The chromite deposits associated with ophiolite complexes, Southeastern Desert, Egypt: Petrological and geochemical characteristics and mineralization

Abstract

The podiform chromitites occur in a well-preserved mantle sequence consisting of lherzolite-harzburgite with abundant lenses of olivine dunite. The podiform chromitite deposits are common as small and irregularly shaped masses in the Southeastern Desert (SED) of Egypt. The podiform chromitites exhibit a wide range of compositions from high Cr to high Al varieties. The Cr of chrome spinel ranges from 0.67 to 0.88 in olivine-dunite, quite similar to that of the high-Cr chromitite, whereas it is around 0.62 in lherzolite-harzburgite. Primary hydrous mineral inclusions, amphibole and phlogopite, in chrome spinel have been reported for the first time from the Pan-African Proterozoic podiform chromitites. On the other hand, petrographic and geochemical evidence suggests that podiform chromitites in the SED of Egypt were formed as a result of crystallization of mafic melts, probably of boninitic composition, the boninitic parental magmas were probably produced by a second stage of melting above a subduction zone. Three types of chromite ores can be distinguished within the SED of Egypt: (a) sulphide-poor podiform ores; (b) brecciated ores; and (c) sulphide-rich ores. Two textural types of inclusions in chromite are distinguished: (1) primary silicate inclusions generally have high Mg-number (>96), Cr and Ni, and are dominated by pargasitic amphibole, forsterite, diopside, enstatite and Na-phlogopite. A diversity of primary and secondary platinum group minerals (PGM) is described from the chromitites, including alloys, sulphides, sulpharsenides and arsenides of Ru, Os, Ir, Rh, Ni, Cu, Fe and Co; (2) in addition to primary PGM and hydrous silicates, the fluids are of low to moderate salinity, sodium-dominated aqueous solutions with complex gas contents. Variable amounts of water, hydrogen, hydrocarbons, carbon dioxides and nitrogen have been determined in inclusion-rich samples. The chondritenormalized PGE patterns of lherzolite-harzburgite and olivine-dunite have negative Ir and Pt, and positive Pd and Au anomalies. Chromitites are homo-geneous in composition but texturally zoned on a large scale. They carry elevated IPGE, manifested in numerous, primary and secondary PGM phases.

Multiple Stages of Carbonation and Element Redistribution during Formation of Ultramafic-Hosted Magnesite in Neoproterozoic Ophiolites of the Arabian-Nubian Shield, Egypt

“…9c, 9d). Boninites, of course, are characteristic of certain stages in the evolution of the forearc of intraoceanic arcs (e.g., Murton 1989;Johnson and Fryer 1990;Bédard 1999;Beccaluva et al 2004), and boninitic affinities have been used to support a forearc setting in a number of other Egyptian ophiolite sequences (e.g., El Sayed et al 1999;Abdel Aal et al 2003;Saleh 2006).…”

Petrological studies of neoproterozoic serpentinized ultramafics of the nubian shield: spinel compositions as evidence of the tectonic evolution of egyptian ophiolites

“…Boninitic affinities of some Egyptian ophiolites have recently been recognized by some authors (e.g. El Sayed et al 1999;Abdel Aal et al 2003;Saleh 2006), and these authors inferred a back-arc or an inter-arc basin origin based on the chemical compositions of the ophiolitic rocks. However, this interpretation conflicts with the observation that most boninites are found in the fore-arcs of intraoceanic arcs (e.g.…”

“…Also, the boninitic affinities of some Eastern Desert ophiolitic rocks support a fore-arc setting (e.g. El Sayed et al 1999;Abdel Aal et al 2003;Saleh 2006) On the Cr# vs. Mg# plot, the fresh chromian spinels show a negative trend (Text- fig. 6d), reflecting a partial melting trend from harzburgite to dunite.…”

Chromian Spinels from Kazanian-Stage Placers in the Southern Pre-Urals, Bashkiria, Russia: Morphological and Chemical Features and Evidence for Provenance

“…Chromian spinels of ophiolitic complexes show widely varied Cr 2 O 3 and Al 2 O 3 contents, due to an almost total Cr 3+ ↔Al 3+ isomorphic substitution, low Fe 3+ and TiO 2 contents and a broadly ranged Mg content (Mg# = 0.4-0.8) [60,61]. Ophiolites include chromite, alumochromite, chrompicotite and picotite mainly, while chromitites from ophiolites commonly contain alumochromite and chromite, i.e., spinels richest in Cr [38,64,65]. Mid-ocean ridge basalts (MORB) and peridotites show a narrower range of Cr 2 O 3 and Al 2 O 3 , while the Al 2 O 3 and Fe 3+ content is higher compared to ophiolites [59,60].…”