METAMORPHISM OF IRON-FORMATION - PARAGENESES IN THE SYSTEM FE-SI-C-O-H

Through the use of phase diagrams previously described for the systems C-O-H and Fe-C-O-H and the mineral assemblages reported from metamorphosed iron-formation, a series of isobaric, isothermal diagrams showing phase relations in the system Fe-Si-C-O-H has been constructed. They indicate that in the presence of CO 2-rich fluids the magnetite + quartz field is bounded by a stability field for siderite and, in the presence of H 2O-rich fluids, by a field for hydrous Fe silicate (grunerite, minnesotaite, or greenalite). The siderite field expands with progressively lower temperatures and under greenschist facies conditions hydrous Fe silicates are stable only in extremely H 2O-rich fluids. The lowest oxygen fugacity under which magnetite + quartz is stable is either that of the quartz-magnetite-fayalite buffer, or, if fayalite is unstable, that of the invariant point involving hydrous Fe silicate, quartz, magnetite, and siderite. Since another invariant point containing hydrous Fe silicate, quartz, siderite, and graphite occurs at low oxygen fugacities, this indicates that graphite should play no role in the metamorphism of iron-formations which contain quartz and magnetite. By assuming that quartz and magnetite are always present, phase assemblages for the system Fe-Si-C-O-H can be projected onto an isobaric T-X CO2, plot which is contoured for oxygen fugacity. This shows that if fluid composition is internally controlled, the only primary mineral assemblages which will not produce the assemblage quartz-magnetite-fayalite (or quartz-magnetite-ferrohypersthene for Mg-bearing systems) at the highest grades of metamorphism are hematite-siderite ± magnetite, where hematite exceeds siderite in abundance, and hematite-quartz ± magnetite. It also indicates that, during prograde metamorphism, the fluid coexisting with a siderite-silicate iron-formation will be buffered to moderate or high X CO2, as siderite and quartz react to silicates. At higher temperatures the fluid will be enriched in H 2O again as grunerite breaks down to fayalite (or ferrohyperthene) + quartz. Due to the buffering of the fluid composition, the assemblage grunerite-fayalite (ferrohyperthene)- quartz-magnetite may be stable over a wide range of temperatures, a fact which could explain its widespread occurrence in nature. © 1979 Society of Economic Geologists, Inc.