Epithermal means "shallow and warm" (epi = near, thermal = heat). These deposits form in roughly the top kilometre of the crust, in volcanic regions — they are essentially the fossil plumbing of ancient hot springs and geysers. (A hot spring is a spot where groundwater, heated by magma below, rises and flows out at the surface; a geyser is a hot spring that every so often erupts a jet of hot water and steam — think of Yellowstone.)

Hot fluids rising toward the surface cool and boil, and boiling makes them drop their gold and silver, often into veins. The result can be spectacularly rich "bonanza" veins — though epithermal deposits are usually smaller than porphyries.

There are two styles — high-sulfidation (harsh, acidic volcanic fluids) and low-sulfidation (milder, deeper-circulating waters). You don't need the chemistry; just know epithermal gold-silver comes in these two flavours.

The big connection

Epithermal deposits are the shallow top of the same magmatic-hydrothermal system that makes porphyries at depth. Picture one tall plumbing system: a porphyry copper deposit forms 2–5 km down around the intrusion, and epithermal gold-silver forms near the surface above it. Same engine — different floor of the building.

Where: the Pacific "Ring of Fire" is epithermal country — the Comstock Lode (Nevada), Yanacocha (Peru), and Hishikari (Japan) are classics.