U-Pb geochronology, REE and trace element geochemistry of zircon from El Fereyid monzogranite, south Eastern Desert, Egypt

The monzogranite of El Fereyid is one of the rare metal-rich granites, where zircon is one of the ore minerals. Thus, studying zircon here is of vital importance. Most of the studied zircon grains are metamict and thus the loss of radiogenic Pb is detected for them. Nevertheless, our study allowed us to obtain the U-Pb (SHRIMP-II) age of magmatic crystallization for the El Fereyid monzogranite: 626 +/- 13 Ma. These data allowed the correction of earlier determined age (K-Ar system in biotite) for the El Fereyid massif. Zircon grains of El Fereyid monzogranite demonstrate heterogeneous structure in CL images, they are rich in rare earth elements (REE, concentration 3000-22500 ppm), trace elements (U: 2000-14800 ppm, Th: 300-2500 ppm), and has low Th/U ratios (average 0.18). Most zircon grains exhibit multiple internal oscillatory zoning in CL images, indicating a typical magmatic origin. The core of zircon grain has a dark tone on CL imaging and magmatic-type REE spectra. Zircon cores are enriched in REEs, U, Th, and Y, thus recording that the magma was rich in incompatible elements. Light CL rims of zircon grains are depleted, relative to the cores in most of the trace elements, except for P, Ca, and Ti. REE spectra in rims show similar patterns that are more often demonstrated by hydrothermal zircon grains. This explains their crystallization in the late-magmatic stage when magma was rich in fluids but depleted in most of the trace elements.

Automated summary

The El Fereyid monzogranite is a rare metal-rich granite containing zircon, highlighting the importance of studying zircon. Analysis of zircon in the monzogranite revealed multiple internal oscillatory zoning and enrichment in rare earth elements in the cores. The zircon rims showed depletion in most trace elements and similar REE spectra to hydrothermal zircon grains, indicating late-magmatic stage crystallization in a fluid-rich but trace element-depleted environment.