Observations on the Contact Metamorphism of the Biwabik Iron-Formation, East Mesabi District, Minnesota
G. B. Morey, J. J. Papike, R. W. Smith, P. W. Weiblen, 1972. "Observations on the Contact Metamorphism of the Biwabik Iron-Formation, East Mesabi District, Minnesota", Studies in Mineralogy and Precambrian Geology, B. R. Doe, D. K. Smith
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A drill core (Drill Hole 17,700) of the middle Precambrian Biwabik Iron-Formation from the contact aureole of the middle Keweenawan Duluth Complex in the East Mesabi district, Minnesota, has been studied by petrographic, x-ray diffraction, and electron microprobe techniques. The core provides 400 ft of iron-formation which is underlain by Pokegama Quartzite and is separated from the overlying Duluth Complex by 500 ft of metagraywacke and argillite (Virginia Formation). The iron-formation now consists of metamorphic assemblages containing grunerite, hornblende, actinolite, hedenbergite, olivine, almandine, andradite, magnetite, and quartz. Relict sedimentary textures that are characterized by specific metamorphic mineral assemblages permit correlations with pre-metamorphic mineral assemblages. The following reactions are suggested (all reactions may involve quartz and/or magnetite): grunerite from minesotaite; hornblende + grunerite from stilpnomelane; actinolite and hedenbergite from ankerite; olivine from siderite; almandine from chamosite; and andradite from a calcite-bearing assemblage.
Perry and Bonnichsen (1966) suggested a temperature range of 400°C to 650°C in the core during the time of contact metamorphism. This study reveals that the core lies outside the stability fields of iron-carbonates and hypersthene. No mineral assemblage variations indicative of facies changes were observed. Compositions of the iron-silicates do not vary systematically with the inferred thermal gradient; they can be correlated, however, with stratigraphy and specific textures.
Estimates of oxygen fugacity based on the inferred temperature of the quartz-magnetite-fayalite buffer assemblage which occurs throughout the core indicate a bulk compositional control on this parameter.