Chapter 6

Minerals as resources

The mineral resources taken from the Earth are now essential for human survival. The key roles they play in production of food and provision of water for domestic use and irrigation mean that they are truly ‘vital’. Complex networks are in place to enable the production and distribution of food and water, as well as the vast range of material goods expected by people, particularly in the developed countries, ranging from cookers and cars, to televisions, computers, and mobile telephones. To take just one example, bread requires crops produced on farms using tractors and other farm machinery themselves made using different metals, paints, plastics, and rubber, as well as mineral fertilizers and, in some cases, irrigation systems. The harvested crops in turn need milling, transport to a bakery with all of its operating equipment, and transport of the baked bread to the shop or supermarket. The distribution network itself requires both the mineral raw materials to manufacture vehicles, and that needed to build the roads and bridges on which the vehicles travel. All of this requires the energy supplied by oil, gas, or some other source and, although the fossil fuels (coal, oil, gas) and many forms of alternative energy (wind, tidal, hydroelectric, solar, etc.) are not mineral based, nuclear power stations require the mining of uranium minerals as their energy source. Also, for all energy sources, mineral raw materials are still required for the equipment used in extraction and distribution.

Not surprisingly, the growth in the consumption of mineral resources in the recent past has been dramatic. A good illustration is provided by world production of iron ore, which in the year 1909 was 126 billion metric tons; by 1959 that had grown to 439 billion metric tons, and by 1999 to 1,020 billion metric tons. It then took just another decade to more than double to 2,240 billion metric tons by 2009. Iron, a raw material sometimes described as the ‘backbone of industry’, accounts for about 94 per cent of all the metals ever mined. Its importance has been strikingly captured in the words of Rudyard Kipling in his poem ‘Cold Iron’:

Gold is for the mistress—silver for the maid,

Copper for the craftsman, cunning at his trade.

‘Good!’ said the Baron, sitting in his hall,

‘But iron—cold iron—is master of them all.’

Iron, particularly when alloyed with relatively small amounts of carbon or of other metals (nickel, cobalt, chromium, vanadium, molybdenum, tungsten, etc.) to give the numerous types of steel, is used in everything from the construction of buildings and bridges, to railway systems, ships, pipelines, motor vehicles, and domestic appliances. The growth in the mining and consumption of iron ore is driven by growing consumer demand, especially in countries like China where rapid industrialization is under way. Ultimately, of course, consumption is driven by a growing world population and the legitimate demands of people in poorer countries for the consumer goods enjoyed by others elsewhere.

A vivid impression of the extent of resource consumption in countries such as Britain and the USA is provided by considering the average ‘consumption’ of certain mineral raw materials by a single person. Most of these materials are not handled directly by us, of course, but used on our behalf in constructing roads, bridges, schools, hospitals, homes, vehicles, and in numerous other material goods. Taking the USA as an example, in an average lifespan of 75 years, an American will consume about 800 metric tons of non-fuel resources and 400 metric tons of fossil fuels. The former includes (as approximate figures) 400,000 kg of stone, 22,000 kg of cement, 11,000 kg of phosphate, 30,000 kg of iron and steel, 1,900 kg of aluminium, 800 kg of copper, 400 kg of zinc, and 4 kg of uranium. Put another way, the present population of the USA will consume a total of more than 330 billion metric tons of resources in their collective lifetimes.

Inevitably, this level of consumption has an impact on what most of us call ‘the environment’ and which Earth scientists now call the ‘critical zone’, meaning the topmost few metres of the solid Earth, the oceans and surface waters, and the lower atmosphere. The use of the word ‘critical’ is a reminder that this zone is the region of the planet on which we humans depend for survival. As long ago as the 1990s, scientists were suggesting that our human activities are moving more material around at the surface of our planet than all natural processes (such as weathering and erosion, volcanic eruptions, earthquakes) combined. The observation that human beings have now become the dominant ‘geological agents’ has prompted some geologists to suggest we change the name of the epoch in which we now live from Holocene (from the Greek meaning ‘wholly recent’) to Anthropocene (the age of humankind). It is not only the extraction of materials from the Earth that causes disruption but also, in certain cases, the use of the resource and disposal of associated wastes. Several categories of minerals are exploited, as I explain below.

Kinds of mineral resources

Ores are rocks containing minerals which, after removal from the Earth using open-pit or underground mining methods, are treated to extract metals from them. Commonly, the valuable ore minerals are sulphides or oxides of one or more metals and occur closely intergrown with other minerals of no value. Processing of the materials taken directly from the ground and which may contain less than 1 per cent of the metal being mined (and, in some cases, far less) initially involves crushing and milling to liberate the valuable minerals, some form of separation and processing to concentrate them, and then extraction of the metal from the mineral concentrate by smelting or a hydrometallurgical process (e.g. dissolving out the metal using a reagent such as a strong acid).

Only six metals have an average concentration in the rocks of the Earth’s crust that is greater than one-tenth of 1 per cent by weight. These are magnesium, aluminium, silicon, titanium, manganese, and iron. For this reason they are known as the abundant metals. More than 30 other metals occur at lower concentrations (at parts per million, or even per billion, levels) and are termed scarce metals. These can be further separated into the precious metals (gold, silver, platinum), base metals such as copper, lead, tin, and zinc, and ferro-alloy metals such as chromium, nickel, tungsten, and molybdenum. The name ‘base metal’ was originally used by alchemists in the Middle Ages because these were the undesirable metals which the alchemists tried to change into gold or silver. Ferro-alloy metals are those used to alloy with iron to make special steels. Another group, known as the special metals, have a variety of applications, mostly in modern industries such as electronics. Examples include gallium, indium, niobium, and tantalum.

Industrial minerals are materials valued for particular physical or chemical properties they possess. As further discussed below, this includes minerals such as diamond, which is mostly valued for its hardness, along with other hard minerals used in abrasives such as garnet; asbestos is valued for its fibrous character, whereas clays such as kaolinite are a pure white, unreactive, fine grained powder used in paper-making, as well as for firing to make high quality ceramics. Other examples are fluorite, used as a flux for steelmaking, and barite for its high specific gravity employed in the ‘heavy mud’ lubricant used when drilling for oil.

Chemical minerals include those such as halite (‘rock-salt’; NaCl), which is used not only as a food preservative and condiment but also as the raw material for the manufacture of chemicals such as hydrochloric acid or caustic soda, or the mineral apatite (calcium phosphate; Ca₅(PO₄)₃(F,Cl,OH)₂), which is used as a source of phosphate for fertilizers.

As well as the many individual minerals exploited by industry, of which those mentioned above are just a few examples, very large amounts of many types of rocks are used in construction as cut (‘dressed’) stone, as aggregates, and as the raw materials for cement and concrete. At the other extreme in terms of value and quantity, as discussed below, gemstones are often rare examples of common minerals of exceptional perfection in terms of brilliance or colour when cut or polished. In the section that follows, we say more about the industrial and chemical minerals that depend on mineral properties for their value. Ores are then the main focus of the sections on mineral deposit formation and on the role of plate tectonics in processes of formation.

Mineral properties and mineral resources

Diamonds to clays

Marilyn Monroe reminded us in song that ‘diamonds are a girl’s best friend’. True, gem diamonds are amongst the most highly valued materials of any sort, but about 80 per cent of the diamonds mined are unsuitable for use as gemstones, being too small or imperfect. The interest in these stones is because of another remarkable property of this mineral; it is by a long way the hardest naturally occurring substance known. Industrially it is used at the ‘sharp end’ of the cutting and grinding tools needed to fabricate numerous products from metals, alloys, and ceramics. As noted in Chapter 1, the reason for the great hardness of diamond, which is chemically just a form of pure carbon, lies in its crystal structure. In diamond, the atoms form a rigid framework in which every carbon atom is linked to another four carbon atoms at the corners of a tetrahedron (see Figure 1c). Equally remarkable is that graphite, another mineral composed only of carbon, is one of the softest minerals known. Again, this is because of its crystal structure (Figure 1b) in which layers of linked carbon atoms have only very weak chemical bonds between the layers, enabling them to ‘slide’ over one another. For this reason graphite is used in lubricants, particularly for the moving parts of certain machines and engines. (Historically, graphite was also mined on a small scale in the English Lake District and used to make pencils.)

The hardness of diamond is a useful property in its role as a gemstone, making it resistant to damage and wear. The optical properties of gems such as diamonds are what makes them objects of beauty when they are cut and polished to show these properties to advantage. In the case of diamond, it is the large variation of refractive index with wavelength of light (‘dispersion’) which produces the characteristic sparkle. The refractive index is a measure of the change in velocity of a beam of light on passing from the air into the mineral; a change that causes the beam of light to bend, and which is exploited when a gem diamond is cut so as to cause multiple reflections of the light entering the stone.

Diamonds are also valued for a range of technological applications. As well as being the hardest natural substance known, diamond has the highest thermal conductivity at room temperature, is highly resistant to attack from chemicals, is an excellent electrical insulator, is transparent to light and X-rays, and can be prepared as a superior semiconductor for electronic devices.

Other gems such as ruby and emerald are prized for their colour. In many cases, a coloured gemstone is just a variety of a common mineral which contains a small amount of a particular impurity substituting in the crystal structure. For example, ruby is a variety of the aluminium oxide mineral corundum (Al₂O₃) which is second only to diamond in hardness and, therefore, also used as an abrasive. Very small amounts of chromium replacing the aluminium atoms in corundum are responsible for the beautiful red colour of a ruby. Emeralds are a variety of the beryllium aluminium silicate mineral, beryl; their beautiful green colour is also attributed to the presence of small amounts of chromium and possibly vanadium.

At the other end of the scale from diamonds and other gemstones are the minerals produced in bulk and at low cost. As we have already noted, the use of clays in brickmaking, pottery, and other ceramics was amongst the first uses by mankind of any minerals. An important property of certain clays, appropriately called fireclays, is a resistance to very high temperatures which makes them ideal for lining the interiors of boilers and furnaces. Silicate minerals that originally formed at high temperatures, such as the olivines, are also heat resistant and used in the production of what are collectively termed ‘refractories’.

Many minerals are used in industrial processes and never seen by the consumer. For example, fluorite (CaF₂) is used as a flux in steelmaking, and baryte (BaSO₄), because it is both of high density and inert, is used in the drilling ‘mud’ pumped down into the drill hole in oil exploration. In this case, it is the density of the baryte that helps to counter the pressures forcing the oil and gas towards the surface. A variety of widely available, relatively low cost, inert, white minerals are well suited to the least glamorous of the many jobs for which minerals are now used. These are the fillers used to provide bulk or other characteristics to a wide range of products; from rubber, plastics, and paints, to detergents, cosmetics, medicines, and toothpastes. The clay mineral kaolinite (Al₄Si₄O₁₀(OH)₈) is a good example of such a mineral. It is used in plastics, adhesives, paint, and particularly, as discussed in Chapter 4, in the paper industry. Kaolinite is particularly well suited to these applications because it is produced as an inert, fine-grained, pure white powder.

Clean-up and catalysis

The examples above focus largely on the physical properties of minerals, but chemical properties are also very important. Certain minerals and their synthetic equivalents play a key role in industry where substances are required to remove impurities from water or to catalyse important chemical reactions. A very good example of this application is provided by members of the silicate mineral group called ‘zeolites’. These are water-containing aluminium silicates of sodium, potassium, and calcium (and to a lesser extent magnesium and barium). A typical example is the mineral chabazite, (Ca,Na)₂Al₂Si₄O₁₂.6H₂O. Zeolites have crystal structures in which the SiO₄ and AlO₄ tetrahedral units are joined together to form a framework in such a way as to leave large cavities containing water molecules. These cavities may be interconnected in one, two, or three directions. When zeolites are heated to about 350°C the water is driven off, leaving a crystal permeated with empty channel systems in up to three directions. These apertures are large enough to allow small atoms and molecules to pass through them, but not large ones. Zeolites can be used to extract impurities from water, whether those impurities are the calcium and magnesium removed to soften drinking water or highly toxic radioactive contaminants such as caesium-137, removed to clean up heavily contaminated water. They are also used to extract CO₂ and H₂S from natural gas, or sulphur and nitrogen oxides from smokestack gases. These pollutants are responsible for acid rain, and for the ‘greenhouse gas’ contamination of the atmosphere implicated in global warming.

Although the industrial applications of zeolites began with the natural (mineral) materials, more important today are the thousands of tons of synthetic zeolites made from the reaction of solutions of sodium hydroxide, sodium silicate, and sodium aluminates. Of the 206 zeolite structures known, only 40 are naturally occurring. These materials can be made with crystal structures tailored to specific applications, notably for use as catalysts. For example, synthetic zeolites with larger empty cavities are used to catalyse the breakdown of large organic molecules in the refining of oil. This ‘catalytic cracking’ is more efficient than heating the oil to cause its break-up and can be used to add hydrogen to the oil. This process of hydrogenation results in an increased yield of petrol (gasoline) from every barrel of oil. Although the use of zeolites in catalysis is big business, the greatest use of these materials today is in household washing powders.

How mineral deposits form

All the minerals and rocks that we exploit have been formed by geological processes. In the case of most rocks, these are major processes such as the emplacement in the crust and cooling of magmas, eruption and deposition of volcanic lavas and ashes, deposition of sediments from waters, whether as chemical precipitates or detrital grains, and the heating and compression of pre-existing rocks during metamorphism. Abundant quantities of the major rock types such as granites, limestones, sandstones, or marbles are available for use in the construction industries. In most cases, such rocks require only cutting up or crushing before being used.

The situation regarding the availability of ‘common’ rocks contrasts with other mineral resources, especially the ‘scarce’ metals. These require unusual processes in order to greatly increase the concentrations of the metal concerned above the average value in the Earth’s crust, sometimes by a factor of many thousands. Although the concentration level required for a particular metal, in order that it be economically worth mining, depends on factors such as the current metal price, ease of extraction, geographical location of the deposit, and political environment in the country concerned, approximate levels can be suggested. Surprisingly perhaps, the highest figure is for mercury (~12,000 times) whereas for gold it is much lower (~250 times)—a reflection of the very high price commanded by the latter. Even the abundant metals need to be significantly enriched over average crustal values to reach concentration levels where mining can be undertaken at a profit. Iron has a crustal abundance of 5.6 per cent, so that a roughly tenfold enrichment is needed. For aluminium (crustal abundance 8.2 per cent), only around fourfold is needed.

In order to form mineral deposits, minerals can be concentrated in one of several ways. The first involves deposition from hot water (‘hydrothermal’) fluids, typically coming from a cooling mass of molten rock such as a granite magma which has forced its way (‘intruded’) into the upper part of the Earth’s crust. As they cool, such magmas expel waters which contain substantial amounts of dissolved metals and other elements which are not components of the main granite minerals (feldspars, micas, and quartz) and so not taken up into those minerals. The heat from the magma may also cause these waters to mix by convection with other waters in the surrounding rocks. The hydrothermal fluids may initially be at temperatures of several hundred degrees, but kept from boiling by being under pressure—rather as in a ‘pressure cooker’. These fluids will then force their way along joints in the cooling rock and along faults and fractures in the surrounding rocks, and with falling temperatures and pressures deposit minerals, including those rich in valuable metals, as veins (Figure 19). The veins (as, for example, in the mineral deposits of Cornwall in the UK) may contain a wide range of minerals exploited for the extraction of metals including copper, tin, tungsten, lead, zinc, and uranium. Nearer to the intrusion, the cooling mixture of fluid and the last portions of crystallizing melt may produce large crystals of both common minerals (feldspars, micas, quartz) and exotic (such as beryllium-bearing or lithium-bearing) minerals in what we call pegmatites.

Granite rocks can be associated with the melting of the continental crust; and rocks related to granites are associated with the melting of a subducted slab, formed when one tectonic plate dives under another at a destructive plate boundary. These magmas are responsible for chains of volcanoes such as those in the Andes of South America. It is in the root zones of such volcanoes that an important kind of deposit known as a porphyry copper deposit is found (and porphyry molybdenum deposits also occur in the same settings). In this case, the ore minerals such as the copper sulphide chalcopyrite or, in some cases, the molybdenum sulphide molybdenite are finely distributed through the rocks, forming a deposit that is a cylindrical mass of ore which is at, or just below, the surface. Such deposits average much less than 1 per cent copper and only became economically viable with the advent of very large-scale mining and processing methods in the mid 20th century. Very large amounts of ore can be removed from open-pit mines using modern methods requiring a comparatively small workforce (Figure 20). In some mines, more than 200,000 metric tons of ore are being extracted in a single day; an amount that would have taken those mining in Cornwall in the early 19th century many years to recover. One other important point about these mines is that their ores also contain small, but economically very significant amounts of other metals such as gold that can be recovered as by-products of the main operation. Such so-called ‘sweeteners’ can ensure the profitability of a mine.

19. A simplified cross-section through the subsurface in a region above a shallow intrusion of an igneous rock such as a granite. Valuable minerals can be concentrated in pegmatite deposits and veins in the rocks surrounding the intrusion

A major advance in our understanding of hydrothermal fluids and their role in forming mineral deposits came from discoveries made in the 1970s in the course of ocean exploration.

The most spectacular of these discoveries was made in the vicinities of mid-ocean ridges, where molten basaltic rock rises from the mantle and spreads outwards forming new ocean floor. Here, hydrothermal fluids are generated as ocean water circulates through the hot rocks at and near the ridges. These fluids, at temperatures of ~350°C, rise rapidly through fractures and form jet-like eruptions called black smokers when they mix with cold ocean water, causing the dissolved material they contain to precipitate. In fact, the plumes of what look like black smoke are fine particles of iron, copper, zinc, and lead sulphide minerals (pyrite, chalcopyrite, sphalerite, galena). Processes such as those associated with the black smokers have occurred in the geological past and produced rich ore deposits, in particular, those known as volcanogenic massive sulphides.

20. A modern large open-pit mining operation

Away from the ridges, sitting on the floor of the deep ocean, is another mineral resource, one of more uncertain origins. Here we find pea-to-grapefruit sized spherical objects called manganese nodules. Internally the nodules have an onion-like layering. These objects probably formed very slowly, and involved much lower temperature fluids, with microbial activity possibly playing a role. As well as iron and manganese oxide minerals, they contain valuable concentrations of nickel and cobalt. Although very challenging, the mining of nodules from the deep ocean is likely to be technically possible. But who owns the mineral resources of the oceans? It will probably come as no surprise to learn that there is no international agreement about the answer to that question, despite decades of discussions.

A second way in which mineral deposits can form is associated with the cooling, crystallization, and solidification of a magma, this time a magma coming originally from the upper mantle of the Earth, and formed by the partial melting of mantle material (possibly due to a local build-up of heat from decay of radioactive minerals). In this case, the magma is comparable in overall composition to the basalts that make up the ocean floor, but it has not reached the surface; instead, it has reached a position in the crust where it will slowly crystallize. This slow cooling allows fractional crystallization (see Chapter 3) to take place, in which different minerals crystallize from the melt and settle to the bottom of the magma chamber in succession, so as to form a layered body. In this way, particular minerals become concentrated in particular layers. Notably, chromite (FeCr₂O₄), the mineral that provides world supplies of chromium, is concentrated in certain layers by this process. Also found concentrated in some layers may be platinum and other platinum group element (palladium, osmium, iridium, rhenium, and ruthenium) minerals. The world’s largest deposits of chromium and the platinum metals formed in this way are found in the Bushveld Complex in the Republic of South Africa.

Whereas the settling of crystals from a molten body of magma to form layers is one form of precipitation, more familiar perhaps is the equivalent process which occurs when seawater in an enclosed water body such as a saline lake or sea (like the Dead Sea on the Israel–Jordan border) evaporates, depositing layers of salts. The most important of the salts making up these evaporite deposits are the minerals gypsum (CaSO₄·2H₂O), carnallite (KCl·MgCl₂·6H₂O), and halite (rock-salt; NaCl) itself. Other economically important minerals precipitated from seawater under appropriate conditions include apatite, Ca₅(PO₄)₃(OH,F), which is a major source of the phosphorous needed for fertilizers. Probably the most important resource now mined almost entirely from deposits formed by sedimentary processes are those of iron; the ironstones and in particular the banded iron formations (BIFs). We are still not sure how BIFs formed. They show very regular layering or irregular banding on a fine scale (millimetres to centimetres) with layers of nearly pure silica (chert) and nearly pure iron oxide minerals: haematite (Fe₂O₃) and magnetite (Fe₃O₄). An important feature of BIFs is that they formed during a period between 2.6 and 1.8 billion years ago when the Earth’s atmosphere was probably very different, with little free oxygen and perhaps more carbon dioxide than today. Under these atmospheric conditions, iron could have been transported much greater distances dissolved in water than would be possible today. The BIFs seem to have formed in broad basins into which soluble iron may have been introduced and then precipitated. The cause of the repetitive precipitation of alternating layers of iron oxides and silica has been a point of much debate, with suggestions including annual climatic changes, cyclic periods of evaporation, episodic volcanism, and even microbial activity.

The formation of evaporites or ironstones such as BIFs involves a chemical precipitation from water of the minerals concerned. Water can also play a role in concentrating minerals when it is flowing and the minerals are already present as detrital grains. To be concentrated in this way, the minerals need to be very resistant to corrosion and denser (‘heavier’) than the average minerals that make up common rocks. The concentration process is like that employed by the old style prospector panning for gold (see Box 5). The gold pan is a large, flat-bottomed metal bowl into which the prospector scoops up water and sediment from a stream. By employing a circular ‘swirling’ motion, the less dense mineral grains are winnowed out and discarded over the edge of the pan until only the densest grains are left. Amongst these the prospector hopes to see the glint of a few grains of gold, washed downstream from the primary source of the metal, the ‘mother lode’. When nature concentrates gold or other minerals by the action of flowing water the result is a placer deposit. Besides gold, other metals extracted by the mining of placer deposits include tin as cassiterite (SnO₂), and titanium as rutile (TiO₂) or ilmenite (FeTiO₃). Other valuable minerals recovered from placers include diamonds, rubies, and sapphires.

A final process to mention is that responsible for the formation of residual mineral deposits. All rocks and minerals exposed at the Earth’s surface are broken down by the weathering action of rain, wind, frost, and the action of microbes including fungi. The materials in a rock which are least stable to such weathering may be transported away, or there may be some redistribution of the material making up the fresh rock. Such processes may result in the selective concentration of valuable minerals. An example is illustrated in Figure 21. Here, peridotite, a rock of a type originating from the upper mantle and rich in minerals such as olivine, (Mg, Fe)₂SiO₄, and pyroxene, (Ca,Mg)SiO₃, has been exposed at the Earth’s surface and is weathering. The fresh olivines and pyroxenes in this rock contain small amounts of nickel (~1 per cent) which is released when these minerals, which are unstable when exposed to the atmosphere, break down. As seen in the depth profile in Figure 21, the heavily weathered zone nearest to the surface is altered to a laterite. This iron-rich, bright orange-red residual material largely made up from iron hydroxide minerals such as goethite (FeOOH) is characteristic of the weathering of rocks such as peridotites in warm tropical regions of high rainfall. Below the zone of residual laterites, and above the fresh peridotite, is a zone of altered peridotite. In this zone, the nickel is concentrated after its release from the olivines and pyroxenes, and is redeposited in the form of a nickel-rich clay mineral called garnierite.

21. A cross-section of the shallow subsurface where the minerals of a peridotite rock are being weathered, resulting in the concentration of mineable quantities of nickel

Although these residual deposits make an important contribution to the world supplies of nickel, the most important such deposits are those of aluminium. The breakdown product in this case is an aluminous laterite called bauxite which is a mixture of several aluminium hydroxide minerals. The aluminium was originally a component of silicate minerals such as feldspars, but under certain weathering conditions, the silica and other components are dissolved away leaving rich ores (sometimes reaching aluminium concentrations of around 40 per cent).

Box 5 The strange story of gold

We have prized gold since the emergence of the first civilizations. As a metal it is soft and malleable, and also extremely resistant to corrosion, and it occurs as the ‘native’ element making it easy to extract and exploit. Gold can be found as veins where it has been deposited from hydrothermal (‘hot water’) solutions in fractures in the vicinity of granites and related rocks. When veins like these are weathered away, the gold may be transported in flowing water in streams and rivers. Being resistant to corrosion and very dense (around 19 times the density of water) it may then become concentrated in parts of a stream where the flow is slowed, such as the inside of a meander loop. Processes like these have had a key role in forming the world’s greatest gold deposits in the Witwatersrand Basin in South Africa. These deposits were discovered in 1886 and rapidly became the main producers in the world, a position they still occupy today. However, during the past 50 years, developments in ‘open-pit’ mining and in ore processing methods have seen increasing competition from very large ‘low grade’ deposits where the gold is a minor component of the extraction operation. For example, the porphyry copper deposits associated with destructive plate boundaries can contain significant gold.

The lure of gold has long been a part of the human condition, notably recorded in the great gold ‘rushes’ like the one in California in 1849. Few who set off to the goldfields made a decent living, let alone a fortune, from mining; arguably those who did best were people like Levi Strauss, a tailor who made his money selling denim trousers (‘jeans’) to the prospectors. The total amount of gold ever mined is remarkably small, estimated to be about enough to fill the space occupied by less than four Olympic sized swimming pools. It is also the case that nearly all of the gold ever mined is still in circulation. The gold ring worn by a reader of this book could well contain atoms of gold that once adorned a famous queen like Cleopatra. Perhaps the strangest aspect of the story of gold is the role it still plays in currency and global finance. Although no longer used in coinage, a very large proportion of the gold we have is buried, not back in the ground but in secure vaults, by governments as a back-up to their currencies.

Plate tectonics and mineral resources

An important question, given the processes of formation of mineral deposits we have described, is: ‘Do these processes relate to the plate tectonic cycle and, if so, how?’

The answer is that many, but certainly not all of these processes can be understood as part of the cycles of formation and destruction of lithospheric plates that we have discussed in Chapter 3. We can consider some examples of mineral deposits mined for metals, examples where plate tectonics provides a global context for understanding what geologists call ‘ore genesis’.

Figure 22 is a much simplified ‘slice’ through part of the lithosphere and uppermost mantle, showing both constructive and destructive plate margins with an indication of the types of deposits found in a particular setting, and the metals extracted from those deposits. In line with the above account of mineral deposit formation, at a constructive margin (mid-ocean ridge), upwelling of magma brings molten rock in contact with seawater; this water is therefore heated and circulates so as to concentrate metals in hydrothermal fluids which are vented into the ocean bottom waters as the metal sulphide-carrying black smokers. This material can accumulate to form rich deposits of copper and zinc ores: the massive sulphide deposits. The ore minerals found in such deposits are dominantly chalcopyrite (CuFeS₂) and sphalerite (ZnS).

Moving away from the ridge to the deep ocean floor we have the manganese nodules, sources of cobalt and nickel as well as manganese (dominantly as manganese oxide minerals), before reaching the subduction zone where ocean crust is dragged down into the mantle. Not surprisingly, the region of the crust where a slab of ocean floor crust is dragged down is one where massive slabs of rock can be thrust up against one another along with modern sediments in a great jumble of rocks called a mélange. In this environment, some of the rocks and mineral deposits that we find actually originate at depth beneath the seafloor in the vicinity of the mid-ocean ridges, and were transported with the ocean crust as it spread. Important examples include deposits of chromium (as chromite; FeCr₂O₄) which originated by fractional crystallization from magmas trapped beneath the surface. They were transported with the spreading ocean floor before being thrust up against the rocks of a continental mass as a stranded piece of ancient ocean floor or ophiolite (from the greek ‘ophis’ meaning ‘snake’, and a reference to the snakeskin-like appearance of the rocks involved).

Figure 22 emphasizes just some of the complexities that can arise at a destructive plate boundary. Here, magmas generated by the partial melting of the down-going slab can form a magmatic (‘island’) arc. In these settings, the interactions between plates can also put the crust under tension and cause stretching of the crust to form (‘back arc’ and ‘outer arc’) basins. These basins can be the sites of mineral deposits which may be in layers parallel to the surrounding strata (stratabound) or may be irregular masses (massive sulphides). These ores are important sources of base metals including copper, lead, and zinc. As noted above, the root zones of the volcanoes of the magmatic arc is where circulating fluids concentrate metals as porphyry copper or porphyry molybdenum deposits. The main ore minerals in porphyry copper deposits are pyrite, chalcopyrite, and bornite (Cu₅FeS₄) and in porphyry molybdenum deposits, molybdenite (MoS₂) is the source of that metal. Minor amounts of native gold are commonly found as ‘sweeteners’ in these deposits.

Finally in Figure 22, the crustal rocks of the continents host plutons of granite and related rocks where veins (see Figure 19) may carry ores which are sources of tin (as cassiterite; SnO₂) and tungsten (as wolframite; (Fe,Mn)WO₄) along with other metals including copper, uranium, and bismuth (as bismuthinite; Bi₂S₃). Where granitic magmas have reacted with the rocks into which they have been intruded (the process of ‘contact metamorphism’; see Chapter 3), a wide range of deposits may be formed, variously providing sources of tin, tungsten, molybdenum, copper, lead, and zinc.

The role of the plate tectonic cycle in the formation of certain mineral deposits has significance in the search for new deposits. It enables the geologist engaged in mineral exploration to focus attention on appropriate areas of the Earth’s crust; areas where the geology is appropriate for the formation of the deposits of interest. In mineral exploration, ancient examples of plate tectonic environments such as subduction zones need to be identified by careful geological mapping and the use of other exploration methods. In response to the age-old saying ‘how do you find an elephant?’ comes the answer ‘look in elephant country’. Mineral exploration is a challenging activity because mineable deposits form only rarely and as the result of a special combination of circumstances. We will return to issues concerning the search for mineral resources, and the scarcity of such resources, in the final chapter of this book.

22. A simplified cross-section of the lithosphere and uppermost mantle illustrating the plate tectonic settings of a number of kinds of mineral deposits