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

Authors: Limtrakun P.; Zaw K.; Ryan C. G.; Mernagh T. P.

Source: Mineralogical Magazine, Volume 65, Number 6, 1 December 2001 , pp. 725-735(11)

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Abstract:

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 (10-15vol.%) 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

Language: English

Document Type: Research article

Publication date: 2001-12-01

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