The deposits form within 1 km of the surface and water temperatures are about 50-200 degrees C. Shallow bodies of magma supply heat. The rising hot water carries dissolved gold and other elements. The water boils about 300 m below the surface and hydrogen sulfide gas escapes. This causes the gold to precipitate. The boiling zone is the target for mineral exploration.
Veins commonly host the economic minerals.
Diagram from ITAM Gold by the Minerals Council of Australia. Lihir Island in Papua New Guinea is an epithermal deposit discovered in 1982. The island is made of three volcanoes including Luise caldera, where the deposit formed. The rocks are trachybasalt lava flows, breccia, and tuffs. The mineralized rocks are highly altered.