B. E. Nesbitt and W. C. Kelly
Metamorphic zonation of sulfides, oxides, and graphite in and around the orebodies at Ducktown, Tennessee
Economic Geology and the Bulletin of the Society of Economic Geologists (November 1980), 75(7):1010-1021

Abstract:
Several distinct zonations of oxide-sulfide-graphite assemblages have been identified in the wall rocks adjoining the metamorphosed massive sulfide deposits at Ducktown, Tennessee. The oxide-sulfide-graphite assemblages used to delimit these zones are as follows: pyrrhotite-ilmenite-graphite (country rocks), pyrrhotite-ilmenite, pyrrhotite-rutile, pyrrhotite-pyrite-rutile (wall rocks), and pyrrhotite-pyrite-magnetite (ore zone). The transition from the pyrrhotite-ilmenite-graphite zone to the pyrrhotite-ilmenite zone can be recognized as far as 90 m from the ore zone. The pyrrhotite-ilmenite to pyrrhotite-rutile transition occurs from 4 to 70 m from the ore. The pyrrhotite-pyrite-rutile zone is discontinuous and is only locally present immediately adjoining the ore zone. Knowledge of the P (6 kb) and T (550 degrees C) of metamorphism allows calculation of equilibria in the system Fe-Ti-O-S from available thermodynamic data. Calculations of the stabilities of various reactions as a function of f (sub S ₂ ) and f (sub O ₂ ) indicate that the log f (sub S ₂ ) varies from --7.0 + or - 0.5 in the country rocks to --2.5 + or - 0.3 in the ore zone. The log f (sub O ₂ ) similarly varies from --21.4 + or - 0.7 in the country rocks to --18.5 + or - 0.5 in the ore zone. These inferred values for f (sub S ₂ ) and f (sub O ₂ ) can be used to calibrate the S ₂ -O ₂ -ferromagnesian silicate reactions which have also taken place in these rocks. The distribution of the oxide-sulfide-graphite zones may be useful in the interpretation of the origin of metamorphosed sulfide bodies, the exploration for undiscovered deposits, and the extension of known deposits.