Browsing by Subject "Mesabi Range"

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    Bulletin No. 38. The Stratigraphy and Structure of the Mesabi Range, Minnesota
    (Minnesota Geological Survey, 1954) White, David A.
    The later Precambrian Animikie group in northeastern Minnesota consists of three sedimentary units: the Pokegama (quartzite), Biwabik (iron-rich rock), and Virginia (argillite) formations. "Mesabi range" designates the preglacial outcrop belt, 1/4 to 3 miles wide and 120 miles long, of the Biwabik formation. Varieties of iron-rich rock ("taconite") are either granular or slaty and consist dominantly of chert, iron silicates, magnetite, and siderite. The Lower Cherty, Lower Slaty, Upper Cherty, and Upper Slaty members of the Biwabik formation, which averages 600 feet in thickness, can be further subdivided as shown on a detailed longitudinal stratigraphic section. These members are fairly uniform along most of the range, but only one cherty and one slaty member exist on the Westernmost Mesabi, where the lithic units are intertongued. The areal distribution of rock units on the Westernmost Mesabi is shown on a geologic map. The Biwabik, Pokegama, and Virginia formations are considered conformable. Mesabi rocks probably correlate with those of the Emily district 30 miles away. Chert, greenalite, minnesotaite, stilpnomelane, magnetite, some hematite, and siderite probably formed either during deposition or diagenesis. The rocks are essentially unmetamorphosed. The Pokegama and Biwabik formations were probably produced by the migration of a series of coexisting environments of deposition during an advance, a retreat, and a second advance of the Animikie sea. The deposits formed, during the retreat, in successive environments seaward from shore, were clastic material, carbonaceous-pyritic mud, chert-siderite, chert-magnetite, and iron silicate. Fine clastics of the Virginia formation, perhaps furnished by an outburst of volcanic activity, spread across the former environments of chemical sedimentation. Possible conditions of iron sedimentation were as follows: derivation of iron and silica by weathering of a low-lying land mass, perhaps under an atmosphere rich in carbon dioxide, and a seasonal climate; tectonic stability; and deposition in a shallow, quiescent epicontinental sea. The Animikie beds strike about N. 75 degrees E. and commonly dip 6-12 degrees SE. A structure contour map on the base. of the Biwabik formation shows numerous small anticlines, synclines, monoclines, and faults. Three major joint sets are present. The few rocks intrusive into the Biwabik formation include diabase sills, the Duluth gabbro, and the Aurora syenite sill. Contact metamorphism by the soda-rich Aurora sill has produced crocidolite in adjacent taconite. Minor internal folding of Animikie beds seems to be more prevalent where the underlying rocks are volcanic or sedimentary rather than granitic. The Mesabi range is covered by glacial drift which thickens southward, commonly from 20 to 200 feet, away from a ridge known as the Giants Range. Drift is as much as 500 feet thick over the Westernmost Mesabi. A map of the thickness of drift shows many drift-buried preglacial bedrock valleys that extend from notches in the Giants Range southward across the Mesabi range. Cretaceous iron-ore conglomerates, which at places overlie the Biwabik formation, occur as erosional remnants on bedrock ridges. The scattered soft iron-ore bodies in the Biwabik formation are residual concentrates of oxidized iron minerals formed by the leaching of silica from the chert and iron silicates in taconite. Conditions favoring ore concentration are thought to be as follows: accentuated fracturing at folds and faults allowing ready circulation of leaching solutions; a high iron content in taconite; reducing rather than oxidizing conditions of deposition of the original taconite; the fine size of the grains in taconite and the intimate intermixing of different minerals; a lack of metamorphism, which coarsens the grain; and the availability of large amounts of solutions. The soft ores may have been concentrated by downward-circulating ground waters.
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    Bulletin No. 43. The Geology of the Metamorphosed Biwabik Iron-Formation, Eastern Mesabi District, Minnesota
    (Minnesota Geological Survey, 1962) Gundersen, James Novotny; Schwartz, George M.
    The construction of large concentrating plants and opening of pits for large-scale mining and milling of taconite during the past decade has emphasized the importance of all aspects of the geology of the Eastern Mesabi district. It had been known from the earliest explorations in the Mesabi district that the eastern twenty-mile portion was characterized by a hard, siliceous, magnetite rock which Winchell called "taconyte." It was soon learned that this rock had not yielded to natural enrichment as had many areas in the main part of the range. Later, as the large complex concentrating plants went into operation, it became evident that intensive study of certain aspects of the geology, including detailed lateral as well as vertical stratigraphic variations, together with mineralogical and petrographic characteristics, would be extremely important to the successful operation of the huge pits and concentrating plants. The only detailed publication on the Eastern Mesabi was by Grout and Broderick in 1919, a study that necessarily depended mainly on outcrops, whereas large amounts of diamond drill core and extensive vertical exposures in the pits are now available. The earlier mineralogical work was done before x-ray methods were well developed and before the complex amphibole and pyroxene groups were well understood. It was therefore obvious to the Director of the Minnesota Geological Survey that a modern detailed study was needed to supplement the excellent earlier work of Grout and Broderick. Fortunately, Dr. E. W. Davis, who had spent much of a lifetime on developing a process to concentrate taconite, was a consultant for the Reserve Mining Company. He fully understood the significance of a detailed knowledge of taconite, and as a result of his suggestions the company established an excellent postgraduate fellowship at the University of Minnesota to aid in fundamental research on the characteristics of taconite. Dr. James N. Gundersen, currently of the Department of Geology, Los Angeles State College, was appointed to the fellowship. The Minnesota Geological Survey agreed to assume field and other expenses and to direct the work. The results published in this bulletin speak for the character of the work accomplished. The bulletin is adapted from Dr. Gundersen's thesis submitted for the degree of Doctor of Philosophy. He deserves the highest praise for the energy and devotion he has given to the problem. The economically important Eastern Mesabi district of Minnesota is the type locality for the iron-formation rock type taconite, a stratified quartzose rock containing significant amounts of iron-bearing oxides, hydroxides, silicates, and, locally west of the district, carbonates. Five basic types of taconite massive, layered, laminated, shaly bedded, and shaly are delineated for detailed classification of the stratified structure and mineralogy of the Biwabik iron-formation in this district.
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    Bulletin No. 45. Progressive Contact Metamorphism of the Biwabik Iron-fomation, Mesabi Range, Minnesota
    (Minnesota Geological Survey, 1968) French, Bevan M.
    The recent, spectacular growth of the taconite industry, and the expansion of taconite operations from the eastern to the central and western parts of the Mesabi range, emphasizes the importance of knowledge of the geology of the range. The earliest taconite plants were established in the Eastern Mesabi district, in areas in which the Biwabik Iron-formation was metamorphosed by the Duluth Gabbro Complex; most of the recent plants are in the Main and Western Mesabi districts, in areas of "unaltered" iron-formation. This report describes the changes in mineralogy and texture from "unaltered" taconite in the Main Mesabi district to highly metamorphosed taconite in the Eastern Mesabi district. It describes not only the silicate minerals, but also the opaque iron oxides, carbonate minerals, and carbonaceous material. Knowledge of the mineralogic changes is extremely important to the practical problems related to beneficiating characteristics of the magnetic taconites. The report is modified from a Ph.D. thesis submitted to the Graduate School at Johns Hopkins University by Bevan M. French. The Biwabik Iron-formation, on the Mesabi range in northern Minnesota, extends for about 120 miles in a generally east-northeast direction, from west of Grand Rapids on the Mississippi River to Birch Lake, east of Babbitt. The formation is the middle unit of the Animikie Group of Middle Precambrian age. On the eastern end of the Mesabi range, the Animikie Group has been metamorphosed by the intrusive Duluth Gabbro Complex; mineralogical changes in the sediments, particularly in the iron-formation, appear to have been caused by the gabbro. From the data of the present study, four metamorphic zones may be distinguished within the Biwabik Iron-formation by changes in mineralogy along the strike of the formation toward the gabbro contact:
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    Information Circular 43. Background Levels of Mercury and Arsenic in Paleoproterozoic Rocks of the Mesabi Iron Range, Northern Minnesota
    (Minnesota Geological Survey, 1999) Morey, G.B.; Lively, R.S.
    This study was primarily designed to calculate background levels of mercury and arsenic in the Biwabik Iron Formation of the Mesabi range. A second objective was to evaluate the ability of various laboratories to provide analytical data for mercury and arsenic that is reasonably priced yet sufficiently reliable for the purpose of environmental screening. Mercury and arsenic where present in sufficient concentrations are considered to be hazardous substances. They may be naturally occurring, of anthropogenic origin, or some combination thereof. To establish natural background levels and to reduce the possibility of anthropogenic sources, 191 samples of the Biwabik Iron Formation were collected from drill-core sites located south of the Mesabi range (Fig. 1). Ten samples of the overlying Virginia Formation were also analyzed for comparison. Four of the selected sites were jointly drilled by the Minnesota Geological Survey and the Iron Range Resources and Rehabilitation Board (IRRRB) in 1966 and 1967; principal facts for these holes are summarized in Pfleider and others (1968). The fifth site, U.S. Steel 17,700 was drilled sometime in the 1950s, and the core was donated to the Minnesota Geological Survey in the late 1960s; principal facts for it are summarized in Morey and others (1972).