ORE GENESIS: Process by which a mineral deposit forms. Metalliferous mineral deposits may be syngenetic (formed at the same time as the host rocks) or epigenetic (deposited later than the host rocks). Deposits may be classified according to their processes of formation into Igneous, sedimentary, metamorphic.
Ore genesis theories generally involve three components:
1 source,
2 transport or conduit
3 trap.
Source is required because metal must come from somewhere, and be liberated by some process
Transport is required first to move the metal bearing fluids or solid minerals into the right position, and refers to the act of physically moving the metal, as well as chemical or physical phenomenon which encourage movement
Trapping is required to concentrate the metal via some physical, chemical or geological mechanism into a concentration which forms mineable ore.
Ore genesis processes
Magmatic mineral deposits concentrated in igneous rocks;
Residual mineral deposits formed by weathering reactions at the earth’s surface.
Sedimentary mineral deposits are precipitated from a solution, typically sea water;
Metamorphic mineral deposits form in association with metamorphism
Hydrothermal mineral deposits form in association with magma and water;
Magmatic processes
Fractional Crystallisation: separates ore and non-ore minerals according to their crystallization temperature. As early crystallizing minerals form, they incorporate certain elements, some of which are metals. These crystals may settle onto the bottom of the intrusion, concentrating ore minerals there. Chromite and magnetite are ore minerals that form in this way.
Liquid immisibility: sulfide ores containing copper, nickel or platinum may form from this process. As a magma changes, parts of it may separate from the main body of magma. Two liquids that will not mix are called immiscible; oil and water are an example. In magmas, sulfides may separate and sink below the silicate-rich part of the intrusion or be injected into the rock surrounding it. These deposits are found in mafic and ultramafic rocks
HYDROTHERMAL PROCESS:
This name is given to any process associated with igneous activity which involves heated or superheated water. Water at a very high temperature is an exceedingly active substances capable of breaking down the silicate and dissolving many other substances which normally thought of as insoluble. Many ore minerals are deposited from hydrothermal solution such as Pb,Zn and Cu
These deposit may fill fissures or may replace the existing rocks. The temperature and pressure at which a hydrothermal ore body is deposited controls the form and mineralogy of the deposits.
1.Hypothermal deposits: These are formed at high Temperatures and Pressure along the fissures and other opening in the earth. Mineralogically quartz is abundant other gangue minerals include garnet, amphibole, pyroxene mica and felspar.
E.g: 1. gold deposits in kolar, Karnataka
3.Epithermal Deposits: These occur in Igneous and sedimentary rocks at shallow depth and are common in regions of late igneous activity where the rocks have not been deeply eroded. They are all of the fissure filling type of deposits like Pb, Zn,Ag,Hg and Au.
4.Telethermal deposits:These are formed from cold meteoric waters remotely connected with igneous bodies and the principle minerals are sulphides of Pb,Zn,Cu and Fe.
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