Economic Geology

Summary

Economic geology is the study of the origin and distribution of mineral deposits of metals, other useful materials (such as building stone and salt), and fossil fuels (petroleum, natural gas, and coal). The economic geologist explores for these materials, predicts the mineral reserves of known deposits, and assesses the economics for the extraction of the ore. For instance, for an iron ore deposit, the geologist needs to assess the kinds and amount of iron minerals present, the depth of the deposit, the location of the deposit, and the ownership of the land so that estimates for the possibility of mining the iron ore profitably may be made.

Definition and Basic Principles

Economic geologists explore for nonrenewable resources that formed so slowly, often over millions to even billions of years, that the processes of formation are much slower than the speed at which the resources can be extracted. For example, much of the planet's oil and gas were formed by the slow, deep burial of plankton that were alive about 66 million to 144 million years ago. These organisms need to be gradually buried to a certain depth for long enough so that they gradually change to petroleum. If they are buried too deeply, the petroleum may break down to form natural gas (methane) or the mineral graphite (pure carbon). If they are not buried deeply enough, then useful petroleum or natural gas will not form.

Nonrenewable resources include abundant metals (such as iron and aluminum), scarce metals (gold and copper), materials used for energy (fossil fuels and uranium minerals), building materials (limestone, crushed stone, sand, and gravel), and other miscellaneous minerals (halite or natural salt). Running water, wind power, and solar power are not included among the materials used for energy because they are renewable sources.

Economic geologists use their knowledge of geology to interpret where certain economic deposits might form. For instance, copper and some other associated elements may occur in what are called porphyry copper deposits. These deposits formed as hot water vapor carried dissolved copper out of granite magma and upward along rock fractures in which the copper precipitated to form a variety of copper minerals. The porphyry copper deposits occur only in association with subduction zones. Therefore, the economic geologist knows to search for them only where ancient or existing subduction zones occur, such as along the west coasts of North and South America.

Background and History

The first use of natural resources was to obtain water, salt, and other natural materials to make tools and weapons. Larger cities were located where there was a source of water, such as a river. Salt was used as a flavoring and to preserve food. A major discovery around 9000 BCE was that clay minerals could be heated to make pottery so that food and water could be stored and transported much more easily.

The first metals used were those such as gold and copper, which sometimes were found as native metals (not combined with other elements). These native elements could be shaped into useful materials by hammering or cutting. By 4000 to 3000 BCE, minerals of copper, zinc, lead, and tin were heated by burning charcoal in a very hot flame so that these elements could be separated from the ore. Much later, this process was applied to iron ore. The purification of iron had to wait until the development of blast furnaces, in which oxygen was blown through molten iron to purify it.

German physician Georgius Agricola wrote about the ways that ore minerals might form in De re metallica (1556; English translation, 1912). He divided the origins of ore deposits into those formed in streams and those formed in place. However, he wrote in Latin, which was the language of scholars, not those actually working with ore deposits, so his writings were largely ignored.

Until the nineteenth century, most ores were discovered accidentally because no one understood how ores and the rocks that contained them were geologically formed. Indeed, many people believed that mysterious celestial powers formed ore minerals. From the late eighteenth to the twentieth centuries, the origin of rocks and ores gradually became understood, so people had a much better understanding of how to search for ores.

How It Works

Selection of the Potential Ore Region. An economic geologist may first examine a geologic map, an aerial photograph, or a satellite photograph of a region, usually where other economic deposits have been found, to see if any geographical features provide clues as to where other deposits might be located. Petroleum and natural gas exploration, for instance, involves understanding that these substances usually occur in sedimentary rocks where certain geologic structures may be seen on photographs or maps. There must be shales that contain organic matter that can potentially be converted to oil or gas if buried to the right depth below the surface. Then, there must be an overlying sedimentary rock, such as a permeable sandstone, sandwiched between two impermeable shales, and the sandstone must have spaces between mineral grains through which the oil may move so that it can flow to where it can be trapped. A variety of traps can be used, but one kind uses an impermeable rock to stop the oil from migrating so that it can collect. Once geologists find these circumstances, they can drill below the ground along the trap to see if any petroleum is present. Narrowing the range of possibilities to find oil is important because drilling a single well may cost more than $1 million.

The method for exploration differs depending on the type of deposit. For example, porphyry copper deposits often occur as veins in igneous rocks that form along subduction zones. Therefore, the exploration takes place along subduction zones. Some of the large iron ore deposits called banded iron formations found in the Lake Superior region formed only in shallow oceans from 1.8 billion to 2.6 billion years ago. Therefore, geologists search for new iron deposits only in other sedimentary rocks of similar ages.

Geophysical Methods. Remote methods of discovering an economic mineral deposit use instruments that can detect variations in magnetism, electrical conductivity, gravity, and radioactivity in rocks at or close to the surface of the Earth. For example, some iron-rich minerals such as magnetite (Fe₃O₄) and ilmenite (FeTiO₃) will produce magnetic attractions that suggest that a region may be hydrothermally mineralized. The variations in magnetism may be detected fairly quickly by flying a magnetic detector over a region in a regular pattern. A Geiger counter may be used to quickly assess if there are any radioactive elements, such as uranium or thorium, present at the Earth's surface. Also, instruments that detect variations in electrical conductivity can be used to determine if certain sulfide minerals might be present.

Petroleum geologists may explode small charges along the surface of the Earth so that sound waves travel through the ground for some distance. The sound waves, which can be detected at a receiver station, travel at varying speeds through different kinds of rocks and geologic structures. A computer program can then be used to construct a cross-section of the characteristics of the rocks below the surface. Sedimentary rocks were deposited in horizontal layers, but in some areas, they may be folded into arches (anticlines), warped downward (synclines), or fractured along faults that displace the sedimentary rock layers. The top portion of an anticline or the side of some fault may provide a possible trap where oil and natural gas can accumulate. Geologists can drill a well into these traps to see if oil or gas is present.

Geochemical Methods. Ore minerals may be weathered out of ore deposits and move into streams, where they slowly move downstream. To search for such deposits, the geologist may collect a series of sediments, often at stream junctions, and look for ore minerals in the sediments or analyze the sediments to see if any metals are present in abnormally high concentrations. This technique was used to discover a number of metal ores, such as the porphyry gold and copper ores in Papua New Guinea.

Once a suspected ore deposit has been found in a certain region, soil samples may be collected in a systematic grid over the area. The soil samples are analyzed for the elements most likely to be present in the potential ore deposit. Areas that have much higher concentrations of these elements may indicate the presence of ore directly below the surface. Geologists may drill directly into the areas with high concentrations of these elements to obtain subsurface samples to confirm the kinds of minerals and the elemental concentrations. This method, for example, found zinc deposits near Queensland, Australia. The results of such a drilling program are used to evaluate whether mining the ore prospect might be worthwhile.

The decision of whether to mine the prospect depends on many factors, such as the size and location of the deposit, the concentration and types of elements present, the finances of the company, the price obtained for the ore, the ownership of the land, and whether open-pit or underground mining can be used. For example, a gold deposit in northern Canada may not be profitable because of the costs of transporting equipment and miners to the area and of transporting the ore to a production facility. If gold prices drastically increase, then a previously uneconomical mine might be able to show some profit. Also, large companies sometimes abandon smaller deposits and try to sell them to smaller companies that might still profitably mine the ore.

Applications and Products

The mineral resources discovered by economic geologists are used by industries to produce the abundant metals, the scarce metals, fossil fuels, and other natural materials. Economic geologists are not directly concerned with the use of these materials, but they must be aware of the demand for these materials so that they know the amount of money that a company may receive for them.

Abundant Metals and Their Uses. The abundant metals are those found in the highest concentrations within the Earth. Iron minerals are used to make steel, which is used to make automobiles, buildings, roads, bridges, major appliances, and construction materials such as nails. The major iron ore minerals, hematite and magnetite (both iron oxides), are converted to steel by heating them to a high temperature with coke and limestone to form molten steel, which contains up to 2 percent carbon to harden the iron.

Aluminum is another abundant metal that is obtained from the ore bauxite, a mix of the minerals boehmite (aluminum oxyhydroxide) and gibbsite (aluminum hydroxide). The production of aluminum is very expensive because the bauxite must be heated with the compound cryolite to make a molten solution in which the aluminum is concentrated by an electric current. Aluminum is used in products such as cans, foil, windows, vehicles, and household items and mixed with other metals in alloys.

Titanium occurs in the minerals rutile (titanium oxide) and ilmenite (iron titanium oxide). Like aluminum, titanium is expensive to produce from its ore. Most titanium used is in the form of titanium oxide, which produces an intense white color in paint and paper. Titanium is also alloyed with aluminum, vanadium, and iron and used in ships, airplanes, and missiles. Magnesium oxide is often alloyed with aluminum to provide corrosion-resistant cans and materials used in vehicles. Magnesium metal is expensive to produce because like aluminum, electricity is used in its production.

Silicon is obtained by melting the mineral quartz (silicon oxide) with iron and coke in an electric furnace. Silicon is often alloyed with iron, aluminum, and copper because it improves the strength of these alloys and guards against corrosion. Silicon is also used in transistors in electronic devices.

Scarce Metals and Their Uses. Scarce metals occur in the Earth in much lower concentrations than the abundant metals. Some minerals, however, may locally concentrate these metals, making them potentially economical to mine. Some scarce metals may be added to steel to give it certain characteristics. For instance, chromium, molybdenum, tungsten, or vanadium may be added to make steel harder, especially at higher temperatures. Stainless steel contains more than 11 percent chromium combined with nickel to help keep steel from rusting. Chromium and nickel may improve the strength of steel. Vanadium added to steel decreases the weight of steel, its strength, ductility, and ease of welding.

In addition, chromium is also used in chromium compounds to produce paint and ink pigments with green, yellow, and orange colors. Molybdenum is used in catalysts, pigments, and lubricants. Tungsten is often combined with carbon to produce a very hard compound, tungsten carbide, which is nearly as hard as diamond.

Copper, lead, zinc, tin, mercury, and cadmium are often grouped together simply because they are not used to alloy with iron. Copper metal conducts electricity well and can be shaped into wires, so it has been used mainly in electric lines and motors. Lead and mercury are used much less than in the past because of their toxicity. Lead is still used in batteries for vehicles, and mercury is used in some batteries, electrical switches, and some chemical compounds. Zinc or tin are used in protective coatings for steel to keep it from rusting. Zinc oxide is added to paint to produce a white color and to a variety of lotions to prevent sunburn. Many so-called tin cans contain tin plated on other metals.

The precious metals are gold, silver, and platinum metals. These metals are often used in jewelry because of their beauty. Gold is also used for money and in industrial materials, such as electronic connectors and dental fillings. Silver is also used in some electrical equipment. Platinum is used as a catalyst, for example, in catalytic converters in cars.

A variety of other metals, such as the rare earth elements, are used for industrial purposes. Rare earth elements have been used as catalysts and to color glass and ceramics and to provide some colors in television screens.

Chemical Minerals and Fertilizers. A number of nonmetallic minerals, such as halite, baking soda, and sylvite, are used in food. Halite and sylvite are salts used for flavoring. Halite is also used to soften water and to melt ice on roads. Borox is a boron compound used in some detergents and cosmetics.

A variety of potassium, nitrogen, and phosphorous minerals are used to fertilize crops. Commonly used nitrate compounds are sodium nitrate and potassium nitrate. Ammonium phosphate and apatite are examples of phosphate minerals.

Gypsum (calcium sulfate) is the main mineral in wallboard. Petroleum-derived sulfur compounds, such as sulphuric acid, are used to make many industrial compounds.

Building Materials. Building materials include building stones, crushed rocks, sand, gravel, cement, plaster, bricks, and glass. Common building stones are granite, limestone, and sandstone. Granite, for example, can be polished to form an attractive surface for building facings and countertops.

Huge quantities of crushed rocks, sand, and gravel are used for roads, building foundations, and concrete. In the United States, more than 100 billion tons of these materials are used yearly.

Gemstones. The most important gems are diamonds, sapphires, rubies, and emeralds. Gems must be harder than most other minerals, and they must be beautiful. Diamonds are composed of the element carbon, and rubies and sapphires are gem-quality varieties of the mineral corundum. Emeralds are gem-quality varieties of the mineral beryl.

Fossil Fuels. Petroleum, natural gas, and coal are the main fossil fuels. Much of the coal is burned to provide electrical power. Much of the natural gas is burned to provide heat in homes and industries. Petroleum products such as gasoline are used in providing power for automobiles and trucks.

Careers and Course Work

A person interested in searching for metals, fossil fuels, and other industrial minerals should major in geology in college with a concentration in economic and exploration geology. Courses such as mineralogy, petrology, economic geology, and structural geology are required to obtain a bachelor's degree. Various mathematics, chemistry, and physics courses should also be taken. Further study for master's and doctoral degrees would enable the student to research a specific economic geology problem. Those interested in solving geochemical aspects of exploration should take many supporting courses in chemistry. Those interested in exploring using geophysical techniques should take supporting courses in mathematics and physics. Advanced degrees give an individual a lot of responsibility if employed by a company searching for natural resources.

Those interested in developing mining operations once a potential site has been discovered can major in mining engineering, with coursework on how to develop mines and supporting courses in geology, physics, and chemistry. Those interested in solving environmental problems associated with an economic site should take various geology, chemistry, water chemistry, and hydrology courses.

Social Context and Future Prospects

The increased demand for most metals, oil, natural gas, coal, and other raw materials such as sand and gravel is likely to continue indefinitely. Construction materials such as sand, gravel, and limestone, which are used in large quantities, need to be sourced close to where they will be used because of the high cost of transporting them. Most metals are used in smaller quantities and can be shipped economically from sites around the world to where they will be used. In industrial societies such as the United States, the amount of money spent on industrial minerals is much greater than that spent on metals.

The use of iron, manganese, aluminum, copper, zinc, gold, graphite, nickel, silver, sulfur, vanadium, and zinc has increased substantially from 1950 into the twenty-first century. The use of some metals, such as lead, increased from the 1950s to the 1980s but subsequently declined because of associated environmental problems.

Worldwide, the distribution of many metals and fossil fuels is uneven, so many countries must import these resources. For example, the United States uses much more petroleum than it can produce, so it must obtain it from countries rich in petroleum. The United States must also import a great deal of aluminum, platinum, manganese, tantalum, and tungsten to meet its demand. The United States, however, has an excess of salt, lead, copper, and iron.

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