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Thejournal of Gemmology The Journal of GemmologO~ly Volume 25 No.7 July 1997 The Gemmological' Association and Gem Testing Laboratory of Great Britain Gemmological Association and Gem Testing Laboratory of Great Britain 27 Greville Street, London ECl N 8SU Tel: 0171 404 3334 Fax: 0171 404 8843 e-mail: [email protected] Website: www.gagtl.ac.uklgagtl President: Professor R.A. Howie Vice-Presidents: E.M. Bruton, A.E. Farn, D.G. Kent, R.K. Mitchell Honorary Fellows: R.A. Howie, R.T. Liddicoat Inr; K. Nassau Honorary Life Members: D.J. Callaghan, E.A Jobbins, H. TiHander Council of Management: CR. Cavey, T.). Davidson, N.W. Deeks, R.R. Harding, J. Thomson, V.P. Watson Members' Council: A). Allnutt, P. Dwyer-Hickey, R. Fuller, J. Greatwood, B. Jackson, ). Kessler, J. Monnickendam, L. Music, J.B. Nelson, P.G. Read, R. Shepherd.. C.H. Winter Branch Chairmen: Midlands - G.M: Green, North West - I. Knight, Scottish - B. jackson Examiners: Aj. Allnutt, M.Se., Ph.D., FGA, S.M. Anderson, B.Se. (Hons), FGA, L. Bartlett, B.Sc., M.Phil., FGA, DGA, E.M. Bruton, FGA, DGA, CR. Cavey, FGA, S. Coelho, B.Sc., FGA, DGA, Prof. AT. Collins, B.Se., Ph.D, A.G. Good, FGA, DGA, C).E. Hall, B.Se. (Hons), FGA, G.M. Howe, FGA, DGA, G.H. jones, B.Se., Ph.D., FGA, M. Newton, SSc., D.Phil., H.L. Plumb, B.Sc., FGA, DGA, R.D. Ross, B.Sc., FGA, bGA, P.A. Sadler, B.Sc., FGA, DGA, E. Stern, FGA, DGA, Prof. I. Sunagawa, D.Se., M. Tilley, GG, FGA, CM. Woodward, e.se., FGA, DGA the .J"Q.tQ.rQf~~jfimJ)l~gy EtJiti!)r~Ori~;R.,,;f1~td;rttg: AsS1sti11J,teiilil@"$':,M.J,. ()~aQfloghue,RG.Read o,5t~r?~~~tt~~~, !:lr 1"E,,'$'higl~Y~~<;lrJ$ba~~iPr6r. Q:.(;,:$Oil,lJI't .arJ$~i.E, '. St~fn. (Lii)f1dQol; Pt¢f.J, $una.:~fJwa (rQk¥Olj,[)tlM1.$vpet-qfcJi;(&tU~~ C~M. Wo~wa:fd (t:Qn~Qh¥ Prodl.litttonEditonM.A. B.urland Vol 25, No.7, July 1997 ISSN: 1355-4565 Sapphires from the Andranondambo mine in SE Madagascar: evidence for metasomatic skarn formation by E.J. Gübelin1 and A. Peretti2 1Lucerne 2GRL Gemresearch Laboratory AG, Lucerne, Switzerland ABSTRACT: Mineral inclusions in gem quality sapphires from the new mining area in southern Malagasy (Andranondambo) include calcite, fluorapatite, fluor-phlogopite, fluorite, baddeleyite, zirconolite, potassium- feldspar, anorthite, scapolite (meionite), thorianite (thorianite-uraninite solid solutions) and varieties of spinel, such as green Mg-Al-spinel, spinel- hercynite and spinel-hercynite-gahnite solid solutions. These minerals are typically found in various rocks in the mining area belonging to the so- called U-Th skarns (various types of calcsilicate marbles). Therefore, these skarns are considered to be the host rocks for sapphire formation. Based on the different generations of primary fluid inclusions in the 389 sapphires, various generations of solid inclusions and the absence of hibonite inclusions, it is concluded that the Andranondambo sapphires have formed during several different metasomatic events. The later blue sapphire generations have grown from CO2-rich fluids of complex chemical composition with minor concentrations of fluorine, water and metals such as aluminium, titanium, iron, chromium and zinc as well as other elements (Mg, Si, Ca, Na, K, P, Zr, U, Th). Introduction undoubtedly comparable with those of Sri Lanka. Beryls of various colours (aquamarine, 7Ê j4ÎFa'a8asy/s (Madagascar) highly emerald, heliodor, morganite), corundums /B/JF variable topography and her (ruby, sapphire), feldspars (amazonite, ortho- J*, WMs, fascinating landscapes indicate clase and labradorite), garnets (almandine, that highly active tectonic powers have rhodolite and spessartine), kunzite, quartz affected her geology, which as a result is very (macro and cryptocrystalline), tourmalines complex. This conjecture has been confirmed (e.g. liddicoatite) and also a large variety of by the results of modern geological research rare gems such as kyanite, danburite, epidote, and by the wealth of gemstones which has hambergite, kornerupine and titanite (sphene) been found over the last hundred years. are being excavated. Furthermore, hibonite 2+ Indeed, Malagasy nowadays has become an (Ca, REE, Th)(AI, Ti, R )12Ol9 with Ti = Ti, Si; important producer of a wide range of R2+ = Mg2+, Fe2+, Zn2+; AI = Al3+, Fe3+ and gemstones (Figure 1), and the occurrences are REE = Sm3+, Nd3+, Ce3+, La3+ - a rare mineral © Gemmological Association and Gem Testing Laboratory of Great Britain ISSN: 1355-456b Figure 1: Map of Malagasy showing some of the most important gem deposits (from the World Map of Gem Deposits by E.J. G übel in). Figure 3: Gem collection at the Andranon­ dambo mine by one of the authors (EG) in early 1996. 389 Figure 2: Main thoroughfare of Andranon- Figure 4: Sapphire samples obtained from a dambo making a lively impression. source in Switzerland (Source C, see text) with eye visible inclusions. On the right, a rock fragment collected at the mines is shown. The first found in meteorites - occurs at section on the left demonstrates how the Andranondambo together with sapphires (for minerals were prepared for SEM-work. a chemical analyis of a hibonite from this locality, see Rakotondrazafy et al., 1996). The classical expert in Malagasy mineralogy, referred to various deposits of cuttable Lacroix (1922), stated that corundum is corundum in the southern part of the island, commonly associated with metamorphic while Koivula et al. (1992) were the first to rocks. Gem quality corundum has, however, mention a source of multicoloured sapphire rarely been found up to now and the south-west of Betroka. production was mainly limited to relatively The recently discovered new deposits of small pieces of low quality. Chikayama (1989) pale to saturated blue sapphire are situated in J. Gemm., 1997, 25, 7, 453-516 Figure 5a: Superb royal blue sapphire of Figure 5b: Faceted stones (about 1 ct) typical about 4 et (not heat-treated) (Source D, of sapphires after heat-treatment (Source D, Bangkok). Bangkok). the south-eastern part of Province Toi iara, near metasomatic events which occurred in the the village of Andranondambo, approximately mining area and which led to the formation of 100 km north-north-east of Fort Dauphin hibonite and other minerals, including (Figures 1 and 2). This source is probably the corundum, in skarns. The present paper first large deposit of cuttable sapphire to be provides further results of inclusion research found on Malagasy. New mines have been relevant to the genesis of the sapphires. The reported by Johnson and Koivula, 1996, and at material originates from the expedition of one 455 least three mining companies (a Swiss, a Thai of the authors (EG) to the gem mines in 389 and a joint venture enterprise) have been Madagascar including those of Andra­ founded to excavate and market the sapphires. nondambo in early June 1996 (Source A) Kiefert et al. (1996), Milisenda and Henn (Figures 3 and 4). Further rough and faceted (1996) and Schwarz et al. (1996) have specimens and rocks containing sapphires described the occurrence, genesis and were obtained from Dr D. Schwarz (Schwarz gemmological properties of the sapphires. But étal., 1996) (Source B), rough material was although very characteristic properties of this obtained from a source in Switzerland (Source new material have been established, consider­ C, Figure 4), faceted samples were acquired in able differences have arisen in the inter­ Bangkok, Thailand (Figures 5a, b) (Source D) pretation of the formation of the sapphires. and rocks containing sapphires were obtained According to Milisenda and Henn (1996), the through Dr K. Schmetzer (Source E, Figure 7b). sapphires were formed by high-grade The research material obtained from sources C granulite faciès metamorphism of altered and D offer a lesser degree of certainty about marbles and calcsilicate-gneisses. In contrast, the veracity of their origin. However, the Kiefert et al. (1996) relate the formation of the authors are confident that all the sapphires are sapphires to pegmatites or pegmatite-like from the new mining area (including the mines veins such as those of the Kashmir mining area mentioned by Johnson and Koivula, 1996). (Peretti ef al.f 1990). Schwarz et al. (1996) The material was tested by different methods, interpreted the genesis of the sapphires as the including XRF-analysis for the chemical result of metasomatism. An in-depth composition, SEM-EDS-analysis of solid petrographical study of the metamorphic and inclusions and immersion microscopic metasomatic events leading to the formation examination for growth structures, and of hibonite has recently appeared spectroscopic analysis. Based on comparison (Rakotondrazafy ef a/v 1996). This work with published results (Milisenda and Henn, provides excellent further data on the 1996; Kiefert etal.f 1996; Schwarz et ai, 1996) Sapphires from the Andranondambo mine in SE Madagascar: evidence for metasomatic skarn formation some of the most typical characteristics have (Kennedy, 1964; Kröner, 1980). In the context been summarized below. However, the present of Gondwanaland, the granulite of Malagasy report concentrates mostly on analyses of the formed the eastern part of the Mozambique inclusions in the sapphires. Based on these belt which is interpreted as the result of a data and the very detailed petrological continent-continent collision that occurred framework of the mining area as presented by about 600 million years ago. Rakotondrazafy et al. (1996), further According to Rakotondrazafy et al. (1996) conclusions are presented here about genetic the Andranondambo mine is situated in the aspects of the sapphire formation. metamorphic rocks of the Proterozoic Tranomaro group located in the south-east of Geological petrographical and the island. This group consists of varied granulite faciès rocks of mainly sedimentary tectonic background origin such as different gneisses (including Malagasy is the world's fourth largest pyroxene scapolite gneisses) and calcitic island.
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