5.5 Mineral resources: formation, mining, environmental impact  (Page 2/43)

Mineral resource principles

A geologist defines a mineral    as a naturally occurring inorganic solid with a defined chemical composition and crystal structure (regular arrangement of atoms). Minerals are the ingredients of rock    , which is a solid coherent (i.e., will not fall apart) piece of planet Earth. There are three classes of rock, igneous, sedimentary, and metamorphic. Igneous rocks form by cooling and solidification of hot molten rock called lava or magma. Lava solidifies at the surface after it is ejected by a volcano, and magma cools underground. Sedimentary rocks form by hardening of layers of sediment (loose grains such as sand or mud) deposited at Earth's surface or by mineral precipitation, i.e., formation of minerals in water from dissolved mineral matter. Metamorphic rocks form when the shape or type of minerals in a preexisting rock changes due to intense heat and pressure deep within the Earth. Ore is rock with an enrichment of minerals that can be mined for profit. Sometimes ore deposits (locations with abundant ore) can be beautiful, such as the giant gypsum crystals at the amazing Cave of the Crystals in Mexico (see Figure Giant Gypsum Crystals ). The enrichment factor    , which is the ratio of the metal concentration needed for an economic ore deposit over the average abundance of that metal in Earth’s crust, is listed for several important metals in the Table Enrichment Factor . Mining of some metals, such as aluminum and iron, is profitable at relatively small concentration factors, whereas for others, such as lead and mercury, it is profitable only at very large concentration factors. The metal concentration in ore (column 3 in Table Enrichment Factor ) can also be expressed in terms of the proportion of metal and waste rock produced after processing one metric ton (1,000 kg) of ore. Iron is at one extreme, with up to 690 kg of Fe metal and only 310 kg of waste rock produced from pure iron ore, and gold is at the other extreme with only one gram (.03 troy oz) of Au metal and 999.999 kg of waste rock produced from gold ore.

Formation of ore deposits

Ore deposits form when minerals are concentrated—sometimes by a factor of many thousands—in rock, usually by one of six major processes. These include the following: (a) igneous crystallization    , where molten rock cools to form igneous rock. This process forms building stone such as granite, a variety of gemstones, sulfur ore, and metallic ores, which involve dense chromium or platinum minerals that sink to the bottom of liquid magma. Diamonds form in rare Mg-rich igneous rock called kimberlite that originates as molten rock at 150–200 km depth (where the diamonds form) and later moves very quickly to the surface, where it erupts explosively. The cooled magma forms a narrow, carrot-shaped feature called a pipe. Diamond mines in kimberlite pipes can be relatively narrow but deep (see Figure A Diamond Mine ). (b) Hydrothermal is the most common ore-forming process. It involves hot, salty water that dissolves metallic elements from a large area and then precipitates ore minerals in a smaller area, commonly along rock fractures and faults. Molten rock commonly provides the heat and the water is from groundwater, the ocean, or the magma itself. The ore minerals usually contain sulfide (S ^2- ) bonded to metals such as copper, lead, zinc, mercury, and silver. Actively forming hydrothermal ore deposits occur at undersea mountain ranges, called oceanic ridges, where new ocean crust is produced. Here, mineral-rich waters up to 350°C sometimes discharge from cracks in the crust and precipitate a variety of metallic sulfide minerals that make the water appear black; they are called black smokers (see Figure Black Smokers ). (c) Metamorphism occurs deep in the earth under very high temperature and pressure and produces several building stones, including marble and slate, as well as some nonmetallic ore, including asbestos, talc, and graphite. (d) Sedimentary processes occur in rivers that concentrate sand and gravel (used in construction), as well as dense gold particles and diamonds that weathered away from bedrock. These gold and diamond ore bodies are called placer deposits . Other sedimentary ore deposits include the deep ocean floor, which contains manganese and cobalt ore deposits and evaporated lakes or seawater, which produce halite and a variety of other salts. (e) Biological processes involve the action of living organisms and are responsible for the formation of pearls in oysters, as well as phosphorous ore in the feces of birds and the bones and teeth of fish. (f) Weathering in tropical rain forest environments involves soil water that concentrates insoluble elements such as aluminum (bauxite) by dissolving away the soluble elements.