Quartz Veined Metamorphic Rock

Description

Quartz veined metamorphic rock refers to any metamorphic rock that has been intruded by or contains discrete bodies of quartz, typically in planar or anastomosing structures known as veins. The metamorphic host rock can be of various types, such as schist, gneiss, slate, phyllite, quartzite, or marble, each exhibiting its characteristic metamorphic fabric (foliation, lineation) and mineral assemblage. The quartz veins themselves are composed predominantly of macrocrystalline quartz (SiO2), which can range from milky white to clear, and may contain minor amounts of other minerals (e.g., sulfides, carbonates, micas) that precipitated from the hydrothermal fluids. The veins often cut across the metamorphic fabric of the host rock, indicating their formation post-dating or syn-dating the main metamorphic event.

How to Identify

Color

The color of the metamorphic host rock varies widely (e.g., grey, green, black, red, white) depending on its mineralogy. The quartz veins are typically white, milky, or translucent to clear, though they can be stained by iron oxides (yellow, brown, red) or contain other colored minerals.

Luster

The luster of the metamorphic host rock varies (e.g., dull, silky, vitreous, pearly) depending on its constituent minerals. The quartz veins exhibit a vitreous (glassy) luster.

Texture

The texture of the metamorphic host rock is typically foliated (e.g., slaty cleavage, schistosity, gneissic banding) or granoblastic (e.g., in quartzite or marble). The quartz veins have a crystalline texture, often massive, granular, or comb-structured, with individual quartz crystals visible.

Crystal Form

Quartz in veins typically forms anhedral to subhedral crystals, often interlocking. Larger, euhedral crystals can occur in vugs or open spaces within the veins. The minerals in the metamorphic host rock exhibit forms characteristic of their metamorphic grade and protolith.

Cleavage

Quartz itself lacks cleavage, exhibiting conchoidal fracture. The metamorphic host rock may exhibit cleavage (e.g., slaty cleavage in slate, schistosity in schist) or no cleavage (e.g., in quartzite or marble).

Geological Environment

Found in regions that have undergone regional metamorphism (e.g., mountain belts, cratonic shields) or contact metamorphism (around igneous intrusions). Veins are common in fault zones, shear zones, and areas of intense fracturing within these metamorphic terrains.

Key Facts

• Hardness: Host rock hardness varies (e.g., 3-7 for common metamorphic minerals); Quartz veins are 7 on Mohs scale.
• Specific Gravity: Host rock specific gravity varies (e.g., 2.6-3.5); Quartz veins are 2.65 g/cm³.
• Crystal System: Host rock minerals vary; Quartz is trigonal.
• Color: Variable for host rock; white, milky, or clear for quartz veins.
• Luster: Variable for host rock; vitreous for quartz veins.
• Transparency: Host rock can be opaque to translucent; Quartz veins are translucent to transparent.
• Fracture: Host rock fracture varies; Quartz exhibits conchoidal fracture.
• Cleavage: Host rock may have cleavage; Quartz has no cleavage.
• Composition: Host rock composition is highly variable (e.g., silicates, carbonates); Quartz veins are primarily SiO2 (silicon dioxide).

Quick Check

• Color: Variable for host rock; white, milky, or clear for quartz veins.
• Luster: Variable for host rock; vitreous (glassy) for quartz veins.
• Streak: White (for quartz and most common metamorphic minerals).

Physical Characteristics

• Crystal Habit: Host rock minerals vary; Quartz in veins is typically massive, granular, or prismatic.
• Cleavage Type: Host rock minerals may exhibit various cleavage types (e.g., basal, prismatic); Quartz has no cleavage.
• Fracture Type: Host rock fracture varies; Quartz exhibits conchoidal fracture.
• Tenacity: Host rock tenacity varies; Quartz is brittle.
• Luster Type: Host rock luster varies; Quartz is vitreous.

Formation

Quartz veins in metamorphic rocks form through the precipitation of silica (SiO2) from hydrothermal fluids circulating through fractures, faults, or shear zones within pre-existing metamorphic rocks. These fluids are typically generated during regional or contact metamorphism, where elevated temperatures and pressures cause dehydration reactions in minerals, releasing water and dissolved silica. The silica-rich fluids migrate along pathways of least resistance, and as pressure and temperature decrease, or as chemical conditions change, quartz crystallizes to fill these open spaces. The metamorphic host rock itself forms from the transformation of pre-existing igneous, sedimentary, or other metamorphic rocks under conditions of elevated temperature and pressure.

Usage

Quartz veined metamorphic rocks are not typically used as a primary resource for the rock itself, but the quartz veins within them can be economically significant. These veins are often sources of industrial quartz (for ceramics, abrasives, electronics), and can also host valuable ore minerals such as gold, silver, copper, and lead, which precipitate alongside or within the quartz. The host metamorphic rock may be used as aggregate or building stone if it possesses suitable physical properties.

Age Distribution

Precambrian to Cenozoic, depending on the metamorphic event and protolith age.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Silicate (for quartz and many host rock minerals)

Group

Tectosilicate (for quartz)

Crystal System

Trigonal (for quartz)

Chemical Formula

SiO2 (for quartz); Host rock is a complex mixture of minerals.

Composition

Primarily silicon dioxide (quartz) within the veins, hosted within a rock composed of various metamorphic minerals (e.g., micas, feldspars, amphiboles, garnets, chlorite, calcite, etc.).

Explore Quartz Veined Metamorphic Rock