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Skarn Deposits
The Power of Fluids
Skarns
Deposits formed at elevated temperatures by the additionand subtraction of material (metasomatism)
Most often developed at the contact of intrusive plutonsand carbonate country rocks. The latter are converted to
marbles, calc-silicate hornfels and/orskarns by contact
metamorphic effects.
Skarn is an old Swedish mining term for silicate gangue Skarns have been termed tactites, hydrothermal
metamorphic, igneous metamorphic, contact metamorphic,
and pyrometasomatic (Lindgren)
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Generalizations
rough temperature of initial skarn formation400-650C
pressures variable, form at depths of 1 to 10+ km some classic skarns are associated with porphyry
copper deposits, indicating a relatively shallowdepth of emplacement
clinopyroxene, garnet and wollastonite commoncalc-silicate minerals
Definitions
skarns can be classified according to therocks they replace, and the terms exoskarn
and endoskarn are applied to replaced oraltered carbonate metasediments and
intrusive rocks, respectively
both endo- and exoskarns may contain ore However in a couplet where the exoskarn is a
converted marble then it usually contains
most or all the economic mineralization
Classification
Exoskarns may be classified accordingto the dominant mineralogy:
magnesium - Mg - forsteritecalcic - Ca - andradite, diopside
Majority of the worlds economic skarndeposits occur in calcic exoskarns
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Classification
Skarn deposits described according to thedominant economic metal or mineral present:
e.g. copper, iron, tungsten, zinc-lead,molybdenum, tin, talc etc.
Generally skarns are smaller than many otherdeposit types such as porphyries, lead-zinc,
sediment-hosted sulfide deposits
Types of Skarn Deposits
Copper skarn deposits, especially those withby-product Au and/or Ag, have been worked
in many parts of the world.
Large deposits may occur associated withporphyry copper deposits, e.g. Twin Buttes,
Arizona and Bingham Canyon, Utah.
Zn-Pb skarn deposits common but small Mo and Sn skarns of little importance
Skarn Zonation
Exoskarns show zoning of both silicate and oreminerals
Endoskarn also displays zonation resulting from theprogressive addition (back flushing) of calcium into
the igneous protolith.
A common zonation outwards towards the marblehost is biotite - amphibole - cpx - garnet. K-feldspar
may disappear but plagioclase commonly survives.
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Gain-loss: Santa Rita stock, NM
limestone
Gain-loss: Santa Rita stock, NM
shale
Evolution of Skarns
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Average Composition of Plutons Tectonic Settings
Tectonic Settings
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Copper Skarn Zinc Skarn
Copper Canyon, Nevada Cu-Ag-Au: Copper Canyon NV
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Gold SkarnGold Skarn X-section
Iron Skarn - Russia Linchberg Mine, NM
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Pine Creek - W Skarn
As a specific example, Pine Creek, CAwas a world class tungsten source for
many years.
Interesting upside-down mine Interesting underground weather Spectacular scenery
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Igneous Rx
Plagioclase Quartz K-feldspar Biotite
Endoskarn
Plagioclase Quartz K-feldspar Diopside Sphene (titanite) Scapolite
Skarn
Garnet Pyroxene Scheelite
Marble
Calcite Diopside Graphite? Vesuvianite? Grossular?
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