CHINESE JOURNAL OF GEOCHEMISTRY Vol. 26 No. 4 2007 DOI: 10.1007/s11631-007-0356-4 Geochemistry of lamprophyres associated with uranium mineralization, Southeastern Desert, Egypt Mohamed E. Ibrahim 1 , Mohamed M. El-Tokhi 2* , Gehad M. Saleh 1 , Mamdouh A. Hassan 1 , and Mohamed A. Rashed 1 1 Nuclear Materials Authority, P. O. Box 530 El Maadi, Cairo, Egypt 2 Geology Deprtment, Faculty of Science, United Arab Emirates University, United Arab Emirates * Corresponding author, E-mail: meltokhi@mans.edu.eg Received August 14, 2006; accepted September 10, 2006 Abstract Two brecciated shear zones (NNW-SSE) are found crosscutting cataclastic rocks. The cataclastic rocks (3.0 km 2 ) occupy the core of the granitic pluton and enclose a roof pendant of mafic-ultramafic rocks. The NNW-SSE-extending lamprophyre dykes vary in thickness from 0.5 m to 1 m and up to 800 m long, cutting the cataclastic rocks and are composed mainly of plagioclases, amphiboles, relics of pyroxenes and K-feldspar phenocrysts embedded in fine-grained groundmass. They are characterized as being peraluminous, calc-alkaline in composition (chemical trap) and enriched in calcite, sulfide and P 2 O 5 . The lamprophyres were affected by hydrothermal alteration (chlorite-carbonate alteration) while the cataclastic rocks were affected by diagenetic alteration (K-feldspar-albite alteration). Uranium mineralization is the product of hydrothermal events and has been investigated by X-ray diffraction (XRD) and environmental scanning electron microscopy (ESEM), involving primary uranium minerals (U 3 O 8 ) and secondary uranium minerals (uranophane and beta-uranophane, kasolite, torbernite, autonite and meta-autonite) in addition to U-bearing minerals (astrocyanite, betafite and fergusonite). The presence of different mineral parageneses associated with clay minerals indicates that the lamprophyres were subjected to acidic and alkaline mineralizing solutions. Moreover, the U-Zr/U, U-Ce/U values show negative correlations, confirming U-enrichment in both cataclastic rocks and shear zones while the Th-eU/eTh, Th-Zr/Th and Th-Ce/Th values show negative correlations, indicating that the U-bearing solutions are rich in Th in the cataclastic rocks only. Key words cataclastic rock; shear zone; lamprophyre; hydrothermal alteration; Southeastern Desert; Egypt 1 Introduction The Abu Rusheid area is one of the most important areas in the Southeastern Desert with respect to its uranium mineralization. Hassan (1964 and 1973), Sabet et al. (1976), El-Gemmizi (1984), Eid (1986), Saleh (1997) and Ibrahim et al. (2000) carried out investigations in this area and revealed some mineralization [e.g. tantalum-niobium, zircon, thorite and secondary uranium minerals (uranophane and beta-uranophane)]. In this paper, we give some light on the U-mineralization and geochemistry of lamprophyres in the Abu Rusheid area. 2 Geological setting and petrography The Abu Rusheid area is located about 45 km southwest of Marsa Alam Town, Southeastern Desert. Detailed geologic survey of the Abu Rusheid area (3.0 km 2 ) was conducted on a 20×20 m grid. The tectonostratigraphic sequence of the Precambrian rocks in the Abu Rusheid area (Fig. 1) is presented as follows: (1) Ophiolitic mélange, consisting of ultramafic rocks and layered metagabbros in metase- dimentary matrix; (2) cataclastic group, consisting of protomylonites, mylonites, ultramylonites and silicified ultramylonites; (3) mylonitic granites; and (4) post-granite dykes and veins (Ibrahim et al., 2004). The protomylonites are coarse- to very coarse- grained (>50 in vol.% porphyroclasts), green to dark greenish-grey in color. They outcrop on the eastward flank of West Abu Rusheid around Khour Abalea as elongated scattered bodies, accounting for about 12.0 in vol.% of the cataclastics. Mylonites cover a large area, amounting to 65.0 in vol.% of the cataclastic rocks with low to medium relief. Feldspathic arenites outcrop as small bands in alternation with mylonite www.gyig.ac.cn www.springerlink.com