World Of Gems Conference 2017 Proceedings Book

FORMATION OF ‘METAMORPHIC’ SAPPHIRES — EXAMPLES FROM SRI LANKA ( ED. Note. The presentation at the World of Gems Conference was by Hanco Zwaan.) O ptical properties, inclusions and geochemistry are com- monly measured and interpreted meticulously to form an opinion on the provenance and origination of sap- phires. The origin of sapphires is often simply referred to as basalt-related or non-basalt related or linked to the mode of for- mation, e.g., syenitic, plumasitic, metamorphic and metaso- matic (e.g., Simonet et al., 2008). While the geology of basalt-related sapphires is often well known, the formation of sapphires related to a metamorphic origin is less understood, because these sapphires are often found in al- luvial deposits and primary rocks are not well known or were not even found. A recently discovered locality of rare in situ sapphire (Liu Shang-I and Zoysa, 2011) has been studied in SE Sri Lanka, at Mirisatahela, 5 km NE of Wellawaya. This locality shows small-scale mining activity and fair outcrops, with large corundum crystals, up to 15 cm. The samples from Wellawaya generally consist of approximately 20-30% large corundum crys- tals, surrounded by plagioclase, K-feldspar and biotite (Figure 1). The large corundum crystals are commonly colorless, with locally a faint blue hue, but may also be very light blue or light purple. These crystals are found at the contact between restitic metapelites (khon- dalites) and a late- to post-tectonic, coarse-grained (pegmatitic) melt of external origin that seems to have acted as a trigger and catalyst for local reactions. The field relations suggest open-system, metasomatic processes at the contact between contrasting lithologies, aided by melts. Pressure-temperature diagrams for specific bulk rock composi- tions (P-T pseudosections) were calculated with Perple_X soft- ware, and gave broad ranges for these high-variance samples, that overlap in a P-T window from 6-9 kbar and 690-83 °C. When compared to the well-established regional P-T path for the High- land Complex (e.g., Dharmapriya et al., 2015), these results show that large-sized corundum (sapphire) was produced after the metamorphic peak (with locally ultra-high temperature (UHT) conditions - 775-925°C, 9.5-11 kbar) and after a subsequent cool- ing and decompression phase. This fits the overall post-tectonic timing of related pegmatitic dykes. The regional distribution of in situ sapphire, suggests a causal link with thrusting of the High- land Complex over the Vijayan Complex. SHRIMP U-Pb zircon geochronology for four corundum-bearing samples gives three data clusters: 1.8-2.1 Ga, probably repre- senting igneous activity in the provenance area of the sedi- ments; 610 and 550 Ma, both interpreted as metamorphic ages. The older of these is interpreted as the regional peak metamor- phism, and the younger is interpreted as the time of sapphire growth. Large sapphire is restricted to samples showing both metamorphic ages. Hence, we postulate that during peak meta- morphism at 600-620 Ma, by melting out silica, rocks obtained the right, high-Al, silica-poor chemistry for corundum growth, and that the later, fluid-rich event at 550 Ma triggered the growth of large crystals of commercial interest. In Sri Lanka, large quantities of gem-quality sapphires are found in secondary alluvial and eluvial deposits. The major gem area is located within the Highland complex, in the Ratnapura- Balangoda region, relatively close to Wellawaya. For long, the whereabouts of primary sources and the growth mechanism WORLD OF GEMS CONFERENCE V - 25 - SEPTEMBER 2017 J.C. (Hanco) Zwaan, Leo M. Kriegsman, Jorien L.N. van der Wal, Richard J.M. Taylor, Regina Mertz-Kraus METAMORPHIC SAPPHIRES ABSTRACT FIGURE 1. Large corundum crystals in a rock, mainly containing feldspar and biotite, found at Mirisatahela, near Wellawaya, Sri Lanka. The corundum crystal on the left has a length of 8.3 cm.