Mineralogical, textural and fluid inclusion studies of corundum and spinel in Sri Lanka

Abstract

Corundum and spinel are the most prominent gem minerals found in Sri Lanka that account for more than half its gem exports. Further, Sri Lanka is a major supplier of top quality sapphire. The main objective of this study was to (i) identify as many properties as possible of Sri Lankan corundum and spinel, not recorded so far and (ii) list the properties supposed to be unique to certain terrains such as alkali magmatic terrains in other countries, which were helpful to understand the origin of corundum and spinel of Sri Lanka. For this study samples representative of the Precambrian metasedimentary terrain of Sri Lanka were collected. For detailed investigations, samples from only Balangoda and Kaltota were chosen, because these areas were underlain by different gem-bearing source rocks. Several features unique to alkali magmatic terrains such as plagioclase crust around the crystal, surface features resembling needle - like patterns, radial cracks around zircon inclusions and inclusions of zircon clusters were observed in relation to Sri Lankan corundum. Chemical fmgerprinting, a methodology adopted to determine the origin of corundum, was carried out for a limited number of selected samples of which, two were identified as magmatic. Several microscopic and macroscopic reaction textures provided evidence for the confirmation of metamorphic growth. These reactions were also confrrmed by EPMA analysis. Fluid inclusions representing the Precambrian rocks of Sri Lanka were scrutinized using petrological microscope, Raman microprobe and thermal stage microscope. During this study it was possible to confirm the theory that Sri Lankan fluid inclusions contained more or less pure CO2, and also the theory that the most common daughter minerals were graphite and diaspore. Necking too was observed and the process was similar to what was mostly observed in the corundum of Malawi. A classification of corundum on the basis of fluid inclusions was formulated based on the shape, size and composition of fluid inclusions. The classification led to four categories. Sapphirine too was found as an inclusion, in contact with spinel, in corundum. Hence it was possible to calculate the sapphirine/spinel thermometry. The calculation revealed that the crystallization temperature of Sri Lankan corundum was 761°C (Fe2+ calculation). Finally the features unique to corundum found in other alkali magmatic terrains of the world, which are now identified in Sri Lanka too are taken into consideration. These features were plagioclase crust around corundum, surface features of needle-like pattern, zircon clusters and radial cracks around zircon inclusions in corundum. Two instances found by chemical finger printing to be of magmatic nature indicated the origin as such, while the reaction textures associated with the corundum bearing rock indicated metamorphic origin. These finding proved that the corundum of Sri Lanka displayed a combination of both magmatic and metamorphic properties. Hence the origin of corundum could not be attributed to metamorphism alone with certainty.