ore deposits

ore deposits Ores are naturally occurring rock materials consisting of one or more minerals from which a valuable constituent or constituents can be extracted for profit. The locations in the Earth's crust where these accumulations occur are collectively referred to as orebodies, ore reserves, or ore deposits. The definition of an ore, in the strictest sense, refers only to mineralized rocks that can be profitably mined. The term is, however, generally applied by geologists to any anomalous accumulation of a valuable constituent or constituents that may have this potential. Whether a deposit can in fact be mined for a profit will depend on such factors as the market price of the constituent, mining costs, taxation, and government environmental policy. The valuable constituents that are extracted from particular ore deposit can include one or more of the metals (e.g., gold, copper, uranium), non-metals (e.g. sulphur, selenium, tellurium), or gemstones (e.g., diamonds and emeralds). The minerals that contain the valuable constituents are called ore minerals; barren minerals associated with these ore minerals are gangue minerals. The presence of an unusual concentration of ore minerals, as compared with the local host rocks, constitutes mineralization, and rocks are said to be mineralized where such a concentration is found.

Many classification schemes for ore deposits have been developed, based variously on morphology, metal association, host rocks, or mode of formation. Many of the older classification schemes relied heavily on the mode of formation. These schemes have consequently become obsolete as ideas about the genesis of certain types of ore deposit have changed. The more recent classifications are likely to be more enduring, since they are based more on host-rock association, with less emphasis on genetic factors. The best classifications are those that provide useful insights into the characteristics of newly discovered mineralization, enabling it to be more efficiently delineated and profitably mined.

Two terms that are still commonly encountered when discussing the classification of ore deposits are syngenetic and epigenetic. Ores formed at the same time as their host rocks are referred to as syngenetic; ores formed after the host rock are called epigenetic. There is a general consensus on whether most types of ore deposit are syngenetic or epigenetic, but debate continues about some. Some ore deposit types, such as volcanogenic massive sulphide deposits, can contain both syngenetic and epigenetic mineralization.

The shape of an ore deposit can have a variety of forms that can reveal much about its genesis. Much epigenetic mineralization is found in tabular-shaped zones called veins, where, ore and gangue minerals have been deposited from heated solutions in vertical, horizontal, or inclined fractures in pre-existing host rocks. Veins can range in width from microscopic dimensions to many metres. In many instances the density of veining determines the viability of an ore deposit. If many cross-cutting veins are present, they form stockwork mineralization, as is common in many porphyry copper deposits. Epigenetic deposits can also occur as tubular bodies or pipes as well as replacement bodies. Here the minerals in the original host rock have been wholly or partially replaced by the ore and gangue minerals, as in skarn deposits. Syngenetic mineralization commonly occurs as beds that lie conformably within the layers of the host rocks and hence are called stratiform deposits.

Magmatic deposits

Magmatic deposits are those formed during the crystallization of a magma; they are therefore syngenetic deposits. Two processes can lead to the concentration of valuable commodities: liquid immiscibility and fractional crystallization. Major deposits of nickel, copper, and platinum-group elements are the result of liquid immiscibility, a process in which a sulphur-rich liquid separates from a cooling silicate magma. These metals tend to migrate from the silicate magma into the sulphide. This sulphide liquid is much denser than the surrounding silicate-rich magma, and it settles and becomes concentrated at the base of the magma chamber. Major deposits of this type are found in Sudbury, Canada; Noril'sk-Talnakh, Russia; and Kambalda, Australia, as well as in numerous smaller occurrences throughout the world. Fractional crystallization also occurs during the cooling of a magma. Early minerals, such as the chromium-bearing mineral chromite, crystallize early and are denser than the surrounding magma. They settle and are concentrated in layers within the magma chamber. These de-posits are high-grade sources of iron, chromium, titanium, vanadium, and platinum-group elements.

Another rare type of magmatic ore deposit is that associated with carbonatites, which are unusual igneous rocks composed primarily of carbonate minerals such as calcite, dolomite, and ankerite (a calcium magnesium iron carbonate). These deposits are major sources of tantalum, niobium, rare-earth elements, and vermiculite. Magmas generated at great depths are the ultimate source of diamonds. When these magmas erupt violently near or at the Earth's surface, they generate kimberlites such as are found in South Africa. Kimberlites occur in many parts in the world, but only rarely do they contain enough diamonds for them to be worth mining.

Hydrothermal deposits

Ores generated by the actions of high-temperature fluids are grouped into a large class known as hydrothermal deposits. This broad group contains many sub-types relating to how and when they were formed. The term hydrothermal refers to the fact that the fluids responsible were composed primarily of hot water containing small amounts of carbon dioxide, sulphur, sodium chloride, and other components. Valuable metals are dissolved in these fluids and are subsequently deposited as a result of changes in the temperature, pressure, or chemical composition of the fluid. Ores that are the direct result of the interplay of high-temperature fluids and intruding magma bodies include pegmatites, skarns, and porphyry-type deposits. Pegmatites are igneous rocks composed of the late differentiates of large magma bodies. As these magmas cool, certain constituents are not fully incorporated in the crystallizing minerals. These become increasingly segregated in the remaining magma, which has a high content of water and silica. This late differentiate is forced into fractures and tabular bodies and cools to form pegmatoidal rocks. Pegmatite orebodies are rich sources of metals such as lithium, beryllium, and the rare-earth elements. Skarn deposits form when magmas are intruded into reactive host rocks such as limestone. The high temperature of the magma and its exsolved fluids causes extensive reactions with the wall rocks and results in the deposition of metals. Many varieties of skarn deposit are found throughout the world and are mined for numerous metals including iron, copper, molybdenum, tin, tungsten, lead, and zinc.

Porphyry-type deposits are the world's primary source of copper and molybdenum. They comprise very large orebodies, which normally range from hundreds to thousands of million of tonnes of ore. Gold can also be an important metal in these deposits and can make an orebody economically mineable when it would not otherwise be worth mining. Porphyry copper deposits are formed when a magma body is intruded close to the Earth's surface. The small proportion of water in the magma remains dissolved at depth because of great pressures. As the magma rises, cools, and begins to crystallize, the water can no longer remain in the melt and is expelled to form a high-temperature concentrated brine. It is generally accepted that this brine contains high concentrations of ore metals. When it rises through fractures in the surrounding solid rocks, these metals are precipitated, normally as the minerals chalcopyrite, bornite, and molybdenite. The cooling intrusion also heats the local water contained in fractures in the cooler host rocks, causing this water to expand and become less dense. Hydrothermal convection cells develop, causing further alteration of the rocks and redistribution of the valuable metals.

Although porphyry deposits are intimately associated with an intruding magma body, there are other types of hydrothermal deposits that appear to be formed as a result of magmatic activity, albeit at a greater distance from the magma. Epithermal deposits are a major source of gold, silver, copper, lead, and zinc. The ore minerals of these metals are deposited in veins or by replacement of the host rocks. The term epithermal is applied because these deposits form near the Earth's surface from hot fluids at temperatures that are generally lower than those encountered in porphyry-type deposits. These epigenetic deposits can be divided into two sub-types according to the ore metals present and the alteration minerals found in the host rocks. High-sulphidation epithermal deposits contain high concentrations of gold, silver, and copper and are formed when gases at high temperature from a nearby cooling intrusion mix with local groundwaters. Rapid chemical changes occur in the groundwater, generating hot fluids that are extremely acidic and cause intense alteration of the host rock. In some instances the alteration is so intense that all that remains of the host is a porous siliceous cap rock. Chemical and isotopic evidence show that a major component of the fluid involved in these deposits is derived directly from the magma, which also appears to be the source of the ore metals. In low-sulphidation epithermal deposits, the fluids are less acidic, and alteration is less intense. These deposits also host major concentrations of gold, silver, and copper. They differ from high-sulphidation deposits in that they can also contain mineable amounts of lead and zinc. The source of fluids for low-sulphidation deposits appears to be mainly from local groundwater which becomes heated by a distant magma body. Economic geologists continue to debate whether this magma body supplies ore metals and other constituents to the mineralizing fluid.

Vein deposits that form at greater depths from fluids at higher temperature than those associated with epithermal deposits are termed mesothermal deposits. These deposits generally consist of large groupings of veins that extend for great depths in highly altered host rocks. The major commodity of interest obtained from mesothermal deposits is gold, but they are also mined for silver and, in some rare instances, for tungsten. Many of the world's largest gold deposits, such as those in Canada, South Africa, Australia, and India are classified as mesothermal deposits. One common feature of these deposits is that they are emplaced in low-grade metamorphic rocks such as greenschists and were formed by hydrothermal fluids rich in carbon dioxide and sulphur. The exact source of the fluids and metals for these deposits remains a matter for debate. Genetic models calling upon magmatic fluids, metamorphic fluids, and even deep circulating groundwater have been suggested. The source of gold in mesothermal deposits continues to be a matter of intense discussion among economic geologists.

In contrast, volcanogenic massive sulphide deposits provide an example of how scientific investigations have revealed the undisputed origin of a major type of ore deposit. These orebodies are composed of massive accumulations of sulphide minerals that are hosts to high concentrations of copper, lead, zinc, and can also yield mineable concentrations of silver and gold. They are generally tabular in form, but many of them have been disrupted by metamorphism and folding of their host rocks. These are invariably volcanic rocks, including rhyolites, andesites, basalts, and their explosive equivalents, pyroclastic rocks. Scientific debate originally focused on whether these deposits were sea-floor deposits or were formed as a result of replacement of the volcanic hosts by the action of hydrothermal fluids during metamorphism. The discovery of ‘black smokers’ on mid-ocean ridges in the late 1970s showed the former model to be correct. Black smokers form on the sea floor at spreading plate boundaries, such as the Mid-Atlantic ridge. In these areas of the Earth's crust, magmatic rocks lie only a short distance below the sea floor. The magmas heat sea water contained in the overlying volcanic rocks, causing convection cells to develop. These convection cells continuely circulate the sea water, which becomes heated and reacts with the volcanic rocks, leaching out metals such as copper, lead, and zinc. The high-temperature fluids ultimately reach the sea floor and are expelled. The immediate drop in temperature and mixing with cold sea water causes metal sulphides to be deposited on the sea floor, building up thick mineralized layers in the course of time. These accumulations eventually become covered by further volcanic eruptions and mineralization ceases.

Carbonate lead–zinc deposits are an example of epigenetic deposits for which hydrothermal fluids at rather low temperatures are responsible. The host rocks for these deposits include limestone and dolomite that have undergone dissolution by low-temperature fluids, either before or during the mineralizing event. This karsting provides highly permeable rocks in which the sulphide minerals sphalerite and galena represent the valuable mineralization. The metals in carbonate lead–zinc deposits are thought to have been carried by fluids expelled from deep sedimentary basins adjacent to the carbonate sediments. Debate continues, however, on the source of the sulphide that is necessary to precipitate the metals from the fluids when they reach the host rocks.

Ores formed at the Earth's surface

All the ore deposit types described above are examples of primary mineralization in which the ore metals were carried from a dispersed source and concentrated in a relatively small area of host rocks. At the surface of the Earth, exposure of such types of ore deposit to erosion can lead to secondary mineralization. Many ore minerals are not chemically stable when exposed to air and surface waters. This causes them to decompose, which results in the production of large amounts of acid. Acidic groundwaters with low pH values are produced and react with the primary minerals, releasing more metals. These become concentrated in the water table when the groundwaters move downwards and are neutralized by water–rock reactions forming supergene mineralization. Some minerals, such as gold and platinum, are particularly resistant to chemical breakdown; when host rocks rich in these minerals are eroded, they are released unharmed. The action of water carries these into streams and rivers. These minerals have much higher densities than the more common minerals, and this causes them to be locally concentrated in the stream and river deposits. Such placer deposits generated the great gold rushes during the nineteenth century in the Yukon and California.

Residual deposits form when unmineralized common rocks are weathered by acidic surface waters. These waters are generated in areas with warm climates with high rainfall and dense vegetation. Decaying vegetation releases large quantities of organic acid, which descends through thin soils and attacks the bedrock. The acid solutions dissolve out many of the major components of the rock, leaving only the most immobile elements such as silicon, aluminum, and iron. Over thousands of years, thick accumulations of residual deposits can accumulate if erosion is minor. Perhaps the most important example of residual deposit is bauxite, which is, at present, the only economically mineable ore of aluminium. Other residual deposits are mined as sources of iron and nickel.

Bruce W. Mountain