ALBERT

Description: 〈span〉〈div〉Abstract〈/div〉The Mong Hsu ruby deposit in Myanmar is a marble-type deposit. It is well-known for attractive crystals with dark violet/blue to black cores and ruby rims, as well as for trapiche rubies. These exceptional rubies have a texture composed of six growth sectors separated by six dendrites running from a central core to the six corners.Fluid inclusions (FI) have been studied in both types of ruby, 〈span〉i.e〈/span〉., trapiche and non-trapiche crystals. Microthermometry combined with Raman spectrometry investigations of primary and secondary FI proved the existence of CO〈sub〉2〈/sub〉-H〈sub〉2〈/sub〉S-COS-S〈sub〉8〈/sub〉-AlO(OH)-bearing fluids with diaspore and native sulfur daughter minerals. The carbonic fluid belongs to the CO〈sub〉2〈/sub〉–H〈sub〉2〈/sub〉S system with CO〈sub〉2〈/sub〉 as a dominant component. Minor COS (1–2 mol.%) and diaspore indicate that H〈sub〉2〈/sub〉O (∼1 mol.%) was present in the paleofluid. The amount of H〈sub〉2〈/sub〉S in FI from trapiche ruby is between 6.9 and 7.4 mol.% for the dendrites and from 7.2 to 15.7 mol.% for the growth sectors. The H〈sub〉2〈/sub〉S content is approximately 10 mol.% in FI from non-trapiche rubies.The presence of dawsonite and previous crushing data carried out on well-cleaned crystals of non-trapiche ruby indicate that an ionic salt melt was present at the time of ruby formation. The non-detection of this melt in trapiche ruby is attributed to the small size of the remnants of molten salts, making it difficult to observe them under the microscope. Following this hypothesis, the FI assemblages in rubies would correspond to the trapping of two immiscible fluids, 〈span〉i.e〈/span〉., a carbonic phase in the CO〈sub〉2〈/sub〉–H〈sub〉2〈/sub〉S–COS–S〈sub〉8〈/sub〉–AlO(OH)-system and molten salts. Such fluid phases are considered to be the product of metamorphism of evaporites during devolatilization of carbonates and thermal-sulfate reduction.The formation of trapiche ruby is discussed in terms of possible variations of the composition of the fluid, temperature, and pressure conditions. The variation in density of the carbonic fluids from the dendrites (0.70 〈 ds 〈 0.78) to the growth sectors (0.56 〈 ds 〈 0.68) records variation of fluid pressure in the metamorphic system. This variation led to episodically local fluid overpressure and hydraulic fracturing in the marble. During such episodes, changes in driving-force conditions allowed for the formation of the trapiche texture in ruby: the development of dendrites and growth sectors occurred under high and low driving-force conditions, respectively. Non-trapiche ruby in veinlets formed concurrently under thermodynamic conditions similar to those registered for the growth sectors in trapiche ruby.〈/span〉

Description: The Namalulu tsavorite (vanadian grossular up to 0.80 wt.% V 2 O 3 ) deposit in northeastern Tanzania occurs in metasomatized graphitic gneiss overlain by a dolomitic marble unit. These rocks belong to the Neoproterozoic metamorphic Mozambique Belt. The graphitic gneiss is composed of quartz, V-bearing kyanite (up to 0.4 wt.% V 2 O 3 ), V-bearing muscovite (up to 1.0 wt.% V 2 O 3 ), V-bearing rutile (up to 1.9 wt.% V 2 O 3 ), and graphite. This rock is affected by calcic metasomatism precipitating calcite and mobilizing the elements for tsavorite crystallization in quartz-calcite veins. The dolomitic marble unit hosts an anhydrite-gypsum-dolomite lens (lens I) and a calcite-scapolite-diopside-sulfides-graphite lens (lens II). This last unit is characterized by the presence of F-bearing minerals (tremolite, phlogopite, tainiolite, titanite), with up to 9.4 wt.% F for tainiolite, and Ba-bearing minerals (feldspar, phlogopite), with up to 7.5 wt.% BaO for feldspar. Lithium (up to 2.0 wt.% Li 2 O) and boron (up to 110 ppm) are also present in tainiolite, as well as in F-bearing tremolite. This is the first description of metamorphic tainiolite. The protoliths of the Namalulu rocks correspond to organic-rich black shales, pure and magnesian carbonates, and evaporites. They were presumably deposited in a marine coastal sabkha located at the eastern margin of the Congo-Kalahari cratons bounded by the Mozambique Ocean during early Neoproterozoic. Tsavorite formed in the Ca-metasomatized gneiss during amphibolite facies retrograde metamorphism at 5.6–6.7 kbar and 630 ± 30 °C.

Description: Emerald at the Lened occurrence in the western Northwest Territories is hosted by quartz veins cutting skarn near the Lened granite pluton and older Selwyn Basin strata. Euhedral beryl crystals (〈0.5 x 5 cm) are present in approximately half of the 26 outcropping veins. Most of the crystals are opaque to translucent and colorless to yellowish and grassy green. Less than 5% of the beryl is transparent, bluish green, and can be considered pale emerald. Using field relationships, Ar-Ar dating, whole-rock geochemistry, stable isotopes (O, H, C, and B), and mineral chemistry, the sources of the emerald-forming fluids and chromophores have been assessed; the results clearly show that the ca . 100 Ma (Ar-Ar muscovite) Lened emerald occurrence is a Type I (igneous) skarn-hosted emerald deposit related to the proximal ca . 100 Ma (Ar-Ar biotite) Lened pluton. Beryllium and other incompatible elements ( i.e. , W, Sn, Li, B, and F) in the emerald, vein minerals, and surrounding skarn were derived during the terminal stages of crystallization of the proximal Lened pluton. Decarbonation during pyroxene-garnet skarn formation in the host carbonate rocks probably caused local overpressuring and fracturing that allowed ingress of magmatic-derived fluids and formation of quartz-calcite-beryl-scheelite-tourmaline-pyrite veins. Channel-water D values in emerald are depleted and range between –87.4 and –62, similar to other granite-related emerald occurrences. The calculated 18 O H2O for the vein fluid (~10, vein quartz, 350 °C) is compatible with a peraluminous granitic fluid source. The calculated 18 O H2O of granite-derived fluids (~12; magmatic quartz, 600–700 °C) is slightly isotopically heavier than the vein fluid, which can be explained by fractionation during vein crystallization or mixing with 18 O-depleted meteoric fluid. The 11 B values of accessory dravite in the emerald veins averages –4.9 ± 0.3 (1, n = 10), which is compatible with a magmatic source, and the Al-Fe-Mg composition is that of tourmaline formed in sedimentary environments, with Mg likely derived from metasomatism of local marine carbonates. The vein fluid was largely igneous in origin, but the dominant emerald chromophore V (emerald vein = avg. 1560 ppm V versus 75 ppm Cr) was mobilized by metasomatism of V-rich sedimentary rocks (avg. 2000 ppm V) that underlie the emerald occurrence. Lened is a unique igneous skarn-hosted emerald occurrence that contributes to the understanding of emerald deposits and emerald exploration criteria in Canada and globally.

Description: Gem-quality corundum (sapphire) occurs in scapolite-rich calc-silicate rock hosted in marble of the Lake Harbour Group near Kimmirut, southern Baffin Island. A deposit of blue and colorless gem corundum (Beluga occurrence) is compared to a similar calc-silicate pod generally lacking corundum but containing nepheline (Bowhead occurrence) and located 170 m to the SSW. Corundum formation was made possible by three equally important sequential metamorphic reactions: (1) formation of nepheline, diopside, and K-feldspar (inferred) at granulite facies peak metamorphic conditions; (2) partial retrograde replacement of the peak assemblage by phlogopite, oligoclase, calcite, and scapolite (Me 50 –Me 67 ) as a result of CO 2 -, H 2 O-, Cl-, F-bearing fluid influx at 1782.5 ± 3.7 Ma ( P-T 〈 720 °C, 6.2 kbar); and (3) retrograde breakdown of scapolite + nepheline (with CO 2 - and H 2 O-bearing fluid) to form albite, muscovite, corundum, and calcite. Late, low-temperature zeolite mineralization is common in corundum-bearing zones. Based on thermodynamic models, the corundum-forming reaction only occurs in a 〈100 °C window with an upper limit determined by scapolite-nepheline stability, and a lower limit determined by the formation of Al-silicate rather than corundum. The protolith is inferred to be dolomitic argillaceous marl with no evidence to suggest the initial presence of evaporites. The enrichment of trace metals V and Cr, and the depletion of Co, Ni, and Mn, suggest reducing diagenetic conditions in the initial sediment. Beluga calc-silicate rock is strongly depleted in REE (Total REE ~ 18 ppm). Oxy-dravite 11 B (+3.9 ± 0.7) is consistent with a marine boron source. The oxygen isotope composition of corundum ( 18 O VSMOW = 16.4 ± 0.1) is comparable to that of corundum in marble or desilicated pegmatite associated with marble. Phlogopite and muscovite 40 Ar/ 39 Ar ages and calculated closure temperatures (considered estimates) are ca . 1640 Ma ( T c = 455 to 515 °C) and 1510 Ma ( T c = 410 to 425 °C), respectively. In the Lake Harbour Group, the most prospective areas for gem corundum exploration are expected to be contiguous to the thrust fault separating the Lake Harbour Group and Narsajuaq terranes, where the retrograde, amphibolite facies overprint of the granulite peak assemblages was most pervasive.

Description: The oxygen isotopic composition of pink to red and other colored spinels hosted by marbles worldwide have been combined with trace elements investigated by electron microprobe analysis (EMPA). For the first time, this database enables the characterization of the geographic origin of the main productive and historical sources of gem spinel. Three consistent sets of 18 O values were found for primary and placer deposits: (1) between 5.6 and 8.6 for the historical Kuh-i-Lal spinels from Tajikistan; (2) from 12.1 to 18.5 for spinels from An Phu and Cong Troi in Vietnam, and Namya from Myanmar; and (3) between 19.0 and 24.2 for spinels from Paigutan in Nepal, Kiswila and Ipanko in Tanzania, Pamreso in Kenya, Mogok in Myanmar, and An Phu in Vietnam. The variation in the 18 O values for most of the deposits can be related to the buffering of the metamorphic fluids by the oxygen isotope composition of the impure marbles. The 18 O values of Kuh-i-Lal spinels result from a metasomatic skarn process between granitic intrusives and marble. Trace elements, such as Fe-Zn-Cr-V-Ti, presented in various new chemical discrimination diagrams allow for the differentiation of all the pink to red spinels. The slight overlaps observed for some chemical domains can be resolved by combination with the O-isotope composition of spinel. The present study shows that it is possible to discriminate gem spinels hosted by marbles from different geographic regions of the world and provides new insights into the traceability of historical spinels such as those from Kuh-i-Lal.

Description: The genetic model previously proposed for tsavorite- (and tanzanite-) bearing mineralization hosted in the Neoproterozoic Metamorphic Mozambique Belt (stretching from Kenya through Tanzania to Madagascar) is refined on the basis of new Sm-Nd age determinations and detailed Sr-O-S isotope and fluid-inclusion studies. The deposits are hosted within meta-sedimentary series composed of quartzites, graphitic gneisses, calc-silicate rocks intercalated with meta-evaporites, and marbles. Tsavorite occurs either in nodules (also called "boudins") oriented parallel to the metamorphic foliation in all of the deposits in the metamorphic belt or in quartz veins and lenses located at the hinges of anticlinal folds (Lelatema fold belt and Ruangwa deposits, Tanzania). Gem tanzanite occurs in pockets and lenses in the Lelatema fold belt of northern Tanzania. The Sm-Nd isotopic data for tsavorites and tanzanites hosted in quartz veins and lenses from Merelani demonstrate that they formed at 600 Ma, during the retrograde metamorphic episode associated with the East African Orogeny. The tsavorites hosted in nodules do not provide reliable ages: their sedimentary protoliths had heterogeneous compositions and their Sm-Nd system was not completely rehomogenized, even at the local scale, by the fluid-absent metamorphic recrystallization. The initial 87 Sr/ 86 Sr isotopic ratios of calcite from marble and tanzanites from Merelani fit with the strontium isotopic composition of Neoproterozoic marine carbonates. Seawater sediment deposition in the Mozambique Ocean took place around 720 Ma. The quartz-zoisite O-isotopic thermometer indicates a temperature of formation for zoisite between 385 and 448 °C. The sulfur isotopic composition of pyrite (between –7.8 and –1.3 V-CDT) associated with tsavorite in the Lelatema fold belt deposits suggests the contribution of reduced marine sulfate. The sulfur in pyrite in the marbles was likely derived from bacterial sulfate reduction which produced H 2 S. Fluid inclusion data from tsavorite and tanzanite samples from the Merelani mine indicate the presence of a dominant H 2 S-S 8 ±(CH 4 )±(N 2 )±(H 2 O)-bearing fluid. In the deposits in Kenya and Madagascar, the replacement of sulfate by tsavorite in the nodules and the boron isotopic composition of tourmaline associated with tsavorite are strong arguments in favor of the participation of evaporites in garnet formation.

Description: Colombian euclase is rare and associated with emerald in medium-temperature hydrothermal veins hosted by Lower Cretaceous black shales (BS). The original sources of euclase production were the mining districts of Gachalá and Chivor in the eastern emerald belt, but in 2016 euclases were also found at the La Marina mine in the western emerald belt. The present study is centered on a chemical and mineralogical examination of zoned Colombian euclase sold on the gem market as "trapiche". Its texture is characterized by growth bands and sectors distinguished by the presence of numerous inclusions (mainly pyrite, carbonates, and organic matter) which represent around 0.2% of the total volume of the crystals. X-ray computed tomography showed that the largest inclusions are randomly located, whereas the small inclusions are concentrated in the center of the crystals, along the crystallographic b axis, between neighboring growth sectors and between growth bands in each sector. The texture cannot be defined as "trapiche", like that of Colombian emeralds, because there is no matrix material from the surrounding BS trapped between the growth sectors and accumulated as dendrites. Three-phase fluid inclusions (FI) containing halite, liquid, and vapor phases are also observed in the euclase, and their volume is identical to that of the inclusions in emerald. Chromium and vanadium are the main chromophores, and the highest concentrations (1240 and 400 ppm, respectively) were found in deep blue-colored zones. Surprisingly, the euclase crystals have high Ge contents, between 230 and 530 ppm. The Rare Earth Element (REE) patterns of euclase are inherited from the enclosed BS or albitized and carbonatized BS. Euclase has the same REE pattern as emerald from the Gachalá mines with an Eu anomaly (Eu/Eu* ~ 0.40) and a depletion in Heavy Rare Earth Elements (HREE). The present study allows for the reconstruction of the formation conditions of "trapiche" euclase and discussion about its probable geographic origin, i.e ., the eastern emerald belt.

Description: Gem-quality (cabochon) ruby-bearing occurrences (here termed PAR-1 and PAR-5) located near Paranesti, north eastern Greece have been systematically studied for the first time in this paper. Tectonically, the occurrences are located within the Nestos Shear Zone (NSZ). The NSZ separates two distinct geological units. The Rhodope Terrane is a heterogeneous unit of gneisses, mafic, ultramafic, and meta-sedimentary rocks in the hanging wall. The footwall Pangaion-Pirin Complex consists of marbles and acid gneisses of a Mesozoic carbonate platform on pre-Mesozoic continental basement. In this paper, a range of petrographic and geochemical techniques were used to determine (1) any similarities and differences to other mafic-ultramafic hosted ruby deposits worldwide; (2) distinctive geochemical fingerprints for Paranesti; and (3) the likely P - T conditions of formation. Detailed petrographic and whole-rock analyses utilizing ICP-MS, XRF, and XRD have found the Paranesti corundum to be of a mafic/ultramafic protolith with approximately 40 wt.% SiO 2 , 16 wt.% Mg, 11000 ppm Cr, and 440 ppm Ni. EMPA major element analysis determined the mineral inclusions within the corundum grains to be picotite and hercynite spinels. Pargasite is the dominant amphibole within the corundum-bearing amphibole schist host. The surrounding non-corundum bearing chlorite schist mainly comprises clinochlore. Petrographic examination of the mineral assemblages within the corundum-bearing schists revealed strong fracturing and alignment (parallel to the main regional foliation) of the corundum grains and margarite reaction rims around the corundum. The surrounding non-corundum amphibolites also contain anorthite, along with relict sillimanite, kyanite, and chlorite/muscovite/epidote overprinting. Detailed LA-ICP-MS trace element analysis of the color range of corundum from the two occurrences showed the corundum to be mainly of metamorphic origin, though pale rubies from PAR-5 suggest some metasomatic influence. The corundum displays distinctive geochemical locality signatures, with a combination of high Cr (average 2300 ppm with 15% sample points on core positions 〉5000 ppm and maximum 8600 ppm); high Si (average 1400 ppm with 40% over 1500 ppm and maximum 2500 ppm), low Mg (average 30 ppm), and very low V, Ti, and Ga. Based on the literature for similar occurrences, and the mineral assemblages observed at Paranesti, the estimated P - T conditions of corundum formation are 〈7 kbar and 〈750 °C, similar to the mafic African amphibolite-hosted rubies. This study has found the Paranesti occurrences to be most similar to the Winza, Tanzania ruby deposit, whilst there are some similarities to other high-Cr ruby deposits, primarily the Fiskenæsset, Greenland and metamorphic amphibolitic schist hosted African deposits. The Paranesti corundum most likely formed during regional amphibolite facies metamorphism which created the Nestos Shear Zone, along with a lesser influence (primarily observed in the PAR-5 occurrence) of more localized metasomatism. Subsequent multiple greenschist facies retrogression of the occurrences resulted in the current-day host amphibole-chlorite schist assemblages.

Description: The Ianapera emerald deposit is hosted in the Neoproterozoic Vohibory domain of southern Madagascar, within a tightly folded metamorphic sequence of mafic and ultramafic rocks (M-UMR), gneiss, and marble, a few kilometers north of the major tectonic break known as the Ianapera-Ampanihy Suture. Late-collisional metaluminous to peraluminous felsic intrusions outcrop in the area. Emerald occurs mostly within metasomatic phlogopitite and quartz-tourmaline veins, developed within lenses of M-UMR. Recent work recognized granitic pegmatites as the source of the emerald-forming fluids, leading to the classification of proximal and distal mineralization styles, based on whether or not pegmatites are associated with the phlogopitite, respectively ( Andrianjakavah et al . 2009 ). Considerations of the chemistry of tourmaline and scapolite associated with emerald and beryl, data from fluid and solid inclusions in these minerals, thermodynamic calculations, and mineral thermobarometric data confirm that Be originated from the pegmatites, transported in a moderately saline aqueous fluid that exsolved at about 600 to 680 °C and 4–6 kbars. This fluid was enriched in alkali elements, incompatible elements and dissolved sulfur, and produced the proximal mineralization. Migration along fractures caused loss of at least part of the pegmatitic elements and local mixing with a metamorphic CO 2 -rich fluid, leading to the more distal mineralization at somewhat lower temperatures (520 to 650 °C). Oxygen and hydrogen stable isotopes were measured from beryl and emerald; the data corroborate a magmatic-hydrothermal origin for the mineralization and a common source for proximal and distal fluids. The calculated 18 O H2O of 12.5 at 520 °C for the distal emerald and the D H2O range measured from the channel of the emerald samples fit within the oxygen and hydrogen isotopic range of values defined for peraluminous granitic magmatism. The occurrence of a distal style of emerald mineralization such as typified by Ianapera suggests that metamorphic origin models for similar occurrences worldwide should be taken with care and possibly reconsidered in terms of source and origin of fluids and metals.