Abstract

Very large pyrite porphyroblasts are a striking feature of the metamorphosed, massive, pyrrhotite-rich sulfide ores at Ducktown, Tennessee. The crystals, up to 300 mm across, are commonly euhedral but exhibit varying degrees of elongation and postgrowth fracturing and granulation. The inclusion patterns within the various porphyroblasts indicate growth before, during, and after the development of schistosity in the silicate gangue. The sphalerite inclusions within single porphyroblasts show no systematic compositional variations from core to rim, partly because the larger porphyroblasts were formed by the aggregation of numerous growing pyrite grains and partly because fractures and chalcopyrite allowed many of the sphalerite inclusions to reequilibrate at lower temperatures and pressures. Sphalerite inclusions surrounding single growth centers, which were selected to avoid reequilibration problems, show that they grew at a nearly constant pressure of 6.8 + or - 0.8 kb. This information is used to relate the growth of the pyrite porphyroblasts to the pressure-temperature evolution of the deposit.Pyrrhotite, which forms the matrix for the porphyroblasts, ranges compositionally from 47.4 at. percent Fe in the matrix to 47.7 at. percent Fe immediately adjacent to the pyrite crystals. Chalcopyrite commonly occurs as fracture fillings and as pressure shadows adjacent to the pyrite porphyroblasts. Minor sphalerite, galena, and magnetite are present. Gangue minerals are dominantly quartz, calcite, sheet silicates, actinolite, and talc.

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