The first occurrence of platinum group minerals (PGM) in a chromite deposit in the Eastern Desert, Egypt

El-Haddad, Mervat

doi:10.1007/s001260050051

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“…The Merensky Reef for most of its strike length overlies an anorthosite and it comprises a lower chromitite layer (0.7-1 cm in thickness), a coarse-grained melanorite (CGM; ~10 cm in thickness), an upper chromitite (~1 cm in thickness) and an overlying melanorite ( Fig 1c); (Ballhaus and Sylvester 2000; Barnes and Maier 2002;Cawthorn and Boerst 2006;Eales and Reynolds 1986;Elhaddad 1996;Maier and Bowen 1996;Naldrett, et al 1986;Naldrett, et al 2009b;Nicholson and Mathez 1991). This study is a detailed comparative study of the lower and upper chromitite layers.…”

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Morphology and microstructure of chromite crystals in chromitites from the Merensky Reef (Bushveld Complex, South Africa)

Vukmanovic

Barnes

Reddy

et al. 2012

Contrib Mineral Petrol

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The Merensky Reef of the Bushveld Complex consists of a lower chromitite layer, a coarsegrained melanorite and upper chromitite layer. Detailed microstructural analysis of chromitite layers using electron backscatter diffraction analysis (EBSD), high-resolution X-ray microtomography and crystal size distribution analyses distinguished two populations of chromite crystals: fine grained idiomorphic and large silicate inclusion bearing crystals. The lower chromitite layer contains both populations, whereas the upper contains only fine idiomorphic grains. Electron backscatter diffraction data shows absence of crystallographic preferred orientation and shape preferred orientation in both layers. Most of the inclusion-bearing chromites have characteristic amoeboidal shapes that have been previously explained as product of sintering of pre-exisiting smaller idiomorphic crystals. Here, two possible scenarios are proposed to explain the sintering process in chromite crystals: 1) amalgamation of a cluster of grains with the same original crystallographic orientation; and 2) sintering of randomly orientated crystals followed by annealing. The EBSD data show no evidence for earlier presence of idiomorphic subgrains spatially related to inclusions, nor for clusters of similarly oriented grains among the idiomorphic population, and therefore argue against a sintering model. An alternative is proposed whereby silicate inclusions are incorporated during maturation and recrystallisation of initially dendritic chromite crystals. Electron backscatter diffraction analysis maps show deformation-related misorientations and curved subgrain boundaries within the large, amoeboidal crystals, and absence of such features in the fine grained population. The deformation record is highly dependent on the size and the shape of the crystals. Microstructures observed in the lower chromitite layer are interpreted as the result of deformation during compaction of the orthocumulate layers, and constitute evidence for the formation of the amoeboid morphologies at an early stage during consolidation.

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