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ore, metal-bearing mineral mass that can be profitably mined. Nearly all rock deposits contain some metallic minerals, but in many cases the concentration of metal is too low to justify mining the ore. Ores are usually found concentrated in deposits with a definite gradation of metal concentration from the ore to the surrounding rock. The ore often occurs in veins, which are ore-filled fissures in the rock. The veins vary in thickness from only fractions of an inch to several hundred feet. Minerals with no commercial value, called gangue minerals, are usually found mixed with the ore in the vein. Some veins are buried deep within the ground, but others lie close to the surface. Veins of ore exposed to weathering are often eroded and redeposited in placers and alluvial deposits, e.g., ore-bearing stream and lake gravels or beach sands. Some ores are simple chemical compounds, while others are chemically complex minerals. Important ores of aluminum, iron, manganese, and tin are oxides; important ores of antimony, copper, lead, mercury, nickel, silver, and zinc are sulfides. Some metals, called native metals, occur uncombined in nature, e.g., copper, gold, platinum, and silver. The recovery of metals from their ores is one area of the field of metallurgy.

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ore

ore OE. ōra unwrought metal (corr. to Du. oer, LG. ūr, of unkn. orig.), repr. by oor(e), (o)ure from XIV to XVII; superseded by the descendant of OE. ār = OS., OHG. ēr, ON. eir, Goth. aiz :- Gmc. *aiz, corr. to L. æs crude metal, bronze, money.

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ore

ore Mineral or combination of minerals from which metals and non-metals can be extracted. It occurs in veins, beds or seams parallel to the enclosing rock or in irregular masses.

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ore

ore / ôr/ • n. a naturally occurring solid material from which a metal or valuable mineral can be profitably extracted.

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ore

ore A mineral or rock that can be worked economically.

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ore

ore A mineral or rock that can be worked economically.

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ore

oreabhor, adore, afore, anymore, ashore, awe, bandore, Bangalore, before, boar, Boer, bore, caw, chore, claw, cocksure, comprador, cor, core, corps, craw, Delors, deplore, door, draw, drawer, evermore, explore, flaw, floor, for, forbore, fore, foresaw, forevermore, forswore, four, fourscore, furthermore, Gábor, galore, gnaw, gore, grantor, guarantor, guffaw, hard-core, Haugh, haw, hoar, ignore, implore, Indore, interwar, jaw, Johor, Lahore, law, lessor, lor, lore, macaw, man-o'-war, maw, mirador, mor, more, mortgagor, Mysore, nevermore, nor, oar, obligor, offshore, onshore, or, ore, outdoor, outwore, paw, poor, pore, pour, rapport, raw, roar, saw, scaur, score, senhor, señor, shaw, ship-to-shore, shop-floor, shore, signor, Singapore, snore, soar, softcore, sore, spore, squaw, store, straw, swore, Tagore, tau, taw, thaw, Thor, threescore, tor, tore, torr, trapdoor, tug-of-war, two-by-four, underfloor, underscore, war, warrantor, Waugh, whore, withdraw, wore, yaw, yore, your

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ORE

ORE occupational radiation exposure

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Ore

Ore

History

Formation of ore

Igneous ore deposits

Hydrothermal ore deposits

Sedimentary ore deposits

Metamorphic ore deposits

Mineral exploration

Mining and processing

Environmental considerations

Resources

Ore is metal-bearing rock that can be mined, transported, processed, and sold at a profit. Although a broader definition includes nonmetallic rocks like rock salt and gypsum, most geologists classify these materials as industrial rocks and minerals.

History

Gold, silver, and copper artifacts left by prehistoric tribes and ancient civilizations attest to an interest in ores extending back to earliest times. Human history is divided into chalcolithic (copper-stone), bronze, iron, and atomic (uranium) ages based on the use of metals. In spite of historical reliance on metals, little was known about the origin of ore until relatively recent times. Greek philosophers believed that metallic veins were living things with roots at depths and near-surface branches of different metals. Astrologers contended that gold, silver, iron, and mercury were formed under the influence of the sun, moon, Mars, and Mercury.

The first major break from this line of thinking came in 1556 with the publication of De Re Metallica by a German physician writing under the Latinized pen name of Georgius Agricola. Agricolas keen observations and naturalistic explanations marked a departure from the speculations of the ancients. His work remains a Renaissance Age classic, and Agricola is recognized as the father of economic geology.

Major advances in the study of ore deposits were made following the discovery and development of the many great deposits in the western United States. An outgrowth of this period was a belief that ores are related to emanations given off by cooling igneous rocks. Although still considered a process of great significance, sedimentary and metamorphic processes are also recognized as important.

Formation of ore

A cubic mile of average rock contains approximately one trillion dollars worth of metal, yet it would be impractical to mine ordinary rock because the expense would be too great. Ore provides a less expensive option. Ore is formed by geologic processes, which concentrate metals to tens or thousands of times their average crustal abundances. Even then, a mine may not prove profitable unless a host of other geologic and nongeologic conditions are met.

Geologic factors include the deposits size, depth, and amenability to processing. Small amounts of arsenic, for example, may increase the cost of processing and waste rock disposal. Higher amounts could mean a profitable arsenic mine. Profitability also ties the definition of ore to a host of nongeologic conditions including demand for the metal, geographic location of the deposit, local labor conditions, local energy costs, governmental regulations, and many other economic factors.

Besides metals, ore commonly contains minerals of no particular value. Gold, for example, occurs in veins composed mostly of quartz. Although not of economic value, so-called gangue minerals can yield valuable information about the origin of the deposit. Quartz, for example, yields information about the temperature at which the ore formed, which could be useful in the search for more gold along the vein. In addition, it is doubtful that the gold mineralization would have been noticed had not quartz caught the eye of a geologist who knew that precious metals sometimes occur in veins of quartz.

Ore deposits are relatively rare and tend to be distributed in an irregular fashion around the globe, but there is nothing unusual about the manner in which they form. They develop from the same geologic processes that form ordinary igneous, sedimentary, and metamorphic rocks.

Igneous ore deposits

Igneous rocks form from the solidification of molten rock called magma. Magmas also contain dissolved gases, and partly solidified magmas contain mineral grains, some metalliferous. As magma solidifies, metallic elements usually remain widely dispersed, but igneous processes can cause their concentration. In rare cases, dense metallic minerals settle to form metal-rich layers at the bottom of the magma chamber. Metal may also separate from the magma if the sulfur content rises to the point where a sulfur-rich magma forms, separates, and sinks. Many metals are naturally attracted to sulfur, and they separate with the new magma. These processes are thought to have formed some chromium, nickel, and platinum-rich layers within igneous rock, but a related process forms a wider variety of ores.

Hydrothermal ore deposits

As magma solidifies, water and other gaseous constituents tend to be concentrated in the decreasing proportion of molten rock. At some point these constituents may literally boil off, penetrate the surrounding rock, and condense to a hot, water-rich fluid. These igneous emanations are termed hydrothermal fluids. They are mobile and capable of dissolving metals from rock through which they pass. They tend to lose the metals they carry and form ore deposits when they encounter a favorable location. Favorable spots include rock fractures or openings along faults where hydrothermal fluids form veins. Other sites include sedimentary rocks like limestone or gypsum. Hydrothermal fluids can chemically react with these rocks to deposit ore. Alternatively, the hydrothermal fluid may simply mix with groundwater, causing the fluids temperature and composition to change and ore to be deposited.

Hydrothermal fluids need not be igneous emanations; they can also consist of groundwater heated by a nearby mass of magma. Hydrothermal waters may reach the surface in hot springs and geysers, as at Yellowstone National Park. Water sampling and drilling at locations similar to Yellowstone has shown that ore minerals are being deposited at depth.

Sedimentary ore deposits

Sedimentary processes form ore either through the selective removal of nonmetallic components or by concentration of metallic minerals. Rock at Earths surface is subjected to weathering and leaching, the process that turns rock into soil. Aluminum resists being leached, and bauxite, the ore of aluminum, is actually an aluminum-rich soil. Bauxite forms in the humid tropics from intense and prolonged weathering of aluminum-bearing rocks.

The concentration of heavy metallic grains in sediments creates placer deposits. Placer gold, for example, accumulates along streams where currents are too weak to carry the heavy flakes of gold but strong enough to winnow away ordinary rock fragments.

Some entire beds of marine sedimentary rocks contain enough metal to be considered ore. Examples include sedimentary beds rich in iron, manganese, and even lead, zinc, and copper. For some, hydrothermal fluids issued from submarine hot springs may have been involved. Others may simply have been deposited directly from metal-rich ocean water.

Metamorphic ore deposits

Metamorphic rocks are formed from heat and fluids near cooling magma (contact metamorphism) and by high temperatures and pressures deep with the crust (regional metamorphism). Although rock metamorphism plays an important role in ore deposition, most of the resulting deposits are classified as hydrothermal. Relatively few ore deposits actually form in regionally metamorphosed rocks, but regional metamorphism drives water and other volatile components from the rock to form hydrothermal fluids responsible for ore deposits elsewhere. Contact metamorphic rocks contain a wide variety of ore deposits, but because hot fluids are commonly involved, they are generally considered to be part of the hydrothermal realm.

Mineral exploration

Mineral exploration has not always been a science. Since ancient times, prospectors relying heavily on luck and persistence have successfully discovered ore deposits of all kinds. The ancient Romans worked every deposit of significance within the bounds of their empire, and it is said in Mexico that if gold and silver ore was at the surface, the Conquistadors found it. The application of modern exploration methods based on geologic principles has led to the development of profitable mines in rocks with no visible indication of metallic ores.

Successful mining companies use a systematic approach based upon knowledge gained from the study of previously discovered and developed ore deposits. It allows them to pinpoint likely areas for more intense exploration, and to find deeply buried deposits from surface geochemical and geophysical indications, or even through remote sensing from aircraft and satellites. The final exploratory phase involves actual drilling and sampling of the postulated deposit. Only then can the final assessment of economic factors be made and the exploration target classified either as ore or just interestingly mineralized rock, perhaps a future ore if economic conditions change.

Mining and processing

Mining removes ore in the least costly manner available. Surface pits are preferred because of their lower costs and safer working conditions, but the shape of many vein deposits require mining via underground shafts and tunnels. Mines of the future may simply inject chemicals or bacteria to dissolve the metal of interest, allowing it to be pumped to the surface. Uranium mining is already done by chemical leaching and most sulfur is mined by dissolution with hot water pumped into the ground. Once above ground, the ore is typically crushed, pulverized, and then upgraded during a process called beneficiation. Beneficiation separates metal and gangue into concentrates and waste material called tailings. The exact beneficiation technique depends on the type of ore being processed and it is usually done at the mine site to avoid transportation

KEY TERMS

Gangue The valueless component of ore, commonly quartz and calcite.

Hydrothermal fluid Hot water-rich fluid capable of transporting metals in solution.

Igneous Formed by solidification of molten rock called magma.

Industrial rocks and minerals Rocks of economic value exclusive of metallic ores, mineral fuels, and gems.

Metamorphic Formed by deformation and/or recrystallization of preexisting rocks.

Ore Rock, usually metallic, that can be mined and processed at a profit.

Sedimentary Formed by accumulation of sediment, mostly commonly by deposition from water.

costs. Concentrates are generally sent to a smelter for further separation of metals.

Environmental considerations

Ore deposits supply much of the raw material on which modern industrial society is based. Even farming would not be possible without metals. Platinum, for example, is used as a catalyst in the chemical reaction that produces nitrogen for fertilizer, and metals are used in the production of pesticides, petroleum, and farm implements. Mining, however, has historically been a dangerous business prone to environmental problems. Mining, beneficiation, and smelting have led to the introduction of unacceptable levels of metals into lakes, streams, and ground water. Underground mining has caused land surface subsidence problems in many parts of the country, principally in coal mining areas. Although problems persist from the early days of mining, modern mining methods are safer and have less environmental impact. Pollution control during the mining and processing of ore, and land reclamation after mine closure are now being considered as one of the economic factors in determining if mineralized rock can be considered ore.

See also Industrial minerals.

Resources

BOOKS

Hartman, H.L. and J.M Mutmansky. Introductory Mining Engineering. Hoboken, New Jersey: Wiley, 2002.

Laznicka, P. Giant Metallic Deposits: Future Sources of Industrial Metals. Berlin: Springer, 2006.

National Research Council. Superfund and Mining Megasites: Lessons from the Coeur Dalene River Basin. Washington, D.C.: National Academies Press, 2006.

Tarbuck, E.J., F.K. Lutgens, and D. Tasa. Earth: An Introduction to Physical Geology. Upper Saddle River, New Jersey: Prentice Hall, 2004.

Eric R. Swanson

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Ore

Ore

Ore is metalliferous rock that can be mined and processed at a profit. Although a broader definition includes nonmetallic rocks like rock salt and gypsum, most geologists classify these materials as industrial rocks and minerals .


History

Gold, silver, and copper artifacts left by prehistoric tribes and ancient civilizations attest to an interest in ores extending back to earliest times. Indeed, human history is divided into chalcolithic (copper-stone), bronze, iron , and atomic (uranium ) ages based on the use of metals. In spite of this, little was known about the origin of ore until relatively recent times. Greek philosophers believed that metallic veins were living things with roots at depths and near-surface branches of different metals. Astrologers contended that gold, silver, iron, and mercury were formed under the influence of the Sun , Moon , Mars , and Mercury.

The first major break from this line of thinking came in 1556 with the publication of De Re Metallica by a German physician writing under the Latinized pen name of Georgius Agricola. Agricola's keen observations and naturalistic explanations marked a departure from the speculations of the ancients. His work remains a Renaissance Age classic, and Agricola is recognized as the father of economic geology .

Major advances in the study of ore deposits were made following the discovery and development of the many great metal-bearing veins in the western United States. An outgrowth of this period was a belief that ores are related to emanations given off by cooling igneous rocks . Although still considered a process of great significance, sedimentary and metamorphic processes are also recognized as important.


Formation of ore

A cubic mile of average rock contains approximately one trillion dollars worth of metal ; yet no one is mining ordinary rock. The expense involved in processing ordinary rock is just too great. Fortunately, ore provides a less expensive option. Ore is formed by geologic processes which concentrate metals to tens or even thousands of times their average crustal abundances. Even then, a mine may not prove profitable unless a host of other geologic and nongeologic conditions are met.

Geologic factors include the deposit's size, depth, and amenability to processing. Small amounts of arsenic, for example, may poison the deal through the added expense of its removal. Higher amounts could mean a profitable arsenic mine. Profitability also ties the definition of ore to a host of nongeologic conditions including demand for the metal, geographic location of the deposit, local labor conditions, local energy costs, governmental regulations, and many other economic factors.

Besides metals, ore commonly contains minerals of no particular value. Gold, for example, occurs in veins composed mostly of quartz. Although not of economic value, gangue minerals can yield valuable information on the origin of the deposit. Quartz, for example, reveals the temperature at which the ore formed, information that could be useful in directing the search for more gold along the vein. In addition, it is doubtful that the gold mineralization would have been noticed had not quartz caught the eye of a geologist who knew that precious metals sometimes occur in veins of quartz.

Ore deposits are relatively rare and tend to be distributed in an irregular fashion around the globe, but there is nothing unusual about the manner in which they form. They develop, in fact, from the same geologic processes that form ordinary igneous, sedimentary, and metamorphic rocks.


Igneous ore deposits

Igneous rocks form from the solidification of molten rock called magma . Magmas also contain dissolved gases, and partly solidified magmas contain mineral grains, some metalliferous. As magma solidifies, metallic elements usually remain widely dispersed, but igneous processes can cause their concentration. In rare cases, dense metallic minerals settle to form metal-rich layers at the bottom of the magma chamber. Metal may also separate from the magma if the sulfur content rises to the point where a sulfur-rich magma forms, separates, and sinks. Many metals are naturally attracted to sulfur, and they separate with the new magma. These processes are thought to have formed some chromium, nickel, and platinum-rich layers within igneous rock, but a related process forms a wider variety of ores.


Hydrothermal ore deposits

As magmas solidify, water and other gaseous constituents tend to be concentrated in the decreasing amount of molten rock. At some point these constituents may literally boil off, penetrate the surrounding rock, and condense to a hot, water-rich fluid. These igneous emanations are termed hydrothermal fluids. They are mobile and capable of dissolving metals from rock through which they pass. They tend to lose the metals they carry and form ore deposits when they encounter a favorable location. Favorable spots include rock fractures or openings along faults were hydrothermal fluids form veins. Other sites include sedimentary rocks like limestone or gypsum. Hydrothermal fluids can chemically react with these rocks to deposit ore. Alternatively, the hydrothermal fluid may simply mix with groundwater , causing the fluid's temperature and composition to change and ore to be deposited.

Hydrothermal fluids need not be igneous emanations, but groundwater heated by a nearby mass of magma. Hydrothermal waters may reach the surface in hot springs and geysers, as at Yellowstone National Park. Water sampling and drilling at locations similar to Yellowstone has shown that ore minerals are being deposited at depth.


Sedimentary ore deposits

Sedimentary processes form ore either through the selective removal of nonmetallic components or by concentration of metallic minerals. Rock at the earth's surface is subjected to weathering and leaching , the process that turns rock into soil . Aluminum resists being leached, and bauxite, the ore of aluminum, is actually an aluminum-rich soil. Bauxite forms in the humid tropics from intense and prolonged weathering of aluminum-bearing rocks. The concentration of heavy metallic grains forms placer ores. Placer gold, for example, accumulates along streams were currents are too weak to carry the heavy flakes of gold but strong enough to winnow away ordinary rock fragments. Some entire beds of marine sedimentary rocks contain enough metal to be considered ore. Examples include sedimentary beds rich in iron, manganese, and even lead, zinc, and copper. For some, hydrothermal fluids issued from submarine hot springs may have been involved. Others may simply have been deposited directly from metal-rich ocean water.


Metamorphic ore deposits

Metamorphic rocks are formed from heat and fluids near cooling magma (contact metamorphism ) and by high temperatures and pressures deep with the crust (regional metamorphism). Although rock metamorphism certainly plays an important role in ore deposition, most of the resulting deposits are classified as hydrothermal. Relatively few ore deposits actually form in regionally metamorphosed rocks, but regional metamorphism drives water and other volatile components from the rock to form hydrothermal fluids responsible for ore deposits elsewhere. Contact metamorphic rocks contain a wide variety of ore deposits, but because hot fluids are commonly involved, they are generally considered to be part of the hydrothermal realm.

Mineral exploration

Mineral exploration has not always been a science. Since ancient times, prospectors relying heavily on luck and persistence have successfully discovered ore deposits of all descriptions. The ancient Romans worked every deposit of significance within the bounds of their empire, and it is said in Mexico that if gold and silver ore was at the surface, the Conquistadors found it. This testament to the Spanish obsession with precious metals has led some to conclude that nothing is left to discover, but that does not discourage the modern exploration geologists. They prefer to say that if ore was not at the surface, the Spanish did not find it. And so the search for metal continues in a scientific way with significant discoveries continuing to be made.

Successful mining companies use a systematic approach based upon knowledge gained from the study of previously discovered and developed ore deposits. It allows them to pinpoint likely areas for more intense exploration, and to find even hidden deposits from telltale surface geochemical and geophysical indications, or even through remote sensing from aircraft and satellites. The final exploratory phase involves actual drilling and sampling of the suspected deposit. Only then can the final assessment of economic factors be made and the exploration target classified either as ore or just interestingly mineralized rock, perhaps a future ore if economic conditions change.


Mining and processing

Mining removes ore in the least costly manner available. Surface pits are preferred, but the shape of many vein deposits require mining via underground shafts and tunnels. It is likely that mines of the future may simply inject chemicals or even bacteria to dissolve the metal of interest, allowing it to be pumped to the surface. Uranium mining is already done by chemical leaching. Once above ground, the ore is typically crushed, pulverized, and then upgraded during a process called beneficiation. Beneficiation separates metal and gangue into concentrates and waste material called tailings. The exact beneficiation technique depends on the type of ore being processed and it is usually done at the mine site to avoid transportation costs. Concentrates are generally sent to a smelter for further separation of metals.

Environmental considerations

Ore deposits supply much of the raw material on which modern industrial society is based. "If you can't grow it, it has to be mined," say mining geologists about the materials consumers use. In addition, much of what is grown would not be possible without metals. Platinum, for example, is used as a catalyst in the chemical reaction that produces nitrogen for fertilizer, and metals are used in the production of pesticides , petroleum , and plows. But the winning of metal has not come without a cost. Mining has historically been a dangerous business and one prone to environmental problems. Mining, beneficiation, and smelting have led to the introduction of unacceptable levels of metals into lakes, streams, and ground water. It has injected dust into the atmosphere and spread metals and acid rain across the land. Subsurface mining has caused surface subsidence problems now plaguing some old mining towns. Mines are an economic necessity and actually occupy only slightly more United States land area than airports. A clean environment is also a necessity for health and quality of life. Although problems from mining's early days remain, modern mining methods are considerably safer and have a much less negative environmental impact. Pollution control during the mining and processing of ore, and land reclamation after mine closure are now being considered as one of the economic factors in determining if mineralized rock is indeed ore.


Future developments

Unlike products from the forest and farm, ores are a nonrenewable resource. The economic survival of industrial societies is linked to the discovery of new supplies of metals and to improved technology for the extraction of metals from ever lower grade deposits. Industry, government, and universities are constantly developing new exploration techniques and more efficient recovery methods.

See also Industrial minerals.

Resources

books

Craig, James, David Vaughan, and Brian Skinner. Resources of the Earth. Englewood Cliffs, NJ: Prentice Hall, 1988.

Evans, Anthony. Ore Geology and Industrial Minerals: An Introduction. Boston: Blackwell Scientific Publications, 1993.

Kesler, Stephen. Mineral Resources, Economics and the Environment. New York: MacMillian College Publishing Company, Inc., 1994.

Klein, C. The Manual of Mineral Science. 22nd ed. New York: John Wiley & Sons, Inc., 2002.


Eric R. Swanson

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Gangue

—The valueless component of ore, commonly quartz and calcite.

Hydrothermal fluid

—Hot water-rich fluid capable of transporting metals in solution.

Igneous

—Formed by solidification of molten rock called magma.

Industrial rocks and minerals

—Rocks of economic value exclusive of metallic ores, mineral fuels, and gems.

Metamorphic

—Formed by deformation and/or recrystallization of preexisting rocks.

Ore

—Rock, usually metallic, that can be mined and processed at a profit.

Sedimentary

—Formed by accumulation of sediment, mostly commonly by deposition from water.

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