Metamorphic sulfide-silicate equilibria in the massive sulfide deposits at Ducktown, Tennessee

Examination of the distribution and composition of the ferromagnesian silicates in the country rocks, wall rocks, and ore zone at Ducktown, Tennessee, indicates that high values for f (sub S ₂ ) and f (sub O ₂ ) during metamorphism have produced distinct transitions in the mineralogy and chemistry of the ferromagnesian silicates in the wall rocks and ore zone. These transitions include (from country rocks to ore zone): (1) appearance of primary chlorite, (2) disappearance of staurolite, and (3) disappearance of garnet. The compositions of the ferromagnesian silicates (country rocks to ore zone) vary as follows: X ^Bt_Fe = 0.62 to 0.19, X ^Gt_Fe = 0.85 to 0.37, X ^Stt_Fe = 0.84 to 0.42, and X ^Chl_Fe = 0.51 to 0.14. These differences parallel the transitions in oxide-sulfidegraphite mineralogy described in Nesbitt and Kelly (1980) in the wall rocks at Ducktown. Application of experimental phase equilibria to the assemblages indicates that the compositions of the ferromagnesian silicates were controlled by complex reactions involving muscovite, pyrrhotite, quartz, and the ferromagnesian silicates. Good agreement is observed between f (sub S ₂ ) -f (sub O ₂ ) conditions inferred from oxide-sulfide-graphite assemblages and ferromagnesian silicate assemblages. The observation of variations in the mineralogy and chemistry of the ferromagnesian silicates around metamorphosed ore deposits can double or triple the target area. In addition, an improved understanding of the variations in f (sub S ₂ ) and f (sub O ₂ ) and the resultant reactions among the ferromagnesian silicate in the wall rocks of metamorphosed ore deposits aids in the interpretation of the origin of such deposits.