U-PB AGES OF ZIRCON INCLUSIONS IN SAPPHIRES FROM RATNAPURA AND BALANGODA (SRI LANKA)

Blue sapphire is one of the most appreciated gemstone and its geographic-geological origin has an important impact on its value. Within a master thesis (Elmaleh, 2015), part of the sapphires collection of the Gübelin Gem Lab Reference Collection from different mining areas was systematically examined. This abstract presents the geochemical and geochronological results of the detailed gemmological characterization of two sapphires from Ratnapura and one from Balangoda (Sri Lanka).

The sapphires from Ratnapura display Ga/Mg ratios of 0.178-0.845, and the Ga/Mg ratio of the sapphire from Balangoda was determined to be 5.034 (Peucat et al., 2007). Both values lie within the range indicated for metamorphic sapphire. In the ternary diagram Fe-Ti*10-Mg*100, all sapphires plot in the field for skarn-related metamorphic sapphires, as well as in the area characteristic for the sapphires from Sri Lanka (Schwarz et al., 2008).

One sapphire from Ratnapura (SASLR050_01) and one from Balangoda (SASLA01) provided homogenous zircon inclusions allowing to be dated with conventional La-ICP-MS. Zircon size ranges between 10 and 30 μm. The zircons show sector zoning under cathodoluminescence. The Raman spectra of the dated zircon SASLA01 of Balangoda shows a pattern characteristic for a metamorphic zircon.

Two U/Pb ages were obtained for the zircon inclusions in the sapphire of Balangoda (SASLA_01) and correspond to a Concordia age of 547.7±5.7 Ma. For the zircons in sapphire SASLR050_1 of Ratnapura, an age of 557 ±8.4 Ma was determined. The zircon ages for the sapphires from both deposits show a similar range of crystallization age. Their formation is probably related to the main phase of granulite facies metamorphism occurring within a suggested time window of 605±4 Ma to 591±57 (Baur et al., 1991) and its relationship to the Pan-African orogeny. The detailed geological setting or whether it is related to a contact metamorphic, a metamorphic-hydrothermal or a regional metamorphic setting remains, however, unclear.

Reference

Baur, N. et al. (1991) The Journal of Geology, 99, 527-545.

Elmaleh, E. (2015) Master thesis, Université de Genève, 425 pages.

Peucat J.J. et al. (2007) Lithos, 98, 261–274.

Schwarz, D. et al. (2008) Gems & Gemology, 44(4), 322–347.