Hydrothermal deposits formed at low temperatures and low pressure. These are associated with large vein systems (Epithermal Veins) found in volcanic rock. The epithermal veins are always located over an intrusive rock structure whch has extruded to the surface in feeder stocks and and dykes. The vent areas are centers of intense fracturing and hydrothermal activitity. The veins normally contain base metal sulphides such as chalcopyrite, galena and sphalerite but are most important as sources for silver and gold which occur as native elements or in cpounds or as impurities in other minerals. The veins can be mined as high grade, low tonnage deposits, often called “bonanza” veins. In some places, the whole shooting match - country rock and veins are mined together in a bult, low grade, large tonnage operation.
