A distinct band of soft, clay-like material or altered rock that forms at the exact contact point between a mineral vein and the host rock. It acts as a lubricating layer where fault movement has ground the rock into a fine paste, or where hydrothermal fluids have chemically altered the wall rock. Miners use this soft boundary to physically separate the valuable ore from the barren rock during extraction.
The vein breaks clean away from the hanging wall right along the selvage.
Also heard as
- gouge
- clay seam
Related terms
Lode
A defined vein or tabular deposit of valuable metallic ore embedded within barren rock.
Country rock
The pre-existing, non-mineralized rock that surrounds and encloses an orebody or igneous intrusion.
Hanging wall
The block of rock that lies above an inclined fault plane or ore body.
Footwall
The block of rock that lies on the underside of an inclined fault or mineral vein.
Gouge
A layer of soft, pulverized rock found along a fault plane or ore contact.