Contact metamorphic efrects of the Stillwater Complex, Montana: the concordant iron-formation: a discussion of the role of buffering in metamorphism of iron-formation

This paper argues that the presence of external control of fluid composition during metamorphism of iron-formation should be indicated by the occurrence of high-grade rocks which either lack the bufering assemblages such as olivine-magnetite-quartz or by the presence of such assemblages in which the silicates show a very restricted range in composition. A review of high-grade iron-formations from contact and regional metamorphic terrains, including the Stillwater iron-formation, the Biwabik iron-formation and ironformations from Western Australia, Labrador and Southwestern Montaria, shows that all were metamorphosed under a variety of oxygen fugacities and that all had bufered the fluid composition during metamorphism.