Abstract
Gem-quality zircon megacrysts occur in placer deposits in the Central Highlands, Vietnam, and have euhedral to anhedral crystal shapes with dimensions of ~3 cm in length. These zircons have primary inclusions of calcite, olivine, and corundum. Secondary quartz, baddeleyite, hematite, and CO2 fluid inclusions were found in close proximity to cracks and tubular channels. LA-ICP-MS U-Pb ages of analyzed zircon samples yielded two age populations of ca. 1.0 Ma and ca. 6.5 Ma, that were consistent with the ages of alkali basalt eruptions in the Central Highlands at Buon Ma Thuot (5.80–1.67 Ma), Pleiku (4.30–0.80 Ma), and Xuan Loc (0.83–0.44 Ma). The zircon geochemical signatures and primary inclusions suggested a genesis from carbonatite-dominant melts as a result of partial melting of a metasomatized lithospheric mantle source, but not from the host alkali basalt. Chondrite-normalized rare earth element patterns showed a pronounced positive Ce, but negligible Eu anomalies. Detailed hyperspectral Dy3+ photoluminescence images of zircon megacrysts revealed resorption and re-growth processes.
Highlights
Zircon (ZrSiO4, tetragonal, I41 /amd) is an accessory mineral in most types of igneous and metamorphic rocks [1]
Zircon placers that are related to basaltic magmatism are often associated with megacrysts of other important gem materials, as reported from various localities, including Australia, Vietnam, Cambodia, Thailand, Minerals 2019, 9, 89; doi:10.3390/min9020089
Zircon megacrysts from the Central Highlands that were investigated in this study had various colors, ranging from colorless, orange, brownish-orange, and dark brown to dark red
Summary
Zircon (ZrSiO4 , tetragonal, I41 /amd) is an accessory mineral in most types of igneous and metamorphic rocks [1]. Zircon megacrysts are often found in placer deposits derived from intraplate basaltic fields as xenocrysts or xenolith debris in alkali basaltic rocks [2]. Zircon material from the Central Highlands, Vietnam, shows brownish-red colors and blue, upon heat-treatment under reducing conditions [12]. This color effect is similar to zircon found in placers in the Ratanakiri district, Cambodia [13]. The Vietnamese zircon deposits are known since the late 1980s, and sporadic mining activities are conducted by local people at numerous small sites. These deposits are exploited by digging to a depth of approximately
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