1 PLATINUM-GROUP ELEMENTS AND GOLD IN Cu Ni MINERALIZED PERIDOTITE AT GABBRO AKAREM, EASTERN DESERT, EGYPT NADIA A. SHARARA Department of Geology, Faculty of Science, Assiut University, Assiut, 71516, Egypt GRAHAM C. WILSON § AND JOHN C. RUCKLIDGE Isotrace Laboratory at the University of Toronto, 60 St. George Street, Toronto, Ontario M5T 2T3, Canada § E-mail address: gcw@quartz.geology.utoronto.ca ABSTRACT The Akarem mafic ultramafic complex is located at 24°01'N, 34°08'E, 130 km east of Aswan, in the southern part of the Eastern Desert of Egypt. The late Proterozoic complex includes an earlier, mostly layered, gabbroic phase and a later peridotite phase. The latter was emplaced in two successive stages, with barren followed by mineralized (Cu Ni sulfide-bearing) peridotites. The gabbroic rocks are largely gabbronorite, olivine gabbronorite, troctolite and hornblende gabbro. Unmineralized peridotite is lherzolite, whereas the mineralized peridotite is dunite and harzburgite. The rocks are highly serpentinized. The estimated mode of the mineralized peridotite includes #35% olivine, 15% serpentine, #3% plagioclase, 16% clinopyroxene, 4% orthopyroxene, 6% amphibole, #15% pyrrhotite, #3% pentlandite and 3% chalcopyrite. The reserves in mineralized peridotite above 130 m depth has been inferred to be -700,000 tonnes. The primary sulfides are pyrrhotite, pentlandite, chalcopyrite and cubanite. These minerals exhibit disseminated, massive and net textures in the olivine-rich ultramafic cumulate host. Secondary minerals include violarite, pyrite, monoclinic pyrrhotite, magnetite, mackinawite and millerite. Analyses of bulk rocks and selected coarse sulfide grains were made by a combination of techniques, including instrumental neutron activation (INAA) and inductively coupled plasma mass spectrometry (ICP MS), electron-probe micro-analysis (EPMA) and accelerator mass spectrometry (AMS). S/Se ratios and * 34 S values of the sulfide-rich rocks, although not a definitive proof, are consistent with a magmatic S source with minimal crustal contamination. In situ Au levels measured by AMS are low: pyrrhotite contains 2 25 ppb, pentlandite 3 8 ppb and chalcopyrite 2 8 ppb. These values are much below bulk-rock levels (60 1080 ppb). Trace Pt (<2 ppb) is detected in the sulfides, also far below bulk levels (20 330 ppb). Coarse pyrrhotite