Abstract:
The Early Proterozoic banded iron formation (BIF) of the Mikhailovskoe iron deposit differs from other known Precambrian BIF by the low Al# and high Fe3+/(Fe3+ + Fe2+) ratios. The elevated oxygen fugacity during the metamorphic event and the Al-poor composition of the rocks were responsible for the origin of minerals high in Fe3+ and the absolute absence of Al-bearing phases. The BIF contains widespread celadonite, tetraferribiotite, Al-free chlorite, riebeckite, and aegirine instead of grunerite, stilpnomelane, minnesotaite, and greenalite, which are minerals usual in BIF elsewhere. Data on the phase equilibria provide information on the origin order of mineral assemblages and the physicochemical conditions of metamorphism. Its early stages were marked by the stability of quartz, carbonates, iron oxides, and potassic micas of specific composition. As the temperature increased, tetraferribiotite, celadonite, and riebeckite crystallized. Depending on the oxygen fugacity (above or below the hematite-magnetite buffer) in discrete layers, the Aeg + Hem and Rbk + Mag assemblages were formed. The metamorphic temperature was estimated at 370-520°C at a pressure of 2-3 kbar, Na activity [log [a (Na +)/a(H+)] = 5.5-6.0, and oxygen fugacity above the hematite-magnetite buffer in layers with the Aeg + Hem assemblage and below this buffer in layers with Rbk + Mag.
