Spectrometric and remote sensing investigations of some granitic rocks in the Egyptian north Eastern Desert: Insights on environmental radiogenic heat production

Abstract

Granitic rocks dominate the Neoproterozoic outcrops in the northern Egyptian Eastern Desert, prominently featuring two main categories: Arc Granitoids (AG) and late-to post-collision granites (LPCG). The AG range from granodiorite and tonalite to quartz-diorite. In contrast, LPCG comprise syenogranite, monzogranite, and alkali-feldspar granite. This study leverages Landsat-8 remote sensing data to effectively discriminate between these rock types using several advanced image processing techniques. False color composite and decorrelation stretch methods highlighted geological and structural features, revealing distinct spectral signatures for each rock type. Principal Component Analysis and band rationing further refined distinguishing various varieties within the LPCG and detailed mapping. Supervised classification using the Support Vector Machine method yielded precise delineation of rock units. The investigated granitic rocks exhibited estimated radiogenic heat production values ranging from 6.02 to 1.41 μW/m³, surpassing the average values observed in the Earth's crust. The reason behind these noteworthy surpassing values of radiogenic heat production can be directly attributed to the relatively high Gamma-ray measurements in the LPCG outcrops. Gamma-ray spectrometric analysis indicated varying distributions of radioelements, particularly between AG and LPCG. The equivalent uranium (eU) concentrations range from 2.8 to 7 ppm in AG, while LPCG exhibited broader variability from 5.1 to 34 ppm. The equivalent thorium (eTh) values range from 16.1 to 34.1 ppm, with an overall average of 23 ppm. Conversely, within the ferruginated-silicified domains, the LPCG display slightly elevated levels of eU, reaching 31.4, 35.5, and 27.8 ppm for the monzogranites, syenogranites, and alkali-feldspar granites, respectively. These elevated levels suggest the potential for iron oxy-hydroxide minerals to adsorb uranium within alteration zones. Additionally, radioactive minerals such as zircon, columbite, uranothorite, allanite, euxenite, and samarskite contribute to the observed spot anomalies.