Economic importance
The economic importance of cumulate rocks is best represented by two classes of mineral deposits found in ultramafic to mafic layered intrusions.
* Oxide mineral cumulates
* Sulfide melt cumulates
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Oxide mineral cumulates
Oxide mineral cumulates form in layered intrusions when fractional crystallisation has progressed enough to allow the crystallisation of oxide minerals which are invariably a form of spinel. This can happen due to fractional enrichment of the melt in iron, titanium or chromium.
These conditions are created by the high-temperature fractionation of highly magnesian olivine and/or pyroxene, which causes a relative iron-enrichment in the residual melt. Then the iron content of the melt is sufficiently high enough, magnetite or ilmenite crystallise and, due to their high density, form cumulate rocks. Chromite is generally formed during pyroxene fractionation at low pressures, where chromium is rejected from the pyroxene crystals.
These oxide layers form laterally continuous deposits of rocks containing in excess of 50% oxide minerals. When oxide minerals exceed 90% of the bulk of the interval the rock may be classified according to the oxide mineral, for example magnetitite, ilmenitite or chromitite. Strictly speaking, these would be magnetite orthocumulate, ilmenite orthocumulate and chromite orthocumulates.
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Sulfide mineral segregations
Sulfide mineral cumulates in layered intrusions are an important source of nickel, copper, platinum group elements and cobalt. These deposits are formed by melt immiscibility between sulfide and silicate melts in a sulfur-saturated magma.
They are not strictly a cumulate rock, as the sulfide is not precipitated as a solid mineral, but rather as immiscible sulfide liquid. However, they are formed by the same processes and accumulate due to their high specific gravity, and can form laterally extensive sulfide 'reefs'. The sulfide minerals generally form an interstitial matrix to a silicate cumulate.
Sulfide mineral segregations can only be formed when a magma attains sulfur saturation. In mafic and ultramafic rocks they form economic Ni, Cu and PGe deposits because these elements are chalcophile and are strongly partitioned into the sulfide melt. In rare cases, felsic rocks become sulfur saturated and form sulfide segregations. In this case, the typical result is a disseminated form of sulfide mineral, usually a mixture of pyrrhotite, pyrite and chalcopyrite, forming Cu mineralisation. It is very rare but not unknown to see cumulate sulfide rocks in granitic intrusions.
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