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


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Limtrakun, P, Zaw, K, Ryan, CG and Mernagh, TP 2001 , 'Formation of the Denchai gem sapphires, northern Thailand: evidence from mineral chemistry and fluid/melt inclusion characteristics' , Mineralogical Magazine, vol. 65, no. 6 , pp. 725-735 , doi: 10.1180/0026461016560004.

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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 I)
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 (~4 wt.%) as well as Ca (~0.5 wt.%), Ti (~1 wt.%), Fe (~2 wt.%), Mn (~0.1 wt.%), V (<0.03 wt.%), Rb (~70 ppm) and Zr (~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.

Item Type: Article
Authors/Creators:Limtrakun, P and Zaw, K and Ryan, CG and Mernagh, TP
Keywords: sapphire, Fluid and melt inclusions, alkaline magmatism, basalt, Thailand.
Journal or Publication Title: Mineralogical Magazine
ISSN: 0026-461X
DOI / ID Number: 10.1180/0026461016560004
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