skarn deposits
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Skarns 10/18/10
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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
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