Diversity of platinum-group minerals in podiform chromitites of the late Proterozoic ophiolite, Eastern Desert, Egypt: Genetic implications

Podiform chromitites are frequently distributed as lensoidal pods in the central and southern parts of the Eastern Desert, Egypt. They are, in most cases, hosted by fully serpentinized peridotite which is a part of dismembered ophiolite complexes of the Pan-African belt of Late Precambrian age. Serpentinites are the predominant components in the ophiolitic melange, either as matrix or as variably sized blocks, and are derived from harzburgite and subordinate dunite. The central Eastern Desert (CED) chromitites have a wide cornpositional range from high-Cr to high-Al varieties, whereas those of the southern Eastern Desert (SED) have a very restricted compositional range. The Cr# of spinel ranges frorn 0.5 up to 0.8 in the former, while it is around 0.8 in the latter. Platinum-group element (PGE) mineralization has been recently reported in podiform chromitites from the late Proterozoic Pan-African ophiolite of the Eastern Desert of Egypt. The populations of platinum-group minerals (PGM) in the studied CED and SED chromitites are quite distinguishable; they are mainly sulfides (Os-rich laurite) in the former, and Os-Ir alloy in the latter. Sulfarsenides and arsenides are also found in subordinate amounts from both chromitites. The most abundant base metal sulfides (BMS) in the Eastern Desert chromitites of Egypt are millerite, heazlewoodite, pentlandite, chalcopyrite and pyrite. The sulfur fugacity and temperature conditions are the main controller of PGE mineralogy in the host chromitite at the initial stage within the upper mantle. Os-rich laurite is stable at high sulfur fugacity and low temperature conditions, whereas Os-Ir alloy is stable at lower sulfur fugacity and higher temperature conditions. The diversity of PGE mineralogy combined with the differences in petrological characteristics of chromian spinels from CED chromitites to SED ones suggests different degrees of partial melting of the mantle rocks of this ophiolite which, in turn may be attributed to different tectonic settings. During the post-magmatic processes, i.e., serpentinization, the primary PGM, e.g., Os-rich laurite and Os-Ir alloy, can be modified at low temperatures to secondary PGM. (0 2007 Elsevier B.V All rights reserved.