Geology: The Science of the Earth's Crust

by William J. Miller

Fig. 80.—Drawing showing details of part of an ore-bearing vein at Pinos Altos, New Mexico. The chalcopyrite and sphalerite are the ores. Somewhat reduced in size. (After Paige, U. S. Geological Survey.)

We shall now very briefly consider several of the greatest iron-mining districts of the United States, giving some idea of the modes of occurrence and origin of the ores. Greatest of all is the Lake Superior region, not far west and south of the lake in Minnesota, Michigan, and Wisconsin. Considerably more than one-half the iron ore mined in the United States comes from the single State of Minnesota, and about one-fourth of it from Michigan. Most of the Minnesota ore by far is obtained from the so-called “Mesaba Range,” which in 1917 produced 41,000,000 tons of hematite ore. The ore deposits are there of irregular shape, lying at or near the surface (usually covered only by glacial deposits). None of them extend downward more than a few hundred feet. The soft, high-grade ore is removed by steam shovels in great open pits. In the several districts of northern Michigan and Wisconsin the ores (nearly all hematite) are associated with more or less highly folded rocks at considerable depths. The Lake Superior iron ores all occur in rocks of Archeozoic and Proterozoic Ages. According to the best explanation of their origin the iron of the ores was once part of a sedimentary series of rocks in the form of iron carbonate and silicate, interstratified with layers of a flintlike rock associated with slate, quartzite, etc. After these rocks were raised into land and subjected to weathering the old iron compounds were altered to oxides, mainly hematite, and somewhat concentrated. Further concentration of the ore was caused by dissolving out the flintlike layers of the old rocks.

The Birmingham, Ala., region is the second most important iron ore producer in the United States, with an output of nearly 6,000,000 tons in 1918. The ore is hematite, forming part of the famous Clinton iron ore deposits of Silurian Age. This deposit, named from Clinton, N. Y., extends through central New York and in more or less interrupted parallel bands through the Appalachian Mountains to near Birmingham where the richest deposits occur. This ore appears to be an original bed (or locally several beds) of fairly rich iron ore deposited on the shallow Silurian sea bottom and then covered by other strata. At the time of the Appalachian Mountain revolution the iron ore was more or less highly folded in with other strata throughout the Appalachians. A remarkable fact regarding the Birmingham district is that in the near vicinity of the ore there are both coal for fuel and limestone flux for smelting the ores.

The next most important mining region of the United States is the Adirondack Mountain region of northern New York, where about 1,000,000 tons of ore are obtained yearly. Magnetite is the ore, and it occurs in more or less irregular lenses and bands in granite and closely associated rocks of pre-Paleozoic Age. One view regarding the origin of this ore is that it segregated during the process of cooling of the molten granite, and another view (recently advocated by the author) is that it was derived from an older iron-rich igneous formation by either the molten granite or very hot solutions from it and concentrated into the ores. About 2,500,000 tons of magnetite were mined in the United States in 1916, nearly one-half of it in the Adirondacks.

The third important iron ore is limonite, nearly 2,000,000 tons of which were produced in the United States in 1916. Most of it came from the Appalachian Mountains. All of this limonite is of secondary origin; that is, it has been derived from certain early Paleozoic iron-bearing limestones either by weathering or solution, and concentrated into ore deposits.

Copper. This is one of the most useful of all metals. Several of its very important uses are as a conductor of electricity in the form of wire; in making alloys such as brass and bronze; in copperplate engraving; and in roofing and plumbing. Various minerals containing copper are found in many parts of the world, but only about six of them are really important as ores. These are native copper, chalcopyrite, chalcocite, azurite, malachite, and cuprite, most of which are described in the chapter on “Mineralogy.” The number of places where they may be profitably mined as ore is distinctly limited. Fifteen or twenty countries produce more or less copper, but the United States is by far the greatest producer, with an output of nearly 2,000,000,000 pounds of copper in 1916, the output having fallen off some in 1918. This was two-thirds of the world’s output and ten times as much as the nearest competitor. The other leading countries are Japan, Chile, Mexico, Spain, and Canada. In 1918 the four leading States in order were Arizona, Montana, Michigan, and Utah, with production ranging from nearly 765,000,000 pounds to about 230,000,000 pounds.

In Arizona several great copper-mining districts lie in the southeastern one-fourth of the State. Almost invariably the ores are directly associated with limestone and an igneous rock (granite), both of late Paleozoic Age. The ores are almost always near the border between the two rocks, mostly as great irregular deposits within the limestone, and less commonly as veins within the granite. The original ores were carried in solution and deposited by hot liquids (or vapors) from the cooling granite. At lower levels the ores are mainly sulphides of copper (e.g., chalcopyrite and chalcocite), while at higher levels they are mostly carbonates (malachite and azurite) and oxides (e.g. cuprite). The difference is due to the fact that the ores nearer the surface have been subjected to weathering and altered from their original condition.

The region around Butte, Mont., is next to the greatest copper producer. Nearly all the ores are sulphides of copper (mainly chalcocite) which occur with quartz in a great system of nearly parallel veins in granite of Tertiary Age. “It is supposed that in the copper veins the hot ore-bearing solutions ascended the fractures in the granite, replacing the rock by ore, and resulting in an intense alteration of the walls.” (Ries.)