GEOLOGICAL ORE DEPOSITS

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GEOLOGICAL ORE DEPOSITS by Mind Map: GEOLOGICAL ORE DEPOSITS

1. Clay deposits

1.1. They form by settling of clay particles in sedimentary basins or through intense weathering of volcanic and granitic rocks.

1.2. The most common clay minerals mined are:

1.2.1. Kaolinite.

1.2.2. Illite.

1.2.3. Montmorillonite.

2. Gold deposits

2.1. Archaean gold deposits

2.1.1. The formation of these deposits is linked to regional-scale processes at mid-crustal levels during regional metamorphism.

2.1.1.1. These deposits account for more than 60% of the world's gold. Over 60% of this comes from the Witwatersrand of South Africa and the remainder from Archaean lode gold deposits in Australia, Canada, southern Africa and South America.

2.2. Epithermal gold deposits

2.2.1. They are volcanic-hosted deposits that are generally relatively recent in age. Generally they are small but rich deposits which consist of vein systems and bonanza ores.

2.2.1.1. The main ore minerals are:

2.2.1.1.1. Native gold and silver.

2.2.1.1.2. Electrum.

2.2.1.1.3. Acanthite and tetrahedrite.

2.2.1.1.4. Silica occurring as quartz, amethyst, opal, chalcedony and cristobalite.

3. Main types

3.1. Metallic deposits.

3.2. Non-metallic deposits.

3.3. Fossil fuel deposits.

4. Placer deposits

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

4.1.1. Alluvial: Detrital material which is transported by a river and usually deposited along the river's pathway.

4.1.2. Colluvial: Weathered material transported by gravity action such as on scree slopes.

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

4.2. The most common placer deposits are those of:

4.2.1. Gold.

4.2.2. Platinum.

4.2.3. Group minerals.

4.2.4. Gemstones.

4.2.5. Tin.

4.2.6. Rutile.

4.2.7. Monazite.

4.2.8. Zircon.

4.3. 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.

5. Sedimentary iron ore and manganese deposits.

5.1. These deposits are very large in size.

5.2. The iron and manganese ores occur as distinct sedimentary layers alternating with iron and manganese-poor sedimentary layers.

5.3. Most of the world's iron and manganese are derived from deposits of this type.

5.4. Iron ore minerals found in these deposits:

5.4.1. Hematite.

5.4.2. Lepidocrocite.

5.4.3. Goethite.

5.5. Manganese ore minerals found in these deposits:

5.5.1. Braunite.

5.5.2. Manganite.

5.5.3. Hausmannite.

6. Oxidised zones

6.1. It's the region above the water-table in an ore.

6.2. It's primarily composed of mixtures of iron oxides/hydroxides and quartz.

6.3. The most common minerals found in oxidised zones are:

6.3.1. Copper: malachite, azurite, chrysocolla.

6.3.2. Gangue minerals: quartz (usually cryptocrystalline), baryte, calcite, aragonite.

6.3.3. Iron: goethite, hematite.

6.3.4. Lead: anglesite, cerussite.

6.3.5. Manganese: pyrolusite, romanechite, rhodochrosite.

6.3.6. Nickel: gaspeite, garnierite.

6.3.7. Silver: native silver, chlorargyrite.

6.3.8. Zinc: smithsonite.

6.4. Below the oxidised zone there's a zone known as the supergene zone.

6.4.1. Here the metals are deposited by fluids and concentrating in a narrow band just below the water table.

6.4.2. The supergene zone is the richest part of an ore deposit but it tends to be very tin or not developed at all.

6.4.3. The most common minerals found in supergene zones are:

6.4.3.1. Copper: chalcocite, bornite.

6.4.3.2. Lead: supergene galena.

6.4.3.3. Nickel: violarite.

6.4.3.4. Silver: acanthite, native silver.

6.4.3.5. Zinc: supergene sphalerite, wurtzite.

7. Mineral deposits.

7.1. Classification

7.1.1. Geologists classify mineral deposits according to:

7.1.1.1. The commodity being mined.

7.1.1.2. The tectonic setting in which the deposit occurs.

7.1.1.3. The geological setting of the mineral deposit.

7.1.1.4. The genetic model for the origin of the ore deposit.

7.2. Types

7.2.1. Orthomagmatic deposits

7.2.1.1. They form from primary magmatic processes. They are hosted in the igneous rocks in which they have formed.

7.2.1.1.1. Orthomagmatic deposits include:

7.2.2. Pneumatolytic and pegmatitic deposits

7.2.2.1. They are formed from volatile-rich high-temperature fluids emanating from igneous intrusions.

7.2.2.1.1. These depostis include:

7.2.3. Hydrothermal deposits

7.2.3.1. They form from hot circulating water-rich fluids.

7.2.3.1.1. These include the two main types of gold deposits: epithermal and lode gold deposits along with replacement deposits in calcareous sequences, base-metal vein deposits and replacement skarn deposits.

7.2.4. Volcanic or extrusive deposits

7.2.4.1. They're associated with volcanic processes and are only found within the volcanic rocks themselves.

7.2.4.1.1. Important deposits of gold, mercury, antimony, copper and base metals are of this type.

8. Evaporite deposits

8.1. An evaporite is a sediment that forms through the evaporation of saline water.

8.2. The most common evaporite deposits are:

8.2.1. Salts.

8.2.2. Gypsum.

8.2.3. Nitrates.

9. Phosphatic deposits

9.1. Phosphorite is a commonly used term for lithified phosphate rock.

9.2. 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

9.3. The island of Nauru in the Pacific is one of the world's largest deposits of phosphorite.

9.4. Phosphate deposits are of three main types:

9.4.1. Primary marine phosphate deposits:

9.4.1.1. These deposits are rare and usually arise from:

9.4.1.1.1. The leaching of the phosphatic limestone.

9.4.1.1.2. Extraction of phosphate at higher levels followed by secondary concentration from downward-percolating groundwaters.

9.4.1.2. These deposits occur under relatively cool conditions in an oxygen-free environment.

9.4.2. Bone beds:

9.4.2.1. These are localized accumulations of fossil deposits of bone, teeth, scales and excreta that are occasionally thick enough to form deposits.

9.4.3. Guano:

9.4.3.1. These are ancient and/or fossil deposits of bird or bat excreta that need a dry climate for their preservation.

9.4.3.2. Guano deposits from birds are most commonly found on oceanic islands.

9.4.3.3. Guano deposits from bats are found in large cave systems.

10. Organic deposits

10.1. Coal

10.1.1. It's stratified accumulations of carbonaceous material derived from vegetation.

10.1.2. Combined processes of compaction and slight heating converts this organic material to black coal.

10.1.3. There are several stages in this process that occur with increasing heat and compaction:

10.1.3.1. Peat.

10.1.3.2. Lignite.

10.1.3.3. Sub-bituminous coal.

10.1.3.4. Bituminous coal.

10.1.3.5. Sub-anthracite.

10.1.3.6. Anthracite.

10.1.3.7. This sequence shows a progressive increase in carbon content with corresponding decrease in volatile content.

10.2. Fireclay

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

10.3. Peat

10.3.1. It's a partially decomposed mass of vegetation that has grown in a shallow lake or marsh.

10.3.2. It contains recognisable vegetable material but very little mineral material.

10.3.3. Peat deposits have long been used as a source of fuel.

10.4. Oil/petroleum

10.4.1. Natural deposits of oil are most commonly found associated with natural gas, salt water, and sometimes, solid hydrocarbons.

10.4.2. Most geologists believe that the oil is derived from organic material by decay.

10.4.3. Most natural accumulations of oil appear to have formed under marine conditions.

10.4.4. For oil to be exploitable, it must be trapped by an impermeable geological barrier.

10.5. Residual deposits

10.5.1. Residual deposits are formed in tropical regions:

10.5.1.1. During the wet season, intense leaching of the rock occurs.

10.5.1.2. 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.

10.5.1.3. 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.

10.5.1.4. The remaining material is usually just iron and aluminium oxides which are concentrated.