Abstract

Ocellar lamprophyre dyke (ENE-WSW) is recorded at Wadi Nugrus, Eastern Desert, Egypt. It cuts across porphyritic biotite granites and varies in thickness from 0.5 to 1.5 m and up to 3 km in length. The lamprophyre dyke is altered and it is characterized by porphyritic and panidiomorphic textures with plagioclase, olivine and augite, constituting the porphyritic phase in a fine groundmass of the same composition. Rutile, titanite, apatite, fluorite, graphite, calcite, allanite, autunite and Fe-Ti oxides are accessory minerals, while kaolinite, chlorite and epidote are secondary minerals. However, carbonitization and hematitization are common. Rounded to sub-round porphyritic and zoned ocelli with radiate or brush-like shapes are generally common and they represent physical traps for mineralization. The ocellar features are interpreted to represent the late stage of magmatic segregation or magmatic crystallization involving two immiscible magmatic liquids. Mineralogical results of altered lamprophyre samples, based on X-ray diffraction (XRD) and environmental scanning electron microscopy (ESEM), indicate the presence of secondary uranium minerals (autunite and uranophane), associated with gold, silver, nickel, atacamite, molybdenum, pyrite and zincite; moreover, the presence of kaolinite indicates a high temperature environment. The mixing of volatile fluids with meteoric water and fluid-wall rock interaction result in changes of pH and oxygen activity and deposition of base metals in reducing environs (graphite and pyrite). Precipitation of hematite probably decreased the pH of the solution and rising acidic fluids. The sudden change in pH and temperature of the fluids leads to destabilization of base metal complexes favoring their deposition. The magma source of the Nugrus lamprophyre is determined to be between calc-alkaline and alkaline and it has HKCAB and shoshonitic nature, which may be derived from decomposition melting of such a metasomatised lithospheric mantle. Olivine is a major fractional phase to account for the rapid decrease in Ni and Fe₂O₃ and increase in SiO₂. Also, clinopyroxene fractionation is important in accounting for the positive correlations of CaO and Cr versus MgO, whereas plagioclase differentiation is insignificant according to the negative correlations between MgO and Al₂O₃ and Sr. Crystal fractionation of accessory minerals such as apatite and Fe-Ti oxides are insignificant because of the less variable P₂O₅ and TiO₂ contents.

Key words: Lamprophyre, calc-alkaline, ocelli, geochemistry, mineralization.