Ore genesis

How the various types of mineral deposits form within the Earth's crust

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.

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.

Metal sources may include a plethora of rocks. However most metals of economic importance are carried as trace elements within rock-forming minerals, and so may be liberated by hydrothermal processes. This happens because of:

Metal can also precipitate when temperature and pressure or oxidation state favour different ionic complexes in the water, for instance the change from sulfide to sulfate, oxygen fugacity, exchange of metals between sulfide and chloride complexes, et cetera.

Metamorphic processes also control many physical processes which form the source of hydrothermal fluids, outlined above.

Ore deposits are usually classified by ore formation processes and geological setting. For example, sedimentary exhalative deposits (SEDEX), are a class of ore deposit formed on the sea floor (sedimentary) by exhalation of brines into seawater (exhalative), causing chemical precipitation of ore minerals when the brine cools, mixes with sea water, and loses its metal carrying capacity.

As they require the conjunction of specific environmental conditions to form, particular mineral deposit types tend to occupy specific geodynamic niches,

Placer deposits are often worked by fossicking, and panning for gold is a popular pastime.

Sulfide phases only form in ultramafic magmas when the magma reaches sulfur saturation. This is generally thought to be nearly impossible by pure fractional crystallisation, so other processes are usually required in ore genesis models to explain sulfur saturation. These include contamination of the magma with crustal material, especially sulfur-rich wall-rocks or sediments; magma mixing; volatile gain or loss.

Nickel deposits are generally found in two forms, either as sulfide or laterite.

Komatiitic nickel-copper sulfide deposits are considered to be formed by a mixture of sulfide segregation, immiscibility, and thermal erosion of sulfidic sediments. The sediments are considered to be necessary to promote sulfur saturation.

Some subvolcanic sills in the Thompson Belt of Canada host nickel sulfide deposits formed by deposition of sulfides near the feeder vent. Sulfide was accumulated near the vent due to the loss of magma velocity at the vent interface. The massive Voisey's Bay nickel deposit is considered to have formed via a similar process.

Copper is found in association with many other metals and deposit styles. Commonly, copper is either formed within sedimentary rocks, or associated with igneous rocks.

Greisen granite is another related tin-molybdenum and topaz mineralisation style.

Carbonatite intrusions are an important source of these elements. Ore minerals are essentially part of the unusual mineralogy of carbonatite.