Iron-oxide mineralogy of banded iron formations in the Takab region, North Western Iran

Siliceous banded and nodular iron ores intercalated in mica schists and amphibolites of probably Late Proterozoic age, were studied from the NW part of Iran (Takab region). The iron ore is predominately composed of quartz and iron-oxides (magnetite, hematite, maghemite, goethite in different proportions). Magnetite was 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 grain boundary migration is also observed. In the nodular ore, grain boundary migration is more pronounced leading also to grain isolation, quartz is partially annealed, and 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, apatite, uraninite, Mn-carbonate and euhedral monazite. Magnetite contains traces of Al. Hematite and goethite show higher Al, Si and Ca contents than the magnetite. Later hydrothermal solutions precipitated goethite surrounding the magnetite-hematite-maghemite grains and replacing hematite. Barite occurs in fractures of iron oxides, Mn-Ba-Pb oxi-hydroxides and scheelite occur interstitial to iron oxides.