Diagenetic and metamorphic history of the Umm Nar BIF, Eastern Desert, Egypt

Abstract

Abstract. The Umm Nar BIF was formed in a sedimentary environment. It is confined to an upper stratigraphic zone of pre-Pan-African metamorphosed shelf deposits. During the Pan-African deformational history, the BIF and the host metasediments were tectonically overlain by ophiolitic m61ange succession. The metasediments and the m61ange were subjected to a major folding phase and then thrust over the "Shaitian" sheared granite, prior to the intrusion of syn-to late-orogenic granitoids. The BIF is divisible into two main types: oxide-bands including magnetite and hematite, and oxide-silicate bands including magnetite, hematite and stilpnomelane. The associated gangues are quartz, calcite, epidote, garnet, plagioclase, graphite and muscovite. Rhythmic banding and lamination, cross-lamination and flaser structure are the most prominent primary features in the IF bands. The iron minerals and the associated gangue show a variety of textural aspects and microscopic interrelationships which indicate successive episodes of mineral accumulation and formation, involving deposition, recrystallization, blastic growths, overgrowths, replacement and deformations, during continuous burial and subsequent tectonic deformations.Banded iron ores (BIF) are known to occur at several localities in the Central Eastern Desert, Egypt. It is generally accepted that BIFs are intimately associated with geosynclinal metavolcanics and metasediments ( = island arc volcanics and volcaniclastics), and pertain to one genetic type. The BIFs in Egypt are assumed to be of Late Proterozoic (Pan-African) age and considered either as (a) chemical marine sediments in a geosynclinal basin (e.g. E1 Shazly 1957; Rasmy 1968) or as (b) subaqueous volcanogenic deposits in an island arc environment (e.g. Ivanov et al. 1973;Garson and Shalaby 1976;Sims and James 1984; E1 Gaby et al. 1988 and El Sharkawi 1990). None of these models provides a comprehensive and convincing view on the sequence of crystallization of the iron minerals and the factors that controlled their formation.The study of the BIF at Umm Nar (Fig. 1) revealed that it differs in many aspects from the other Egyptian BIF occurrences described from other localities. The present work summarizes the results of megascopic and microscopic investigations carried out on the Umm Nar BIF, Eastern Desert, Egypt (Fig. 1). Attention was concentrated on the paragenesis of the BIF intervals, morphology of the iron minerals and associated gangues, and modifications resulting from metamorphism and deformation. The chronologic sequence of the mineral constituents and their modes of formation are discussed. A large number of oriented samples were collected, consideration being given to the setting of these samples in respect to the original bedding habitat and deformational features of the country rocks. The samples were petrographically examined under polarizing and reflecting microscopes. The identity of the iron silicates was confirmed by X-Ray diffractions (XRD).In order to clarify the chemical composition of th...

Pan-African skarn deposits related to banded iron formation, Um Nar area, central Eastern Desert, Egypt

“…The skarn contains 43 wt.% Fe 2 O 3 and 70 ppm Cu in average. The Um Nar skarns are characterized by higher Cu, Zn and Pb contents than those reported by El Aref et al (1993b) for the Um Nar BIF. Ni, Ti, Mn, Sr and Ba are also relatively high in the studied skarn samples, a feature common in Fe-skarn.…”

Textural complications of banded iron formation and the potential production of nano-magnetite: a case study from the Central Eastern Desert of Egypt

“…The net result of this regional deformation exposes as two overturned folds, comprising the aforementioned BIF-mica schist-bearing anticline and another ophiolitic mélange-bearing syncline, that were thrust over the Shaitian granite. Afterwards, the study area was intruded by varieties of granitic rocks including grey and pink granites 7 , 8 , 10 , 23 , 24 .

Figure 1 Geological map of the Um Nar area 23 ( a ), with close-up field views of BIF successions ( b ) that are invaded by quartz veins (white in color) ( c and d ).

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Comparative study for reducibility of various Egyptian iron ores

“…It is generally accepted that BIFs are intimately associated with geosynclinalmetavolcanics and metasediments (island arc volcanics and volcaniclastics), and pertain to one genetic type. The BIFs in Egypt are assumed to be of Late Proterozoic (Pan-African) age and considered either as (a) chemical marine sediments in a geosynclinal basin [10] or as (b) subaqueous volcanogenic deposits in an island arc environment [11][12][13][14][15][16][17].…”