Mineral chemistry of monazite from the black sand deposits, northern Sinai, Egypt: a provenance perspective

摘要

Electron microprobe analysis (EMPA) and microscopic investigations were performed on monazite grains from the black sand deposits of northern Sinai beach. Electron microprobe backscattered electron images, X-ray mapping and wavelength dispersive spectroscopy line scans showed some grains with sector zoning and others with thorite inclusions. Based on the EMPA data, the studied monazite is grouped into monazite-(Ce) and Th-rich monazite. Monazite-(Ce) is enriched in REE and P, whereas Th-rich monazite is enriched in Th, U, Ca, Y, Si and Fe. The compositional variations of monazite are governed by the substitution of REE by Th, U, Ca and Y. The monazite grains show enriched chondrite-normalized REE patterns represented mainly by LREE and only Gd from the HREE. These patterns demonstrate negative Nd and Eu anomalies. The floods associated with the pluvial periods which prevailed in Egypt during the Pleistocene, were able to erode the source rocks and liberate heavy minerals including monazite. The mineral grains were transported through several wadis and tributaries to the main channel of the River Nile. At the confluence sites, these heavy minerals were mixed with Ethiopian and central African heavy mineral assemblages. The grains continued to move together downriver until being deposited in their current locations. The analytical results suggest that pegmatites and granites of the Eastern Desert are the most likely source of the monazite. However, due to the large area of the Nile watershed, other undiscovered sources are possible.