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Lamprophyres () are uncommon, small-volume ultrapotassic igneous rocks primarily occurring as dikes, , , stocks, and small intrusions. They are alkali metal silica-solid solution mafic or ultramafic rocks with high magnesium oxide, >3% potassium oxide, high sodium oxide, and high nickel and chromium.
Lamprophyres occur throughout all geologic eras. Archean examples are commonly associated with lode gold deposits. Cenozoic examples include magnesian rocks in Mexico and South America, and young ultramafic lamprophyres from Gympie in Australia with 18.5% MgO at ~250 Ma.
Lamprophyres are not amenable to classification according to modal proportions, such as the system QAPF diagram, because of their peculiar mineralogy, nor compositional discrimination diagrams, such as TAS, because of their peculiar geochemistry. They are classified under the IUGS Nomenclature for Igneous Rocks (Le Maitre et al., 1989) separately; this is primarily because they are rare, have peculiar mineralogy and do not fit classical classification schemes. For example, the TAS scheme is inappropriate due to the control of mineralogy by potassium, not by calcium or sodium.
Mitchell has suggested that rocks belonging to the "lamprophyre facies" are characterized by the presence of phenocrysts of mica and/or amphibole together with lesser clinopyroxene and/or melilite set in a groundmass which may consist (either singly or in various combinations) of plagioclase, alkali feldspar, , carbonate, monticellite, melilite, mica, amphibole, pyroxene, perovskite, Fe-Ti oxides and glass.
Classification schemes which include genetic information may be required to properly describe lamprophyres (Tappe et al., 2005).
Mitchell considered the lamprophyres as a "facies" of igneous rocks created by a set of conditions (generally; late, highly volatile differentiates of other rock types). Either scheme may apply to some, but not all, occurrences and variations of the broader group of rocks known as lamprophyres and melilitic rocks.
Leaving aside complex petrogenetic arguments, the essential components in lamprophyre genesis are:
Individual examples thus may have a wide variety of mineralogy and mechanisms for formation. Rock considered lamprophyres to be derived from deep, volatile-driven melting in a subduction zone setting. Others such as Mitchell consider them to be late offshoots of plutons, etc., though this can be difficult to reconcile with their primitive melt chemistry and mineralogy.
Biotite (usually phlogopite) and amphibole (usually pargasite or other magnesian hornblende) are panidiomorphic; all are euhedral, well formed. Feldspar is restricted to the ground mass. In many lamprophyres the pale quartz and felspathic ingredients tend to occur in rounded spots, or ocelli, in which there has been progressive crystallization from the margins towards the center. These spots may consist of radiate or brush-like feldspars (with some phlogopite and hornblende) or of quartz and feldspar. A central area of quartz or of analcite probably represents an original miarolitic cavity infilled at a later period.
The presence or absence of the four dominant minerals, orthoclase, plagioclase, biotite and hornblende, determines the species:
Each variety of lamprophyre may and often does contain all four minerals but is named according to the two which predominate.
These rocks contain also (usually titaniferous), apatite, sometimes sphene, augite, and olivine. The hornblende and biotite are brown or greenish-brown, and as a rule their crystals even when small are very perfect and give the thin section views an easily recognizable character. Green hornblende occurs in some of these rocks. Augite exists as euhedral crystals of pale green color, often zonal and readily weathering. Olivine in the fresh state is rare; it forms rounded, corroded grains; in many cases it is decomposed to green or colorless hornblende in radiating nests (pilite). The plagioclase occurs as small rectangular crystals; orthoclase may have similar shapes or may be fibrous and grouped in sheaf-like aggregates that are narrow in the middle and spread out towards both ends. As all lamprophyres are prone to alteration by weathering a great abundance of secondary minerals is usually found in them; the principal are calcite and other , limonite, Chlorite group, quartz and kaolin.
Ocellar structure is common; the ocelli consist mainly of orthoclase and quartz, and may be up to one quarter of an inch in diameter. Another feature of these rocks is the presence of large foreign crystals, or , of feldspar and of quartz. Their forms are rounded, indicating partial resorption and the quartz may be surrounded by corrosion borders of minerals such as augite and hornblende produced where the magma is attacking the crystal.
Lamprophyres (including minette) traditionally have been defined as:Le Maitre, R. W., editor, 2002, Igneous Rocks: A Classification and Glossary of Terms. Recommendations of the International Union of Geological Sciences, Subcommission of the Systematics of Igneous Rocks. Cambridge University Press.
On a purely chemical basis, an extrusive lamprophyre (sp. minette) might be classified as potassic trachybasalt, shoshonite, or latite using the total alkali-silica diagram (see TAS classification), or as absarokite, shoshonite, or banakite using a classification sometimes applied to potassium-rich lavas. Such chemical classifications ignore the distinctive textures and mineralogies of lamprophyres.
Modern nomenclature has been derived from an attempt to constrain some genetic parameters of lamprophyre genesis.Mitchell, R.H., 1994b. Suggestions for revisions to the terminology of kimberlites and lamprophyres from a genetic viewpoint. In Proc. Fifth Int. Kimberlite Conf. 1. Kimberlites and Related Rocks and Mantle Xenoliths (H.O.A. Meyer & O.H. Leonardos, eds.). Companhia de Pesquisa de Recursos Minerais (Brasilia), Spec. Publ. 1/A, 15-26. This has, by and large, dispensed with the previous provincial names of lamprophyre species, in favor of a mineralogical name. The old names are still used for convenience.
Examples include minettes in the Navajo Volcanic Field (e.g. dikes near Shiprock and Mitten Rock, NM) of the Colorado PlateauRoden, M. F. and Smith, D., 1979, Field geology, chemistry, and petrology of Buell Park minette diatreme, Apache County, Arizona: In Kimberlites, Diatremes, and Diamonds: Their Geology, Petrology, and Geochemistry, Boyd, F. R., and Meyer, H. O. A., eds., American Geophysical Union: Proceedings of the Second International Kimberlite Conference, v 1, pp. 364–381. and in the Mexican Volcanic Belt.Wallace, P., and Carmichael, I. S. E., 1989, Minette lavas and associated leucitites from the Western Front of the Mexican Volcanic Belt: petrology, chemistry, and origin. Contributions to Mineralogy and Petrology, v 103, pp. 470–492.
Lamprophyres are also known to be spatially and temporally associated with gold Ore genesis, for example Orogeny gold deposits.Müller D., Groves D.I. (2019) Potassic igneous rocks and associated gold-copper mineralization (5th edition). Mineral Resource Reviews. Springer-Verlag Heidelberg, 398 pp Rock (1991) considered lamprophyres to be possible source rocks for the gold, but this view is not generally supported. The more reasonable explanation for the correlation is that lamprophyres, representing "wet" melts of the asthenosphere and mantle, correlate with a period of high fluid flow from the mantle through the crust, during subduction-related metamorphism, which drives gold mineralisation.
Non-melilitic lamprophyres are found in many districts where granites and diorites occur, such as the Scottish Highlands and Southern Uplands of Scotland;Thorpe R.S., Gaskarth J.W. & Henney P.J., 1993. Composite Ordovician lamprophyre (spessartite) intrusions around the Midlands Microcraton in central Britain. Geological Magazine, volume 130, pp. 657-663, 1993.Rock, N.M.S, Gaskarth J.W., Rundle C.C., 1986. Late Caledonian dyke-swarms in southern Scotland: A regional zone of primitive K-rich Lamprophyres and associated vents. Journal of Geology, volume 94, pp. 505-522, 1986. the Lake District of northwest England; Ireland; the Vosges Mountains of France; the Black Forest and Harz mountain regions of Germany; Mascota; Jamaica and in certain locations of British Columbia, Canada.Adams , M., Lentz, D.R., Shaw, C., Williams, P., Archibald, D.A., Cousens, B., 2005. Eocene Lamprophyre Dykes intruding the Monashee Complex, B.C.: Petrochemical to Petrogenetic Relationships with the Kamloops Group Volcanic Sequence. Canadian Journal of Earth Sciences, v. 42, p. 11-24.
(2025). 9783540279464, Springer, Berlin, Heidelberg. ISBN 9783540279464
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