Abstract The western Vermilion district is located in northeastern Minnesota, about 18 mi (30 km) north of the towns of Virginia and Aurora on the Mesabi iron range (Fig.1). The major towns in the western Vermilion district are Cook, Tower, Soudan, and Ely. The area to be described here lies within the Tower and Soudan 7¼-minute Quadrangles (Fig. 1).

Abstract Electrical resistivity, magnetic susceptibility, and gamma-ray intensity measurements were made of iron formations and associated rocks in the Cuyuna and Mesabi iron ranges of Minnesota, the Gogebic iron range of Wisconsin, and the Iron River district of Michigan. The measurements were made in drill holes and on core samples in the laboratory to provide information on physical properties for evaluating surface geophysical methods of prospecting. Electrical resistivity of the iron formations varies widely and, in general, is a function of the porosity of the rock and type of iron material in the rock. Rocks containing iron carbonate and iron silicate minerals have high resistivity (400 to 1,000 ohm-m), whereas most rocks containing secondary hematite have moderately low resistivity (20 to 100 ohm-m) because of higher porosity. The chert-, magnetite-, and specularite-bearing taconites of the western Gogebic range have extremely low resistivity (less than one ohm-meter) because of electrical conduction through metallic mineral grains rather than through pore waters. Associated silicate-magnetite rocks have high resistivity (500 to 1,000 ohm-m). In general, rocks adjacent to iron formation have much higher resistivity (1,000 to 5,000 ohm-m) than rocks in the iron formation; an exception is the conductive graphitic slate in the Iron River-Crystal Falls district. Magnetic susceptibility measurements, in the drill holes and in the laboratory, show no direct relation between magnetite content and resistivity. In many places, the in-hole measurements are valuable as a supplement to regular core analysis and description.

ABSTRACT The 100-mile-long Mesabi Iron Range contains the Biwabik Iron Formation, the largest of the Lake Superior–type iron-formations in the United States, deposited on the northern edge of the Paleoproterozoic Animikie Basin. This basin has been interpreted as a foreland basin that developed north of the Penokean Fold-and-Thrust Belt (ca. 1850 Ma), or alternatively, as a backarc basin north of the Wisconsin magmatic terrane. The basal unit in the basin, the siliciclastic Pokegama Formation, was deposited upon the ca. 2700 Ma granitic-volcanic basement. It is conformably overlain by the Biwabik Iron Formation, 200–750 ft thick, which consists of four members: lower cherty, lower slaty, upper cherty, and upper slaty. There are two prominent stromatolite zones. Both of the above formations contain attributes of deposition in a tidally influenced environment. The Biwabik is conformably overlain by the Virginia Formation, a thick turbiditic sequence of interbedded black shale, graywacke, and ash beds. All three formations dip southeastward at 10°–20°. The iron-formation (taconite) generally consists of 20%–30% Fe present in carbonates, silicates, and oxides, and 70%–80% SiO 2 . Direct shipping ores, also called natural ores, were originally mined on the Mesabi Iron Range and were instrumental in making the United States an industrial giant and in the winning of WWI and WWII. These ores originated along fault zones in the iron-formation where silica was removed leaving high-grade oxidized hematite-goethite natural ore bodies of 50%–55% Fe. Processing of low-grade magnetic taconite began in 1952, passed the natural ores in tonnage in 1967, and is now totally dominant. Field trip stops will include all three formations, with emphasis on the iron-formation. The final stop is in the folded Thomson Formation, the southerly equivalent of the Virginia Formation.

Abstract Approximately 3.4 billion tons (Gt) of iron ores containing >50 percent Fe were produced from U.S. mines in the Lake Superior region from 1848 until they were exhausted 20 to 30 years ago. The Vermilion Range in Minnesota produced nearly 100 million tons (Mt) of this ore from Archean greenstone belt-hosted iron formation. The remaining production has come from Proterozoic strata including 2.3 Gt from the Mesabi and 100 Mt from the Cuyuna Ranges in Minnesota while Michigan and Wisconsin contributed 230 Mt from the Marquette Range, 290 Mt from the Menominee Range, and 325 Mt from the Gogebic Range. The protore of these direct-shipping ores are carbonate- or oxide-facies banded iron formations that contained 25 to 35 percent Fe prior to undergoing leaching (desilicification), oxidation, and volume loss. The conventional model ascribing these changes to supergene processes has recently been challenged by research showing that hypogene fluids, channeled by faults into structurally favorable horizons and settings, have played a dominant role in producing some of the high-grade (>60% Fe) ores that are presently providing much of the world's iron ore. Descriptions of the North American iron ores, generally starting with the U.S. Geological Survey monographs published at the beginning of the 20 th century provide many tantalizing clues, suggesting that hypogene fluids have indeed played an important role in the evolution of some of these districts. Application of modern geophysical techniques and structural and geochemical analyses may well guide the discovery of new high-grade ores either below or adjacent to the historic mining areas. The time seems to be ripe for exploration to return to the area that can claim to have begun geologists' understanding of this most important ore deposit type.