The northern periphery of G. Gattar granites is mineralogically studied. The study area exhibits significant potential for radioactive occurrences especially in the strongly altered granites at the intersection between the basic dykes striking N30 0 E and a sinestral strike-slip fault trending N-S. These occurrences are mostly of epigenetic origin. They are closely associated with the hydrothermal alterations; hematitization, kaolinitization and episyenitization causing marked enrichment of some elements notably, Zr, Y, Zn, Ba and REEs. The uranium occurrences vary considerably in their contents from 68 ppm to 4800 ppm, whereas the thorium contents reach up to 600 ppm especially in the desilicified granites. These radioactive occurrences may be due to the existence of some primary and secondary radioactive minerals. The primary minerals comprising uraninite and thorianite whereas the secondary ones including altered aggregated fragments of coffinite, becquerelite and gummite which were probably formed in the early stages due to uraninite dissolution. Furthermore, uranophane, kasolite, weeksite and autunite are the most conspicuous supergene uranium minerals which are mostly formed as a result of the extensive alterations of the pre-existing primary ones. The latter four minerals display mutual genetic association, hence, it is suggested that the heterogeneous hydrothermal solutions may play a crucial role in formation of the observed secondary U-minerals association in the northern part of Gattar granites pluton.
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.
The present work aims to study the mineralogy as well as the radioactivity of the acidic Dokhan volcanic in Gabal Nuqara area to identify the minerals that responsible for the radioactivity. The Dokhan volcanic of Gabal Nuqara, Central Eastern Desert are related to fissure-type eruption and are subdivided into intermediate and acidic volcanics. The acidic Dokhan volcanics rocks are located as two elongated flows of rhyolite, dacite and their tuffs, trending NW-SE direction and cut by younger granites and younger gabbros. Radiomatically, tuffs and dacite rocks show low radioactivity, while the concerned rhyolite exhibits relatively high eTh and eU contents (140 ppm and 89 ppm in average respectively). Moreover, an anomalous content was recorded in rhyolite rocks where eTh and eU contents are 745 ppm and 280 ppm in average, respectively. The detailed mineralogical studies, using ESEM and XRD techniques, indicate the presence of some primary radioactive minerals that are responsible for the radioactivity of the Nuqara rhyolites (e.g. thorite and uranothorite). Furthermore, samarskite, zircon and allanite occur as uraniferous accessory minerals. The mineral chemistry by EPMA analysis of thorite reflects the main following components; ThO 2 (38 %), SiO 2 (33 %) and UO 2 (6 %), while the main constituents of uranothorite are ThO 2 (40 %), SiO 2 (20 %), and UO 2 (12 %). Beside, the studied zircon crystals display a large chemical variability between core and rim. Zircon and thorite from Nuqara rhyolite are demonstrably magmatic origin, can be attributed concentrated in the rhyolite extrusive rocks in the late stage of the volcanoes in the study area.
The studied granitic rocks are mainly distinguished into granodiorites and syenogranites based on their field relationship and petrography. The syenogranites are unconformably overlain by Nubian sandstones from the west. They exhibit an extensive alteration nearby the jasperoid veins such as ferrugination, sericitization, kaolinization, silicification and black manganese dendritic staining. The mineralized reddish brown jasperoid veins intrude the syenogranites and are mostly observed at the highly fractured parts. Some of these jasperoid veins stained with black and reddish brown Mn and Fe oxides associated with bright yellow color of secondary uranium minerals. The radiometric measurements show that the granodiorites and syenogranites exhibits moderate uranium and thorium contents. They display 6 ppm eU and 15 ppm eTh in average for the first and 19 ppm eU and 30 ppm eTh for the later. Unlike, the jasperoid veins attains highest values (eU=250 ppm, eTh=72 ppm) in average. The enrichment of U in the jasperoid veins is probably because of mobilization of U from the syenogranite. Detailed microscopic examinations, X-ray diffraction (XRD) and Environmental Scanning Electron Microscope (ESEM) with EDX microanalysis revealed that the presence of secondary uranium minerals as carnotite and uranophane as well as U-bearing accessory minerals such as betafite, samarskite, zircon and violet fluorite are responsible for radioactivity.
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