Iron-oxide mineralogy of banded iron formations in the Takab region, North Western Iran
Résumé
Siliceous banded and nodular iron ores intercalated in mica schists and marbles of Precambrian age, were studied from the NW part of Iran (Takab region). The iron ore is composed of magnetite, which was partly transformed into maghemite and hematite along fractures and cleavages during a major oxidizing deformation event. Quartz shows undulose extinction. Both quartz and iron oxides partly form 120° triple junctions, but essentially grain boundary migrations. In the nodular ore, grain boundary migration is more pronounced leading also to grain isolation quartz is partially annealed. Iron oxides are agglomerated, elongated and disrupted. These finding indicate a longer recrystallization episode in the nodular ore. The iron oxide grains contain inclusions of euhedral, zoned and corroded zircons and euhedral monazite. Magnetite contains traces of Al. Hematite and goethite show higher Al, Si and Ca contents. Later hydrothermal solutions precipitated goethite surrounding the magnetite-hematite-maghemite grains and replacing hematite. Barite and uraninite occur in fractures of iron oxides, Mn-Ba-Pb oxi-hydroxides and scheelite occur interstitial to iron oxides.
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