Petrology of the Sokoman Iron Formation in the Howells River area, at the western edge of the Labrador Trough

The Proterozoic Sokoma-n Iron Formation in the Howells River area, at the western edge of the Labrador Trough, has undergone extensive diagenesis but probably only very low grade metamorphism. Comparison with oxygen isotope data for very similar assemblages in the Biwabik Iron Formation suggests that the iron-formation of this study has been subjected to temperatures of about 150 degrees C or somewhat less.The iron-formation assemblages can be divided into four facies: sulfide, silicate, magnetite-carbonate, and hematite-carbonate. In almost all of the assemblages strong recrystallization has taken place of the original, sedimentary precursors. In some assemblages, however, as in the underlying orthoquartzites and the green chert member of the iron-formation, very fine, fibrous and colloidal textures are still preserved in chalcedony and chert. Representative assemblages in the various facies are as follows: Sulfide facies-chamosite-siderite-ankerite-pyrite-chert-carbon; ankerite-siderite-chamosite-Al greenalite-chert-pyrite-carbon. Silicate facies-magnetite-greenalite-stilpnomelane-chert - minnesotaite - ankerite - siderite; chert - ankerite-siderite-stilpnomelane-magnetite; greenalite-stilpnomelane-minnesotaite-ankerite-magnetite-siderite-chert. Magnetite-carbonate facies-quartz-magnetite-hematite-siderite-stilpnomelane; chert-ankerite-siderite-magnetite-stilpnomelane; chert-magnetite-siderite-calcite-hematite. Hematite-carbonate facies-quartz - magnetite-hematite-calcite-ankerite-stilpnomelane; quartz-hematite-magnetite-dolomite-stilpnomelane.Minnesotaite, in the above assemblages, is considered to be a late diagenetic or very low grade metamorphic reaction product. All other phases are concluded to be recrystallization products of sedimentary precursors. Electron microprobe analyses of the co-existing phases in the above assemblages in general show internally consistent element fractionation as shown by the general lack of tie-line crossings in graphical representations. This indicates that, even at very low temperatures, equilibrium was approached or in part attained. The textures and assemblages are interpreted in terms of possible sedimentary precursors.