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Material World
Deposit type

Epithermal vein

Ore deposited from hot water in a fracture near the surface — where mercury, antimony and much of the world's gold and silver are found.

Ore

An epithermal deposit is what a hot spring leaves behind underground. Water heated by a magma body at depth circulates upward through fractures, dissolving metals as it goes, and drops them where the pressure falls and the fluid boils — within a kilometre or so of the surface, at temperatures well under 300 °C.

The result is a vein: a fracture filled with quartz and whatever the fluid was carrying, sharply bounded against the rock on either side. That sharpness is why these were the first deposits mined anywhere. A vein is visible, it can be followed, and the grade inside it can be a hundred times the rock around it.

How it forms

Boiling is the mechanism that matters. Metals travel in solution as complexes with sulfur or chlorine, and those complexes are stable only while the fluid stays under pressure. Where a rising fluid crosses the boiling point the gas separates, the chemistry changes abruptly, and gold, silver and the sulfides come out of solution together over a very short vertical interval.

That is why epithermal deposits are so strongly zoned, and why a mine can pass from ore to nothing in a few tens of metres. It is also why the surface expression — sinter, silicified ground, a hot spring still running — is one of the more reliable exploration signals there is.

Mercury sits at the top of the sequence, being volatile and travelling furthest; antimony and arsenic below it; then silver and gold; then the base metals deeper still.

Economic significance

Epithermal veins supplied most of the precious metal in history. Comstock, Cripple Creek, Potosí, Hishikari and the Carpathian districts are all this kind of deposit, and the reason gold and silver were mined thousands of years before copper was mined at scale is that a vein can be worked with hand tools and a porphyry deposit cannot.

They are small by modern standards and rich, which inverts the economics of a bulk mine: an epithermal operation is selective, underground, and vulnerable to the ore simply stopping.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

is composed of

  • Quartz — mineral · the gangue that fills most of the vein, and the reason a quartz vein is what a prospector looks for
  • Cinnabar — mineral · at the top of the sequence, mercury being volatile enough to travel furthest before the fluid drops it
  • Stibnite — mineral · with arsenic below the mercury and above the precious metals, in the same zoned column
  • Acanthite — mineral · the silver sulfide, deposited where the fluid boiled — which is why epithermal silver ore stops so abruptly with depth
  • Cobaltite — mineral · in the deeper and hotter veins, with the nickel and arsenic minerals it usually accompanies
  • Fluorite — mineral · common gangue in the cooler veins, and mined from them where there is enough of it
  • Baryte — mineral · the other common gangue, and the reason vein deposits supply a share of the world's drilling-mud barite

is sourced from

  • Gold — element · the deposit type most of the gold mined before the twentieth century came out of, and still a major share

Sources

  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
  • Material World
    Our own writing

Where it comes from, and what it becomes

Follow Epithermal vein back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Epithermal vein → is sourced from (the deposit type most of the gold mined before the twentieth century came out of, and still a major share) → Gold → is sourced from (a significant share of world gold arrives this way, recovered from copper anode slimes at no mining cost of its own) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation
  • Epithermal vein → is composed of (the gangue that fills most of the vein, and the reason a quartz vein is what a prospector looks for) → Quartz
  • Epithermal vein → is composed of (common gangue in the cooler veins, and mined from them where there is enough of it) → Fluorite
  • Epithermal vein → is composed of (at the top of the sequence, mercury being volatile enough to travel furthest before the fluid drops it) → Cinnabar
  • Epithermal vein → is composed of (in the deeper and hotter veins, with the nickel and arsenic minerals it usually accompanies) → Cobaltite
  • Epithermal vein → is composed of (with arsenic below the mercury and above the precious metals, in the same zoned column) → Stibnite

These are the most distinct paths back. Epithermal vein can be traced through others besides.