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GENERAL GEOLOGY

Correlation

Although the general geology of the Mesabi Range has been described most exhaustively in U. S. Geological Survey Monographs Nos. 43 and 52, and elsewhere, a short synopsis may be a necessary preliminary to the detailed discussion of the Range presented in this paper, because many of the readers of this article may be unfamiliar with the above publications or the general geology of the Mesabi district.

The topographic feature of the Mesabi Range is a line of fairly prominent and continuous hills ranging in elevation from 1,400 to 1,900 ft. above sea level, composed of a complex of granites, greenstones, green schists, slates, graywacke and conglomerate. Resting unconformably upon this basement complex, and sloping away at a gentle angle to the southeast, is a series of sedimentary rocks, the middle member of which is iron-bearing. The outcrop of this iron-bearing member is the geologic feature known as the Mesabi (or Missabe) Iron Range. Within this formation the iron orebodies are found. Its outcrop has been traced by explorations from Sec. 12, T. 142 N., R. 25 W., northeastward to Birch Lake, in Sec. 26, T. 61 N., R. 12 W., a distance along the strike of about 112 miles. Its width varies from 3% to 3 miles, due to variation in the dip and thickness. The sedimentary series above mentioned consists of a basal quartzite, named Pokegama, an intermediate iron-formation, named Biwabik, and a top black slate, named Virginia slate.

The location and general extent of the Range is shown on the Map, Fig. 1. The general relations of the iron-bearing member to associated rocks are shown by the cross-section, Fig. 2, better than further description could explain.

Instead of the complete correlation table as given by the U. S. Geological Survey, a simplified

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geologic column is herewith given, which is adequate for the engineer in the district.

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Unconformity

Upper Huronian Series.....

(Sedimentary)

Unconformity

Algonkian and Archean Series....

Conglomerates and shales, 0 to 50 ft.

Duluth Gabbro

Embarrass Granite

East end of range.

Virginia slate 0 ft. to great thickness. Biwabik Iron-formation, 475 to 775 ft. Pokegama Quartzite, 50 to 150 ft.

Basement complex of Slate-graywacke-conglomerate series, granites, greenstones, greenschists and porphyries.

From a commercial standpoint the Upper Huronian series, Virginia Slate, Biwabik Iron-formation, Pokegama Quartzite are the important rocks of the district.

The characteristics of the different formations need not be discussed. This will be done for the iron-formation later on.

Orebodies

The Upper Huronian Quartzite-Iron-formation-Slate series is an ordinary (except for the iron) series of clastic sediments, deposited in fairly shallow water, contemporaneous with the middle member of which was deposited or precipitated out of solution an enormous quantity of iron, chiefly in the form of a ferrous silicate, FeSiO., occurring as coarse and minute green granules of the mineral greenalite, embedded in a matrix of chert. Iron carbonate, grunerite, amphibole, actinolite, and hematite occur in small quantities. Original magnetite in bands and finely disseminated grains occurs in much larger quantity than has been recognized heretofore, in all parts of the district. The bedding-planes of all three members are approximately parallel. Interbedded with the iron-formation in certain horizons are numerous slate layers, varying in thickness from a few inches to many feet. The iron-formation from Virginia slate to quartzite varies in thickness from 475 to 775 ft., the average being about 600 ft.

After this series of sediments had been laid down, earth movements raised them above water level, allowing erosive agencies to cut through the overlying slate into the underlying formations. These earth movements warped the formations and cracked the brittle iron-formation quite extensively, allowing surface waters to enter its upturned edges. Especially where such cracking was pronounced, the ground-waters entering the iron-formation, carrying carbon dioxide in solution, attacked the ferrous iron compounds and oxidized them. In such localities much of the ferrous silicate has been changed to a hematite-chert rock, the hematite occurring as bands and as disseminated particles in the chert, the rock still retaining its solidity. The whole iron-formation, whether

thus altered or not, is a ferruginous chert with interbedded slate layers and locally is called "taconite." This term should apply strictly to the ferruginous chert and not to the slate layers, though in some horizons the slate bands and chert layers are so intimately interbedded as to make a distinction quite impossible.

Where the cracking has been most intense the circulation of groundwater has been most vigorous, the solvents in the ground-waters have leached out the silica and other minor constituents of the rocks and have oxidized and left in place the iron. Such residual material now constitutes the orebodies. They are surrounded on all sides by the rock walls of the iron-formation from which they are derived. Pore-space was developed by this removal of silica and the settling or slumping in place of the layers of the orebodies is a characteristic feature. The typical orebody thus developed has a trough structure and an irregular trough shape. Orebodies vary in size from a few acres to several hundred acres, and from a few feet to several hundred feet in thickness. In many

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FIG. 3.-PLAT SHOWING SHAPE AND AREA OF TYPICAL OREBODIES.

places small troughs unite to form a large one. Fig. 3 shows the shape and area of a typical orebody. While the trough orebody is the typical one, there are two other types, namely, the flat-layered body and the fissure-type orebody. The former is either the remnant left by the erosion of a former trough-body or it is an ore layer continuing down the dip from a trough-body. Usually such a layer has a rock (slate) capping. The fissure-type orebody is an incompletely developed trough-body and is usually associated with a larger trough orebody. Fig. 4 shows the development of a trough orebody on the axis of an anticline, where cracking has been pronounced. All four stages here shown can be observed in the field. Notice the slumping of ore layers near the rock walls. The features of the three different types of orebodies were discussed at length in the Engineering and Mining Journal, July 17 to Aug. 7, 1915, and will not be repeated here.

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FIG. 4.-CROSS-SECTIONS SHOWING STAGES IN DEVELOPMENT OF A TROUGH OREBODY ON THE AXIS OF A GENTLE ANTICLINE.

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