Layered intrusion

Orebodies of Nickel-Copper-Platinum group elements (Ni-Cu-PGE), chromite, magnetite, and ilmenite are often associated with base metal Sulfide mineral assemblages within these rare intrusions.

For instance, in most Archaean cratons, greenstone belts correlate with voluminous dike injections as well as usually some form of larger intrusive episodes into the crust.

The problem of creating space for such intrusions is easily explained by the extensional tectonics in operation; extensional or listric faults operating at depth can provide a triangular space for keel-shaped or boat-shaped intrusions such as the Great Dyke of Zimbabwe, or the Narndee-Windimurra Complex of Western Australia.

It is also possible that what we see as a cratonic margin today were created by the action of a plume event initiating a continental rifting episode; therefore the tectonic setting of most large layered complexes must be carefully weighed in terms of geochemistry and the nature of the host sequence, and in some cases a mixed mechanism cause is possible.

The causes of layering in large ultramafic intrusions include convection, thermal diffusion, settling of phenocrysts, assimilation of wall rocks and fractional crystallization.

This will tend to increase the silica content of the melt, which will eventually prompt a mineral to reach the liquidus for that magma composition.

Chromite layers are associated with platinum-palladium group element (PGE) deposits, the most famous of these being the Merensky Reef in the Bushveld Igneous Complex.

[10][11][12][13] Layered intrusions have potential to be economically significant for the occurrence of Nickel-Copper-Platinum group element (Ni-Cu-PGE), Chromitite, and Ilmenite (Fe-Ti oxide) Ore deposits.

[8][6] The standard magmatic sulfide assemblage is composed of Pyrrhotite, Pentlandite, and Chalcopyrite, with lesser to trace amounts of Pyrite, Cubanite and magnetite.

[15] In local portions of the intrusive suite or in systems lacking chromium, it may occur as chromitite clasts associated with base metal magmatic sulfides.

[5] Similar to chromium occurrences, iron and titanium rich systems may form discrete cumulate layers composed primarily of magnetite and ilmenite.

Chromitite and anorthosite layered igneous rocks in Critical Zone UG1 of the Bushveld Igneous Complex at the Mononono River outcrop, near Steelpoort, South Africa
Map displaying the locations of intrusions hosting reef-type PGE and contact-type NI-Cu-PGE deposits. Image courtesy of the U.S. Geological Survey.
Cut drill core displaying massive sulfide within an ultramafic rock. The massive sulfide is primarily composed of pyrrhotite with trace amounts of chalcopyrite and pentlandite.