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Quartz veined basalt is a dark, fine-grained extrusive igneous rock characterized by the presence of distinct, lighter-colored veins composed predominantly of quartz. The basalt matrix is typically black to dark gray, often aphanitic (fine-grained) to porphyritic, and may exhibit vesicular or amygdaloidal textures. The quartz veins can vary in thickness from hairline fractures to several centimeters wide, appearing white, clear, or sometimes stained by iron oxides (yellow, brown, red). The contact between the quartz vein and the basalt can be sharp or show evidence of alteration (e.g., chloritization, sericitization) in the basalt adjacent to the vein.
How to Identify
- Color
- Basalt matrix: Dark gray to black. Quartz veins: White, clear, sometimes milky, or stained by iron oxides (yellow, brown, red).
- Luster
- Basalt matrix: Dull to sub-vitreous. Quartz veins: Vitreous (glassy).
- Texture
- Basalt matrix: Aphanitic (fine-grained), sometimes porphyritic, vesicular, or amygdaloidal. Quartz veins: Crystalline, often granular or massive.
- Crystal Form
- Basalt: Microscopic crystals of plagioclase, pyroxene, olivine. Quartz veins: Anhedral to euhedral quartz crystals, often interlocking.
- Cleavage
- Basalt: No distinct cleavage in the rock mass, but constituent minerals (e.g., pyroxene, plagioclase) have cleavage. Quartz: No cleavage (conchoidal fracture).
- Geological Environment
- Volcanic terrains, rift zones, oceanic crust, large igneous provinces, areas subjected to post-magmatic hydrothermal activity, often associated with fault zones or shear zones that act as conduits for hydrothermal fluids.
Key Facts
- Hardness: Basalt matrix: 5-6 (Mohs). Quartz veins: 7 (Mohs).
- Specific Gravity: Basalt: 2.8-3.0 g/cm³. Quartz: 2.65 g/cm³.
- Crystal System: Basalt: Polycrystalline, constituent minerals are typically triclinic (plagioclase), monoclinic (pyroxene), or orthorhombic (olivine). Quartz: Trigonal.
- Color: Dark gray to black (basalt), white to clear (quartz).
- Luster: Dull to sub-vitreous (basalt), vitreous (quartz).
- Transparency: Opaque (basalt), transparent to translucent (quartz).
- Fracture: Conchoidal to uneven (basalt), conchoidal (quartz).
- Cleavage: None in the rock mass; constituent minerals have cleavage. Quartz has no cleavage.
- Composition: Basalt: Predominantly plagioclase feldspar, pyroxene, and often olivine, with minor iron-titanium oxides. Quartz veins: SiO2 (silicon dioxide).
Quick Check
- Color: Dark matrix with white/clear veins
- Luster: Dull to sub-vitreous for basalt, vitreous for quartz
- Streak: White (for quartz), typically no streak for basalt unless weathered
Physical Characteristics
- Crystal Habit: Basalt: Aphanitic to porphyritic, sometimes vesicular. Quartz: Massive, granular, or prismatic crystals within veins.
- Cleavage Type: Basalt: Not applicable to the rock as a whole; constituent minerals have varying cleavage. Quartz: None.
- Fracture Type: Basalt: Uneven to conchoidal. Quartz: Conchoidal.
- Tenacity: Brittle (both basalt and quartz).
- Luster Type: Basalt: Dull to sub-vitreous. Quartz: Vitreous.
Formation
Basalt is an extrusive igneous rock formed from the rapid cooling of basaltic lava at or near the Earth's surface. Quartz veins form later through hydrothermal processes. After the basalt has solidified and fractured, hot, silica-rich fluids circulate through these fractures. As these fluids cool, dissolved silica (SiO2) precipitates out, filling the cracks and forming quartz veins. The silica can originate from the basalt itself through alteration, or from external sources.
Usage
Primarily as a geological indicator for hydrothermal activity and potential mineralization (e.g., gold, silver, copper often associated with quartz veins in volcanic terrains). Less commonly used as an ornamental stone or aggregate, though the quartz veins can add aesthetic appeal. The basaltic component can be used as crushed stone for construction, road aggregate, or railway ballast.
Age Distribution
Variable, depending on the age of the basaltic extrusion and subsequent hydrothermal activity. Can range from Precambrian to Cenozoic.
Where to Find
Mid-Ocean Ridges
Hydrothermal vents on mid-ocean ridges are active sites of basalt alteration and quartz precipitation, though often in the form of chert or disseminated silica rather than macroscopic veins.
Volcanic Arcs
Regions with active or ancient subduction zones, where volcanic activity is prevalent, often exhibit extensive hydrothermal systems leading to quartz veining in volcanic rocks like basalt.
Continental Flood Basalt Provinces
Large areas covered by extensive basalt flows (e.g., Deccan Traps, Columbia River Basalt Group) can show localized hydrothermal alteration and quartz veining, particularly along fault systems.
Ophiolite Complexes
Remnants of ancient oceanic crust obducted onto continents, where basaltic rocks have undergone significant hydrothermal alteration.
Finding Tips
Look for Outcrops
Search in areas known for basaltic volcanism and evidence of past or present hydrothermal activity. Road cuts, stream beds, and coastal exposures can reveal fresh surfaces.
Identify Fractures
Quartz veins typically form in pre-existing fractures or fault zones within the basalt. Look for linear features or networks of lighter-colored material cutting across the darker basalt.
Examine Alteration Zones
The basalt immediately adjacent to quartz veins may show signs of hydrothermal alteration, such as discoloration, softening, or the presence of other secondary minerals (e.g., chlorite, epidote, pyrite).
Check for Associated Minerals
In some cases, the quartz veins may contain other minerals, including sulfides (pyrite, chalcopyrite), carbonates, or even precious metals, which can be indicators of economic mineralization.
Similar Rocks
Basalt
Basalt
Also known as: Volcanic rock
Andesite with Quartz Veins
Andesite with secondary quartz mineralization
Also known as: Silicified Andesite
Diorite with Quartz Veins
Diorite with secondary quartz mineralization
Also known as: Silicified Diorite
Scientific Classification
- Mineral Class
- Igneous rock (basalt) with silicate mineral (quartz) infillings
- Group
- Volcanic rock (basalt), Tectosilicate (quartz)
- Crystal System
- Basalt: Polycrystalline (various systems). Quartz: Trigonal.
- Chemical Formula
- Basalt: Complex silicate mixture (e.g., (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 for pyroxene, (Ca,Na)(Al,Si)4O8 for plagioclase). Quartz: SiO2.
- Composition
- Basalt: Mafic, low silica content (45-52% SiO2), rich in iron and magnesium. Quartz: Pure silica.
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