sphalerite geobarometry in the Balmat-Edwards district , New yorkl
Sphaleri te-pyri te-pyrrhoti te barometry was undertaken in the Balmat-Edwards distr ict in the NW Adirondacks to determine the pressures in the metamorphosed ore body. Metamorphic temperatures were estimated at 625 t 25"C using calcite-dolomite thermometry. Sphaleri tes with extensive exsolut ion of chalcopyrite are very inhomogeneous, even within ten microns of the po + py + sp tr iple point, and have iron contents of 16-12 mole percent FeS, corresponding to 3-7 kbar (Scott, 1973). This local variat ion in pressure is unreasonable; either the system did not equi l ibrate during metamorphism, errat ical ly reequil ibrated afterwards, or exsolved chalcopyrite has variably affected the barometer. A few sphaleri tes without microscopic chalcopyrite blebs yielded consistent FeS contents of 13-12 mole percent where touching pyrrhoti te-pyri te and correspond to pressures of 6.5-7.5 kbar. These data and the presence of si l l imanite, indicative of pressures less than 6-6.5 kbar at 600-650.C, in the adjacent paragneisses uggest hat the Balmat distr ict was subject to a regional metamorphism of 6.5 a 0.5 kbar. Incautious application of the sphaleri te barometer based on a few samples or on heterogeneous materials can lead to large errors in est imated pressures.
New Data on Phase Relations in the Cu-Fe-S System*
The phase relations in the central portion of the Cu-Fe-S "dry" system have been studied at 100oC and 600oC. The 600oC isothermal section is presented and low- temperature phase relations based on synthetic and natural assemblages are discussed. At low temperatures, the minerals chalcopyrite (CuFeS2), cubanite (CuF%Sa), talnakhite (CuoesSo), mooihoekite (Cuoeoo), and haycockire (Cu4FesSs) are stable phases. Chalcopyrite, talnakhite, and mooihoekite have been synthesized, while attempts to synthesize cubanite and haycockite have been unsuccessful. Synthetic talnakhite (/-phase), on heating,. shows transformations at ,186oC and at ,230oC to intermediate high-temperature phases, I and II, and finally transforms at 520 to 525oC to a face-centered cubic ((cc) sphalerite-like structure (iss). None of these phases are quenchable. Synthetic mooihoekite (-/-phase) transforms to intermediate high-temperature phase A at ,167oC, and this phase changes at ,236oC to the (cc sphalerite-like structure (iss). Both high-temperature forms of mooihoekite are un- quenchable. For the haycockite composition, an unquenchable high-temperature phase is formed between 20 and 200oC which appears isostructural with (cc high-temperature mooihoekite and iss. On quenching, this phase changes to a structure which can be indexed as having a primitive cubic cell with a  5.32A. Cubanite transforms fo a (cc phase at 200 to 210oC which appears isostructural The studies made to date have already shown that the central area of the Cu-Fe-S system is char- acterized by a large solid solution field at elevated temperatures which breaks up into five distinct phases at low temperatures. The picture is com- plicated by numerous phase transformations, un- quenchable phases, and phases with closely related crystal chemistry, resulting in their having similar physical appearance and X-ray diffraction powder patterns. The slow rate of most of the reactions at low temperatures also makes it difficult (in some cases impossible) to achieve equilibrium in the labo- ratory. In this study it was decided to approach an understanding of the low-temperature phase relations by a thorough study of the naturally occurring min- erals suspected to exist in the chalcopyrite composi- tion area because they have had a better chance to equilibrate and crystallize. Such mineralogical studies on talnakhite, mooihoekite, and haycockite have been reported above. With the realization that most of the low-temperature experiments would not reach equilibrium, careful electron probe analyses were
Solid Solutions in the System Cu-Fe-S, Part I; The Cu-S and CuFe-S Joins
The data of Rau (1967) on the H 2 S/H 2 ratios of gas in equilibrium with high digenite solid solutions permit the calculation of the activity of Cu 2 S as a function of temperature and composition. The activity of Cu 2 S falls to 0.5 when the mole fraction (in the system Cu 2 S-S) drops only to 0.87. These calculations also permit the estimation of the activity of S 2 -temperature curve for the hexagonal chalcocite-high digenite reaction and a revised standard free energy for covellite.New data on the variation of the activity of S 2 in equilibrium with various metal/sulfur ratios for Cu = Fe in the intermediate solid solution permit the calculation of the activity of CuFeS 2 and show that it may drop as low as 0.2 at a mole fraction of 0.90 (system CuFeS 2 -Cu 2 Fe 2 S 2 ). The behavior of the copper-bearing solid solutions closely parallels that of pyrrhotite (Toulmin and Barton, 1964) in which the activity of FeS falls to 0.4 at a mole fraction of 0.9. A phase diagram is given for the CuFe-S join between 400 degrees and 700 degrees C.