World Of Gems Conference 2017 Proceedings Book

WORLD OF GEMS CONFERENCE V - 26 - SEPTEMBER 2017 that could produce such large quantities, were enigmatic (e.g., Kumaratilake & Ranasinghe, 1992; Zwaan, 1982, 1986; Pardieu et al., 2012). However, the proposed mechanism for large corundum growth, with a precursor regional peak-metamor- phic event of melting out silica, indicates the possibility of corundum growth on a larger scale, producing larger quanti- ties. The chemistry of the Wellawaya crystals corresponds to the chemistry of (very) light blue to blue sapphires found in al- luvial deposits around Ratnapura (including Balangoda; e.g., Abduriyim and Kitawaki, 2006; Peucat et al., 2007), which would support the likelihood of similar origin (Fig. 2). Ga/Mg versus Fe and Fe/Ti versus Cr/Ga diagrams show compositions of Wellawaya corundum plotting largely within the composi- tional ranges of sapphires found in the Ratnapura-Balangoda region, which correspond to the compositional range of meta- morphic sapphires in general, differing greatly with those of sapphires with a basalt-related origin. We conclude that gem-quality sapphire growth in Sri Lanka was essentially metasomatic, promoted by a sequence of events in- cluding UHT metamorphism priming the rocks, a tectonic con- tact creating space, and fluid/pegmatitic melt transfer from underthrusted gneisses. Details of this study and more data will be published in a comprehensive forthcoming publication. u METAMORPHIC SAPPHIRES FIG. 2. Compositions of large corundum crystals fromWellawaya, respectively 8 cm across (Wella 63 - green lozenge shapes) and 4 cm (13HZ30 - blue triangles), in relation to metamorphic and magmatic sapphires, and in particular to sapphires from the Ratnapura-Balangoda gem gravels (shaded compositional ranges), as shown by Peucat et al., 2007 (left) and Abduriyim and Kitawaki, 2006 (right). REFERENCES Abduriyim, A. & Kitawaki, H., 2006. Determination of the origin of blue sapphire using laser Ablation Inductively Coupled Plasma mass Spectrometry (LA-ICP-MS). J. Gemmology, 30, 1/2, 23-36. Dharmapriya, P.L, Malaviarachchi, S.P.K., Galli, A., Ben-Xun Su, Subasinghe, N.D. Dissanayake, C.B., 2015. Rare evidence for formation of garnet + corundum during isobaric cooling of uHT meta-pelites: New insights for retrograde P-T trajectory of the Highland Complex, Sri Lanka. Lithos, 220, 300-317. Kumaratilake, W. L. D. R. A., and u. N. Ranasinghe (1992). unusual corundum-bearing gem pockets at Avissawella and Getahetta, Sri Lanka, Zeitschrift der Deutschen Gemmologis- chen Gesellschaft, 41, 1, 7–16. Liu Shang-I, E. and Zoysa, E.G., 2011. The Mirisatahela primary sapphire deposit in Sri Lanka. Incolor, winter, 34-39. Pardieu, V., Dubinsky, E.V., Sangsawong,S. Chauviré, B., 2012. Sapphire rush near Kataragama, Sri Lanka (February-March 2012). GIA News from Research, May 2012, http://www.giathai.net . Peucat J.J., Ruffault P., Fritsch E., Bouhnick-le Coz M., Simonet C., Lasnier B., 2007. Ga/Mg ratio as a new geochemical tool to differentiate magmatic from metamorphic blue sapphires. Lithos, 98: 261 – 274. Simonet, C., Fritsch, E., & Lasnier, B. (2008). A classification of gem corundum deposits aimed towards gem exploration. Ore Geology Reviews, 34(1-2), 127–133. Zwaan P. C. (1982). Sri Lanka: The gem island. Gems & Gemology, 18, 2, 62–71. Zwaan P. C. (1986). Gem minerals from the Embilipitya and Kataragama areas in Sri Lanka, Australian Gemmologist, 16, 2, 35–40.