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Concept
Ore genesis
Résumé
Various theories of ore genesis explain how the various types of mineral deposits form within Earth's crust. Ore-genesis theories vary depending on the mineral or commodity examined.
Ore-genesis theories generally involve three components: source, transport or conduit, and trap. (This also applies to the petroleum industry: petroleum geologists originated this analysis.)
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 their current position, and refers to the act of physically moving the metal, as well as to chemical or physical phenomena which encourage movement.
Trapping is required to concentrate the metal via some physical, chemical, or geological mechanism into a concentration which forms mineable ore.
The biggest deposits form when the source is large, the transport mechanism is efficient, and the trap is active and ready at the right time.
Fractional crystallization: separates ore and non-ore minerals according to their crystallization temperature. As early crystallizing minerals form from magma, 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 immiscibility: 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.
These processes are the physicochemical phenomena and reactions caused by movement of hydrothermal water within the crust, often as a consequence of magmatic intrusion or tectonic upheavals. The foundations of hydrothermal processes are the source-transport-trap mechanism.
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