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In St. Lawrence and Jefferson Counties, New York, there are 19 known occurrences of pyrite concentration of sufficient magnitude to warrant consideration as possible sources of sulfur. The important deposits occur in a belt about 35 miles long and three to four miles wide extending northeast from the vicinity of Antwerp. This belt lies within the Grenville Low-lands region of the northwest Adirondacks, southeast of the St. Lawrence River.The bedrock is principally Precambrian Grenville metasediments and associated granitic rocks, but abundant small outliers of flat-lying Cambrian Potsdam sandstone occur throughout the region. The rocks of the isoclinally folded Grenville series have a marked foliate structure parallel to the northeast regional trend of the lithologic units and major folds, and generally dip northwest within the pyrite belt. The Grenville series is 15,000-16,000 feet thick in the Lowlands region.The earliest pyrite mining in the region was in 1883-4 at the Stella mine; it ceased in 1921, at which time about 600,000 long tons of concentrates averaging about 40 percent sulfur had been produced. Pyrite and lesser amounts of pyrrhotite are essentially the only sulfides present. The deposits occur in rusty-weathering pyritic gneisses, which are essentially chloritic and graphitic quartz-feldspar-mica gneisses containing more or less disseminated pyrite. The ore veins represent concentrations of pyrite within the rusty gneisses and occur as thin sheets parallel to the gneissic foliation. Thirteen analyses indicate a sulfur content from 20.2 to 35.6 percent, and average 25.6 percent sulfur.The pyrite of the ores crystallized later than the gangue minerals, with the possible exception of sericite, and partially replaces them, chlorite being the most readily replaced. The graphite content of the ores characteristically parallels increases in the pyrite content.The pyrite deposits are considered to be concentrations of iron sulfides formed from inherent constituents of the parent sediments from which the rusty gneisses were derived. It is believed that the parent sediments were argillaceous sandstones containing abundant organic impurities and some intercalated calcareous layers. The late-stage formation of abundant chlorite and the evident mobilization and recrystallization of sulfides and graphite well after the period of metamorphism are attributed to the circulation of aqueous solutions.