Lamprophyre

Lamprophyres (Greek Lampros, "bright", and the terminal part of the word "porphyry", meaning "rocks containing bright porphyritic crystals")

Lamprophyres are uncommon, small volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks and small intrusions. They are alkaline silica-undersaturated, ultramafic rocks with high magnesium oxide, >3% potassium oxide, high sodium oxide and high nickel and chromium.

Lamprophyres occur throughout all geologic eras. Archaean 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.

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Petrology

Modern science treats lamprophyres as a grab-bag term for ultrapotassic mafic igneous rocks which have primary mineralogy consisting of amphibole or biotite , and with feldspar in the groundmass.

Lamprophyres are not amenable to classification according to modal proportions, such as the system QAPF due to 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 ^[1] 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, feldspathoids, carbonate, monticellite, melilite, mica, amphibole, pyroxene, perovskite, Fe-Ti oxides and glass.

Classification schemes which do not fit the IUGS scheme, which include genetic information, may be required to properly describe lamprophyres.

Genesis

Rock ^[2] considered lamprophyres as part of a "clan" of rocks, with similar mineralogy, textures and genesis. Rock considered lamprophyres similarly to lamproites and kimberlites. While modern concepts see orangeites, lamproites and kimberlites as separate, a vast majority of lamprophyres have similar origins to these other rock types.

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, it is fair to say that the essential components in lamprophyre genesis are;

• high depth of melting, which yields more mafic magmas;
• low degrees of partial melting, which yields magmas rich in the alkalis (particularly potassium);
• lithophile element (K, Ba, Cs, Rb) enrichment, high Ni and Cr,
• high potassium and sodium concentrations (silica undersaturation is common)
• some form of volatile enrichment, to provide the biotite (phlogopite) and amphibole (pargasite) mineralogy
• lack of fractional crystallisation (generally; there are exceptions)
• high Mg# (MgO//FeO + Fe₂O₃)

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.

Petrography

Lamprophyres are a group of rocks containing phenocrysts, usually of biotite and amphibole (with bright cleavage surfaces), and pyroxene, but not of feldspar. They are thus distinguished from the porphyries and porphyrites in which the feldspar has crystallized in two generations. They are essentially dike rocks, occurring as dikes and thin sills, and are also found as marginal facies of plutonic intrusions.

They are usually dark in color, owing to the abundance of ferro-magnesian silicates, of relatively high specific gravity and liable to decomposition. For these reasons they have been defined as a melanocrate series (rich in the dark minerals); and they are often accompanied by a complementary leucocrate series (rich in the white minerals feldspar and quartz) such as aplites, porphyries and felsites.

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. Minette contains biotite and orthoclase; kersantite, biotite and plagioclase. Vogesite contains hornblende and orthoclase; spessartite, hornblende and plagioclase. Each variety of lamprophyre may and often does contain all four minerals but is named according to the two which preponderate.

These rocks contain also iron oxides (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 micro-sections an easily recognizable character. Green hornblende occurs in some of these rocks. The augite builds 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 carbonates, limonite, chlorite, quartz and kaolin.

Ocellar structure is common; the ocelli consist mainly of orthoclase and quartz, and may be a quarter-of-an-inch in diameter. Another feature of these rocks is the presence of large foreign crystals, or xenocrysts, 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.

Distribution

Non-melilitic lamprophyres are found in many districts where granites and diorites occur, such as the Scottish Highlands and Southern Uplands^[3], the Lake district, Ireland, the Vosges, Black Forest, Harz, Mascota Mexico, Jamaica, and in certain locations of British Columbia, Canada ^[4].

Lamprophyres are usually associated with voluminous granodiorite intrusive episodes^ . They occur as marginal facies to some granites, though usually as dykes and sills marginal to and crosscutting the granites and diorites^ . In other districts where granites are abundant no rocks of this class are known. It is rare to find only one member of the group present, but minettes, vogesites, kersantites, etc., all appear and there are usually transitional forms.

Lamprophyres are also known to be spatially and temporally associated with gold mineralisation. Rock (1991) considered them possible source rocks, 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.

Above Rocks Lamprophyres, Jamaica

Minettes (biotite-orthoclase) and spessartite (amphibole-orthoclase) lamprophyre dykes are reported from the Above Rocks granodiorite in Jamaica. Jackson et al. (1998)^  report that the volumetrically minor minettes are composed of biotitie-phlogopite and pyroxene phenocrysts in an alkali feldspar rich groundmass. The spessartites contain euhedral TiO₂-rich calcic amphibole and pyroxee phenocrysts and feldspar xenocrysts in a plagioclase-orthoclase, sphene, barite, magnetite groundmass. The lamprophyres have an Mg# of 65 and are olivine and nepheline normative, with extreme LILE (Ba, Rb, Cs), CO₂ and volatile enrichment. Jackson et al. (1998) consider these lamprophyres as indicative of evolution of volatile-enriched magma from a heterogeneous (veined) mantle, which was enriched by fluids driven off a subducted slab. The lamprophyres are classed according to Rock's (1991) Type A association as a back-arc magma suite. The age of the lamprophyres is Cretaceous, around 60 Ma.

Wandagee Lamprophyre Suite, Western Australia

The Wandagee Lamprophyre Suite in Western Australia is host to diamond-bearing picritic monchiquite lamprophyre plugs. There are at least 14 known sills, dykes and carrot-shaped diatremes mostly concealed beneath thin colluvium, and are of Jurassic age (~160 Ma). The formation of these intrusions is considered equivalent to that of the kimberlite and lamproite pipes of the Kimberley region, and they are focused along the Wandagee Fault.

Petrographically they are olivine megacrystic with a groundmass of pale green diopside, chromian spinel, accessory biotite and kaersutite. Heavy mineral indicators include Cr-spinel, pyrope garnet and diamond. Geochemically these rocks are primitive and ultramafic (21-28% MgO, 850-1200ppm Ni, 1500-2200ppm Cr) with low alkalis (NaO & K₂O <2.5%) and low incompatible elements and light REE.

References

1. ^  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.
2. ^  Jackson T.A., Lewis J.F., Scot P.W., Manning P.A.S., 1998. The Petrology of Lamprophyre Dykes in the Above Rocks Granitoid, Jamaica: Evidence of rifting above a subduction zone during the early Tertiary. Caribbean Journal of Science, vol. 34, no. 1-2, pp. 1-11, 1998.
3. ^  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.
4. ^  Rock, N.M.S., 1991. Lamprophyres. Blackie, Glasgow, UK
5. ^ 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, vol. 94, pp. 505-522, 1986.
6. ^  Thorpe R.S., Gaskarth J.W. & Henney P.J., 1993. Composite Ordovician lamprophyre (spessartite) intrusions around the Midlands Microcraton in central Britain. Geology Magazine, vol. 130, pp. 657-663, 1993.