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Skarn Deposits

The Power of Fluids

Skarns •  Deposits formed at elevated temperatures by the addition

and subtraction of material (metasomatism) •  Most often developed at the contact of intrusive plutons

and carbonate country rocks. The latter are converted to marbles, calc-silicate hornfels and/or skarns by contact metamorphic/metasomatic 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 formation 400-650°C

•  pressures variable, form at depths of 1 to 10+ km •  some classic skarns are associated with porphyry

copper deposits, indicating a relatively shallow depth of emplacement

•  clinopyroxene, garnet and wollastonite common calc-silicate minerals

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Definitions •  skarns can be classified according to the

rocks they replace, and the terms exoskarn and endoskarn are applied to replaced or altered metasediments (usually carbonate) 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

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Classification-1

•  Exoskarns may be classified according to the dominant mineralogy: – magnesium - Mg - forsterite – calcic - Ca - andradite, diopside

•  Majority of the worlds economic skarn deposits occur in calcic exoskarns

Classification-2

•  Skarn deposits described according to the dominant economic metal or mineral present: –  e.g. copper, iron, tungsten, zinc-lead,

molybdenum, tin, talc etc.

•  Generally skarns are smaller than many other deposit types such as porphyries, lead-zinc, sediment-hosted sulfide deposits –  But they may carry quite high-grade ore

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Types of Skarn Deposits

•  Copper skarn deposits, especially those with by-product Au and/or Ag, have been worked in many parts of the world. – Large deposits may occur associated with

porphyry 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

(Mn)

(Fe+2)

(Mn + Fe+2)

(Fe+3) (Mg) (Al)

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Skarn Zonation •  Exoskarns show zoning of both silicate and ore

minerals

•  Endoskarn also displays zonation resulting from the progressive addition (“back flushing”) of calcium and CO2 into the igneous protolith.

•  A common zonation outwards towards the marble host is biotite - amphibole - cpx - garnet. K-feldspar may disappear but plagioclase commonly survives.

“Isochemical” Reaction or Bimetasomatic

Small-scale fluid

movement

‘All-in’ fluid buffered system

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Gain-loss: Santa Rita stock, NM

limestone

Gain-loss: Santa Rita stock, NM

shale

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Evolution of Skarns

Average Composition of Plutons

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Copper Skarn

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Zinc Skarn

Copper Canyon, Nevada

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Cu-Ag-Au: Copper Canyon NV

Iron Skarn - Russia

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Linchberg Mine, NM

Pine Creek - W Skarn

•  As a specific example, Pine Creek, CA was a world class tungsten source for many years.

•  Interesting “upside-down” mine •  “Special” underground weather •  Spectacular scenery

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Isotopic Reservoir Mixing

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Time-Temperature Evolution