Ore

In most cases, an ore does not consist entirely of a single mineral, but is mixed with other valuable minerals and with unwanted or valueless rocks and minerals. The part of an ore that is not economically desirable and that cannot be avoided in mining is known as gangue. or ore dressing. Mineral processing consists of first liberation, to free the ore from the gangue, and concentration to separate the desired mineral(s) from it. Once processed, the gangue is known as tailings, which are useless but potentially harmful materials produced in great quantity, especially from lower grade deposits. == Ore deposits ==

An ore deposit is an economically significant accumulation of minerals within a host rock. This is distinct from a mineral resource in that it is a mineral deposit occurring in high enough concentration to be economically viable. Most ore deposits are named according to their location, or after a discoverer (e.g. the Kambalda nickel shoots are named after drillers), or after some whimsy, a historical figure, a prominent person, a city or town from which the owner came, something from mythology (such as the name of a god or goddess) or the code name of the resource company which found it (e.g. MKD-5 was the in-house name for the Mount Keith nickel sulphide deposit). Classification Ore deposits are classified according to various criteria developed via the study of economic geology, or ore genesis. The following is a general categorization of the main ore deposit types: Magmatic deposits Magmatic deposits are ones which originate directly from magma • Pegmatites are very coarse grained, igneous rocks. They crystallize slowly at great depth beneath the surface, leading to their very large crystal sizes. Most are of granitic composition. They are a large source of industrial minerals such as quartz, feldspar, spodumene, petalite, and rare lithophile elements. • Carbonatites are an igneous rock whose volume is made up of over 50% carbonate minerals. They are produced from mantle derived magmas, typically at continental rift zones. They contain more rare earth elements than any other igneous rock, and as such are a major source of light rare earth elements. • Magmatic Sulfide Deposits form from mantle melts which rise upwards, and gain sulfur through interaction with the crust. This causes the sulfide minerals present to be immiscible, precipitating out when the melt crystallizes. Magmatic sulfide deposits can be subdivided into two groups by their dominant ore element: • Ni-Cu, found in komatiites, anorthosite complexes, and flood basalts. These highly mafic intrusions are a source of chromite, the only chromium ore. They are so named due to their strata-like shape and formation via layered magmatic injection into the host rock. Chromium is usually located within the bottom of the intrusion. They are typically found within intrusions in continental cratons, the most famous example being the Bushveld Complex in South Africa. • Podiform Chromitites are found in ultramafic oceanic rocks resulting from complex magma mixing. They are silicates derived from the recrystallization of carbonates like limestone through contact or regional metamorphism, or fluid related metasomatic events. Not all are economic, but those with potential value are classified depending on the dominant element such as Ca, Fe, Mg, or Mn among many others. Porphyry copper deposits These are the leading source of copper ore. Porphyry copper deposits form along convergent boundaries and are thought to originate from the partial melting of subducted oceanic plates and subsequent concentration of Cu, driven by oxidation. These are large, round, disseminated deposits containing on average 0.8% copper by weight. • Mississippi Valley-Type (MVT) deposits precipitate from relatively cool, basal brinal fluids within carbonate strata. These are sources of lead and zinc sulphide ore. These are the second most common source of copper ore after porphyry copper deposits, supplying 20% of the worlds copper in addition to silver and cobalt. • Orogenic gold deposits are a bulk source for gold, with 75% of gold production originating from orogenic gold deposits. Formation occurs during late stage mountain building (see orogeny) where metamorphism forces gold containing fluids into joints and fractures where they precipitate. These tend to be strongly correlated with quartz veins. Banded iron formations (BIFs) are the highest concentration of any single metal available. Their deposition occurred early in Earth's history when the atmospheric composition was significantly different from today. Iron rich water is thought to have upwelled where it oxidized to Fe (III) in the presence of early photosynthetic plankton producing oxygen. This iron then precipitated out and deposited on the ocean floor. The banding is thought to be a result of changing plankton population. Sediment Hosted Copper forms from the precipitation of a copper rich oxidized brine into sedimentary rocks. These are a source of copper primarily in the form of copper-sulfide minerals. Placer deposits are the result of weathering, transport, and subsequent concentration of a valuable mineral via water or wind. They are typically sources of gold (Au), platinum group elements (PGE), sulfide minerals, tin (Sn), tungsten (W), and rare-earth elements (REEs). A placer deposit is considered alluvial if formed via river, colluvial if by gravity, and eluvial when close to their parent rock. They are formed by a combination of diagenetic and sedimentary precipitation at the estimated rate of about a centimeter over several million years. The average diameter of a polymetallic nodule is between 3 and 10 cm (1 and 4 in) in diameter and are characterized by enrichment in iron, manganese, heavy metals, and rare earth element content when compared to the Earth's crust and surrounding sediment. The proposed mining of these nodules via remotely operated ocean floor trawling robots has raised a number of ecological concerns. == Extraction ==

, Mexico The extraction of ore deposits generally follows these steps. • Prospecting to find where an ore is located. The prospecting stage generally involves mapping, geophysical survey techniques (aerial and/or ground-based surveys), geochemical sampling, and preliminary drilling. • After a deposit is discovered, exploration is conducted to define its extent and value via further mapping and sampling techniques such as targeted diamond drilling to intersect the potential ore body. This exploration stage determines ore grade, tonnage, and if the deposit is a viable economic resource. Methods can be generally categorized into surface mining such as open pit or strip mining, and underground mining such as block caving, cut and fill, and stoping. • Reclamation, once the mine is no longer operational, makes the land where a mine had been suitable for future use. With rates of ore discovery in a steady decline since the mid 20th century, it is thought that most surface level, easily accessible sources have been exhausted. This means progressively lower grade deposits must be turned to, and new methods of extraction must be developed. == Hazards ==

Some ores contain heavy metals, toxins, radioactive isotopes and other potentially negative compounds which may pose a risk to the environment or health. The exact effects an ore and its tailings have is dependent on the minerals present. Tailings of particular concern are those of older mines, as containment and remediation methods in the past were next to non-existent, leading to high levels of leaching into the surrounding environment. Additional elements found in ore which may have adverse health affects in organisms include iron, lead, uranium, zinc, silicon, titanium, sulfur, nitrogen, platinum, and chromium. Exposure to these elements may result in respiratory and cardiovascular problems and neurological issues. When water becomes contaminated it may transport these compounds far from the tailings site, greatly increasing the affected range. == History ==

Metallurgy began with the direct working of native metals such as gold, lead and copper. Placer deposits, for example, would have been the first source of native gold. These were the easiest to work, with relatively limited mining and basic requirements for smelting. Lead production from galena smelting may have been occurring at this time as well. This has led to an ever-growing search for REE ore and novel ways of extracting said elements. == Trade ==

Ores (metals) are traded internationally and comprise a sizeable portion of international trade in raw materials both in value and volume. This is because the worldwide distribution of ores is unequal and dislocated from locations of peak demand and from smelting infrastructure. Most base metals (copper, lead, zinc, nickel) are traded internationally on the London Metal Exchange, with smaller stockpiles and metals exchanges monitored by the COMEX and NYMEX exchanges in the United States and the Shanghai Futures Exchange in China. The global Chromium market is currently dominated by the United States and China. Iron ore is traded between customer and producer, though various benchmark prices are set quarterly between the major mining conglomerates and the major consumers, and this sets the stage for smaller participants. Other, lesser, commodities do not have international clearing houses and benchmark prices, with most prices negotiated between suppliers and customers one-on-one. This generally makes determining the price of ores of this nature opaque and difficult. Such metals include lithium, niobium-tantalum, bismuth, antimony and rare earths. Most of these commodities are also dominated by one or two major suppliers with >60% of the world's reserves. China is currently leading in world production of Rare Earth Elements. The World Bank reports that China was the top importer of ores and metals in 2005 followed by the US and Japan. == Important ore minerals ==

For detailed petrographic descriptions of ore minerals see Tables for the Determination of Common Opaque Minerals by Spry and Gedlinske (1987). Below are the major economic ore minerals and their deposits, grouped by primary elements. == See also ==