Mineral Compositions and Equilibria in the Metamorphosed Iron Formation of the Gagnon Region, Quebec, Canada

Forty-seven specimens of the Wabush Iron Formation were collectedfrom ten outcrop areas. Twenty-five specimens contain the assemblage(1), quartz+clinopyroxene+calcite with or without orthopyroxene,grunerite, magnetite, ankerite, and siderite. Five specimenscontain assemblage (2), quartz+clinopyroxene+actinolite+calcite+magnetite+hematite,and two contain assemblage (3), quartz+orthopyroxene+actinolite+magnetite+hematite.In three specimens of assemblage (1), graphite occurs in theabsence of magnetite; pyrrhotite and pyrite occur separatelyor together in specimens with assemblage (1). Thirty-nine clinopyroxenes, 38 orthopyroxenes, 18 grunerites,7 actinolites, 16 calcites, 1 ankerite, and 1 siderite wereanalyzed for iron, manganese, and calcium by X-ray emissionspectrography. Magnesium contents were estimated by assumingstoichiometric proportions. Minerals occurring with hematite show low Fe/(Fe+Mg) ratios,and those in the other assemblages show higher values with awide range of variation. In orthopyroxene, Fe/(Fe+ Mg) rangesfrom 0·17 (with hematite) to 0·77. Regularity in the distributions of Fe, Mn, and Ca between pairsof coexisting minerals shows that equilibrium was attained inmost of the rocks studied. This regularity is also accomplishedin the distribution of Mn between calcite and coexisting silicatesas well as between the silicates themselves. Small differencesin the distributions of Ca and Fe depend on both outcrop areaand mineral assemblage. Phase rule considerations suggest that the specimens with dolomite-ankeriteor magnesitesiderite do not represent equilibrium assemblages.Variations in orthopyroxene compositions in assemblages withpyrite or pyrrhotite, or both, and magnetite indicate non-equilibrationof sulfides with silicates. The presence of the oxygen buffer,magnetite+hematite, attests to the immobility of oxygen duringmetamorphism. Within each outcrop area, over which the temperature and pressureare assumed to have been uniform, variations in the compositionsof the silicates in the sub-assemblages quartz+ orthopyroxene+gruneriteand quartz+orthopyroxene+clinopyroxene+calcite indicate gradientsof µ_H2O µ_CO2 and respectively. As characterizedby the composition of orthopyroxene, both gradients are relativelylow along strike, and high across strike. The direction of gradientsacross strike is almost without reversals, which is consistentwith intergranular diffusion of H₂O and CO₂. Phase rule restrictionsfor a majority of assemblages are not in accord with the simultaneousimposition of µ_H2O and µ_CO2 gradients on the rocks,nor the formation of an H₂O-CO₂ fluid phase during metamorphism.