veins

veins Veins are sheet-like, usually discordant, bodies formed by the infilling by minerals of fractures, such as joints or fault planes, bedding planes or solution cavities. They are epigenetic, that is, they were formed after the rocks in which they occur, and can be found in almost all types of rock. They are highly variable in width, ranging from a few millimetres to several metres, and may extend for considerable distances both vertically and horizontally. The walls, especially of secretion veins, may be parallel but are more commonly branching and display pinch-and-swell structure which depends on the lithology of the host rock and the nature of the fracture-forming process. Veins commonly display a regularity of pattern over wide areas (the vein system) which is related to that of the fractures or joints produced by extensional, shear, or hybrid stress systems.

The simplest veins are those formed by lateral secretion of material that has migrated out of the enclosing rock from the immediate vicinity of the vein. They thus reflect the nature of the host rock and are usually monomineralic. Quartz is the dominant mineral in veins in siliceous rocks, calcite in limestones, and gypsum in gypsiferous sediments. The minerals in these simple veins commonly show growth fibres whose orientation is related to the origin of the fractures that they occupy. For example, in extensional situations the fibres are usually perpendicular to the vein walls, but along shear fractures the fibres are oblique to the walls, indicating the orientation of the opening of the fracture; they may be curved, reflecting a change in orientation of the openings as crystallization proceeded.

Most veins, however, contain minerals deposited from solution migrating into the system from outside the area, either from above (supergene) or below (hypogene), or sometimes from both directions. The hypogene veins are deposited from water at temperatures varying from about 100 °C to 500 °C and are called hydrothermal veins. Here, too, the veins may be monomineralic, commonly quartz or calcite, but more usually are polymineralic with a mixture of metalliferous sulphide or oxide (ore) minerals and gangue minerals. The gangue minerals are usually of limited or no commercial value, but some, such as fluorite or barite, may be valuable. The minerals seen at or near the surface may not be those originally deposited but may instead be the result of alteration by near-surface oxidation processes.

These hydrothermal veins are often complex and banded, revealing a long history of repeated opening and reflecting the changing composition of the fluids with time. The walls are the nucleating surfaces with the first minerals growing on them; ore and gangue minerals may grow sequentially and symmetrically from the walls inwards producing crustification veins. Cavities or vugs in the centre of the veins make it possible for minerals to grow into free space forming large and well-formed crystals.

Veins may be zoned not only internally but also on a regional scale, their mineralogy changing both laterally and vertically away from the source body (usually a granite) as a result of declining temperature of the mineralizing fluids. A good example of this zoning and of the pattern of the vein systems is found in south-west England.

The temperature of formation of the minerals and the composition of the fluids can be determined by studying fluid inclusions, which are small bubbles of liquid trapped on the surface of the growing minerals at the time of formation. The origin of the vein-forming fluids—whether magmatic, meteoric, or connate—may also be determined from a study of the oxygen isotopes of the inclusions.

R. Bradshaw

Bibliography

Park, C. F. and and MacDiarmid, R. A. (1975) Ore deposits. W. H. Freeman, San Francisco.