20) Mbuji-Mayi. Brazil: (21) São Luis-Juina River. Colombia: (22) Mercaderes-Rio Mayo. United States: Mayo. United States: (23) (Corundum Hill; Cowee Valley; Buck Creek); (24) (23) North Carolina (Corundum Hill; Cowee Valley; Buck Creek); (24) Montana (Rock Creek). Canada: (Rock Creek). Canada: (25) Revelstoke. Greenland: (26) South of Nuuk region (Fiskenæsset district- (25) Revelstoke. Greenland: (26) South of Nuuk region (Fiskenæsset district-Aappaluttoq); North of Aappaluttoq); North of Nuuk (Storø; Kangerdluarssuk). Switzerland: (27) Campo Lungo. France: (28) NuukBrittany (Storø; (Champtoceaux); Kangerdluarssuk). (29) Switzerland: French Massif (27) Central Campo Lungo.(Haut-Allier-Chantel; France: (28) Brittany Peygerolles; (Champtoceaux); Lozère- (29)Aveyron-Marvejols; French Massif Central Vialat-du-Tarn); (Haut-Allier-Chantel; (30) Pyrenées Peygerolles; (Arignac). Loz Spain:ère-Aveyron-Marvejols; (31) Alboran sea; Marrocos: Vialat-du-Tarn); (32) (30)Beni Pyren Bousera.ées (Arignac). Italy: (33) Spain:Piedmont. (31) Norway: Alboran (34) sea; Frol Marrocos:and. Finland: (32) Beni(35) Kittilä Bousera.. Macedonia: It aly: (33) (36) Piedmont. Prilep. Norway:Greece: (37) (34) Gorgona-Xanthi; Froland. Finland: Paranesti-Drama. (35) Kittilä. Macedonia: Tajikistan: (38) (36) Snezhnoe, Prilep. Greece: Turakuloma, (37) Gorgona-Xanthi; Badakhshan. Paranesti-Drama.: (39) Tajikistan:Jegdalek, Kash. (38) Snezhnoe,Pakistan: (40) Turakuloma, Hunza valley; Badakhshan. (41) Batakundi, Afghanistan: Nangimali; (39) (42) Jegdalek, Dir. Kash.India: Pakistan: (43) Orissa, (40) HunzaKalahandi; valley; (44) (41) Karnakata Batakundi, (Mys Nangimali;ore); (45) Andhra (42) Dir. Pradesh India: (43)(Salem Orissa, district). Kalahandi; Sri (44)Lanka: Karnakata (46) Ratnapura, (Mysore); (45)Polonnaruwa; Andhra Pradesh Elahera. (Salem Nepal: district). (47) Chumar, Sri Lanka: Ruyil. (46) Chin Ratnapura,a: (48) Qinghai; Polonnaruwa; (49) Elahera.Yuan Jiang. Nepal: (50) (47)Muling. Chumar, : Ruyil. (51) China:Luc Yen-Ye (48)n Qinghai; Bai; (52) (49)Quy YuanChau. Jiang.Myanmar: (50) (53) Muling. Mogok, Vietnam: (54) (51)Namya; Luc Yen-Yen (55) Mong Bai; Hsu. (52) QuyThailand: Chau. (56) Myanmar: Chanthabur (53)i-Trat Mogok, (Bo Waen, (54) Namya; Bo Na (55)Wong, Mong Wat Hsu. Tok Phrom, Thailand: (56)Bo Chanthaburi-Trat Rai, Nong Bon). (BoCambodia: Waen, Bo (57) Na Pailin, Wong, Samlaut. Wat Tok : Phrom, (58) Bo Ida. Rai, NongAustralia: Bon). (59) : Lava Plains; (57) (60) Pailin, Samlaut.Anakie Japan:fields-Rubyvale; (58) Ida. Australia:(61) New England (59) Lava fields Plains; (Inverell); (60) Anakie (62) Macquarie-Cudgegong, fields-Rubyvale; (61)New Barrington England fieldsTops-Yarrowitch; (Inverell); (62) Macquarie-Cudgegong,(63) Tumbarumba; (64) BarringtonWestern Me Tops-Yarrowitch;lbourne fields; (65) (63) Hart Tumbarumba;s range; (66) (64) Poona. Western MelbourneNew Zealand: fields; (67) (65) Westland Harts range; (Hokitika). (66) Poona. Russia: New (68) Zealand: Polar (67)Urals Westland (Hit Island); (Hokitika). (69) Ural Russia: mountains (68) Polar Urals(Kuchin-Chuksin-Kootchin (Hit Island); (69) Ural mountainsskoye); (70) (Kuchin-Chuksin-Kootchinskoye); Karelia. (70) Karelia. High-value Mineralsrubies2020 ,are10, 597 important mineral resources that contribute significantly6 of 83 to the gross domestic product of many countries. In 2005, Africa was officially (statistics are rare on the production of High-value rubies are important mineral resources that contribute significantly to the gross most ruby deposits)domestic responsible product of many for countries. around In 2005,90% Africa of world was officially ruby (statistics production are rare on the(in production weight, not in quality; 8000 tons [28]; Figureof most 3) ruby and deposits) this was responsible before for around 2009 90% when of world the ruby Montepuez production (in weight,ruby notdeposits in quality; were discovered 8000 tons [28]; Figure 3) and this was before 2009 when the Montepuez ruby deposits were discovered in Mozambique [21–23].in Mozambique [21–23].

Figure 3. Ruby production worldwide in 2005 showing the production (in %) of some African countries, Figure 3. Ruby productionand others; modified worldwide from [28 ].in 2005 showing the production (in %) of some African countries, and others; modified from [28]. The highest-quality ruby crystals come from Central and South-East Asia and Eastern Africa ([27]; see Figure 1C). Myanmar, with the Mogok Stone Tract (see Figure 1A), is famous for having produced rubies The highest-qualityof the finest ruby quality crystals since 600 CEcome [5,29,30 from]. These Central are often calledand “pigeonSouth-East blood”, aAsia term of and Myanmar Eastern Africa ([27]; origin popularized by English traders in the 19th century [31]. Other Asian producers including Thailand see Figure 1C). Myanmar,(Chanthaburi-Trat with mines) the andMogok Cambodia Stone (Pailin Trac and Samlaut)t (see emergedFigure as 1A), serious is players famous in the for 1970s, having produced rubies of the finestbut theirquality production since has 600 declined CE dramatically [5,29,30] over. These the past are 20 years often [6]. Rubycalled deposits “pigeon in Vietnam blood”, a term of (Luc Yen, Yen Bai, and Quy Chau areas), Afghanistan (Jegdalek), Pakistan (Hunza valley), Azad-Kashmir Myanmar origin (Batakundipopularized and Nangimali), by English and Tajikistan traders (Kukurt, Turakoluma,in the 19th and Badakshan) century have [31]. become Other significant Asian producers including Thailandproducers (Chanthaburi-Trat since the end of the 1980s, mines) but their and production Cambodia remains limited.(Pailin In theand early Samlaut) 1990s Mong emerged Hsu as serious in Eastern Myanmar emerged as a major deposit but its production has declined since the early 2000s [6]. players in the 1970s,Today but their the high production production and has quality declin ofed rubies dramatically from Mozambique over (and the to past a lesser 20 years [6]. Ruby deposits in Vietnamextend Madagascar)(Luc Yen, has changedYen Bai, the way and rubies Quy are traded Chau and withareas), the arrival Afghanistan of these new players, (Jegdalek), the Pakistan production of many other deposits collapsed [6]. Production from the Winza deposit diminished to the (Hunza valley), Azad-Kashmirpoint of essentially disappearing, (Batakundi and the and Rockland Nangimali), (former John Saul) and mine Tajikistan in Kenya stopped (Kukurt, producing. Turakoluma, and Badakshan) haveIn become Africa (Figure significant4) gem rubies areproducers mined in Kenya si (Mangarence the and end Baringo), of Tanzaniathe 1980s, (Umba-Kalalani, but their production Morogoro, Mahenge, and Loolera), Malawi (Chimwadzulu), Madagascar (Vatomandry, Andilamena, Didy, remains limited. Zahamena,In the early and Ilakaka), 1990s and Mong Mozambique Hsu (Montepuez, in Eastern Ruambeze, Myan andmar M’sawize). emerged The Montepuez as a major deposit but its production hasmine declined is the most since highly mechanizedthe early ruby 2000s mine worldwide[6]. [6,26]. In Southern Greenland True North Gems Inc. started working on the Aappaluttoq deposit near Fiskenæsset in 2005 (see Figure 1F) [32–37] Today the highand a mineproduction (operated by “Greenlandand quality Ruby”) enteredof rubies into production from inMozambique 2017. (and to a lesser extend Madagascar) has changed the way rubies are traded and with the arrival of these new players, the production of many other deposits collapsed [6]. Production from the Winza deposit diminished to the point of essentially disappearing, and the Rockland (former John Saul) mine in Kenya stopped producing. In Africa (Figure 4) gem rubies are mined in Kenya (Mangare and Baringo), Tanzania (Umba-Kalalani, Morogoro, Mahenge, and Loolera), Malawi (Chimwadzulu), Madagascar (Vatomandry, Andilamena, Didy, Zahamena, and Ilakaka), and Mozambique (Montepuez, Ruambeze, and M’sawize). The Montepuez mine is the most highly mechanized ruby mine worldwide [6,26]. In Southern Greenland True North Gems Inc. started working on the Aappaluttoq deposit near Fiskenæsset in 2005 (see Figure 1F) [32–37] and a mine (operated by “Greenland Ruby”) entered into production in 2017. Minerals 2020, 10, 597 7 of 83

Figure 4. In East Africa and Madagascar rubies have b beeneen and are produced fromfrom both primary and secondary deposits.deposits. The The most most important important deposits deposits are those are located those in located the Neoproterozoic in the Neoproterozoic Metamorphic MozambiqueMetamorphic Belt.Mozambique Of much lesser Belt. importanceOf much lesser are deposits importance in which are the deposits ruby is entrainedin which asthe xenocrysts ruby is inentrained Cenozoic as xenocrysts alkali basalts; in Cenozoic these are alkali the Baringobasalts; these and Antanifotsyare the Baringo (Soamiakatra) and Antanifotsy deposits (Soamiakatra) in Kenya anddeposits Madagascar, in Kenya respectively. and Madagascar, (A) Desilicated respectively. pegmatite (A) Desilicated in ultramafic pegmatite rocks in of ultramafic the Rockland rocks (ex-John of the Saul)Rockland mine (ex-John in the Mangare Saul) mine area. in the Photo Mangare width area 7 cm.. Photo Photo: width Gaston 7 cm. Giuliani; Photo: Gaston (B) a ruby Giuliani; hexagonal (B) a prismruby hexagonal (5 cm across) prism in anyolite(5 cm across) rock (with in anyolite zoisiterock (with and blackgreen amphibole) zoisite andfrom black Longido, amphibole) Tanzania. from Photo:Longido, Gaston Tanzania. Giuliani; Photo: (C )Gaston rubies Giuliani; and two ( coloredC) rubies and two (3 colored cm across) sapphires from (3 the cm Winza across) deposit from inthe Tanzania, Winza deposit which in occurs Tanzania, in metamorphosed which occurs in leucogabbro metamorphosed and amphibolite.leucogabbro and Photo: amphibolite. Vincent Pardieu; Photo: (VincentD) ruby Pardieu; (5 cm long) (D) ruby in metasomatized (5 cm long) in amphibolite metasomatized at M’sawize. amphibolite Photo: at M’sawize. Vincent Pardieu Photo: Vincent© GIA; (PardieuE) corundum© GIA; (E (1) pink cm across) corundum from placer(1 cm atacross) Bemainty, from Madagascar. placer at Bemainty, Photo: Vincent Madagascar. Pardieu ©Photo:GIA; andVincent (F) rubyPardieu and sapphires© GIA; and recovered (F) ruby from and placersapphires at Didy, recovered Madagascar. from placer Photo: at Vincent Didy, Pardieu.Madagascar. Photo: Vincent Pardieu. 3. The Mineralogical, Physical, and Chemical Properties of Ruby 3. The Mineralogical, Physical, and Chemical Properties of Ruby 3.1. Mineralogical and Physical Properties

3.1. MineralogicalCorundum( andα-Al Physical2O3) is trigonal Properties and crystallizes in R3c with approximate unit-cell parameters a = 4.76 Å and c = 12.99 Å [38]. In corundum the oxygen atoms are in a hexagonal Corundum (α-Al2O3) is trigonal and crystallizes in space group 𝑅3𝑐 with approximate unit-cell close packing arrangement corresponding to an ABAB stacking of alternate anion layers (Figure 5). parameters a = 4.76 Å and c = 12.99 Å [38]. In corundum the oxygen atoms are in a hexagonal close This arrangement creates octahedral sites of which 2/3 are occupied by Al as suggested by the packing arrangement corresponding to an ABAB stacking of alternate anion layers (Figure 5). This cation-anion ratio in the formula. Each Al atom is coordinated by three oxygen atoms from layer A and arrangement creates octahedral sites of which 2/3 are occupied by Al as suggested by the cation-anion ratio in the formula. Each Al atom is coordinated by three oxygen atoms from layer A and three from layer B. The structure is thus based on pairs of edge-sharing octahedra in sheets parallel to {0001}. In Minerals 2020, 10, 597 8 of 83 three from layer B. The structure is thus based on pairs of edge-sharing octahedra in sheets parallel to {0001}. In each pair, one octahedron shares a face with an octahedron in the sheet below, while the each pair, one octahedron shares a face with an octahedron in the sheet below, while the other other octahedronoctahedron has a a shared shared face face with with an octahedron an octahedron in the sheet in the above sheet [39]. above [39].

Figure 5.FigureThe 5. crystal The crystal structure structure of corundum.of corundum. ((AA)) A single single layer layer of spheres of spheres representing representing oxygen oxygen atoms atoms in hexagonal close packed coordination. In a second layer the spheres would be oriented over the B in hexagonal close packed coordination. In a second layer the spheres would be oriented over the B locations, and a third layer would be over the first (A locations); (B) six oxygen atoms coordinate Al locations, and a third layer would be over the first (A locations); (B) six oxygen atoms coordinate Al atoms at the C positions forming an octahedron (the tetrahedral coordinated site is vacant). Only two atoms atout the of Cevery positions three Al forming sites are occupied an octahedron in the Al (the layers; tetrahedral (C) the crystal coordinated structure of site corundum is vacant). looking Only two out of everydown threec; and Al(D) sites perspective are occupied view of the in thecrystal Al structure layers; ( Cof) corundum the crystal [5,40]. structure of corundum looking down c; and (D) perspective view of the crystal structure of corundum [5,40]. If there are no cation substitutions in the structure, corundum is colorless. However when If therechromophorous are no cationelements substitutions replace aluminum in in the the structure, octahedral site, corundum the crystal is colored. colorless. The substitution However when chromophorousof Al3+ by Cr elements3+ results replacein pink to aluminum red , independing the octahedral on the Cr site, content. the crystal The pink is colored.corundumThe variety substitution is called “pink sapphire” or “pink ruby”, and the red variety, with higher Cr contents (0.1 < Cr2O3 < 3.0 wt of Al3+ by Cr3+ results in pink to red colors, depending on the Cr content. The pink corundum variety %; [5]), is called “ruby”. However, the distinction between these two varieties is not yet well defined and is calledis “pink debated sapphire” by gem dealers or “pink and laboratories. ruby”, and As the such, red in variety, the present with work, higher pink corundum Cr contents crystals (0.1 that< Cr are2O 3 < 3.0 wt %; [5associated]), is called with “ruby”. deep red However, ruby in some the deposits distinction worldwide between will be these considered two varieties rubies from is not here yet onward. well defined and is debatedConcentrations by gem of 9.4 dealers wt % Cr and2O3 were laboratories. measured in As ruby such, from in Kareli the presenta in Russia work, [41] and pink up to corundum 13 and 13.4 crystals that arewt associated % respectively, with in deep ruby redfrom ruby Westland in some in New deposits Zealand worldwide [42], and in ruby will in beclusions considered in diamond rubies from from here placers associated with the Juina kimberlite [43]. onward. Concentrations of 9.4 wt % Cr2O3 were measured in ruby from Karelia in Russia [41] and up The optical absorption spectrum of ruby shows two large absorption bands (Figure 6) with to 13 and 13.4 wt % respectively, in ruby from Westland in New Zealand [42], and in ruby inclusions in transmission windows at 480 nm (blue visible light) and at 610 nm (red visible light). diamond fromThe placersmost desired associated color for rubies with is the known Juina in the kimberlite trade as “pigeon’s [43]. blood”. While there is no globally Theaccepted optical definition absorption for it, spectrumit is usually ofused ruby for st showsones with two a vivid large red absorption color with sometimes bands (Figure a small 6) with transmissionamount windows of blue (just at enough 480 nm to (bluebalance visible the light) from and the at gold 610 on nm which (red the visible stone might light). be set). The Thepresence most desiredof Cr at the color octahedral for rubies site also is knowngives rise in to thestrong trade fluorescence as “pigeon’s when the blood”. stone is While exposed there to is no globallyultraviolet accepted light definition (365 nm). for The it, intens it is usuallyity of fluorescence used for is stones a function with of aCr vivid concentration red color and with the sometimesCr/Fe a ratio [43], because the presence of Fe or an excess of Cr tends to eliminate or quench the fluorescence in small amountruby. In ofsome blue stones, (just the enough fluorescence to balance can even the be yellowseen when from exposed the goldto the onultraviolet which component the stone in might be set). Thesunlight, presence and this of Cr makes at the the ruby’s octahedral color more site intense, also gives increasing rise toits value. strong This fluorescence fluorescence is when sometimes the stone is exposedobserved to in small lightpinkish (365 rubies, nm). from The marble intensity deposits, of fluorescencethat contain little is ato function no iron, including of Cr concentration those and thetraditionally Cr/Fe ratio from [43 ],Myanmar because (Mog the presenceok, Mong ofHsu, Fe and or anNamya), excess and of Crrecently tends from to eliminate Vietnam and or quench the fluorescenceAfghanistan in [44]. ruby. Some In rubies some from stones, amphibolite the fluorescence (Mozambique) canand alkali-basalts even be seen (Thailand) when have exposed also to the been described as “pigeon’s blood” in laboratory reports but this is controversial because most reputable ultraviolet component in sunlight, and this makes the ruby’s color more intense, increasing its value. laboratories limit their use of the term to strongly fluorescent rubies (which means the low-iron type, This fluorescence is sometimes observed in small pinkish rubies, from marble deposits, that contain little to no iron, including those traditionally from Myanmar (Mogok, Mong Hsu, and Namya), and recently from Vietnam and Afghanistan [44]. Some rubies from amphibolite (Mozambique) and alkali-basalts (Thailand) have also been described as “pigeon’s blood” in laboratory reports but this is controversial because most reputable laboratories limit their use of the term to strongly fluorescent Minerals 2020, 10, 597 9 of 83 rubies (which means the low-iron type, which is typically from marble-type deposits). The intensity of fluorescence is a function of the Fe content of ruby and three categories are distinguished: Burma which is typically from marble-type deposits). The intensity of fluorescence is a function of the Fe content Pigeon’swhichof ruby Blood is and typically Strongthree fromcategories Fluorescent, marble-type are distinguished: Mozambique deposits). The Burma Typeintensity Pigeon’s I Pigeon’s of fluorescence Blood Blood Strong is Fluorescent, a Fluorescent, function of andthe Mozambique Fe Mozambique content TypeofType II ruby Pigeon’s I Pigeon’s and three Blood Blood categories Fluorescent Fluorescent, are distinguished: [and44] Mozambique (Figure Burma7). Type Pigeon’s II Pigeon’s Blood Blood Strong Fluorescent Fluorescent, [44] Mozambique (Figure 7). Type I Pigeon’s Blood Fluorescent, and Mozambique Type II Pigeon’s Blood Fluorescent [44] (Figure 7).

FigureFigure 6. Ultraviolet, 6. Ultraviolet, visible, visible, and and near near infrared infrared (UV-Vis-NIR)(UV-Vis-NIR) spectra spectra of ofrubies rubies from from the theManinge Maninge Nice Nice Figureand Mugloto 6. Ultraviolet, deposits, visible, Mozambique; and near modifiedinfrared (UV-Vi from [26].s-NIR) The spectra three spectra of rubies show from the the Cr Maninge3+ absorption Nice and Mugloto deposits, Mozambique; modified from [26]. The three spectra show the Cr3+ absorption andbands Mugloto at 410 and deposits, 560 nm, Mozambique; and the Cr3+ modified“doublet” from at 694 [26]. nm. The Spectrum three spectra (A) of ashow ruby the from Cr Mugloto3+ absorption also bands at 410 and 560 nm, and the Cr3+ “doublet” at 694 nm. Spectrum (A) of a ruby from Mugloto bandsshows atthe 410 shoulder and 560 at nm, 330 and nm thebut Cr in3+ addition “doublet” a single at 694 Fe nm.3+ feature Spectrum at 377, (A) 388,of a rubyand 450 from nm Mugloto that reflects also 3+ also shows the shoulder at 330 nm but in addition a single3+ Fe feature at 377, 388, and 450 nm that showsa higher the Fe shoulder content inat 330this nmruby. but Spectrum in addition (B )a of single a ruby Fe from feature Maninge at 377, Nice 388, showsand 450 a nmshoulder that reflects at 330 reflectsanm higher a caused higher Fe bycontent Fe Fe content3+ pairs.in this in Spectrum thisruby. ruby. Spectrum (C Spectrum) of a(B ruby) of (a Bfrom ruby) of aMugloto from ruby Maninge from differs Maninge Nice from shows the Nice others a showsshoulder mainly a shoulder at by 330 a at 3+ 330 nmnmFe2+ caused caused Ti4+ bychargeby FeFe3+ transfer pairs.pairs. Spectrum band Spectrum centered (C ()C of) at ofa 580ruby a ruby nm from and from Mugloto increasing Mugloto differs abso diff rptionfromers from the above others the 600 others mainlynm. mainly by a by a Fe2+ Fe2+Ti 4+ Ticharge4+ charge transfer transfer band band centeredcentered at at 580 580 nm nm and and increasing increasing abso absorptionrption above above 600 nm. 600 nm. →

Figure 7. FeO versus Cr2O3 diagram of different pigeon’s blood color of rubies worldwide (modified Figure 7. FeO versus Cr2O3 diagram of different pigeon’s blood color of rubies worldwide (modified Figurefrom 7. FeO[44]. versusThe color Cr of2O the3 diagram gem is defined of diff erentas vivid pigeon’s with high blood intensity color and ofrubies low tone. worldwide The distinction (modified fromfromof [44 rubies]. [44]. The betweenThe color color of the Myanmarof the gem gem is Pigeon’s definedis defined asBlood as vivid vivid Stro with withng Fluorescent high high intensityintensity (in and marble low low fromtone. tone. TheMyanmar The distinction distinction and of of rubies between Myanmar Pigeon’s Blood Strong Fluorescent (in marble from Myanmar and rubiesVietnam), between Mozambique Myanmar Pigeon’s Type I BloodPigeon’s Strong Blood Fluorescent Fluorescent, (in and marble Mozambique from Myanmar II Pigeon’s and Blood Vietnam), Vietnam),Fluorescent Mozambique (both in amphibolite) Type I Pigeon’s is correlated Blood with Fluorescent, the amount and of Fe Mozambique in the ruby, respectivelyII Pigeon’s BloodFeO < Mozambique Type I Pigeon’s Blood Fluorescent, and Mozambique II Pigeon’s Blood Fluorescent Fluorescent0.1 wt %; 0.1 (both < FeO in < amphibolite)0.35 wt %; and is correlated0.35 < FeO with< 0.6 thewt %amount (data fromof Fe [44]). in the ruby, respectively FeO < (both in amphibolite) is correlated with the amount of Fe in the ruby, respectively FeO < 0.1 wt %; 0.1 wt %; 0.1 < FeO < 0.35 wt %; and 0.35 < FeO < 0.6 wt % (data from [44]). 0.1 < FeO < 0.35 wt %; and 0.35 < FeO < 0.6 wt % (data from [44]). Minerals 2020, 10, 597 10 of 83

Euhedral crystals of ruby can exhibit several faces (Figure 8) that correspond to the following Euhedral crystals of ruby can exhibit several faces (Figure 8) that correspond to the following crystalline forms [38]: pinacoid {0001}, first-order hexagonal prism {1010}, second-order hexagonal crystallineEuhedral forms crystals [38]: pinacoid of ruby { 0001},can ex first-orderhibit several hexagonal faces (Figure prism {10 8)1 th0},at second-order correspond hexagonalto the following prism prism{11crystalline {1120},2 0},hexagonal hexagonal forms [38]:dipyramid pinacoid dipyramid {hh {20001},hl}, {hh rhombohedron first-order2hl}, rhombohedron hexagonal {h0ℎ𝑙 }, andprism {h0 ditrigonalhl {10},1 and0}, second-order scalenohedron ditrigonal scalenohedronhexagonal {hkil}. prism {hkil}. {1120}, hexagonal dipyramid {hh2hl}, rhombohedron {h0ℎ𝑙 }, and ditrigonal scalenohedron {hkil}.

FigureFigure 8. Crystalline 8. Crystalline forms forms of the of32 the/m class32/m of class the rhombohedral of the rhombohedral system, aftersystem, [ 38 ].after (A) positive[38]. (A) rhombohedron; positive (B) negativerhombohedron;Figure 8. rhombohedron; Crystalline (B) negative forms (C ofrhombohedron;) hexagonalthe 32/m class dipyramid; (C of) hexagonal the rhombohedral (D) dipyramid; pinacoid; system, (ED)) first-orderpinacoid; after [38]. (E hexagonal)( Afirst-order) positive prism; (F) second-orderhexagonalrhombohedron; prism; hexagonal (B ()F negative) second-order prism; rhombohedron; (G) dihexagonal hexagonal (C prism; ) hexagonal and(G) (dihexagonalHdipyramid;) ditrigonal (prism;D scalenohedron.) pinacoid; and ( H(E)) ditrigonalfirst-order scalenohedron.hexagonal prism; (F) second-order hexagonal prism; (G) dihexagonal prism; and (H) ditrigonal Rubiesscalenohedron. from Mong Hsu in Myanmar show a number of different habits (Figure 9)[30,45] and are boundedRubies by pinacoidal from Mong {0001} Hsu in faces Myanmar and hexagonal show a number dipyramidal of different faces habits {hh (Figure2hl}, sometimes 9) [30,45] and associated are boundedRubies by pinacoidalfrom Mong {0001} Hsu facesin Myanmar and hexagonal show a dipyramidal number of different faces {ℎℎ2 habitsℎ𝑙}, sometimes (Figure 9) associated [30,45] and with are with small rhombohedral {1011} faces. Second-order prism {1020} faces are also observed in some rubies. smallbounded rhombohedral by pinacoidal {101 {0001}1} faces. faces Second-order and hexagonal prism dipyramidal {1020} faces faces are { ℎℎ2alsoℎ𝑙 observed}, sometimes in some associated rubies. withThe The smallersmall rhombohedral dipyramidal dipyramidal faces{101 faces1 are} faces. { are224 Second-order {223}, whereas43}, whereas the prism larger the {102 dipyramidal larger0} faces dipyramidal are onesalso observedare often ones inreported are some often rubies. as reported{448 The1} as {4481}orsmaller or{14 {14 14 dipyramidal 1428 28 3}, 3},or less or facesless commonly commonlyare {224 {2243}, 1whereas {22} or4 {1}112 or the1}. {11 Thelarger21}. {14 Thedipyramidal 14 28 {14 3} 14 indices28 ones3} are indicesare too often high are reported for too smooth high as { for448faces, smooth1} faces,andor and {14 it could14 it could28 be 3}, possible or be less possible commonly that they that cons {224 theyist1 }of or consist alternating {1121}. of The alternatingmi {14crosteps 14 28 between 3} microsteps indices {112 are0 too} between and high {0001} for {11smooth faces20} [46,47]. andfaces, {0001} facesRubiesand [46 ,it47 could from]. Rubies beMogok possible from are that Mogokgenerally they arecons tabular generallyist of alternatingwith tabularpredominant microsteps with pinacoids predominant between and {112 rhombohedral pinacoids0} and {0001} and faces{101 rhombohedral [46,47].1} and {1011}dipyramidalRubies and dipyramidalfrom {Mogok1121} faces.are {11 generally2 Other1} faces. habits tabular Other (Figure with habits predominant9) have (Figure been pinacoids9reported) have been forand marble-hosted rhombohedral reported for rubies { marble-hosted1011 }from and rubiesMorogorodipyramidal from Morogoro in Tanzania{1121} faces. in as Tanzaniaflattened Other habitsto as tabular flattened (Figure pseudo-c 9) to have tabularubes, been pseudocubes pseudo-cubes, reported accordingfor marble-hosted pseudocubes to the relative rubies according size from of to the relativetheMorogoro six rhombohedral size in Tanzania of the six faces as rhombohedralflattened and six additionalto tabular faces pseudo-c prism and faces sixubes, { additional102 pseudocubes1} with a prism lath-like according faces form to {10 [48],the2 1}relative and with Jegdalek size a lath-like of inthe Afghanistan six rhombohedral [5]. Rubies faces in andalkali six basalt additional from Tha prismiland faces and { 102Cambodia1} with are a lath-like tabular andform have [48], combinations and Jegdalek form [48], and Jegdalek in Afghanistan [5]. Rubies in alkali basalt from Thailand and Cambodia are ofin rhombohedralAfghanistan [5]. {101 Rubies1} and in alkalipinacoid basalt {0001} from faces Tha iland(Figure and 9). Cambodia are tabular and have combinations tabularof rhombohedral and have combinations {1011} and pinacoid of rhombohedral {0001} faces (Figure {1011} 9). and pinacoid {0001} faces (Figure 9).

Figure 9. Main habits of ruby from different marble-hosted deposits: Mong Hsu and Mogok in FigureMyanmar,Figure 9. Main9. MainMorogoro habits habits ofin ofTanzania, ruby ruby from from and didifferent Jegdalekfferent marble-hostedmarble-hosted in Afghanistan. deposits: deposits:The rubies Mong Mongfrom Hsu the Hsu andPailin andMogok deposit Mogok in in Myanmar,(Cambodia)Myanmar, Morogoro Morogoro are from in alkali-basalt Tanzania,in Tanzania, andpl andacer Jegdalek Jegdalek deposits. in From Afghanistan. [5,30,45,48]. The The rubies rubies from from the thePailin Pailin deposit deposit (Cambodia) are from alkali-basalt placer deposits. From [5,30,45,48]. (Cambodia) are from alkali-basalt placer deposits. From [5,30,45,48]. Minerals 2020, 10, 597 11 of 83

To explain these morphological differences, Sorokina et al. [49] introduced the concept of crystal “ontogeny”To thatexplain encompasses these morphological nucleation, differences, growth, Sorokina and alteration et al. [49] of introduced single crystals. the concept According of crystal to their data,“ontogeny” the variations that encompasses of crystal habits nucleation, in rubies growth, from and deposits alteration in of Africasingle crystals. and South-East According Asiato their can be data, the variations of crystal habits in rubies from deposits in Africa and South-East Asia can be directly correlated with Cr and Fe contents and changes in P–T conditions during crystal growth. directly correlated with Cr and Fe contents and changes in P–T conditions during crystal growth. 3.2. The Trapiche Texture of Ruby 3.2. The Trapiche Texture of Ruby The conditionsThe conditions under under which which the the ruby ruby growsgrows can give give rise rise toto a particular a particular texture texture called called ‘trapiche’ ‘trapiche’ (Figure(Figure10A). 10A). This textureThis texture was firstwas describedfirst described by [50 by] for [50] an for unusual an unusual Colombian Colombian emerald that resemblesthat the spokesresembles of a the milling spokes wheel of a milling used to wheel process used sugar to process cane. Thesugar term cane. “trapiche” The term was“trapiche” used exclusivelywas used for emeraldsexclusively until the for1990s, when until trapiche the 1990s, rubies when appeared trapiche rubies in the Thaiappeared and Burmesein the Thai gem and marketsBurmese [gem47,51 ,52]. Themarkets trapiche [47,51,52]. texture is composed of growth sectors separated by more or less sharp boundaries of inclusionsThe trapiche [53,54], texture known is composed as “sector of boundaries”. growth sectors separated These boundaries by more or separateless sharp aboundaries central “core” (correspondingof inclusions to [53,54], the pinacoidal known as growth “sector sectors) boundaries”. from the These surrounding boundaries growth separate sectors. a central The boundaries“core” are generally(corresponding inclined to withthe pinacoidal respect to thegrowth c axis. sector Thiss) texturefrom the should surrounding not be confused growth withsectors. that The seen in boundaries are generally inclined with respect to the c axis. This texture should not be confused with -pink and blue sapphires (Figure 10B) and resulting from the distribution of different inclusions that seen in purple-pink and blue sapphires (Figure 10B) and resulting from the distribution of in alternatingdifferent inclusions portions in of alternating the crystal portions and called of the “trapiche-like” crystal and called [55 “trapiche-like”,56]. [55,56].

FigureFigure 10. The10. The trapiche trapiche texture texture of corundum. (A ()A Photomicrograph) Photomicrograph in transmitted in transmitted light of a light trapiche of a ruby trapiche rubyfrom from Mong Mong Hsu, Hsu, Myanmar Myanmar (Photo: (Photo: Virginie Virginie Garnier) Garnier) and (B and) aspect (B) of aspect a “trapiche-like” of a “trapiche-like” sapphire from sapphire fromsouthern southern Africa Africa (2 cm (2 × 1.8 cm cm ×1.8 0.2 cm, cm 13.460.2 cts). cm, Photo: 13.46 Louis-Dominique cts). Photo: Bayle Louis-Dominique © le Règne Minéral. Bayle © le × × Règne Minéral. The trapiche texture is best seen in cross-sections perpendicular to the c axis. The presence and Thesize of trapiche the core texture depends is on best the seen orientation in cross-sections of the cross-section perpendicular in the crystal to the [57]c axis.(Figure The 11A). presence Thus, and two cross-section appearances are possible: (1) the core is present and the sector boundaries surround size of the core depends on the orientation of the cross-section in the crystal [57] (Figure 11A). Thus, it and develop from its edges to the rim of the crystal, separating six trapezoidal growth sectors two cross-section appearances are possible: (1) the core is present and the sector boundaries surround (Figure 11B) and (2) the core is absent and the sector boundaries intersect at a central point, giving it andrise develop to triangular from growth its edges sectors to the(Figure rim 11C). of the crystal, separating six trapezoidal growth sectors (Figure 11VariableB) and (2)contents the core of ischromophores, absent and the in particular sector boundaries Cr but also intersect Ti and atV ahave central been point, detected giving in rise to triangulardifferent growthrubies as sectors well as (Figurewithin the11 C).same samples [52,58,59]. This can explain why the core can be Variablered (Cr > Ti) contents or blackof (Ti chromophores, > Cr) [60], although in the particular core is sometimes Cr but also yellowish/white-to- Ti and V have been like detected the in differentsector rubies boundaries. as well For as both within core theand same sector samples boundaries, [52 ,the58, yellowish/white-to59]. This can explain brown why color the is coredue to can be red (Crthe> presenceTi) or black of , (Ti > dolomite,Cr) [60], althoughcorundum, the uniden coretified is sometimes silicate inclusions yellowish containing/white-to-brown K-Al-Fe-Ti, like the sectorand boundaries. Fe-bearing minerals. For both Iron core /hydroxides and sector boundaries, formed theduring yellowish late weathering/white-to that brown was favored color is by due to the high porosity of the sector boundaries (Figure 12) [52,57]. The sector boundaries develop along the presence of calcite, dolomite, corundum, unidentified silicate inclusions containing K-Al-Fe-Ti, and the ‹110› directions and can contain other solid inclusions such as plagioclase, , titanite, Fe-bearing minerals. Iron oxides/hydroxides formed during late weathering that was favored by the margarite, , etc. [57–59], but they do not contain matrix material from the host rock. This highabsence porosity is ofan theimportant sector feature boundaries differentiating (Figure trapiche12)[52,57 rubies]. The from sector trapiche boundaries emeralds develop [61]. along the ‹110› directions and can contain other solid inclusions such as plagioclase, rutile, titanite, margarite, zircon, etc. [57–59], but they do not contain matrix material from the host rock. This absence is an important feature differentiating trapiche rubies from trapiche emeralds [61]. Minerals 2020, 10, 597 12 of 83

FigureFigure 11. VariationsVariations 11. Variations of the of the ruby ruby trapiche trapiche texture. texture. (( AA)) Variation Variation of of of the the the core, core, core, which which corresponds corresponds to the to the pinacoidal growth sectors (in ). These sectors have a tapered shape and the core size varies as a pinacoidal growth sectors (in grey). These These sectors sectors have have a a tapered tapered shape shape and the core size varies as a function of the orientation of the cross-section; (B) aspect of the section near the vertex of the pyramid; function of the the orientation orientation of the the cross-section; cross-section; ( B) aspect aspect of of the the section section near near the the vertex vertex of of the the pyramid; pyramid; and (C) aspect of the section if cut in the vertex plane. Photos by Isabella Pignatelli of two trapiche and (C) aspect of the section if cut in the vertex plane. Photos by Isabella Pignatelli of two trapiche and (Crubies) aspect from of Mong the sectionHsu, Myanmar. if cut in the vertex plane. Photos by Isabella Pignatelli of two trapiche rubies from Mong Hsu, Myanmar.

Figure 12. X-ray computed tomography image showing the high porosity (in black) of a trapiche ruby from Luc Yen, North Vietnam [57]. The porosity corresponds to 0.6% of the ruby volume and is developed in the sector boundaries. Photo: Christophe Morlot. Figure 12. 12. X-rayX-ray computed computed tomography tomography image image showing showing the thehigh high porosity porosity (in black) (in black) of a trapiche of a trapiche ruby fromruby Tube-like fromLuc Yen, Luc Yen,voidsNorth North filled Vietnam Vietnamwith [57].carbonates, [57 The]. The porosity liquid, porosity and/orcorresponds corresponds gas may to tobe0.6% 0.6%observed of of the the inruby ruby the volumegrowth volume sectorsand and is developed(Figure 13) in [51,57,62]. the sector Theseboundaries. can develop Photo: Christophefrom Christophe the core Morlot. Morlot. or sector boundaries and extend into the growth sectors, running perpendicular to the sectors with an inclination of 5° relative to the {0001} Tube-likeruby faces. voids Sometimes filledfilled the with tube-like carbonates, voids can liquid, be parallel and/or and/or to gas the may c axis. be observed in the growth sectors (Figure 1313)Garnier)[ 51[51,57,62].,57 ,et62 al.]. These[58,59] These found can can develop single-phasedevelop from from (liquid), the the core two-phascore or sector or esector (liquid boundaries boundaries+ gas), and three-phase extend and extend into (liquid the into growth + the gas + solid) inclusions between the tube-like voids and/or their extensions into the sector boundaries. growthsectors, sectors, running running perpendicular perpendicular to the sectors to the with sectors an inclination with an inclination of 5◦ relative of 5° to relative the {0001} to rubythe {0001} faces. rubySometimes Microthermometryfaces. Sometimes the tube-like and the voids Ramantube-like can spectrometry be voids parallel can indicate tobethe parallel cthat axis. these to the inclusions c axis. correspond to the trapping of two immiscible fluids during the ruby formation: a carbonic fluid in the CO2-H2S-COS-S8-AlO(OH) Garnier etet al.al. [[58,59]58,59] foundfound single-phasesingle-phase (liquid),(liquid), two-phasetwo-phase (liquid(liquid ++ gas), and three-phase (liquid + system and molten salts [62–64]. These fluids are considered the product of metamorphism of evaporites gas + solid) inclusions between the tube-like voids and/or their extensions into the sector boundaries. gas +duringsolid) the inclusions devolatization between of carbonates the tube-like and thermochemical-sulfate voids and/or their extensions reduction. into the sector boundaries. Microthermometry and Raman spectrometry indicate that these inclusions correspond to the trapping of two immiscibleimmiscible fluids fluids during during the the ruby ruby formation: formation: a carbonic a carbonic fluid influid the COin 2the-H 2COS-COS-S2-H2S-COS-S8-AlO(OH)8-AlO(OH) system systemand molten and molten salts [62 salts–64]. [62–64]. These fluidsThese are fluids considered are cons theidered product the product of metamorphism of metamorphism of evaporites of evaporites during duringthe devolatization the devolatization of carbonates of carbonates and thermochemical-sulfate and thermochemical-sulfate reduction. reduction. Minerals 2020, 10, 597 13 of 83

Trapiche rubies are found in Myanmar,Myanmar, Vietnam,Vietnam, Tajikistan,Tajikistan, Pakistan,Pakistan, and Nepal, but have never been described by geologists in situ in primaryprimary deposits.deposits. The trace element contents of the Myanmar and Vietnamese trapiche rubies prove thatthat theythey originatedoriginated fromfrom marble-typemarble-type depositsdeposits [[57–59].57–59]. Moreover, similar mineralogical mineralogical and and chemical chemical features features have have been been observed observed for trapiche for trapiche and andnon-trapiche non-trapiche rubies rubies from fromMong MongHsu in Hsu Myanmar in Myanmar and Luc and Yen Luc in Vietnam, Yen in Vietnam, confirming confirming that they thatformed they in formed the same in geological the same geologicalcontext, i.e., context, marble-hosted i.e., marble-hosted ruby deposits ruby [57,58,62,65]. deposits [57 The,58 rubies,62,65 ].formed The rubies in the formed amphibolite in the facies, amphibolite during facies,retrograde during metamorphism retrograde metamorphism at 620 < T < 670 at°C 620and< 2.6T < P670 < 3.3◦C kbar and [65,66]. 2.6 < P < 3.3 kbar [65,66].

Figure 13. Solid and fluid fluid inclusions trapped by trapiche rubies. ( A) Primary fluid fluid inclusions trapped along the sector boundary of a trapichetrapiche rubyruby fromfrom MongMong Hsu,Hsu, Myanmar.Myanmar. The fluidfluid was trapped withinwithin tube-liketube-like structuresstructures formedformed byby dolomitedolomite (Dol).(Dol). ItIt is is a multi-phase fluidfluid inclusioninclusion containing a two-phase (liquid + vapor) CO2-H2S-S8-bearing fluid and minerals including diaspore containing a two-phase (liquid + vapor) CO2-H2S-S8-bearing fluid and minerals including diaspore (Dsp)(Dsp) andand rutilerutile (Rt).(Rt). VV = vapor phase, l = liquidliquid phase; from [[62].62]. Photo: Gaston Giuliani; ( B) X-ray computed tomographytomography image image of of a trapichea trapiche ruby ruby from from Luc Luc Yen, Yen, north north Vietnam, Vietnam, showing showing tube-like tube-like voids thatvoids contain that contain solid andsolid fluid and inclusion fluid inclusion cavities; cavities; from [ 57from]. Photo: [57]. Photo: Christophe Christophe Morlot. Morlot.

In this geologicalgeological context, the trapiche textur texturee developed very quickly, as witnessed by the trapping of severalseveral solidsolid andand fluidfluid inclusionsinclusions andand thethe formationformation ofof tube-liketube-like voids.voids. The growthgrowth episodes in in minerals minerals are are due due to to changes changes in inthe the driv driving-forceing-force conditions conditions [47]. [ 47In ].solution In solution growth, growth, mass masstransfer transfer plays a plays key role. a key The role. driving The force driving is defi forcened isby definedthe difference by the in di concentrationfference in concentration between the betweenbulk and thesubsurface bulk and supersaturation, subsurface supersaturation, i.e., the concentration i.e., the concentration gradient within gradient the boundary within the layer boundary [46]. layerThree [growth46]. Three episodes growth due episodes to changes due toin driving changes forc in drivinge conditions force were conditions identifi wereed by identified [47] (Figure by [14).47] (FigureFirstly, 14the). Firstly,core formed the core under formed low under driving-force low driving-force conditions conditions via layer-by-layer via layer-by-layer growth growth (Step 1). (Step Its 1).development It s development was hindered was hindered by the byrapid the formation rapid formation of sector of sectorboundaries, boundaries, which whichenveloped enveloped it and itconstitute and constitute the “skeleton” the “skeleton” of the trapiche of the trapiche ruby (Step ruby 2). (Step These 2). boundaries These boundaries formed when formed the when driving- the driving-forceforce conditions conditions increased increased and adhesive-type and adhesive-type growth took growth place. took Finally, place. a Finally, decrease a decreasein the driving- in the driving-forceforce conditions conditions allowed allowedthe formation the formation of growth of sectors growth and sectors the corresponding and the corresponding crystal faces crystal through faces throughordinary ordinarylayer-by-layer layer-by-layer growth (Step growth 3). (StepThe growth 3). The growthsectors filled sectors the filled interstices the interstices left by leftthe bysector the sectorboundary, boundary, completing completing the trapiche the trapiche texture. texture.

3.3. Chemical Composition an andd Geographic Origin of Ruby Currently, the the accurate accurate chemical chemical composition composition of rubies of rubies can be candetermined be determined by several by analytical several analyticaltechniques, techniques, such as X-ray such fluorescence as X-ray fluorescence (XRF), elec (XRF),tron microprobe electron microprobe analysis (EMPA), analysis laser (EMPA), ablation- laser ablation-inductivelyinductively coupled plasma-mass coupled plasma-mass spectrometry spectrometry (LA-ICP-MS), (LA-ICP-MS), and secondary and secondary mass spectrometry ion mass spectrometry(SIMS). Chemical (SIMS). data Chemical facilitate data the facilitate differentiation the differentiation between natural between and natural synthetic and synthetic rubies [67] rubies (Figure [67] (Figure15) and15 help) and to helpdetermine to determine their geological their geological origin originworldwide worldwide [11,68–72]. [11,68 –72]. Minerals 2020, 10, 597 14 of 83

Figure 14. Schematic diagram showing growth rate versus driving force of the three development Figure 14. Schematic diagram diagram showing showing growth growth rate vers versusus driving force of the three development stagesstages of trapiche of trapiche ruby. ruby. The The diagram diagram shows shows the the didifffferenterent types of of crystal crystal growth growth mechanisms, mechanisms, modified modified stagesfrom of trapiche [47]: ruby. growth, The diagram two-dimensional shows the diffgrowerentth, andtypes adhesive-type of crystal growth growth. mechanisms, The increasing modified from [47]: spiral growth, two-dimensional growth, and adhesive-type growth. The increasing intensity from intensity[47]: spiral of the growth, driving forcetwo-dimensional is presented betweengrowth, the and critical adhesive-type points X and growth. Y, where The nucleation increasing of the driving force is presented between the critical points X and Y, where nucleation occurred. intensityoccurred. of the The driving curves representforce is presentedgrowth rate between versus driving the critical force relations points forX andthe threeY, where mechanisms. nucleation The curves represent growth rate versus driving force relations for the three mechanisms. The curve occurred.The curve The (incurves blue) representshows the spiral growth growth rate mechanism; versus driving the curve force (in red)relations represents for the two-dimensional three mechanisms. (in blue) shows the spiral growth mechanism; the curve (in red) represents the two-dimensional The curvenucleation (in blue) growth shows mechanism; the spiral thgrowthe curve mechanism; (in green) denotes the curve the (inadhesive-type red) represents mechanism. the two-dimensional A rough nucleation growth mechanism; the curve (in green) denotes the adhesive-type mechanism. A rough nucleationinterface growth is expected mechanism; above thpointe curve Y and (in itgreen) corresponds denotes to the the adhesive-type dendritic stage mechanism. of trapiche-ruby A rough interface is expected above point Y and it corresponds to the dendritic stage of trapiche-ruby formation. interfaceformation. is expected The nucleation above occurredpoint Y betweenand it correX andsponds Y domains to thewhere dendritic the growth stage sectors of formedtrapiche-ruby in The nucleationtrapiche texture. occurred Finally, between a smooth X and interface Y domains is expected where the below growth X where sectors nucleation formed inby trapichedislocation texture. formation. The nucleation occurred between X and Y domains where the growth sectors formed in Finally,occurred a smooth either interface for non-trapiche is expected ruby belowor in overgrowth X where zone nucleation in trapiche by dislocationruby. occurred either for trapiche texture. Finally, a smooth interface is expected below X where nucleation by dislocation non-trapiche ruby or in overgrowth zone in trapiche ruby. occurred either for non-trapiche ruby or in overgrowth zone in trapiche ruby.

Figure 15. Gallium (Ga)–vanadium (V)–iron (Fe) ternary showing the distinction between natural and synthetic rubies, modified from [67].

Sutherland et al. [73–75] used a Fe2O3/TiO2 versus Cr2O3/Ga2O3 diagram to decipher the metamorphic Figure 15. GalliumGallium (Ga)–vanadium (Ga)–vanadium (V)–iron (V)–iron (Fe) ternary showing the distinction between natural and versus magmatic origin of corundum. This diagram is commonly used for the separation of blue synthetic rubies, modified from [67]. syntheticsapphires rubies, from several modified deposits from [67].worldwide [76]. The corundum crystals termed “metamorphic” have Sutherlandpastel (blue, et al.pink, [73 –and75] usedorange) a Fe toO red/TiO colors,versus and Cr areO /richGa Oin diagramchromium to decipherand poor the in metamorphicgallium 22 33 22 22 3 2 33 Sutherland(Cr2O3/Ga2O et3 ratio al. [73–75] > 3). The used corundum a Fe O /TiO crystals versus defined Cr Oas /Ga“magmatic”O diagram are toxenocrysts decipher of the blue-green- metamorphic versus magmatic origin of corundum. This diagram is commonly used for the separation of blue sapphires versusyellow magmatic (BGY) originsapphires of corundum.with a Cr2O This3/Ga2 Odiagram3 ratio < is1. commonlyThe diagram used was forapplied the separation to rubies and of blue from several deposits worldwide [76]. The corundum crystals termed “metamorphic” have (blue, sapphiressapphires from related several to depositsalkali basalt worldwide from the provinces [76]. Th eof corundum crystals (the Soamiakatra, termed “metamorphic” Kianjanakanga, have pink,pasteland and(blue, Ambatomainty ) pink, to and red deposits) colors,orange) and toand arered Toamasina rich colors, in chromium and(the areVatomandry andrich poorin chromiumand in galliumAndilamena and (Cr2 Opoordeposits)3/Ga in2O 3 galliuminratio > 3). The corundum crystals defined as “magmatic” are xenocrysts of blue-green-yellow (BGY) sapphires (Cr2O3/Ga2O3 ratio > 3). The corundum crystals defined as “magmatic” are xenocrysts of blue-green- with a Cr O /Ga O ratio < 1. The diagram was applied to rubies and sapphires related to alkali basalt yellow (BGY)2 3 sapphires2 3 with a Cr2O3/Ga2O3 ratio < 1. The diagram was applied to rubies and sapphiresfrom the provinces related to of alkali Antananarivo basalt from (the the Soamiakatra, provinces of Kianjanakanga, Antananarivo (the and AmbatomaintySoamiakatra, Kianjanakanga, deposits) and and Ambatomainty deposits) and Toamasina (the Vatomandry and Andilamena deposits) in Minerals 2020, 10, 597 15 of 83

ToamasinaMadagascar (the [77,78] Vatomandry (Figure and16). AndilamenaNote that all deposits) of the points in Madagascar related to [77 rubies,78] (Figure plot in16 the). Notemetamorphic that all of thefield. points They related overlap to data rubies for plot rubies in the from metamorphic Andilamena, field. which They is overlap located data in the for north rubies of from the Vatomandry Andilamena, whicharea, and is located where inthe the corundum north of theis hosted Vatomandry in differ area,ent types and where of rocks the (viz. corundum metamorphosed is hosted in mafic different and typesultramafic of rocks rocks (viz. (M-UMR) metamorphosed and plumasites mafic and in ultramafic M-UMR). rocksThe majority (M-UMR) of and the plumasites sapphires inplot M-UMR). in the Themagmatic majority domain. of the The sapphires distribution plot in of the sapphires magmatic from domain. the Vatomandry The distribution area is of heterogeneous, sapphires from and the Vatomandrywhile most are area located is heterogeneous, in the magmatic and while domain most areyet locatedothers inare the in magmaticthe metamorphic domain yetone. others The arelatter in thecorresponds metamorphic to colorless, one. The light latter to corresponds dark green, to and colorless, light lightto purplish to dark blue green, sapphires and light with to purplish significant blue sapphires with significant Cr2O3 contents (150 ppm < Cr2O3 < 2100 ppm). Cr2O3 contents (150 ppm < Cr2O3 < 2100 ppm).

Figure 16.16. CrCr22O3/Ga/Ga22OO33 versusversus Fe Fe22OO3/TiO3/TiO2 2diagramdiagram for for rubies rubies and and sapphires sapphires associated with alkali basalts from the AntananarivoAntananarivo andand ToamasinaToamasina ProvincesProvinces inin Madagascar.Madagascar. The metamorphic and magmatic boxes defineddefined for Australian sapphires areare fromfrom [[68,75].68,75].

Giuliani et et al. al. [72] [72] and and Uher Uher et et al. al. [79] [79] proposed proposed a a new new chemical classification classification diagram for gem corundum basedbased on on the the di differentfferent types types of primaryof primary corundum corundum deposits deposits worldwide worldwide (see [40(see,80 [40,80]).]). It used It aused database a database of 2000 of EMPA 2000 EMPA analyses analyses of ruby of and ruby sapphire and sapphire that were that obtained were obtained under the under same analyticalthe same conditions.analytical conditions. The FeO + TheTiO2 FeO+ Ga +2 OTiO3 versus2 + Ga FeO–Cr2O3 versus2O3 –MgO–VFeO–Cr2O2O3–MgO–V3 diagram2O in3 diagram Figure 17 inuses: Figure (1) the17 FeOuses: content(1) the FeO to discriminate content to discriminate between iron-poor between rubiesiron-poor in marbles rubies in and marbles iron-rich and rubiesiron-rich in rubies M-UMR in andM-UMR (2) the and second (2) the discriminator second discriminator between ruby between and sapphire ruby and is thesapphire addition is tothe (Y-axis addition parameter) to (Y-axis or subtractionparameter) or from subtraction (X-axis parameter) from (X-axis FeO parameter) of trace elements FeO of trace associated elements preferentially associated withpreferentially sapphire (TiOwith2 sapphireand Ga2 (TiOO3) or2 and ruby Ga (Cr2O32) Oor3 ,Vruby2O (Cr3, and2O3, MgO).V2O3, and The MgO). fields The of the fields diff erentof the gemdifferent corundum gem corundum deposits are:deposits for rubies, are: for (R1) rubies, marble; (R1) (R2) marb Johnle; (R2) Saul John mine-type; Saul mine-type; (R3) M-UMR; (R3) (R4)M-UMR; metasomatites; (R4) metasomatites; for sapphires, for (S1)sapphires, syenitic (S1) rocks; syenitic (S2) rocks; metasomatites; (S2) metasomatites; and (S3) alkali-basalt and (S3) alkali-basalt and lamprophyre. and lamprophyre. Finally, the Finally, chemical the diagramchemical permitsdiagram a permits clear separation a clear separation with limited with overlap limited of theoverlap main of types the main of deposit types found of deposit worldwide. found worldwide. Minerals 2020, 10, 597 16 of 83

FigureFigure 17.17. FeO-CrFeO-Cr2O2O3-MgO-V3-MgO-V2O23O versus3 versus FeO FeO + TiO+ TiO2 + 2Ga+2OGa3 2diagramO3 diagram based based on a ondatabase a database of 2000 of 2000electron electron microprobe microprobe analysis analysis (EMPA) (EMPA) analyses analyses (in (in wt wt % %oxides) oxides) showing showing the the main main chemical fieldsfields defineddefined forfor didifferentfferent typestypes ofof corundumcorundum depositsdeposits worldwideworldwide [[72].72]. DiDifferentfferent geochemicalgeochemical domainsdomains areare defineddefined (see(see text).text).

TheThe chemicalchemical diagramdiagram providesprovides susufficientfficient discriminationdiscrimination to distinguishdistinguish between the didifferentfferent typestypes ofof corundum corundum deposits deposits for for the the following following reasons: reasons: (1) the(1) databasethe database contains contains more thanmore 2000 than EMPA 2000 analysisEMPA analysis representative representative of all of of the all types of the of types primary of primary deposits deposits and (2) theand magmatic (2) the magmatic sapphire sapphire domain (S1)domain is defined (S1) is by defined the geology by the and geology chemistry and of chem corundum.istry of Thecorundum. sapphires The are sapphires hosted in syeniticare hosted rocks in orsyenitic occur asrocks megacrysts or occur in as syenitic megacrysts xenoliths in syenitic carried byxenoliths alkali basalts. carried Most by alkali of the basalts. sapphires Most in placers of the linkedsapphires to the in erosionplacers oflinked alkali to basalts the erosion (domain of alkali S2) overlap basalts the (domain domain S2) S1. overlap The main the superpositiondomain S1. The of geologicalmain superposition domains is of for geological ruby of metasomatic domains is (-metamorphic) for ruby of metasomatic origin that overlaps(-metamorphic) the domain origin of rubythat associatedoverlaps the with domain M-UMR of ruby (strictly associated metamorphic); with M-UM (3)R the (strictly diagram metamorphic); uses Ga and (3) Mg the despite diagram their uses high Ga detectionand Mg despite limits, their and thehigh two detection elements limits, are opposed:and the tw Gao elements is added are to Ti opposed: while Mg Ga is is added added to to Cr Ti andwhile V. ThisMg is choice added corresponds to Cr and V. with This the choice fact that corresponds Ga and Ti arewith concentrated the fact that in Ga magmatic and Ti are sapphires concentrated (syenitic in rocks)magmatic while sapphires Mg, Cr, and (syenitic V are concentratedrocks) while inMg, metamorphic Cr, and V rubiesare concentrated hosted in Mg-Cr-(V)-rich in metamorphic protoliths. rubies hostedA statisticalin Mg-Cr-(V)-rich classification protoliths. by discriminant factors analysis using concentrations (Ti, Mg, V, Cr, Fe,A andstatistical Ga) of classification more than 900 by corundum discriminant crystals factors from analysis the main using primary oxide concentrations deposits worldwide (Ti, Mg, was V, proposedCr, Fe, and by Ga) [72 of]. more The deposits than 900 were corundum classified crystals into difromfferent the types:main primary (1) for ruby deposits as marble, worldwide M-UMR, was andproposed metasomatites by [72]. The and deposits (2) for sapphires were classified as syenitic into rocks,different metasomatites, types: (1) for plumasites, ruby as marble, and skarns. M-UMR, and metasomatitesThe discriminating and (2) factors for sapphires F1 and F2 as calculated syenitic rocks, for the metasomatites, different types plumasites, of ruby deposits and skarns. (known classes)The with discriminating their respective factor centroids F1 and for F2 each calculated class are for as the follows: different types of ruby deposits (known classes) with their respective centroid for each class are as follows: Factor 1 = 0.271691 (Cr O 0.294456)/0.376245 + 0.796762 (FeO 0.26294)/0.224015 × 2 3 − × − +Factor0.0206904 1 = 0.271691(Ga O× (Cr0.0112047)2O3 − 0.294456)/0.376245/0.0124095 + 0.136135 + 0.796762(MgO × (FeO − 0.26294)/0.224015 × 2 3 − × − 0.00545581)+ 0.0206904/0.00526035 × (Ga2O0.1243673 − 0.0112047)/0.0124095(TiO 0.0274651) /0.0539087+ 0.1361350.108332 × (MgO(V O − − × 2 − − × 2 3 0.00545581)/0.005260350.0102233)/0.0111099 + −0.124367 × (TiO2 − 0.0274651)/0.0539087 + −0.108332 × (V2O3 − − 0.0102233)/0.0111099 Factor 2 = 0.124957 (Cr O 0.294456)/0.376245 0.121871 (FeO 0.26294)/0.224015 × 2 3 − − × − Factor0.287007 2 = 0.124957(Ga O × (Cr0.0112047)2O3 − 0.294456)/0.376245/0.0124095 0.0527783 + −0.121871(MgO × (FeO − 0.26294)/0.224015 − × 2 3 − − × − 0.00545581)+ −0.287007/0.00526035 × (Ga2+O0.05848023 − 0.0112047)/0.0124095(TiO 0.0274651) +/0.0539087 −0.05277830.439517 × (MgO(V O− × 2 − − × 2 3 0.00545581)/0.005260350.0102233)/0.0111099 + 0.0584802 × (TiO2 − 0.0274651)/0.0539087 + −0.439517 × (V2O3 − − 0.0102233)/0.0111099 The discriminating factors F1 and F2 calculated for the different types of sapphire deposits (known classes) with their respective centroid for each class are as follows: Minerals 2020, 10, 597 17 of 83

The discriminating factors F1 and F2 calculated for the different types of sapphire deposits (known classes) with their respective centroid for each class are as follows:

Factor 1 = 0.23095 (Cr O 0.0613814)/0.0596296 0.723839 (FeO Factor 1 = 0.23095 ×× (Cr22O3 3 − 0.0613814)/0.0596296 −+ −0.723839× × (FeO− − 0.489593)/0.372367 0.0403934 (Ga O 0.0159195)/0.00946033 + 0.0767757 0.489593)/0.372367 + −0.0403934− × (Ga× 2O32 − 30.0159195)/0.00946033− + 0.0767757 × × (MgO 0.00613983)/0.0040002 0.00901195 (TiO 0.0236271)/0.0567399 + (MgO − 0.00613983)/0.0040002− + −−0.00901195 × ×(TiO2 2−− 0.0236271)/0.0567399 + 0.0791335 (V O 0.00402119)/0.00329669 0.0791335 × (V2×O3 −2 0.00402119)/0.003296693 − Factor 2 = 0.530426 (Cr O 0.0613814)/0.0596296 0.638704 (FeO Factor 2 −= −0.530426× × 2 (Cr3 −2O3 − 0.0613814)/0.0596296− + −×0.638704− × (FeO − 0.489593)/0.372367 0.00893226 (Ga O 0.0159195)/0.00946033 0.200622 (MgO 0.489593)/0.372367 −+ −0.00893226× × (Ga22O33 − 0.0159195)/0.00946033 +− −0.200622 ×× (MgO 0.00613983)/0.0040002 + 0.296868 (TiO 0.0236271)/0.0567399 0.165072 (V O − 0.00613983)/0.0040002 + 0.296868 ×× (TiO2 2−− 0.0236271)/0.0567399 + −-0.165072 ×× (V2O2 3 −3 − 0.00402119)/0.003296690.00402119)/0.00329669 Finally, predictionprediction diagramsdiagrams were proposed for the didifferentfferent typestypes ofof depositsdeposits andand appliedapplied to placers of alkalialkali basalt-relatedbasalt-related deposits [[72].72]. Figure 18 shows how such a diagram can be used to determine the most probableprobable depositdeposit typetype forfor rubyruby fromfrom thethe SoamiakatraSoamiakatra depositdeposit inin Madagascar.Madagascar.

Figure 18. DiscriminantDiscriminant factors factors analysis analysis using using oxide oxide concentrations concentrations in ruby; in ruby; modified modified from from [72]. [72(A].) (DiagramA) Diagram used used to distinguish to distinguish between between the the different different types types of of ruby ruby deposit deposit (marble, (marble, metamorphosed maficmafic andand ultramafic ultramafic rocks rocks (M-UMR), (M-UMR), and and metasomatite) metasomatite) with with thecentroids the centroids of the of known the known deposit deposit classes andclasses (B) and prediction (B) prediction of the geologic of the origingeologic of rubiesorigin fromof ru thebies Soamiakatra from the Soamiakatra deposit in thedeposit Antanarivo in the provinceAntanarivo of Madagascar.province of Madagascar.

Peucat et al. [[70]70] obtainedobtained LA-ICP-MS LA-ICP-MS analyses analyses of of corundum corundum from from diff differenterent geological geological settings settings and suggestedand suggested using using their their Fe, Ti, Fe, Cr, Ti, Ga, Cr, and Ga, Mg and contents Mg cont andents chemical and chemical ratios suchratios as such Ga/ Mg,as Ga/Mg, Fe/Ti, Fe Fe/Ti,/Mg, andFe/Mg, Cr/ Gaand to Cr/Ga determine to determine their geological their geological origins. Theorigins. Ga/Mg The ratio Ga/Mg versus ratio the versus Fe content the Fe has content proven has to beproven an effi tocient be toolan efficient for discriminating tool for discriminating between some between metamorphic some andmetamorphic magmatic blueand sapphires,magmatic evenblue ifsapphires, some localities even if overlap some andlocalities are di overlapfficult to and discriminate. are difficult Natural to discriminate. rubies have Natural normally rubies limited have Ga (normally<200 ppm) limited and low Ga Ga(<200/Mg ppm) ratios and (<10) low and Ga/Mg in some ratios cases (<10) their and trace-element in some cases contents their trace-element are unusual andcontents they are plot unusual in the field and forthey sapphire plot in [the81]. field for sapphire [81]. The use of Fe versus Ga/Mg Ga/Mg for ruby is is presented presented here here for for different different types types of of deposits deposits (Figure (Figure 19).19). The main types ofof deposits areare discriminateddiscriminated asas aa functionfunction ofof thethe FeFe contentcontent ofof thethe ruby:ruby: (1) range 10,000–100010,000–1000 (ppm) (ppm) for for ruby ruby in in M-UMR M-UMR (metamorphism (metamorphism sensu sensu stricto stricto and and metamorphic metamorphic metasomatic) metasomatic) with with the example of different ruby deposits in Southern Madagascar (Vohitany, Anavoha, and Andriba), Greenland (Aappaluttoq), Mozambique (Maninge Nice at Montepuez), ruby as xenocrysts in alkali basalts (examples include Pailin, Macquarie River, New England, and Chanthaburi-Trat) and kimberlite (Mbuji-Mayi) or alluvial from kimberlites (Juina, Brazil); (2) range 1000–10 ppm for Minerals 2020, 10, 597 18 of 83 the example of different ruby deposits in Southern Madagascar (Vohitany, Anavoha, and Andriba), Greenland (Aappaluttoq), Mozambique (Maninge Nice at Montepuez), ruby as xenocrysts in alkali basalts (examples include Pailin, Macquarie River, New England, and Chanthaburi-Trat) and kimberlite (Mbuji-Mayi) or alluvial from kimberlites (Juina, Brazil); (2) range 1000–10 ppm for ruby in marbles (Lu Yen-Yenruby in Baimarbles and Quy (Lu ChauYen-Yen in Vietnam, Bai and Mogok Quy Chau and Mongin Vietnam, Hsu in Myanmar);Mogok and and Mong (3) lessHsu than in Myanmar); 10 ppm for metasomatitesand (3) less than as plumasites10 ppm for (Johnmetasomatites Saul mine, as Kenya). plumasites (John Saul mine, Kenya).

FigureFigure 19.19. FeFe (ppm)(ppm) versusversus GaGa/Mg/Mg discriminationdiscrimination diagram for ruby (this work; modifiedmodified from [[69]),69]), showingshowing thethe variation variation of of Fe Fe content content for for some some rubies rubies relative relative to their to their geological geological environments. environments. The boxes The forboxes magmatic for magmatic sapphire sapphire related related to alkali to basaltsalkali basalt are reporteds are reported for comparison for comparison with the with plot the of plot Pailin of sapphirePailin sapphire in Cambodia in Cambodia (data from(data [ 72from,82 ])[72,82]) as well as as well the as box the of box metamorphic of metamorphic sapphires sapphires (drawn (drawn with thewith range the range of values of values defined defined in the in the tables tables of [of70 ]).[70]). The The following following di differfferentent types types ofof rubyruby andand/or/or theirtheir chemicalchemical fieldsfields areare shown:shown: (1) thethe chemicalchemical domaindomain ofof rubiesrubies associatedassociated withwith M-UMRM-UMR (red(red domains)domains) from (i) Madagascar Madagascar [72] [72 ]with with the the occurrences occurrences of ofVohitany Vohitany (analysis, (analysis, n =n 25),= 25), Anavoha Anavoha (n = (6),n = and6), Andriba and Andriba (n = 6); (n (ii)= 6); Greenland (ii) Greenland and the and chemical the chemical domain domain of Aappaluttoq of Aappaluttoq ruby deposit ruby deposit (n = 24, (datan = 24,from data [83]); from (iii) [ 83Mozambique]); (iii) Mozambique (Montepuez (Montepuez mining district) mining with district) the colluvial with the deposit colluvial of Maninge deposit ofNice Maninge from (Pardieu, Nice from unpublished (Pardieu, unpublished data); (2) inclusions data); (2) of inclusions ruby in of ruby in fr diamondsom Juina fromin Brazil Juina (red in Brazilhexagon) (red [43]; hexagon) (3) xenocrysts [43]; (3) xenocrystsof ruby from of the ruby Mb fromuji-Mayi the Mbuji-Mayi kimberlite in kimberlite the Democratic in the DemocraticRepublic of RepublicCongo (red of Congosquare) (red [84]; square) (4) xenocrysts [84]; (4) of xenocrysts rubies in alluvial of rubies of in alkali alluvial basalts of alkali such basaltsas Pailin, such Chantaburi- as Pailin, Chantaburi-Trat,Trat, and Macquarie and MacquarieRiver and RiverNew England and New in England Austra inliaAustralia (red star) (red [82,85,86]; star) [82 (5),85 ,rubies86]; (5) in rubies marble in marblefrom Luc from Yen-Yen Luc Yen-Yen Bai represented Bai represented by single by measurements single measurements ( circle) (violet and circle) Quy and Chau Quy (blue Chau domain) (blue domain)in Vietnam in Vietnam[57,66], Mogok [57,66], (black Mogok domain) (black domain)[87], and [Mong87], and Hsu Mong (orange Hsu domain) (orange [88]; domain) and (6) [88 ];rubies and (6)from rubies the former from the John former Saul John(now SaulRockland) (now Rockland)mine in Kenya mine (green in Kenya domain) (green from domain) desilicated from desilicatedpegmatites pegmatites(Giuliani et (Giulianial., unpublished et al., unpublished data). These data).rubies Theseshow high rubies variations show high in Fe, variations which permits in Fe,which discrimination permits discriminationof their geological of their type; geological this is characterized type; this is by characterized a decrease of by Fe a decreasefrom rubies of Fe in fromM-UMR rubies to marble in M-UMR and tometasomatic marble and rocks metasomatic (plumasite rocks and (plumasite shear-zones). and shear-zones). The Ga/Mg ratio The Gaof /rubiesMg ratio from of rubiesM-UMR from is different M-UMR isfrom diff erentmagmatic from sapphires magmatic but sapphires is within butthe isrange within of va thelues range of metamorphic of values of sapphires metamorphic as defined sapphires by [69]. as definedThe domains by [69 represent]. The domains the distributi representon theof several distribution spot analyses. of several The spot others analyses. analyses The othersare the analyses average are the average values of punctual measurements. values of punctual measurements.

The Ga/Mg ratio is highly variable, between 0.1 and 10, and some rubies such as those occurring as xenocrysts in alkali basalt (Chanthaburi-Trat and New England) and M-UMR (Madagascar) plot in the magmatic field of sapphire (right side of Figure 19). Such an overlap opens the question of the significance of such geochemical signatures and will be discussed elsewhere. Geographic origin determination of ruby has become a great challenge during recent decades [72]. The sampling of rubies from reliable sources worldwide [20,36,89] has facilitated the assembly of reliable reference collections in the gem laboratories [90]. The mineral assemblages and inclusions Minerals 2020, 10, 597 19 of 83

The Ga/Mg ratio is highly variable, between 0.1 and 10, and some rubies such as those occurring as xenocrysts in alkali basalt (Chanthaburi-Trat and New England) and M-UMR (Madagascar) plot in the magmatic field of sapphire (right side of Figure 19). Such an overlap opens the question of the significance of such geochemical signatures and will be discussed elsewhere. Geographic origin determination of ruby has become a great challenge during recent decades [72]. The sampling of rubies from reliable sources worldwide [20,36,89] has facilitated the assembly of reliable reference collections in the gem laboratories [90]. The mineral assemblages and inclusions combinedcombined withwith tracetrace elementelement chemistrychemistry sometimessometimes makemake itit possiblepossible toto decipherdecipher thethe geographicgeographic originorigin ofof rubies.rubies. Trace elementselements areare usedused byby mostmost ofof thethe laboratorieslaboratories andand thethe datadata areare collectedcollected withwith severalseveral analyticalanalytical techniquestechniques [[91]91] mainlymainly LA-ICP-MS.LA-ICP-MS. RepresentativeRepresentative analysesanalyses ofof rubiesrubies originatingoriginating fromfrom depositsdeposits ofof didifferentfferent geologicalgeological typestypes areare presentedpresented inin TablesTables1 1and and2 2.. TheThe quantificationquantification ofof thethe variousvarious tracetrace elementselements isis donedone usingusing commerciallycommercially availableavailable calibrationcalibration standardsstandards suchsuch asas thosethose providedprovided byby thethe NationalNational InstitutesInstitutes ofof StandardsStandards andand TechnologyTechnology (NIST)(NIST) oror thethe UnitedUnited StatesStates GeologicalGeological SurveySurvey (USGS).(USGS). TheThe didifferencefference inin compositioncomposition betweenbetween the the standards standards and and the analyzedthe analyzed corundum corundum has led has to some led uncertaintiesto some uncertainties in the data generatedin the data by thegenerated different by laboratories. the different The labora Gemmologicaltories. The Gemmological Institute of America Institute laboratory of America developed laboratory matrix-matched developed corundummatrix-matched standards corundum of ruby standards and sapphire of andruby the and standard sapphire deviation and the of standard the analyzed deviation ruby became of the negligibleanalyzed ruby and became therefore negligible can be comparedand therefore with can a be database compared of stoneswith a withdatabase known of stones provenance with known [92]. Theprovenance application [92]. of thisThe analyticalapplication protocol of this to analytical ruby made protocol it possible to toruby confirm made the it geologicalpossible to classification confirm the ofgeological gem corundum classification (ruby of and gem pink corundum corundum) (ruby for and (1) Fe-poorpink corundum) ruby hosted for (1) in Fe-poor marble andruby (2) hosted Fe-rich in rubymarble in and M-UMR (2) Fe-rich (Figure ruby20). in The M-UMR geographic (Figure determination 20). The geographic is not determination based on the Cr is not content based of on ruby the becauseCr content it appears of ruby by because experience it appears that it isby not experience useful for that origin it is certification not useful [for11]. origin The discrimination certification [11]. for Fe-richThe discrimination rubies in M-UMR for Fe-rich is based rubies on thein M-UMR contents is of based V-Mg-Fe on the as shown contents in Figureof V-Mg-Fe20 for as deposits shown inin Mozambique,Figure 20 for deposits Madagascar, in Mozambique, and Thailand Madagascar,/Cambodia. and Thailand/Cambodia.

FigureFigure 20.20. TheThe geographicgeographic originorigin ofof rubiesrubies fromfrom Mozambique,Mozambique, Madagascar,Madagascar, andand ThailandThailand/Cambodia/Cambodia usingusing VV versusversus MgMg andand FeFe versusversus VV diagrams,diagrams, modifiedmodified fromfrom [11[11].]. TheThe limitslimits ofof detectiondetection ofof thethe analysesanalyses byby laserlaser ablation-inductively ablation-inductively coupled coupled plasma-mass plasma-mass spectroscopy spectroscopy (LA-ICP-MS) (LA-ICP-MS) are are for for Mg, Mg, V, andV, and Fe respectively,Fe respectively, 0.1–0.3, 0.1–0.3, 0.03–0.2, 0.03–0.2, and and 5–20 5–20 ppm. ppm.

TheThe geographicgeographic originorigin ofof rubyruby cancan usuallyusually bebe achievedachieved basedbased onon inclusionsinclusions inin associationassociation withwith tracetrace elementelement chemistrychemistry despitedespite somesome overlapsoverlaps [[1111–14].–14]. TheThe oxygenoxygen isotopicisotopic compositionscompositions ofof thesethese rubiesrubies overlapoverlap stronglystrongly andand areare notnot usefuluseful forfor geographicgeographic origin origin determination determination [ 93[93].]. Minerals 2020, 10, 597 20 of 83

Table 1. Representative oxide (wt %) EMPA analyses of rubies originating from deposits of different geological types relative to the new enhanced geological classification for ruby.

Type of Deposit Deposit Color Corundum Country Al2O3 MgO TiO2 V2O3 (in wt %) Cr2O3 FeO Ga2O3 Total References deep red 98.16 na 0.03 0.060 0.858 0.52 0.01 99.64 Type IA Somiakatra Madagascar [94] pink 98.75 na 0.01 0.000 0.243 0.26 0.01 99.28 Type IB Mbuji Mayi fuchsia RDC 93.76 0.01 0.01 0.029 5.641 0.47 0.01 99.94 [84] Paranesti red Greece 97.58 bdl 0.01 0.010 2.650 0.39 0.07 100.71 [95] Montepuez red Mozambique 99.38 bdl 0.01 na 0.080 0.45 bdl 99.92 [96] Type IIA1 Vohitany red Madagascar 99.06 0.01 0.01 0.000 0.219 0.59 0.01 99.90 [72] Ronda red Spain 99.33 0.02 0.01 na 0.120 0.19 na 99.67 [97] Luc Yen red to pink Vietnam * 99.42 0.02 0.02 0.010 0.370 0.02 0.01 99.87 [98] Mogok red 99.01 0.00 0.01 0.047 0.794 0.01 0.01 99.88 Type IIA2 Myanmar Mong Hsu red 99.20 0.01 0.08 0.041 0.521 0.00 0.00 99.85 [72] Nangimali pink Pakistan 99.85 0.01 0.02 0.005 0.055 0.00 0.01 99.94 Anavoha pink Madagascar 98.76 0.00 0.00 0.002 0.023 0.45 0.00 99.25 Naxos pink Greece 100.70 0.35 0.27 0.090 0.180 0.40 0.15 102.14 [95] Type IIB1 Polar Urals red Russia 95.16 bdl bdl na 3.860 0.44 na 99.46 [99] John Saul pink Kenya 100.09 0.03 0.04 0.016 0.595 0.00 0.04 100.80 [72] (Kimbo) Zazafotsy red Madagascar 98.63 0.01 0.03 0.005 0.188 0.30 0.01 99.17 [100] Type IIB2 Hokitika red New Zealand 90.65 na bdl 0.050 9.050 0.18 na 99.93 [42] red 98.43 0.02 0.04 0.040 0.761 0.52 0.02 99.83 Vatomandry pink 99.29 0.03 0.04 0.005 0.161 0.44 0.02 99.99 Madagascar [94] Type IIIA red brown 97.68 0.03 0.04 0.053 0.561 0.63 0.03 99.02 Antsabotraka red brown 99.48 0.02 0.02 0.010 0.130 0.39 0.02 100.06 Ambatomainty red brown 99.50 0.03 0.02 0.012 0.127 0.21 0.01 99.91 Macquarie red Australia 99.07 0.09 bdl na 0.450 0.22 na 99.83 [75] Ilakaka pink Madagascar 99.28 0.01 0.01 0.001 0.178 0.29 bdl 99.77 [72] Type IIIB Quy Chau red to pink Vietnam ¤ 99.28 0.00 0.03 0.020 0.580 0.06 0.01 99.98 [98] Luc Yen red to pink Vietnam ¤ 99.76 0.00 0.01 0.070 0.540 0.01 0.01 100.40 na = not analyzed; *, ¤ = number of analyses for calculating the presented average composition of ruby or pink corundum with respectively; * n = 29, ¤ n = 40 and n = 16; bdl = below detection limit. Minerals 2020, 10, 597 21 of 83

Table 2. Representative analyses (in ppm) obtained by LA-ICP-MS of rubies originating from deposits of different geological types relative to the new enhanced geological classification for ruby.

Type of Deposit or Mining V Number of Country Mg Ti Fe Ga Cr References Deposit District (in ppma) Analyses

Type IIA1 Longido Tanzania (7–42) np (3–25) (62–544) (6–10) (1604–5059) 16 [101] in M-UMR Winza (0–118) np (0–1) (405–1596) (4–11) (161–1094) 14 Chimwadzulu Malawi (10–39) np (1–10) (953–2760) (5–15) (269–1816) 18 Type IIB1 Aappaluttoq Greenland (2–81) (15–210) (0.1–14) (655–2516) (0.7–25) (50–2871) 17 [102] Type IIA2 Mogok (Mo) Myanmar (5–75) (7–80) (48–1089) (bdl-301) (2–51) np 65 = Mo + MH [11] in marble Dattaw (Mo) (bdl) (105–702) (66–97) (bdl-237) * (32–42) (105–702) 4 [103] Kadoke Tat (Mo) (bdl–25) (bdl-580) (150–1472) (bld-644) * (11–140) (61–2963) 13 Baw-Padan (Mo) (33–40) (70–74) (206–347) (270–442) (106–166) (29–2339) 2 [87] Namya (bdl–66) (79–739) (184–489) (bdl-234) * (23–107) (479–6864) 12 [103] Mong Hsu (MH**) (48–115) (778–1815) (324–417) (bdl-29) (66–89) (2026–7015) 65 = Mo + MH [88] not precise Vietnam (1–130) (bdl-1214) (2–214) (bdl-28) (1–122) np 93 [11] Jegdalek Afghanistan (7–380) (9–486) (4–135) (bdl-1128) (5–35) np 74 Snezhnoe Tajikistan (6–58) (bdl–579) (24–112) (bdl-155) (14–28) np 48 (4–72) (6–75) (51–122) Bdl * (61–83) (1696–4204) 6 [104] Type IIIA Pailin Cambodia (97–258) (32–128) (5–22) (818–1935) (5–11) np 34 [11] in alkali (118–226) (95–210) (16–33) (2454–3620) (18–34) (450–7761) 14 [105] basalt Chantaburi- Thailand (126–181) (32–138) (4–14) (756–1442) ((5–10) np 7 [11] Trat (102–163) (81–219) (9–30) (2175–3794) (16–32) (955–2856) 10 [105] New England Australia (9–55) (6–203) (4–14) (2560–3150) (170–310) (870–3370) ? [81] Type IIIB Mugloto Mozambique (11–50) np (2–7) (851–2034) (5–12) (506–1737) 14 [101] in M-UMR Maninge Nice (19–47) np (1–6) (261–402) (6–9) (1886–4941) 5 Montepuez (8–65) (bdl-59) (bdl–10) (231–2154) (5–14) np 85 [11] Type IIIB Zahamena Madagascar (13–61) np (10–100) (284–1994) (10–30) (135–3922) 14 [101] in MetamR Didy (0–67) np (2–17) (533–1274) (3–17) (32–2698) 21 Andilamena (19–53) np (4–29) 559–1559) (7–21) (53–1569) 14 np = not precise because Cr is no longer used in the GIA laboratory as a discriminant for geographic origin determination; * = iron was measured by EMPA; MH** = range of averages composition of core and rim for two samples (n = 33 analyses) from Mong Hsu (MH); MetamR = metamorphic rocks environment; M-UMR = mafic-ultramafic rocks; Type IIA2 refers to the proposed new geological classification for ruby; bdl = below detection limit. Minerals 2020, 10, 597 22 of 83

4. Geological Environment and Age of Primary Ruby Deposits The global distribution of ruby deposits is closely linked to the Wilson cycle, i.e., collision, rift and subduction geodynamics [80,106,107]. Ruby forms in mafic, felsic, and metamorphosed carbonate platform geological environments, but it is always associated with rocks depleted in silica and enriched in alumina. In the presence of silica, Al is preferentially incorporated into Al-Si-bearing minerals such as and micas. The mineral association of corundum commonly consists of plagioclase, sapphirine, biotite, phlogopite, amphibole, pyroxene, and carbonates [108]. Two major geological environments are favorable for the presence of ruby, specifically amphibolite to medium-pressure granulite facies metamorphic belts, and alkaline basaltic volcanism in continental rifting environments. Three questions arise concerning ruby in metamorphic environments, which formed primarily via changes in temperature and pressure and fluid–rock interaction such as diffusion or percolation (metasomatism): (1) the origin and role of the parental fluids; (2) the nature and importance of the protolith; and (3) the characteristics of the paleogeography of the depositional sedimentary environment (in the case of carbonate platforms). Gem ruby is rare because it requires the existence of several conditions: (1) an environment containing aluminum or the circulation of an Al-bearing metasomatic fluid; (2) a parental fluid issued either from the gem host-rock environment or exotic fluid circulations at medium to high temperatures (T > 500 ◦C); (3) a seed surface and sufficient space for the growth of the crystal; (4) the incorporation of trace elements especially Cr, Fe, and V from the parental fluid in the unit cell of the mineral; and (5) the absence of internal crystalline deformation during and after growth. In the amphibolite to medium-pressure granulite facies metamorphic belts, the gem corundum pressure-temperature field corresponds to a pressure domain above 2 kbar, and temperatures between 500 and 850 ◦C (Figure 21). The host lithologies are alumina-rich and/or silica-poor rocks such as marble, aluminous gneiss, metamorphosed M-UMR, or juxtaposed meta-felsic and silica-poor rocks affected by the circulation of fluids at their contact and altered by metasomatism (metasomatites such as desilicated pegmatite called plumasite, skarn, etc.). Marble is a silica- and alumina-depleted rock, and under these conditions the transport of alumina by a fluid phase seems necessary for the crystallization of corundum; however, Al is usually considered to be an immobile element but investigations on different ruby deposits associated with fluid circulation and metasomatism have shown that Al was mobile [83]. Ruby-bearing skarns have not been clearly identified but the presence of silica enrichments in certain rubies from Mogok [87,103] led some authors to suspect the influence of thermal anomalies accompanying the different Si-rich intrusions affecting the mining district. This opens debate on the accuracy of determination of Si content in ruby, and the use of accurate ruby standards is necessary to decrease the detection limit of Si as done by [87,92]. Stone-Sundberg et al. [92] measured by LA-ICP-MS analysis Si ranging from 70 to 305 ppm in rubies from Mogok as well as other elements such as B (11–82 ppm), Ga (280–800 ppm), and Sn (2–35 ppm) that recorded potential skarn trace-element enrichments [88]. Emmett et al. [109] focused on the low mass resolution of quadrupole ICP-MS as the cause of that technique inability to measure accurately Si in corundum. Another problem is that ICP-MS occurs in a silica glass torch, which causes large variations in the Si background leading to high detection limits [109]. The results published up to now for different rubies [91,104] remain questionable. Measurements of Si by SIMS could be a more accurate method as explained by [109]. In alkali basalt volcanism, rubies are xenocrysts carried to the surface by the magmas. The corundum is found either as xenocrysts or crystal within xenoliths in lava flows and plugs of subalkaline olivine basalts, high alumina alkali basalt, and basanite. These magmas occur in crustal extensional environments impacted by the rise of upwelling mantle plumes. Ruby-bearing xenoliths are very rare because the mother rocks generally melt or disintegrate during storage in the magma chamber or during the magma transport. In Madagascar, xenoliths of ruby-bearing clinopyroxenite and metagabbro have been found in the Soamiakatra deposit. The metamorphic ruby formed in the lower crust at a temperature around 1100 ◦C and a pressure of about 20 kbar [77,94]. Minerals 2020, 10, 597 23 of 83

FigureFigure 21.21. Pressure–temperature (P–T) formation conditionsconditions of metamorphicmetamorphic gem rubyruby (modified(modified fromfrom [[40,110]).40,110]). TheThe P–T P–T fields fields of: of: (1) (1) the the marble marble type type deposits deposits from from Mong Mong Hsu Hsu [60 ],[60], Urals Urals [111 ],[111], Luc YenLuc andYen Jegdalekand Jegdalek [65], [65], Hunza Hunza [112 ],[112], Revelstoke Revelstoke [113 [113],], Mahenge Mahenge [114 [114],], and and Snezhnoe Snezhnoe [115 [115]] are are represented represented by violetby violet rectangles rectangles and (2) and the (2) M-UMR the M-UMR deposits, deposits, metamorphic metamorphic sensu stricto sensu or associated stricto or with associated metasomatic with metamorphismmetasomatic metamorphism are represented are by represented red rectangles: by red Greenland-Kangerdluarssuk rectangles: Greenland-Kangerdluarssuk [116], Aappaluttoq [116], 1Aappaluttoq [34] and 2 [183 [34]]; Southern and 2 [83]; Kenya Southern [117 Kenya,118]; [ Montepuez117,118]; Montepuez [96]; Mangare [96]; Mangare [119]; Winza [119]; [Winza120]; North [120]; CarolinaNorth Carolina [121]; and[121]; Vohibory and Vohibory [122]. The[122]. P–T The fields P–T fiel of rubyds of andruby pink and corundumpink corundum in clinopyroxenites in clinopyroxenites and metagabbroand metagabbro xenoliths xenoliths associated associated with alkali with basalts alkaliare ba notsalts shown. are not The shown. schematic The fieldsschematic of Greenschist, fields of Amphibolite,Greenschist, Amphibolite, and Granulite and facies Granulite are reported. facies are reported.

TheThe agesages ofof corundumcorundum formationformation are defineddefined by indirectindirect dating of aa seriesseries ofof mineralsminerals (zircon,(zircon, monazite,monazite, rutile, titanite, and micas) present either in the hosthost rocksrocks oror asas syngeneticsyngenetic inclusionsinclusions inin thethe corundum.corundum. These minerals have didifferentfferent blockingblocking temperaturestemperatures for closureclosure toto isotopicisotopic migrationmigration thatthat makemake itit possiblepossible toto establishestablish aa coolingcooling historyhistory forfor thethe corundum.corundum. Four mainmain periodsperiods ofof gemgem corundumcorundum formationformation that include include ruby ruby and and sapphire sapphire and and encompass encompass the the main main economic economic production production worldwide, worldwide, are arepresented presented in Figure in Figure 22 22[80,106,107].[80,106,107 The]. The others others primary primary ruby ruby occu occurrencesrrences known known worldwide worldwide as asin inAustralia, Australia, Norway, Norway, Finland, Finland, United-States, United-States, and andother other countries countries (see Figure (see Figure 2) were2) werenot considered not considered in the inmain the periods main periods of gem ofruby gem formation. ruby formation. They are They either are uneconomical either uneconomical or without or determination without determination of ages. of ages.The oldest deposits were formed in the Archean (2.97–2.6 Ga). An example is the Aappaluttoq rubyThe deposit oldest in deposits the 2.97 were Ga Fiskenæsset formed in the anorthos Archeanite(2.97–2.6 complex, Ga). Greenland An example (Figure is the 2). AappaluttoqThe deposit rubyoccurs deposit at the contact in the 2.97between Ga Fiskenæssettwo types of anorthositerocks, i.e., ultramafic complex, rocks Greenland (meta-peridotite (Figure 2). or The dunite) deposit and occursan anorthite-rich at the contact rock between (anorthosite two types and of leucogabbro) rocks, i.e., ultramafic [34,83]. rocksRuby (meta-peridotiteis hosted in phlogopite-calcic or dunite) and anamphibole anorthite-rich metasomatic rock (anorthosite rocks formed and by leucogabbro) fluid–rock interaction [34,83]. Rubyat the iscontact hosted of in the phlogopite-calcic two lithologies. amphiboleThe metasomatic metasomatic fluids were rocks triggered formed by by fluid–rock the intrusion interaction of pegmatites at the contact dated between of the two 2.72 lithologies. and 2.70 TheGa with metasomatic U–Pb zircon fluids geochronology were triggered by by LA-ICP-MS the intrusion [83]. of pegmatites U/Pb dating dated of monazite between 2.72gave and age 2.70 at 2.64 Ga withGa [34] U–Pb and zircon a direct geochronology Pb-Pb isochron by LA-ICP-MSage of 2.68 Ga [83 was]. U /obtainedPb dating on of ruby monazite by Thermal gave age Ionisation at 2.64 Ga Mass [34] andSpectrometry a direct Pb-Pb[103]. The isochron age of ageruby of is2.68 correlated Ga was to obtainedthe age of onthe rubypegmatites by Thermal intrusion; Ionisation consequently Mass Spectrometryan average age [103 at ].2.71 The Ga age is considered of ruby is correlated for ruby formation to the age of[83]. the pegmatites intrusion; consequently an average age at 2.71 Ga is considered for ruby formation [83]. Minerals 2020, 10, 597 24 of 83

Figure 22.22. TheThe spiral spiral time time diagram diagram of corundum form formationation that includes economic ruby (and sapphire) deposits.deposits. Four Four main main economic economic corundum corundum periods periods have beenhave definedbeen defined worldwide worldwide by different by methodsdifferent ofmethods indirect of dating indirect of corundum. dating of Thecorundum oldest deposit. The oldest is Aappaluttoq deposit isin Aappaluttoq Greenland where in Greenland the ruby formedwhere the during ruby theformed Archean during at 2.71 the Ga.Archean The main at 2.71 period Ga. The of gemmain rubies period formation of gem rubies was the formation Pan-African was orogenythe Pan-African (750–450 orogeny Ma). This (750–450 includes Ma). primary This ruby includ depositses primary in the Pan-Africanruby deposits tectonic-metamorphic in the Pan-African belttectonic-metamorphic such as the John Saul belt mine such in as Kenya, the John Longido Saul mine in Tanzania, in Kenya, and Longido the deposits in Tanzania, of Southern and Madagascar,the deposits allof Southern formed at Madagascar, around 610 Ma. all formed The third at periodaround corresponds 610 Ma. The to third the Cenozoic period corresponds Himalayan to orogeny the Cenozoic (55 Ma toHimalayan the Quaternary) orogeny with (55 theMa famousto the marble-hostedQuaternary) with ruby the deposits famous in marble-hosted Central and South-East ruby deposits Asia asin inCentral the deposits and South-East of Thureing Asia Taung as in at the Mogok deposits and Mongof Thureing Hsu in Taung Myanmar at Mogok [88]. The and fourth Mong period Hsu in is dominatedMyanmar [88]. by the The extrusion fourth period of alkali is dominated basalts in the by Cenozoicthe extrusion (65 Maof alkali to Quaternary) basalts in wherethe Cenozoic ruby (and (65 sapphire)Ma to Quaternary) were transported where ruby as xenocrysts (and sapphire) or in xenoliths were transported by the magmas. as xenocrysts The age or of in the xenoliths parental by rocks the ofmagmas. the ruby The is unknownage of the but parental a metamorphic rocks of origin the ruby is preferentially is unknown evidenced. but a metamorphic Most of the origin ages are is minimumpreferentially cooling evidenced. ages except Most those of the obtained ages are onminimum zircon by cooling the U /agesPb method. except those obtained on zircon by the U/Pb method. The second period of corundum formation was the Pan-African orogeny (750–450 Ma). This includesThe second primary period rubyof corundum and sapphire formation deposits was inthe the Pan-African gemstone orogeny belt of eastern (750–450 Africa, Ma). This Madagascar, includes India,primary and ruby Sri and Lanka sapphire that are deposits linked toin collisionalthe gemstone processes belt of betweeneastern Africa, Eastern Madagascar, and Western India, Gondwana and Sri duringLanka that Pan-African are linked tectonic-metamorphic to collisional processes events between [123]. Eastern The metamorphic and Western ruby Gondwana and sapphire during deposits Pan- inAfrican Southern tectonic-metamorphic Madagascar have events numerous [123]. The geological metamorphic similarities ruby and with sapphire those in deposits Eastern in Africa, Southern Sri Lanka,Madagascar and have Southern numerous India geological (see [40]). similarities U/Pb dating with ofthose zircon in Eastern from theAfrica, John SriSaul Lanka, mine andin Southern Kenya (612India (see6 Ma; [40]). [118 U/Pb]), Longido dating of in zircon Tanzania from (610 the John6 Ma; Saul [124 mine]), and in theKenya Vohibory (612 ± 6 deposits Ma; [118]), in Madagascar Longido in ± ± (612Tanzania5 Ma; (610 [125 ± 6]) Ma; reveal [124]), similar and the periods Vohibory of formation deposits relatedin Madagascar to the East (612 African ± 5 Ma; orogeny. [125]) reveal The precisesimilar ± periods of formation related to the East African orogeny. The precise age of formation for the ruby deposit of Montepuez in Mozambique is unknown but a Pan-African age is probable [126]. Moreover, the U/Pb ages of zircon coeval with sapphire in the Andronandambo skarn deposit in Southern Madagascar range Minerals 2020, 10, 597 25 of 83 age of formation for the ruby deposit of Montepuez in Mozambique is unknown but a Pan-African age is probable [126]. Moreover, the U/Pb ages of zircon coeval with sapphire in the Andronandambo skarn deposit in Southern Madagascar range between 523 and 510 Ma [127], and the 40Ar/39Ar ages of biotite-phlogopite syngenetic with sapphire and ruby from the Ambatomena, , and Zazafotsy deposits in Madagascar are between 494 and 487 Ma [128]. LA-ICP-MS dating of rutile inclusions in ruby from the John Saul and Aqua mines in the Mangare area gave ages of 533 and 526 Ma, respectively [129]. These cooling ages, lower that those obtained by the U/Pb method, must be considered as minimum ages but as confirming the existence of a metamorphic corundum episode during the late Pan-African orogenic cycle (Cambrian) and corresponding to the Kuunga orogeny [130]. The third period corresponds to the Cenozoic Alpine orogeny with tectonics events registered in the Alps; Rhodopes and Himalayas (55 Ma to the Quaternary). Examples include the marble-hosted ruby deposits in Campo Lungo (Switzerland) and Xanthi (Greece), and in Central and South-East Asia. In the Himalayas, the ruby deposits in marble occur in metamorphic blocks that were affected by major tectonic events during the collision of the Indian and Asian plates [131–134]. The ruby has been indirectly dated by 40Ar/39Ar stepwise heating experiments performed on single grains of coeval phlogopite, and by ion-probe U–Pb analyses of zircon included in the corundum [40,88,106,135]. Other indirect dating for the Snezhnoe deposits in Central Pamir was realized on bulk rock (ruby-bearing phlogopite-plagioclase rocks) by the Rb-Sr method [134]. The Rb–Sr errorchron age yielded 23 1.6 Ma, ± which had to be considered as a minimum age for ruby formation. All of the Eocene to Pliocene ages (55–5 Ma) are consistent with extensional tectonic events that were active from Afghanistan to Vietnam in the ruby-bearing metamorphic belt. The fourth main period of gem economic ruby is dominated by the extrusion of alkali basalts in the Cenozoic (65 Ma to Quaternary). Gem corundum occurs worldwide as xenocrysts or megacrysts in xenoliths or enclaves incorporated in basaltic magmas during their ascent. Such sapphire and ruby deposits occur from Tasmania through Eastern Australia, South-East Asia, and Eastern China to far Eastern Russia [68,85,88,106]. They are also found in Nigeria and Cameroon in the Aïr and Hoggar regions, the French Massif Central in the Limagne rift, and in Northern, Central, and Eastern Madagascar [39]. In the Australian West Pacific margins intraplate basaltic fields, the U/Pb ages on are widespread between 60 and 1 Ma [69,73,85,86,106]. Nevertheless, U/Pb dating of zircons xenocrysts in the Barrington Tops deposits in New South Wales indicated multiple episodes of corundum formation and eruptive discharges between 60 and 3 Ma [136]. Few U/Pb data gave ages of 150 and 180 Ma for zircons from the deposit of Cudgegong [136], and 400 Ma for those from Tumbarumba [137]. These oldest ages signify that the Australian West Pacific margins has been essentially a passive margin in a couple hundred million years until the Cenozic basalts related to continental rifting brought corundum to the surface.

5. Classification of Ruby Deposits

5.1. Genetic Classifications Gem corundum deposits are classified as either primary or secondary deposits. The primary deposits contain corundum either in the rock where it crystallized or as xenocrysts and in xenoliths in the rock that carried it from the zone of crystallization in the crust or mantle to the Earth’s surface [39]. The secondary deposits, i.e., present-day placers, are of three types [138]: (1) eluvial concentrations derived by in situ weathering, or weathering plus gravitational movement, or accumulation; (2) colluvial deposits corresponding to decomposed primary deposits which have moved vertically and laterally down-slope as the hillside eroded; and (3) alluvial deposits resulting from erosion of the host rock and transport of corundum by streams and rivers. Concentration occurs where water velocity drops at a slope change in the hydrographical profile of the river, e.g., at the base of a waterfall, broad gullies, debris cones, meanders, and inflowing streams. Sometimes the corundum placers are mined in beach sands [139,140]. Minerals 2020, 10, 597 26 of 83

Classification systems for corundum deposits have evolved over time and are based on different mineralogical and geological features including: (1) the morphology of the corundum [141]; (2) the geological context of the deposits [5]; (3) the lithology of the host rocks [142]; (4) the genetic processes responsible for corundum formation [110]; (5) the genetic type of the deposit [143], (6) the geological environment and nature of the corundum host-rock [40,80,108,140]; (7) the oxygen isotopic composition of the corundum [144], and (8) discriminant factors developed using oxide concentrations in 900 corundum samples from the main primary deposits [72]. Classification systems for ruby deposits have not been previously proposed.

5.2. An Enhanced Classification for Ruby Deposits In this work we proposed classifying ruby from primary and secondary deposits into three main types (Table 3). Primary ruby deposits were subdivided into two types based on their geological environment of formation: (Type I) magmatic and (Type II) metamorphic. (Type I) Magmatic-related with two sub-types: (1) Sub-Type IA: Rubies either as xenocrysts or in xenoliths hosted by magmatic rocks such as alkali basalts (Madagascar, and others; see Figure 2); (2) Sub-Type IB: Xenocrysts of ruby in kimberlite (Democratic Republic of Congo). (Type II) Metamorphic-related with two sub-types: (1) Sub-Type IIA: Metamorphic deposits sensu stricto in metamorphosed mafic and ultramafic rocks (M-UMR; Sub-Type IIA1) and marble (Sub-Type IIA2). Examples of Sub-Type IIA1 include the ruby deposits in M-UMR from Montepuez (Mozambique), Bekily-Vohibory region (Madagascar), and others, examples of Sub-Type IIA2 concern the ruby deposits in marble from the Mogok Stone Track, and others. (2) Sub-Type IIB: Metamorphic-metasomatic deposits characterized by high fluid–rock interaction and metasomatism (Sub-Type IIB1), i.e., plumasite or desilicated pegmatites as at the former John Saul mine in Kenya, and metasomatites in M-UMR as at Aappaluttoq (Greenland), and marble, and shear zone-related or fold-controlled metasomatic-metamorphic deposits in different substrata, corundum-bearing Mg-Cr-biotite schist and gneiss or marble (Sub-Type IIB2). Examples are respectively the ruby occurrences at Sahambano, Zazafotsy, and Ambatomena in Madagascar, and Mahenge and the Uluguru Mountains in Tanzania. Secondary ruby deposits are defined as Type III, i.e., sedimentary-related. They are divided into three sub-types hosted in sedimentary rocks: (1) Sub-Type IIIA: Gem placers in alkali basalt environments (Eastern Australia, Central Madagascar, South-East Asia, and others). (2) Sub-Type IIIB: Gem placers in metamorphic environments (Montepuez in Mozambique and the Mogok Stone Track in Myanmar, and others). (3) Sub-Type IIIC: Gem placers with ruby originating from multiple and unknown sources, such as Ilakaka in Madagascar, Tunduru and Songea in Tanzania, and others. Minerals 2020, 10, 597 27 of 83

Table 3. Typological classification of ruby occurrences and deposits worldwide.

Type of Deposit Magmatic-Related Metamorphic-Related Sedimentary-Related * Geological Magmatic Metamorphic Sedimentary Environment Metamorphic Amphibolite to Granulite or Eclogite Greenschist–Amphibolite–Granulite Facies No Metamorphism Conditions Facies Host-Rocks Magmatic and Metamorphic Rocks Metamorphic Rocks Detritic Rocks Type IIB Metamorphic-Metasomatism (Fluid-Rock Types of Deposit TYPE I Magmatic-Metamorphic Type IIA Metamorphic sensu-stricto TYPE III (Placers) Interaction)

Sub-Types TYPE IA TYPE IB TYPE IIA1 TYPE IIA2 Type IIB1 Type IIB2 Type IIIA Type IIIB Alkali Basalt Kimberlite * Mafic-Ultramafic Rocks (M-UMR) * Marble Plumasite * or Metasomatite Shear-Fault-fold Rudites–Arenites–Lutites Host-rock of Ruby in Gneiss, Schist, Calc-Silicate Xenocryst-Xenolith Xenocryst "Verdite" "Anyolite" Metamorphosed Mafic M-UMR Marble M-UMR Alkali Basalt- Metamorphic Rocks M-UMR Gneiss Kimberlite Fields Environement Metamorphic (and Corundum Metamorphicruby Ruby** Ruby Ruby Ruby Ruby* Ruby** Ruby Ruby Ruby** magmatic?) ruby DRC Tanzania Afghanistan South Africa Tanzania Madagascar Australia (Lava Sri Lanka Zimbabwe Tanzania India Deposits Worldwide (Mbuji-Mayi) (Winza). (Jegdalek). (Transvaal). (Mahenge ¤¤, (Sahambano *; Plains; (Ratnapura; Madagascar Nepal (Chumar; Zimbabawe Zazafotsy *; xenocrysts (O'Briens) (Longido) (Mysore). Kitwalo and Anakie fields; Elahera, (Andriba; Ruyil). (O'Briens). Ionaivo *, Pakistan (Hunza Kenya (Mangare South Africa Bekily-Vohibory (Ejeda; Greyson mines). Ambatomena **). New England fields; Polonnaruwa). All in alkali basalt valley; area, environment with the Batakundi; Aqua; Penny (Barberton) Ianapera; Fotadrevo; India (Kerala). Macquarie-Cudgegong; Myanmar (Mogok; presence of either Nangimali). lane; disseminated ruby Tajikistan Rockland-John Kenya (Mangare Anavoha, Gogogogo, Barrington Tops; Mong Hsu). xenocrysts in (Turakuloma**; Saul mine; area). volcanoclastics, Tanzania Madagascar Maniry; Vohitany; Badakhshan). Hard Rock). Tumbarumba; epiclastics, diatremes, (Morogoro, (Ilakaka****; or in ruby-bearing Myanmar Tanzania Ethiopia (Kibre mengist). Mahenge; Western Melbourne Andilamena; Didy; xenoliths of (Mogok; (Umba-Kalalani). pyroxenite and Madagascar Uluguru Kenya (Kitui; Mangare). Mong Hsu). fields). Zahamena). metagabbro (Vohibory area - Mountains ***). (Antanifotsy, Bekily, Soamiakatra*, Malawi (Chimwadzulu). China (Qinghai; Anavoha***; China (Muling). Vietnam (Luc Yen; Madagascar) Vohitany; Yen Bai; Quy Mozambique (Montepuez; Yuan Jiang). Andilamena). Thailand (Chantaburi- Chau). Russia (Polar M'sawize; Ruambeze). Vietnam (Luc Yen; Trat); Tanzania (Tunduru; Urals, Hit Island; Yen Bai; Quy France (Brittany; Lozère; Karelia). Cambodia (Pailin). Songea; Winza; Chau). Australia (Poona Tanzania French Massif Central, ****; Harts Madagascar Umba valley,. (Morogoro; Range¤). United-States Morogoro; Haut-Allier, Peygerolles). Mahenge). (Ankaratra massif; (Corundum Hill). Mahenge). Kenya (West France (Haut Mozambique Italy (Piemont). Vatomandry). Pokot). Allier, Chantel) (Montepuez; Canada Greenland M'sawize; Norway (Froland). Kenya (Baringo). (Revelstoke ***). (Aappaluttoq; Ruambeze). Macedonia Malawi Finland (Kittilä). Nuuk-Stovø); Democratic Republic of (Prilep). (Chimwadzulu). Greece (Paranesti). Switzerland Kangerdluarssuk). Congo (Mbuji-Mayi). United-States (North Pakistan (Dir). (Campo Lungo). India (Kerala) Brazil (São Luis).*** Carolina-Cowee United-States India (Orissa; Kerala). France (Pyrenees). Creek) (North Carolina). New Zealand (Hokitika) Russia (Urals). Spain (Alboran Sea) Greece (Xanthi). New Zealand (Hokitika). Minerals 2020, 10, 597 28 of 83

Table 3. Cont.

Type of Deposit Magmatic-Related Metamorphic-Related Sedimentary-Related * Geological Magmatic Metamorphic Sedimentary Environment Metamorphic Amphibolite to Granulite or Eclogite Greenschist–Amphibolite–Granulite Facies No Metamorphism Conditions Facies Host-Rocks Magmatic and Metamorphic Rocks Metamorphic Rocks Detritic Rocks Type IIB Metamorphic-Metasomatism (Fluid-Rock Types of Deposit TYPE I Magmatic-Metamorphic Type IIA Metamorphic sensu-stricto TYPE III (Placers) Interaction)

Sub-Types TYPE IA TYPE IB TYPE IIA1 TYPE IIA2 Type IIB1 Type IIB2 Type IIIA Type IIIB Alkali Basalt Kimberlite * Mafic-Ultramafic Rocks (M-UMR) * Marble Plumasite * or Metasomatite Shear-Fault-fold Rudites–Arenites–Lutites Host-rock of Ruby in Gneiss, Schist, Calc-Silicate Xenocryst-Xenolith Xenocryst "Verdite" "Anyolite" Metamorphosed Mafic M-UMR Marble M-UMR Alkali Basalt- Metamorphic Rocks M-UMR Gneiss Kimberlite Fields Environement * desilicated *very rare economic *very rare *including metamorphosed mafic-ultramafic rocks, * include pink ***** ruby * in biotitites with * netotectonics and **** placers are Remarks -pegmatite or deposit occurrence mafic corundum. associated sapphires basins. hosted -gneiss gneisses, and special cases of retromorphosed UMR as ** Al-rich and other -felsic ** circulation of ** include pink with emerald. by detritic rocks of verdites metapelite or metamorphic fluids in MR sapphire. metabauxite **** circulation of and anyolite. rocks. ¤ in amphibolite. *** inclusion of ruby the Triassic Isalo intercalated fluids ** sometimes ¤¤metasomatism ** most of the time "corundum". in marble in folds hinge. in diamond. sedimentary serie, corundumite. of *** metapelite *** rocks called mafic dyke in unknown origin for intercalated sakenites by marble. ruby. in marble Lacroix (1922). carving, Economy non-economic economic non-economic economic economic economic economic economic and gems ± for some deposits for some deposits for some deposits for some deposits for most of the deposits Minerals 2020, 10, 597 29 of 83

6. The Different Types of Ruby Deposits We will now examine the main ruby deposits worldwide using the above classification.

6.1. Magmatic-Related (Type I)

6.1.1. Sub-Type IA: Metamorphic Rubies either as Xenocrysts or Xenoliths in Magmatic Rocks such as Alkali Basalts (See Table 3) During recent decades, studies of sapphire and ruby xenocrysts in alkali basalts focused on the nature of solid and fluid inclusions [11,75,85,145–150], trace element chemistry [70,72–74,81,150], oxygen isotopes [40,144,151–153], and the nature of their parental xenoliths [75,77,81,145,154] to show that gem corundum may have different magmatic and metamorphic origins. Whatever their origin, these rubies are classified in the Type I category because the classification is based on their geological environment and not on their genetic origin. The origin of ruby has long been debated but its metamorphic origin was accepted based on the work done by Australian researchers on different deposits from eastern Australia [68,106], work that focused on trace element diagrams, mineral inclusions, and oxygen isotopes [155]. A metamorphic origin has been questioned once more by recent studies showing melt inclusions in ruby from the Chanthaburi-Trat region of Thailand and Pailin in Cambodia [105]. The metamorphic origin of xenocrystic rubies as in Eastern Australia, the Kanchanaburi-Bo Rai and Nam Yuen deposits in Thailand, and Pailin in Cambodia, and Central and Eastern Madagascar is characterized by their trace element distributions and inclusions. The classical Fe/Ti versus Cr/Ga chemical variation diagram proposed by [74] permits characterization of “metamorphic” ruby and pink corundum, which are rich in Cr and poor in Ga (Cr2O3/Ga2O3 ratio > 3), and contain inclusions of chromiferous spinel, pleonaste, Al-rich , and sapphirine. Recently, [81] characterized rubies (up to 3370 ppm Cr) and pink corundum (up to 1520 ppm Cr) from New England placers, which have distinctive trace element features compared to those found elsewhere in Eastern Australia. In particular they exhibit very high contents of Ga (up to 310 ppm) and Si (up to 1820 ppm). High Ga and Ga/Mg values are unusual in ruby, and the trace elements suggest a magmatic–metasomatic influence for the genesis of New England rubies. The oxygen isotopes (δ18O = 4.4 0.4%, n = 22) fit within the range ± of δ18O values defined for ruby hosted in M-UMR. Zoned ruby and pink corundum (Figure 23A) have high Mg, Ga, and Cr and the Fe versus Ga/Mg diagram suggests magmatic affinity (Figure 23B). The geological origin of New England rubies before their transport by alkali basalts remains a great mystery. Sutherland et al. [81] proposed for their formation either the interaction of intrusive fractionated I-type granitoid magmas, during the New England orogeny, with Cr-bearing ophiolites, or ruby-bearing eclogites and -cliopyroxenites in metamorphosed M-UMR. It is important to note that the different chemical parameters (such as Ga/Mg, Fe/Ti, Fe/Mg, and Cr/Ga) proposed by [70] were designed only for origin determination of blue sapphires. In some cases, the use of these chemical diagrams is not useful for discriminating geologic or geographic origin of corundum. In the case of Australian rubies, the majority of the data are scattered in the field of metamorphic sapphires, but those of New England overlap the field of magmatic ones. The New England case, and other as for Chanthaburi-Trat and Pailin rubies illustrate the complexity and ambiguity of the use of trace element chemistry [11]. These chemical domains have no genetic significance for some cases and they illustrate only the variation of metals in the parental fluids before their incorporation in the structure during crystal growth, i.e., a kind of chemical color signature. On the contrary, the Fe (ppm) versus Ga/Mg discrimination diagram applied to ruby (see Figure 19) shows that the high variation of Fe in ruby correlates with the geological classification of ruby deposits characterized by decreasing Fe in ruby from M-UMR to marble and plumasite. The overlap is not extensive, as it is for sapphires and the diagram can apparently be used for deciphering the geological origins of ruby. Minerals 2020, 10, 597 30 of 83 Minerals 2019, 9, x FOR PEER REVIEW 30 of 82

FigureFigure 23. 23. RubiesRubies from from the theNew New England England locality locality of Eastern of Eastern Australia. Australia. (A) Apex ( Aculet) Apex view culet of an viewoval-cut of an2.31 oval-cut carats zoned 2.31 ruby carats showing zoned alte rubyrnating showing growth alternating zones passing growth through zones violet passing to white through and pink violet bands to whiteand to andruby pink on the bands extreme and margin to ruby of onthe the top extremetable, Collection margin Australian of the top Museum. table, Collection Photo: Gayle Australian Webb; Museum.(B) Fe (ppm) Photo: versus Gayle Ga/Mg Webb; diagram (B) Fefor (ppm) ruby and versus pink, Ga violet,/Mg diagramand white for corundum ruby and from pink, New violet, England and whiteruby (NER) corundum and other from Eastern New England Australian ruby localities; (NER) andchem otherical data Eastern from Australian [86]. The red localities; symbols chemical indicate ruby, data fromthe blue [86]. triangles The red symbolsmetamorphic indicate sapphire ruby, thesuites, blue and triangles the blue metamorphic squares magmatic-transitional sapphire suites, and sapphire the blue squaressuites. The magmatic-transitional different localities are: sapphire NSW = suites.New South The diWales;fferent BAR localities = Barrington are: NSW Tops;= NewYAR = South Yarrowitch; Wales; BARC-G == Cudgegong-Gulgong;Barrington Tops; YAR MAR= Yarrowitch; = Macquarie C-G River;= Cudgegong-Gulgong; WR = Weld River, NE MAR Tasmania;= Macquarie PAC = River;Pailin, WRCambodia;= Weld BPT River, = Bo NE Phloi, Tasmania; Thailand; PAC HSL= =Pailin, Huai Sai, Cambodia; Laos; CSC BPT = Changle,= Bo Phloi, Shandong, Thailand; China; HSL DNV= =Huai Dak Sai,Nong, Laos; Vietnam; CSC = CTHChangle, = Chanthaburi-Trat, Shandong, China; Thailand; DNV RMC= Dak = Rio Nong, Mayo, Vietnam; Colombia. CTH = Chanthaburi-Trat, Thailand; RMC = Rio Mayo, Colombia. Mineral inclusions are also key features in the debate about metamorphic versus magmatic origin of rubies.Mineral The inclusionsmost common are also solids key featuresare clinopyroxene in the debate (Al-rich about metamorphic diopside), plagioclase versus magmatic (bytownite origin ofto rubies.andesine), The K--spinel most common solids associat are clinopyroxeneion [148,149], garnet (Al-rich (pyrope, diopside), ± almandine), plagioclase sapphirine, (bytownite scapolite, to andesine), Cr- K-feldspar-spinel association [148,149], garnet (pyrope, almandine), sapphirine, scapolite, Cr-spinel, spinel, meionite, phlogopite, anatase, apatite, sillimanite,± and sulphides [5,68,77,105,145,146,154,156]. All meionite,of these minerals phlogopite, are described anatase, in apatite, rubies sillimanite,found in metamorphic and sulphides environment,[5,68,77,105 although,145,146 some,154,156 of ]them. All are of thesealso found minerals in magmatic are described assemblages. in rubies The found discovery in metamorphic of melt inclusions environment, in ruby although reopened some the debate of them about are alsotheir foundpossible in magmatic magmatic origin assemblages. [105,148,149]. The Glassy discovery melt of inclusions melt inclusions are described in ruby frequently reopened for the sapphires debate aboutin alkali their basalts possible [157] magmatic and lamprophyres origin [105 [147,158],148,149 ].but Glassy not commonly melt inclusions in rubies. are describedMelt inclusions frequently in rubies for sapphiresfrom Thailand in alkali and basalts Cambodia [157] were and lamprophyres reported by [ [16],147 ,recognized158] but not by commonly [156], and in confirmed rubies. Melt by inclusions[148,149]. inTwo rubies types from of melt Thailand inclusions and Cambodiawere observed were in reported the Thai byand [16 Cambod], recognizedian rubies by [156[106]:], and(1) glassy confirmed melt byinclusions [148,149 with]. Two negative types crystal of melt morphology, inclusions weregenera observedlly 25–100 in µm the in Thaisize, and with Cambodian a contraction rubies bubble; [106]: µ (1)(2) glassypolycrystalline melt inclusions aggregates, with 200–500 negative µm crystal across, morphology, considered generallyto be recrystallized 25–100 m melt in size, inclusions. and with The a µ contractionEMPA analyses bubble; of two (2) polycrystallinemelt inclusions aggregates, in ruby from 200–500 Thailandm across,and Cambodia considered gave to besimilar recrystallized chemical melt inclusions. The EMPA analyses of two melt inclusions in ruby from Thailand and Cambodia gave compositions in wt % oxides: 53.4. < SiO2 < 55; 23.6 < Al2O3 < 24; 4.9 < CaO < 6.2; 4.4 < Na2O < 5.2; 0.1 < K2O similar chemical compositions in wt % oxides: 53.4. < SiO < 55; 23.6 < Al O < 24; 4.9 < CaO < 6.2; < 3.1; 1.2 < FeO < 1.6; 1.4 < MgO < 1.9; 0.1 < TiO2 < 0.2, and 6.32 < volatiles < 7.5.2 3These chemical data are 4.4comparable< Na2O < to5.2; those 0.1 < foundK2O

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The main discussion that arises from this genetic scenario is that the proposed anorthosite protolith for ruby formation has been affected by deformation and consequent metamorphism at high temperature. Under these conditions, how can we define the exact nomenclature and the genesis of ruby, i.e., magmatic origin (anorthosite) or metamorphic (clinopyroxenite or metagabbro in the eclogite facies),Minerals or both? 2019, 9 The, x FOR following PEER REVIEW studies are examples of the ongoing debate: 31 of 82 (1) Sutthirat et al. [145] reported ruby (0.27 wt % Cr O ) included in a clinopyroxene (0.22 wt % ruby, i.e., magmatic origin (anorthosite) or metamorphic2 3(clinopyroxenite or metagabbro in the Cr O ; 16.7 wt % Al O ; and 1.8 wt % Na O) xenocryst in the Nong Bon alkali basalt, one of the 2 eclogite3 facies), or2 both?3 The following studies2 are examples of the ongoing debate: Cenozoic Chantaburi-Trat basalts of Eastern Thailand. Ruby from the alluvial deposits contains (1) Sutthirat et al. [145] reported ruby (0.27 wt % Cr2O3) included in a clinopyroxene (0.22 wt % inclusionsCr2O3; of16.7 garnet wt % (pyrope),Al2O3; and sapphirine, 1.8 wt % Na and2O) xenocryst Al-Na-clinopyroxene. in the Nong Bo Thermodynamicn alkali basalt, one calculationof the constrainedCenozoic the Chantaburi-Trat temperatures ofbasalts ruby +ofclinopyroxene Eastern Thailand. crystallization Ruby from tothe between alluvial 800deposits and 1150contains100 C ± ◦ and pressuresinclusions of garnet between (pyrope), 10 and sapphirine, 25 kbar, correspondingand Al-Na-clinopyroxene. to depths Thermodynamic of 35–90 km. calculation The pyrope + rubyconstrained assemblages the formed temperatures at depths of ruby greater + clinopyroxene than 60 km crystallization and the source to between rock proposed800 and 1150 was ± 100 a mafic ruby-pyrope-pyroclasite°C and pressures of between or ruby-pyrope-clinopyroxenite. 10 and 25 kbar, corresponding to Sutthirat depths of et 35–90 al. [ 145km.] The rejected pyrope a + magmatic ruby crystallizationassemblages for formed the ruby at depths and this greater was confirmedthan 60 km byand later the source studies rock of Thaiproposed ruby was [148 a, 149mafic,154 ruby-]. pyrope-pyroclasite or ruby-pyrope-clinopyroxenite. Sutthirat et al. [145] rejected a magmatic (2) Sutherland et al. [75] studied a suite of alluvial-sourced pink corundum and ruby (up to crystallization for the ruby and this was confirmed by later studies of Thai ruby [148,149,154]. 0.6 wt %(2) Cr Sutherland2O3) from et the al. paleodrainage[75] studied a suite deposits of alluvi alongal-sourced the pink Cudgegong-Macquarie corundum and ruby (up River to 0.6 inwt New South% Wales, Cr2O3) from Australia. the paleodrainage The ruby containeddeposits along inclusions the Cudgegong-Macquarie of clinopyroxene River (0.1 in wt New % Cr South2O3 ;Wales, 20.3 wt % Al2O3Australia.; and 1.0 The wt ruby % Nacontained2O), meionite, inclusions and of clinopyroxene anatase. Primary (0.1 wt % fluid Cr2O3 inclusions; 20.3 wt % Al are2O3 either; and 1.0 two-phase wt % (liquidNa+2O),gas) meionite, or three-phase and anatase. (liquid Primary+ gas fluid+ inclusionsdaughter are phases), either two-phase but melt (liquid inclusions + gas) were or three-phase not reported. The comparison(liquid + gas of+ daughter clinopyroxene-ruby phases), but melt thermometry inclusions were at Cudgegong-Macquarie not reported. The comparison suggested of clinopyroxene- ruby formation at temperaturesruby thermometry between at Cudgegong- 1000 and 1300Macquarie◦C. The suggested P–T diagram ruby formation showing at thermometric temperatures between paths for 1000 Al-rich diopside-rubyand 1300 °C. pairs The from P–T diagram different showing ruby deposits thermometric in Thailand paths for and Al-rich Australia diopside-rub representedy pairs in from Figure different24 [75 ,81]. Saminpanyaruby deposits et al. [in146 Thailand] confirmed and Australia the presence represente of inclusionsd in Figure of garnet24 [75,81]. (pyrope), Saminpanya Al-rich et diopside, al. [146] and confirmed the presence of inclusions of garnet (pyrope), Al-rich diopside, and sapphirine in Thai rubies. sapphirine in Thai rubies. They proposed that the rubies crystallized in high-pressure metamorphic rocks They proposed that the rubies crystallized in high-pressure metamorphic rocks of UMR composition in of UMRthe compositionupper mantle. in the upper mantle.

FigureFigure 24. Thermometric24. Thermometric traces traces for for mineral mineral inclusioninclusion pairs pairs for for rubies rubies from from Australia Australia (Ruby (Ruby Hill, Hill, Barrington Tops, and Macquarie River) and Thailand (Nong Bon), modified from [81]. The P (GPa)– Barrington Tops, and Macquarie River) and Thailand (Nong Bon), modified from [81]. The P (GPa)–T T (°C) diagram is based on Al- in diopside/garnet and sapphirine/spinel pairs, after [75,136]. The ( C) diagram is based on Al- in diopside/garnet and sapphirine/spinel pairs, after [75,136]. The traces ◦ traces intersect the T range for Barrington Tops rubies (vertical lines), the East Australian ruby- intersectbearing the xenolith-derived T range for Barrington Mesozoic Topsgarnet rubies granulite (vertical (RH) and lines), late theCenozoic East Australian(SEA) geotherms, ruby-bearing the xenolith-derivedgarnet to spinel Mesozoic pyroxenite garnet transition granulite of [159] (RH) (H), and and the late garnet-clinopyrox Cenozoic (SEA)ene-plagioclase geotherms, transition the garnet to spinelof pyroxenite [160] (I). The transition red area represents of [159] (H),P 6–8 and kbar the for garnet-clinopyroxene-plagioclasethe Barrington Tops ruby suite. The extreme transition Al-rich of [ 160] (I). Thediopside red area and represents meionite-bearing P 6–8 kbar Macquarie for the Barrington River ruby Tops suggests ruby suite.a P–T The crystallization extreme Al-rich between diopside and meionite-bearingthermometric trace Macquarie (MR) and the River meionite ruby suggestsstability boundary a P–T crystallization (Me). between thermometric trace (MR) and the meionite stability boundary (Me). (3) Sutthirat et al. [154] studied different types of xenoliths contained in basalts from Bo Rai viz. (1) peridotites; (2) clinopyroxenites; and (3) ruby-bearing mafic granulites with plagioclase-clinopyroxene-

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(3) Sutthirat et al. [154] studied different types of xenoliths contained in basalts from Bo Rai viz. (1) peridotites; (2) clinopyroxenites; and (3) ruby-bearing mafic granulites with plagioclase-clinopyroxene- spinel-garnet. Peridotitic xenoliths have characteristics of depleted mantle (lherzolite to harzburgite and dunite) andMinerals the 2019 P–T, 9, x estimates FOR PEER REVIEW yielded approximately 1000–1300 ◦C at 1.6–1.9 GPa. Clinopyroxenite32 of 82 and mafic granulites xenoliths are mantelic cumulates formed at P-T, respectively, between 950 and 1200 ◦C and spinel-garnet. Peridotitic xenoliths have characteristics of depleted mantle (lherzolite to harzburgite and 1.6 and 2.2 GPa, and 700 and 1000 ◦C and 0.95 and 1.3 GPa. The xenoliths suffered a few metamorphic dunite) and the P–T estimates yielded approximately 1000–1300 °C at 1.6–1.9 GPa. Clinopyroxenite and events andmafic the granulites authors xenoliths argued are that mant rubyelic formationcumulates formed is clearly at P-T, related respectively, to mafic between granulite 950 and xenoliths 1200 °C (which appear toand have 1.6 and formed 2.2 GPa, at and about 700 30–50and 1000 km °C depths and 0.95 before and 1.3 transportGPa. The xenoliths by basaltic suffered eruption). a few metamorphic (4) Theevents Soamiakatra and the authors ruby argued deposit that ruby in theformation Antsirabe is clearly area, related Antananarivo to mafic granulite province, xenoliths was (which an economic in situ miningappear to site have such formed as at about Pailin, 30–50 Cambodia, km depths before where transport ruby was by basaltic found eruption). either as xenocrysts in alkali basalt or in xenoliths(4) The Soamiakatra brought ruby to deposit the surface in the Antsirabe by the Cenozoic area, Antananarivo Ankaratra province, volcanism was an economic [77]. The in deposit situ mining site such as at Pailin, Cambodia, where ruby was found either as xenocrysts in alkali basalt or at Moraranoin xenoliths known brought as Soamiakatra to the surface by [95 the,161 Cenozoic–163] Ankaratra was discovered volcanism in [77]. 1997. The deposit Ruby hasat Morarano been extracted by panningknown from as Soamiakatra alluvial and [95,161–163] soils, butwas prospectingdiscovered in 1997. and Ruby mechanization has been extracted of operations by panning from exposed the weatheredalluvial primary and soils, deposit but prospecting at the top and of mechanization the basaltic of operations (Figure exposed25A). the The weathered ruby-bearing primary clay and phylliticdeposit material at the of top whitish of the basaltic color (20plug m (Figure thick) 25A). was Th exploitede ruby-bearing for clay two and years phyllitic before material bedrock of whitish was reached (Figure 25colorB). (20 The m secondarythick) was exploited deposit for assured two years an before easy bedrock and e ffwasective reached ruby (Figure production 25B). The ofsecondary high quality. deposit assured an easy and effective ruby production of high quality.

Figure 25.FigureThe 25. Soamiakatra The Soamiakatra ruby ruby deposit deposit (Madagascar). (Madagascar). (A ()A View) View of the of quarry the quarry showing showing the contact the contact of the alkali basalt (β) plug with the Neoproterozoic metamorphic formations (Nmf). The economic of the alkali basalt (β) plug with the Neoproterozoic metamorphic formations (Nmf). The economic weathered upper zone of the alkali basalt (β) was mined in the past up to the non-weathered basalt; weathered(B) aspect upper ofzone the ruby-bearing of the alkali weathered basalt basaltic (β) was zone, mined around in 20 them thick, past mined up to in thebenches; non-weathered (C) basalt; (Bruby-bearing) aspect of metagabbro the ruby-bearing xenolith (mg) weathered carried by basaltic the alkali zone, basalt around (β); (D) 20 macroscopic m thick, minedaspect of in a benches; (C) ruby-bearingruby (Crn)-bearing metagabbro metagabbro xenolith xenolith (mg) (mg); carried (E) xenocryst by the alkali of ruby basalt with an (β olivine); (D) (Ol) macroscopic crystal in the aspect of a ruby (Crn)-bearingalkali basalt (β metagabbro); and (F) ruby-garnet xenolith (Grt)-plagioclase (mg); (E) xenocryst (Pl)-bearing of ruby metagabbro with an crosscut olivine by (Ol) the alkali crystal in the basalt (β) flow. Photos: Gaston Giuliani and Saholy Rakotosamizanany. alkali basalt (β); and (F) ruby-garnet (Grt)-plagioclase (Pl)-bearing metagabbro crosscut by the alkali basalt (β) flow. Photos: Gaston Giuliani and Saholy Rakotosamizanany.

Minerals 2020, 10, 597 33 of 83 Minerals 2019, 9, x FOR PEER REVIEW 33 of 82

The alkali basalt dyke intruded graphitic gn gneisses,eisses, khondalites, and migmatites of the Neoproterozoic AmbatolampyAmbatolampy andand TolongoinaTolongoina series.series. TheThe alkalialkali basaltbasalt hostshosts numerousnumerous enclaves and xenoliths (Figure 25C),25C), 2–20 cm across, of crustal (gneiss,(gneiss, migmatite, and granitoid) and mafic mafic (peridotite, clinopyroxenite, gabbro, gabbro, and and garnetite) garnetite) origin. origin. Ruby Ruby was was found: found: (1) (1)as xenocrysts as xenocrysts up upto 2 to cm 2 cmacross across in the in alkalithe alkali basalt basalt (Figure (Figure 25E);25 (2)E); in (2) clinopyroxenite in clinopyroxenite xenoliths xenoliths composed composed of plagioclase of plagioclase ( (labradorite and bytownite),and bytownite), scapolite scapolite (meion (meionite),ite), garnetgarnet (pyrope), (pyrope), clinopyroxene clinopyroxene (Al-diopside), (Al-diopside), orthopyroxene orthopyroxene (85 mol. (%85 En), mol and % En pargasite;), and pargasite; and (3) in and banded (3) in metagabbro banded metagabbro (Figure 25D,F) (Figure composed25D,F) composed of alternating of alternating dark and greyishdark and ruby-bearing greyish ruby-bearing bands. The bands.dark bands The darkcontai bandsn pyrope contain (Figure pyrope 26A,C), (Figure plagioclase26A,C), (labradorite- plagioclase (labradorite-bytownite),bytownite), Mg (±Cr)-spinel, Mg scapolite ( Cr)-spinel, (meionite), scapolite Al-diopside, (meionite), and Al-diopside, orthopyrox andene (85% orthopyroxene mol. En). The (85 ± greyishmol % En).band is The made greyish up of bandlabradorite-bytownite, is made up of labradorite-bytownite, diopside, magnesian spinel, diopside, sapphirine magnesian (Figure spinel, 26D), somesapphirine calcite, (Figure and pyrope.26D), Diopside some calcite, formed and as pyrope.a clean and Diopside clearly formedvisible corona as a clean around and both clearly corundum visible andcorona Mg around (±Cr)-spinel both corundum(Figure 26D). and Two Mg generations ( Cr)-spinel of (Figure clinopyroxene26D). Two are generationsdiscriminated of (Figure clinopyroxene 26A,B): are(1) ± earlydiscriminated Al-rich crystals (Figure (13–17.526A,B): (1)wt early % Al Al-rich2O3), 1–2 crystals mm (13–17.5across, associated wt % Al2O with3), 1–2 plagioclase, mm across, garnet, associated and withscapolite plagioclase, (Cpx1), and garnet, (2) late and and scapolite less Al-rich (Cpx1), crystals and (2) (5–8.8 late wt and % less Al2O Al-rich3, Cpx2), crystals 10–40 (5–8.8µm across, wt % formed Al2O3, aroundCpx2), 10–40Cpx1 (Figureµm across, 26B) formed or as inclusions around Cpx1in ruby. (Figure 26B) or as inclusions in ruby.

Figure 26. PetrographicPetrographic aspects aspects of ofthe the metagabbroic metagabbroic xenoliths xenoliths in the in theSoamiakatra Soamiakatra ruby ruby deposit. deposit. (A) (PlagioclaseA) Plagioclase (Pl)-bearing (Pl)-bearing ruby ruby (Crn) (Crn) within within garnet garnet (Grt). (Grt). Garnet Garnet (pyrope) (pyrope) associated associated with with an an Al-rich clinopyroxene (Cpx1) (Cpx1) and and showing showing a a keliphitic keliphitic texture. texture. The The keliphitic keliphitic halo halo is iscomposed composed of ofsmall small grains grains of chloriteof chlorite (Chl). (Chl). The spinel The spinel (Spl) and (Spl) Cpx1 and are Cpx1 bordered are bordered by a second by ageneration second generation of clinopyroxene of clinopyroxene (Cpx2); (B) (Cpx2);association (B) ruby-Cpx1-plagioclase association ruby-Cpx1-plagioclase (Pl)-spinel. The (Pl)-spinel. ruby formed The upon ruby Cpx1 formed while upon small Cpx1 crystals while of smallCpx2 formedcrystals later; of Cpx2 (C) formedinclusions later; of (Cpx1C) inclusions in pyrope of (Grt); Cpx1 inand pyrope (D) association (Grt); and spinel-ruby (D) association with spinel-ruby Cpx1 with withplagioclase. Cpx1 with Spinel plagioclase. is bordered Spinel by sapphiri is borderedne (Spr). by Photos: sapphirine Saholy (Spr). Rakotosamizanany. Photos: Saholy Rakotosamizanany. Mineral inclusions in the ruby consist of phlogopite, rutile with some lamellar exolutions of Mineral inclusions in the ruby consist of phlogopite, rutile with some lamellar exolutions of ilmenite, zircon, albite, Al-pyroxene, garnet (pyrope), and Mg-spinel. The trace element chemistry of ilmenite, zircon, albite, Al-pyroxene, garnet (pyrope), and Mg-spinel. The trace element chemistry of the ruby is characterized by low Ga2O3 (between 70 and 110 ppm) and high Cr2O3/Ga2O3 ratios (see the ruby is characterized by low Ga2O3 (between 70 and 110 ppm) and high Cr2O3/Ga2O3 ratios (see Figure 16). Vanadium varies from less than 22 to 860 ppm. ranges between 60 and 940 ppm, Figure 16). Vanadium varies from less than 22 to 860 ppm. Titanium ranges between 60 and 940 ppm, and Cr between 350 and 3830 ppm. The Soamiakatra rubies fall into the metamorphic domain defined and Cr between 350 and 3830 ppm. The Soamiakatra rubies fall into the metamorphic domain defined by [68] when plotted on the Fe2O3/TiO2 versus Cr2O3/Ga2O3 diagram (see Figure 16). by [68] when plotted on the Fe2O3/TiO2 versus Cr2O3/Ga2O3 diagram (see Figure 16). Petrographic examinations showed two main stages for the mineral assemblages: (1) the first stage Petrographic examinations showed two main stages for the mineral assemblages: (1) the first is typified by the association Cpx + garnet + scapolite + ruby, and spinel + ruby + garnet. The spinel stage is typified by the association Cpx + garnet + scapolite + ruby, and spinel + ruby + garnet. The spinel is always surrounded by a of ruby, clinopyroxene, and sapphirine, and the mineral association

Minerals 2020, 10, 597 34 of 83 is always surrounded by a ring of ruby, clinopyroxene, and sapphirine, and the mineral association is in turn haloed by coronitic garnet. Thermodynamic calculations constrained the P–T estimates of the assemblages to around 1100 ◦C and 2 GPa; (2) the second stage results in the destabilization of scapolite to anorthite + calcite with the destabilization of some Al-diopside in pargasite, and also the reaction spinel + ruby to sapphirine (T 1100 ◦C and P 15 GPa). The P–T conditions of around 1100 ◦C and 2 GPa clearly show that the xenoliths originated from the upper mantle and that the metagabbros experienced the eclogite facies. During decompression the mineral assemblage was marked by the destabilization of Cpx1 and the formation of Cpx2 (in the granulite facies). Rakotosamizanany et al. [77] hypothesized that the ruby formed in M-UMR at the base of the lower crust-upper mantle and was later transported to the surface by the alkali basalt. The oxygen isotopic composition of the rubies (1.25% < δ18O < 5%, n = 5) is within the range of values defined for ruby in basaltic environments (1.25% < δ18O < 5.9%, n = 4; mean δ18O = 3.1 1.1%, n = 80) and in ± metamorphosed M-UMR (0.25% < δ18O < 6.8%, n = 19; [163]. Rubies from Chantaburi-Trat volcanics have similar O-isotopic ranges (1.3% < δ18O < 4.2%;[152]) and the P–T conditions are identical to those proposed for the Soamiakatra rubies. (5) Occurrences of ruby in the Neoproterozoic Vohibory formation from Southern Madagascar (see Figure 1E) are found in M-UMR complexes of meta-peridotite, -gabbro, and -troctolite transformed into amphibolites and with anorthositic dykes [116]. The ruby occurrences include Anavoha, Gogogogo, Maniri, Vohitany, Ianapera, and Marolinta [164]. The ruby frequently occurs in sapphirine or gedrite and locally garnet (pyrope-almandine)-bearing amphibolites and locally anorthosite [122]. These rocks are associated with garnet-bearing granulites (salite + almandine + hornblende + andesine). At Anavoha, ruby occurs with (1) sapphirine or gedrite + garnet, and (2) anorthosite with ruby + garnet + anorthite. These ruby-bearing rocks are associated with a coronitic metatroctolite. The transition is sharp between the metatroctolite and the sapphirine or gedrite amphibolites and gradual between the garnet (50% almandine-35% pyrope)-ruby-bearing anorthosite and amphibolites. The ruby is surrounded by a corona of green spinel and anorthite and the mineralogical association is gedrite + anorthite + garnet (60% pyrope) + ruby spinel. Sometimes the ruby is totally replaced by ± spinel. The P–T conditions of ruby formation during the Neoproterozoic metamorphism was estimated at P = 0.9–1.15 GPa and 750–800 ◦C[116], similar to those reported by [75] for the ruby-bearing xenolith of Ruby Hill in Australia. At Vohitany and Anavoha, ruby is also found in desilicated pegmatites at the contact of amphibolites. We previously examined the origin of ruby transported to the surface by basaltic volcanism by studying examples from Thailand, Australia [81], and Madagascar. Mafic cumulate xenoliths are rare worldwide and [154] showed that mantle and deep crustal xenoliths from the Bo Rai ruby deposit were the probable source of “basaltic rubies” mined in placers from Thailand and Cambodia. The Cpx-Spl-Grt (Ol) (Opx) and Pl-Cpx-Spl (Grt) (Crn) assemblages were considered to ± ± ± ± be two different types of mafic cumulates with high Al2O3 contents that suffered metamorphic overprints [154]. P–T constraints based on textural observations gave estimated depths of formation between 30 and 80 km. These xenoliths are similar to those reported for ruby-related alkali basalts from Madagascar [77]. At Soamiakatra, the estimated temperature 1100 ◦C and pressure 2 GPa of ruby formation are comparable to those proposed for mafic cumulates from Bo Rai. Al-rich clinopyroxene-pyrope-anorthite is the main assemblage of these ruby-bearing metagabbros and clinopyroxenites. Spinel and ruby are associated but ruby also formed at the expense of clinopyroxene. The probable original source of the ruby in the mafic cumulates carried by Cenozoic alkali basalt plugs in the Ankaratra massif is visible in the Neoproterozoic metamorphic rocks of the Vohibory formation in Southern Madagascar. The ruby-bearing amphibolites and anorthosites are the fruit of the metamorphic transformation of a magmatic cumulate complex. The presence of melt inclusions as described by [105,148,149] opened the debate about the possibility of “magmatic” ruby. The genetic model proposed by [105], i.e., an anorthosite Minerals 2020, 10, 597 35 of 83 converted at higher pressure to a garnet-clinopyroxenite, is debatable but not applicable for ruby in Madagascar. The melt inclusions can result either from an incongruent melting of plagioclase during metamorphism [165] or directly through a peritectic melting reaction involving plagioclase or other al-rich minerals. In consequence, most rubies in alkali basalts can be considered actually Minerals 2019, 9, x FOR PEER REVIEW 35 of 82 as metamorphic in origin but the presence of primary melt inclusions in ruby [105] indicates that aduring free magma ruby growth. phase Additional existed during studies ruby characterizing growth. Additional carefully studiesthe inclusions characterizing in more basalt-related carefully the inclusionsrubies would in more be helpful basalt-related to shed rubies more would light beon helpful the two to possible shed more origins, light on i.e., the metamorphic two possible origins,versus i.e.,magmatic. metamorphic The ruby-bearing versus magmatic. mineral The assemblages ruby-bearing described mineral assemblagesin M-UMR, describedmetamorphosed in M-UMR, and metamorphosedrecrystallized at depth and recrystallized within the mantle, at depth at Beni within Bousera, the mantle, Morocco at Beni [166,167] Bousera, and MoroccoRonda, Spain [166, 167[97],] andand Ronda,considered Spain of [97 metamorphic], and considered origin, of metamorphiccan be described origin, as can a potential be described restitic as a potentialcomponent restitic of a componentpartially melted of a partially mafic M-UMR melted protolith? mafic M-UMR protolith? 18 TheThe δδ18O values of rubyruby fromfrom thethe Soamiakatra,Soamiakatra, BoBo Rai,Rai, Pailin,Pailin, Barrington,Barrington, Anavoha,Anavoha, andand BeniBeni BouseraBousera occurrences fit fit within within the the worldwide worldwide range range defined defined for forruby ruby in basaltic in basaltic environments environments (1.25 18 18 (1.25%< δ18O < <5.9‰,δ O mean< 5.9% δ18O, = mean 3.5 ± 1.3‰,δ O =n =3.5 57) and1.3% ruby, n associated= 57) and with ruby M-UMR associated (1.25 with< δ18O M-UMR < 7.0‰, 18 18 ± 18 (1.25%mean δ18

FigureFigure 27.27. Oxygen isotope ranges defined defined for didifferenfferentt rubyruby andand pinkpink corundumcorundum depositsdeposits andand occurrencesoccurrences relatedrelated toto metamorphosedmetamorphosed mafic-ultramaficsmafic-ultramafics andand alkalialkali basaltsbasalts worldwideworldwide asas wellwell asas thethe kimberlitekimberlite atat Mbuji-Mayi,Mbuji-Mayi, modifiedmodified fromfrom [[84].84]. TheThe datadata areare reportedreported inin thethe conventionalconventional deltadelta notationnotation relativerelative toto V-SMOWV-SMOW (Vienna(Vienna Standard Standard Mean Mean Ocean Ocean Water). Water). The Theδ δ1818O values for zircon, clinopyroxene, andand garnetgarnet megacrystsmegacrysts fromfrom thethe Mbuji-MayiMbuji-Mayi kimberliteskimberlites areare fromfrom [[163].163]. TheThe depositsdeposits thatthat occuroccur inin amphibolitesamphibolites areare AnavohaAnavoha andand Vohitany (Madagascar),(Madagascar), VialaViala andand PeygerollesPeygerolles (France),(France), Kitui (Kenya), andand WinzaWinza (Tanzania).(Tanzania). TheThe HokitikaHokitika (New(New Zealand)Zealand) andand BeniBeni BouseraBousera (Morocco)(Morocco) occurrencesoccurrences areare linkedlinked toto metamorphosedmetamorphosed M-UMR.M-UMR.

6.1.2. Sub-Type IB: Metamorphic Rubies as Xenocrysts in the Mbuji-Mayi Kimberlite (Democratic Republic of Congo; See Figure 2, Table 3) Ruby has been described as micrometric crystal inclusions in diamond at Juina in Brazil [43,169– 171]. Crystals of ruby and Cr-bearing corundum (up to 2 cm) xenocrysts have been found in the Mbuji-Mayi kimberlite in the Democratic Republic of Congo [84]. The kimberlite contains megacrysts of garnet and diopside [172], zircon and baddeleyite, and also Mg-rich ilmenite, Nb- or Cr-rich rutile, corundum, kyanite, Mg-rich chlorite, and rutile-silicate intergrowths [173]. The 0.5–2 cm-sized corundum megacrysts are:

Minerals 2020, 10, 597 36 of 83

6.1.2. Sub-Type IB: Metamorphic Rubies as Xenocrysts in the Mbuji-Mayi Kimberlite (Democratic Republic of Congo; See Figure 2, Table 3) Ruby has been described as micrometric crystal inclusions in diamond at Juina in Brazil [43,169–171]. Crystals of ruby and Cr-bearing corundum (up to 2 cm) xenocrysts have been found in the Mbuji-Mayi kimberlite in the Democratic Republic of Congo [84]. The kimberlite contains megacrysts of garnet and diopside [172], zircon and baddeleyite, and also Mg-rich ilmenite, Nb- or Cr-rich rutile, corundum, kyanite, Mg-rich chlorite, and rutile-silicate intergrowths [173]. The 0.5–2 cm-sized corundum megacrysts are: (1) Gem rubies from red to fuchsia and flashy-pink deep pink rubies (n = 3) that contain Cr-bearing spinel (with Cr2MineralsO3 up 2019 to, 9, x 1.5 FOR wtPEER%), REVIEW chromite, and rutile. The rubies have very high36 of contents 82 of Cr2O3

(0.71–4.59 wt %) and(1) Gem FeO rubies (0.22–0.66 from red wtto fuchsia %). Vanadium and flashy-pink is deep between pink rubies 986 (n and = 3) 1867that contain ppm, Cr- and Ti less than 68 ppm. The contentsbearing spinel of MgO (with Cr (31–1002O3 up to ppm)1.5 wt %), and chromite, Ga2O and3 (50–193 rutile. The ppm)rubies have range very tohigh moderate contents levels with a low Ga/Mg ratioof Cr (0.6–2.6,2O3 (0.71–4.59 see wt Figure %) and FeO19). (0.22–0.66 wt %). Vanadium is between 986 and 1867 ppm, and Ti less than 68 ppm. The contents of MgO (31–100 ppm) and Ga2O3 (50–193 ppm) range to moderate In the diagramlevels with FeO-Cr a low Ga/Mg2O3-MgO-V ratio (0.6–2.6,2O see3 versus Figure 19). FeO + TiO2 + Ga2O3 [84], the three rubies plot in the domain R3 of M-UMRIn the diagram (Figure FeO-Cr282O3-MgO-V): 2O3 versus FeO + TiO2 + Ga2O3 [84], the three rubies plot in the domain R3 of M-UMR (Figure 28):

Figure 28. ChemicalFigure 28. variation Chemical variation of ruby of ruby and and pinkish pinkish corundum from fromthe Mbuji-Mayi the Mbuji-Mayi kimberlite in the kimberlite in the Democratic Republic of Congo. (A) The FeO-Cr2O3-MgO-V2O3 versus FeO + TiO2 + Ga2O3 (analysis in Democratic Republicwt %) classification of Congo. diagram, (A) modi Thefied FeO-Cr from [84].2O The3-MgO-V different chemical2O3 versus fields of the FeO studied+ TiO corundum2 + Ga 2O3 (analysis in wt %) classificationare reported diagram, with respectively modified those for ruby from (samples [84]. CO1, The CO2, di andfferent CO3) and chemical pinkish corundum fields of the studied (CO, yellowish field); (B) Field of chemical compositions of the ruby and pink sapphires from Mbuji- corundum are reportedMayi. Different with chemical respectively fields are reported those for ruby for rubyfrom Beni (samples Bousera in CO1,Morocco CO2, (BB), Soamiakatra and CO3) and pinkish corundum (CO,(SOAM), yellowish Anavoha field); (ANA), Vohitany (B) Field (VOH), of and chemical Andriba (AND) compositions from Madagascar. of theOtherruby representative andpink sapphires from Mbuji-Mayi.compositions Different of ruby chemical from occurrences fields in kimberlite are reported and diamond, for rubyalkali basalt, from and Beni metamorphic Bousera in Morocco rocks are also shown; modified from [84,164]. (BB), Soamiakatra (SOAM), Anavoha (ANA), Vohitany (VOH), and Andriba (AND) from Madagascar. Other representative compositions of ruby from occurrences in kimberlite and diamond, alkali basalt, and metamorphic rocks are also shown; modified from [84,164]. Minerals 2020, 10, 597 37 of 83

(a) The field of sample CO1 overlaps the plots of rubies from Andriba in Madagascar hosted in ultramafics [174], the Winza deposit in Tanzania located in amphibolite and metagabbro [120], high-grade Proterozoic orthoamphibolites of the Bamble area in Norway [43], and ultrahigh pressure ruby-rich garnetite layers in garnet lherzolite in the Sulu terrane of China [175].

(b) The field of sample CO2 is defined by the very high Cr2O3 content of ruby up to 4.6 wt %. Chromium-rich ruby is rare and found in the Brazilian São Luiz alluvial deposit and in the Juina kimberlite [43,170], the Moses Rock Dyke kimberlite in Utah [176], the metagabbros at Karelia in Russia [41], and in the at Hokitika in New Zealand [42]. (c) The field of sample CO3 is comparable to the chemical composition of ruby-bearing garnetite xenolith in garnet peridotite from the Sulu terrane in China [175].

The rubies have a restricted range of δ18O values from 5.5% to 5.6% (n = 3; Figure 27). They fit within the worldwide O-isotopic range of rubies associated with metamorphosed M-UMR. (2) Cloudy pinkish to greyish corundum (n = 10), which are host to inclusions of Mn-rich ilmenite, margarite, and rutile. They are richer in FeO (0.38–0.75 wt %) than rubies and they always contain some Cr2O3 (0.08–0.58 wt %) and V (318–1128 ppm). The Ga/Mg ratio (1.95–3.95) is in the range of, or above, the Ga/Mg ratio of ruby. They have δ18O values in the range 4.3–5.4% with a mean δ18O value of 4.9 0.4% (n = 10; Figure 27), within the accepted range of mantle values [168]. These oxygen ± isotope values are within the range of values defined for ruby xenocrysts or megacrysts carried to the surface by alkali basalts [84]. They are also similar to those found for ruby in eclogites derived from oceanic crust [166] and hosted by clinopyroxene xenocrysts from Bo Rai alkali basalt alluvials as well as those from the Soamiakatra metagabbros. Kimberlites, like alkali basalts, are conveyors of gems (respectively diamond and corundum) from the asthenosphere and/or lithosphere to the upper crust [86,177]. Ruby and sapphire inclusions in diamond are rare and corundum xenocrysts or megacrysts in kimberlites are exceptional. On the contrary, diamond has never been found as inclusions in corundum or within xenocrysts in alkali basalts. The chemistry and oxygen isotope composition of ruby from Mbuji Mayi allow for different possible origins: (1) An upper mantle-derived material (garnet lherzolite, garnetite and pyroxenite) that could have been infiltrated and metasomatized by the kimberlite melt, or a high-pressure orogenic peridotite; or (2) eclogite-type xenoliths (omphacitic clinopyroxene-bearing metagabbro and/or clinopyroxenite) affected by a retrograde metamorphic stage in the granulite facies at the base of the continental crust (Figure 29). The pinkish to greyish corundum could also have different probable origins: either as subducted oceanic crust transformed into eclogite, or as clinopyroxenite layers/horizons in peridotite.

6.2. Metamorphic-Related (Type II)

6.2.1. Sub-Type IIA: Metamorphic Deposits Sensu Stricto with Two Different Types Formed in Amphibolite to Granulite Facies (See Table 3)

Sub-Type IIA1 includes ruby in metamorphosed mafic and ultramafic rocks (M-UMR) as found at Montepuez in Mozambique (see Table 3). Ruby and pink corundum in metamorphosed M-UMR called “ruby in amphibolite” by some gemmologists originate from the metamorphism of gabbroic and dunitic rocks from ophiolites or volcano-sedimentary sequences. These deposits are the main sources of pink and red corundum but generally not of gem quality material. Minerals 2020, 10, 597 38 of 83 Minerals 2019, 9, x FOR PEER REVIEW 38 of 82

FigureFigure 29. Schematic29. Schematic genetic genetic models models for gem gem corundum corundum related related to kimberlite to kimberlite (A) and (A alkali) and basalt alkali (B basalt) environments, modified from [178–180]. The drawings show the different types of corundum (B) environments, modified from [178–180]. The drawings show the different types of corundum occurrences hosted in Precambrian basements in the crust, and lherzolites in the lithosphere. Selected occurrences hosted in Precambrian basements in the crust, and lherzolites in the lithosphere. Selected ruby occurrences are indicated (with their corresponding δ18O composition, in ‰) including the Beni ruby occurrences are indicated (with their corresponding δ18O composition, in %) including the Beni Bousera (BB) garnet clinopyroxenite, the Soamiakatra (SO) garnet metagabbro and -clinopyroxenite, Bouseraand (BB) the garnetChanthaburi-Trat clinopyroxenite, (CT) pyroxene the Soamiakatra inclusion (SO) in garnetruby, with metagabbro their retrograde and -clinopyroxenite, metamorphic and the Chanthaburi-Tratlocation in the granulite (CT) pyroxene facies. Panel inclusion (B) shows in ruby, ruby with deposits their retrograde associated metamorphicwith alkali basalt location in in the granulitecontinental facies. extension Panel domains (B) shows characterized ruby deposits by ther associatedmal anomalies with alkali linked basalt to mantle in continental plumes and extension rift domainsvalley characterized fracturing and by volcanism thermal anomaliesin the upper linked part of to the mantle crust. plumest1 and t2 and are rifttwo valleysuccessive fracturing ascent and volcanismstages inof the the alkali upper basalt part magmatism. of the crust. t1 and t2 are two successive ascent stages of the alkali basalt magmatism. The crystals are used for cut and ornamental artifacts such as the famous ‘anyolite’ Thefromcrystals Longido arein Tanzania used for [181–183] cabochon (see cut Figure and 4B ornamental). Such ruby artifacts deposits suchin amphibolite as the famous and gabbro ‘anyolite’ fromare Longido known in (see Tanzania Figure 2) [181 from–183 Kittilä] (see in Figure Finland4B). [184], Such Sittampundi ruby deposits in India inamphibolite [185], Chantel and in France gabbro are known[186], (see Paranesti Figure 2 in) fromGreece Kittilä [95,187], in FinlandLossogonoi [184 in], Ta Sittampundinzania [181], inKitui India in Kenya [185], [188], Chantel Chimwadzulu in France [ 186], in Mozambique [189,190], the Vohibory formations in Madagascar [119,122,164] (see Figure 1E), the Paranesti in Greece [95,187], Lossogonoi in Tanzania [181], Kitui in Kenya [188], Chimwadzulu in Harts Range in Australia [191,192], Dir in Pakistan [193], Hokkaido in Japan [194], and some of the Mozambiquedeposits in [189 North,190 ],Carolina the Vohibory in the USA formations [5,121,195]. in Madagascar In Africa, the [ruby119,122 deposits,164] (seediscovered Figure in1E), the the last Harts Rangedecade in Australia are highly [191 economic,192], Dir for in Pakistantheir color, [193 clar], Hokkaidoity, and quantity; in Japan these [194 ],are and Winza some in of Tanzania the deposits in North[120,196,197] Carolina (see in theFigure USA 4C), [5 ,121and, 195at Montepuez,]. In Africa, Ruambeze, the ruby depositsand M’sawize discovered in Mozambique in the last [21– decade are highly23,25,26,96,198] economic (see for Figure their 1C). color, clarity, and quantity; these are Winza in Tanzania [120,196,197] (see FigureThe4 C),common and atmineral Montepuez, assemblage Ruambeze, of the ruby-bearing and M’sawize amphibolite in Mozambique includes corundum, [21–23 ,anorthite,25,26,96 ,198] (see Figureamphibole1C). (gedrite, pargasite), and margarite [21,22,121,122]. Others minerals including sapphirine, Thepyrope-almandine common mineral garnet, assemblage spinel, kornerupine, of the ruby-bearing plagioclase, amphibolite phlogopite, and includes zoisite corundum, may be present. anorthite, The initial basic composition of the protolith includes gabbro, dunite, and troctolite [121,122,194,199]. amphibole (gedrite, pargasite), and margarite [21,22,121,122]. Others minerals including sapphirine, (1) The ruby deposits at Longido and Lossogonoi in Northeastern Tanzania are hosted in pyrope-almandine garnet, spinel, kornerupine, plagioclase, phlogopite, and zoisite may be present. amphibolite dykes that crosscut metamorphosed M-UMR (Figure 30). At Longido, the ruby is located The initialin amphibolite basic composition dykes up to of 1the m thick protolith that are includes oriented gabbro, parallel dunite, to the andmetamorphic troctolite foliation [121,122 [118].,194, 199]. (1)The Thedykes ruby crosscut deposits a serpentinite at Longido with relics and of Lossogonoi olivine and orthopyroxene. in Northeastern They Tanzania are retromorphosed are hosted in amphiboliteto a rockdykes composed that of crosscut zoisite and metamorphosed ruby called “anyolite” M-UMR (green (Figure stone30 in). Massai At Longido, language). the The ruby contact is located in amphiboliteof the dyke dykeswith the up serpentinite to 1 m thick is outlined that are by orientedthe presence parallel of phlogopite to the metamorphicand Mg-amphibole foliation schist. [ 118]. The dykes crosscut a serpentinite with relics of olivine and orthopyroxene. They are retromorphosed to a rock composed of zoisite and ruby called “anyolite” (green stone in Massai language). The contact of the dyke with the serpentinite is outlined by the presence of phlogopite and Mg-amphibole schist. Minerals 2020, 10, 597 39 of 83 Minerals 2019, 9, x FOR PEER REVIEW 39 of 82

The mineral assemblage inin thethe amphibolite isis anorthiteanorthite+ + corundum ++ amphibamphiboleole + greengreen spinel spinel while while the anyolite contains zoisite + corundum . The age of formation of the ruby obtained on the anyolite contains zoisite + corundum± ± tschermakite. The age of formation of the ruby obtained onzircon zircon by theby the U/Pb U/Pb method method is 610 is 610 Ma Ma [124 [124].]. (2) At the Winza deposit in central Tanzania, high-quality ruby and sapphires have been recovered from an eluvial deposit. Mining reveal revealeded that the gem corundum crystals are embedded in dark dark amphibolite amphibolite [120,196,197]. [120,196,197 The]. The corundum corundum is locally is locally associated associated with areas with of areas brown of to brown orangey to orangey garnet feldspar, with accessory Cr-spinel, mica, kyanite, and allanite. The ruby garnet ± feldspar, with± accessory Cr-spinel, mica, kyanite, and allanite. The ruby and sapphire crystals are closelyand sapphire associated crystals with are dykes closely of a associated garnet- and with pargasite-bearing dykes of a garnet- rock and crosscutting pargasite-bearing amphibolite rock (Figurecrosscutting 31A). amphiboliteThe central part (Figure of the31 dykeA). Theis comp centralosed partof garnet of the (pyrope-almandine) dyke is composed + pargasite of garnet ± (pyrope-almandine) + pargasite plagioclase corundum spinel apatite. Metamorphic conditions plagioclase ± corundum ± spinel± ± apatite. ±Metamorphic± conditions± estimated from the garnet- estimated from the garnet-amphibole-corundum equilibrium assemblage are T 800 50 C and P amphibole-corundum equilibrium assemblage are T 800 ± 50 °C and P 8–10 kbar [120].± The◦ rubies and8–10 pink kbar [corundum120]. The rubies are zoned and pink with corundum rhombohedral are zoned twin with planes. rhombohedral The rubies twin exhibit planes. a The suite rubies of inclusionsexhibit a suitewith amphibole, of inclusions garnet, with apatite, amphibole, and opaque garnet, minerals apatite, (chalcocite). and opaque Medium minerals and (chalcocite). vivid red rubiesMedium have and vividorange-brown red rubies hair-like have orange-brown inclusions hair-likeof unknown inclusions composition of unknown [120]. composition The chemical [120]. compositionsThe chemical of compositions the Winza ruby of the and Winza pink rubysapphires and pink (Figure sapphires 31B) include (Figure moderate31B) include contents moderate of Cr (0.1–0.8contents wt of % Cr Cr (0.1–0.82O3) and wt Fe % (0.2–0.8 Cr2O3 )wt and % Fe 2O (0.2–0.83), very wt low % to Fe low2O3 ),amounts very low of toTi low(55–192 amounts ppm TiO of Ti2) and(55–192 V (up ppm to 164 TiO 2ppm) and V V2O (up3), and to 164 low ppm to moderate V2O3), and Ga low(64–146 to moderate ppm Ga2 GaO3). (64–146 ppm Ga2O3).

Figure 30.30. TheThe ruby-bearingruby-bearing amphibolite amphibolite from from Longido, Longido, Northern Northern Tanzania. Tanzania. (A) The(A) amphiboliteThe amphibolite dyke, dyke,which which is up to is 1up m to thick 1 m and thick parallel and parallel to the metamorphicto the metamorphic foliation, foliation, crosscuts crosscuts a serpentinite. a serpentinite. The contact The contactzone between zone between the two the rocks two rocks is marked is marked by theby th presencee presence of of a a phlogopite phlogopiteand and amphibole-bearing schist. Photo: Photo: Elisabeth Elisabeth Le Le Goff; Goff ;((BB)) anyolite anyolite with with patches patches of of ruby ruby (Crn) (Crn) and and retrograde retrograde zoisite zoisite (Zo). (Zo). Remnants of the amphibolite (Amph) highlight the metamorphic foliation. Photo: Photo: Elisabeth Elisabeth Le Le Goff; Goff; (C) representative samples of anyolite with ruby andand black amphibole (tschermakite). Photo: Vincent Vincent Pardieu; ((DD)) rubyruby crystalscrystals extracted extracted from from new new veins veins that that are are very very diff erentdifferent to the to classicalthe classical massive massive ruby rubyof Longido. of Longido. Photo: Phot Vincento: Vincent Pardieu. Pardieu.

(3) The majority majority of of gem-quality gem-quality rubies rubies from from Africa are are from deposits located in metamorphosed metamorphosed M-UMR from the Neoproterozoic Metamorphic Mo Mozambiquezambique Belt (NMMB). The Rockland mine was the main producer of cabochon-grade ruby ruby until until the the discovery of of the the Winza deposit in 2007. Then, Then, the confirmed presence of ruby in the Niassa National Reserve in Mozambique in 2008 opened the economic reign of Mozambique over the production of very high quality rubies worldwide as in the

Minerals 2020, 10, 597 40 of 83 the confirmed presence of ruby in the Niassa National Reserve in Mozambique in 2008 opened the economic reign of Mozambique over the production of very high quality rubies worldwide as in the Montepuez mining district (Figure 32)[21,22,198,200,201]. A review and summary of the history, Minerals 2019, 9, x FOR PEER REVIEW 40 of 82 geology, and gemmological properties of ruby from Mozambique was recently published by [26]. TheMontepuez NMMB mining is subdivided district (Figur into severale 32) [21,22,198,200,201]. complexes grouped A review in four and geological summary entities,of the history, which are fromgeology, west to eastand gemmological [202–204]: (1) properties the Ponta of Messula ruby from complex; Mozambique (2) the was Unango recently and published Maruppa by [26]. complexes; (3) the NampulaThe NMMB complex; is subdivided and (4)into the several east–west complexes Pan-African grouped in overthrustsfour geological (e.g., entities, Nairoto, which Meluco,are Muaquia,from west M’sawize, to east [202–204]: Xixano, (1) Lalamo, the Ponta and Messul Montepuez),a complex; which(2) the Unango form the and CaboMaruppa Delgado complexes; complex overlying(3) the the Nampula Marrupa complex; complex and (4) (see the Figureseast–west2 –Pan-Af42 fromrican [40 overthrusts]). The ruby (e.g., deposits Nairoto, Meluco, are within Muaquia, the Cabo M’sawize, Xixano, Lalamo, and Montepuez), which form the Cabo Delgado complex overlying the Delgado complex: Marrupa complex (see Figures 2–42 from [40]). The ruby deposits are within the Cabo Delgado (a) The Ruambeze (or Luambeze) deposit, in Niassa province, is located between Marrupa and complex: Meculo along(a) The the Ruambeze Luambeze (or River. Luambeze) It was deposit, apparently in Niassa discovered province, in is 1992 located [21] between and it has Marrupa produced and dark red toMeculo brown along cabochon the Luambeze grade material. River. It was apparently discovered in 1992 [21] and it has produced (b)dark The red M’sawize to brown cabochon deposit, grade in Niassa material. province, is located in the Niassa National Reserve, about 43 km southeast(b) The M’sawize of M’sawize deposit, village in Niassa [21]. province, The M’sawize is located complex in the Niassa is composed National of Reserve, orthogneiss about and paragneiss.43 km southeast The M’sawize of M’sawize ruby deposit village is[21]. hosted The M’sawize by metagabbro complex and is gabbroiccomposedgneiss of orthogneiss [22]. The and ruby is associatedparagneiss. with actinolite,The M’sawize anorthite, ruby deposit scapolite, is hosted epidote, by metagabbro and diopside, and gabbroic as well asgneiss with [22]. eluvial The ruby-richruby soil [22is ].associated Ruby has with Fe andactinolite, Cr oxide anorthite, contents scapolite, between epidote, 0.28 andand 0.33diopside, and 0.12as well and as 0.48 with wt eluvial %, respectively, ruby- rich soil [22]. Ruby has Fe and Cr oxide contents between 0.28 and 0.33 and 0.12 and 0.48 wt %, which fall in the range of the Winza ruby from Tanzania [23]. respectively, which fall in the range of the Winza ruby from Tanzania [23].

FigureFigure 31. The 31. Winza The Winza ruby deposit ruby deposit in Tanzania. in Tanzania. (A) The primary(A) The rubyprimary and ruby sapphire and (Crn)sapphire mineralization (Crn) is hostedmineralization by an orangey is hosted brown by an garnet-rich orangey brown rock (Grt)garnet-rich intercalated rock (Grt) in a intercalated fine-grained in amphibolitea fine-grained (Am). amphibolite (Am). Photo: Patrick Lagrange; (B) the Cr2O3 versus Fe2O3 diagram showing the chemical Photo: Patrick Lagrange; (B) the Cr2O3 versus Fe2O3 diagram showing the chemical composition of composition of the Winza rubies compared with those from other ruby deposits hosted in the Winza rubies compared with those from other ruby deposits hosted in metamorphosed M-UMR metamorphosed M-UMR in the Neoproterozoic Metamorphic Mozambique Belt: Chimwadzulu in the Neoproterozoic Metamorphic Mozambique Belt: Chimwadzulu (Malawi), Mangare (Kenya), (Malawi), Mangare (Kenya), Montepuez (Mozambique), and the placers at Songea (Tanzania). Montepuez (Mozambique), and the placers at Songea (Tanzania). (c) The Namahumbire and Namahaca deposits in Delgado province are together called the (c)Montepuez The Namahumbire deposits. The andCabo NamahacaDelgado nappe deposits complex in represents Delgado provincethe remnant are of togetherNeoproterozoic called the Montepuezvolcanic deposits. arcs and a Themicrocontinent Cabo Delgado after 596 nappe ± 11 Ma complex [205]. The represents lithological the su remnantccession is of very Neoproterozoic complex volcanicdue arcsto intense and a deformation microcontinent but structural after 596 trends11 Ma can [be205 drawn]. The from lithological satellite images succession (Figure is very 32; [25]). complex ± due toThe intense ruby area deformation includes high-grade but structural metamorphic trends can rocks be drawnsuch as fromgneisses, satellite migmatites, images marbles, (Figure and32;[ 25]). The rubyamphibolites area includes [96]. At high-grade the end of 2018, metamorphic mining was rocks being such done as by gneisses, companies migmatites, such as Fura marbles, Gems, and amphibolitesBeleza Ruby, [96]. Gemfields, At the end Mustang, of 2018, and mining Redstone was being[25], and done by bynumerous companies small-scale such as miners Fura Gems,working Beleza Ruby,either Gemfields, in community Mustang, areas and such Redstone as at Nacaca [25], or and illegally by numerous inside other small-scale company minersconcessions. working either in Two types of deposits are known: (1) extensive secondary deposits, i.e., colluvials, which are the community areas such as at Nacaca or illegally inside other company concessions. main source of faceted-grade material [25] (also see Chapter 6.3.2. Sub-Type IIIB) and (2) primary Two types of deposits are known: (1) extensive secondary deposits, i.e., colluvials, which are the deposits in amphibolite [96,198,200]. main sourceThe ofprimary faceted-grade deposit is materialmade up of [25 ruby-] (also and see pink Chapter corundum-bearing 6.3.2. Sub-Type lenses IIIB)of amphibolites and (2) primary in depositsmigmatitic in amphibolite gneiss. They [96 ,contain198,200 amphibole]. (pargasite), anorthite, and mica [22,96,200] (Figure 33). The primary deposit is made up of ruby- and pink corundum-bearing lenses of amphibolites in migmatitic gneiss. They contain amphibole (pargasite), anorthite, and mica [22,96,200] (Figure 33).

Minerals 2020, 10, 597 41 of 83 Minerals 2019, 9, x FOR PEER REVIEW 41 of 82 Minerals 2019, 9, x FOR PEER REVIEW 41 of 82

Figure 32. The Montepuez mining district with the different ruby mines of primary and mostly Figure 32. The Montepuez mining district with the different ruby mines of primary and mostly secondary deposits (drawn as red spots). The structural trends observed from both satellite imagery secondary deposits (drawn as red spots). The structural trends observed from both satellite imagery Figureand 32. field The works Montepuez are shown. mining The ruby district mineralization with the defines different a N110°E ruby trending mines zone of primarythat extends and for mostly and fieldsecondary40 works km; depositsmodified are shown. from(drawn [25]. The as Crystal rubyred spots). of mineralization ruby The (siz estructural 1 cm long). defines trends Photo: a N110 observedVincent◦E Pardieutrending from both© GIA. zone satellite that imagery extends for 40 km;and modified field works from are [ 25shown.]. Crystal The ruby of ruby mineralization (size 1 cm defines long). a Photo: N110°E Vincent trending Pardieu zone that© extendsGIA. for 40 km; modified from [25]. Crystal of ruby (size 1 cm long). Photo: Vincent Pardieu © GIA.

Figure 33. The Mozambican ruby deposits. (A) Ruby-bearing amphibolite (Am) at the M’sawize deposit, Niassa Province, 2009. The weathered amphibolite (wAm) is crosscut by whitish plagioclase veins (Plv); (B) the Maninge Nice deposit, Concession Gemfields, Montepuez, 2012. The ruby (Crn) is surrounded by a plagioclase corona (Pl, anorthite) in a weathered amphibolite (wAm); (C) euhedral pargasite (Prg) in ruby. Field of view 1.5 mm; (D) inclusions of actinolite (Act) in ruby. Field of view 2.5 mm; (E ) crystal of FigureFigure 33.pargasite 33.The The Mozambicanassociated Mozambican with mica ruby (Mca) deposits. deposits. in ruby. (A Field) (Ruby-bearingA )of Ruby-bearing view 1.1 mm; amphibolite (F) a amphibolite melted (Am) mica at (Mca) the (Am) M’sawize surrounded at the deposit, M’sawize Niassaby Province,a halo of 2009.fluid inclusionsThe weathered (Flinc) amphibolitearranged in a(wAm “butterfly) is crosscut wing” indicative by whitish of decrepitationplagioclase veinsand (Plv); deposit, Niassarecrystallization Province, phenomena. 2009. TheThe pargasite weathered is euhedral amphibolite and devoid (wAm) of deformation. is crosscut Field of by view whitish 1.7 mm. plagioclase (B) the Maninge Nice deposit, Concession Gemfields, Montepuez, 2012. The ruby (Crn) is surrounded by veins (Plv);Photos (B) A the and Maninge B: Vincent NicePardieu deposit, © GIA. Photos Concession C to F: Jonathan Gemfields, Muyal © Montepuez, GIA. 2012. The ruby (Crn) is a plagioclase corona (Pl, anorthite) in a weathered amphibolite (wAm); (C) euhedral pargasite (Prg) in surrounded by a plagioclase corona (Pl, anorthite) in a weathered amphibolite (wAm); (C) euhedral ruby. Field of view 1.5 mm; (D) inclusions of actinolite (Act) in ruby. Field of view 2.5 mm; (E) crystal of pargasitepargasite (Prg) associated in ruby. with Field mica of view(Mca) 1.5in ruby. mm; Field (D) of inclusions view 1.1 mm; of actinolite (F) a melted (Act) mica in (Mca) ruby. surrounded Field of view 2.5 mm;by a (E halo) crystal of fluid of pargasite inclusions associated (Flinc) arranged with micain a “butterfly (Mca) in ruby.wing” Field indicative of view of decrepitation 1.1 mm; (F) aand melted mica (Mca)recrystallization surrounded phenomena. by a halo The of pargasite fluid inclusions is euhedral (Flinc) and devoid arranged of deformation. in a “butterfly Field wing”of view indicative1.7 mm. of decrepitationPhotos A and B: recrystallization Vincent Pardieu © phenomena. GIA. Photos C The to F: pargasiteJonathan Muyal is euhedral © GIA. and devoid of deformation. Field of view 1.7 mm. Photos A and B: Vincent Pardieu © GIA. Photos C to F: Jonathan Muyal © GIA.

The texture is granoblastic with 120◦ triple junctions between amphibole and anorthite. The mineral assemblage formed during prograde metamorphism. The ruby often shows a corona Minerals 2020, 10, 597 42 of 83 of spinel + anorthite formed during the retrograde stage [96]. The ruby porphyroblasts have sizes between 1 mm and 1 cm. The P–T conditions of ruby formation (P = 10–11.5 kbar and T = 450–600 ◦C), calculated from P–T pseudosections, show the lowest temperature of formation compared to similar ruby-bearing amphibolites (see Figure 21) such as at Mangare [119], Winza [120], North Carolina [121], and Vohibory [122]. These calculated P–T conditions obtained by pseudo-sections [97] for the formation of ruby of Montepuez is somewhat questionable because clinochlore (and ) is a retrograde mineral in M-UMR and not in equilibrium with ruby. Geochemical analyses of amphibolites indicated that the protolith was basaltic and associated with subduction-related arc magmatism [96], following the model proposed by [206] for the arc assembly and continental collision in the Neoproterozoic East African Orogen. Sub-Type IIA2 includes rubies in marble such as those of the Mogok Stone Track (Myanmar), and others from central and eastern Asia, Canada, and Tanzania, (see Table 3). Ruby and pink corundum deposits hosted in marbles are known from Asia [5,30,65,103,112,115,134,207–210], North America [113,211], Europe [95,110], and Africa [48,110,114,182] (see Figure 2). These ruby deposits are hosted by meta-pure or impure limestone of carbonate platforms and are of two types: (1) in marble where the gem corundum crystallized as a result of prograde and retrograde isochemical metamorphic reactions, mainly in a closed system, and where the ruby occurs as disseminated crystals in marble, in Central and Southeast Asia and other places worldwide [65]; and (2) in impure marble containing gneiss and silicate layers where the ruby crystallized at the peak of prograde metamorphism, such as at Revelstoke in Canada [113,212], Snezhnoe in Tadjikistan [115,134], and Morogoro in Tanzania [114,163]. (1) Ruby mines in marble in Central and South-East Asia. These are one of the main worldwide sources of high-quality ruby with intense color and high transparency [5]. There is extensive literature on the geology and gemmology of these deposits; see [5,29,40,65,80,87,88,110,133,143,156]. Below we will present a short overview of the geology of these ruby deposits and focus on the significance of the mineral associations and geochemistry of the paleofluids in order to discuss the genesis of the corundum. These deposits occur from Afghanistan to Southern China (see Figure 2) in marbles that are intercalated with garnet biotite-sillimanite- or biotite-kyanite-bearing gneisses, amphibolites, and (Figure 34A), which are sometimes intruded by granitoids, as in the Mogok district [87,88,213]. The marble units consist of discontinuous horizons up to 300 m in thickness, oriented parallel to the main regional foliations, thrusts, or shear zones related to the Cenozoic Himalayan orogenesis between 45 and 5 Ma [88,133]. The ruby mineralization is restricted to impure marble horizons. The protolith of the ruby-bearing metamorphic rocks comprises of carbonates enriched in detrital clays and organic matter, and intercalated evaporitic layers (Figure 34B). Ruby crystals occur: (1) disseminated within marbles and associated with phlogopite, muscovite, scapolite, margarite, spinel, titanite, pyrite, and graphite, as at Jegdalek, Afghanistan; Chumar and Ruyil, Nepal; Hunza and Nangimali, Pakistan; Mogok and Mong Hsu, Myanmar; Luc Yen, Vietnam; and Yuan Jiang in China; (2) in veinlets or gash veins, as in some occurrences in Northern Vietnam, and associated with phlogopite, margarite, titanite, graphite, and pyrite, and sometimes related to micro-shear zones, as at Nangimali in Pakistan (Figure 34C); and (3) in pockets associated with K-feldspar, phlogopite, margarite, graphite, and pyrite in some occurrences in Northern Vietnam. The thermobarometric conditions of mineral equilibrium have shown that ruby could have grown during both prograde and retrograde metamorphic events. The most common reaction is the formation of ruby by the destabilization of spinel (Figure 35A):

spinel + calcite + CO corundum + dolomite 2 ↔

Ruby formed during the retrograde metamorphism stage at T 620–670 ◦C and P 2.6–3.3 kbar [65]. The aluminum and the chromophorous elements in the ruby originated from marbles (Al up to 1000 ppm, V and Cr between 5 and 30 ppm for the Nangimali deposit). Minerals 2020, 10, 597 43 of 83 Minerals 2019, 9, x FOR PEER REVIEW 43 of 82

The variations variations of of local local chemistry chemistry of ofthe the marbles marbles along along several several decimeters decimeters are due are to due lateral to lateral facies 2 faciesvariations variations of the of protoliths the protoliths which which resulted resulted in ina succession a succession of of different different F-Na-SO42−− -rich-rich mineral associations: (1) (1) F-aspidolite F-aspidolite (Na (Na2O-rich2O-rich phlogopite; phlogopite; Figure Figure 34D)34D) associated associated with with F-phlogopite F-phlogopite and and F- F-paragoniteparagonite in inthe the different different mineralized mineralized zones, zones, as asseen seen in the in the Nangimali Nangimali deposi depositt in Pakistan in Pakistan [214]; [214 (2)]; (2)anhydrite anhydrite either either associated associated with with F-tremolite, F-tremolite, F-edenite, F-edenite, F-pargasite, F-pargasite, and carbonates and carbonates in samples in samples from fromthe Hunza the Hunza Valley, Valley, Pakistan Pakistan and Luc and Yen, Luc Vietnam, Yen, Vietnam, or included or included in ruby in with ruby relics with of relics spinel of (Figure spinel (Figure35A,B); 35(3)A,B); anhydrite (3) anhydrite and salt and crystals salt crystals (CaCl2, (CaCl NaCl,2, NaCl,and KCl) and as KCl) solids as solidsin the in Nangimali, the Nangimali, Luc Yen, Luc Yen,and Mogok and Mogok deposits; deposits; and (4) and F-bearing (4) F-bearing dravite-uvite dravite-uvite tourmaline and apatite, and apatite, indicating indicating a low activity a low activityof water of in water the fluid. in the fluid.

Figure 34. 34. RubyRuby in metamorphosed in metamorphosed carbonate carbonate platform platformss from Central from Central and South-East and South-East Asia. (A) Asia. The (NangimaliA) The Nangimali formation, formation, Pakistan, Pakistan, showing showing the location the location of the of theruby ruby deposit deposit in inthe the metamorphic metamorphic pile. pile. Abbreviations: a: a: ; quartzite; b: b: amphibolite; amphibolite; c: mica-bearingc: mica-bearing marble; marble; d: pyrite-bearing d: pyrite-bearing marble; marble; e: calc-schist e: calc- andschist garnet-biotite and garnet-biotite micaschist; micaschist; f: ruby-anhydrite-bearing f: ruby-anhydrite-bearing yellow yellow Mg-marble; Mg-marble; g: white g:dolomite white dolomite and/or calciteand/or marble; calcite (marble;B) the succession (B) the succession of marble (white) of marble and (white) garnet-sillimanite-bearing and garnet-sillimanite-bearing calc-schist and calc-schist micaschist (brown);and micaschist the unique (brown); ruby-bearing the unique marble ruby-b isearing reported marble as Rm; is (reportedC) gem ruby-bearingas Rm; (C) gem gash ruby-bearing vein: (Crn), phlogopitegash vein: (Phl)(Crn), and phlogopite pyrite (Py) (Phl) in marble;and pyrite and (Py) (D) back-scatteredin marble; and electron (D) back-scattered images of coexisting electron phlogopite images of andcoexisting aspidolite phlogopite with their and X-ray aspidolite images with for potassium their X-ra (K)y images and sodium for potassium (Na) [214 (K)]. Photos:and sodium Gaston (Na) Giuliani. [214]. Photos: Gaston Giuliani. Paleo-fluids trapped as primary fluid inclusions in the minerals during their growth allow access to thePaleo-fluids composition trapped of the mineralizingas primary fluid fluids. inclusions The combination in the minerals of these during data their with growth those obtainedallow access for theto the stable composition isotope ratios of the of mineralizing elements such fluids. as oxygen, The combination hydrogen, sulphur, of these and data boron with onthose minerals obtained coeval for withthe stable the gems isotope allows ratios for of characterization elements such as of oxyge the originn, hydrogen, and source sulphur, of the and diff boronerent chemicalon minerals elements. coeval Microthermometrywith the gems allows studies for characterization combined with of Ramanthe orig spectroscopyin and source of ofprimary the different fluid chemical inclusions elements. in ruby revealedMicrothermometry the coeval trappingstudies combined of two types with of carbonicRaman spectroscopy fluid inclusions of [primary62,63]: mono- fluid inclusions to two-phase in fluidruby inclusionsrevealed the (Figure coeval35 D)trapping in the systemof two COtypes-H ofS carbCOSonic Sfluid(1 inclusions< H O < 10 [62,63]: mol %), mono- and polycrystalline to two-phase 2 2 ± ± 8 ± 2 fluid inclusions (Figure 35D) in the system CO2-H2S ± COS ± S8 ± (1< H2O < 10 mol. %), and polycrystalline fluid inclusions (Figure 35C) in the system Na-K-Ca-CO3-SO4-NO3-Cl-F ± CO2-H2S ± H2O. The

Minerals 2020, 10, 597 44 of 83

Minerals 2019, 9, x FOR PEER REVIEW 44 of 82 fluid inclusions (Figure 35C) in the system Na-K-Ca-CO -SO -NO -Cl-F CO -H S H O. The different 3 4 3 ± 2 2 ± 2 differentsolids in solids the polycrystalline in the polycrystalline fluid inclusions fluid inclusions are mixtures are mixtures of carbonates of carbonates with Ca-Na-Alwith Ca-Na-Al cations, cations, such suchas shortite as shortite and dawsonite, and dawsonite, sulphates sulphates (mainly (mainl anhydritey anhydrite and barite), and phosphatesbarite), phosphates (F-apatite), (F-apatite), nitrates, nitrates,fluorides fluorides (fluorite), (), and chlorides and chlorides (halite, Ca (halit ande, K Ca chlorides). and K chlorides). These solids These are solids daughter are daughter minerals mineralsof ionic liquids of ionic formed liquids duringformed theduring metamorphism the metamorphism of evaporites of evaporites and limestones and limestones [65]. In [65]. ruby, In ruby, these thesepolycrystalline polycrystalline fluidinclusions fluid inclusio are rarens are because rare because if the salts if the are salts not are immediately not immediately trapped trapped by the crystals, by the crystals,and they and dissolve they dissolve because ofbecause their high of their solubility high solubility in water-rich in water-rich fluids. fluids.

Figure 35. MineralMineral assemblage assemblage and and fluid fluid inclusions in in ruby-bearing marble marble deposits from from Central and South-East Asia. Asia. (A (A) )Association Association ruby ruby (Crn), (Crn), dolomite dolomite (Dol), (Dol), calcite calcite (Cal), (Cal), and and spinel spinel (Spl) (Spl) illustrating illustrating the formationthe formation of ruby of ruby by the by chemical the chemical reaction: reaction: spinel spinel + calcite+ calcite + CO2+ ↔CO corundumcorundum + dolomi+ dolomite,te, Hunza Hunzavalley 2 ↔ (Pakistan);valley (Pakistan); (B) inset (B of) Figure inset of 35A Figure (detail35A B) (detail showing B) showingthe presence the of presence anhydrite of anhydrite(Anh) with (Anh) spinel with inclusions spinel (Spl).inclusions Spinel (Spl). is a pre-ruby Spinel phase is a pre-ruby that has phasehigh Cr that (up hasto 19 high wt % Cr Cr (up2O3) toand 19 Zn wt (up % Crto 102O wt3) and% ZnO) Zn (upcontents; to 10 (wtC) %primary ZnO) polycrystalline contents; (C) primary fluid inclusion polycrystalline cavity in ruby fluid containing inclusion different cavity in solids ruby (molten containing salts), di ffwhicherent aresolids mixtures (molten of salts),carbonates, which with are mixturesCa-Na-Al ofcations carbonates, such as with calcite Ca-Na-Al (Cal), dawsonite cations such (Dw), as shortite, calcite (Cal), and apatitedawsonite (Ap), (Dw), fluorite shortite, (Fl), andhalite apatite (Hl), (Ap),graphite fluorite (C), (Fl),and halitea CO (Hl),2-H2S-bearing graphite (C),fluid and phase a CO (CO2-H2),S-bearing Mogok, Myanmar;fluid phase and (CO (D2),) three Mogok, phase Myanmar; fluid inclusions and (D with) three CO phase2 and H fluid2S liquid inclusions (l) + vapor with (V) CO phases2 and Hand2S native liquid sulphur(l) + vapor (S8)(V) trapped phases by and ruby native in a rutile sulphur (Rt)-bearing (S8) trapped cavity, by rubyMong in Hsu a rutile deposit (Rt)-bearing Myanmar. cavity, Photos Mong A and Hsu B: Virginiedeposit Myanmar.Garnier, [65]; Photos Photos A C and and B: D: Virginie Gaston Garnier,Giuliani, [[62,63].65]; Photos C and D: Gaston Giuliani, [62,63].

Crushing and and leaching leaching of of rubies rubies [62] [62 showed] showed that that chloride chloride is the is the dominant dominant anion anion (25 (25to 53 to mol. 53 mol %) %)with with sulphate sulphate (2 to (2 36 to mol. 36 mol %), %),nitrate nitrate (2 to (2 17 to mol. 17 mol %), %),and and fluoride fluoride (0 to (0 25 to mol. 25 mol %). %).Sodium Sodium is the is dominantthe dominant cation cation (16 to (16 42 to mol. 42 mol%). The %). presence The presence of nitrate of nitrate is a strong is a strong argument argument for a continental for a continental input toinput the tooriginal the original sediment sediment as, generally, as, generally, nitrate nitrate salts saltsprecipitate precipitate in closed in closed basin basin playas playas or salars. or salars. In addition,In addition, the the isotopic isotopic variation variation of ofsulphur sulphur in in anhydrite anhydrite included included in in ruby ruby and and marble marble defines defines two two sets sets 34 of δ34SS (V-CDT) (V-CDT) values: the first,first, betweenbetween 27%27 and and 23‰, 23% for, for a amarine marine anhydrite; anhydrite; the the second, second, between between 4.84.8% and and 1.6‰, 1.6% for, fora continental a continental source source [65]. [65 ]. The presencepresence ofof sulphate-carbonate-fluorine sulphate-carbonate-fluorine mixtures mixtures decreased decreased the meltingthe melting temperature temperature of halite of haliteand other and salts,other and salts, allowed and allowed the formation the formation of chlorine- of andchlorine- fluorine-bearing and fluorine ionic-bearing liquids. ionic Fluorine liquids. of Fluorine of continental origin probably played an important role in the extraction of the Al present in impurities (clays) from the impure limestone. The existence of an ionic liquid trapped in the form of polycrystalline solids in ruby explains the color and clarity of the ruby by (1) the mobilization of

Minerals 2020, 10, 597 45 of 83 continental origin probably played an important role in the extraction of the Al present in impurities (clays) from the impure limestone. The existence of an ionic liquid trapped in the form of polycrystalline solids in ruby explains the color and clarity of the ruby by (1) the mobilization of Al, Cr, and V contained in the meta-limestone, and (2) their incorporation in an isotropic environment allowing crystalline growth with a minimum of defects. The presence of evaporites of either continental or marine origin in the Ca-Mg protoliths is the key feature in the metamorphic model for ruby in marble. Ruby formed during the metamorphism, in the amphibolite facies, of carbonates interbedded with mudstones and containing intercalations of sulphates-chlorides-nitrates-borates of impure evaporites. The lithological control of the mineralization is a function of the paleogeography of the carbonate platform environment. The original sedimentary landscape converges to an epeiric carbonate platform succession with a combination of salt and evaporite mudflats of gypsum and anhydrite [62]. (2) Ruby and pink corundum in marble with intercalated silicate and gneiss layers. (i) The Revelstoke occurrence is in the Monashee Complex of the Omineca belt of the Canadian Cordillera, British Columbia [113,212]. Ruby (Cr O 0.21 wt %) occurs 2 3 ≤ in folded and stretched layers associated with green Cr-V-Ti-Ba-rich muscovite + Ba-bearing K-feldspar + anorthite phlogopite Na-poor scapolite. Other silicate layers within the marble ± ± are: (1) diopside + tremolite quartz and (2) almandine-rich garnet + Na-rich scapolite + diopside + ± tremolite + (Na,K)-amphiboles. Corundum formed at the peak of metamorphism (T 650–700 ◦C and at P 8.5–9 kbar) and the sources of the Cr are the silicate layers and gneiss. (ii) The Snezhnoe deposit in Tadjikistan [115,134] is localized in the Muzkol-Rangkul anticlinorium within the Cimmerian zone of the Central Pamir. The Muzkol metamorphic series consists of calcitic marbles present as layers intercalated with amphibole-pyroxene and scapolite calciphyres, gneisses, Ti-Ba-rich muscovite schists, and quartzites. These calcitic marbles host the Snezhnoe deposit, formed by micaceous ruby-bearing lenses arranged as discrete en echelon lenses along the marble foliation. Ruby is found in four mineral assemblages as (i) scapolite + phlogopite + muscovite + margarite; (ii) plagioclase + muscovite + margarite; (iii) muscovite + phlogopite + margarite; and (iv) calcite. The main economic paragenesis is associated with ruby + calcite. The temperature of the metamorphic processes was estimated at 760 30 C using Zr-in-rutile geothermometry [134]. Ancient Al-enriched ± ◦ sediments such as metapelite or meta-bauxite intercalated in the marbles are suggested to be the possible protolith for the ruby-parent rocks. (iii) At the Ngong’Oro mine, Morogoro district in Tanzania [182], ruby is located in metasomatic zones concordant to the metamorphic foliation, and formed at the contact between marble and biotite gneiss layers. The ruby is associated with spinel and sapphirine.

6.2.2. Sub-Type IIB: Metamorphic-Metasomatic Deposits Formed by Fluid–Rock Interaction and Metasomatism (See Table3)

1. Sub-Type IIB1 corresponds to desilicated pegmatites (plumasite) and associated metasomatites in M-UMR such as at the Rockland mine (Kenya) or Polar Urals and Karelia (Russia), and others (see Table 3). This sub-type also concerns deposits associated with M-UMR intercalated with Si-Al rocks (gneiss, leuco-gabbro, or anorthosite), which have suffered fluid percolation and consequent metasomatism such as at the Aappaluttoq ruby deposit in Greenland. These deposits formed at medium to high temperatures during hydrothermal–metamorphic fluid circulation and bi-metasomatism that developed at the contact of two contrasting lithologies, i.e., either granitoid or pegmatite or gneiss adjoining different rocks, such as metamorphosed M-UMR or marble [83,110]. The metasomatic reactions are related to the infiltration of post- or magmatic or metamorphic solutions, which originated from the same or from other magmatic or metamorphic events. The geochemical mechanism of dissolution of quartz (desilication) by a loss of a SiO2-bearing rock, involves diffusion of Si from a pegmatite vein to a M-UMR for example (which plays the role of SiO2 sink) at a rate more rapid than the diffusion of Al. The Al:Si ratio increases in the Minerals 2020, 10, 597 46 of 83 pegmatite vein, and a metasomatic plagioclase-corundum association in which the mass of alumina per unit volume is much greater than in the initial rock may develop if the volume of rock decreases at the same time and is underlain by a zone of quartz dissolution. These desilicated pegmatite or granitoidic veins are called plumasite following the definition of [215] that where formerly described as an oligoclase-corundum rock [216] forming a dyke in a serpentinized peridotite at Plumas, California. Gordon [215] noted that they were described from several countries (United-States, South Africa, and Russia) as “an abnormal group of granitic pegmatites, composed either of plagioclase, usually albite, aloneMinerals (albitite), 2019, 9, x FOR of PEER plagioclase REVIEW and corundum or largely corundum, all of which46 of 82 occcuring exclusively in peridotites or their altered equivalents”. Gordon [215] indicated that the desilication occurred byCalifornia. fluid–rock Gordon interaction [215] noted alongthat they the were contact described between from several the pegmatite countries (United-States, and the intruded South M-UMR. Africa, and Russia) as “an abnormal group of granitic pegmatites, composed either of plagioclase, The mineralogicalusually albite, composition alone (albitite), of of the plagioclase desilicated and corundum pegmatite or largely and corundum, contact rocks all of which depends occcuring of the degree of desilication.exclusively Some in peridotites desilicated or pegmatitestheir altered equivalents”. present metasomatic Gordon [215] indicated zoning that characterized the desilication by different mineral assemblagesoccurred by fluid–rock from the interaction centre toalong the the periphery contact between of the the dyke pegmatite as described and the intruded by [195 ]M- at the ruby CorundumUMR. Hill The mine, mineralogical in Macon composition County (United-States).of the desilicated pegmatite and contact rocks depends of the degree of desilication. Some desilicated pegmatites present metasomatic zoning characterized by World-classdifferent gemmineral corundum assemblages samples, from the includingcentre to the tabular periphery ruby of the in dyke desilicated as described pegmatites, by [195] at have been describedthe from ruby the Corundum Umba Hill and mine, Kalalani in Macon deposits, County (United-States). which are located in serpentinite bodies [217] in Tanzania. World-classWorld-class rubiesgem corundum were mined samples, in including the Mangare tabular arearuby in desilicated southern pegmatites, Kenya, including have from the well-knownbeen described (and formerly)from the Umba John and SaulKalalani mine deposits [118, ,which119]; are and located the Polarin serpentinite Urals inbodies Russia [217] [218] and in Tanzania. World-class rubies were mined in the Mangare area in southern Kenya, including from Karelia [41the]. Anotherwell-known unusual (and formerly) case of John bi-metasomatism Saul mine [118,119]; that and is notthe Polar a desilicated Urals in Russia pegmatite, [218] and is related to the intrusionKarelia of ultramafic[41]. Another dykes unusual in case marble of bi-metasomatis as observedm atthat the is not Kitwalo a desilicated ruby pegmatite, occurrence is related in the Mahenge district (Tanzania).to the intrusion At Kitwaloof ultramafic the dykes ruby in is marble hosted as observed by phlogopite at the Kitwalo schists ruby and occurrence formed in via the fluid–rock interactionMahenge at the contactdistrict (Tanzania). between At marble Kitwalo and the anruby ultramafic is hosted by dyke phlogopite (Figure schists36)[ and182 ].formed via fluid–rock interaction at the contact between marble and an ultramafic dyke (Figure 36) [182].

Figure 36. The ruby occurrence at Kitwalo, Mahenge district, Tanzania. (A) Amphibolite vein (Amph) Figure 36. The ruby occurrence at Kitwalo, Mahenge district, Tanzania. (A) Amphibolite vein crosscutting a dolomitic marble (Mb) and (B) detail of the contact between the two rocks showing a (Amph) crosscuttingphlogopitite (Phl), a dolomitic a result of marble fluid–rock (Mb) intera andction. (B )The detail amphibolite of the exhibits contact a betweenhigh degree the of two rocks showing aphlogopitization phlogopitite (Phl), (PhlAmph) a result associat of fluid–rocked with the interaction.ruby deposition The (Crn). amphibolite Photos: Elisabeth exhibits Le Goff. a high degree of phlogopitization (PhlAmph) associated with the ruby deposition (Crn). Photos: Elisabeth Le Goff. We proposed to examine different deposit cases in order to characterize the infiltrational fluid We proposedmetasomatism to examineof M-UMR diduringfferent medi depositum to high cases grade in metamorphism: order to characterize the infiltrational fluid (1) In the United-States, these deposits are associated with M-UMR intercalated with Si-Al rocks metasomatismmetamorphosed of M-UMR in the during amphibolite medium to granulite to high facies. grade The contact metamorphism: between meta-dunite and amphibolite (1) Inwith the gneisses United-States, at different thesescales is deposits underlined are by metamorphic-metasoma associated with M-UMRtic zones. intercalated The Corundum with Si-Al rocks metamorphosedHill mine at Macon in County the amphibolite in North Carolina to granulitepresents different facies. types The of co contactntacts and between distributions meta-dunite and amphiboliteof ruby [195]: with gneisses at different scales is underlined by metamorphic-metasomatic zones. (a) A gradual transition between ruby accumulations and meta-dunite with some remnants of The CorundumM-UMR Hill in the mine ruby atzone Macon (Figure County 37A). in North Carolina presents different types of contacts and distributions of(b)ruby The contact [195]: zone is characterized by a metasomatic column, of 6–7.5 m width, symmetrical (a) Afrom gradual the periphery transition to the between centre (Figure ruby 37B) accumulations with dunite, schist, and meta-dunitefibrous enstatite, with green somechloritite, remnants of M-UMR inchloritite-bearing the ruby zone ruby, (Figure and spinel37A). (1.8–2.4 m wide). At Hunters mine, the amphibolite presents a plagioclase vein with symmetrical metasomatic zoning, 0.9–1.2 m wide, from the outer to inner zones the following mineral assemblage: amphibolite, actinolitite, ruby-bearing margarite-vermiculite, and ruby-bearing feldspar vein altered into kaolin (Figure 37C). These different types of contact indicate

Minerals 2020, 10, 597 47 of 83

(b) The contact zone is characterized by a metasomatic column, of 6–7.5 m width, symmetrical from the periphery to the centre (Figure 37B) with dunite, talc schist, fibrous enstatite, green chloritite, chloritite-bearing ruby, and spinel (1.8–2.4 m wide). At Hunters mine, the amphibolite presents a plagioclase vein with symmetrical metasomatic zoning, 0.9–1.2 m wide, from the outer to inner zones the following mineral assemblage: amphibolite, actinolitite, ruby-bearing margarite-vermiculite, and ruby-bearing feldspar vein altered into kaolin (Figure 37C). These different types of contact indicate the circulationMinerals of metamorphic-metasomatic 2019, 9, x FOR PEER REVIEW fluids between two rocks of different47 of competency82 and chemistry. Thethe considerable circulation of metamorphic-me width oftasomatic somemetasomatic fluids between two columns, rocks of different up competency to 7.5 m, and indicates a clear infiltrational metasomaticchemistry. The processconsiderable as width described of some metaso for thematic Polar columns, Urals up [to99 7.].5 m, indicates a clear infiltrational metasomatic process as described for the Polar Urals [99].

Figure 37. Metasomatic zoning in ruby-bearing metamorphosed dunites and amphibolites in the Figure 37. Metasomaticdeposits of North zoning Carolina, in USA, ruby-bearing modified from [195]. metamorphosed (A) The contact between dunites dunite andand gneiss amphibolites is in the deposits of Northoutlined Carolina, by corundumite, USA, Corundum modified Hill mine, from Macon [195 County.]. (A) The The scale contact is not qualitative; between (B) ruby- dunite and gneiss is outlined bybearing corundumite, metasomatic zones Corundum in dunite at Hill the Corundum mine, Macon Hill mine. County. The central part The contains scale green is not qualitative; chlorite (Chl) + ruby (Crn) + spinel (Spl) and the other zones are symmetrically disposed from this (B) ruby-bearingcentre metasomatic part. En = enstatite; zones Tlc = talcose in dunite rock; (C at) ruby-bearing the Corundum feldspar vein Hill in amphibolite mine. The at Hunters, central part contains green chlorite (Chl)Iredell County.+ ruby A (Crn)metasomatic-metamorphic+ spinel (Spl) symmetric and theal otherzoning is zones developed are around symmetrically the vein. From disposed from the centre to the periphery: (1) a zone composed of vermiculite (Vrm)-bearing ruby with some this centre part.margarite En = (Mrg)enstatite; and (2) an Tlc actinolite= talcose (Act) zo rock;ne at the (C contact) ruby-bearing with the amphibolite. feldspar vein in amphibolite at Hunters, Iredell County. A metasomatic-metamorphic symmetrical zoning is developed around the (2) In Kenya, rubies were discovered in the Mangare area, Voi mining district, in 1973 by John vein. From the centre to the periphery: (1) a zone composed of vermiculite (Vrm)-bearing ruby with Saul [183]. Mining in the John Saul mine (recently renamed the Rockland mine) provided a regular some margaritesupply (Mrg) of either and (2) grade an actinoliteor cabochon (Act) rubieszone from 1993 at the to 2012. contact The withJohn Saul, the Penny amphibolite. Lane, and Hard Rock deposits occur in lenses of serpentinized ultramafic rock (UMR) bound by shear zones in (2) In Kenya,a metasedimentary rubies were series discovered composed of ingraphitic the Mangare gneiss and felsic area, gneiss Voi (Figure mining 38) [118]. district, Fe-poor in 1973 by John ruby (FeO < 0.06 wt %) and pink corundum is located in (i) micaceous lenses and pockets along the Saul [183]. Miningcontact in of the a shear John zone Saul with mine the metasomatized (recently renamedUMR. The ruby the is Rockland associated with mine) ± spinel provided ± a regular supply of eithersapphirine facet grade + mica or or in some cabochon cases with rubies phlogopite from + Mg-chlorite; 1993 to (ii) 2012. in plumasitic The Johnveins that Saul, crosscut Penny Lane, and

Hard Rock deposits occur in lenses of serpentinized ultramafic rock (UMR) bound by shear zones in a metasedimentary series composed of graphitic gneiss and felsic gneiss (Figure 38)[118]. Fe-poor ruby (FeO < 0.06 wt %) and pink corundum is located in (i) micaceous lenses and pockets along the contact of a shear zone with the metasomatized UMR. The ruby is associated with spinel sapphirine + mica ± ± or in some cases with phlogopite + Mg-chlorite; (ii) in plumasitic veins that crosscut the serpentinites Minerals 2020, 10, 597 48 of 83

Minerals 2019, 9, x FOR PEER REVIEW 48 of 82 where the ruby is associated with zoisite and anorthite muscovite [119]. The P–T conditions for ruby the serpentinites where the ruby is associated with zoisite± and anorthite ± muscovite [119]. The P–T formation are betweenconditions 700for ruby and formation 750 ◦C are and between 8 and 700 10andkbar 750 °C in and the 8 and metamorphic 10 kbar in the metamorphic granulite facies. granulite facies.

Figure 38. Geological map of the John Saul (now Rockland) mine and its serpentinized ultramafic Figure 38. Geologicalrock body maplocated ofin the the metamorphic John Saul formations (now of Rockland) Mgama-Taita, mine modified and after its [118]. serpentinized The Kimbo ultramafic rock body locatedPit extension in the metamorphicfollows the structural formations direction of th ofe desilicated Mgama-Taita, pegmatite modifiedvein, i.e., a ruby-bearing after [118 ]. The Kimbo plumasite, which formed at the contact between the graphitic gneisses and the ultramafic body. Pit extension follows the structural direction of the desilicated pegmatite vein, i.e., a ruby-bearing plumasite, whichAs formedseen in the at Kimbo the contact Pit at the between John Saul mine the graphitic(Figure 38), gneissesthe ruby-bearing-plagioclasite and the ultramafic veinbody. is always present at a depth of 40 m and has a longitudinal extent of 100 m. The contact of the vein with the UMR is outlined by a phlogopite schist with apatite and Mg-chlorite [118]. The plagioclasite As seen inis the coarse-grained Kimbo Pit and at is composed the John of Sauloligoclase-andesine mine (Figure (Figure38 39),A). the Locally, ruby-bearing-plagioclasite a huge concentration vein is always presentof at ruby a depth (around of 70% 40 min volume), and has green a longitudinal tourmaline (uvite-dravite), extent of and 100 blue m. apatite The contactwith some of the vein with the UMR is outlinedphlogopite by is aobserved phlogopite (Figure 39B). schist In other with places, apatite the ruby and exhibited Mg-chlorite zoned scalenohedron [118]. crystals The plagioclasite is (Figure 39C) or “mushroom” (so-called by John Saul) habits up to 3 cm across (Figure 39D). The coarse-grained andproduction is composed of ruby from ofJuly oligoclase-andesine 1995 to April 1997 was estimated (Figure to have39 beenA). 36 Locally, kg/m3 [219]. a hugeThe U/Pb concentration of ruby (around 70%age inobtained volume), on zircons green from tourmaline the plumasite (uvite-dravite), was 612 ± 0.6 Ma and and the blue P–T conditions apatite withfor ruby some phlogopite formation were estimated to have been 500 °C and 5 kbar [118]. is observed (Figure (3)39 B).The Inruby other occurrences places, in theNorthwestern ruby exhibited Karelia in zoned Russia, scalenohedronlocated between thecrystals Kola and (Figure 39C) or “mushroom” (so-calledKarelia Archean by Johnblocks, Saul)is another habits case of up desilication to 3 cm of acrosspegmatite (Figure and gneisses39 D).due Theto the productionfluid– of ruby rock interaction [143,220]. The deposits, which are located in a 3.0–2.5 Ga3 polymetamorphic Archean from July 1995complex, to April are 1997 in ruby-bearing was estimated paragneisses, to amphib haveolites, been and 36 amphibole kg/m schists.[219]. Several The Ucollisional/Pb age obtained on zircons from theepisodes plumasite occurred was but612 at 1.9–1.80.6 Ga Ma metamorphism and the P–T and conditionsregional intrusions for rubywith pegmatites formation were were estimated ± to have been 500responsibleC and for 5 the kbar ruby [ formation118]. through metasomatic processes. The ruby-bearing plumasite veins crosscut◦ biotite-hornblende gneiss and amphibolite. The veins range from several cm to 10 m thick (3) The rubyand up occurrences to 60 m long, and in are Northwestern parallel to the meta Kareliamorphic foliation. in Russia, The plumasite located is composed between of the Kola and Karelia Archeanplagioclase blocks, (oligoclase-andesine), is another case phlogopite, of desilication tourmaline, of zoisite, pegmatite epidote, and and chlorite. gneisses The corundum due to the fluid–rock crystals have prismatic and bladed shapes, 5 cm long and up to 3 cm in diameter. The ruby contains interaction [143up,220 to 9.4]. wt The % Cr2 deposits,O3. Ruby and pink which corundum are locatedfrom the Proterozoic in a 3.0–2.5 gneisses Gahave polymetamorphicthe lowest-known Archean complex, are inδ ruby-bearing18O compositions ( paragneisses,−1.5 to −27‰) for amphibolites,terrestrial corundum and [221], amphibole while the corundum schists. in Several the collisional amphibolite has δ18O contents between −2.3 and 7‰ [1] (Figure 40). The plagioclase and amphibole episodes occurredhave similar but at O-isotope 1.9–1.8 values Ga as metamorphism the ruby. The highly depleted and regional δ18O values intrusions are due to the withcirculation pegmatites were responsible for the ruby formation through metasomatic processes. The ruby-bearing plumasite veins crosscut biotite-hornblende gneiss and amphibolite. The veins range from several cm to 10 m thick and up to 60 m long, and are parallel to the metamorphic foliation. The plumasite is composed of plagioclase (oligoclase-andesine), phlogopite, tourmaline, zoisite, epidote, and chlorite. The corundum crystals have prismatic and bladed shapes, 5 cm long and up to 3 cm in diameter. The ruby contains up to 9.4 wt % Cr2O3. Ruby and pink corundum from the Proterozoic gneisses have the lowest-known δ18O compositions ( 1.5% to 27%) for terrestrial corundum [221], while the corundum in the − − amphibolite has δ18O contents between 2.3% and 7% [1] (Figure 40). The plagioclase and amphibole − have similar O-isotope values as the ruby. The highly depleted δ18O values are due to the circulation of meteoric glacial subwaters in a rift zone at around 2.4 Ga, which affected the initial isotopic composition of the protolith prior to ruby formation [41,222]. MineralsMinerals 2019 2019, ,9 9, ,x x FOR FOR PEER PEER REVIEW REVIEW 4949 ofof 8282 ofMineralsof meteoricmeteoric2020, 10 glacialglacial, 597 subwaterssubwaters inin aa riftrift zonezone atat aroundaround 2.42.4 Ga,Ga, whichwhich affectedaffected thethe initialinitial isotopicisotopic49 of 83 compositioncomposition of of the the protolith protolith prior prior to to ruby ruby formation formation [41,222]. [41,222].

FigureFigure 39. 39. TheThe ruby-bearing ruby-bearingruby-bearing rocksrocks of ofof the thethe John JohnJohn Saul SaulSaul mine, mine,mine, Voi VoiVoi area, area,area, Kenya KenyaKenya [118 [118,163].[118,163].,163]. (A ()(AA The)) TheThe Kimbo KimboKimbo Pit PitandPit and and its its ruby-bearingits ruby-bearing ruby-bearing plumasite plumasite plumasite showing showing showing the mineralthethe mineral mineral assemblage assemblage assemblage ruby ruby ruby (Crn), (Crn), (Crn), plagioclase plagioclase plagioclase (Pl), and(Pl), (Pl), someand and sometourmalinesome tourmalinetourmaline (Tur); (Tur); ((Tur);B) concentration ((BB)) concentrationconcentration of ruby, ofof green ruby,ruby, tourmaline, greengreen tourmatourma andline, blueline, and apatiteand blueblue (Ap) apatiteapatite in the (Ap)(Ap) plagioclase inin thethe plagioclaseveinplagioclase of the vein vein Kimbo of of the the Pit; Kimbo Kimbo (C) ruby Pit; Pit; ( showing(CC)) ruby ruby showing showing the scalenohedron the the scalenohedron scalenohedron habit in habit habit the plagioclasein in the the plagioclase plagioclase matrix matrix matrix of the ofplumasite,of the the plumasite, plumasite, Kimbo Kimbo Kimbo Pit; and Pit; Pit; (D and and) mushroom ( (DD)) mushroom mushroom habit ofhabit habit a ruby, of of a a 3ruby, ruby, cm across, 3 3 cm cm across, across, from the from from John the the Saul John John mine. Saul Saul Photosmine. mine. PhotosA–C:Photos Gaston A–C: A–C: Gaston Gaston Giuliani; Giuliani; Giuliani; Photo D:Photo Photo Courtesy D: D: Courtesy Courtesy John Saul. John John Saul. Saul.

FigureFigure 40. 40.40. OxygenOxygen isotopicisotopic compositioncomposition ofof ruby ruby from fromfrom various various Russian Russian occurrences. occurrences. (( AA)) Oxygen OxygenOxygen isotopicisotopic composition composition of of ruby ruby in in marble, marble, breccia breccia in in mica mica schist schist and and marble, marble, and and plagioclase-dominated plagioclase-dominated metasomaticmetasomatic rocksrocks fromfrom the the Urals, Urals, and and gneisses gneisses an andandd amphibolites amphibolites fromfrom northwestnorthwest Karelia,Karelia, Russia,Russia, modifiedmodifiedmodified from from [153] [[153]153] and and ( (BB)) the thethe oxygen oxygenoxygen isotopic isotopic range range of of values valuesvalues for for ruby ruby worldwide worldwide associated associated with with primaryprimary deposits deposits [40,41,93,163]. [[40,41,93,163].40,41,93,163].

(4)(4) Ruby-bearing Ruby-bearing plagioclasite plagioclasite vein vein vein at at Makar Makar Ruz’ Ruz’ in in the the Ray-Iz Ray-IzRay-Iz ophiolitic ophioliticophiolitic belt belt of of Polar PolarPolar Urals, Urals,Urals, Russia RussiaRussia (see(see Figure FigureFigure 2)22))[ [143,223].[143,223].143,223]. TheThe Paleozoic Paleozoic ophiolitic ophiolitic mama massifssifssif consistsconsists mainlymainly ofof dunite dunite and and harzburgite harzburgite that that underwentunderwent polyphasepolyphase metamorphismmetamorphism withwith with aa high highhigh degree degree ofof serpentinization.serpentinization.serpentinization. Chromitite Chromitite in in dunite dunite yieldedyielded an an an Ordovician Ordovician Ordovician Re-Os Re-Os Re-Os age age age of of 470 of470 470Ma Ma Ma[224] [224] [224 whil whil] whileee zircon zircon zircon from from fromthe the ruby-bearing ruby-bearing the ruby-bearing plagioclasite plagioclasite plagioclasite gave gave a a SiluriangaveSilurian a SilurianU/Pb U/Pb between between U/Pb between380 380 [218] [218] 380 and and [ 218398 398] Ma andMa [225]. [225]. 398 Ma TheThe [ 225vein vein]. wasThewas completely veincompletely was completely minedmined outout minedand and is is outnono longer longer and is visible.novisible. longer ScherbakovaScherbakova visible. Scherbakova (1975,(1975, inin (1975,[99])[99]) recognized inrecognized [99]) recognized twtwoo typestypes two of typesof plagioclasite:plagioclasite: of plagioclasite: (i)(i) plagioclase-richplagioclase-rich (i) plagioclase-rich andand and phlogopite-rich. The plagioclasite vein, 10–12 m thick, presented a clear symmetric metasomatic

Minerals 20192020,, 910, x, 597FOR PEER REVIEW 5050 of of 82 83 phlogopite-rich. The plagioclasite vein, 10–12 m thick, presented a clear symmetric metasomatic zoning aszoning also observed as also observed in emerald-bearing in emerald-bearing plagioclasit plagioclasitee hosted in hostedmetamorphosed in metamorphosed M-UMR [226]. M-UMR From [226 the]. centreFrom theto the centre outer tometasomatic the outer metasomaticzones (Figure zones41) this (Figure is: (i) plagioclasite41) this is: (andesin (i) plagioclasitee to oligoclase), (andesine 1.5–2.5 to moligoclase), wide, with 1.5–2.5 amphibole m wide, (actinolite-tremolite, with amphibole pa (actinolite-tremolite,rgasite) and without pargasite)ruby; (ii) a ruby and without(0.3 < Cr2 ruby;O3 < 7.5 (ii) wt a %)-bearingruby (0.3 < phlogopite-richCr2O3 < 7.5 wt zone %)-bearing (with Na-rich phlogopite-rich phlogopite, zone i.e., aspidolite (with Na-rich in Cr-spinel; phlogopite, [99]), i.e., 0.3–1 aspidolite m thick, within Cr-spinel; plagioclase, [99 ]),Cr-spinel 0.3–1 m (0.46 thick, < Cr# with < plagioclase,0.86). The ph Cr-spinellogopite is (0.46 Ba-rich< Cr# (0.8< <0.86). BaO < The 2.7 phlogopitewt %, [218]). is ParagoniteBa-rich (0.8 (0.8< BaO < Sr << 2.72.3 wt %,%), [218margarite,]). Paragonite and zoisite (0.8 always< Sr < 2.3occur wt between %), margarite, the ruby and and zoisite plagioclase, always andoccur are between probably the retromorphic ruby and plagioclase, phases. The and mean are probablymodal composition retromorphic is plagioclase phases. The (32–67 mean vol. modal %), paragonitecomposition (7–31 is plagioclasevol. %), phlogopite (32–67 (10–22 vol %), vol. paragonite %), and Cr-spinel (7–31 vol (5–7 %), vol. phlogopite %); (iii) plagioclase, (10–22 vol amphibole %), and (pargasite),Cr-spinel (5–7 clinochlore, vol %); (iii) Cr-spinel, plagioclase, talc, amphiboleand augite (pargasite),in a 7–8 m thick clinochlore, zone; (iv) Cr-spinel, finally talc,the dunite, and augite which in exhibiteda 7–8 m thick three zone; sub-zones, (iv) finally from the the dunite, inner whichto the exhibitedouter, respectively: three sub-zones, (a) chlorite from + the talc inner + Cr-spinel; to the outer, (b) chloriterespectively: + talc (a)+ carbonate chlorite ++ talcserpentine+ Cr-spinel; + olivine; (b) chloriteand (c) amphibole+ talc + carbonate (actinolite-tremolite)+ serpentine ++ chloriteolivine; + andCr- spinel(c) amphibole + talc + serpentine (actinolite-tremolite) + olivine. + chlorite + Cr-spinel + talc + serpentine + olivine.

Figure 41. 41. TheThe ruby-bearing ruby-bearing plagioclasite plagioclasite dyke dyke in in a a serp serpentinizedentinized dunite dunite from from the the Makar Makar Ruz’ Ruz’ deposit deposit inin the Ray-Iz ophiolitic belt of PolarPolar Urals,Urals, Russia,Russia, modifiedmodified from [[99].99]. The The plagioclasite shows shows a symmetricalsymmetrical metasomatic metasomatic zoning zoning with with sharp contact zones characteristic of metasomatic zoning and percolation of fluids. fluids. Ruby Ruby (Crn) is hosted in thethe plagioclase (anorthite)-phlogopite (Phl)-Cr-spinel (Cr-Spl) inner inner zone zone of of the the metasomatic metasomatic column. column. Abbreviations: Abbreviations: Am Am = amphibole,= amphibole, Act Act = actinolite,= actinolite, Tr =Tr tremolite,= tremolite, Prg Prg = pargasite,= pargasite, Tlc Tlc = talc,= talc, Srp Srp = =serpentinite,serpentinite, Ol Ol = =olivine,olivine, Pl Pl == plagioclase,plagioclase, Chl Chl == chlorite,chlorite, Aug = augite,augite, and and Cb Cb == carbonate.carbonate.

2 ε Aqueous fluidfluid inclusionsinclusions inin thethe plagioclaseplagioclase are are rich rich in in HCO HCO3−3,−, SO SO442−−,, and and Cl−.. Zircon Zircon εHfHf values values are between 11 and +13; the calculated εNd (380 Ma) value is +3.3 for the whole-rock plagioclasite [227] are between− −11 and +13; the calculated εNd (380 Ma) value is +3.3 for the whole-rock plagioclasite [227]and theandε theNd ε (400Nd (400 Ma) Ma) values values are are+6.4 +6.4 and and+6.6, +6.6, respectively respectively for for dunitedunite andand harzburgite (Ronkin, (Ronkin, 2000 in [99]). [99]). Based on these considerations, different different authors ag agreeree that (i) the vein minerals resulted from thethe fluid–rock fluid–rock interaction with with the the dunite and not from the fractional crystallization of ultramafic ultramafic magma as as proposed proposed by by[228]; [228 (ii)]; the (ii) source the source of Cr is of the Cr chromitite is the chromitite in the dunite; in the and dunite; (iii) the andmetasomatic (iii) the columnsmetasomatic are columnsdue to fluid are due circulation, to fluid circulation, which is whichconsidered is considered subduction-zone-derived subduction-zone-derived and from and metasomatismfrom metasomatism in the in mantle the mantle wedge. wedge. The The associat associationion plagioclase-corundum plagioclase-corundum gave gave P–T P–T estimates estimates at at 1.0–1.1 GPa and between 600 and 700 °C◦C[ [99].99]. The The formation formation of of ruby ruby and and pink pink corundum corundum is is attributed attributed toto desilicationdesilication of of the the dunite dunite by theby Na-Al-Sithe Na-Al-Si fluid circulationfluid circulation [218] rather [218] thanrather derivation than derivation from precursor from precursorpegmatite pegmatite [100]. Such [100]. metasomatic Such metasomatic zones that displayzones that clear display zonation clear with zonation very sharp with metasomatic very sharp metasomaticfronts, i.e., metasomatic fronts, i.e., columnsmetasoma (seetic columns Figure 37 (seeC), formedFigure 37C), by infiltrational formed by processesinfiltrational following processes the following the rules of [229], are also seen in the Haut-Allier, France [40,186], at Corundum Hill mine in North Carolina [195], and in the Vohibory unit in Southern Madagascar [164].

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Mineralsrules of 2019 [229, 9,], x FOR are PEER also REVIEW seen in the Haut-Allier, France [40,186], at Corundum Hill mine in51 North of 82 Carolina [195], and in the Vohibory unit in Southern Madagascar [164]. (5)(5) In In Southern Madagascar, Madagascar, desilicated desilicated rocks rocks called called “sakenites” “sakenites” by by [230] [230] occur occur at at Sakeny, Sakeny, northwest of of . Ihosy. Sakenite Sakenite is a is whitish a whitish to green to greenishish rock composed rock composed of anorthite of anorthite ± corundumcorundum (mostly ± sapphire(mostly sapphireand sometimes and sometimes ruby) ± spinel ruby) ± sapphirinespinel ±sapphirine phlogopite ±phlogopite amphibole (edenite)amphibole ± clinopyroxene (edenite) ± ± ± ± ± ±clinopyroxene zircon, and is foundzircon, in and boudinaged is found in bands boudinaged or benches bands in or the benches Precambrian in the Precambrian high-grade high-grade granulitic ± domaingranulitic of domain Southern of Southern Madagascar. Madagascar. The “sakenites The “sakenites”” are intercalated are intercalated with with Al-rich Al-rich paragneisses, paragneisses, amphibolites, and and clinopyroxenites. The The benches benches ca cann attain attain 10 10 m m width and display petrographic variations within a single bench, with local local predominance predominance of one mineral species. [230] [230] distinguished distinguished sapphirine, spinel, spinel spinel ++ sapphirine-bearingsapphirine-bearing “sak “sakenites”,enites”, and and sometimes sometimes “anorthitic “anorthitic corundumite” corundumite” (anorthite + corundumcorundum or or ± spinel)spinel) and and “corundumite” “corundumite” occurring occurring as as pebbles pebbles and and rounded rounded blocks blocks in ± riversrivers [231] [231] (Figure (Figure 42A).42A). Raith Raith et et al. al. [232] [232] revisited revisited the the occurrence occurrence an andd described described sakenites sakenites associated associated with phlogopitites,phlogopitites, calc-silicate calc-silicate rocks, rocks, and and marbles. marbles. The P–T The conditions P–T conditions of formation of formation of the intercalated of the intercalatedAl-rich paragneisses Al-rich paragneisses were estimated were at estimated 800 ◦C and at 6–7 800 kbar °C and [232 6–7]. kbar [232].

Figure 42. TheThe sakenites sakenites from from Sakeny and othe otherr sites in SouthernSouthern Madagascar.Madagascar. ( A)) Corundumite composedcomposed of of ruby (Crn) and spinel (Spl).(Spl). CollectionCollection Lacroix, MNHN, MusMuséuméum National d’Histoired'Histoire Naturelle de Paris; ( B) sakenite, i.e., plumasite from the AnavohaAnavoha occurrence. The The ruby crystals are disseminated in in a a matrix matrix of of anorthite anorthite (An) (An) and and pyroxene pyroxene (Px). (Px). Collection Collection Lacroix, Lacroix, MNHN MNHN Paris; Paris; (C) the(C) corundumite the corundumite from from Sakeny. Sakeny. A corona A corona of sapphirin of sapphirinee (Spr) and (Spr) spinel and is spinel present is presentaround the around whitish the sapphirewhitish sapphire (Crn). Collection (Crn). Collection Lacroix, Lacroix,MNHN Paris; MNHN (D Paris;) sakenite (D) sakenitevein with vein anor withthite, anorthite, corundum corundum + spinel +at spinelBekinana at Bekinana (Tranomaro (Tranomaro area). The area). corundum The corundum generally generallyshows a showscorona aof corona black ofspinel. black Photos: spinel. GastonPhotos: Giuliani. Gaston Giuliani.

At lower P–T conditions (750–700 °C◦C and and 6 6 Kbar) Kbar) the the sakenite sakenite was was affected affected by the circulation of potassic fluids. fluids. The corundum + anorthiteanorthite assembla assemblagege was was transformed transformed into into phlogopite and Mg-Al minerals producing corundum-spinel-sapphirine-bear corundum-spinel-sapphirine-bearinging phlogopitites.phlogopitites. Subsequently the corundum corundum was replaced by spinel, spinel-sapphirine, or sapphirinesapphirine at lower temperatures andand pressurespressures [[232].232]. Other occurrences in southern Madagascar include Vohidava, Anavoha (Figure 42B),42B), and Bekinana [164]. [164]. These These rocks rocks are are anorthitites anorthitites showin showingg corona corona textures textures due due to to the the partial partial or or complete complete replacementreplacement ofof corundum corundum porphyroblasts porphyroblasts by spinelby spin andel sapphirineand sapphirine (Figure (Figure42C,D), 42C,D), or spinel or+ spinelhibonite, + hibonite,spinel + sapphirine, spinel + sapphirine, K-feldspar +K-feldsparspinel, and + anorthite spinel, and+ sapphirine. anorthite Lacroix+ sapphirine. considered Lacroix “sakenites” considered to be “sakenites”a product of high-gradeto be a product metamorphism of high-grade of clay-rich metamorphism marls. [233] favored of clay-rich metamorphism marls. /[233]metasomatism favored metamorphism/metasomatismof clay-rich limestones. Raith et al.of [clay-rich232] suggested limestones. kaolinite-rich Raith et sediments al. [232] orsuggested calcareous kaolinite-rich bauxites as a sediments or calcareous bauxites as a protolith. The chemical composition of the “anorthitic corundumite” [230], i.e., 12.3 wt % SiO2 and 73.6 wt % Al2O3, implies a loss of silica with drastic enrichment of alumina, with mafic rocks (amphibolites and clinopyroxenites) or marble and calc- silicate rocks playing the role of SiO2 sink at a rate more rapid than the diffusion of Al.

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protolith. The chemical composition of the “anorthitic corundumite” [230], i.e., 12.3 wt % SiO2 and 73.6 wt % Al2O3, implies a loss of silica with drastic enrichment of alumina, with mafic rocks (amphibolites Mineralsand clinopyroxenites) 2019, 9, x FOR PEER or REVIEW marble and calc-silicate rocks playing the role of SiO2 sink at a rate more52 rapid of 82 than the diffusion of Al. (6) In In Southwest Southwest Greenland, Greenland, numerous numerous ruby ruby occurrences occurrences have have been been discovered discovered and and mapped mapped in thein the Fiskenæsset Fiskenæsset Anorthosite Anorthosite Complex Complex [33,34,83,234,235]. [33,34,83,234,235 Fagan]. Fagan [34] [stated34] stated that thatthere there are at are least at least 534 individual534 individual corundum corundum occurrences occurrences in the in thearea area and and an an open-pit open-pit ruby ruby mine mine recently recently opened opened at at the Aappaluttoq locality (Figure 4343A).A). The deposit has defined defined minable reserves of 59.2 million grams of pink corundum and ruby [[35].35].

Figure 43. 43. (A(A) The) The ruby ruby mine mine at Aappaluttoq. at Aappaluttoq. Photo: Photo: Vincent Vincent Pardieu; Pardieu; (B) gem ( Bcorundum) gem corundum in outcrop. in Photo:outcrop. Vincent Photo: Pardieu; Vincent ( Pardieu;C) ruby rough. (C) ruby Photo: rough. Vincent Photo: Pardieu; Vincent ( Pardieu;D) a selection (D) a of selection cut gems of cut(0.5–0.75 gems ct)(0.5–0.75 from the ct) Greenland from the Greenland deposits [34] deposits, courtesy [34], courtesyof True North of True Gem North Inc. Gem Inc.

The 2.97 2.97 Ma Ma Fiskenæsset Fiskenæsset Complex Complex is isone one of ofthe the be bestst preserved preserved layered layered Archean Archean intrusions intrusions in the in worldthe world ([236] ([236 and] and references references therein). therein). It Itconsists consists of ofa aca. ca. 550-m-thick 550-m-thick association association of of anorthosite, leucogabbro, gabbro, and metamorphosed UMR (dunit (dunite,e, peridotite, pyroxeni pyroxenite,te, and and hornblendite), which are found in thethe miningmining areaarea atat AappaluttoqAappaluttoq (Figure(Figure 4444).). DespiteDespite amphiboliteamphibolite to granulitegranulite facies metamorphism (that peaked at at 2.80 Ga ([235] ([235] and references therein)) and polyphase deformation, primary cumulate textures and igneous layering are well-preservedwell-preserved [[236].236]. The rubies rubies occur occur in in a a reaction reaction metasomatic metasomatic zone zone (Figure (Figure 43B)43B) formed formed at atthe the contact contact between between an anorthite-richan anorthite-rich rock, rock, such such as asanorthosite anorthosite or orleuc leucogabbro,ogabbro, and and an an UMR, UMR, such such as as a metamorphosed dunite or or peridotite. peridotite. The The metasomatic metasomatic zones zones resulting resulting from from fluid fluid percolation percolation are typically are typically 0.5–5 0.5–5m wide m wideand contain and contain ruby, ruby,phlogopite, phlogopite, cordierite, cordierite, spinel, spinel, sapphirine, sapphirine, calcic calcicamphibole, amphibole, sillimanite, sillimanite, and kornerupineand kornerupine [83]. The [83]. ruby The reaction ruby reaction zone consists zone consistsof three ofmain three groups: main (i groups:) phlogopitite (i) phlogopitite with 80% withmica 80%(eastonite-phlogopite) mica (eastonite-phlogopite) and up to and 20% up tocorundum; 20% corundum; (ii) sapphirine-gedrite (ii) sapphirine-gedrite rocks, rocks, and and(iii) (iii)anthophyllite-cummingtonite anthophyllite-cummingtonite (-pargasite) (-pargasite) rocks. rocks. The The rubies rubies formed formed at approximately at approximately 640 640 °C◦ Cand and 7 kbar,7 kbar, when when pegmatites pegmatites intruded, intruded, at at 2.71 2.71 Ga, Ga, the the anorthite-rich anorthite-rich rock rock and and juxtaposed juxtaposed UMR UMR [83,235]. [83,235]. This caused the anorthite to alter to calcic amphibole, releasing Al, and Cr from the UMR to be mobilized by the pegmatitic fluids.fluids. The ruby-formi ruby-formingng reaction occurred at upper amphibolite facies conditions and under high fluidfluid fluxflux interactioninteraction between twotwo rocksrocks ofof contrastingcontrasting chemistry.chemistry. Keulen et al. [83] [83] proposed to name this kind of deposit as ruby-bearing ultramafic ultramafic complexes. complexes. The description description of of the the deposit deposit confirms confirms that that it formed it formed through through an intense an intense fluid–rock fluid–rock interaction interaction that resulted in large amounts of K-metasomatism (ruby-phlogopitite zone). In consequence, the Aappaluttoq deposit might be classified in the Sub-type IIB1 proposed by the enhanced classification for ruby deposits (see Table 3). The nature of the protoliths can vary worldwide from mafic to ultramafic rocks for low Si and Cr-bearing rocks, and from gneiss, granitoid to anorthosite rocks for Si-Al-K-rich rocks.

Minerals 2020, 10, 597 53 of 83 that resulted in large amounts of K-metasomatism (ruby-phlogopitite zone). In consequence, the Aappaluttoq deposit might be classified in the Sub-type IIB1 proposed by the enhanced classification for ruby deposits (see Table 3). The nature of the protoliths can vary worldwide from mafic to ultramafic rocks for low Si and Cr-bearing rocks, and from gneiss, granitoid to anorthosite rocks for

Si-Al-K-richMinerals 2019 rocks., 9, x FOR PEER REVIEW 53 of 82

FigureFigure 44. Geological 44. Geological map map of the of Aappaluttoqthe Aappaluttoq ruby ruby mine mine [34 [34].]. SapGed: SapGed: sapphirine-gedrite; sapphirine-gedrite; Phlog-Parg:Phlog- phlogopite-pargasite;Parg: phlogopite-pargasite; Gn: orthogneiss; Gn: orthogneiss; SGc andSGc SGf:and SGf: coarse coarse (c) (c and) and finefine (f) sapphirine-gedrite; sapphirine-gedrite; L: L: leucogabbro; M: melanogabbro; UM: ultramafic rock; and P: pegmatite (yellow) or phlogopitite leucogabbro; M: melanogabbro; UM: ultramafic rock; and P: pegmatite (yellow) or phlogopitite (marron). (marron). According to [83] rubies from the Fiskenæsset Complex (Figure 43C) are distinguished by their According to [83] rubies from the Fiskenæsset Complex (Figure 43C) are distinguished by their high Cr, intermediate Fe, low V, Ga, and Ti, low O isotope values (1.6–4.2%), and rare mineral high Cr, intermediate Fe, low V, Ga, and Ti, low O isotope values (1.6–4.2‰), and rare mineral inclusionsinclusions (e.g., (e.g., anthophyllite anthophyllite+ biotite)+ biotite) and and growthgrowth features. Numerous Numerous faceted faceted pink pink corundum corundum to to rubyruby more more than than 1 carat 1 carat have have been been cut cut (Figure (Figure43 43D)D) and and the the largest largest piecepiece of Cr-corundum Cr-corundum recovered recovered to date isto thedate carved is the carved opaque opaque 440 ct 440 Kitaa ct Kitaa Ruby Ruby [35 ].[35]. ThereThere are are also also a a numbernumber of of occurrences occurrences in Greenl in Greenlandand north of norththe Fiskenæsset of the FiskenæssetComplex. Keulen Complex. et Keulenal. [83] et al. listed [83 (approximately] listed (approximately from north to from south): north Ujarassuit to south): Nunaat, Ujarassuit Maniitsoq Nunaat, NW, Kangerluarsuk, Maniitsoq NW, Kangerluarsuk,several occurrences several on occurrences the island of on Storø, the islandand Kapisillit. of Storø, and Kapisillit. At Kangerluarsuk,At Kangerluarsuk, ruby ruby occurs occurs in in schist schist with with plagioclase, plagioclase, both surrounded both surrounded by biotite, by with biotite, minor with orthopyroxene, amphibole, and olivine ([237] and references therein). The ruby formed by a metamorphic minor orthopyroxene, amphibole, and olivine ([237] and references therein). The ruby formed by reaction between a metaperidotite and a kyanite-quartz-garnet schist (with minor plagioclase, spinel, a metamorphicand mica) at reactionamphibolite-facies between conditions a metaperidotite and a kyanite-quartz-garnet schist (with minor plagioclase,At spinel,Ujarassuit and Nunaat, mica) at an amphibolite-facies ultramafic enclave conditions.experienced amphibolite-grade metamorphism Atresulting Ujarassuit in gabbro Nunaat, anorthosite an ultramaficwith amphibole-calcite enclave experienced plagioclase and amphibolite-grade a small amount of red metamorphism corundum resultingovergrowing in gabbro the anorthosite latter [238]. with amphibole-calcite plagioclase and a small amount of red corundum overgrowingOn Storø the latter Island, [238 similar]. to Kangerluarsuk, the ruby occurs in a mica schist layer up to 3 m thick Onand Storøformed Island, by a similarmetamorphic to Kangerluarsuk, reaction between the a ruby metaperidotite occurs in aand mica a sillimanite-biotite-garnet schist layer up to 3 m thick and formedschist (with by a minor metamorphic plagioclase) reaction at amphibolite-facies between a metaperidotite conditions [237]. and a sillimanite-biotite-garnet schist (with minorAt plagioclase)both Kangerluarsuk at amphibolite-facies and Storø Island, conditions the ruby [237 is ].thought to have resulted from the breakdown of kyanite and sillmanite at elevated temperatures due to the removal of SiO2. The reaction was facilitated by a chemical potential gradient between the relatively Si- and Al-rich metasedimentary rocks and the low-Si ultramafic rocks. According to [237], the additional required Al2O3 was supplied via metasomatism, with the Al mobilized by complexation with hydroxide at alkaline conditions or transported as K-Al-Si-O polymers at deep crustal levels.

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At both Kangerluarsuk and Storø Island, the ruby is thought to have resulted from the breakdown of kyanite and sillmanite at elevated temperatures due to the removal of SiO2. The reaction was facilitated by a chemical potential gradient between the relatively Si- and Al-rich metasedimentary rocks and the low-Si ultramafic rocks. According to [237], the additional required Al2O3 was supplied via metasomatism, with the Al mobilized by complexation with hydroxide at alkaline conditions or transported as K-Al-Si-O polymers at deep crustal levels. Minerals 2019, 9, x FOR PEER REVIEW 54 of 82 Poulsen et al. [239] described several occurrences of pink corundum near Nattivit in Southeast Greenland.Poulsen The et corundum al. [239] described occurs several where occurrences late-stage felsicof pink pegmatites corundum near crosscut Nattivit and in Southeast interact with metamorphosedGreenland. The ultramafic corundum rocks. occurs The where source late-stage of Al is the felsic plagioclase pegmatites in thecrosscut pegmatite and interact with removal with of Si bymetamorphosed interaction with ultramafic the UMR rocks. [239]. The source of Al is the plagioclase in the pegmatite with removal 2.ofSub-Type Si by interaction IIB2 is with characterized the UMR [239]. by ruby and pink corundum in shear zone-related or fold-controlled deposits in2. different Sub-Type substrata IIB2 is (M-UMR, characterized gneiss, by schist, ruby and and pink marble) corundum with fluid in shear circulation zone-related and metasomatism. or fold- Thesecontrolled deposits deposits formed in different at medium substrata to high(M-UMR, temperatures gneiss, schist, during and metasomatic-metamorphicmarble) with fluid circulation fluid circulation.and metasomatism. They include ruby or pink corundum-bearing metamorphosed and metasomatized M-UMR and marbleThese (see deposits Table 3 )formed such as at thosemedium from to high the Neoproterozoictemperatures during occurrences metasomatic-metamorphic of Zazafotsy, Sahambano, fluid and Ambatomenacirculation. They (Southern include ruby Madagascar), or pink coru Keralandum-bearing (southern metamorphosed India), and Mahenge and metasomatized and the Uluguru M- MountainsUMR and (Tanzania). marble (see Table 3) such as those from the Neoproterozoic occurrences of Zazafotsy, (1)Sahambano, For Madagascar, and Ambatomena the geology (Southern and genesis Madagascar), of these Kerala types (southern of deposits India), was and described Mahenge by and [40, 164]. the Uluguru Mountains (Tanzania). These studies focused on the Sahambano and Zazafotsy deposits that produced sapphires (rather than (1) For Madagascar, the geology and genesis of these types of deposits was described by [40,164]. rubies)These from studies feldspar-rich focused on mafic the Sahambano gneisses. and Fractures Zazafotsy and deposits tension that gashes produced associated sapphires with (rather shear than zones favoredrubies) fluid from circulation feldspar-rich and mafic biotitization gneisses. ofFractures the host and rocks. tension At gash Sahambano,es associated sapphires with shear and zones pink to reddishfavored corundum fluid circulation (1000 < andCr2 biotitizationO3 < 2000 ppm)of the host occur rocks. in biotitites At Sahambano, with sillimanitesapphires and and pink spinel, to reddish and also in gneissescorundum with (1000 sapphirine, < Cr2O3 < K-feldspar,2000 ppm) occur biotite, in biotitites sillimanite, with sillimanite spinel, garnet, and spinel, and and albite. also The in gneisses corundum formedwith at sapphirine, T 650 ◦C andK-feldspar, P 5 kbar biotite, [240 ,sillimanite,241]. At Zazafotsy, spinel, garnet, the corundumand albite. The deposit corundum lies informed the Zazafotsy at T shear650 zone °C and system P 5 kbar [242 [240,241].]. The corundum At Zazafotsy, occurs the corundum in lenses deposit with diliesff erentin the mineralZazafotsy assemblages: shear zone system (i)[242]. Almandine The corundum garnet, occurs biotite, in lenses plagioclase, with different and mineral sillimanite, assemblages: with a corona of spinel and K-feldspar (i) Almandine garnet, biotite, plagioclase, and sillimanite, with a corona of spinel and K-feldspar formed around corundum (Figure 45A). The resulting mineral assemblage can be described by the formed around corundum (Figure 45A). The resulting mineral assemblage can be described by the following reaction: following reaction: biotite + corundum spinel + K-feldspar + H2O biotite + corundum→ → spinel + K-feldspar + H2O The association of corundum, garnet, spinel, and sillimanite in some samples results from the reaction: The association of corundum, garnet, spinel, and sillimanite in some samples results from the reaction: garnet + corundum spinel + sillimanite garnet + corundum→ → spinel + sillimanite (ii) In(ii) biotite In biotite schist schist with with biotite, biotite, corundum, corundum, spinel,spinel, and minor minor grandidierite grandidierite (Figure (Figure 45B). 45 B).

FigureFigure 45. 45.The The Zazafotsy Zazafotsy rubyruby and sapphir sapphiree deposit, deposit, northeast northeast of Ihosy, ofIhosy, Southern Southern Madagascar. Madagascar. (A) (A) PurplePurple sapphiressapphires (Crn) (Crn) in in a agarnet garnet (Grt)-bearing (Grt)-bearing biotite biotite schist schist (Bt). The (Bt). crystals The crystals are embedded are embedded in a corona of K-feldspar (Kfs) and spinel (Spl) and (B) pink corundum hosted by a biotitite (Bt). Photo: in a corona of K-feldspar (Kfs) and spinel (Spl) and (B) pink corundum hosted by a biotitite (Bt). Gaston Giuliani. Photo: Gaston Giuliani. The Ambatomena ruby deposit was exploited by a private mining company between 2000 and 2001. The production was of very good quality with euhedral crystals up to 2 cm across and 3 cm long. The ruby deposit is located in the Androyan granulitic metamorphic series, which is composed of para-and orthogneisses, marble, granite, clinopyroxenite, and quartzite. The four ruby deposits (called D1 to D4) formed lenses of clinopyroxenite (D1 and D2), sapphirinite (D3), and plagioclasite (D4) intercalated with

Minerals 2020, 10, 597 55 of 83

The Ambatomena ruby deposit was exploited by a private mining company between 2000 and 2001. The production was of very good quality with euhedral crystals up to 2 cm across and 3 cm long. The ruby deposit is located in the Androyan granulitic metamorphic series, which is composed of para-and orthogneisses, marble, granite, clinopyroxenite, and quartzite. The four ruby deposits (called D1 to D4) formedMinerals lenses 2019, 9, of x FOR clinopyroxenite PEER REVIEW (D1 and D2), sapphirinite (D3), and plagioclasite (D4)55 of 82 intercalated with garnet-bearing granofels [243] (Figure 46). The formation of ruby is linked to the percolation of garnet-bearing granofels [243] (Figure 46). The formation of ruby is linked to the percolation of fluids at fluids at the contactMinerals 2019 between, 9, x FOR PEER the REVIEW lenses and the granofels. The ruby (2180 < Cr255O of3 82< 6670 ppm) is the contact between the lenses and the granofels. The ruby (2180 < Cr2O3 < 6670 ppm) is found in found in phlogopitized clinopyroxenites (G2), sapphirinites (G3 and G4; Figure 47A), and plagioclasites phlogopitizedgarnet-bearing clinopyroxenites granofels [243] (G2), (Figure sapphirinites 46). The formation (G3 and of G4; ruby Figure is linked 47A), to thande percolation plagioclasites of fluids (G4; at Figure (G4; Figure47B).47 B).ThetheThe mineralcontact mineral between assemblage assemblagethe lenses of the and ruby the of isgranofels. the spinel, ruby TheK-feldspar,is ruby spinel, (2180 plagioclase, < K-feldspar, Cr2O3 < 6670clinopyroxene, ppm) plagioclase, is found sapphirine, in clinopyroxene, sapphirine,phlogopite, phlogopite,phlogopitized sillimanite, sillimanite, clinopyroxenites cordierite, cordierite, (G2), titanite, sapphirinites titanite, ruti le,(G3 and rutile, G4; hibonite Figure and 47A),hibonite (Figure and plagioclasites 47C,D). (Figure The(G4;47C,D). Figureretrograde The retrograde 47B). The mineral assemblage of the ruby is spinel, K-feldspar, plagioclase, clinopyroxene, sapphirine, metamorphicmetamorphic stage is stage characterized is characterized by by the the destablization destablization of ofruby ruby into intospinel spinel in the inplagioclasites the plagioclasites and and sapphirinephlogopite, in the clinopyroxenites,sillimanite, cordierite, each titanite,mineral ruti formingle, and a coronahibonite ar (Figureound ruby 47C,D). crystals. The retrograde sapphirine in themetamorphic clinopyroxenites, stage is characterized each mineral by the destablizatio formingn of a ruby corona into spinel around in the ruby plagioclasites crystals. and sapphirine in the clinopyroxenites, each mineral forming a corona around ruby crystals.

Figure 46. PanoramicFigure 46. view Panoramic and geologicalview and geological cross-section cross-section of of the theAmbatomena Ambatomena ruby ruby deposit, deposit, Betroka Betroka region, Figureregion, 46. Panoramic Southern viewMadagascar, and geological modified cross-sectionfrom [243]. The of correlationthe Ambatomena between rubythe ruby-bearing deposit, Betroka Southern Madagascar,region,occurrences Southern modified(G1–G4) Madagascar, and the from garnet-bearingmodified [243]. from The hornfels [ correlation243]. and The pyroxenites correlation between can be seenbetween the in the ruby-bearing cross-section.the ruby-bearing occurrences (G1–G4) andoccurrences the garnet-bearing (G1–G4) and the hornfels garnet-bearing and pyroxeniteshornfels and pyroxenites can be seen can be in seen the in cross-section. the cross-section.

Figure 47. The Ambatomena ruby deposit, south of Betroka, Southern Madagascar. (A) Ruby (Crn)- bearing sapphirinite (Spr); (B) plagioclasite (Pl) with whitish corundum (Crn) showing a corona of spinel (Spl); (C) microscopic image of ruby-bearing sapphirinite with a granoblastic texture of K- feldspar (Kfs), sapphirine (Spr), sillimanite (Sil), and rutile (Rt). The ruby sometimes has a corona of Figure 47.FigureThesapphirine 47. Ambatomena The Ambatomena and K-feldspar; ruby ruby (D) microscopic deposit, south image south of of Betroka,the of corundum-b Betroka, Southernearing Southern Madagascar. plagioclasite. Madagascar. (TheA) textureRuby (Crn)- (A) Ruby bearingis sapphirinitegranoblastic decussate (Spr); ( Bwith) plagioclasite plagioclase (Pl) (Pl) and with corundum. whitish The corundum plagioclase (Crn) is associated showing with a some corona of (Crn)-bearing sapphirinite (Spr); (B) plagioclasite (Pl) with whitish corundum (Crn) showing a corona spinel sapphirine(Spl); (C) andmicroscopic clinopyroxene image (Cpx). of ruby-bearingThe corundum issapphirinite embedded in with a corona a granoblastic of green spinel texture with of K- some hibonite (Hib). Photos: Alfred Andriamamonjy. of spinel (Spl);feldspar ( C(Kfs),) microscopic sapphirine (Spr), image sillimanite of ruby-bearing (Sil), and rutile sapphirinite(Rt). The ruby sometimes with a granoblastic has a corona of texture of K-feldsparsapphirine (Kfs), sapphirine and K-feldspar; (Spr), (D) sillimanite microscopic image (Sil), andof the rutile corundum-b (Rt).earing The ruby plagioclasite. sometimes The texture has a corona of sapphirineisand granoblastic K-feldspar; decussate (D )with microscopic plagioclase (Pl) image and corundum. of the corundum-bearing The plagioclase is associated plagioclasite. with some The texture is granoblasticsapphirine decussate and clinopyroxene with plagioclase (Cpx). The (Pl) corundum and corundum. is embedded The in plagioclasea corona of green is associated spinel with with some some hibonite (Hib). Photos: Alfred Andriamamonjy. sapphirine and clinopyroxene (Cpx). The corundum is embedded in a corona of green spinel with some hibonite (Hib). Photos: Alfred Andriamamonjy. Minerals 2020, 10, 597 56 of 83

(2) For Tanzania, the ruby deposits in the Uluguru and Mahenge Mountains are located, respectively, in dolomite-Minerals 2019 and, 9, x calcite-dominated FOR PEER REVIEW marbles but deformation controlled the mineralization,56 of 82 which is located in saddle reef structures within fold hinges [114] (Figure 48A). The ruby is metamorphic (2) For Tanzania, the ruby deposits in the Uluguru and Mahenge Mountains are located, and formedrespectively, between in dolomite- 620 and and 580 calcite-dominated Ma. At Mahenge marbles the but deposits deformation are controlled Kitwalo, the Ipango, mineralization, Matote, and Lukande.which The is located Morogoro in saddle mining reef district structures consists within of the fold Kibuko, hinges Mwalazi, [114] (Figure Nyama 48A). Nyama, The ruby Visaki, is and Msongemetamorphic deposits. and The formed marbles between belong 620 to and the 580 Neoproterozoic Ma. At Mahenge cover the deposits sequence are thatKitwalo, was Ipango, thrust onto the gneissicMatote, and basement Lukande. of The the Morogoro Eastern Granulites.mining district At consists Uluguru, of the the Kibuko, mineral Mwalazi, assemblage Nyama isNyama, dolomite, phlogopiteVisaki, andspinel, Msonge pargasite, deposits. scapolite, The marbles plagioclase, belong to margarite, the Neoproterozoic chlorite, andcover tourmaline. sequence that was ± Atthrust Mahenge, onto the the gneissic paragenesis basement is of calcite, the Eastern plagioclase, Granulites. phlogopite At Uluguru,dolomite, the mineral pargasite, assemblage sapphirine, is ± titanite,dolomite, and tourmaline. phlogopite ± Two spinel, episodes pargasite, of rubyscapolite, formation plagioclase, were margarite, defined bychlorite, [114]: and tourmaline. At Mahenge, the paragenesis is calcite, plagioclase, phlogopite ± dolomite, pargasite, sapphirine, (i) Ruby formed during the prograde metamorphic stage at T 750 ◦C and P 1.0 GPa by the titanite, and tourmaline. Two episodes of ruby formation were defined by [114]: breakdown of either diaspore or via the chemical reaction: (i) Ruby formed during the prograde metamorphic stage at T 750 °C and P 1.0 GPa by the breakdown of either diaspore or via the chemical reaction: margarite anorthite + corundum + H2O (1) margarite↔ ↔ anorthite + corundum + H2O (1) The metamorphicThe metamorphic mineralizing mineralizing fluids fluids circulated circulated along along the zonesthe zones of higher of higher permeability permeability at granulite at faciesgranulite conditions. facies conditions.

FigureFigure 48. (A48.) The(A) The schematic schematic structural structural control control ofof rubyruby mineralization in inthe the Uluguru Uluguru and andMahenge Mahenge mountains, Southern Tanzania, modified from [114]. Ruby formed in zones of higher permeability near mountains, Southern Tanzania, modified from [114]. Ruby formed in zones of higher permeability fold hinges; (B) the structural control of ruby formation by folding and opening of fractures is also observed near fold hinges; (B) the structural control of ruby formation by folding and opening of fractures is at the Lukande ruby deposit in the Mahenge Mountains. The ruby mineralization is located in the hinge of also observeda fold that affected at the Lukande both marble ruby and deposit amphibolite. in the The Mahenge contact between Mountains. the two Thecontrasting ruby mineralization rocks hosts a is locatedmetasomatic in the hinge zone of composed a fold that of ca arbonates,ffected both phlogopite, marble and and Mg-amphibole. amphibolite. Photo: The contactGaston Giuliani. between the two contrasting rocks hosts a metasomatic zone composed of carbonates, phlogopite, and Mg-amphibole.

Photo:(ii) Gaston Changes Giuliani. in the T-XCO2 conditions of the metamorphic fluid triggered the ruby deposition through two main reactions: (ii) Changes in the T-XCO2 conditions of the metamorphic fluid triggered the ruby deposition spinel + calcite + CO2 ↔ corundum + dolomite (2) through two main reactions:

margarite + calcite ↔ corundum + scapolite + H2O (3) spinel + calcite + CO2 corundum + dolomite (2) The sources of Cr (and V) were not discussed↔ by [114] but two possibilities can be proposed: (i) a sedimentary source, i.e., limestone or Mg-limestone (muds or clays interbedded with impure carbonates) margarite + calcite corundum + scapolite + H2O (3) from the carbonate platform of the Neoproterozoic↔ Mozambican ocean. Such a source was also proposed Thefor the sources of mineralization Cr (and V) werein Northern not discussed Tanzania by[244]. [114 Both] but tsavorite two possibilitiesat Merelani and can ruby be in proposed: the (i) a sedimentaryMahenge and Uluguru source, i.e.,Mountains limestone formed or in Mg-limestone a similar geological (muds setting or clays and interbeddedduring the East with African impure carbonates)orogeny; from (ii) a mafic the carbonate source such platform as amphibolite of the that Neoproterozoic was infiltrated by Mozambican the fluids circulating ocean. along Such zones a source was alsoof higher proposed permeability for the during tsavorite folding mineralizationand the formation in of Northernsaddle reef structures. Tanzania [244]. Both tsavorite at MerelaniAt and the ruby Lukande in the deposit, Mahenge the marbles and Uluguru are intercal Mountainsated with formed Cr-bearing in a amphibolites similar geological that were setting folded and fractured (Figure 48B). Both types of rocks have suffered fluid percolation and ruby- and during the East African orogeny; (ii) a mafic source such as amphibolite that was infiltrated by bearing metasomatic zones formed at the marble-amphibolite contact. The mineral assemblage is the fluids circulating along zones of higher permeability during folding and the formation of saddle ruby-carbonate-phlogopite-magnesian amphibole. The style of deformation and the nature of the reef structures.protoliths are similar to those proposed for the formation of tsavorite vein deposits from the Merelani At the Lukande deposit, the marbles are intercalated with Cr-bearing amphibolites that were folded and fractured (Figure 48B). Both types of rocks have suffered fluid percolation and Minerals 2020, 10, 597 57 of 83 ruby-bearing metasomatic zones formed at the marble-amphibolite contact. The mineral assemblage is ruby-carbonate-phlogopite-magnesian amphibole. The style of deformation and the nature of the protoliths are similar to those proposed for the formation of tsavorite vein deposits from the Merelani area [244]. The metamorphic formations were affected by tight ductile isoclinal folding and shearing, and the tsavorite occurs at the hinges of sheared isoclinal folds within saddle-reef structures. Minerals 2019, 9, x FOR PEER REVIEW 57 of 82

6.3. Sedimentary-Relatedarea [244]. The metamorphic (Type III): formations Placer Deposits were affected by tight ductile isoclinal folding and shearing, Placersand the tsavorite are hosted occurs in sedimentaryat the hinges of rocks sheared (soil, isoclinal rudite, fold arenite,s within and saddle-reef silt) that structures. formed via erosion, gravity effect, mechanical transport, and sedimentation along slopes or basins related to neotectonics. 6.3. Sedimentary-Related (Type III): Placer Deposits Climate is a major factor in the formation of secondary deposits. In tropical areas, rocks are exposed to meteoricPlacers alteration, are hosted resulting in sedimentary in an rocks assemblage (soil, rudite, of arenite, clay minerals, and silt) that Fe formed and Mn via erosion, oxides, gravity and other supergeneeffect, mechanical phases. Corundum transport, and and sedimentation zircon are resistant along slopes minerals or basins found related in to soils, neotectonics. laterites, Climate and gravelly is a major factor in the formation of secondary deposits. In tropical areas, rocks are exposed to meteoric levels overlying bedrock. In high-altitude areas such as in Nepal, Pakistan, and Azad-Kashmir the alteration, resulting in an assemblage of clay minerals, Fe and Mn oxides, and other supergene phases. alteration is mechanical and ruby is found in boulders accumulated on the lower part of the slopes as Corundum and zircon are resistant minerals found in soils, laterites, and gravelly levels overlying for thebedrock. Nangimali In high-altitude deposit areas [245]. such Present-day as in Nepal, placersPakistan, are and illustrated Azad-Kashmir in Figurethe alteration49, which is mechanical follows the classificationand ruby proposedis found in byboulders [138]: accumulated on the lower part of the slopes as for the Nangimali deposit (i)[245]. Eluvial Present-day concentrations placers are derived illustrated by in in Figure situ weathering 49, which follows or weathering the classification plus gravitational proposed by movement[138]: or accumulation.(i) Eluvial Eluvial-deluvial concentrations depositsderived by are in on situ slopes weathering and in karstor weatheri cavitiesng (marble plus gravitational type). Colluvial depositsmovement correspond or accumulation. to decomposed Eluvial-deluvial primary deposits,deposits are which on slopes have and moved in karst vertically cavities and(marble laterally down-slopetype). Colluvial as the hillside deposits eroded. correspond to decomposed primary deposits, which have moved vertically (ii)and Alluvial laterally depositsdown-slope resulting as the hillside from theeroded. erosion of the corundum-bearing host rock and transport (ii) Alluvial deposits resulting from the erosion of the corundum-bearing host rock and transport by streams and rivers. Concentration occurs where water velocity drops at a slope change in the by streams and rivers. Concentration occurs where water velocity drops at a slope change in the hydrographical profile of the river, e.g., at the base of a waterfall, broad gullies, debris cones, meanders, hydrographical profile of the river, e.g., at the base of a waterfall, broad gullies, debris cones, and inflowingmeanders, streams.and inflowing Terminal streams. placers Terminal [138] formedplacers [138] by deltaic, formed aeolian, by deltaic, and aeolian, marine and accumulations marine are unknownaccumulations for ruby are unknow deposits.n for ruby deposits.

FigureFigure 49. The49. The diff differenterent types types of of placer placer depositsdeposits formed on on the the continental continental surface surface and anddeltaic deltaic or or marinemarine basins, basins, modified modified from from [138 [138].]. The The majoritymajority of these these types types of of placer, placer, except except for foraeolian aeolian and andmarine marine placers,placers, occur occur for gemfor gem corundum corundum deposits deposits worldwide. worldwide.

The placerThe placer deposits deposits are are divided divided in in three three mainmain sub-types (Table (Table 3).3).

6.3.1.6.3.1. Sub-Type Sub-Type IIIA: IIIA: Gem Gem Placers Placers in in an an Alkali Alkali BasaltBasalt Environment (Eastern (Eastern Australia, Australia, Pailin- Pailin-Chanthaburi-Trat,Chanthaburi-Trat, Eastern Eastern and andCentral Central Madagascar, Madagascar, and Others) and Others) Ruby-bearingRuby-bearing placers placers linked linked to to volcanic volcanic rocksrocks are fo foundund in in continental continental alkali alkali basalt basalt extrusions extrusions [68] [68] (Figure(Figure2). These 2). These deposits deposits are are sporadic sporadic and and economically economically quite quite rarerare when compared compared to to sapphire sapphire deposits deposits of the sameof the type, same except type, except for the for ruby the depositsruby deposits at Pailin at Pa andilin and Chanthaburi-Trat Chanthaburi-Trat [145 [145].]. In In Australia, Australia, RayRay etet al.al. [246] [246] showed that within the New South Wales deposits, in 420 listed gem corundum occurrences, 10% had ruby. However, the production realized on the short-term depended on worldwide demand.

Minerals 2020, 10, 597 58 of 83 showed that within the New South Wales deposits, in 420 listed gem corundum occurrences, 10% had Minerals 2019, 9, x FOR PEER REVIEW 58 of 82 ruby. However, the production realized on the short-term depended on worldwide demand. Ruby placersplacers inin alkalialkali basalt basalt environments environments include include the the deposits deposits in easternin eastern Australia Australia[81, 106[81,106,247–,247–250]; Asia,250]; Asia, with with deposits deposits at Pailin, at Pailin, Cambodia Cambodia [5 ,[5,74,82,248]74,82,248] and and the the provinces provinces of of Chanthaburi-Trat and Kanchanaburi [[145,146,148,149,154,251];145,146,148,149,154,251]; ChinaChina withwith thethe MulingMuling deposit in Shandong Shandong province province [252]; [252]; Eastern Africa, in Kenya with the Baringo deposit, which is linked to the volcanism of the East African rift [[118];118]; and in Madagascar Madagascar at at Antsirabe Antsirabe in in An Antananarivotananarivo province province [95,161,231] [95,161,231] and Vatomandry in Toamasina provinceprovince [ 77[77,78].,78]. (1) In EasternEastern Australia,Australia, ruby placers linked to volcanicvolcanic rocks are foundfound inin continentalcontinental alkalialkali basaltbasalt extrusions [68] [68] (Figure 22).). They occuroccur as scatteredscattered depositsdeposits that areare subordinatesubordinate toto sapphiressapphires [[81,85]81,85] (Figure 5050).). Rubies are found in alluvial placers shed from Cenozic lava fieldsfields within an eruptiveeruptive zone extending from Northeast Queensland to Southeast Tasmania. The The main deposits are located in New South Wales, i.e.,i.e., BarringtonBarrington Tops,Tops, Macquarie-CudgegongMacquarie-Cudgegong Rivers, Rivers, and and New England (Figure 50A).50A). Occurrences are also also found found in in South South Australia Australia at at Adelaide, Adelaide, Victoria Victoria around around Melbourne, Melbourne, and and Tasmania. Tasmania. In New South Wales thethe depositsdeposits areare foundfound [[81,85,86]:81,85,86]: (i) AtAt BarringtonBarrington Tops, Tops, erosion erosion and and transport transport by drainages by drainages of the of eroded the eroded Barrington Barrington Tops volcano Tops gavevolcano rise gave to the rise formation to the formation of mineable of mineable placers inplacers the river. in the Ruby river. was Ruby found was in found 1851 andin 1851 alluvial and prospectingalluvial prospecting was done was over done a long over period a long between period the between 1960s andthe 1960s 1990s. and The 1990s. geology The of geology the Barrington of the TopsBarrington Plateau Tops was Plateau described was in described the early in 2000s the earl [253y] 2000s and mining [253] and at Upper mining Manning at Upper River Manning took River place fromtook place early 2005from until early 2008 2005 (Figure until 200850B). (Figure The ruby 50B). is associated The ruby with is associated other pink, with purple, other and pink, pastel purple, blue corundumand pastel blue used corundum for jewelry used (Figure for 50jewelryC). (Figure 50C).

Figure 50. (A) LocationLocation ofof rubyruby deposits deposits in in New New South South Wales, Wales, modified modified after after [81 ,[81,86];86]; (B) ( openingB) opening of the of Barringtonthe Barrington Tops Tops mine, mine, March March 2005, upper2005, upper Manning Manning flats. Photo: flats. CluPhoto:ff Resources Cluff Resources Pacific M Pacific/L, Courtesy M/L, LinCourtesy Sutherland; Lin Sutherland; (C) faceted (C 1) caratfaceted ruby, 1 carat 6 mm ruby,4 6 mm x 4 across,mm across, Gloucester Gloucester Tops, Tops, SE Barington SE Barington Tops × plateau.Tops plateau. Photo. Photo. Gayle Gayle Webb. Webb.

(ii) At Yarrowitch, the alluvial gem field is more restricted than at Barrington Tops and the ruby is associated with sapphires, red zircon, and spinel. The alluvial occurrences have not been mined. (iii) At Macquarie-Cudgegong Rivers, extensive alluvial exploitation took place in the rivers from 1851 to 1941. The placers contain ruby, sapphires, zircon, gold, and diamond; their original source is unknown, despite the presence of numerous basaltic vents in the area.

Minerals 2020, 10, 597 59 of 83

(ii) At Yarrowitch, the alluvial gem field is more restricted than at Barrington Tops and the ruby is associated with sapphires, red zircon, and spinel. The alluvial occurrences have not been mined. (iii) At Macquarie-Cudgegong Rivers, extensive alluvial exploitation took place in the rivers from 1851 to 1941. The placers contain ruby, sapphires, zircon, gold, and diamond; their original source is unknown, despite the presence of numerous basaltic vents in the area. (iv) At Tumbarumba, ruby is minor and the placers contain blue to green sapphires, which are corroded and worn. (v) At New England, the Inverell-Glen Innes area is known for its sapphire placers. Ruby is scarce but is recovered from all the rivers. The crystals grade from red into pink and purple corundum [86]. Sometimes, theMinerals crystals 2019, 9, x are FOR PEER zoned REVIEW with red to pink cores, a white peripheral zone,59 of 82 and a violet rim.

Several large gem(iv) crystals At Tumbarumba, have been ruby cut,is minor such and asthe theplacers 48 contain carat blue “Aurora” to green sapphires, zoned stonewhich are found near Glen Innes in 1988,corroded which and was worn. cut into a 17 carats red, a 9 carats blue, and a 2 carats pink stones [249]. Other stones have been(v) At New faceted, England, such the asInverell-Glen a 2.13 carats Innes purplearea is known red for oval its cut,sapphire a 0.31 placers. carats Ruby light is red baguette scarce but is recovered from all the rivers. The crystals grade from red into pink and purple corundum cut, and a 0.22[86]. carats Sometimes, red marquise the crystals cutare zoned [81]. with red to pink cores, a white peripheral zone, and a violet In Queensland,rim. Several rubies large are gem very crystals rare have and been the cut, depositssuch as the 48 are carat mainly “Aurora” composed zoned stone offound sapphires. near In Victoria Glen Innes in 1988, which was cut into a 17 carats red, a 9 carats blue, and a 2 carats pink stones [249]. and Tasmania, pinkOther stones to red have corundum been faceted, are such rare as a and 2.13 thecarats high purple content red oval ofcut, Fe a 0.31 turns carats the light color red to brown [254]. (2) The presencebaguette cut, of and corundum a 0.22 carats red in marquise alluvial cut placers [81]. from Madagascar has been known since the In Queensland, rubies are very rare and the deposits are mainly composed of sapphires. In Victoria beginning of theand 20th Tasmania, century pink to [ 231red corundum]. The depositsare rare and formedthe high content by the of Fe erosion turns the color of corundum-bearingto brown [254]. Cenozic lava fields within(2) eruptive The presence zones of corundum extending in alluvial from plac theers from centre Madagascar of the has island, been known in thesince Ankaratrathe massif, Antananarivo province,beginning of the to 20th the century north [231] in. The the deposits Montagne formed by d’Ambre, the erosion of Antsirananacorundum-bearing provinceCenozic (Figure 51). lava fields within eruptive zones extending from the centre of the island, in the Ankaratra massif, The productionAntananarivo of ruby province, in Madagascar to the north in became the Montag significantne d’Ambre, Antsiranana after the province discovery (Figure 51). in The 2000 of two ruby deposits in theproduction province of ruby of Toamasina in Madagascar namedbecame significant Andilamena after the discovery [164] in and 2000 Vatomandry of two ruby deposits [255 in ]. the province of Toamasina named Andilamena [164] and Vatomandry [255].

Figure 51. The volcanic provinces of Madagascar, modified from [256] with the localities of the basaltic Figure 51. Thecorundum volcanic deposits provinces related to of the Madagascar, Cenozoic basalts modified in the provinces from of Antsiranana, [256] with Antananarivo, the localities and of the basaltic corundum depositsToamasina related [161,162]. to The the deposits Cenozoic of gem corundum: basalts no inrth in the Antsiranana provinces Province, of the Antsiranana, mining districts Antananarivo, and Toamasinaof Ambondromifehy [161,162]. The and Anivorano deposits in the of Montagne gem corundum:d’Ambre, Ambato northpeninsula inand AntsirananaBefotaka in Nosy Province, the Be island; Central Madagascar in Antananarivo Province, the mines of Soamiakatra, Mandrosohasina– mining districtsKianjanakanga of Ambondromifehy (Antsirabe area) in th ande volcanic Anivorano Ankaratra massif; in the andMontagne east in Toamasina d’Ambre, Province, with Ambato the peninsula and Befotakadeposits in Nosy of Vatomandry Be island; in the Takarindiona CentralMadagascar and Vohibalaina basaltic in Antananarivo areas. The geographic Province, coordinates the mines of are related to the Laborde projection. Soamiakatra, Mandrosohasina–Kianjanakanga (Antsirabe area) in the volcanic Ankaratra massif; and east in Toamasina Province, with the deposits of Vatomandry in the Takarindiona and Vohibalaina basaltic areas. The geographic coordinates are related to the Laborde projection. Minerals 2019, 9, x FOR PEER REVIEW 60 of 82

In 2005 Madagascar was the third largest ruby-exporting country after Kenya and Tanzania (see Figure 3). The secondary deposits, i.e., alluvial placers and paleoplacers are located (i) to the north and Mineralseast of2020 Antsirabe, 10, 597 city in Antananarivo province (Ambatomainty, Antsabotraka, and Soamiakatra60 of 83 deposits) and (ii) to the north of Vatomandry in Toamasina province. In this last province the alluvial and paleoplacer deposits border the Takarindiona and Vohibalaina basaltic massifs (Figure 51). The mining sites Inare 2005 Amfao, Madagascar Sahanonoka, was and the thirdAntsidikana largest near ruby-exporting the commune country of Amboditavolo. after Kenya They and Tanzania are located (see in Figurea number3). of The watersheds secondary and deposits, major river i.e., alluvialsystems placers downstream and paleoplacers of the Vohibalaina are located volcanic (i) to massif, the north and andthose east of ofAntanambao, Antsirabe city Mahatsara, in Antananarivo and Ankazombanga province (Ambatomainty, on the southern Antsabotraka, and eastern and borders Soamiakatra of the deposits)Takarindiona and massif (ii) to [94]. the north of Vatomandry in Toamasina province. In this last province the alluvialAt andVatomandry, paleoplacer the deposits ruby borderis associated the Takarindiona with crystals and Vohibalainaof sapphire basaltic and red massifs zircon (Figure in some51). Thepaleoplacers mining sites but aremostly Amfao, in alluvials. Sahanonoka, Generally, and Antsidikana the gem gravels near the consist commune of 0.5–2 of Amboditavolo. m thick sediments They areoverlying located bedrock in a number in the of watershedsriverbeds. The and ruby major crystals river systems are 5 mm downstream to 3 cm ofacross, the Vohibalaina rounded and volcanic with massif,different and colors those (red, of Antanambao, red purple, red Mahatsara, brown, and Ankazombangapink), but a red-purple on the southern color is dominant and eastern [77]. borders of theThe Takarindiona most common massif inclusions [94]. in ruby from the Sahanonoka deposit are rutile and zircon (Figure 52A),At which Vatomandry, are cogenetic the ruby and is arranged associated in with clusters crystals [161,162,255]. of sapphire Ilmenite, and red zirconrutile, inand some Al-silicates paleoplacers also butoccur mostly as inclusions in alluvials. (Figure Generally, 52B). the The gem other gravels solids consist are oftalc 0.5–2 (Figure m thick 52C), sediments phlogopite overlying (Figure bedrock 52D), inpentlandite, the riverbeds. plagioclase, The ruby titanite, crystals ilmenorutile, are 5 mm to 3 , cm across, and rounded Al-silicates. and with Apatite different was colors observed (red, redby purple,[78]. Rutile red brown,crystals andwere pink), identified but a red-purpleas a daughter color phase is dominant in the cavities [77]. of fluid inclusions trapped by the ruby.The mostThe fractures common in inclusionsthe rubies ar ine rubyfilled fromby limonite the Sahanonoka and kaolinite deposit [78]. are rutile and zircon (FigureThe52 originalA), which source are of cogenetic these rubies and is arrangedunknown inneither clusters xenocrysts [161,162 nor,255 corundum]. Ilmenite, xenoliths rutile, have and Al-silicatesever been observed also occur in as the inclusions basalts. The (Figure placers52B). th Theat are other proximal solids and are talcdownstream (Figure 52 ofC), the phlogopite different (Figurebasaltic 52volcanics,D), pentlandite, ruby crystals plagioclase, associated titanite, with ilmenorutile, fragments of hematite, basalt, and and red Al-silicates. to orangey Apatite zircon wasand observedsapphires by leave [78]. a Rutilehigh probability crystals were that identified the rubies as we a daughterre connected phase to the in the different cavities basaltic of fluid flows inclusions of the trappedTakarindiona by the and ruby. Vohibalaina The fractures basaltic in the massifs. rubies are filled by limonite and kaolinite [78].

FigureFigure 52.52. InclusionsInclusions inin rubyruby fromfrom thethe SahanonokaSahanonoka deposit in thethe VatomandryVatomandry areaarea [[94].94]. ((AA)) ZirconZircon (Zrn)(Zrn) associated with with rutile rutile (Rt); (Rt); (B (B) )rutile, rutile, ilmenite ilmenite (Ilm), (Ilm), and and Al-silicate Al-silicate (Al-Si); (Al-Si); in ruby in ruby (Crn); (Crn); (C) (associationC) association of rutile of rutile and and talc talc (Tlc) (Tlc) in ruby; in ruby; and and (D) ( Dphlogopite) phlogopite (Phl) (Phl) crystal crystal included included in a in ruby. a ruby.

TheThe originalFe contents source of ofthe these rubies rubies (1500–8460 is unknown ppm neither) are always xenocrysts higher nor than corundum Cr and xenoliths V. The Cr have2O3 evercontents been differ observed with color: in the (i) basalts. red brown The placers(4760–6670 that ppm), are proximal (ii) red (3500–3800 and downstream ppm), (iii) of the pink di ff(1390–erent basaltic11,670 ppm), volcanics, and ruby(iv) red crystals to purple associated (720–2000 with fragmentsppm). The of chemical basalt, and diagram red to orangeyfor classification zircon and of sapphirescorundum leave deposits a high (Figure probability 53) show thats a the distinctive rubies were bimodal connected chemical to the distribution different basaltic for the flowsrubies of from the TakarindionaVatomandry, andwhich Vohibalaina plot in the basaltic metamorphosed massifs. M-UMR (R3) and metasomatic (R4) domains. The first Thefield Fe in contents the R3 ofdomain the rubies (M-UMR (1500–8460 signature) ppm) arecontains always red higher and thanred brownish Cr and V.The rubies; Cr2 Othe3 contents second difieldffer in with the color: R4 domain (i) red brown(metasomatic (4760–6670 such ppm), as plumasite (ii) red (3500–3800 or metasomatite ppm), (iii)in M-UMR) pink (1390–11,670 contains red ppm), to and (iv) red to purple (720–2000 ppm). The chemical diagram for classification of corundum deposits (Figure 53) shows a distinctive bimodal chemical distribution for the rubies from Vatomandry, which plot in the metamorphosed M-UMR (R3) and metasomatic (R4) domains. The first field in the R3 Minerals 2020, 10, 597 61 of 83

Minerals 2019, 9, x FOR PEER REVIEW 61 of 82 domain (M-UMR signature) contains red and red brownish rubies; the second field in the R4 domain purple(metasomatic and pink such stones. as plumasite The bimoda or metasomatitel distribution in is M-UMR) also present contains for the red rubies to purple from and Soamiakatra pink stones. in AnkaratraThe bimodal and distribution Andimalena, is also while present the for rubies the rubies from fromAntsabotraka Soamiakatra and in Ambatomainty Ankaratra and Andimalena, plot in the metasomaticwhile the rubies domain. from Antsabotraka and Ambatomainty plot in the metasomatic domain. δ18 The δ18OO values values for for rubies rubies from from the theVatomandry Vatomandry mining mining district district span spanthe range the range2.7–6.7‰ 2.7–6.7% (n = 8) (n = 8) with a mean value of δ18O = 5.1 1.4%. They fit within the worldwide range defined for with a mean value of δ18O = 5.1 ± 1.4‰.± They fit within the worldwide range defined for ruby in ruby in basaltic environments (1.3% < δ18O < 5.9%, mean δ18O = 3.1 1.1%, n = 57; [80]) and ruby basaltic environments (1.3 < δ18O < 5.9‰, mean δ18O = 3.1 ± 1.1‰, n =± 57; [80]) and ruby associated δ18 withassociated M-UMR with (0.25 M-UMR < δ18O (0.25%< 6.8‰,

2 3 2 3 2 2 3 Figure 53. ((A)) Chemical diagramdiagram FeO-CrFeO-Cr2OO3-MgO-V2O3 versus FeO + TiO2 + GaGa2OO3 showingshowing the the main main chemical fieldsfields defined defined for for di ffdifferenterent types types of corundum of corundum deposits deposits worldwide worldwide [72,77]; [72,77]; (B) field ( ofB) chemicalfield of chemicalcompositions compositions of ruby from of ruby the Vatomandry from the Vatomand depositsry (Toamasina deposits (Toamasina Province) compared Province) with compared the primary with thedeposit primary at Soamiakatra, deposit at Soamiakatra, and the placer and deposits the plac of Antsabotrakaer deposits of andAntsabotraka Ambatomainty and Ambatomainty (Antananarivo (AntananarivoProvince). The VatomandryProvince). The and Vatomandry Soamiakatra depositsand Soamiakatra show a distinct deposits bimodal show chemical a distinct distribution bimodal chemicalin the M-UMR distribution (R3) and in the metasomatite M-UMR (R3) (R4) and domains, metasomatite respectively. (R4) domains, The deposit respectively. at Andilamena The deposit in atToamasina Andilamena Province in Toamasina is apparently Province not relatedis apparently to alkali not basalts related but to is alkali likely basalts derived but from is likely plumasite derived in fromM-UMR plumasite because in field M-UMR work because has shown field the work presence has show of inn situ the highly presence altered of in mafic situ highly rocks, biotitealtered schists, mafic rocks,and kaolinite-bearing biotite schists, and rocks. kaolinite-bearing rocks.

6.3.2. Sub-Type Sub-Type IIIB: Gem Placers in Meta Metamorphicmorphic Environment in M-UMR (Montepuez, Mozambique) and Marble (Mogok Stone Track, Myanmar, and Others) Mozambique) and Marble (Mogok Stone Track, Myanmar, and Others) (1) At Montepuez in Mozambique, the main source of facet-grade material in the Gemfields (1) At Montepuez in Mozambique, the main source of facet-grade material in the Gemfields Group Group Ltd. properties is secondary deposits in M-UMR environments [25,26] (see Figures 32 and 54). Ltd. properties is secondary deposits in M-UMR environments [25,26] (see Figures 32 and 54). The placer The placer deposits were previously interpreted as alluvial and related to a flood event in the river deposits were previously interpreted as alluvial and related to a flood event in the river systems [27,200,257,258]. However, Simonet [25] concluded that the deposits were not alluvial since there was no evidence of systematic rounded pebbles, variations in the grain sizes and graded bedding in the sediments.

Minerals 2020, 10, 597 62 of 83 systems [27,200,257,258]. However, Simonet [25] concluded that the deposits were not alluvial since there was no evidence of systematic rounded pebbles, variations in the grain sizes and graded bedding in theMinerals sediments. 2019, 9, x FOR PEER REVIEW 62 of 82

FigureFigure 54. 54.The The placer placer deposits deposits in in the the Montepuez Montepuez miningmining district in in Mozambique. Mozambique. (A (A) Aerial) Aerial view view of of thethe Mugloto Mugloto Pit Pit operated operated by by the the Gemfields Gemfields GroupGroup Ltd.Ltd. The miners miners are are using using a aback-filling back-filling technique technique (top)(top) and and using using tree tree plantations plantations for for rehabilitation rehabilitation (bottom); (bottom); ( B(B)) the the ruby-rich ruby-rich gravel gravel layerlayer atat MuglotoMugloto in thein Montepuez the Montepuez mining mining district. district. The detriticThe detritic levels le containvels contain sub-angular sub-angular quartz quartz and clayand pebblesclay pebbles located betweenlocated 4 between and 10 m 4 underand 10 the m surface.under the Gemfields surface. Ge removesmfields the removes overburden the overburden and uses anand excavator uses an to collectexcavator this gravel to collect layer, this that gravel is then layer, sent that to the is then wash sent plant to tothe extract wash rubies;plant to ( Cextract) small rubies; scale miners(C) small near Nacacascale village,miners near an area Nacaca reserved village, for an locals area outside reserved the for southeast locals outside border the of thesoutheast Gemfields border concession, of the whereGemfields a few thousandconcession, people where have a few worked thousand since people 2012. Athave this worked location since small 2012. rubies At this are foundlocation in small gravels underrubies to are 2–5 found m of overburden.in gravels under The to Nacaca2–5 m of stones overburden. are usually The Nacaca smaller stones and are contain usually more smaller inclusions and thancontain those more from inclusions Mugloto andthan their those iron from content Mugloto is similar and their to that iron of content the material is similar from to Maninge that of the Nice; (D)material parcel offrom rough Maninge rubies Nice; from (D Mugloto) parcel of Pit rough near rubies Montepuez from Mugl (aboutoto Pit 0.5 near g per Montepuez stone) at a(about Gemfields 0.5 g per stone) at a Gemfields rough ruby auction in Singapore. The rubies coming from a gravel layer rough ruby auction in Singapore. The rubies coming from a gravel layer range from bright to dark red range from bright to dark red (depending on their iron contents), and present some abrasion features. (depending on their iron contents), and present some abrasion features. The area was discovered in The area was discovered in early 2014 and probably more than 90% of Gemfields income since then early 2014 and probably more than 90% of Gemfields income since then has been extracted from this has been extracted from this deposit. Photo: Vincent Pardieu © GIA. deposit. Photo: Vincent Pardieu © GIA.

TheThe secondary secondary deposits deposits are are mainly mainly colluvialcolluvial withwith limited horizontal tran transportsport (more (more sub-angular sub-angular fragments) and a few alluvial pebbles (smooth and round appearance). Most of the mineralized horizons fragments) and a few alluvial pebbles (smooth and round appearance). Most of the mineralized are stone lines, i.e., weathering-resistant rock fragments formed over millions of years of erosion [25]. The horizons are stone lines, i.e., weathering-resistant rock fragments formed over millions of years of stone lines, which are composed of angular fragments of quartz and pegmatite, are excellent traps for erosion [25]. The stone lines, which are composed of angular fragments of quartz and pegmatite, smaller grains of high-density minerals such as ruby. The colluvial deposits (Figure 54) are either are excellent traps for smaller grains of high-density minerals such as ruby. The colluvial deposits associated with the proximal primary deposits (as at the Maninge Nice/glass mines) or disconnected (as (Figureat the54 Mugloto) are either mine). associated Simonet [25] with proposed the proximal an exploration primary scheme deposits for (as the at co thelluvial Maninge deposits Nice in /theglass mines)Montepuez or disconnected area based (ason detailed at the Mugloto mapping mine). of the primary Simonet deposits [25] proposed (stressing an the exploration importance scheme of the strike for the colluvialand dip deposits of the mineralized in the Montepuez structures) area and based erosion on detailedthrough mappingtime (Figure of the55). primaryThe distance deposits between (stressing the theprimary importance deposit of thein strikeoutcrop and and dip the of edge the mineralized of the colluvial structures) accumulation and erosion increases through during time the (Figure erosion55 ). Theprocess. distance Mapping between of the the stone primary lines depositand outcrops in outcrop of ruby-bearing and the rocks edge could of the be colluvial an important accumulation tool for increasesfuture ruby during placer the exploration erosion process. in the Montepuez Mapping mining of the stone area. lines and outcrops of ruby-bearing rocks could be an important tool for future ruby placer exploration in the Montepuez mining area.

Minerals 2020, 10, 597 63 of 83 Minerals 2019, 9, x FOR PEER REVIEW 63 of 82

Minerals 2019, 9, x FOR PEER REVIEW 63 of 82

Figure 55. Schematic time-relationship of the formation and position of a colluvial placer through Figure 55.Figure Schematic 55. Schematic time-relationship time-relationship of of the the formation formation and and position position of a of colluvial a colluvial placer throughplacer through erosion ofof anan angle-dippingangle-dipping primaryprimary depositdeposit [[25].25]. erosion of an angle-dipping primary deposit [25]. (2) EconomicEconomic rubyruby placerplacer depositsdeposits inin marblemarble environmentsenvironments ofof South-EasternSouth-Eastern AsiaAsia (Figure(Figure 22).). (2) EconomicIn thethe MogokMogok ruby Stoneplacer Stone Track Track deposits in in Myanmar Myanmar in marble (Figure (Figure environments56A), 56A), gem-bearing gem-bearing of levels South-Eastern levels enriched enriched in pebbles, Asia in pebbles, (Figure sand, 2). In thesilt,sand, clay,Mogok silt, and clay, ironStone and oxides iron Track oxides are calledin areMyanmar ‘byon’called ‘byon’ [259 (Figure,260 [259,260]] (Figure 56A), (Figure56 B,C).gem-bearing 56B,C). levels enriched in pebbles, sand, silt, clay, and iron oxides are called ‘byon’ [259,260] (Figure 56B,C).

Figure 56. DifferentDifferent aspects aspects of of the the gem gem placers placers in inthe the Mo Mogokgok Stone Stone Track, Track, Myanmar Myanmar (in January (in January 2015). 2015). (A) (PanoramicA) Panoramic view view of the of primary the primary deposits deposits mined minedin marble in marbleon the flank on the of the flank hills of (foreground the hills (foreground view) and view)secondary and deposits, secondary i.e., deposits,placers, mined i.e., placers,in the valley mined (background); in the valley (B) image (background); of the gem-bearing (B) image material of the gem-bearingcalled ‘byon’ enriched material in called gravels ‘byon’ and enrichedsand. The in concentration gravels and of sand. heavy The minerals concentration is visible of and heavy is composed minerals isof visibleiron oxides, and iscorundum, composed quartz, of iron and oxides, pieces corundum, of marble; quartz, (C) concentration and pieces of of gem marble; minerals (C) concentration (ruby, spinel, ofand gem sapphire) minerals in the (ruby, pan of spinel, a miner; and and sapphire) (D) sedimentary in the panbreccia of aformed miner; in anda cave (D placer) sedimentary karst. The brecciabreccia formed from in a cavean argillite placer containing karst. The crystals breccia of formed spinel, plates from anof argillitegraphite, containingand fragments crystals of marble of spinel, and Figure 56. Different aspects of the gem placers in the Mogok Stone Track, Myanmar (in January 2015). (A) platesquartz. of Size graphite, of the breccia and fragments sample around of marble 15 cm and long. quartz. Photo: Size Gaston of the Giuliani. breccia sample around 15 cm long. PanoramicPhoto: view Gaston of the Giuliani. primary deposits mined in marble on the flank of the hills (foreground view) and secondarySimilar deposits, deposits i.e., placers, are found mined at Namya in the invalley the nort (background);hern part of (theB) imagecountry of wherethe gem-bearing the monsoon material is calledan ‘byon’effective enriched erosion in mechanism gravels and in sand. low-altitude The concentration environments of heavy [5,108,261]. minerals The is gem visible concentration and is composed is of ironclosely oxides, related corundum, to the formation quartz, ofand karst pieces and ofchemical marble; weathering (C) concentration of marble of and gem associated minerals rocks,(ruby, i.e., spinel, and schists,sapphire) gneisses, in the panand ofgranitoids a miner; (Figure and (D )56D). sedimentary The cave breccia placers formedin karst incontain a cave sedimentary placer karst. breccias The breccia formed from an argillite containing crystals of spinel, plates of graphite, and fragments of marble and quartz. Size of the breccia sample around 15 cm long. Photo: Gaston Giuliani.

Similar deposits are found at Namya in the northern part of the country where the monsoon is an effective erosion mechanism in low-altitude environments [5,108,261]. The gem concentration is closely related to the formation of karst and chemical weathering of marble and associated rocks, i.e., schists, gneisses, and granitoids (Figure 56D). The cave placers in karst contain sedimentary breccias

Minerals 2020, 10, 597 64 of 83

Similar deposits are found at Namya in the northern part of the country where the monsoon is an effective erosion mechanism in low-altitude environments [5,108,261]. The gem concentration is closely relatedMinerals 2019 to the, 9, x formation FOR PEER REVIEW of karst and chemical weathering of marble and associated rocks, i.e., schists,64 of 82 gneisses, and granitoids (Figure 56D). The cave placers in karst contain sedimentary breccias formed byformed argillites by argillites with ruby, with sapphire, ruby, sapphire, spinel, graphite, spinel, graphite, and fragments and fragments of marble of and marble quartz. and The quartz. ruby andThe otherruby and gems other (sapphire gems (sapphire and spinel) and are spinel) trapped are in trapped eluvial, in colluvial, eluvial, alluvial,colluvial, fracture-filling, alluvial, fracture-filling, and cave depositsand cave (Figuredeposits57 (Figure)[29]. 57) [29].

Figure 57.57. The main types of ruby-bearing placers in the Mogok Stone Track, Myanmar;Myanmar; modifiedmodified from [[29].29]. The primary ruby depositsdeposits are located in marble and the secondary deposits are residual, colluvial, alluvial, fracture-filling,fracture-filling, and cave placersplacers inin karstkarst morphologies.morphologies. The figurefigure represents thethe ruby deposit ofof Dattaw inin marble (on the left) and the sapphire deposit of Thurein-taung (on(on thethe right).right). At Mogok,Mogok, the didifferentfferent typestypes ofof primaryprimary depositsdeposits of rubyruby andand sapphiressapphires resultedresulted in thethe minglingmingling of gems from didifferentfferent sourcessources inin singlesingle secondarysecondary accumulations.accumulations.

In Vietnam, thethe placersplacers areare proximalproximal toto the main primary deposits of ru rubyby and colored sapphires inin marble.marble. They They are are located located in in the the Luc Luc Yen Yen and Yen BaiBai provincesprovinces inin thethe RedRed RiverRiver shearshear zonezone areaarea and in the Quy Chau ruby deposit (Nghe An province) in the Bu Kang metamorphic zone in Central Vietnam [98,210]. [98,210]. The The placers placers are are exploited exploited at atLuc Luc Yen, Yen, An An Phu, Phu, Khoang Khoang Thong, Thong, and andNuoc Nuoc Ngap Ngap (Luc (LucYen district); Yen district); at Truc at Truc Lau, Lau, Yen Yen Thai, Thai, Tan Tan Huon Huong,g, Tan Tan Dong, Dong, Hoa Hoa Cuong, Cuong, and and Tran Tran Yen Yen (Yen Bai district); and at at Quy Quy Hop Hop and and Quy Quy Chau Chau (Nghe (Nghe An An district). district). The The paleoplacer paleoplacer at atTruc Truc Lau Lau consists consists of of10 10m of m sediments of sediments overlying overlying bedrock. bedrock. The rubies The rubies and blue and sapphires blue sapphires are contained are contained in 5 m inthick 5 m gravel thick gravellayer overlain layer overlain by a 3.5 by m a 3.5thick m be thickd of bed Quaternary of Quaternary sediments sediments and 1–1. and5 1–1.5m of msoil. of soil.In 2002, In 2002, up to up two to twoboulders boulders (1–2 (1–2kg) of kg) pink of pinksapphire sapphire and star and ruby star rubywere were recovered recovered every every month month in the in paleoplacer. the paleoplacer. The Theplacer placer deposits deposits consist consist of gravel of gravel concentrations concentrations in karst in pockets karst pockets (Figure (Figure 58A) and58 A)alluvial and alluvial fans (Figure fans (Figure58B) found58B) all found over all the over Luc the Yen Luc region Yen region [261,262 [261]. ,The262]. gem-bearing The gem-bearing valleys valleys are often are often narrow narrow and 2 andcorrespond correspond to small to small depressions, depressions, which which typically typically rangerange in area in from area 2 from to 3 2km to2 3(Figure km (Figure 58D). These58D). Theseplacers placers furnish furnish a variety a variety of gem-qu of gem-qualityality materials materials for a market, for a market,which has which been has open been daily open in Luc daily Yen in Luccity Yensince city 1987 since (Figure 1987 (Figure58C). The58C). rubies The rubiesin the inplacers the placers have a have proximal a proximal origin origin as confirmed as confirmed by the by 18 theoxygen oxygen isotopic isotopic values values of ofthe the crystals: crystals: δδ18OO ranges ranges between 15.5%15.5 and and 22.3‰, 22.3% ,which which overlaps overlaps the worldwideworldwide oxygenoxygen isotopicisotopic rangerange (16–23%(16–23‰)) defineddefined forfor rubyruby hostedhosted inin marblemarble [[40,93].40,93]. Mining is conducted on on a a small small scal scale,e, from from Bai Bai Lai Lai in in the the northeast northeast of ofLuc Luc Yen Yen to tothe the Na Na Ha Ha area area in inthe the south, south, near near the the banks banks of ofThac Thac Ba BaLake. Lake. The The most most active active sites sites are are along along the the streams streams of Cong of Cong Troi, Troi, in Nuoc Ngap (An Phu area). Material is also removed from old mines in areas such as Minh Tien, Khoan Thong, and Bai Chuoi, where big companies mined the most accessible and profitable material, often missing areas between marble pinnacles (Figure 58E) that excavators could not easily reach. As a result, some gem-rich ground was left behind; this is now collected by locals with shovels and buckets.

Minerals 2020, 10, 597 65 of 83 in Nuoc Ngap (An Phu area). Material is also removed from old mines in areas such as Minh Tien, Khoan Thong, and Bai Chuoi, where big companies mined the most accessible and profitable material, often missing areas between marble pinnacles (Figure 58E) that excavators could not easily reach. As a Mineralsresult, some2019, 9, gem-rich x FOR PEER ground REVIEW was left behind; this is now collected by locals with shovels and buckets.65 of 82

Figure 58. 58. Ruby-bearingRuby-bearing placers placers hosted hosted in inmarble marble from from northern northern Vietnam Vietnam (Luc (Luc Yen Yenarea; area; 2009). 2009). (A) Exploitation(A) Exploitation by locals by locals of ruby-bearing of ruby-bearing gravel gravel concentration concentration in in karst karst pockets; pockets; (B (B) )artisanal artisanal exploitation exploitation of ruby-bearing alluvialalluvial fansfans concentrated concentrated in in narrow narrow valleys valleys bordered bordered by riceby rice fields; fields; (C) the(C) famousthe famous gem gemmarket market at Luc at Yen Luc city; Yen authorization city; authorization for the photograph for the photograph of the Luc of Yen the market Luc Yen by filed market gemmologist by filed gemmologistM. Shuan, and M. ( DShuan,) the gem-bearing and (D) the gem-bearing valleys are often valleys narrow are often and correspondnarrow and tocorrespond small depressions, to small 2 depressions,which typically which range typically in area range from 2 in to area 3 km from. These 2 to valleys3 km2. These are defined valleys by are shear defined zones by and shear faults zones that andcrosscut faults the that marble crosscut units. the The marble karst units. geomorphology The karst geomorphology is characterized byis charac an alignmentterized by of marblean alignment domes offorming marble cones domes and forming lines of cones karst and hills; lines (E) of excavation karst hills; of ( ruby-bearingE) excavation areasof ruby-bearing continued upareas to thecontinued marble uppinnacles, to the marble but the pinnacles, missed gem but areas the missed between gem the area pinnacless between is exploited the pinnacles by locals is exploited with shovels, by locals picks withand buckets.shovels, Photos:picks and Vincent buckets. Pardieu Photos:© VincentGIA. Pardieu © GIA. 6.3.3. Sub-Type IIIC: Gem Placers with Ruby Originating from Multiple and Unknown Sources 6.3.3. Sub-Type IIIC: Gem Placers with Ruby Originating from Multiple and Unknown Sources Gem placers of economic interest are also found in Madagascar (Ilakaka, Andilamena, Gem placers of economic interest are also found in Madagascar (Ilakaka, Andilamena, Bemainty, Bemainty, Didy, and Zahamena) and Tanzania (Tunduru, Songea, and Umba). The historical, Didy, and Zahamena) and Tanzania (Tunduru, Songea, and Umba). The historical, geological, and geological, and mineralogical features of these deposits are described in different publications; mineralogical features of these deposits are described in different publications; see see [5,19,20,108,161,164,263–266]. These placers contain ruby, sapphires, and other minerals (garnets, [5,19,20,108,161,164,263–266]. These placers contain ruby, sapphires, and other minerals (garnets, alexandrite, , zircon, spinel, and tourmaline) that originate from multiple unknown sources. We alexandrite, topaz, zircon, spinel, and tourmaline) that originate from multiple unknown sources. We can note that at Songea and Andilamena, corundum is found both in placers and in situ in highly can note that at Songea and Andilamena, corundum is found both in placers and in situ in highly weathered rocks. weathered rocks In the Ilakaka area (see Figure 4), gem corundum is found at three gravel levels in two main alluvial In the Ilakaka area (see Figure 4), gem corundum is found at three gravel levels in two main terraces deposited on the Triassic Isalo sandstone [108]. The terraces are weakly consolidated, but alluvial terraces deposited on the Triassic Isalo sandstone [108]. The terraces are weakly consolidated, correspond to paleoplacers. The terraces are the result of the erosion and stripping of the sandstones. but correspond to paleoplacers. The terraces are the result of the erosion and stripping of the The deposit is not well consolidated and is composed of quartz-rich sands containing pebbles of sandstones. The deposit is not well consolidated and is composed of quartz-rich sands containing ferruginous laterite, rounded blocks of Isalo sandstone and quartz, and quartzite and schist pebbles. pebbles of ferruginous laterite, rounded blocks of Isalo sandstone and quartz, and quartzite and schist pebbles. The Ilakaka deposits produce very fine blue, blue-violet, violet, purple, orange, yellow, and translucent sapphire crystals along with pink corundum and rare rubies, zircon, alexandrite, topaz, garnet, spinel, andalusite, and tourmaline. In 2002, test mining by Gem Mining Resources Company over 38 days resulted in approximately 43 kilos of gems, comprising of 4% semi-precious stones, volcanic glasses, and ruby, and 96% sapphire comprising 58% pink gem corundum, 30% sapphire, and 8% other colors of sapphire (e.g., yellow, orange, padparadscha, and green). Microscopic examination

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The Ilakaka deposits produce very fine blue, blue-violet, violet, purple, orange, yellow, and translucent sapphire crystals along with pink corundum and rare rubies, zircon, alexandrite, topaz, garnet, spinel, andalusite, and tourmaline. In 2002, test mining by Gem Mining Resources Company over 38 days resulted in approximately 43 kilos of gems, comprising of 4% semi-precious stones, volcanic glasses, and ruby, and 96% sapphire comprising 58% pink gem corundum, 30% sapphire, and 8% other colors Minerals 2019, 9, x FOR PEER REVIEW 66 of 82 of sapphire (e.g., yellow, orange, padparadscha, and green). Microscopic examination of respectively of30 respectively ruby and 58 30 blue ruby sapphire and 58 crystalsblue sapphire showed crystals that all showed of the rubiesthat all have of the the rubies same have gemmological the same gemmologicalfeatures, i.e., growth, features, twinning, i.e., growth, color twinning, bands, and bands, inclusions, and solid but thatinclusions, the blue but sapphires that the define blue sapphirestwo different define mineralogical two differen groupst mineralogical [40]. groups [40]. 18 18 The ruby crystals have δ 18O values between 2.6% and 3.8% 18(δ Omean = 3.5 0.5%, n = 4) that The ruby crystals have δ O values between 2.6 and 3.8‰ (δ Omean = 3.5 ± 0.5‰,± n = 4) that are withinare within the theisotopic isotopic range range defined defined for for primary primary ruby ruby deposits deposits in in metamorphosed metamorphosed M-UMR M-UMR from 18 SouthernSouthern Madagascar (2.5 (2.5% < δ <18Oδ

FigureFigure 59. PlacerPlacer depositsdeposits in in the the eastern eastern part part of Madagascar. of Madagascar. (A) General (A) General view ofview the rubyof the and ruby sapphire and sapphiredeposit in deposit the jungle in the of jungle Didy. Photo:of Didy. Nirina Photo: Rakotosaona; Nirina Rakotosaona; (B) crystals (B) ofcrystals ruby and of ruby sapphires and sapphires from the fromDidy the deposit. Didy Thedeposit. crystals The arecrysta aroundls are 8 around mm in size;8 mm photo: in size; Vincent photo: PardieuVincent© PardieuGIA; and © GIA; (C) sapphire and (C) sapphireand pink and corundum pink corundum placer deposits placer indeposits the jungle in the near jungle Bemainty. near Bemainty. Photo: Vincent Photo: Pardieu. Vincent Pardieu.

RubyRuby is is generally generally associated associated with with sapphires sapphires at Vatomandry at Vatomandry [77,78], [77,78 Didy], Didy [265] [ 265(Figure] (Figure 59), and59), Bemaintyand Bemainty [265] [but265 is] butexploited is exploited as a main as a gem main mineral gem mineral from the from Andilamena the Andilamena and Zaha andmena Zahamena deposits [266]. Up until the present, geological information is lacking, but visits to the mining district of Andilamena by V. Pardieu allowed him to observe the corundum host rocks, which were highly weathered. They comprised fuchsite-bearing altered mafic rocks, biotite schists, and kaolinite- bearing rocks. The rubies are found in mafic rocks and/or biotite schists. Sapphires are restricted to veins of quartz-free kaolinite-bearing rocks that crosscut the mafic rocks. Debate on the geologic and geographic origins of rubies from Andilamena and Vatomandry involved:

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deposits [266]. Up until the present, geological information is lacking, but visits to the mining district of Andilamena by V. Pardieu allowed him to observe the corundum host rocks, which were highly weathered. They comprised fuchsite-bearing altered mafic rocks, biotite schists, and kaolinite-bearing rocks. The rubies are found in mafic rocks and/or biotite schists. Sapphires are restricted to veins of quartz-free kaolinite-bearing rocks that crosscut the mafic rocks. Minerals 2019, 9, x FOR PEER REVIEW 67 of 82 Debate on the geologic and geographic origins of rubies from Andilamena and Vatomandry involved:

(i) The The chemistry chemistry of of these these rubies rubies in inthe the diagram diagram FeO FeO-Cr-Cr2O3-MgO-V2O3-MgO-V2O3 versus2O3 FeOversus + TiO FeO2 + Ga+ 2TiOO3 is2 +similarGa2O 3andis similar characterized and characterized by an overlap by anof two overlap chemical of two domains chemical (bimodal domains distribution; (bimodal distribution; see Figure 53).see FigureOn the53 contrary,). On the the contrary, diagram the V diagram versus Fe V (Fig versusure Fe60) (Figure shows60 a )clear shows difference a clear di inff erencethe V contents in the V ofcontents ruby for of both ruby deposits for both depositsthat allows that the allows allocation the allocation of the geographic of the geographic origin of origin the Vatomandry of the Vatomandry rubies relativerubies relative to Andilamena to Andilamena and also and the also main the ruby main deposits ruby deposits worldwide; worldwide; 18 (ii) The The δδ18OO values values for for rubies rubies from from the the Vatomandry Vatomandry mi miningning district district span span the the range range 2.7–6.7‰ 2.7–6.7% (n 18 (=n 8)= with8) with a mean a mean value value of δ18 ofO =δ 5.1O ±= 1.4‰5.1 (n1.4% = 5). Those(n = 5).for Thoserubies from for rubies the Andilamena from the Andilamena deposit are ± 18 betweendeposit are0.5 betweenand 4‰ with 0.5% a andmean 4% value with of aδ18 meanO = 2.9 value ± 1.5‰ of δ [128].O = Both2.9 ranges1.5% of[128 δ18].O Bothvalues ranges overlap of 18 ± andδ O fit values within overlap the worldwide and fit within range the defined worldwide for ruby range in basaltic defined environments for ruby in basaltic (1.3 < environmentsδ18O < 5.9 ‰, 18 18 (1.3%mean δ <18δO =O 3.1< 5.9%± 1.1‰,, mean n = 57);δ O [80])= 3.1 and1.1% ruby, nassociated= 57); [80 ])with and metamorphosed ruby associated with M-UMR metamorphosed (0.25 < δ18O 18 ±

Figure 60. VV versus Fe Fe (in ppm) for the main ruby depositsdeposits of Madagascar (Vatomandry, Andilamena, Didy, Zahamena)Zahamena) and otherother depositsdeposits worldwideworldwide (Kenya,(Kenya, Baringo;Baringo; Greenland, Aappaluttoq; Tanzania, Winza; Mozambique, Mozambique, Montepuez; Montepuez; Malawi, Malawi, Chimwadzul Chimwadzulu;u; Cambodia Cambodia and Thailand; and Thailand; and finally and deposits finally ofdeposits marble of marbletype from type Central from Central and Southeastern and Southeastern Asia). Asia). Chemical Chemical data data from from [200,267] [200,267 ]and and Pardieu unpublished data. data. The The chemical chemical domain domain of ruby of ruby from from Vatomandry Vatomandry is clearly is clearly separated separated from fromall the all other the onesother and ones this and parameter this parameter permits permits to precise to precise its geographic its geographic origin. origin.The name The of name ruby ofdeposits ruby deposits related

respectively,related respectively, to M-UMR to M-UMRis written isin written red (Types in red IIA (Types1, IIB1), to IIA marble1, IIB1 deposits), to marble in violet deposits (Type in IIB violet2), to alkali- (Type basaltIIB2), toin alkali-basaltblue (Type IIIA), in blue and (Typeto metamo IIIA),rphic and environment to metamorphic in black environment (Type IIIB). in black (Type IIIB). 7. Conclusions and Perspectives 7. Conclusions and Perspectives In the present paper, the ruby occurrences and deposits were classified into three main types: In the present paper, the ruby occurrences and deposits were classified into three main types: (Type (Type I) magmatic-related, (Type II) metamorphic-related, and (Type III) sedimentary-related (Figure 61). I) magmatic-related, (Type II) metamorphic-related, and (Type III) sedimentary-related (Figure 61). The economic ruby deposits, in terms of volume and quality, are the placers that in the new The economic ruby deposits, in terms of volume and quality, are the placers that in the new ruby ruby deposits classification are called “Sedimentary-related (Type III)”. The hardness and density deposits classification are called “Sedimentary-related (Type III)”. The hardness and density of ruby of ruby are the main physical parameters that enable it to survive the erosion of primary deposits, are the main physical parameters that enable it to survive the erosion of primary deposits, mechanical mechanical transport, and accumulation along slopes or sedimentation in basins. Today Africa is the transport, and accumulation along slopes or sedimentation in basins. Today Africa is the top gem ruby producer in the world, via secondary deposits in metamorphic environments (Sub-Type IIIB) resulting from the erosion of primary deposits in metamorphosed mafic and ultramafic rocks (M- UMR; Sub-Types IIA1 and IIB1). Over the last decade the main discoveries of ruby have been in Mozambique and Madagascar. It is not the end but the beginning of a new era for understanding the formation of such primary deposits and prospecting their associated placers. At the Montepuez mining district, the colluvial deposits are highly economic and characterized by limited transport. Most of the mineralized horizons are stone lines but the placers are proximal to the primary deposits. The exploration scheme

Minerals 2020, 10, 597 68 of 83 top gem ruby producer in the world, via secondary deposits in metamorphic environments (Sub-Type IIIB) resulting from the erosion of primary deposits in metamorphosed mafic and ultramafic rocks (M-UMR; Sub-Types IIA1 and IIB1). Over the last decade the main discoveries of ruby have been in Mozambique and Madagascar. It is not the end but the beginning of a new era for understanding the formation of such primary deposits and prospecting their associated placers. At the Montepuez mining district, the colluvial deposits are Mineralshighly 2019 economic, 9, x FOR and PEER characterized REVIEW by limited transport. Most of the mineralized horizons are68 stone of 82 lines but the placers are proximal to the primary deposits. The exploration scheme proposed by [25] proposedfor the colluvial by [25] deposits for the basedcolluvial on detaileddeposits mapping based on of detailed the primary mapping deposits of the and primary erosion deposits through timeand erosionis of high through importance. time is Itof includeshigh importance. the study It of in thecludes paleogeography the study of the of eroded paleogeography areas (plateaux of eroded and areasbasins) (plateaux in Eastern and Africa basins) where in Eastern ruby occurrences Africa wher aree known,ruby occurrences and the geology are kn ofown, mineralized and the geology structures of mineralizedin metamorphic structures belts. in metamorphic belts.

Figure 61. 61. New Newtypological typological classification classification for ruby for occurrences ruby occurrences and deposits and worldwide. deposits worldwide.M-UMR = metamorphosedM-UMR = metamorphosed mafic-ultramafic mafic-ultramafic rocks. rocks.

The extent ofof erosionerosion throughthrough time time can can be be assessed assessed by by dating dating the the colluvial colluvial or or alluvial alluvial material material by 10 bymeasuring measuringBe 10 (cosmogenicBe (cosmogenic isotope) isotope) in recent in recent quartz-rich quartz sediments-rich sediments or in pebbles or in pebbles of quartz of (alluvial quartz (alluvialor not), which or not), allows which the allows quantification the quantification of mean denudationof mean denudation rates, in alluvialrates, in terraces alluvial and terraces abrasion, and abrasion,across a watershed across a watershed or an ancient or basin.an ancient Cosmogenic basin. Cosmogenic isotopes are producedisotopes are on produced the Earth’s on surface the Earth’s by the 10 surfaceinteraction by ofthe cosmic interaction rays and of theircosmic products rays and with their matter products ( Be, T1with/2 1.5 matter Ma). ( Such10Be, isotopesT1/2 1.5 accumulateMa). Such progressivelyisotopes accumulate in rocks progressively near the surface, in rocks depending near the surface, on altitude depending and erosion on altitude rates. and The erosion cosmogenic rates. Theisotopic cosmogenic concentration isotopic of theconcentration sediment provides of the sediment access to provides the average access erosion to the rate average of the watershed erosion rate at itsof thesource. watershed Fission trackat its studiessource. on Fission zircon track (and apatite)studies mayon zircon be used (and in sediment apatite) may provenance be used determination in sediment provenanceand in terms determination of the time of coolingand in belowterms of 300 the◦C time in the of zircon-bearing cooling below protolith 300 °C in before the zircon-bearing erosion. protolithThe before new and erosion. major discoveries of ruby and sapphire placers in Eastern Madagascar at Didy, Zahamena,The new and and Bemainty, major discoveries located in tropical of ruby forests, and sapphire are economic. placers They in Eastern were found Madagascar by local peopleat Didy, in Zahamena,areas, which and are Bemainty, crosscut bylocated numerous in tropical NE-SW forests, faults are separating economic. small They alluvialwere found and by diluvial local people basins. Thesein areas, zones which are are diffi crosscutcult to accessby numerous and are NE-SW protected faults forest separating areas; therefore small alluvial prospecting and diluvial has not basins. been Thesedone butzones the are combination difficult to ofaccess photosatellite and are protecte imagesd with forest geological areas; therefore maps, lookingprospecting for metamorphic has not been doneM-UMR but formations,the combination will help of photosatellite in the discovery images of new with placers. geological maps, looking for metamorphic M-UMROther formations, ruby-bearing will Neoproterozoichelp in the discovery metamorphic of new placers. M-UMR in East Africa have been productive and someOther of ruby-bearing them still produce Neoproterozoic a few rubies metamorphic as in Tanzania M-UMR (Winza, in East Longido), Africa Kenyahave been (Mangare productive area, andKitui), some Malawi of them (Chimwadzulu), still produce a and few Southern rubies as Madagascar in Tanzania (Vohibory(Winza, Longido), region). InKenya 2005 (Mangare these countries area, Kitui),assumed Malawi most (Chimwadzulu), of the worldwide and production. Southern TheseMadagascar primary (Vohibory deposits region). in M-UMR In 2005 can these be found countries in all assumedmetamorphosed most of volcano-sedimentary the worldwide production. sequences These from primary the Archean deposits to the in Cenozoic.M-UMR can The be discovery found in and all productionmetamorphosed of ruby volcano-sedimentary at the Aappaluttoq depositsequences (south from of the Nuuk; Archean Fiskenæsset to the district)Cenozoic. and The the discovery presence andof new production occurrences of ruby north at of the Nuuk Aappaluttoq (Kangerdluarssuk) deposit (south is an excellentof Nuuk; exampleFiskenæsset that district) must encourage and the presencea better understanding of new occurrences of their north formation of Nuuk and (Kange their prospectingrdluarssuk) asis an a function excellent of example the latitude that ofmust the encouragedifferent countries a better understanding (tropical versus of glacial their formation and temperate and their climates). prospecting as a function of the latitude of the different countries (tropical versus glacial and temperate climates). The second important type of ruby deposit includes those located in marble (Sub-Type IIA2) in Central and Southeastern Asia. These Cenozoic deposits are always one of the main worldwide sources of high quality ruby with intense color and high transparency, such as the famous Mogok pigeon blood rubies. The mining district of Mogok is rich in different types of gems including red spinel and ruby in marble. Genetic models are proposed for their genesis. They formed during the metamorphism, in the amphibolite facies, of carbonates interbedded with mudstone and evaporitic (salts and sulfates) intercalations. Lithological control of the mineralization is a function of the paleogeography of the carbonate platform environment before metamorphism. The role of evaporites

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The second important type of ruby deposit includes those located in marble (Sub-Type IIA2) in Central and Southeastern Asia. These Cenozoic deposits are always one of the main worldwide sources of high quality ruby with intense color and high transparency, such as the famous Mogok pigeon blood rubies. The mining district of Mogok is rich in different types of gems including red spinel and ruby in marble. Genetic models are proposed for their genesis. They formed during the metamorphism, in the amphibolite facies, of carbonates interbedded with mudstone and evaporitic (salts and sulfates) intercalations. Lithological control of the mineralization is a function of the paleogeography of the carbonate platform environment before metamorphism. The role of evaporites is proposed as key for the genesis of ruby in marble but Al-enriched sediments such as metapelite or meta-bauxite intercalated in the marbles are in some cases also possible protolith for the ruby-parent rocks (Revelstoke and Snezhnoe deposits). The Mogok mining district remains a suitable site for developing further geological research considering the complexity of the relationships between the metamorphic rocks and granitoid intrusions. The formation of ruby-bearing skarns is possible, given the recrystallization of some carbonates, with crystals up to 10 cm across, present in the Dattaw mine. Multi-stage mineralization and telescoped episodes of thermal anomalies related to regional or contact metamorphism are highly probable. Future efforts will move towards developing more effective exploration guidelines that consider new geological factors responsible for the formation of ruby at Mogok and the timing of formation of these multi-stage gem deposits. The timing between deformation, metamorphism, and ruby formation in these marble deposits is of importance, as shown in some deposits from Tanzania that are associated with folding, saddle-reef structure, and fluid circulation in zones of higher permeability. This scheme is applicable for all deposits, whatever their ages, but must be prospected in the Neoproterozoic Metamorphic Belt. The classification proposed for ruby occurrences and deposits is a necessary step to build a new scheme for understanding ruby-mineralization from the field to the laboratory. The determination of the different sub-types based on the host rock of ruby and the knowledge on the genesis of most of the deposits will help us to understand the mineral associations and the chemistry of rubies. The combination of all these data linked to a specific type of deposit will serve as a basis for the determination of the geographical origin of rubies. The diversification of known ruby sources in the world during these last two decades has increased considerably the challenge of gem ruby geographic origin determination [11–15]. The gemmological laboratories amassed rubies with samples obtained by professional field gemmologists [9,21,37,90,200]. The precise gem localization combined with accurate chemical analysis obtained by LA-ICP-MS calibrated by natural standards [92], enriched the collections of standard stones for different types of ruby deposits worldwide. The combination of inclusions and trace element chemistry makes the geographic origin determination more reliable currently, although it can remain inconclusive [11]. The case of ruby and sapphire in placers from basaltic environments is illustrative and the geographic origin is always matter of debate; the geologic origin is often confronted to different possible possibilities, i.e., metamorphic sensu stricto, metamorphic–metasomatic, and magmatic for gem material [11,268]. The V versus Fe diagram of ruby from several economic deposit (see Figure 60) confirms clearly the separation of the two main types of primary ruby deposits worldwide, i.e., those hosted in M-UMR (Types IIA1 and IIB1) and in marble (IIB2), which are typically metamorphic. The origin of ruby from secondary deposits, i.e., placers in the alkali basalt environment (Type IIIA) is debatable because melt inclusions are present in some rubies from Thailand, Cambodia, and Montana. A magmatic origin is probable but the V versus Fe diagram shows that their chemical domain overlaps that of rubies from metamorphic origin. The discussion presented here provides a basis for further investigations into the origin of these rubies keeping in mind that they were incorporated by ascending basaltic magmas as xenocrysts from possible different geological sources. Rubies are classified by most of the gemmological laboratories into two groups based on their trace element chemistry [11]: (i) low (below 200 pm) and high iron (above 400 ppm) rubies. The marble-hosted metamorphic rubies (Type IIA2) have a very low Fe content when compared to all the other types Minerals 2020, 10, 597 70 of 83

of rubies except those from the former John Saul mine (see Figure 19, e.g., plumasite of Type IIB1). Nevertheless rubies in marble deposits have similar inclusion characteristics and their chemical domain overlap as well as their oxygen isotopic values. This remark opens the debate about the reality of determining the geographic origin of a gem when disconnected from its geological environment. Some stones and sometimes of top-quality are mingled with other ones and finally whenever the ruby is Fe-poor, the geographic determination is inconclusive. Myanmar stones are always highly prized on the market for their color and transparency. The Mogok and Mong Hsu rubies are identified based on their internal features and trace element profiles [11]. High-iron rubies (Thailand, Cambodia, Madagascar, Mozambique, Kenya, and Greenland) are usually basically identified by their inclusions and trace element profiles (V, Mg, and Fe but not Cr, see Figure 20;[11,82,83]). The main question of geographic attribution remains for rubies between these two ranges of Fe contents. The new geological classification of ruby based on the geological environment and the gem host-rock proposed three main types and eight sub-types of deposits. The classification system reflects the interplay between tectonic with magmatic and metamorphic events during the history of the Earth, and through continental plate tectonics within Wilson cycles, over circa 150 millions of years. The final point concerns the basic erosion of ruby-bearing rocks at the surface, and the transport and sedimentation of the detrital material in basins. The different sub-types for primary deposits are based on the gem host-rock of ruby, which is different from place to place in the crust, the final petrographic product depending on the nature of the fluids, the timing of their circulation and the intensity of fluid–rock interaction. Desilication and/or metasomatism phenomena are effective processes able to extract not only the chromophores Cr, V, and Fe but also Al from M-UMR for the formation of ruby. The different sub-types of deposits will help either the geologist to understand the genesis of new deposits and to define zones of interest for exploration or the gemmologist for geographic origin determination. Fingerprinting of ruby used by gemmologists is a tool derived from geological and mineralogical investigations: (i) Trace elements (V, Fe, Ga, and Mg) are used as previously reported. Ti is also an interesting element that can be used for determining the geological history of the gem-bearing sample as shown by [269] for corundum. The combination of different ratio such as Mg/Mg+Fe versus Cr/Cr+V will permit to trace the source of these elements, i.e., ultramafic versus mafic rocks as done for emerald by [270]; (ii) the combination of Fe, Ti, Ga, Cr, V, and Mg with oxygen isotope ratios of ruby provide a powerful tool for the determination of geologic origin [40,82,87,93,144,151,152]. The oxygen isotopes separate clearly three types of ruby deposits: (a) marble, (b) the John Saul mine type, and (c) metamorphosed M-UMR. The analysis of oxygen isotopes by SIMS is semi-destructive as is LA-ICP-MS for trace elements analysis but can be applied on cut and polished rubies; and (iii) inclusions are helpful in origin determination but similar geologic environments can produce gems of similar inclusion suites [269]. Determining the geological origin of corundum mingled in huge metamorphic placers remains a very difficult task, as for the Ilakaka deposit in Madagascar. The objective geological criteria synthesized in the present review through scientific works and experience open a new framework for mineral exploration guidelines. The final conclusion is that the main economic ruby deposits are the result of continental plate tectonics. Ruby deposits are found where continental plate collision formed mountain ranges, as was the case in the Neoproterozoic with the Metamorphic Mozambican Belt, or with the Himalayas. Continental collision increases the thickness of the continental lithosphere, which is accompanied by an increase of pressure and temperature producing a zone of higher grade metamorphism (amphibolite to granulite facies), and sometimes melting rocks to form felsic magmas. Extensive deformation at high temperature with kilometer-scaled migration of material and fluids along shear zones (e.g., in the case either of the Red River shear zone in Southern China and Northern Vietnam or the Mogok Metamorphic Belt, where ruby is found in placers defining the trace of shear zones in Northern Vietnam) and faults results in the remobilization and precipitation of elements, and tectonic melange zones. These processes are heterogeneous, and proceed by dissolution and precipitation, as well as by fusion and crystallization, Minerals 2020, 10, 597 71 of 83 where elements (such as chromium) are unusually concentrated in the crust. The return to isostatic equilibrium (30 km thick for the continental crust) and erosion processes through time brings to the surface the metamorphic roots (belts) of these ancient mountain chains. This is currently the case with the Mozambican metamorphic belt where high-temperature gems such as ruby, spinel, tsavorite, and other garnets are found. The main production of gem ruby is provided by this Pan-African metamorphic belt. All ruby deposits are of metamorphic origin in the broad sense. These mineral deposits are associated either with regional metamorphism sensu stricto or through fluid–rock interactions accompanied by metasomatism, contemporary with, or slightly later than, metamorphism. Their dismantling by current erosion creates important reserves and the exploration of continental placers in tropical landscapes where regoliths have been present for several thousand years should be studied with new approaches.

Author Contributions: All the authors contributed with their field and laboratory work and experience, and the works of worldwide researchers for over one century with the first detailed description, at the beginning of the 20th century, of the Malagasy deposits by A. Lacroix (MNHN Paris), to the realization of this description of the state-of-the-art of the geology and genesis of ruby deposits worldwide. G.G., L.A.G. and V.P. collected the studied samples in the field as well as realized photographs on specific geological and gemmological cases. G.G. conducted geological mapping in several areas worldwide and G.G., L.A.G., I.P. and V.P. obtained and evaluated part of the mineralogical and geochemical data. I.P. conducted cristallography on corundum crytals as well as the study on trapiche rubies from differents deposits. A.E.F. and G.G. obtained and evaluated the oxygen isotope data. V.P. obtained and evaluated part of the gemmological data. Investigations in the different specific domains were realized by the authors in their own laboratory. G.G. realized some software for corundum chemical analysis as well as the original draft preparation and the supervision of the paper project with approval of the co-authors. The writing was realized by all the autors in their area of competence with the supervision by G.G and language corrections by A.E.F. Review and editing was done by G.G. All authors have read and agreed to the published version of the manuscript. Funding: The present paper was realized without any direct funding. The work is based on the knowledge and funding obtained in the course of more than 30 years work on the geology of gems. Acknowledgments: The authors want to thank the Editor of Minerals and Guest Editors, Frederick Lin Sutherland and Khin Zaw, for invitation to contribute in a special issue on “Mineralogy and Geochemistry of Ruby”, and for their valuable suggestions. We are also thankful to four anonymous reviewers for improving the text by pertinent and constructive comments on the article. The authors want also to thank D. Schwarz, E. Le Goff, C. Simonet for their discussion and collaborations on the scientific topic. G.G. wants to thank the Institut de Recherche pour le Développement (IRD, France) and the French CNRS and the Unversities of Lorraine and Paul Sabatier (CRPG/CNRS-Université de Lorraine and GET/IRD-Université Paul Sabatier) for academic and financial support for research on the geology of gems over the past 30 years. Conflicts of Interest: The authors declare no conflict of interest.

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