How do metallic ore deposits form

For oil to be exploitable, it must be trapped by an impermeable geological barrier. The oil-bearing strata itself has to be pervious or porous so that the oil can flow freely through it. Oil usually contains many different hydrocarbons that have differing boiling temperatures. Those that have the lowest boiling temperature become accumulations of natural gas which, being lighter than the oil, forms a gas-cap on top of the oil accumulation.

The pressure of this gas cap on the overlying strata and the upward pressure of any underlying water below the oil layer are used to drive the oil towards the surface without the need for pumping. Residual deposits are formed in tropical regions. During the wet season, intense leaching of the rock occurs. Then, during the dry season, the solution containing the leached ions is drawn towards the surface by capillary action where it evaporates leaving behind salts that are washed away in the next wet season.

Eventually, the whole zone down to the base of the water table is leached of relatively mobile ions such as sodium, potassium, calcium and magnesium. With leaching under the right pH conditions, silica is also dissolved and removed from the system. The remaining material is usually just iron and aluminium oxides which are concentrated. Many fossil laterites are known and these provide evidence of former tropical environments.

Basic and ultrabasic rocks tend to form laterites while granitic rocks lead to the development of bauxite. In some tropical regions, where the laterite is developed on ultrabasic rocks, it forms important deposits of nickel, usually at the base of the laterite zone. In New Caledonia, these deposits are extensively mined and the main nickel-bearing phases are amorphous nickel silicates.

Other areas which contain extensive laterite nickel mineralisation are the Norseman-Wiluna greenstone belt of Western Australia and Central Africa. Some of these laterites also contain elevated levels of the platinum-group elements which are an important by-product of the mining.

The problem with these deposits is that the nickel is often very difficult to extract from the laterite. The Australian Museum respects and acknowledges the Gadigal people as the First Peoples and Traditional Custodians of the land and waterways on which the Museum stands. Image credit: gadigal yilimung shield made by Uncle Charles Chicka Madden.

This website uses cookies to ensure you get the best experience on our website. Learn more. Skip to main content Skip to acknowledgement of country Skip to footer On this page Geological ore deposits are of many different types and occur in all geological environments. The main types of geological ore deposits of importance can be divided into: metallic deposits non-metallic deposits fossil fuel deposits Finding ore deposits Geologists are always searching for more ore deposits to meet constant demand.

Economic viability of ore deposits Many factors control the economic viability of an ore deposit but the most important are: grade i. The most common minerals found in oxidised zones are: Copper: malachite, azurite, chrysocolla Gangue minerals: quartz usually cryptocrystalline , baryte, calcite, aragonite Iron: goethite, hematite Lead: anglesite, cerussite Manganese: pyrolusite, romanechite, rhodochrosite Nickel: gaspeite, garnierite Silver: native silver, chlorargyrite Zinc: smithsonite Immediately below the oxidised zone is sometimes a zone known as the supergene zone where metals are deposited by fluids percolating downwards from the oxidised zone and concentrating in a narrow band just below the water table.

The most common minerals found in supergene zones are: Copper: chalcocite, bornite Lead: supergene galena Nickel: violarite Silver: acanthite, native silver Zinc: supergene sphalerite, wurtzite Classification and types of mineral deposits Geologists classify mineral deposits in many different ways, according to the: commodity being mined tectonic setting in which the deposit occurs geological setting of the mineral deposit genetic model for the origin of the ore deposit The most commonly used scheme is the genetic classification scheme.

These deposits include: Orthomagmatic deposits are those that form from primary magmatic processes i. Orthomagmatic deposits include: chromium titanium iron nickel copper platinum-group elements diamonds Pneumatolytic and pegmatitic deposits are formed from volatile-rich i.

These are important sources for: tin rare-earth elements tantalum beryllium lithium molybdenum tungsten Hydrothermal deposits cover a wide range of different deposits types but all form from hot circulating water-rich fluids.

These include the two main types of gold deposits - epithermal and lode gold deposits along with replacement deposits in calcareous sequences Mississippi Valley deposits , base-metal vein deposits and replacement skarn deposits Volcanic or extrusive deposits are associated with volcanic processes and are only found within the volcanic rocks themselves.

Remobilisation of ore deposits As many ore deposits formed many hundreds to thousands of millions of years ago, many have experienced numerous episodes of deformation and metamorphism which left them with few characteristics of their original form.

Epithermal gold deposits These deposits are related to convergent-margin tectonic settings such as that of the Andes, South America.

The main ore minerals are: native gold and silver electrum acanthite and tetrahedrite silica occurring as quartz most often forming comb-like aggregates , amethyst, opal, chalcedony and cristobalite. Sedimentary iron ore and manganese deposits Most of the world's iron and manganese are derived from deposits of this type.

Iron ores of this type are commonly termed the banded iron formation or simply abbreviated as BIF All ore minerals in these deposits are oxides and hydroxides. Placer deposits These are made of alluvial, colluvial and eluvial material, which contain economic quantities of some valuable minerals: Alluvial: Detrital material which is transported by a river and usually deposited along the river's pathway, either in the riverbed itself or on its floodplain.

Colluvial: Weathered material transported by gravity action such as on scree slopes. Eluvial: Weathered material still at or near its point of formation. Clay deposits These either form by settling of clay particles in sedimentary basins or through intense weathering of volcanic and granitic rocks.

Useful clay types include: China Clay: deposits of kaolin produced by hydrothermal decomposition or deep weathering of feldspar minerals in granites. Evaporite deposts An evaporite is a sediment that forms through the evaporation of saline water. Phosphatic deposits Phosphorite is a commonly used term for lithified phosphate rock. Contact metamorphic deposits result from hot solutions that migrate from a cooling intrusion and deposit minerals in cracks in the surrounding country rock.

Hydrothermal veins are also mineral deposits in faults and cracks but are not necessarily related to an intrusive body. The fluid can be meteoric water that has moved downward toward a heat source, been heated, and ascended, leaching metals along its path.

The sulfides are later deposited a considerable distance from the heat source. Some of the richest gold and silver deposits in the world are hydrothermal veins. Disseminated deposits are those in which the metal is evenly distributed in generally low concentrations throughout large masses of rock.

An important type of disseminated deposit is the porphyry copper deposit , in which copper and molybdenum are found in porphyritic intrusive rocks. Hot springs deposits are minerals that formed in response to hot spring activity at the surface of the earth. These can be rich in gold, silver, antimony, arsenic, and mercury. Ore deposits can form also by other processes at the earth's surface.

Mining, especially public land mining, is sometimes a controversial environmental issue. For example, the Mining Law gives U. Miners pay no royalties, in contrast with those who develop coal, oil, or gas on public lands. So, the law is a federal subsidy for mining. Many environmental groups want to see Congress change the law. They argue that public lands are for the public, not mining corporations, and they point out that mining is incompatible with other uses such as wildlife habitat, hiking, and camping.

Furthermore, mining leaves scars on the land and may cause long-term environmental degradation. The mining industry argues that we need the law to ensure a flow of mineral resources to our citizens.

They point out the importance of mining to some local western economies and say they can mine in an environmentally friendly way. The mining industry is correct when they argue that we need mineral resources and they have to come from somewhere.

A visit to active or abandoned mines confirms this. Besides scars on the land, less obvious problems include air, water, and soil pollution. All these problems can be limited, but not eliminated, by following the best mining practices. The real questions are where are they going to come from and how much are we willing to pay?

Those who seek reform of the law argue that some areas should be off limits to mining, that mining companies should pay more in royalties, and that there should be strict antipollution and land reclamation requirements. If enacted by Congress, these changes might affect the price of mineral commodities, but most economists think the effect would be very small.

We saw a histogram of this distribution in Figure 2. It is no wonder, then, that humans have developed ways to use these elements in industry, agriculture, and manufacturing. Less abundant elements have also become important to modern society. These include metals, radioactive elements such as uranium or thorium, and fertilizer components including, most importantly, nitrogen and phosphorous. The economical concentration factor listed in the table above is the ratio of typical minimum economical ore concentration to average crustal concentration.

For example, the average crustal abundance of chromium is about 0. The necessary concentration factor is therefore nearly 3, — chromium must be concentrated at least 3, times to create profitable ore. The table compares economical concentration factors for a dozen different metals. They are ordered from those most abundant top to those that are rare bottom. Concentration factors range from 4 for aluminum and iron, to nearly 3, for tin, chromium and lead. Elements that occur in high abundance do not need a high concentration factor to make mining economical.

In contrast, less common chromium, lead, tin, and zinc require great concentrations to be profitably mined see the table above. We mine relatively common elements, such as iron and aluminum, in many places worldwide; we mine rarer elements, including tin, chromium, or lead, in far fewer places. Although the table does not include prices, there is a correlation between the economical ore grades and the price of a given resource.

Gold, for example, is much more expensive than the metals listed, although the demand for gold is less than for the others. This price difference exists because the natural processes that concentrate most commonly used metals are much more common than the processes that concentrate gold, so there are fewer high-quality gold deposits than there are other kinds of deposits.

Many gold mines can remain profitable even if the ore contains less than 0. The gold flakes are small — the entire photo is less than 2 cm across. The market for metals can be extremely volatile. Geopolitics, wars, economic sanctions, and other things may cause major market disruptions. But, trends in technology may, over the long run, be even more significant. For example, beginning about 5 years ago, many predicted that the demand for electric vehicles EV was going to skyrocket.

A growing EV industry means that demand for lithium-ion batteries will increase. So, in , the average market price for lithium began rising and doubled in two years. But, lithium-ion batteries also include other key metals besides lithium, for example cobalt. Why did this happen? Several things are undoubtably important. Perhaps most significant is that the projected increase in EV sales and demand for lithium-ion batteries did not occur as rapidly as predicted. At the same time, smaller independent operators started new mines.

So now we have a market surfeit of cobalt, and prices are about the lowest they have been in a decade. Still, market prognosticators say that with the inevitable increase in demand for EVs, and for rechargeable batteries in general, prices for cobalt, nickel and graphite, and other key components of lithium-iron batteries can be expected to increase soon. Geological processes that concentrate minerals are not unusual.

But, the processes that create economically productive ore deposits are rare. If they were not, market prices would fall, decreasing profits and putting some mines out of business. The largest and most easily produced mines control market prices. Old mines shut down and new mines open up when new discoveries are made.

Today, however, new discoveries are generally smaller than in the past because the largest deposits, which are more easily found than smaller deposits, have already been developed. Because the geology of Earth varies, the distribution of ore deposits around the globe is uneven, and the minerals industry flourishes in some places and not in others.

Some regions of the world contain most of the supply of certain commodities; this can affect international politics. This mine has historically been the largest producer of molybdenum in the world. Production started in , but the mine temporarily shut down between and due to low molybdenum prices. Nonetheless, the US has sufficient supply of molybdenum. Unfortunately, many other important minerals are not produced in the United States. We call these minerals critical minerals , or strategic minerals See Box Additionally, we import many mineral commodities, that we might produce ourselves, because it would cost too much to mine them in our own country; tungsten is a good example.

The largest pillow structure is about 65 cm in long dimension. Most South African ore deposits are associated with regions called Precambrian greenstone belts , ancient volcanic terranes.

Figures 9. Various other types of geological terranes are associated with ore deposits, too. Most economical metal and semimetal deposits are found near margins of continents, or the former margins of continents, where mountain building and igneous activity have occurred.

Still, other types of deposits are found in continental interiors. We use many different minerals and metals to maintain our lifestyles and provide military security. Some of these commodities are not found or produced in the United States in sufficient quantities to meet demand.

Consequently, we must import them from other nations. And sometimes supplies are problematic. During the Cold War, for example, the former Soviet Union and its allies stopped exporting minerals commodities to the Untied States. So, sources of strategic metals are controlled by politics as well as geology.

Consider the push to expand the use of electric vehicles EVs. The Netherlands, United Kingdom, France and some other countries have announced ambitious plans to completely eliminate gasoline and diesel vehicles.

China is moving in that direction as well. But EVs need batteries and, although battery technology continues to evolve, lithium and cobalt are key components. Only eight countries produce lithium, and most of it comes from only three countries. Cobalt supplies are even more limited. The last functioning rare earth mine in North America closed for financial reasons in Figure 9. Thus, the United States is heavily reliant on just a few countries if we are to expand electric vehicle technology and keep up with the rest of the world.

Further complicating the picture is that the United States is both an importer and an exporter of some key metals and minerals. Today, the US relies entirely on imports for the following important element and mineral commodities:. The United States is partially reliant on imports to meet needs for these element and mineral commodities:. Ores and ore minerals vary greatly in quality.

Such ores do not exist, but some come close. Native copper, for example, is an ideal copper ore mineral. Ideal ore minerals are, however, uncommon. The most commonly mined ores are not ideal. Instead they are rich in ore minerals that can be processed relatively inexpensively to isolate desired components. The table seen here lists common ore minerals for various metals. The minerals include the native metals copper and gold, and many sulfides, oxides, and hydroxides.

Minerals in these groups are generally good ore minerals because they contain relatively large amounts of the desired elements. Furthermore, processing and element extraction are usually straightforward and relatively inexpensive.

That is why we mine Cu and Cu-Fe sulfides for their copper content and iron oxides for their iron content. Silicate minerals, although common, are generally poor ore minerals and are not included in the table. For example, although aluminum is found in many common silicates, tight bonding makes producing metallic aluminum from silicates uneconomical. We obtain most aluminum from Al-hydroxides found in bauxite deposits.

We discussed igneous and sedimentary minerals in previous chapters. In the following section, we focus on economic minerals that belong to other groups. Native elements have high value because they may require no processing before being used in manufacturing, as currency, or for other purposes.

The first metals ever used by humans were native minerals. Only later did humans develop refining techniques for the extraction of elements from more complex minerals. We conveniently divide native elements into metals, semimetals, and nonmetals based on their chemical and physical properties.

The table to the right includes the most common minerals of each group. Within the metal group, the principal native minerals are gold, silver, copper, and platinum. These four minerals all contain weak metallic bonds.

Gold, silver, and copper have further commonality in their chemical properties because they are in the same column of the periodic table. Gold and silver form a complete solid solution; we call compositions containing both gold and silver electrum. But, because copper atoms are smaller than gold and silver atoms, solutions are limited between copper and the precious metals. Native gold, silver, and copper may contain small amounts of other elements.

For example, native copper frequently contains arsenic, antimony, bismuth, iron, or mercury. Native platinum is much rarer than gold, silver, or copper. It typically contains small amounts of other elements, especially palladium. The native semimetals arsenic, antimony, and bismuth are also rare. Native copper, gold, silver, and platinum have atomic structures with atoms arranged in a cubic pattern Figure 9. Iron does, too, although native iron is rare, except in meteorites, and the atomic arrangement in native iron is not quite the same as in the other metals.

Nonetheless, euhedral crystals of any of these minerals may be cubic or, as we will explain in the next chapter, octahedral. More typically, however, these minerals crystallize in less regular shapes. Native zinc, a very rare mineral, has a hexagonal atomic arrangement and so forms crystals of different shapes. The photos below Figures 9. Gold, sometimes mined as nuggets or flakes see the example in Figure 9.

Large, visible specimens, like the one seen below in Figure 9. Most gold and other precious metal ores contain very fine subhedral metal grains, often microscopic. Silver sometimes occurs in a wire-like or arborescent tree-like form Figure 9. It also easily tarnishes and so has a gray color in this photo. Most bedrock gold and silver deposits are in quartz-rich hydrothermal veins.

Besides hard-rock deposits, gold and silver are also found in placers accumulations in river, stream, or other kinds of sediments , and native silver is found in several other types of deposits. Box below describes the Witwatersrand gold deposits, the largest gold deposits in the world. Section 9. The sample is 4 cm tall. The largest are about 2 mm across. Copper is found as branching sheets, plates, and wires, and as massive pieces. In Figure 9. We mine native platinum primarily from ultramafic igneous rocks, but platinum is also found in placers — Figure 9.

Platinum is also a secondary product of Cu- or Ni-sulfide refining. Native antimony in Figure 9. It is usually in solution with arsenic and may contain small amounts of other metals.

Untarnished specimens are metallic and silvery, but antimony typically tarnishes to a gray color as seen in this photo. Graphite, diamond, and sulfur are examples of nonmetallic native elements. Figure 3. Sulfur deposits are associated with volcanoes, often concentrated at fumaroles. Sulfur is also found in veins in some sulfide deposits and in sedimentary rocks where it is found with halite, anhydrite, gypsum, or calcite.

Most of the rest is separated from sulfides during processing to recover metals. Both graphite and diamond consist only of carbon. We discussed the nature of the two minerals in Box of Chapter 3. Graphite is common as a minor mineral in many kinds of metamorphic rocks, including marbles, schists, and gneisses. The origin of the carbon is usually organic material in the original sediments.

Graphite also occurs in some types of igneous rocks and in meteorites. Diamond only forms at very high pressures associated with the lowermost crust or mantle of Earth. We mine it from kimberlite pipes, where rapidly moving, sometimes explosive, mafic magmas have carried it up to the surface.

After formation, diamond sometimes concentrates in river and streambeds where we mine it from placer deposits. Although some diamonds are of gem quality, most are not.

We call lower-quality diamonds industrial diamonds or bort if the diamonds are small and opaque. See section 9. Gold occurs in many different ore deposits. The yellow grains are gold, and the black material is uraninite. This ore, like many Witswatersrand ores, is quite radioactive. The Witwatersrand deposits are paleoplacer deposits, meaning that they were placers when originally deposited.

They occur in an area about km by 40 km. The origin of the Witwatersrand deposits is a bit of a mystery. Placers form when hard-rock deposits are eroded, and sedimentary processes concentrate ore. Yet today we know of no hard-rock gold deposits of sufficient size to account for the volume of the Witwatersrand placers. The Witwatersrand gold prospects were discovered in , but the discovery was kept secret.

It was not until that significant production began. A booming mining industry led to the rapid growth of Johannesburg, a central town in frontier South Africa. Within a decade, Johannesburg was the largest city in the country. When miners reached a zone of pyrite in , the mining slowed because it was not known how to extract gold from sulfides at the time.

Subsequently, John MacArthur, Robert Forrest and William Forrest, three Scotsmen working for the Tennant Company in Glasgow, developed a dissolution process involving cyanide that would extract gold from sulfides.

So, Johannesburg flourished once more. Most are quite rare. The table seen here lists the more important species. Pyrite iron sulfide is most common. Other relatively common sulfides include chalcopyrite copper iron sulfide , molybdenite molybdenum sulfide , sphalerite zinc sulfide , galena lead sulfide , and cinnabar mercury sulfide. The others in the table are less abundant but are occasionally concentrated in particular deposits. Sulfide minerals such as pyrite contain one or several metallic elements and sulfur as the only nonmetallic element.

Bonding is either covalent, metallic, or a combination of both. Other very uncommon minerals grouped with the sulfides because of similar properties contain selenium the selenides , tellurium the tellurides , or bismuth the bismuthides instead of sulfur. A related group of minerals, the sulfosalts , contains the semimetals arsenic and antimony in place of some metal atoms.

Because many sulfides have similar atomic arrangements, solid solutions between them are common. The same holds true for the sulfosalts. Metal deposits are mined in a variety of different ways depending on their depth, shape, size, and grade.

Relatively large deposits that are quite close to the surface and somewhat regular in shape are mined using open-pit mine methods Figure Creating a giant hole in the ground is generally cheaper than making an underground mine, but it is also less precise, so it is necessary to mine a lot of waste rock along with the ore.

Relatively deep deposits or those with elongated or irregular shapes are typically mined from underground with deep vertical shafts , declines sloped tunnels , and levels horizontal tunnels Figures In this way, it is possible to focus the mining on the ore body itself. However, with relatively large ore bodies, it may be necessary to leave some pillars to hold up the roof. In many cases, the near-surface part of an ore body is mined with an open pit, while the deeper parts are mined underground Figures A typical metal deposit might contain a few percent of ore minerals e.

Other sulphide minerals are commonly present within the ore, especially pyrite. When ore is processed typically very close to the mine , it is ground to a fine powder and the ore minerals are physically separated from the rest of the rock to make a concentrate. At a molybdenum mine, for example, this concentrate may be almost pure molybdenite MoS 2. The rest of the rock is known as tailings.

It comes out of the concentrator as a wet slurry and must be stored near the mine, in most cases, in a tailings pond. The tailings are contained by an embankment. Also visible in the foreground of Figure Although this waste rock contains little or no ore minerals, at many mines it contains up to a few percent pyrite. The tailings and the waste rock at most mines are an environmental liability because they contain pyrite plus small amounts of ore minerals.

When pyrite is exposed to oxygen and water, it generates sulphuric acid — also known as acid rock drainage ARD. Acidity itself is a problem to the environment, but because the ore elements, such as copper or lead, are more soluble in acidic water than neutral water, ARD is also typically quite rich in metals, many of which are toxic.