Basalt petrology, zircon ages and sapphire genesis from Dak Nong, southern Vietnam

Abstract

Basalts associated with sapphire deposits are widespread in Europe, Africa, Asia and Australia. In Vietnam, blue, green and yellow sapphires are recovered from eluvial and alluvial placers hosted in basaltic fields of the Dak Lak Province. Two distinct basalt suites are recognized in this field: a tholeiitic suite without any xenocrysts and an alkaline suite with mantle and lower crustal xenocrysts. The sapphires are enriched in Fe (0.43 to 1.26 wt.%), with moderate contents of Cr (33–1582 ppm), Ti (35–1080 ppm), Ga (149–308 ppm) and V (28–438 ppm) and they are poor in Zn and Mg. Their O-isotope compositions range from 6.0 to 6.9% and are not in equilibrium with the alkali basalts which have δ18O values between 5.0 and 5.7%. The U-Pb dating of zircons recovered from the basaltic placers provides evidence of two eruptional events: one at ~6.5 Ma followed by another one at ~1 Ma. The petrography of the basalts and the oxygen isotopic composition of the sapphires suggest that the sapphires are xenocrysts and that they crystallized in a deep magma chamber, at the lower continental lithosphere and the upper mantle boundary, in evolved melts issued from the fractionation of alkali basaltic magmas contaminated with lower crustal fluids.

Contrasts in gem corundum characteristics, eastern Australian basaltic fields: trace elements, fluid/melt inclusions and oxygen isotopes

“…Thus, the crystallizing conditions were probably buffered by ultramafic bodies of different settings, as described by Giuliani et al (2005) and Garnier et al (2005).…”

Sapphires related to alkali basalts from the Cerová Highlands, Western Carpathians (southern Slovakia): composition and origin

“…Sutherland et al 2002Sutherland et al , 2009Garnier et al 2005;Zaw et al 2006;Peucat et al 2007). Generally, Fe and Ga reveal higher concentrations in magmatic sapphires (~1800 to 13,000 ppm Fe and ~70 to 570 ppm Ga), than in metamorphic ones which have Fe and Ga contents less than 3000 and 75 ppm, respectively.…”

“…The 18 O value in gem corundum represents a useful tool for deciphering its geological origin (Yui et al 2003Giuliani et al 2005Giuliani et al , 2007Giuliani et al , 2009Garnier et al 2005;Zaw et al 2006;Sutherland et al 2009). As the mantle and crustal rocks show distinct oxygen isotope compositions, the 18 O value enables us to investigate the origin and source of corundum.…”

Oxygen isotopic composition as an indicator of ruby and sapphire origin: A review of Russian occurrences

“…5. Pink corundum from the Proterozoic gneiss of the northwestern Karelia shows anomalous δ 18 Osmow between −26.3‰ and +3.0‰ (36 analyses; Garnier et al, 2005;Ustinov et al, 2008;Bindeman et al, 2010Bindeman et al, , 2014Bindeman and Serebryakov, 2011;Vysotskiy et al, 2014). 6.…”