Why can epithermal veins contain very rich gold and silver ores?

Epithermal veins are hydrothermal deposits that form at shallow depths, typically beneath a boiling hot spring system. These veins are often referred to as "bonanzas" because they can be exceedingly rich in gold and silver.1

The high concentration of precious metals in these deposits is primarily driven by the behavior of the hydrothermal solutions as they rise toward the surface. As these solutions flow through open fissures, the precipitation of minerals—which concentrates the gold and silver—is triggered by specific physical and chemical changes, most notably boiling, cooling, or chemical reactions between the solution and the surrounding rock. While these deposits often lack great vertical continuity, the efficiency of these precipitation processes in shallow, boiling environments allows for the formation of exceptionally high-grade ore zones.1