Iron deposits

Iron, one of the most abundant metals in the earth’s crust, is invaluable in modern industry and architecture because it is the primary component in steel. Locating iron deposits requires onsite testing and interpretation of satellite and other data. Deposits occur in many forms and are extracted all over the world in pit or tunnel mines.

What Is Iron Ore?

The planet Mars derives its red appearance from the iron oxides in its crust. Iron is one of the most abundant metals in the earth’s crust, too, and it is the magnetism of the iron in the earth’s core that creates the North Pole and the South Pole. Iron, along with cobalt and nickel, is one of three elements that is naturally magnetic, and of the three, iron is the most magnetic.

Iron ores are compounds of iron and other elements, and they can occur in a variety of forms. Ferric iron is iron with an oxidation number of +3, while ferrous iron has an oxidation number of +2. A positive oxidation number results when an iron atom loses electrons from joining with another element, such as oxygen or sulfur. For instance, hematite has an oxidation number of +3. Iron has more than two oxidation states, but +2 and +3 are the most common.

A number of frequently occurring iron compounds exist. Hematite, also called bloodstone, is reddish in color, while magnetite is usually gray or black. Both of these are iron oxides, which means they contain both iron and oxygen. Siderite is an iron carbonate, made of iron and carbon. Iron also can join with sulfur to form a sulfide such as pyrite. These ores are differentiated by the amount of iron they contain. Hematite, for example, contains 70 percent iron, while magnetite contains 72 percent iron.

Iron-rich soil is easily identified by its reddish color. However, though common, it would be impractical to extract that usable iron from soil because of the cost and difficulty of the process. Iron (or any other metal) that is found in a form that can be extracted profitably is called a reserve. A resource, though, is simply the presence of iron (or any other metal), whether or not it is usable. In time, iron deposits that are more accessible and more highly concentrated are exploited. Mining poorer or less accessible deposits requires more energy and larger mining operations, but mining and processing technologies are constantly being improved.

Taconite, which is mined in the United States, contains only 20 to 30 percent iron. It used to be considered a resource because its concentration of iron was too low to make it worth mining, but new technology has allowed it to be processed more efficiently; now, large taconite deposits are considered reserves rather than resources.

The Formation of Iron Deposits

In the past, iron ores called bog iron and ironstones were mined. Bog iron formed (and still forms) in lakes and swamps when chemical reactions precipitate iron from solution, producing thin deposits. Ironstones, which are usually pellets of goethite and hematite, formed in shallow ocean environments during periods of tectonic change and warm climate. Neither of these sources is commonly mined.

Banded iron formations (BIFs), found throughout the world, compose the type of deposit most commonly mined today. BIFs are made up of iron ores, such as magnetite, hematite, pyrite, and siderite, in alternating layers with chert (a silicate). BIFs were formed by sedimentary processes—iron precipitating from shallow ocean waters—in the Precambrian period, primarily 1.8 to 2.5 billion years ago. Because conditions for the formation of BIFs no longer exist, humans can only hypothesize the specifics of their formation.

It is thought that the oxygen-poor conditions of the atmosphere and oceans of the past created ideal conditions for the formation of BIFs because photosynthetic microorganisms flourished. These organisms created oxygen as a waste product, and the oxygen bonded with dissolved iron before the sediment settled on the ocean floors. The population of microorganisms increased and decreased cyclically, causing a corresponding cyclical accumulation of sediment, which could explain the alternating iron-rich and iron-poor layers.

Iron deposits often form at tectonic plate boundaries, where cold water passes through basaltic rocks and is heated by magma. These metal-rich liquids are called hydrothermal solutions. The heated water leaches metals from the basaltic rock and then deposits them in formations called black smokers, which are chimney-like formations reaching heights of up to 60 meters (197 feet). Another hydrothermal process is the formation of ferromanganese nodules, which occur in open ocean conditions where currents prevent dense accumulation of sediment. The formation of ferromanganese nodules occurs slowly. These resources cannot be profitably mined because of the low concentrations of iron and the relative inaccessibility of the deposits.

Identifying Iron Deposits

For a deposit to be profitably mined, it must contain large amounts of ore (in the case of hematite, tens of millions of tons). It might seem like these giant deposits would be easy to find, but the process requires gathering extensive data and then testing to make sure a deposit will be worth mining.

Iron deposits can be identified in many other ways. Magnetometers can measure both the strength and the direction of magnetic fields. Gravity meters on the ground can locate areas of gravitational attraction created by the presence of heavy elements. Ground crews can also test soil, rivers, and streams for iron particles, which result from the weathering and erosion of iron-containing rock. Taking readings from airplanes or satellites, a process called remote sensing, also can provide geologists with important data.

One method of remote sensing is called hyperspectral imaging. The rocks on the surface of the earth have magnetic, reflective, and radioactive properties that can be measured from the air. Every metal reflects light and emits electromagnetic radiation, and each can be identified by what is called a spectral signature. A spectral signature is similar to a fingerprint in humans: It is a unique identifier. Instead of a pattern in the skin, the spectral signature comprises absorption bands in the visible, infrared, and ultraviolet spectra. In a laboratory, identification is relatively straightforward because conditions are controlled, but the process of identifying metals in situ is complicated by atmospheric attenuation, the presence of vegetation, variations in topography, the quality of the satellite data, and the organic and inorganic elemental make up of rocks and soils.

The use of satellite technology to locate potential iron reserves is relatively new. As late as the 1980s, few mining companies had the money, equipment, or expertise to acquire and interpret satellite imagery, but agencies such as the National Aeronautics and Space Administration were developing hyperspectral imaging that could accurately identify mineral species.

Both ferric and ferrous forms of iron are highly identifiable with hyperspectral imaging because they have broad absorption bands. Hematite and goethite, for instance, have distinct spectral signatures. Limonite encompasses several different types of ferric iron (including hematite and goethite). Identification of surficial limonite is useful for locating iron deposits because the presence of limonite often indicates hydrothermal processes, which are associated with the accumulation of iron.

Mining Iron Ore

All the methods above can indicate the general area in which iron deposits are present, but to confirm the exact location of the deposits, the land must be drilled for rock samples. Test drilling helps to define the area to be mined by showing the extent and shape of the deposit. A shallower, broader deposit can be exploited more easily with a pit mine, but a deeper, narrower deposit requires tunneling. In the United States, both types of mines are commonly used to obtain iron ore.

Pit mines are generally dug in a bench formation with slightly graded walls to prevent collapse or slides. Because they spread out instead of down, pit mines can become large. The Hull-Rust-Mahoning Mine in Hibbing, Minnesota, one of the largest open pit mines in the world, is more than 5 kilometers (3 miles) long, 3 km (2 mi) wide, and 163 m (535 ft) deep at its most extreme points. It is so large that the town of Hibbing had to be relocated to accommodate the mine. Because of its long mining history, the mine and surrounding area is now a tourist attraction and a national historic landmark.

Shaft or underground mining requires tunneling into the earth and bringing the iron ore to the surface. A shaft is a vertical or near-vertical tunnel, which is excavated adjacent to the deposit. At intervals along the shaft, tunnels called drifts and crosscuts are excavated to allow access for the miners. The drifts and crosscuts are connected by vertical tunnels called raises. This network of passages allows workers to blast into the deposit and remove materials for transport to the surface.

Some iron deposits (soft hematite) are not competent, which means they will not remain in place when the material beneath them is excavated. For this type of ore, a method of extraction called block caving must be used. In block caving, an excavation is made beneath the deposit, and the deposit is allowed to cave in to the level below.

The largest producers of iron ore are Russia, Brazil, China, Australia, India, South Africa, and the United States. Worldwide, high-grade ores have been largely depleted and lower-grade ores are taking their place in the mining and manufacturing cycle. The US Geological Survey reports that primary North American iron reserves will be depleted sometime before 2050. The International Metallic Association cautioned that a shortage of commercially available DR-grade iron ore pellets is likely to occur by the end of the 2030s. Such a shortage is problematic because the demand for DR-grade iron is expected to significantly increase as the world progresses to net-zero emissions.

Technology may make underwater mining profitable. Areas of high iron content would be located by surveying with submersibles, instead of with airplanes or satellites, and then drilling for samples. It would not be economical to mine iron in this manner. Because so much iron already has been brought to the surface, recycling is more likely to replace mining as a way to supply human needs. Although not much scrap iron is recycled, more than 67 percent of steel, mostly from the auto industry, is salvaged and reprocessed.

Environmental Issues Related to Mining and Processing

Pit mines are less costly than underground mines, but they also have a higher environmental impact. Dust is stirred up on-site during clearing of the land and during blasting, drilling, crushing, and transportation of ore. The machinery used in the iron mining process is also energy intensive and produces emissions. As with all forms of ore mining, buried rock that is exposed to air and water leaches metals into nearby water systems, creating a substance known as acid mine drainage, which can have far-reaching implications when it enters rivers and streams.

A particular iron deposit may be exploited for decades before it is depleted, but when a mine is no longer viable, the mining company must do a number of things to restore the landscape and to prevent pollution and other side effects of the mining process.

A pit mine can be filled in with mullock or overburden (the rock removed to access the ore) after the mine is closed. Mine walls can be reshaped and planted with vegetation. Toxic water and tailings must be contained by dams and neutralized in treatment pools. A possible alternative option is to use bioleaching bacteria to process contaminated water.

In the case of underground mines, mine shafts and adits (horizontal access tunnels) must be capped or filled. If bats are present, bat grates must be used to seal tunnel openings while still allowing bat ingress and egress. Subsidence hazards also must be addressed. Mine exhaustion also has social and economic consequences because towns are built to support the labor and other needs of a mine. When the mine closes, the town’s industries and workers are no longer needed by the mining company.

Environmental issues also affect processing and manufacturing. During the manufacture of steel, particulates are kept from the air with dust catchers, electrostatic precipitators, and wet scrubbing systems. Settling basins and clarifiers remove oil and solids from water before it is returned to the local water system. Water used in processing can also be cleaned and recycled within the plant.

Principal Terms

alloy: a metal composed of two or more elements

banded iron formations: the type of iron deposit most commonly mined; composed of iron ores in alternating layers with chert

ferric: describes iron compounds with an oxidation number of +3

ferrous: describes iron compounds with an oxidation number of +2

goethite: a yellowish-brown or reddish-brown iron oxide-hydroxide

hematite: an iron oxide that is usually gray or black, but can also be red; contains 70 percent iron

limonite: an iron oxide-hydroxide of varying composition; includes hematite and goethite

magnetite: an iron oxide that is usually black; contains 72 percent iron

siderite: an iron carbonate that contains 48 percent iron

taconite: a low-grade ore that contains 20 to 30 percent iron

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