In the Balmat area in northern New York, tabular deposits of sulfide minerals parallel the layering in folded, siliceous magnesian marbles of a metamorphic complex commonly referred to as the Precambrian Granville Series. Sphalerite, pyrite, and, locally, pyrrhotite and galena have replaced the carbonate minerals in parts of the marble units. The contacts between ore and marble are, in general, ill-defined; scattered grains of sulfides are present from several inches to hundreds of feet from the massive portions of ore. Access to the ore is provided through the Balmat No. 2 and No. 3 mines.

The isotopic composition of lead from primary galena is uniform within an individual mine. The model age of this lead agrees with the age of the mineralization determined by other means—about a billion years. The isotopic composition of the leads in the marble is not uniform today, and calculations indicate that it was probably not uniform a billion years ago. Unless the lead in the ores is a uniform mixture of lead isotopes from an isotopically poorly mixed source, it is doubtful that the lead in the ores was derived from the surrounding marbles.

Cobalt and nickel concentrations in pyrite from grains disseminated in the metasedimentary rocks away from the ore bodies are each greater than 200 ppm. Most samples of pyrite from the ore bodies at the No. 2 and No. 3 mines contain less than 50 ppm each of cobalt and nickel. Therefore the author believes it unlikely that the pyrite of the ores is genetically related to the pyrite in the metasedimentary rocks.

Textural relationships suggest that pyrrhotite formed after most of the sphalerite, which in turn formed after most of the pyrite in the ore bodies. By use of the experimentally determined systems FeS-ZnS and FeS-FeS2, it is inferred from the amounts of iron in sphalerite and sulfur in pyrrhotite that the bulk of the sulfide minerals in the No. 2 mine formed above 320° C. The absolute temperature of formation of pyrrhotite indicated by the FeS-ZnS system is about 150° higher than that indicated by the FeS-FeS2 system. The former system probably gives the more reliable estimate.

The concentrations of individual minor elements in sphalerite and pyrite range considerably among specimens of the same sulfide mineral from the same level and ore body. An exception is cadmium in sphalerite which has a narrow concentration range around 1400 ppm in both the No. 2 and No. 3 mines. The ratio of the concentrations of minor elements between sphalerite-pyrite pairs varies considerably also. This variation probably indicates that exchange of minor elements between pyrite and sphalerite durin g the formation of the ores was very slow and incomplete.

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