Geological, Mineralogical and Geochemical Aspects of the Granitic Rocks at Umm Nafie Area, North Eastern Desert, Egypt

Abstract

Wadi Umm Nafie area is located between lat. 26˚18`-27˚05`N and long. 33˚23`and 33˚29`E. The rocks cropping out in the area are older granitoids and younger granites. The studied area displays secondary structures which are represented by joints and faults. The most predominant fault sets are trending in NE-SW, NNW-SSE and NW-SE directions. Petrographically, the older granitoids are classified as quartz diorites and the younger granites as syenogranites. The radioactivity of the syenogranites is significantly high, comparing with the older granitoids. The anomalous syenogranite (about 1x5m dimensions) exhibits equivalent uranium occurrences that vary considerably in their contents from 115.5 to 125 ppm with an average of 122.8 ppm and from 139.2 to 168.6 ppm equivalent thorium with an average of 155.4 ppm. Th-U, Zr-U, Zr-Th, , Nb-U, Nb -Th relationships show ill-defined trends, suggesting that radioelements distribution aren't controlled by accessory minerals but essentially related to the latter hydrothermal solutions. Anomalous syenogranite is affected by various phases of hydrothermal alteration processes along brittle structures, comprising hematitization, chloritization, epidotization, silicification and kaolinitization. Unusual REEs patterns and non-CHARAC ratios of isovalents confirm that the anomalous syenogranite is affected by late stage hydrothermal solutions resulting fluid-rock interaction and M-type tetrad effect. U and REEs could be leached from the sheared syenogranite at low pH conditions and precipitated in alkaline environments by hematitization process. The main minerals occur in the highly radioactive syenogranite are thorite, uranothorite, betafite, yttrocolumbite, samarskite, ishikawaite, polycrase and fergusonite, in addition to zircon, xenotime, allanite, cerite and monazite.