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

¹ School of Earth Sciences, University of Tasmania, GPO Box 252-79, Hobart, TAS 7001, Australia
² Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, TAS 7001, Australia
³ CSIRO Exploration and Mining, PO Box 136, North Ryde, NSW 2113, Australia
⁴ Australian Geological Survey Organisation, Canberra, ACT 2601, Australia

^* E-mail: plimtrak{at}postoffice.utas.edu.au

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.

KEYWORDS: sapphire, fluid and melt inclusions, alkaline magmatism, basalt, Thailand

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