Minerals and Mining

Minerals are defined as naturally occurring solids found in the earth that are composed of matter other than plants or animals. Ore is a naturally occurring source of minerals, such as a rock. A mineral can be composed from one element, such as diamond, which contains only carbon, or several elements, such as quartz, which contains silicon and oxygen. An element is a substance that cannot be divided by ordinary chemical means. Even ice is considered a mineral. Minerals are found everywhere on Earth, from the bottom of the ocean to the highest mountains. Mineral deposits are frequently located underground, and thus they must be mined. South Africa and Russia hold the largest amount of minerals in the world. Minerals are vital to people's lives, and many of these minerals are critical to countries' industries and economies.

The United States is relatively poor in critical minerals, including platinum, cobalt, and gold, but there are sand deposits of titanium ore in Florida and the Pacific Northwest. In the central United States, minerals that contain lead and gallium (used in computer chips) are abundant, and iron ore is found in the states near the Great Lakes. Most of the diamonds are mined in Africa, as is gold, although gold is found in many other locations as well.

Importance of minerals

Minerals are essential in every aspect of life for humans. Humans need to ingest minerals in order for our bodies to function normally. Most of these required minerals come from the foods people eat. Gold and silver have been valued by civilization since ancient times. Metals became useful for purposes other than money during the Bronze Age, when weapons and tools could be made from metal as people became more educated with how to process the minerals and extract (remove) metals. As metals were not evenly dispersed around the globe, the more powerful nations became that way through military might from weapons made from metal.

Industry also depends upon minerals. Without aluminum (mined mostly in Jamaica), people could not manufacture airplanes, much less soda-pop cans. Titanium is used in the aerospace industry for constructing spacecraft and in medicine for the construction of artificial limbs and joints. Copper is required to make wire that carries electricity to homes and factories.

Mineral reserves are of great importance in the marketplace and it is not uncommon to stockpile (save) certain metals extracted from mineral ores. Because large mineral deposits are located in regions of the world that are, at times, politically and economically unstable, the supply of critical minerals is not guaranteed. The United States stockpiles metals such as platinum, palladium, cobalt, chromium, manganese, and vanadium. These metals are used in the high-technology industries and the military. Chromium, for instance, is used to produce stainless steel. Vanadium is used, along with aluminum, to make forms of titanium that are resistant to fracture (breaking), enabling the manufacture of jet planes that can withstand extreme conditions. Platinum is used in removing the impurities from oil. Palladium is used in the exhaust systems of automobiles to reduce the amount of pollutants. It is advantageous for a highly-industrialized country such as the United States to have these resources at hand, and to purchase reserves when prices are low.

Water-laid ores and minerals

The formation of mineral deposits always involves water. Water is part of the chemical processes of mineral formation and also changes the mineral content of rocks by dissolving certain elements in the ore and transporting them elsewhere. Heat is another ingredient in the formation of many mineral deposits.

Manganese Nodules

Manganese nodules (solid, raised bumps) are tennis-ball sized mineral nodules that litter the ocean floor, mostly in the Pacific Ocean. They form much like pearls, with a small center that can be a grain of sand or even a tooth from a fish, and over millions of years, manganese, along with iron and other minerals build-up as shells. There are numerous manganese nodules yet they are hard to extract, largely due to the depths from which they must be brought to the surface. This is an expensive proposition, and until other sources of the minerals contained in manganese nodules are exhausted, will not likely be a source of minerals for human use.

Because the nodules are in international waters, there has been debate over who can claim ownership. Many nations formed the United Nations Convention on the Law of the Sea, which in turn led to the formation of the International Seabed Authority in 1994, which controls international ocean mining rights. The reason for making these laws and forming a governing body was to ensure that all nations could share in the wealth stored in the nodules. The International Seabed Authority has granted several areas for exploration and recovery to many public and private concerns, but the United States does not abide by these directives because, as of 2004, it does not observe the Law of the Sea.

Hydrothermal deposits. Many metal-bearing ores are found in veins (cracks in rock filled with minerals) that cut through surrounding rock. In these cases, very hot water reacted with elements and other minerals in the rock, and burst through the layers of rock where there was a weakness. These mineral deposits are called hydrothermal deposits. Hydrothermal deposits form gold, silver, and the platinum-group metals, which are commonly found in veins. The metals themselves are hosted in a vein that is often quartz. Miners follow the vein, extract the ore, and remove the host rock to extract the metals contained within. Mining minerals from veins is an expensive process that is seldom used today.

When hot water flows through porous rock (rock with many small holes), the rock can become a host to a kind of deposit known as porphyry. The host rock containing a porphyry deposit is filled with small veins of (usually) quartz that contain the minerals. Although the mineral content is low, porphyry deposits are large, and most of the copper that is mined comes from these unique deposits. Fool's gold (iron sulfide) is often found in porphyry deposits as well.

Volcanogenic deposits. Volcanogenic deposits form when magma (molten or melted rock beneath the Earth's surface) from miles (kilometers) down in the earth is transported to the surface in volcanoes. There are two kinds of volcanic eruptions that most concern scientists. One of these brings iron-rich magma to the surface (such as in Hawaii) and one brings explosive plumes of ash and magma to the surface (such as in Mt. Saint Helens). Elements in the water that is in contact with the magma, along with the rock through which the water travels on its way to the surface, determine the kinds of minerals found in volcanogenic deposits. For the most part, lead and iron ore are found in volcanogenic deposits, along with smaller amounts of cadmium, antimony, and copper.

On the floor of the ocean, the same kinds of deposits can form where magma seeps through a crack in the seabed. These features are called black smokers, because the iron-rich magma makes the plume appear like black smoke. The mineral deposits collect near the smoker until the hole becomes plugged or the magma is diverted elsewhere.

Mining for minerals

The process of mining for minerals begins after a mineral deposit has been identified. The common types of mines used to excavate minerals are open-pit mines, strip mines, and stope and adit mines.

• Open-pit mines: These mines are large craters dug into the earth to extract ore that is near the surface. Open pit mines are usually associated with porphyry deposits, and minerals such as galena (which contains lead), chalcopyrite (which contains copper), and sphalerite (which contains zinc) are commonly mined at open pits. The open pit is excavated using very powerful and large earth-moving equipment, and processing of the ore (crushing, grinding, partial refinement) is often done near the open pit. Open pits are less environmentally friendly than conventional mines because any native vegetation in the area is lost, and abandoned open pit mines eventually pool waters that are frequently contaminated. Open-pit mines are used for copper in Arizona.
• Strip mines: These mines are large swaths dug through ore-rich zones (common for coal). Most mines are located below the surface of the Earth, and require drilling shafts to enable workers to reach the ore below and transport it to the surface. Strip mines are used for coal in many states and other areas where the ore is buried deeply, as in Montana, where platinum and palladium ore is extracted (removed).
• Stope and adit mines: These mines are bored into the ground. Shafts are bored vertically, and horizontal offshoots (adits) from the shafts that lead into ore-containing portions of the subsurface are dug. Large mining vehicles that crush the rock move along veins of ore, and this byproduct is transported to the surface by rail or cable lifts. Temperatures in conventional mines are high, and the conditions are dangerous—cave-ins occur often—so safety measures are very strict. The shafts and other structures are reinforced with concrete or metal supports.

Other ore and mineral deposits

Placer deposits are concentrated metals that have been transported to streambeds (the channel through which the stream runs) or beaches. The most famous placer deposits are gold nuggets, although silver is sometimes found as well. Placer metals must be resistant to water, or they would dissolve again. The usual way to extract the placer deposits is to scoop sediment (particles of rock, clay, or silt) from the stream and sift it, leaving the larger rock behind and making the gold easier to spot. California, Alaska, Oregon, and Idaho have all had significant placer deposits of gold.

Evaporite deposits form by evaporation. As waters that contain dissolved mineral species evaporate, the minerals remain in solid form. Minerals found in evaporites include potassium chloride, sodium chloride (halite or table salt), calcium sulfate (gypsum), barium sulfate (barite), and potassium nitrate (saltpeter). Most of these deposits are near the surface and are scooped from the ground with large earth-moving equipment. Gypsum is used to make sheetrock, which is used to construct the walls of homes and buildings. It is fire-retardant and easily cast into shapes. Barium sulfate is used as drilling mud in oil-producing wells because it is very dense, and prevents oil gushers from erupting as the drill is lowered. Barium itself has use in medicine. Saltpeter is used as an ingredient in gunpowder and fertilizers.

Placer deposits and the California Gold Rush

On January 24, 1848, placer gold was discovered in Sutter's Mill, California, setting off what would become the "Gold Rush." People from all over the United States came to California to seek their fortune in gold and silver mining in stream beds. These people were known as prospectors and San Francisco was a major center to prospectors. The Gold Rush was so large, the population of California exploded almost instantly.

Where did the gold come from? To the East, in the Sierra Nevada mountain range, gold-containing rocks eroded (wore away) over tens of thousands of years. Streams transported gold dust and nuggets to northern California where the gold was lying in streams, waiting to be collected.

Panning for gold was the usual way to collect the placer deposits. Like a strainer from the kitchen, the pan let water and small particles through and did not allow larger particles to pass. In this way, mud and other sediments could be washed away, and the resulting rocks examined to see if any were gold. Shaker tables were also used. This is a slanted surface that mechanically vibrated, sending the less dense (heavy or thick per unit of volume) rocks to the bottom of the table while the more dense rocks, such as gold, stayed near the top.

In the oceans, concentrations of dissolved mineral ingredients are very high. Evaporites of the chloride type are most common, and they occur in areas where seawater collects in shallow areas that are confined. Thus, a pool of salt-rich water forms, evaporation speeds up the process, and salt deposits result. Another widespread mineral formed in marine environments is limestone, or calcium carbonate. This mineral is formed in the same way as the chloride salts, but also includes another source, organisms whose skeletons are made from calcium carbonate. These organisms die and collect on the sea floor, where they add to the content of calcium carbonate. Limestone is used in constructing buildings and as an ingredient in concrete.

Todd Minehardt, Ph.D.

For More Information

Books

Postel, Sandra, and Brian Richter. Rivers for Life: Managing Water for People and Nature. Washington, DC: Island Press, 2003.

Websites

"Geoenvironmental Model of Volcanogenic Massive Sulfide Deposits." Government of British Columbia Ministry of Energy and Mines.http://www.em.gov.bc.ca/Mining/Geolsurv/MetallicMinerals/metallogeny/O98_abstract_alpers.HTM (accessed onAugust 27, 2004).

"The Gold Rush." Oakland Museum of California.http://www.museumca.org/goldrush (accessed on August 27, 2004).

"Manganese Nodules." Wikipedia.http://www.fact-index.com/m/ma/manganese_nodule.html (accessed on August 27, 2004).

"Rivers, Dams, and Climate Change." International Rivers Network.http://www.irn.org/programs/greenhouse/ (accessed on August 27, 2004).