Mineralogy and Petrology of the Metamorphosed Wabush Iron Formation, Southwestern Labrador

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Abstract
The Wabush Iron Formation, of late Precambrian (Proterozoic) age is part of the Labrador Trough in southwestern Labrador, Canada. It is the regionally metamorphosed equivalent of low grade metamorphic (chlorite zone) iron-rich sediments of the central part of the Labrador Trough. The metamorphic grade is kyanite-staurolite zone, as concluded from conformably underlying pelitic schist assemblages. Sedimentary textural features such as very pronounced banding and a very rare occurrence of relic granules are still preserved. The iron formation consists mainly of quartz, specularite, magnetite, cummingtonite-grunerite, and ferrodolomite-ankerite. Less common are actinolite, anthophyllite, riebeckitetremolite, magnesioriebeckite, ferrosalite, orthopyroxene, aegirine-augite, aegirine, rhodonite, garnet (almandine, spessartine, calderite), siderite, rhodochrosite, calcite, and kutnahorite. Conventional wet chemical analyses or electron microprobe analyses have been made of thirty-four phases belonging to the above list. Six additional electron probe analyses have been made of phases from the underlying pelitic schists. All conventionally analyzed phases are characterized by complete optical, unit cell parameter, and density measurments. The analyzed assemblages from the silicate and silicate-carbonate iron formation include grunerite-ferrosalite, grunerite-eulite-siderite, grunerite-actinolite, grunerite-almandine, cummingtonite-spessartine, rhodonite-kutnahorite-calderite, aegirine-augite-riebeckite-tremolite, magnesioriebeckite-cummingtonite-rhodonite, aegirine-augite-rhodonite-rhodo-chrosite, and aegirine-rhodonite-calderite-rhodochrosite. The assemblages are concluded to be equilibrium assemblages. Of the volatile components, O2, CO2, and H2O, O2, is concluded to have behaved as an inert (buffered) component. Variations in the activity of CO2 are concluded to have existed between silicate-oxide and carbonate-oxide members of the iron formation. It is not clear, however, whether CO2 has acted as a perfectly mobile component with strong aco2 gradients throughout the area, or as an inert component in some parts of the area. H2O is considered to have been perfectly mobile. An increase in Mg/(Mg+Fe) ratio in ferromagnesian silicates is correlated with an increase in the oxidation state of the assemblage. A similar increase in (Mg+Mn)/(Mg+Mn+Fe) is found in manganoan ferromagnesian silicates with increasing activity of O2. A number of ferromagnesian silicates contain large amounts of Na+ and Fe3+ as a result of the very low Al2O3 content of the iron formation. The P and T conditions of metamorphism are deduced from experimental studies applicable to the underlying pelitic schists.

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