On the basis of new demagnetization and rock magnetic experiments, a scattered, two-polarity remagnetization extending throughout the contact metamorphic aureole surrounding the Jurassic Notch Peak granitic stock in western Utah is now thought to reside in metamorphic pyrrhotite rather than in fine-grained hematite. In part, the pyrrhotite formed by in situ reduction of magnetite by pyrite and organic compounds; it could not have been formed by the large-scale introduction of reducing fluids, as isotopic evidence indicates that these metalimestones remained impermeable. At higher grades, some pyrrhotite probably also formed by the reaction of pyrite and Fe-bearing silicates. As magnetite destruction and pyrrhotite formation extends into rocks that are not obviously metamorphosed, the reactions responsible could proceed at very low (less than ~200¿C) temperatures, and the feasibility of such low-temperature reactions in a closed system is shown by detailed thermodynamic calculations. This study demonstrates that under reducing conditions in the presence of sufficient pyrite, pyrrhotite can form at the expense of magnetite under very mild conditions; indeed, magnetite may be destroyed merely by deep burial. Such a mechanism may help explain the paucity of primary magnetite in ancient sedimentary rocks, particularly since the single-domain and pseudo-single-domain grains capable of retaining a stable paleomagnetic signature over geologic time are extremely fine-grained and hence particularly vulnerable to destruction.