Lamprophyre
Mafic to ultramafic porphyritic dike rock (mica/amphibole phenocrysts)
A dark, mineral-rich dike rock with abundant mica or amphibole phenocrysts set in a fine groundmass, often associated with gold and diamonds.
- Mohs hardness
- 5-6
- Color
- Dark grey to greenish-black
- Type
- igneous
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Overview
Lamprophyre is a group of dark, porphyritic igneous rocks that typically occur as dikes and small intrusions. They are defined by abundant phenocrysts of mafic minerals (especially biotite/phlogopite mica and amphibole, sometimes pyroxene or olivine) set in a fine-grained groundmass, with feldspar or feldspathoid largely confined to the groundmass.
The group includes named varieties such as minette, kersantite, vogesite, and spessartite, distinguished by their mafic and feldspar mineralogy. Lamprophyres are unusually rich in potassium, water, and incompatible elements.
They are geologically significant because they often signal deep mantle melting and are spatially linked to gold and diamond deposits.
Formation & geology
Lamprophyres form from volatile-rich, mantle-derived magmas that ascend rapidly along fractures, emplacing as dikes, sills, and small plugs rather than large bodies. Their high water and potassium contents reflect melting of enriched or metasomatized mantle, sometimes related to subduction or rifting.
Because they tap deep sources and rise quickly, lamprophyres are often associated with other deep magmas such as kimberlite and lamproite, and they commonly intrude orogenic gold districts, making them useful exploration indicators.
How to identify it
Look for a dark grey-to-greenish-black, heavy rock showing a porphyritic texture: shiny black mica flakes or stubby amphibole crystals visibly set in a fine dark matrix, with no feldspar phenocrysts. Carbonate-filled amygdules (gas cavities) are common.
Hardness is roughly 5-6. Distinguish lamprophyre from basalt by its abundant mica/amphibole phenocrysts and lack of feldspar phenocrysts, and from kimberlite and lamproite by mineral details that usually require thin-section study. Its occurrence as narrow dikes is a strong contextual clue.
Uses & significance
Lamprophyre has little direct commercial use, though it is occasionally crushed for aggregate. Its main importance is scientific and economic as an exploration guide: lamprophyre dikes frequently accompany orogenic gold deposits and, like kimberlite, can be associated with diamonds.
It carries no gemstone or metaphysical tradition. For geologists, lamprophyres are valuable samples of volatile-rich, deep mantle melts.
Frequently asked questions
What defines a lamprophyre?
Abundant mafic phenocrysts of mica or amphibole in a fine groundmass, with feldspar restricted to the groundmass, and high potassium and water content.
What are the main types of lamprophyre?
Common varieties include minette, kersantite, vogesite, and spessartite, named for their mica, amphibole, and feldspar mineralogy.
Why do geologists care about lamprophyres?
They are deep mantle-derived magmas often associated with gold deposits and sometimes diamonds, making them useful exploration indicators.
How is lamprophyre different from basalt?
Lamprophyre has prominent mica or amphibole phenocrysts and no feldspar phenocrysts, while basalt is feldspar-bearing and usually lacks such mafic megacrysts.
Lamprophyre guides
In-depth guides for identifying, valuing, and understanding Lamprophyre.
Lamprophyre identified by the community
Real specimens identified with Rock Identifier.
