Trace elements and oxygen isotopes of gem spinels in marble from the Luc Yen - An Phu areas, Yen Bai province, North Vietnam

Vietnam Journal of Earth Sciences, 40(2), 165-177, Doi:10.15625/0866-7187/40/2/12241 165 (VAST) Vietnam Academy of Science and Technology Vietnam Journal of Earth Sciences Trace elements and oxygen isotopes of gem spinels in marble from the Luc Yen - An Phu areas, Yen Bai province, North Vietnam Pham Van Long*1, Gaston Giuliani2, Anthony E. Fallick3, Andrian J. Boyce3, Vincent Pardieu4 1Institute for Gems and Gold Research of VINAGEMS (IGG), Hanoi, Vietnam 2Université de Lorraine, IRD and CRPG UMR 7358 CNRS-UL, BP 20, 15 rue Notre-Dame-des-Pauvres, 54501 Vandœuvre-lès-Nancy, France 3Isotope Geosciences Unit, S.U.E.R.C., Rankine Avenue, East Kilbride,Glasgow G75 0QF, Scotland, United Kingdom 4Field gemmologist, Bangkok, Thailand Received 16 January 2018; Received in revised form 16 March 2018; Accepted 22 March 2018 ABSTRACT Trace elements investigated by electron microprobe analysis (EMPA) have been combined with oxygen isotopic composition of pink, red and other colored spinels (blue, purple, brown, orange, lavender) hosted by marbles and found in placers from Luc Yen and An Phu deposits, Yen Bai province, North Vietnam. The deposits are those from Nuoc Ngap, Cong Troi, Bai Son and different placers from the An Phu area. Trace elements such as Fe-Zn-Cr-V in red and pink gem spinels permit to separate those from Cong Troi and those from the others deposits of the An Phu area. Spinels from Cong Troi have low to extremely low Zn (< 500 ppm) and high Fe contents (3,000 to 16,000 ppm) while those from An Phu area are Zn-rich (up to 11,000 ppm). Iron is the dominant element for the other colored spinels whereas Zn, Cr and V contents are extremely variable. The Bai Son blue spinel is Fe-rich (5,000 to 7,200 ppm) with some V (950 to 1,830 ppm), Cr (270 to 480 ppm), Co (240 to 400 ppm) and Ni (550 to 950 ppm). The O-isotope composition of the whole spinels ranges between 12.1 and 24.2‰ (n = 25). Within each deposit, the range of 18O values for red, pink and colored spinels is usually similar. However, the red and pink spinels from An Phu present two distinct sets of 18O values, respectively between 13.2 to 17.0‰ (n = 7) and 22.5<18O < 24.2 (n = 5). Those from Cong Troi are from 14.8 to 17.7‰ (n = 3) and their range overlaps that of An Phu. The use of O-isotopes is not useful for distinguishing between the deposits, but the low to extremely low Zn content of the Cong Trois spinels is a discriminant. The variation of 18O values (12.1 <18O < 24.2‰) of the whole spinels indicates that the oxygen isotopic compositions of the metamorphic fluids were probably buffered by the local 18O values of the im- pure host marbles. Keywords: marble; gem spinel; trace elements; oxygen isotopes; geographic origin; Luc Yen - An Phu; Vietnam. ©2018 Vietnam Academy of Science and Technology 1. Introduction1 Gem spinels have compositions close to MgAl2O4 sensustricto and the color depends *Corresponding author, Email: long@vinagems.vn on the incorporation in various proportions of bivalent and/or trivalent chromophores (Fe2+, Fe3+, Cr and V) in the octahedral and tetrahe- dral sites of the crystal structure. Spinel offers a wide palette of colours extending from pink Pham Van Long, et al./Vietnam Journal of Earth Sciences 40 (2018) 166 to red, violet to purple tinges, orange brown to yellow, and also cobalt blue (Figure 1). World-famous and economic deposits of Cr- V-bearing red and pink spinels occur in Hima- layan and East African marbles that have un- dergone high-temperature metamorphism (Pardieu & Hughes 2008; Giuliani et al., 2017). Vietnam is the main producer of gem spinels found in primary (marble type) and secondary (placer) deposits. Spinel is exploit- ed in small-scale mines and the most signifi- cant deposits are found in North Vietnam along the Red River Shear Zone (Huong et al., 2012; Pham et al., 2013). Vietnamese spinels have been investigated by different authors in terms of geological setting as for the discov- ery of the Lang Chap deposit in the Tan Hu- ong-Truc Lau area (Malsy et al., 2012), gem- mology (Pham et al., 2014), trace elements chemistry (Pham et al., 2004; Malsy&Klemm, 2010; Kleišmantas&Daukšyte, 2016; Giuliani et al;, 2017) and genesis (Hauzenberger et al., 2001; Chauviré et al., 2015). Figure 1. Spinels on the gem market of Luc Yen. A, B, C- Several gem cut coloured spinels. D- Three of rough sky blue spinels from Bai Son area. Photographs: Pham Van Long The present study describes the use of trace elements and oxygen isotopic ratios (18O/16O) of lattice oxygen in pink and red spinels as well as other coloured varieties, to provide a gem geographic identification. This study differs from the previous paper devoted to the characterization of pink and red spinels hosted by marbles worldwide (Giuliani et al., 2017), when considering the following issues: (i) an update of spinel mining activities and types of deposits in the Luc Yen - An Phu ar- eas; (ii) trace elements combined to oxygen isotopes exclusively to Vietnamese gem spinels; (iii) an in-depth discussion of the data to precise that the O-isotope composition of spinel is not useful in discriminating their ge- ographic origin while trace elements are rele- vant for indicating their mine origin. 2. Geological setting The primary spinel deposits are located in the Himalayan mountain belt which formed during Tertiary collision of the Indian plate northward into the Eurasian plate. They are spatially related to ruby deposits hosted by platform carbonate series metamorphosed to the amphibolite facies (Garnier et al., 2008). The marble units are intercalated generally with biotite-garnet-sillimanite or -kyanite gneisses, and granitoids. Marble paragenesis consists of calcite, dolomite, spinel, forsterite, clinohumite, apatite, margarite, pargasite, chlorite such as sheridanite-clinochlore, ± graphite, ± pyrite, ± pyrrhotite (Garnier et al., 2008). In Vietnam, the primary deposits occur in the Lo Gam tectonic zone in the Thac Ba and An Phu metasedimentary sequences of Cambrian age, comprising marble and overly- ing biotite-garnet-sillimanite gneiss and schist (Figure 2). The Lo Gam zone is limited by two left-lateral faults which belong to the Red River Shear Zone, and is intruded by granit- oids of Triassic age (Garnier et al., 2005). Spinel (Spl) occurs either as crystals dissemi- nated in marble (Figure 3A, C) or impure marbles associated with mostly calcite (Cal) - dolomite (Dol), forsterite (Fo), clinohumite (Chu), pargasite (Figure 3E), phlogopite (Phl) and chlorite (Figure 3B, D). Hauzenberger et al. (2001) obtained a minimum temperature of about 700°C for the formation of spinel based on the mineral assemblage Chu-Cal-Fo-Dol- Spl at water-rich fluid composition. Such temperature conditions are different from those defined for gem ruby (Crn) formation Vietnam Journal of Earth Sciences, 40(2), 165-177 167 which occurred during retrograde metamor- phism (620<T<670°C; Garnier et al., 2008). The main equilibrium reaction between co- rundum and spinel during prograde metamor- phism is the following: Crn + DolSpl + Cal + CO2. This reaction explains why gem co- rundum and spinel are generally not associat- ed in the same outcrop. In addition, in situ U/Pb dating of zircons included in spinel from An Phu (Garnier et al., 2005) indicated a crys- tallization age during the Permian (256 Ma) with a reopening of the U/Pb system in the Early Triassic (232 Ma). These two ages con- trast with the U/Pb ages of 38 Ma found for zircons included in ruby from the same area. This discrepancy signifies that at least two metamorphic episodes occurred in the Indo- china block before the Himalayan orogeny, and most likely some of the spinels are older than the associated rubies. Figure 2. Location of the spinel deposits from the Luc Yen area in northern Vietnam (modified from Garnier 2003) Pham Van Long, et al./Vietnam Journal of Earth Sciences 40 (2018) 168 Figure 3. Spinels in marble. A- Pink spinels (Spl) disseminated in a calcitic (Cal) marble from Cong Troi, Luc Yen area. B- Spinel (Sp) associated with clinohumite (Chu), edenite (Ed) in a calcitic marble (Cal) from An Phu quarries. C- Crystals of red spinel around 1 cm across in a marble from Luc Yen. D- Pinkish spinel associated with phlogopite (Phl) and edenite (Ed) in a marble from Cong Troi. D- Pargasite (Prg) in a saccharoidal marble from An Phu. E- Pale blue spinel from the Luc Yen area. Photographs A-B-D-E: G. Giuliani/IRD; C-F: V. Pardieu/GIA. Mineral abbrevia- tions after Kretz (1983) 3. Spinel mining activities in Luc Yen district Gem spinels, rubies and sapphires were discovered in Luc Yen in 1987 (Pham et al., 2013). Today, the deposits which extend be- tween Luc Yen and An Phu localities are found either in marble or placers (Figure 4). Primary deposits concern the Cong Troi mining district where spinels occur in differ- ent colours (red, pink, brown, violet) and the Bai son mine where spinel is blue. These de- posits are characterized by crystals of spinel widespreading in dolomitic marble with Vietnam Journal of Earth Sciences, 40(2), 165-177 169 sometimes a Ca- and Mg-Si-rich paragenesis such as dolomite, calcite, phlogopite, forster- ite, clinohumite and edenite-pargasite (Figure 3), and sulphides and graphite. Spinel, edenite-pargasite and phlogopite are distribut- ed in bands and strips parallel to the foliation of the marbles. The crystals vary in size from 1x1 cm to 3x3 cm, and present the common octahedral habit and its red to brownish red colour. The biggest crystals are usually trans- lucent to opaque and are used only as miner- alogical collection samples. Figure 4. Views of spinel mining in the Luc Yen and An Phu areas.A- The Cong Troi spinel mining quarry and camp. B- Opening of a spinel-bearing alluvial in a rice field. C- Mining spinels in colluvial soil and sediments. D- Prospecting spinel in a rice field. E- Gem spinel mining in a karst formed in marble. Photographs: V. Pardieu The spinel and pargasite mining activities are focused in the Cong Troi area since 2000s (Figure 4A). In 2009-2010, the mining licenc- es in the region were allocated to a local com- Pham Van Long, et al./Vietnam Journal of Earth Sciences 40 (2018) 170 pany for exploiting white marble intended for export. They started to carry out drilling oper- ations in the area, so the exploitation of gems by local people was suspended up to late 2010. Currently, there are two or three differ- ent groups of miners with a total of twenty people are working at the mine site. The spi- nel mining area is often spread over a large area and covers the whole Cong Troi moun- tain from the base to the top. The spinels in the Cong Troi area present different colours, but the most common ones are brownish red, reddish brown, brown, purple, pink, and pur- plish pink. The size varies from 1×1 mm to 50 × 50 mm (Figure 3A, D). The crystals present the octahedral habit with spinel twin law. Blue spinel is found in the white marble along Bai Son valleys. The blue spinels from placer are generally of gem quality (Figure 1D), however the blue spinel crystals found in marble are often in dull coloured, of low transparency, and not of gem quality. Spinel is associated with phlogopite, clinohumite and sometimes pyrite. The pargasite crystals pre- sent prismatic and elongated shapes. The col- our of the amphibole varies from green, light green to yellow green but the crystals are not transparent. They can form groups of crystals with a size between 0.5 × 2 cm to 1.5 × 5 cm. Placer deposits are mined along the streams or alluvial by local farmers (Figure 4B to D). Spinels are generally of gem-quality and used for gem cutting. They exhibit red, brownish red, pink, purple, sea blue, and sky blue colours (Figure 3F). The red, brownish red and pink spinels are found in An Phu, Minh Tien, and Khoan Thong areas (Figure 2), and pink and purple spinels are found in the Cong Troi area. Sea blue spinel is mined in Co Ngan area located at about 5 km north of An Phu commune and sometimes in Bai Son. Sky blue colour spinel is only found in Bai Son and Lung Thin area in an under- ground development about 5-8 m deep, where a blue spinel of 97 cts was recovered. Mining is carried out along the streams of Cong Troi, Nuoc Ngap (An Phu area) and sometimes in the removing spoil tips of old mining areas such as in Minh Tien, Khoan Thong, Bai Chuoi areas producing light blue spinel. In some places, the local people mined in karst or caves formed in white marbles (Figure 4E; Cong Troi 2, Cong Troi 3, May Thuong and May Ha deposits). 4. Analytical Techniques For this study, 32 crystals of spinels col- lected from primary and secondary deposits of Cong Troi, An Phu commune, Nuoc Ngap and Bai Son (see Tables 1 and 2 for the type of deposit) were used for examination through EMPA (n = 32, and 220 analysis) and oxygen isotopes analyses (n = 25). Spinel samples were collected by the authors from different field work programs. EMPA data of the spinels were obtained using the CAMECA SX100 facility at the University of Lorraine, Nancy, France. The operating conditions were the following: ac- celerating voltage of 15 kV for Mg, Al and Fe, beam current 20 nA, raster length 0.05 µm, collection time 20 s; and accelerating voltage of 25 kV for the other trace elements, beam current 100 nA, raster length 0.05 µm, collection time of 30 s for Ti, Mn and Zn and 60 s for Ca, V, Cr, Co and Ni. Mineral and synthetic standards used were Mg (olivine), Al (corundum), Fe (hematite), Ti and Mn (manganese titanite), V (V-SX9 metal), Cr (chromite), Co (Co metal), Ni (NiO alloy) and Zn (sphalerite). The detection limits for trace elements in mass percentage ratio (ppm) were 37 for Ti, 60 for V, 30 for Cr, 130 for Mn, 90 for Co and Ni, and 180 for Zn. Data reduction was performed with the PAP program (Pouchou and Pichoir, 1991). Oxygen isotope analyses were performed on different coloured spinels at S.U.E.R.C Glasgow (Scotland) using a modification of the laser-fluorination technique (see Giuliani et al., 2005). Data are reported in the conven- tional 18O notation as per mil (‰) relative to Vietnam Journal of Earth Sciences, 40(2), 165-177 171 the Vienna Standard Mean Ocean Water (V- SMOW). 5. Results 5.1. EMPA analysis Representative EMPA data of spinels are reported in Table 1. The highest concentra- tions in trace elements were obtained for Cr, V, Fe and Zn whereas Ti and Mn contents were low to very low and sometimes below the detection limits. Co and Ni were only de- tected in blue spinels. The chemical variations between pink to red spinels (variety 1) with other colours (variety 2) will be examined separately. Variety 1: The pink and red spinels are characterized by the presence of Fe, Cr and V. Zn is not a chromophore but its concentration is very high for some spinels from An Phu (up to 11,000 ppm). The Zn vs. Fe diagram dis- criminates the spinels from Cong Troi from those of An Phu and Nuoc Ngap (Figure 5). The spinels from Cong Troi are Fe-rich and Zn-poor. They show a vertical trend starting from 3,000 and going up to 16,000 ppm of Fe but with very low Zn contents (Zn < 500 ppm). On the contrary, the An Phu spinels present four chemical fields. They are Zn-rich and with Fe values between 4,000 and 11,000 ppm. Figure 5. Zn vs. Fe (in ppm) diagram for pink and red spinels from the mining sites of Cong Troi, An Phu and Nuoc Ngap Table 1. Representative electron microprobe analyses of pink, red and coloured spinels from An Phu, Cong Troi, Bai Son and Nuoc Ngap deposits Deposit Type of Sample Colour MgO Al2O3 FeO TiO2 V2O3 Cr2O3 MnO ZnO CoO NiO Total deposit (weight %) An Phu primary AP4 red 27.64 70.86 0.57 0.00 0.00 0.18 0.00 0.04 0.00 0.00 99.29 placer VIET1 red 27.55 70.25 0.55 0.02 0.37 1.15 0.00 0.80 0.00 0.00 100.69 " VIET2 pale red 27.56 71.10 0.41 0.02 0.52 0.12 0.00 0.48 0.00 0.00 100.21 " VIET3 red 27.92 70.64 0.92 0.00 0.10 0.23 0.01 1.27 0.00 0.00 101.08 " VIET8 pale pink 28.34 70.91 0.48 0.01 0.16 0.17 0.00 0.66 0.00 0.00 100.73 " ANP5 lavender 27.77 71.61 0.66 0.01 0.05 0.05 0.01 0.12 0.00 0.00 100.28 " VIET23 purple 27.39 71.69 0.79 0.00 0.00 0.00 0.01 0.32 0.00 0.00 100.20 to brown placer VIET24 purple 25.97 71.89 3.41 0.00 0.01 0.10 0.02 0.05 0.00 0.00 101.5 to brown placer VIET27 brownish to 27.74 71.46 0.57 0.01 0.01 0.57 0.09 0.28 0.00 0.00 100.7 orange Cong Troi placer VIET9 pink 27.36 72.62 0.97 0.00 0.00 0.01 0.00 0.02 0.00 0.00 100.98 primary CT1 pink 28.54 70.75 0.50 0.01 0.20 0.06 0.01 0.00 0.00 0.00 100.13 " CT4 pink 28.12 70.34 1.41 0.02 0.52 0.10 0.01 0.05 0.00 0.00 100.71 " VIET31 pink 27.70 71.83 0.41 0.02 0.25 0.18 0.00 0.01 0.00 0.00 100.41 " VIET33 pink 27.43 71.32 0.58 0.00 0.04 1.13 0.02 0.06 0.00 0.00 100.58 " VIET1 pale purple 28.97 71.27 0.48 0.00 0.09 0.01 0.01 0.00 0.00 0.00 100.9 placer VIET11 purple 28.33 70.58 0.57 0.01 0.17 0.51 0.00 0.67 0.00 0.00 100.8 " VIET14 purple 28.41 71.55 0.73 0.00 0.06 0.02 0.00 0.00 0.00 0.00 100.8 " VIET15 deep purple 27.03 70.95 1.58 0.00 0.00 0.01 0.01 1.31 0.00 0.00 100.9 " VIET16 pale purple 28.89 72.19 0.39 0.00 0.03 0.01 0.00 0.00 0.00 0.00 101.5 primary VIET32 dark brown 28.10 70.88 1.45 0.02 0.41 0.09 0.01 0.00 0.00 0.00 101.0 Bai Son placer VIET19 pale blue 27.52 72.36 0.75 0.00 0.00 0.01 0.00 0.34 0.00 0.00 101.0 " VIET20 purple 28.10 71.13 0.90 0.01 0.14 0.07 0.00 0.00 0.00 0.00 100.4 " VIET21 blue 27.88 72.95 0.51 0.01 0.14 0.07 0.01 0.02 0.05 0.12 101.8 Nuoc Ngap placer V41a1 pinkish 27.14 71.11 0.91 0.02 0.01 0.01 0.09 0.12 0.00 0.00 99.41 " V41a2 pink 26.56 71.89 0.78 0.02 0.02 0.01 0.10 0.17 0.00 0.00 99.53 Pham Van Long, et al./Vietnam Journal of Earth Sciences 40 (2018) 172 The FeO-Cr2O3 vs. 2ZnO + V2O3 diagram (Figure 6A) shows that the chemical fields of spinels from Cong Troi and Nuoc Ngap plot mainly in the Fe>Cr and V>Zn boxes. Some samples of Cong Troi and An Phu plot in the box of Fe<Cr. The Cong Troi spinels have higher Cr (between 70 and 9,000 ppm) than V (between 70 to 3,670 ppm). The ternary dia- gram Fe-V-Zn (Figure 7A) corroborates these different chemical trends and also discrimi- nates their geographic origin. The ternary dia- gram Cr-V-Zn (Figure 7B) confirms the chemical differences between spinels from Cong Troi, An Phu and Nuoc Ngap. Figure 6. FeO - Cr2O3 vs. 2ZnO + V2O3 (in wt %) diagram for A: pink and red spinels, and B: other coloured spinels from Cong Troi, An Phu, Nuoc Ngap and Bai Son mines Vietnam Journal of Earth Sciences, 40(2), 165-177 173 Figure 7. Chemical ternary diagrams for pink and red spinels from Cong Troi, An Phu and Nuoc Ngap. A: Fe-V-Zn ternary diagram that discriminates amongst the different deposits. B: Cr-V-Zn ternary diagram for each spinel deposit Variety 2: The chemical compositions of coloured spinel reported in Table 1 are pre- sented in Figure 6B. Iron is the dominant ele- ment whereas Zn, Cr and V contents are ex- tremely variable. The Bai Son blue spinel is Fe-rich (5,000 to 7,200 ppm) with some V (950 to 1,830 ppm), Cr (270 to 480 ppm), Co (240 to 400 ppm) and Ni (550 to 950 ppm). Purple spinels from An Phu are characterized by Fe > Zn > V > Cr whereas light to deep purple ones from Cong Troi are Fe-rich but with extreme variation in Zn (a few to 10,100 ppm), and with Cr up to 6,350 ppm and V up to 1,300 ppm. Brown to orange spinels from An Phu have Fe > Zn with extremely variable Cr and V contents (Table 2) whereas dark brown spinels from Cong Troi are Zn-free but with concentrations in Fe, V and Cr of respec- tively, 14,500, 2,800 and 600 ppm. 5.2. Oxygen isotope analyses The O-isotope composition of Vietnamese spinels from primary and secondary deposits ranges between 12.1 and 24.2‰ (n= 32; Table 2). Spinels can be classified heuristically into tw odistinct sets of 18O values (Figure 8): (i) The first 18O set, between 12.1 and 18.6‰ (n = 19) includes red, pink and col- oured spinels: (i) the pink and red spinels from the primary and placer deposits of Cong Troi and An Phu have 18O values in the range of respectively, 14.7 to 17.7‰ (n=3) and 13.2 to 17‰ (n=7); (ii) the other coloured spinels from Cong Troi have 18O values be- tween 15.5 and 18.6‰ (n = 3) whereas those of An Phu are in the range 14.2‰ to 14.9‰ (n = 4). The Bai Son blue dark spinel has a rela- tively low O-isotopic composition of 12.1‰. Pham Van Long, et al./Vietnam Journal of Earth Sciences 40 (2018) 174 (ii) Spinels from the second set of 18O value sare characterized by higher values (19 <18O < 24.2‰, n= 13): (i) the pink to red spinels from An Phu (22.5<18O < 24.2, n = 5); (ii) the coloured spinels, either orangey or brown, from An Phu have a 18O range be- tween 21.2 and 24.1‰ (n = 5). The Bai Son light blue and purplish spinels have O-isotopic values in the range 19.4 to 22.7 (n= 2) and a single Cong Troi purple spinel has a value of 22.6‰. 6. Discussions The use of trace elements such as Fe, Zn, Cr and V allowed to characterize the geo- graphic origin i.e. the mine locality or mining district,of the red and pink spinels (variety 1) from Cong Troi, An Phu and Nuoc Ngap (Figure 6A and 7A). The low to extremely low Zn content of the Cong Trois pinels is a discriminant compared to other deposits. On the contrary, the O-isotope composition of spinels is not useful for geographical origin determination because the range of 18O val- ues from Cong Troi (14.7 to 17.7‰, n =3) overlaps the first range of 18O values from An Phu (13.2 to 17.0‰, n=7). As regards spinels of variety 2, they are Fe-rich (Fe > Cr) and present variation in the content of Zn and V. To the contrary of varie- ty 1, the spinels from Cong Troi are enriched in Zn and V (with Zn > V) while those of An Phu are depleted (with V > Zn). The blue spinels from Bai Son have a chemical field that is similar to the purple ones from An Phu or Cong Troi (with V > Cr) but they contain Co and Ni. The spinels pre- sent highly variable O-isotope compositions between 12.7 and 22.7‰. Chauviré et al.(2015) assumed that the cobalt blue colour is due predominantly to Co2+ substituting for Mg2+ in the spinel structure even if iron is more abundant. To the contrary, shades of paler blue in spinel with insignificant Co con- tents are due to electronic transitions in Fe (D'Ippolitoet al., 2015). Huong et al.,2012) have shown that: (i) Co2+ and Cr3+ were locat- ed in the octahedral site of Al3+ in the spinel structure; and (ii) Co2+, Fe2+ and Cr3+ ions contributed to colour variation. Within each deposit, the range of 18O val- ues for red, pink and coloured spinels is simi- lar indicating that O-isotope compositions are independent of the nature of the chemical el- ements incorporated in the spinel structure. The 18O values of spinel from variety 2 over- lap those of variety 1 for An Phu and Cong Troi (Figure 8), except for one purple spinel of Cong Troi which has a 18O value of 22.6‰. Figure 8. Distribution of the 18O values (‰, V- SMOW) of spinels in marble from the Cong Troi, An Phu and Bai Son deposits. Numbers enclosed in paren- theses are the number of analyses Spinels are considered to be of metamor- phic origin (Hauzenberger et al., 2001; 2014; Garnier 2003; Giuliani et al., 2017) but their O- isotope composition is highly variable (12.1 <18O < 24.2‰). Spinel is either disseminated in the marble or associated with other minerals such as clinohumite, phlogopite, pargasite, chlorite (Table 2) but the 18O value of spinel is independent of the colour of the crystal and the nature of its paragenesis. The O-isotope variability is probably related to the initial O- isotopic composition of the host marble. The Vietnam Journal of Earth Sciences, 40(2), 165-177 175 18O values for spinel (and/or ruby)-bearing marbles from the Lo Gam zone range between 18 and 27.1‰ (Garnier, 2003; Garnier et al., 2008). The depletion in oxygen-isotope values for the majority of these marbles cannot be re- lated to batch or Rayleigh decarbonation pro- cesses of marble during metamorphism, be- cause the observed isotopic variations are too large (see the discussion in Valley, 1986). It is more likely that the O-isotope heterogeneity of the impure marbles is linked to their heteroge- neous compositional chemistry and oxygen isotopic ratios acquired during sedimentation and diagenesis of the protolith. Additionally, the study of Yuiet al. (2008) has shown that the O-isotope fractionation 18O(cc-sp) between co- existing calcite and spinel, at Mogok in My- anmar, was 3.1 and 4.0‰ (n = 2 samples). Such O-isotope fractionation between calcite and spinel could explain the observed variation of O-isotopes for these spinels and current study on spinel in marbles worldwide indicates that the O-isotope fractionation is around 3.6‰ (manuscript in preparation). Table 2. Oxygen isotope composition (in ‰, V-SMOW) accompanied by the V/Cr ratio and Zn content of spinels from An Phu, Cong Troi and Bai Son deposits 7. Conclusions The EMPA and oxygen isotope ratio anal- yses of gem spinels from Luc Yen -An Phu areas have shown that: (i) the use of trace el- ements such as Fe-Zn-Cr-V in red and pink gem spinels permits to discriminate the geo- graphic origin of Cong Troi and An Phu de- posits; (ii) on the contrary, the O-isotope composition of spinels is not useful in this re- Deposit Type of Sample Colour Mineral assemblage d18O V/Cr Zn deposit in the marble (V-SMOW) (ppm) (‰) An Phu alluvial ANP1 reddish 14.3 na na " ANP2 reddish to pinkish 16.2 " " " ANP3 pink 14.4 " " " ANP4 pinkish 13.2 " " " ANP5 lavender 14.5 0.97 1080 " VIET1 red 23.8 0.31 6358 " VIET2 pale red 22.5 5.26 3588 " VIET3 red 22.8 0.47 10,488 " VIET5 light red 15.3 - 1390 " VIET7 pale pink 24.2 na na " VIET8 pale pink 22.9 0.96 5543 " VIET23 purple to brown 22.4 0.03 2624 " VIET24 purple to brown 21.3 0.09 362 " VIET27 brownish to orange 24.1 0.02 2162 " VIET28 brownish to orange 21.2 na na " VIET29 dark orange 22.9 " " " VIET30 dark orange 14.8 " " An Phu primary Apa deep purple pargasite, clinohumite 14.9 " " " AP2 pale purple forsterite, edenite, graphite 14.2 " " " AP4 red edenite 16.8 0.03 321 " V41a pale pink pyrite, graphite 17.0 na na Cong Troi placer VIET9 pink 17.7 - 175 " VIET11 purple 22.6 0.31 5632 " VIET14 purple 15.5 5.81 0 " VIET15 purple 18.5 0.10 10,560 " VIET16 pale purple 18.6 2.90 0 primary VIET31 pale pink no associated mineral 14.7 1.2 72 " VIET32 dark brown clinohumite 17.0 5.54 40 " VIET33 red no associated mineral 14.8 0.08 460 Bai Son placer VIET19 pale blue 22.7 - 2624 " VIET20 purple 19.4 1.81 0 " VIET21 blue 12.1 5.28 102 Pham Van Long, et al./Vietnam Journal of Earth Sciences 40 (2018) 176 gard. Indeed, the 18O range of values for spinels from Cong Troi overlaps that of An Phu but the low to extremely low Zn and high Fe contents of spinels from Cong Troi com- pared to An Phu permit to discriminate these two main Vietnamese deposits; (iii) the varia- tion of 18O values (12.1 <18O < 24.2‰) of spinel indicates that the oxygen isotopic com- position of the metamorphic fluids were prob- ably buffered by the local 18O values of the impure host marbles. Future work combining more analysis of trace elements and O-isotopes on spinels from Cong Troi with its considerable quantity of blue spinels, An Phu and other Vietnamese deposits will improve the preliminary chemi- cal and O-isotope database presented in this work. Acknowledgements The authors thank the Institut de Recher- che pour le Développement (UPS-GET/IRD, Toulouse, France) and the University of Lor- raine (UL-CRPG/CNRS, Nancy, France) for the financial support for field and laboratory works. We thank Prof. Lee Groat, Chief- Editor of the journal The Canadian Mineralo- gist, for authorizing us to use the data relative to the Vietnamese spinels, already published in a paper from The Canadian Mineralogist, for writing the present work for the Vietnam Journal of Earth Sciences. This paper is sup- ported by NAFOSTED of Vietnam, code 105.02-2010.11. References Chauviré B., Rondeau B., Fritsch E., Ressigeac Ph., Devidal J.-L., 2015. Blue spinel from the Luc Yen district of Vietnam. Gems & Gemology, 51, 2-17. D'Ippolito V., Andreozzi G.B., Hålenius H., Skogby H., Hametner K., Günther D., 2015. Colour mechanisms in spinel: cobalt and iron interplay for the blue col- our. Physics and Chemistry of Minerals, 42, 431-439. Garnier V., 2003. Les gisements de rubis associés aux marbres de l’Asie Centrale et du Sud-est: genèse et caractérisation isotopique. PhD thesis INPL, Nancy, France, 373p. Garnier, V., Ohnenstetter, D., Giuliani, G., Maluski, H., Deloule, E., Phan Trong Trinh, Pham Van Long, Hoang Quang Vinh, 2005. Age and significance of ruby-bearing marbles from the Red River shear zone, northern Vietnam. The Canadian Mineralogist, 43, 1315-1329. Garnier V., Giuliani G., Ohnenstetter D., Fallick A.E., Dubessy J., Banks D., Hoang Quang Vinh, Lhomme Th., Maluski H., Pêcher A., Bakhsh K.A., Pham Van Long, Phan Trong Trinh, Schwarz D., 2008. Marble- hosted ruby deposits from central and Southeast Asia: towards a new genetic model. Ore Geology Reviews, 34, 169-191. Giuliani G., Fallick A.E., Garnier V., France-Lanord Ch., Ohnenstetter D., Schwarz D., 2005. Oxygen isotope composition as a tracer for the origins of ru- bies and sapphires. Geology, 33(4), 249-252. Giuliani G., Fallick A.E., Boyce A.J., Pardieu V., Pham Van Long, 2017. Pink and red spinels in marble: trace elements, oxygen isotopes, and sources. The Canadian Mineralogist, 55, 743-761. Hauzenberger C.A., Häeger T., Baumgartner L.P., Hof- meister W., 2001. High-grade metamorphism and stable isotope geochemistry of N-Vietnamese gem- bearing rocks. In: Proceedings of the Workshop on gems and minerals of Vietnam, Hanoi, 124-138. Hauzenberger C.A., Bagola C., Häeger T., Muellen C., Nguyen Ngoc Khoi, Le Thi Thu Huong, 2014. Min- eralogy and petrology of the An Phu marble hosted spinel and corundum deposit, Luc Yen, N-Vietnam. In Proceedings of the 4th International Gem and Jewelry Conference, Chiang Mai, Thailand, 76-78. Kleišmantas A., Daukšyte A., 2016. The influence of Vietnam and Sri Lanka spinel mineral chemical el- ements on colour. Chemija, 27, 45-51. Kretz R., 1983. Symbols for rock-forming minerals. American Mineralogist, 68, 277-279. Le Thi Thu Huong, Häeger T., Hofmeister W., Hauzen- berger C., Schwarz D., Pham Van Long, Wehmeis- ter U., Nguyen Ngoc Khoi, Nguy Tuyet Nhung, 2012. Gemstones from Vietnam: An update. Gems & Gemology, 48, 158-176. Vietnam Journal of Earth Sciences, 40(2), 165-177 177 Malsy A., Klemm L., 2010. Distinction of gem spinels from the Himalayan mountain belt. Chimia, 64(10), 741-746. Malsy A., Karampelas S., Schwarz D., Klemm L., Arm- bruster T., Tuan Do Anh, 2012. Orangey-red to or- angey-pink gem spinels from a new deposit at Lang Chap (Tan Huong - Truc Lau), Vietnam. The Jour- nal of Gemmology, 33, 19-27. Pham Van Long, Hoang Quang Vinh, Garnier V., Giuli- ani G., Ohnenstetter D., Lhomme,T., Schwarz D., Fallick A.E., Dubessy J., Phan Trong Trinh, 2004. Gem corundum deposits in Vietnam. Journal of Gemmology, 29, 129-147. Pham Van Long, Pardieu V., Giuliani G., Nguy Tuyet Nhung, Pham Thi Thanh Hien, Pham Duc Anh, Nguyen Ngoc Khoi, Hoang Quang Vinh, 2014. Gemmological characteristics of spinel from Luc Yen, Yen Bai. Journal of Geology, 340, 29-36. Pham Van Long, Pardieu V., Giuliani G., 2014. Update on gemstone mining in Luc Yen, Vietnam. Gems & Gemology, 49, 233-245. Pouchou J.L., Pichoir F., 1991. Quantitative analysis of homogeneous or stratified microvolumes applying "PAP" In Electron Probe Quantification (K.F.J. Heinrich & D.E. Newbury eds.). Plenum Press, New York, USA, 31-75. Valley J.W., 1986. Stable isotope geochemistry of met- amorphic rocks. Reviews in Mineralogy, 16, 445-481. Yui T.F., Khin Zaw, Wu C.-M., 2008. A preliminary stable isotope study on Mogok ruby, Myanmar. Ore Geology Reviews, 34, 182-199.