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

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: Fe 2 O 3 (0.32À1.98 wt.%), TiO 2 (0.01À0.23 wt.%), Cr 2 O 3 (<0.01 wt.%), Ga 2 O 3 (0.01À0.03 wt.%) and V 2 O 5 (<0.03 wt.%). Optical studies of sapphires revealed three types of primary fluid/melt inclusions. CO 2 -rich inclusions (Type I) contain three phases (LH 2 O + LCO 2 + V) with the vapour phase comprising <10À15 vol.%. The presence of CO 2 was confirmed by microthermometry and laser Raman analysis. Polyphase inclusions (Type II) (vapour + liquid + solid) contain a fluid bubble (20À30 vol.%), an aqueous phase (10À15 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 Ga 2 O 3 abundances and Cr 2 O 3 /Ga 2 O 3 values (<1) of the sapphires favour their formation by magmatic processes. The presence of CO 2 -rich fluids and high K concentrations in the silicate melt inclusions link the origin of the Denchai gem sapphires to CO 2 -rich alkaline magmatism.

Petrogenesis of alkaline basalt-hosted sapphire megacrysts. Petrological and geochemical investigations of in situ sapphire occurrences from the Siebengebirge Volcanic Field, Germany

“…Of special interest are secondary corundum deposits that are associated with rift-related alkaline basaltic volcanism as these are among the world's largest gem corundum suppliers (e.g., Guo et al 1996;Limtrakun et al 2001;Shore and Weldon 2009) and are hence economically of great importance. Such basaltic secondary deposits are located all over the world, most notably in Asia (e.g., Guo et al 1996, and in eastern Australia, at the western Pacific margin (e.g., Coenraads 1990;Graham et al 2008;Sutherland 1996;Sutherland et al 1998aSutherland et al , 2002Sutherland et al , 2009bSutherland et al , 2015Sutherland et al , 2017Saminpanya 2001;McGee 2005;Abduriyim et al 2012), but also in Europe (e.g., Malikova 1999;Giuliani et al 2009;Uher et al 2012), Africa (Kiefert and Schmetzer 1987;Krzemnicki et al 1996;Schwarz et al 2000;Pardieux et al 2014), in South America , and in North America (e.g., Garland 2002;Berg 2007Berg , 2014Palke et al 2017).…”

Petrochemistry of Mafic Granulite Xenoliths from the Chantaburi Basaltic Field: Implications for the Nature of the Lower Crust beneath Thailand

“…Unraveling the origin of gem-quality corundum in the region of Thailand has been a major goal of several investigations (e.g., Coenraads et al, 1995;Levinson and Cook, 1995;Guo et al, 1996;Sutherland et al, 1998;Limtrakun et al, 2001;Sutthirat et al, 2001). Despite these efforts, however, there is still no consensus regarding the genesis of these gem-quality corundum megacrysts, partly because of the apparent absence of corundum-bearing rocks that could provide detailed petrologenetic information.…”

“…Previously, petrochemical investigations were aimed at understanding the genesis of gem-quality corundum (ruby and sapphire) through studies of basalts, as well as inclusions in corundum megacrysts that occur abundantly in placer deposits near these basaltic fields (e.g., Vichit et al, 1978;Barr and MacDonald, 1981;Guo et al, 1996;Intasopa et al, 1999;Sutthirat et al, 2001;Limtrakun et al, 2001). Only a few studies have been performed on mantlederived megacrysts and peridotite xenoliths (Barr and Dostal, 1986;Promprated et al, 1999).…”

Trace-element compositions of sapphire and ruby from the eastern Australian gemstone belt

“…These inclusions suggest mixing between a silicic component and a carbonatitic/Si-poor component at crustal levels or crystallization from felsic magmas (Guo et al, 1996;Sutherland et al, 1998a). Sapphires in Thailand and Vietnam contain high-K and high-Na silicate melt inclusions and CO 2 -rich fluid inclusions, which suggest a magmatic source (Limtrakun et al, 2001;Izokh et al, 2010). In contrast, the association of sapphirine, Al-rich diopside, spinel and garnet with high-Cr/Ga corundum in some SE Asian and eastern Australian alluvial deposits has been cited to suggest a metamorphic origin for these corundum occurrences (Sutherland and Coenraads, 1996;Sutherland et al, 1998a;Saminpanya and Sutherland, 2011).…”