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A belt of pyrite deposits about 40 miles long occurs in the Grenville Series of the northwest Adirondacks, New York. The pyrite deposits are conformable and are restricted to certain stratigraphic zones. Graphite is a consistent accessory. The trace element characteristics of the pyrite, very low tellurium and a high nickel to cobalt ratio, are typical of pyrite from known sedimentary occurrences. The massive pyritic zones contain isotopically light sulfur (~ - 10 %0 average) and exhibit similar sulfur isotope characteristics to the Wabana, Newfoundland, and Nairne, Australia, deposits, both of inferred sedimentary origin. The pyrite sulfur probably underwent bacterial reduction from aqueous sulfate. On the basis of the stratigraphic, geochemical, and isotopic evidence, the Adirondack pyrite deposits are inferred to have been initially bacterial, sedimentary-diagenetic in origin.

The pyrite-bearing rocks have been strongly metamorphosed, on a regional scale, in the upper almandine-amphibolite facies and hornblende granulite subfacies. Most of the host rocks are migmatitic gneisses. In the vicinity of the pyritic zones, the mineralogy and textures of the regionally metamorphosed host rocks have been altered by subsequent hydrothermal activity. Dravite is present in concentrations of up to several percent in some of the pyrite deposits; texturally, the dravite exhibits replacement relationships with the host rock silicate minerals. The ground-mass of the pyrite concentrations consists largely of quartz and very line-grained nearly isotropic chlorite; sericite is abundant locally. Massive, coarsely crystalline pyrite aggregates and nodular pyrite are common rock fabric elements. The nodular structure consists of concentric spherical shells, entirely of pyrite, or largely of chlorite and quartz. Chemically, the groundmasses of the pyritic ores are high in H20, Mg0, and C (graphite) and low in Na20, Ca0, and K20 in comparison to the regional gneisses. The pyritic deposits thus have undergone at least a three-stage history: sedimentation-diagenesis; high grade metamorphism; and localized hydro-thermal metamorphism.

Pyrite and ore sulfides from the pyritic-zinc deposit at Balmat, New York, contain isotopically heavy sulfur (~ + 15 %0). Mt. Isa, Australia, Pine Point, Canada, and Meggen, Germany, are similar to Balmat in their stratabound characteristics, abundance of pyrite with zinc or lead or both, and isotopically heavy sulfur. On the basis of published data, circulating thermal brines coupled with bacterial or organic chemical reduction of sulfate could be a source of sulfide sulfur with a relatively heavy and uniform isotopic composition.

Sulfur isotope data on the Lockatong (Pennsylvania), Wabana (Newfoundland), and Wauseca (Michigan) pyrite concentrations were determined for this study and given for comparison.

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