Gaafar, et al., reported in 2014 that the present work aims at identifying Nb-Ta-, Zr-Hf-, REE-, Th-U-bearing two-mica granite from geological, geophysical cross-sections and mineral chemistry studies from three boreholes at G. El Sela shear zone. Microscopically, the three boreholes are composed mainly of two-mica granite. They are composed of K-feldspar, plagioclase, quartz, biotite and muscovite. Accessories are pyrite, zircon, fluorite, rutile, monazite with Th-U-mineralization identified by scanning electron microscope (SEM) and electron probe-microanalyses (EPMA). Chlorite, muscovite, sericite, kaolinite are secondary minerals. Geochemically, two-mica granite boreholes are A-type granites and peraluminous characteristics. They are enriched in large ion lithophile elements (LILE; Ba, Rb and Sr), high field strength elements (Y, Zr and Nb), and LREE but depleted in HREE with negative Eu anomaly.
U-enrichment associated with chloritization, muscovitization, albitization, sericitization, kaolinization and argillization results from convective hydrothermal circulation of fluids through brittle structures along the ENE-WSW main shear zone. The ratios Nb/Ta (7.7 - 17.7) and Zr/Hf (16.9 - 26.4) are relatively enriched in the lighter isovalents Ta and Hf. The accessory minerals observed in the two-mica granites are represented by metallic sulfides (pyrite, arsenopyrite, chalcopyrite, galena and sphalerite), Nb-rutile, Hf-zircon, fluorite, monazite, columbite, betafite, thorite, phosphothorite, uranothorite, brannerite, uraninite, coffinite and pitchblende at G. El Sela shear zone.
Uraninite with a low Th content indicates a hydrothermal origin of U-mineralization, Thorite, uranothorite, monazite and zircon is the main uranium bearing minerals of magmatic origin within the enclosing granite. The primary U-mineralization has been observed in two boreholes. In order to illustrate the geophysical signature of El Sela U-mineralization, the radiometric, magnetic, and VLF-EM data as well as radon concentration are included. The magnetic, electrical conductivity and radiometric profiles were produced from detailed ground surveys. The shear zone is characterized by relatively weak levels for both K and eTh, but very high eU anomalies (<3500 ppm),
Therefore, the Sela shear zone acts as a good trap for U-mineralization. The Sela Shear zone coincides with positive conductivity anomalies, which are the most prominent features on the respective profiles. The magnetic field over the Sela shear zone is also conspicuous by the sharp contrast which makes with the strong negative signatures of the altered microgranite. The radon distribution map showed the presence of seven high anomalies that are mostly controlled by the structures due to the easy movement of radon through them.
The authors concluded that primary hydrothermal U-mineralization was confirmed in drilled core samples. The continuation of the uranium mineralization at depth is evidenced by the Track Etch Survey along the El Sela main shear zone with radon emission from 960 to 300,300 Bq/m3 The γ-ray spectrometric profiles across the Sela shear zone indicate the strong increase of eU and, eU/eTh and eU/K ratios and a decrease of eTh and eK anomaly in relation with the enrichment of the shear zone in quartz. The strong direct increase of eU/eTh ratio is typical of hydrothermal uranium enrichment with no mobility of Th.
The VLF-EM profiles show marked high positive conductivity correlated with the shear zone within the granitic body, indicating higher fracturation and clay mineral content of the shear zone. The magnetic profile passing through W1 and W3 shows clearly an increase of total magnetic field above the El Sela shear zone associated with a magnetic low at its southern margin. To the opposite the magnetic profile passing through W2 showed only a strong magnetic low above the El Sela shear zone. Therefore, the association of spectrometric, radon, magnetic and electrical conductivity anomalies for the Sela shear zone clearly identifies the uranium anomalous zones which can be selected for more drilling.
The occurrence of U-oxides along the margin of the pyrite crystals may indicate that they have played a role on the reduction of U6+ from the uranyl ion ( 2 UO2+ ) dissolved in the hydrothermal fluids into U4+ which precipitates as, brannerite, coffinite and pitchblende. Pb tends to migrate outside the uranium mineral structure. In the present study Pb released from U-oxides commonly forms galena. The occurrence of brannerite indicates a first phase of U remobilization at relatively high temperature, brannerite being stable above 250ËšC - 300ËšC. The U-oxides are also locally altered to coffinite. Pitchblende occurs as botryoidal form or vein deposited on pyrite.
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