Ore forming processes
Magmatic is when liquid magna cools and solidifies to form igneous rock, it forms ore of metals, gems and other precious resources.
Magma of the mafic nature solidifies in layers. Every layer has different mineralization. Some layers may have rich deposits. There are three type of processes which produces layered plutons
- As magma solidifies (plutonic) its temperature gradually decreases. Minerals crystalize first in the liquid magma. Some crystals are denser than the magma which will settle down and is concentrated at the bottom which is also called crystal settling.
- In the mafic magma crystallization usually starts from the bottom so early forming minerals are concentrated at the base.
- When these processes occur and form minerals, at some time different magma can enter these mineraliztion which can change the early minerals formed.
Kimberlites are igneous rocks that are produced in the mantle and are the main diamonds source.
Volcanic vent deposits
Volcanic vent deposits are from the gases escaping from the volcano where it precipitates such as sulphur. Sulphur usually deposits as a pure yellow deposition from the escaping gases and are used in sulphuric acid manufacturing.
Hydrothermal process is the most common ore forming process. Water is heated by the magma chamber and this water dissolves metals. Metals are dissolved in the water and this solution seeps through cracks, fractures and through permeable rocks until they are precipitated and form a deposit. There are three types of water sources
- Magmas of granitic composition when solidifies leaves a water-rich residual fluid which precipitates and form ore body.
- In active volcanic activities when ground water seeps down through the crust, these magma chamber heats the water which provides the hydrothermal solution. This hydrothermal solution is at shallow depths and its precipitation makes an ore.
- Sea water is heated when it seeps along cracks in the oceanic crust mostly at the mid oceanic ridges and submarine volcanoes.
There are several types of hydrothermal mineralization
- Hydrothermal vein deposits are formed when these hydrothermal solutions enter a country rock along cracks and fractures. It precipitates in the parent rock in a vein like structure.
- The hydrothermal solution in vein deposits can also soak through the country rock which for disseminated ore deposits. This is less concentrated but as formed with the vein together they form economical deposit.
- Disseminated copper depoits are associated with porphyry copper deposits. This depoist is associated with granitic to dioritic composition.
Sedimentary process includes sedimentary sorting and precipitation.
Sedimentary sorting is the settling down of heavy minerals first and later of the less denser particles. example of such sorting is settling of gold in a stream when water slows down. As streams flows it carries clay, sand and gold particles. Gold settles down and conceive in the bedrock into coarser sediments. Gold concentrates in the bedrock and coarser gravel and is placer deposits.
Water seeping through the soil and bedrock dissolve ions which then flows into streams and oceans providing sodium and chloride ions which makes water salty. these can deposit only by the evaporation process which is not possible in the ocean however, lakes developed in the deserts evaporates and evaporites deposits are formed by precipitating crystals.
Environments of high rainfall, water dissolves most soluble ions forms the soil and exposed surface rocks where insoluble ions are left as a residual. Iron and aluminium have less solubility so these ions are left behind which forms the bauxite. Bauxite is the principal aluminium providing residual but in come cases iron can also accumulate enough to be economical deposit. Weathering process can also provide metals to ores which are produced by other processes. Disseminated deposits contain metals but are not economical to mine so ground water and rainfall weathers these metals. In some cases metals react to weathering instead of removal which creates a supergene ore rich in metals above low grade mineralization.