Geophysical Mapping, Geochemical Evidence and Mineralogy for Nuweibi Rare Metal Albite Granite, Eastern Desert, Egypt

Abstract

The present study aims to shed light on the rare metals of Nuweibiareaalbite granite in the Eastern Desert through the chemical analyses of the two types of fine-grained albite granite (FAG) and medium-grained albite granite (MAG) in addition to mineralogical studies as well as ground spectrometric survey and aeromagnetic mapping. On the basis of ground spectrometric measurements K, eUand eTh distribution maps were obtained. The concentration of K, U and Th content shows maxima (4.5%, 13 ppm and 27 ppm on average, respectively) in the FAG, and (4.5%, 10 ppm and 35 ppm on average) in the MAG. The eU/eTh ratio significantly increases in FAG with higher magma differentiation than MAG reaching 0.63. This paper uses magnetic geophysical methods to investigate geometry and sense of motion across the Nuweibi area. The interpreted structures from the magnetic maps are characterized by two main intersecting sets of NW-SE and NE-SW trending faults in addition to other three minor faults that trend in N-S, NNW-SSE and ENE-WSW directions. The NW-SE trending faults represent the recent sets in the study area where they are dissected and displaced by the other old faults. The Werner depth map shows the interface depths of the granite and basement rocks that extend to great depths ranging from 10 to 380 m. FAG is extended underneath most of the surrounding schist rocks because of their attributed low magnetic intensity that confirmed also with drilling. Microscope and Microprobe analyses indicated that the most important radioactive minerals include uranothorite, thorite, zircon, and monazite. Columbite group minerals represent the most common Nb-Ta host in Nuweibi-albite granites that contain significant levels of Ta (up to 65.4 wt. % Ta 2 O 5 ) and Nb (up to 60 wt. % Nb 2 O 5 ), with Ta/(Ta+Nb) ratio ranging from 0.17 to 0.84. Columbite group minerals are represented mostly by columbite-(Mn) and tantalite-(Mn), with Mn/(Mn+Fe) ratio ranging from 0.42 to 0.89. Ixiolite, wodgnite and tapiolite-(Mn) were found only in the FAG indicating the final stages of the evolution of parental granitic magma. The U-Th and U-K variation diagrams suggested that magmatic processes controlled the distribution of these elements. The Scanning Electron-microprobe analyses reveal variable compositions and extents between the MAG and FAG in the Nb, Ta-Ti, Sn-Fe, Mn triangular plot. It is worthy to be noted that because of the higher Ta/Nb ratio in the tapiolite-Mn and ixiolite of FAG in comparison with the coexisting Mn-columbite in the MAG, levels of HfO 2 greater than 15% and even attaining 23%, characterized the hafnium zircon in the Nwueibialbite-enriched facies. There is a close correlation between Hf/(Hf + Zr) and Ta/(Nb + Ta) which seems mainly associated with the FAG.