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2 2023

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Thorite inclusions in zircon of the monzogranite, Lower Nubia, SW Egypt

Hassan Kamaleldin M., Brunarska Irena

Mineralogia

2023

Vol. 54, iss. 1

69--77

This report presents the results of a petrographical and mineralogical (optical microscopy, BSE-EDS image analysis) study of zircon in samples from the Lower Nubian monzogranite. The mineral occurs as smaller grains (< 80 µm) in a finegrained quartz-feldspar-matrix, coexisting with other accessory minerals including biotite, clinochlore, titanite, and britholite-(Ce). Zircon is also present within biotite and britholite-(Ce), suggesting that it started crystallization at about the same time as these two minerals. Two types of thorite inclusions within zircon are reported in this study. Type 1 is widely distributed throughout the zircon grain, forming crystallites of which each is typically < 1 µm in size. Type 2 is relatively larger (5-15 µm) and occurs only in one part of the grain. For the thorite inclusions, three possible origins are briefly discussed: (1) exsolution of thorite from zircon; (2) dissolution/reprecipitation of zircon; and (3) growth syngenetically with zircon. Of these potential hypotheses, the syngenetic growth model seems more favorable for zircon-thorite intergrowths than the other two hypotheses. Thorite inclusions and their host zircons seem to have grown from magma rather than hydrothermal or supercritical solutions. Here, they have primary textures and consistent chemical compositions that are consistent with whole-rock geochemistry.

Britholite-(Ce) from the metaluminous granite of SW Egypt

Hassan Kamaleldin M.

Mineralogia

2023

Vol. 54, iss. 1

11--17

Optical microscopy and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) analyses were used to characterize britholite-(Ce) of the metaluminous granite (Proterozoic) intruded into the Nubian Formation in southwestern Egypt and having a relatively high content of radioactive elements (potassium = 4.4 wt%, thorium = 52 ppm, uranium = 10 ppm). The britholite-(Ce) studied here incorporates light rare earth elements (LREE) including lanthanum, cerium, praseodymium, and neodymium as well as thorium. The mineral forms as tabular crystals in a fine-grained quartz-K-feldspar-plagioclase matrix, coexisting with other accessory minerals including biotite, zircon, clinochlore, titanite, and magnetite. Primary britholite-(Ce) usually includes titanite, zircon, and magnetite and is occasionally included in biotite. The inclusion of britholite-(Ce) in biotite suggests that the two minerals crystallized approximately coevally. Petrographical and SEM-EDS data indicate breakdown of the primary magmatic britholite-(Ce) in the samples. This study constitutes the first report for a sole presence of britholite-(Ce) as LREE phase in metaluminous granite samples from Egypt.