A Study of Phase Equilibria in a Metamorphic Iron Formation

The paper is based on a study of minerl assemblages in metamorphiciron formations in northere Quebec. Thirty-four mineral analyseshave been performed, using photometric methods of silicate analysis. Three facies of iron formation are represented, namely, quartz-speculariteiron formation, quartz-magnetite iron formation, and quartz-silicateiron formation. The silicate iron formation is made up mainlyof quartz, ferrohypersthene, ferroaugite, cummingtonite, andcalcite. Arfvedsonite and aegirine-hedenbergite occur in magnetite-richrocks. The analysed coexisting minerals are from the silicate ironformation and from overlying mafic rocks of igneous origin.The analysed assemblages include ferrohypersthene-ferro-augite-cummingtonite,ferrohypersthen-ferroaugite, cummingtonite-ferroaugite, diopside-hornblende,anthophyllite-hornblende, hypersthene-diopside-hornblende-garnet,and olivine-hypersthene-chlorite. The analyses show that (1) the minerals are nearly in equilibriumas shown by parallel tie lines, (2) cummingtonite generallyhas a higher MgO/FeO ratio than coexisting ferrohypersthene,(3) the phase higher in calcium has the higher MgO/FeO ratio,(4) the solubility gap between ferrohypersthene and ferroaugiteis 43 mole percent. The stability relationships of pyroxenes and amphiboles arediscussed in view of the new data. It is shown that FeO-CaO-SiO₂systems often behave differently from MgO-CaO-SiO₂ systems.Hedenbergite is probably not stable in the presence of excessCO₂ in the area. The low variance ferrohypersthene-ferroaugite-cummingtonite-calcite-quartzassemblage indicates that the rocks were closed to H₂O and CO₂.The behaviour of the volatile components is discussed in viewof this fact. The source of the silicate minerals is assumed to be hydrousiron silicates and carbonates. A model of the sequence of chemicalreactions taking place in carbonate-silicate iron formationsduring metamorphism is suggested. ¹Present address: Bolidens Gruvaktiebolag, Boliden, Sweden.