The distribution and variation of sulfide constituents of the Great Smoky Group, Ducktown area, Tennessee, with respect to metamorphic grade

The metasedimentary rocks of the Precambrian Great Smoky Group in the Ducktown mining district show a Barrovian metamorphism from the chlorite zone through the staurolite zone. Modal data of sulfide distribution and abundance in these metasedimentary rocks indicate that with increasing metamorphic grade: (a) pyrite is converted to pyrrhotite and (b) total sulfide content decreases. Premetamorphic cubic pyrite grains have pressure shadows of quartz, calcite, and pyrrhotite which parallel the S₁ cleavage. A synmetamorphic origin of pyrrhotite is suggested by the alignment of pyrrhotite parallel to S₁ and S₂ cleavages. The absence of premetamorphic pyrite in lithologies from the garnet and staurolite zones therefore implies that the transition of pyrite to pyrrhotite is complete at garnet grade conditions. Mobilization of sulfur by some metamorphic process can explain the noted decrease in average sulfide content as a function of metamorphic grade. Comparison of average ratios of major element content versus aluminum content derived from whole-rock chemical analyses of representative samples does not discern mobilization of elemental constituents during metamorphism. However, because of the large variation of ratios in each metamorphic zone, the absence of significant trends of elemental ratios cannot be used to support or repudiate the decrease in sulfide content with increasing grade as defined by modal data. Correlation coefficient analysis of S with other major elements shows that S content is not related to the Si, Ti, Al, Fe, Mn, Mg, Ca, K, and P content in these metasedimentary rocks.

The decrease in average sulfide content between lithologies in the chlorite zone through the garnet zone can be explained by volumetric decreases caused by the transition of pyrite to pyrrhotite in a reaction of: 2FeS2 + 2H2O + C (double arrow) 2FeS + 2H2S + CO2 in an open metamorphic system. The removal of sulfide constituents in the development of differentiated cleavages during prograde metamorphism can account for the decrease in average sulfide content of lithologies in the garnet zone versus lithologies in the staurolite zone. The above data indicates that S was mobile during prograde metamorphism and that significant amounts of sulfide material within the Ducktown ore deposits could have been derived by this process.