Abstract
Sphalerite-pyrite-pyrrhotite barometry was undertaken in the Balmat-Edwards district in the NW Adirondacks to determine the pressures in the metamorphosed ore body. Metamorphic temperatures were estimated at 625 ± 25°C using calcite-dolomite thermometry. Sphalerites with extensive exsolution of chalcopyrite are very inhomogeneous, even within ten microns of the po + py + sp triple point, and have iron contents of 16-12 mole percent FeS, corresponding to 3-7 kbar (Scott, 1973). This local variation in pressure is unreasonable; either the system did not equilibrate during metamorphism, erratically reequilibrated afterwards, or exsolved chalcopyrite has variably affected the barometer. A few sphalerites without microscopic chalcopyrite blebs yielded consistent FeS contents of 13-12 mole percent where touching pyrrhotite-pyrite and correspond to pressures of 6.5-7.5 kbar. These data and the presence of sillimanite, indicative of pressures less than 6-6.5 kbar at 600-650°C, in the adjacent paragneisses suggest that the Balmat district was subject to a regional metamorphism of 6.5 ± 0.5 kbar. Incautious application of the sphalerite barometer based on a few samples or on heterogeneous materials can lead to large errors in estimated pressures.
