The first appearance of pyrrhotite in the Barrovian-type metamorphic succession of the Blue Ridge province of southeastern Tennessee and southwestern North Carolina is interpreted to represent a metamorphic isograd located in the upper chlorite zone slightly west of the biotite isograd. At lower grades of metamorphism, pyrite is the only recognizable iron sulfide, and it is characterized by textures indicating an arrested state of growth. Concentrations of pyrite are commonly aligned parallel to bedding planes. Pyrrhotite appears as streaks along slaty cleavage and is preferentially developed in pyrite-bearing zones. Hexagonal pyrrhotite is dominant, but monoclinic and monoclinic + hexagonal intergrowths also occur. Where pyrrhotite is present, coexisting pyrite is typically replaced by quartz, chlorite, and biotite. The sulfur and some of the iron contained in pyrrhotite is interpreted to have been derived locally from the replaced pyrite simultaneously with the development of metamorphic minerals.

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