Epithermal Veins

Epithermal vein deposits are large vein systems, usually in volcanic rocks. However, the deposits, the volcanic rocks that hold them, and the intrusive rocks that are always beneath them are the product of a single process. The volcanic rocks are extruded at the surface, but the volcanic vents have feeder stocks and dykes that ultimately cool to form intrusive rocks. The vent areas are centers of intense fracturing and hydrothermal activity, just like the areas around an intrusion.

The mineralized zones are usually veins in radial or concentric fractures, but it is also common for the mineralization to extend into the country rocks as disseminations and replacement bodies. The veins normally contain base metal sulphides like chalcopyrite, galena and sphalerite, but they are most important as sources of gold and silver, which occur as native elements, or in compounds or as impurities in other minerals. The veins also have worthless gangue material, like quartz, fluorite, calcite or dolomite.

Sometimes the narrow veins are mined as high-grade, low-tonnage deposits, and are often called “bonanza” veins for these high grades. In other places, the vein and the country rocks are mined together in bulk in large, low-grade operations.