Formation of the Denchai gem sapphires, northern Thailand: evidence from mineral chemistry and fluid/melt inclusion characteristics

The Denchai gem sapphire deposits in Phrae Province, northern Thailand are closely associated with late Cenozoic alkaline basaltic rocks. The sapphires occur in alluvial placer deposits in palaeo-channels at shallow depths. Electron microprobe analysis of minor and trace element contents (Fe, Ti, Cr, Ga and V) of the sapphires indicate the following oxide abundances: Fe2O3 (0.32-1.98 wt.%), TiO2 (0.01-0.23 wt.%), Cr2O3 (<0.01 wt.%), Ga2O3 (0.01-0.03 wt.%) and V2O5 (<0.03 wt.%). Optical studies of sapphires revealed three types of primary fluid/melt inclusions. CO2-rich inclusions (Type 1) contain three phases (LH2O + LCO2 + V) with the vapour phase comprising < 10-15 vol.%. The presence Of CO2 was confirmed by microthermometry and laser Raman analysis. Polyphase inclusions (Type II) (vapour + liquid + solid) contain a fluid bubble (20-30 vol.%), an aqueous phase (10-15 vol.%) and several solid phases. Silicate-melt inclusions (Type III) comprise vapour bubbles, silicate glass and solid phases. Proton-induced X-ray emission (PIXE) analysis revealed high concentrations of K (similar to4 wt.%) as well as Ca (similar to0.5 wt.%), Ti (similar to1 wt.%), Fe (similar to2 wt.%), Mn (similar to0.1 wt.%), V (<0.03 wt.%), Rb (similar to 70 ppm) and Zr (similar to 200 ppm) in the silicate glass. The Ga2O3 abundances and Cr2O3/Ga2O3 values (<1) of the sapphires favour their formation by magmatic processes. The presence Of CO2-rich fluids and high K concentrations in the silicate melt inclusions link the origin of the Denchai gem sapphires to CO2-rich alkaline magmatism.