"The pyrite-pyrrhotite belt is a northeast-southwest trending area about 35 miles long and three to four miles wide located in Jefferson and western St. Lawrence counties, New York, within the metamorphosed Grenville Lowland just northwest of the Adirondack massif.
Outcropping within this belt are a number of thin discontinuous quartz-biotite-oligoclase gneisses with an unusually high graphite, chlorite, and pyrite content, known as "rusty gneisses." Sulfides are concentrated in these gneisses into zones of up to 50% or more pyrite. The pyrite is granular or crystalline and is invariably accompanied by large amounts of chlorite and graphite and local minor segregations of pyrrhotite. Sphalerite and chalcopyrite, the only other sulfides present, are trivial in amount. There are two types of pyritic concentrations: The first is a simple variation of the rusty gneiss marked by abundant chlorite and graphite and thick disseminations or clusters of pyrite grains, or sheeted zones of pyrite. The second type is found in well defined vein-like bodies in which the pyrite is intimately folded and contorted with vein quartz. Local pegmatitic lithologies are also present.
The deposits are all characterized by their restriction to one lithology, the rusty gneiss, and by their distinct lack of true veining. They are located at the base of a thick marble unit which overlies a thick gneiss of tuffaceous composition.
Results of this study show that the ores were strongly affected by metamorphism and late hydrothermal remobilization. Geothermometric determinations obtained from pyrrhotite indicate that at least this sulfide formed from 475°C to 535°C within the regional metamorphic range of temperature.
A volcanic-sedimentary origin is proposed for these deposits. The sedimentary nature of the ores is indicated by their conformable relations, restriction to one lithology, minor amounts of sulfides other than iron sulfides, large amounts of associated graphite, low Co:Ni ratios, and local presence of pyrite spheroids.
The volcanic nature of the deposits is indicated by the high trace element content, the abnormally large amounts of iron and sulfur, and is suggested by the large amount of underlying rock of tuffaceous composition. Sulfur isotope ratios from the deposits are typical of both volcanic and biogenic sulfur.
The deposits were subjected to intense regional metamorphism as indicated by the folded structures of many deposits, the sheeted pyrite zones, and the injection of subsequently deformed quartz veins. The ore textures are in part typically metamorphic, and the pyrrhotite temperatures fall in the range of peak metamorphic temperatures for the area.
Late hydrothermal effects are evidenced by local veining by pyrite, formation of pyrite metacrysts in abundant introduced replacing chlorite, and the local transgressive nature of much of the remobilized sulfides. The pyrrhotite represents a separate phase of late mineralization caused by the local melting of pyrite.
Latest supergene alteration has affected pyrrhotite more than it has pyrite, forming marcasite and pyrite alterations associated with veinlets of limonite and hematite"--Abstract, pages ii-iv.
Hagni, Richard D.
Grant, S. Kerry
Geosciences and Geological and Petroleum Engineering
M.S. in Geology
University of Missouri at Rolla
xi, 120 pages
Jefferson County (N.Y.)
Saint Lawrence County (N.Y.)
© 1967 Dennis O'Leary, All rights reserved.
Thesis - Open Access
Gneiss -- New York (State)
Geology -- New York (State)
Petrology -- New York (State)
Pyrites -- Analysis.
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Electronic OCLC #
Link to Catalog Record
O'Leary, Dennis, "A study of the genesis of the pyritic gneiss in the Grenville series, St. Lawrence and Jefferson counties, New York" (1967). Masters Theses. 3108.