Abstract

The unleached sedimentary oxide facies of unmetamorphosed Precambrian iron-formation generally consists of a finely banded assemblage of chert, jasper or quartz, hematite or magnetite, or both, and locally some hydrous iron oxides. Upon regional metamorphism to kyanite or sillimanite grade, recrystallization of the chert and iron oxides takes place, accompanied by a marked increase in grain size of the constituent phases, especially quartz. Generally no reactions take place among these minerals and original sedimentary textures are frequently preserved in the recrystallized assemblage. The resulting, banded, metamorphic quartz-specularite-magnetite horizons, with quartz-specularite and quartz-magnetite bands alternating on a scale of less than 1 mm, indicate that oxygen has behaved as an internally controlled (buffered) component in these occurrences during metamorphism.The banded sedimentary carbonate facies generally consists of one or more carbonates (calcite, members of the ferroan dolomite-ankerite series, and siderite), lesser amounts of magnetite, chert, or quartz, and locally some iron silicates. If the original carbonate facies consists only of carbonate, magnetite, and quartz, and if the chemical potential of CO 2 during subsequent metamorphism remains locally high enough to prevent the breakdown of the carbonates, no reactions occur among the coexisting phases. Only recrystallization and increased grain size are noted. If, on the other hand, the chemical potential of CO 2 is reduced, chert (or quartz) will react with the carbonates to form new silicates. For example :Ca(Fe,Mg) (CO 3 ) 2 + 2SiO 2 = Ca(Fe,Mg)Si 2 O 6 + 2CO 2 ferroan dolomite clinopyroxeneIf the chemical potential of H 2 O is high, while the chemical potential of CO 2 is low during metamorphism, the following type of reaction occurs:5Ca(Fe, Mg) (CO 3 ) 2 + 8SiO 2 + H 2 O = Ca 2 (Fe,Mg) 5 Si 8 O 22 (OH) 2 + 3CaCO 3 + 7CO 2 ferroan dolomite actinoliteAs a result of medium- to high-grade metamorphic conditions (kyanite to sillimanite zones), much of the banded carbonate facies contains metamorphic silicates, indicating a generai loss of CO 2 during metamorphism. Some parts of the same carbonate facies in the same metamorphic area may, however, still consist of the original, although recrystallized, quartz-carbonate-magnetite assemblage. This implies that the chemical potential of CO 2 has been locally variable in the iron-rich carbonate rocks and that CO 2 , therefore, cannot at all times be considered as a perfectly mobile component. Such carbonate-quartz-magnetite recrystallization has taken place in a system essentially closed to CO 2 .The original silicate facies consists of a complex mixture of hydrous Fe-silicates (greenalite, stilpnomelane, and minnesotaite), carbonates (members of the dolomite-ankerite series, siderite, and calcite), chert or quartz, and iron oxides. This part of the iron-formation shows most consistently the results of general decarbonatization and dehydration during progressive metamorphism. Not only do the carbonates and chert react to form abundant silicates, such as members of the cummingtonite-grunerite series, but also the original silicates give way to clino- and orthoamphiboles, garnet, and ortho-and clinopyroxenes. The production of olivine (fayalite) has been described only from contact metamorphic occurrences such as along the contact of the Duluth Gabbro with the Biwabik Iron Formation in Minnesota. Fayalite assemblages represent conditions of low relative chemical potential of O 2 . Manganese-rich horizons are frequently present. Original mixtures of Mn-oxides, hydroxides, and carbonates (rhodochrosite and kutnahorite) give way, upon metamorphism, to assemblages containing rhodonite, Mn-garnet (spessartite or calderite), manganoan cummingtonite, Mn-rich pyroxenes (such as manganoan aegrine-augite and hedenbergite), bustamite, and rhodochrosite. In the metamorphic assemblages of the Labrador Trough, many Mn-rich horizons of the iron-formation are also Na-rich, as shown by assemblages such as Mn-aegirine-rhodonite-specularite-rhodochrosite.

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