Prograde skarn stage• Fluid exsolution from cooling magma• Reaction of the magmatic fluid, usually Si- and Fe-rich, with

carbonatesCaO(s) + FeO(aq) + 2SiO2(aq) --> CaFeSi2O6

hedenbergite 3CaO(s) + Fe2O3(aq) + 3SiO2(aq) --> Ca3Fe2(SiO4)3

andradite

• Formation of anhydrous Ca-silicates (mostly garnet, pyroxene) at T ~750°-400°C. Magnetite forms in magnesian skarns

• Mineralogical zoning• Fluid salinities >30% eq. NaCl in garnet and pyroxene • Oxygen isotopes of garnet and pyroxene compatible with

magmatic fluid

• Skarn alteration of the country rock, generally carbonate

• May be classified according to the dominant mineralogy

• Most of the world’s major skarn deposits are in calcic exoskarns

• Usually show a zoning of both silicate and ore minerals

Exoskarn

• Skarn alteration of intrusive rock

• In classic carbonate intrusive contact, this would be the intrusive rock altered to calc-silicate

• May show zoning from potassic minerals (primary?) to calcic at contact

• Biotite -> amphibole -> pyroxene -> garnet (towards marble)

• May merge with the exoskarn

Endoskarn

Prograde stage mineral zoning

Cu skarnsproximal distal

GARNETbrown green-yellow

PYROXENElight color(high-Mg)

dark color(high-Fe)

Retrograde skarn stage

Cooling, involvement of meteoric fluids, oxidizing, CO2-rich prograde anhydrous minerals substituted by hydrous silicates (e.g., chlorite, epidote, amphibole), iron oxides (magnetite, hematite), calcite, quartz

5 CaFeSi2O6 + H2O + 3CO2 --> Ca2Fe5Si8O22(OH)2 + 3CaCO3 + 2SiO2

hedenbergite actinolite

3CaFeSi2O6 + 1/2O2 + 3CO2 --> Fe3O4 + 3CaCO3 + 6SiO2

hedenbergite magnetite

Ca3Fe2(SiO4)3 + 3CO2 --> Fe2O3 + 3CaCO3 + 3SiO2

andradite hematite

Continental Mine (New Mexico, USA). Intense retrograde envelopes on veins cutting through prograde garnet (red-brown color) and pyroxene (light tan color) skarn. In more widespread retrograde alteration, the vein envelopes would coalesce and completely obliterate the prograde garnet and pyroxene.

Retrograde Alteration 

Retrograde skarn stage

T<400°C

Salinities <25% eq. NaCl

Oxygen and hydrogen isotopes of retrograde minerals indicate participation of meteoric fluids

Irregular geometry, often larger volume than prograde skarn

Skarn types

Calcic skarns: Ca- and Ca-Fe(-Mg)-silicates (wollastonite, vesuvianite, andradite, hedenbergite)

Magnesian skarns: Mg- and Ca-Mg(-Fe)-silicates and oxides/hydroxides (forsterite, diopside, talc, brucite, periclase, magnetite)

Skarn types

Reduced skarnsFe2+-bearing silicates (hedenbergite>>andradite)

FeO(aq) + 2SiO2(aq) + CaCO3 --> CaFeSi2O6 + CO2

diopside

Oxidized skarnsFe3+-bearing silicates (andradite>> hedenbergite)

2FeO(aq) + 3SiO2(aq) + 3CaCO3 + 1/2O2 --> Ca3Fe2(SiO4)3 + 3CO2

andradite

Reduced vs. oxidized skarns

Organic matter

CaO(s) + C(s) + FeO(aq) + 2SiO2(aq) + O2 --> CaFeSi2O6(s) + CO2(g)

hedenbergite

3CaO(s) + 2FeO(aq) + 3SiO2(aq) + 1/2O2 --> Ca3Fe2(SiO4)3(s)

andradite

Magma chemistry

Magma fO2

fO2

Hematite

Titanite

Magnetite

Ilmenite

Most porphyry-Cu plutons

Bio hfelsPx hfels

Calcareoussiltstone

limestone

dolomite

gar

gar

woam

ph

px

px

ol ol

serp

serp

Ca-sil (gar)

Px h

fels

Reduced environment Oxidized environment

ZONATION AND HOST ROCKS

Space-Time zonationMineralogy

• Gar/px ratio• Gar color +

composition• Px color +

composition• Distal pyroxenoid-

vesuvianite

Metal ratios

• Proximal: Cu±Mo±W

• Distal: Mn-Pb-Zn-Ag-Au

Present in almost all skarns

Garnet

Pyroxene

Amphibole

Quartz 

Other Common

Olivine

Wollastonite

Epidote

Actinolite 

More Local

Scapolite

Zeolite

plagioclase

Skarn Mineralogy 

Ore stages in skarn• Prograde stage: scheelite, magnetite

(magnesian skarns), sulfides (after scheelite) CaCO3 + H2WO4 --> CaWO4 + H2O +CO2

• Retrograde stage: most sulfides, gold and 2nd generation scheelite

Many (associated with porphyry copper deposits)

Calc-alkaline

Granodiorite to monzogranite stocks

Continental marginsMesozoic and tertiary western AmericasCarboniferous arcs in western Russia

Island arcsQuartz diorite to monzogranite

Copper Skarns

Cu skarn zonation

Skarn geotectonic setting