Abstract

Fe-Ti oxides and Co-Ni-Cu sulfides hosted by younger gabbroic rocks occurring in Imliq, El-Khamila, Wadi Tweiba, Nakhil and Rahaba localities in south Sinai, consist ilmenite, magnetite and titanomagnetite and pyrite, chalcopyrite, pyrrhotite and pentlandite. They often show fine intergrowths, exsolutions or composite grains. Pyrrhotite is replaced by pentlandite or arranged along the amphibole cleavages. The sulfides occur as monomineralic grains included in silicates or attached to the grain boundaries of Fe-Ti oxides. A few sulfide grains are included in or sandwiched by Fe-Ti oxides. The Fe-Ti oxides and sulfides are located in the intercumulus spaces of the host rocks. Exsolution lamellae of magnetite in titanomagnetite or granular exsolution of ilmenite around grains of magnetite were developed by oxidation during cooling of the intrusions. The formation of ilmenite trellis and lamellae in magnetite and titanomagnetite indicates again an oxidation process due to excess of oxygen contained in titanomagnetite; trapped and external oxidizing agents. This indicates the highPH₂O and oxygen fugacity of the parental magma, as suggested by modal increasing of hornblende and biotite and the presence of hypersthene and pigeonite. Addition of crustal sulfur is required to explain the present values of the mineralized gabbros. Sulfide mineralization in present rocks is believed to have resulted from the emplacement of sulfide-saturated tholeiitic and calc-alkaline basaltic magmas that had experienced varying degrees of silicate and sulfide fractionation. These sulfides can be interpreted to have formed by accumulation of immiscible magmatic sulfide droplets. The tholeiitic gabbros dominated in El-Khamila, Wadi Tweiba and Nakhil are characterized by Ni-rich pyrite and chalcopyrite as compared with that of the calc-alkaline gabbros occurring in Imliq and Wadi Nakhil (Ni-poor pyrite and chalcopyrite). A multistage model of ore genesis, involving two stages of crustal contamination, two magma chambers, and multiple pulses of geochemically distinct magma, is entirely consistent with the geochemical data from south Sinai younger gabbros. The Fe-Ti oxides have been formed under temperature of ~ 800°C for ilmenite, and ~ 600°C for magnetite, while the sulfide assemblage is crystallized below 600°C, with final re-equilibration temperature above 140°C.

Key words: Younger gabbros, Fe-Ti oxides, sulfides, microprobe, P-T conditions, south Sinai, Egypt.