VMS

A Volcanogenic Massive Sulfide (VMS) deposit results from volcanic action, usually on the sea floor, that concentrates sulfide ores. The lava is typically of felsic (high silica) composition and thus forms rhyolite. Ores include chalcopyrite, sphalerite/galena, and even some native gold, in order of decreasing depth. These usually occur in the ocean, but sometimes also in folded mountain ranges, where tectonic forces have brought the ancient sea floor into the upper reaches of the crust.