Epithermal vein deposits are large vein systems, most of the time in volcanic rocks. however, the deposits and the volcanic rocks that hold them, and the intrusive rocks that are always under 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 create intrusive rocks. the vent areas are centers of intense fracturing and hydrothermal activity, very similar to the areas which surround an intrusion.
The mineralized areas are most of the time veins in concentric or radial fractures, but it is common for the mineralization to extend itself on the country rocks as dissemiations and replacement bodies. The veins most of the time contain base metal sulphides like sphalerite, galena and chalcopyrite, but they are most important as sources of silver and gold, which occur as native elements, or in compounds, or as impurities in other minerals. The veins also have gangue material which is considered worthless, like dolomite, calcite, quartz or fluorite.
There are times that the narrow veins are mined as high-grade, low-tonnage deposits, and often happen to be called “bonanza” veins for these high grades. In other sectors or areas, the veins and the country rocks are mined together in bulk in large, low-grade operations.