THE GENINA GHARBIA CU-NI-PGE MINERALIZATION, EASTERN DESERT, EGYPT: AN EXTENSIVE LATE-MAGMATIC HISTORY

The Genina Gharbia intrusion is a small late Precambrian mafic-ultramafic complex in the Eastern Desert of Egypt. It comprises hornblende harzburgite, hornblende-bearing peridotite and hornblende pyroxenite and gabbros. The intrusion is not metamorphosed but highly affected by faulting and shearing. High modal content of hydrous phases is characteristic. The Cu-Ni ore forms either disseminations in hornblende harzburgite or massive batches in hornblende gabbro and consists of pyrrhotite>> pentlandite°Ý chalcopyrite>> pyrite°Ý violarite°Ýcubanite and minor cobaltite-gersdorfite, niccolite, sphalerite, molybdenite and valleriite. Intense alteration commonly associates sulfides in gabbroic rocks where the silicate assemblage is dominated by actinolite, chlorite, epidote, albite and quartz.

Platinum group minerals (PGM) are only Pd-bismuthotellurides; i.e., michenerite, merenskyite and Pd-Bi-melonite. PGM are usually associated with hessite, altaite, tsumoite, sylvanite and native-Te. Ninety percent of PGM and tellurides grains are located at sulfide-silicate contacts and as inclusions in altered silicates. The total Cu + Ni locally reaches up to 1.5 wt%, with Cu/Ni ratios <1. Anomalous platinum group elements (PGE) concentrations (maximum 260 ppb Pd, 65 ppb Pt, 9 ppb Rh, 38 ppb Ir, 10 ppb Ru, 7 ppb Os) were detected in sulfide bearing samples. The Pd/Pt ratio increases from the hornblende harzburgite to hornblende gabbro.

The mineralogical and chemical characteristics of the Genina Gharbia mineralization are best explained by a three-stage process: (1) a stage of magmatic crystallization in which, metals were concentrated in a sulfide melt largely in the hornblende harzburgite, (2) a late-magmatic stage in which base and precious metals were concentrated in a volatile-rich fluid, and (3) a post-magmatic faulting and shearing stage which shortly remobilized metals and concentrated them along shear zones. In stage (1) PGE were hosted in base metal sulfides, mainly pentlandite and cobaltite-gersdorfite. The availability of semimetals (Te, Bi) in the late-magmatic fluid deposited PGM in stage (2).