Sapphire crystallization, age and origin, Ban Huai Sai, Laos: age based on zircon inclusions

Abstract

Sapphire from the Ban Huai Sai alluvial gemfield, Laos, derives from local Cenozoic basalts. Syngenetic mineral inclusions in the sapphire include zircon, cheralite, albite and ?euxenite-(Y). Sensitive high resolution ion microprobe (SHRIMP) analysis of zircon inclusions show enrichment in Hf, Y and rare earth elements (REE), with REE ranging into the highest abundances in such zircons (up to 5785ppm). This, with Eu depletion and the other inclusion mineralogy, suggests the sapphire crystallized from highly evolved alkaline melts, before corrosive transport in basalt magma. U–Pb isotope (SHRIMP) determinations on zircon inclusions give measured zircon ages of 1.2–1.3±0.3Ma. This is taken to mark the time of host sapphire crystallization, within error of a basalt K–Ar age from province. The suggested age for Ban Huai Sai zircon/sapphire crystallization matches that given by a zircon–sapphire–magnetite xenolith from Khao Wau, Thailand (1–2Ma). These results favour sapphire genesis linked to processes associated with young alkali basaltic magmatism in South East Asia.