ABSTRACT

Mineral assemblages and the Fe/(Fe+Mg) ratio in minnesotaite are evidently different for some low-grade metamorphic Precambrian iron formations in spite of very similar metamorphic conditions (temperature, probably pressure, and rock chemistry). Since carbonates, siderite and ankerite are commonly found, such differences are considered to have been caused by a difference in the degree of the escape of carbon dioxide during metamorphism. In comparison with the iron formations, the greatest degree of the escape is found in the Sokoman and Biwabik Iron Formations, the lowest degree in the Brockman Iron Formation, and the intermediate in the Gunflint Iron Formation. The variation of Fe/(Fe+Mg) in minnesotaite in each iron formation reflects that of oxygen fugacity during metamorphism. The ratio increases with decreasing oxygen fugacity. The fugacity change is probably due to the degree of consuming carbonaceous matters at constant temperature. Fe/(Fe+Mg) in greenalite probably reflects that in a primary one. Greenalite seems to be easily oxidized and to be decomposed to form iron talc with increasing P_CO2 at lower temperatures because of very conservative substitution of Fe²⁺ by Mg. The ratios in siderite and ankerite do not change so significantly as that in minnesotaite, although no sufficient data is available. But they might have resulted from the ratios in carbonates, which were involved prior to recrystallization or ankeritization. It is therefore proposed that Fe/(Fe+Mg) in minnesotaite is a good indicator to reveal the magnitudes of partial pressures of oxygen and carbon dioxide during low-grade metamorphism of Precambrian iron formations.