Skarn
Limestone recrystallised and chemically rebuilt where an intrusion cut through it — the world's main source of tungsten, and a significant one for copper, iron, zinc and molybdenum.
A skarn is what happens when a body of magma intrudes limestone. Heat alone would produce marble; what makes a skarn is that hot fluids leave the intrusion carrying silicon, aluminium, iron and metals, and react with the carbonate rock rather than simply baking it. The limestone's calcium stays put and everything else is exchanged, producing a coarse rock of garnet, pyroxene and other calc-silicate minerals that looks nothing like either parent.
The exchange is what concentrates the ore. Metals that were dispersed through a large volume of magma at parts-per-million levels are deposited into a narrow reaction zone, and the resulting grades can be very high over widths of only a few metres.
How it forms
The process is metasomatism — chemical replacement in the solid state, driven by fluid rather than by melting. It works because carbonate rock is unusually reactive. Silica-bearing fluid meeting calcite breaks the carbonate down, releases carbon dioxide and builds calcium silicates in its place, and the reaction front advances outward from the contact.
Which metals arrive depends on the intrusion. Granites that are chemically reduced and relatively evolved tend to produce tungsten and tin skarns; more oxidised intrusions give copper and iron; and the zinc and lead skarns generally sit furthest from the contact, where the fluid had cooled. That zoning is systematic enough to be used as an exploration guide — the mineral assemblage tells you which direction the intrusion lies in.
Economic significance
Tungsten is the clearest case of a metal whose supply is a skarn story. Scheelite in skarn is the dominant source, and the largest known deposits are of this type, so what the world does with tungsten carbide tooling ultimately rests on a particular kind of contact between granite and limestone.
Skarn deposits are also awkward to mine. They are irregular in shape, follow the geometry of a contact rather than a plane or a pipe, and can change grade over a few metres — so they reward selective underground methods far more than bulk open-pit extraction.
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
- Calcite — mineral · 5–40% · relict from the limestone the skarn replaced, surviving where the reaction ran short of silica
- Scheelite — mineral · the tungsten mineral, and the reason skarn is the dominant source of that metal worldwide
- Magnetite — mineral · the ore of the iron skarns, formed where an oxidised intrusion met carbonate rock
- Chalcopyrite — mineral · the copper of the proximal skarns, closest to the intrusion that supplied the fluid
- Sphalerite — mineral · with galena in the distal zones, deposited furthest out where the fluid had cooled
- Galena — mineral · the lead of the distal zones, generally alongside sphalerite rather than on its own
- Fluorite — mineral · where the intruding granite was fluorine-rich, which is also the case that favours tungsten
is sourced from
- Limestone — rock · carbonate rock rebuilt where an intrusion supplied heat and reactive fluid — the same protolith as marble, taken further
Sources
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- Material WorldOur own writing