FLUORESCENCE AND LACK THEREOF IN ZIRCONS FROM THE FREEMAN ZIRCON MINE, ZIRCONIA DISTRICT, NORTH CAROLINA

Large euhedral to subhedral zircons, averaging 0.25-0.64 cm in diameter, occur in a zoned syenitic-pegmatite near Zirconia, North Carolina, and were mined for zirconium in the late 19th and early 20th centuries. Zircon concentrates taken from four sites at the Freeman mine dump have been classified as either strongly or poorly fluorescent under shortwave UV light. Fluorescence activation in zircon has been attributed to high concentrations of U, Hf, and REE (particularly Dy), and these two distinct groups of highly fluorescent and poorly fluorescent zircons provide an opportunity to correlate fluorescence with a number of variables.

Strongly fluorescent zircons in this study are a light brown in color, whereas poorly fluorescent samples are darker brown and tend to be coated with an iron stain. X-ray powder diffraction studies indicates that neither is significantly metamict, and there are no major differences in unit cell dimensions or crystallinity between the populations. Bulk analyses of 0.2 grams of both concentrates from the four sites, indicate that the poorly fluorescent zircons have generally higher average concentrations of Fe₂O₃ (5% vs 1.3%), TiO₂ (1.17% vs 0.09%), Hf (8068 ppm vs 7585 ppm), U (1319 ppm vs 785 ppm), P (938 ppm vs 131 ppm) and total REE + Y (3680 ppm vs 2355 ppm), including Dy (204 ppm vs 157 ppm) than the strongly fluorescent group, respectively, although some of the ranges overlap for samples collected at the same sites. Zr levels in the highly fluorescent samples are significantly greater (averaging 517,968 ppm vs 443,565 ppm).

Results appear to indicate an inverse relationship between higher concentrations of U, Hf, and REE + Y and fluorescence activation, and the possible effect of Fe-quenching needs to be considered. Additional work in progress, including elemental mapping of individual crystals may shed some light on the cause of the difference in fluorescence in these zircons.