Mineral inclusions in sapphire from basalt - related deposit in Southern Vietnam

Abstract

Sapphires in Southern Vietnam have been discovered in alluvial deposit derived from Cenozoic basalts. Sample collections for this study were taken from the most significant gem fields including Pleiku, Dak Nong, Di Linh, Binh Thuan, and Krong Nang. Several syngenetic mineral inclusions in these sapphires were classified as alkali feldspar, zircon, ferrocolumbite, pyrochlore, and iron oxides. Mineral chemical characteristics of these inclusions appearred to have evolved from alkaline felsic suite.

Oxide mineral inclusions occurred in sapphire included hercynite, wüstite, and ilmenite. Ilmenite ranged in composition between Il49-54He34-40Mt7-10 and Il24-30He36-38Mt35-40 falling in ilmenite-hematite series. Wüstite with nonstoichiometry, (Fe2+0.3-0.9)(Ti3+<0.179Al3+≤0.6Cr3+<0.1 Fe3+≤0.46)£≤0.23O, was usually associated with hercynite inclusion clearly indicating cogenetic sapphire formation. Wüstite and sapphire appeared to have formed from the breakdown reaction of hercynite (hercynite = sapphire+wüstite). Titanohematite series and titanomagnetite series might have crystallized during iron-titanium reequilibration via subsolidus exsolution.

U-Pb geochronology of zircon inclusion in these sapphires yielded between 35.5 ± 1.6 Ma (~35 Ma) and 14.73 ± 0.29 Ma (~15 Ma) for Krong Nang sapphire, 5.94 ± 0.13 Ma (~6 Ma) for Binh Thuan and Dak Nong sapphire, and 5.491 ± 0.077 Ma (~5.5 Ma) for Di Linh sapphire. These age constraints relate to the main Cenozoic basaltic eruptions in this region. Crystallization temperature of sapphire formation was estimated at about 561- 7810C using Ti-in-zircon thermometry.