Metamorphic zonation of sulfides, oxides, and graphite in and around the orebodies at Ducktown, Tennessee

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 (sub 2) ) and f (sub O (sub 2) ) indicate that the log f (sub S (sub 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 (sub 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 (sub 2) ) and f (sub O (sub 2) ) can be used to calibrate the S (sub 2) -O (sub 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.