Geological ore deposits

Geological ore deposits are of many different types and occur in all geological environments.

The main types of geological ore deposits of importance can be divided into:

  • metallic deposits
  • non-metallic deposits
  • fossil fuel deposits

Finding ore deposits

Geologists are always searching for more ore deposits to meet constant demand. This has become more difficult with time as easily accessible ore deposits close to the Earth's surface have already been exploited by humans in the past. Therefore, more complex techniques have been developed to locate new deposits. However, better and more efficient processing techniques now mean that we can exploit ore deposits which were previously uneconomic.

Economic viability of ore deposits

Many factors control the economic viability of an ore deposit but the most important are:

  • grade (i.e. amount of metal per ton of rock)
  • size of the deposit (i.e. tonnage)
  • easy access to infrastructure such as roads and rail for transportation
  • current price for the commodity
  • demand

Oxidised zones of ore deposits

The region above the water-table in an ore deposit is known as the oxidised zone as it is the zone of oxidation of the primary ore minerals. This oxidised zone is primarily composed of mixtures of iron oxides/hydroxides and quartz which we call gossan.

Most primary ore minerals (particularly the sulfide minerals) are only stable in anaerobic dry environments. With the rise and fall of the water-table and downward percolating rainwater (containing dissolved oxygen), these minerals dissolve and new minerals (oxide zone minerals) are precipitated in the gossan. With the dissolution of sulfide minerals, the water becomes acidic, further enhancing the dissolution of the ore.

Most of the spectacular minerals we see from ore deposits are those formed in the oxidised zone. When the oxidised zone is well developed and the secondary minerals sufficiently concentrated, it is a highly profitable zone to mine as the processing is much cheaper and easier and the metals more concentrated. However, most oxidised zones have been mined in the past because they formed outcrops of easily identifiable stained gossans. The most common minerals found in oxidised zones are:

  • Copper: malachite, azurite, chrysocolla
  • Gangue minerals: quartz (usually cryptocrystalline), baryte, calcite, aragonite
  • Iron: goethite, hematite
  • Lead: anglesite, cerussite
  • Manganese: pyrolusite, romanechite, rhodochrosite
  • Nickel: gaspeite, garnierite
  • Silver: native silver, chlorargyrite
  • Zinc: smithsonite

Immediately below the oxidised zone is sometimes a zone known as the supergene zone where metals are deposited by fluids percolating downwards from the oxidised zone and concentrating in a narrow band just below the water table. The supergene zone is the richest part of an ore deposit but in many instances, is either only very thin or not developed at all. The most common minerals found in supergene zones are:

  • Copper: chalcocite, bornite
  • Lead: supergene galena
  • Nickel: violarite
  • Silver: acanthite, native silver
  • Zinc: supergene sphalerite, wurtzite

Classification and types of mineral deposits

Geologists classify mineral deposits in many different ways, according to the:

  • commodity being mined
  • tectonic setting in which the deposit occurs
  • geological setting of the mineral deposit
  • genetic model for the origin of the ore deposit

The most commonly used scheme is the genetic classification scheme. These deposits include:

Orthomagmatic deposits are those that form from primary magmatic processes (i.e. magmas). They are hosted in the igneous rocks in which they have formed. Most of the world's nickel, chromium and platinum-group elements are derived from these deposits. The largest deposits of platinum-group elements and chromium come from the 2055 million year old Bushveld Complex of northern South Africa. Orthomagmatic deposits include:

  • chromium
  • titanium
  • iron
  • nickel
  • copper
  • platinum-group elements
  • diamonds

Pneumatolytic and pegmatitic deposits are formed from volatile-rich (i.e. rich in water, fluorine, boron) high-temperature fluids emanating from igneous intrusions. Some of these deposits occur as pipe-like bodies or breccia pipes. These are important sources for:

  • tin
  • rare-earth elements
  • tantalum
  • beryllium
  • lithium
  • molybdenum
  • tungsten

Hydrothermal deposits cover a wide range of different deposits types but all form from hot circulating water-rich fluids. These include the two main types of gold deposits - epithermal and lode gold deposits along with replacement deposits in calcareous sequences (Mississippi Valley deposits), base-metal vein deposits and replacement skarn deposits

Volcanic or extrusive deposits are associated with volcanic processes and are only found within the volcanic rocks themselves. Important deposits of gold, mercury, antimony, copper and base metals are of this type. The largest deposits of this type are known as kuroko or volcanogenic massive sulfide deposits (commonly abbreviated as VMS deposits). These have formed on the ocean floor by circulating hydrothermal fluids emanating from a volcanic vent which leach metals from the surrounding volcanic rocks. They are currently forming on the sea floor and these are commonly referred to as black smokers.

Remobilisation of ore deposits

As many ore deposits formed many hundreds to thousands of millions of years ago, many have experienced numerous episodes of deformation and metamorphism which left them with few characteristics of their original form. Many of the ore components were remobilised (i.e. moved) either by physical (i.e. deformation) or chemical (i.e. dissolution and precipitation) mechanisms. The Broken Hill lode in far western New South Wales is an excellent example of this. It has undergone numerous episodes of deformation and metamorphism with the ore bodies themselves being strongly folded, faulted and metamorphosed.

Archaean gold deposits

Archaean gold deposits account for more than 60% of the world's gold. Over 60% of this comes from the Witwatersrand of South Africa (a hydrothermally remobilised palaeo-placer gold deposit) and the remainder from Archaean lode gold deposits in Australia, Canada, southern Africa and South America. The Archaean granite-greenstone belts of the world are characterised by numerous lode gold deposits, the largest of which is the Superior Province of Canada which has produced more than 170 million ounces of gold (a troy ounce is 31.1 grams).

In Australia, most Archaean lode gold deposits occur in the Norseman-Wiluna greenstone belt of Western Australia (this includes the famous Kalgoorlie Golden Mile containing the Super Pit, an open-cut gold operation that has dimensions of 3 km x 1 km x 1 km). These gold deposits are generally of large tonnage (millions of tonnes), and low-grade (commonly 1.5 - 3 g/T), confined to very narrow, mostly steeply-plunging near-vertical structures and are sulfide-deficient. They are confined to the volcanic-intrusive-sedimentary sequences of the greenstone belts and not the granites.

There is a very strong structural control to Archaean lode gold deposits with most being related to regional fault systems. The gold mineralisation usually occurs at the contact between the veins and wallrocks. Most of the ore in these deposits is of the disseminated type with the gold mineralisation generally being invisible to the naked eye. The formation of these deposits is linked to regional-scale processes at mid-crustal levels during regional metamorphism.

Epithermal gold deposits

These deposits are related to convergent-margin tectonic settings such as that of the Andes, South America. They are volcanic-hosted deposits (consisting of both subvolcanics and sub-aerial pyroclastics) that are generally relatively recent in age (e.g. Mesozoic to the present-day). In general, they are small (less than a million tons) but rich (up to 500g/T) deposits which consist of vein systems and bonanza ores (e.g. the El Indio deposit of Chile contains 120 000 tons at 250g/T and 3.2 million tons at 12.3g/T). Epithermal gold deposits occur over a considerable length (tens of kilometres) but are of limited vertical extent (on average, 350 m). They are ancient volcanic structures, most commonly collapsed craters and calderas.

Epithermal gold deposits essentially form by the boiling and mixing of primary hydrothermal fluids at temperatures of 230° C - 260° C. The hydrothermal fluids themselves are derived from deeper subvolcanic or plutonic rocks. Initially, fracturing of the host rock occurs under high fluid pressures and these fractures then act as channel-ways for the hydrothermal fluids.

The ore itself most commonly occurs in veins, breccia zones, stockworks and replacement zones. The ore minerals are generally fine-grained, and quartz pseudomorphs after calcite are characteristic of these deposits. Hydrothermal alteration of the volcanic host rocks is a characteristic of these deposits, and the gold is usually associated with zones of silicification.

The main ore minerals are:

  • native gold and silver
  • electrum
  • acanthite and tetrahedrite
  • silica occurring as quartz (most often forming comb-like aggregates), amethyst, opal, chalcedony and cristobalite

Sedimentary iron ore and manganese deposits

Most of the world's iron and manganese are derived from deposits of this type. These deposits are very large in size (thousands of millions of tons) and are usually mined by open-cut methods. The iron and manganese ores occur as distinct sedimentary layers alternating with iron and manganese-poor sedimentary layers - the whole mass is sometimes mined. Iron ores of this type are commonly termed the banded iron formation or simply abbreviated as BIF

All ore minerals in these deposits are oxides and hydroxides. The most common iron ore minerals are hematite, lepidocrocite and goethite while the most common manganese ore minerals are braunite, manganite and hausmannite. These deposits are believed to have formed in shallow marine basins and are confined to Proterozoic rock sequences. They are believed to have formed as chemical precipitates on the floor of the shallow oceanic basins in a highly oxidising environment. As they are very ancient, most of these deposits have been intensely deformed and metamorphosed.

Bog Iron ore was formerly mined in Europe hundreds of years ago when the large iron ore deposits had not been discovered. It consists of iron hydroxide (goethite) deposited in swamps and lakes as a product of bacterial action.

Placer deposits

These are made of alluvial, colluvial and eluvial material, which contain economic quantities of some valuable minerals:

  • Alluvial: Detrital material which is transported by a river and usually deposited along the river's pathway, either in the riverbed itself or on its floodplain.
  • Colluvial: Weathered material transported by gravity action such as on scree slopes.

  • Eluvial: Weathered material still at or near its point of formation.

The most common placer deposits are those of gold, platinum, group minerals, gemstones, tin, rutile, monazite and zircon.

For a mineral to be concentrated in a placer deposit, it must be resistant to weathering and erosion and have a relatively high specific gravity. The placer deposits usually form from primary deposits in which the ore mineral is widely disseminated and uneconomic. Concentration occurs when the surrounding rock is eroded away and the heavy ore mineral becomes concentrated by erosional processes. During transport of this material, concentration can occur within depressions, such as where a river changes its speed or in river bends.

Recently formed marine placer deposits of rutile, monazite and zircon are currently being exploited along the coast of eastern Australia. These are very important as they are our main source for the elements titanium and zirconium. Fossil placer deposits are also of great importance, with the world's largest gold deposit, the Witwatersrand reefs of South Africa, an excellent example.

Clay deposits

These either form by settling of clay particles in sedimentary basins or through intense weathering of volcanic and granitic rocks. They generally occur as lens-shaped bodies and are most commonly mined by open-cut methods. The most common clay minerals mined are kaolinite, illite and montmorillonite.

Useful clay types include:

China Clay: deposits of kaolin produced by hydrothermal decomposition or deep weathering of feldspar minerals in granites.

Fullers Earth: an aluminium-poor montmorillonite clay which is highly absorbent (very useful for absorbing contaminant oil).

Evaporite deposits

An evaporite is a sediment that forms through the evaporation of saline water. The most common evaporite deposits are salts (most commonly sylvite and halite), gypsum, and nitrates. Most evaporites are derived from bodies of sea-water, but under special conditions, inland lakes may also give rise to evaporite deposits, particularly in regions of low rainfall and high temperature. The original character of most evaporite deposits has been destroyed by replacement through circulating fluids.

An ideal evaporite sequence (in decreasing order of solubility) is as follows:

  • Potassium and magnesium salts (kainite, carnallite, sylvite) type 1
  • Rock salt (halite) - type 2
  • Gypsum (below 42° C) or anhydrite (above 42° C) - type 3
  • Calcite and dolomite - type 4

As evaporite beds of types 1 and 2 consist of highly soluble minerals, they are commonly re-dissolved by the influx of new salt-water. To be preserved, they must be covered over quickly by an impervious layer.

Since sea-water only contains 31 parts per thousand of dissolved salts, even evaporation of large areas of sea-water will only result in the deposition of a thin evaporite layer. For thick, economically viable evaporite layers to be deposited, a continuous evaporation-replenishment system must operate.

Salt domes: under conditions of high pressure, salt deforms plastically and behaves like a magma deforming and piercing through the overlying sediments. They are of economic importance as they form oil traps and can sometimes contain economic deposits of sulfur and boron.

Phosphatic deposits

Phosphorite is a commonly used term for lithified phosphate rock. The island of Nauru in the Pacific is one of the world's largest deposits of phosphorite and has been mined since at least the 1950s. The mineralogy of phosphate deposits is very complex. They usually consist of fine-grained mixtures of various calcium phosphates with the most common mineral being apatite. Collophane is an amorphous calcium phosphate that is also commonly found in phosphate deposits.

Phosphate deposits are of three main types:

  1. Primary marine phosphate deposits: All marine sediments, particularly limestones, contain some phosphate, which under particular conditions may rise to a greater concentration than normal (phosphatic limestone), but rarely reaching an economically extractable concentration. These deposits are rare and usually arise from either the leaching of the phosphatic limestone (dissolving away the calcium carbonate and leaving behind the detrital phosphate) or the extraction of phosphate at higher levels followed by secondary concentration from downward-percolating groundwaters. These deposits occur under relatively cool conditions in an oxygen-free environment.
  2. Bone beds: These are localised accumulations of fossil deposits of bone, teeth, scales and excreta (i.e. coprolites) that are occasionally thick enough to form economic deposits. These have mostly been mined in the past. A good example of bone beds is the marsupial-rich bone phosphate deposits of the Wellington Caves near Dubbo, New South Wales.
  3. Guano: These are ancient and/or fossil deposits of bird or bat excreta. Guano deposits from birds are most commonly found on oceanic islands. Guano deposits from bats are found in large cave systems. Guano deposits need a dry climate for their preservation.

Organic deposits

Coal

Coal is a general name given to stratified accumulations of carbonaceous material derived from vegetation. The starting material is usually peat or some other form of partially decayed organic material (commonly leaves and branches of trees). Combined processes of compaction (from overlying sediments) and slight heating converts this organic material to black coal. There are several stages in this process that occur with increasing heat and compaction:

  • Peat
  • Lignite (also known as brown coal)
  • Sub-bituminous coal
  • Bituminous coal
  • Sub-anthracite
  • Anthracite

This sequence shows a progressive increase in carbon content with corresponding decrease in volatile content. The percentage of carbon in a dry mineral-free coal is called the rank. Individual components of coal are known as macerals, of which there are three main types, vitrinite, exinite and inertinite.

Fireclay

Fireclay is a fossil clay-rich soil associated with coal deposits. It is useful as a refractory material.

Peat

Peat is a partially decomposed mass of vegetation that has grown in a shallow lake or marsh. Peat contains recognisable vegetable material but very little mineral material. The main plants which make-up peat are the peat mosses, along with rushes, sedges and horse-tails. Peat deposits may be tens of metres thick and cover a large area. Peat deposits form in areas of high rainfall, usually under temperate to cold climatic conditions. Peat deposits have long been used (particularly in Northern Europe) as a source of fuel.

Oil/petroleum

Natural deposits of oil are most commonly found associated with natural gas (which is itself derived from the heating-up of the oil), salt water, and sometimes, solid hydrocarbons. The origin of natural oil is still disputed but most geologists believe that it is derived from organic material by decay.

Most natural accumulations of oil appear to have formed under marine conditions (i.e. marine basins) though some has also formed in estuarine and deltaic environments. Natural accumulations of oil are rarely found where they initially formed because the oil is a fluid and migrates easily through any openings in rocks such as pores in sedimentary rocks, joint planes, cleavage planes, bedding planes and fractures. During migration, the oil-water system breaks-up with the oil floating on the water. Unless this migration of the oil is prevented it will flow to the surface with the liquid parts evaporating, leaving behind the solid hydrocarbons such as waxes and asphalts.

For oil to be exploitable, it must be trapped by an impermeable geological barrier. There are two main types of such traps, structural traps (such as folds or faults) and stratigraphic/lithological traps (such as salt domes, overlying impermeable sedimentary layers). The oil-bearing strata itself has to be pervious or porous so that the oil can flow freely through it. Oil usually contains many different hydrocarbons that have differing boiling temperatures. Those that have the lowest boiling temperature become accumulations of natural gas which, being lighter than the oil, forms a gas-cap on top of the oil accumulation. The pressure of this gas cap on the overlying strata and the upward pressure of any underlying water below the oil layer are used to drive the oil towards the surface without the need for pumping.

  • Kerogen: a solid organic material that yields petroleum-type hydrocarbons on heating and distillation.
  • Tar pits: small areas where soft asphaltic tar wells up to the surface and fills a hollow.
  • Oil shale: this is a fine-grained black or dark grey clay-rich sedimentary rock that yields liquid hydrocarbons upon distillation (i.e. heating). Most oil shale deposits are associated with coal. Though oil shales were exploited in the early parts of the twentieth century, they are now uneconomic because of the low price of petroleum and the high cost of processing.

Residual deposits

Residual deposits are formed in tropical regions. During the wet season, intense leaching of the rock occurs. Then, during the dry season, the solution containing the leached ions is drawn towards the surface by capillary action where it evaporates leaving behind salts that are washed away in the next wet season. Eventually, the whole zone down to the base of the water table is leached of relatively mobile ions such as sodium, potassium, calcium and magnesium. With leaching under the right pH conditions, silica is also dissolved and removed from the system. The remaining material is usually just iron and aluminium oxides which are concentrated. Many fossil laterites are known and these provide evidence of former tropical environments. Basic and ultrabasic rocks tend to form laterites while granitic rocks lead to the development of bauxite.

In some tropical regions, where the laterite is developed on ultrabasic rocks, it forms important deposits of nickel, usually at the base of the laterite zone. In New Caledonia, these deposits are extensively mined and the main nickel-bearing phases are amorphous nickel silicates. Other areas which contain extensive laterite nickel mineralisation are the Norseman-Wiluna greenstone belt of Western Australia and Central Africa. Some of these laterites also contain elevated levels of the platinum-group elements which are an important by-product of the mining. The problem with these deposits is that the nickel is often very difficult to extract from the laterite.


Ian Graham
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