Cumulate rock

Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating.

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Formation

Cumulate rocks are the typical product of precipitation of magmatic crystal products from a fractionating magma chamber. These accumulations typically occur on the floor of the magma chamber, although they are possible on the roofs if anorthite plagioclase is able to float free of a denser melt.

Cumulates are typically found in ultramafic intrusions, in the base of large ultramafic lava tubes in komatiite and magnesium rich basalt flows and also in some granitic intrusions.

Terminology

Cumulates are named according to their dominant mineralogy and the percentage of crystals to their groundmass.

• Adcumulates are rocks containing ~100-93% accumulated magmatic crystals in a fine grained groundmass.
• Mesocumulates are rocks with between 93-85% accumulated minerals in a groundmass.
• Orthocumulates are rocks containing between 85-75% accumulated minerals in groundmass.

Cumulate rocks are typically named according to the cumulate minerals in order of abundance, and then cumulate type (adcumulate, mesocumulate, orthocumulate), and then accessory or minor phases. For example:

• a layer with 50% plagioclase, 40% pyroxene, 5% olivine and 5% groundmass (in essence a gabbro) would be termed a plagioclase-pyroxene orthocumulate with accessory olivine.
• a rock consisting of 80% olivine, 5% magnetite and 15% groundmass is an olivine mesocumulate, (in essence a peridotite).

Cumulate terminology is appropriate for use when describing cumulate rocks. In intrusions which have a uniform composition and minimal textural and mineralogical layering or visible crystal accumulations it is inappropriate to describe them according to this convention. This is particularly true if a gabbro is in fact a gabbro.

Geochemistry

Cumulate rocks, because they are fractionates of a parental magma, should not be used to infer the composition of a magma from which they are formed. For instance, a magma of basalt composition that is precipitating cumulates of plagioclase plus pyroxene is changing composition by the removal of the precipitated minerals. The rock that is made of the accumulated minerals will not have the same composition as the magma: such cumulate rocks can have plagioclase/(plagioclase plus pyroxene) anywhere in the range from zero to one.

One way to infer the composition of the magma that created the cumulate rocks is to measure groundmass chemistry, but that chemistry is problematic or impossible to sample. Otherwise, complex calculations of averaging cumulate layers must be required, which is a complex process. Alternatively, the magma composition can be estimated by assuming certain conditions of magma chemistry and testing them on phase diagrams using measured mineral chemistry. These methods work fairly well for cumulates formed in volcanic conditions (ie; komatiites). Investigating magma conditions of large layered ultramafic intrusions is more fraught with problems.

These methods have their drawbacks, primarily that they must all make certain assumptions which rarely hold true in nature. The foremost problem is the fact that in large ultramafic intrusions, assimilation of wall rocks tends to alter the chemistry of the melt as time progresses, so measuring groundmass compositions may fall short. Mass balance calculations will show deviations from expected ranges, which may infer assimilation has occurred, but then further chemistry must be embarked upon to quantify these findings.

Secondly, large ultramafic intrusions are rarely sealed systems and may be subject to regular injections of fresh, primitive magma, or to loss of volume due to further upward migration of the magma (possibly to feed volcanic vents or dyke swarms). In such cases, calculating magma chemistries may resolve nothing more than the presence of these two processes having affected the intrusion.

Economic importance

The economic importance of cumulate rocks is best represented by three classes of mineral deposits found in ultramafic to mafic layered intrusions.

• Silicate mineral cumulates
• Oxide mineral cumulates
• Sulfide melt cumulates

Silicate mineral cumulates

Silicate minerals are rarely sufficiently valuable to warrant extraction as ore. However, some anorthosite intrusions contain such pure anorthite concentrations that they are mined for feldspar, for use in refractories, glassmaking and other sundry uses (toothpaste, cosmetics, etcetera).

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. When 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.

Sulfide mineral segregations

Sulfide mineral cumulates in layered intrusions are an important source of nickel, copper, platinum group elements and cobalt. Deposits of a mixed massive or mixed sulphide-silicate 'matric' of pentlandite, chalcopyrite, pyrrhotite and/or pyrite are formed, occasionally with cobaltite and platinum-tellurium sulphides. 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.

See also

• Igneous differentiation
• Igneous rocks
• Ultramafic to mafic layered intrusions
• Ultramafic rocks
• List of rock textures
• Komatiite

References

• Blatt, Harvey and Robert J. Tracy, 1996, Petrology: Igneous, Sedimentary and Metamorphic, 2nd ed., pp. 123-132 & 194-197, Freeman, ISBN 0-7167-2438-3
• Ballhaus, C.G. & Glikson, A.Y., 1995, Petrology of layered mafic-ultramafic intrusions of the Giles Complex, western Musgrave Block, central Australia. AGSO Journal, 16/1&2: 69-90.