Platinum-group elements and gold in Cu-Ni-mineralized peridotite at Gabbro Akarem, Eastern Desert, Egypt
Sharara, NA; Wilson, GC; Rucklidge, JC
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 have 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 δ34S 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 carries more Rh (20-120 ppb, mean 60 ppb) than bulk rock (mean 20 ppb) and other major sulfides. Ir is similar; other sulfides and bulk samples carry much less (a few ppb) than coarse pyrrhotite (30-100 ppb, mean 60 ppb). Coarse granular pentlandite has the highest Pd contents, up to 1030 ppb. Mass-balance calculations suggest that most Au and Pt probably form discrete grains of native Au and PGM. In contrast, most of the Pd, Rh and Ir is present within Fe-Ni-Cu sulfides, probably incorporated in the lattice of their host(s). PGE distributions can be explained by fractional crystallization of mss from parental sulfide liquid.