Banded Iron Formations from the Eastern Desert of Egypt: A new type of Ore ?
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Banded iron formations (BIFs) occur in thirteen localities in an area approximately 30,000 km2
within the eastern desert of Egypt. With the exception of the southernmost deposit of Um Nar
which is suspected to be pre-Panafrican, all other BIFs are considered Neoproterozoic in age.
The iron ore occurs as rhythmically layered bands, groups of bands or separate lenses that
reach a maximum thickness of 100 m, and which are intercalated with volcanic arc assemblages
dominated by andesitic lava flows, tuffs and lapilli tuffs, and basaltic pyroclastics. In most cases,
the BIFs contain syn-sedimentary structures such as bedding and lamination. The entire
sequence of BIFs and host rocks is strongly deformed and regionally metamorphosed under
greenschist to amphibolite facies conditions.
All thirteen deposits are comprised of an oxide facies consisting of magnetite and hematite,
and a silicate facies consisting of quartz with subordinate amounts of one or more of the
minerals: chlorite (ripidolite - clinochlore), greenalite, stilpnomelane, garnet (grossular –
almandine), carbonate (mainly calcite), epidote, hornblende, or plagioclase. With the exception
of the northernmost jaspilite type deposit of Hadrabia, magnetite is the predominant oxide,
where it seems to be primary, even when martitized. Major and trace element compositions of
the Egyptian BIFs show significant variations from one deposit to another. The most intriguing
geochemical feature of the investigated BIFs is their high Fe/Si ratio in comparison with Algoma
and Superior types. Based on Fe/Si ratios, these deposits are classified into two groups; a) fresh
BIFs with Fe/Si ratio < 2.3 (e.g. Um Nar, Gebel El Hadid and Wadi El Dabbah) and b) altered BIFs
with Fe/Si ratio > 3.0 (e.g. Gebel Semna, Hadrabia and Abu Merwat).
The relatively small nature of individual deposits, strong variations in Fe2O3(t) and SiO2
contents and the enrichment in Cr, V and Ni (for a few deposits) support a volcanic exhalative
source for Fe and Si, leading most scientists to classify them as “Algoma type BIFs”. On the other
hand, the lack of sulfides, varve – like nature of some deposits, and lack of a distinct enrichment
in Co, Ni, Cu, As, and Sr are at odds with such a classification. Finally, the Neoproterozoic age of
Egyptian BIFs, high Fe and P contents, and presence of diamictites intercalated with at least one
of these deposits compels a comparison with the Rapitan type deposits.
The presence of laminations and absence of wave generated structures in most Egyptian BIFs
indicate subaqueous precipitation below wave base. The formation of authegenic primary
magnetite as the most abundant mineral instead of hematite reflects precipitation away from
the shore and under slightly euxinic conditions in basins where S and CO2 activities were low.
The paucity of primary sulfides and pure siderite in the Egyptian BIFs support this interpretation
and may also indicate formation away from the deepest parts of the basin. Accordingly, we
suggest that the Egyptian BIFs formed in the deepest “shelf – like” environments of fore-arc and
back-arc basins. These characteristics may indeed justify the definition of a new type of BIF.
within the eastern desert of Egypt. With the exception of the southernmost deposit of Um Nar
which is suspected to be pre-Panafrican, all other BIFs are considered Neoproterozoic in age.
The iron ore occurs as rhythmically layered bands, groups of bands or separate lenses that
reach a maximum thickness of 100 m, and which are intercalated with volcanic arc assemblages
dominated by andesitic lava flows, tuffs and lapilli tuffs, and basaltic pyroclastics. In most cases,
the BIFs contain syn-sedimentary structures such as bedding and lamination. The entire
sequence of BIFs and host rocks is strongly deformed and regionally metamorphosed under
greenschist to amphibolite facies conditions.
All thirteen deposits are comprised of an oxide facies consisting of magnetite and hematite,
and a silicate facies consisting of quartz with subordinate amounts of one or more of the
minerals: chlorite (ripidolite - clinochlore), greenalite, stilpnomelane, garnet (grossular –
almandine), carbonate (mainly calcite), epidote, hornblende, or plagioclase. With the exception
of the northernmost jaspilite type deposit of Hadrabia, magnetite is the predominant oxide,
where it seems to be primary, even when martitized. Major and trace element compositions of
the Egyptian BIFs show significant variations from one deposit to another. The most intriguing
geochemical feature of the investigated BIFs is their high Fe/Si ratio in comparison with Algoma
and Superior types. Based on Fe/Si ratios, these deposits are classified into two groups; a) fresh
BIFs with Fe/Si ratio < 2.3 (e.g. Um Nar, Gebel El Hadid and Wadi El Dabbah) and b) altered BIFs
with Fe/Si ratio > 3.0 (e.g. Gebel Semna, Hadrabia and Abu Merwat).
The relatively small nature of individual deposits, strong variations in Fe2O3(t) and SiO2
contents and the enrichment in Cr, V and Ni (for a few deposits) support a volcanic exhalative
source for Fe and Si, leading most scientists to classify them as “Algoma type BIFs”. On the other
hand, the lack of sulfides, varve – like nature of some deposits, and lack of a distinct enrichment
in Co, Ni, Cu, As, and Sr are at odds with such a classification. Finally, the Neoproterozoic age of
Egyptian BIFs, high Fe and P contents, and presence of diamictites intercalated with at least one
of these deposits compels a comparison with the Rapitan type deposits.
The presence of laminations and absence of wave generated structures in most Egyptian BIFs
indicate subaqueous precipitation below wave base. The formation of authegenic primary
magnetite as the most abundant mineral instead of hematite reflects precipitation away from
the shore and under slightly euxinic conditions in basins where S and CO2 activities were low.
The paucity of primary sulfides and pure siderite in the Egyptian BIFs support this interpretation
and may also indicate formation away from the deepest parts of the basin. Accordingly, we
suggest that the Egyptian BIFs formed in the deepest “shelf – like” environments of fore-arc and
back-arc basins. These characteristics may indeed justify the definition of a new type of BIF.
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