Bulletin No. 38. The Stratigraphy and Structure of the Mesabi Range, Minnesota

Abstract

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.