15.1: Mining

Mining is defined as extracting valuable materials from the Earth for society's use. Usually, these include solid materials such as gold, iron, coal, diamond, sand, and gravel, but can also include fluid resources such as oil and natural gas. Modern mining has a long relationship with modern society. The oldest mine dates back 40,000 years to the Lion Cavern in Swaziland where there is evidence of concentrated digging into the Earth for hematite, an important iron ore used as red dye [4]. Resources extracted by mining are generally considered to be nonrenewable.

Figure \(\PageIndex{1}\): Map of world mining areas.

Renewable versus Nonrenewable Resources

Resources generally come in two major categories: renewable and nonrenewable. Renewable resources can be reused over and over or their availability replicated over a short human life span; nonrenewable resources cannot.

Renewable resources are materials present in our environment that can be exploited and replenished. Some common renewable energy sources are linked with green energy sources because they are associated with relatively small or easily remediated environmental impact. For example, solar energy comes from fusion within the Sun, which radiates electromagnetic energy. This energy reaches the Earth constantly and consistently and should continue to do so for about 5 billion more years [5]. Wind energy, also related to solar energy, is maybe the oldest form of renewable energy, and is used to sail ships and power windmills. Both solar and wind-generated energy are variable on the Earth’s surface. These limitations are offset because we can use energy-storing devices, such as batteries or electricity exchanges between producing sites. The Earth's heat, known as geothermal energy, can be viable anywhere that geologists drill deeply enough. In practice, geothermal energy is more useful where heat flow is great, such as volcanic zones or regions with a thinner crust [6]. Hydroelectric dams provide energy by allowing water to fall through the dam under gravity, which activates turbines that produce energy. Ocean tides (tidal energy) can also be a reliable energy source. All of these renewable resources provide energy that powers society. Other renewable resources are plant and animal matter, which are used for food, clothing, and other necessities, but are being researched as possible energy sources.

Figure \(\PageIndex{2}\): Hoover Dam provides hydroelectric energy and stores water for southern Nevada.

Nonrenewable resources cannot be replenished at a sustainable rate. They are finite within a human lifetime. Many nonrenewable resources come from planetary, tectonic, or long-term biologic processes, and include materials such as gold, lead, copper, diamonds, marble, sand, natural gas, oil, and coal. Most nonrenewable resources include specific concentrated elements listed on the periodic table; some are compounds of those elements. For example, if society needs iron (Fe) sources, then an exploration geologist will search for iron-rich deposits that can be economically extracted. Nonrenewable resources may be abandoned when other materials become cheaper or serve a better purpose. For example, coal is abundantly available in England and other nations, but because oil and natural gas are available at a lower cost and lower environmental impact, coal use has decreased. Economic competition among nonrenewable resources is shifting use away from coal in many developed countries.

Figure \(\PageIndex{3}\): Natural, octahedral shape of diamond.

Ore

Earth’s materials include the periodic table elements. However, it is rare that these elements are concentrated to the point where it is profitable to extract and process the material into usable products. Any place where a valuable material is concentrated is a geologic and geochemical anomaly. A body of material from which one or more valuable substances can be mined at a profit, is called an ore deposit. Typically, the term ore is used for only metal-bearing minerals, but it can be applied to valuable nonrenewable resource concentrations such as fossil fuels, building stones, and other nonmetal deposits, even groundwater. If a metal-bearing resource is not profitable to mine, it is referred to as a mineral deposit. The term natural resource is more common than the term ore for non-metal-bearing materials.

Figure \(\PageIndex{4}\): Banded-iron formations are an important ore of iron (Fe).

It is implicit that the technology to mine is available, economic conditions are suitable, and political, social and environmental considerations are satisfied in order to classify a natural resource deposit as ore. Depending on the substance, it can be concentrated in a narrow vein or distributed over a large area as a low-concentration ore. Some materials are mined directly from bodies of water (e.g. sylvite for potassium; water through desalination) and the atmosphere (e.g. nitrogen for fertilizers). These differences lead to various methods of mining, and differences in terminology depending on the certainty. Ore mineral resource is used for an indication of ore that is potentially extractable, and the term ore mineral reserve is used for a well-defined (proven), profitable amount of extractable ore.

Figure \(\PageIndex{5}\): Diagram illustrating the relative abundance of proven reserves, inferred reserves, resources, and undiscovered resources. (Source: Chris Johnson)

Figure \(\PageIndex{6}\): McKelvey diagram showing different definitions for different degrees of concentration and understanding of mineral deposits.

Mining Techniques

The mining style is determined by technology, social license, and economics. It is in the best interest of the company extracting the resources to do so in a cost-effective way. Fluid resources, such as oil and gas, are extracted by drilling wells and pumping. Over the years, drilling has evolved into a complex discipline in which directional drilling can produce multiple bifurcations and curves originating from a single drill collar at the surface. Using geophysical tools like seismic imaging, resources can be pinpointed and extracted efficiently.

Figure \(\PageIndex{7}\): Bingham Canyon Mine, Utah. This open-pit mine is the largest man-made removal of rock in the world.

Solid resources are extracted by two principal methods, of which there are many variants. Surface mining is the practice of removing material from the outermost part of the Earth. Open-pit mining is used to target shallow, broadly disseminated resources. Open-pit mining requires careful study of the ore body through surface mapping and drilling exploratory cores. The pit is progressively deepened through additional mining cuts to extract the ore. Typically, the pit's walls are as steep as can safely be managed. Once the pit is deepened, widening the top is very expensive. A steep wall is thus an engineering balance between efficient and profitable mining (from the company’s point of view) and mass wasting (angle of repose from a safety point of view) so that there is less waste to remove. The waste is called non-valuable rock or overburden and moving it is costly. Occasionally landslides do occur, such as the very large landslide in the Kennecott Bingham Canyon Mine, Utah, in 2013. These events are costly and dangerous. The job of engineering geologists is to carefully monitor the mine; when company management heeds their warnings, there is ample time and action to avoid or prepare for any slide.

Strip mining and mountaintop mining are surface mining techniques that are used to mine resources that cover large areas, especially layered resources like coal. In this method, an entire mountaintop or rock layer is removed to access to the ore below. The environmental impacts of surface mining are usually much greater due to the larger surface footprint that is disturbed [7].

Figure \(\PageIndex{8}\): A surface coal mine in Wyoming.

Underground mining is often used for higher-grade, more localized, or very concentrated resources. For example, geologists mine some underground ore minerals by introducing chemical agents, which dissolve the target mineral. Then, they bring the solution to the surface where precipitation extracts the material. But more often, a mining shaft tunnel or a large network of these shafts and tunnels is dug to access the material. The decision to mine underground or from Earth’s surface is dictated by the ore deposit's concentration, depth, geometry, land-use policies, economics, surrounding rock strength, and physical access to the ore. For example, to use surface mining techniques for deeper deposits might require removing too much material, or the necessary method may be too dangerous or impractical, or removing the entire overburden may be too expensive, or the mining footprint would be too large. These factors may prevent materials from being mined from the surface, and cause a project to be mined underground. The mining method and its feasibility depends on the price of the commodity and the cost of the technology needed to remove it and deliver it to market. Thus, mines and the towns that support them come and go as the commodity price varies. And conversely, technological advances and market demands may reopen mines and revive ghost towns.

Figure \(\PageIndex{9}\): Underground mining in Estonia of oil shale.

Concentrating and Refining

All ore minerals occur mixed with less desirable components called gangue. The process of physically separating gangue minerals from ore-bearing minerals is called concentrating. Separating the desired element from a host mineral by chemical means, including heating, is called smelting. Finally, taking a metal such as copper and removing other trace metals such as gold or silver is done through the process of refining. Typically, refining is done one of three ways:

1. Materials can either be mechanically separated and processed based on the unique physical properties of the ore mineral, like recovering placer gold based on its high density.
2. Materials can be heated to chemically separate desired components, such as refining crude oil into gasoline.
3. Materials can be smelted, in which controlled chemical reactions unbind metals from the minerals they are contained in, such as when copper is taken out of chalcopyrite (CuFeS₂).

Mining, concentrating, smelting and refining processes require enormous energy. Continual advances in metallurgy and mining practices strive to develop ever more energy-efficient and environmentally benign processes and practices.

Figure \(\PageIndex{10}\): A phosphate smelting operation in Alabama, 1942.

References

4. Vogel, J. C. Groningen radiocarbon dates IX. Radiocarbon 12, 444–471 (1970).

5. Schröder, K.-P. & Connon Smith, R. Distant future of the Sun and Earth revisited. Mon. Not. R. Astron. Soc. 386, 155–163 (2008).

6. Duffield, W. A. Volcanoes, geothermal energy, and the environment. Volcanoes and the Environment 304 (2005).

7. Palmer, M. A. et al. Science and regulation. Mountaintop mining consequences. Science 327, 148–149 (2010).