干旱区生态环境知识资源中心

Intensive investigation of the basement rock units of Abu Marawat-Semna area in the northern part of Central Eastern Desert of Egypt and their gold-bearing alteration zones using remote sensing techniques is accomplished. The study area consists of a Pan-African tectono-stratigraphic sequence, where allochthonous sheets and slices of ophiolitic serpentinite-talc-carbonate components were thrusted onto an island-arc metavolcanics. These rocks were later intruded by gabbro-diorite and biotite granite and invaded by a network of quartz veins, which are locally charged with gold. The visible near-infrared (VNIR) and short-wave infrared (SWIR) reflectance data of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imagery and Landsat 8 data were employed in the detailed geological mapping and detection of mineralized alteration zones. The results show that Landsat 8 band ratio image (b6/b2, b6/b7, and b6/b5xb4/b5) distinguishes the lithological units. Two different types of alteration zones (1 and 2) have been recognized in the study area using ASTER spectral characteristic analysis (SCA), band ratios, principle component analysis (PCA), Minimum Noise Fraction (MNF) techniques and ascertained by field verification, XRD, and XRF investigations. Type 1 (carbonate are the main phases) is mainly confined to the ophiolitic serpentinite-talc-carbonate rocks, whereas type 2 (quartz, sericite, and kaolinite-rich alteration zone are the main phases) is located within island-arc metavolcanics. The geochemical studies recorded anomalous contents of Cr and Ni in the alteration zone type 1 while Cr reaches 1,549 ppm and Ni abundance attains about 1,052 ppm. The investigated alteration zone type 2 in the metavolcanics recorded the presence of high anomalous contents of gold (up to 36 ppm), silver (up to 200 ppm), zinc (up to 3,992 ppm), and copper (up to 83,720 ppm). The geochemical study indicates that the detected alteration zones in the metavolcanics and the ophiolitic ultramafic rocks are promising and need more detailed exploration for Au and Ag mineralization. The study proves the potentiality of the alteration zones in and around the ophiolitic serpentinite-talc-carbonate rocks and their intimately associated island-arc metavolcanics. The image processing of the remote sensing data as proposed in the present study proves its high capability in detecting the mineralized alteration zones in other tectonically and geologically similar arid regions in the Arabian-Nubian Shield (ANS) and worldwide.