Introduction to Earth Science - OER textbook - Chapter 16.pdf

16. ENERGY AND MINERAL RESOURCES Learning Objectives By the end of this chapter, students should be able to: Describe how a renewable resource is different from a nonrenewable resource. Compare the pros and cons of extracting and using fossil fuels and conventional and un...

16. ENERGY AND MINERAL RESOURCES Learning Objectives By the end of this chapter, students should be able to: Describe how a renewable resource is different from a nonrenewable resource. Compare the pros and cons of extracting and using fossil fuels and conventional and unconventional petroleum sources. Describe how metallic minerals are formed and extracted. Understand how society uses nonmetallic mineral resources. This text has previously discussed geology’s pioneers, such as scientists James Hutton and Charles Lyell, but the first real “geologists” were the hominids who picked up stones and began 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 geo- logic study, and it dates back 2.6 million years to east Africa. In modern times, geologic knowledge is important for locating economi- cally valuable materials for society’s use. In fact, all things we use come from only three sources: they are farmed, hunted or fished, or mined. At the turn of the twentieth century, speculation was rampant that food sup- plies would not keep pace with world demand, suggesting the need to Figure 16.1: A Mode 1 Oldowan tool used for chopping. develop artificial fertilizers. Sources of fertilizer ingredients are: nitrogen is processed from the atmosphere, using the Haber process for the manufacture of ammonia from atmospheric nitrogen and hydrogen; potassium comes from the hydrosphere, such as lakes or ocean evaporation; and phosphorus is mined from the lithosphere, such as minerals like apatite from phosphorite rock, which is found in Florida, North Carolina, Idaho, Utah, and around the world. Thus, without mining and processing of natural materials, modern civilization would not exist. Indeed, geologists are essential in this process.  ENERGY AND MINERAL RESOURCES | 429 16.1 Mining.M. "",~ CON" : ~::,.. Nllui..·.-...... ·--............ C , ~utwm ,......... 0 Q ~ CD GROSS CALORIFIC VALUE (BTU/LB ON A MOIST, * *"' ,.. MINERAL-MATTER-FREE BASIS) Figure 16.20: USGS diagram of different coal rankings.  ENERGY AND MINERAL RESOURCES | 437 Peat itself is an economic fuel in some locations like the British Isles and Scandinavia. 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 become graphite. Humans have used coal for at least 6,000 years, mainly as a fuel source. Coal resources in Wales are often cited as a primary reason Figure 16.21: Peat (also known as turf) consists of partially for Britain’s rise, and later, for the decayed organic matter. The Irish have long mined peat to United States’ rise during the be burned as fuel though this practice is now discouraged for environmental reasons. Industrial Revolution. According to the US Energy Information Administration, US coal production has decreased due to competing energy sources’ cheaper prices and due to society recognizing its negative environmental impacts, including increased very fine-grained particulate matter as an Figure 16.22: Anthracite coal, the highest grade of coal. air pollutant, greenhouse gases, acid rain, and heavy metal pollution. Seen from this perspective, the coal industry as a source of fossil energy is unlikely to revive. As the world transitions away from fossil fuels including coal, and manufacturing seeks strong, flexible, and lighter mate- rials than steel including carbon fiber for many applications, current research is exploring coal as a source of this carbon. Take this quiz to check your comprehension of this section. If you are using an offline version of this text, access the quiz for section 16.2 via the QR code. An interactive H5P element has been excluded from this version of the text. You can view it online here: https://pressbooks.lib.vt.edu/introearthscience/?p=973#h5p-108 438 | ENERGY AND MINERAL RESOURCES 16.3 Mineral Resources Mineral resources, while principally nonrenewable, are gener- ally placed in two main categories: metallic, which contain met- als, and nonmetallic, which contain other useful materials. Most mining has been traditionally focused on extracting metal­ lic minerals. Human society has advanced significantly because we’ve developed the knowledge and technologies to yield metal from the Earth. This knowledge has allowed humans to build the machines, buildings, and monetary systems that dominate our world today. Locating and recovering these metals has been a key facet of geologic study since its inception. Every ele­ ment across the periodic table has specific applications in human civilization. Metallic mineral mining is the source of many of these elements. Figure 16.23: Gold-bearing quartz vein from California. 16.3.1 Types of Metallic Mineral Deposits The various ways in which 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. In the following section, we describe some of the more common deposit types along with their associated elemental concentrations and world class occur- rences. Magmatic Processes When a magmatic body crystallizes and differentiates (see chapter 4), it can cause certain minerals and elements to concentrate. Layered intru- sions, typically ultramafic to mafic, can host deposits that contain cop- per, nickel, platinum, palladium, rhodium, and chromium. The Stillwater Complex in Montana is an example of economic quantities of layered mafic intrusion. Associated deposit types can contain chromium or tita- nium-vanadium. The largest magmatic deposits in the world are the chromite deposits in the Bushveld Igneous Complex in South Africa. These rocks have an areal extent larger than the state of Utah. The chromite occurs in layers, which resemble sedimentary layers, except these layers occur within a crystallizing magma chamber. Figure 16.24: Layered intrusion of dark Water and other volatiles that are chromium-bearing minerals, Bushveld Complex, South Africa. not incorporated into mineral crystals when a magma crystal- lizes can become concentrated around the crystallizing magma’s margins. Ions in these hot fluids are very mobile and can form exceptionally large crystals. Once crystallized, these large crystal masses are then called pegmatites. They form from magma fluids that are expelled from the solidifying magma when nearly the entire magma body has crystallized. In addition to minerals that are predominant in the Figure 16.25: This pegmatite contains lithium-rich green elbaite (a tourmaline) main igneous mass, such as quartz, feldspar, and mica, pegmatite bodies may and purple lepidolite (a mica). 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.  ENERGY AND MINERAL RESOURCES | 439 An unusual magmatic process is a kimberlite pipe, which is a Zone Tuff Ring Erosion Level volcanic conduit that transports ultramafic magma from within Depth (km) the mantle to the surface. Diamonds, which are formed at great C rater 0 - - Or,1pa temperatures and pressures of depth, are transported by a Kim­ berlite pipe to locations where they can be mined. The process that created these kimberlite ultramafic rocks is no longer com- mon on Earth. Most known deposits are from the Archean Eon. Oiatreme Root Dike Figure 16.26: Schematic diagram of a kimberlite pipe. Hydrothermal Processes Figure 16.27: The complex chemistry around mid-ocean ridges. Fluids rising from crystallizing magmatic bodies or that are heated by the geothermal gradient cause many geochemical reactions that form various mineral deposits. The most active hydrothermal process today produces volcanogenic mas­ sive sulfide (VMS) deposits, which form from black smoker hydrothermal chimney activity near mid-ocean ridges all over the world. They commonly contain copper, zinc, lead, gold, and silver when found at the surface. Evidence from around 7000 BC in a period known as the Chalcolithic shows copper was among the earliest metals smelted by humans as means of obtaining higher temperatures were developed. The largest of these VMS deposits occur in Precambrian period rocks. The Jerome deposit in central Arizona is a good example. 440 | ENERGY AND MINERAL RESOURCES Another deposit type that draws on magma-heated water is a porphyry deposit. This is not to be confused with the por­ phyritic igneous texture, although the name is derived from the porphyritic texture that is nearly always present in the igneous rocks associated with a porphyry deposit. Several types of porphyry deposits exist, such as porphyry copper, porphyry molybdenum, and porphyry tin. These deposits contain low-grade disseminated ore minerals closely associ- ated with intermediate and felsic intrusive rocks that are present over a very large area. Porphyry deposits are typically the largest mines on Earth. One of the largest, richest, and possibly best studied mine in the world is Utah’s Kennecott Bingham Canyon Mine. It’s an open pit mine, which, for over 100 years, has produced several elements including copper, gold, molybdenum, and silver. Underground carbonate replacement deposits produce lead, zinc, gold, silver, and copper. In the mine’s past, the open pit predominately produced copper and gold from chalcopyrite and bornite. Gold only occurs in minor quantities in the copper-bearing minerals, but because the Kennecott Bingham Canyon Mine produces on such a large scale, it is one of the largest gold mines in the US. In the future, this mine may produce more copper and molyb- denum (molybdenite) from deeper underground mines. Most porphyry copper deposits owe their high metal content, and hence, their economic value to weathering processes called super­ gene enrichment which occurs when the deposit is uplifted, eroded, and exposed to oxidation. This process occurred millions of years after the initial igneous intrusion and hydrothermal expulsion ends. When the deposit’s upper pyrite-rich portion is exposed to.......~ rain, the pyrite in the oxidizing zone creates an extremely acid con- dition that dissolves copper out of copper minerals, such as chal- I. ~- 6 ".;" ~... copyrite, and converts the chalcopyrite to iron oxides, such as hematite or goethite. The copper minerals are carried downward ,- ----- -.., , in water until they arrive at the groundwater table and an environ- Figure 16.28: The Morenci porphyry is oxidized toward its top ment where the primary copper minerals are converted into sec- (as seen as red rocks in the wall of the mine), creating ondary higher-copper content minerals. Chalcopyrite (35% Cu) is supergene enrichment. converted to bornite (63% Cu), and ultimately, chalcocite (80% Cu). Without this enriched zone, which is two to five times higher in copper content than the main deposit, most por­ phyry copper deposits would not be economic to mine. If limestone or other calcareous sedimentary rocks are near the magmatic body, then another type of ore deposit called a skarn deposit forms. These metamorphic rocks form as magma-derived, highly saline metalliferous fluids react with carbonate rocks to create calcium-magnesium-silicate minerals like pyroxene, amphibole, and gar- net, as well as high-grade iron, copper, zinc minerals, and gold. Intrusions that are genetically related to the intrusion that made the Kennecott Bingham Canyon deposit have also produced copper-gold skarns, which were mined by the early European settlers in Utah. When iron and/or sulfide deposits undergo metamorphism, the grain size commonly increases, which makes separating the gangue from the Figure 16.29: Garnet-augite skarn from desired sulfide or oxide minerals much easier. Italy.  ENERGY AND MINERAL RESOURCES | 441 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-grade hydrothermal reactions, generally in the realm of dia­ genesis, 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 rocks situated with a high geothermal gra­ dient. The Mercur deposit in Utah’s Oquirrh Mountains was this type’s earliest locally mined deposit. There, almost a million ounces of gold was recovered between 1890 and 1917. In the 1960s, a metallurgical process using cyanide was developed for these low-grade ore types. These deposits are also called Car­ lin-type deposits because the disseminated deposit near Carlin, Nevada, is Figure 16.30: In this rock, a pyrite cube has dissolved (as seen with the negative “corner” where the new technology was first applied and where the first definitive scien- impression in the rock), leaving behind small tific studies were conducted. Gold was introduced into these deposits by specks of gold. hydrothermal fluids that reacted with silty calcareous rocks, removing carbon­ ate, 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 disseminated gold deposits in Nevada. Similar deposits, but not as large, have been found in China, Iran, and Macedonia. Non-magmatic Geochemical Processes Geochemical processes that occur at or near the surface without magma’s aid 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 places are considered reducing environments. Uranium deposits are an example of where redox concentrated the metal. Uranium is soluble in oxidizing groundwater environments and precipitates as uraninite when encountering reducing conditions. Many of the deposits across the Col- orado Plateau, such as in Moab, Utah, were formed by this method. Figure 16.31: Underground uranium mine near Moab, Redox reactions are also responsible for creating banded iron forma­ Utah. tions (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. Cycles of oxygenating iron-rich waters initiated precipitation of the iron beds. Because BIFs are gener- ally Precambrian in age, happening at the event of atmospheric oxygenation, they are only found in some of the older exposed rocks in the United States, such as in Michigan’s upper peninsula and northeast Minnesota. 442 | ENERGY AND MINERAL RESOURCES ~~di~:~f-h~!~~eJl~!d-ri~~ d~a;~i;i~om1nated : ~i'::~~;~~~im~~~;:dType L500,000 Km2, (310,685.596mi) and the Zambian Copper Belt in Africa. Soils and mineral deposits that are exposed at the surface experience deep and intense weathering, which can form surficial deposits. Baux­ ite, an aluminum ore, is preserved in karst topography and laterites, which are soils formed in wet tropical environments. Soils containing '.,_ aluminum concentrate minerals, such as feldspar, and ferromagnesian ,.I. minerals in igneous and metamorphic rocks, undergo chemical ~.-.f weathering processes that concentrate the metals. Ultramafic rocks ' that undergo weathering form nickel-rich soils, and when the mag- netite and hematite in banded iron formations undergo weathering, it ~ ---~.&.&.&11 forms goethite, a friable mineral that is easily mined for its iron content. Figure 16.33: A sample of bauxite. Note the unweathered igneous rock in the center.  ENERGY AND MINERAL RESOURCES | 443 Surficial Physical Processes At the Earth’s surface, mass wasting and moving water can cause hydraulic sorting, which forces high-density minerals to concentrate. When these minerals are concentrated in streams, rivers, and beaches, they are called placer deposits, and occur in modern sands and ancient lithified rocks. Native gold, native platinum, zircon, ilmenite, rutile, mag- netite, 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 Figure 16.34: Lithified heavy mineral sand (dark layers) from a beach deposit in India. Metallic mineral min­ ing’s primary impact comes from the mining itself, including disturbing the land surface, covering landscapes with tailings impoundments, and increasing mass wasting by accelerating erosion. In addition, many metal deposits contain pyrite, an uneconomic sulfide mineral, that when placed on waste dumps, generates acid rock drainage (ARD) during weathering. In oxygenated water, sulfides such as pyrite react and undergo complex reactions to release metal ions and hydrogen ions, which lowers pH to highly acidic levels. Mining and processing of mined materials typically increase the surface area to volume ratio in the material, causing chemical reactions to occur even faster than would Figure 16.35: Acid mine drainage in the Rio occur naturally. If not managed properly, these reactions lead to acidic streams Tinto, Spain. and groundwater plumes that carry dissolved toxic metals. In mines where limestone is a waste rock or where carbonate minerals like calcite or dolomite are present, their acid neutralizing potential helps reduce acid rock drainage. Although this is a natural process too, it is very important to isolate mine dumps and tailings from oxygenated water, both to prevent the sulfides from dis- solving and subsequently percolating the sulfate-rich water into waterways. Industry has taken great strides to prevent contamination in recent decades, but earlier mining projects are still causing problems with local ecosystems. 444 | ENERGY AND MINERAL RESOURCES 16.3.3 Nonmetallic Mineral Deposits While receiving much less attention, nonmetallic mineral resources, also known as industrial minerals, are just as vital to ancient and mod- ern society as metallic minerals. The most basic is building stone. Limestone, travertine, granite, slate, and marble are common build- ing 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, which is used in concrete, roads, and foun- dations. Gravel is one of the more common aggregates. Evaporites Evaporite deposits form in restricted basins where water evaporates faster than it recharges, such as the Great Salt Lake in Utah, or the Dead Sea, Figure 16.36: Carrara marble quarry in Italy, source to which borders Israel famous sculptures like Michelangelo’s David. and Jordan. As the waters evaporate, soluble minerals are concentrated and become super- Figure 16.37: Salt-covered plain known as the saturated, at which point they precipitate from the now highly-saline Bonneville Salt Flats, Utah. waters. If these conditions persist for long stretches, thick rock salt, rock gypsum, and other mineral deposits accumulate (see chapter 5). Evaporite minerals, such as halite, are used in our food as common table salt. Salt was a vitally important food preservative and economic resource before refrigeration was developed. While still used in food, halite is now mainly mined as a chemical agent, water softener, or road de-icer. Gypsum is a common nonmetallic mineral used as a building material; it is the main component in dry wall. It is also used as a fertil- izer. Other evaporites include sylvite—potassium chloride, and bischof- ite—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 fer- tilizer. In hyper-arid locations, even more rare and complex evaporites, Figure 16.38: Hanksite, Na22K(SO4)9(CO3)2Cl, one of the few minerals that is considered a carbonate and a like borax, trona, ulexite, and hanksite are mined. They can be found in sulfate. places such as Searles Dry Lake and Death Valley, California, and in the Green River Formation’s ancient evaporite deposits in Utah and Wyoming.  ENERGY AND MINERAL RESOURCES | 445 Phosphorus 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 an important component of bone and DNA. Bone ash and guano are natural sources of phosphorus. Figure 16.39: Apatite from Mexico. Take this quiz to check your comprehension of this section. If you are using an offline version of this text, access the quiz for section 16.3 via the QR code. I ~ _ _ _ _I An interactive H5P element has been excluded from this version of the text. You can view it online here: https://pressbooks.lib.vt.edu/introearthscience/?p=973#h5p-109 Summary Energy and mineral resources are vital to modern society, and it is the role of the 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 the deposits 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 renewable 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.