selvage

In mining, a selvage is the band of altered or mineralized rock that forms along the contact between a vein or lode and the surrounding host rock, known as country rock. It marks the boundary zone where hydrothermal fluids have chemically modified the material on either side of the ore body. Selvages vary in width from millimeters to meters, depending on the intensity and duration of the mineralizing process.

The composition and character of a selvage depend on what fluids passed through and what host rock they encountered. A quartz vein cutting through granite might develop a selvage of clay minerals and sericite where acidic fluids leached feldspars. Base metal veins in limestone often show a selvage of skarn, a calcium silicate rock formed by reaction between the intrusive fluids and carbonate host rock. Some selvages are barren; others carry economic minerals in concentrations rivaling the main vein.

Recognition and practical significance

Geologists and mine geotechnicians use selvage zones as markers when mapping and sampling. A well-developed selvage makes the vein boundary sharp and traceable; its absence or weakness may indicate a vein that dies out or becomes too narrow to mine economically. Assayers sometimes sample the selvage separately from the core of a vein, because it can show whether mineralization extends beyond the visible ore or whether it represents a barren halo.

In hard rock mining, selvage strength affects stability. A clay-rich selvage may be weaker and more prone to slippage than fresh country rock, which influences design of stope walls and pillar dimensions. In underground coal mining, the term is sometimes applied to the thin band of shale or dirt directly above or below the coal seam, though this usage is regional and less precise than the metal mining definition.

The word comes from the textile industry, where selvage (or selvedge) refers to the finished edge of a woven fabric. The transfer to mining is logical: both denote the defined boundary or edge of a useful material, the point where one substance meets another.