16 Energy and Mineral Resources

The nugget has cube shapes.
The Latrobe Gold Nugget, as seen on display in the London Natural History Museum, is 717 grams and displays the rare cubic form of native gold. Most gold, even larger nuggets, grow in confined spaces where the euhedral nature of the mineral is not seen.

16 Energy and Mineral Resources

KEY CONCEPTS

The rock has a smooth side and a sharp side.
A Mode 1 Oldowan tool used for chopping
This text has discussed pioneers in the scientific study of geology like James Hutton and Charles Lyell, but the first “geologists” were the hominids who picked up stones, beginning the stone age. Maybe stones were first used as curiosity pieces, maybe as weapons, but ultimately, they were used as tools. This was the Paleolithic Period, the beginning of the study of geology and it goes back 2.6 million years ago to east Africa .

In modern times, an important use for geologic knowledge is locating economically valuable materials for use in society. All items we use can come from only three sources: they can be farmed, hunted or fished, or they can be mined. At the turn of the Twentieth Century, speculation was rampant that food supplies would not keep pace with world demand, and artificial fertilizers would need to be developed . The ingredients for fertilizers are mined: nitrogen from the atmosphere using the Haber process , potassium from the hydrosphere (lakes or oceans) by evaporation, and phosphorus from the lithosphere (minerals like apatite from phosphorite rock, found in Florida, North Carolina, Idaho, Utah, and around the world).  Thus, without mining, modern civilization would not exist. Geologists are essential in the process of mining.

16.1 Mining

The map shows many different materials that are mined across the world.
Map of world mining areas.
Mining is defined as the extraction, from the Earth, of valuable material for societal use. Usually, this includes solid materials (e.g. gold, iron, coal, diamond, sand, and gravel), but can also include fluid resources such as oil and natural gas. Modern mining has a long relationship with modern society. The oldest evidence of mining, with a concentrated area of digging into the Earth for materials, has a history that may go back 40,000 years to the hematite (used as red dye) of the Lion Cave in Swaziland . Resources extracted by mining are generally considered to be nonrenewable.

16.1.1. Renewable vs. nonrenewable resources

Resources generally come in two major categories: renewable, which can be reused over and over, or replicate over the course of a short (less than a human life span) time, and nonrenewable, which cannot.

The dam has a large lake behind it
Hoover Dam provides hydroelectric energy and stores water for southern Nevada.
Renewable resources are items that are present in our environment which can be exploited and replenished. Some of the more common energy sources in this category are linked with green energy resources because they are associated with environmental impacts that are relatively small or easily remediated. Solar energy is the energy that comes from fusion within the Sun, which radiates electromagnetic energy. This energy reaches the Earth constantly and consistently, and should continue to do so for about 5 billion more years . Wind energy is maybe the oldest form of renewable energy, used in sailing ships and windmills.  Both solar and wind generated energy are variable on Earth’s surface.  These limitations may be offset through the use of energy storing devices such as batteries or electricity exchanges between producing sites. The heat of the Earth, known as geothermal, can be viable anywhere if drilling goes deep enough. In practice it is more useful where heat flow is great, such as volcanic zones or regions with thinner crust . Hydroelectric dams provide energy by allowing water to fall through the dam activating turbines that produce the energy. Ocean tides can also be a reliable source of energy. All of these types of renewable resources can provide the energy that powers society. Other renewable resources that are not directly energy related are plant and animal matter, which are used for food, clothing, and other necessities.

The diamond is clear and pyramidal.
Natural, octahedral shape of diamond.
Nonrenewable resources cannot be replenished at a sustainable rate.  They are finite within a human lifetime.  Many nonrenewable resources are chiefly a result of planetary, tectonic, or long-term biologic processes, and include items such as gold, lead, copper, diamonds, marble, sand, natural gas, oil, and coal. Most nonrenewable resources are utilized for their concentration of specific elements on the periodic table. For example, if society needs sources of iron (Fe), then it is the exploration geologist who will search for iron-rich deposits that can be economically extracted. Non-renewable resources may be abandoned when other materials become cheaper or serve their purpose better.  For example, abundant coal is available in England, but the availability of North Sea oil and natural gas (at lower cost and environmental impact) led to the decrease in coal usage.

16.1.2. Ore

The rock shows red and brown layering.
Banded-iron formations are an important ore of iron (Fe).
The elements of the periodic table are found within the materials that make up the Earth. However, it is rare for the amount of the element to be concentrated to the point where the extraction and processing of the material into usable product becomes profitable. Any place where the amount of valuable material is concentrated is a geologic and geochemical anomaly.  If the material can be mined at a profit, the body constitutes an ore deposit. Typically, the term ore is used for only metal-bearing minerals, though the concept of ore as a non-renewable resource can be applied to valuable concentrations of fossil fuels, building stones, and other non-metal deposits, even groundwater. The term “natural resource” is more common than ore for these types of materials.

Diagram shows the small box of "reserves" within a larger box of "resources". There is also an "inferred resources" box that is slightly larger than "proven reserves" box and an "undiscovered resources" box slightly larger than the resources box.
Diagram illustrating the relative abundance of proven reserves, inferred reserves, resources, and undiscovered resources. (Source: Chris Johnson)

It is implicit that the technology to mine is available, economic conditions are suitable, and political, social and environmental considerations are satisfied in order to classify a  natural resource deposit as ore.  Depending on the substance, it can be concentrated in a narrow vein or distributed over a large area as a low-concentration ore. Some materials are mined directly from bodies of water (e.g. sylvite for potassium; water through desalination) and the atmosphere (e.g. nitrogen for fertilizers).  These differences lead to various methods of mining, and differences in terminology depending on the certainty. Ore mineral resource is used for an indication of ore that is potentially extractable, and the term ore mineral reserve is used for a well defined (proven), profitable amount of extractable ore.

The chart shows reserves vs. resources
McKelvey diagram showing different definitions for different degrees of concentration and understanding of mineral deposits.

16.1.3. Mining Techniques

The image is a large hole in a mountainside.
Bingham Canyon Mine, Utah. This open pit mine is the largest man-made removal of rock in the world.
The style of mining is a function of technology, social license, and economics. It is in the best interest of the company extracting the resources to do so in a cost-effective way. Fluid resources, such as oil and gas, are extracted by drilling wells. Over the years, drilling has evolved into a complex discipline in which directional drilling can produce multiple bifurcations and curves originating from a single drill collar at the surface. Using geophysical tools like seismic imaging, resources can be pinpointed and extracted efficiently.

A large machine is removing coal.
A surface coal mine in Wyoming.
Solid resources are extracted by two principal methods, of which there are many variants. Surface mining is the practice of removing material from the outermost part of the Earth. Open pit mining is used to target shallow, broadly disseminated resources. Typically, the pit progressively deepens through additional mining cuts to extract the ore, and the walls of the pit are as steep as can safely be managed. A steep wall means there is less waste (non-valuable) rock or overburden to remove and is an engineering balance between efficient mining and mass wasting. Occasionally landslides do occur, including a very large landslide that occurred in the Bingham Canyon mine in 2014. These events are costly and dangerous, though careful monitoring gave the Bingham Canyon mine ample warning time. Strip mining and mountaintop mining are surface mining techniques also used for resources that cover large areas, especially layered resources like coal. In this case, an entire mountaintop or rock layer is removed to gain access to the ore below. The environmental impacts of surface mining are usually greater due to the larger surface disturbance footprint .

A large truck is loading material underground.
Underground mining in Estonia of Oil Shale.
Underground mining is often used for higher-grade, more localized, or very concentrated resources. Some ore minerals are mined underground by introducing chemical agents that dissolve the target mineral followed by solution extraction and subsequent precipitation in a surface operation, but more often a mining shaft/tunnel (or a large network of these shafts and tunnels) is dug to access the material. Whether mining occurs underground or from Earth’s surface is dictated by ore deposit depth, geometry, land use policies, economics, strength of the surrounding rock, and physical access to the ore to be mined. For example, deeper deposits might require removal of too much material, it may be too dangerous or impractical to remove, or it may be too expensive to remove the entire overburden. These factors may prevent materials from being mined from the surface, and cause a project to be mined underground. Also, places where the mining footprint can not be large may force underground mining to occur. The method of mining and whether mining is feasable depends on the price of the commodity and the cost of available technology to remove it and deliver it to market.  Thus mines and the towns that support them come and go as the price of the commodity varies.  Technological advances and market demands may reopen mines and revive ghost towns.

16.1.4. Concentrating and Refining

A man is operating a large machine that looks like a blast furnace.
A phosphate smelting operation in Alabama, 1942.
All ore minerals are mixed with less desirable components called gangue. The process of physically separating gangue minerals from ore bearing minerals is call concentrating. Separating a desired element from a host mineral by chemical means (including heating in the presence of other minerals) is called smelting.  Finally, taking a metal such as copper and removing other trace metals such as gold or silver is done through the process of refining. Typically, this is done one of three ways: 1. items can either be mechanically separated and processed based on the unique physical properties of the ore mineral, like recovering  placer gold based on its high density; 2. items can also be heated to chemically separate desired components, like refining crude oil into gasoline; or 3. items can be smelted, in which controlled chemical reactions unbind metals from the minerals they are contained in, such as when copper is taken out of chalcopyrite (CuFeS2). Mining, concentrating, smelting and refining processes require enormous amounts of energy.  Continual advances in metallurgy and mining practice aim to develop ever more energy-efficient and environmentally benign processes and practices.

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16.2. Fossil Fuels

The power plant has smoke coming from it
Coal power plant in Helper, Utah.
Fossils fuels are extractable sources of stored energy created by ancient ecosystems. The natural resources that typically fall under this category are coal, oil (petroleum), and natural gas. This energy was originally formed via photosynthesis by living organisms such as plants, phytoplankton, algae, and cyanobacteria. Sometimes this is known as fossil solar energy, since the energy of the sun in the past has been converted into the chemical energy within a fossil fuel. Of course, as the energy is used, just like respiration from photosynthesis that occurs today, carbon can enter the atmosphere, causing climate consequences (see ch. 15). Fossil fuels account for a large portion of the energy used in the world.

The reef has many intricacies.
Modern coral reefs and other highly-productive shallow marine environments are thought to be the sources of most petroleum resources.
The conversion of living organisms into hydrocarbon fossil fuels is a complex process. As organisms die, decomposition is hindered, usually due to rapid burial, and the chemical energy within the organisms’ tissues is added to surrounding geologic materials.  Higher productivity in the ancient environment leads to a higher potential for fossil fuel accumulation, and there is some evidence of higher global biomass and productivity over geologic time . Lack of oxygen and moderate temperatures seem to enhance the preservation of these organic substances . Heat and pressure that is applied after burial also can cause transformation into higher quality materials (brown coal to anthracite, oil to gas) and/or migration of mobile materials .

16.2.1. Oil and Gas

Darker countries are higher in oil
World Oil Reserves in 2013. Scale in billions of barrels.
Petroleum, with the liquid component commonly called oil and gas component called natural gas (mostly made up of methane), are principally derived from organic-rich shallow marine sedimentary deposits . As the rock, (which is typically shale, mudstone, or limestone) lithifies, the oil and gas leak out of the source rock due to the increased pressure and temperature, and migrate to a different rock unit higher in the rock column. Similar to the discussion of good aquifers in chapter 11, if the rock is a sandstone, limestone, or other porous and permeable rock, then that rock can act as a reservoir for the oil and gas.

The rock layers are folded, and the petroleum is pooling toward the top of the fold.
A structural or anticline trap. The red on the image represents pooling petroleum. The green layer would be a non-permeable rock, and the yellow would be a reservoir rock.
A trap is a combination of a subsurface geologic structure and an impervious layer that helps block the movement of oil and gas and concentrates it for later human extraction . The development of a trap could be a result of many different geologic situations. Common examples include: an anticline or domal structure, an impermeable salt dome, or a fault bounded stratigraphic block (porous rock next to non-porous rock). The different traps have one thing in common: they pool the fluid fossil fuels into a configuration in which extraction is more likely to be profitable. Oil or gas in strata outside of a trap renders extraction is less viable.

Onlap is sediments moving toward the land. Offlap is moving away.
The rising sea levels of transgressions create onlapping sediments, regressions create offlapping.
A branch of geology that has grown from the desire to understand how changing sea level creates organic-rich shallow marine muds, carbonates, and sands in close proximity to each other is called sequence stratigraphy . A typical shoreline environment has beaches next to lagoons next to coral reefs. Layers of beach sands and lagoonal muds and coral reefs accumulate into sediments that form sandstones, good reservoir rocks, next to mudstones next to limestones, both potential source rocks. As sea level either rises or falls, the location of the shoreline changes and the locations of sands, muds, and reefs with it.  This places oil and gas producing rocks (like mudstones and limestones) next to oil and gas reservoirs (sandstones and some limestones). Understanding the interplay of lithology and ocean depth can be very important in finding new petroleum resources, because using sequence stratigraphy as a model can allow predictions to be made about the locations of source rocks and reservoirs.

Tar Sands

The sandstone is black with tar.
Tar sandstone from the Miocene Monterrey Formation of California.
Conventional oil and gas (pumped from a reservoir) are not the only way to obtain hydrocarbons. The next few sections are known as unconventional petroleum sources, though, they are becoming more important as conventional sources increase in scarcity. Tar sands, or oil sands, are sandstones that contain petroleum products that are highly viscous (like tar), and thus, can not be drilled and pumped out of the ground, unlike conventional oil. The fossil fuel in question is bitumen, which can be pumped as a fluid only at very low rates of recovery and only when heated or mixed with solvents.  Thus injections of steam and solvents, or direct mining of the tar sands for later processing can be used to extract the tar from the sands. Alberta, Canada is known to have the largest reserves of tar sands in the world . Note: an energy resource becomes uneconomic once the total cost of extracting it exceeds the revenue which is obtained from the sale of extracted material.

Oil Shale

Oil shale has dramatically increased starting around 1945.
Global production of Oil Shale, 1880-2010.
Oil shale (or tight oil) is a fine-grained sedimentary rock that has a significant quantity of petroleum or natural gas. Shale is a common source of fossil fuels with high porosity but it has very low permeability. In order to get the oil out, the material has to be mined and heated, which, like with tar sands, is expensive and typically has a negative impact on the environment .

Fracking

The image shows fracking fluids cracking the rock, allowing methane to escape.
Schematic diagram of fracking.
Another process which is used to extract the oil and gas from shale and other unconventional tight resources is called hydraulic fracturing, better known as fracking . In this method, high pressure injections of water, sand grains, and added chemicals are pumped underground, creating and holding open fractures in the rocks, which aids in the release of the hard-to-access fluids, mostly natural gas. This is more useful in tighter sediments, especially shale, which has a high porosity to store the hydrocarbons but low permeability to transmit the hydrocarbons. Fracking has become controversial due to the potential for groundwater contamination  and induced seismicity , and represents a balance between public concerns and energy value.

16.2.2. Coal

The chart shows many different coal rankings
USGS diagram of different coal rankings.
Coal is the product of fossilized swamps , though some older coal deposits that predate terrestrial plants are presumed to come from algal buildups . It is chiefly carbon, hydrogen, nitrogen, sulfur, and oxygen, with minor amounts of other elements . As this plant material is incorporated into sediments, it undergoes a series of changes due to heat and pressure which concentrates fixed carbon, the combustible portion of the coal. In this sense, the more heat and pressure that coal undergoes, the greater is its fuel value and the more desirable is the coal. The general sequence of a swamp turning into the various stages of coal are: Swamp => Peat => Lignite => Sub-bituminous => Bituminous => Anthracite => Graphite. As swamp materials collect on the floor of the swamp, they turn to peat. As lithification occurs, peat turns to lignite. With increasing heat and pressure, lignite turns to sub-bituminous coal, bituminous coal, and then, in a process like metamorphism, anthracite. Anthracite is the highest metamorphic grade and most desirable coal, since it provides the highest energy output. With even more heat and pressure driving out all the volatiles and leaving pure carbon, anthracite can turn to graphite. 

It is very black and shiny.
Anthracite coal, the highest grade of coal.
Coal has been used by humans for at least 6000 years , mainly as a fuel source. Coal resources in Wales are often cited as a primary reason for  the rise of Britain (and later, the United States) in the Industrial Revolution . According to the US Energy Information Administration, the production of coal in the US has decreased due to cheaper prices of competing energy sources and recognition of its negative environmental impacts, including increased very fine-grained particulate matter, greenhouse gases , acid rain , and heavy metal pollution . Seen from this point of view, the coal industry is unlikely to revive.

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16.3 Mineral Resources

The yellow gold is inside white quartz.
Gold-bearing quartz vein from California.
Mineral resources, while principally nonrenewable, are generally placed in two main categories: metallic (containing metals) or nonmetallic (containing other useful materials). Most mining is focused on metallic minerals. A significant part of the advancement of human society has been developing the  knowledge and technologies that yielded metal from the Earth and allowed the machines, buildings, and monetary systems that dominate our world today. The location and recovery of these metals has been a key facet of the study of geology since its inception. Every element across the periodic table has specific applications in human civilization. Metallic mineral mining is the source of many of these elements.

16.3.1. Types of Metallic Mineral Deposits

The number of ways that minerals and their associated elements concentrate to form ore deposits are too complex and numerous to fully review in this text. However, entire careers are built around them.  Some of the more common types of these deposits are described, along with their associated elemental concentrations and world class occurrences.

Magmatic Processes

The rock has several layers, with the dark layers being the ones with value.
Layered intrusion of dark chromium-bearing minerals, Bushveld Complex, South Africa
Crystallization and differentiation (see chapter 4) of a magmatic body can cause the concentration of certain minerals and elements. Layered intrusions (typically ultramafic to mafic) can be host to deposits that contain copper, nickel, platinum-palladium-rhodium, and chromium. The Stillwater Complex in Montana is an example of an economic layered mafic intrusion . Associated deposit types can contain chromium or titanium-vanadium. The largest magmatic deposits in the world are the chromite deposits in the Bushveld Igneous Complex in South Africa . Rocks of the Bushveld Igneous Complex have an areal extent larger than the state of Utah. The chromite occurs in layers, which resemble sedimentary layers, except this occurred within a crystallizing magma chamber.

The rock is mostly green and purple
This pegmatite from Brazil contains lithium-rich green elbaite (a tourmaline) and purple lepidolite (a mica).
Water and other volatiles that are not incorporated into mineral crystals while a magma crystallizes become concentrated around the margins of these crystallizing magmas. Ions in these hot fluids are very mobile and can form exceptionally large crystals.  Once crystallized, masses of these large crystals are called pegmatites that form from the concentration of magma fluids near the end of crystallization when nearly the entire magma body has crystallized. In addition to minerals that are predominant in the main igneous mass, such as quartz, feldspar, and mica, pegmatite bodies may also contain very large crystals of unusual minerals that contain rare elements like beryllium, lithium, tantalum, niobium, and tin, as well as native elements like gold . Such pegmatites are ores of these metals.

The pipe is deep and narrow.
Schematic diagram of a kimberlite pipe.
An unusual magmatic process is a kimberlite pipe, which is a volcanic conduit that transports ultramafic magma from depths in the mantle to the surface. Diamonds, which are formed at great temperature and depth, are transported this way to locations where they can be mined. The process that emplaced these kimberlite (ultramafic) rocks is no longer common on Earth, and most of the known deposits are Archean .

Hydrothermal Processes

The diagram shows water going into the ground and coming out, with many different reactions.
The complex chemistry around mid-ocean ridges.
Fluids rising from crystallizing magmatic bodies or heated by the geothermal gradient cause a wide range of geochemical reactions that can form a variety of mineral deposits. The most active hydrothermal process today produces volcanogenic massive sulfide (VMS) deposits, which form from black smoker activity near mid-ocean ridges all over the world, and commonly contain copper, zinc, lead, gold, and silver when found on the surface . The largest of these deposits occur in Precambrian age rocks. The Jerome deposit in central Arizona is a good example.

The diagram shows the zones of mineralization.
USGS schematic of a Porphyry copper deposit.
Another type of deposit which draws on heated water from magma is a porphyry deposit. This is not to be confused with the igneous texture porphyritic, although the name is derived from the porphyritic texture that is nearly always present in the igneous rocks in a porphyry deposit. Several types of porphyry deposits exist: porphyry copper, porphyry molybdenum, and porphyry tin. They are characterized by the presence of low-grade disseminated ore minerals closely associated with intermediate and felsic intrusive rocks over a very large area . Porphyry deposits are typically the largest mines on Earth. One of the largest, richest, and possibly best studied mines in the world is Utah’s Bingham Canyon open pit mine, which has had over 100 years of high production of several elements including copper, gold, molybdenum, and silver.  Associated underground carbonate replacement deposits have produced lead, zinc, gold, silver, and copper . Past open pit production at this mine was dominated by copper and gold from chalcopyrite and bornite. Gold occurs in minor quantities in the copper-bearing mineral, but the large scale of production makes Bingham Canyon one of the largest gold mines in the U.S. Future production may be more copper and molybdenum (molybdenite) from deeper underground mines.

The mine contains grey rocks, which are not enriched, and red rocks, which is where the enrichment occurs.
The Morenci porphyry is oxidized toward its top (as seen as red rocks in the wall of the mine), creating supergene enrichment.
The majority of porphyry copper deposits owe their economic value to concentration by weathering processes occurring millions of years after the hosting intrusion called supergene enrichment. These occur once the hydrothermal event has ceased and the ore body has been uplifted, eroded, and exposed to oxidation .  When the upper pyrite-rich portion of the deposit is exposed to rain, pyrite in the oxidizing zone creates an extremely acid condition which dissolves copper out of copper minerals such as chalcopyrite, converting the chalcopyrite to iron oxides like hematite or goethite.  The copper is carried downward in solution until it arrives at the groundwater table and a reducing environment where the copper precipitates, converting primary copper minerals into secondary higher-copper content minerals.  Chalcopyrite (35% Cu)  is converted to bornite (63% Cu) and ultimately chalcocite (80% Cu). Without this enriched zone (2 to 5 times higher in copper content than the main deposit) most porphyry copper deposits would not be economic.

Calcite is blue, augite green, and garnet brown/orange in this rock.
Garnet-augite skarn from Italy.
If limestone or other calcareous sedimentary rocks are present adjacent to the magmatic body, then another type of ore deposit called a skarn deposit can form. These metamorphic rocks form as magma-derived, highly saline metalliferous fluids react with carbonate rocks, creating calcium-magnesium-silicate minerals like pyroxene, amphibole, and garnet, as well as high grade zones of iron, copper, and zinc minerals and gold . Intrusions that are genetically related to the intrusion that made the Bingham Canyon deposit have also produced copper-gold skarns that were mined by the early European settlers in Utah . Metamorphism of iron and/or sulfide deposits commonly results in an increase in grain size that makes separation of gangue from the desired sulfide or oxide minerals much easier.

The rock is red.
In this rock, a pyrite cube has dissolved (as seen with the negative “corner” impression in the rock), leaving behind small specks of gold.
Sediment-hosted disseminated gold deposits consist of low concentrations of microscopic gold as inclusions and disseminated atoms in pyrite crystals. These are formed via low-level hydrothermal reactions (generally in the realm of diagenesis) that occur in certain rock types, namely muddy carbonates and limey mudstones. This hydrothermal alteration is generally far-removed from a magma source, but can be found in extended rocks with a high geothermal gradient. The earliest locally mined deposit of this type was the Mercur deposit in the Oquirrh Mountains of Utah where almost one million ounces of gold were recovered between 1890 and 1917. In the 1960s a metallurgical processes using cyanide was developed for these types of low grade ores.  These deposits are also called Carlin-type deposits because the disseminated deposit near Carlin, Nevada is where the new technology was first applied and because the first definitive scientific studies were conducted there . Gold was introduced by hydrothermal fluids which reacted with silty calcareous rocks, removing carbonate, creating additional permeability, and adding silica and gold-bearing pyrite in the pore space between grains. The Betze-Post mine and the Gold Quarry mine on the “Carlin Trend” are two of the largest of the disseminated gold deposits in Nevada. Similar deposits, but not as large, have been found in China, Iran, and Macedonia .

Non-magmatic Geochemical Processes

A dark shaft runs into the mountain.
Underground uranium mine near Moab, Utah.
Geochemical processes that occur at or near the surface without the aid of magma also concentrate metals, but to a lesser degree than hydrothermal processes. One of the main reactions is redox (short for reduction/oxidation) chemistry, which has to do with the amount of available oxygen in a system. Places where oxygen is plentiful, as in the atmosphere today, are considered oxidizing environments, while oxygen poor environments are considered reducing. Uranium deposition is an example of redox mobilization. Uranium is soluble in oxidizing groundwater environments and precipitates as uraninite when reducing conditions are encountered. Many of the deposits across the Colorado Plateau (e.g. Moab, Utah) were formed by this method .

Redox reactions were also responsible for the creation of banded iron formations (BIFs), which are interbedded layers of iron oxide (hematite and magnetite), chert, and shale beds. These deposits formed early in the Earth’s history as the atmosphere was becoming oxygenated. Cyclic oxygenation of iron-rich waters initiated the precipitation of the iron beds. Because BIFs are generally Precambrian in age, they are only found in the some of the older exposed rocks in the United States, in the upper peninsula of Michigan and northeastern Minnesota .

The are globally distributed.
Map of Mississippi-Valley type ore deposits.
Deep, saline, connate fluids (trapped in the pore spaces), within sedimentary basins may be highly metalliferous. When expelled outward and upward during basin compaction, these fluids may form lead and zinc deposits in limestone by replacement or by filling open spaces (caves, faults) and in sandstone by filling pore spaces. The most famous of these are called Mississippi Valley-type deposits . Also known as carbonate-hosted replacement deposits, they are large deposits of galena and sphalerite (lead and zinc ores) which form from fluids in the temperature range of 100 to 200°C. Although they are named for occurrences along the Mississippi River Valley in the United States, they are found world wide.

Sediment-hosted copper deposits occurring in sandstones, shales, and marls are enormous in size and their contained resource are comparable to porphyry copper deposits.  These were most-likely formed diagenetically by groundwater fluids in highly-permeable rocks . Well-known examples are the Kupferschiefer in Europe, which has an areal coverage of >500,000 Km2, and the Zambian Copper Belt in Africa.

The outside of the rock is tan and weathered, the inside is grey.
A sample of bauxite. Note the unweathered igneous rock in the center.
Deep and intense weathering of soils and mineral deposits exposed at the surface can result in the formation of surficial deposits. Bauxite, an ore of aluminum, is preserved in karst topography and laterites (soils formed in wet tropical environments) . Aluminum concentrates in soils as feldspar and ferromagnesian minerals in igneous and metamorphic rocks undergo chemical weathering processes. Weathering of ultramafic rocks results in the formation of nickel-rich soils and weathering of magnetite and hematite in banded iron formation results in the formation of goethite, a friable mineral that is easily mined for its iron content.

Surficial Physical Processes

The tan rock has dark streaks of minerals.
Lithified heavy mineral sand (dark layers) from a beach deposit in India.
At the earth’s surface, the physical process of mass wasting or fluid movement concentrates high-density minerals by hydraulic sorting. When these minerals are concentrated in streams, rivers and beaches, they are called placer deposits, whether in modern sands or ancient lithified rocks . Native gold, native platinum, zircon, ilmenite, rutile, magnetite, diamonds, and other gemstones can be found in placers. Humans have mimicked this natural process to recover gold manually by gold panning and by mechanized means such as dredging.

16.3.2. Environmental Impacts of Metallic Mineral Mining

The water in the river is bright orange.
Acid mine drainage in the Rio Tinto, Spain.
The primary impact of metallic mineral mining comes from the mining itself, including disturbance of the land surface, covering of landscapes by tailings impoundments, and increased mass wasting by accelerated erosion . In addition, many metal deposits contain pyrite, an uneconomic sulfide mineral placed on waste dumps, which may generate acid rock drainage (ARD) during weathering. In the presence of oxygenated water, sulfides such as pyrite react undergo complex reactions to release metal ions and hydrogen ions, lowering pH to highly acidic levels. Mining and processing of mined materials typically increase the surface area to volume ratio in the material, causing reactions to occur even faster than what would occur naturally. If not managed properly, these reactions may lead to acidification of streams and groundwater plumes that can carry dissolved toxic metals. In mines where limestone is a waste rock or carbonate minerals like calcite or dolomite are present, their acid neutralizing potential helps reduce the likelihood of generating ARD. Although this is a natural process too, it is very important to isolate mine dumps and tailings from oxygenated water, both to prevent the dissolution of sulfides and subsequent percolation of the sulfate-rich water into waterways. Industry has taken great strides in preventing contamination in recent decades, but earlier mining projects are still causing problems with local ecosystems.

16.3.3. Nonmetallic Mineral Deposits

The image shows a hillside with blocks of marble removed.
Carrara marble quarry in Italy, source to famous sculptures like Michelangelo’s David.
While receiving much less attention, nonmetallic mineral resources (also known as industrial minerals) are just as vital to ancient and modern society as metallic minerals. The most basic of these is building stone. Limestone, travertine, granite, slate, and marble are common building stones, and have been quarried for centuries. Even today, building stones from slate roof tiles to granite countertops are very popular. Especially-pure limestone is ground up, processed, and reformed as plaster, cement and concrete. Some nonmetallic mineral resources are not mineral specific; nearly any rock or mineral can be used. This is generally called aggregate, and is used in concrete, roads, and foundations. Gravel is one of the more common aggregates.

Evaporites

The ground is white and flat for a long distance.
Salt-covered plain known as the Bonneville Salt Flats, Utah.
Evaporite deposits form in restricted basins, such as the Great Salt Lake or the Dead Sea, where evaporation of water exceeds the recharge of water into the basin . As the waters evaporate, soluble minerals are concentrated and become supersaturated, at which point they precipitate from the now highly-saline waters. If these conditions persist for long stretches of time, thick deposits of rock salt and rock gypsum and other minerals can accumulate (see chapter 5).

The mineral is hexagonal and clear.
Hanksite, Na22K(SO4)9(CO3)2Cl, one of the few minerals that is considered a carbonate and a sulfate
Evaporite minerals like halite are used in our food as common table salt.  Salt was a vitally important economic resource prior to refrigeration as a food preservative. While still used in food, now it is mainly mined as a chemical agent, water softener, or a de-icer for roads. Gypsum is a common nonmetallic mineral used as a building material, being the main component of dry wall. It is also used as a fertilizer. Other evaporites include sylvite (potassium chloride) and bischofite (magnesium chloride), both of which are used in agriculture, medicine, food processing and other applications.  Potash, a group of highly soluble potassium-bearing evaporite minerals, is used as a fertilizer. In hyper arid locations, even more rare and complex evaporites, like borax, trona, ulexite, and hanksite, are found and mined. They can be found in such localities as Searles Dry Lake and Death Valley, California, and in ancient evaporite deposits of the Green River Formation of Utah and Wyoming.

Phosphorus

The crystal is hexagonal and light green.
Apatite from Mexico.
Phosphorus is an essential element that occurs in the mineral apatite, which is found in trace amounts in common igneous rocks. Phosphorite rock, which is formed in sedimentary environments in the ocean , contains abundant apatite and is mined to make fertilizer.  Without phosphorus, life as we know it is not possible. Phosphorous is a major component of bone and a key component of DNA. Bone ash and guano are natural sources of phosphorus. 

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Summary

Energy and mineral resources are vital to modern society, and it is the role of a geologist to locate these resources for human benefit. As environmental concerns have become more prominent, the value of the geologist has not decreased, as they are still vital in locating and identifying the least intrusive methods of extraction.

Energy resources are general grouped as being renewable or nonrenewable. Geologists can aid in locating the best places to exploit renewables resources (e.g. locating a dam), but are commonly tasked with finding nonrenewable fossil fuels. Mineral resources are also grouped in two categories: metallic and nonmetallic. Minerals have a wide variety of processes that concentrate them to economic levels, and are usually mined via surface or underground methods.

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