2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 3
Presentation Time: 1:30 PM


AHMED, Ahmed Hassan and ARAI, Shoji, Department of Earth Sciences, Kanazawa University, Kakuma 920-1192, Kanazawa, Japan, Kanazawa, 920-1192, Japan, ahmed@earth.s.kanazawa-u.ac.jp

The PGE in ophiolites are mainly concentrated as discrete mineral inclusions in chromian spinel of chromitite (Os-Ir-Ru phases) or as sulphide-bearing mafic-ultramafic rocks (Rh-Pt-Pd phases). Some ophiolites show (Os-Ir-Ru)-dominant PGE pattern, while others show enrichment in both (Os-Ir-Ru)- and (Rh-Pt-Pd)-PGE patterns.

The mantle rocks (harzburgite-dunite-chromitite) of the Pan-African ophiolite complexes of Egypt are severely serpentinized, and partly being enriched in both IPGE (Os, Ir, Ru) and PPGE (Rh, Pt, Pd). We obtain the highest PGE content (3197 ppb) from a chromitite pod in the central Eastern Desert of Egypt, which has (Rh+Pt+Pd)=604 ppb, (Os+Ir+Ru)=2593 ppb, and a gentle negative slope of PGE pattern. Dunite envelopes around such PGE-rich chromitite pods are also high-PGE, up to 2258 ppb total PGE, which has (Rh+Pt+Pd)=1781 ppb, (Os+Ir+Ru)=477 ppb, and a positive slope of PGE pattern. Many other chromitite pods in both central and southern parts of Eastern Desert are intermediate to low-PGE with normal ophiolitic negative slope of PGE patterns.

The mantle section of the Oman ophiolite, on the other hand, comprises harzburgite with small amounts of dunite and chromitite. PGE-rich chromitites only found in the deep part of the mantle section at Wadi Hilti and Wadi Rajmi, northern Oman, contains up to 1476 ppb of PGE, being highly enriched in IPGE (Os+Ir+Ru=1450 ppb) and extremely depleted in PPGE (Pt+Pd=6 ppb). Other types of chromitite in the northern part of the Oman ophiolite, the mantle PGE-poor and Moho transition zone (MTZ) chromitites, are intermediate to low-PGE. The mantle PGE-poor chromitites show a very gentle negative slope of PGE pattern, while the MTZ chromitites show much more fractionated patterns than the mantle chromitites. The chromitite pods in the southern part of the Oman ophiolite show intermediate to low PGE contents (up to 200 ppb) with a convex-upward shape from Os to Pd. The mantle harzburgites in both ophiolites show very low PGE contents with approximately unfractionated PGE patterns.

The differences in PGE concentrations and distribution patterns from place to place in both the Proterozoic ophiolite complex, Egypt, and Phanerozoic ophiolite, Oman, may reflect the diversity of tectonic setting and mantle processes involved in the formation of podiform chromitites.