Stability of the association fayalite + calcite within metamorphosed banded iron formations: The case of the magnetite ores of Edough massif (N-E Algeria)

We interpret the mineral assemblages observed in iron magnetite deposits, and in particular the association fayalite + calcite, which result from the metamorphism of iron carbonate. The variety of mineral assemblages is explained in terms of compositional differences in the parent rock as well as by the intensity of both the reduction and decarbonation processes. The Ca-Fe-Si-C-O system is considered. Phase diagrams are constructed with μO2 and μCO2 as coordinates (mobile components), the elements Ca, Fe and Si being inert. The method is applied to the Edough massif, the easternmost crystalline massif of the Algerian coast. This massif hosts ancient deposits of magnetite ± hematite ores hosted in kyanite-staurolite-garnet schists. The main minerals of the ore bodies are magnetite, hematite, siderite, quartz, calcite, fayalite, hedenbergite and garnet, resulting from the metamorphism of siderite + calcite + quartz rocks containing variable proportions of the starting minerals. The stability of the fayalite + calcite assemblage coexisting with magnetite and other minerals is suggested from phase relations observed in the iron formations of the Edough massif. Although this association is generally regarded as impossible, it has been described in a number of studies. The thermodynamic calculations reported here demonstrate the existence of a stability domain for the association fayalite + calcite.