UNIT 11 GIFTS FROM THE EARTH: MINERAL RESOURCES

Overview

Products made from the mineral resources of the Earth are so familiar in our industrialized society that we tend to take them for granted. In fact, most of the industrialized nations owe their present positions to an abundance of a broad range of mineral resources. In this unit you will examine the various uses that we make of important minerals, their worldwide distribution, and various mechanisms by which minerals are generally believed to be concentrated in some localities and not in others.

B. THE TYPES AND USES OF MINERALS

1.  Metals

The eminent philosopher Bertrand Russell once wrote, "It is to steel and oil and uranium, not to martial ardor, that modern nations must look for victory in war." And, he might have added, for prosperity in peace. The modern industrial nation makes prodigious use of mineral resources from the Earth. Our buildings and highways are made from concrete (cement from limestone, sand and gravel from stream deposits) and steel (iron and a variety of alloying metals); our automobiles and other machines are made of iron, specialty steels, and aluminum, with sizable amounts of copper devoted to wiring. Much of our food is prepared in metal containers and packaged in cans of aluminum or tin-plated steel, or in metal foils. Even the pen with which you are (hopefully!) taking notes may be made of a half-dozen different metals encased in a variety of plastics produced from another resource from the Earth -- petroleum.

There is no question that the search for minerals has profoundly affected human history. In their classic textbook on economic mineral deposits, Mead L. Jensen and Alan M. Bateman give the following perspective:

The quest for the wealth of minerals wrested from the Earth for man's vanity, necessities, or comforts has ever been a powerful incentive to discovery, exploration, and trade. Their search has given rise to voyages of discovery and settlement of new lands. Their ownership has resulted in industrial development and in commercial or political supremacy, and has also caused strife and war. In the ancient country of Saomes, the winter torrents brought down gravels containing gold, which the barbarians passed through inclined troughs lined with sheeps' fleeces to catch the gold. The fleeces that were hung on trees to dry, so that the fine gold could be beaten out of them, spurred Jason and the Argonauts in the ship Argo to seek the Golden Fleece near the shore of the Euxine. This is the earliest record of a placer gold rush and a poetic expression of an early mining venture. It was tin that drew the Phoenicians and Romans to Britain; it was gold and silver that lured the Spanish Conquistadores to the settlement of the New World. The gold rush of 1849 led to the settlement of California and then to the acquisition of the western part of the United States from Mexico and Spain.

Let us look at some of the most important materials that we take from the Earth and how we use them. Many of these, but by no means all, are metals. They seldom come from the ground as pure metals, but rather are found as chemical compounds -- metallic elements bound to other elements such as sulfur (to form sulfides) or oxygen (to form oxides). These compounds, along with the relatively few elements that occur in pure form, are referred to as minerals, in that they have definite chemical compositions. Rocks are heterogeneous mixtures of minerals that can have a wide variety of different compositions, and ores are rocks in which certain minerals are concentrated by natural processes to such an extent that it is profitable to mine them.

A number of the metals are relatively abundant: iron, aluminum, manganese, magnesium, and titanium. Of these, iron is the most important, accounting for more than 95% of all the metals used in our civilization, and is the principal constituent in the making of steel. You need only look around you to see the myriad uses for this versatile, strong, and inexpensive metal. Nowadays, iron is seldom used in its pure form, but is usually alloyed with other metals: nickel, chromium, tungsten, vanadium, cobalt, and manganese. Thus, the steelmaking industry is dependent upon supplies of all of these metals as well.

Aluminum is light and strong, and in recent years has taken over many uses formerly reserved for iron. Many beverage cans, for instance, are now made from aluminum rather than from tin-plated steel. Aluminum is replacing iron in many automobile parts such as engines and bumpers, as automakers strive to increase the fuel efficiency of their cars by making them lighter. It has good resistance to corrosion and so finds many uses in building construction and window frames. Its light weight makes it ideal for use in the construction of aircraft.

Manganese is seldom used by itself, but it is an indispensable ingredient in the production of many kinds of steel, including all carbon steels. It is also used in specialty steels that are extremely hard and tough and serve well in such uses as structural steel, gears, armor plate, and safes. Magnesium, not to be confused with manganese, oxidizes at low temperatures, burning with a bright white light. Fine powders and filaments of this metal are used in fireworks and flashbulbs. Magnesium is the lightest metal known, and so finds use in aircraft, automobiles, and instrument parts.

Titanium is not quite as light as aluminum but is stronger and resists corrosion better. It is difficult to separate from its compounds, however, and so has not seen widespread use as a pure metal in spite of its abundance. At the present time, it is used mostly in its form of titanium oxide (TiO₂), as a white pigment for paints.

The other metals are much more scarce in the crust: copper, lead, zinc, nickel, molybdenum, mercury, chromium, tin, tungsten, and uranium. The last, though a metal, is better treated as an energy resource, and we shall defer most discussion of uranium until the next unit.

Copper is an excellent conductor of electricity and can be drawn easily into flexible wires. As a result it is essential to our electrified civilization, carrying electrical power and communication signals throughout a web of wires that form the technological nervous system of the developed nations. It is also used in brass (copper and zinc) and bronze (copper, tin, and zinc).

Lead is used principally in storage batteries, but it finds other uses in bullets, solder, and-because of its extreme resistance to corrosion-as protective sheathing for electrical cables. Lead is toxic and long since has been removed from most paints for this reason. For the same reason, a common use of lead today, as tetraethyl lead -- an antiknock compound in gasoline -- is gradually being phased out in several countries in order to protect the environment from local lead contamination. Nonetheless, there are many uses of lead that are environmentally sound, and its high salvage value means that with care it can be recycled through such uses with a minimum amount finding its way into the general environment.

Zinc has excellent corrosion resistance and is electrolytically deposited on sheet metal to form galvanized iron. Its greatest use, however, is in diecasting alloys. Dies are the tough metal stamps and forms used to punch out and shape complex sheet metal parts such as automobile fenders. We have already mentioned the use of zinc in brass and bronze. Nickel lends toughness, strength and anticorrosion properties to steel, and its principal use is in alloys such as stainless steel. It is also used for nickel-plating and in coinage.

Molybdenum is used almost entirely as an alloying agent in steels, to which it imparts strength and ductility (the ability to be drawn into rods). Tungsten has similar properties in a steel alloy, also imparting great hardness. As a result, it is used in high-speed cutting tools that are used to shape other steels in lathes and mills. It is also used as tungsten carbide, the hardest known cutting agent after diamond. A minor, though familiar, use is as a filament in ordinary light bulbs.

Chromium is used as an ingredient of stainless steel and other steel alloys. Chromium-plating was formerly used widely on automotive bumpers and trim, though this use is decreasing. It continues in many uses as a tough, attractive coating for other metals. The mineral ore chromite, which is an oxide of chromium and other metals, is an excellent refractory used in furnace linings.

Mercury is the only metal that is liquid at room temperature. It was known to the ancients and in the Middle Ages, fascinated alchemists with its ability to alloy (amalgamate) with many other metals, including gold and silver. For hundreds of years it has been used in the recovery of these two precious metals. Mercury, also known as quicksilver, has myriad other uses in thermometers, electrical switches, pharmaceuticals, insecticides and fungicides, explosives, and antifouling paints. Its use that comes closest to home, however, is in dental amalgam, the metallic substance used to fill a cavity. In spite of its toxicity, the mercury remains safely trapped within the amalgam and only very minute amounts escape into our bodies.

Tin may have been one of the first metals used by mankind. Its use has been traced back to 3700 BC in Egypt, and its use in bronze affected an entire period of human history -- the Bronze Age. In more recent times, its uses in pewter and tin roofing have given way to other metals, and today its principal use is in tinplate, solder, bearing alloys, and bronze.

The precious metals, gold, silver, and platinum have captured the fancies -- and greed -- of men and women for thousands of years, but in addition to jewelry, coinage, and bullion, they have many practical uses as well. Gold is used in electrical contacts, in plating, and in dentistry. Silver finds use in silverware utensils, other plated objects, and in the photographic industry. New uses are in the production of printed circuits for computers and other electronic devices. Platinum sees far more industrial than monetary or jewelry use, and finds employment in electrodes, crucibles, electrical thermometers, and medical and dental devices. A recent use that has spurred demand is as the catalyst in catalytic converters installed in the exhaust systems of automobiles that are designed to convert harmful emissions to more benign gases.

Because of its persistent value and the fact that it is nearly indestructible, most of the gold mined throughout all of history is still in use or in stockpiles of various sorts. It offers us the ultimate example of a recycled resource, in which part of an Egyptian king's adornment may now reside in one of your teeth or on one of your fingers.

2.  Industrial Minerals

In addition to the minerals discussed so far, there is a large group of mineral resources known collectively as industrial rocks and minerals. These comprise a myriad of diverse materials such as asbestos, clays, graphite, lithium, talc, and vermiculite, and although less well-known and less glamorous than the metals, modern industrial society cannot function without them. Moreover, their total value far outstrips that of the metals.

Most important of these materials are those that make up concrete: sand, gravel, and cement which in turn is made by mixing and heating limestone and shale, the products of mines and quarries.

As concrete is literally our most important heavy construction material, so sulfuric acid is perhaps the most important industrial chemical. Sulfur, used to make sulfuric acid, is used somewhere in the production chain of nearly every important item used by modern man from oil refining products to plastics to steel. However, the single most important use of sulfuric acid is to combine it with ammonia and phosphate to form ammonium sulfate and super- and triple-super phosphate fertilizers. We derive our sulfur from the flanks of volcanoes, from salt domes, sulfide minerals such as pyrites, and sour gas.

Various clays are used extensively for pottery, chinaware and other ceramics. Less well-known are specialty clays which are used to filter and clarify many liquids, including beer and vegetable oils; kaolin, used as a filler and coating on papers, in refractories, and in rubber; bentonite, used as a drilling mud in the petroleum industry; and fire clays used metallurgically.

Fluorspar (CaF₂), another widely used mineral, is the raw material for hydrofluoric acid, essential in the manufacture of the fluorocarbon compounds, so vital to our consumer economy. Borax, whose most advertised use is as a cleanser, finds its major use as an indispensable ingredient of high-temperature glasses and fiber glass. Another important element is lithium, which comes to us from natural brines and pegmatites, is a critical material in the manufacture of aluminum and finds extensive use as an ingredient in low-temperature greases.

A number of materials contain what are known as the rare earths, comprising such elements as europium, gadolinium, cerium, and zirconium. The United States is the largest producer and consumer of the rare earths which find such diverse uses as petroleum catalysts, super alloys, color TV tube phosphors and X-ray screen intensifiers, and specialty magnets.

Saline minerals, such as those containing sodium carbonate, impact our lives daily. Nearly half the mined product goes into the manufacture of glass and the chemical industry consumed much of the rest of such products as detergents and soaps.

3.  Other Minerals

Certain elements are of vital importance to agriculture: nitrogen, phosphorus, and potassium. Nitrogen is abundant in the atmosphere, but plants are not able to use nitrogen gas until it has been "fixed" or incorporated into a soluble form as a nitrate (KNO₃ or NaNO₃) or as an ammonia compound such as ammonium sulfate ((NH₄)2SO₄). Bacterial action in soil and certain plants can fix nitrogen, but modern agriculture relies heavily on chemical fertilizers. The nitrogen in these is obtained from the atmosphere and combined with hydrogen to form ammonia. Phosphorus is obtained from the mineral apatite (Ca₅(PO₄)3OH), which occurs in certain phosphate rocks or in marine sedimentary deposits called phosphorites. Potassium is obtained from salts left behind in the evaporation of seawater.

An "edible" mineral is common table salt (NaCl), which is found in thick sedimentary beds, salt domes, or may be obtained from evaporated seawater. Most salt is used not for eating but in the chemical industry and for road salt to melt snow and ice in northern climates.

Finally, there are "wearable" minerals: diamonds and gemstones. Diamonds are a high-pressure form of carbon, formed deep within the Earth. Rubies and sapphires are composed of aluminum oxide; emeralds are a beryllium aluminum silicate; amethyst is quartz (silicon dioxide). The distinctive colors of each gem are determined in some cases by minute amounts of impurities and in others by slight defects in their crystal structure.

Although man-made diamonds are making inroads on the natural stone for industrial use, natural diamonds are a critical industrial mineral widely used for cutting, grinding, and polishing. The drilling of many holes for petroleum and mineral exploration would be impossible without diamond drill bits.

C. WORLDWIDE DISTRIBUTION AND ABUNDANCE OF METALS

The Earth's crust contains all of the stable elements and some of the radioactive ones as well. Figure 11-1 shows the relative abundances of the most prevalent elements in continental crust and in seawater (excluding the hydrogen and oxygen of the water). In the crust, silicon and oxygen are the most abundant elements, reflecting the fact that the most common rock-forming minerals are silicates. The abundant metallic elements shown in the diagram -- aluminum, iron, calcium, magnesium, sodium, potassium, and titanium -- are for the most part bound up in the various silicates, carbonates, oxides, and other rocky materials that make up the crust.

•A few very rare elements and short-lived radioactive elements are omitted. These data from Mineral and Energy Resources, Douglas G. Brookins, (1990).

Figure 11-1  Terrestrial Abundances of the Elements

Source: Crustal elements, Mineral and Energy Resources, Douglas G. Brookins, (1990). Major ions in seawater from Planet Earth, Casare Emiliani, (1992).

 

Silicates are extremely stable in the chemical sense, and it requires prodigious amounts of energy to dissociate ordinary rocks in order to retrieve pure metals from them. While it is possible to obtain mineral resources in this way, until we find cheaper energy sources, it is not profitable to do so. Instead, as has been the practice since the dawn of civilization, it is easier to mine deposits in which nature has produced local concentrations of one or more minerals far in excess of what is found in ordinary rocks.

Perhaps the most striking characteristic of ore deposits is that they are far from being equably distributed throughout the world. Many countries may have few or no economic deposits, while others may be blessed with fabulous mineral wealth. Even on a very local scale, ore deposits may be found in one county or parish but not in its neighbor.

You probably have gold in your backyard. The real question, though, may be put as follows: Is there enough of it in a high enough concentration so that it is profitable to mine? Perhaps you noted that the word "profit" appeared in the last section when we defined what we meant by the term "ore." It is quite impossible to separate economics from any discussion of resources, and in fact the study of mineral resources is often called "Economic Geology."

The price of gold strongly influences the number of operating gold mines. When the cost of producing gold from ore at a mine rises above the value of the gold produced, the operation becomes unprofitable and either higher gold prices, higher grade ores, or improved extraction technology are needed for the mine to stay in business.

The use of iron ore in the United States serves as an illustration of how both economic and technical factors can affect the availability of mineral resources. Three essential ingredients are necessary to the making of iron: iron ore, coal, and limestone, which is used as a flux to remove impurities. Iron ore contains iron oxides, which are very stable, and only by the application of high temperatures in an atmosphere that is deficient in oxygen can metallic iron be separated in pure form. A blast furnace is filled with a charge of the three ingredients and ignited. Air or oxygen is forced through the charge in order to bring the temperature up to the point where metallic iron is reduced by some of the coal, melted, and can be tapped off from the bottom of the furnace. The oxygen from the air is consumed in the burning process, producing the required oxygen deficiency.

Early centers of steelmaking-such as Pittsburgh in the United States, Birmingham in England, and in Germany and Sweden arose in places where all three ingredients were found together or in close proximity, resulting in low transportation costs. In the Pittsburgh area, coal, iron ore, and limestone were all found in the same immediate area. The local supplies of iron ore soon ran out, however, and ore had to be imported from other regions -- in this case, from the rich deposits of the Mesabi Range in Minnesota. These deposits contained more than 50% iron and were profitably utilized even though the ore had to be transported long distances.

As early as 1908, however, steelmaker Andrew Carnegie warned that these high-grade deposits were in danger of running out. Imminent shortage following World War II was overcome when it was discovered that lower-grade deposits, called taconite ores, could be treated in such a way as to increase their iron content from as low as 20% to more than 60%-higher than that of high-grade ores. These upgraded ores so improved the efficiency of the steelmaking process that the costs of treating the ores were more than recovered. Because of the development of the concentration process, iron ore reserves in the United States have been vastly increased.

It is necessary to make a distinction between reserves and resources. Reserves are usually defined as known deposits from which minerals can be extracted profitably using existing technology and under present economic and political conditions. Because economic, technology, and legal/political approvals are essential parts of this definition, world reserves of some minerals could increase or decrease significantly with no significant change in the actual amount of that mineral that is known to be in the ground. There are a number of deposits that can be economically mined, but are precluded from production for legal/political reasons, in many cases related to environmental concerns.

Figure 11-2  World Distribution of Metal Reserves

Source: Mineral and Energy Resources, Douglas G. Brookins, (1990).

 

Resources, on the other hand, are known potential sources of extractable minerals that might be used in the future if changes in technology or economic and legal conditions allow. For this reason, the term often appears as potential resources. In a sense, reserves are birds in hand, while resources are birds in a bush -- we may or may not someday actually mine resources. Also, you should realize that the two categories are not fixed and immutable. If mineral prices fall, as they have in recent years, some marginal reserves may slip into the category of potential resources; when prices improve, they may shift back into the active reserves.

With this distinction in mind, let us look at the distribution of reserves for a number of important metals among the countries and regions of the world. At first thought, you might expect the countries with the largest areas to contain the most mineral wealth, and to a certain extent this is true. The former Soviet Union (F.S.U.). and the United States certainly are among the leaders in mineral reserves. But there are some surprises as well. Figure 11-2 and Figure 11-3 show the distribution of reserves for eight important metal ores. Iron ore is distributed more or less as we might expect on the basis of area, though Africa has clearly been slighted. On the other hand, the United States and South America have a disproportionate share of copper resources.

Figure 11-3  World Distribution of Metal Reserves, Continued

Source: Mineral and Energy Resources, Douglas G. Brookins, (1990).

 

Though shown in Figure 11-2 as reserves, recent economic conditions have shifted sizable amounts of United States copper into the category of resources, resulting in a domestic copper industry that is extremely depressed. This is an excellent but unfortunate example of the extreme volatility of reserve estimates due to economic conditions, and of the high-risk situation that is a continual part of life in the mineral industry. The United States holds more than half the total world's reserves of molybdenum, but has almost no reserves of chromium, manganese, and tin. Alaska, however, holds ample tin resources.

Even more surprising is the dominance of some small countries for certain metals: Cuba accounts for nearly a third of all the nickel reserves in the world. Four small countries, Malaysia, Indonesia, Thailand, and Bolivia, account for nearly three-quarters of the world tin reserves, and South Africa, in addition to its gold and diamond mines, has a third of world manganese reserves and more than three-quarters of world chromium reserves.

Why should mineral reserves be so unequally divided? And what is the mechanism that concentrates minerals in ore deposits, sometimes achieving concentrations that are many thousands of times richer than those found in ordinary rocks? We shall take the two questions in reverse order and treat them in the next two sections.

D. PROCESSES OF MINERAL CONCENTRATION

Suppose I were to mix thoroughly a pinch of ground pepper with a spoonful of salt, and then ask you to separate the pepper from the mixture. It would not take you long to realize how to do it: simply place the mixture in warm water, mix it until the salt is dissolved, and pour off the solution, leaving the pepper grains behind.

Or perhaps you might have thought of a different method. If the mixture is slowly poured in a thin stream from a container while you gently blow across it from the side, the lighter pepper grains will be deflected more than the salt grains and will tend to concentrate in the downwind direction. In this case, however, the pepper will not be completely separated from the salt, but will become concentrated considerably over its earlier dispersed state. Ancient methods of winnowing grain work in the same way, with the heavier wheat being separated from the lighter chaff by the effects of an air current.

The process of concentration of minerals can proceed in nature in a number of different ways, just as in our experiment with salt and pepper. Different processes of ore concentration may work with different minerals, or with the same mineral under different circumstances. Sometimes it is difficult to determine just which process has acted to concentrate a particular ore, but general knowledge of the workings of these concentration mechanisms has improved over the years. The principal mechanisms that have produced valuable mineral deposits are magmatic concentration, hydrothermal processes, replacement, sedimentation, evaporation, residual concentration, and mechanical concentration into placer deposits. Let us examine each in turn.

Magmatic concentration refers to the process by which molten rock at depth (magma) segregates into different constituents as it cools and solidifies. An analogous situation in the kitchen is the process of clarifying butter. In this procedure, butter is melted and allowed to stand for a few minutes. The milk solids precipitate out and settle to the bottom of the container, allowing the clear (clarified) butterfat to be skimmed off the top and separated. One might look at it as a process for concentrating milk solids at the bottom of the container that previously had been dispersed throughout the butter.

In a similar manner, when a body of magma far underground begins to cool, certain minerals (often the more mafic ones) will crystallize first and, being denser than the surrounding fluid, will sink to the bottom of the magma chamber. The residual magma will become increasingly felsic, as was described in Unit 3, Plate Tectonics, in relation to the chemical differentiation processes that occur at oceanic ridges and in subduction zones. Ore minerals frequently have different chemical affinities for the different silicates that are involved in this differentiation process, and so will be concentrated either among the early crystals or in the portion of the magma that is late in solidifying.

In some cases, the last remaining fluids, together with a slurry of crystals already solidified, may be injected into cracks or fissures in the surrounding rock to form pegmatite dikes, coarsely crystalline masses that are often rich in metals and, sometimes, gemstones. The term dike refers to a tabular sheet of rock that has been injected into older, already existing rocks.

Magmatic concentration or segregation appears to be important in some deposits of chromium, iron, titanium, platinum, nickel, and copper.

Among the most important concentrating mechanisms are the hydrothermal processes. "Hydrothermal" simply means "hot water". We may draw upon another kitchen analogy to explain how these processes work. You probably know that if you boil vegetables too long, many of the vitamins, which are soluble, will be lost to the water. This is fine if you are making soup, but not good if you plan to discard the water and eat only the vegetables.

In the geological situation, hot water circulating among cracks will dissolve minerals from the rock and carry them away from the source rock to where the minerals can be deposited. Solution may take place at depth where temperature and pressure are high. Under these circumstances, the ability of water to dissolve minerals is enhanced, especially if the fluid contains chlorine or fluorine in solution. The metal atoms are transported in the hydrothermal fluid as metal chlorides or fluorides until they meet an external source of sulfur, perhaps in the form of hydrogen sulfide that may be derived from magma. A chemical reaction between the fluid and the sulfur creates metal sulfides that are highly insoluble in water. These precipitate out, taking the metal out of solution and concentrating it into an ore deposit. This may deposit mineral veins in fissures or pores of the surrounding rock, or perhaps the hydrothermal fluid may react chemically with the rock, altering it and producing a deposit via the process of replacement.

In replacement deposits, rocks are altered when they come into contact with hot fluids. These fluids are commonly rich in hot water, gases exhaled from the magma, or rarely magma itself. Some elements in the rock become dissolved in the fluid and other elements carried in the fluid replace them, chemically altering the rocks in the process. In this way, mineral ores can be formed, especially in the hot region immediately surrounding a body of magma underground.

Replacement can form very rich deposits. Although some of these are large and important, others are often very local and may be hard to find. They can also be exasperating to mine, because they may look very promising at first, but then may peter out abruptly as the mine is extended. Ores formed by replacement often contain iron, copper, zinc, lead, tin, tungsten, molybdenum, graphite, gold, silver, manganese, and corundum (an abrasive).

In the units on the oceans, we saw that the process of hydrothermal activity is taking place in the oceanic spreading ridges. Seawater, filtering through cracks in the rock, reacts with magma from the mantle, dissolves minerals from it, and then is ejected from hot springs in the ocean floor. On encountering the cold water of the ocean bottom, minerals are precipitated from solution, producing in some cases the black smokers referred to in Unit 6, Dynamics of the Oceans. Local mineral concentrations are created in this way, and it is now realized that some mineral deposits on the continents may have had their origins in hydrothermal activity associated with the ocean ridges. Other deposits formed from groundwater (mostly derived from rainwater), acting hydrothermally in the vicinity of cooling magma bodies. Many natural hot springs, such as those found in health spas, are noted for their "mineral waters," and the colorful deposits that often form around hot springs in Yellowstone Park or in Iceland or Japan attest to the mineral concentrating mechanisms at work in hydrothermal fluids.

Hydrothermal activity accounts for a number of important deposits of gold, silver, copper, lead, zinc, tin, tungsten, mercury, antimony, cobalt, and germanium.

The processes of sedimentation can also act to concentrate minerals. Exposed to weathering -- the effects of rainfall, freezing and thawing, chemical reactions with air and water, and mechanical breakdown during floods or landsliding events -- rocks eventually become reduced to sedimentary detritus. This takes the form of small grains of sediment with widely varying composition. Transported by water, sediment is deposited on river banks, floodplains, in the deltas of major river systems, or on the sea floor. Throughout the process there is ample opportunity for minerals to become dissolved in the transporting water. As an example, when river water encounters the colder ocean there is a tendency for dissolved minerals to precipitate out of solution, helping to cement the deposited sediments into rock and sometimes forming widespread layered mineral deposits. Bacterial action may also come into play in aiding the mineral concentration process during and after deposition.

Hydrothermal fluids described above are also discharged directly onto the sea floor where chemical precipitation of their dissolved metals forms metal-enriched chemical sedimentary strata known as "exhalites". Sedimentary processes produce deposits of iron ore, manganese, phosphorus, sulfur, copper, cobalt, lead, zinc, silver, gold, uranium, limestone, and clay.

Evaporation is a familiar process for concentrating minerals. A glassful of tap water that is allowed to evaporate will leave behind a slight film of minerals on the walls and bottom of the glass. When ocean water is evaporated, salt deposits reflecting the concentration of elements shown in Figure 11-1 will form. Lakes that have no outlet to the ocean, such as the Great Salt Lake of Utah or the Dead Sea in the Middle East, collect and concentrate minerals from the river water draining into them. If such a lake completely dries up, an enormous store of mineral deposits is left behind to be incorporated into the local rock strata as evaporites. In some cases, basins with a weak connection to the ocean can dry up from time to time, creating extensive deposits. Recall from Unit 6, Dynamics of the Oceans, that the Mediterranean Sea once dried up totally, producing immense deposits of salts that became buried in beds of sediment beneath its floor. The sediment layers now protect the salt beds from being redissolved in the present waters of that sea.

Evaporite deposits include gypsum, common salt (NaCl), potash (potassium ore) from the evaporation of seawater, and borax from the evaporation of saline lakes on the continents. When thick beds of salt are buried by overlying sediments, as in the Mediterranean and in the Gulf of Mexico, the extremely low density of the salt produces a curious and important effect. Since the overlying sediment is more dense, it tends to sink, displacing the salt upwards in rising columns called salt domes. These features (Figure 12-5) are of considerable importance to the petroleum industry, as we shall see in the next unit, and as sites for the deposition of sulfur.

Processes of weathering can operate in more than one way. In addition to carrying away desirable minerals for deposit elsewhere, weathering can remove common rock minerals, leaving concentrations of less easily-weathered minerals behind. This is called residual concentration of minerals, and can result in deposits of iron ore, manganese, bauxite, nickel, and clay.

Bauxite is the principal ore of aluminum, and it tends to form as a red aluminum-rich soil in tropical climates with high average temperatures and abundant rainfall. Source rocks rich in aluminum are also necessary for the creation of bauxite.

We conclude our list of mineral enrichment mechanisms with mechanical concentration. Heavy metals such as gold that do not react with oxygen or water may be released from deposit by weathering of the rocks and may then be transported in streams. Because of their density, the metal grains will not be carried as readily as the silicate sediment grains and will tend to become deposited in the sands and gravels of the stream bed that drains the source rock, or "mother lode". Concentrations of this kind are called placer deposits (the word placer, by the way, is pronounced with a short "a", as in the word "act.")

Placer deposits may contain gold, platinum, tin, titanium, rare earths, diamonds, and other gemstones. The stereotyped grizzled prospector with his mule and pan is long since gone in the United States, though in some parts of the world he remains an important part of today's exploration team. Many people today follow in his footsteps, panning stream deposits for recreation and occasional modest profits. The process of panning essentially duplicates the mechanical concentration that nature used to produce the placer deposit. Sediment and water from the stream bed are placed in the pan and swirled around. The water current and the centrifugal force of the swirling motion separates the lighter sediment from the gold, forcing the former over the rim of the pan and leaving any flecks of gold behind. More serious operations use dredges and water sluices to accomplish the same thing, continuing the ancient tradition that began with the Golden Fleece.

E. PLATE TECTONICS AND OCCURRENCE OF MINERAL DEPOSITS

Many of the processes for mineral concentration that were described in the previous section depend upon the presence of magma bodies underground. In Unit 3, Plate Tectonics, you learned that magma often results from tectonic activity near plate boundaries, and so it should seem reasonable that there are links between the occurrence of mineral deposits and the plate tectonic history of a region. In particular, magma generation is associated with oceanic spreading ridges and with subduction zones. Let us examine the deposits that are likely to result from each of these settings.

1.  Mineral Deposits at the Oceanic Ridges

We have already discussed the process by which hydrothermal fluids can concentrate metal sulfide deposits at the oceanic ridges. Ocean water circulating through cracks in the new basaltic sea floor created at spreading ridges is heated by contact with hot rocks. Metals are dissolved in the saline water and precipitate near the discharge points of the hot springs when they combine with hydrogen sulfide to form metal sulfides. In this manner copper, iron, lead, and zinc sulfide minerals precipitate from solution. When the mineralized water encounters and mixes with cold ocean water, oxides of iron and manganese may precipitate.

At the sites along the East Pacific Rise where the submersible Alvin was used to investigate the spectacular hot spring vents described in Unit 6, Dynamics of the Oceans, the black smoker chimneys were found to consist largely of metal sulfides. Chimneys have been discovered 15 meters (50 ft) tall resting on a metal sulfide mound of possibly equal height and up to 30 meters (100 ft) across. Such chimney-mound structures may weigh several thousand tons and one that was sampled was found to contain 14% iron, 0.7% copper, and 31% zinc, along with some cobalt, silver, and gold.

Only a small portion of the oceanic ridge system has been investigated in detail to date, but on the basis of present evidence, it seems that the extremely active chimney-vent type of activity may be restricted to fast-spreading ridges such as the East Pacific Rise, where new sea floor is being added at a rate of about 18 cm (8 in) per year. This type of vigorous sea floor spreading activity is reflected in the hydrothermal regime present at the ridge crests.

Sea-floor spreading rates are much slower in other parts of the world, and this seems to influence the style of hydrothermal activity as well. For instance, at sites investigated on the Mid-Atlantic Ridge, where the spreading rate is on the order of two to three centimeters (one inch) per year, surface deposits consist of nearly pure manganese oxide encrusting the ocean floor. Where deeper layers of the sea floor are exposed, metallic sulfide deposits are found, indicating that the hydrothermal fluids have had a chance to mix thoroughly with ocean water before emerging from cracks in the walls of the central rift valley at the crest of the ridge.

Still another type of spreading boundary environment is encountered in the Red Sea mineral deposits. The Red Sea is a new ocean, formed by the rifting of the Arabian peninsula away from Africa. Its deposits were discovered in the 1960s during an international oceanographic expedition when echo sounders on the research vessels recorded an unusual reflection within the ocean water some distance above the seafloor. When the water was sampled at that depth, very high salinities were found, indicating that the sonar reflection marked a boundary between normal seawater and denser salt brine that was warm and rich in metals. The brine was collected in a series of pools located along the axis of the Red Sea. When cores were taken of the sea floor sediments, it was found that they contained layers of metal-rich sediment ten meters or more thick.

The Red Sea deposits are rich in iron, copper, zinc, and small amounts of silver and gold. It is estimated that the largest of these pools, the Atlantis II Deep, contains three million tons of metals, not counting the iron minerals. The two countries flanking that part of the Red Sea, Saudi Arabia and Sudan, have formed a joint commission to study the feasibility of mining this deposit. If the venture appears to be profitable, the Red Sea may be the site for the first commercial application of deep-sea mining. The technique most likely to be used is a dredge combined with a powerful suction device controlled from a sea-surface vessel. The device would scoop up loose sediment from the ocean floor and pump it up as a slurry to a ship on the surface.

In a few places, bits of oceanic crust have been scraped off the lithosphere at a subduction zone and added to continental crust, making them easily accessible. These hunks of displaced ocean floor are called ophiolites and have been mined for their stores of copper and other metals since antiquity. The best-known example is the Troodos Massif on the island of Cyprus in the Mediterranean Sea.

2.  Mineral Deposits Related to Subduction Zones

Important occurrences of copper ore are found in what are called porphyry copper deposits. These form by magmatic concentration with hydrothermal alteration and replacement in magma intrusions into continental crust. Figure 11-4 shows the worldwide distribution of regions containing major porphyry copper deposits as shaded zones. Note that many occur in close association with subduction zones, such as those in South America, the Philippines, and the Middle East. In other cases, such as in western North America, eastern Australia, and in the Ural Mountains of Russia, the deposits possibly were associated with subduction that took place in the past.

Figure 11-4  Porphyry Copper Deposits in Relation to Plate Boundaries

Recall that in a subduction zone, oceanic crust on the downgoing slab is heated, melted, and chemically differentiated, producing relatively felsic magma that is supplied to the volcanic arc above the subduction zone. The subduction process not only supplies the magma intrusions to fuel the hydrothermal process, but because the subducted ocean floor was already enriched in metal deposits by the processes described previously, it taps a source that has an abundant supply of metals.

Porphyry copper deposits tend to be large and relatively low grade, containing from 0.2% to 2% copper. Fortunately, the refining process is relatively easy, and because of the size and grade of the deposits, they are often profitable to mine. A single porphyry deposit may contain up to several million tons of copper, though most are considerably smaller.

In South America, the subduction zone associated with the Peru-Chile Trench is hard against the continental coastline, while in many places along the northwest margin of the Pacific Ocean, subduction takes place some distance offshore from the continent of Asia, forming a volcanic island arc separated from the main continental landmass by an oceanic basin in which slow sea-floor spreading may be taking place. The Japan Sea is an example in which spreading has widened the separation of the Japanese islands from China. Figure 11-5 shows the relation between this spreading basin and the subduction zone.

Figure 11-5  Back-Arc Spreading Shown in Relation to a Subduction Zone

A number of different mineral deposits are associated with back-arc spreading, resulting from similar concentrating mechanisms to those active on the other oceanic spreading ridges. Chief among these are the Kuroko-type massive sulfide deposits found in northern Japan and elsewhere that contain copper, zinc, lead, gold, and silver concentrations. Although smaller than the porphyry copper deposits, these are often significantly higher-grade ores and are of economic value.

In some places, the extensional or pulling-apart forces that generate back-arc basins can operate on a continent as well, so long as active subduction takes place nearby. This forms continental rift systems much like the oceanic rifts, but within the continent. Where these rifts are flooded, either with inflow of seawater or as lakes, like the great lake system of the East African Rift, subaqueous hydrothermal activity may form other types of lead-zinc-silver-rich deposits in a manner like that described for the Red Sea.

The western part of the United States was the scene of continuous subduction of Pacific Ocean floor along the west coast until about 26 million years ago. Continental rifting occurred inland from the subduction, similar to back-arc spreading and, where this was accompanied by certain types of igneous intrusions, resulted in hydrothermal emplacement of large molybdenum deposits in the front ranges of Colorado. These give the United States its dominant position in reserves of this important steel-making element, as shown in Figure 11-3.

F. SOME NOTABLE MINERAL DEPOSITS

This is a good time for us to put together the various concepts discussed so far in terms of the origin and setting for different kinds of deposits.

1.  The Bushveld Igneous Complex, South Africa

The Bushveld igneous complex in South Africa is magmatic in origin and consists of a series of layered igneous rocks that contain vast reserves of chromium along with platinum, nickel, and iron. It is the largest single deposit of chromium ore found anywhere in the world and has been estimated to contain reserves in the range of 6,000 million tons of chromite.

The Bushveld complex is a good example of magmatic concentration as described earlier, in which chromite (chromium ore) accumulates toward the bottom of a layered magma intrusion due to its high density and tendency to crystallize before the rest of the magma. The metals are concentrated in a series of relatively thin layers ranging in thickness from centimeters to as much as one meter. They extend over an area of many thousands of square kilometers, however, accounting for large reserves.

Radioisotope dating of the Bushveld complex yields ages on the order of 2 billion years, placing its origin nearly half way between the origin of Earth and the present. The complex occurs as a series of nearly circular igneous intrusions that are relatively isolated in that the surrounding rock is much older and essentially undisturbed. It does not appear to be associated with either subduction or sea-floor spreading and so its origin is still a bit of a mystery, although an ancient form of continental rifting may have been involved. Two interesting hypotheses have been advanced -- one is that the intrusion is due to hot spot activity, and the other is that the circular igneous complexes resulted from the impacts of fragments of an asteroid.

2.  The Troodos Massif in Cyprus

The Troodos Massif in the western part of the Mediterranean island of Cyprus contains the Troodos ophiolite segment of ocean floor produced by sea-floor spreading in Cretaceous time. It contains massive sulfide deposits of iron, copper, zinc, and cobalt that were concentrated by sea-floor hydrothermal activity at an ocean ridge, as well as magmatic nickel sulfide and chromium deposits that were formed in deeper intrusive rocks at the ridge. The copper deposits on Cyprus have been worked since antiquity, and indeed the metal derives its name from this locality, having been called the "Cyprian metal."

Ophiolites around the world have their origins in the ocean floor and in addition to their mineral deposits are of interest to geologists because they provide dry-land exposures where the structure of what was once oceanic crust may be studied with relative ease.

3.  Kuroko Massive Sulfide Deposits, Japan

We have already mentioned the Kuroko-type deposits in the previous section, relating their origin to back-arc sea-floor spreading. The Kuroko massive sulfide deposits are found in the northern portion of the main Japanese island of Honshu and contain zinc, copper, and lead along with minor amounts of precious metals. The concentrating mechanism was hydrothermal activity on the sea floor approximately 12 to 15 million years ago, making this a relatively young deposit, but there are many similar, much older deposits in more ancient rocks in other parts of the world.

4.  The Noranda District of Quebec, Canada

The Horne Mine and others at Noranda in northern Quebec is an example of a layered massive sulfide deposit of considerably greater age, dating back to the early Precambrian. It is a classic example of a submarine volcanic process forming layered sulfide deposits over the hot springs on the ocean floor, and is similar to the Kuroko deposits of Japan. It produces copper, zinc, gold, and silver.

5.  Uranium Deposits in Saskatchwan, Canada

Uranium deposits are generally found in sedimentary environments and are of several types, but their exact origin is still a matter of debate. Some geologists believe that the concentration mechanism is chemical in nature, due to circulating groundwater that collects dispersed uranium from the sediments and deposits it in favorable settings.

G. SEARCHING FOR MINERALS

Our greatly improved knowledge of how mineral deposits form has strongly affected the approach used in locating them. If we return to our grizzled prospector with his pan and mule, we can gain some perspective on how far we have come. For the most part, these hardy fortune-seekers had limited knowledge of geology or mineralogy. They were simply looking for a particular type or color of rock. Not that they expected gold or other mineral deposits to appear so obviously, but they knew that many of the minerals that they sought were to be found in rocks that had been hydrothermally altered and were rich in sulfides. The prospector, then, was looking for an environment favorable for mineralization, even though he knew little of the mineral concentration mechanisms.

Modern prospecting is involved in similar searches, but in a more sophisticated manner. An important first step is in the production of geologic maps of a region, which identifies the rock types at the surface (or outcrop) in each area of the map. The geologic structure of an area can provide important clues as to where mineralization may occur. Faults and fracture zones in the rock can provide pathways for hydrothermal fluids and such linear features are often found in association with mineralized zones. But these are all surface features, and many mineral deposits are covered by unmineralized rocks or surface debris. How may these be found?

Dense or magnetic mineral deposits may be located by using gravity or magnetic surveys, in which the Earth's gravity and magnetic fields are measured and mapped to high precision. For example, if the gravity field shows an anomalously high value over a portion of ground, this probably indicates that dense rock is buried below.

Geochemical surveys provide other techniques for literally "sniffing out" buried minerals. The concentration mechanisms that we discussed may tend to act in a somewhat diffuse manner, placing most of the deposit in one restricted location, but spreading smaller amounts of the mineral throughout the rock layers over a much wider area. Leaching of minerals by groundwater subsequent to the formation of a deposit may also serve to provide widespread indications of its presence.

Samples of rock and soil are chemically analyzed and examined for traces of compounds that indicate the presence of particular minerals. Even vegetation may yield important clues, since trace minerals in the ground may be taken up by the plant's roots and incorporated into wood and leaves. These analyses are usually carried out in sophisticated chemical laboratories, but new technology has provided portable and even airborne instruments that allow analyses to be done on the spot, with greater efficiency and less cost.

1.  Mineral Exploration

The use of genetic models-which relate the geologic processes such as have been discussed to the specific geologic environment that is most likely to host the particular kind of ore body being sought-is an important first step in the exploration process. Choosing this geologic environment -- "getting in close" -- is the "art" of exploration, the successful practice of which requires a broad knowledge of the science and of the ore deposits, the real-life ore models.

Once the geologist has "gotten in close," largely by the use of general geologic principles, augmented by fieldwork, the target commonly can be narrowed by geochemical and geophysical surveys. Although a geochemical anomaly may be a direct reflection of a buried deposit, a geophysical anomaly is only an indirect reflection of mineral deposits. Geophysical methods, by and large, measure electrical responses and magnetic properties and responses that are not specific to a given mineral or ore.

Some of the metallic mineral deposits have conductive properties that, when excited by an induced current, generate a magnetic field which can be recorded by sophisticated instrumentation. A number of electrical-magnetic prospecting methods and instruments are in wide use, both as ground and airborne systems, and many hidden massive sulfide ore bodies have been discovered around the world by their use.

The natural radioactivity of rocks can sometimes be more valuable a clue to mineral concentration than those of the more "classical" physical properties. Apart from uranium and thorium and the products of their radioactive decay, the only naturally occurring radioactive element of any importance in prospecting is potassium.

Most rocks contain some uranium, thorium, and potassium, so radioactivity surveys, both ground and airborne, can be of great use in general geologic mapping. Such surveys, of course, are of direct use in prospecting for uranium deposits.

Radioactivity sensors can also be lowered into drill holes to detect various elements and properties in such holes not otherwise directly detectable.

Since none of the geochemical and geophysical tools allow the geologist to see what is below the surface, his most important exploration tool is the drill. It alone allows him to "see into the ground" by taking samples from below the surface. Since a drill hole through rock is expensive, and "sees" only the rock penetrated by the hole, the geologist must place his holes judiciously. Moreover, since the hole may narrowly miss a deposit, he must study the samples closely and he must recognize the "alteration halo" around the hidden deposit, in the same -- but more sophisticated -- way as the prospector recognized the obvious altered rocks.

In summary, recent advances in scientific knowledge of how and where mineral deposits form have given today's exploration geologist the intellectual tools unavailable to both his predecessor and the prospector. These tools have allowed the geologist to narrow the search area. Theoretical and technological advances in geochemistry and geophysics have allowed him to focus the search. However, no methods yet known enable us to remotely detect a hidden ore deposit. Only the expensive drill hole can make the discovery. The single most important element of exploration, therefore, has been and remains, the drill hole.

Important as they are to the existence, well-being and improvement of modern society, mineral deposits occupy much less that 1% of the Earth's surface. Since they occur only where the vagaries of geologic processes placed them, if society is to continue to depend on an ever-increasing supply of minerals for its well-being, it must carefully weigh the benefits to be gained from exploration for and extraction of minerals against the benefits from other uses or designations for the same land that would exclude these activities.

2.  Remote Sensing

Some of the most intriguing new developments in mineral exploration do not even require the presence of a geologist at the time a site is being investigated. Remote sensing, the analysis of the surface of Earth from aircraft or spacecraft, has provided another method of exploration geology.

The first application of remote sensing was apparently made by Galileo, who turned his telescope on the Moon as it rose during daytime from behind a stone wall in his garden. He noted that the wall, which was illuminated by sunlight, appeared brighter than the surface of the Moon. He concluded that the lunar surface must reflect less light than terrestrial rocks and so must be made of darker material.

Remote sensing, as we have already seen, was developed to high levels of sophistication during the investigation of the solar system by spacecraft carrying a wide variety of instruments. That technology has been turned back on Earth itself in a series of satellites designed to provide a new look at our own planet from a much larger perspective than the ground-based observer ever could obtain. In 1972, the first in a series of Earth-resource satellites, called LANDSAT, was launched. Three additional Landsats have followed, providing unbroken surveillance for more than a decade.

The first three Landsats carried a multispectral scanner, or MSS. This is a device that scans a narrow strip of the surface beneath the satellite as it moves along its orbit. The strip is 185 km (115 mi) wide, and the scanner is able to resolve features that are only 200 meters (660 ft) or less in size. The unique feature of the MSS, however, is its ability to record the image as seen through four different filters, each admitting radiation of a particular color or wavelength range. Two of these spectral ranges are in visible light (green and red) and two are in the infrared portion of the spectrum (see Figure 7-3). The images are digitized and radioed to Earth, where they are processed and reconstructed into pictorial images. Modern satellite images can now record 255 wavelengths of radiation with spatial resolutions of 10 meters. 

A more recent version of these satellites contains a more advanced version of the MSS, called a thematic mapper that uses seven spectral bands instead of four, and is capable of even higher resolution (LANDSAT 7), approximately 20 meters per pixel side. 

When different materials reflect sunlight, they selectively reflect more or less of each wavelength of light in a pattern that is often unique to that material. This reflectance spectrum may then be used to identify the material that is illuminated. The four (or seven) spectral bands measured by the Landsats allows this kind of analysis to be made remotely from space. You have probably seen the brightly colored images of portions of Earth from Landsat, in which the four spectral images are superposed and printed in four false colors. The false color requirement arises from the fact that infrared is invisible to the human eye, and so these images are printed in arbitrary colors that have been chosen to make them stand out against the visible light images. A variety of computer enhancements have been applied to these images in order to emphasize the differences between rock and soil types, vegetative cover, and so on.

Landsat images have been used to study the shape of landscape features, geologic structures such as folds and faults, and to produce rough geologic maps of broad areas that have not yet been mapped in detail by traditional methods. The exploration geologist can use Landsat images in a variety of ways to aid in the search. For instance, mineral concentration zones may reveal themselves via the distinctive colors and reflectance spectra of the altered rock that often surrounds them. Uranium deposition in sedimentary rocks is sometimes marked by distinctive colors in the rocks. Landsat's sensitivity to the reflectance of chlorophyll can also be used to detect characteristic vegetation patterns over mineralized zones. These kinds of data have provided geologists with new tools and new perspectives for mineral exploration that can be applied worldwide and, in some cases, at relatively little cost.

Another new remote sensing technique is provided by Side-Looking Airborne Radar (SLAR). Radar is microwave radiation, with the unique capability of penetrating clouds. Unlike Landsat, which is dependent upon clear weather for its observations of Earth's surface, radar can function through overcast and view the surface without interference from the weather.

SLAR imagery shows the shape of the landscape so clearly that many geological structures may be clearly seen. Linear alignments of valleys, cliffs, or other structures, called lineaments, commonly betray the existence of faults and fractures within the rocks that may be sites of mineralization.

Synthetic Aperature Radar (SAR) remote sensing instruments use a similar microwave radiation source. However, later additional computer processing builds a "virtual antenna" equivalent to a kilometers-long space craft. Fully processed images, such as those from the European Space Agency ERS-1 satellite, have resolutions of approximately 10 meters. Related instruments have also been flown in the Space Shuttle. These Shuttle Imaging Radars (SIR-A,B,C) have been useful in studying geological and structures worldwide.

These new remote sensing techniques have provided a broad-scale view of the geology of a region that offers a totally new perspective for mineral and oil prospecting and geological studies. With the lessening of the Cold War, the former Soviet Union has recently begun to release data from previously classified satellites. These images, which are digital and can be considerably enhanced using computer processing techniques, have resolutions of between 4 and 2 meters per picture element. Intelligence agencies in the United States have begun to release even higher resolution data after a case-by-case review process.

H. PRESENT SUPPLIES AND PROSPECTS FOR THE FUTURE

In discussing the availability or scarcity of various minerals, a distinction should be made between taking a local or worldwide view of the situation. You should recall the situation presented in Figure 11-2 and Figure 11-3. As an example, chromium has worldwide reserves adequate for the immediate future, but an industrialized country like the United States must import virtually all of this metal to meet its needs. No satisfactory substitute for chromium has been found, and so this makes the industrialized world dependent on only a few exporting countries such as South Africa, Zimbabwe, and Turkey. Russia and Albania are also significant producers.

Assessing the future supplies for any of these minerals is a hazardous business at best. Projections of exponentially escalating demand and imminent shortages in a number of minerals were common in the middle 1970s, but worldwide economic slowdown turned shortage into glut and today many mineral companies have shut down or are struggling to make a profit. If economic improvement continues, the situation is likely to turn around again, and perhaps critical shortages will once again be a concern.

Another uncertainty in forecasting mineral supply is the near impossibility of predicting major new discoveries. However, many knowledgeable observers are pessimistic about truly radical changes in known mineral reserves, and feel that the principal changes will be driven by economics and not by real changes in the resource base. Exploration techniques have developed to the point where they feel that most of the major surface deposits of high-quality mineral ores have already been found in most parts of the world.

At present rates of consumption, most scarce metal reserves should last another 20 to 100 years. Metals that are most likely to become scarce during your lifetime are silver, mercury, tin, and tungsten. The common metals, iron, aluminum, manganese and, to a lesser extent, copper and nickel, are relatively abundant, but many of the higher-grade ore deposits are becoming exhausted. The result is that lower-grade ores will have to be mined and refined, at greater cost. The prognosis in the long run, at least, seems to be gradually rising prices for these metals.

Figure 11-6 presents a model for the consumption of a limited resource within an industrialized country. Initially, the number of operating mines and the production of metal increase together. Eventually, as more deposits are worked out, the number of operating mines decreases, followed by the production. If demand continues, a greater and greater proportion of the metal must be imported from other countries.

Figure 11-6  A Rough Model for the Consumption of a Limited Resource

Brian J. Skinner, Earth Resources, 2nd Ed., ©1976. p. 81. Reprinted by permission of Prentice-Hall Inc., Englewood Cliffs.,NJ .

 

If we consider total mineral production, Russia, with its vast mineral reserves, is still in the stage of expansion at the left of the diagram; the United States is somewhere near peak production, with imports rising; and England has long since exhausted many of its resources and is heavily dependent on imports of raw materials. Japan joins England on the right side of the curve, having a smaller resource base to begin with and having passed through the process much more rapidly, partly due to its large population and partly due to its prodigious use of resources during World War II.

There is a significant disparity in resource use between the developed and the less-developed nations. At the present time 5% of the world's population consumes 90% of world mineral production. This divides the world into two camps: the highly industrialized consuming nations and the less-developed exporting nations. To a considerable extent, the industrialized nations are those that had a broad and rich resource base to begin with. The nations of Europe built the industrial revolution on domestic resources of minerals and energy, and even today total mineral production is dominated by only five nations: Russia, the United States, Canada, Australia, and South Africa.

As the developed nations exhaust their domestic reserves, they are turning increasingly to mineral imports from their less-developed neighbors. To some, this smacks of exploitation of Third World countries, but to many of those countries, the exporting of minerals is their principal means of obtaining capital for development, jobs, taxes, and a favorable influence on their balance of trade.

I. MEETING FUTURE MINERAL DEMANDS

There is sharp dispute even when mineral forecasts are made for 20 years into the future; attempting to see farther into the future can be little more than guesswork. We have seen that economics are a strong part of estimating reserves, and it turns out that the cost of energy is a strong part of the economic picture in the mineral industry. Refining metals from ore requires energy, and lower-grade ores require more energy. The combination of rising energy prices and the need to mine lower-grade ore deposits can have a dramatic effect on metal prices. This effect is partly offset, however, by technological breakthroughs that can increase the efficiency of extraction and refining.

Finding new sources for minerals is another distinct possibility for the future. We have already seen that steps are being taken toward the development of mineral deposits on the floor of the Red Sea, and it is clear that the sea floors hold large deposits of a number of minerals. The question is, will it ever be economical to mine them?

Serious consideration has been given to mining a mineral resource that is reasonably concentrated and accessible on the ocean floors. Large parts of the Pacific Ocean floor are covered with manganese nodules concretions of manganese and iron oxides along with copper, nickel, and other scarce metals that were discovered during the voyage of the research ship H.M.S. Challenger.M.S.; in the 1870s. Already trial dredging operations have been conducted, and total reserves are estimated to be huge. In the cases of manganese and copper, the resources present in manganese nodules may surpass present continental reserves. Development of these resources has a number of problems associated with it, however. Resource analyst Brian J. Skinner points out:

What bottom dredging of the ocean will do to sea life, who really has the right to recover the material, and how mining is to be monitored and policed are vast problems for the future. An entire new field of legal expertise will apparently have to develop.

There are still a number of regions of the world that have seen little if any mineral development. This is true especially for the polar regions. Technical advances are slowly making it possible to operate in the frigid reaches of northern Canada, Siberia, and Antarctica, where significant mineral deposits have been found and more are likely to turn up with intensive exploration. Antarctica, however, is presently off limits for commercial development of any kind under the terms of the Antarctic Treaty, which dedicates that continent solely to scientific research.

If prudence makes it unwise to predict the mineral situation for no more than a few decades into the future, it is only natural to speculate on the longer-term needs of humankind. We would like to think of our civilization as enduring for hundreds or thousands of years, yet the finite nature of mineral deposits casts doubt on our ability to maintain high consumption patterns for very long.

Some have envisioned the mining of extraterrestrial sources for future mineral supplies, and serious thought has been given to mining techniques that might be used on the Moon to supply any colonies that might be established there. Space stations might be constructed from minerals mined from small asteroids, making it unnecessary to supply the materials from Earth, and even more importantly, saving the energy necessary to lift large masses of metal into orbit.

Whether or not such sources are eventually developed, the most likely scenario for insuring near-future mineral supplies is through substitution and conservation. At present, the scarce metals are being used far out of proportion to their abundances, and efforts are constantly being made to substitute cheaper and more abundant materials for the expensive and scarce. Conservation is a reliable means of reducing demand for raw materials. The design of products that are more durable and more easily repaired, and implementation of effective recycling of materials are approaches that are likely to become more desirable as raw material prices rise. The recycling of lead, gold, and aluminum is a substantial industry, and the recovery of many metals from junked automobiles is already profitable.

Eventually, we may turn to our garbage dumps as high-grade "ores" of the next century. In the distant future, it may prove necessary to extract minerals from seawater itself, which probably contains more total metals in all the oceans than may be found on the continents. At present, the cost of extracting these very low concentrations makes this impractical.

RECOMMENDED READING

For a basic description of resources check Skinner (1976) or Park and MacDiarmid (1976). For specific deposits refer to Dixon (1979) or Kunz (1967). The geology of these deposits and their relation to plate tectonic processes is well described in Bates (1969), Brookings (1990), and Sawkins (1990).

• Robert L. Bates, Geology of the Industrial Rocks and Minerals, Dover, 1969.
• D. G. Brookins, Mineral and Energy Deposits, Merrill, 1990.
• C. J. Dixon, Atlas of Economic Mineral Deposits, Cornell University Press, 1979.
• George F. Kunz, Gems and Precious Stones of North America, Dover, 1967.
• C. F. Park Jr., and R. A. MacDiarmid, Ore Deposits, W. H. Freeman, 1975.
• F. J. Sawkins, Metal Deposits in Relation to Plate Tectonics, Springer-Verlag, 1990.
• Brian J. Skinner, Earth Resources, Prentice Hall, 1976.