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Veined metamorphic rock is a broad descriptive term for any metamorphic rock that contains distinct, often planar or anastomosing, infillings of minerals, most commonly quartz (SiO2) or calcite (CaCO3), within fractures or shear zones. The host rock can be any type of metamorphic rock, such as schist, gneiss, slate, marble, quartzite, or amphibolite. The veins represent pathways where hydrothermal fluids migrated and deposited dissolved minerals. The texture of the veins can range from massive to crystalline, and their thickness can vary from hairline fractures to several meters wide. The contrast in color and texture between the vein material and the host rock often makes these features visually prominent.
How to Identify
- Color
- Host rock color varies widely (e.g., grey, black, green, red, white). Quartz veins are typically white, milky, or translucent. Calcite veins are typically white, cream, or light grey.
- Luster
- Host rock luster varies (e.g., dull, silky, vitreous, pearly). Quartz veins exhibit a vitreous (glassy) luster. Calcite veins exhibit a vitreous to dull luster.
- Texture
- Host rock texture is metamorphic (e.g., foliated, lineated, granoblastic). Veins are typically massive, granular, or crystalline, cutting across the host rock fabric.
- Crystal Form
- Quartz in veins often forms anhedral to subhedral crystals, sometimes euhedral if space allowed. Calcite in veins can form rhombohedral crystals, granular aggregates, or fibrous forms. Host rock minerals exhibit metamorphic crystal forms.
- Cleavage
- Quartz has no cleavage, but conchoidal fracture. Calcite has perfect rhombohedral cleavage in three directions. Host rock minerals will exhibit their characteristic cleavage patterns.
- Geological Environment
- Common in regions that have undergone significant tectonic deformation and metamorphism, such as mountain belts, subduction zones, and ancient cratons. Veins are often associated with fault zones, shear zones, and areas of intense fluid flow.
Key Facts
- Hardness: Quartz: 7 on Mohs scale. Calcite: 3 on Mohs scale. Host rock hardness varies.
- Specific Gravity: Quartz: 2.65 g/cm³. Calcite: 2.71 g/cm³. Host rock specific gravity varies.
- Crystal System: Quartz: Trigonal. Calcite: Trigonal. Host rock minerals vary.
- Color: Veins typically white, milky, or translucent. Host rock color is highly variable.
- Luster: Vitreous (quartz) to vitreous/dull (calcite).
- Transparency: Translucent to opaque (quartz and calcite veins).
- Fracture: Conchoidal (quartz). Uneven to subconchoidal (calcite).
- Cleavage: None (quartz). Perfect rhombohedral in 3 directions (calcite).
- Composition: Veins primarily SiO2 (quartz) or CaCO3 (calcite). Host rock composition is highly variable, reflecting its protolith and metamorphic grade.
Quick Check
- Color: Veins typically white/milky (quartz) or white/cream (calcite), contrasting with host rock.
- Luster: Vitreous (quartz) to vitreous/dull (calcite).
- Streak: White for both quartz and calcite.
Physical Characteristics
- Crystal Habit: Massive, granular, crystalline, sometimes euhedral crystals within veins. Host rock minerals exhibit metamorphic textures (e.g., schistose, gneissic, granoblastic).
- Cleavage Type: Quartz: None. Calcite: Perfect rhombohedral. Host rock minerals vary.
- Fracture Type: Quartz: Conchoidal. Calcite: Uneven to subconchoidal.
- Tenacity: Brittle (quartz and calcite).
- Luster Type: Vitreous (quartz) to vitreous/dull (calcite).
Formation
Veined metamorphic rocks form when pre-existing rocks (igneous, sedimentary, or other metamorphic rocks) are subjected to intense heat, pressure, and/or chemically active fluids, leading to recrystallization and the development of new mineral assemblages. The veins themselves form through the precipitation of minerals (most commonly quartz or calcite) from hydrothermal fluids circulating through fractures and fissures within the host metamorphic rock. These fluids are often generated during metamorphism, magmatic intrusion, or tectonic activity.
Usage
Veined metamorphic rocks are not typically used as a primary resource for the host rock itself, but the veins can be economically significant. Quartz veins can host valuable ore minerals (e.g., gold, silver, copper, lead, zinc) or industrial minerals (e.g., high-purity quartz for electronics). Calcite veins can be a source of calcium carbonate for cement, agriculture, or as decorative stone. The host rock, if durable and attractive, might be used as dimension stone or aggregate.
Age Distribution
Precambrian to Cenozoic, depending on the specific metamorphic event and host rock age.
Where to Find
Appalachian Mountains, USA
Extensive metamorphic terrains with numerous quartz and calcite veins, often associated with gold mineralization.
Canadian Shield, Canada
Ancient cratonic areas with widespread metamorphic rocks and associated vein systems, including significant gold and base metal deposits.
Alps, Europe
Young mountain belt with diverse metamorphic rocks and abundant quartz and calcite veins, some containing alpine-type mineralizations.
Western Cordillera, USA
Regions like the Sierra Nevada and Rocky Mountains exhibit extensive metamorphic rocks with various vein types, often associated with precious metal deposits.
Finding Tips
Look for Contrasting Textures
Veins often stand out due to their different color, texture, and mineralogy compared to the surrounding host rock. Look for linear or anastomosing features cutting across the rock fabric.
Identify Host Rock
Understanding the type of metamorphic host rock (e.g., schist, gneiss, marble) can provide clues about the geological setting and potential vein mineralogy.
Test Vein Minerals
Perform simple field tests: scratch test for hardness (quartz is harder than steel, calcite is softer), and acid test for calcite (calcite effervesces with dilute HCl). This helps distinguish quartz from calcite veins.
Examine for Associated Minerals
Look for other minerals within the veins or altered zones adjacent to the veins, as these can indicate the presence of valuable ore minerals or specific hydrothermal processes.
Safety Precautions
When collecting samples, be aware of unstable rock faces, especially in areas with extensive fracturing and veining. Always wear appropriate safety gear, including eye protection and gloves. If collecting in old mine workings, be extremely cautious of unstable ground and potential hazardous gases. Some vein minerals can contain trace amounts of heavy metals; wash hands after handling.
Similar Rocks
Hydrothermal Veins in Igneous Rocks
Varies depending on host rock and vein mineralogy
Also known as: Veined Igneous Rock
Hydrothermal Veins in Sedimentary Rocks
Varies depending on host rock and vein mineralogy
Also known as: Veined Sedimentary Rock
Pegmatite
Pegmatite
Also known as: Giant-grained igneous rock
Scientific Classification
- Mineral Class
- Quartz: Silicate. Calcite: Carbonate.
- Group
- Quartz: Tectosilicate. Calcite: Carbonate.
- Crystal System
- Quartz: Trigonal. Calcite: Trigonal.
- Chemical Formula
- Quartz: SiO2. Calcite: CaCO3.
- Composition
- Veins are predominantly silicon dioxide (quartz) or calcium carbonate (calcite). The host metamorphic rock's composition is diverse, depending on its protolith and metamorphic history.
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