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Geology
The consensus amongst scientists is that our solar system began about 5,000 million years ago, and planet Earth formed from a superheated cloud of dust and gas (following the 'Big Bang').
The Earth is believed to comprise a deep interior (thecore), surrounded by a zone of heavy rock (the mantle) and a thin outer skin (the crust). Cooling from the core outwards sets up convection currents, and as these reach the crust the patterns they set up have been instrumental in forming a series of interlocking crustal plates.
Plate tectonics is a relatively new theory and has revolutionised the way geologists think about the Earth. The size and position of the plates change over time. The edges of the plates, where they move against each other (the so-called mobile belts) are sites of intense geologic activity; such as earthquakes, volcanoes and mountain building (and mineralisation). Periods of mountain-building are referred to as orogeny, the most recent of which started 200 million years ago (about the time the first mammals appeared).
Where plates 'collide', one plate might slide beneath (subduction), or ride above (obduction), another plate. Such movements are often accompanied by the intrusion into the crust of molten rock (magma) from the mantle. The magma cools to form igneous rock.
There are broadly two types of igneous rock:
Light (felsic) rocks that are rich in silica and aluminium, eg granites.
Dark and heavy (mafic) rocks rich in iron and magnesia, eg gabbro.
Where the magma reaches the surface of the crust it is extruded and cools very quickly as lava to form fine-grained volcanic rocks, eg basalt.
The igneous activity associated with mobile belts is often accompanied by the introduction of hydrothermal fluids rich in minerals, giving rise to some of the world's biggest mineral deposits (eg the copper deposits of the Andes). A present-day example of such hydrothermal activity is in the South Pacific where 'black smokers' on the seafloor are currently depositing metal sulphides along the junction of two tectonic plates.
Over geological time, some plates fuse together and new ones form, and, away from the edges of existing plates, the older rocks form the ancient 'basement', or cratons (also termed shields). 'Fossilised' mobile belts are preserved in cratons as Greenstone belts, which are a major source of gold deposits (eg in Western Australia, the Canadian Shield and West Africa).
The crust and its plates are subject to constant erosion and the resultant material is re-deposited as sediment in rivers, lakes and seas, eventually consolidating into layers or strata - sedimentary rocks.
These sedimentary rocks fall into two categories:
| Clastic | Fragments brought together by ice, water or wind (eg sandstone). |
| Chemical | Precipitation of dissolved materials (eg forming limestone); with evaporates (eg rock salt from sea water) being a particular type. |
In many areas of the globe, sedimentary rocks cover the basement and cratons entirely. Minerals contained in the sediments may accumulate in economic quantities.
Over millions of years, sedimentary rocks are subject to heat and pressure as a result of igneous intrusions, mountain-building activity or the weight of the overlying sediments, to form metamorphic rocks. Hence a limestone becomes a marble, shale becomes a slate, sandstone becomes a quartzite, etc. The gold deposits of South Africa's Witwatersrand and the iron-ore deposits in the Pilbara district of Western Australia are examples of sedimentary deposits that have been metamorphosed.
According to the amount of heat and pressure, the original sediments can eventually be metamorphosed to schists, and volcanic/igneous rocks can be metamorphosed to form gneisses. Metamorphism often remobilises and reconcentrates the contained metals to form new deposits. The world's cratons consist entirely of metamorphic rocks.
General Deposit Types
Sedimentary deposits can be in the form of lenses and pods, often
deposited along bedding planes or in fractures, faults and fissures.
Under certain conditions, eg warm climate and shallow seas, sediments
accumulate in large basins, and minerals become increasingly
concentrated as salts as a result of evaporation. Many of the world's
large deposits of potash, nitrate, phosphate and rock salt have formed
in this fashion.
Deposits in igneous rocks can also occur as lenses and pods, and in fractures, faults and fissures. They can also be distributed through the rock as fine disseminations and in small quartz veinlets as stockworks (typical of porphyry copper). Such deposits tend to be of large size and low grade. They often possess a surface (or supergene) zone that has been enriched in metals as a result of weathering. Beneath this zone, the ore unaffected by weathering is termed primary (or hypogene).
Massive deposits (see below) are of higher grade and consist almost entirely of sulphide minerals. They are generally associated with metamorphic terrain. Where their deposition is associated with volcanic activity, they are termed volcanic massive sulphides (VMS). Where deposits associated with volcanic activity are stratified they have been referred to as sedimentary exhalative (sedex) deposits.
A number of the world's most important deposits of nickel, chromite, copper and platinum occur in mafic rocks (see above) in layered igneous intrusions. The metals occur at distinct horizons, reflecting the pressure and temperature at which they formed as the magma cooled down. The platinum and palladium deposits of the Bushveld complex in southern Africa are of this type.
One particular type of mafic rock, kimberlite, is the world's principal source of diamonds. Diamonds are formed (from carbon) in the mantle under extreme temperature and pressure, and are carried to the surface in kimberlite pipes. These occur throughout the world but very few contain diamonds, and even fewer have diamond concentrations of economic interest (as noted above).
Alluvial deposits are formed where material resulting from weathering and erosion is transported by rivers and streams and re-deposited. The mineral must be chemically stable and physically resistant to survive the process (restricting such deposits to precious metals, diamonds and other gemstones). Alluvial deposits are relatively recent in age and are generally unconsolidated.
Laterite deposits are a product of tropical weathering and comprise a mixture of oxide and hydroxide minerals and clays. Bauxite, the chief ore of aluminium, is a laterite, and there are vast deposits in Brazil and Guinea. There are also important deposits of nickel laterite (eg in New Caledonia and Cuba).
Where mineral deposits are formed at the same time as the host rock they are termed syngenetic. Where they have been introduced afterwards, they are termed epigenetic.
Deposit Summary
Diamond Pipes: Formed at least 150 km below the surface (where temperatures and pressures are extreme enough to create diamonds, rather than graphite or coal, from the element carbon). These kimberlite mineral accumulations only become economic when they are brought to the surface by volcanic activity.
Epithermal: Formed by hydrothermal volcanic activity that pushes magma (and the contained minerals) through vents (to form extensive vein systems). An important source of gold and silver, normally as 'native' metal rather than in a mineral (and are the most likely type of deposit for high-grade, 'bonanza'-type discoveries).
Laterites: A deeply weathered mixture of oxide and hydroxide minerals and clays (usually found in the tropics). These form the main orebodies for aluminium, and an increasingly important source of nickel (although recovery of the latter is a still problematic process).
Lode: Found in Greenstone belts (see above), these deposits are an important source of precious metals and cluster around large regional fault zones. Although usually narrow and inconsistent (and so hard to identify) they can extend to great depths.
Magmatic:
As molten rock cools, the minerals crystallise and sink to the base.
They are usually tabular, or lens-like, in shape, and form many of the
world's great base-metal sulphide deposits, especially copper and
nickel (and also some oxide deposits of iron, titanium and chrome).
Massive:
Nothing to do with size, rather a mineralisation (made up almost
entirely of sulphides) that is homogeneous and conforms to the host
rock's structure (usually indicating that it was formed at the same
time). These orebodies are relatively easy to understand and mine.
Placer: Minerals that have been eroded from the primary source and transported (normally by water action) and then deposited in a sedimentary bed. The mineral must be chemically stable and physically resistant to survive this process (restricting such deposits to precious metals and gemstones).
Porphyry: Typical of deposits (especially copper) formed by igneous activity, with both the intrusion and host rock being severely fractured, with the mineralisation forming veins. The deposits are usually large but low grade, although subsequent leaching and precipitation can form areas of substantially higher grades (supergene enrichment).
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