Mineral inclusions in ruby and sapphire from Bo Welu and Bo Rai deposits, eastern Thailand

Abstract

Corundum deposits located in several places around the world which appear to have different genesis models. The origins of corundum can be distinguished by mineral inclusions. In this study, most of the rough gem corundum from Bo Rai deposit, Trat Province, and Bo Welu deposit, Chanthaburi Province, in Eastern Thailand show etched or dissolved surfaces that were resulted from dissolution by hot basaltic magma during transportation to the surface. Moreover, these gem corundums also show absorption spectral characteristics related to trace elements which typical for corundum from this area. Corundums from Bo Rai deposit in Trat Province, and from Bo Welu deposit in Chanthaburi Province were collected for this study. Various mineral inclusions in ruby were discovered, particularly high Al diopside (fassaite pyroxene), pyropic garnet, plagioclase feldspar, sillimanite, spinel, and sulphide are also observed in Bo Welu ruby similar to Bo Rai ruby. However, anatase inclusion is only identified in Bo Rai. On the other hand, sapphirine, quartz, nepheline, and anhydrite are only identified as inclusions in Bo Welu ruby. Mineral inclusions in sapphire from Bo Welu deposit are zircon, alkali feldspar, sulphide, monazite, columbite. Based on chemical composition of mineral inclusion, rubies from both Bo Rai and Bo Welu deposits may have originally formed within a similar environment, mafic granulites rocks, probably within lower crust to upper mantle. Blue and bluish green sapphires from Welu deposit may have crystallized from a highly evolved, alkali-rich and silica-poor magma at a shallower level in the lithosphere. Although, the formation of ruby located at deeper levels (upper mantle) than the sapphire (crust/upper mantle), some similar mineral inclusions in both ruby and sapphire may indicate co-crystallization between ruby and sapphire in some places.