ABSTRACT

Rare metals mineralization in Jabal Gattar, N. E. Egypt, occurs at the sheared tectonic contact between a younger granite and Hammamat sediments. The contact is affected by a local reverse fault directed N75°E and dipping 45°SSE. The Hammamat sediments consist substantially of rhythmic siltstones and immature greywackes. The paragenetic history of the mineralized sediments involves many types of alteration, such as dissolution of primary quartz (episyenitization), sericitization, albitization, hematization, pyritization, silicification, kaolinization and bleaching. The calculation of the mass-balance indicates preferential mobilization of REE, Rb, Cs, Be and S from Gattar granite to Hammamat sediments in the alteration zone. The elements; U, Y, W, Nb, Cu, Pb, Sb and Zn are enriched in both granite and sediments, but with higher magnitude for the latter. The hematized sediments are generally better accumulators for rare metals. The Gattar granite is classified as a meta-aluminous leucogranite, containing low Ca and high alkali. The low Ba and Sr and the high REE + Y reflect a highly fractionated (low P) A-type granite. The mineralized granite is affected by several metasomatic alterations. The mineralization causes enrichment in the content of U, Nb, W, Y and the chalcophile elements. The REE content displays portioning towards preferential mobilization of the LREE to the adjacent mineralized Hammamat sediments while higher quotient of the HREE is adopted within the secondary fluorite. The volume loss of granite due to leaching reactions (e.g., episyenitization) causes apparent enrichment of the immobile elements such as Sc, Th and Zr. The mineralization was caused by alkaline and oxidizing hot fluids, with contribution of meteoric water volume heated by convection. The hot fluids flowed up along ENE-WSW and NE-SW faults to cause a rather complicated series of metasomatic reactions. The rare metals were mobilized from the younger granite pluton of Gattar and concentrated in the sheared tectonic contact. The reaction of the mineralizing solutions with the wall rocks and the pseudomorphic oxidation of pyrite caused reduction and fixation of uranium. The study area can be considered as a possible potential deposit for U, REE + Y and other rare metals as well.