Gabbro Akarem, Eastern Desert, Egypt: Cu-Ni-PGE mineralization in a concentrically zoned mafic-ultramafic complex

The Gabbro Akarem (Late Precambrian) intrusion is concentrically zoned with a dunite core surrounded by Iherzolite-clinopyrexenite enveloped by olivin e-plagioclase hornblendite and plagioclase hornblendite. Cu-Ni-PGE minerali zation is closely associated with peridotite, especially in the inner, oliv ine-rich core (dunite pipes) where net-textured and massive sulfides (pyrrh otite, pentlandite, chalcopyrite) are found in association with Al-Mg-rich spinel and Cr-magnetite. Primary magmatic textures are well preserved; howe ver, deformation and mobilization due to shearing are locally observed. Pla tinum-group minerals (PGM) documented from the deposit are: merenskyite (Pd Te2) and michenerite (PdTeBi), as well as palladian bismuthian melonite (Ni ,Pd) (Te,Bi)(2). These minerals occur in intimate association with hessite (Ag2Te) and electrum (Au0.65Ag0.31Bi0.04) in two distinct textural position s: (1) as inclusions in pyrrhotite, pentlandite, and rarely chalcopyrite an d (2) at sulfide-silicate grain boundaries and on microfractures in base-me tal sulfides (BMS) and olivine associated with serpentine and secondary mag netite. Textural features suggest that PGM were exsolved from monosulfide s olid solution over a wide range of temperatures. Late-stage, low-temperatur e hydrothermal solutions led to redistribution of PGE. Mineralized samples show Ni/Cu ratios ranging from 0.2 to 2 with an average of 1.0. The (Pt + P d + Rh)/(Os + Ir + Ru) ratio is generally >6 in most samples, and Os, Ru, a nd Ir are below the detection limit (2 ppb). The PGE contents show positive correlation with S only at low sulfur contents. The PGE patterns of Gabbro Akarem are similar to those of Alaskan-type deposits. Compared with strati form deposits, Gabbro Akarem is depleted in PGE. The consistently low PGE c ontents of the mineralization and their uniform distribution in the ultrama fic rocks despite the high sulfur content of the rock is attributed to rapi d crystallization of sulfides in a highly dynamic environment.