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Browsing by Author "Listerud, William Harold"

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    Geology of a Sulfide Deposit in Lower Precambrian Metavolcanic-Metasedimentary Rocks near Birchdale, Koochiching County, Minnesota
    (1974-02-22) Listerud, William Harold
    The Birchdale area lies near the southern edge of the Superior Province of the Canadian Shield. The rocks are felsic-intermediate metatuffs, metasediments, granite, and mafic intrusives. The rocks are part of a northwest dipping, northeast trending monocline which is on the southern margin of a small granitic pluton, the Birchdale Granite. The greenstone rocks have been metamorphosed twice, first to upper greenschist or lower amphibolite facies during the regional metamorphism, and then to albite-epidote hornfels facies as a result of the intrusion of the Birchdale Granite. The two metamorphic events have obscured much of the original textures and structures of the rocks and made the origin of the sulfides more difficult to determine. The sulfides of the Birchdale anomaly area are pyrrhotite, pyrite, chalcopyrite, sphalerite, pentlandite, and marcasite. The main sulfide horizon is in a series of interbedded metatuffs and graphitic metasediments, although disseminated sulfides are found throughout the section. Pyrrhotite is the principal sulfide, with only minor amounts of the other phases. The main sulfide horizon thickens, increases in sulfide content, and increases in graphitic material northeastward along strike. Trace element work shows apparent increases in cobalt and nickel, and an enrichment of copper over zinc northeastward. The sulfides appear to be syngenetic, based on their position in the sequence and their relationships with the enclosing rocks. The sulfides now show mineralogy, textures, and structures related to the metamorphic episodes.