Distribution and Composition of Sulfide Minerals at Balmat, New York

Part I. In the Balmat area, northern New York, tabular deposits of sulfide minerals are found parallel to the layering in folded, siliceous magnesian marbles of a metamorphic complex commonly referred to as the Pre-Cambrian Grenville 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, diffused over distances of inches to feet. Access to the ore is provided through the Balmat No. 2 and No. 3 mines. The isotopic composition of lead from primary galena is homogeneous within an individual mine. The model age of this lead is in agreement with the age of the mineralization determined by other means about a billion years. The isotopic composition of lead and the concentrations of lead and uranium in two samples of marble were investigated. The isotopic composition of the leads in the marble is not homogeneous. Calculations indicate that the isotopic composition of lead in the marble was probably not homogeneous a billion years ago. Unless the isotopic composition of lead in the ores is a homogeneous mixture of lead from an isotopically inhomogeneous 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 greater than 200 ppm. Most samples of pyrite from the ore bodies of the No. 2 and No. 3 mines contain less than 50 ppm each of cobalt and nickel. From these differences in concentrations, the author believes that it is unlikely that the pyrite of the ores is genetically related to the pyrite in the metasedimentary rocks. Studies of textural relations 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-S 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. This estimation assumes a total pressure of 3 kilobars, The absolute temperature of formation of pyrrhotite indicated by the FeS-ZnS-S system is about 150° higher than that indicated by the FeS-FeS2 system. 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 the minor elements did not achieve a well-developed equilibrium distribution between pyrite and sphalerite during the formation of the ores. Part 2: The sphalerite and pyrite from the No. 2 and No. 3 mines at Balmat, New York, and pyrite from the surrounding country rock are analyzed for minor and trace elements. The iron content was determined in 18 samples of sphalerite associated with pyrite and pyrrhotite, and in 177 samples of sphalerite associated with pyrite without pyrrhotite. Analyses for minor and trace elements were made of 74 samples of sphalerite from the mining areas, 32 samples of pyrite from the mining areas, and nine specimens of pyrite from the surrounding metasedimentary rock. In addition, trace and minor elements were determined in nine samples of calcite, seven samples of dolomite, and three samples of pyritic schist from four meta-sedimentary units. The isotopic composition of lead was determined in seven specimens of galena and in two samples of marble. The compositional studies of minerals are related to geologic features of the ore bodies and the surrounding metasediments in order to clarify the (1) relative and absolute age of the deposition of the sulfides, (2) temperature and pressure at which the sulfides were emplaced, (3) source and composition of the ore forming fluid, and (4) causes of the zonal pattern of the minerals in the ore bodies. The primary mineralization occurred during a period of hydration of the surrounding metasediments, after the growth of tremolite and before the formation of talc. Tremolite and talc formed during the metamorphism and retrograde metamorphism of the Grenville series near Balmat. The metamorphism is thought to have occurred about 1050 million years ago. The ore fluid successively deposited pyrite, then galena and iron-rich sphalerite followed by pyrrhotite and chalcopyrite. Subsequently much of the pyrrhotite was altered to marcasite. The marcasite is thought to have formed during the deposition or remobilization of anhydrite and barite. The mineralization concluded with the deposition of iron-poor sphalerite. Recurrent microbrecciation of the metasediments and ores can be correlated with the time sequence of formation of the various ore minerals in various parts of the mines. The ore fluid followed the separate zones of microbrecciation and deposited the sulfides at nearly the temperature of the country rock. On the basis of the space and time relationship between the zones of microbrecciation and different ore minerals, it is inferred that the composition of the ore fluid reaching the site of deposition of the sulfides changed with time. The isotopic composition of lead from galena indicates that the source of the galena is compatible with the following model - a homogenized substratum in which radiogenic lead from the decay of uranium and thorium has been added to lead of meteoritic composition without disruption from the time of origin of the earth until the time of extraction of the ore fluid. A part of the pyrrhotite found in the No, 2 mine was formed in an area containing sphalerite and pyrite. The iron content of sphalerite (12.5 ? 1 percent FeS by weight) associated with pyrrhotite and pyrite is found to be significantly higher than in areas where sphalerite occurs without pyrrhotite. It is inferred that in the region containing pyrrhotite the sphalerite became saturated with FeS at the temperature of formation of pyrrhotite. From the estimates of others on similar areas, the total pressure during the deposition of the ores at Balmat is assumed to be 3 +/- 1 kilobars. By use of the phase diagram for the FeS-ZnS system, the temperature of deposition of pyrrhotite is found to be 510°. The composition of pyrrhotite was investigated by the measurement of lattice spacings. Only one specimen of pyrrhotite was found to contain the necessary hexagonal structure for use with published plots of lattice spacing versus composition. By use of the phase diagram for the FeS-FeS2 system, a temperature of formation of pyrrhotite based on one specimen is indicated to be 340°C. The discordance in the temperature estimate using two different geothermometers is thought to be due at least in part to exsolution of pyrite from pyrrhotite at a temperature lower than that of the formation of pyrrhotite. The variation in the iron content of sphalerite associated with pyrite is thought to be mainly due to variations in the fugacity of sulfur species rather than changes in temperature. By use of the phase diagram of the ZnS-FeS2-S system, the temperature of formation of sphalerite (6-9 percent FeS by weight) associated with pyrite in the No. 2 mine is indicated to be greater than 320°C. Sphalerite associated with pyrite in the No. 3 mine contains 2-3 percent FeS by weight. Calculations based on the partitioning of trace and minor elements between sphalerite and pyrite give a first approximation of the temperature at which equilibrium was approached in the No. 3 mine of 300-350°C. Once sphalerite acquires a particular concentration of iron, it is quite resistant to change due to following geologic events. At the temperatures of formation of the sulfides at Balmat, however, reaction between sphalerite and pyrrhotite must have been comparitively rapid. Alternation of pyrrhotite to marcasite did not seem to affect the concentration of iron in sphalerite. Supergene mineralization of magnetite, hematite, and chlorite leached iron from sphalerite grains, but the leached zone is usually 0.1 millimeter thick or less and only rarely attains a thickness of one millimeter. On the other hand, the content of manganese in sphalerite from the zones of supergene mineralization is much lower than sphalerite away from the zones. The concentration of cadmium in sphalerite from both mines falls in the range 1000 to 2000 parts per million with an average value of 1400 parts per million. The content of cadmium in sphalerite does not seem to be sensitive to temperature or accompanying mineral phases or the supergene mineralization.