The Relationship between the Cloud Zone and Basal Zone Cu-Ni Sulfides, and the Significance of Mafic Pegmatites, Minnamax Property, Duluth Complex, Minnesota
1988-05
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The Relationship between the Cloud Zone and Basal Zone Cu-Ni Sulfides, and the Significance of Mafic Pegmatites, Minnamax Property, Duluth Complex, Minnesota
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1988-05
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Abstract
The intrusive rocks and associated sulfides of the Partridge River troctolite are examined in two drill cores (82 and 363) from the Minnamax property in northeastern Minnesota. Each drill core is approximately 1700 feet (~525 meters) in length. The predominantly troctolitic rocks are divided into four highly generalized rock units, primarily for the purposes of discussion. These have been named the upper mixed unit (UMU), main unit (MU), lower mixed unit (LMU), and basal unit (BU). The UMU and LMU are moderately inhomogeneous units containing troctolitic rocks, mafic pegmatite, hornfels inclusions, and irregularly distributed sulfides. The MU divides the upper mixed unit from the lower mixed unit and is characterized by relatively homogeneous medium- to coarse-grained troctolitic anorthosite anorthositic troctolite. In the MU, mafic pegmatite, hornfels, and sulfide are more uncommon, especially in DDH 363. The basal unit is a highly heterogeneous zone in the lowermost 100-150 feet (30-50 meters) of the drill core. Mafic pegmatite, hornfels, and sulfide are ubiquitous and abundant. The basal zone and cloud zone sulfides in drill cores 82 and 363 are hosted by the LMU/BU and UMU, respectively. The basal and cloud zones are separated from each other by the dominantly sulfide-free MU. In both zones, sulfide occurs as disseminated, fine to medium grains within the troctolitic rocks, or as coarse to pegmatitic interstitial grains in pegmatitic gabbroic anorthosite - anorthositic gabbro. The pegmatite commonly contains a much greater percentage of sulfides (5- 30 %) than the troctolites (< 1%). Intervals of sulfide-bearing mafic pegmatite (and hornfels inclusions) are more common in the LMU, and especially the BU, than in the UMU, contributing to the greater abundance of sulfide in the basal zone. The cloud zone sulfide mineralogy is dominated by copper sulfide; chalcopyrite is commonly more abundant than cubanite. The basal zone mineralization is divided into an upper basal zone and a lower basal zone on the basis of a change in relative abundances of the major sulfide minerals. The upper basal zone (corresponding roughly to LMU) is characterized by a greater abundance of cubanite relative to chalcopyrite and a low abundance of pyrrhotite. The lower basal zone mineralization (corresponding roughly to BU) contains pyrrhotite as the dominant sulfide mineral. The striking association of maf ic pegmatite and sulfide mineralization is briefly explored. Given the common proximity of Virginia Formation xenoliths to pegmatite, it is suggested that, by giving up volatile constituents (which includes S and C) to the troctolitic magma, these xenoliths promoted both the formation of pegmatite and of sulfide saturation of the melt.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Sarah Mills Ervin in partial fulfillment of the requirements for the degree of Master of Science, May 1988.
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